Assessing Permafrost Stability
The Arctic is one of the fastest warming regions on the planet. Currently, much of the Arctic is covered by permafrost, which contains approximately 1,700 gigatons of organic carbon. Permafrost thaw could release a substantial amount of this carbon as greenhouse gases into the atmosphere through microbial decomposition, potentially dramatically amplifying anthropogenic warming. However, the risk of permafrost thaw is uncertain, with models exhibiting a wide range of possibilities. Assessing the stability of permafrost during past interglacial periods enables evaluation of the sensitivity of permafrost to warming. Cave mineral deposits (speleothems) in areas currently covered with permafrost can act as a proxy for past permafrost thaw, as liquid water is one criterion for speleothem growth and thus speleothem deposition implies thawed ground conditions. Previous uranium-thorium (U-Th) dating of speleothems (n=73) from a wide range of latitudes and permafrost zones across the southern Canadian Rockies, Northwest Territories, and northern Yukon indicate that most of these formations exceed the U-Th dating limit of 500 ka. In this study, I apply uranium-lead (U-Pb) geochronology to several of these speleothems to extend the record of speleothem growth further back in time. Results include a U-Pb age of 412.8 ± 1.3 ka that replicates a previous U-Th age of 415.0 ± 11.5 ka, demonstrating the reliability of the U-Pb methodology. Additionally, U-Pb ages on six other speleothems are determined to be 876 ± 9 ka, 1501 ± 31 ka, 1570 ± 66 ka, 2046 ± 106 ka, 7636 ± 184 ka, and 7697 ± 185 ka. Further application of this technique could result in long records of past permafrost thaw and Arctic terrestrial climate extending back millions of years.