laser step heating 40ar39ar age spectra from early archean 3 5 ga barberton greenstone belt sediments a technique for detecting cryptic tectono thermal events abstract samples from sediments of the fig tree group located in the central part of the circa 3 2 to 3 5 ga barberton greenstone belt bgb have been analyzed by the 40 ar 39 ar laser step heating technique this technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include reworked chemical sediments barite clastic sediments sandstone shale and one stromatolite in most cases the apparent age plateaux can be identified by corresponding ca k and cl k ratio plots as belonging to separate mineral phases most of the samples exhibit simple ar diffusion loss during the low temperature part of the experiments while occasionally showing excess ar or possible 39ar recoil effects the various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 ma one barite sample gave t0 original blocking age 2673 3 ma 1 which is close to a calculated model ar diffusion loss age for the same sample of 2688 3 ma 1 this age is interpreted as representing final granitoid activity adjacent to the bgb and or craton scale tectonism associated with the limpopo orogeny two samples gave t0 ages of 2350 2400 ma which may reflect increased thermal gradients associated with the formation of the thick transvaal sedimentary basin that may once have covered the bgb the dominant apparent age plateaux together with modelled diffusive ar loss ages of 2025 2090 ma are thought to represent regional thermal anomalies related to large scale tectono thermal activity in the kaapvaal craton of which the bushveld complex which covers a surface area of 60 000 km2 and formation of the vredefort structure are obvious manifestations the strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere i e 40 ar 36 ar ratios

the flank margin model for dissolution cave development in carbonate platforms the bahama islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level the development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens lens position is controlled by sea level which in stable carbonate platforms like the bahamas is a function of glacioeustatic sea level still stands caves in the bahamas that are currently subaerial must have developed during past higher sea levels during the late quaternary sea levels higher than present have been relatively short lived and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation the bahama islands are low lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level past high sea level events greatly reduced the exposed land area of the bahama islands thus also limiting both the catchment for and size of freshwater lenses caves must be younger than the rock in which they are developed most subaerial bahamian caves are found in limestones that are less than 150000 years old development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism the mixing of discharging freshwater with tide pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands

rb sr and sm nd isotope geochemistry and chronology of cherts from the onverwacht group 3 5 ae south africa we report rb sr and sm nd isotopic analyses of chert samples from the geluk subgroup previously called the mafic to felsic unit mfu of the onverwacht group in south africa these archean cherts have the same characteristic rb and sr concentrations as phanerozoic cherts with high rb sr ratios the dense massive essentially low carbonate cherts from the onverwacht group show a very large range of 87 rb 86 sr ratios from 0 859 to 7 99 one sample of chert leached in acid yielded 87 rb 86 sr 7 04 the data define a very good correlation line in the 87 rb 87 sr evolution diagram which corresponds to an age of 2119 44 m y with an initial 87 rb 86 sr ratio of 0 72246 16 exchange of sr in the cherts with the associated carbonate appears very limited the correlation line is interpreted as reflecting the age of rehomogenization of the sr in the protolith and the recrystallization of these cherts due to circulating hydrothermal fluids during regional metamorphism about 1 4 ae after deposition of the onverwacht group it may also correspond to the closure of the rb sr system in the cherts when tectonic activity stopped in the region 2200 m y the initial 87 rb 86 sr ratio obtained for the cherts requires the protolith which provided the sr to have rb sr 1 the sm nd systematics do not provide any strict chronologic information but appear to reflect an ancient source age 3 5 ae the nd at 3 54 ae for the cherts are grouped around zero i e close to the initial value observed for volcanics of the same age and provide no evidence for the presence of abundant crustal material older than 3 5 ae

porosity development in coastal carbonate aquifers geochemical mixing theory suggests that the mixing of seawater and calcite saturated fresh ground water can result in a solution that is undersaturated with respect to calcite previous studies of the mixing of such waters in carbonate rocks along certain coastlines have indicated that this mixing effect may be responsible for significant amounts of calcite dissolution and porosity development in this study potential rates of porosity development by calcite dissolution are assessed by combining geochemical mixing theory with the hydrodynamics of fresh water salt water mixing zones in a coupled reaction transport model results from the reaction path model phreeqe are used with a variable density ground water flow and solute transport model to simulate an idealized cross section of a coastal carbonate aquifer results of the simulations indicate that the dissolution process is sensitive to fresh water chemistry ground water velocities and sea level movement dissolution potential was evaluated at three field sites and evidence from those sites is in general agreement with the simulation results dissolution rates indicated by the model show that under the proper conditions this dissolution mechanism can produce significant increases in porosity over relatively short spans of geologic time tens of thousands of years

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

groundwater geochemistry of the yucatan peninsula mexico constraints on stratigraphy and hydrogeology summary we report 87sr 86sr and ion concentrations of sulfate chloride and strontium in the groundwater of the northern and central yucatan peninsula mexico correlation between these data indicates that ejecta from the 65 95 m y old chicxulub impact crater have an important effect on hydrogeology geomorphology and soil development of the region ejecta are present at relatively shallow subsurface depths in north central yucatan and at the surface along the rio hondo escarpment in southeast quintana roo where they are referred to as the albion formation anhydrite gypsum and by inference celestite are common in impact ejecta clasts and in beds and cements of overlying paleocene and lower eocene rocks cored around the margin of the crater the sulfate rich minerals that are found in rocks immediately overlying the impact ejecta blanket may either be partially mobilized from the ejecta layer itself or may have been deposited after the k t impact event in an extensive pre oligocene shallow sea these deposits form a distinctive sedimentary package that can be easily traced by the eocene cretaceous 87sr 86sr signal a distinct sr isotopic signature and high so4 cl ratios are observed in groundwater of northwestern and north central yucatan that interacts with these rocks moreover the distribution of the gypsum rich stratigraphic unit provides a solution enhanced subsurface drainage pathway for a broad region characterized by dissolution features poljes extending from chetumal quintana roo to campeche campeche the presence of gypsum quarries in the area is also consistent with a sulfate rich stratigraphic package that includes ejecta the distinctive chemistry of groundwater that has been in contact with evaporite ejecta can be used to trace flow directions and confirms a groundwater divide in the northern peninsula information about groundwater flow directions and about deep subsurface zones of high permeability is useful for groundwater and liquid waste management in the area where it discharges at the coast the unique chemistry of the groundwater that has interacted with the evaporite ejecta strata may also have significant geomorphologic implications while groundwater seawater mixing at the coast has been shown to dissolve and erode limestone phreeqc modeling shows that mixing of water nearly saturated in caso4 with seawater has a less vigorous dissolution effect due to its high ca content

geochemical investigations of saltwater intrusion into the coastal carbonate aquifer of mallorca spain abstract coastal aquifers often display seawater intrusion resulting in the formation of a salty water wedge progressing inland this study investigates the mass transfers in the mixing zone at the freshwater seawater interface where the water is out of equilibrium with the rock forming carbonates investigations were conducted in two boreholes separated by 5 m at the ses sitjoles test site mallorca island spain where repeated electrical conductivity logs of the formation and the saturating fluid as well as regular pore water sampling and permanent downhole multi parameter monitoring of the water were performed over a period of 9 a in the mixing zone the significant acidification the calcite saturation index profile and the ca concentration profile cannot be explained by conservative mixing nor by dissolution precipitation reactions only conversely the analysis of organic c content and of the distinctly different time resolved ph profiles measured in the two boreholes suggests the development of perennial biomass that triggers calcite dissolution moreover the presence of biomass seems to be correlated with the permeability and vertical connectivity at the meter scale it is speculated that the mechanism could be self activated because the microbiological activity induces calcite dissolution and tends to increase porosity and permeability that favors biomass development

evaluating formation fluid models and calibrations using clumped isotope paleothermometry on bahamian dolomites abstract the use of stable oxygen isotopes to understand the mechanisms of dolomite formation has been hampered by the inability to precipitate well ordered dolomite under normal earth surface conditions several studies have attempted to address this problem either by precipitating high temperature ordered dolomites and extrapolating the data to low temperatures or by using more disordered very high mg calcites as a proxy for low temperature dolomites the result is eight equations that disagree significantly from each other by as much as 3 6 in the 18o value of the precipitating fluid at 25 c and produce differences which can greatly affect the interpretation of the formation mechanisms for low temperature dolomites however by combining the recently developed clumped isotope paleothermometer an independent isotopic measurement 47 that directly relates to the temperature of formation to late miocene to pleistocene aged dolomites from the bahamas with a well constrained thermal and fluid history we have attempted to narrow down the viable equations used to interpret the 18o values of dolomites the clumped isotope temperatures measured on the bahamian dolomites 16 37 c agrees with the range of temperatures expected in the bahamas pairing these temperatures with geological and mineralogical arguments we favor the equation suggested by matthews and katz 1977 as it is the only one that produces realistic 18o fluid values across the range of clumped isotope temperatures both the clumped isotope temperatures and 18o values of the precipitating fluid show a strong positive covariance that we have interpreted as reflecting the mixing of surface brines that have undergone varying amounts of evaporation and normal seawater the different mechanisms driving these fluids included formation by normal marine seawater driven by the compensatory flow of the mixing zone bank wide kohout convection and evaporative brine reflux

cathodoluminescence petrography and isotope geochemistry of kt impact ejecta deposited 360 km from the chicxulub crater at albion island belize the depositional and diagenetic history of cretaceous tertiary kt impact ejecta deposited 360 km from the chicxulub crater at albion island belize has been investigated using integrated cathodoluminescence and isotopic analyses a quarry exposes 26 m of upper cretaceous barton creek formation dolomitized marine limestone overlain by 16 m of dolomitized albion formation impact ejecta the albion formation consists of a lower finegrained 1 m thick spheroid bed and an upper 15 m thick coarse conglomeratic diamictite bed a 14 event paragenetic sequence has been documented and used as a temporal framework to interpret chemostratigraphic trends in bulk rock d 18 o d 13 c and 87 sr 86 sr the uppermost surface of the barton creek formation was subaerially exposed before the kt impact as indicated by a brecciated palaeosol that caps upsection decreases in d 13 ca nd d 18 o small 1 cm diameter spheroids in the spheroid bed exhibit vermicular crystalline textures but lack the concentric zonations common to accretionary lapilli these spheroids are hypothesized originally to have been impact glass or reactive ca and mg oxide dusts that adhered to water vapour particles condensing from the cooling impact vapour cloud the spheroids were dolomitized soon after deposition the earliest dolomitization in the matrix sediments of the albion formation was also post depositional replacing clays formed by devitrification of impact glass dolomite and clay 87 sr 86 sr exhibit a distinct symmetrical distribution in the spheroid bed ranging from 0ae707745 to 0ae707872 although unproven this may represent primary changes in the chemical composition of the impact glass the limestone clasts in the diamictite bed were dolomitized before the kt impact and exhibit upsection decreases in bulk rock 87 sr 86 sr this suggests that the clasts were excavated from strata equivalent in age or older than the barton creek formation at locations closer to or in the chicxulub crater

