Redox Proxies Research Articles

Reconstructing oxygen deficiency in the glacial Gulf of Alaska: Combining biomarkers and trace metals as paleo-redox proxies

Marine anaerobic oxidation of ammonium (anammox) plays a central role in the nitrogen cycle of modern Oxygen Deficient Zones (ODZs). The newly developed bacteriohopanetetrol stereoisomer (BHT-x) biomarker for anammox, which is largely unaffected by early diagenesis, allows for the reconstruction of the presence and dynamics of past ODZs from the sedimentary record of continental margins. In this study, we investigate the development and dynamics of the ODZ in the Gulf of Alaska (GOA) between 60 and 15 cal ka BP using records of redox sensitive trace metals (TM) and the BHT-x anammox biomarker from IODP Site U1419 (~700 m water depth). The biomarker record indicates that the ODZ in the GOA was in concert with global climate fluctuations in the late Pleistocene. Anammox was more pronounced during warmer periods and diminished during cooler periods, as indicated by correlation with the δ18O signal obtained by the North Greenland Ice core Project (NGRIP). Trace metal enrichments, however, do not match the trend in BHT-x. Systematic metal enrichments in intervals where biomarkers point to more intense water column deoxygenation are not observed. We suggest that this proxy discrepancy was caused by environmental factors, other than water column redox conditions, with opposing effects on the TM and biomarker records. Two of the most widely used redox indicators, Mo and U, are not significantly enriched throughout the sediment record at Site U1419. Site U1419 experienced some of the highest sedimentation rates (100–1000 cm ka−1) ever reported for late Pleistocene continental margin sediments, leading to a continuous and rapid upward migration of the sediment-water interface. We suggest that despite water column and seafloor oxygen depletion, significant sedimentary enrichments of these redox sensitive trace metals were prevented by a limited time for their diffusion across the sediment-water interface and subsequent enrichment as authigenic phases. Thus, depositional conditions were ideal for biomarker preservation but prevented significant authigenic trace metal accumulations. Similar discrepancies between organic and inorganic redox proxies could exist in other high sedimentation rate environments, potentially putting constraints on paleo-redox interpretations in such settings if they are based on trace metal enrichments alone.

Marinoan glacial aftermath in South China: Paleo-environmental evolution and organic carbon accumulation in the Doushantuo shales

The paleo-environment and organic matter accumulation after the Marinoan “Snowball Earth” event is of great significance to understand the Earth system and petroleum geology during this critical period. However, the present studies in South China are not well constrained due to the lack of continuous sedimentary records. Here, we present an integrated petrological and geochemical study of a unique, continuous, and complete outcrop in the northeastern Sichuan Basin, SW China. Results show that the postglacial Doushantuo shales in the study area can be divided into three intervals (from bottom to top: Unit1–Unit2–Unit3) according to geochemistry evolutionary trend. Multiple redox proxies (VEF, UEF, MoEF, Corg/P, Th/U, and UMo covariation pattern) indicate the bottom redox condition changed from oxic to suboxic, and back to weakly oxic stratigraphically. Babio and Ba/Al paleoproductivity proxies suggest that the primary productivity was middle–high and it was slightly higher in Unit 2. The CIAcorr values of Doushantuo shales range from 72.2 to 86.8 and gradually increase from Unit 1 to Unit 3, indicating relatively intense weathering conditions and global warm and humid paleoclimatic conditions. The Doushantuo shales in our studied outcrop is approximately 100 m in thickness. They are at over mature stage with average random reflectance of 1.86–2.14%, which makes the recovered original organic matter abundance up to ~2.0 wt%. The kerogen of the shales is type II. The accumulation of organic matter was mainly controlled by the preservation environments, with the primary productivity level as a secondary control. These data were further compared with other outcrops reported in previous studies for a comprehensive understanding. The variation of organic carbon contents in the Doushantuo shales hold similar features across the entire South China, with the middle part producing the highest-quality source rocks with conventional and unconventional hydrocarbon geological significance. This suggests that changes in sea level and paleo-weathering after the Marinoan glaciation controlled the enrichment of organic carbon in shales. The enrichment is a large-scale geological event across basins rather than a local one. This might be general and referenced for the postglacial and/or interglacial shales.

