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

Pollen, plant macrofossils, phytoliths, carbon isotopes? and alluvial histo ry from sediments exposed along the South Fork of the Big Nemaha River, sou theastern Nebraska, USA, provide an integrated reconstruction of changes in Holocene vegetation, climate, and fluvial activity. From 9000 to 8500 unca librated C-14 yr BP, climate became more arid and the floodplain and alluvi al fans in the main valley aggraded rapidly, upland deciduous forest declin ed, and prairie attained its Holocene dominance. From 8500 to 5800 yr BP. u pland forest elements disappeared, and even riparian trees were sparse unde r dry climatic conditions. Alluvial fans continued to aggrade but aggradati on 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 w ere intermittent. Alluviation was slower and punctuated by two major episod es of channel incision and terrace formation in the main valley. Aggradatio n on alluvial fans slowed and finally ceased near the end of this period. D uring a short dry interval from 3100 to 2700 yr BP riparian tries (except e lm) disappeared, and prairie and weedy species became more abundant. This i nterval is represented by the organic Roberts Creek Member, and the alluvia l setting was a slightly incised meandering channel belt. Habitats became s imilar to presettlement conditions during the last 2700 yr BP. Weedy taxa d ominate modern samples, reflecting widespread disturbance. Alluvial fans an d terrace surfaces were stable during the last 2500 years, but episodes of floodplain aggradation were punctuated by incision of the main channel. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved.