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1.
A comparative study of three methods for the determination of iodate-iodine in seawater is described. In one method the iodate is determined polarographically while in the others the iodate is determined colorimetrically as iodonium ions. In one of the colorimetric methods each sample is pre-treated with excess iodine-water in an attempt to eliminate suspected interference from naturally occurring reducing agents. The tests were conducted on a selection of open-ocean and near-shore waters with iodate concentrations ranging from 0 to 60 μg 1?1-I. The tests indicated that the polarographic method and the colorimetric method without iodine-water give the more reliable measurement of iodate concentration. Also, the method with iodine water was found to be in error especially at low iodate concentrations. Reducing agents, if present, were found not to interfere significantly.  相似文献   

2.
Existing methods for the determination of the various forms of iodine present in seawater are laborious or unwieldy in use. This paper describes modified forms of the iodate and total iodine methods described by Barkley and Thompson (1960), a spectrophotometric procedure for iodate determination derived from Johannesson's (1958) work, and an automatic method for total iodine determination. Procedures for iodate and total-iodine determination which are suitable for use aboard ship, are recommended. Both procedures for iodate determination returned a standard deviation close to 0.5 μg/l when several replicate samples of a seawater containing approximately 30 μg/l of iodate-iodine were analysed. The automatic method for total iodine determination yielded a standard deviation of 0.8 μg/l by the repeated analysis of a seawater containing a total of approximately 50 μg/l of iodine. Suitable methods for the filtration and storage of seawater are also described.  相似文献   

3.
Iodide oxidation to iodate in near-surface waters of the open oceans is an elusive process, and an unequivocal demonstration of it would simplify modelling of the marine iodine system. In the open ocean, the upward advection of iodate complicates any mathematical treatment of the problem. In this context, the high concentration (0.1 μM) of iodate in the Black Sea surface waters suggested that this Sea might be a place where oxidation might be demonstrated. Hydrologically, the surface waters of the Black Sea appear to be downstream of the deeper waters and, given the latter's anoxicity, the surface waters seemed likely to gain most of their iodine as iodide by upward advection. To test this further, prior to experimentation, an iodine budget for the near-surface waters, based upon the latest hydrological model of the Sea was prepared; this predicts a minimum oxidation flux of 3.89×10−4 mol I m−2 a−1. The chemistry of this oxidation is discussed in the light of existing knowledge of the sulfide system. It is argued that as the redox potential of the IO3−/I− and I2/I− couples at pHs typical of the Black Sea (7.75) are much higher than that of the sulfate–sulfide couple, iodide is probably oxidized in the near-surface domain. This contrasts with sulfide oxidation in the suboxic zone. The possible role of nitrifying bacteria in the oxidation is discussed.  相似文献   

4.
Shipboard incubations from the US JGOFS cruise to the Arabian Sea (TN045) March, 1995 showed evidence of iodate reduction in 0.45 μ (Gelman Supor membrane) filtered seawater samples collected from intermediate depths (200–600 m) within the oxygen minimum zone (OMZ). Inorganic chemical reduction of iodate in these samples was ruled out as no free sulfide was measurable and concentrations of ammonia and nitrite were found to be less than 5 μM. To examine whether the reduction of iodate observed at sea could have been the result of bacterial metabolism, reduction of iodate (IO3−) to iodide (I−) by Shewanella putrefaciens strain MR-4 was studied in artificial seawater using electrochemical methods. MR-4 is a ubiquitous marine bacterium which may be of considerable importance when considering redox zonation in the water column because it is a facultative anaerobe and may switch amongst a suite of electron acceptors to support metabolism. In all experiments MR-4 reduced all iodate to iodide. The rate of formation of [I−]in the culture followed pseudo-first order kinetics. This is the first report of the marine bacterial reduction of iodate where the concentrations of iodide and iodate were measured directly. Our results may help to explain the depth distribution of iodine speciation reported in productive waters like the Arabian Sea and for the first time couple iodine speciation with bacterial productivity in the ocean.  相似文献   

