首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 218 毫秒
1.
Organic carbon isotope composition was studied in the sedimentary cover of the southern Siberian Platform and its surrounding fold systems. The rocks experienced catagenesis, metamorphism, and metasomatism. The chloroform bitumoid (CB) has a stable carbon isotope composition within a wide range of postsedimentation transformations. The average values of δ13C in CB of the sedimentary cover are ?29.5‰. Metamorphism and, especially, ore metasomatism, at the Sukhoi Log deposit caused a 2‰ increase in the heavy carbon isotope concentration of CB as compared to that of the platform deposits. The narrow variations in carbon isotope composition of the bitumoid are defined by their derivation from lipids, whose components are almost insusceptible to changes in the PT conditions. Kerogen from platform deposits is more strongly depleted than CB in the heavy carbon isotope (δ13Cav ? 32.2‰). The insoluble carbonaceous matter (ICM) of the metamorphic shales is significantly enriched in the heavy carbon isotope (δ13Cav ? 21.9‰). The highest changes in carbon isotope composition were found in concentrates of ICM from metasomatically altered rocks of the Sukhoi Log deposit (δ13Cav ? 17.5‰). The heavier carbon isotope composition caused by metamorphism and metasomatism is evidently defined by isotopic exchange between the carbonate carbon and CO2 of metasomatic solutions, on one hand, and ICM of shales, on the other.  相似文献   

2.
Monitoring and sampling of main plants,soil CO2,soil water,bedrock,spring water,drip water and its corresponding speleothem were performed at four cave systems of Guizhou,Southwest China,from April 2003 to May 2004,in order to understand stable carbon isotope ratios variations of dissolved inorganic Carbon(DIC) in cave percolation waters(δ13CDIC) and their implications for paleoclimate.Stable carbon isotopic compositions and ions(Ca,Mg,Sr,SO4,Cl etc.) were measured for all samples.The results indicate that there are significant differences among the δ13CDIC values from inter-cave,even inter-drip of intra-cave in the four caves.The δ13CDIC values from the Liangfeng Cave(LFC) is lightest among the four caves,where vegetation type overlying the cave is primary forest dominated by tall trees with lighter average δ13C value(–29.9‰).And there are remarkable differences in δ13CDIC values of different drip waters in the Qixing Cave(QXC) and Jiangjun Cave(JJC),up to 6.9‰ and 7.8‰,respectively.Further analyses show that the δ13CDIC values in cave drip waters are not only controlled by vegetation biomass overlying the cave,but also hydro-geochemical processes.Therefore,accurate interpreting of δ13C recorded in speleothems cannot be guaranteed if these effects of the above mentioned factors are not taken into consideration.  相似文献   

3.
Carbon stable-isotope compositions of coexisting carbon dioxide and methane from geothermal springs across the Central Andes of northern Chile and Bolivia are reported. A total of 60 samples were analyzed for δ13CCO2 and, of these, 10 were selected for δ13CCH4 analyses. The Central Andes are characterized by an active volcanic arc and an unusually thick (up to 75 km) continental crust behind the arc, beneath the high plateau region of the Altiplano. Furthermore, helium-isotope evidence suggests active mantle degassing in a 350-km-wide zone beneath the thick continental crust in the Central Andes (Hoke et al., 1994).

The present results show a wide range of δ13CCO2 (-14.9 to -0.6‰) and a surprisingly heavy δ13CCH4 (?20.9 to ?12.3‰). The difference between δ13CCO2 and δ13CCH4 (δ13CCO2-CH4 ) for individual samples varies between 1.5‰ and 13.5‰. The δ13CCO2 results show wide and overlapping ranges in the samples collected from the Precordillera, the Volcanic Arc (or Western Cordillera), the Altiplano, and the Eastern Cordillera. The widest ranges occur in the Eastern Cordillera (?15.0 to ?4.8‰) and the Altiplano (?20 to ?6‰). The δ13CCO2 results for geothermal samples from the Volcanic Arc range between ?8.0‰ (Surire) and ?0.6‰ (Abra de Nappa), whereas δ13CCO2 measured in gases collected from geothermal springs in the Precordillera range from ?10 to ?5‰.

