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1.
Measured mole fractions ( X) and δ 13C values of the Fe(CO 3)OH component in pedogenic goethite from a mid-latitude Oxisol of Early Eocene age (≈52 Ma B.P.) range from 0.0014 to 0.0064 and −20.1 to −15.4‰, respectively. These values of X imply that concentrations of CO 2 gas in the paleosol were ≈7400 to ≈34,000 ppm. δ 13C and 1/ X are correlated and define a linear, soil-CO 2 diffusive mixing line with a positive slope. Such positive slopes are characteristic of mixing of two isotopically distinct CO 2 endmembers (atmospheric CO 2 and CO 2 from oxidation of soil organic matter). From the intercept of the mixing line, it is calculated that the δ 13C value of organic matter in the ancient soil was ≈−28.0‰. The magnitude of the slope implies an Early Eocene atmospheric CO 2 concentration of ≈2700 ppm.A simple model for forest soils suggests that a “canopy effect” may cause atmospheric CO 2 concentrations deduced from pedogenic minerals to underestimate the actual concentrations of atmospheric CO 2. If a significant forest canopy were present at the time of formation of pedogenic goethite in the Ione Fm, the concentration of 2700 ppm calculated for atmospheric CO 2 could be slightly low, but the underestimate is expected to be < ≈300 ppm (i.e., less than the analytical uncertainty). The relatively high concentration of 2700 ppm inferred for atmospheric CO 2 at ≈52 Ma B.P. would have been coincident with the Early Eocene climatic optimum. This result seems to support the case for an important role for variations of atmospheric CO 2 in the modification of global paleoclimate. 相似文献
2.
Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO 2 components—atmospheric CO 2 and CO 2 from in situ oxidation of organic matter. The δ 13C values measured for the Fe(CO 3)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(CO 3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO 2 components. δ 13C 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 Pco 2 value of about 1 × PAL, but the scatter in the measured δ 13C values of organic matter permits a calculated maximum Pco 2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO 3)OH in MCGoeth and SAP correspond to soil CO 2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO 2 concentrations are similar to modern soils in warm, wet environments.The average δ 13C 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 δ 13C value for all profiles of −5.4‰. Thus, the value of Δ 13C 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 Pco 2 and organic matter δ 13C values were the same for both paleosol types. Furthermore, the atmospheric Pco 2 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 Pco 2 (permitted by one standard deviation of the organic matter δ 13C value) of ∼5 × PAL.If, however, measured average δ 13C 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 Pco 2 using these 13C-enriched organic values would underestimate the actual atmospheric Pco 2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO 2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO 2.The Fe(CO 3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO 3)OH in goethites from soils in which there are mixtures of two isotopic CO 2 components. Field-relationships and the δ 13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO 2 components at the time of goethite crystallization. The three components were probably atmospheric CO 2, CO 2 from in situ oxidation of organic matter and CO 2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO 2 components, as recorded by Fe(CO 3)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 Pco 2. 相似文献
3.
The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate. 相似文献
4.
Evidence from laboratory experiments indicates that fractionation against the heavy stable isotope of carbon (Δ 13C) by bryophytes (liverworts and mosses) is strongly dependent on atmospheric CO 2. This physiological response may therefore provide the basis for developing a new terrestrial CO 2 proxy [Fletcher, B.J., Beerling, D.J., Brentnall, S.J., Royer, D.L., 2005. Fossil bryophytes as recorders of ancient CO 2 levels: experimental evidence and a Cretaceous case study. Global Biogeochem. Cycles19, GB3012]. Here, we establish a theoretical basis for the proxy by developing an extended model of bryophyte carbon isotope fractionation (BRYOCARB) that integrates the biochemical theory of photosynthetic CO 2 assimilation with controls on CO 2 supply by diffusion from the atmosphere. The BRYOCARB model is evaluated against measurements of the response of liverwort photosynthesis and Δ 13C to variations in atmospheric O 2, temperature and irradiance at different CO 2 concentrations. We show that the bryophyte proxy is at least as sensitive to variations in atmosphere CO 2 as the two other leading carbon isotope-based approaches to estimating palaeo-CO 2 levels ( δ13C of phytoplankton and of paleosols). Mathematical inversion of BRYOCARB provides a mechanistic means of estimating atmospheric CO 2 levels from fossil bryophyte carbon that can explicitly account for the effects of past differences in O 2 and climate. 相似文献
5.
