CALIBRATING BENTHIC MG/CA PALEOTHERMOMETERS FOR HIGH LATITUDE STUDIES

The northern North Atlantic Ocean is a key player in the global oceanic circulation system. Reconstructions of both temperature and salinity are needed to reconstruct its history of thermohaline convection. Knowing both surface and bottom water conditions will facilitate the reconstruction. Several proxies are available for reconstructing sea surface temperature, but many of them are inapplicable to bottom water. The Mg/Ca method, based on temperature dependent Mg/Ca ratios in biogenic calcite, may be used for both. Contrary to d¹⁸O the Mg/Ca signal is believed to be primarily temperature dependent and unlikely to vary on glacial-interglacial time scales. Given that Mg/Ca provides independent temperature reconstructions, paired d¹⁸O and Mg/Ca measurements allow salinity to be reconstructed. So far, very few Mg/Ca calibrations exist for high latitude foraminifera. The objective of this pilot study is to determine whether Mg/Ca ratios in shell calcite of three arctic, benthic foraminifera species can be used as a paleothermometer in high latitude regions.

Modern oceanographic conditions on the Iceland shelf (water depth 100-400 m) range from Atlantic water of 7°C on the SW shelf to Arctic water of 0°C on the N shelf. This temperature range makes the Iceland shelf an ideal location for a cool water Mg/Ca calibration study. We use a set of surface (grab) samples collected in 1997 from the Iceland shelf (Cruise B997). Due to changing environmental conditions it is difficult to focus on just one benthic species for downcore records. Therefore, three infaunal species; Melonis barleeanus, Cassidulina neoteretis and Islandiella norcrossi were selected for analysis. Minor and trace elements were measured on an ICP MS at the Univ. of California, Santa Barbara. Before analysis samples were subjected to rigorous cleaning procedure established by Boyle and Keigwin (1985/86). Isotope analysis was performed at the Kiel University.

Mg/Ca and d¹⁸O were compared to bottom water winter and summer temperatures in search for a temperature relationship. First analysis of the data indicates a clear species-specific relationship as has been previously shown to occur in benthic foraminifera by Lear et al. (2000).

Boyle, E. & Keigwin, L. D. (1985/1986): EPSL 76, 135-150. Lear, C. H., Elderfield, H. & Wilson, P. A. (2000): Science, 287(269-272)