HR: 0800h
AN: PP31A-1503    [Abstracts]
TI: Quantitative Evaluation of a Rapid and Severe Early Holocene Drought in NW-Montana
AU: * Sperazza, M
EM: sperazza@mso.umt.edu
AF: Department of Geology University Of Montana, 32 Campus Drive, Missoula, MT 59812 United States
AU: Hofmann, M H
EM: michael.hofmann@umontana.edu
AF: Department of Geology University Of Montana, 32 Campus Drive, Missoula, MT 59812 United States
AU: Moore, J N
EM: Johnnie.moore@umontana.edu
AF: Department of Geology University Of Montana, 32 Campus Drive, Missoula, MT 59812 United States
AU: Hendrix, M S
EM: marc.hendrix@umontana.edu
AF: Department of Geology University Of Montana, 32 Campus Drive, Missoula, MT 59812 United States
AB: Extreme abrupt climate changes are common throughout the Holocene and have been recognized from several regions around the world. Here we present a preliminary quantitative evaluation of a severe early Holocene drought as recorded in sediments of Flathead Lake, Montana. Flathead Lake is a large (496km²) open lake system that drains >18,000km² of high valley plains and mountain terrain. In order to study the records of Pleistocene deglaciation and Holocene climate change, we recovered 19 piston cores from the lake bottom at locations that were tied to a set of 3.5 kHz seismic reflection data. Seismic data and cores obtained within Big Arm Bay, a shallow embayment in the western part of the lake, indicate the presence of an erosional unconformity that we interpret as indicating a significant lowering of lake level at about 7600 cal yr. BP. Sub-unconformity reflectors are clearly truncated and the unconformity itself is onlapped by overlying reflectors. The uppermost onlapped reflector coincides with a ~12cm accumulation of Mt. Mazama tephra (7,630±80 cal yr. BP), providing a temporal constraint on the lowstand. Morphometric analysis of the lake indicates that during this lowstand the surface of the lake dropped about 15m below the modern lake's natural bedrock spillway and Flathead Lake lost about 25% of its present volume. Interestingly none of our core data suggest an increase in CaCO₃ at this unconformity. Sedimentologic evidence for a lake lowstand comes from changes in early and middle Holocene grain size observed in several cores. For example, grain size data from core FL-00-9P indicates a significant increase in clay fraction just prior to the Mt Mazama tephra. We interpret the increase in clay as resulting from reduced inflow into Flathead Lake. An increase in grain size, right after the deposition of the Mt Mazama tephra in turn coincides with the actual lake level lowstand and the refilling of the lake. Based on these observations we created a hydrologic surface flow and geochemical model, using MINTEQ and WATEQ, to quantify the average precipitation and evaporation rate at ~7600 cal yr. BP. Model results suggest that the lake lowering had to occur over a very short period of time to avoid precipitation of CaCO₃. More specifically, the draw down had to occur in less than 20 years to keep the concentration factor in the lake below saturation index. We suggest that the surface inflow into Flathead Lake was reduced to an annual average of about 10% of its modern day annual average flow. Our observational and modeling results are consistent with a variety of studies suggesting an ice-free NW-Montana during the early and middle Holocene with a significantly thinner winter snowpack and significantly lower river discharges. The average summer precipitation-derived flow into the lake during this time had to only be reduced by ~50%, assuming negligible spring runoff. Our hydrologic modeling efforts suggest that such a low inflow requires a dramatic drop in annual net precipitation to as low as 4cm/year, for several years, and an increase in evaporation rates 10 to 20% higher than today`s rates.
DE: 0545 Modeling (4255)
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 1630 Impacts of global change (1225)
DE: 1637 Regional climate change
DE: 9350 North America
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005