Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland

[1] We have reconstructed chronology for the disturbed bottom parts of the GRIP and GISP2 ice cores using the combined paleoatmospheric records of CH4 concentration and delta(18)O(atm) in the trapped gases. Our reconstructed ages for basal ice samples are based on comparison of published measurements of CH4 and delta(18)O(atm) from the disturbed section of the GRIP and GISP2 cores with the same properties in the Vostok ice core. NGRIP delta(18)O(ice) values are also used to constrain the chronology during the end of marine isotope stage 5e. For each sample, we assign an age that represents the unique or most probable time of gas trapping, given its gas composition. Of 157 samples with CH4 and delta(18)O(atm) data, 10 give unique ages. Twenty-five newly measured values of the triple isotope composition of O-2 from the disturbed section of the GISP2 core add a third time-dependent gas property that agrees with our reconstruction. Our reconstruction supports earlier conclusions of Landais et al. ( 2003) that the disturbed section primarily includes ice from the last interglacial (MIS 5e) and the penultimate glacial period ( MIS 6). The oldest ice in the basal layer of GISP2 and GRIP has an age >= 237 ka. The climate history we derive suggests that the last interglacial at Summit, Greenland, around 127 ka was slightly warmer than the current interglacial period. Reduction of various ion concentrations in ice and thickening of the ice sheet during Termination II was similar to that in Termination I.