Weathering Reactions Research Articles

Variations in the radiophysical properties of tesserae and mountain belts on Venus: Classification and mineralogical trends

Numerous studies show that major Venus highlands display anomalously high radar reflectivity and low radar emissivity relative to the planetary average. This is thought to be the result of the formation of minerals having high dielectric constants via weathering reactions occurring between the surface and the deep atmosphere in these elevated terrains, where temperatures are lower. These reactions are a function of rock composition, atmospheric composition, and degree of weathering, or age. Here, we examine the Magellan radar emissivity, altimetry and backscatter data for all mapped tesserae and mountain belts on Venus. We characterize and classify each contiguous highland according to its pattern of the variation of radar emissivity with increasing altitude. The highlands can be assigned to 7 distinct patterns of emissivity that correspond to at least 2 discrete types of mineralogy based on the altitude (and temperature) of the emissivity changes from the global average (excursions). The majority of the emissivity changes occur at altitudes above 6053 km (temperature below 726 K). The emissivity signature of the major tesserae of Aphrodite Terra, Beta Regio and Phoebe Regio are consistent with the presence of ferroelectric minerals in their rocks (Curie temperatures of ~700–720 K). Fortuna tesserae and the mountains belts (Maxwell, Freyja, Akna and Danu montes) in Ishtar Terra are consistent with the presence of semiconductor minerals. Some tesserae in Ishtar Terra (Clotho, Itzpapatotl and Jyestha tesserae) lie at altitudes over 6055 but lack the emissivity excursions seen in Fortuna tesserae and the mountains at same altitudes and thus may represent a third type of tessera composition. Finally, the spatial distribution of radar emissivity classes correlates to different geologic settings which may reflect differences in the mantle dynamics. Alternatively, this variability could be ascribed to changes in the atmospheric conditions.

Gold nuggets: the inside story

Gold nuggets have long captured the imagination of geologists, prospectors and the public alike, but their origin remains disputed. Supporting a supergene origin, most gold nuggets in Australia have been found at or near the soil surface. Many are intimately associated with, and even appear to enclose, soil materials and weathered rock. Even large nuggets (e.g., >2 kg) have surface features and/or gross morphologies that suggest chemical reworking in the regolith. Conversely, other nuggets have been found at depths of ten metres or more in the regolith, and large masses of gold have been encountered at considerable depth, in completely unweathered, hypogene environments. Nuggets and particulate gold from many deposits in Australia, New Guinea, SE Asia and Brazil have been examined by optical and electron-optical techniques to determine characteristics that may indicate their genesis and stability in the regolith. The specimens have been collected at or close to the surface but all nuggets (mass range ~1 gm to >8 kg) and many smaller grains appear to be hypogene. They have nearly homogeneous Ag contents, mostly in the range 3 to 20 wt%, although some have no detectable Ag. One specimen also contains up to 3 wt% Hg, but no other alloyed metals > 0.1 wt% were detected. Enclosed minerals are rare – with only galena, Bi sulphide, galenobismutite and complex Ag-Hg tellurides in a few samples. The internal structure of the nuggets comprises nearly equigranular, randomly-oriented crystal domains. Many crystals display coherent twins and/or short incoherent twins that terminate within the crystal, all typical of thermal annealing at temperatures >250°C. Some small nuggets from SE Asia, also with annealing fabrics are possibly the product of hydrothermal remobilization and re-precipitation. In comparisons, some specimens from New Guinea contain 10->30 wt% Ag and have internal structures such as zoning and ‘fern-like’ crystal habits. These are derived from epithermal deposits and have not been deformed or recrystallized since initial deposition. Even the largest nuggets have internal evidence of weathering. Many have secondary minerals such as Fe oxides, clays and calcite within them, but none of these is fully enclosed. Rather, they are all open to the outer margin of the nuggets, situated in interconnecting voids along crystal boundaries. These boundaries also exhibit Ag depletion, similar to the depletion rims on the external surface. EBSD analysis shows there is no variation in crystallographic orientation across, or into, the depletion zones. These characteristics show that nuggets are dissolving in the surface environment, not forming, with weathering reactions initiated on the external surface and, internally, along crystal boundaries.

