HR: 0800h
AN: PP31B-0421    [Abstracts]
TI: Mg/Ca Ratios in Planktonic Foraminifera Across a Strong Salinity Gradient in the Mediterranean Sea
AU: * Ferguson, J E
EM: julie.ferguson@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Henderson, G M
EM: gideon.henderson@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Kucera, M
EM: michal.kucera@uni-tuebingen.de
AF: Institut für Geowissenschaften, Eberhard-Karls Universität Tübingen, Sigwartstrasse 10, Tübingen, D-72076, Germany
AU: Rickaby, R E
EM: rosr@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AB: The Mediterranean Sea provides an ideal environment to conduct a field study of the effect of salinity on Mg incorporation into planktonic foraminifera. Its semi-enclosed nature results in the development of a considerable gradient (5 psu) in salinity while its west to east alignment leads to relatively small sea surface temperature gradients (a maximum of 8° C in the summer). Eleven box core tops were sampled from across the Mediterranean Sea. Orbulina universa, Globigerinoides ruber (white) and (pink) and Globigerinella siphonifera, were picked and thoroughly cleaned using standard techniques for trace-metal work. Samples were analysed for Mg/Ca, Cd/Ca, U/Ca, Sr/Ca, Fe/Ca, Al/Ca and Mn/Ca. Samples were screened for contamination using Mn/Ca ratios for ferro-manganese crusts and Fe/Ca and Al/Ca for clays. SEM images and sequential dissolution experiments indicated that most samples did not feature inorganic encrusting high-Mg calcite (which has previously been found in some high salinity regions such as the Red Sea). After rejecting samples that failed these screening tests, Mg/Ca ratios in all species are higher, for their calcification temperatures, than seen in previous studies. There is an overall increase in Mg/Ca from west to east and percentage increases per ° C are unusually high. Mg/Ca ratios are particularly high in samples from the high-salinity eastern Mediterranean. We present arguments to suggest that these high values are not the result of nutrient or carbonate ion changes, with both of these variables being similar to those in the open ocean. Correlations of Mg/Ca with salinity are generally more significant than with temperature, with Mg/Ca increases per salinity unit of 15-60% - much higher than suggested in previous culture studies. Our results suggest that salinity may act as a significant secondary control on Mg/Ca ratios in addition to the dominant temperature control. A re-analysis of previous calibrations from other settings also suggests that salinity changes may contribute to the variations seen in the open ocean. We suggest that caution should be used when using Mg/Ca as a palaeotemperature proxy in areas of high salinity, such as the Caribbean, or where large salinity changes have occurred in the past.
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1065 Major and trace element geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting