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1.
The evolution of early diagenetic calcite cements in microbial mats of recent supratidal sediments of the southern North Sea is modeled in a two-dimensional microscale approach by a cellular automaton model (CAM). Calcite is traced out in the model by virtual calcium distribution patterns obtained from runs under different assumptions concerning sediment-intrinsic conditions. For justification of the CAM, real calcium distribution patterns, documented by scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDX), are quantitatively compared with the virtual patterns on the basis of multifractal analyses. The formation of high magnesian calcite as a consequence of biogenic anaerobic decomposition of organic matter starts at certain initial calcite domains. In this stage an inhomogeneous and multifractal calcium distribution is characteristic. Nearly complete remineralization of organic matter leads to monofractal behavior of generalized fractal dimensions (DB(q) ±1.84). The CAM results confirm that calcite formation is a self-determining morphogenetical process and diffusive transport processes of reactants within the mat affect the biogenic calcite formation.  相似文献   

2.
Stoichiometric solubility constants of calcite in initially supersaturated solutions of various magnesium to calcium concentration ratios but identical ionic strength were determined at 25°C and one atmosphere total pressure.The thermodynamic solubility constant of calcite is used with ion pairing equations to interpret the data reported in this study. Results indicate that even though magnesian calcites, rather than pure calcite, precipitate from seawater solutions containing magnesium ions, the incorporation of MgCO3 in the calcite crystal lattice does not extensively alter the equilibrium calcium carbonate activity product.The equilibrium activity of the ionic species in solution and the composition of magnesian calcite overgrowths precipitated from solutions of similar composition are used to calculate the solubility of magnesian calcites. The values for magnesian calcite solubilities obtained by this approach are lower than those obtained from the dissolution kinetics of biogenic carbonates.  相似文献   

3.
Late Cretaceous economic phosphorites from the Red Sea, Nile Valley, and Abu Tartur areas, Egypt, show distinct variations in the lithology of associated sediments, mineralogy of nonphosphatic constituents, and distributions of major and trace elements. In the Red Sea area, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and calcite, ankerite, and pyrite cements. In the Nile Valley, the phosphorite beds are intercalated with chert, marl, and sandstone and the nonphosphatic constituents are detrital quartz and calcite and chalcedony cements. In the Abu Tartur Plateau, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and ankerite and pyrite cements. The phosphorites studied also show distinct variations in major- and trace-element concentrations. The Abu Tartur phosphorites have higher contents of TiO2, Al2O3, Fe2O3, K2O, Co, Nb, Pb, Sr, Th, Y, and Zr and lower SiO2, Ba, and U contents as compared to those in the Red Sea and Nile Valley areas. The positive correlations between Al2O3 and TiO2, K2O, Nb, Y, and Zr suggest the detrital origin of these constituents.Similarity in the phosphatic constituents, which were derived from outside the depositional sites, and variations in the lithology of associated sediments and the mineralogy and geochemistry of the nonphosphatic constituents, which reflect the conditions at the depositional sites, suggest that the variations in the depositional environment of the phosphorites are the potential controlling factor of the compositional variations among these phosphorites. The abundance of black shales in the Red Sea and Abu Tartur areas, as well as the occurrence of ankerite and pyrite as cementing materials for the phosphatic constituents, might reflect reducing conditions in these areas, while the abundance of siliciclastic sediments and calcite and chalcedony cements suggests oxidizing conditions in the Nile Valley. The reducing conditions in the Red Sea and Abu Tartur areas were probably developed within the pre-existing depressions in a shelf environment. These depressions might have formed as a result of a change in the movements of the North Atlantic, Eurasian, and African Plates during the late Santonian, which led to transgressive inversion of rifts along northern Egypt and consequent folding in the continental interior. The higher contents of detrital components in the Abu Tartur phosphorites compared to the Red Sea and Nile Valley areas suggest more detrital inputs during the deposition of the phosphorites in Abu Tartur. The products of the diagenesis and weathering of these deposits also reflect the variations in the depositional conditions.  相似文献   

