AN ANALYSIS OF POTENTIAL REGIONAL SOURCES OF ATMOSPHERIC CA2+ AND MG2+ SUSPECTED TO CONTRIBUTE TO THE FORMATION OF DECOMPOSITION CHIMNEYS IN MOUNT BALDY DUNE AT INDIANA DUNES NATIONAL LAKESHORE

Dune decomposition chimneys are temporarily stable voids in modern dunes that form by the decay and collapse of trees buried during dune migration. Argyilan et al (2015) investigated these features at the Mount Baldy dune of the Indiana Dunes National Lakeshore (INDU) on the southern coast of Lake Michigan. An unexpected finding was the presence of authigenic carbonate precipitating in the siliciclastic sands directly in contact with buried trees. A carbonate-rich deposit was also observable after heavy rainfall events, occurring as both a surface coating and in association with sedimentary features (pin stripe laminations) in foreset beds, suggesting an atmospheric source of Ca²⁺ and Mg²⁺ to the system. This study investigates possible sources of Ca²⁺ and Mg²⁺, focusing on ion analyses of local precipitation. Two precipitation datasets were utilized; annual precipitation-weighted means (1985-2013) from the National Atmospheric Deposition Program for sites surrounding INDU and precipitation samples collected via a civilian sampling network established throughout northwest Indiana. The maximum annual mean concentration of Ca²⁺ (0.676 mg/L) occurred in 2012 for INDU and the data suggest that widespread drought conditions contributed to the elevated ion concentration. However, cluster analysis and an ANOVA test on the full time series indicated that values from INDU are statistically different from nearby stations. Liquid phase ion chromatography yielded concentrations for Ca²⁺ and Mg²⁺ ions in precipitation samples collected across the civilian sites. Elevated concentrations of Ca²⁺ and Mg²⁺ likely reflect atmospheric absorption of byproducts released from industrial sources including coal-fired power plants, cement factories, and coke and steel manufacturing in the region. Another possible local source of Ca²⁺ and Mg²⁺ is a basal fossiliferous clay layer that could contribute to authigenic carbonate formation via ongoing weathering, erosion, dissolution, and re-precipitation within dune sediments. These data can aid in authenticating the initial hypothesis that a localized atmospheric source of Ca²⁺ and Mg²⁺ is contributing to the formation of decomposition chimneys at Mount Baldy. Findings will impact risk assessment for similar migrating dunes along the Great Lakes and elsewhere.