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1.
The carbon (δ13 C) and oxygen (δ18O) isotopic composistion in mollusc shells in mainly determined by the isotopic composition of water and dissolved bicarbonate. The δ18O values of water show a good correlation with the salinity of the Baltic. This correlation served as a basis for reconstructing palaeosalinity and for stratifying the marine sediments according to the δ18O values of the carbonate skeletons of subfossil shells. The δ13C values in shells are mainly determined by the isotopic composition of land-originating bicarbonate, especially in the carbonate skeleton of Lymnaea balthica , which inhabits the immediate coastal zone. According to the δ18O data, salinity in the investigated area (the coastal area of W and NW Estonia) was highest (about 9–11%) during the Littorina stage. The Limnae a stage had, in general, a salinity similar to the contemporary one, but during some phases possibly exceeding it by 2–3%.  相似文献   

2.
Results from detailed pollen and 18O/16O studies on two sediment profiles from small Swiss lakes are reported. 18O/16O records in lacustrine carbonate contain paleoclimatic information because they reflect mainly the isotope ratio in rain and snow which is correlated to temperature. Several transitions between different climatic periods determined palyno-logically are also indicated by marked changes in the isotope ratios in both profiles, namely the transitions Oldest Dryas - Bøiling and Allerød - Younger Dryas - Preboreal. 18o/16O was 2 to 3 %0 lower during Younger Dryas than during the adjacent periods, corresponding to a temperature drop of a few degrees Centigrade according to a tentative estimate.  相似文献   

3.
Abstract. Carboniferous-Permian limestones of the Akiyoshi Plateau, in the Inner Zone of southwestern Japan, are composed of essentially pure calcium carbonate containing only small amounts of other elements, and they are accompanied by marble and copper skarn deposits near the contact with late Cretaceous granitoids. The δ18O values of the Akiyoshi limestones range widely from 7.6 to 28.3% and are mostly lower than those of other areas of the same age (23–29%), whereas the differences among the δ13C values are small. The δ18O values are negatively correlated with Mn and Fe contents. Samples with high δ18O (>25%) and δ13C (>2%) values do not contain Fe, Zn, or Pb, but those with low δ18O values tend to be rich in these elements, indicating that these elements were introduced by interaction with H2O dominant fluids, possibly of magmatic origin. Potential scores for evaluating the degree of interaction with hydro thermal fluids were calculated for δ18O, δ13C, Fe, Mn, Zn, Pb, and Sr. Higher scores implying much hydrothermal interaction were evident in the Mt. Hananoyama area, where there are many skarn deposits, and along faults oriented mainly NNW-SSE. Therefore, these are promising areas for exploring for blind deposits. It is likely that the hydrothermal fluid traveled through the limestones along fractures at the time of the granitic intrusions. However, the potential scores here are much smaller than those in the Pb-Zn mineralized area of the Kamioka mine, so more detailed petrological and mineralogical investigations are necessary.  相似文献   

4.
Abstract: Carbonate rocks of Cambrian (18 samples) and lower-middle Ordovician (11 samples) ages from South Korea were analyzed for sulfur contents of structurally substituted sulfate (SSS) and sulfides and their δ34S values. The δ34S values of SSS ranging from +25.9 to +45.2 permil, are averaged as +33.6 and +33.5 permil for the Cambrian and Ordovician rocks, respectively, which indicate high δ34S values of the Cambro-Ordovician seawater. The SSS contents in the carbonate rocks are low being 2.9 to 17.3 ppm S (averaged as 7.0 ppm S). Sulfide sulfur, on the contrary, is much abundant containing 3 to 1,880 ppm S and the δ34S values range widely between –17.6 and +31.1 permil. Sulfide sulfur of the studied rocks excluding impure carbonates has an average content of 187 ppm S and δ34S value of +12.8 permil (n=24). The estimated δ34S (sulfate–sulfide) values, which range from 13.8 to 25.4 permil in general with a few exceptions from 36.5 up to 52.3 permil for some impure carbonates, may provide evidence for the persistent oceanic anoxia with its temporary recovery during the Cambro-Ordovician time.
The SSS and sulfide sulfurs have often higher δ34S values than the Mesozoic-Cenozoic ore sulfur (Ishihara et al., 2000). Since carbonate rocks are very reactive with circulating hydrothermal ore solution, high δ34S values of the Korean ore deposits might be caused to some extent by 34S enrichment from the host carbonates, resulting in the low SSS contents observed.  相似文献   