4.
The International Ocean Discovery Program (IODP) Expedition 367/368 reported massive Upper Miocene deep-sea turbidite in the northern South China Sea basin. The Upper Miocene turbidite sand-bodies at Site U1500 were examined with detrital zircon U-Pb dating to conduct the source-to-sink analysis. This study shows that the U-Pb age spectrums of Site U1500 sample are similar to those detrital zircons from the Miocene Qiongdongnan Basin and the Pearl River Mouth Basin. Multidimensional scaling (MDS) plot also shows that the turbidite sand-bodies at Site U1500 are closely related to the sediments in the Pearl River Mouth Basin and Qiongdongnan Basin. It is likely that the thick deep-sea turbidite succession in the deep-water basin of northern South China Sea was formed by a mixed provenance pattern during the late Miocene. On the one hand, terrigenous sediments from the west of the South China Sea were transported along the Central Canyon to the eastern South China Sea deep-sea basin in the form of turbidity current. On the other hand, terrigenous sediments were also transported from the Pearl River through the slope canyon system to the northern South China Sea in the form of gravity flow . Those mixed sediments from two different source areas have collectively deposited at the deep-sea basin and thus, give rise to turbidite sequence of hundred meters. Provenance analysis of the thick turbidites sand-bodies in the deep-sea basin is of great significance to the profound understanding of the tectonic evolution, filling processes, provenance evolution, and the palaeogeographic characteristics of the Cenozoic basins of the South China Sea.  相似文献   

5.
IODP367/368航次在南海北部深海盆地多个站位发现上中新统厚达数百米的大规模深海浊积岩。采用碎屑锆石U-Pb年龄谱系分析方法对U1500站上中新统浊积砂体进行源汇对比分析。研究结果表明U1500站上中新统浊积岩碎屑锆石年龄谱系与其西侧琼东南盆地和北侧珠江口盆地中新世沉积物特征类似。多维排列分析(MDS)结果也显示,该站位样品与珠江口盆地、琼东南盆地沉积物关系密切,表明南海北部深海盆地内厚达数百米的上中新统浊积砂体为南海北部物源和南海西部物源混合堆积形成。南海西部陆源输入物质以浊流搬运的方式,沿中央峡谷从西到东搬运至南海东部深海盆地;南海北部珠江物源以重力流的形式,经南海北部陆坡峡谷搬运至深海盆地中,两种来源的沉积物在深海盆地发生混合沉积,形成U1500站厚达数百米的浊积砂体。南海北部深海盆地厚层浊积砂体物质来源的准确识别,对深刻理解南海新生代盆地的构造演化、沉积物充填过程、物源演变以及古地理特征均具有重要意义。  相似文献   

6.
The supply and accumulation of silica in the marine environment   总被引：4，自引：0，他引：4  
Rivers and submarine hydrothermal emanations supply 6.1 × 1014g SiO2/yr to the marine environment. Approximately two-thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep-sea deposits. Siliceous deep-sea sediments located beneath the Antarctic Polar Front (Convergence) account for over a fourth (1.6 × 1014g SiO2/yr) of the silica supplied to the oceans. Deep-sea sediment accumulation rates beneath the Polar Front are highest in the South Atlantic with values as large as 53cm/kyr during the last 18.000 yr. Siliceous sediments in the Bering Sea, Sea of Okhotsk, and Subarctic North Pacific accumulate 0.6 × 1014g SiO2/yr or 10% of the dissolved silica input to the oceans. The accumulation of biogenic silica in estuarine deposits removes a maximum of 0.8 × 1014g SiO2/yr. Although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters, the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters. Siliceous sediments from continental margin upwelling areas (e.g. Gulf of California, Walvis Bay, or Peru-Chile coast) have the highest biogenic silica accumulation rates in the marine environment (69 g SiO2 cm2/kyr). Despite the rapid accumulation of biogenic silica, upwelling areas account for less than 5% of the silica supplied to the marine environment because they are confined laterally to such small areas.  相似文献   

7.
The quantitative distribution of planktonic foraminifera, pteropods, and coccolithophorids, as well as oxygen-isotope variations were analyzed in four deep-sea cores from the Gulf of Aqaba (Elat) and the northernmost Red Sea. The core record covers about 150,000 yr. Detailed stratigraphic subdivision is facilitated by combining all calcareous plankton groups. Time-stratigraphic correlation and dating beyond the radiocarbon range are possible by comparison of the oxygen-isotope curves. During the glacial maximum salinity rose to more than 50‰, while winter temperature of the upper waters fell by at least 4°C compared to the present. The rise in salinity can be accounted for by sea-strait dynamics and lowering of sea level. The Gulf of Aqaba and the Red Sea were continuously connected through the Straits of Tiran, and there is no indication of desiccation during the glacial maximum.  相似文献   

