Controls on riverine magnesium isotopic composition: Evidence from mono-lithological catchments

Non‐traditional stable isotopes of metals were recently shown as new dietary tracers in terrestrial and marine mammals. Whether these metal stable isotopes can be used to understand feeding habits in stream food webs is not known yet. In this study, we explored the potential of stable isotopes of essential Mg (δ26Mg) and Zn (δ66Zn) as a new tool in stream ecology. For this purpose, we determined δ26Mg and δ66Zn values of stream organisms and their potential metal sources in upper and lower reaches of two streams in the Lake Biwa catchment, Central Japan. Our goals were (1) to explore variations in δ26Mg and δ66Zn across organisms of different feeding habits and (2) to understand Mg and Zn sources to stream organisms. Overall, δ26Mg and δ66Zn values of organisms were neither related to each other, nor to δ13C and δ15N values, indicating different elemental sources and factors controlling isotopic fractionation depending on element and taxa. Low δ26Mg values in filter‐feeding caddisfly larvae and small gobies indicated aqueous Mg uptake. Higher δ26Mg values in leaf‐shredding crane fly and grazing mayfly larvae suggested Mg isotopic fractionation during Mg uptake from the diet. While the δ26Mg values of stonefly nymphs reflected those of caddisfly larvae as a potential prey, the highest δ26Mg values found in dobsonfly nymphs can be explained by 26Mg enrichment during maturing. δ66Zn values of caddisfly and mayfly larvae indicated Zn was a mixture of aqueous and dietary available Zn, while higher δ66Zn values in crane fly larvae pointed to Zn isotopic fractionation during Zn uptake from plant litter. δ66Zn values in stonefly and dobsonfly nymphs were often in the range of those of caddisfly larvae as their prey, while dragonfly nymphs and small goby were depleted in 66Zn relative to their dietary Zn sources. We conclude that δ26Mg is a promising indicator to assess Mg sources in stream ecology depending on taxa, while the use of δ66Zn is limited due to the complexity in Zn sources.

Biogeochemical cycling of Mg and its isotopes in a sugar maple forest in Québec

Revisiting magnesium isotope constraints on the petrogenesis of the Fe-Ti oxide-bearing mafic–ultramafic intrusions

Differential weathering of basaltic and granitic catchments from concentration–discharge relationships

The magnesium (Mg) isotopic composition of marine authigenic carbonates is considered as promising archive of ancient seawater geochemistry and paleoenvironments. Previous experimental and theoretical work has shown that Mg isotope fractionation during carbonate mineral formation is a function of mineralogy and precipitation rate. However, information on Mg isotope fractionation is limited for well-defined precipitation rates in natural settings. Here, we investigate pore waters from sediments of an area of active methane seepage in the South China Sea. Low δ13C values (< −48.3‰ VPDB) of dissolved inorganic carbon (DIC) near the sulfate-methane transition zone (SMTZ) indicate that sulfate-driven anaerobic oxidation of methane (SD-AOM) is the predominant biogeochemical process. Pore water composition of dissolved Mg, calcium (Ca), and strontium (Sr) agrees with aragonite as the dominant carbonate mineral at the site ROV1, and high Mg-calcite at sites ROV2 and ROV4. Calculated carbonate precipitation rates are 0.92 μmol cm−2 yr−1 for site ROV2 and 1.24 μmol cm−2 yr−1 for site ROV4; these estimates are similar to previous calculations for seeps from other areas. The pore water δ26Mg values (−0.88‰ to −0.71‰) obtained for the three study sites are similar to those of seawater, in accord with a minor effect of Rayleigh fractionation due to abundant supply of Mg from seawater and insignificant consumption of Mg during carbonate precipitation. The modeled Mg isotope fractionation (ϵ = −2.0‰ to −1.0‰ for core ROV2; ϵ = −1.3‰ to −0.3‰ for core ROV4) can be explained by kinetic isotope fractionation during carbonate precipitation. The calculated carbonate precipitation rates and the degree of fractionation of Mg isotopes support the notion that fractionation is small at high precipitation rates. However, the carbonate precipitation rates calculated for the studied seep environments are much smaller than those in laboratory experiments, documenting a discrepancy of isotopic fractionation between carbonate authigenesis in laboratory experiments and natural environments. These results, including the modeled precipitation rates, provide new constraints for Mg isotope fractionation in natural settings.

