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 31 毫秒
1.
Recent (<50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean δ18O value of ?5–9‰ PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (?7–5‰SMOW) assuming a mean water temperature of 9°C. The mean δ18O of lake data, ?4–5‰ PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to ?12‰ PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light ‘soil zone’ carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO2 and aqueous HCO?3. We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary δ18O values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.  相似文献   

2.
The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ13C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ18O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ18O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ13C ? 6-8‰.  相似文献   

3.
Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ13C=?28·7‰ PDB and δ18O=?1·6‰ PDB through to δ13C=?6·9‰ PDB and δ18O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ13C=?12·0 to ?10·1‰ PDB and δ18O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ34S=+24·8‰ CDT and δ18O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ34S=+6 to +11‰ CDT and δ18O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.  相似文献   

4.

Sideritic ironstones in Tertiary lacustrine oil shale from the Lowmead and Duaringa Basins in Queensland, contain two distinctive types of siderite in the ironstone bands: sphaerosiderite in the mudstone and coal, and finely crystalline siderite in the lamosite. The petrological evidence indicates that the siderite in the ironstone bands formed eogenetically by growing displacively within the soft sediment. Chemically the siderite is very pure though the sphaerosiderite sometimes shows compositional zoning. Stable oxygen and carbon isotope analyses of the siderite show a wide range of values from ‐12.8‰ to ‐2.4 %0 δ18O (PDB) and ‐5.5‰ to +12.9‰ δ13C (PDB) for the Lowmead Basin; and ‐9.6‰ to ‐1.2‰ δ18O (PDB) and ‐18.6‰ to +16.4‰ δ13C (PDB) for the Duaringa Basin. The oxygen isotope data indicate that the siderite formed in freshwater environments but not in isotopic equilibrium with the formation waters. Kinetic factors offer the most plausible explanation for the anomalously light δ18O values of many of the siderites. The carbon isotope data show that the carbonate for the formation of the siderite originated predominantly from methanogenic fermentation processes but there was also the varying influence of bacterial oxidation processes. The different petrological and isotopic characteristics of the ironstones broadly reflect variations in their depositional environments and the variable eogenetic conditions in which the siderite formed. There is no suitable single model to explain the genesis of all the different types of ironstones other than that a synsedimentary iron‐enrichment process is involved.  相似文献   

5.
The clay fractions of sedimentary kaolin deposits representing different ages (Carboniferous and Cretaceous), types (pisolitic flint and plastic), and localities (Sinai and Aswan) from Egypt were analyzed for their H and O isotopic compositions to examine the paleoclimate conditions during their formation. The δD values of the Carboniferous deposits in Sinai range between −67‰ and −88‰, while the values for the Cretaceous deposits in Sinai range between −59‰ and −75‰. The δ18O values of the Carboniferous deposits range from 17.9‰ to 19.4‰ and the values for the Cretaceous deposits range between 19.2‰ and 20.4‰. The relatively low δD and δ18O values of the Carboniferous deposit at the Abu Natash area (−67‰ and 17.9‰, respectively) compared to other Carboniferous deposits (averages of −83.3‰, and 18.8‰ for δD and δ18O, respectively) could be due to isotopic exchange between this deposit and the adjacent dolomite and/or the enclosed hydrothermally-formed Mn ores of the Carboniferous Um Bogma Formation. The δD and δ18O values of the Cretaceous pisolitic flint kaolin deposit from Aswan (averages of −65‰ and 20.3‰, respectively) and plastic kaolin from the same area (averages of −66‰ and 19.5‰, respectively) are almost identical. The differences in the δ18O values between the clay fractions of the pisolitic flint kaolin (20.3‰) and the previously analyzed bulk kaolin of the same deposit (average of 17.5‰) suggest a significant effect of non-clay minerals on the isotopic compositions of the kaolin deposits.The H and O isotopic compositions plot close to the kaolinite line that marks the isotopic composition of kaolinite in equilibrium with meteoric water at 20 °C. This indicates that the kaolinite from both the Carboniferous and Cretaceous deposits in Egypt formed by meteoric water weathering of the source rock(s). The δD and δ18O values also suggest that kaolinite of these deposits formed under warm-temperate to tropical conditions. The slight deviations of some samples from the kaolinite line suggest post-depositional modifications of the isotopic compositions of studied deposits probably due to the interaction between earlier-formed kaolinite and downward percolating meteoric water.The δD and δ18O values of the Cretaceous and Carboniferous deposits from all localities suggest that both deposits formed under similar climatic conditions due to the location of Egypt at almost the same distance from the equator either to the south during the Carboniferous or to the north during the Cretaceous.  相似文献   

