Particulate Cadmium Accumulation in the Mesopelagic Ocean

Observations of dissolved cadmium (dCd) and phosphate (PO₄) suggest an unexplained loss of dCd to the particulate phase in tropical oxyclines. Here, we compile existing observations of particulate Cd and phosphorus (P), and present new data from the US GEOTRACES GP15 Pacific Meridional Transect to examine this phenomenon from a particulate Cd perspective. We use a simple algorithm to reproduce station depth profiles of particulate Cd and P via regeneration and possible subsurface accumulation. Our examination of regeneration reveals decoupling of particulate Cd and P driven by variable partitioning between two particulate pools with differing labilities. Further, we identify evidence for subsurface particulate Cd accumulation at 31 stations. Subsurface particulate Cd accumulation occurs most consistently in the mesopelagic tropical Pacific but can be found in all examined ocean basins. This accumulation is not well-correlated with dissolved oxygen or particulate sulfide concentration. Instead, we observe that particulate Cd accumulation occurs in regions where the concentration of dCd is relatively high compared to dissolved zinc (dZn) and speculate that it is the result of enhanced dCd biological uptake in response to the subsurface micronutrient balance.