Global Redox Conditions Research Articles

The Global Biogeochemical Cycle of Rhenium

AbstractThis paper is the first comprehensive synthesis of what is currently known about the different natural and anthropogenic fluxes of rhenium (Re) on Earth's surface. We highlight the significant role of anthropogenic mobilization of Re, which is an important consideration in utilizing Re in the context of a biogeochemical tracer or proxy. The largest natural flux of Re derives from chemical weathering and riverine transport to the ocean (dissolved = 62 × 106 g yr−1 and particulate = 5 × 106 g yr−1). This review reports a new global average [Re] of 16 ± 2 pmol L−1, or 10 ± 1 pmol L−1 for the inferred pre‐anthropogenic concentration without human impact, for rivers draining to the ocean. Human activity via mining (including secondary mobilization), coal combustion, and petroleum combustion mobilize approximately 560 × 106 g yr−1 Re, which is more than any natural flux of Re. There are several poorly constrained fluxes of Re that merit further research, including: submarine groundwater discharge, precipitation (terrestrial and oceanic), magma degassing, and hydrothermal activity. The mechanisms and the main host phases responsible for releasing (sources) or sequestrating (sinks) these fluxes remain poorly understood. This study also highlights the use of dissolved [Re] concentrations as a tracer of oxidation of petrogenic organic carbon, and stable Re isotopes as proxies for changes in global redox conditions.

Variable local basin hydrography and productivity control the uranium isotope paleoredox proxy in anoxic black shales

Geochemical proxies—such as the isotopic compositions and abundances of redox-sensitive metals, organic biomarkers, and sedimentological indicators—can be used qualitatively and quantitatively to reconstruct marine redox landscapes through Earth history. Isotopic proxies with residence times longer than ocean mixing times, including uranium (238U/235U, commonly denoted as δ238U), are a promising but still developing approach to constrain global changes in redox conditions. Our current understanding of the controls on δ238U variability and associated isotopic fractionations is limited, complicating the interpretation of δ238U records through Earth history. With these gaps in mind, we investigate the major controls on the expression of uranium (U) isotope fractionation within two basins of the Miocene Monterey Formation (ca. 18–6 Ma) that represent contemporaneous deposition under different environmental conditions. Our finding that the isotopic offset in the productive and anoxic Santa Barbara Basin is similar to that in modern euxinic settings suggests that highly productive settings without persistent euxinia can also exert significant leverage on seawater δ238U. Distinct patterns in U concentrations and δ238U between basins demonstrate that local depositional controls can impart a strong influence on the U isotope offset in reducing settings, as has been observed in modern sediment datasets. High productivity and low sedimentation rates are predicted to result in a diagnostic inverse relationship between U enrichment and δ238U, which can partially explain the overall weak negative correlation between [U] and δ238U in the distal Naples Beach section in the Santa Barbara Basin. In a core from a more restricted proximal setting in the San Joaquin Basin, we propose that changes in basin hydrography led to a potential relationship between [U] and δ238U. These interpretations are consistent with the characteristic depositional conditions interpreted independently for each basin (e.g., persistence of anoxia, productivity, sedimentation rates, phosphate content, and basin hydrography) and are supported by modeling results of U enrichment and isotope fractionation under these conditions. In particular, our results suggest that the isotope offset of U in reducing sinks varies and has done so over Earth history and in different paleogeographic locations as a result of local productivity and basin restriction. These variations indicate the need to consider these uncertainties in quantitative models of global redox conditions from δ238U records—and specifically in using δ238U to distinguish between euxinic and oxic redox states without considering other oxygen-depleted environments as part of a continuum of settings that can impact δ238U if spatially widespread. This study also highlights the utility of using different lithologies to evaluate global versus local controls on δ238U records as well as multi-proxy approaches to constrain ancient redox landscapes.

