field derived relationships for flow velocity and resistance in high gradient streams summary we measured velocity and channel geometry in 10 reaches bed gradient 0 08 0 21 of a predominantly step pool channel the rio cordon italy over a range of discharges 3 80 of the bankfull discharge the resulting data were used to compute flow resistance at a station hydraulic geometry relations indicate that in most reaches the exponent describing the rate of velocity increases with discharge was between 0 48 and 0 6 which is within the range of published values for pool riffle channels the rio cordon data are also combined with published hydraulics data from step pool streams to explore non dimensional relationships between velocity and flow resistance and factors including unit discharge channel gradient and step geometry multiple regression analysis of this combined field dataset indicated that dimensionless unit discharge q is the most important independent variable overall in explaining variations in velocity and flow resistance followed by channel slope and the ratio of step height to step length empirical equations are provided both for dimensionless velocity and flow resistance but prediction of the former variable appears more reliable

sedimentary facies distribution and genesis of a recent carbonate rich saline lake gallocanta lake iberian chain ne spain abstract the study focuses on the holocene sedimentary infill of the gallocanta lacustrine basin in the iberian chain ne spain the gallocanta lake is a saline wetland with a maximum length of 7 5 km and a maximum width of 2 85 km the water depth varies significantly from a maximum depth of 2 m to completely dry in the central areas central subenvironment sapropels and salts develop with halite gypsum anhydrite dolomite aragonite calcite magnesite and lesser amounts of quartz and clay minerals cyanobacteria filaments are related to the aragonite and dolomite crystals the marginal subenvironment either has a gradual or a sharp change from that of the central subenvironment an inner area with desiccated light grey lutites is present in this marginal subenvironment in sem and x ray diffraction analyses quartz clay minerals aragonite calcite and small quantities of dolomite gypsum anhydrite and halite can be identified this inner area is surrounded by an external fringe composed of light brown lutites and a high concentration of salicornia meadwod and microbial mats this zone is only occasionally submerged and contains sandy and conglomerate islets active palustrine areas are flood zones where grey lutites with a significant quantity of vegetation such as reeds are common in general this entire sector is being modified by human action five sedimentary facies have been defined and this has enabled the identification of three distinct stages in the general evolution of the basin the first stage is an alluvial period developing during an arid climate after this a more humid stage facilitated the installation of a shallow carbonate rich lake a reduction of the water level probably due to a more arid stage induces a salinity increase of the lacustrine brine and the change to the third stage which corresponds to the present conditions the water level experienced frequent oscillations and alternations between humid conditions with a high production of organic matter which favors carbonate formation and water level dropdowns even to total dryness with saline sedimentation

mineral biosignatures in evaporites presence of rosickyite in an endoevaporitic microbial community from death valley california rosickyite is a rare form of sulfur sulfur monoclinic symmetry that is not thermodynamically predicted to be stable at earth s surface temperatures instead it reverts to the more common sulfur form orthorhombic symmetry here we show for the first time that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of death valley california we hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum caso4 2h2o to sulfide and reoxidation of the sulfide to elemental sulfur rosickyite within a stable oxygen sulfide gradient maintained by the organisms furthermore we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

channel width flow discharge relationships for rills and gullies abstract research conducted during the first half of the last century has shown that a strong power relationship exists between channel width and total flow discharge in streams recent studies have shown that this power relationship can be theoretically derived for bankfull discharge in channels the relationship has been extended empirically to rills and gullies revealing that the discharge exponent for rills and gullies is significantly smaller than that for rivers however water flow in rills and gullies is only rarely bankfull indicating that the theoretical explanation for the power relationship found for rivers does not apply to rills and gullies in order to investigate the width discharge relationships for rills and gullies a new method is proposed based on field measurements of widths of concentrated flow erosion channels both upstream and downstream of channel junctions although the method only allows the determination of the exponent of the power relationship it is easy and inexpensive to apply a total of 322 rill and gully channel junctions with various soils and land use types were investigated in belgium italy and spain the obtained data confirmed the existence of the power relationship for rills and gullies with the exponent varying from 0 43 for small rills about 3 cm in width to 0 5 for gullies about 50 to 100 cm in width the data did not allow deciding whether the exponent varies consistently with channel width or in a step wise fashion the exponent values obtained in this study are larger than those reported in previous studies but this may result from differences in the definition of the discharge that eroded the channel to its current width

dynamics of intensified downwelling circulation over a widened shelf in the northeastern south china sea abstractthis study reveals the dynamics behind the intensified downslope cross isobath transport over a widened shelf narrowing downwave in the northeastern south china sea nscs during a downwelling event utilizing a three dimensional numerical model over an idealized nscs shelf this study identified the forcing process and derived an analytical understanding of the invoked dynamics for the intensified downslope cross isobath transport this study found that the transport was formed by an amplified geostrophic transport because of an increased positive along isobath pressure gradient force pgf py and by an enhanced bottom ekman transport due to converging flow over the widened shelf based on the depth integrated vorticity dynamics py was linked with net frictional stress curl in the water column and the curl was primarily associated with the shear vorticity field induced by downwelling jet the increased positive vorticity seaside of the jet provided positive py for the downslope geostrop