Materials and pathways of the organic carbon cycle through time

The cycle of organic carbon through the atmosphere, oceans, continents and mantle reservoirs is a hallmark of Earth. Over geological time, chemical exchanges between those reservoirs have produced a diversity of reduced carbon materials that differ in their molecular structures and reactivity. This reactive complexity challenges the canonical dichotomy between the surface and deep, short-term and long-term organic carbon cycle. Old and refractory carbon materials are not confined to the lithosphere but are ubiquitous in the surface environment, and the lithosphere hosts various forms of reduced carbon that can be very reactive. The biological and geological pathways that drive the organic carbon cycle have changed through time; from a synthesis of these changes, it emerges that although a biosphere is required to produce organic carbon, mortality is required to ensure its export to the lithosphere, and graphitization is essential for its long-term stabilization in the solid Earth. Among the by-products of the organic carbon cycle are the accumulation of a massive lithospheric reservoir of organic carbon, the accumulation of dioxygen in the atmosphere and the rise of a terrestrial biosphere. Besides driving surface weathering reactions, free dioxygen has allowed the evolution of new metabolic pathways to produce and respire organic carbon. From the evolution of photosynthesis until the expansion of biomineralization in the Phanerozoic, inorganic controls on the organic carbon cycle have diversified, tightening the connection between the biosphere and geosphere. A review of the organic carbon cycle explores the interactions between the Earth’s surface and deeper reservoirs, the expanding inorganic controls on the organic carbon cycle, and how these links have strengthened through geological time.

Using plug-flow column reactor data to constrain calcic mineral weathering rates from watershed mass-balance methods: Lithogenic apatite dissolution and phosphorus fluxes into the Loch Vale Watershed ecosystem, Colorado, USA

Sources of phosphorus (P5+) to ecosystems can be very difficult to identify and quantify due to P5+ being monoisotopic. Previous field-based, watershed mass-balance calculations of the lithogenic P5+ flux in the alpine/subalpine Loch Vale watershed (LVW) of Colorado, USA suggested that weathering was controlled by one of three acid-weathering scenarios: (1) All minerals experience only carbonic acid weathering; (2) all minerals experience both carbonic acid and sulfuric acid weathering; and (3) only silicate minerals experience sulfuric acid weathering. We used a plug-flow column reactor to provide additional constraints that strongly indicate scenario (1) (carbonic acid-only weathering) explains the stream chemistry at the LVW outlet (1984–2008 period of sampling). Therefore, sulfuric acid generated by pyrite oxidation in the LVW is not significantly involved in chemical weathering reactions. In the column reactor, calcite dissolution by carbonic acid seemed to suppress apatite dissolution with the likelihood of this suppression confirmed by thermodynamic modeling and comparison to kinetic data. The comparability between column reactor and watershed-scale data likely reflects the LVW landscape being dominated by relatively high rates of mechanical weathering. The lithogenic P5+ flux from the LVW is solely attributable to apatite (Ca5(PO4)3(OH,F,Cl)) dissolution and is ~47 mol ha−1 yr−1, ~18 times greater than the modeled global average. The relatively high solubility of apatite in the Loch Vale watershed likely results from substantial quantities of radionuclides hosted as substituted elements in the crystal lattice, which may cause loss of crystallinity due to self-irradiation. Aeolian inputs to landscapes may supplement any P5+ from primary phosphate mineral dissolution. These natural sources of P5+ are likely contributing to the increases in algal biomass observed today.

Probing silicate weathering reactions in soils with B isotopes

The determination of the mechanisms and extent of soil mineral weathering can be challenging, and the caveats reside in 1) difficulty identifying minerals that are actually involved in weathering reactions, 2) non-stoichiometric release of cations during weathering processes due to coupled dissolution, precipitation and transformation reactions and, 3) impact of vegetation activity on elemental cycles in upper soil horizons.To better characterize mechanisms controlling mineral weathering in soils and trace the evolution of B concentration and isotope ratios during chemical weathering, quantitative mineralogical analyses were coupled to B isotopes in a group of minerals (biotite, muscovite, K-feldspar and albite). Samples were selected along an Alocrisol (Alumic Cambisol, WRB FAO) soil profile from the bedrock (at 130 cm depth) up to 20 cm depth, developed on granitic bedrock in the Breuil-Chenue forest (France). The samples consist of residual primary minerals associated with weathering secondary phases (vermiculite, kaolinite…) in varying proportions.The B isotopic compositions of the most pristine minerals span a very narrow range of values (around −31‰), whereas all secondary phases point to a much heavier value (around −16‰), regardless of mineralogy. Our results also show a mineral-dependent evolution of B concentration or isotopic composition as weathering progresses: no variation is observed during dissolution of K-feldspars; B behaves like a very mobile element in micas (biotite and muscovite), whereas it concentrates in weathered products derived from albite. However, rates of B concentrations and changes in isotopic compositions appear to be much faster than those inferred from mineralogy or major element concentrations determined by XRD and bulk chemical analyses, respectively. These results indicate that B is involved in very early weathering reactions and raises the question of its actual location in the structure of the various soil minerals as well as its pathway to solution.

