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We evaluate the factors influencing the abundance, [Li], and isotopic composition of riverine Li delivered to the oceans through analyses and modeling of [Li] and δ7Li in streams and groundwaters draining a single continental lithology, the Columbia River Basalts (CRBs). The streams were sampled in different climate zones that lie east (dry), and west (wet) of the Cascades Mountains, and during two different seasons (summer and late winter) in order to evaluate climatic and seasonal influences on Li isotopes in rivers. Dissolved Li (δLidis7=+9.3 to +30.4) is systematically heavier than that of fresh or weathered CRBs (-4.7 to +6.0, Liu et al., 2013), suspended loads (-5.9 to -0.3), and shallow groundwaters (+6.7 to +9.4), consistent with previous studies showing that Li isotope fractionation is affected by equilibration between stream water and secondary minerals. However, the lack of correlation between δ7Lidis and climate zone, the uniform secondary minerals and bedrock, coupled with the highly variable (>20‰) δLidis7 indicate that other factors exert a strong control on δ7Lidis. In particular, the heavier Li in streams compared to the shallow groundwaters that feed them indicates that continued isotopic fractionation between stream water and suspended and/or bed loads has a major influence on riverine δ7Li. Seasonal δ7Li variation is observed only for streams west of the Cascades, where the difference in precipitation rate between the dry and wet seasons is greatest. Reactive transport model simulations reveal that riverine δ7Li is strongly controlled by subsurface residence times and the Li isotope fractionation occurring within rivers. The latter explains why there is no positive correlation between δ7Li and traditional weathering proxies such as Si or normalized Si in rivers, as riverine Li isotope fractionation drives δ7Li to higher values during transport, whereas the concentrations of major cations and anions are diluted. The varying residence time for groundwaters feeding the western streams in summer (long residence times, higher δ7Li, greater weathering) and winter (short residence times, lower δ7Li, less weathering) explains the observed seasonal variations. A global, negative correlation between δ7Li and Li/Na for streams and rivers draining basaltic catchments reflects the overall transport time, hence the amount of silicate weathering. Based on our results, the increase of δ7Li in seawater during the Cenozoic is unlikely related to changing climate, but may reflect mountain building giving rise to increased silicate weathering.

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]

We combine spectroscopic and geochemical approaches to interpret the fate of potassium (K) during forest soil development along a 4-million-year chronosequence sampled from relatively undisturbed rainforests in Hawai'i. Potassium derived from weathering of lava is dominant in the youngest site (0.3 ky), but its contribution to the soil K budget declines as weathering progresses. Sites older than 0.3 ky are characterized by substantial K depletion (τ K,Nb ∼−1), with soil isotopic composition (41K/39K, δ41K) varying from −1.91 ± 0.08‰ to −0.09 ± 0.08‰, relative to the homogeneous basaltic substrate (−0.48‰). Exchangeable and interlayered K show δ41K values ranging from −1.32 ± 0.06‰ to 0.06 ± 0.08‰, higher than their corresponding bulk δ41K values. The δ41K patterns of soils and exchangeable components are vertically similar, implying similar environmental controls. The variability in K phase and isotope composition reflects the accumulative effect of different processes. Chemical weathering and plant cycling retain isotopically light K in soils, in particular for 20–150 ky sites. In contrast, atmospheric inputs of marine aerosols (0.14‰) and mineral aerosols (−0.44‰) add heavier K (than native basalts) and crust-like K (similar to basaltic δ41K) in soils, respectively. In sites older than 150 ky, nearly complete depletion of basaltic K and reduced plant K imprints result in the dominance of atmospheric inputs in soils. In sum, this study emphasizes the sensitivity of δ41K to terrestrial biotic and abiotic K cycles. [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]

