Investigating the behaviour of Mg isotopes during the formation of clay minerals

Wimpenny, Josh; Colla, Christopher A.; Yin, Qing‐Zhu; Rustad, James R.; Casey, William H.

doi:10.1016/j.gca.2013.12.012

Cited by 156 publications

(121 citation statements)

References 75 publications

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“…Soil Mg isotopes can only be explained if δ 26 Mg in secondary minerals (δ 26 Mg sec ) ≥ 4 ‰. To our knowledge, such high clay δ 26 Mg values have never been reported to date (Wimpenny et al, 2014). These clay δ 26 Mg values would require that Mg clays precipitate from dissolved Mg with a δ 26 Mg of ∼ 3.5 ‰, which has never been observed either (Tipper et al, 2012, and references therein).…”

Section: Mg Isotope Fractionation By Clay Formationmentioning

confidence: 72%

“…3) can only be caused by the preferential uptake of heavy Mg isotopes by trees. We can exclude that the development of such an isotopically light exchangeable Mg compartment throughout the regolith is due to fractionation during adsorption (Opfergelt et al, 2014) as the associated fractionation factor is close to 0 ‰ (Wimpenny et al, 2014). Also, Bullen and Chadwick (2016) have shown that isotopic fractionation during adsorption onto clay minerals is absent for other bivalent cations.…”

Section: Nutrient Uplift From the Deep Saprolitementioning

confidence: 98%

“…Neoformation of Mg clays is a mechanism that preferentially removes 26 Mg from soil solution and enriches this heavy isotope in Mg clays such as smectite, illite or vermiculite (Wimpenny et al, 2014;Ryu et al, 2016). Clay formation is thus a potential cause for the low δ 26 Mg diss observed in the Providence Creek streams.…”

Section: Mg Isotope Fractionation By Clay Formationmentioning

confidence: 99%

“…The absence of Mg clays and the presence of kaolinite was confirmed by published clay contents of less than 10 % in the soils (Dahlgren et al, 1997) and by thermodynamic modelling (PhreeqC). Because kaolinite has a low Mg content (< 0.03 wt %; Wimpenny et al, 2014) and a relatively low adsorption cation exchange capacity (CEC) of < 10 cmol c kg −1 (Wimpenny et al, 2014) it is unlikely to incorporate Mg in such amounts that the Mg mass balance in soil is affected. Therefore, neoformation of Mg clays is not the mechanism that preferentially removes 26 Mg from soil solution.…”

Section: Appendix B: Xrd Analyses and The Potential Incorporation Of mentioning

confidence: 99%

“…Unless the formation of secondary minerals is significant (Wimpenny et al, 2014), the main shift affecting the 26 Mg / 24 Mg ratio in weathering systems is the isotopic fractionation towards high ratios during nutrient uptake by plants (Black et al, 2008;Bolou-Bi et al, 2012;Mavromatis et al, 2014), such that the residual dissolved Mg is shifted towards the complementary low ratio. These two isotopically distinct compartments will remain separated if a fraction of the Mg accumulates in wood of a regrowing forest after clear cutting or if a fraction of the Mg is eroded after utilisation and is not released back into solution.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Abstract. Plants and soil microbiota play an active role in rock weathering and potentially couple weathering at depth with erosion at the soil surface. The nature of this coupling is still unresolved because we lacked means to quantify the passage of chemical elements from rock through higher plants. In a temperate forested landscape characterised by relatively fast ( ∼ 220 t km −2 yr −1 ) denudation and a kinetically limited weathering regime of the Southern Sierra Critical Zone Observatory (SSCZO), California, we measured magnesium (Mg) stable isotopes that are sensitive indicators of Mg utilisation by biota. We find that Mg is highly bio-utilised: 50-100 % of the Mg released by chemical weathering is taken up by forest trees. To estimate the tree uptake of other bioutilised elements (K, Ca, P and Si) we compared the dissolved fluxes of these elements and Mg in rivers with their solubilisation fluxes from rock (rock dissolution flux minus secondary mineral formation flux). We find a deficit in the dissolved fluxes throughout, which we attribute to the nutrient uptake by forest trees. Therefore both the Mg isotopes and the flux comparison suggest that a substantial part of the major element weathering flux is consumed by the tree biomass. The enrichment of 26 Mg over 24 Mg in tree trunks relative to leaves suggests that tree trunks account for a substantial fraction of the net uptake of Mg. This isotopic and elemental compartment separation is prevented from obliteration (which would occur by Mg redissolution) by two potential effects. Either the mineral nutrients accumulate today in regrowing forest biomass after clear cutting, or they are exported in litter and coarse woody debris (CWD) such that they remain in "solid" biomass. Over pre-forest-management weathering timescales, this removal flux might have been in operation in the form of natural erosion of CWD. Regardless of the removal mechanism, our approach provides entirely novel means towards the direct quantification of biogenic uptake following weathering. We find that Mg and other nutrients and the plant-beneficial element Si ("bio-elements") are taken up by trees at up to 6 m depth, and surface recycling of all bio-elements but P is minimal. Thus, in the watersheds of the SSCZO, the coupling between erosion and weathering might be established by bio-elements that are taken up by trees, are not recycled and are missing in the dissolved river flux due to erosion as CWD and as leaf-derived bio-opal for Si. We suggest that the partitioning of a biogenic weathering flux into eroded plant debris might represent a significant global contribution to element export after weathering in eroding mountain catchments that are characterised by a continuous supply of fresh mineral nutrients.

