Re-Evaluating Hydrogen Sulfide as a Sink for Cadmium and Zinc in the Oxic to Suboxic Upper Water Column of the Pacific Ocean

Hydrogen sulfide is produced by heterotrophic bacteria in anoxic waters and via carbonyl sulfide hydrolysis and phytoplankton emissions under oxic conditions. Apparent losses of dissolved cadmium (dCd) and zinc (dZn) in oxygen minimum zones (OMZs) of the Atlantic and Pacific Oceans have been attributed to metal-sulfide precipitation formed via dissimilatory sulfate reduction. It has also been argued that such a removal process could be a globally important sink for dCd and dZn. However, our studies from the North Pacific OMZ show that dissolved and particulate sulfide concentrations are insufficient to support the removal of dCd via precipitation. In contrast, apparent dCd and dZn deficits in the eastern tropical South Pacific OMZ do reside in the oxycline with particulate sulfide maxima, but they also coincide with the secondary fluorescence maxima, suggesting that removal via sulfide precipitation may be due to a combination of dissimilatory and assimilatory sulfate reduction. Notably, dCd loss via precipitation with sulfide from assimilatory reduction was found in upper oxic waters of the North Pacific. While dissimilatory sulfate reduction may explain local dCd and dZn losses in some OMZs, our evaluation of North Pacific OMZs demonstrates that dCd and dZn losses are unlikely to be a globally relevant sink. Nevertheless, metal sulfide losses due to assimilatory sulfate reduction in surface waters should be considered in future biogeochemical models of oceanic Cd (and perhaps Zn) cycling. The Pacific Ocean hosts multiple oxygen minimum zones (OMZs), chiefly in the Northeast Pacific and the Eastern Tropical Pacific. Despite studies identifying apparent deficits of dissolved cadmium (dCd) and zinc (dZn) relative to established relationships with macronutrients within OMZ waters and hypothesizing that precipitation of insoluble metal sulfides is the cause, our measurements of particulate metal sulfides are too small to support this removal hypothesis. However, we demonstrate that a possible loss of dCd related to metal-sulfide precipitation is actually more likely in the surface oxygenated ocean rather than in OMZ waters. Sulfide concentrations are too low to support the 0.1 nM deficit observed in dissolved Cd in the low oxygen North Pacific Assimilatory and dissimilatory sulfate reduction can explain the dissolved metal deficits in the shallow Eastern Tropical South Pacific OMZ Particulate sulfide maxima coincide with the largest apparent dissolved Cd deficits observed in the subsurface waters of the Pacific Ocean