Advance and Retreat of Grinnell Glacier During the Last Glacial Maximum

ADVANCE AND RETREAT OF GRINNELL GLACIER DURING THE LAST GLACIAL MAXIMUM, YOUNGER DRYAS, AND EARLY HOLOCENE AS RECORDED IN LAKE JOSEPHINE SEDIMENTS, GLACIER NATIONAL PARK, MONTANA A thesis submitted in partial fulfillment of the requirements for graduation with Honors in Geology. ii TABLE OF CONTENTS LIST OF FIGURES............................................................................................................iv ABSTRACT.........................................................................................................................v 1. INTRODUCTION...........................................................................................................1 2. GEOLOGIC BACKGROUND........................................................................................4 2.1 GEOGRAPHIC AND GEOLOGIC SETTING.................................................4 2.2 REGIONAL CLIMATIC AND GLACIAL HISTORY....................................5 3. METHODS......................................................................................................................6 4. RESULTS......................................................................................................................10 4.1 CORE SEDIMENTATION.............................................................................10 4.2 ORGANIC CONTENT, CARBONATE CONTENT, AND BULK DENSITY..............................................................................................................10 5. DISCUSSION................................................................................................................12 5.1 CARBONATE CONTENT AND BULK DENSITY......................................12 5.2 CONSIDERATION OF NON-CARBONATE SEDIMENTS........................13 5.3 CARBONATES AND ORGANIC MATTER................................................15 5.4 SITE-TO-SITE COMPARISONS...................................................................18 CONCLUSION.................................................................................................................20 ACKNOWLEDGMENTS.................................................................................................20 REFERENCES CITED......................................................................................................21 iii LIST OF FIGURES Figure 1. Map of Many Glacier field area...........................................................................3 Figure 2. Map of ice extent during the LGM.......................................................................5 Figure 3. Coring craft...........................................................................................................7 Figure 4. Livingstone corer..................................................................................................7 Figure 5. Core image............................................................................................................8 Figure 6. Organic matter, carbonate minerals, and bulk density v. age.............................11 Figure 7. Bulk density v. carbonate minerals....................................................................14 Figure 8. Organic matter v. carbonate minerals.................................................................16 Figure 9. Site-to-site carbonate comparison......................................................................18 iv ABSTRACT Glacier National Park in Montana is known for its spectacular landscape, carved by valley glaciers. Understanding the complex pattern of glacial advance and retreat may provide important insights into the relationship between climate and glacial cycles in alpine environments, and may help us to better predict the future of such landscapes. In the Grinnell Glacier valley, the remaining glacier sits atop a carbonate formation of the Belt Supergroup. Previous studies of lake sediment cores within the drainage system (MacGregor et al., 2011; Schachtman et al., in press) have found higher concentrations of carbonate sediments in parts of the core inferred to be deposited during times of glacial advance, suggesting that the concentration of carbonate in sediments may be used as a proxy for periods when the Grinnell Glacier advanced. In this study, we tested this hypothesis by measuring carbonate concentrations in sediments deposited during the Pleistocene and Holocene (a period of over 6,000 years) from a glacially fed lake closer to the Grinnell Glacier. Carbonate concentrations in sediments are higher during inferred periods of cooling, such as the Younger Dryas, when the Grinnell Glacier is expected to have been advancing. Conversely, carbonate concentrations decrease during warming periods, such as the Bølling-Allerød and early Holocene. This supports our hypothesis that carbonate concentration may be