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

November 29, 2012 • Filed under: News — melanie.rohr

Ben Gaglioti uses a lake sediment record to examine the dynamics of permafrost carbon to past warming events.

Friday Seminar Series

  • What: Radiocarbon Ages Offsets in Arctic Lake Sediments Describe the Vulnerability of Permafrost Carbon to Past Climate Warming
  • Who: Ben Gaglioti
  • When: 3:30-4:30 p.m. Friday, Nov. 30
  • Where: 531 Duckering

Warming temperatures in the future could subject permafrost carbon to decomposition and release to the atmosphere causing a positive feedback to climate warming. An effective approach to better understand this problem is to observe how permafrost carbon 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 carbon 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 that was eroded or leached from peatlands, soils, and thawing permafrost in the watershed. Changes in the magnitude between the age of deposition (“true age”) and the radiocarbon age of the same sediment layer is called a radiocarbon age offset, which can serve as a gauge for the amount of permafrost carbon release from the watershed. We analyzed the sediments of Lake of the Pleistocene (LOP), which provide a record of how age offsets varied over the last ca. 14,500 calendar years. The study area is located in the northern foothills region of Northern Alaska where the watershed is underlain by continuous permafrost and contains extensive, frozen peatlands. The lake is partially drained, which allowed us to excavate a wide swath of the sediment bed to collect hundreds of willow twigs and sediment samples and construct a high-resolution age offset chronology. Well-preserved willow twigs that are directly blown into the lake offer a “true” 14C age of when that particular layer was deposited. The 14C of the lake sediment organic matter from the same layer provides the age of both primary productivity from within the lake and old DOC and POC 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 during the Younger Dryas cold interval. The age offsets we describe from the LOP record reveals strong effects of warming on C release during the Allerod/Bolling warm period and during the early Holocene Thermal Maximum. The peat that now covers most of the watershed is maintaining low temperatures in the ground and likely stabilizing permafrost C in the face of warming climate. Based on this record, we hypothesize that upon thawing a significant amount of permafrost C gets stored in depositional environments such as lake sediments where it may be prone to methanogenesis or long-term sequestration.

Field workers measure the sampling depth on exposed sediment section on a bug-free day in northern Alaska

Image Left: Field workers measure the sampling depth on exposed sediment section on a bug-free day in northern Alaska

(Image by Ben Gaglioti)