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The snowball Earth hypothesis predicts that continental chemical weathering diminished substantially during, but rebounded strongly after, the Marinoan ice age some 635 Mya. Defrosting the planet would result in a plume of fresh glacial meltwater with a different chemical composition from underlying hypersaline seawater, generating both vertical and lateral salinity gradients. Here, we test the plumeworld hypothesis using lithium isotope compositions in the Ediacaran Doushantuo cap dolostone that accumulated in the aftermath of the Marinoan snowball Earth along a proximal-distal (nearshore-offshore) transect in South China. Our data show an overall decreasing δ7Li trend with distance from the shoreline, consistent with the variable mixing of a meltwater plume with high δ7Li (due to incongruent silicate weathering on the continent) and hypersaline seawater with low δ7Li (due to synglacial distillation). The evolution of low δ7Li of synglacial seawater, as opposed to the modern oceans with high δ7Li, was likely driven by weak continental chemical weathering coupled with strong reverse weathering on the seafloor underneath silica-rich oceans. The spatial pattern of δ7Li is also consistent with the development and then collapse of the meltwater plume that occurred at the time scale of cap dolostone accumulation. Therefore, the δ7Li data are consistent with the plumeworld hypothesis, considerably reduced chemical weathering on the continent during the Marinoan snowball Earth, and enhanced reverse weathering on the seafloor of Precambrian oceans. [ABSTRACT FROM AUTHOR]

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Elevated temperatures persisted for an anomalously protracted interval following pulsed volcanic carbon release associated with the end-Permian mass extinction, deviating from the expected timescale of climate recovery following a carbon injection event. Here, we present evidence for enhanced reverse weathering--a CO2 source--following the end-Permian mass extinction based on the lithium isotopic composition of marine shales and cherts. We find that the average lithium isotopic composition of Lower Triassic marine shales is significantly elevated relative to that of all other previously measured Phanerozoic marine shales. Notably, the record generated here conflicts with carbonate-based interpretations of the lithium isotopic composition of Early Triassic seawater, forcing a re-evaluation of the existing framework used to interpret lithium isotopes in sedimentary archives. Using a stochastic forward lithium cycle model, we demonstrate that elevated reverse weathering is required to reproduce the lithium isotopic values and trends observed in Lower Triassic marine shales and cherts. Collectively, this work provides direct geochemical evidence for enhanced reverse weathering in the aftermath of Earth's most severe mass extinction. [ABSTRACT FROM AUTHOR]

Better understanding the conditions of formation of authigenic Mg‐silicates and their reactivity is key to interpret the palaeoenvironmental message carried by the sedimentary record and evaluate the effect of reverse weathering, a process involved in long‐term climate evolution. Microbialites from most alkaline crater lakes in Mexico contain Mg‐silicates except those in Lake Alchichica, where concentration of orthosilicic acid is low (<26 μm). This study investigated the first metre of sediments in Lake Alchichica in order to check how their mineralogy compared with that of shoreline microbialites. The mineralogy and chemistry of the sediment column were determined, together with the pore water chemistry, providing insights on the processes occurring during early diagenesis. Below ca 3 cm in depth, diatom frustules are progressively pseudomorphized into Al‐poor Mg‐silicates with a composition corresponding to stevensite. This diagenetic process is massive and the resulting silicate represents between 30 and 53 wt.% of the sediment content at all depths. This observation questions the possibility to infer lake palaeochemistry from the presence/absence of Mg‐silicates in the sedimentary record. Moreover, it allowed refinement of the conditions under which Mg‐silicates authigenesis occurs: the saturation of the solution should be higher or equal to the solubility of a Mg‐silicate phase close to that of 'amorphous sepiolite'. Although the solubility of authigenic silicates is a key parameter of reverse weathering modelling during geological times, it is still debated. In this study, a solubility constant deduced from a natural system is proposed that should be considered when modelling the formation of Mg‐silicates in a natural environment. The proportion of reverse weathering associated with this solubility constant could be higher than previously predicted based on experiments and thus have a greater impact on climate stability over geological timescales. [ABSTRACT FROM AUTHOR]

Present estimates of the biogeochemical cycles of calcium, strontium, and potassium in the ocean reveal large imbalances between known input and output fluxes. Using pore fluid, incubation, and solid sediment data from North Pacific multi‐corer cores we show that, contrary to the common paradigm, the top centimeters of abyssal sediments can be an active site of authigenic precipitation of clay minerals. In this region, clay authigenesis is the dominant sink for potassium and strontium and consumes nearly all calcium released from benthic dissolution of calcium carbonates. These observations support the idea that clay authigenesis occurring over broad regions of the world ocean may be a major buffer for ocean chemistry on the time scale of the ocean overturning circulation, and key to the long‐term stability of Earth's climate. Key Points: North Pacific red clay sediments are a sink for marine calcium, strontium, and potassiumAuthigenic formation of clay minerals is prevalent in pelagic sediments throughout the North PacificThe main mechanism for clay formation is recrystallization of aluminosilicates, neoformation can occur in biogenic silica rich sediments [ABSTRACT FROM AUTHOR]

