Tracing iron along the flowpath of East Australian Current using iron stable isotopes

Abstract

The mechanisms delivering iron (Fe) to Southern Ocean surface waters and resulting capacity for the oceanic drawdown of CO2 are important to constrain in a changing climate. The East Australian Current (EAC) is a major western boundary current flowing south along the eastern margin of Australia, hypothesized to play an important role in entraining Fe from a variety of potential sources and supplying Fe to large annual spring phytoplankton blooms in HNLC waters. We report Fe concentration and Fe isotope (δ56Fe) profiles for both dissolved and particulate phases at stations within the EAC, in the frontal mixing zone, and in subantarctic waters along the southward flowpath of the EAC in early spring. Mixed layer dFe concentrations declined from EAC source waters (0.32–0.53 nmol kg−1) into subantarctic waters (0.27–0.42 nmol kg−1) with a concurrent deepening of the ferricline. Particulate trace metal concentrations indicate robust inputs of lithogenic particles to subtropical EAC waters, potentially impacted by advection of shelf sedimentary particles and evidence of a significant supply from local aerosol deposition. The isotopic composition of pFe in the EAC (0.3 ± 0.1‰) is similar to prior reports of aerosol δ56Fe and upper ocean δ56pFe in the western Pacific and isotopically heavier than average values for the bulk continental crust. In EAC surface waters, isotopically light δ56dFe values (0.11–0.16‰) are observed likely due to reductive release of dFe from lithogenic particles in the photic zone with a particle-dissolved phase fractionation (Δ56FedFe–pFe) of −0.1 to −0.4‰. At depth, elevated dFe concentrations and a heavy δ56dFe signature suggest a non-reductive release of dFe from lithogenic particles via desorption or dissolution processes with a Δ56FedFe–pFe of +0.3‰. During southward transit of EAC waters, mixed layer dFe pools become increasingly heavy due to biological uptake through the frontal mixing zone and into subantarctic waters. We also observe differences in particulate trace metal concentrations in intermediate waters between subantarctic stations that could result from entrainment of sedimentary particles during transit over the relatively shallow South Tasman Rise.