theoretical regime equations for mobile gravel bed rivers with stable banks a system of rational regime equations is developed for gravel bed rivers with stable banks using the optimality theory ot the optimality theory is based on the premise that equilibrium river geometry is characterised by an optimum configuration defined here as maximum sediment transport efficiency theoretical dimensionless equations are derived for width depth slope width depth ratio and meandering braiding transition independent dimensionless variables comprise discharge sediment concentration and relative bank strength which is defined as the ratio of the critical shear stresses for the bank and bed sediments discharge exponents and general form of the equations agree well with previously developed empirical relations relative bank strength is used to parameterise the influence of riparian vegetation on bank strength and is evaluated by calibrating against observed width depth ratio once calibrated the hydraulic geometry of natural gravel rivers is well described by the theoretical equations including discrimination between meandering and braiding channels the results provide strong support for the assumption that equilibrium or regime river behavior is equivalent to an optimal state and underline the importance of bank strength and sediment load as controls on hydraulic geometry

microbial dolomite crusts from the carbonate platform off western india the occurrence of late pleistocene dolomite crusts that occur at 64 m depth on the carbonate platform off western india is documented dolomite is the most predominant mineral in the crusts in thin section the crust consists of dolomitized microlaminae interspersed with detrital particles under scanning electron microscopy these laminae are made up of tubular filaments or cellular structures of probable cyanobacterial origin dolomite crystals encrust or overgrow the surfaces of the microbial filaments and or cells progressive mineralization obliterates their morphology well preserved microbial mats sulphide minerals pyrrhotite and marcasite and the stable isotope composition of dolomite in the crusts indicate hypersaline and anoxic conditions during dolomite formation the crusts are similar to dolomite stromatolites and biogeochemical processes related to decaying microbial mats under anoxic conditions probably played an important role in dolomite precipitation the dolomite is therefore primary and or very early diagenetic in origin the dolomite crusts are interpreted to be a composite of microbial dolomite overprinted by early burial organic dolomite the results of this study suggest that a microbial model for dolomite formation may be relevant for the origin of ancient massive dolomites in marine successions characterized by cryptalgal laminites the age of the crusts further suggests that the platform was situated at shallow subtidal depths during the last glacial maximum

granulite facies metamorphism in the anatectic complex of toledo spain late hercynian tectonic evolution by crustal extension the anatectic complex of toledo consists of a heterogeneous suite of high grade metamorphic rocks mainly granulitic migmatites of pelitic composition and orthogneisses and different types of syn orogenic granitoids with minor associated basic rocks none of the metamorphic rock types records evidence of the early prograde history and only the peak and retrograde p t conditions can be estimated the intrusion of the syn orogenic granitoids is either slightly before or synchronous with the metamorphic climax these rock types thus can be used to make p t estimates of the peak and retrograde conditions microtextural analysis of reaction textures in conjunction with a petrogenetic grid has enabled construction of an incomplete clockwise p t path that is characterized by an isothermal decompression after or during the metamorphic peak temperatures the peak estimates from different geo thermobarometers are 800 50 c and 4 6 kbar the highest temperatures recorded in this part of the iberian hercynian belt it is proposed that this high t low p metamorphism is at least partially related to an extensional process possibly due to gravitational collapse of previously thickened crust

soil shear strength its measurement and soil detachability abstract recent research on soil erosion mechanics has pointed out that the condition at which rill flow becomes erosive is controlled by soil surface shear strength torri et al 1987a rauws govers 1988 also splash detachment processes are strictly linked to soil shear strength as shown by nearing bradford 1985 torri et al 1987b a comparison of the proposed threshold conditions for incipient rilling immediately shows that they differ in the two quoted papers the same situation characterizes the relations proposed between splash detachment and soil shear strength those differences may depend on the different instruments used for measuring soil shear strength a geonor model g 200 laboratory cone penetration apparatus a laboratory vane tester and a pocket vane tester those instruments differ in shape pocket and laboratory vane testers or in the physics involved in their functioning vane tester and drop cone penetrometer the experimental results show that the drop cone penetrometer is characterized by a low repeatability moreover it is sensitive to small variations in soil texture the laboratory vane apparatus seems to determine a surface of shear close to that of a sphere circumscribed to the vane the pocket vane apparatus instead is characterized by a cylindrical surface of shear those results seem to explain the main differences in soil detachment in particular incipient rilling conditions are clearly reduced to a single relation between critical shear velocity and soil shear strength it can consequently be concluded that soil detachability is strictly linked to soil shear strength

environmental controls on the late holocene carbonate sedimentation of a karstic lake in the middle atlas mountains lake afourgagh morocco the lake afourgagh sediment record and facies successions provide an outstanding example of environmentally controlled carbonate sedimentation afourgagh is a small shallow permanent lake located in the middle atlas mountains in morocco in a karstic context it is fed by ground waters that are relatively enriched in mg resulting from the leaching of the jurassic dolomitic bedrock of the catchment this eutrophic lake is episodically restricted and characterized by alkaline waters with a fluctuating high mg ca ratio the maximum extension of the holocene shoreline coincides with evidence of a lake stabilization level corresponding to the outflow of the lake through a wadi lakeshore terrace sediments deposited on an alluvial fan siltstone during the past ca 2500 cal yr bp comprise four main facies a littoral crust palaeosols palustrine silts and charophyte tufas which reflect different environments from the shoreline toward the deeper water in the more distal parts the charophyte tufas display a well expressed lamination punctuated by the development of microstromatolites on algae thalli the mineralogical composition of the carbonates is linked to the facies while the charophyte tufas are characterized by a relatively high content in aragonite in addition to low mg calcite the littoral crust is mainly composed of magnesite this pattern is related to the evolving chemistry of water due to the influence of charophyte proliferation during dry summers calcium carbonate precipitation on algae thalli both bioinduced and microbially mediated progressively induces an increase in the mg ca ratio of the lake water while the capillary evaporation of shallow ground waters causes precipitation of a magnesite precursor on the shoreline producing magnesite during early diagenesis this effect is characteristic of two episodes part of the roman warm period and the beginning of the dark age cold period the carbonate mineralogy of the different depositional sequences at afourgagh indicates lake level and water chemistry fluctuations under a climatic influence therefore among other regional records the lake afourgagh sedimentary record provides useful evidence for reconstructing these environmental changes

