Terrestrial Permafrost Research Articles

Methane at the NW of Weddell Sea, Antarctica

The presence of gaseous hydrocarbons (from methane to n-pentane) in the seabed sediments and the bubbling of methane may suggest the presence of gas accumulations in the substrate of the NW Weddell Sea, Antarctica. The release of methane from the frozen ocean substrate adjacent to Seymour Island would be linked to climate instability during Late Cenozoic, when vast areas of the Antarctic continental shelf were flooded during the marine transgression that occurred c. 18,000 years ago, after the Last Glacial Maximum (LGM). As the ice melted, the sea again occupied the regions which it had abandoned. As the transgression was relatively rapid, the sub-air relief was not destroyed but was submerged and the ground had frozen (permafrost) along with it. Thus, the heat flow from the sea to the marine substrate, now flooded, would have destabilized frozen gas accumulations, which were originally formed into terrestrial permafrost during the LGM, similarly to what would have happened in the Arctic.

Microbial lipid signatures and substrate potential of organic matter in permafrost deposits: Implications for future greenhouse gas production

AbstractA terrestrial permafrost core from Buor Khaya in northern Siberia comprising deposits of Late Pleistocene to Early Holocene age has been investigated to characterize living and past microbial communities with respect to modern and paleoclimate environmental conditions and to evaluate the potential of the organic matter (OM) for greenhouse gas generation. Microbial life markers—intact phospholipids and phospholipid fatty acids—are found throughout the entire core and indicate the presence of living microorganisms also in older permafrost deposits. Biomarkers for past microbial communities (branched and isoprenoid glycerol dialkyl glycerol tetraether as well as archaeol) reveal links between increased past microbial activity and intervals of high OM accumulation accompanied by increased OM quality presumably caused by local periods of moister and warmer environmental conditions. Concentrations of acetate as an excellent substrate for methanogenesis are used to assess the OM quality with respect to microbial degradability for greenhouse gas production. For this purpose two acetate pools are determined: the pore water acetate and OM bound acetate. Both depth profiles reveal similarities to the OM content and quality indicating a link between the amount of the stored OM and the potential to provide substrates for microbial greenhouse gas production. The data suggest that OM stored in the permafrost deposits is not much different in terms of OM quality than the fresh surface organic material. Considering the expected increase of permafrost thaw due to climate warming, this implies a potentially strong impact on greenhouse gas generation from permafrost areas in future with positive feedback on climate variation.

Arctic Deltaic Lake Sediments As Recorders of Fluvial Organic Matter Deposition

Arctic deltas are dynamic and vulnerable regions that play a key role in land-ocean interactions and the global carbon cycle. Delta lakes may provide valuable historical records of the quality and quantity of fluvial fluxes, parameters that are challenging to investigate in these remote regions. Here we study lakes from across the Mackenzie Delta, Arctic Canada, that receive fluvial sediments from the Mackenzie River when spring flood water levels rise above natural levees. We compare downcore lake sediments with suspended sediments collected during the spring flood, using bulk (% organic carbon, % total nitrogen, 13C, 14C) and molecular organic geochemistry (lignin, leaf waxes). High-resolution age models (137Cs, 210Pb) of downcore lake sediment records (n=11) along with lamina counting on high-resolution radiographs show sediment deposition frequencies ranging between annually to every 15 years. Down-core geochemical variability in a representative delta lake sediment core is consistent with historical variability in spring flood hydrology (variability in peak discharge, ice jamming, peak water levels). Comparison with earlier published Mackenzie River depth profiles shows that (i) lake sediments reflect the riverine surface suspended load, and (ii) hydrodynamic sorting patterns related to spring flood characteristics are reflected in the lake sediments. Bulk and molecular geochemistry of suspended particulate matter from the spring flood peak and lake sediments are relatively similar showing a mixture of modern higher-plant derived material, older terrestrial permafrost material, and old rock-derived material. This suggests that deltaic lake sedimentary records hold great promise as recorders of past (century-scale) riverine fluxes and may prove instrumental in shedding light on past behaviour of arctic rivers, as well as how they respond to a changing climate.

