Concentrations Of Redox-sensitive Trace Elements Research Articles

Constraining reducing conditions in the Prague Basin during the late Silurian Lau/Kozlowskii extinction event

The Silurian was marked by repeated extinctions, carbon cycle volatility and significant intervals of climatic change. The most notable of these events were the Ludfordian Lau/Kozlowskii extinction and the associated Mid-Ludfordian Lau C isotope excursion, both of which have been linked to a period of global cooling and expanded reducing conditions in the global ocean. We present new data that characterize the marine palaeoredox conditions of the Prague Basin, a peri-Gondwanan terrane. We use I/Ca ratios to assess the local redox conditions in a shallow water carbonate succession and Fe speciation and redox-sensitive trace element concentrations to assess the local redox conditions of a deeper water sequence. Consistently low I/Ca values in the shallow water section suggest either persistent local low-oxygen conditions or possibly diagenetic overprinting. Fe speciation data suggest that the bottom water redox conditions in the deeper shelf setting were consistently anoxic with possible intermittent euxinia. Concentrations of redox-sensitive trace elements consistently higher than upper continental crust values also indicate persistent reducing conditions in the deeper part of the basin. These local redox proxy data from the Prague Basin, including trends in new pyrite S isotope ( δ 34 S pyr ) data, are consistent with previous findings of the expansion of anoxic and/or euxinic oceanic conditions. These data, derived from a mid-palaeolatitude marine setting, fill an important gap in our current global dataset for this interval of the late Silurian. Supplementary material: All the geochemical data reported and discussed herein are available in the Supplemental Data Tables at https://doi.org/10.6084/m9.figshare.c.7008107 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Trace element, rare earth element and trace carbon compounds in Subglacial Lake Whillans, West Antarctica

Whillans Subglacial Lake (SLW) lies beneath 801 m of ice in the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and active subglacial drainage network. Here, the geochemical characterization of SLW rare earth elements (REE), trace elements (TE), free amino acids (FAA), and phenolic compounds (PC) measured in lakewater and sediment porewater are reported. The results show, on average, higher values of REEs in the lakewater than in the porewater, and clear changes in all REE concentrations and select redox sensitive trace element concentrations in porewaters at a depth of ~15 cm in the 38 cm lake sediment core. This is consistent with prior results on the lake sediment redox conditions based on gas chemistry and microbiological data. Low concentrations of vanillyl phenols were measured in the SLW water column with higher concentrations in porewater samples and their concentration profiles in the sediments may also reflect changing redox conditions in the sediments. Vanillin concentrations increased with depth in the sediments as oxygenation decreases, while the concentrations of vanillic acid, the more oxidized component, were higher in the more oxygenated surface sediments. Collectively these results indicate redox changes occurring with the upper 38 cm of sediment in SLW and provide support for the existence of a seawater source, already hypothesized, in the sediments below the lowest measured depth, and of a complex and dynamic geochemical system beneath the West Antarctic Ice Sheet.Our results are the first to detail geochemical properties from an Antarctic subglacial environment using direct sampling technology. Due to their isolation from the wider environment, subglacial lakes represent one of our planets last pristine environments that provide habitats for microbial life and natural biogeochemical cycles but also impact the basal hydrology and can cause ice flow variations.

Spatial and temporal variations in the isotope hydrobiogeochemistry of a managed river draining towards the southern Baltic Sea

The flow path of a river draining a lowland into the southern Baltic Sea, the Warnow River, was investigated to evaluate its freshwater composition and potential to be a source of dissolved substances for regional coastal waters. A spatial study was conducted in April 2019 to follow the variations from the source to the estuary. A temporal study of the composition, for five years (2017–2021), was carried out at a site just before the river reaches the estuary. Surface water was sampled to analyse major and trace elements, stable (H, C, O, S), and short-lived (Ra) isotopes. The results show that the composition of the Warnow River along the flow path is controlled by a complex interplay between in-situ processes, exchange with the atmosphere, diffuse groundwater, and surface water inlets. On a temporal scale, pH, nutrient, and redox-sensitive trace element concentrations are strongly impacted by pelagic primary production in spring. During summer and autumn, influences occurred due to benthic microbial activity, associated diffusive release from soils/sediments, and surface water inlets. Throughout the investigation period, the Warnow River was a source of isotopically light CO2 to the atmosphere and DIC to the estuarine waters. The delivered DIC concentrations seemed to vary with season due to changes in biological pelagic and benthic activities. DOC was derived from a mixture of dominated C3 organic sources and potentially from fertilizers. From concentration-discharge relationships, examples of dilution, mobilization, and chemostasis trends were found. Discharge-controlled seasonal trends were superimposed by system internal processes and the hydrological consequences of the river management. Our analysis thus provides new insights into the controls on the variations of water and solutes in a managed river at the land-sea interface as part of the regional hydrological cycle of a lowland catchment-coastal water system.

