Enrichment Of Redox-sensitive Elements Research Articles

Coupling data-driven geochemical analysis and ensemble machine learning for automatic identification of oceanic anoxic events

Oceanic Anoxic Events (OAEs) have been recorded across the Phanerozoic and linked with catastrophic events in geological records, including the massive release of CO2 into the atmosphere and mass extinctions of marine animals, particularly during the Cretaceous period (e.g., OAE-2). Overall, the occurrence of OAEs were typically identified based on the deposition of organic-rich black shales, carbon isotopic excursions, and enrichment of redox-sensitive elements. While various OAE intervals have been extensively studied across the Tethys using multiproxy geochemical records, recognizing the expression, and understanding the duration of these events are rather challenging and a subject of active debate. This is further compounded by the time-consuming and expert-demanding analysis to interpret complex geochemical records associated with OAEs. To address these issues, we propose a novel approach by coupling data-driven geochemical analysis and ensemble machine learning to recognize and predict the occurrence of OAE-2 in the Upper Cretaceous based on key geochemical records (δ13Corg, TOC, Mo, V, U) collected from different areas geographically. Considering variation in data availability and completeness, we performed machine learning-based data imputation to fill the gaps in geochemical records without perturbing the overall trends and patterns. With this, our prediction of OAE in various locations using ensemble machine learning, achieving an accuracy of up to 90% in the validation and 78% in the blind test predictions. The model could also match the interpreted OAE-2 intervals from different locations with higher resolution prediction based on the δ13Corg and the TOC as the most important parameters followed by the sensitive redox elements. This suggests that the model utilized similar parameters used by geologists in identifying OAEs, increasing the model interpretability. Application of machine learning and data-driven geochemical analysis could help in providing a robust and time-efficient identification of OAE and find new unexplored OAEs along the stratigraphic records.

Changes of bottom water oxygenation during the last half millennium in the Gulf of Tehuantepec (Eastern Tropical North Pacific): A multiproxy approach

The Gulf of Tehuantepec (GoT), in the Eastern Tropical North Pacific Ocean, is home to one of the largest and more intense Oxygen Minimum Zones (OMZ). To identify the OMZ variability during the last five hundred years in the GoT, we analyzed the temporal variations of benthic foraminiferal (BF) assemblages, enrichment of redox-sensitive elements (Mo, V, Cd, U, and Re) and δ15Nsed, on a laminated sediment core. During the Little Ice Age (LIA; ∼1500 to ∼1860 CE), the BF assemblages showed high dominance, and the most abundant BF species in the sediments were Epistominella sandiegoensis, Takayanagia delicata, and Buliminella tenuata. This assemblage can withstand the lowest dissolved oxygen conditions, indicating an intensified OMZ consistent with the enrichment of Mo, Re, Cd, U, and enhanced denitrification. During the Current Warm Period (CWP; ∼1860 CE to present), the BF assemblages were more diverse, and the most abundant species were Bolivina seminuda, Epistominella sp.1, Gyroidina nitidula, and Suggrunda eckisi, an assemblage less tolerant to low dissolved oxygen conditions, together with lower concentrations of redox-sensitive elements and δ15Nsed, suggest weakening of the reducing conditions within the OMZ. Additionally, our BF data showed evidence of the influence of the Pacific Decadal Oscillation on bottom-water oxygenation. Bottom-water oxygenation changes appear to respond to increased solar forcing caused by Intertropical Convergence Zone migration and strengthened Pacific Walker Circulation, which in turn modulated productivity, organic matter flux to the seabed, and oxygen consumption in the bottom water. Our findings agree with previous studies further north from the study site, indicating these changes' regional scale.

Reconstructing redox variations in a young, expanding ocean basin (Cretaceous Central Atlantic)

