Gravitational Fractionation Research Articles

The influence of layering and barometric pumping on firn air transport in a 2-D model

Abstract. Ancient air trapped in ice core bubbles has been paramount to developing our understanding of past climate and atmospheric composition. Before air bubbles become isolated in ice, the atmospheric signal is altered in the firn column by transport processes such as advection and diffusion. However, the influence of low-permeability layers and barometric pumping (driven by surface pressure variability) on firn air transport is not well understood and is not readily captured in conventional one-dimensional (1-D) firn air models. Here we present a two-dimensional (2-D) trace gas advection–diffusion–dispersion model that accounts for discontinuous horizontal layers of reduced permeability. We find that layering or barometric pumping individually yields too small a reduction in gravitational settling to match observations. In contrast, when both effects are active, the model's gravitational fractionation is suppressed as observed. Layering focuses airflows in certain regions in the 2-D model, which acts to amplify the dispersive mixing resulting from barometric pumping. Hence, the representation of both factors is needed to obtain a realistic emergence of the lock-in zone. In contrast to expectations, we find that the addition of barometric pumping in the layered 2-D model does not substantially change the differential kinetic fractionation of fast- and slow-diffusing trace gases. Like 1-D models, the 2-D model substantially underestimates the amount of differential kinetic fractionation seen in actual observations, suggesting that further subgrid-scale processes may be missing in the current generation of firn air transport models. However, we find robust scaling relationships between kinetic isotope fractionation of different noble gas isotope and elemental ratios. These relationships may be used to correct for kinetic fractionation in future high-precision ice core studies and can amount to a bias of up to 0.45 °C in noble-gas-based mean ocean temperature reconstructions at WAIS Divide, Antarctica.

Origin of nelsonite and Fe–Ti oxides ore of the Damiao anorthosite complex, NE China: Evidence from trace element geochemistry of apatite, plagioclase, magnetite and ilmenite

Nelsonite and Fe–Ti oxides ore are common in Proterozoic massif-type anorthosites and layered intrusions. Their geneses have long been controversial, with existing hypotheses including liquid immiscibility between Si-rich and Fe–Ti–P-rich melts and gravitational fractionation among apatite, magnetite, ilmenite and silicates. In this paper, we report detailed field geology and mineral geochemical studies of the nelsonite and Fe–Ti oxides ore from the Damiao anorthosite complex, NE China. Geological observations indicate that the nelsonite and Fe–Ti oxides ore occur as irregularly inclined stratiform-like or lensoid or veins, and are in sharp contact with the anorthosite and gabbronorite. The widespread veins and lenses structure of the Damiao nelsonite and Fe–Ti oxides ore in the anorthosite indicates their immiscibility-derived origin. The apatite in the nelsonite and gabbronorite shows evolution trends different from that in the gabbronorite in the diagrams of Sr versus REEs and Eu/Eu*, suggesting that petrogenesis of the nelsonite and gabbronorite is different from the gabbronorite. Compared with the gabbronorite, the nelsonite and Fe–Ti oxides ore have magnetite high in Cr, plagioclase high in Sr and low in An, and apatite high in Sr, low in REEs with negative Eu anomaly. The evidence permits us to propose that the Damiao Fe–Ti oxides ore/nelsonite and gabbronorite were derived from different parental magmas. The gabbronorite was formed by solidification of the interstitial ferrodioritic magma in the anorthosite, which was the residual magma after extensive plagioclase and pyroxene crystallization and was carried upward by the plagioclase crystal mesh. In contrast, the Fe–Ti oxides ore and nelsonites and mangerite were produced by crystallization of the Fe–Ti–P-rich and SiO2-rich magmas, respectively, due to the liquid immiscibility that occurred when the highly evolved ferrodioritic magma mixed with newly replenished magmas. The variation from Fe–Ti oxides ore to nelsonite and gabbro-nelsonite upwards (as apatite content increases with height) in the steeply inclined Fe–Ti oxides orebodies suggest that gravity fractionation may have played important roles during the crystallization of the Fe–Ti–P-rich magma.

