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.
At the last deglaciation Earth's largest biome, mammoth-steppe, vanished. Without knowledge of the productivity of this ecosystem, the evolution of man and the glacial–interglacial dynamics of carbon ...storage in Earth's main carbon reservoirs cannot be fully understood. Analyzes of fossils 14C dates and reconstruction of mammoth steppe climatic envelope indicated that changing climate wasn't a reason for extinction of this ecosystem. We calculate, based on animal skeleton density in frozen soils of northern Siberia, that mammoth-steppe animal biomass and plant productivity, even in these coldest and driest of the planet's grasslands were close to those of an African savanna. Numerous herbivores maintained ecosystem productivity. By reducing soil moisture and permafrost temperature, accumulating carbon in soils, and increasing the regional albedo, mammoth-steppe amplified glacial–interglacial climate variations. The re-establishment of grassland ecosystems would slow permafrost thawing and reduce the current warming rate. Proposed methods can be used to estimate animal density in other ecosystems.
► Animal in mammoth steppe was as high as in African savannah. ► This ecosystem played important role in shaping planets environment. ► Mammoth steppe went extinct not because of climate. ► Restoration of this ecosystem is possible.
Large uncertainties in the budget of atmospheric methane, an important greenhouse gas, limit the accuracy of climate change projections. Thaw lakes in North Siberia are known to emit methane, but the ...magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable. Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia. We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated. Extrapolation of these fluxes indicates that thaw lakes in North Siberia emit 3.8 teragrams of methane per year, which increases present estimates of methane emissions from northern wetlands (< 6-40 teragrams per year; refs 1, 2, 4-6) by between 10 and 63 per cent. We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent. Furthermore, the Pleistocene age (35,260-42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost.
Polar ice-core records suggest that an arctic or boreal source was responsible for more than 30% of the large increase in global atmospheric methane (CH₄) concentration during deglacial climate ...warming; however, specific sources of that CH₄ are still debated. Here we present an estimate of past CH₄ flux during deglaciation from bubbling from thermokarst (thaw) lakes. Based on high rates of CH₄ bubbling from contemporary arctic thermokarst lakes, high CH₄ production potentials of organic matter from Pleistocene-aged frozen sediments, and estimates of the changing extent of these deposits as thermokarst lakes developed during deglaciation, we find that CH₄ bubbling from newly forming thermokarst lakes comprised 33 to 87% of the high-latitude increase in atmospheric methane concentration and, in turn, contributed to the climate warming at the Pleistocene-Holocene transition.
Zimov and colleagues reconstitute the mammoth ecosystem in one modest parcel of the northern Siberian region of Yakutia. They call their project Pleistocene Park. Its primary scientific goal is to ...determine more precisely the role that Pleistocene animals played in maintaining their own ecosystem.
With increasing air temperatures and changing precipitation patterns forecast for the Arctic over the coming decades, the thawing of ice-rich permafrost is expected to increasingly alter hydrological ...conditions by creating mosaics of wetter and drier areas. The objective of this study is to investigate how 10 years of lowered water table depths of wet floodplain ecosystems would affect CO2 fluxes measured using a closed chamber system, focusing on the role of long-term changes in soil thermal characteristics and vegetation community structure. Drainage diminishes the heat capacity and thermal conductivity of organic soil, leading to warmer soil temperatures in shallow layers during the daytime and colder soil temperatures in deeper layers, resulting in a reduction in thaw depths. These soil temperature changes can intensify growing-season heterotrophic respiration by up to 95 %. With decreased autotrophic respiration due to reduced gross primary production under these dry conditions, the differences in ecosystem respiration rates in the present study were 25 %. We also found that a decade-long drainage installation significantly increased shrub abundance, while decreasing Eriophorum angustifolium abundance resulted in Carex sp. dominance. These two changes had opposing influences on gross primary production during the growing season: while the increased abundance of shrubs slightly increased gross primary production, the replacement of E. angustifolium by Carex sp. significantly decreased it. With the effects of ecosystem respiration and gross primary production combined, net CO2 uptake rates varied between the two years, which can be attributed to Carex-dominated plots' sensitivity to climate. However, underlying processes showed consistent patterns: 10 years of drainage increased soil temperatures in shallow layers and replaced E. angustifolium by Carex sp., which increased CO2 emission and reduced CO2 uptake rates. During the non-growing season, drainage resulted in 4 times more CO2 emissions, with high sporadic fluxes; these fluxes were induced by soil temperatures, E. angustifolium abundance, and air pressure.
Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and ...may cause positive feedback to warming as CO2 is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15°C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO2 with radiocarbon (14C) ages ranging between∼21 and 24 ka bp was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q10 value of 1.9±0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions of Δ14CO2 values in respiration CO2 and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO2 in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5°C, about 1 Pg C will be initially released from labile C pools, followed by respiration of∼40 Pg C to the atmosphere over a period of four decades.
