Redox-sensitive Metals Research Articles

Mercury evidence of intense submarine volcanism and hydrothermal activity during a mid-Tournaisian anoxic event in the Carnic Alps

The mid-Tournaisian Event, or Lower Alum Shale Event (LASE), is linked to severe facies changes including pelagic carbonates and widespread black siliceous shales and radiolarites in many parts of the world. For the first time, high-resolution inorganic geochemistry and framboidal pyrite analyses were applied to decipher depositional condition changes during the mid-Tournaisian event in the Carnic Alps. The predominance of tiny pyrite framboids, high Corg/P, U/Th, and V/Cr ratios, generally higher enrichments of redox-sensitive metals such as Mo, U, V, and depletion in Mn suggest anoxic/euxinic conditions at the sea bottom. Moreover, we found very large anomalous Hg spikes (with maximum values reaching 3650 ppb) for the first time in the mid-Tournaisian deep-water marine succession of the Carnic Alps. Our results suggest intense volcanism during the LASE, which triggered the mid-Tournaisian anoxic event. The most likely candidate for the observed Hg anomalous contents is submarine arc volcanism and hydrothermal activity connected with the collision of the peri-Gondwanan terranes Paleo-Adria and probably Armorican Terrane Assemblage (ATA) and/or Noric terrane during the first phase of the Variscan orogeny.

No effect of thermal maturity on the Mo, U, Cd, and Zn isotope compositions of Lower Jurassic organic-rich sediments

AbstractThe isotope ratios of redox-sensitive metals in organic-rich rocks are critical tools for quantifying the timing and severity of deoxygenation and nutrient cycling in Earth's past. The resilience of isotopic data to thermal alteration of the host sediments over millions of years of burial is, however, largely unknown. We present molybdenum, uranium, cadmium, and zinc stable-isotope data from two stratigraphic successions of the same Lower Jurassic Posidonienschiefer unit in the Lower Saxony Basin of northern Germany that were affected by different burial histories. We show that thermal maturity had no effect on the isotopic compositions of these elements but does appear to have increased their concentrations in the rock. The data corroborate the results of laboratory-based maturation studies and provide constraints on the Mo, U, Cd, and Zn isotopic compositions of ca. 182 Ma seawater in the Lower Saxony Basin.

Covariation of Deep Antarctic Pacific Oxygenation and Atmospheric CO2 during the Last 770 kyr

Abstract We present new geochemical evidence of changes in oxygenation of the deep Antarctic Pacific over the last 770 kyr. Our data are derived from redox-sensitive metals and export production proxies extracted from gravity core ANT34/A2-10 at 4217 m water depth. Our results show that oxygen levels in the deep Antarctic Zone (AZ) varied in line with the release of deeply sequestered remineralized carbon to the atmosphere during glacial–interglacial (G–IG) cycles, with lower oxygen concentrations and more carbon storage during glacial periods. Subsequent reductions in the amount of carbon stored at depth were closely associated with improved ventilation during glacial terminations. The systematic and repeated glacial-to-interglacial increases in export production in the AZ region indicate a robust pattern of enhanced Southern Ocean (SO) ventilation during interglacial periods. In addition to the decline in atmospheric CO2 caused by iron fertilization in the Subantarctic AZ (SAZ) during the latter half of the glacial progression, decreases in productivity in the central AZ suggest that the weakening of SO ventilation induced deep AZ carbon sequestration and that this might have made a continuous additional contribution to the CO2 decline from each interglacial peak to glacial maximum. Observed variations in the degree of deep oxygenation and “organic carbon pump” efficiency in the central AZ might be driven primarily by physical “ventilation” processes (i.e., overturning circulation, mixing, and/or air–sea gas exchange). Our records of abyssal oxygenation in the central AZ, which vary in concert with atmospheric CO2 levels over the last several G–IG cycles, provide strong evidence that SO ventilation plays a significant role in controlling variations in both the amount of respired carbon sequestered in the deep ocean and atmospheric CO2 concentrations on G–IG timescales. Specifically, we suggest that the “organic carbon pump” (OCP) in the SAZ and the physical ventilation processes in the AZ (the “carbon venting valve”) acted together synergistically, but dominated at different intervals over G–IG cycles, to repeatedly switch the SO between carbon sink and carbon source, thereby modulating the atmospheric CO2 over the last 770 kyr. These findings provide new insights into the role of the AZ in controlling deep SO carbon sequestration and atmospheric CO2 levels in G–IG cycles.

Evidence for the oxidation of Earth\u2019s crust from the evolution of manganese minerals

Analysis of manganese mineral occurrences and valence states demonstrate oxidation of Earth’s crust through time. Changes in crustal redox state are critical to Earth’s evolution, but few methods exist for evaluating spatially averaged crustal redox state through time. Manganese (Mn) is a redox-sensitive metal whose variable oxidation states and abundance in crustal minerals make it a useful tracer of crustal oxidation. We find that the average oxidation state of crustal Mn occurrences has risen in the last 1 billion years in response to atmospheric oxygenation following a 66 ± 1 million-year time lag. We interpret this lag as the average time necessary to equilibrate the shallow crust to atmospheric oxygen fugacity. This study employs large mineralogical databases to evaluate geochemical conditions through Earth’s history, and we propose that this and other mineral data sets form an important class of proxies that constrain the evolving redox state of various Earth reservoirs.

Palaeoredox reconstruction in the eastern Arabian Sea since the late Miocene: Insights from trace elements and stable isotopes of molybdenum (δ98/95Mo) and tungsten (δ186/184W) at IODP Site U1457 of Laxmi Basin

The present study investigates the oxygenation history of the northeastern Arabian Sea since the late Miocene using redox sensitive elemental and metal stable isotopic signatures in the deep-sea sediments. To achieve this, the sediment core samples collected at Site U1457 (67°55.80′E, 17°9.95′N, water depth 3534 m) of Laxmi Basin in the northeastern Arabian Sea during the International Ocean Discovery Program (IODP) Expedition 355 were analysed for a suite of elemental (Mo, W, U, V, Ba, Cd and P) and stable molybdenum (Mo) isotope (δ 98/95 Mo relative to NIST SRM 3134 lot No. 130418) as well as stable tungsten (W) isotope (δ 186/184 W relative to NIST SRM 3163 lot No. 080331) composition. Sedimentary δ 98/95 Mo values (−0.70‰ to +1.18‰) at IODP Site U1457 in the northeastern Arabian Sea indicated partial authigenic Mo component. In contrast, the sedimentary δ 186/184 W values (−0.02‰ to +0.21‰) were in the range similar to that of lithogenous material suggesting dominance of detrital composition. The study reveals that the water column in the eastern Arabian Sea was oxic during the late Miocene and Pliocene while oxic to suboxic condition prevailed during the Pleistocene. The study also explores that under oxic to suboxic condition with limited particle shuttling, the W isotopes do not undergo significant fractionation, and their isotope ratios reflect the detrital source signature. This study reports the first results on isotopic compositions of Mo and W in sediments of the northeastern Arabian Sea since the late Miocene to investigate the palaeoredox conditions on a million-year time scale. • First results of δ 98/95 Mo and δ 186/184 W in the late Miocene Arabian Sea sediments. • Oxic condition during late Miocene & Pliocene, and suboxic during Pleistocene. • δ 98/95 Mo values (−0.70 to +1.18‰) exhibits partial authigenic component. • δ 186/184 W values (−0.02 to +0.21‰) exhibit dominant detrital control. • Under oxic setting with limited particle shuttling, tungsten isotopes do not fractionate.

