High-Resolution Lacustrine Records of Historical Environmental Change in Glacier National Park, Montana, USA

摘要

Multi-proxy geochemical analyses of lacustrine sediment cores from a chain of lakes in east Glacier National Park, Montana reflect increasing human activity, glacier response, and regional climate change over the past ~1400 years. We analyzed sediments from four lake cores from three lakes for carbon/nitrogen (C:N), carbon and nitrogen isotopes values (δ13C and δ15N), mass accumulation rate (MAR), and percent total organic and inorganic carbon (%TOC and %TIC). An abrupt increase in %TIC, C:N and δ13C in Lake Josephine at 1650 A.D. clearly defines the initiation of the Little Ice Age (LIA) in the valley, with a subsequent drop in %TIC at 1850 marking the retreat of Grinnell Glacier from its LIA moraine. We posit the %TIC record directly reflects erosion and transport of dolomite, which is present in outcrop only in Grinnell Glacieru27s cirque basin at the top of the valley.All four sites show a response to climate change and regional human impact. The farthest down-valley core, proximal to buildings and roads, also preserves a record of large flooding events and more localized human activity. All four sites display decreasing C:N and δ13C values toward the present, indicating increasingly algal sources of organic material. MAR increases throughout the 20th century at all three sites, suggesting an influx in erosion associated with construction and human activity in the Park. Peaks in MAR, C:N, %TOC and δ13C at the down-valley, development proximal site correspond to major flooding events in the valley. These peaks are not evident in the up-valley cores, suggesting reduced soil stability and · increased run-off at the farthest down-valley site. All four lake cores exhibit decreasing δ15N values within the last two hundred years, at least 2-3 times below δ15N values from the previous ~1400 years. This suggests a regional anthropogenic forcing, consistent with previous records of atmospheric nitrogen deposition found in other pristine alpine lakes in the Rocky Mountains. Stochasticity in the Lower Grinnell Lake record is likely due to geomorphic activity relating to the nearby retreating glacier and hillslope failure due to the steep valley walls surrounding this lake. This record exhibits similar trends to the three down-valley sites through 1990, but all geochemical values display significant peaks between ~1990 and 2008 that arenu27t seen at any other sites in the valley. This spike is comprised of at least three points and extends over the span of at least a decade in all data sets, and therefore cannot be explained by a single event.