Swann, G.E.A.; (2007) Diatom oxygen isotopes and biogenic silica concentrations: an examination of their potential for reconstructing palaeoenvironmental change. Doctoral thesis , University of London.

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Abstract

Diatoms represent between 25% and 45% of all marine primary productivity. As yet, however, the potential of using diatom isotopes, in particular oxygen (518Odiatom), in palaeoceanographic reconstructions has yet to be fully assessed. Work here within this thesis demonstrates a method for extracting pure, mono or near mono species specific, diatom samples from marine sediments. Over two time intervals, the potential of using 8l8Odiatom in palaeoceanographic reconstructions is then shown at ODP site 882 in the North West Pacific Ocean over the onset of major Northern Hemisphere Glaciation (2.73 Ma) and over the last 200 kyr BP. During both periods, major changes in the halocline/stratification of the water column, relating to significant freshwater input to the region, are reconstructed. These results indicate a significant role for the region in initiating glacier growth across the North American continent and in causing changes in atmospheric pC02. Strong evidence exists, however, that both an inter- and intra-species vital/species effect of up to 3.5l%o may be present in 518Odiatom. As such, this may limit future applications of 518Odiatom to samples which are dominated by only one taxa. Section two of the thesis investigates the potential for wet-alkaline measurements of BSi to provide insights into the global silicon cycle. A sequential Si/Al technique, which directly accounts for levels of clay/aluminosilicate digestion, is demonstrated to produce lower and more accurate measurement of BSi in samples for which a rapid non-BSi digestion phase occurs. However, comparisons between wet-alkaline BSi measurements and diatom biovolume measurements, which more accurately reflect the true amount of BSi within a sample, show a poor relationship between the two variables. This is particularly true in samples where diatom dissolution is high. Consequently, diatom biovolume measurements may be better suited for making quantitative palaeoenvironmental reconstructions.

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