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The ratio of atmosphere-derived¹⁰Be to continent-derived⁹Be in marine sediments has been used to probe the long-term relationship between continental denudation and climate. However, its application is complicated by uncertainty in⁹Be transfer through the land-ocean interface. The riverine dissolved load alone is insufficient to close the marine⁹Be budget, largely due to substantial removal of riverine⁹Be 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 the⁹Be 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 reconstruct 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 oxygen 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 oxidised 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 sediments 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 estimated 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 productivity. 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 conditions 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 sedimentological conditions. Thus, careful assessment of the impact of organic matter on Cr is required for reconstructions of redox conditions with sedimentary records. 

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/PO 4 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 × 10 7  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 114/110 Cd NIST-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.