Changes in palaeoclimate and palaeoenvironment in the Upper Yangtze area (South China) during the Ordovician–Silurian transition

The Ordovician–Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician–early Silurian interval, we conducted mineralogical, geochemical, and isotopic analyses of three formations (Wufeng, Guanyinqiao and Longmaxi formations) in the Xindi No. 2 well. The ternary and bivariate diagrams indicate that the provenance is mainly felsic igneous rocks and originated mainly from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values in the lower Wufeng and Longmaxi formations are relatively high (67.48–73.57), indicating a warm and humid climate. In contrast, the CIA values declined rapidly (58.30–64.66) during the late Katian to early Hirnantian, which had a fluctuating cold and dry climate and was interrupted by a transient warm and humid climate. The palaeoredox indices (Mo concentrations and Moauth/Uauth, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician–early Silurian indicate two cycles of water column euxinia. The first cycle occurred during Wufeng Formation deposition, with bottom waters evolving from oxic-suboxic to suboxic-anoxic. Most samples show relatively low redox-sensitive trace element concentrations during the Guanyinqiao Formation, pointing to oxic-suboxic conditions. The second cycle, during the late Hirnantian, transitioned from oxic to euxinic water conditions. Our δ13Corg data are comparable to previously reported records and exhibit a strong correlation between the Hirnantian isotopic carbon excursion (HICE), climate change, and redox conditions. We suggest that the variations in the δ13C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions, and (2) increased carbonate weathering exposed by the glacio-eustatic sea- level. In addition, the high δ13Corg values might indicate a more shelf-proximal setting during Xindi No. 2 well deposition. The δ13Corg isotopic data effectively constrain the timing of the Late Ordovician mass extinction (LOME) and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.

Redox conditions on the Anadarko Shelf of Oklahoma during the deposition of the “Mississippian Limestone”

A multiproxy approach, including stable nitrogen isotopes and redox sensitive trace elements, was employed in order to evaluate the impact of depositional redox conditions on organic matter accumulations within the Mississippian Limestone interval in the Anadarko Shelf of Oklahoma and also to investigate hydrocarbon source rock potential of the Mississippian Limestone in the study area. Nitrogen isotopes (δ 15 N), total organic carbon (TOC), and concentrations of redox-sensitive trace elements (Cu, Mn, Mo, Ni, U, V) were measured on samples from a core drilled in Canadian County, Oklahoma. Values of TOC are between 0 and 2.5 wt. % and average 0.8 wt. % in the studied interval of the core. The δ 15 N data (3‰–14‰, average 8 ± 1‰) indicate sediment deposition under oxic to suboxic water conditions. This interpretation of depositional redox conditions is supported by elemental redox proxies which show no enrichment relative to average carbonate. Our geochemical data suggest that the Mississippian Limestone is unlikely to be an oil source rock due to pervasively oxic to suboxic depositional redox conditions which would have resulted in oxidation of organic matter. • Water column and benthic redox conditions at time of deposition were oxic-suboxic. • Depositional redox does not influence organic matter accumulation in study site. • Mississippian Limestone sediments have poor oil source rock potential due to oxygenated conditions during deposition.

Nitrogen and carbon cycle perturbations through the Cenomanian-Turonian oceanic anoxic event 2 (~94 Ma) in the Vocontian Basin (SE France)

Oceanic Anoxic Event 2, spanning the Cenomanian-Turonian boundary, was an episode of major perturbations in global biogeochemical cycles, characterized by the widespread burial of organic carbon in both oceans and epicontinental basins. In the Vocontian Basin (SE France), this event was expressed by the deposition of the Niveau Thomel, a succession of interbedded dark shales, grey marls and bioturbated carbonates as observed in the Lambruisse section. Analyses of the total organic carbon (TOC) content, combined with the concentrations of redox-sensitive trace elements, revealed that conditions fluctuated from oxic to suboxic-anoxic during the deposition of the Niveau Thomel. Transient periods of deoxygenation were related to perturbations in nutrient cycling reflected by near-zero to negative δ15N signatures indicative of increased diazotrophy.The development of deoxygenated conditions in the Vocontian Basin was likely preconditioned by hydrographically restricted conditions, as confirmed by Mo vs. U enrichments, reducing bottom-water ventilation associated with water-column stratification. Intervals of reoxygenation were likely linked to climate-induced changes. In particular, a phase of regional or global cooling associated with a drop in pCO2, called the Plenus Cold Event (PCE) resulted in vigorous reoxygenation of the Vocontian Basin.

