RESEARCH HIGHLIGHT

The highest concentrations of dissolved methane ever measured in the Arctic Ocean found beneath the sea-ice on the Laptev Sea shelf

May 4, 2007

The Arctic Ocean is a vulnerable environment with unique ecosystems that are adapted to harsh conditions. Enormous stores of methane gas (CH4), referred to commonly as the “Arctic Carbon Hyper Pool”, are present in the Arctic Ocean sediments. These stores include three basic reservoirs of methane: methane trapped within seabed permafrost (sediments with a 2-year mean temperature below 0˚C), methane stored in a form of gas hydrate deposit beneath the permafrost, and thermogenic methane (free gas) that comes upward from deep within the earth. the amount of methane within seabed permafrost is unknown but it is presumed to be significant. Gas hydrate deposits beneath the Siberian Arctic shelf are predicted to contain about 60 Giga-tons (Gt) (1Gt=1015g) of C-CH4 (carbon within the methane deposits); the amount of free gas underlying the gas hydrates is likely to be one half to two thirds the amount of gas stored in gas hydrate. 

This “mega-pool” of methane substantially exceeds the atmospheric methane reservoir of 3.6Gt. The release of a very small fraction of the methane currently sequestered in Siberian Arctic shelf (~1% or 1 Gt of CH4) would raise the current atmospheric concentrations by almost 30%, which is similar to the increase in atmospheric methane observed during the 20th century (IPCC, 2001).

Accelerated warming temperatures and the resultant thawing of seabed permafrost may increase methane emission gradually or there may be “tipping point”, after which methane emissions will increase dramatically. Recentdata, obtained during the cruises on the East Siberian Arctic shelf (ESAS), show that concentrations of methane in sea water in some areas of the shelf reach up to 700 nM (that is 200 times higher than a concentration in equilibrium with the atmosphere). This means that the ESAS acts as a source of methane to the overlying atmosphere during the ice-free period of year. These findings raise important questions for researchers: Is the enormous amounts of methane stored within methane reservoirs being released into sea water and subsequently into the atmosphere? Which methane reservoir involved is the primary source? Does methane release occur seasonally or year-round? Does it happen locally or spread widely? How large is the portion of methane ready to be released currently and what is the timeline for the possible release?

To answer one of these questions (seasonal variability in methane release) an expedition led by Igor Semiletov was conducted over the Laptev Sea shelf. A tractor–sledge caravan started from Tiksi, Russia on March 30th (Fig.1). April 1st-April 15th was spent in the south-eastern part of the Laptev Sea eastward of the Lena River delta where 53 complex oceanogra­phic stations (CTD/water/sediment sampling) were done through 2m fast ice along the ~1,700km route. Methane measurements, gas extraction, pH and filtering samples were also processed in a mobile laboratory. Extremely high concentrations of methane (up to 10 μM – two orders of magnitude higher compared to summer concentrations) were measured in the water sampled from beneath the sea ice over the Ust’-Lena rift at the 72-73˚N latitude belt. Researchers suppose that seabed permafrost might be completely thawed in this area due to upward geothermal heat flux and heating effect of the Lena River runoff. This potentially allows methane from all three reservoirs to move upwards, reach the sea surface, and accumulate beneath the sea ice. Accumulation of methane occurs in the form of bubbles which can reach up to 1 m in diameter (Fig.2). All this methane trapped within bubbles and accumulated beneath the sea ice will release into the atmosphere when the ice melts in summer. Marine methane studies will be continued at IARC in our effort to understand atmospheric dynamics and natural forces that are contributing to our changing climate.

Contacts:

Igor Semiletov
Natalia Shakhova