DEPTH-TO-CARBONATE (DTC) FROM MODERN SOILS AND ESTIMATIONS OF MEAN ANNUAL PRECIPITATION (MAP) FROM PALEOSOLS: ONE SIZE DOES NOT FIT ALL!

Depth-to-carbonate (DTC) is one of the most widely used methods for estimating mean annual precipitation (MAP) from paleosols. Modern soil climofunctions developed from the literature and from the USDA Natural Resources Conservation Service (NRCS) database, suffer from lack of attention to important details such as soil orders, textures, drainage classes, slopes, ages, and parent materials. A further problem is that a systematic definition for “depth-to-carbonate” has never been established in prior studies. As a consequence of these problems, statistical fit between DTC and MAP is typically moderate at best. We address these problems by examining a climosequence in the Coast Prairie of Texas for Vertisols developed on fluvio-deltaic deposits of the late Pleistocene Beaumont Formation. We tested Vertisol microlows and microhighs vs MAP under well-drained conditions using depths to 1%, 2%, 5%, and maximum abundances, for nodules and calcium carbonate equivalent. The best fit (positive) for Vertisol microlow pedons between MAP and DTC was defined by the highest occurrence of either 2% or 5% nodules (R² = 0.70 and 0.68, respectively). The DTC-MAP curve from previous universal data sets that include multiply soil orders with a preponderance of loamy textures, is much steeper and terminates before 1000 mm MAP; whereas all carbonate forms persist in Vertisols to a MAP of 1500 mm. These results are significant and suggest that DTC-MAP climofunctions based on modern soils and applied to paleosols must be developed more judiciously, applied to specific soil orders (as best can be determined in the rock record), and that one size does not fit all! MAP estimations for paleosols based on previous climofunctions may therefore require re-examination.