Geochemical analyses of air from an ancient debris-covered glacier, Antarctica

We examined air trapped in ancient ice from three shallow cores (<35 m deep) recovered from stagnant portions of the Mullins glacier, an 8 km long debris-covered alpine glacier in the McMurdo Dry Valleys that is overlain by several in-situ volcanic ash-fall deposits. Previously reported 40Ar/39Ar dates on ash-fall in the vicinity of the core sites average 4.0 Ma, and underlying ice is presumably as old in some areas. We analyzed the elemental and isotopic composition of O2, N2, and Ar and total air content of the glacial ice. We also dated the trapped air directly to an uncertainty of ±220 kyr (1σ) by measuring its 40Ar/36Ar and 38Ar/36Ar ratios. Our results suggest that the air analyzed is likely a mixture of ancient atmosphere trapped at the time of ice formation and more recent air introduced via cracks in the ice that penetrate to at least 33 m. The isotopic signatures of gases have been complicated by gas loss, as well as a mixture of thermal and gravitational fractionation. The oldest age estimated for the trapped air dates to 1.6 Ma, indicating that the original air is at least as old as 1.6 ± 0.2 Ma. A convergence to older ice ages with increasing depth in the deepest core analyzed (33 m) hints at the possibility that pristine air might be recovered at greater depths. Minor interstitial debris present in the glacial ice (<1%), along with geochemical evidence for in-situ microbial respiration, prohibit direct analysis of CO2. We measured the triple isotopic composition of O2 as a proxy for CO2 and infer that, in the air represented in our ice samples, CO2 concentrations are within the range observed over the last 800 ka.

GRAVITATIONAL FRACTIONATION OF ISOTOPES AND DISSOLVED COMPONENTS AS A FIRST-ORDER PROCESS IN CRUSTAL FLUIDS

Abstract The origin of ore-forming hydrothermal fluids and mechanisms of ore formation are commonly inferred from chemical and isotopic signatures. Fluid mixing and fluid-rock interaction are well-known processes that modify and produce these signatures, respectively. We propose that gravitational fractionation is an additional, hitherto not considered process that can significantly affect fluid signatures. Gravitational fractionation is the tendency for heavier components in a fluid to concentrate at the base of a reservoir. In stagnant fluids, gravitational fractionation can lead to significant changes in isotopic signatures, but also halogen ratios, within tens of millions of years. Rising fractionated fluids can give the same signatures that are classically and potentially erroneously interpreted as indicating mixing of surface-derived and deeper fluids. Recognition of gravitational fractionation as an additional first-order process affecting fluid composition necessitates a thorough reevaluation of models for crustal fluid flow, especially for the formation of unconformity-related hydrothermal ore deposits.

Glacial–interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-δ&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N measurements

Abstract. Correct estimation of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice core studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: outputs of a firn densification model, and measurements of δ15N of N2 in air trapped in ice core, assuming that δ15N is only affected by gravitational fractionation in the firn column. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi-coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available ice core air-δ15N measurements from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rates and temperature conditions. Our δ15N profiles reveal a heterogeneous response of the firn structure to glacial–interglacial climatic changes. While firn densification simulations correctly predict TALDICE δ15N variations, they systematically fail to capture the large millennial-scale δ15N variations measured at BI and the δ15N glacial levels measured at JRI and EDML – a mismatch previously reported for central East Antarctic ice cores. New constraints of the EDML gas–ice depth offset during the Laschamp event (~41 ka) and the last deglaciation do not favour the hypothesis of a large convective zone within the firn as the explanation of the glacial firn model–δ15N data mismatch for this site. While we could not conduct an in-depth study of the influence of impurities in snow for firnification from the existing datasets, our detailed comparison between the δ15N profiles and firn model simulations under different temperature and accumulation rate scenarios suggests that the role of accumulation rate may have been underestimated in the current description of firnification models.

