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Abstract: The recent increase in dune studies in North America has been heavily focused in the Great Plains, while less attention has historically been given to the dune fields east of the Mississippi River. Here we report ages and suggest a potential sediment source for sand dunes in the Green River Lowland, Illinois, which may provide a better understanding of the dynamic interactions between eolian, glacial, lacustrine and fluvial processes that shaped the landscapes of the upper Midwest. Seven coherent optically stimulated luminescence ages (OSL, or optical ages) obtained from four sites suggest that major dune construction in the Green River Lowland occurred within a narrow time window around 17,500ago. This implies either an enhanced aridity or an episodic increase of sediment supply at 17,500years ago, or combination of the both. Contrary to previous assertions that dune sand was sourced from the deflation of the underlying outwash sand deposited when the Lake Michigan Lobe retreated from the area, we propose that Green River Lowland dunes sand originated from the Green Bay Lobe through the Rock River. Specifically, sediment supply increased in the Rock River valley during drainage of Glacial Lake Scuppernong, which formed between ∼18,000 and 17,000years ago, when the Green Bay Lobe retreated from its terminal moraine. The lake drained catastrophically through the Rock River valley, providing glacial sediment and water to erode the preexisting sandy sediments. Throughout the remainder of the late Pleistocene, the Laurentide Ice Sheet drained into larger more northerly glacial lakes that in turn drained through other river valleys. Therefore, the dunes in the Green River Lowland formed only during the catastrophic drainage of Glacial Lake Scuppernong, but were stabilized through the remainder of the Pleistocene. This scenario explains the abrupt dune construction around 17,500years ago, and explains the lack of later dune activity up to the Pleistocene–Holocene transition. OSL and radiocarbon ages also indicate that dunes were reactivated during the early, middle and late Holocene. Some eolian activation occurred within well-defined dry intervals in the upper Midwest, suggesting that increased aridity may have been the primary driver in mobilizing sand. However, many ages do not correspond to drier periods. In contrast to the relative coherency of the Pleistocene OSL ages from multiple study sites, the Holocene OSL ages do not overlap from one site to another, suggesting that increased aridity alone cannot explain the multiple phases of dune reactivation in the Holocene. Therefore, we conclude that the combined effect of localized disturbances and greater aridity acted in concert to increase eolian sand activity in the Holocene. The multiple periods of eolian activity during the Holocene suggest a high potential for future sand activation in the region, and these results are informative for environmental prediction and potential future mitigation. [Copyright &y& Elsevier]

Abstract: Various lines of evidence support conflicting interpretations of the timing, abruptness, and nature of climate change in the Great Plains during the Pleistocene–Holocene transition. Loess deposits and paleosols on both the central and northern Great Plains provide a valuable record that can help address these issues. A synthesis of new and previously reported optical and radiocarbon ages indicates that the Brady Soil, which marks the boundary between late Pleistocene Peoria Loess and Holocene Bignell Loess, began forming after a reduction in the rate of Peoria Loess accumulation that most likely occurred between 13.5 and 15cal ka. Brady Soil formation spanned all or part of the Bølling-Allerød episode (approximately 14.7–12.9cal ka) and all of the Younger Dryas episode (12.9–11.5cal ka) and extended at least 1000 years beyond the end of the Younger Dryas. The Brady Soil was buried by Bignell Loess sedimentation beginning around 10.5–9cal ka, and continuing episodically through the Holocene. Evidence for a brief increase in loess influx during the Younger Dryas is noteworthy but very limited. Most late Quaternary loess accumulation in the central Great Plains was nonglacigenic and was under relatively direct climatic control. Thus, Brady Soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis, probably through relatively high effective moisture. Optical dating of loess in North Dakota supports correlation of the Leonard Paleosol on the northern Great Plains with the Brady Soil. Thick loess in North Dakota was primarily derived from the Missouri River floodplain; thus, its stratigraphy may in part reflect glacial influence on the Missouri River. Nonetheless, the persistence of minimal loess accumulation and soil formation until 10cal ka at our North Dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored Brady Soil formation. Burial of both the Brady Soil and the Leonard Paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity. Overall, the loess–paleosol sequences of the central and northern Great Plains record a broad peak of high effective moisture across the late Pleistocene to Holocene boundary, rather than well-defined climatic episodes corresponding to the Bølling-Allerød and Younger Dryas episodes in the North Atlantic region. [Copyright &y& Elsevier]

This paper reexamines the stratigraphy, sources, and paleoclimatic significance of Holocene Bignell Loess in the central Great Plains. A broadly similar sequence of loess depositional units and paleosols was observed in thick Bignell Loess sections up to 300 km apart, suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis. New optical ages, together with previously reported radiocarbon ages, indicate Bignell Loess deposition began 9000–11,000 yr ago and continued into the late Holocene; some Bignell Loess is <1000 yr old. There is little evidence that Holocene Loess was derived from flood plain sources, as previously proposed. Instead, thick Bignell Loess occurs mainly near the downwind margins of inactive dune fields, particularly atop escarpments facing the dunes. Thus, the immediate loess source was dust produced when the dunes were active. Previous work indicates that widespread episodes of dune activity are likely to have resulted from drier-than-present climatic conditions. The regionally coherent stratigraphy of Bignell Loess can be interpreted as a near-continuous record of climatically driven variation in dune field activity throughout the Holocene. [Copyright &y& Elsevier]

Loess deposits of the central Great Plains, USA, and their intercalated soils provide a detailed record of climatically driven changes within the aeolian system during the Pleistocene-Holocene transition and the Holocene. Here we present a detailed optically stimulated luminescence (OSL) chronology as well as multi-proxy analysis obtained for the first time on the Enders section, located in southwestern Nebraska, central Great Plains. The section records multiple episodes of rapid loess deposition alternating with soil formation. Rapid accumulation of Late Pleistocene Peoria Loess was replaced around 13–14 ka by formation of the Brady Soil until 9.5 ± 0.6 ka. The Holocene Bignell Loess then buried the Brady Soil and accumulated episodically throughout the Holocene. The loess-paleosol stratigraphy since the Late Pleistocene at the Enders site is very similar to that at other sites in western Nebraska, and the newly developed OSL chronology (based on three grain size classes) adds new confidence to earlier dating. The high-resolution grain size profile from Enders shares many features with similar data from the previously studied Wauneta site, including three peaks of fine-grained material just above and within the Brady Soil, likely representing response to millennial-scale climatic changes during the Pleistocene-Holocene transition. This study demonstrates the potential for developing high-resolution, well-dated paleoclimatic records from the loess of the central Great Plains. Contrasts between Great Plains and Eurasian loess records reflect differences in the Late Pleistocene to Holocene climatic evolution and other factors influencing the loess system. • Multi-proxy paleoclimatic record obtained from loess sections of Great Plains, USA. • New multi-method OSL dating adds confidence to Great Plains loess chronology. • Brady Soil (12.7 ± 0.8–9.5 ± 0.6 ka) buried by episodic Holocene loess deposition. • Isotopic analysis of carbonates in this loess may provide paleoclimatic records. • Contrasts with Eurasian loess reflect regional variation in climate evolution. [ABSTRACT FROM AUTHOR]