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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.
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   

3.
ABSTRACT The carbon-isotope signature of terrestrial organic matter (OM) offers a valuable tool to develop stratigraphic correlations for near-shore deposits. A mid-Cretaceous coastal succession of the western Algarve Basin, Portugal, displays a marked negative δ13C excursion ranging from − 21.2‰ to − 27.8‰ in the Early Aptian followed by two shifts towards higher values (up to − 19.3‰) during the Early and Late Aptian, respectively. The dominance of cuticle and leaf debris in the bulk OM fraction is confirmed by optical studies, Rock-Eval pyrolysis and by comparison with the δ13C signature of four different types of fossilized land-plant particles. Correlation of two terrestrial δ13Cbulk OM records from different study sites leads to a significant enhancement of the intrabasinal stratigraphic correlation within the Algarve Basin. Three prominent excursions in the Portuguese records can be correlated with existing δ13C curves from pelagic and terrestrial environments. The general carbon-isotope pattern is superimposed by small-scale fluctuations which can be explained by compositional variations within the OM.  相似文献   

4.
Oxygen isotope profiles along the growth axis of fossil bivalve shells of Macoma calcarea were established to reconstruct hydrographical changes in the eastern Laptev Sea since 8400 cal yr B.P.. The variability of the oxygen isotopes (δ18O) in the individual records is mainly attributed to variations in the salinity of bottom waters in the Laptev Sea with a modern ratio of 0.50‰/salinity. The high-resolution δ18O profiles exhibit distinct and annual cycles from which the seasonal and annual salinity variations at the investigated site can be reconstructed. Based on the modern analogue approach oxygen isotope profiles of radiocarbon-dated bivalve shells from a sediment core located northeast of the Lena Delta provide seasonal and subdecadal insights into past hydrological conditions and their relation to the Holocene transgressional history of the Laptev Sea shelf. Under the assumption that the modern relationship between δ18Ow and salinity has been constant throughout the time, the δ18O of an 8400-cal-yr-old bivalves would suggest that bottom-water salinity was reduced and the temperature was slightly warmer, both suggesting a stronger mixture of riverine water to the bottom water. Reconstruction of the inundation history of the Laptev Sea shelf indicates local sea level ∼27 m below present at this time and a closer proximity of the site to the coastline and the Lena River mouth. Due to continuing sea level rise and a southward retreat of the river mouth, bottom-water salinity increased at 7200 cal yr B.P. along with an increase in seasonal variability. Conditions comparable to the modern hydrography were achieved by 3800 cal yr B.P.  相似文献   

5.
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   

6.
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   

7.
Two sections of the Upper Cenomanian and Lower Turonian in central and south-east Poland were investigated for foraminifers, CaCO3content, carbon content insoluble in HCl (Corg) and in the carbonates (Ccarb), carbon and oxygen isotopic composition of bulk-rock carbonates and elemental abundances. The Cenomanian/Turonian boundary interval is characterized by the appearance of more marly facies, a δ13C and δ18O stable isotope anomaly, a considerable increase in Corg content and decrease in Ccarb content and substantial changes in the foraminiferal assemblages. A major carbon stable isotope excursion with a shift of +2 (PDB) occurs in the lowermost Whiteinella archaeocretacea Zone. The late Cenomanian δ13C anomaly is associated with heavy δ18O values. The peak value of δ13C corresponds to the minima in P/B ratio and in diversity of foraminiferal assemblages. A late Cenomanian anoxic event is thought to be responsible for changes in foraminiferal assemblages. However, elemental abundance analyses do not show changes in the concentrations of trace elements. This may be explained by the long distance between studied area and a source of enrichment which was probably located in the western hemisphere.  相似文献   

8.
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   

9.
The Archean mafic–ultramafic complex of Lac des Iles, Ontario, Canada, hosts economic platinum group elements (PGE)-Au-Cu-Ni mineralization in the Roby Zone. All lithologies in the North Roby Zone have been affected by hydrothermal alteration. The alteration products include talc (the most dominant mineral), anthophyllite, serpentine, actinolite, tremolite, chlorite, hornblende, zoisite, clinozoisite, epidote and sericite. In the altered rocks, light rare earth elements (La, Ce, Nd, Sm), Pb, Rb, Ba, Cs, S and possibly Y have been added by hydrothermal solution whereas Eu and heavy rare earth elements (Yb, Gd, Dy, Er) remained immobile. There are five types of fluid inclusions in the pegmatitic plagioclase with homogenization temperature and salinity ranging from 240°C to 445°C and 15.37 to 48.52 wt% equivalent NaCl, respectively. The δ18O and δD of talc range form 6.2‰ to 6.9‰ and −28‰ to −48‰, respectively. δ18O and δD water in equilibrium with talc during the hydrothermal alteration suggest a modified source for the hydrothermal solution. Microthermometry and stable isotope studies suggest that high temperature–high salinity fluid was diluted by, and mixed with, low temperature–low salinity meteoric solution. This mechanism precipitated the hydrothermal assemblage and redistributed trace elements during and after pegmatite formation in the North Ruby Zone.  相似文献   