Dynamic dissolution of halite rock during flow of diluted saline solutions

The dynamic dissolution pattern of halite salt rocks taken from coreholes near the Dead Sea was studied in laboratory‐scale experiments. When unsaturated solution (with respect to halite) flowed through salt cores, dissolution developed along preferential flow pathways in a channel structure. The channel structure was related to the salt's properties and internal heterogeneities, flow velocity and impact of gravity. Preferential dissolution pathways developed in areas of minimum resistance to flow, such as large‐pore networks and cracks. Nevertheless, in many cases no structural heterogeneity was observed along the dissolution channels prior to the experiments. The initial formation of channels took place above a critical flow velocity; below this threshold, dissolution developed as a slowly propagating front. In these cases, salt re‐precipitation resulted in clogging and cessation of flow through a few of the salt cores. Solution density was found to be important, as evidenced by the fact that more channels developed upward than downward, due to gravitational fractionation. The development of dissolution channels could have very important implications for the overall permeability of the salt layer in general, and the use of salt formations for industrial waste storage and the development of sinkholes along the Dead Sea shore in particular.

Stable carbon isotope ratio analyses on trace methane from ice samples

Measurement of the concentration and stable isotope ratios of methane from air enclosures in ancient ice can aid in understanding the temporal dynamics of methane sources and sinks. These measurements can reconstruct past atmospheric compositions on scales of tens-to-hundreds of years with adequate precision and accuracy. We present an improved micro-extraction gas-chromatography continuous-flow isotope ratio mass spectrometer (CF-IRMS) method to extract and measure the methane 13C/ 12C ratio (δ 13CH 4) of air occluded in glacial ice. The technique described uses a unique post-combustion trapping of the methane signal to produce a high amplitude sample peak increasing the signal to noise ratio of the sample. The technique requires only ca. 165 pmol of CH 4 (ca. 30–150 g of ice depending on methane concentration). The small sample requirement permits very fine temporal sampling resolution and the possibility of routine duplicate measurements from multi-parameter ice core samples. The technique was applied to atmospheric air, artificial ice samples, and samples from the GISP2 ice core. The precision of our technique was evaluated using both atmospheric air and artificial ice samples and is less than ± 0.3‰ (1σ). Our test measurements of shallow, poorer quality GISP2 ice from 225 ± 20 yr BP give a δ 13CH 4 value of − 47.37 ± 0.6‰ (corrected for gravitational fractionation and referenced to VPDB), which is in excellent agreement with other published data.

Role of general melt convection in the development of cryptic layering of intrusive complexes at cratons

The role of melt movement is discussed with reference to the development of phase and cryptic magmatic layering. Differences in the evolution of unmoving and moving melt are controlled by the corresponding types of local equilibria: mosaic and volumetric, respectively. Unmoving melt crystallizes within its marginal gradient zone containing suspension of crystals, and this process gradually extends to the whole melt volume in the chamber. Mineral grains nucleate in the homogeneous melt ahead of the crystallization front. Each crystallization center is immediately surrounded by a locally equilibrated isolated diffusion cell, and hence, only complementary gravitational fractionation of the solid phase from a melt of unchanging composition can take place in unmoving melt. This is the reason for the absence of cryptic layering in the crystallization products. Phase layering is generated by the overlap of batch fractionation with the participation of phase convection, which disturbs the even character of the former, on the continuous process of melt equilibrium crystallization within isolated diffusion cells. Moving melt undergoes in-situ fractional crystallization within a narrow marginal gradient layer. Its composition and temperature systematically change in the course of its consolidation, and this results in the cryptic layering of the crystallization products. The phase layering is controlled by the crystallization sequence of phases on the liquidus at decreasing temperature and by the self-sustained oscillatory process of nucleation in the undercooled melt in the course of cotectic crystallization. It is demonstrated that the origin of layered rock series of the Moncha Tundra and Monchegorsk massifs and one of the world’s largest Stillwater Complex was formed by the crystallization of unmoving melt. Intrusive massifs in the Imandra gabbroid complex in the Monchegorsk district and the Pilgujarvi gabbro-wehrlite complex in the Pechenga district, Kola Peninsula, the Burakovka Massif in southern Karelia, and the Bushveld Complex in South Africa were formed with the participation of overall melt convection. The compositional variations of bodies filling fractures in the root zone of the Monchegorsk Pluton were controlled by the fractional crystallization of mantle melt at the walls of magma conduits during melt ascent to upper crustal levels and provide evidence of variations in the melt velocity.

