Late Pleistocene Palynology and Biogeography of the Western Missouri Ozarks

Abstract

Palynological investigations of spring deposits were conducted as part of an interdisciplinary research effort into late Pleistocene archaeology, geology, and paleoecology of the western Missouri Ozarks. The springs, located in the Pomme de Terre River valley in Benton and Hickory Counties, Missouri, lie 400 km south of the maximum extent of the Wisconsin ice sheet on the present Great Plains—Eastern Deciduous Forest border. The valley will soon be flooded by a government impoundment. Two springs were completely excavated and three others sampled, including a spring originally excavated by Albert Koch in 1840. The pollen record, with associated plant macrofossils, is divided into three distinct zones: a lower nonarboreal pollen (NAP) and pine zone, a middle spruce— (Picea) dominated zone, and an upper zone of spruce with deciduous elements. Radiocarbon dates show that the NAP—pine zone was established prior to 40,000 BP and existed until 20,000—25,000 BP; it was deposited during the mid—Wisconsin interstade. The mid—Wisconsin interstadial vegetation of the Ozarks is interpreted as an open pine—parkland. Plant macrofossils of Pinus banksiana in this zone indicate that jack pine was present. Spruce was probably absent as no spruce macrofossils and only traces of spruce pollen were found. The associated mid—Wisconsin fauna is dominated by mastodon (Mammut americanum) with horse (Equus) and musk—ox (Symbos). With the onset of late Wisconsin full—glacial conditions, variously dated at 20,000—25,000 BP, pollen dominance shifts from NAP—pine to spruce. This is interpreted as the replacement of the pine—parkland by boreal spruce forest. Two pollen zones are apparent within the full—glacial: an older zone with up to 92% spruce pollen occurring after the transition from NAP—pine to spruce, and a younger zone with less spruce (38%) and more thermophilous deciduous tree pollen. The change to less spruce and more deciduous tree pollen apparently reflects slightly improved climatic conditions during a late phase of the full—glacial; the zone is radiocarbon dated at 16,500 BP on spruce logs. It contains abundant spruce and larch (Larix) macrofossils associated with the remains of at least 30 mastodons plus giant beaver (Castoroides), ground sloth (Paramylodon), tapir (Tapirus), deer (Odocoileus), and horse (Equus). The western Missouri Ozarks were covered by open pine—parkland from at least 40,000 BP until the start of the full—glacial 20,000—25,000 BP, then boreal spruce forest until at least 13,500 BP. Deciduous elements became more prominent in the spruce forest in its latter phase. The present oak—hickory forest developed after the decline of the spruce and is thus no older than postglacial.