| Summary: | Diatoms are key primary producers and therefore make an important contribution to carbon export. These phytoplankton utilise dissolved silica (DSi), sourced from riverine, atmospheric, sedimentary and hydrothermal inputs, and convert it to biogenic silica (bSiO2). Some of this bSiO2 dissolves in the water column, regenerating the DSi pool, whereas the remaining fraction settles out to the sediments (e.g. via aggregates) to form bSiO2 accumulations. These occur primarily in the Southern Ocean and provide a sedimentary archive regarding export production over hundreds to millions of years. Although atmospheric inputs of soluble Si to the surface ocean contribute to nutrient DSi, findings in this thesis reveal that they represent just ~0.2 – 0.4% of total DSi inputs to the Global Ocean. It is also shown that aerosol Si inputs originate primarily from Saharan dust, but that the soluble fraction is small (0.12% - 0.29%) in comparison to total aerosol Si. These solubilities are an order of magnitude lower than that used in an existing hypothesis that proposed a global dust increase of 2 – 8 times present values could explain glacial - interglacial pCO2 transitions. Hence, it is shown that supply would actually need to increase by a factor of 50 – 300 for this hypothesis to hold. Data obtained during the CROZEX cruise showed that DSi supports high diatom productivity to the north of the Crozet islands in the Southern Ocean, where iron inputs from underlying sediments/plateau relieves High Nutrient Low Chlorophyll (HNLC) conditions. Results presented in this thesis show that a phytoplankton bloom in October 2004 was characterised by large, heavily silicified diatoms. As nutrients were depleted there was a shift towards smaller and more lightly silicified cells that dissolved within the euphotic zone. In contrast, more silicified cells tended to be exported, often as aggregated bSiO2, particularly to the south of the plateau where Fe limitation meant that low productivity was characterised by heavily silicified diatoms ...
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