Geologic versus Ecologic Gas Seeps in Alaska and Greenland lakes

February 7, 2013 • Filed under: News — melanie.rohr

Katey Walter Anthony will share her findings on subacap methane seeps at this week’s WERC seminar, Friday, February 8, 2013.

Friday Seminar Series

  • What: Geologic vs. ecologic gas seeps in Alaska and Greenland lakes
  • Who: Katey Walter Anthony
  • When: 3:30-4:30 p.m. Friday, Feb. 8
  • Where: 531 Duckering

Glaciers, ice sheets, and permafrost form a ‘cryosphere cap’ that traps methane formed in the subsurface, restricting its flow to the Earth’s surface and atmosphere. Despite model predictions that glacier melt and degradation of permafrost open conduits for methane’s escape, there has been a paucity of field evidence for ‘subcap’ methane seepage to the atmosphere as a direct result of cryosphere disintegration in the terrestrial Arctic. Here, we document for the first time the release of sub-cryosphere methane to lakes, rivers, shallow marine fjords and the atmosphere from abundant gas seeps concentrated along boundaries of receding glaciers and permafrost thaw in Alaska and Greenland. Through aerial and ground surveys of 6,700 lakes and fjords in Alaska we mapped >150,000 gas seeps identified as bubbling-induced open holes in seasonal ice. Using gas flow rates, stable isotopes, and radiocarbon dating, we distinguished recent ecological methane from subcap, geologic methane. Subcap seeps had anomalously high bubbling rates, 14C-depletion, and stable isotope values matching microbial sources associated with sedimentary deposits and coal beds as well as thermogenic methane accumulations in Alaska. Across Alaska, we found a relationship between methane stable isotopes, radiocarbon age, and distance to faults. Faults appear to allow the escape of deeper, more 14C-depleted methane to the atmosphere, whereas seeps away from faults entrained 14C-enriched methane formed in shallower sediments from microbial decomposition of younger organic matter. Additionally, we observed younger subcap methane seeps in lakes of Greenland’s Sondrestrom Fjord that were associated with ice-sheet retreat since the LIA. These correlations suggest that in a warming climate, continued disintegration of glaciers, permafrost, and parts of the polar ice sheets will weaken subsurface seals and further open conduits, allowing a transient expulsion of methane currently trapped by the cryosphere cap.

Methane-induced melt-hole on a frozen lake in the Alaska Brooks Range, April 2011.

Photo top: Methane-induced melt-hole on a frozen lake in the Alaska Brooks Range, April 2011.

(Photo by Katey Walter Anthony)