Van Horsten. 2016, Dr Hélène Planquette, Dr Géraldine Sarthou, Ryan-Keogh T J, Dr Nolwenn Lemaitre, Mtshali T., Roychoudhury A. N., Dr Eva Bucciarelli

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 (Baxs) concentrations were measured for the first time, along 30◦ E in the southern Indian Ocean. Winter Baxs concentrations of 59 to 684 pmol/L were comparable to those observed throughout other seasons. The expected decline of the mesopelagic Baxs 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 Baxs data, including data from this study, shows an accumulation rate of ∼ 0.9 μmol/m2/d from September to July that correlates with temporally integrated remotely sensed primary productivity (PP) throughout the SO from data spanning ∼ 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 ∼ 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 Baxs stock, we could obtain better estimates of carbon export and remineralisation signals within the SO on annual and basin scales.

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Integrated mesopelagic Baxs stock plotted against integrated remotely sensed PP from the preceding September up to one month prior to sampling, all available literature data and winter data from this study, (a) South of the PF (SPF, black squares) and (b) North of the PF (NPF, black circles). Red open squares are data points from our winter dataset where there was not sufficient remote sensing PP data to integrate up to 1 month prior to sampling and available data up to 3 months prior to sampling was plotted but not included in the statistical analysis.