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 epoch. However, the same thermokarst lakes can also sequester carbon, 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 accumulation in deep thermokarst-lake sediments since the last deglaciation is 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 to immediate radiative warming, carbon uptake in peat-rich sediments occurs over millennial timescales. We assess thermokarst-lake 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 ± 10 grams of carbon per square metre per year; mean ± standard error) were driven by thermokarst erosion and deposition of terrestrial organic matter, by nutrient release from thawing permafrost that stimulated lake productivity and by slow decomposition in 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 disappears, potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene.

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Permafrost thaw exposes previously frozen soil organic matter to microbial decomposition. This process generates methane and carbon dioxide, and thereby fuels a positive feedback process that leads to further warming and thaw1. Despite widespread permafrost degradation during the past ~40 years2, 3, 4, the degree to which permafrost thaw may be contributing to a feedback between warming and thaw in recent decades is not well understood. Radiocarbon evidence of modern emissions of ancient permafrost carbon is also sparse5. Here we combine radiocarbon dating of lake bubble trace-gas methane (113 measurements) and soil organic carbon (289 measurements) for lakes in Alaska, Canada, Sweden and Siberia with numerical modelling of thaw and remote sensing of thermokarst shore expansion. Methane emissions from thermokarst areas of lakes that have expanded over the past 60 years were directly proportional to the mass of soil carbon inputs to the lakes from the erosion of thawing permafrost. Radiocarbon dating indicates that methane age from lakes is nearly identical to the age of permafrost soil carbon thawing around them. Based on this evidence of landscape-scale permafrost carbon feedback, we estimate that 0.2 to 2.5 Pg permafrost carbon was released as methane and carbon dioxide in thermokarst expansion zones of pan-Arctic lakes during the past 60 years

Permafrost carbon feedback (PCF) modeling has focused on gradual thaw of near-surface permafrost leading to enhanced carbon dioxide and methane emissions that accelerate global climate warming. These state-of-the-art land models have yet to incorporate deeper, abrupt thaw in the PCF. Here we use model data, supported by field observations, radiocarbon dating, and remote sensing, to show that methane and carbon dioxide emissions from abrupt thaw beneath thermokarst lakes will more than double radiative forcing from circumpolar permafrost-soil carbon fluxes this century. Abrupt thaw lake emissions are similar under moderate and high representative concentration pathways (RCP4.5 and RCP8.5), but their relative contribution to the PCF is much larger under the moderate warming scenario. Abrupt thaw accelerates mobilization of deeply frozen, ancient carbon, increasing 14C-depleted permafrost soil carbon emissions by ~125–190% compared to gradual thaw alone. These findings demonstrate the need to incorporate abrupt thaw processes in earth system models for more comprehensive projection of the PCF this century.

[1] Ebullition is an important but highly heterogeneous mode of methane emission in lakes. Variability in both spatial distribution and temporal flux creates difficulty in constraining uncertainties in whole lake emission estimates. Analysis of short-and long-term flux measurements on 162 ebullition seeps in 24 panarctic lakes confirmed that seep classes, identified a priori according to bubble patterns in winter lake ice, have distinct associated fluxes irrespective of lake or region. To understand the drivers of ebullition's spatial variability and uncover ways to better quantify ebullition in field work, we combined point-process modeling with field measurements of 2679 GPS-marked and classified ebullition seeps in three Alaskan thermokarst (thaw) lakes that varied by region, permafrost type, and seep distribution. Spatial analysis of field data revealed that seeps cluster above thawed permafrost soil mounds in lake bottoms. Seep density and clustering, determined from field observations, were used as parameters in a Poisson cluster process model to simulate seeps across entire lake surfaces. Sampling results indicated that (1) applying seep-class mean flux values to unmeasured seeps counted on ice-bubble surveys does not compromise accuracy of whole lake flux estimates; (2) three distributed 50 m 2 ice-bubble survey transects more accurately estimate mean lake ebullition than 17 dispersed 0.2 m 2 bubble traps; and (3) the uncertainty associated with whole lake mean ebullition estimated by lake-ice survey transects is inversely related to seep density. Findings suggest that transect field data collected on a large number of widely distributed lakes can be combined to provide a well-constrained, bottom-up estimate of regional lake ebullition.Citation: Walter Anthony, K. M., and P. Anthony (2013), Constraining spatial variability of methane ebullition seeps in thermokarst lakes using point process models,