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1.
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   

2.
The carbonate carbon isotope record of the Phanerozoic is marked by repeated high-amplitude excursions towards more positive values. Although the interpretation of C-isotope events remains controversial, they are regarded as a proxy of the global C-cycle. Using δ13COrg-measurements of samples from DSDP Site 416, it is shown that a classic example of platform drowning coincided with the onset of the mid-Valanginian carbon-cycle excursion. Widespread carbonate platform drowning would have contributed to the observed positive shift in the C-isotope record. As choking of carbonate production was compensated by increased organic production, both processes favoured a shift in marine carbon partitioning from the oxidized to the reduced carbon reservoir. This would have resulted in an increased flux of CO2 from the atmosphere into the marine and sedimentary carbon reservoir.  相似文献   

3.
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   

4.
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   

5.
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   

6.
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   

7.
Studies on Cretaceous pelagic limestones have shown a close correspondence between putative sea-level curves and the carbon-isotope profile, suggesting a possible relationship between the two phenomena. Such a relationship could relate to shelf-sea area governing the global burial rate of organic carbon which, in turn, controls the 13C/12C ratio of dissolved inorganic carbon in the oceanic reservoir. In order to see whether or not this relationship might be of more general applicability, the major mid-Oxfordian (Jurassic) transgression is documented to zonal level and appropriate carbon-isotope data are presented. Although the applicability of ammonite zonation to the mid Oxfordian is problematic on a regional scale, a clear δ13C excursion is present in the trunsversariurn Zone, which is coincident with regional overstep and/or evidence of shoreline retreat and/or evidence of bathymetric deepening in Europe and elsewhere. The use of the carbon-isotope curve as a proxy for shelf-sea area or relative sea level is worthy of further exploration.  相似文献   

8.
The carbon isotope measurements, carried out on subsurface carbonate samples from Oxfordian Jaisalmer Formation, western India, yield positive d13C values up to +3.17%. The most positive Oxfordian C-isotope value corresponds to the carbon isotope excursion measured in samples from from other late Jurassic basins of world. The latest Oxfordian C-isotope values of Jaisalmer Basin fluctuate around 2% while the C-isotope values of 1.50% mark the base of Kimmeridgian. The Oxfordian C-isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level.  相似文献   

9.
The Toarcian oceanic anoxic event ( ca 183 Ma) coincides with a global perturbation marked by enhanced organic carbon burial and a general decrease in calcium carbonate production, probably triggered by changes in the composition of marine plankton and elevated carbon dioxide levels in the atmosphere. This study is based on high-resolution sampling of two stratigraphic successions, located in Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps), Italy, which represent expressions of the Toarcian oceanic anoxic event in deep-water pelagic sediments. These successions are characterized by the occurrence of black shales showing relatively low total organic carbon concentrations (compared with coeval strata in Northern Europe), generally < 2%, and low hydrogen indices. On this basis, they are similar to other Toarcian black shales described from the Tethyan region. The positive and negative carbon-isotope records from the two localities permit a high-resolution correlation such that ammonite biostratigraphy information from Valdorbia can be transferred to those parts of the Monte Mangart section that lack these fossils. Spectral analyses of δ13Corg values and of CaCO3 percentages from the sedimentary records of both the Valdorbia and Monte Mangart sections reveal a strong cyclic pattern, best interpreted as an eccentricity signal which hence implies a duration of ca 500 kyr for the negative carbon-isotope excursion. Based on the carbon-isotope curves obtained, the high-resolution correlation between the Italian successions and a section in Yorkshire (Northern Europe) confirms the supposition that the apparent mismatch between the dating of the Toarcian oceanic anoxic event in the Boreal and Tethyan realms is an artefact of biostratigraphy.  相似文献   

10.
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   

11.
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   

12.
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   

13.
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   

14.
R. Graziano 《地学学报》1999,11(6):245-250
Drowning successions which cap carbonate platforms and flanks bear palaeoenvironmental information which is useful for genetic stratigraphy; they constitute predictive key-markers in regional to global correlations. An Early Cretaceous platform-to-basin transition has been investigated in Apulia (southern Italy) and two drowning unconformities, dated as early Valanginian and late early Aptian, have been documented. They occur at the base of thick pelagic tongues wedging toward the platform and mark the base of two depositional sequences showing distinct transgressive–regressive cycles. Timing of drowning processes, based on biostratigraphy and dynamic stratigraphy, allows the correlation of unconformities with global-scale palae- oceanographic events marked, among others, by positive spikes of well-established δ13C curves. Drowning signatures in the Apulia carbonates fit the stratigraphic, palaeoecological and possibly geochemical evidence found in global records at the same stratigraphic levels. Moreover, it is proposed that the observed drowning events were caused by palaeoceanographic crises affecting factory productivity.  相似文献   

