Title

A shift of thermokarst lakes from carbon sources to sinks during the Holocene epoch

Abstract

Thermokarst lakes formed across vast regions of Siberia and Alaska during the last deglaciation and are thought to be a net source of atmospheric methane and carbon dioxide during the Holocene epoch1–4. However, the same thermokarstlakes can also sequester carbon5 , and it remains uncertain whether carbon uptake by thermokarst lakes can offset their greenhouse gas emissions. Here we use field observations of Siberian permafrost exposures, radiocarbon dating and spatial analyses to quantify Holocene carbon stocks and fluxes in lake sediments overlying thawed Pleistocene-aged permafrost.We find that carbon accumulationin deep thermokarst-lake sediments since thelast deglaciationis about 1.6 times larger than the mass of Pleistocene-aged permafrost carbon released as greenhouse gases when the lakes first formed. Although methane and carbon dioxide emissions following thaw lead toimmediate radiative warming, carbon uptakein peat-rich sediments occurs over millennial timescales. We assess thermokarstlake carbon feedbacks to climate with an atmospheric perturbation model and find that thermokarst basins switched from a net radiative warming to a net cooling climate effect about 5,000 years ago. High rates of Holocene carbon accumulation in 20 lake sediments (47 6 10 grams of carbon per square metre per year; mean 6 standard error) were driven by thermokarst erosion and deposition of terrestrial organic matter, by nutrient release from thawing permafrost that stimulatedlake productivity and by slow decompositionin cold, anoxic lake bottoms.When lakes eventually drained, permafrost formation rapidly sequestered sediment carbon. Our estimate of about 160 petagrams of Holocene organic carbon in deep lake basins of Siberia and Alaska increases the circumpolar peat carbon pool estimate for permafrost regions by over 50 per cent (ref. 6). The carbon in perennially frozen drained lake sediments may become vulnerable to mineralization as permafrost disappears7–9, potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene.

Publication Date

7-2014

Journal Title

Nature

Publisher

Nature Publishing Group

Digital Object Identifier (DOI)

10.1038/nature13560

Document Type

Letter to the Editor