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Stratigraphic ages from conventional glauconite geochronology are commonly younger than those obtained via high temperature chronometers. The widely used glauconite reference material GL‐O, for example, has a K‐Ar age (95.03 ± 1.11 Ma) ~ 5 Ma younger than its expected stratigraphic age. To identify the influences on glauconite ages and assess the suitability of GL‐O as a reference material for in situ Rb‐Sr geochronology, we separated GL‐O grains based on colour and morphology. Each fraction was characterised petrographically and compositionally before in situ Rb‐Sr dating. Separate aliquots were dated via conventional isotope dilution (ID) Rb‐Sr geochronology. We find a ~ 10 Ma spread in the in situ Rb‐Sr ages of GL‐O fractions, where more rapid maturation and isotopic closure of darker grains yields ages closer to the depositional age, whereas smaller, more porous light green grains show evidence for delayed maturation leading to continued Rb uptake during burial as well as Sr isotope exchange with connate fluids. Discrepancies between ID and in situ Rb‐Sr ages are explained by (i) core‐rim age zonation, (ii) the presence of alteration resistant, Sr‐rich apatite inclusions, (iii) differences in laser‐induced fractionation. We recommend additional purification steps before use of GL‐O as reference material for in situ Rb‐Sr geochronology. [ABSTRACT FROM AUTHOR]

In contrast to the prevalence of dolomite [CaMg(CO3)2] in the geological record, there are few instances of recent formation. This discrepancy occurs despite supersaturation with respect to dolomite in many modern marine and lacustrine environments. Additionally, laboratory experiments have struggled to precipitate dolomite at ambient temperatures (<40°C) even under highly saturated conditions. However, recent work has found dissolved silica to be an effective catalyst for the direct precipitation of dolomite. To test this hypothesis, the hydrology and mineralogy of Deep Springs Lake, a playa setting with primary dolomite precipitation has been explored. In this study, the central playa sites with the highest sedimentation rates were found to have dissolved silica concentrations close to saturation. Rietveld refinement showed that bulk mineral assemblages of core samples contain 20 to 40% fine‐grained, partially ordered dolomite along with a 10 to 20% poorly crystalline clay fraction, while transmission electron microscopy imaging found a co‐precipitation relationship between the fine‐grained dolomite and Mg‐rich smectite clays, where the Mg‐rich smectite clays act as a sink for the excess dissolved silica during evaporation. Transmission electron microscopy results indicate that the nanodolomite crystals formed through surface‐induced nucleation and growth processes in the presence of dissolved silica as a catalyst. The broad coincidence of conditions for the direct precipitation of dolomite and Mg‐rich smectite clays provides evidence that silica may be a key to the dolomite problem, as well as constraining palaeoenvironments such as the Pre‐salt Barra Velha Formation. [ABSTRACT FROM AUTHOR]