effects of rock fragments incorporated in the soil matrix on concentrated flow hydraulics and erosion rock fragments can act as a controlling factor for erosional rates and patterns in the landscape thus the objective of this study is to better understand the role that rock fragments incorporated into the soil matrix play in concentrated flow hydraulics and erosion laboratory flume experiments were conducted with soil material that was mixed with rock fragments rock fragment content ranged from 0 to 40 per cent by volume other treatments were slope 7 and 14 and flow discharge 5 7 and 11 4 l min 1 an increase in rock fragment content resulted in lower sediment yield and broader width of flow rock fragment cover at the soil surface i e surface armour increased with time in experiments with rock fragments flow energy was largely dissipated by rock fragment cover for more turbulent flow conditions when roughness elements were submerged in the flow hydraulic roughness was similar for different rock fragment contents in experiments with few or no rock fragments a narrow rill incised flow energy was dissipated by headcuts total sediment yield was much larger than for experiments with rock fragments in the soil adding just a small number of rock fragments in the soil matrix resulted in a significant reduction of sediment yield copyright 2007 john wiley sons ltd

estimation of runoff critical shear stress for soil erosion from soil shear strength abstract critical shear stress is an important soil parameter governing detachment by runoff which appears in numerous erosion models soil shear strength has been often presented as the best soil property to predict critical shear stress however only a few studies have documented the relationship between soil shear strength and critical shear stress and the results obtained have sometimes seemed contradictory this is why we have put together all available data about the relationship between critical shear stress and soil shear strength in order to analyse and discuss its significance and its generality results showed that contradictions between different studies mainly arise because total shear stress is a poor predictor of detachment on rough soils when grain shear stress is used rather than total shear stress a significant relationship between saturated soil shear strength and critical shear stress does exist soil shear strength may thus be used to predict critical grain shear stress however it is recalled that in general only total shear stress is predicted by erosion models which makes detachment predictions difficult on rough natural soils

spatial variation of bed roughness in eroding rills and gullies when overland flow concentrates rill and gully channels can be formed if a series of thresholds are exceeded these thresholds are more or less explicitly linked to the erosion resistance of the topsoil moreover flow velocity and channel width depend on total flow discharge more recently also bed roughness in eroding channels has been attributed to the eroding effect of flow discharge while channel width was shown to be the result of the interplay between erosion resistance of the topsoil and flow discharge see channel junction approach torri et al 2006 which could be described by a modified leopold and maddock s relationship the objective of this paper is to investigate the spatial variability of channel bed roughness in rills and gullies using an approach based on these findings field data confirm the various aspects reported so far hence these were used to develop a new equation allowing one to predict bed roughness in eroding rill and gully channels each of the new aspects introduced into the channel width flow discharge equation by the channel junction approach is discussed and verified with new data the validity of this approach is tested against channel data from mars finally an equation predicting bed roughness and based on stream power is developed and compared with measured rill and gully bed roughness successfully confirming that 1 bed roughness if generated by concentrated flow increases with stream power 2 channel width local bed slope topsoil cohesion at saturation and grain size are all important factors controlling channel bed roughness and 3 these variables as well as soil characteristics all measurable in the field after a rill or gully forming event are sufficient to determine channel bed roughness therefore one may expect that bed roughness of an eroded channel and consequently hydraulic roughness or friction will generally increase with bed gradient erosion resistance of the soil and grain size following a logic which is best expressed by the final equations described in this paper

cyanobacterial precipitation of gypsum calcite and magnesite from natural alkaline lake water results from transmission electron microscopy provide direct evidence for cyanobacterial biomineralization of gypsum and calcite in aquatic environments laboratory simulations using filter sterilized natural lake water inoculated with synechococcus sp isolated from fayette ville green lake new york revealed epicellular biomineralization of gypsum calcite and magnesite experimental electron microscopical and sedimentological evidence indicates that synechococcus is responsible for a major proportion of the marl sediment and carbonate bioherms in green lake the elucidated role of synechococcus in biomineralization and its ubiquitous distribution in nature have widespread implications for cyanobacterial mineralization in marine and freshwater environments since late archean time

petrology and major element geochemistry of peru margin phosphorites and associated diagenetic minerals authigenesis in modern organic rich sediments abstract new petrographic and major element geochemical data from modern peru margin upper slope outer shelf phosphorites are presented which provide insight into their origin and paragenetic relationship with other authigenic minerals glauconite pyrite and dolomite occurring in organic rich sediments glauconites are precipitated relatively early following the partial reduction of ferric iron and following this process phosphate pyrite and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate as in many ancient economic phosphorite deposits the phosphatic facies here consist of nodules crusts coatings and strata composed of phosphatic pelletal grains ooids structureless grains intraclasts clumps and biogenic grains in association with organic rich biosiliceous sediments all are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment water interface important factors that influence which morphology will tend to develop include the amount of available pore space the presence of suitable nucleation sites the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions bacterial mediation may play an important but as yet unspecified role in the precipitation process textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion free na f mg co3 substituted pore water precipitates whereas ooids are inclusion rich pore water precipitates poor in lattice substituted components variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore water carbonate ion concentrations and thus relative degrees of organic matter degradation phosphate and dolomite are intimately mixed yet mineralogically distinct phases in phosphatized dolomicrites depth stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate below depths of a few centimeters excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite periodic sediment reorganization bioturbation current winnowing and erosion mass wasting etc plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment as well as mixing ordered mineral parageneses into complicated sequences