Demystifying provenance signatures and paleo-depositional environment of mudrocks in parts of south-eastern Nigeria: Constraints from geochemistry

Geochemical signatures of sedimentary rocks provide reliable proxies that allow a better understanding of sedimentary processes. The geochemical compositions of Awgu Shale (southern Benue Trough) and Enugu Shale (Anambra Basin), south-eastern Nigeria, were investigated to appraise the degree of source area weathering and reconstruct the provenance and paleo-depositional environment. High values of several weathering indices such as Chemical index of alteration (CIA; 91.26 ± 3.26), Chemical Index of Weathering (CIW; 98.78 ± 1.42), Plagioclase Index of Alteration (PIA; 98.64 ± 1.62) and the Al2O3-(CaO + Na2O)–K2O ternary relationship indicate intense weathering in the source area. The remarkable enrichment in light rare-earth elements (LREE), the negative Eu anomalies (0.77 ± 0.06) and elemental ratios (La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, and Th/Sc) diagnostic of provenance suggest derivation from predominantly felsic sources. The results also provide clues of the tectonic settings and suggest deposition on a passive margin. The signals of redox proxies (Ni/Co, V/Cr, Cu/Zn, U/Th, Ce/Ce*) reflect predominantly oxic conditions while the Sr/Ba ratios (0.25–2.04) suggests alternated marine and continental paleo-depositional settings. The inorganic geochemical proxies of weathering, provenance, salinity, and redox conditions reflect an insignificant disparity between the pre-Santonian Awgu Shale and the post-Santonian Enugu Shale.

Post-depositional REE mobility in a Paleoarchean banded iron formation revealed by La-Ce geochronology: A cautionary tale for signals of ancient oxygenation

Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce⁎ that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 ± 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation.

Elemental and Mineral Composition of the Barents Sea Recent and Late Pleistocene−Holocene Sediments: A Correlation with Environmental Conditions

A comprehensive examination of the elemental (including radionuclides and heavy metals), mineral, and grain-size composition of sediments from different areas of the Barents Sea was performed. Sediment cores were sampled in the Central Deep, Cambridge Strait (Franz Josef Land Archipelago), Russkaya Gavan’ Bay (Novaya Zemlya Archipelago), and Bear Island Trough. We aim to evaluate how the modern and more ancient environmental conditions are reflected in the elemental and mineral composition, as well as to test indicative elemental ratios. The applied methods include elemental analysis using gamma-ray spectroscopy, X-ray fluorescence (XRF), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and X-Ray Difractometry XRD analysis of minerals. Difference in sedimentation rates, grain-size composition, and sources of material, are reflected in downcore variation of Si/Al, Mn/Fe, P/Al, Ti/K, and quartz-feldspar ratios. At boundary Early Holocene/Late Deglaciation, intensive bottom currents from the West-Southern shelf areas contributed to increase of Si/Al and Zr/Ca ratios. Distinct growth of the Si/Fe ratio within the sediments deposited over Late Pleistocene to Mid Holocene may be caused by increased contents of the coarse sand material, as well as by abundant fluxes of clay-mineral-loaded glacial meltwater during the main deglaciation phase. The Mn/Fe ratio used as redox proxy, displayed peaks at different depths related to oxygen concentration growth in bottom water.

Bottom-water deoxygenation at the Peruvian margin during the last deglaciation recorded by benthic foraminifera

Abstract. Deciphering the dynamics of dissolved oxygen in the mid-depth ocean during the last deglaciation is essential to understand the influence of climate change on modern oxygen minimum zones (OMZs). Many paleo-proxy records from the eastern Pacific Ocean indicate an extension of oxygen-depleted conditions during the deglaciation, but the degree of deoxygenation has not been quantified to date. The Peruvian OMZ, one of the largest OMZs in the world, is a key area to monitor such changes in near-bottom-water oxygenation in relation to changing climatic conditions. Here, we analysed the potential to use the composition of foraminiferal assemblages from the Peruvian OMZ as a quantitative redox proxy. A multiple regression analysis was applied to a joint dataset of living (rose-bengal-stained, fossilizable calcareous species) benthic foraminiferal distributions from the Peruvian continental margin. Bottom-water oxygen concentrations ([O2]BW) during sampling were used as the dependant variable. The correlation was significant (R2=0.82; p<0.05), indicating that the foraminiferal assemblages are rather governed by oxygen availability than by the deposition of particulate organic matter (R2=0.53; p=0.31). We applied the regression formula to three sediment cores from the northern part of the Peruvian OMZ between 3 and 8∘ S and 997 and 1250 m water depth, thereby recording oxygenation changes at the lower boundary of the Peruvian OMZ. Each core displayed a similar trend of decreasing oxygen levels since the Last Glacial Maximum (LGM). The overall [O2]BW change from the LGM and the Holocene was constrained to 30 µmol kg−1 at the lower boundary of the OMZ.

