INVESTIGATING THE ENVIRONMENTAL IMPACTS AND CULTURAL RESPONSES TO THE MAZAMA ASHFALL ON THE NORTHERN PLAINS

The cataclysmic Mazama eruption occurred about 7600 years ago and spread a thick layer of volcanic ash across much of northwestern North America. When exposed in sections in southern Alberta, the ash may be several centimeters thick and is often clearly visible as a distinct white band several metres below ground surface. In southern Alberta, about 1000 km north-east of the vent, the ash fell on a prairie landscape that was occupied by hunter-gatherers whose subsistence pattern focussed on bison hunting. Then as now, the northern plains experienced year-to-year spatial variability in moisture inputs, affecting resource availability. Some places offered more predictable and reliable resources. These included isolated uplands, such as the Cypress Hills. Here, occupation levels at the multicomponent Stampede Site (DjOn-26) extend to at least 8,000 yr BP. Landscape response to Mazama ash can be reconstructed by analogy with the recent well-studied Mount St Helens 1980 (MSH) ashfall. Immediate effects would have been substantial, with ash forming a blanket up to 15 cm thick, coating vegetation and clogging water courses. In forested areas, fuel build-up resulted in wildfires, although lower fuel loads on the plains would have precluded widespread fires. Short-term and longer-term (several years) climate cooling occurred. Research at MSH shows considerable ecosystem resiliency, with biotic “hotspots” particularly important for vegetation recovery. Nevertheless, immediate impacts would have caused resource scarcity over a wide area, necessitating movement by people out of the main ashfall envelope. Available archaeological evidence suggests a hiatus in occupation of the ash-affected area for perhaps 200 years, or about 10 generations. At the Stampede Site, it is remarkable that people returned to re-occupy exactly the same spot after the hiatus. An event of this magnitude requires resiliency and adjustment on a time-scale of decades to centuries, even in distal areas.