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Rare earth elements are required for the energy transition and new sources will be needed to meet projected demand. Coal combustion by-products could be a promising, near-term source of rare earth elements.

Phosphorus (P) is typically considered to be the ultimate limiting nutrient for Earth's biosphere on geologic timescales. As P is monoisotopic, its sedimentary enrichment can provide some insights into how the marine P cycle has changed through time. A previous compilation of shale P enrichments argued for a significant change in P cycling during the Ediacaran Period (635-541 Ma). Here, using an updated P compilation-with more than twice the number of samples-we bolster the case that there was a significant transition in P cycling moving from the Precambrian into the Phanerozoic. However, our analysis suggests this state change may have occurred earlier than previously suggested. Specifically in the updated database, there is evidence for a transition ~35 million years before the onset of the Sturtian Snowball Earth glaciation in the Visingsö Group, potentially divorcing the climatic upheavals of the Neoproterozoic from changes in the Earth's P cycle. We attribute the transition in Earth's sedimentary P record to the onset of a more modern-like Earth system state characterized by less reducing marine conditions, higher marine P concentrations, and a greater predominance of eukaryotic organisms encompassing both primary producers and consumers. This view is consistent with organic biomarker evidence for a significant eukaryotic contribution to the preserved sedimentary organic matter in this succession and other contemporaneous Tonian marine sedimentary rocks. However, we stress that, even with an expanded dataset, we are likely far from pinpointing exactly when this transition occurred or whether Earth's history is characterized by a single or multiple transitions in the P cycle.

Sedimentary manganese carbonate deposits, the major economic source of Mn globally, are the product of complex interactions that occur in the marine environment, including both biological Mn(II) oxidation and Mn(IV) reduction. Precise and accurate age constraints for Mn carbonate deposits have been difficult to obtain, hindering the understanding of possible correlations between Mn metallogenic and paleoenvironmental processes at regional to global scale. The involvement of organic matter during Mn carbonate mineralization, however, allows for the Re-Os system, an ideal geochronological tool for determining the depositional or alteration ages of organic-rich rocks, to be applied. Here we present the first Re-Os systematics of Mn carbonate ores from the giant Ortokarnash Mn deposit in the West Kunlun orogenic belt, Xinjiang, China. The use of the Re-Os geochronometer, along with petrographic, whole-rock total organic carbon, and major element analyses, allows for the depositional age and mineralizing processes to be directly constrained. The Mn carbonate ores with relatively homogeneous initial 187Os/188Os values yield a robust mineralization age of 320.3 ± 6.6 Ma (Model 1; Isoplot regression) or 321.8 ± 14.5 Ma (Monte Carlo simulation). This age correlates well with U-Pb ages of the youngest detrital zircon group from the footwall volcanic breccia-bearing limestone and a newly obtained Re-Os age from the hanging-wall marlstones. Enrichment of hydrogenous Re and Os in the Ortokarnash Mn carbonate ores is likely related to the variable redox environments during Mn carbonate mineralization, where Re tends to be preserved in the organic matter that persists following the diagenetic reduction of the Mn(IV) oxyhydroxides in suboxic or anoxic sediment pore water. Conversely, Os was likely absorbed by Mn(IV) oxyhydroxides in oxic seawater during Mn(II) oxidation. Elevated Osinitial(i) for the Mn carbonate ores relative to that of the coeval global seawater value suggests that an increased riverine flux may have been a contributing factor leading to Mn mineralization.

The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ56Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ13C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

Iron formations (IFs) are iron-rich chemical sediments that were widespread throughout the Precambrian. The deposition of IFs has provided key evidence about the prevalence of anoxic and ferruginous marine conditions throughout Earth’s history. Here, we describe the Shikebutai IF, hosted in a Late Carboniferous volcano-sedimentary sequence in the Western Tianshan region of northwest China. The IF is largely composed of hematite, jasper, and siderite. The chemical composition of the Shikebutai IF is dominated by Fe2O3 and SiO2, and very low concentrations of Al2O3, TiO2, Th, Nb, and Sc were observed, indicative of an authigenic origin. High Fe/Ti, Fe/Al, Co/Zn, and Ni/Zn ratios, along with positive Eu anomalies and eNd(t) values point to a significant submarine volcanic and hydrothermal contribution. The absence of negative Ce anomalies and heterogeneous δ56Fe values suggest progressive oxidation of hydrothermally sourced Fe(II) and deposition in an anoxic to low oxygen basin. Low P/Fe ratios in the Shikebutai IF, comparable to those found in the Precambrian, are consistent with Si inhibition of P sorption and P drawdown with high hydrothermal inputs. The intimate association of this Late Carboniferous IF with submarine volcanic activity suggests an origin that is analogous to Precambrian Algoma-type IFs. Further, the Shikebutai IF provides evidence for an episode of basin-scale anoxic, ferruginous conditions coinciding with the generally well-oxygenated deep oceans of the Phanerozoic, with deposition driven by enhanced submarine volcanic and hydrothermal activity in the Western Tianshan region during the Late Carboniferous. This demonstrates how, even with near modern surface oxygen levels, enhanced hydrothermal activity can shape the marine redox landscape and cause the development of ferruginous conditions without quantitative sulfate drawdown.

