When was it wet, when was it dry, and why?  A Holocene Climate Record from the Northern Rocky Mountains of Montana

Matt Finkenbinder, Assistant Professor of Geology, Wilkes University

Long term records of natural climate variability are essential to give context to the relatively short instrumental record of climate and to place recent changes into an appropriate geological perspective.  Lakes and their sediments provide an excellent means to reconstruct long-term terrestrial climatic and paleoenvironmental change because they integrate changes in the surrounding watershed and atmosphere, and further respond rapidly and are sensitive to climatic change. In addition, lake sediments can provide continuous archives, are easily constrained with radiometric dating techniques, and can be analyzed for multiple indicators or proxies of environmental change. In this talk, I will present an analysis of sediments collected from Rock Lake, a small closed-basin lake located in the Northern Rocky Mountains of Montana.  A series of sediment cores were collected along a water depth transect, from the shoreline to central depocenter, to reconstruct Holocene lake level variations using sedimentary facies analysis.  To further document decadal to centennial variations in hydroclimate change, carbonate oxygen isotope analyses were conducted on the deep-water core sequence that spans the last 11,000 years.  Results from both sets of analyses indicate the lowest lake levels and overall driest conditions occurred across several centuries after the Mazama climatic eruption (~ 7,600 yr BP), supported by the highest oxygen isotope values during the entire record and sedimentological features in shallow water cores including mudcracks and paleosols.  A transition to higher lake levels and wetter conditions occurred over the next millennia, defined by an abrupt decrease in oxygen isotope values and the accumulation of fine-grained lacustrine sediment in shallow water cores.  The last ~ 5,000 years are characterized by centennial variations in oxygen isotope values and high but variable lake levels.  The implications of the lake level record, along with possible forcing mechanisms that could explain the variations, will be discussed in the context of other paleoclimate records from the region.