SOUTHERN ROCKY MOUNTAIN CLIMATIC STABILITY DURING THE MIDDLE AND LATE HOLOCENE DETERMINED FROM LAKE-BOTTOM SEDIMENTS AND SOIL: THE LOCH VALE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, COLORADO, USA

Lake-bottom sediments are frequently investigated to reconstruct paleoenvironmental conditions of a region. However, unconformities prevent collection of observational and quantitative data for the time represented by the hiatus. The youngest (uppermost) lake-bottom sediments of The Loch, an open lake located at the outlet of the alpine-subalpine Loch Vale watershed (LVW), Colorado USA, yields a modelled radiocarbon age of 6517 ± 139 cal yr BP. The bathymetry of The Loch reflects accommodation space still being available for further sediment deposition. With no lacustrine sediment deposition in The Loch since ~6500 years ago, the age of the upstream cryic soils adjacent to The Loch was determined. Solute-based, small watershed mass-balance methods were used to calculate the weathering rates of oligoclase and biotite, the two primary minerals that weather to secondary clay minerals comprising the soil. For the 1984-2008 sample period, the oligoclase to kaolinite weathering rate was 193 ± 7 moles hectare^-1 year^-1, and the biotite to smectite-illite weathering rate was 46 ± 2 moles hectare^-1 year^-1. The LVW forest and meadow soil age calculated from the oligoclase and biotite weathering rates were 4330 ± 120 years and 6510 ± 190 years, respectively. To achieve complete mineral destruction and soil formation, the slowest rate prevails yielding a soil age of ~6500 years. The comparability between the ages of the youngest lake-bottom sediments in The Loch and the soil permits the interpretation that landscape stability within the LVW commenced ~6500 years ago. This onset of landscape stability coincides with the beginning of the Middle Holocene in the southern Rocky Mountains. At this time summer insolation and temperature declined, the intensity of spring melt was consequently reduced, and decreased erosion yielded a landscape sufficiently stable for soil development. The lack of sediment deposition in The Loch since ~6500 years ago indicates that the climatic stability responsible for LVW landscape stability within the LVW has fluctuated relatively little during the Middle and Late Holocene.