Deciphering the upper ordovician Wufeng siliceous shale depositional environments (Wuxi, NE Chongqing) based on multi-proxy record

Abstract Based on stratigraphic variations of petrology, geochemistry (major, trace elements), and pyrite framboids, we study the sedimentary environment of siliceous rocks of the Wufeng Formation in Wuxi, Northeastern Chongqing, China. Coupled Al2O3/(Al2O3+Fe2O3), SiO2/Al2O3, with Al–Fe–Mn values indicate that Wufeng Formation are deposited in a continental margin and influenced by detrital input. UEF-MoEF and V/Cr–U/Th cross-plots suggest that the siliceous shale was formed in dysoxic to anoxic conditions. Redox proxies imply that the lower, the middle to upper, and the top part of the Wufeng sediments were deposited in an anoxic, dysoxic and oxic environment, respectively. Accordingly, the average size of pyrite framboids are gradually increased from bottom to top, showing that increased oxidisability. A complete vibratory third-order cycle of sea level fluctuations during the Wufeng deposition can be identified. During Wufeng to Longmaxi transition, the sedimentary environment exhibits a short-scale oscillatory pattern and was probably transformed from an outer shelf to inner shelf. Coupled multi-proxies are considered more reliable proxies for deciphering redox conditions in fine-grained sediment.

Weathering, alteration and reconstructing Earth's oxygenation.

Deciphering the role-if any-that free oxygen levels played in controlling the timing and tempo of the radiation of complex life is one of the most fundamental questions in Earth and life sciences. Accurately reconstructing Earth's redox history is an essential part of tackling this question. Over the past few decades, there has been a proliferation of research employing geochemical redox proxies in an effort to tell the story of Earth's oxygenation. However, many of these studies, even those considering the same geochemical proxy systems, have led to conflicting interpretations of the timing and intensity of oxygenation events. There are two potential explanations for conflicting redox reconstructions: (i) that free oxygen levels were incredibly dynamic in both time and space or (ii) that collectively, as a community-including the authors of this article-we have frequently studied rocks affected by secondary weathering and alteration (particularly secondary oxidation) while neglecting to address the impact of this alteration on the generated data. There are now multiple case studies that have documented previously overlooked secondary alteration, resolving some of the conflicting constrains regarding redox evolution. Here, an analysis of a large shale geochemistry database reveals significant differences in cerium (Ce) anomalies, a common palaeoredox proxy, between outcrop and drill core samples. This inconsistency provides support for the idea that geochemical data from altered samples are frequently published in the peer-reviewed literature. As individuals and a geochemical community, most of us have been slow to appreciate how pervasive the problem is but there are examples of other communities that have faced and met the challenges raised by such quality control crises. Further evidence of the high potential for alteration of deep-time geochemical samples, and recognition of the manner in which this may lead to spurious results and palaeoenvironmental interpretations, indicate that sample archiving, in publicly accessible collections needs to become a prerequisite for publication of new palaeoredox data. Finally, the geochemical community need to think about ways to implement additional quality control measures to increase the fidelity of palaeoredox proxy work.

