PALEOENVIRONMENTAL RECORD FROM A LATE GLACIAL TO EARLY HOLOCENE BEAVER POND IN EAST-CENTRAL WISCONSIN

The waning stages of the late Wisconsin were a time of rapid and pronounced climate change, and therefore, when pond sediment and peat exposed in a farm-pond excavation in east-central Wisconsin were found to contain abundant plant macrofossils, including many trunks and branches of trees that preserve beaver tooth marks (North American beaver, Castor canadensis), that date to this time period, we realized that this could be an important site at which to investigate macro- and microfossil records. Radiocarbon dates on wood indicate that sediment began accumulating at the site 14,000 cal yr B.P. in a small (30 by 50 meters) kettle and ceased after 10,740 cal yr B.P.

At two levels in the pond sediment, tree trunks (mostly Picea [spruce]) and branches were woven together in a structure resembling a dam. In other parts of the exposure, concentrations of beaver-chewed twigs and sand and gravel suggest the remains of beaver lodges. Other common plant macrofossils are spruce needles, cones of black spruce (P. mariana) and tamarack (Larix laricina), and seeds from the aquatic plants yellow water lily (Nuphar), slender pondweed (Potamogeton filiformis), and water nymph (Najas flexilis). Muskgrass (Chara) oospores indicate hard-water conditions in the pond. Invertebrates preserved include water flea (Daphnia) egg cases, gastropods (including Gyraulis), and bivalves (including fingernail clams, Pisidium). A muskrat (Ondatra zibethicus) femur also was recovered. The peat is composed almost entirely of the calciphilic aquatic moss Calliergon trifarium, a moss of mineral-rich fens.

The single major climatic event preserved in the sediment is represented by the change from mud deposition to peat at 11,000 cal yr B.P. indicating declining water level in the pond. This change is the culmination a drying trend that began shortly after 14,000 cal yr B.P. and is evidenced by decreasing conifer needle concentrations and increasing charcoal concentrations. Additional evidence for this change in climate is provided by exposure damage recorded in the rings of a number of fossil tree roots.