Ecosystem flux measurements using the eddy covariance (EC) technique were undertaken in 4 subsequent years during summer for a total of 562 days in an arctic wet tundra ecosystem, located near ...Cherskii, Far-Eastern Federal District, Russia. Methane (CH₄) emissions were measured using permanent chambers. The experimental field is characterized by late thawing of permafrost soils in June and periodic spring floods. A stagnant water table below the grass canopy is fed by melting of the active layer of permafrost and by flood water. Following 3 years of EC measurements, the site was drained by building a 3 m wide drainage channel surrounding the EC tower to examine possible future effects of global change on the tundra tussock ecosystem. Cumulative summertime net carbon fluxes before experimental alteration were estimated to be about +15 g C m⁻² (i.e. an ecosystem C loss) and +8 g C m⁻² after draining the study site. When taking CH₄ as another important greenhouse gas into account and considering the global warming potential (GWP) of CH₄ vs. CO₂, the ecosystem had a positive GWP during all summers. However CH₄ emissions after drainage decreased significantly and therefore the carbon related greenhouse gas flux was much smaller than beforehand (475 ± 253 g C-CO₂-e m⁻² before drainage in 2003 vs. 23 ± 26 g C-CO₂-e m⁻² after drainage in 2005).
This study reports an atmospheric methane (CH4) source term previously uncharacterized regarding strength and isotopic composition. Methane emissions from 14 Siberian lakes and 9 Alaskan lakes were ...characterized using stable isotopes (13C and D) and radiocarbon (14C) analyses. We classified ebullition (bubbling) into three categories (background, point sources, and hot spots) on the basis of fluxes, major gas concentrations, and isotopic composition. Point sources and hot spots had a strong association with thermokarst (thaw) erosion because permafrost degradation along lake margins releases ancient organic matter into anaerobic lake bottoms, fueling methanogenesis. With increasing ebullition rate, we observed increasing CH4 concentration of greater radiocarbon age, depletion of 13CCH4, and decreasing bubble N2 content. Microbial oxidation of methane was observed in bubbles that became trapped below and later within winter lake ice; however, oxidation appeared insignificant in bubbles sampled immediately after release from sediments. Methanogenic pathways differed among the bubble sources: CO2 reduction supported point source and hot spot ebullition to a large degree, while acetate fermentation appeared to contribute to background bubbling. To provide annual whole‐lake and regional CH4 isofluxes for the Siberian lakes, we combined maps of bubble source distributions with long‐term, continuous flux measurements and isotopic composition. In contrast to typical values used in inverse models of atmospheric CH4 for northern wetland sources (δ13CCH4 = −58‰, 14C age modern), which have not included northern lake ebullition as a source, we show that this large, new source of high‐latitude CH4 from lakes is isotopically distinct (δ13CCH4 = −70‰, 14C age 16,500 years, for North Siberian lakes).
Summary
Synthesis of results from several Arctic and boreal research programmes provides evidence for the strong role of high‐latitude ecosystems in the climate system. Average surface air ...temperature has increased 0.3 °C per decade during the twentieth century in the western North American Arctic and boreal forest zones. Precipitation has also increased, but changes in soil moisture are uncertain. Disturbance rates have increased in the boreal forest; for example, there has been a doubling of the area burned in North America in the past 20 years. The disturbance regime in tundra may not have changed. Tundra has a 3–6‐fold higher winter albedo than boreal forest, but summer albedo and energy partitioning differ more strongly among ecosystems within either tundra or boreal forest than between these two biomes. This indicates a need to improve our understanding of vegetation dynamics within, as well as between, biomes. If regional surface warming were to continue, changes in albedo and energy absorption would likely act as a positive feedback to regional warming due to earlier melting of snow and, over the long term, the northward movement of treeline. Surface drying and a change in dominance from mosses to vascular plants would also enhance sensible heat flux and regional warming in tundra. In the boreal forest of western North America, deciduous forests have twice the albedo of conifer forests in both winter and summer, 50–80% higher evapotranspiration, and therefore only 30–50% of the sensible heat flux of conifers in summer. Therefore, a warming‐induced increase in fire frequency that increased the proportion of deciduous forests in the landscape, would act as a negative feedback to regional warming.
Changes in thermokarst and the aerial extent of wetlands, lakes, and ponds would alter high‐latitude methane flux. There is currently a wide discrepancy among estimates of the size and direction of CO2 flux between high‐latitude ecosystems and the atmosphere. These discrepancies relate more strongly to the approach and assumptions for extrapolation than to inconsistencies in the underlying data. Inverse modelling from atmospheric CO2 concentrations suggests that high latitudes are neutral or net sinks for atmospheric CO2, whereas field measurements suggest that high latitudes are neutral or a net CO2 source. Both approaches rely on assumptions that are difficult to verify. The most parsimonious explanation of the available data is that drying in tundra and disturbance in boreal forest enhance CO2 efflux. Nevertheless, many areas of both tundra and boreal forests remain net sinks due to regional variation in climate and local variation in topographically determined soil moisture. Improved understanding of the role of high‐latitude ecosystems in the climate system requires a concerted research effort that focuses on geographical variation in the processes controlling land–atmosphere exchange, species composition, and ecosystem structure. Future studies must be conducted over a long enough time‐period to detect and quantify ecosystem feedbacks.
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