RAPID LATE PLEISTOCENE AND HOLOCENE CLIMATE CHANGES IN NW-USA REVEALED BY GLACIAL LOBE FLUCTUATIONS AND LAKE LEVEL CHANGES OF FLATHEAD LAKE, MONTANA

During the summers of 2000 and 2003, we recovered several piston cores from Flathead Lake, a large (496km²) open lake system in NW-Montana. Core locations were selected on the basis of 3.5 kHz seismic reflection data and designed to yield the most information about the Holocene lake-level history and the late Pleistocene deglaciation record.

Particularly interesting for the analysis of lake level history are seismic data and cores obtained within Big Arm Bay, a shallow embayment in the western part of the lake. Seismic data in this part of the lake indicate the presence of an erosional unconformity that relates to a significant lowering in lake level. Sub-unconformity reflectors are clearly truncated and the unconformity itself is onlapped by overlying reflectors. This unconformity coincides with the deposition of the Mt Mazama tephra (7,630±80 cal yr BP) and provides information about a significant Holocene lake-level lowstand, during which the surface of the lake dropped about 15m below the modern lake level and Flathead Lake lost about 25% of its present volume. Although no major increase in CaCO₃ content was observed along this unconformity, simple hydrologic and geochemical models suggest that the lake level lowering is related to a decrease in precipitation, related in part to the Mt Mazama volcanic eruption. Furthermore our modeling efforts suggest that the drop in lake level had to occur over a short period of time, likely less than 150 years, to avoid CaCO₃ precipitation in the lake basin.

Further downsection our sediment cores contain an important record of deglaciation for the SE part of the Cordilleran Ice Sheet. This record consists of a series of upward-thinning glacial varves that we are able to individually correlate across the lake. We interpret the upward thinning character of the varves to reflect the retreat of the Flathead Lobe upvalley, away from our core sites. Wood that we recovered from the uppermost part of the varved sequence reveals the minimum age of the varved section to be 14,150±150 cal yr BP. The varved sequence is abruptly overlain by a series of upward-fining coarse silt beds that we interpret as ‘event beds' deposited by high discharge pulses of glacial meltwater. In general these event beds record the change from a glacially influenced lake system to the post-glacial lake system present today.