HOLOCENE PALEOCLIMATE VARIATIONS AND EVIDENCE OF HUMAN DISTURBANCE RECORDED IN LAMINATED SEDIMENTS FROM BIG SODA LAKE, NEVADA

Paleolimnological research in the Great Basin has produced a rich record of late Quaternary environmental change. Most studies have focused on climate implications of past lake level changes. Few of these studies have focused on high resolution, i.e. sub-decadal scale records. Preliminary results of our climate proxy analyses of a 2-meter laminated sediment core recovered from Big Soda Lake, a maar near Fallon, Nevada indicate that the laminations average 2-4 mm in thickness over the last 2000 years. The pollen data reflect changes in the composition of the local vegetation during the recent past with the first appearance of non-native Euro-Asian weeds (e.g. Rumex actosella, Plantago lanceolata, and Salsola kali) around A.D. 1900, coinciding with the onset of agriculture in the area. Changing frequencies of the major pollen types (Sacrobatus, Artemisia, and Chenopodiaceae) are probably related to climate fluctuations and more specifically to changes in the areal extent of lakes in the Carson Sink. The geochemical and mineralogical evidence (XRF and XRD) records late 19^th and early 20^th century sodium salt extraction; return of brine to the lake, produced a large calcium/calcium carbonate peak. The construction of Lahontan Dam on the Carson River in 1915 and the subsequent development of irrigated agriculture in the area led to a rise in the groundwater table. As a result fresh groundwater raised lake level 18 m by 1930, and produced a negative oxygen isotope shift in calcium carbonate. Marked changes in Ti and Fe concentrations (from continuous XRF core scanning) indicate several substantial changes in clastic content during the past two thousand years. Such changes may reflect changes in erosion rates or land disturbance and are likely to provide information on paleodroughts. Further work using carbon and nitrogen isotopes will provide data on lake productivity and evidence of changes in lake level.