wave induced advective transport below a rippled water sediment interface the exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements in this paper we examine quantitatively the effects of modulations in the profile of the water sediment interface on this exchange and on the advective transport inside the sediment bed the flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using darcy s law the forcing is due to progressive gravity waves in the water above the results of two different models for the pressure variation imposed on the upper boundary are compared the two pressure profiles are derived from potential flow theory and from a numerical solution to the navier stokes equations for the oscillatory flow over a rippled bed from an analytic solution to the two dimensional model the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed the shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface the total exchange across the water sediment interface averaged over one wave period is significantly higher across a rippled interface than across a flat bed this difference increases with increasing ripple slope and the strength of the wave motion and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave since rippled bed forms are commonly found in coastal waters the increase in the total exchange across a rippled water sediment boundary can enhance the exchange of solutes due to wave pumping immediately below the water sediment interface circulation cells with net advective transport over a wave period are found such net advection patterns can lead to spatial in the horizontal direction inhomogeneities of the vertical concentration or temperature profiles if the overlying water column and or the sediment bed act s as source or sink this gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements

stratigraphy diagenesis and geological evolution of the paleoproterozoic roraima basin guyana links to tectonic events on the amazon craton and assessment for uranium mineralization potential abstract the paleoproterozoic roraima supergroup is a siliciclastic succession exposed in guyana venezuela and brazil that filled the extensive roraima basin on the amazon craton sequence stratigraphy mineral geochemistry and geochronology are integrated to understand the evolution of the basin within the tectonic framework of the amazon craton and fluid flow events to evaluate its uranium resource potential three depositional sequences comprise the roraima supergroup in guyana these are bounded by laterally extensive sequence boundaries marking both fluvial incision and source area uplift followed by aggradation and basin subsidence the roraima supergroup experienced at least three major fluid events following deposition associated with 1 early burial 1820 ma 2 metasomatism during intrusion of the avanavero suite mafic sills 1799 9 ma to 1779 10 ma and 3 late burial diagenesis 1756 5 ma to 1723 10 ma the events are identified based on petrography 207pb 206pb and 40ar 39ar ages of fluorapatite and white mica and the isotopic compositions of h c o and sr in white mica and calcite despite having multiple characteristics indicating that it could host unconformity related u deposits the roraima basin suffered extensive permeability loss during early burial and then with avanavero suite intrusion as a result the probability that high grade unconformity related u deposits formed in the study area is unlikely

daytime rock surface temperature variability and its implications for mechanical rock weathering tenerife canary islands summary summer and winter rock surface temperature measurements are presented from three sites along an altitudinal gradient significant between and within site variability is recognised related to insolation cloud cover and air temperature differences maritime influences and time of year the variability argues against the use of a single rock temperature regime to characterise hot desertic environments in for example weathering simulation studies superimposed upon diurnal patterns of heating and cooling are numerous short 3 15 minutes and medium term 1 2 hours fluctuations related to wind speed and cloud cover variations rates of surface temperature change are very high during these fluctuations 3 c minute and it is proposed that they may enhance the operation of a number of mechanical rock weathering mechanisms

mixing of heterogeneities in the mantle effect of viscosity differences the effect of viscosity variations associated with large scale chemical heterogeneities on mixing in the mantle is studied for a model two dimensional problem low viscosity regions are rapidly deformed develop long tendrils and are entrained towards regions of surface divergence very viscous regions are only slowly stretched thus geochemical reservoirs can persist relatively undisturbed for long periods of time if they are o 10 100 times more viscous than the surrounding mantle high viscosity ratio blobs have a tendency to aggregate leading to the formation of large scale heterogeneities from smaller ones