Arctic soils contain a large fraction of Earth’s stored carbon. Temperature increases in the Arctic may enhance decomposition of this stored carbon, shifting the role of Arctic soils from a net sink to a new source of atmospheric CO 2. Predicting the impact of Arctic warming on soil carbon reserves requires knowledge of the composition of the stored organic matter. Here, we employ solid state 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared-photoacoustic spectroscopy (FTIR-PAS) to investigate the chemical composition of soil organic matter collected from drained thaw-lake basins ranging in age from 0 to 5500 years before present (y BP). The 13C NMR and FTIR-PAS data were largely congruent. Surface horizons contain relatively large amounts of O-alkyl carbon, suggesting that the soil organic matter is rich in labile constituents. Soil organic matter decreases with depth with the relative amounts of O-alkyl carbon decreasing and aromatic carbon increasing. These data indicate that lower horizons are in a more advanced stage of decomposition than upper horizons. Nonetheless, a substantial fraction of carbon in lower horizons, even for ancient thaw-lake basins (2000-5500 y BP), is present as O-alkyl carbon reflecting the preservation of intrinsically labile organic matter constituents. Climate change-induced increases in the depth of the soil active layer are expected to accelerate the depletion of this carbon. 相似文献
6.
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 δ 13Cm values of the Fe(CO 3)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 δ 13Cm vs. 1/ Xm suggest the existence of two generations of goethite in the pisolites. The apparently younger generation of goethite (“generation 2”) evolves CO 2 at 170 °C, while the older generation (“generation 1”) evolves CO 2 at 220 °C. The distribution of the data suggests that generation 2 goethite is a proxy for mixing of CO 2 from three distinct CO 2 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 CO 2 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 δ 13Cm 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 CO 2 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 CO 2 (∼1900 ppmV) is consistent with the idea that variations of atmospheric CO 2 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
7.
This article focuses on the contribution of natural ecosystems (forests, grasslands) and agrosystems to carbon sequestration either in biomass or in soil. Carbon stocks are important (650 Gt in biomass, 1500 to 2000 Gt in soils as compared with 750 for atmospheric CO 2), and also fluxes that led to CO 2 emissions in the past (due to deforestation or cultivation) and which now turn to carbon sequestration (2 Gt C/year). This article shows great spatial variations in stocks and fluxes and great measurement difficulties, especially for stock variations. Anthropic actions such as reforestation (mainly in the North), changes in land use or in crop management, can increase carbon sequestration in biomass or soil, with a residence time of several decades, which is not insignificant with respect to the Kyoto protocol and which also has other environmental benefits. To cite this article: M. Robert, B. Saugier, C. R. Geoscience 335 (2003). 相似文献
8.
