Early winter barium excess in the Southern Indian Ocean as an annual remineralisation proxy (GEOTRACES GIPr07 cruise)

Abstract. The Southern Ocean is of global importance and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region is of substantial interest. During this study the proxy barium excess which is utilised to shed light on mesopelagic remineralisation was measured at seven stations along 30° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge this is the first reported winter study utilising this proxy in the Southern Ocean. Concentrations of 59 to 684 pmol L⁻¹ were comparable to those observed throughout other seasons, indicating that this proxy has a longer timescale than previously thought. Background barium excess values observed in deep waters were also similar to previous studies, not having declined down to an expected true Southern Ocean background value. It is apparent that processes driving the mesopelagic barium excess signal are still underway during early winter. Indicating that continuous remineralisation is sustained at levels comparable to summer, well after bloom termination. Moreover, linking integrated remote sensing primary production to the mesopelagic barium excess signal reiterates a longer timescale. The significant positive correlations obtained in the Antarctic and Subantarctic zones suggest that mesopelagic barium excess stock can be used as a remineralisation proxy on an annual timescale and possible inference of carbon remineralisation from remote sensing data on an annual and basin scale.