Relating the depth of the water table to the depth of weathering

AbstractWeathering of bedrock creates and occludes permeability, affecting subsurface water flow. Often, weathering intensifies above the water table. On the contrary, weathering can also commence below the water table. To explore relationships between weathering and the water table, a simplified weathering model for an eroding hillslope was formulated that takes into account both saturated and unsaturated subsurface water flow (but does not fully account for changes in dissolved gas chemistry). The phreatic line was calculated using solutions to mathematical treatments for both zones. In the model, the infiltration rate at the hill surface sets both the original and the eventual steady‐state position of the water table with respect to the weathering reaction front. Depending on parameters, the weathering front can locate either above or below the water table at steady state. Erosion also affects the water table position by changing porosity and permeability even when other hydrological conditions (e.g. hydraulic conductivity of parent material, infiltration rate at the surface) do not change. The total porosity in a hill (water storage capacity) was found to increase with infiltration rate (all else held constant). This effect was diminished by increasing the erosion rate. We also show examples of how the infiltration rate affects the position of the water table and how infiltration rate affects weathering advance. Published 2020. This article is a U.S. Government work and is in the public domain in the USA

New insights into Paleoproterozoic surficial conditions revealed by 1.85 Ga corestone-rich saprolith

Spheroidally weathered corestones, which are remnant pieces of bedrock surrounded by progressively weathered saprolite, preserve an ideal, small-scale natural interface for examining incipient to intermediate weathering reactions. Ancient corestones are preserved in some Precambrian paleosols but have remained surprisingly understudied. Here detailed mineral-chemical trends are examined across corestone-saprolith interfaces in ca. 1.85 Ga dolerite-hosted paleosol from the Flin Flon-Creighton area (Manitoba and Saskatchewan, Canada). The study presents the first comprehensive paleo-redox tracer suite (Fe-Mn-Mo-U-V-Cr-Ce) for the classic Flin Flon paleosol, which formed during a crucial period in the Paleoproterozoic for which marine sedimentary archives infer a return to low oxygen levels after the Great Oxidation Event (GOE).The textural and mineralogical progression across corestone-saprolith interfaces documented with petrography and scanning electron microscope-mineral liberation analysis (SEM-MLA) show many features (e.g., development of weathering rindlets and solution channels) strikingly reminiscent of modern mafic rock-hosted saprolite, despite later overprint. Albite-dominated cores preferentially preserving carbonate and sulfide (pyrite, chalcopyrite) are progressively altered outwards to a finer-grained saprolith rich in chlorite, illite, and muscovite, with embaying rindlets bearing cryptocrystalline hematite-quartz-illite.These mineral-textural observations guided sub-sampling for bulk Fe(II) and solution ICP-MS ultra-trace and major element analysis, and mineral-scale LA-ICP-MS analysis. Many high-field-strength elements (Al, Ti, Zr, Nb, Hf, Ta, and Th), remained immobile across the interface and ascertain the homogeneity of the parent dolerite. Progressive weathering from corestone to saprolith was quantified with chemical index of alteration minus K (CIA-K) values that fall into three zones (incipient (1): 45–55; modest (2): 55–65; and moderate (3): 65–75). Mass balance and spatial geochemical analysis revealed the following features: outward migration of Fe(II) and Mn from corestone to saprolith, with partial oxidation of Fe(II) to Fe(III) in rindlets; minimal REE mobility with no pronounced Ce anomalies, but well-developed, unidirectional Y/Ho fractionation; significant Cr and V mobility from corestones outwards with enrichment in saprolith; minimal dm-scale cycling and limited loss of Mo and U from saprolith; and post-depositional enrichment of K, Rb, Cs, Tl, Ba, Be, and W. Using semi-quantitative LA-ICP-MS elemental maps, it was possible to contextualize the mineralogical controls on chemical weathering reactions. Iron, Mg, and Mn are coupled within chlorite (representing former pedogenic phyllosilicates), which also scavenged Cr. The REEs are predominantly hosted by apatite and titanite. The Ti-phases ilmenite and titanite are the predominant U- and Mo-bearing hosts.Collectively, the insights from the Flin Flon paleosol converge with those from other ca. 1.90–1.85 Ga terrestrial and marine deposits in inferring an oxygen-limited atmosphere capable of efficiently oxidizing Fe and S, but not Mn, in terrestrial environments. The Cr and V distributions are most consistent with small-scale solubilization and redeposition that appears to be linked to the weatherability of protolith minerals and locally-generated acid- and/or ligand-rich conditions rather than oxidation. Although normally sensitive indicators of oxidative weathering, the inhibited release of Mo and U from the corestone was instead primarily dictated by the weathering resistance of their host minerals. Oxygen-limited weathering in saprolite supplied sulfate to the oceans, but the continental flux of redox-sensitive trace elements from this zone as well as more weathered substrates was limited, ultimately contributing to sustaining a largely redox-dynamic, weakly buffered, and nutrient-limited ocean.

The average Phanerozoic CO2 degassing flux estimated from the O-isotopic composition of seawater

The degassing rate of CO2 from the solid Earth, and matching burial rate, is critical to understanding the long-term carbon and oxygen cycles and the maintenance of a habitable climate. However, estimates of modern CO2 degassing rates vary by more than a factor of four and uncertainties are even larger further back in Earth history. Both the crustal C-inventory and C-isotope mass balance suggests that over long timescales ∼70-90% of the CO2 degassed from the solid Earth is drawn-down through silicate weathering reactions (subaerial and submarine). These reactions involve dissolution of silicate minerals, that typically formed at relatively high temperatures, and formation of silicate and carbonate minerals in low temperature aqueous environments. These new low temperature minerals have much higher δ18O than their precursors; thus the rate of degassing of CO2 is tied to the rate of removal of materials with high δ18O from the hydrosphere into weathering products. Maintaining a near-constant O-isotopic composition of the ocean over the Phanerozoic means that this weathering sink of isotopically heavy O must have equaled the source of isotopically heavy O to the ocean. This isotopically heavy oxygen largely comes from high-temperature hydrothermal vent fluids at mid-ocean ridges. New O-isotope data for the previously under-sampled lower oceanic crust from fast-spreading ridges, along with a compilation of published O-isotopic compositions of other sections of oceanic crust altered at high-temperatures, allows the magnitude of this high δ18O flux to be quantified. Modelling the coupled O-isotope and CO2 mass balance constrains suggests that the average Phanerozoic solid Earth CO2 degassing rate was ∼3.4 Tmolyr−1 (10th to 90th percentile of 1.8-5.3 Tmolyr−1). This is substantially lower than has been suggested by some recent studies, and is at or below the lower end of the range of previous estimates, with wide ranging implications for our understanding of the long term C cycle.

Lithium and strontium isotope dynamics in a carbonate island aquifer, Rottnest Island, Western Australia.

Water-rock interactions in aquifer systems are a key control on water quality but remain poorly understood. Lithium (Li) isotopes are useful for understanding water-rock interactions, but there are few data available for groundwater aquifers. Here we present a Li isotope dataset for rainfall and groundwater samples from a carbonate island aquifer system: Rottnest Island, Western Australia. This dataset was complemented by strontium (Sr) isotope and major and trace element data for groundwaters, and leaching experiments on bedrock samples. The δ7Li values and 87Sr/86Sr ratios of fresh groundwaters ranged from +23 to +36‰ and 0.709167 to 0.709198, respectively. Mass balance calculations indicated that silicate weathering supplied ~60 and 70% of dissolved Li and Sr in fresh groundwaters, respectively, with the remainder provided by atmospheric input, and carbonate weathering; for major cations, the majority of calcium and sodium (Na) are supplied by carbonate weathering and atmospheric input, respectively. The estimated low proportion of Sr produced by carbonate weathering was surprising in a carbonate aquifer, and the 87Sr/86Sr data indicated that the silicate Sr source had low Rb/Sr and 87Sr/86Sr ratios. There was an increase in the maximum δ7Li values in fresh groundwaters (+36‰) relative to the maximum value in rainfall and seawater (ca. +31‰). As clay minerals are undersaturated in fresh groundwaters, this increase may be explained by Li isotope fractionation associated with ion-exchange reactions on clays and iron(oxy)hydroxides. In the more saline groundwaters, the minimum δ7Li values decreased with depth to +14.5‰, suggesting increased silicate mineral dissolution in the deeper aquifer. These results reveal the importance of water-rock interactions in a coastal carbonate aquifer, and demonstrate the usefulness of Li isotopes for tracing weathering reactions in an environmental setting where traditional weathering tracers, such as sodium and Sr isotopes, are less appropriate.

