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1.
Anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide (CO2) through the carbonation of magnesium silicate minerals to form magnesium carbonate minerals. The hydromagnesite [Mg5(CO3)4(OH)2·4H2O] playas of Atlin, British Columbia, Canada provide a natural model to examine mineral carbonation on a watershed scale. At near surface conditions, CO2 is biogeochemically sequestered by microorganisms that are involved in weathering of bedrock and precipitation of carbonate minerals. The purpose of this study was to characterize the weathering regime in a groundwater recharge zone and the depositional environments in the playas in the context of a biogeochemical model for CO2 sequestration with emphasis on microbial processes that accelerate mineral carbonation.Regions with ultramafic bedrock, such as Atlin, represent the best potential sources of feedstocks for mineral carbonation. Elemental compositions of a soil profile show significant depletion of MgO and enrichment of SiO2 in comparison to underlying ultramafic parent material. Polished serpentinite cubes were placed in the organic horizon of a coniferous forest soil in a groundwater recharge zone for three years. Upon retrieval, the cube surfaces, as seen using scanning electron microscopy, had been colonized by bacteria that were associated with surface pitting. Degradation of organic matter in the soil produced chelating agents and acids that contributed to the chemical weathering of the serpentinite and would be expected to have a similar effect on the magnesium-rich bedrock at Atlin. Stable carbon isotopes of groundwater from a well, situated near a wetland in the southeastern playa, indicate that  12% of the dissolved inorganic carbon has a modern origin from soil CO2.The mineralogy and isotope geochemistry of the hydromagnesite playas suggest that there are three distinct depositional environments: (1) the wetland, characterized by biologically-aided precipitation of carbonate minerals from waters concentrated by evaporation, (2) isolated wetland sections that lead to the formation of consolidated aragonite sediments, and (3) the emerged grassland environment where evaporation produces mounds of hydromagnesite. Examination of sediments within the southeastern playa–wetland suggests that cyanobacteria, sulphate reducing bacteria, and diatoms aid in producing favourable geochemical conditions for precipitation of carbonate minerals.The Atlin site, as a biogeochemical model, has implications for creating carbon sinks that utilize passive microbial, geochemical and physical processes that aid in mineral carbonation of magnesium silicates. These processes could be exploited for the purposes of CO2 sequestration by creating conditions similar to those of the Atlin site in environments, artificial or natural, where the precipitation of magnesium carbonates would be suitable. Given the vast quantities of Mg-rich bedrock that exist throughout the world, this study has significant implications for reducing atmospheric CO2 concentrations and combating global climate change.  相似文献   

2.
The Lake Afourgagh sediment record and facies successions provide an outstanding example of environmentally controlled carbonate sedimentation. Afourgagh is a small, shallow permanent lake located in the Middle‐Atlas Mountains in Morocco in a karstic context. It is fed by ground waters that are relatively enriched in Mg resulting from the leaching of the Jurassic dolomitic bedrock of the catchment. This eutrophic lake is episodically restricted and characterized by alkaline waters with a fluctuating high Mg/Ca ratio. The maximum extension of the Holocene shoreline coincides with evidence of a lake stabilization level corresponding to the outflow of the lake through a wadi. Lakeshore terrace sediments deposited on an alluvial fan siltstone during the past ca 2500 cal yr bp comprise four main facies: a littoral crust, palaeosols, palustrine silts and charophyte tufas, which reflect different environments from the shoreline toward the deeper water. In the more distal parts, the charophyte tufas display a well‐expressed lamination punctuated by the development of microstromatolites on algae thalli. The mineralogical composition of the carbonates is linked to the facies. While the charophyte tufas are characterized by a relatively high content in aragonite, in addition to low‐Mg calcite, the littoral crust is mainly composed of magnesite. This pattern is related to the evolving chemistry of water due to the influence of charophyte proliferation during dry summers. Calcium‐carbonate precipitation on algae thalli (both bioinduced and microbially mediated) progressively induces an increase in the Mg/Ca ratio of the lake water, while the capillary evaporation of shallow ground waters causes precipitation of a magnesite precursor on the shoreline, producing magnesite during early diagenesis. This effect is characteristic of two episodes: part of the Roman Warm Period and the beginning of the Dark Age Cold Period. The carbonate mineralogy of the different depositional sequences at Afourgagh indicates lake‐level and water‐chemistry fluctuations under a climatic influence. Therefore, among other regional records, the Lake Afourgagh sedimentary record provides useful evidence for reconstructing these environmental changes.  相似文献   