Spatial heterogeneity of redox-sensitive trace metal enrichments in upper Ediacaran anoxic black shales

The Ediacaran radiation of metazoans is widely thought to have been triggered by an increase in atmospheric and oceanic oxygen levels. Although supported by other proxies, rising oxygen levels were deduced to a significant degree from sedimentary enrichments of redox-sensitive trace elements (RSTEs). However, some organic-rich shales of this period show only minor enrichments in RSTEs, leaving the significance of RSTE data for palaeo-oxygenation interpretations in doubt. We measured and compiled proxies for marine redox conditions (Fe species, RSTEs), total organic carbon (TOC) and water mass restriction (Mo/TOC and Co × Mn v. Cd/Mo) in the intra-shelf Jiulongwan and basinal Sandu sections of the Nanhua Basin in South China. Compared with the same proxies from coeval sections in the Nanhua Basin and globally, our results document a strong spatial heterogeneity of RSTE enrichments in anoxic black shales during the late Ediacaran. We infer that RSTE enrichments were strongly influenced by local factors, such as basinal restriction, seawater RSTE concentrations, and differential elemental responses to redox conditions and other influences. The broader significance of our findings is that they highlight the difficulty of investigating global redox conditions based on an analysis of local RSTE proxies within a single depositional basin or a limited number of study sections. Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion Supplementary material : Tables S1–S3 are available at https://doi.org/10.6084/m9.figshare.c.5325047

Chromium isotope systematics and the diagenesis of marine carbonates

Stable chromium (Cr) isotopes have emerged as a new tool for tracking broad-scale changes in Earth's surface oxygen levels. Carbonates are one proposed sedimentary Cr isotope archive. In order to contribute to the development of a robust framework for interpreting carbonate Cr isotopic compositions and evaluating their ability to record the global redox state, we explored Cr isotope systematics of modern and Archean carbonate successions—representing end member oxic and anoxic Earth system states. We generated new data from carbonate platform sediments from the Great Bahama Bank with variable post-depositional histories to better understand the effects of diagenetic alteration on carbonate-bound Cr and its isotopic composition in a modern well-oxygenated ocean-atmosphere system. More specifically, we investigated the effects of dolomitization and aragonite-to-calcite neomorphism in marine and meteoric fluids. We also present δ53Cr values from three carbonate successions, the ∼3.0 Ga Chobeni Formation (South Africa), ∼2.8 Ga Mosher Carbonate Formation (Canada), and ∼2.65 Ga Cheshire Formation (Zimbabwe), that were deposited under an anoxic atmosphere. We find that modern Bahamian carbonates have a large range of almost exclusively positive δ53Cr values (from −0.04 to 2.88‰). The δ53Cr values appear to be altered during both meteoric diagenesis and dolomitization but there may also be instances of rock-buffered conditions that may best preserve depositional δ53Cr values. These observations provide a baseline for carbonate Cr isotope behavior in a well-oxygenated ocean-atmosphere system. Our Archean carbonate successions contain both positively fractionated and crustal δ53Cr values, ranging between −0.37 and 0.89‰. The Cr isotope fractionation observed in Archean strata is most plausibly linked to either non-redox dependent Cr isotope fractionation, local Cr redox cycling, late-stage diagenetic alteration, or some combination thereof. Given the significant effects of post-depositional alteration on the isotopic composition of carbonate-bound Cr, we suggest that Cr isotope values from ancient carbonate sediments should be generally interpreted with caution and cannot be straightforwardly linked to changes in atmospheric oxygen abundance.

Mesoproterozoic paleo-redox changes during 1500–1400 Ma in the Yanshan Basin, North China

