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1.
In situ measured microprofiles of Ca2+, pCO2, pH and O2 were performed to quantify the CaCO3 dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m−2 d−1 and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m−2 a−1. The metabolic CO2, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca2+ microprofiles were 0.6 mmol m−2 d−1 for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO2 produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O2, pH and Ca2+ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw−1 a−1 (mol per kg water and year), which equals 95% d−1. This rate constant is at the upper end of reported in situ values.  相似文献   

2.
Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3−, NH4+, Fe2+, and Mn2+ and SO42− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, NO3−, Fe(OH)3 and SO42−, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm−2 yr−1, of which 77% were due to O2, 17% to NO3− and 3% to Fe(OH)3 and 3% to SO42−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d−1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m−2 yr−1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m−2 yr−1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.  相似文献   

3.
We present a numerical model to quantify calcite dissolution in the guts of deposit feeding invertebrates. Deposit feeder guts were modeled as constantly stirred reactors (CSTRs) following terminology from digestion theory. Saturation state and dissolution of calcium carbonate were calculated from changes in total dissolved carbon dioxide and alkalinity resulting from sediment passage through the digestive tract, while accounting for dissolution of calcite and respiration of organic carbon. Typical dissolution rates for a gut volume of 1 ml ranged between 0.5-4 mg calcite d−1. Sensitivity analysis revealed gut pH, sediment organic matter (OM) content and OM reactivity to be the critical parameters determining calcite dissolution rate. Carbonate dissolution rate was inversely related to gut pH. However, calcite dissolution was found to be possible even at alkaline gut pH due to respiration by intestinal microbes. The kinetics of calcite dissolution had only marginal influence on daily calcite dissolution rates: Varying the calcite dissolution rate constant κ by six orders of magnitude affected calcite dissolution rates by less than a factor of 10. Calcite dissolution rates were calculated for 4 different hydrographic regimes that differed in their content of sedimentary calcite and OM and furthermore in their OM reactivity. Highest dissolution rates were calculated for the shallow water setting, where relatively high OM content facilitated high microbial respiration rates depressing gut pH. However, dissolution rates for the deep sea setting were only slightly lower, due to greatly elevated ingestion rates resulting from low OM content. As a consequence of much higher faunal abundances, shallow-water benthos is likely to contribute the vast majority of gut-mediated carbonate dissolution. Nevertheless, the fraction of sedimentary calcite that dissolves during one gut passage is probably too small to be observable by simple gravimetric analysis. This may explain the notable scarcity of evidence for gut-mediated carbonate dissolution in the literature to date. Assuming depth-dependent calcite dissolution rates and deposit feeder abundances, we estimate gut-mediated carbonate dissolution to contribute approximately 5% of the annual global sedimentary carbonate dissolution rate, which corresponds to an average calcite dissolution rate of approximately 0.5 mg m−2 d−1 for the entire ocean floor.  相似文献   

4.
蚯蚓肠道内小分子有机酸与摄入的土壤矿物相互作用,加速矿物溶解。摄入的土壤在蚯蚓肠道内平均停留时间约为12 h,不足以使土壤矿物产生显著的溶解特征,因此这一过程难以在蚯蚓体内进行评估。本研究通过体外实验控制pH值和有机酸浓度,模拟蚯蚓肠道中有机酸对土壤中常见矿物的溶解反应,探讨了方解石和钾长石在蚯蚓肠道环境中的初始溶解动力学。研究发现,矿物在混合有机酸中的溶解速率比在纯水中高一个数量级,说明有机配体和质子促进了矿物溶解。溶解速率及粒度分析表明,方解石(CaCO3)溶解速率不受溶解过程中粒度变化的影响,而钾长石(KAlSi3O8)粒度在溶解期间未出现显著变化。在此基础上,采用初始速率法模拟了钾长石的初始溶解动力学,计算得出的溶解速率表明钾长石在溶解初期主要为表面K~+的释放。使用缩核模型(shrink core model)和Hixson-Crowell模型对方解石溶解过程进行动力学解析,发现方解石的溶解主要受溶液中反应物内扩散的速率影响。这定量描述了两种矿物在有机酸溶液和纯水中的溶解差异。现有研究表明,有机配体和质子协同促...  相似文献   