used as proxy for glacial extent within the Grinnell Glacier valley. v 1. INTRODUCTION Determining the timing, magnitude, and spatial extent of climate shifts in the late Quaternary is an essential component of understanding controls on climate and predicting future climate change. Glacial extent is directly related to climate and can thus serve as a primary indicator of climate change (Mayewski et al., 2004). Small alpine glaciers respond to climate variability with time lags on the order of decades, making them particularly useful for characterizing these fluctuations (Jóhannesson et al., 1989). An improved understanding of alpine glacier dynamics may therefore offer insight into paleoclimate variability. Because glaciers significantly affect their landscape, glacial fluctuations are often well-recorded. Records of glacial fluctuations have been derived from a variety of sources including landforms, tree rings, and lacustrine sediments. Moraines and tree rings can provide well-constrained estimates of maximum glacial extent and timing of glacial advance, respectively. However, because of the tendency of moraines to be overridden during subsequent glaciations, in most cases only moraines of the most extensive and most recent advances are preserved (Gibbons et al., 1984). Tree ring records are also limiting in that glacial maxima and intervals of retreat are not represented (Carrara and McGimsey, 1981). Lake sediments can preserve a high-resolution record of glacial fluctuations because glacial activity exerts a first-order control on the character of sedimentation in downstream lakes (Karlén, 1981). Because of this, lake sediments have provided some of the most complete records of glacial dynamics through the Holocene (e.g., Leonard and Reasoner, 1999; Munroe et al., 2012). Pristine, glaciated, sub-alpine 1 environments, such as national parks throughout the North American Rockies, provide ideal localities from which we can extract such records. Previous work on lake sediment cores in the Grinnell Glacier Valley in Glacier National Park has suggested that carbonate concentrations in lake sediments can be used as a proxy for increased activity of the Grinnell Glacier due to the presence of dolomitic limestone from the Helena Formation (Fig.1) in contact with the glacier (MacGregor et al., 2011; Schachtman et al., in press). During periods of expected glacial advance, increased erosion of the Helena Formation and greater hydrologic energy resulted in delivery of greater amounts of carbonate to lakes within the drainage. In addition, the removal of upstream lakes as sediment sinks, and closer terminus position of the glacier, enhanced carbonate delivery. However, this previous work focused on cores only from Swiftcurrent Lake, the lowest in a series of four lakes in the drainage system of the Grinnell Glacier Valley (Fig. 1). The sedimentary records of three upstream lakes (Upper Grinnell Lake, Lower Grinnell Lake, and Lake Josephine) that act as sediment sinks have not been investigated for the late Pleistocene, when the Grinnell Glacier was certainly larger than it is today. In order to better use carbonate as a tracer to constrain the timing of glacial fluctuations in the valley, we focus on the sedimentary record in an upstream lake to gain an understanding of how carbonate moves through the system during times of glacial advance and retreat. 2 Figure 1. Map of the Many Glacier region of Glacier National Park, Montana, including the Grinnell Glacier and its upper drainage system. The lake coring site for this study (GNPJOS14-2A) and two others addressed in this research (GNP-JOS14-1B, GNP-SWF10-3D) are indicated along with the extent of the Helena Formation (the primary source of carbonate sediment in the cores). 3 In this study, we quantified detrital carbonate concentrations in a sediment core from Lake Josephine to test the hypothesis that carbonate concentrations in the lake sediments are a proxy for the size and erosional footprint of the Grinnell Glacier. We are focusing on a time period between ~13,500 and 7,600 ybp, spanning the end of the Last Glacial Maximum, the Younger Dryas, and the beginning of the Holocene when northern hemisphere glaciers and ice sheets were advancing and retreating in response to climate change; this time period is bracketed by volcanic ash units of known age. Because Lake Josephine is farther upvalley than Swiftcurrent Lake, with one fewer lake acting as a sediment sink, we expect carbonate to be more abundant in Josephine and for it to record glacial behavior more sensitively than does Swiftcurrent Lake (Schachtman et al., in press). 