The Mesoproterozoic (1600–1000 million years ago, Ma) is thought to have experienced important changes in ocean chemistry and eukaryote evolution. Our understanding of global redox conditions remains in its infancy, and redox data derived from the Mesoproterozoic thus far indicate heterogeneity and complexity in the realm of ocean redox, which hampers our understanding of the potential significance of variations in ocean redox on eukaryote evolution. In this study, we report analyses of Fe species and redox-sensitive trace metals of U and Mo from the Jixian Group, including the Wumishan, Hongshuizhuang and Tieling formations, as well as the Xiamaling Formation, in the Yanshan Basin in North China. The Fe speciation data from the Wumishan, Hongshuizhuang, and Tieling formations first reported in this study help bridge the global data gap of Fe species data during 1500–1400 Ma. Our data suggest likely oxic bottom water conditions for the Wumishan Formation and ferruginous bottom water conditions for the Tieling Formation. The Fe species data can distinguish ferruginous conditions for the upper Hongshuizhuang from sulfidic depositional conditions for the lower-middle Hongshuizhuang Formation. Based on Fe species and trace metal data, redox conditions likely varied from anoxic conditions with transient euxinia in the lower-middle Xiamaling Formation to oxic conditions in the upper Xiamaling Formation. The new Fe speciation data from the Yanshan Basin, when combined with previously published data, show a surprisingly high proportion of middle Proterozoic samples with very low highly reactive to total iron ratios. This suggests either a relatively large extent of oxic seafloor, or non-uniformitarian iron cycling. Resolving this discrepancy will be key to understanding the extent of suitable habitats for complex eukaryotes.

Constraining oceanic oxygenation during the Shuram excursion in South China using thallium isotopes.

Ediacaran sediments record an unusual global carbon cycle perturbation that has been linked to widespread oceanic oxygenation, the Shuram negative C isotope excursion (NCIE). However, proxy-based estimates of global ocean redox conditions during this event have been limited largely due to proxy specificity (e.g., euxinic sediments for Mo and U isotopes). Modern global seawater documents a homogenous Tl isotope composition (ε205 Tl=-6.0) due to significant manganese oxide burial, which is recorded in modern euxinic sediments. Here, we provide new data documenting that sediments deposited beneath reducing but a non-sulfidic water column from the Santa Barbara Basin (ε205 Tl=-5.6±0.1) also faithfully capture global seawater Tl isotope values. Thus, the proxy utilization of Tl isotopes can extend beyond strictly euxinic settings. Second, to better constrain the global redox conditions during the Shuram NCIE, we measured Tl isotopes of locally euxinic and ferruginous shales of the upper Doushantuo Formation, South China. The ε205 Tl values of these shales exhibit a decreasing trend from ≈-3 to ≈-8, broadly coinciding with the onset of Shuram NCIE. There are ε205 Tl values (-5.1 to -7.8) during the main Shuram NCIE interval that approach values more negative than modern global seawater. These results suggest that manganese oxide burial was near or even greater than modern burial fluxes, which is likely linked to an expansion of oxic conditions. This ocean oxygenation may have been an important trigger for the Shuram NCIE and evolution of Ediacaran-type biota. Subsequently, Tl isotopes show an increasing trend from the modern ocean value to values near the modern global inputs or even heavier (ε205 Tl≈-2.5~0.4), occurring prior to recovery from the NCIE. These records may suggest that there was a decrease in the extent of oxygenated conditions in the global oceans during the late stage of the Shuram NCIE.

Uranium reduction and isotopic fractionation in reducing sediments: Insights from reactive transport modeling

Uranium concentrations and isotopic ratios (238U/235U, denoted as δ238U) have been used to provide quantitative information about the degree of oxygenation and de-oxygenation of past oceans. The potential to constrain changes in global redox conditions, in contrast to many other proxies that reflect local conditions, is a particular strength of the uranium isotope approach. Because uranium reduction primarily occurs in sediments underlying anoxic water columns rather than in the anoxic water column itself, the removal of uranium in organic-rich shales is the largest lever on seawater δ238U. Accordingly, accumulation and isotopic fractionation are modulated by local variations in productivity, basin connectivity, sedimentation rate, and bottom-water redox conditions. To isolate the processes at the sediment-water interface that control δ238U and uranium accumulation in reducing sediments, we constructed a reactive transport model that couples biogeochemical reactions to diffusive transport and the burial of solutes and minerals.Using the model framework, we test the sensitivity of authigenic uranium isotopic fractionation and accumulation to oxygenation, permeability, sedimentation rate, organic carbon delivery, and basin restriction. Our results demonstrate that these external forcings produce diagnostic patterns in isotopic fractionation. Specifically, the model predicts that authigenic δ238U is sensitive to productivity because of the associated organic carbon burial rate. Moreover, our results suggest that the isotopic offset does not vary significantly due to changing bottom-water O2 concentrations, but the amount of accumulation does—a result that differs from previous estimates. Water column uranium reduction adds additional complexity to the ultimate δ238U value. The predictive patterns derived from model results can offer insight into local depositional conditions, such as sedimentation patterns. Collectively, these effects—including bottom-water redox conditions and related reducing sediments—alter the isotopic signature of the overlying water column according to the authigenic δ238U value and the diffusive fluxes arising from porewater concentration gradients. More broadly, this work provides important new constraints on the major controls on the δ238U of sediments while also supporting its use as a proxy for global marine redox conditions.

