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Two redox-sensitive metalloids, arsenic (As) and antimony (Sb), are examined here to determine what can be their help in the deciphering of past depositional conditions. The enrichment factors of the two elements are compared for a set of geological formations and marine deposits covering a relatively wide range of paleoenvironmental settings, from oxic to euxinic conditions. This work confirms that As and Sb are not robust paleoredox proxies but examining their relative enrichment may be useful. These preliminary results indicate that a co-enrichment of both elements with Sb being more enriched than As could be the mark of the so-called particulate shuttle effect. Notably, Sb would be more sensitive to Mn-shuttling than As. If confirmed, this trend could be used to further identify the cause of As-enrichment in marine sediments impacted by cold seepage fluids.

Methane release from marine sediments strongly influences the local seafloor environment and ecosystems, and may impact Earth's climate system. Recent studies revealed anomalous molybdenum (Mo) enrichment in seep sediments, which was linked to methane release events. Marine seep sediments are a potential sink of Mo for the global ocean, while the mechanisms leading to local Mo enrichment are not fully understood. The sediments from a gas hydrate-bearing area of the South China Sea analyzed herein reveal authigenic Mo (Mo auth) contents ranging from 0 and 31.4 µg/g and δ98Mo auth values ranging from 0.18 ‰ to 3.31 ‰. The range of δ98Mo auth values of seep sediments is therefore similar to values of modern iron-rich sediments with low concentrations of dissolved porewater hydrogen sulfide and sediments deposited under weakly euxinic sediments. Among the obtained South China Sea data, the more positive δ98Mo auth values (>ca. 1.5 ‰) are interpreted to reflect diffusion of seawater Mo into the sediment at moderate seepage rates and Mo isotope fractionation during the formation of thiomolybdates in the sulfidic seep environment. The lower δ98Mo auth values (

While a "volcanic Pompeii" model is well accepted as the cause of mass mortality and exceptional preservation of the Jehol Biota, no consensus has been reached thus far regarding the cause of mass mortality of fossils found in interbedded finely laminated sediments. We propose that volcanically driven, lacustrine redox fluctuations were responsible for the formation of the Jehol Lagerstätte, based on data obtained from a new drill core of Lower Cretaceous sedimentary rocks in the Chaoyang Basin, Northeast China. We found manganese-rich carbonates are interbedded with molybdenum-rich black mudstones and tuffs, and phosphatized fossils in the drill core. The elemental enrichment factors for molybdenum are much higher than for uranium in the black mudstones, suggesting the role of the manganese particulate-shuttle in deposition of this element. The manganese-rich carbonates are formed by the reaction of manganese oxides with organic matter, indicating an oxidized water column. This process accelerates the enrichment of molybdenum under anoxic non-sulfidic conditions. The interbedded manganese carbonates and molybdenum-rich mudstones thus record redox fluctuations in the lacustrine depositional environment. Given the numerous tephra in the black mudstones and soft-sediment deformation structures ostensibly associated with seismicity, we propose that the redox fluctuations were caused by lake eutrophication and turnover driven by volcanism that was ultimately related to the destruction of the North China Craton. These volcanically induced redox fluctuations, along with the toxicity of the volcanic gases, may have caused the simultaneous mass mortality of aquatic, terrestrial, and avian fauna. Micro-X-ray fluorescence, energy dispersive X-ray spectroscopy, and Raman mapping show that the preservation of fossils was mainly due to phosphatization. Fluctuating redox conditions facilitated phosphate enrichment in sediments. Both the oxic and anoxic non-sulfidic conditions were conducive to the preservation of organisms by phosphatization, and oxidation was favorable for phosphate mineralization, both contributing to the exceptional preservation of the Jehol Biota fossils. • "Fe-Mn particulate shuttle" controls elements cycle and enrichment in deep time lacustrine sediments. • Tectonic volcanism drives lake redox fluctuations and terrestrial ecosystem collapse. • Fluctuating redox conditions promote phosphate mineralization of Jehol fossils. [ABSTRACT FROM AUTHOR]

Abstract Molybdenum isotopes are considered to be a promising paleoredox proxy in reducing facies for evaluating either global-ocean redox conditions (given quantitative Mo uptake) or local redox conditions (given non-quantitative Mo uptake). Here, we examine the Mo-isotopic composition of black to gray shales from the Permian-Triassic boundary (PTB) section at Chaohu, eastern China, in order to assess the utility of this proxy under a range of paleoredox conditions. We evaluated local redox conditions using a combination of redox-sensitive trace-element concentrations (Mo, U) and Fe T /Al ratios, recognizing six intervals of more reducing conditions at the PTB and Induan-Olenekian boundary (IOB), in the mid-Smithian, at the Smithian-Spathian boundary (SSB), and in the mid- and upper Spathian. These six intervals are associated with variably weak to moderate enrichments of Mo, and they show variable shifts away from background δ98Mo values of ca. −0.4 to +0.1‰ (recording detrital Mo-isotopic compositions) toward δ98Mo values as high as +2.3 ± 0.2‰. Based on an analysis of local redox proxies versus δ98Mo as well as of sample-to-sample variance in δ98Mo values, we conclude that only one of the six reducing intervals in the study section is likely to record contemporaneous seawater δ98Mo (as a result of quasi-quantitative Mo uptake), i.e., the interval from the Hindeodus typicalis conodont Zone close to the PTB, for which δ98Mo values of +2.3 ± 0.2‰ suggest average global-ocean redox conditions similar to those of the modern ocean. For the remaining five reducing intervals, non-quantitative Mo uptake led to Mo-isotope signatures reflecting local, mostly suboxic redox conditions. The highest Mo concentrations (17–23 ppm) were yielded by the mid- and upper Spathian intervals, and the lack of concurrent U enrichment in the mid-Spathian interval suggests the local operation of a Mn-Fe particulate shuttle, an inference that is consistent with its relatively low δ98Mo values (0 ± 0.3‰). The lack of evidence for strong oceanic anoxia in the South China Craton following the PTB crisis may have been a factor in the relatively rapid recovery of marine ecosystems in this region during the Early Triassic. The present contribution serves as a case study of the application of Mo-isotopes as a paleoredox proxy in low-[Mo] sediments, which requires a careful assessment of the relative importance of global versus local redox influences based on additional proxies and evaluation criteria. Highlights • Case study of use of Mo-isotopes in low-[Mo] sediments for analysis of paleoredox conditions • Six intervals of more reducing conditions at the PTB, IOB, mid-Smithian, SSB, middle Spathian, and upper Spathian • Peak δ98Mo of +2.3±0.2 ‰, observed at the PTB, may be a global-ocean signal preserved by quantitative Mo uptake. • Peak Mo concentrations, in the middle and upper Spathian, indicate local operation of a Mn-Fe particulate shuttle. • The Mo-enriched Spathian beds associated with a Mn-Fe particulate shuttle yielded low δ98Mo values (0±0.3 ‰) [ABSTRACT FROM AUTHOR]

Hypersaline seafloor environments formed by brine seepage represent role model examples of extreme marine ecosystems; they may for example help to identify potential habitats for life on other planets. Trace and rare earth elements (REEs) are crucial components enabling microbial life at brine and hydrocarbon seeps. However, the behavior of trace elements at brine seeps is currently poorly understood. Because authigenic carbonates forming at brine seeps archive information on the composition of seepage fluids, we have analyzed carbonates collected from a brine seep site (Mississippi Canyon 709, MC709 for short) of the northern Gulf of Mexico (GoM). The dominance of low-magnesium (Mg) calcite among the carbonate minerals, moderately low δ13C carb values (average: −26.1‰, VPDB), as well as the presence of oil in carbonates suggest concomitant seepage of brine fluids and crude oil at this site. High δ18O carb values (up to +4.5‰, VPDB) suggest the involvement of 18O-rich fluids during carbonate precipitation, likely resulting from mineral-water reactions in deeper sediments. Significant enrichments of barium (Ba), strontium (Sr), and lithium (Li) in the samples suggest that deep ascending brine fluids contributed to carbonate formation. Enrichments in molybdenum (Mo), arsenic (As), and antimony (Sb) and a correlation between Mo, As, and Sb enrichment factors (EF) and the authigenic iron fraction (Fe/Al ratios) suggest that iron particulate shuttle processes were involved in scavenging of dissolved Mo, As, and Sb from seawater. This notion is strengthened by the co-variation of Mo EF and U EF and the shale-normalized REE patterns of the brine-seep carbonates. This study allows to constrain the enrichment mechanisms of trace and REE elements at brine seeps, providing the foundation for an improved understanding of the distribution and behavior of trace metals, as well as their role in microbial metabolisms in the local hypersaline environment. • Authigenic carbonates from a brine seep are dominated by low-magnesium calcite. • Enrichments of Sr, Ba, and Li in carbonates derive from parent fluids ascending from greater depths. • Enrichments of Mo, As, Sb are ascribed to the particulate shuttle process. • Trace elements and REEs are important for microbial metabolism at brine seeps. [ABSTRACT FROM AUTHOR]