5.
Séranne 《地学学报》1999,11(4):135-140
The post-rift stratigraphy on the west African margin is characterized by aggradation of a carbonate ramp during Late Cretaceous to Eocene epochs and progradation of a terrigenous wedge from Oligocene to the Present. Such first-order structure has been attributed in the past to geodynamic forcing. However, comparison of the stratigraphic record of the margin with eustasy, δ18O and 87Sr/86Sr curves, shows a close temporal relationship with the Tertiary climate cooling, an increase of continental weathering, and a long-term lowering of sea level. We suggest that the transition from low-amplitude, high-frequency sea-level changes during the greenhouse period to high-amplitude, high-frequency sea-level changes during the icehouse period may account for: (i) the switch from an aggrading carbonate ramp to a prograding clastic wedge, and (ii) the enhanced continental weathering and increased terrigenous influx to the margin.  相似文献   

6.
Carbon isotope measurements carried out on 201 carbonate samples from the early Proterozoic of the Kola Peninsula, N. Karelia and Norway yield δ13C (PDB) spanning - 20.5% to + 11%. A general δ13C secular trend shows that prior to 2.33 Ga values are typically 'normal' marine, averaging around - 3%0. Between 2.33 and 2.06 Ga, in Jatulian time, there follows a rapid excursion to positive δ13C of around + 6%. Post-Jatulian time is characterized by δ13C of sedimentary carbonates fluctuating between - 5% and +3%; also it is remarkable for the first pronounced development of diagenetic carbonates, which have δ13C between - 14 % and - 6% . The c. 6% positive δ13C shift with a duration of about 270 Myr coincides with a maximum in the diversity and abundance of stromatolites, and with widespread development of 'red beds', but does not coincide with the maximum of buried Corg mass. The Fennoscandian Shield represents the largest isotoically anomalous carbonate province yet reported, and the positive δ13C excursion together with a series of major global palaeoenviromental changes seems to be more intense than the Precambrian/Cambrian transition events. However, it is still not clear what kind of mechanism this phenomenon could be attributed to. An increase of the 'Ronov ratio', and/or 'Broecker ratio' and other possible models are discussed as the target for future investigations.  相似文献   

7.
Oxygen and carbon isotopes were measured to a high depth resolution in a nannoplankton carbonate sequence spanning the Cretaceous-Tertiary boundary at Koshak Hill, Mangyshlak Peninsula, Kazakhstan. The boundary is characterized by the presence of a ∼ 1 cm thick clay layer having a sharp peak in iridium concentration with a maximum value of 3.7 ng g-1. The δ18O data reveal rapid (∼103 years) excursion of sea-surface temperature at the boundary where an initial cold pulse is followed by a persistent warm period. The δ13C data, in contrast, indicate only a gradual change in productivity across the boundary. The observations suggest a biogeochemical scenario for the boundary event pertaining to shallow epicontinental seas.  相似文献   

8.
ABSTRACT The Tripoli Formation (Lower Messinian) in Sicily includes diatomites irregularly alternating with marl and carbonate beds and lies, stratigraphically, between the Tortonian pelagic marls and the evaporitic Calcare di base. The relationships between mineralogy, textural features and oxygen-carbon isotopic compositions of carbonate components point to a wide variability of depositional conditions and suggest that Tripoli sedimentation occurred in small basins characterized by periodic and marked restriction from the open sea.
The isotopic values of calcite and dolomite in the diatomites suggest an evolution from normal marine towards more restricted environments. Evaporating conditions are also indicated by the occurrence of anhydrite, length-slow chalcedonic quartz and moulds of gypsum. In a more advanced stage, the precipitation of heavy δ180 dolomite in the interstitial pores of fossil-poor diatomites denotes an environment with highly evaporated water. Mixing of meteoric and marine waters, on the other hand, might have favoured the precipitation of a dolomite characterized by relatively low δ180 and δ13C values.
The deposition of marl and carbonate beds alternating with or overlying the diatomites took place in an environment with highly evaporated marine waters on the basis of δl18O values of dolomite (up to + 9.10‰) and aragonite (up to + 5.83‰), occurrence of evaporitic minerals and lack of fossils. The presence at these levels of calcite with extremely negative δ13C values (down to - 38.40‰), filling gypsum moulds, suggests activity of sulphate-reducing bacteria. Some aragonitic marls, however, bear evidence of deposition in relatively normal marine conditions.  相似文献   