In situ high-resolution two-dimensional profiles of redox sensitive metal mobility in sediment-water interface and porewater from estuarine sediments

Metals in contaminated sediments may present high environmental risks and ecological threats to benthic organisms. Redox sensitive elements with different oxidation states show variations in solubility as a function of redox status of the sediment water environment. The novel high-resolution ZrO-Chelex-AgI diffusive gradients in thin film (HR-ZCA DGT) technique provided sensitive in situ mapping of metals in the estuarine sediments. The present study investigated the sub-millimeter two-dimensional distributions of DGT-labile S(-II), P(V), and six redox sensitive metals (Fe, Mn, V, Cu, Ni, and Zn) across sediment-water interface (SWI) severely influenced by anthropogenic activity. We for the first time used the V-turning value (the V/Fe ratios at ~0.03) to accurately identify the actual SWI. The diffusion boundary layer (DBL) thickness of Ni, Cu and Zn was consistent with those identified by the dissolved oxygen microelectrode method, and was 3–6 mm above the SWI. No significant release of dissolved Fe and P from sediments into the overlying water was found by diffusion process. The estimated fluxes (Fdif) of Ni, Cu, and Zn at DBL were 4.0–176, −1.1–235, and 5.0–108 μg m−2 d−1, respectively, and were significantly higher in sediments near the industrial effluent dumping sites than those in sediments impacted by domestic wastewater releases. Metal diffusion flux was mainly controlled by the particulate matter on the surface sediment and organic degradation. Traditional diffusion flux may have underestimated the flux of metals from the surface sediments. The discharge of hypoxic tributary was an important source of metal pollution in the contaminated estuarine sediments.

Drivers and Seasonal Variability of Redox-Sensitive Metal Chemistry in a Shallow Subterranean Estuary

The subterranean estuary (STE) has been historically defined in terms of the mixing of saline and fresh water, in an analogy to surface estuaries. However, redox gradients are also a defining characteristic of the STE and influence its role as a source or sink for metals in the environment. Approaching the STE from a redox-focused biogeochemical perspective (e.g., considering the role of microbial respiration and availability of organic matter) provides the ability to quantify drivers of metal transport across spatial and temporal scales. This study measured the groundwater composition of a shallow STE over 2 years and used multiple linear regression to characterize the influence of salinity and redox chemistry on the behavior of redox-sensitive metals (RSMs) including Mo, U, V, and Cr. Molybdenum and uranium were both supplied to the STE by surface water, but differed in their removal mechanisms and seasonal behavior. Molybdenum showed non-conservative removal by reaction with sulfide in all seasons. Sulfide concentrations at this site were consistently higher than required for quantitative reaction with Mo (10 µM sulfide), evidently leading to quantitative removal at the same depth regardless of season. In contrast, U appeared to depend directly on microbial activity for removal, and showed more extensive removal at shallower depths in summer. Both V and Cr were elevated in meteoric groundwater (2.5–297 nM and 2.6–236 nM, respectively), with higher endmember concentrations in summer. Both V and Cr also showed non-conservative addition within the STE relative to conservative mixing among the observed endmembers. The mobility of V and Cr in the STE, and therefore their supply to the coastal ocean, was controlled by the availability of dissolved organic matter and Fe, suggesting V and Cr were potentially complexed in the colloidal fraction. Complexation by different organic matter pools led to seasonal variations in V but greater interannual variability of Cr. These results reveal distinct behaviors of RSMs in response to seasonal biogeochemical processes that drive microbial activity, organic matter composition, and complexation by inorganic species.

Persistent deep water anoxia in the eastern South Atlantic during the last ice age

During the last glacial interval, marine sediments recorded reduced current ventilation within the ocean interior below water depths of approximately >1,500 m [B. A. Hoogakker etal., Nat. Geosci. 8, 40-43 (2015)]. The degree of the associated oxygen depletion in the different ocean basins, however, is still poorly constrained. Here, we present sedimentary records of redox-sensitive metals from the southwest African margin. These records show evidence of continuous bottom water anoxia in the eastern South Atlantic during the last glaciation that led to enhanced carbon burial over a prolonged period of time. Our geochemical data indicate that upwelling-related productivity and the associated oxygen minimum zone in the eastern South Atlantic shifted far seaward during the last glacial period and only slowly retreated during deglaciation times. While increased productivity during the last ice age may have contributed to oxygen depletion in bottom waters, especially on the upper slope, slow-down of the Late Quaternary deep water circulation pattern [Rutberg etal., Nature 405, 935-938 (2000)] appears to be the ultimate driver of anoxic conditions in deep waters.

