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Late Holocene dune activity in the Eastern Platte River Valley, Nebraska
Geomorphology 103 (2009) 555–561
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h
Late Holocene dune activity in the Eastern Platte River Valley, Nebraska P.R. Hanson a,⁎, R.M. Joeckel a,b, A.R. Young a, J. Horn b a b
University of Nebraska-Lincoln, School of Natural Resources, Hardin Hall, Lincoln, Nebraska 68583-0996, USA University of Nebraska-Lincoln, Department of Geosciences, USA
a r t i c l e
i n f o
Article history: Received 21 December 2007 Received in revised form 30 July 2008 Accepted 31 July 2008 Available online 8 August 2008 Keywords: Late Holocene Eolian activity Drought Optical dating Platte River Loup River
a b s t r a c t Large-scale dune activity in the Nebraska Sand Hills and elsewhere on the western Great Plains has been linked to prehistoric “megadroughts” that triggered the activation of regional dune fields. The effect of megadroughts on the smaller dune fields east of the Nebraska Sand Hills has never been assessed, however. This study focuses on the Duncan dune field near the confluence of the Loup and Platte rivers in eastern Nebraska. Seventeen optically stimulated luminescence age estimates were obtained and reveal two periods of dune activation that occurred between 4.4 to 3.4 ka and 0.8 to 0.5 ka. Significantly, both periods chronologically overlap large-scale dune activity identified in the Nebraska Sand Hills. Geochemical evidence indicates that the Duncan dunes received sand not only from the terrace underlying them, but also from the Loup River. These data link dune activity in the Duncan area, at least indirectly, to increased sediment supply from streams that drain the Sand Hills during megadroughts, implying the activation of the dunes occurred as an indirect response to regional megadroughts. Calculations of dune migration rates, however, argue in favor of local, drought-driven hydrologic changes as a causative factor in dune activation, in other words, a direct effect of megadroughts. Whether the impact was direct or indirect, it is highly likely that the repeated reactivation of the Duncan dunes resulted in some way from regional, large-magnitude droughts. Other paleoclimate proxies from the Great Plains tend to support this conclusion. We conclude that the megadroughts that have been identified in the Sand Hills and other Great Plains dune fields were indeed regional events with far-reaching effects. Published by Elsevier B.V.
1. Introduction Holocene dune sands and loess provide important information about large-magnitude prehistoric drought events (‘megadroughts’) on the central Great Plains (Muhs et al., 1997; Stokes and Swinehart, 1997; Muhs and Zárate, 2001; Forman et al., 2001; Goble et al., 2004; Mason et al., 2004; Sridhar et al., 2006; Miao et al., 2007). Megadroughts stand out in the regional climate record as major shifts in the states of biological and geological systems, particularly dune fields, in which changes in the availability of water determine the nature of vegetational cover and therefore the relative stability of dunes. Indeed, megadroughts could ultimately be characteristic features of the long-term climate on the North American Plains. Recent research on megadroughts and dune activation has focused on areas west of the 99th Meridian (Fig. 1). Miao et al. (2007), for example, interpreted four periods of increased dune activity between approximately 9 and 6.6 ka, and centered around 3.8 ± 0.3, 2.5 ± 0.1, and 0.67 ± 0.1 ka (ages here and throughout the text are given in calendar years) in the Nebraska Sand Hills. The youngest of the events identified by Miao et al. (2007) corresponds to periods of drought
⁎ Corresponding author. E-mail address: [email protected] (P.R. Hanson). 0169-555X/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.geomorph.2008.07.018
and dune activation across the central Great Plains, including northeastern Colorado (Clarke and Rendell, 2003), and the Great Bend Sand Prairie in Kansas (Arbogast, 1996). The identification of widespread megadrought effects has potentially dramatic implications in understanding the chronology of dune activity on the Great Plains. Nonetheless, the full geographic extent of the megadroughts' effects, particularly east of the Nebraska Sand Hills, remains unknown, and even the most basic aspects of Holocene dunes at the eastern margin of the Great Plains are insufficiently documented. The Duncan dune field is located at 97.5° W in Nebraska's Platte River Valley at the easternmost edge of the Great Plains. The dunes lie over 80 km east of the Sand Hills (Fig. 1), and N150 km east of dunes dated in previous studies (i.e. Forman et al., 2005; Miao et al., 2007). The purposes of this study are to: (1) identify when dune activity occurred in this setting, and (2) evaluate potential causes of dune activation. Like most of the relatively few and small eastern Nebraska dune fields, the Duncan dune field is located adjacent to streams (in this case, the Loup and Platte Rivers) that receive drainage from the Sand Hills (Fig. 1). Unlike in larger dune systems such as the Sand Hills where increased dune activity can be directly related to drought, the development and migration of dunes near major river systems can be an indirect response to drought conditions, such as an increase in sediment availability from streams (Muhs et al., 1996). Therefore, the second goal of this study is to evaluate the sediment source for these
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Fig. 1. Duncan dunes and other dunefields in Nebraska. All previously-dated dunes from Nebraska lie west of 99° W.
river-valley dunes, and determine whether their formation can be either directly or indirectly attributed to drought conditions. 2. Regional setting The Nebraska Sand Hills are the dominant geological and geomorphic feature of central Nebraska. However, east of this area several smaller dune fields occur along the Elkhorn, Loup, and Platte Rivers that range in size from ~70 to 660 km2 (Fig. 1). These eolian landforms are primarily found on the southern sides of river valleys, either on alluvial terraces or overlying loess-covered uplands. The Duncan dune field mantles a set of alluvial terraces at ~ 470 to 455 m in elevation, between the Loup and Platte Rivers (Fig. 2A, B). The treads underlying the eastern and western ends of the dunes are ~ 5–10 m higher relative to its center (Fig. 2B). This difference in relief was most likely caused by entrenchment of the Platte River through the Qat2 terrace tread creating the younger inset Qat1 tread (Fig. 2B). Dunes and low-relief sand sheets are present over most of these terrace surfaces, some 170 km2 in area, producing one of the largest dune fields in eastern Nebraska (Fig. 1). Although some dunes exceed 25 m in height, the total relief of most dunes ranges from 10–15 m. Irregular, but thin (generally b2 m), sand sheets locally occur between the larger dunes. Where dune slip faces are readily identifiable, it appears that the dunes migrated from the northwest toward the southeast (Fig. 2B). This is evident as the darker dune slip faces are on the southeastern edges of individual dunes (Fig. 2B). Annual precipitation averages about 600 mm in the area of the Duncan dunes (Loerch et al., 1988). In contrast, precipitation in the Sand Hills ranges from 580 mm to 400 mm on the eastern and western edges, respectively (Wilhite and Hubbard, 1990). The Duncan dunes are currently inactive and are completely covered by grass, except where human activity and overgrazing have locally damaged the vegetational cover. Although some blowouts appear locally in the Duncan dunes, they are rarer than in the drier Sand Hills. Aerial photographs dating to the later 1930s indicate that no dune activation in the Duncan area occurred during the 20th century. Furthermore, general land office (GLO) surveys from the early 1860s indicate that the entire area was suitable for pasture, but not farming. In addition, no areas of bare, unvegetated sand were identified (Nebraska State Surveyor's Office, undated). Therefore, the current vegetated and inactive state of the dunes has existed for at least a century and a half.
