Marine biogeochemical cycling of cadmium and cadmium isotopes in the extreme nutrient-depleted subtropical gyre of the South West Pacific Ocean

Abstract

The Cd isotopic analysis of waters from an increasing number of oceanic regions has provided a wealth of new information on the oceanic cycling of Cd, revealing the complex interplay of a multitude of different biogeochemical processes. In this study, paired Cd concentration and Cd isotopic measurements were made on samples collected during the GEOTRACES GP-13 zonal section in the South West Pacific Ocean. The South Pacific subtropical gyre is the most oligotrophic gyre in the global ocean and a unique area to study the Cd isotope systematics associated with phytoplankton productivity under ultra-low nutrient concentrations. The dissolved Cd and PO4 concentrations of the study area are well correlated and can be expressed by two different linear relationships, as observed in other oceanic regions. The near quantitative biological uptake of Cd in the upper water column and the mixing of different water masses with different pre-formed Cd/PO4 ratios likely produces the ‘kink’ in the Cd–PO4 relationship. Across the GP-13 zonal section, the Cd isotopic composition of deep waters is relatively constant, as observed in other regions, and is centred around a δ114Cd value of 0.26 ± 0.11‰ (2 SD, n=40). In contrast, across the thermocline depth range, extending from 150 to 1500 m depth, the Cd concentration and δ114Cd values are negatively correlated and best described by Cd isotope fractionation under open-system conditions with continuous replenishment of the Cd source/s and a fractionation factor of 1.0006 ± 0.0002. This contrasts with the closed-system conditions without Cd replenishment that have been used to describe some other open ocean settings. Below 500 m depth, the Cd isotope systematics can largely be explained by three component mixing between key water masses with different pre-formed Cd isotope signatures. However, above 500 m, the Cd isotope systematics appear to be influenced by both water mass mixing and the biological uptake of isotopically light Cd in local and remote surface waters and the regeneration of Cd from sinking organic material deeper in the water column. Additionally, an unusual positive correlation was observed between Cd concentration and Cd isotopic composition in the upper water column of the South Pacific Ocean, from 15 to 150–200 m depth. These systematics can potentially be explained by one, or a combination, of the following processes: (i) a dominant role of supply-limited conditions during Cd uptake, (ii) partitioning of Cd into ligand phases, and/or (iii) atmospheric sources of Cd in these oligotrophic waters. Although subject to uncertainty, flux calculations suggest that atmospheric input could contribute 10–83% of the total Cd input to the surface waters of the subtropical South Pacific gyre.