Vanadium isotopic fractionation during the formation of marine ferromanganese crusts and nodules

Vanadium (V) isotopes can be significantly fractionated during the delivery, cycling, and burial to the ocean. The utilization of V isotopes might provide a useful geochemical tracer for the evolution of the oceans. This study provides the first detailed investigation of V isotopes in ferromanganese (Fe-Mn) crusts and nodules, which are widely distributed in the modern oxic ocean. Our results show a large variability of V isotope compositions in Fe-Mn crusts and nodules with a δ51V range from −0.89 to −1.65‰, much greater than current analytical uncertainty (±0.10‰, 2 SD). Therein, the most recent layers of hydrogenetic Fe-Mn crust and hydrogenous nodules have a narrower V isotope range (−0.89‰ to −1.25‰), with no correlation to their collection location or water depth. Thus, our results suggest a relatively homogeneous V isotope composition of marine hydrogenetic Fe-Mn crusts and nodules, with an average δ51V of −1.05 ± 0.16‰ (2 SD). The hydrogenetic Fe-Mn crusts and nodules are depleted in 51V compared with the recently reported seawater value (0.2 ± 0.15‰) by ∼1.2 ± 0.2‰ (2 SD). This can be explained by isotope fractionation during the adsorption of V onto Fe-Mn oxyhydroxides. The various δ51V in Fe-Mn nodules (−0.98‰ to −1.65‰) might be caused by diagenetic precipitation of V from pore fluids with lighter isotope composition compared to seawater, thus driving a negative V isotope shift recorded in Fe-Mn nodules with a diagenetic imprint. Additionally, a depth profiles across an Fe-Mn crusts reveals systematic changes in δ51V from −1.04 ± 0.13‰ (2 SD) in upper layers to 1.32 ± 0.06‰ (2 SD) in lower layers representing the relatively older deposition. The observed δ51V depth profiles might record the changes of V isotope composition of seawater. The proposed temporal variations of δ51V in seawater could be controlled by the isotope fractionation and fluxes of various V sources and sinks to the ocean that is likely related to the global redox state of the oceans. Another potential interpretation of this isotopic shift in the crusts could be related to the modification of the primary V isotope signature due to diagenetic remobilization and reorganization. However, there is no obvious evidence that definitively documents a diagenetic control or signal. This study highlights the burial of V with Fe-Mn oxyhydroxide as an important control on the V isotope composition of seawater, and the potential application of Fe-Mn crusts to track the temporal V isotope variations of seawater.

Geochemical evidence for expansion of marine euxinia during an early Silurian (Llandovery–Wenlock boundary) mass extinction

Repeated biotic crises have become the hallmark for the Silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations. Causal mechanisms linking these marine extinction events with positive carbon isotope (δ13C) excursions, paleoceanographic change, and climate remain poorly constrained. Here, we examine the positive δ13C excursion across the Llandovery/Wenlock boundary, and the associated Ireviken extinction event. This positive δ13C excursion has been interpreted to reflect a major change in global oceanographic-climate state and enhanced organic carbon burial. New geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon- and sulfur-isotopes (δ34S), and iodine (I/Ca+Mg) proxies. The high-resolution δ13C and δ34S data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles, with minimal temporal offsets between the two systems. Numerical box modeling of δ13C and δ34S data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite, which are most likely due to enhanced burial under euxinic (anoxic and sulfidic) conditions. Independently, I/(Ca+Mg) values point to locally anoxic bottom waters in the distal and deeper basinal setting in Nevada before, during, and after the Ireviken positive δ13C excursion. I/(Ca+Mg) values in the proximal shelf setting in Tennessee show relatively oxic waters during the onset of peak δ13C values, after which bottom-water oxygen concentrations dropped throughout the remainder of the excursion. This multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the Silurian biotic event and positive δ13C excursion. Integration of these geochemical data for local- and global-scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea-level rise points toward a shoaling of anoxic and/or euxinic waters onto the shelf as a driver for the Ireviken extinction event.

