Detection of CH₄ Containing Aged C Released from Thermally-degrading Permafrost using ¹⁴CH₄ Measurements

M. Dyonisius¹, S. Lehman¹, J. Miller², C. Wolak¹, S. Morgan¹ and P. Cappa¹

¹Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309; 857-200-8396, E-mail: michael.dyonisius@colorado.edu
²NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305

Arctic regions have warmed 2-4 times faster than the global average, raising concerns that large, perennially frozen carbon stocks that have been stored in permafrost may become remobilized and emitted to the atmosphere as CO₂ and CH₄ – leading to the so-called “permafrost carbon-climate feedback”. Atmospheric radiomethane (¹⁴CH₄) is a sensitive tracer of emissions from aged sources indicative of this feedback process. Currently, background levels of atmospheric ¹⁴CH₄ are maintained at high levels by emissions from nuclear power plant(Δ¹⁴CH₄ bg ≈ +350 ‰). In contrast, CH₄ emissions associated with microbial sources accessing aged, previously frozen carbon in permafrost will be depleted in ¹⁴C (Δ¹⁴CH₄ < 0‰) because of radioactive decay during soil carbon storage, producing detectably large ¹⁴C depletion signals in atmospheric Δ¹⁴CH₄. We present ~10-year-long records of atmospheric Δ¹⁴CH₄ observations from two Arctic sites: Utqiaġvik (formerly Barrow), Alaska (BRW; 71.32^oN, 156.61^oW) and the CARVE (Carbon in Arctic Reservoirs Vulnerability Experiment) tower (CRV; 64.99^oN, 147.60^oW) near Fairbanks. Preliminary results from a one-dimensional analytical framework propagating estimated uncertainties in background CH₄ mole fraction, δ¹³C-CH₄ source signatures, and combined atmospheric and soil sink fractionations indicate flux-weighted Arctic wetland Δ¹⁴C-CH₄ source signatures in the range of -130 ± 124 ‰ (BRW, ±1σ Monte Carlo derived uncertainty) and -132 ± 160 ‰ (CRV). These isotopic source signatures are comparable to those obtained from bottom-up measurements and are equivalent to ~34% of summertime CH₄ emissions being sourced from recently remobilized permafrost carbon (assuming a representative end-member age of ~4000 years for old permafrost carbon). Although we note a clear and consistent warm season ¹⁴C depletion signal, longer observational records are needed to determine whether the flux-weighted CH₄ source signatures we observe represent a steady-state signal or the first signs of newly mobilized old carbon sources associated with accelerated permafrost degradation.

Figure 1. Measured (red and blue dots) and modeled (solid black line, with shaded grey region for 95% confidence interval) Δ¹⁴C-CH₄ values for two sites in Arctic Alaska, along with estimated warm-season CH₄ source ¹⁴C signatures obtained from a simple one-dimensional model used to isolate the CH₄ mole fraction signal arising from wetland emissions and the range of associated best-fit source values. Source Δ¹⁴C values represent the flux-weighted mean of all source components (both aged and modern).