reach scale channel geometry of mountain streams abstract the basic patterns and processes of steep channels remain poorly known relative to lower gradient channels in this analysis characteristics of step pool plane bed and pool riffle channels are examined using a data set of 335 channel reaches from the western united states nepal new zealand and panama we analyzed differences among the three channel types with respect to hydraulics channel geometry boundary roughness and bedforms step pool channels have significantly steeper gradients coarser substrate higher values of shear stress and stream power for a given discharge and larger ratios of bedform amplitude wavelength h l pool riffle channels have greater width depth ratios and relative grain submergence r d84 than the other channel types plane bed channels tend to have intermediate values for most variables examined relative form submergence r h is statistically similar for step pool and pool riffle channels despite the lesser relative grain submergence and greater bedform amplitude of step pool channels mean values of darcy weisbach friction factor do not change in response to changes in relative grain submergence these patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types

determining the mean depth of overland flow in field studies of flow hydraulics in field studies of interrill overland flow conducted on runoff plots the mean depth of flow is usually obtained by averaging measurements of flow depth at regularly spaced points across the plot including points where there is no flow although this procedure may be appropriate in studies aimed at runoff prediction it is not in studies concerned with flow hydraulics in the latter studies more realistic results can be obtained by excluding those points where the depth is zero and reducing the width of flow in proportion to the number of points excluded failure to do this results in the underestimation of the actual mean flow depth reynolds number and friction factor where discharges are small or microtopography is large this underestimation is likely to be significant

mg silica precipitation in fossilization mechanisms of sand tufa endolithic microbial community mono lake california abstract the detection of mg silica associated with endolithic biofilms of cyanobacteria and bacterial communities living in lake shore sand tufa formations in mono lake california has been observed using scanning electron microscopy with backscattered electron imaging sem bse and energy dispersive x ray spectroscopy eds mg accumulated in cell walls of living cyanobacteria is interpreted to promote mg silicification with structural preservation while non cyanobacterial prokaryotes are not fossilized however extracellular polymeric substances around the living communities were prominent as nucleation sites for authigenic mineral precipitation a mg silicate served to stabilize the biofilm remains further diagenesis of these textures resulted in occlusion of the cytoplasm pore spaces by carbonate precipitates this precipitate damaged the microfossil structures but the mg silica fabric was still present the paleontological significance of mg silica precipitates as a possible biosignature of the former presence of endolithic biofilms is addressed

hydromagnesite replacement of biomineralized aragonite in a new location of holocene stromatolites lake walyungup western australia holocene stromatolites are described from lake walyungup a coastal hyposaline lake in south western australia at summer low water this groundwater fed depression comprises two permanent shallow water bodies and an ephemeral southern pool set within an areally extensive littoral zone of variably cemented carbonate crust up to 5 m of organic rich carbonate mud has been deposited within each of these basins in less than 7000 years stromatolites rim the water bodies with individual columns up to 2 m tall stromatolite capped tepee structures in subparallel alignment are widespread in the littoral crust suggesting a linkage between stromatolite growth and zones of groundwater discharge lake walyungup stromatolites regardless of external morphology and setting are coarsely laminated and have aragonitic mesoclot microfabrics these microfabrics are similar to those from lithified portions of active thrombolitic microbialites from nearby lake clifton hydromagnesite is a minor to subdominant phase up to 47 wt of the carbonate mineral assemblage in lake walyungup it occurs mainly in the littoral zone as a diagenetic replacement of precursor aragonite particularly within the mesoclot fabric of stromatolites but also in sediments strandline and dune sand crusts derived mainly from erosion of stromatolites in contrast with nonreplaced and impermeable inorganic aragonitic cements stromatolite mesoclots are microper meable micropermeability is inferred to facilitate hydromagnesite diagenesis dolomite is also present in minor amounts as a pore fill in stromatolites and as a subdominant to dominant up to 100 wt phase in thin mudcracked micrite layers within the crust package the layered dolomite may be precipitated directly from the lake water major element abundance of the lake water is na mg2 k ca2 for cations and cl so42 hco3 co32 for anions compared to other nearby coastal lakes lake walyungup has a high ph 9 0 and an extremely high molar mg ca ratio of 90 groundwater in the area has a mg ca ratio generally less than 1 the unusual mg ca ratio in lake walyungup is partially a result of in lake processes with additional minor contribution of mg2 sourced from basal marine sand because no mg rich bedrock source has been found in the region

rainfall induced consolidation and sealing effects on soil erodibility during concentrated runoff for loess derived topsoils flume experiments simulating concentrated runoff were carried out on remolded silt loam soil samples 0 36 0 09 0 09 m3 to measure the effect of rainfall induced soil consolidation and soil surface sealing on soil erosion by concentrated flow for loess derived soils and to establish a relationship between soil erodibility and soil bulk density soil consolidation and sealing were simulated by successive simulated rainfall events 0 600 mm of cumulative rainfall alternated by periods of drying soil detachment measurements were repeated for four different soil moisture contents 0 04 0 14 0 20 and 0 31 g g 1 whereas no effect of soil consolidation and sealing is observed for critical flow shear stress cr soil erodibility kc decreases exponentially with increasing cumulative rainfall depth the erosion reducing effect of soil consolidation and sealing decreases with a decreasing soil moisture content prior to erosion due to slaking effects occurring during rapid wetting of the dry topsoil after about 100 mm of rainfall kc attains its minimum value for all moisture conditions corresponding to a reduction of about 70 compared with the initial kc value for the moist soil samples and only a 10 reduction for the driest soil samples the relationship estimating relative kc values from soil moisture content and cumulative rainfall depth predicts kc values measured on a gradually consolidating cropland field in the belgian loess belt reasonably well mef 0 54 kc is also shown to decrease linearly with increasing soil bulk density for all moisture treatments suggesting that the compaction of thalwegs where concentrated flow erosion often occurs might be an alternative soil erosion control measure in addition to grassed waterways and double drilling copyright 2007 john wiley sons ltd

introduction to the special issue signatures of microbes and microbial mats and the sedimentary record abstract the present special issue originated from a session ssp4 5 at the egu general assembly 2010 vienna microbes and microbial mat signatures and the sedimentary record this special issue comprises a set of papers that focus on several aspects of microbial and microbial mat signatures grouped into four major fields 1 microbial biomineralisation 2 microbial communities and mat related structures 3 organo sedimentary accretion and microbial deposits and 4 processes and products in submarine cold seeps and terrestrial mud volcanoes

biosedimentary and geochemical constraints on the precipitation of mineral crusts in shallow sulphate lakes abstract seasonal desiccation of mg2 na ca2 so42 cl saline lakes in la mancha central spain that host microbial mats led to the precipitation of hydrated na mg sulphates and gypsum sulphates precipitated in the submerged conditions form extensive biolaminites whilst in marginal areas they produce thin crusts sedimentological mineralogical petrographic and high resolution textural studies reveal that the crusts were formed within the benthic microbial mats that thrive at salinities ranging from 160 to 340 g l 1 the minerals of the crusts are primary bloedite na2mg so4 2 4h2o epsomite mgso4 7h2o gypsum caso4 2h2o and mirabilite na2so4 10h2o as well as secondary hexahydrite mgso4 6h2o and thenardite na2so4 primary bloedite crystals which form the framework of surficial and submerged crusts are seen to nucleate subaqueously and grow incorporatively within the matgrounds displacive and incorporative epsomite grows on previous bloedite crystals and also on the ground mirabilite is precipitated rapidly at the brine air interface over bloedite and epsomite hexahydrite and thenardite are formed due to dehydration of epsomite and mirabilite respectively hydrochemical modeling with phreeqc indicated that evaporitic biolaminites are forming from brines undersaturated with respect to bloedite epsomite and mirabilite which suggests that the microorganisms contribute to the heterogeneous nucleation of the sulphates in the microbial mats unlike carbonates the influence of microbes on the growth and morphology of complicated double salts such as bloedite has not been documented previously and provides a new perspective on the formation of hydrated sulphate minerals that are common on earth as well as other planets

meteoric calcitization of magnesite in miocene lacustrine deposits calatayud basin ne spain abstract sedimentary magnesite deposits are commonly present in evaporite successions in the tertiary calatayud basin ne spain outcropping evaporite sequences mainly composed of secondary gypsum after anhydrite glauberite show centimetre thick layers of magnesite passing laterally into calcite pseudospar petrographic and stable isotope data indicate that calcite pseudospar formed via late stage calcitization of magnesite under near surface conditions calcitization occurred by the interaction between magnesite and ca enriched waters derived from the dissolution of gypsum and or glauberite the geochemical simulation of this process petrographic evidence and the correlation between the magnesite calcite ratio and the 18o values of both magnesite and calcite samples across the reaction front are in agreement with the existence of neoformed 18o depleted magnesite during the waning stages of the calcitization process

soil erosion by surface water flow on a stony semiarid hillslope soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows this study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment a field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface the method allowed for measurements in this study of flow erosion at a much wider range of slopes 2 6 to 30 1 per cent and unit discharge rates 0 0007 to 0 007 m2 s 1 than have been previously feasible flow velocities were correlated to discharge and hydraulic radius but not to slope the lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity the detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected flow detachment rates were better correlated to a power function of either shear stress r2 0 51 or stream power r2 0 59 published in 1999 by john wiley sons ltd

mediation of endoevaporitic microbial communities in early replacement of gypsum by dolomite a case study from miocene lake deposits of the madrid basin spain abstract based on petrographic mineralogical isotope and facies assemblage analysis a microbial origin is established for the formation of dolomite associated with gypsum in miocene evaporite lake deposits of the madrid basin central spain in these deposits dolomite is present as both intercalated carbonate beds locally showing domal stromatolite structures between packages of selenite christmas tree like gypsum and patches replacing macrocrystalline gypsum texture of the dolomite is characterized by crystal aggregates showing a variety of crystal sizes and morphologies e g platelets rhombs micro rods and rings whilst larger crystals are commonly spherical and or wheat grain shaped organic remains in the form of filaments shrubs micro fibrils and strands are also common and contain significant amounts of carbon these textural features are also recognized in dolomite replacing gypsum where fe oxide and sulfide as well as celestite are ubiquitous mixed with the dolomite groundmass the dolomite whether primary or replacing gypsum is poorly ordered and slightly ca rich thus non stoichiometric stable isotope compositions are characterized by negative values for both oxygen and carbon dolomite beds featuring domal stromatolites have 18o values ranging from 2 99 and 3 79 and 13c values ranging from 4 67 and 7 35 whilst 13c values determined in the dolomite replacive of gypsum shows a small range of variation between 5 70 and 6 96 by contrast 18o values of replacive dolomite oscillate in a wider range from 3 04 to 7 99 formation of the dolomite was associated mainly with microbial mats having taken place in relatively diluted lake water further evaporative concentration resulted in precipitation of gypsum crystals sealing the mats and creating endoevaporitic microenvironments in which endolithic cyanobaterial activity produced extensive boring and corrosion of the gypsum crystals hiatuses in gypsum growth caused an intensification of the corrosion process and favored the precipitation of dolomite mediated by microbes resulting in pervasive replacement of the sulfate

a 100 ka record of water tables and paleoclimates from salt cores death valley california abstract sedimentary and petrographic features of evaporites and associated sediments from a 185 m deep core taken in death valley ca together with uranium series dating have been used to reconstruct the history of water table fluctuations and climate changes in death valley for the past 100 ka death valley has been arid during the holocene 0 10 ka with predominantly mudflat and saline pan subenvironments a perennial lake up to 90 m deep existed in death valley from 10 to 35 ka saline pan and mudflat subenvironments dominated death valley from 35 to 100 ka the chronology of changing subenvironments and water table fluctuations in death valley generally correlates with other climate records in the western us owens lake and searles lake ca browns room cave calcite nv the marine oxygen isotope record and the vostok ice core record core intervals through saline pan sediments are composed of interbedded halite chaotic muddy halite and mud the halite contains abundant vertical dissolution pipes cemented with clear halite these sediments record repeated flooding by dilute waters dissolution of subaerially exposed surface salt crusts deposition of mud from suspension precipitation of halite during the saline lake phase and cementation by diagenetic halite mudflat sediments consist of clayey silt with sand patches and mud cracks which document long periods of desiccation and the formation of efflorescent salt crusts from the evaporation of groundwater brines saline pan and mudflat deposits formed during periods when death valley was relatively arid similar to the modern climate lacustrine deposits consist of mudhalite cycles accumulated during the early lake stage bedded thenardite na 2 so 4 and mud above and a cap of massive halite formed during the latest lake stage all of which record fluctuating salinities and lake levels in a perennial system such deposits document a relatively wet climate with a high ratio of water inflow to evaporation ostracodes in mud layers represent the least saline deepest lake phases halite layers are made of fine grained cumulates and clear vertically oriented crystals precipitated during shallower perennial lake stages of significance is the nearly complete absence of syndepositional dissolution of saline minerals in the lacustrine interval indicating that accumulated salts were permanently protected from dissolution by saline lake waters such evidence strongly suggests that lakes existed continually without desiccating for 25 ka between 10 and 35 ka b p

longitudinal velocity patterns and bed morphology interaction in a rill present day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills the latter are dif cult to collect with traditional techniques due to the very low water depths and the relatively high ow velocities in rills the objectives of this paper were to investigate the feasibility of miniaturized acoustic doppler velocimeter madv measurements in rill ow and to explore longitudinal variations in ow velocities and their relationship with rill bed morphology detailed data on longitudinal ow velocity were required to achieve these objectives a 1 8 m long rill was formed freely in a ume at 5 slope and 0 001 m3 s 1 discharge rill topography was characterized by an alternation of steps and pools the ume surface was then xed to preserve rill roughness a topographical scanning of the entire ume surface was made velocity was measured with a madv along the rill and at different depths flow depth in a longitudinal direction was also measured using an elevation gauge a strong relationship exists between rill topography and ow hydraulics over steps ow was unidirectional and rapidly accelerating until a threshold froude number fn value between 1 3 and 1 7 was reached and a hydraulic jump occurred leading to the formation of a pool in the pool the ow pattern was multidirectional and complex the ow was subcritical when leaving the pool and accelerated over the next step until the threshold froude number value was again reached energy loss in the rill was concentrated in the pools mainly due to the action of a hydraulic jump this mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution copyright 2004 john wiley sons ltd

initiation and evolution of gullies along the shoreline of lake huron over one hundred gullies are deeply incised into algonquin bluff along the eastern shoreline of lake huron aerial photographs between 1930 and 1992 were used to measure temporal changes in area and length of a sample of forty four gullies regression analysis shows that gully area and length have increased in a linear fashion and that gully initiation occurred in the nineteenth century gully initiation coincides with deforestation and settlement of the area surface drainage was redirected into drainage ditches concentrating the flow and enhancing gully erosion natural processes associated with climatic fluctuations are probably insufficient to cause gully initiation but may contribute to ongoing gully expansion

mineralogical footprints of microbial life earth s geosphere is intimately tied to its biosphere a major link between the two lies in the microbial realm microorganisms grow in and upon rocks and minerals often relying on their substratum for critical compounds needed in order to produce cellular energy the presence of a metabolizing cell on a mineral substrate has a significant effect on the mineral texture and on the geochemistry of the surrounding microenvironment in nature microorganisms exist in microbial commu nities as mats or biofilms growing upon a solid substrate as such they cover a vast surface area both within and below the surface of earth s land and sea the following review will provide a glimpse into the latest findings in the field of geomicrobiology and is intended to convey a sense of the profound influence microorganisms can have upon the geological environment they inhabit

ravine formation on steep slopes forward versus regressive erosion some case studies from rwanda summary observations on steep slopes in rwanda reveal that human interventions on surficial drainage can introduce gullying on places where runoff normally leads to diffuse or ephemeral rill erosion in such cases an initial runoff incision or slide scar respectively due to an artificial concentration of runoff or underflow grows in downslope direction mainly by runoff incision this type of progressive or forward gully development contrasts with the more classical regressive development here only observed in places where the flow gradient is less than 7 8

