PRESENCE OF PRAIRIE DURING THE MID-HOLOCENE AT WINNEBAGO CREEK, SOUTHEASTERN MINNESOTA

The northwest-southeast trending prairie-forest ecotone in the Upper Midwestern United States has shifted in response to climate change throughout the Holocene. The position of the Holocene ecotone is well understood in central Minnesota because of numerous limnological studies conducted on post-glacial lake and bog deposits. In comparison, the position of the Holocene ecotone is difficult to determine in the pseudo-driftless area of southeastern Minnesota because of the few lake and bog environments. In the pseudo-driftless area, Holocene paleoecology can be reconstructed from macrofossils preserved in alluvial sediments that have remained below the water table since deposition.

The goal of this project is to record the position of the ecotone during the mid-Holocene at Winnebago Creek in Houston County, Minnesota, by collecting, identifying, and dating macrofossils. Mid-Holocene alluvial cutbanks were identified in the field based upon the degree of pedogenic alteration. Macrofossil-rich soils were removed by shovel and returned to the lab in 4 liter plastic bags. Soil samples were sieved through a 0.5 mm mesh sieve and macrofossils were removed by hand. Using a dissecting microscope, plant macrofossils were identified by comparison to modern samples and reference publications. Pre-settlement vegetation and geologic maps were constructed from GIS data made available by the Minnesota Geologic Survey.

The presence of prairie species such as Andropogon gerrardii, Amorpha spp., Monarda fistulosa, and Helianthus spp. indicates the presence of prairie at Winnebago Creek during the mid-Holocene. The lack of deciduous taxa at this level also implies prairie conditions. At the time of settlement, about 150 years before present, the Winnebago Creek drainage basin was approximately 45% deciduous forest, 45% prairie, and 10% oak savanna. The fossil assemblage at Winnebago Creek during the mid-Holocene suggests a patch of prairie extending directly east to west, cutting across the prairie-forest boundary. The isolated tongue of prairie at Winnebago Creek speaks to the mosaic nature of the ecotone and its diffuse, rather than sharp, boundary. This patch of prairie at Winnebago Creek persisted until the time of settlement.