Search

Rare earth elements (REE) are well known tracers of geochemical processes in marine shales and have been used extensively in provenance studies and as markers of paleo-oceanographic conditions at the time of deposition. The application of REE hinges on the premise that REE are relatively immobile during sedimentary processes associated with sediment transport and deposition. However, recent studies have shown that diagenesis can substantially affect REE and trace element distribution patterns in sediments, leading to their non-conservative behaviour. This study investigates the patterns of diagenetic remobilization of REEs in the whole-rock and labile fraction of black shales of the Upper Ordovician Utica Shale magnafacies (USM) of Québec, Ontario, and New York. The time correlative shale units are of different thermal grades, ranging from thermally immature in Québec (T max 20–50 °C), to mature oil-bearing in Ontario (T max 50–140 °C), to post-mature gas-bearing in New York (T max > 200 °C). The results show that the whole-rock maintains a flat 'continental-type' rare earth element abundance pattern, whereas the labile fraction, comprising carbonate, phosphate, sulfide minerals, and organic matter, shows variable middle-REE (MREE) enriched patterns relative to the light and heavy rare earth elements (LREE + HREE). The MREE enrichment increases with level of thermal maturity, suggesting that the overall REE patterns in the labile fraction of these black shales may instead reflect diagenetic remobilization rather than paleo-seawater REE patterns. Correlation of trace element paleoredox indicators (Ni/Co, U/Th, and V/Cr) with MREE enrichment tracers (La/Sm and Gd/Yb), as well as Ce/Ce*, suggests diagenetic overprint of traditionally used paleoproxies due to mobilization of trace elements during thermal maturation. The results reported here suggest that fractionation of trace elements including REE during thermal maturation of black shales have significant effects on REE patterns in whole-rock and labile shale fractions. This finding has important implications for the use of trace elements in paleoenvironmental reconstructions for sedimentary rocks that have undergone thermal maturation. [ABSTRACT FROM AUTHOR]

The influence of paleoredox conditions and water-mass restriction on trace metal (TM) accumulation in the lower (LBS) and upper black shales (UBS) of the Devonian–Mississippian (D–M) Bakken Formation was assessed utilizing V/Cr, V/(V + Ni), Ni/Co, U/Th, and enrichment factors (EF) for Mo vs. U. These assessments were compared to previously published estimates based on degree of pyritization (DOP T), sulfur‑iron and C–S–Fe relationships, and sedimentological data. Bi–metal ratios suggest >80 % of samples were deposited under suboxic to anoxic/euxinic conditions, whereas DOP T results suggest >60 % of samples were deposited under dysoxic (0.42–0.75 DOP T) bottom–water conditions. DOP T results are in good agreement with C–S–Fe and total S vs. Fe assessments of paleoredox conditions and sedimentological evidence. Good agreement is also seen between Mo/TOC and Mo EF/U EF that both suggest the Bakken shales were deposited under a relatively unrestricted water mass conditions resulting in consistent renewal of TMs into the basin. With TM resupply outpacing drawdown, the basin could support enhanced primary productivity as well as high concentrations of TMs. Trace metal concentrations for the Bakken Fm. show considerable range for Co (0–10,324 ppm), Mo (0–2018 ppm), Ni (0–1574 ppm), U (0–1604 ppm), and V (0–3194 ppm), and bi–metal ratios for the Bakken Fm. are up to 5× greater than those reported for other D–M black shale formations. Bivariate and principal component analysis indicate Mo, Ni, U, and V accumulation was more closely associated with the organic fraction than with reducing conditions, thus accumulation of organic matter during deposition and generation, migration, and biodegradation of hydrocarbons during diagenesis may have influenced the accumulation and distribution of these TMs. Furthermore, changes in biogeochemical processes during the D–M transition may have fundamentally altered marine chemistry, with the evolution of vascular land plants and enhanced terrestrial weathering leading to an increased influx of nutrients into marine waters resulting in the Annulata, Dasberg, and Hangenberg black shale events. Factors controlling TM accumulation during time of deposition (e.g., TM availability, bottom-water redox conditions, adsorption onto organic matter) and during diagenesis and catagenesis (e.g., alteration and break down of organic matter, movement of fluid hydrocarbons or other basinal fluids) likely contribute to the lack of agreement between redox proxies, and subsequently, the lack of applicability of bi–metal ratios in assessing bottom–water conditions for the Bakken shales. Therefore, it is suggested that these bi-metal redox proxies are not an effective means to describe bottom-water conditions during the deposition of the Bakken shales. [ABSTRACT FROM AUTHOR]

Fossil bone from deep time is not only permineralized but strikingly preserves its histological integrity. Exceptions are rare and have not been systematically studied, especially regarding the effects of late diagenesis and low-grade metamorphism experienced by many fossils from Early Mesozoic and Paleozoic rocks and from orogenic belts. Bone samples of 26 specimens of marine reptiles from two Middle and Late Triassic black shale Lagerstätten (Xingyi Fauna and Guanling Biota) of southwest China were examined from a mineralogical perspective by polarized light microscopy, BSE-SEM imaging including cathodoluminescence, EPMA element mapping, and Raman spectroscopy. In addition, paleotemperature was determined from carbonaceous matter via Raman spectroscopy. Preservation ranged from typical fossil bone fluorapatite (FAp) to heavy alteration with loss of histological structure. The mineral fluorite in the Xingyi samples and a continuous bone apatite phase transformation from FAp to recrystallized apatite (RAp) were observed in fossil bone for the first time. Peak diagenetic temperatures were elevated beyond the oil window to low-grade metamorphism (177–209 °C), consistent with previous regional studies of coal rank in Guizhou Province. Our results suggest a correlation between the duration of exposure to high temperature and increased alteration levels of bone apatite crystals. We defined five stages of preservation in the two lagerstätten and propose a model for increasing diagenesis and metamorphic alteration of bone histology. Stage I represents no alteration beyond diagenetic mineral precipitation in bone pores. Stage II shows slight loss of apatite crystallite birefringence and beginning of fluorite precipitation. Stage III encompasses the migration of kerogen from host shale into fossil bone, further loss of bone histological structure, and more fluorite precipitation. Stage IV shows recrystallization of FAp to idiomorphic RAp. Stage V is the complete loss of bone matrix structure and replacement by RAp. We also address the correlation of alteration processes with temperature and the sources of F and Ca that formed the fluorite. This study provides reference results for different stages of bone alteration during real geo-history (as opposed to experimental or archaeological time frames), which is essential for studying bone diagenetic processes resulting from protracted exposure to high temperatures and fluids. [Display omitted] • Fossil bone alteration associated with low-grade metamorphism is described. • Continuous apatite phase transformation is reported first time in fossil bones. • We detect fluorite mineralization in fossil bone for the first time. • We establish a sequential model with five alteration stages of the fossils. • Hydrothermal fluids and "baking time" determine the different alteration stages. [ABSTRACT FROM AUTHOR]

The latest Proterozoic to early Cambrian is characterized by major global phosphogenic events and the diversification of complex multicellular life. However, the feedback mechanism of atmospheric oxidation and upwelling processes to form voluminous phosphorite deposits is unclear. South China is a key example of the sedimentary spectrum related to the Neoproterozoic to Early Cambrian bloom of oxygenic photosynthesis, with alginite deposits (sapropelite; "stone coal"), barite, polymetallic black shales and phosphorites along the passive continental paleo-margin of the Yangtze craton. Here, we present a comprehensive geochemical analysis of drill core samples from the Zhijin phosphorite deposit, including Cr isotope, Fe speciation, and bulk elemental composition of dolostone, phosphorite and black shale from the Ediacaran to Early Cambrian sequence. The positively fractionated δ53Cr values of 0.11 to 1.81 ‰ of leachates from Ediacaran dolostone and Terreneuvian phosphorite indicate intensive oxidative terrestrial weathering. The phosphorite samples exhibit REE distribution patterns similar to modern seawater and strongly negative Ce anomalies (0.47–0.61), suggesting an oxic, open ocean environment. In contrast, the absence of Ce anomaly, high Fe HR /Fe T , and Fe py /Fe HR ratios, enhanced redox-sensitive element concentrations, high Mo/TOC and TS/TOC, and high molar C org :P ratios suggest euxinic to anoxic conditions for the overlying black shale sedimentation in a restricted sedimentary marine environment. Bulk-rock shale samples retain the positively fractionated Cr isotope pattern. The dramatic change from oxic to anoxic conditions aligns with the fossil record, which shows a proliferation of small shelly fossils in the phosphorite and their extinction in the overlying black shales, likely by hydrogen sulfide poisoning. We infer that nutrient-rich, oxidized upwelling seawater transported phosphorus and other nutrients to the photic zone, from where organic matter settled down to the seafloor and formed thick phosphorite beds at the oxic-anoxic boundary at and below the seafloor. Our study highlights that atmospheric and seawater oxygenation accelerated the Early Cambrian bio-diversification, and the subsequent restricted basin situation is responsible for the extinction at the Neoproterozoic-Cambrian boundary. [ABSTRACT FROM AUTHOR]

The Bakken Formation is a hydrocarbon-rich, metalliferous shale with subordinate sandstone and limestone that was deposited in the Williston Basin in Canada and the USA. Deposition of the Bakken Formation spanned the Devonian-Carboniferous boundary – a key interval in Earth's history representing profound biotic and geochemical changes in the terrestrial and marine realms, leading to deposition of metal-rich shales in the lower part of the sequence. Most studies of metal enrichment in the Bakken Formation have focused solely on the bulk rock litho-geochemistry, with little attention given to the organic matter, mineralogical, and diagenetic controls on metal sequestration and distribution. The current study combined bulk sulfur isotope and litho-geochemistry, along with reflected light and scanning electron microscopy (SEM) petrography to elucidate organic matter and mineralogical controls on trace element geochemistry. We have also used laser ablation-inductively couple plasma-mass spectrometry (LA-ICP-MS) to quantitatively assess the distribution of trace elements among different mineral phases, shale matrix, and organic matter. We show that sphalerite is the main zinc host, and that sphalerite sequesters Cu, Ga, Ge, and Cd within its crystal structure. Pyrite is the main Fe mineral and has typical abundances for sedimentary pyrite, however, the total bulk rock abundances of Ni, Mo, and Cu cannot be accounted for in pyrite alone, and indeed, are present within early mature organic matter in the shale matrix. Vanadium (V) is associated with organic carbon but is also high in aluminosilicates within the matrix. Collectively, this study demonstrates the need for detailed petrographic analyses even in seemingly monotonous black shales. [ABSTRACT FROM AUTHOR]

