Non-exchangeable stable hydrogen isotope ratios in clay minerals and soil clay fractions: A method test

Stable hydrogen isotope ratios (delta H-2 values) in structural hydroxyl groups of pedogenic clay minerals are inherited from the surrounding water at the time of their formation. Only non-exchangeable H preserves the environmental forensic and paleoclimate information (delta H-2(n) value). To measure delta H-2(n) values in structural H of clay minerals and soil clay fractions, we adapted a steam equilibration method by accounting for high hygroscopicity. Our delta H-2(n) values for USGS57 biotite (-95.3 +/- SD 0.9 parts per thousand) and USGS58 muscovite (30.7 +/- 1.4 parts per thousand) differed slightly but significantly from the reported delta H-2 values (-91.5 +/- 2.4 parts per thousand and -28.4 +/- 1.6 parts per thousand), because the minerals contained 1.1%-4.4% of exchangeable H. The low SD of replicate measurements (n = 3) confirmed a high precision. The clay separation method including destruction of Fe oxides, carbonates and soil organic matter, and dispersion did not significantly change the delta H-2(n) values of five different clay minerals. However, we were unable to remove all organic matter from the soil clay fractions resulting in an estimated bias of 1 parts per thousand in two samples and 15 parts per thousand in the carbon-richest sample. Our results demonstrate that delta H-2(n) values of structural H of clay minerals and soil clay fractions can be reliably measured without interference from atmospheric water and the method used to separate the soil clay fraction.

Automated summary

Stable hydrogen isotope ratios in structural hydroxyl groups of pedogenic clay minerals can be reliably measured using a steam equilibration method. The δ H-2(n) values of clay minerals and soil clay fractions remain consistent after separation methods.