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Pollen diagram from the Nebraska Sandhills and the age of the dunes
QUATERNARY
RESEARCH
24, 115-120 (1985)
Pollen Diagram from the Nebraska Sandhills and the Age of the Dunes H. E. WRIGHT, JR.,* J. C. ALMENDINGER,*
AND J. GR~GER?
*Litnnological Research Center9 University of Minnesota, Minneapolis, Minnesota 55455? and +Abteilung fiir Palynologie der Universittit, Unterekarspiile 2, D-34 Gtittingen, West Germany Received July 6. 1984 Radiocarbon dates of organic alluvium beneath as much as 40 m of dune sand along the Dismal River have led to the suggestion that the Nebraska Sandhills date from the Holocene rather than the last glacial period. On the other hand, the basal layers of lake and marsh deposits in interdune depressions at three localities date in the range of 9000 to 12,000 yr B.P., implying a pre-Holocene age for the sand dunes. A pollen diagram for one of these sites, Swan Lake, indicates prairie vegetation throughout the last 9000 yr, with no suggestion that the landscape was barren enough to permit the shaping of the massive dunes characterizing the area. Sand was not transported across the site during the Holocene, either during the marsh phase, which lasted until 3700 yr BP, or during the subsequent lake phase. The sand that buries the alluvium along the Dismal River may represent only local eohan activity, or it may indicate that the younger of the two main dune series identified by H. T. U. Smith (1965, Journal ofGeo[ogy 73, 557-578) is Holocene in age, and the older one Late Wisconsin in age. ‘D I985 University of Washqtm
INTRODUCTION
One of the most striking geomorphic features of midcontinental North America is the Nebraska Sandhills, which cover an area of about 400 x 200 km in northwestern Nebraska. The area consists of a complex of sand dunes, with individual transverse dunes (megabarchans) as much as 8 km long and 80 m high. Smith (1965) postulated three phases of dune formation on the basis of the pattern of megadunes and superposed microdunes. Warren (1976), however, interpreted the dune forms and structure as a product of a single episode of dominant northwesterly winds with subsidiary southwesterly winds. Wells (1983) also attributes the dunes to northwesterly winds. Both the age of the dunes and the source of the dune sands are controversial. Some postulate that the dunes are entirely or mostly of Holocene age and are derived from underlying alluvial sands. The evidence for this age assignment comes from radiocarbon dates of organic alluvium
buried beneath as much as 40 m of dune sand along the Dismal and Middle Loup rivers in the central part of the Sandhills (Fig. 1); eight dates are in the range of 3000 to 5000 yr B.P., and two older dates are between 7000 and 8500 yr B.P. (Ahlbrandt ef al. 1983). All imply substantial sand movement, especially during the late Holocene. Others suggest that the dunes were formed by periglacial winds during the time of maximum extent of the Laurentide Ice Sheet in the Late Wisconsin or earlier glacial episodes and that the sand is derived from the alluvial deposits and eroded hill slopes of the White River Valley north of the Sandhills (Wright, 1970). Evidence for this viewpoint comes from the close proximity of the Sandhills to the loess blanket to the southeast, which is dated as Late Wisconsin and exhibits a southeasterly decrease in thickness and grain size. Calculations based on rates of sand transport in desert areas with dunes of comparable size indicate that the Sandhills megabarchans must have taken a great many thousand years to form and 115 0033-5894185 $3.00 Copyright c 1985 by the University of Wahington. All rights of reproduction in my form reserved.
116
WRIGHT,
ALMENDINGER,
AND GRUGER
42O-
FIG. 1. Map of Nebraska Sandhihs showing the locations of Rosebud and other dated interdune depressions (H, Hackbetry and Beaver Lakes; K, Krause Ranch; S, Swan Lake) and of alluvial sites buried by sand dunes along the Dismal River (between D and D) and the Middle Loup River(L). The longitudinal dunes in the east belong to the younger of the two main dune series described by Smith (1965). Dune and loess patterns were taken from Thorp and Smith (1952).
