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1.
A mid-Cretaceous (∼95 Ma) laterite in southwestern Minnesota contains pisolites that consist primarily of gibbsite, quartz, and kaolinite with smaller amounts of goethite (α-FeOOH) and hematite. The presence of minor berthierine (an Fe(II) sheet silicate) suggests that this Cenomanian laterite experienced some degree of low temperature reductive diagenesis during its burial history. The prospects for extracting useful paleoenvironmental information from the pisolitic goethite were explored by studying measured mole fraction (Xm) and δ13Cm values of the Fe(CO3)OH component in solid solution in the goethite using the method of incremental vacuum dehydration-decarbonation.Data arrays that occupy distinctly different domains in plots of δ13Cm vs. 1/Xm suggest the existence of two generations of goethite in the pisolites. The apparently younger generation of goethite (“generation 2”) evolves CO2 at 170 °C, while the older generation (“generation 1”) evolves CO2 at 220 °C. The distribution of the data suggests that generation 2 goethite is a proxy for mixing of CO2 from three distinct CO2 sources in an oxidative environment which post-dated the reductive diagenesis. The small amount of generation 1 goethite seems to have persisted through the reductive diagenesis, and nine of the generation 1 goethite data appear to define a proxy, two-endmember, soil CO2 mixing line. Such two-component mixing is consistent with expectations for a highly evolved, carbonate-free laterite (i.e., the pre-diagenetic Cenomanian weathering system). The δ13Cm values of these nine data points range from −23.1‰ to −13.7‰, whereas Xm values range from 0.0007 to 0.0222. Linear regression of these nine data yields a slope of 0.0064, which corresponds to an ancient tropospheric CO2 concentration of about 1900 ppmV.Isotopic data from pisolitic kaolinite indicate a paleotemperature of about 24 °C at a paleolatitude of ∼40°N. This is substantially warmer than modern continental temperatures at such latitudes and is consistent with published indications of a generally warmer mid-Cretaceous climate. Moreover, the correspondence of a warmer mid-Cretaceous climate with the inferred, relatively high concentration of Cenomanian tropospheric CO2 (∼1900 ppmV) is consistent with the idea that variations of atmospheric CO2 have a relation to climate change. The results of this study emphasize the importance of careful evaluation of incremental dehydration-decarbonation data from natural goethites to assess the possibility that more than one generation of goethite is present in a sample. However, the results also indicate that the carbon isotope information recorded in admixed goethite generations may be sorted out and used in paleoenvironmental interpretations. 相似文献
2.
Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The δ13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. δ13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco2 value of about 1 × PAL, but the scatter in the measured δ13C values of organic matter permits a calculated maximum Pco2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments.The average δ13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ13C value for all profiles of −5.4‰. Thus, the value of Δ13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco2 and organic matter δ13C values were the same for both paleosol types. Furthermore, the atmospheric Pco2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco2 (permitted by one standard deviation of the organic matter δ13C value) of ∼5 × PAL.If, however, measured average δ13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric Pco2 using these 13C-enriched organic values would underestimate the actual atmospheric Pco2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2.The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the δ13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco2. 相似文献
3.
Crayton J. Yapp 《Geochimica et cosmochimica acta》2003,67(11):1991-2004
“Plateau” δ18O values of CO2 that evolved from the Fe(CO3)OH component during isothermal vacuum dehydrations (200-230 °C) of 18 natural goethites range from 8.2 to 28.1‰. In contrast, the measured δ18O values of the goethite structural oxygen range from −11.3 to 1.7‰. The results of this study indicate that the apparent oxygen isotope fractionation factor (18αapp) between plateau CO2 and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the 18αapp values are predicted by a relatively simple mass balance model with the following assumptions: (1) the rate of isothermal vacuum dehydration of goethite (for the interval from 0 to ∼60 to 80% loss of structural hydroxyl hydrogen) can be reasonably well represented by first-order kinetics and (2) isotopic exchange between evolving H2O vapor and solid occurs only in successive, local transition states. The generally good correspondence between the model predictions and the experimental data seems to validate these assumptions. Thus, the 18O/16O ratios of the evolved CO2 can act as probes into the transient processes operating at the molecular level during the solid-state goethite-to-hematite phase transition. For example, the activation energy for the rate constant associated with the transition state, oxygen isotopic exchange between solid and H2O vapor, is tentatively estimated as 28 ± 11 KJ/mol. Such knowledge may be of consequence in understanding the significance of 18O/16O ratios in hematites from some natural environments (e.g., Mars?).Kinetic data and δ18O values of CO2 are routinely obtained in the course of measurements of the abundance and δ13C values of the Fe(CO3)OH in goethite. The observed correlation between 18αapp and dehydration rates suggests that plateau δ18O values of evolved CO2 may provide complementary estimates of the δ18O values of total goethite structural oxygen (O, OH, CO2) with an overall precision of about ±1‰. However, because of isotopic exchange during the dehydration process, δ18O values of the evolved CO2 do not reflect the original δ18O values of the CO2 that was occluded as Fe(CO3)OH in goethite. 相似文献
4.
Twelve goethite samples with different degrees of substitution of Al for Fe were synthesized at 22-48 °C and pH values of 1.5-14 under closed system conditions and used to study the effects of Al substitution on the hydrogen isotopic fractionation between goethite and its ambient water. The syntheses followed two pathways: (1) Fe3+ hydrolysis in high pH aqueous solutions; (2) oxidation of Fe2+ to Fe3+ in mid to low pH solutions. XRD and SEM analyses indicated that, irrespective of temperature and pH, goethite was the predominant product of the syntheses in all of the experiments (with degrees of Al substitution as high as ∼13 mol %). “High temperature nonstoichiometric” (HTN) water is present in all of the samples and rapidly exchanges D/H with ambient vapor at room temperature. Uncertainties in the value of the apparent D/H fractionation factor (αe-v) between HTN water and ambient exchange water at 22 °C lead to significant uncertainties in determinations of the δD values of structural hydrogen (δDs) in goethites which contain high proportions of HTN water. As determined for the samples of this study, αe-v has a nominal value of 0.942 (±0.02). δDs values determined using an αe-v value of 0.942 indicate that Al substitution increases the δD value of structural hydrogen in goethite by about 1.4 (±0.4)‰ for each increase in Al of 1 mol %. This dependence on Al is of the same sign as, but somewhat larger in magnitude than, the effect of Al predicted by a published model (∼0.7‰ per mol % Al). The overall uncertainties in the current results suggest that an increase of ∼1‰ per mol % Al, as adopted by previous studies, may be a reasonable estimate with which to adjust δ Ds values of natural goethites to those of the pure FeOOH endmember and could be valid for degrees of Al substitution of up to at least 15 mol %. These synthesis experiments also yield a hydrogen isotopic fractionation factor (DαG-W) between pure goethite (α-FeOOH) and liquid water of 0.900 (±0.006), which is analytically indistinguishable from the published value of 0.905 (±0.004). Thus, use of an DαG-W value of 0.905 in applications to the FeOOH component of natural goethites is supported by the current study. 相似文献
5.
Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO2 and H2O. “Plateau” F (CO2/H2O ratios) and δ13C values of the CO2 derived from gibbsite were attained over the dehydration interval, Xv(H2) = 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 ± 0.0003, while the corresponding δ13C value of evolved CO2 was −16.0‰±0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate + 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of 13C-depleted CO2 (−46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or δ13C for CO2 evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO2 occluded in gibbsite may preserve information on δ13C values of CO2 in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO2 occluded in gibbsite and CO2 in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems. 相似文献
6.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt. 相似文献
7.
