Paper No. 9
Presentation Time: 10:30 AM


THACKRAY, Glenn D., Department of Geosciences, Idaho State University, 921 South 8th Ave., Box 8072, Pocatello, ID 83209,

Variations in precipitation and temperature before, during, and after the ice sheet Last Glacial Maximum affected western glacial systems in multiple ways, dependent largely on their climatic setting. Thorough understanding of regional climatic processes thus requires a broad view of glaciation in space and time.

Ice advances during late MIS 3 (30-40 ka) are a key indicator. Such advances are clearly seen on the West Coast and may have occurred in interior ranges as well. In the Olympic Mountains, the most extensive ice of the last 50 ka occurred 30-35 ka, and LGM-correlative ice position represented a ca. 30% reduction in glacier. An advance of alpine glaciers in the North Cascades range may have been correlative with the Olympic advance. In the Sierra Nevada, the earliest Tioga advance (aka Tenaya) occurred 30-35 ka, but its evidence is preserved only locally and it was likely a lesser advance. In interior ranges, isolated CRN and OSL ages, coupled with relationships from range front fault offset rages, suggest that selected ranges in Idaho and Wyoming ranges also preserve a 30-35 ka advance. A regionally extensive MIS 3 advance, if clearly demonstrated, might indicate strong Pacific Ocean climatic forcing favoring glacier growth at that time, a strong and regionally pervasive LGM ice sheet anticyclone limiting LGM glacier growth, or both. The latitudinal extent of the anticyclone influences remains elusive, but may be revealed by further dating of glacial sequences.

During the LGM, PMIP model outputs suggest that the main belt of Pacific moisture was diverted south into the Great Basin. That diversion may have limited ice extent in northern ranges and enhanced ice extent in parts of the Sierra Nevada, and in the Great Basin. Glaciers in several interior ranges (e.g., Wasatch, Uinta, Ruby, Sawtooth, Teton, and Wind River ranges) persisted near full-glacial limits until 17-14 ka and, in fact, maximum ice extent is represented by moraines abandoned ca. 17 ka, i.e., after the LGM sensu stricto. Differences between LGM-correlative ice maxima and 17 ka ice maxima may reflect important paleoclimatic contrasts, or simply minor fluctuations around persistent maximum positions.