The redox sensitive elements (RSE) are indicators of organic matter (OM)-rich rocks and provide valuable information on paleoenvironmental parameters, including organic productivity, chemical evolution of water masses, ocean residence times and sea-bottom redox conditions. Yet, their potential to understand global environmental changes is hindered by the scarcity of continuous geological records, often highly fragmented in space and time. In this contribution, a large dataset from five closely spaced sedimentary sections in the Western African margin is used to reconstruct a 40 myr-long record of the redox conditions in a time spanning from the early Aptian to early–middle Campanian. Studied intervals were originally located in different positions of the embryonic Atlantic Ocean, and intercept four separated oceanic anoxic events (OAE1c, OAE1d, OAE2 and OAE3). The complete geochemical characterization revealed the changes of Mn, V, Ni, Cu, Zn, Mo, U and Se during the deposition of OM-rich sediments, as a function of their depositional environments, redox conditions, and time. It is shown that, although enrichments of RSE are triggered by increasingly reducing seafloor conditions, the extent to which these elements are retained within the sedimentary record depend on both local variations in detrital inputs and global perturbations of the marine RSE inventories. The analyses show that RSE are characteristically enriched with respect to global shale values, having geochemical characteristics (i.e., high Mo/TOC and high Mo/U) suggestive of suboxic to sulfidic conditions from the late Albian to Campanian.

Redox-stratified seawater during the GOE: Evidences from rare earth elemental and C-O isotopic compositions of Paleoproterozoic BIF and carbonate rocks from the Taihua Group, North China Craton

The Paleoproterozoic Great Oxidation Event (GOE) is known for the dramatic change in atmospheric composition that led to oxygen becoming a persistent component of global surface processes. Drastic environmental changes greatly affected ocean and atmosphere chemistry. However, the distribution of dissolved oxygen in the ocean during the GOE is still unclear as rare contemporary sediment-records have been found. In order to identify the redox state of both shallow and deep water, and reveal the ocean redox structure during this time, we present new major and trace elemental and C-O isotopic data of bulk samples of the banded iron formation (BIF), dolomitic marble, and graphite-bearing marble of the early Paleoproterozoic upper Taihua Group in the Lushan area, southern margin of the North China Craton (NCC). The BIF samples exhibit typical major and trace element characteristics akin to very pure marine chemical sediments, such as the shale-normalized positive La, Ga, and Y anomalies, indicative of a marine depositional setting. In these BIF samples, both negative Ce anomalies and the lack of Ce anomalies reflect the presence of both oxic and suboxic/anoxic water in the ocean. Samples of the dolomitic marble are characterized by extremely low amounts of crustal material (such as Al, Ti, Zr, Hf) and high Y/Ho ratios (46–66), also implying a pure marine chemical origin. Excluding some contaminated samples, the rest of the graphite-bearing marble samples display similar features of the dolomitic marble. Significant positive Ce anomalies and high light to heavy REE (Pr/Yb(SN)) ratios in both dolomitic marble and graphite-bearing marble are essentially consistent with the 2.2–2.1 Ga carbonates from the Guanmenshan Formation of the Liaohe Group in the NE Sino-Korean Craton. Combined with the Ce anomalies in the BIF samples, we consider that the seawater when the upper Taihua Group deposited was redox-stratified from oxic shallow water to deeper anoxic water. Besides, the graphite-bearing marble record coeval surface water in the column that was enriched in nutritive materials and beneficial to microbiology and high productivity. Moreover, the early Paleoproterozoic IFs in the Songshan Group and the Lüliang Group in the NCC also exhibit significant both positive and negative Ce anomalies and enrichment of redox sensitive elements. These evidences, therefore, suggest that redox-stratified seawater occurred extensively during the GOE.

Precessional pacing of early Proterozoic redox cycles

Regularly alternating reduction-oxidation (redox) patterns attributed to variations in the Earth's orbit and axis (Milankovitch cycles) are widely recorded in marine sediment successions of the Phanerozoic and attest to a dynamic history of biospheric oxygen in response to astronomically forced climate change. To date, however, such astronomical redox control remains elusive for much older, Precambrian intervals of the geological record that were characterized by a globally anoxic and iron-rich ocean, i.e., prior to Earth's atmospheric oxygenation (ca. 2.4–2.2 billion years ago). Here we report a detailed cyclostratigraphic and geochemical investigation of marine-sedimentary redox cycles identified in the ca. 2.46 billion-year-old Joffre Member of the Brockman Iron Formation, NW Australia, suggesting the imprint of Earth's climatic precession cycle. Around the base and top of regularly intercalated mudrock layers, we identify sharp enrichments in redox sensitive elements (Fe, S, Ca, P) that appear to represent chemical reaction fronts formed during nonsteady state diagenesis. Using a reactive transport model, we find that the formation of characteristic double S peaks required periods of increased organic matter deposition, coupled to strongly declining Fe2+ concentrations in the overlying water column. This combination, in turn, implies a periodic deepening of the iron chemocline due to enhanced oxygenic photosynthesis in marine surface waters, and is interpreted as the result of precession-induced changes in monsoonal intensity that drove variations in runoff and associated nutrient delivery. Our study results point to a dynamic redox evolution of Precambrian oceanic margin environments in response to Milankovitch forcing, and offer a temporal framework to investigate linkages between biological activity and the early build-up of oxygen in Earth's ocean-atmosphere system.