fluid inclusion analysis of twinned selenite gypsum beds from the miocene of the madrid basin spain implication on dolomite bioformation this research work is centred on continental lacustrine gypsum deposits of miocene age cropping out in the easternmost part of the madrid basin these gypsum deposits accumulated in a continental saline lake are characterized by a spectacular distinctive christmas tree morphology and a peculiar dolomite replacement a combination of microscopic petrography and scanning electron microscopy and analytical techniques fluid inclusion microthermometry x ray energy dispersive spectroscopy and x ray diffractometry was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum the objectives were to characterize the continental brine from which the mineral precipitated and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid lake water of low to moderate total salinity between 20 and 90 g l nacl secondary fluid inclusions represent interstitial lake brine in contact with gypsum slightly enriched in total salt content as crystal formation proceeded textural ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes

hydraulics and erosion in eroding rills rills often act as sediment sources and the dominant sediment and water transport mechanism for hillslopes six experiments were conducted on two soils and a uniform sand using three experimental methodologies the results of this study challenge the assumption often used in hydrologic and erosion models that relationships derived for sheet flow or larger channel flow are applicable to actively eroding rills velocity did not vary with slope and reynolds number was not a consistent predictor of hydraulic friction this result was due to interactions of slope gradient flow rate erosion and the formation of rill roughness bed structures and head cuts a relationship for rill flow velocities was proposed stream power was found to be a consistent and appropriate predictor for unit sediment load for the entire data set while other hydraulic variables were not the data for stream power and sediment load fit the form of a logistic curve r2 0 93 which is promising relative to recently proposed erosion models which are based on probabilistic particle threshold theory

bacterially induced mg calcite formation role of mg2 in development of crystal morphology abstract the production of several morphologies of magnesian calcite crystals by the soil bacterium myxococcus xanthus is reported for the first time mg calcite crystallization occurred in agar agar gel culture media in the presence of the live bacteria the agar agar used to solidify the nutritive microbial solution acted as a porous system that allowed slow counterdiffusion of cations and anions and of the bacterial metabolites produced under these conditions crystal nucleation and growth occurs apparently as a consequence of the localized ion supersaturation produced by the microbial metabolites and by microbial supply of heterogeneous nuclei for crystallization several morphologies of mg calcite typical of those formed under biotic and abiotic conditions developed simultaneously the crystals produced were not compositionally zoned and showed no significant variation in mg content probably as a consequence of the sponge like character of the precipitates

spatial distribution of gully head activity and sediment supply along an ephemeral channel in a mediterranean environment abstract in this study we examined the factors that control the spatial distribution of bank gully heads along a reach of an ephemeral river rambla salada in an area threatened by desertification in southeast spain the activity of 458 gully heads was assessed in the field by pre defined criteria such as sharp edges presence of a plunge pool tension cracks recent deposited sediments flow marks and vegetation re growth the results showed that land use has a significant impact on bank gully head activity recent land use changes involving the extension of almond cultivation appears to intensify bank gully head activity also lithology has a clear impact on the bank gully extension it was further investigated whether the gully heads were important sediment sources that contributed to reservoir sedimentation the density of very active bank gully heads along the study reach was one per 17 m of channel length average annual retreat volumes were derived from measurements at 46 active gully heads 4 0 m3 y 1 by selecting all the channel sections in the catchment of the puentes reservoir with a similar pattern of bank gullies using aerial photographs an estimate of basin wide sediment production of bank gully heads was established it was estimated that the retreat of active bank gully heads alone in the 12 760 ha study area representing 12 of the total catchment area of the puentes reservoir produced 6 of the sediment filling up the reservoir considering that the sediment is also derived from other sources such as channel walls channel beds and hillslopes the overall conclusion is that bank gully expansion in southeast spain is a major point source of sediment and therefore a major process of land degradation

lower miocene gypsum palaeokarst in the madrid basin central spain dissolution diagenesis morphological relics and karst end products the miocene sedimentary record of the madrid basin displays several examples of palaeokarstic surfaces sculpted within evaporite formations one of these palaeokarstic surfaces represents the boundary between two main lithostratigraphic units the miocene lower and intermediate units of the madrid basin the palaeokarst formed in lacustrine gypsum deposits of aragonian age and corresponds to a surface palaeokarst epikarst further buried by terrigenous deposits of the overlying unit karst features are recognized up to 5ae5 m beneath the gypsum surface exokarst and endokarst zones are distinguished by the spatial distribution of solution features i e karren dolines pits conduits and caves and collapse breccias sedimentary fills and alteration of the original gypsum across the karst profiles the development of the gypsum palaeokarst began after drying out of a saline lake basin as supported by recognition of root tubes later converted to cylindrical and funnel shaped pits at the top of the karstic profiles the existence of a shallow water table along with low hydraulic gradients was the main factor controlling the karst evolution and explains the limited depth reached by both exokarst and endokarst features synsedimentary fill of the karst system by roughly laminated to massive clay mudstone with subordinate carbonate and clastic gypsum reflects a punctuated sedimentation regime probably related to episodic heavy rainfalls typical of arid to semi arid climates duration of karstification is of the order of several thousands of years which is consistent with previous statements that gypsum karstification can develop rapidly over geologically short time

