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The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆ 47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18O fluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18O fluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆ 47 -derived temperatures , it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids. [ABSTRACT FROM AUTHOR]

Abstract Equilibrium Mg isotope fractionation properties between aqueous Mg2+ and minerals are the key for the applications of Mg isotopes in geochemistry. This study conducts first-principles molecular dynamics (FPMD) simulations for aqueous Mg2+ based on the density functional theory (DFT). Thirty-five snapshots are extracted from the FPMD trajectories after equilibration for the calculations of the reduced partition function ratio (β factor or 103lnβ). Combining with the β factors of minerals, we found that the β factor decreases in the sequence of lizardite > brucite > aqueous Mg2+ > dolomite > magnesite > calcite > aragonite. Our calculations also confirm the effect of Mg concentration on the β factor of calcite, and further show that the concentration effect is negligible when Mg/(Mg + Ca) (atomic ratio hereafter) is lower than 1/32. Our results depict significant equilibrium Mg isotope fractionations between minerals and aqueous Mg2+ (103lnα minerals-Mg_aq), which are dominantly controlled by the average force constant of Mg in these phases. Compared to aqueous Mg2+, carbonates are enriched in light Mg isotopes but brucite and lizardite show enrichment in heavy Mg isotopes. Among all minerals, 103lnα aragonite-Mg_aq is the largest, which is up to −14‰ at 300 K. Notably, 103lnα calcite-Mg_aq is not only controlled by temperature, but also significantly affected by Mg content in calcite. 103lnα calcite-Mg_aq is negatively correlated with Mg content in calcite. The experimentally measured Mg isotope fractionations between minerals (dolomite, magnesite, brucite) and solution at equilibrium are consistent with our predicted results, showing the accurate description of FPMD simulations for aqueous Mg2+ and the reliability of our calculations. Overall, the calculated equilibrium Mg isotope fractionations between minerals and aqueous Mg2+ provide a guideline for applications of Mg isotopes in aqueous geochemical processes. [ABSTRACT FROM AUTHOR]

Abstract Magnesium isotope behavior in dissolution and precipitation reactions during chemical weathering have been well documented. However, mechanisms of mineral dissolution and precipitation impact on the riverine Mg isotope composition under different climatic and geology background are not well constrained, which limits Mg isotope application in weathering research. Mg isotopic compositions for solute and suspended sediments in Jinsha River Basin, located in Tibetan Plateau, China, were examined to address this issue. The δ26Mg values for dissolved loads range from −1.67‰ to −0.5‰, and the suspended loads show systematically heavier Mg isotope compositions (−1.15‰ to −0.06‰). Conservative mixing between different rock weathering end-members fails to fully explain Mg isotopic composition variation of Jinsha River waters based on mass balance and mixing model with the river geochemistry data. Mg in rivers draining dominantly carbonate and evaporite is isotopically heavier compared with the value of catchment bedrocks, and water pH and δ26Mg values are negatively correlated in carbonate (calcite and dolomite) oversaturated river waters, which suggests the precipitation of secondary carbonate as an important mechanism driving the δ26Mg of dissolved loads heavier. For carbonate unsaturated waters, Mg concentrations and δ26Mg values are intermediate between those of silicate dominated basins and carbonate oversaturated waters. The results suggest two possible different mechanisms controlling river solute δ26Mg values: fractionation during carbonate precipitation incorporating of Mg; and conservative mixing between a solute end-member formed from carbonate precipitation and end-members from rock weathering. This study provides new evidence and insights in carbonate precipitation processes regulating river Mg isotope signature and its potential influence on Mg cycling. [ABSTRACT FROM AUTHOR]

Shallow-water carbonate sediments constitute one of the most abundant and widely used archives of Earth’s surface evolution. One of the main limitations of this archive is the susceptibility of the chemistry of carbonate sediments to post-depositional diagenesis. Here, we develop a numerical model of marine carbonate diagenesis that tracks the elemental and isotopic composition of calcium, magnesium, carbon, oxygen, and strontium, during dissolution of primary carbonates and re-precipitation of secondary carbonate minerals. The model is ground-truthed using measurements of geochemical proxies from sites on and adjacent to the Bahamas platform (Higgins et al., 2018) and authigenic carbonates in the organic-rich deep marine Monterey Formation (Blättler et al., 2015). Observations from these disparate sedimentological and diagenetic settings show broad covariation between bulk sediment calcium and magnesium isotopes that can be explained by varying the extent to which sediments undergo diagenesis in seawater-buffered or sediment-buffered conditions. Model results indicate that the covariation between calcium and magnesium isotopes can provide a semi-quantitative estimate of the extent and style (fluid-buffered vs. sediment-buffered) of early marine diagenesis. When applied to geochemical signatures in ancient carbonate rocks, the model can be used to quantify the impact of early marine diagenesis on other geochemical proxies of interest (e.g. carbon and oxygen isotopes). The increasing recognition of early marine diagenesis as an important phenomenon in shallow-water carbonate sediments makes this approach essential for developing accurate records of the chemical and climatic history of Earth from the chemical and isotopic composition of carbonate sediments. [ABSTRACT FROM AUTHOR]

Magnesium isotopes in early diagenetic dolomite have been proposed as a potential tracer for seawater chemistry and global Mg cycles. However, the applicability of Mg isotopes of early diagenetic dolomite in studies of ancient seawater requires a detailed understanding of the behavior of Mg isotopes during dolomitization in a variety of geological settings. Hinterland attached carbonate platform is an important sink of seawater Mg through dolomitization, and basin restriction is a common feature in hinterland attached carbonate platforms, yet its effects on Mg isotope systematics in carbonates have not been well documented. The upper Albian Hevyon Formation in southern Israel was deposited in a typical hinterland attached carbonate platform setting and provides an ideal case for investigating the Mg isotope behavior during dolomitization in environments of frequent basin restrictions. The abundance of dolomite increases up-section in the Hevyon Formation and correlates with the appearance of microbial deposits and disappearance of metazoans, reflecting water level fluctuations in multiple exposure and recharging events. In conjunction with sedimentary facies analysis, multiple geochemical proxies (Ni/Co, V/Cr, Ce/Ce ∗ , and δ 13 C) indicate the development of anoxic conditions. These two lines of evidence, together with textural indicators and 87 Sr/ 86 Sr ratios, suggest dolomitization was microbial and penecontemporaneous in a shallow low circulation water body. δ 26 Mg of dolomite increases from −1.99 ± 0.12‰ in the lower part of the sequence to a value of −1.52 ± 0.02‰ in the middle interval, then decreases back to −2.06 ± 0.18‰ in the upper unit. Overall, Mg isotope compositions co-vary with δ 13 C, Mg/Ca ratios, as well as crystal size of dolomite, which implies that variations in δ 26 Mg value reflect syn-depositional changes in water chemistry as a result of dolomitization in episodically restricted basins. δ 26 Mg tracks the fluctuating marine connection and provides a powerful tracer for the level of restriction. Furthermore, by pairing Mg isotope variability of the dolomite with a Rayleigh distillation model and elemental mass balance in aqueous solution, it is possible to constrain the Mg/Ca ratio of seawater. Based on the records from the Hevyon Formation, Mg/Ca ratio of seawater during the Albian is constrained between 1.5 and 1, which is consistent with results from other records. [ABSTRACT FROM AUTHOR]

Changing conditions including steep redox gradients result in different Ca-Mg-carbonates forming at methane seeps as a consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Mg 2+ is one of the main cations in Ca-Mg-carbonates, Mg isotope composition may reflect the mode of mineral authigenesis at seeps. Seep carbonates were collected from the Shenhu area and the Southwest (SW) Taiwan basin in the South China Sea. The carbonate phases in Shenhu samples are dolomite accompanied by accessory Mg-calcite, while SW Taiwan samples consist of Mg-calcite and dolomite. Correspondingly, Shenhu carbonates show systematically higher Mg/Ca ratios. Low δ 13 C values of seep carbonates confirm their derivation from the oxidation of methane, δ 18 O values reflect formation in equilibrium with coeval seawater. The δ 26 Mg value of a reference sample of biodetrital carbonate (−4.28‰) is lower than those of seep carbonates (−3.25 to −2.95‰). Since only little variability of δ 26 Mg values of pore waters is expected based on previous work, differences in the δ 26 Mg values of seep carbonates were apparently caused by changing degrees of isotopic fractionation during precipitation. Trends between δ 26 Mg values and Mg/Ca ratios and between δ 26 Mg and δ 13 C values suggest that Mg isotope fractionation was controlled by a kinetic mechanism affecting the incorporation of Mg 2+ ions into the carbonate lattice in the course of SD-AOM. By consuming sulfate and by producing sulfide, SD-AOM reduces the energy differences for the dehydration of ions of light and heavy Mg isotopes, lowering isotope fractionation. Additionally, the two trends have been found to be steeper for Shenhu samples, suggesting more pronounced SD-AOM in the Shenhu area. Our study indicates that Mg isotope composition of methane-derived carbonates is affected by the process that drives carbonate precipitation – SD-AOM. Future work is required to confirm the utility of Mg isotopes as a new proxy for this major biogeochemical process. [ABSTRACT FROM AUTHOR]

