Increasing Signature of Old Carbon in Headwater Streams Following 14 Years of Permafrost Thaw

Authors

Allison K. Kelley* (akk259@nau.edu), Chris Ebert, Edward (Ted) A. G. Schuur

Institutions

Center for Ecosystem Science and Society, Northern Arizona University–Flagstaff, AZ

URLs

• http://www2.nau.edu/schuurlab-p/CiPEHR.html

Abstract

Soils in the permafrost regions comprise only 15% of the global soil area yet contain an estimated 33% of global soil carbon. These soils have accumulated almost three times the amount of carbon currently in the atmosphere as a result of limited microbial decomposition and water flow in subzero conditions. However, recent analyses indicate that the Arctic is warming at more than twice the rate of the global average and that there is an accelerated decomposition and export of ancient permafrost as this soil thaws. Though permafrost organic carbon has often been viewed as less decomposable, with deeper, older layers containing more recalcitrant carbon, the vulnerability of stored soil carbon to release is quite variable and dependent on inherent differences in the decomposability of individual molecules and changes to the landscape. Increasing atmospheric release has been well-documented for decades; however, streams are more difficult to monitor, and thus, their role in the Arctic carbon cycle has often been overlooked. This study assessed the contribution that Arctic headwaters play in long-term permafrost carbon release at the Arctic Carbon and Climate (ACCLIMATE) observatory, located in the discontinuous permafrost zone of interior Alaska.

Researchers compared the age of carbon exported vertically, through respiration of carbon dioxide with that exported laterally, as dissolved organic carbon (DOC) over 18 years. Age was estimated using radiocarbon (¹⁴C) dating. The team found that both vertical and lateral signals follow similar release patterns, releasing large, ancient pulses of permafrost carbon at the same time. This indicates that landscape-level permafrost degradation signals can be tracked contemporaneously both in streams and the atmosphere. Further, it was found that permafrost carbon contributions to both ecosystem respiration and DOC fluxes have increased over 14 years as changing ecosystem conditions increasingly favored permafrost carbon release. However, the contribution of permafrost carbon has increased three times more quickly within DOC fluxes than it has within ecosystem respiration fluxes (22‰ year^-1 ∆¹⁴C of DOC vs. 6.5‰ year^-1 ∆¹⁴C of ecosystem respiration), highlighting the emerging importance of headwaters in the export of ancient, permafrost-derived carbon. Overall, whether the release of permafrost carbon be terrestrially derived or aquatic, this increase in ancient carbon release signifies that permafrost systems in this region are destabilizing as a result of an accelerating climate change feedback.