Hg species distribution in South Atlantic Ocean along 40°S parallel

Abstract:

Ocean water masses are globally important vectors, reservoirs and transformation basins of various Hg species. However, Hg ocean cycling is not yet completely understood, partly because many regions remain undersampled. During UK-GEOTRACES sampling campaign GA10 water has been collected along the 40° parallel in the South Atlantic Ocean.

Total mercury (THg), monomethylmercury (MeHg), dissolved gaseous mercury (DGM) concentrations were determined at very high vertical frequency of 20-24 depths per station. THg was uniformly distributed throughout the water column with an average of 1.6 ± 0.99 pM. Surface waters were depleted in comparison to deep and intermediate waters, where concentrations increased slightly westwards from Mid-Atlantic Ridge.

An increase of MeHg (up to 0.15 pM) in surface waters from 0°W to 20°W was observed, co-occurring with increased Chlorophyll a abundance. MeHg was locally increased also at depths corresponding to North Atlantic Deep Water, which may be carrying anthropogenic signal of intensive Hg use on northern hemisphere.

DGM was strongly depleted in surface waters, indicating they are source of Hg⁰ to the atmosphere. Concentrations ranged from 0.01 pM to 0.55 pM and represented between 0.9% and 56% of THg. Highest concentrations were found in Antarctic Intermediate Water and Antarctic Bottom Water. Upper Circumpolar Deep Water, which was rich in nutrients, showed also higher DGM concentrations. In addition, we have observed an interesting correlation between nitrate and DGM, both increasing with depth.