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The North American Midcontinent Sea (NAMS) covered a vast area during Late Pennsylvanian glacio-eustatic highstands, providing a laboratory for analysis of the internal watermass dynamics of large cratonic seas (of which few exist in the Recent). In this study, a novel proxy, copper (Cu) isotopes, was used to gain a better understanding of NAMS watermass dynamics. We analyzed the 63-cm-thick Stark Shale (Dennis Formation, Missourian Stage, Upper Pennsylvanian) in the Iowa Riverton core (IRC) at a centimeter scale in order to reconstruct secular variations in Cu-isotopic compositions (δ65Cu) and other geochemical proxies. The gray shale facies yielded δ65Cu of +0.02 ± 0.06‰ (2σ, n = 14), similar to detrital Cu in modern marine sediments (δ65Cu = +0.08 ± 0.20‰; 2σ, n = 42). In contrast, the black shale facies, in which the proportion of authigenic Cu hosted by the organic fraction is 50–100% of total Cu content, exhibits heavier δ65Cuauth values, mostly between +0.09 and +0.43‰. In modern marine systems, one of the main processes leading to 65Cu-enriched sediment compositions is adsorption of aqueous Cu onto Fe-Mn particulates. The black shale facies of the Stark Shale exhibits four peaks in Mo/U ratios that are indicative of an active particulate shuttle because of the tendency of Mo to adsorb more strongly than U to Fe-Mn particulates. The Sr/Ba ratio, a paleosalinity proxy, shows correlated variations, with high values (indicative of more fully marine conditions) linked to low Mo/U and light δ65Cu, and low values (indicative of brackish conditions) linked to high Mo/U and heavy δ65Cu. These considerations suggest that the flux of isotopically heavy Cu to the sediment–water interface via Fe-Mn cycling was enhanced during brackish intervals, with subsequent reductive dissolution of Fe-Mn particulates allowing transfer of hydrogenous Cu to the organic fraction of the sediment. Control of δ65Cu by Fe-Mn cycling is supported by the results of a sequential extraction experiment showing that organic δ65Cu is positively correlated with Mo/U. These findings provide evidence of large fluctuations in watermass salinity and sea-level elevation within the NAMS at timescales of ∼104 yr during core shale deposition, as previously inferred but not conclusively demonstrated until now. The dynamics of particulate shuttle operation suggest that sea-level fluctuations must have had amplitudes of at least a few tens of meters. These fluctuations ended during the final stages of black shale deposition, probably owing to overall sea-level fall and permanent shallowing of the study site into the oxic, low-salinity surface layer of the NAMS.

International audience; The Vaca Muerta Formation is a famous source rock being largely present in Argentina. The formation was examined in the southern part of the Neuquén Basin through clay-mineral assemblage composition, as well as organic and inorganic geochemical characteristics. In total, 259 samples were analyzed on both sides of the Huincul Arch (major E-W structure that divided the southern part of the basin into two depocentres). South to this structure (Picún Leufú area), the total organic carbon content (TOC) can reach 20% but is generally ranging from 0.1 to 6%. To the north (Covunco area), average TOC does not exceed 0.5%. Clay-mineral assemblages indicate an evolution of the sources of clastic supply through time in response to evolving weathering/erosion at the regional scale. Redox-sensitive trace-element distribution shows that the particulate iron shuttle process operated, with an initially-euxinic depositional environment that evolved rapidly to oxygenated seawater and suboxic sediments. Our results suggest an episodically restricted circulation pattern that occurred at the beginning of the deposition of the Vaca Muerta Formation (highest TOC content).

International audience; 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.

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.

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 Fjacka 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 Fjacka 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.

Muds deposited in large-scale epicontinental seaways provide deep insights into palaeoclimates, biogeochemical cycles, sedimentation processes, and organic carbon burial during exceptionally warm periods throughout the Phanerozoic. Temporal changes can be monitored at single locations but the key, larger-scale oceanographical and related biogeochemical processes are likely to be more clearly revealed by comparisons between individual sub-basins within seaways. Here, we compare inorganic geochemical records from the Jurassic (upper Pectinatites wheatleyensis to lower Pectinatites pectinatus ammonite zones) of the Swanworth Quarry 1 Core from the Wessex Basin (Dorset, UK) to time-equivalent records from the Ebberston 87 Core in the Cleveland Basin (Yorkshire, UK), 400 km apart. Our synthesis shows that while the Dorset sediments were deposited in an energetically more dynamic setting than the Yorkshire sediments, the overarching climatic and oceanographical processes responsible for variations in organic carbon enrichment and sedimentation, were similar. Intervals of coeval organic carbon-rich sedimentation occurred in both basins, and a particulate shuttle was intermittently active in both basins. Consistent with recent climate simulations, we conclude that tropical climate conditions, associated with enhanced nutrient supply, were key drivers of sedimentation between the Jurassic Wessex and Cleveland Basins.

