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 (δ 65 dCu) 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 δ 65 dCu values ranging from 0.25 ‰ to 0.31 ‰. Mixed layer δ 65 dCu 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, δ 65 dCu profiles showed significant variability in the upper water column (200 m). The δ 65 dCu composition was heavier at depths of chlorophyll a maxima, particularly for the high productivity stations, attributed to either biological uptake and/or scavenging. A heavier surface δ 65 dCu 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) δ 65 dCu 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 δ 65 Cu dataset, affording new insights into the biogeochemical cycling of Cu in the ocean. • Nine copper concentration and isotope depth profiles are present for waters east of Australia extending into the Southern Ocean. • Copper isotope compositions were generally heavier at the depth of the chlorophyll maximum, especially for high-productivity stations. • Potential for benthic input of isotopically light copper for deep water close to the East Tasman Rise.