Mercury and Methylmercury Distributions Along a Longitudinal Transect of the North Atlantic Ocean

Abstract:

From August 1 to September 23, 2013 the A16N Repeat Hydrography Cruise (US CLIVAR program) conducted a nearly longitudinal (~20-25°W) sampling transect of the Atlantic Ocean (63°N to 6°S) originating in Reykjavik, Iceland. Water column profiles were taken at 145 locations along this transect, and at 13 of these locations samples for total mercury (THg) and methylmercury (MeHg) were also collected. Although profile depths for this cruise reached nearly 6000 m, Hg and MeHg sampling was limited to 12 discrete samples ranging from the surface to about the 1000 m. All samples were collected while observing strict adherence to trace-metal-free protocols. Upon collection from the rosette, each whole-water sample was immediately acidified to 1% HCl, thus our analysis of THg and MeHg includes both particulate and filter-passing fractions. For MeHg, our results represent a composite of both monomethyl and dimethyl mercury species. All samples were analyzed at the USGS Mercury Research Lab.

Our results show several clear and interesting trends. First, an examination of THg versus MeHg reveals a bimodal distribution. Samples with THg > 0.03 pM show a highly correlated and linear distribution (R²=0.72) and a high mean percentage of MeHg (49%). On the other hand, samples with THg < 0.03 pM show a poor relation with MeHg and much lower %MeHg (9%). A depth discrimination of samples falling into these two regions of the bimodal THg versus MeHg plot shows that the vast majority of the results falling in the highly correlated region are from depths greater than 200 m, whereas the low %MeHg and poorly correlated samples were dominantly from 0-200 m depth. Similar to observations from other ocean basins, profiles of MeHg show clear subsurface peaks occurring between about 400-800 m depth. When examining the profiles in a north-to-south arrangement, an increasing trend is evident in the mean MeHg concentration of the subsurface peak, with maximum levels observed in profiles collected between 9 and 14°N. This region of the North Atlantic Ocean is associated with an oxygen minimum and PCO₂ maximum zone both driven by degradation of sinking algal-derived organic matter. Last, the overall results from the Atlantic Ocean show a clear MeHg enrichment compared to similar data collected in the Pacific and Southern Indian Oceans.