Documenting the glacial cycle in a temperate southeastern Alaska fjord
by Kathleen McCoy |
Dr. Bryce A. Willems, assistant professor with UAA's Department of Geological Sciences, is currently studying Hubbard Glacier, North America's largest tidewater glacier. According to the Field Notes blog on the Polar Field Services newsletter, "Unlike most glaciers, which have thinned and retreated during the last century, Hubbard Glacier is thickening and advancing. In the last 25 years, the Hubbard has twice advanced to close off the narrow gap between Russell Fiord and Disenchantment Bay, creating the largest glacier-dammed lake in North America, Russell Lake.
"During both damming events, in 1986 and 2002, flow out of Russell Fiord slowed long enough to raise water levels and significantly decrease water salinity, threatening the fiord's sea life. In 1986, water levels behind the dam rose 25 meters. In both cases, large floods rolled into Disenchantment Bay when the dam finally broke.
"If Hubbard Glacier continues advancing, Russell Fiord could become dammed again. Although the last two dams broke within one season, they managed to raise water levels behind the dam to dangerous levels in a very short period of time, creating the largest glacial-lake outburst floods ever recorded. If another dam forms, Russell Lake could overflow backwards and flood westward down the Situk River valley, threatening a vital fresh-water ecosystem and possibly submerging the Yakutat airport."
While still affiliated with the Department of Geology and Environmental Sciences, and Analytical Center for Climate and Environmental Change at Northern Illinois University, Willems co-authored the article Fjords as temporary sediment traps: History of glacial erosion and deposition in Muir Inlet, Glacier Bay National Park, southeastern Alaska that was published in the Geological Society of America Bulletin 2010, v. 122. The full article was also published online and can be read here.
According to the article, "Most glaciers in Alaska are currently thinning, and their termini are in retreat. In Glacier Bay, this retreat began at the end of the Little Ice Age (LIA) and opened up an extensive, bifurcated fjord system in less than 250 years. The eastern arm of this system, Muir Inlet, is a deep, narrow fjord containing 10 basins separated by bedrock sills, some of which are buried below younger fjord-floor sediment. Muir Glacier has a well-documented historical retreat beginning with observations of the ice cliff by John Muir and ending when it became grounded as a terrestrial terminus in 1993... the early history of glacial fluctuations is not well preserved in coastal Alaska because most glaciers reached their greatest Holocene extent in the LIA, destroying earlier evidence.
"Muir Inlet is an ideal natural laboratory because extant tidewater glaciers have allowed us to directly observe sedimentary processes and quantify rates of debris transfer to the fjord. The retreat of Muir Glacier and associated migration of the active depocenter through the silled fjord basins is documented at nearly annual resolution for over 110 years. These data allow us to interpret the glacial history and calculate the sediment flux from high-resolution, seismic-reflection profiles of the fjord basin fill with a high degree of certainty."