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1.
Stable isotopic signature of a palaeoaquifer, Mississippian Alamogordo Member limestones, Sacramento Mountains, New Mexico, USA 总被引:1,自引:0,他引:1
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 δ18 O and constant δ13 C values, whereas those in the sections solely composed of lime mudstone and wackestone have, in general, relatively uniform δ18 O and δ13 C 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 δ13 C and downward decrease in δ18 O 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 δ18 O and δ13 C 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
2.
JAAN-MATI PUNNING TÖNU MARTMA HELGI KESSEL REIN VAIKMÄE 《Boreas: An International Journal of Quaternary Research》1988,17(1):27-31
The carbon (δ13 C) and oxygen (δ18 O) isotopic composistion in mollusc shells in mainly determined by the isotopic composition of water and dissolved bicarbonate. The δ18 O 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 δ18 O values of the carbonate skeletons of subfossil shells. The δ13 C 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 δ18 O 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
3.
Oxygen Isotopic Characteristics of Granitic Intrusions Related to Haobugao Zn-Pb-Cu-Sn Skarn Deposit, East Inner Mongolia, China 总被引:2,自引:0,他引:2
Jingbin Wang Lijuan Wang Yuwang Wang Tiejun Li Hidehiko Shimazaki 《Resource Geology》2004,54(2):205-208
Abstract. Granitic rocks related to the formation of Haobugao Zn-Pb-Cu-Sn skarn deposit, Inner Mongolia, China, show unusual low whole-rock δ18 O values down to -8.8 % (V-SMOW), whereas separated quartz crystals from those rocks give positive δ18 O values of+4.1 to +9.9 %. Chemical analyses and microscopic observation of those granitic rocks confirm that they suffered hydrothermal alteration. Some skarn specimens and quartz from the Haobugao deposit also show negative δ18 O values. The isotopic evidence indicates that intensive meteoric water circulation occurred at the time of granitic intrusion, and caused the pervasive hydrothermal alteration of granitic rocks and the precipitation of skarn deposit in this area. 相似文献
4.
Danuta Peryt Krystyna Wyrwicka Charles Orth Moses Attrep Jr Leonard R. Quintana 《地学学报》1994,6(2):158-165
Two sections of the Upper Cenomanian and Lower Turonian in central and south-east Poland were investigated for foraminifers, CaCO3 content, 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 δ13 C and δ18 O 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 δ13 C anomaly is associated with heavy δ18 O values. The peak value of δ13 C 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
5.
Seasonal and interannual variability of Siberian river discharge in the Laptev Sea inferred from stable isotopes in modern bivalves 总被引:2,自引:0,他引:2
THOMAS MUELLER-LUPP HELMUT ERLENKEUSER HENNING A. BAUCH 《Boreas: An International Journal of Quaternary Research》2003,32(2):292-303
Stable oxygen and carbon isotope profiles from modern bivalve shells were investigated in order to reconstruct short-term hydrographical changes in the river-shelf system of the Laptev Sea. Oxygen isotopic profiles obtained from the aragonitic species Astarte borealis exhibit amplitude cycles interpreted as annual hydrographical cycles. These records reflect the strong contrast between summer and winter bottom water conditions in the Laptev Sea. The seasonal variations in δ18 O are mainly controlled by the riverine freshwater discharge during summer with 0.5‰ per salinity unit. Corrected for a defined species-dependent fractionation offset of -0.37‰, time-dependent salinity records were reconstructed from these δ18 O profiles. They indicate a good correspondence to seasonal hydrographic changes and synoptical data. Persistent trends with shell growth towards more negative δ13 C values are observed in all specimens and appear to be related to metabolic changes of the bivalves during ontogeny. In contrast, short-term fluctuations are likely linked to seasonal variabilities of the river water outflow patterns and enhanced phytoplankton productivity during summer. This is corroborated by a clear watermass-related distinction of the various δ13 C records made on the basis of water depth and distance from the riverine source. 相似文献
6.
A breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein-fill is coarse grained and equigranular, suggesting precipitation in a fluid-filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ13 C values of -11.1 ± 0.1% and δ18 O values of 6-10%o , whereas the host metapelite has δ13 C values of -10.62 and -10.11% and δ18 O values of 14-15%o . These values are consistent with an igneous-derived, H2 O-dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large-volume fluid flow within the shear zone. 相似文献
7.
Geochemical Process of Gas Hydrate Formation in the Nankai Trough Based on Chloride and Isotopic Anomalies in Interstitial Water 总被引:2,自引:0,他引:2
Abstract: Interstitial water expelled from gas hydrate-bearing and -free sediments in the Nankai Trough are analyzed in terms of Cl-, SO4 2 -, δ18 O and δD. The baselines for the Cl- concentration and δ18 O 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 δ18 O and δD values are both high, suggesting that the water forming these gas hydrates was poor in Cl- and enriched in 18 O and D during gas hydrate formation. Calculation of the gas hydrate saturations using Cl "and δ18 O anomalies gives results of up to 80 % in sand, and shows that the δ18 O baseline is not consistent with the Cl" baseline. The δ18 O 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 18 O are responsible for the increase in the δ18 O 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
8.
Many different types of water and processes have been proposed for the formation of dolomites. The three phases of hydrothermal dolomites in the Middle Atlas Causse were investigated to elucidate their formation processes. The first two of these are associated with sphalerite and galena in stratiform and open space-filling deposits. These formed early in the history of the deposition of the Pb–Zn mineralization and commonly reveal a paragenetic overlap. A later phase, post-dating Pb–Zn mineralization, is reflected in saddle dolomite.
All three phases show a decrease in δ18 O and δ13 C values passing from sterile (unmineralized) to mineralized rocks, and isotopic signatures are independent of the carrier facies. However, early-formed dolomites can be separated into two distinct groups on the basis of δ18 O values. Type 1 dolomites host stratiform ore deposits, whereas type 2 dolomites host an open space-filling ore-body. Later saddle dolomites are more depleted in 18 O than either of these.
The early hydrothermal and saddle dolomites precipitated from similar fluids during three distinct events, but formed by two mechanisms: replacement (hydrothermal dolomite) and cement precipitation (saddle dolomite). They show different isotopic signatures and apparently formed at different temperatures. Field data, petrographic and stable isotope results suggest a continuum of replacement, during the Carixian for the early hydrothermal dolomite 1, and during the Toarcian for early hydrothermal dolomite 2, followed by a cement precipitation phase for saddle dolomite. 相似文献
All three phases show a decrease in δ
The early hydrothermal and saddle dolomites precipitated from similar fluids during three distinct events, but formed by two mechanisms: replacement (hydrothermal dolomite) and cement precipitation (saddle dolomite). They show different isotopic signatures and apparently formed at different temperatures. Field data, petrographic and stable isotope results suggest a continuum of replacement, during the Carixian for the early hydrothermal dolomite 1, and during the Toarcian for early hydrothermal dolomite 2, followed by a cement precipitation phase for saddle dolomite. 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 (δ13 C = -1 to -2% 0 ; δ18 O=+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 (δ13 C=+3 to +4%0 ; δ18 O=+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
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 (δ
10.
Stable Isotope Study of the Langshan Polymetallic Mineral District, Inner Mongolia, China 总被引:3,自引:0,他引:3
Abstract: A comprehensive stable isotope investigation was carried out to clarify the geneses of the ore deposits in the Langshan Pb-Zn mineral district. The lead isotope study shows that these deposits were probably formed from 2. 0 to 1. 5 Ga, and were deformed and metamorphosed 1. 45 Ga. Ore lead could be a mixture of mantle lead and crustal lead. The C and S isotope results indicate that these deposits were precipitated in closed or semi-closed rift basins, and the source of sulfur might be Proterozoic ocean sulfate. The H and O isotope results indicate that the δD and δ18 O values of rocks were changed by water-rock interaction during metamorphism and hydrothermal alteration. The scale of δD and δ18 O shift of rocks reflects the grade of metamorphism and alteration as well as the water-rock ratios. However, the water-rock ratios in the metamorphic processes of Langshan mineral district were relatively low, and the source of water during metamorphism is suggested to be ancient meteoric water. Based on isotopic results and the geological background, it is concluded that these deposits may belong to Proterozoic sedimentary exhalative (SEDEX) type. 相似文献
11.