The terminal Ediacaran (∼551–541 Ma) was a critical period in Earth's history witnessing the coupled evolution of tectonism, climate, marine environment, and biodiversity, during which a global pronounced positive carbon isotope excursion (i.e., EP3) was recorded in marine carbonates. However, there is still a lack of systematic research on its triggering mechanism. Here, we present high-resolution C–Sr–Li isotopic records from a forced regressive to transgressive microbialite-dominated dolostone succession in the Xigou section of Aksu, northwestern Tarim Basin, aiming to reflect coeval hydrogenous (i.e., seawater-sourced) signals. The forced regressive succession shows distinct positive δ13C carb , δ13C org , and 87Sr/86Sr carb excursions, and negative δ7Li carb excursions, reflecting increased terrestrial flux into the ocean and enhanced chemical weathering, which caused an increase in primary productivity and 12C-enriched organic carbon burial, as well as incremental inorganic carbon isotope of shallow seawater. The intense continental weathering in this succession is mainly attributed to sea-level fall under the combined control of the Gondwana assemblage and the Luoquan glaciation. The overlying transgressive succession presents relatively stable high δ13C carb , low 87Sr/86Sr carb , and positive δ7Li carb excursions, implying the weakened continental weathering, which is probably due to sea-level rise as Luoquan glaciers began to partially melt, and the dominance of reverse weathering. During the early stage of transgression, upwelling transported deep marine nutrients to the photic zone, allowing the high primary productivity of shallow water to sustain for a long time, resulting in the continuous burial of organic carbon with the mass formation of clay minerals and stable high δ13C carb values. Subsequently, as reverse weathering continues, primary productivity decreases, shallow marine oxygen levels decline, and inorganic carbon isotopes gradually fall. This study demonstrates the great potential of Precambrian dolostones as archives of coeval seawater Li isotope, and provides an insight into the co-evolution of carbon cycle, marine redox, and Ediacaran biota from Shuram excursion to EP3 event. • Li and Sr isotopes indicate the transition from chemical to reverse weathering. • Weathering transition was mainly controlled by the Luoquan glacio-eustatic change. • Two weathering processes triggered the positive CIE through increasing OM burial. • EP3 triggered the evolution of Ediacaran biota from White Sea to Nama assemblage. • Precambrian dolostones are potential archives of coeval seawater Li isotope. [ABSTRACT FROM AUTHOR]

Sedimentary records suggest that the mid-Proterozoic (ca. 1.8–0.8 Ga) was persistently characterized by a greenhouse climate despite significantly lower solar luminosity compared to modern levels. To maintain greenhouse conditions, the partial pressure of carbon dioxide (pCO2) must have remained elevated, possibly indicative of key differences in the complexities of the carbon cycle compared to the modern. Modeling has suggested that high pCO2 was likely maintained by elevated rates of "reverse weathering:" marine authigenic clay formation, a process that consumes alkalinity and generates CO2. This process is kinetically slow in modern marine environments, yet is hypothesized to have been enhanced during the mid-Proterozoic due to the greater availability of important species for clay authigenesis such as silica and ferrous iron. This hypothesis is testable using the geological record, because enhanced reverse weathering would lead to the formation of abundant marine authigenic clays. However, the distribution of marine authigenic clays in the Proterozoic sedimentary record has not been paid sufficient attention. In this study, we report the presence of authigenic clays (glauconite and berthierine) from the Xiamaling Formation (ca. 1.4 Ga), North China. The glauconite-berthierine horizons occur as millimeter- to centimeter-thick laminae interbedded with muddy siltstone and feature detrital grains supported by the clay matrix. In places, these layers were partially reworked to form soft and cohesive intraclastic sands, suggesting a syndepositional origin. We hypothesize that marine iron cycling in the iron- and silica-rich mid-Proterozoic oceans may have facilitated the formation of authigenic iron-rich clay during the deposition of the Xiamaling Formation. The formation of iron-hydroxides on the seafloor—and the local increase in pH caused by subsequent dissimilatory iron reduction—could have resulted in the absorption of SiO2, Al(OH)3, and Fe(OH)2 to form soft, cohesive and noncrystalline Fe(OH)3-SiO2-Al(OH)3-Fe(OH)2 gels. These gels would have subsequently converted to glauconite/berthierine through aging. The transformation from glauconite-rich layers to berthierine-rich laminae was likely facilitated by a greater availability of Fe(II), and therefore higher Fe(II)/TFe and Fe/ Si ratios. We suggest that the relatively rapid formation of syndepositional, seafloor berthierine and glauconite layers in the basal Xiamaling Formation is the result of enhanced reverse weathering during this time. This study provides an important geological support for carbon cycle models that invokes enhanced reverse weathering rates in the mid-Proterozoic ocean that may have helped to maintain a high-baseline pCO2 during this time. [ABSTRACT FROM AUTHOR]

