Mg isotope composition of runoff is buffered by the regolith exchangeable pool

Abstract

In a small, forested catchment underlain by gneiss (Conventwald, Black Forest, Germany), we found that the magnesium isotope composition (δ26Mg) of creek water did not show seasonal variability, despite variations in dissolved Mg concentrations. To investigate the potential controlling factors on water δ26Mg values, we studied the Mg isotope composition of solid samples (bedrock, bulk soil, clay-sized fraction of soil, separated minerals, the exchangeable fraction of regolith) and water samples comprising time series of creek water, groundwater and subsurface flow. Subsurface flow from 0–15 cm depth (−0.80 ± 0.08‰) and 15–150 cm depth (−0.66 ± 0.17‰), groundwater (−0.55 ± 0.03‰), and creek water (−0.54 ± 0.04‰) are all depleted in heavy Mg isotopes compared to bedrock (−0.21 ± 0.05‰). Subsurface flow samples have similar δ26Mg values to the regolith exchangeable fraction at the respective sampling depths. Also, groundwater and creek water show δ26Mg values that are identical to those of the exchangeable fraction in the deep regolith. We suggest, therefore, that cation-exchange processes in the regolith control Mg concentrations and δ26Mg values of creek water at our study site. This assumption was further verified by batch adsorption-desorption experiments using soil samples from this study, which showed negligible Mg isotope fractionation during adsorption-desorption. We propose that the exchangeable fraction of the regolith buffers dissolved Mg concentrations by adsorbing and storing Mg when soil solutions are high in concentration in the dry season and desorbing Mg when rainfall infiltrates and percolates through the regolith in the wet season. This mechanism may explain the near chemostatic behavior of Mg concentrations and the invariance of δ26Mg values in creek water. In addition, the depth distribution of exchangeable Mg concentration and isotope composition in the regolith reflects mineral dissolution and secondary mineral formation in deep regolith (>3 m) and biological cycling in shallower depth (0–3 m). Magnesium stable isotopes thus provide an accurate snapshot of the geogenic (weathering) and the organic (bio-cycled) nutrient cycle.