geochemistry of precambrian carbonates 3 shelf seas and non marine environments of the archean a comprehensive whole rock study of mineralogical chemical and isotopic attributes of archean carbonates suggests that their lithologies and facies have been controlled by tectonic setting in the first two papers of this series we have shown that the dominant lithology of sedimentary carbonates in greenstone belt settings is limestone fe rich dolostone of ankeritic composition in older examples in this paper we suggest that the archean shelf sequences are mostly dolostone and the contemporaneous lacustrine playa lakes are characterized by limestone facies the present study is of the shelf environments of the archean represented by the pongola supergroup of south africa and the hamersley group wittenoom and carawine dolomite of australia the lacustrine playa examples have been sampled from the ventersdorp supergroup of south africa and the fortescue group of australia geological trace element and oxygen isotope considerations of the shelf carbonates suggest that their original mineralogy may have been aragonite and that the pongola dolostones probably represent a direct dolomitization product of this precursor in contrast the stabilization of the hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone prior to dolomitization subsequently both sequences but the pongola supergroup much more so were subjected to pervasive thermal events which overprinted their chemical and isotopic signatures consideration of the best preserved samples yields 0 9 2 1 and 6 l pdb for the 13c and 18o respectively for the initial archean signature if compared to phanerozoic carbonates the archean 13c values are comparable but the oxygen isotope measurements again support the primary nature of 18o depletion in ancient marine sediments regardless of its ultimate cause the 87sr86sr of 2 5 ga old sea water was not more than 0 7033 if this value reflects the contemporaneous sea water it would indicate the commencement of a significant 87sr input from the growing and aging continents it is however entirely possible that the slight enrichment in radiogenic 87sr relative to the contemporaneous mantle represents only an unresolved artefact of post depositional alteration if so the 87sr86sr of the 2 5 ga old sea water may still have been within a mantle range the lacustrine limestones retain a strong facies control in their trace element attributes they are also 4 depleted in 13c compared to marine carbonates and strongly depleted in 18o 19 l pdb these light 18o values are somewhat enigmatic unless the lakes were of the high altitude latitude variety the 87sr86sr of 0 711 for the ventersdorp lakes suggests that their drainage basin incorporated not only volcanic rocks but also aged continental crust archean carbonates of all lithologies and facies are enriched in mn and fe compared to their phanerozoic counterparts

water chemistry and caco3 dissolution in the saline part of an open flow mixing zone coastal yucatan peninsula mexico along the caribbean coast of the yucatan peninsula brackish ground water mixed fresh water and sea water is channeled through upper pleistocene limestone via fracture controlled caverns in caves cenotes and caletas at xcaret yalku and tancah this open flow coastal mixing zone comprises three major layers 1 an upper dilute zone of gradually increasing salinity with depth slow mixing 2 a thin intermediate zone of rapidly increasing salinity with depth rapid mixing and 3 a lower saline zone of gradually increasing salinity with depth the intermediate layer occurs at different absolute salinities at different localities and it generally corresponds to the level of a notch apparently related to dissolution in the wall rock of the caverns chemical composition of the mixed ground water is consistent with that predicted from theoretical conservative mixing of fresh water and sea water end members sampled in the yucatan coastal region inorganic c alkalinity mg ca and sr ca and 18o and 13c of the mixed water give no indication of dissolution or precipitation of cac03 as the ground water flows rapidly seaward field and petrographic observations however show that aragonite is preferentially dissolving throughout the modern mixing zone and in particular in the saline portion comparison of saturation indices to calculated mixing curves shows that the coastal mixing zone of the eastern yucatan peninsula is undersaturated with respect to cac03 over a wide range of salinity including mixtures of as much as 95 sea water for aragonite and 90 sea water for calcite supersaturation with respect to stoichiometric dolomite is maintained over the whole range of salinities but super saturation with respect to calcian dolomite ca57mg43 can be confined to high salinity mixtures depending on the thermodynamic data base used in calculations these data suggest that 1 dissolution of coastal limestone can take place in the sea water dominated portion of the mixing zone and 2 the mixing zone model for dolomitization can be extended to much higher salinities than is indicated by the widely accepted dorag model

the early archaean nondweni greenstone belt southern kaapvaal craton south africa part i stratigraphy sedimentology mineralization and depositional environment abstract the nondweni greenstone belt is an exceptionally well preserved 3 4 ga succession of predominantly volcanic rocks near the southern margin of the kaapvaal craton nondweni group rocks show both similarities and differences to the barberton sequence of similar age 300 km to the north in contrast with the latter none of the nondweni lavas exceed 23 mgo and spinifex textures are exclusively of pyroxene type the characteristics of the lavas and magma genesis are discussed in part ii of this series of papers three formations are recognized in the nondweni group the lowermost mogongolozi formation comprises basaltic andesites komatiitic andesites and basalts with intercalated coarser grained pyroxenites and gabbros the toggekry formation is a felsic unit of quartz sericite schists with small cu zn massive sulphide ore bodies of volcanogenic origin the uppermost witkop formation comprises mainly basalts komatiites and komatiitic basalts intercalated with chert layers and capped by a clastic sedimentary succession the total exposed thickness of the nondweni group is 16 500 m is the result of duplication of stratigraphy and tectonic thickening during early d1 thrusting in the bedding plane of much thinner lithological units tuffaceous sediments comprise a relatively minor component of the stratigraphy but are indicative of depositional environment banded chert layers are largely derived from accumulations of accretionary lapilli tuffs although a diverse origin is indicated for some varieties at the base of the witkop formation a number of undeformed banded coarse grained graphitic quartzite bodies entirely devoid of sedimentary structures are considered to have originated as hydrothermal deposits resulting from fumarolic activity selective brecciation of layers resulted from explosive release of fluids and gases the textures of these bodies are in marked contrast to the highly rounded pseudo conglomerates from the clastic sedimentary succession at the top of the witkop formation this 500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits into a zone of evaporites and stromatolites swallow tail twin crystals of barite and an evaluation of crystal interfacial angles are indicative f replacement of primary evaporative gypsum most primary features of the nondweni greenstone belt suggest a shallow water or subaerial depositional environment and together with characteristics of the magma composition indicate a tectonic setting in a subduction related continental margin setting

aragonite to calcite transformation during fresh water diagenesis of carbonates insights from pore water chemistry dissolved strontium and calcium concentrations in fresh water lenses fwl and associated mixing zones mz on two small holocene ooid sand islands in the schooner cays bahamas were monitored during a 1 yr period to quantitatively analyze the transformation of aragonite to calcite the observed characteristics of this mass transfer are functions of climate and hydrology aragonite to calcite transformation in all hydrologic zones is primarily associated with meteoric recharge the transformation occurs throughout the fwl and in the mz to relative salinities of 19 and 36 sea water on the two islands rates of transformation are rapid in all zones and are greatest in the fwl a limestone composed of 100 calcite should form from an aragonite precursor within 4 700 to 15 600 yr in the fwl and within 8 700 to 60 000 yr in the upper mz efficiencies of transformation can vary between hydrologic zones due to pco2 effects yet the efficiency of the entire system fwl mz is high 87 this indicates that most caco3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh water system or upper mixing zone co2 effects fresh water sea water mixing and the differing solubilities between aragonite and calcite all drive the mass transfer the latter is the most significant accounting for up to nine times more mass transfer than co2 effects and at least ten times more mass transfer than fresh water sea water mixing differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle but it apparently becomes ineffective after the rainy season possibly due to the inhibition of calcite precipitation

microfossils and possible microfossils from the early archean onverwacht group barberton mountain land south africa there is widespread textural evidence for microbial activity in the cherts of the early archean onverwacht group layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic hooggenoeg and kromberg formations in rare cases filamentous microfossils are associated with the laminae the morphologies of the fossils as well as the texture of the encompassing laminae suggest an affinity to modern mat dwelling cyanobacteria or bacteria a variety of spheroidal and ellipsoidal structures present in cherts of the hooggenoeg and kromberg formations resemble modern coccoidal bacteria and bacterial structures including spores the development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions including the effects of very large meteorite impacts on the young earth

tidal asymmetry in estuaries with mixed semidiurnal diurnal tides 1 tidal asymmetry in estuaries with mixed mainly semidiurnal tides arises from both the interaction of principal tides and the higher harmonics generated by distortions within the estuary the duration asymmetry in rise and fall of water level caused by principal tides on the west coast of the continental united states is ebb dominant and so the tide entering estuaries is also ebb dominant prior to any internal distortions within the estuary the interaction of higher harmonics with principal constituents either augments or cancels the duration asymmetry in the principal tides in estuaries where tidal elevation and velocity phase are near quadrature 90 out of phase the duration asymmetry in tidal elevation leads to asymmetries in tidal current magnitude asymmetry can be conveniently quantified in terms of the sample skewness 1 the normalized third sample moment about the mean an analytic approximation to the skewness shows that traditional metrics of asymmetry namely the ratio of constituent amplitudes and the relative constituent phase difference arise from calculating the third sample moment observations from three california estuaries of different morphologies are presented as an illustration of how skewness can be used to quantify asymmetry in real systems as in semidiurnal systems morphology is a good predictor of whether higher harmonics engender ebb dominance or flood dominance however asymmetry imposed by principal tides at the mouth must first be overcome and so there is a spatial evolution in the total asymmetry quantifying observations via skewness should be considered in addition to traditional metrics in estuaries with mixed tides

mixing zone diagenesis in the subsurface of florida and the bahamas abstract three cores from great bahama bank and southern florida contain examples of mixing zone diagenesis where typical shallow water aragonite rich sediments were altered to low mg calcite the 40 50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric 18o 3 2 0 7 13c 1 6 1 8 to enriched marine isotopic values 18o 1 0 0 3 13c 1 9 0 7 the age of the mixing zones varies from late pliocene to pleistocene so these cores provide three independent examples of mixing zone diagenesis mixing zone diagenesis produced a limestone composed of low mg calcite with petrographic fabrics including moldic porosity blocky to dogtooth calcite cementation and recrystallization of lime mud to micrite and microspar the mixing zone diagenetic model predicts alteration in only part of the mixing zone because the alteration recorded in these cores shows continuous alteration through the entire mixing zone we consider the mixing of waters itself to be largely irrelevant rather diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter the mixing zone has been proposed as a likely location for dolomitization with the exception of some clearly sea floor dolomite in one core unda the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone the lack of dolomite in these mixing zones combined with the extensive literature questioning the chemical reasoning for mixing zone dolomite supports rejection of the mixing zone dolomite model