5.
Cai, B., Zhu, J., Ban, F. & Tan, M. 2011: Intra‐annual variation of the calcite deposition rate of drip water in Shihua Cave, Beijing, China and its implications for palaeoclimatic reconstructions. Boreas, Vol. 40, pp. 525–535. 10.1111/j.1502‐3885.2010.00201.x. ISSN 0300‐9483. Monthly in situ monitoring of the calcite deposition rate, drip‐water chemistry and surrounding cave environment was carried out at Shihua Cave, Beijing, China, through two hydrological years (from January 2006 to February 2008) to determine the seasonal variability and mechanisms of stalagmite growth in Shihua Cave. Calcite deposition rates exhibit significant intra‐annual variation, with the lowest values during the summer monsoonal rainy season (July–August) and peak values from autumn to spring. The temporal change in the calcite deposition rate is negatively correlated with the drip rate, cave‐air PCO2 (CO2 partial pressure) and Ca concentration, and positively correlated with the pH of the feeding drip water. The seasonal recharge regime of drip water is likely to be the primary control on the drip‐water quality and quantity, which, in turn, control the calcite deposition rate in Shihua Cave. During the summer rainy season, periodic and intense rainstorms increase the drip rate and cave‐air PCO2, leading to drip water with a lower pH and saturation index of calcite, thereby reducing the calcite precipitation. It seems that the high cave‐air PCO2 is the dominant control on the calcite deposition rate during the rainy season. Our previous study on the dissolved organic carbon of drip water concluded that the thin luminescent bands in stalagmite laminae from Shihua Cave form during the rainy season. The lower calcite deposition rate during the rainy season further supports this suggestion. The significant intra‐seasonal variability of the calcite deposition rate implies that the seasonal bias of δ18O of stalagmites should be considered when stalagmite δ18O is used as a high‐resolution palaeoclimatic archive.  相似文献   

6.
This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg–1 pH of 7.6 and p(CO2) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg–1, pH of 7.70 and p(CO2) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO2) up to 5.6 times the atmospheric P(CO2).  相似文献   

7.
Studies of the δ13C of pore water dissolved inorganic carbon (δ13C-DIC) were carried out in shallow water carbonate sediments of the Great Bahamas Bank (GBB) to further examine sediment-seagrass relationships and to more quantitatively describe the couplings between organic matter remineralization and sediment carbonate diagenesis. At all sites studied δ13C-DIC provided evidence for the dissolution of sediment carbonate mediated by metabolic CO2 (i.e., CO2 produced during sediment organic matter remineralization); these observations are also consistent with pore water profiles of alkalinity, total DIC and Ca2+ at these sites. In bare oolitic sands, isotope mass balance further indicates that the sediment organic matter undergoing remineralization is a mixture of water column detritus and seagrass material; in sediments with intermediate seagrass densities, seagrass derived material appears to be the predominant source of organic matter undergoing remineralization. However, in sediments with high seagrass densities, the pore water δ13C-DIC data cannot be simply explained by dissolution of sediment carbonate mediated by metabolic CO2, regardless of the organic matter type. Rather, these results suggest that dissolution of metastable carbonate phases occurs in conjunction with reprecipitation of more stable carbonate phases. Simple closed system calculations support this suggestion, and are broadly consistent with results from more eutrophic Florida Bay sediments, where evidence of this type of carbonate dissolution/reprecipitation has also been observed. In conjunction with our previous work in the Bahamas, these observations provide further evidence for the important role that seagrasses play in mediating early diagenetic processes in tropical shallow water carbonate sediments. At the same time, when these results are compared with results from other terrigenous coastal sediments, as well as supralysoclinal carbonate-rich deep-sea sediments, they suggest that carbonate dissolution/reprecipitation may be more important than previously thought, in general, in the early diagenesis of marine sediments.  相似文献   

8.
This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg?1 pH of 7.6 and p(CO2) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg?1, pH of 7.70 and p(CO2) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO2) up to 5.6 times the atmospheric P(CO2).  相似文献   

9.
Pore water chemistry and 234U/238U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent reactive transport code Crunch. The reactive transport model includes a solid-solution formulation to enable the use of the 234U/238U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still exists when a mechanistic reactive transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m2/g based on both grain geometry and on the depletion of 234U/238U in the sediment via α-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water 234U/238U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10-20% of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 102 to 105 times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The reactive transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (105), less substantial with the Al-inhibition formulation (103), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (102). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of reactive sites on silicate surfaces with time. The results imply that methods of mineral surface characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.  相似文献   

10.
A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., Corg:S ratio, Corg:Porg ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters.The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N2 production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.  相似文献   

11.
Zou  Lin  Dong  Lin  Ning  Meng  Huang  Kangjun  Peng  Yongbo  Qin  Shujian  Yuan  Honglin  Shen  Bing 《中国地球化学学报》2019,38(4):481-496

The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate.

