PRELIMINARY SHORELINE-BASED 14C LAKE AREA CHRONOLOGY FOR PLUVIAL LAKE CHEWAUCAN, SOUTHERN OREGON, USA

The shoreline deposits of pluvial Lake Chewaucan, located in the northwestern Great Basin, are an important archive of late Pleistocene paleoclimate for the northwestern USA. At ~42.5°N latitude, the basin occupies a critical location for investigating regional precipitation in the northwestern US during the late Pleistocene to present. Considerable core- and section-based research has been done in the Summer Lake subbasin of the two-basin system, but relatively little work on shoreline deposits exists. As part of ongoing work, we present radiocarbon dates on aquatic mollusks and inorganic lacustrine CaCO₃ (tufa and beach gravel cement) from the Summer Lake and Abert Lake subbasins that record lake level change during the latest Pleistocene. Because lake areas of closed-basin lake systems are indicators of precipitation/evaporation ratio, we also present lake areas and associated lake area ratios (lake area/total basin area, expressed as Aw) derived from a digital elevation model for the basin. We have constructed the chronology based on shorelines in the Abert Lake basin from near the modern lake level at 1295 m asl (Aw=0.043) to the spilling threshold with the Summer Lake subbasin at 1338 m asl, and shorelines from 1341-1346 m asl in both subbasins which represent the integrated paleolake Chewaucan. Our results for the integrated paleolake show good agreement in both basins, indicating Lake Chewaucan was at an elevation of 1344-1346 m asl (+79-81 m above Summer Lake, Aw=0.304) at 14.5±0.5 ka BP. Lake level in the Abert Lake subbasin dropped below the spilling threshold at 1338 m asl and was at an elevation of ~1321 m asl (+26 m above Abert Lake, Aw=0.158) by 13.5±0.2 ka BP to 13.8±0.2 ka BP. Previous work indicates further regression to 1310 m asl (+15 m above Abert Lake, Aw=0.101) occurred shortly thereafter by 13.5±0.3 ka. The highest elevation shoreline dates likely represent the time of regression from the Late Pleistocene highstand, which is similar to the regression from the highstand of nearby Lake Lahontan, and from the Provo shoreline of Lake Bonneville.