must be of Pleistocene rather than Holocene age (Wells, 1982). Another potential means of dating the end of dune formation is provided by organic sediments in lakes or marshes located in interdune swales. Five sites have been dated (Fig. 1). The first two are Hackberry Lake, south of Valentine in the northern Sandhills (Sears, 1961) where the base of a core was dated as 5040 ? 95 yr B.P. (Y-912), and nearby Beaver Lake, where the base of a 6-m core was dated as 6690 ? 80 yr B.P. (WE-1622). The adjacent dunes could have formed in the late Pleistocene or early Holocene. The third such dated site is a marsh near Rosebud near the northern edge of the Sandhills (Watts and Wright, 1966). Here the basal dates are 12,630 ? 160 yr BP. (Y-1360) and 12,580 ? 160 yr B.P. (Y-1359). Spruce pollen exceeding 60% in the dated samples indicates that the area was forested at that time. The dates imply that the dunes bounding the depression had been formed
by strong Late Wisconsin periglacial winds, rather than by Holocene winds. It is known from pollen studies in northeastern Kansas (Grtiger, 1973) that spruce forest dominated in that area throughout the Late Wisconsin, and spruce occupied the Late Wisconsin moraines in South Dakota and North Dakota as the ice retreated. It is postulated that the forest temporarily invaded at least the northern edge of the Sandhills from the northeast as the ice sheet withdrew and the winds subsided. A fourth site is a marsh at the Krause Ranch near the western edge of the Sandhills, where organic matter near the base of a deposit of peat and marl 4.3 m thick yielded a date of 12,080 5 380 yr B.P. (OWU-85A; Ogden and Hay, 1965). Finally, at Swan Lake in the southwestern Sandhills the organic sediment at the base of a 13-m core was dated by Stuiver (1969, p. 578) as 8950 2 160 yr BP (Y-1984). A pollen study of this core is described below.
NEBRASKA
The dates for the Rosebud, Krause, and Swan Lake sites all suggest that the dunes formed before 9000-12,000 yr ago, i.e., that the dunes are Late Wisconsin in age and not Holocene. On the other hand, Ahlbrandt er ui. (1983) contend that the stratigraphic position of the dated organic material at these three sites is uncertain, so that the time of dune formation cannot be determined from the dates. This problem of dating is further discussed after a description of the pollen diagram from Swan Lake. SWAN
LAKE
Swan Lake is an elongated two-basin lake located just west of the Crescent Lake Wildlife Refuge in the western part of the Sandhills (41’ 43’ N, 102’ 30’ W., Crescent Lake Quadrangle, Garden Co., Nebraska). It occupies a swale between two dunes more than 30 m high. The eastern basin has an area of about 80 ha and is 1.4 m deep at the core locality near the center. A core was taken with a piston corer by H. E. Wright through a thin ice cover. The upper sediment consists of pinkish-brown soft gyttja to a depth of about 8.2 m below the water surface, below which is black peat or detritus gyttja to a depth of 14.85 m. The core bottomed abruptly on sand. The basal 10 cm of organic sediment provided the radiocarbon date of 8950 ? 160 yr B.P. mentioned above. A second date at 8.0-8.2 m just above the peaty sediment is 3680 ? 70 yr B.P. (WIS-1537). A preliminary pollen diagram was prepared by J. Griiger, and additional counts were made by J. C. Almendinger (Fig. 2). The pollen sum amounted to 300-700 grains in the upper half of the diagram and usually 100-250 grains in the lower half, where the pollen concentration was lower and where the pollen of sedges and other semiaquatic plants (excluded from the sum except for their own percentage calculations) increased the count by 50- 100 grains for a total generally of at least 200. Preservation was satisfactory except for a few levels in the peaty lower part of the core.
SAND
DUNES
I17
The pollen diagram is dominated by herbs throughout, implying vegetation in the Sandhills much like that of today, as recorded by surface samples elsewhere in the Sandhills area (Watts and Wright, 1966) or farther north in the South Dakota prairie (McAndrews and Wright, 1969). Tree pollen at most levels amounts to about 15-25%. mostly Pinus, presumably derived from ponderosa pine trees that are common on bluffs west of the Sandhills. No explanation is offered for the spike of Pinus at the top of the peaty section. Westerly winds must account also for the pollen grains of Piceo (spruce) and Abies (fir) probably originating from the Rocky Mountains. Juniperu.s-type pollen is more common in the upper half of the sequence, as is Popuh~. Juniper trees grow on bluffs throughout the western part of the Great Plains, and Pup14144s occurs as P. grandidentutu (cottonwood) in a few marshy areas. Cyperaceae (sedge family) has very high values in the lower half of the sequence, along with Typha (cattail) and other semi-aquatic types. The site was apparently a sedge/cattail marsh from its inception until about 3700 years ago, when it became a permanent lake. Because the water level in depressions in the highly permeable sand dunes is determined largely by the groundwater level, it would seem that the conversion of a marsh to a lake 3700 years ago must indicate that the groundwater level was lower during the early and middle Holocene and accordingly that the climate was relatively dry during that time. The slightly lower percentage of tree pollen at this time, even if not locally derived: is additional evidence for dry conditions. On the other hand, the upper half of the diagram has higher percentages of Artemi.siu than below, and this genus is more closely associated with the drier western plains than the eastern plains (McAndrews and Wright, 1969). A possible explanation here is that the relatively low Arfemisiu values for the marsh phase may simply reflect higher percentages for Poaceae (grasses) and chenopods. some of
118
Pwcent of polle” S”nl
I Excluded from pollen sum
FIG. 2. Pollen diagram for the sediment of Swan Lake, Garden County, Nebraska, located in an interdune depression in the southwestern part of the Nebraska Sandhills.
