Etude pedogenetique et isotopique des neoformations de calcite dans un sol sur craie: Caracteristiques et origines     

L. Dever  R. Durand  J.Ch. Fontes  P. Vachier 《Geochimica et cosmochimica acta》1983,47(12):2079-2090

Cryoturbated facies are found at the boundary between soil horizons and Cretaceous chalk. Several types of secondary calcite appear in soil horizons: orange coloured and rounded (partially dissolved) nodules, deeply coloured angular aggregates, transparent isolated rhombs and polycrystalline nodules, needles. The carbon and oxygen isotope compositions of these calcites are correlated: δ¹³C = 4.9δ¹⁸O_PDB + 15.9End members of this correlation are the orange rounded nodules (δ¹³C ? + 8%., $\text{δ}{}^{18}\text{O ? ?1.5‰}$) and the transparent angular polycrystalline nodules (δ¹³C ? ?13; δ¹⁸O ? ?6).Partially dissolved nodules have formed under periglacial climatic conditions. Crystallisation would have occurred under the following (equilibrium) environmental conditions: δ¹⁸O_SMOW (soil solution) ?7, δ¹³C (gaseousCO₂) ? ?5.2, t ? ?2°C. Soil solution was enriched in ¹⁸O by evaporation and atmospheric CO₂ was enriched in ¹³C as compared to present day. Transparent polycrystalline nodules are compatible with present day environmental conditions: δ¹⁸O (soil solution) ranging from ?9 to ?4 and δ¹³C (soil CO₂) ranging from ?24.5 to ?23. These nodules crystallize between May and October at soil temperatures ranging from 10 to 25°C, from evaporated soil solutions. Angular coloured aggregates may form under present day winter conditions for temperatures between 0 and 10°C. However they may also result from present accretion of fragments of periglacial nodules.All recent secondary calcite results from CO₂ degassing and evaporation of soil solutions. Degassing is controlled by the gradient of CO₂ partial pressure within the soil profile. During winter this gradient is low and the resulting calcite precipitation is not significant. During summer a large difference in pCO₂ appears between the root zone and deep soil horizons. The degassing accounts for an increase of about 2‰ in δ¹³C of the total dissolved inorganic carbon and of the related solid carbonate. Evaporation is the main driving force for secondary calcite precipitation.  相似文献   

$\text{sol}{}^{18}\text{O}{}^{16}\text{O}$ of Archean clastic metasedimentary rocks: a petrogenetic indicator for Archean gneisses?     

Frederick J. Longstaffe  Henry P. Schwarcz 《Geochimica et cosmochimica acta》1977,41(9):1303-1312

The oxygen isotopic compositions of over 100 Archean clastic metasedimentary, felsic metavolcanic and gneissic rocks from selected areas within the Superior Province have been determined. δ¹⁸O values of low grade, immature clastic metasediments range from 8.0 to 13.3%. and hence are somewhat depleted in ¹⁸O relative to other clastic metasediments. The lower ¹⁸O content is attributed to the large proportion of unaltered rock fragments in the Archean metasediments. Felsic metavolcanics have a similar range of δ¹⁸O values (7.3 to 13.0%..δ¹⁸O values for the middle amphibolite facies Pakwash paragneiss (8.8–11.7%.) are higher than those determined for the amphibolite to granulite facies Twilight paragneiss (7.3–9.2%.). Archean orthogneisses such as the Clay Lake gneiss (7.0–9.0%.), the Cedar Lake gneiss (7.4–8.7%.) and the Footprint gneiss (5.9-8.8%.) have δ¹⁸O values similar to those of the Twilight paragneiss. Therefore, at least at metamorphic grades higher than middle amphibolite facies, Archean metasedimentary gneisses may have δ¹⁸O values that are indistinguishable from those of Archean orthogneisses, and hence are no longer isotopically recognizable as metasedimentary rocks.  相似文献   

Stable isotope geochemistry of deep sea cherts   总被引：1，自引：0，他引：1  

Yehoshua Kolodny  Samuel Epstein 《Geochimica et cosmochimica acta》1976,40(10):1195-1209

Seventy four samples of DSDP recovered cherts of Jurassic to Miocene age from varying locations, and 27 samples of on-land exposed cherts were analyzed for the isotopic composition of their oxygen and hydrogen. These studies were accompanied by mineralogical analyses and some isotopic analyses of the coexisting carbonates. δ¹⁸O of chert ranges between 27 and 39%. relative to SMOW, δ¹⁸O of porcellanite—between 30 and 42%.. The consistent enrichment of opal-CT in porcellanites in ¹⁸O with respect to coexisting microcrystalline quartz in chert is probably a reflection of a different temperature (depth) of diagenesis of the two phases.δ¹⁸O of deep sea cherts generally decrease with increasing age, indicating an overall cpoling of the ocean bottom during the last 150 m.y. A comparison of this trend with that recorded by benthonic foraminifera (Douglas and Savin, 1975) indicates the possibility of δ¹⁸O in deep sea cherts not being frozen in until several tens of millions of years after deposition. Cherts of any Age show a spread of δ¹⁸O values, increasing diagenesis being reflected in a lowering of δ¹⁸O. Drusy quartz has the lowest δ¹⁸O values.On-land exposed cherts are consistently depleted in ¹⁸O in comparison to their deep sea time equivalent cherts.Water extracted from deep sea cherts ranges between 0.5 and 1.4 wt %. δD of this water ranges between ?78 and ?95%. and is not a function of δ¹⁸O of the cherts (or the temperature of their formation).  相似文献   

Oxygen and hydrogen isotopic studies of ore deposition and metamorphism at the Raul mine,Peru     

Edward M Ripley  Hiroshi Ohmoto 《Geochimica et cosmochimica acta》1979,43(10):1633-1643

The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ¹⁸O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ¹⁸O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ¹⁸O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ¹⁸O_H2O and δD_H2O values range from 8 to 12%. and ?3 to +42%., respectively.δ¹⁸O_H2O values calculated from δ¹⁸O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ¹⁸O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are ¹⁸O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ¹⁸O_H2O and δD_H2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. ¹⁸O-enrichment in andesites may be produced by isotopic exchange with high ¹⁸O waters at elevated temperatures and sufficiently high water/rock ratios.  相似文献   

The isotopic composition of nitrate produced from nitrification in a hardwood forest floor   总被引：1，自引：0，他引：1  

J. Spoelstra  S.L. Schiff  D.S. Jeffries 《Geochimica et cosmochimica acta》2007,71(15):3757-3771

Dual isotopic analysis of nitrate (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) is increasingly used to investigate the environmental impacts of human-induced elevated atmospheric nitrate deposition. In forested ecosystems, the nitrate found in surface water and groundwater can originate from two sources: (1) atmospheric deposition, and (2) nitrate produced from nitrification in forest soils (microbial nitrate). Application of the dual nitrate isotope technique for determining the relative importance of nitrate sources in forested catchments requires knowledge of the isotopic composition of microbial nitrate. We excluded precipitation inputs to three zero-tension lysimeters installed below the F-horizon (Oe) at the Turkey Lakes Watershed (TLW) in order to measure the isotopic composition of microbial nitrate produced in situ. To our knowledge, this is the first in situ study of the isotopic composition of microbial nitrate in forest soils. Over a 2-week period, nitrate produced by nitrification was periodically flushed to the lysimeters by watering the area with a nitrogen-free solution. Nitrate produced in the forest floor had δ¹⁸O values ranging from +3.1‰ to +10.1‰ with a mean of +5.2‰. These values were only slightly higher than from the expected value of +1.0‰ calculated for chemolithoautotrophic nitrification, which depends on the δ¹⁸O of available O₂ and H₂O. In addition to nitrate, we also collected soil gas to determine if soil respiration and O₂ diffusion affected soil gas δ¹⁸O-O₂, which is typically assumed to be identical to atmospheric O₂ (+23.5‰) when calculating microbial nitrate δ¹⁸O values. No significant difference in δ¹⁸O-O₂ from atmospheric O₂ was found in forest soils to a depth of 55 cm, and therefore ¹⁸O-enrichment of soil gas O₂ could not explain the modest enrichment of nitrate ¹⁸O. Evaporative ¹⁸O-enrichment of soil water available to nitrifiers in the forest floor is a plausible mechanism for slightly elevated nitrate δ¹⁸O values. However, the observed nitrate δ¹⁸O values could also be explained by a minor contribution of nitrate from heterotrophic nitrifiers. The δ¹⁵N of nitrate produced ranged from −10.4 to −7.3‰ and, as expected, was depleted in ¹⁵N relative to soil organic nitrogen. Microbial nitrate produced in the forest floor was also significantly depleted in ¹⁵N relative to microbial nitrate exported in groundwater and headwater streams at the TLW. We hypothesize that ¹⁵N-depleted forest floor nitrate is not detected in groundwaters largely because of: (1) the immobilization of forest floor nitrate in the mineral soil and (2) the mixing of the remaining forest floor nitrate with nitrate generated in the mineral soil, which is expected to have higher δ¹⁵N values. This study demonstrates that current methods of calculating a priori the δ¹⁸O of microbial nitrate provide a reasonable value for nitrate produced by nitrification at the TLW.  相似文献   

