The pattern and timing of the last Pleistocene glaciation in northeastern Utah: evidence of an ancient lake effect

Setting

During the last Pleistocene glaciation, the highest mountains in northern Utah were blanketed by snow and ice, accumulating in broad cirques to form valley glaciers. The Great Salt Lake rose more than 300 meters above its modern surface, flooding its neighboring valleys to form Lake Bonneville. This vast body of water covered nearly half of Utah and attained a surface area close to that of modern day Lake Michigan (Figure 1). The abundance of surface water and ice in this region of the western United States was indeed a stark contrast to its semi-arid environment of the present. The last glaciation is therefore an important chapter of geologic history in northeastern Utah, and is clearly represented by the geomorphology of the modern landscape.

The record of Pleistocene glaciers in the Wasatch and Uinta Mountains includes scenic alpine cirques, broad glacial troughs, and looping moraine ridges. Throughout western Utah, wave-cut terraces and shoreline deposits at or near altitude 1550 meters above sea level mark the limit of Lake Bonneville during its highstand. At the western toe of the Wasatch Mountains, frontal slopes of moraine ridges display wave-cut notches and are overlain by lacustrine gravels, remnants of the highest stand of Lake Bonneville (Figure 2). Observing glacial and lacustrine features in close proximity and in clear stratigraphic order raises questions about the relationship of glaciers and the lake in northern Utah; for example, were glaciers still present at the foot of the mountains when Lake Bonneville formed? Was there a hydrologic connection between them? How can we use these records to understand climate during glaciations?

Climate Change and Glacial Geomorphology

One could begin addressing these questions by observing the landscape. For example, the topography of glacial valleys offers clues to the past extent of mountain glaciers, which is a function of temperature, precipitation, and solar radiation. End moraines mark the down valley extent of glaciers, while lateral moraines, trimlines (clear lines with contrasting surface appearance on a valley slope), and striated ledges (bedrock surfaces displaying glacial striations) limit the vertical extent of ice in the upper parts of valleys.

Tracing these features on topographic maps (Figure 2B) and aerial photographs allows the past extent of a mountain glacier to be reconstructed, and from it the location of the glacier's equilibrium-line altitude (ELA) can be estimated. The glacier equilibrium line is a hypothetical line on a glacier surface that separates the upper zone of accumulation from the lower zone of ablation. At the equilibrium line, net accumulation of snow and ice during winter is exactly balanced by melting during summer. Therefore, its altitude provides a first-order estimate of temperature depression and effective precipitation changes during a past glaciation.

How can one estimate the ELA of a vanished glacier? There are several methods for doing this, some of which are reviewed by Meierding (1982). Glaciologists (scientists who study the physics of glacier movement) have postulated that a valley glacier at steady state – one that gains as much mass as it loses over one year – has an accumulation-area ratio of 0.65. This means that the surface area of the accumulation zone represents approximately 65% of the total surface area of the glacier and, correspondingly, the ablation area represents 35% of the total surface area. If the total surface area is known, the location and altitude of the equilibrium line can be approximated according to this ratio.

ELAs in northeastern Utah

Reconstructions of ice extent during the last glaciation in northern Utah reveal a compelling pattern of glaciation, especially when considered with Lake Bonneville (Figure 3). Glacier ELAs were lowest in the western Wasatch Mountains and Uinta Mountains, closest to and downwind of Lake Bonneville, and rose eastward by more than 800 m over a distance of ~175 km. If the temperature depression during the last glaciation in this area (at latitude N40.5°) was nearly uniform, then this pattern of ELAs reflects a west to east decline in precipitation. Munroe and Mickelson (2002) estimated based on these ELA differences that glaciers in the western Uinta Mountains received about 1000 mm more winter precipitation than those in eastern valleys, which greatly exceeds the modern precipitation difference between these two areas. They infer that Lake Bonneville provided a local moisture source for glaciers located to the east and downwind, but only if the lake and glaciers attained their maximum extents at approximately the same time.

