UPDATING OUR UNDERSTANDING OF COLLAPSE FEATURES IN A MIGRATING COASTAL DUNE, INDIANA DUNES NATIONAL LAKESHORE

Temporarily stable voids, or “decomposition chimneys,” occur in a modern, migrating dune along the southern coast of Lake Michigan and represent a new geologic hazard. The holes were discovered and publicized in 2013 after a six-year old boy was buried, and ultimately rescued, from a void that extended more than 3.5 m below the surface of the Mount Baldy dune at the Indiana Dunes National Lakeshore. Research following the incident identified that the voids are caused by the decay and collapse of oak trees buried during landward dune migration over forested relict dunes. Ongoing research focuses on understanding the details of how and where the voids form in Mount Baldy. Reconstruction and analyses of dune migration since the mid-1800’s were accomplished using historical shoreline photographs, aerial photography, topographic maps, and LIDAR data. Mapping of the internal stratigraphy of the dune was completed using ground penetrating radar profiles and GeoProbe cores. Radiocarbon and optically stimulated luminescence ages from surface and core samples provided a detail chronology. These data indicate that Mt. Baldy initially formed about 1900 and then migrated inland ~4 m/yr, progressively burying the 19^th and 20^th century surface, including soil, trees, and structures. New voids continue to be found, and some of which are visible at the surface while others are recognized only when the veneer of sand that covers the void is disturbed. Most of the voids occur in areas where sand is less than 12 m thick above a soil horizon where rooted trees were buried for more than 70 years. Ongoing changes in the dune’s morphology continue to exhume trees that were buried during the past few decades, producing a ghost forest. The research regarding risk management continues to examine the role of other factors, such as transitioning fungal communities within trees pre- and post-burial, biomineralization within the organic matter and its influence on hole stability, and the significance of rates of dune movement and the duration of burial.