A HOLOCENE RECORD OF VEGETATION AND CLIMATE CHANGE FROM PYRAMID LAKE, NEVADA

Closed basin lakes along the eastern Sierra Nevada Range, including Pyramid Lake, preserve a record of lake-level fluctuations associated with wet and dry climatic episodes. Pleistocene Lake levels are recorded in wave-cut terraces and lake-deposited tufas above the modern shoreline, but the Holocene climate record is preserved primarily in the sedimentary record. Pyramid Lake receives the majority of its input from the Truckee River, which heads in the Sierra Nevada. It is the only large closed-basin lake in the western Great Basin to have escaped desiccation during the Holocene. For this reason, it is an ideal site for reconstructing Holocene climate change.

Three sediment cores spanning the last 7500 calendar years before present (cal yr BP) were analyzed for pollen at decadal to century-scale resolution. Dry periods were defined using the ratio of Chenopodiaceae (saltbush) pollen to Artemisia (sagebrush). This ratio suggests that the mid-Holocene (7500 – 6300 cal yr B.P.) was the warmest and driest portion of the record. Pyramid Lake was greatly reduced in size and the surface of Lake Tahoe was ~4 m below its modern elevation. Dry conditions persisted until ~5000 cal yr B.P. at which time there was a gradual but erratic increase in precipitation that continued as a generally wetter climate through 3500 cal yr B.P. A return to very dry conditions is seen between 2500 and 1800 years B.P. The last 1800 years have seen repeated wet-dry cycles, however none of the dry periods were as intense as those occurring during earlier periods. The record supports the evidence for prolonged droughts centered at ~800 and ~600 cal yr BP identified from submerged stumps at Mono Lake.

The drought record reconstructed from fossil pollen correlates with the high-resolution record of multidecadal and multicentennial droughts identified in the oxygen isotope record from Pyramid Lake and compares favorably with other proxy records from the Great Basin. The timing of major shifts in climate also corresponds to the timing of rapid changes in drift-ice accumulation identified in the North Atlantic. These changes have been documented to coincide with changes in solar output. Our results imply that similar mechanisms may be responsible for Great Basin Holocene climate changes.