Foraminifer test preservation and diagenesis: comparison of high latitude Eocene sites
Paul N. Pearson, Catherine E. Burgess, 2008. "Foraminifer test preservation and diagenesis: comparison of high latitude Eocene sites", Biogeochemical Controls on Palaeoceanographic Environmental Proxies, W. E. N. Austin, R. H. James
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Foraminifer tests are prone to diagenetic recrystallization on a micron scale that can affect their geochemical composition, hence it is important to identify fossil material that is well-preserved. Here we illustrate the textures of tests from several high-latitude Eocene sites using a combination of Reflected Light Microscopy and high-resolution Scanning Electron Microscopy. The sites are Ocean Drilling Program (ODP) Site 647 in the Labrador Sea (53°N), ODP Site 689 at Maud Rise in the Weddell Sea (64°S), ODP Site 1135 (Kerguelen Plateau c. 59°S) and outcrop samples from Hampden Beach, New Zealand (palaeolatitude c. 55°S). The foraminifera studied from Site 647 and Hampden Beach have glassy, transparent tests that show only minor signs of diagenetic alteration, whereas the foraminifera from Sites 690 and 1135 are opaque and recrystallized. We associate the better state of foraminifer preservation at the former sites with the high clay content of the sediments. Our observations suggest that Site 647 and Hampden Beach may be useful for the establishment of high-latitude sea-surface temperatures and hence global temperature gradients in the Eocene.
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Most of our information about the evolution of Earth’s ocean-climate system comes from the analysis of sediments laid down in the past. For example, the microfossil assemblage reflects the temperature, salinity and nutrient abundance of the water in which the organisms lived, while the chemical and isotopic composition of biogenic carbonates may be used to reconstruct past variations in the operation of the carbon cycle, as well as changes in ocean circulation.
Nevertheless, understanding the link between these sediment variables (or ‘proxies’) and environmental conditions is not straightforward. This volume adopts a novel approach by bringing together palaeontologists, geochemists and palaeoceanographers, who contribute evidence that is required to better constrain these proxies. Topics include: (i) processes of biomineralization, and their effect on the chemical and isotopic composition of different organisms; (ii) proxy validation, including field, laboratory and theoretical studies; (iii) the links between modern and fossil organisms.