Madhusoodhan Satish-Kumar 《Geochimica et cosmochimica acta》2005,69(15):3841-3856
Graphite in deep crustal enderbitic (orthopyroxene + garnet + plagioclase + quartz) granulites (740°C, 8.9 kb) of Nilgiri hills, southern India were investigated for their spectroscopic and isotopic characteristics. Four types of graphite crystals were identified. The first type (GrI), which is interstitial to other mineral grains, can be grouped into two subtypes, GrIA and GrIB. GrIA is either irregular in shape or deformed, and rough textured with average δ13C values of −12.7 ± 0.4‰ (n = 3). A later generation of interstitial graphite (GrIB) shows polygonal crystal shapes and highly reflecting smooth surface features. These graphite grains are more common and have δ13C values of −11.9 ± 0.3‰ (n = 14). Both subtypes show well-defined Raman shifts suggesting a highly crystalline nature. Cores of interstitial graphite grains have, on average, lower δ13C values by ∼0.5‰ compared to that of the rim. The second type of graphite (GrII) occurs as solid inclusions in silicate minerals, commonly forming regular hexagonal crystals with a slightly disordered structure. The third type of graphite (GrIII) is associated with solid inclusions (up to 100 μm) that have decrepitation halos of numerous small (<15 μm) satellite fluid inclusions of pure CO2 with varying density (1.105 to 0.75 g/cm3). The fourth type of graphite (GrIV) is found as daughter crystals within primary type CO2-fluid inclusions in garnet and quartz. These fluid inclusions have a range of densities (1.05 to 0.90 g/cm3), but in general are significantly less dense than graphite-free primary, pure CO2 fluid inclusions (1.12 g/cm3). Raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature (∼ 500°C). The precipitation of graphite probably occurred during the isobaric cooling of CO2-rich peak metamorphic fluid as a result of oxyexsolution of oxide phases. The oxyexsolution process is evidenced by the magnetite-ilmenite granular exsolution textures and the systematic presence of numerous micron-sized rutile and other oxide inclusions in association with fluid inclusions within garnet, plagioclase, and quartz.The carbon isotope compositions of coexisting CO2 (in fluid inclusions) and graphite show a fractionation (α2CO−gr) of ∼6‰ in garnet, consistent with the existing theoretical estimates of α2CO−gr at 800°C. A subsequent generation of CO2 inclusions trapped in matrix quartz and quartz segregation have higher δ13C values, −4‰ and −2.9‰ respectively. Graphite in quartz segregations also has higher δ13C values (−9.8‰) than those in enderbite (−12.7‰). Micro-graphite crystals included in garnet, quartz (enderbite), and quartz (segregation) have average δ13C values of −11.1, −10.4, and −8.7‰ respectively, indicating progressive enrichment in 13C with a decrease in temperature of recrystallization of respective minerals. This progressive enrichment is also observed in carbon isotope compositions of fluid inclusion CO2, suggesting isotopic equilibrium during graphite precipitation from CO2 fluids. Thus, the carbon isotope record preserved in these rocks by the interstitial graphite, CO2 fluid in enderbite, graphite microcrystals, graphite in quartz segregation, and CO2 fluid in quartz segregation, suggests a temperature-controlled isotopic evolution. This evolution is in accordance with a closed system Rayleigh-type graphite precipitation process which progressively enriched residual CO2 in 13C. 相似文献
8.
We determined total CO2 solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H2O, OH−, molecular CO2, and CO32−. CO2 solubility was determined in hydrous andesite glasses and we found that H2O content has a strong influence on C-O speciation and total CO2 solubility. In anhydrous andesite melts with ∼60 wt.% SiO2, total CO2 solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO2 solubility increases by about 0.06 wt.% per wt.% of total H2O. As total H2O increases from ∼0 to ∼3.4 wt.%, molecular CO2 decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO32− increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO2 increases as the calculated mole fraction of CO2 in the fluid increases, showing Henrian behavior. In contrast, CO32− decreases as the calculated mole fraction of CO2 in the fluid increases, indicating that CO32− solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO2 degassing. If substantial H2O is present, total CO2 solubility is higher and CO2 will degas at relatively shallow levels compared to a drier melt. Total CO2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H2O contents (<1 wt.%). We found that total CO2 solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO2 is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations. 相似文献
9.
Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ18O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ18O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100-200 cm (for MAT = −6 °C) or ∼250-450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time.The δ18O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ18O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern.The δ13C value of the Fe(CO3)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO2 derived primarily from oxidation of relict Eocene wood with δ13C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of 13C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ18O record of the latest Miocene/Pliocene Arctic climate. 相似文献
10.