which may be local wet-ground plants and thus not a record of plants growing on the dunes, to which Artemisia was confined. In any case, the inferred marshy condition and low water table is consistent with the interpretation for the eastern part of the prairie: pollen and plant-macrofossil diagrams for Pickerel Lake in northeastern South Dakota also show that the early Holocene was relatively dry (Watts and Bright, 1968). It is not consistent, however, with the conclusion of Ahlbrandt et al. (1983) that greater aridity in the late Holocene (5000-3000 yr B.P.) resulted in major dune activity throughout the Sandhills, AGE OF THE DUNES The radiocarbon dates of buried organic matter along the Dismal River and other localities strongly suggest major sand movement during the late Holocene, and Ahlbrandt et af. (1983) make the case that the entire Sandhills is the product not of Pleistocene eolian action but of Holocene action. On the other hand, at the Rosebud, Krause, and Swan Lake sites the radio-
carbon dates of 12,600 to 9000 yr ago on unburied lake deposits in existing interdune depressions suggest that the dunes have been essentially stable throughout the Holocene. Ahlbrandt er al. (1983, p. 390) object that “at none of these sites was the stratigraphic relationship of the dated horizon to nearby dune sand documented.” Only two alternative explanations can be envisioned if the dunes next to Swan Lake are to be considered as of late Holocene rather than Late Wisconsin age: (1) The sand blew across the lake from one dune to the other without leaving a trace in the intervening depression. It is hard to visualize how massive dunes up to 60 m high could be constructed without leaving some sand in the interdune depression, however, especially if the depression was covered with marsh vegetation. The existence of a marsh at Swan Lake is indicated by the sediment lithology and by the pollen stratigraphy for the lower part of the core. Such a marsh should inhibit the traction of sand grains and should result in their deposition. But the marsh sediments are sand free. After 3700 yr B.l? the per-
NEBRASKA
SAND
DUNES
119
manent lake occupying the depression stratigraphic arrangement can only be conserved as a further barrier to significant firmed by extensive borings in transects exsand transport. Even if the sand blew in the tending from the depressions into the adwinter across the ice, which is short-lived jacent dunes. at best, some grains should be left on the At Swan Lake, however, the soft algal ice surface and be dropped into the sedi- gyttja extends to the surface of the sediment at breakup. In a somewhat compament and is being deposited in the present rable sand-dune setting in central Minneday. The softness of the entire sediment is sota, a lake bordering a small patch of ac- indicated by the ease with which one tive mid-Holocene sand dunes has a scatter person was able to obtain a core 13 m long of sand grains throughout the fine-grained with a 5-cm piston corer without difficulty. gyttja and even has thin sand lenses, The sediment was never compacted by a formed by sand grains carried in suspension load of sand, out into the lake or by traction across the A possible resolution of the apparently ice by strong northwest winds (Keen, contradictory evidence for dating the sand 1985)-a clear stratigaphic record of sand dunes might rest on the suggestion that the movement contemporaneous with dune for- principal dune forms are of Pleistocene age mation. A transect of cores showed that the but that intensive Holocene reactivation sand content diminished away from the permitted even deep burial of interdune ordune field, and no dunes were formed on ganic deposits in some areas, such as along the other side of the lake. the deeply dissecting Dismal River, where ln the case of the Krause site, the 4.3 m lateral migration of the river across its of sediment postdating 12,040 yr B.P. is de- floodplain could undercut the dunes and scribed as peat and marl (Ogden and Hay, then retreat to allow dunes to spill onto the 1965). No intermixed sand is mentioned. At margin of the floodplain. The map of Smith Rosebud the spruce pollen zone dating (1965; Thorp and Smith, 1952) shows two from about 12,600 yr ago is overlain by 5 principal dune series (Fig. 1). The older of m of sandy organic sediment with poorly the two, formed by northerly or