T285 Examining Big Questions or Paradigms with Geochronology

Wednesday, 7 November 2018
1:30 PM - 5:30 PM
Room 136-137

HOLOCENE LANDSCAPE EVOLUTION AND BIOCLIMATIC CHANGE IN THE REPUBLICAN RIVER VALLEY, SOUTH-CENTRAL NEBRASKA: IMPLICATIONS FOR GEOARCHAEOLOGY IN THE CENTRAL GREAT PLAINS

Paper number: T285-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

The Republican River valley in south-central Nebraska has a long history of stratigraphic and archaeologic research. Stratigraphic investigations conducted throughout the late 20th century proposed a sequence consisting of two distinct Holocene terraces. This differentiation, however, was based on limited chronologic control and is problematic. Accurately determining the timing and nature of Holocene landscape evolution is needed in order to assess the geologic potential for buried prehistoric cultural deposits in the region. Here, we present results from a systematic study of the stratigraphy of different alluvial landform sediment assemblages (LSAs) at 22 localities in the Republican River valley. A suite of nearly 50 bulk soil 14C ages provides chronological control for periods of landscape stability and soil formation.

A mosaic of Holocene alluvial deposits occurs in the Republican River valley. Specifically, the valley floor consists of a broad floodplain complex (T-0a, T-0b, T-0c), a single alluvial terrace (T-1), and large coalescing alluvial fans that grade to the T-1 surface. Radiocarbon ages suggest that the T-1 fill and alluvial fans aggraded between ca. 9000 and 1000 14C yr B.P. Hence, nearly all of the Holocene alluvium in the Republican River valley is represented by these LSAs. Sedimentation, however, was interrupted by several periods of landscape stability and soil development. Preliminary 14C ages from these buried soils cluster around ca. 6800, 4500, 1800, and 1200 14C yr B.P. Entrenchment of the Republican River occurred around 1000 14C yr B.P. and was soon followed by aggradation and the accumulation of the T-0c fill.

Bioclimatic change was inferred from the δ13C composition of soil organic matter. Preliminary δ13C data show an increase from ~40% C4 biomass at ca. 6800 14C yr B.P. to ~85% at ca. 4500 14C yr B.P., and a second peak of ~85% at ca. 1200 14C yr B.P. These increases in C4 biomass are interpreted as reflecting a shift to warmer and likely drier conditions during the Altithermal and immediately prior to the Medieval Climatic Anomaly, respectively. The proposed timing and nature of landscape change is consistent with regional records from the central Great Plains and provides an important model for identifying where specific cultural deposits are likely to be preserved in buried contexts.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-319737

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