| Summary: | Thesis (MSc)--Stellenbosch University, 2020. ENGLISH ABSTRACT: The carbon cycle in the Southern Ocean (SO) plays a very important role for all life on earth as it regulates carbon fluxes, transport nutrients to marine organisms and it is linked to the amount of available oxygen in the atmosphere. The conditions driving the carbon cycle dramatically changed over the past few decades which directly effects marine phytoplankton. Phytoplankton are key role players in the marine food web as they utilize atmospheric carbon dioxide transported into the surface ocean through different chemical exchanges. Fractionation of stable carbon isotopes occurs during the biological uptake of inorganic carbon, although all the influential forces behind the fractionation are not yet fully understood. Here we examine the potential physical- and chemical driving factors for carbon isotopic signature of particulate organic carbon (δ13CPOC) in the surface water of the SO. Winter and summer results are compared along the Bonus Good Hope line as well as a section in the Indian Ocean. The aim is to determine potential influences of the physical- and chemical driving factors on phytoplankton δ13CPOC, because it is possible to measure δ13CPOC directly from underlying sediments. Understanding the driving factors of δ13CPOC, will help the interpretation of sedimentary δ13CPOC in paleo-reconstruction studies, such as reconstructing past atmospheric partial pressure of carbon dioxide (pCO2 concentrations). Our results indicate that silicic acid and nitrate concentrations (μM), cyanobacteria and diatoms contributions to total chl-a (%) and temperature (ºC) could potentially be primary driving factors of δ13CPOC during the summer along the Bonus Good Hope line, as all these factors are significantly correlated to δ13CPOC. Winter results in the Atlantic sector of the SO (Winter Cruise 2015) did not show any potential driving factor. This is in contrast with results found in the Indian sector of the SO (Winter Cruise 2017), where all four ...
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