geochemical processes evidence and thermodynamic behavior of dissolved and precipitated carbonate minerals in a modern seawater freshwater mixing zone of a small tropical island abstract the geochemical processes and thermodynamic behavior of dissolved and precipitated carbonate minerals controlling the hydrochemistry of an aquifer in the seawater freshwater mixing zone of a small island are identified field and laboratory analyses geochemical modeling phreeqc and multivariate statistical analysis msa provide a quantitative interpretation for the geochemistry of the carbonate dominated aquifer geochemical analyses and modeling results show that dissolution and re precipitation of caco3 are the prevalent processes governing geochemical reactions in the mixing zone furthermore this was confirmed by coherent statistical output that incorporates principle component analysis pca and k means cluster analysis k ca generally the composition of the lowland sandy soil was rather homogeneous and was primarily composed of quartz aragonite calcite and mg calcite thermodynamic model calculations indicate that the carbonate minerals calcite aragonite and dolomite are supersaturated in the mixing zone nevertheless powder x ray diffraction pxrd and scanning electron microscope sem examination verified the occurrence of low mg calcite lmc and the absence of dolomite attributed to thermodynamic kinetic hindrance cation disorder and the presence of dolomite crystal growth rate inhibitors such as so4 the results suggest that dissolution of aragonite and precipitation of lmc drives the solid phase geochemistry in the small tropical island aquifer

differential dissolution of a pleistocene reef in the ground water mixing zone of coastal yucatan mexico a geochemical explanation is provided for the extensive dissolution observed along the carbonate coast of the yucatan peninsula mexico mixing of fresh ground water with subterranean caribbean seawater generates a highly reactive geochemical zone that enhances aragonite and calcite dissolution and permits neomorphism of aragonite mixing zone dissolution caused by ground water discharge is a major geomorphic process in developing caves coves and crescent shaped beaches along the yucatan coast such dissolution has probably been a significant control on permeability and porosity distribution in carbonate rocks in the geologic record

reactive transport modeling of calcite dissolution in the fresh salt water mixing zone abstract dissolution of calcite in the saltwater mixing zone of coastal carbonate aquifers has been the subject of numerous studies they conclude that dissolution is driven by mixing of freshwater and saltwater which may lead to undersaturation even if both end members are saturated however as transport in the mixing zone is rather complex a fully coupled reactive transport model is needed to properly understand the interplay between transport and chemistry we perform 1 and 2d simulations to gain insight into the processes controlling dissolution rate exploring its sensitivity to the chemistry of end members and the dispersivity of the medium we conclude that the results of sanford and konikow sanford w e konikow l f 1989 simulation of calcite dissolution and porosity changes in saltwater mixing zones in coastal aquifers water resources research 25 655 667 which used a two step method solely transporting the amount of calcite dissolved are essentially correct but their solution is essentially kinetic with a rate dependent on the time increment used the 1d simulations allow us to show that maximum dissolution rates do occur neither at the location nor for the end members of maximum undersaturation for non reactive mixing in the 1d case enhanced transport at the freshwater end causes dissolution to concentrate there however in the 2d case representative of coastal aquifers maximum dissolution occurs at the saline side near the discharge zone because of a very active convection cell that develops there which enhances mixing and thus reaction rates this mechanism may explain observations at blue holes and should display a positive feedback with dissolution causing an increase in permeability that would lead to increased seawater flux and mixing thus enhancing further dissolution

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

seawater intrusion in the guanahacabibes peninsula pinar del rio province western cuba effects on karst development and water isotope composition the water resources in the guanahacabibes peninsula are distributed in two areas the northeastern area is characterized by swamps wetlands and lagoons with a low contribution of seawater whereas the area in the southwestern plain shows a considerable development of the karst structures that limits the existence of superficial waters but permits the ingression of the surrounding seawater in this latter area the groundwater showed a marked increase in salinity with the depth in particular groundwater with a seawater fraction of 0 1 had the lowest ca mg carbonates saturation indexes calculated by modeling the mixing between freshwater and seawater using different software thermodynamic databases and equations for activity coefficients generally seawater and groundwaters with an added seawater fraction above 0 60 0 65 showed similar oversaturated indexes in high mg calcites and pure ca carbonates calcite and aragonite differently in the groundwater that showed carbonates undersaturation generally with a seawater fraction between 0 02 and 0 60 the saturation indexes in high mg calcites were 0 2 lower than pure ca carbonates locally the bacterial reduction of the dissolved sulfate enhanced the dissolution of the limestone contributing to the increased development of the karst structures and the seawater intrusion finally the presence near the coastline of fresh ca and na bicarbonate waters was in accordance with the upward flow of the shallow freshwater during the formation of the saline wedge however the oxygen and hydrogen stable isotope composition of the waters showed a probable contribution to the area from a deep aquifer that is recharged in the highest reliefs of the province cordillera de guaniguanico

carbonate dissolution and precipitation in coastal environments laboratory analysis and theoretical consideration 1 we have conducted laboratory experiments to examine caco3 dissolution and precipitation in saltwater freshwater mixing zones with a view to understanding and predicting porosity changes in coastal environments mixing of seawater or saline subsurface water with fresh water can be of major importance in the chemical diagenesis of carbonate rocks and sediments we used artificial seawater and nacl solutions of different concentrations under different co2 partial pressures and with different mixing ratios two dimensional flow cells filled with glass beads and crushed calcium carbonate rock were used to measure calcium carbonate precipitation and dissolution respectively an important feature of these experiments is that the results are shown to agree well with a relatively simple transport theory describing mineral precipitation dissolution that results from the nonlinear dependence of caco3 saturation upon electrolyte concentration the theory demonstrates that the rate of dissolution or precipitation depends on the curvature and sign of the solubility as a function of salinity the square of the salinity gradient and the macroscopic dispersion coefficient the theory is largely scale independent and depends upon field parameters that can be determined analysis of data from three field sites yucatan peninsula bermuda and mallorca demonstrates excellent agreement between field observations and theory

heterogeneous distributions of co 2 may be more important for dissolution and karstification in coastal eogenetic limestone than mixing dissolution mixing dissolution a process whereby mixtures of two waters with different chemical compositions drive undersaturation with respect to carbonate minerals is commonly considered to form cavernous macroporosity e g flank margin caves and banana holes in eogenetic karst aquifers on small islands macroporosity commonly originates when focused dissolution forms globular chambers lacking entrances to the surface suggesting that dissolution processes are decoupled from surface hydrology mixing dissolution has been thought to be the primary dissolution process because meteoric water would equilibrate rapidly with calcium carbonate as it infiltrates through matrix porosity and because pco2 was assumed to be homogeneously distributed within the phreatic zone here we report data from two abandoned well fields in an eogenetic karst aquifer on san salvador island bahamas that demonstrate pco2 in the phreatic zone is distributed heterogeneously the pco2 varied from less than log 2 0 to more than log 1 0 atm over distances of less than 30 m generating dissolution in the subsurface where water flows from regions of low to high pco2 and cementation where water flows from regions of high to low pco2 using simple geochemical models we show dissolution caused by heterogeneously distributed pco2 can dissolve 2 5 to 10 times more calcite than the maximum amount possible by mixing of freshwater and seawater dissolution resulting from spatial variability in pco2 forms isolated globular chambers lacking initial entrances to the surface a morphology that is characteristic of flank margin caves and banana holes both of which have entrances that form by erosion or collapse after cave formation our results indicate that heterogeneous pco2 rather than mixing dissolution may be the dominant mechanism for observed spatial distribution of dissolution cementation and macroporosity generation in eogenetic karst aquifers and for landscape development in these settings copyright 2015 john wiley sons ltd

physical and chemical stratigraphy suggest small or absent glacioeustatic variation during formation of the paradox basin cyclothems the paradox basin cyclothems previously have been interpreted as milankovitch style glacial interglacial cycles from the late paleozoic ice age but an unambiguous test for a glacioeustatic origin has not been conducted a high resolution coupled chemical and physical stratigraphic analysis of two outcrop sections and three core segments provides new evidence that supports either minor sea level change of several meters or an autocyclic mechanism for parasequence formation high amplitude sea level change is ruled out by the scale of thin top negative isotopic meteoric diagenesis trends associated with parasequence tops and subaerial exposure fabrics isotopic gradients from shelf light to basin heavy indicate that parasequences are deposited diachronously with isotopes of more distal sections recording increased basin restriction these results support the idea that the late pennsylvanian was a prolonged period of relatively static eustasy agreeing with recent studies in the western usa the methods provide a new set of tools and context for extracting environmental information from cyclic upward shallowing carbonate parasequences

fragmentation and dispersal of komatiitic pyroclasts in the 3 5 3 2 ga onverwacht group barberton greenstone belt south africa the 3 5 3 2 ga onverwacht group of the barberton greenstone belt south africa contains thick sequences of komatiitic tuff lapilli tuff and lapillistone representing proximal to distal volcanic settings volcaniclastic komatiites in the southern part of the belt include 1 areally extensive sheets of silicified massive to normally graded ash and accretionary lapilli interpreted as pyroclastic fall deposits and 2 thick widespread carbonatized beds of lapilli and minor ash deposited by subaqueous sediment flow and pyroclastic fallout lithofacies 1 clasts are aphyric and poorly to nonvesicular implying rapid quench rates and fragmentation primarily through magma water contact low water magma ratios initial melt fracturing through minor volatile exsolution and high magma fluxes were probably the main factors that enabled the production of buoyant eruption columns and widespread ash distribution these tuffs were generated during vent shoaling in water that was probably