While marine sediments have been used to constrain a history of redox chemistry throughout the Precambrian, far fewer data have been generated from lakes. With major biological innovations thought to have occurred in Proterozoic lakes, understanding their chemistry is critical for understanding the evolution of eukaryotic life. We use sediment geochemistry to characterize the redox conditions of the Nonesuch Formation (~1.1 Ga) and a modern analogue for the Proterozoic: the Middle Island Sinkhole in Lake Huron (USA). Iron speciation, Mo contents, and Mo‐U covariation demonstrate oxic and anoxic—not euxinic—environments, with no clear indicators of enhanced biological productivity in the Nonesuch Formation. Moderate Mo enrichments observed in the Nonesuch Formation are not attributed to euxinia, but instead to an authigenic particulate shuttle. We suggest that the Fe and Mo sediment geochemistry of these lacustrine systems reflect only local water column and sediment burial conditions and not atmospheric oxygenation. Plain Language Summary: Lakes are proposed to have been critical environments for the evolution of life during the Proterozoic (~2.5 to 0.5 billion years ago). However, relatively little is known about the chemistry of ancient lakes, including the availability of oxygen for biological productivity, and how local oxygen availability can be extrapolated to understand global oxygen availability. In addition, with no lakes remaining from the Proterozoic, the only way to study ancient lakes is to use the chemistry of the sediments left behind. This study uses the sediment chemistry of elements that are sensitive to oxygen to understand oxygen availability in a Proterozoic lake environment. These data were then compared to modern lake environments with known chemistry and oxygen levels in order to interpret the results better. We found that oxygen availability in the Proterozoic lake was variable, with no clear indicators of abundant biological productivity. We conclude that ancient lake sediments only constrain the chemistry of the local environment, with no major implications for global or even regional atmospheric oxygenation. Key Points: Fe, Mo, and U sediment geochemistry of the Nonesuch Formation (~1.1 Ga; USA) indicate fluctuating oxic and anoxic redox chemistryMo and U covariation in the Nonesuch Formation and modern analogue sediments confirm euxinia is not necessary for moderate Mo burialComparison of Nonesuch Formation and modern analogue indicates that Proterozoic lakes are unlikely to constrain atmospheric oxygen [ABSTRACT FROM AUTHOR]

The Kimmeridge Clay Formation (KCF) is a laterally extensive, total-organic-carbon-rich succession deposited throughout northwest Europe during the Kimmeridgian–Tithonian (Late Jurassic). It has recently been postulated that an expanded Hadley cell, with an intensified but alternating hydrological cycle, heavily influenced sedimentation and total organic carbon (TOC) enrichment by promoting primary productivity and organic matter burial in the UK sectors of the Boreal Seaway. Consistent with such climate boundary conditions, petrographic observations, total organic carbon and carbonate contents, and major and trace element data presented here indicate that the KCF of the Cleveland Basin was deposited in the Laurasian Seaway under the influence of these conditions. Depositional conditions alternated between three states that produced a distinct cyclicity in the lithological and geochemical records: lower-variability mudstone intervals (LVMIs) which comprise clay-rich mudstone and higher-variability mudstone intervals (HVMIs) which comprise TOC-rich sedimentation and carbonate-rich sedimentation. The lower-variability mudstone intervals dominate the studied interval but are punctuated by three ∼ 2–4 m thick intervals of alternating TOC-rich and carbonate-rich sedimentation (HVMIs). During the lower-variability mudstone intervals, conditions were quiescent with oxic to suboxic bottom water conditions. During the higher-variability mudstone intervals, highly dynamic conditions resulted in repeated switching of the redox system in a way similar to the modern deep basins of the Baltic Sea. During carbonate-rich sedimentation, oxic conditions prevailed, most likely due to elevated depositional energies at the seafloor by current–wave action. During TOC-rich sedimentation, intermittent anoxic–euxinic conditions led to an enrichment of redox-sensitive and sulfide-forming trace metals at the seafloor and a preservation of organic matter, and an active Mn–Fe particulate shuttle delivered redox-sensitive and sulfide-forming trace metals to the seafloor. In addition, based on TOC–S–Fe relationships, organic matter sulfurization appears to have increased organic material preservation in about half of the analysed samples throughout the core, while the remaining samples were either dominated by excess Fe input into the system or experienced pyrite oxidation and sulfur loss during oxygenation events. New Hg/TOC data do not provide evidence of increased volcanism during this time, consistent with previous work. Set in the context of recent climate modelling, our study provides a comprehensive example of the dynamic climate-driven depositional and redox conditions that can control TOC and metal accumulations in a shallow epicontinental sea, and it is therefore key to understanding the formation of similar deposits throughout Earth's history. [ABSTRACT FROM AUTHOR]

Abstract Certain trace metals are anomalously enriched in sediments and pore fluids at cold hydrocarbon seeps. Recent studies ascribed such anomalies, particularly those of molybdenum (Mo), arsenic (As), and antimony (Sb), to so-called particulate shuttle processes by manganese and iron oxyhydroxides. However, there are still some uncertainties about the prevalence of this mechanism, and it is difficult to generally rule out an influence of deep ascending fluids on trace metal enrichments. The Pliocene Chiahsien seep carbonates from the southern Western Foothills of Southwestern Taiwan were precipitated through anaerobic oxidation of methane contained in fluids that yielded no evidence for an admixture of deep ascending fluids. These authigenic carbonates consequently represent a good example to explore the role of particulate shuttle processes on the enrichments of trace metals. The Chiahsien seep carbonates reveal strong enrichment of authigenic Mo relative to uranium (U), reflected in high Mo/U ratios that are much higher than those of seawater. Such a pattern of Mo U covariation is best explained by a weak particulate shuttle process that operated during the formation of the Chiahsien seep carbonates. Furthermore, the seep carbonates show exceptional Mo, As, and Sb enrichments with no U enrichment and strong covariations between the former three trace metals and reactive iron. These findings indicate that iron oxyhydroxide particulate shuttles indeed played an intrinsic role in the enrichments of Mo, As, and Sb at the Pliocene seeps. Our findings confirm the potential importance of iron particulate shuttles on the biogeochemistry of marine cold seeps, but the factors governing the presence or absence of particulate shuttles are insufficiently understood and require more research on trace metal sequestration at seeps in the future. Graphical abstract Image 1 Highlights • The seep carbonates formed from methane-rich fluids not influenced by deep fluids. • Strong enrichment of Mo auth relative to U auth suggests a particulate shuttle process. • The seep carbonates reveal anomalous Mo, As and Sb enrichments with no U enrichment. • Mo, As and Sb contents display good correlations with reactive iron. • Iron shuttle played an intrinsic role in the enrichment of some trace metals. [ABSTRACT FROM AUTHOR]

Abstract: Although oxic surface, ferruginous deep, and euxinic intermediate waters have been documented, the redox heterogeneity of early Cambrian oceans remains largely unclear, precluding our understanding of the relationship between marine redox evolution and early animal diversification. In this study, we analyzed iron species, redox‐sensitive trace elements, and S‐N‐C isotopes of deepwater black shales in the Silikou section in the continental‐margin Nanhua Basin (South China), which represent extensive clastic input mainly from the Cathaysia Block during the early Cambrian (~541–509 Ma). Integrated data reveal a continuous shift in bottom‐water redox conditions at Silikou from euxinic in the lowermost black‐shale interval (BS1, 0–37.4 m) to oxic in the uppermost black‐shale interval (BS4, 296–372.9 m) accompanying a progressive movement of the O2/H2S redoxcline into the sediment. In between, most importantly, the BS2 interval (91–154 m) accumulated under manganous‐ferruginous conditions (i.e., Fe‐Mn reduction zone) characterized by an active Fe‐Mn particulate shuttle, and the BS3 interval (204–260 m) under nitrogenous conditions (i.e., nitrate reduction zone) characterized by strong denitrification processes, suggesting highly complex redox conditions in early Cambrian oceans. The observed sequence of redox conditions supports a strong lateral and depth‐related redox stratification in the early Cambrian ocean, including (from surface to deep and from nearshore to offshore) oxic, nitrate reduction, Fe‐Mn reduction, and euxinic zones, which were sequentially recorded at Silikou likely due to secular expansion of the oxic surface layer and/or long‐term relative sea level fall. Our study highlights the need for continued paleo‐redox studies to explore the redox heterogeneity of early Cambrian oceans. [ABSTRACT FROM AUTHOR]

In the present study, we carried out trace element analyses of black shales of the Ediacaran Doushantou Formation from two sections (Jiulongwan, Baiguoyuan) in Hubei Province, South China. Mo-U characteristics of black shales from the two sections and compiled Mo-U data of Doushantuo black shales from sections of a variety of sedimentary facies described the temporal/spatial variability in the redox conditions of paleo-seawater during deposition of the Doushantuo Formation. Changes in Mo-U patterns of the Doushantuo Member II (DST2) shales of open marine environments are consistent with a shift from a predominately oxic to a predominately anoxic ocean during their deposition. Mo-U patterns of the DST2 black shales from intra-shelf sections reflect basin restriction may have happened in the intra-shelf basin and are compatible with the redox-stratified model of the intra-shelf basin. Mo-U patterns of black shales of the Doushantuo Member IV (DST4) reveal that the shales from intra-shelf sections have more pronounced Mo enrichment and more significant enrichment of Mo over U than the slope shales, indicating the operation of a Mn particulate shuttle in the intra-shelf basin. High Mo/TOC ratios of the DST4 at the intra-shelf sections, in combination with similar Mo-TOC patterns of the DST4 from both intra-shelf and slope sections, indicate the intrashelf basin was well connected to the open ocean during deposition of the DST4. [ABSTRACT FROM AUTHOR]

