Reconstructing Pedogenic Processes, Weathering Environments, and Paleoclimate Using the Chemical Composition of Paleosols

Paleosols (fossil soils) are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows. However, until recently, paleosols were studied using primarily qualitative methods. In recent years, paleopedology has shifted from being a field based on qualitative comparisons with modern analogues to an increasingly quantitative endeavor. The new techniques and proxies are relevant to understanding a wide range of paleoenvironmental and paleoclimatic conditions and processes. This is a significant development because, in contrast to many marine proxies for paleoclimate, soils form at the Earth's surface, in direct contact with the atmospheric, environmental, and climatic conditions at the time of their formation, and thus, their isotopic and bulk chemical composition is also controlled directly by those conditions.

The whole-rock chemical composition (major, trace, and REE) of paleosols may be used to distinguish pedogenic processes, weathering environment factors, and paleoclimatic conditions. A variety of major and trace elemental ratios have been proposed as means of quantifying pedogenic processes in paleosols. Critical evaluation of the proposed proxies indicates that ratios for hydrolysis (Al/Si, ΣBases/Al), provenance and acidification (Ti/Al, La/Ce, Sm/Nd, U/Th) perform well for most settings, while ratios for salinization and leaching (Ba/Sr) are not universally applicable. In addition, the derivation and limitations of some widely applied climofunctions for paleotemperature and paleoprecipitation will be discussed. Case studies of both Cenozoic and older paleosols will be used to demonstrate the pedogenic process ratio and climofunction approaches. In the process of discussing the case studies, two new proxies will also be introduced, one for quantifying long-term chemical weathering trends and one for controlling for the effects of potassium metasomatism in moderately to deeply buried paleosols. In general, all of this work suggests the need for a multi-proxy approach rather than relying on any one single proxy.