3. Materials and methods Hand augers were used to collect samples from both eolian sand in the dunes and the underlying alluvial deposits. Eolian sand samples were taken from large, high relief dunes, primarily at or near the dune crest. The alluvial deposits underlying the dunes were sampled by augering through the thinner sand sheets that overlie the alluvium. Dune crests were exclusively chosen for sampling in order to determine when large-scale duneforms were active. Exposures in blowouts were avoided to prevent the irrelevant documentation of highly localized erosion. In total, 13 auger holes were excavated to depths ranging from 1.6 to 6.6 m; 11 of these holes were sampled for geochemical analyses and optical dating. Optical dating samples were collected by pounding opaque tubes into sand collected in the bucket augers. Twenty-three samples were processed and dated using the single aliquot regenerative (SAR) method (Murray and Wintle, 2000) on 90– 150 μm quartz grains. Quartz sand was isolated using sodium polytungstate treatments to remove heavy minerals and hydrofluoric acid treatments to remove feldspars and etch quartz grains. Equivalent dose (De) values were determined on Risø model DA 15 and DA 20 TL/ OSL readers. Preheat and cutheat temperatures of 220 °C were chosen based on preheat plateau experiments. Aliquots for all samples were created by spraying the inner 5 mm of 10 mm aluminum disks with a medical grade silicone spray. These disks hold approximately 1200 grains each. Five of the six fluvial samples were also run by spraying the inner 2 mm of the disk with silicone, yielding ~200 grains per disk. Using either technique, final age estimates were based on a minimum of 20 aliquots and De values were calculated using the central age model of Galbraith et al. (1999). Aliquots were rejected if recycling ratios were N10% from unity, or if aliquots had measurable signals during stimulation with IR diodes. To avoid over-estimating optical ages, aliquots having De values that were N3σ from the mean were also rejected. Environmental dose rate values were calculated from concentrations of K, U, and Th taken from bulk sediment samples as determined by inductively coupled plasma-mass spectrometry. Dose rate calculations assumed 10% errors for K, U, and Th values. Moisture contents were measured in situ from bulk sediment samples taken adjacent to the optical dating sample. The cosmogenic dose rate contribution was estimated using equations from Prescott and Hutton (1994).
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Fig. 2. (A) Shaded-relief digital elevation model showing Duncan dunes and loess-covered uplands (Ql), alluvial terrace without loess cover (Qat), and Holocene alluvium (Qa). Terrace scarps indicated by arrows. (B) Auger holes in Duncan dunes and sampling sites for sediments of the Platte (Px) and Loup (Lx) rivers. Other features are: lower (Qat1) and higher (Qat2) terrace treads underlying the dunefield (see Fig. 4). Sampling Site P1 is located ~ 20 km upstream from Site P2.
Bulk sediment samples were collected for elemental analyses in order to identify potential source sediments for the Duncan dunes. Similar applications of elemental comparisons have been used effectively to identify sources for eolian sand units in Nebraska (Muhs et al., 1997, 2000) and the Great Bend Sand Prairie in Kansas (Arbogast and Muhs, 2000). ICP-MS and ICP-AES were used to determine K2O (%) and Rb (ppm) contents, and ±2.5% and 4% errors are assumed for these elements, respectively. These results were collected for dose rate data from each optical dating sample, providing us with geochemistry values for the eolian deposits and alluvium underlying the Duncan Dunes. We also sampled deposits from three modern Platte and seven Loup River sites, including abandoned alluvial sediments from terraces along the Loup River that lie adjacent
to the Duncan dunes (Fig. 2B). In order to more accurately compare the elemental signals from alluvial and eolian sediments, the alluvial fractions were sieved to remove particles that exceeded 1000 μm. These comparisons were used to assess whether the sediments within the Duncan dunes originated from (a) the Loup River, (b) the alluvial fill underlying the dunes, or (c) the Platte River. 4. Results 4.1. Geomorphic mapping The surficial geology surrounding the Duncan dunes is dominated by alluvium and eolian sediments (Fig. 2A, B). Eolian sediments
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Fig. 3. Auger hole stratigraphy and optical age chronology in east–west transect of Duncan dune field.
include Peoria loess that was deposited before ~14 ka (Bettis et al., 2003), and dunes and/or eolian sand sheets that are commonly found on Quaternary terrace deposits (Qat1 and Qat2). Neither the Qat1 and Qat2 deposits nor the more recent alluvium (Qa) of the Loup and Platte Rivers are covered by loess, and the latter are not significantly impacted by eolian activity and primary alluvial morphologies are commonly visible. 4.2. Dune stratigraphy In auger holes, eolian sands were distinguished from alluvial sediments by direct landform association (i.e., occurrence in an existing dune form), good sorting, and texture (fine to medium sand). Alluvial sediments were distinguished from eolian sands either by the
occurrence of pebbles, which are not likely to have been moved appreciable distances by winds, or by interbedded silts and very fine sands. The latter sedimentary facies are analogous to some of the sediments encountered at purely alluvial sites in the Platte and Loup River valleys, but could be interpreted as interdune alluvial deposits. Of the eleven auger holes studied, five contained between 1 and 3.5 m of dune sand over alluvial sediments that ranged from 0.5 to 4 m in thickness (Fig. 3: A–D, F), whereas the other holes did not fully penetrate eolian sand (Fig. 3: E, G–K), even at depths of greater than 6 m below the modern land surface (Fig. 3: I and K). In the Duncan dunes, buried A horizons, which were identified on the basis of darker colors, were encountered in five of the holes (Fig. 3: Holes F, G, I, K). The degree of development evident in both modern and buried soils is minimal and B horizons are absent, although clay
Fig. 4. K2O (%) vs. Rb (ppm) values for alluvium and eolian sediments. Analytical errors for K2O and Rb are 2.5% and 4%, respectively.
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Table 1 Equivalent dose, dose rate data, and optical age estimates for Duncan dunefield Profile
A B
UNL Lab # UNL-1636 UNL-1631 UNL-1632 UNL-1633 UNL-1637 UNL-1634 UNL-1635 UNL-1638 UNL-1466 UNL-1467 UNL-1471 UNL-1468 UNL-1469 UNL-1470 UNL-1338 UNL-1342 UNL-1346 UNL-1347 UNL-1348 UNL-1349 UNL-1343 UNL-1344 UNL-1345
C D E F G
H I J K
a b c
Depth
U
Th
K2O
In situ H2O
Dose rate
De (Gy)
Aliquots
Optical age
m
ppm
ppm
wt. %
%a
Gy/ka
± 1 S.E.
nb
±1σ
26/30 23/28 23/28 22/28 25/30 28/31 25/30 23/30 20/23 20/23 25/30 20/23 20/21 23/28 20/29 20/24 22/28 21/23 21/23 22/24 26/31 21/23 21/23
13,360 ± 1450 3640 ± 280 4170 ± 410 14,240 ± 1300 15,310 ± 1480 4360 ± 360 12,600 ± 1300 16,110 ± 1590 830 ± 90 490 ± 50 4980 ± 570 720 ± 70 3440 ± 310 5070 ± 430 690 ± 60 670 ± 60 700 ± 60 690 ± 70 670 ± 60 3720 ± 340 590 ± 50 560 ± 50 3990 ± 350
1.5 1.0 1.5 3.2 5.9 1.5 4.6 2.4 0.9 2.6 1.6 1.0 2.7 3.7 0.9 3.9 1.3 5.9 1.4 4.9 1.4 2.3 5.2
0.9 1.0 1.0 1.5 1.0 1 1.3 0.7 0.9 0.8 0.7 0.6 0.7 0.7 0.6 0.6 0.8 0.8 0.7 0.8 0.9 1.0 0.9
4.6 5.3 5.0 6.3 4.8 4.7 6.1 3.3 4.3 4.0 3.3 3.3 3.4 3.8 3.2 4.1 3.8 3.9 4.0 3.8 4.5 5.0 4.4
1.9 2.0 1.9 2.2 2.0 1.9 2.0 1.8 1.9 1.9 2.1 1.8 1.7 1.8 1.7 1.9 1.8 1.8 1.9 1.9 1.9 1.9 1.9
4.0 3.5 8.9 6.3 3.6 5.4 6.6 5.8 6.6 5.0 4.4 3.0 5.2 4.5 3.7 4.0 2.1 3.8 2.2 5.5 3.9 5.3 5.0
c
2.14 ± 0.14 2.31 ± 0.14 2.06 ± 0.18 2.54 ± 0.18 2.26 ± 0.09 2.13 ± 0.14 2.31 ± 0.17 1.85 ± 0.14 2.10 ± 0.12 2.03 ± 0.10 2.13 ± 0.10 1.97 ± 0.07 1.79 ± 0.09 1.93 ± 0.09 1.82 ± 0.07 1.99 ± 0.09 2.05 ± 0.07 1.96 ± 0.08 2.10 ± 0.07 1.96 ± 0.10 2.13 ± 0.09 2.17 ± 0.11 2.06 ± 0.10
28.6 ± 1.6 8.4 ± 0.2 8.6 ± 0.2 36.2 ± 1.3 34.6 ± 1.3c 9.3 ± 0.2 29.1 ± 1.6c 29.8 ± 1.7c 1.74 ± 0.11 1.0 ± 0.04 10.6 ± 0.7c 1.42 ± 0.09 6.16 ± 0.17 9.80 ± 0.19 1.26 ± 0.02 1.34 ± 0.05 1.43 ± 0.05 1.35 ± 0.08 1.40 ± 0.05 7.30 ± 0.20 1.26 ± 0.04 1.22 ± 0.05 8.22 ± 0.17
Dose rate estimate assumes ±100% variability in measured moisture values. Accepted disks/all disks. 2 mm mask.