The Lower Cambrian black shale has attracted much attention due to its role in elucidating the chemical evolution and sulfur cycles of the global ocean, as well as its economic significance as a source of phosphate, barite, and high-grade Ni Mo polymetallic sulfide ores. However, the occurrence of abnormally high metal accumulation within thin ore layers across extensive areas remains contentious. Pyrite, that occurs as framboids or fine- to coarse-grained euhedral-subhedral crystals, is a ubiquitous sulfide found in both host black shale and metalliferous ore horizons. This study delineates a three-stage growth of pyrite within the Chuanyanping and Sancha Ni Mo sulfide layers in western Hunan Province, South China, based on the micro-textures, chemical compositions, and sulfur isotopes. The earliest pyrite (PyI) appears as framboids (3–7 μm) or as small euhedral-subhedral grains (<20 μm). It exhibits a depletion in trace elements and negative δ34S V-CDT values spanning from −34.2 to −17.4 ‰, indicating the formation in a euxinic water column via microbial sulfate reduction (MSR) during syngenesis. The later pyrite (PyII), which is associated with gersdorffite, appears as thin overgrowth rings encircling PyI or euhedral to subhedral crystals displaying complex oscillatory and lacy zoning patterns. It is characterized by elevated Cu (up to 5.84 wt%), Ni (up to 3.56 wt%), and As (up to 1.70 wt%) concentrations and highly variable sulfur isotopic compositions. The overgrowth rings exhibit δ34S V-CDT values spanning from −9.6 to 12.4 ‰, while the euhedral to subhedral crystals have inhomogeneous sulfur isotopic signatures, with the cores enriched in heavy sulfur (31.7 to 33.7 ‰ δ34S V-CDT) and the rims marked by comparatively light sulfur (−7.1 to 7.2 ‰ δ34S V-CDT). This isotopic variation reflects the joint effect of thermochemical sulfate reduction (TSR) and hydrothermal-derived sulfur. Specifically, the initial influx of high-temperature hydrothermal fluids produced minimal isotopic fractionation between sulfide and seawater sulfate via TSR, facilitating the formation of heavy‑sulfur PyII. Subsequently, hydrothermal sulfur became the dominant source for the deposition of light‑sulfur PyII, with a minor contribution from dissolution of PyI. Given the close association of organic matter with ores, hydrocarbons may significantly facilitate metal transport via organometallic complexing, with Ni primarily derived from hydrothermal fluids and Mo from seawater as indicated by previous Ni and Mo isotopic analyses. The barren pyrite (PyIII), which presents as overgrowth matrix cementing early PyI and PyII, has positive δ34S V-CDT values (15.5 to 34.6 ‰), suggesting formation via MSR in closed, sulfate-limited environment, most likely following deep burial. Collectively, the chemical and sulfur isotopic evidence, coupled with petrographic observations, highlights the crucial role of hydrothermal impulse in the formation of hyper-enriched black shale. [Display omitted] • Three-stage episodes of pyrite growth observed during the syngenesis to diagenesis. • Pyrite enriched in Cu and Ni was produced by hydrothermal fluid influx. • Lower Cambrian hyper-enriched black shale formed from interactions among seawater, hydrothermal fluids, and hydrocarbons. [ABSTRACT FROM AUTHOR]

Stratigraphic successions are rarely sampled at sufficiently dense spacing to permit evaluation of paleoenvironmental variation at the shortest fundamental frequency operative in the depositional system of interest, often because that frequency remains undetermined. Here, we provide an example of a sampling strategy optimized for recovery of high-frequency paleoenvironmental signals, based on (1) full-core analysis using a continuous survey tool such as X-radiography, followed by (2) fine sampling of selected small-scale cycles that record significant short-term paleoenvironmental variation. We show that this strategy results in recovery of high-frequency paleoenvironmental signals that are completely missed by low-density sampling protocols, permitting key insights into depositional and early diagenetic processes that may otherwise by overlooked. The present case study is based on C-S-Fe systematics of the Cleveland and Sunbury shales, which belong to the Upper Devonian-lowermost Carboniferous black shale succession of the Central Appalachian Basin in eastern North America. In these units, the fundamental scale of paleoenvironmental variation is recorded by pervasive decimeter-scale (~5- to 20-cm-thick) compositional cycles that are readily visible in outcrop, in which high-organic layers weather out as resistant ledges. We identified these cycles in drillcores using X-radiography, which proved to be an effective proxy for compositional variation owing to control of bulk density primarily by total organic carbon (TOC) content. We then sampled selected dm-scale cycles (n = 21) at a centimeter spacing, generating short, high-resolution, multiproxy chemostratigraphic records including TOC, sulfur and Fe concentrations, Fe py /Fe HR (a redox proxy), δ34S pyrite , and organic maceral content. This analysis yielded the novel insight that all proxies related to authigenic iron sulfides (e.g., S total and S pyrite , Fe total and Fe pyrite , Fe py /Fe HR , and δ34S pyrite) covary negatively with TOC within the dm-scale cycles. We interpret the unusual pattern of negative TOC-pyrite covariation in the Cleveland and Sunbury shales to have resulted from differential distributions of labile organic matter (OM), which drove H 2 S production via microbial sulfate reduction (MSR), and reactive Fe, which controlled pyrite formation. Labile OM was concentrated in high-TOC layers whereas reactive Fe was concentrated in low-TOC layers (possibly as Fe-oxyhydroxide coatings on silt grains), as a consequence of which part of the H 2 S generated in the high-TOC layers diffused upward into the low-TOC layers before becoming fixed as pyrite. This process is evidenced by several contrasts between high-TOC and low-TOC layers: (1) higher S pyrite concentrations and Fe py /Fe HR values in the latter, (2) concentration of pyrite in the lower part of low-TOC layers, immediately above high-TOC layers, and (3) higher δ34S pyrite values in the latter, reflecting late diagenetic formation of the H 2 S that diffused upward into the low-TOC layers. To our knowledge, this process has not been documented previously in ancient marine shales, probably owing to insufficiently high-resolution analysis in earlier studies. • High-resolution C-S-Fe analysis of D-C Cleveland and Sunbury shales, Appalachian Basin • Optimized sampling protocol for analysis of high-frequency paleo-environmental dynamics • Higher TOC linked to more oxidized and degraded OM, showing dilution control of OM • Upward H 2 S diffusion led to peak pyrite formation and δ34Spyrite in low-TOC layers. • Dm-scale cyclicity driven by ~20-kyr climate fluctuations of precipitation and runoff [ABSTRACT FROM AUTHOR]

Reconstructing fluctuations in marine and atmospheric redox conditions during the late Precambrian/Early Cambrian is essential to better our understanding of the link between oxygen and biological evolution. This study presents REE+Y and Cr isotopic data together with TOC data and redox-sensitive trace metal concentrations of marine black shales and cherts from the Cambrian Jiumenchong Formation and Ediacaran Liuchapo Formation from South China, in order to improve our understanding of the redox conditions in the Nanhua basin during this critical time period. The Liuchapo Formation records negative Ce anomalies and highly fractionated δ53Cr values of up to 1.06 ± 0.09‰, suggesting deposition under oxic conditions and implying high oxidative weathering rates during the end of the Ediacaran. The overlying Jiumenchong Formation is characterised by a stratigraphic upward decrease in δ53Cr values to typical igneous values as well as a decrease in negative Ce anomalies and an increase in positive Eu anomalies, suggesting an increase in detrital content due to more aeolian weathering and deposition under anoxic conditions at the beginning of the Cambrian. The latter is also confirmed by the trends in V/Cr and Th/U ratios, which further indicate an eventual return to (dys-)oxic seawater conditions during the Early Cambrian. • At the End of the Ediacaran oxic slope seawater conditions and oxidative weathering dominated in South China. • More aeolian weathering resulted in more detrital influx to the oceans at the beginning of the Cambrian. • The seawater above the slope of the Nanhua Basin cycled to anoxic and back to oxic at conditions during the Cambrian. [ABSTRACT FROM AUTHOR]

The early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes. To unravel the oceanic redox conditions, high-resolution analyses of iron speciation and redox-sensitive trace elements (Mo and U) within the framework of sequence stratigraphy were carried out on the Niutitang Formation (~528–521 Ma). The two examined sections, the Daotuo and Bahuang sections, were respectively located in a mid-upper slope setting and a lower slope to basin settings behind a seaward submerged sill on the middle Yangtze Block, South China. At Daotuo, the Fe py /Fe HR values display moderate positive covariances with the total organic carbon contents (TOC) and the Mo/TOC ratios, notably in the basal part of the Niutitang Formation. Therefore, at this locality the euxinic water mass wedge was developed in association with a high primary organic productivity/burial rate, likely within a high-productivity zone. In contrast, at Bahuang, the Fe py /Fe HR values show weak to negative covariances with the Mo/TOC ratios and TOC contents in the basal part, indicating that the biogeochemical cycles of Fe, Mo with C were decoupled in a lower-productivity, ferruginous deeper basinal setting. These spatial changes in marine redox structures and biogeochemical cycles can be reasonably explained by the existence of oceanic upwelling in the presence of strong offshore currents and a seaward submarine sill, reconciling the oxygen minimum zone (OMZ) in modern oceanic margins to some extent. This spatial redox pattern also accounts well for previous data based on which the euxinic state intermittently invaded upward onto the shelf margin and evolved into a ferruginous-dominant anoxia in the inner shelf subbasins. In addition, the euxinic wedge dynamically fluctuated upslope and downslope along the transect from the ferruginous outer shelf slope to the basin in response to rise and fall of sea-level. Consequently, the temporal evolution of the redox conditions was driven in part by eustatic variations. Moreover, the paleogeographic position of the Yangtze Block in the mid-lower latitudes suggests that the block was associated with relatively strong offshore currents induced by trade winds, which notably enhanced the oceanic upwelling near the transgression maximum. On the other hand, the dominance of deep ferruginous waters with the local occurrence of a euxinic wedge on the outer shelf slope indicates relatively low concentrations of seawater sulfate and atmospheric oxygen during the deposition of the Niutitang black shales. This redox model thus highlights the important roles of paleogeographic, paleoclimatic and eustatic sea-level changes in controlling the spatiotemporal fluctuations in marine redox conditions and biogeochemical cycling in the early Cambrian ocean. [ABSTRACT FROM AUTHOR]

The Early Toarcian, organic-rich, black shales of the Cleveland Basin, Yorkshire UK, are the type sediments for the supposed early Toarcian oceanic anoxic event. The sediments have values of Cd/Mo that are <0.1 and values of Co (mg/kg) × Mn (%) that are >0.4. These values are typical of sediment deposited in modern basins that are hydrographically restricted and show that the Cleveland Basin was hydrographically restricted when depositing organic-rich sediments. These palaeo-proxies confirm earlier interpretations, based on Mo/TOC values, that argued for hydrographic restriction. The term Toarcian oceanic anoxic event can now be discarded. [ABSTRACT FROM AUTHOR]

