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1.
Denis Scholz Christian Mühlinghaus Augusto Mangini 《Geochimica et cosmochimica acta》2009,73(9):2592-695
We derive equations describing the evolution of the carbon and oxygen isotope composition of the bicarbonate in a calcite precipitating solution on the surface of a stalagmite using a classical Rayleigh approach. The combined effects of calcite precipitation, degassing of CO2 and the buffering effect of the water reservoir are taken into account. Whereas δ13C shows a progressive increase to a final constant value, δ18O shows an initial isotopic enrichment, which exponentially decays due to the buffering effect of the water reservoir. The calculated evolution is significantly different for both carbon and oxygen isotopes than derived in a recent paper [Dreybrodt W. (2008) Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H2O-CO2-CaCO3 solution and the related isotopic composition of calcite in stalagmites. Geochim. Cosmochim. Acta72, 4712-4724.].Furthermore, we discuss the isotopic evolution of the bicarbonate in the solution for long residence times on the stalagmite surface, i.e., for t→∞. The equilibrium isotope ratio of the bicarbonate is then determined by isotopic exchange between the cave atmosphere and the bicarbonate in the solution and can be calculated by equilibrium isotope fractionation. For strongly ventilated caves exchange with the cave atmosphere will result in higher δ13C and δ18O values than those observed in a pure Rayleigh distillation scenario, for sparsely ventilated caves it will result in lower δ13C and δ18O values. 相似文献
2.
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 (δ18Ocel), relative humidity (RH), precipitation δ18O (δ18Oppt), and mean annual temperature (MAT) at the continental scale, and the general range of variability in δ18Ocel 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 δ18Ocel. At the continental scale, δ18Ocel was strongly influenced by modeled δ18Oppt (R2 = 0.80, p < 0.001). Average summer minimum RH (RHmin) combined with δ18Oppt explained more of the variability (R2 = 0.93, p < 0.001) in δ18Ocel across North American and Caribbean forests. MAT and δ18Ocel were also strongly correlated across North America (R = 0.91 and 0.95, p < 0.001, for angiosperms and conifers, respectively). The difference between δ18Oppt and δ18Ocel is not constant (varying from 35-44‰) and is inversely correlated with δ18Oppt. The relationships among δ18Oppt, RHmin, δ18Ocel, and MAT established for North America and the Caribbean applied reasonably well when δ18Ocel was used to estimate MAT and δ18Oppt in Asia, Europe, and South America, but there were important exceptions. The most accurate predictions of MAT and δ18Oppt from δ18Ocel require RHmin. Predictions of δ18Oppt and MAT made from δ18Ocel alone produced errors of up to 8‰ and 16 °C, respectively. 相似文献
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5.
Kalmychkov G. V. Hachikubo A. Pokrovsky B. G. Minami H. Yamashita S. Khlystov O. M. 《Lithology and Mineral Resources》2020,55(6):439-444
Lithology and Mineral Resources - We studied the composition and isotopic characteristics (δ13C and δD) of hydrocarbon gases from the Kotelnikovsky, Zmeiny, and Goryachnisky hot springs... 相似文献
6.
Matthew J. Kohn 《Contributions to Mineralogy and Petrology》1993,113(2):249-261
A general model has been developed to calculate changes of 18O of minerals in addition to their composition and modal abundance in metamorphic systems. A complete set of differential equations can be written to describe any chemical system in terms of the variables dP, dT, dX, dM, and d18O (X, M, and 18O refer to the chemical composition, number of moles, and oxygen isotope composition of each phase respectively). This set is composed of the differentials of five subsets of equations: (1) conditions of heterogeneous equilibrium; (2) compositional stoichiometry for each mineral; (3) mass balance for each oxide component; (4) oxygen isotope partitioning between phases; (5) conservation of the oxygen isotope ratio of the system. The variance of the complete set of equations is 2, and changes of 18O, composition, and modal abundance for each mineral can be calculated for arbitrary changes of P and T. Applications to a typical pelitic bulk composition at amphibolite and lower granulite facies conditions suggest that for systems dominated by continuous reactions such as: (a) chlorite + quartz = garnet+H2O; (b) staurolite + biotite = garnet + muscovite + H2O; or (c) garnet + muscovite = sillimanite + biotite, isopleths of mineral 18O are nearly independent of pressure, and have a spacing of about 0.1 per 10–20°C. For nearly discontinuous reactions such as: (d) garnet + chlorite + muscovite = biotite + staurolite+H2O; (e) staurolite + muscovite = biotite + aluminosilicate + garnet+H2O; or (f) muscovite + quartz = sillimanite + K-feldspar+H2O, isopleths of mineral 18O have slopes more nearly parallel to endmember reaction boundaries and 18O of phases can have a greater temperature dependence (e.g., 0.1 per 2°C for reaction d). This behavior results from relatively large amounts of reaction progress for small changes of P or T. However, the calculated exhaustion of a reactant within 0.1–5°C ensures that the predicted effects of such reactions on mineral 18O will not exceed 0.25 for typical bulk compositions. Models that allow for fractional crystallization of garnet suggest that prograde garnet zoning in pelitic assemblages will be relatively smooth until staurolite becomes unstable. At higher temperatures, garnet may develop a step of as much as 0.6 in its core-rim zoning as a result of combined garnet resorption during the continuous reaction garnet + muscovite = sillimanite + biotite and repartitioning of the garnet rim composition to relatively heavy 18O. The models are insensitive to the degree to which garnet fractionally crystallizes and to the isotope fractionation factors used; only extreme changes in modal abundance or bulk composition for a given mineral assemblage can produce significant changes in the predicted trends. In the absence of infiltration, isotopic shifts resulting from net transfer reactions for minerals in typical amphibolite, eclogite, and lower granulite facies metapelites and metabasites are inferred from the models to be 1 or less for 150°C of heating. 相似文献
7.
Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ13C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ13CDIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ13CDIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ13CDIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ13CDIC = −2.49[DIC] + 0.64, r2 = 0.84). A shallow pool fed by multiple drips shows a bimodal δ13CDIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO2 fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ13CDIC variability. Measured values of high [DIC] and low δ13CDIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ13CDIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO2 and precipitated CaCO3(1000lnα[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of 14C mass balance and paired 14C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from 14C-dead dolomite cap while the remainder of ∼77% is derived from 14C-live biomass. δ13C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ13C values calculated using the shallow pool summer/early fall data thus confirming the δ13C seasonality in both drips and coeval aragonite. δ13C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ13C records of stalagmites at DeSoto Caverns with resolving power >10−1 year will likely carry the imprints of drip annual means that record climate-driven δ13C seasonal biases. 相似文献
8.
Ashley K. Russell Kouki Kitajima Ariel Strickland L. Gordon Medaris Jr. Daniel J. Schulze John W. Valley 《Contributions to Mineralogy and Petrology》2013,165(1):103-116
In situ analysis reveals that eclogite-facies garnets are zoned in δ18O with lower values in the core and rims that are ~1.5 to 2.5 ‰ higher. This pattern is present in 9 out of 12 garnets analyzed by SIMS from four orogenic eclogite terranes, and correlates with an increase in the mole fraction of pyrope and Mg/Fe ratio from core to rim, indicating prograde garnet growth. At the maximum temperatures and the time-scales experienced by these garnets, calculated intragranular diffusion distances for oxygen are small (<5 μm), indicating that δ18O records primary growth zoning and not diffusive exchange. The oxygen isotope gradients are larger than could form due to temperature changes during closed-system mineral growth. Thus, gradients reflect the compositions of fluids infiltrating during prograde metamorphism. Values of δ18O in garnet cores range from ?1 to 15 ‰, likely preserving the composition of the eclogite protoliths. Two garnet cores from the Almenningen eclogite in the Western Gneiss Region, Norway, have δ18O ~?1 ‰ and are the first negative δ18O eclogites identified in the region. In contrast with orogenic eclogites, seven high δ18O garnets (>5 ‰) from two kimberlites are homogeneous in δ18O, possibly due to diffusive exchange, which is possible for prolonged periods at higher mantle temperatures. Homogeneity of δ18O in garnets outside the normal mantle range (5–6 ‰) may be common in kimberlitic samples. 相似文献
9.
Thomas Rigaudier Véronique Gardien François Martineau 《Geochimica et cosmochimica acta》2011,75(16):4637-4652
Deuterium and oxygen isotope fractionations between liquid and vapor water were experimentally-determined during evaporation of a NaCl solution (35 g L−1) as a function of water temperature and wind velocity. In the case of a null wind velocity, slopes of δD-δ18O trajectories of residual waters hyperbolically decrease with increasing water temperatures in the range 23-47 °C. For wind velocities ranging from 0.8 to 2.2 m s−1, slopes of the δD-δ18O trajectories linearly increase with increasing wind velocity at a given water temperature. These experimental results can be modeled by using Rayleigh distillation equations taking into account wind-related kinetics effects. Deuterium and oxygen isotope compositions of water inclusions trapped by the precipitated halite crystals were determined by micro-equilibration techniques.These isotopic compositions accurately reflect those of the surrounding residual waters during halite growth. Isotopic compositions of water inclusions in twenty natural halites from the Messinian Realmonte mine in Sicily suggest precipitation temperatures of that match the homogenization temperatures obtained by microthermometry (median = 34 ± 5 °C). The similarity between the measured and experimental slopes of the δD-δ18O evaporation trajectories suggests that the effect of wind was negligible during the genesis of these halite deposits. Hydrogen and oxygen isotope compositions of water inclusions from Realmonte halite also define a linear trend whose extrapolation until intersection with the Mediterranean Meteoric Water Line allows the characterization of the water source with δD and δ18O values of −70 ± 10‰ and −11.5 ± 1.5‰, respectively. These results reveal that the huge amounts of salts deposited in Sicily result from the evaporation of seawater mixed with a dominant fraction (?50%) of meteoric waters most likely deriving from alpine fluvial discharge. 相似文献
10.