The iron paleoredox proxies: A guide to the pitfalls, problems and proper practice

Oceanic anoxia—including euxinic settings defined by the presence of water column hydrogen sulfide (H2S)—is minor in the ocean today. Such conditions, however, were common or even dominant in the past, particularly during the Precambrian and Phanerozoic oceanic anoxic events. The latter are associated with massive petroleum and mineral reserves and many of the major extinction events in the paleontological record. Our ability to recognize ancient oxygen deficiencies relies strongly on paleontological data viewed in combination with geochemical tracers, and geochemistry is typically our only window onto ancient marine redox during the Precambrian when diagnostic skeletal and behaviorial traces of oxygen-dependent animals are mostly missing. So far no approach has gained wider acceptance than the iron proxies, which rely generally on quantification of the extent to which reactive iron (as oxides principally) is converted to pyrite. The promise of these approaches lies in part with the relative ease of measurement, but it is this ease and the corresponding widespread use that has also led to misuses. Much of the recent confidence in the iron paleoredox proxies lies with sophisticated deconstruction of the reactive Fe pool via mineral-calibrated wet chemical speciation. These validations and calibrations, mostly in the modern ocean, expose the challenges, while at the same time opening other doors of opportunity as the catalog of controlling factors extends beyond water column redox to include sedimentation rate, sedimentary Fe remobilization, signals of oscillatory redox, and hydrothermal versus other primary Fe inputs to the ocean, among other factors. Also key is a deep understanding of the limitations imposed—or at least the due diligence required—as linked to mineral transformations during burial and metamorphism. This review seeks to highlight many of the key issues, including appropriate sample choices, as a roadmap for those keen to apply Fe proxies in their studies of ancient oceans and their relationships to co-evolving life. Among the critical messages to take away is the value of robust Fe-based measures of local redox that, when combined with elemental mass balances and isotopic proxies dependent on those local conditions, can shed light on the global redox state of the oceans through time and related implications for the history of life on Earth.

Evidence for local and global redox conditions at an Early Ordovician (Tremadocian) mass extinction

Profound changes in environmental conditions, particularly atmospheric oxygen levels, are thought to be important drivers of several major biotic events (e.g. mass extinctions and diversifications). The early Paleozoic represents a key interval in the oxygenation of the ocean–atmosphere system and evolution of the biosphere. Global proxies (e.g. carbon (δ13C) and sulfur (δ34S) isotopes) are used to diagnose potential changes in oxygenation and infer causes of environmental change and biotic turnover. The Cambrian–Ordovician contains several trilobite extinctions (some are apparently local, but others are globally correlative) that are attributed to anoxia based on coeval positive δ13C and δ34S excursions. These extinction and excursion events have yet to be coupled with more recently developed proxies thought to be more reflective of local redox conditions in the water column (e.g. I/Ca) to confirm whether these extinctions were associated with oxygen crises over a regional or global scale.Here we examine an Early Ordovician (Tremadocian Stage) extinction event previously interpreted to reflect a continuation of recurrent early Paleozoic anoxic events that expanded into nearshore environments. δ13C, δ34S, and I/Ca trends were measured from three sections in the Great Basin region to test whether I/Ca trends support the notion that anoxia was locally present in the water column along the Laurentian margin. Evidence for anoxia is based on coincident, but not always synchronous, positive δ13C and δ34S excursions (mainly from carbonate-associated sulfate and less so from pyrite data), a 30% extinction of standing generic diversity, and near-zero I/Ca values. Although evidence for local water column anoxia from the I/Ca proxy broadly agrees with intervals of global anoxia inferred from δ13C and δ34S trends, a more complex picture is evident where spatially and temporally variable local trends are superimposed on time-averaged global trends. Stratigraphic sections from the distal and deeper part of the basin (Shingle Pass and Meiklejohn Peak) preserve synchronous global (δ13C and δ34S) and water column (I/Ca) evidence for anoxia, but not at the more proximal section (Ibex, UT). Although geochemical and paleontological evidence point toward anoxia as the driver of this Early Ordovician extinction event, differences between I/Ca and δ13C–δ34S signals suggest regional variation in the timing, extent, and persistence of anoxia.

