Holocene paleoenvironments in southeastern Minnesota – chasing the prairie-forest ecotone

Abstract

This paper uses a multi-proxy approach involving pollen, plant macrofossils, speleothem isotopes, and alluvial history of streams to reconstruct the history of prairie expansion and contraction along the prairie-forest border of southeastern Minnesota, USA. Early Holocene forests were replaced by prairie along this border, but eastward expansion of prairie stalled for 2000 yr when the prairie-forest ecotone stabilized. Prairie invaded the area from the west 8000–9000 yr B.P., but mesic forest remained less than 100 km to the east until about 6000 yr B.P. Changes in δ 13C values in speleothem calcite, that reflect the rise of C 4 grasses, correlate well with the presence of C 4 grass species identified in the plant macrofossil record. Periods of large floods correlate with speleothem evidence of dry summers, increased cool-season precipitation (both resulting in less plant cover to absorb moisture), and change to prairie vegetation.