Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO₂ concentrations     

Neil J. Tabor  Crayton J. Yapp 《Geochimica et cosmochimica acta》2004,68(7):1503-1517

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   

Paleoenvironmental implications of concentration and C/C ratios of Fe(CO₃)OH in goethite from a mid-latitude Cenomanian laterite in southwestern Minnesota     

Weimin Feng  Crayton J. Yapp 《Geochimica et cosmochimica acta》2009,73(9):2559-7116

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 (X_m) and δ¹³C_m 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 δ¹³C_m vs. 1/X_m 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 δ¹³C_m values of these nine data points range from −23.1‰ to −13.7‰, whereas X_m 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   

Indoor and outdoor urban atmospheric CO₂: Stable carbon isotope constraints on mixing and mass balance     

Yurena Yanes  Crayton J. Yapp 《Applied Geochemistry》2010

From July to November 2009, concentrations of CO₂ in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ¹³C values varied from −8.9‰ to −19.4‰. The CO₂ from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ¹³C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO₂ exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ¹³C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO₂ gas. Collectively, the δ¹³C values of the indoor CO₂ samples were linearly correlated with the reciprocal of CO₂ concentration, exhibiting an intercept of −21.8‰, with r² = 0.99 and p < 0.001 (n = 78). This high degree of linearity for CO₂ data representing 18 interior spaces (with varying numbers of occupants), and the coincidence of the intercept (−21.8‰) with the average δ¹³C value for human-exhaled CO₂ demonstrates simple mixing between two inputs: (1) outdoor CO₂ introduced to the interior spaces by ventilation systems, and (2) CO₂ exhaled by human occupants of those spaces. If such simple binary mixing is a common feature of interior spaces, it suggests that the intercept of a mixing line defined by two data points (CO₂ input from the local ventilation system and CO₂ in the ambient air of the room) could be a reasonable estimate of the average δ¹³C value of the CO₂ exhaled by the human occupants. Thus, such indoor spaces appear to constitute effective “sample vessels” for collection of CO₂ that can be used to determine the average proportions of C₃ and C₄-derived C in the diets of the occupants. For the various groups occupying the rooms sampled in this study, C₄-derived C appears to have constituted ∼40% of the average diet.  相似文献   

A pedogenic goethite record of soil CO₂ variations as a response to soil moisture content     

Erik L. Gulbranson  Neil J. Tabor 《Geochimica et cosmochimica acta》2011,75(22):7099-7116

The terrestrial carbon cycle and the role of atmospheric CO₂ concentrations in controlling global temperatures can be inferred from the study of ancient soils (paleosols). Soil-formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric pCO₂ from ancient terrestrial records. In the case of goethite, optimum sampling strategies for reconstructing pCO₂ focus on the portion of the soil profile that displays steep gradients in both soil CO₂ concentration and δ¹³C values of soil CO₂ such that a keeling plot can be developed for a given soil and atmospheric pCO₂ can be calculated from it. We report data from a Carboniferous paleosol that depart from the expected linear trends. The results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil CO₂, over a range of timescales, and can record these variations in the carbon isotope composition and mole fraction of Fe(CO₃)OH in solid solution with goethite. We explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired CO₂ or in the residence time of carbon in soils. The implications of this result are overestimation of paleoatmospheric pCO₂ from pedogenic goethite.  相似文献   

Incremental vacuum dehydration-decarbonation experiments on a natural gibbsite (α-Al(OH₃)): CO₂ abundance and δC values     

Neil J. Tabor  Crayton J. Yapp 《Geochimica et cosmochimica acta》2005,69(3):519-527

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, X_v(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   

Determination of carbon fractionation factor between aqueous carbonate and CO₂(g) in two-direction isotope equilibration     

P.M. Le?niak  P. Zawidzki 《Chemical Geology》2006,231(3):203-213

The experiments were conducted in the open CO₂ system to find out the equilibrium fractionation between the carbonate ion and CO₂(g). The existence of isotopic equilibrium was checked using the two-direction approach by passing the CO₂−N₂ gases with different δ¹³C compositions (− 1.5‰ and − 23‰) through the carbonate solution with δ¹³C = − 4.2‰. The Δ_{CO3T²⁻−CO2(g)} equilibrium fractionation is given as 6.03 ± 0.17‰ at 25 °C. Discussion is provided about the significance of carbonate complexing in determination of Δ_{CO3T²⁻−CO2(g)} and Δ_{HCO3T⁻−CO2(g)} fractionations. Finally, an isotope numerical model of flow and kinetics of hydration and dehydroxylation is built to predict the isotopic behaviour of the system with time.  相似文献   

