Cold climate conditions as a driver of alluvial fan deposition in the Lost River Range, Idaho, USA

Numerous large alluvial fans sit along the western front of the Lost River Range (LRR) in east-central Idaho, USA (Figure 1). These alluvial fans form where streams exit confined basins within the mountain range, spread out, and lose their ability to transport sediment (Figure 2). Although these fans are some of the most significant geomorphic features in the basin, they are almost entirely inactive in our modern climate. This suggests that different climatic conditions in the past drove the formation of fans in the LRR; but what were the climate conditions (i.e. cold or warm, wet or dry) during periods of major deposition? Answering this question requires knowing when sediment deposition occurred and what the climatic conditions were like during periods of sediment deposition.

To acquire numerical ages for the alluvial fans of the LRR, we used optically stimulated luminescence dating (OSL). This method has been increasingly used to date fluvial and alluvial deposits because (1) it relies on silt to fine sand-sized quartz grains (present in nearly all sediment) and, (2) it provides an age for the actual timing of deposition, rather than only a minimum or maximum age like many other dating methods. During transport by wind or water, the luminescence signal of quartz grains is bleached by exposure to sunlight. After deposition and burial, the luminescence signal of the quartz begins to accumulate again with exposure to ambient radiation, the signal increasing with time.

Samples for OSL dating are collected without exposing the sediment to light, typically by pounding an opaque tube horizontally into a thick sand lens to remove a core. Once in the lab, the quartz grains within the sample are isolated and stimulated by blue green light so that the natural luminescence signal can be measured. Then the quartz grains are given varying doses of radiation and the resulting luminescence signal measured. The resulting dose-response relationship is used to estimate the total natural dose of radiation the quartz received since deposition.

In order to calculate an age, the dose rate of the sample must be estimated. The dose rate of the sample is the amount of radiation the sample was exposed to per thousand years (measured in grays per thousand years, or Gy/ka). The dose rate depends on factors that include the chemistry of the sediment and the latitude, longitude, and elevation of the sample location. The age of the deposit is then calculated as

age (ka) = total dose (Gy)/ dose rate (Gy/ka)

Deposits of LRR alluvial fans are composed predominantly of coarse gravels and cobbles and lack thick sand lenses typically sampled for OSL (Figure 3). Unable to pound tubes into these gravelly deposits for sampling, we collected samples without exposing them to light by excavating sediment into containers at night or while under light-safe tarps. We collected a total of 32 OSL samples from five LRR fans and two additional samples from terraces of the East Fork Big Lost River (Figure 1).

We used the resulting OSL ages to help correlate surfaces of similar age on individual fans and between fans throughout the basin for geomorphic mapping. OSL ages (Figure 4) and maps (example in Figure 5) suggest that there have been five distinct intervals of deposition on LRR fans in last 120 ka. The most recent interval occurred during the Holocene in the last 10 ka (Figure 4), but produced little deposition compared to older intervals. Three different intervals during the late Pleistocene (10-20 ka, 20-35 ka, and 35-60 ka; Figure 4) resulted in significant deposition that produced many of the large, distinct surfaces that characterize LRR fans (Figure 5). An older interval of deposition approximately 90-120 ka is suggested by several ages from Upper Cedar Creek Fan (Figure 4), but the extent of this episode is unknown because the deposits are buried by younger sediment (Figure 6).

Climate within the region is the most probable mechanism driving this largely synchronous deposition on alluvial fans throughout the LRR. This is because climatic conditions are largely the same at any given time over the spatial extent of the LRR. Precipitation and temperature can drive deposition by influencing (1) how much sediment is available and (2) the amount of water available in streams to move that sediment out of the mountains onto the fans. Comparing OSL ages and geomorphic maps for LRR fans to records of past climate for the region (Figure 5) shows that the major deposition between 10-60 ka occurred when climate conditions were cool to very cold (i.e. glacial). A variety of records, including dates for glacier advances, pollen records, water levels in lakes, and loess accumulation, show glacial conditions in the LRR between approximately 12-24 ka, and cooler climate between approximately 24-60 ka (Figure 4). However, it is not clear if these cooler to cold temperatures were also accompanied by increased precipitation. In contrast, climate records show that the comparatively minor deposition from 0-10 ka occurred when climate conditions were generally warmer and drier (Figure 4).

How then, might cooler climate conditions (but not necessarily increased precipitation) enhance deposition on LRR alluvial fans? First, cooler temperatures decrease water losses from evaporation and plant transpiration, increasing the water available in streams to move sediment. In addition, colder temperatures result in longer winters and the accumulation of a deeper snowpack. Snowmelt-driven flows in the summer are then larger and capable of moving more sediment out of contributing basins for deposition on the fans. Though not clear from the available climate records, increased annual precipitation at the same time would further enhance these effects of cold temperatures on streamflow. Second, a cooler climate may enhance sediment production within contributing basins in a variety of ways. One way is by glaciation, as glaciers produce large volumes of sediment. However, only two the five LRR fans studied (Willow Creek and Birch Springs) had significant glaciation within their contributing basin during the late Pleistocene. This suggests that a cold climate can increase sediment production in other ways as well, such as bedrock weathering. Rates of bedrock weathering could be enhanced by the greater availability of soil moisture with less water loss to evaporation or transpiration. Cooler temperatures could also increase sediment production from the bedrock by making frost weathering processes more effective (e.g. Hales and Roering, 2009).

The results from this study show that in the LRR (and perhaps surrounding region), cool to cold climate conditions greatly enhance the production of sediment within alluvial fan-contributing basins as well as the movement of that sediment out of the basins for deposition on the fans. In contrast, warmer climate conditions produce little sediment production or movement, resulting in largely inactive alluvial fans.

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