8.
Studies on surface water characters of the estuarine environment of Iraq, northwestern Arabian Gulf, define three subdivisions of salinity: oligohaline, polyhaline, and euhaline. Textural analysis reveals that surface sediments covering the bottom are composed of six distinct classes: silty clay, clayey silt, sand-silt-clays, clayey sand, silty sand and sand. Five physiographic subdivisions are identified and described fluvial-estuarine, tidal mud flats (subtidal flat, lower intertidal flat, upper intertidal flat, supratidal flat), sand bars, Abdallah-Shetana channel and submerged estuarine distributary channel and bar systems. The content of organic matter in the sediments ranges from 0.24-3.69 per cent by weight. High values were recorded from the Abdallah-Shetana channel while lower values are confined to sand bars and submerged estuarine distributary channel and bar systems. Carbonates, quartz, feldspar, and halite are the main mineral constituents of the non-clay grade sediments; carbonates are present mainly as low-magnesium calcite followed by dolomite and high-magnesium calcite. Aragonite is present only in trace amounts. Both detrital and biogenic sources are suggested for low-magnesium calcite. A detrital source for dolomite and a biogenic source for high-magnesium calcite and aragonite are proposed.  相似文献   

9.
Free-drift dissolution data and inverse time plots were used to evaluate the stabilities of synthetic and biogenic magnesian calcites in aqueous solutions at 25°C and 1 atm total pressure. Synthetic phases with MgCO3 concentrations below 6 mole percent have stoichiometric ion activity products that are less than the value for calcite, whereas the values for phases with higher concentrations are greater than that of calcite. For synthetic phases, stability is a smooth function of composition and all phases (up to 15 mole percent MgCO3) have values of ion activity products less than that for aragonite. These results agree with those of Mucci and Morse (1984) derived from precipitation of magnesian calcites in aqueous solutions. “Average” seawater at 25° and 1 atm total pressure is supersaturated with respect to all synthetic phases in the compositional range studied.Biogenic samples are less stable than synthetic phases of similar Mg concentrations and stability is not a smooth function of composition. Biogenic materials with compositions greater than 11–13 mole percent MgCO3 have ion activity products greater than that for aragonite.The difference in stability between biogenic materials and synthetic phases is due to greater variation in chemical and physical heterogeneities found for the biogenic samples. If it is assumed that the results of the dissolution experiments reflect only differences in Gibbs free energies of formation between synthetic phases and biogenic materials of similar Mg concentration, the biogenic materials are 200–850 j/mol less stable than the synthetic phases. Only the results of synthetic dissolution experiments should be used to model the thermodynamic behavior of the magnesian calcite solid solution. The results for the synthetic phases, however, may not be appropriate to use for interpreting diagenetic reaction pathways for magnesian calcites in modern sediments, except as a basis of comparison with the behavior of natural materials.  相似文献   

10.
An ecological transfer function based on the distribution of planktonic foraminifera in 66 Mediterranean and 8 North Atlantic surface-sediment samples is used to estimate sea-surface temperatures and salinities for the eastern Mediterranean during the last glacial maximum (18,000 yr B.P.). The present-day distribution of planktonic foraminifera can be explained by four faunal assemblages, each of which has diagnostic environmental preferences. Factor 1 is a tropical-subtropical assemblage; factor 2 is a transitional assemblage; factor 3 is a low-salinity assemblage; and factor 4 is a subpolar assemblage. The geographic distribution of these faunal assemblages reflect the variation in overlying hydrographic conditions. The 18,000-yr B.P. samples were selected based on total faunal stratigraphy, oxygen-isotope stratigraphy, and previously determined radiometric dates for eastern Mediterranean volcanic ash layers. Estimated temperature and salinity patterns show that the greatest change between present-day and 18,000-yr B.P. sea-surface conditions existed in the Aegean Sea and immediately south of Crete. The winter temperature anomaly (18,000 yr B.P.-present) within the Aegean Sea is 6°C cooler than present. In contrast to this, the maximum summer temperature anomaly exists to the south of Crete, where sea-surface temperatures were 4°C cooler than present. Estimated sea-surface salinities also show that the greatest change took place within the Aegean Sea, being 5‰ less saline than present. The estimated temperature and salinity patterns seem to reflect changing drainage patterns during glacial times and the diversion of cool, low-salinity water into the Aegean Sea. The source of this glacial runoff appears to be large freshwater lakes that existed during this time over parts of eastern Europe and western Siberia.  相似文献   

