MID-HOLOCENE DIATOM RECORD FROM LAKE ITASCA, CLEARWATER COUNTY, MINNESOTA

By comparing the record of diatom assemblages from large, shallow Lake Itasca in north-central Minnesota to the well-known record from the nearby small and deep Elk Lake, we hope to address the question of the influence of basin morphometry and lake dynamics on paleolimnological records of climate change. As these two lakes are in the same geological setting and responding to the same Holocene climatic changes, any differences between their diatom assemblages should reflect, at least in part, the impact of basin dynamics on the sedimentary record. Multi-proxy evidence from Elk Lake shows three phases in the lake sediment record (Wright, 1976; Bradbury and Dieterich-Rurup, 1993): the postglacial lake (10-8.2 ka), the prairie lake (8.2-4.0 ka), and the mesic-forest lake (4.0 ka-present). Other paleolimnologic evidence supports the idea that during the mid-Holocene (4-8 ka) the north-central United States was drier than it is today allowing prairie vegetation to migrate eastward. The diatom record from Elk Lake shows a number of changes in assemblages and flux. During the mid-Holocene prairie period Stephanodiscus minutulus is the dominant species indicating that the late spring and early summer were cold and dry. From 6.4-4 ka a surge in the Aulacoseira ambigua population implies increased turbulence due to strong winds and storms. The influx of diatoms is highest during the mid Holocene reflecting the more rapid circulation of nutrients. The evidence does not support any drastic chemical or physical limnologic changes during this time. In the late Holocene flux is reduced and Fragilaria crotonensis takes over as the dominant species suggesting reduced spring circulation and increased precipitation. The diatom record in the 15.5 meter livingstone core from Lake Itasca is well preserved through the early and mid Holocene. The same species can be found throughout the core, however the abundance and dominant species fluctuate. The upper 3 meters of the core have poor preservation and lack a significant number of diatoms.