The Upper Carboniferous Ortokarnash manganese ore deposit in the West Kunlun orogenic belt of the Xinjiang province in China is hosted in the Kalaatehe Formation. The latter is composed of three members: (1) the 1st Member is a volcanic breccia limestone, (2) the 2nd Member is a sandy limestone, and (3) the 3rd Member is a dark gray to black marlstone containing the manganese carbonate mineralization, which, in turn, is overlain by sandy and micritic limestone. This sequence represents a single transgression-regression cycle, with the manganese deposition occurring during the highstand systems tract. Geochemical features of the rare earth elements (REE+Y) in the Kalaatehe Formation suggest that both the manganese ore and associated rocks were generally deposited under an oxic water column with Post-Archean Australian Shale (PAAS)-normalized REE+Y patterns displaying characteristics of modern seawater (e.g., light REE depletion and negative Ce anomalies). The manganese ore is dominated by fine-grained rhodochrosite (MnCO3), dispersed in Mn-rich silicates (e.g., friedelite and chlorite), and trace quantities of alabandite (MnS) and pyrolusite (MnO2). The replacement of pyrolusite by rhodochrosite suggests that the initial manganese precipitates were Mn(IV)-oxides. Precipitation within an oxic water column is supported by shale-normalized REE+Y patterns from the carbonate ores that are characterized by large positive Ce (>3.0) anomalies, negative Y (~0.7) anomalies, low Y/Ho ratios (~20), and a lack of fractionation between the light and heavy rare earth elements ((Nd/Yb)PAAS ~0.9). The manganese carbonate ores are also 13C-depleted, further suggesting that the Mn(II) carbonates formed as a result of Mn(III/IV)-oxide reduction during burial diagenesis.

Several Late Devonian sedimentary successions host pyrite with highly positive sulfur isotope values (δ34Spyrite). These anomalous values have been linked to marine anoxia, low sulfate concentrations in seawater, or aerobic re-oxidation of dissolved sulfide within well-oxygenated bottom waters in a local depositional environment. Implicit to these previous models is the assumption of a biogenic genesis from microbial sulfate reduction (MSR) of these pyrites, which, in turn, can be used to understand the biogeochemical sulfur cycle and reconstruct paleoenvironmental conditions. In South China, the Late-Devonian bulk-rock sulfur isotope excursion occurs in the lower Fammenian Wuzhishan Formation of the Youjiang Basin, in both the sedimentary limestone that corresponds to the Late triangularis conodont zone at the Fuhe section and the sediment-hosted Xialei Mn deposit that lies between the Late triangularis and crepida conodont zones. To further constrain the significance of these 34S-enriched pyrites, we carried out detailed textural and in-situ sulfur isotope examinations of pyrites in the Xialei Mn deposit. We found one type of pyrite in siliceous limestone referred to as the host rocks and three distinct pyrite types in Mn ores: Type I – pyrites with Mn-carbonate inclusion-rich cores; Type II – pyrite overgrowths surrounding an “inclusion-rich” core; and Type III – relatively inclusion-poor and mainly subhedral disseminated pyrite grains. Type I pyrites in Mn ores have negative δ34Spyrite values from −7.8‰ to −2.0‰. Types II and III pyrites, and pyrites in the host rock are characterized by highly positive δ34Spyrite values (+4.2‰ to +37.6‰), with some values being higher than that of contemporaneous seawater sulfate. Type I pyrites with negative δ34S values likely formed via MSR in a closed porewater system, driven by increased sedimentation rates during a sea level lowstand. The other pyrites with highly positive δ34S values appear to have formed during secondary hydrothermal alteration, with their sulfur sourced through thermochemical sulfate reduction (TSR) of sulfate from dissolved barites in the underlying Liujiang Formation. This implies that the positive δ34S values of the Late Devonian pyrites from the Wuzhishan Formation represent a later alteration event caused by post-depositional hydrothermal fluids, and may lack a direct biogeochemical connection to the surface sulfur cycle. These results provide an important perspective on the origin of variability in sulfur isotope records and suggest the need for careful petrographic screening and micrometer-scale analysis of sedimentary units used to reconstruct paleoenvironmental conditions.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. It has commonly been proposed that the development of complex life is tied to increases in atmospheric oxygenation. However, there is a conspicuous gap in time between the oxygenation of the atmosphere 2.4 billion years ago (Ga) and the first widely-accepted fossil evidence for complex eukaryotic cells . At present the gap could either represent poor sampling, poor preservation, and/or difficulties in recognizing early eukaryote fossils, or it could be real and the evolution of complex cells was delayed due to relatively low and/or variable O2 levels in the Paleoproterozoic. To assess the extent and stability of Paleoproterozoic O2 levels, we measured chromium-based oxygen proxies in a core from the Onega Basin (NW-Russia), deposited billion years ago—a few hundred million years prior to the oldest definitive fossil evidence for eukaryotes. Fractionated chromium isotopes are documented throughout the section (max. ‰ ), suggesting a long interval (possibly >100 million years) during which oxygen levels were higher and more stable than in the billion years before or after. This suggests that, if it is the case that complex cells did not evolve until after 1.7 Ga, then this delay was not due to O2-limitation. Instead, it could reflect other limiting factors—ecological or environmental—or could indicate that it simply takes a long time—more than the tens to >100 million years recorded in Onega Basin sediments—for such biological innovations to evolve.

Ancient iron formations - iron and silica-rich chemical sedimentary rocks that formed throughout the Precambrian eons - provide a significant part of the evidence for the modern scientific understanding of palaeoenvironmental conditions in Archaean (4.0–2.5 billion years ago) and Proterozoic (2.5–0.539 billion years ago) times. Despite controversies regarding their formation mechanisms, iron formations are a testament to the influence of the Precambrian biosphere on early ocean chemistry. As many iron formations are pure chemical sediments that reflect the composition of the waters from which they precipitated, they can also serve as nuanced geochemical archives for the study of ancient marine temperatures, redox states, and elemental cycling, if proper care is taken to understand their sedimentological context.

Ratios of phosphorous (P) to iron (Fe) in Precambrian banded iron formations (BIFs) have previously been used to estimate dissolved seawater phosphate concentrations in the ancient oceans. Such studies rely on an assumed composition of the primary iron minerals, the concentrations of the major ions in seawater, and empirical partitioning coefficients for phosphate sorption to Fe(III) (oxyhydr)oxides. There is limited data, however, regarding the post-depositional stability of phosphate associated with presumed primary BIF iron minerals, such as ferrihydrite under low-grade metamorphic temperature and pressure conditions. Here we experimentally formed ferrihydrite in the presence of silica, which was abundant in the Precambrian oceans, and then incubated it at 170 °C and 1.2 kbar in the presence or absence of organic carbon (Corg; either glucose or microbial biomass) as a proxy for ancient planktonic biomass. We found that the post-metamorphic mineral assemblage resulting from thermochemical Fe(III) reduction of Si-doped ferrihydrite depended on Corg reactivity: In the presence of highly reactive glucose, siderite, magnetite, and vivianite were formed, with less than 1.2 mol% of phosphate (0.5 M NaCl extractable) being mobilized. In contrast, the reaction of Si-doped ferrihydrite with less reactive microbial biomass resulted in the formation of hematite and siderite, but not vivianite, and approximately 10 mol% of phosphate was remobilized into the pore fluids. Collectively, our data suggest that the fidelity with which BIFs record ancient oceanic phosphate concentrations depends on the mineralogy and diagenetic history of individual BIFs but should be reliable within 10%.

This study investigated the biogeochemistry of uranium (U), and the co-occurring elements nickel (Ni) and arsenic (As), in colloids and sediments from two meromictic mine pit lakes that have considerably different depths and geochemistry. In order to characterize the processes controlling metal speciation and cycling in the pits, the distribution and speciation of the elements in colloidal size fractions were analyzed using micro- and ultrafiltration in combination with transmission electron microscopy. Sediment traps collected fresh sediments over the course of one year below the chemocline of the pit lakes and were subsequently analyzed by scanning electron microscopy (SEM) for morphology and chemical digestions. The most common particles found in the shallower pit consisted of Ca-O and Fe-O colloids, while the particles in the deeper pit were composed of Ca-S-O. Filtration results showed a higher abundance of metals in larger colloidal fractions in aged samples, suggesting that colloids can act as metal accumulators. Sediment traps showed the formation of Fe-O, Fe-S, Al-Si, and Ce-P phases, which were observed to sorb U and Ni. The overall U removal was calculated to be 0.9 g/m2/year in both pits, despite considerably different geochemical conditions between the two, and the maximum removal rates for As (shallow pit) and Ni (deeper pit) were 4.7 g/m2/year and 0.6 g/m2/year, respectively. Bottom sediments were also collected from both pits, and characterized using sequential extractions, SEM, synchrotron-based X-ray absorption spectroscopy and Laue diffraction techniques. These techniques showed that the stability of metals in the sediment follows the order Ni

Highlights • The adsorption of Co, Ni, Cu, and Zn to Synechococcus sp PCC 7002 was studied using a surface complexation modelling approach. • A surface complexation model was developed to determine the thermodynamic binding constants of Co, Ni, Cu, and Zn to Synechococcus. • The surface complexation model was able to accurately predict the competitive adsorptionof the four metals to Synechococcus. • Synechococcus could have been an important exit channel for trace elements into ancient sediments such as BIF Marine bacterial plankton play a key role in elemental cycling through their ability to bind, assimilate, metabolize, and modify the redox state of trace metals in seawater. Of those processes, arguably the least studied are the mechanisms underpinning trace metal adsorption to planktonic marine bacteria, despite a plethora of literature pertaining to terrestrial species. Recently, Liu et al. (2015) demonstrated that the marine cyanobacterium Synechococcus sp. PCC 7002 has the capacity to remove appreciable amounts of Cd2+, a proxy for other divalent cations, from seawater by adsorption. In this study, we build on that work and employ a surface complexation modelling (SCM) approach using titration and pH adsorption edge experiments to calculate the thermodynamic binding constants of four bioessential transition metals (Co, Ni, Cu, Zn) to Synechococcus in simulated seawater. Based on the titration results, the major functional groups involved in metal binding were carboxyl groups with a pKa of 5.59 and phosphoryl groups with a pKa of 7.61. Metal adsorption experiments indicate that Synechococcus can bind considerable concentrations of Zn, Cu, Ni, and Co at pH 8. When all four metals are simultaneously added to solution, the same adsorption pattern of Zn > Cu > Ni > Co is maintained, and accurately predicted by the SCM. Based on average marine cell densities and turnover rates of Synechococcus cells in the photic zone, we calculate that Synechococcus, in the absence of competing ligands such as dissolved organic matter (DOM), has the theoretical capacity to remove nearly all of the free metal cations from seawater. These observations highlight the surface reactivity of marine cyanobacteria as a potentially important vector for the transfer of dissolved metals from the photic zone to deeper waters or the seafloor in modernoceans, but they also have implications for the Precambrian oceans as sinking cyanobacteria could have acted as an exit channel for trace elements into ancient sediments including banded iron formations (BIF).