Clays are understood to form the majority of fluid‐flow barriers in faulted reservoirs and numerous fault gouge and fault seal studies have quantified the volumes of smeared and abraded clays create fluid‐flow barriers along fault surfaces. However, clay‐related permeability adjacent to the fault surface, including in the fault damage zone, has largely been neglected. Previous studies have shown the morphology and distribution of unfaulted authigenic clays, and not just clay volume, exert a significant control on the magnitude of permeability. However, fault‐related studies have neither characterized deformed authigenic clays nor addressed their influence on fluid‐flow. In this study laboratory permeabilities of faulted, authigenic clay bearing sandstones sampled from the Otway basin (Australia) and the Orcadian basin (UK) present trends which; (a) do not correspond to expected patterns of fluid‐flow in faulted clay‐bearing sandstones and, (b) cannot be explained using published models of permeability related to changing clay volume. Microscopic analysis shows that faulting has disaggregated authigenic clays and, similarly to framework grain deformation, comminuted and sheared clay grains. However, instead of impeding fluid‐flow, analysis of pore networks (using mercury injection porosimetry) showed that faulting of authigenic clays has increased pore connectivity, contributing to increased magnitude of permeability and development of permeability anisotropy. Contrary to published results of faulting and fluid‐flow in impure sandstones, our results show that fault related processes involving the formation of clays in the fault zone can increase permeability and reduce the capillary threshold pressures of fault rocks relative to the unfaulted host rock. Plain Language Summary: Predictions of fluid leakage in geological reservoirs are essential for energy extraction (e.g., geothermal, hydrocarbons), development and maintenance of sustainable energy resources (e.g., carbon dioxide sequestration and nuclear waste storage). Reservoir‐bounding faults commonly form fluid‐flow barriers as the process of faulting breaks grains and smears clays along fault surfaces creating a seal. Therefore, the volume of clay in faulted rocks is an important parameter in relation to fault sealing capacity. Authigenic clays are minerals that commonly grow in spaces between grains, choking fluid pathways and reducing flow. In this study, microscopic analysis of two faulted sandstones containing authigenic minerals (chlorite and kaolinite) showed that faulting also breaks authigenic clay grains. However, instead of impeding fluid‐flow, laboratory measurements of faulted sandstones showed that deformation of authigenic minerals can influence flow direction and even increase the magnitude of fluid‐flow. Consequently, rather than representing fluid barriers, some faults may actually form important fluid draining structures. These novel results prompt questions about the robustness of parameters currently used to model fluid‐flow around faults and, with global need for sustainable energy demanding that we can safely store fluids in subsurface reservoirs, should motivate geologists to take a fresh look at fault seal analysis. Key Points: Tectonically deformed authigenic clays have been identified in faulted sandstonesThe morphology, aggregate structure and distribution of authigenic clay can contribute to increased permeability in faulted sandstonesIn contrast to accepted fault seal models, this study shows that clay‐rich fault zone rocks can be the locus of better fluid‐flow [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]

• Authigenic Mg-clays are widely found in the Ediacaran Doushantuo Formation. • The Doushantuo Formation experienced an expansive restricted, evaporitic environment. • Petrography work with novel SEM-EDS technique can help to distinguish clay origins. • Clay mineralogy can be used as environment indicators in the Earth System science. The carbonate-rich shelf facies of the Ediacaran Doushantuo Formation (South China) have produced a broad array of microfossil and isotope proxy records which underpin much of our understanding of environmental change and biospheric evolution during this key time period. Recent reports of locally abundant authigenic, Mg-rich saponite clay in the Yangtze Gorges Area hint at potentially widespread evaporitic conditions in a lagoonal setting. Despite the implications for interpretation of proxy records and the environmental setting of early metazoans, the spatiotemporal extent of restricted, evaporitic conditions across the Yangtze Block remains largely unexplored. Here we use mineralogical and petrographic techniques to document the spatial and stratigraphic distribution of authigenic clay minerals across seven sites, representing a transect from shallow proximal shelf to deep basinal settings. Our results demonstrate the widespread and persistent occurrence of authigenic saponite in proximal shelf settings, whereas talc is identified at more distal shelf sites, consistent with Mg-clay authigenesis in an evaporitic lagoon with spatially variable detrital aluminosilicate input. Slope and basinal sites contain no Mg-clays and are instead characterized by abundant authigenic illite, consistent with an open marine setting. Stratigraphically, illite was found in the basal cap carbonate and uppermost Doushantuo Formation, with Mg-clays only present in the interval in-between, suggesting gradual development of an extensive carbonate rimmed lagoon over the Yangtze shelf that restricted seawater exchange, followed by stronger marine influence due to erosion of the carbonate rim and subsequent marine transgression. We conclude that restricted, evaporitic conditions were much more expansive than previously assumed, potentially providing favorable conditions for the Doushantuo Biota. Further, the widespread presence of authigenic clays across the shelf supports the case for enhanced reverse weathering before the advent of biosilicification. [ABSTRACT FROM AUTHOR]

