Euxinic Research Articles

North‐East Peri‐Tethyan Water Column Deoxygenation and Euxinia at the Paleocene Eocene Thermal Maximum

AbstractThe Paleocene–Eocene Thermal Maximum (PETM) is associated with climatic change and biological turnover. It shares features with the Oceanic Anoxic Events (OAEs) of the Mesozoic, such as transient global warming and biogeochemical perturbations. However, the PETM experienced a more muted expansion of marine anoxia compared to the Mesozoic OAEs (especially OAE2), with geographically limited evidence for photic zone euxinia (PZE). We explore the extent and drivers of marine deoxygenation during the PETM using biomarkers for water column euxinia and anoxia as well as an intermediate complexity Earth system model (cGEnIE). These reveal that the water column in the North‐East Peri‐Tethys became anoxic, with euxinic conditions reaching the photic zone (PZE) during the PETM. Our model shows that euxinia developed due to a global increase in the ocean nutrient inventory with concomitant oxygen consumption, similar to findings for OAE2. The particularly strong regional response in the NE Peri‐Tethys appears to arise from a combination of global CO2‐weathering forcing, regionally restricted circulation and upwelling of sulphidic thermocline waters. Unlike OAE2, anoxia and PZE do not become widespread in our PETM simulations, consistent with new and existing geochemical and biological proxy data. This globally muted response could result from reduced biogeochemical feedbacks to climate forcing relative to the mid‐Cretaceous climate. Our observations suggest that similar mechanisms operated in response to disparate Cenozoic (PETM) and Mesozoic (OAEs) transient global warming events, while also highlighting that background conditions are crucial in modulating the sensitivity of Earth's system to them.

Re-Os geochronology and geochemical evolution of late Cambrian to Middle Ordovician Alum and Tøyen shales, Sweden

The limited number of accurate and precise radiometric ages through the ∼100 Myr span of the Cambrian and Ordovician impedes reliable age determinations for stage boundaries in these periods. Here, we fill significant gaps in the early Paleozoic chronostratigraphy by providing precise Re-Os time-pins. Sample selection is linked to a firm biostratigraphic framework built on the appearance and distribution of trilobites, graptolites, and conodonts. A Furongian (upper Cambrian) Alum Shale section (Andrarum-3 drill core, Scania, Sweden) at the onset of the Steptoean Positive Carbon Isotopic Excursion (SPICE) yields highly non-isochronous Re-Os isotopic data from a section with wildly fluctuating δ13Corg; however, selected data from a narrow sediment band with steady carbon isotope stratigraphy provides an imprecise Re-Os age of 497 ± 28 Ma (2σ; Model 3; n = 3), with an initial 187Os/188Os ratio (Osi) of 0.74 ± 0.05. Organic-rich Alum Shale (Tomten-1 drill core, Västergötland, Sweden) from ∼120 cm below the Cambrian-Ordovician boundary yields a Model 1 age of 488.6 ± 5.1 Ma (2σ; MSWD = 1.5; n = 25) and an Osi of 0.82 ± 0.04 for Stage 10, uppermost Cambrian. Biostratigraphic data indicate the dated Alum Shale is from an interval slightly below the Top Of Cambrian Excursion (TOCE) and slightly above the First Appearance Datum (FAD) of the agnostoid Lotagnostus americanus. Organic-rich Tøyen Shale (Lerhamn drill core, Scania, Sweden) yields a precise Model 1 Re-Os age of 469.7 ± 1.4 Ma (2σ; MSWD = 1.0; n = 10) and Osi of 0.802 ± 0.002 for the maximum age of the Floian–Dapingian stage boundary (Lower–Middle Ordovician boundary). The Os isotopic composition of seawater from the latest Ediacaran through the Cambrian to Early-Middle Ordovician hovers around 0.8 but falls to 0.54 by early Silurian. This significant decrease in seawater 187Os/188Os is consistent with reduced chemical weathering and cooler seawater temperatures through the Middle–Late Ordovician. Overall, Redox Sensitive Element (RSE; Re, Os, Mo, U) abundances correlate positively with Total Organic Carbon (TOC), suggesting efficient removal of these elements from an anoxic water column by organic matter. However, these relationships break down for high TOC (>10%) shales depositing under euxinic conditions. The RSE-TOC relationship breakdown supports enhanced metal drawdown from the water column with local pyrite accumulation. Geochemical data suggest the deposition of Alum and Tøyen shales under hydrographically restricted settings with increased primary productivity along the Baltica's margin during the latest Cambrian to Early-Middle Ordovician.