Active-Layer Thickness Measurements Using a Handheld Penetrometer at Boreal and Tundra Sites in Eastern Siberia

Active-layer thickness (ALT) is one of the most robust measures used to assess the impact of climate change on terrestrial permafrost. Testing of a handheld dynamic cone penetrometer showed that it was capable of measuring ALT with the same level of accuracy as conventional methods in boreal and tundra sites in eastern Siberia. The penetrometer also characterised the vertical structure of ground hardness within the active layer. The vertical profile of penetrometer measurements corresponded closely with soil plasticity and the liquid limit in high-centred polygons produced by thermokarst subsidence in dry grassland areas at a boreal site at Churapcha. The ALT was markedly deeper (>70 cm) at gravelly slope points adjacent to a wet tundra plain (<50 cm) in a CALM grid (R8) at Tiksi. Overall, the penetrometer is considered to provide an accurate and informative proxy for rapidly assessing the spatial heterogeneity and interannual changes in ALT. Copyright © 2016 John Wiley & Sons, Ltd.

Coastal dynamics and submarine permafrost in shallow water of the central Laptev Sea, East Siberia

Abstract. Coastal erosion and flooding transform terrestrial landscapes into marine environments. In the Arctic, these processes inundate terrestrial permafrost with seawater and create submarine permafrost. Permafrost begins to warm under marine conditions, which can destabilize the sea floor and may release greenhouse gases. We report on the transition of terrestrial to submarine permafrost at a site where the timing of inundation can be inferred from the rate of coastline retreat. On Muostakh Island in the central Laptev Sea, East Siberia, changes in annual coastline position have been measured for decades and vary highly spatially. We hypothesize that these rates are inversely related to the inclination of the upper surface of submarine ice-bonded permafrost (IBP) based on the consequent duration of inundation with increasing distance from the shoreline. We compared rapidly eroding and stable coastal sections of Muostakh Island and find permafrost-table inclinations, determined using direct current resistivity, of 1 and 5 %, respectively. Determinations of submarine IBP depth from a drilling transect in the early 1980s were compared to resistivity profiles from 2011. Based on borehole observations, the thickness of unfrozen sediment overlying the IBP increased from 0 to 14 m below sea level with increasing distance from the shoreline. The geoelectrical profiles showed thickening of the unfrozen sediment overlying ice-bonded permafrost over the 28 years since drilling took place. We use geoelectrical estimates of IBP depth to estimate permafrost degradation rates since inundation. Degradation rates decreased from over 0.4 m a−1 following inundation to around 0.1 m a−1 at the latest after 60 to 110 years and remained constant at this level as the duration of inundation increased to 250 years. We suggest that long-term rates are lower than these values, as the depth to the IBP increases and thermal and porewater solute concentration gradients over depth decrease. For the study region, recent increases in coastal erosion rate and changes in benthic temperature and salinity regimes are expected to affect the depth to submarine permafrost, leading to coastal regions with shallower IBP.

Threshold sensitivity of shallow Arctic lakes and sublake permafrost to changing winter climate

AbstractInteractions and feedbacks between abundant surface waters and permafrost fundamentally shape lowland Arctic landscapes. Sublake permafrost is maintained when the maximum ice thickness (MIT) exceeds lake depth and mean annual bed temperatures (MABTs) remain below freezing. However, declining MIT since the 1970s is likely causing talik development below shallow lakes. Here we show high‐temperature sensitivity to winter ice growth at the water‐sediment interface of shallow lakes based on year‐round lake sensor data. Empirical model experiments suggest that shallow (1 m depth) lakes have warmed substantially over the last 30 years (2.4°C), with MABT above freezing 5 of the last 7 years. This is in comparison to slower rates of warming in deeper (3 m) lakes (0.9°C), with already well‐developed taliks. Our findings indicate that permafrost below shallow lakes has already begun crossing a critical thawing threshold approximately 70 years prior to predicted terrestrial permafrost thaw in northern Alaska.

