WINDOWS OF OPPORTUNITY: PALEONTOLOGICAL, CHRONOLOGICAL AND TAPHONOMIC PERSPECTIVES ON PALEOCOLOGY AFTER THE LAST GLACIAL MAXIMUM IN COASTAL BRITISH COLUMBIA AND WASHINGTON

Pacific Northwest ecosystems after the last glacial maximum responded to dramatic changes in temperature, Cordilleran Ice Sheet extent, and relative sea level; as well as differential dispersal rates between constituent species, including humans. Discoveries from the San Juan Islands, Vancouver Island, and Olympic Peninsula document terrestrial megafauna (ground sloth, mastodon, bison, mountain goat, giant short-faced bear, brown bear, and other species) by or before 11,000 radiocarbon yr BP, some by 12,500 yr BP. Coastal sites were colonized by sea lions and water birds as early as 12,000 yr BP. Given widespread modern forests and soil acidity their remains must be sought in taphonomic “windows” such as caves or wetland deposits buffered by shelly marls. Evidence shows that Last-Glacial advance and retreat times differed between SE Alaska and SW British Columbia/NW Washington; glaciers were advancing in the latter when already retreating in the former. Hence ecosystem history and human migration cannot be modeled by an “open and shut” coastal migration scenario: for example, a temporal “window of opportunity” may have allowed southward movement ca. 16,000 yr BP, linking patchy refugia. For an unknown time after the eustatic minimum, migratory birds and sea mammals could have used emergent seamounts as well as coastal sites for stopping-points: their abundance and maintenance of migratory routes did not simply depend upon coastal refugia and recolonization of coastal lands would have been rapid. People moving down the coast would have found varied biotic resources, including terrestrial megafauna as they neared the southern ice margin. Challenges remain in comparing chronologies from area to area and species to species; for example, authors have used differing values for marine reservoir corrections for dates on marine biota. Presence of terrestrial megafauna opens the likelihood that they influenced ecological succession in a top-down trophic cascade. Vegetation changes were not simply responses to climate change, with vegetation as a “container” for incoming fauna: rather, such changes may have been engineered in part by the megafauna, ultimately also signaling their disappearance.