Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Subject "Biogeochemical cycles -- Seasonal variations"

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    Insights into early winter Southern Indian Ocean dissolved iron distributions and remineralisation using excess barium (GEOTRACES GIPr07 cruise)
    (Stellenbosch : Stellenbosch University, 2022-04) Van Horsten, Natasha; Roychoudhury, Alakendra N.; Sarthou, Géraldine; Mtshali, Thato; Bucciarelli, Eva; Planquette, Hélène; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Southern Ocean is of central importance to the global ocean, connecting the Atlantic, Pacific and Indian basins via the Antarctic Circumpolar Current, which carries large amounts of heat, carbon and nutrients. Thus, this oceanic province partly regulates the Earth's climate, present and future. One of the factors influencing the biological carbon pump of the Southern Ocean is the availability of iron, which is an essential nutrient for phytoplankton. However, observations in this region have mainly been conducted in spring and summer, leaving significant gaps in our knowledge of the biogeochemical cycle of iron, in particular on the seasonality of internal processes such as mesopelagic remineralisation. During this study, the distributions of dissolved iron and excess barium (Baxs), a mesopelagic remineralisation proxy, were measured throughout the water column, at seven stations along 30 ° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge, this is the first winter study on these two parameters and in this sector of the Southern Ocean. A more in-depth evaluation of the Baxs proxy was conducted by comparing integrated remote sensing primary production data and the Baxs signal, for all observations available in the Southern Ocean, on an annual timescale. Our results suggest that remineralization is maintained at comparable levels as observed during summer, well after the bloom decline, resulting in the mesopelagic Baxs signal, at the onset of winter. Regarding dissolved iron, the winter concentrations were not significantly different to summer observations in the western Southern Indian Ocean, remaining at sub-nanomolar concentrations, from 0.02 to 0.73 nmol L-1. Concentrations in surface water were depleted, resembling late summer observations, indicating that in July there has not yet been a replenishment of the dissolved iron stock in surface waters. The distributions of dissolved iron and Baxs exhibited peaks in intermediate waters on the water column profiles, indicating iron remineralization. Using the apparent oxygen utilization proxy, we estimated that a large fraction (58 ± 1 to 160 ± 2%) of the measured mesopelagic dissolved iron stock was provided by remineralisation. Iron remineralisation fluxes were largest in the subantarctic zone and decreased to a minimum in the oligotrophic Subtropical zone. This study contributes to the emerging understanding of winter trace metal distributions and internal cycling, giving insight into the replenishment of the winter dFe reservoir, as well as obtaining better estimates of export and remineralisation on annual and basin scales, within the Southern Ocean.