The role of CO 2 in karst has been of interest for decades, and emphasized by IGCP 379, International Geoscience Programme, UNESCO started in 1995. There are still open questions about the dynamics of carbon in karst systems, particularly the flux of carbon between the surface and subsurface and between different components in the karst subsurface. This research report focuses on the variations of hydrochemistry and PCO 2 (partial pressures of carbon dioxide) in subtropical karst groundwater, using high-resolution auto-monitoring hydrochemical data (15-min intervals). The aim of this study was to understand how hydrochemistry and PCO 2 in karst systems respond to recharge over different time scales and what the controlling factors are. An auto-monitoring hydrochemistry station was installed about 300 m upstream from the exit in the active stream channel of Xueyu Cave, a typical subtropical karst cave. Four years of high-resolution continuous pH, specific conductivity (Spc), temperature and water-level data were collected. A thermodynamic model was used to link the continuous data to monthly water quality data, allowing the calculation of CO 2 partial pressures and calcite saturation (SIc) levels on a continuous basis. Seasonal, diurnal and storm-scale variations were captured in the hydrochemistry and calculated PCO 2 records, indicating that the cave stream is a dynamic and variable system. Seasonal features (higher specific conductivity and lower pH in summer; lower specific conductivity and higher pH in winter) tend to covary with temperature which influences the production of CO 2 in soils, thus being the driving force for the variations (the soil CO 2 effect). Due to the buffer effect of a thick vadose zone and large void cave space, diurnal variations are not obvious compared with epikarst springs in SW China. Storm-scale fluctuations due to storm events occur during the summer rainy season. Piston flow effects, dilution and soil CO 2 effects determine the variations in different storm events. At the beginning of the rains, the piston effect drives the variations, characterized by increase in Spc, SIc and pH in the cave stream and decrease in PCO 2. With heavy rainfall, decrease in Spc shows control by the dilution effect, while decrease in SIc and pH and increase in PCO 2 indicates the greater influence of soil CO 2. These results imply that the soil and cave voids are important factors influencing the hydrochemical evolution of karst groundwater. Future works need to use such high-resolution technology widely for tracing the PCO 2 and hydrochemical variations in different karst aquifers. 相似文献
9.
The first and possibly only major rise of atmospheric oxygen, from pO2 ≤ 0.1% PAL (the present atmospheric level) to pO2 ≥ 10% PAL, appears to have occurred sometime before 2 Ga ago, although the exact time of and the cause(s) for the rise have been hotly debated. Equally important questions on the atmospheric oxygen concern its stability, especially the mechanisms regulating the atmospheric pO2 level and the causes and magnitude of pO2 variations since the first major rise of atmospheric oxygen. Previous efforts to model the pO2 variation during the Phanerozoic time have typically relied on secondary information, such as the carbon and sulfur isotopic records of sedimentary rocks, and on simple dynamics of the geochemical cycles of O, C, S, and P based on box-type models. As a result, many kinetic questions about the variation and stability of atmospheric oxygen could not have been answered. Here we quantitatively evaluate the dynamics and stability of atmospheric O 2 and CO 2, using recent experimental data, field observations, and a new model for the C-O coupled geochemical cycles. We examine the change with time in the fluxes of various compounds (O 2, CO 2, phosphate, organic C, carbonate C, C-bearing reduced volcanic gases, and C-free reduced volcanic gases) among the various reservoirs (atmosphere, soil, surface ocean, deep ocean, the lower crust and mantle, and upper crust) under a variety of scenarios. Our model does not assume steady-state fluxes for any of the reservoirs. Rather, the model incorporates the kinetic experimental data on oxidation of coal, a proxy for kerogen, the dynamics of soil formation and erosion, the kinetics of decomposition of organic matter in the Oceans by aerobic and anaerobic bacteria, the equilibrium ocean-atmosphere carbonate model, the observed relationships among the organic burial flux, dissolved O 2 content of deep ocean, and sedimentation rates, and the three-box model ocean. The important parameters that strongly influence the dynamics of atmospheric O 2, are found to be (a) the total area of soil formation on Earth; (b) the average soil depth; (c) the average rate of physical erosion of soils, which is linked to the average rate of accumulation of clastic sediments in the oceans; (d) the composition and flux of volcanic gas; and (e) the level of atmospheric CO 2. We develop kinetic equations linking these parameters to the production and consumption fluxes of atmospheric oxygen and also to stable pO2 values. Considering the likely ranges of variations in these parameters in geologic history, we suggest that the atmospheric pO2 level is likely to have stayed within a very narrow range of 0.6-2 PAL and that the entire ocean, except for local euxinic basins, is likely to have been basically oxygenated since the first major rise of atmospheric oxygen more than 2 Ga ago. 相似文献
10.
