Since the discovery of cable bacteria more than a decade ago, these multicellular, filamentous sulfur-oxidizing bacteria have been found in a range of sedimentary environments. However, their abundance, diversity, and activity in continental margin sediments overlain by oxygen-deficient waters at water depths of > 100 m remain poorly known. Here we address this by studying five basins along the coasts of California and Mexico. All sediments are organic carbon rich (2.5 wt %-7.5 wt %) and characterized by active iron and sulfur cycling. Nitrate is present in the bottom water at all sites. Results of fluorescence in situ hybridization (FISH) indicate a low areal abundance of cable bacteria (0.2 to 19 m cm-2) in sediments of the hypoxic San Clemente, Catalina, and San Pedro basins and the anoxic San Blas basin. In the anoxic Soledad basin, in contrast, we found abundant cable bacteria near the sediment surface (129 m cm-2). DNA amplicon sequencing detected cable bacteria reads in sediments of the hypoxic San Pedro basin and the anoxic Soledad and San Blas basins. Phylogenetic analysis indicated that the diversity of the amplicon sequence variants (ASVs) was spread across the Candidatus Electrothrix lineage, including multiple ASVs closely related to Electrothrix gigas, a recently discovered species of giant cable bacteria. Additionally, multiple sequences retrieved from the Soledad and San Blas basins revealed affiliation with a clade sister to Electrothrix, hypothesized as a novel genus, suggesting possible relic or novel adaptations of cable bacteria to these anoxic and nitrogenous environments. The areal abundance of cable bacteria was negatively related to sediment Fe / S ratios, suggesting a control by sulfide availability. However, free sulfide in the porewater was only detected at the anoxic Soledad site. Micro-profiling of pH and electric potential points toward a lack of cable bacteria activity at the time of sampling, possibly due to a limitation by a suitable electron donor and/or acceptor. Periodically enhanced organic matter input and associated sulfate reduction and/or inflows of oxic water could alleviate the deficiency, creating the observed niche for diverse cable bacteria.

Iron (Fe) availability impacts marine primary productivity, potentially influencing the efficiency of the biological carbon pump. Stable Fe isotope analysis has emerged as a tool to understand how Fe is sourced and cycled in the water column; however its application to sediment records is complicated by overlapping isotope signatures of different sources and uncertainties in establishing chronologies. To overcome these challenges, we integrate Fe and osmium isotope measurements with multi-element geochemical analysis and statistical modeling. We apply this approach to reconstruct the history of Fe delivery to the South Pacific from three pelagic clay sequences spanning 93 million years. Our analysis reveals five principal Fe sources-dust, distal background, two distinct hydrothermal inputs, and a magnesium-rich volcanic ash. Initially, hydrothermal inputs dominated Fe deposition, but as the sites migrated away from their respective mid-ocean ridges, other sources became prominent. Notably, from 66 to 40 million years ago (Ma), distal background Fe was the primary source before a shift to increasing dust dominance around 30 Ma. This transition implies that Fe in South Pacific seawater has been dust-dominated since approximate to 30 Ma, despite extremely low dust deposition rates today. We speculate that the shift to episodic and low Fe fluxes in the South Pacific and Southern Ocean over the Cenozoic helped shape an ecological niche that favored phytoplankton that adapted to these conditions, such as diatoms. Our analysis highlights how Fe delivery to the ocean is driven by large-scale tectonic and climatic shifts, while also influencing climate through its integral role in marine phytoplankton and Earth's biogeochemical cycles.

Thallium (Tl) isotopic values (epsilon 205Tl) appear to track changes in marine manganese oxide deposition, with these isotope signatures having been utilized as a proxy for rapid oceanic seafloor (de-)oxygenation events. With a residence time longer than ocean mixing time and the ability to track the deposition of manganese oxides, epsilon 205Tl may effectively record the earliest global transitions in the extent of ocean oxygenation. However, some uncertainty remains for the minor Tl sinks in degree of fractionation from seawater values, if any, with the limited data currently available suggesting at least a 6 epsilon unit range in fractionation from seawater values in low oxygen environments. This study provides Tl data for sediment cores from a range of low-oxygen marine environments. With these data, we identify potential processes that impact the range of Tl isotope variations within the sediments, which are not all due to local Mn oxide cycling. Previous work indicates that euxinic (anoxic and sulfidic water column) conditions and early diagenetic pyrite formed under consistently anoxic sediments record seawater values with no (or not measurable) fractionation during absorption to pyrite. Our new data provide downcore confirmation. Meanwhile, only limited data from 'suboxic' environments (those with low oxygen but likely not permanently anoxic conditions) has been analyzed. Thus, isotopic data for suboxic systems is needed to refine the current mass balance. Several sites off the California, Mexico, and Peru coasts with a range of redox states from oxic to perennially anoxic were selected to allow for a comparison across a range of open ocean bottom water conditions. The locations with oxic sediments tend to document more positive values compared to seawater, as expected due to local manganese oxide incorporation. Sediments from more 'suboxic' (manganous to ferruginous) sites tend to have invariable downcore geochemical signatures that are between marine inputs (-2) and modern seawater (-6) values, indicating a mixing of epsilon 205Tl signatures from different authigenic phases; however, these are not primarily due to the incorporation of Mn oxides as the concentrations are low and uncorrelated. The anoxic sites record Tl isotope compositions near seawater values, confirming that early diagenetically formed pyrite (and precursor minerals) record seawater epsilon 205Tl signatures under permanent anoxia. Importantly, these permanently anoxic localities have minor Mn contents, which suggest no local Mn oxide Tl isotope signatures. Therefore, unlike the anoxic sediments, the 'suboxic' sediments have a Tl isotope value that is slightly offset from seawater without significant Mn oxide deposition, thus suggesting there could be a fractionation for this process. Our observations provide improved constraints on the Tl isotope system, especially on a poorly constrained aspect of the mass balance, which will be important for deep-time applications.

Wind-borne dust supply of iron (Fe) to the oceans plays a crucial role in Earth’s biogeochemical cycles. Iron, a limiting micronutrient for phytoplankton growth, is fundamental in regulating ocean primary productivity and in turn the global carbon cycle. The flux of bioavailable Fe to the open ocean affects oscillations in atmospheric CO2 due to its control on inorganic carbon fixation into organic matter that is eventually exported to the sediments. However, the nature of dust-delivered Fe to the ocean and controls on its bioavailability remain poorly constrained. To evaluate the supply of wind-borne bioavailable Fe and its potential impact on Fe-based climate feedbacks over the last 120,000 years, we examine sediment profiles from four localities that define a proximal to distal transect relative to Saharan dust inputs. Bulk δ56Fe isotope compositions (average = -0.05‰) and FeT/Al ratios suggest crustal values, thus pointing to a dominant dust origin for the sediments at all four sites. We observed no variability in grain size distribution or in bioavailable Fe supply at individual sites as a function of glacial-versus-interglacial deposition. Importantly, there is no correlation between sediment grain size and Fe bioavailability. Spatial trends do, however, suggest increasing Fe bioavailability with increasing distance of atmospheric transport, and our sediments also indicate the loss of this Fe and thus potential bioavailability utilization once deposited in the ocean. Our study underscores the significance of Fe dynamics in oceanic environments using refined speciation techniques to elucidate patterns in Fe reactivity. Such insights are crucial for understanding nutrient availability and productivity in various ocean regions, including the Southern Ocean, where wind-delivered Fe may play a pivotal role. It is expected that dust delivery on glacial-interglacial timescales would be more pronounced in these high-latitude regions. Our findings suggest that studies linking Fe availability to marine productivity should benefit significantly from refined Fe speciation approaches, which provide insights into the patterns and controls on Fe reactivity, including atmospheric processing. These insights are essential for understanding the impacts on primary production and thus carbon cycling in the oceans and consequences for the atmosphere.