In the aftermath of Marinoan global glaciation (∼650–635 Ma), the deglacial intense continental weathering tended to dramatically lower the atmospheric pCO 2 level, potentially driving the Earth back to the glacial climatic condition. However, the resultant global cooling and glaciation did not occur. The CO 2 drawdown by continental weathering might have been compensated by additional CO 2 release via reverse weathering in the ocean, as evidenced by abundant precipitations of authigenic clay minerals, for example, Mg-rich saponite and clinochlore in the lower part of Doushantuo Formation (635–551 Ma) in South China. Massive precipitations of Mg-rich authigenic clays imply a distinct marine Mg cycle in the early Ediacaran ocean, but their origins have been debated. Here, we measured the Mg isotopic compositions of authigenic clays from the Doushantuo Formation. Both saponite and clinochlore are extremely 26Mg-enriched, with Mg isotopes of saponite (δ26Mg sap) up to +0.39‰ and clinochlore up to +1.26‰. Considering the Mg isotopic fractionation in saponite precipitation ranging from −0.95‰ to −1.45‰, high δ26Mg sap values requires extremely high seawater Mg isotopic composition (δ26Mg sw), which cannot be resolved by direct precipitation with seawater Mg supply. Instead, it requires other diagenetic processes that elevated the porewater Mg isotopic composition (δ26Mg pw). A numerical model was applied to quantify the Mg isotopic fractionation in diagenesis. The modeling result indicates an earlier dolomitization relative to saponite precipitation might have elevated the porewater Mg isotopes, followed by the precipitation of extremely 26Mg-enriched saponite. The coupled carbonate-silicate diagenesis is a key mechanism sustaining stable seawater Mg concentration and δ26Mg, highlighting authigenic clay mineral as a buffer for Mg and C cycles counterbalancing massive weathering input after Marinoan glaciation. • Authigenic clay mineral with extremely high δ26Mg value in Doushantuo formation • Dolomitization increase Mg isotope difference between authigenic mineral and seawater. • Coupled carbonate-silicate diagenesis is critical buffer for seawater δ26Mg. [ABSTRACT FROM AUTHOR]

In this study, to better understand the factors controlling the concentration and isotope composition of lithium (Li) in the ocean, we investigated the behaviour of Li during interaction of kaolinite with artificial seawater. Dissolution of kaolinite in Li-free seawater at acidic conditions (exp. 1) results in a strong preferential release of light Li isotopes, with △7Li aq-kaol ∼ −19‰, likely reflecting both the preferential breaking of 6Li O bonds over 7Li O bonds and the release of Li from the isotopically lighter AlO 6 octahedral sites. Sorption experiments on kaolinite (exp. 2) revealed a partition coefficient between kaolinite and fluid of up to 28, and an isotopic fractionation of −24‰. Thermodynamic calculation indicates authigenic smectites formed from the dissolution of kaolinite in seawater at pH 8.4 (exp. 3). The formation of authigenic phase strongly removed Li from the solution (with a partition coefficient between the solid and the fluid equal to 89) and led to an increase of ca. 25‰ in seawater δ7Li. This fractionation can be described by a Rayleigh fractionation model at the early stage of the experiment during rapid clay precipitation, followed, at longer reaction time, by equilibrium isotope fractionation during the much slower removal of aqueous Li via co-precipitation and adsorption. Both processes are consistent with a fractionation factor between the solid and the aqueous solution of ∼−20‰. These experiments have implications for interpreting the Li isotopic composition of both continental and marine waters. For instance, the preferential release of 6Li observed during kaolinite far-from-equilibrium dissolution could explain the transient enrichments in 6Li observed in soil profiles. With regard to the evolution of seawater δ7Li over geological time scales, our experimental results suggest that detrital material discharged by rivers to the ocean and ensuing "reverse chemical weathering" have the potential to strongly impact the isotopic signature of the ocean through the neoformation of clay minerals. [ABSTRACT FROM AUTHOR]

