An isotopic record of mercury in San Francisco Bay sediment

Abstract

We analyzed eight sediment cores from wetland and subtidal locations around San Francisco (SF) Bay, terrace sediment from the Yuba River, CA and precipitation from the SF Bay region. We defined the Hg isotopic composition of uncontaminated SF Bay sediment, two contributing endmember sediments contaminated by distinct Hg sources, and regional precipitation. Deep subtidal sediments with pre-mining THg concentrations (less than 60ng/g) have δ202Hg of −0.98±0.06‰ and Δ199Hg of 0.17±0.03‰ (1 s.d.; n=5). The δ202Hg of subtidal sediment in SF Bay systematically changed between pre-mining, circa 1960, and surface sediment. In circa 1960 sediment δ202Hg ranges from −0.63 to −0.32‰ (±0.10‰) with higher δ202Hg in the south and lower δ202Hg in the north; however in surface sediment δ202Hg is nearly constant and averages −0.52±0.04‰ (1 s.d.; n=6). These latter values for SF Bay surface sediment are similar to those for terrace sediment along the Yuba River that have δ202Hg of −0.57±0.10‰ and Δ199Hg of 0.05±0.01‰ (1 s.d.; n=2) and are consistent with sediment contaminated by a metallic Hg source that was derived from Hg ore in the California Coast Ranges and used in Sierra Nevada gold mining. Wetlands adjacent to south and central SF Bay have high THg in deeper sediment layers (>3000ng/g) that vary by ~0.6‰ in δ202Hg, presumably reflective of various anthropogenic Hg sources. The three sediment endmembers (metallic Hg, Hg mining and background Hg) were defined using δ202Hg and THg concentrations. Based on the estimated contribution of endmember sediment to subtidal and intertidal locations, we suggest that the subtidal Hg stable isotope record in sediment cores is consistent with historical sediment transport to, and deposition in, SF Bay. Thus, Hg stable isotopes can be used to trace sediment transport in locations that are impacted by multiple anthropogenic Hg sources.