Soil CO2 flux measurements in volcanic and geothermal areas

The accumulation chamber methodology allows one to obtain reliable values of the soil CO₂ flux, ϕ_{soil CO2}, in the range 0.2 to over 10 000 g m⁻² d⁻¹, as proven by both laboratory tests and field surveys in geothermal and volcanic areas. A strong negative correlation is observed between Δϕ_{soil CO2}/Δt and ΔP_atm/Δt. Maps of classes of log ϕ_{soil CO2} for the northern sector of Vulcano Island, Solfatara of Pozzuoli, Nea Kameni Islet and Yanbajain geothermal field evidence that active faults and fractures act as uprising channels of deep, CO₂-rich geothermal or magmatic gases. The total diffuse CO₂ output was evaluated for each surveyed area.     

Correlation analysis of NDVI dynamics and hydro-meteorological variables in growth period for four land use types of a water scarce area

As the characterization of primary productivity of wetland ecosystem, the Normalized Difference Vegetation Index (NDVI) plays an important role in local ecosystem conservation for environmental management. In this paper, the correlations of NDVI and hydro-meteorological variables were studied in a water scarce area with emphasis on different land use types, namely water, wetland, residential land and farmland, during the growing seasons of 1999 and 2000. The significant NDVI changes were detected between spring and summer for all land use types. The correlation analysis revealed that the NDVI-temperature correlation (P?P?P?farmland > R_wetland > R_{residential land} > R_water for NDVI and precipitation correlations (P?water > R_wetland > R_residential land > R_farmland for NDVI and temperature correlations (P?相似文献   

Noble gases as proxies of mean ocean temperature: sensitivity studies using a climate model of reduced complexity

Past global mean ocean temperature may be reconstructed from measurements of atmospheric noble gas concentrations in ice core bubbles. Assuming conservation of noble gases in the atmosphere-ocean system, the total concentration within the ocean mostly depends on solubility which itself is temperature dependent. Therefore, the colder the ocean, the more gas can be dissolved and the less remains in the atmosphere. Here, the characteristics of this novel paleoclimatic proxy are explored by implementing krypton, xenon, argon, and N₂ into a reduced-complexity climate model. The relationship between noble gas concentrations and global mean ocean temperature is investigated and their sensitivities to changes in ocean volume, ocean salinity, sea-level pressure and geothermal heat flux are quantified. We conclude that atmospheric noble gas concentrations are suitable proxies of global mean ocean temperature. Changes in ocean volume need to be considered when reconstructing ocean temperatures from noble gases. Calibration curves are provided to translate ice-core measurements of krypton, xenon, and argon into a global mean ocean temperature change. Simulated noble gas-to-nitrogen ratios for the last glacial maximum are δKr_atm = ?1.10‰, δXe_atm = ?3.25‰, and δAr_atm = ?0.29‰. The uncertainty of the krypton calibration curve due to uncertainties of the ocean saturation concentrations is estimated to be ±0.3 °C. An additional ±0.3 °C uncertainty must be added for the last deglaciation and up to ±0.4 °C for earlier transitions due to age-scale uncertainties in the sea-level reconstructions. Finally, the fingerprint of idealized Dansgaard-Oeschger events in the atmospheric krypton-to-nitrogen ratio is presented. A δKr_atm change of up to 0.34‰ is simulated for a 2 kyr Dansgaard-Oeschger event, and a change of up to 0.48‰ is simulated for a 4 kyr event.     

Improving noble gas based paleoclimate reconstruction and groundwater dating using Ne/Ne ratios

The interpretation of noble gas concentrations in groundwater with respect to recharge temperature and fractionated excess gas leads to different results on paleo-climatic conditions and on residence times depending on the choice of the gas partitioning model. Two fractionation models for the gas excess are in use, one assuming partial re-equilibration of groundwater supersaturated by excess air (PR-model, Stute et al., 1995), the other assuming closed-system equilibration of groundwater with entrapped air (CE-model, Aeschbach-Hertig et al., 2000). In the example of the Continental Terminal aquifers in Niger, PR- and CE- model are both consistent with the data on elemental noble gas concentrations (Ne, Ar, Kr, and Xe). Only by including the isotope ratio ²⁰Ne/²²Ne it can be demonstrated that the PR-model has to be rejected and the CE-model should be applied to the data. In dating applications ³He of atmospheric origin (³He_atm) required to calculate ³H-³He water ages is commonly estimated from the Ne excess presuming that gas excess is unfractionated air (UA-model). Including in addition to the Ne concentration the ²⁰Ne/²²Ne ratio and the concentration of Ar enables a rigorous distinction between PR-, CE- and UA-model and a reliable determination of ³He_atm and of ³H-³He water ages.     

