Ice ages: Evidence for past melting at the base of the GISP2 ice core from uranium-thorium disequilibria dating

摘要

In order to further develop U-238-U-234-Th-230 disequilibria techniques for dating polar ice, we measured these nuclides by mass spectrometric methods for silty ice samples from the base of the GISP2 central Greenland ice core. We separated the samples by filtration and analyzed the <50 nm (truly dissolved + particulate) and >200 nm (particulate) filtered fractions. Th-23(0)/U-234 activity ratios are quite low in the <50 nm fraction (0.18-0.24), and since natural liquid water is characterized by Th-23(0)/U-234 activity ratios <= 1, these results suggest that recent melting/freezing event(s) have occurred at the base of the GISP2 core. The particulate fraction is characterized by Th-23(0)/U-234 activity ratios of 1.12-1.23 which are also consistent with recent (<380 ka) fractionation of thorium from uranium in these ice samples.