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

The Southern Ocean (SO) is of global importance to the carbon cycle, and processes such as mesopelagic remineralisation that impact the efficiency of the biological carbon pump in this region need to be better constrained. During this study early austral winter barium excess (Ba-xs) concentrations were measured for the first time, along 30 degrees E in the southern Indian Ocean. Winter Ba-xs concentrations of 59 to 684 pmol L-1 were comparable to those observed throughout other seasons. The expected decline of the mesopelagic Ba-xs signal to background values during winter was not observed, supporting the hypothesis that this remineralisation proxy likely has a longer timescale than previously reported. A compilation of available SO mesopelagic Ba-xs data, including data from this study, shows an accumulation rate of similar to 0.9 mu mol m(-2) d(-1) from September to July that correlates with temporally integrated remotely sensed primary productivity (PP) throughout the SO from data spanning similar to 20 years, advocating for a possible annual timescale of this proxy. The percentage of mesopelagic particulate organic carbon (POC) remineralisation as calculated from estimated POC remineralisation fluxes over integrated remotely sensed PP was similar to 2-fold higher south of the polar front (19 +/- 15 %, n = 39) than north of the polar front (10 +/- 10 %, n = 29), revealing the higher surface carbon export efficiency further south. By linking integrated remotely sensed PP to mesopelagic Ba-xs stock, we could obtain better estimates of carbon export and remineralisation signals within the SO on annual and basin scales.