5.
Dissolved iodine in waters overlying and in the Orca Basin,Gulf of Mexico   总被引：1，自引：0，他引：1  
The distribution and speciation of iodine, a biophilic redox-sensitive trace element, in waters overlying and in the Orca Basin, Gulf of Mexico, which contains hypersaline, anoxic and yet non-sulfide-bearing brine have been determined. The distribution of iodate and iodide in the oxic waters overlying the anoxic brine are similar to those reported in other oceans. However, in the oxic-anoxic mixing zone, iodate disappears while the concentration of iodide reaches a maximum of 8.1 μM, the highest concentration ever reported in open oceans. There is also a maximum in specific iodine of 30.7 nM‰?1 at this depth. Specific iodine in oxic seawater is only about 10–14 nM ‰?1. These features may be explained by the preferential dissolution of biogenic particles that have accumulated in a strong pycnocline. In the anoxic brine proper, the concentration of iodide is 3.8 μM and can be explained almost entirely by the simultaneous mobilization of chloride and iodide during the dissolution of evaporite beds as the specific iodine of 14.5 nM‰?1 is only slightly higher than those observed in the oxic waters.  相似文献   

6.
Dissolved iodate and total iodine were studied in St Helena Bay, South Africa, during a period of acute hypoxia, following upwelling off Cape Columbine. Despite the generally high concentrations of chlorophyll α (10–30 mg m−3) total iodine concentration was essentially constant in the main part of the Bay, and similar to that found elsewhere in the oceans. Occasional, lower concentrations of total iodine (0·28 to 0·42 μM) were found with exceptionally high chlorophyll α concentrations (500 mg m−3) in shallow waters. In contrast, iodate was found to be reduced to iodide at both the surface and the bottom of the Bay. The implications of these changes are discussed, given that the surface waters reflect sustained eutrophication while the bottom waters are hypoxic as a result of the organic-rich sediment from the waters above.  相似文献   

7.
The recently developed 2–5 L small-volume MnO2 coprecipitation method for determining 234Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of 234Th and 234Th-based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO2 precipitate onto a 25-mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 min for unfiltered waters from the South China Sea. In addition, we have modified the MnO2 purification procedure to allow for alpha spectrometric measurements of 234Th recoveries. Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO4 and 0.1 mg MnCl2·4H2O per liter of sample, while still maintaining high 234Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of the small-volume method.  相似文献   

8.
A widely used method for determining iodate in seawater involves reaction of the IO3? with excess I?, under acid conditions, to form I2 which is measured colorimetrically. It is known that when the acidification is by mineral acid, nitrite can interfere with the method by oxidising I? to I2 in an analogous way to IO3?. Some workers have used sulphamic acid for acidification since it is reported to destroy nitrite in solution.We have investigated the extent of nitrite interference in the colorimetric determination of iodate using both mineral acid and sulphamic acid for acidifying the reaction mixture. In seawater the extent of the interference is equivalent to 0.212 μM 1?1 IO3? per μM 1?1 NO2? with sulphuric acid, and 0.128 μM 1?1 IO3? per μM 1?1 NO2? with sulphamic acid. This means that for a seawater containing 0.31 μM 1?1 (39 μg 1?1) IO3?, the presence of 0.5 μM 1?1 NO2? will lead to an error of 25% in the iodate determined colorimetrically in methods using sulphuric acid, and 15% in methods in which sulphamic acid is used.  相似文献   

9.
The distributions of iodide, iodate and total iodine were determined along a transect from the Sargasso Sea and across the Gulf Stream to the continental shelf of the South Atlantic Bight during November 1990. The western boundary of the Gulf Stream at the outer shelf-upper slope was characterized by steeply sloping isotherms and isopleths of iodide and iodate, resulting from a dome of cold water that was rich in iodate and nearly devoid of iodide at the slope. Both the mid and the inner shelf were relatively well mixed vertically. The concentration of iodate in the surface waters decreased shoreward from >0.3 μM in the Sargasso Sea/Gulf Stream/outer shelf, to 0.29 μM in the midshelf, 0.19 μM in the outer-inner shelf and 0.11 /IM in the inner-inner shelf. Concomitantly, the concentration of iodide increased from <161 nM to 175 nM, 257 nM and 300 nM. The concentration changes were more abrupt in the inner-inner shelf within about 30 km from the shore. There was no evidence of significant concentrations of organic iodine. These distributions of iodide and iodate suggest that the South Atlantic Bight may act as a geochemical processor of dissolved iodine. Iodate is added to the shelf during topographically induced upwelling and frontal exchange with the Gulf Stream. In the shelf waters, iodate is reduced to iodide in situ. Iodide is exported from the shelf to the Gulf Stream which may eventually further transport it to the ocean interior. A ☐ model calculation suggests that 28% and 43% of the iodate added to the Bight and the inner shelf, respectively, are converted to another form in these waters, almost all of which is iodide. About a third of the reduction of iodate to iodide in the Bight occurs in the inner shelf. Thus, the inner shelf may be the most geochemically active zone within the Bight. The residence times of iodide relative to its production and that of iodate relative to its removal are 3.1 and 3.6 months in the Bight and 0.9 and 1.8 months in the inner shelf.  相似文献   

