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The Mesoproterozoic Era (1.6–1.0 Ga), long regarded as an interval of sluggish biotic evolution and persistently low atmospheric-oceanic oxygen levels, has become the subject of recent controversy regarding putative large-scale oxygenation events. In this study, we conducted a comprehensive investigation of redox, productivity, seawater sulfate concentrations, and hydrographic conditions for the ∼1.4–1.32-Ga Xiamaling Formation in the shallow Hougou and mid-depth Huangtugui sections in the Yanshan Basin (North China). Integrated Fe-trace metal data suggest that bottom-water redox conditions mainly underwent a three-stage secular evolution: (1) suboxic (-oxic), (2) predominantly anoxic, and (3) oxic. Integrated productivity (TOC, P/Al, and Cu-Zn-Ni enrichment factors) and depositional system (major and trace elemental enrichment patterns) data suggest varying productivity levels during deposition of the Xiamaling Formation, with high productivity fueled mainly by intense open-marine upwelling. In combination with published data from the deep Xiahuayuan and Chicheng sections, a refined stratigraphic correlation framework was developed for the Xiamaling Formation based on the basinal upwelling event and existing lithostratigraphic framework. Within this new framework, sulfur concentration and pyrite δ34S (δ34S py) data suggest a rise in marine sulfate concentrations at ∼1.36 Ga. Furthermore, strong spatiotemporal redox heterogeneity was observed within the ∼1.4–1.32-Ga Yanshan Basin, i.e., strong bottom-water anoxia (or euxinia) in deep sections and less anoxic (even oxic) conditions in shallow sections, although all sections experienced high productivity owing to oceanic upwelling. We propose a productivity-driven shelf oxygenation model to explain this spatiotemporal redox heterogeneity within the context of eustatic changes, an upwelling event, and the rise of oceanic sulfate concentrations at ∼1.36 Ga. In this model, from initially low atmospheric oxygen levels, increasing surface productivity in shelf areas due to elevated nutrient supplies from upwelling not only intensified bottom-water anoxia through organic matter export but also stimulated expansion of the ocean-surface oxic layer through photosynthetic O 2 release, contributing to the development of contrasting bottom-water redox conditions between shallow and deep sections during the upwelling event identified in this study. Our model helps reconcile conflicting observations concerning Mesoproterozoic marine redox conditions, and provides new insights into putative Mesoproterozoic oceanic oxygenation events and their relationship to the evolution of early eukaryotes. [ABSTRACT FROM AUTHOR]

Abstract As a self-contained source-reservoir petroleum system, shale gas plays have different generation mechanisms from conventional natural gas plays, especially with respect to the effects of residual hydrocarbons on shale gas generation. In this study, closed-system pyrolysis experiments were conducted on a suite of residual shales obtained by the semi-closed pyrolysis of shales from the Neoproterozoic Xiamaling Formation under various conditions. The effects of oil expulsion on the geochemical characteristics of residual organic matter (both bitumen and kerogen) in shale and on shale gas generation in the high maturity stage were investigated. The results indicated that shales with high oil expulsion efficiencies should be depleted in the aliphatic fractions in residual shale. In contrast, most of the aliphatic fractions should remain in shales with low oil expulsion efficiencies. Additionally, oil expulsion can further affect the chemical and isotopic compositions of the gas generated at later stages of thermal maturation. With increasing oil expulsion efficiency, C 1 , C 2 , C 3 , C 1–5 , and C 2–5 gas yields and gas wetness decrease, while C 1 , C 2 , and C 3 carbon isotopic values become heavier. Highlights • The roles of residual hydrocarbons differ between conventional and shale gas plays. • The effects of oil expulsion on shale chemistry and gas generation were explored. • Increased oil expulsion efficiencies result in decreased gas yield and wetness. • High expulsion efficiencies promote the cracking of long-chains in kerogen. • Mixing of kerogen- and bitumen-derived gas impacts shale gas isotopic composition. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an essential element for life, and the oceanic P cycle is closely coupled with the global carbon and oxygen cycles via the role of P as a limiting nutrient. Since P has only one stable isotope, tracking the P cycle through geological history is challenging. However, phosphate oxygen isotopes (δ18O p) represent a valuable tool for deciphering various P-related reactions, such as intracellular P turnover and enzymatic P regeneration, and have been successfully applied to modern soils and marine sediments. The marine P cycle prior to the Neoproterozoic Era remains controversial due to the scarcity of phosphorite and large uncertainties in estimating seawater phosphate levels. Here, we present δ18O p of carbonate fluorapatite (CFA) from the Mesoproterozoic Gaoyuzhuang and Xiamaling Formations in North China. Despite considerable uncertainties, the derived authigenic δ18O p values are markedly lower than those from modern sites with similar depth and latitude, which imply a warmer climate during the Mesoproterozoic Era compared to the present day. Based on thermodynamic calculations of saturation state with respect to CFA, we suggest that the elevated temperatures could have hindered P authigenesis, partially accounting for the general absence of phosphorite in contemporaneous successions. • Phosphorus occurs as tiny CFA spheroids in Mesoproterozoic organic-rich sediments. • Phosphate oxygen isotopes reveal warm climates during the Mesoproterozoic Era. • The high temperature of Mesoproterozoic seawater inhibited phosphorus authigenesis. [ABSTRACT FROM AUTHOR]

