French, M;
(2009)
Silica: Cycling in the Southern Ocean and Atmospheric Inputs to the Global Ocean.
Doctoral thesis , Universtiy of East Anglia.
Abstract
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 typical of HNLC conditions. The importance of aggregation, diatom species, physiological state and temperature in bSiO2 dissolution was investigated experimentally. Findings presented in this thesis revealed that i) the rate of dissolution of aggregates formed using the highly silicified, typically HNLC diatom Fragilariopsis kerguelensis was significantly lower than that for the lesser silicified Chaetoceros debilis, ii) dissolution increased markedly with temperature, and iii) dissolution of exponentially grown C.debilis aggregates began with ~14 day lag in comparison to their senescent counterparts.
| Type: | Thesis (Doctoral) |
|---|---|
| Title: | Silica: Cycling in the Southern Ocean and Atmospheric Inputs to the Global Ocean. |
| Event: | Universtiy of East Anglia |
| Additional information: | Held at UEA library: Accession Number:uea.003790677. http://search.ebscohost.com/login.aspx?direct=true&db=cat01883a&AN=uea.003790677&site=eds-live&scope=site |
| Keywords: | Silica, Southern Ocean, diatoms, dissolution |
| UCL classification: | UCL > Provost and Vice Provost Offices UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Inst for Risk and Disaster Reduction |
| URI: | https://discovery.ucl.ac.uk/id/eprint/1462267 |
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