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

Abstract

Stable hydrogen isotope ratios (δ2H 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 (δ2Hn value). To measure δ2Hn values in structural H of clay minerals and soil clay fractions, we adapted a steam equilibration method by accounting for high hygroscopicity. Our δ2Hn values for USGS57 biotite (−95.3 ± SD 0.9‰) and USGS58 muscovite (30.7 ± 1.4‰) differed slightly but significantly from the reported δ2H values (−91.5 ± 2.4‰ and −28.4 ± 1.6‰), 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 δ2Hn 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‰ in two samples and 15‰ in the carbon‐richest sample. Our results demonstrate that δ2Hn 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. Highlights We tested steam equilibration to determine stable isotope ratios of structural H in clay. Gas‐tight capsule sealing in Ar atmosphere was necessary to avoid remoistening. Our steam equilibration method showed a high accuracy and precision. The clay separation method did not change stable isotope ratios of structural H in clay.

Equilibrium isotope fractionation factors of H exchange between steam and soil clay fractions

“…This is corroborated by the increasing χ e values with increasing latitude in this group of soil clay fractions (Table 1), because it is known that the χ e values decrease in the order: swellable three-layer clay minerals > nonswellable three-layer clay minerals (illite) > two-layer clay minerals > mica. 23,24 The α ex-w values of the group of soil clay fractions with a CEC pot > 400 mmol c kg À1 , presumably dominated by swellable three-layer clay minerals, correlated with the δ 2 H p values, but the slope was flatter than that for the group of soil clay fractions with a CEC pot of 160-400 mmol c kg À1 (Figure 3). We attribute this finding to the fact that the group of soil clay fractions with a CEC pot > 400 mmol c kg À1 contained a less heterogeneous mixture of minerals, possibly dominated by a single three-layer clay mineral.…”

“…[2][3][4][5] The latter can be reached by closing the samples in airtight tin capsules under an Ar atmosphere, which require less mechanical force for crimping than Ag capsules. [23][24][25] For samples with a high χ e value, the calculated δ 2 H n values respond sensitively to variations in the α ex-w value, because the α ex-w value influences both the slope and the intercept of the regression line of δ 2 H t values of the studied sample on the δ 2 H w values used for the exchange (Equation [1]). 8,13,19,22,26 Thus, the knowledge of a sample-specific α ex-w value might improve the measurement of δ 28 and a minor contribution of rockderived clay minerals that were not formed under the current climatic conditions.…”

“…The sample DE-KA1 originated from the 7-13 cm depth layer of a Gleysol near Karlsruhe, Germany, with a clay content of 58% and an organic C concentration of 55 g kg À1 . 23 The signature "EC" refers to samples from tropical forest sites in south Ecuador. 55,56,58 EC-BOM was collected from the 0-25 cm layer of a Cambisol at an elevation of$3000 m above sea level and EC-SF from the 0-15 cm layer of a Cambisol at an elevation of $2000 m above sea level.…”

“…Analogous to the approach of Schimmelmann, 6 from the intersection point of the pair of regression lines with different slopes resulting from two different drying conditions, α ex-w can be calculated, provided the difference in the slopes is sufficiently large to precisely determine the intersection point 6. In a previous work, Merseburger et al23 tested whether the classic clay separation treatment, including the removal of Fe oxides and carbonates, reduction of soil organic matter (SOM), and dispersion of the remaining material, affects the δ 2 H n values of clay minerals. In the absence of a known α ex-w value, the equilibrium fractionation between the exchangeable H pool and the steam-H was ignored (i.e., α ex-w = 1).…”

“…In the absence of a known α ex-w value, the equilibrium fractionation between the exchangeable H pool and the steam-H was ignored (i.e., α ex-w = 1). Merseburger et al23 found that the differences in δ 2 H n values between clay minerals (kaolinite, illite, montmorillonite, and vermiculite) subjected and not subjected to the clay separation treatment were not significant and concluded that the clay separation treatment did not affect the δ 2 H n values of clay minerals and soil clay fractions. However, this conclusion can be maintained only if the clay separation treatment did not change α ex-w , which remains to be tested.Ruppenthal et al 8 reported a correlation between the modeledδ 2 H values of local mean annual precipitation and the δ 2 H n values of a global set of bulk soil samples.…”