Mg isotopic systematics during early diagenetic aragonite-calcite transition: Insights from the Key Largo Limestone

The Mid‐Brunhes Event (MBE, ∼430 ka) represents a pivotal climatic transition in the late Pleistocene, while monsoon‐driven impacts on low‐latitude regions during this interval remain debated. Sediments archived in the MD05‐2901 gravity core from the South China Sea (SCS) record the influence of the East Asian summer monsoon (EASM) on regional climate dynamics. In this study, we employ major and trace element systematics and magnesium isotope analyses (δ26Mg) of clay‐sized (<2 μm) sediments to reconstruct EASM variability, offering a novel perspective on its evolution over the past 430 ka. Upper Continental Crust normalized rare earth element and major elemental depletion patterns are consistent with established sequences of elemental mobility during silicate weathering, thus these sediments are produced by silicate weathering. A temporal decline in δ26Mg values is observed, correlating with reduced smectite and chlorite proportions in the total mineral assemblage. This coupled trend deviates from expectations for silicate weathering sequences, necessitating a re‐evaluation of sediment provenance. Provenance analysis demonstrates that smectite and chlorite predominantly derive from mafic rock weathering in the southern SCS. We propose that the observed mineralogic and isotopic shifts reflect a temporal decrease of highly weathered siliciclastic materials from southern provenances presumably related to a weakened clockwise oceanic circulation system, which is a coupled response to the long‐term weakening of EASM and intermittent inundation of the Sunda Shelf over the past 430 Ka. Our results underscore the interconnected marine‐terrestrial climatic feedbacks modulating tropical monsoon systems on orbital timescales.

Speleothems are an important archive for reconstructing past climate variability. The Magnesium isotope proxy tested so far in limestone-hosted caves provides the possibility of reconstructing climate conditions from changes in the silicate-to-carbonate weathering ratio. Other caves, however, are situated in dolostone host rock. Consequently, the Mg content of the host rock is much higher than that of limestone. Dripwater monitoring in a set of dolostone-dominated caves in Germany and Morocco, as well as the collection of soil (silicate minerals), host rock (carbonate), and speleothem samples, aims to apply the Mg isotope proxy in dolostone-hosted caves. The time-series analyses of the Mg isotope composition of dripwaters revealed, for most dripwater sites, significant variations in δ26Mg values, which can be related to changes in the silicate-to-carbonate weathering ratio. Silicate weathering is enhanced under dry, warm conditions, whereas cold, wet conditions favour carbonate weathering. Due to significant differences in the Mg isotope composition of silicate (soil) and carbonate (host rock) minerals, changes in the weathering regime are detectable in drip-water Mg isotope ratios in both climate regions. In German caves, where changes in temperature are more pronounced than changes in rainfall amount, the weathering ratio is driven by temperature variations. In Morocco, however, both temperature and rainfall amount complement each other and drive changes in the silicate-to-carbonate weathering ratio. Furthermore, the different transfer times at each drip site ranged from a few months to at least a year. Some drip water sites show no variation in Mg isotope composition. In these cases, the signal of the weathering ratio is strongly buffered by longer water transfer times/residence times and mixing of waters in the aquifer. Although possible, no dependence of Mg isotope variations in the dripwaters on prior calcite precipitation was observed. Corresponding speleothems from the monitored dripwater sites exhibit varying Mg isotope compositions of calcite and aragonite. There is no overprint of other factors during carbonate precipitation; thus, these variations are solely due to changes in the silicate-to-carbonate weathering ratio and, consequently, changes in temperature and rainfall amount. Furthermore, observations on the Mg isotope fractionation factor of aragonite-dominated samples revealed a smaller Δ26Mg than for calcite speleothem samples. Generally, the Mg isotope proxy is a valuable tool for reconstructing past climate conditions in both limestone- and dolostone-dominated caves.