Mud volcanoes and ice-keel ploughmarks, Beaufort Sea shelf, Arctic Canada

The shallow shelf waters of the Beaufort Sea have experienced marine transgression during the Holocene (Hill et al. 1985; Taylor et al. 2013). This has led to a warming of what was terrestrial permafrost by water incursion, and to the dissociation of subsurface gas hydrates which now vent into marine waters. Accompanying this change is the development of conical submarine landforms produced through the extrusion of sediments, combined with the continuing reworking of the seafloor by the ploughing action of the keels of drifting ice (Fig. 1). Fig. 1. Multibeam swath bathymetry and bathymetric profiles of mud volcanoes and ice-keel ploughmarks on the Beaufort Sea shelf, Arctic Canada. ( a ) Sun-illuminated multibeam-bathymetric image showing mud volcanoes and ice-keel ploughmarks. Acquisition system Kongsberg EM302. Frequency 30 kHz. Grid-cell size 5 m. White arrows denote a curvilinear ploughmark cross-cutting its previous track. The image is a compilation of data from CCGS Amundsen -based surveys in 2009 and 2011. The NW–SE narrow, linear artefacts arise from instrumentation employed in the initial survey. Wide, curvilinear artefacts arise from different instrumentation employed in a subsequent survey. ( b ) Sun-illuminated multibeam-bathymetric image showing a swarm of mud volcanoes. Acquisition system Kongsberg EM3002. Frequency 300 kHz. …

Evolution of subsea permafrost landscapes in Arctic Siberia since the Late Pleistocene: a synoptic insight from acoustic data of the Laptev Sea

Using high-resolution seismic data, this study aims at investigating the evolution and morphological diversity of subsea permafrost features on the eastern Laptev Sea shelf, Arctic Siberia. Several seismic facies were recognized. These relate to the major environmental changes, which affected the Laptev Sea area before, during, and after the last global transgression. Because this shallow shelf was part of the Beringian landmass, we consider a prominent subsurface seismic basal reflector as the top of the former terrestrial permafrost table. Five zones differing in geometry, reflection patterns, depths, and continuity of the permafrost top are identified. Where visible, the upper 70 m of the sediments consists of epigenetically and syngenetically frozen ice-poor sandy deposits at the base, possibly of early last glacial age, marine isotope stages (MIS) 5 and 4. These are followed by late glacial, ice-rich facies interpreted to be MIS 3 to 2. The early Holocene (MIS 1) features well-stratified lagoonal and taberal deposits. As verified by radiocarbon-dated sediment cores, these deposits are overlain by middle to late Holocene sediments with an increasingly marine signature.

The Effects of Perchlorates on the Permafrost Methanogens: Implication for Autotrophic Life on Mars.

The terrestrial permafrost represents a range of possible cryogenic extraterrestrial ecosystems on Earth-like planets without obvious surface ice, such as Mars. The autotrophic and chemolithotrophic psychrotolerant methanogens are more likely than aerobes to function as a model for life forms that may exist in frozen subsurface environments on Mars, which has no free oxygen, inaccessible organic matter, and extremely low amounts of unfrozen water. Our research on the genesis of methane, its content and distribution in permafrost horizons of different ages and origin demonstrated the presence of methane in permanently frozen fine-grained sediments. Earlier, we isolated and described four strains of methanogenic archaea of Methanobacterium and Methanosarcina genera from samples of Pliocene and Holocene permafrost from Eastern Siberia. In this paper we study the effect of sodium and magnesium perchlorates on growth of permafrost and nonpermafrost methanogens, and present evidence that permafrost hydogenotrophic methanogens are more resistant to the chaotropic agent found in Martian soil. In this paper we study the effect of sodium and magnesium perchlorates on the growth of permafrost and nonpermafrost methanogens, and present evidence that permafrost hydogenotrophic methanogens are more resistant to the chaotropic agent found in Martian soil. Furthermore, as shown in the studies strain M2T M. arcticum, probably can use perchlorate anion as an electron acceptor in anaerobic methane oxidation. Earth’s subzero subsurface environments are the best approximation of environments on Mars, which is most likely to harbor methanogens; thus, a biochemical understanding of these pathways is expected to provide a basis for designing experiments to detect autotrophic methane-producing life forms on Mars.