northwestpreserved pollen, presumably a marsh de- erly winds, is represented by dune patterns posit in which some eolian sand accumumostly in the northern and western part of lated. The quantity of sand, however, is in- the Sandhills. The younger of the two, sufficient to suppose that major dunes mi- formed dominantly by more westerly winds grated across the site in the Holocene, and locally crosscutting the older more Thus full-scale transport of sand across massive dunes with longitudinal forms, oclakes or marshes in the Nebraska Sandhills curs in the eastern half of the Sandhills, inseems unlikely. The pollen sequence at cluding the Dismal River and Middle Loup Swan Lake does not suggest at any time River areas where the Holocene dates of since 9000 yr ago the kind of barren land- Ahlbrandt et ~1. (1983) were acquired. ,41scape believed by Smith (1965) and Wells though Warren (1976), in his more detailed (1982) to be required for free sand moveanalysis of dune forms, does not make such ment and the migration of major transverse an age distinction, his study area was in the region of Smith’s older dune series, and he sand dunes. (2) The organic sediment in the interconcludes that winds here were primarily dune depressions extends laterally under from the northwest. the adjacent Holocene dunes (in the Additional radiocarbon dates from lake manner of the Dismal River sections) and or marsh sediments in the interdune depreshas since been exhumed in the interdune sions in different parts of the Sandhills may depressions, where it occurs at the surface. resolve the question and permit more acAhlbrandt et af. (1983) indicate that such a curate environmental reconstructions for
WRIGHT,
120
both the Pleistocene area.
and Holocene
ALMENDINGER,
of this
ACKNOWLEDGMENTS Completion of this project was supported by NSF Grant BSR 8311473. Contribution 269 of the Limnological Research Center, University of Minnesota.
REFERENCES Ahlbrandt, T. S., Swinehart, J. B., and Maroney, D. G. (1983). The dynamic Holocene dune tields of the Great Plains and Rocky Mountain basins, U.S.A. In “Eolian Sediments and Processes” (M. E. Brookfield and T. S. Ahlbrandt, Eds.), pp. 379-406. Elsevier, Amsterdam. Griiger, J. (1973). Studies on the late-Quaternary vegetation history of northeastern Kansas. Geo/ogica/ Society of America Bulletin 84, 237-250. Keen, K. L. (1985). “Sand Dunes of the Anoka Sandplain, Minnesota.” Unpublished MS. thesis, University of Minnesota. McAndrews, J. H., and Wright, H. E., Jr. (1969). Modem pollen rain across the Wyoming basins and the northern Great Plants. Review ofPak7eobotany and Palynology 9, 17-43. Ogden, J. G., III, and Hay, R. J. (1965). Ohio Wesleyan University Natural Radiocarbon Measurements. II. Rudiocarbon 7, 116- 173. Sears, P. B. (1961). A pollen profile from the grassland province. Science (Washington, D.C.) 134, 20382040. Smith, H. T. U. (1965). Dune morphology and chro-
AND
GRUGER
nology in central and western Nebraska. Journa/ of Geology 73, 557-578. Stuiver, M. (1969). Yale natural radiocarbon measurements. IX. Radiocarbon 2, 545-658. Thorp, J., and Smith, H. T. U. (1952). “Pleistocene Eolian Deposits of the United States, Alaska, and Parts of Canada.” Geological Society of America, map, scale 1:2,500,000. Warren, A. (1976). Morphology and sediments of the Nebraska Sand Hills in relation to Pleistocene winds and the development of aeolian bedforms. Journa! of Geology 84, 685-700. Watts, W. A., and Bright, R. C. (1968). Pollen, seed, and mollusk analysis of a sediment core from Pickerel Lake, northeastern South Dakota. Geologica/ Society of America Bulletin 79, 855-876. Watts, W. A., and Wright, H. E., Jr. (1966). Late-Wisconsin pollen and seed analysis from the Nebraska Sandhills. Ecology 47, 202-210. Wells, G. L. (1982). Dynamic considerations for the age of the Nebraska Sand Hills. 11th International Congress on Sedirnentology (Hamilton, Ontario, 1982), Abstracts, p. 172. Wells, G. L. (1983). Late-glacial circulation over central North America revealed by aeolion features. In ‘*Variations in the Global Water Budget” (A. StreetPerrott, M. Beran, and R. Ratchffe, Eds.), pp. 317330. Wright, H. E., Jr. (1970). Vegetational history of the central plains. Zn “Pleistocene and Recent Environments of the Central Plains” (W. Dart, Jr. and J. K. Jones, Jr., Eds.), pp. 157-172. Univ. of Kansas Press, Lawrence, Kansas.