• First paired δ7Li data of different components were reported for the cap carbonates. • Marine clay authigenesis is intense during the late neoproterozoic. • δ7Li data indicate highly congruent weathering in the Marinoan aftermath. The termination of the Cryogenian period marks a transition from an extreme ice- to a greenhouse climate, likely initiated by increased atmospheric CO 2 , however uncertainties persist on continental weathering regarding this climate transition. Here we present paired lithium isotope data from carbonate (δ7Li carb) and carbonate-hosted silicate (δ7Li sili) components of the Marinoan cap carbonates to constrain C and Si cycles in the deglacial period. We find that the silicate component of cap carbonates is dominated by marine authigenic clays with minor contributions from terrestrially detrital materials and oxides. Built from δ7Li sili data, large variations of seawater δ7Li suggest a heterogeneous marine Li reservoir due to a stratified ocean in the deglacial period. Overall, widespread clay authigenesis and low seawater δ7Li values indicate active carbon-silicon cycling and congruent silicate weathering due to warm climate and exposure of fresh rock surface in the Marinoan aftermath, which promotes the deposition of cap carbonates and stabilization of Earth's surface environment. [ABSTRACT FROM AUTHOR]

We measure the oxygen and hydrogen stable isotope composition of authigenic clays from Himalayan foreland sediments (Siwalik Group), and from present day small stream waters in eastern Bhutan to explore the impact of uplift of the Shillong Plateau on rain shadow formation over the Himalayan foothills. Stable isotope data from authigenic clay minerals (<2 μm) suggest the presence of three paleoclimatic periods during deposition of the Siwalik Group, between ∼7 and ∼1 Ma. The mean δ18O value in paleometeoric waters, which were in equilibrium with clay minerals, is ∼2.5‰ lower than in modern meteoric and stream waters at the elevation of the foreland basin. We discuss the factors that could have changed the isotopic composition of water over time and we conclude that (a) the most likely and significant cause for the increase in meteoric water δ18O values over time is the "amount effect," specifically, a decrease in mean annual precipitation. (b) The change in mean annual precipitation over the foreland basin and foothills of the Himalaya is the result of orographic effect caused by the Shillong Plateau's uplift. The critical elevation of the Shillong Plateau required to induce significant orographic precipitation was attained after ∼1.2 Ma. (c) By applying scale analysis, we estimate that the mean annual precipitation over the foreland basin of the eastern Bhutan Himalayas has decreased by a factor of 1.7–2.5 over the last 1–3 million years. Key Points: Isotopic composition of authigenic clays in sediments of the Himalayan foreland basin indicates environmental changes since Late MioceneSurface uplift of the Shillong Plateau led to the formation of a rain shadow in its lee, in the Himalayan forelandThe mean annual precipitation over the foreland basin of the eastern Bhutan Himalayas has decreased by a factor of 1.7–2.5 [ABSTRACT FROM AUTHOR]

Authigenic clay (Aclay) is common in grain-supported tight reservoirs and exerts a primary control on reservoir’s percolation properties. In this study, we investigated the impacts of Aclay on pore connectivity, pore size distribution (PSD), and rock compressibility, to the resulting impacts on storage and percolation in order to arrive at a genetic classification of tight sandstones. QEMSCAN® combined with scanning electron microscopy was employed to identify the distribution and content of Aclay in a set of the Cretaceous tight gas sandstones, China. The size distribution of clay-related pores and their contributions to storage and percolation were distinguished from the interparticle pores by combining rate-controlled mercury porosimetry (RCP) and nuclear magnetic resonance (NMR). We found that Aclay evolved from predominantly pore-lining clays to predominantly pore-filling types as its content increased. Along this transition, clay-related pores gradually play a significant role in controlling the storage and percolation of tight sandstone reservoirs. There is very little change in pore volume and a sharp decrease in percolation threshold (PT) and permeability as Aclay content increases, which is mainly attributed to the dissolution that leads to the clay precipitation. On the other hand, compaction results in the reduction in PSD, porosity, PT and permeability together, however, the reduction rate caused by compaction become smaller as Aclay content increases, due to the limited compressibility of massive Aclay in grain-supported rocks. The hybrid effects of Aclay and compaction result in the formation of tight sandstones with various pore structures, corresponding to distinct pore types and reservoirs performances. We propose a threefold genetic classifications of tight sandstones: either from compaction alone, from Aclay cementation that plugs the original interparticle pores, or from a hybrid effect of these two processes. [ABSTRACT FROM AUTHOR]