Analytical solution to 1D coupled water infiltration and deformation in two‐layer unsaturated soils

An analytical solution to 1D coupled water infiltration and deformation in layered soils is derived using a Laplace transformation. Coupling between seepage and deformation, and initial conditions defined by arbitrary continuous pore‐water pressure distributions are considered. The analytical solutions describe the transient pore‐water pressure distributions during 1D, vertical infiltration toward the water table through two‐layer unsaturated soils. The nonlinear coupled formulations are first linearized and transformed into a form that is solvable using a Laplace transformation. The solutions provide a reliable means of comparing the accuracy of various numerical methods. Parameters considered in the coupled analysis include the saturated permeability (k_s), desaturation coefficient (α), and saturated volumetric water content (θ_s) of each soil layer, and antecedent and subsequent rainfall infiltration rates. The analytical solution demonstrates that the coupling of seepage and deformation plays an important role in water infiltration in layered unsaturated soils. A smaller value of α or a smaller absolute value of the elastic modulus of the soil with respect to a change in soil suction (H) for layered unsaturated soils means more marked coupling effect. A smaller absolute value of H of the upper layer soil also tends to cause more marked coupling effect. A large difference between the saturated coefficients of permeability for the top and bottom soil layers leads to reduced rainfall infiltration into the deep soil layer. The initial conditions also play a significant role in the pore‐water pressure redistribution and coupling effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Use of SF6 to label drilling air in unsaturated,fractured rock studies: risk of over-purging

Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a borehole in the unsaturated zone of a partially-welded, fractured tuff at Apache Leap, Arizona. One-meter intervals were later isolated at multiple depths and pumped to purge drilling air from each interval. The volume of air purged (at 1 atm, 20 °C), ranged from a low of 0.3 m³ in an unfractured interval, to a high of 252 m³ in a highly fractured interval. The concentration of SF₆ remained high throughout the purge in all intervals and often increased over time. Measurements of δ¹³C, ¹⁴C and CO₂ concentration indicated that atmospheric air was eventually drawn into several of the intervals in spite of the fact that SF₆ concentrations remained high. Possible explanations include mixing of atmospheric air drawn through fractures with partially-purged matrix air, and delayed removal of SF₆ relative to atmospheric gases due to adsorption of SF₆ within the tuff matrix, dissolution into pore water, or diffusion from dead-end pores with restricted openings. In this system, following a long delay between drilling and purging, it was found that the risk of contamination from surface air by over-purging was substantially greater than the risk of contamination from residual drilling air by under-purging.     

The role of pore water in the mechanical behavior of unsaturated soils

Deformation and failure of soils are governed by the stresses acting on the soil skeleton. The isotropic stress acting on the soil skeleton can be divided into two components. One is the stress component which is transmitted through the soil skeleton. This skeleton stress is influenced by the pore water (bulk water) in the soil. The other is the internal stress component which does not contribute to equilibrium with a given external force. The internal stress is induced by the capillary tension of meniscus water clinging to the contact point of soil particles and acts so as to connect the soil particles tightly. Therefore, in modeling the stress and strain relations for unsaturated soils, it is of much importance to quantitatively evaluate how the pore water exists in the soil. This paper discusses the role of pore water on the mechanical behaviour of the soil. In particular, the significance of the water retention curve is emphasized from a mechanical viewpoint. Essential features required in modeling of the constitutive relations for unsaturated soils are discussed and presented.     