2. GEOLOGIC BACKGROUND 2.1 Geographic and Geologic Setting The Many Glacier region of Glacier National Park is situated in northern Montana, just east of the Continental Divide (Fig. 1). Grinnell Glacier terminates in Upper Grinnell Lake (1975 m above sea level, ASL), which drains into Lower Grinnell Lake (1510 m ASL), Lake Josephine (1490 m ASL), and Swiftcurrent Lake (1485 m ASL) successively. The drainage basin is underlain by the Middle Proterozoic Belt Supergroup, which is comprised primarily of sandstones, siltstones, and shales (Whipple, 1992). Grinnell Glacier sits on the Helena Formation, which includes dolomitic limestone, stromatolitic limestone, and calcitic argillite, and is likely the primary bedrock source of carbonate in the valley (Whipple, 1992; MacGregor et al., 2011) (Fig.1). 4 2.2 Regional Climatic and Glacial History During the Last Glacial Maximum (LGM) (~ 20 ka), much of northwest Montana including Glacier National Park, was covered by extensive icefields and valley glaciers that covered the present day Livingston and Lewis mountain ranges (Carrara, 1987). West of the Continental divide these glaciers merged with the Flathead Lobe of the Cordilleran Ice Sheet (Fig. 2). East of the divide, valley glaciers flowed out onto the plains of Montana but appear not to have merged with the Shelby Lobe of the Laurentide Ice Sheet, which lay farther east (Carrara, 1987). Following the LGM, ice sheets Figure 2. Generalized map of maximum ice extent during the LGM in Northwestern Montana including the Cordilleran Ice Sheet, the Laurentide Ice Sheet, and icefields and valley glaciers that existed between the two (Carrara, 1986). 5 retreated and the region experienced warmer, drier summers and wetter winters, resulting in the retreat of valley glaciers toward the end of the Pleistocene (Bartlein et al., 1998). In the Northern Rockies, this retreat has been shown to be complex and non-uniform (Licciardi et al., 2004; Young et al., 2011). However, deposits of Glacier Peak G ash (erupted from Glacier Peak in Washington ~13.5 ka) found on 10 different postglacial deposits throughout Glacier National Park indicate that by the end of the Pleistocene, substantial de-glaciation had occurred and the remaining glaciers in the area were confined to local valleys (Carrara, 1987; Mehringer et al., 1984). The Younger Dryas (12.9 ka - 11.6 ka) was a period of intense cooling in the northern hemisphere that resulted in glacial advance throughout the Northern Rockies (Reasoner et al., 1994; Clark and Gillespie, 1997). In the western part of Glacier National Park, lake sediment records and moraines proximal to modern glaciers indicate that glaciers in the park experienced advance during the Younger Dryas (MacLeod et al., 2006). Following this period of cooling and advance, climate warmed and stabilized during the early Holocene, leading to variable but limited glacial activity in the region (Beierle et al., 2003; Reasoner and Huber, 1999). 3. METHODS Core GNP-JOS14-2A was collected from a floating platform (Fig. 3) in July 2014 using the Livingstone-type (Fig. 4) and Bolivia-type coring techniques (Wright, 1967, 1991). This core was collected downvalley from another in Lake Josephine (JOS14-1B) in order to assess sediment transport within the lake. The core was logged, split, digitally photographed, and described using smear slides (Rothwell, 1989) at the National 6 Lacustrine Core Facility (LacCore), University of Minnesota. Bulk density, magnetic susceptibility, and color reflectance analyses were performed at the lab. The composite core (created from four separate overlapping core sections) was then subsampled at 1 cm depth increments between 428 and 693 cm below the sediment-water interface (Fig. 5). 2 1 Figure 3. Pontoon coring craft with metal casings (1) used to guide coring equipment and center opening between boards (2) through which cores were taken. Photo Credit: Karly Clippinger Figure 4. Disassembled Livingston corer consisting of a head (1), square rod (2), barrel (3), piston (4), and threaded connector (5) that attaches the square rod to drive rods (not pictured). Photo Credit: Amy Myrbo 7 Mazama Ash Glacier Peak G Ash Figure 5. Image of the four core sections from Lake Josephine (GNP-JOS14-2A). The core progresses from the top of section A (~420 cm) to the bottom of section D (~694 cm) The analyzed section spans from just below the homogenous tan Mazama ash at 428 cm to just above the thin white Glacier Peak G ash at 693 cm. 8 This section of the core was chosen for analysis for two reasons. First, the section is bracketed by the Glacier Peak G ash (13.71 ka - 13.41 ka; Kuehn et al., 2009) and the Mazama ash (7.63 ka +/- 150 years; Zdanowicz et al., 1999); these tephras constrain our age estimates for the section in the absence of radiocarbon ages. Additionally, this period spans a climatically well-documented sequence of events: the Bølling-Allerød (a period of warming and regional glacial retreat), the Younger Dryas (a cold period of abrupt onset when the glacier advanced; Schachtman et al., in press), and warming in the Holocene (when Grinnell Glacier