Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska

Abstract

Pollen, plant macrofossils, phytoliths, carbon isotopes, and alluvial history from sediments exposed along the South Fork of the Big Nemaha River, southeastern Nebraska, USA, provide an integrated reconstruction of changes in Holocene vegetation, climate, and fluvial activity. From 9000 to 8500 uncalibrated 14 C yr BP, climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly, upland deciduous forest declined, and prairie attained its Holocene dominance. From 8500 to 5800 yr BP. upland forest elements disappeared, and even riparian trees were sparse under dry climatic conditions. Alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr BP. From 5800 to 3100 yr BP, riparian forests returned to prominence, and droughts were intermittent. Alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley. Aggradation on alluvial fans slowed and finally ceased near the end of this period. During a short dry interval from 3100 to 2700 yr BP riparian trees (except elm) disappeared, and prairie and weedy species became more abundant. This interval is represented by the organic Roberts Creek Member, and the alluvial setting was a slightly incised meandering channel belt. Habitats became similar to presettlement conditions during the last 2700 yr BP. Weedy taxa dominate modern samples, reflecting widespread disturbance. Alluvial fans and terrace surfaces were stable during the last 2500 years, but episodes of floodplain aggradation were punctuated by incision of the main channel.