Multi-proxy analysis of organic matter accumulation in the Upper Ordovician–Lower Silurian black shale on the Upper Yangtze Platform, south China

Abstract There is no doubt that organic matter plays an important role in shale gas accumulation and storage. Thus, successful evaluation and production strategies of organic-rich shale deposits require an understanding of the range of factors that contribute to the accumulation and preservation of organic matter in these deposits. This paper reports results of a multi-faceted study of two Ordovician–Silurian transition sections of the Upper Yangtze Platform of South China. Organic and inorganic geochemical data are used to reconstruct paleoenvironmental changes across this critical interval of Earth history and to identify those factors that were most important to organic matter accumulation. Deposition of organic-rich shale of the Katian lower Wufeng Formation was favored by rising sea level and diminished clastic sediment content. Black shale of the upper Wufeng preceded Hirnantian glaciation whereas the overlying lower Longmaxi Formation accumulated in association with a post-glacial rise of sea level as well as the peak of Caledonian tectonism. The carbonaceous deposits appear to reflect the combined effects of elevated primary productivity as suggested by Si/Al and Ti/Al ratios, especially in the upper Wufeng, and consequent depletion of water column oxygen (preservation). Consideration of the relationships of redox-sensitive trace element concentrations and total organic carbon content suggest that bottom water conditions fluctuated between anoxic and euxinic during this period, perhaps related to rapid oscillations of sea level. Moreover, the basin appears to have remained connected with the global ocean in spite of Caledonian uplift of bordering areas. Thus, black shale sedimentation on the Upper Yangtze Platform at the Ordovician-Silurian boundary reflects the interplay of climate change and tectonism and their influences on sea level, clastic sediment content, and paleo-productivity.

Chemostratigraphic resolution of widespread reducing conditions in the southwest Pacific Ocean during the Late Paleocene

Paleogene strata throughout much of the southwest Pacific is comprised of thick successions of fine-grained siliceous to calcareous sediments. However, during the Late Paleocene the south Pacific experienced widespread deposition of organic-rich and δ13Corg-enriched sediments that are recorded in the Waipawa Formation and the correlative Waipawa Organofacies. The mechanisms responsible for the elevated input and preservation of organic matter are still poorly understood. Using a multi-proxy chemostratigraphic approach we assess paleoceanographic conditions, including paleo-redox states and paleo-productivity of the Waipawa Formation. Eight key sections from the East Coast Basin, New Zealand were sampled and analysed for multi-element geochemistry and bulk pyrolysis. Sedimentological, stratigraphic and chemostratigraphic evidence indicates the south Pacific underwent an abrupt regression event during the Late Paleocene resulting in on-shelf erosion and/or bypassing of continental sourced sediments from the shelf into deeper waters. The change in sediment delivery further increased nutrients and terrestrial organic matter loading to the site of deposition. Coeval excursions in Ti/Zr and Rb/Sr indicate a shift to an aluminosilicate-dominated sediment supply, resulting in a modest increase in marine productivity that cannot account for the anomalous organic matter enrichment. Changes in the hydrogen index show a transition from terrestrial to mixed terrestrial–marine organic matter occurred during the Late Paleocene, in which high concentration of Type II-III kerogens were buried and preserved in association with enhanced water column reducing conditions in deep marine settings. Elevated concentrations of redox-sensitive trace elements (e.g. U, Mo, Fe, S) are consistent with alternating phases of dysoxic to oxic conditions suggesting an expansion of an oxygen minimum zone, enhancing preservation and delivery of organic matter to the seafloor, leading to the subsequent burial and preservation of organic matter within anoxic sediments. These new data provide insight into the depositional conditions and the petroleum potential of prospective source rocks in this basin using an integrated organic and inorganic geochemical approach to refine our knowledge of the depositional environment and paleoceanographic changes in shelfal to bathyal marine sediments of the Late Paleocene.

