Gradients in Strong and Weak Organic Copper-Binding Ligands in the Eastern Tropical South Pacific

Abstract:

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), and a subset were analyzed using multiple competition strengths of the added ligand salicylaldoxime (1, 2.5, 5, 10, and 25 µM). Titration data was processed using newly available multiple analytical window data processing techniques, which unify the multiple window dataset as a whole. Multiple competition strengths of the added ligand enabled the detection of an additional weaker class of copper-binding ligand, compared to the two stronger ligand classes which have been measured previously in the open ocean. The strongest ligand class (L₁) ranged in concentration from 1-10 nmol L^-1 and had a conditional stability constant (logK) ranging from approximately 15.0-16.0. The weaker ligand classes (L₂, and L₃) were present in much higher concentrations even in surface waters, with concentrations ranging from 5-50 nmol L^-1 and conditional stability constants ranging from 8.6-12.5. The elevated ligand concentrations, both in surface and deep waters, lead to extremely low concentrations of Cu²⁺ throughout the transect, possibly influencing important biogeochemical processes such as inducible iron acquisition by diatoms, and ammonium oxidation in the oxygen minimum zone.