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1.
Four cores of anoxic sediments were collected from the Seine estuary to assess the early diagenesis pathways leading to the formation of previously reactive phase. Pore waters were analyzed for dissolved iron (Fe) and manganese (Mn) and different ligands (e.g., sulfate, chloride, total inorganic carbon). The anoxic zone is present up to the first centimeter depth, in these conditions the reduction of Mn and Fe oxides and SO4 2− was verified. The sulfate reduction was well established with a subsequent carbon mineralization in the NORMAI94 core. The chemical speciation of Mn and Fe in the dissolved and solid phases was determined. For the dissolved phase, thermodynamic calculations were used to characterize and illustrate the importance of carbonate and phosphate phases as sinks for Fe and Mn. The ion activity product (IAP) of Fe and Mn species was compared to the solubility products (Ks) of these species. In the solid phase, the presence of higher concentration of calcium carbonate in the Seine sediments is an important factor controlling Mn cycle. The carbonate-bound Mn can reach more than 75% of the total concentration. This result is confirmed by the use of electron spin resonance (ESR) spectroscopy. The reduction of Fe is closely coupled to the sulfate reduction by the formation of new solid phases such as FeS and FeS2, which can be regarded as temporal sinks for sulfides. These forms were quantified in all cores as acid volatile sulfide (AVS: FeS+ free sulfide) and chromium reducible sulfide (CRS: FeS2+elemental sulfur S0).  相似文献   

2.
Pore water profiles of total-CO2, pH, PO3?4, NO?3 plus NO?2, SO2?4, S2?, Fe2+ and Mn2+ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O2 > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO2 concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.  相似文献   

3.
Free and sulfur-bound biomarkers in sediments deposited in the northern proto North Atlantic (Newfoundland Basin, ODP Site 1276) during the Cenomanian–Turonian oceanic anoxic event 2 (OAE-2) were studied. The δ13C records of phytane and lycopane confirmed the stratigraphic position of the positive carbon isotope excursion associated with OAE-2, previously reported for total organic carbon (TOC) and β,β-homohopane. Sediments before and after the OAE-2 interval were poor in organic matter (OM) and comprised numerous gravity flow deposits. The interval itself was composed of pelagic sediments with occasionally a much higher TOC content of up to 12.7%. The OAE-2 sediments were characterized by a low amount of terrestrial OM since the dominant biological sources of the biomarkers were aquatic in origin. High hopane, pentamethylicosane (PMI), and squalane abundances in the OM-rich sediments pointed to a relatively high input of prokaryotes, partly derived from cyanobacteria, as suggested by the occasional occurrence of 2-methylhopanes. PMI comprised both the regular and irregular isomer and changes in the δ13C of PMI are thought to reflect contributions from methanogenic and methanotrophic archea. The high relative concentration of lycopane indicated that bottom water conditions were anoxic during large parts of the OAE-2 interval. In one horizon, trace amounts of isorenieratane provided evidence for the occasional occurrence of photic zone anoxia. Taken together, the data imply that oceanic anoxia, and probably also high productivity, reached the northernmost part of the proto-North Atlantic during OAE-2, albeit that photic zone anoxia was much less common than in the southern proto-North Atlantic.  相似文献   

4.
The Dariyan Formation (southwestern Iran) records the characteristic features of an oceanic anoxic event with organic- and radiolarian-rich hemipelagic intervals. A biostratigraphic study based on benthic and planktonic foraminifers provides an early Aptian age for the organic-rich succession and an Aptian age for the entire Dariyan Formation in this area. In this study, we report the first stable carbon isotope curve for the Dariyan Formation which is characterized by a pronounced negative spike (reaching ?2?‰) at the base of the section followed by a subsequent positive excursion (4?‰) and a plateau with values fluctuating around 3?‰. The integration of the δ13C record with the previously published litho- and biostratigraphy provides the characteristic features of the oceanic anoxic events (OAE) 1a interval. This detailed curve improved the stratigraphic resolution in this area and allowed the establishment of a temporal framework which showed good correlations with other OAE 1a sections worldwide. High production of organic matter and abundance of radiolarians and planktonic foraminifers suggest high-nutrient fluxes and meso- to eutrophic conditions at the time of deposition of the organic-rich interval of the Dariyan Formation. This is in agreement with enhanced greenhouse conditions. The facies distribution (from shallow to deep water environments) and the paleogeography of Arabian Plate during the early Aptian suggest that increasing continental runoff was a primary trigger of high trophic level conditions. Redox conditions, estimated from manganese (Mn) behavior, indicate dysoxic to anoxic conditions within the basin during OAE 1a.  相似文献   

