Copper distribution and speciation across the International GEOTRACES Section GA03

Copper (Cu) distribution and speciation were characterized along a zonal section in the North Atlantic Ocean from Lisbon, Portugal, to Woods Hole, Massachusetts as part of the U.S. GEOTRACES program. Dissolved Cu profiles displayed many of the same features identified by other researchers, including sub-surface scavenging and a linear increase with depth, but many also exhibited unique properties and geographic trends. Concentrations ranged from 0.43nM at the surface to 3.07nM near the seafloor. The highest concentrations were measured in deep waters to the west of Cape Verde and northwest of the Canary Islands while the lowest concentrations were measured in upper waters, mostly between Mauritania and Cape Verde. The westernmost sampling sites overlying or adjacent to the U.S. east coast continental shelf featured surface maxima that decreased in magnitude moving east toward Bermuda, reflecting declining inputs from Cu-enriched coastal waters and North American aerosols. Free Cu (Cu2+) concentrations were tightly controlled by organic complexation and scavenging across the section with values varying between 1.54fM and 1.07pM. These results provide the first evidence that Cu2+ concentrations are strongly complexed throughout the water column, even in boundary zones where dissolved Cu concentrations are elevated because of local sources. Strong organic ligands (L) acted as a buffer for Cu2+, restricting concentrations to a narrow range (10–100fM) throughout most of the water column. Cu2+ and dissolved Cu were strongly scavenged by suspended particulate matter within several benthic nepheloid layers and a hydrothermal plume above the Trans-Atlantic Geotraverse (TAG) vent field on the Mid-Atlantic Ridge (MAR).