was likely smaller or non-existent; Carrara, 1987). This high degree of variability in climatic conditions is important to our understanding of how carbonate concentration is tied to glacier dynamics. Using dates of 7630 ybp for the Mazama ash and 13,500 ybp for the Glacier Peak G ash and assuming a constant rate of sedimentation, we created a simple age-depth model for the core. The 1-cm resolution subsamples from this section were analyzed at Whitman College for organic matter and carbonate mineral content using loss on ignition (LOI; Heiri et al., 2001). Percent organic matter and percent carbonate minerals were measured every centimeter using a Lindberg/Blue M 1100 Box Furnace. Samples were dried at 100º Celsius, and then heated at 550ºC and 1000ºC to incinerate organic matter and carbonate minerals respectively. Samples were weighed between each burn and percentages of the two components were calculated using a Microsoft Excel macro developed at LacCore. This analysis was carried out in accordance with the standard operating procedure for LOI used at LacCore. Qualitative XRD analysis was performed on 6 cm of the core on an Agilent Xcalibur Nova X-ray Diffractometer. Samples were ground in acetone and, once 9 homogenized, were further sub-sampled and mounted in oil on micromesh. Jade 2010 was used to analyze the XRD results and outputs were compared to Jade's minerals database for mineral identification. 4. RESULTS 4.1 Core sedimentation From our age model, we estimated a sedimentation rate of ~0.4 mm/yr in Lake Josephine between 13.5 and 7.6 ka. XRD analysis confirmed the presence of quartz, calcite, dolomite, and a variety of clay minerals in the core sediments, in addition to organic carbon identified in the field. 4.2 Organic Content, Carbonate Content, and Bulk Density Percent organic matter, percent carbonate, and bulk density measurements are shown in Figure 6. Percent organic matter ranges from 1.4% to 8.9% (Fig. 6a), averaging 4.2%; percent carbonate varies between 1.4% and 37% (Fig. 6b), averaging 9.8%; and density ranges between 1.2 and 2.0 g/cm3 (Fig. 6c), averaging 1.6 g/cm3. We divide these data into four zones corresponding to significant changes in percent carbonate values. In Zone 1, carbonate decreases, with a low of 1.8% at about 12.6 ka (Fig. 6b). Organic carbon is consistently around 3%, but increases slightly between 13.5 ka and 13.3 ka (Fig. 6a). Bulk density decreases during this period, though less dramatically than carbonate, reaching a minimum of 1.4 g/cm3 around 12.5 ka (Fig. 6c). In Zone 2, carbonate increases to a maximum of 37% with several prominent peaks (Fig. 6b). Organic carbon peaks at 7.4% and 8.9% near the beginning of this zone, 10 but thereafter fluctuates around 3% (Fig. 6a). Bulk density increases during this period similar to increases observed in carbonate (Fig. 6c). Variability in bulk density aligns with percent carbonate except for a dramatic dip at 12.2 ka (Fig. 6). In Zone 3, carbonate drops substantially from the maximum reached in Zone 2, peaking around 10.1 ka (Fig. 6b). Average organic matter increases 2-3% between about Figure 6. (a) Organic content (%), (b) carbonate content (%), and (c) bulk density (g/cm3) from core GNP-JOS14-2A. Mazama and Glacier Peak G ashes are identified by lines at 7600 YBP and 13,500 YBP respectively. The graph is divided up into four zones based on significant trend changes in carbonate values. Estimated ages assume a constant rate of sedimentation (~0.4mm/yr). 11 11 ka and 10 ka (Fig. 6a), and bulk density decreases during this period (Fig. 6c). In Zone 4, carbonate decreases from about 5% to about 3% over time, with little variability (Fig. 6b). In contrast, organic carbon is highly variable, with a slight decrease over time from an average of 6% to 5% (Fig. 6a). Average bulk density decreases slightly during this period, from 1.5 to 1.45 g/cm3 (Fig. 6c). 5. DISCUSSION 5.1 Carbonate and Bulk Density Decreasing carbonate values in Zone 1 (Fig. 6b) may correspond to the BøllingAllerød, a period from 14.7 ka - 12.7 ka of northern hemispheric warming in the late Pleistocene following the Last Glacial Maximum (Yu and Eicher, 2001; Bartlein et al., 1998). Increasing carbonate in Zone 2 (Fig. 6b) may overlap with the Younger Dryas - a period of known cooling and glacial advance in the region that lasted from about 12.8 11.5 ka (Muscheler et al., 2008). Finally, Zones 3 and 4 (Fig. 6b) line up with a period of warming following the Younger Dryas and climate stabilization into the Holocene (Beierle et al., 2003; Reasoner and Huber, 1999). This suggests that percentage of carbonate in Lake Josephine sediments reflects the position and extent of the Grinnell Glacier during the late Pleistocene and early Holocene. This claim is further supported by the fluctuations seen in bulk density that match fluctuations in carbonate content (Fig. 7). Besides