On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period (original) (raw)

2000, Global Biogeochemical Cycles

We present high resolution records of atmospheric methane from the GISP2 (Greenland Ice Sheet Project 2) ice core for four rapid climate transitions that occurred during the past 50 ka: the end of the Younger Dryas at 11.8 ka, the beginning of the Bolling-Aller0d period at 14.8 ka, the beginning of interstadial 8 at 38.2 ka, and the beginning of intersradial 12 at 45.5 ka. During these events, atmospheric methane concentrations increased by 200-300 ppb over time periods of 100-300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with largescale decomposition of methane hydrates in sediments. With additional results from the Taylor Dome Ice Core (Antarctica) we also reconstruct changes in the interpolar methane gradient (an indicator of the geographical distribution of methane sources) associated with some of the rapid changes in atmospheric methane. The results indicate that the rise in methane at the beginning of the B011ing-Aller0d period and the later rise at the end of the Younger Dryas were driven by increases in both tropical and boreal methane sources. During the Younger Dryas (a 1.3 ka cold period during the last deglaciation) the relative contribution from boreal sources was reduced relative to the early and middle Holocene periods. Paper number 1999GB001182. 0886-6236/00/1999GB001182512.00 associated with orbital forcing. A 20,000 year periodicity in the Vostok record was attributed to the effects of solar insolation variations on the tropical monsoon cycle [Chappellaz et al., 1990; Petit-Maire et al., 1991 ], or alternatively, to temperature and precipitation variations in high latitude wetland regions [Crowley, 1991]. More recently, higher-resolution records have become available from cores through the rapidly accumulating Greenland Ice Sheet. Methane records from the Greenland Ice Sheet Project 2 (GISP2) and Greenland Ice Core Program (GRIP) ice cores support the general patterns revealed at Vostok [Chappellaz et al., 1993a; Brook et al., 1996a]. They also reveal that atmospheric methane levels varied significantly on millennial timescales during the last glacial period. The millennial variations were coeval with millennial-scale warmings and coolings (interstadial events) inferred from oxygen isotope records from central Greenland ice cores, proxies for surface temperature [Johnsen et al., 1993; Grootes et al., 1993]. The ice core methane record therefore indicates a close relationship between interstadial climate and changes in terrestrial methane emissions [Chappellaz et al., 1993a]. This conclusion augments a large body of evidence suggesting a correlation between rapid climate change in Greenland ice core records and terrestrial climate change in a variety of locations around the world [e.g., Grimm et al., 1993; Broecker et al., 1998; Gasse and Van Campo; 1998, Grigg and Whitlock, 1988; Allen et al., 1999]. Gaps in our understanding of millennial-scale shifts in methane concentration include incomplete knowledge of (1) the detailed timing and magnitude of the rapid concentration changes, (2) the possible role of massive releases of clathratebound methane from marine sediments and permafrost, and (3) the methane source locations and how they changed with time. In this paper we examine these issues using methane records covering the past 50,000 years from two ice cores, the GISP2 559 560 BROOK ET AL: RAPID METHANE VARIATIONS DURING THE LAST GLACIAL PERIOD core from central Greenland and the Taylor Dome core from the Ross Sea sector of Antarctica. We use the results to constrain the rate and magnitude of methane concentration change associated with rapid climate shifts during the last glacial period and deglaciation. We also determine the interpolar methane gradient, a possible proxy for the geographical distribution of methane sources [Rasmussen and Khalil, 1984], at selected time intervals, The GISP2 ice core was recovered in 1993 at 72ø36'N, 38ø30'W in central Greenland. We have extended our original GISP2 data set [Brook et al., 1996a] by analyzing samples from 129 additional depths in the upper 2438 m, which represents the past 50 ka. We focused on abrupt transitions in the record, including the termination of the Younger Dryas period at •-11.8 ka, the beginning of the B011ing-Aller0d period at --14.5 ka, and the onsets of interstadials 8 and 12, rapid warming events that occurred at--38.2 and 45.5 ka in the GISP2 isotope record. The GISP2 methane data set over the 0-50 ka interval now includes 289 sample depths. The Taylor Dome ice core was recovered in 1994 at 77ø48'S, 159ø53'E on Taylor Dome, a small ice dome in the Ross Sea sector of Antarctica. The Taylor Dome paleoclimate record extends to greater than 130 ka at a depth of 554 m [Grootes and Steig, 1994; Steig, 1996; Steig et al., 2000] but we focus here on methane measurements of 137 samples in the upper 445 m (0-50 ka), with high resolution measurements over the time intervals listed above. Data are available from the PBO IACP , , ß . . ; ß : ,. ß .. ; ,. ß ß ß Older Dryas Oldest Dryas ß ß •t•.t'• , . J. Severinghaus,