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Potassium isotopes may provide a novel approach for fingerprinting recycled sediments in the mantle due to the significant differences in K abundance and isotopic ratio between subducting sediment and the mantle. However, the behavior of K isotopes in sediments during subduction zone metamorphism is still unknown. Here we investigate K isotopic composition of a set of well-characterized high- to ultrahigh-pressure metasediments from the Schistes Lustrés nappe (western Alps), which represents marine sediments subducted down to ~90 km depth in a cold subduction zone, and their protoliths from the Lavagna nappe (Apennines, Italy). The metasediments display δ41KSRM 3141a values from –0.76‰ to –0.48‰, which are on average lower than the mantle value (–0.43‰) but similar to those of non-metamorphic equivalents (–0.79‰ to –0.49‰). No systemic variation of δ41K with metamorphic grade is observed, suggesting negligible K isotope fractionation in these sediments during prograde metamorphism. This is in accord with the limited loss of K during the entire metamorphic history as evidenced by the constancy of K/Rb and K/Cs ratios between metamorphic and non-metamorphic sediments and the absence of correlations of δ41K with K/Rb and K/Cs. The heterogeneous δ41K values of metasediments are most likely inherited from their protoliths, which experienced different degrees of chemical weathering depending on their provenances. Our results demonstrate that the variable and light K isotopic signatures in subducting sediments could be preserved to depths of at least 90 km along a cold geotherm gradient, indicating that the introduction of sediments into the mantle could produce K isotope heterogeneity in the source regions of mantle-derived lavas. [ABSTRACT FROM AUTHOR]

The article focuses on a study which described high-precision measurements that reveal variability in oceanic plates and offers insights into the oceanic K cycle and mantle heterogeneity. Results of the study are discussed. Materials and methods used in the study are described. The direction and extent of associated franctionations are also examined.

Marine ecosystem recovery after the latest Permian mass extinction (LPME) was a protracted process during the Early Triassic (∼252–247 Ma) owing to repeated climatic and environmental perturbations. Chemical weathering can supply nutrients to the ocean and may have played an important role in the Early Triassic carbon cycle and biological recovery. However, only limited geochemical records of chemical weathering during the Early Triassic have been presented to date, and the relationship between changes in weathering intensity and the slow recovery of marine ecosystems is effectively unknown. Here, we report magnesium (Mg) isotopic compositions of the siliciclastic components from shallow-marine carbonates in two well-studied Upper Permian-Lower Triassic sections in Iran and South China to track changes in chemical weathering intensity after the LPME. Both sections display a wide range of δ26Mg values (−2.09‰ to +1.10‰ at Zal, −2.30‰ to +0.33‰ at Zuodeng). We identified two distinct stages (I and II) in each section based on δ26Mg values and major elemental ratios. Variations of δ26Mg values in Stage I (Changhsingian to mid-Dienerian) are mainly controlled by mineralogical composition that can obscure weathering signals. By contrast, δ26Mg variations in Stage II (upper Dienerian to upper Spathian) are independent of lithology and inferred to reflect control by chemical weathering intensity. The trends in chemical weathering intensity within Stage II correspond to first-order variations in climate and carbon cycling. Marine ecosystem recovery during the Early Triassic may have been linked to recurrent episodes of intense chemical weathering caused by CO 2 degassing and climate warming. This study demonstrates the potential utility of the Mg isotopic compositions of the silicate fraction in marine carbonates as a proxy for chemical weathering intensity, laying the groundwork for general applications of this method to deep-time Earth systems. [ABSTRACT FROM AUTHOR]

Potassium isotopic compositions of profiles through saprolites developed on a diabase in South Carolina, U.S.A., and on a granite in Guangdong, China, allow characterization of the behavior of K isotopes during continental weathering. Saprolites from the diabase profile are heavily weathered with chemical index of alteration (CIA) values up to 95; however, their K isotopic variation is limited, with δ41K ranging from −0.475 ± 0.028‰ in the unweathered diabase to −0.407 ± 0.021‰ for the saprolites. The lack of significant K isotope fractionation mainly reflects the conservative behavior of K in the diabase weathering profile, with >50% of the original K remaining in the saprolites. By contrast, K isotopes are fractionated during granite weathering and correlate with sample depth, CIA, and kaolinite abundance, with δ41K decreasing from −0.493 ± 0.030‰ in the unweathered granites at the bottom to −0.628 ± 0.021‰ in the most weathered saprolite close to the surface. These observations suggest the preference of light K isotopes in saprolites relative to fluids, which is further supported by the overall isotopically heavy nearby stream water samples (δ41K = −0.709 ± 0.017 to −0.339 ± 0.018‰). These results demonstrate that continental weathering plays an important role in the global K isotopic budget through the formation of isotopically heterogeneous rivers and weathered regolith. Recycling of K-rich crustal materials with distinct K isotopic signatures may produce distinct mantle K isotopic end members. [ABSTRACT FROM AUTHOR]