the dramatic dip in bulk density at 12.2 ka (likely due to partial sediment loss at the end of a core segment that occurred during extraction), fluctuations in both variables are similar through all four Zones (Fig. 7). During periods of expected glacial advance, an increase in glacial flour production, along with an increase in summer melt, could increase the delivery of very fine-grained sediment to ice-proximal lakes, which 12 results in an increase in sediment density due to more efficient compaction of this very fine sediment (Munroe, 2012). Bulk density may therefore also be used as a proxy for Grinnell Glacier extent. The lag in percent carbonate and bulk density increase relative to the onset of the Younger Dryas may be explained by a later onset of Younger Dryas cooling in this area, a lag in glacial response to the onset of cooling, or uncertainties in our age model. 5.2 Consideration of non-carbonate sediments A more expansive glacier may be expected to deliver a sediment load with decreased carbonate concentrations due to its coverage and erosion of non-carbonate bearing bedrock units (those below the Helena Formation). However, evidence to the contrary observed in the relationship between carbonate concentration and times of expected glacial advance may be explained by a combination of several factors. First, the glacier may erode a sufficiently large area of the Helena Formation to overwhelm the volumes of other detrital sediments that are eroded. Second, argillite located below the Helena Formation and exposed downvalley of the cirque may not be as efficiently eroded by the glacier due to the dip of its bedding in the opposite direction of ice flow. Third, as carbonates from the Helena Formation are transported the greatest distance of eroded sediments in the valley, they likely make up a higher proportion of silt-sized sediments that settle in the lower-energy lacustrine environment of Lake Josephine. 13 Figure 7. Bulk density (g/cm3) vs. carbonate content (%) from 13.5 ka to 7.6 ka. A positive correlation (y = 37.755x - 50.139) between these two variables suggests that bulk density may also be used as a proxy for glacial extent. 14 5.3 Carbonates and Organic Matter Comparison of carbonate and organic matter records (Fig. 8) reveals important information about the nature of sedimentation in the lake. During the late Pleistocene (Zone 1), average organic matter was lower than during the early Holocene (Zone 4), which may reflect decreased organic production during colder time periods in the region. This hypothesis is supported by previous work showing a correlation between solar forcing and percent organic matter in Swiftcurrent Lake between 7.6 ka and the present (MacGregor et al., 2011). However, other work has shown that organic matter decreases in alpine lakes during periods of ice advance because clastic sediment flux increases, effectively ‘swamping’ the organic content (e.g., Leonard and Reasoner, 1999; Rodbell et al., 2008; Schacthman et al., in press). Additionally, increased clastic sediment flux reduces clarity in the water, and thus organic production due to decreased solar radiation through the water column. 15 Figure 8. Organic content (%) vs. carbonate content (%) from 13.5 ka to 7.6 ka. Points from the late Pleistocene (red) are relatively spread out and not correlated. Points from the early Holocene (blue) show a negative correlation (y= -0.8857x + 9.6945) between % organics and % carbonate. 16 The relationship between carbonate and organic matter shifts from non-correlated through the late Pleistocene and Younger Dryas (scattered red data points, Fig. 8), to a negatively-correlated relationship in the early Holocene (clustered blue data points, Fig. 8). This pattern suggests that increased sediment flux (reflected in increased percent carbonate) may be diluting organic matter during dynamic periods of glacial advance and retreat, leading to the non-correlated relationship in the Pleistocene. In contrast, during periods of climate stability and decreased sediment flux (such as the Holocene), a distinct negative correlation between organic carbon and carbonate indicates that during times of less dramatic sediment flux, smaller fluctuations in climate can be correlated with trends in carbonate versus organics: cooler periods yield higher carbonate (due to increased erosion and sediment transport) and lower organic matter (due to colder temperatures). Further age constraints and mass accumulation rates are needed to refine the relationship between organic carbon and carbonate and determine if clastic swamping controls organic matter during cooler periods. 