Crustal metasomatism by subduction is considered as an important mechanism for generating mantle heterogeneity through infiltration of different metasomatic agents into the mantle. As a consequence of the subduction of oceanic crust (including oceanic basaltic rocks and seafloor sediments), both carbonate and silicate metasomatism are expected to occur at the slab-mantle interface in an oceanic subduction channel. This is demonstrated by an integrated study of major and trace elements, stable Mg-isotopes and radiogenic Sr-Nd-Hf isotopes in Cenozoic and Mesozoic alkali basalts from the West Qinling orogen, China. Although the two series of continental basalts show ocean island basalt (OIB)-like trace element distribution patterns and relatively depleted Sr- Nd-Hf isotope compositions, they exhibit differences in other geochemical variables. The Cenozoic basalts have low SiO2, but high CaO and MgO concentrations and high CaO/Al2O3 ratios but low d26Mg values of -0.54 to -0.32‰. They have relatively high abundances of melt-mobile incompatible trace elements such as the light rare earth elements (LREE) and most large ion lithophile elements (LILE), but low abundances of K, Pb, Zr, Hf and Ti, with high (La/Yb)N values but low Ti/Eu and Hf/Sm ratios. In contrast, the Mesozoic basalts have relatively high SiO2, but low CaO and MgO concentrations and low CaO/Al2O3 ratios, but high d26Mg values of-0.35 to -0.21‰. They have relatively low abundances of melt-mobile incompatible trace elements such as the LILE and LREE, but are relatively enriched in Zr, Hf and Ti, with low (La/Yb)N values but high Ti/Eu and Hf/Sm ratios. These observations indicate a significant difference in the composition of the mantle sources between the two periods of alkali basalt magmatism. Whereas the low d26Mg values of the Cenozoic basalts indicate the involvement of sedimentary carbonate in their mantle source, the high d26Mg values of the Mesozoic basalts point to a primary contribution from a silicate component. Metasomatic reaction of depleted mid-ocean ridge basalt (MORB)-source mantle peridotite with carbonate and silicate melts, respectively, at the slab-mantle interface in the Paleotethyan oceanic subduction channel are responsible for the generation of their mantle sources. Both carbonate and silicate melts have been incorporated into the depleted MORB-source mantle wedge producing 'crustal metasomatis'm in an oceanic subduction channel. We suggest that the type of metasomatic agent in the mantle sources is the key to the composition of the resultant alkali basalts. [ABSTRACT FROM AUTHOR]