Chemostratigraphy as a tool for sequence stratigraphy in the Devonian Hare Indian Formation in the Mackenzie Mountains and Central Mackenzie Valley, Northwest Territories, Canada

The Hare Indian Formation (HIF) is a late Eifelian to Givetian organic-rich mudstone constituting the lower portion of the Horn River Group (HRG), which has been minimally scrutinized in the literature. This paper proposes depositional environments and a sequence stratigraphic framework for the HIF. Using composition data collected via energy-dispersive X-ray fluorescence, geochemical proxies inform detrital input, silica source, and paleoredox conditions. Cross-plots and chemostratigraphic profiles of detritally sourced Al, Ti, and K and redox-sensitive Mo and V inform depositional and stratigraphic constraints. Silica proportions vary, indicating that sediment was derived from detrital and biogenic sources. Al, Ti, and K distributions increase upwards, showing increased continentally sourced minerals. Redox-sensitive metals are highest in the Bluefish Member (BM), suggesting intermittent euxinia. Based on the presence of continental and pelagic sediments, the sedimentary environment is interpreted as proximal- to mid-shelf. These proxies guide systems tract interpretations. Si and redox-sensitive metal concentrations peak higher in the BM, accompanied by lowered concentrations of Al, Ti, and K, suggesting a maximum flooding surface. At the top of the Prohibition and Bell Creek members, redox-sensitive enrichments are lower with higher concentrations of Al, Ti, and K, suggesting a maximum regressive surface. Transgression occurred during the initial deposition of the BM, followed by regression for the remainder of the HIF. The sedimentology of the HIF can be difficult to decipher; the use of chemostratigraphy supports its geological history (including sedimentation trends and a local record of relative sea level) using methods that may be applied to other fine-grained successions.

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.

Copper recycling and redox evolution through progressive stages of oceanic subduction: Insights from the Izu-Bonin-Mariana forearc

Addition of subducted materials from the slab to the mantle wedge is often thought to elevate the oxygen fugacity of arc magmas and also to fertilize the mantle wedge in metals for subduction-related Cu-Au deposits. However, it remains controversial if slab-driven metal addition is effective and whether it occurs at all stages of subduction. The Izu-Bonin-Mariana (IBM) forearc preserves a full record of arc development from the initiation of subduction of the Pacific plate to mature arc volcanism. Here, we study the ore-forming and redox-sensitive metal Cu and its isotopes (δ65Cu) in the type-locality forearc basalts (FABs), boninites and high-Mg andesites from the IBM forearc, as well as MORBs from the East Pacific rise. Overall, the FABs display variably high Cu contents and MORB-like δ65Cu, consistent with limited Cu isotopic fractionation during partial melting of typical MORB mantle sources. Beginning with the boninites, the magma products from the IBM record strong slab signals, high water contents and high fO2 (ΔFMQ > +1). However, the oxidized, hydrous boninites and subsequent high-Mg andesites display low Cu contents, and mantle-like Cu/Sc ratios and δ65Cu, with no clear indication of the addition of Cu from the slab. The boninites show noticeably lower TiO2, Yb and Cu contents than the FABs, consistent with refractory mantle sources. Combined with available data from boninites and arc basalts worldwide, our results lead to a general conclusion that subducted oceanic slabs from initial to mature subduction contribute little Cu to the mantle wedge. This is further supported by the compiled Cu contents of arc peridotites. Adding Cu-poor, water-rich slab melts to the mantle wedge at any stage of Pacific oceanic subduction causes limited release of Cu, which remains trapped predominantly by reduced sulfides in the subducting slab. This likely reflects the overall reducing nature of slab materials added to the mantle wedge. However, we propose that the subsequent reaction of such reducing, hydrous slab melts with peridotites and flux melting produce oxidized primitive arc magmas deep in the mantle wedge. The reactive process promotes sulfide dissolution and metal release from the mantle wedge itself to oxidized arc magmas with high sulfur solubility, which explains the inheritance of mantle-like δ65Cu and the sulfide-undersaturated early-stage evolution in boninites and arc basalts. This study elucidates the role of subducted oceanic slabs in metal transfer and redox evolution and implies no significant Cu enrichment in the metasomatized mantle sources of magmatic arcs.

