Paleoecological Evidence of Milankovitch and Sub-Milankovitch Climate Variations in the Western U.S. During the Late Quaternary.
Cathy Whitlock and Laurie D. Grigg (in press),
IN: The Roles of High and Low Latitudes in Millenial Scale Global Climate Change.
R.S. Webb, P.U. Clark, and L.D. Kaigwin (Editors)
American Geophysical Union Monograph.

Abstract:-and-nbsp; Vegetation history provides a record of responses to paleoenvironmental variations occurring across a number of spatial and temporal scales. Effects at particular scales cannot be identified easily from a single site, and a single pollen spectrum cannot be interpreted without understanding the higher and lower frequencies of variation. The vegetation history of the western U.S. shows a strong response to Milankovitch forcing, including the effects that variations in insolation and ice sheet size have had on atmospheric circulation, the position of the jet stream, and subregional precipitation regimes. The response to millennial-scale changes is contained in the residual signal. While the registration of Dansgaard-Oeschger (D-O) and Heinrich events is clear in terrestrial records from regions surrounding the North Atlantic, the signal is greatly attenuated in other parts of the world. In the western U.S., many sites show millennial-scale variability, but few exhibit a spatially coherent pattern that can be ascribed to specific D-O cycles or Heinrich events. Well-dated sites in the Pacific Northwest with close-interval sampling, however, indicate climatic oscillations associated with H1, H2, and H3. Based on these records, the vegetational response may have been greatest when the Laurentide Ice Sheet was at its maximum size, possibly because the climate changes in the North Atlantic were amplified regionally by the presence of the glacial anticyclone. The Younger Dryas cool event (12.9-11.6 Cal ka) is evident in coastal sites and high-elevation sites inland. In the Pacific Northwest the cooling event began later at 12.4 Cal ka and lasted until 11 Cal ka. Millennial-scale climate changes are not evident in most Holocene pollen records from the western U.S., except in those from treeline locations. Charcoal records of past fire occurrence may provide a more sensitive climate proxy during interglacial periods.