Geochemical studies of the Green River Formation in the Piceance Basin, Colorado

Hierarchical cluster analysis (HCA) was applied to a geochemical dataset representing the Eocene Green River Formation in the Piceance Basin of Colorado to identify chemofacies in core and outcrop samples from the basin margin and the basin center. The input dataset consisted of inductively coupled plasma optical emission spectroscopy and mass spectrometry and total organic carbon (TOC) content analyses of 186 basin margin outcrop samples and 190 basin center core samples discussed in Part 1 of this study (this volume). TOC values and twenty-five major and trace elements were used as variables to define statistical clusters of samples for the overall dataset, for the two basin center cores, and for each separate core or outcrop dataset by HCA applying Euclidean distance and Ward’s method algorithms. For each dataset, five cluster-defined chemofacies were identified. The chemofacies for each dataset show chemical affinities with five informally defined rock types– mudstone, marlstone, carbonate-rich mudstone, siliciclastic-rich mudstone/siltstone/sandstone, and Na-rich (saline) mudstone, with each showing variations in TOC content and abundance of redox sensitive minor and trace elements. A close relationship between enrichment of redox sensitive elements, particularly As and Mo, and TOC is identified in the basin center. Whereas enrichment factors (relative to average shale) are relatively low for many Period IV (PIV) transition metals, as discussed in Part 1 of this study, their consistent coherence in enrichment or depletion in HCA-defined chemofacies demonstrates the expected relationship between redox state and organic richness. Enrichment in PIV transition metals also shows a correlation to enrichment in elements with affinity for siliciclastic sediment. Enrichment/depletion among several groups of redox indicators is not everywhere consistent, with some chemofacies showing, for example, enrichment of PIV transition metals and depletion of sulfur and arsenic. Early timing of saline conditions in the basin margin is clearly displayed in the chemofacies log display, consistent with observations based on geochemical interpretations of concentrations and elemental ratios discussed in Part 1. Overall, the chemofacies are consistent with major mineralogical units and lake history stages defined in previous work, but provide more detail on the fluctuations in lake chemistry that occurred during deposition of Green River oil shale in the Piceance Basin.

Main Controlling Factors and Models of Organic Matter Accumulation in Lower Carboniferous Dawuba Formation Shale in Southern Guizhou, China.

A set of high-quality marine facies organic-rich shales developed in the Lower Carboniferous Dawuba Formation, which is considered to be the main target of shale gas exploration and development in Guizhou Province. In this paper, 53 samples from Well ZY1 are selected, and the core observation data, field-emission scanning electron microscopy (FE-SEM) images, and geochemical data of these samples are analyzed. On the basis of these data, the main influencing factors of organic matter enrichment in the Dawuba Formation shale were identified and an organic matter accumulation model was established. The results show that total organic carbon (TOC) values of the Dawuba Formation in the ZY1 well vary between 1.97 and 4.11%, with high values appearing at the depths of 2796–2814 m (3.00–4.11) and 2877–2894 m (1.97–3.49). The redox-sensitive element enrichments are generally low, indicating that these samples were deposited under oxic–suboxic conditions. The micronutrients (Zn, Cu, and Ni), biological Ba (BaXS), and P/Al also show low values, indicating low primary productivity. The chemical index of alteration (CIA) and terrigenous clastic input index (Ti/Al) showed two obvious high-value zones, indicating that shale in the study area was affected by terrigenous inputs. Similarly, the calculation results show that Fe/Mn and Rb/K values have two abnormal data segments at the same depth. The anomaly of these data at the same depth section further suggests that the shale was affected by terrigenous input during deposition. Moreover, the terrigenous input reaches the maximum in the above TOC high-value region, and it is inferred by combining with the core observation results that the gravity flow occurs in this depth. The carbon isotope of kerogen (δ13Corg) ranges from −26.84 to −24.36%, indicating that the source of organic matter is likely to be terrestrial plants. This is further supported by the widespread presence of filamentous organic matter using FE-SEM, despite the low productivity and poor preservation conditions during deposition of the Dawuba Formation; the enhanced terrigenous input may have provided additional sources of organic matter for the Dawuba shale.

