Biogeochemical cycling of dissolved Cu along the East Australian Current

Copper (Cu) is an important micronutrient that is involved in multiple metabolic processes in marine phytoplankton. However, Cu concentrations that exceed an organism-specific tolerance level can be toxic. The measurement of stable Cu isotopic composition in seawater is an effective tool to better understand the biogeochemical cycling of this micronutrient in the marine environment. Here, we report the dissolved Cu (dCu) concentration and isotope composition of waters of the East Australian Current (EAC), the western boundary current in the south Pacific. The voyage was undertaken in 2018 during the austral spring as GEOTRACES process study GPpr13 consisting of a north-south transect categorised by the transition from warm, salty subtropical (ST) waters to the cold, fresh subantarctic (SA) waters in the Pacific sector of the Southern Ocean. The average dCu isotope composition (δ⁶⁵dCu) of the upper water column increased by ∼0.3 ‰ from north to south. The northern EAC stations generally had isotopically lighter dCu in surface waters, with mixed layer δ⁶⁵dCu values ranging from 0.25 ‰ to 0.31 ‰. Mixed layer δ⁶⁵dCu values ranged from 0.20 ‰ to 0.47 ‰ at the southern extension of the EAC and from 0.54 ‰ to 0.63 ‰ in the subantarctic zone. Generally, δ⁶⁵dCu profiles showed significant variability in the upper water column (200 m). The δ⁶⁵dCu composition was heavier at depths of chlorophyll a maxima, particularly for the high productivity stations, which can be attributed to either biological uptake and/or scavenging. A heavier surface δ⁶⁵dCu isotopic signature relative to deeper waters was also observed at stations with low chlorophyll a concentrations due to strong organic complexation of the heavy isotope in the upper water column. The deep ocean (≥2000 m) δ⁶⁵dCu at one of the subantarctic stations (station 10) was considerably lighter (0.25 ± 0.06 ‰), indicating a possible benthic supply of isotopically light Cu. This study provides a high-resolution δ⁶⁵Cu dataset, affording new insights into the biogeochemical cycling of Cu in the ocean.