Changes in Holocene Climate, Fire and Vegetation from the Northeastern Great Basin: A 13,500 Year Sedimentary Record from Swan Lake, ID.

Precipitation patterns in the western US are characterized by a north-south dipole, typically manifesting as wet conditions in the northwest and a dry southwest. This pattern is, in large part, determined by the strength and position of the Pacific subtropical jet, which is responsible for the generation and trajectory of winter storms that provide the majority of annual moisture. Modern climate variability on interannual/decadal timescales results in latitudinal shifts in the boundary between the wet-north/dry-south; strong ENSO activity typically results in drying in the north and increased precipitation in the south. Previous paleoclimate work in the Great Basin has shown coherence in the timing of major climatic shifts in the Holocene, yet past spatial variability of the dipole remains little studied. Here we present new data from a site that lies within the transition zone of the precipitation dipole.

Swan Lake, located in southeastern Idaho along the northeast edge of the Great Basin, was formed in the spillway channel created by the catastrophic flooding of Lake Bonneville ~14,500 yrs BP. This study seeks to provide insight into the timing and magnitude of late-glacial and Holocene climate variability in the northeastern Great Basin in order to better understand past spatial variability of precipitation patterns. Charcoal, pollen and sedimentological data from a 7.65 m sediment core from Swan Lake are used to reconstruct fire history and vegetation change in the area. Age control is provided by 15 AMS radiocarbon determinations. Results are placed in the context of regional paleoclimate studies. These data build on earlier work by Bright (1966) who reported on pollen, macrofossils and sedimentology from Swan Lake, as well as characterizing the modern vegetation biomes within this area and the climate conditions necessary for their occurrence.

Our preliminary data suggest dramatic reduction in fire frequency coinciding with results from nearby studies that point to either cold or very wet climate phases. Principal cold/wet phases are: 12.5-6.5 and 4.6-1.0 ka BP. Quantitative analysis of charcoal influx data and comparison of pollen assemblages with modern biome/climate relationships provide a basis for testing this hypothesis of alternating climatic conditions.