LACUSTRINE LOSS-ON-IGNITION RECORDS FROM THE SOUTHERN UINTA MOUNTAINS

Continuous sediment cores were retrieved from 8 lakes in the southern Uinta Mountains. The lakes range in elevation from 2960 to 3475 m, and two of the lakes are located above the modern treeline. A minimum of four AMS dates on terrestrial macrofossils and daphnia ephippia per core provide age control and confirm that the records extend into the latest Pleistocene (11 to 15 ka BP). Loosely consolidated sediment near the sediment-water interface was not retrieved, however the uppermost sampled sediment dates to 500 to 2000 cal yrs BP. Percent loss-on-ignition (LOI) was determined at 1-cm intervals for each core, corresponding to 30 to 90 years per sample. Mean LOI values generally decrease with elevation, ranging from 11% to 22%. Maximum values per core range from 14 to 40%. Five of the cores captured the onset of organic sedimentation in their respective lakes; sediment deposited prior to this transition has LOI values <10%. Two of the cores record an LOI maximum in the early Holocene, possibly in response to the insolation maximum. In contrast, five cores record generally rising LOI during the Holocene, and one core (from the highest lake) records a nearly constant (~11%) LOI since the onset of organic sedimentation. Comparison of cores from adjacent lakes reveals a high degree of variability between records, likely reflecting the control of lake-specific features (including bathymetry and the presence or absence of bedrock-floored outlets) on LOI records. Ongoing acquisition and analysis of additional cores from lakes at the eastern end and on the north side of the range should allow range-wide patterns to be distinguished from local effects.