A methane-based time scale for Vostok ice

Tuning the Vostok methane signal to mid-July 30degreesN insolation yields a new ice-core gas time scale. This exercise has two rationales: (1) evidence supporting Kutzbach's theory that low-latitude summer insolation in the northern hemisphere controls the strength of tropical monsoons, and (2) interhemispheric CH4 gradients showing that the main control of orbital-scale CH4 variations is tropical (monsoonal) sources. The immediate basis for tuning CH4 to mid-July insolation is the coincident timing of the most recent (pre-anthropogenic) CH4 maximum at 11,000-10,500 calendar years ago and the most recent July 30degreesN insolation maximum (all ages in this paper are in calendar years unless specified as C-14 years). The resulting CH4 gas time scale diverges by as much as 15,000 years from the GT4 gas time scale (Petit et al., Nature 399 (1999) 429) prior to 250,000 years ago, but it matches fairly closely a time scale derived by tuning ice-core delta(18)O(atm) to a tagged insolation signal (Shackleton, Science 289 (2000) 1897). Most offsets between the CH4 and delta(18)O(atm) time scales can be explained by assuming that tropical monsoons and ice sheets alternate in controlling the phase of the delta(18)O(atm) signal. The CH4 time scale provides an estimate of the timing of the Vostok CO2 signal against SPECMAP marine delta(18)O, often used as an index of global ice volume. On the CH4 time scale, all CO2 responses are highly coherent with SPECMAP 6180 at the orbital periods. CO2 leads delta(18)O by 5000 years at 100,000 years (eccentricity), but the two signals are nearly in-phase at 41,000 years (obliquity) and 23,000 years (precession). The actual phasing between CO2 and ice volume is difficult to infer because of likely SST overprints on the SPECMAP delta(18)O signal. CO2 Could lead, or be in phase with, ice volume, but is unlikely to lag behind the ice response. (C) 2002 Elsevier Science Ltd. All rights reserved.