Preferential exchange rate effect of isotopic fractionation in a melting snowpack

Stable isotope exchange processes between solid and liquid phases of a natural melting snowpack are investigated in detail by separating the liquid water from snow grains at different depths of the snowpack and collecting the bottom discharge using a lysimeter. In the melting–freezing mass exchange process between the two phases, the theoretical slope of the δD? δ¹⁸O line for newly refrozen ice is calculated to be nearly that of pore water. However, based on observations of the isotopic evolution and snow grain coarsening of the snowpack, it is demonstrated that the slope of the δD? δ¹⁸O line for newly refrozen ice is equal to that of the original ice. This is proved to be due to preferential water flow in the snowpack, which leads to relatively more deuterium and less oxygen‐18 in the mobile water than the immobile water because of the kinetic effect. Higher mass exchange rate in the mobile water region results in excess deuterium in the bulk refrozen ice, compared with the fractionation of uniform fractionation factors and exchange rate. This effect, which is termed the ‘preferential exchange rate effect of isotopic fractionation’, is shown to be larger in the lower part than the upper part of the snowpack. Copyright © 2008 John Wiley & Sons, Ltd.     

Sr isotopic fractionation in Allende chondrules: A reflection of solar nebular processes

True relative Sr isotopic compositions, determined by the double-spike technique, are reported for 8 olivine chondrules from Allende and a single chondrule from Richardton. The Richardton chondrule has an Sr composition identical with the whole meteorite, but the Allende chondrules are up to 1.4‰ per mass unit light-isotope enriched, closely similar to Ca-Al inclusions (CAI) from the same individual stone. The correspondence of the patterns for chondrules and CAI suggests that both groups of objects derived their fractionated Sr in similar ways. The lack of any detectable non-linear Sr isotopic anomaly in the objects suggests that their Sr compositions did not have some exotic or extrasolar origin, but were derived from normal solar system Sr by mass fractionation. The consistent light-Sr enrichment of Allende objects may be explained by several schemes, and all are heavily model-dependent. Most plausible to the author is that the CAI and chondrules derived their fractionated Sr from a region of the nebula made isotopically light by partial kinetic mass separation of elements in the vapour phase. Later, the solid objects may have moved to an isotopically more normal region, where the Allende matrix accreted.     

Recharge processes during snowmelt: An isotopic and hydrometric investigation

Results from hydrometric and isotopic investigations of unsaturated flow during snowmelt are presented for a hillslope underlain by well-sorted sands. Passage of melt and rainwater through the vadose zone was detected from temporal changes in soil water ²H concentrations obtained from sequential soil cores. Bypassing flow was indicated during the initial snowmelt phase, but was confined to the near-surface zone. Recharge below this zone was via translatory flow, as meltwater inputs displaced premelt soil water. Estimates of premelt water fluxes indicate that up to 19 per cent of the premelt soil water may have been immobile. Average water particle velocities during snowmelt ranged from 6.2 × 10^?7 to 1.1 × 10^?6 ms^?1, suggesting that direct groundwater recharge by meltwater during snowmelt was confined to areas where the premelt water table was within 1 m of the ground surface. Soil water ²H signatures showed a rapid response to isotopically-heavy rain-on-snow inputs late in the melt. In addition, spatial variations in soil moisture content at a given depth induced a pronounced lateral component to the predominantly vertical transport of water. Both factors may complicate isotopic profiles in the vadose zone, and should be considered when employing environmental isotopes to infer recharge processes during snowmelt.     

Soil organic carbon dynamics study bias deduced from isotopic fractionation in corn plant

Carbon stable isotope techniques were extensively employed to trace the dynamics of soil organic carbon(SOC)across a land-use change involving a shift to vegetation with different photosynthetic pathways.Based on the isotopic mass balance equation,relative contributions of new versus old SOC,and SOC turnover rate in corn fields were evaluated world-wide.However,most previous research had not analyzed corn debris left in the field,instead using an average corn plant δ~(13)C value or a measured value to calculate the proportion of corn-derived SOC,either of which could bias results.This paper carried out a detailed analysis of isotopic fractionation in corn plants and deduced the maximum possible bias of SOC dynamics study.The results show approximately 3‰ isotopic fractionation from top to bottom of the corn leaf.The ~(13)C enrichment sequence in corn plant was tassel﹥stalk or cob﹥root﹥leaves.Individual parts accounting for the total dry mass of corn returned distinct values.Consequently,the average δ~(13)C value of corn does not represent the actual isotopic composition of corn debris.Furthermore,we deduced that the greater the fractionation in corn plant,the greater the possible bias.To alleviate bias of SOC dynamics study,we suggest two measures:analyze isotopic compositions and proportions of each part of the corn and determine which parts of the corn plant are left in the field and incorporated into SOC.     