Methane release from pingo‐like features across the South Kara Sea shelf, an area of thawing offshore permafrost

AbstractThe Holocene marine transgression starting at ~19 ka flooded the Arctic shelves driving extensive thawing of terrestrial permafrost. It thereby promoted methanogenesis within sediments, the dissociation of gas hydrates, and the release of formerly trapped gas, with the accumulation in pressure of released methane eventually triggering blowouts through weakened zones in the overlying and thinned permafrost. Here we present a range of geophysical and chemical scenarios for the formation of pingo‐like formations (PLFs) leading to potential blowouts. Specifically, we report on methane anomalies from the South Kara Sea shelf focusing on two PLFs imaged from high‐resolution seismic records. A variety of geochemical methods are applied to study concentrations and types of gas, its character, and genesis. PLF 1 demonstrates ubiquitously low‐methane concentrations (14.2–55.3 ppm) that are likely due to partly unfrozen sediments with an ice‐saturated internal core reaching close to the seafloor. In contrast, PLF 2 reveals anomalously high‐methane concentrations of &gt;120,000 ppm where frozen sediments are completely absent. The methane in all recovered samples is of microbial and not of thermogenic origin from deep hydrocarbon sources. However, the relatively low organic matter content (0.52–1.69%) of seafloor sediments restricts extensive in situ methane production. As a consequence, we hypothesize that the high‐methane concentrations at PLF 2 are due to microbial methane production and migration from a deeper source.

Utilization of ancient permafrost carbon in headwaters of Arctic fluvial networks.

Northern high-latitude rivers are major conduits of carbon from land to coastal seas and the Arctic Ocean. Arctic warming is promoting terrestrial permafrost thaw and shifting hydrologic flowpaths, leading to fluvial mobilization of ancient carbon stores. Here we describe 14C and 13C characteristics of dissolved organic carbon from fluvial networks across the Kolyma River Basin (Siberia), and isotopic changes during bioincubation experiments. Microbial communities utilized ancient carbon (11,300 to >50,000 14C years) in permafrost thaw waters and millennial-aged carbon (up to 10,000 14C years) across headwater streams. Microbial demand was supported by progressively younger (14C-enriched) carbon downstream through the network, with predominantly modern carbon pools subsidizing microorganisms in large rivers and main-stem waters. Permafrost acts as a significant and preferentially degradable source of bioavailable carbon in Arctic freshwaters, which is likely to increase as permafrost thaw intensifies causing positive climate feedbacks in response to on-going climate change.

In the Warming Arctic Seas

In the Warming Arctic Seas

RETRACTED: A global survey of gas hydrate development and reserves: Specifically in the marine field

Gas hydrates, also known as methane hydrates, are formed due to the high hydraulic pressures present under the cold seabed over long periods of time. Gas hydrates are mainly composed of methane produced in the seabed by bacteria in the use of the remains of animals and plants as food. Often appearing as translucent or opaque ice, gas hydrates can be separated into water and methane gas, which can be burned at normal temperatures and pressures, giving this substance the nickname “combustible ice.” As global oil reserves continue to be depleted, scientists are regarding methane hydrates as a new energy source that is very likely to replace oil in the 21st century. According to reports by the United States Geological Survey, the potential natural gas energy that can be recovered from global methane hydrate formations is two times the amount of fossil fuel energy available to the world. Therefore, many countries that are deeply engaged in the development of gas hydrates, such as the United States, Japan, Canada, China, India, and Taiwan, hope that this new energy source can become a substitute for more conventional petroleum sources. Japan—the first country to develop methane hydrates—will be ready for commercial mass production in the eastern Japanese Nankai Trough prior to 2018, according to Japan׳s Methane Hydrate R&D Program-MH 21. However, the exploitation of methane hydrates in terrestrial permafrost requires less technical risk and costs. Joint explorations in areas of Alaska by the United States, Japan, and Canada will enter the preparation phase for commercial output as early as 2015. In Taiwan, cooperation with Germany and the United States has led to methane hydrate exploration and the initiation of drilling sampling in the South China Sea that is expected to be completed in 2016, with commercial production ready as soon as 2026.

Impact of Lake-Level and Climate Changes on Microbial Communities in a Terrestrial Permafrost Sequence of the El'gygytgyn Crater, Far East Russian Arctic