Large amounts of fluid, bound up in the hydrated upper layers of the ocean crust, are consumed at convergent margins and released in subduction zones through devolatilization. The liberated fluids may play an integral role in subduction zone processes, including the generation of arc-magmas. However, exhumed subduction zone rocks often record little evidence of large-scale fluid flow, especially at deeper levels within the subduction zone. Basaltic pillows from the high-pressure Corsican and Zermatt-Saas ophiolites show a range of δ18 O values that overall reflect seafloor alteration prior to subduction. However, comparison between the δ18 O values of the cores and rims of the pillows suggests that the δ18 O values of the pillow rims at least have been modified during subduction and high-pressure metamorphism. Pillows that have not undergone high-pressure metamorphism generally have rims with higher δ18 O values than their cores, whereas the converse is the case in pillows that have undergone high-pressure metamorphism. This reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid–rock interaction occurring during subduction and high-pressure metamorphism. However, the preservation of different δ18 O values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled. Resetting of the δ18 O values in the pillow rims was probably due to fluid-hosted diffusion that occurred over relatively short time-scales (<1 Myr). 相似文献
12.
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 δ18 0 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 δ18 0 and δ13 C 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 δl18 O 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 δ13 C 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
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 δ
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 δl
13.
KLAUS FELIX KAISER UELI EICHER 《Boreas: An International Journal of Quaternary Research》1987,16(3):293-303
Pollen and mollusc deposits in a sedimentation series laid down in the Dattnau valley during the Late Glacial and early Post-glacial were studied. For the first time δ13 C and δ18 O in land-snail shells were measured. It was possible to reconstruct vegetational and climatic developments from the Bölling ( c . 12,500 B.P.) until well into the early Boreal ( c . 9,000 B.P.). The two sets of findings agree well. The Late Glacial is seen to comprise two intervals: the continuous, locally rather moist, warm Bölling/Alleröd period, and the subsequent dry . cold Younger Dryas. The profile ceases after the changeover from the Late Glacial to the Post-glacial. The transitions from the Alleröd to the Younger Dryas and hence to the Preboreal and Boreal are both clearly identifiable in the pollen diagram; the mollusc record, however, ceases to be interpretable shortly before the climatic change to the Post-glacial. The δ18 O curve shows a clear distinction between the AllerÖd and the Younger Dryas. The Gerzensee fluctuation, immediately before this transition. is evident as a negative deviation. 相似文献
14.
Carbon-isotope stratigraphy in shallow-water carbonates: implications for Cretaceous black-shale deposition 总被引:4,自引:0,他引:4
A Barremian to Albian succession on Mount Kanala, part of a Tethyan isolated carbonate platform, was investigated for its δ13 C 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 δ13 C 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 δ13 C variations for stratigraphic purposes in shallow-water limestones. Using the δ13 C 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 δ13 C excursions. This, and the fact that positive shifts in the δ13 C 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 δ13 C 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
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 δ
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 δ13 C values prior to the rapid δ13 C excursion. Associated with the 613 C 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 δ13 C values in the Tethys suggests that the P-E δ13 C 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.
Carbon isotope measurements carried out on 201 carbonate samples from the early Proterozoic of the Kola Peninsula, N. Karelia and Norway yield δ13 C (PDB) spanning - 20.5% to + 11%. A general δ13 C 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 δ13 C of around + 6%. Post-Jatulian time is characterized by δ13 C of sedimentary carbonates fluctuating between - 5% and +3%; also it is remarkable for the first pronounced development of diagenetic carbonates, which have δ13 C between - 14 % and - 6% . The c. 6% positive δ13 C 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 δ13 C 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
17.
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 δ13 C 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 δ13 C signature of four different types of fossilized land-plant particles. Correlation of two terrestrial δ13 Cbulk 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 δ13 C 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
18.
G. R. T. JENKIN C. M. FARROW A. E. FALLICK D. HIGGINS 《Journal of Metamorphic Geology》1994,12(3):221-235
Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of δ18 O within grains, and the bulk δ18 O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the δ18 O values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values ('slow cooling'), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures ( T c ) for the two phases and the mode of the rock. Adding a third phase into a system which undergoes 'slow' cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two-phase system. In some systems oxygen isotope reversals can be developed. If cooling is not 'slow', then the mineral δ18 O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral δ18 O values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems. 相似文献
19.
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 δ18 O and δ13 C 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
20.
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 δ18 O 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 δ13 C 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