On the nature of the dirty ice at the bottom of the GISP2 ice core

We present data on the triple Ar isotope composition in trapped gas from clean, stratigraphically disturbed ice between 2800 and 3040 m depth in the GISP2 ice core, and from basal dirty ice from 3040 to 3053 m depth. We also present data for the abundance and isotopic composition of O 2 and N 2, and abundance of Ar, in the basal dirty ice. The Ar/N 2 ratio of dirty basal ice, the heavy isotope enrichment (reflecting gravitational fractionation), and the total gas content all indicate that the gases in basal dirty ice originate from the assimilation of clean ice of the overlying glacier, which comprises most of the ice in the dirty bottom layer. O 2 is partly to completely depleted in basal ice, reflecting active metabolism. The gravitationally corrected ratio of 40Ar/ 38Ar, which decreases with age in the global atmosphere, is compatible with an age of 100–250 ka for clean disturbed ice. In basal ice, 40Ar is present in excess due to injection of radiogenic 40Ar produced in the underlying continental crust. The weak depth gradient of 40Ar in the dirty basal ice, and the distribution of dirt, indicate mixing within the basal ice, while various published lines of evidence indicate mixing within the overlying clean, disturbed ice. Excess CH 4, which reaches thousands of ppm in basal dirty ice at GRIP, is virtually absent in overlying clean disturbed ice, demonstrating that mixing of dirty basal ice into the overlying clean ice, if it occurs at all, is very slow. Order-of-magnitude estimates indicate that the mixing rate of clean ice into dirty ice is sufficient to maintain a steady thickness of dirty ice against thinning from the mean ice flow. The dirty ice appears to consist of two or more basal components in addition to clean glacial ice. A small amount of soil or permafrost, plus preglacial snow, lake or ground ice could explain the observations.

Controlling the Formation of Silver Nanoparticles on Silica by Photochemical Deposition and Other Means†

Silver nanoparticles (Ag-NP) on silica were produced in aqueous solution by deposition of silver on colloidal silica in a small cuvette using radiation from a xenon-mercury lamp. Ag-NP were also synthesized on a much larger scale with low-level, long-term visible light irradiation for several months. In both cases, the nanoparticle production was monitored by the appearance of the surface plasmon resonance (SPR) band at around 410 nm. The growth of the nanoparticles was directly related to the time exposed to radiation, which could be tracked spectrophotometrically over time. We also investigated the possibilities of rapid nanoparticle production without the assistance of radiation though silver oxide by adding alkali hydroxide, which is a relatively unexplored approach for syntheses of Ag-NP on silica. The SPR absorption of Ag-NP was used as a tool in evaluating the size and shape of the resulting nanoparticles along with dynamic light scattering and transmission electron microscopy data. In order to better utilize and understand Ag-NP, we present various ways to control their production through initial concentration adjustments, irradiation effects, gravitational fractionation, sonication and silver oxide formation.

Effect of Reaction Parameters on Size Distribution of Emulsion-Polymerized Polystyrene Latex Beads Studied by Gravitational Flow-Flow Fractionation (GrFFF)

A gravitational field-flow fractionation (GrFFF) was employed to study the influence of various reaction parameters in emulsion polymerization on size distribution of polymeric latex beads. Micron-sized polystyrene (PS) latex beads were synthesized by conventional emulsion polymerization. The reaction parameters such as the stirring rate (rpm), reaction time, concentration of azobisisobutyronitrile (AIBN) and polyvinylpyrrolidone (PVP), were systematically varied. Then the PS beads were analyzed by GrFFF for size determination. It was found that the mean diameter and the size distribution of the PS latex beads change as some of the reaction parameters change. No clear trends were observed in the mean diameter and the coefficient of variation (CV) with the stirring rate in the range of 60 to 300 rpm, or with the reaction time in the range of 13 to 24 hrs. A clear trend of increase in the mean diameter with the AIBN concentration was observed, suggesting the particle size can be controlled by varying the AIBN concentration without affecting the size distribution much. It was also found that the mean diameter tends to decrease as the PVP concentration increases while the CV tends to increase. Results obtained from GrFFF were compared with those obtained from photon-correlation spectroscopy (PCS) and optical microscopy (OM).

