Effects of chemical pretreatments on the hydrogen isotope composition of 2:1 clay minerals

Abstract

Clays are a commonly occurring authigenic mineral within soils, and the hydrogen isotope ratios of their structural hydroxyl groups can serve as a useful environmental proxy. Chemical and physical treatments are necessary to eliminate contaminating materials from clays, but they have the potential to alter their isotopic composition. Several pure, commonly occurring 2:1 clay mineralogies were treated with typical chemical procedures for the removal of carbonates, organic matter, and amorphous hydroxides. Samples were heated under vacuum to remove the adsorbed and interlayer water that occurs universally in natural clays. The samples were then analyzed for hydrogen content and isotope ratios through thermal conversion elemental analyzer/isotope ratio mass spectrometry (TC-EA/IRMS). No effects on hydrogen composition were found for most of the analyzed mineralogies. Smectite samples showed significant alteration following treatments for carbonate and organic matter. In addition, the smectites showed variable levels of hydrogen exchange with isotopically labeled H2 O. Smectites show significant and highly variable alteration following common chemical treatments, and analysis for paleoclimate reconstruction is problematic. A minor amount of H isotope exchange is likely at temperatures sufficient to remove adsorbed water, and efforts must be made to minimize and quantify this exchange during analyses of clay samples.