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

• Published in: Earth and Planetary Science Letters
• Main Authors: Janssen, David J., Abouchami, Wafa, Galer, Stephen J. G., Purdon, Kathryn B., Cullen, Jay T.
• Other Authors: Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2019
• Subjects: cadmium isotopes; GEOTRACES; trace metal; northeast Pacific; biogeochemical cycles; [SDU]Sciences of the Universe [physics]; Pacific; Subarctic
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-03586646; https://doi.org/10.1016/j.epsl.2019.03.006

International audience 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 δ 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 (p δ 114 / 110 Cd) distributions limits our ability to use stable Cd isotopes to better understand Cd cycling in the global ocean. Here we present two p δ 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 p δ 114 / 110 Cd , while lighter than surface d δ 114 / 110 Cd, is heavy relative to Pacific deepwater and crustal p δ 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 p δ 114 / 110 Cd of around -0.5‰, among the lightest values reported in natural telluric samples. This p δ 114 / 110 Cd 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 δ 114 / 110 Cd above the winter mixed layer, rather than the formation of especially light surface p δ 114 / 110 Cd , may help to maintain the observed surface-to-deep d δ ...