O18O16 of feldspars in carbonate rocks     

J.R Lawrence  M Kastner 《Geochimica et cosmochimica acta》1975,39(2):97-102

Six authigenic feldspars and three detrital feldspars in limestones and dolostones of Eocene to Preeambrian ages were analyzed for their $\text{O}{}^{18}\text{O}{}^{16}$ content. The difference in δO¹⁸ between the authigenic feldspars (δO¹⁸range = + 18.2 to + 24.7%.) and carbonate host rocks, both limestones and dolostones, was found to be ?0.5 to ?1.4%. Detrital feldspars (δO¹⁸ = + 11.2, + 22.5 and + 17.0%.) exhibit Δ^feldspar_carbonate values of ?12.0, ?2.4 and ?1.6‰, respectively, and appear to have undergone increased isotopic exchange as a function of decreased grain size under solid-state conditions.  相似文献   

Kiglapait geochemistry VI: Oxygen isotopes     

Ruth I. Kalamarides 《Geochimica et cosmochimica acta》1984,48(9):1827-1836

The Kiglapait layered intrusion is the first major intrusion found to have all whole rock and calculated liquid δ¹⁸O values close to a normal uncontaminated gabbroic value of 6.0. The intrusion experienced no detectable oxygen isotope exchange with its surrounding rocks and cooling of the magma was conductive. The δ¹⁸O values of average whole rocks vary smoothly from 6.0 at the base of the Lower Zone to 6.3 at the top of the Upper Zone. The calculated liquid δ¹⁸O values lie practically superimposed on the whole rock trend. The whole-rock data and the modelled δ¹⁸O of the magma and cumulates rigorously demonstrate that the effect of incoming cumulus phases such as magnetite and augite on the δ¹⁸O of the liquid and rocks during fractional crystallization is negligible. The cancelling effects of complementary modal variations among the mafic mineral phases and feldspar, keep the δ¹⁸O of the whole rocks constant to within ±0.1 %.. The minor change in δ¹⁸O that does occur with fractionation is consistent with the enrichment of residual liquids in feldspar component and the increasing fractionation factor δ Liquid-Fsp with falling temperature.The δ¹⁸O values of the country rocks bracket the estimated δ¹⁸O of the Kiglapait magma. Modelling with oxygen isotopes indicates that contamination of the intrusion, indicated by published radiogenic Sr and Nd isotopic data, was minor. The most probable contaminant had δ¹⁸O?7.7 and the contamination most likely occurred at >99% solidified. Subsolidus oxygen isotope exchange with an external source appears to have been very minor.  相似文献   