Abstract As reverse weathering has been shown to impact long‐term changes in atmospheric CO2 levels, it is crucial to develop quantitative tools to reconstruct marine authigenic clay formation. We explored the potential of the beryllium (Be) isotope ratio (10Be/9Be) recorded in marine clay‐sized sediment to track neoformation of authigenic clays. The power of such proxy relies on the orders‐of‐magnitude difference in 10Be/9Be ratios between continental Be and Be dissolved in seawater. On marine sediments collected along a Chilean margin transect we chemically extracted reactive phases and separated the clay‐sized fraction to compare the riverine and marine 10Be/9Be ratio of this fraction. 10Be/9Be ratios increase fourfold from riverine to marine sediment. We attribute this increase to the incorporation of Be high in 10Be/9Be from dissolved biogenic opal, which also serves as a Si‐source for the precipitation of marine authigenic clays. 10Be/9Be ratios thus sensitively track reverse‐weathering reactions forming marine authigenic clays.

As reverse weathering has been shown to impact long‐term changes in atmospheric CO2 levels, it is crucial to develop quantitative tools to reconstruct marine authigenic clay formation. We explored the potential of the beryllium (Be) isotope ratio (10Be/9Be) recorded in marine clay‐sized sediment to track neoformation of authigenic clays. The power of such proxy relies on the orders‐of‐magnitude difference in 10Be/9Be ratios between continental Be and Be dissolved in seawater. On marine sediments collected along a Chilean margin transect we chemically extracted reactive phases and separated the clay‐sized fraction to compare the riverine and marine 10Be/9Be ratio of this fraction. 10Be/9Be ratios increase fourfold from riverine to marine sediment. We attribute this increase to the incorporation of Be high in 10Be/9Be from dissolved biogenic opal, which also serves as a Si‐source for the precipitation of marine authigenic clays. 10Be/9Be ratios thus sensitively track reverse‐weathering reactions forming marine authigenic clays. Plain Language Summary: Clay minerals can form on land by the chemical breakdown of rock‐forming minerals, but clays can also form in the ocean. When clay formation takes place in the ocean, CO2 is released. To date, there is no method that can easily measure the amount of clay minerals formed in the ocean. We used two isotopes of the same element, beryllium (Be), with the atomic mass of 9 and 10 to test whether this isotope system can be used to measure marine clay formation. The abundance of these isotopes differs majorly on land and in the ocean. We measured beryllium isotopes in river sediment and ocean‐bottom sediment offshore the Chile coast and compared the ratios of the isotopes (10Be/9Be). The ratio is four times higher in ocean sediment, when compared to river sediment. We interpret this increase to be due to the formation of clay minerals in the ocean, which include the high 10Be/9Be ratio during their formation. We conclude that the beryllium‐isotope system can be used to measure the formation of even very small amounts (less than 2%) of marine clay minerals. This is important, as the clay‐forming chemical reactions release CO2 which has a long‐term effect on global climate. Key Points: We explored the potential of the beryllium isotope ratio to track neoformation of marine authigenic claysBeryllium isotope ratios increase fourfold from riverine to marine sediment due to the presence of marine Be incorporated in authigenic clayBeryllium isotope ratios sensitively track reverse‐weathering reactions forming marine authigenic clays [ABSTRACT FROM AUTHOR]

Abstract The geological cycles of carbon and silicon are linked via the silicate weathering feedback, but growing understanding of reverse weathering—the formation of authigenic clay minerals via back reaction of silica with cations—indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed. Recent carbon cycle modeling has suggested that enhanced reverse weathering during Precambrian time could have played an important role in regulating climate. The topology of the Precambrian silica cycle, including the size of the authigenic clay sink and its influence on the carbon cycle, is challenging to reconstruct due to inherent preservation biases in the rock record. Here we used an alternative approach using Si isotope values to invert for the magnitudes of Precambrian silica fluxes, by assuming that the silica cycle must be in mass balance with the bulk silicate Earth over geological time scales with respect to Si isotopes. To estimate uncertainties associated with these flux reconstructions, we used Monte Carlo simulations based on the natural variation exhibited in Si isotope ratio data from the Precambrian sedimentary record (n = 2118, spanning ~3.8 Ga to ~525 Ma) and modern authigenic clays (n = 123). The perspective of Si isotope mass balance shows that the previously observed increase in chert δ30Si values across Precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature. Results yielded a secular increase in the relative contribution of the authigenic clay sink across most of Precambrian time—this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater. This result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust. This approach also highlights key areas where improving Si isotope records could further enhance our constraints on the Precambrian silica cycle, and its relationship with the carbon cycle and climate stability. [ABSTRACT FROM AUTHOR]