6.
Upper Visean limestones in the Campine Basin of northern Belgium are intensively fractured. The largest and most common fractures are cemented by non-ferroan, dull brown-orange luminescent blocky calcite. First melting temperatures of fluid inclusions in these calcites are around -57°C, suggesting that precipitation of the cements occurred from NaCl-CaCl2-MgCl2 fluids. The final melting temperatures (Tmice) are between -5 and -33°C. The broad range in the Tmice data can be explained by the mixing of high salinity fluids with meteoric waters, but other hypotheses may also be valid. Homogenization temperatures from blocky calcite cements in the shelf limestones are interpreted to have formed between 45 and 75°C. In carbonates which were deposited close to and at the shelf margin, precipitation temperatures were possibly in the range 70-85°C and 72-93°C, respectively. On the shelf, the calcites have a δ18O around -9.3‰ PDB and they are interpreted to have grown in a fluid with a δ18O between −3.5 and +1.0‰ SMOW. At the shelf margin, blocky calcites (δ18O∼ - 13.5‰ PDB) could have precipitated from a fluid with a δ18O betweenn -4.0 and -1.1‰ SMOW. The highest oxygen isotopic compositions are comparable to those of Late Carboniferous marine fluids (δ18O= - 1‰ SMOW). The lowest values are more positive than a previously reported composition for Carboniferous meteoric waters (δ18O= -7‰ SMOW). Precipitation is likely to have occurred in marine-derived fluids, which mixed with meteoric waters sourced from near the Brabant Massif. Fluids with a similar negative oxygen isotopic composition and high salinity are actually present in Palaeozoic formations. The higher temperature range in the limestones near the shelf margin is explained by the upward migration of fluids from the ‘basinal’ area along fractures and faults into the shelf.  相似文献   

7.
Two types of ‘pseudobreccia’, one with grey and the other with brown mottle fabrics, occur in shoaling‐upward cycles of the Urswick Limestone Formation of Asbian (Late Dinantian, Carboniferous) age in the southern Lake District, UK. The grey mottle pseudobreccia occurs in cycle‐base packstones and developed after backfilling and abandonment of Thalassinoides burrow systems. Burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence. Mottling formed when an early diagenetic ‘aerobic decay clock’ operating on buried organic material was stopped, and sediment entered the sulphate reduction zone. This probably occurred during progradation of grainstone shoal facies, after which there was initial exposure to meteoric water. Microspar calcites then formed rapidly as a result of aragonite stabilization. The precipitation of the main meteoric cements and aragonite bioclast dissolution post‐date this stabilisation event. The brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete, which developed beneath palaeokarstic surfaces capping cycle‐top grainstones and post‐date all depositional fabrics, although they may also follow primary depositional heterogeneities such as burrows. They consist of coarse, inclusion‐rich, microspar calcites that are always very dull to non‐luminescent under cathodoluminescence, sometimes with some thin bright zones. These are interpreted as capillary rise and pedogenic calcrete precipitates. The δ18O values (?5‰ to ?8‰, PDB) and the δ13C values (+2‰ to ?3‰, PDB) of the ‘pseudobreccias’ are lower than the estimated δ18O values (?3‰ to ?1‰ PDB) and δ13C values of (+2‰ to +4‰ PDB) of normal marine calcite precipitated from Late Dinantian sea water, reflecting the influence of meteoric waters and the input of organic carbon.  相似文献   

