sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

constancy of silicate mineral weathering rate ratios between natural and experimental weathering implications for hydrologic control of differences in absolute rates abstract inverse models which apportion watershed effluxes geochemical mass balance over estimated mineral surface area give weathering rates for individual silicate minerals e g feldspar 1 3 orders of magnitude slower than laboratory rates comparisons of rates calculated from several recent watershed geochemical mass balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data thus the coefficient which corrects the weathering rate for any mineral from a laboratory data set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets the correction factor linking experimental and natural rates must be independent of composition physical e g hydrologic controls rather than compositional or chemical controls cause the difference between weathering rates in nature and the laboratory because flow in natural weathering profiles is spatially heterogeneous not all of the potentially available surface in natural systems actually participates in reactions with pore fluids

anthropogenic impacts on mineral weathering a statistical perspective abstract correspondence analysis was adopted as tool for investigating the statistical structure of hydrochemical and weathering datasets of groundwater samples with the main purpose of identifying impacts on mineral weathering caused by anthropogenic activities namely fertilizing of farmlands the hydrochemical dataset comprised measured concentrations of major inorganic compounds dissolved in groundwater namely bicarbonate silica usually by products of chemical weathering chloride sulphate and nitrate typically atmospheric plus anthropogenic inputs the weathering dataset consisted of calculated mass transfers of minerals being dissolved in loess sediments of a region located in sw hungary szigetvar area namely na plagioclase calcite and dolomite and of pollution related concentrations of sodium magnesium and calcium a first run of correspondence analysis described groundwater composition in the study area as a system of triple influence where spots of domestic effluents dominated chemistries are surrounded by areas with agriculture dominated chemistries both imprinted over large regions of weathering dominated chemistries a second run revealed that nitrification of n fertilizers is promoting mineral weathering by the nitric acid reaction anthropogenic pathway in concurrence with the retreating of weathering by carbonic acid natural pathway it also indicated that dolomite and calcite are being players in a dedolomitization process driven by dissolution of gypsum fertilizers and nitrification of n fertilizers

putting transport into water rock interaction models new quantitative models for water rock interaction are needed that explicitly couple fluid flow and chemical reactions one such model is based on the finite difference method and is applicable to both one and two dimensional reactive flow in applying the model we find that ignoring the coupling between hydrodynamic dispersion and chemical reactions leads to incorrect predictions of the rates of reactions along fluid flow paths and in cases of which reactions will occur

role of hydraulic diffusivity in the decrease of weathering rates over time summary springs emerging within massifs of crystalline rocks were monitored for discharge rate q and the q values combined with geomorphic and hydrographic parameters in a hydrologic model to calculate hydraulic conductivity k and effective porosity n e of the spring watersheds the spring waters several borehole waters and rain water were analyzed for major dissolved compounds strontium and isotopes 18 o 2 h 13 c and 87 sr 86 sr with a shift to less negative values 18 o and 2 h were fitted by a line approximately parallel to the gmwl but no significant dependence on altitude was found the 18 o and 2h values correlate better with those of precipitation amount the 87 sr 86 sr ratios in drilled well waters correlate positively with the depth of water circulation reported in the borehole logs the corresponding regression equations were used to extrapolate the depth of hydraulic circuits within the spring watersheds the previous data together with groundwater travel times calculated by a water balance model and with reactions of granite metassediment plagioclase and biotite precipitating halloysite gibbsite and vermiculite were assembled in a mass balance model to calculate solute flux weathering rates of plagioclase w pl the w pl s were described as a function of d k n e where d is the hydraulic diffusivity the discrepancies between the w pl values and solid state rates based on the differences between elemental isotopic and mineral compositions measured in present day regoliths and in the assumed protolith were assigned to a decrease in d over time from values in the protolith to values in the weathered aquifer

determining mineral weathering rates based on solid and solute weathering gradients and velocities application to biotite weathering in saprolites abstract chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths an increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change the solid state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration these relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients contemporary weathering rates based on solute residence times can be directly compared to long term past weathering based on changes in regolith composition both rates incorporate identical parameters describing mineral abundance stoichiometry and surface area weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the piedmont of northern georgia usa and in luquillo mountains of puerto rico solid state weathering gradients for mg and k at panola produced reaction rates of 3 to 6 10 17 mol m 2 s 1 for biotite faster weathering rates of 1 8 to 3 6 10 16 mol m 2 s 1 are calculated based on mg and k pore water gradients in the rio icacos regolith the relative rates are in agreement with a warmer and wetter tropical climate in puerto rico both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering

modelling weathering processes at the catchment scale the witch numerical model a numerical model of chemical weathering in soil horizons and underlying bedrock witch has been coupled to a numerical model of water and carbon cycles in forest ecosystems aspects to simulate the concentration of major species within the soil horizons and the stream of the strengbach granitic watershed located in the vosges mountains france for the first time simulations of solute concentrations in soil layers and in the catchment river have been performed on a seasonal basis the model is able to reproduce the concentrations of most major species within the soil horizons as well as catching the first order seasonal fluctuations of aqueous calcium magnesium and silica concentrations however the witch model underestimates concentrations of mg2 and silica at the spring of the catchment stream and significantly underestimates ca2 concentration the deficit in calculated calcium can be compensated for by dissolution of trace apatite disseminated in the bedrock however the resulting increased ca2 release yields important smectite precipitation in the deepest model layer in contact with the bedrock and subsequent removal of large amount of silica and magnesium from solution in contrast the model accurately accounts for the concentrations of major species ca mg and silica measured in the catchment stream when precipitation of clay minerals is not allowed the model underestimation of mg2 and h4sio4 concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is overestimated and that more soluble poorly crystallized and amorphous materials may be forming in agreement with observations on other watersheds draining granitic rocks this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the strengbach watershed

on the description of the basin effective drainage structure abstract the relationships between the local properties of the morphological elements of a drainage basin and its drainage structure are analysed two theoretical approaches are used for which hydro dynamics and basin morphology are respectively assumed to represent the lowest and the highest starting scale level the results show a convergence toward the idea of slope scaling with contributing area in river networks this concept is then used as an indicator of channel initiation and maintenance for the identification of the effective drainage structure of the basin on the basis of existing digital elevation models dems integral properties of the single dem pixel are finally introduced to reduce the noise owing to a possible dem low accuracy e g in the estimation of the local slope value from the elevation data set the resulting network is characterized by a non uniform drainage density as expected for natural river networks

hydraulic head response of a confined aquifer influenced by river stage fluctuations and mechanical loading summary the response to river stage fluctuation of a drilled well penetrating a confined aquifer was simulated using a stream aquifer interaction algorithm because the confined aquifer is overlaid by a water table aquifer the algorithm was coupled with formulae used to correct the heads for mechanical loading the coupling of stream aquifer interaction and mechanical loading models was tried for the first time in this study the test site was a drilled well installed on the montalto uffugo aquifer located at the calabria university groundwater test field calabria region south of italy this aquifer comprises a 44 m thick sand bank bounded on bottom and top by clay layers and covered by a 7 m thick sandy conglomerate being adjacent to the mavigliano river overall the head changes caused by a river stage raise represented a contribution of 49 3 57 8 to the total head while mechanical loading accounted for the remaining 50 7 32 2 the loading was triggered by a sequence of short spaced rainfall events lasting for a total of 167 days which caused recharge to the unconfined aquifer thickening the water column by some 3 1 m

chemical weathering in granitic environments abstract factors controlling chemical weathering in granitic environments are deduced using an extensive database from the literature based on a transition state theory model we evaluated the effects on chemical weathering of runoff temperature and ph the dependence between temperature and runoff and chemical weathering is complicated by other factors such as the presence depth of soil cover soil cover favors chemical weathering when it is thin and rich in weatherable minerals in contrast it reduces chemical weathering when is thick and poor in primary minerals processes that increase the contact time and the surface of the water rock interactions such as physical denudation increase chemical weathering rates in granitic environments the effect of temperature can be quantified assuming an apparent activation energy value of 48 7 kj mol to describe chemical weathering of silica in granitic crystalline environment under high runoff conditions