lamellae were noted in a small blowout near Holes I and J (Fig. 2B). Lamellae were not readily identified in the sub-surface in our bucket augers, and we expect that lamellae are more common in the dunes than our study would suggest. Most of the buried soils occur within eolian sands (Fig. 3: Holes G, I, K), but one is developed atop alluvium that is covered by a slopewash deposit (Fig. 3: Hole F). The lack of a modern A horizon developed in this unit suggests that the slopewash deposit is historic in age (Fig. 3; Hole F). 4.3. Dune sand provenance A comparison of potassium (% K2O) and Rb (ppm) analyses indicates differences between modern alluvium, terrace sediments, and eolian sands (Fig. 4). Platte River sediments have significantly higher K2O and Rb values than those of the Loup River (Fig. 4). We assume that Loup River sands are similar in composition to the eolian sands of their Nebraska Sand Hills source (Fig. 1), therefore our trends are consistent with those identified by Muhs et al. (1997) which showed both higher K2O and Rb values for Platte River sands relative to those from the Sand Hills. Our K2O and Rb data also demonstrate that the alluvium from the terrace underlying the Duncan dunes clusters between values derived from Loup River and Platte River sediments (Fig. 4). Eolian sands from the Duncan dunes, however, plot between, and statistically overlap with both the underlying terrace alluvium and modern Loup River sediments (Fig. 4). Our geochemical results are unlikely to be biased by sediment texture because there is no statistical difference in the K2O or Rb values from both alluvial-terrace silts and pebbly sands underlying the dunes (Table 1; Fig. 3). Moreover, our data are very similar to those published by Muhs et al. (1997), which clearly differentiated Platte River Valley sediments from sands in the Nebraska Sand Hills. 4.4. Optical age chronology A total of 23 optical ages were generated for dune sands and underlying alluvial sediments from the Duncan dunes (Table 1). A dose-recovery test was performed on 10 aliquots of sample UNL-1631
to determine if the sands were amenable to optical dating procedures (Murray and Olley, 2002). In the laboratory we subjected the aliquots to two 30 °C bleaches using blue-green diodes then applied a beta dose of 8.28 Gy. Using the SAR procedure we recovered 8.30 ± 0.21 Gy from this sample suggesting that the samples are well behaved for optical dating. A total of six age estimates were determined for samples taken from alluvium that directly underlies the dunes. Five of these samples were run using both 5 and 2 mm aliquots holding ~ 1200 and 200 grains per disk, respectively, and Table 2 shows a comparison of these results. In four of the five cases, 2 mm De values were lower than the 5 mm De values, with 2 mm/5 mm ratios ranging from 0.89–1.09. Importantly, the age estimates calculated from the 2 and 5 mm data fall within 1σ errors in all but one case. Although there are no differences in our interpretations using one or the other, we prefer the somewhat younger age estimates calculated using the 2 mm data simply because they are less likely to show the influences of any partial bleaching problems. Hence, the alluvial ages in Table 1 and Fig. 3 are based on the 2 mm results. Three optical age estimates were taken from Holes B and C on the Qat2 surface on the western end of the Duncan dunes (Figs. 2b and 3). These age estimates range from
Table 2 Comparison of 2 mm and 5 mm De values for fluvial samples Disk size
2 5 2 5 2 5 2 5 2 5
mm mm mm mm mm mm mm mm mm mm a
UNL Lab #
UNL-1636 UNL-1637 UNL-1635 UNL-1638 UNL-1471
De (Gy)
Aliquots a
2 mm/5 mm
Optical age
±1 S.E.
n
De
± 1σ
28.6 ± 1.6 30.1 ± 1.1 34.6 ± 1.3 36.6 ± 1.3 29.1 ± 1.6 32.2 ± 1.5 29.8 ± 1.7 34.2 ± 1.2 10.6 ± 0.7 10.2 ± 0.4
26/30 25/28 25/30 22/25 25/30 22/31 23/30 23/27 25/30 22/31
0.98
13,360 ± 1450 14,040 ± 1180 15,310 ± 1480 16,170 ± 1390 12,600 ± 1300 13,920 ± 1350 16,110 ± 1590 18,490 ± 1690 4980 ± 570 4800 ± 440
Accepted disks/nnall disks.
0.95 0.92 0.89 1.09
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15.3 to 12.6 ka (Fig. 3). Three optical age estimates for the alluvium taken from within the fill of the Qat1 terrace range from 16.1 to 5.0 ka (Fig. 3: Holes A, D, and F). The 17 optical age estimates taken from dune sands ranged from 5.1 to 0.5 ka. The optical ages recovered from the eolian sands fall into two groups between 4.4–3.4 and 0.8–0.5 ka. One age estimate is an apparent outlier at 5.1 ± 0.4 ka, and was obtained from sand underlying a buried A horizon (Fig. 3: Hole G). Six of the optical age estimates taken from five holes cluster between 4.4 and 3.4 ka. Two of these age estimates are found below buried soils (Fig. 3: Holes G and K), and three of them are found below younger dune sand (Fig. 3: Holes G, J, and K). The youngest ages cluster between ~ 830 and 500 yrs ago (Fig. 3), and include eleven estimates taken from six holes (Fig. 3: Holes E, G, H, I, J, and K). These latest ages are best represented in our samples, and with the exception of Holes B and C (Fig. 3), each of the holes with age control yielded ages that fall within this interval. 5. Discussion The effects of partial bleaching can result in optical age overestimations in some alluvial environments (Olley et al., 1999). Both the local loess stratigraphy and analysis of the De distributions, however, suggest that partial bleaching is not a significant problem in this setting. The absence of loess on the Qta1 and Qta2 surfaces (Fig. 2B) suggests that the surficial deposits should be younger than 14 ka, the estimated termination of loess deposition in the region (Bettis et al., 2003). With one exception (16,100 ± 1600), each of our age estimates is either younger than or falls within 1σ errors of this threshold. Analysis of the De distributions can also be used to analyze problems with partial bleaching. For five of the six alluvial samples we analyzed, both 5 and 2 mm aliquots were used to produce age estimates (Table 2). Three aliquots of samples UNL-1635 and UNL1638 were run using a single-grain laser to estimate the number of grains contributing to the luminescence signal. For this test the aliquots were bleached twice at room temperature, given a 5 Gy dose, and the grains were bleached with the laser. For both samples approximately 10–12% of the grains gave a luminescence signal above background levels, and 3–4% of the grains dominated the total signal. Based on these findings, the 5 mm aliquots would have signal dominated by between ~120 and 50 grains while the 2 mm aliquots would have signal dominated by between ~ 20 and 10 grains, approaching results that would be achieved through single-grain analysis. While this difference in the number of significant grains is large, the resultant difference in the De values is relatively minor (Table 2), and most of the ages generated using the two different aliquot sizes were within 1σ errors. This evidence suggests that partial bleaching is not a significant problem for these samples. Five of the six alluvial age estimates taken from within the terrace fills indicate that the alluvium was deposited between 16.1 and 12.6 ka, while one age is significantly younger indicating that deposition occurred at ~ 5.0 ka (Fig. 3; Hole F). Although we believe that the measurable effects of partial bleaching are negligible in these samples the young age estimate for sample UNL-1471 (~5.0 ka; Fig. 3; Hole F) is problematic. This age estimate lies well outside of the 3σ errors given for the other alluvial samples taken from the fill underlying the Qat1 surface (Table 1). This comparatively young age could simply be erroneous, but we instead suggest that it results from one of the following potential scenarios. One possible explanation is that the age is too young due to post-burial disturbance, such as bioturbation in the shallow subsoil (Bateman et al., 2003; Feathers, 2003). Alternatively, the age could be correct, and dates fluvial activity that is significantly younger than the other alluvial ages from this study. The Qat1 tread was cut at ~ 5.0 ka as suggested by sample UNL-1471, and the older ages from the Qat1 surface are actually exhumation ages resulting from the erosion of older sediments that underlie the terrace tread (Fig. 2B). Finally, the age may be correct
and result from alluvial deposition in an interdune environment. Regardless which of these scenarios are correct, this single sample has negligible bearing on our eolian age estimates which are the focus of this study. A total of seventeen optical age estimates indicate that dunes were largely stabilized over the past five centuries but were active during two periods over the last ~5 kyr. The clustering of these ages indicates that dunes were active during two major periods between 4.4–3.4 and 0.8–0.5 ka. These two recent dune activation events recorded in the Duncan dunes correspond well with other drought records from the Great Plains (Fig. 5). Each of the optical ages clustering within the period of dune activity identified between 4.4 and 3.4 ka overlaps within 2σ errors of the 4.3–4.1 ka drought recognized from multiproxy evidence on the Great Plains (Booth et al., 2005). Each of the Duncan dune ages also overlaps within 1σ of the ~3.8 ka drought identified in the Sand Hills and adjacent loess regions in Nebraska (Miao et al., 2007; Fig. 5). The youngest event recorded from the Duncan dunes also corresponds well with regional drought records. Miao et al. (2007) showed that dune activation in the Nebraska Sand Hills and associated loess deposition occurred from ~ 1.2–0.6 ka (Fig. 5). Similar periods of dune activity were also identified in the Great Bend Sand Prairie in Kansas (Arbogast, 1996), and the Fort Morgan dune field in northeastern Colorado (Clarke and Rendell, 2003). This latest period of dune activity identified in the Duncan dunes and many other dunefields on the Great Plains correlates with increased aridity associated with the Medieval Warm Period (MWP) (Daniels and Knox, 2005). One notable difference between our results and those from the Nebraska Sand Hills is that our earliest dune age, which dates to 5.1 ka, does not appear to correspond to significant drought events from the Great Plains. Based on evidence of dune activity from the Nebraska Sand Hills, Miao et al. (2007) showed that drought conditions existed between 9 and 6 ka, and that dunes were stable until ~3.8 ka. Assuming that the age is accurate our lone 5.1 ka age estimate from this time period may reflect a local blowout or a large-scale dune activation event with limited preservation. Future work in these dunes may help us to better characterize regional dune activity during this time period. Another difference between the Duncan dunes and the Nebraska Sand Hills is the evidence for extensive dune activity around 2.5 ka in the Sand Hills (Miao et al., 2007), contrasted with the apparent lack of dune activity at that time in the Duncan dunes (Fig. 5). This difference could be related to either preservation or sampling biases. Alternatively, and probably more likely, the wetter climate of the Duncan dunes may have given greater vegetative cover and therefore buffered the dunes during some megadroughts. Thus, more easterly dune fields may be expected to show greater stability and less vulnerability to megadroughts. This may explain the apparent stability of these dunes between 3 and 2 ka when records from the Nebraska Sand Hills show dune activation and loess deposition (Miao et al., 2007). Future studies
Fig. 5. Optical age estimates for eolian samples from Duncan dune field. Gray bars represent megadroughts identified in Nebraska Sand Hills and nearby areas (Miao et al., 2007).
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in the region's dunes will allow us to better address this apparent dichotomy. Overall, the chronology of dune activity in the Duncan dunes is similar to those which have been reconstructed in some other Great Plains dune fields (Clarke and Rendell, 2003; Arbogast, 1996; Miao et al., 2007). While in most of these larger dunefields dune activity can be directly related to widespread drought conditions, dune activation in lowland settings such as the Duncan dunes could be attributed to multiple causes, including both direct and indirect responses to megadroughts. Specifically, dune activity near streams like the Loup River that drain major dune fields such as the Sand Hills can result from: (1) the lowering of the local water table under a terrace tread by fluvial incision, followed by deflation of sands above the lowered water table; (2) the increased availability of sand to the Loup River through activation of the Nebraska Sand Hills during megadroughts (an indirect response to regional hydrologic drought); or (3) repeated dune reactivation solely within the Duncan dunes due to megadroughts (a direct result of regional hydrologic drought). In theory, case (1) should be manifested in the sedimentary record as a single period of dune activity, without buried soils, large-scale truncation surfaces, or multiple dune ages. Both our geochronologic and geomorphic results indicate that this scenario is not applicable to the Duncan dunes, suggesting that dune mobilization is better explained by either case (2) or case (3). Existing data are insufficient to adequately test hypotheses presented by the two remaining cases. K2O (%) and Rb (ppm) analyses, for example, indicate that the Duncan eolian sands, regardless of proximity to the Loup River, are derived both from the river itself and from the sub-dune terrace (Fig. 4). These data tend, in themselves, to support case (2). But, if the dunes received sand primarily from the Loup River, they would need to have moved 5 km from the river to the southeastern edge of the dune field (Fig. 2B). At migration rates of 20– 30 m/yr (cf. Long and Sharp, 1964; Fryberger et al., 1984), an absolute minimum of 170 to 250 years would have been required to traverse the dune field. This estimated time period is, potentially, equivalent to the duration of each of the late Holocene megadroughts, but sustained dune migration at such high rates is highly unlikely. In opposition to the geochemical data, these projections suggest that case (3) is more likely than case (2). Therefore, existing data, while effectively arguing for a relationship between dune formation and hydrologic drought, present a paradox. A full test of hypotheses of indirect vs. direct dune responses to regional megadroughts will require a comprehensive study of dune fields at the eastern margin of the central Great Plains. 6. Conclusions Seventeen optical age estimates indicate that dunes near Duncan, Nebraska, although stable for long periods during the Holocene, were activated at least twice in the past ~ 5 ka, including between 4.4–3.4 and 0.8–0.5 ka. Our results, compared with regional chronologies of dune activity, suggest that reactivation of the Duncan dunes was either a direct or indirect result of regional megadroughts. The correspondence of the last two episodes of dune activity in the Duncan dunes (4.4–3.4 and 0.8–0.5 ka) with activation in other dune fields reinforces the conclusion that megadroughts, events only recently implicated in system change on the Great Plains (Miao et al., 2007), also impacted the far eastern margin of that region, and perhaps beyond. Acknowledgements This project emerged from geologic mapping funded by the U.S. Geological Survey STATEMAP cooperative mapping program at the Conservation and Survey Division of the School of Natural Resources at the University of Nebraska-Lincoln. The manuscript was improved by input from three anonymous reviewers.