15.
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   

16.
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   

17.
An exhaustive 14C dating programme of molluscs from the Fossvogur sediments in Reykjavik. Iceland is presented. For the first time all the fossiliferous units of the sediments are dated. The results confirm earlier conclusions of a widespread occurrence of marine sediments of Allerød age in Reykjavik. The set of dates from the Fossvogur sediments shows a narrow 14C age distribution (standard deviation of ±235 years) of molluscs from all localities and from successive marine units in vertical sections. The weighted mean conventional 14C age is 11,400 BP. Assuming a reservoir effect of 400 years. this corresponds to a reservoir-corrected age of I1,000 BP. i.e. the Allerød- Younger Dryas transition for the sampled units, These new 14C dates from Fossvogur confirm the need for a revision of the Upper Pleistocene chronology of the Reykjavik region. They also have a bearing on the Late Weichselian record of glacier readvances and sea-level changes in the area. The dates suggest that the marine units in Fossvogur accumulated within a restricted time-span of a few hundred years. The sediments in Fossvogur are of volcaniclastic origin and are extremely lithified, indicating local geothermal activity soon after their deposition. This may explain anomalously high D/L amino acid ratios measured in molluscs from the Fossvogur sediments. δ13C and δ18O results suggest that temperatures may have ranged up to 60°C.  相似文献   

18.
We present an approach for tracing the fate of anthropogenic CO2, compiling a large data set of stable organic carbon isotope ratios from surface sediments, plankton, and sinking matter in the Atlantic Ocean. The δ13C values of sinking matter are generally lower by 0.5–4.6‰ compared to the surface sediments. This difference increases with increasing latitude, which is explained by a stronger modern increase in surface water [CO2 (aq)] in the Southern Ocean relative to the Tropical/Subtropical Ocean. Preindustrial dissolved CO2 concentrations in Atlantic surface waters, estimated from the δ13Corg of surface sediments, are compared to recently measured surface water [CO2 (aq)] values taken from literature. We obtain only a slight increase in [CO2 (aq)] at lower latitudes but a significant change of about 7 ± 2 μ m in high latitudinal surface waters which we attribute to anthropogenic perturbation. Our results suggest that CO2 released by human activities has been stored in Southern Ocean surface waters.  相似文献   

19.
In order to evaluate the sensitivity of aquatic and terrestrial ecosystems to climatic changes, lithological (sediment structure and color, grain size, physical properties) and biochemical (TOC, TOC/TN, δ13C of TOC and carbonates) investigations were carried out on an 11.12 m-long sediment core from Lama Lake (Central Siberia, Putorana Plateau). According to the pollen data, the sequence represents the termination of the Pleistocene, and the entire Holocene. It is composed of highly terrigenous and stratified clays and silts. Sediment structure, grain-size distribution, carbonate contents and physical properties of the sediment indicate that glaciers were present in the catchment area of Lama Lake during the period Oldest Dryas to AllerØd. For the same time period, δ13C values of TOC give indications of a perennial ice cover. Since the AllerØd, organic matter accumulation has increased, caused by an increasing input of land vegetation and aquatic primary production as revealed by relations TOC contents, TOC/TN ratios and δ13C values of TOC. During the Holocene climatic optimum, in late Preboreal and Boreal times, biogenic primary production in Lama Lake reached its maximum and the vegetation in the catchment area changed from grassy tundra to a dense forest. From the Atlantic period to the present, small variations in δ13C values of TOC and TOC contents are probably related to the location of Lama Lake on the border between grass and wooded steppe, leading to responses of environmental conditions to even small changes in climate.  相似文献   

20.
The formulas for correcting 14C measurements are explained. The expected ranges of the δ13C values for some types of samples are given. It is pointed out that the corrections are approximate because of the choice of correction formula, and that all samples arc normalized to wood samples instead of to carbon dioxide of the air. Some results from the Uppsala laboratory for plant materials and carbonates are given.  相似文献   

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