on the use of the boussinesq equation for interpreting recession hydrographs from sloping aquifers 1 the method of recession analysis proposed by brutsaert and nieber 1977 remains one of the few analytical tools for estimating aquifer hydraulic parameters at the field scale and beyond in the method the recession hydrograph is examined as dq dt f q where q is aquifer discharge and f is an arbitrary function the observed function f is parameterized through analytical solutions to the one dimensional boussinesq equation for unconfined flow in a homogeneous and horizontal aquifer while attractive in its simplicity as originally presented it is not applicable to settings where slope is an important driver of flow or where hydraulic parameters vary greatly with depth we compare analytical solutions to the linearized one dimensional boussinesq equation for a sloping aquifer to numerical solutions of the full nonlinear equation the behavior of the nonlinear boussinesq equation is also assessed when the aquifer is heterogeneous wherein the lateral saturated hydraulic conductivity k varies as a power law with height z above the impermeable layer k zn n constant 0 all of the analytical solutions differ in key aspects from the nonlinear solution when plotted as dq dt f q and thus are inappropriate for a brutsaert and nieber type analysis however new analytical solutions for a sloping aquifer are derived empirically from the numerical simulations that are applicable during the late period of recession when the recession curve converges to dq dt aqb where b 2n 1 n 1 and a is a function of the dimensions and hydraulic properties of the aquifer

is high obliquity a plausible cause for neoproterozoic glaciations 1 the main features of the low latitude neoproterozoic glaciations remain the subject of controversial debates concerning possible triggers here we use an agcm coupled with a slab ocean to test one of the earliest and simplest explanation for tropical glaciations a higher obliquity of the earth s rotation axis we show that high obliquity may result in an extensive glaciation during the sturtian episode 750 ma due to the location of continental masses in the tropical areas but cannot produce a large glaciation in the case of mid to high latitude paleogeographies such as those thought to typify the varangian vendian episodes 620 580 ma

the impact of climate change human interference scale and modeling uncertainties on the estimation of aquifer properties and river flow components summary within the period 1978 2006 climate change and human interferences produced noticeable impacts on the hydrology of a small watershed known as the beca river basin climate change was characterized by a persistent raise in temperature 0 78 c decade 1 and a drop in the annual rainfall 300 mm decade 1 human interferences included the construction of a dam for electric power generation in 1998 and since then the transference of beca river flows from the dam lake to the adjacent t mega river the impacts on catchment hydrology comprised a decline of aquifer hydraulic conductivity and effective porosity by approximately one order of magnitude related to a water table lowering of about 17 m within the bedrock aquifer composed of weathered and fractured hercynian granites and paleozoic metassediments and of saprolite layers derived therefrom aquifer property estimates were compared across spatial scales namely the beca river and the nested sub basins scale sub basin aquifers are more porous and permeable than the basin aquifer because corresponding hydraulic circuits are shallower comparisons were also made between aquifer properties derived from measured and simulated stream flows which revealed effects of modeling uncertainties on the results river flows also suffered a substantial decrease in the course of climate change and human interference especially the overland flows 4 5 decrease and the base flows 2 3 decrease the inter flows were less affected 1 3 decrease because they were partly fed with water from the aquifer storage which in turn underwent depletion the hydrologic changes in the beca river basin anticipate important impacts on the local use of natural water in this context the aforementioned water table lowering may have caused limited access to shallow groundwater for activities such as crop irrigation from dug wells whereas the severe decline in overland flows and base flows had certainly reduced the availability of surface water for the refilling of dam lakes and of groundwater for the supply of public and private boreholes