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 FeT/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.

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.

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.

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 ( 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.

The Lower Jurassic Agua de Madeiros (AM Fm) and Vale das Fontes (VF Fm) Formations, and the Upper Jurassic Cabacos Formation (Cab Fm) are considered the most important units with petroleum source-rock potential in the Lusitanian Basin, and the likely sources for its widespread hydrocarbon occurrences. Nonetheless, the bottom-water redox conditions during their deposition remain debatable. To resolve this controversy, we investigated whole-rock inorganic geochemical data of the most significant organic-rich levels of the AM, VF and Cab Fms cropping out at basin scale. Significant variations in TOC, TS and redox-sensitive elements contents and their ratios are evident within and between the organic-rich levels of the studied units. A positive covariation between primary productivity proxies, TOC enrichment and paleo-redox proxies for the Lower Jurassic units clearly suggest an interdependence between enhanced primary productivity, organic matter accumulation and bottom-water redox conditions. For the AM Fm, the paleo-redox proxies indicate that the black shales deposition took place beneath bottom-waters that ranged from suboxic to strongly euxinic conditions, with a clear predominance of the latter. A TOC threshold of ~6–7 wt% clearly separates samples deposited under suboxic to anoxic conditions from those deposited under true euxinic conditions, and is interpreted as the minimum amount of organic matter (OM) accumulation within sediments required to sustain H2S production at rates that outpaced its consumption through precipitation of sulphides and/or OM sulphurization, allowing the diffusion of H2S to the bottom-waters. For the Vale das Fontes Fm, the paleo-redox proxies suggest a prevalence of suboxic to anoxic bottom-waters, despite all black shales displaying TOC contents greater than the above threshold. The less reducing conditions relative to the AM Fm euxinic black shales are attributed to: i) a larger proportion of continental OM; ii) lower reactivity of the OM reaching the sediment-water interface (decreasing oxygen consumption); and iii) higher availability of reactive iron (increasing the buffer effect to H2S produced through OM remineralization by sulphate-reducing bacteria). Both units display Mo-EF vs U-EF covariation patterns that deviate from the field of sediments deposited under unrestricted open-marine conditions, towards higher Mo enrichment relative to that of U, indicating an accelerated transport of aqueous Mo to the sediments, consistent with the operation of a metal-oxyhydroxide particulate shuttle linked to Mn and Fe redox cycling within the water column in a weakly-restricted basin. The Mo-COT regression-line slope indicates a moderate degree of bottom-water mass restriction, with [Mo]aq concentration of ~60–70% of the present-day seawater value and bottom-water renewal times of ~10–20 years, akin to the present-day Cariaco Basin. The redox conditions of the Cab Fm organic-rich levels are dependent on the location of the depositional environment in the basin, with deposition taking place beneath oxygenated bottom-waters at Cabo Mondego, and under suboxic conditions at Vale de Ventos and Pedrogao, the latter denoting a larger degree of oxygen depletion than the former. In this context, the overall potential for OM preservation should increase from Cabo Mondego to Vale de Ventos and Pedrogao.

