New spatial Mg/Ca-temperature calibrations for three Arctic, benthic foraminifera and reconstruction of north Iceland shelf temperature for the past 4000 years

Abstract

We have developed cold-end Mg/Ca-temperature calibrations for three common Arctic benthic foraminifera, Islandiella norcrossi/helenae, Melonis barleeanus, and Cassidulina neoteretis, and compare the three calibrations in a late Holocene downcore record (0-4000 cal yr B.P.). The calibration and downcore trends for the three Arctic species extend the observation that Mg incorporation into benthic foraminifera is species specific. For the calibration we use a set of CTD casts, bottom water δ18Oseawater measurements, and surface grab-samples collected from the Iceland margin (cruise B997) and the Greenland margin (cruise BS1191). Water depth of sites used ranges from 165 to 656 m, while spatial bottom temperature ranges from 0 to 7°C. Mg/Ca values ranged from 1.02 to 1.47 for I. norcrossi/helenae, 0.64 to 2.21 for M. barleeanus, and 0.93 to 1.38 mmol/mol for C. neoteretis. We calibrated Mg/Ca content against isotopic calcification temperature (calculated using T = 16.9 - 4.0*(δ18O calcite corrected for vital effect - δ18O seawater)). Exponential calibrations for the three species are as follows: I. norcrossi/helenae Mg/Ca = 1.051 ± 0.03 * exp(0.060 ± 0.011 * T), M. barleeanus Mg/Ca = 0.658 ± 0.07 * exp(0.137 ± 0.020 * T), and C. neoteretis Mg/Ca = 0.864 ± 0.07 * exp(0.082 ± 0.020 * T). On the basis of Mg/Ca in these benthic species the downcore record from core MD99-2269 is reconstructed. Bottom temperature values are interpreted to reflect variable inflow of Atlantic and Arctic water to the north Iceland shelf during the last 4000 cal yr B.P. All three reconstructions show a decline by 0.1°C per century from circa 1500-0 cal yr B.P., which coincides with an increase in Arctic benthic foraminifera abundances and a rise in sea ice proxies in the same core. Intriguingly, C. neoteretis diverges periodically to higher average temperature (Atlantic water conditions) than shown by M. barleeanus or I. norcrossi/helenae (which both show Arctic water temperature) circa 1500-4000 cal yr B.P. Copyright 2007 by the American Geophysical Union.