modeling rock weathering in small watersheds summary many mountainous watersheds are conceived as aquifer media where multiple groundwater flow systems have developed toth 1963 and as bimodal landscapes where differential weathering of bare and soil mantled rock has occurred wahrhaftig 1965 the results of a weathering algorithm pacheco and van der weijden 2012a 2014 which integrates topographic hydrologic rock structure and chemical data to calculate weathering rates at the watershed scale validated the conceptual models in the river sordo basin a small watershed located in the marao cordillera north of portugal the coupling of weathering groundwater flow and landscape evolution analyses as accomplished in this study is innovative and represents a remarkable achievement towards regionalization of rock weathering at the watershed scale the river sordo basin occupies an area of approximately 51 2 km 2 and was shaped on granite and metassediment terrains between the altitudes 185 1300 m the groundwater flow system is composed of recharge areas located at elevations 700 m identified on the basis of 18 o data discharge cells comprehend terminations of local intermediate and regional flow systems identified on the basis of spring density patterns infiltration depth estimates based on 87 sr 86 sr data and spatial distributions of groundwater ph and natural mineralization intermediate and regional flow systems defined where infiltration depths 125 m develop solely along the contact zone between granites and metassediments because fractures in this region are profound and their density is very large weathering is accelerated where rocks are covered by thick soils being five times faster relative to sectors of the basin where rocks are covered by thin soils differential weathering of bare and soil mantled rock is also revealed by the spatial distribution of calculated aquifer hydraulic diffusivities and groundwater travel times

contributions of water rock interactions to the composition of groundwater in areas with a sizeable anthropogenic input a case study of the waters of the fundao area central portugal we used a combination of a grouping algorithm and a weathering algorithm to assess the contributions made by chemical weathering and anthropogenic inputs to the groundwater composition in a granitoid area the first algorithm is based on the mathematical concept of equivalent relations between objects and is used to find groups of water samples using the grouping algorithm we identified groups with similar chemistries in a set of data relating to the water composition in 160 springs and wells in the fundao area portugal the second algorithm is based on stoichiometries mass and charge balances in weathering reactions and is used to relate the water composition of each identified group to water mineral interactions in the area background information on the petrology and mineralogy of the area allowed us to choose the most realistic water mineral interactions we also had information about the use and composition of fertilizers in this agricultural area in applying the weathering algorithm we considered all dissolved silica and bicarbonate to be produced by chemical weathering and all dissolved chloride sulphate and nitrate to be derived from other sources mainly from fertilizers the anthropogenic contributions to the water chemistries in the area are high despite the high background concentrations derived from pollution we were able to assess the contribution of weathering to the water chemistries we obtained realistic results with the two algorithms supported by the fact that the spatial distribution of the samples belonging to the various groups shows a good correlation with the geology and petrology of the area

drying front in a sloping aquifer nonlinear effects 1 the profiles for the water table height h x t in a shallow sloping aquifer are reexamined with a solution of the nonlinear boussinesq equation we demonstrate that the previous anomaly first reported by brutsaert 1994 that the point at which the water table h first becomes zero at x l at time t t c remains fixed at this point for all times t t c is actually a result of the linearization of the boussinesq equation and not as previously suggested brutsaert 1994 verhoest and troch 2000 a result of the dupuit assumption rather by examination of the nonlinear boussinesq equation the drying front i e the point x f at which h is zero for times t greater than or equal to t c actually recedes downslope as physically expected this points out that the linear boussinesq equation should be used carefully when a zero depth is obtained as the concept of an average depth loses meaning at that time

integrating topography hydrology and rock structure in weathering rate models of spring watersheds weathering rate models designed for watersheds combine chemical data of discharging waters with morphologic and hydrologic parameters of the catchments at the spring watershed scale evaluation of morphologic parameters is subjective due to difficulties in conceiving the catchment geometry besides when springs emerge from crystalline massifs rock structure must be accounted in formulas describing the area of minerals exposed to the percolating fluids for a realistic evaluation of the rates these particular features are not included in the available approaches and for that reason a new model was developed coined throw model this is a lumped approach that integrates t opography h ydrology ro ck structure and w eathering in a single algorithm the study area comprises several stream watersheds and spring sites of the vouga river basin northern portugal shaped on granites firstly the throw model couples a terrain modeling analysis with hydrologic models based on discharge rates to determine hydraulic conductivities k effective porosities ne and annual recharges vr at the stream watershed scale subsequently these parameters are used in a water balance model to estimate concomitant groundwater travel times t the mean k 4 7 3 2 10 7 m s 1 and ne 2 0 1 3 10 2 values are adopted as proxies for the spring watersheds and a firm regression equation is defined between time and stream watershed area a secondly two more runs of terrain modeling analysis are executed to extrapolate morphologic parameters for the spring watersheds the first run hinges on scaling properties of the drainage networks known as horton laws and is used to scale watershed areas across stream orders i the scaling function is described by another regression equation the second run evaluates the order of a spring watershed defined as equivalent order ieq and equated to the mean order of the surrounding stream watersheds having calculated the ieq spring watershed areas and travel times were downscaled using the regression equations a 10 km2 and t 1 4 2 8 year standing on the physical and hydrologic parameters of the spring watersheds the throw model finally calculates plagioclase weathering rates in the vicinity of the spring sites the sib model pacheco and van der weijden 1996 was used before to estimate the contribution of plagioclase dissolution to the chemical composition of these springs van der weijden and pacheco 2006 the chemical data were now coupled with k ne and t in a rate equation to estimate chemical weathering rates of plagioclase in the basin in the throw model the rate equation describes the exposed surface area as a function of fracture spacings openings and porosities pacheco and alencoao 2006 the calculated rates wpl 2 5 1 2 10 14 mol m 2 s 1 are consistent with previous reports and with results of experimental kinetic models the sib results predict formation of halloysite and gibbsite along the flow path which were indeed close to equilibrium with the dissolved al and si activities o develop a weathering model that incorporates rock structure in the rate equation o conceive a weathering model especially designed for fracture artesian springs o create a model that integrates topography hydrology rock structure and weathering