Redox-driven geochemical partitioning of metal(loid)s in the iron-rich anoxic sediments of a recently flooded lignite mine pit: Lake Medard, NW Czechia

We evaluated the geochemical partitioning of Fe, Mn, As, V, and REE in sediments of a recently flooded open-cast lignite mine to interpret their response to recently established anoxia, and minor variations in the redox potential of its ferruginous bottom water column. Results from a sequential extraction scheme targeting reactive Fe mineral phases are combined with an assessment of sediment enrichment factors and REE systematics. Across the sediment pile metal(loid)s are being released from pre-existing authigenic phases as minor shifts in redox potential induce elemental immobilisation through co-precipitation with Fe into mineral phases of diverse reactivity. REE systematics confirms that minor oscillations in the water column's redox state can trigger swift changes in the speciation of the redox-sensitive elements. The observed metal(loid)s enrichment in the sediments can either be considered an ecological risk, if a management scenario involving solely recreational purposes is conceived, but it can also be seen as a feasible source of critical metals (e.g., REE and V) that are amenable to sustainable, secondary metal recovery endeavours. By anticipating the fate of redox-sensitive metal(loid)s in our study site, we provide parameters useful to delineate management programmes of this and similar post-mining lakes.

Redox dynamics of later Cambrian oceans

A growing body of evidence suggests that the deep oceans during the early Paleozoic Era were widely oxygen deficient, despite evidence for increased marine oxygenation during the Neoproterozoic. However, the temporal and geographic extents and dynamics of reducing marine conditions within these oceans are not well understood. Here, we investigate marine redox history during the Drumian through the earliest Jiangshanian International Stages of the Cambrian Period, using concentrations of redox-sensitive metals (vanadium, uranium, and molybdenum), iron speciation, and Mo isotope stratigraphy of the Alum Shale Formation of Scandinavia. These data suggest a major perturbation occurred in trace metal cycling during the later Cambrian Period that was linked to a transient change in marine redox conditions coincident with the well-known Steptoean Positive Isotope Excursion or SPICE. The δ98Mo measurements of the Alum shale show systematic variations during the interval that contains the SPICE which are broadly consistent with a transient expansion of sulfidic, reducing marine environments — indicating a significant exacerbation of an already-common condition during the Cambrian Period. Additionally, iron speciation data record a local transition from predominantly anoxic, ferruginous (Fe+2 containing) to anoxic, euxinic (sulfide containing) water column conditions near the initiation of the SPICE. Trace metal abundances, however, appear to decline well before the start of the SPICE, suggesting an earlier initiation of the global expansion of reducing environments. More broadly, our data and modeling support the notion that significant portions of the oceans remained oxygen deficient throughout the later portion of the Cambrian, and that these oceans were also prone to transient intervals of more reducing conditions similar to the Oceanic Anoxic Events of the Mesozoic.

Benthic redox conditions and nutrient dynamics in the ca. 2.1 Ga Franceville sub-basin