Primary productivity recovery and shallow-water oxygenation during the Sturtian deglaciation in South China

The Cryogenian Period (ca. 720–635 Ma) marks a key node in Earth's history, characterized by two global glaciations when the paleo-tropical regions were covered by glaciers, namely the Sturtian and Marinoan Snowball Earth events. Within the Cryogenian, the evolution of primary producers persisted, evidenced by the worldwide discovery of diverse microfossils and the proliferation of algae in the interglacial oceans. The Cryogenian radiation of algae facilitated the emergence and evolution of early eumetazoan animals. However, the paleoredox reconstruction of the interglacial Cryogenian presents more challenges. The open oceans were predominantly and permanently ferruginous throughout the Cryogenian Period, and persistently euxinic conditions developed in restricted basins, such as the Nanhua Basin. Some of the studies suggested a possible fully‑oxygenated interval in the aftermath of the Sturtian glaciation. Therefore, a knowledge gap lies between the biological prosperity and severe habitability in the interglacial oceans. In order to fill this gap, we report a newly discovered interglacial deposition – the Datangpo Formation – of shallow water depth from the Guitouwan section, Shennongjia area, South China. A combination of iron speciation, total organic matter (TOC), and pyrite sulfur isotopes (δ34Spy) analyses are conducted, along with numerical simulations, for reconstructions of paleoredox conditions and paleoproductivity level. Additionally, we collected Fe chemistry, TOC, and pyrite sulfur data from multiple successions of the Datangpo Formation in South China to reconstruct the paleoredox landscape. The spatial TOC comparison suggests that the interglacial interval is marked by a TOC increase at the base, and the Guitouwan succession represents the early deposition during the wake of Sturtian deglaciation. Fe speciation suggests locally (dys)oxic-ferruginous condition in the Shennongjia area, while the δ34Spy systematics indicate a transition from ferruginous to euxinic condition and a sharp increase in primary productivity during the Sturtian deglaciation. With respect to the high δ34Spy values, the observed discrepancy suggests that the iron proxy might be inapplicable during fast environmental changes, especially during the deglaciation episodes. The comparison of Fe chemistry and pyrite sulfur isotopes between different water-depth settings informs a redox-stratified ocean in the Nanhua Basin. The newly discovered interglacial deposition fills the blank in understanding the shallow-water oxygenation in the Sturtian glacial aftermath, recording the transition from (dys)oxic-ferruginous to ferruginous-euxinic and from low to high productivity levels. These findings shed new light on the interpretation of Fe chemistry and δ34Spy in the geological past, and provide a novel perspective on investigating productivity levels.

Cascading sulfur cycling in simulated oil sands pit lake water cap mesocosms transitioning from oxic to euxinic conditions

Base Mine Lake (BML), the first full-scale demonstration of oil sands tailings pit lake reclamation technology, is experiencing expansive, episodic hypolimnetic euxinia resulting in greater sulfur biogeochemical cycling within the water cap. Here, Fluid Fine Tailings (FFT)-water mesocosm experiments simulating the in situ BML summer hypolimnetic oxic-euxinic transition determined sulfur biogeochemical processes and their controlling factors. While mesocosm water caps without FFT amendments experienced limited geochemical and microbial changes during the experimental period, FFT-amended mesocosm water caps evidenced three successive stages of S speciation in ∼30 days: (S1) rising expansion of water cap euxinia from FFT to water surface; enabling (S2) rapid sulfate (SO42−) reduction and sulfide production directly within the water column; fostering (S3) generation and subsequent consumption of sulfur oxidation intermediate compounds (SOI). Identified key SOI, elemental S and thiosulfate, support subsequent SOI oxidation, reduction, and/or disproportionation processes in the system. Dominant water cap microbes shifted from methanotrophs and denitrifying/iron-reducing bacteria to functionally versatile sulfur-reducing bacteria (SRB) comprising sulfate-reducing bacteria (Desulfovibrionales) and SOI-reducing/disproportionating bacteria (Campylobacterales and Desulfobulbales). The observed microbial shift is driven by decreasing [SO42−] and organic aromaticity, with putative hydrocarbon-degrading bacteria providing electron donors for SRB. Comparison between unsterile and sterile water treatments further underscores the biogeochemical readiness of the in situ water cap to enhance oxidant depletion, euxinia expansion and establishment of water cap SRB communities aided by FFT migration of anaerobes. Results here identify the collective influence of FFT and water cap microbial communities on water cap euxinia expansion associated with sequential S reactions that are controlled by concentrations of oxidants, labile organic substrates and S species. This emphasizes the necessity of understanding this complex S cycling in assessing BML water cap O2 persistence.

