Increasing Organic Carbon Biolability With Depth in Yedoma Permafrost: Ramifications for Future Climate Change.

Permafrost thaw subjects previously frozen organic carbon (OC) to microbial decomposition, generating the greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4) and fueling a positive climate feedback. Over one quarter of permafrost OC is stored in deep, ice‐rich Pleistocene‐aged yedoma permafrost deposits. We used a combination of anaerobic incubations, microbial sequencing, and ultrahigh‐resolution mass spectrometry to show yedoma OC biolability increases with depth along a 12‐m yedoma profile. In incubations at 3 °C and 13 °C, GHG production per unit OC at 12‐ versus 1.3‐m depth was 4.6 and 20.5 times greater, respectively. Bacterial diversity decreased with depth and we detected methanogens at all our sampled depths, suggesting that in situ microbial communities are equipped to metabolize thawed OC into CH4. We concurrently observed an increase in the relative abundance of reduced, saturated OC compounds, which corresponded to high proportions of C mineralization and positively correlated with anaerobic GHG production potentials and higher proportions of OC being mineralized as CH4. Taking into account the higher global warming potential (GWP) of CH4 compared to CO2, thawed yedoma sediments in our study had 2 times higher GWP at 12‐ versus 9.0‐m depth at 3 °C and 15 times higher GWP at 13 °C. Considering that yedoma is vulnerable to processes that thaw deep OC, our findings imply that it is important to account for this increasing GHG production and GWP with depth to better understand the disproportionate impact of yedoma on the magnitude of the permafrost carbon feedback. Plain Language Summary: Despite occupying only 7% of global permafrost area, yedoma permafrost contains over a quarter (~398 Gt) of the estimated global permafrost organic carbon (OC) pool. It has been suggested that microbial decomposition of thawed, reactivated OC from yedoma permafrost and subsequent greenhouse gas production may be a "tipping point" for future climate warming. Our research shows that greenhouse gas production potentials increase with depth in yedoma permafrost and, importantly, that following thaw OC from deeper yedoma is more prone to being microbially processed into methane, a potent greenhouse gas with 34 times more global warming potential than carbon dioxide on a century time scale. Considering that yedoma permafrost is particularly vulnerable to processes that deeply thaw permafrost OC, it is crucial to account for this increasing greenhouse gas production and global warming potential per unit OC with depth to better understand how thawing yedoma permafrost will impact global carbon cycling and future climate warming. Key Points: Organic matter quality and greenhouse gas production potentials increase with depth in yedoma permafrostCO2:CH4 production potentials decrease with depth in yedoma permafrost soilsWater‐extractable organic matter composition depth trends suggest long‐term processing under reduced conditions [ABSTRACT FROM AUTHOR]