Anomalously high enrichment of mercury in early Cambrian black shales in South China

The widespread lower Cambrian black shales reflect a significant anoxic event during a global transgression, recording synergistic changes in the marine environment and biological evolution. Extremely high Hg (714–6187 ppb, average is 3380 ppb) contents and Hg to total organic carbon ratios (Hg/TOC: 120–765 ppb/wt.%) were observed in the Danquan section of South China, suggesting that extensive volcanic activities may occur during the early Cambrian. The redox condition of the Danquan section is euxinic as evidenced by multiple redox proxies, which include U/Th, V/Cr, Corg/P and authigenic enrichment of redox sensitive elements (MoEF, UEF). The Co*Mn – Al crossplot revealed that black shale developed in different geographical settings (upwelling setting for the Interval I, and restricted condition for the Interval II). Besides, the sedimentary Hg is hosted in different phases in the two intervals: in organic matter for the Interval I, and in sulfides for the Interval II. Moreover, the anomalous enrichment of Hg in both intervals document the Hg controlled by geographical settings and host phases. Thus, volcanic activities and geographical settings both constrain the evolution of life, as well as redox conditions.

Persistent oxic deep ocean conditions and frequent volcanic activities during the Frasnian-Famennian transition recorded in South China

Abstract The Frasnian-Famennian (F F) transition of Late Devonian was a critical episode in geological history, recording a major mass extinction event. In this study, we focus on an F F succession from a deep marine context in Bancheng, southern Guangxi, South China, to investigate coeval changes in pelagic environments of the Paleo-Tethys Ocean. The studied succession is exclusively composed of bedded cherts intercalated with multiple siliceous volcanic ash beds. A SIMS zircon U Pb Concordia age of 367.8 ± 2.5 Ma is reported for a tuffaceous layer slightly above the F F boundary. Geochemical ratios of Al/(Al + Fe + Mn), Ce/Ce*, Y/Ho, and Al, Fe contents in bedded cherts indicate that they are of predominantly biogenic/chemical origin with some terrigenous inputs. Negligible enrichment of redox sensitive elements (Mo, U, V) and low V/Cr ratios ( F transition. These findings call into question the widely held hypothesis that marine anoxia was the primary killing mechanism for the F F crisis. In contrast, multiple tuffaceous layers throughout the F F boundary succession indicate frequent volcanic activity, which could have released massive amounts of greenhouse gases into the atmosphere, inducing climate warming. This scenario may have increased continental weathering and riverine fluxes into the ocean, reconciling the increases in Al2O3 content and Al/(Al + Fe + Mn) ratio across the F F boundary. Documentation of persistently oxic conditions and frequent volcanic activitiy provides new perspectives on the inter-relationship between volcanism, climate, and oceanic redox fluctuation during the F F biotic crisis.

Major and trace element characterization of Oceanic Anoxic Event 1d (OAE 1d): Insight from the Umbria-Marche Basin, central Italy

The Pialli Level in the Umbria-Marche Basin (central Italy) correlates with the lowermost part of the positive carbon isotopic excursion characterizing the late Albian–early Cenomanian Oceanic Anoxic Event 1d (OAE 1d). High-resolution litho-, bio- and chemostratigraphic data from the Monte Petrano and Le Brecce sections allow for a bed-by-bed comparison of the two successions, and discriminate local from basin-scale signals. We present new X-ray fluorescence, ICP-MS and TOC data for both the limestones and the Pialli Level shales integrated with available carbonate carbon isotopes and nannofossil temperature and nutrient indices. Data indicate a homogenous background sedimentation dominated by pelagic carbonates, biogenic silica with little contribution by clays. The limited variation in lithogenic elements points to an essentially homogeneous detrital source area with limited fluvial terrigenous input. Higher Mn concentrations coupled with low enrichments in redox-sensitive elements, such as U, Fe, S, Re, Mo, Ag, suggest that the Pialli Level shales represent temporary suboxia without reaching anoxia. Furthermore, P, authigenic Ba, Cd and Ni enrichments, together with nannofossil nutrient index indicate generally low primary productivity conditions along the entire succession, with only minor increases for some of the black-to-dark grey shales of the Pialli Level. The nannofossil temperature index highlights a warm climate during the OAE 1d, with the warmest conditions experienced during the deposition of the Pialli Level shales. During the late Albian, the warm and humid climate was interrupted by brief episodes of relatively warmer and less saline surface waters ensuring slower rates of bottom water renewal and producing temporary suboxic conditions. Such paleoceanographic dynamics would be the continuation of episodic warmer and humid pulses characterizing the late Albian interval in the Umbria-Marche Basin. As such, the Pialli Level can be considered the result of a last episode closing a cycle before the establishment of a steadier climate during the early Cenomanian.