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h
Late Holocene dune activity in the Eastern Platte River Valley, Nebraska P.R. Hanson a,⁎, R.M. Joeckel a,b, A.R. Young a, J. Horn b a b
University of Nebraska-Lincoln, School of Natural Resources, Hardin Hall, Lincoln, Nebraska 68583-0996, USA University of Nebraska-Lincoln, Department of Geosciences, USA
a r t i c l e
i n f o
Article history: Received 21 December 2007 Received in revised form 30 July 2008 Accepted 31 July 2008 Available online 8 August 2008 Keywords: Late Holocene Eolian activity Drought Optical dating Platte River Loup River
a b s t r a c t Large-scale dune activity in the Nebraska Sand Hills and elsewhere on the western Great Plains has been linked to prehistoric “megadroughts” that triggered the activation of regional dune fields. The effect of megadroughts on the smaller dune fields east of the Nebraska Sand Hills has never been assessed, however. This study focuses on the Duncan dune field near the confluence of the Loup and Platte rivers in eastern Nebraska. Seventeen optically stimulated luminescence age estimates were obtained and reveal two periods of dune activation that occurred between 4.4 to 3.4 ka and 0.8 to 0.5 ka. Significantly, both periods chronologically overlap large-scale dune activity identified in the Nebraska Sand Hills. Geochemical evidence indicates that the Duncan dunes received sand not only from the terrace underlying them, but also from the Loup River. These data link dune activity in the Duncan area, at least indirectly, to increased sediment supply from streams that drain the Sand Hills during megadroughts, implying the activation of the dunes occurred as an indirect response to regional megadroughts. Calculations of dune migration rates, however, argue in favor of local, drought-driven hydrologic changes as a causative factor in dune activation, in other words, a direct effect of megadroughts. Whether the impact was direct or indirect, it is highly likely that the repeated reactivation of the Duncan dunes resulted in some way from regional, large-magnitude droughts. Other paleoclimate proxies from the Great Plains tend to support this conclusion. We conclude that the megadroughts that have been identified in the Sand Hills and other Great Plains dune fields were indeed regional events with far-reaching effects. Published by Elsevier B.V.
1. Introduction Holocene dune sands and loess provide important information about large-magnitude prehistoric drought events (‘megadroughts’) on the central Great Plains (Muhs et al., 1997; Stokes and Swinehart, 1997; Muhs and Zárate, 2001; Forman et al., 2001; Goble et al., 2004; Mason et al., 2004; Sridhar et al., 2006; Miao et al., 2007). Megadroughts stand out in the regional climate record as major shifts in the states of biological and geological systems, particularly dune fields, in which changes in the availability of water determine the nature of vegetational cover and therefore the relative stability of dunes. Indeed, megadroughts could ultimately be characteristic features of the long-term climate on the North American Plains. Recent research on megadroughts and dune activation has focused on areas west of the 99th Meridian (Fig. 1). Miao et al. (2007), for example, interpreted four periods of increased dune activity between approximately 9 and 6.6 ka, and centered around 3.8 ± 0.3, 2.5 ± 0.1, and 0.67 ± 0.1 ka (ages here and throughout the text are given in calendar years) in the Nebraska Sand Hills. The youngest of the events identified by Miao et al. (2007) corresponds to periods of drought
⁎ Corresponding author. E-mail address: [email protected] (P.R. Hanson). 0169-555X/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.geomorph.2008.07.018
and dune activation across the central Great Plains, including northeastern Colorado (Clarke and Rendell, 2003), and the Great Bend Sand Prairie in Kansas (Arbogast, 1996). The identification of widespread megadrought effects has potentially dramatic implications in understanding the chronology of dune activity on the Great Plains. Nonetheless, the full geographic extent of the megadroughts' effects, particularly east of the Nebraska Sand Hills, remains unknown, and even the most basic aspects of Holocene dunes at the eastern margin of the Great Plains are insufficiently documented. The Duncan dune field is located at 97.5° W in Nebraska's Platte River Valley at the easternmost edge of the Great Plains. The dunes lie over 80 km east of the Sand Hills (Fig. 1), and N150 km east of dunes dated in previous studies (i.e. Forman et al., 2005; Miao et al., 2007). The purposes of this study are to: (1) identify when dune activity occurred in this setting, and (2) evaluate potential causes of dune activation. Like most of the relatively few and small eastern Nebraska dune fields, the Duncan dune field is located adjacent to streams (in this case, the Loup and Platte Rivers) that receive drainage from the Sand Hills (Fig. 1). Unlike in larger dune systems such as the Sand Hills where increased dune activity can be directly related to drought, the development and migration of dunes near major river systems can be an indirect response to drought conditions, such as an increase in sediment availability from streams (Muhs et al., 1996). Therefore, the second goal of this study is to evaluate the sediment source for these
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Fig. 1. Duncan dunes and other dunefields in Nebraska. All previously-dated dunes from Nebraska lie west of 99° W.
river-valley dunes, and determine whether their formation can be either directly or indirectly attributed to drought conditions. 2. Regional setting The Nebraska Sand Hills are the dominant geological and geomorphic feature of central Nebraska. However, east of this area several smaller dune fields occur along the Elkhorn, Loup, and Platte Rivers that range in size from ~70 to 660 km2 (Fig. 1). These eolian landforms are primarily found on the southern sides of river valleys, either on alluvial terraces or overlying loess-covered uplands. The Duncan dune field mantles a set of alluvial terraces at ~ 470 to 455 m in elevation, between the Loup and Platte Rivers (Fig. 2A, B). The treads underlying the eastern and western ends of the dunes are ~ 5–10 m higher relative to its center (Fig. 2B). This difference in relief was most likely caused by entrenchment of the Platte River through the Qat2 terrace tread creating the younger inset Qat1 tread (Fig. 2B). Dunes and low-relief sand sheets are present over most of these terrace surfaces, some 170 km2 in area, producing one of the largest dune fields in eastern Nebraska (Fig. 1). Although some dunes exceed 25 m in height, the total relief of most dunes ranges from 10–15 m. Irregular, but thin (generally b2 m), sand sheets locally occur between the larger dunes. Where dune slip faces are readily identifiable, it appears that the dunes migrated from the northwest toward the southeast (Fig. 2B). This is evident as the darker dune slip faces are on the southeastern edges of individual dunes (Fig. 2B). Annual precipitation averages about 600 mm in the area of the Duncan dunes (Loerch et al., 1988). In contrast, precipitation in the Sand Hills ranges from 580 mm to 400 mm on the eastern and western edges, respectively (Wilhite and Hubbard, 1990). The Duncan dunes are currently inactive and are completely covered by grass, except where human activity and overgrazing have locally damaged the vegetational cover. Although some blowouts appear locally in the Duncan dunes, they are rarer than in the drier Sand Hills. Aerial photographs dating to the later 1930s indicate that no dune activation in the Duncan area occurred during the 20th century. Furthermore, general land office (GLO) surveys from the early 1860s indicate that the entire area was suitable for pasture, but not farming. In addition, no areas of bare, unvegetated sand were identified (Nebraska State Surveyor's Office, undated). Therefore, the current vegetated and inactive state of the dunes has existed for at least a century and a half.