10.
The temporal variation of iodate and total iodine in the Menai Straits and Irish Sea is discussed together with station data for the Atlantic Ocean. Surprisingly little temporal variation was found in either iodate or total iodine even though seasonal nutrient cycling occurred within these water bodies. The iodate concentrations of the temperate Atlantic Ocean, Irish Sea and Menai Straits surface waters were significantly different. These results, together with the known hydrology of the Menai Straits, make it likely that iodate-reducing substances present in terrestrial run-off will be responsible for relatively low iodate concentrations in shallow-seawaters, such as the Irish Sea. However, it is suggested that the low iodate concentrations of tropical surface waters are more likely to have a biological origin. The possibility that the marine iodine system is open to a thermo-dynamical interpretation, as some workers have suggested, is refuted in the case of temperate surface waters as well as deep ocean waters (>200 m). It is shown that existing analyses indicate that below 200 m at oceanic stations, iodate does not vary significantly with depth.  相似文献   

11.
During the August 1993 Intergovernmental Oceanographic Commission's Contaminant Baseline Survey cruise to the high latitude North Atlantic, determinations of total dissolved sulfide (TDS=free sulfide, H2S(g)+HS−+S2−, plus dissolved metal–sulfide complexes), free sulfide, and carbonyl sulfide (OCS) were made along a horizontal transect and at six vertical profile stations. Unlike data from lower latitudes, the distributions of OCS and TDS were remarkably uniform, with surface water OCS averaging 108 pmol/l and TDS averaging 58 pmol/l; free sulfide was below the detection limits of 5 pmol/l at all stations. The vertical profiles of both OCS and TDS show surface maxima and rapid decreases into the major thermocline. For OCS this is indicative of production via photolysis of dissolved organic sulfur compounds, while TDS may be produced from the hydrolysis of OCS. The concentrations of OCS are similar to those found in coastal waters, and suggests that these sub-polar regions may be large OCS sources to the troposphere during summer. However, it is unclear whether higher concentrations of OCS precursors, a long photo period during summer, or slow rates of removal by hydrolysis due to low temperatures are responsible for the elevated OCS levels. TDS concentrations are primarily controlled by the rate of OCS hydrolysis, production by phytoplankton, and oxidative loss by oxygen and iodate. Both of the losses are affected by trace metal complexation, and to examine this, freshly collected seawater was amended by hydrogen sulfide gas and trace metal additions, and the concentration of free sulfide monitored as a function of metal concentration. This allowed the determinations of conditional stability constants for metal sulfides, with the log Kcond of Cd(HS)+ being 8.0±0.5, 7.0±0.6 for Ni(HS)+, and 7.4±0.7 for Zn(HS)+; attempts at measuring the Kcond of Cu(HS)+ were thwarted by the apparent reduction of Cu(II) to Cu(I) by sulfide. Using these constants in an equilibrium speciation model indicates that on average about 75% of the measured TDS was free, with the remaining fraction complexed with Ni, Cd, and Zn (in order of decreasing percentages). While closer to the field observations than would be found with stability constants reported by other workers, these values are still at variance with the actual speciation (i.e., <30% free). This suggests that the stability constants for Cd, Ni, and Zn are somewhat higher than found, thus reducing the concentration of free sulfide. Nevertheless, these speciation data are important for balancing the TDS budget since the loss by iodate oxidation of free sulfide exceeds all production estimates.  相似文献   