3.
The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO2‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.  相似文献   

4.
The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.  相似文献   

5.
Abstract In mid‐Middle Cambrian time, shallow‐water sedimentation along the Cordilleran passive margin was abruptly interrupted by the development of the deep‐water House Range embayment across Nevada and Utah. The Marjum Formation (330 m) in the central House Range represents deposition in the deepest part of the embayment and is composed of five deep‐water facies: limestone–argillaceous limestone rhythmites; shale; thin carbonate mud mounds; bioturbated limestone; and cross‐bedded limestone. These facies are cyclically arranged into 1·5 to 30 m thick parasequences that include rhythmite–mound, rhythmite–shale, rhythmite–bioturbated limestone and rhythmite–cross‐bedded limestone parasequences. Using biostratigraphically constrained sediment accumulation rates, the parasequences range in duration from ≈14 to 270 kyr. The mud mounds are thin (<2 m), closely spaced, laterally linked, symmetrical domes composed of massive, fenestral, peloidal to clotted microspar with sparse unoriented, poorly sorted skeletal material, calcitized bacterial(?) filaments/tubes and abundant fenestrae and stroma‐ tactoid structures. These petrographic and sedimentological features suggest that the microspar, peloids/clots and syndepositional micritic cement were precipitated in situ from the activity of benthic microbial communities. Concentrated growth of the microbial communities occurred during periods of decreased input of fine detrital carbonate transported offshore from the adjacent shallow‐water carbonate platform. In the neighbouring Wah Wah Range and throughout the southern Great Basin, coeval mid‐Middle Cambrian shallow‐water carbonates are composed of abundant metre‐scale, upward‐shallowing parasequences that record high‐frequency (104?105 years) eustatic sea‐level changes. Given this regional stratigraphic relationship, the Marjum Formation parasequences probably formed in response to high‐frequency sea‐level fluctuations that controlled the amount of detrital carbonate input into the deeper water embayment. During high‐frequency sea‐level rise and early highstand, detrital carbonate input into the embayment decreased as a result of carbonate factory retrogradation, resulting in the deposition of shale (base of rhythmite–shale parasequences) or thin nodular rhythmites, followed by in situ precipitated mud mounds (lower portion of rhythmite–mound parasequences). During the ensuing high‐frequency sea‐level fall/lowstand, detrital carbonate influx into the embayment increased on account of carbonate factory pro‐ gradation towards the embayment, resulting in deposition of rhythmites (upper part of rhythmite–mound parasequences), reworking of rhythmites by a lowered storm wave base (cross‐bedded limestone deposition) or bioturbation of rhythmites by a weakened/lowered O2‐minimum zone (bioturbated lime‐ stone deposition). This interpreted sea‐level control on offshore carbonate sedimentation patterns is unique to Palaeozoic and earliest Mesozoic deep‐water sediments. After the evolution of calcareous plankton in the Jurassic, the presence or absence of deeper water carbonates was influenced by a variety of chemical and physical oceanographic factors, rather than just physical transport of carbonate muds.  相似文献   

6.
《Sedimentology》2018,65(1):303-333
Calathid–demosponge carbonate mounds are a feature of Early to Middle Ordovician shallow‐marine carbonate depositional environments of tropical to subtropical palaeolatitudes. These mounds contain an important amount of autochthonous non‐skeletal microcrystalline calcium‐carbonate (automicrite) conventionally considered microbial in origin. Here, the automicrite of calathid–demosponge carbonate mounds (Tarim Basin, north‐west China) is broken down into five distinct fabrics: an in situ peloidal–spiculiferous fabric (AM‐1), an in situ peloidal fabric (AM‐2), an aphanitic–microtubular fabric (AM‐3), a minipeloidal fabric (AM‐4) and a laminoid–cerebroid fabric (AM‐5). Type AM‐1 occurs with AM‐2 being succeeded by an assemblage of AM‐3 and AM‐4. Types AM‐4 and AM‐5 are separated by an erosional disconformity. A good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite, probably via the permineralization of non‐living organic substrates adsorbing dissolved metal–humate complexes. Using a spreadsheet with six parameters and 17 characters, AM‐1 to AM‐4 turn out to be non‐microbial in origin. Instead, these automicrites represent relics of calcified metazoan tissues, such as siliceous sponges, non‐spiculate sponges or the basal attachment structures of stalked invertebrates. Fabric AM‐5 is a microbial carbonate but is post‐mound in origin forming a drape within a reefal framework established by AM‐4. The five automicritic fabrics, individually or as an assemblage, are a common element of Ordovician calathid–demosponge carbonate mounds in general. The reassessment of the origins of these automicritic fabrics holds consequences for understanding of the Great Ordovician Biodiversification Event in terms of community structure, reef ecology and reef evolution. Episodically, these fabrics are also present in other carbonate build‐ups stretching from the Neoproterozoic over the entire Phanerozoic Eon. The massive calcification of metazoan soft tissue (AM‐1 to AM‐4) characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of p CO2, Ca concentration and Mg/Ca ratio at the scale of individual sedimentary basins.  相似文献   

