Pedogenic carbonate nodules from a forested region of humid climate in central Tennessee, USA, and their implications for interpretation of C3-C4 relationships and seasonality of meteoric precipitation from carbon isotope (δ13C) data

Abstract

Pedogenic carbonate materials are found most commonly in regions of dry climate, and rarely in forested humid regions. This paper nonetheless reports pedogenic carbonate nodules from central Tennessee in eastern North America, a forested region with average atmospheric precipitation exceeding 1300 mm/yr. Values of δ13C from these calcitic nodules range from −11.6 to −9.6‰ relative to VPDB, consistent with values from other continental carbonates (stalagmites) in eastern Tennessee and consistent with an origin under the region’s C3 vegetation. These low values of δ13C from a region of exceptionally great precipitation provide an uncommon input for understanding environmental controls on isotopic compositions of pedogenic carbonates. In that regard, comparison of values of δ13C from the Tennessee nodules with those of pedogenic carbonates across the continental United States supports the view that seasonality, rather than amount, of atmospheric precipitation is the principal determinant of δ13C of pedogenic carbonates and thus presumably of the photosynthetic pathway of the vegetation under which they form.