The Jiertieke-Zankan area lies in the Taxkorgan region (West Kunlun Mountains) of Xinjiang province, northwestern China. The area contains the largest commercial iron ore belt in the region. Furthermore, this iron ore belt hosts the youngest banded iron formation identified to date (the Jiertieke iron deposit), which was deposited during the Early Cambrian. Previous studies have mainly provided evidence regarding the depositional environment and formational age of this area, but the tectonic setting and geochronology of the ore deposits remains poorly studied. Here, we provide an integrated examination of high-precision SIMS zircon U-Pb dating, along with the major and trace element geochemistry of the metasedimentary rock (a biotite-quartz schist) that host the iron ores. The Jiertieke area is characterized by bedded metapelites that contain detrital zircons as young as 546 Ma, whereas the Zankan area contains metapsammites, in addition to the metapelites, as young as 541 Ma. Multiple detrital zircons age groups ranging from 700–1100 Ma, 1600–1850 Ma, and 2250–2500 Ma, indicate the input of ancient clastic material to the basin. Based on provenance indicators and Nd isotopic data, they suggest that the metasedimentary rocks are the product of an undifferentiated felsic terrain, likely ancient upper continental crust, being eroded. Given the geochronological and geochemical features of the metasedimentary rocks, and previously identified bimodal volcanics within the region, we suggest that the tectonic setting of the Zankan-Jiertieke area was a back-arc rift basin during the Late Neoproterozoic to Early Cambrian.

The ancestors of cyanobacteria generated Earth’s first biogenic molecular oxygen but how they dealt with its toxicity remains unconstrained. Here we investigated when superoxide dismutase enzymes (SODs) capable of removing superoxide free radicals evolved. We found phylogenetic evidence that ancestral cyanobacteria used SODs with copper and zinc cofactors (CuZnSOD) during the Archaean. By the Paleoproterozoic, they became genetically capable of using iron, nickel, and manganese as cofactors (FeSOD, NiSOD, and MnSOD respectively). The evolution of NiSOD is particularly intriguing because it has been previously hypothesized that declining seawater Ni concentrations at the end of the Archaean caused a fundamental shift in the marine biosphere away from methanogenesis towards oxygenic photosynthesis. Our novel analyses of enzymes dealing with O2 toxicity now demonstrate that the beneficiaries of this chemical change - marine planktonic cyanobacteria - were able to utilize the remaining Ni from seawater 0.9-0.8 Ga to supplement their existing metabolic capabilities.

The Zaonega Formation in northwest Russia (~2.0 billion years old) is amongst the most complete successions that record the middle of the Palaeoproterozoic era. As such, geochemical data from the formation have played a central role in framing the debate over redox dynamics in the aftermath of the Great Oxidation Event (GOE). However, uncertainty over local redox conditions and the degree of hydrographic restriction in the formation has led to contradictory interpretations regarding global oxygen (O2) fugacity. Here, we provide new iron (Fe) isotope data together with major and trace element concentrations to constrain the local physiochemical conditions. The Zaonega Formation sediments show authigenic Fe accumulation (Fe/Al ≫ 1 wt.%/wt.%) and δ56Fe ranging from −0.58‰ to +0.60‰. Many of the data fall on a negative Fe/Al versus δ56Fe trend, diagnostic of a benthic Fe shuttle, which implies that Zaonega Formation rocks formed in a redox-stratified and semi-restricted basin. However, basin restriction did not coincide with diminished trace metal enrichment, likely due to episodes of deep-water exchange with metal-rich oxygenated seawater, as evidenced by simultaneous authigenic Fe(III) precipitation. If so, the Onega Basin maintained a connection that allowed its sediments to record signals of global ocean chemistry despite significant basinal effects.

Geobiology, and the subdiscipline of geomicrobiology, are focused on understanding the interactions between life and the solid Earth. This encompasses the entirety of life's interaction with the planet and covers a diverse set of topics including: the diversity of microbial life, the metabolisms of microbial life within the natural environment, the cycling of trace elements, and the evolution of life over Earth's 4-billion-year history. Geomicrobiology, as a subdiscipline, is inherently more focused on the interactions of microbial life with the environment. This is an aspect of geology that is fundamental to our understanding of the Earth system as microbial life has the power to influence a wide variety of processes in both surface and sub-surface environments. In this context, an area of geobiology that has received much focus is how microbes have shaped Earth's surface environments over the last 4.6 billion years and, in turn, how this has affected the evolution of life on Earth. While the field of geobiology continues to grow, having benefited from advances in technology for in-situ measurements, the diversity in topics addressed also continues to expand. In recent years, geobiology has begun to employ traditional geochemical techniques to address medical problems, inform the search for life on Mars and other exoplanets, and provide an important context for assessing future climate scenarios. Given the breadth of topics that geobiology and geomicrobiology seek to address, these are two highly interdisciplinary fields that have led to new insights into how life and the Earth have interacted in the past and will continue to interact in the future.