Weathering-induced organic matter enrichment in marine-continental transitional shale: A case study on the early Permian Taiyuan Formation in the Ordos Basin, China

A comparative analysis of the factors controlling organic matter (OM) enrichment between marine-continental transitional (transitional hereafter) and marine or lacustrine shales is lacking. The early Permian Taiyuan Formation in the Ordos Basin, deposited during a shift from marine to continental settings in northern China, provides a unique opportunity to unravel the differences in OM enrichment mechanisms among these shales. The Taiyuan Formation is characterized by high TOC content (average 4.50%) and kerogen type II2-III. Most samples are thermally mature with a few high to post-mature samples relating to the Late Jurassic–Early Cretaceous Yanshanian magmatism. Rare earth elements and yttrium (REY) are dominated by light- and medium-types enrichments, with distinctly positive Gd anomaly, likely due to seawater incursion. A warm and humid climate prevailed during deposition of the Taiyuan Formation, consistent with the tropical-subtropical location of the North China Plate in the early Permian. The climatic conditions promoted intense continental weathering as reflected by high Th/Sc ratios, chemical index of alteration values, and feldspar alteration to scaly kaolinite. The V/(V + Ni) ratio is inconsistent with the other redox proxies, presumably due to variations in the redox buffer supply in the transitional facies (e.g., OM and pyrite), varying burial rates and dissimilar redox potential of different elements. Hence, this proxy should be interpreted with caution in such settings. Most redox proxies indicate oxic bottom water during deposition of the Taiyuan Formation transitional shale, in contrast to typical OM enriched marine and lacustrine shales where redox stratification or euxinic conditions are common. Instead, the dominant factor for OM enrichment in transitional shales appears to have been a high influx of terrestrial weathering products, including abundant higher-plants OM, associated with preservation of OM due to rapid burial. This process minimizes the detrimental effects of oxic conditions on OM accumulation in the transitional shale facies. This mechanism may hold relevance for analogous basins elsewhere.

Geochemistry of cherts from the northern Jiangxi region, South China: Implication for paleoenvironment

The extensive bedded cherts deposited during the Ediacaran–Cambrian (E–C) transition period play a crucial role in understanding the geological evolution of this period, yet the origin of these cherts remains disputed. Here, we present new geochemical data for cherts of the Piyuancun (PYC) Formation deposited during the Late Ediacaran and the Hetang (HT) Formation deposited during the Early Cambrian in northern Jiangxi region, Lower Yangtze region, South China. The PYC cherts contain a small amount of monaxons sponge spicules and radiolarian fragments, while the HT cherts lack siliceous organism evidence. Major and trace element analysis, coupled with discriminant diagrams, indicate a possible shift in redox conditions of seawater during the E–C transition in the northern Jiangxi region. The shift suggests a change from weakly–moderately restricted euxinic conditions to strongly restricted euxinic conditions. Furthermore, the location of both cherts are distant from the source area of siliceous organisms. Fossil evidence, as well as the values of Fe/Ti and Fe/(Mn+Ti), Eu anomalies, Post-Archean Australian Shale (PAAS) normalized REE+Y patterns, and various discriminant diagrams, support the conclusion that the PYC and HT cherts originated primarily from direct seawater precipitation, with the PYC cherts exhibiting weak hydrothermal evidence. Upwelling contributes to the formation of HT cherts and organic matter (OM) accumulation. Ocean acidification, triggered by OM degradation and biodegradation processes during the E–C transition period, leads to the extensive silica precipitation and preservation. These results enhance our understanding of the geological processes during the E–C transition.

The depositional redox conditions of Fe‐speciation reference materials (BHW and WHIT) using redox‐sensitive trace metal enrichment

Iron speciation has emerged as a robust proxy for discerning oceanic redox conditions; nonetheless, it is subject to certain limitations. Specifically, the applicability of the degree of pyritization is contingent upon the presence of unequivocal evidence of an anoxic water column and its discriminatory capacity is limited to distinguish between ferruginous (anoxic) and euxinic conditions. This study highlights that through the integration of redox‐sensitive trace metal enrichment data with Fe‐speciation data, the depositional redox conditions for marine sediments can be established with greater certainty. Recently, a set of dedicated geological reference materials (BHW and WHIT) have been developed for validating the Fe‐speciation analytical results for redox reconstruction studies; however, to the best of our knowledge, these reference materials are not characterized for trace and rare earth elements (REEs). In this connection, the BHW (oxic) and WHIT (anoxic) reference materials are measured for major, trace and REEs. After careful statistical considerations for these reference standards, a complete set of trace and REEs is reported. Furthermore, considering BHW and WHIT as oxic and anoxic end‐members, respectively, the utility of trace metal enrichment and Fe‐speciation data in combination has been discussed. The trace and REE concentrations of BHW and WHIT reported in this study will enhance their applicability as a reference material to understand ocean chemistry and the oxidation state of the ancient oceans.