A high-TOC shale in a low productivity world: The late Mesoproterozoic Arctic Bay Formation, Nunavut

The latest Mesoproterozoic Arctic Bay Formation (Borden Basin, Nunavut, Canada) is up to ∼1130 m-thick and contains a significant proportion of unusually organic-rich black shale (up to 12.3 wt% total organic carbon). Insofar as increased biological productivity is related to organic matter burial, this organic-rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the Proterozoic. To better understand the conditions leading to development of this organic-rich unit, we explore the redox geochemistry of the Arctic Bay Formation using a multi-proxy approach (nitrogen isotopes, iron speciation, total organic carbon, total sulphur, and trace metal abundances). Redox proxy data support a stratified water column, with oxic surface waters underlain by intermittently euxinic waters, which are in turn underlain by persistently ferruginous deeper waters. The highly alkaline, restricted marine basin in which the Arctic Bay Formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals, resulting in an ‘excess’ of sulphur that resulted in sulphurisation of organic matter. Estimates for organic matter burial rates during deposition of the Arctic Bay Formation suggest that they were perhaps ∼5–6 times mid-Proterozoic average values (although there are permissible scenarios in which it was extremely productive), underscoring that such organic-rich sedimentary rocks could be produced in a low productivity world.

Assessment of the controls on (234U/238U) activity ratios recorded in detrital lacustrine sediments

Understanding landscape evolution on geological time scales has become increasingly important in the light of recent climate warming and intensified land use. Uranium isotopes ((234U/238U) activity ratios) can be used to reconstruct catchment-wide erosion from fine-grained detrital matter, as the lighter nuclide 234U is lost from grains <63 μm into surrounding pore space in a weathering profile, during transportation, and after final deposition. Thus, (234U/238U) activity ratios are a measure for the time elapsed since comminution of bedrock into detrital sediment. When applied to detrital grains in sedimentary deposits, the comminution age allows derivation of past variations in sediment residence time, i.e. the amount of time for which sediments are stored on hillslopes and transported.Loss of 234U is mainly attributed to recoil of 234Th during α-decay of 238U. However, mobilisation of 234U is also controlled by preferential oxidation and leaching of 234U from detrital grains. Further considerations are required concerning the sediment mineralogy and uranium bound to authigenic or endogenic matter.Here, we extend the discussion about the controls on previously published (234U/238U) activity ratios and uranium concentrations of detrital matter along a 5.43 m-long, Late Glacial to Holocene sedimentary record from Lake Ohrid (North Macedonia, Albania). Lake Ohrid is chosen to study how erosion responds to anthropogenic disturbances and climate variability, which are common through the Late Pleistocene and Holocene in the Mediterranean Region. We compared uranium concentration and isotope data to redox-sensitive, mineralogical and biogeochemical proxies. Mineralogical and biogeochemical data show no control on uranium concentration and (234U/238U) activity ratios. Rock magnetic redox proxies indicate uranium mobilisation from detrital matter in more oxic environments. No correlation between (234U/238U) activity ratios and rock magnetic data is observed, which implies that the redox environment has no control on the isotope signal. Monte Carlo simulations reveal that post-depositional preferential mobilisation of 234U has only negligible impact on the calculated sediment residence times. Our model implies that pre-depositional leaching can results in shorter sediment residence times but low (234U/238U) activity ratios during cold and dry intervals imply that recoil is likely the main processes explaining loss of 234U from the detrital grain. This is explained by the time scales studied herein in (>10,000 years), during which preferential leaching is seen to be not significant.

Petrographic, mineralogical and geochemical characterization of fine-grained rocks of the Pilmatué Member (Upper Valanginian-lower hauterivian) of the Neuquén Basin (Argentina): Implications for siliciclastic input, carbonate productivity and redox conditions

Petrographic, mineralogical and geochemical analyses of mixed siliciclastic-carbonate offshore deposits of the Pilmatué Member (Neuquén Basin) were carried out to define two scenarios for deposition: a confined ramp setting (CRS) and an open ramp setting (ORS). The CRS involves marly to carbonate-rich fine-grained mudstones, mostly laminated and non-bioturbated, enriched in redox proxies and depleted in rare earth elements (REE). The ORS comprises abundant siliciclastic-rich mudstones, mostly coarser-mudstones, depleted in redox proxies and with REE pattern similar to the PAAS. These variations are explained through fluctuations in carbonate productivity/terrigenous input, and changes in the circulation pattern of the basin. Within the CRS, high carbonate productivity would be favored due to the general greenhouse condition for the Early Cretaceous and the scarcity of terrigenous supply, related to the landward migration of the shoreline and low effectiveness of sediment distribution by marine currents. Furthermore, high organic matter productivity should lead to an oxygen-depletion close to the sediment-water interface. Instead, increasing sediment supply and dissolved oxygen in the ORS is related to shoreline progradation in the south and the consequent installation of across shelf flows that allowed vertical water circulation and the oxygen replenishment in benthic environment.