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.

Empirical links between trace metal cycling and marine microbial ecology during a large perturbation to Earth's carbon cycle

Understanding the global redox state of the oceans and its cause-and-effect relationship with periods of widespread organic-carbon deposition is vital to interpretations of Earth's climatic and biotic feedbacks during periods of expanded oceanic oxygen deficiency. Here, we present a compilation of new and published data from an organic-rich locality within the proto-North Atlantic Ocean during the Cenomanian–Turonian boundary event that shows a dramatic drawdown of redox-sensitive trace elements. Iron geochemistry independently suggests euxinic deposition (i.e., anoxic and sulfidic bottom waters) for the entire section, thus confirming its potential as an archive of global marine metal inventories. In particular, depleted molybdenum (Mo) and vanadium (V) concentrations effectively record the global expansion of euxinic and oxygen-deficient but non-sulfidic waters, respectively. The V drawdown precedes the OAE, fingerprinting an expansion of oxygen deficiency prior to an expansion of euxinia. Molybdenum drawdown, in contrast, is delayed with respect to V and coincides with the onset of OAE2. Parallel lipid biomarker analyses provide evidence for significant and progressive reorganization of marine microbial ecology during the OAE in this region of the proto-North Atlantic, with the smallest relative eukaryotic contributions to total primary production occurring during metal-depleted intervals. This relationship may be related to decreasing supplies of enzymatically important trace elements. Similarly, box modeling suggests that oceanic drawdown of Mo may have approached levels capable of affecting marine nitrogen fixation. Predictions of possible nitrogen stress on eukaryotic production, locally and globally, are consistent with the low observed levels of Mo and a rise in 2-methylhopane index values during the peak of the OAE. At the same time, the environmental challenge presented by low dissolved oxygen and euxinia coincides with increased turnover rates of radiolarian clades, calcareous nanofossils, and foraminifera, suggesting that the temporal patterns of anoxia/euxinia and associated nutrient limitation may have contributed to the fabric of OAE2-related turnover.

Selective and Reversible Approaches Toward Imaging Redox Signaling Using Small-Molecule Probes.

Recent research has identified key roles for reactive oxygen species (ROS)/reactive nitrogen species (RNS) in redox signaling, but much remains to be uncovered. Molecular imaging tools to study these processes must not only be selective to enable identification of the ROS/RNS involved but also reversible to distinguish signaling processes from oxidative stress. Fluorescent sensors offer the potential to image such processes with high spatial and temporal resolution. A broad array of strategies has been developed that enable the selective sensing of ROS/RNS. More recently, attention has turned to the design of reversible small-molecule sensors of global redox state, with a further set of probes capable of reversible sensing of individual ROS/RNS. In this study, we discuss the key challenges in achieving simultaneous detection of reversible oxidative bursts with unambiguous determination of a particular ROS/RNS. We have highlighted key design features of small-molecule probes that show promise in enabling the study of redox signaling, identifying essential parameters that must be assessed for any new probe. Antioxid. Redox Signal. 24, 713-730.