Multiple negative molybdenum isotope excursions in the Doushantuo Formation (South China) fingerprint complex redox-related processes in the Ediacaran Nanhua Basin

Abstract The Ediacaran Doushantuo Formation offers one of the most complete and extensively studied records of end-Neoproterozoic biotic and environmental change. Here, we report multiple coeval negative molybdenum (Mo) isotope excursions (to as low as δ98MoNIST+0.25 = −2.24 ± 0.10‰; 2SD) in shales from four separate sites in South China (Rongxi, Taoying, Wuhe, and Yuanjia) that preserve the Doushantuo Formation. The negative δ98Mo excursions appear coincident with previously discovered and seemingly peculiar redox-sensitive element (RSE) patterns in the same sedimentary rocks. We propose that these geochemical trends can be explained by some combination of (a) enhanced local marine oxygenation in the sedimentary basin where the Doushantuo Formation was originally deposited (the Nanhua Basin) and (b) changes in the degree of connectivity between this paleo basin and the open ocean. Enhanced local marine oxygenation, by exposing more sediments in the Nanhua basin to H2S-poor conditions, could have hindered quantitative tetrathiomolybdate formation within these sediments. Local marine oxygenation could have also stimulated the operation of a Mn oxide shuttle. Today, both of these processes are shown to promote the retention of lighter-mass Mo isotopes in sediments and also govern RSE enrichment patterns. Alternatively, or in addition, the Nanhua Basin may not have maintained an uninterrupted connection with the open ocean during the entirety of the Ediacaran Period. The negative δ98Mo excursions occur coincident with sea level highstands that could have also exposed more sediments in the basin to H2S poor conditions and/or catalyzed the operation of a local Mn oxide shuttle. When trying to infer temporal changes in ancient global ocean redox, it is important to consider the influence of sea level changes and associated variations in local depositional conditions on stratigraphic trends in RSE enrichments and isotope compositions.

Geochemical characteristics of trace elements and carbon isotopes in Taiyuan Formation: a case study of Huainan Coalfield

This paper reports systematically the redox sensitive elements (Mo, V, Cd, Cr, U and Th) and carbon and oxygen isotope data of the Late Carboniferous Taiyuan Formation limestone in the Huainan Coalfield in the southern margin of North China Platform, combined with the observation of limestone slice, and tries to explain the sedimentary environment of each layer of limestone. Through the analysis of the diagenesis of each layer of limestone, except for L7 and L12 limestone, the lithogenesis change of the remaining limestone becomes weak, which generally reflects the geochemical characteristics of the paleosol during sedimentation. There are three negative shifts of δ13C in Taiyuan Formation limestone, which may be related to the enrichment of redox sensitive elements (Cd, Cr, Mo and V) in limestone. In addition, the value of the above redox sensitive elements shows a negative drift. Combining the low V/Cr ratio and high Th/U ratio, it can be concluded that limestone of the Taiyuan Formation is likely to be deposited under an oxidizing condition.

Provenance and depositional environment of organic-rich calcareous black shale of the Late Ordovician Macasty Formation, western Anticosti Basin, eastern Canada

The organic-rich Macasty shale in the Gulf of St. Lawrence was deposited in the Late Ordovician during the Taconic orogeny. The orogeny involved explosive volcanism and thrusting of allochthonous rocks in the eastern margin of North America. Neodymium isotope compositions of the shale show that the provenance is predominantly Grenvillian granite–gneissic rocks, which were widely exposed north of the basin, with little contribution from Taconic igneous rocks. The bulk composition and the presence of detrital kaolinite suggest that the Grenvillian source rocks underwent intense weathering before erosion. Fine-grained detritus was deposited in the Anticosti Basin, where abundant organic activity kept the sediment–water interface under anoxic conditions. This proposed interpretation is supported by the enrichment of redox-sensitive elements, such as As, V, and U, and by high δ34S for pyrite. Calcite cement formed in the pore space of sediments during the diagenesis at temperatures below 60 °C. The low-temperature diagenetic conditions are consistent with the preservation of abundant organic matter in the shale.