Nitrogen is an essential nutrient for all life on Earth. It is, and likely has been, a critical limiting nutrient controlling primary production rates in the oceans. The marine N budget is regulated by the balance between nitrogen loss as N 2 (and N 2 O) in low-oxygen settings in the oceans including marine sediments and coastal oxygen-minimum zones and nitrogen inputs, mainly from nitrogen fixation. How this balance has changed through Earth history, particularly considering changing degrees of ocean oxygenation, is a frontier research area. Here, we explore ancient ocean nitrogen dynamics as revealed through stable nitrogen isotopes in the ca. 1400 million year old (Ma) Xiamaling Formation of North China. We compare these dynamics to other Mesoproterozoic-aged settings and to modern environments. Through these comparisons, we see that Mesoproterozoic (1600 to 1000 Ma) oceans experienced both nitrogen replete- and nitrogen-deplete conditions with isotope systematics very similar to modern analogues. We provide models explaining the ancient ocean nitrogen dynamics, and we conclude that Mesoproterozoic oceans generally maintained extensive regions of nitrate-replete conditions. These nitrate-deplete conditions could have corresponded with extensive areas of deoxygenated seawater or vast nitrate-replete marine environments similar to modern oxygen-minimum zone environments. We also evaluate, generally, what nitrogen isotope systematics can and cannot tell us about nitrogen limitation in ancient oceans. [ABSTRACT FROM AUTHOR]

To refine the use of graptolite and solid bitumen as thermal proxies at overmature conditions, we evaluated their evolution via Raman and infrared (IR) spectroscopies, reflectance, and geochemical screening using high-temperature pyrolysis experiments in comparison to naturally matured samples. Naturally matured samples included marine shales from the overmature Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations (herein referred to as Wufeng-Longmaxi) of the Sichuan Basin, China. Immature samples for pyrolysis experiments included Mesoproterozoic (Ectasian) Xiamaling marine shale from Hebei, China (graptolite absent) and graptolite-bearing Ordovician (Tremadocian) Alum marine shale from Grönhögen, Sweden. Pyrolysis experiments at 360 °C for 10 days and 27 days (hydrous conditions), and 450 °C 3 days, 550 °C 3 days, and 550 °C 10 days (all anhydrous) created Xiamaling and Alum residues with solid bitumen and graptolite, respectively, of similar and higher maturity compared to the naturally matured Wufeng-Longmaxi samples. Compositional proxies from IR spectroscopy were inconclusive. Raman spectral properties including Raman band separation (RBS) and the full-width at half-maximum of the G-band (G-FWHM) exhibited robust positive and inverse relationships with increasing reflectance in pyrolysis residues, respectively. RBS values were systematically lower in artificially matured samples versus naturally matured samples with equivalent reflectance values, whereas G-FWHM values were systematically higher. This observation indicates that the limited duration or low internal reactor pressure of pyrolysis experiments is insufficient to allow alignment or ordering of aromatic carbon arrays in both organic matter types, relative to natural maturation occurring at geologic time scales. Reflectance of graptolite was typically higher than co-occurring solid bitumen in Wufeng-Longmaxi samples and the untreated Alum sample, suggesting inherently higher aromaticity. However, aromaticity in pyrolysis residues was systematically higher in solid bitumen compared to graptolite with equivalent reflectance values, indicating a higher kinetic barrier to molecular rearrangement in graptolite versus solid bitumen. These data further clarify differences in the molecular evolution of sedimentary organic matter in natural versus analogue laboratory environments and distinguish the properties of individual organic matter types in response to thermal stress. [ABSTRACT FROM AUTHOR]

