EVOLUTION OF SMALL COASTAL DUNE LAKES ALONG THE SOUTHEASTERN SHORE OF LAKE MICHIGAN: CLUES FROM SILICEOUS MICROFOSSILS

Siliceous microfossils (diatoms and chrysophyte stomatocysts) have been examined from 20 intervals in a 2.6 meter long core from Gilligan Lake, a small lake at the eastern edge of the coastal dune complex near Holland, Michigan. Radiocarbon ages on terrestrial plant fossils collected from the core indicate that it spans 5700 years of deposition. Among the most important diatoms present are benthic naviculoids (Navicula spp, Achnanthidium spp), Eunotia (E. incisa, E. cf. flexuosa), planktonic centrics (Cyclotella stelligera, Aulacoseira spp) and Fragilaria spp. The lower portion of the core is sand rich. The very base of this section has low relative abundance of benthic naviculoids, Eunotia spp. and planktonic centrics but high Fragilaria abundance with smaller amounts of Rhopalodia and Epithemia. Higher in the sandy segment Fragilaria abundance decreases, Eunotia abundances increases and Rhopalodia and Epithemia nearly disappear. This indicates a possible shift from a shallow alkaline wetland to a more acidic lake which may mark the damming of a drowned stream valley by a growing coastal dune complex shortly after the Nipissing Transgression. Sapropelic sediments dominate the top of the core. Eunotia abundances decrease in the lower sapropels and then level off; naviculoids are the dominant benthic diatoms throughout most of this interval. Planktonic centric diatoms and chrysophyte stomatocysts are more abundant in the upper than the lower core. The shift in the benthic community, together with the increase in the abundance of planktonic forms, indicates an increase in the pH of the lake accompanied by either an increase in water depth or a decrease in water clarity.