In the marine environment, nickel distributions are linked to the biogeochemical cycling of both organic matter and manganese. Thus, Ni and its isotopes have the potential to be used to reconstruct changes of bioproductivity and oxygenation in paleoenvironments. However, their utility relies on an understanding of the behaviour of Ni in modern marine environments. Here we investigate the distribution of Ni under a range of oxidation-reduction conditions, with organic carbon (Corg) burial rates that range from approximately 0.08 to 8.4 mmol m−2 d−1, none of the sites show deep (> 5 cm) penetration of dissolved oxygen into the sediment. We present Ni concentrations for sediments and pore fluids, as well as Ni isotope compositions from pore fluids, from the California and Mexico continental margins. The sites are sufficiently reducing that solid-phase Mn concentrations are typically <1000 ppm, but two of the stations, where oxygenated bottom water bathes the underlying sediment, have near-surface sediment Mn concentrations that reach up to 2.3%.Dissolved Ni is lower in pore fluids for stations with high solid phase Corg compared to those stations with oxygenated bottom water and high solid phase Mn concentrations. The calculated benthic fluxes of Ni at all stations are small relative to their burial rates, which implies a high Ni burial efficiency (> 85%). Pore fluid δ60Ni values range from approximately −0.39 to +2.36 ‰, with the higher δ60Ni values occurring at the Corg-rich station, and the lower values at the Mn-rich stations. At the station with the highest Corg content, the distribution of solid-phase Ni as a function of Corg content is consistent with the strong association of Ni with Corg seen at open-ocean upwelling margins globally. In contrast, the other stations investigated here clearly do not show such an association. Our data offer support for the notion that Ni accumulation within sediments is linked to organic matter accumulation in regions of high photic zone productivity and where the sediments are reducing. However, at the sites in our study where Mn oxidation-reduction reactions occur near the sediment-water boundary, any simple relationship between Ni and Corg burial is obfuscated. With a sufficiently deep oxygen penetration depth and the formation of a solid-phase Mn oxide layer, Ni burial within the sediment can be highly efficient. Importantly, Ni is well preserved in sediments deposited under the full range of conditions studied here. This observation of high Ni preservation is an important constraint if Ni is to be used as a proxy to reconstruct paleoenvironmental conditions.

Iron isotopes are a valuable tool for evaluating processes that control Fe redox cycling in modern and ancient environmental settings. However, robust evaluation of Fe isotope compositions in natural samples requires that fractionations associated with key (bio)geochemical reactions are well-defined. The reductive dissolution of Fe (oxyhydr)oxide minerals mediated by dissolved sulfide exerts a major influence on solid phase Fe mineralogy and dissolved porewater Fe profiles during early diagenesis of organic-rich sediments, but to date, no studies have investigated Fe isotope fractionations during this process. Here, we report the results of laboratory sulfidation experiments, examining apparent Fe isotope fractionations for a variety of Fe (oxyhydr)oxide minerals. The iron isotope compositions of reaction products were determined for both the reduction-dominated and dissolution-dominated steps of the reaction. The reductive step for lepidocrocite and hematite produced Fe(II) that was up to 0.25 parts per thousand heavier than the bulk starting mineral. By contrast, the reduction of ferrihydrite produced isotopically light Fe(II), with isotope compositions -0.1 to -0.6 parts per thousand lower than the initial mineral. Consistent with previous studies of the reductive dissolution of Fe (oxyhydr)oxide minerals via abiological and biological pathways, the lighter isotope was preferentially released from the mineral surface during the dissolution phase for all minerals, with dissolved Fe2+ isotope compositions up to similar to 2.0 parts per thousand lower than the surface bound Fe(II). The magnitude of isotopic fractionation during both of these steps is directly related to rates of reaction, and is thus controlled by factors such as sulfide concentration, mineral concentration, crystal structure, surface area and pH. Our data demonstrate that dissolved Fe2+ with delta Fe-56 compositions approaching -1.0 parts per thousand is readily generated during the overall reaction, suggesting that sulfide-promoted reductive dissolution of Fe (oxyhydr)oxide minerals may contribute significantly to the generation of light Fe isotope compositions in anoxic settings.