11.
米粒状白云石及其出溶成因   总被引：4，自引：0，他引：4  
米粒状白云石产于奥陶系近岸碳酸盐岩序列的云斑(生屑)泥晶灰岩、砾屑灰岩和叠层石灰岩中,在虫孔和砾屑粒间最为富集,外形为伸长菱面体,薄片中呈米粒状.晶体无环带或雾心,但内部含大量呈规则分布的细小方解石“包体”,结晶C轴垂直于晶体延长方向.与背景方解石泥晶相比,Fe、Mn、Na和Ba含量较高,Sr含量较低.C轴的固定取向排除了石膏假晶的可能,也不是普通白云石化或交代钡白云石的结果,推测是沉淀镁方解石重结晶后在压力增加的条件下经出溶作用形成.  相似文献   

12.
The textural and geochemical aspects of the sediments of subtropical mangrove ecosystem and surrounding areas have been studied and discussed. Forty sediment samples were collected from different areas of mangrove environment and surrounding areas along the Egyptian Red Sea coast. The sediments of each study areas are characterized by the abundance of sand with minor amounts of mud and gravel. The mean size of the sediments ranged from medium grained to fine grained at the study areas. Cluster analysis showed that the distribution of gravel, sand, and mud fractions is related to bottom facies and type of sediment source. Generally, sand fraction is the main category among the three constituents. Carbonate content recorded minor values in the study areas. The CaCo3 content of the sediments ranged from 4.7 % at Hamata area to 64.9 % at Erier area. Terrigenous and biogenic components are the factor controlling of the carbonate content of studies sediments. The organic carbon content ranged from 1.10 at Hamata area to 3.1 % at Sharm el-Qibli and Erier areas, which is controlled by particle size of the sediments. Phosphorus content in the different studied localities is related to the sources of phosphorus to the area. Our observations provide evidence that there are no significant differences in environmental texture and geochemistry of the sediments of subtropical mangrove ecosystem and surrounding areas of the Egyptian Red Sea coast.  相似文献   

13.
Selected geochemical parameters and siliceous microfossil assemblages in Baltic Sea sediments are presented which reflect past variations in redox conditions, salinity and primary production. The sediments were deposited during the freshwater Ancylus Lake (9500-8000 14C BP) and brackish Litorina Sea (8000-3000 BP) stages of the Baltic. The diatom record shows that surface-water salinity increased further at c . 7000-6500 BP, although smaller amounts of brackish water entered the basin from c . 8000 BP onwards. Attempts to use exchangeable Mg as a palaeosalinity indictor were not applicable. Gross primary productivity increased along with salinity, which has been interpreted as an effect of nutrient enrichment in the photic zone. This led between c . 6500 and 4500 BP to a high accumulation of organic carbon, anoxic or nearly anoxic bottom conditions and formation of laminated deposits. Certain laminae consist of alternating layers of organic and minerogenic material and were probably formed annually, i.e. in the manner of varves. The laminated successions are distinguished by enrichments of V, Cu and especially Mo. The highest Mo content occurs in the core from the greatest water depth, an effect of anoxic conditions during deposition. The Fe/Mn ratio was shown to be ambiguous as an indicator of past redox conditions. Since biogenic silica shows large variablity in contemporaneously deposited sediments, this parameter cannot be used as a proxy for the past production of siliceous algae in the Baltic Sea.  相似文献   

14.
Relics of a thick, widely spread, fluvial sequence of Early Miocene age are scattered throughout southern Israel, eastern Sinai, the Dead Sea Rift Valley and the western margins of the Jordanian Plateau. These relics are mainly preserved in structural lows, karstic systems, and abandoned stream valleys. The paleogeography of this fluvial system was reconstructed based on the relations between the sequence remnants and the main structural and morphological features of the southeastern Levant region.Three sedimentary associations were identified in the Miocene sequence: a lower part dominated by locally derived clastic sediments; a thicker middle part, composed mostly of far-field allochthonous clastic sediments; and an upper part composed of local as well as allochthonous sediments. The two lower parts are regionally distributed whereas the upper part is syn-tectonic and confined to the Dead Sea basin and the Karkom graben in the central Negev. The composition of the far-field allochthonous sediments points to a provenance of Precambrian crystalline rocks of the Arabo-Nubian massif that were exposed along the uplifted shoulders of the Red Sea Rift as the upper drainage basin of the fluvial system. The diverse mammal remains found in this fluvial sequence suggest a complex of savanna, forests and fluvial habitats similar to those of present East Africa, with monsoon-type rains, which were the dominant water source of the rivers.The thickness of the Miocene sequence in the central Negev is at least 1700 m, similar to that of the subsurface sequence encountered in the Dead Sea basin. This similarity suggests that both were parts of an extensive subsiding sedimentary basin that developed between the Neo-Tethys and the uplifted margins of the Red Sea.The relations between the reconstructed pre-depositional landscape of southern Israel during the Early Miocene and the overlying fluvial sequence indicate that the entire area was buried under several hundred meters of fluvial sediments, reflecting a subsidence of the northern margins of the African continent (Arabian plate) before its breakup and the splitting of the Sinai–Israel subplate by the Dead Sea Transform.During the early Middle Miocene the subsidence was inversed as the mountainous backbone of Israel was uplifted. The uplift triggered a large scale denudation that removed the thick Early Miocene fluvial sequence from the Negev and transported the eroded sediments northwestward toward the eastern Mediterranean basin. Additional uplift during the late-Middle Miocene was associated with entrenchment of the Be’er Sheva Valley between the Judea Mountains in the north and the Negev Highlands in the south. This valley was flooded by the sea during the Late Miocene.We suggest that the formation of the Early Miocene subsiding basin at the northern edge of the Arabian sub-plate predated the breakup of the Arabian plate by the DST. The inversion of the subsiding regime, which led to the establishment of the Negev Highlands seems to be intimately related to the detachment of the Sinai–Israel sub-plate from the Arabian plate during the Middle Miocene.  相似文献   