Continental-margin sediments account for ~ 50% of the oceanic biogenic silica burial despite covering < 10% of its area. In Mississippi Sound, a coastal lagoon in the northern Gulf of Mexico (nGoM), we measured sediment biogenic silica at sites removed from major freshwater discharge sources using the traditional method and a method that has been modified for deltaic systems to quantify other reactive silica pools, specifically those involved in the process of reverse weathering. The magnitude of authigenically-altered biogenic silica during our study was significant and represented, on average, 33% of the total sediment biogenic silica among core depths and sites. Additionally, there was a significant relationship between the degree to which the biogenic silica pool was authigenically altered and the source of the sediment organic matter, with lower modification in sediments corresponding with higher terrestrial organic matter. We observed no positive correlation between the magnitude of authigenic modification and sediment clay content. Thus, our findings suggest that these processes may occur within a variety of sediment compositions and add to a growing body of evidence suggesting that reverse weathering of silica in coastal systems is a significant pathway in the global silica budget. [ABSTRACT FROM AUTHOR]

Burial of biogenic silica (bSitotal) in high sedimentation rate continental margins remains highly uncertain. Cosmogenic 32Si (t1/2~140 years) can be used to trace the fates of bSitotal postdeposition, including as opal (bSiopal) and diagenetically altered opal (bSialtered), the latter dominantly authigenic clay (bSiclay). To determine the magnitude and form of bSitotal storage in coastal sediments, conventional operational leaches targeting bSiopal and bSialtered (including bSiclay) were modified for large-scale samples necessary for measurement of 32Si. The 32Si activity was used to estimate total biogenic silica burial (bSitotal = bSiopal + bSialtered) in several depositional settings: Gulf of Papua, Gulf of Mexico, Long Island Sound, and in the previously studied Amazon-Guianas deltaic system. In subtropical and temperate regions, 32Si was detected in both traditional biogenic silica leaches (bSiopal) and residual authigenic clays. Traditional bSiopal and modified operational leaches designed to target the most reactive authigenic silicates (~bSialtered) consistently underestimate authigenic clay formation (bSiclay) and thus the magnitude of bSitotal burial in temperate coastal zones and subtropical deltas by 2-4-fold. In tropical deltas, 32Si activities in the residual fraction after removal of bSiopal demonstrate rapid and almost complete alteration of initial bSiopal to new forms, most likely bSiclay. Globally, 4.5-4.9 Tmol/yr Si may be trapped in marine nearshore deposits as rapidly formed clay (bSiclay), 100% of the 'missing silica sink' in the marine silica budget. [ABSTRACT FROM AUTHOR]

The Oceanic Anoxic Event 2 (OAE 2; ca. ∼94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO 2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (△ ε Hfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (∼62°S) during the late Cretaceous. The reliability of △ ε Hfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low △ ε Hfclay values (−5.9 ± 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in △ ε Hfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced △ ε Hfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest δ 13 C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO 2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments. • Combined Nd-Hf isotopes as a proxy for continental weathering during OAE 2. • Negligible influence of reverse weathering on Hf-Nd isotopes in marine clays. • Intensifying weathering and river discharge in SW Australia during peak OAE 2 warmth. • Southward shift of westerlies drove enhanced weatherability in southern high-latitude regions. [ABSTRACT FROM AUTHOR]

Dissolved Rare Earth Elements (REE) and radiogenic neodymium (Nd) isotope composition (ε Nd) of seawater are widely used geochemical tools in studying marine processes, but their modern ocean budgets are poorly understood. Recent discoveries of large benthic fluxes of REE with unique ε Nd signatures from marine sediments, particularly in the deep-sea, have led to a "bottom-up" hypothesis, which suggests that early diagenesis below the sediment-water interface (SWI) controls the ocean's REE and ε Nd budgets. To investigate such sedimentary processes, we created a reactive-transport model for the biogeochemical cycling of Nd and ε Nd in marine sediments. Here, we attempt to quantify the roles of authigenesis, marine silicate weathering and reverse weathering in the diagenetic cycling of Nd and ε Nd at a deep-sea (3000 m) site on the Oregon margin. Our model predicts that, at this site, Nd carried by Fe/Mn oxides into sediments eventually transforms to authigenic Nd-phosphate, during which ∼9% of the incoming solid Nd flux is released as a dissolved benthic flux back to the overlying bottom water. We also find that the classic reversible scavenging formulation applied to Nd co-cycling with Fe/Mn oxides is inconsistent with the data. Rather, a co-precipitation formulation, assuming Nd is structurally incorporated into Fe/Mn oxides, successfully simulates the data. The model also shows that authigenesis alone cannot explain the pore water and authigenic ε Nd , which are both more radiogenic than bottom water at this site. However, the weathering of volcanic silicates sourced from the local subduction zone can successfully explain ε Nd. We suggest that, because reverse weathering by authigenic clay formation maintains the under-saturation of primary silicates in pore water, marine silicate weathering can proceed. The processes we model likely affect the sedimentary cycling of many other trace elements and isotopes, with much broader implications for the understanding of ocean biogeochemistry. • We present a reactive-transport model of the early diagenesis of Nd and ε Nd. • Pore water Nd concentration is explained by Fe/Mn cycling and authigenic phosphate. • Reversible scavenging cannot explain pore water Nd concentration profile. • Weathering of volcanic silicates explains radiogenic pore water/authigenic ε Nd. • Reverse weathering is a driving force for marine silicate weathering. [ABSTRACT FROM AUTHOR]