Continental crust loses Mg by lower crustal foundering and chemical weathering to become its intermediate-silicic composition. Both processes should preferentially retain heavy Mg isotopes in the crust, yet the estimated Mg isotopic ratio for the bulk continental crust is indistinguishable from that for unmodified mantle. This can be elucidated by two notions that are not mutually exclusive: (i) the basaltic parent to the crust has Mg isotopes lighter than the mantle and has since become mantle-like as a result of Mg loss by igneous and weathering fractionation, and (ii) heavy Mg isotopes in the continental crust are constantly balanced by a hidden input of light Mg isotopes. Arc magmas are arguably building blocks of the continental crust and their Mg isotopic signature could be used to place limits on the above views. Here, we conducted a Mg isotopic study of the Makran arc, a rare continental arc in Iran within the Alpine-Himalayan orogenic belt. The measured Mg isotopic ratios for most mafic and intermediate samples are mantle-like with δ26Mg values ranging from −0.32‰ to −0.20‰. After excluding dubious samples affected by alteration, the silicic samples span a broad range of isotopic ratios with δ26Mg values ranging from −0.32‰ to +0.32‰, beyond that of unmodified mantle (δ26Mg = −0.25 ± 0.04‰). The isotopic data form a trend from mantle-like δ26Mg at relatively high MgO towards elevated δ26Mg at low MgO, requiring the isotopic variability be related to differentiation in an open system involving assimilation of high-δ26Mg crustal rocks. Our results are generally consistent with Mg isotopic data compiled for global arcs where magmas have Mg isotopic ratios ranging from mantle-like to ∼0.7 per mil above the mantle range, and compositions substantially below that range are rare. Thus, heavy Mg isotopes in the continents predicted by igneous and weathering fractionation might have been compensated by a hidden input of light Mg isotopes, which is presumably in the form of carbonates, leading to a bulk crustal Mg isotopic composition that is mantle-like. [ABSTRACT FROM AUTHOR]

Substantial quantities of oceanic slabs are known to be recycled into the mantle through subduction. However, tracing their fate in the mantle through studies of mantle-derived samples is not always straightforward. As K represents an omnipresent element in subducting crustal materials, its two stable isotopes (39K and 41K) display outstanding potential in tracing recycled crustal materials in the mantle. Here we report the first set of K isotopic data for a suite of well-characterized continental basaltic lavas from Northeast China. The δ41K of these lavas exhibit large variation from −0.81‰ to −0.15‰, compared with the narrow range of unaltered oceanic basalts (−0.43 ± 0.03‰). The isotopic variations, together with geochemical data obtained in earlier studies, cannot be ascribed to chemical weathering, crystal fractionation, crustal contamination, or partial melting, but are consistent with the presence of diverse crustal materials in magma sources. Recycled oceanic crust and sediments both have contributed to the contrasting K isotopic signature observed in these lavas. Our study here highlights the potential of using K isotopes to trace different types of recycled crustal materials in the mantle and places new constraints on the generation of enriched mantle reservoirs. [ABSTRACT FROM AUTHOR]

