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Particulate cadmium stable isotopes in the subarctic northeast Pacific reveal dynamic Cd cycling and a new isotopically light Cd sink

Abstract : The nutrient-type distribution of dissolved cadmium concentrations (dCd) reflects a biological control in the global ocean, with uptake of dissolved Cd into biogenic particles in surface waters and regeneration of particulate Cd at depth. Depth profiles of dissolved Cd stable isotope composition (dδ 114 / 110 Cd), while sparse in coverage, exist for most of the major ocean basins, with spatial coverage improving through the efforts of the GEOTRACES program. However, a dearth of similarly resolved particulate δ 114 / 110 Cd (pδ 114 / 110Cd) distributions limits our ability to use stable Cd isotopes to better understand Cd cycling in the global ocean. Here we present two pδ 114 / 110 Cd depth profiles from the subarctic northeast Pacific which demonstrate more complex δ 114 / 110 Cd cycling than dissolved profiles would suggest.

Surface pδ 114 / 110 Cd , while lighter than surface dδ 114 / 110Cd, is heavy relative to Pacific deepwater and crustal pδ 114 / 110 Cd components. Surface particulate and dissolved δ 114 / 110 Cd distributions are not well explained by closed-system Rayleigh fractionation following a single fractionation factor, in agreement with other recent studies in the Atlantic and Pacific Oceans. These variable fractionation trends in surface waters complicate the potential utility of δ 114 / 110 Cd as a paleoproductivity proxy. Particulate δ 114 / 110 Cd becomes lighter as particulate Cd is remineralized in the nutricline, reaching a minimum pδ 114 / 110 Cd of around -0.5‰, among the lightest values reported in natural telluric samples. This pδ 114 / 110Cd trend within the nutricline might be explained by (1) multiple pools of particulate Cd with different isotopic compositions and labilities, or (2) by fractionation during particulate Cd remineralization. The observed shallow loss of heavy pδ 114 / 110 Cd above the winter mixed layer, rather than the formation of especially light surface pδ 114 / 110 Cd , may help to maintain the observed surface-to-deep dδ 114 / 110 Cd gradient. Below the mid-depth pδ 114 / 110 Cd minimum, pδ 114 / 110 Cd increases with depth toward the deepwater dδ 114 / 110 Cd value, possibly reflecting an isotopic equilibration between the particulate and dissolved phases. Dissolved δ 114 / 110 Cd profiles show uniform isotope composition at intermediate depths, while calculated remineralized pδ 114 / 110 Cd is isotopically variable and distinct from the bulk dissolved pool. This suggests that one-dimensional particle export and regeneration is not the primary control on dδ 114 / 110 Cd in the Pacific Ocean, but rather that regenerated δ 114 / 110 Cd is spatially or temporally variable and an advected dδ 114 / 110 Cd signal from subsurface Southern Ocean waters controls deep North Pacific dδ 114 / 110 Cd . Our results imply that export of isotopically light pδ 114 / 110 Cd to shelf sediments may act as an important oceanic sink, helping to balance the known sources and sinks of Cd with the global deepwater dδ 114 / 110 Cd .

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https://hal-insu.archives-ouvertes.fr/insu-03586646
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Submitted on : Thursday, February 24, 2022 - 8:36:37 AM
Last modification on : Saturday, February 26, 2022 - 3:08:36 AM

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David J. Janssen, Wafa Abouchami, Stephen J. G. Galer, Kathryn B. Purdon, Jay T. Cullen. Particulate cadmium stable isotopes in the subarctic northeast Pacific reveal dynamic Cd cycling and a new isotopically light Cd sink. EARTH AND PLANETARY SCIENCE LETTERS, 2019, 515, pp.67-78. ⟨10.1016/j.epsl.2019.03.006⟩. ⟨insu-03586646⟩

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