The formation of caliche in soils of the Mojave Desert,California     

William H. Schlesinger 《Geochimica et cosmochimica acta》1985,49(1):57-66

Radiocarbon and ²³⁰Th-²³⁴U dates of calcic horizons from calciorthid soil profiles in the Mojave Desert were used to calculate the rate of deposition of pedogenic CaCO₃. A major period of CaCO₃ deposition appears to have occurred about 20000 yBP forming calcic horizons below 100-cm depth during a climatic regime with greater effective rainfall than in the present. The overall rate of deposition has been 1.0 to 3.5 g CaCO₃/m²/yr during soil formation. This rate is consistent with present-day rates, assuming that the atmospheric deposition of Ca limits the process. Stable isotope ratios in calcic horizons indicate that CaCO₃ precipitated from a soil environment with CO₂ of ? 15.5%. ${}^{13}\text{C}{}^{12}\text{C}$ (vs. PDB) and H₂O of + 2.0%. ${}^{18}\text{O}{}^{16}\text{O}$ (vs. SMOW). These values suggest that CaCO₃ precipitates when seasonal drought simultaneously lowers soil pore pCO₂ and enriches soil water ¹⁸O by evaporation. The role of soil calcic horizons in the global geochemical cycle of carbon is discussed.  相似文献   

The fate of nitrogen and sulfur in hard-rock aquifers as shown by sulfate-isotope tracing     

Hélène Pauwels  Virginie Ayraud-Vergnaud  Luc Aquilina  Jérôme Molénat 《Applied Geochemistry》2010

Stable SO₄ isotopes (δ³⁴S_-SO4 and δ¹⁸O_-SO4), and more occasionally δ¹⁵N_-NO3 were studied in groundwater from seven hard-rock aquifer catchments. The sites are located in Brittany (France) and all are characterized by intensive agricultural activity. The purpose of the study was to investigate the potential use of these isotopes for highlighting the fate of both SO₄ and NO₃ in the different aquifer compartments. Nitrate-contaminated groundwater occurs in the regolith; δ³⁴S fingerprints the origin of SO₄, such as atmospheric deposition and fertilizers, and δ¹⁸O_-SO4 provides evidence of the cycling of S within soil. The correlation between the δ¹⁸O_-SO4 of sulfates and the δ¹⁵N_-NO3 of nitrates suggests that S and N were both cycled in soil before being leached to groundwater. Autotrophic and heterotrophic denitrification was noted in fissured aquifers and in wetlands, respectively, the two processes being distinguished on the basis of stable SO₄ isotopes. During autotrophic denitrification, both δ³⁴S_-SO4 and δ¹⁸O_-SO4 decrease due to the oxidation of pyrite and the incorporation of O from the NO₃ molecule in the newly formed SO₄. Within wetlands, fractionation occurs of O isotopes on SO₄ in favour of lighter isotopes, probably through reductive assimilation processes. Fractionation of S isotopes is negligible as the redox conditions are not sufficiently reductive for dissimilatory reduction. δ³⁴S_-SO4 and δ¹⁸O_-SO4 data fingerprint the presence of a NO₃-free brackish groundwater in the deepest parts of the aquifer. Through mixing with present-day denitrified groundwater, this brackish groundwater can contribute to significantly increase the salinity of pumped water from the fissured aquifer.  相似文献   

Laser oxygen isotope analysis of weathering goethite from the lateritic profile of Yaou,French Guiana: paleoweathering and paleoclimatic implications     

