Of Mice and Ice in the Late Pliocene of North America

Résumé

Between 2.5 to 1.8 million years ago changes in arctic climate and in meadow mouse dispersal routes correlate with part of the history of uplift and glacial erosion of the Chugach and Saint Elias mountains in Alaska and adjacent Canada. Earlier meadow mice dispersing from Asia to central North America followed a southward coastal route between these mountains and the Pacific Ocean, appearing first in the United States Pacific Northwest. Two and a half million years ago, accelerated uplift of the Chugach and Saint Elias mountains milked Pacific westerly winds, enlarging the ice fields in these mountains so that they then flowed to the sea. This blocked the coastal dispersal for 600,000 years, when no new immigrant meadow mice appeared in the conterminous United States. The uplift also restrained westerly winds that crossed Canada, permitting moister air from the subtropical Atlantic and from an unfrozen Arctic Ocean to produce significant continental glaciation, centered in eastern Canada. By 2.0 million years ago, glacial erosion had lowered these mountains again, letting relatively dry Pacific westerlies extend across Canada, reducing the encroachment of moist Atlantic and arctic air, and ending continental glaciation to the east. The simultaneous reduction of glacial activity in the cordillera allowed meadow mice to renew southward dispersal. Additionally, the lowered mountains remained a rain shadow, causing grassland in the Great Plains of Canada. Thus a new dispersal route to the United States was opened for grazing meadow mice and for the first time their earliest records were in the Great Plains. Loss of the continental ice sheet and an unfrozen Arctic Ocean facilitated the northward spread of warm and moist air from the North Atlantic subtropical high; it flowed northward up the eastern front of the Rocky Mountains in the United States and, mingling with the dry westerlies, northeastward across Canada to northernmost Greenland, where trees then grew. About 1.8 million years ago the Great Plains of the United States were subtropical savannah and remained so until the beginning of the Ice Age 850,000 years ago.