Soils of the Chinese Loess Plateau(CLP)contain substantial amounts of soil inorganic carbon(SIC),as well as recent and ancient soil organic carbon(SOC).With the advent of the Anthropocene,human perturbation,including excavation,has increased soil CO 2 emission from the huge loess carbon pool.This study aims to determine the potential of loess CO 2 emission induced by excavation.Soil CO 2 were continuously monitored for seven years on a newly-excavated profile in the central CLP and the stable C isotope compositions of soil CO 2 and SOC were used to identify their sources.The results showed that the soil CO 2 concentrations ranged from 830μL·L -1 to 11190μL·L -1 with an annually reducing trend after excavation,indicating that the human excavation can induce CO 2 production in loess profile.Theδ 13 C of CO 2 ranged from–21.27‰to–19.22‰(mean:–20.11‰),with positive deviation from top to bottom.The range of δ 13CSOC was–24.0‰to–21.1‰with an average of–23.1‰.Theδ 13 C-CO 2 in this study has a positive relationship with the reversed CO 2 concentration,and it is calculated that 80.22%of the soil CO 2 in this profile is from the microbial decomposition of SOC and 19.78%from the degasification during carbonate precipitation.We conclude that the human excavation can significantly enhance the decomposition of the ancient OC in loess during the first two years after perturbation,producing and releasing soil CO 2 to atmosphere. 相似文献
11.
Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of δ 18O and δ 13C observed in terrestrial caliche. Within each volcanic field, δ 18O is broadly co-variant with δ 13C and increases as δ 13C increases. The most 18O and 13C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most 18O and 13C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has δ 18O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low δ 13C characteristic of microbial soil CO 2. Subaerial caliche has formed from 18O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated 13C enrichment is due either to removal of 12C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a “warm, wet” early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for δ 18O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche. 相似文献
12.
Annually laminated carbonates, known as tufas, commonly develop in limestone areas and typically record seasonal patterns of oxygen- and carbon-isotope compositions. δ 18O values are principally controlled by seasonal changes of water temperature, whereas δ 13C values are the result of complex reactions among the gaseous, liquid, and solid sources of carbon in the system. We examined the processes that cause the seasonal patterns of δ 13C in groundwater systems at three tufa-depositing sites in southwestern Japan by applying model calculations to geochemical data. Underground inorganic carbon species are exchanged with gaseous CO 2, which is mainly introduced to the underground hydrological system by natural atmospheric ventilation and by diffusion of soil air. These processes control the seasonal pattern of δ 13C, which is low in summer and high in winter. Among the three sites we investigated, we identified two extreme cases of the degree of carbon exchange between liquid and gaseous phases. For the case with high radiocarbon composition (Δ 14C) and low pCO 2, there was substantial carbon exchange because of a large contribution of atmospheric CO 2 and a small water mass. For the other extreme case, which was characterized by low Δ 14C and high pCO 2, the contribution of atmospheric CO 2 was small and the water mass was relatively large. Our results suggest that at two of the three sites water residence time within the soil profile was longer than 1 year. Our results also suggested a short residence time (less than 1 year) of water in the soil profile at the site with the smallest water mass, which is consistent with large seasonal amplitude of the springwater temperature variations. The Δ 14C value of tufas is closely related to the hydrological conditions in which they are deposited. If the initial Δ 14C value of a tufa-depositing system is stable, 14C-chronology can be used to date paleo-tufas. 相似文献
13.
Concentrations of CO 2 in soil atmosphere and CO 2 efflux were measured across a marine terrace soil chronosequence near Santa Cruz, California. Soil development, specifically the formation of an argillic horizon, has created a two-tier soil gas profile in the older terrace soils. The soil above the argillic horizon has seasonal variations in soil CO 2 associated with plant respiration. The older soils with dense argillic horizons maintain a year round ~1%CO 2 below the argillic horizon. The CO 2efflux during the growing season is higher on the older terraces. 相似文献
14.