on the nature of tidal asymmetry in the gulf of khambhat arabian sea using hf radar surface currents abstract the hourly ocean surface current observations from available high frequency radars hfr have been analyzed to i quantify the tidal constituents ii investigate the role of the bathymetry on shallow water tides and iii identify the nature of tidal asymmetry in the inverted funnel shaped gulf of khambhat gokh on the western shelf of india during may 2012 the hfr observations captured strong currents 2 0 m s 1 showing significant spring neap variations tidal harmonic analysis of the hfr currents showed that the tidal currents at the mouth are dominated by the m2 constituent with amplitude 4 5 times higher than that of k1 among the shallow water constituents m4 dominates the entire gulf with significant amplitudes 10 cm s 1 and gets amplified towards the head of the gulf 30 cm s 1 followed by ms4 comparisons with sea level heights from tide gauges show a higher correlation for the major 0 97 and shallow water 0 96 tidal constituents in terms of the order of amplitudes of the tidal constituents the circulation pattern in the gulf is predominantly driven by tidal currents with the total tidal variance being 90 of the total current variance the major contributions are from the m2 tidal currents 70 90 except at the head where the m4 component contributes significantly up to 40 the interactions with shallow bathymetry are responsible for the amplification of tidal currents in this converging channel with negligible impact from stratification during may the harmonic analysis derived relative surface phase 2m2 m4 and higher m4 m2 amplitude ratio at the head of the gulf reveal that the gulf is highly tidally asymmetric and flood dominant indicating stronger non linear tidal distortions in the nearshore regions a generalized skewness based analysis confirms the flood dominance with positive values of a morphology dependent asymmetry factor and skewness two tidal combinations m2 m4 overtide and m2 s2 ms4 compound tide are identified as the major contributors to the flow velocity skewness in the gokh with mixed mainly semi diurnal tidal regime

the umsoli chert turbidite testament for a major phreatoplinian event at the onverwacht fig tree transition swaziland supergroup archaean south africa abstract the volcanogenic origin of accretionary grains in the oolitic umsoli chert is proved by the unequivocal identification of constituent vesicular and pumiceous ash particles by the discovery of authigenic alkali feldspars and by the k meta bentonite parentage of associated illites celadonites ash particle morphology and ash grain size distribution suggest a phreato magmatic process of eruption sedimentary structures indicate that the felsic ash precursor of this ancient chert was deposited by turbidity currents the sedimentologic model invokes the transition of parts of pyroclastic falls and base surges into turbidites accretionary lapilli forming a major constituent of this silicified tuff oolite may be indicators of subaerial eruption but they can probably not be used as evidence for subaerial deposition

subaqueous deposition of accretionary lapilli significance for palaeoenvironmental interpretations in archaean greenstone belts abstract accretionary lapilli from phreatomagmatic eruptions are abundant in the late archaean fortescue group of the pilbara region western australia where the sequence is uncleaved and gently folded well preserved depositional structures include densely packed accretionary lapilli in trough cross beds and sparse accretionary lapilli in trough and wedge to tabular cross beds accretionary lapilli also occur in conglomerate layers and localised pebbly channel fills associated with granite and pegmatite clasts up to 10 cm in diameter these lapilli are clearly robust being capable of withstanding fairly energetic aqueous transport in contrast to the opinion of many authors who consider that all accretionary lapilli would disintegrate if settled through extensive water columns or incorporated in turbidite deposits accretionary lapilli indicate subaerial or very shallow water eruption but their presence cannot be used alone to support very shallow water or subaerial deposition accretionary lapilli are found throughout many volcanic piles and are probably widespread in archaean greenstone successions but their environmental significance has to be assessed carefully using a variety of associated features

volcanic evolution of the upper onverwacht suite barberton greenstone belt south africa abstract the volcanic stratigraphy of the upper onverwacht suite in the southeastern part of the paleoarchean barberton greenstone belt has been investigated in 18 sections through parts of the hooggenoeg kromberg and lower mendon complexes the ca 2700 m thick volcanic sequence of the lowest tectonostratigraphic unit the hooggenoeg complex hc can be subdivided into 9 stratigraphic units representing major eruptive phases each separated by silicified sedimentary and volcaniclastic rocks the thicknesses of the units vary from the lavas of the hc and kc are predominantly pillowed and massive flows whereas those of the ncakini section are exclusively massive the massive and pillowed lava are commonly organized in cyclic units from 3 m to 32 m thick and consisting of massive lava or large pillows that are succeeded by progressively smaller pillows cyclic units are inferred to have resulted from individual eruptions with decreasing rates of effusion the pillow lavas of both the hc and kc contain

carbonate geochemistry and the concentrations of aqueous mg2 sr2 and ca2 western north coast of the yucatan mexico abstract we have measured concentrations of major cations sr 2 and alkalinity in groundwater pore water and surface water samples collected from the western north coast of the yucatan peninsula mexico thermodynamic calculations indicate that almost all water samples from the study area are supersaturated with respect to calcite aragonite high mg calcite hmc and dolomite analysis of the data suggests that the geochemical processes controlling mg 2 sr 2 and ca 2 concentrations differ in different environments mass balance modeling has been used to calculate the amount of dissolution and or precipitation of aragonite low mg calcite lmc and hmc and or dolomite required to produce waters with the observed mg 2 sr 2 and ca 2 concentrations mass balance calculations indicate that hmc and aragonite are dissolving whereas lmc is precipitating from most groundwater and inland surface waters the chemistry of surface waters from an extensive saline coastal swamp suggests that lmc and aragonite in surficial swamp sediments are converted to hmc or dolomite while swamp pore water chemistry suggests that hmc is dissolving and lmc and aragonite are precipitating in deeper swamp sediments the chemistry of pore waters suggests that they are derived from saline surface waters and that fresh to brackish groundwater from the shallow confined aquifer does not flow into swamp sediments

carbonate minerals in the global carbon cycle abstract carbonate minerals constitute earth s largest c reservoir this reservoir is considered unimportant to the global c cycle over long periods of time e g 10 6 yrs because it provides balanced sources and sinks of atmospheric co 2 as carbonate minerals precipitate and dissolve from carbonic acid respectively however carbonate mineral reactions may impact the carbon cycle at short time scales 10 2 to 10 5 yrs particularly when atmospheric pco 2 changes by several hundred ppm as exemplified by the paleocene eocene thermal maximum at transitions between glacial interglacial times and over the past century from anthropogenic inputs here i review recent literature about mechanisms in which carbonate minerals may impact the global carbon cycle carbonate mineral dissolution which increases dissolved inorganic carbon dic concentrations may represent a net atmospheric co 2 sink as primary productivity consumes dic resulting in organic c burial in fluvial systems of carbonate terrains alternatively carbonate minerals dissolved by sulfuric and nitric acids may represent an atmospheric source of co 2 with no rapid balancing atmospheric sink sulfuric acid dissolves carbonate minerals of modern carbonate platform sediments and water filled sinkholes and in the deep sea at the base of carbonate platform scarps in terrestrial landscapes sulfuric acid dissolves carbonate minerals of karst terrains hypogene caves and in comminuted glacial sediments nitric acid forms from oxidation of reactive n which has doubled in the past hundred years as humans fix atmospheric n 2 similar to some reactions involving sulfuric acid nitric acid dissolution of carbonate minerals represents a net source of atmospheric co 2 this source is concentrated in fertilized agricultural fields in carbonate terrains each of these processes represents a small flux relative to major c exchanges such as volcanic sources and silicate mineral weathering but collectively they could impact the c cycle at magnitudes similar to these major processes these impacts may increase as humans further alter elemental cycles of s and n

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

do graded units of accretionary spheroids in the barberton greenstone belt indicate archaean deep water environment we report here a re analysis of the sedimentology of the msauli chert oolite which shows that environmental conditions during the early depositional phase of the fig tree group in the barberton greenstone belt were of a deep water nature the unit is compared with quaternary sediments deposited in the pacific ocean at bathyal to abyssal depths

the geochemistry of archaean shales derived from a mafic volcanic sequence belingwe greenstone belt zimbabwe provenance source area unroofing and submarine versus subaerial weathering shales of the 2 7 ga zeederbergs formation belingwe greenstone belt zimbabwe form thin 0 2 2 m horizons intercalated with submarine lava plain basalts shales of the overlying cheshire formation a foreland basin sedimentary sequence form 1 100 m thick units intercalated with shallow water carbonates and deep water resedimented basalt pebble conglomerates zeederbergs shale is characterized by high contents of mgo and transition metals and low concentrations of k2o and lile as compared to average phanerozoic shale indicative of an ultramafic to mafic source terrain cheshire shales have similar major and trace element contents but mgo and transition metals are less enriched and the lile are less depleted zeederbergs shales have smoothly fractionated ree patterns lan ybn 2 84 4 45 and no significant eu anomaly eu eu 0 93 0 96 ree patterns are identical to those of the surrounding basaltic rocks indicating local derivation from submarine reworking cheshire shales have rather flat ree patterns lan ybn 0 69 2 19 and a small negative eu anomaly average eu eu 0 85 indicative of a mafic provenance with minor contributions of felsic detritus a systematic change in ree patterns and concentrations of transition metals and hfse upwards in the sedimentary succession indicates erosion of progressively more lree depleted basalts and ultramafic volcanic rocks followed by unroofing of granitoid crust weathering indices confirm the submarine nature of zeederbergs shale whereas cheshire shale was derived from a source terrain subjected to intense chemical weathering

formation of phreatic caves in an eogenetic karst aquifer by co2 enrichment at lower water tables and subsequent flooding by sea level rise formation of extensive phreatic caves in eogenetic karst aquifers is widely believed to require mixing of fresh and saltwater extensive phreatic caves also occur however in eogenetic karst aquifers where fresh and saltwater do not mix for example in the upper floridan aquifer these caves are thought to have formed in their modern settings by dissolution from sinking streams or by convergence of groundwater flow paths on springs alternatively these caves have been hypothesized to have formed at lower water tables during sea level low stands these hypotheses have not previously been tested against one another analyzing morphological data and water chemistry from caves in the suwannee river basin in north central florida and water chemistry from wells in the central florida carbonate platform indicates that phreatic caves within the suwannee river basin most likely formed at lower water tables during lower sea levels consideration of the hydrological and geochemical constraints posed by the upper floridan aquifer leads to the conclusion that cave formation was most likely driven by dissolution of vadose co2 gas into the groundwater sea level rise and a wetter climate during the mid holocene lifted the water table above the elevation of the caves and placed the caves tens of meters below the modern water table when rising water tables reached the land surface surface streams formed incision of surface streams breached the pre existing caves to form modern springs which provide access to the phreatic caves phreatic caves in the suwannee river basin are thus relict and have no causal relationship with modern surficial drainage systems neither mixing dissolution nor sinking streams are necessary to form laterally extensive phreatic caves in eogenetic karst aquifers dissolution at water tables potentially driven by vadose co2 gas offers an underappreciated mechanism to form cavernous porosity in eogenetic carbonate rocks copyright 2012 john wiley sons ltd

