SIMILARITIES AND DIFFERENCES BETWEEN LACUSTRINE AND VEGETATION HISTORIES OF THE BONNEVILLE BASIN DURING THE LATE PLEISTOCENE, AND THEIR POTENTIAL CAUSES

Over the past 125 years, scientists studying lake sediments and shoreline features have developed a detailed chronology of the rise, persistence, and eventual decline of Pleistocene Lake Bonneville, the deepwater lake that occupied the basin where Great Salt Lake now resides. Studies of pollen preserved in Lake Bonneville sediments, coupled with plant macrofossils from nearby packrat middens, provide a similarly detailed history of vegetation change in the area immediately surrounding the lake. Comparison of lacustrine and vegetation changes through the late Pleistocene provides clues on the climatic and nonclimatic factors that may have influenced lake levels and plant distributions in similar or divergent ways.

A shallow lake occupied the Bonneville Basin from ~40 to ~30 ka, after which lake waters started a long period of deepening, and by ~18 ka Lake Bonneville was ~270 m deep and covered much of northwestern Utah. At that point the lake reached an outlet, which experienced a catastrophic collapse, and the lake depth decreased to ~170 m, and this level was maintained until ~15 ka. The lake declined to near modern levels (< 10 m deep) by ~12 ka. Vegetation and lacustrine changes were similar from ~40 to ~26 ka, with changes from sagebrush to pine dominance in the pollen record correlated with the initial lake-level rise. There is less agreement for the period from ~26 to ~15 ka, as the pollen data suggest drier conditions during the deepwater phases. The discordance is greatest during the initial decline of Lake Bonneville after 15 ka, when the lower elevational limits of limber pine moved downslope, which would seem to indicate wetter-than-previous conditions while lake levels were lowering.

The initial similarities between the vegetation and lacustrine histories suggest that both were then controlled primarily by decreased summer insolation and by changes in atmospheric circulation related to the growth of the Laurentide Ice Sheet. The subsequent divergence may be related to differing sensitivities to greater summer insolation, but also to the effects of changing levels of atmospheric CO₂ on water use efficiency of many plant species (with higher CO₂ levels permitting plants to live on drier sites).