15.
The Red Sea represents an extreme marine environment, with high salinity, high temperature and low level of nutrients, complicating the application of standard geochemical palaeotemperature proxies. In order to investigate the applicability of the TEX86 (TetraEther indeX of GDGTs with 86 carbons) proxy for sea surface temperature (SST) in the Red Sea, the distribution of glycerol dialkyl glycerol tetraether membrane lipids (GDGTs) in sediments from the Red Sea and the Gulf of Aden were examined. Against expectations, TEX86 values for the Red Sea do not show a simple linear relationship with SST and deviate from the global core top calibration. In the northern Red Sea, at temperatures between 25 and 28 °C, the values increase linearly with SST, whereas in the southern Red Sea, at temperatures above 28 °C, TEX86 decreases with increasing temperature. Factors like seasonality and depth of production, salinity and nutrient availability, as well as diagenetic overprint or influence of allochtonous terrestrial lipids, cannot explain this pattern. However, the observed TEX86 relationship with SST could be explained by the presence of a hypothetical endemic Crenarchaeota population in the Red Sea with a specific TEX86 vs. SST relationship. In the Southern Red Sea, a two-component mixing model implies an exponential decrease in the endemic population towards the Gulf of Aden. Thus, the application of the TEX86 as a palaeotemperature proxy in the Red Sea is likely only possible for the northern Red Sea area with the specific SST vs. TEX86 relationship determined in this study and potentially for the whole Red Sea basin during glacials, when water exchange with the Indian Ocean was more restricted than today and the endemic archaeal lipid distribution was not affected by transport from the Indian Ocean. Our results suggest that distinct populations of Crenarchaeota in extreme environments such as evaporitic basins may have different membrane composition, necessitating application of another calibration than the global core top calibration for TEX86 palaeothermometry.  相似文献   

16.
Almost all Red Sea deeps contain metal-rich sediments covered by brine pools. It is generally agreed that these metal-rich deposits precipitated from overlying metal-rich brines that originated from migrating hydrothermal fluids. No brine pool has ever been reported in Thetis Deep, inciting us to evaluate if such a brine layer ever occurred in the deep during the past. In order to address that questioning, a study combining mineralogical, geochemical (major-, minor-, rare-earth elements) and isotopic (Sr, Nd, Pb) approaches was completed on cored sediments and extracted interstitial waters from inside and outside the deep.The sediments have an overall hydrothermal origin, as shown by the REE concentrations and patterns, metal contents, and Pb–Nd isotopic data, all pointing to a mantle signature. The intensity of the hydrothermal activity varied with time in the deep; the most intense episode resulting in an almost pure Fe-oxi-hydroxide layer. Varied chemical arguments, especially the Zr and REE data of the sediments, favor the fact that the whole sedimentation in Thetis Deep occurred in the absence of a stable, salt-rich and mineralized brine pool, and that no brine layer ever existed. This conclusion is supported by the constant Sr isotope composition of the sediment and its interstitial waters that are almost identical to that of the Red Sea seawater. The study also suggests that hydrothermal activity monitored fluid supplies that interacted differently with seawater in the different Red Sea deeps, resulting in an overall formation of metal-rich sediments, but along varied local conditions.  相似文献   