modelling the chemical weathering fluxes at the watershed scale in the tropics mule hole south india relative contribution of the smectite kaolinite assemblage versus primary minerals we investigate the chemical weathering processes and fluxes in a small experimental watershed sew through a modelling approach the study site is the mule hole sew developed on a gneissic basement located in the climatic gradient of the western ghats south india the model couples a lumped hydrological model simulating the water budget at the watershed scale to the witch model estimating the dissolution precipitation rates of minerals using laboratory kinetic laws forcing functions and parameters of the simulation are defined by the field data the coupled model is calibrated with stream and groundwater compositions through the testing of a large range of smectite solubility and abundance in the soil horizons we found that despite the low abundance of smectite in the dominant soil type of the watershed 4 vol their net dissolution provides 75 of the export of dissolved silica while primary silicate mineral dissolution releases only 15 of this flux overall smectites modelled as montmorillonites are not stable under the present day climatic conditions furthermore the dissolution of trace carbonates in the saprolitic horizon provides 50 of the calcium export at the watershed scale modelling results show the contrasted behavior of the two main soil types of the watershed red soils 88 of the surface are provider of calcium while black soils smectite rich and characterized by a lower drainage consumes calcium through overall carbonate precipitation our model results stress the key role played by minor accessory minerals and by the thermodynamic properties of smectite minerals and by the drainage of the weathering profiles on the weathering budget of a tropical watershed c 2010 elsevier b v all rights reserved

ground surface slope as a basin scale parameter a simple and relatively rapid method of determining the characteristic or typical slope of a basin is proposed the expression for the parameter which is given by savg l hsu hso2 a cos was developed from considerations of basin geometry and basic geomorphologic facts and involves only quantities that can be easily extracted from topographic maps characteristic basin slope values of 19 appalachian plateau watersheds which range in size from 7 4 to 178 2 km2 were determined by the proposed method these values were compared with corresponding values obtained by horton s intersection line method and with those derived from county soil survey reports and maps the comparisons showed very close agreement between the three sets of values indicating the high accuracy of the method developed here similar comparisons involving values determined by previous slope methods indicated that these values are at best poorly related to actual average basin slope and that most of them are incapable of detecting small differences in slope between similar watersheds moreover the proposed method significantly reduces the amount of work required by other slope methods of comparable accuracy

phase relations of tourmaline leucogranites and the significance of tourmaline in silicic magmas by means of melting and recrystallization tourmaline seeded experiments phase relations of two natural tourmaline bearing fe rich members of the schorl dravite series leucogranites have been determined between 1 and 3 kbar under h2o saturation with fo2 close to ni nio the presence of tourmaline slightly lowers the solidus temperatures of both samples from 5 to 20 c compared to the corresponding tourmaline free solidus quartz k feldspar and plagioclase phase relations are not significantly dependent on the presence of tourmaline tourmaline is the last major phase to disappear for both samples and a demonstrated liquidus phase exists for one in the recrystallization experiments the compositions of the tourmalines growing from the melt change from more mg rich at high temperatures to more fe rich at lower temperatures the tourmalines are always richer in mg than the melt is but fe and mg are not especially partitioned between crystallizing tourmaline and melt melt compositions do not depart

