| Summary: | This project has two distinct parts; Project A, an acidification experiment intended to illustrate the effects of ocean acidification on the isotopic signature of foraminiferal calcite during the Palaeocene/Eocene thermal maximum (PETM) and Project B, an ecological study of the benthic foraminifera of Plymouth Sound. The PETM was a period of significant global warming, estimated to be between 4-9°C approximately 55Ma. This event has been attributed to a massive release of isotopically light carbon into the atmosphere which resulted in a considerable negative carbon isotope excursion (CIE) recorded by both marine and terrestrial proxies. The magnitude of this CIE is disputed; terrestrial higher plant proxies indicate a negative CIE of 4-6%o, while marine foraminifera record a negative shift of 2-4%o. It has been suggested that the isotopic signature of many of the foraminifera tests fi-om the PETM used for palaeoceanographic reconstructions have been subjected to dissolution induced fractionation of the lighter isotopes of carbon and oxygen as a result of CO2 driven oceanic acidification; as this may account for some of the discrepancy noted between the marine and terrestrial proxies This study was designed to assess the effect of ocean acidification on the isotopic signature of foraminiferal calcite post-mortem. An acidification experiment was conducted where foraminifera tests of the species Ammonia beccarii were subjected to different sea water conditions with pH values of 7.4, 7.6, 7.8 and 8.1 for a period of 1, 2, 4 and 7 days. S'^O and 5'^C isotopic analysis of the foraminifera after the experiments revealed no significant trends to suggest dissolution induced fractionation had affected the isotopic signature of the specimens used in this experiment. It is suggested that the theory behind the experiment is yet to be fully realised and that the results are an artefact of insufficient acidity and exposure time during the experiment; and that the natural isotopic variability of the population used for the experiment was too large and may have absorbed any small but significant trends. The ecological study of Plymouth Sound focused upon the benthic foraminiferal community with reference to previous work. Samples were collected during February and June from 16 localities within Plymouth Sound. The localities were situated along two transects traversing different substrate types; some of the locations sampled in previous studies were also revisited. The substrate types, where comparable, were foimd to be the same as when mapped and sampled in the early 1990's, which suggests there has been no major changes in the sediment types or energy regimes of Plymouth Sound in the past 17 years. A variety of sediment types were recorded, from muds containing some sand and gravel to well washed gravels comprised of shell fragments. This reflects the diverse range of energy regimes at work in Plymouth Sound. The species that characterize the living population of the Plymouth Sound assemblage are Ammonia beccarii, Cibicides lobatulus, Elphidium crispum and Gaudryina rudis. The dead population is dominated by Ammonia beccarii, Cibicides lobatulus, Elphidium crispum and Quinqueloculina species. The species represented in the Plymouth Sound assemblage have not changed significantly in the past 40 years. The living population was extremely low in diversity and abundance. Species richness appears to increase with decreasing median grainsize and total carbon levels in the sediment; and increase with increasing total nitrogen levels in the sediment. The dead assemblage in contrast is abundant and diverse. This pattern is suggested to reflect the general level of productivity of the benthic foraminifera outpacing the relatively low sedimentation rates in Plymouth Sound, coupled with sampling error and natural distributional patchiness of fauna. Faculty of Science
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