Red-bed bleaching in a CO2 storage analogue: Insights from Entrada Sandstone fracture-hosted mineralization

ABSTRACTImproving our ability to predict the interactions between CO2 and reservoir rocks at geological time scales is of key importance if carbon capture and storage (CCS) is to have a role in climate-change mitigation, particularly in the light of likely regulatory requirements. Understanding and identifying the relevant geological processes over long time scales can be obtained only at natural-analogue sites. At one such site, in the Salt Wash Graben area of Utah, USA, widespread bleaching affects the Middle Jurassic red-bed “wet dune” Entrada Sandstone. Previous work has proposed a genetic link between the bleaching and spatially concomitant recent and modern CO2-rich fluids. The results presented here challenge some of the previous models and come from a detailed petrographic examination of mineralized fractures in the Entrada Sandstone that are centered in vertical extensions to the bleaching. These fractures typically contain complex mineralization assemblages. Pyrite was a paragenetically early phase, identifiable from common pseudomorphs of mixed iron oxides and oxyhydroxides that rarely contain relict pyrite. The pyrite contains up to 3 wt% arsenic. The volume of fracture-adjacent bleached sandstone is sufficient to have been the source of iron for the pyrite originally present in the fracture. The pyrite pseudomorphs occur at the center of fracture- and pore-filling cements that comprise intergrowths of hematite–goethite–jarosite–gypsum, an assemblage that suggests that their formation resulted from the oxidative alteration of pyrite, a genetic link supported by the arsenic present in the iron-bearing minerals. The presence of jarosite and proximal removal of earlier, sandstone-hosted carbonates are consistent with, and indicative of, the low-pH conditions associated with pyrite oxidation reactions. Calcite- and gypsum-cemented fractures crosscut, and contain fragments of, the pyrite-pseudomorphic and -oxidation assemblages, proving that they postdate pyrite formation and its subsequent oxidation, and that pyrite oxidation was not a result of modern weathering reactions. In outcrop, some calcite- and gypsum-cemented fractures link with travertine deposits associated with the modern and recent CO2-rich fluids. The mineral assemblages observed here, and the paragenetic sequence that we have inferred, suggest that the fracture-associated bleaching patterns result from the fracture-fed movement of sulfur-bearing reducing fluids, with hydrogen sulfide the most likely bleaching agent. We conclude that bleaching adjacent to fractures is not genetically related to modern CO2-bearing fluids despite the spatial relationship. The bleaching was already present when the modern fluids utilized the same fracture-based fluid pathways. We suggest that the more widespread regional bleaching formed contemporaneously with the fracture bleaching and followed similar mechanisms. This study highlights the complexity of interpreting analogue sites and the importance of using field and petrographic observations to unravel textures and events that are juxtaposed spatially but not temporally.

Geochemistry and conceptual model of thermal waters from Erciş - Zilan Valley, Eastern Turkey

Faults related to the neotectonic stress regime of Eastern Anatolia permit the rise and outflow of thermal waters in the Zilan Valley located 30 km north of the Erciş, Van province. Thermal springs discharge in a volcanic terrain, often in clusters, along faults in two separate areas: In the vicinity of Taşkapı (Şörköy) and Gergili villages, which lie in the north of the area; and in the area of Hasanabdal (Doğancı) thermal springs that lie to the south of the Zilan Valley. Temperatures of the springs range from 20 to 78 °C, with discharge rates of 4−20 L/s. Thermal waters of the Zilan region are of different types, from Na−HCO3 to Na-Cl, with dissolved ion concentrations varying over a wide range between 484–4572 mg/L. Calcite dissolution and silicate hydrolysis are the dominant water-rock interactions. Constituents with minor and trace element concentrations reinforce conclusions drawn from major-ion composition that the major hydrogeochemical processes are: mixing between cold-shallow and hot-deep waters, loss of energy by heat conduction, and steam-heating of perched-aquifers. The cold spring and Zilan Creek waters are of the Ca−HCO3 and Na−HCO3 type respectively, and their dissolved ion concentrations are low. Hydrogeochemical processes for the cold waters involve carbonate dissolution and silicate weathering reactions. Chemical geothermometers are used to estimate the temperature of the deep fluids using the chemical composition of the less modified fluids. They indicate that fluids emerging from volcanic rocks in the Zilan valley reach temperatures of around 150 °C. Thermal waters, generated through conductive heat transfer or the input of geothermal vapor or gases from below, delineate the extent of the geothermal reservoir(s) at depth.Deuterium and oxygen-18 signatures of thermal waters indicate that they are at least partially derived from thermally heated water through a process of 18O enrichment owing to interaction with the marine limestones through which the water passes to the surface springs.Both geochemical and isotope data indicate that the Zilan Valley hosts a low temperature, fracture-zone system with aconvective up-flow and dominant conductive heat transfer from a resource base in the upper crust to the surface.Ore deposits in the Zilan Valley, hydrothermal alteration in the Şörköy lava, and high concentrations of As, Au, Hg, Zr, Ba, Ni, Sb and other trace elements that are present in the travertine is the evidence for a heat source formed at a shallower depth. A conceptual model of the geothermal system of Zilan is proposed based on adata gathered.