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

regional hydrogeochemistry of a modern coastal mixing zone in west central florida groundwater samples were collected along flow paths in the unconfined upper floridan aquifer that cross the inland freshwater recharge area and the coastal discharge area a groundwater flow and solute transport model was used to evaluate groundwater flow and mixing of fresh and saline groundwater along a cross section of the unconfined upper floridan aquifer results show that between 8 and 15 of the fresh and 30 31 of the saline groundwater penetrates to the depth in the flow system where contact with and dissolution of gypsum is likely the deeply circulating fresh and saline groundwater returns to the near surface environment discharging caso4 rich water to the coastal area where it mixes with fresh cahco3 groundwater resulting in a prediction of calcite precipitation in the modern mixing zone

petrography of finely crystalline cenozoic dolostones as revealed by backscatter electron imaging case study of the cayman formation miocene grand cayman british west indies abstract finely crystalline cenozoic island dolostones like those found in the cayman formation on grand cayman are commonly assumed to be petrographically and compositionally homogeneous backscatter electron images bsei however show that the constituent dolomite crystals 3 content moreover such images allow 1 depiction of growth patterns in the constituent crystals irrespective of their origin 2 recognition of replacive dolomite as opposed to dolomite cement and 3 delineation of stratigraphic packages in the dolomite cements that reflect different episodes of cementation integration of this information forms the basis for paragenetic interpretations of the dolostones on grand cayman the miocene cayman formation can be divided into friable high porosity dolostones and hard low porosity dolostones backscatter electron images show that the hard dolostones are characterized by complex arrays of zoned dolomite cements that have occluded most of the pores caymanite an internal sediment has occluded many of the larger cavities in contrast the high porosity dolostones contain little cement and no internal sediments precipitation of the cements and the deposition of internal sediments were related to the passage of large volumes of water through some of the dolostones thus the hard low porosity dolostones are found in the cap rock of the formation in coastal locations and in areas close to solution widened fractures conversely the friable high porosity dolostones form the lower porous unit of the formation in the interior of the island where the passage of water was more restricted

diagenesis of middle tertiary carbonates in the toa baja well puerto rico this is the publisher s version also available electronically from http onlinelibrary wiley com

island groundwater resources impacts of abstraction and a drying climate rottnest island western australia abstract coastal aquifers provide a source of water for more than one billion people with island freshwater lenses being some of the most vulnerable coastal groundwater systems due to their susceptibility to saltwater intrusion basic hydrogeological and hydrochemical knowledge regarding the recharge and salinisation processes of freshwater lenses is important to ensure sustainable utilisation especially considering possible climate change effects this paper makes an assessment of the fate of a freshwater lens in a drying climate through a comparison of current and historic hydrochemical data which to the author s knowledge is unique to this study fresh groundwater stable isotope signatures 18 o 2 h reflect local amount weighted rainfall signatures 18 o 3 8 2 h 15 1 and confirm rainfall as the origin of fresh groundwater 18 o 4 47 to 3 82 2 h 20 0 to 16 6 mixing with seawater was identified through enriched groundwater 18 o and 2 h signatures maximum values of 0 36 and 1 4 respectively compared to local rainfall and higher salinity maximum 29 267 mg l total dissolved solids tds in a number of monitoring wells around the freshwater lens enhanced seawater intrusion detected in the northern section of the lens area was identified through significantly increased tds values over the last 20 40 years with increases of up to 3000 observed between 1990 and 2014 a reduction in the extent of freshwater by approximately 1 km 2 was identified since 1977 which was found to be primarily caused by a reduction in recharge to the freshwater lens due to a 20 decline in winter rainfall in the south west western australian region since the mid 1960s groundwater abstraction was found to equate to between 5 and 9 of the estimated recharge for the island and is not a significant factor in the reduction of the lens extent compared to the observed decline in rainfall recharge interestingly seawater intrusion into the fresh water lens was found to occur by older seawater 0 03 0 09 tu in regions of the lens that were previously fresh or slightly brackish while one sample 0 67 tu suggests either modern seawater intrusion or mixing of older saline groundwaters 60 years with rainfall recharge the use of tritium dating in this island aquifer was essential in identifying older seawater that was previously unidentified until now the isotopic and hydrochemical tools used in this paper quantify the effects of groundwater abstraction and climate variability on the freshwater lens and have implications for the sustainable management of the groundwater resource on rottnest island and elsewhere

processes affecting groundwater chemistry in a zone of saline intrusion into an urban sandstone aquifer abstract samples have been collected from inflows into railway tunnels in the triassic sandstone aquifer beneath liverpool and the mersey estuary england u k these provide a profile through a saline freshwater mixing zone analyses were made of major anions and cations 34s and 18o in so4 13c in dissolved inorganic c and 87sr 86sr the data demonstrate that the presence of a low permeability fault exerts a strong control on the local groundwater chemistry on the estuary side of the fault groundwater chemistry is dominated by mixing of intruding estuary water which is modified by so4 reduction and calcite dissolution with fresh groundwater the environment of so4 reduction in the tidal estuary is one of repeated reduction and re oxidation of s in an open system and has resulted in virtually no change in s isotopic composition but an enrichment in residual so4 18o of 1 5 groundwater chemistry on the landward side of the fault is primarily the result of recharge in an urban environment there is also evidence that saline water has been present in this region of the aquifer in the past and that this has now been flushed by fresh groundwaters this saline water was either transported along the landward side of the fault from nearer the estuary or more probably transmitted across the fault both mechanisms would have been driven by large landward head gradients caused by heavy industrial abstraction earlier this century this has produced a zone of groundwaters depleted in ca and radiogenic sr and enriched in na as a result of ion exchange between the fresh groundwaters and the aquifer previously occupied by more saline water sulphur isotopic composition however shows no variation since so4 does not undergo significant ion exchange a tracer test from a borehole to the tunnels showed multiple breakthroughs to some locations indicating a number of different flow paths through the aquifer the maximum flow velocity recorded in this test was 140 m d suggesting flow along fractures

changes in tidal characteristics as a result of the construction of sea dike sea walls in the mokpo coastal zone in korea abstract it is shown by numerical analysis via the adoption of a fully implicit scheme that the tidal choking is much less significant after the construction of sea dike sea walls in the mokpo coastal zone tidal records show an amplification of tidal range extreme high tide is 60 cm higher and extreme low tide is 43 cm lower this amplification may cause serious inundation in the event of a typhoon another problem is the reduction in tidal velocity which may cause deterioration of the coastal environment in order to determine the ebb dominance near the mokpo coastal zone the extent of ebb dominance of each region was quantified by analyzing the m 2 and m 4 tidal constituents and in addition the variation of ebb dominance in mokpo harbor was observed after the construction of the sea dike sea walls ebb dominance appears to be more serious with each successive construction this is probably caused by the elimination of tidal choking at mokpogu the transport mechanism of sediment and pollutants has been altered because of increased ebb dominance however ebb dominance will be somewhat lessened after the construction of the fourth sea wall

mixing driven diagenesis and mineral deposition caco3 precipitation in salt water fresh water mixing zones 1 we perform a novel set of laboratory experiments that depict caco3 precipitation upon mixing between saturated fresh and salt water solutions thus simulating mixing diagenesis in a coastal aquifer the experimental results are shown to agree with the result predicted from a relatively simple mathematical model thus suggesting that the model may be extrapolated to natural environments application of the model to the coastal aquifer of mallorca spain indicates calcite precipitation is reducing porosity at a rate of 13 per 10 000 years consideration of precipitation processes can thus explain contradictory interpretations of natural evolution in carbonate formations

hydrogeochemistry of grand cayman british west indies implications for carbonate diagenetic studies abstract groundwater in the dolostone aquifers of the bluff group oligocene miocene on grand cayman is divided into fresh lightly and highly brackish and saline type i and ii zones according to chemical characteristics that were determined during a 3 year 1985 1988 monitoring program brackish and type i saline waters display the greatest variation in chemical properties whereas the type ii saline water has the most stable chemical characteristics most groundwaters from these dolostone aquifers are thermodynamically capable of precipitating calcite and or dolomite the saturation indices for these minerals however vary through time and space even in the context of small water lens simple mixing of fresh and sea water cannot explain the chemistry of the water found in the joint and karst controlled dolostone aquifers of grand cayman deviation from a simple mixing model is due to variations caused by tidal fluctuation the rate of rain water recharge influx of ca rich groundwater from the surrounding limestone aquifers influx of co 2 rich surface water from sinkholes and swamps and water rock interactions dissolution and precipitation of calcite and dolomite sustained groundwater abstraction from a lens can significantly alter the hydrochemistry of the water lens this suggests that hydrochemical characterization of small fresh water lenses like those on grand cayman cannot be based on spot or short term sampling interpretation of such fluids in terms of calcite dolomite precipitation and or dissolution must be treated with caution if the data base has not been derived from long term monitoring

isovolumetric silicification of early archean komatiites geochemical mass balances and constraints on origin stratiform quartz phyllosilicate carbonate alteration zones are a common feature of archean greenstone terranes and determining their origin is an important aspect of understanding the nature of shallow crustal alteration during the early history of the earth komatiites and komatiitic basalts of the uppermost onverwacht group ca 3 4 3 5 ga barberton greenstone belt south africa were regionally metasomatized prior to major tectonic deformation to the assemblage quartz dioctahedral mica chlorite pm dolomite subsequent metamorphism of these rocks has been restricted to greenschist facies and they thus record processes that may be obscured in analogous sequences from more highly metamorphosed terranes the isovolumetric nature of much of the alteration and the relative immobility of elements such as al zr and y permit a quantitative assessment of elemental gains and losses during metasomatism for each m 3 of komatiite altered approximately 10 3 kg one ton of mgo were removed

origin and mechanisms of k si metasomatism of ca 3 4 3 3ga volcaniclastic deposits and implications for archean seawater evolution examples from cherts of kittys gap pilbara craton australia and msauli barberton greenstone belt south africa abstract the effects of k si metasomatism during the formation of early archean replacement cherts have been quantified in this study by the investigation of two well known stratigraphic sections the msauli chert mc barberton greenstone belt south africa and the kittys gap chert kgc pilbara craton western australia the kgcs have a dacitic precursor similar to duffer formation dacites pilbara craton while the mcs are derived from al depleted komatiites similar to those from the weltevreden formation barberton greenstone belt mass balance calculations reveal that the volcaniclastic deposits had initial porosities of up to 85 vol for the kgc and of 65 vol for the mc secondary porosities 27 vol mc 8 vol kgc produced during k metasomatism are proportional to the dissolution of fe ca mg rich glass and precursor minerals komatiites have a higher chemical exchange potential than dacites each gram releasing 1 2 mmol fe2 2 8 mmol mg2 1 4 mmol ca2 and 1 1 mmol na to seawater together with 4 4 mmol o2 k metasomatism of 1 g of komatiite further implies an uptake of 0 67 mmol of k and 2 7 mmol of h the highest silica uptake is achieved for the kgc 82 mmol g of precursor this silica enrichment most likely operated in the water column and at the sediment water interface by sorption mechanisms on the surface of detrital particles and particulate organic matter as a result of seawater silica saturation acidic conditions ph 5 5 6 5 and hot temperatures 70 c favored the formation of k rich phyllosilicates by interaction with seawater during the early diagenetic alteration of the volcaniclastic particles the widespread occurrence of k si metasomatism in volcanic and sedimentary rocks can be regarded as a general alteration process of the early archean seafloor with a major influence on seawater composition the highly k selective metasomatism confirms previous studies suggesting that the archean ocean was acidic and probably in equilibrium with a co2 rich atmosphere

