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

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.