Deciphering the role—if any—that free oxygen levels played in controlling the timing and tempo of the radiation of complex life is one of the most fundamental questions in Earth and life sciences. Accurately reconstructing Earth's redox history is an essential part of tackling this question. Over the past few decades, there has been a proliferation of research employing geochemical redox proxies in an effort to tell the story of Earth's oxygenation. However, many of these studies, even those considering the same geochemical proxy systems, have led to conflicting interpretations of the timing and intensity of oxygenation events. There are two potential explanations for conflicting redox reconstructions: (i) that free oxygen levels were incredibly dynamic in both time and space or (ii) that collectively, as a community—including the authors of this article—we have frequently studied rocks affected by secondary weathering and alteration (particularly secondary oxidation) while neglecting to address the impact of this alteration on the generated data. There are now multiple case studies that have documented previously overlooked secondary alteration, resolving some of the conflicting constrains regarding redox evolution. Here, an analysis of a large shale geochemistry database reveals significant differences in cerium (Ce) anomalies, a common palaeoredox proxy, between outcrop and drill core samples. This inconsistency provides support for the idea that geochemical data from altered samples are frequently published in the peer-reviewed literature. As individuals and a geochemical community, most of us have been slow to appreciate how pervasive the problem is but there are examples of other communities that have faced and met the challenges raised by such quality control crises. Further evidence of the high potential for alteration of deep-time geochemical samples, and recognition of the manner in which this may lead to spurious results and palaeoenvironmental interpretations, indicate that sample archiving, in publicly accessible collections needs to become a prerequisite for publication of new palaeoredox data. Finally, the geochemical community need to think about ways to implement additional quality control measures to increase the fidelity of palaeoredox proxy work.

The organic geochemistry of the Paleogene-aged second and first members of the Kongdian Formation (Ek2 and Ek1) and the third member of the Shahejie Formation (Es3) in the Cangdong sag, Bohai Bay Basin, China, are characterized and the hydrocarbon generating potential is evaluated. The Ek2 source rocks have a good to very good oil-generating potential, as indicated by the high abundance and good quality of organic matter within a mature stage. Although the Ek1 mudstones have entered the oil window, they are organic lean and are dominantly composed of Type III kerogen. Accordingly, the Ek1 mudstones are considered to be poor source rocks. The Es3 member is thermally mature and contains several intervals that have high contents of good-quality organic matter (mixed Type I and II kerogens), suggesting that these intervals also have a good potential for oil-generation. Given this potential, new exploration plays sourced from the Es3 member could be possible in the Cangdong sag. Biomarker distributions in the Ek2 and Es3 source rocks are both characterized by a dominance of medium- to high-molecular-weight n-alkanes (n-C18 to n-C29), a low sterane/hopane ratio, and a low diasterane abundance. The Ek2 samples have low Pr/Ph ratios ( 0.1) and β-carotane, trace amounts of C30 4α-methyl-24-ethlycholestanes, and a predominance of C29 regular steranes. Collectively, these characteristics indicate that abundant prokaryotic biomass mixed with some terrigenous organic matter was deposited under reducing conditions within a stratified water column. In contrast, the Es3 source rocks are characterized by variable Pr/Ph ratios, low gammacerane and β-carotane, comparable C27 and C29 regular steranes, and relatively high C30 4α-methyl-24-ethlycholestanes. These properties indicate that the Es3 source rocks contain significant contributions from prokaryotic and algal sources, and were deposited under reducing to suboxic conditions within a relatively less stratified water column.

Biochar has emerged as a useful amendment to release nutrients into agricultural soil, to increase crop productivity, and as a sorbent to remediate metals and organics contamination. Since soils have heterogeneous physical properties across a given crop field, and even over a growing season, it is imperative to select the most appropriate biochar for the intended purpose and in defining the amendment level. In this study, we investigate the adsorption of Cd(II) and Se(VI) as model pollutant cations and anions, respectively, to two agricultural soils amended with a wood pin chip biochar (WPC). The proton reactivity of each sorbent was determined by potentiometric titration, and single-metal, single-sorbent experiments were conducted as a function of pH. The resulting data were modeled using a non-electrostatic surface complexation modeling (SCM) approach to determine the proton and metal binding constants and surface functional group concentrations of each soil and WPC. The SCM approach is a considerable advance over empirical modeling approaches because SCM models can account for changes in pH, ionic strength, temperature, and metal-to-sorbent ratio that may happen over the course of a growing season. The constants derived from the single-metal, single-sorbent experiments were then used to predict the extent of metal adsorption in more complex mixtures of Cd, Se, soil and WPC. Overall the SCM approach was successful in predicting metal distribution in multi-component mixtures. In cases where the predictions were poorer than expected, we identify reasons and discuss future experiments needed to further the application of SCM to sorbent mixtures containing biochar.

