HR: 0800h
AN: PP11B-1447    [Abstracts]
TI: Evaluation of Benthic Foraminiferal Mg/Ca and δ¹⁸O: Paleoceanographic Application
AU: * Fukuda, K
EM: kenichif@alkali.otago.ac.nz
AF: University of Otago, Department of Chemistry, P.O Box 56, Dunedin, 8001 New Zealand
AU: Frew, R D
PP11B-1447 AF: University of Otago, Department of Chemistry, P.O Box 56, Dunedin, 8001 New Zealand
AU: Fordyce, R E
PP11B-1447 AF: University of Otago, Depatment of Geology, P.O Box 56, Dunedin, 8001 New Zealand
AB: Using several different analytical approaches on the same samples is crucial for reducing uncertainties in paleoceanographic studies. We examined two different sequences near Oamaru, New Zealand to evaluate a combination of Mg/Ca and δ¹⁸O techniques on benthic foraminifera. As a trial, we chose well-preserved material from the Altonian stage (-18 Ma) while as an application, cemented/altered material in Whaingaroan/Runangan stage (-34 Ma) was selected. For the Altonian, Mg/Ca in Notorotalia spinosa and Cibicides spp. were analysed by ICP-OES throughout the fossiliferous sequence and then paleotemperatures were estimated by our modern Mg/Ca calibration curves. The δ¹⁸O in N. spinosa and some Cibicides were also measured from the same stations for pairing with Mg/Ca results. Further, to evaluate paleotemperature estimates from the whole tests, spots analyses of Mg/Ca were taken through the successive chambers for the two species using Electron Probe Micro Analysis (EPMA). Paleotemperatures through the successive chambers, which should be related to their life spans, were estimated by the modern calibration curves established from EPMA analysis. Results show that Notorotalia may retain at least an annual record while the signal in Cibicides may retain a part of season. There is distinctive seasonality observed in this period and the δ¹⁸O_seawater estimates paired with Mg/Ca in N. spinosa are comparable with published estimates. For the Whaingaroan/Runangan, Mg/Ca in Cibicides parki (ICP) shows relatively low values (cool) through this sequence in agreement with EPMA analysis. However, δ¹⁸O-derived temperatures from C. parki imply warmer conditions prevailed. In addition, Mg/Ca and δ¹⁸O from Cribrorotalia (closely related to Notorotalia) provide similar temperature estimates to the C. parki isotope results. It appears that Mg/Ca in certain species are susceptible to post-mortem alteration resulting in lower apparent temperatures. Spot analyses in Cribrorotalia show no distinctive seasonality and the δ¹⁸O_seawater estimates indicate ice-free conditions. We conclude that pairing Mg/Ca with δ¹⁸O allows the estimation of δ¹⁸O_seawater, but only if well-preserved and annual recorder specimens are examined. Combination with EPMA analysis may provide insight into seasonal variability.
DE: 4870 Stable isotopes (0454, 1041)
DE: 4875 Trace elements (0489)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005