The ratio of atmosphere-derived 10Be to continent-derived 9Be in marine sediments has been used to probe the long-term relationship between continental denudation and climate. However, its application is complicated by uncertainty in 9Be transfer through the land-ocean interface. The riverine dissolved load alone is insufficient to close the marine 9Be budget, largely due to substantial removal of riverine 9Be to continental margin sediments. We focus on the ultimate fate of this latter Be. We present sediment pore-water Be profiles from diverse continental margin environments to quantify the diagenetic Be release to the ocean. Our results suggest that pore-water Be cycling is mainly controlled by particulate supply and Mn-Fe cycling, leading to higher benthic fluxes on shelves. Benthic fluxes may help close the 9Be budget and are at least comparable to, or higher (~2-fold) than, the riverine dissolved input. These observations demand a revised model framework, which considers the potentially dominant benthic source, to robustly interpret marine Be isotopic records.

In the marine sediment record, concentrations and isotope ratios of chromium (Cr) can be used to recon-struct ocean biogeochemical conditions. These reconstructions rely on a detailed understanding of the chemical pathways that Cr undergoes as it is transferred from the water column to the sediment record. We examined Cr concentrations in marine pore fluids and sediments from six continental margin sites, which can be grouped into two basic environments: (1) sites where sediments are oxygenated and rich in solid phase Mn (herein termed oxic), and (2) sites where sediments are organic C (Corg)-rich and oxy-gen is depleted (anoxic). We found Cr concentrations to be lower (maximum of 12 nM in pore fluids and 124 ppm sediment solid phase) at oxic sites compared with anoxic sites (maximum of 77 nM and 184 ppm). Our findings confirm previously published interpretations of dissolved Cr in pore fluids (Brumsack and Gieskes, 1983; Shaw et al., 1990). In oxic surface sediments, particulate Cr(III) can be oxi-dised by Mn oxides, which leads to elevated concentrations of dissolved Cr co-occurring at the same depth as elevated Mn concentrations in the sediment. Under these oxidising conditions, down-core sed-iments contain relatively low solid-phase Cr concentrations. In oxic sediments, Cr speciation reveals that most of the pore fluid Cr is in the Cr(VI) state. At the site where Mn oxide-rich sediments rest below an oxic water column, oxidative loss of Cr from the sediment to the bottom water leads to the lowest esti-mated Cr burial efficiency of the sites examined here. Under anoxic Corg-rich conditions, both pore fluids and sediment solid phases contain high Cr concentrations, with 40-80% of dissolved pore fluid Cr present as Cr(III). This enrichment of Cr appears to be tightly linked to the presence of high total organic carbon (TOC) content and scavenging of Cr by (organic) particles in the water column. Combined, these data highlight the strong dependence of Cr on both sedimentary redox conditions as well as biological produc-tivity. Based on the data from modern continental margin sediments, we propose that Cr concentrations and isotope compositions of the authigenic sediment fraction may record a combination of redox condi-tions and biological productivity in the water column. If confirmed by Cr isotope analyses, these findings will add support for the notion that Cr may serve as a proxy for ocean biological and chemical sedimen-tological conditions. Thus, careful assessment of the impact of organic matter on Cr is required for recon-structions of redox conditions with sedimentary records. (c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

The Amundsen Sea in the Pacific sector of West Antarctica receives meltwater from the fastest retreating Antarctic glaciers, and its coastal polynyas host the highest primary productivity per unit area observed on the Antarctic continental shelf. Polynya productivity provides the base for a robust, diverse ecosystem and is controlled primarily by light and the availability of the micronutrient iron (Fe). While the sources of Fe in the region are not yet certain, Fe could be transported within modified Circumpolar Deep Water (mCDW) that intrudes onto the retrograde shelf and into ice shelf cavities, where it gains buoyancy through the addition of glacial meltwater and is injected into the upper water column when it exits the cavity. Thus, fluxes of dissolved Fe from the seafloor into in-flowing mCDW may ultimately be a source of Fe to the euphotic zone in the Amundsen Sea. To investigate the surface sediment biogeochemistry and the potential for a significant benthic flux of Fe to the waters on the Amundsen Sea shelf, sediment cores were collected at two sites close to the calving fronts of the Pine Island and Thwaites Glacier ice shelves. Pore water was analyzed for trace element content, and sediment was analyzed for physical and chemical properties including organic carbon and trace elements. Using a novel approach based on hypothesized Fe speciation and colloidal particle radius, theoretical Fe fluxes were calculated from pore water gradients and porosity. The fluxes reveal a spatially variable Fe input to the lower water column that could ultimately fertilize primary productivity. Supported by geochemical and physical evidence, we conclude that submarine weathering of volcanic glass grains observed and quantified in seabed sediments at the Pine Island site drives nonreductive Fe fluxes that are 100-fold higher than at the Thwaites site. This study highlights the need for further investigations of benthic-pelagic coupling in the Amundsen Sea region, which will likely be impacted in coming decades by accelerating glacial melting.

