Tundra and boreal forest of interior Alaska during terminal MIS 6 and MIS 5e

Two sites within the boreal forest of interior Alaska shed light on the climate and vegetation of terminal marine isotope stage (MIS) 6 (ca. 140–130 kyr ago) and MIS 5e (125–116 kyr ago). The Birch Creek and Koyukuk localities are river-cut exposures with sediments dating from the penultimate glaciation (at least) to the present. Plant macrofossils, pollen, and beetles were analyzed at these sites. Terminal MIS 6-aged samples indicate a cooler than modern climate and the presence of shrub tundra. During MIS 5e, boreal forest grew at the sites and temperatures were similar to modern times. However, the forest may also have been more mesic than today, as indicated by relatively abundant ferns. Winters may have been warmer than today, as suggested by beetle-based climatic reconstructions as well as the presence of two extralimital taxa that today live in regions where winter temperatures are up to 15 °C warmer than at the site localities.