Philip StackLuc Rock 《Applied Geochemistry》2011,26(8):1478-1487
A map of the spatial variation in isotopic composition of a substrate, or isoscape, provides a tool to address a range of research questions, such as the use of isotope fingerprinting to identify the origin of compounds. The focus of this study was to establish a soil S isotope map for Northern Ireland in order to investigate spatial variability within the isotopic composition of total soil sulphur (S). The sample resolution was about one sample every 141 km2. δ34S values were grouped based on soil type and soil parent material. Total soil S δ34S data were grouped according to soil type, to evaluate whether a soil’s characteristics affect its isotopic composition. Gleyed soils had a mean δ34S value of +6.3 ± 6.1‰, lower than the means for other soil types. A trend towards higher δ34S values (mean δ34S of +13.8 ± 6.0‰) was observed in soils with a high organic matter content. No trends in δ34S value were observed for other soil types. There were no discernible correlations between total soil S δ34S and soil parent material, with the exception of shale and mudstones. The majority of soils derived from shale and mudstones had δ34S values at or close to the mean of +4.9‰. A spatially coherent pattern of relatively high soil total S δ34S values in the west and north, and low in the SE, is discernible in Northern Ireland. Based on the observed spatial distribution, it has been concluded that the quantity and source (anthropogenic or marine) of wet atmospheric S deposition is a controlling factor on regional variation of soil δ34S in Northern Ireland. This S isoscape will assist studies of S cycling in Northern Ireland and may assist in the creation of an ‘isotopic fingerprint’ for a potential ‘input’ source needed to interpret data in traceability studies. 相似文献
11.
We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (εa) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of 2H and 18O associated with evaporation of lake waters. The most negative εa values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum εa values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ18O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau. 相似文献
12.
Doklady Earth Sciences - 相似文献
13.
Birgit Hagedorn Ronald S. Sletten David F. McTigue 《Geochimica et cosmochimica acta》2010,74(2):435-173
Soils in the McMurdo Dry Valleys, Antarctica contain ice and considerable amounts of salt. Ice often occurs at shallow depth throughout the Dry Valleys and other areas of hyperarid permafrost, notably on Mars. This common occurrence of shallow ice is enigmatic; however, since according to published sublimation models it should disappear relatively quickly (at rates of order 0.1 mm a−1) due to vapor loss to the atmosphere. This loss may be offset by recharge from snowmelt infiltrating and freezing in the soil. Herein, we present a first quantitative estimate of this recharge based on measured vertical profiles of δD and δ18O that reveal considerable detail about the sources and sinks of ice. We model these profiles, taking into account the salt content and a soil temperature record along a 1.6 m depth profile of ∼10 ka old ice-cemented soils in Victoria Valley, Antarctica. The stable isotopes of ice are enriched in heavy isotopes at the top of the ice cement (20 cm depth); both δD and δ18O values plotted against depth exhibit a concave upward curve. At depth, the isotope composition is similar to that of Lake Victoria and modern meteoric water. The concave shape of the isotope profile is suggestive of downward advection-dispersion of snowmelt water enriched in heavy isotopes into the ice cement. Our advection-dispersion model, coupled with field data, enables us to quantify the advective flux and dispersion of melt water into the ice. The advective velocity and dispersion coefficient depend on the time since advection began and the ice-to-brine ratio; they are, respectively, of the order of 10−11-10−10 m s−1 and 10−12-10−11 m2 s−1. These values suggest that over the ∼10 ka time period, a total of 190 mm water infiltrated into the ice-cemented ground. The isotope composition and deuterium excess values of the uppermost ice cement can be modeled from snowmelt water enriched in salts using open system-Rayleigh fractionation. To develop the isotopic signature of the upper ice cement requires evaporation of ∼95% of the snowmelt water. Based on 190 mm brine infiltrating into the soil requires an initial total of ∼4 m of snowmelt water. This corresponds to ∼0.4 mm a−1 suggesting that, under the current climate condition, water from snowmelt is sufficient to compensate modeled sublimation rates, and therefore conserve ground ice in Victoria Valley. 相似文献
14.
Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave.Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s−1 in winter and 0.4 m s−1 in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO2. A clear relationship is found between calcite δ13C and cave air ventilation rates estimated by proxies pCO2 and 222Rn. Calcite δ13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ13CCaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ13C records in ancient speleothems. Calcite δ13CCaCO3 may be a proxy not only for atmospheric CO2 or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows.Farmed calcite δ18O was found to exhibit a +0.82 ± 0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ13CCaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ18OCaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water-calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments:
1000lnα=16.1(103T-1)-24.6 相似文献
15.
《Quaternary Science Reviews》1999,18(8-9):1021-1038
Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ18O trends that are almost exactly out of phase with coupled δ18O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ18O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ18O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ18O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ18O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ18O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ18O, probably indicative of warm and/or dry conditions. Exceptionally low δ18O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ18O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ18O and extension rate. Instead, its extension rate correlates roughly with δ13C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ18O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ18O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ18O and extension rate. Had drip-water availability been the dominant control on δ18O at this semi-arid site then higher δ18O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed. 相似文献
16.
In freshwater settings, dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophs and heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (δD) should be valuable proxies for paleohydrological reconstruction. However, because the purity required for hydrogen isotope measurements is difficult to achieve using standard wet chemical purification methods, their potential as a paleohydrological proxy is rarely exploited. In this study, we tested δD values of dinosterol in both particulate organic matter (POM) and sediments of stratified tropical freshwater lakes (from Cameroon) as a paleohydrological proxy, the lakes being characterized by variable degrees of eutrophication. In POM and sediment samples, the δD values of dinosterol correlated with lake water δD values, confirming a first order influence of source water δD values. However, we observed that sedimentary dinosterol was D enriched from ca. 19 to 54‰ compared with POM dinosterol. The enrichment correlated with lake water column conditions, mainly the redox potential at the oxic–anoxic interface (Eh OAI). The observations suggest that paleohydrologic reconstruction from δD values of dinosterol in the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water δD values, with more positive dinosterol δD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C34 botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and ββ-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesterol and dinosterol) correlated with Eh OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterol δD values to evaluate the effect of lake trophic status on sedimentary dinosterol δD values, as a basis for accurately reconstructing tropical lake water δD values. 相似文献
17.
Mudrocks from three lower Paleozoic basins in the British Caledonides (southern Lake District, northern Lake District and Southern Uplands) were investigated to determine the influence of sub-greenschist facies metamorphism on Li and the factors that control Li in fine-grained terrigenous sedimentary rocks. Metamorphic grade, as determined by KI (Kübler index) does not correlate with Li content ([Li]) and δ7Li, indicating that sub-greenschist facies metamorphism has negligible effect on Li in these rocks. Collectively, the data for all three basins show a negative correlation between [Li] and δ7Li and a positive correlation between [Li] and the Chemical Index of Alteration (CIA), suggesting that provenance exerts the greatest control on Li in mudrocks. Samples from the northern Lake District, which were deposited in an extensional basin, have homogeneous REE patterns, similar to shale composites (PAAS), the highest CIA, Th/U and [Li] and the lowest δ7Li and εNd, consistent with their derivation from a highly weathered, ancient continental source. By contrast, mudrocks from the Southern Uplands range to the lowest CIA, Th/U and [Li] and have the highest δ7Li and εNd. These samples were deposited in a forearc basin on the southern margin of the Laurentian craton and contain volcanic detritus. Their REE patterns are the most variable, ranging from average shale-like patterns to less LREE-enriched patterns. The compositional heterogeneity within the Southern Uplands mudrocks is consistent with a mixed provenance that includes juvenile crustal materials (lower [Li], εNd and Th/U, higher δ7Li), likely derived from the arc, as well as more highly weathered continental detritus. Mudrocks from the southern Lake District were deposited in a foreland basin, and exhibit geochemical characteristics intermediate between the northern Lake District and the Southern Uplands mudrocks, indicating their derivation from a mixed source. Our study shows that Li concentrations and δ7Li can provide additional information on the degree of weathering of the provenance of mudrocks. 相似文献
18.
