ATMOSPHERIC DEPOSITION OF CALCIUM AND MAGNESIUM AS A CONTRIBUTOR TO THE FORMATION OF DUNE DECOMPOSITION CHIMNEYS IN A MODERN COASTAL DUNE OF THE INDIANA DUNES NATIONAL LAKESHORE

Authigenic calcium carbonate was unexpectedly found in quartz-dominated sands of a migrating dune at the Indiana Dunes National Lakeshore and was associated with the formation of hazardous dune decomposition chimneys. Structures like calcretes and tree casts have been recognized in carbonate-rich environments, but are less common in siliciclastic systems. Following a severe storm, a carbonate-rich coating was observed on exposed surfaces and in association with sedimentary features (pin stripe laminae) at Mount Baldy, suggesting an atmospheric source. Samples from five distinct sedimentary features permitted investigation of the origin of carbonate by evaluating mineralogy, grain properties, and texture using thin section and SEM analyses with EDS. Carbonate was abundant (~30% modal analysis) in the laminae; sedimentary layers composed largely of opaque minerals and garnet. Similarly, carbonate was abundant in the “crust” sample (~28%); a precipitated sediment protruding from the interface of the pin stripe laminae and sands within foreset beds after the storm. This amount of carbonate exceeds previously reported values for dunes along the southern coast of Lake Michigan and is notably higher than the trace amount (<1%) observed in interlaminae dune sand. The carbonate appears to form as a precipitate. Even the limited carbonate in the “cement” sample appears on the surface of detrital grains. We propose that grain-size differences between interlaminae and laminae sands facilitated the observation of the in-situ precipitation of carbonate from atmospheric water enriched in Ca²⁺ and Mg²⁺ ions. SEM analysis reveals coal spherioles in the “crust” and “cement” samples, strong evidence for atmospheric sourcing. Additionally, ongoing shoreline erosion is eroding a carbonate-rich lacustrine clay that underlies Mount Baldy. We suspect that carbonate has not been observed in this abundance in previous studies that characterize detrital grains because it can dissolve and remobilize. These results suggest that dune decomposition chimneys are forming in a siliciclastic dune where abundant atmospheric input of Ca²⁺ and Mg²⁺ promotes ongoing fungal decomposition of buried trees and precipitation of authigenic carbonate in a process similar to the formation of rhizocretes in carbonate environments.