Thermal control of low-pressure fractionation processes

Thermal models detailing the solidification paths for shallow basaltic magma chambers (both open and closed systems) were calculated using finite-difference techniques. The total solidification time for closed chambers are comparable to previously published calculations; however, the temperature-time paths are not. These paths are dependent on the phase relations and the crystallinity of the system, because both affect the manner in which the latent heat of crystallization is distributed.In open systems, where a chamber would be periodically replenished with additional parental liquid, calculations indicate that the possibility is strong that a steady-state temperature interval is achieved near a major phase boundary. In these cases it is straightforward to analyze fractionation models of the basaltic liquid evolution and their corresponding cumulate sequences. This steady thermal fractionating state can be invoked to explain large amounts of erupted basalts of similar composition over long time periods from the same volcanic center and some rhythmically layered basic cumulate sequences.     

Crystal fractionation of granitic magma during its non-transport processes: A physics-based perspective

Granitic continental crust distinguishes the Earth from other planets in the Solar System. Consequently, for understanding terrestrial continent development, it is of great significance to investigate the formation and evolution of granite.Crystal fractionation is one of principal magma evolution mechanisms. Nevertheless, it is controversial whether crystal fractionation can effectively proceed in felsic magma systems because of the high viscosity and non-Newtonian behavior associated with granitic magmas. In this paper, we focus on the physical processes and evaluate the role of crystal fractionation in the evolution of granitic magmas during non-transport processes, i.e., in magma chambers and after emplacement. Based on physical calculations and analyses, we suggest that general mineral particles can settle only at tiny speed(~10~(-9)–10~(-7) m s~(-1))in a granitic magma body due to high viscosity of the magma; however, the cumulating can be interrupted with convection in magma chambers, and the components of magma chambers will tend to be homogeneous. Magma convection ceases once the magma chamber develops into a mush(crystallinity, F~40–50%). The interstitial melts can be extracted by hindered settling and compaction, accumulating gradually and forming a highly silicic melt layer. The high silica melts can further evolve into high-silica granite or high-silica rhyolite. At various crystallinities, multiple rejuvenation of the mush and the following magma intrusion may generate a granite complex with various components. While one special type of granites, represented by the South China lithium-and fluoride-rich granite, has lower viscosity and solidus relative to general granitic magmas, and may form vertical zonation in mineral-assemblage and composition through crystal fractionation. Similar fabrics in general intrusions that show various components on small lengthscales are not the result of gravitational settling. Rather, the flowage differentiation may play a key role. In general, granitic magma can undergo effective crystal fractionation; high-silica granite and volcanics with highly fractionated characteristics may be the products of crystal fractionation of felsic magmas, and many granitoids may be cumulates.     

Hydrologic implications of the isotopic kinetic fractionation of open-water evaporation

The kinetic fractionation of open-water evaporation against the stable water isotope H_2 ~(18)O is an important mechanism underlying many hydrologic studies that use ~(18)O as an isotopic tracer. A recent in-situ measurement of the isotopic water vapor flux over a lake indicates that the kinetic effect is much weaker(kinetic factor 6.2‰) than assumed previously(kinetic factor14.2‰) by lake isotopic budget studies. This study investigates the implications of the weak kinetic effect for studies of deuterium excess-humidity relationships, regional moisture recycling, and global evapotranspiration partitioning. The results indicate that the low kinetic factor is consistent with the deuterium excess-humidity relationships observed over open oceans.The moisture recycling rate in the Great Lakes region derived from the isotopic tracer method with the low kinetic factor is a much better agreement with those from atmospheric modeling studies than if the default kinetic factor of 14.2‰ is used. The ratio of transpiration to evapotranspiration at global scale decreases from 84±9%(with the default kinetic factor) to 76±19%(with the low kinetic factor), the latter of which is in slightly better agreement with other non-isotopic partitioning results.     

Temperature dependence of the oxygen isotopic fractionation between diatom silica and water

We present a new paleotemperature scale, based on the oxygen isotopic ratio of the non-exchangeable fraction of the oxygen from diatom silica. The equation t = 17.2 − 2.4 (δ¹⁸O_silica − δ¹⁸O_water − 40) − 0.2 (δ¹⁸O_silica − δ¹⁸O_water − 40)² was derived using recent sediment samples from different oceanic areas, the temperature and isotopic composition of the local surface water. Comparison of our results with other relationships established for quartz-water or amorphous silica-water at higher temperature suggests no difference in isotopic fractionation between quartz-water and biogenic silica-water couples.     

Carbon isotopic fractionation in the process of Fischer-Tropsch reaction in primitive solar nebula

Carbon isotopic fractionation of low-weight molecules of the products of Fischer-Tropsch reaction under the conditions of Earth's accumulation region in primitive solar nebula has been experimentally studied. Among the reaction products, carbon dioxide has the heaviest isotopic composition; carbon isotopic composition of methane has a large variation; different from biogenic natural gases, carbon isotopic compositions among ethane, propane and butane yield a reverse distribution pattern. It suggests that primordial hydrocarbons that were captured in the interior of the Earth during its accretion possess a reverse carbon isotopic distribution pattern. Project supported by the National Natural Science Foundation of China (Grant No. 49233060).     