Uranium enrichment driven by paleomarine condition and event deposition: Insights from the lower Cambrian Niutitang Formation organic-rich shales in northeastern Guizhou, South China

Organic-rich shale related uranium (U) mineralization has attracted widespread attention as a strategic and clean alternative to fossil energy and a paleoceanic redox proxy. A comprehensive study of mineralogy, organic geochemistry and elemental geochemistry of the lower Cambrian Niutitang Formation organic-rich shales in northeastern Guizhou, South China was conducted to address this issue. SEM-EDS results showed that the U occurrence state in the Niutitang Formation shales is dominated by nanoscale U minerals, mainly including coffinite, brannerite, U-bearing apatite and U-bearing xenotime. Various states of U are closely associated with organic matter, apatite, pyrite and clay minerals. Based on geochemical indexes, paleomarine condition and event deposition jointly drove the U enrichment in the Niutitang shales, in which hydrothermal activities and/or volcanic eruptions increased U fluxes in seawater and euxinic environment with high primary productivity provided a favorable condition for U reduction. The adsorption, complexation and reduction of UO22+ by organic matter and the formation of uranyl phosphate complexes by the combination of phosphate and UO22+ accelerated U immobilization and enrichment from bottom water to sediments. The microbial activities (e.g., BSR) in this process created favorable euxinic conditions and released inorganic-phase P to provide available ligands for UO22+. The U enrichment models of the lower Cambrian Niutitang shales in northeastern Guizhou were established. This paper provides a new insight into the U enrichment mechanism in organic-rich shales, and may also have implications for the applicability of U as redox proxy and the exploration of organic-rich shale related U deposits.

Statistical estimation of the early to middle Ediacaran ocean redox architecture in the Yangtze block of South China

Early to middle Ediacaran organic-rich black shales host several famous fossil biotas and provide key evidence for understanding the coevolution of multicellular organisms and palaeoceanic environment. The water column redox states play critical roles in organic-rich shale deposition. Amongst multiple geochemical redox indexes, iron-speciation chemistry (FeHR/FeT, Fepy/FeHR) in shales constitutes a reliable proxy to reconstruct the first-order redox framework of the ocean basin. However, iron-speciation is a local marine redox proxy, and only compiled data collected from multiple different sedimentary facies record statistically significant changes of oxidation states within a depositional basin. Here we compiled the spatiotemporal distribution of iron-speciation data in early to middle Ediacaran black shales of the Yangtze block in South China. New high-resolution analysis on two outcrops and one drill-core reflects overall ferruginous condition and dominant euxinic condition locally interrupted by oxic states, respectively, generally indicating pervasive anoxic depositional environment for the Member II shales of the Ediacaran Doushantuo Formation in the Lower Yangtze block. Compilation of fourteen Ediacaran sections from shallow-, slope- and deep-water facies demonstrates frequent spatiotemporal oscillation of the redox conditions throughout the Yangtze block. Widespread ferruginous states, accompanied with euxinic zones mainly focusing in lower slope facies, are detected during the early Ediacaran period, in contrast with more widespread euxinic settings in the middle Ediacaran period. Statistical data including new high-resolution results from the Lower Yangtze block show generally low enrichment for redox sensitive elements (RSEs, e.g., Mo, V and U) in the Member II shales probably due to their contemporary low seawater concentrations. An increase of RSEs concentrations occurs in the Member IV shales, which probably reflects more oxidized Earth surface environment during middle Ediacaran period. The increasing Earth oxygenation documents the occurrence of widespread euxinic states during deposition of the Member IV shales. Compilation of spatial distribution of TOC contents in black shales demonstrates their apparent relevance to spatial distribution of the euxinic rather than ferruginous states, which is likely due to the favorable role of sulfurization process on burial and preservation of organic matters. The euxinic states may also facilitate the exceptional preservation of the soft-bodied Ediacaran fossils.

Paleoenvironment in the circum-arctic region from the Middle Jurassic to Early Cretaceous: Trace element and stable isotope geochemistry of the Agardhfjellet Formation, Svalbard

From the Middle Jurassic to Early Cretaceous, the mudstone-dominated Agardhfjellet Formation was deposited during the prolonged shelf dysoxic-anoxic event (SDAE) recorded predominantly in the circum-arctic polar regions. Commonly, oceanic anoxic events (OAEs) throughout Earth's history are linked to the emplacement of large igneous provinces (LIPs). OAEs and LIPs, alone or jointly, are inferred to be triggers for severe biotic crises in Earth's history. However, the causes for SDAEs remain uncertain. To elucidate these causes, we document the geochemical features of the Agardhfjellet Formation using trace element concentrations and stable isotope ratios. The Agardhfjellet Formation, which consists of the Oppdalen, Lardyfjellet, Oppdalssåta, and Slottsmøya members in ascending order, formed during ∼13 Myr of prolonged dysoxic, anoxic, and euxinic conditions. The onset of shelf anoxia took place between deposition of the Oppdalen and Lardyfjellet Members. The extent of sea floor anoxia in the basin varied within the Lardyfjellet Member but likely persisted throughout most of the Slottsmøya Member. Mo/TOC and MoEF/UEF ratios in the sedimentary rocks are consistent with the anoxic events. Enhanced continental runoff marked the Oppdalen, Oppdalssåta, and Slottsmøya members, contrasting with most of the Lardyfjellet Member, in which authigenic phases were widespread under anoxic conditions. Our results show that, compared to other OAEs, such as the Toarcian OAE (∼183 Ma) or the OAE2 (∼94 Ma), the Late Jurassic – Early Cretaceous SDAE recorded in the Agardhfjellet Formation is characterized by prolonged suboxic–anoxic condition, substantial water-mass stratification, and increased continental runoff.

