Holocene Age Methane and Carbon Dioxide Dominate Northern Alaska Thaw Lake Emissions

Abstract:

Lakes expanding into ice-rich permafrost can rapidly re-introduce large quantities of ancient organic carbon (C) to the atmosphere as carbon dioxide (CO₂) or the more powerful greenhouse gas methane (CH₄). Quantifying the sources of greenhouse gas emissions from arctic lakes will reduce large uncertainties in the magnitude and timing of the C-climate feedback from the Arctic, and thus trajectories of climate change.

This work provides the first regional assessment of integrated whole-lake radiocarbon (¹⁴C) ages of dissolved CH₄ and CO₂ as a proxy for C emission sources in northern Alaska. We collected water samples from below ice along two 170 km north-south transects on the Arctic Coastal Plain (ACP) of Alaska in April 2012 and 2013. These lakes represent a network monitored by the US-NSF funded project, Circum-Arctic Lakes Observation Network (CALON), http://www.arcticlakes.org/. Dissolved CH₄ and CO₂ were extracted and analyzed for their ¹⁴C content. The presence of winter ice on the surface of lakes obstructs the emission of CH₄ and CO₂ originating from the perennially thawed sub-lake sediments. The trapped gases are forced to mix, thus measured ¹⁴C ages are integrated signatures representing the whole-lake emissions.

Dissolved CH₄ and CO₂ ages do not correlate with latitude, yet seem to be driven by surficial geology. Of nearly 150 ¹⁴C measurements, below-ice dissolved CH₄ is the oldest (around 2145 ± 15 ¹⁴C YBP) in a lake residing on “peaty, sandy lowland” on the northern ACP near the town of Barrow. Modern CH₄ and CO₂ dominate emissions from “eolian sandy lowlands” in the interior of the ACP. Across all lakes, dissolved CH₄ (avg. 836 ¹⁴C YBP) is older than dissolved CO₂ (avg. 480 ¹⁴C YBP) by a regional average of ca. 360 ¹⁴C YBP. Results from this study indicate that decomposing Holocene-age organic material is the primary source of CH₄ and CO₂ emissions from the Alaskan ACP. This baseline dataset provides the foundation for future regional lake monitoring in the warming arctic climate. Our whole-lake proxy facilitates lake-to-lake comparison and regional up scaling, thereby strengthening quantitative understanding of how environmental drivers (water body size, climate, surficial geology) affect the magnitude and sources of biogenic CH₄ and CO₂ emissions.