Inconsistent Climate Inferences between Pollen and other Paleontological, Geochemical, and Geophysical Proxies in Late Pleistocene Lacustrine Sediments from Summer Lake, Oregon, Western Great Basin

Abstract:

This study has established a high resolution paleoclimate record from western Great Basin pluvial Summer Lake, Oregon during the late Pleistocene Mono Lake Excursion (~34 ka), Dansgaard-Oeschger (D-O) interstadials 7 and 8, and the end of Heinrich Even 4 (~38 ka). Proxies of grain-size, carbon/nitrogen (C/N) ratio, ostracode analysis and palynology from a depocenter core show new results regarding high amplitude, high frequency changes in lake level, precipitation and temperature which correspond directly with colder/warmer and respectively drier/wetter climates as documented with Northern Atlantic Greenland ice core data.

The granulometry, geochemical, and ostracode results consistently demonstrate the correspondence of low lake conditions and colder water temperatures during D-O stadials and warmer/wetter climate during interstadials. These results are contradicted by the pollen results. Existence of cold temperature spores Botrychium and Selaginella coincide with increases in Artemisia, Atriplex, Sarcobatus, Cyperaceae and decreases in Pinus, also suggesting periods of colder/drier climate and shallower lake levels but the timing does not match that of those conditions inferred by the other methods.

Granulometry, geochemical, and ostracode proxies denote cold periods and low lake levels roughly between 37.5-35.6 ka and 34.6-33.8 ka. Pollen analysis suggests near-opposite time intervals with cold periods roughly 38-37.5 ka, 35.6-35 ka. This pollen inconsistency suggests the possibility of (1) a millennial-scale lag response of vegetation to climate change, (2) runoff from stadial precipitation causing influx in pollen abundances and variety found in the depocenter core, or (3) turbulent mixing from shallow lake level causing resuspension and redeposition of pollen (Bradley 1999).