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