Clinton Rissmann Bruce Christenson Cynthia Werner Matthew Leybourne Jim Cole Darren Gravley 《Applied Geochemistry》2012
Carbon dioxide emissions and heat flow have been determined from the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand following 20 a of production (116 MWe). Soil CO2 degassing was quantified with 2663 CO2 flux measurements using the accumulation chamber method, and 2563 soil temperatures were measured and converted to equivalent heat flow (W m−2) using published soil temperature heat flow functions. Both CO2 flux and heat flow were analysed statistically and then modelled using 500 sequential Gaussian simulations. Forty subsoil CO2 gas samples were also analysed for stable C isotopes. Following 20 a of production, current CO2 emissions equated to 111 ± 6.7 T/d. Observed heat flow was 70 ± 6.4 MW, compared with a pre-production value of 122 MW. This 52 MW reduction in surface heat flow is due to production-induced drying up of all alkali–Cl outflows (61.5 MW) and steam-heated pools (8.6 MW) within the Ohaaki West thermal area (OHW). The drying up of all alkali–Cl outflows at Ohaaki means that the soil zone is now the major natural pathway of heat release from the high-temperature reservoir. On the other hand, a net gain in thermal ground heat flow of 18 MW (from 25 MW to 43.3 ± 5 MW) at OHW is associated with permeability increases resulting from surface unit fracturing by production-induced ground subsidence. The Ohaaki East (OHE) thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production, with an observed heat flow of 26.7 ± 3 MW and a CO2 emission rate of 39 ± 3 T/d. The negligible change in the thermal status of the OHE thermal area is attributed to the low permeability of the reservoir beneath this area, which has limited production (mass extraction) and sheltered the area from the pressure decline within the main reservoir. Chemistry suggests that although alkali–Cl outflows once contributed significantly to the natural surface heat flow (∼50%) they contributed little (<1%) to pre-production CO2 emissions due to the loss of >99% of the original CO2 content due to depressurisation and boiling as the fluids ascended to the surface. Consequently, the soil has persisted as the major (99%) pathway of CO2 release to the atmosphere from the high temperature reservoir at Ohaaki. The CO2 flux and heat flow surveys indicate that despite 20 a of production the variability in location, spatial extent and magnitude of CO2 flux remains consistent with established geochemical and geophysical models of the Ohaaki Field. At both OHW and OHE carbon isotopic analyses of soil gas indicate a two-stage fractionation process for moderate-flux (>60 g m−2 d−1) sites; boiling during fluid ascent within the underlying reservoir and isotopic enrichment as CO2 diffuses through porous media of the soil zone. For high-flux sites (>300 g m−2 d−1), the δ13CO2 signature (−7.4 ± 0.3‰ OHW and −6.5 ± 0.6‰ OHE) is unaffected by near-surface (soil zone) fractionation processes and reflects the composition of the boiled magmatic CO2 source for each respective upflow. Flux thresholds of <30 g m−2 d−1 for purely diffusive gas transport, between 30 and 300 g m−2 d−1 for combined diffusive–advective transport, and ?300 g m−2 d−1 for purely advective gas transport at Ohaaki were assigned. δ13CO2 values and cumulative probability plots of CO2 flux data both identified a threshold of ∼15 g m−2 d−1 by which background (atmospheric and soil respired) CO2 may be differentiated from hydrothermal CO2. 相似文献
11.
Sylvain Huon Francis E. GroussetDidier Burdloff Gérard BardouxAndré Mariotti 《Geochimica et cosmochimica acta》2002,66(2):223-239
Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios (δ13C, δ15N) of fine (<50 μm) size fractions of deep-sea sediments from the central North Atlantic were employed to identify changes in sources of organic matter over the past 50 ka BP. Ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs (averages ± 1σ: OC/TN = 7.6 ± 0.8; δ13C = −22.8 ± 1.0‰; δ15N = 5.5 ± 0.6‰). δ13C, OC, and TN concentrations shift to higher values during the Holocene, indicating a gradual decrease of fine terrigenous supply to the North Atlantic. The unchanged δ15N record between last glacial and Holocene stages indicates that the central North Atlantic region remained oligotrophic at least during the past 50 ka BP, but additional studies are required to support this result in terms of nitrogen oceanic budget. During the phases of enhanced ice-rafted detrital supply corresponding to prominent Heinrich events (HL1, HL2, HL4, and HL5), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower δ13C (down to −28‰) and δ15N (down to 1.6‰) values and high OC:TN ratios (up to 14.7 ± 1.1) are found for HL1, HL2, and with lesser extent for HL4. These values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter-bearing rock sources of the underlying geological basement. During HL5, only the δ13C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL5 to HL1 (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant δ13C, increasing OC/TN and decreasing δ15N values. 相似文献
12.