Microbial permafrost communities play an important role in carbon cycling and greenhouse gas fluxes. Despite the importanceof theseprocesses, thereisa lack of knowledge about how environmental and climatic changesaffect the abundance and composition of microorganisms. Here, we investigated the changing distribution of permafrost microorganisms in response to climate and lake-level changes. The permafrost core was drilled at the near shore of Lake El’gygytgyn, Far East Russian Arctic, and a combined microbiological and lipid biomarker approach was applied. The lower part of the permafrost core, deposited under subaquatic conditions, contains only small amounts of microbial signals; total organic carbon (TOC) content is sparse. After exposure of the site to subaerial conditions during the Allerod, the abundance of Bacteria and Archaea started to increase and the lake-level change is especially evidenced by the relative proportion of archaeal biomarkers. This increase is supported by rising bacterial and archaeal 16S ribosomal ribonucleic acid (rRNA) gene copy numbers and significant amounts of TOC during the late Allerod. After a small decrease during the colder Younger Dryas,the TOC content and the microbial signals stronglyincreaseduringthe Holocene, presumably stimulated by pedogenesis. The occurrence of intact phospholipids indicates the presence of living microorganisms in these deposits. Our data suggest that methane formation is mainly expected for the subaerial interval, especially the Holocene where methanogens were identified by fingerprinting. This study emphasises the role of the uppermost permafrost deposits as a hotspot of carbon cycling in arctic environments, especially in the light of expected future global warming. Copyright © 2014 John Wiley & Sons, Ltd.

Ebullition and storm-induced methane release from the East Siberian Arctic Shelf

Vast quantities of carbon are stored in shallow Arctic reservoirs, such as submarine and terrestrial permafrost. Submarine permafrost on the East Siberian Arctic Shelf started warming in the early Holocene, several thousand years ago. However, the present state of the permafrost in this region is uncertain. Here, we present data on the temperature of submarine permafrost on the East Siberian Arctic Shelf using measurements collected from a sediment core, together with sonar-derived observations of bubble flux and measurements of seawater methane levels taken from the same region. The temperature of the sediment core ranged from -1.8 to 0 degrees C. Although the surface layer exhibited the lowest temperatures, it was entirely unfrozen, owing to significant concentrations of salt. On the basis of the sonar data, we estimate that bubbles escaping the partially thawed permafrost inject 100-630 mg methane m(-2) d(-1) into the overlying water column. We further show that water-column methane levels had dropped significantly following the passage of two storms. We suggest that significant quantities of methane are escaping the East Siberian Shelf as a result of the degradation of submarine permafrost over thousands of years. We suggest that bubbles and storms facilitate the flux of this methane to the overlying ocean and atmosphere, respectively.

Influences of salinity and temperature on the stable isotopic composition of methane and hydrogen sulfide trapped in pressure-vessel hydrates

The stable isotopic composition of carbon (δ13CCH4) and hydrogen (δ2HCH4) in methane and sulfur (δ34SH2S) in hydrogen sulfide can be used to infer the source of volatile molecules encaged in gas hydrates. Differentiation of methane and hydrogen sulfide from microbial and thermal origins provides valuable information for hydrocarbon exploration and for climatic models assessing the role of gas hydrates during climate change. In astrobiological studies, δ13CCH4, δ2HCH4, and δ34SH2S values will be critical in deciphering the origin of methane and hydrogen sulfide molecules if gas hydrates are detected within the cryosphere of Mars or associated with ice-covered oceans on Europa or Enceladus. It is challenging, however, to apply isotope systematics to hydrate-forming systems due to complex influences on nucleation and decomposition under varying conditions of salinity, pressure, and temperature. Few laboratory studies have evaluated the effect of hydrate formation, on isotopic composition of free, encaged, and dissolved gas molecules. In this study, pressure-vessel hydrates were nucleated under conditions inferred for marine continental margins and terrestrial permafrost: low temperatures, moderate pressures, saturation of methane and/or hydrogen sulfide saturation, and varying concentration of sodium chloride (NaCl) and magnesium sulfate heptahydrate (MgSO4·7H2O). Methane experiments show less than 1‰ differences in values of δ13CCH4 between free and encaged molecules and up to 6.5‰ variations in values of δ2HCH4 between free and encaged molecules. In hydrogen-sulfide hydrates, δ34SH2S values show less than 4‰ differences between free and encaged molecules, but up to 14‰ differences between dissolved and free molecules and between dissolved and encaged molecules. Results presented here indicate that shifts found for free and encaged values of δ13CCH4 and δ2HCH4 are small and do not complicate interpretation of gas provenance. Conversely, in hydrate systems containing H2S molecules values of δ34SH2S need to be interpreted with caution. Although isotopic fractionation between free- and encaged-sulfur molecules is mild during hydrate formation, values of dissolved δ34SH2S are substantially fractionated and necessitate careful examination of sulfur isotopic values. Because dissolved H2S could potentially be recycled by oxidation and reduction processes during hydrate formation events, use of δ34SH2S values might complicate assessment of biosignatures for other planetary bodies.