In order to acknowledge the enrichment mechanism of organic matter in the black shale of the Middle Ordovician Yanxi Formation in central and southern Hunan area, the shale samples of the Yanxi Formation in the Maduqiao, Jinweixiang, Hejiadong and Mianhuaping are selected to test abundance of organic matter, major, trace and rare earth elements. The results show that the average total organic carbon (TOC) of black shale in study area is 2.03%, indicating the high abundance of organic matter in the Yanxi Formation. The CIA values and w(Sr)/w(Cu) ratios show that the Yanxi Formation had a warm and humid climate. The concentrations of Zn, V, Ni and Cu imply a medium primary productivity in the Yanxi Formation, whereas they have weak correlations with TOC. The element concentrations of V, Cr, Ni, and Mo reveal that the black shales were deposited in anoxic-euxinic conditions. The accumulation of organic matter is jointly affected by paleoclimate, primary productivity, redox conditions, sea level change and terrigenous debris. The organic matter enrichment model of the Yanxi Formation shales is a 'deepwater retention basin' controlleded by anoxic conditions. [ABSTRACT FROM AUTHOR]

Capturing the loss of mass-independent sulphur isotope fractionation (MIF-S), the correlative South African Duitschland and Rooihoogte formations are widely held to bear the isotopic fingerprint of the first atmospheric oxygenation at the onset of the so-called Great Oxidation Event (GOE). Surprisingly, however, while the multiple sulphur isotope systematics of these formations remain central to our understanding of the GOE, until now, comparatively little work has been done to elucidate the repercussions within the marine realm. Here we present chemostratigraphic records from four drill cores covering a large area of the Transvaal Basin, transcending these crucial units and continuing into the overlying Timeball Hill Formation (TBH), that document the immediate, yet counterintuitive, marine response to atmospheric oxygenation. Specifically, irrespective of the interpretative framework employed, our basin-wide redoxsensitive trace element data document an environmental change from oxic/suboxic conditions within the lower and middle parts of the Duitschland and Rooihoogte formations to suboxic/anoxic conditions within their upper reaches. Interestingly, in concert with a ~35‰ negative δ34S excursion that implicates increased sulphate availability and bacterial sulphate reduction, δ98/95Mo3134+0.25 values increase by ~1.0 to 1.5‰. Combining these observations with increased Fe/Mn ratios, elevated total sulphur and carbon contents and a trend towards lower δ13Corg values imply a shift toward less oxygenated conditions across the Transvaal Basin. The combined observations in the mentioned parameters expose a geobiological feedback-driven causality between the earliest oxygenation of the atmosphere and decreased redox potentials of medium to deep marine environments, at least within the Transvaal Basin. [ABSTRACT FROM AUTHOR]

The sedimentary Mo and U isotope systems have been commonly used as novel global ocean redox tracers due to their long oceanic residence times and redox-sensitive behavior. However, local sedimentary environments and global ocean redox conditions both influence the Mo and U isotope compositions of euxinic organic-rich mudrocks (ORM). Here, we further develop the coupled use of Mo and U isotope data from euxinic ORM to more robustly infer coeval global ocean redox conditions. We measured δ238U from eight late Neoproterozoic to middle Paleozoic ORM units that have previously reported Mo isotope and Fe speciation data. Integration of our new data with previously published Proterozoic and Phanerozoic Mo and U isotope data reveals that there is no overall correlation between the Mo and U isotope compositions of euxinic ORM. This observation confirms that the extent to which local versus global environments influenced the preserved Mo and U isotope compositions in ORM was variable. Individual ORM units can have negative, positive, or no correlation between δ98Mo and δ238U. A negative correlation between δ98Mo and δ238U in the Upper Devonian Kettle Point Formation is similar to the observations from modern euxinic basins, reflecting a major control on the Mo-U isotope systematics by changes in the local depositional environment, such as bottom-water sulfide concentrations. A positive correlation between δ98Mo and δ238U observed in the Upper Ordovician Fjäcka Shale is best explained by changes in global ocean redox conditions that simultaneously shifted the Mo and U isotope compositions of the global seawater and the Fjäcka Shale ORMs in the same direction. No correlations between δ98Mo and δ238U for euxinic ORM may be caused by specific local depositional changes, a lack of or a combination of local and global environmental changes, and/or is an artifact of limited data. For example, a vertical trend (variable δ98Mo but similar δ238U) is shown by most samples from Member IV of the Ediacaran Doushantuo Formation, implying a strong influence on the Mo isotope data by an Fe-Mn particulate shuttle. A horizontal trend (similar δ98Mo but variable δ238U) is observed from the Paleoproterozoic Zaonega Formation, implying that relatively constant bottom water sulfide concentrations caused similar magnitudes of Mo isotope fractionations whereas other factors (e.g., U reduction pathways, aqueous U species, productivity) were responsible for variable U isotope fractionations. Relatively constant elemental concentrations and isotope compositions from the Tanezzuft Formation are indicative of stable conditions at local and global scales. We further propose a method to estimate the coeval seawater Mo and U isotope compositions based on a coupled Mo-U isotope mass balance model and the observations from modern euxinic basins. The coupled Mo-U isotope data from euxinic ORMs provide more insights on the local and global environmental controls on the preservation of both isotope systems than previously realized. Our study highlights the importance of examining the local depositional environment and using large datasets of coupled Mo-U isotope compositions from euxinic ORM intervals to reconstruct paleocean redox conditions. [ABSTRACT FROM AUTHOR]

Abstract: Cold seeps, located on sea floors and frequently fueled with methane, have been much studied for the formation of carbonate nodules, crusts or masses resulting from the activity og communities of anaerobic bacteria. In this paper, we examine the impact of cold seepage on their caging sediments, as far as their geochemical composition and microfaunal content are concerned. To this end, two fossil cold seeps of the Jurassic Beauvoisin site (Vocontian Trough, SE-France) are studied, focusing on the redox-sensitive and/or sulfide-forming trace metals. We observe that: 1) the carbonate edifices of the cold seeps are enriched in specific elements, namely, Sr, Mo As and Sb, compared to their caging marlstones, and 2) the benthic foraminifer populations were present close to the edifices. The Sr enrichment is ascribed to a contamination of the seeping fluids at the contact with underlying evaporites; the Mo–As–Sb enrichment may have several, non-exclusive, origins. We retain two scenarios; in the first one, the enrichment is due to the ascending fluids that could collect these elements when passing through thick shaly formations; in the second scenario, Mo, As and Sb were supplied to the sites of carbonate precipitation together with Fe- and Mn-oxy-hydroxides (the so-called particulate shuttle process). We favor the second scenario (possibly coupled to the first one) because the rather uncommon Mo–As–Sb association is also observed for reducing sediments deposited under the influence of Fe–Mn-shuttling. [Copyright &y& Elsevier]

Abstract The Devonian–Carboniferous boundary was coeval with the Hangenberg Crisis, which is regarded as a first-order mass extinction event related with large sea-level changes and widespread anoxia. This study aims to trace the geochemical paleoproductivity and paleoredox proxies across the Devonian–Carboniferous carbonate-dominated successions of the Namur-Dinant Basin (Belgium, northern France) and the Moravo-Silesian Basin (Czech Republic), located in the Variscan Rhenohercynian Zone. The research was focused on the distribution of oxides (SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, Na 2 O, K 2 O, TiO 2 , MnO) and trace elements (Th, U, V, Zr, Mo, Zn, Pb, Ni, Cu) measured by ICP-OES and ICP-MS, respectively. The enrichment factors of Mo and U and their ratios were used as the main paleoredox proxies, whereas the enrichment factors of Zn, Cu, Pb, and Ni were employed to trace the paleoproductivity changes. High values of Zr/Al 2 O 3 (>0.001), TiO 2 /Al 2 O 3 (>0.06), and Fe/Ti (>20) and low values of Al/(Al + Fe + Mn) (<0.35) coinciding with an increase of paleoredox proxies suggest a possible volcanic and hydrothermal source of nutrients, related to Late Devonian and Early Carboniferous extensional magmatism in the Rhenohercynian domain, although a relationship between higher paleoproductivity and enhanced continental runoff of volcanic material cannot be excluded. The studied Devonian–Carboniferous boundary sections from the Namur-Dinant Basin (Gendron-Celles, Ardennes quarry) reveal three levels with similar vectors of Mo and U enrichments. The pre-crisis Famennian pattern is typical for weakly restricted basins with Fe-Mn redox cycling accompanying vertical fluctuations of the oxic/anoxic chemocline close to the sediment/water interface with the influence of particulate shuttle. The Mo EF and U EF patterns corresponding to the unrestricted or weakly restricted basin with alternating suboxic to anoxic conditions are considered to represent the transgressive Hangenberg Black Shale Event. The lower Tournaisian Mo EF and U EF patterns fall along a vector in the direction of the strongly restricted marine conditions with prevailing suboxic conditions. In the Moravo-Silesian Basin (Lesní lom section), the pre-crisis Famennian Mo EF and U EF vector indicates an unrestricted marine trend which converges with that of restricted systems. Redox conditions range from suboxic to euxinic. In the Tournaisian, Mo EF and U EF oscillate between oxic and anoxic conditions and fall along a vector of strongly restricted marine conditions. Increased isolation of both studied basins in the Early Tournaisian is regarded as a reflection of a significant eustatic fall during the Hangenberg Crisis, linked to a severe episode of the Gondwana glaciation. These paleohydrographic patterns have correlative potential that should be considered for a revised Devonian-Carboniferous Boundary definition. Graphical abstract Unlabelled Image Highlights • Mo EF and U EF revealed changes in redox and paleohydrographic conditions. • Both basins were moderately restricted in the latest Famennian. • High restriction in the Early Tournaisian resulted from Hangenberg sea-level fall. • Increased hydrothermal input proxies co-occur or precede high productivity and anoxia. • Fluctuations of redox conditions range from oxic to euxinic. [ABSTRACT FROM AUTHOR]