Characterization of polysulfides (Sx2−) in seawater euxinic conditions by electroanalytical methods

Polysulfides (Sx2−) are important reduced sulfur species (RSS) that play a role in numerous environmental processes. A sound analytical method for the measurement of Sx2− in euxinic waters is lacking. In this work, differential pulse voltammetry (DPV) at the Hg electrode was used to measure the presence of Sx2− in a model seawater solution and an euxinic marine lake (Rogoznica Lake - RL Croatia), in which the concentration of RSS, mainly HS−, varies between 100 and 4000 μM. In the DPV, an adsorption phenomenon associated with Sx2− reduction on Hg produces a characteristic current minimum at −1.0 V (vs. Ag/AgCl), the magnitude of which is proportional to the concentration of polysulfidic sulfur.The DPV current minima were recorded in the model solution K2Sx NaCl/NaHCO3 (pH ∼ 8.2) in a concentration range from 10 to 100 μM of polysulfidic sulfur. Total RSS was measured by cyclic voltammetry, and sampled DC voltammetry showed the ratio between HS− and S0 within the Sx2−. Using the same methodology, the presence of Sx2− below the chemocline enriched by photoptrophic sulfur bacteria was measured in the euxinic layer of RL at concentrations of up to 70 μM of polysulfidic sulfur. The results suggest that euxinic RL samples can be considered as polydisperse solutions of S0 or of S-rich compounds that can change their physicochemical properties and speciation during sample manipulation. These changes can be detected by electrochemistry. Atomic force microscopy proved the release of bacterial cellular S0 during the acidification and purging step in the electrochemical measurements, which contributed to the voltammetric RSS signal.

The paleoredox context of early eukaryotic evolution: insights from the Tonian Mackenzie Mountains Supergroup, Canada.

Tonian (ca. 1000-720 Ma) marine environments are hypothesised to have experienced major redox changes coinciding with the evolution and diversification of multicellular eukaryotes. In particular, the earliest Tonian stratigraphic record features the colonisation of benthic habitats by multicellular macroscopic algae, which would have been powerful ecosystem engineers that contributed to the oxygenation of the oceans and the reorganisation of biogeochemical cycles. However, the paleoredox context of this expansion of macroalgal habitats in Tonian nearshore marine environments remains uncertain due to limited well-preserved fossils and stratigraphy. As such, the interdependent relationship between early complex life and ocean redox state is unclear. An assemblage of macrofossils including the chlorophyte macroalga Archaeochaeta guncho was recently discovered in the lower Mackenzie Mountains Supergroup in Yukon (Canada), which archives marine sedimentation from ca. 950-775 Ma, permitting investigation into environmental evolution coincident with eukaryotic ecosystem evolution and expansion. Here we present multi-proxy geochemical data from the lower Mackenzie Mountains Supergroup to constrain the paleoredox environment within which these large benthic macroalgae thrived. Two transects show evidence for basin-wide anoxic (ferruginous) oceanic conditions (i.e., high FeHR/FeT, low Fepy/FeHR), with muted redox-sensitive trace metal enrichments and possible seasonal variability. However, the weathering of sulfide minerals in the studied samples may obscure geochemical signatures of euxinic conditions. These results suggest that macroalgae colonized shallow environments in an ocean that remained dominantly anoxic with limited evidence for oxygenation until ca. 850 Ma. Collectively, these geochemical results provide novel insights into the environmental conditions surrounding the evolution and expansion of benthic macroalgae and the eventual dominance of oxygenated oceanic conditions required for the later emergence of animals.

Sediment-redox dynamics in an oligotrophic deep-water lake in Tierra del Fuego: insights from Fe isotopes