RESEARCH
24, 115-120 (1985)
Pollen Diagram from the Nebraska Sandhills and the Age of the Dunes H. E. WRIGHT, JR.,* J. C. ALMENDINGER,*
AND J. GR~GER?
*Litnnological Research Center9 University of Minnesota, Minneapolis, Minnesota 55455? and +Abteilung fiir Palynologie der Universittit, Unterekarspiile 2, D-34 Gtittingen, West Germany Received July 6. 1984 Radiocarbon dates of organic alluvium beneath as much as 40 m of dune sand along the Dismal River have led to the suggestion that the Nebraska Sandhills date from the Holocene rather than the last glacial period. On the other hand, the basal layers of lake and marsh deposits in interdune depressions at three localities date in the range of 9000 to 12,000 yr B.P., implying a pre-Holocene age for the sand dunes. A pollen diagram for one of these sites, Swan Lake, indicates prairie vegetation throughout the last 9000 yr, with no suggestion that the landscape was barren enough to permit the shaping of the massive dunes characterizing the area. Sand was not transported across the site during the Holocene, either during the marsh phase, which lasted until 3700 yr BP, or during the subsequent lake phase. The sand that buries the alluvium along the Dismal River may represent only local eohan activity, or it may indicate that the younger of the two main dune series identified by H. T. U. Smith (1965, Journal ofGeo[ogy 73, 557-578) is Holocene in age, and the older one Late Wisconsin in age. ‘D I985 University of Washqtm
INTRODUCTION
One of the most striking geomorphic features of midcontinental North America is the Nebraska Sandhills, which cover an area of about 400 x 200 km in northwestern Nebraska. The area consists of a complex of sand dunes, with individual transverse dunes (megabarchans) as much as 8 km long and 80 m high. Smith (1965) postulated three phases of dune formation on the basis of the pattern of megadunes and superposed microdunes. Warren (1976), however, interpreted the dune forms and structure as a product of a single episode of dominant northwesterly winds with subsidiary southwesterly winds. Wells (1983) also attributes the dunes to northwesterly winds. Both the age of the dunes and the source of the dune sands are controversial. Some postulate that the dunes are entirely or mostly of Holocene age and are derived from underlying alluvial sands. The evidence for this age assignment comes from radiocarbon dates of organic alluvium
buried beneath as much as 40 m of dune sand along the Dismal and Middle Loup rivers in the central part of the Sandhills (Fig. 1); eight dates are in the range of 3000 to 5000 yr B.P., and two older dates are between 7000 and 8500 yr B.P. (Ahlbrandt ef al. 1983). All imply substantial sand movement, especially during the late Holocene. Others suggest that the dunes were formed by periglacial winds during the time of maximum extent of the Laurentide Ice Sheet in the Late Wisconsin or earlier glacial episodes and that the sand is derived from the alluvial deposits and eroded hill slopes of the White River Valley north of the Sandhills (Wright, 1970). Evidence for this viewpoint comes from the close proximity of the Sandhills to the loess blanket to the southeast, which is dated as Late Wisconsin and exhibits a southeasterly decrease in thickness and grain size. Calculations based on rates of sand transport in desert areas with dunes of comparable size indicate that the Sandhills megabarchans must have taken a great many thousand years to form and 115 0033-5894185 $3.00 Copyright c 1985 by the University of Wahington. All rights of reproduction in my form reserved.
116
WRIGHT,
ALMENDINGER,
AND GRUGER
42O-
FIG. 1. Map of Nebraska Sandhihs showing the locations of Rosebud and other dated interdune depressions (H, Hackbetry and Beaver Lakes; K, Krause Ranch; S, Swan Lake) and of alluvial sites buried by sand dunes along the Dismal River (between D and D) and the Middle Loup River(L). The longitudinal dunes in the east belong to the younger of the two main dune series described by Smith (1965). Dune and loess patterns were taken from Thorp and Smith (1952).