selective weathering of cross bedded layers forming shelters and small caves on akopan tepui venezuela field laboratory and experimental evidence about diagenesis and weathering of the mataui formation arenites roraima supergroup middle proterozoic abstract south american table top mountains tepuis are formed by quartzites and sandstones of mataui formation belonging to the roraima supergroup lower to middle proterozoic field observations in the northern part of akopan tepui in venezuela revealed that unlike the beds with flat bedding the cross bedded layers are commonly affected by selective weathering expressed by depressions shelters and even small caves all the arenites are fine grained most of them are likely of aeolian origin hardness measurements confirmed that the cross bedded layers are softer than the flat bedded or structureless beds occurring above and below them petrographic analysis showed that the rocks are mostly fine grained quartzites to quartz sandstones with original sand grains overgrown by syntaxial quartz cement and bearing signs of compaction manifested by local pressure solution some intergranular spaces are filled by newly formed phyllosilicates paragonite locally intergrown with muscovite in the overlying beds no porosity was observed by microscopic study and under sem whereas in the underlying rocks signs of slight weathering and etching of phyllosilicates was observed together with locally widened spaces along the grain to grain boundaries the samples from cross bedded layers display weathering the degree of which depends on the present access of flowing water where water has periodical access to the arenite it shows dissolution of quartz cement arenization together with complete leaching and removal of the phyllosilicates due to lateritization the intergranular spaces are considerably widened and quartz grains bear signs of strong skeletal etching and pitting samples from the sites where the water access ceased show mostly leaching of phyllosilicates with weak skeletal dissolution of quartz grains without pitting to imitate the origin of structures observed in the field experimental modelling was performed using artificially layered and compacted fine grained sands on which sodium silicate solution was dripped during soaking downward preferential penetration of the solution along the cross bedding laminae funnel flow was observed in the cross bedded layer the resulting structures display good lithification of horizontally layered sands and relatively poorer lithification of the cross bedded sediment between them showing triangular shaped overhangs and shelters which mimic those on the akopan tepui differential weathering of otherwise petrologically very similar arenites indicates that there had to be a difference in the initial porosity in the time of the rock denudation a preserved porosity in the cross bedded layers was necessary for the water rock contact to perform dissolution

a hybrid model to evaluate subsurface chemical weathering and fracture karstification in quartz sandstone abstract a hybrid numerical finite differences and semi analytical empirical model has been developed to evaluate the spatial and time scale of chemical weathering along a fracture in quartz sandstones the model is based on the diffusion transport occurring from the bedrock mass toward the fracture walls driven by the sio2 concentration gradient between water in the bedrock pores high silica content and water flowing in the fracture low silica content because of molecular diffusion intergranular water becomes undersaturated with respect to silica this promotes dissolution of quartz at grain boundaries and results in a porosity profile decreasing from the fracture walls toward the rock interior bedrock individual grains are released and gradually removed by the water flowing into the fracture when a critical value of porosity is reached grain release threshold grt this process drastically increases the fracture enlargement rate the model outcomes establish a minimum time of 90 ka to reach the grt this time is independent of length aperture and dip of the fracture but is controlled by parameters such as the initial fracture aperture water temperature quartz grain size and initial bedrock porosity it is attested that the water flowing in the fracture remains undersaturated with respect to silica over very long timescales in the order of 105 years and over very long flow paths in turn this suggests that the extremely slow reaction between quartz and water is the key factor for the formation of subterranean karst like features in quartz sandstone otherwise silica saturation would be reached after short distances and deep weathering in this lithology would be prevented finally the model outcomes were compared to field data from the gran sabana caves venezuela and other quartz sandstone weathering landforms elsewhere showing that dissolution diffusion in the rock matrix is a reliable mechanism for the formation of these peculiar karst like features

the diffusive tortuosity of fine grained unlithified sediments abstract use of fick s first law to estimate solute fluxes is arguably the most common and important calculation related to the geochemistry and environmental chemistry of fine grained unlithified sediments in such porous media the diffusion coefficient in this law must be reduced by the square of the tortuosity to account for the longer diffusive paths experimental methods to estimate tortuosity are specialized or time consuming and not routinely done an analysis of available data shows that tortuosity is accurately correlated to readily measured porosity values through the simple relation 2 1 l n 2

