Biogeochemistry of dissolved arsenic in the temperate to tropical North Atlantic Ocean

The biogeochemical cycle of arsenic was examined in the water column across the North Atlantic from 39 degrees to 17 degrees N as part of the US GEOTRACES North Atlantic study (GEOTRACES Section GA03). Results show limited nutrient-like distribution of As5+, and upper ocean maxima in As3+ 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 instead of methylation as detoxification of As5+ taken up during phosphorus (P) limitation due to their chemical similarities. The depth-integrated average concentrations in the mixed layer depth of As3+ in the western and eastern Atlantic Ocean were 130 + 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 Ass (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 As3+, we conclude that As3+ 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 Ass+ can compensate for the losses via export fluxes and microbial reduction to As3+. The cycling of As3+ is more complex, with sources from Ass+ reduction and losses due to photochemical and microbial-induced oxidation. The resulting residence time of As3+ with respect to these processes can be as short as 0.7-3 days. Unlike As5+, atmospheric inputs of As3+ cannot balance the oxidative losses and the short residence time further limits horizontal and vertical advective/diffusive inputs. It appears that reduction of Ass coupled with detoxification and general microbial reduction are the sources of As3+ in the oceanic mixed layer. While As3+ production during As5+ detoxification has been well studied, the generic microbial reduction of Ass to As3+ requires a more thorough investigation. (C) 2014 Elsevier Ltd. All rights reserved.