12.
Toward assessing the biogeochemical significance of seaweeds in relation to dissolved iodine in seawater, the effect of whole seaweeds (Laminaria digitata and Fucus serratus) upon iodide and iodate, at essentially natural concentrations, has been studied. The weeds were carefully removed from the sub-littoral zone of the Menai Straits and exposed to iodide and iodate at their natural temperature (6 °C), but under continuous illumination. Laminaria digitata was found to decrease the concentration of iodate with an exponential rate constant of 0.008–0.24 h−1. This is a newly discovered process which, if substantiated, will require an entirely new mechanism. Generally, apparent iodide concentration increased except in a run with seawater augmented with iodide, where it first decreased. The rate constant for loss of iodide was 0.014–0.16 h−1. Meanwhile, F. serratus was found not to decrease iodate concentrations, as did L. digitata. Indeed, after ∼30 h iodate concentrations increased, suggesting that the weed may take in iodide before oxidising and releasing it. If substantiated, this finding may offer a way into one of the most elusive of processes within the iodine cycle – iodide oxidation. With both seaweeds sustained long-term increases of apparent iodide concentration are most easily explained as a secretion by the weeds of organic matter which is capable of reducing the Ce(IV) reagent used in determination of total iodine. Modelling of the catalytic method used is provided to support this contention. The possibility of developing this to measure the strain that seaweeds endure in this kind of biogeochemical flux experiment is discussed. A Chemical Oxygen Demand type of approach is applied using Ce(IV) as oxidant. The results of the iodine experiments are contrasted with the several investigations of 131I interaction with seaweeds, which have routinely used discs of weed cut from the frond. It is argued that experiments conducted with stable iodine may measure a different variable to that measured in radio-iodine experiments.  相似文献   

13.
《Marine Chemistry》2002,77(1):1-6
An anion chromatography with ultraviolet detection (IC/UV) method was developed to simultaneously measure NO3−, NO2−, and HS− concentrations in saline (pore)waters. This method achieves nanomolar detection limits without the need for a Cd/Cu reducing column and requires <100 μl volume of sample, making it ideal for use in porewaters, where high sulfide concentrations can be present. Bromide and iodide among other anions are also measurable by this technique because of their UV absorption properties. Sample filtration is the only sample treatment required before analysis.  相似文献   

14.
A procedure is described for the analysis of the stable carbon isotopic composition of dissolved organic carbon (DOC) in natural waters from marine and higher-salinity environments. Rapid (less than 5 min) and complete oxidation of DOC is achieved using a modification of previous photochemical oxidation techniques. The CO2 evolved from DOC oxidation can be collected in less than 10 min for isotopic analysis. The procedure is at present suitable for oxidation and collection of 1–5 μmol of carbon and has an associated blank of 0.1–0.2 μmol of carbon.Complete photochemical oxidation of DOC standards was demonstrated by quantitative recovery of CO2 as measured manometrically. Isotopic analyses of standards by photochemical and high-temperature sealed-tube combustion methods agreed to within 0.3.. Photochemical oxidation of DOC in a representative sediment pore-water sample was also quantitative, as shown by the excellent agreement between the photochemical and sealed-tube methods. The δ13C values obtained for pore-water DOC using the two methods of oxidation were identical, suggesting that the modified photochemical method is adequate for the isotopically non-fractionated oxidation of pore-water DOC.The procedure was evaluated through an analysis of DOC in pond and pore waters from a hypersaline microbial mat environment. Concentrations of DOC in the water column over the mat displayed a diel pattern, but the isotopic composition of this DOC remained relatively constant (average δ13C = −12.4.). Pore-water DOC exhibited a distinct concentration maximum in the mat surface layer, and δ13C of pore-water DOC was nearly 8. lighter at 1.5–2.0-cm depth than in the mat surface layer (0–0.5-cm depth). These results demonstrate the effectiveness of the method in elucidating differences in DOC concentration and δ13C over biogeochemically relevant spatial and temporal scales. Carbon isotopic analysis of DOC in natural waters, especially pore waters, should be a useful probe of biogeochemical processes in recent environments.  相似文献   

15.
Dissolved oxygen in seawater has been determined by using the Winkler's reaction scheme for decades. An interference in this reaction scheme that has been heretofore overlooked is the presence of naturally occurring iodate in seawater. Each mole of iodate can result in an apparent presence of 1.5 mol of dissolved oxygen. At the concentrations of iodate in the surface and deep open ocean, it can lead to an overestimation of 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg− 1 of oxygen in these waters respectively. In coastal and inshore waters, the effect is less predictable as the concentration of iodate is more variable. The solubility of oxygen in seawater was likely overestimated in data sources that were based on the Winkler's reaction scheme for the determination of oxygen. The solubility equation of García and Gordon [Garcia H.E., Gordon, L.I., 1992. Oxygen solubility in seawater: Better fitting equations. Limnol. Oceanogr. 37, 1307–1312] derived from the results of Benson and Krause [Benson, F.B., Krause, D. Jr., 1984. The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnol. Oceanogr. 29, 620–632] is free from this source of error and is recommended for general use. By neglecting the presence of iodate, the average global super-saturation of oxygen in the surface oceans and the corresponding efflux of oxygen to the atmosphere both have been overestimated by about 8%. Regionally, in areas where the degree of super-saturation or under-saturation of oxygen in the surface water is small, such as in the tropical oceans, the net air–sea exchange flux can be grossly under- or overestimated. Even the estimated direction of the exchange can be reversed. Furthermore, the presence of iodate can lead to an overestimation of the saturation anomaly of oxygen in the upper ocean attributed to biological production by 0.23 ± 0.07%. AOU may have been underestimated by 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg− 1 in the surface mixed layer and deep water, while preformed phosphate and preformed nitrate may have been overestimated by 0.004 ± 0.001 and 0.06 ± 0.02 μmol kg− 1 in the surface mixed layer, and 0.005 ± 0.0004 and 0.073 ± 0.006 μmol kg− 1 in the deep water. These are small but not negligible corrections, especially in areas where the values of these parameters are small. At the increasing level of sophistication in the interpretation of oxygen data, this source of error should now be taken into account. Nevertheless, in order to avoid confusion, an internationally accepted standard needs to be adopted before these corrections can be applied.  相似文献   