10.
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   

11.
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   

12.
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   

13.
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   

14.
Abstract. Primary fluid inclusions in quartz and carbonates from the Kanggur gold deposit are dominated by aqueous inclusions, with subsidiary CO2-H2O inclusions that have a constant range in CO2 content (10–20 vol %). Microthermometric results indicate that total homogenization temperatures have a wide but similar range for both aqueous inclusions (120 to 310C) and CO2-H2O inclusions (140 to 340C). Estimates of fluid salinity for CO2-H2O inclusions are quite restricted (5.9∼10.3 equiv. wt% NaCl), whereas aqueous inclusions show much wider salinity ranging from 2.2 to 15.6 equivalent wt %NaCl.
The 6D values of fluid inclusions in carbonates vary from -45 to -61 %, in well accord with the published δD values of fluid inclusions in quartz (-46 to -66 %). Most of the δ18O and δD values of the ore-forming fluids can be achieved by exchanged meteoric water after isotopic equilibration with wall rock by fluid/rock interaction at a low water/rock ratio. However, the exchanged meteoric water alone cannot explain the full range of δ18O and δD values, magmatic and/or meta-morphic water should also be involved. The wide salinity in aqueous inclusions may also result from mixing of meteoric water and magmatic and/or metamorphic water.  相似文献   

15.
Expanded sedimentary records from the Tethys reveal unique faunal and isotopic changes across the Palaeocene-Eocene (P-E) transition. Unlike in the open oceans, the Tethys exhibits a gradual decrease of 1.5% in δ13C values prior to the rapid δ13C excursion. Associated with the 613C excursion is a decrease in calcite burial, increase in detrital content and appearance of a unique opportunistic planktic foraminifera1 assemblage (e.g. compressed acarininids). The existence of a prelude decrease in δ13C values in the Tethys suggests that the P-E δ13C excursion may have occurred in two steps and over a few hundred thousand years, rather than as one step over a few thousand years as previously suggested. This slower excursion rate is readily explained by changing organic carbon weathering or burial rates and avoids the need of invoking ad hoc scenarios.  相似文献   

16.
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   

17.
The cause of the middle Miocene Badenian salinity crisis in the Central Paratethys is addressed by examining the palaeotemperature evolution of Badenian waters before and after the deposition of evaporites. Selected foraminifer taxa ( Globigerinoides spp., Globigerina bulloides , and Uvigerina ) characterizing, respectively, the near-surface, intermediate, and bottom layers of the water column, were studied in two boreholes of SW Poland. The δ18O and δ13C values for these taxa show distinct differences which can be explained by the temperature difference between surface and bottom waters during deposition. These values also show temporal changes corresponding to the water temperature evolution in the Badenian basin. Different and quickly changing environmental conditions have been inferred from changes in foraminifer assemblages. They explain why biostratigraphic subdivisions based on well-recognized assemblages are the most accurate approach for determining the biostratigraphy of middle Miocene deposits in the Central Paratethys. The results of isotopic studies indicate that evaporites occur in a part of the Badenian section that was characterized by the lowest temperatures in the studied sections.  相似文献   

18.
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   

19.
Topaz granite is alkali-feldspar granite that contains essential albite, quartz, K-feldspar, lithium-mica, and topaz. As a group topaz granites are characterized by their extreme enrichment in F (up to 3 wt%) and a wide variety of lithophile elements. They can be subdivided into a 'low-P2O5 subtype' (P2O5 < 0.1 wt%, Al2O3 < 14.5 wt%, SiO2 > 73 wt%) and a 'high-P2O5 subtype' (P2O5 > 0.4 wt%, Al2O3 > 14.5 wt%, SiO2 < 73 wt%), the δ18O values of which indicate a dichotomy of source rock: the low-P2O5 subtype (δ18O < 10‰) having a meta-igneous protolith and the high-P2O5 subtype (δ18O > 10 ‰) a source with a significant component of pelitic material. The unusually high F contents enhance the efficacy of melt segregation and crystal-melt fractionation and so facilitate extreme differentiation in topaz granite magmas. Very low melt volumes restrict the bulk composition of the partial melts regardless of the nature of the source; and extreme fractionation forces them along a path of magmatic convergence, to produce a group of granitic rocks with near-minimum compositions so enriched in a variety of lithophile elements (Li, Nb, Ta, Sn) that economic mineralization often results.  相似文献   

20.
This contribution describes the field geometry, petrography and geochemistry of a well-exposed dolomitization front in Upper Jurassic carbonates, and attempts to highlight the sedimentological, structural and relative sea-level controls on multiphase dolomitization and related diagenetic events. The data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body, whose dimensions are well defined in the field. Local microbial intraclastic dolomites of Late Tithonian age accumulated in a hypersaline lagoon during relative sea-level fall. These pre-date beige hydrothermal dolostones (51 to 55 mol% CaCO3; δ 18O: −9·3 to −4·0‰ V-PDB; δ 13C: −1·5 to +2·1‰ V-PDB; 87Sr/86Sr: 0·70742; matrix porosity: ≈6%; Klinkenberg permeability: ≈0·5 mD), whose dolomitizing fluid circulated along faults and invaded the nearby facies. First, the burrows were dolomitized, then the bulk rocks, resulting in the investigated 'tongue'-shaped dolomite body. Upon Late Jurassic–Early Cretaceous uplift, near-surface water percolated through – and altered – the underlying beige dolostones. This event was followed by a ferroan dolomite cement phase, which occurred during further burial. This contribution, featuring a well-defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set, may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization.  相似文献   

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