  相似文献   

12.
The North Appalachian Experimental Watershed in Coshocton, Ohio, USA has recorded average pH of precipitation of 4.7 over a 30-year period. The area lies within the Pennsylvanian siltstones and shale, dominated by aluminosilicates and <5% calcite. A study was conducted to determine the evolution of acid deposition through an unsaturated to saturated zone composed of siltstone and shale in an isolated hill, precluding lateral flow and seepage. The results from water–rock chemical reactions modeled using PHREEQM demonstrate the percolating precipitation water is neutralized to pH 7.5 within the top 1.5 m. The model suggests that, along with calcite, dissolution of albite, illite, and kaolinite are the dominant mechanisms of neutralization. The cation exchange capacity of the siltstone and shale, in the range 54.6–386 meq/100 g, appears to be a function of high organic carbon content of 2.0–3.2%. While cation exchange is responsible for some of the Na+ in solution, it is not the primary source of Ca2+, Mg2+, or K+ ions. Exchange onto clays is occurring, but is secondary to exchange on organic matter. Chemical composition of groundwater perched within a coal seam is controlled by oxidation and dissolution of pyrite, returning pH to approximately 4.0.  相似文献   

13.
 Pore water concentration profiles of sediments at a site on the Amazon Fan were investigated and simulated with the numerical model CoTReM (column transport and reaction model) to reveal the biogeochemical processes involved. The pore water profiles for gravity core GeoB 4417-7 showed a distinct sulfate–methane transition zone in which deep sulfate reduction occurs. Only a small sulfide peak could be observed at the reaction zone. Due to high amounts of iron minerals, the produced sulfide is instantaneously precipitated in form of iron sulfides. We present a simulation which starts from a steady state system with respect to pore water profiles for methane and sulfate. Furthermore, sulfide, iron, pH, pE, calcium and total inorganic carbon (TIC) were included in the simulation. The program calculated mineral equilibria to mackinawite, iron sulfides (more stable than mackinawite), iron hydroxides and calcite via saturation indices (SI) by a module incorporating the program PHREEQC (Parkhurst 1995). The measured sulfide and iron profiles are obtained in the simulation output by using a constant SI (=0) for mackinawite and calcite, while a depth dependent SI distribution is applied for the PHREEQC phases “Pyrite” and “Fe(OH)3(a)”, representing a composition and the kinetics of different iron sulfides and iron hydroxides. These SI distributions control the results of sulfide and iron pore water profiles, especially conserving the sulfide profile at the reaction zone during the simulation. The results suggest that phases of iron hydroxides are dissolved, mackinawite is precipitated within, and other iron sulfides are precipitated below the reaction zone. The chemical reactivity of iron hydroxides corresponds to the rate of sulfide production. The system H2O–CO2–CaCO3 is generally successfully maintained during the simulation. Deviations to the measured pH profile suggest that further processes are active which are not included in the simulation yet. Received: 9 November 1998 / Accepted: 26 October 1999  相似文献   

14.
Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (δ44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of δ44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, δ44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the δ44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.  相似文献   

15.
碳酸盐胶结物是东营凹陷中央隆起带砂岩储层中重要的自生矿物,但其形成机制目前尚无深入研究.首先在岩相观察下针对东营凹陷中央隆起带沙河街组的碳酸盐胶结物进行期次划分并归纳其发育特征,再依据各期碳酸盐胶结物的共生矿物、碳氧同位素组成、流体包裹体温度、元素化学成分等信息对其形成机制进行研究.研究结果表明:研究区碳酸盐胶结物的碳不仅受到有机质成熟过程中排放的有机酸的影响,还受到沙四段沉积的湖相碳酸盐岩溶解的影响.研究区沙河街组主要发育四期碳酸盐胶结物:第一期碳酸盐胶结物以白云石为主,其形成过程与甲烷细菌对有机质的分解作用有关;第二期碳酸盐胶结物以方解石为主,与第一期碳酸盐胶结物之间夹有一层绿泥石薄膜,胶结物的形成与孔隙流体的浓缩导致的Ca、HCO3-过饱和现象相关;第三期碳酸盐胶结物主要为方解石、白云石和铁白云石,以充填长石溶孔和原生孔隙为特征,其物质来源于长石的溶蚀及泥岩的脱水作用;第四期碳酸盐胶结物多为铁方解石和铁白云石,以充填早期碳酸盐溶蚀后形成的次生孔隙为特征,物质来源于粘土矿物的转化,多与黄铁矿颗粒共生,其形成过程受到烃类流体活动的影响.   相似文献   

