LAST GLACIAL MAXIMUM POLLEN AND PLANT MACROFOSSILS FROM THE PLATTE RIVER VALLEY IN SOUTH-CENTRAL NEBRASKA, USA

There are very few Pleistocene plant macrofossil and pollen localities in the Central Great Plains region. During subsurface investigations in support of a geologic mapping project (USGS STATEMAP) we recovered a 9.1 m core from a loess-mantled alluvial surface in the big bend portion of the Platte River valley in south-central Nebraska. The core revealed ~6.3 m of Peoria loess overlying 3 m of silty alluvium and possible loess. The core terminated in alluvial sand and gravel. The lower portion of the core (8.1 to 8.7 m bgs) is organic-rich and contains well-preserved plant remains. Samples of spruce needles from the upper and lower levels of this portion yielded calibrated AMS ¹⁴C ages of ~22.6 and ~23.4 ka, respectively. We have identified this deposit in additional cores, demonstrating that it extends for at least 1.6 km² and is up to 5 m thick. AMS ¹⁴C analyses on wood sampled from a recent core yielded calibrated ages of ~22.7 and ~26.7 ka on the upper and lower portions of the deposit.

The deposit is minerogenic, but contains abundant plant macrofossils, mostly Picea needle fragments, Carex and Pilea fontana seeds, abundant wood fragments, and a large amount of unidentifiable plant debris. We also collected insect remains, including an intact rove beetle (Staphylinidae). We are currently completing pollen analyses on samples from the deposit. Our preliminary observations include tree and shrub pollen, mostly Picea and Pinus, with minor amounts of pollen of deciduous genera such as Salix, Betula, Ulmus, Alnus and possibly Cupressaceae. Non-arboreal pollen includes grasses (Poaceae), sedges (Carex, Cyperaceae), and other herbaceous plants (Asteraceae, Lamiaceae, and Liguliflorae).

The presence of coniferous stomates indicates that the trees most likely grew on the adjacent landscape. Our preliminary data suggest a last glacial maximum environment of open, spruce-deciduous parkland with interspersed wetland and grassland areas.