Highly heterogeneous “poikiloredox” conditions in the early Ediacaran Yangtze Sea

Although there has been a sustained effort to reconstruct ocean-redox conditions throughout the Ediacaran, a lack of high-resolution data across time-equivalent strata (shelf to basin) has hindered our ability to understand redox dynamics at the basinal scale. We measured iron species, trace- and major-element concentrations, TOC, and Corg/P ratios of black shale from six high-resolution sections from the early Ediacaran Yangtze Sea (i.e., the inner-shelf Jiulongwan, outer-shelf Zhongling, slope Siduping and Mingle, and basinal Yuanjia and Long’e sections), which were integrated with published data from these sections and seven other correlative sections from the same region for a basinwide perspective on redox tracers. These results document strong spatiotemporal redox variability during the early Ediacaran, including transitions between ferruginous, euxinic, suboxic, and oxic conditions at high frequencies and short distances (a pattern for which we coin the term “poikiloredox conditions”). Our results also suggest that multiple geochemical proxies including iron species, redox sensitive trace element concentrations and enrichments, and Corg/P ratios show different responses to poikiloredox conditions, accounting for conflicting redox interpretations in early Ediacaran studies. This study thus demonstrates the importance of multiple proxies, high stratigraphic resolution, and a basin-scale perspective in paleomarine redox studies, underlining the potential for overgeneralization of redox patterns based on analysis of a single or a limited number of proxies or sections.

Xrf-based Chemostratigraphy Between and Across Two Disconformities in the Ordovician Trenton Group and Utica Shale of Central New York, U.s.a.

Abstract Thick mudstone successions contain a wealth of elemental proxy information than can be readily accessed by hand-held X-ray fluorescence (XRF) spectroscopy. Mudstones of the Upper Ordovician Trenton Group and Utica Shale record drowning of a carbonate-shelf in a peripheral foreland basin during the Taconic Orogeny. Two discrete episodes of shelf erosion, the Thruway and Honey Hill disconformities, followed by deepening occurred during this period. XRF-based bulk-rock elemental data, coupled with detailed lithologic descriptions and XRD analysis, highlight previously unidentified changes in the composition of siliciclastic detritus across these disconformities. This likely relates to variation in the relative contribution of sediment derived from the cratonic interior, eroded carbonate-shelf, and rising orogenic wedge, as well as possible exhumation of the Adirondack Massif. The data also indicate that mudstones of the lower Indian Castle Shale, the interval of the Utica Shale deposited between the Thruway and Honey Hill disconformities, are more carbonate-rich than the underlying mudstones of the Trenton Group. This is surprising given the large volume of limestone in the Trenton Group, and attests to the considerable amount of intrabasinal carbonate winnowed from the shelf during formation of the Thruway Disconformity. Drowning, following erosion along the Honey Hill Disconformity, diminished this sediment source and led to a significant reduction in the amount of carbonate mudstone present in the upper Indian Castle Shale. The concentrations of redox-sensitive trace elements normalized to Al increase sharply across the Thruway Disconformity, suggesting increased bottom-water anoxia and organic-matter preservation in a deepening basin. Increases in the S and Fe concentrations above the Honey Hill Disconformity suggest a shift to even more strongly reducing conditions. Results of this study are consistent with previous interpretations, indicating that higher-resolution XRF-based elemental concentration data may reveal more cryptic changes in sediment provenance and bottom-water redox conditions in the Taconic foreland basin, and in other thick mudstone successions.

Geochemical and faunal proxies in the Westphalian A (Langsettian) marine horizon of the Lublin Coal Basin

The uppermost Westphalian marine horizon ( Dunbarella horizon) is especially important for correlation of Carboniferous deposits in the Lublin Coal Basin and in other basins of the Northwest European Carboniferous Basin. The Dunbarella horizon is characterized by cyclic sedimentation and consequent faunal spectrum variability, typical for Westphalian marine horizons of northwestern Europe. Palaeontological study of the Dunbarella horizon showed the presence of macrofauna representing different palaeoenvironments, from marine to brackish (non-marine) and freshwater conditions. The vertical sea level fluctuations and changes in seawater salinity resulting in palaeontological record changes do not link with geochemical proxies. TOC, redox-sensitive trace element concentrations, and V/Cr, Ni/Co and V/(V + Ni) ratios generally suggest that the Dunbarella horizon sediments were deposited under predominantly oxic conditions (with local exceptions during the initial phase of the Dunbarella ingression; Kopina 1 borehole).

Organic petrology and micro-spectroscopy of Tasmanites microfossils: Applications to kerogen transformations in the early oil window

The transformation of kerogen to hydrocarbons in the early stages of oil generation is critical for understanding the resource potential of liquid-rich shale plays. Organic petrology commonly is used for visual evaluation of type, quality, and thermal maturity of organic matter, but the relationship of visual petrographic changes to chemical transformations is not well characterized. To improve understanding of these processes, organic-walled microfossils of the unicellular green alga Tasmanites (composed of algaenan) in Upper Devonian Ohio Shale (Huron Member, Appalachian Basin) were analyzed by micro-spectroscopy techniques including micro-Fourier transform infrared (micro-FTIR), X-ray photoelectron (XPS), electron probe microanalysis (EPMA), and fluorescence. Immature to mid-oil window maturation sequences of core and outcrop samples with solid bitumen reflectance (BR) and vitrinite reflectance (VR) values ranging from 0.45 to 0.80%Ro were used. Hydrous pyrolysis was applied to low-maturity (BR: 0.25–0.39%Ro) Huron and time-correlative New Albany shale samples to create similar artificial maturation sequences for comparison. Micro-FTIR spectroscopy revealed a decrease in the CH2/CH3 ratio with increasing maturity, indicating Tasmanites aliphatic chains become shorter and more branched. Oxygenated functional groups decreased relative to aliphatic stretching bands and increased aromaticity was noted at the highest maturities. In samples that were pyrolyzed for 72h at temperatures of 300–320°C (BR: 0.56–0.68%Ro), Tasmanites showed similar trends, whereas at pyrolysis temperatures of 340°C and higher (BR>1.0%Ro), Tasmanites was pseudomorphed by accumulations of solid bitumen, carbonate and sulfide. Replacement of Tasmanites by these phases in hydrous pyrolysis experiments ≥340°C and its absence at higher maturities (peak oil, VR and BR≥0.9%Ro) in naturally matured samples, as documented in a previous study, implies that a large fraction of the algaenan component of original organic carbon is converted to petroleum during thermal maturation. XPS analysis indicated the molar proportion of aliphatic carbon increases with increasing thermal maturity, accompanied by decreases in oxygenated functional groups and olefinic carbon. EPMA of Tasmanites showed highest concentrations of S, with concentrations of redox-sensitive trace elements U, Mo, Ni and V generally at or below detection limits. Decrease in organic S with increasing thermal maturity may be related to cleavage of Tasmanites at CS linkages; however, this relationship was inconsistent and presence of adjacent or entrained nanoscale silicate or sulfide phases may impact measured trace element concentrations. Fluorescence microscopy and spectroscopy showed a red shift in spectral maxima and decreased emission intensities with increasing maturity, interpreted as due to non-radiative energy loss possibly because of increased aromaticity. Collectively, these results provide new insights into the in situ chemical transformations that accompany petrographic changes as oil-prone kerogen converts to petroleum with thermal advance from immature conditions into the mid-oil window.

Transient deep-water oxygenation in the early Cambrian Nanhua Basin, South China

Many late Neoproterozoic to early Cambrian fossils of multicellular eukaryotes, including those of benthic animals, are found preserved under anoxic and even euxinic bottom-water conditions, which is contradictory to the consensus that oxygen is essential to eukaryotes. To investigate this conundrum, we conducted an integrated study of iron speciation, redox-sensitive trace elements, and Mo isotopes (δ98Mo) on the black shale interval of the lower Cambrian Hetang Formation (∼535–521Ma) at Lantian, South China, in which benthic sponge fossils are abundant in the lower member (LM) but absent in the upper member (UM). Iron speciation data point to uniformly anoxic-ferruginous conditions in the LM and euxinic conditions in the UM, whereas the trace-element and δ98Mo data show greater secular variation in redox conditions. The LM shows higher mean trace element concentrations (Mo: 108ppm, U: 36ppm, V: 791ppm) and lower and more variable δ98Mo (+0.13 to +1.76‰) relative to the UM (Mo: 45ppm, U: 18ppm, V: 265ppm, δ98Mo: +1.59 to +1.67‰), and ratios of redox-sensitive trace element concentrations to total organic carbon are significantly more variable and higher on average in the LM relative to the UM. The appearance of sponge fossils and lower δ98Mo values correlate strongly with gray (i.e., lighter-colored) layers in the LM. These patterns can best be interpreted as recording mainly euxinic conditions throughout deposition of the study units, with more intense background euxinia in the LM relative to the UM, but also with frequent transient oxygenation events in the LM that do not appear in the UM. The transient oxygenation events of the LM led to the initial colonization of the deep Nanhua Basin by sponges, and the termination of these events in the UM caused sponge faunas to disappear until a general rise in O2 levels later in the Cambrian permitted their return to deeper-water habitats. Our study also illustrates that multiple geochemical and paleobiological proxies exhibit different responses in ‘poikiloredox’ environments (i.e., characterized by small-scale spatial and high-frequency temporal variations), which can lead to apparent contradictions between metazoan fossil occurrences and their inferred watermass redox conditions.