drainage of a horizontal boussinesq aquifer with a power law hydraulic conductivity profile 1 solutions to the boussinesq equation describing drainage into a fully penetrating channel have been used for aquifer characterization two analytical solutions exist for early and late time drainage from a saturated homogeneous and horizontal aquifer following instantaneous drawdown the solutions for discharge q can be expressed as dq dt aq b where a is constant and b takes on the value 3 and 3 2 for early and late time respectively though many factors can contribute to departures from the two predictions we explore the effect of having permeability decrease with depth as it is known that many natural soils exhibit this characteristic we derive a new set of analytical solutions to the boussinesq equation for k oc z n where k is the saturated hydraulic conductivity z is the height above an impermeable base and n is a constant the solutions reveal that in early time b retains the value of 3 regardless of the value of n while in late time b ranges from 3 2 to 2 as n varies from 0 to oo similar to discharge water table height h in late time can be expressed as dh dt ch d where d 2 for constant k and d oo as n in theory inclusion of a power law k profile does not complicate aquifer parameter estimation because n can be solved for when fitting b to the late time data whereas previously b was assumed to be 3 2 however if either early or late time data are missing there is an additional unknown under appropriate conditions water table height measurements can be used to solve for an unknown parameter

hydrogeochemistry in the vouga river basin central portugal pollution and chemical weathering abstract to quantify and explain the contributions by pollution and chemical weathering to their composition we studied the chemistries of springs and surface waters in the mountainous part of the vouga river basin water samples were collected during a number of consecutive summer campaigns recharge rates were derived from monitored discharge rates within the basin very large contributions by meteoric agricultural and domestic sources to the water chemistries were found identified by the chloride sulfate and nitrate concentrations on average only 1 4 to 1 3 of the solutes could be attributed to chemical weathering two petrologic units characterize the river basin granites and metasediments the waters collected within metasediment units are distinct from those in granite terrain by a higher magnesium concentration on that basis it could be estimated that the rio vouga when leaving the mountainous part of the basin has for some 2 5 a signature determined by chemical weathering in the metasediments the dominant primary minerals subject to chemical weathering are plagioclase pl and biotite in granite or pl and chlorite in metasediment kaolinite gibbsite and vermiculite are the major weathering products where annual precipitation p 1000 mm y 1 and kaolinite vermiculite and smectite where p was lower using an algorithm based on the ratio of dissolved silica to bicarbonate the contributions of chemical weathering of primary minerals could be unraveled the results show that in granite the export rate as mol ha 1 y 1 wt mineral 1 of oligoclase pl with an10 30 was 5 0 2 6 and of biotite 3 2 2 6 while in metasediment these rates for albite pl with an0 10 are 16 5 8 9 and for chlorite are 0 5 0 5 the observed decrease of dissolved silica in surface waters relative to springs was ascribed to summer uptake by aquatic biota

theoretical approach to evaluating plagioclase dissolution mechanisms abstract the more rapid dissolution of ca rich feldspars relative to na k rich feldspars has been attributed to the preferential leaching of al deep within the feldspar structure evidence from surface microanalysis e g hellmann et al 2003 however shows that preferential dissolution of al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution it is thus critical to examine the extent of preferential al removal here we present a theoretical study of plagioclase dissolution behavior using parameterized monte carlo simulations two different dissolution mechanisms a mechanism involving preferential leaching of al and an interfacial dissolution reprecipitation mechanism are tested using compositions representing the entire plagioclase solid solution series our modeling results indicate that under the control of the preferential al leaching mechanism the influence of al si disorder on the dissolution rate is significant at a fixed composition an increase in the degree of al si disorder yields an increased dissolution rate with an 8 fold increase in dissolution rate observed for highly disordered albite an 0 compared to low albite increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder the difference in the dissolution rate of 293 tested oligoclase configurations with a composition of an 20 is 3 fold and the difference is reduced to 2 fold among 107 andesine configurations of an 30 furthermore feldspar configurations with completely disordered al si distributions yield a consistent log linear dependence of dissolution rate on the anorthite content an while other feldspar configurations with modest degrees of al si disorder exhibit rates less than this trend in contrast when al removal is confined to the top surface layers a variety of feldspar configurations with different al si disorder but a single fixed composition have similar dissolution rates and the dissolution rate of ca rich feldspars departs positively from its log linear relationship with anorthite content this departure occurs around an 80 and is in good agreement with previous experimental studies subsequent modeling results of aluminum inhibition g dependence and formation of altered surface layers in the framework of the interfacial dissolution reprecipitation mechanism are all comparable with experimental investigations and these results suggest that an interfacial dissolution reprecipitation mechanism governs the dissolution of plagioclase feldspars