Assessing the K isotopic composition of the upper continental crust is important for understanding the processes by which the crust is generated and modified, and constraining the K isotopic budget of the silicate Earth. High-precision K isotopic data are reported for 70 well-characterized individual and composite samples from the upper continental crust, including diorite, granodiorite, granite, loess, shale, graywacke, pelite, and tillite, to constrain its K isotopic composition. δ41K varies significantly in eight I-type and two S-type granites from Australia (−0.57 to −0.40‰), nine A-type granites (−0.53 to −0.38‰), and three granitoid composites including diorite, granodiorite, and granite (−0.50 to −0.37‰) from China, mainly reflecting source heterogeneity. The 22 post-Archean Australian shales (PAAS) (δ41K = −0.68 to −0.12‰) and the 12 sedimentary composites including graywacke, pelite, and tillite from China (δ41K = −0.57 to −0.23‰) have heterogeneous K isotopic compositions while the 12 loess samples from around the world display more limited K isotopic variation (δ41K = −0.47 to −0.35‰). δ41K values display a smaller dispersion in loess compared to shales, which have comparatively more intense weathering and higher chemical index of alteration (CIA). δ41K correlates with Fe 2 O 3 /Al 2 O 3 and Fe 2 O 3 /K 2 O in shales and Al 2 O 3 /SiO 2 , K 2 O/Al 2 O 3 , and δ7Li in loess, suggesting that K isotopes are fractionated during chemical weathering. Overall, the upper continental crust has a heterogeneous K isotopic composition, ranging from −0.68 to −0.12‰ with an average δ41K of −0.44 ± 0.05‰ (2SD, n = 88), which is indistinguishable from the mantle. [ABSTRACT FROM AUTHOR]

Cryogenian (~720-635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth's history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)--indicative of an episode of intense chemical weathering--occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan Island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ 26Mg=−0.36‰), the overlying saprolites are strongly depleted in Mg (i.e., τ Th,Mg=−99.1% to −92.9%), and display highly variable δ 26Mg, ranging from −0.49‰ to +0.40‰. Magnesium concentration and δ 26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes: (1) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (2) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ 26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ 26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters. [Copyright &y& Elsevier]

Abstract: High-precision Mg isotopic data are reported for ∼100 well-characterized samples (granites, loess, shales and upper crustal composites) that were previously used to estimate the upper continental crust composition. Magnesium isotopic compositions display limited variation in eight I-type granites from southeastern Australia (δ26Mg=−0.25 to −0.15) and in 15 granitoid composites from eastern China (δ26Mg=−0.35 to −0.16) and do not correlate with SiO2 contents, indicating the absence of significant Mg isotope fractionation during differentiation of granitic magma. Similarly, the two S-type granites, which represent the two end-members of the S-type granite spectrum from southeastern Australia, have Mg isotopic composition (δ26Mg=−0.23 and −0.14) within the range of their potential source rocks (δ26Mg=−0.20 and +0.15) and I-type granites, suggesting that Mg isotope fractionation during crustal anatexis is also insignificant. By contrast, δ26Mg varies significantly in 19 A-type granites from northeastern China (−0.28 to +0.34) and may reflect source heterogeneity. Compared to I-type and S-type granites, sedimentary rocks have highly heterogeneous and, in most cases, heavier Mg isotopic compositions, with δ26Mg ranging from −0.32 to +0.05 in nine loess from New Zealand and the USA, from −0.27 to +0.49 in 20 post-Archean Australian shales (PAAS), and from −0.52 to +0.92 in 20 sedimentary composites from eastern China. With increasing chemical weathering, as measured by the chemical index of alternation (CIA), δ26Mg values show a larger dispersion in shales than loess. Furthermore, δ26Mg correlates negatively with δ7Li in loess. These characteristics suggest that chemical weathering significantly fractionates Mg isotopes and plays an important role in producing the highly variable Mg isotopic composition of sedimentary rocks. Based on the estimated proportions of major rock units within the upper continental crust and their average MgO contents, a weighted average δ26Mg value of −0.22 is derived for the average upper continental crust. Our studies indicate that Mg isotopic composition of the upper crust is, on average, mantle-like but highly heterogeneous, with δ26Mg ranging from −0.52 to +0.92. Such large isotopic variation mainly results from chemical weathering, during which light Mg isotopes are lost to the hydrosphere, leaving weathered products (e.g., sedimentary rocks) with heavy Mg isotopes. [Copyright &y& Elsevier]