A model for O/O variations in CO₂ evolved from goethite during the solid-state α-FeOOH to α-Fe₂O₃ phase transition     

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 (¹⁸α_app) between plateau CO₂ and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the ¹⁸α_app 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 ¹⁸α_app 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   

Genesis of fumarolic emissions as inferred by isotope mass balances: CO₂ and water at Vulcano Island, Italy     

A. Paonita  R. FavaraP.M. Nuccio  F. Sortino 《Geochimica et cosmochimica acta》2002,66(5):759-772

We have developed a quantitative model of CO₂ and H₂O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H₂O and CO₂ and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H₂O and CO₂ content and the δD, δ¹⁸O, and δ¹³C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ¹³C_CO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO₂ concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ¹³C_CO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ¹³C of the entire hydrothermal system, including mixed H₂O-CO₂ vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO₂. On these grounds, we think that Mediterranean marine water (δD_H2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO₂ and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.  相似文献   

Coexisting goethite and gibbsite from a high-paleolatitude (55°N) Late Paleocene laterite: Concentration and C/C ratios of occluded CO₂ and associated organic matter     

Neil J. Tabor  Crayton J. Yapp 《Geochimica et cosmochimica acta》2005,69(23):5495-5510

A Late Paleocene (∼60 Ma BP) lateritic soil from Northern Ireland (the Antrim paleosol, herein referred to as Nire) contains coexisting goethite, gibbsite, phyllosilicate, and hematite. The Fe(III) oxides exhibit pisolitic and Liesegang-type morphologies that are mutually exclusive in hand specimens. X-ray diffraction (XRD) measurements of Al substituted for Fe in goethite indicate two populations: (1) low-Al, Liesegang-type goethites (∼0 mol% Al) and (2) high-Al, pisolitic goethites (∼9 to ∼24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and δ¹³C values of CO₂ occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO₃)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO₂ in the ancient soil. The δ¹³C values of CO₂ occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO₂.The measured mole fractions (X) of Fe(CO₃)OH in the high-Al goethites range from 0.0059 (±0.0005) to 0.0077 (±0.0006) and correspond to soil CO₂ concentrations of ∼28,000 to ∼37,000 ppmV. The average values of X and δ¹³C for the four high-Al goethites are 0.0067 ± 0.0007 and −20.1 ± 0.5‰, respectively. The δ¹³C value of the organic matter undergoing oxidation in this midlatitude (∼55°N) Late Paleocene soil appears to have been ∼ −28.2‰. Taken together, these data indicate an atmospheric CO₂ concentration of ∼2400 ppmV (± ∼1200 ppmV) at ∼60 Ma BP. The inferred high concentration of atmospheric CO₂ would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO₂ plays an important role in climate variation.  相似文献   

Abundance of mass 47 CO₂ in urban air, car exhaust, and human breath     

Hagit P. Affek  John M. Eiler 《Geochimica et cosmochimica acta》2006,70(1):1-12

Atmospheric carbon dioxide is widely studied using records of CO₂ mixing ratio, δ¹³C and δ¹⁸O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally ¹³C¹⁸O¹⁶O) provides an additional constraint. In particular, the mass 47 anomaly (Δ₄₇) can distinguish between CO₂ produced by high temperature combustion processes vs. low temperature respiratory processes. Δ₄₇ is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured ¹³C and ¹⁸O values. In this study, we estimate the δ¹³C (vs. VPDB), δ¹⁸O (vs. VSMOW), δ47, and Δ₄₇ values of CO₂ from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO₂ from these sources. δ47 is defined as , where is the R⁴⁷ value for a hypothetical CO₂ whose δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 0, and Δ₄₇ = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO₂] from 383 to 404 μmol mol⁻¹, in δ¹³C and δ¹⁸O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ₄₇ from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ¹³C (similar to the δ¹³C of local gasoline), δ¹⁸O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ₄₇ of 0.41 ± 0.03‰. Both δ¹⁸O and Δ₄₇ values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO₂ and water generated by combustion of gasoline-air mixtures. Samples of CO₂ from human breath were found to have δ¹³C and δ¹⁸O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ₄₇ in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.  相似文献   

The role of sulfur in chemical weathering and atmospheric CO₂ fluxes: Evidence from major ions, δC_DIC, and δS_SO4 in rivers of the Canadian Cordillera     

