Characterising copper biogeochemistry in marine systems

Abstract

Copper (Cu) has often been termed a Goldilocks element regarding its role in marine systems. This is because copper is beneficial at low free Cu(II) ion concentrations and toxic at higher free Cu(II) ion concentrations to many phototrophic primary producers. Thus, there is a defined window where Cu concentrations are optimal for biological uptake by marine algae. Complexation of dissolved Cu by natural organic ligands is vital in defining the free Cu(II) ion concentration window but critically, the distribution of organic Cu species, both geographically and within the water column, is presently unclear. The aim of this project was to investigate organic Cu speciation in seawater by developing and using a new Cu isotopic technique to compare results against corresponding Cu speciation measurements. By concurrently measuring these two parameters in seawater, we aimed to reveal new aspects of marine Cu biogeochemistry and develop a new perspective from which to better understand the processes that determine and regulate organic Cu speciation in the ocean. Seawater samples were collected from the Tasman Sea and contrast the oligotrophic northern region with the comparatively eutrophic and productive southern region. Organic Cu speciation was investigated at two sites in the Tasman Sea and revealed that the northern site was characterised by detection of a single, strong, class of ligand with a uniformly distributed conditional stability constant (log K = 12.97 +/- 0.26). At the two shallowest depths of the southern site, two ligands were detected. Below 120 m at this site, a single, strong, ligand class was detected with a uniformly distributed conditional stability constant (log K = 13.32 +/- 0.21). A solvent-extraction technique was then developed to detect isotopic Cu compositions in seawater and was used to measure three Tasman Sea profiles to 3500 m depth. Two of the profiles showed maximum variability in the surface waters (0 - 130 m depth), which was attributed to biological uptake and scavenging by the particulate phase. The Cu isotope composition of all 3 profiles was relatively homogenous below {u0303}130 m depth. This work confirmed that organic Cu complexation plays a major role in the cycling of this metal in open-ocean settings and showed that the profile trends of organic Cu species and isotopic Cu signatures were comparable. Two interpretations for this similarity are that 1) the homogenised d65Cu composition of the dissolved phase is due to the considerably greater concentration of dissolved Cu relative to particulate Cu in the ocean and/or 2) that the d65Cu composition of dissolved Cu is directly influenced by organic Cu complexation which accounts for > 99 % of dissolved Cu and which stabilises the d65Cu composition within strong organic bonds. These measurements have significantly contributed to our understanding of the distribution of organic Cu species in seawater and have laid some important foundations for the future direction of stable Cu isotope geochemistry in seawater. Show more