influence of sea level rise on iron diagenesis in an east florida subterranean estuary abstract subterranean estuary occupies the transition zone between hypoxic fresh groundwater and oxic seawater and between terrestrial and marine sediment deposits consequently we hypothesize in a subterranean estuary biogeochemical reactions of fe respond to submarine groundwater discharge sgd and sea level rise porewater and sediment samples were collected across a 30 m wide freshwater discharge zone of the indian river lagoon florida usa subterranean estuary and at a site 250 m offshore porewater fe concentrations range from 0 5 m at the shoreline and 250 m offshore to about 286 m at the freshwater saltwater boundary sediment sulfur and porewater sulfide maxima occur in near surface oc rich black sediments of marine origin and dissolved fe maxima occur in underlying oc poor orange sediments of terrestrial origin freshwater sgd flow rates decrease offshore from around 1 to 0 1 cm day while bioirrigation exchange deepens with distance from about 10 cm at the shoreline to about 40 cm at the freshwater saltwater boundary doc concentrations increase from around 75 m at the shoreline to as much as 700 m at the freshwater saltwater boundary as a result of labile marine carbon inputs from marine sgd this labile doc reduces fe oxides which in conjunction with slow discharge of sgd at the boundary allows dissolved fe to accumulate upward advection of fresh sgd carries dissolved fe from the fe oxide reduction zone to the sulfate reduction zone where dissolved fe precipitates as fe sulfides saturation models of fe sulfides indicate some fractions of these fe sulfides get dissolved near the sediment water interface where bioirrigation exchanges oxic surface water the estimated dissolved fe flux is approximately 0 84 m fe day per meter of shoreline to lagoon surface waters accelerated sea level rise predictions are thus likely to increase the fe flux to surface waters and local primary productivity particularly along coastlines where groundwater discharges through sediments

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

reactive transport modeling of iron diagenesis and associated organic carbon remineralization in a florida usa subterranean estuary article iron oxides are important terminal electron acceptors for organic carbon oc remineralization in subterranean estuaries particularly where oxygen and nitrate concentrations are low in indian river lagoon florida usa terrestrial fe oxides dissolve at the seaward edge of the seepage face and flow upward into overlying marine sediments where they precipitate as fe sulfides the dissolved fe concentrations vary by over three orders of magnitude but fe oxide dissolution rates are similar across the 25 m wide seepage face averaging around 0 21 mg cm 2 yr the constant dissolution rate but differing concentrations indicate fe dissolution is controlled by a combination of increasing lability of dissolved organic carbon doc and slower porewater flow velocities with distance offshore in contrast the average rate constants of fe sulfide precipitation decrease from 21 9 10 8 s 1 to 0 64 10 8 s 1 from the shoreline to the seaward edge of the seepage face as more oxygenated surface water circulates through the sediment the amount of oc

weathering of sandstone by the combined action of frost and salt preliminary results from a continuing series of laboratory experiments designed to examine the combined effects of salt and frost weathering indicate that some salts greatly enhance the breakdown of rocks by frost samples of ardingly sandstone from southeast england were soaked in saturated solutions of sodium chloride or sodium sulphate and subjected to alternating cycles of freezing and thawing rapid disintegration of the sandstone occurred within twenty cycles in each of the salt solutions in contrast samples subjected to freezing and thawing in deionized water suffered very little damage unless they were saturated under vacuum when samples were subjected to twenty cycles of wetting and drying at temperatures above 0 c those soaked in deionised water or sodium chloride suffered no appreciable damage but those soaked in sodium sulphate rapidly disintegrated tentative explanations of these results are offered the enhancement of frost weathering by salt appears to have been overlooked by many writers yet it is likely to be an important geomorphological process in those areas of mid and high latitudes where rocks are saturated with sodium salts

the role of microorganisms and biofilms in the breakdown and dissolution of quartz and glass three different types of silica 1 quartz sand 2 crystalline scepter quartz and 3 commercial glass subjected to biological growth were investigated for evidence of biologically induced breakdown the results of laboratory experiments with biofilms on quartz sand and glass were compared with material from the field microscopic optical and sem analysis of quartz sand weathered in vitro by a microbial community of cyanobacteria diatoms and heterotrophic bacteria demonstrate grain diminution of a sand fraction of north sea sediment the microbial community changes the minerals in the vicinity of the living cells which leave their mark on the quartz surface in form of imprints depressions and pits subsequently microbial growth on the surface brought about a general decrease of the grain size in the layer beneath a biofilm imprints of diatoms on glass surfaces colonized by diatoms and heterotrophic bacteria confirm the chemical etching activity of these organisms mixed cultures of diatoms and bacteria show depressions corresponding to the shape of individual cells our experiments confirm that diatoms heterotrophic bacteria and cyanobacteria from natural biofilms can actively attack quartz and glass microscopic analysis of an idiomorphic scepter quartz crystal from a tepui weathering environment reveals that the associated biofilms can create a local shift in the ph from 3 4 ph of water on the tepui to evidently higher than 9 necessary for quartz dissolution the quartz covered with a biofilm is partially perforated to a depth of more than 4 mm we conclude that biofilm growth in marine sub aquatic and terrestrial sub aerial conditions can significantly increase the breakdown of silica in the amorphous glass sub crystalline chert crystalline and granular forms of quartz microbial growth may therefore substantially modify the processes of transformation of a major rock forming mineral of great chemical and physical resistance

