Palynological and tephra correlations among deep wells in the modern Great Salt Lake, Utah, USA : implications for a neogene through pleistocene climatic reconstruction

Abstract

Palynological and tephra information provides the foundation for correlation of five deep wells from the Great Salt Lake. Pollen correlations are primarily resolved using pollen percentage diagrams. Analysis of the individual tephra units allow correlation of tephras based on chemical similarities. Known regional tephras recognized in one or more of the wells are the Lava Creek B ash bed, the Bishop ash bed, the Huckleberry Ridge ash bed, the Santee ash bed and the Walcott ash bed. Absolute ages associated with these ash beds and three other units subsequently allow for the calculation of long term sediment accumulation rates at each well location. Sediment accumulation rates, averaged over the last 4 to 5 million years of record, range between 0.18 mm/yr and 0.43 mm/yr, with fairly constant rates within each well. The two longest records, which represent an approximated 16 million years, provide average sedimentation rates over this entire period of 0.23 mm/yr and 0.13 mm/yr. Neogene through Pleistocene paleoclimatic conditions are estimated using pollen percentages based on the pollen counts from all five wells. Quantitative climatic interpretations based on the palynology suggest close to modern, but slightly warmer and drier conditions from ca. 7 million years ago (Ma) to ca. 3.5 Ma. The period lasting from ca. 3.5 Ma to ca. 0.7 Ma was characterized by a clearly hotter and more arid climate compared to today; though a short cool period may have occurred between 2.7 and 2.9 Ma. Conditions over the last 0.7 Ma were generally cooler and moister than the previous period, and at times may have been significantly cooler and moister than the modern climate. A qualitative estimate of precipitation conditions based on Pinus percentages suggests higher than modern mean annual precipitation in the mid-Miocene, with an overall decrease through the Pliocene.