Tracking the onset of Phanerozoic-style redox-sensitive trace metal enrichments: New results from basal Ediacaran post-glacial strata in NW Canada

A global rise in oxygen levels has been proposed to coincide with the Ediacaran to Cambrian radiation of animals, yet the precise timing and nature of this change remains unresolved. One hypothesis is that the ocean/atmosphere system became temporarily well-oxygenated in the earliest Ediacaran, directly following the Marinoan Snowball Earth glaciation (~635Ma). The evidence for oxygenation is based on large enrichments of redox-sensitive trace elements in black shale from South China presumably deposited under a euxinic water column. These enrichments meet or exceed the bulk concentrations of redox-sensitive trace elements found in Phanerozoic shale deposited under euxinic water columns. Here we test the early Ediacaran post-Snowball oxygenation hypothesis with new data from a high-resolution, multi-proxy geochemical and sedimentological study of three stratigraphic sections in the earliest Ediacaran Sheepbed Formation of the Mackenzie and Wernecke Mountains, NW Canada. Iron speciation data from all sections suggest that the local water column was dominantly ferruginous, with a notable exception of probable euxinic conditions recorded in part of one section. Redox-sensitive elements show no appreciable enrichments in the basal Sheepbed Formation, with maximum concentrations of molybdenum, vanadium, uranium and chromium only slightly above world average shale values. The lack of substantial elemental enrichments within the Sheepbed Formation is consistent with results from coeval ferruginous strata in Svalbard. The dominance of local ferruginous conditions in NW Canada cannot alone account for the muted redox-sensitive element enrichments as V, U, and Cr are thought to be enriched under these conditions, and samples deposited under euxinic conditions also lack Phanerozoic-style enrichments. These contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution.

Multiple sulfur isotopes (δ34S, Δ33S) and trace elements (Mo, U, V) reveal changing palaeoenvironments in the mid-Carboniferous Chokier Formation, Belgium

In this study, multiple sulfur isotopes are combined with widely-used redox-sensitive trace element concentrations (Mo, V, and U) in order to further validate the use of δ34S and Δ33S as a paleoredox proxy. The approach is applied to the mid-Carboniferous Chokier Formation, for which a detailed microfacies study identified a marked change in the environmental conditions during sediment deposition.Sediments from the upper part of the Chokier Formation exhibit negative δ34S (mean −24.6‰) and positive Δ33S (mean +0.040‰) values that are attributed to microbial sulfate reduction (MSR) and/or microbial sulfur disproportionation (MSD). Samples from the lower part of the Chokier Formation display less negative δ34S (mean −9.7‰) and negative Δ33S (mean −0.029‰) values. Paired δ34S-Δ33S values are located on a mixing curve between two endmembers: (1) sulfur generated by MSR and/or MSD, and (2) seawater sulfate. This latter observation is attributed to a restricted seawater exchange that limited the sulfate supply, thus, promoting closed-system conditions. Consequently, sulfur isotope values approached the isotopic composition of contemporaneous seawater.Trace element abundances exhibit a stronger enrichment in the upper Chokier Formation (EFMo=9–77, EFV=1–4; EF=Enrichment Factor) than in the lower Chokier Formation (EFMo=16–21, EFV ≈1). However, a hydrographic rather than a redox control is inferred for both sections based on Mo/TOC≤15 and Mo/U ratios well below seawater values. For the upper Chokier Formation, Mo/TOC≈15 indicates a moderately restricted basin, whereas the lower part is more strongly restricted (Mo/TOC≪15).Multiple sulfur isotopes and trace element abundances/ratios reveal a consistent picture of restricted basinal conditions during deposition of the lower Chokier Formation and more open conditions for the upper Chokier Formation. Hence, it can be concluded that results support the application of multiple sulfur isotope analyses for revealing paleoenvironmental conditions.

Preliminary nannofossil and geochemical data from Jurassic black shales from the Qiangtang Basin, northern Tibet

This paper presents new biostratigraphic and geochemical data from the Biluo Co section in northern Tibet, which exposes Jurassic black organic-rich shales, locally containing abundant coccoliths. Because of a general lack of macrofossils, the stratigraphic ages have been a matter of debate. However, coccoliths suggest an Early Bajocian through Bathonian to possibly Early Callovian age (Middle Jurassic) for the middle–upper part of the section. In this study, a range of trace-metal paleoredox proxies is used to assess how seawater oxygen levels varied both locally and globally during the deposition of these shales. The redox-sensitive elements V, Cr, U, Ni, Cu, Mo, Co, Cd and Zn exhibit relatively high concentrations and element/Al ratios. In particular, the Ni compositions fluctuate between ∼75ppm and ∼106ppm and Mo between ∼1ppm and ∼7ppm: values that are higher than those of the post-Archean Average Shale. Palaeoproductivity proxies, such as Zn, P and Cd, which can be fixed in elevated concentrations in sediments deposited under generally reducing conditions, are also relatively enriched. Furthermore, the U–Mo concentrations and Enrichment Factors (EFs) are consistent with deposition under predominantly suboxic to weakly anoxic conditions. Scattered bivalves, however, point to at least intermittent oxic conditions on the sea floor. Based on the redox-sensitive trace-element concentrations, together with ratios (V/(V+Ni), Ni/Co and V/Cr), the formation of the Biluo Co black shales, in Tibet was probably caused by increased productivity and organic-matter flux, leading to enhanced preservation of organic material under low-oxygen conditions.