For long time in the history of Earth, ferruginous conditions governed the oceans. With the rise of oxygen during the Proterozoic era and the subsequent evolution of living organisms, worldwide deposition of iron formations occurred. These sedimentary units reveal the transition into oxic oceans, passing by local and transitory euxinic conditions, especially in coastal shelves. Constraining the iron cycle and the biogeochemical processes occurring in present and past ferruginous basins helps answering some of the question regarding global oxygenation, the evolution of life and past climate changes. Therefore, Fe speciation and Fe isotopes in both Proterozoic and recent sedimentary records have been widely used to reconstruct past basin dynamics and redox conditions in the sediment–water interface. However, sedimentation and early diagenesis can alter paleoredox proxies and their primary climate signals. In this work, we disentangled alteration processes occurring at the redox front below the sediment–water interface of a ventilated deep-water lake (Lago Fagnano, Argentina/Chile). A sequential extraction protocol was applied to characterize two reactive Fe pools: Fe oxyhydroxides and reduced iron. Subsequently, Fe isotopes were constrained to determine the main processes mobilizing Fe. At the redox front, ferric minerals reach a δ56Fe value of − 1.3‰ resulting from oxidation of dissolved Fe likely following a Rayleigh distillation effect. Dissolved Fe is produced right below via Fe reduction, as shown by the low ferric Fe content. Our observations delineate a redox cycle and a redox horizon undergoing constant upward migration, initiated by regular sedimentation. However, during events of increased rapid sedimentation (e.g., seismites) this dynamic cycle is interrupted inducing full or partial preservation of the Fe-rich redox front. In such case, oxidation of dissolved Fe is interrupted and can be recycled in ferrous minerals, such as Fe monosulfides and amorphous phases with δ56Fe values down to − 1.7 ‰. These findings have significant implications for the recording of biogeochemical cycles in the geological past, the use of Fe isotopes in freshwater-lake sediments for paleoclimate studies, and the progress of our knowledge regarding the geochemistry of past oceans.

Pyrite morphological evidence for sedimentary conditions of the Wufeng formation–Longmaxi formation in south-east Chongqing area–insights for high-quality shale gas reservoir formation mechanism

As one of the important minerals in marine shale reservoirs, the development characteristics of pyrite can provide guidance for the exploration of deep shale gas resources and the study of high-quality reservoir development mechanisms. In this study, samples of the Wufeng and Longmaxi Formation in the southeast Chongqing area were selected, and the development characteristics of framboidal pyrite were revealed through a combination of qualitative and quantitative method. High-quality reservoirs of the upper Wufeng Formation and the bottom of the Longmaxi Formation are developed with smaller sizes, weaker variation and a narrower size distribution of pyrite framboids compared with other units of the strata, suggesting relatively stable euxinic conditions during deposition. The development characteristics of framboidal pyrite and some of the key reservoir evaluation parameters such as organic matter content and brittle mineral content, etc., are controlled by similar factors. Therefore, pyrite morphological evidence can be used as a potential indicator of high-quality shale reservoirs.

The Bowland Shale Formation in the Blacon Basin: syngenetic processes, stacking patterns and heat productivity

Abstract We conducted a high-resolution multi-disciplinary analysis of two core sections in the borehole Ellesmere Port-1, Cheshire, UK. Biostratigraphic analysis indicates that the core sections are Kinderscoutian and late Arnsbergian–Chokierian in age, respectively. Both cores are assigned to the Bowland Shale Formation (Holywell Shale). Coupled core scan and discrete geochemical analysis enables interpretation of syngenetic processes at a high stratigraphic resolution. Both cores exhibit the classic cyclicity of limestones, calcareous to non-calcareous mudstones and siltstones, interpreted to represent sediment deposition during fourth-order sea-level fluctuation. Machine learning of the well log data coupled to the core scan data enabled prediction of the key lithofacies through the entire Bowland Shale interval in Ellesmere Port-1. The machine predictions show that the Bowland Shale is interfingered with three turbiditic leaves of the Cefn-y-Fedw Sandstone Formation and contains at least 12 complete fourth-order cycles. The Bowland Shale exhibits high radiogenic heat productivity in comparison with other sedimentary rocks, due primarily to relative U enrichment under intermittently euxinic conditions. Thermal modelling, however, shows that the radiogenic heat productivity of the Bowland Shale contributes a negligible source of additional heat at the scale of hundreds of metres.

Geochemical Characteristics of Mesoproterozoic Source Rocks in North China: Insights for Organic Matter Enrichment and Thermal Evolution