3. Materials and methods Hand augers were used to collect samples from both eolian sand in the dunes and the underlying alluvial deposits. Eolian sand samples were taken from large, high relief dunes, primarily at or near the dune crest. The alluvial deposits underlying the dunes were sampled by augering through the thinner sand sheets that overlie the alluvium. Dune crests were exclusively chosen for sampling in order to determine when large-scale duneforms were active. Exposures in blowouts were avoided to prevent the irrelevant documentation of highly localized erosion. In total, 13 auger holes were excavated to depths ranging from 1.6 to 6.6 m; 11 of these holes were sampled for geochemical analyses and optical dating. Optical dating samples were collected by pounding opaque tubes into sand collected in the bucket augers. Twenty-three samples were processed and dated using the single aliquot regenerative (SAR) method (Murray and Wintle, 2000) on 90– 150 μm quartz grains. Quartz sand was isolated using sodium polytungstate treatments to remove heavy minerals and hydrofluoric acid treatments to remove feldspars and etch quartz grains. Equivalent dose (De) values were determined on Risø model DA 15 and DA 20 TL/ OSL readers. Preheat and cutheat temperatures of 220 °C were chosen based on preheat plateau experiments. Aliquots for all samples were created by spraying the inner 5 mm of 10 mm aluminum disks with a medical grade silicone spray. These disks hold approximately 1200 grains each. Five of the six fluvial samples were also run by spraying the inner 2 mm of the disk with silicone, yielding ~200 grains per disk. Using either technique, final age estimates were based on a minimum of 20 aliquots and De values were calculated using the central age model of Galbraith et al. (1999). Aliquots were rejected if recycling ratios were N10% from unity, or if aliquots had measurable signals during stimulation with IR diodes. To avoid over-estimating optical ages, aliquots having De values that were N3σ from the mean were also rejected. Environmental dose rate values were calculated from concentrations of K, U, and Th taken from bulk sediment samples as determined by inductively coupled plasma-mass spectrometry. Dose rate calculations assumed 10% errors for K, U, and Th values. Moisture contents were measured in situ from bulk sediment samples taken adjacent to the optical dating sample. The cosmogenic dose rate contribution was estimated using equations from Prescott and Hutton (1994).
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Fig. 2. (A) Shaded-relief digital elevation model showing Duncan dunes and loess-covered uplands (Ql), alluvial terrace without loess cover (Qat), and Holocene alluvium (Qa). Terrace scarps indicated by arrows. (B) Auger holes in Duncan dunes and sampling sites for sediments of the Platte (Px) and Loup (Lx) rivers. Other features are: lower (Qat1) and higher (Qat2) terrace treads underlying the dunefield (see Fig. 4). Sampling Site P1 is located ~ 20 km upstream from Site P2.
Bulk sediment samples were collected for elemental analyses in order to identify potential source sediments for the Duncan dunes. Similar applications of elemental comparisons have been used effectively to identify sources for eolian sand units in Nebraska (Muhs et al., 1997, 2000) and the Great Bend Sand Prairie in Kansas (Arbogast and Muhs, 2000). ICP-MS and ICP-AES were used to determine K2O (%) and Rb (ppm) contents, and ±2.5% and 4% errors are assumed for these elements, respectively. These results were collected for dose rate data from each optical dating sample, providing us with geochemistry values for the eolian deposits and alluvium underlying the Duncan Dunes. We also sampled deposits from three modern Platte and seven Loup River sites, including abandoned alluvial sediments from terraces along the Loup River that lie adjacent
to the Duncan dunes (Fig. 2B). In order to more accurately compare the elemental signals from alluvial and eolian sediments, the alluvial fractions were sieved to remove particles that exceeded 1000 μm. These comparisons were used to assess whether the sediments within the Duncan dunes originated from (a) the Loup River, (b) the alluvial fill underlying the dunes, or (c) the Platte River. 4. Results 4.1. Geomorphic mapping The surficial geology surrounding the Duncan dunes is dominated by alluvium and eolian sediments (Fig. 2A, B). Eolian sediments
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Fig. 3. Auger hole stratigraphy and optical age chronology in east–west transect of Duncan dune field.
include Peoria loess that was deposited before ~14 ka (Bettis et al., 2003), and dunes and/or eolian sand sheets that are commonly found on Quaternary terrace deposits (Qat1 and Qat2). Neither the Qat1 and Qat2 deposits nor the more recent alluvium (Qa) of the Loup and Platte Rivers are covered by loess, and the latter are not significantly impacted by eolian activity and primary alluvial morphologies are commonly visible. 4.2. Dune stratigraphy In auger holes, eolian sands were distinguished from alluvial sediments by direct landform association (i.e., occurrence in an existing dune form), good sorting, and texture (fine to medium sand). Alluvial sediments were distinguished from eolian sands either by the
occurrence of pebbles, which are not likely to have been moved appreciable distances by winds, or by interbedded silts and very fine sands. The latter sedimentary facies are analogous to some of the sediments encountered at purely alluvial sites in the Platte and Loup River valleys, but could be interpreted as interdune alluvial deposits. Of the eleven auger holes studied, five contained between 1 and 3.5 m of dune sand over alluvial sediments that ranged from 0.5 to 4 m in thickness (Fig. 3: A–D, F), whereas the other holes did not fully penetrate eolian sand (Fig. 3: E, G–K), even at depths of greater than 6 m below the modern land surface (Fig. 3: I and K). In the Duncan dunes, buried A horizons, which were identified on the basis of darker colors, were encountered in five of the holes (Fig. 3: Holes F, G, I, K). The degree of development evident in both modern and buried soils is minimal and B horizons are absent, although clay
Fig. 4. K2O (%) vs. Rb (ppm) values for alluvium and eolian sediments. Analytical errors for K2O and Rb are 2.5% and 4%, respectively.
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Table 1 Equivalent dose, dose rate data, and optical age estimates for Duncan dunefield Profile
A B
UNL Lab # UNL-1636 UNL-1631 UNL-1632 UNL-1633 UNL-1637 UNL-1634 UNL-1635 UNL-1638 UNL-1466 UNL-1467 UNL-1471 UNL-1468 UNL-1469 UNL-1470 UNL-1338 UNL-1342 UNL-1346 UNL-1347 UNL-1348 UNL-1349 UNL-1343 UNL-1344 UNL-1345
C D E F G
H I J K
a b c
Depth
U
Th
K2O
In situ H2O
Dose rate
De (Gy)
Aliquots
Optical age
m
ppm
ppm
wt. %
%a
Gy/ka
± 1 S.E.
nb
±1σ
26/30 23/28 23/28 22/28 25/30 28/31 25/30 23/30 20/23 20/23 25/30 20/23 20/21 23/28 20/29 20/24 22/28 21/23 21/23 22/24 26/31 21/23 21/23
13,360 ± 1450 3640 ± 280 4170 ± 410 14,240 ± 1300 15,310 ± 1480 4360 ± 360 12,600 ± 1300 16,110 ± 1590 830 ± 90 490 ± 50 4980 ± 570 720 ± 70 3440 ± 310 5070 ± 430 690 ± 60 670 ± 60 700 ± 60 690 ± 70 670 ± 60 3720 ± 340 590 ± 50 560 ± 50 3990 ± 350
1.5 1.0 1.5 3.2 5.9 1.5 4.6 2.4 0.9 2.6 1.6 1.0 2.7 3.7 0.9 3.9 1.3 5.9 1.4 4.9 1.4 2.3 5.2
0.9 1.0 1.0 1.5 1.0 1 1.3 0.7 0.9 0.8 0.7 0.6 0.7 0.7 0.6 0.6 0.8 0.8 0.7 0.8 0.9 1.0 0.9
4.6 5.3 5.0 6.3 4.8 4.7 6.1 3.3 4.3 4.0 3.3 3.3 3.4 3.8 3.2 4.1 3.8 3.9 4.0 3.8 4.5 5.0 4.4
1.9 2.0 1.9 2.2 2.0 1.9 2.0 1.8 1.9 1.9 2.1 1.8 1.7 1.8 1.7 1.9 1.8 1.8 1.9 1.9 1.9 1.9 1.9
4.0 3.5 8.9 6.3 3.6 5.4 6.6 5.8 6.6 5.0 4.4 3.0 5.2 4.5 3.7 4.0 2.1 3.8 2.2 5.5 3.9 5.3 5.0
c
2.14 ± 0.14 2.31 ± 0.14 2.06 ± 0.18 2.54 ± 0.18 2.26 ± 0.09 2.13 ± 0.14 2.31 ± 0.17 1.85 ± 0.14 2.10 ± 0.12 2.03 ± 0.10 2.13 ± 0.10 1.97 ± 0.07 1.79 ± 0.09 1.93 ± 0.09 1.82 ± 0.07 1.99 ± 0.09 2.05 ± 0.07 1.96 ± 0.08 2.10 ± 0.07 1.96 ± 0.10 2.13 ± 0.09 2.17 ± 0.11 2.06 ± 0.10
28.6 ± 1.6 8.4 ± 0.2 8.6 ± 0.2 36.2 ± 1.3 34.6 ± 1.3c 9.3 ± 0.2 29.1 ± 1.6c 29.8 ± 1.7c 1.74 ± 0.11 1.0 ± 0.04 10.6 ± 0.7c 1.42 ± 0.09 6.16 ± 0.17 9.80 ± 0.19 1.26 ± 0.02 1.34 ± 0.05 1.43 ± 0.05 1.35 ± 0.08 1.40 ± 0.05 7.30 ± 0.20 1.26 ± 0.04 1.22 ± 0.05 8.22 ± 0.17
Dose rate estimate assumes ±100% variability in measured moisture values. Accepted disks/all disks. 2 mm mask.