must be of Pleistocene rather than Holocene age (Wells, 1982). Another potential means of dating the end of dune formation is provided by organic sediments in lakes or marshes located in interdune swales. Five sites have been dated (Fig. 1). The first two are Hackberry Lake, south of Valentine in the northern Sandhills (Sears, 1961) where the base of a core was dated as 5040 ? 95 yr B.P. (Y-912), and nearby Beaver Lake, where the base of a 6-m core was dated as 6690 ? 80 yr B.P. (WE-1622). The adjacent dunes could have formed in the late Pleistocene or early Holocene. The third such dated site is a marsh near Rosebud near the northern edge of the Sandhills (Watts and Wright, 1966). Here the basal dates are 12,630 ? 160 yr BP. (Y-1360) and 12,580 ? 160 yr B.P. (Y-1359). Spruce pollen exceeding 60% in the dated samples indicates that the area was forested at that time. The dates imply that the dunes bounding the depression had been formed
by strong Late Wisconsin periglacial winds, rather than by Holocene winds. It is known from pollen studies in northeastern Kansas (Grtiger, 1973) that spruce forest dominated in that area throughout the Late Wisconsin, and spruce occupied the Late Wisconsin moraines in South Dakota and North Dakota as the ice retreated. It is postulated that the forest temporarily invaded at least the northern edge of the Sandhills from the northeast as the ice sheet withdrew and the winds subsided. A fourth site is a marsh at the Krause Ranch near the western edge of the Sandhills, where organic matter near the base of a deposit of peat and marl 4.3 m thick yielded a date of 12,080 5 380 yr B.P. (OWU-85A; Ogden and Hay, 1965). Finally, at Swan Lake in the southwestern Sandhills the organic sediment at the base of a 13-m core was dated by Stuiver (1969, p. 578) as 8950 2 160 yr BP (Y-1984). A pollen study of this core is described below.
NEBRASKA
The dates for the Rosebud, Krause, and Swan Lake sites all suggest that the dunes formed before 9000-12,000 yr ago, i.e., that the dunes are Late Wisconsin in age and not Holocene. On the other hand, Ahlbrandt er ui. (1983) contend that the stratigraphic position of the dated organic material at these three sites is uncertain, so that the time of dune formation cannot be determined from the dates. This problem of dating is further discussed after a description of the pollen diagram from Swan Lake. SWAN
LAKE
Swan Lake is an elongated two-basin lake located just west of the Crescent Lake Wildlife Refuge in the western part of the Sandhills (41’ 43’ N, 102’ 30’ W., Crescent Lake Quadrangle, Garden Co., Nebraska). It occupies a swale between two dunes more than 30 m high. The eastern basin has an area of about 80 ha and is 1.4 m deep at the core locality near the center. A core was taken with a piston corer by H. E. Wright through a thin ice cover. The upper sediment consists of pinkish-brown soft gyttja to a depth of about 8.2 m below the water surface, below which is black peat or detritus gyttja to a depth of 14.85 m. The core bottomed abruptly on sand. The basal 10 cm of organic sediment provided the radiocarbon date of 8950 ? 160 yr B.P. mentioned above. A second date at 8.0-8.2 m just above the peaty sediment is 3680 ? 70 yr B.P. (WIS-1537). A preliminary pollen diagram was prepared by J. Griiger, and additional counts were made by J. C. Almendinger (Fig. 2). The pollen sum amounted to 300-700 grains in the upper half of the diagram and usually 100-250 grains in the lower half, where the pollen concentration was lower and where the pollen of sedges and other semiaquatic plants (excluded from the sum except for their own percentage calculations) increased the count by 50- 100 grains for a total generally of at least 200. Preservation was satisfactory except for a few levels in the peaty lower part of the core.
SAND
DUNES
I17
The pollen diagram is dominated by herbs throughout, implying vegetation in the Sandhills much like that of today, as recorded by surface samples elsewhere in the Sandhills area (Watts and Wright, 1966) or farther north in the South Dakota prairie (McAndrews and Wright, 1969). Tree pollen at most levels amounts to about 15-25%. mostly Pinus, presumably derived from ponderosa pine trees that are common on bluffs west of the Sandhills. No explanation is offered for the spike of Pinus at the top of the peaty section. Westerly winds must account also for the pollen grains of Piceo (spruce) and Abies (fir) probably originating from the Rocky Mountains. Juniperu.s-type pollen is more common in the upper half of the sequence, as is Popuh~. Juniper trees grow on bluffs throughout the western part of the Great Plains, and Pup14144s occurs as P. grandidentutu (cottonwood) in a few marshy areas. Cyperaceae (sedge family) has very high values in the lower half of the sequence, along with Typha (cattail) and other semi-aquatic types. The site was apparently a sedge/cattail marsh from its inception until about 3700 years ago, when it became a permanent lake. Because the water level in depressions in the highly permeable sand dunes is determined largely by the groundwater level, it would seem that the conversion of a marsh to a lake 3700 years ago must indicate that the groundwater level was lower during the early and middle Holocene and accordingly that the climate was relatively dry during that time. The slightly lower percentage of tree pollen at this time, even if not locally derived: is additional evidence for dry conditions. On the other hand, the upper half of the diagram has higher percentages of Artemi.siu than below, and this genus is more closely associated with the drier western plains than the eastern plains (McAndrews and Wright, 1969). A possible explanation here is that the relatively low Arfemisiu values for the marsh phase may simply reflect higher percentages for Poaceae (grasses) and chenopods. some of
118
Pwcent of polle” S”nl
I Excluded from pollen sum
FIG. 2. Pollen diagram for the sediment of Swan Lake, Garden County, Nebraska, located in an interdune depression in the southwestern part of the Nebraska Sandhills.