Sedimentary rocks and minerals formed during the final two-hundred million years of the Archean Eon (2.7 to 2.5 billion years ago, or Ga) are more depleted in Fe-56 than at any other time in Earth's past. Three hypotheses are proposed to explain these Fe-56 depletions: (1) a very negative late-Archean seawater delta Fe-56 value, (2) "shuttling" of isotopically light Fe across the chemocline in redox-stratified settings, and (3) pyrite formation in an Fe(II)-rich ocean. Each of these scenarios has different implications for the initial oxidation of Earth's surface, the climax of which - the Great Oxidation Event - immediately postdates the appearance of these 56Fe depletions in the rock record. To help inform this debate, we measured the Fe isotope ratios of 120 shale and pyrite samples from Western Australia (Mt. McRae Shale and Jeerinah Formation) and South Africa (Klein Naute Formation) deposited between-2.65 Ga and-2.50 Ga. As in previous studies, we also find very strong sedimentary Fe-56 depletions, to as low as delta Fe-56 =-2.06 +/- 0.08% in bulk shales and delta Fe-56 =-2.31 +/- 0.08% in pyrite. Some, but not all, of the severest (56F)e depletions appear alongside evidence of an Fe shuttle and local pyrite formation. These processes need not be mutually exclusive, and some combination of them likely played a partial, probably faciliatory role in driving some strong 56Fe depletions in our dataset. Most interestingly, and with little exception, the severest Fe-56 depletions appear in samples deposited farther from shore under H2S-rich and anoxic ("euxinic") conditions. We find it difficult to explain this connection without invoking the persistent presence of a very negative global seawater delta Fe-56 value during the latest Archean, one that was most consistently captured in sediments formed in distal euxinic settings. In order to impart this isotopic effect on seawater, the global seawater Fe(II) reservoir needed to have been partially oxidized during at least the final few hundreds of millions of years leading up to the Great Oxidation Event. Our new data add support to the idea that Earth's initial oxidation was a long and protracted process rather than a rapid event. (C) 2021 Elsevier Ltd. All rights reserved.

Uranium isotopes (delta U-238 values) in ancient sedimentary rocks (shales, carbonate rocks) are widely used as a tool to reconstruct paleo-redox conditions, but the behaviour of U isotopes under modern non-sulfidic anoxic vs. oxic conditions remains poorly constrained. We present U concentration and isotope data for modern sediments from the Peruvian margin, a highly productive open ocean environment with a range of redox conditions. To investigate U in different host fractions of the sediment (reactive, silicate, and HNO3-soluble fraction), we conducted a series of sequential extractions. Detrital-corrected authigenic U isotope compositions (delta U-238(auth)) in sediments deposited beneath an oxic water column show little deviation from the dissolved seawater U source, while anoxically deposited sediments have delta U-238(auth) values that are up to 0.4 parts per thousand heavier compared to seawater delta U-238. Under anoxic, non-euxinic conditions, the U isotope offset between sediment and seawater is larger compared with oxic, but significantly smaller when compared with euxinic conditions from the literature. The results from sequential extractions show that the reactive sediment fraction records more pronounced differences in delta U-238(reactive) than delta U-238(auth) values depending on the oxidation state of the overlying water column. Furthermore, we found a strong correlation between total organic carbon (TOC) and both U concentrations (U-auth) and delta U-238(auth) values (R-2 = 0.70 and 0.94, respectively) at the persistently anoxic site that we examined. These correlations can be caused by several processes including U isotope fractionation during microbially-mediated U reduction at the sediment-water interface (diffusive U input), during sorption onto and/or incorporation into organic matter in the water column (particulate U input) and diagenetic redistribution of U, or a combination of these processes. Our data show that several factors can influence delta U-238 values including oxidation state of U, the presence or absence of hydrogen sulfide and organic matter. These findings add new constraints to the degree of U isotope fractionation associated with U incorporation into sediments in different low-oxygen environments, thus aiding in interpretation of ancient paleo-redox conditions from U isotope data.