5.
The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ18O values of −5.4‰ (VSMOW) and are generally depleted in 18O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ18O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ18O values of +8.4‰ accounts for a permanent enrichment of porewater 18O and observed δ18O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO2 with δ18O values of +8.9‰ released oxygen enriched in 18O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO2 accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ18O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ18O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO2 reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.  相似文献   

6.
The Early Cretaceous carbonate carbon isotope stratigraphy established in pelagic limestones is marked by several pronounced excursions towards positive δ13C values. We investigated a biostratigraphically and palaeomagnetically calibrated Aptian section to see whether C isotope stratigraphy could be recognized in shallow-water carbonates. The chosen carbonate platform sections are located in southern Italy and have been dated by biostratigraphy. Bulk samples, chosen from the Barremian-Albian part of the sequence, were analysed for their O and C isotope compositions. The C isotope curve established shows two major positive excursions which can be correlated with the synchronous and globally recognized Aptian C isotope events. The data provide evidence that C isotope stratigraphy can be used as a powerful correlation tool between pelagic and shallow-water limestone sequences.  相似文献   

7.
Orca Basin, an anoxic, brine-filled depression at a depth of 2200 m in the Northwestern Gulf of Mexico continental slope, has been studied with respect to its uranium geochemistry. Uranium concentration profiles for four cores from within the basin were determined by delayed-neutron counting. Uranium concentrations ranged from 2.1 to 4.1 ppm on a salt-free and carbonate-corrected basis. The highest uranium concentrations were associated with the lowest percentage and δ13C organic carbon values.For comparison, cores frm the brine-filled Suakin and Atlantis II Deeps, both in the Red Sea, were also analyzed. Uranium concentrations ranged from 1.2 to 2.6 ppm in the Suakin Deep and from 8.0 to 11.0 ppm in the Atlantis II Deep. No significant correlation was found between uranium concentrations and organic carbon concentrations and δ13C values for these cores.Although anoxic conditions are necessary for significant uranium uptake by non-carbonate marine sediments, other factors such as dilution by rapidly depositing materials and uranium supply via mixing and diffusion across density gradients may be as important in determining uranium concentrations in hypersaline basin sediments.  相似文献   

8.
The Parnok deposit is made up of stratiform lodes of iron (magnetite) and manganese (oxide-carbonate, carbonate, and carbonate-silicate) ores localized among terrigenous-carbonate sediments (black shales) on the western slope of the Polar Urals. The lithological study showed that ore-bearing sediments were accumulated in a calm hydrodynamic setting within a relatively closed seafloor area (trap depressions). Periodic development of anaerobic conditions in the near-bottom seawater was favorable for the accumulation of dispersed organic matter in the terrigenous-carbonate sediments. Carbon required to form calcium carbonates in the ore-bearing sediments was derived from carbon dioxide dissolved in seawater. In the organic-rich sediments, carbonates were formed with the participation of carbon dioxide released by the destruction of organic matter. However, δ13C values (from 0.5 to ?4.4‰ PDB) suggest a relatively low fraction of the isotopically light biogenic carbon in the host calcite. The most probable sources of Fe and Mn were hydrothermal seepages at the seafloor. The Eh-pH conditions during stagnation were favorable for the precipitation of Fe and accumulation of Mn in a dissolved state. Transition from the stagnation regime to the concentration of oxygen in near-bottom waters was accompanied by oxidation of the dissolved Mn and its precipitation. Thus, fluctuations in Eh-pH parameters of water led to the differentiation of Fe and Mn. Initially, these elements were likely precipitated as oxides and hydroxides. During the subsequent lithification, Fe and Mn were reduced to form magnetite and rhodochrosite. The texture and structure of rhodochrosite aggregates indicate that manganese carbonates already began to form at the diagenetic stage and were recrystallized during the subsequent lithogenetic stages. Isotope data (δ13C from ?8.9 to ?17.1‰ PDB) definitely indicate that the oxidized organic matter of sediment served as the main source of carbon dioxide required to form manganese carbonates. Carbonates from host rocks and manganese ores have principally different carbon isotopic compositions. Unlike carbonates of host rocks, manganese carbonates were formed with an active participation of biogeochemical processes. Further processes of metagenesis (T ≈ 250–300°C, P ≈ 2 kbar) resulted in the transformation of textures, structures, and mineral composition of all rocks of the deposit. In particular, increase in temperature and pressure provided the formation of numerous silicates in manganese ores.  相似文献   

