2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 310-3
Presentation Time: 9:45 AM


FRAHM, Jacob Y. and THEISSEN, Kevin M., Geology, University of St. Thomas, Mail# OWS 153, 2115 Summit Ave, Saint Paul, MN 55105, frah3712@stthomas.edu

Previously, we presented a 2400-year stable isotopic record (δ18O and δ13C) from a single 14C age-dated sediment core from Lower Pahranagat Lake (LPL) , a very shallow (< 1 m), alkaline, closed-basin lake in central Nevada. Here, we report a refined paleohydrological record with newly correlated sediment cores sampled across LPL. Previous work on our sediment cores has shown that LPL sediments are dominated by Mg- rich calcite (typically >60% by weight). Thin (<2 mm), filamentous laminae within the cores suggests microbially-influenced carbonate precipitation. The LPL δ18O record indicates several multi-century dry/wet periods, consistent with other findings across the Great Basin during the late Holocene. Lack of correlation between δ18O and δ13C in the LPL record during the Medieval Climate Anomaly (MCA) suggests that a significant hydrological reorganization occurred at that time. To explore this question, we took additional sediment cores from the northern and central areas of LPL for further clues to the changing hydrology through time. We carefully described the facies of the old and newly sampled cores and visually correlated these to determine the depositional environments across LPL. Coupling core visual descriptions with sediment microscopy and geochemical results we distinguish four lake stages in LPL basins. In order of relative water depth, shallowest to deepest, these are: 1) sub-aerial/wetland, 2) transitional stage, 3) shallow water level, and 4) deep water level. The sub-aerial/wetlands is characterized by a “spongy” carbonate mud (40-70% CaCO3) with no laminations and relatively high in both clastic and biogenic content. Transitional stages are characterized by higher biogenic content and intermittent or no microbially-laminated carbonate mud (60-70% CaCO3). Shallow water levels are characterized by microbially-laminated carbonate mud (60-90% CaCO3). The deep water environment is characterized by massive-bedded carbonate mud (50-60% CaCO3). Our reconstruction indicates that during the MCA, unlike other time periods represented in this record, LPL was likely divided into two isolated sub-basins with a subaerial/wetland condition in the north and a shallow water condition in the south. This aligns with the idea that a hydrological reorganization occurred during this time.