Redox state and salinity are fundamental properties of watermasses, and in modern environments, detailed analysis of spatial variability in redox and salinity is possible through direct measurement. Watermass reconstruction is difficult in deep-time systems, however, because the sedimentary record of ancient watermasses is often incomplete or difficult to access on spatial scales large enough to permit basin-scale reconstruction. During the Middle Devonian to Early Mississippian periods, the Appalachian Basin of eastern North America was characterized by a variably restricted watermass, leading to extensive deposition of organic-rich black shale associated with the regional development of bottom-water anoxia. Watermass reconstruction during deposition of these units has previously been attempted, but only on small spatial scales, with most studies focused on trace element proxies applied to individual study sections. Here, we focus instead on a broader geographic reconstruction of watermasses in the Lower Mississippian (Tournaisian) Sunbury Shale and present a new geochemical dataset that spans five drill cores across a basin transect from northeastern Ohio to southern Kentucky (USA). Iron speciation and trace metal abundances reveal a strong paleoredox gradient that can be related to water depth and basin hydrography. In the northeastern-most core of Ohio, iron speciation reveals largely ambiguous redox conditions; however, oxic conditions are potentially indicated by very low total organic carbon and trace metal contents. These oxygenated conditions were likely related to proximity to the Catskill deltaic wedge complex that fed freshwater into the basin and lowered the position of the regional pycnocline. In contrast, in central and southern Ohio, paleoredox indicators reveal the development of ferruginous conditions associated with rapid deepening to the south into the central axis of the Appalachian Basin. Further to the south in Kentucky, our data reveal dominantly euxinic conditions, which developed as environments shoaled towards the basin-bounding Cumberland Sill. We also investigated paleosalinity across the Appalachian Basin using strontium/barium (Sr/Ba) ratios. Our data reveal a subtle paleosalinity gradient from low-brackish (i.e., close to freshwater) conditions close to the Catskill Delta complex to increasingly brackish conditions to the southwest. This trend was disrupted by large paleosalinity variation across a shallow mixing zone in the southwestern-most core. Lastly, strong enrichments of redox-sensitive trace metals in both ferruginous and euxinic environments are parsimoniously attributed to the operation of an iron-manganese particulate shuttle that continuously supplied oxide-sensitive metals to the Appalachian Basin. Ultimately, this study is among the first to present detailed watermass reconstruction in an ancient epeiric sea using both redox and salinity proxies applied over a large lateral transect. We suggest that a similar approach may be useful in other deep-time systems of geobiological, paleoclimatic, and paleoceanographic significance.

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 (δ98Mo = −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 (δ98Mo = +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 O2/H2S chemocline from the water column to the sediment. Large δ98Mo fluctuations in the LM may record variations of H2S concentrations in the mid-depth euxinic waters. The intermediate and relatively uniform δ98Mo 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 δ98Mo 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 H2S concentrations in the euxinic watermass. By compiling published data, we found that the large sedimentary δ98Mo 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.

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.

Organic- and sulfide-rich sediments have formed in oxygen-depleted environments throughout Earth&rsquo, 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&mdash, at least partially owing to a depositional process called the &ldquo, particulate shuttle&rdquo, &mdash, 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.

The Kimmeridge Clay Formation (KCF) is a laterally extensive, total organic carbon-rich succession deposited throughout Northwest Europe during the Kimmeridgian–Tithonian (Late Jurassic). Here we present a petrographic and geochemical dataset for a 40 metre-thick section of a well-preserved drill core recovering thermally-immature deposits of the KCF in the Cleveland Basin (Yorkshire, UK), covering an interval of approximately 800 kyr. The new data are discussed in the context of depositional processes, sediment source and supply, transport and dispersal mechanisms, water column redox conditions, and basin restriction. Armstrong et al. (2016) recently postulated that an expanded Hadley Cell, with an intensified but alternating hydrological cycle, heavily influenced sedimentation and total organic carbon (TOC) enrichment, through promoting the 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 distal part of the Laurasian Seaway. Depositional conditions alternated between three states that produced a distinct cyclicity in the lithological and geochemical records: lower variability mudstone intervals (LVMIs) which comprise of clay-rich mudstone, 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 (here termed higher variability mudstone intervals, HVMIs). During the lower variability mudstone intervals, conditions were quiescent with oxic to sub-oxic 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, anoxic-euxinic conditions led to an enrichment of redox sensitive/sulphide forming trace metals at the seafloor and a preservation of organic matter, and an active Mn-Fe particulate shuttle delivered redox sensitive/sulphide forming trace metals to the seafloor. In addition, based on TOC–S–Fe relationships, organic matter sulphurisation 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 sulphur 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 the distal part of a shallow epicontinental sea, and is therefore key to understanding the formation of similar deposits throughout Earth's history.

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.

International audience; The distribution of, Co, Mo, Zn, Ni, Cd, U, Cu, Mn, V, Al, Fe, Ti, Total Organic Carbon (TOC) and Total Sulphur (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 cruses 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 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 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.