Abstract: Reactors containing frustules of the cultured diatom Thalassiosira punctigera suspended in seawater were incubated with or without added sediment from the Mississippi River Delta or the Congo River Fan. The diatom frustules were separated from the terrigenous sediments by a dialysis membrane, thereby only allowing the exchange of dissolved species. One series of incubations was carried out in the laboratory, at room temperature (21°C) and for a period of 10months. Another series of reactors was deployed along a mooring in the Mozambique Channel at three water depths (500, 1250, and 2000m), for a period of 22months. Chemical analyses after total destruction of frustules collected at the end of the incubations showed elemental transfer from seawater (Mg and K) and the sediments (Al, Fe, Mn, P and Ca) to the frustules. In the presence of the terrigenous sediments, the dissolved silicate concentrations at the end of the incubations were systematically lower that those measured in the incubations without the sediments. In addition, electron microscopy revealed the formation of new mineral precipitates. These included amorphous deposits on the frustules containing Si, Fe, Al, Mg, K and P, as well as euhedral clay crystallites. Differences were observed between the incubations performed in the laboratory and those deployed at sea, likely as a result of differences in redox conditions, temperature and reaction time. Overall, the interactions between biogenic silica, seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by siliceous organisms. These interactions take place on relatively short time scales (months to years), and affect not only the marine cycle of silicon, but also those of other major and minor elements, such as Al, Fe, Mn, K, and Mg. [Copyright &y& Elsevier]

Abstract: Previous studies have shown how biogenic silica particles undergo conversion to aluminosilicate phases in large tropical deltaic systems, thus affecting the world ocean budget of major seawater cations. This study tackles the important question of the silica budget in the coastal zone of the Mississippi River Delta, providing evidence for the role of biogenic silica diagenesis in this subtropical system from direct examination of individual diatom particles, sediment leachates and pore-water composition. The estimated reactive silica stored in the study area (5990km2) is based on operational leaches that account for altered biogenic silica particles and other authigenic aluminosilicate phases in addition to fresh biogenic silica. Early diagenesis of silica in the delta front occurs mainly where more siliceous material is deposited. An inner-shelf area, where hypoxic conditions are found, significantly contributes to the formation of authigenic products of Si alteration. Data suggest that the limiting factor of silica alteration processes is the availability of detrital phases such as Al and Fe. The estimated total reactive silica accumulation in the study area is 1.45×1010 molSiyear−1, representing ∼2.2% of the long-term bulk sediment accumulation. On the basis of a conservative appraisal, the authigenic mineral components account for ∼40% of the long-term reactive silica storage. This study shows that non-tropical deltaic systems are significantly more important sinks of silica than previously thought and that, where conditions are favourable, a consistent portion of reactive silica not leaving the shelf is stored within the delta in the form of authigenic components. [Copyright &y& Elsevier]

Solid-phase chemistry of modern carbonate–siliciclastic marine sediments from the San Blas Archipelago, Panama, reveals the occurrence of an Fe–Mg authigenic clay facies that is dominated by a ∼7.1-A˚ ferric clay mineral. These Fe-rich clays are highly susceptible to HCl-attack and can comprise a large fraction (up to 5.4 wt.% Fe) of the sediment. Some of the clays are clearly forming in the marine environment as they coat grains, infill carbonate micropores, and replace faecal pellets and carbonate shells. The mode of occurrence, structure, and ferric nature of the clay assemblage identifies the facies as verdine, which has odinite, a dioctohedral–trioctohedral Fe3+-rich 1:1 clay mineral as its key member.The verdine facies in San Blas occurs in suboxic sediments where iron reduction can be a prevalent organic carbon oxidation pathway. Sediment radiocarbon ages show that the San Blas verdine clays formed within the last few thousand years, and in some cases, the clays have been extensively converted into pyrite. The verdine clays most likely grew by incorporating terrestrially derived Fe and Al, seawater Mg, and pore water Si derived from biogenic opal dissolution. Similar clay formation processes may be common in other tropical nearshore environments where sediments are derived from highly weathered lateritic soils and may prove to be an important oceanic elemental sink. [Copyright &y& Elsevier]

