Dissolved copper isotope biogeochemistry in the Tasman Sea, SW Pacific Ocean

Abstract

Described as a ‘Goldilocks element’ in seawater, the free copper ion (Cu2+) concentration in open ocean waters is generally low enough to avoid causing toxicity to marine algae and bacteria, but high enough to be utilised in a range of bio-essential metallo-proteins and enzymes. The measurement of stable copper (Cu) isotopic compositions in seawater is proposed as a means to better understand the biogeochemical cycling of this important micronutrient in marine systems. Processes including changes in oxidation state, biological uptake and scavenging have all been shown to fractionate Cu isotopes and it is hoped that their measurement in seawater will shed some light on the processes that govern the regulation of the bioavailable pool of Cu in the ocean. However, measurement of Cu isotopic compositions in seawater is analytically challenging. This work utilised solvent-extraction coupled with anion-exchange and multi-collector inductively-coupled plasma mass spectrometry to spatially characterise their distribution in the Tasman Sea, south-west Pacific Ocean. Three profiles were measured to 3500m depth along a broad transect covering the oligotrophic north Tasman Sea, the Tasman Front and the productive waters of the Southern Ocean in the south. The Cu isotope composition of all 3 profiles was relatively homogenous and corresponding measurements of organic Cu complexation (Thompson et al., 2014. Dissolved copper speciation in the Tasman Sea, SW Pacific Ocean) showed similarly homogenous conditional stability constants for the complexation of Cu by natural organic ligands. Greater than 99% of dissolved Cu is complexed to natural organic ligands; thus, the strong complexation by organic Cu-binding ligands likely stabilises the heavy isotopic composition of dissolved Cu in the Tasman Sea at around 0.70±0.23‰ (n=47, 2SD). It is hypothesised that the breakdown of dissolved organic Cu complexes may provide a source of isotopically heavy Cu that could be removed from the oceanic system as a sink in certain environments such as anoxic basins.