Deciphering eolian sand depositional records
western Great Plains (past 2000 years)
landscape response to extreme drought
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Stabilized
dune fields are common throughout the Great Plains of North America. These landforms
attest to periods of pronounced aridity during the Holocene that exceeded or
equaled drought conditions in the 20th century. Field research has focused on
deciphering the activity of parabolic and barchanoid-ridge paleodunes in western
Nebraska in the past 2000 years. The internal stratigraphy of dunes is well exposed
in sections adjacent to lakes, with many of the lakes formed with dune damning
of drainages. These sections reveal intercalated weak soils (A/C or A/Bw) within
eolian sand sequences that indicate repeated dune reactivation during the late
Holocene. Quartz grains from these eolian sediments are directly dated using
recent advances in optically stimulated luminescence, single aliquot regeneration
(OSL-SAR) protocols providing decadal to century precision for the past 2 ka.
Stratigraphic analyses combined with OSL-SAR ages identify eolian sand depositional
events separated by paleosols at 60 ± 20, 150 ± 20, 460 ± 30,
and 1310 ± 100 cal. yr BP which are coincident with the limited 14C age
control and with droughts inferred from the tree-ring or paleolimnologic record.
One of the thickest (>4 m) eolian sand deposits in western Nebraska, exhibiting
clear cross-stratification indicative of large-scale dune movement and is dated
by OSL-SAR to the late 16th century, which is coincident with a well recognized
and widespread North American “megadrought” in the tree-ring record.
SAR-OSL ages also place deposition of cover sand in the mid-19th century associated
with a persistent drought identified in dendroclimatic records from the Rocky
Mountains and western Great Plains. There is also compelling geomorphic and stratigraphic
evidence for the reactivation of dunes during the 1930’s drought, and is
confirmed by aerial photography from 1939 that shows abundant sand blows, incipient
parabolic dunes and dried lakes and wetlands. Dune reactivation of various magnitudes
occurred at least five times in the past 1500 years and appears to be coincident
with many continental-scale droughts identified in the tree-ring record. |
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S. L. Forman, J. Pierson, R. Webb (NOAA-Boulder) and G. H. Miller (Univ. of Colorado-Boulder)
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