The Early-Middle Jurassic (∼201–161.5 Ma) was characterized by major fluctuations in Earth's climate, tectonic activity, and large-scale magmatic events. There has been considerable focus on understanding the mechanisms that triggered the Early Jurassic Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma)—a warming episode associated with a perturbation of the carbon cycle. This was marked by a negative δ13C isotope excursion (CIE) and CO 2 release from the Karoo-Ferrar Large Igneous Province (KF-LIP)—resulting in widespread organic carbon burial under strong euxinic marine conditions. Despite extensive study of the TOAE from the NW European epicontinental shelf, less focus has been placed on the influence of North Sea Dome (NSD) uplift (late Toarcian-early Aalenian) and eruption (Bajocian-Bathonian) on Jurassic paleoceanography. The NSD is a local feature commonly invoked as a driver of regional paleoclimatic conditions—a bathymetric barrier between warm Tethyan Ocean and cooler Boreal Sea waters. This study contributes new, high-resolution geochemical data derived from Viking Corridor (VC) cores and cuttings that fills a critical spatial and temporal gap in records spanning the Early-Middle Jurassic. Redox-sensitive trace elements (RSTEs; Mo/Al, U/Al, V/Al) as well as Cd*Mn and Cd/Mo ratios were used to reconstruct paleoceanographic and redox conditions. We track magmatic events and their implications using the relative enrichment of Hg, and attempt to decouple global versus regional/local volcanic systems with the enrichment factor values of mafic trace metals (MTM-EFs; Co, Cu, Ni, and Cr). Varying concentrations of RSTEs and a particulate shuttle mechanism during the Early-Middle Jurassic suggest that the VC had fluctuating paleoredox conditions, and that the NSD did not fully restrict the VC. We document a negative CIE as well as high Hg influxes during the TOAE—likely associated with KF-LIP eruptions (Early Jurassic). In addition, pulses of elevated Hg and MTM-EF values were observed during the Aalenian and Bajocian (Middle Jurassic) that were likely a result of NSD uplift and eruptions. The subaerial exposure of the NSD created progressively restricted conditions during the Aalenian-Bajocian, but the seaway between the VC and European epicontinental shelf remained open. Together, this work provides evidence that the NSD had a critical role in driving paleobathymetric and paleoceanographic conditions during the Early-Middle Jurassic. Furthermore, this study provides a geochemical insight into the interplay between δ13C isotope excursions, the KF-LIP, the NSD, and Hg-RSTE-MTM systems during an important transition in the Earth's biogeochemical history. • Multiproxy analysis of fluctuating redox conditions in Viking Corridor, North Sea. • Presence of the Toarcian and Bajocian carbon isotope excursions. • North Sea Dome is a driver of paleobathymetric and paleoceanographic conditions. • Limited restriction between warm Tethyan Ocean and cooler Boreal Sea. • Use of mafic trace mineral enrichment factors to identify volcanism. [ABSTRACT FROM AUTHOR]

The basal unit of the Early Cambrian black shale sequence of South China hosts sulfide-rich polymetallic units, non-sulfidic vanadium-rich black shales, sapropelic alginite (combustible shale), barite, and phosphorite. This rock spectrum occurs in a paleoceanographic similar, and stratigraphically correlated, transgressive upwelling setting on the passive continental margin of the Neoproterozoic Yangtze Platform. Several centimeter-thick polymetallic sulfidic units (3–13 wt% Mo+Ni, 100–600 ppm U) have relatively light Mo (δ98/95Mo = 1.1 ± 0.2‰) and relatively heavy U isotope composition (δ238/235U = 0.2 ± 0.1‰). Several meter-thick V-rich shales with multiple ore-grade layers (0.1–0.8 wt% V, < 100 ppm U, Mo and Ni ~ 100 ppm) have isotopically lighter Mo (δ98/95Mo = 0.3 ± 0.4‰) and heavier U composition (δ238/235U = 0.4 ± 0.2‰ and up to 0.7‰). The inverse Mo versus U isotope correlation suggests that both metals were enriched by removal from anoxic to strongly euxinic bottom water in restricted basins along the rifted continental margin. Metal replenishment probably occurred via the cycling of Fe–Mn-oxyhydroxide particles across the redox boundary, with sorption/desorption of Mo (and likely Ni) in a stratified water column. In contrast, V enrichment with much lower Mo, Ni, and U contents, but more fractionated Mo and U isotope composition, reflects non-sulfidic anoxic depositional conditions in a partly open system with higher bottom water renewal rates. While Mo isotope fractionation likely occurred in the water column, U isotope fractionation may dominantly have occurred at the water-sediment interface, perhaps in a benthic organic flocculent layer. These findings indicate that local hydrodynamic control and stratified water column redox conditions may explain the observed variation of metal enrichment (Mo–Ni versus V) in the black shales. Furthermore, the high δ98/95Mo values up to 2.6‰ of the black shales studied and the correlated U and Mo isotope data suggest that Early Cambrian seawater was at least episodically broadly similar to modern seawater. [ABSTRACT FROM AUTHOR]

The Precambrian‐Cambrian (Pc‐C) boundary marks significant biological, atmospheric, and oceanic changes. These changes include extinction of the Ediacaran fauna, initiation of complex lifeforms, and oxygenation of the atmosphere and oceans. In this contribution, elemental and Mo‐S isotopic compositions of organic‐rich shales overlying the Pc‐C boundary from the Tal Formation, Lesser Himalaya, have been investigated. These datasets are used to reconstruct past oceanic redox state and sulfate concentrations. The principal component analysis of the elemental dataset identifies six major factors, with factors associated with organic matter and sulfide phases accounting for about half of the total variance. Iron speciation and Mo/U data suggest that the shales were deposited in anoxic and ferruginous deep water conditions. The δ98Mo data (1.5 ± 0.2‰) and their mass balance calculations indicate that the areal extent of sulfidic waters and pyrite burial rates were about 2–4 times higher during the Pc‐C transition than in the modern ocean. The pyrite‐δ34S values (3.6–8.3‰) for the Tal shales are isotopically heavier compared to modern‐day sedimentary pyrites (∼−21‰). Calculations involving earlier‐reported δ34S values for early Cambrian seawater and our measured pyrite‐δ34S data estimate the seawater sulfate concentration (8 ± 3 mM) during their deposition. This sulfate value for the Tal basin is higher than that reported for the late Neoproterozoic ocean (<5 mM), attributable to increasing oxygen availability and continental supply during this time. The observed basinal conditions and high terrestrial input may have influenced metazoan diversification. Key Points: Chemical and Mo‐S isotopic study of black shales overlying the Pc‐C boundary from the Tal Formation (Lesser Himalaya) was carried outTrace element, Fe‐speciation and Mo isotopic data suggest a stratified basin with anoxic and ferruginous bottom water conditionsHigher seawater sulfate (∼8 mM) during the Pc‐C transition (than Proterozoic) linked to atmospheric oxygenation and enhanced riverine input [ABSTRACT FROM AUTHOR]

Port sediments are often contaminated with metals and organic compounds from anthropogenic sources. Remobilization of sediment during a planned expansion of Port Everglades near Fort Lauderdale, Florida (USA) has the potential to harm adjacent benthic communities, including coral reefs. Twelve sediment cores were collected from four Port Everglades sites and a control site; surface sediment was collected at two nearby coral reef sites. Sediment cores, sampled every 5 cm, were analyzed for 14 heavy metals using inductively coupled plasma-mass spectrometry. Results for all three locations yielded concentration ranges (mg/g): As (0.607-223), Cd (n/d-0.916), Cr (0.155-56.8), Co (0.0238-7.40), Cu (0.004-215), Pb (0.0169-73.8), Mn (1.61-204), Hg (n/d-0.736), Mn (1.61-204), Ni (0.232-29.3), Se (n/d-4.79), Sn (n/d-140), V (0.160-176), and Zn (0.112-603), where n/dDnon-detected. The geo-accumulation index shows moderateto-strong contamination of As and Mo in port sediments, and potential ecological risk indicates moderate-to-significantly high overall metal contamination. All four port sites have sediment core subsamples with As concentrations above both threshold effect level (TEL, 7.24 μg/g) and probable effect level (PEL, 41.6 μg/g), while Mo geometric mean concentrations exceed the background continental crust level (1.5 μg/g) threshold. Control site sediments exceed TEL for As, while the reef sites has low to no overall heavy metal contamination. Results of this study indicate there is a moderate to high overall ecological risk from remobilized sediment due to metal contamination. Due to an imminent dredging at Port Everglades, this could have the potential to harm the threatened adjacent coral communities and surrounding protected habitats. [ABSTRACT FROM AUTHOR]