Soils overlying two porphyry Cu deposits (Spence, Gaby Sur) and the Pampa del Tamarugal, Atacama Desert, Northern Chile were collected in order to investigate the extent to which saline groundwaters influence “soil” chemistry in regions with thick Miocene and younger sediment cover. Soil carbonate (calcite) was analyzed for C and O isotopes and pedogenic gypsum for S isotopes. Soil calcite is present in all soils at the Spence deposit, but increases volumetrically above two fracture zones that cut the Miocene gravels, including gravels that overlie the deposit. The C isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the Spence deposit; all δ 13C VPDB values fall within a narrow range (1.40–4.23‰), consistent with the carbonate having formed in equilibrium with atmospheric CO 2. However, δ 18O VPDB for carbonate over both fracture zones is statistically different from carbonate elsewhere (average δ 18O VPDB = 0.82‰ vs. −2.23‰, respectively), suggesting involvement of groundwater in their formation. The composition of soils at the Tamarugal anomaly has been most strongly affected by earthquake-related surface flooding and evaporation of groundwater; δ 13C VPDB values (−4.28‰ to −2.04‰) are interpreted to be a mixture of dissolved inorganic C (DIC) from groundwater and atmospheric CO 2. At the Spence deposit, soils only rarely contain sufficient SO 4 for S isotope analysis; the SO 4-bearing soils occur only above the fracture zones in the gravel. Results are uniform (3.7–4.9‰ δ 34S CDT), which is near the middle of the range for SO 4 in groundwater (0.9–7.3‰). Sulfur in soils at the Gaby Sur deposit (3.8–6.1‰ δ 34S CDT) is dominated by gypsum, which primarily occurs on the flanks and tops of hills, suggesting deposition from SO 4-rich fogs. Sulfate in Gaby Sur deposit gypsum is possibly derived by condensation of airborne SO 4 from volcanic SO 2 from the nearby Andes. At the Gaby Sur deposit and Tamarugal anomaly, pedogenic stable isotopes cannot distinguish between S from porphyry or redeposited SO 4 from interior salars.The three sites studied have had different histories of salt accumulation and display variable influence of groundwater, which is interpreted to have been forced to the surface during earthquakes. The clear accumulation of salts associated with fractures at the Spence deposit, and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process. The interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism. 相似文献
15.
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 δ 13C values of CO 2 occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO 3)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO 2 in the ancient soil. The δ 13C values of CO 2 occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO 2.The measured mole fractions (X) of Fe(CO 3)OH in the high-Al goethites range from 0.0059 (±0.0005) to 0.0077 (±0.0006) and correspond to soil CO 2 concentrations of ∼28,000 to ∼37,000 ppmV. The average values of X and δ 13C for the four high-Al goethites are 0.0067 ± 0.0007 and −20.1 ± 0.5‰, respectively. The δ 13C 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 CO 2 concentration of ∼2400 ppmV (± ∼1200 ppmV) at ∼60 Ma BP. The inferred high concentration of atmospheric CO 2 would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO 2 plays an important role in climate variation. 相似文献
16.
Accurate measurements of soil CO 2 concentrations (pCO 2) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO 2, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO 2 distribution can predictably vary with topographic position. We measured soil pCO 2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO 2 (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO 2 generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO 2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO 2 diffusive fluxes. The highest pCO 2 values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO 2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO 2 because they are shallower (≤0.6 m) and drier, resulting in high CO 2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO 2: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO 2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO 2 was similar across the watershed, the amplitude of the pCO 2 peak was higher in the deep soils due to more variable moisture content. The high pCO 2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO 2 alone. CO 2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO 2 dynamics would be improved by including landscape controls on soil pCO 2. 相似文献
17.
Stable isotope ratios of oxygen and carbon were determined for CO 2 in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO 2 concentrations detected previously by McCarthy et al. (1986). Values of δ13C in soil gas CO 2 from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ13C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C 3 (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C 3 and C 4 (Hatch-Slack) plants, together with atmospheric CO 2 are the likely sources of carbon in soil gas CO 2. Values of δ18O in soil-gas CO 2 range from 32 to 38‰ (SMOW). These δ18O values are intermediate between that calculated for CO 2 gas in isotopic equilibrium with local groundwaters and that for atmospheric CO 2. The δ 18O data indicate that atmospheric CO 2 has been incorporated by mixing or diffusion. Any CO 2 generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO 2 taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ13C value of soil-gas CO 2 was observed, contrary to what would be expected if the anomalous CO 2 were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C 3 plant material were responsible for most CO 2 generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO 2 anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO 2 concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases. 相似文献
18.
