A methane-based time scale for Vostok ice

Tuning the Vostok methane signal to mid-July 30°N 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 CH 4 gradients showing that the main control of orbital-scale CH 4 variations is tropical (monsoonal) sources. The immediate basis for tuning CH 4 to mid-July insolation is the coincident timing of the most recent (pre-anthropogenic) CH 4 maximum at 11,000–10,500 calendar years ago and the most recent July 30°N insolation maximum (all ages in this paper are in calendar years unless specified as 14C years). The resulting CH 4 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 δ 18O atm to a lagged insolation signal (Shackleton, Science 289 (2000) 1897). Most offsets between the CH 4 and δ 18O atm time scales can be explained by assuming that tropical monsoons and ice sheets alternate in controlling the phase of the δ 18O atm signal. The CH 4 time scale provides an estimate of the timing of the Vostok CO 2 signal against SPECMAP marine δ 18O, often used as an index of global ice volume. On the CH 4 time scale, all CO 2 responses are highly coherent with SPECMAP δ 18O at the orbital periods. CO 2 leads δ 18O 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 CO 2 and ice volume is difficult to infer because of likely SST overprints on the SPECMAP δ 18O signal. CO 2 could lead, or be in phase with, ice volume, but is unlikely to lag behind the ice response.