16.
Geochemical and rock-magnetic investigations were carried out on a sediment core collected from the SE Arabian Sea at 1420 m depth in oxygenated waters below the present-day oxygen minimum zone. The top 250 cm of the core sediments represent the last 35 kaBP. The · 18O values of Globigerinoides ruber are heaviest during the Last Glacial Maximum (LGM) and appear unaffected by low-saline waters transported from the Bay of Bengal by the strong northeast monsoon and West Indian coastal current. The signatures of Bølling-Allerød and Younger Dryas events are distinct in the records of magnetic susceptibility, organic carbon (OC) and · 18O. Glacial sediments show higher OC, CaCO3, Ba, Mo, U and Cd, while the early-to-late Holocene sediments show increasing concentrations of OC, CaCO3, Ba, Cu, Ni and Zn and decreasing concentrations of Mo, U and Cd. Productivity induced low-oxygenated bottom waters and reducing sedimentary conditions during glaciation, and productivity and oxygenated bottom waters in the Holocene are responsible for their variation. The core exhibits different stages of diagenesis at different sediment intervals. The occurrence of fine-grained, low-coercivity, ferrimagnetic mineral during glacial periods is indicative of its formation in organic-rich, anoxic sediments, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea and Japan Sea. The glacial sediments exhibiting reductive diagenesis with anoxic sedimentary environment in this core correspond to reductive diagenesis and intermittent bioturbation (oxygenation) reported in another core in the vicinity. This suggests that the poorly oxygenated bottom water conditions during glacial times should not be generalized, but are influenced locally by productivity, sedimentation rates and sediment reworking.  相似文献   

17.
Depth profiles of the naturally-occurring radionuclides 238U, 234U, 226Ra, 228Ra and 228Th were obtained in two diverse anoxic marine environments; the permanently anoxic Framvaren Fjord in southern Norway and the intermittently anoxic Saanich Inlet in British Columbia. Concentrations of total H2S were over three orders of magnitude greater in the anoxic bottom waters of Framvaren Fjord compared to those in Saanich Inlet.In Framvaren Fjord, the O2/H2S interface was located at 17 m. While dissolved 238U behaved conservatively throughout the oxic and anoxic water columns, concentrations based on the 238U/salinity ratio in oxic oceanic waters were almost 30% lower. Dissolved 226Ra displayed a sharp maximum just below the O2/H2S interface, coinciding with dissolved Mn (II) and Fe (II) maxima in this zone. It is suggested that reductive dissolution of Fe-Mn oxyhydroxides remobilizes 226Ra in this region.In Saanich Inlet, the O2/H2S interface was located at 175 m. Dissolved 238U displayed a strongly nonconservative distribution. The depth profiles of dissolved 226Ra and 228Th correlated well with the distribution of dissolved Mn (II) in the suboxic waters above the O2/H2S interface, suggesting that reduction of particulate Mn regulates the behavior of 226Ra and 228Th in this region.Removal residence times for dissolved 228Th in the surface oxic waters of both systems are longer than those generally reported for particle-reactive radionuclides in coastal marine environments. In the anoxic waters of Framvaren Fjord and Saanich Inlet, however, the dissolved 228Th removal residence times are quite similar to values reported for dissolved 210Pb in the anoxic waters of the Cariaco Trench and the Orca Basin. This implies that the geochemistries of Th and Pb may be similar in anoxic marine waters.  相似文献   

