GEOTRACES 2009 Intercalibration: Low-level Determination of Monomethylmercury in Seawater

Abstract

<p>Monomethylmercury (MMHg) is the form of Hg that accumulates in biota and poses the greatest health risk to humans. However, concentrations of MMHg in seawater are often either near or less than limits of detection with contemporary methods and instrumentation (about 50 fM), which has resulted in a paucity of knowledge regarding the oceanic distribution and cycling of this toxic compound. We have developed a new method, employing a derivatization/purge-and-trap technique, that has lowered the detection limit for MMHg to ~2 fM in seawater and thereby permits accurate quantification of both MMHg and dimethylmercury (DMHg) at trace levels. The method was used to conduct shipboard analyses of alkylmercury species during the 2009 GEOTRACES Intercalibration cruise in the North Pacific to provide the first highresolution, full-depth profile in a major ocean basin. Dissolved (< 0.2 μm filtered) MMHg is low in the mixed layer (~20 fM), increased markedly in oxic North Pacific Intermediate Water (NPIW, 45 fM) and the oxygen minimum (1000 m, 95 fM), and uniformly low at depths below the thermocline (27-31 fM). DMHg has a comparable vertical distribution, with marked increases in both NPIW and the oxygen minimum, although levels were ~2-4x less than MMHg. The vertical distribution of alkylmercury species in the ocean suggests that both MMHg and DMHg are produced in lowoxygen waters of the thermocline and, surprisingly, under oxic conditions in NPIW. Moreover, the uniform distribution and low concentrations of MMHg in deep water exclude deep-sea sediment and hydrothermal processes as substantial sources to the mixed layer and associated biota.</p><p>This presentation occurred at the Wright State University Campus-Wide Celebration of Research, Scholarship and Creative Activities on April 16, 2010</p>