9.
The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.  相似文献   

10.
The structure of the glacial ocean was significantly different to that of the present day with intermediate to mid-depth waters being notably more δ13C enriched than deep waters. This contrast was especially pronounced in the South Atlantic suggesting the development of a sharp chemical divide, or ‘chemocline’, at around 2500 m water depth between upper and lower layers, with implications for deep-ocean carbon storage [Hodell et al., 2003. Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry, Geophysics, Geosystems, 4(1): doi: 1004 10.1029/2002GC000367.]. We evaluate existing benthic foraminiferal δ13C, Cd/Ca and derived carbon isotope air–sea exchange signature (δ13Cas) data sets for the Atlantic during the Last Glacial Maximum (LGM), and Marine Isotope Stages (MIS) 6 and 8 in order to examine the regional extent of the chemocline in the South Atlantic. Benthic δ13C data north of the approximate latitude of the LGM Subantarctic Front (LGM-SAF, 43°S) linearly decrease with water depth, indicative of mixing between upper ‘well’ and lower ‘poorly’ ventilated water masses, with little evidence of the sharp chemical divide. Conversely, benthic δ13C data south of the LGM-SAF below 2500 m water depth are uniformly around ?0.8‰. The apparent δ13C gradient across the LGM-SAF suggests enhanced mid-depth ventilation north of the SAF and reduced ventilation to the south. From this pattern we conclude that the regional chemocline in the South Atlantic constituted a dominantly meridional, rather than a vertical gradient, and was developed during at least the past three glacial periods. Benthic Cd/Ca data indicate that the gradient was not nutrient related, although further data from the South Atlantic are needed for a better assessment of this suggestion. The combined benthic δ13C and Cd/Ca data indicate the source of well-ventilated upper waters in the South Atlantic changed from Northern Component Water (NCW) during early glacial phases to Upper Southern Component Water (USCW) during mid-to-late glacial phases when the Southern Ocean may have become isolated. USCW maintained a positive δ13C and δ13Cas signature simulating a North Atlantic origin that has been implicated in previous studies. The data demonstrate that secular imprints on δ13C must be taken into consideration when assessing the implications of the vertical δ13C gradient. This data also supports a variable water column architecture and modes of water mass formation as primary means to draw down atmospheric CO2 and storage in the abyssal ocean by involving processes occurring on either side of the SAF in the glacial Southern Ocean.  相似文献   

11.
Geochemistry of Peruvian near-surface sediments   总被引：6，自引：0，他引：6  
Sixteen short sediment cores were recovered from the upper edge (UEO), within (WO) and below (BO) the oxygen minimum zone (OMZ) off Peru during cruise 147 of R/V Sonne. Solids were analyzed for major/trace elements, total organic carbon, total inorganic carbon, total sulfur, the stable sulfur isotope composition (δ34S) of pyrite, and sulfate reduction rates (SRR). Pore waters were analyzed for dissolved sulfate/sulfide and δ34S of sulfate. In all cores highest SRR were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and/or diffusion of sulfate from bottom water. δ34S of dissolved sulfate showed only minor downcore increases. Strong 32S enrichments in sedimentary pyrite (to −48‰ vs. V-CDT) are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction. Manganese and Co are significantly depleted in Peruvian upwelling sediments most likely due to mobilization from particles settling through the OMZ, whereas release of both elements from reducing sediments only seems to occur in near-coastal sites. Cadmium, Mo and Re are exceptionally enriched in WO sediments (<600 m water depth). High Re and moderate Cd and Mo enrichments are seen in BO sediments (>600 m water depth). Re/Mo ratios indicate anoxic and suboxic conditions for WO and BO sediments, respectively. Cadmium and Mo downcore profiles suggest considerable contribution to UEO/WO sediments by a biodetrital phase, whereas Re presumably accumulates via diffusion across the sediment-water interface to precipitation depth. Uranium is distinctly enriched in WO sediments (due to sulfidic conditions) and in some BO sediments (due to phosphorites). Silver transfer to suboxic BO sediments is likely governed by diatomaceous matter input, whereas in anoxic WO sediments Ag is presumably trapped due to sulfide precipitation. Cadmium, Cu, Zn, Ni, Cr, Ag, and T1 predominantly accumulate via biogenic pre-concentration in plankton remains. Rhenium, Sb, As, V, U and Mo are enriched in accordance with seawater TE availability. Lead and Bi enrichment in UEO surface sediments is likely contributed by anthropogenic activity (mining). Accumulation rates of TOC, Cd, Mo, U, and V from Peruvian and Namibian sediments exceed those from the Oman Margin and Gulf of California due to enhanced preservation off Peru and Namibia.  相似文献   

12.
The isotopic evolution of δ13C and δ18O is reported for the Jurassic and early Cretaceous in two pelagic sections of the External Zones in the Betic Cordilleras (SE Spain). Stable isotope curves from pelagic trough and swell sections display similar patterns. Variations in δ18O and δ13C values from strata at equivalent age probably reflect both early diagenetic cementation and later temperature‐related burial diagenesis. Comparison of global isotope curves with those presented in this work allows the differentiation of global from local events. Thus, the anoxic event during the early Toarcian (falciferum Zone) is characterized by elevated δ13C and depressed δ18O values. The isotopic record also allows the detection of the middle Oxfordian transgression. There are other peaks for the late Toarcian, early Bajocian, Callovian and early Berriasian that can also be used as tools for correlation purposes. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

13.
The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ56Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ56Fe values with increasing manganese contents. The low δ56Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.  相似文献   

14.
Abstract A detailed carbon isotope study has been carried out on a Santonian (Upper Cretaceous) carbonate platform succession that crops out at Monte Sant'Erasmo (southern Italy). Previous centimetre‐scale studies on this succession have shown that high‐frequency eustatic changes, resulting from the Earth's orbital fluctuations, controlled the hierarchical organization of the depositional and early diagenetic features in elementary cycles, bundles (groups of three to five elementary cycles) and superbundles (groups of three or four bundles). The elementary cycles, which correspond to single beds, suggest a control caused by Earth's precession; the bundles and superbundles record the short (≈ 100 kyr) and long (≈ 400 kyr) eccentricity periodicity respectively. The δ13C signal of the Monte Sant'Erasmo succession is cyclic in nature and may be considered to be a reliable proxy for the sedimentary evolution (and related sea‐level history) of the analysed sequence. The carbon isotope cyclicity is recorded at bundle and superbundle level, but it is not evident at the scale of the elementary cycles, at least with the sampling interval used in this study. Spectral analysis of the δ13C record shows two main peaks corresponding to the short‐ and long‐eccentricity periodicity, whereas the precession signal is not evident in the power spectrum. In addition, lithofacies analysis shows that, in each bundle (and superbundle), higher C isotope values occur in sediments characterized by marine cements, whereas lower values are normally found in more restricted deposits overprinted by early meteoric diagenesis. Early diagenesis, driven by periodic sea‐level fluctuations, developed in either shallow‐subtidal (marine diagenesis) or subaerial‐exposed (meteoric overprint) sedimentary environments and directly influenced the carbon isotope signature. As a consequence, the δ13C record at Monte Sant'Erasmo reflects high‐frequency climatic oscillations controlling both environmental and early diagenetic changes. The long‐term isotopic record is similar to that of contemporaneous pelagic sections in England and elsewhere in Italy. It is concluded that the δ13C signature of shallow‐water carbonates, such as those of Monte Sant'Erasmo, offers great potential for correlation with coeval sections, including those of the pelagic realm.  相似文献   

