The hydrogen and oxygen isotope geochemistry of pedogenic clay minerals: principles and theoretical background

Abstract

The 18O/ 16O and D/H ratios of the structural oxygen and hydrogen of naturally occurring clay minerals carry information about the conditions under which the minerals formed. Clays generally form in isotopic equilibrium with the environment, and hence have isotopic compositions that reflect the temperature of formation and the 18O/ 16O and D/H ratios of water in the micro-environment. The isotopic signature of water in the weathering environment is a reflection of the climate. Once formed, clays are resistant to isotopic alteration under surface and near-surface conditions, except when they undergo subsequent mineralogical alteration. In some instances, this alteration may be subtle, as in the case of conversion of kaolinite with a high degree of crystalline order (as measured by the Hinckley index) to kaolinite of low crystalline order in a pedogenic profile. Isotopic studies of pedogenic clays may be expected to yield more information about pedogenesis and paleoclimate when both 18O/ 16O and D/H ratios of the clays are measured. However, few studies have incorporated the use of the two isotope ratios.