Collaborative Research: Degrading offshore permafrost as a source of methane on the East Siberian Arctic Shelf

This project is a represents an international collaborative effort between the Joye group and Drs. Igor Semiletov and Natasha Shakohova (University of Alaska), Dr. Christof Meile (UGA), and scientists from Russia (Dr. D. Nicolsky, M. Grigoriev, O. Dudarev, and R. Pekant, and A. Kholodov). The following group members are involved in this work:

****A PhD student position is currently available on this project. If you are interested, please contact Dr. Joye (mjoye (at) uga (dot) edu) or Dr. Samarkin****

What is the problem…

The Arctic Ocean is vulnerable environment with unique ecosystems that are adapted to harsh conditions. Global warming has driven drastic large-scale environmental change in Arctic environments. The Arctic Ocean contains abundant offshore and onshore permafrost that is degrading at increasing rates under warmer conditions. The most pronounced warming is in the East Siberian region, where surface air temperature increased by 5ºC between 2000-2005 (http://www.eoearth.org/article/State_of_the_Arctic_Report). Enormous stores of methane (CH4) gas are present in the Arctic tundra soils and marine sediments. Most of this CH4 is stored as offshore gas hydrate. Accelerated warming and the resultant thawing of offshore permafrost may increase CH4 emission gradually or there may be a ‘tipping point’, at which time CH4 emissions increase dramatically.

Biological processes in the Arctic are generally slower relative to those occurring in warmer seas areas, so perturbations to the Arctic system may require a significantly longer recovery period, if the system recovers at all. The proposed work aims to further our knowledge of the processes mediating methane dynamics in Arctic sediments. The Arctic environment plays both a critical role in the regulation of global climate, and is sensitive to the increased global temperatures resulting from increased concentrations of greenhouse gases.

The Arctic is a critical component of the global carbon cycle

Biogenic CH4 is produced in shallow, e.g. river deltas, estuaries, and harbors, and deep, e.g. continental shelves and slopes, sediments on continental margins worldwide, when sufficient organic matter is available. Methane production proceeds in earnest in sediments where sulfate is absent; in organic rich sediments, methanogenesis can occur in shallow (even surficial) sediments. Recent estimates suggest that globally, about 20 Tg of CH4 per year escapes from the seabed into the atmosphere. This amounts to ~15% of global CH4 emission from natural sources. This estimate is based on limited data and major knowledge gaps exist in polar regions when it comes to constraining the amount of gas stored in the sediments, its current and potential release rates into water column and atmosphere, and the factors that regulate CH4 production and release.

Arctic shallow seabed sediments have received little attention with regard to their contribution to the global carbon budget and, particularly, the marine CH4 budget, because of their small areal extent. However, recent work has highlighted the potential for significant CH4 release from Arctic sediments and suggests CH4 release from the Arctic is increasing over time, at least in the Beaufort Sea region. The low temperatures that characterize Arctic sediments are not considered conducive to methanogenesis, but psychrophilic methanogens do exist. In case of the East Siberian Arctic Shelf (ESAS), shallow sediments have not been considered a CH4 source to the hydrosphere or atmosphere because submarine (offshore) permafrost, defined as sediments with a 2-year mean temperature below 0°C, acts as an impermeable lid, preventing CH4 escape.

Fig. 1. Dissolved CH4 in surface seawater (nM, upper) and the overlying atmosphere (ppm, lower) along the East Siberian shelf (from 10/2005). Matching areas of increased CH4 concentration noted by red circles.

However, recent data (2003-2005) from the ESAS showed extreme surface water CH4 supersaturation (up to 22,000% saturation), implying high sea-to-air fluxes. Elevated CH4 concentrations in the air above the sea surface along the ship route was detected in the SE Laptev Sea (Fig. 1). Moreover, extremely high dissolved CH4 concentrations (up to 20 µM) were detected in the surface layer of seawater beneath the sea ice during winter. Understanding the factors responsible for the observed high CH4 concentrations in the ESAS surface waters requires additional research.