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]

The distribution and amount of clay rim cements are different for Permian Rotliegend and Lower Triassic Bunter sandstones in the Southern Permian and German Triassic Basins, respectively. Both are similar fluvial-aeolian deposits in hot-arid endorheic basins. In both Permian and Triassic sandstones clay grain cutans can be present, but clay rim cement is often lacking or rare in Bunter sandstones. At first sight it would appear that the presence of cutans is thus not relevant for the development of clay cementation. However, at closer inspection, it appears that authigenic clay is indeed present in both cases. In Rotliegend sandstones, the authigenic clay mainly developed as rims around the clay grain cutans, which are often thin and well microlaminated with few intra-micropores and composed of platy particles. The clay crystals in the rim decrease the pore interconnectivity and lower permeability significantly. In Bunter sandstones, most of the authigenic clay developed within the cutans. These cutans are less well microlaminated, have ample micropores, and are composed of more equant-shaped particles. The latter looser structure facilitated clay authigenesis within the cutans and their micropores. This often led to exfoliation of the cutan laminae and expansion of the entire cutan. These expanded cutans also lower pore connectivity. The presence and thickness of clay rim cement on top of cutans and grains is thus not correlated with the thickness of the cutans but with the texture of the cutans. The latter determined where authigenic clays precipitated and the thickness of cutans is partly the result of clay authigenesis within the cutans. This demonstrates that the composition and the texture of sedimentary components constrained and controlled burial diagenesis. • Clay grain cutans yield the mass for clay authigenesis during burial, forming complex coatings on grains. • Authigenic clay (mainly illite and chlorite) precipitates on top of grain cutans forming rim cement or within the cutans. • Cutans originate from clay infiltration in sandy sediment during exposure or soil formation. • Susceptible components in cutans become unstable during burial and yield material for clay authigenesis. • Clay grain cutans may resemble detrital matrix after clay authigenesis. [ABSTRACT FROM AUTHOR]

The results of this study showed how clay mineral assemblages from alluvial sediments were used to deduce the environmental impact of anthropogenic modifications in the Upper Chicamocha River Basin (UCRB) in Colombia. Two dams were investigated: La Playa dam (with anthropogenic input from farm activities and wastewater) and Sochagota Lake dam (with input from natural hydrothermal waters) and found to contain organic-rich deposits modified under highly saline, eutrophic conditions. Results from X-ray diffraction (XRD), scanning electron microscope (SEM) and analytical high resolution electron microscope AEM-HRTEM analyses indicated that the mineral assemblage in the sediments trapped in La Playa dam in the upper part of the basin is characterized by the presence of Fe-bearing smectite (up to 0.4 atoms per formula unit (a.p.f.u.)), whereas sediments downstream the Sochagota Lake dam are characterized by the presence of illite and dispersed fragments of pyrite framboids. Intense eutrophication due to organic compounds and nutrient loads from urban wastewater and agricultural activities in the area above La Playa dam created a reducing environment in which sediments promoted Fe-uptake in authigenic smectite and efficiently trapped it in the sediments. Periodic release of water from Sochagota Lake into the Chicamocha River allowed the incorporation of detrital illite grains and fragments of pyrite framboids (formed by low temperature illitization and Fe-uptake in the lake sediments) into the alluvial sediments in the lower section of the UCRB. • Chicamocha River anthropic changes influenced the mineralogy of the sediments. • Saline waters and organic matter control mineral authigenic formation. • Smectite is efficiently trapped in La Playa dam. [ABSTRACT FROM AUTHOR]