• We investigate authigenic processes using rocks from a palaeo-delta formation. • Authigenic clay formation in deltas fractionate sediment Li and Si isotopes. • Deltaic processes result in a coupling between the Li and Fe cycle. • Deltaic processes are part of the "reverse weathering" reactions. • Li isotopes can be a tracer to distinguish the marine and continental facies. Marine authigenic clay formation has long been postulated as a major process to explain the mass budgets of some elements in seawater, and might act a reverse reaction for the neutralization of atmospheric acidity by soil forming reactions on land. Nevertheless, to date, a handful of studies have directly investigated the effect of reverse weathering due to the challenges associated with sampling complexity. Deltas are thought to be one of the possible environments where reverse weathering reactions may occur because of the abundant influx of weathering derived materials and dynamic activities. In this study, we use a unique combination of three isotope systems (Si, Li, and Nd) as evidence of authigenic clay formation using sediments collected from a ∼40 Ma-old delta complex (Ainsa Basin, Spain). Sediments were collected along the land to sea depositional continuum, from alluvial and coastal plains to marine environments. Direct comparison between alluvial, coastal, and marine sediments allows for observing the potential effect of reverse weathering. Systematic differences in Si and Li isotope composition exist between marine and continental sediments, revealing the formation of an authigenic phase sequestrating light Li and Si as associated with the diagenesis of Fe in reducing deltaic environments. Finally, this study proposes a geochemical tool for helping distinguishing marine and continental origins of sedimentary rocks when lithofacies and biofacies prove to be ineffective. [ABSTRACT FROM AUTHOR]

• Both mineralogical and geochemical analysis suggest a shift towards more extensive reverse weathering within the uppermost portion of the glaciogenic Nantuo Formation. • This reverse weathering was likely driven by a combination of rising temperatures and pH and high dissolved silica concentrations. • The carbon cycle modeling implies that widespread reverse weathering could have driven a protracted (millions of years) carbon dioxide drawdown following deglaciation. Snowball Earth glaciations are the most extreme climate perturbations recorded in Earth's history. It has been argued that the termination of these events was characterized by a single rapid transition from near-global ice coverage to an ice-free greenhouse climate state. Notably, this deviates with more extended transition periods of ice sheet waxing and waning typical of Phanerozoic glaciations. Using a coupled mineralogical and Mg and Li isotopic approach, we explore the role that authigenic clay formation within the seafloor may have played on Earth's climate during deglaciation of the Marinoan Snowball Earth event. Marine authigenic clay formation—a process referred to as reverse weathering—recycles carbon within the ocean–atmosphere system and acts to elevate atmospheric CO 2 levels. The results indicate a shift towards more extensive reverse weathering within the uppermost portion of the glaciogenic Nantuo Formation in South China. Carbon cycle modeling indicates that widespread reverse weathering could have driven a protracted (millions of years) carbon dioxide drawdown following high carbon dioxide levels expected during deglaciation. [ABSTRACT FROM AUTHOR]

• A biogeochemical baseline of sediment geochemistry at potential offshore CCS sites. • Diagnostic indicators of CO 2 leakage based on stoichiometry of porewater chemistry. • Porewater chemistry is modified by reverse weathering processes at Goldeneye site. Injection of carbon dioxide (CO 2) into subseafloor reservoirs is gaining traction as a strategy for mitigating anthropogenic CO 2 emissions to the atmosphere. Yet, potential leakage, migration and dissolution of externally-supplied CO 2 from such reservoirs are a cause for concern. The potential impact of CO 2 leakage on the biogeochemistry of sediments and overlying waters in the North Sea was studied during a controlled subsurface CO 2 release experiment in 2019 at a potential carbon capture and storage site (Goldeneye). This study describes the natural (unperturbed) biogeochemistry of sediments. They are classified as muddy sand to sandy mud with low organic carbon content (∼0.6 %). Distributions of dissolved inorganic carbon (DIC) and total alkalinity (TA) in sediment porewaters are reported in addition to in situ benthic fluxes of dissolved nutrients and oxygen between the sediments and the overlying water. Oxygen fluxes into the sediment, measured using benthic chambers and eddy covariance, were 6.18 ± 0.58 and 5.73 ± 2.03 mmol m−2 d-1, respectively. Diagnostic indicators are discussed that could be used to detect CO 2 enrichment of sediments due to reservoir leakage at CCS sites. These include the ratio TA and ammonium to sulfate in sediment porewaters, benthic fluxes and chloride-normalized cation distributions. These indicators currently suggest that the organic carbon at Goldeneye has an oxidation state below zero and is mainly degraded via sulfate reduction. Carbonate precipitation is apparently negligible, whereas decreases in Mg2+ and K+ point toward ongoing alteration of lithogenic sediments by reverse weathering processes. [ABSTRACT FROM AUTHOR]