Understanding the influence of terrestrial soil-atmosphere-biosphere exchanges on Li geochemical behaviors is vital before using Li isotopes as a weathering tracer. We investigated Li geochemistry of the humid and arid regolith profiles formed on the Pololu lavas, the Kohala Mountain, Hawaii. The shallow regolith (0–1 m depth) retains Li (τ Li,Nb > 0) and displays peak Li accumulation in biologically-active, near-surface soil layers (humid, τ Li,Nb = 10.9; arid, τ Li,Nb = 2.8), with heavy Li isotopic compositions (humid, 4.7–9.9‰; arid, 4.0–13.9‰) with respect to the basalt signal (2.2‰). The deep regolith (>1 m) demonstrates δ7Li (ave. 3.3‰) comparable to the composition of the underlying parent basalt (2.2‰). Decoupling of Li abundance and isotopic composition in the shallow regolith from those of the deep regolith implies different regolith controls on Li chemistry at vertical locations. The Li geochemistry in the shallow regolith has been substantially influenced by: (i) atmospheric deposition, (ii) plant cycling, and (iii) secondary mineral formation. In addition to weathering alteration, our data show that dust addition mainly influences δ7Li in the humid regolith, and marine aerosol largely affects δ7Li in the arid regolith. In the humid regolith, downward seepage migration allows for deeper and more advanced weathering, whereas occasional wetting events followed by rapid drying likely dominate in the arid regolith. Thus, biological cycling and the deposition of Asian dust and volcanic ash from the more recent Hawi eruptions are responsible for upward Li enrichment in soils and increases of soil δ7Li composition in the shallow regolith. This is particularly important in the humid site with dense plant coverage and heavy rainfall. By contrast, the deep regolith of the humid and arid sites is mostly affected by pore fluid percolation and accumulation, thus inheriting heavy Li isotope signals from pore fluids. Our study emphasizes climate-regulated and long-neglected biological controls on terrestrial Li cycling during chemical weathering. [ABSTRACT FROM AUTHOR]

The feedback between CO 2 sources and sinks through chemical weathering is one of the important reasons why Earth has maintained a habitable climate for over four billion years. The lithium (Li) isotopic system is a promising tracer of silicate weathering, but the mechanisms causing its isotope fractionation during weathering remain ambiguous. Here, we performed batch experiments of Li adsorption to one of the common clay minerals -kaolinite in three sets, including the time-series, pH-dependent, and concentration-control sets. Our results demonstrate that the liquid-solid Li isotope fractionation reaches up to 36‰, with up to 99% initial Li being adsorbed on kaolinite. The magnitudes of Li adsorption and isotope fractionation increase with reaction time, and reach the steady-state after ∼1000 min. The magnitude of Li isotope fractionation increases with the adsorption ratio of Li, in positive relationships with solute pH and ionic strength. At constant solute pH = 8.5 and ionic strength of 0.001 M, the adsorption ratio and isotope fractionation of Li on kaolinite reach the maximum with the lowest initial Li concentration of 2 μM. In experiments, Li is removed by kaolinite as the inner-sphere and/or outer-sphere complexes, likely followed by structural occupation as supported by incomplete desorption. We model the Li isotope fractionation in all three sets, which can be best described by Rayleigh distillation models. In summary, significant Li isotope fractionation occurs following a kinetic law in closed-systems during adsorption on kaolinite. Adsorption-driven isotope fractionation conforms to a kinetic solid-liquid isotope fractionation factor α ∼ 0.992, consistent with theoretical ion-desolvation between complexed and dissolved ions (α = 0.9925, Hofmann et al., 2012). This study reveals a vital kinetic role of clay uptake in Li isotope fractionation during chemical weathering, suggesting rapid forward adsorption reaction versus relatively slow backward reaction. The dominance of the kinetic mechanism over the equilibrium mechanism further explains field observations from rivers worldwide. Given above, the outcome of this study calls for additional attention on low-temperature kinetic Li isotope fractionation at Earth's surface and further refinement of quantitative models using geological Li records to trace weathering and reconstruct climate. [ABSTRACT FROM AUTHOR]