Response of methanogenic archaea to Late Pleistocene and Holocene climate changes in the Siberian Arctic

In order to investigate the link between the methane dynamics in permafrost deposits and climate changes in the past, we studied the abundance, composition, and methane production of methanogenic communities in Late Pleistocene and Holocene sediments of the Siberian Arctic. We detected intervals of increased methane concentrations in Late Pleistocene and Holocene deposits along a 42 ka old permafrost sequence from Kurungnakh Island in the Lena Delta (northeast Siberia). Increased amounts of archaeal life markers (intact phospholipid ethers) and a high variety in genetic fingerprints detected by 16S ribosomal ribonucleic acid gene analyses of methanogenic archaea suggest presently living and presumably active methanogenic archaea in distinct layers predominantly in Holocene deposits, but also in deep frozen ground at 17 m depth. Potential methanogenic activity was confirmed by incubation experiments. By comparing methane concentrations, microbial incubation experiments, gene analysis of methanogens, and microbial life markers (intact phospholipid esters and ethers) to already partly degraded membrane lipids, such as archaeol and isoprenoid glycerol dialkyl glycerol tetraethers, we demonstrated that archaeol likely represents a signal of past methanogenic archaea. The archaeol signal was used to reconstruct the response of methanogenic communities to past temperature changes in the Siberian Arctic, and the data suggest higher methane emissions occurred during warm periods, particularly during an interval in the Late Pleistocene and during the Holocene. This new data on present and past methanogenic communities in the Siberian terrestrial permafrost imply that these microorganisms will respond to the predicted future temperature rise in the Arctic with increasing methane production, as demonstrated in previous warmer periods.

Detecting unfrozen sediments below thermokarst lakes with surface nuclear magnetic resonance

AbstractA talik is a layer or body of unfrozen ground that occurs in permafrost due to an anomaly in thermal, hydrological, or hydrochemical conditions. Information about talik geometry is important for understanding regional surface water and groundwater interactions as well as sublacustrine methane production in thermokarst lakes. Due to the direct measurement of unfrozen water content, surface nuclear magnetic resonance (NMR) is a promising geophysical method for noninvasively estimating talik dimensions. We made surface NMR measurements on thermokarst lakes and terrestrial permafrost near Fairbanks, Alaska, and confirmed our results using limited direct measurements. At an 8 m deep lake, we observed thaw bulb at least 22 m below the surface; at a 1.4 m deep lake, we detected a talik extending between 5 and 6 m below the surface. Our study demonstrates the value that surface NMR may have in the cryosphere for studies of thermokarst lake hydrology and their related role in the carbon cycle.

Design and Experimental Study of the Steam Mining System for Natural Gas Hydrates

The mining technique for natural gas hydrates is an essential technology to enable natural gas hydrates to replace conventional energy sources. This paper presents a steam mining system for natural gas hydrates (SMSGH) for exploiting natural gas hydrates in terrestrial permafrost regions. The system comprises of the surface steam heating device, underground mining, sealing device, surface gas collection, and ignition device. The working principle, design and calculation process, and structure composition of the system are described. The heating effect of the system is verified by laboratory experiments. The optimal heating parameters are proposed; steam open/closed pressure is 1.0/0.5 MPa. A field experiment was conducted in drilling wells of natural gas hydrates in Qilian Shan permafrost in Qinghai province, China. A total of 3.28 m3 of natural gas was successfully exploited, and the ignition experiment was completed. This is the first study conducted in China that has successfully exploited natural gas hydrates in terrestrial permafrost.

Past extreme warming events linked to massive carbon release from thawing permafrost

Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend. The first and largest of these events, the Palaeocene-Eocene Thermal Maximum (PETM), is characterized by a massive input of carbon, ocean acidification and an increase in global temperature of about 5 °C within a few thousand years. Although various explanations for the PETM have been proposed, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETM and successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate-ecosystem-soil simulations accounting for rising concentrations of background greenhouse gases and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of petagrams (10(15) grams) of carbon to the atmosphere-ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event, while providing a sensitive carbon reservoir for the next hyperthermal. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth's orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.