Impact-induced devolatilization and hydrogen isotopic fractionation of serpentine: implications for planetary accretion

The degree of impact-induced devolatilization of nonporous serpentine, porous serpentine, and deuterium-enriched serpentine was investigated using two independent experimental methods, the gas recovery method and the solid recovery method, yielding consistent results. The gas recovery method enables determination of the chemical and hydrogen isotopic composition of the recovered gases. Experiments on deuterium-enriched serpentine unambiguously identify the samples as the source of the recovered gases, as opposed to other possible contaminants. For shock pressures near incipient devolatilization (Pinitial = 5.0 GPa), the hydrogen isotopic composition of the evolved gas is similar to that of the starting material. For higher shock pressures the bulk evolved gas is significantly lower in deuterium than the starting material. There is also significant reduction of H2O to H2 in gases recovered at higher shock pressures, probably caused by reaction of evolved H2O with the metal gas recovery fixture. The hydrogen isotopic fractionation between the evolved gas and the residual solid indicates nonequilibrium, kinetic control of gas-solid isotopic ratios. In contrast, gaseous H2O-H2 isotopic fractionation suggests high temperature (800-1300 K) isotopic equilibrium between the gaseous species, indicating initiation of devolatilization at sites of greater than average energy deposition (i.e., shear bands). Impact-induced hydrogen isotopic fractionation of hydrous silicates during accretion can affect the distribution of hydrogen isotopes of planetary bodies during accretion, leaving the interiors enriched in deuterium. The significance of this process for planetary development depends on the models used for extrapolation of the observed isotopic fractionation to devolatilizations greater than those investigated experimentally and assumptions about timing and rates of protoatmosphere loss, frequency of multiple impacts, and rates of gas-solid or gas-melt isotopic re-equilibration. A simple model indicates that substantial planetary interior enrichments of D/H relative to that of the incident material can result from impact-induced hydrogen fractionation during accretion.     

Transport,biodegradation and isotopic fractionation of chlorinated ethenes: modeling and parameter estimation methods

An analytical, one-dimensional, multi-species, reactive transport model for simulating the concentrations and isotopic signatures of tetrachloroethylene (PCE) and its daughter products was developed. The simulation model was coupled to a genetic algorithm (GA) combined with a gradient-based (GB) method to estimate the first order decay coefficients and enrichment factors. In testing with synthetic data, the hybrid GA-GB method reduced the computational requirements for parameter estimation by a factor as great as 300. The isotopic signature profiles were observed to be more sensitive than the concentration profiles to estimates of both the first order decay constants and enrichment factors. Including isotopic data for parameter estimation significantly increased the GA convergence rate and slightly improved the accuracy of estimation of first order decay constants.     

Effects of various transport processes on the streaming ion density during the first stage of plasmaspheric refilling

The dependence of the first stage H⁺ plasmaspheric refilling density on various parameters is examined using a kinetic transport model. The first stage of refilling is defined as the time for the source cone to reach a quasi-steady-state level. Three influencing factors are examined in detail. The first two factors are actually studying numerical influences of physical phenomena. That is, the method of including these processes in the calculation is varied to determine the importance of calculational rigor. The two processes of interest are self-collisional feedback and the ambipolar electric field. The third influencing factor to be examined is the effect of coexistent energetic populations of the refilling rate. It is found that the results greatly depend on the method of incorporating self-collisions into the model, as the scattering and loss processes of the low-energy proton population interacting with itself has a significant influence on the early stage density. This interaction is particularly strong in the low-altitude region where the densities are high enough to substantially alter the distribution function. It is also found that the ambipolar electric field is the dominant force term, increasing the densities in the plasmasphere by accelerating the particles through the low-altitude scattering zone. The hot populations are found to have only a minor influence near the equatorial region, where they slow the H⁺ streams down and cause the density to slightly increase. The effects of hot ions are more pronounced in the streaming velocity and the temperature anistropy, but still confined to the equatorial region.     

Numerical simulation of water exchange processes in Lake Baikal

A review of the study of water exchange processes in Lake Baikal is given. The major attention is given to the mathematical simulation of the processes of density stratification processes and the effect of different hydrophysical factors on water exchange processes, in particular, the formation of thermobar and vertical circulations in the lake.     