Depositional environment and geomechanical properties of Cambay Shale: potential reservoir for shale oil and gas in India

This study aims to investigate mineralogy, depositional environment and rock mechanical properties of Cambay Shale Formation in Jambusar-Broach block of South Cambay Basin, India, by an integrated approach of combining wireline logs and laboratory studies. The study involves the analysis of spectral gamma ray (SGR) logs and laboratory studies of core samples using X-ray diffraction (XRD), thin-section analysis and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS). The results show that in the Cambay Shale, mineral components are dominated by clay minerals, quartz and siderite with minor amounts of feldspar, mica and pyrite. Both thorium (Th)–potassium (K) cross-plots derived from SGR logs and XRD analysis have revealed that kaolinite and chlorite are the dominant clay minerals. The dominant clay mineralogy and the analysis of thorium to uranium ratio plots indicate the deposition of the shale in marginal marine to marine environment. Pyrite framboid size distribution, used as a redox proxy, indicates deposition under anoxic conditions. The geomechanical property of Cambay Shale has been analysed based on the overall mineral composition of the core samples. Mineralogy-based brittleness index (BI) has been evaluated and found to vary between 0.21 and 0.66, suggesting low to moderately high brittleness. Kaolinite and chlorite, the dominant clay minerals, are the less swelling clays and exhibit a lower plasticity. It may be concluded that these factors might facilitate the intrinsic fracking properties of Cambay Shale.

Tin stable isotopes in magmatic-affected coal deposits: Insights in the geochemical behavior of tin

Tin (Sn) is a redox-sensitive element and geochemically incompatible that accumulates in evolved magmas and associated hydrothermal fluids. Here, a Sn stable isotope approach is used to characterize sources and geochemical behaviors of Sn in magmatic intruded coal deposits by combining with petrographic and elemental analysis. Sn isotope ratios of twelve coal samples with various magmatic-affected degrees show a δ120/118Sn variation from −0.06 to 0.22‰ (2σ of 0.07‰, relative to NIST 3161a). Basing on Sn isotope data and geochemical characteristics of the samples, we suggest binary source mixing and redox state related isotope shift as two mechanisms accounting for the isotope variations. The first mechanism is an admixture of magmatic-hydrothermal fluids, characterized by high δ120/118Sn, into coal deposits with low δ120/118Sn. A binary mixing model is used to estimate Sn contributions of the two end-members. The second process results in a positive isotope shift caused by an oxidation of sample states when they are affected by magmatic intrusions. In fact, magmatic intrusions could trigger the oxidation of Sn, resulting in heavy Sn isotope enrichment in the magmatic affected samples. The change in coal redox-state is supported by a concomitant change of redox proxies, such as V/(V + Ni), Mn/Cr and V/Cr. The enrichment of heavy Sn isotopes in samples impacted by magmatic hydrothermal fluids compared to normally-deposited coals could be explained by the conversion of dominant Sn species from Sn (II)–S to Sn (IV)–O under the influence of oxidative magma, which favors the enrichment of heavier Sn isotopes in Sn (IV)–O due to its shorter bond length. This study shows that Sn isotope ratios are useful tools for investigating Sn geochemical behavior in different Earth reservoirs and during redox sensitive processes.

Comparison of Ediacaran platform and slope δ238U records in South China: Implications for global-ocean oxygenation and the origin of the Shuram Excursion