The relationships between 3He/4He, δ13CCO2 , and δ13CCH4 are used to distinguish between crustal and mantle origins. The wide (21‰) range in the is interpreted to reflect contributions from different CO2 sources that include organic and inorganic crustal and mantle carbon. Assuming isotopic equilibrium between coexisting methane and carbon dioxide, Δ13CCO2-CH4 suggests very high equilibrium temperatures, in excess of 530°C, for some geothermal systems that also are characterized by a high (up to 63%) mantle-derived helium component.

δ13CCH4 results suggest that methane has not formed by bacteriogenic processes or by thermal decomposition of organic matter, but rather abiogenically through the high-temperature reaction between H2 and CO2. The δ13CCH4 results for the samples from the Volcanic Arc and from two CO2-rich geothermal springs in the Altiplano (Coipasa-2 and Belen de Andamarca) are similar to those reported from hydrothermal fluids emitted from the East Pacific Rise (Welhan, 1988) and White Island, New Zealand (Hulston and McCabe, 1962), suggesting a mantle-derived carbon component in the methane.  相似文献   

4.
The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ13C values from–6.6 to 1.3‰ and δ18O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ13C from–5.2 to–2.9 and δ18O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ13C from–16.4 to–13.1 and δ18O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ13C from–13.0 to–10.4‰ and δ18O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ18O from 18.7 to 17.6‰ and δ13C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO2. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.  相似文献   

5.
The carbon isotopic composition of CO2 inclusions trapped in minerals reflects the origin and evolution of CO2-bearing fluids and melts, and records the multiple-stages carbon geodynamic cycle, as CO2 took part in various geological processes widely. However, the practical method for determination isotope composition of individual CO2 inclusion is still lacking. Developing a microanalytical technique with spatial resolution in micrometers to precisely determinate the δ13C value of individual CO2 inclusion, will make it possible to analyze a tiny portion of a zoning mineral crystal, distinguish the differences in micro-scale, and possible to find many useful information that could not be obtained with the bulk extraction and analysis techniques. In this study, we systematically collected Raman spectra of CO2 standards with different δ13C values (?34.9 ‰ to 3.58 ‰) at 32.0 °C and from ～7.0 MPa to 120.0 MPa, and developed a new procedure to precisely determinate the δ13C value of individual CO2 inclusion. We investigated the relationship among the Raman peak intensity ratio, δ13C value, and CO2 density, and established a calibration model with high accuracy (0.5 ‰?1.5 ‰), sufficient for geological application to distinguish different source of CO2 with varying δ13CO2. As a demonstration, we measured the δ13C values and the density of CO2 inclusions in the growth zones of alkali basalt-hosted corundum megacrysts from Changle, Shandong Province. We found the significant differences of density and δ13C between the CO2 inclusions in the core of corundum and those inclusions in the outer growth zones, the δ13C value decreases from core to rim with decreasing density: δ13C values are from ?7.5 ‰ to ?9.2 ‰ for the inclusions in the core, indicating the corundum core was crystallized from mantle-derived magmas; from ?13.5 ‰ to ?18.5 ‰ for CO2 inclusions in zone 1 and from ?16.5 ‰ to –22.0 ‰ for inclusions in zone 2, indicating the outer zones of corundum grew in a low δ13C value environment, resulted from an infilling of low δ13C value fluid and/or degassing of the ascending basaltic magma.  相似文献   

