Deuterium and oxygen-18 correlation: Clay minerals and hydroxides in Quaternary soils compared to meteoric waters

δD and δO18 of clay minerals and hydroxides in Quaternary soils of the United States correlate directly with δD and δO18 of corresponding present-day meteoric waters. Samples from Montana and Idaho are much lower in both δD and δO18 than those from coastal areas. The Hawaiian and Gulf Coast samples are highest in δD and δO18. Soils rich in gibbsite or Al-Fe-Si hydroxides are isotopically distinct from clay-rich soils. These systematics indicate that isotopic equilibrium is approximately attained between the weathering products and their coexisting waters. Isotopic fractionation factors estimated from the data for kaolinite-H2O and montmorillonite-H2O at earth surface temperatures are very similar, with αclay-H2Ohy ≈ 0.970 and αclay-H2Oox ≈ 1.027, whereas for gibbsite-H2O αhy ≈ 0.985 and αox ≈ 1.018.