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 (U-238/U-235, commonly denoted as delta U-238), are a promising but still developing approach to constrain global changes in redox conditions. Our current understanding of the controls on delta U-238 variability and associated isotopic fractionations is limited, complicating the interpretation of delta U-238 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 delta U-238. Distinct patterns in U concentrations and delta U-238 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 delta U-238 which can partially explain the overall weak negative correlation between [U] and delta U-238 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 delta U-238. 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 delta U-238 records and specifically in using delta U-238 to distinguish between euxinic and oxic redox states without considering other oxygen-depleted environments as part of a continuum of settings that can impact delta U-238 if spatially widespread. This study also highlights the utility of using different lithologies to evaluate global versus local controls on delta U-238 records as well as multi-proxy approaches to constrain ancient redox landscapes. (C) 2021 Elsevier Ltd. All rights reserved.

The uranium (U) content, and more recently, the ratio between 238U and 235U in black shales are commonly applied as a proxy to determine redox conditions and infer organic-richness. Uranium contents typically display a linear relationship with total organic carbon (TOC) in shales. This relationship is due to the processes and mechanisms responsible for the incorporation of U into the sediment during the deposition and remineralization of organic matter. This U/TOC relationship can vary, however, and some shales display uncharacteristically low U content despite having high TOC content, while others show large enrichments of U relative to TOC. Here we examine the U to TOC ratios and U-isotope compositions of three Upper Devonian-Lower Mississippian shales: the Woodford Shale, the Cleveland Shale, and the Bakken Shale, with two study sites in Oklahoma, one site in eastern Kentucky, and three sites in eastern Montana and western North Dakota, respectively. The U/TOC ratios of each shale are distinct from one another exhibiting average ratios ranging from 3 in the Cleveland Shale, to over 10 in the Bakken Shale. The distinct geochemical composition of the three shales suggests that, although lithologically similar, each study site represents a markedly different and dynamic depositional environment. The low average U/TOC (~3) along with the relatively high δ238U values (~0.03‰) of the Cleveland Shale core suggests deposition along the basin margin under normal marine conditions with periods of reduced bottom water oxygenation, likely due to fluctuations in the location of the pycnocline. The Woodford Shale on the other hand, shows higher U/TOC ratios (~4, George core, ~9, Poe core) and δ238U (~0.02‰ average, George core, ~0.06‰ average, Poe core), which suggests an unrestricted setting with intermittent euxinic conditions. In contrast, high U/TOC ratios (2–15), and very high δ238U values (up to 0.55‰) in the Bakken Shale cores indicate intense metal draw-down into sediments under sulfidic waters. The results show that when the U/TOC ratios and U-isotopic compositions of each studied shale are compared to modern anoxic basins and upwelling areas, it allows for an enhanced understanding of the paleoenvironmental conditions such as basin restriction and redox state of waters within the Late Devonian epicontinental seas of North America.

The Amundsen Sea (AS) hosts the highest primary productivity per unit area observed on the Antarctic continental shelf, concentrated in coastal polynyas near the outlets of the fastest melting glaciers on Antarctica. These polynyas are highly dynamic carbon sinks where production is controlled by light and the availability of iron (Fe). Previous studies indicate that the majority of the Fe in the polynya surface waters is delivered through upward transport of modified Circumpolar Deep Water that invades the deep shelf, acquires additional Fe, and enters the ice shelf cavities, where it is injected into the upper water column by the addition of meltwater buoyancy (the “meltwater pump”). While an Fe source from shelf sediments is suggested by observations and modeling to exceed a direct glacial meltwater source, the distribution and magnitude of the purported benthic Fe flux is unknown. To investigate the surface sediment biogeochemistry and the potential for a dominant benthic flux of Fe to the AS, sediment cores were collected at two sites close to the calving face of the Pine Island and Thwaites ice shelves in late summer 2020, and subsampled for pore water and solid sediment. Pore water was analyzed for Fe and other trace elements. Solid sediment was analyzed for porosity, along with grain size, organic carbon content, and total trace metal contents. Dissolved Fe fluxes were calculated from porewater gradients and porosity, revealing a spatially variable but potentially important input to the lower water column that could ultimately fertilize primary productivity in the polynyas via the meltwater pump. High flux and burial of labile organic carbon in the rapidly deposited sediments may increase benthic Fe reduction, creating a positive feedback loop between primary production and sediment Fe release. The

