A 2400-YEAR δ18O AND δ13C STABLE ISOTOPIC RECORD FROM LOWER PAHRANAGAT LAKE SUGGESTS HYDROLOGICAL REORGANIZATION DURING THE MEDIEVAL CLIMATE ANOMALY

We present a 2400-year stable isotopic record from Lower Pahranagat Lake (LPL), a very shallow (< 1 m) alkaline system in central Nevada. Previous work established that LPL sediments are dominated by clotted, probably peloidal high Mg-calcite (typically >60% by weight). Localized marginal microbial mats and thin (< 2 mm), filamentous laminae within the core suggest microbially-influenced carbonate precipitation. LPL is a closed basin and studies of water stable isotopic values show the occurrence of a very large δ¹⁸O evaporative enrichment accompanying seasonal hydrological changes in the lakes of the Pahranagat Valley. We analyzed the <75 µm carbonate fraction for δ¹⁸O and δ¹³C from a radiocarbon age-dated sediment core, along with surface samples from transects of the lake basin and margins. Both δ¹⁸O and δ¹³C values are sensitive proxies, with values varying by up to 6 ‰ over the record. We interpret the δ¹⁸O values recorded in lake sediments to primarily reflect changes in basin hydrology and the regional climate. The δ¹⁸O record suggests several multi-century dry/wet periods, consistent with other findings across the Great Basin during this period. However, the relationship between changing climate and the isotopic values is not straightforward. The Medieval Climate Anomaly (MCA) is a period characterized by drought in many Great Basin paleoclimate records including a previously reported pollen record for LPL (Wigand, 1997). In the LPL stable isotopic record, however, the MCA period shows relatively low values that could be interpreted to represent wetter conditions. Groundwater strongly influences LPL water chemistry. We suspect that during periods of especially low lake levels, such as the MCA, the influence of spring water with relatively low δ¹⁸O values feeding the lake may override the signal of any evaporative isotopic enrichment. While the δ¹⁸O and δ¹³C records show a strong, positive correlation typical of hydrologically-closed basins for the record as a whole, there is no correlation between the two during the MCA. This supports the idea of a significant hydrological change in the basin during this period. Elevated δ¹⁸O values in the most recent part of the record reflect human controls on the hydrology of the Pahranagat Valley that have largely reduced the amount of water reaching LPL over the last century.