15.
The Nanhuan manganese deposits in the southeastern Yangtze Platform occur in the black shale series in the lower part of the Datangpo Formation. In order to constrain the genesis of the deposits, a detailed study was undertaken that involved field observations, major and trace element analyses, organic carbon analyses, and isotope analyses (C, O, S). The major findings are as follows. (1) The ore-bearing rock series, morphology of the ore bodies, and characteristics of ores in several deposits are similar. The ore minerals are rhodochrosite and manganocalcite. The gangue minerals are mainly quartz, feldspar, dolomite, and illite. Minor apatite and bastnaesite occur in the manganese ores. (2) The ores are enriched in Ca and Mg, whereas they are depleted in Si, Al, K, and Ti compared to wall rocks. The ores normalized to average Post-Archean Australian shale (PAAS) are enriched in Co, Mo, and Sr. The chondrite-normalized rare earth element (REE) patterns for ores and wall rocks are between those of typical hydrogenous and hydrothermal type manganese deposits. Additionally, the ores have positive Ce anomalies with an average Ce/Ce* of 1.23 and positive Eu anomalies with an average Eu/Eu* of 1.18 (normalized to PAAS). (3) The average content of organic carbon is 2.21% in the samples, and the average organic carbon isotopic value (δ13CV-PDB) is − 33.44‰. The average inorganic carbon isotopic value (δ13CV-PDB) of carbonates in Gucheng is − 3.07‰, while the values are similar in the other deposits with an average of − 8.36‰. The oxygen isotopic compositions (δ18OV-PDB) are similar in different deposits with an average of − 7.72‰. (4) The sulfur isotopic values (δ34SV-CDT) of pyrite are very high and range from + 37.9‰ to + 62.6‰ (average of 52.7‰), which suggests that the pyrite was formed in restricted basins where sulfate replenishment was limited. The sulfate concentrations in the restricted basins were extremely low and enriched in δ34S, which resulted in the very high δ34S values for the pyrite that formed in the manganese deposits. Therefore, a terrigenous weathering origin for manganese can be excluded; otherwise, the sulfate would have been introduced into the basins together with terrigenous manganese, which would have decreased the δ34S values of pyrites. The manganese, which originated from hydrothermal processes, was enriched in the restricted and anoxic basins, and then, it was oxidized to manganese oxyhydroxide in the overlying oxic waters whereby the products precipitated into the sediments. The manganese oxyhydroxide in the sediment was then reduced to Mn2 + and released to the pore waters during the process of diagenesis. Some organic carbon was oxidized to CO32 −, which made the depletion of 13C in manganese carbonates. Therefore, we suggest that the Nanhuan manganese deposits are hydrothermal–sedimentary/diagenetic type deposits.  相似文献   

16.
New stable carbon and oxygen isotope data from an Upper Cretaceous section in Tibet are presented, and compared to carbon isotope records from England, Italy, and Germany. Together with a stratigraphic re-interpretation of published carbon isotope data from a nearby section in Tibet, our data can surprisingly well be correlated with the European sections. This indicates that, similar to the distinct positive carbon isotope excursion at the Cenomanian-Turonian boundary, also the broad positive carbon isotope shift in the middle-late Coniacian and early Santonian reflects a major perturbation of the carbon cycle on a global scale, even though organic-rich sediments related to the OAE3 appear to be mainly restricted to the Atlantic Ocean and adjacent basins. The data further show that, apart from the broad Coniacian-Santonian carbon isotope excursion, also isotopic shifts on a smaller scale in the Turonian and Coniacian, such as the Round Down, Pewsey, and Hitchwood Events, can be correlated over both hemispheres. This demonstrates that the development of global oceanic anoxic conditions and associated burial of large amounts of organic carbon do not constitute a prerequisite for globally reflected carbon isotopic shifts. The data from Tibet support the concept of a relation between main carbon isotope excursions and major sea-level variations. Cyclic fluctuations of geochemical and lithological parameters are likely to be orbitally driven. These cycles appear to be preferably reflected in the sediments during periods of lower or variable sea-level, whereas the ocean-atmosphere system seems to have operated in a different mode during long phases of high, stable sea-level, as during the Coniacian-Santonian OAE3.  相似文献   