Wanning nickel (Ni) abundances in the Neoarchean may have led to a decline in marine methanogenic bacteria due to the loss of a key micronutrient, causing a collapse in atmospheric methane levels and the subsequent rise of atmospheric oxygen – the Great Oxidation Event ∼2.5 Ga. However, this hypothesis currently lacks geological evidence that corroborates the processes that led to a decline in the supply of Ni to the oceans at that time. Here we investigate temporal variations of Ni concentrations in the likely source rocks – various volcanic lithologies - spanning the past 3.5 Byr. By applying a spatially gridded resampling to geochemical data from a global compilation of ∼96,000 continental volcanic rocks, we show that komatiites and basaltic-andesitic rocks largely controlled Ni fluxes to the Earth's surface during the Archean but had a waning influence thereafter due to the secular cooling of the mantle. A new compilation of marine shales (397 samples) and an updated compilation of banded iron formations (2037 samples) further confirms a declining clastic and dissolved supply of terrestrial Ni, respectively. Interestingly, we further observed a stable marine Ni concentration for the last 2 Byr. To explain this unexpected trend, we provide a computational model of mantle melting demonstrating that increasing melting pressures likely inhibited a further decline of Ni concentration in volcanic rocks and led to this relative consistency in Ni supply.

The oceans probably remained well-oxygenated for millions of years after the Palaeoproterozoic Lomagundi-Jatuli Event, according to high concentrations and isotope signatures of redox-sensitive metals in the 2-billion-year-old Zaonega Formation, Russia. The approximately 2,220-2,060 million years old Lomagundi-Jatuli Event was the longest positive carbon isotope excursion in Earth history and is traditionally interpreted to reflect an increased organic carbon burial and a transient rise in atmospheric O-2. However, it is widely held that O-2 levels collapsed for more than a billion years after this. Here we show that black shales postdating the Lomagundi-Jatuli Event from the approximately 2,000 million years old Zaonega Formation contain the highest redox-sensitive trace metal concentrations reported in sediments deposited before the Neoproterozoic (maximum concentrations of Mo = 1,009 mu g g(-1), U = 238 mu g g(-1) and Re = 516 ng g(-1)). This unit also contains the most positive Precambrian shale U isotope values measured to date (maximum U-238/U-235 ratio of 0.79 parts per thousand), which provides novel evidence that there was a transition to modern-like biogeochemical cycling during the Palaeoproterozoic. Although these records do not preclude a return to anoxia during the Palaeoproterozoic, they uniquely suggest that the oceans remained well-oxygenated millions of years after the termination of the Lomagundi-Jatuli Event.

Banded iron formations are critical to track changes in Archaean to Palaeoproterozoic ocean chemistry, with deposition triggered by water column iron oxidation. Recently, however, it was suggested that reduced iron minerals were the primary precipitates, and these were subsequently oxidized by oxygen-bearing groundwater. If true, this would cast doubt on our understanding of how banded iron formations were deposited and their ability to record early ocean chemistry. Here we present a hydrogeological box model, based on the approximately 2.5 billion year old Hamersley Basin of Western Australia, developed to evaluate the plausibility of secondary iron oxidation. The box model calculates the time required for groundwater to flux enough oxygen through the basin to oxidize a given amount of ferrous iron. Less than 9% of nearly four million model iterations returned oxidation times less than the age of the basin. Successful simulations required simultaneously steep hydraulic gradients, high permeability and elevated oxygen concentrations. Our simulations show that the postdepositional oxidation of large banded iron formations is unlikely, except on a limited scale (that is, during secondary ore formation), and that oxidized iron phases were probably the precursor to large Palaeoproterozoic banded iron formations. Banded iron formations could not have formed by postdepositional oxidation, according to four million hydrogeological box model iterations that failed to reproduce secondary oxidation on reasonable timescales.

International audience; While we appreciate Lascelles’ interest in our work, we find most of the points raised to be either poorly-supported or arising from a significant misunderstanding.

Measuring the metal binding potential and reactivity of bacterial mats is challenging in alkaline and carbonate-rich systems. Traditional methods used to measure these parameters, such as potentiometric titrations and metal adsorption pH edges, are difficult to implement due to the presence of the carbonate minerals that buffer pH and prevent assessment of mat surface reactivity. Additionally, under alkaline conditions metals may form hydroxide and/or carbonate precipitates. In this study we examined the metal binding capacity of four distinct bacterial mats collected from Fairmont Hot Springs, BC, Canada. To prevent metal precipitation, the bacterial mat concentration was varied under a constant initial cadmium (Cd) concentration of 8.89 μM and at pH 8. In addition to the intact bacterial mats, a carbonate mineral sample and two bacterial mats in which the carbonate mineral was removed via acid-treatment, were used as end-members to assess the mechanisms of reactivity in the whole system. Freundlich adsorption isotherms were used to fit metal adsorption data and directly compare surface reactivity among intact mats and mat components. Two of the intact mats exhibited a higher affinity for Cd compared to the mineral at metal equilibrium concentrations above 2.5 μM, while the other two intact mats had lower affinities under all experimental conditions. Generally, we found the acid-treated mats had higher Cd adsorption capacities than the carbonate mineral. When compared to their equivalent intact mats, only one acid-treated mat had a higher affinity for Cd. Further, we modeled whether metal adsorption in the intact mats, containing microbes and carbonate mineral, could be explained by a linear combination of the observed metal uptake by the organic and inorganic components through end-member experiments. Metal adsorption additivity results were mixed. Metal uptake by one intact mat was found to be additive, while for the other mat the additive model significantly underestimated the observed Cd accumulation. Our study demonstrates the potential, as well as the limitations, of using modified metal adsorption edges to determine the metal binding affinity and surface reactivity of bacterial mats in alkaline and carbonate-rich systems.

