Lake Bonneville Spits as Paleoclimate Wind Socks

Lake Bonneville is the largest of several pluvial lakes (formed during a period of elevated precipitation) that formed in the Great Basin of the western United States during the Last Glacial Maximum 32,000 to 10,000 ago. At its greatest extent, the lake covered 50,000 km² and had a maximum depth of ~300 m. The lake stabilized for varying lengths of time at several prominent elevations which today are marked by shorelines at ~1550 m (the Bonneville shoreline), ~1450 m (the Provo shoreline), ~1370 m (the Stansbury shoreline), and ~1295 m (the Gilbert shoreline) although a number of lesser shorelines are also visible (Figure 1). All of these shorelines represent prolonged periods of stability of the lake level.

G. K. Gilbert, one of the greatest geologists of the late 19^th and early 20^th centuries, studied Lake Bonneville in great detail. Gilbert recognized and described not only the major shorelines in Lake Bonneville, but also a number of important geomorphic features including spits, bars, and baymouth barriers (basically a bar that stretches across a small valley). Applying the Principle of Uniformitariansim ("the present is the key to the past") to a lake that no longer existed, Gilbert knew that many of these features were the result of very large storms and the resulting high wave energy capable of moving large quantities of sediment.

What might Lake Bonneville geomorphic features tell us about the climate at the time of Lake Bonneville? The peculiar geometry of spits offers particular promise in this regard. A spit forms when wave trains (groups of waves moving in the same direction) produced by strong winds approach a land mass at an angle of less than 90^o (Figure 2). The waves impart a shear stress (force per unit area) to the shorezone and cause longshore transport that moves sediment along the coast. Upon reaching a change in the orientation of the coastline, the sediment may be deposited to form a spit. In a sense, spits are like "sediment flags" in a water body. The direction of the spit gives a rough indication of the direction that waves were coming from. A spit points in the opposite direction of the strong winds that formed the spit just like a flag points in the opposite direction of the wind.

Applying this general concept to Lake Bonneville has produced a very interesting result. In order to place Lake Bonneville spits in a paleoclimatic context, the first question that must be addressed is: which spits are appropriate for this analysis? In general, wave energy is a function of three factors: fetch (length of water over which the wind blows), wind duration, and wind magnitude. Although we cannot make exact determination of wind magnitude and duration during the Late Pleistocene (12,000 -18,000 years ago) over Lake Bonneville, we can determine the maximum fetch direction. A scientific test of the "sediment flag" hypothesis must therefore examine only those spits for which the fetch was approximately the same in most directions. This criterion is met in five small mountain ranges that were islands in the central part of Lake Bonneville during the Late Pleistocene.

17 spits meeting this criterion and confirmed following examination in the field show a general (although by no means perfect) trend toward southerly orientation, implying that the strongest winds were from the north (Figure 3). What does this mean for the general climate of the Great Basin during the Last Glacial Maximum?

Modern wind records from the eastern Great Basin show that strong winds are predominantly from the south (Figure 4). This is the result of significant energy from storms being funneled around the southern end of the Sierra Nevada Mountains (Shafer and Steenburgh, 2008). The direction of strongest winds during the Last Glacial Maximum was thus clearly different. Why? One explanation might be that persistent high atmospheric high pressure over the continental ice sheet to the north may have led to katabatic winds: winds that are pushed down ice sheets toward areas of lower pressure (Jewell, 2007). Such winds are common off the modern Greenland and Antarctic ice sheets and may well have existed in the Great Basin during the Late Pleistocene. However, the cause and timing of these changes in wind direction in the Great Basin remains an unresolved research question (Figure 5).

• Jewell, P. W., 2007, Morphology and paleoclimate significance of Pleistocene Lake Bonneville spits: Quaternary Research, v. 68, p. 421-430.
• Shafer, J. C., and W. J. Steenburgh, 2008. Climatology of strong intermountain cold fronts. Monthly Weather Review, 136, 784-807.