Introduction


During the beginning of the last deglaciation between 17,500 and 14,500 years ago atmospheric CO2 concentrations began rising from about 190 ppm in glacial times to approximately 270 ppm by the beginning of the present warm period. There is evidence that the rise in CO2 was from an old carbon pool that was formed from a biological source. Two possible sources are the southern ocean and permafrost soils. A large amount of carbon, of the same order of magnitude as is contained in the atmosphere, was deposited in permafrost soils during the last ice age. In addition the oceans contain about ten times as much CO2 as the atmosphere. It has been suggested that if melting permafrost did initially contribute to rising CO2 levels during the last deglaciation, the ventilation of CO2 from the oceans must have amplified this effect.

When permafrost melts, methane and other greenhouse gases are emitted. The conversion of old soil carbon deposited during the last ice age to greenhouse gases is difficult to measure in most terrestrial environments because greenhouse gases are also generated by modern plant and fauna decomposition.

However, the lakes formed by melting thermafrost provide a way of measuring the emissions of greenhouse gases from old soils without this complication. When permafrost soils melts, the ground surface collapses and lakes called thermokarst lakes are formed in the depressions. Permafrost soils thawing beneath thermokarst lakes emit methane which forms bubbles in the lakes. Ultimately the methane is emitted into the atmosphere. These emissions can be detected and measured. Because of this process whereby soil containing carbon captured during the last ice age decomposes and emits methane, thermokarst lakes can be used to quantify the relationship between permafrost melting and greenhouse gas release.

Observations


In this article radiocarbon dating has been applied to methane in lake bubbles and soil organic carbon for lakes in Alaska, Canada, Sweden and Siberia. Methane emissions and radiocarbon ages were measured in 60-year thermokarst expansion zones and stable open-water zones of a variety of lake types in Alaska and Siberia spanning different latitudes (63°–71° N), ecosystems, and permafrost types.

Methane forms bubbles in lake sediments. Newly formed bubbles follow escape pathways through sediments called seeps. This results in point sources of continuous methane seepage into the water. During the ice-free season, bubbles rise through the water and escape to the atmosphere. In winter, lake ice traps the seep bubbles. The majority of winter bubble methane escapes from the lakes in spring when the ice melts.

Seep density, GPS-mapped seeps, and measured flows and collected gas using submerged bubble traps were used to quantify methane emissions. 9,102 individual seeps were surveyed in different lakes. This included removing snow from early winter lake ice to expose bubble clusters trapped in ice for seep classification. The radiocarbon age of methane was measured in lake bubbles collected from seeps, background bubble traps and stirred sediments.

Soil profiles adjacent to lakes were sampled in late summer 2008 and 2010 to obtain permafrost soil samples ranging from 3.7 to 5.6  meters deep and organic carbon density (kilograms of carbon per cubic meter or kg C /m3) and carbon dated age were measured.

It was found that the methane age from lakes is nearly identical to the age of permafrost soil carbon thawing around them thus confirming that the methane is entirely from an old carbon source and not from modern plant decay.

Geospatial analysis


They then used remote sensing to measure the increase in extent of thermokarst lakes in the Arctic regions of the Earth. Aerial photos from the 1950s were overlaid with shorelines identified in modern high-resolution satellite imagery to quantify the increase in thermokarst zones over the past 60 years. Black and white aerial photos from circa 1950 were acquired from the USGS EROS Data Center and high-resolution panchromatic satellite imagery from 2010 from Digital Globe satellites.

Geospatial analysis was combined with physical modelling of permafrost thaw to relate rates of methane emission from lakes to the soil carbon inputs in zones of lakes that changed from land to water via thermokarst expansion during the past 60 years. Based on an existing model of permafrost thawing beneath these lakes, the volume of permafrost soils that thawed and eroded into lakes during the 60 years observation period was calculated.

The volume of permafrost soil that thawed and eroded into lakes during the 60 year observation period was estimated using lake depth measurements taken in the field and the shoreline positions of lakes 60 years ago determined from remote sensing imagery. Average lake expansion rates were calculated by combining the lake expansion over 60 years, the present lake water depth at the point where the shoreline was 60 years ago determined by field measurements, and the volumetric ice content of permafrost surrounding lakes determined from past regional studies.

Results


Based on this analysis, it was estimated that 0.2 to 2.5 Pg (0.2 to 2.5 gigatonnes) of permafrost (old) carbon was released as methane and carbon dioxide in thermokarst expansion zones of pan-Arctic lakes during the past 60 years. For comparison global carbon emissions from fossil fuel use were 9.795 gigatonnes (Gt) in 2014. The comparison indicates that over the past 60 years permafrost thawing has not contributed significantly to increases in atmospheric greenhouse gases.

However, it is estimated that 1,400 petograms (Pg), or 1,400 gigatonnes, of organic carbon were captured in permafrost soils as a result of the decay of plants and fauna during the last ice age. Currently there is roughly 800 gigatonnes of carbon in the atmosphere. If a significant amount of the carbon captured in permafrost soils were released, it would contribute to accelerating the increase in atmospheric greenhouse gas concentrations.

Source


Methane emissions proportional to permafrost carbon thawed in Arctic lakes since the 1950s, Katey Walter Anthony, Ronald Daanen, Peter Anthony, Thomas Schneider von Deimling, Chien-Lu Ping, Jeffrey P. Chanton & Guido Grosse, Nature Geoscience 9, 679–682(2016) doi:10.1038/ngeo2795

Permafrost carbon: Catalyst for deglaciation, Andrew H. MacDougall, Nature Geoscience 9, 648–649 (2016) doi:10.1038/ngeo2802