Biogeochemistry of dissolved arsenic in the temperate to tropical North Atlantic Ocean. (June 2015) - Wurl, Oliver
Abstract: The biogeochemical cycle of arsenic was examined in the water column across the North Atlantic from 39° to 17°N as part of the US GEOTRACES North Atlantic study (GEOTRACES Section GA03). Results show limited nutrient-like distribution of As 5+, and upper ocean maxima in As 3+ and methylated As as found in many other studies In the oligotrophic water masses, microbial communities, i.e. phytoplankton, appear to favor the reduction to As 3+ instead of methylation as detoxification of As 5+ taken up during phosphorus (P) limitation due to their chemical similarities. The depth-integrated average concentrations in the mixed layer depth of As 3+ in the western and eastern Atlantic Ocean were 1.30±1.14 nmol L −1 ( n =4) and 0.65 ( n =2), respectively, and rose to 3.30 nmol L −1 ( n =2) in the Central Atlantic Ocean. No pattern was observed for As 5+ (15.7±2.8 nmol L −1, n =8) and methylated species were detected occasionally below 0.41 nmol L −1 in the mixed layer. Based on significant correlations between phosphate, alkaline phosphate activity (APA), a conventional proxy for P limitation, and As 3+, we conclude that As 3+ is a good proxy for P limitation within the upper water column similar to our earlier evaluation of surface data. Mass balances for the mixed layer show that atmospheric inputs of As 5+ can compensate for the losses via export fluxes and microbial reduction to As 3+ . The cycling of As 3+ is more complex, with sources from As 5+ reduction and losses due to photochemical and microbial-induced oxidation. The resulting residence time of As 3+ with respect to these processes can be as short as 0.7–3 days. Unlike As 5+, atmospheric inputs of As 3+ cannot balance the oxidative losses and the short residence time further limits horizontal and vertical advective/diffusive inputs. It appears that reduction of As 5+ coupled with detoxification and general microbial reduction are the sources of As 3+ in the oceanic mixed layer. While As 3+ production during As 5+ detoxification has been well studied, the generic microbial reduction of As 5+ to As 3+ requires a more thorough investigation.