In recent years, the exploration of oil and gas in China’s Precambrian strata has garnered significant attention, leading to notable advancements in exploration play assessment. However, there is a dearth of published literature on Proterozoic source rocks’ organic sources, sedimentary environments, marine hydrochemistry, and other attributes. This study focuses on investigating potential source rocks within the Hongshuizhuang and Xiamaling Formations in the Jibei Depression of North China. A comprehensive analysis was conducted to evaluate hydrocarbon generation characteristics, using hydrocarbon biomarkers and polar compounds as geochemical indicators for precursor biota and maturity levels. The results indicate high organic matter abundance with predominantly type I-II1 organic matter composition in the studied source rocks. These samples are at an immature–low mature stage, with the potential for primarily generating aromatic crude oil. The parent material is mainly attributed to lower aquatic organisms, such as bacteria and algae. The sedimentary environment exhibits marine facies, characterized by high evaporation rates, salinity levels, and strong euxinic conditions, that led to sulfur incorporation into the organic matter matrix. It should be noted that correlations between biomarker parameters and maturity may not be fully applicable to ancient source rocks; however, the methyldibenzothiophene ratio (MDR) demonstrates a strong correlation with Tmax. The compounds and their total monoisotope ions abundance (TMIA) were primarily identified and analyzed using FT–ICR MS. It was observed that these compounds were influenced by the depositional environment and organic matter maturity. Importantly, it was clearly demonstrated that the DBE and carbon number range of CH compounds gradually increased with maturity, due to the removal of N, S, and O functional groups. Specifically, N1 compounds predominantly consisted of carbazoles with short alkyl side chains which readily converted into N1Ox compounds. On the other hand, O1 compounds mainly comprised benzofurans with low abundance, indicating a reducing sedimentary environment, as suggested by their low TMIA values. Furthermore, S1 compounds were primarily thiophenes whose DBE range and carbon number increased with maturity, possibly suggesting an abiotic input of inorganic sulfur. Notably, the maturity indices (MAT) proved suitable for Mesoproterozoic source rocks while exhibiting strong linear relationships.

Productivity and organic carbon loading control uranium isotope behavior in ancient reducing settings: Implications for the paleoredox proxy

Uranium isotopes in ancient sedimentary rocks have emerged as a powerful proxy for the oxygenation history of Earth surface environments. Proper quantitative interpretation of δ238U data hinges, however, on an understanding of isotopic fractionation associated with uranium removal to sediments under different redox conditions. Whereas oxygenated environments are plentiful in the modern ocean, euxinic settings are scarce today, and persistently ferruginous settings are not a feature in the modern ocean, leaving lakes as our best analogs for iron-rich waters. These challenges to studying δ238U behavior under contemporaneous oxic, euxinic (sulfidic), and ferruginous (iron-rich) marine conditions limit our ability to apply the isotope proxy to ancient oceans. Here, we present δ238U data from five coeval cores through the Lower Mississippian Sunbury Shale of the semi-restricted Appalachian Basin (North America), which were deposited across a strong redox gradient from intermittently oxic conditions proximal to the Catskill Delta, to ferruginous conditions in the central basin axis and euxinic conditions along the basin-bounding Cumberland Sill. Overall, we find that both euxinic and ferruginous environments are capable of imparting high degrees of uranium isotope fractionation and that δ238U values covary more strongly with total organic carbon (TOC) and trace metal abundances than they do with the presence of euxinic versus ferruginous conditions. We suggest uranium sorption or incorporation into sinking organic matter in the water column as a mechanism for generating positive covariation between δ238U and TOC, particularly at lower TOC concentrations. We also compare our δ238U data to broadly coeval carbonates from Nevada, which can be used to estimate Early Mississippian seawater δ238U values. We find that Sunbury Shale cores deposited under stably anoxic conditions (both euxinic and ferruginous) and under consistently brackish salinities away from freshwater sources record an increasing δ238U trend through time that loosely mirrors the record for carbonates (and thus estimates for seawater) with an offset of ∼0.4‰ to 1.1‰, depending on the inferred offset between carbonate and seawater. In contrast, cores deposited close to the Catskill Delta and in a shallow oceanic-restricted basin transition across the Cumberland Sill record a decreasing δ238U trend through time, opposite the inferred seawater trend and suggestive of entirely local controls on δ238U values. Ultimately, these data suggest that highly productive environments with high rates of organic carbon loading can exert a strong control on the global δ238U mass balance regardless of whether euxinic or ferruginous conditions are present. These results necessitate a partial reassessment of the processes controlling δ238U variability in seawater and sedimentary rocks through Earth history.

A volatile sulfur sink aids in reconciling the sulfur isotope mass balance of closed basin lakes