A gas chromatography/combustion/isotope ratio mass spectrometry system for high‐precision δ13C measurements of atmospheric methane extracted from ice core samples

Past atmospheric composition can be reconstructed by the analysis of air enclosures in polar ice cores which archive ancient air in decadal to centennial resolution. Due to the different carbon isotopic signatures of different methane sources high-precision measurements of delta13CH4 in ice cores provide clues about the global methane cycle in the past. We developed a highly automated (continuous-flow) gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) technique for ice core samples of approximately 200 g. The methane is melt-extracted using a purge-and-trap method, then separated from the main air constituents, combusted and measured as CO2 by a conventional isotope ratio mass spectrometer. One CO2 working standard, one CH4 and two air reference gases are used to identify potential sources of isotope fractionation within the entire sample preparation process and to enhance the stability, reproducibility and accuracy of the measurement. After correction for gravitational fractionation, pre-industrial air samples from Greenland ice (1831 +/- 40 years) show a delta13C(VPDB) of -49.54 +/- 0.13 per thousand and Antarctic samples (1530 +/- 25 years) show a delta13C(VPDB) of -48.00 +/- 0.12 per thousand in good agreement with published data.

Comparison of Gravitational SPLITT Fractionation with Gravitational Settling for Separating Micron Size Particles

The performance of gravitational Split‐flow thin‐channel (GrSPLITT) fractionation has been compared with conventional gravitational settling (GrSettling) method. Two modes of GrSPLITT (full‐feed depletion (FFD) mode and transport (TS) mode) were employed. Silica and natural river particles were used to test these techniques. The particles were fractionated at the following cutoff diameters (d c ) 2, 4, 6, 10, 14, and 20 µm. The fractions with diameter <d c were analyzed for their particle size distribution by both optical or scanning electron microscopy and image analysis. Several parameters were used to objectively compare the efficiency of the methods. Almost all of the tests showed that TS‐GrSPLITT had the highest separation efficiency. The ranking of FFD‐GrSPLITT and GrSettling were considered to be of equal ranking as the order varied for the different parameters and samples used.

A microscopic and chemical study of airborne coarse particles with particular reference to sea salt in chile at 30° s

Along a transect ranging from the Pacific to the summit of Cerro Tololo at 2200 m above sea level (a.s.l.) in Chile at 30 °S, coarse particle concentration was measured at three sites in a one-year study from December 2002 to 2003. The climate of this area is characterised by semi-arid conditions. Intensive thermally induced valley winds occurring particularly in summer, possibly transporting polluted air masses form the coastal area into the mountain region. This may question the background characteristics of Cerro Tololo as suitable site in Chile for a monitoring station of the Global Atmospheric Watch (GAW) program. Particle sampling by a passive sampling technique was employed to analyse composition of particles, find out effective sources and trace possible transport of suspended particles in this area. Temporal resolution of sampling was one week. Composition and size distribution of weekly deposition samples were determined by microscopic single particle analysis and bulk ionchromatographic analysis. Coarse particle concentration decreases differently along the transect as a function of occurring sources and depending on reactivity of components. The percentage of a black particle component remains at the same level of about 4% at all the three sites, confirming biomass burning as a wide-spread pollution. The percentage of sea salt, by contrast, amounts on average about 50% of total coarse particle concentration at the coastal area, quickly decreasing to about 10% along the transect due to losses by particle deposition and chemical reaction during transport. Ammonium shows a different behaviour, characterised by enhanced values at the central part of the transect, due to emissions from agricultural activities in this area. A shift in size distribution between the different sites is explained not only by gravitational fractionation but also by chemical conversion during transport. As a result, it is concluded that an impact of contaminated air at Cerro Tololo cannot be ruled out at a low level particularly in summer. However, the amount, composition and size distribution of the coarse fraction rather confirms the pristine area characteristics of Cerro Tololo and its background conditions.

