PALEOCLIMATIC IMPLICATIONS OF LATE PLEISTOCENE SEDIMENT YIELD RATES FOR THE BONNEVILLE BASIN, NORTHERN UTAH

Climate factors such as precipitation and temperature as well as clima te-driven geomorphic processes such as glaciation affect rates of eros ion and the amount of sediment supplied from a drainage basin (sedimen t yield). Thus, sediment yield rates provide paleoclimatic implication s, although very little work has been published on ancient sediment yi eld rates. Late Pleistocene Lake Bonneville (Utah, Idaho and Nevada), with its well-documented lake level history and well-preserved deltaic sequences, provides a unique opportunity to estimate ancient sediment yield rates. Two end-member delta systems, coarse-grained (gravel) cl assic Gilbert-type deltas and fine-grained (sand, silt and clay) delta s, are exposed along the eastern margin of paleolake Bonneville. One e xample of each end-member delta is examined in this study (American Fo rk and Weber River deltas). The established Bonneville hydrograph can be used to determine time of deposition for a given package of sedimen ts because these shoreline deltaic sediments were deposited in shallow water. The geomorphic expressions of these deltas are well-preserved and their sediment volumes can be determined. River drainage basin are a can be combined with time constraints and delta volumes to estimate sediment yield rates. Sediment yield rate estimates for the coarse-gra ined American Fork and fine-grained Weber River deltas are 478 m(3) km (-2) yr(-1) +/-34% and 705 m(3) km(-2) yr(-1) +/-15%, respectively. Th e similarity of these values is somewhat surprising due to the large d ifference in drainage basin sizes (American Fork approximate to 160 km (2) and Weber River approximate to 3328 km(2)). However, this similari ty suggests that precipitation may be the dominant control on sediment yield. Using sediment yield/precipitation regression equations develo ped for similar modern basins in the Southern Alps of New Zealand, a f irst-order approximation of paleoprecipitation is 1.6 m/yr (+/-14% for American Fork delta) and 1.9 m/yr (+/-7% for Weber River delta). Thes e paleoprecipitation value are comparable to modern precipitation valu es of 0.4-1.5 m/yr (range of mean values over both drainage basins). A maximum late Pleistocene precipitation increase of 33% over the moder n day value is suggested. This is determined by the difference between the highest paleoprecipitation rate (1.9 m/yr+7% of the Weber River d elta) and the modern value (1.5 m/yr). Previous climate models and thi s 33% paleoprecipitation increase, suggest a maximum decrease in basin floor paleotemperature of approximately 13 degrees C. This paleotempe rature decrease is consistent with previous estimates of late Pleistoc ene temperatures (using amino acid epimerization rates and periglacial wedges) in the Bonneville Basin and adjacent areas. These estimates s uggest that although late Pleistocene precipitation rates may have bee n slightly higher than present (up to 33% higher), late Pleistocene te mperature and evaporation rates were more important in the development of Lake Bonneville and associated glaciers.