Speaker: Claudia Czimczik (UC Irvine)
Title: Closing the Winter Gap and Digging Deeper: A Radiocarbon View of Permafrost Carbon Loss
Abstract:
The Arctic holds roughly twice as much carbon in permafrost soils as the current atmosphere, much of it Pleistocene-age. As this ground thaws, whether the region amplifies warming depends not just on how much CO₂ is released but on its source: recently fixed carbon, or legacy carbon long locked away? Estimates still disagree on whether the permafrost region is a net CO₂ sink or source, limited by sparse cold-season data and an oversimplified picture of the soils involved. Radiocarbon speaks directly to this, separating plant from microbial respiration and dating the carbon being lost.
I will show how a rugged, passive ¹⁴CO₂ sampler enables year-round measurements that close the long-standing winter gap in Arctic flux records, revealing a seasonal shift toward older carbon sources in fall and winter. A 25-year snow-addition experiment extends this from seasonal to decadal time scales: a deeper, more insulating snowpack sustained warmer soils and accelerated the loss of legacy carbon.
I will then argue that the standard two-layer picture, a seasonally thawing active layer over permafrost, hides a climatically labile “transition zone.” Change-point detection on radiocarbon depth profiles resolves active, transient, intermediate, and permafrost layers, and shows we may need to sample twice as deep as active-layer observations suggest. Both deeper snow and wildfire collapse this zone, melting ground ice and bringing Pleistocene-age carbon to the modern surface. Shallow, growing-season sampling therefore under-represents the transition zone, biasing the age and decomposability assumptions that models use to project permafrost carbon release.