The macronutrient phosphorus is thought to limit primary productivity in the oceans on geological timescales. Although there has been a sustained effort to reconstruct the dynamics of the phosphorus cycle over the past 3.5 billion years, it remains uncertain whether phosphorus limitation persisted throughout Earth's history and therefore whether the phosphorus cycle has consistently modulated biospheric productivity and ocean-atmosphere oxygen levels over time. Here we present a compilation of phosphorus abundances in marine sedimentary rocks spanning the past 3.5 billion years. We find evidence for relatively low authigenic phosphorus burial in shallow marine environments until about 800 to 700 million years ago. Our interpretation of the database leads us to propose that limited marginal phosphorus burial before that time was linked to phosphorus biolimitation, resulting in elemental stoichiometries in primary producers that diverged strongly from the Redfield ratio (the atomic ratio of carbon, nitrogen and phosphorus found in phytoplankton). We place our phosphorus record in a quantitative biogeochemical model framework and find that a combination of enhanced phosphorus scavenging in anoxic, iron-rich oceans and a nutrient-based bistability in atmospheric oxygen levels could have resulted in a stable low-oxygen world. The combination of these factors may explain the protracted oxygenation of Earth's surface over the last 3.5 billion years of Earth history. However, our analysis also suggests that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic eon (between 800 and 635 million years ago), coincident with a previously inferred shift in marine redox states, severe perturbations to Earth's climate system, and the emergence of animals.

Life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes. These requirements differ between organisms and have evolved over geological time, likely guided in some part by environmental conditions. Until recently, most of what was understood regarding trace element concentrations in the Precambrian oceans was inferred by extrapolation, geochemical modeling, and/or genomic studies. However, in the past decade, the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new, and potentially more direct, insights into secular changes in seawater composition – and ultimately the evolution of the marine biosphere. Compiled records of many bioessential trace elements (including Ni, Mo, P, Zn, Co, Cr, Se, and I) provide new insight into how trace element abundance in Earth's ancient oceans may have been linked to biological evolution. Several of these trace elements display redox-sensitive behavior, while others are redox-sensitive but not bioessential (e.g., Cr, U). Their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere-ocean redox conditions that are linked to biological evolution, for example, the activity of oxygen-producing, photosynthetic cyanobacteria. In this review, we summarize available Precambrian trace element proxy data, and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life. We also examine several biologically relevant and/or redox-sensitive trace elements that have yet to be fully examined in the sedimentary rock record (e.g., Cu, Cd, W) and suggest several directions for future studies.

Banded iron formations (BIFs) are enigmatic chemical sedimentary rocks that chronicle the geochemical and microbial cycling of iron and carbon in the Precambrian. However, the formation pathways of Fe carbonate, namely siderite, remain disputed. Here, we provide photomicrographs, Fe, C and O isotope of siderite, and organic C isotope of the whole rock from the ~2.52 Ga Dagushan BIF in the Anshan area, China, to discuss the origin of siderite. There are small magnetite grains that occur as inclusions within siderite, suggesting a diagenetic origin of the siderite. Moreover, the siderites have a wide range of iron isotope compositions (&delta, 56FeSd) from &minus, 0.180&permil, to +0.463&permil, and a relatively negative C isotope composition (&delta, 13CSd = &minus, 6.20&permil, to &minus, 1.57&permil, ). These results are compatible with the reduction of an Fe(III)-oxyhydroxide precursor to dissolved Fe(II) through microbial dissimilatory iron reduction (DIR) during early diagenesis. Partial reduction of the precursor and possible mixing with seawater Fe(II) could explain the presence of siderite with negative &delta, 56Fe, while sustained reaction of residual Fe(III)-oxyhydroxide could have produced siderite with positive &delta, 56Fe values. Bicarbonate derived from both DIR and seawater may have provided a C source for siderite formation. Our results suggest that microbial respiration played an important role in the formation of siderite in the late Archean Dagushan BIF.

Banded iron formations (BIF) represent one of the few chemical sedimentary archives available for exploring the redox state of the Precambrian atmosphere-ocean system and the marine availability of biologically critical trace nutrients through time. Temporal records compiled for a number of trace elements show a relatively wide spread in concentrations spanning several orders of magnitude within a single deposit. It has been suggested that this spread is present in both bulk rock digestion and laser ablation data, however, the comparability of data from the two methods has not been thoroughly evaluated. Here, we provide a statistical assessment of how comparable laser ablation data of individual BIF hematite and magnetite grains are to one another, as well as how closely averages of laser ablation data approximate bulk rock values. To do so, we use (laser ablation)-high resolution inductively coupled plasma mass spectrometry ((LA)-HR-ICP-MS) data from 14 BIF slabs and corresponding powders that span four different formations. We focus in particular on BIF from the ≥3.75 Ga Nuvvuagittuq Supracrustal Belt (Quebec, Canada) and multiple BIF deposits of the 2.6–2.45 Ga Hamersley Group (North Western Australia), representing deposits separated by over a billion years and of differing metamorphic grade. The scaling between selected transition metals is also evaluated in light of expectations for possible delivery mechanisms of trace elements from seawater to BIF precursor sediments – the adsorption to hydrous ferric oxides, biological vectors, and detrital influence. Our findings show that average trace element concentrations between bulk versus laser ablation data typically vary within an order of magnitude. Furthermore, reproducibility between the two methods appears to be independent of formation age, metamorphic grade, or depositional setting. These findings underscore the overall fidelity of the BIF record at capturing seawater compositional signatures at various scales. We demonstrate that evaluations of ancient seawater chemistry using the BIF record are best served by a combined approach utilizing both bulk rock and laser ablation data, as bulk methods offer the most conservative proxy data for ancient seawater composition but may mask significant enrichments or depletions that are detectable at the grain-scale by laser ablation.