The mechanisms of water diffusion in polymerized silicate melts

Diffusion of water was experimentally investigated for melts of albitic (Ab) and quartz-orthoclasic (Qz₂₉Or₇₁, in wt %) compositions with water contents in the range of 0 to 8.5 wt % at temperatures of 1100 to 1200 °C and at pressures of 1.0 and 5.0 kbar. Apparent chemical diffusion coefficients of water (D _water) were determined from concentration-distance profiles measured by FTIR microspectroscopy. Under the same P-T condition and water content the diffusivity of water in albitic, quartz-orthoclasic and haplogranitic (Qz₂₈Ab₃₈ Or₃₄, Nowak and Behrens, this issue) melts is identical within experimental error. Comparison to data published in literature indicates that anhydrous composition only has little influence on the mobility of water in polymerized melts but that the degree of polymerization has a large effect. For instance, D_water is almost identical for haplogranitic and rhyolitic melts with 0.5–3.5 wt % water at 850 °C but it is two orders of magnitude higher in basaltic than in haplogranitic melts with 0.2–0.5 wt % water at 1300 °C. Based on the new water diffusivity data, recently published in situ near-infrared spectroscopic data (Nowak 1995; Nowak and Behrens 1995), and viscosity data (Schulze et al. 1996) for hydrous haplogranitic melts current models for water diffusion in silicate melts are critically reviewed. The NIR spectroscopy has indicated isolated OH groups, pairs of OH groups and H₂O molecules as hydrous species in polymerized silicate melts. A significant contribution of isolated OH groups to the transport of water is excluded for water contents above 10 ppm by comparison of viscosity and water diffusion data and by inspection of concentration profiles from trace water diffusion. Spectroscopic measurements have indicated that the interconversion of H₂O molecules and OH pairs is relatively fast in silicate glasses and melts even at low temperature and it is inferred that this reaction is an active step for migration of water. However, direct jumps of H₂O molecules from one cavity within the silicate network to another one can not be excluded. Thus, we favour a model in which water migrates by the interconversion reaction and, possibly, small sequences of direct jumps of H₂O molecules. In this model, immobilization of water results from dissociation of the OH pairs. Assuming that the frequency of the interconversion reaction is faster than that of diffusive jumps, OH pairs and water molecules can be treated as a single diffusing species having an effective diffusion coefficient . The shape of curves of D_water versus water content implies that increases with water content. The change from linear to exponential dependence of D_water between 2 and 3 wt % water is attributed to the influence of the dissociation reaction at low water content and to the modification of the melt structure by incorporation of OH groups. Received: 26 March 1996 / Accepted: 23 August 1996     

Analytical solution to 1D coupled water infiltration and deformation in unsaturated soils

An analytical solution to 1D coupled water infiltration and deformation is derived using a Fourier integral transform. Exponential functional forms are used to represent the hydraulic conductivity–pore‐water pressure relationship and the soil‐water characteristic curve. Fredlund's incremental‐linear constitutive model for unsaturated soils is adopted. The analytical solution considers arbitrary initial pore‐water pressure distributions and flux and pressure boundary conditions. The corresponding analytical solutions to coupled steady‐state problems are also obtained. The analytical solutions demonstrate that the coupling of seepage and deformation plays an important role in water infiltration in unsaturated soils. In the early stages of infiltration, the difference between uncoupled and coupled conditions becomes marked over time, and in late stages, the difference caused by the coupling effects diminishes toward the steady state. The difference between the uncoupled and coupled conditions increases with decreasing desaturation coefficient (α). Pore‐water pressure or deformation changes caused by the coupling effects are mainly controlled by the degree of soil volume change due to a change in soil suction (H). The smaller the absolute value of H, the greater the effect of coupling on the infiltration and deformation. The ratio of rainfall intensity to saturated permeability (q/k_s) also has a strong influence on the coupled seepage and deformation. Copyright © 2008 John Wiley & Sons, Ltd.     

