SEDIMENTATION PATTERNS IN A SMALL COASTAL DUNE LAKE: FLUVIAL VERSUS AEOLIAN INPUTS

Goshorn Lake occurs at the eastern edge of the coastal dune complex along Lake Michigan just north of the Kalamazoo River mouth. Goshorn Creek enters the northeastern arm of this Y-shaped lake while the western arm abuts the base of a large parabolic dune. Vibracores were sampled continuously in one cm3 blocks for carbonaceous organic sediment, determined by loss-on-ignition (520-600°C), and for sand and mud (silt/clay) ratios by sieving through a 63 micrometer mesh. Vibracore GOSH06-1, obtained from the western edge is 3.79 m long with a basal 0.12 m-thick marl abruptly overlain by a sand bed 0.29 m thick. Wood from the sand bed gives a calibrated radiocarbon age of 5900 to 5610 YBP. The rest of the core consists of a dark sapropel with occasional sand lenses. Peaks in sand concentration range up to 80% of the sample, while the abundance of mud and carbonaceous organic material (LOI) vary independently of each other. The bottom portion of this core appears to mark the transition from a wetland to a deeper lake perhaps due to the damming of the creek by migrating dunes. Vibracore GOSH06-2 obtained from the northeastern edge of the lake near the mouth of Goshorn Creek consists entirely of a dark sapropel. Within it are several smaller peaks in sand abundance ranging up to 15 % of the sample but generally contains less than 5% sand. The abundance of the mud fraction is marked by peaks of up to 80% and troughs down to 10 % compared to the background levels of 50-60% of the sample. There is a strong inverse correlation between LOI and mud but there is no consistent correlation with the sand fraction. These results suggest that the sources of most of the sand in the lake are the dunes along the western edge and that increases in sand concentration are proxies for growth and migration of these dunes. On the other hand, the terrigenous portion of the mud fraction at the northeastern edge may reflect fluvial input into the lake and might serve as a proxy for the rate of erosion in the watershed.