Magnesium is a macro-nutrient for all living organisms, and its stable isotope ratios (δ26Mg) can be used for analyzing feeding habits and the dynamics of elements within the body. However, it was still unclear what kind of conditions for feeding and the surrounding seawater affect the Mg isotope ratios of marine fish. In this study, we report on the Mg isotopic composition of different organs of three fish species, blue mackerel (Scomber australasicus), black rockfish (Sebastes schlegeli), and Pacific bluefin tuna (Thunnus orientalis), as well as the magnitude of isotopic fractionation associated with Mg uptake in one-year-old T. orientalis. By using a delipidation treatment before acid samples digestion, the δ26Mg value of each organ was compared before and after removing the lipids and body fluids. In T. orientalis and S. schlegel, the δ26Mg of muscle and most internal organs is similar, whereas gills, fins, and bones are often 0.5‰ lower than the internal organs. We examined the variation in δ26Mg values for each organ using multiple T. orientalis individuals, and found that muscle tissue showed the least variable values among individuals. The δ26Mg of blood of one-year-old T. orientalis was clearly lower than the seawater value, and the value was close to that of its feed. This is thought to be the results that Mg2+ in seawater and blood plasma are separated each other in order to regulate osmotic pressure, and the contribution of Mg derived from feed is high. Isotopic fractionation of -0.40‰ was observed between the muscle of one-year-old T. orientalis and formula feed, similar to a cellular isotope fractionation between methanogen Methanothermobacter marburgensis and incubation medium. This is thought to reflect the selectivity of Mg uptake into the cell via membrane proteins.

Chemical weathering and associated CO 2 consumption in the Godavari river basin, India

The influence of peritectic garnets on magnesium isotopic composition during crustal anatexis: Constraints from TTG-like leucosomes from the North Qaidam orogen, China

Calcium isotope fractionation during tropical weathering of granites

The Qaidam Basin, located on the northern margin of the Qinghai-Tibet Plateau in China, is the second-largest inland basin in the country. Hosting numerous salt lakes rich in mineral resources, with particularly prominent lithium reserves. The Nalenggele River, the largest inland river within the Qaidam Basin, hosts the most extensive brine-type lithium deposit in China in its terminal salt lake region. With lithium resources reaching 2.3 million tons, this deposit accounts for approximately 87% of Chinese total lithium reserves. The genetic mechanism of brine in this terminal salt lake area is closely linked to hydrogeochemical processes. Previous studies have primarily focused on the qualitative analysis of hydrogeochemical processes, while relatively few have quantitatively assessed the impact of different hydrogeochemical processes. In this study, river water and groundwater from the mountainous areas to the basin within the watershed are selected as the research objects. Multiple isotopic tracers (δ²H, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ11B and δ7Li) are employed to trace the material sources and evolutionary processes of elements. The material sources, controlling factors and evolutionary mechanisms of hydrochemistry in the Nalenggele River Basin are clarified. The Positive Matrix Factorization model is applied to quantitatively identify the recharge sources of different water bodies, trace the material sources of major ions, and elucidate the evolutionary processes of the watershed hydrological cycle. The results show that: (1) The hydrochemical compositions of both river water and groundwater are dominated by Na⁺ and Cl⁻. The hydrochemical types evolve from mixed Cl·HCO₃·SO₄-Na·Ca type in the upstream rivers to Cl-Na type in the downstream waters. Analyses of ionic ratios and strontium isotope data confirm that water-rock interaction is the primary controlling factor of hydrochemical compositions, which is characterized by silicate weathering as the dominant process, supplemented by carbonate weathering and evaporite (halite, gypsum, mirabilite) dissolution. Cation exchange exhibits spatial heterogeneity: forward exchange (Ca²⁺/Mg²⁺ vs. Na⁺/K⁺) occurs in the upstream and downstream areas, while reverse exchange takes place in the midstream area. (2) Evidence from δ²H and δ¹⁸O indicates that river water is mainly recharged by atmospheric precipitation from the southern mountainous areas with the elevation of 4700m. Groundwater has close hydraulic connectivity with river water, showing bidirectional recharge-discharge interactions. (3) The observed B and Li isotopic footprints in the Nalenggele River Catchment are significantly depleted in heavy isotopes compared with those in other geological systems dominated by natural weathering processes. In the upper reaches of the Nalenggele River, the concentrations of B and Li increase sharply, while the δ¹¹B and δ⁷Li values decrease gradually. The mechanism responsible for the B and Li enrichment is mainly associated with the Li-B supply potential of material sources, favorable tectonic conduits for water circulation, and high evaporation rates. (4) The Positive Matrix Factorization model quantitatively reveals the contribution rates of different hydrogeochemical processes during the hydrological cycle, specifically: evaporite mineral dissolution (28%), mixed evaporite dissolution (25%), agricultural activity (17%), and silicate weathering (30%). This study provides a comprehensive framework for integrating multi-isotope tracers and statistical models to quantify hydrochemical processes in arid inland basins.

Chemical weathering of small catchments on the Southeastern Tibetan Plateau I: Water sources, solute sources and weathering rates

Tree-ring based minimum temperature reconstruction on the southeastern Tibetan Plateau