A 19,000-year vegetation and climate record for Bear Lake, Utah and Idaho

Pollen analysis of sediments from core BL96-2 at Bear Lake (42°N, 111°20′W), located on the Utah-Idaho border in America’s western cordillera, provides a record of regional vegetation changes from full glacial to the late Holocene. The reconstructed vegetation records are mostly independent of Bear Lake’s hydrologic state and are therefore useful for identifying times when climate forcing contributed to lake changes. The Bear Lake pollen results indicate that signifi cant changes in the Bear Lake vegetation occurred during the intervals 15,300–13,900, 12,000–10,000, 7500– 6700, 6700–5300, 3800–3600, and 2200–1300 cal yr B.P. These intervals coincide with regional shifts in vegetation and climate, documented in pollen, isotope and biogeographic records in the Basin and Range region, suggesting that large-scale climate was the primary forcing factor for these intervals of change. Maximum aridity and warmth is indicated from 12,000 to 7500 cal yr B.P., followed by intervals of generally more mesic and cool conditions, especially after 7500 cal yr B.P. Doner, L.A., 2009, A 19,000-year vegetation and climate record for Bear Lake, Utah and Idaho, in Rosenbaum, J.G., and Kaufman, D.S., eds., Paleoenvironments of Bear Lake, Utah and Idaho, and its catchment: Geological Society of America Special Paper 450, p. 213–223, doi: 10.1130/2009.2450(09). For permission to copy, contact editing@geosociety.org. ©2009 The Geological Society of America. All rights reserved. INTRODUCTION Paleoenvironmental histories are often derived from multiproxy studies of lake and wetland sediments. The aquatic environment tends to preserve characteristics in the sediments that are otherwise lost through oxidation or erosion. Deep lakes, in particular, can provide high-resolution records because of reduced wind-induced sediment mixing and profundal anoxia. Environmental proxies include diatoms, ostracodes, mollusks, oxygen and carbon isotopes, carbon content, particle-size, geochemistry, sedimentary magnetism, and pollen. In highly variable lake environments, the climatic signals of these proxies can be masked or confounded by internal forcing factors or strongly mitigated by within-lake environmental conditions. For example, in the past 19,000 years, Bear Lake’s chemistry has varied widely as a result of changes in Bear River infl ux, resulting in relatively large magnitude changes in oxygen isotope ratios (Dean, this volume). The infl uence of temperature on these isotopic ratios is mixed with, and hidden by, these hydrologic events. Although pollen grains are subject to current-driven sorting and preservation conditions in the lake, most of the source area is terrestrial and dominantly extra-local, especially for large lakes (Tauber, 1977; Bradshaw, 1994; Jackson, 1994). Long-distance transport of pollen in river systems, or redeposition of pollen that has been exposed to an oxygenated environment, is easily distinguished from airborne, rapidly deposited pollen by its preservation state (Cushing, 1967). Pollen data can thus serve as a bellwether for distinguishing local-lake hydrologic changes from regional climate changes. One of a series of papers in this volume examining Quaternary sediment records from Bear Lake, Utah and Idaho, this paper focuses on post-glacial vegetation reconstructions from *E-mail: donerl@mac.com 214 Doner spe 450-09 2nd pgs page 214 pollen analyses and identifi cation of climate forcing of sedimentary changes. The overall aim of the Bear Lake project is to reconstruct local and regional changes associated with tectonic basin subsidence, Bear River migration, and climate change. Few continuous vegetation records from the glacial maximum to the late Holocene in the Basin and Range region of the western United States exist because of erosion of the long-term sediment record at low elevations from lake desiccation and at high elevations from mountain glaciations. Proximal to Bear Lake, pre-Holocene paleovegetation records have been developed from the Great Salt Lake/Bonneville Basin, covering the last 15 m.y. (Davis and Moutoux, 1998), from a 70,000 yr vegetation reconstruction in a modern marsh at Grays Lake, Idaho, ~80 km northeast of Bear Lake (Beiswenger, 1991), and from a deglacial sequence at Rapid Lake, Wyoming (Fall et al., 1995). Holocene records in the region also come from these records plus peat deposits in the Uinta Mountains, southeast of Bear Lake (Munroe, 2003). A new, multi-glacial pollen record from the GLAD800 drilling project at Bear Lake reveals regional vegetation sensitivity to summer insolation, global ice volume, and some Heinrich events (Jimenez-Moreno et al., 2007). Although Bear Lake sediments show clear responses to the dominant climate changes of glacial-interglacial cycles and transition intervals (Jimenez-Moreno et al., 2007; Kaufman et al., this volume), interpretation of Bear Lake’s response to the milder climate changes of the late Quaternary is complicated. Tectonic activity along the graben, diversion of the Bear River out of the lake watershed, and varying inputs from multiple groundwater sources caused strong variations in lake level and water source during the last 25 k.y. (Reheis et al., this volume). These hydrologic changes in the late Pleistocene confuse climate interpretations from diatoms, ostracodes, isotopes, and carbonates (Moser and Kimball, this volume; Bright, this volume, Chapter 8; Dean, this volume). Sedimentary magnetism also shows a strong glacial-interglacial component from transport of sediments to Bear Lake by the Bear River but after diversion of the river, this climate signal is lost by post-depositional destruction of Fe-oxide minerals (Rosenbaum and Heil, this volume). In this situation, where changes in water and sediment sources and chemistries create uncertainties in identifi cation of forcing factors by alternative proxies, the onus to demonstrate climate forcing of late Quaternary changes in the sediments is on the pollen record.