The Mg and Sr isotopic compositions (δ 26 Mg and 87 Sr/ 86 Sr) of pore fluids and bulk carbonates from Ocean Drilling Project Site 1171 (South Tasman Rise; 2148.2 m water depth) are reported, in order to evaluate the potential of diagenesis to alter carbonate-based geochemical proxies in an open marine system. Given the trace amounts of Mg in marine carbonates relative to coexisting pore fluids, diagenesis can alter carbonate δ 26 Mg, a promising proxy for seawater δ 26 Mg that may help elucidate long-term changes in the global Mg cycle. Constraints on the effect of diagenetic recrystallization on carbonate δ 26 Mg are therefore critical for accurate proxy interpretations. This study provides context for assessing the fidelity of geochemical proxy-reconstructions using the primary components (i.e., foraminiferal tests and nannofossils) of bulk carbonate sediments. We find that pore fluid δ 26 Mg values (on the DSM3 scale) at Site 1171 increase systematically with depth (from −0.72‰ to −0.39‰ in the upper ∼260 m), while the δ 26 Mg of bulk carbonates decrease systematically with depth (from −2.23‰ to −5.00‰ in the upper ∼260 m). This variability is ascribed primarily to carbonate recrystallization, with a small proportion of the variability due to down-hole changes in nannofossil and foraminiferal species composition. The inferred effect of diagenesis on bulk carbonate δ 26 Mg correlates with down-core changes in Mg/Ca, Sr/Ca, Na/Ca, and 87 Sr/ 86 Sr. A depositional reactive-transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate δ 26 Mg. Model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work, assuming calcite recrystallization rates of ⩽7%/Ma constrained by Sr geochemistry. In addition, either partial dissolution or a distinctly different previous diagenetic regime must be invoked in order to explain aspects of the elemental chemistry and 87 Sr/ 86 Sr of relatively deep sediments from Holes A and C. This study indicates that the dynamics of a given sedimentary system can significantly alter bulk carbonate geochemistry, and presents a framework for considering the potential impact of such alteration on picked archives such as foraminiferal tests and nannofossils. Ultimately, this study contributes to the development of δ 26 Mg as a proxy for seawater δ 26 Mg by quantifying the susceptibility of carbonate δ 26 Mg to diagenetic alteration, particularly in sediments in open marine systems. This study suggests that because of the sensitivity of carbonate δ 26 Mg to diagenetic recrystallization, it can, in certain systems, be used to quantify the impact of diagenesis on carbonate-based geochemical proxies. [ABSTRACT FROM AUTHOR]

The magnesium isotope compositions of sedimentary carbonates are widely used to investigate the geochemical cycling of this element in seawater. Density Functional Theory techniques and experimental data from this study and literature on Mg isotope fractionation between sedimentary minerals and fluids have been used to derive equilibrium fractionation factors and to quantify the impact of aqueous magnesium speciation on the isotopic composition of its aquo ion and Mg in the precipitated carbonates. Although aqueous Mg 2 + undergoes hydrolysis to a lesser extent than divalent transition metals, it nevertheless forms relatively strong complexes with inorganic and organic ligands including bicarbonate/carbonate, sulfate and carboxylate. Furthermore, aqueous Mg 2 + undergoes a significant contraction of its coordination sphere when it reacts with bicarbonate and carbonate ions to form the MgHCO 3 + and MgCO 3 ° complexes which favors the preferential partitioning of heavy Mg into these species. Calculated values of the reduced partition function ratios for Mg 2 + (aq) and Mg 2 + bound to a number of inorganic and organic ligands show a strong enrichment of 26 Mg in MgCO 3 °, MgHCO 3 + and MgSO 4 ° compared to Mg 2 + ( i.e. 1000lnβ 26/24MgCO3° -1000lnβ 26/24Mg 2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate) 2 − , Mg(oxalate) 2 2 − , Mg(citrate) − ) or depletion (Mg(EDTA) 2 − , Mg(citrate) 2 4 − ) of 26 Mg in Mg-carboxylate complexes compared to Mg 2 + . Analysis of experiments from this study and the literature on Mg isotope fractionation between aqueous solution and Mg carbonate and hydroxide minerals validate the reduced partition functions for Mg 2+ , Mg bicarbonate/carbonate and carboxylate couples generated in the present work and confirms the significant impact of carbonate and carboxylic ligands on the isotope composition of precipitated Mg-bearing minerals. This study thereby provides new insights into the parameters controlling the isotope composition of aqueous Mg 2+ in natural fluids as well as improved tools to reconstruct paleo-environmental conditions from the magnesium isotope compositions recorded in carbonate sediments. [ABSTRACT FROM AUTHOR]

