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 (dδ114/110Cd), 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 δ114/110Cd(pδ114/110Cd) distributions limits our ability to use stable Cd isotopes to better understand Cd cycling in the global ocean. Here we present two pδ114/110Cd depth profiles from the subarctic northeast Pacific which demonstrate more complex δ114/110Cd cycling than dissolved profiles would suggest. \nSurface pδ114/110Cd, while lighter than surface dδ114/110Cd, is heavy relative to Pacific deepwater and crustal pδ114/110Cd components. Surface particulate and dissolved δ114/110Cd 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 δ114/110Cd as a paleoproductivity proxy. Particulate δ114/110Cd becomes lighter as particulate Cd is remineralized in the nutricline, reaching a minimum pδ114/110Cdof around −0.5‰, among the lightest values reported in natural telluric samples. This pδ114/110Cd 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 pδ114/110Cd above the winter mixed layer, rather than the formation of especially light surface pδ114/110Cd, may help to maintain the observed surface-to-deep dδ114/110Cd gradient. Below the mid-depth pδ114/110Cd minimum, pδ114/110Cd increases with depth toward the deepwater dδ114/110Cd value, possibly reflecting an isotopic equilibration between the particulate and dissolved phases. Dissolved δ114/110Cd profiles show uniform isotope composition at intermediate depths, while calculated remineralized pδ114/110Cd 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 dδ114/110Cd in the Pacific Ocean, but rather that regenerated δ114/110Cdis spatially or temporally variable and an advected dδ114/110Cd signal from subsurface Southern Ocean waters controls deep North Pacific dδ114/110Cd. Our results imply that export of isotopically light pδ114/110Cdto shelf sediments may act as an important oceanic sink, helping to balance the known sources and sinks of Cd with the global deepwater dδ114/110Cd.