Weimin Feng 《Geochimica et cosmochimica acta》2009,73(20):6249-6263
Kaolinite, gibbsite and quartz are the dominant minerals in samples collected from two outcrops of a Cenomanian (∼95 Ma) laterite in southwestern Minnesota. A combination of measured yields and isotope ratios permitted mass balance calculations of the δD and δ18O values of the kaolinite in these samples. These calculations yielded kaolinite δD values of about −73‰ and δ18O values of about +18.7‰. The δD and δ18O values appear to preserve information on the ancient weathering system.If formed in hydrogen and oxygen isotope equilibrium with water characterized by the global meteoric water line (GMWL), the kaolinite δD and δ18O values indicate a crystallization temperature of 22 (±5) °C. A nominal paleotemperature of 22 °C implies a δ18O value for the corresponding water of −6.3‰. The combination of temperature and meteoric water δ18O values is consistent with relatively intense rainfall at that mid-paleolatitude location (∼40°N) on the eastern shore of the North American Western Interior Seaway. The inferred Cenomanian paleosol temperature of ∼22 °C is in general accord with published mid-Cretaceous continental mean annual temperatures (MAT) estimated from leaf margin analyses of fossil plants.When compared with results from a published GCM-based Cenomanian climate simulation which specifies a latitudinal sea surface temperature (SST) gradient that was either near modern or smaller-than-modern, the kaolinite paleotemperature of 22 °C is closer to the GCM-predicted MAT for a smaller equator-to-pole temperature difference in the mid-Cretaceous. Moreover, the warm, kaolinite-derived, mid-paleolatitude temperature of 22 °C is associated with proxy estimates of high concentrations of atmospheric CO2 in the Cenomanian. The overall similarity of proxy and model results suggests that the general features of Cenomanian continental climate in that North American locale are probably being revealed. 相似文献
13.
Giora Proskurowski Marvin D. Lilley E.J. Olson 《Geochimica et cosmochimica acta》2008,72(8):2005-2023
A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH4 and CO2 in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in 13C by ∼10‰ in values of δ13C of CH4, and by ∼0.55‰ in values of δ13C of CO2, relative to the values of the high-temperature source fluid (δ13C of CH4 =−20.1 ± 1.2‰, δ13C of CO2 =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in 13C of both CH4 and CO2 at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and 13C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH4 and CO2 concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable 13C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis. 相似文献
14.
A ∼17-m paleosol sequence at Schagen, South Africa, which developed on a serpentinized dunite intrusion in a granite-gneiss terrain ∼2.6 Ga ago, is characterized by an alternating succession of thick (∼1-3 m) carbonate-rich (dolomite and calcite) zones and silicate-rich (serpentines, talc, and quartz) zones; the upper ∼8 m section is especially rich in organic C (up to ∼1.4 wt.%). Petrologic and geochemical data suggest the upper ∼8 m section is composed of at least three soil profiles that developed on: (i) silicate-rich rock fragments (and minerals) that were transported from local sources (serpentinite and granite) by fluvial and/or eolian processes; and (ii) dolomite and calcite zones that formed by locally discharged groundwater. The Mg and Fe in the paleosol sequence were largely supplied from local sources (mostly serpentinite), but the Ca, Sr, and HCO3− were supplied by groundwater that originated from a surrounding granite-gneiss terrain. In the uppermost soil profile, the (Fe is retained, the Fe3+/Fe2+ ratio increases, and ferri-stilpnomelane is abundant. These data suggest the atmospheric pO2 was much greater than ∼10−3.7 atm (>0.1% present atmospheric level [PAL]).The carbonaceous matter in the soils is intimately associated with clays (talc, chlorite, and ferri-stilpnomelane) and occurs mostly as seams (20 μm to 1 mm thick) that parallel the soil horizons. These occurrences, crystallographic structures, H/C ratios, and δ13Corg values (−17.4 to −14.4‰ PDB) suggest that the carbonaceous matter is a remnant of in situ microbial mats, originally ∼1 to ∼20 mm thick. The microbial mats developed: (a) mostly on soil surfaces during the formation of silicate-rich soils, and (b) at the bottom of an evaporating, anoxic, alkaline pond during the precipitation of the Fe-rich dolomite. These δ13Corg values are difficult to be explained by a current popular idea of a methane- and organic haze-rich Archean atmosphere (Pavlov et al., 2001); these values, however, can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment. 相似文献
15.