Sedimentary rocks from the early Eocene Green River Formation comprise the largest known lacustrine oil shale deposits, contain remarkably well-preserved fossils, and provide a unique record of climate evolution across the Early Eocene Climate Optimum, a period of high atmospheric CO2. The depositional environment of these intermountain lakes spanned from relatively fresh and fluvially influenced to expanded and stratified saline closed basin lakes under the influence of the Laramide orogeny and alternating humid and arid climatic conditions. As the surface area of the lakes expanded, alkalinity and salinity increased, with depositional cycles that linked to evaporative cycles and marked by an increasing abundance of organic matter-rich shales (TOC > 10 wt%). Simultaneously, an intriguing 20 ‰ positive shift in the sulfur isotope composition of sulfide minerals and organic sulfur is observed. Given that this trend cannot be simply explained by a change in the source of sulfate delivered to the basin, the evolution of biogeochemical sulfur cycling and the balance of fluxes in response to basin evolution remain unresolved. Here, we combine the sulfur isotope compositions of pyrite, organic sulfur, and carbonate-associated sulfate and molecular proxies of euxinia in samples from the Uinta basin's depocenter. We find that organic matter-rich sediments reflect deposition in a stratified water column with enhanced burial of pyrite and sulfurized organic matter, while organic-lean facies present evidence of, at least transiently, euxinic conditions reaching the photic zone during arid conditions presumably because of evaporation. As the lake became both saline-stratified and euxinic, we observe that δ34S values of all measured sulfur-bearing sedimentary proxies increase and evolve along a 1:1 line, a trend independent of facies that we interpret as reflecting a sulfate-limited system despite saline conditions. The isotopic mass balance of sulfur fluxes implies the existence of a sink of sulfur depleted in 34S that is spatially decoupled from burial in the depocenter. Modeling sulfur biogeochemical processes in a saline stratified lake system allows us to estimate that at least 50 % of the sulfur entering the lake could have been lost from the upper part of the euxinic water column where the fractionation factor imparted by microbial sulfate reduction is expressed. We propose that the overall isotopic enrichment of the system was caused by H2S degassing during arid climate intervals, presumably enhanced by transient water column mixing events. Further, episodic intrusion of euxinic bottom waters into the upper part of the water column might have triggered mass fish and plankton mortality, consequently facilitating the formation of these exceptionally fossiliferous and organic matter-rich rocks. Our study finds that volatile outgassing may be an underappreciated mechanism for the sulfur mass balance of stratified lacustrine systems.

Mediterranean-like “fall dump” events in the Baltic Sea

In the Mediterranean Sea, organic carbon-rich sapropels have been deposited periodically over the last fifteen million years. Some sapropels are characterized by high contents of the mat-forming, planktonic diatom Pseudosolenia calcar-avis and the planktonic diatom Thalassionema nitzschioides as a result of their mass sinking in autumn (the so-called “fall dump”). The present study shows that fall dump events also occurred in the brackish Baltic Sea around 6300–5800 calibrated years before present (cal yr BP; present = AD 1950). In sediments from the northern Baltic Sea these events are evidenced by high contents of a lipid specific for P. calcar-avis (a C25:2 highly branched isoprenoid alkene) and corroborated by fossil remains of both P. calcar-avis and T. nitzschioides. A biomarker index based on long-chain alkyl diol lipids indicates that mat-forming Proboscia diatoms were also present. High contents of calanoid resting eggs further suggest that the copepod population was stressed being unable to feed on such large diatoms. The fall dump events occurred during a complete stratification of the water column and euxinic conditions, as reflected by the redox-sensitive trace metal uranium, allowing upward diffusion of nutrients and the growth of rhizosolenid mat-forming diatoms in a deep chlorophyll maximum. Synchronous remains of P. calcar-avis in the central Baltic Sea further suggest that these events occurred on a basin-wide scale.

Reconfiguring oxygenation at ∼1.4 Ga: New constraints as informed by the ancient oceanic sulfur cycle

The extent of atmospheric and oceanic oxygenation during the Mesoproterozoic remains an area of active debate. Here, we report major and trace elements, organic sulfur (OS) speciation, sulfur isotopes in multiple phases [pyrite (py), OS, and carbonate-associated sulfate (CAS)], and a numerical model of sulfate concentrations from one of the Mesoproterozoic's best-preserved geochemical archives, the Xiamaling Formation (ca. 1.4 Ga) in the Yanliao Basin. These data reveal a previously unrecognized ocean-atmosphere oxygenation process. Starting with Unit 3, intense sulfurization increased organic carbon burial and thus oxygen release, resulting in a deep-water oxidation event within the Yanliao Basin that could be regarded as a transient restricted oxygen oasis based on low pyrite content, elevated kerogen S:C ratios, higher δ34Spy than δ34SOS (Δδ34SOS-py < 0), and a low calculated sulfate concentration of ∼0.07–0.14 mM. During Unit 2, the basin was reconnected to the open ocean with the deposition of organic-rich shale across northern China and northern Australia, accompanied by a shift in Δδ34SOS-py to 5.0‰ and a calculated seawater sulfate concentration of ∼0.1–1.14 mM. Such large-scale burial of organic matter released oxygen into the atmosphere, accompanied by significant sulfate delivery to the basin and euxinic conditions characterized by Mo concentrations and 34S-depleted pyrite at the topmost of Unit 2. A subsequent globally synchronized atmospheric and deep ocean oxygenation event may have occurred during Unit 1. This event was recorded by a progressive increase in Δδ34SOS-py (up to +9.6‰), increased oxidized OS species, and positive Ce anomalies (attributed to the reductive dissolution of the Ce-enriched Mn oxides), all of which were caused by increased sulfate concentrations (calculated to be ∼1.15–2.31 mM) and a spatial descent of the redox interface. The combination of these lines of evidence illustrates the sensitivity of the oceanic sulfur cycle to the ocean-atmosphere system and provides novel constraints on oxygenation at ∼1.4 Ga.