9.
ABSTRACT
The mineralogy and isotope geochemistry of carbonate minerals in the Coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water. The different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral. In addition, the dolomite was not formed by simple solid state cation exchange.
The occurrence of two types of dolomite was shown by isotope analysis and SEM observations. The dolomite, which is isotopically light (δ13C = -1 to -2% 0 ; δ18O=+3 to +5%0) and of fine grain size (˜ 0·5 μm) probably precipitated under the influence of evaporitically modified continental water. Coarser grained dolomite (up to 4 μm) is isotopically heavier (δ13C=+3 to +4%0; δ18O=+5 to + 6%0) contains Mg in excess of Ca and was formed in or close to equilibrium with atmospheric CO2 probably by the dolomitization of aragonite.  相似文献   

10.
We report silicon isotopic determinations for USGS rock reference materials BHVO-1 and BHVO-2 using a Nu Plasma multi-collector (MC)-ICP-MS, upgraded with a new adjustable entrance slit, to obtain medium resolution, as well as a stronger primary pump and newly designed sampler and skimmer cones ("B" cones). These settings, combined with the use of collector slits, allowed a resolution to be reached that was sufficient to overcome the 14N16O and 14N2 interferences overlying the 30Si and the 28Si peaks, respectively, in an earlier set-up. This enabled accurate measurement of both δ30Si and δ29Si. The δ value is expressed in per mil variation relative to the NBS 28 quartz reference material. Based on data acquired from numerous sessions spread over a period of six months, we propose a recommended average δ30Si of −0.33 ± 0.05‰ and −0.29 ± 0.11‰ (2se) for BHVO-1 and BHVO-2, respectively. Our BHVO grand mean silicon isotope composition (δ30Si =−0.31 ± 0.06‰) is significantly more negative than the only published value for BHVO-2, but is in very good agreement with the recently established average value of ocean island basalts (OIB), confirming the conclusion that the OIB reservoir has a distinct isotopic composition from the solar reservoir as sampled by chondrites.  相似文献   

11.
A Barremian to Albian succession on Mount Kanala, part of a Tethyan isolated carbonate platform, was investigated for its δ13C variations. The limestone sequence is composed of a series of peritidal shallowing-upward cycles with clear petrographic evidence for strong early diagenetic overprinting related to repeated subaerial exposure. Despite significant impact of diagenesis, the observed changes in δ13C can be very well correlated with deep-water sections from different ocean basins and shallow water carbonate platforms in the Middle East. This lends further support to the applicability of δ13C variations for stratigraphic purposes in shallow-water limestones. Using the δ13C signal, time resolution in Lower Cretaceous platform carbonates can be significantly increased, independent of bio-zonations often hampered by ecological variability.
Cyclostratigraphic analysis of the Aptian part of the section shows that strong positive excursions of the cumulative departure from mean cycle thickness of the peritidal shallowing-upward cycles coincide with global positive δ13C excursions. This, and the fact that positive shifts in the δ13C record are preserved within shallow water limestones, provide evidence that black-shale accumulation in the ocean basins occurred during sea-level rise and flooding of platform tops. Integration of carbon-isotope-, cyclo- and sequence-stratigraphic results from different carbonate platforms indicate that strong positive global δ13C shifts and concurrent organic-carbon burial during black-shale deposition are ultimately caused by rapid rises of eustatic sea level. Hence, the rate of change of eustatic sea level is considered to play a crucial role in black-shale accumulation in the global ocean basins during the Cretaceous.  相似文献   