The Ediacaran Shuram negative carbon isotope excursion (SE) records major paleoceanographic changes during the late Neoproterozoic, possibly linked to a global oceanic oxygenation event, yet its cause(s) remain uncertain. Earlier studies of the upper Ediacaran Doushantuo Formation in South China based on local redox proxies have documented strong spatial redox heterogeneity along shelf-to-basin transects, but variations of δ238U (a global redox proxy) have not yet been examined in deep-water SE carbonates. In this study, we examined δ238U variations through the SE in the upper slope Siduping section. Similar to platform SE sections, Siduping exhibits a shift toward higher δ238U values correlative with the peak of the SE (i.e., maximum negative δ13Ccarb), confirming inferences of global ocean oxygenation during the SE. This raises an apparent paradox, because a global negative carbon isotope excursion implies net oxidant consumption, requiring an ocean-based oxygenation mechanism. We hypothesize that an increase in the efficiency of phosphorus burial due to a plankton-driven shift from dominantly dissolved organic matter (DOM) cycling to greater particulate organic matter (POM) export depleted the ocean of nutrient phosphorus. By producing a steep redox gradient close to the sediment-water interface, we suggest that ocean oxygenation also triggered a globally simultaneous diagenetic event in which isotopically light δ13Ccarb was precipitated in authigenic carbonate minerals. This scenario can account for δ238U differences between shallow-water and deep-water carbonates, which reflect precipitation of relatively larger amounts of authigenic carbonate minerals in shallow-water settings, generating both a larger negative δ13Ccarb shift and a larger early diagenetic δ238U offset.

Organic geochemistry of a lacustrine shale across the Toarcian Oceanic Anoxic Event (Early Jurassic) from NE China

Thirty-nine shale samples of the Lower Jurassic Beipiao Formation from drill core ZK01 in the Jinyang Basin, NE China were investigated by comprehensive organic geochemical methods in this study. A prominent negative excursion in δ 13 C org values (~ 2‰) was observed in the Beipiao Formation, which is most likely associated with Toarcian Oceanic Anoxic Event (T-OAE) negative carbon isotope excursion (CIE) that is recorded globally in marine and terrestrial sediments constrained by previous studies. Elemental and Rock-Eval analyses of Beipiao shales demonstrate that the total organic carbon (TOC) ranges from 0.56 to 4.33 wt% (average of 1.40 wt%) and S 2 values range from 0.12 to 6.91 mg/g (average of 1.3 mg/g) and implicate fair to good source rock potential. A majority of the T max values of the Beipiao shales range from 456 to 465 °C, indicating that the organic matter is within the mature oil window apart from few overmature samples with extreme high T max values that might have been affected by local magmatic intrusions. Elemental analysis suggests that the kerogens are mainly Type II-III. Unlike many marine organic-rich sediments deposited during OAEs, redox proxies indicate no significant upper water column oxygen-depletion during the formation of Beipiao lacustrine shale. The decreasing upward trend of organic source indicators such as tricyclic terpanes/C 30 hopane values, relative distributions of regular C 27 steranes to C 29 steranes, and steranes/hopanes in the Beipiao Formation consistently suggest a shift to a lower abundance of eukaryotic organisms relative to bacterial community abundance during the onset of the T-OAE. Meanwhile, gradually decreasing values of gammacerane/C 30 hopane in the Beipiao Formation during the negative CIE suggest decreasing salinity and increased mixing of the Jinyang paleo-lake system, potentially corresponding with the shift toward an accelerated hydrological cycle with enhanced fresh water input to this lacustrine system. Moreover, increasing C 30 moretane/C 30 hopane values contemporaneous with the negative CIE also suggest enhanced land erosion during putative Toarcian climatic warming, which could have increased delivery of riverine nutrients and elevated regional primary productivity, eventually resulting in massive organic matter accumulation in the Beipiao shales. • A prominent negative excursion in δ 13 C org values (~ 2‰) was observed in the Toarcian Beipiao lacustrine shale. • A shift to lower abundance of eukaryote relative to bacterial community in lacustrine system during the onset of the T-OAE. • Progressive accelerated hydrological cycle enhanced fresh water input and increased mixing of NE China lacustrine system. • The organic enrichment and good hydrocarbon generation potential resulted from enhanced delivery of riverine nutrients.

Geochemical signatures of transgressive shale intervals from the 811 Ma Fifteenmile Group in Yukon, Canada: Disentangling sedimentary redox cycling from weathering alteration

