HOLOCENE PRECIPITATION VARIATIONS IN THE WESTERN UNITED STATES

Holocene climate patterns across the western US are controlled by complex ocean/atmosphere dynamics including (i) strength and position of the Aleutian Low and North Pacific High pressure systems which determine the magnitude, duration, and position of winter storm systems, and (ii) the North American Monsoon which is driven by Northern Hemisphere summer warming. On annual-to-decadal time scales, ENSO- and PDO-controlled precipitation variability is expressed as a north-south dipole in the western US, with a transition zone that separates a wet Pacific Northwest and a dry Southwest.

Most lacustrine paleoclimate records suggest that water levels were lower across the western US, due to warmer, drier conditions during the middle Holocene. Lake levels rose with increased precipitation and decreased evaporation in the late Holocene. The magnitude of the lake level changes and the timing of these transitions vary with latitude and local conditions such as proximity to the coast and elevation.

Favre Lake (2899 masl), located in the Ruby Mountains of northeastern Nevada, filled gradually between 7625 and 5600 cal yr BP, after which lake level remained relatively stable for the remainder of the Holocene. Medicine Lake (2033 masl), lying within the caldera of Medicine Lake Volcano (northeastern California) filled to a maximum level between 11,400 and 6000 cal yr BP and then fluctuated by several meters during the remainder of the Holocene. The lowest elevation lake, Swamp Lake (1545 masl), exhibited the strongest record of changes in seasonality over the Holocene. Warmer conditions began at ~10,800 cal yr BP, with maximum drying between 7400-5500 cal yr BP, followed by a gradual increase in moisture for the remainder of the middle Holocene. A major shift to wetter conditions occurred at about 3100 cal yr BP. Lower Bear Lake (2065 masl) in the San Bernardino Mountains east of Los Angeles is most directly influenced by marine conditions. The sediment archive from Lower Bear Lake shows an overall gradual increase in salinity during the Holocene, suggesting dryer conditions. Several centuries-long pluvial events punctuate the record.