The Distribution and Biogeochemistry of Transition Metal - Organic Complexes in Marine Waters

This thesis addresses certain problems in chemical oceanography that pertain to. the chemical speciation of transition metals dissolved in seawater. Knowledge of the chemical speciation of dissolved trace metals is important for understanding their role in various biogeochemical processes. Historically, the concept that dissolved organic matter (DOM) may complex metal ions in seawater has often influenced interpretation of observations in the oceanography disciplines. Unfortunately, the extent of metal complexation and the nature of the organic ligands involved are poorly understood. This thesis involves an investigation of the interaction between DOM and dissolved transition metals, particularly copper, in marine waters. Analytical techniques, including reverse phase liquid chromatography (C18-RPLC), heated graphite atomizer atomic absorption spectrophotometry (HGA-AAS), and high performance liquid chromatography (HPLC), have been applied for measurement, and characterization, respectively, the of isolation, organically complexed metals dissolved in seawater. Dissolved metals have also been determined by HGA-AAS after preconcentration by co-precipitation with cobalt and ammonium pyrrolidine dithiocarbamate. Although other metals (iron, cadmium, zinc and manganese) have been investigated, only copper and nickel were significantly complexed by the isolated organic matter. The geochemistry of copper and iron was studied in the estuarine waters of the Connecticut River, Connecticut, U.S.A. in July 1981. Field measurements indicated that dissolved copper and iron were nonconservative in the estuary. Laboratory estuarine mixing experiments indicated that the low salinity iron and copper removal mechanism, flocculation of riverine colloidal hydrous ferric oxides and humic acids with associated copper, was similar to that reported for other estuarine systems. The organically complexed copper isolated by C18-RPLC comprised-24 to 59% of the dissolved copper and was mostly conservative in the estuary and in the laboratory estuarine mixing experiment. Riverine DOM, similar to terrigenous fulvic acid was responsible for the observed copper complexation. The shelf and slope waters of the Middle Atlantic Bight were vertically stratified during a seven station transect in July 1980. The dissolved copper and nickel concentrations in the lower salinity surface waters were higher than in the cool subpycnocline and slope waters. A significant fraction of the dissolved copper (9-60%) and smaller amounts of the dissolved nickel (0-8%) were complexed by DOM. The percentage of organically complexed copper was linearly correlated with apparent oxygen utilization (AOU) indicating that biological production and consumption may considerably alter the copper speciation of seawater by simply forcing shifts in the carbonate equilibria and variations in pH. Riverine organic matter was also responsible for the copper complexation observed in the Middle Atlantic Bight. The vertical distributions of dissolved copper and organically complexed copper species isolated by C18-RPLC were investigated during July 1980, at four stations in the Northwestern Atlantic Ocean. The vertical profiles of dissolved copper were similar to those previously reported for the North Atlantic Ocean. Oceanic dissolved copper concentrations increased gradually with depth from 0.8 to 1.6 nmol/kg in near surface waters up to 2.3 nmol/kg at 5 km depth in the Sargasso Sea. Organically complexed copper species were present in all oceanic waters sampled and comprised 9 to 64% of the dissolved copper. The vertical distributions of organically complexed copper were related to other hydrographic features, characterized by low concentrations in near surface waters, increasing to euphotic zone maxima at the depth of the seasonal thermocline and gradually increasing concentrations in deeper waters. Refractory organic matter of possible terrigenous origin may be responsible for the uniform complexation of copper observed in the deep ocean (average 28% organically complexed copper). Organic ligands of more recent biological origin may contribute to the higher levels (up to 46%) of organically complexed copper found in the euphotic zone maxima. A pH dependent equilibrium copper speciation model was used to predict the concentrations of organically complexed copper measured analytically by C18-RPLC in well characterized samples of Narragansett Bay seawater collected from several Marine Ecosystem Research Laboratory (MERL) microcosms in August 1981. An equilibrium model, incorporating the complexation of copper by humic ligands, accounted for most of the organically complexed copper isolated by and the copper organically complexed (26-40%) dependent upon the pH and alkalinity in a predictable manner