Saturday, 15 July 2006

Methane and Carbon Dioxide Release from Eroding Coastline of North Slope, Alaska.

Gary J. Michaelson1, Ping Chien-Lu1, M. Torre Jorgenson2, Fugen Dou1, Yuri Shur1, and Laodong Guo1. (1) University of Alaska Fairbanks, Ag/Forestry Exp Station, 533 E Fireweed, Palmer, AK 99645, (2) Alaska Biological Research Inc, PO Box 80410, Fairbanks, AK 99708

Recent changes in sea ice conditions have resulted in more open water along the coastline of Alaska and thus more exposure of the coastline to erosion from storm surges and wave action. Studies are currently underway to better understand the transfer of soil materials, such as sediments, carbon and nutrients from terrestrial arctic tundra ecosystems to the near-shore waters of the Arctic Ocean. As part of these coastal erosion/deposition studies, complete (surface to sea level) soil profile exposures were studied and sampled from twenty-nine shoreline sites. Sites were distributed across 250 km of Alaska coastline from Barrow to the Colville River delta and represented major coastal exposure types and soil conditions. Core samples were collected at 20-cm increments from the soil surface to sea level with exposures ranging from nearly sea level to 350 cm in height. Cores were kept sealed and cool or frozen in the case of permafrost samples. Methane and carbon dioxide release was measured as the cores were thawed to 20oC in sealed chambers. Amounts of methane released from the soil increased with depth with a maximum reached in the zone where the active layer, and upper permafrost table meet. Average releases of 0.34, 0.91, and 0.58 g CH4 m-3 of soil layer were measured for the cores from depths of 0-50, 50-100, and 100-350 cm depths, respectively. These ranges correspond roughly to the active layer, lower active layer/intermediate layer and upper permafrost respectively. Maximum methane release values found for the above depth ranges increased in a similar pattern with depth at 3.06, 7.70, and 4.51 g CH4 m-3 for each increased depth category respectively. The CO2 gas released from the thawed soil cores followed a different pattern and was highest in the near-surface active layer and dropped to lower levels for the two deeper layers. Averages of, 61, 49, and 45 g CO2 m-3 and maximums of 442, 200, and 247 g CO2 m-3 were measured for each depth category, respectively. The relative release rates of both gases were related to the factors as soil texture and water content, and organic carbon content and quality. High correlations were observed for these soil factors at specific sites, such as the Colville River delta sediments, where soil texture and organic carbon were evenly distributed and of similar quality with depth. For these sediments, as volumetric water content increased from 45 to 96%, CO2 release decreased from 83 to 11 g CO2 m-3 (R2=0.84***). At the same time CH4 release increased sharply from an average of 0.19 to 2.43 g CH4 m-3 as the soil crossed the saturation threshold of 80% water/ice content. Significance of the amounts of C released as CH4 and CO2 could be assessed as erosion rates become better known and a balance can be determined between the amounts of losses incurred for the more actively gas producing soil layers versus the losses of the more stable frozen (locked-in) gas containing permafrost layers. With large proportions of eroding soil being permafrost (approximately 50% within a 1-m exposure), there could be increasingly significant CH4 effluxes with increased erosion rates. While the results can be used to calculate total potential release rates, the percentage of methane that might be oxidized within the more aerobic, thawed soil environment as the peat becomes exposed on the high-energy beach environment remains uncertain.

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