Radiocarbon Age Offsets in Arctic Lake Sediments Describe the Vulnerability of Permafrost Carbon to Past Climate Warming

A warming climate in the future could release permafrost carbon (C) as carbon dioxide and methane to the atmosphere, causing a positive feedback to climate warming. An effective approach to better understanding this problem is to observe how permafrost C dynamics responded to past warming events. Here we use the sediment record of a lake basin in northern Alaska as a long-term archive for past permafrost C release from the surrounding watershed. The age of deposition and burial for an arctic lake sediment horizon is often younger than the 14C age because of old, 14C-depleted C eroded or leached from peat, soil, and thawing permafrost in the watershed. Changes in the magnitude between the age of deposition ("true age") and the radiocarbon age of the bulk sediment from the same layer is the radiocarbon age-offset, which serves as a gauge for the relative amount of permafrost C released from the watershed. We analyzed the sediments of Lake of the Pleistocene (LOP), a partially-drained lake basin located in the northern flank of the Brooks Range that contains continuously deposited sediments spanning the last 14,500 calendar years. The LOP watershed is underlain by continuous permafrost and contains extensive, frozen peatlands. We were able to excavate a wide swath of the former lakebed and collect hundreds of willow twigs and sediment samples to construct a high-resolution age-offset chronology. We dated well-preserved willow twigs that are directly blown into the lake to obtain a "true" 14C age and the age of bulk lake-sediment organic matter from the same layer that records the age of both primary productivity from within the lake and of the old dissolved and particulate organic carbon reworked from the watershed. Today, the radiocarbon age of the surface sediments of LOP is 2,000 calendar years old, which is roughly the same as the age offset during the Younger Dryas cold interval. During the warmer than present Holocene Thermal Maximum (HTM; ~11,700-9,000 calendar years ago) age offsets nearly doubled to around 4,000 calendar years. Because most of the deepest and oldest organic-rich soils in the LOP watershed are located in topographical lowlands in the northern Brooks Range region today, we hypothesize that these regions are where ancient organic matter was released during the HTM. The thick peat that now covers these lowlands is insulating the permafrost and likely stabilizing permafrost C in the face of recent warming causing lake sediment age offsets to be similar to those offsets during a period of relatively cold, and unproductive soils. The vulnerability of permafrost C is evidenced by the limit of peat stabilization that was apparently crossed during the ~ 2-3° C of rapid warming that occurred at the end of the Younger Dryas.