Paleoredox conditions of the Middle-Upper Jurassic black shales in the Blue Nile Basin, Ethiopia

Redox-sensitive elements (Mo, V, U, Re, Cd, Co, As, Sb, Tl, Ni, Cr, Mn, Fe, Pb, Cu, Zn, Se) and proxies for detrital clastic sedimentation (Al, Ti, Sc, Th) along with total organic carbon (TOC) and total sulfur (TS) were analysed to constrain the paleoredox conditions of the Middle-Upper Jurassic black shales in the Blue Nile (Abay) Basin, Ethiopia. These samples were collected from the deep marine Antalo Limestone (Oxfordian-Kimmeridgian), Gohatsion Formation (Bathonian) and glauconitic shale-mudstone unit (Aalenian-Toarcian). Significant variations in redox-sensitive elements concentration within and between these shales were evident for variable redox conditions. In general, the concentration of proxies for detrital clastic sedimentation increases from the Antalo Limestone to Gohatsion Formation and further to glauconitic shale-mudstone unit shales, respectively. The Antalo Limestone shales show slightly higher enrichment in redox-sensitive elements than the Gohatsion Formation and glauconitic shale-mudstone unit shales. The Antalo Limestones shales have higher TOC content than the glauconitic shale-mudstone unit shales and become very low in the Gohatsion Formation shales. The TS content in contrast, is relatively lower in the Antalo Limestone shales, intermediate in the Gohatsion Formation shales and becomes higher in the glauconitic shale-mudstone unit shales. The Antalo Limestone shales were deposited under anoxic-suboxic conditions whereas the Gohatsion Formation and glauconitic shale-mudstone unit shales were deposited under suboxic-oxic conditions. The TOC content and redox conditions of the Antalo Limestone shales implies favourable organic matter preservation and future source rock explorations in the basin should be targeted there.

Climatic and oceanic changes during the Middle-Late Ordovician transition in the Tarim Basin, NW China and implications for the Great Ordovician Biodiversification Event

The Middle-Late Ordovician transition (Darriwilian to Sandbian Age) witnessed a major pulse of the Great Ordovician Biodiversification Event (GOBE) and distinctive oceanic geochemical fluctuations, such as coeval negative C and Sr isotope excursions. In this study, investigations into geochemical variations, notably the Hg abundance (or Hg/TOC), have been carried upon the organic-rich black shale of the Middle-Upper Ordovician Saergan Formation to unravel the causes of this pulse. Based on these data, three phases were identified. Phase 1 (0 to 3 m) is characterized by rising Hg/TOC (up to 138 ppb/wt%) and Ti/Al values as well as high CIAcorr (corrected chemical index of alteration) values (68.9–72.3) with negligible enrichment of redox sensitive elements (RSE) and nutrient elements (e.g. U ≤ 5.2 ppm, V ≤ 153 ppm, Mo ≤ 1.8 ppm, P2O5 ≤ 0.2%), suggesting intensified volcanism, which could have emitted significant amounts of greenhouse gases, thereby leading to climate warming. In contrast, Phase 2 (3 to 11 m) is characterized by decreasing Hg/TOC and Ti/Al ratios, relatively low though slightly fluctuating CIAcorr values, generally depleted in RSE (except moderately enriched U up to 14.6 ppm) and increased P/Al and Ba/Al ratios, implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling. Phase 3 (Sandbian Age: 11–13 m) witnessed continuous decrease in Hg/TOC ratio, an increase in Ti/Al and CIAcorr values, fairly low values of RSE enrichment and P/Al and Ba/Al ratios, indicating recurrent climate warming, and the potential for slowed oceanic circulation and attenuated upwelling of nutrient-rich deep waters onto the shallow shelf. These changes could have diminished bioproductivity and organic output onto the seafloor. This study offers insights into volcanic-climatic-oceanic interactions during a major pulse of the GOBE around the Middle-Late Ordovician transition while black shales were extensively deposited.