6.
The first detailed isotope-geochemical study of carbonate deposits has been performed in the Lower Famennian stratotype section of the northwestern Kuznetsk Basin (Kosoy Utyos), which was localized in the middle latitudes of the Northern Hemisphere in the Late Devonian. The δ13Ccarb, δ13Corg, and δ18O variation curves were constructed for the section deposits. Geochemical and petrographic studies of carbonates allowed allocation of samples that underwent postsedimentation alteration and exclude them from further interpretation. Compared with coeval sections in the other world's regions, the Kosoy Utyos section is characterized by higher δ13Ccarb values, up to 5.4‰, whereas the maximum value in subequatorial area sections is 4‰. The isotope shift amplitude of the studied section reaches 4.6‰, which is 1.5‰ higher than those in other regions. The δ18O values are 3‰ lower than the ones of the world's coeval sections. The results obtained show that δ13C and δ18O variation trends differ from those of coeval subequatorial sections. The high shift amplitude and maximum δ13Ccarb values in the Kosoy Utyos section are due to the shallow-water carbonate sedimentation environments on the Siberian continental shelf and, probably, the lower temperatures of waters in the middle latitudes as compared with the subequatorial areas.  相似文献   

7.
An in situ measurement method for simultaneous determination of carbon and nitrogen isotope ratios in organic matter was developed by secondary ion mass spectrometry with a spatial resolution of ~ 12 μm. Secondary ion intensities of 12C?2, 12C13C?, 12C14N? and 12C15N? were simultaneously measured by three Faraday cups and one electron multiplier. Ions of 12C2H? were measured to monitor hydride interferences and to correct for mass bias. The analytical precisions of δ13C and δ15N values of a reference material (UWLA‐1 anthracite) were 0.16‰ and 0.56‰, respectively (2s). A negative correlation was observed between the mass bias of carbon and 12C2H?/12C?2 ratios of examined reference materials. In contrast, there was no correlation between mass bias and hydrogen concentration for nitrogen. The δ13CVPDB and δ15NAir values of twenty‐two individual globules of organic matter, found in carbonate rock of the 1878 Ma Gunflint Formation, were determined by the new procedure, ranging from ?33.8‰ to ?33.3‰ and +4.2‰ to 5.8‰, respectively. Means of δ13CVPDB and δ15NAir values, ?33.5 ± 0.25‰ and +5.2 ± 0.81‰, are consistent with reported values from bulk sample analysis within analytical precision.  相似文献   

8.
Stable isotopes of injected CO2 act as useful tracers in carbon capture and storage (CCS) because the CO2 itself is the carrier of the tracer signal and remains unaffected by sorption or partitioning effects. At the Ketzin pilot site (Germany), carbon stable isotope composition (δ13C) of injected CO2 at the injection well was analyzed over a time period of 4 months. Occurring isotope variances resulted from the injection of CO2 from two different sources (an oil refinery and a natural gas-reservoir). The two gases differed in their carbon isotope composition by more than 27‰. In order to find identifiable patterns of these variances in the reservoir, more than 250 CO2-samples were collected and analyzed for their carbon isotope ratios at an observation well 100 m distant from the injection well. An isotope ratio mass spectrometer connected to a modified Thermo Gasbench system allowed quick and cost effective isotope analyses of a high number of CO2 gas specimens. CO2 gas from the oil refinery (δ13C = −30.9‰, source A) was most frequently injected and dominated the reservoir δ13C values at the injection site. Sporadic injection of the CO2 from the natural gas-reservoir (δ13C = −3.5‰, source B) caused isotope shifts of up to +5‰ at the injection well. These variances provided a potential ideal tracer for CO2 migration behavior. Based on these findings, tracer input signals that were injected during the last 2 years of injection could be reconstructed with the aid of an isotope mixing model and CO2 delivery schedules. However, in contrast to the injection well, δ13C values at the observation well showed no variances and a constant value of −28.5‰ was measured at 600 m depth. This is in disagreement with signals that would be expected if the input signals from the injection would arrive at the observation well. The lack of isotope signals at the observation well suggests that parts of the reservoir are filled with CO2 that is immobilized.  相似文献   

