Geoscience Reference
In-Depth Information
cooling of the Arctic Ocean from 33.5 Ma ago onwards. The cooling culminated in
the relatively rapid accumulation of ice across the Laurentian shield around 2.5 Ma
ago, resulting in the formation of a major ice cap. The waxing and waning of suc-
cessive ice caps was modulated by changes in the earth's orbital geometry, with the
23 and 19 ka precessional cycles dominant until 2.6 Ma, the 41 ka obliquity cycle
dominant between 2.5 and 0.7 Ma, and the 100 ka orbital eccentricity cycle dominant
thereafter. Deposition of loess and wind-blown desert dust was characteristic of cold,
dry periglacial environments, with dunes active during glacial times. Lakes in the
Great Basin were generally high during times of maximum glaciation, although there
were regional differences in the timing of late Pleistocene high lake levels as a result
of differences in precipitation source areas. The most recent ages obtained from late
Pleistocene high lake levels do not support the once widely accepted hypothesis of a
southward displacement of the westerlies during glacial times. In the presently very
arid deserts of northern Mexico and the American Southwest, the macrofossil remains
in packrat middens indicate relatively mild and moist conditions during the LGM,
in opposition to an early view espousing a cold, dry LGM climate in the American
Southwest. Both lake shorelines and fossil soils that developed on alluvial fan deposits
indicate that the climate was wetter during earlier glacial cycles than it was during
the LGM. The precise causes of megafaunal extinctions in North America remain
enigmatic, with evidence from ancient DNA and from cave speleothems pointing to
an important role played by climate as opposed to simply attributing all extinctions
to the arrival of the Clovis hunters.