Regional background
34S values of pyrite-(marcasite) nodules throughout the Zechstein basin in Poland have been measured to help estimate the proportion of externally derived sulphur in the Kupferschiefer Cu-Ag ores. The
34S values of the 17 FeS2 nodules measured range widely, from -25.2 to -51.9%., similar to the previously published -28 to -43%. range in disseminated pyrite in the Kupferschiefer. The wide variation cannot be attributed to pyrite versus marcasite mineralogy, amount of contained chalcopyrite or sphalerite, carbonate versus shale host rock, early versus late formation, percent of included calcite, or to size, shape, or texture. There is also no relation with proximity to the centres of copper mineralization in southwestern Poland where sulphides are typically isotopically heavier. The
34S values do, however, vary directly with percent of host-rock fragments included in the nodules. Repeat samples that were washed with acid or hot water show the same wide variation, indicating that contamination by sulphate sulphur in the host rock is not a factor. Neither is organic sulphur because of its small volume. Instead, the sulphur composition may be fundamentally controlled by the formation mechanism of the nodule, whereby 34S-rich sulphide is preferentially concentrated, possibly replacing anhydrite lenses. Alternatively, a network of host rock inclusions might act as a more accessible conduit for later, 34S-rich fluids to infiltrate the nodule and add to earlier, 34S-poor pyrite.In the ore deposits, higher
34S values of ore nodules suggest less indigenous sulphur in limestone than shale lithologies. An isotopic temperature of 61 °C from a chalcopyrite-galena pair agrees with other estimates of <105°C. Higher values in ore nodules/veinlets than in adjacent disseminations, and the calculated
34Spy value from a pyrite-bornite mixture support the idea that metal-bearing 34S-rich fluids penetrated the Kupfer-schiefer through a network of fractures.Contribution to IGCP Project 254 Metalliferous Black Shales 相似文献
19.
Carbon isotope ratio (δ13C) values of lipid biomarkers from plants can be used to assess water use efficiency and to reconstruct environmental conditions in the past. We assessed the effect of salinity on the δ13C values for leaf wax n-C31 and n-C33 alkanes, bulk leaf matter and leaf total lipid extracts from Avicennia marina (gray mangrove) trees growing along the Brisbane River estuary in Queensland, Australia. We observed an increase in 0.19 ± 0.053‰ (R2 0.61, p 0.008) and 0.16 ± 0.052‰ (R2 0.55, p 0.01) per salinity unit for the two n-alkanes, respectively, and of 0.087 ± 0.028‰ (R2 0.41, p 0.009) for whole leaves per salinity unit, indicating that water use efficiency of A. marina increased with the salt content of water. There was no correlation between δ13C values of total lipid extracts and salinity, perhaps because of a decrease in lipid concentration at higher salinity or because of varying contributions of different lipid classes to the extract. The robust relationship between salinity and δ13C values of leaf wax lipids provides a means of quantitatively reconstructing past salinity from carbon isotope ratios of mangrove lipid biomarkers in sediments. When paired with measurements of the hydrogen isotope ratio values of the same compounds, the approach should facilitate quantitative reconstruction of the hydrogen isotope composition of environmental water. In order for the method to successfully reconstruct past salinity and water isotopes, a mangrove source for leaf wax would need to be confirmed by palynological or other evidence, or the isotopic composition of a more source specific biomarker, such as taraxerol, would need to be measured. 相似文献
20.
YANG Yuncheng SHEN Zhaoli WEN Dongguang HOU Guangcai SHE Hongquan ZHAO Zhenhong WANG Dong LI Jingwen 《《地质学报》英文版》2010,84(2):432-440
<正>The Ordos Cretaceous Groundwater Basin,located in an arid-semiarid area in northwestern China,is a large-style groundwater basin.SO_4~(2-) is one of the major harmful components in groundwater.Dissolved SO_4~(2-) concentrations,andδ~(34)S-SO_4~(2-) andδ~(18)O-SO_4~(2-) in groundwater from 14 boreholes and in gypsum from aquifer were analyzed.Results show that SO_4~(2-) in shallow groundwaters originates from precipitation,sulfide oxidation,and dissolution of stratum sulphate,with a big range ofδ~(34)S values,from-10.7‰to 9.2‰,and addition of SO_4~(2-) in deep groundwater results from dissolution of stratum sulphate,with biggerδ~(34)S values,from 7.8‰to 18.5‰,compared with those in shallow groundwater.This research also indicates that three types of sulphate are present in the strata,and characterized by highδ~(34)S values and highδ~(18)O values-style,highδ~(34)S values and middleδ~(18)O valuesstyle, middleδ~(34)S values and lowδ~(18)O values-style,respectively.Theδ~(34)S-SO_4~(2-) andδ~(18)O-SO_4~(2-) in groundwater have a good perspective for application in distinguishing different groundwater systems and determining groundwater circulation and evolution in this area. 相似文献