Stable carbon isotope ratios of carbon dioxide from EDC and Berkner Island ice cores for 40-50 ka BP

The stable carbon isotopic signature of carbon dioxide (d13CO2) measured in the air occlusions of polar ice provides important constraints on the carbon cycle in past climates. In order to exploit this information for previous glacial periods, one must use deep, clathrated ice, where the occluded air is preserved not in bubbles but in the form of air hydrates. Therefore, it must be established whether the original atmospheric d13CO2 signature can be reconstructed from clathrated ice. We present a comparative study using coeval bubbly ice from Berkner Island and ice from the bubble-clathrate transformation zone (BCTZ) of EPICA Dome C (EDC). In the EDC samples the gas is partitioned into clathrates and remaining bubbles as shown by erroneously low and scattered CO2 concentration values, presenting a worst-case test for d13CO2 reconstructions. Even so, the reconstructed atmospheric d13CO2 values show only slightly larger scatter. The difference to data from coeval bubbly ice is statistically significant. However, the 0.16 per mil magnitude of the offset is small for practical purposes, especially in light of uncertainty from non-uniform corrections for diffusion related fractionation that could contribute to the discrepancy. Our results are promising for palaeo-atmospheric studies of d13CO2 using a ball mill dry extraction technique below the BCTZ of ice cores, where gas is not subject to fractionation into microfractures and between clathrate and bubble reservoirs.

Stable isotope data and calcification temperature from planktic foraminifera in sediment core BOFS31/1K

Determining the past record of temperature and salinity of ocean surface waters is essential for understanding past changes in climate, such as those which occur across glacial-interglacial transitions. As a useful proxy, the oxygen isotope composition (delta18O) of calcite from planktonic foraminifera has been shown to reflect both surface temperature and seawater delta18O, itself an indicator of global ice volume and salinity (Shackleton, 1974; Rostek et al., 1993, doi:10.1038/364319a0). In addition, magnesium/calcium (Mg/Ca) ratios in foraminiferal calcite show a temperature dependence (Nürnberg, 1995, doi:10.2113/gsjfr.25.4.350; Nürnberg et al., 1996, doi:10.1016/0016-7037(95)00446-7; Lea et al., 1999, doi:10.1016/S0016-7037(99)00197-0) due to the partitioning of Mg during calcification. Here we demonstrate, in a field-based calibration experiment, that the variation of Mg/Ca ratios with temperature is similar for eight species of planktonic foraminifera (when accounting for Mg dissolution effects). Using a multi-species record from the Last Glacial Maximum in the North Atlantic Ocean we found that past temperatures reconstructed from Mg/Ca ratios followed the two other palaeotemperature proxies: faunal abundance (CLIMAP, 1981; Mix et al., 1999, doi:10.1029/1999PA900012) and alkenone saturation (Müller et al., 1998, doi:10.1016/S0016-7037(98)00097-0 ). Moreover, combining Mg/Ca and delta18O data from the same faunal assemblage, we show that reconstructed surface water delta18O from all foraminiferal species record the same glacial-interglacial change-representing changing hydrography and global ice volume. This reinforces the potential of this combined technique in probing past ocean-climate interactions.