Evidence of microbial consortia metabolizing within a low-latitude mountain glacier

Carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ) concentrations of ice samples from a South American mountain glacier are larger than those in coeval polar ice cores. Of the six possible mechanisms that may cause the composition of air measured in ice cores to depart from that of the overlying troposphere (gravitational fractionation, thermal fractionation, gases trapped in refrozen meltwater, atmospheric heterogeneity, abiotic chemical reactions, and biologically mediated chemical reactions), only metabolism of a consortium of microorganisms within the ice can explain our data.

Isotopic Fractionation by Gravitational Escape

Present natural data bases for abundances of the isotopic compositions of noble gases, carbon and nitrogen inventories can be found in the Sun, the solar wind, meteorites and the planetary atmospheres and crustal reservoirs. Mass distributions in the various volatile reservoirs provide boundary conditions which must be satisfied in modelling the history of the present atmospheres. Such boundary conditions are constraints posed by comparison of isotopic ratios in primordial volatile sources with the isotopic pattern which was found on the planets and their satellites. Observations from space missions and Earth-based spectroscopic telescope observations of Venus, Mars and Saturn’s major satellite Titan show that the atmospheric evolution of these planetary bodies to their present states was affected by processes capable of fractionating their elements and isotopes. The isotope ratios of D/H in the atmospheres of Venus and Mars indicate evidence for their planetary water inventories. Venus’ H2O content may have been at least 0.3% of a terrestrial ocean. Analysis of the D/H ratio on Mars imply that a global H2O ocean with a depth of ≤ 30 m was lost since the end of hydrodynamic escape. Calculations of the time evolution of the 15N/14N isotope anomalies in the atmospheres of Mars and Titan show that the Martian atmosphere was at least ≥ 20 times denser than at present and that the mass of Titan’s early atmosphere was about 30 times greater than its present value. A detailed study of gravitational fractionation of isotopes in planetary atmospheres furthermore indicates a much higher solar wind mass flux of the early Sun during the first half billion years.

Gravitational Enrichment of 84 Kr/ 36 Ar Ratios in Polar Ice Caps: A Measure of Firn Thickness and Accumulation Temperature

Measurements of 84 Kr/ 36 Ar ratios in Greenland ice show that gravitational separation in the firn layer is responsible for the enrichments relative to atmospheric ratios. The 84 Kr/ 36 Ar ratio is enriched by 12.8 per mil and is 24 times the 18 O/ 16 O enrichment in trapped O 2 , as predicted for gravitational fractionation. Because gravitational enrichment depends on firn thickness, which in turn depends on annual mean temperature, noble gas ratios provide a method for determining paleotemperatures and ancient firn thicknesses in polar ice caps. The gravitational effects are modulated by about 10 to 15 percent by atmospheric concentration changes caused by temperature effects on oceanic gas solubilities. The availability of five noble gases should make it possible to deconvolute the solubility and gravitational enrichments for calibration of 18 O paleotemperatures throughout the polar ice sheets.

Fractionation of soil gases by diffusion of water vapor, gravitational settling, and thermal diffusion