Significance of paleo-glaciers and lakes in northeastern Utah

What was the relative timing of glacier and lake maxima? The chronology of Lake Bonneville has been developed since the advent of radiocarbon dating in the 1960s, and recently has been more precisely limited. The hydrologic maximum of the lake – the period during which it overflowed at a topographic threshold – was attained from 18,300 to 14,500 years ago (Oviatt, 1997), which is after the time of the global Last Glacial Maximum, 26,500 to 19,000 years ago. The chronology of glacial deposits in the Wasatch and Uinta Mountains is more difficult to limit by radiocarbon dating, but is limited by cosmogenic ¹⁰Be surface-exposure dating of moraines. This method of numerical dating of surficial deposits was applied broadly by Laabs et al. (2009) to moraines in the Wasatch and Uinta Mountains. Terminal moraines in the Wasatch and western Uinta Mountains yield ¹⁰Be surface-exposure ages of 18,000 to 15,000 years ago, whereas moraines in the eastern Uinta Mountains yield ages of 22,000 to 20,000 years ago (Figure 4). Laabs et al. (2009) interpret these ages to represent the time when glaciers began retreating from their terminal moraines, and suggest that glaciers in the Wasatch and western Uinta Mountains persisted at their terminal moraines for as much as 4000 years after the end of the global Last Glacial Maximum. Retreat of glaciers in the eastern Uinta Mountains from their terminal moraines was approximately in step with the start of global ice retreat.

Why did Lake Bonneville and downwind glaciers persist at their maximum extents while glaciers elsewhere in the northern hemisphere were retreating? Again, precipitation may be the answer. The increase in precipitation that accompanied the expansion of Lake Bonneville has been attributed to the mean position of the jet stream and accompanying storm tracks, which may have been situated over northern Utah due to the presence of North American ice sheets. Although lakes are believed to be more sensitive to precipitation changes than glaciers, increased snow fall may have provided sufficient mass to sustain glaciers in the Wasatch and western Uinta Mountains during an interval of warming. The relatively low glacier ELAs in these two areas may reflect amplified precipitation brought about by lake-effect moisture derived from Lake Bonneville, similar to the precipitation phenomena observed downwind of the modern Great Lakes.

The idea that Lake Bonneville provided lake effect precipitation for downwind glaciers is a working hypothesis. Indeed, questions still remain regarding the hydrologic relationship between the lake and glaciers in northeastern Utah. Consider some other ways that a large lake, comparable in size to the modern Great Lakes of the northern United States, could have affected climate. How might the lake have affected cloud cover (and, therefore, incoming solar radiation) in the region? What if the lake was frozen for most of the year during the late Pleistocene? Using geomorphology in combination with other geologic records may help to answer these questions, and improve the understanding of paleoclimate in the region.

• Laabs, B.J.C., Refsnider, K.A., Munroe, J.S., Mickelson, D.M., Applegate, P.J., Singer, B.S., and Caffee, M.W. Latest Pleistocene glacial chronology of the Uinta Mountains: support for moisture-driven asynchrony of the last deglaciation. Quaternary Science Reviews, 28, 1171-1187 (2009).
• Meierding, T. Late Pleistocene Glacial Equilibrium-Line Altitudes in the Colorado Front Range: A Comparison of Methods. Quaternary Research, 18, 289-310 (1982).
  
• Munroe, J.S., Laabs, B.J.C., Shakun, J.D., Singer, B.S., Mickelson, D.M., Refsnider, K.A., and Caffee, M.W. Latest Pleistocene advance of alpine glaciers in the southwestern Uinta Mountains, Utah, USA: Evidence for the influence of local moisture sources. Geology, 34, 841-844 (2006).
• Munroe, J.S., and Mickelson, D.M. Last Glacial Maximum equilibrium-line altitudes and paleoclimate, northern Uinta Mountains, Utah, U.S.A.: Journal of Glaciology, 48 (161), 257-266 (2002).
• Oviatt, C.G. Lake Bonneville fluctuations and global climate change: Geology, 25(2), 155-158 (1997).
• Scott, W.E. and Shroba, R.R. Surficial geologic map of an area along the Wasatch Fault Zone in the Salt Lake Valley, Utah. U.S. Geological Survey Open File Report, 85-448, 19 pp (1985).