17.
Micropalaeontological and isotopic studies of the upper Cenomanian turbiditic/hemipelagic sediments from the High-Tatric unit (Central Western Carpathians; Polish part of the Tatra Mountains) has been undertaken to characterize the sedimentary conditions in the Tatric basin, a part of the Western Tethys, related to the interval preceding the late Cenomanian oceanic anoxic event (OAE2). The deposition of these sediments, including organic-rich layers (TOC up to 0.7%), corresponds to the Rotalipora cushmani foraminiferal Zone. Microfacial, foraminiferal and palynological analyses show that the sea floor was located at upper bathyal depths and the water column was poorly oxygenated. The intrabasinal carbonate material indicates moderate primary productivity with rare periods of upwellings. The scarcity of marine fossils in redeposited material and features of carbonate lithoclasts suggest very low productivity in the nearshore surface water, most probably due to a low-density hyposaline cap as surface runoff from the southern margin of the basin. The carbon isotopic study documents the negative values of δ13Ccarb in the whole section as an effect of transfer of isotopically light carbon sourced from various sources. Such negative values of δ13Ccarb are characteristic of the upper Cenomanian sediments, deposited in relatively shallow water basins, characterized by input of terrestrial organic matter and/or carbonate particles known from the Western Interior sections, the Atlantic coastal plain, the northwestern African margin, the eastern margin of the Apulian Platform and shelf sediments in the NW Europe and Tethyan Himalayas. Most probably, all of these events could be related to the global sea level fluctuations that occurred ca. 95.5–94.5 Ma comparing with the Haq (2014) eustatic curve.  相似文献   

18.
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19.
刘振  马志鑫  刘伟  凌云 《沉积学报》2021,39(3):515-524
在系统分析重庆秀山小茶园锰矿区大批钻孔数据资料的基础上,基于野外地质调查和室内沉积学、地球化学等分析得出,盆地中心相的沉积物厚度大,TOC、U、Mo、V、δ34Spy的含量高,草莓状黄铁矿丰富,指示了缺氧的水体条件,且锰矿的品位和厚度大。向盆地边缘相、斜坡相,沉积物厚度减小,TOC、U、Mo、V、δ34Spy含量降低,指示水体缺氧程度减弱,同时锰矿的品位和厚度减小。锰矿体厚度的变化与含锰岩系厚度的变化规律一致,二者呈正相关关系。小茶园矿区矿体展布方向与小茶园次级地堑盆地长轴方向基本一致,为NE60°左右。因此,小茶园矿区锰矿床矿体产出受到次级地堑盆地和沉积相带的控制,缺氧的盆地中心相成矿最佳,其次为盆地边缘相,斜坡相成矿较差。  相似文献   

20.
Total organic carbon (TOC) and total nitrogen (TN) concentrations are analyzed with high temporal resolution (ca. 100 years) for cores MD179-3304 and MD179-3312 taken from the Japan Sea off Joetsu City. The temporal changes in TOC and TN concentrations vary quasi-regularly in similar patterns. The age models are formed on the basis of the dates of 14C dating, marker tephra beds, TL layers, and marine isotope events with depth. TOC concentration is high in MIS 1 and 5, low in MIS 2 and 4, and slightly elevated in MIS 3 with frequent short fluctuations. This general trend is very similar to LR04 curve, except for the reduced dominance of TOC around the MIS 5.5 substage. As shown typically in MIS 3, there are many peaks of TOC in a short interval. The details of these TOC peaks can be correlated with the warm interstadials of the Greenland ice core. We can identify a sawtooth-like decreasing trend of TOC in MIS 3. In contrast, the decreasing trend of δ18O in the ice core corresponds to an increasing trend of TOC in MIS 5.The most plausible explanation for the correlation of climate changes between East Asia and the North Atlantic is oscillation of the Arctic polar front through time. The detailed correspondence of TOC concentrations of the Japan Sea sediments to other common paleoclimate proxies means that the TOC concentrations of these sediments is an excellent paleoclimate record in Far East Asia, although the genetic relationship between air temperature and biological productivity in the Japan Sea is as yet unknown.  相似文献   

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