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Late Quaternary landscape evolution and bioclimatic change in the central Great Plains, USA

Anthony L. Layzell and Rolfe D. Mandel
Late Quaternary landscape evolution and bioclimatic change in the central Great Plains, USA
Geological Society of America Bulletin (April 2020) Pre-Issue Publication

Abstract

A systematic study of floodplains, terraces, and alluvial fans in the Republican River valley of south-central Nebraska provided a well-dated, detailed reconstruction of late Quaternary landscape evolution and resolved outstanding issues related to previously proposed Holocene terrace sequences. Stable carbon isotope (delta 13C) values determined on soil organic matter from buried soils in alluvial landforms were used to reconstruct the structure of vegetation communities and provided a means to investigate the relationships between bioclimatic change and fluvial activity for the period of record. Our study serves as a model for geomorphological and geoarcheological investigations in stream valleys throughout the central Great Plains and wherever loess-derived late Quaternary alluvial fans occur, in particular. Holocene alluvial landforms in the river valley include a broad floodplain complex (T-0a, T-0b, and T-0c), a single alluvial terrace (T-1), and alluvial fans that mostly grade to the T-1 (AF-1) and T-0c (AF-0c) surfaces. Remnants of a late Pleistocene terrace (T-2), mantled by Holocene (Bignell) loess, are also preserved, and some Holocene alluvial fans (AF-2) grade to T-2 surfaces. Radiocarbon ages suggest that the T-1 fill and AF-1 fans aggraded between ca. 9000-1000 yr B.P. Hence, nearly all of the Holocene alluvium in the river valley is stored in these landforms. Sedimentation, however, was interrupted by several periods of landscape stability and soil formation. Radiocarbon ages from the upper A horizons of buried soils in the T-1 and AF-1 fills, indicating approximate burial ages, cluster at ca. 6500, 4500, 3500, and 1000 yr B.P. Also, based on the radiocarbon ages, the T-0c fill and AF-0c fans were aggrading between ca. 2000-900 yr B.P. Given that the T-0c fill and upper parts of the T-1 fill were both aggrading after ca. 2000 yr B.P, we suggest that the T-1 surface was abandoned between ca. 4500-3500 yr B.P., but subsequent aggradation of both the T-1 and T-0c fills occurred due to large-magnitude flood events during the late Holocene. The delta 13C data indicate a shift from approximately 40% C4 biomass at ca. 6000 to approximately 85% at ca. 4500 yr B.P. We propose a scenario where (1) a reduction in C3 vegetation after 6000 yr B.P. destabilized the uplands, resulting in an increase in sediment supply and aggradation of the T-1 fill and AF-1 fans, and (2) the establishment of C4 vegetation by ca. 4500 yr B.P. stabilized the uplands, resulting in a reduction in sediment supply and subsequent incision and abandonment of the T-1 and most AF-1 surfaces. The proposed timing and nature of landscape and bioclimatic change are consistent with regional records from the central Great Plains.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: Pre-Issue Publication
Title: Late Quaternary landscape evolution and bioclimatic change in the central Great Plains, USA
Affiliation: University of Kansas, Kansas Geological Survey, Lawrence, KS, United States
Published: 20200410
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 66
Accession Number: 2020-039390
Categories: Quaternary geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 1 table, sketch map
N40°00'00" - N40°22'00", W99°30'00" - W99°10'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 2020
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