High-precision double-spike Cd isotopic measurements of seawater by MC-ICP-MS and its application to seawater affected by hydrothermal vent fluids

Cadmium (Cd) isotopes in seawater have been proven as an important geochemical tool for tracing ocean Cd circulation in the modern ocean. In this study, we evaluated a new method to separate Cd (~ 60 ng) from seawater using Chelex resin (1.0 g) coupled with AG-MP-1M resin. The results show that the Chelex resin is sufficient to remove Cd from Na and Mg matrix with Cd recoveries at 98.3 ± 3.5 % (2SD, N = 6); while AG-MP-1M resin could separate Cd from the residual Na, Mg, and isobaric inferences. The total Cd recoveries of the method are 96.3 ± 1.5 % (2SD; N = 4) and the salinity of the samples has no significant impacts on Cd recovery. Cd isotope ratios were measured using a Nu Plasma III MC-ICP-MS and ¹¹¹Cd–¹¹⁰Cd double spike technique. By comparing the δ^114/110Cd values (0.00 ± 0.06‰) of synthetic seawaters doped with Cd isotope standard (NIST-3108; treated by Chelex + AG-MP-1M resin) and the reference value (~ 0.00 ‰), no variations were observed. We also analyzed the Cd isotope compositions of three deep seawaters from a column at the Southwest Indian Ocean Ridges (SWIR). The δ^114/110Cd values of the column are decreased from 1.05 ± 0.05 ‰ at 3200 m to 0.36 ± 0.05 ‰ at 2800 m, differing from reported δ^114/110Cd values of deep seawater in other oceans. Considering the spatial distance between the column and active hydrothermal vents in SWIR (~ 13 km), we propose that such positive δ^114/110Cd values of deep seawater were likely contaminated by vent fluids, which could provide heavy Cd isotope to deep seawaters. This study demonstrates that Cd isotope is more sufficient to distinguish the impact of plumes on deep seawater.