Is the Coastal Ocean a Source of Mercury to Marine Advective Fog

Abstract:

Marine advective fog is a common feature along the California coast during the summer season. This fog provides an important water source to many endemic fauna and flora. Studies are underway to better understand the chemical makeup of Pacific marine fog as it is an important input to the hydrologic cycle. We report results from our study focused on investigating the potential for coastal ocean upwelling to contribute volatile organic mercury to the overlying atmosphere where it could be incorporated into cloud droplets as monomethyl mercury (MMHg). Preliminary research by this group has indicated that fog water inputs to certain coastal locations may contribute up to 99% of the MMHg flux to land compared to the MMHg flux in rain. Mercury measurements, including total mercury (Hgt), MMHg, elemental mercury (Hg⁰), and dimethyl mercury (DMHg), were made to unfiltered water collected from depth profiles at 12 stations from Big Sur to Trinidad Head over the California shelf during summer 2014. Profiles of Hgt ranged from 0.3-2.4 pM and were similar to other reported measurements of Hgt for the North Pacific. A large range in concentration was observed for MMHg (10-540 fM) with elevated values generally occurring below the oxycline (>50m). Concentrations of Hg⁰ were 0.06 to 0.57 pM with elevated concentrations at depth relative to surface values. Depth profiles of DMHg were similar to MMHg and concentrations were measured from 10-295 fM with highest concentrations observed below the oxycline. Surface concentrations of DMHg averaged 40 ± 22 fM. Given the observed profiles for DMHg and the fact that it is sparingly soluble in water, a net flux of DMHg to the atmosphere is likely occurring. Based on these findings and the fact that MMHg and DMHg concentrations in the coastal ocean were highest in the low oxygen zone, we speculate that mercury is methylated in the water column and/or sediments as DMHg and that this water is upwelled seasonally in the coastal zones and contributes organic mercury to overlying cloud banks, which has the potential to deposit onto upland terrestrial ecosystems. Research is ongoing through the establishment of a network of 7 coastal terrestrial sites from Big Sur to Trinidad Head and through coastal oceanographic expeditions planned through the summer of 2015.