Stable isotopes were measured in the carbonate and organic matter of palaeosols in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy records for climatic and ecological changes in this area during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition was not accompanied by significant change in δ 18O of pedogenic carbonate. This could be explained by changes in evaporation rate and in isotope fractionation between water and precipitated carbonate with temperature, which counterbalanced the expected change in isotope composition of meteoric water. Because of the rise in temperature and humidity and the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in δ 18O of pedogenic carbonate recorded after this eruption. The δ 13C values of organic matter and pedogenic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO 2, and the pedogenic carbonate. Carbon isotopes suggest prevailing C 3 vegetation and negligible mixing with volcanogenic or atmospheric CO 2. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
19.
Several recent studies have highlighted the importance of soil organic matter (SOM) mineralization at high latitudes during winter for ecosystem carbon (C) balances, and the ability of the soil to retain unfrozen water at sub-zero temperatures has been shown to be a major determinant of C mineralization rates. Further, SOM is believed to strongly influence the liquid water contents in frozen surface layers of boreal forest soils and tundra, but the mechanisms and specific factors involved are currently unknown. Here we evaluate the effects of the chemical composition of SOM on the amount of unfrozen water, the pore size equivalents in which unfrozen water can exist, and the microbial heterotrophic activity at sub-zero temperatures in boreal forest soils. To do this, we have characterized the chemical composition of SOM in forest soil samples (surface O-horizons) using solid state CP-MAS (cross polarization magic angle spinning) NMR spectroscopy. The acquired information was then used to elucidate the extent to which different fractions of SOM can explain the observed variations in unfrozen water content, pore size equivalents, and biogenic CO 2 production rates in the examined soil samples under frozen conditions (−4 °C). The data evaluation was done by the use of principal component analysis (PCA) and projections to latent structures by means of partial least square (PLS). We conclude that aromatic, O-aromatic, methoxy/ N-alkyl and alkyl C are the major SOM components affecting frozen boreal forest soil’s ability to retain unfrozen water and sustain heterotrophic activity (95% confidence level). Our results reveal that solid carbohydrates have a significant negative impact (95% confidence level) on CO 2 production in frozen boreal spruce forest soils, in contrast to the positive effects of carbohydrate polymers during unfrozen conditions. We conclude that the hierarchy of environmental factors controlling SOM mineralization changes as soils freeze. The effect of SOM composition on pore size distribution and unfrozen water content has a superior influence on SOM mineralization and hence on heterotrophic CO 2 production of frozen soils. 相似文献
20.
土壤呼吸是陆地植物固定CO 2尔后又释放CO 2返回大气的主要途径,是与全球变化有关的一个重要过程。综述了全球变化下CO 2浓度上升、全球增温、耕作方式的改变及氮沉降增加的土壤呼吸效应。大气CO 2浓度的上升将增加土壤中CO 2的释放通量,同时将促进土壤的碳吸存;在全球增温的情形下,土壤可能向大气中释放更多的CO 2,传统的土地利用方式可能是引发温室气体CO 2产生的重要原因,所有这些全球变化对土壤呼吸的作用具有不确定性。认为土壤碳库的碳储量增加并不能减缓21世纪大气CO 2浓度的上升。据此讨论了该问题的对策并提出了今后土壤呼吸的一些研究方向。其中强调,尽管森林土壤碳固定能力有限,但植树造林、森林保护是一项缓解大气CO 2上升的可行性对策;基于现有田间尺度CO 2通量测定在不确定性方面的进展,今后应继续朝大尺度田间和模拟程序方面努力;着重回答全球变化条件下的土壤呼吸过程机理;区分土壤呼吸的不同来源以及弄清土壤呼吸黑箱系统中土壤微生物及土壤动物的功能。当然,土壤呼吸的测定方法尚有待改善。 相似文献
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