《Applied Geochemistry》1997,12(2):163-174

Oxygen isotopic compositions of weathering goethite pseudomorphs after pyrite in the lateritic profile of Yaou, French Guiana, were measured by laser fluorination. The laterite at Yaou is composed of a 25 m thick saprolite overlain by a 3 m thick latosol. Petrographic observations indicate that the pseudomorphic replacement of pyrite by goethite occurs at or near the weathering front and is complete. The goethite pseudomorphs are essentially devoid of Al and are progressively dissolved and partially replaced by a microcrystalline plasma of kaolinite and goethite in the upper horizons (latosol) of the profile.The laser technique used in this study permitted the determination of the δ¹⁸O value of individual grains of goethite and the investigation of grain-to-grain variation within a single sample (horizon) as well as vertical variation with depth. In the latosol, the goethite δ¹⁸O values range from 0.8 to 3.2%‰ and exhibit significant grain-to-grain and vertical variation. This most likely reflects undetected intragrain contamination with microcrystalline kaolinite. In the saprolite, the goethite δ¹⁸O values are consistent, ranging from 1.0 to 1.8%‰ between 3 and 18 m depth, and from 2.5 to 3.3‰ between 23 m depth and the weathering front at 28 m. Oxygen isotope compositions of present groundwater, “soil water” and rainfall at Yaou were also measured, and compared with calculated isotopic compositions for goethite-forming waters. Goethite pseudomorphs in the lower part of the saprolite are in isotopic equilibrium with present groundwater, indicating that they may be forming today or that they formed in the recent past under climatic conditions similar to present conditions. In contrast, goethite pseudomorphs found higher in the profile are not in isotopic equilibrium with present water but with a water depleted in¹⁸O by ≈ 1.50% relative to present groundwater. It is suggested that these low-δ¹⁸O pseudomorphs are probably older and formed under different paleoclimatic conditions, at a time when climatic regimes were possibly more monsoonal than today. This is in agreement with models of climatic evolution derived from pedological considerations.The results of this study suggest that goethite pseudomorphs in ancient saprolites may preserve their original O isotopic composition acquired at the weathering front, and may, therefore, be used as indicators of changes in weathering-climatic conditions during the evolution of a profile.  相似文献   

Continental-scale patterns in modern wood cellulose δO: Implications for interpreting paleo-wood cellulose δO     

S.L. Richter  A.H. Johnson  M.M. Dranoff  K.D. Taylor 《Geochimica et cosmochimica acta》2008,72(12):2735-2743

The isotopic composition of ancient wood may be a useful archive of some climatic or geochemical conditions of the past, but presently there are many uncertainties that constrain such interpretations. We sampled naturally growing, predominantly native trees in forested regions of North America and the Caribbean to evaluate the strength of the relationships among cellulose δ¹⁸O (δ¹⁸O_cel), relative humidity (RH), precipitation δ¹⁸O (δ¹⁸O_ppt), and mean annual temperature (MAT) at the continental scale, and the general range of variability in δ¹⁸O_cel associated with site hydrologic conditions and species differences. We found up to 4‰ differences among different species growing at the same site, that conifer cellulose at a site is more enriched than angiosperm cellulose by 1.5‰ (p < 0.001), and that differences in landscape position, reflecting differing access to the water table, produced differences of <1‰ in δ¹⁸O_cel. At the continental scale, δ¹⁸O_cel was strongly influenced by modeled δ¹⁸O_ppt (R² = 0.80, p < 0.001). Average summer minimum RH (RH_min) combined with δ¹⁸O_ppt explained more of the variability (R² = 0.93, p < 0.001) in δ¹⁸O_cel across North American and Caribbean forests. MAT and δ¹⁸O_cel were also strongly correlated across North America (R = 0.91 and 0.95, p < 0.001, for angiosperms and conifers, respectively). The difference between δ¹⁸O_ppt and δ¹⁸O_cel is not constant (varying from 35-44‰) and is inversely correlated with δ¹⁸O_ppt. The relationships among δ¹⁸O_ppt, RH_min, δ¹⁸O_cel, and MAT established for North America and the Caribbean applied reasonably well when δ¹⁸O_cel was used to estimate MAT and δ¹⁸O_ppt in Asia, Europe, and South America, but there were important exceptions. The most accurate predictions of MAT and δ¹⁸O_ppt from δ¹⁸O_cel require RH_min. Predictions of δ¹⁸O_ppt and MAT made from δ¹⁸O_cel alone produced errors of up to 8‰ and 16 °C, respectively.  相似文献   

The use of stable sulfur and oxygen isotope ratios for interpreting the mobility of sulfate in aerobic forest soils     

《Applied Geochemistry》1995,10(2):161-173

The isotope compositions of sulfate in bulk precipitation near Munich (Germany) and of seepage water and soil sulfate in five acid forest soils representative of southern Germany were determined in order to ascertain the sources and dynamics of sulfur. While the δ³⁴ S-values of inorganic sulfate in soil solution and solid phases were found to be nearly identical to those of precipitation sulfate, a depletion of several per mil was observed for the δ¹⁸ O-values of sulfate within the uppermost 30 cm of the investigated soils. Mineralization of carbon-bonded sulfur to SO₄²⁻ in the forest floor and humic mineral soil horizons is the only known process which can explain the observed shifts in δ¹⁸O_sulfate. The fact that this¹⁸O-depleted sulfate recharges the groundwater under forests must be considered, when sulfur and oxygen isotope data of sulfate are used for interpretations of the past geochemistry of groundwater systems.Since the δ³⁴S-values of precipitation sulfate were barely altered during percolation through the soils, sulfate mobilities were inferred from a lysimeter experiment with undisturbed soil cores from the same sites, using the stable isotope composition of the irrigation sulfate as a tracer. Fifteen cores of each of the five forest soils, were repeatedly irrigated over 20 months with³⁴S- and¹⁸O-enriched sulfate in three different treatments (35, 63, and 131 kg S ha⁻¹ respectively). Despite the fact that the mean residence time of the seepage water was of the order of only a few months, the throughput of irrigation sulfate did not exceed 34% for all soils and irrigation treatments during the experiment. The low recovery of irrigation sulfate in the seepage water implies mean residence times for sulfur in the uppermost 60 cm of the forest soils of the order of decades, much longer than previously suggested.  相似文献   

A review of diatom δ18O in palaeoceanography     

George E.A. Swann  Melanie J. Leng 《Quaternary Science Reviews》2009,28(5-6):384-398

  相似文献   

Stable isotope studies of fluid inclusions in speleothems and their paleoclimatic significance     

Henry P Schwarcz  Russell S Harmon  Peter Thompson  Derek C Ford 《Geochimica et cosmochimica acta》1976,40(6):657-665

Fluid inclusions found trapped in speleothems (cave deposited travertine) are interpreted as samples of seepage water from which enclosing calcium carbonate was deposited. The inclusions are assumed to have preserved their D/H ratios since the time of deposition. Initial ¹⁸O/¹⁶O ratios can be inferred from δD because rain- and snow-derived seepage waters fall on the meteoric water line (δD = 8δ¹⁸O + 10). Estimates of temperature of deposition of the carbonate can be calculated from inclusion D/H ratios and δ¹⁸O of enclosing calcite in Pleistocene speleothems. For most speleothems investigated (0–200,000 yr old) δ¹⁸O of calcite appears to have decreased with increasing temperature of deposition indicating that the dominant cause of climate-dependent change in δ¹⁸O of calcite was the change in K_cw, the isotope fractionation equilibrium constant, with temperature; δ¹⁸O of meteoric precipitation generally increased with increasing temperature, but not sufficiently to compensate for the decrease in K_cw.  相似文献   

The transfer of oxygen isotopic signals from precipitation to drip water and modern calcite on the seasonal time scale in Yongxing Cave,central China     

Quan Wang  Yongjin Wang  Kan Zhao  Shitao Chen  Dianbing Liu  Zhenqiu Zhang  Wei Huang  Shaohua Yang  Yijia Liang 《Environmental Earth Sciences》2018,77(12):474

Stable isotope data of precipitation (δ¹⁸O_p and deuterium excess), drip water (δ¹⁸O_d), and modern calcite precipitates (δ¹⁸O_c and δ¹³C_c) from Yongxing Cave, central China, are presented, with monthly sampling intervals from June 2013 to September 2016. Moderate correlations between the monthly variation of δ¹⁸O_p values (from ??11.5 to ??0.7‰) and precipitation amount (r = ??0.59, n?=?34, p?<?0.01) and deuterium excess (r?=?0.39, n?=?31, p?<?0.01) imply a combined effect of changes in precipitation amount and atmospheric circulation. At five drip sites, the δ¹⁸O_d values have a much smaller variability (from ??9.1 to ??7.5‰), without seasonal signals, probably a consequence of the mixing in the karst reservoir with a deep aquifer. The mean δ¹⁸O_d value (??8.4‰) for all drip waters is significantly more negative than the mean δ¹⁸O_p value (??6.9‰) weighted by precipitation amount, but close to the wet season (May to September) mean value (??8.3‰), suggesting that a threshold of precipitation amount must be exceeded to provide recharge. Calculation based on the equilibrium fractionation factor indicates that the δ¹⁸O_c values are not in isotopic equilibrium with their corresponding drip waters, with a range of disequilibrium effects from 0.4 to 1.4‰. The δ¹⁸O_c and δ¹³C_c values generally increase progressively away from the locus of precipitation on glass plates. The disequilibrium effects in the cave are likely caused by progressive calcite precipitation and CO₂ degassing related to a high gradient of CO₂ concentration between drip waters and cave air. Our study provides an important reference to interpret δ¹⁸O_c records from the monsoon region of China.  相似文献   

