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

Abstract

In order to further develop 238U-234U-230Th 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. 230Th/234U activity ratios are quite low in the <50 nm fraction (0.18-0.24), and since natural liquid water is characterized by 230Th/234U 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 230Th/234U activity ratios of 1.12-1.23 which are also consistent with recent (<380 ka) fractionation of thorium from uranium in these ice samples.We have modeled these results using a two component mass balance calculation, with dissolved and particulate pools for each radionuclide. Although the model results vary slightly depending on assumptions, corrected dates for when the samples were last frozen range from 3500-8300 years ago, with typical 2σ uncertainties of 600-2300 years. These results are generally consistent with other geochemical and chronological data for Greenland ice cores and the notion of ice melting at the base of large continental ice sheets, even in areas that are currently frozen. Based on our U-series results and gas data in the literature, prior melting of the GISP2 ice core has evidently occurred for most of the 13 m thick silty basal layer but is most pronounced for the deepest 2 m. We also suggest that this new 230Th ingrowth method for dating previously melted ice may be more generally applicable to samples with a range of particulate matter concentrations.