8.
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   

9.
Three categories of fibrous calcite from early to middle Caradoc platform-marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C-O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ13C and intermediate δ18O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn-rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ13C and negative δ18O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ13C and δ18O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ13C=1% PDB and δ18O=?4 to ?5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ18O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than ?3‰ SMOW. Assuming an Ordovician seawater δ18O value of ?1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ18O value of ?2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a ?3‰ SMOW value yields temperatures of 18–26 °C.  相似文献   

10.
南海台西南区是中国南海中天然气水合物赋存的最有利场所。研究表明,该区的碳酸盐岩主要以结壳、烟囱的形式出现,结壳的裂隙或孔洞中常常充填有淡黄-白色的文石晶体。碳酸盐岩中自生碳酸盐矿物主要为文石、高镁方解石,少量白云石、铁白云石和菱铁矿。扫描电子显微镜(SEM)分析表明,文石主要呈针状、长柱状、放射束状,高镁方解石呈颗粒状。碳酸盐岩的碳同位素δ13C值主要在-56·878‰~-32·829‰PDB之间,大多数小于-40‰PDB,显示了生物甲烷成因碳源的特征;氧同位素δ18O值在2·1875‰~5·045‰PDB之间,主要在4‰PDB以上,这种较重的氧同位素比值表明,天然气水合物分解产生的富18O水体可能是碳酸盐岩沉淀的流体源。矿物学和碳氧稳定同位素研究表明,南海台西南区的碳酸盐岩为细菌性甲烷成因碳酸盐岩,可能与天然气水合物有关,显示了该区水合物存在的可能性很大。  相似文献   

11.
The Newania carbonatite complex of Rajasthan, India is one of the few dolomite carbonatites of the world, and oddly, does not contain alkaline silicate rocks thus providing a unique opportunity to study the origin and evolution of a primary carbonatite magma. In an attempt to characterize the mantle source, the source of carbon, and the magmatic and post-magmatic evolution of Newania carbonatites, we have carried out a detailed stable carbon and oxygen isotopic study of the complex. Our results reveal that, in spite of being located in a metamorphic terrain, these rocks remarkably have preserved their magmatic signatures in stable C and O isotopic compositions. The δ13C and δ18O variations in the complex are found to be results of fractional crystallization and low temperature post-magmatic alteration suggesting that like other carbonatites, dolomite carbonatites too fractionate isotopes of both elements in a similar fashion. The major difference is that the fractional crystallization of dolomite carbonatites fractionates oxygen isotopes to a larger extent. The modes of δ13C and δ18O variations in the complex, ?4.5?±?1‰ and 7?±?1‰, respectively, clearly indicate its mantle origin. Application of a multi-component Rayleigh isotopic fractionation model to the correlated δ13C versus δ18O variations in unaltered carbonatites suggests that these rocks have crystallized from a CO2 + H2O fluid rich magma, and that the primary magma comes from a mantle source that had isotopic compositions of δ13C ~ ?4.6‰ and δ18O ~ 6.3‰. Such a mantle source appears to be a common peridotite mantle (δ13C = ?5.0?±?1‰) whose carbon reservoir has insignificant contribution from recycled crustal carbon. Other Indian carbonatites, except for Amba Dongar and Sung Valley that are genetically linked to Reunion and Kerguelen plumes respectively, also appear to have been derived from similar mantle sources. Through this study we establish that dolomite carbonatites are generated from similar mantle source like other carbonatites, have comparable evolutionary history irrespective of their association with alkaline silicate rocks, and may remain resistant to metamorphism.  相似文献   

