PALEOENVIRONMENTAL CHANGE IN MORAINE-DAMMED ALPINE WETLANDS OF THE MITCHELL LAKES BASIN, SAN JUAN MOUNTAINS, COLORADO, USA

An ongoing paleoenvironmental and geomorphological study examines paleoenvironmental change of two moraine-dammed alpine wetlands in the Mitchell Lakes Basin (MLB), San Juan Mountains, Colorado. Radiocarbon ages, charcoal size and presence, and pollen analysis at three sample depths in each of two cores reconstruct paleoenvironmental change within MLB. Research questions include: What do these analyses tell us about paleoenvironmental change? Were the moraines constructed by small proto-cirque glaciers that house the wetlands, or by lateral moraines related to the larger Animas Valley Glacier (AVG)? In Wetland 2 (W2), we recovered an age of ~16,885 14C yr BP at a depth of 3.44m. In Wetland 3 (W3) we recovered a basal age of ~21,504 14C yr BP at a depth of 2.37m. Pollen data from W2 show Pleistocene presence of sporormiella, often related to large herbivore and possibly megafauna dung, which then disappears after ~10,779 ± 58 14C yr B.P., coinciding with the Pleistocene-Holocene transition. Further pollen data from W2 show high concentrations of upland herbs such as Artemisia and low counts of Pinus ~16,885 and ~10,766 14C yr BP with a much higher amount of Pinus ~6,470 14C yr BP suggesting a rise in treeline between ~10,766 and ~6,470 14C yr BP. An 800% increase in macrocharcoal in the upper sample also suggests an increase in local fires in the Holocene. Basal pollen and charcoal data from W3 reveal presence of drought resistant Artemesia and Amaranthaceae and high levels of microcharcoal (relative to macrocharcoal) suggesting the pollen was not local and glacial oligotrophic lake/pond environments existed ~21,504 yr BP. The presence of Bakers Bridge granite samples in moraines suggests they are laterals of the AVG, while samples of the Pennsylvania Hermosa Group and Cutler Formation also indicates local glacial flow from the proto-cirques above the MLB. This combination suggests moraines were formed by complex glacial activity.