Relative timing and variability of atmospheric methane and GISP2 oxygen isotopes between 68 and 86 ka

Abstract

The global biogeochemical cycle of methane has received wide attention because of methane's role as a greenhouse gas. Measurements of methane in air trapped in Greenland ice cores provide a high‐resolution record of methane levels in the atmosphere over the past ∼100 ka, providing clues about what controls the methane cycle on geologic timescales. Remarkable similarity between local temperature recorded in Greenland ice cores and changes in global methane concentrations has been noted in previous studies, with the inference that the local temperature variations have global significance, but the resolution of sampling and measurement precision limited fine‐scale comparison of these variables. In this work a higher‐precision (∼2 ppb) methane data set was obtained from the Greenland Ice Sheet Project 2 (GISP2) ice core for the time interval between 86 and 68 ka, encompassing three large abrupt warming events early in the last glacial period: Dansgaard‐Oeschger (D‐O) events 19, 20, and 21. The new data set consists of duplicate measurements at 158 depths, with average time resolution of 120 years. Such detailed measurements over D‐O 21, the longest in Greenland records, have not yet been reported for other ice cores. The new data set documents short‐term variability (∼20 ppb typical amplitude), which is remarkably persistent, and in many cases similar features are observed in the most detailed published δ18Oice record. High‐precision GISP2 δ15N data show that changes in Greenland temperature are synchronous with the methane variations at the onset of D‐O events 19, 20, and 21, supporting previous results from the Greenland Ice Core Project ice core for D‐O 19 and 20. Cross‐spectral analysis quantifies the extremely close similarity between the new methane record and the δ18Oice record. Because methane sources are widely distributed over the globe, this work further validates δ18Oice at Greenland summit as a geographically broad climate indicator on millennial to multicentennial timescales.