Controls on granitic weathering fronts in contrasting climates

Granitic weathering profiles display highly diverse morphologies, reflecting the complex relationships between climate and weathering rates. Some profiles exhibit abrupt transitions from fresh bedrock to highly weathered material over short (<1 m) distances, while others exhibit only limited weathering extending 10s of meters into the bedrock. Although granitic weathering processes have been well studied, the controls on profile morphology and weathering rates within granitic, and many other lithologies remain poorly understood; these are likely influenced by a range of both intrinsic and extrinsic factors, which in turn will have crucial implications for understanding, for example, climate-weathering feedbacks. In this study we present multi-scale elemental and mineralogical analyses of a >30 m granitic weathering profile from the cool, temperate, Lysina catchment in the NW Czech Republic, from which we calculated mass transfer, weathering indices, and mineral specific weathering rates. The Lysina profile exhibits limited weathering extending >30 m into fractured bedrock, dominated by albite weathering at a rate of 9.3 × 10−17 mol m2 s−1.To identify environmental and geological controls on weathering front morphology and chemical weathering rates, Lysina was compared to previously published granitic weathering profiles from around the world. Weathering front morphology and weathering rates were calculated for the additional sites from published data and were correlated to mean annual precipitation (MAP), mean annual temperature (MAT), and erosion rates, with MAP having the strongest relationship. Higher MAP likely promotes lower saturation indices in pore waters, allowing weathering reactions to occur further from equilibrium. Comparison of erosion rates amongst the granitic catchments revealed an inconsistent effect on chemical weathering rates, but high erosion rates may promote weathering by reducing the thickness of the regolith and exposing the bedrock to reactive fluids. Mean annual temperatures appear to only have significant impacts on weathering fronts in environments with high precipitation and high erosion rates. Fractured bedrock profiles (Lysina and Río Icacos) have higher weathering intensities, than the other sites studied here. High connected porosity in fractured rocks enhances water movement allowing more efficient removal of weathering products, thus reducing thermodynamic saturation, increasing weathering rates, and producing sharper weathering gradients. These findings indicate that CO2 drawdown on geological timescales is also likely to be governed by precipitation rates, as well as temperature, and that much of the climate-significant weathering may occur within very narrow zones of the Earth's surface.

Understanding river – subsurface water interactions in upper Ganga basin, India

ABSTRACTIn the upper Ganga basin particularly in Himalayan terrain, precipitation and/or snow/glacial melt contribute towards the flow of the Ganga river in the form of surface and sub-surface water. In the present study, river–subsurface water chemistry is compared to infer river–subsurface water interactions in higher Himalayan region. Forty-one water samples from the Ganga river and its small tributaries between Gomukh and Dabrani section as well as subsurface water were collected during the months of March, May, September, October and November 2016 with in-situ measurements of pH, Electric Conductivity, temperature and bicarbonates. The samples were analysed for major ions and isotopes of oxygen and hydrogen. The results revealed that major ion chemistry of sub-surface water and river water is influenced by seasonal mineral dissolution and rock weathering reactions in the sub-surface water. A dominance of Calcium (36–43% in surface and 61–71% in sub-surface water), bicarbonate (13–66% in surface water and 45–85% in sub-surface water) and sulphate (31–86% in surface water; 15–47% in sub surface) is observed. High carbonate dominance is observed in surface water during the pre-monsoon (May 2016) suggesting more carbonate dissolution while high sulphates are found in March and September. The isotopes characterization of the surface and sub-surface water indicated highly evaporated sub-surface water in pre-monsoon season while in post monsoon season the slope and intercept values of δ18O and δD plot was more close to that of surface water suggesting its influence. The study helps understand to river–subsurface water interactions in the Himalayan terrain of the Ganga basin. The understanding of river–subsurface water interactions can help in management of the Ganga river water and in addressing downstream environmental issues associated with the river.