accretionary lapilli associated with archaean banded iron formations of the kraaipan group amalia greenstone belt south africa abstract the kraaipan group in the amalia greenstone belt south africa consists of an archaean volcano sedimentary assemblage of mafic metavolcanic rocks and banded iron formations together with subordinate greywacke slate and carbonate rocks the sequence is deformed and intruded by a variety of granites and gneisses interlayered with the volcano sedimentary rocks in close proximity to a small gold prospect are pyroclastic rocks in which are described well developed coated or rim type accretionary lapilli of a variety hitherto rarely if ever recorded in association with oxide facies banded iron formations the accretionary lapilli are discussed in terms of their significance and as palaeoenvironmental indicators it is concluded that the accretionary lapilli probably developed in a proximal setting with respect to an ancient eruptive centre and that the lapilli themselves do not constitute exceptional indicators of sedimentary environments but are useful mainly as time or stratigraphic markers their association with banded iron formations suggests the accretionary lapilli here described were deposited from volcanic centres close to some shallow marine or barred lagoonal basin the latter possibly analogous to a playa lake complex developed in a primitive inter arc or back arc basin setting in existence during the evolution of the kraaipan volcano sedimentary assemblage found in the amalia greenstone belt

an unusual occurrence of mafic accretionary lapilli in deep marine volcaniclastics on eua tonga palaeoenvironment and process reports of occurrences of accretionary lapilli on earth whether in historic time or in the geological record are restricted to subaerial environments or to shallow marine environments when faunal evidence exists to determine palaeodepths the proximity of the deep ocean to subduction zones island arcs where moist explosive volcanism conducive to ash aggregate formation is common makes this surprising in this paper accretionary lapilli are reported within middle miocene mafic glass rich volcaniclastics on eua the island closest to the tonga trench a persistent high in the frontal arc basin the glass in the accretionary lapilli has been subjected to advanced palagonitisation but concentric layers marked by micro aggregates containing shard shaped particles survive to determine one group of occurrences as layered accretionary lapilli the palaeoenvironment as established by pelagic microfauna is clearly deep marine not less than 1600 m the host rocks typically gravel sand in grain size contain sedimentary structures normal grading to inverse and normal to inverse grading lack of grading large scale cross bedding slump bedding and sedimentary dykes suggesting that the full spectrum of sediment gravity flow types including less ordered debris flows has been active in an island arc environment a range of sediment gravity flow types can be initiated some by pyroclastic flows entering the sea however the thin beds of accretionary lapilli do not exhibit features of sediment gravity flow deposits or those of submarine pyroclastic flows possible transport processes must account for the matrix between the ash aggregates which is either coarse grained or absent modelling of particle descent times to 1600 m through a sea water column provides one explanation for the features displayed

tourmaline mineralization in the barberton greenstone belt south africa early archean metasomatism by evaporite derived boron tourmaline rich rocks are common in the lowgrade interior portions of the barberton greenstone belt of south africa where shallow marine sediments and underlying altered basaltic and komatiitic lavas contain up to 50 tourmaline the presence of tourmaline bearing rip up clasts intraformational tourmalinite pebbles and tourmaline coated grains indicates that boron mineralization was a low temperature surficial process the association of these lithologies with stromatolites evaporites and shallow water sedimentary structures and the virtual absence of tourmaline in correlative deep water facies rocks in the greenstone bels strengthens this model

subaqueous intermediate to silicic composition explosive volcanism a review abstract mass flow deposits of pyroclastic debris commonly consist of two parts a lower coarse grained basal section and an upper finer grained bedded section subaqueous pyroclastic flow deposits particularly those derived from a vertically directed eruption appear better sorted than their subaerial counterparts because of the efficient sorting characteristics of water compared to air subaqueous deposits frequently are depleted in bubble wall pumice that floats off and vitric ash that remains in suspension hence the deposits show a relative enrichment of crystals especially at the bases from 1 flotation and suspension of the vitric material and 2 ingestion of water at the head of the flow a subaqueous volcaniclastic flow deposit may contain evidence indicative of both turbulent and non turbulent movement the flow may have had a turbulent head and non turbulent body or flow transformations may have occurred from 1 collapse of the eruption column 2 changes in slope and in the velocity of the flow and or 3 the amount of water mixed into the flow retention of heat in the subaqueous environment may be enhanced by lateral eruptions high eruption rates large eruptive volumes and little or no mixing of pyroclastic debris with water the lack of welded subaqueous deposits in the geologic record may be a result of 1 effusive rather than explosive eruptions in a deep water environment 2 explosive phreatic disintegration of pyroclastic flows in a shallow subaqueous environment and or 3 the inability of subaerial pumiceous pyroclastic flows to penetrate the land water interface characteristics of explosive intermediate to silicic composition subaqueous volcanic eruptions also have been summarized in terms of 1 environment of the eruption 2 orientation of the eruption and 3 composition of the juvenile debris these three aspects of explosive subaqueous volcanism are ideal end members and gradations no doubt occur

geochemical investigation of salt water intrusion into a coastal carbonate aquifer mallorca spain geochemical processes occurring within the mixing zone on mallorca spain were investigated in relation to the diagenesis of carbonate minerals and to the development of porosity and permeability within the pleistocene limestone aquifer rock core and ground water samples were obtained from the fresh water zone the mixing zone and the top of the sea water zone at two sites near the coast ground water withdrawals in the vicinity of one study site depressed the water table to below sea level resulting in sea water encroachment and a thin mixing zone laboratory tests conducted on the rock core indicated that porosity and hydraulic conductivity were not enhanced within the transitory location of the present day mixing zone vuggy zones in the core correspond to the approximate location of the water table because the ground water was supersaturated with respect to the carbonate minerals dissolution of the limestone aquifer is no predicted for the present day mixing zone results of mass balance and source rock calculations indicated an excess of calcium and strontium in the mixing zone waters in relation to what was expected from conservative mixing these elevated concentrations are hypothesized to result from limestone dissolution in the vadose zone

trace element geochemistry of groundwater in a karst subterranean estuary yucatan peninsula mexico ndseg nsf chemical oceanography program oce 0751525 coastal ocean institute ocean and climate change institute at woods hole oceanographic institution

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

geochemical tracing and hydrogeochemical modelling of water rock interactions during salinization of alluvial groundwater upper rhine valley france in the southern upper rhine valley groundwater has undergone intensive saline pollution caused by the infiltration of mining brines a consequence of potash extraction carried out during the 20th century major and trace elements along with sr and u isotopic ratios show that groundwater geochemical characteristics along the saline plumes cannot reflect conservative mixing between saline waters resulting from the dissolution of waste heaps and one or more unpolluted end members the results imply the occurrence of interactions between host rocks and polluted waters and they suggest that cationic exchange mechanisms are the primary controlling process a coupled hydrogeochemical model has been developed with the numerical code kirmat which demonstrates that cationic exchange between alkalis from polluted waters and alkaline earth elements from montmorillonite present in the host rock of the aquifer is the primary process controlling the geochemical evolution of the groundwater the model requires only a small amount of montmorillonite between 0 75 and 2 25 which is in agreement with the observed mineralogical composition of the aquifer the model also proves that a small contribution of calcite precipitation dissolution takes places whereas other secondary mineral precipitation or host rock mineral dissolution do not play a significant role in the geochemical signature of the studied groundwater samples application of the model demonstrates that it is necessary to consider the pollution history to explain the important cl na and ca concentration modifications in groundwater samples taken over 2 km downstream of waste heaps additionally the model shows that the rapidity of the cationic exchange reactions insures a reversibility of the cation fixation on clays in the aquifer

stratigraphic and sedimentological evidence bearing on structural repetition in early archean rocks of the barberton greenstone belt south africa abstract the 3 5 3 3 ga old swaziland supergroup in the barberton greenstone belt south africa includes a lower predominantly volcanic unit the onverwacht group and an upper largely sedimentary section the fig tree and moodies groups although viljoen and viljoen concluded that the onverwacht group exceeds 15 km in stratigraphic thicknesses more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting our mapping indicates that the upper half of the group includes at least 8 km of unrepeated stratigraphic section inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the onverwacht group on the west limb of the onverwacht anticline shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform highly complex zone of deformation up to 2 km wide the displacement across the zone is unknown but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the group has not been significantly altered structures previously mapped as major fold hinges within the upper part of the onverwacht group can be shown to have formed by local deformation and reflect neither large scale folding nor structural repetition of the rocks several kilometers higher in the section rocks of the uppermost onverwacht and fig tree groups are stacked and repeated in a series of thrust nappes individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks they were apparently emplaced by faulting or gravity sliding at or near the earth s surface their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying much weaker serpentinized ultramafic komatiites locally the sheets were deformed to form fold nappes the lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact the thrust faults are the loci of small hypabyssal igneous bodies that appear to be co magmatic with a suite of upper fig tree volcanic rocks this style of faulting has not affected rocks of the overlying moodies group although estimates of the timing of deformation are as yet imprecise these features suggest that it occurred during a late fig tree phase of tectonism and volcanism