Abstract: High-precision Mg isotopic analysis was performed on a suite of well-characterized I-type granitoids and associated hornblende and biotite minerals from the Dabie Orogen in central China, to address the behavior of Mg isotopes during granite differentiation. Although these granitoids formed through different degrees of partial melting and fractional crystallization, with large variations in elemental and mineral compositions, their δ 26Mg values vary from −0.26 to −0.14 and are indistinguishable within our analytical precision (±0.07‰; 2SD). Coexisting hornblendes and biotites in these granitoids display similar Mg isotopic composition, with δ 26Mg ranging from −0.31 to −0.14 in hornblendes and −0.23 to −0.12 in biotites. The inter-mineral fractionation factors (Δ26MgHbl-Bt = δ 26MgHbl − δ 26MgBt) vary from −0.10 to −0.02, with an average=−0.06±0.08 (2SD). The limited inter-mineral fractionation agrees with the theoretic prediction that Mg cations in both hornblende and biotite are octahedrally coordinated with oxygen, which restricts the magnitude of equilibrium isotope fractionation. Overall, data from both bulk granitoids and associated mineral separates suggest that Mg isotope fractionation during I-type granite differentiation is limited. Collectively, granitoids studied here have Mg isotopic composition similar to that of terrestrial basalts and peridotites (δ 26Mg=−0.21±0.07 vs. −0.25±0.07; 2SD), confirming that magmatic processes do not significantly fractionate Mg isotopes. The continental crust in the Dabie Orogen, as sampled by these I-type granitoids, has a mantle-like Mg isotopic composition. Given that significant Mg isotope fractionation occurs during chemical weathering processes, Mg isotopes may potentially be used for tracing granite genesis, in particular, if sedimentary materials are involved in granite sources. [Copyright &y& Elsevier]

• Comprehensive study of Mg isotopic behavior in progressive weathering. • Phyllosilicates control the Mg isotopic fractionation during weathering of granite. • δ 26 Mg can substantially increase since the onset of incipient weathering. • Incorporation of saprock could form high- δ 26 Mg aeolian sedimentary rocks. • Interaction with saprock can significantly lower δ 26 Mg in riverine system. Continental weathering is a fundamental process in releasing magnesium (Mg) from crystalline rocks to the hydrosphere and biosphere. Mg isotopes can be substantially mobilized, re-distributed, and fractionated during weathering, and therefore can be used as a powerful tool to trace the biogeochemical cycle of Mg. Causes of significant Mg isotopic fractionation and behaviors during silicate weathering are still not well understood, hindering further application of the Mg isotopes to probe different geological processes. In this study, we demonstrate that dissolution and formation of phyllosilicates are the main control of Mg isotopic fractionation during sub-tropical weathering of granite. Furthermore, different formation and dissolution mechanisms for the same mineral phase could also cause variations in magnitude and directionality of fractionation. In incipient weathering, supergene phyllosilicates form mainly through topotactic transformation. Vermiculitization of parental chlorite tends to release 24Mg and causes significant 26Mg enrichment in the saprock. In an advanced stage of weathering, Mg isotopic compositions of supergene phyllosilicates are more influenced by the interaction with the soil solutions. Minerals formed mainly through a dissolution-precipitation mechanism with Mg in neoformed phyllosilicates dominantly sourced from the contemporary soil solutions. 26Mg would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in 24Mg with depth in the pedolith, from which relatively 24Mg-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged 24Mg, enriching the soil solutions with 26Mg. Varying relative abundances of different phyllosilicate minerals along the profile could cause large variations in the Mg isotopic compositions of regolith. Our study shows that Mg isotopic composition of the slightly weathered materials could be significantly heavy. Hence, entrainment of 26Mg-rich but slightly weathered materials could be an alternative to explain the high δ 26 Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ 26 Mg widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and 26Mg scavenging during phyllosilicate transformation, instead of severe depletion of 26Mg in soil solutions due to intense weathering and vast formation of secondary minerals, as previously suggested. Comprehensive characterization of the weathering processes and the resultant products is essential to interpret the observed Mg isotopic fractionation and trace the biogeochemical cycle of Mg. [ABSTRACT FROM AUTHOR]

The Triassic-Jurassic boundary is one of the most important geological boundaries in the Earth's evolutionary history. Previous paleoclimate studies focused primarily on Tethys realm at low and middle latitudes, while the northern hemisphere has yet to be studied. Here, we report Mg-Zn-Cu isotopic data for a continuous terrestrial sedimentary section in the southern margin of the Junggar Basin to constrain chemical weathering intensity and climatic change during this critical interval at high latitude. This section is enriched in heavy Mg isotopes with δ26Mg ranging from 0.08‰ to 0.43‰. It displays a decreasing Zn isotopic composition with δ66Zn ranging from −0.05‰ to 0.24‰, as well as limited Cu isotope fractionation (δ65Cu = −0.06‰ to 0.09‰). These isotopic results, combined with major and trace elemental variations, suggest a reduced sedimentary environment and increasingly intensive chemical weathering across the boundary, responding to a warmer and more humid paleoclimate. Compared with other places located around Tethys, this regional climatic change in high latitude areas can be linked to known global climate change during this period. Our study provides new insights from high-latitude Asia to reconstruct the global pattern of climate change at the Triassic-Jurassic boundary. [ABSTRACT FROM AUTHOR]

