4.1. Common Features of the GPR Profiles
Interpretation of the GPR transect collected across shorelines of pluvial Lake Clover reveals several features that are consistent between many, if not all, of the ridges. First, all ridges are comprised of convex-upward sedimentary packages sitting atop a lakeward-dipping, high-amplitude reflector. Many hand excavations into the crests of individual beach ridges encountered a carbonate-cemented horizon at depth; at four sites the depth of this horizon was used to determine the dielectric permittivity of these materials (Figure 5
). The apparent slope of this reflector is 0.0126 (dropping 5.5 m over 437 m), matching the gradient of the alluvial fan surface upslope from the highest shoreline. This correspondence suggests that the reflector is the original land surface upon which the shorelines were constructed, or a layer of caliche that formed within the soil profile on the former landscape. Beach ridges are clearly resolved as accumulations of sand and gravel concentrated by wave action at the point where the waters of Lake Clover met this pre-lake landscape.
Second, all of the GPR profiles contain packages of lakeward-dipping strata beneath the former beach face. On the basis of observations made in hand excavations into the beach ridge sediments, these are likely layers of gravelly sand interbedded with more sand-dominated sediment. Together these materials are typical of a wave-washed, high-energy environment and are similar to those reported from other pluvial lake shorelines in the Great Basin [11
Third, some GPR profiles reveal packages of landward dipping reflectors beneath the landward side of the beach crest. These are presumably overwash deposits emplaced by storm waves that overran the beach crest and transported sediment to the landward side of the shoreline [18
Fourth, no beach ridges contain evidence of erosional unconformities or depositional hiatuses that would suggest submergence by later high water after ridge formation. Although this is negative evidence, it is consistent with sequential ridge formation during overall lake regression.
Finally, many of the ridges contain, at their core, a package of sediment distinguished by opposing lakeward and shoreward-dipping reflectors. In cross section, these packages mimic the overall convex-upward shape of the larger beach ridge and appear to be proto-beach berms that were buried by later sedimentation [18
]. In all cases, the crests of these buried berms are located landward from the crest of the overall beach ridge, indicating a lakeward shift in focused sediment deposition during beach ridge growth.
4.2. Model for Beach Ridge Construction
Studies of beaches built by other pluvial lakes, as well as the modern Laurentian Great Lakes, support a model of the events responsible for the construction of beach ridges like those produced by Lake Clover. Formation of a beach ridge as a preservable landform with positive relief is dependent on the rate of change of water level and the availability of sediment in the near-shore system [24
]. In the case of Lake Clover, it is unlikely that sediment supply varied significantly during the episode of beach construction; there are no lakes upstream that would have altered the amount of sediment reaching Lake Clover, glaciers in the Lake Clover watershed were restricted in extent, and no large rivers entered the lake that could have impacted the delivery of sediment to the near-shore system. Instead, variations in the rate of water level change were likely the more important variable in the formation of these beaches.
When the rate of change of water level is zero (or near zero) and the water level is stable (or nearly stable), then the repeated action of waves will winnow fines and concentrate coarse sediment at the water’s edge [25
]. Slow rise of the water level, matched by an increase in sediment supply, will cause this shoreline sediment to aggrade vertically forming a beach ridge. However, if the water level rises too quickly, then the beach ridge will be submerged, and a new ridge will be built at a higher elevation farther up the slope. In contrast, continued stability or slow rates of water level fall result in progradation of the shoreline in a lakeward direction as additional material is added incrementally to the beach face [18
]. More rapid rates of water level fall can exceed the ability of the waves to concentrate material at a particular location. Beach sediment will, therefore, be distributed uniformly across the slope as the water retreats to a lower elevation. If the water level stabilizes again, a new beach ridge will be constructed farther downslope [25
This general model of beach ridge formation fits neatly with the evidence from Lake Clover. The buried berms imaged at the core of many of the beach ridges represent the first concentration of material produced when the regressing lake paused at a particular elevation (or, in the case of the highstand shoreline, when that elevation was first reached). Stability of the water level, or even slight water level rise, drove slight upward aggradation that buried these original berms. Overwash resulting from particularly large storm waves further buried these berms by transporting material up and over to the landward side. Continued water level stability, or perhaps water level fall at a slow rate, caused the shoreline to prograde lakeward, building the series of lakeward-dipping reflectors beneath the beach face that were observed in all shorelines [18
]. Eventually, accelerating water level fall, perhaps combined with changes in the availability of sediment, terminated formation of a particular shoreline as the water level descended faster than shoreline construction could proceed. Resumed stability (or a reduced rate of regression) of the water level at a new, lower elevation, then initiated another cycle of (1) initial berm construction, (2) aggradation and burial, (3) followed by lakeward progradation, and 4) eventual abandonment. The sequence of shorelines seen in the study area indicates that this overall pattern was repeated multiple times during the regression of Lake Clover.