Consolidation in spatially random unsaturated soils based on coupled flow‐deformation simulation

This paper integrates random field simulation of soil spatial variability with numerical modeling of coupled flow and deformation to investigate consolidation in spatially random unsaturated soil. The spatial variability of soil properties is simulated using the covariance matrix decomposition method. The random soil properties are imported into an interactive multiphysics software COMSOL to solve the governing partial differential equations. The effects of the spatial variability of Young's modulus and saturated permeability together with unsaturated hydraulic parameters on the dissipation of excess pore water pressure and settlement are investigated using an example of consolidation in a saturated‐unsaturated soil column because of loading. It is found that the surface settlement and the pore water pressure profile during the process of consolidation are significantly affected by the spatially varying Young's modulus. The mean value of the settlement of the spatially random soil is more than 100% greater than that of the deterministic case, and the surface settlement is subject to large uncertainty, which implies that consolidation settlement is difficult to predict accurately based on the conventional deterministic approach. The uncertainty of the settlement increases with the scale of fluctuation because of the averaging effect of spatial variability. The effects of spatial variability of saturated permeability k_sat and air entry parameters are much less significant than that of elastic modulus. The spatial variability of air entry value parameters affects the uncertainties of settlement and excess pore pressure mostly in the unsaturated zone. Copyright © 2016 John Wiley & Sons, Ltd.     

An experimental investigation on diffusion of water in haplogranitic melts

  The diffusivity of water has been investigated for a haplogranitic melt of anhydrous composition Qz₂₈Ab₃₈Or₃₄ (in wt %) at temperatures of 800–1200°C and at pressures of 0.5–5.0 kbar using the diffusion couple technique. Water contents of the starting glass pairs varied between 0 and 9 wt %. Concentration-distance profiles for the different water species (molecular water and hydroxyl groups) were determined by near-infrared microspectroscopy. Because the water speciation of the melt is not quenchable (Nowak 1995; Nowak and Behrens 1995; Shen and Keppler 1995), the diffusivities of the individual species can not be evaluated directly from these profiles. Therefore, apparent chemical diffusion coefficients of water (D _water) were determined from the total water profiles using a modified Boltzmann-Matano analysis. The diffusivity of water increases linearly with water content <3 wt % but exponentially at higher water contents. The activation energy decreases from 64 ± 10 kJ/mole for 0.5 wt % water to 46 ± 5 kJ/mole for 4 wt % water but remains constant at higher water contents. A small but systematic decrease of D _water with pressure indicates an average activation volume of about 9 cm³/mole. The diffusivity (in cm²/s) can be calculated for given water content (in wt %), T (in K) and P (in kbar) by

Field experiments yield new insights into gas exchange and excess air formation in natural porous media

Gas exchange between seepage water and soil air within the unsaturated and quasi-saturated zones is fundamentally different from gas exchange between water and gas across a free boundary layer, e.g., in lakes or rivers. In addition to the atmospheric equilibrium fraction, most groundwater samples contain an excess of dissolved atmospheric gases which is called “excess air”. Excess air in groundwater is not only of crucial importance for the interpretation of gaseous environmental tracer data, but also for other aspects of groundwater hydrology, e.g., for oxygen availability in bio-remediation and in connection with changes in transport dynamics caused by the presence of entrapped air bubbles. Whereas atmospheric solubility equilibrium is controlled mainly by local soil temperature, the excess air component is characterized by the (hydrostatic) pressure acting on entrapped air bubbles within the quasi-saturated zone. Here we present the results of preliminary field experiments in which we investigated gas exchange and excess air formation in natural porous media. The experimental data suggest that the formation of excess air depends significantly on soil properties and on infiltration mechanisms. Excess air was produced by the partial dissolution of entrapped air bubbles during a sprinkling experiment in fine-grained sediments, whereas similar experiments conducted in coarse sand and gravel did not lead to the formation of excess air in the infiltrating water. Furthermore, the experiments revealed that the noble gas temperatures determined from noble gases dissolved in seepage water at different depths are identical to the corresponding in situ soil temperatures. This finding is important for all applications of noble gases as a paleotemperature indicator in groundwater since these applications are always based on the assumption that the noble gas temperature is identical to the (past) soil temperature.     

Helium isotopes in gases of Mineral Waters in the western Caucasus

The He isotope composition as an indicator of mantle-derived component was studied in gases from mineral springs, stratal waters, and mud volcanoes developed west of the Teberda River valley (10 objects) and two springs in the central segment of the Greater Caucasus orogen between the active El’brus and Kazbek volcanoes. In the western segment of the orogen, the values of ³He/⁴He = R_corr vary in the range of (46–114) × 10⁻⁸ = (0.33–0.81)R_atm, where R_atm =1.4 × 10⁻⁶ is the atmospheric ratio. They are substantially lower as compared with values in the vicinity of El’brus and Kazbek and close to those in samples from the central segment equal to (70–134) × 10⁻⁸ = (0.50–0.96)R_atm, as well as to the values previously recorded in the Caucasian Mineral Waters (CMW) area. Moreover, the content of ³He in them is notably higher as compared with its crustal radiogenic level characteristic of mud volcanoes in the Taman Peninsula, where R = (1.4–2.8) × 10⁻⁸ = (0.01–0.02)R_atm. Nitrogen-methane gas from northern piedmonts of the western Caucasus also contain nonatmogenic components, including the radiogenic ⁴⁰Ar (⁴⁰Ar/³⁶Ar = 900), “excess” nitrogen (∼87% of the total N₂ concentration in sample) and the mantle He. These data specify the distribution of mantle derivates along the orogen strike and age of intrusive magmatic activity in its different segments.     

Effects of carbon dioxide variations in the unsaturated zone on water chemistry in a glacial-outwash aquifer

The research site at Otis Air Base, Cape Cod, Massachusetts, has been developed for hydrogeological and geochemical studies of sewage-effluent contaminated ground water since 1982. Research of hydrologic properties, transport, and chemical and biological processes is ongoing, but the origin of background water chemistry has not been determined.The principal geochemical process giving rise to the observed background water chemistry is CO₂-controlled hydrolysis of Na feldspar. Geochemical modeling demonstrated that CO₂ sources could vary over the project area. Analyses of unsaturated zone gases showed variations in CO₂ which were dependent on land use and vegetative cover in the area of groundwater recharge. Measurements of CO₂ in unsaturated-zone gases showed that concentrations of total inorganic C in recharge water should range from about 0.035 to 1.0 mmoles/L in the vicinity of Otis Air Base. Flux of CO₂ from the unsaturated zone varied for 4 principal land uses, ranging from 86 gC/m²/yr for low vegetated areas to 1630 gC/m²/yr for a golf course. Carbon dioxide flux from woodlands was 220 gC/m²/yr, lower than reported fluxes of 500 to 600 gC/m²/yr for woodlands in a similar climate. Carbon dioxide flux from grassy areas was 540 gC/m²/yr, higher than reported fluxes of 230 to 490 gC/m²/yr for grasslands in a similar climate.     

Noble gas and carbon isotopes in Mariana Trough basalt glasses

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The ³He/⁴He ratios of 8.22 and 8.51 R_atm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 R_atm), whereas a mean ratio of 8.06±0.35 R_atm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of ²⁰Ne and ²¹Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess ¹²⁹Xe is observed in the sample which also shows ²⁰Ne and ²¹Ne excesses. Observed δ¹³C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO₂/³He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.     

CaO-MgO-Al2o3-SiO2 liquids: chemical and isotopic effects of Mg and Si evaporation in a closed system of solar composition

A method is shown for calculating vapor pressures over a CMAS droplet in a gas of any composition. It is applied to the problem of the evolution of the chemical and Mg and Si isotopic composition of a completely molten droplet having the composition of a likely refractory inclusion precursor during its evaporation into the complementary, i.e. modified solar, gas from which it originally condensed, a more realistic model than previous calculations in which the ambient gas is pure H_2(g). Because the loss rate of Mg is greater than that of Si, the vapor pressure of Mg_(g) falls and its ambient pressure rises faster than those of SiO_(g) during isothermal evaporation, causing the flux of Mg_(g) to approach zero faster and MgO to approach its equilibrium concentration sooner than SiO₂. As time passes, δ²⁵Mg and δ²⁹Si increase in the droplet and decrease in the ambient gas. The net flux of each isotope crossing the droplet/gas interface is the difference between its outgoing and incoming flux. δ²⁵Mg and δ²⁹Si of this instantaneous gas become higher, first overtaking their values in the ambient gas, causing them to increase with time, and later overtaking their values in the droplet itself, causing them to decrease with time, ultimately reaching their equilibrium values. If the system is cooling during evaporation and if mass transfer ceases at the solidus temperature, 1500 K, final MgO and SiO₂ contents of the droplet are slightly higher in modified solar gas than in pure H_2(g), and the difference increases with decreasing cooling rate and increasing ambient pressure. During cooling under some conditions, net fluxes of evaporating species become negative, causing reversal of the evaporation process into a condensation process, an increase in the MgO and/or SiO₂ content of the droplet with time, and an increase in their final concentrations with increasing ambient pressure and/or dust/gas ratio. At cooling rates <∼3 K/h, closed-system evaporation at P^tot ∼ 10⁻³ bar in a modified solar gas, or at lower pressure in systems with enhanced dust/gas ratio, can yield the same δ²⁵Mg in a residual CMAS droplet for vastly different evaporated fractions of Mg. The δ²⁵Mg of a refractory residue may thus be insufficient to determine the extent of Mg loss from its precursor. Evaporation of Mg into an Mg-bearing ambient gas causes δ²⁶Mg and δ²⁵Mg of the residual droplet to fall below values expected from Rayleigh fractionation for the amount of ²⁴Mg evaporated, with the degree of departure increasing with increasing fraction evaporated and ambient pressure of Mg. δ²⁶Mg and δ²⁵Mg do not depart proportionately from Rayleigh fractionation curves, with δ²⁵Mg being less than expected on the basis of δ²⁶Mg by up to ∼1.2‰. Such departures from Rayleigh fractionation could be used in principle to distinguish heavily from lightly evaporated residues with the same δ²⁵Mg.     

A 17-Year Record of Environmental Tracers in Spring Discharge,Shenandoah National Park,Virginia, USA: Use of Climatic Data and Environmental Conditions to Interpret Discharge,Dissolved Solutes,and Tracer Concentrations

A 17-year record (1995–2012) of a suite of environmental tracer concentrations in discharge from 34 springs located along the crest of the Blue Ridge Mountains in Shenandoah National Park (SNP), Virginia, USA, reveals patterns and trends that can be related to climatic and environmental conditions. These data include a 12-year time series of monthly sampling at five springs, with measurements of temperature, specific conductance, pH, and discharge recorded at 30-min intervals. The monthly measurements include age tracers (CFC-11, CFC-12, CFC-113, CFC-13, SF₆, and SF₅CF₃), dissolved gases (N₂, O₂, Ar, CO₂, and CH₄), stable isotopes of water, and major and trace inorganic constituents. The chlorofluorocarbon (CFC) and sulfur hexafluoride (SF₆₎ concentrations (in pptv) in spring discharge closely follow the concurrent monthly measurements of their atmospheric mixing ratios measured at the Air Monitoring Station at Big Meadows, SNP, indicating waters 0–3 years in age. A 2-year (2001–2003) record of unsaturated zone air displayed seasonal deviations from North American Air of ±10 % for CFC-11 and CFC-113, with excess CFC-11 and CFC-113 in peak summer and depletion in peak winter. The pattern in unsaturated zone soil CFCs is a function of gas solubility in soil water and seasonal unsaturated zone temperatures. Using the increase in the SF₆ atmospheric mixing ratio, the apparent (piston flow) SF₆ age of the water varied seasonally between about 0 (modern) in January and up to 3 years in July–August. The SF₆ concentration and concentrations of dissolved solutes (SiO₂, Ca²⁺, Mg²⁺, Na⁺, Cl^?, and HCO₃ ^?) in spring discharge demonstrate a fraction of recent recharge following large precipitation events. The output of solutes in the discharge of springs minus the input from atmospheric deposition per hectare of watershed area (mol ha^?1 a^?1) were approximately twofold greater in watersheds draining the regolith of Catoctin metabasalts than that of granitic gneisses and granitoid crystalline rocks. The stable isotopic composition of water in spring discharge broadly correlates with the Oceanic Niño Index. Below normal precipitation and enriched stable isotopic composition were observed during El Niño years.     

Meteoric fluid‐rock interaction in Variscan shear zones

Variscan shear zones in the Armorican Massif represent sites of strong fluid‐rock interaction. The hydrogen isotope composition of muscovite (δD_Ms) from syntectonic leucogranite allows to determine the source of fluids that infiltrated the footwall of three detachment zones and the South Armorican Shear Zone. Using temperatures of hydrogen isotope exchange estimated from microstructural data, we calculate the hydrogen isotope ratios of water (δD_water) present within the shear zones during high‐temperature deformation. A ~40‰ difference in δD_water values from deep to shallow crustal level reveals a mixing relationship between deep crustal fluids with higher δD values that range from ?34 to ?33‰, and meteoric fluids with δD values as low as ?74‰ in the upper part of detachment footwalls.     

δ18O and δ13C in fossil shells of Radix sp. from the sediment succession of a dammed palaeo‐lake in the Yarlung Tsangpo valley,Tibet, China

A series of confirmed and suspected dammed palaeo‐lake sedimentary successions is scattered within the middle Yarlung Tsangpo valley in Tibet. However, the chronology, the genesis of the dam and its location, the water level of the dammed lake, the process of dam failure and the spatiotemporal relationships between the sedimentary successions remain controversial. Here, we focus on one sedimentary succession of the suspected dammed palaeo‐lake at Xigazê. We measured the grain‐size distribution, magnetic susceptibility, organic and inorganic carbon content, and δ¹³C_org and δ¹⁵N_total ratios of the sediments. In addition, we measured the δ¹⁸O_shell and δ¹³C_shell values of modern and fossil Radix sp. shells, and the δ¹⁸O_water and δ¹³C_DIC values of the ambient water with different hydrological regimes. The results indicate that the δ¹⁸O_shell values of modern Radix sp. and the δ¹⁸O_water of the ambient water body significantly depend on its hydrological status. In addition, a strong positive relationship was observed between δ¹⁸O_shell values of modern Radix sp. shells and the δ¹⁸O_water of the ambient water on the Tibetan Plateau. According to this correlation, the δ¹⁸O_water values of the palaeo‐water body are reconstructed using the δ¹⁸O_shell values of Radix sp. fossil shells in the Xigazê section. Further, based on the δ¹⁸O_shell values of fossil Radix sp., the reconstructed δ¹⁸O_water of the palaeo‐water body and the specific habitats of Radix sp., we infer that the sedimentary succession in the Xigazê broad valley was mainly formed within the backwater terminal zone of a dammed palaeo‐lake and that the elevation of the water level of the lake was approximately 3811 m a.s.l. AMS ¹⁴C dating indicates that the deposits of the dammed palaeo‐lake were formed at about 33–22 cal. ka BP. Finally, the presence of Radix sp. fossil shells within the Xigazê section suggests that Radix sp. survived the late Last Glacial Period on the Tibetan Plateau.     

Hydrogen isotope fractionation factor for mixed-layer illite/smectite at 60° to 150°C: new data from the northeast Texas Gulf Coast

The δD values of water and clay collected from 0.6 to 5.5 km deep wells in 9 oil and gas fields within the Houston salt basin located in the northeast Texas Gulf Coast were used to reevaluate the temperature dependence of hydrogen isotope fractionation between mixed layer illite/smectite (I/S) and water, and the equations of Yeh (1980), Geochim. Cosmochim. Acta, 42:140-143) and Capuano (1992), Geochim. Cosmochim. Acta, 56:2547-2554) for the calculation of αI/S-water, both of which are commonly cited in the literature. δD values of water and clay in the normally pressured and geopressured sections are different. In the normally pressured section (<2.6 km), δD_water is constant (−15 ± 1 (1σ)‰) while δD_clay increases linearly from −59 to −43‰ with increasing depth. In contrast, in the geopressured section δD_water decreases linearly from −7 to −26‰ with increasing depth while δD_clay is nearly constant (−36 ± 3 (1σ)‰). The opposite trends are a product of D/H exchange between clay and water in a water-dominated system within the normally pressured section and rock-dominated system in the geopressured section. Assuming hydrogen isotope equilibrium between sediments and enclosed pore water, the new δD_water and δD_clay data were used to calculate the hydrogen isotope fractionation factor between I/S and water (αI/S-water), which showed a good fit to the equation derived by Capuano (1992), but not to the equation derived by Yeh (1980), both for the normally and geopressured data despite that both data sets are strikingly different. This suggests that hydrogen isotope equilibrium has been achieved in these two different regimes and that the equation by Capuano (1992) is valid and has wider applicability.