9.
Mineralization of groundwater in volcanic aquifers is partly acquired through silicates weathering. This alteration depends on the dissolution of atmospheric, biogenic, or mantellic gaseous CO2 whose contributions may depend on substratum geology, surface features, and lava flow hydrological functionings. Investigations of $ {\text{P}}_{{{\text{CO}}_{ 2} {\text{eq}}}} $ and δ13CTDIC (total dissolved inorganic carbon) on various spatiotemporal scales in the unsaturated and saturated zones of volcanic flows of the Argnat basin (French Massif Central) have been carried out to identify the carbon sources in the system. Mantellic sources are related to faults promoting CO2 uplift from the mantle to the saturated zone. The contribution of this source is counterbalanced by infiltration of water through the unsaturated zone, accompanied by dissolution of soil CO2 or even atmospheric CO2 during cold periods. Monitoring and modeling of δ13CTDIC in the unsaturated zone shows that both $ {\text{P}}_{{{\text{CO}}_{ 2} {\text{eq}}}} $ and δ13CTDIC are controlled by air temperature which influences soil respiration and soil-atmosphere CO2 exchanges. The internal geometry of volcanic lava flows controls water patterns from the unsaturated zone to saturated zone and thus may explain δ13C heterogeneity in the saturated zone at the basin scale.  相似文献   

10.
Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time,as long as they are acquired from waters where the dissolved inorganic carbon(DIC)is in isotope equilibrium with the atmospheric CO2.However,in shallow water platforms and epeiric settings,the influence of local to regional parameters on carbon cycling may lead to DIG isotope variations unrelated to the global carbon cycle.This may be especially true for the terminal Neoproterozoic,when Gondwana assembly isolated waters masses from the global ocean,and extreme positive and negative carbon isotope excursions are recorded,potentially decoupled from global signals.To improve our understanding on the type of information recorded by these excursions,we investigate the pairedδ^13Ccarb andδ^13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior:the basal Bambui Group.This succession represents a 1~(st)-order sedimentary sequence and records two majorδ^13Ccarb excursions in its two lowermost lower-rank sequences.The basal cap carbonate interval at the base of the first sequence,deposited when the basin was connected to the ocean,hosts antithetical negative and positive excursions forδ^13Ccarb andδ^13Corg,respectively,resulting inΔ^13C values lower than 25‰.From the top of the basal sequence upwards,an extremely positiveδ^13Ccarb excursion is coupled toδ^13Corg,reaching values of+14‰and-14‰,respectively.This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambui Group that occurs with only minor changes inΔ^13C values,suggesting change in the DIC isotope composition.We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles.This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis,favored by the basin restriction.The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere,resulting in a^13C-enriched DIC influenced by methanogenic CO2.Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere,potentially affecting both the global carbon cycle and the climate.  相似文献   

11.
Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78–79% methane and 2–19% C2+ HC. Its dryness coefficient (C1/C1–5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH4 and heavy hydrocarbons (C2+) and higher contents of non-hydrocarbons (e.g. CO2), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (δ13C1: −45‰ to −36‰) and heavier values for ethane (δ13C2: −30‰ to −25‰). The case is similar for gas in fluid inclusions, but δ13C1 = −36‰ to −45‰ and δ13C2 = −24.8‰ to −28.1‰, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has δ13CCO2 values of −15.6‰ to −5.6‰, while the gas in fluid inclusions has δ13CCO2 values of −16.6‰ to −9‰, indicating its organic origin. Geochemical comparison shows that CO2 captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO2. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin.  相似文献   

