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  1. The distribution of dissolved iodine in seawater is sensitive to multiple biogeochemical cycles, including those of nitrogen and oxygen. The iodine-to-calcium ratio (I/Ca) of marine carbonates, such as bulk carbonate or foraminifera, has emerged as a potential proxy for changes in past seawater oxygenation. However, the utility of the I/Ca proxy in deep-sea corals, natural archives of seawater chemistry with wide spatial coverage and radiometric dating potential, remains unexplored. Here, we present the first I/Ca data obtained from modern deep-sea corals, specifically scleractinian and bamboo corals, collected from the Atlantic, Eastern Pacific, and Southern Oceans, encompassing a wide range of seawater oxygen concentrations (10–280 μmol/kg). In contrast to thermodynamic predictions, we observe higher I/Ca ratios in aragonitic corals (scleractinian) compared to calcitic corals (bamboo). This observation suggests a strong biological control during iodate incorporation into deep-sea coral skeletons. For the majority of scleractinian corals, I/Ca exhibits a covariation with local seawater iodate concentrations, which is closely related to seawater oxygen content. Scleractinian corals also exhibit notably lower I/Ca below a seawater oxygen threshold of approximately 160 μmol/kg. In contrast, no significant differences in I/Ca are found among bamboo corals across the range of oxygen concentrations encountered (15–240 μmol/kg). In the North Atlantic, several hydrographic factors, such as temperature and/or salinity, may additionally affect coral I/Ca. Our results highlight the potential of I/Ca ratios in deep-sea scleractinian corals to serve as an indicator of past seawater iodate concentrations, providing valuable insights into historical seawater oxygen levels.

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    Free, publicly-accessible full text available November 7, 2024
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  3. Abstract

    The flux of terrestrial material from the continents to the oceans links the lithosphere, hydrosphere, and biosphere through physical and biogeochemical processes, with important implications for Earth's climate. Quantitative estimates of terrigenous fluxes from sources such as rivers, aeolian dust, and resuspended shelf sediments are required to understand how the processes delivering terrigenous material respond to and are influenced by climate. We compile thorium‐230 normalized232Th flux records in the tropical Atlantic to provide an improved understanding of aeolian fluxes since the Last Glacial Maximum (LGM). By identifying and isolating sites dominated by aeolian terrigenous inputs, we show that there was a persistent meridional gradient in dust fluxes in the eastern equatorial Atlantic at the LGM, arguing against a large southward shift of the intertropical convergence zone during LGM boreal winter. The ratio of LGM to late‐Holocene232Th fluxes highlights a meridional difference in the magnitude of variations in dust deposition, with sites <10°N showing larger changes over time. This supports an interpretation of increased trade wind strength at the LGM, potentially combined with differential changes in soil moisture and reductions in higher altitude summer winds. Our results also highlight the persistent importance of continental margins as sources of high terrigenous flux to the open ocean. This is especially evident in the western tropical Atlantic, where study locations reveal the primary influence of the South American continent up to >700 km away, characterized by232Th fluxes approximately twice as large as aeolian‐dominated sites in the east.

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  4. Abstract

    The Antarctic Cold Reversal (ACR; 14.7 to 13 thousand years ago; ka) phase of the last deglaciation saw a pause in the rise of atmospheric CO2and Antarctic temperature, that contrasted with warming in the North. A reexpansion of sea ice and a northward shift in the position of the westerly winds in the Southern Ocean are well‐documented, but the response of deep‐sea biota and the primary drivers of habitat viability remain unclear. Here, we present a new perspective on ecological changes in the deglacial Southern Ocean, including multifaunal benthic assemblage (foraminifera and cold‐water corals) and coral geochemical data (Ba/Ca and δ11B) from the Drake Passage. Our records show that, during the ACR, peak abundances of thick‐walled benthic foraminiferaUvigerina bifurcataand corals are observed at shallow depths in the sub‐Antarctic (∼300 m), while coral populations at greater depths and further south diminished. Our ecological and geochemical data indicate that habitat shifts were dictated by (a) a northward migration of food supply (primary production) into the sub‐Antarctic Zone and (b) poorly oxygenated seawater at depth during this Antarctic cooling interval.

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  5. Abstract

    230Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of230Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of230Th as a constant flux proxy. Anomalous230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).

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