In this study, we identify and quantify processes that lead to sedimentary potassium (K) sequestration and phosphorus (P) release on the Amazon shelf. To this end, seven short sediment cores were recovered from the Amazon shelf during R/V Meteor cruise M147. All of the sediment cores investigated in this study are characterized by elevated K to aluminum (Al) ratios compared to Amazon riverine suspended matter, which indicates that seawater K+ is incorporated into the solid phase on the entire Amazon shelf. Pore water silica (Si) profiles are characterized by irregularly increasing concentrations and plateaus, thus, deviating from the asymptotic shape that is typically found in continental margin sediments. At one site, a dissolved Si plateau coincides with a K+ minimum suggesting that these solutes are incorporated into authigenic minerals, a process referred to as reverse weathering. Previous flux estimates for elements that participate in reverse weathering on the Amazon shelf were derived from pore water diffusive fluxes, reaction rates estimated from sediment incubations and solid phase extractions. In this study, we took an alternative approach, which is based on the concentration difference between shelf sediments and river suspended particles. The resulting K flux due to reverse weathering of 1.7 ∙ 1011 mol yr−1 is in agreement with previous estimates and corresponds to 13% of the global riverine dissolved K+ input. Previous studies demonstrated that Amazon riverine particulate P is partly solubilized on the Amazon shelf. However, these results are exclusively based on sediment data close to the river mouth and no distinction between terrestrial and marine sediment components was made. Here, we quantify P release from Amazon shelf sediments by comparing terrestrial P concentrations in shelf sediments with P concentrations in river suspended particles. The resulting solubilized P flux of 2.2 ∙ 1010 mol yr−1 is about five to six times higher than previous estimates and about seven times the Amazon riverine dissolved P discharge. The magnitudes of the presented fluxes imply that the alteration of riverine shelf sediments significantly affects the mean concentrations of dissolved K+ and P in the global ocean. • Amazon shelf sediments take up seawater potassium (K) due to reverse weathering. • Amazon shelf sediments release terrigenous phosphorus (P) during resuspension. • Updated estimates of sedimentary K uptake and P release on Amazon shelf are presented. • Sedimentary K uptake on Amazon shelf corresponds to 13% of global riverine K input. • Sedimentary P release is ~5 times higher than dissolved P discharge of Amazon River. [ABSTRACT FROM AUTHOR]

• Higher dissolved silica in Precambrian ocean may have increased reverse weathering. • Elevated reverse weathering in the Proterozoic could provide ∼5 K climate warming. • Large uncertainties in model parameters mean negligible warming is also possible. • Constraints on parameters or new geological proxies are required for progress. The balance between carbon outgassing and carbon burial controls Earth's climate on geological timescales. Carbon removal in carbonates consumes both atmospheric carbon and ocean carbonate alkalinity sourced from silicate weathering on the land or seafloor. Reverse weathering (RW) refers to clay-forming reactions that consume alkalinity but not carbon. If the cations (of alkalinity) end up in clay minerals rather than in the carbonates, carbon remains as atmospheric CO 2 , warming the climate. Higher silicate weathering fluxes and warmer temperatures are then required to balance the carbon cycle. It has been proposed that high dissolved silica concentrations resulting from the absence of ecologically significant biogenic silica precipitation in the Precambrian drove larger RW fluxes than today, affecting the climate. Here, we present the first fully coupled carbon-silica cycle model for post-Hadean Earth history that models climate evolution self-consistently (available as open source code). RW fluxes and biogenic silica deposition fluxes are represented using a sediment diagenesis model that can reproduce modern conditions. We show that a broad range of climate evolutions are possible but most plausible scenarios produce Proterozoic warming (+5 K relative to without RW), which could help explain the sustained warmth of the Proterozoic despite lower insolation. RW in the Archean is potentially more muted due to a lower land fraction and sedimentation rate. Key model uncertainties are the modern reverse weathering flux, the rate coefficient for RW reactions, and the solubility of authigenic clays. Consequently, within the large uncertainties, other self-consistent scenarios where Proterozoic RW was unimportant cannot be excluded. Progress requires better constraints on parameters governing RW reaction rates including explicit consideration of cation-limitations to Precambrian RW, and perhaps new inferences from Si or Li isotopes systems. [ABSTRACT FROM AUTHOR]