Uranium isotope variations in a dolomitized Jurassic carbonate platform (Tithonian; Franconian Alb, Southern Germany)

The uranium (U) isotopic composition (δ238U) of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean. However, many limestones have undergone significant dolomitization during diagenesis. To assess the potential impact of diagenetic changes on the U isotope composition of dolomitized rocks, we examined the uranium isotopic composition of a Jurassic carbonate platform of the Franconian Alb in Southern Germany. This platform underwent dolomitization during shallow burial due to the supply of magnesium-rich fluids in the form of slightly modified seawater. This type of dolomitization is common in the geologic record. The carbonate platform of the Franconian Alb can therefore serve as an example for many dolomitized carbonate platforms.A positive correlation between the concentrations of redox sensitive elements (e.g., Re and V) and δ238U confirms that the authigenic uptake of U under reducing conditions preferentially incorporates U enriched in 238U. This positive correlation between redox sensitive elements and δ238U does not change within the dolomitized interval, suggesting that the δ238U is not altered during shallow burial dolomitization of limestones. Therefore, our results indicate that dolostones can retain information about secular variations in seawater δ238U.The diagenetic uptake of U shifts δ238U of bulk carbonate sediments to values heavier than contemporaneous seawater. The magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set. Samples with high Re and V concentrations and high Re/Mo ratios are the most enriched in 238U. This positive correlation could be used to constrain the amount of diagenetic offset of bulk δ238U values from seawater. This is particularly important in carbonate systems that had a primary calcitic mineralogy. Primary calcite precipitates have low U concentrations and so small amounts of 238U uptake during diagenesis can have a significant impact on the bulk composition.The samples with the most depleted δ238U values have high Mo concentrations and low Re/Mo ratios. As carbonate sediments are generally Mo poor, the high Mo concentrations suggests that the depleted δ238U values might be linked to a manganese cycle that operated across the sediment-water interface during the deposition of this Jurassic carbonate platform.

Oceanic redox condition during the late Ediacaran (551–541 Ma), South China

The late Ediacaran (551–541 Ma) oceanic environment could represent a significant preparation for the enrichment of redox sensitive elements during the early Cambrian and even for the evolution of life. However, the marine environment during the late Ediacaran has not been thoroughly characterized to date. To better understand late Ediacaran oceanic redox conditions, we investigated the Fe and S isotopic compositions of cherts from the slope (Yinjiang) and basin (Silikou) sections of the Nanhua basin in South China. The slope (Yinjiang) section is characterized by a wide range of δ56FeT values (−0.12‰ to 1.06‰) and by continuously decreasing δ34SPy values from 40.4‰ to 4.1‰. The variable δ56FeT values in the Yinjiang section primarily reflect the various oxidation degrees of dissolved Fe2+ in ferruginous seawater. The homogeneous δ56FeT signatures (0.02–0.40‰) along the Silikou section and the decreasing trend of δ56FeHR values from 0.86‰ to near zero primarily depend on the mixing of Fe3+ hydroxide/oxide precipitates in a ferruginous water column, possibly followed by partial reduction, the authigenic framboidal pyrite trapped within an intermittent euxinic condition and detrital silicate Fe minerals. A 20–30‰ discrepancy in δ34SPy values between the Yinjiang and Silikou sections may be ascribed to the formation of diagenetic pyrites and framboidal pyrites, respectively. Therefore, the continuously decreasing δ34SPy values in the Yinjiang and Silikou sections may reflect a significant increase of sulfate diffusion into sediments at the slope location and the enlarged marine sulfate reservoir at the basin location. These new results indicate that the deep water in the Nanhua basin is primarily characterized by a ferruginous condition with intermittent euxinic conditions in the basin location during the late Ediacaran.

Diagenetic effects on uranium isotope fractionation in carbonate sediments from the Bahamas

