HOLOCENE AQUATIC ECOSYSTEM VARIABILITY FROM BIRCH LAKE AND LOST LAKE, ALASKA USING VISIBLE REFLECTANCE SPECTROSCOPY

Though it had been thought that the Holocene has had a relatively stable climate, it is now clear from the geologic record that widespread droughts have been common. Developing tools to understand these changes and reconstruct natural climate variations over this period is therefore essential to provide a context for recent 20^th century changes linked to anthropogenic disturbances. In this study, sediment cores from Birch Lake and Lost Lake in the interior of Alaska are analyzed for physical properties and diffuse spectral reflectance (DSR) to reconstruct changes in environment through time. The derivative of the DSR values are calculated, in a method called visible derivative spectroscopy (VDS). VDS has not been widely used for analyzing lacustrine sediments to study paleoclimate, but recently it has been shown to be an effective tool that compliments traditional multi-proxy geochemical analyses. Varimax-rotated principal component analysis (VPCA) is then used to identify the components contributing to the color of the sediment, such as phytoplankton composition and mineralogy, by comparing the values to previously published pigments or known mineral standards. Temporal variations in the VDS values indicate various kinds of environmental change.

Preliminary results from these analyses show substantial changes in clastic sediment composition and aquatic ecosystem variability during periods of variable and stable lake levels, which suggests complex environmental forcings. There is not a prominent pigment trough expressed in the reflectance data, but spectral decomposition of the data set allows it to be partitioned into multiple components to separate the lithologic from the organic downcore variability.

Finding patterns in the climate and understanding their effects on the environment is crucial for both understanding how our climate has changed in the past and how it will continue to in the future, and the location of these lakes can give a unique insight of how these changes effect the subarctic region.