Exactly how long the water level must remain stable in order to construct a preservable shoreline at a certain elevation is unclear. However, it is worth noting that beach berms were built by the modern Great Salt Lake in Utah in response to just a few years of high water in the 1980s [26
]. Thus, the Lake Clover beach berms do not necessarily imply stability of lake level over long (>>~101
year) time intervals.
4.3. Implications for Relative Age and Paleoclimate Conditions during Beach Ridge Formation
The beach ridges built by pluvial Lake Clover are unequivocal evidence for profound hydroclimate change. As such, determining when they were constructed, and how hydroclimate conditions evolved during the episode of beach formation, are important objectives in better understanding the paleoclimate history of this region. As noted above, the internal stratigraphy of the ridges, combined with the lack of evidence for submersion and erosion, are best explained by ridge formation in response to monotonic lake regression. In other words, in terms of relative age, Lake Clover built its highest shoreline first, then constructed sequentially lower shorelines step by step until the lake disappeared or became too small to support the wave energy necessary to construct preservable shoreline landforms. This model implies that the overall peak in effective moisture responsible for the Lake Clover highstand was followed by a decrease in effective moisture and increasing aridity that shrank the lake before eliminating it entirely.
Absolute age control helps to put this history of Lake Clover into a chronologic context. Five radiocarbon dates are available for gastropod shells obtained from two different locations along the highstand shoreline of Lake Clover. These ages fall into two non-overlapping clusters, one with calibrated ages ca. 19.5 ka and another ca. 17 ka BP [20
]. The simplest, although surprising, interpretation of these ages is that the lake coincidentally reached its highstand elevation twice during the last glacial cycle; the highstand shoreline, therefore, is a compound feature. On the other hand, no evidence was noted in the GPR profile across the highstand ridge to suggest that this feature was built in stages by separate episodes of high water (Figure 7
). A buried berm is present at the core of the highstand beach ridge, and it is possible that this feature represents the older episode of highwater at this elevation. However, buried berms are present at the core of several other ridges, and it is unlikely that they are all compound features. The gastropod shells yielding the two clusters of radiocarbon ages were collected from different sites, and all of the older samples came from a greater depth below the surface. This correspondence raises the possibility that the older shells were obtained from sediments that stratigraphically pre-date the ridge built at 17 ka BP. Future GPR work focused on the location where these radiocarbon ages were obtained could clarify whether the older ages reflect an older landform buried beneath the highstand shoreline.
Other information about the absolute ages of the Lake Clover shorelines is provided by luminescence dating of quartz and K-feldspar sand grains collected from pits excavated into beach ridge crests in the immediate vicinity of the GPR transects [19
] and Munroe et al. in review. These results confirm an age of 16–17 ka BP for the highstand ridge, document regression of the lake over several thousand years in the latest Pleistocene, and reveal that the lowest shoreline was constructed in the earliest Holocene.
Previous work has proposed a variety of mechanisms for the increased effective moisture in the Great Basin that drove pluvial lakes to their late Pleistocene highstands. Whether this moisture was a response to southward deflection of the prevailing storm track by the Laurentide Ice Sheet [28
], steering of storms inland from the Pacific [31
], or even a response to northward moisture transport [32
], remains a topic of debate. What is clear, however, is the synchrony of many lake highstands across the entire latitudinal sweep of the Great Basin with Heinrich Stadial I (H1) in the North Atlantic region [20
]. H1 was a massive release of icebergs from the Laurentide Ice Sheet, which drove dramatic millennial-scale climate changes of at least hemispheric extent [33
]. Melting of these icebergs apparently altered ocean currents in the North Atlantic [35
], leading to cooling of near-surface waters [36
] and tremendous sea-ice expansion [37
]. Farther afield, these changes impacted the average position of the Aleutian Low in the northern Pacific [38
], which helped to steer moisture into the southwestern US [39
]. As a result, effective moisture increased rapidly in the Great Basin in response to H1, and pluvial lakes quickly expanded to larger surface areas in equilibrium with a wetter climate.