18.
Actinide speciation in aquatic systems   总被引：1，自引：0，他引：1  
Nuclear test explosions and reactor wastes have deposited an estimated 16 × 1015 Bq of plutonium into the world's aquatic systems. However, plutonium concentration in open ocean waters is on the order 10− 5 Bq/kg, indicating that most of the plutonium is quite insoluble in marine waters and has been incorporated into sediments. Actinide ions often are not in a state of thermodynamic equilibrium and their solubility and migration behavior is related to the form in which the nuclides were introduced into the aquatic system.Actinide solubility depends on such factors as the pH (hydrolysis), Eh (oxidation state), reaction with complexants (e.g., carbonate, phosphate, humic acid, etc.), sorption to surfaces of minerals and/or colloids etc., in the water. The most significant of these variables is the oxidation state of the metal ion. The simultaneous presence of more than one oxidation state for some actinides (e.g., plutonium) in a solution complicates actinide environmental behavior. Both Np(V)O2+ and Pu(V)O2+, the most significant states in natural, oxic waters are relatively noncomplexing and resistant to hydrolysis and subsequent precipitation. The solubility of NpO2+ can be as high as 10− 4 M while that of PuO2+ is limited by reduction to the insoluble tetravalent species, Pu(OH4), (pKsp = 56). The net solubility of hexavalent UO22+ in sea water is also limited by hydrolysis; however, it has a relatively high concentration due to formation carbonate complexes. The insoluble trivalent americium hydroxocarbonate, Am(CO)3(OH), is limiting species for the solubility of Am(III) in sea water. Thorium is found exclusively as the tetravalent species and its solubility is limited by the formation of quite insoluble Th(OH)4.The chemistry of actinide ions in the environment is reviewed to show the spectrum of reactions that can occur in natural waters which must be considered in assessing the environmental behavior of actinides. While much is understood about sorption of actinides on surfaces, the mode of migration of actinides in such waters and the potential effects of these radioactive species on marine bioto, much more is needed for a satisfactory understanding of the behavior of the actinides in the environment.  相似文献   

19.
A modified procedure has been proposed for the colorimetric determination of dissolved oxygen in seawater to improve its precision and accuracy. When a pickled sample is acidified, iodine liberated in the iodometric reaction is measured by direct spectrophotometry at 456 nm. Loss of molecular iodine by volatilization is eliminated by transferring the sample to a flow cuvette without contact with air. The method was calibrated for oxygen by spiking known amounts of potassium iodate. Precision was found at better than 0.2% r.s.d. (full scale). Evaluation of accuracy was made by comparison with calculated oxygen solubilities, which shows a relative bias of no more than 0.5% for oxic waters. The analytical throughput was much faster than that of the standard titration procedure.  相似文献   

20.
Several diagenetic models have been proposed for Middle and Upper Jurassic carbonates of the eastern Paris Basin. The paragenetic sequences are compared in both aquifers to propose a diagenetic model for the Middle and Late Jurassic deposits as a whole. Petrographic (optical and cathodoluminescence microscopy), structural (fracture orientations) and geochemical (δ18O, δ13C, REE) studies were conducted to characterize diagenetic cements, with a focus on blocky calcite cements, and their connection with fracturation events. Four generations of blocky calcite (Cal1–Cal4) are identified. Cal1 and Cal2 are widespread in the dominantly grain-supported facies of the Middle Jurassic limestones (about 90% of the cementation), whereas they are limited in the Oxfordian because grain-supported facies are restricted to certain stratigraphic levels. Cal1 and Cal2 blocky spars precipitated during burial in a reducing environment from mixed marine-meteoric waters and/or buffered meteoric waters. The meteoric waters probably entered aquifers during the Late Cimmerian (Jurassic/Cretaceous boundary) and Late Aptian (Early Cretaceous) unconformities. The amount of Cal2 cement is thought to be linked to the intensity of burial pressure dissolution, which in turn was partly controlled by the clay content of the host rocks. Cal3 and Cal4 are associated with telogenetic fracturing phases. The succession of Cal3 and Cal4 calcite relates to the transition towards oxidizing conditions during an opening of the system to meteoric waters at higher water/rock ratios. These meteoric fluids circulated along Pyrenean, Oligocene and Alpine fractures and generated both dissolution and subsequent cementation in Oxfordian vugs in mud-supported facies and in poorly stylolitized grainstones. However, these cements filled only the residual porosity in Middle Jurassic limestones. In addition to fluorine inputs, fracturation also permitted inputs of sulphur possibly due to weathering of Triassic or Purbeckian evaporites or H2S input during Paleogene times.  相似文献   

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