Abstract Rare earth element (REE) signatures are often applied to interpret paleoenvironmental conditions in sedimentary basins, however the complicated diagenetic histories in dynamic depositional environments can affect interpretation of measured REE signatures. Prior studies on REE content in shales and limestones indicated that REE in specific mineral phases may provide unique information concerning diagenetic reactions occurring in sedimentary rocks during burial and compaction. Application of sequential extraction techniques to target REE signatures in specific mineral fractions may provide greater insight into the complex processes that occurred during diagenesis and catagenesis. Thus, using this technique, this study provides a detailed account for REE, 87Sr/86Sr, and δ13C in the carbonate fraction of the Marcellus Shale and adjacent formations in the Appalachian Basin. A suite of 49 rock samples collected from two cores recovered from Monongalia County, West Virginia, USA was analyzed. The results showed that the carbonate concretions in calcareous shales and carbonate cements in black shales were very distinct. The REE plus yttrium (REY) were more concentrated in the carbonate concretions than in the carbonate cements. The carbonate concretions displayed REY patterns that are closely similar to modern seawater, while MREE-enrichment was observed in the carbonate cements. Likewise, the 87Sr/86Sr values in the carbonate concretions were similar to those measured for unaltered Middle Devonian carbonates, while the 87Sr/86Sr in the carbonate cements were much more radiogenic. This observation indicates that the carbonate cement could have been inherited radiogenic 87Sr expulsed from clays during illite-smectite transition. Overall, this study demonstrated two distinct processes involved with controlling the carbonate geochemistry within Appalachian Basin shales and limestones: one fraction displaying minimal diagenetic alteration relative to depositional conditions (carbonate concretions in calcareous shales and limestone carbonate in limestones), and another fraction displaying the evidence for extensive chemical alteration during illite-smectite transition and catagenesis (carbonate cements in black shales). In addition, highly variable and significant enrichments of U and Mo demonstrate that the Union Spring member and the Lower Oatka Creek member of the Marcellus Shale in the southwestern Appalachian Basin (MSEEL site) were deposited under mainly anoxic environment whereas intermitted episodes of dysoxic to perhaps oxic conditions occurred during the deposition of the Upper Oatka Creek member. Graphical abstract Unlabelled Image Highlights • A sequential extraction method was applied to separate authigenic minerals. • Authigenic components provide more detailed insights onto depositional environments and diagenesis in sedimentary rocks. • Carbonate cement in black shales is significantly altered during illite-smectite transition and catagenesis. • Carbonate minerals in limestones is minimally altered during burial compared to carbonate concretions and carbonate cement. • Intermitted episodes of dysoxic/oxic environment occurred during the deposition of the Upper Oatka Creek member. [ABSTRACT FROM AUTHOR]

The Bakken black shales represent a unique sedimentary system characterized by high total organic carbon contents (TOC; Lower Bakken shale = 10.8 and Upper Bakken shale = 11.8 avg. wt%) and enrichment factors (EF) of various trace metals (TMs) such as Cu (6.2–7.7), Mo (219.7–237.8), Ni (9.4–10.2), U (20.6–29.3), V (9.9–14.2), and Zn (10.4–12.2) that are considerably higher than many other Devonian-Mississippian (D–M) black shales. In this study, raw distributions of elemental concentrations combined with bivariate and principal component analysis (PCA) were used to clarify processes that contributed to high TM EF values in the Bakken shales. Total organic carbon shares heavier PCA component loadings (>0.445) and stronger Pearson correlation coefficients (r) with Cu, Mo, Ni, U, V, and Zn than with pyrite-associated (As, Co, Fe, and S) elements, suggesting that organic matter (OM) played a primary role in the scavenging and accumulation of TMs in the sediments. Reducing conditions within bottom waters or sediment pore waters may have accelerated the accumulation of redox-sensitive Cu, Mo, Ni, V, and Zn introduced into the sediments via an OM detritus host and most likely played a secondary role in their enrichment. Deep-water renewal times for the Bakken have been estimated to be ∼10–30 yrs. based on Mo/TOC results. The high TM EF values observed in the Bakken shales may reflect frequent resupply of TMs into basin waters, enhanced primary productivity that is necessary in scavenging TMs from the water column, the presence of H 2 S within sediment pore or bottom waters, or possibly secondary processes associated with basin-wide fluid and hydrocarbon migration. High TOC concentrations may be the result of a possible shallow water column (∼150 m), which would limit exposure time of OM to oxidation and degradation, coupled with intermittent fluctuations between dysoxic and anoxic/euxinic bottom-water redox conditions, the latter of which would reduce rates of OM degradation by benthic organisms. • Bakken shales are characterized by high TOC and trace metal (TM) contents. • Cu, Mo, Ni, U, V, and Zn are strongly associated with TOC. • Organic matter scavenging may have been a primary mechanism for TM accumulation. • Depositional redox conditions and later diagenetic changes are secondary factors. • Enrichment of Cu, Mo, Ni, V, and Zn is higher than in many other D-M black shales. [ABSTRACT FROM AUTHOR]

Carbonate-hosted Zn Pb deposits are a major source of zinc, lead, and some critical raw metal resources worldwide (e.g., germanium and gallium). However, the timing and geodynamics setting of their mineralization remain enigmatic, leading to ongoing debates about their origins. To shed light on these issues, we utilized LA-ICP-MS in situ U Pb dating of ore-related calcites to determine the timing and geodynamic setting of mineralization events in the world-class Huayuan carbonate-hosted Zn Pb ore field in South China. The petrographic and cathodoluminescence features, fluid inclusion characteristics, and the Pb isotope data indicate that the tested calcites were formed from the same ore-forming fluids as the intergrown sphalerite and were not overprinted by possible later hydrothermal fluids. Our results show that the calcite U Pb ages of 510–495 Ma refute the previously proposed Mississippi Valley-type (MVT) model, which suggested that the younger Caledonian orogeny (460–420 Ma) played a significant role in the genesis of the Huayuan Zn Pb ore field. Instead, our findings link the formation of this ore field to crustal extension and high heat flux resulting from mantle upwelling events in northeast Gondwana during the Cambrian. The findings indicate that deep mantle processes can trigger shallow crustal fluid circulation to create a giant carbonate-hosted Zn Pb ore field in a long-dormant rift. We present a genetic model proposing that the extensive carbonate platform of the Qingxudong Formation was likely a "brine factory" that supplied the brines to the Proterozoic basement through reactivated syn -rift faults. The high heat flux from upwelling mantle not only heated these brines to high temperatures but also caused early maturation of organic matter in the lower Cambrian Niutitang black shales to generate hydrocarbons. The hydrocarbons migrated into the limestone of the Qingxudong Formation. Meanwhile, the hot brines leached Zn and Pb from the Proterozoic basement rock. These Zn Pb bearing brines ascended to the Qingxudong Formation where they mixed with the hydrocarbons. These hydrocarbons acted as reducing agents for Zn Pb sulfide precipitation, forming the Huayuan giant Zn Pb ore field. Furthermore, U Pb dating results of syn -mineralization calcite, together with the relationships between the ores and host rocks, indicate that Zn Pb mineralization occurred over timespans during or shortly after host-rock deposition and continuing during lithification for several million years in this ore field. The similar prolonged duration of mineralization may be relatively prevalent in other carbonate-hosted Zn Pb deposits/ore fields. We therefore recommend that reliable age dating for all mineralization stages is necessary to establish genetic models for carbonate-hosted Zn Pb deposits. [ABSTRACT FROM AUTHOR]

The Alum Shale Formation, a widespread metal-rich black shale in Baltoscandia, was deposited during the Cambrian–Ordovician transition. Given that it straddles a period of alternating extinction/recovery events following the Cambrian Explosion and prior to the Great Ordovician Biodiversification Event, the Alum Shale in Scandinavia has been extensively studied to assess the effect of oceanic redox variations on these biotic events. However, there is an on-going debate about the depositional environment and the prevailing redox state. An Early Ordovician (Tremadocian) equivalent of the Scandinavian Alum Shale, locally known as graptolite argillite, can be found in Estonia and NW Russia, in the easternmost extension of the 'Alum Shale Sea'. In this contribution, we studied the redox-sensitive metal distribution and accumulation mechanisms in graptolite argillite successions in NW Estonia. Our aim was to provide better constraints on redox and depositional environments in the Baltic Palaeobasin. Our findings reveal that despite a consistent mineral and major elemental composition, V, Mo, Re and U contents vary systematically within the stratigraphy of the up to 7-m-thick black shale unit, with element contents reaching up to hyper-enrichment at the base (Mo > 200 mg/kg, U > 100 mg/kg and V > 1000 mg/kg) followed by a declining trend towards the upper part of the succession. This implies that temporal changes in redox conditions or sedimentation rates controlled trace element accumulation in the graptolite argillite. Although the redox-sensitive trace elements are hyper-enriched (particularly Mo), which has previously been interpreted to reflect euxinic depositional conditions, the V/Al vs. U/Al and V/Al vs. Mo/Al ratios indicate a non-euxinic anoxic depositional environment within a perennial oxygen minimum zone for the graptolite argillite. Further evidence of an unrestricted, relatively open marine depositional environment possibly influenced by upwelling is suggested by Cd/Mo and Co × Mn relationships. These results are supported by U EF and Mo EF ratios that follow a seawater enrichment trend. The observed redox-sensitive element contents could be explained via diffusion-controlled uptake owing to the low sedimentation rates of the graptolite argillite (3–5 mm/kyr). These results highlight considerable redox variability within the Baltic Palaeobasin underscoring the need for a careful and critical reassessment of global redox interpretations based on Alum Shale data and emphasise the importance of using multiple independent redox and depositional environment proxies to determine palaeoredox conditions. • Despite uniform mineral and major element, V, Mo, Re and U contents vary systematically within the Tremadocian shale. • Multiple redox-sensitive element proxies were applied to reconstruct the palaeo-environment duringshale deposition. • We suggest an unrestricted, relatively open marine depositional environment, possibly influenced by upwelling. • High redox-sensitive element contents are attributed to diffusion-controlled uptake owing to low sedimentation rates. [ABSTRACT FROM AUTHOR]

Abstract Titanium (Ti) is typically hosted in detrital minerals in marine sediments and has long been considered to be immobile during diagenesis. In this study, the authigenic titania minerals, brookite and anatase, are observed in early Cambrian carbonaceous shales from the Meishucun and Zhajin sections of South China, respectively. Black shales in the Meishucun section have total organic carbon (TOC) contents from 1.6 to 3.9 wt% and HI (hydrogen index) values from 3.8 to 20 mg HC/g TOC, whereas black shales in the Zhajin section have much higher TOC (7.1–15.6 wt%) but lower HI (<2.0 mg HC/g TOC) and contain abundant bitumen (~3 vol%). Brookite in black shales from the Meishucun section crystallized invariably along cleavages of detrital biotite. This intimate association suggests that the Ti required to form titania minerals was derived from detrital biotite and that Ti was mobilized only on a nano- to micro-meter scale. In contrast, anatase aggregates in black shales from the Zhajin section are intergrown with bitumen. It is proposed that Ti in these shales was mobilized in low pH organic-rich fluids and, subsequently, preferentially precipitated as anatase with increasing pH. The mobilization of inert Ti in black shales is indicative of a possible diagenetic or post-diagenetic elemental redistribution in carbonaceous rocks. Our study further demonstrates that proxies based on geochemical component of black shales can be employed to estimate redox state of ancient oceans, but must be applied with caution. [ABSTRACT FROM AUTHOR]

