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 (similar to 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 & PLUSMN; 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 & PLUSMN; 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 Cd-111-Cd-110 double spike technique. By comparing the delta Cd-114/110 values (0.00 & PLUSMN; 0.06 parts per thousand) of synthetic seawaters doped with Cd isotope standard (NIST-3108; treated by Chelex + AG-MP-1M resin) and the reference value (similar to 0.00 parts per thousand), 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 delta Cd-114/110 values of the column are decreased from 1.05 & PLUSMN; 0.05 parts per thousand at 3200 m to 0.36 & PLUSMN; 0.05 parts per thousand at 2800 m, differing from reported delta Cd-114/110 values of deep seawater in other oceans. Considering the spatial distance between the column and active hydrothermal vents in SWIR (similar to 13 km), we propose that such positive delta Cd-114/110 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.