Jody Spence  Kevin Telmer 《Geochimica et cosmochimica acta》2005,69(23):5441-5458

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, K⁺, Na⁺), δ¹³C of dissolved inorganic carbon (δ¹³C_DIC), and δ³⁴S of dissolved sulfate (δ³⁴S_SO4) to quantify chemical weathering rates and exchanges of CO₂ between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ³⁴S_SO4 (−8.9 to 14.1‰_CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ¹³C_DIC values (−9.8 to −3.7‰ _VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰).δ³⁴S_SO4 is negatively correlated with δ¹³C_DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ¹³C_DIC of rivers. The negative correlation between δ³⁴S_SO4 and δ¹³C_DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas.Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10³ mol C · km⁻² · yr⁻¹) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10³ mol C · km⁻² · yr⁻¹). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO₂ to carbonate rocks, sulfide-carbonate weathering can liberate CO₂ from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO₂ drawdown by silicate weathering in the region.  相似文献   

O/O and D/H in goethite from a North American Oxisol of the Early Eocene climatic optimum     

Crayton J. Yapp 《Geochimica et cosmochimica acta》2008,72(23):5838-5851

An Early Eocene Oxisol in the Ione Formation of California formed in a coastal continental weathering environment at a paleolatitude of ∼38°N. The dominant minerals in the Oxisol are goethite, quartz, and kaolinite. Material balance calculations were applied to new measurements of chemical composition, D/H, and ¹⁸O/¹⁶O ratios of Oxisol samples to determine the δD (−150 ± 3‰) and δ¹⁸O (−2.4 ± 0.3‰) values of the goethite (α-FeOOH). These data, in combination with the global meteoric water line (MWL), yielded an isotopic temperature of 21(±4) °C. The nominal value of 21 °C contrasts with the modern mean annual temperature (MAT) of 16 °C in that area. The warmer temperature is consistent with formation of the goethite during the Early Eocene climatic optimum. The isotopic composition of the goethite and a temperature of 21 °C imply ancient water with a δD value of −61(±4)‰ and a δ¹⁸O value of −8.9(±0.5)‰. This Early Eocene δ¹⁸O (or δD) value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a MAT of 21 °C.At times of warm global climates, the location of a near-surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions. A simple Rayleigh-type condensation model indicates that, if one “follows the isotherm”, the associated scatter in δD and δ¹⁸O of precipitation in very warm global climates should shift (for a given isotherm) to more negative values that may be detectable in proxy records. The isotopic results from the goethite of the Early Eocene Oxisol appear to add to evidence in support of this idea.  相似文献   

Oxygen isotope exchange between H₂O and CO₂ at elevated CO₂ pressures: Implications for monitoring of geological CO₂ storage     

G. Johnson  B. Mayer 《Applied Geochemistry》2011,26(7):1184-1191

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.  相似文献   

Graphite-bearing CO₂-fluid inclusions in granulites: Insights on graphite precipitation and carbon isotope evolution     

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 (Gr_I), which is interstitial to other mineral grains, can be grouped into two subtypes, Gr_IA and Gr_IB. Gr_IA 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 (Gr_IB) 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 (Gr_II) occurs as solid inclusions in silicate minerals, commonly forming regular hexagonal crystals with a slightly disordered structure. The third type of graphite (Gr_III) 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 (Gr_IV) 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   

Environmental memory and a possible seasonal bias in the stable isotope composition of (U-Th)/He-dated goethite from the Canadian Arctic     

Crayton J. Yapp  David L. Shuster 《Geochimica et cosmochimica acta》2011,75(15):4194-4215

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   

CO₂ solubility in Martian basalts and Martian atmospheric evolution     

Ben D. Stanley  Marc M. Hirschmann  Anthony C. Withers 《Geochimica et cosmochimica acta》2011,75(20):5987-6003

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.  相似文献   

Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: Implications for detection of microbial CO₂ signatures using δC-DIC     

Mark Skidmore  Martin Sharp 《Geochimica et cosmochimica acta》2004,68(21):4309-4317

Laboratory experiments on reagent-grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric CO₂. There is preferential dissolution of Ca¹²CO₃ during hydrolysis, resulting in δ¹³C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as ε_carb. ε_carb is greater on average for glacial sediments (−17.4‰) than for calcium carbonate (−7.8‰) for the < 63 μm size fraction, a sediment concentration of 5 g L⁻¹ and closed system conditions at 5°C. This difference is most likely due to the preferential dissolution of highly reactive ultra-fine particles with damaged surfaces that are common in subglacial sediments. The kinetic isotopic fractionation has a greater impact on δ¹³C-DIC at higher CaCO₃:water ratios and is significant during at least the first 6 h of carbonate dissolution driven by atmospheric CO₂ at sediment concentrations of 5 g L⁻¹. Atmospheric CO₂ dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least ∼ 6 h after the start of the experiment at 5°C. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the δ¹³C-DIC signal in natural environments where rock:water contact times are short <6-24 h (e.g., glacial systems, headwaters in fluvial catchments) and there is an excess of carbonate in the sediments. It will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial CO₂ signatures in DIC from δ¹³C-DIC data.  相似文献   