a global review of solutional weathering forms on quartz sandstones abstract solutional landforms in limestone have been described for over a hundred years but landforms of similar morphology on highly siliceous sandstones and quartzites have also been identified in a wide variety of environments and generally termed pseudokarst these include large bedrock pinnacles and towers caves corridors grikes solution basins and runnels and even silica speleothems quartzites and quartz sandstones have been held to be amongst the most chemically resistant of rocks but the similarity both in morphology and genetic process of many landforms developed from them to features of known solutional origin on limestone has prompted some authors to refer to these quartzose landforms as true karst the most detailed studies of quartzose karst landforms have been in present day tropical regions or areas believed to have been tropical in the geologically recent past this concentration of research in hot wet areas allied with the long held assertion of the insolubility of silica especially quartz has led to a belief that tropical climatic conditions are necessary for karstic solution of these rocks however the existence of quartzose karst landforms in temperate and even sub polar latitudes especially where there is no evidence of prior tropical conditions suggests that the requirement of tropical weathering is no longer tenable the reports of these quartzose solutional landforms are widely scattered through the geomorphological and geological literature but a comprehensive world wide review of the range of solutional landforms on quartzose rocks has not previously been published because of the increasing awareness in this karst type such a summary is sorely overdue

a multi level pore water sampler for permeable sediments abstract the construction and operation of a multi level piezometer multisampler designed to collect pore water from permeable sediments up to 230 cm below the sediment water interface is described multisamplers are constructed from 1 inch schedule 80 pvc pipe one quarter inch flexible pvc tubing leads from eight ports at variable depths to a 1 inch tee fitting at the top of the pvc pipe multisamplers are driven into the sediment using standard fence post drivers water is pumped from the pvc tubing with a peristaltic pump field tests in banana river lagoon florida demonstrate the utility of multisamplers these tests include collection of multiple samples from the permeable sediments and reveal mixing between shallow pore water and overlying lagoon water

funneled flow mechanisms in layered soil field investigations abstract the movement of water and potential pollutants in the vadose zone of fluvial deposits is often difficult to predict because fine over coarse layers may behave as capillary barriers funneling water and dissolved solutes into concentrated preferential flow paths capillary barriers have been studied in laboratory experiments and by mathematical analysis with well defined boundaries but little is known about water and solute movement in naturally layered soils this paper demonstrates how naturally occurring capillary barriers affect water and solute movement experiments were carried out in a river valley near amherst ma and on a prehistoric beach near cornell university in ithaca ny dye tracer and chloride were initially applied near the soil surface followed by intermittent rainfall over several weeks after 2 6 weeks the blue dye soil water content and chloride concentration distributions in the soil profile were examined by excavating a trench and photographing and sampling the exposed soil face at both sites the infiltrating water was diverted and flowed laterally over a relatively coarse layer at the cornell site flow broke through the fine coarse interface into the coarse layer at a few points along the barrier the primary breakthrough at the cornell site occurred at a horizontal section of the interface 2 m down slope from the point of dye application the capillary diversions generally agreed with current theory however the flow patterns were difficult to predict accurately without more detailed information about the soil layering characteristics

pathways of organic carbon oxidation in three continental margin sediments we have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of denmark and norway rates of nh4 and sigma co2 liberation sediment incubations were used with o2 penetration depths to conclude that o2 respiration accounted for only between 3 6 17 4 of the total organic carbon oxidation dentrification was limited to a narrow zone just below the depth of o2 penetration and was not a major carbon oxidation pathway the processes of fe reduction mn reduction and sulfate reduction dominated organic carbon mineralization but their relative significance varied depending on the sediment where high concentrations of mn oxide were found 3 4 wt mn only mn reduction occurred with lower mn oxide concentrations more typical of coastal sediments fe reduction and sulfate reduction were most important and of a similar magnitude overall most of the measured o2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon we suspect that the importance of o2 respiration in many coastal sediments has been overestimated whereas metal oxide reduction both fe and mn reduction has probably been well underestimated