S. Shilobreeva I. Martinez V. Busigny P. Agrinier 《Geochimica et cosmochimica acta》2011,75(9):2237-57
Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with δD values from −64‰ to −25‰. All samples are enriched in water relative to fresh basalts. The δD values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO2, extracted by step-heating technique, ranges between 564 and 2823 ppm with δ13C values from −14.9‰ to −26.6‰. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with δ13C = −4.5‰ and (2) an organic compound with δ13C = −26.6‰. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO2 yield after H3PO4 attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO2 degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO2 is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 °C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when “fresh” oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO2, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at ∼ −4.7‰, similar to the δ13C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 ± 0.4 × 1012 molC/yr. 相似文献
16.
Stephen T Grimes David P Mattey Margaret E Collinson 《Geochimica et cosmochimica acta》2003,67(21):4033-4047
Understanding past climate change is critical to the interpretation of earth history. Even though relative temperature change has been readily assessed in the marine record, it has been more difficult in the terrestrial record due to restricted taxonomic distribution and isotopic fractionation. This problem could be overcome by the use of multiple paleoproxies. Therefore, the δ18O isotopic composition of five paleoproxies (rodent tooth enamel, δ18OPhosphate = +17.7 ± 2.0‰ n = 74 (VSMOW); fish scale ganoine δ18OPhosphate = +19.7 ± 0.7‰ n = 20 (VSMOW); gastropod shell δ18OCalcite = −1.7 ± 1.3‰ n = 50 (VPDB); charophyte gyrogonite δ18OCalcite = −2.4 ± 0.5‰ n = 20 (VPDB); fish otolith δ18OAragonite = δ18O = −3.6 ± 0.6‰ n = 20 (VPDB)) from the Late Eocene (Priabonian) Osborne Member (Headon Hill Formation, Solent Group, Hampshire Basin, UK) were determined. Because diagenetic alteration was shown to be minimal the phosphate oxygen component of rodent tooth enamel (as opposed to enamel carbonate oxygen) was used to calculate an initial δ18OLocal water value of 0.0 ± 3.4‰. However, a skewed distribution, most likely as a result of the ingestion of evaporating water, necessitated the calculation of a corrected δ18OLocal water value of −1.3 ± 1.7‰ (n = 62). This δ18OLocal water value corresponds to an approximate mean annual temperature of 18 ± 1°C. Four other mean paleotemperatures can also be calculated by combining the δ18OLocal water value with four independent freshwater paleoproxies. The calculated paleotemperature using the fish scale thermometry equations most likely represents the mean temperature (21 ± 2°C) of the entire length of the growing season. This should be concordant with the paleotemperature calculated using the Lymnaea shell thermometry equation (23 ± 2°C). The lack of concordance is interpreted to be the result of diagenetic alteration of the originally aragonitic Lymnaea shell to calcite. The mean paleotemperature calculated using the charophyte gyrogonite thermometry equation (21 ± 2°C), on the other hand, most likely represents the mean temperature of a single month toward the end of the growing season. The fish otolith mean paleotemperature (28 ± 2°C) most likely represents the mean temperature of the warmest months of the growing season. An approximate mean annual temperature of 18 ± 1°C, in addition to a mean growing season paleotemperature of 21 ± 2°C (using fish scale only) with a warmest month temperature of 28 ± 2°C, and high associated standard deviations suggest that a subtropical to warm temperate seasonal climate existed during the deposition of the Late Eocene Osborne Member. 相似文献
17.
Hanns-Peter LiermannJibamitra Ganguly 《Geochimica et cosmochimica acta》2002,66(16):2903-2913
The diffusion coefficients of Fe2+ and Mg in aluminous spinel at ∼20 kb, 950 to 1325°C, and at 30 kb, 1125°C have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO2, was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe2+ and Mg at ∼20 kb, 950 to 1325°C, fit well the Arrhenian relation, D = D0exp(−Q/RT), where Q is the activation energy, with D0(Fe) = 1.8 (±2.8) × 10−5, D0(Mg) = 1.9 (±1.4) × 10−5 cm2/s, Q(Fe) = 198 ± 19, and Q(Mg) = 202 ± 8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of ∼5 cm3/mol. From analysis of compositional zoning in natural olivine-spinel assemblages in ultramafic rocks, previous reports concluded that D(Fe-Mg) in spinel with Cr/(Cr + Al) ≤0.5 is ∼10 times that in olivine. The diffusion data in spinel and olivine have been applied to the problems of preservation of Mg isotopic inhomogeneity in spinel within the plagioclase-olivine inclusions in Allende meteorite and cooling rates of terrestrial ultramafic rocks. 相似文献
18.
