Drivers of Surface Ocean Mercury Concentrations and Air-Sea Exchange in the West Atlantic Ocean

Accurately characterizing net evasion of elemental mercury (Hg-0) from marine systems is essential for understanding the global biogeochemical mercury (Hg) cycle and the pool of divalent Hg (He-II) available for methylation. Few high resolution measurements of Hg-0 are presently available for constraining global and regional flux estimates and for understanding drivers of spatial and temporal variability in evasion. We simultaneously measured high-resolution atmospheric and surface seawater Hg-0 concentrations as well as the total Hg-0 distribution during six cruises in the West Atlantic Ocean between 2008 and 2010 and examined environmental factors affecting net Hg-0 formation and evasion. We observed the lowest fraction of Hg as He-0 (7.8 +/- 2.4%) in the near-coastal and shelf areas that are influenced by riverine inputs. Significantly higher %Hg-0 observed in open ocean areas (15.8 +/- 3.9%) may reflect lower dissolved organic carbon (DOC) in offshore environments, which is known to affect both the reducible Hg-II pool and redox kinetics. Calculated Hg-0 evasion changed by more than a factor of 3 between cruises (range: 2.1 +/- 0.7 to 6.8 +/- 5.1 ng m(-2) h(-1)), driven mainly by variability in Hg-0 and wind speed. Our results suggest that further mechanistic understanding of the role of DOC on Hg-0 redox kinetics in different types of marine environments is needed to explain variability in Hg concentrations and improve global estimates of air-sea exchange.