MINERALOGY AND CHEMISTRY OF SEDIMENTS FROM A SMALL COASTAL DUNE LAKE ALONG THE SOUTHWESTERN SHORE OF LAKE MICHIGAN

The mineralogy of sediments in a 3 m-long vibracore from Gilligan Lake was investigated by X-ray diffraction and chemistry by Proton Induced X-ray Emission (PIXE). Gilligan Lake (<.1 km²) is along the eastern margin of the Lake Michigan coastal dune complex southwest of Holland, Mi. Radiocarbon ages from terrestrial detritus in the core indicates deposition spanned 5700 calendar years. Organic matter was removed by treatment with hydrogen peroxide and sediments were centrifuged to split them into a clay size fraction (< 5 microns in diameter) and a sand/silt size fraction. The sediments fall into three major categories. Exogeneous clastic sediments (ECS), dominated by quartz with subordinate amounts of K-feldspar and plagioclase, reflect the composition of the sand in nearby dunes. Illite peaks are evident in clay and, to a lesser extent, sand/silt size fractions. Relatively high bulk K/Ca (~1) indicates the importance of K-feldspar. Endogenic biogenic sediments (EBS) are characterized by high amounts of amorphous silica (diatoms and sponge spicules are prominent), indicated by a broad hump on the XRD pattern, that converts to cristobalite upon heating to 1200 C. X-ray patterns indicate the presence of whewellite, a calcium oxalate (CaC₂O₄.H₂O) produced by some aquatic macrophytes. Sediments with whewellite have relatively low bulk K/Ca (~0.2) and Fe/Ca (~1). Endogenic ferruginous sediments (EFS) turn red upon treatment with hydrogen peroxide, contain hematite and other ferric compounds, and have relatively high Fe/Mn (>200) and Fe/Ca (5-25) ratios.

ECS dominates the lower 80 cm of the core, a period when sedimentation was dominated by sand inputs from the adjacent dunes. EFS appears at a depth of 220 cm ( ~ 5000 cal YBP) and declines in abundance again at 180 cm ( ~ 4000 cal YBP). During this interval iron entering the lake, perhaps dissolved in groundwater, was precipitated as ferric compounds, perhaps as a consequence of the mixing of oxygen rich and oxygen poor waters during seasonal turnover. EBS increases in abundance above 180 cm and dominate the sediments in the interval between 100 (~ 1800 cal YBP) and 20 cm (~200 cal YBP), a period also marked by a lack of aeolian activity in the surrounding dunes. Both ECS and EBS increase at the very top of the core corresponding with a widespread increase in dune mobility.