The controls on potassium (K) isotope fractionation during chemical weathering are evaluated using two regolith profiles developed over ∼350 kyr on the humid and arid sides of Kohala Mountain, Hawai'i. The humid regolith shows 145% K enrichment relative to the basaltic parent in shallow (≤1 m) horizons, but losses of up to 90% in the deeper horizons. By contrast, the arid regolith has lost between 60 and 90% K from the top 1 m of the soil with the least depletion in the deeper horizons due to limited chemical weathering. This apparent contradiction can be explained by enhanced accumulation of K-bearing mineral aerosols in the humid regolith. Bulk δ41K varies from −0.76 ± 0.08 to −0.31 ± 0.06‰ in the humid regolith compared with −0.48‰ for the underlying basalt. In contrast, the arid regolith shows δ41K values ranging from −0.39 ± 0.10 to −0.02 ± 0.05‰, heavier than that of their basaltic parent. Exchangeable (NH 4 Ac extracts) δ41K of the humid and arid regoliths ranges from −0.63 ± 0.07 to 0.11 ± 0.07‰ and −0.01 ± 0.05 to 0.04 ± 0.08‰, respectively. Exchangeable K has δ41K higher than (or similar to) the bulk values in most samples, reflecting a potential contribution of marine aerosols to the labile (plant available) K pool. In the shallow regolith, K derived from mineral aerosols is significant, especially for the humid site, and this idea is supported by enriched quartz, radiogenic Nd-Sr isotope values towards the surface, and increasing δ41K close to the upper crustal composition (an analogue of the dust). The enrichment of K in humid surface soils, an upward decrease in exchangeable δ41K in the humid regolith and plant-like δ41K in the topmost, organic-rich soils may reveal the contribution of plant cycling. Low δ41K in deep, humid regolith relative to δ41K Basalt appears to be driven by clay incorporation of isotopically light K. In comparison, higher δ41K in the arid regolith than δ41K Basalt likely reflects an interplay between preferential clay 41K sorption in alkaline environments and preservation of seawater-derived K in forms of clay adsorbed complex and carbonate phases (via adsorption and/or incorporation). Our results reveal that the K isotope composition in Hawaiian regoliths depends on climate, while it is complicated by the interaction among weathering, plant cycling and addition of marine and mineral aerosols. [ABSTRACT FROM AUTHOR]

[Display omitted] Clay adsorption is a critical process responsible for the mobilization and cycling of potassium (K) on Earth's surface. Recent studies emphasized the potential of using stable K isotopes (δ41K) to understand chemical weathering. However, the direction, degree, and mechanism of K isotopic fractionation linked to clay K uptake during chemical weathering remain poorly constrained. This work investigated the mechanism of K adsorption on clays (kaolinite and smectite) and the isotopic fractionation in three experimental sets with K-containing solutions. The time-series experiments revealed that the adsorption and isotope equilibria were attained after less than 12-hour reaction. Potassium adsorption rate slowed down and its isotopic fractionation approached the steady-state during 15-day reaction. The pH-dependent experiments demonstrated that the percentage of clay K adsorption and the isotopic composition of adsorbed K (and aqueous K) display negative linear correlations. Net isotopic fractionation between adsorbed and aqueous phases (Δ41K ad-aq) remained near-constant (0.6–0.8‰), regardless of variations in pH ranging from 4 to 10. The concentration-control experiments demonstrated that the percentage of K adsorption decreased with increasing KCl concentrations from 0.005 to 20 mM. The δ41K values of aqueous K reached the minimum of −0.53‰ after 92.7% K adsorbed (initial KCl of 0.005 mM). Potassium adsorption was substantially suppressed as ionic strength (fixed by Na+) increased from 0.001 to 0.5 M without apparent Δ41K ad-aq variations. The K K -edge XANES demonstrated that primary K incorporated in clay lattice and surface KCl derived from sorbed K+ and Cl− synchro-dehydration can be identified after drying of clays. These features indicate that adsorbed K+ was bounded onto clays as outer-sphere complexes, which can be replaced with excess Na+ at high ionic strength. Based on experimental results, we cannot distinguish specific mineralogy regulation on K isotopic fractionation. In sum, isotopically heavy K is preferentially sorbed on clay minerals. The results confirm an equilibrium fractionation path independent of reaction time, pH, ionic strength, and initial KCl concentration. Observed K isotopic fractionation is best fitted by an equilibrium isotopic fractionation law with a fractionation factor α ad-aq of 1.00075. We highlight the opposite direction of K isotopic fractionation in clay adsorption and structural incorporation during chemical weathering, and their comparative contributions should be considered for future field investigations. [ABSTRACT FROM AUTHOR]