17 Figure 9. Carbonate content (%) in the Josephine Upvalley site (GNP-JOS14-1B), the Josephine Downvalley site (GNP-JOS14-2A), and the Swiftcurrent site (GNP-SWF103D). While values are variable between GNP-JOS14-2A and GNP-JOS13-1B, they follow the same general trends. Percent carbonate values in GNP-SWF10-3D are much lower. However, the primary time frame when an inorganic carbon signal is picked up (12.7 ka 11.3 ka) corresponds to the increase in % carbonate seen in the other two records. 5.4 Site-to-site Comparisons Comparison of carbonate between this site (GNP-JOS14-2 (Site 2)), an upstream site in Josephine (GNP-JOS14-1 (Site 1); Clippinger, unpublished), and a third in downstream Swiftcurrent Lake (GNP-SWF10-3 (Site 3); Schachtman et al., in press) (Fig. 9), provides further support for carbonate content as a proxy for glacial fluctuations. Sedimentation rates vary greatly between the two sites in Lake Josephine with a rate of 0.6 mm/yr at Site 1 (more proximal to the source) compared to 0.4 mm/yr at Site 2. While these sites have different amounts of carbonate and varying magnitudes 18 of fluctuation in carbonate, the fluctuations generally follow the same trends temporally, despite different rates of sedimentation. The overall higher carbonate concentrations between 13.7 and 10.2 ka (Zones 1-3; late Pleistocene) at Site 2 are initially surprising, considering Site 2 is more distal to the sediment source than Site 1. We suspect that fine-grained carbonate abraded from the cirque of the Grinnell Glacier is deposited in downvalley Lake Josephine, whereas coarser sediments have settled in Lower Grinnell Lake. Because of this, we might expect that during times when the glacier was closer to or terminating in Lake Josephine (13.5 ka - 10.5 ka), higher hydrologic energy from increased meltwater might have swept finer sediments farther out into the lake (toward Site 2) before they settled, while coarser sediments settled out at Site 1. Shallower conditions at Site 1 may have also contributed to finer carbonates being swept downstream by meltwater currents. Higher values at Site 1 during the early Holocene could be explained by a substantially smaller and more distal glacier with lower rates of erosion, sediment production, and lower meltwater discharge, yielding calmer conditions more conducive to carbonate settling at this site. Low carbonate concentrations in Swiftcurrent Lake compared to Lake Josephine reflect the efficient sediment-trapping ability of Lake Josephine. However, even in the Swiftcurrent record, there are carbonate concentrations as high as 1.5% during the Younger Dryas, when carbonate increases at both sites in Lake Josephine. While the values in Swiftcurrent Lake increase slightly earlier than in Lake Josephine, this can likely be explained by variance in sedimentation rates that increase uncertainties in both age models during this time period. Uncertainties in age models based on variable sedimentation rates between the two sites in Lake Josephine may also help to explain the 19 offset of spikes in carbonate concentrations (such as the two seen in both records between 9.5 ka and 10.5 ka) that are likely representative of the same carbonate influx events or periods. 6. CONCLUSION Analysis of lacustrine sediments from Lake Josephine in the Grinnell Glacier Valley reveals that carbonate concentrations are a good proxy for glacial extent within the valley during the late Pleistocene and early Holocene. Carbonate percentages in sediments decrease in concert with warming trends during the late Pleistocene (when we would expect the glacier to be receding), increase throughout the Younger Dryas cooling (when we would expect the glacier to be advancing), and decrease and stabilize into the early Holocene when climate warmed and stabilized. Similar trends in bulk density support this conclusion. Additionally, non-correlated inorganic carbon versus organic carbon concentrations during the Pleistocene contrasted with negatively-correlated concentrations in the Holocene indicate that the organic signal may be swamped during periods of increased sediment flux associated with glacial advance. ACKNOWLEDGMENTS Thank you to the Keck Geology Consortium for this opportunity, Nick Bader and Bob Carson for their guidance and advice while researching and writing, Doug Juers for his help on XRD analysis, my exceptional field team for their tenacity and good humor, Lucy Andrews for all the delicious meals and moral support, Amy Myrbo and Jessica Rodysill for their leadership on the lake and in the lab, Kelly MacGregor for her endless support, enthusiasm, and guidance, and wolverines for being who they are. 20 REFERENCES CITED Bartlein, P.J., Anderson, K.H., Anderson, P.M., Edwards, M.E., Mock, C.J., Thompson, R.S., Webb, R.S., Webb III, T., Whitlock, C., 1998, Paleoclimate simulations for North America over the past 21,000 years: Features of the simulated climate and comparisons with paleoenvironmental data: Quaternary Science Reviews, v. 17, p. 549-585. Beierle, B. D., Smith, D. G., Hills, L. 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