A Reconstruction of Seawater δ44Ca From Foraminiferal Records of the Western Equatorial Pacific Ocean

We present δ⁴⁴Ca records of three different foraminifera species (G. ruber/subquadratus, G. trilobus and Globigerinella spp.) from the western equatorial Pacific (ODP Site 871 and 872) corresponding to the last about 25 Ma. Assuming a constant calcium isotope fractionation factor (α) between seawater and the foraminiferal calcium carbonate the δ⁴⁴Ca of the past seawater (δ⁴⁴Ca_sw) can be reconstructed. The δ⁴⁴Ca_sw records of G.ruber/subquadratus and of Globigerinella spp. are similar. The two records show a decrease of the δ⁴⁴Ca_sw between 25 and 16 Ma of about 0.5 ‰ followed by an increase of about 0.5 ‰ between 16 and 3 Ma. Between 3 Ma and the present the δ⁴⁴Ca_sw decreases again by about 0.25 ‰ . The δ⁴⁴Ca_sw calculated from the G. trilobus record shows a similar trend between 22 and 6 Ma but is isotopically lighter by about 0.2 ‰ compared to the other two records: The δ⁴⁴Ca_sw of Site 871 is positively related to its ⁸⁷Sr/⁸⁶Sr record. This indicates that the δ⁴⁴Ca_sw is triggered by the balance of the input of continental weathering products and submarine volcanism and the output of biogenically driven calcium carbonate precipitation.\From G. sacculifer being closely related to G. trilobus it is known that calcium isotope fractionation is temperature dependent (Nägler et al. 2000; Gussone et al. 2002). Thus the differences between the δ⁴⁴Ca_sw from G. trilobus and the mean of the δ⁴⁴Ca_sw of G. ruber/subquadratus and Globigerinella spp. can be interpreted as a temperature related signal. Using the correlation of the fractionation factor (α) and temperature (T) of G. sacculifer the calculated temperature varies between 26.5 °C and 29 °C over the past 23 Ma. The calculated temperatures of the G. trilobus record show a cooling trend between 23 and 16 Ma followed by slight warming between 16 and 3 Ma. Between 3 and 1.5 Ma a rapid warming can be observed. These observed temperature variations are in the order of 1-2 °C. In contrast, the δ¹⁸O record of Site 871 and the global evolution of the δ¹⁸O record of benthic foraminiferas show a cooling between 16 Ma and 1.5 Ma being in contradiction to the δ⁴⁴Ca based temperature reconstructions of the G. trilobus record. References:\Gussone N., Eisenhauer A., Dietzel M., Heuser A., Spero H., Bijma J., Böhm F., Nägler Th. F. (2002) Geophysical Research Abstracts 4, (EGS02-A-02944)\Nägler T. F., Eisenhauer A., Müller A., Hemleben C., and Kramers J. (2000) Geochem. Geophys. Geosyst. 1, 2000GC000091