The role of the tropopause in stratospheric-tropospheric exchange processes

Summary In a previous studyReiter andMahlman [37]³) have estimated the amount of stratospheric air intruding into the stable layer of the jet stream front in a case of cyclogenesis not accompanied by surface radioactive fallout. In the present report the same case is examined on a more general basis. Outflow from, as well as inflow into, the stratosphere is estimated over the entire thickness of the tropopause gap.The research reported in this paper was carried out under contract AT(11-1)-1340 with the U.S. Atomic Energy Commission.     

From precipitation to runoff: stable isotopic fractionation effect of glacier melting on a catchment scale

Stable isotope variability and fractionation associated with transformation of precipitation/accumulation to firn to glacial river water is critical in a variety of climatic, hydrological and paleoenvironmental studies. This paper documents the modification of stable isotopes in water from precipitation to glacier runoff in an alpine catchment located in the central Tibetan Plateau. Isotopic changes are observed by sampling firnpack profiles, glacier surface snow/ice, meltwater on the glacier surface and catchment river water at different times during a melt season. Results show the isotopic fractionation effects associated with glacier melt processes. The slope of the δD‐δ¹⁸O regression line and the deuterium excess values decreased from the initial precipitation to the melt‐impacted firnpack (slope from 9.3 to 8.5 and average d‐excess from 13.4‰ to 7.4‰). The slope of the δD‐δ¹⁸O line further decreased to 7.6 for the glacier runoff water. The glacier surface snow/ice from different locations, which produces the main runoff, had the same δD‐δ¹⁸O line slope but lower deuterium excess (by 3.9‰) compared to values observed in the firnpack profile during the melt season. The δD‐δ¹⁸O regression line for the river water exhibited a lower slope compared to the surface snow/ice samples, although they were closely located on the δD‐δ¹⁸O plot. Isotope values for the river and glacier surface meltwater showed little scatter around the δD‐δ¹⁸O regression line, although the samples were from different glaciers and were collected on different days. Results indicate a high consistency of isotopic fractionation in the δD‐δ¹⁸O relationships, as well as a general consistency and temporal covariation of meltwater isotope values at the catchment scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of ion exchange on dissolved load of alpine meltwaters

Meltwaters were collected in Switzerland at the border of two alpine glaciers, the Tsanfleuron glacier resting on limestones and the Tsijiore Nouve glacier flowing over gneissic rocks. Waters were analysed by atomic absorption spectrophotometry for the four major cations. Cation exchange appears to be an essential mechanism in the explanation of the dissolved cationic content of meltwaters in the frontal zone of glaciers. This is indicated by a study of the rate and characteristics of water enrichment. The rate-determining step in the process seems to be film diffusion. The influence of this diffusion is put forward in an analysis of the rates of change in concentration of the four major cations with discharge. Glacier grinding is considered as a factor favouring cation exchange in a proglacial environment.     

Isotopic fractionation of argon during stepwise release from shungite

In previous attempts to determine the⁴⁰Ar³⁶Ar ratio in the ancient atmosphere, the only direct measurement yielding a value below the atmospheric value of today is for argon released at low temperatures from a pre-Cambrian shungite, an amorphous carbon mineral [1]. The present work confirms a low value for⁴⁰Ar³⁶Ar in gas released from a type I shungite at low temperatures. But quantitative scrutiny of the accompanying³⁸Ar³⁶Ar ratios and the enhanced ratio of⁴⁰Ar³⁶Ar for the fractions released at high temperatures shows convincingly that the effect seen here is an artifact of the stepwise heating and the argon diffusion mobilized thereby. The low⁴⁰Ar³⁶Ar previously obtained is very likely from the same cause rather than reflecting the isotopic composition of the pre-Cambrian atmosphere. The vitreous character of and the sharp, conchoidal fractures seen in the specimens of type I shungite suggest that the substance may exhibit simple volume diffusion over macroscopic dimensions as glasses do. If so, the diffusion parameters (D_∞ = 3 × 10^?4cm²/s andE = 11kcal/mole) obtained from the data imply rapid exchange with the atmosphere for any argon initially trapped in centimeter-thick veins of the material.     

A theoretical investigation of tropospheric ozone and stratospheric-tropospheric exchange processes

Summary Based on the global distribution of various surface types the mean tropospheric residence time of ozone is estimated as a function of latitude. Due to the land-sea distribution varies from 50 days in the northern hemisphere to 190 days in the southern hemisphere. For the stratospheric-tropospheric exchange a sinusoidal variation with season is assumed. The annual variation of tropospheric ozone thus gets a sine function from mean, amplitude and phase of whch the injection function for the particular latitude can be determined.     

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in ³He and ⁴He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in ³He/⁴He should be correlated with larger variations (factor of 5) in ⁴He/⁴⁰Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s ⁴He/⁴⁰Ar and ³He/⁴He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of ³He relative to ⁴He and of ⁴He relative to ⁴⁰Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low ⁴He/⁴⁰Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low ⁴He/⁴⁰Ar, the complement of the high ⁴He/⁴⁰Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high ³He/⁴He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.