High-resolution interpolar difference of atmospheric methane around the Last Glacial Maximum

Abstract

Reconstructions of past atmospheric methane concentrations are available from ice cores from both Greenland and Antarctica. The difference observed between the two polar methane concentration levels represents a valuable constraint on the geographical location of the methane sources. Here we present new high-resolution methane records from the North Greenland Ice Core Project (NGRIP) and the European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (EDML) ice cores covering Termination 1, the Last Glacial Maximum, and parts of the last glacial back to 32 000 years before present. Due to the high resolution of the records, the synchronisation between the ice cores from NGRIP and EDML is considerably improved, and the interpolar concentration difference of methane is determined with unprecedented precision and temporal resolution. Relative to the mean methane concentration, we find a rather stable positive relative interpolar difference throughout the record with its minimum value of 3.7 ± 0.7 % between 21 900–21 200 years before present, which is higher than previously estimated in this interval close to the Last Glacial Maximum. This implies that Northern Hemisphere boreal wetland sources were never completely shut off during the peak glacial, as suggested from previous bipolar methane concentration records. Starting at 21 000 years before present, i.e. several millennia prior to the transition into the Holocene, the relative interpolar difference becomes even more positive and stays at a fairly stable level of 6.5 ± 0.8 % during Termination 1. We thus find that the boreal and tropical methane sources increased by approximately the same factor during Termination 1. We hypothesise that latitudinal shifts in the Intertropical Convergence Zone (ITCZ) and the monsoon system contribute, either by dislocation of the methane source regions or, in case of the ITCZ, also by changing the relative atmospheric volumes of the Northern and Southern Hemispheres, to the subtle variations in the relative interpolar concentration difference on glacial/interglacial as well as on millennial time scales

Limited response of peatland CH<sub>4</sub> emissions to abrupt Atlantic Ocean circulation changes in glacial climates

“…The inferred source changes for northern sources from recent data of Baumgartner et al (2012) (in agreement with inferences of Brook et al, 2000) suggest that northern sources were approximately halved during the last glacial period. The strong reduction in northern peatland emissions in LPJWHyMe is consistent with this inference, but it is not possible to differentiate between the boreal and subtropical northern sources using the ice-core interpolar gradient, so quantitative comparison between LPJ-WHyMe and the ice-corebased inference is difficult.…”

“…Ice-core data on the interpolar gradient of CH 4 as well as its isotopic signature allow for top-down estimates of the changes in sources during past climate and in general suggest that wetland emissions played a significant role in past atmospheric CH 4 variations. Recent improvements in the determination of the interpolar gradient of CH 4 from ice-core measurements suggest that low-latitude sources made the dominant contribution to abrupt changes in atmospheric CH 4 during the last glacial period (Baumgartner et al, 2012) in agreement with Brook et al (2000). This result updates older measurements by Dällenbach et al (2000) which had previously strongly implied that high-latitude sources played an important role during these abrupt events.…”

“…Comparing the LPJWHyMe changes in CH 4 emissions across all of the simulations summarised in Table 3, it is clear that the simulated changes will not explain a significant component of the observed D-O event abrupt changes in CH 4 . This is consistent with a dominant contribution of tropical sources to abrupt CH 4 changes as inferred by Baumgartner et al (2012). Furthermore, the relatively minor changes in emissions between the different LPJ-WHyMe simulations are at least partially consistent with the relatively stable contribution from the northern sources through the latter part of the last glacial period as inferred by Baumgartner et al (2012).…”

“…The recent interpolar gradient data from Baumgartner et al (2012) (from NGRIP and EDML) and prior work of Brook et al (2000) based on GISP2 and Taylor Dome ice cores, both suggest modest emission increases at high latitudes during D-O events, with a more important contribution from subtropical and northern tropical regions. Comparing the LPJWHyMe changes in CH 4 emissions across all of the simulations summarised in Table 3, it is clear that the simulated changes will not explain a significant component of the observed D-O event abrupt changes in CH 4 .…”

Methanogens in the Environment: An Insight of Methane Yield and Impact on Global Climate Change

“…Wetlands also contribute to the atmospheric methane concentration (Baumgartner et al, 2012) and the largest supplier of methane including bogs, peatlands, swamps and marshes. It contributes one quarter of global methane emissions (Kirschke et 2000) concluded that the CH 4 production in wetlands is suffering from the acetate provide through acetate fermentation and the CO 2 reduction potential.…”