The Mg/Ca ratio of seawater increased from ∼1.5 to its present value of 5.4 over the Neogene, indicating a fundamental imbalance in the Mg–Ca cycle during the late Cenozoic. The imbalance in Mg–Ca cycles since the late Cenozoic, however, is contrasted by an arguable constancy in the Mg isotope compositions of seawater (δ 26 Mg sw) reconstructed from several low-resolution carbonate records. Addressing such Cenozoic Seawater Conundrum requires reliable seawater Mg isotopic records. The Xisha Islands in the South China Sea host large Cenozoic dolostone sequences that record seawater chemistry over the past 23 Myr. This study reports the Mg isotopic and trace elemental compositions of island dolostones in the well-studied Xike-1 drill core from the Xisha Islands. Petrographic, C–O–Sr isotopic, and paleomagnetic data from the drill core samples collectively support a "fluid-buffered", syn-depositional origin for the dolostones. The dolostones from the Xike-1 core have a narrow range of δ 26 Mg (− 2.83 ‰ ± 0.12 ‰), confirming that δ 26 Mg sw have been stable around the modern value (−0.83‰) since the late Cenozoic. This record provides a new opportunity to better understand the Mg–Ca cycle during the late Cenozoic. We ran numerical models of global seawater Mg–Ca contents and δ 26 Mg sw with the most recently published flux and isotope fractionation data. The results show that the contrast between the constancy of δ 26 Mg sw values and the dramatic increase in seawater Mg/Ca ratios cannot be explained by a single mechanism (i.e., a decrease in the rate of phyllosilicate formation on the seafloor alone). Instead, processes involving different Mg fluxes need to be coupled to explain the observed trends in Mg content and isotopic compositions of Cenozoic seawater. The solution to the Cenozoic Seawater Conundrum includes simultaneous decreases in the rates of both clay formation and dolomitization on the seafloor, or an increase in the rate of both continental weathering together with decreased seafloor clay formation. Our study underlines the sensitivity of the Mg cycle to the interactions between climate, continental weathering, and seafloor processes during the late Cenozoic. • Seawater δ 26 Mg sw values can be reconstructed from tropical island dolostones. • Record of Xisha Island dolostones shows a stable δ 26 Mg sw for the past 23 million years. • A sharp contrast between stable δ 26 Mg and rising Mg/Ca of seawater in the late Cenozoic. • Changes in dolomitization intensity canceled the Mg isotope effect of changes in clay formation intensity. [ABSTRACT FROM AUTHOR]

Abstract: Magnesium (Mg) isotopes can be a proxy for directly constraining the sources of riverine Mg, but the dominant controls on riverine Mg isotope ratios are still uncertain. Here, we report Mg isotope ratios for river waters, experimental leachates and digestions, bulk rocks, and fertilizers in the Han River (HR), South Korea. The HR is composed of two lithologically distinct tributaries: the North Han River (NHR) that flows over only silicate rocks, and the South Han River (SHR) that flows over silicate and carbonate rocks. The lithological differences between the NHR and SHR are reflected in major ion, 87Sr/86Sr, and δ26Mg geochemistry. In particular, the NHR has lower major ion concentrations but higher 87Sr/86Sr ratios and δ26Mg values than the SHR. Simple mass balances and mixing equations indicate that if the riverine δ26Mg values in the HR system are mainly controlled by conservative mixing between silicate and carbonate weathering, the average carbonate end-member δ26Mg value should be unlikely lower than what are measured in this study. Although multiple process-related fractionations occur in the HR system, the enrichment of 24Mg in river waters relative to silicate rocks they drain could be mostly controlled by either fractionation or mixing between isotopically distinct reservoirs, such as minerals or fractions (labile and structural Mg), during dissolution, while the little depletion of 24Mg in the SHR waters relative to carbonate rocks they drain could be likely due to the input of groundwater with lower δ26Mg value rather than fractionation. However, it is difficult to identify the contribution of anthropogenic inputs to riverine δ26Mg because their effects are little. This study suggests that the potential of Mg isotopes for constraining Mg sources in a lithologically varied river basin can be enhanced with a better understanding of process-related fractionation. [Copyright &y& Elsevier]

Abstract: To constrain how Mg isotopes behave during chemical interactions and physical transport in carbonate-rich settings, we measured δ 26Mg values of surface water, groundwater, and dolomite samples from the Madison Aquifer, South Dakota. Groundwater in the Madison Aquifer chemically evolves by dedolomitization during transport along a 236km flow path. Surface streams recharging the aquifer have δ 26Mg values of −1.08 and −1.18‰. Following recharge, groundwater δ 26Mg values vary between −1.10 and −1.63‰ up to a distance of 20km. Between 20 and 189km, δ 26Mg values remain nearly constant at −1.40‰, and a final sample at 236km shows an increase to −1.09‰. Dolomite exhibits a wide range of δ 26Mg values between −2.21 and −1.27‰. Reactive-transport modeling and isotope mixing calculations employing previously published major ion mass-balances, 87Sr/86Sr ratios, and δ 44Ca values were used to determine whether dedolomitization reactions, namely dolomite dissolution, calcite precipitation, and Mg-for-Na ion-exchange, fractionate Mg isotopes. We tentatively attribute the final δ 26Mg value to preferential uptake of 24Mg during Mg-for-Na ion-exchange. Otherwise, we find little evidence of isotopic fractionation and observe instead that δ 26Mg conservatively traces lithologic and hydrologic sources. Either isotope exchange between dolomite and water, with a fractionation factor of 0‰, or mixing between different water sources establishes the δ 26Mg value of −1.40‰ at 20km. This value remains unchanged for the next 169km of water transport because dolomite adds Mg with an average δ 26Mg value near −1.40‰, and no other processes cause fractionation. Calcite precipitation is unimportant either because calcite is not a significant sink for Mg or because Mg uptake during calcite precipitation under conditions of chemical equilibrium does not fractionate Mg isotopes. This study suggests Mg isotopes undergo conservative transport in carbonate-rich settings where waters are in chemical equilibrium with respect to major sources and sinks of Mg. [Copyright &y& Elsevier]

