Physical and biogeochemical controls on the distribution of dissolved cadmium and its isotopes in the Southwest Pacific Ocean

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Cadmium stable isotope ratios (δ114Cd) have become a useful tool for oceanographers investigating the biogeochemical and physical processes that affect the nutrient-like distribution of the bioactive trace metal cadmium (Cd) throughout the oceans. Here, we present a meridional transect of dissolved Cd and δ114Cd from Japanese GEOTRACES section GP19 along 170°W from 64°S in the Southern Ocean to the equatorial Pacific. Along the GP19 section, the deep ocean (>1500 m) shows small variability in dissolved Cd (0.75–0.9 nmol kg−1) and a homogeneous δ114Cd signature (+0.26 ± 0.06‰, 2SD, n = 60; relative to NIST SRM-3108). Adding these data to previously published work allows us to calculate a deep Pacific and Southern Ocean (>1500 m) mean δ114Cd of +0.26 ± 0.10‰ (2SD, n = 436). Higher in the water column, depth profiles of Cd along the GP19 section exhibit a strong vertical gradient from a maximum (up to 0.9 nmol kg−1) at 1500–2000 m up to depleted surface waters (<0.001 nmol kg−1 in the equatorial Pacific). This gradient in dissolved Cd concentration is associated with changes in dissolved δ114Cd, with values higher (+0.4 to +0.6‰) than the deep ocean average at intermediate depths (300–1500 m), and then a further increase towards high δ114Cd values (up to +0.9‰) in the surface ocean. Both patterns could be explained by one-dimensional biological cycling including preferential uptake of isotopically light Cd by phytoplankton, and such processes likely explain the surface patterns. At intermediate depths, however, the observed strong vertical Cd concentration and isotopic gradients instead result from the lateral isopycnal transport of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), both of which carry distinctly lower pre-formed Cd concentrations and higher δ114Cd values. These pre-formed signatures, which are imparted during water-mass formation in the Southern Ocean, are clearly conserved into the lower latitude Pacific as these water masses travel northward.Overall, the distribution of Cd and δ114Cd along the GP19 section is remarkably well explained by large scale mixing of water mass endmembers with defined δ114Cd signatures, emphasizing the importance of surface Southern Ocean processes for the distribution of trace metals such as Cd in the subsurface Southwest Pacific. At the regional scale, however, two other processes may overprint this mixing relationship. First, by comparison with the nearby Southeast Pacific GP16 section, we find that the δ114Cd signature of equatorial intermediate water masses shows little zonal variation across the equatorial Pacific, despite becoming enriched in dissolved Cd due to remineralization. We propose that this uniformity is explained by complete utilization of Cd in the surface tropical Pacific and remineralization of Cd with an isotopic signature similar to intermediate waters, therefore conserving the southern-sourced isotopic signature. Similarly, the observed increase of about 30% in deep ocean Cd concentrations from the South to the North Pacific is associated with a near-constant δ114Cd signal. These observations enable us to constrain the net δ114Cd of Cd added by remineralization to the deep ocean, with the caveat that such a signal is integrated over the entire Pacific, and that remineralization under different oceanic regimes such as HNLC areas may add Cd with different isotopic compositions to deep waters. Second, at GP19 stations close to the equator, subtle Cd depletion (relative to phosphate) is observed associated with low-oxygen subsurface waters, consistent with other studies from the North Pacific. Discerning the effects of such processes on Cd isotopic distributions is an important step to a more detailed understanding of the biogeochemical cycling of Cd in the modern ocean, and the application of δ114Cd as a tracer of past deep water circulation.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.