TERRESTRIAL PALEOCLIMATE FUTURES: GAMBLING ON INVESTMENTS IN MODERN ANALOGS

This year’s GSA theme “The Past is the Key to the Future” indicates that geoscientists are able to improve our understanding of global change issues from what is learned of the past. Climate reconstruction by paleopedologists, however, is predicated on pedologists developing reliable modern analogs linking soil properties to climate variables. Here we: (1) review soil analogs between isotopic, chemical, and morphological properties and mean annual temperature (MAT) and mean annual precipitation (MAP); and (2) test the strength of these proxy on examples from Quaternary and pre-Quaternary buried soils. The δ¹³C of bulk soil organic matter reflects summer temperatures through contributions by C₄ plants. This method is limited, however, to the Neogene after the evolution of the C₄ photosynthetic pathway. A late Quaternary temperature curve from the Great Plains using this method corroborates a cool Younger Dryas, a warm Great Plains Excursion (P-H boundary), and warm Altithermal. The δ¹⁸O of pedogenic carbonate is also useful for estimating MAT, assuming formation in equilibrium with temperature-dependent meteoric waters without diagenetic overprinting. An example from a Cretaceous-Tertiary (K-T) paleosol succession in west Texas shows that δ¹⁸O-derived temperatures are similar to those calculated by the NAK (Na₂O+K₂O/Al₂O₃) whole-rock geochemical weathering index. For MAP, we applied the familiar CIA-K (Al₂O₃/Al₂O₃+CaO+Na₂O) and CALMAG (Al₂O₃/Al₂O₃+CaO+MgO) weathering indexes to the same K-T paleosol succession. Even though results are similar, it appears that a family of soil-specific curves are needed to estimate MAP more accurately and precisely. Depth-to-carbonate is used as a morphological proxy for MAP, but erosion during burial, rhizocretions after plant roots, and sediment compaction diminish reliable depth-to-carbonate measurements. The sum of these isotopic and geochemical methods applied to the west Texas K-T paleosol succession suggest that the previously recognized mid Maastrichtian and Late Maastrichtian greenhouse events were warm-wet and warm-dry, respectively. It is imperative that geoscientists rapidly improve existing modern analogs and develop new ones as we seek an improved understanding of the past in preparation for what may be a much different future.