Highlights • A multiproxy study of oceanic palaeoredox conditions. • Restriction of oxic conditions to shallow coastal waters in a Mesoproterozoic basin. • Dominance of anoxic ferruginous waters over an 800 million year period. • An enhanced Fe oxide particulate shuttle resulted in moderate Mo enrichments. Abstract The apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the Proterozoic Eon (2.5–0.542 Ga). It is generally accepted that from ∼1.85 Ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous (Fe(II)-rich), with episodic euxinia confined to productive continental margins and intracontinental basins. The geochemical record that informs this picture, however, is currently sparse, and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time. Here, we report Fe-S-C systematics, alongside major (Al, Mn) and trace metal (Mo, U) data for ∼1.6–0.8 Ga marine sediments from the Bashkir Meganticlinorium (BMA), Southern Urals, Russia. Our Fe speciation data reveal a water column dominated by ferruginous conditions, no evidence for euxinia, and oxygenated waters confined to only the shallowest, near-coastal settings. Trace metal data support these findings, with Mo and U enrichment factors in anoxic sediments implying the operation of a particulate Fe (oxyhydr)oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic Mo enrichments. Sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the BMA succession, which, in combination with a low organic carbon loading, promoted the development of ferruginous, rather than euxinic, water column conditions. The restriction of oxic conditions to the shallowest, near-coastal depositional settings within the Bashkir basin contrasts to other redox studies from this period. Such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the Mesoproterozoic Era, and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period. [ABSTRACT FROM AUTHOR]

Elemental vanadium (V), an essentially redox-sensitive metal in seawater, has had a significant impact on the understanding of the evolution of the atmosphere-ocean system throughout the history of the Earth. In fact, the geochemical cycle of V in early Cambrian seawater may have had an influence on the Chengjiang Biota in South China; however, it has not yet been well established. Given the authigenic vanadium accumulation is sensitive to the redox conditions of seawater, here, to constrain the geochemical cycle of V in seawater during the early Cambrian, the Mo, U and total organic carbon (TOC) distributions with high-resolution samples from both the outer shelf and slope facies (e.g., the Duoding and Longbizui sections), are applied to evaluate the redox conditions of ambient seawater. The Mo-U relationships indicate that the redox conditions of the mid-depth seawater evolved in a systematic way in South China, transitioning from an Fe-Mn reduction zone to anoxic/intermittently euxinic states and then to oxic conditions during the early Cambrian. As a consequence, the authigenic V enrichment, constrained by the marine redox conditions, was mainly controlled by the Fe-Mn particulate shuttle and the reduction and adsorption of organic matter in anoxic/euxinic conditions. However, the decoupling among V, Mo, U and TOC under anoxic/euxinic conditions suggests a dwindling vanadium concentration in the early Cambrian seawater of South China. The scavenging efficiency of V from seawater is much higher than those of Mo and U under anoxic/euxinic conditions. Ultimately, these trace elements (e.g., Mo, U, and especially V) in seawater could effectively be regulated and adjusted to a reasonable level under the widespread anoxic/euxinic conditions. The drawdown of trace elements in seawater might provide an early-stage preparation of the marine environment for the subsequent Chengjiang Biota. [ABSTRACT FROM AUTHOR]

Organic-rich continental shale, widespread in the Sichuan Basin during the deposition of the Jurassic Dongyuemiao Member (J1d), is considered the next shale hydrocarbon exploration target in southern China. To identify a shale gas sweetspot and reduce exploration risk, it is of great significance to determine the organic matter (OM) enrichment mechanism of J1d shale. In this study, based on sedimentological characteristics and organic matter content, high-resolution major and trace elements were systematically analyzed to demonstrate terrigenous influx, paleoredox, paleosalinity, paleoproductivity, and paleoclimate. The 1st section interval of the J1d 1st submember is dominated by shallow lake subfacies, while the other intervals have the characteristic of semideep to deep lake subfacies. The 1st submember interval of J1d lacustrine shale is characterized by the warmest-humid paleoclimate, strongest weathering degree, highest terrigenous input, moderate paleoproductivity, and paleoredox condition. Within the Dongyuemiao 1st submember, the 4th section interval has the highest paleoproductivity and the most oxygen-deficient condition in bottom water. During the deposition period of the 2nd submember, the sedimentary environment turned to a cold-dry paleoclimate, weak weathering degree, low terrigenous input, low paleosalinity, and high paleoproductivity. Under the background of semideep and deep lake, the terrigenous OM input plays the most critical role in controlling OM enrichment. Moreover, the high primary productivity of lake surface water and the suboxic condition of lake bottom water contribute to the formation of relatively higher TOC lacustrine shale interval in the 4th section of 1st submember. [ABSTRACT FROM AUTHOR]

Most scholars believe that transgression events can contribute positively to organic matter enrichment and shale gas sweet spot development, while whether or not regression events are conducive to shale gas sweet spot development remains to be further discussed. Variations in organic carbon content (TOC), and major and trace elements at the Rhuddanian–Aeronian stage in the Luzhou area, the southern margin of Sichuan Basin, were analyzed in this paper. We discuss differences in paleoenvironment organic matter enrichment and shale sweet spot development during transgression and regression. A transgressive system tract (TST) occurred during the early Rhuddanian stage, while early highstand system tracts (Ehst-1 and Ehst-2) occurred during the late Rhuddanian stage and Aeronian stage, and a late highstand system tract (LHST) developed during the late Aeronian stage. A rise in sea level during the TST in the upper Yangtze resulted in an anoxic environment, where a continuous upwelling current brought about a large number of nutrients in the seawater, significantly increasing paleoproductivity. Strong tectonic subsidence, weak chemical weathering, and a rising sea level together led to a low terrigenous debris supply in the catchment area. Therefore, paleoproductivity and redox conditions were the primary controlling factors of organic matter enrichment at the TST stage, with a clastic supply of secondary importance. With the advance of the Guangxi orogeny, the organic matter enrichment at the EHST-1, EHST-2, and LHST stages was mainly controlled by redox conditions and debris supply. A comparison of the key physical parameters and geochemical indicators of shale reservoirs from these four system tracts suggests that two shale sweet spot types (type I and II) were developed during the Rhuddanian–Aeronian stage, occurring in the TST and EHST-2 stages. High TOC and high microcrystalline quartz content are key to developing type I sweet spots, while enhanced anoxic conditions in the bottom water caused by ephemeral, small-scale sea level rises are the main determinant of class II sweet spots in the later EHST stage. [ABSTRACT FROM AUTHOR]

The Lower Permian Fengcheng formation is a significant source of rocks and a reservoir for the Mahu Sag in the Junggar Basin. Recently, the paleo-environment deposition factors of the P1f formation have become a popular research topic. This research was conducted using data from the F7 well of Mahu Sag, based on the geochemical analysis results of TOC (total organic carbon), REE (rare earth elements), and major and trace elements of 53 samples from P1f (Lower Permian Fengcheng formation), and some deposition factors are discussed. The P1f deposition process was classified into four stages based on paleo-environment elemental indicators. This research describes the deposition process of the evolution of alkaline lakes. The early and preliminary stages of alkali lake evolution are considered as late P1f1 to middle P1f2; the paleoclimate of this process was dry, the reduction conditions increased, and the paleo-productivity and lake salinity were enhanced. The terminal stage of alkali lake evolution is considered as late P1f2 to middle P1f3; in this period, the paleoclimate changed with seasonal cycles, resulting in a decrease in water salinity and an increase in oxidation; the paleo-productivity of the alkaline lake was at a medium level. Until the end of P1f3, the salinity of the lake decreased, and the water body became anoxic and weakly alkaline. Furthermore, the research on TOC and sedimentary parameters confirmed that the deposition of P1f organic matter is affected by multiple types of factors. A relatively warm climate, lack of oxygen, fresh water–brackish water, suitable debris flow, and high primary productivity conditions promoted organic matter deposition. [ABSTRACT FROM AUTHOR]

The redox condition of the ocean played an important role in the evolution of the Ediacaran biota, but it remains largely unexplored, particularly for the deep ocean environment. In this study, we present a comprehensive study of Fe speciation and trace elements of the Doushantuo black shales from the Longe and Jinjiadong sections in South China, to investigate the redox condition of basinal environments in Ediacaran. In general, both sections show high Fe HR /Fe T ratios (>0.38), low Fe Py /Fe HR ratios (<0.7), suggesting a predominately anoxic and ferruginous condition in the deep water of Nanhua basin during the Doushantuo period. A few high Fe Py /Fe HR ratios (>0.7) in the Jinjiadong section indicating occasional euxinia in the basin facies. Fe Py /Fe HR ratios show an increase in the middle part both in the Longe and Jinjiadong sections, indicating enhanced production of H 2 S via bacteria sulfate reduction (BSR) in the deep anoxic water, which was likely responsible to the upper Doushantuo negative carbon isotope excursion. The enrichment factors (EFs) of most redox sensitive trace elements in the sediments of both sections are low (≤1), while EF-Mo shows moderately enriched (∼3–60), which is similar to the modern Cariaco Basin. Because of paleogeographic restriction or persistent ocean stratification, the trace elements renewal would be mostly obstructed in the isolated deep water, and their inventory could become depleted, which cause the low EFs of most trace elements in sediments. However, for the non-euxinic deep ocean, a “particulate shuttle” would transport Mo from shallow water to deep water through adsorption-desorption on the Mn–Fe-oxyhydroxides, which maintained moderate Mo enrichment in seawater and related sediments. [ABSTRACT FROM AUTHOR]

Organic- and sulfide-rich sediments have formed in oxygen-depleted environments throughout Earth's history. The fact that they are generally enriched in redox-sensitive elements reflects the sedimentary environment at the time of deposition. Although the modern ocean is well oxidized, oxygen depletion occurs in certain areas such as restricted basins and high-productivity zones. We measured bulk chemical compositions (major and trace elements, total organic carbon, and total sulfur) of organic- and sulfide-rich sediments collected from eight areas having oxygen-depleted water to discuss relationships between geochemical features and sedimentary environments. Major elemental compositions generally show mixtures of terrigenous detritus and biogenic carbonate. Some redox-sensitive elements might be controlled by organic matter content, whereas others could be contained in sulfide minerals in sediments. In particular, Mo and U show a characteristic trend; areas with higher Mo and U—at least partially owing to a depositional process called the "particulate shuttle"—generally correspond to regions influenced by the open ocean. In contrast, areas with lower Mo and U are more restricted marine environments. This suggests that the degree of Mo and U enrichment reflects the geography in terms of proximity to the open ocean, or the degree of the supply of these elements from the open ocean. [ABSTRACT FROM AUTHOR]

