Biogeochemical cycling of hydrogen sulfide in the oxic Pacific Ocean

Hydrogen sulfide is an inorganic ligand that is present in oxic waters at pico- to nanomolar concentrations and can affect trace metal cycling. The chemical speciation of dissolved H₂S can be broken down into two main categories: Free sulfide and total dissolved sulfide (TDS = free sulfide, H₂S_aq+HS^-+S^2-, plus metal-sulfide complexes). TDS concentrations in the open ocean are primarily controlled by production via carbonyl sulfide (OCS) hydrolysis and phytoplankton emissions, and oxidative loss by oxygen and iodate.

To investigate processes affecting hydrogen sulfide in the upper Pacific Ocean, samples were collected on the 2018 US GEOTRACES Pacific Meridional Transect (GP15) from 56°N to 20°S along 152°W. Filtered water samples were analyzed using an acidification, cryogenic trapping and GC/flame photometric method; this quantifies dissolved sulfide as free ions and metal-sulfide complexes (TDS). At 34 stations along the transect, TDS ranged from 1.4–347 pM and averaged 107 pM in the upper 100 m for the northern-most coastal waters, and averaged 24 pM in the open ocean. Overall, TDS decreased with depth and from the Alaskan shelf moving southward. Free sulfide was undetectable at nearly all stations, implying that all of the TDS was complexed with trace metals such as Zn, Cd, and Ni.

OCS, a major source of hydrogen sulfide in the upper ocean, ranged from 17–121 pM, averaged 52 pM, and did not show much latitudinal variation. In terms of a mass balance, TDS can undergo oxidation by iodate > oxygen depending on its chemical speciation and represents a major sink. In this respect, while iodate oxidation is very rapid for most forms of sulfide, the fraction that is complexed with zinc is essentially inert. Production via OCS hydrolysis can be easily calculated, but phytoplankton fluxes depend on the actual species present. This talk will evaluate the various sources and sinks encountered on this 7400 km transect and discuss its implications for trace metal cycling.