This is a collaborative proposal between Principal Investigators at the University of California - San Diego and Washington State University. They will develop a large-volume atmospheric archive at the margin of the Greenland ice sheet in order to test two competing hypotheses for the cause of the abrupt increases in atmospheric methane concentration that punctuated the last glacial period. The main objective is to measure the carbon-14 (14C) content of atmospheric methane before and after these events. These events have been attributed to 1) destabilization of sedimentary methane hydrate (clathrate) or 2) increased methane production in terrestrial wetlands due to warmer and wetter climatic conditions. The carbon-14 content of atmospheric methane should distinguish between clathrate and wetland sources because clathrates are made from carbon-14-free sedimentary organic matter, whereas the carbon-14 content of wetland-derived methane is essentially the same as that of atmospheric carbon dioxide. This test is important because the wetland hypothesis has been used to infer large-scale tropical abrupt climate change, with broad implications for our understanding of the potential for rapid change in both past and future climates. The main obstacle to this measurement is that it requires ~200 liters of air, much more than is available from ice core samples. This work will use ice that outcrops due to ablation and ice flow at the margin of the Greenland ice sheet at a location called Pakitsoq, enabling virtually unlimited sample size. Preliminary fieldwork has demonstrated that ice of the appropriate age (11,000-15,000 years ago) can be found, and that it is not contaminated with respect to methane or gas isotopes, by comparison with the known atmospheric record from the Greenland Ice Sheet Project 2 (GISP2) ice core. Ice of Eemian (last Interglacial) age also appears to be present at this site, and this work will use gas measurements to establish the chronology of the entire margin ice section including a thick Holocene section.