which may be local wet-ground plants and thus not a record of plants growing on the dunes, to which Artemisia was confined. In any case, the inferred marshy condition and low water table is consistent with the interpretation for the eastern part of the prairie: pollen and plant-macrofossil diagrams for Pickerel Lake in northeastern South Dakota also show that the early Holocene was relatively dry (Watts and Bright, 1968). It is not consistent, however, with the conclusion of Ahlbrandt et al. (1983) that greater aridity in the late Holocene (5000-3000 yr B.P.) resulted in major dune activity throughout the Sandhills, AGE OF THE DUNES The radiocarbon dates of buried organic matter along the Dismal River and other localities strongly suggest major sand movement during the late Holocene, and Ahlbrandt et af. (1983) make the case that the entire Sandhills is the product not of Pleistocene eolian action but of Holocene action. On the other hand, at the Rosebud, Krause, and Swan Lake sites the radio-
carbon dates of 12,600 to 9000 yr ago on unburied lake deposits in existing interdune depressions suggest that the dunes have been essentially stable throughout the Holocene. Ahlbrandt er al. (1983, p. 390) object that “at none of these sites was the stratigraphic relationship of the dated horizon to nearby dune sand documented.” Only two alternative explanations can be envisioned if the dunes next to Swan Lake are to be considered as of late Holocene rather than Late Wisconsin age: (1) The sand blew across the lake from one dune to the other without leaving a trace in the intervening depression. It is hard to visualize how massive dunes up to 60 m high could be constructed without leaving some sand in the interdune depression, however, especially if the depression was covered with marsh vegetation. The existence of a marsh at Swan Lake is indicated by the sediment lithology and by the pollen stratigraphy for the lower part of the core. Such a marsh should inhibit the traction of sand grains and should result in their deposition. But the marsh sediments are sand free. After 3700 yr B.l? the per-
NEBRASKA
SAND
DUNES
119
manent lake occupying the depression stratigraphic arrangement can only be conserved as a further barrier to significant firmed by extensive borings in transects exsand transport. Even if the sand blew in the tending from the depressions into the adwinter across the ice, which is short-lived jacent dunes. at best, some grains should be left on the At Swan Lake, however, the soft algal ice surface and be dropped into the sedi- gyttja extends to the surface of the sediment at breakup. In a somewhat compament and is being deposited in the present rable sand-dune setting in central Minneday. The softness of the entire sediment is sota, a lake bordering a small patch of ac- indicated by the ease with which one tive mid-Holocene sand dunes has a scatter person was able to obtain a core 13 m long of sand grains throughout the fine-grained with a 5-cm piston corer without difficulty. gyttja and even has thin sand lenses, The sediment was never compacted by a formed by sand grains carried in suspension load of sand, out into the lake or by traction across the A possible resolution of the apparently ice by strong northwest winds (Keen, contradictory evidence for dating the sand 1985)-a clear stratigaphic record of sand dunes might rest on the suggestion that the movement contemporaneous with dune for- principal dune forms are of Pleistocene age mation. A transect of cores showed that the but that intensive Holocene reactivation sand content diminished away from the permitted even deep burial of interdune ordune field, and no dunes were formed on ganic deposits in some areas, such as along the other side of the lake. the deeply dissecting Dismal River, where ln the case of the Krause site, the 4.3 m lateral migration of the river across its of sediment postdating 12,040 yr B.P. is de- floodplain could undercut the dunes and scribed as peat and marl (Ogden and Hay, then retreat to allow dunes to spill onto the 1965). No intermixed sand is mentioned. At margin of the floodplain. The map of Smith Rosebud the spruce pollen zone dating (1965; Thorp and Smith, 1952) shows two from about 12,600 yr ago is overlain by 5 principal dune series (Fig. 1). The older of m of sandy organic sediment with poorly the two, formed by northerly or northwestpreserved pollen, presumably a marsh