The lithium isotope system can be an important tracer for various geological processes, especially tracing continental weathering. The key to this application is the accurate and precise determination of lithium isotopic composition. However, some of the previously established column separation methods are not well behaved when applied to chemically diverse materials, due to the significant variations in matrix/lithium ratios in some materials. Here, we report a new dual‐column system for lithium purification to achieve accurate and precise analysis of lithium isotopic compositions using a multi‐collector inductively coupled plasma‐mass spectrometer (MC‐ICP‐MS). Compared with single‐column systems, our dual‐column system yielded a consistent elution range of the lithium‐bearing fraction (7–16 ml) for samples with a large range of lithium loads and matrix compositions, so that column re‐calibration is not required. In addition, this method achieved complete lithium recovery and low matrix interference (e.g., Na/Li ≤ 1) with a short elution time (~ 6 h, excluding evaporation), with the entire procedure completed in 1.5 days. We report high precision Li isotopic compositions in twelve chemically diverse materials including seawater, silicates, carbonates, manganese nodules and clays. New recommended Li isotopic values and associated uncertainties are presented as reference values for quality control and inter‐laboratory calibration for future research and were consistent with previously published data. However, significant lithium isotopic variances (~ 1‰) in BHVO‐2 from different batches suggest Li isotopic heterogeneity in this reference material and that Li isotopic studies using this reference material should be treated with caution. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) signatures in the dissolved loads of rivers serve as excellent monitors of weathering processes within a basin. However, quantifying REE behavior spatially (along a river's profile) and temporally (seasonable variations) is rare. This investigation reports REE concentrations of dissolved river loads collected during four sampling seasons at five monitoring stations along the Neuse River, North Carolina. The areal percentage of the Neuse River's catchment lithology composition, Post Archean Australian Shale (PAAS) normalized REE patterns, inter-REE ratios, and calculated anomalous REE values (i.e. cerium anomaly, gadolinium anomaly) were determined at each site in four seasons over a year. Our results show slight differences among inter-REE relationships and anomalous REE values in the dissolved loads along the length of the river. PAAS normalized REE patterns of dissolved water loads vary slightly spatially and temporally among the five sampling stations, and experience varying values of anomalous REE values. Collected data indicates that both variations in localized REE patterns and local bedrock occur along the river profile. Significant gadolinium (Gd) anomalies varied seasonally and spatially at select sites, suggesting changes in REE behavior along a river result from both natural and anthropogenic processes. [ABSTRACT FROM AUTHOR]

• We evaluate seawater K uptake via early diagenesis using modern marine sediments. • Continental weathering and reverse weathering mainly control sediment δ 41 K values. • Glauconite is an important sink of seawater K with preference to isotopically lighter K. The sedimentary fluxes controlling potassium (K) budget of the oceans and the isotope composition of K (δ 41 K) potentially yield valuable information about the global K cycling. However, at present sedimentary diagenesis in the oceans is one of the least well-known components of the seawater K budget. This study presents a dataset of the modern (<50 cm) sediments from a range of hydrographic regimes including continental margin settings (the Peru Margin, California Borderland, and Mexican Margin) and deep-sea environments (the Equatorial Pacific). We determine the K isotope compositions and origins of K in the sediments and compare them with the results of elemental abundances and spectroscopic analyses. The wide variability in sedimentary K/Al and Rb/K ratios, K-phase distributions, and K isotope compositions support the interplay between continental weathering, which provides clastic inputs, and sedimentary diagenesis, which constitutes an important sink of seawater K. The net result of these processes potentially alters the K elemental and isotopic budgets in sediment strata of various marine environments. We demonstrate that K is dominantly hosted by illite, glauconite, and feldspars, and regional K-phase partitioning modulates marine sedimentary K budget. For example, considerable amounts of K hosted in illite (up to 90%) primarily of terrestrial origins increase Rb/K ratios, producing low δ 41 K in California Borderland sediments (−0.57 to −0.40‰), the Pescadero Slope from the Mexican Margin (−0.42 to −0.39‰), and Equatorial Pacific deep-sea basins (−0.56 to −0.44‰). In contrast, glauconite authigenesis causes decreases in the Rb/K ratio and relatively high δ 41 K at a site along the Peru Margin (−0.35 to −0.29‰), as well as at two other sites along the Mexican Margin (−0.39 to −0.27‰). Despite the presence of feldspar-K (<20%), δ 41 K in the sediments reflects dominant contributions of detrital illite (δ 41 K illite ∼ −0.56‰) and authigenic glauconite (δ 41 K glauconite ∼ −0.18‰). We conclude that the formation of glauconite in the sediments takes up isotopically light K from seawater and serves as an important sink of seawater K (including K in seawater-derived porewater) (∼1 to 16, average ∼6 Tg K⋅yr−1) during early diagenesis, especially on continental margins. [ABSTRACT FROM AUTHOR]