16.
《Geochimica et cosmochimica acta》1999,63(19-20):3349-3356
The calcium cycle, in particular carbonate dissolution, was analyzed in two deep eutrophic lakes, Lago di Lugano (288 m maximum depth) and Sempachersee (87 m) located in Switzerland. A box model approach was used to calculate calcite dissolution in the water column and at the sediment-water interface based on various lake monitoring data such as sediment traps, sediment cores, water and pore-water analysis. A model for stationary conditions allowing the calculation of calcite dissolution in the water column for a given particle size distribution was developed. The relative values of the simulated flux were consistent with sediment trap observations. The best fit of the dissolution rate constant of sinking calcite in Lago di Lugano was on the same order of magnitude (3 · 10−10 kg1/3 s−1) as published laboratory values for this surface controlled process.Both lakes show a similar specific calcite precipitation rate of 170 g Ca m−2 a−1. The diffusive flux across the sediment-water interface amounts to about 15 and 10% of total calcite precipitation in Sempachersee and Lago di Lugano, respectively. However, 61% of the precipitated calcite is dissolved in the water column of Lago di Lugano compared to only 13% in Sempachersee. These results point towards the importance of grain size distributions and settling times in stratified deep waters as the two most important factors determining calcite retention in sediments of hard water lakes.  相似文献   

17.
Mid-shelf sediments off the Oregon coast are characterized as fine sands that trap and remineralize phytodetritus leading to the consumption of significant quantities of dissolved oxygen. Sediment oxygen consumption (SOC) can be delayed from seasonal organic matter inputs because of a transient buildup of reduced constituents during periods of quiescent physical processes. Between 2009 and 2013, benthic oxygen exchange rates were measured using the noninvasive eddy covariance (EC) method five separate times at a single 80-m station. Ancillary measurements included in situ microprofiles of oxygen at the sediment–water interface, and concentration profiles of pore water nutrients and trace metals, and solid-phase organic C and sulfide minerals from cores. Sediment cores were also incubated to derive anaerobic respiration rates. The EC measurements were made during spring, summer, and fall conditions, and they produced average benthic oxygen flux estimates that varied between ?2 and ?15 mmol m?2 d?1. The EC oxygen fluxes were most highly correlated with bottom-sensed, significant wave heights (H s). The relationship with H s was used with an annual record of deepwater swell heights to predict an integrated oxygen consumption rate for the mid-shelf of 1.5 mol m?2 for the upwelling season (May–September) and 6.8 mol m?2 y?1. The annual prediction requires that SOC rates are enhanced in the winter because of sand filtering and pore water advection under large waves, and it counters budgets that assume a dominance of organic matter export from the shelf. Refined budgets will require winter flux measurements and observations from cross-shelf transects over multiple years.  相似文献   

18.
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   

19.
Reservoir quality is critical for sweet-spot evaluation in tight sandstone plays, but few studies have focused on the origin of authigenic minerals in tight sandstones and their impact on reservoir quality. This study integrates petrographic analysis, carbon and oxygen isotopic data and mercury injection capillary pressure data of the Upper Triassic Chang-7 tight sand reservoir samples in the Ordos Basin, China to understand the origin of authigenic minerals and the impact of authigenic minerals on reservoir quality. Carbonate minerals, including calcite, ferroan calcite and dolomite, and kaolinite, are the major authigenic minerals in the Chang-7 sandstone. They were derived from the chemical diagenetic alteration of detrital feldspar and biotite, with the involvement of ions that are believed to be primarily from the connate water in interstitial pore space of the interbedded mudstones. Meteoric water and organic fluids from hydrocarbon generation may also have been involved in the alteration processes of the minerals in the Chang-7 tight sandstone. The origin of kaolinite indicates that the porosity was increased by dissolution of detrital grains, offsetting the porosity loss from burial compaction. Authigenic minerals appear to have a weak correlation with pore structure, suggesting that the pore structure of the Chang-7 tight sandstone is not only controlled by major authigenic minerals but also affected by other geological factors.  相似文献   

20.
The dissolution of Iceland spar in CO2-saturated solutions at 25°C and 1 atm total pressure has been followed by measurement of pH as a function of time. Surface concentrations of reactant and product species have been calculated from bulk fluid data using mass transport theory and a model that accounts for homogeneous reactions in the bulk fluid. The surface concentrations are found to be close to bulk solution values. This indicates that calcite dissolution under the experimental conditions is controlled by the kinetics of surface reaction. The rate of calcite dissolution follows an empirical second order relation with respect to calcium and hydrogen ion from near the initial condition (pH 3.91) to approximately pH 5.9. Beyond pH 5.9 the rate of surface reaction is greatly reduced and higher reaction orders are observed. Calculations show that the rate of calcite dissolution in natural environments may be influenced by both transport and surface-reaction processes. In the absence of inhibition, relatively short times should be sufficient to establish equilibrium.  相似文献   

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