Magnesium isotope fractionation during shale weathering in the Shale Hills Critical Zone Observatory: Accumulation of light Mg isotopes in soils by clay mineral transformation
Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ26Mg values (− 0.6‰ to − 0.1‰) of stream and soil pore waters are about ~ 0.5‰ to 1‰ lighter than the shale bedrock δ(26Mg values of + 0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg2 + concentrations but lower δ26Mg values (− 1.1‰) in groundwater, ~ 1.5‰ lighter than the bedrock. δ26Mg values (+ 0.2‰ to + 0.4‰) of soil samples on the planar hillslope are either similar or up to ~ 0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ26Mg values of + 0.3‰ to + 0.5‰) accumulated on the valley floor are ~ 0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir.
Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier.