Miocene floristic change at 15 Ma, Nevada to Washington, U.S.A.

Abstract

Mid-Miocene floras from the western interior regularly have numerous exotics allied to broad-leaved deciduous hardwoods and conifers now in the eastern United States and Asia. Successive floras from local areas show that exotics were eliminated first from western Nevada, later in Oregon and Washington to the north where precipitation was higher, as it is today. Precipitation requirements of allied modern species suggest that the exotics were reduced significantly at 15 Ma as summer rainfall (May-August) decreased to <35 per cent of the annual total. The decrease seems attributable to the spread of the East Antarctic ice sheet (15 Ma) and resultant cold water upwelling that produced drier summers. Commencing 8-7 Ma, the spread of the West Antarctic ice sheet and Arctic polar ice, with resultant further cold water and drier summers, continued to reduce exotics and confine them to relict sites in mild coastal areas down to the close of the Tertiary.