Uranium isotope variations (δ238U) recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time. However, the effects of diagenetic alteration on the U isotopic composition in carbonate sediments, which are crucial to understand the accurate reconstruction of marine δ238U, are currently poorly constrained. Here we examine the effects of the aragonite-to-calcite transition in the Pleistocene Key Largo Limestone of South Florida, and assess the effects of vadose meteoric, phreatic meteoric, and phreatic marine diagenesis on U isotope fractionation in carbonate sediments from the Bahamas Transect, including the well-studied Clino, Unda, and ODP Site 1006 drill cores.Our results suggest that early diagenetic processes in Bahamas carbonate sediments fractionate U isotopes by an average of 0.27 ± 0.14‰ (1 SD) heavier than contemporaneous seawater. Downcore variations of δ238U in slope and basin sediments display little, if any, correlation with U concentration and common geochemical indicators of diagenesis (δ13C, δ18O, Mn/Sr, Mg/Ca, Sr/Ca), enrichments of redox-sensitive elements, or rare earth elements anomalies. We propose two possible mechanisms to interpret the positive change in the δ238U during carbonate diagenesis: authigenic enrichment of isotopically positive U(IV) in carbonates and preferential incorporation of isotopically positive aqueous U(VI) species into carbonates. These processes likely operate during early (syndepositional) diagenesis on the banktop. Further diagenesis during deeper burial is limited by the low solubility of U(IV) under reducing pore water conditions.The early diagenetic behavior of U isotopes in Bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past. We suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater δ238U from ancient carbonates. Furthermore, early diagenesis induces significant statistical variability in sediment δ238U values, pointing to the need for large, high resolution data sets in order to average out stochastic variations in individual bulk sediment samples.

Response of calcareous nannoplankton to the Late Cretaceous Oceanic Anoxic Event 2 at Oued Bahloul (central Tunisia)

Oued Bahloul is a reference section for the Cenomanian–Turonian that was located on the North African palaeomargin, where Oceanic Anoxic Event 2 (OAE2) is represented by black limestones and grey marls. Biostratigraphic study allowed the identification of calcareous nannofossil UC3 to UC6 zones. Quantitative analyses of calcareous nannofossil assemblages indicate that high-fertility surface-water conditions predominated throughout most of the OAE2 interval. The most eutrophic conditions probably developed during deposition of the lowermost part of the Whiteinella archaeocretacea Zone (UC3 to UC4 zones), where nannofossil-derived fertility indices reach high values and the small Zeugrhabdotus gr. predominates over Biscutum spp. The temperature index based on calcareous nannofossil assemblages indicates a warming at the beginning of the OAE2, followed by progressive cooling (the same trend indicated by δ18Obulk values). The significant proportion of holococcoliths suggests high-stress conditions in surface waters during most of the OAE2 interval. The enrichment in redox-sensitive elements, together with the abundance of low-oxygen tolerant benthic foraminifera, indicates dysoxic conditions both in the deep-water column and sea-bottom at the beginning of the OAE2 (W. archaeocretacea Zone). Geochemical detrital proxies indicate an enhanced rate of terrigenous supply during the OAE2 interval resulting from increased riverine influx related to a more humid climate. Enhanced fluvial run-off increased nutrient input to marine surface waters contributing to eutrophication (recorded by calcareous nannoplankton and planktic foraminifera). Plankton consumption of surface water nutrients increased organic matter production, whose decay contributed to the decrease in dissolved oxygen, generating increasingly dysoxic/anoxic conditions and favoring preservation of organic matter. Although low-latitude nannofossil assemblages such as those of Oued Bahloul reflect an increase in nutrient availability during the OAE2 interval, assemblages from mid-latitude areas record a decreased nutrient content.

Oceanic oxygenation events in the anoxic Ediacaran ocean.

The ocean-atmosphere system is typically envisioned to have gone through a unidirectional oxygenation with significant oxygen increases in the earliest (ca. 635Ma), middle (ca. 580Ma), or late (ca. 560Ma) Ediacaran Period. However, temporally discontinuous geochemical data and the patchy metazoan fossil record have been inadequate to chart the details of Ediacaran ocean oxygenation, raising fundamental debates about the timing of ocean oxygenation, its purported unidirectional rise, and its causal relationship, if any, with the evolution of early animal life. To better understand the Ediacaran ocean redox evolution, we have conducted a multi-proxy paleoredox study of a relatively continuous, deep-water section in South China that was paleogeographically connected with the open ocean. Iron speciation and pyrite morphology indicate locally euxinic (anoxic and sulfidic) environments throughout the Ediacaran in this section. In the same rocks, redox sensitive element enrichments and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran-early Cambrian ocean. This dynamic redox landscape contrasts with a recent view of a redox-static Ediacaran ocean without significant change in oxygen content. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well-oxygenated Phanerozoic oceans. Anoxic events caused mass extinctions followed by fast recovery in biologically diversified Phanerozoic oceans. In contrast, oxygenation events in otherwise ecologically monotonous anoxic Ediacaran-early Cambrian oceans may have stimulated biotic innovations followed by prolonged evolutionary stasis.