Copper Speciation Results From The U.S. GEOTRACES Eastern Tropical South Pacific Cruise

Dissolved organic copper-binding ligands were examined on the U.S. GEOTRACES zonal transect in the Eastern Tropical South Pacific from Peru to Tahiti. All samples were measured using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV), the bulk in duplicate titrations at a single analytical window (5 µM) using the added ligand salicylaldoxime (SA). A subset of samples were also analyzed using multiple competition strengths (1, 2.5, 5, 10, and 25 µM added SA), along with equilibration tests at each analytical window. Titration data was processed using newly available software for single and multiple analytical window data. Equilibration tests conducted at the various analytical windows showed no significant differences in ligand concentration and binding strength between overnight vs. 15-30 minute equilibration times. Samples analyzed at a single window reveal excess strong ligands in the coastal region over the oxygen minimum zone with a conditional stability constant (logK) around 14. Multiple competition strengths of the added ligand enabled the detection of up to three classes of copper-binding ligands, with conditional stability constants ranging from 8.6-16.0, and high concentrations of weaker ligands throughout the water column. The presence of strong copper-binding ligands across the transect led to low free copper concentrations, which can be limiting to some phytoplankton. Analysis is ongoing and this presentation will summarize the status of this unique data set.