Carbonate weathering regulates the short-term carbon (C) cycle and global climate due to its fast response to hydrological processes. The carbonate weathering flux needs to be well constrained to better understand the climate change at short time scale. Riverine magnesium (Mg) isotopes are sensitive to primary mineral dissolution and so have great potential to trace carbonate weathering. Global large rivers draining continental crust dominate weathering flux to the oceans, but how riverine Mg isotopes respond to carbonate weathering remains unclear. The Yangtze River drainage basin (YRDB) was selected to test the robustness of riverine Mg isotopes (δ26Mg) in tracing continental carbonate weathering because it spans a wide range in lithology, geomorphology and climate. The riverine δ26Mg values within the YRDB show a decreasing trend from the headwater to the mainstream ranging from −1.36‰ to −0.59‰. The dissolved δ26Mg have strong negative correlations with carbonate weathering rate and intensity within the YRDB, indicating a sensitive response of riverine δ26Mg to the carbonate weathering flux. In a compilation of Mg fluxes and δ26Mg in the world's largest rivers, there is similar dominance of carbonate weathering on riverine Mg fluxes and isotopes. Therefore, we propose that riverine δ26Mg in large rivers are a robust tracer of carbonate weathering intensity. Intensifying carbonate weathering under global warming tends to increase riverine Mg and C fluxes to the oceans and thus the atmospheric CO 2 sink at the millennial time scale. • The first systematic riverine Mg and Sr dataset within the Yangtze River drainage basin was provided. • Carbonate weathering dominates Mg isotopes in global large rivers, with limited fractionation. • Riverine Mg isotopes of global large rivers can trace carbonate weathering intensity at the continental scale. [ABSTRACT FROM AUTHOR]

This paper integrates Mg-, C-, O-, Sr-isotopes, fluid-inclusions microthermometry, and radiometric dating of dolostones and vug-filling saddle dolomite and calcite of the Upper Cretaceous Bekhme Formation, the Kurdistan Region of Iraq, provide new insights into the origin, timing and geochemical evolution of hydrothermal dolomitizing fluids. Similar isotopic compositions and fluid-inclusion microthermometry of the dolostones and saddle dolomite suggest formation by similar fluids. The U–Pb ages of dolostones and saddle dolomite suggest that dolomitization (74.8 Ma) was accomplished in 300 Ka at near Earth's surface by flow of hydrothermal fluids followed by dolostone dissolution and precipitation of vug-filling saddle dolomite (73.8 Ma). Mg isotopic compositions of the dolomites (26Mg DSM3= −0.81‰ to −0.76‰) are interpreted to indicate derivation of Mg from marine connate waters entrapped in overpressured basinal mudstones. Relatively high Sr-isotope ratios of the dolomites precludes the role of ophiolites as an important source of Mg in the basin. • Similar ranges of isotopes and microthermometry values of the dolostones and saddle dolomite suggest formation by similar fluids. • Dolomitization was accomplished in 300 Ka at near earth's surface by flow of hydrothermal fluids. • The source of Mg is interpreted to be basinal marine connate waters. [ABSTRACT FROM AUTHOR]

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing δ13C values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, δ26Mg values decrease from periphery to inner portion in one tubular carbonate (−2.63‰ to −3.24‰), while δ26Mg values are close to constant for the second tubular carbonate (−3.42‰ to −3.37‰); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between δ26Mg and δ13C values and a negative correlation between δ26Mg and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling. [ABSTRACT FROM AUTHOR]