The geological significance of novel anthropogenic materials: Deposits of industrial waste and by-products

Industrial wastes and by-products are increasingly (re-)used as filling material in constructions. To enhance awareness among the geological community of the growing and widespread occurrence of deposits that contain these “novel anthropogenic materials”, this paper reviews three volumetrically important materials: (1) ash remaining from the incineration of household waste (Municipal Solid Waste Incineration bottom ash), (2) slag from steel production, and (3) the stony aggregate fraction of construction and demolition waste. We review their origin, main geochemical weathering reactions, and influence on the natural geogenic environment. These materials have properties that set them apart from the geogenic materials that they replace and overlie. They are formed under high-temperature conditions, are thermodynamically unstable, and may exhibit physical and chemical changes on relatively short timescales, an aspect relatively new for mapping geologists. Because knowledge of deposits in the shallow urban subsurface is increasingly important for urban planning, more articulation and detail in the classification of novel anthropogenic materials is necessary. © 2019 The Author(s)

Modelling the riverine δ7Li variability throughout the Amazon Basin

The present study investigates the processes controlling the elementary and isotopic cycle of the lithium over the Amazon basin. A numerical model is developed to simulate two major processes that have been proposed as key controls of the river lithium isotopic composition: weathering reactions inside the regolith, accounting for secondary phase formation, and interactions between riverine water and secondary phases in floodplain. Both processes generate fractionation of lithium isotopes (“batch” fractionation and “Rayleigh” distillation respectively) that potentially control the riverine isotopic composition of the Amazon and its tributaries.A study of the model parameters shows that two different regimes are impacting the lithium isotopic composition of the rivers within the Amazon catchment. In the South (Madeira and its tributaries), the lithium isotopic signature of river waters can be explained by lithium release and fractionation during weathering reactions in the regolith, followed by “Rayleigh distillation” in the floodplain increasing progressively the lithium isotopic composition, in agreement with a previously published hypothesis. In contrast, the lithium isotopic composition of rivers located in the northern part of the Amazon watershed (Solimoes and tributaries) cannot be simulated by the model assuming the same processes than in the southern part. Model optimization suggests than the nature of the material being eroded and weathered is important. In the North, fresh source rocks of volcanic origin releases large amount of Li and promotes rapid smectite precipitation, allowing the riverine δ7Li to rise before flowing through floodplains. This result suggests that the environments able to generate high riverine δ7Li (higher than 25‰) are complex and not firmly identified yet.

Room temperature hydrocarbon generation in olivine powders: Effect of mechanical processing under CO2 atmosphere

The present work focused on the weathering reaction of olivine under CO2 atmosphere to produce light hydrocarbons via mechanical process by using ball milling apparatus. The evolution of methane and light hydrocarbons with different amount of water is reported in detail, and the process efficiency compared with the thermally activated one. Methane and light hydrocarbons productions of at least three orders of magnitude larger, are detected for the mechanically activated process. Such significant improvement can be attributed to the extreme conditions experienced by the powders processed at each impact during ball milling, whit an estimated local temperature of 648 °C. Structural and microstructural characterization reveals the formation of serpentine and magnetite-based minerals, and the occurring of the carbonation process, which competes with serpentinization, confirming that carbon capture can take place during ball milling of olivine under low pressure of CO2.

Soil Fluid Biogeochemical Response to Climatic Events

AbstractPredicting fluid biogeochemistry in the vadose zone is difficult because of time‐dependent variation in multiple controlling factors, such as temperature, moisture, and biological activity. Furthermore, soils are multicomponent, heterogeneous porous media where manifold reactions may be affecting solution chemistry. We postulated that ecosystem‐scale processes, such as carbon fixation and ecohydrologic partitioning, control subsurface biogeochemical reactions, including mineral weathering. To test this hypothesis, we applied a novel “instrumented pedon” research approach. Analysis of the data streams demonstrates the interactions between pulsed wetting events and biogeochemical processes in the soil profile, and along groundwater flow paths. Rapid wetting front propagation into dry soil resulted in a pulsed increase in CO2 partial pressure in deeper soil layers, whereas wetting front propagation into a premoistened soil profile showed the opposite effect. The apparent respiratory quotient (ARQ), calculated from CO2 and O2 fluxes, deviated from expected oxidative ratios particularly during soil wetting events. These deviations were correlated in time with pore water geochemical responses, revealing that a fraction of the respired CO2 was consumed locally in pulsed silicate weathering events that accompanied wetting‐front propagation. However, most of this CO2 was dissolved in the soil pore water and transported downgradient, and along the soil‐bedrock interface, where a portion of it was further consumed in silicate weathering reactions, and another portion was degassed to the atmosphere. These results highlight the tight coupling that exists between physical, biological, and chemical processes, on event time scales, during incremental co‐evolution of the critical zone, particularly in water‐limited systems.