Abstract Radium in hydraulic fracturing wastewaters derives from two isotopically distinct end-members in the shale, labile 228Ra hosted by mineral surfaces (226Ra/228Ra atom ratio ~250) and exchangeable 226Ra hosted by organic surfaces (226Ra/228Ra ~10,000). Here we use mass balance and isotope mixing models to reconcile extraction of Ra from these phases with mechanisms of Marcellus wastewater production. Radium isotopic mass balance requires that the characteristic water-rock ratio between wastewater and shale is exceedingly low, on the order of 0.04, and that this ratio decreases with time during wastewater production. An evolving water-rock interaction drives increasing Ra concentrations (=[Ra]) and 226Ra/228Ra ratios during wastewater production, all mediated by increasing [Ca2+] that favors desorption of 226Ra from organics. Our observations and models of Ra isotope geochemistry are best reconciled with observations of water and salinity mass balance, δ 18O, Na-Br-Cl, and 87Sr/86Sr if wastewater is produced by mixing of injected fluids with a limited volume of pore brine (on the order of 13% by volume), accompanied by contemporaneous extraction of excess alkaline earth elements by water-rock exchange. Validated using Ra isotope data, this model attributes the extreme salinity and [Ra] in wastewaters to the progressive, hydrologic enrichment of injected fluids during hydraulic fracturing. [ABSTRACT FROM AUTHOR]

Abstract The Early Cambrian was a key period in Earth's history, and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period. New Early Cambrian data for Re–Os and Mo isotopic compositions and redox-sensitive elements (RSE) are reported here for the Niutitang Formation, SE Chongqin, SW China. A Re–Os age of 520 ± 30 Myr (Model 3, 2σ, n = 21, MSWD = 62) and a (187Os/188Os) i ratio of 0.79 ± 0.11 were obtained, consistent with previously reported data for equivalent strata from other areas. Separate regression of samples with (187Os/188Os) i of 0.72–0.79 and 0.81–0.89 produce more precise ages of 520.1 ± 9.5 Myr (Model1, 2σ, n = 7, MSWD = 1.0) and 513 ± 10 Myr (Model 1, 2σ, n = 8 MSWD = 0.96) respectively, suggesting heterogeneity in the primary source of Os as the dominant cause of scatter and uncertainty. High initial 187Os/188Os of 0.79 ± 0.11 may indicate a high Os input from oxidative weathering of upper continental crust for Early Cambrian ocean. Based on RSE data such as Mo, U, V enrichment and Ni/Co, V/Cr, V/(V + Ni), Mo EF /U EF , Re/Mo ratios, two geochemically distinct zones (upper and lower) can be delineated in the sedimentary sequence, as reflected in drillcore sections. The upper section (n = 9) is characterized by low total organic carbon (TOC) contents (mean 1.8 ± 1.5 wt%), low RSE contents (Mo (0.008 ± 0.006) × 10−3 g/g; V (0.22 ± 0.15) × 10−3 g/g; U (0.008 ± 0.004) × 10−3 g/g), and low V/Cr (2.0 ± 1.7), Ni/Co (5.9 ± 3.5), Mo EF /U EF (2.6 ± 1.8), and Mo/TOC (6.0 ± 3.9) ratios; in contrast, the lower core section (n = 13) is characterized by high TOC contents (4.4 ± 2.2 wt%), high RSE contents (Mo (0.10 ± 0.08) × 10−3 g/g; V (1.0 ± 1.0) × 10−3 g/g; U (0.05 ± 0.04) × 10−3 g/g), high V/Cr (6 ± 5), Ni/Co (24 ± 14), Mo EF /U EF (6.3 ± 2.2), and Mo/TOC (21 ± 9) ratios (uncertainties are ±1SD here). Significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic/euxinic to oxic condition. All samples display extremely low Re/Mo ratios of 0.2 × 10−3–1.9 × 10−3, indicating the absence of intermediate reducing conditions, possibly reflecting rapid ocean oxygenation. The Mo isotopic composition in the sedimentary succession of the YC9 core exhibits a large variation, with δ98/95Mo values of 0.04–2.00‰. Upper section δ98/95Mo values display an opposite trend to Mn content and negative correlation with Mo/TOC ratios, indicating Mo fractionation associated with adsorption on Mn oxides or oxyhydroxides. Lower section δ98/95Mo values are positively correlated with Mo/TOC ratios and negatively with Re/Mo ratios, suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions. High δ98/95Mo of 2.00‰ and high average Mo/TOC ratios of 21 (10−6 g/g)/wt% were observed in euxinic shales in the Niutitang Formation, suggesting extensive ocean oxygenation may have taken place at ca. 520 Ma. [ABSTRACT FROM AUTHOR]

Abstract Hydraulic fracturing of the Marcellus Shale produces wastewaters that are hypersaline and highly enriched in isotopes of radium. Radium is understood to derive from the Marcellus Shale itself, but its source phases and their contributions to wastewater production have not been described. Using sequential extractions and experimental leachates, we characterize two distinct end-members that could contribute Ra to wastewaters, (1) a mineral phase, which hosts labile 228Ra and has 226Ra/228Ra atom ratios ~250, and (2) an organic phase, which hosts exchangeable 226Ra and has 226Ra/228Ra ~10,000. In leaching experiments we observed rapid extraction of Ra from these phases, with high ionic strength solutions leaching up to 14% of Ra from the shale in just hours. Radium concentrations and 226Ra/228Ra ratios increase with [Ca2+] of the leaching solution, and solutions approaching 1 M Ca2+ produce 226Ra/228Ra ratios compatible with Marcellus wastewaters. In contrast, pure water removes <0.5% of Ra from the shale with low 226Ra/228Ra ratios incompatible with wastewaters. Experimental results and wastewater data together provide a coherent picture, that the distinctive Ra isotopic signature of Marcellus wastewaters results from contemporaneous water-rock interactions that promote desorption of 226Ra from organics during hydraulic fracturing. Graphical abstract Unlabelled Image Highlights • Leachable radium in Marcellus Shale occurs in two isolated surface domains. • Labile Ra on mineral surfaces has 226Ra/228Ra atom ratios ~250. • Exchangeable Ra on organic surfaces has atom ratios ~104. • Increasing Ca2+ releases 226Ra over 228Ra to produce wastewater 226Ra/228Ra ratios. • Wastewater production requires the introduction of Ca2+ to unwetted organic surfaces. [ABSTRACT FROM AUTHOR]

Sub-aerial weathering of black shales drives the gradual leaching of sulphur- and organic-bound heavy metal elements, which are usually abundant in these rocks due to depositional conditions. The formation of oxygen functional groups in kerogen, however, can lead to an opposing mechanism - metal adsorption and binding, similar to a process common in soils. An increase in cation exchange capacity (CEC) measured previously using metal complexes on black shales oxidised under laboratory conditions implies that the same phenomenon may occur in a naturally oxidised black shale. This idea was tested on a unique, well-developed and -preserved Permian palaeoweathering profile containing two neighbouring but diverse black shales from the Devonian/Carboniferous boundary in the Holy Cross Mountains (Poland). In the studied black shale beds, the oxygen groups formed in kerogen in the partially-weathered zone were found to be responsible for significant changes in adsorption properties measured using hexamminecobalt(III) and Cu(II)‑triethylenetetramine cations, which are common probes for CEC. Compared to a pristine part of black shales, the partially weathered zone was depleted of total organic carbon (TOC), sulphur, and sulphur- and organic-bound metals, and highly enriched in Cu, which is generally present in low levels in the nascent shales. In the partially weathered zone, where TOC content is reduced, apparent CEC values surpass the CECs predicted from the contents and structures of clay minerals, and correlate linearly with the content of oxygen groups developed during weathering. The adsorption properties of carboxyl groups in the oxidised kerogen are suggested as being responsible for the syn - or post-weathering enrichment in Cu caused by the remobilisation of older Cu-sulphide ores present in the area. As opposed to natural weathering, aggressive oxidation, e.g. under laboratory conditions produces a high proportion of cross-linked oxygen groups that do not participate in metal cation adsorption. The CEC values of artificially oxidised samples reached a limit corresponding to those of naturally oxidised shales. [ABSTRACT FROM AUTHOR]

The high geogenic levels of trace metal(loid)s preserved within black shale can be released during natural weathering, causing global concern. Currently, the accumulation, speciation and potential mobilization of two highly toxic metal(loid)s, arsenic (As) and antimony (Sb), from black shale to soils remain unresolved, which hinders the evaluation of their potential risk to surrounding environments. Here, we demonstrate the geochemical enrichment, species transformation, and leaching behavior of As and Sb by applying weathering-induced alteration, sequential extraction and synchrotron-based X-ray absorption near edge spectroscopy (XANES) in the analysis of a range of black shale and weathered samples from two profiles collected from typical Cambrian sedimentary strata of the Pearl River Delta, southern China. The results showed significant accumulation of trace metal(loid)s, including As (43.8–458.0 mg/kg), Sb (7.2–34.5 mg/kg), Cu (8.8–451.0 mg/kg) and Ni (22.2–108.0 mg/kg), in the black shale and weathered products, and high enrichment factors for As (22.0–266.8) and Sb (1.2–6.4) were observed, consistent with the very high degrees of weathering (chemical index of alteration: 83–91). Arsenic in the shale was dominated by arsenate (46–85%), as identified using XANES, and this was resulted from extensive weathering, which facilitated the oxidation of arsenite and arsenopyrite species. Accordingly, significant decreases in sulfide-associated As and Sb were observed in highly weathered products. As a result, As and Sb were primarily in the immobilizable fractions associated with amorphous and well-crystallized Fe/Mn oxides, largely due to the increased formation of secondary Fe (oxyhydr)oxides, including goethite-like minerals, which mainly derived from the oxidative weathering of chlorite-like minerals. Nevertheless, high concentrations of As (<64 μg/L), Sb (14–70 μg/L), and Ni (3–92 μg/L) still leached from black shale samples exposed to synthetic acid rain in the 18-h batch leaching experiment. Our combined characterization results indicated that a small percentage of the high contents of geogenic As and Sb could potentially be released. However, soluble trace metal(loid)s in shale leachate could still endanger surrounding ecosystems, and continual monitoring during long-term natural weathering processes is merited for global black shale regions. • Natural weathering governed the accumulation of trace metal(loid)s in soils derived from black shale • Arsenate dominated in black shale-associated products and soils due to oxidation of reduced As species in oxic environment • In situ-formed secondary Fe-bearing minerals sequestered the pre-released As and Sb from black shale weathering • Low-level but persistent dissolved trace metal(loid)s from black shales still pose ecological risks [ABSTRACT FROM AUTHOR]

The pelagic barite precipitation in the seawater column is tightly associated with the biological pump, and thus the barite Ba isotope composition (δ138/134Ba) has been advocated as a novel paleo-productivity proxy. However, pelagic barites in sediments are susceptible to diagenetic processes that may modify primary δ138/134Ba signals and hamper the application. To constrain the diagenetic effect, we report δ138/134Ba data of diagenetic barites from the black shales of the lower Cambrian Yurtus Formation in the Aksu area of the Tarim Basin. The δ138/134Ba values vary within a large range from −0.15‰ to +0.49‰ and show a generally positive correlation with the 87Sr/86Sr ratios, indicating the mixing of two end-members. The samples with 87Sr/86Sr ratios close to the coeval seawater value record relatively low δ138/134Ba values from −0.15‰ to +0.14‰, which are inferred to inherit from the primary barites; whereas the samples with 87Sr/86Sr ratios exceeding the coeval seawater value record higher δ138/134Ba values from −0.05‰ to +0.49‰, which may have resulted from the alteration by the host black shales during early diageneses. Based on these investigations, we propose that pore waters below the sulfate-methane transition zone may provide an additional source of Ba (enriched in heavy isotopes) for the formation of diagenetic barites. We suggest to combine barite 87Sr/86Sr ratio with δ138/134Ba value to evaluate diagenetic influence on primary Ba isotope signatures especially for an over-determined system, which is important for applying Ba isotope compositions of barites to understand paleo-oceanic evolution. [ABSTRACT FROM AUTHOR]