Trace and rare earth elemental geochemistry of carbonate succession in the Middle Gaoyuzhuang Formation, Pingquan Section: Implications for Early Mesoproterozoic ocean redox conditions

The concentrations of redox-sensitive trace elements, such as uranium (U), vanadium (V), molybdenum (Mo), cobalt (Co), chromium (Cr) and rare earth elements (REE+Y) were determined in a given carbonate succession in the Gaoyuzhuang Formation (~1.56 Ga), which spans depths from outer shelf to intertidal, to explore the Early Mesoproterozoic ocean redox conditions. The values of the Zr-normalized redox-sensitive trace element concentrations and some relevant ratios show obvious changes from bottom to top in the succession. Samples from the outer shelf setting (M1 interval) demonstrate significantly enhanced values in Zr-normalized redox-sensitive trace element concentrations and relevant ratios (the peaks of Mo/U, V/Cr and Ni/Co ratios larger than 8, 4.25, and 7, respectively). Authigenic framboidal pyrites were also found within oncolite-like carbonate concretions and surrounding host rocks in this interval. These all indicate a euxinic state in the outer shelf environment. Less enrichment of Zr-normalized redox-sensitive elemental abundances and a mild decrease in the values of geochemical ratios were present in the inner shelf environment (M2 interval) (the V/Cr and Ni/Co ratios fall into a range of 2.5-4.25 and 4-5, respectively), suggesting dysoxic conditions dominant in the inner shelf setting. Samples from the shallower subtidal and intertidal settings (M3 and M4 intervals) are mostly invariable with much lower values of Zr-normalized redox-sensitive elements and relevant ratios, with the V/Cr and Ni/Co ratios typically near or less than 2 and 5 respectively, indicative of oxic conditions in the high-energy subtidal/intertidal zones. A remarkable negative Ce anomaly exhibited in the shale-normalized REE+Y diagram in the M4 interval may provide evidence in support of the hypothesis. Taken together, our results suggest a relatively shallow chemocline in the Early Mesoproterozoic ocean: the transitions between euxinic, dysoxic and oxic may occur in quiet-water outer shelf and highenergy subtidal zone, respectively. The presence of euxinic ocean bottom waters is compatible with low concentrations of seawater sulfate and reduced levels of atmospheric oxygen during this period. The extreme environmental conditions induced by these anoxic oceans could have been responsible for the delayed oxygenation of the biosphere and hindered the evolution of multicellular life.

Hydrological connectivity and mixing of Lake Towuti, Indonesia in response to paleoclimatic changes over the last 60,000 years

The Indo-Pacific Warm Pool (IPWP) is an important driver of global climate, but its response to and involvement in paleoclimate change is poorly constrained. We generated a new record of sediment geochemistry from Lake Towuti (2.5°S, 121.5°E), Indonesia, located in the heart of the IPWP, to investigate changes in hydrological connectivity with upstream lakes and the extent of lake mixing and oxygenation during paleoclimate changes over the last 60,000years BP (60ka). Lake Towuti is located at the downstream end of the Malili Lakes, a chain of large, ancient, and biologically diverse tectonic lakes occupying a geologically heterogeneous terrain in central Sulawesi, Indonesia. Major and trace element data from river and lake sediments suggest no changes in sediment provenance during the Last Glacial Maximum (LGM), indicating that some of the Malili Lakes remained hydrologically open despite a regionally drier climate. However, samples from the LGM are uniformly less enriched in trace elements than samples from the Holocene and Marine Isotope Stage 3 (MIS3), which suggests a decrease in weathering intensity during the LGM, likely in response to decreased precipitation and temperature. Changes in Fe and other redox-sensitive trace element concentrations indicate changes in water column oxygenation, with the highest oxygen availability occurring during the LGM (15–35ka) likely due to more frequent and/or deeper mixing of Lake Towuti's water column. The glacial–interglacial trend in lake oxygenation corresponds with changes in regional precipitation and associated changes in the seasonal cycle. The high degree of faunal endemism in these lakes may be related to changes in the lake geochemistry associated with glacial–interglacial environmental variability driven by changing inputs and redox variability.