Colin G. Macpherson David R. Hilton Tibor J. Dunai 《Geochimica et cosmochimica acta》2005,69(24):5729-5746
New volatile data (CO2, H2O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO2 and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ13C values of dissolved CO2 or coupled CO2-Ar variations. This suggests that δ13C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H2O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO2 with δ13C of −6‰, 0.1 to 0.35 wt.% H2O, 3He/4He ∼11 RA, and CO2/3He of ∼2 × 109. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the 40Ar/36Ar may be low. The 3He/4He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 RA) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of 3He/4He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone. 相似文献
19.
The patterns of dissolved inorganic C (DIC) and aqueous CO2 in rivers and estuaries sampled during summer and winter in the Australian Victorian Alps were examined. Together with historical (1978–1990) geochemical data, this study provides, for the first time, a multi-annual coverage of the linkage between CO2 release via wetland evasion and CO2 consumption via combined carbonate and aluminosilicate weathering. δ13C values imply that carbonate weathering contributes ∼36% of the DIC in the rivers although carbonates comprise less than 5% of the study area. Baseflow/interflow flushing of respired C3 plant detritus accounts for ∼50% and atmospheric precipitation accounts for ∼14% of the DIC. The influence of in river respiration and photosynthesis on the DIC concentrations is negligible. River waters are supersaturated with CO2 and evade ∼27.7 × 106 mol/km2/a to ∼70.9 × 106 mol/km2/a CO2 to the atmosphere with the highest values in the low runoff rivers. This is slightly higher than the global average reflecting higher gas transfer velocities due to high wind speeds. Evaded CO2 is not balanced by CO2 consumption via combined carbonate and aluminosilicate weathering which implies that chemical weathering does not significantly neutralize respiration derived H2CO3. The results of this study have implications for global assessments of chemical weathering yields in river systems draining passive margin terrains as high respiration derived DIC concentrations are not directly connected to high carbonate and aluminosilicate weathering rates. 相似文献
20.
Adriana Heimann Brian L. Beard Clark M. Johnson 《Geochimica et cosmochimica acta》2008,72(17):4379-4396
Highly differentiated igneous rocks can, in some cases, have 56Fe/54Fe ratios that are significantly higher than those of mafic- to intermediate-composition igneous rocks. Iron isotope compositions were obtained for bulk rock, magnetite, and Fe silicates from well-characterized suites of granitic and volcanic rocks that span a wide range in major- and trace-element contents. Sample suites studied include granitoids from Questa, N.M. (Latir volcanic field) and the Tuolumne Intrusive Series (Sierra Nevada batholith), and volcanic rocks from Coso, Katmai, Bishop Tuff, Grizzly Peak Tuff, Seguam Island, and Puyehue volcano. The rocks range from granodiorite to high-silica granite and basalt to high-silica rhyolite. The highest δ56Fe values (up to +0.31‰) are generally restricted to rocks that have high Rb (>100 ppm), Th (>∼15 ppm) and SiO2 (>70 wt.%) but low Fe (<2 wt.% total Fe as Fe2O3) contents. Magnetite separated from these rocks has high δ56Fe values, whereas Fe silicates have δ56Fe values close to zero. Although in principle crystal fractionation might explain the high δ56Fe values, trace-element ratios in high-δ56Fe igneous rocks indicate that crystal fractionation is an unlikely explanation. The highest δ56Fe values occur in volcanic and plutonic rocks that contain independent evidence for fluid exsolution, including sub-chondritic Zr/Hf ratios, suggesting that loss of a low-δ56Fe ferrous chloride fluid is the most likely explanation for the high δ56Fe values in the bulk rocks. Based on magnetite solubility in chloride solutions and predicted Fe isotope fractionations among Fe silicates, magnetite, and ferrous chloride fluids, the increase in δ56Fe values of bulk rocks may be explained by isotopic exchange between magnetite and , which predicts an increase in the δ56Fe values of magnetite upon fluid exsolution. This model is consistent with the δ56Fe values measured in this study for bulk rocks, as well as magnetite and Fe silicates. Our results suggest that fluid exsolution from siliceous hydrous magmas, which sometimes produce porphyry-style Cu, Mo, or Cu-Au mineralization, may be traced using Fe isotopes. 相似文献