Methane Emissions are Predominantly Derived from Contemporary Carbon from a Thawing Permafrost Peatland in Canada

Abstract:

Rapid warming at high northern latitudes is resulting in permafrost degradation. When permafrost thaws in wetlands, the peat plateaus tend to collapse, bringing the water table closer to the surface. Methane (CH₄) fluxes increase post-thaw, but the extent to which this is caused by CH₄ release from the decomposition of previously-frozen, old carbon, deep within the soil profile, versus waterlogging near the soil surface resulting in anaerobic decomposition of more recent inputs, is unclear. Quantifying the relative contributions of these contrasting CH₄ sources is essential for predicting future rates of CH₄ release from thawing permafrost wetlands.

The most definitive test of whether old permafrost-derived C contributes substantially to CH₄ release post thaw, would be to measure the radiocarbon (¹⁴C) content of the CH₄. However, until recently this was very challenging in remote locations. Using new techniques that overcome previous limitations, we were able to measure the ¹⁴C content of CH₄ being released from thawing wetlands in northern Canada. We hypothesised that time since plateau collapse would affect the amount of old CH₄ being released, and so sampled in locations where collapse had occurred at different times. Samples were collected from collars that either included or excluded CH₄ from deep within the peat profile, and CH­₄ from a depth of 100 cm was collected by sampling soil water.

We demonstrate that millennium-old CH₄ was being produced at 100 cm. However, CH₄ being released from the surface had contemporary ¹⁴C signatures. Using our collar treatments, we were able to calculate that deep CH₄ contributed less than 10% of the surface flux, and that this contribution did not vary with time since collapse. The effect of permafrost thaw on CH₄ fluxes in these peatlands appears to be more closely related to changes in near surface conditions than to increases in anaerobic decomposition of previously frozen C. This indicates permafrost thaw does not necessarily lead to increased emissions of old C via the CH₄ pathway as many have assumed. This is not generally reflected in Earth system models.