Pleistocene Permafrost Thawing History of Alaska, the Yukon, and the Northwest Territories from U-Th Dating of Cave Speleothems

Abstract:

Permafrost, or permanently frozen ground, is widespread in the Arctic and contains twice as much carbon as the atmosphere in the form of frozen organic matter. This carbon may be vulnerable to thaw and release to the atmosphere as methane under a warming climate, making permafrost thaw one of the potentially most significant amplifying feedbacks to anthropogenic warming. Nonetheless, permafrost can be slow to respond to warming, the short instrumental record may not adequately capture long-term trends, and the modest temperature changes of the past few millennia provide poor analogues to understand the possibility of crossing climate thresholds in the next century or beyond. One way to address this problem is to assess the stability of permafrost during previous interglacial periods of varying levels of warmth, which provide natural experiments to examine the Arctic’s sensitivity to warming.

Cave speleothems in the Arctic are relics of past periods of thaw that enabled meteoric water to seep into caves and deposit calcite. We employed uranium-thorium (U-Th) dating to constrain the chronology and extent of permafrost thaw in the North American Arctic during the past 600,000 years. We sampled caves from a range of permafrost zones and latitudes, including the Fishing Branch Territorial Park, Yukon (66.5°N, continuous permafrost), White Mountains National Recreation Area, Alaska (65°N, discontinuous permafrost), Yukon-Charley Rivers National Preserve, Alaska (64.5°N, discontinuous permafrost), and Nahanni National Park Reserve, Northwest Territories (62°N, discontinuous permafrost). Thirty-five samples from 30 speleothems have been analyzed at this point, with 23 samples lying beyond the U-Th dating range, and finite ages tending to cluster near Marine Isotope Stage 11 (MIS 11) within uncertainty, as well as perhaps MIS 9, 13, and 15. This preliminary dataset, coupled with a similar study in Siberia (Vaks et al., 2013), is thus suggestive of an episode of widespread thaw during the MIS 11 interglacial about 400,000 years ago when global temperature was only ~1°C warmer than pre-industrial temperatures. We anticipate dating more speleothems in the months prior to AGU, and exploring the use of U-Pb for older samples.