Copper speciation and distribution in the Atlantic sector of the Southern Ocean

The distribution and speciation of dissolved copper (Cu) was investigated in the Atlantic sector of the Southern Ocean in austral autumn of 2008 as part of the IPY GEOTRACES expedition ZERO & DRAKE. Distribution measurements focused on two transects across the major frontal systems along the Zero Meridian and across the Drake Passage whereas speciation work was investigated in the Drake Passage and the ice covered Weddell Sea. Along the two transects the dissolved Cu concentration exhibited a gradient in the surface with Cu values increasing poleward. Vertical profiles of Cu showed in general monotonic increases with depth, which correlated slightly with silicate but poorly with phosphate. Benthic sources of Cu were observed along the Zero Meridian transect and close to the Antarctic Peninsula for the Drake Passage. Dissolved Cu (DCu) appears to be removed from surface waters through adsorption/complexation by particulate matter. Speciation measurements on DCu indicated a relatively uniform distribution of 25-50 nM of L2 (log K ~ 11.8) class ligands. No L1 class ligands (log K > 13) were detected in any of the samples examined. Estimates of free Cu (pCu = 13.5 to 12.5) showed only small variations in the upper water column and was generally uniform in deep waters. These findings indicate that there is insufficient free copper to account for the rapid reactivity of superoxide (O2−) with Cu in these waters (Heller and Croot, 2010c) suggesting that the organic Cu complexes present in seawater can undergo rapid redox reactions.