12.
Twenty geological reference samples have been analyzed for selenium using thermal neutron activation followed by radiochemical separation of 75Se. Data are presented for 3 NBS, 11 USGS, and 6 CCRMP materials.  相似文献   

13.
Abstract: Interstitial water expelled from gas hydrate-bearing and -free sediments in the Nankai Trough are analyzed in terms of Cl-, SO42-, δ18O and δD. The baselines for the Cl- concentration and δ18O value are close to seawater values (530 mM and 0%), indicating that the interstitial water is of seawater origin. The δD values decrease with depth, implying isotopic exchange of hydrogen between upwelling biogenic methane depleted in D and interstitial water. The Cl- concentrations in gas hydrate-bearing sediments are anomalously low, while the δ18O and δD values are both high, suggesting that the water forming these gas hydrates was poor in Cl- and enriched in 18O and D during gas hydrate formation. Calculation of the gas hydrate saturations using Cl "and δ18O anomalies gives results of up to 80 % in sand, and shows that the δ18O baseline is not consistent with the Cl" baseline. The δ18O baseline increases by +1% in gas hydrate-free clay and silt. This is considered to be caused by clustering of water molecules after gas hydrate dissociation in response to the upward migration of the base of gas hydrate stability, as indicated by the presence of a double bottom-simulating reflector at this site. The water clusters enriched in 18O are responsible for the increase in the δ18O baseline with normal Cl". The abrupt shallowing of the base of gas hydrate stability may induce the dissociation of gas hydrates and the accumulation of gases in the new stability zone, representing a geological process that increases gas hydrate saturation.  相似文献   

14.
Stalagmite SV1 from Grotta Savi, located at the SE margin of the European Alps (Italy), is the first Alpine speleothem that continuously spans the past c . 17kyr. Extension rate and δ18Oc record for the Lateglacial probably reflect a combination of temperature and rainfall, with rainfall exerting the dominant effect. Low speleothem calcite δ18 Oc values were recorded from c . 14.5 and 12.35 kyr, during GI-1 (Bølling— Allerød) interstadial, which in our interpretation, was warm and wet. The GS-1 (Younger Dryas) was characterized by a shift to heavier δ18 Oc, coinciding with δ13Cc enrichment and extremely low extension rate (<8 μm/year). These characteristics indicate that GS-1 climate was cool and dry in the SE Alps. Calibration using historical data revealed that there is a positive δ18Oc/dT relationship. A 1°C rise in mean annual temperature should correspond to c . 2.85% increase of SV-1 δc18Oc. We reconstructed a slow and steady temperature rise of c . 0.5°C since 10 kyr BP, in broad agreement with reconstructions from pollen data for SE Europe. Stalagmite SV1 indicates that climate variability in the SE Alps has been influenced by the Mediterranean Sea for the past c . 17 kyr.  相似文献   

15.
The Fairholme carbonate complex is part of the extensively dolomitized Upper Devonian carbonate reefs in west-central Alberta. The studied formations contain moulds (up to 10 cm in diameter), which are filled partially with (saddle) dolomite, quartz and calcite cements. These cements precipitated from a mixture of brines that acquired high salinity by dissolution of halite and brines derived from evaporated sea water. The fluids were warm (homogenization temperature of primary fluid inclusions of 76 to 200 °C) and saline (20 to 25 wt% NaCl equivalent) and testify to thermochemical sulphate reduction processes. The latter is deduced from S in solid inclusions, CO2 and H2S in volatile-rich aqueous inclusions and depleted δ13C values down to −26‰ Vienna Pee Dee Belemnite. High 87Sr/86Sr values (0·7094 to 0·7110) of the cements also indicate interaction of the fluids with siliciclastic sequences. The thermochemical sulphate reduction-related cements probably formed during early Laramide burial. Another (younger) calcite phase, characterized by depleted δ18O values (−23·9‰ to −13·9‰ Vienna Pee Dee Belemnite), low Na (27 to 37 p.p.m.) and Sr (39 to 150 p.p.m.) concentrations and non-saline (∼0 wt% NaCl equivalent) fluid inclusions, is attributed to post-Laramide meteoric water.  相似文献   