an updated global review of solutional weathering processes and forms in quartz sandstones and quartzites abstract quartz is considered one of the less soluble minerals of the earth s crust and thus hardly affected by chemical weathering despite this for more than forty years it is clear that the formation of caves and peculiar solutional weathering dominated landforms in quartz rich lithologies is common and shares several similarities to the well known karstic ones in carbonate rocks in the last thirty years great strides have been made in furthering our knowledge of the distribution of these forms around the world and the geochemical processes involved these studies have clearly shown that solutional weathering is a fundamental process acting through intergranular dissolution of quartz increasing the rock porosity and decreasing the rock strength to erosion this process has been described in the concepts of arenization and phantomization and the widespread evidences of the fundamental role of quartz solution in landform genesis has even developed to the extent of several geomorphologists reassessing the definition of the term karst and its application to these peculiar lithologies nonetheless the process is complicated by several factors related both to environmental conditions water chemistry and availability as well as to the compositional and textural characters of the lithology presence of clays iron hydroxides carbonate cement etc all these aspects have to be taken carefully in consideration in order to understand if solution is a dominant or accessory process in the landscape evolution in this review the state of knowledge on the relevant chemical processes weathering mechanisms and speleogenesisk involved in the surface and underground karstification and clear examples of quartz solution and solutional landforms from different world locations are outlined and discussed

reply to the comment on sandstone caves on venezuelan tepuis return to pseudokarst abstract in aubrecht et al 2011 we brought new data showing that the arenitic caves in the venezuelan tepuis did not necessarily originate through quartz cement dissolution arenization but the main portion of the caves originated because of poor lithification prior to erosion an additional important process is lateritization which is responsible for at least 30 of the caves volume sauro et al 2012 in their comment support the arenization theory quartz cement dissolution and provide several arguments against some points of our research their main objections and arguments relate to the following topics 1 the validity of the arenization theory 2 the origin and importance of the finger flow pillars 3 the importance of the schmidt hammer measurements and 4 metamorphism of the mataui formation in our reply we present further documentation that 1 the quartz cement dissolution theory of cave evolution was far less documented petrographically than the new theory presented by us although the presence of quartz dissolution is evident there is no current evidence that it is widespread or that it plays a trigger role many of the presented examples of corrosion on quartz grains could have been caused by local alkalization in addition the hydrogeochemical data presented in the comment to support the arenization theory do not in fact contradict our speleogenetic interpretations 2 finger flow pillars are speleogenetic indicators and not speleogenetic factors their absence in some caves provides no evidence against our theory 3 schmidt hammer measurements only objectivize the data on contrasting hardness in the mataui formation beds they do not say anything about the origin of this difference 4 the arguments of sauro et al 2012 involve the metamorphic overprint of the mataui formation shown by the presence of pyrophyllite and quartz mobilizations because quartz mobilization also occurs in hypergenic conditions it may be neglected as a metamorphic indicator pyrophyllite is mentioned as a metamorphic indicator in the literature anchizone to epizone but it does not explain why there is still a vast quantity of kaolinite that did not undergo this reaction in the soft arenitic beds there is an evidence that pyrophyllite may originate at lower temperatures some phases of the alumino silicate mineral phases existing at atmospheric pressure and 25 c temperature show that h 4 sio 4 concentration is a critical factor in kaolinite pyrophyllite transition an increase in this concentration can theoretically cause transformation of kaolinite to pyrophyllite without increased temperature or pressure

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

sandstone caves on venezuelan tepuis return to pseudokarst abstract venezuelan table mountains tepuis host the largest arenite caves in the world the most frequently used explanation of their origin so far was the arenization theory involving dissolution of quartz cement around the sand grains and subsequent removing of the released grains by water new research in the two largest arenite cave systems churi tepui system in chimanta massif and ojos de cristal system in roraima tepui showed that quartz dissolution plays only a minor role in their speleogenesis arenites forming the tepuis are not only quartzites but they display a wide range of lithification and breakdown including also loose sands and sandstones speleogenetic processes are mostly concentrated on the beds of unlithified sands which escaped from diagenesis by being sealed by the surrounding perfectly lithified quartzites only the so called finger flow pillars testify to confined diagenetic fluids which flowed in narrow channels leaving the surrounding arenite uncemented another factor which influenced the cave forming processes by about 30 was lateritization it affects beds formed of arkosic sandstones and greywackes which show strong dissolution of micas feldspars and clay minerals turning then to laterite barro rojo the main prerequisite to rank caves among karst phenomena is dissolution as the dissolution of silicate minerals other than quartz appears to play not only a volumetrically important role but even a trigger role these arenitic caves may be ranked as karst

