Evidence for Past Melting at the Base of the GISP2 Ice Core from Uranium-Thorium Disequilibrium Measurements
Abstract
We measured 238U-234U-230Th disequilibria by mass spectrometric methods for silty ice samples from the base of the GISP2 Greenland ice core, at a depth of 3040-3052 m. The expected age of these samples is >150 kyr based on counting ice layers. We separated the samples into several fractions by filtration and analyzed the <50 nm (truly dissolved + particulate) and >200 nm (particulate) filtered fractions. In the <50 nm fractions, low Th/U mass ratios of 0.51-0.65 indicate that a large portion (86-89%) of U is truly dissolved and not associated with particles. In addition, 230Th/234U activity ratios are quite low (0.18-0.24), suggesting either recent 230Th loss and/or 234U addition to the samples. This Th/U fractionation is not consistent with an age >150 kyr. Since 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 (>200 nm) fraction is characterized by Th/U ratios of ~4.4-4.9, 234U/238U activity ratios of 1.049-1.056, and 230Th/234U activity ratios of 1.12-1.23. The U-Th disequilibrium in the particles is consistent with recent (<350 kyr) fractionation of U-Th in these ice samples. We have modeled these results using a two component mass balance calculation, with dissolved and particulate pools for each radionuclide. The main assumption is that all of the 232Th in the <50 nm fraction is due to particles, due to the low aqueous solubility of 232Th. By mass balance, 230Th/234U activity ratios for the truly dissolved fraction are 0.031-0.062, and dates for when the samples were last frozen are 3.5-6.9 kyr. These results are consistent with the notion of ice melting at the base of large continental ice sheets, with recent evidence of large sub-glacial lakes in Antarctica and active melting at the base of the nearby N-GRIP Greenland ice core. There also appears to be a significant difference in age for the deepest ice sample (3.5 kyr; 3052 m) and the other samples (5.2-6.9 kyr; 3040-3048 m), suggesting that the deepest part of the ice core adjacent to bedrock at 3053.3 m depth may have existed as liquid water for at least several hundred - few thousand years.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V11D2540G
- Keywords:
-
- 0724 CRYOSPHERE / Ice cores;
- 0726 CRYOSPHERE / Ice sheets;
- 1115 GEOCHRONOLOGY / Radioisotope geochronology;
- 1120 GEOCHRONOLOGY / Isotopic disequilibrium dating