12.
We discuss water oxygen isotopes (δ18Ow) and carbon isotopes of dissolved inorganic carbon (δ13CDIC) of brine‐enriched shelf water (BSW) from Storfjorden (southern Svalbard) in comparison to Recent benthic foraminiferal δ18Oc and δ13Cc calcified in the same water. We determined relatively high δ18Ow values of 0.15±0.03‰ VSMOW in BSW below sill depth at temperatures below ?1.8 °C, and high δ18Oc values of 3.90±0.18‰ VPDB. Such high BSW δ18Ow cannot significantly deplete 18Ow contents of Arctic Ocean deep water; furthermore, such high δ18Oc cannot be distinguished from δ18Oc values of 3.82±0.12‰, calcified in warmer Arctic and Nordic seas intermediate and deeper waters. Today, in Storfjorden low benthic δ13Cc and high δ18Oc reflect the low δ13CDIC and relatively high δ18Ow of BSW. High benthic δ18Oc is in contrast to expected low δ18Oc as brine rejection is widely thought to predominantly take place in surface water diluted by meteoric water with very low δ18Ow. Low epibenthic δ13Cc values of 0.50±0.12‰ partly reflect low δ13CDIC caused by enhanced uptake of atmospheric low δ13CCO2 decreased by anthropogenic activities. An adjustment for preindustrial higher values would increase δ13Cc by about 0.6‰. Therefore, in Storfjorden brine formed before the industrial era would be characterized by both high δ13Cc as well as high δ18Oc values of benthic foraminiferal calcite. Our data may cast doubt on scenarios that explain negative excursions in benthic foraminiferal stable isotope records from the Atlantic Ocean during cold stadials in the last glacial period by enhanced brine formation in Nordic seas analogously to modern processes in Storfjorden.  相似文献   

13.
《Applied Geochemistry》2004,19(5):695-707
Taiwan is located at the collision boundary between the Philippine Sea Plate and the Asian Continental Plate and is one of the most active orogenic belts in the world. Fluids sampled from 9 sub-aerial mud volcanoes distributed along two major geological structures in southwestern Taiwan, the Chishan fault and the Gutingkeng anticline, were analyzed to evaluate possible sources of water and the degree of fluid-sediment interaction at depth in an accretionary prism. Overall, the Taiwanese mud volcano fluids are characterized by high Cl contents, up to 347 mM, suggesting a marine origin from actively de-watering sedimentary pore waters along major structures on land. The fluids obtained from the Gutingkeng anticline, as well as from the Coastal Plain area, show high Cl, Na, K, Ca, Mg and NH4, but low SO4 and B concentrations. In contrast, the Chishan fault fluids are much less saline (1/4 seawater value), but show much heavier O isotope compositions (δ18O=5.1–6.5 ‰). A simplified scenario of mixing between sedimentary pore fluids and waters affected by clay dehydration released at depth can explain several crucial observations including heavy O isotopes, radiogenic Sr contents (87Sr/86Sr=0.71136–0.71283), and relatively low salinities in the Chishan fluids. Gases isolated from the mud volcanoes are predominantly CH4 and CO2, where the CH4–C isotopic compositions show a thermogenic component of δ13C=−38 ‰. These results demonstrate that active mud volcano de-watering in Taiwan is a direct product of intense sediment accretion and plate collision in the region.  相似文献   

14.
The hydrogeochemical and carbon isotope characteristics of the Krka River, Slovenia, were investigated to estimate the carbon transfer from the land ecosystem in the watershed. During the 3-year sampling period (2008–2010), temperature, pH, electrical conductivity, major ion content, dissolved inorganic carbon (DIC) and dissolved organic carbon content, and the isotopic composition of DIC (δ13CDIC) were monitored in the main stream of the Krka River and its tributaries. The major solute composition of analysed waters is dominated by an input of HCO3 ?, Ca2+ and Mg2+ originating from carbonate dissolution. The Mg2+/Ca2+ and Mg2+/HCO3 ? molar ratio values ranging from 0.24 to 0.71 and 0.05 to 0.30, respectively, indicate a high degree of dolomite dissolution relative to calcite. Dissolved CO2 concentrations in the river were up to tenfold supersaturated relative to the atmosphere, resulting in supersaturation with respect to calcite and degassing of CO2 downstream. The δ13C values in river water range from ?15.6 to ?9.4 ‰ and are controlled by the input of tributaries, exchange with atmospheric CO2, degradation of organic matter, and dissolution of carbonates. The mass balance calculations for riverine DIC suggest that the contribution from carbonate dissolution and degradation of organic matter have major influence, whereas the exchange with atmospheric CO2 has minor influence on the inorganic carbon pool in the Krka River.  相似文献   