The Devonian–Carboniferous transition marks a fundamental shift in the surface environment primarily related to changes in ocean–atmosphere oxidation states1,2, resulting from the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering3,4, glacioeustasy5,6, eutrophication and anoxic expansion in epicontinental seas3,4, and mass extinction events2,7,8. Here we present a comprehensive spatial and temporal compilation of geochemical data from 90 cores across the entire Bakken Shale (Williston Basin, North America). Our dataset allows for the detailed documentation of stepwise transgressions of toxic euxinic waters into the shallow oceans that drove a series of Late Devonian extinction events. Other Phanerozoic extinctions have also been related to the expansion of shallow-water euxinia, indicating that hydrogen sulfide toxicity was a key driver of Phanerozoic biodiversity.A spatial and temporal compilation of geochemical data documents a stepwise transgression of toxic euxinic waters into shallow oceans that drove a series of Late Devonian extinction events. [ABSTRACT FROM AUTHOR]

Molybdenum (Mo) concentrations and Mo isotopes have been widely used as proxies for local and global redox conditions in early oceans (>520 Ma) that were stratified and characterized by dynamically euxinic mid-depth waters. However, the nature of the Mo cycle and accompanying isotopic fractionations in such oceans remain poorly known. To fill this gap, we conducted an integrated study of Mo isotopes and redox-sensitive trace element (RSTE) abundances in the Lower Cambrian Niutitang Formation at Yangjiaping, South China. This section accumulated on the northern shelf margin of the Nanhua Basin, a failed intracontinental rift basin with a good connection to the open ocean during the early Cambrian. The Niutitang Formation contains a ∼18-m-thick lower black shale member, and a ∼56-m-thick upper gray silty shale member. The lower member (LM) is moderately to strongly enriched in Mo, U and V, and heterogeneous in Mo isotopic composition (δ 98 Mo = −0.65‰ to +2.14‰), indicative of dominantly euxinic depositional conditions punctuated by ferruginous episodes (as shown by previously reported Fe speciation data). The upper member (UM) shows lesser enrichment of Mo, U and V, higher Mo/U ratios, and intermediate and more uniform Mo-isotopic compositions (δ 98 Mo = +1.16‰ to +1.71‰), indicative of weakly oxic to anoxic-euxinic depositional conditions. Geochemical profiles suggest that the LM-to-UM transition reflects a shift of the O 2 /H 2 S chemocline from the water column to the sediment. Large δ 98 Mo fluctuations in the LM may record variations of H 2 S concentrations in the mid-depth euxinic waters. The intermediate and relatively uniform δ 98 Mo values of the UM are attributed to the effects of a local Fe–Mn particulate shuttle. In light of the observations at Yangjiaping and other contemporaneous sections in the Nanhua Basin, we propose a new marine Mo biogeochemical model for the early Cambrian ocean. In this model, an Fe–Mn reduction zone may have developed above mid-depth euxinic waters in nearshore areas of the Nanhua Basin due to relatively high availability of fluvial Fe–Mn oxides and their subsequent reduction below the chemocline. Within this redox framework, large sedimentary δ 98 Mo variations observed for the Niutitang shales can be explained through the combined effects of Mo isotopic fractionation associated with adsorption onto Fe–Mn-oxides and spatially variable H 2 S concentrations in the euxinic watermass. By compiling published data, we found that the large sedimentary δ 98 Mo variations previously observed in the early Cambrian Nanhua Basin and in other Neoproterozoic-Cambrian marine basins can be similarly explained. Our model provides a novel interpretation of the Mo biogeochemistry of early Earth oceans, within the framework of which Mo isotopes may serve to further our understanding of the oxygenation history of the Earth-surface system. [ABSTRACT FROM AUTHOR]

Previous studies have generated conflicting interpretations of the nature of oceanographic conditions on the Early Cambrian Yangtze Platform. We investigated paleo-environmental conditions there through an analysis of major elements and redox-sensitive trace elements in the lower Cambrian Niutitang Formation from two new drillcores (RY1 and RY2) in Guizhou Province. High TOC and excess Ba concentrations record elevated productivity in the study area. Redox variation was documented based on enrichment factors (EFs) for redox-sensitive trace elements (Mo, U, V, Cr, Ni, and Co) and C org :P ratios. These proxies indicate that the Lower Unit of the Niutitang Formation was deposited under fluctuating but mostly anoxic conditions, whereas the Upper Unit was deposited under predominantly suboxic conditions. Mo/TOC ratios suggest persistently moderate to strong basinal restriction, although Mo–U covariation patterns show no evidence for operation of a particulate shuttle despite the semi-restricted, anoxic nature of the deep watermass. Episodic deep water oxygenation events occurred within the Yangtze Platform sea, and longer-term secular variation may record eustatic influences. Additionally, patterns of trace-metal enrichment record evidence of submarine hydrothermal inputs. We propose an integrated paleoenvironmental model to account for the observed productivity, redox, and hydrographic patterns in the Early Cambrian Yangtze Platform. On the basis of paleo-marine environmental analysis, we ascribe the scarcity of trace fossils in the bottom to oxygen deficiency, and deny considering the Songlin Biota as the onset of the intense biogenic evolution after global atmospheric and oceanic oxygenation. [ABSTRACT FROM AUTHOR]

This study presents results of the petrographic, mineralogical, stable isotopes of oxygen and carbon, and trace element investigation of authigenic carbonates collected at newly discovered active cold seeps on the Laptev Sea continental slope at ∼300 m water depth. These carbonates are mainly represented by Mg-calcite with MgCO 3 content from 9.1 mol% to 14.0 mol%. The low δ13C values of carbonates ranging from −50.6 ‰ to −32.4 ‰ (V-PDB) indicate that they were formed from anaerobic oxidation of biogenic methane and minor participation of other carbon sources. The difference between measured (from 4.7 ‰ to 5.5 ‰) and calculated (4.0 ‰) δ18O carb values might be inherited from fluids enriched in 18O due to dissociation of gas hydrates, which could be the source of methane. The carbonates exhibit weak enrichment in Co, moderate and strong enrichments in As, Mo, and Sb, and strong enrichment in U. Interestingly, As, Sb, and Co correlate with the pyrite content. This indicates that authigenic iron sulfides promote the immobilization of these redox-sensitive elements in seep sediments. The (Mo/U) EF values and anomalies of concentration of Mo and U probably indicate variations in the redox conditions during carbonate formation due to episodically seepage activity changes. Ascending methane-bearing fluids were the main contributor to the enrichment of cold-seep carbonates in As, Mo, Sb, and U at the Laptev Sea continental slope. However additional input from the particulate shuttle process can not be ruled out. • Cold-seep carbonates from Laptev Sea continental slope are mainly represented by high-magnesium calcite. • High-magnesian calcite formed from anaerobic oxidation of biogenic methane and minor participation of other carbon sources. • Carbonates are enriched by As, Mo, Sb, and U. • Ascending methane fluids are the main contributor to the enrichment of cold-seep carbonates by trace elements. [ABSTRACT FROM AUTHOR]

Any effort to reconstruct Earth history using variations in authigenic enrichments of redox-sensitive and biogeochemically important trace metals must rest on a fundamental understanding of their modem oceanic and sedimentary geochemistry. Further, unravelling the multiple controls on sedimentary enrichments requires a multi-element approach. Of the range of metals studied, most is known about the behavior of Fe, Mn, and Mo. In this study, we compare the authigenic enrichment patterns of these elements with a group whose behavior is not as well defined (Cd, Cu, Zn, and Ni) in three oxygen-poor settings: the Black Sea, the Cariaco Basin (Venezuela), and the Peru Margin. These three settings span a range of biogeochemical environments, allowing us to isolate the different controls on sedimentary enrichment. Our approach, relying on the covariation of elemental enrichment factors [EF, defined for element X as: EFX = (X/Al)sample/(X/Al)lithogenic], has previously been applied to Mo and U to elucidate paleoenvironmental information on, for example, benthic redox conditions, the particulate shuttle, and the evolution of water mass chemistry. We find two key controls on trace metal enrichment. First, the concentration of an element in the lithogenic background sediment (used in calculating EFX) controls the magnitude of potential enrichment. Maximum enrichment factors of 376 and 800 are calculated for Mo (~1 ppm in detrital sediments) and Cd (~0.3 ppm), respectively, compared to values not greater than 17 in any setting for the other five metals (~45 ppm to -4.5 wt.% in detrital sediments). Second, there is a relationship between the aqueous concentration of the element in overlying seawater and its degree of enrichment in the sediment. We further identify four important processes for delivery of trace metals to the sediment. These are: (1) cellular uptake (especially important for Zn and Cd), (2) interaction/co-precipitation with sulfide (Mo, Cu, and Cd), (3) passive scavenging via the traditional particulate shutde (Mo, Ni, and Cu), and (4) an association with the benthic Fe redox shuttle (Mn, Ni). Finally, we summarize the oceanic mass balance of Cd and Mo and place the first constraints on the contribution of reducing sediments to the oceanic mass balance of Cu, Zn, and Ni. We show that reducing sediments are the ultimate repository for up to half the total output flux of these elements from the oceanic dissolved pool. [ABSTRACT FROM AUTHOR]

