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.