small scale modelling of cementation by descending silica bearing fluids explanation of the origin of arenitic caves in south american tepuis abstract geoscientific research was performed on south american table mountains tepuis and in their sandstone cave systems to explain speleogenesis in these poorly soluble rocks two theories were introduced a arenization theory implying selective weathering of quartz along grain boundaries and releasing of sand grains b selective lithification theory implying cementation by descending silica bearing fluid flow the latter theory presumes that the descending fluid flow becomes unstable on the interface between two layers with different porosity and splits to separate flow channels so called finger flow the arenites outside these channels remain uncemented to verify the latter theory small scale modelling was performed using layered sands and sodium silicate solution fine to medium sand was used 0 08 0 5 mm along with a coarse sand fraction 0 5 1 5 mm the sands were layered and compacted in a transparent plastic boxes three liters of sodium silicate solution so called water glass were left to drip for several hours to the top of the sediment the fine grained layers were perfectly laterally impregnated whereas the descending fluid flows split to fingers in the coarse grained layers due their higher hydraulic conductivity this small scale laboratory simulation mimics the real diagenesis by descending silica bearing fluids and matches the real phenomena observed on the tepuis the resulting cemented constructions closely mimic many geomorphological features observed on tepuis and inside their caves e g finger flow pillars overhangs imperfectly formed aborted pillars in forms of hummocks hanging from ceilings locally also thicker central pillars that originated by merging of smaller fluid flow channels the modelling showed that selective lithification theory can explain most of the geomorphological aspects related to the speleogenesis in tepuis

organic matter mineralization in sediment of a coastal freshwater lake and response to salinization solid phase and pore water chemical data collected in a sediment of the haringvliet lake are interpreted using a multi component reactive transport model this freshwater lake which was formed as the result of a river impoundment along the southwestern coast of the netherlands is currently targeted for restoration of estuarine conditions the model is used to assess the present day biogeochemical dynamics in the sediment and to forecast possible changes in organic carbon mineralization pathways and associated redox reactions upon salinization of the bottom waters model results indicate that oxic degradation 55 denitrification 21 and sulfate reduction 17 are currently the main organic carbon degradation pathways in the upper 30 cm of sediment unlike in many other freshwater sediments methanogenesis is a relatively minor carbon mineralization pathway 5 because of significant supply of soluble electron acceptors from the well mixed bottom waters although ascorbate reducible fe iii mineral phases are present throughout the upper 30 cm of sediment the contribution of dissimilatory iron reduction to overall sediment metabolism is negligible sensitivity analyses show that bioirrigation and bioturbation are important processes controlling the distribution of organic carbon degradation over the different pathways model simulations indicate that sulfate reduction would rapidly suppress methanogenesis upon seawater intrusion in the haringvliet and could lead to significant changes in the sediment s solid state iron speciation the changes in fe speciation would take place on time scales of 20 100 years

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

structural and lithological guidance on speleogenesis in quartz sandstone evidence of the arenisation process a detailed petrographic structural and morphometric investigation of different types of caves carved in the quartz sandstones of the tepui table mountains in venezuela has allowed identification of the main speleogenetic factors guiding cave pattern development and the formation of particular features commonly found in these caves such as funnel shaped pillars pendants and floor bumps samples of fresh and weathered quartz sandstone of the mataui formation roraima supergroup were characterised through wds dispersive x ray chemical analyses picnometer measurements edax analyses sem and thin section microscopy in all the caves two compositionally different strata were identified almost pure quartz sandstones with content of silica over 95 and high primary porosity around 4 and phyllosilicate rich quartz sandstone with contents of aluminium over 10 and low primary porosity lower than 0 5 phyllosilicates are mainly pyrophyllite and kaolinite sem images on weathered samples showed clear evidence of dissolution on quartz grains to different degrees of development depending on the alteration state of the samples grain boundary dissolution increases the rock porosity and gradually releases the quartz grains suggesting that arenisation is a widespread and effective weathering process in these caves the primary porosity and the degree of fracturing of the quartz sandstone beds are the main factors controlling the intensity and distribution of the arenisation process weathering along iron hydroxide or silt layers which represent inception horizons or a strata bounded fracture network predisposes the formation of horizontal caves in specific stratigraphic positions the loose sands produced by arenisation are removed by piping processes gradually creating anastomosing open fracture systems and forming braided mazes geometric networks or main conduit patterns depending on the local lithological and structural guidance on the weathering process this study demonstrates that all the typical morphologies documented in these quartz sandstone caves can be explained as a result of arenisation which is guided by layers with particular petrographic characteristics primary porosity content of phyllosilicates and iron hydroxides and different degrees of fracturing strata bounded fractures or continuous dilational joints