Carbon isotope evidence implying high O₂/CO₂ ratios in the Permo-Carboniferous atmosphere     

D.J. Beerling  J.A. LakeR.A. Berner  L.J. HickeyD.W. Taylor  D.L. Royer 《Geochimica et cosmochimica acta》2002,66(21):3757-3767

Theoretical models predict a marked increase in atmospheric O₂ to ∼35% during the Permo-Carboniferous (∼300 Ma) occurring against a low (∼0.03%) CO₂ level. An upper O₂ value of 35%, however, remains disputed because ignition data indicate that excessive global forest fires would have ensued. This uncertainty limits interpretation of the role played by atmospheric oxygen in Late Paleozoic biotic evolution. Here, we describe new results from laboratory experiments with vascular land plants that establish that a rise in O₂ to 35% increases isotopic fractionation (Δ¹³C) during growth relative to control plants grown at 21% O₂. Despite some effect of the background atmospheric CO₂ level on the magnitude of the increase, we hypothesize that a substantial Permo-Carboniferous rise in O₂ could have imprinted a detectable geochemical signature in the plant fossil record. Over 50 carbon isotope measurements on intact carbon from four fossil plant clades with differing physiological ecologies and ranging in age from Devonian to Cretaceous reveal a substantial Δ¹³C anomaly (5‰) occurring between 300 and 250 Ma. The timing and direction of the Δ¹³C excursion is consistent with the effects of a high O₂ atmosphere on plants, as predicted from photosynthetic theory and observed in our experiments. Preliminary calibration of the fossil Δ¹³C record against experimental data yields a predicted O₂/CO₂ mixing ratio of the ancient atmosphere consistent with that calculated from long-term models of the global carbon and oxygen cycles. We conclude that further work on the effects of O₂ in the combustion of plant materials and the spread of wildfire is necessary before existing data can be used to reliably set the upper limit for paleo-O₂ levels.  相似文献   

Iron weathering products in a CO₂ + (H₂O or H₂O₂) atmosphere: Implications for weathering processes on the surface of Mars     

V. Chevrier  P.-E. Mathé  O. Grauby  F. Trolard 《Geochimica et cosmochimica acta》2006,70(16):4295-4317

Various iron-bearing primary phases and rocks have been weathered experimentally to simulate possible present and past weathering processes occurring on Mars. We used magnetite, monoclinic and hexagonal pyrrhotites, and metallic iron as it is suggested that meteoritic input to the martian surface may account for an important source of reduced iron. The phases were weathered in two different atmospheres: one composed of CO₂ + H₂O, to model the present and primary martian atmosphere, and a CO₂ + H₂O + H₂O₂ atmosphere to simulate the effect of strong oxidizing agents. Experiments were conducted at room temperature and a pressure of 0.75 atm. Magnetite is the only stable phase in the experiments and is thus likely to be released on the surface of Mars from primary rocks during weathering processes. Siderite, elemental sulfur, ferrous sulfates and ferric (oxy)hydroxides (goethite and lepidocrocite) are the main products in a water-bearing atmosphere, depending on the substrate. In the peroxide atmosphere, weathering products are dominated by ferric sulfates and goethite. A kinetic model was then developed for iron weathering in a water atmosphere, using the shrinking core model (SCM). This model includes competition between chemical reaction and diffusion of reactants through porous layers of secondary products. The results indicate that for short time scales, the mechanism is dominated by a chemical reaction with second order kinetics (k = 7.75 × 10⁻⁵ g⁻¹/h), whereas for longer time scales, the mechanism is diffusion-controlled (De_A = 2.71 × 10⁻¹⁰ m²/h). The results indicate that a primary CO₂- and H₂O-rich atmosphere should favour sulfur, ferrous phases such as siderite or Fe²⁺-sulfates, associated with ferric (oxy)hydroxides (goethite and lepidocrocite). Further evolution to more oxidizing conditions may have forced these precursors to evolve into ferric sulfates and goethite/hematite.  相似文献   

Surface heat flow and CO₂ emissions within the Ohaaki hydrothermal field,Taupo Volcanic Zone,New Zealand     

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 MW_e). 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