Magnesium isotope fractionation during shale weathering in the Shale Hills Critical Zone Observatory: Accumulation of light Mg isotopes in soils by clay mineral transformation

Abstract

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: δ 26 Mg values (−0.6‰ to −0.1‰) of stream and soil pore waters are about 0.5‰ to 1‰ lighter than the shale bedrock δ( 26 Mg 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 Mg 2+ concentrations but lower δ 26 Mg values (− 1.1‰) in groundwater,~1.5‰ lighter than the bedrock. δ 26 Mg 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 δ 26 Mg 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 (δ 26 Mg 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.

Quantifying nutrient uptake as driver of rock weathering in forest ecosystems by magnesium stable isotopes

“…Those data can be interpreted to imply natural erosion of Sr in plant litter and CWD. The same interpretation is possible for the data from the Shale Hills Critical Zone Observatory, where a similar deficit in heavy Mg isotopes was found in stream and soil water (Ma et al, 2015). In that study the bio-cycling hypothesis was dismissed on the grounds of missing accumulation of Mg in the organic-rich portions of the soil.…”

Magnesium isotope fractionation during microbially enhanced forsterite dissolution

“…Surface adsorption has been shown to have a variable impact on Mg isotope fractionation. Studies using natural samples (Brewer et al, ; Ma et al, ; Opfergelt et al, , ) and laboratory experiments (Wimpenny et al, ) have found that minerals with high fractions of adsorbed Mg, such as kaolinite, preferentially retain 24 Mg. However, other studies have concluded that adsorbed Mg, on gibbsite, for example, is isotopically heavy (Huang et al, ; Liu et al, ; Pogge von Strandmann et al, ).…”

“…Bacillus subtilis endospore surfaces preferentially adsorb aqueous 24 Mg, much like some mineral surfaces (Brewer et al, ; Ma et al, ; Opfergelt et al, , ; Wimpenny et al, ). In the biotic dialysis assay, the endospores are separated from the forsterite powder by dialysis tubing and cannot interact with the minerals directly.…”

“…The aqueous phase tends to be correspondingly isotopically light depending upon the degree of weathering and additional factors, such as weathering profile lithology (Huang, Teng, Wei, Ma, & Bao, ; Liu et al, ; Pogge von Strandmann et al, ; Pogge von Strandmann et al, ; Teng, Li, Rudnick, et al, ; Wimpenny et al, , ). The formation of secondary minerals can have a variable effect on Mg isotopes, with carbonate precipitates typically exhibiting a light Mg isotope composition (as light as −5.6‰; Wombacher et al, ), while clay minerals generally preferentially incorporate 26 Mg into their structural sites and are therefore often isotopically heavy compared with the aqueous phase (Huang et al, ; Li et al, ; Ma et al, ; Opfergelt et al, ; Tipper et al, ; Wimpenny et al, ). Magnesium adsorbed onto mineral surfaces has been found to be variably heavy (Huang et al, ; Liu et al, ; Pogge von Strandmann et al, ) or light (Brewer, Teng, & Dethier, ; Ma et al, ; Opfergelt et al, , ; Wimpenny et al, ), and the mechanisms responsible for these differences are not well understood.…”

“…The formation of secondary minerals can have a variable effect on Mg isotopes, with carbonate precipitates typically exhibiting a light Mg isotope composition (as light as −5.6‰; Wombacher et al, ), while clay minerals generally preferentially incorporate 26 Mg into their structural sites and are therefore often isotopically heavy compared with the aqueous phase (Huang et al, ; Li et al, ; Ma et al, ; Opfergelt et al, ; Tipper et al, ; Wimpenny et al, ). Magnesium adsorbed onto mineral surfaces has been found to be variably heavy (Huang et al, ; Liu et al, ; Pogge von Strandmann et al, ) or light (Brewer, Teng, & Dethier, ; Ma et al, ; Opfergelt et al, , ; Wimpenny et al, ), and the mechanisms responsible for these differences are not well understood. Magnesium isotope fractionation during adsorption and desorption is therefore an area of active investigation.…”