7.
There are 59 springs at the Gevas–Gurp?nar–Güzelsu basins, 38 of these springs emerge from the fractured karst aquifers (recrystallized limestone and travertine) and 21 emerge from the Yuksekova ophiolites, K?rkgeçit formation and alluvium. The groundwater samples collected from 38 out of the total of 59 springs, two streams, one lake and 12 wells were analyzed physico-chemically in the year 2002. EC and TDS values of groundwater increased from the marble (high altitude) to the ophiolites and alluvium (toward Lake Van) as a result of carbonate dissolution and connate seawater. Five chemical types of groundwater are identified: Ca–Mg–HCO3, Mg–Ca–HCO3, Mg–Na–HCO3, Na–Ca–HCO3 and Mg–Ca–Na–HCO3. The calculations and hydrochemical interpretations show that the high concentrations of Ca2+, Mg2+ and HCO3 ? as predominant ions in the waters are mainly attributed to carbonate rocks and high pCO2 in soil. Most of the karst springs are oversaturated in calcite, aragonite and dolomite and undersaturated in gypsum, halite and anhydrite. The water–rock interaction processes that singly or in combination influence the chemical composition of each water type include dissolution of carbonate (calcite and dolomite), calcite precipitation, cation exchange and freshening of connate seawater. These processes contribute considerably to the concentration of major ions in the groundwater. Stable isotope contents of the groundwater suggest mainly direct integrative recharge.  相似文献   

8.
This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a ??26Mg offset of 0.8?C1.4??? is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a ??26Mg 0.9?C1.1??? more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, 2012a. 76, 161?C174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg5(CO3)4(OH)2·5(H2O)) and hydromagnesite (Mg5(CO3)4(OH)2·4H2O) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process.  相似文献   

9.
Abstract Cangrejo and Bulkhead Shoals are areally extensive, Holocene biodetrital mud‐mounds in northern Belize. They encompass areas of 20 km2 and 35 km2 in distal and proximal positions, respectively, on a wide and shallow‐water, microtidal carbonate shelf where storms are the major process affecting sediment dynamics. Sediments at each mound are primarily biodetrital and comprise part of a eustatically forced, dominantly subtidal cycle with a recognizable deepening‐upward transgressive systems tract, condensed section and shallowing‐upward highstand systems tract. Antecedent topographic relief on Pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7·6 m thick at Cangrejo and 4·5 m thick at Bulkhead. Despite differences in energy levels and location, facies and internal sedimentological architectures of the mud‐mounds are similar. On top of Pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels. Deposition of these basal transgressive, premound facies tracked the rapid rate of sea‐level rise from about 6400–6500 years BP to 4500 years BP, and the thin basal sedimentation unit of the overlying mound‐core appears to be a condensed section. Following this, the thick and complex facies mosaic comprising mound‐cores represents highstand systems tract sediments deposited in the last ≈ 4500 years during slow and decelerating sea‐level rise. Within these sections, there is an early phase of progradationally offlapping catch‐up deposition and a later (and current) phase of aggradational keep‐up deposition. The mound‐cores comprise stacked storm‐deposited autogenic sedimentation units, the upper bounding surfaces of which are mostly eroded former sediment–water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with Thalassia‐colonized surfaces. Vertical stacking of these units imparts a quasi‐cyclic architecture to the section that superficially mimics metre‐scale parasequences in ancient rocks. The locations of the mud‐mounds and the tidal channels transecting them have apparently been stable over the last 50 years. Characteristics that might distinguish these mud‐mounds and those mudbanks deposited in more restricted settings such as Florida Bay are their broad areal extent, high proportion of sand‐size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas.  相似文献   