The onset of the Middle Permian Emeishan Large Igneous Province (ELIP), is commonly implicated in the Guadalupian mass extinction, one of the largest Phanerozoic extinction events, yet the contribution of the ELIP to this biotic event remains unresolved because of a lack of appropriate age constraints. In this study, we examined claystone beds overlying the cherty limestone of the Maokou Formation and underlying the flood basalts of the ELIP that were emplaced around the Guadalupian-Lopingian Boundary. Integrated petrographic, mineralogical, elemental, and isotopic features indicate that the studied claystones contain significant components from mafic volcanic ash that was likely associated with the emplacement of the ELIP. Zircons from the claystones yield U Pb ages of ~262.5 Ma, which predates the subaerial eruptive phase of the ELIP (259.1–260.4 Ma), but is comparable to the J. altudaensis conodont biostratigraphic zone (~263 Ma, Middle Capitanian Stage) which is contemporaneous with the emplacement of early-stage Emeishan subaqueous pillow basalts and mafic volcaniclastics. Our data provide a crucial new constraint on the lower age limit for the onset of the ELIP, which was likely associated with violent submarine eruptions and thus could have contributed to the loss of marine fauna during the Guadalupian mass extinction.

International audience; The Great Oxidation Event (GOE) at 2.45 Ga facilitated the global expansion of oxidized compounds in seawater. Here, we demonstrate that the GOE coincided with a sharp increase in arsenate and arsenic sulfides in marine shales. The dramatic rise of these oxygen-sensitive tracers overlaps with the expansion of key arsenic oxidants, including oxygen, nitrate, and Mn(IV) oxides. The increase in arsenic sulfides by at least an order of magnitude after 2.45 Ga is consistent with the proposed transition to mid-depth continental-margin sulfide-rich waters following the GOE. At the same time, the strong increase in arsenate content, to ~60% of the total arsenic concentration in shales, suggests that the oxidative component of the arsenic cycle was established for the first time in Earth's history. These data highlight the global emergence of a new selective pressure on the survival of marine microbial communities across the GOE, the widespread appearance of toxic, oxidized chemical species such as arsenate in seawater.

The Boolgeeda Iron Formation and overlying Turee Creek Group, Hamersley Basin, Western Australia, represent a conformable succession of sediment deposited between 2.45 and 2.22 Ga. This interval of geologic history is of significant interest because it spans the Great Oxidation Event (GOE), when oxygen first accumulated in Earth’s atmosphere. Here we present geochemical and petrographic data from the uppermost 45 m of the Boolgeeda Iron Formation and an additional 30 m of the overlying Kungarra Formation mudstones, as sampled from the Turee Creek Drilling Project 1 drill core (TCDP1). This core captures the termination of BIF deposition in the Hamersley Basin and coincides with a global decline in BIF deposition in the Paleoproterozoic. We provide a continuous, high resolution chemostratigraphic dataset of major and trace element concentrations, as well as Fe speciation data, to assess the relationship between the rise of atmospheric oxygen and the subsequent decline in BIF deposition. We also highlight the interplay between local and global controls on the preservation of redox signatures, including the rise and fall of local base-level, input of weathered continental material, influx of reduced sulfur species associated with the continental weathering, and global increases in atmospheric oxygen. We interpret Boolgeeda deposition to have taken place under shallow water, oxic conditions overlying anoxic ferruginous deep water. Intermittent periods of oxidative weathering led to influxes of redox sensitive trace elements from land as continental sulfide minerals were weathered. This also led to the temporary disappearance of MIF-S, but O2 remained below the threshold capable of completely eliminating S isotope signatures associated with the MIF-S recycling.

It has been proposed that anoxic and iron-rich (ferruginous) marine conditions were common through most of Earth history. This view represents a major shift in our understanding of the evolution of marine chemistry. However, thus far, evidence for ferruginous conditions comes predominantly from Fe-speciation data. Given debate over these records, new evidence for Fe-rich marine conditions is a requisite if we are to shift our view regarding evolution of the marine redox landscape. Here we present strong evidence for ferruginous conditions by describing a suite of Fe-rich chemical sedimentary rocks—banded iron formation (BIF)—-deposited during the Early Cambrian in western China. Specifically, we provide new U-Pb geochronological data that confirm a depositional age of ca. 527 Ma for this unit, as well as rare earth element (REE) data are consistent with anoxic deposition. Similar to many Algoma-type Precambrian iron formations, these Early Cambrian sediments precipitated in a back-arc rift basin setting, where hydrothermally sourced iron drove the deposition of a BIF-like protolith, the youngest ever reported of regional extent without direct links to volcanogenic massive sulphide (VMS) deposits. Their presence indicates that marine environments were still characterized by chemical- and redox-stratification, thus supporting the view that—despite a dearth of modern marine analogues—ferruginous conditions continued to locally be a feature of early Phanerozoic seawater.