Proglacial freshwaters are significant and previously unrecognized sinks of atmospheric CO2

Carbon dioxide (CO2) emissions from freshwater ecosystems are almost universally predicted to increase with climate warming. Glacier-fed rivers and lakes, however, differ critically from those in nonglacierized catchments in that they receive little terrestrial input of organic matter for decomposition and CO2 production, and transport large quantities of easily mobilized comminuted sediments available for carbonate and silicate weathering reactions that can consume atmospheric CO2 We used a whole-watershed approach, integrating concepts from glaciology and limnology, to conclusively show that certain glacier-fed freshwater ecosystems are important and previously overlooked annual CO2 sinks due to the overwhelming influence of these weathering reactions. Using the glacierized Lake Hazen watershed (Nunavut, Canada, 82°N) as a model system, we found that weathering reactions in the glacial rivers actively consumed CO2 up to 42 km downstream of glaciers, and cumulatively transformed the High Arctic's most voluminous lake into an important CO2 sink. In conjunction with data collected at other proglacial freshwater sites in Greenland and the Canadian Rockies, we suggest that CO2 consumption in proglacial freshwaters due to glacial melt-enhanced weathering is likely a globally relevant phenomenon, with potentially important implications for regional annual carbon budgets in glacierized watersheds.

Weathering in a world without terrestrial life recorded in the Mesoproterozoic Velkerri Formation

Today the terrestrial surface drives biogeochemical cycles on Earth through chemical weathering reactions mediated by the biological influence of soils. Prior to the expansion of life on to land, abiotic weathering may have resulted in different boundary conditions affecting the composition of the biosphere. Here we show a striking difference in weathering produced minerals preserved in the Mesoproterozoic Velkerri Formation. While the bulk chemistry and mineralogy is dominated by illite similar to many modern mudstones, application of a novel microbeam technology reveals that the initial detrital minerals were composed of mica (28%) and feldspar (45%) with only a trace amount (<2%) of typical soil formed clay minerals. The majority of illite and the high Al2O3 fraction previously interpreted as a weathering signal, is present as a replacement of feldspar and mica. These sediments record physical erosion with limited pedogenic clay mineral formation implying fundamentally different weathering pathways.

Insights on catchment-wide weathering regimes from boron isotopes in riverine material

Chemical weathering contributes to the regulation of the global carbon cycle and biogeochemical cycles. Accordingly, the identification of the parameters that control weathering reactions and transport of weathering signals at the catchment scale is essential. The use of boron (B) isotopes have been shown to be a useful proxy in tracing weathering reactions due to large isotope fractionation during weathering processes. However, our knowledge of how boron isotopes record the weathering regime at the catchment scale and how that weathering signal is transported from source areas to the depositional environment remains limited. Here we characterize B isotope and major element behavior during chemical weathering and transport by analyzing the B isotopic (δ11B) and element compositions of riverine material (riverbank sands (<63 µm), clay fractions (<2 µm) extracted from sands, and dissolved load) along the course of the Murrumbidgee River (NSW, Australia), its upstream tributaries, and monolithologic subcatchments. In the Murrumbidgee, two distinct weathering regimes are present, one where mineral dissolution is associated with minimal neoformation at higher elevations and another where mineral neoformation dominates at lower elevations and in granitic lithologies. Significant B isotope difference between the clay fraction and the bedrock (Δ11Bclay-bedrock) is observed in most monolithological catchments at high mean elevation (excluding granites), which correlates with a large B depletion. Smaller isotope difference between the clay fraction and bedrock is observed in monolithological catchments at lower elevations as well as in granitic catchments at all elevations and is associated with limited B removal. These results suggest that lithology and catchment topography influence B mobility during weathering and the isotopic composition of weathering products. By mass balance calculation, the B isotope and chemical composition of the clay and sand fractions in the Murrumbidgee River can be explained as a mixture of the clays and sands produced throughout the catchment delivered to the main channel by the tributaries. These results indicate that there is little or no chemical and isotopic modification of the river sediment during fluvial transport and that weathering signal produced in the sediment source areas is transferred to the depositional environment without significant modification. The boron content of the clay-sized fraction (∼40 ppm) is several orders of magnitude greater than that of the dissolved load while B isotope compositions of the clay-sized fraction are isotopically much lighter (up to 40‰). Because a maximum isotopic difference of 30‰ between the dissolved and solid phases is expected during adsorption processes, the observed isotope compositions in the dissolved load and the sediment clay fraction cannot be explained by pH-driven B partitioning. These observations suggest that clays are not directly precipitating from solutions compositionally similar to surface waters; deeper soil solutions are expected to play a significant role in clay formation. This research highlights the potential of B isotopes in river sediments to describe the present and past weathering regimes at the catchment scale, including possible paleoenvironment reconstruction as the B isotope signature of riverine material records the conditions of its formation.