Last glacial–interglacial environments in the southern Rocky Mountains, USA and implications for Younger Dryas-age human occupation

a r t i c l e i n f o Keywords: Last glacial–interglacial transition Younger Dryas Chronozone Vegetation and fire history Abrupt climate change Folsom-age Paleoindians The last glacial-interglacial transition (LGIT; 19–9 ka) was characterized by rapid climate changes and significant ecosystem reorganizations worldwide. In western Colorado, one of the coldest locations in the continental US today, mountain environments during the late-glacial period are poorly known. Yet, archaeological evidence from the Mountaineer site (2625 m elev.) indicates that Folsom-age Paleoindians were over-wintering in the Gunnison Basin during the Younger Dryas Chronozone (YDC; 12.9–11.7 ka). To determine the vegetation and fire history during the LGIT, and possible explanations for occupation during a period thought to be harsher than today, a 17-ka-old sediment core from Lily Pond (3208 m elev.) was analyzed for pollen and charcoal and compared with other high-resolution records from the southern Rocky Mountains. Widespread tundra and Picea parkland and low fire activity in the cold wet late-glacial period transitioned to open subalpine forest and increased fire activity in the Bølling–Allerød period as conditions became warmer and drier. During the YDC, greater winter snowpack than today and prolonged wet springs likely expanded subalpine forest to lower elevations than today, providing construction material and fuel for the early inhabitants. In the early to middle Holocene, arid conditions resulted in xerophytic vegetation and frequent fire. Introduction The transition between glacial and Holocene conditions (the last glacial–interglacial transition [LGIT]; 19–9 ka) has received much attention in investigations of environmental and human responses in the face of rapid climate change (Alley, 2000; Shuman et al., 2002; Meltzer and Holliday, 2010). The Gunnison Basin of Colorado is a closed, high-elevation basin that experiences today some of the coldest temperatures in the continental US. During the Younger Dryas Chronozone (YDC; 12.9– 11.7 ka), Folsom Paleoindian groups occupied several sites in the Basin, most prominently the Mountaineer site (Fig. 1a). Located atop Tenderfoot Mesa (~2625 m elev.) and radiocarbon dated to 12.3 ka, excavations at the site have yielded remnants of several substantial house structures (Stiger, 2006). In an effort to understand the climatic and environmental context of the Paleoindian occupation of the Gunnison Basin, a sediment core was extracted from Lily Pond (38°56′06′′N, 106°38′37′′W, 3208 m elev.; Fig. 1b; Table 1), located in Taylor Park ~50 km northeast of the Mountaineer site. Results of the pollen, charcoal and lithological analyses help reconstruct the environmental …