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Section: Mg Isotope Fractionation By Clay Formationmentioning

confidence: 72%

Section: Nutrient Uplift From the Deep Saprolitementioning

confidence: 98%

Section: Mg Isotope Fractionation By Clay Formationmentioning

confidence: 99%

Section: Appendix B: Xrd Analyses and The Potential Incorporation Of mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC‐ICPMS

Teng

Yin

Ullmann

et al. 2015

Geochem Geophys Geosyst

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To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3& in 26 Mg/ 24 Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.

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Constraining dolomitization by Mg isotopes: A case study from partially dolomitized limestones of the middle Cambrian Xuzhuang Formation, North China

Yang

Shen

Lang

et al. 2016

Geochem Geophys Geosyst

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The “dolomite problem” refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record, as well as failure of laboratory inorganic dolomite precipitation at near Earth‐surface temperature. Novel Mg isotope systematics provides a promising tool in resolving the “dolomite problem”. Here, we develop a protocol to place constraints on the dolomitization process by using Mg isotopes. In this study, we measured Mg isotopic compositions ( δ26Mg) of two batches of partially dolomitized limestone samples from the middle Cambrian Xuzhuang Formation in North China. δ26Mg varies between −0.55‰ and −3.18‰, and shows a negative linear correlation with 1true[Mgtrue], suggesting that δ26Mg can be described by a binary mixing between the calcite and dolomite components. Mg isotopic composition of the dolomite component ( δ26Mgdol) for the lower sample set that is collected from a 4 m stratigraphic interval containing three high‐frequency ribbon rock‐packstone cycles is −1.6‰, while δ26Mgdol for the upper sample set (from a thick sequence of ribbon rock) is significantly higher (−0.3‰). However, neither mineralogical and elemental compositions, carbon and oxygen isotopes, nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples. δ26Mgdol of the Xuzhuang limestone is simulated by the Advective Flow (AF) and the Diffusion‐Advection‐Reaction (DAR) models. The AF model assumes that Mg is transported by advective fluid flows, while the DAR model simulates a contemporaneous seawater dolomitization process, in which Mg is delivered by diffusion. The AF modeling result indicates that δ26Mg of the dolomitization fluid is +0.4‰ and +1.7‰ for the lower and upper sample sets, respectively. These values are significantly higher than modern and Cenozoic seawater Mg isotopic composition, suggesting that the dolomitization fluid is not contemporaneous seawater. The AF model also predicts spatially heterogeneous δ26Mgdol with progressive enrichment in 26Mg along the fluid flow pathway. In the DAR model, both dolomite content and δ26Mgdol of the lower sample set can be simulated by using seawater Mg isotopic composition of −0.75‰, thus contemporaneous seawater dolomitization may explain δ26Mgdol of the Xuzhuang limestone. Furthermore, the DAR model demonstrates spatially homogeneous δ26Mgdol. To differentiate the AF and DAR models, samples from multiple sections are required. Nevertheless, this study implies that Mg isotope might be a useful tool in the study of dolomitization.

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Investigating the behaviour of Mg isotopes during the formation of clay minerals

Cited by 156 publications

References 75 publications

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

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

Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC‐ICPMS

Constraining dolomitization by Mg isotopes: A case study from partially dolomitized limestones of the middle Cambrian Xuzhuang Formation, North China

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