The Toarcian Oceanic Anoxic Event (T-OAE) and its corresponding Carbon Isotope Excursion (CIE) have been reported widely across the Tethyan region and globally. In Arabia, and based on ammonite dating, the time window of the T-OAE coincided with the deposition of the reddish siliciclastic unit of the Marrat Formation. However, no evidence of the T-OAE/CIE was ever reported from Arabia because these red beds were previously interpreted as continental deposits. Recently, these red beds have been recognized as shallow marine deposits which opened an opportunity to assess the occurrence and expression of T-OAE–CIE in Arabia. In this study, a multiproxy geochemical characterization was performed on the Toarcian Marrat Formation to infer the chemistry of the paleowater column and identify intervals of possible T-OAE/CIE in Arabia. While the low concentrations of redox-sensitive elements (Mo, U, V, Cr) may indicate a shallow oxic marine settings, the coupled negative δ13Corganic excursion and apparent increase in the chemical weathering suggests that the deposition of Marrat red beds coincided with the development of T-CIE and possibly time-equivalent to the T-OAE globally. The origin of reddening is interpreted to have occurred during the middle Marrat deposition due to the stabilization of unstable hydrous iron oxides to hematite under oxic marine conditions. The proposed model further indicates the possible development of source rocks in the deep, anoxic environment counterpart where the T-OAE may be expressed. Since our study documents the first record of the T-CIE and discuss the origin of shallow marine siliciclastic red beds in the Arabian Plate, this will have significant implications for the overall understanding of the T-CIE globally and for hydrocarbon exploration through realizations of potential new source rocks associated with the OAEs in the Toarcian and other time intervals. [ABSTRACT FROM AUTHOR]

Early Cambrian black shales on the Yangtze Platform host a regionally distributed highly metalliferous sulfide-rich carbonaceous unit which has been the subject of extensive debate. This marker unit, with a few centimeters or tens of centimeters in thickness, displays extreme enrichment in Mo and Ni (wt% range), and in a broad spectrum of other metals such as As, Au, PGE, Re, Cu, Zn, Cd, Ag, Sb, Se, Tl, and Hg, and occurs discontinuously along the western passive margin of the Yangtze Platform. It grades laterally in stratigraphically equivalent meter-thick vanadium-rich shale and tens-of-meter-thick sapropelite (combustible shale). New Cu and Zn isotope data, combined with existing Cd, Cr, Ni, Mo, Hg, and Se isotope and other chemical data, allow to attempt an integrated view on the formation of this intriguing unit of hyper-enriched metalliferous black shale. The authigenic Cu enrichment in the 1000-ppm range has produced no or little Cu isotope fractionation (0.03 ± 0.26 ‰ δ65Cu) beyond the lithogenic background. Heavy zinc and cadmium isotope enrichment in the sulfidic samples (1.11 ± 0.18 ‰ δ66Zn, 0.31 ± 0.10 ‰ δ114Cd) is controlled by sulfide fractionation and contrasts with V-rich and barren shale (0.60 ± 0.18 ‰ δ66Zn, 0.00 ± 0.14 ‰ δ114Cd). The distinctly negative Ni isotope composition of the metalliferous unit (−0.84 ± 0.05 ‰ δ60Ni) with Ni in the percent range has been interpreted as due to hydrothermal activity related to the leaching of mafic rocks and their sulfides. Sorption processes (Fe-oxyhydroxides) and redox cycling in the water column and the bottom sediment with microbial activity could be an alternative interpretation. The extreme metal enrichment can be understood as due to a process chain, from high biological productivity in the oxic photic zone to sulfate reduction in the deeper sulfidic water column and upper sediment layer. Key to the metal enrichment seems to be extremely low clastic sedimentation and advanced carbon destruction by anaerobic oxidation. Hydrothermal input of basinal brines along the rifted margin of the Yangtze Platform was likely a part of this scenario. [ABSTRACT FROM AUTHOR]

Ultramafic xenoliths from southeastern Arizona, USA, provide evidence for Cu‐isotope heterogeneity in the lithospheric mantle. We report new data on Type I (Cr‐, Mg‐rich) peridotites, but also the first Cu‐isotope data for Fe‐Ti‐Al‐rich Type II pyroxenite (±amphibole) xenoliths. Whole rock δ65Cu values of the pyroxenites and cryptically metasomatized Type I lherzolites range to isotopically heavier compositions than asthenospheric mantle (i.e., up to +1.44‰ and +1.12‰, respectively, vs. ∼0‰ ± 0.2‰). Copper leached from the xenoliths using aqua regia, assumed to be hosted in interstitial sulfides, is even more variable (δ65Cu −0.78 to +3.88‰), indicating considerable isotopic heterogeneity within individual samples. Host basalts have low δ65Cu (−0.23‰ to −1.30‰), so basalt—xenolith interactions are not responsible for the compositional variations observed. While mass‐dependent fractionation may be partly responsible, metasomatism by fluids derived from recycled crustal materials is the predominant control on isotopic variations observed. Amphibole megacrysts and amphiboles separated from Type II amphibole‐bearing clinopyroxenite have normal, mantle‐like 18O/16O ratios but H‐isotope compositions (δ2HSMOW −82‰ to −45‰) that range between that of nominally anhydrous mantle (−80 ± 10‰) and seawater (0‰). Host basalts are also enriched in 34S relative to depleted asthenospheric mantle, having δ34SCDT values up to +8‰, i.e., compositions commonly attributed to a component of recycled seawater or hydrated oceanic crust. These new data suggest that formation of Type II metasomes in the lithospheric mantle beneath the Basin and Range Province was associated with subduction of the Farallon plate and not alkali basalt magmatism associated with Basin and Range extension. Plain Language Summary: The Cu‐isotope composition of Earth's mantle is believed to be homogenous, with δ65Cu varying between −0.2‰ and +0.2‰ (where δ65Cu is the 65Cu/63Cu ratio relative to the NIST976 copper standard). However, samples of the lithospheric mantle entrained as inclusions in Plio‐Pleistocene age basalts from SE Arizona, USA, record a much wider compositional range of −0.78‰ to +3.88‰. The greatest compositional range (−0.08‰ to +1.44‰) is observed in Fe‐, Ti‐, and Al‐rich pyroxenites (±amphibole) that formed by fractional crystallization of basaltic melts in the mantle. While mass‐dependent fractionation in the mantle may be responsible for some of the isotopic variation, metasomatism during subduction, by fluids derived from recycled crustal materials, is proposed as the dominant mechanism. These new data also suggest that formation of enriched veins in the lithospheric mantle beneath the Basin and Range is associated with subduction of the Farallon plate, and not the alkali basalt magmatism caused by Basin and Range extension, as had previously been proposed. Key Points: The Cu‐isotope composition of lithospheric mantle is heterogeneousMetasomatism of this mantle by fluids/melts derived from crustal materials recycled via seduction is the principal source of the heterogeneityFe‐Al‐rich pyroxenites, representative of geochemically enriched mantle metasomes, exhibit greater isotopic variation than metasomatized peridotites [ABSTRACT FROM AUTHOR]

The Cenomanian–Turonian boundary (ca. 93.5 Ma) is marked by an episode of profound environmental change, including a major perturbation of the carbon cycle and an Oceanic Anoxic Event (OAE-2). Here, we present molybdenum (Mo) isotope variations within the OAE-2 interval for four sections from the western Tethys (Furlo and La Contessa) and the North-Atlantic (ODP site 1276 and DSDP site 367). The main target of this study is to investigate the extent of reducing conditions (truly global in extent or restricted to poorly-ventilated restricted deep basins), with particular reference to the relationship between the change in the oxygenation state of the ocean and the link to global perturbations of the carbon cycle recorded in carbon isotopes. All four sections show fluctuations in the redox sensitive trace metal (RSTE) distribution, suggesting rapid variations in local redox conditions, ranging from anoxic to euxinic. The RSTE enrichment factors (EFs) also suggest different depositional conditions and paleoceanographic processes in the western Tethys versus the North Atlantic. Whereas the North Atlantic sites show evidence of weak watermass restriction associated with the action of a particulate shuttle within the water column, the EFs of the Tethyan sections are characteristic of unrestricted marine systems. Mo isotopes show surprisingly negative values through the Tethyan sections. At the onset of OAE-2, an increasing trend in δ Mo 98 is observed, with values ranging from −0.6 to 0.6‰. During the second half of OAE-2, the δ Mo 98 curve shows a progressive shift towards more negative values. In the North Atlantic, δ Mo 98 signatures from ODP site 1276 show a similar behaviour as observed in the western Tethys. At DSDP site 367, Mo isotopes are generally heavier during OAE-2, fluctuating around an average value of 1.1‰. This is consistent with fully euxinic conditions and the black shales deposited may have recorded the seawater signature during OAE-2. The Mo isotope signature is lighter than the modern ocean value (2.3‰) and suggests a higher proportion of bottom water euxinia (∼10%). Although this estimate is greater than the euxinic fraction in the modern ocean, our results challenge the suggestion of a globally euxinic deep waters during OAE-2. The variations in Mo / TOC ratios within the North Atlantic and Tethys oceans are in good agreement with a general drawdown of Mo within the OAE interval, suggesting a general increase in anoxic conditions during OAE-2. Our results highlight the role of local parameters in the development of euxinia, even during period of widespread anoxia such as OAEs. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of, Co, Mo, Zn, Ni, Cd, U, Cu, Mn, V, Al, Fe, Ti, Total Organic Carbon (TOC) and Total Sulfur (TS) was studied in surface sediments accumulated at the upper limit of the oxygen minimum zone in the northern Humboldt Current (Mejillones Bay), using samples taken during two cruises in May 2002 and October 2004. The time series of dissolved oxygen revealed a seasonal fluctuation of the upper limit of the oxygen minimum zone into the bay. Similar bathymetric distribution curves of the metals measured in the two studied periods in the OMZ off northern Chile suggest that the factors responsible for this distribution are persistent over time. Authigenic metal content was calculated using lithogenic background by determining Metal/Al ratio from local aeolian dust, instead of average shale values, which are classically used for background correction. This more precise assessment allows examination of the source–sink relation of each metal in this coastal zone. Mo preserved in the bottom sediments originates from seawater and accumulates via particulate shuttle, whereas Cd is pre-concentrated in biodetritus. Ni, Mn, V and Fe show a combined behavior. Sulfate reduction is not the most important mechanism affecting metal accumulation in Mejillones Bay, probably due to the presence of Thioploca, which inhibits the pyrite formation and the metal-sulfide reaction. We propose the use of the authigenic Mo/U ratio and a preliminary statistical model to reconstruct the levels of paleoxygenation in the subsurface waters of Mejillones Bay, by analyzing the same metals measured throughout the sedimentary sequences (and using the same statistical procedure), at least to identify extreme variations of the OMZ in this bay of northern Chile. [Copyright &y& Elsevier]

The Arabian Sea encompasses oxygen minimum zone with denitrifying conditions. For the present study, sediments were collected across three transects off Goa transect (GT), Mangalore transect (MT) and Kochi transect (KT) in contact with water column dissolved oxygen (DO) range of 1.4–118.0 µM. Sediments were investigated for texture, clay mineralogy, total organic carbon (Corg), total nitrogen, CaCO3, δ15N, δ13C, metal content to infer their distribution with changing DO and their use as possible palaeo-proxies. The Corg (0.9–8.6%) is largely marine and δ15N from GT and MT preserves signatures of higher water column denitrification. Al normalised ratios of Cu, Ni, Zn, V and Mo are GT > MT > KT. Nickel could be a productivity proxy for DO <80 µM; Cr, U could be a redox proxy for DO <20 µM; Co and Mn could be a better proxy for DO <80 µM; U/Mo vs. DO could separate sediments with DO less than and >80 µM. We observed that eastern Arabian Sea dataset does not always fall within the defined proxies of the other anoxic basins. The metal concentrations in modern day sediments from the eastern Arabian Sea may be used as a proxy to infer the ancient sediment redox conditions. Highlights: Organic carbon distribution in anoxic sediments. Provenance of sediments. Nitrogen isotopes and geochemistry as palaeo-proxy for anoxic basins. [ABSTRACT FROM AUTHOR]

The Triassic Boreal Ocean was a shallow epicontinental basin and the sink of the World's largest delta plain known to date. Nutrient and freshwater supply from this delta have been regarded as important causes for high productivity and water mass stratification, forming Middle Triassic oil‐prone source rocks. Recent studies attribute upwelling and a productivity‐induced oxygen minimum zone as important factors. A multi‐elemental chemostratigraphic study of a Spathian–Carnian mudstone succession exposed in eastern Svalbard was performed to investigate their formation. This includes 89 samples from three localities, from which 34 elements were acquired using combustion and X‐ray fluorescence analyses. The goal is to provide a correlation framework and infer the role of productivity, redox and water mass restriction on organic matter accumulation and source rock formation. These processes had major impact on the source potential. The Spathian Vendomdalen Member suggests deposition during intermittent benthic euxinia and low productivity, corresponding with a reported deep thermocline that obstructed upwelling. The lower Anisian lower–middle Muen Member shows negligible enrichment in redox‐sensitive elements but in situ phosphate nodules, consistent with developing upwelling and moderate productivity. The middle Anisian upper Muen Member formed during high productivity and phosphogenesis and is linked with basin‐wide upwelling. Productivity, phosphate and redox proxies are all strongly enriched in the upper Anisian–Ladinian Blanknuten Member. In the south‐western Barents Sea, the pro‐deltaic environment of the emerging Triassic Boreal Ocean delta system had terminated these conditions. The upper Ladinian upper Blanknuten Member formed within intermittent euxinic bottom waters due to the shallowing sea level. The Carnian Tschermakfjellet Formation marks the dominance of the prograding delta system and the end of Triassic oil‐prone source rock formation in Svalbard. This study investigates the elemental composition of Lower–Upper Triassic marine organic‐rich mudstones from the eastern part of Svalbard, the high‐Arctic archipelago in the north‐western Barents Sea. The aim is to apply elemental chemostratigraphic variations as proxies to understand how palaeoproductivity and palaeoredox conditions affected the petroleum source rock formation in the region. Results from this study indicate that palaeooceanographic reconfiguration and onset of extensive pro‐deltaic sedimentation at roughly the Anisian–Spathian and the Ladinian–Carnian boundaries, respectively, largely influenced the observed palaeoproductivity, benthic palaeoredox conditions and sedimentation rate in the region, ultimately resulting in variable Lower–Upper Triassic source rock properties. [ABSTRACT FROM AUTHOR]

Abstract: The use of molybdenum isotope data (δ98Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H2S]aq >11μM) water column to the sediment, thus preserving the seawater δ98Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ98Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ98Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ98Mo values (+1.1 to +1.8‰). These results contrast with the pattern of redox — δ98Mo covariation in modern marine environments, in which the heaviest δ98Mo values are found in the most intensely euxinic facies. We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater–freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ98Mo by water-column redox cycling of Mn, with particulate Mn-oxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ98Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ98Mo trends within the Hushpuckney Shale, seawater δ98Mo must have varied by ~1.2‰ at a ~100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was <100kyr. Although both the second and third hypotheses are viable based on the present limited δ98Mo dataset, we discuss how each model might be tested through additional Mo isotope data. [Copyright &y& Elsevier]

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The Lower Member of the Longmaxi Formation is generally dominated by siliceous shale, but recently we found some siltstone-mudstone rhythm sections developed in the Lower Member of the Longmaxi Formation. The study of formation mechanism of siltstone-mudstone rhythmic sedimentary sections may provide new insights into the shale sedimentary environment. Therefore, we studied the characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Member of the Longmaxi Formation in the Changning area based on core observation, thin section identification, major elements and trace elements analysis. The results show the following: 1) Two siltstone-mudstone rhythmic sedimentary sections are characterized by frequent interbed between black or gray-black shale and light gray siltstone, abundant argillaceous laminas and silty laminas, with obvious lithological boundaries having developed. Horizontal laminas and rhythmic laminas are well-developed in the shale layer, while the wavy laminas are well-developed in the siltstone layer. 2) The major compositional elements are SiO2, Al2O3 and CaO, followed by Fe2O3, MgO, K2O and Na2O. 3) Compared with the world average shale, these siltstone-mudstone rhythmic sedimentary sections are rich in Mo, U and Ba, but less in V, Co, Ni, Cu. Compared with the shale layer, the siltstone layer has lower contents of V, Co and Ni. 4). The geochemical redox indices, Mo-U and CIA values suggest the formation of the siltstone-mudstone rhythmic sedimentary sections are related to influences from bottom currents in an oxic condition with a warm and humid paleoclimate. [ABSTRACT FROM AUTHOR]

Marine red beds are usually interpreted as indicating water column oligotrophy, good bottom‐water oxygenation and redox conditions. Lower Devonian successions of the Prague Basin, Czechia, exhibit a distinct centimetre to metre‐scale alternation of layers of marine red beds, grey carbonates, marls and black shales. In order to understand why the redox potential fluctuated so rapidly, reflectance spectroscopy, microscopy, elemental geochemistry data and stable isotopes of Mo have been analysed in this paper. Whilst the grey and black facies only contain goethite, the marine red beds are enriched with synsedimentary and early diagenetic, submicronic hematite, which is present in micrite, skeletal interiors, microstromatolites and oncoids. It was formed by microbially mediated precipitation, the replacement of detrital Fe phyllosilicates, and/or by the oxidation of microbially precipitated Fe‐bearing aluminosilicate precursors. The marine red beds are frequently enriched in Fe, depleted in U, V, Mo and Cu, and show negative δ98Mo values indicating oxic conditions. Peloidal micrite, microbial coatings and cements with the marine red beds exhibit positive (up to 9) Ce/Ce* anomalies. The non‐red facies show opposite patterns. This geochemical variability is probably related to Mn oxyhydroxide cycling and organic matter remineralization along the sediment subsurface redox gradient, particularly by reactions between pore water and various elemental pools. These patterns, combined with the centimetre‐scale colour alternation of the sediments, may reflect redox zonation that has been preserved beneath the ancient seafloor. Four zones are recognized: (i) the oxic zone of Fe‐oxide precipitation (marine red beds); suboxic zones of (ii) Fe enrichment, and (iii) U‐Mo enrichment; and (iv) suboxic–anoxic zone of Cu, V (± Mo) enrichment. The presented model of a marine red bed origin from redox reactions in the sediment subsurface contradicts models of the formation of marine red beds through iron enrichment from Fe2+ supersaturated ocean waters following periods of ocean anoxia. [ABSTRACT FROM AUTHOR]