17.
The spatial distribution patterns of surficial sediment samples from different sedimentary domains(shallow to deep-sea regions) of the eastern Arabian Sea were studied using sediment proxies viz.environmental magnetism,geochemistry,particle size and clay mineralogy.Higher concentrations of magnetic minerals(high x_(lf)) were recorded in the deep-water sediments when compared with the shallow water sediments.The magnetic mineralogy of one of the shallow water samples is influenced by the presence of bacterial magnetite as evidenced from the x_(ARM)/x_(lf)~(vs).x_(arm_/x_(fd)biplot.However,the other samples are catchment-derived.The high correlation documented for x_(lf).anhysteretic remanent magnetisation(x_(arm)) and isothermal remanent magnetisation(IRM) with Al indicates that the deep-sea surficial sediments are influenced by terrigenous fluxes which have been probably derived from the southern Indian rivers,the Sindhu(the Indus) and the Narmada-Tapti rivers.A lower Mn concentration is recorded in the upper slope sediments from the oxygen minimum zone(OMZ) but a higher Mn/Al ratio is documented in the lower slope and deep-sea sediments.Clay minerals such as illite(24-48.5%),chlorite(14.1-34.9%),smectite(10.6-28.7%) and kaolinite(11.9-27.5%) dominate the sediments of shallow and deep-sea regions and may have been derived from different sources and transported by fluvial and aeolian agents.Organic carbon(OC) data indicate a low concentration in the shallow/shelf region(well oxygenated water conditions) and deeper basins(increased bottom-water oxygen concentration and low sedimentation rate).High OC concentrations were documented in the OMZ(very low bottom-water oxygen concentration with high sedimentation rate).The calcium carbonate concentration of the surface sediments from the continental shelf and slope regions( 1800 m) up to the Chagos-Laccadive Ridge show higher concentrations(average = 58%) when compared to deep basin sediments(average = 44%).Our study demonstrates that particle size as well as magnetic grain size,magnetic minerals and elemental variations are good indicators to distinguish terrigenous from biogenic sediments and to identify sediment provenance.  相似文献   

18.
A bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite. Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO3 and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO3. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization. The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen-depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the sea-floor.  相似文献   

19.
Many fabrics in Corallian (Upper Jurassic) carbonates in England, France and Switzerland are bigenetic, forming by solution/precipitation and recrystallization processes. Early precipitated cements are non‐ferroan, whereas those formed later are decidedly ferroan. Mossbauer spectroscopy has shown that the iron in the carbonates is largely divalent, substituting for calcium in the calcite lattice. The cements may be subdivided into those forming in an oxidizing environment, and those forming in a reducing environment. Fabric evidence indicates that a solution period separates the two cement phases. The diagenetic history is linked with the sedimentation pattern. Thus, under progressive build up of sediments into shallow‐water or supratidal conditions, the first‐phase cement probably precipitated from non‐marine, partially oxygenated vadose water. Subsequent subsidence and sedimentation resulted in the depression of partially cemented sediments into zones within the sedimentary pile conducive first to pyrite precipitation, and then to ferroan calcite precipitation. Fabrics can therefore be used to interpret the diagenetic environment of regressive sedimentary associations.  相似文献   

20.
Origin of authigenic carbonates in sediment from the deep Bering Sea   总被引：1，自引：0，他引：1  
Forty beds of authigenic carbonate were identified from the deep Bering Sea in cores taken on Leg 19 of the Deep Sea Drilling Project. Carbonate minerals were mainly high-magnesium calcite and protodolomite, less commonly siderite, rhodo-chrosite, low-magnesium calcite, and manganosiderite. Authigenic carbonates cement and replace diatom ooze, ash and bentonite beds, and, less commonly, clastic beds. Replacement zones are as much as 60 cm thick. Eighty-five per cent of carbonate beds occurred below 400 m sub-bottom depth and 70% in sediment older than 4 m.y. δ13C values averaged -17.200/00 PDB and δ18O ranged from 18.59 to 34–110/00SMOW. The carbon was derived from oxidation of organic matter under anaerobic conditions during bacterial reduction of sulphate, or from CO2 produced in concert with CH4 during degradation of organic matter. The cations (Ca, Mg, Fe, Mn) were derived from alteration of ash beds. In Bering Sea deposits, ash beds altered to smectite within about 3–5 m.y. Carbonate precipitated simultaneously at different stratigraphic levels within the 627–1057 m sections at temperatures of 7–85°C. No apparent calcite precursor of biogenic origin was found for these authigenic carbonates.  相似文献   

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