Marine Mo biogeochemistry in the context of dynamically euxinic mid-depth waters: A case study of the lower Cambrian Niutitang shales, South China

Molybdenum (Mo) concentrations and Mo isotopes have been widely used as proxies for local and global redox conditions in early oceans (>520Ma) that were stratified and characterized by dynamically euxinic mid-depth waters. However, the nature of the Mo cycle and accompanying isotopic fractionations in such oceans remain poorly known. To fill this gap, we conducted an integrated study of Mo isotopes and redox-sensitive trace element (RSTE) abundances in the Lower Cambrian Niutitang Formation at Yangjiaping, South China. This section accumulated on the northern shelf margin of the Nanhua Basin, a failed intracontinental rift basin with a good connection to the open ocean during the early Cambrian. The Niutitang Formation contains a ∼18-m-thick lower black shale member, and a ∼56-m-thick upper gray silty shale member. The lower member (LM) is moderately to strongly enriched in Mo, U and V, and heterogeneous in Mo isotopic composition (δ98Mo=−0.65‰ to +2.14‰), indicative of dominantly euxinic depositional conditions punctuated by ferruginous episodes (as shown by previously reported Fe speciation data). The upper member (UM) shows lesser enrichment of Mo, U and V, higher Mo/U ratios, and intermediate and more uniform Mo-isotopic compositions (δ98Mo=+1.16‰ to +1.71‰), indicative of weakly oxic to anoxic-euxinic depositional conditions. Geochemical profiles suggest that the LM-to-UM transition reflects a shift of the O2/H2S chemocline from the water column to the sediment. Large δ98Mo fluctuations in the LM may record variations of H2S concentrations in the mid-depth euxinic waters. The intermediate and relatively uniform δ98Mo values of the UM are attributed to the effects of a local Fe–Mn particulate shuttle.In light of the observations at Yangjiaping and other contemporaneous sections in the Nanhua Basin, we propose a new marine Mo biogeochemical model for the early Cambrian ocean. In this model, an Fe–Mn reduction zone may have developed above mid-depth euxinic waters in nearshore areas of the Nanhua Basin due to relatively high availability of fluvial Fe–Mn oxides and their subsequent reduction below the chemocline. Within this redox framework, large sedimentary δ98Mo variations observed for the Niutitang shales can be explained through the combined effects of Mo isotopic fractionation associated with adsorption onto Fe–Mn-oxides and spatially variable H2S concentrations in the euxinic watermass. By compiling published data, we found that the large sedimentary δ98Mo variations previously observed in the early Cambrian Nanhua Basin and in other Neoproterozoic-Cambrian marine basins can be similarly explained. Our model provides a novel interpretation of the Mo biogeochemistry of early Earth oceans, within the framework of which Mo isotopes may serve to further our understanding of the oxygenation history of the Earth-surface system.

Mo marine geochemistry and reconstruction of ancient ocean redox states

Molybdenum (Mo) proxies, including bulk concentration and isotopic composition, have been increasingly used to reconstruct ancient ocean redox states. This study systematically reviews Mo cycles and their accompanying isotopic fractionations in modern ocean as well as their application in paleo-ocean redox reconstruction. Our review indicates that Mo enrichment in sediments mainly records the adsorption of Fe-Mn oxides/hydroxides and chemical bonding of H2S. Thus, Mo enrichment in anoxic sediments generally reflects the presence of H2S in the water column or pore waters. In addition to the effect of euxinia, sedimentary Mo enrichment is related to the size of the oceanic Mo reservoir. Given these primary mechanisms for oceanic Mo cycling, Mo abundance data and Mo/TOC ratios acquired from euxinic sediments in geological times show that fluctuations of the oceanic Mo reservoir are well correlated with oxygenation of the atmosphere and oceans and suggest that oxygenation occurred in phases. Mo proxies suggest that Mo isotopes in strongly euxinic sediments reflect the contemporaneous Mo isotopic composition of seawater, but other processes such as iron-manganese (Fe-Mn) adsorption and weak euxinia can result in different fractionations. Diagenesis may complicate Mo enrichment and its isotopic fractionation in sediments. With appropriate constraints on the Mo isotopic composition of seawater and various outputs, a Mo isotope mass-balance model can quantitatively reconstruct global redox conditions over geological history. In summary, Mo proxies can be effectively used to reconstruct oceanic redox conditions on various timescales due to their sensitivity to both local and global marine redox conditions. However, given the complexity of geochemical processes, particularly the effects of diagenesis, further work is required to apply Mo proxies to ancient oceans.

