Pedogenic carbonate nodules from a forested region of humid climate in central Tennessee, USA, and their implications for interpretation of C-3-C-4 relationships and seasonality of meteoric precipitation from carbon isotope (delta C-13) data

摘要

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 delta C-13 from these calcitic nodules range from -11.6 to -9.6 parts per thousand relative to VPDB, consistent with values from other continental carbonates (stalagmites) in eastern Tennessee and consistent with an origin under the region's C-3 vegetation. These low values of delta C-13 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 delta C-13 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 delta C-13 of pedogenic carbonates and thus presumably of the photosynthetic pathway of the vegetation under which they form.