The Arctic region contains a huge amount of organic carbon buried inland and within the Arctic Ocean sedimentary basin (“Arctic carbon hyper pool”). The Russian Arctic shelf acts as an estuary of the Great Siberian Rivers and has the highest known coastal sedimentation rates, about 10×106 t Corg yr-1, which approximately equals the amount of sediment accumulated over the entire pelagic zone of the World Ocean. The rapid sedimentation rates typical of the ESAS means that much of the deposited organic carbon survives decomposition and is buried within seabed sediments. Some of these sediments are seasonally or annually frozen (˜offshore permafrost), representing a substantial reservoir of potentially labile organic carbon. Upon permafrost melting, the old organic carbon they contain will be reintroduced into the active biogeochemical C cycle and may potentially fuel in situ methanogenesis providing an additional and new source of CH4 to the region.

A second, potentially more dynamic, sedimentary CH4 pool exists in deeper layers beneath the permafrost. Methane generated by microbial methanogenesis and/or the thermogenic breakdown of organic matter migrates through deep sediments towards the seafloor. This CH4 can be sequestered in gas hydrates or may escape through natural gas seeps. Previous researchers have estimated that natural gas seeps contribute 30 to 50 Tg of CH4 per year to the seabed and that 10 to 30 Tg of this CH4 enters the atmosphere. This CH4 source represents an increase to the current total “natural” CH4 sources to the atmosphere (~120 Tg yr-1) by up to 25%. A recent global study by Rigby et al. (2008) noted that the first increase in atmospheric CH4 levels this century — by about 28 million tonnes since mid-2006 — was due in part to CH4 release in and near the Arctic. This CH4 spike appears to be generated by the rapid melting of sea ice across the Arctic circle and thawing of permafrost, driven by higher than average summer temperatures.

Could offshore permafrost be a methane source to overlying sediments and the atmosphere?

No data are available to constrain the distribution, thickness and thermal conditions of offshore permafrost along the ESAS. Estimates for the depth of the permafrost upper boundary (permafrost table), its distribution and its degradation rate range from <10 m to about 100 m, 0 to 1400 m, and 0 to several m per 1000 yrs, respectively. The general assumption is that permafrost is distributed continuously along the shallow Laptev shelf; this permafrost did not thaw completely during the last glacial-interglacial transgression, except along active fault zones with high geothermal heat flux. Emission of CH4 from permafrost gas hydrates is believed to occur only along the deeper shelf (>70 m), with the exception of taliks, thawed zones that form within fault zones along the shallow shelf and provide conduits for the release of CH4-rich groundwater. Interestingly, release of permafrost CH4 from seafloor vents (called ‘pingos’) along the Beaufort Shelf was reported recently.

Repeated expeditions to the eastern Laptev Sea and western East Siberian Sea the mid-1990s showed that CH4 concentrations in the ESAS were usually less than 15 nM. Since 2003, we have documented an increasing number of “hotspots” of CH4 release. This year’s the International Siberian Shelf Expedition brought a broad combination of state-of-the-art sampling and analytical techniques to the region. While there are inter-annual variations, with lower concentrations some years, the long-term data demonstrates higher levels at locations that are re-visited. Moreover, pingo-like structures, and acoustical gas chimneys were found recently in the shallow and mid-depths (40-70m) of the ESAS where “continuous permafrost” exists according to a general thermodynamic model of permafrost freezing/thawing.

Right now, there is an incomplete understanding of the position of the permafrost table in the ESAS; it may vary significantly over 10’s of meters. A large number of gas seeps, mostly located in depressions above a prominent reflector, noted in the area suggests that submarine permafrost is a CH4 source to the overlying water and possibly to the atmosphere. These seeps represent CH4 escape routes for gaseous and dissolved CH¬4, which may bubble vigorously from "chimneys" or seep diffusely from faults through the consolidated sediments. Such “acoustic” gas chimneys were observed throughout the permafrost in the Laptev Sea. Our recent ocean surveys accomplished in September 2007 and 2008 reported ''whole fields of methane bubbles'' in the East Siberian Sea and Laptev seas, with CH4 levels up to 10,000 times higher than previously observed.