12.
The oxygen isotopic composition of Stenomelania gastropod shells was investigated to reconstruct Holocene palaeoclimate change at Lake Kutubu in the southern highlands of Papua New Guinea. Oxygen isotope (δ18O) values recorded in aquatic gastropod shells change according to ambient water δ18O values and temperature. The gastropod shells appear to form in oxygen isotopic equilibrium with the surrounding water and record a shift in average shell oxygen isotopic composition through time, probably as a result of warmer/wetter conditions at ca. 600–900 and 5900–6200 cal a bp. Shorter term fluctuations in oxygen isotope values were also identified and may relate to changes in the intensity or source of rainfall. Further δ18O analyses of gastropod shells or other carbonate proxies found in the Lake Kutubu sediments are warranted. © 2020 John Wiley & Sons, Ltd.  相似文献   

13.
Early concretionary and non-concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85–97 mol% FeCO3, and are commonly enriched in manganese. δ13C (PDB) values are highly variable, ranging from - 18·4 to +2·9‰, whereas δ18O (PDB) values are very consistent, ranging from - 14·0 to - 10·2‰ (mean= - 11·9 ± 1·0‰). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non-marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low δ13C values demonstrate that, contrary to current views, highly 13C-depleted siderite can be produced at shallow burial depths in anoxic non-marine sediments.  相似文献   

14.
A carbonate buildup of Middle Triassic age, the Esino Limestone, outcrops in the Southern Calcareous Alps of Lombardy (N Italy). Along its margin and within the open subtidal facies, the Esino Limestone contains calcite cement-filled cavities of cm to m size. These features, known as evinosponges, may form pervasive networks within the host rock. The filling consists of concentric, isopachous layers of fibrous low-Mg calcite crystals characterized by strong undulose extinction and bent cleavages. The cement crusts are non-luminescent under cathodoluminescence, but both cements and host rock are cross-cut by micro-fractures filled with bright-luminescent calcite, related to late void-filling sparite. Mixing of different carbonates is reflected in stable isotope data. On the hand specimen scale, the oxygen and carbon isotope compositions of cements and host rock show little variation. When compared on a regional scale, the values cover a broad range from δ18O(PDB)=?5‰ to ?12‰ and from δ13O =0‰ to +3‰. The linear covariant trends defined by the oxygen and carbon isotope data for different sampling regions reflect the admixture of late, isotopically depleted calcite with an isotopically enriched non-luminescent calcite of early diagenetic origin. The Esino Limestone fibrous cements, which were probably precipitated in the marine or marine-meteoric phreatic environment, were affected by late diagenetic processes that caused mineral deformation and isotopic depletion through recrystallization and the admixture of a later calcite. These later calcites precipitated from penetrative fluids possibly related to Late Triassic volcanic activity and/or to the Late Cretaceous/Early Palaeogene alpine orogeny.  相似文献   

15.
刘文彬 《地层学杂志》1993,17(2):141-144
<正> 新疆乌鲁木齐西南自柳树沟—祁家沟—塔什库拉沟—井井子沟—郭家沟出露厚达80000m的石炭—二叠纪沉积,早二叠世以前大都为海相,其后为陆相,很多学者进行详细的工作,在沉积方面存在以下争议。一种是中石炭世—早二叠世该地区为一套海退相序的沉积,即由浅海—潮坪相或海陆过渡相—三角洲相。另一种是中石炭世至石炭纪末  相似文献   

16.
REE－fluorocarbonates as major REE minerals in the Bayan Obo deposit,the largest REE deposit in the world,were analyzed for their stable isotopic compositions,The δ^13 C and δ^18 O values of huanghoite,cebaite and bastnaesite from late-stage veins vary in the ranges of 7.8--4.0‰ and 6.7-9.4‰,respectively,These data are relatively similar to those of bastnaesites from banded ores:δ^13C-5.6--5.2‰ andδ^18O3.6-5.5‰.The REE fluorocarbonates from both late-staege veins and banded ores are characterized by lower δ^13 C and δ^18O values,especially the δ^18O values of bastnaesites from banded ores.Compared with them,the disseminated bastnaesits the dolomite-type ores possess rather highδ^13 C and δ^18O values,i.e.,-2.1-0.4‰ and 8.6-12.9‰ respectively.The high values are typical of the sedimentary host dolomite rocks as well as of the dolomite-type-ores.The carbon and oxygen isotopic characteristics of REE fluorocarbonate minerals provide new evidence for the hypothesis on the origin of Bayan Obo deposit-epigenetic hydrothermal metasomatism.  相似文献   