Abstract: We report SHRIMP U–Pb zircon ages from K-bentonite beds from different locations in the Xiamaling Formation near Beijing at the northern margin of the North China Craton (NCC). The 1379±12 Ma and 1380±36 Ma ages obtained in our study correlate with similar data (1368±12 Ma) obtained from recent studies and assign a Mid-Mesoproterozoic (Ectasian Period) age for the formation. In addition, a detailed bed-by-bed study of the K-bentonite sequence indicates that the present dated successions northwest of Beijing can be correlated fairly well with that of the well-known Meso- to Neoproterozoic standard section in Jixian to the east of Beijing. The new data call for a major revision of the previous Meso- to Neoproterozoic chronostratigraphic subdivision of the northern NCC, with a revised calibration into the early to middle Mesoproterozoic Era. Our data suggest that the Tawuia-bearing assemblage in the lower part of the Xiamaling Formation in the NCC could be older than ~1380 Ma, and is not limited to a representative feature of the Neoproterozoic Little-Dal-Group-type macroalgal biota. Combined with other similar discoveries, we conclude that the macro-evolution of Proterozoic eukaryotic biotas, represented by macroalgal assemblages, would have probably been a long, protracted, and somewhat monotonous process. On the other hand, reassessment of the paleomagnetic database for these successions, combined with stratigraphic correlations and similarities in the macroalgal assemblages on the NCC and Laurentia, support a configuration in which the NCC was in close proximity to the Laurentia–Siberia–Baltica grouping during much of the Mesoproterozoic to Neoproterozoic time. Furthermore, our study suggests recent assumptions linking δ 34SCAS and δ 13Ccarbonate fluctuations in the Jixian section with the assembly and breakup of Rodinia must now be reconsidered, as these fluctuations may correlate better with the tectonics associated with the Columbia supercontinent cycle. [Copyright &y& Elsevier]

The overmature Precambrian to Lower Paleozoic marine shales from China contain relatively simple organic matter (OM) composition, which is dominated by in-source solid bitumen (i.e., solid bitumen in the hydrocarbon source rocks). The thermal evolution behavior of the original OM and the determination of thermal maturity in the Precambrian to Cambrian marine shales have been challenging for decades. The vitrinite-like maceral (VLM) is widely present in these marine shales, and its origin is still unknown. To address these issues, the immature Proterozoic Xiamaling shales from China and the immature Cambrian Alum shales from Sweden, and a Chinese immature Carboniferous coal were heat-treated at temperature range of 300 °C to 550 °C. The carbonized residue of the artificially matured samples was examined for their morphological and reflectance variation, and the results were compared with data on the other overmature natural shales from China and Sweden. OM components are similar in the Xiamaling and Alum immature oil shales, consisting of filamentous algae, matrix bituminite, bituminite, VLM and liptodetrinite, and rare thucholites are present in the Xiamaling shales. The algal-derived OM decomposed gradually due to hydrocarbon generation at 300–350 °C. OM is mainly composed of the in-source solid bitumen in the artificially heated shales after 350 °C, similar to the overmature Precambrian to Cambrian natural shales, and the in-source solid bitumen gradually loses its mass with increasing thermal maturity. The in-source solid bitumen is derived from the thermal cracking of the retained oil or the direct conversion of algal-derived liptinite macerals (e.g., the bituminite) or their mixture. VLM in the Xiamaling oil shales can not be observed after 350 °C, but VLM is still present in the Alum oil shales. It can be inferred that there is a different source of VLM in these shales, and VLM in the Xiamaling oil shales contains more volatile material. The VLM in the Xiamaling shales may be biodegradation products of liptinites under anoxic environments. The origin of VLM in the Cambrian Alum shales requires further study to be verified, although it is certain that graptolites are not its sources. The reflectance of in-source solid bitumen (SBR o) increases with heat-treated temperature in both the Xiamaling shales and the Alum shales, but at different rates, which may be due to the difference of the original kerogen composition in these shales. VLM reflectance (VLMR o) and SBR o in sections perpendicular to bedding can be used to determine the maturation level of the Precambrian-Cambrian sediments. Their relation to equivalent vitrinite reflectance (EqVR o) can be expressed by the following equations: EqVR o = 1.07 × VLMR o – 0.18, EqVR o = 0.87 × SBR o + 0.25 (in the Precambrian sediments) and EqVR o = 1.15 × SBR o + 0.01 (in the Cambrian sediments). • The vitrinite-like macerals have different thermal evolution behavior and origin. • The formation mechanism of in-source solid bitumen has been determined. • Thermal maturity indicators have been studied for the Precambrian-Cambrian shales. • In-source solid bitumen is the predominant composition in the postmature shales. [ABSTRACT FROM AUTHOR]