Rock magnetic and geochemical record in a sediment core from the eastern Arabian Sea: Diagenetic and environmental implications during the late Quaternary

Rock magnetic concentration, grain size and mineralogy parameters together with organic carbon, calcium carbonate, redox-sensitive elements, δ 18O of Globigerinoides ruber and radiocarbon dating were carried out on a 445 cm long sediment core collected at 1380 m depth off Mangalore, southwestern margin of India. The top 290 cm sediments of the core correspond to the last 18 kaBP. The δ 18O and magnetic records exhibit major events at ∼ 16 kaBP, 14.5 kaBP, 11.5 kaBP and 9.8/8.6 kaBP related to start and intensity of the summer monsoon and climate change, and are synchronous with that of the western Arabian Sea and North Atlantic. The sediments with high magnetic susceptibility correlate with high sedimentation rates. The sediments are dominated by fine-grained magnetite, but intervals of 1.2–3.8 kaBP and 10–13.5 kaBP were subjected to diagenetic changes, resulting in the dissolution of fine-grained magnetites and enrichment of redox-sensitive trace elements (Cu, Ni, Zn, V, Mo and U). The sediments between 290 cm and 445 cm correspond to 18–27 kaBP and are characterized by distinct decrease in magnetic concentration, grain size and mineralogy parameters, high organic carbon, low concentrations of redox-sensitive trace elements and abundant pyritized tubules. The reductive diagenetic conditions indicated by rock magnetic properties are in contrast with the weak sub-oxic conditions revealed by low concentrations of trace elements in the sediments. The seasonal organic matter flux produced during the winter monsoon and moderate sedimentation rates favoured reductive diagenesis in the sediments at and below the last glacial maximum (LGM). Intermittent bioturbation, however, allowed oxidants to penetrate into the sediments, remobilized redox-sensitive trace elements into the water column and modified the primary geochemical signal of the sedimentary environment.

The Cenomanian/Turonian oceanic anoxic event in the South Atlantic: New insights from a geochemical study of DSDP Site 530A

One of the key objectives of Deep Sea Drilling Project (DSDP) Leg 75 was to shed light on the underlying causes of Cretaceous oceanic anoxia in the South Atlantic by addressing two major hypotheses: productivity productivity-driven anoxia vs. enhanced ocean stratification leading to preservation of organic matter and black shale deposition. Here we present a detailed geochemical dataset from sediments deposited during the Cenomanian/Turonian (C/T) transition and the global oceanic anoxic event 2 (OAE 2) at DSDP Site 530A, located off-shore Namibia (southeast Angola Basin, north of Walvis Ridge). To characterise the succession of alternating black and green shales at this site and to reconstruct the evolution of their paleoenvironmental setting, we have combined data derived from investigations on bulk organic matter, biomarkers and the inorganic fraction. The location of the C/T boundary itself is biostratigraphically not well constrained due to the carbonate-poor (but organic matter-rich) facies of these sediments. The bulk δ 13C org record and compound-specific δ 13C data, in combination with published as well as new biostratigraphic data, enabled us to locate more precisely the C/T boundary at DSDP Site 530A. The compound-specific δ 13C record is the first of this kind reported from C/T black shales in the South Atlantic. It is employed for paleoenvironmental reconstructions and chemostratigraphic correlation to other C/T sections in order to discuss the paleoceanographic aspects and implications of the observations at DSDP Site 530A in a broader context, e.g., with regard to the potential trigger mechanisms of OAE 2, global changes in black shale deposition and climate. On a stratigraphic level, an approximation and monitoring of the syndepositional degree of oxygen depletion within the sediments/bottom waters in comparison to the upper water column is achieved by comparing normalised concentrations of redox-sensitive trace elements with the abundance of highly source specific molecular compounds. These biomarkers are derived from photoautotrophic and simultaneously anoxygenic green sulphur bacteria (Chlorobiacea) and are interpreted as paleoindicators for events of photic zone euxinia. In contrast to a number of other OAE 2 sections that are characterised by continuous black shale sequences, DSDP Site 530A represents a highly dynamic setting where newly deposited black shales were repeatedly exposed to conditions of subtle bottom water re-oxidation, presumably leading to their progressive alteration into green shales. The frequent alternation between both facies and the related anoxic to slight oxygenated conditions can be best explained by variations in vertical extent of an oxygen minimum zone in response to changes in a highly productive western continental margin setting driven by upwelling.