lamellae were noted in a small blowout near Holes I and J (Fig. 2B). Lamellae were not readily identified in the sub-surface in our bucket augers, and we expect that lamellae are more common in the dunes than our study would suggest. Most of the buried soils occur within eolian sands (Fig. 3: Holes G, I, K), but one is developed atop alluvium that is covered by a slopewash deposit (Fig. 3: Hole F). The lack of a modern A horizon developed in this unit suggests that the slopewash deposit is historic in age (Fig. 3; Hole F). 4.3. Dune sand provenance A comparison of potassium (% K2O) and Rb (ppm) analyses indicates differences between modern alluvium, terrace sediments, and eolian sands (Fig. 4). Platte River sediments have significantly higher K2O and Rb values than those of the Loup River (Fig. 4). We assume that Loup River sands are similar in composition to the eolian sands of their Nebraska Sand Hills source (Fig. 1), therefore our trends are consistent with those identified by Muhs et al. (1997) which showed both higher K2O and Rb values for Platte River sands relative to those from the Sand Hills. Our K2O and Rb data also demonstrate that the alluvium from the terrace underlying the Duncan dunes clusters between values derived from Loup River and Platte River sediments (Fig. 4). Eolian sands from the Duncan dunes, however, plot between, and statistically overlap with both the underlying terrace alluvium and modern Loup River sediments (Fig. 4). Our geochemical results are unlikely to be biased by sediment texture because there is no statistical difference in the K2O or Rb values from both alluvial-terrace silts and pebbly sands underlying the dunes (Table 1; Fig. 3). Moreover, our data are very similar to those published by Muhs et al. (1997), which clearly differentiated Platte River Valley sediments from sands in the Nebraska Sand Hills. 4.4. Optical age chronology A total of 23 optical ages were generated for dune sands and underlying alluvial sediments from the Duncan dunes (Table 1). A dose-recovery test was performed on 10 aliquots of sample UNL-1631
to determine if the sands were amenable to optical dating procedures (Murray and Olley, 2002). In the laboratory we subjected the aliquots to two 30 °C bleaches using blue-green diodes then applied a beta dose of 8.28 Gy. Using the SAR procedure we recovered 8.30 ± 0.21 Gy from this sample suggesting that the samples are well behaved for optical dating. A total of six age estimates were determined for samples taken from alluvium that directly underlies the dunes. Five of these samples were run using both 5 and 2 mm aliquots holding ~ 1200 and 200 grains per disk, respectively, and Table 2 shows a comparison of these results. In four of the five cases, 2 mm De values were lower than the 5 mm De values, with 2 mm/5 mm ratios ranging from 0.89–1.09. Importantly, the age estimates calculated from the 2 and 5 mm data fall within 1σ errors in all but one case. Although there are no differences in our interpretations using one or the other, we prefer the somewhat younger age estimates calculated using the 2 mm data simply because they are less likely to show the influences of any partial bleaching problems. Hence, the alluvial ages in Table 1 and Fig. 3 are based on the 2 mm results. Three optical age estimates were taken from Holes B and C on the Qat2 surface on the western end of the Duncan dunes (Figs. 2b and 3). These age estimates range from
Table 2 Comparison of 2 mm and 5 mm De values for fluvial samples Disk size
2 5 2 5 2 5 2 5 2 5
mm mm mm mm mm mm mm mm mm mm a
UNL Lab #
UNL-1636 UNL-1637 UNL-1635 UNL-1638 UNL-1471
De (Gy)
Aliquots a
2 mm/5 mm
Optical age
±1 S.E.
n
De
± 1σ
28.6 ± 1.6 30.1 ± 1.1 34.6 ± 1.3 36.6 ± 1.3 29.1 ± 1.6 32.2 ± 1.5 29.8 ± 1.7 34.2 ± 1.2 10.6 ± 0.7 10.2 ± 0.4
26/30 25/28 25/30 22/25 25/30 22/31 23/30 23/27 25/30 22/31
0.98
13,360 ± 1450 14,040 ± 1180 15,310 ± 1480 16,170 ± 1390 12,600 ± 1300 13,920 ± 1350 16,110 ± 1590 18,490 ± 1690 4980 ± 570 4800 ± 440
Accepted disks/nnall disks.
0.95 0.92 0.89 1.09
560
P.R. Hanson et al. / Geomorphology 103 (2009) 555–561
15.3 to 12.6 ka (Fig. 3). Three optical age estimates for the alluvium taken from within the fill of the Qat1 terrace range from 16.1 to 5.0 ka (Fig. 3: Holes A, D, and F). The 17 optical age estimates taken from dune sands ranged from 5.1 to 0.5 ka. The optical ages recovered from the eolian sands fall into two groups between 4.4–3.4 and 0.8–0.5 ka. One age estimate is an apparent outlier at 5.1 ± 0.4 ka, and was obtained from sand underlying a buried A horizon (Fig. 3: Hole G). Six of the optical age estimates taken from five holes cluster between 4.4 and 3.4 ka. Two of these age estimates are found below buried soils (Fig. 3: Holes G and K), and three of them are found below younger dune sand (Fig. 3: Holes G, J, and K). The youngest ages cluster between ~ 830 and 500 yrs ago (Fig. 3), and include eleven estimates taken from six holes (Fig. 3: Holes E, G, H, I, J, and K). These latest ages are best represented in our samples, and with the exception of Holes B and C (Fig. 3), each of the holes with age control yielded ages that fall within this interval. 5. Discussion The effects of partial bleaching can result in optical age overestimations in some alluvial environments (Olley et al., 1999). Both the local loess stratigraphy and analysis of the De distributions, however, suggest that partial bleaching is not a significant problem in this setting. The absence of loess on the Qta1 and Qta2 surfaces (Fig. 2B) suggests that the surficial deposits should be younger than 14 ka, the estimated termination of loess deposition in the region (Bettis et al., 2003). With one exception (16,100 ± 1600), each of our age estimates is either younger than or falls within 1σ errors of this threshold. Analysis of the De distributions can also be used to analyze problems with partial bleaching. For five of the six alluvial samples we analyzed, both 5 and 2 mm aliquots were used to produce age estimates (Table 2). Three aliquots of samples UNL-1635 and UNL1638 were run using a single-grain laser to estimate the number of grains contributing to the luminescence signal. For this test the aliquots were bleached twice at room temperature, given a 5 Gy dose, and the grains were bleached with the laser. For both samples approximately 10–12% of the grains gave a luminescence signal above background levels, and 3–4% of the grains dominated the total signal. Based on these findings, the 5 mm aliquots would have signal dominated by between ~120 and 50 grains while the 2 mm aliquots would have signal dominated by between ~ 20 and 10 grains, approaching results that would be achieved through single-grain analysis. While this difference in the number of significant grains is large, the resultant difference in the De values is relatively minor (Table 2), and most of the ages generated using the two different aliquot sizes were within 1σ errors. This evidence suggests that partial bleaching is not a significant problem for these samples. Five of the six alluvial age estimates taken from within the terrace fills indicate that the alluvium was deposited between 16.1 and 12.6 ka, while one age is significantly younger indicating that deposition occurred at ~ 5.0 ka (Fig. 3; Hole F). Although we believe that the measurable effects of partial bleaching are negligible in these samples the young age estimate for sample UNL-1471 (~5.0 ka; Fig. 3; Hole F) is problematic. This age estimate lies well outside of the 3σ errors given for the other alluvial samples taken from the fill underlying the Qat1 surface (Table 1). This comparatively young age could simply be erroneous, but we instead suggest that it results from one of the following potential scenarios. One possible explanation is that the age is too young due to post-burial disturbance, such as bioturbation in the shallow subsoil (Bateman et al., 2003; Feathers, 2003). Alternatively, the age could be correct, and dates fluvial activity that is significantly younger than the other alluvial ages from this study. The Qat1 tread was cut at ~ 5.0 ka as suggested by sample UNL-1471, and the older ages from the Qat1 surface are actually exhumation ages resulting from the erosion of older sediments that underlie the terrace tread (Fig. 2B). Finally, the age may be correct