10.
It has been suggested that the highly hydrated character of the Mg2+ ion in aqueous solution is responsible for the often encountered difficulty of precipitating stable, anhydrous phases of magnesium carbonate and calcium-magnesium carbonate. In an effort to investigate this, a study of magnesite crystallization kinetics was undertaken, utilizing the reaction of hydromagnesite plus CO2 to yield magnesite at 126°C. The reactions were characterized by prolonged initial quiescent periods prior to the onset of detectable crystallization. The length of the initial period was found to vary with Mg concentration, pCO2 and ionic strength. Contrary to classical kinetics, the reaction studied was inhibited by increased Mg concentration. Ionic strength and pCO2 acted as positive catalysts.  相似文献   

11.
ABSTRACT Gravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic-rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydromagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by ? 0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgCO3 mole per cent data of the dolomite. Detailed X-ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in the co direction (co= 16.09 Å) relative to ideal dolomite (co= 16.02 Å) and contracted in the ao direction (ao= 4.796 Å) relative to ideal dolomite (ao= 4.812 Å). The mol fraction of MgCO3 in the upper dolomite shows up to 4.0 ±M 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium-rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak. Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. δ18O ～+ 7.55%o, δ13C ～+ 4.10%o), yet show three upward-lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain-water dilution of the brine at times when the water in the lake was at its shallowest levels. Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expanded co-axis. The margin dolomite is Ca-rich, has a more homogeneous microstructure, and has expanded ao and co axes. The abundance of relatively soluble Mg-bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment. This process may leave a finely laminated dolomicrite deposit which retains little, if any, evidence of evaporite minerals.  相似文献   

12.
Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO3 waters containing little Ca (<2 mg l?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO2 degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca2+ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.  相似文献   

13.
The metasomatic nature of magnesite formation, sequence and timing of geological processes, and solution sources have been established by comprehensive geological and geochemical study of the typical Satka and Ismakaevo deposits of sparry magnesite in the South Ural province. The hydrothermal metasomatic formation of magnesite is related to injection of high-Mg evaporite brine into heated carbonate rocks within permeable rift zones. The numerical physicochemical simulation of solution–rock interaction allowed us to determine the necessary prerequisites for sparry magnesite formation: the occurrence of marine salt solutions with a high Mg/Ca ratio and heating of solutions before or during their interaction with host carbonate rocks. The contribution of compositionally various solution sources, the temperature variation regime, proportions of CO2 and H2S concentrations in solution created specific features of particular deposits.  相似文献   

14.
Groundwater samples (n = 163) were collected across Kashmir Valley in 2010 to assess the hydrogeochemistry of the groundwater in shallow and deep aquifers and its suitability for domestic, agriculture, horticulture, and livestock purposes. The groundwater is generally alkaline in nature. The electrical conductivity (EC) which is an index to represent the total concentration of soluble salts in water was used to measure the salinity hazard to crops as it reflects the TDS in groundwater ranging from 97 to 1385 μS/cm, except one well in Sopore. The average concentration of major ions was higher in shallow aquifers than in deeper aquifers. In general, Ca2+ is the dominant cation and HCO \(_{3}^{-}\) the dominant anion. Ca–HCO3, Mg–HCO3, Ca–Mg–HCO3, Na–HCO3 were the dominant hydrogeochemical facies. High concentration of HCO3 and pH less than 8.8 clearly indicated that intense chemical weathering processes have taken place in the study area. The groundwater flow pattern in the area follows the local surface topography which not only modifies the hydrogeochemical facies but also controls their distribution. The groundwater in valley flows into four directions, i.e., SW–NE, NE–W, SE-NW and SE–NE directions. The results suggest that carbonate dissolution is the dominant source of major ions followed by silicate weathering and ion-exchange processes. The concentrations of all the major ions determined in the present study are within the permissible limits of WHO and BIS standards. The results of Total Hardness, SAR, Na%, Kelly Index, USDA classification, Magnesium absorption ratio, residual sodium carbonate, and PI suggested that groundwater is good for drinking, livestock, and irrigation purposes.  相似文献   

15.
李状  苏晶文  董长春  叶永红  杨洋 《中国地质》2022,49(5):1509-1526
【研究目的】 了解长江中下游平原地区地下水流系统并深入分析其地下水水化学特征及其演化机制。【研究方法】 综合马鞍山市当涂地区的水文地质条件、水动力场等,基于研究区水化学基本特征,运用多元统计分析、水化学图件、离子比值和反向水文地球化学模拟等方法对该地区浅层地下水水化学演化进行分析。【研究结果】 结果表明：（1）研究区地下水主要为低矿化度偏碱性水,地下水组分中阳离子以Ca2+和Mg2+为主,阴离子以HCO3-和SO42-为主。（2）研究区地下水水化学类型主要可分为7类,其中松散岩类孔隙含水岩组和碎屑岩类孔隙裂隙含水岩组的水化学类型主要为HCO3-Ca型、HCO3-Ca·Na型、HCO3·Cl-Ca·Na型以及HCO3-Ca·Mg型;基岩类裂隙含水岩组的化学类型主要为HCO3·SO4-Ca·Mg型和SO4·HCO3-Ca·Mg型。（3）研究区浅层地下水水样超标率为46%,总体水质较差,超标率较高的组分依次为Mn、高锰酸盐指数（CODMn）、硝酸盐（以N计）、Fe、As、氨氮（以N计）等。（4）研究区地下水的化学组分主要受到岩石风化作用的控制;此外,还存在Na-Ca的正向阳离子交替吸附作用。反向水文地球化学模拟结果进一步定量论证了水岩相互作用对本区浅层地下水组分的形成和演化起着主导作用。【结论】 研究区地下水主要为低矿化度偏碱性,主要可分为松散岩类孔隙水、碎屑岩类孔隙裂隙水和基岩类裂隙水。主要离子比例和反向水文地球化学模拟揭示了本区浅层地下水化学组分主要是地下水溶滤方解石、白云石等碳酸盐矿物、石英、长石等硅酸盐矿物,高岭土等黏土矿物以及岩盐、石膏等达到过饱和之后形成的。  相似文献   

16.
The valley plain of Lhasa City is located on the Qinghai-Tibetan Plateau, which is one of the most developed and densely populated areas in Tibet. Groundwater is an important water supply source and plays an irreplaceable role in the social and economic development of Lhasa City. This study has investigated the dynamic characteristics of groundwater in the valley plain of Lhasa City through the methods of mathematical statistics and hydrochemical analysis. The results showed that local topography, climate, and urbanization substantially influenced the groundwater dynamics. Under the combined influences from urbanization and climate, the groundwater level decreased over three time periods, but the groundwater-level configuration has not shown significant changes in over 15 years. From 1997 to 2015, the hydrochemical type of groundwater has changed from HCO3–Ca to HCO3·SO4–Ca·Mg and HCO3·SO4–Ca. The concentrations of Cl?, Mg2+, and SO42? in groundwater increased, but the concentrations of other ions were relatively stable. Water–rock interaction was the main mechanism controlling the groundwater chemistry in the study area, and it was mainly associated with the dissolution of silicate, carbonate, and halite.  相似文献   

17.
Holocene stromatolites are described from Lake Walyungup, a coastal hyposaline lake in south-western Australia. At summer low water, this groundwater-fed depression comprises two permanent shallow water bodies and an ephemeral southern pool, set within an areally extensive littoral zone of variably cemented carbonate crust. Up to 5 m of organic-rich carbonate mud has been deposited within each of these basins in less than 7000 years. Stromatolites rim the water bodies with individual columns up to 2 m tall. Stromatolite-capped tepee structures in subparallel alignment are widespread in the littoral crust, suggesting a linkage between stromatolite growth and zones of groundwater discharge. Lake Walyungup stromatolites, regardless of external morphology and setting, are coarsely laminated and have aragonitic mesoclot microfabrics. These microfabrics are similar to those from lithified portions of active thrombolitic microbialites from nearby Lake Clifton. Hydromagnesite is a minor to subdominant phase (up to 47 wt%) of the carbonate mineral assemblage in Lake Walyungup. It occurs mainly in the littoral zone as a diagenetic replacement of precursor aragonite, particularly within the mesoclot fabric of stromatolites, but also in sediments (strandline and dune sand, crusts) derived mainly from erosion of stromatolites. In contrast with nonreplaced and impermeable inorganic aragonitic cements, stromatolite mesoclots are microper- meable. Micropermeability is inferred to facilitate hydromagnesite diagenesis. Dolomite is also present in minor amounts as a pore fill in stromatolites, and as a subdominant to dominant (up to 100 wt%) phase in thin, mudcracked micrite layers within the crust package. The layered dolomite may be precipitated directly from the lake water. Major element abundance of the lake water is: Na+ > Mg2+ » K+ > Ca2+ for cations, and Cl? » SO42? ≈ HCO3? > CO32? for anions. Compared to other nearby coastal lakes, Lake Walyungup has a high pH (> 9·0), and an extremely high molar Mg/Ca ratio of > 90. Groundwater in the area has a Mg/Ca ratio generally less than 1. The unusual Mg/Ca ratio in Lake Walyungup is partially a result of in-lake processes with additional minor contribution of Mg2+ sourced from basal marine sand because no Mg-rich bedrock source has been found in the region.  相似文献   