16.
The δ13Ccarb and 87Sr/86Sr secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local δ13Ccarb fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative δ13Ccarb excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590–544 Myr interval, and two age-groups at 660–610 and 740–690 Myr can be resolved.  相似文献   

17.
Nine stratigraphic sections, each ≈5 m thick, were sampled from the Alamogordo Member limestones of the Lake Valley Formation, Sacramento Mountains, New Mexico, USA. Four stratigraphic sections consist entirely of lime mudstone and wackestone, whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base. Stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in δ18O and constant δ13C values, whereas those in the sections solely composed of lime mudstone and wackestone have, in general, relatively uniform δ18O and δ13C values. The diagenesis of the Alamogordo Member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre-Pennsylvanian exposure surface ≈100 m above this unit. However, the uniform δ13C and downward decrease in δ18O values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the Alamogordo Member, rather than descended from the overlying exposure surface. This indicates that the basal packstones were probably a conduit for meteoric water. This is further supported indirectly by the relatively uniform δ18O and δ13C values of the lime mud in the sections without basal packstones. The implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers, flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer.  相似文献   

18.
Refinements have been made to achieve over 99% yield in the conversion of CO to CO2 in order to improve the reproducibility and accuracy of δ18 O measurements in sulfates. BaSO4 (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO2; the former gas was simultaneously converted to CO2 by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ18 O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO2 produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO2 equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from 18O/16O of the working calibrator we used CO2 gas prepared from ocean water sulfate for this purpose. The calibrated δ18 O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively.  相似文献   

19.
Lower Cretaceous pelagic carbonates outcropping along the Southern Alps of northern Italy provide a record of Tethyan palaeoceanography as well as of low frequency fluctuations in the global carbon cycle. The carbonate C-isotope stratigraphy established at five selected localities in the Southern Alps allows an accurate picture to be drawn of the duration and amplitude of the Valanginian C-isotope event. δ13C values near 1.25–1.50% determined in Berriasian and lower Valanginian sediments are replaced by more pdsitive δ13C values near 3% in the late Valanginian. The carbonate C-isotope excursion ends in the early Hauterivian with values fluctuating between 1.5% and 2%. The carbonate C-isotope excursion is accompanied by a positive excursion in the total organic carbon C-isotope curve. The Valanginian C-isotope excursion identified in Tethyan sediments correlates with a C-isotope excursion recorded in the western North Atlantic, in the Gulf of Mexico, and in the Central Pacific (DSDP Sites 534,391,535 and 167). By analogy with the Aptian stage, also marked by a significant positive C-isotope excursion, the time of positive δ13C values is regarded as a time of accelerated carbon cycling coupled with increased burial rates of organic carbon and detrital material in oceanic sediments. A warm and humid climate, possiblycoupled with a high atmospheric CO2 content and a high global sea-level, may have triggered the acceleration of the global carbon cycling. In this case the Valanginian C-isotope event would reflect a first episode of Greenhouse Earth conditions during the Cretaceous.  相似文献   

20.
Values of δ13C obtained from conventional bulk sediment radiocarbon dates encompassing the Pleistocene Holocene boundary have been compiled and plotted against 14C age. In all. 286 lake sediment dates from southern Sweden in the range 8.000 to 13.000 BP have been evaluated. A significant decrease in δ13C values, initiated shortly before 10.000 RP and amounting to 5%, is distinguished. This change is accompanied by increased limnic productivity. decreased erosive input and increased organic carbon content of the sediments. A probable explanation for the δ13C decline in organic material is decreased importance of dissolution of silicates at the transition to the Holocene. During the Late Weichselian. extensive weathering of exposed minerogenic material with subsequent input of bicarbonate to the lake water may have caused a relative enrichment of 13C in dissolved inorganic carbon. Furthermore, the early Holocene increase in terrestrial vegetation cover probably led to an increased supply of 13C depleted carbon dioxide to the lake water by root respiration. Altered limnic vegetation, presumably towards increased production of phytoplankton. could also have contributed to the observed decreasing δ13C trend. The importance of these processes compared to other possible influencing factors. mainly endogenic carbonate production and changes in the global carbon cycle. is discussed.  相似文献   

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