the unit response of groundwater outflow from a hillslope subsurface flow from a hillslope can be described by hydraulic groundwater theory as formulated by the boussinesq equation solution of its linear version brings out some of the essential features of this phenomenon arbitrary inputs can be accommodated by simple convolution of the sudden drawdown problem the relative magnitude of the driving mechanisms namely the streamwise pressure gradient resulting in diffusion and gravity i e slope resulting in advection can be determined by a simple dimensionless parameter

streamflow recession in basins with multiple water storages abstract a streamflow recession formula for natural basins is derived by linear hydrologic routing of the sum of inflows from discrete independent water storages through a channel storage water storages include depression detention snow and ice channel bank aquifer and cavern evapotranspiration is incorporated in the recession formula but seasonal effects and basin wetness and storage conditions before storms are not considered data from seven basins involving 156 precipitation free recessions are used to test the performance of two and three parameter approximations to the theoretical recession formula recessions are defined to last at least 2 weeks and to begin 2 days after the peak of a basin outflow hydrograph or at a point on the falling limb 2 days after any precipitation occurs recessions end when precipitation recommences or when daily flow data stop decreasing an inverse square formula explains most variance in the two parameter case in contrast to the simple exponential formula usually recommended for recession modelling with three parameters equally good performance is shown by formulae involving a constant plus an inverse square or inverse cubic or simple exponential term these theoretically based formulae should be useful at basin scale for interpolating or extrapolating recession data particularly in basins where little is known about water storage behaviour

quantitative model prediction of the combined effect of moisture content and temperature on purple mudstone decay in south western china abstract rock decay is mainly the result of the combined effects of moisture content and temperature but little is known about the quantitative relationship between these variables and the rate of rock decay in this study we develop quantitative calculation models of rock decay rate under laboratory conditions and validate the efficiency of these models by comparing the predicted rock decay mass and that measured for rock exposed in the field rainfall and temperature data in the field were standardised to a dimensionless moisture content and temperature variables respectively and then the predicted rock decay mass was calculated by the models the measured rock decay mass was determined by manual sieving based on our previously determined relationship between a single factor moisture content or temperature and the rate of rock decay in the laboratory power function models are developed results show that the rock decay mass calculated by the model was comparable with field data with averaged relative errors of 1 53 9 00 and 11 82 for the tuodian group j 3 t matoushan group k 2 m and lufeng group j 1 l respectively which are mainly due to inaccurate transformation of field rainfall into the rock moisture content and artificial disturbance when the samples were sieved in the field our results show that the developed models based on laboratory derived rates can accurately predict the decay rates of mudstones exposed in the field

lithologic control on the multifractal spectrum of river networks previous observations on some natural river networks showed the similar values of the support fractal dimension and the information entropy in basins having similar source rocks in the present work lithologic control on river network multifractality is investigated through the reconstruction of the spectra belonging to two classes of different lithology the plutonic metamorphic class and the coherent sedimentary one two computational methods of the generalised fractal dimensions and multifractal spectra are illustrated and applied to river networks of the calabria region southern italy it is shown that an increment of both the fractal dimensions and the lipschitz holder exponents of singularity occurs when passing from a coherent sedimentary to a plutonic metamorphic river network the importance of this finding emerges from the direct link between flood hydrographs and multifractal spectra as expressed in the multifractal instantaneous unit hydrograph miuh the increment of the multifractal parameters produces an increment of the flood peaks