Together the combination of luminescence age results with the GPR profiles reveals just how unusual the moisture pulse associated with H1 was: after spiking rapidly to its highstand elevation, Lake Clover steadily regressed as the climate transitioned into the aridity that characterized the Holocene. Aside from the somewhat equivocal cluster of older radiocarbon dates, there is no stratigraphic, geophysical, or geomorphic evidence to suggest that Lake Clover was near its highstand elevation multiple times. There is evidence that some pluvial lakes were high during other glacial cycles of the Pleistocene [4
], however these may also have been driven by teleconnections with the North Atlantic [41
]. Either way, the late Pleistocene highstand of Lake Clover and the lower shorelines are, therefore, not the result of repeated climatic fluctuations that alternatingly drove the lake to higher and lower elevations, building, submerging, and re-exposing beach ridges during multiple transgressions and regressions. Rather, the rise of the lake to its highstand was an unusual, perhaps singular event (at least in the context of the last glacial cycle), that was followed by a long interval of lake regression.
At the same time, the presence of shorelines below the highstand ridge does indicate that the rate of water-level lowering slowed or stopped multiple times during overall regression. One possible explanation for that behavior is that the overall transition toward greater aridity after Heinrich Stadial I was not steady, and that conditions occasionally stabilized long enough for water balance of the lake to come into equilibrium. Support for this theory comes from luminescence ages indicating that after regressing rapidly into the warm Bølling/Allerød interval, Lake Clover built some of its lower shorelines during the cooler Younger Dryas stadial [19
] and Munroe et al. in review.
On the other hand, the presence of shorelines does not necessarily require that a climatic change temporarily stalled the desiccation of the lake and held the water level at a constant elevation. Figure 16
illustrates that that the area-altitude distribution beneath the highstand beach ridge is not uniform. Rather, there are inflections and irregularities that would have altered the rate at which the water level was falling even if lake area was steadily decreasing. Therefore, even if the reduction of the lake surface area proceeded at a constant rate in response to climate forcing, the rate of water level fall could still have varied due to the hypsometry of the basin.
This analysis is furthered by determining the vertical lowering of the water surface that would have accompanied each increment of area loss (in vertical m/1% of total lake area). In Figure 17
the result of this calculation is presented, and the position of the mapped shorelines is highlighted. This analysis is somewhat circular, since the hypsometry of the basin is calculated from the modern topography where the shorelines are present. Nonetheless, most beach ridges are clearly associated with reduced rates of water-level fall. Local accelerations in the rate of water-level lowering, reflecting elevation bands through which the water-level would have lowered more rapidly, tend to occur between the preserved shorelines. The salient conclusion is that recessional shorelines could have been constructed at times when the rate of water-level lowering slowed in response to the overall shape of the lake basin. The presence of these shorelines may certainly reflect a climatic cause, but this is not the only possible explanation.
4.4. Limitations and Directions for Further Research
The GPR data collected in this project provide valuable information about the subsurface stratigraphy of these beach ridges, information that could otherwise be attained only through expensive excavations. Nonetheless, there are some clear limitations to these data that should be considered, some of which provide useful guidelines for planning similar projects in the future.
One limitation is that the depth estimates for contacts and other features seen in the GPR data are entirely dependent on the assumed value of dielectric permittivity. This value was derived from GPR data acquired over prominent stratigraphic contacts with known depths at a few representative locations (Figure 5
), so it is likely reasonable for this study area. However, because the properties of the surveyed sediments vary along each GPR transect, ranging from dense silt to loose gravelly sand, application of a single dielectric value for all the GPR data introduces error into the depth estimates. Future work, perhaps involving mechanical augering to add ground truth from other locations, could help refine the assumed dielectric value. If it is demonstrated that the dielectric permittivity varies greatly between the silty sediments and the material comprising the ridges, then it might be better to present each transect in subsections, each with its own dielectric value, rather than applying a single value to the entire dataset.
Second, the GPR data are inherently 2-dimensional, because they were collected as transects oriented roughly normal to the beach ridges. True dip of subsurface reflectors cannot be determined from a single 2-D cross section. As a result, the direction of dip (for instance, lakeward) is constrainable, but the actual magnitude of the dip is not. Future work could employ 3-D techniques [16
] to gain additional understanding of how inclined packages of sand and gravel are arranged beneath a beach face.
Finally, a familiar challenge in GPR studies is the reality that the central frequency of the selected antenna controls the depth of EM penetration and the scale of resolvable features in inverse ways [44
]. The 400-Mz antenna utilized in this study was adequate for resolving near-surface features at scales down to ~10 cm, but penetration to >3 m was minimal. Future work could employ additional antenna frequencies, for instance a lower frequency that might be able to resolve stratigraphy at greater depths, or a higher frequency that could reveal finer-scale details of the sedimentary architecture within each ridge crest.