NEW EVIDENCE FOR AN EXTENDED OCCUPATION OF THE PROVO SHORELINE AND IMPLICATIONS FOR PALEOENVIRONMENTAL CHANGE FROM PLEISTOCENE LAKE BONNEVILLE, UTAH

The shorelines of ancient Lake Bonneville document environmental change on centennial to millennial timescales. Refining the record of lake level is key to understanding climate change in the Bonneville Basin during the Pleistocene to Holocene transition. Throughout most of its existence, Lake Bonneville functioned as a closed-system, oscillating markedly in response to changes in the local hydrologic balance. However, wetter and/or cooler climate conditions caused the lake to transgress intermittently to the point of overflow. The Provo shoreline formed during the lake's open-basin phase, following the Bonneville flood. Past hydrographic models depict an approximate 500 year occupation of the Provo shoreline from about 14,500 ¹⁴C yr B.P. to 14,000 ¹⁴C yr B.P., followed by a gradual regression to modern levels by about 12,000 ¹⁴C yr B.P. New radiocarbon and stratigraphic data from shoreline deposits, examined in the context of previous studies, suggest that the lake actually remained at or near the level of the Provo shoreline for a much longer period of time. Data show that the lake oscillated near the Provo level for more than 2500 ¹⁴C years and then rapidly regressed at a rate of approximately 25 cm/yr, reaching modern Great Salt Lake levels by about 11,500 ¹⁴C yr B.P. These observations, coupled with independent evidence from vegetation and glacial records, suggest that climate conditions in the Bonneville Basin remained relatively wetter and/or cooler until about 12,000 ¹⁴C yr B.P. and that the onset of warmer and/or drier conditions was quite sudden in this region.