Lithogenic particle flux to the subantarctic Southern Ocean : a multi-tracer estimate using sediment trap samples

The delivery of lithogenic material from atmospheric dust to remote regions of the Southern Ocean is thought to be a key source of micronutrients, particularly iron, essential for phytoplankton. Here, we present a time series of lithogenic flux estimates from 2010 to 2019 using sinking particles collected by sediment traps at 1000m at the Southern Ocean Time Series mooring station in the subantarctic Southern Ocean. Lithogenic flux estimates were made based on aluminium, titanium, iron and thorium concentration in sinking particles and showed good agreement with one another. A multi-tracer flux estimate was calculated using an average of all individual tracer flux estimates in order to reduce biases associated with each element's reactivity in seawater. This mean lithogenic flux exhibited a strong seasonality, with two peaks in late spring and in summer. The magnitude of our multi-tracer lithogenic flux was comparable to previous sediment trap-based fluxes reported in the Southern Ocean and to 230Th-normalised lithogenic fluxes from core-top sediments at the same study location. A lack of lithogenic tracer enrichment in our samples compared to the average upper continental crust values indicated that lithogenic material dominates iron supply in this region. Sinking particle samples from SOTS showed similar Fe enrichment to aerosols collected over southern marine regions around Australia, suggesting that Australian dust may be the primary lithogenic source to SOTS sinking particles. Evidence of lead (Pb) enrichment in sediment trap samples at 1000m highlighted a non-negligible contribution from anthropogenic particles in SOTS sediments, the later source which could represent a secondary Fe supply to the samples. Therefore, we concluded that aeolian emissions from Australia likely compose the primary source of lithogenic particles to SOTS subantarctic Southern Ocean 1000m-deep waters. This hypothesis was further supported by maximum lithogenic particle flux at 1000m depth in late spring preceded by seasonal atmospheric aerosol loading from aerosol optical depth and satellite reanalysis modelled dust deposition maxima that occurred during the Australian dust storm season in spring across the study period. While carbon export at 1000m showed good correlation with surface chlorophyll concentrations at SOTS across the study period, a lack of linear relationship between lithogenic fluxes at 1000m and productivity leaves the link between marine productivity and 1000m-deep sinking particles in this subantarctic region of the Southern Ocean unclear.