Diatom indicators of peatland development at Pogonia Bog Pond, Minnesota, USA

Sediment cores were taken from the central pond and fringing mat of a Sphagnum peatland located in the Big Woods vegetation type. Pollen analysis of the pond core spanning 12 000 cal. BP to the present indicated a mid-postglacial warm period (10 000 to 3900 cal. BP at this site), when the area was covered by open oak woodland and prairie. From 3900 to 300 cal. BP oak forest expanded. At 300 cal. BP (AD 1650) Big Woods species (e.g., Tilia, Ostrya, Ulmus and Acer) increased as a result of 'Little Ice Age' cooling. Diatoms indicating water of high acid-neutralizing capacity (e.g., Stephanodiscus hantzschii) dominated the lake during the mid-Holocene. At 2100 cal. BP these species were succeeded by Aulacoseira distans and A. perglabra, indicating water of low acid-neutralizing capacity and marking peatland initiation. Transfer functions derived from diatoms in surface-sediment samples from Minnesota and the Upper Peninsula of Michigan suggest that this change was associated with reduced acid-neutralizing capacity in Pogonia Bog Pond. This decline was probably caused by peat accumulation around the Pond. The development of peatland at Pogonia Bog is late relative to sites farther north and east, indicating that the climate of the site has become conducive to peat accumulation in the last 2200 years and that the initiation of peatland development represents a time-transgressive boundary that has moved across central North America.