Air sampled from the moist unsaturated zone in a sand dune exhibits depletion in the heavy isotopes of N 2 and O 2. We propose that the depletion is caused by a diffusive flux of water vapor out of the dune, which sweeps out the other gases, forcing them to diffuse back into the dune. The heavy isotopes of N 2 and O 2 diffuse back more slowly, resulting in a steady-state depletion of the heavy isotopes in the dune interior. We predict the effect's magnitude with molecular diffusion theory and reproduce it in a laboratory simulation, finding good agreement between field, theory, and lab. The magnitude of the effect is governed by the ratio of the binary diffusivities against water vapor of a pair of gases, and increases ~linearly with the difference between the water vapor mole fraction of the site and the advectively mixed reservoir with which it is in diffusive contact (in most cases the atmosphere). The steady-state effect is given by δ i = [ i / j i o / j o − 1 ] 10 3 ‰ ≅ [ ( 1 − x H 2 O 1 − x H 2 O 0 ) ( D j − H 2 O / D i − H 2 O ) − 1 − 1 ] 10 3 ‰ , where δ i is the fractional deviation in permil of the gas i/gas j ratio from the advectively mixed reservoir, x H 2O and x H 2O 0 are respectively the mole fractions of water vapor at the site and in the advectively mixed reservoir, and D i−H 2O is the binary diffusion coefficient of gas i with water vapor. The effect is independent of scale at steady state, but approaches steady state with the time constant of diffusion set by the length scale. Exploiting the mechanism, we make an experimental estimate of the relative diffusivities of O 2 and N 2 against water vapor, finding that O 2 diffuses 3.6 ± 0.3% faster than N 2 despite its greater mass. We also confirm in the study dune the presence of two additional known processes: gravitational fractionation, heretofore seen only in the unconsolidated firn of polar ice sheets, and thermal diffusion, well described in laboratory studies but not seen previously in nature. We predict that soil gases in general will exhibit the three effects described here, the water vapor flux fractionation effect, gravitational fractionation, and thermal diffusion. However, our analysis neglects Knudsen diffusion and thus may be inapplicable to fine-grained soils.

広島花こう岩―巨大な垂直方向累帯深成岩体

The Hiroshima Granite is Late Cretaceous large batholithic intrusion in Sanyo belt of southwestern Japan. It is composed of the following rock types; quartz diorite (QD), medium-grained hornblende biotite granite (MG), medium to coarse-grained biotite granite (CG) and fine-grained biotite granite (FG). QD occurs as xenolith in MG. The main parts are composed of upper CG and lower MG, and CG intrudes into MG. FG, the latest stage intrusion, is located at the upper most part beneath the roof. Thus, the Hiroshima Granite is vertically zoned. SiO2 decreases from 76 to 71 wt% downward from the roof. This vertical zonation may have been firstly formed by gravitational fractionation. Then, more differentiated liquid moved under the roof and resulted in the upper most FG.

Changes in tropospheric methane between 1841 and 1978 from a high accumulation-rate Antarctic ice core

To determine in detail how the concentration of tropospheric methane has changed from preindustrial until recent times, an ice core with remarkably fine air-age resolution was investigated. The core, called DE08, contains air from as recent as 1978 with an age resolution (80% air-age distribution width) of about 14 years. It was drilled from a region of Law Dome, Antarctica with extremely high snow-accumulation rate (1160 kg m -2 yr -1 ). The ice chronology was determined from the observed chemical and isotopic seasonal variations, verified against a volcanic horizon. The calculated air-age includes the effects of bubble close-off, sealing of the firn and diffusive mixing of air in the firn. The mean air-age was 35 years younger than the host ice except for air in summer ice layers which was 37 years younger than the host ice. The extracted ice-core air was analysed for methane using gas chromatography with flame-ionisation detection. Adjustments of – 6 ppbv and + 0.3% were made to the measured concentrations to allow for the effects of the extraction process and gravitational fractionation, respectively. Methane concentrations in the DE08 record increased from 823 parts per billion by volume (ppbv, in dry air) in 1841 to 1481 ppbv in 1978. The measurement precision was ± 22 ppbv (1σ). The similarity of the methane records from the DE08 ice core and from Cape Grim, Tasmania implies that there was insignificant modification during the enclosure of air in the ice or during its recovery and analysis. Methane concentrations in the period from 1951 to 1978, which were previously estimated from sporadic and inferred data, are particularly well defined in this core. The DE08 record shows that methane growth rates have generally increased since the onset of the industrial revolution to a level of 14 ppbv year -1 (about 1% per year) by the 1970s. The exception was between about 1920-1945 when the growth rate stabilised at about 5 ppbv year -1 . DOI: 10.1034/j.1600-0889.1992.t01-3-00006.x