17.
We report here for the first time geochemical, mineralogical and stable carbon and oxygen isotopic data on the crystalline basement rocks of the 1993 Killari earthquake region of Maharashtra (India), which is covered by a thick suite of Deccan volcanics. Our results revealed the hitherto unknown amphibolite–granulite nature of the 2.5?Ga basement, which contains 2.00–2.28?wt% of CO2. The stable carbon (δ13C) and oxygen (δ18O) isotopic measurements on carbonates separated from two basement samples KIL-13 (440.5?m depth) and KIL-20 (499.6?m depth) collected from the KLR-1 borehole drilled in the epicentral region showed the respective values of ?6.23 and ?6.22‰ versus PDB for δ13C and 7.94 and 8.11‰ versus SMOW for δ18O. The samples plot in the primary igneous carbonatite field, indicating the mantle origin of the carbonates, derived through the process of mantle metasomatism from the deep mantle carbon reservoir. This would suggest large-scale crust-mantle thermal fluid interaction beneath the Killari seismogenic region, which is characterized by massive upwarping of the high-velocity mafic crust and retrograde metamorphism.  相似文献   

18.
This paper investigates the stable isotopic composition from late Pleistocene–Holocene (~ 13 to ~ 10.5 cal ka BP) shells of the land snail Helix figulina, from Franchthi Cave (Greece). It explores the palaeoclimatic and palaeoenvironmental implications of the isotope palaeoecology of archaeological shells at the time of human occupation of the cave. Modern shells from around the cave were also analysed and their isotopic signatures compared with those of the archaeological shells. The carbon isotope composition of modern shells depicts the consumption of C3 vegetation. Shell oxygen isotopic values are consistent with other Mediterranean snail shells from coastal areas. Combining empirical linear regression and an evaporative model, the δ18Os suggest that modern snails in the study area are active during periods of higher relative humidity and lower rainfall δ18O, probably at night. Late glacial and early Holocene δ18Os show lower values compared to modern ones. Early Holocene δ18Os values likely track enhanced moisture and isotopic changes in the precipitation source. By contrast, lower late glacial δ18O could reflect lower temperatures and δ18Op, compared to the present day. Shell carbon isotope values indicate the presence of C3 vegetation as main source of carbon to late glacial and early Holocene snails.  相似文献   

19.
《Geochimica et cosmochimica acta》1999,63(13-14):1981-1989
In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ18O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in 18O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (∼8.6) and increased [CO32−] (∼50 μmol/l) in the surface waters. The resulting weighted average δ18O value for the studied period is −9.6‰, compared with a calculated equilibrium δ18O value of −9.4‰. These data convincingly demonstrate that δ18O of calcite are, for the most part, a very reliable proxy for temperature and δ18O of the water.  相似文献   

20.
Detailed isotopic study (δ13C, δ18O) of Lower Proterozoic sedimentary carbonates from biostratigraphically dated successions in Karelia (Russia) revealed a large positive δ13C anomaly (average δ13C-value constitutes approximately +10‰ PDB) at 2.3 Byr ago and a huge drop of δ13C-values in the subsequent ‘black shale period’(2.2 Byr ago). These data are interpreted to be a result of large-scale changes in the oxygen content of the atmosphere. According to this model the positive carbon isotopic anomaly of sedimentary carbonates at about 2.3 Byr ago reflects a very high O2-content of the atmosphere at this time; the subsequent drop in δ13C-values equates with a sharp decrease of the O2-content of the atmosphere.  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