genesis of giant sinkholes and caves in the quartz sandstone of sarisarinama tepui venezuela abstract quartz sandstone of the sarisarinama massif in venezuela hosts the world biggest collapse dolines in quartz rich lithologies with volumes up to some millions of cubic meters due to extremely complex logistics required to reach the massif the genesis of these depressions and of the underlying caves has never been studied in detail the lack of field campaigns and extended data has fostered a decade long scientific debate on whether their origin was due to epigenic or hypogenic processes this study integrates petrological structural and hydrochemical observations including analyses of silica concentration ph conductivity of surface and cave waters ec to investigate the speleogenetic processes acting underground petrographic and compositional analyses of the host rock mataui formation show that in the sarisarinama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides compared to surface waters subsurface infiltration along vertical fractures and fault planes show enrichment in silica higher ph and lower ec confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities the weathering of the clay and iron hydroxide interlayers guides the speleogenesis weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata the initial void created by piping of the loose sand released by quartz sandstone weathering can migrate upwards by means of roof and wall breakdown this chain of events eventually triggers a collapse at the surface which generates a circular or squared sinkhole the weathering acts mainly along the dominant fracture networks showing a clear guidance by regional tectonics these speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas suggesting a primary role of long term epigenic chemical weathering and mechanical erosion guided by joints weak clay and iron hydroxide interlayers followed by subsequent massive collapses

sandstone weathering a century of research and innovation abstract a review of sandstone weathering research particularly in the past 100 years reveals a trajectory of enquiry from early description and classification of features to development of process based explanations to decreasing scales of investigation and a disparity between understanding of process es and explanations of the genesis of sandstone weathering features developments in expositions on mesoscale weathering features on sandstone surfaces are discussed demonstrating a range of approaches to weathering phenomena field based and laboratory based that must be linked to provide an explanation of observed features on a landform scale throughout the twentieth century a thematic chronology highlights certain trends in research description of forms often in arid and semi arid environments single process form models an emphasis on experimentation difficulties in measuring weathering rates and a persistent emphasis on physical causes of breakdown a new research agenda is promoted in which biodeterioration and chemical processes gain parity a holistic approach based on conceptual modeling of weathering systems gains prominence and scale issues are addressed more rigorously

funneled flow mechanisms in a sloping layered soil laboratory investigation artificial capillary barriers are being used to divert water away from sensitive underground regions conversely funneled flow over natural capillary barriers may increase the danger of groundwater contamination by decreasing the travel time and contact area there have been relatively few experimental studies of capillary barrier flow patterns in this study water was applied uniformly across the top surface of a backlit tilting chamber 1 cm thick 110 cm high and 180 cm long in which a coarse sand layer was imbedded in a fine sand bedding slope and water application rates were varied between 0 and 12 and 1 and 3 cm h 1 respectively after attaining steady state matric potential was measured along the textural interface and photos of dye traces were taken in order to visualize streamlines the funneled flow was characterized by three discrete regions an initial capillary diversion a breakthrough region and a toe diversion the breakthrough region consisted of a significant zone of partial breakthrough where the vertical flux into the coarse layer was less than the water application rate the lateral distance of the capillary diversion was explained well by previously published relationships when the water entry value at the textural interface was replaced by lower observed matric potential at which breakthrough occurred at the most upslope point the length of the capillary diversion was overpredicted using the air entry value finally the toe of the coarse layer had significant observed effects on funneled flow patterns which have previously received little if any attention the results of this study imply that the slope of the coarse layer and infiltration rate will largely govern the effectiveness of capillary barriers and that capillary barriers are less effective than previously assumed

an emp and tem aem study of margarite muscovite and paragonite in polymetamorphic metabauxites of naxos cyclades greece and the implications of fine scale mica interlayering and multiple mica generations