The co-existence of motile macroorganisms and mat-building cyanobacteria in the Paleoproterozoic FB2 Member of the Franceville sub-basin, Gabon, points to the possible emergence of multi-trophic-level biological interaction by 2.1 billion years (Ga) ago. However, it is uncertain how these shallow-marine communities acquired and cycled nitrogen, a key, biolimiting nutrient required to sustain life at all trophic levels. Here, we use carbon and nitrogen isotope data from ancient microbial mats and host sediments, in combination with bottom-water redox proxies, to constrain biogeochemical processes operating in these settings. In this shallow-marine upwelling zone, iron speciation data and redox-sensitive metal concentrations point to oxygen-deficient bottom waters, which were episodically renewed with upwelling deep anoxic waters rich in nutrients and manganese. Organic carbon and nitrogen isotopes show little difference between the mat-related structures (MRS) and host sediments, suggesting either that similar metabolisms operated in benthic and planktonic microbial communities or that benthic carbon fixation contributed organic matter to the host sediments. The isotopic fractionation between organic and inorganic carbon is as large as 44‰, implying the involvement of multiple levels of heterotrophic carbon processing, linked to phototrophy, secondary productivity, and methanotrophy. Whole-rock nitrogen isotope values in the range of −3.5 to + 1.9‰ are consistent with microbial community nitrogen fixation in a nitrate-limited ecosystem. These data suggest that nitrogen fixation, common in photosynthetic microbial mats in modern environments, operated in benthic settings in the coastal area of the mid-Paleoproterozoic Franceville sub-basin. The upwelling of deep, anoxic waters invoked for deposition of the upper part of the underlying FB1 Member suggests that basin-scale redox structure modulated nitrate availability in this otherwise oxic, shallow-marine basin shelf environment.

Oxidative dissolution of TcO2 by Mn(III) minerals under anaerobic conditions: Implications on technetium-99 remediation

Manganese oxides are prolific minerals with strong oxidizing properties causing them to strongly influence the stability of any redox sensitive metal and metalloid reduced species in the environment. Their potential to oxidize reduced species is disproportionate to their concentration due to regeneration of Mn(III) mineral formed by abiotic processes or by microorganisms even under suboxic conditions. Tc-99 is produced during uranium fission in nuclear power generation and it is a risk driving environmental contaminant because it is found as highly soluble Tc(VII)O4− under oxidizing conditions. Current remediation efforts have focused on the immobilization of highly soluble Tc(VII) to sparingly soluble Tc(IV) under anoxic conditions. This publication investigates for the first time the impact of Mn(III) minerals on the oxidative dissolution of TcO2 under anaerobic conditions. α-Mn(III)2O3 and γ-Mn(III)OOH were used in batch experiments in contact with TcO2 under anaerobic conditions at different pH values and solid:liquid ratios and in the presence of cations common in groundwater. Both minerals incurred significant oxidation of TcO2 and consequently, release of pertechnetate in the aqueous phase at pH 6.5, and negligible oxidation of TcO2 at pH 8.0. For both minerals, increasing the solid:liquid ratio increased the extent of TcO2 oxidation. γ-Mn(III)OOH was insensitive to the presence of Ca2+ and Mn2+ ions, whereas α-Mn(III)2O3 sorbed Ca2+, which substantially reduced the mineral's oxidative capacity. These experimental findings demonstrate the liability of manganese minerals in oxidizing Tc(IV) insoluble phases to highly soluble Tc(VII) even in the absence of oxygen and highlights the need to be taken into account when it comes to remediation efforts for the abatement of Tc-99 in the environment.

Reconstructing Paleo‐oxygenation for the Last 54,000 Years in the Gulf of Alaska Using Cross‐validated Benthic Foraminiferal and Geochemical Records

AbstractHolocene and Pleistocene marine sediment records in the North Pacific record multiple dysoxic events proximal to continental margins and oxygen minimum zones (OMZs). High‐resolution paleoenvironmental studies in the Gulf of Alaska (GoA) were previously restricted to the last ∼17,000 years, limiting our knowledge of oxygenation in the high latitude North Pacific. Here we develop a ∼54,000‐year‐long record of co‐registered benthic foraminiferal assemblages and redox sensitive metal concentrations (Mo/Al and U/Al) at Site U1419 in the upper OMZ of GoA to reconstruct the history of OMZ extent and intensity at multi‐centennial resolution. Using multivariate analyses of total benthic foraminiferal assemblages, we develop quantitative dissolved oxygen estimates that are robust to differences in the benthic foraminiferal size fraction analyzed, replicate modern oxygenation patterns in the GoA, and are cross‐validated by redox sensitive metal concentrations. We identify dysoxic events in the early Holocene and in the Bølling‐Allerød (B/A), consistent with previous studies, as well as two dysoxic events during MIS 3 that are comparable in severity to the B/A event and lower in oxygen than the modern GoA OMZ. We further record short‐duration (<300 years) dysoxic events during glacial times similar to those recorded at more southern latitudes. Rates of oxygenation change can be abrupt with transitions exceeding 1 ml/L O2 in 100 years. Quantitative estimates of paleo‐oxygenation, such as those possible with benthic foraminiferal assemblages, are important for forecasting future oxygenation changes in OMZs and their potential impacts on the marine ecosystems.