15.
The hydrogeochemistry and isotope geochemistry of groundwater from 85 wells in fractured dolomite aquifers of Central Slovenia were investigated. This groundwater represents waters strongly influenced by chemical weathering of dolomite with an average of δ13CCARB value of +2.2 ‰. The major groundwater geochemical composition is HCO3 ? > Ca2+ > Mg2+. Several differences in hydrogeochemical properties among the classes of dolomites were observed when they were divided based on their age and sedimentological properties, with a clear distinction of pure dolomites exhibiting high Mg2+/Ca2+ ratios and low Na+, K+ and Si values. Trace element and nutrient concentrations (SO4 2?, NO3 ?) were low, implying that karstic and fractured dolomite aquifers are of good quality to be used as tap water. Groundwater was generally slightly oversaturated with respect to calcite and dolomite, and dissolved CO2 was up to 46 times supersaturated relative to the atmosphere. The isotopic composition of oxygen (δ18OH2O), hydrogen (δDH2O) and tritium ranged from ?10.3 to ?8.4 ‰, from ?68.5 to ?52.7 ‰ and from 3.5 TU to 10.5 TU, respectively. δ18O and δD values fell between the GMWL (Global Meteoric Water Line) and the MMWL (Mediterranean Meteoric Water Line) and indicate recharge from precipitation with little evaporation. The tritium activity in groundwater suggests that groundwater is generally younger than 50 years. δ13CDIC values ranged from ?14.6 to ?9.3 ‰ and indicated groundwater with a contribution of degraded organic matter/dissolved inorganic carbon in the aquifer. The mass balances for groundwater interacting with carbonate rocks suggested that carbonate dissolution contributes from 43.7 to 65.4 % and degradation of organic matter from 34.6 to 56.3 %.  相似文献   

16.
To understand deep groundwater flow systems and their interaction with CO2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ13CDIC) and DIC concentrations of the sampled waters show that magma-derived CO2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ13CDIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ18O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ13CDIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.  相似文献   

17.
《International Geology Review》2012,54(15):1909-1921
This paper reports the carbon and oxygen isotope compositions of lacustrine carbonate sediments from the Palaeogene Shahejie Formation, Qikou depression, Bohaiwan Basin, with the aim of determining the palaeoenvironmental conditions in the region. Results from Es2, the second member of the Shahejie Formation, showed values of δ13C and δ18O from –1.2‰ to +2.4‰ (average +0.6‰) and from –6.8‰ to –4.7‰ (average –5.7‰), respectively, suggesting a relatively hot climate attending deposition. The slightly closed nature of the lake, which contains brackish water, resulted in higher carbonate δ13C and δ18O values than in a meteoric environment. The values of δ13C and δ18O preserved within the carbonates of the overlying lower Shahejie I (Es1) varied between +1.3‰ and +4.9‰ (average +3.2‰) and from ?4.4‰ to ?1.8‰ (average ?3.1‰), respectively, indicating that the climate became colder at that time. Subsequently, a marine transgression caused the salinity of the lake water to increase. The values of δ13C and δ18O were controlled by salinity. The high δ13C values were also influenced by the rapid burial of the lake organisms and by algal photosynthesis. Values of δ13C and δ18O from carbonates in upper Es1 ranged from ?8.0‰ to +11.0‰ (average +10.1‰) and from ?5.0‰ to ?1.5‰ (average ?3.4‰), respectively, indicating a slight increase in the temperature over time. In the closed and reducing environment, extremes in δ13C values resulted from biochemical fermentation. The positive δ13C excursion recorded in the carbonates of the Shahejie Formation in the Qikou depression indicates that the palaeoclimate underwent a significant transformation during the Eocene and the Oligocene.  相似文献   