We apply trace element analysis of syn -sedimentary pyrite to track the redox and nutrient variations in the Ediacaran-Cambrian Bambuí Group of eastern Brazil. Sedimentary pyrite from black shales interleaved within the basal cap carbonate of the 635–600 Ma Pedro Leopoldo Member, deposited in an open marine setting connected to the global sea, preserve high Mo up to >200 ppm, erratic enrichment of micronutrients such as Cu, Ni, Ba, Zn and uniform enrichment of Se. This indicates the importance of oxidative continental weathering and development of an enhanced Redox Sensitive Elements (RSE) dissolved reservoir on the aftermath of the Marinoan glaciation, with atmospheric O 2 levels, calculated from Se/Co ratios of pyrites, around 2.5–3.5 wt%. A shallow oxic layer over deep euxinic realms made possible by high concentrations of dissolved sulfate was probably maintained up to the late Ediacaran, during deposition of the Cloudina-bearing Lagoa Santa Member (585–540 Ma). Upsection, however, radical shifts to muted Mo concentrations and overall low micronutrient concentrations are recorded in the Serra de Santa Helena Formation, with O 2 levels plunging below 1 wt%. This indicates the development of ferruginous bottom water conditions in a sulfate-poor setting, due to restriction of the basin by the encircling Brasiliano mountains that formed Gondwana. The restricted, oxygen-poor and nutrient-depleted conditions caused the demise of complex life forms and hampered the development of typical Ediacaran-Cambrian ecosystems. Basin re-connection might have occurred during deposition of the Lagoa do Jacaré Formation, with independent proxies such as local tidal facies indicating a return to more ventilated conditions in the upper Bambuí Group. Our results highlight the importance of basin restriction and connection over coupled redox-nutrients variations during the development of Ediacaran-Cambrian ecosystems. • Sedimentary pyrite trace element concentrations of the Bambuí Group. • Basin restriction and connection controls coupled redox-nutrients variations. • Radical shifts in nutrient and oxygen levels in the Ediacaran-Cambrian transition. • Redox-nutrient variations control the development of complex life forms. [ABSTRACT FROM AUTHOR]

The essential role of the fine mineral fraction in V hyper-enrichment in black shales has been suggested in previous studies; however, 'the operational details' of such metallogenic systems remain poorly understood. This study addresses the syngenetic enrichment pathways of V in black shales from the Lower Ordovician in the inner shelf of the Baltic Palaeobasin, showing very high content of redox-sensitive elements, including that of V. X-ray fluorescence spectroscopy (XRF) and organic element analysis (CHNS-O), combined with bivariate and multivariate data analyses, were employed to analyse high-resolution geochemical profiles (> 360 samples) of thermally immature black and grey shales, as well as black shale containing siltstones, from two drill cores from NE Estonia. The detected V (up to 2300 ppm), Mo (up to 4500 ppm), and U (up to 500 ppm) enrichment lacked well-defined co-variance, indicating a complex redox environment and element-specific enrichment pathways. While the maximum V and Mo concentrations were confined to the basal part of the black shale–black shale complex, V hyper-enrichment was also observed in the Mo-U-poor black shale interbeds in the siltstones. Co-variance analyses in metalliferous horizons revealed strong positive relationships between V and organic matter, Al, Ti, and K. Cr, characterised by a high affinity for natural sorbents, demonstrated an almost perfect correlation with V (r = ∼95), despite its low concentrations. These patterns are hypothesised to reflect V capture via reduced V(IV)-organic complexes adsorbed to fine clay particles after reduction of V(V) in the seawater column rich in dissolved organic matter. These data suggest that the specific surface area of clay particles acted as the primary limiting parameter for V capture, and the particle flux to the shallow sea bottom defined the degree of V hyper-enrichment in the studied settings. [Display omitted] [ABSTRACT FROM AUTHOR]

Extremely elevated Hg levels up to the μg g − 1 range were found in sulfide-rich black shale and phosphorite in the lowermost part of the Early Cambrian black shale sequences on the Yangtze Platform in South China. In this study, mercury isotopes were used to help resolve the origin of this anomalous Hg enrichment. Mass independent fractionation signatures of Hg isotopes with positive Δ 199 Hg values of 0.13 to 0.24‰ and positive Δ 200 Hg values of 0.05 to 0.10‰ were observed, indicating seawater is an important source of Hg. We hypothesize that upwelling of nutrient-rich waters from the open ocean resulted in high bioproductivity in the photic zone with concomitant scavenging of Hg from seawater. Advanced decay of biomass and remineralization under anoxic to euxinic conditions combined with very low clastic input led to enrichment of Hg and a broad spectrum of other redox-sensitive and biogenic metals up to 10 7 compared to modern seawater. Such spectacular hydrogenous metal enrichment has previously been observed in the oxic deep-sea environment only, i.e. manganese crusts/nodules. [ABSTRACT FROM AUTHOR]

Organic-rich, fine-grained sedimentary rocks, such as black shales, are important geochemical archives providing information on the evolution of seawater composition and biological activity over the past 3 billion years. While biological productivity and sedimentation rates greatly affect the organic matter content in these rocks, mechanisms linking these two processes remain poorly resolved. Here, we examine the interactions of clay minerals with the marine planktonic cyanobacterium Synechococcus sp. PCC 7002. We suggest that clays settling through the water column could influence carbon and trace metal burial in three ways: (1) the interaction of reactive clay surfaces with the bacterial cells increases organic matter deposition via mass increase in a seawater growth medium by several orders of magnitude; (2) reactive bacterial cells become completely encased within a clay shroud, enhancing the preservation potential of this organic matter; and (3) the trace metal content of the biomass buried along with metals sorbed to the clay particles contributes to the trace metal concentrations of the black shale precursor sediments. Significantly, our findings imply that the chemical composition of ancient, organic-rich, fine-grained deposits are not only archives of ancient seawater composition and redox state, but they also provide a record of the degree of biological activity in the water column through geological time. [ABSTRACT FROM AUTHOR]

Black shales of the Late Devonian to Early Mississippian Bakken Formation are characterized by high concentrations of organic carbon and the hyper-enrichment (> 500 to 1000s of mg/kg) of V and Zn. Deposition of black shales resulted from shallow seafloor depths that promoted rapid development of euxinic conditions. Vanadium hyper-enrichments, which are unknown in modern environments, are likely the result of very high levels of dissolved H 2 S (~ 10 mM) in bottom waters or sediments. Because modern hyper-enrichments of Zn are documented only in Framvaren Fjord (Norway), it is likely that the biogeochemical trigger responsible for Zn hyper-enrichment in Framvaren Fjord was also present in the Bakken basin. With Framvaren Fjord as an analogue, we propose a causal link between the activity of phototrophic sulfide oxidizing bacteria, related to the development of photic-zone euxinia, and the hyper-enrichment of Zn in black shales of the Bakken Formation. [ABSTRACT FROM AUTHOR]

Uranium (U) isotope signatures (δ238U) recorded in sedimentary archives provide insight into the paleo redox state of the ocean. But the robust interpretation of these sedimentary U isotope records requires characterisation of the U isotope signature of rivers, the main source of U to the ocean. The main controlling factors on riverine δ238U remain poorly constrained. Here, we investigate the elemental and isotope signatures of uranium in the dissolved and solid loads of a well-characterised river, the Mackenzie Basin (Canada). In the Mackenzie Basin, the solid load δ238U shows a positive relationship with U and vanadium (V) contents, consistent with the suggestion that particulate δ238U are explained by variable contributions via erosion of silicate and black shale. The δ238U of the dissolved and solid loads are correlated which, at first sight, suggests no U isotope fractionation during chemical weathering, and a purely lithological control on both the river dissolved and solid δ238U. Moreover, relationships between dissolved U and δ238U and major elements such as calcium and sulfate, also support the idea of a lithological control. However, the δ238U of end members inferred from mixing relationships are not consistent with binary mixing of two sources, suggesting some potential U isotope fractionation during weathering. In fact, the abundance of U in the river dissolved load is always lower than that predicted by silicate rock weathering. This suggests that 1) the weathering of silicate only can explain the abundance of U in the river dissolved load and 2) secondary weathering processes scavenge a proportion of the U released by primary mineral breakdown. The broad negative relationship between δ238U and the depletion of dissolved U is also consistent with the control of dissolved δ238U by secondary weathering processes following silicate mineral breakdown. The relationships observed between dissolved U, δ238U and the large-scale environmental controls on weathering processes (such as weathering intensity or runoff) support this hypothesis. Overall, our interpretations of the variation in the river dissolved δ238U challenge the common assumption of the control of dissolved U by black shale and carbonate weathering. In addition, we suggest that the extent of secondary weathering processes can imprint on the U isotope signature of rivers, now and in the past. [ABSTRACT FROM AUTHOR]

The U-isotope system is a well-established palaeo-redox proxy that potentially constrains the global extent of marine anoxia during average as well as extreme redox events throughout Earth's history. A typical archive that forms underneath a reducing water column and acts as an intense U sink is organic-rich black shale. However, the degree to which black shale archives reflect the marine U-isotope signature is not well understood because U-isotope fractionation between U(VI)-bearing seawater and U(IV)-bearing black shales may vary as a function of local environmental conditions. Here, we present a combination of U-isotope and elemental concentration datasets, supported by a complementary Mo-isotope record, for the Furlo sedimentary section in Marche–Umbria, Italy and interrogate the combined systematics to unravel the mechanisms controlling the U-isotope fractionation factor between black shales and ambient seawater. We examine black shales deposited before and during Oceanic Anoxic Event 2 (OAE 2), which was one of the most extreme climatic perturbations of the Mesozoic Era that took place around the Cenomanian–Turonian boundary (Late Cretaceous, c. 94 Ma). The results of this study show that the U-isotope signature in the black shales deposited before OAE 2 was controlled by different mechanisms than the U-isotope ratios recorded in black shales deposited during OAE 2, with both stratigraphic intervals likely influenced by local environmental conditions. Probable local environmental changes include increased U reduction associated with biomass at or above the sediment–water interface and varying dissolved hydrogen sulphide concentrations in the water column and sediment. The overall results of this study confirm that black shales are a highly complex archive for U-isotope studies of past oceanic redox conditions, due to the sensitivity of the U-isotope fractionation mechanism to local environmental conditions, which are difficult to constrain. We propose the application of a ∆238U shale-seawater of 0.6 ± 0.1 ‰ to black shale records deposited under locally constant euxinic conditions at non-restricted settings. • Comparison of U-isotope ratios recorded in black shales deposited before and during OAE 2. • A pronounced difference in the U- and Mo-isotope systematics between the pre-OAE 2 and OAE 2 black shales is observed. • Ideal black shale records have been deposited under locally constant euxinic conditions at non-restricted settings. • We recommend the application of a U-isotope fractionation factor of 0.6 ± 0.1 ‰ to constantly euxinic black shale records. [ABSTRACT FROM AUTHOR]

