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

Munroe, Jeffrey S., Geology Department, Middlebury College, Middlebury, VT 05753, jmunroe@middlebury.edu

Lake coring efforts during the summers of 2004 and 2005 have resulted in the retrieval of continuous sediment cores from eighteen high-elevation lakes in the Uinta Mountains of northeastern Utah. The west-east orientation of the range offers the possibility of investigating post-glacial paleoclimate records from contrasting moisture regimes including the monsoon-dominated southeastern Uintas, and the upwind, western, end of the range that is dominated by Pacific moisture. Ten lakes from the southern Uinta Mountains were investigated first and are reported on here. The lakes range in elevation from 2960 to 3475 m, and two are located above the modern treeline. Sixty AMS radiocarbon dates on terrestrial macrofossils, Daphnia ephippia, and pollen concentrates provide age control and confirm that the records extend into the latest Pleistocene. Loosely consolidated sediment near the sediment-water interface was not retrieved, however the uppermost sampled sediment dates to 500 to 2000 cal yrs BP. Organic content was determined by loss-on-ignition (LOI) at 1-cm intervals for each core (2239 samples), each corresponding to 20 to ~100 yrs. Mean LOI values per lake generally decrease with elevation, ranging from 9% to 24%. Maximum values per core range from 14 to 40%. Most of the cores captured the onset of organic sedimentation in their respective lakes; sediment deposited prior to this transition has LOI values <10%. Three of the lakes from the monsoon-dominated southeastern Uintas show a period of enhanced LOI variability from ~9750 to 5500 BP, which may reflect increased inwash of terrestrial organic matter during a time of heightened monsoon intensity. Three other lakes are in the southwestern Uintas, where winter precipitation comprises more than half of the total annual precipitation. Lake records there show synchronous negative LOI drops, possibly representing drought events. The remaining four lakes are located in the south-central Uintas, near the modern boundary between summer-wet (monsoonal) and winter-wet climates (Pacific frontal). Two of these lakes show an increasing LOI trend until 6 ka BP, possibly reflecting the warmer temperatures of the early Holocene Altithermal. The other two lakes show a nearly constant LOI signal throughout the Holocene, possibly because they are too deep or located at too high an elevation to register a response to minor forcing. Consideration of all of the records highlights the importance of local geomorphic setting in controlling LOI variability.