de- erly winds, is represented by dune patterns posit in which some eolian sand accumumostly in the northern and western part of lated. The quantity of sand, however, is in- the Sandhills. The younger of the two, sufficient to suppose that major dunes mi- formed dominantly by more westerly winds grated across the site in the Holocene, and locally crosscutting the older more Thus full-scale transport of sand across massive dunes with longitudinal forms, oclakes or marshes in the Nebraska Sandhills curs in the eastern half of the Sandhills, inseems unlikely. The pollen sequence at cluding the Dismal River and Middle Loup Swan Lake does not suggest at any time River areas where the Holocene dates of since 9000 yr ago the kind of barren land- Ahlbrandt et ~1. (1983) were acquired. ,41scape believed by Smith (1965) and Wells though Warren (1976), in his more detailed (1982) to be required for free sand moveanalysis of dune forms, does not make such ment and the migration of major transverse an age distinction, his study area was in the region of Smith’s older dune series, and he sand dunes. (2) The organic sediment in the interconcludes that winds here were primarily dune depressions extends laterally under from the northwest. the adjacent Holocene dunes (in the Additional radiocarbon dates from lake manner of the Dismal River sections) and or marsh sediments in the interdune depreshas since been exhumed in the interdune sions in different parts of the Sandhills may depressions, where it occurs at the surface. resolve the question and permit more acAhlbrandt et af. (1983) indicate that such a curate environmental reconstructions for
WRIGHT,
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both the Pleistocene area.
and Holocene
ALMENDINGER,
of this
ACKNOWLEDGMENTS Completion of this project was supported by NSF Grant BSR 8311473. Contribution 269 of the Limnological Research Center, University of Minnesota.
REFERENCES Ahlbrandt, T. S., Swinehart, J. B., and Maroney, D. G. (1983). The dynamic Holocene dune tields of the Great Plains and Rocky Mountain basins, U.S.A. In “Eolian Sediments and Processes” (M. E. Brookfield and T. S. Ahlbrandt, Eds.), pp. 379-406. Elsevier, Amsterdam. Griiger, J. (1973). Studies on the late-Quaternary vegetation history of northeastern Kansas. Geo/ogica/ Society of America Bulletin 84, 237-250. Keen, K. L. (1985). “Sand Dunes of the Anoka Sandplain, Minnesota.” Unpublished MS. thesis, University of Minnesota. McAndrews, J. H., and Wright, H. E., Jr. (1969). Modem pollen rain across the Wyoming basins and the northern Great Plants. Review ofPak7eobotany and Palynology 9, 17-43. Ogden, J. G., III, and Hay, R. J. (1965). Ohio Wesleyan University Natural Radiocarbon Measurements. II. Rudiocarbon 7, 116- 173. Sears, P. B. (1961). A pollen profile from the grassland province. Science (Washington, D.C.) 134, 20382040. Smith, H. T. U. (1965). Dune morphology and chro-
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nology in central and western Nebraska. Journa/ of Geology 73, 557-578. Stuiver, M. (1969). Yale natural radiocarbon measurements. IX. Radiocarbon 2, 545-658. Thorp, J., and Smith, H. T. U. (1952). “Pleistocene Eolian Deposits of the United States, Alaska, and Parts of Canada.” Geological Society of America, map, scale 1:2,500,000. Warren, A. (1976). Morphology and sediments of the Nebraska Sand Hills in relation to Pleistocene winds and the development of aeolian bedforms. Journa! of Geology 84, 685-700. Watts, W. A., and Bright, R. C. (1968). Pollen, seed, and mollusk analysis of a sediment core from Pickerel Lake, northeastern South Dakota. Geologica/ Society of America Bulletin 79, 855-876. Watts, W. A., and Wright, H. E., Jr. (1966). Late-Wisconsin pollen and seed analysis from the Nebraska Sandhills. Ecology 47, 202-210. Wells, G. L. (1982). Dynamic considerations for the age of the Nebraska Sand Hills. 11th International Congress on Sedirnentology (Hamilton, Ontario, 1982), Abstracts, p. 172. Wells, G. L. (1983). Late-glacial circulation over central North America revealed by aeolion features. In ‘*Variations in the Global Water Budget” (A. StreetPerrott, M. Beran, and R. Ratchffe, Eds.), pp. 317330. Wright, H. E., Jr. (1970). Vegetational history of the central plains. Zn “Pleistocene and Recent Environments of the Central Plains” (W. Dart, Jr. and J. K. Jones, Jr., Eds.), pp. 157-172. Univ. of Kansas Press, Lawrence, Kansas.