• Well-preserved organic-walled microfossils from the early Mesoproterozoic Xiamaling Formation in North China. • In total, 36 species of 28 genera including 1 new species are described. • The identification of 12 eukaryotic eukaryotic taxa indicates a moderate eukaryotic diversity in this formation. • The presence of Jixiania in North China strengthens its potential utility in biostratigraphy. The mid-Proterozoic (ca. 1.85–0.85 Ga) might have been environmentally "boring", but was a crucial interval of time for the early establishment of eukaryotic life on Earth. The Mesoproterozoic shale-dominated Xiamaling Formation (ca. 1.4–1.35 Ga) in North China preserved abundant organic-walled microfossils, some of which were of eukaryotic affinity but their taxonomic diversity and evolutionary implications remain unclear. In this paper, we present a palynological study of the lower Xiamaling Formation in the northern Tianjin Municipality. Similar to other Mesoproterozoic microbiotas, this assemblage consists of well-preserved and diverse spheroidal to ellipsoidal vesicles, filamentous microfossils and cellular aggregates. In total, 36 species belonging to 28 genera are identified, including 1 new species Quadrimurus clavatus gen. et sp. nov., and 5 unnamed forms. Majority of these taxa are recognised for the first time from this formation, indicating a richer assemblage than previously known. Although most taxa are of unknown biological affinities, this microbiota is thought to be composed of prokaryotic and eukaryotic microorganisms, in which 12 taxa are inferred to be eukaryotic. These microfossils indicate a moderate diversity of eukaryotic life documented in the Xiamaling Formation, which is consistent with other microbiotas in the mid-Proterozoic. The presence of Jixiania lineata Yan, 1986 (= Lineaforma elongata Vorob'eva and Sergeev, 2015) in North China extends its geographic distribution and further enhances its potential as index fossil for the early Mesoproterozoic strata. The Xiamaling assemblage provides one of the broader pictures of the Mesoproterozoic biosphere, in which eukaryotes were predominantly protistan-grade microorganisms. [ABSTRACT FROM AUTHOR]

Fossil life in the Precambrian is commonly preserved as carbonaceous compressions, where Pre-Cryogenian carbonaceous compressions mainly occur in organic-poor rocks. Black-shale-preserved carbonaceous compressions have become more common since the Ediacaran Period. To explore the depositional context of carbonaceous compressions, micro X-ray fluorescence spectroscopy (μ-XRF) was used to image elemental distributions in carbonaceous compressions and surrounding rocks in Mesoproterozoic and Ediacaran-aged rocks. The carbonaceous compressions preserved in the Xiamaling green silty mudstones (1.40 Ga) and Lantian black shales (0.63 Ga) were selected and scanned, respectively. The behavior of redox-sensitive elements (Mo, U, V, Zn, Cu) in the Xiamaling and Lantian rocks and carbonaceous compressions suggest that the overlying water conditions during their sedimentation were not sulfidic. However, the co-enrichments of Fe and S in Xiamaling and Lantian carbonaceous compressions indicate pyrite permineralization. The Lantian carbonaceous compressions have a higher pyrite permineralization degree and a lower organism decomposition degree than those of the Xiamaling carbonaceous compressions. We speculate that these differences are related to a higher sulfate reduction rate and a lower dissimilatory iron reduction rate in the sediments hosting the Lantian carbonaceous compressions. Such conditions might be achieved through rapid burial and encircling by clay minerals. Overall, our results shed new light on understanding the preservation processes of the Precambrian life and the influence of environmental factors. [Display omitted] • Multi-element imaging of the Precambrian carbonaceous compressions using μ-XRF • The Xiamaling and Lantian carbonaceous compressions formed in non-sulfidic water. • Rapid burial by clay minerals and pyritization help the preservation of organisms. [ABSTRACT FROM AUTHOR]

• δ13C of 2,3,6-TMAI ranged −19.8‰ to −21.3‰ in north China Mesoproterozoic sediments. • Distribution of green sulfur bacteria (GSB) exhibited dynamic variations with redox. • Contributions of GSB to the Xiamaling sedimentary organic matter were limited. The redox state of the marine photic zone is crucial to the evolution of life, which is further linked to the degree of oxygenation of the atmosphere and ocean systems. Hydrocarbon molecular fossils (biomarkers) and isotopic fingerprints of anoxygenic photoautotrophs can provide direct evidence for seawater redox conditions within the photic zone. In this study, we identified a series of 2,3,6-trimethyl aryl isoprenoids (2,3,6-TMAI) from low-maturity sedimentary organic matter in the Xiamaling Formation (XML), deposited 1.4 billion years ago in the Xiahuayuan area of northern China. The δ13C values of C 18 and C 19 TMAI ranged from −19.8‰ to −21.3‰. These 13C-enriched values indicate their origin in anaerobic photosynthetic green sulfur bacteria (GSB). The episodic enrichments of 2,3,6-TMAI in the XML Formation suggest dynamic marine geochemistry. Specifically, low concentrations of 2,3,6-TMAI at the bottom of the studied profile reflect an oxygenated deposition environment (unit 4), where the organic matter was almost all decomposed in the oxic water column during deposition, while the gradually increasing 2,3,6-TMAI concentrations from the middle to the upper XML reflects alteration of oceanic redox condition from an oxygen minimum zone (OMZ) in unit 3 through periods of euxinia during the deposition of units 2 and 1. The absence of co-variation between total organic carbon (TOC) and enriched 13C values indicates that anoxygenic photoautotroph activities might have contributed little to sedimentary organic matter burial in the Mesoproterozoic XML sediments. [ABSTRACT FROM AUTHOR]