SAND, DIATOMS, AND POLLEN FROM A SMALL COASTAL DUNE LAKE IN SOUTHWESTERN MICHIGAN: POSSIBLE LINKS WITH DUNE HISTORY

Sediment size, diatoms, and pollen in a 290 cm long vibracore (GL 205-6) from Gilligan Lake are being studied for proxies of paleo-aeolian dune activity. This small (< 0.1 km2) lake is on the downwind side (east) of a coastal-dune complex, southwest of Holland, MI. Five radiocarbon ages from terrestrial detritus indicate that the core spans 5,700 calendar years. Sand and sandy sapropels were deposited during an early period of extensive dune growth and migration from 5700 to ~ 4000 years. The upper portion of the core is dominated by sapropel with peaks in sand concentration correlating to periods of mobility in the surrounding dunes. Proportions of birch and hickory pollen are low in the lower (<2 % apiece) versus the upper (~ 5% apiece) portions of the core, and hemlock pollen are above 30% at the bottom of the core, declining to less than 15 % towards the middle, and then increasing to 29 % at the top. Preliminary results suggest that ratios of tricolporate (oak + maple) pollen to winged vesiculate (pine + spruce) pollen are lower during periods of dune mobility than during periods of dune stability. This may reflect a lower proportion of climax to early ecological succession vegetation during dune migration.

Thus far, twenty-seven diatom genera have been identified from the upper 200 cm of the core. Rhopalodia and Epithemia are important minor constituents in the interval between 170-200 cm depth but almost completely disappear [< 0.5 %] higher in the core, while Camplyodiscus appears only at the top (2 cm depth) of the core. Eunotia and Navicula are dominant in most samples, together usually making up over 40% of the diatoms. Relatively high abundances of Eunotia tend to be correlated with periods of dune activity (high proportion of sand in lake sediments) while high abundances of Navicula tend to be correlated with periods of dune stability (low proportions of sand in lake sediments). Abundances of the planktonic, centric genus Cyclotella also tend to increase in sediment deposited during periods of dune stability. Although preliminary, diatoms that record changes in water chemistry, clarity and/or depth in the lake appear responsive to changes in the environment correlated with episodes of dune growth and migration. We conclude that sand content, diatoms, and pollen may be used as proxies for paleo-aeolian dune activity.