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

evolution of porosity in the saltwater freshwater mixing zone of coastal carbonate aquifers an alternative modelling approach summary dissolution of calcium carbonate in the saltwater freshwater mixing zone in carbonate islands and coastal aquifers up to now has been treated by coupling geochemical equilibrium programmes to a reactive transport model the result is a complex nonlinearly coupled set of differential transport advection equations which need high computational efforts if dissolution rates of calcite are sufficiently fast such that one can assume the solution to be in equilibrium with respect to calcite a highly simplified modelling approach can be used by use of the concept of phillips in his book flow and reactions in permeable rock cambridge university press new york 1991 and its more general formulation by de simoni et al de simoni m carrera j sanchez vila x guadagnini a 2005 a procedure for the solution of multicomponent reactive transport problems water resource research 41 w11410 it is possible to decouple the complex set of equations to calculate initial changes of porosity in the rock matrix one needs solely to solve the advection transport equation for salinity s current codes on density driven flow such as seawat can be used to obtain the dissolution capacity of the mixed saltwater freshwater solution the calcium equilibrium concentration c eq s is obtained as a function of salinity by phreeqc 2 initial porosity changes can then be calculated by a simple analytical expression of the gradient of the spatial distribution s x y of salinity the distribution of flow fluxes q x y and the second derivative of the calcium equilibrium concentration c eq s with respect to salinity s this approach does no longer require large dispersivities on the order of meters but reasonable results are obtained with dispersivity on the scale of the pore size of the porous rock which correctly describes mixing on the molecular scale the method is applied to a simple 1 d domain without convective flow and a constant gradient of salinity as presented by rezaei et al rezaei m sanz e raeisi e ayora c vazquez sune e carrera j 2005 reactive transport modeling of calcite dissolution in the fresh saltwater mixing zone journal of hydrology 311 1 4 282 298 our results exhibit a sharper dissolution zone close to the freshwater side of the domain which is caused by the properties of the second derivative of c eq s recently singurindy et al water resources research 40 2004 and correction in water resources research 41 2005 performed dissolution experiments in a saltwater freshwater mixing zone in a sandbox the results from our model simulations are in good agreement to their experimental data finally the modelling approach is employed to porosity evolution in homogeneous and heterogeneous carbonate islands the results are in general agreement to earlier work due to the properties of the second derivative however the regions of active dissolution are restricted to more narrow regions

skeletal aragonite dissolution from hypersaline seawater a hypothesis abstract hypersaline seawater has often been invoked as a mechanism to explain the pervasive dolomitization of ancient platform carbonates but its potential in causing skeletal aragonite dissolution of these carbonates has rarely been investigated previous experimental and theoretical studies have demonstrated that hypersaline seawater is undersaturated with respect to aragonite when evaporation reaches a certain degree it is contended here that similar undersaturation could also have occurred in ancient evaporitic seas geological evidence from the miocene carbonates of se spain gulf of suez red sea and iraq suggests that this may have been the case despite differences in their geological settings these carbonates have common diagenetic features including 1 widespread dissolution of skeletal aragonite with little or no calcite cementation and 2 pervasive dolomitization dolomite occurs as both a replacement of mg calcite and mouldic pore filling cement the association of the dolomites with evaporites their relatively heavy oxygen isotopic values and widespread distribution suggest a hypersaline seawater origin of these dolomites petrographic data indicate that skeletal aragonite was dissolved during dolomitization because the aragonite fossil moulds contain dolomite cements but no pre dolomitization calcite cements implying that the dolomitizing fluids hypersaline seawater were probably undersaturated with respect to aragonite the dissolved calcium may have been subsequently incorporated into the dolomite this may help to explain the lack of calcite cementation despite the extensive skeletal aragonite dissolution

heterogeneity induced mixing and reaction hot spots facilitate karst propagation in coastal aquifers the data provided in the included folders are results directly taken from simulations originating from comsol multiphysics all data are provided as text files each text file includes coordinate data and the raw values for the variable of interest presented in the manuscript and supporting information the files provided are separated into folders for homogeneous equivalent cases and heterogeneous cases for each of these folders the data is further subdivided by the variance of the hydraulic conductivity field finally the data is organiyed according to the output variable of interest

early archaean spherule beds in the barberton mountain land south africa no evidence for impact origin abstract detailed petrographical and geochemical studies are reported for several well documented occurrences of spherule rich layers in the barberton mountain land south africa which were previously inferred by lowe and co workers to be the result of early archaean meteorite impacts in contrast we made the following observations the textures of most spherules are not necessarily the result of impact but could instead be the product of radial or intersertal growth of crystals during secondary mineral formation there is no difference in the content of siderophile elements between spherule layers and layers devoid of spherules the siderophile element abundances are high where sulphide minerals e g pyrite gersdorffite and chalcopyrite and or chromite are present independent of the presence or absence of spherules high contents of e g ir up to 2700 ppb ni 0 96 wt and cr 1 6 wt were found in various samples abundances of these elements in chondritic meteorites are approximately 600 ppb 1 4 wt and 0 35 wt respectively resulting in respective meteoritic components in these samples of 450 70 and 460 we do not accept these high concentrations as primary meteoritic signatures impact melt rocks have typically 1 of a meteoritic component furthermore spherules of any kind are very rare in known impact deposits including the k t boundary if present at all they are not usually associated with any significant ir or pge anomaly the high abundances of the siderophile elements in some barberton samples and their enrichment in secondary minerals indicate that these elements have been remobilized and reconcentrated the pge interelement ratios have changed during remobilization as well thus the pge abundance patterns and ratios are not primary and cannot be used as an argument in favour of an impact origin either nickel rich cr spinels which are found in some of the barberton spherule samples have low fe 3 fe total ratios these ratios are incompatible with an extraterrestrial or impact origin because such impact derived spinels including those found at the k t boundary are highly oxidized and also have different chemical compositions there is also no indication of any evidence of shock metamorphism associated with the barberton spherule layers which is the commonly accepted definitive criterion for recognition of an impact origin this is unusual because such evidence is preserved even in heavily altered samples from deeply eroded archaean impact structures we conclude that while these spherule layers are clearly unusual and deserve further attention there is no convincing evidence for an origin by impact we suggest that they were formed by volcanic processes followed by extensive hydrothermal alteration

how can we improve a global ocean tide model at a regional scale a test on the yellow sea and the east china sea a global ocean model has been developed within the context of topex poseidon t p on the basis of a finite element hydrodynamic modeling approach combined with data assimilation based on the representer method the solution produced at global scale represents a spectacular improvement over what was available before the era of t p typically the mean discrepancy between the main tidal components and a hundred in situ tide gauges is 1 7 cm for m2 and 1 cm or less for the other components however the accuracies of this solution and of that produced by different authors within the t p tide working group are all worse near coastlines and over continental shelves this is the case for our finite element solution fes fes94 1 solution over the yellow sea and the east china sea ys ecs where the discrepancy is 33 cm for the m2 tide and 9 cm for the k1 tide when compared to a set of 192 tide gauges distributed along the coastlines this is due largely to the complex geometry of the basin and the limited knowledge of the bathymetry the aim of this paper is to investigate how our fes model can be improved with a dedicated application focused on one of the most energetic coastal basins the ys ecs 180 gigawatts for m2 i e 8 of the global energy dissipated in the whole ocean for this application a finite grid was implemented with a resolution down to 5 km along the coasts and over the continental shelf break particular attention was paid to bathymetry by complementing the etop05 database with regional maps sensitivity tests to the tuning of the bottom friction and the nonlinear interactions between the diurnal and semidiurnal components allow us to investigate the impact of dissipation parameterization and to produce an optimal solution without any data assimilation for nine main components of the tidal spectrum improvement by a factor of 2 m2 distance to the observations is now 17 5 cm global variance of 92 cm and k1 is 4 cm global variance of 21 cm as nonlinear components are significant over these basins the two main quarterdiurnal components are calculated the improvements are explained by five points 1 the choice of a mixed friction 2 a refinement of the mesh 3 a choice of reliable boundary conditions 4 a refined topography and 5 the use of a specific friction coefficient 1 5 10 3 surprisingly the energy budget for the m2 component leads to a dissipation similar to the value estimated by the global fes94 1 this is a major result of this study which leads us to the conclusion that when forcing a regional model with sea level boundary conditions along the open limits the simulated velocity field adjusts to the tuning of friction coefficients in order to dissipate the same amount of energy made available at its open boundaries

carbonate dissolution in a modern mixing zone the potential for dissolution of calcite and aragonite and precipitation of dolomite by the mixing of fresh and saline ground waters has been demonstrated theoretically1 3 although geologists have used these processes in the explanation of the diagenesis of recent4 7 and ancient8 12 carbonates there has been only limited investigation of the geochemical processes of modern mixing zones13 14 here we report the results of chemical and rock profiling through a modern mixing zone on andros island in the bahamas our results demonstrate undersaturation of the waters in the mixing zone with respect to aragonite and calcite and corresponding pervasive dissolution of the carbonate rocks at this level undersaturation is significantly enhanced by bacterial processes which are also responsible for precipitation of an iron rich crust despite chemical supersaturation precipitation of dolomite was not observed thereby casting some doubt on the mixing zone model of dolomitization

carbonate platform stratal geometries and the question of subaerial exposure abstract integrated petrographic geochemical and fluid inclusion analyses indicate that the most prominent exposure unconformity at the southern margin of the vercors carbonate platform in southeastern france does not coincide with the major break in depositional geometry therefore changes in sediment supply and dispersal strongly influenced the resulting stratal geometries and prevent direct reconstructions of sea level from the stratal patterns platform to basin transects exposed in the cirque d archiane contain two prograding grainstone tongues separated by a wedge of finer grained slope sediments that thin toward the platform diagenetic analyses have been done at several strategic platform top bedding planes to determine the extent to which these sedimentologic breaks represent subaerial exposure while many bedding planes contain some evidence of meteoric alteration one of these bedding surfaces is unique in that it exhibits several generations of overprinted meteoric calcite cements bladed to blocky morphology precipitation in biomolds extinct to zoned cathodoluminescence low mg and 18 o variable 13 c freshwater fluid inclusions that are cross cut by marine borings eroded dolomitized clasts derived from this exposure surface occur in the uppermost portion of the finer grained wedge this combined evidence indicates that a significant episode of subaerial exposure took place at this horizon prior to deposition of the overlying platform top sediments the most significant change in depositional style and therefore the most likely position for a sequence boundary defined by stratal geometry is the top of the lower grainstone tongue however the intensely altered exposure surface and its basinward equivalent in the wedge lie 50 m above the top of the lower grainstone tongue therefore the surface containing the most extensive record of meteoric diagenesis does not stratigraphically coincide with the surface representing the most significant lateral shift in depositional facies

