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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]

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]

• 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]

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]