The alteration of carbonate, caused by organic acid release during hydrocarbon generation, is usually neglected in carbonate chemistry, since the unaltered carbonate, which may reflect the (syn)depositional environments, can dilute and conceal the modification signals (e.g., δ13C carb and elemental compositions). In carbonate-deficient black shales, however, active hydrocarbon generation may provide sufficient organic acid to dissolve the original minerals, and the geochemical signals may remain evident in the altered carbonate. Moreover, laminae represent the smallest phase in sedimentary rock and can be distinguished by their chemical and mineral compositions, which reflect the alteration reactions and products. Thus, microscale investigation on laminae would help clarify alteration pathways during burial diagenesis and its geochemical effects. In order to understand the diagenetic reactions and organo-modifications of carbonate minerals, we examined both the mineral and geochemical characteristics of the carbonate-deficient, organic-rich, laminated black shales from the Yanchang Formation of the Ordos Basin, North China. Our study demonstrates that the carbonate content is negatively correlated with the ion concentrations within the carbonate lattice (e.g., Sr carb), providing evidence that carbonate phases have undergone considerable late diagenetic alteration during deep burial. This is consistent with the argument that stronger alteration indicators for carbonate should be found in rocks with lower carbonate content. In addition, the (re)precipitation of carbonate minerals, accompanied by corrosion of siliciclastic minerals, is also observed in thin sections, supporting the deep alteration scenario. By confirming the presence of deep diagenetic alteration of carbonate during deep burial and investigating this alteration in detail, this study provides new insights into interpretations of carbonate chemistry. [ABSTRACT FROM AUTHOR]

Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean. The stable isotope systematics of transitional metals (e.g., Cu and Zn) may be used as a proxy for evaluating their geochemical cycling. To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering, in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China. The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like δ 65 Cu (+ 0.14 ± 0.09‰, 1σ) but heavy δ 66 Zn (0.51 ± 0.11‰, 1σ). The elevated δ 66 Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter (OM) adsorption. The Zn isotopic compositions of these metalliferous shales are different from those of the ‘normal’ shales, highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems. The weathered shales and cherts have an extreme δ 65 Cu range from − 6.42‰ to + 19.73‰ and a modest δ 66 Zn range of + 0.25‰ to + 0.78‰. The strongly weathered samples have lower Cu and Zn concentrations and lighter isotopic compositions compared to the weakly weathered samples. The leaching of Cu- and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation, with the huge Cu isotope variation generated by multistage redox leaching. In general, δ 66 Zn values of the weathered shales shift towards light values compared to the unweathered protoliths, suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average (+ 0.33‰). Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering. These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans. [ABSTRACT FROM AUTHOR]

Molybdenum (Mo) isotope variations recorded in black shales provide important constraints on marine paleoredox conditions. However, suitable shales are not ubiquitous in the geologic record. Moreover, reliable reconstruction of Mo isotope records from shales requires deposition from a water column containing very high concentrations of sulfide—a condition which is both rare and difficult to verify with certainty when examining preserved sediments. The utility of Mo isotopic records could be improved if reconstructions were possible using alternative lithologies, such as marine carbonates, which are more abundant in the geologic record. Here, we focus on the role of early diagenesis in determining the Mo isotopic composition preserved in shallow-water carbonate sediments from four push cores collected in different shallow-water depositional environments in the Bahamas. In contrast with carbonate primary precipitates, which generally contain < 0.2 ppm Mo, we find that carbonate sediments deposited under fully oxic shallow bottom water can experience extensive syndepositional authigenic Mo enrichment (1 to > 10 ppm Mo). The extent of this authigenic enrichment appears to be driven by high concentrations of hydrogen sulfide in the porewaters. In cores with the least authigenic Mo enrichment and lowest pore water sulfide, Mo isotopes are ~ 1–1.2‰ lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2–2.5‰), even under oxic bottom water conditions. However, the sensitivity of bulk carbonate δ 98 Mo to syndepositional diagenetic conditions potentially complicates interpretation of a carbonate Mo isotope paleoredox proxy. Robust reconstruction of seawater Mo isotopic composition from carbonates will thus require the ability to place constraints on early diagenetic conditions of pore waters at the time of deposition. We show that in order to record seawater Mo isotope values, carbonate pore waters must contain 50–100 μM H 2 S aq , which is achieved only in organic- and sulfide-rich carbonate sediments. [ABSTRACT FROM AUTHOR]

In order to advance our understanding of the emerging chromium (Cr) isotope system as a paleoredox proxy, we measured the 53 Cr/ 52 Cr of black shales deposited before, during, and after Cretaceous Oceanic Anoxic Event 2 (OAE2). We observed a > 1‰ coherent negative 53 Cr/ 52 Cr excursion and significant drawdown in Cr enrichments during OAE2 coincident with a large positive carbon isotope excursion. Our observed negative 53 Cr/ 52 Cr excursion during OAE2 is most easily linked to an increase in the ratio of euxinic to reducing conditions. Additional work on other OAE sections is needed to determine the spatial significance of this Cr isotope trend. [ABSTRACT FROM AUTHOR]

Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. However, many geological reservoirs and isotope fractionation processes are still not well understood. The purpose of this contribution is to move forward our understanding of (1) the Earth's high temperature Cr isotope inventory and (2) Cr isotope fractionations during subduction-related metamorphism, black shale weathering and hydrothermal alteration. The examined basalts and their metamorphosed equivalents yielded δ 53 Cr values falling within a narrow range of − 0.12 ± 0.13‰ (2SD, n = 30), consistent with the previously reported range for the bulk silicate Earth (BSE). Compilations of currently available data for fresh silicate rocks (43 samples), metamorphosed silicate rocks (50 samples), and mantle chromites (39 samples) give δ 53 Cr values of − 0.13 ± 0.13‰, − 0.11 ± 0.13‰, and − 0.07 ± 0.13‰, respectively. Although the number of high-temperature samples analyzed has tripled, the originally proposed BSE range appears robust. This suggests very limited Cr isotope fractionation under high temperature conditions. Additionally, in a highly altered metacarbonate transect that is representative of fluid-rich regional metamorphism, we did not find resolvable variations in δ 53 Cr, despite significant loss of Cr. This work suggests that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. Oxidative weathering of black shale at low pH creates isotopically heavy mobile Cr(VI). However, a significant proportion of the Cr(VI) is apparently immobilized near the weathering surface, leading to local enrichment of isotopically heavy Cr (δ 53 Cr values up to ~ 0.5‰). The observed large Cr isotope variation in the black shale weathering profile provides indirect evidence for active manganese oxide formation, which is primarily controlled by microbial activity. Lastly, we found widely variable δ 53 Cr (− 0.2‰ to 0.6‰) values in highly serpentinized peridotites from ocean drilling program drill cores and outcropping ophiolite sequences. The isotopically heavy serpentinites are most easily explained through a multi-stage alteration processes: Cr loss from the host rock under oxidizing conditions, followed by Cr enrichment under sulfate reducing conditions. In contrast, Cr isotope variability is limited in mildly altered mafic oceanic crust. [ABSTRACT FROM AUTHOR]

Earth's atmosphere and surface ocean were pervasively and mildly oxygenated after the ca. 2.4 Ga Great Oxidation Event (GOE), which induced dramatic environmental and biological changes. Positive nitrogen isotopic compositions of Paleoproterozoic marine deposits reveal aerobic nitrogen cycling and the widespread availability of bioavailable nitrate, but the nature of nitrogen cycling in freshwater environment remains unclear. To elucidate the redox conditions and bioavailability of nitrogen, redox-sensitive element and nitrogen isotopic compositions, as well as the depositional age, were determined for black shales of the late Paleoproterozoic Embury Lake Formation, Flin Flon belt, Canada. This formation accumulated in a sulfate-poor freshwater basin isolated from the open ocean, as indicated by its low total‑sulfur/total-organic‑carbon ratio of ∼0.05. Our U-Pb ages for detrital zircons show that the formation is younger than 1862.2 ± 2.6 Ma. Redox-sensitive elements (V and U) in the black shales show positive correlation with detrital tracers Al and Ti. Low enrichment factors for V and U indicate minor authigenic accumulations of the two elements. Moreover, the black shales are depleted in Mn, suggesting deposition under suboxic to anoxic conditions. δ15N values of the black shales are generally < +1‰, indicating a nitrate-limited environment, and are distinct from those of previously reported coeval marine deposits. It is proposed that the nitrate was exhausted by nitrogen-loss processes such as denitrification in the freshwater basin, but was available for nitrate-assimilating organisms in the ocean during the late Paleoproterozoic. [ABSTRACT FROM AUTHOR]

Many paleo-karstic bauxite deposits contain elevated concentrations of Ni and Co. It has been known that Co and Ni are dominantly hosted in sulfides, but it remains unclear how Ni and Co were incorporated into sulfides and what kind of physiochemical conditions were involved. The Maochang bauxite deposit, as one of the large Ni- and Co-bearing paleo-karstic bauxite deposits in Guizhou Province, southwest China, was selected to tackle these questions. Detailed mineralogical and sulfur isotopic analyses using integrated EBSD, EPMA, LA-ICPMS, and LA-MC-ICPMS methods revealed three stages of sulfide mineralization: stage I with pyrite (PyI), stage II with pyrite (PyII), millerite, sphalerite, chalcopyrite and violarite, and stage III with pyrite (PyIII), chalcopyrite and galena. The fine-grained, euhedral PyI has up to 0.79 wt% Ni and up to 0.53 wt% Co. In contrast, PyII, which replaced or overgrew PyI, exhibits high but variable Ni, Co and As contents, and is further divided into three sub-types: Ni-rich PyII-a (up to 16.54 wt% Ni, 0.94 wt% Co, and 0.38 wt% As) and PyII-b (up to 3.30 wt% Ni, 1.41 wt% Co, and 0.36 wt% As), and Co-rich PyII-c (up to 0.99 wt% Ni, 7.57 wt% Co, and 1.81 wt% As). PyIII is Ni- and Co-poor. Ni increases from PyI to PyIIa and PyIIb, and then decreases to PyIIc and PyIII, whereas Co remains at similar levels for PyI, PyIIa and PyIIb, significantly increases to PyIIc, and then drops abruptly to PyIII. δ34S V-CDT values vary dramatically from negative values for PyI (−6.9 to −16.6‰) to positive values for PyII-b (+19.4 to +22.8‰), sphalerite (+17.2 to +19.7‰), and PyIII (+25.3 to +32.3‰). Based on these results, and considering that the protoliths of the bauxites were likely Ni- and Co-rich black shales according to previous studies, a four-step model is proposed to explain the enrichment of Ni and Co in the bauxites: 1) during weathering and bauxite formation on the surface, Ni and Co were leached from the black shale together with sulfate; the metals were dissolved in the solution and partly adsorbed by Fe-oxides/hydroxides under moderate to high f O2 ; 2) during early diagenesis, the sulfate was partially reduced to sulfide via the bacterial sulfate reduction (BSR) process and formed PyI; much of the Ni and Co, together with sulfate, remained in the solution; 3) with increasing burial, temperature increased and f O2 decreased, and the remaining sulfate (with elevated δ34S V-CDT) was reduced to sulfide via the same BSR process and formed PyII; Ni was largely consumed and incorporated into PyIIa and PyIIb, whereas Co remained in the solution until precipitation of PyIIc; 4) by the time of PyIII precipitation, both Ni and Co had been taken by PyII, leaving a Ni- and Co-poor solution and PyIII. Step 1 occurred in a relatively open system, whereas steps 2–4 likely occurred in a closed system. [Display omitted] [ABSTRACT FROM AUTHOR]