Molybdenum evidence for expansive sulfidic water masses in ~ 750 Ma oceans

The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO 2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. δ 98 Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1–4% of the global seafloor, corresponding to a ~ 20–80 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800–750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O 2 level, consistent with the hypothesis that pO 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.

Modulation of Chlamydomonas reinhardtii flagellar motility by redox poise.

Redox-based regulatory systems are essential for many cellular activities. Chlamydomonas reinhardtii exhibits alterations in motile behavior in response to different light conditions (photokinesis). We hypothesized that photokinesis is signaled by variations in cytoplasmic redox poise resulting from changes in chloroplast activity. We found that this effect requires photosystem I, which generates reduced NADPH. We also observed that photokinetic changes in beat frequency and duration of the photophobic response could be obtained by altering oxidative/reductive stress. Analysis of reactivated cell models revealed that this redox poise effect is mediated through the outer dynein arms (ODAs). Although the global redox state of the thioredoxin-related ODA light chains LC3 and LC5 and the redox-sensitive Ca2+-binding subunit of the docking complex DC3 did not change upon light/dark transitions, we did observe significant alterations in their interactions with other flagellar components via mixed disulfides. These data indicate that redox poise directly affects ODAs and suggest that it may act in the control of flagellar motility.

Production of CO2 and H2 by Diking-Eruptive Events at Mid-Ocean Ridges: Implications for Abiotic Organic Synthesis and Global Geochemical Cycling

We have calculated the amounts of CO2 and H2 produced by complete degassing of mid-ocean ridge basalt (MORB) magma, and by degassing during transient diking-eruptive events. Our CO2 calculations are based on a model estimate of an initial CO2 content of 1800 ppm in MORB magma, which is equivalent to 2.2 × 1012 mol CO2 per year for magma production at worldwide ocean ridges. Observations indicate that many MORB magmas are emplaced in numerous small pulses of dikes and associated lava flows with very short emplacement times, which would result in release of relatively large amounts of CO2 over short intervals. For example, a dike injected into the oceanic crust that extends from the top of its magma chamber at 2 km depth to the seafloor would degas 2.3 × 104 mol CO2 per m2 surface area of dike, and produce another 4.0 × 104 mol CO2 per m2 on complete crystallization. Unlike CO2, which is not strictly governed by crystallization-alteration processes, H2 is produced from MORB by the reduction of H2O by ferrous iron in the magma to form magnetite and H2 as the magma cools and crystallizes. From published paired analyses of MORB glass and crystalline rock, we estimate that the amount of H2 produced from one cubic meter of rock averages 301 mol. We suggest that the oxidizing agent is H2O dissolved in the magma, which results in rapid generation of H2. The amount of pre-alteration oxidation may be limited by the amount of H2O dissolved in the magma; thus relatively water-rich magmas will undergo greater oxidation. For the case of the two-kilometer-high dike reaching the seafloor, the amount of H2 released is 6.2 × 105 moles H2 per m2 surface area of the dike. This is 10 times greater than the total CO2 released by degassing and crystallization of the dike. Assuming that the H2 generation rate for the entire basaltic layer of the oceanic crust is the same as for MORB lavas (312 mol/m3), then the annual global H2 production rate is 6.3 × 1012 mol H2 per year. This amount is about three times greater than our calculated annual CO2 production from MORBs. Given that the annual CO2 production rate from MORBs over 3.3 Ga can account for all CO2 found in the Earth's crust, hydrosphere, and atmosphere, it is likely that the H2 produced at mid-ocean ridges plays a significant role as a reducing agent in the global redox state of the Earth's surface. In contrast to time-averaged global production rates, the rapid release of CO2 and H2 in diking-eruptive events may locally result in formation of a separate gas phase containing H2-CO2-H2O in that order of abundance. The amounts of CO2 and H2 produced could provide a significant energy source for autotrophic microorganisms. It has been demonstrated that such a CO2-H2-H2O gas mixture yields methanol in magnetite-surface catalyzed reactions at seafloor hydrothermal conditions. Such abiotic synthesis reactions could have been important in early Earth processes.