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

• Published in: Biogeosciences
• Main Authors: van Horsten, Natasha René, Planquette, Hélène, Sarthou, Géraldine, Ryan-Keogh, Thomas James, Lemaitre, Nolwenn, Mtshali, Thato Nicholas, Roychoudhury, Alakendra, Bucciarelli, Eva
• Other Authors: Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Institut Universitaire Européen de la Mer (IUEM), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Council for Scientific and Industrial Research Cape Town (CSIR), Ministery of Science and Technology, Department of Earth Sciences Stellenbosch, Stellenbosch University, Institute of Geochemistry and Petrology ETH Zürich, Department of Earth Sciences Swiss Federal Institute of Technology - ETH Zürich (D-ERDW), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology Zürich (ETH Zürich)-Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology Zürich (ETH Zürich), Auteur indépendant, Co-funded by a grant from the French government under the programme “Investissements d’Avenir”. International collaboration was made possible by funding received by the French–South African National Research Foundation (NRF) Collaboration (PROTEA; FSTR180418322331), NRF funding (SNA170518231343 and UID 110715) including funding from South African Department of Science and Innovation, French Ministry of National Education, Higher Education and Research, and the French Ministry of Foreign Affairs and International Development., ANR-17-EURE-0015,ISBlue,Interdisciplinary Graduate School for the Blue planet(2017)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: barium; biological pump; mesopelagic zone; primary production; remineralization; remote sensing; winter; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Southern Ocean; Austral; Indian
• Online Access: https://hal.science/hal-03811499; https://hal.science/hal-03811499/document; https://hal.science/hal-03811499/file/bg-19-3209-2022.pdf; https://doi.org/10.5194/bg-19-3209-2022

International audience 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−1 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 m−2 d−1 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.