Despite iron and trace element proxies informing much of our insight into Earth’s oxygenation history, the processes that controlled their accumulation and distribution in ancient sedimentary environments are not fully understood. Furthermore, deciphering between primary, depositional signals and oxidative weathering alteration poses a substantial challenge to reconstructing Earth’s redox history using its sedimentary record. Here, we present a multi-proxy geochemical investigation of three transgressive shale intervals from the ca. 811 Ma Reefal assemblage (upper Fifteenmile Group) in Yukon, Canada to help resolve these issues. Systematic stratigraphic trends from low to high FeHR/FeT and FeT/Al compositions across shale intervals represent shoaling of a discrete redoxcline that separated oxygenated surface waters from anoxic, ferruginous deep waters during marine transgression. In some cases, these trends are followed by a symmetric fall, which represents a full “transgressive-regressive cycle” of the redoxcline. The high proportion of glauconite to total illite indicate short-term oscillating redox conditions as the redoxcline migrated across the sediment-water interface during relative sea-level rise. These conditions invigorated dissimilatory iron reduction (DIR) within anoxic sediment pore waters, which released Fe(II) into the overlying water column where it was oxidized to iron oxy(hydr)oxide and shuttled downslope. High rates of DIR within sediment replete with highly reactive iron oxy(hydr)oxide and organic matter removed isotopically light, bio-available iron, rendering the residual sediment depleted in FeT/Al relative to the detrital baseline and enriched in 56Fe. The low iron content of the detrital flux to the basin (Fe/Al ∼ 0.3) rendered the sediment susceptible to changes in its bulk δ56Fe composition by modification of its authigenic components alone and enabled fractionation from DIR to drive bulk-rock δ56Fe values up to 0.8‰. Anomalously high concentrations of redox-sensitive elements in three samples (up to 5792 ppm Cr, 586 ppm Mn, 726 ppm Mo, and 3509 ppm Ni) and their relative distribution patterns show similarities to co-enrichment due to particulate shuttling within a weakly restricted basin. However, SEM-EDS element maps and large-area BSE image mosaics show the remains of framboidal pyrite “ghosts” with secondary enrichment of redox-sensitive elements. Thus, these geochemical trends in redox-sensitive trace metals most likely reflect mobilization and re-accumulation following pyrite dissolution during outcrop weathering. The low primary pyrite content and local transformation of pyrite iron to iron oxy(hdr)oxide suggest that δ56Fe and FeHR/FeT data were unaltered despite indications of pyrite weathering. Ultimately, this dataset elucidates key aspects of sedimentary redox cycling directly preceding evolutionary milestones and a major perturbation to the global carbon cycle and also provides a template for evaluating the effects of outcrop weathering on commonly used sedimentary redox proxies.

Arsenic in marine sediments: how robust a redox proxy?

Arsenic (As) is reputed to be a redox-sensitive element but has not been fully explored yet as a proxy in marine sediments and sedimentary rocks, compared to extensively examined elements such as molybdenum (Mo) and uranium (U). Here the behavior of As is compared to that of Mo and U in a number of paleo-environmental situations that have been studied before and are, therefore, rather well constrained. The results indicate that attention must be paid to the reference values used to normalize sedimentary As content in geochemical studies. The most-commonly used references (namely the Post-Archean Australian Shale and average upper crust of Earth, to be opposed to the Average Shale) tend to overestimate slightly As enrichments as shown by As enrichment factors being quasi systematically larger than 1, whatever the geological setting. The results also show that As enrichments are less straightforward compared to those of Mo and U, when using these elements as redox proxies. In addition, the depositional milieus that endured the most hydrologically-restricted and/or reducing (euxinic) conditions are not those with the highest As enrichments. Thus, arsenic shows a redox-sensitive behavior but is not adequate as a redox proxy. However As enrichments are correlated with Mo enrichments in two distinct types of situations: settings impacted 1) by particulate iron shuttling processes and, 2) by diagenetic fluid circulation (cold seeps). In both cases, the Mo-As correlation is induced by the strong relationship between these trace metals and reactive iron: Fe conveys both As and Mo to the sediment where they are trapped. Thus arsenic has a promising potential to allow recognition of past depositional environments associated with iron shuttling and (cold) seep circulations. Lastly, high-TOC, high-Mo, low-As situations point out possible occurrences of sulfurized organic matter in sediments. High Mo concentrations indicate reducing, sulfidic conditions prone to organic-matter accumulation (high TOC values), but low As concentrations indicate that reactive iron was a limiting factor, preventing As capture. Under such conditions, free H2S was not associating with iron and could react with organic matter, causing its sulfurization.

