WHAT MAKES LAKE MICHIGAN’S COASTAL DUNES MIGRATE? INVESTIGATING THE TRIGGERS OF DUNE MOBILITY IN LAKE MICHIGAN’S DUNES USING A MULTI-PROXY STUDY OF A COASTAL DUNE LAKE

Aeolian studies of Lake Michigan’s coastal dunes have shown periods of dune mobility and stability in the last 6,000 years. However, the triggering mechanisms for these changes in aeolian activity are not well understood. Small lakes located downwind of dune complexes contain aeolian sand and biological proxies that can provide both dune activity and independent paleoenvironmental data. We undertook a multi-proxy study of a lake sediment core collected from Gilligan Lake, located adjacent to a large coastal dune complex in Allegan County, Michigan, to reconstruct these histories. A 7.5-cm-diameter vibracore was obtained and subsampled in 1 cm³ aliquots for percent sand by weight using loss on ignition, charcoal abundance, and pollen identification analyses. Many of the sand peaks appear to either correlate with or immediately follow the charcoal peaks. Increases in birch pollen and higher hickory pollen abundances appear to be associated with and follow the charcoal peaks. Lower abundances of hemlock, spruce and fir pollen are generally noted during intervals with higher sand and charcoal peaks. The higher charcoal peaks suggest a higher frequency of fires, indicating drier conditions conducive to fire. The higher hickory pollen abundances and the decreases in hemlock, spruce and fir pollen also imply drier conditions. As a pioneer species, birch is typically one of the first tree species to revegetate a burned area. Hence, higher birch pollen abundances associated with the charcoal peaks might suggest the regrowth of birch in the surrounding area following fires. Both dry conditions and fire could decrease or remove beach grasses and other vegetation stabilizing the sand dunes, allowing greater sand mobility in the dune complex. Sand in actively migrating sand dunes is deposited in small coastal lakes downwind of the dunes. Hence, this increase in dune mobility is reflected by the increase in sand percentages in the cores. Our preliminary results then indicate a possible link between drier climate conditions and greater dune mobility in the Lake Michigan coastal dunes.