Chromium (Cr) is a redox sensitive trace metal and its variations in concentrations and isotope compositions ( δ 53 Cr) in sedimentary rocks have been used to reconstruct past ocean chemical conditions. However, little is known about the transformations Cr is undergoing during early diagenesis and the mechanism by which it is incorporated into the marine sedimentary record. Here, we investigate Cr concentrations and isotope compositions in pore fluids and sediments to examine the effect of early diagenesis on Cr burial. Our study sites target the California and Mexico continental margins, which span a range of marine and sedimentary chemical conditions. In particular, these sites exhibit a range in bottom water oxygen concentrations from <0.1 to 130 µM, and in organic carbon burial rates from 0.08 to 8.4 mmol m -2 d -1 . Additionally, two stations show high solid Mn concentrations (up to 2.3 %) near the sediment surface. Under sulfidic, organic-rich sedimentary conditions, dissolved Cr in pore fluids increases with depth (>50 nM), which we interpret as Cr release from decomposing organic matter. In contrast, under oxic, organic-poor conditions, with low to high Mn concentrations, dissolved Cr typically remains below 10 nM. Hence, reductive dissolution of Mn oxides does not appear to release dissolved Cr. Preliminary d53Cr values in sediment extracts (1 M HCl) are highest (nearly 1 ‰) in reducing organic-rich sediments, whereas the δ 53 Cr values at our other study sites are lower, with an average value of approximately 0.2 ‰. Collectively, our data suggest that organic matter burial and potentially the presence of Mn oxides play key roles in setting the Cr isotope sedimentary record.

Cadmium is a trace metal of interest in the ocean partly because its concentration mimics that of phosphate. However, deviations from the global mean dissolved Cd/PO4 relationship are present in oxygen deficient zones, where Cd is depleted relative to phosphate. This decoupling has been suggested to result from cadmium sulphide (CdS) precipitation in reducing microenvironments within sinking organic matter. We present Cd concentrations and Cd isotope compositions in organic-rich sediments deposited at several upwelling sites along the northeast Pacific continental margin. These sediments all have enriched Cd concentrations relative to crustal material. We calculate a net accumulation rate of Cd in margin settings of between 2.6 to 12.0 × 107 mol/yr, higher than previous estimates, but at the low end of a recently published estimate for the magnitude of the marine sink due to water column CdS precipitation. Cadmium in organic-rich sediments is isotopically light (δ 114/110CdNIST-3108 = +0.02 ± 0.14‰, n = 26; 2 SD) compared to deep seawater (+0.3 ± 0.1‰). However, isotope fractionation during diagenesis in continental margin settings appears to be small. Therefore, the light Cd isotope composition of organic-rich sediments is likely to reflect an isotopically light source of Cd. Non-quantitative biological uptake of light Cd by phytoplankton is one possible means of supplying light Cd to the sediment, which would imply that Cd isotopes could be used as a tracer of past ocean productivity. However, water column CdS precipitation is also predicted to preferentially sequester light Cd isotopes from the water column, which could obfuscate Cd as a tracer. We also observe notably light Cd isotope compositions associated with elevated solid phase Fe concentrations, suggesting that scavenging of Cd by Fe oxide phases may contribute to the light Cd isotope composition of sediments. These multiple possible sources of isotopically light Cd to sediments, along with evidence for complex particle cycling of Cd in the water column, bring into question the straightforward application of Cd isotopes as a paleoproductivity proxy.