Seasonal dynamics of coastal water masses in a Scottish fjord and their potential influence on benthic foraminiferal shell geochemistry
Alix G. Cage, William E. N. Austin, 2008. "Seasonal dynamics of coastal water masses in a Scottish fjord and their potential influence on benthic foraminiferal shell geochemistry", Biogeochemical Controls on Palaeoceanographic Environmental Proxies, W. E. N. Austin, R. H. James
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Pronounced seasonal heating of middle and high latitude shelf seas results in large temporal changes in seawater temperature that have the potential to be recorded by benthic foraminifera. Predicted oxygen isotope composition for calcite precipitating in equilibrium conditions with seawater suggest that pronounced ‘seasonal isotope effects’ may be encountered in the growth history of benthic foraminifera. Such ‘seasonal’ effects can be difficult to distinguish from so-called ‘vital effects’, where shell and equilibrium calcite values are offset by a constant difference in oxygen isotope values. Preliminary findings suggest that benthic foraminifera may have more than one phase of growth, for example Ammonia becarii calcifies in spring and late summer, potentially introducing apparent intra-annual and inter-annual temperature variations of >1°C into palaeoclimatic reconstructions when mixed-season populations are sampled. We highlight the need to select species-specific palaeotemperature equations to establish reliable isotopic disequilibria and illustrate the importance of understanding the ‘seasonal isotope effect’ when considering disequilibrium effects in foraminifera which have grown in seasonally-changing environments.
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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.