We evaluate the factors influencing the abundance, [Li], and isotopic composition of riverine Li delivered to the oceans through analyses and modeling of [Li] and δ 7 Li in streams and groundwaters draining a single continental lithology, the Columbia River Basalts (CRBs). The streams were sampled in different climate zones that lie east (dry), and west (wet) of the Cascades Mountains, and during two different seasons (summer and late winter) in order to evaluate climatic and seasonal influences on Li isotopes in rivers. Dissolved Li ( δ Li dis 7 = + 9.3 to +30.4) is systematically heavier than that of fresh or weathered CRBs (−4.7 to +6.0, Liu et al., 2013 ), suspended loads (−5.9 to −0.3), and shallow groundwaters (+6.7 to +9.4), consistent with previous studies showing that Li isotope fractionation is affected by equilibration between stream water and secondary minerals. However, the lack of correlation between δ 7 Li dis and climate zone, the uniform secondary minerals and bedrock, coupled with the highly variable ( > 20 ‰ ) δ Li dis 7 indicate that other factors exert a strong control on δ 7 Li dis . In particular, the heavier Li in streams compared to the shallow groundwaters that feed them indicates that continued isotopic fractionation between stream water and suspended and/or bed loads has a major influence on riverine δ 7 Li. Seasonal δ 7 Li variation is observed only for streams west of the Cascades, where the difference in precipitation rate between the dry and wet seasons is greatest. Reactive transport model simulations reveal that riverine δ 7 Li is strongly controlled by subsurface residence times and the Li isotope fractionation occurring within rivers. The latter explains why there is no positive correlation between δ 7 Li and traditional weathering proxies such as Si or normalized Si in rivers, as riverine Li isotope fractionation drives δ 7 Li to higher values during transport, whereas the concentrations of major cations and anions are diluted. The varying residence time for groundwaters feeding the western streams in summer (long residence times, higher δ 7 Li, greater weathering) and winter (short residence times, lower δ 7 Li, less weathering) explains the observed seasonal variations. A global, negative correlation between δ 7 Li and Li/Na for streams and rivers draining basaltic catchments reflects the overall transport time, hence the amount of silicate weathering. Based on our results, the increase of δ 7 Li in seawater during the Cenozoic is unlikely related to changing climate, but may reflect mountain building giving rise to increased silicate weathering. [ABSTRACT FROM AUTHOR]

Abstract: Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8–9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including “mobile” and some “immobile” elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and ε Nd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20–60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and ε Nd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust. [Copyright &y& Elsevier]

This study experimentally investigates the isotopic behaviors of Li and K during the dissolution of silicate rocks (i.e., basalt and granite). Proton-driven dissolution (in 0.8 M HNO 3) and ligand-driven dissolution (in 5 mM critic acid or oxalic acid) experiments were performed in batch-closed systems over 15 days. We provide a time-series interpretation of Li and K isotope fractionation during silicate dissolution in ultra-acidic (unidirectional) and near-natural (biologically affected) environments. As the reaction progressed, we measured large isotope fractionation between the liquid (l) phase and the pristine silicate (s) phase, ranging from −10.3 to 0.1‰ (Δ7Li l-s) and from −1.01 to −0.11‰ (Δ41K l-s) through the early stage of dissolution (<24 h). The enrichment of lighter Li and K isotopes in the solutions rapidly diminished as rock dissolution continued and gradually approached equilibrium to the end of experiments. In contrast, resorption of pre-leached isotopically lighter Li on silicate residuals during ligand-driven dissolution produced lighter isotope enrichment in the solutions compared to the initial rock by up to 2.8‰. Despite the preferential dissolution of specific minerals, the isotope fractionation patterns of Li and K do not vary with lithology, indicating limited inter-mineral isotopic differences. During the experiments, the Li and K isotopic pattern could be divided into two-to-three stages. The initial enrichment of light isotopes in the liquids can be ascribed to the kinetic isotope effect, confounded by diffusion and ion solvation. A later transition towards no isotope fractionation of Li and K may be explained by (i) the masking effect from dissolution, and (ii) an imprint from the destruction of 7Li/41K-enriched surface layers. Lateral resorption of solute Li after ~100 h reaction could be facilitated by the electrostatic attraction from increasing surface negative charges and active hydroxyls with increasing pH during ligand-driven dissolution (pH ~ 4) relative to proton-driven dissolution (pH ~ 0.2). Therefore, the presence of organic ligands impacts dissolution stoichiometry, and potentially modifies Li isotope fractionation in natural weathering environments. In comparison, K isotope fractionation driven by rock dissolution stops immediately (within days) after starting the experiments. This research helps to understand the mechanisms of Li and K isotope fractionation during chemical weathering and trace long-term climate change using geological records. [ABSTRACT FROM AUTHOR]