Million-year secular variations in the elemental geochemistry of Devonian marine records and a link to global climate and bioevents: Prague Basin, Czechia

The sedimentary record is affected by periodic and/or random variations in Earth's near-surface conditions, such as the Milankovitch band variations, which are responsible for small-scale cyclicity in distal marine settings. However, the controlling factors of normal field-scale sedimentary cycles operating on Myr-scale that dominate the stratigraphic record remain largely enigmatic. The Lower to Middle Devonian succession (Lochkovian to Givetian) of the Prague Basin, Czechia, is represented by an alternation of largely deep-marine carbonates and shales with typical duration of ~5–7 Myr. We studied the elemental geochemistry of bulk rock samples supported by XRD mineralogy and TOC concentrations. Stratigraphic distribution of the organic-productivity proxies, redox proxies, Ce/Ce* and Pr/Pr* anomalies, and TOC, supported by lithology, allochem composition and field gamma-ray spectrometry, indicates that the Prague Basin was governed by two alternating depositional modes. The oligotrophic mode is characterised by low values of productivity proxies and U/Th ratios, good bottom water oxygenation, heterotrophic skeletal producers, and facies typical of homoclinal ramp settings. The mesotrophic mode is characterised by high U/Th, elevated organic productivity, less oxic bottom conditions, relatively abundant autotrophic skeletal producers and deposition on a distally steepened ramp. The modes were in phase with the Devonian climatically driven environmental changes. We assume that the elevated silicate weathering rates during warmer periods, as previously reported, resulted in higher nutrient input to the seas setting the marine system into the mesotrophic mode. Cooler periods led to oligotrophic water conditions. The global Devonian bioevents show recurrence intervals of the same order as the trophic modes but they typically coincided with the transition between the modes. It is suggested that marine biotic assemblages and the associated carbonate production became unstable during perturbations of the carbon cycle.

Equilibrium fractionation and isotope exchange kinetics between aqueous Se(IV) and Se(VI)

The selenium (Se) isotope system has been proposed as a redox proxy in environmental and paleoceanographic studies. However, Se isotope exchange among various Se species can potentially interfere with redox-related isotope signatures, and is still poorly understood. In this work, we investigated Se isotope exchange kinetics and equilibrium fractionations between aqueous Se(IV) and Se(VI) under various experimental conditions. At pH = 7, low-Se concentration experiments (0.026 mM Se(IV) and 0.026 mM Se(VI)) at 25 °C, 38 °C and 60 °C were conducted for 900 days, while high-Se concentrations (0.13 mM Se (IV) and 0.14 mM Se(VI); 1.3 mM Se(IV) and 1.4 mM Se(VI)) at 60 °C were conducted for 1547 days. All experiments did not reach isotopic equilibrium, with observed Se isotope fractionations <0.20‰. Adding an electronic shuttle (Anthraquinone-2, 6-disulfonate) did not increase the isotope exchange rate. These results show that under the experimental conditions examined, the isotope exchange reaction between aqueous Se(IV) and Se(VI) is extremely slow.The exchange kinetics between Se(IV) and Se(VI) were also investigated using a 82Se tracer. The exchange rates (R) at 0.13 mM Se(IV) and 0.13 mM Se(VI) at 25 °C, 38 °C and 60 °C were determined to be ≤6.34 × 10−10 M day−1, ≤1.12 × 10−09 M day−1 and ≤1.17 × 10−09 M day−1, respectively. Using the upper bound for the isotope exchange rate at 25 °C and theoretically calculated equilibrium fractionations, and assuming a first order isotope exchange reaction between Se(IV) and Se(VI) by analogy to the sulfur system, the timescale of isotope exchange between aqueous Se (IV) and Se (VI) in a natural lake (Sweitzer Lake, Colorado, USA) was estimated. The minimum half-time (t1/2, time to reach 50% isotopic equilibrium) and the minimum time for detectable isotope exchange (tmin) are ≥440,000 and ≥18,000 years, respectively. In the modern oceans, t1/2 and tmin are ≥51 million and ≥3.6 million years, respectively. These timescales are much longer than the residence time of Se in Sweizer Lake (2.4 years) and the modern ocean (26,000 years). Therefore, when using Se isotopes to trace the biogeochemical cycle of Se in lakes and oceans, the effect caused by isotope exchange between aqueous Se(IV) -Se(VI) systems is insignificant.

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.