Taliks formed beneath thermokarst lakes serve as a source of Pleistocene CH4 to the overlying waters and atmosphere. While developing, these taliks melt through the permafrost, providing a conduit for rapid release of ascending gas. The range of variability in CH4 concentrations dissolved in the waters of thermokarst lakes is comparable to that in offshore waters (about 20 µM, where the existence of taliks is hypothesized. The CH4 concentration of water of Ugly Laguna (west coast of the Lena Delta), where open taliks presumably exist, was up to 480 µM. We hypothesize that taliks act as a “conduits” for release of CH4 of mixed biogenic-thermogenic origin, which provides a positive feedback on climate warming via the release of the old carbon previously stored in permafrost.

What we hope to do…

We are planning a multidisciplinary, comprehensive study to better understand CH4 dynamics in the ESAS region. This study includes intensive field sampling and experimentation linked to an integrative, predictive modeling framework and will be a cooperative effort between Russian and US scientists. To evaluate the role of changing sub-sea permafrost in the regional carbon budget of the ESAS, we can examine the current and potential release of CH4 from different types of seabed sediments, obtained from various bottom landscapes and representing different stages of permafrost development or degradation. The ESAS area is significantly impacted by current and future global warming. We will estimate the current methane potential (CMP) of sediments by determining the CH4 content at specific depth intervals and integrating the total amount of CH4 over depth. The full methane potential (FMP) will be estimated as net CH4 production by methanogenic bacteria from the same sediment cores. In addition to these direct measurements, we will establish a model describing the transformation CMP into FMP under different climate conditions, i.e. at increased temperature. Spatial extrapolation will be applied across areas of defined sediments characteristics to estimate CH4 sources at a regional scale. We will use a suite of sensitive radiotracer methods for quantifying rates of CH4 production and oxidation as well as other organic carbon oxidation processes (i.e., sulfate reduction) in deep modern, talik and permafrost sediments. We will determine the δ13C, δD and Δ14C signatures of CH4, organic carbon and DIC to distinguish the contribution of modern biogenic, old biogenic and thermogenic CH4 to the total sediment CH4 inventories and flux.

Some of the research questions we will ask:

Context

During interglacial periods, seafloor permafrost along the Siberian shelf may experience a pronounced change in thermal regime. These temperature changes may affect permafrost and ASSH in several ways, ultimately leading to its degradation thereby enhancing CH4 release.

Research Area 1 - Permafrost Stability

Are changing thermal conditions of ESAS causing the permafrost to thaw? Do accelerated rates of permafrost degradation lead to development of taliks that act as CH4 vents to the overlying seawater and potentially the atmosphere?

Research Area 2 - Biological methane production

Does biological CH4 production occur in permafrost and the overlying sediments? How do CH4 oxidation rates compare to CH4 production rates? What regulates net CH4 cycling rates? How does biological CH4 production in permafrost and overlying sediments vary with temperature? Does methanogenesis in thawing permafrost reflect production of biogenic CH4 from “old” organic carbon?

Research Area 3 - Methane release from thawing permafrost and decomposing hydrate

What is the current CH4 potential (CH4 available for immediate release) of offshore permafrost and how will it change with rising temperatures? What is the relative importance of biogenic versus thermogenic CH4 in the CH4 budget of the ESAS? What is the CH4 flux from the landscape scale and how does it compare to CH4 fluxes measured in other regions?

We thank the National Science Foundation (NSF) for funding this research.

**Disclaimer** The content of this page is based in part on work supported by the NSF. Any opinions, findings, and conclusions or recommendations expressed here are those of the author (Mandy Joye) and do not necessarily reflect the views of the NSF.