spatio temporal variability of tidal asymmetry due to multiple coastal constructions along the west coast of korea abstract at least 19 remarkable dikes and land reclamations have been constructed since 1970 along the west coast of korea which resulted in a reduction in tidal flat area of almost 50 both the reduction in tidal flats and the artificially simplified coastal line have distorted the spatio temporal tidal hydrodynamics to quantify this we analyzed and evaluated tidal asymmetry by phase differences of the principal semi diurnal lunar constituent m 2 and its first over tide m 4 moreover we applied the adcirc model to quantitatively investigate near and far field impacts on tidal variations using the gamma parameter tidal energy flux and dissipation rate through this study we found that the tidal regime around the incheon harbor area in gyeonggi bay has changed from ebb to flood dominant due to multiple nearby reclamations in particular to the construction of the siwha dike the saemangeum dike caused near field de amplification of m 2 but far field amplification in the shandong area of china in addition to allowing a traditional asymmetry approach using phase difference analyses based on the gamma parameter and tidal energy variations could distinctly improve spatial understanding of anthropogenic impacts on coastal tidal hydrodynamics

geology and tectonostratigraphy of the onverwacht suite barberton greenstone belt south africa abstract a synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern barberton greenstone belt into seven units with different geological histories current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained we argue here that the status of the traditional formations of the onverwacht group of the barberton greenstone belt be changed to complexes and collectively these complexes be referred to as the onverwacht suite the total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km respectively the precise age range of the rocks present in each complex is largely unknown the original spatial relationships between the complexes can in most cases only be inferred all of the complexes include significant volumes of intrusions that were emplaced into the volcanic and volcaniclastic host rocks the ages of the intrusions in the different complexes vary in some complexes the intrusions and volcanic rocks have similar geochemistry and are probably co magmatic others provide stitching ages between complexes four of the complexes display internal relationships that resemble ophiolites basaltic pillowed and massive lavas are the most common rock types sheeted dykes are generally absent whilst multiple mafic sills are common and dominate one of the complexes the volcanology and geochemistry of the complexes suggest that they formed predominantly in oceanic spreading and intra oceanic arc environments at water depths of 1 7 4 km the complexes were tectonically stacked during the final stages of their development and during two subsequent orogenic events one complex the hooggenoeg complex was uplifted and deeply eroded between about 3460 and 3470 ma with a minimum uplift rate ca 0 5 mm yr similar to that during emplacement of phanerozoic ophiolites unconformably overlying sub aerial coarse clastic sedimentary rocks part of the newly distinguished noisy complex overlap in age with felsic intermediate igneous activity dated between 3445 and 3460 ma that in turn is genetically related to tonalite trondjemite plutons exposed farther to the south the emergence of the deep water hooggenoeg complex above sea level the start of its erosion and fluvial sedimentation on top of it at ca 3458 ma represents the onset of its obduction onto older complexes some of which are now preserved as enclaves in the granitoid terrains to the south other oceanic slabs the kromberg and the mendon complexes were subsequently emplaced onto the noisy complex during latitudinal displacements ca 12 cm yr that were not notably faster than today exhumation with up to 18 km of unroofing along the southern margin of the greenstone belt took place during a second orogenic episode of subduction and accretion between 3140 and 3230 ma this episode was associated with folding thrusting and molasse type sedimentation within the greenstone belt coupled to extension and ductile faulting at a lower crustal level during this period younger complexes were assembled and the older complexes re cycled and eroded at rates and with geotherms comparable to those in modern orogenic belts in the barberton region between 3 2 and 3 5 ga tectonic processes appear to have operated at rates similar to those within modern evolving plate boundaries

geochemistry of meteoric calcite cements in some pleistocene limestones in this study the stable isotope and trace element geochemistries of meteoric cements in pleistocene limestones from enewetak atoll western pacific ocean cat island bahamas and yucatan were characterized to help interpret similar cements in ancient rocks meteoric calcite cements have a narrow range of 18o values and a broad range of 13c values in each geographical province these pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location enewetak calcite cements have a mean 18o composition of 6 5 0 pdb and 13c values ranging from 9 6 to 0 4 0 pdb sparry calcite cements from cat island have a mean 18o composition of 4 1 0 and 13c values ranging from 6 3 to 1 1 0 sparry cements from yucatan have a mean 18o composition of 5 7 0 and 13c values of 8 0 to 2 7 0 the mean 18o values of these pleistocene meteoric calcite cements vary by 2 4 0 due to climatic variations not related directly to latitude the 13c compositions of meteoric cements are distinctly lower than those of the depositional sediments variations in 13c are not simply a function of distance below an exposure surface meteoric phreatic cements often have 13c compositions of less than 4 0 0 which suggests that soil derived co2 and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements concentrations of strontium and magnesium are quite variable within and between the three geographical provinces mean strontium concentrations for sparry calcite cements are for enewetak atoll 620 ppm 510 ppm for cat island 1200 ppm 980 ppm and for yucatan 700 ppm 390 ppm equant cements intraskeletal cements and bahamian cements have higher mean strontium concentrations than other cements equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements magnesium concentrations in pleistocene meteoric cements are similar in samples from enewetak atoll mean 1 00 mol mgco3 0 60 mol mgco3 and cat island mean 0 84 mol mgco3 0 52mol mgco3 but yucatan samples have higher magnesium concentrations mean 2 20 mol mgco3 0 84mol mgco3 higher magnesium concentrations in some yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water

a study of non linear tidal propagation in shallow inlet estuarine systems part i observations the offshore tide becomes strongly distorted as it propagates into shallow estuarine systems observations of sea surface elevation and horizontal currents over periods ranging from three days to one year at nine stations within nauset inlet estuary document the non linear interaction of the off shore equilibrium tidal constituents despite strong frictional attenuation within the estuary the overtides and compound tides of m2 s2 and n2 in particular reach significant amplitude resulting in strong tidal distortion high frequency forced constituents in sea surface are phase locked consistently leading the forcing tides by 60 70 resulting in a persistent distortion where falling tide is longer than rising tide forced constituents in currents are more nearly in phase with equilibrium constituents producing flood currents which are shorter but more intense than ebb currents a compound fortnightly tide msf modulates the mean water level such that lowest tides occur during neap phase instead of spring phase this fortnightly tide can be contaminated by storm surge changing the phase characteristics of this constituent implications of the overtides compound tides and lower frequency tides on near bed suspended and dissolved material transport are profound

hydrogeology of grand cayman british west indies a karstic dolostone aquifer abstract on grand cayman freshwater bodies present in the bluff formation are typically small and occur as thin lenses floating on top of dense saline water evaluation of the water resource potential of these freshwater lenses is difficult because of their variable hydrological conditions complex paleohydrogeology and aquifer heterogeneity secondary porosity created by preferential dissolution of aragonitic fossil components is common open fissures and joints developed under tectonic stress and karst development associated with sea level fluctuations are however the two most important causes of porosity and permeability in the aquifers on grand cayman fracture and karst porosity control the lens occurrence by 1 acting as avenues for the intrusion of seawater or upward migration of saline water 2 acting as recharge focal points 3 enhancing hydrodynamic dispersion 4 defining lens geometry 5 facilitating carbonate dissolution along joints and fissures a clear understanding of the hydrological and geological conditions is important in developing small lenses in a setting similar to that on grand cayman this pragmatic approach can help identify the optimum location of the well field and avoid areas particularly susceptible to saline water intrusion

sr concentrations and isotope ratios as tracers of ground water circulation in carbonate platforms examples from san salvador island and long island bahamas abstract the depth to which seawater and fresh water circulate through modern carbonate platforms may be estimated with 87 sr 86 sr isotope ratios of dissolved sr 2 that is enriched through carbonate mineral dissolution and recrystallization in 23 water samples from onshore san salvador island and long island bahamas carbonate mineral dissolution and aragonite to calcite transformations elevate sr 2 concentrations to twice seawater values in water with near seawater salinity and to about 130 times the expected value for seawater that has been mixed with fresh water carbonate mineral dissolution enriches ca 2 concentrations to around 30 times seawater concentrations only in the mixed waters water with seawater salinity has approximately seawater ca 2 concentrations assuming two end member mixing between seawater sr 2 and mineral derived sr 2 model estimates indicate that mineral derived sr 2 of 19 samples have 87 sr 86 sr ratios equivalent to modern seawater within error of the measurement indicating alteration of shallow buried late pleistocene to holocene carbonate minerals four samples have mineral derived sr 2 with 87 sr 86 sr ratios lower than modern seawater value these low ratios reflect alteration of carbonate minerals that were deposited around 1 mybp although the measured 87 sr 86 sr values could reflect ages as great as 4 6 ma considering the analytical uncertainty of the measurements these estimated ages are likely to be minimum values because alteration of modern carbonate minerals at the surface would provide an unknown but probably large amount of sr 2 with modern seawater isotope signatures thereby overprinting any low 87 sr 86 sr ratios of non modern mineral derived sr 2 three of the four samples with low 87 sr 86 sr ratios have high salinities and were collected from the interior of the islands they reflect seawater flow paths at least tens of meters deep may link the ocean to water several kilometers inland the fourth sample is from the fresh water lens below a pleistocene beach ridge 125 ka only 100 m from the shore line this sample suggests the lens may be thicker than expected based on estimates of recharge hydraulic conductivity and size of the ridge

vadose co2 gas drives dissolution at water tables in eogenetic karst aquifers more than mixing dissolution most models of cave formation in limestone that remains near its depositional environment and has not been deeply buried i e eogenetic limestone invoke dissolution from mixing of waters that have different ionic strengths or have equilibrated with calcite at different pco2 values in eogenetic karst aquifers lacking saline water mixing of vadose and phreatic waters is thought to form caves we show here calcite dissolution in a cave in eogenetic limestone occurred due to increases in vadose co2 gas concentrations and subsequent dissolution of co2 into groundwater not by mixing dissolution we collected high resolution time series measurements 1 year of specific conductivity spc temperature meteorological data and synoptic water chemical composition from a water table cave in central florida briar cave we found spc pco2 and calcite undersaturation increased through late summer when briar cave experienced little ventilation by outside air and decreased through winter when increased ventilation lowered cave co2 g concentrations we hypothesize dissolution occurred when water flowed from aquifer regions with low pco2 into the cave which had elevated pco2 elevated pco2 would be promoted by fractures connecting the soil to the water table simple geochemical models demonstrate that changes in pco2 of less than 1 along flow paths are an order of magnitude more efficient at dissolving limestone than mixing of vadose and phreatic water we conclude that spatially or temporally variable vadose co2 g concentrations are responsible for cave formation because mixing is too slow to generate observed cave sizes in the time available for formation while this study emphasized dissolution gas exchange between the atmosphere and karst aquifer vadose zones that is facilitated by conduits likely exerts important controls on other geochemical processes in limestone critical zones by transporting oxygen deep into vadose zones creating redox boundaries that would not exist in the absence of caves copyright 2014 john wiley sons ltd