18.
In the light of progressive depletion of groundwater reservoir and water quality deterioration of the Neyveli basin, an investigation on dissolved major constituents in 25 groundwater samples was performed. The main objective was detection of processes for the geochemical assessment throughout the area. Neyveli aquifer is intensively inhabited during the last decenniums, leading to expansion of the residential and agricultural area. Besides semi-aridity, rapid social and economic development stimulates greater demand for water, which is gradually fulfilled by groundwater extraction. Groundwaters of the study area are characterized by the dominance of Na?+?K over Ca?+?Mg. HCO3 was found to be the dominant anion followed by Cl and SO4. High positive correlation was obtained among the following ions: Ca–Mg, Cl–Ca,Mg, Na–K, HCO3–H4SiO4, and F–K. The hydrochemical types in the area can be divided into two major groups: the first group includes mixed Ca–Mg–Cl and Ca–Cl types. The second group comprises mixed Ca–Na–HCO3 and Ca–HCO3 types. Most of the groundwater samples are within the permissible limit of WHO standard. Interpretation of data suggests that weathering, ion exchange reactions, and evaporation to some extent are the dominant factors that determine the major ionic composition in the study area.  相似文献   

19.
Dolomitization often plays a critical role in the pore network development of platform carbonates, with implications for reservoir quality distribution. Understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies. Here, the role of secular variations in seawater Mg/Ca as a control on dolomitization and early porosity modification was evaluated using one‐dimensional reactive transport models and fluids based on modern (aragonite sea), Mississippian and Aptian (calcite sea) seawaters. The sensitivity of dolomitization to a range of extrinsic controls (brine salinity, temperature, fluid flow rate and pCO2) and to intrinsic reactivity of the sediments (effective reactive surface area) was also explored. Simulations suggest faster calcite replacement by dolomite for seawaters with higher Mg/Ca, indicating that dolomitization potential is determined more by Mg/Ca rather than saturation index. Increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density‐driven fluid flux. In addition to brine composition, effective surface area of precursor sediments and temperature exert a critical control on replacement rate, while secular variations of pH and carbonate alkalinity associated with changes in pCO2 are only secondary controls. Above flow rates of 0·01 m yr?1 replacive dolomitization is reaction‐limited rather than flux limited, favouring alteration of fine‐grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low. Post‐replacement dolomite cementation is flux dependent, and thus favoured in areas of high head gradient and high permeability sediments and, contrary to replacement, supersaturation is a more important driver than Mg/Ca. While uncertainties remain regarding low‐temperature dolomitization kinetics, the capability of numerical simulations to decouple individual controls provides new insights which can be used, in conjunction with traditional comparative sedimentology, to generate more rigorous conceptual models for individual reservoir settings.  相似文献   

20.
This study used a mass-balance simulation approach in conjunction with geochemical, mineralogical, thermodynamic and isotopic constraints, to assess the origins of NaSO4(±HCO3) type groundwater and springwater associated with smectitic sulphide-mineral-bearing unconsolidated surficial sediments and the underlying Paskapoo Formation in south-central Alberta. Results indicate that alteration of albite to kaolinite and alteration of kaolinite to Na-smectite are the primary controls on dissolved Na and SiO2 concentrations in groundwater and springwater. Concentrations of dissolved Ca and Mg are controlled by reactions involving carbonate minerals and possibly cation exchange. Dissolved SO4 is generated primarily through oxidation of pyrite. Most H+ generated by oxidation of pyrite is consumed in aluminosilicate alteration reactions. The carbon isotopic composition of CO2 gas required in mass-balance simulations suggests the presence of an isotopically heterogeneous environment with respect to 13C. This apparent isotopic heterogeneity may result from the presence of varying fractions of atmospheric and microbially respired CO2.  相似文献   

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