• We report high-precision K isotopes of two weathering profiles developed on basalts. • Deposition of aeolian dust and formation of secondary minerals control K isotopic composition of bauxite profiles. • Weathering of igneous rocks leads to loss of isotopically heavy K from the continents. • Heavy K isotopic composition in modern seawater results from both continental weathering and silicate fixation. Non-traditional stable isotopes (e.g. , Li, Mg, and Si) are increasingly used as tracers for studying Earth's surface processes. The isotopes of potassium (K), a highly soluble and mobile element during weathering, could be a promising new tracer for continental weathering; however, the K isotopic variations in weathering profile has not been directly studied due to previous analytical difficulties. Recent high-precision measurements revealed that K isotopes in global river waters are fractionated from the Bulk Silicate Earth (BSE) value, indicating they are influenced by chemical weathering of the crust. Isotopic fractionation during chemical weathering is one of several processes that could ultimately lead to ∼0.6‰ difference of δ 41 K between the BSE and modern seawater. In order to determine the direction and controlling factors of K isotopic fractionation during basalt weathering, especially under intense weathering conditions, we measured K isotopic compositions in two sets of bauxite developed on the Columbia River Basalts, together with fresh parental basalt and aeolian deposit samples using a recently developed high-precision method. Results show that K isotopic variations among fresh basalts and aeolian dust are limited, close to the BSE value. Extreme K depletion (>99%) and K isotopic fractionation (δ 41 K up to 0.5 ‰) are observed in bauxite drill cores due to intense chemical weathering. The top of the weathering profiles shows less depletion in K abundances and the δ 41 K values are closer to those of the fresh basalts and aeolian dusts, likely due to addition of aeolian dust at the tops of both profiles. The weathered products are generally depleted in heavy K isotopes, which is consistent with heavier K isotopic compositions observed in river water and seawater. The δ 41 K in bauxites displays a positive correlation with K 2 O contents as well as δ 7 Li, indicating the behaviors of K and Li isotopes are comparable during chemical weathering. This study shows that K concentrations and its isotopic compositions are sensitive tracers of chemical weathering and could be good weathering proxies over Earth's history. [ABSTRACT FROM AUTHOR]

To investigate the use of cerium (Ce) in the upper continental crust (UCC) for tracing oxidative weathering, we measured the Ce anomaly (Ce/Ce*), the Ce stable isotope composition (δ142Ce), and the average Ce valence state (Ce#) in twenty-four glacial diamictite composites with depositional ages ranging from ∼2.9 to 0.3 Ga. We observe no distinguishable secular changes in Ce/Ce* (Ce/Ce* = 0.87–0.97) or δ142Ce (−0.092 ± 0.033 to 0.028 ± 0.043‰, 2 S.D.) of the diamictites. Limited elemental partitioning and isotopic fractionation of Ce can be ascribed to the immobilization of REEs, including Ce, in the diamictites during oxidative weathering. We calculated the present-day UCC δ142Ce to be −0.025 (median) ± 0.018‰ (MAD) or −0.029 (mean) ± 0.058‰ (2 S.D.). We assigned average valence state data of Ce (Ce#) in the diamictites into three age groups relative to the timing of the Great Oxidation Event (GOE): pre-GOE (2.9–2.4 Ga; Ce# = 3.09 to 3.22; average of 3.14), syn -GOE (2.4–2.2 Ga; Ce# = 3.18 to 3.73; average of 3.29), and post-GOE ages (0.8–0.3 Ga; Ce# = 3.21 to 3.68; average of 3.36). Our results show distinguishable correlations between Ce# and common weathering (major element ratios and Li isotopes) and redox tracers (e.g., Mn, Mo, V and Th/U) due to increasingly oxidative weathering with time. These results suggest that the valence state of Ce is a sensitive indicator of redox, and advise against relying solely on element- or isotope-based proxies. Combining redox proxies with valence state information can strengthen and complement the interpretation of past redox conditions. [ABSTRACT FROM AUTHOR]