Oxygen isotopic fractionation between oxygen of different sites in hydroxyl-bearing silicate minerals     

M.S. Hamza  S. Epstein 《Geochimica et cosmochimica acta》1980,44(2):173-182

A method has been developed for extraction of hydroxyl oxygen from hydroxyl-bearing silicate minerals for oxygen isotopic analysis.The δO¹⁸ of oxygen of the OH groups is significantly different from that in the rest of the mineral structure. The isotopic fractionation between the two types of sites has the potential to be a sensitive geothermometer.Several δO¹⁸ values were obtained for oxygen of the OH attached to different silicate structures as well as for two muscovite samples with quantitatively estimated different temperatures of formation.The 1000 ln α (mineral-OH) values ranged from 5.2%. for muscovite to about 12.6%. for kaolinite and chlorite.  相似文献   

Hydration, ¹⁸O enrichment and oxidation during ocean floor hydrothermal metamorphism of ophiolitic metabasic rocks from E. Liguria,Italy     

E.T.C. Spooner  R.D. Beckinsale  P.C. England  A. Senior 《Geochimica et cosmochimica acta》1977,41(7):857-871

During ocean-floor hydrothermal metamorphism of a 225 m thick allochthonous Jurassic sequence of ophiolitic pillow lavas and underlying material in E. Liguria, Italy, the rocks were hydrated, enriched in ¹⁸O and oxidised. H₂O⁺ contents increased from ~0.3 to 3.8 wt.%, δ¹⁸O values increased from ~ +6‰ to values as high as +13.2‰, and ($\text{Fe}{}_2\text{O}{}_3\text{FeO}$1) increased from 0.18 to ratios as high as 1.0. Both δ¹⁸O values and oxidation ratios decrease in the original direction of increasing depth. Pillow margins are consistently more enriched in ¹⁸O and are more oxidised than cores.These observations are qualitatively interpreted in terms of a non-isothermal, heterogeneous reaction model of interaction of basalt with oxygen-bearing sea water during flow through the packed bed of pillows. Fluid flow approximated undirectional downward motion, and occurred in the recharge part of a cycle of single pass convection. Mass transfer through the pile was by flow (infiltration metasomatism), whereas intra-pillow mass transfer was diffusional. Oxygen isotope exchange and oxidation did not occur under conditions of perfect incremental equilibrium. An integrated bulk volumetric water/rock ratio of ~2 × 10³:1 is estimated from the oxidation profile.  相似文献   

Reconstruction of seasonal temperature variability in the tropical Pacific Ocean from the shell of the scallop, Comptopallium radula     

Julien Thébault  Laurent Chauvaud  Jennifer Guarini  Renaud Fichez  Eric Morize 《Geochimica et cosmochimica acta》2007,71(4):918-928

We investigated the oxygen isotope composition (δ¹⁸O) of shell striae from juvenile Comptopallium radula (Mollusca; Pectinidae) specimens collected live in New Caledonia. Bottom-water temperature and salinity were monitored in-situ throughout the study period. External shell striae form with a 2-day periodicity in this scallop, making it possible to estimate the date of precipitation for each calcite sample collected along a growth transect. The oxygen isotope composition of shell calcite (δ¹⁸O_{shell calcite}) measured at almost weekly resolution on calcite accreted between August 2002 and July 2003 accurately tracks bottom-water temperatures. A new empirical paleotemperature equation for this scallop species relates temperature and δ¹⁸O_{shell calcite}: