Distinguishing Climate in the Soil Record Using Chemical Trends in a Vertisol Climosequence from the Texas Coast Prairie, and Application to Interpreting Paleozoic Paleosols in the Appalachian Basin, U.S.A.

摘要

A suite of Vertisols (clay-rich soils with high shrink-swell potential) were examined across a climosequence (climatic transect) in twelve soil pits from the Coast Prairie of Texas in order to determine if mean annual precipitation (MAP) exerts a control on the chemistry of these soils, and if the observed chemical trends are useful for interpreting paleoclimate records of paleoVertisols in the geologic record. The precipitation regime of the climosequence spans a range between 144 and 86 cm/year, with moisture regimes classified as udic, udic-ustic, ustic, and aridic-ustic, in a general northeast to southwest direction. Other soil-forming factors, such as soil age (<35–40 ka), parent material (fluviodeltaic Beaumont Formation of late Pleistocene age), landscape (low-relief coastal plain), and vegetation (prairie or mixed woody shrubs), are relatively constant across the climosequence. Climate-sensitive chemical proxies of MAP identified include dithionite citrate-extractable Fe (Fe_dith), acid oxalate-extractable Fe (Fe_oxal), CaCO₃ equivalent (CaCO_3equiv), S, and ammonium acetate-extractable Na, K, and Mg (Na_acet, K_acet, and Mg_acet, respectively), which vary across the climosequence because of differences in effective depths of leaching and intensity of wetting and drying cycles. These standard USDA wet-chemical climate proxies are related to bulk (oxide or element) chemistry of soils and paleosols measured using XRF, which supports the use of geochemical climate proxies for interpreting the paleoclimate records of paleoVertisols. Application of the chemical index of alteration minus potash (CIA-K) geochemical climofunction to late Mississippian paleosols from the Appalachian basin of the eastern U.S. demonstrates evidence for a shift from a lower to a higher MAP paleoclimate that is consistent with previous paleoclimate models and with observed morphological changes in the paleosols. We advocate actualistic research using bulk chemistry of soils and paleosols as a means of obtaining soil information useful for interpreting paleosols in the geological record.