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

Abstract

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.