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere–ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere–ocean system decreased considerably after ca. 2.06 billion years ago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85 Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06 Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88 Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93 Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90 million years before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event. [ABSTRACT FROM AUTHOR]

Proterozoic sedimentary deposits from the Vindhyan Supergroup, central India are excellent archives of paleo-environmental conditions. Meaningful interpretation of these geological signatures and their global correlation requires depositional age information for the sediments, data on which are limited for the Upper Vindhyan sediments. In this work, Re–Os isotopic study of the Bijaigarh shale, Kaimur Group from the Upper Vindhyan has been carried out to infer their depositional age and paleo-hydrological conditions. The Re–Os isochron for the Bijaigarh shale, a lagoonal deposit with an open ocean connection, provides a depositional age of 1210 ± 52 Ma (Model 3; MSWD = 69; n = 15; 2σ). This age for the Kaimur Group is younger by ~ 400 Ma than the underlying Semri Group (age ~ 1600 Ma), confirming existence of a long sedimentary hiatus between the two groups. The isochron-derived initial 187 Os/ 188 Os (Os i ) ratio for these shales (~ 0.6) is higher than those reported for Mesoproterozoic marine shales (~ 0.3) from other global margins. Os i ratios of shales could be regulated by regional paleo-hydrological conditions; osmium mass balance calculations for the restricted basin of the Bijaigarh shale show that a minor increase in freshwater influx (as low as ~ 2.4% i.e. salinity change by ~ 0.8 psu) to the euxinic deep water of the basin is sufficient to elevate Os i ratio from 0.3 (marine value) to ~ 0.6 (Os i ratio of the lagoonal shale). In contrast to lagoonal shales, the Os i ratios of marine shales seem to mimic the seawater 187 Os/ 188 Os during their deposition period. Compilation of marine Os i ratios shows coarse synchrony between temporal changes in Os i during the Proterozoic eon and the atmospheric oxygen level, pointing to the dominant role of oxidative continental weathering on paleo seawater 187 Os/ 188 Os ratio. [ABSTRACT FROM AUTHOR]

The role that marine organic-rich sediments play in the surficial osmium and rhenium cycles has been assumed to be important due to the high Re/Os ratios and high concentrations of these elements in such reducing sediments compared to the average continental crust. The high Re/Os leads to rapidly increasing 187 Os/ 188 Os ratios. We determine how large a role these sediments play in the surficial osmium and rhenium budgets by compiling a geochemical database to quantify the mean 187 Re/ 188 Os and 187 Os/ 188 Os ratios of organic-rich sediments. We then utilize estimates of the abundance of organic-rich sediments in the geologic record of different ages to estimate an average 187 Os/ 188 Os for these sediments. We find that the calculated 187 Os/ 188 Os value of organic-rich sediments varies between 2.22 and 2.27 depending on the volume–age model used, with error estimates of 2.07 to 2.42 and 2.12 to 2.48, respectively. Despite the fact that these sediments constitute a relatively small proportion of the upper continental crust, they contribute a disproportionately large amount (> 50%) to the upper continental crustal rhenium and, to a lesser extent (5–10%), osmium inventories owing to their high concentrations of these elements. [ABSTRACT FROM AUTHOR]

The Cryogenian interglacial period have witnessed dramatic changes in clim ate, oceanic environment and biological evolution. The nitrogen isotopic composition, as an important biogeochemical proxy, has the potential to track both the nutrient cycling and redox conditions in the past. However, nitrogen isotopic data during this interglacial time is rather limited. Here, we present integrated data for nitrogen isotopes (δ15N), as well as organic carbon isotopes (δ13C org), iron (Fe) speciation, pyrite morphology and trace elements from the Cryogenian interglacial Datangpo Formation derived from a drill core from South China to figure out the nitrogen cycling and coeval redox states. The results reveal a gradual oxygenation bottom waters along with deepening O 2 -H 2 S chemocline from euxinic black shale in the lower unit (LU), to oxic siltstone in the upper unit (UU), which is consistent with the redox variation inferred in previous studies. Sedimentary δ15N values constantly remain positive (mean value> + 4‰) throughout the Datangpo Formation and display an increase from the LU to the UU. We interpreted the positive δ15N values in the LU as a result of denitrification related to the development of euxinic conditions. The further increase towards the UU associated with limited nutrient supply may indicate the existence of a stable nitrate reservoir in the upper water column, which was consistent with an increased oxygenation during the interglacial time. The elevated O 2 levels and bioavailable nitrogen in seawater may have further contributed to the appearance of the earliest metazoan. In addition, our data lay good foundation for further global δ15N correlations among other interglacial successions. [ABSTRACT FROM AUTHOR]

The black shale in the Lower Cambrian Niutitang Formation in Southwest China is anomalously enriched in sedimentary exhalative Ni- and Mo-sulfides. The occurrence of large fluctuation of ocean redox conditions and thriving of benthic animals highlight the uniqueness of the stratum and its link to the Cambrian Explosion. Here, we report our observation of the high-resolution structures of the Ni- and Mo-extremely enriched sulfidic spheroids and measurements of their Ni- and Mo-isotopic compositions. Mo-sulfides appear as clusters of oval spheroids disseminated with pyrite framboids inside; differently, Ni-sulfides fill fractures or appear as coronas of the Mo-sulfidic spheroids. These observations indicate that the Mo-sulfide and the Ni-sulfide are mineralized by different processes. The Ni isotopic compositions of the spheroids and the groundmass fall on the mixing line between seawater (low [Ni]/[Mo] and heavy Ni isotopes) and hydrothermal fluids (high [Ni]/[Mo] and light Ni isotopes), confirming the hydrothermal-fluid-addition of Ni to the deposit. In contrast, the Mo isotopic compositions of the spheroids and the groundmass consistently exhibit their seawater-source. Along with our analyses of biomarkers and X-ray absorption fine structure of Mo, we deduce that Mo in the seawater was reduced and deposited as sulfidic spheroids, while Ni had an additional source from hydrothermal fluids that enhanced the biogeochemical cycles during their depositions. [ABSTRACT FROM AUTHOR]

Abstract: The dispersion of acidic solutions with high levels of metals/metalloids, as a result of oxidative weathering of pyritic geomaterials, is a major environmental problem in areas where these materials are widely distributed and/or were historically mined. In this study, four types of materials encountered in an old black-shale mining area (unweathered black shale, weathered black shale, burnt black shale, and lime-mixed burnt black shale) were subjected to a long-term (up to 137weeks) humidity cell test (HCT) combined with sequential chemical extractions (SCE), with the aim of examining geochemical controls on the release of Ni, U and As in this kind of pyritic settings. By combining the results of HCT and SCE as well as previously collected groundwater data, it is clearly shown that the degree of pyrite oxidation is the only major factor controlling the release of Ni, resulting in its highly elevated concentrations in acidic groundwaters. Although U followed a similar leaching pattern as observed for Ni and occurred abundantly in acidic groundwaters, a major decrease in the chemical fraction targeting exchangeable and carbonate phases, and a correlation of U concentrations with redox potential in groundwaters collectively suggest that the release of U was largely controlled by the solubilization of sorbed/carbonate U phases by oxidation to the highly soluble form (UO2 2+). As compared to the HCT, the SCE procedures used in this study delivered equally good estimates of Ni, U and S cumulatively leached, suggesting the strength of the SCE in terms of quantification of these elements during the weathering of pyritic geomaterials. Arsenic X-ray absorption near-edge structure spectroscopy shows that during the HCT (oxidation and leaching) of unweathered black shale, As was oxidized from its reduced form (having the oxidation state of −1 and most probably occurs as arsenian pyrite) to As(+5). Compared to the two cationic metals, As was released to a very limited extent and was not detectable in the leachates having pH between 6 and 3. This is because As was speciated exclusively as negatively-charged oxyanions in these leachates as predicted by MINTEQ modeling, thus was effectively attenuated by concurrently formed iron minerals. These minerals include mainly schwertmannite and K-jarosite as observed by SEM–EDS and also predicted by MINTEQ modeling. Elevated levels of As exclusively occurred in the groundwaters from one tube strongly impacted by seawater intrusion. This was regarded as a reflection of loosely-sorbed As oxyanions reliberated through ion exchange with seawater chloride. In this context, sea-level rise on a global scale as a potential driver for arsenic remobilization in low-lying coastal areas deserves further attention. [Copyright &y& Elsevier]

The 250–300-m-thick Carajás Formation in the Carajás mineral province, northern Brazil, consists of banded iron formation (including giant high-grade iron-ore deposits) and minor black shale, overlying a thick pile (2–3km) of about 2.75-Ga-old metabasalt. Carbonaceous shale with pyrite- and locally pyrrhotite-rich patches from drillcore of the Serra Sul exploration project has up to 29ppm Mo; iron-speciation analysis indicates essentially ferruginous and for some samples likely euxinic depositional conditions. Positive δ34S-isotope ratios of TRIS are between +0.3 to +10.7‰, with heavy data restricted to pyrrhotite-free samples. The data suggest microbial sulfate reduction under, at least partially, sulfate-limiting conditions with later overprint by migrating solutions. The black shale is affected by pronounced low-temperature potassium metasomatism (K2O/Na2O>100; up to 10wt.% K2O as adularia) related to diagenetic processes at <100°C, and low-grade metamorphic overprint. We studied a 20-cm-black-shale drillcore interval from the middle part of the Serra Sul BIF sequence in detail. Five samples with the most euxinic signature give a Re–Os regression of 2710±38Ma (2σ) with an initial ratio of −0.37±0.40 (MSWD=3.3), whereas the full data set (n=11), including black-shale samples from the top and bottom of the BIF sequence, gives a regression of 2661±110Ma (MSWD=121). Molybdenum-isotope patterns suggest mixing between a clastic end member, with about 0.2‰ δ98/95Mo for continental input (TDM of 2.8–3.1Ga according to Nd-isotope data), and 0.9‰ δ98/95Mo for a hydrogenous component. Black-shale samples from the bottom of the BIF sequence have heavy Mo-isotope composition of up to 1.8‰ δ98/95Mo. The significant Mo-isotope fractionation is either the result of an early and transient “whiff of oxygen” at 2.7Ga, or the result of hydrothermal fluid overprint. [ABSTRACT FROM AUTHOR]

