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

The Republican River valley in south-central Nebraska has a long history of stratigraphic and archaeologic research. Stratigraphic investigations conducted throughout the late 20^th 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.