The Role of Acantharia in Southern Ocean Strontium Cycling and Carbon Export: Insights From Dissolved Strontium Concentrations and Seasonal Flux Patterns

Dissolved strontium (Sr) concentrations from Southern Ocean water samples and Sr export fluxes from sediment trap moorings at 1,000 m were used to assess particulate organic carbon (POC) export associated with Acantharia for 2010, 2018, and 2020. The dissolved Sr data revealed a prominent vertical gradient with lower surface Sr concentrations depleted up to 1.5% relative to deep waters. A strong latitudinal surface gradient was observed, ranging from 86.8 μmol kg⁻¹ near the northern end to 88.0 μmol kg⁻¹ near the southern end of a transect through the Australian sector of the Southern Ocean. The vertical and latitudinal gradients are associated with celestite (SrSO₄) precipitation by Acantharia and subsequent export to depth. Seasonal variability in Sr export fluxes can be large, particularly during intense events in summer, and reaches a maximum of 11.7 mmol Sr m⁻² yr⁻¹, contributing up to 7% of the POC export flux. The coincidence of Sr flux with the second peak of POC export flux implies an association of Acantharia biomass with summertime productivity.