Abstract: The Paleoproterozoic Era was a time of remarkable importance in the redox evolution of Earth's atmosphere and oceans. Here, we present a multi-proxy study of Mo, U and Fe isotopes together with Fe speciation of black shales and siltstones from the upper Zaonega Formation of the Onega Basin in Karelia. We attempt to better understand oceanic redox conditions during the 2.05Ga Shunga Event as the next step following the Great Oxidation Event (GOE) and the Lomagundi carbon isotope excursion Event. A cautious examination of the Fe speciation data shows that the studied section was deposited under dominantly euxinic conditions (anoxic and sulfidic) and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite. During this episode, the system was closed with respect to Fe but not sulfur. The Mo and U isotopic compositions (corrected for detrital input) were not affected by the metamorphism and loss of S and are fairly uniform throughout the entire section. The Fe isotope compositions are exceptionally heavy in the lower part of the section (up to δ56FeIRMM-14 =0.83‰) and become lighter towards the upper intervals, which also show significant [Mo] and [U] enrichments. We suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column. The upper part was deposited within a locally euxinic portion of the basin where H2S availability was highest, removal of Mo and U was more efficient and precipitated pyrite captured relatively non-fractionated dissolved Fe. In other words, quantitative uptake of Fe was favored. In contrast, the lower interval was deposited on the lower margin of a euxinic wedge where H2S availability was lower, and removal of Mo and U was less efficient. Pyrite precipitation in this part of the water column reflected a more fractionated dissolved Fe reservoir due to more protracted, non-quantitative Fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of Fe. The U isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co-precipitation into carbonates was the main process of U removal at this time. We estimate the Mo isotope composition of the contemporaneous ocean to be δ98MoSW =0.85±0.21‰. This is the lowest value yet reported for the Proterozoic ocean, suggesting that the oceanic Mo cycle was dominated by euxinic and anoxic sinks with negligible Mo removal into oxic environments. Recent studies have proposed a sharp increase in ocean–atmosphere oxygen levels during the Lomagundi Event followed by a dramatic crash. Our results from black shales of the 2.05Ga Shunga Event are consistent with a post-Lomagundi decrease in biospheric oxygen levels. [Copyright &y& Elsevier]

Abstract: Clay minerals are exceptionally well preserved in marine black shale of the ca. 2.1Ga Francevillian Group in southeastern Gabon. The FB Formation of the Francevillian Group is characterized by smectite-rich clay minerals including randomly ordered (R0-type) and ordered (R1-type) mixed layer illite/smectite (I/S). The preservation of R0-type clay minerals suggests unexpectedly slow mineral transformation and a moderate degree of diagenesis, which is unique, considering the Paleoproterozoic age of the sedimentary rocks. R0- and R1-type, smectite-rich particles occur in stratigraphic intervals with high organic carbon content and are associated with carbonaceous filamentous structures, suggesting formation of clay–organic matter complexes. Our data suggests that clay minerals may have enhanced organic matter preservation, providing the oldest example where a link between clay minerals and organic matter sequestration can be established. Our findings are consistent with the hypothesis that clay minerals enhanced organic carbon burial and aided in atmospheric oxygen accumulation through time. [Copyright &y& Elsevier]

Abstract: Elevated molybdenum (Mo) contents in organic-rich sediments are indicative of deposition from an anoxic and sulfide-rich (euxinic) water-column. This can be used for tracing past euxinic conditions in ancient oceans from sedimentary archives. Conventional analytical detection of elevated molybdenum levels is, however, expensive and cannot be directly performed in the field. Here, we show that handheld x-ray absorption spectroscopy (HHXRF) on both rock-powders and rock surfaces provides a powerful tool for laboratory or field-based, geochemical characterization of Mo content and the tracing of euxinic depositional conditions. We further present a sedimentary Mo database, compiled from over 300 globally distributed modern marine sites, to show that modern euxinic marine basins are characterized by molybdenum enrichment (>25ppm) relative to average crust (~1.5ppm) and river suspended matter (~3ppm). Only Mn-oxide rich deposits from oxygenated deep sea settings match such high Mo enrichments, but these are easily distinguished from Mo-rich sediments from the continental shelves. Hence, past and present organic rich sediments enriched in Mo are indicative of (at least intermittent) water-column euxinia. We analyzed the molybdenum content of powdered and homogenized samples of Holocene, Miocene, Triassic–Jurassic, Ordovician and Cambrian age by HHXRF and Inductively-Coupled-Plasma-Mass-Spectrometry (ICPMS). We further analyzed 2 certified NIST standard references (NIST-2781 with Mo: 47ppm and NIST-2702 with Mo: 10ppm) for analytical control. Analytical precision (1σ) after 30, 120, and 300seconds of measuring time was 4, 2, and 1ppm, with a respective detection limit of 11, 5, 3ppm (3σ, noise level). The data were accurate to within the given precision (1σ) after a daily calibration to samples covering a range of Mo concentrations from 0 to >30ppm. Hand-held XRF equipment also allows Mo measurements directly on fresh rock surfaces, both in the field and under laboratory conditions. Rock-samples from a Cambrian drill core closely match ICPMS obtained Mo concentrations from homogenous powders of the same rocks. Collectively, our data show that HHXRF as a quick and reliable method for the precise and accurate quantification of molybdenum concentrations in both rock powders and at fresh rock surfaces. The non-destructive nature of the HHXRF measurements is combined with low (per sample) analytical costs, minor sample preparation and easy applicability in the field. [Copyright &y& Elsevier]

Abstract: The trace and minor element and Re–Os geochemistry of the Upper Devonian Woodford Shale are analyzed in order to characterize elemental abundances, to identify associations among trace elements and to constrain paleoceanographic conditions and depositional processes. This organic-carbon-rich mudstone in the Permian Basin, west Texas, is a major source of hydrocarbons in the basin and is coeval with many other Upper Devonian shales in North America. The Woodford lacks enrichment in many trace metals. Only Mo, U, S and Se are significantly enriched. Other redox sensitive elements are depleted or similar to average shale composition, including Pb, Bi, Cr, Ti, Cu, Zn, Co, and V. Elements associated with granitic sources such as rare earths, Th, Ce, and TiO2 are also depleted relative to average shale; this appears to be related to be a source control. A strong basin reservoir effect is noted among several redox sensitive elements, including Mo, Cu and Ni, which largely accounts for the depletion. Dilution by biogenic silica had an additional effect on metal concentrations. Multivariate factors analysis identified associations between elements, including groupings of: rare earth elements; elements enriched in granitic crust; silica, varying antithetically with elements in carbonate minerals; organic carbon, Mo and U; V; phosphate; Fe and S. Noteworthy among the results are the different behavior of redox-sensitive elements, suggesting different precipitation mechanisms or varying dependence on reservoir effects. A strong redox effect is noted in the TOC/Ptot ratio at approximately the Frasnian–Famennian boundary, indicating an abrupt transition to an anoxic column boundary that coincides with a short-term significant fall in sea level. This suggests that anoxia was induced by isolation of the basin from the global ocean. However with the exception of the uppermost Famennian, initial 187Os/188Os values determined from Re–Os geochronology for the Permian Basin are similar to correlative sections of the Appalachian and Peace River Basins of North American and the Rhenohercynian basin of Europe. This indicates that although the Permian Basin became restricted during the upper Devonian and early Mississippian, ocean connectivity remained between regional and global basins. [Copyright &y& Elsevier]

Abstract: Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz, illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite. During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures. Some of these pores are either filled with organic matter or air-filled but remain unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface, mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity, ii) greater average length of connected pores, and iii) a more branched pore network compared to the unweathered sample. This work highlights the impact of shale–water–O2 interactions in near-surface environments: (1) black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and (2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low-temperature processes in our field study. [Copyright &y& Elsevier]

Abstract: This study presents Re Os isotopes and major and trace elemental geochemistry of carbonaceous (black) shales and a weathering profile developed on them from the Paleoproterozoic deposits of the Jhamarkotra Formation, Lower Aravalli Group, India. These measurements provide information on the impact of post-depositional processes, particularly hydrothermal fluid interactions, on the 187Re 187Os geochronology and element mobility in shales. In contrast to previously reported compositions for black shales, the Re concentrations of the Jhamarkotra shales are in general lower than their corresponding Os concentrations. The 187Os/188Os ratios of the black shales vary widely, from 3.5 to 10.5, without any discernible systematic trend with 187Re/188Os (range: 0.4 to 17.7; excluding an outlier); these 187Re/188Os ratios are lower by a few orders of magnitude than those generally reported for “fresh” black shales. These observations indicate that the Jhamarkotra shales, though they seem visually well preserved, have undergone post-depositional modification, resulting in “open system” behavior of Re and Os, with Re more mobile than Os. Higher SiO2/TiO2 and K2O/Na2O ratios of the Jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal (possibly Si and K-rich) fluids. The SiO2/TiO2 ratios (an index for hydrothermal alteration) show negative correlation with Re/Corg and positive correlation with Corg/N ratios (42–154), indicating loss of Re and nitrogen during hydrothermal fluid interaction. Earlier studies have shown the presence of Re porphyrins (N-containing organic compounds) in black shales. Thus, the observed co-variations of SiO2/TiO2 with Re/Corg and Corg/N ratios suggest mobilization of Re from the porphyrins in the shales. Further, positive correlation between SiO2/TiO2 and 187Os/188Os ratios indicates that significant amounts of radiogenic Os component were re-distributed throughout the Jhamarkotra shales during the post-depositional processes. In the weathering profile developed on black shales, the Al2O3-normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering. On the contrary, the Al2O3-normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile, suggesting limited mobility over the depth of the profile. The depth profiles of Re/Al2O3 and 188Os/Al2O3 and 187Os/188Os ratios in the weathering profile by and large reflect compositions of their presumed parent shales, which have gone through significant post-depositional changes. [Copyright &y& Elsevier]

Abstract: The distribution of redox-sensitive elements and their stable isotope variations in marine sediments has been employed to track the possible oxygenation of the atmosphere at, and before, the Great Oxidation Event (GOE; 2.4–2.2Ga). Contrasting datasets have been used to advocate for and against the rise of free oxygen in the atmosphere prior to 2.4Ga based on various geochemical tracers, and evidence for a partially oxidized environment remains elusive. Herein, we present stable isotope datasets for Mo and Cr, in conjunction with major and trace elements from four black shale horizons spanning a time interval from 3.46 to 2.76Ga in order to ascertain atmospheric oxygen levels through the Archean. The Mo, Cr and U elemental signatures within all sedimentary units are dominated by continental input. Both Mo isotopic values and Mo and U elemental signatures suggest anoxic conditions with no indication of redox cycling of these elements. This contradicts previous interpretations of an oxidized atmosphere based on published sulfur isotopic data (Ohmoto et al., 2006) and oxidized hematite occurrences at ~3.45Ga (Hoashi et al., 2009). Cr concentration patterns within the sample suites are controlled by continental provenance composition rather than by an authigenic Cr seawater contribution. This interpretation is supported by the relatively homogeneous Cr isotopic compositions across all sample suites and is consistent with high temperature controlled Cr isotopic ratios. Identification of redox cycling processes within detrital dominated sediments with Cr isotopes is likely to be less sensitive compared to Mo isotopes, due to the much smaller authigenic/crustal enrichment factors for Cr. [Copyright &y& Elsevier]