Globigerina bulloides : Preservation state and stable isotope variation
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A 'preservation index' has been developed that enables selective population sampling of the planktic foraminiferal species Globigerina bulloides for geochemical and isotopic analyses. Specimens from deep-sea core horizons were separated into 5 grades, based on the degree of transparency exhibited under the transmitted light microscope (grade 1 = thick-walled, non-transparent tests, to grade 5 = thin-walled, entirely transparent tests). 'Preservation indices' were then derived for a down-hole sequence of 23 samples from deep-sea core SO 136-005 (Eastern Tasman Sea, 958m). These indices indicate a general increase in dissolution downhole and significant variation in preservation between alternate sampled horizons. Fragmentation counts were made using the same samples and indicate increased fragmentation with depth. Analysis of δ18O showed a significant difference in concentration of that isotope (0.2 to 1.2‰) between grade 1 and grade 5 specimens (where usually, a divergence of >0.3‰ is considered significant). The fact that it is possible to derive contrasting δ18O results from specimens of one foraminiferal species extracted from the same sample has far reaching implications for paleoceanography in general, and oxygen isotopic studies in particular. These results show that significant variation in δ18O signal can be entirely owing to preservation level. Collection of samples exhibiting the same preservation state, or systematic selection of the same number of specimens from different grades for all horizons in a sediment pile, should lead to increased accuracy with respect to the resultant oxygen isotope data for paleoceanography studies. In many cases, foram tests used in this study also exhibited a secondary calcite layer on the external surface. The fact that the remains of nannofossils were occasionally found submerged in the same layer is interpreted as a potential source of contamination of the primary oxygen isotope signal. The ease with which this secondary layer of calcite, on the exterior surfaces of foram tests, could be removed through acid etching was also examined. Foram tests were exposed to acetic acid solutions of pH 5.5-6.5 for differing exposure times and the effects of the acid on tests examined under the Scanning Electron Microscope. An optimum exposure time of 2 days at pH 5.5 was derived for total removal of the surface layer. A number of recommendations are made with regard to areas of future research resulting from the present study.