THE ORIGIN AND DYNAMICS OF SOIL INORGANIC CARBON IN GLACIAL DRIFT IN SOUTHERN MICHIGAN, USA

Pedogenic carbonate is typically found in arid drylands. In the humid continental climate of southern Michigan, we identified soil inorganic carbon (SIC) at depths > 50 cm in glacial drift. The morphology of the SIC indicates in situ, pedogenic formation, including rinds forming on the bottom of clasts and rhizoliths. Given the unusual setting for authigenic SIC, we investigated the formation conditions using environmental monitoring, stable isotope analyses (δ¹³C, δ¹⁸O, ∆₄₇), and ¹⁴C dating. The δ¹⁸O values of the SIC (-8.3 to -5.4 ‰ VPDB, n = 19, depths of 50 ­to 375 cm) suggest that it could have feasibly formed in equilibrium with modern meteoric and soil waters. However, ∆₄₇-temperatures are 22 to 33 °C and δ¹³C values range from -7.9 to 0.2 ‰ VPDB. In a C₃-dominated ecosystem, these warm temperatures and δ¹³C values would require soil carbonate formation at soil CO₂ of <750 ppm or in a mixed C₃-C₄ environment. These conditions are not fully compatible with the modern, as soil CO₂ is >5000 ppm even in winter and the site is a C₃-dominated forest. The ∆₄₇-temperatures match and exceed feasible modern summer soil temperature and are thus inconsistent with equilibrium formation in a colder, pre-Holocene (>11 ka) climate. Instead, the SIC may (1) represent a mix of pedogenic carbonate and detrital limestone or (2) disequilibrium formation. These two scenarios are supported by the most positive δ¹³C and δ¹⁸O values being found in diffuse, matrix carbonate (which may be entrained in the rinds) and by the old ages of two rinds (23.7 and 17.4 kcal yr BP, i.e., when the site was glaciated and near or just after glacial retreat, respectively). Regardless of the formation conditions, the apparent in situ formation of rinds and rhizoliths supports SIC storage in a wider range of ecosystems than is typically recognized, though perhaps only transiently as carbonate-rich till undergoes chemical weathering in postglacial landscapes.