The increase in marine Li isotope composition (δ 7 Li, ∼9‰) since the Cenozoic is possibly related to continental chemical weathering associated with tectonic uplift (e.g., the Tibetan Plateau [TP]). However, the reasons for the spatiotemporal variations of δ 7 Li in the rivers flowing through the TP are still under debate, hindering the understanding of the changes in seawater δ 7 Li. Herein, various geological samples, i.e. river waters, river suspended particulate materials, river sediments, hot springs and silicate rocks, from the Yarlung Tsangpo River basin, the largest river system in the TP, have been analyzed to understand the source and isotopic fractionation of dissolved Li in rivers flowing through high-altitude and tectonic regions. The Yarlung Tsangpo River has an unusually high dissolved Li concentration ([Li] dis) (0.5–313 μg/L, mean 58.4 μg/L, n = 93) and low δ 7 Li dis values (+1.0‰–+14.7‰, mean +6.4‰, n = 92) compared with most river waters. These results can be explained by the input of geothermal water with extremely high [Li] dis (5.5–34.4 mg/L, mean 15.7 mg/L, n = 9) and low δ 7 Li dis (−1.7‰–+3.1‰, mean +1.0‰). Silicate weathering is probably responsible for elevated δ 7 Li dis in river water compared to geothermal water, but the binary mixture model results of silicate weathering and geothermal water do not support this speculation. Ongoing Li isotope fractionation between dissolved loads and secondary minerals has been suggested to be the main reason for the increased δ 7 Li dis in river water. Field study and adsorption experiment results support the view of continuous Li isotope fractionation in rivers. Physical erosion and chemical weathering processes that provide fresh secondary minerals to rivers as well as dissolved Li from geothermal water transported in rivers over long residence times promote Li isotope fractionation. Hence, with the emergence of a continental geothermal system, the tectonic activity may directly or indirectly induce the simultaneous increase in dissolved Li flux and δ 7 Li dis in rivers that eventually flow into the ocean. This can partially explain the increase in seawater δ 7 Li in the Late Cenozoic. • Li isotope composition is analyzed in the largest river system in Tibetan Plateau. • Dissolved load has very high [Li] dis , high Li/Na ratios, and low δ 7 Li dis values. • Geothermal water is responsible for the high [Li] dis and low δ 7 Li dis in river water. • Continental geothermal water increases in the Li flux and δ 7 Li dis in river estuaries. • The results help to explain the increase of seawater δ 7 Li in the late Cenozoic. [ABSTRACT FROM AUTHOR]

During subaerial chemical weathering, submarine weathering and other intracrustal processes, Sm Nd chemical fractionation remains uncertain, while a little is known about the conditions, scale and magnitude, specific processes and actual mechanism for possible Sm Nd fractionation. Here, an in situ terra rossa weathering profile developed on dolostone is chosen for the study. Based on sequential extraction experiments and measurements of the mineral and geochemical compositions including Sr Nd isotopic compositions, we suggest that subaerial chemical weathering not only leads to considerable Sm Nd mobilization, redistribution and secondary enrichment, but also gives rise to extreme Sm Nd fractionation, apparent 143Nd isotope homogenization, nearly complete resetting of the 147Sm 143Nd isotope system, anomalous initial 143Nd/144Nd ratios, and meaningless 143Nd model age (T DM) for weathering products. We found that the lanthanide tetrad effect, rather than mineralogical control, is responsible for the observed substantial Sm Nd fractionation and for the other rare earth element fractionation. Under domination of the lanthanide tetrad effect, the mobility of Sm is preferential to that of Nd during their migration in the form of dissolved organic complexes. Our results also illustrate that resetting of the 147Sm 143Nd isotope system in weathering products is controlled by the combined effect of the extraneous dissolved Sm Nd input and additional Sm Nd fractionation. Finally, we argue that similar scenarios may occur in submarine weathering, diagenesis and other geological processes. [ABSTRACT FROM AUTHOR]