This study reports that substantial chemical weathering occurs at subzero temperatures in ice-and-salt-rich permafrost in the McMurdo Dry Valleys, Antarctica. Chemical weathering is documented in a 30.0-m core collected in Beacon Valley by measuring the ionic composition, pH, and Mg isotopes of water extracted from thawed ice-rich sediment. Evidence of rock weathering is revealed by coinciding increases in the Mg isotopic composition and pH values. The primary factor that controls weathering is the salt content that leads to unfrozen brine; this is most apparent in the upper 7.0 m where salt content is high, temperatures rise above −21 °C and modeled unfrozen water reaches up to 4.0% of ice-content. In the upper 7.0 m, up to 60% of soluble Mg in the thawed permafrost ice is sourced from Ferrar Dolerite (δ26Mg = −0.22 ± 0.07‰) weathering, resulting in δ26Mg values ranging from of −0.82 ± 0.05‰ to −0.64 ± 0.05‰. Below 7.0 m, temperatures remain below −21 °C, unfrozen water is less than 2.0% of ice-content, and on average, 5% of soluble Mg is sourced from dolerite weathering with δ26Mg values ranging from −1.05 ± 0.05‰ to −0.76 ± 0.05‰. Regions of the core that are modeled to have no unfrozen water show little or no evidence of chemical weathering and relatively constant δ26Mg values close to Taylor Glacier and Beacon Valley snowfall values (–0.93 ± 0.06‰). This study demonstrates that significant chemical weathering occurs at subzero temperatures in permafrost where liquid brines form. [ABSTRACT FROM AUTHOR]

Bacillus subtilis endospore‐mediated forsterite dissolution experiments were performed to assess the effects of cell surface reactivity on Mg isotope fractionation during chemical weathering. Endospores present a unique opportunity to study the isolated impact of cell surface reactivity because they exhibit extremely low metabolic activity. In abiotic control assays, 24Mg was preferentially released into solution during forsterite dissolution, producing an isotopically light liquid phase (δ26Mg = −0.39 ± 0.06 to −0.26 ± 0.09‰) relative to the initial mineral composition (δ26Mg = −0.24 ± 0.03‰). The presence of endospores did not have an apparent effect on Mg isotope fractionation associated with the release of Mg from the solid into the aqueous phase. However, the endospore surfaces preferentially adsorbed 24Mg from the dissolution products, which resulted in relatively heavy aqueous Mg isotope compositions. These aqueous Mg isotope compositions increased proportional to the fraction of dissolved Mg that was adsorbed, with the highest measured δ26Mg (−0.08 ± 0.07‰) corresponding to the highest degree of adsorption (~76%). The Mg isotope composition of the adsorbed fraction was correspondingly light, at an average δ26Mg of −0.49‰. Secondary mineral precipitation and Mg adsorption onto secondary minerals had a minimal effect on Mg isotopes at these experimental conditions. Results demonstrate the isolated effects of cell surface reactivity on Mg isotope fractionation separate from other common biological processes, such as metabolism and organic acid production. With further study, Mg isotopes could be used to elucidate the role of the biosphere on Mg cycling in the environment. [ABSTRACT FROM AUTHOR]

The K isotopic composition of modern seawater is an important reference point to consider when using K isotopes to trace silicate weathering records in deep time; however, it is not yet well-constrained. The K isotopic composition of seawater also plays a pivotal role in understanding the global K budget, where seawater constitutes the isotopically heaviest terrestrial K reservoir at ~0.6‰ higher than common igneous rocks [[1]]. The high SP 41 sp K value of seawater obtained by our study suggests that K isotope fractionation during these processes needs further investigation to better constrain oceanic K-cycling. 3 (Color online) Potassium concentration [K] (ppm) plotted against the 41K (‰) of the mantle and the three primary K reservoirs: rivers, oceans and the upper continental crust. [Extracted from the article]