Cycling of redox-sensitive trace metals in barrier island freshwater lenses

Freshwater lenses connect the terrestrial and marine realm via groundwater discharge at the edges of islands and serve as drinking water resources. We studied the redox-sensitive metals U, Mo, V, and Tl along the redox gradient of fresh groundwater lenses on Spiekeroog Island, northern Germany. Groundwater solute concentrations were linked to groundwater age and redox characteristics. We further quantified the contribution of precipitation, sea spray, and aquifer matrix to the groundwater metal concentrations and evaluated the sink and source function of the aquifer under oxic and reducing conditions.We found that biogeochemical processes altered the concentrations of the trace metals. In young, oxygen to nitrate reducing zones, the aquifer matrix represented the major metal source to the groundwater. For Tl, rain was an additional important (anthropogenic) source. Under manganese and iron oxide to sulfate reducing conditions, U and Tl were sensitive to redox dependent removal, whereas Mo and V were less affected by reductive precipitation/adsorption. In detail, 99% of dissolved Tl, 88% of U, 66% of Mo, and 44% of V were removed to the solid phase in comparison to values from less reducing zones. Large parts of the western freshwater lens on Spiekeroog were anoxic. For this reason, the delivery of aquifer derived metals to the ocean via fresh groundwater discharge appeared to be limited. Higher U, Mo, V, and Tl concentrations were observed in the presently developing young freshwater lens in the east of Spiekeroog Island. This suggests that less reducing groundwater lenses may be a source of these metals to the adjacent beach/coastal seawater. Especially for V, freshwater discharge from sandy coastal aquifers may be important, as groundwater concentrations exceeded seawater concentration under oxic as well as anoxic conditions. Regarding the suitability of the freshwater as drinking water, all measured trace metal concentrations were classified as uncritical.

The mid-Tournaisian (Early Carboniferous) anoxic event in the Laurussian shelf basin (Poland): An integrative approach

A wide range of proxies (organic and inorganic geochemistry, gamma-ray spectrometry, microfacies, framboidal pyrite analysis) were applied in order to decipher changes in depositional conditions during the mid-Tournaisian anoxic event, referred to as the Lower Alum Shale Event (LASE) in the Polish part of the Laurussia basin. The LASE part of the section, unlike older Tournaisian deposits, is characterised by high concentrations of isorenieratane, palaeorenieratane, and their diagenetic derivatives, the occurrence of 2-methyl-3-iso-butyl-maleimides, and high values of the sterane-to-hopane ratio. The predominance of tiny pyrite framboids, inorganic data such as low values of the Th/U and high values of the Corg/P and V/Cr ratios, and generally higher enrichments of redox-sensitive metals such as U, V, Mo, Pb, and Zn and depletion in Mn, all suggest anoxic/euxinic conditions on the sea-bottom. The results of our investigations, especially Co, Mn, Cd, and Mo distribution patterns, demonstrate that enhanced primary productivity in the water column was responsible for a high degree of organic carbon burial rather than preservation in restricted conditions on the seafloor. This elevated productivity may have been generated by the upwelling of nutrient-rich deep water, an increase in volcanic activity and volcanogenic oceanic fertilisation, or both. Volcanically driven mid-Tournaisian climatic changes (warming), as manifested by high Zr concentrations and Hg anomalies, may have been responsible for the deglaciation of Gondwana resulting in the global crenulata transgression.The global rise in sea level and high level of organic productivity were responsible for the development of anoxic conditions and the halt of carbonate production, as reflected by drastic changes of facies not only in the sections investigated, but in many other parts of the world as well.