18.
The surface sediments of two mud mounds (“Mound 11” and “Mound 12”) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δ18Ocarbonate values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δ13Ccarbonate values from ?52.2 to ?14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The δ18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (δ18Oporefluid = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δ18Oporefluid ≈5 ‰) in Mound 11. A positive correlation between δ13Ccarbonate and δ18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δ13Cporefluid ≈0 ‰) and (2) bicarbonate which formed during the AOM (δ13Cporefluid ≈?70 ‰). Furthermore, the δ18Oporefluid composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δ13CCH4 = ?38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).  相似文献   

19.
It is shown that the gas and water phases of the thermal nitrogen–methane waters in the Talysh fold zone of the Lesser Caucasus mountain system contain helium and strontium with mantle isotope signatures (3Не/4Не from 200 × 10–8 to 401 × 10–8 and 87Sr/86Sr from 0.70490 to 0.70562). At the same time, clear signs of the mantle component in other gases (nitrogen, methane, and carbon dioxide) are absent. The δ15N value in nitrogen varies from +0.3 to +1.7‰, methane is mainly characterized by δ13C from–57.4 to–38.0‰, while δ13C(CО2) varies from–24.4 to–11.3‰. An increase of the CО2 content is accompanied by the decrease of δ13C in CО2, against the background of increasing SO4 content in the salt composition of waters. This indicates a microbial nature of CO2 in the studied gases. Thus, the presence of mantle helium and strontium in the thermal waters is likely related to their leaching from the Pleogene–Neogene host volcanic rocks. The studies of the oxygen and hydrogen isotope composition in water revealed quite different mechanisms for the formation of cold and thermal waters of the region. The cold waters are mainly fed by local infiltration, whereas the feeding of thermal nitrogen–methane waters is strongly provided by transit atmogenic waters (>50%), which are formed in the mountain ranges at altitudes no less than 1600 m and spaced at 20–40 km or more from the thermal discharge sites.  相似文献   

20.
《Gondwana Research》2014,25(3):1057-1069
The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed the oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding the co-evolution of the Earth's environment and life. Here, we first report on coupled organic carbon and nitrogen isotope chemostratigraphy from the entire Ediacaran to Early Cambrian period by using drill core samples from the Yangtze Platform, South China. The results show that δ15NTN values were high (~ + 6‰) until middle Ediacaran, gradually dropping down to − 1‰ at the earliest Cambrian, then rising back to + 4‰ in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with a relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest that the δ15NTN and C/N trends mainly reflect secular changes in nitrogen cycling in the Yangtze Platform. Onset of the observed negative N isotope excursion coincided with a global carbon isotope excursion event (Shuram excursion). Before the Shuram event, the high δ15N probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the 15N-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of the Shuram event, both δ13Ccarb and δ15NTN values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by an anomalously low C/N ratio, implying that enhanced respiration resulted in selective loss of carbon as CO2 with recycled organic nitrogen. After the Shuram event, the δ15N value continued to decrease despite that δ13Ccarb rose back to + 4‰. The continued δ15N drop appears to have coincided with a decreasing phosphorus content in carbonate. This suggests that ocean oxygenation may have generated a more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative δ13Ccarb event in the Canglanpuan stage of the Early Cambrian is also characterized by carbon isotopic decoupling as well as the low C/N ratio. The results strongly support that the two stages of the decoupled negative δ13Ccarb excursions reflect a disappearance of a large organic carbon pool in the ocean. The two events appear to relate with the appearance of new metazoan taxa with novel feeding strategies, suggesting a link between ocean oxygenation, nutrient cycling and the appearance and adaptation of metazoans. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号