and result from alluvial deposition in an interdune environment. Regardless which of these scenarios are correct, this single sample has negligible bearing on our eolian age estimates which are the focus of this study. A total of seventeen optical age estimates indicate that dunes were largely stabilized over the past five centuries but were active during two periods over the last ~5 kyr. The clustering of these ages indicates that dunes were active during two major periods between 4.4–3.4 and 0.8–0.5 ka. These two recent dune activation events recorded in the Duncan dunes correspond well with other drought records from the Great Plains (Fig. 5). Each of the optical ages clustering within the period of dune activity identified between 4.4 and 3.4 ka overlaps within 2σ errors of the 4.3–4.1 ka drought recognized from multiproxy evidence on the Great Plains (Booth et al., 2005). Each of the Duncan dune ages also overlaps within 1σ of the ~3.8 ka drought identified in the Sand Hills and adjacent loess regions in Nebraska (Miao et al., 2007; Fig. 5). The youngest event recorded from the Duncan dunes also corresponds well with regional drought records. Miao et al. (2007) showed that dune activation in the Nebraska Sand Hills and associated loess deposition occurred from ~ 1.2–0.6 ka (Fig. 5). Similar periods of dune activity were also identified in the Great Bend Sand Prairie in Kansas (Arbogast, 1996), and the Fort Morgan dune field in northeastern Colorado (Clarke and Rendell, 2003). This latest period of dune activity identified in the Duncan dunes and many other dunefields on the Great Plains correlates with increased aridity associated with the Medieval Warm Period (MWP) (Daniels and Knox, 2005). One notable difference between our results and those from the Nebraska Sand Hills is that our earliest dune age, which dates to 5.1 ka, does not appear to correspond to significant drought events from the Great Plains. Based on evidence of dune activity from the Nebraska Sand Hills, Miao et al. (2007) showed that drought conditions existed between 9 and 6 ka, and that dunes were stable until ~3.8 ka. Assuming that the age is accurate our lone 5.1 ka age estimate from this time period may reflect a local blowout or a large-scale dune activation event with limited preservation. Future work in these dunes may help us to better characterize regional dune activity during this time period. Another difference between the Duncan dunes and the Nebraska Sand Hills is the evidence for extensive dune activity around 2.5 ka in the Sand Hills (Miao et al., 2007), contrasted with the apparent lack of dune activity at that time in the Duncan dunes (Fig. 5). This difference could be related to either preservation or sampling biases. Alternatively, and probably more likely, the wetter climate of the Duncan dunes may have given greater vegetative cover and therefore buffered the dunes during some megadroughts. Thus, more easterly dune fields may be expected to show greater stability and less vulnerability to megadroughts. This may explain the apparent stability of these dunes between 3 and 2 ka when records from the Nebraska Sand Hills show dune activation and loess deposition (Miao et al., 2007). Future studies
Fig. 5. Optical age estimates for eolian samples from Duncan dune field. Gray bars represent megadroughts identified in Nebraska Sand Hills and nearby areas (Miao et al., 2007).
P.R. Hanson et al. / Geomorphology 103 (2009) 555–561
in the region's dunes will allow us to better address this apparent dichotomy. Overall, the chronology of dune activity in the Duncan dunes is similar to those which have been reconstructed in some other Great Plains dune fields (Clarke and Rendell, 2003; Arbogast, 1996; Miao et al., 2007). While in most of these larger dunefields dune activity can be directly related to widespread drought conditions, dune activation in lowland settings such as the Duncan dunes could be attributed to multiple causes, including both direct and indirect responses to megadroughts. Specifically, dune activity near streams like the Loup River that drain major dune fields such as the Sand Hills can result from: (1) the lowering of the local water table under a terrace tread by fluvial incision, followed by deflation of sands above the lowered water table; (2) the increased availability of sand to the Loup River through activation of the Nebraska Sand Hills during megadroughts (an indirect response to regional hydrologic drought); or (3) repeated dune reactivation solely within the Duncan dunes due to megadroughts (a direct result of regional hydrologic drought). In theory, case (1) should be manifested in the sedimentary record as a single period of dune activity, without buried soils, large-scale truncation surfaces, or multiple dune ages. Both our geochronologic and geomorphic results indicate that this scenario is not applicable to the Duncan dunes, suggesting that dune mobilization is better explained by either case (2) or case (3). Existing data are insufficient to adequately test hypotheses presented by the two remaining cases. K2O (%) and Rb (ppm) analyses, for example, indicate that the Duncan eolian sands, regardless of proximity to the Loup River, are derived both from the river itself and from the sub-dune terrace (Fig. 4). These data tend, in themselves, to support case (2). But, if the dunes received sand primarily from the Loup River, they would need to have moved 5 km from the river to the southeastern edge of the dune field (Fig. 2B). At migration rates of 20– 30 m/yr (cf. Long and Sharp, 1964; Fryberger et al., 1984), an absolute minimum of 170 to 250 years would have been required to traverse the dune field. This estimated time period is, potentially, equivalent to the duration of each of the late Holocene megadroughts, but sustained dune migration at such high rates is highly unlikely. In opposition to the geochemical data, these projections suggest that case (3) is more likely than case (2). Therefore, existing data, while effectively arguing for a relationship between dune formation and hydrologic drought, present a paradox. A full test of hypotheses of indirect vs. direct dune responses to regional megadroughts will require a comprehensive study of dune fields at the eastern margin of the central Great Plains. 6. Conclusions Seventeen optical age estimates indicate that dunes near Duncan, Nebraska, although stable for long periods during the Holocene, were activated at least twice in the past ~ 5 ka, including between 4.4–3.4 and 0.8–0.5 ka. Our results, compared with regional chronologies of dune activity, suggest that reactivation of the Duncan dunes was either a direct or indirect result of regional megadroughts. The correspondence of the last two episodes of dune activity in the Duncan dunes (4.4–3.4 and 0.8–0.5 ka) with activation in other dune fields reinforces the conclusion that megadroughts, events only recently implicated in system change on the Great Plains (Miao et al., 2007), also impacted the far eastern margin of that region, and perhaps beyond. Acknowledgements This project emerged from geologic mapping funded by the U.S. Geological Survey STATEMAP cooperative mapping program at the Conservation and Survey Division of the School of Natural Resources at the University of Nebraska-Lincoln. The manuscript was improved by input from three anonymous reviewers.
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