Elemental and isotopic compositions of trench-slope black shales, Bohemian Massif, with implications for oceanic and atmospheric oxygenation in early Cambrian

This study examines a lower Cambrian pyrite-bearing black shale–graywacke succession in the Czech Republic interpreted as infill of a deep-marine, extensional trench-slope basin on top of an accretionary wedge that developed during subduction of an oceanic plate beneath the northern margin of Gondwana. The new U–Pb detrital zircon geochronology of the graywacke constrains the maximum age of deposition to 533 +5/−6 Ma, whereas the Re–Os dating of pyrite yielded a younger age of 507 ± 16 Ma. Distribution of major and trace elements and calculated enrichment factors (EF) indicate that this succession was deposited under changing redox conditions over a short time span and that the basin was presumably controlled by tectonic subsidence and varying supply of terrigenous arc-derived material. This depositional setting is reflected by largely variable EFs and δ98Mo and δ53Cr values. Euxinic conditions detected at the base of the sampled black shale interval are documented by the highest values of EFs of redox-sensitive metals (e.g., Mo, U, V, Ni, Co, As) and also the Corg/P (>1000) and DOPT (>0.7) values. Black shales show lower δ56Fe values due to the excess of authigenic pyrite-Fe with a mean δ56Fe value of −0.02‰ over detrital Fe in graywacke with a mean δ56Fe value of +0.25‰. However, the Fe isotopic signatures of the black shales are not consistent with iron shuttling, mixing of authigenic and detrital sources, or hydrothermal metal enrichment. Instead, they most likely resulted from partial oxidation of pyrite through the syndepositional oxidizing hydrothermal fluids (Si–Ba enrichment), which resulted in precipitation of isotopically heavy Fe-oxyhydroxides. We propose that our maximal recorded δ98Mo value (+ 0.98‰) might represent the best estimate for the ancient local seawater Mo composition at around 533 Ma and argues against deep-ocean oxygenation in the early Cambrian. On the other hand, the Se/Co ratios of synsedimentary pyrite indicate a mean atmosphere O2 value of ~27%.

Geochemical Controls on Release and Speciation of Fe(II) and Mn(II) From Hyporheic Sediments of East River, Colorado

Hyporheic zones act as critical ecological links between terrestrial and aquatic systems where redox-sensitive metals of iron (Fe) and manganese (Mn) significantly impact nutrient cycling and water quality. However, the geochemical controls on the release and speciation of Fe(II) and Mn(II) in these biogeochemical hotspots are still poorly understood. Here we conducted batch incubation experiments and analyzed Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy data using sediment samples from a hyporheic zone of the East River floodplain in Colorado to understand the production, release and speciation of Fe(II) and Mn(II) in groundwater. Our results indicate that the production and release of Fe(II) and Mn(II) vary with sediment reducing conditions and subsurface positions, and the rates were determined either by a zero- or first-order rate equation. The sediments with higher Fe(II) production did not necessarily result in higher release of dissolved Fe(II), and ≥97% Fe(II) is accumulated in solid phase. We found that the majority of Fe(II) exists as siderite (FeCO3), Fe(II)-natural organic matter (NOM) complexes and ferrosmectite, and the equilibrium concentrations of dissolved Fe(II) are controlled primarily by siderite solubility, and enhanced greatly by formation of strong Fe(II)-NOM complexes as dominant aqueous Fe(II) species. By contract, dissolved Mn(II) increases slowly and linearly, and an equilibrium concentration was not reached during the incubation period, and the roles of rhodochrosite (MnCO3) and Mn(II)-NOM complexes are insignificant. Furthermore, we reviewed and calibrated the literature reported binding constants (log K) of Fe(II)-NOM complexes which successfully predicted our experimental data. This work reveals that siderite and dissolved NOM are the controlling phases in release and speciation of dissolved Fe(II), and the finding is expected to be applicable in many hyporheic zones and subsurface environments with similar geochemical conditions.