effects of grass roots on the erodibility of topsoils during concentrated flow abstract traditional vegetative techniques to control gully development rely mainly on the effects of above ground biomass whereas little attention has been given to the role of below ground biomass yet in a context where above ground biomass may temporarily or spatially disappear e g due to fire or grazing roots can play an important role in protecting soil against erosion few studies have investigated the impacts of roots of natural vegetation such as grass on the resistance of topsoils in concentrated flow erosion zones although grasses grow in many environments therefore the objective of this study is to investigate the impact of root density and root length density of grass on the erodibility of root permeated saturated topsoils three plots were established on a sandy loam their treatments were 1 bare 2 low density drilled grass and 3 high density drilled grass simulating different root densities after one month topsoil samples were taken and subjected to concentrated flow using a hydraulic flume in the laboratory slope flow discharge mean velocity water temperature and sediment concentration were measured root density and root length density values were assessed relative soil detachment rates and mean flow shear stresses were calculated the results indicate a negative exponential relation between the relative soil detachment rate and root density as well as root length density independent of the applied flow shear stresses however the best relationship fitting the data is the hill curve indicating that relative soil detachment rates decrease to very low values 0 05 with an increase in root density from 0 to 4 kg m 3 or root length density from 0 to 400 km m 3 a comparison between the effects of vegetation cover on sheet and rill erosion rates and those of the root area ratio of grass roots on relative soil detachment rates reveals that grass roots are very effective in reducing soil detachment rates the equations obtained can be used to predict the effect of grass roots on soil erosion rates during concentrated runoff and to evaluate the ability of roots to increase topsoil resistance against erosion by concentrated flow calculations of relative erosion rates using the equations from the rusle and wepp models indicate that the observed trend is better predicted with the rusle model and the wepp model for croplands than with the wepp model for rangelands

search for signs of ancient life on mars expectations from hydromagnesite microbialites salda lake turkey the 9white rock9 constituting a portion of what may be a lacustrine sedimentary sequence near the margin of a crater c 90 km across in sabaea terra mars measures 18 15 km 180 540 m high it is re interpreted as a lens of magnesium carbonate precipitated where ground waters seeped into an ancient evaporating crater lake were life to have emerged on mars as seems feasible then the 9white rock9 might be expected to comprise a complex of stromatolitic mounds salda golu lake in turkey is taken as an analogue this enclosed lake is nearly surrounded and underlain by partially serpentinized harzburgite hydromagnesite stromatolites microbialites up to 7 m high coalesce to form a group of small islands 200 m across the microbialites are seen to be growing near the mouth of the usually dry salda river in the southwestern sector smaller developments of hydromagnesite encircle the lake and image processing of satellite data reveals a second extensive zone beneath the lake surface over a delta in the southeast individual columns a few centimetres high constitute bulbous mounds which are about 2 m in diameter these columns terminate in domes a centimetre or so across the domes are often annulated and are covered with a green biofilm a few millimetres thick comprised of cyanobactrial filaments the columns consist of alternating fine and coarse hydromagnesite layers differentiated on a millimetric scale the coarser layers near the surface still contain traces of the biofilm fossil microbialites were also discovered in the friable hydromagnesite cliffs shoreward of the main developments though the structures of the individual microbes have not survived instead the vestiges of microbialites are easily recognized and delineated by their coarse grain size and high porosity annular structures on their upper surfaces can be seen in places the intervening and overlying material also comprised of hydromagnesite is a semi lithified mud bulbous megascopic structures separated by finer grained magnesium carbonate mudstone within strata in the 9white rock9 would be strong evidence of a photosynthetic microbial genesis another deposit of white rock on the western margin of juventae chasma could have a similar origin

orbital identification of carbonate bearing rocks on mars geochemical models for mars predict carbonate formation during aqueous alteration carbonate bearing rocks had not previously been detected on mars surface but mars reconnaissance orbiter mapping reveals a regional rock layer with near infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the nili fossae region the carbonate is closely associated with both phyllosilicate bearing and olivine rich rock units and probably formed during the noachian or early hesperian era from the alteration of olivine by either hydrothermal fluids or near surface water the presence of carbonate as well as accompanying clays suggests that waters were neutral to alkaline at the time of its formation and that acidic weathering proposed to be characteristic of hesperian mars did not destroy these carbonates and thus did not dominate all aqueous environments

endomycorrhizae in miocene paleosols implications in biotite weathering and accumulation of dolomite in plant roots sw madrid basin spain abstract this work provides evidence of arbuscular mycorrhizae or endomycorrizhae and their role in mineral weathering and soil carbonate production from miocene paleosols formation of the paleosols bearing mycorrhizae took place in marginal lacustrine and distal fan deposits in the madrid basin endomycorrizhae fossils are preserved as carbonate in biotite filled rhizoliths fossilized mycorrhizae are morphologically identical to those of living counterparts and consist of two linked parts the extraradicular segment surrounding the root is represented by a calcitic mycelium comprising spores and two types of hyphae that are seen to attach and to corrode the mineral grains the intraradicular part is made of a network of branched filaments hyphal coils arbuscules and likely vesicles that are mixed with the cortical root cells of the plants fossilized as dolomicrosparite aggregates the preferential formation of the dolomite aggregates on biotite grains in rhizoliths is interpreted to reflect a focused interest of the plants on this mineral and indicates the mineral may have been used as a source of plant nutrients the close association of the carbonate with the silicate further suggests that the precipitation of dolomite in the root cells and the weathering of the biotite resulted from interrelated processes main mechanisms to produce mineral dissolution include organic acids and chelate secretions by the plant roots and fungi which would remove nutrients these results confirm that carbonate paleosols provide an excellent reservoir of information on ecological interactions and biogeochemical cycling and expand the range of biogenic processes and their resultant products involved in the formation of carbonate soils this approach has broad applications given the abundance of carbonate paleosols and the endomycorrhiza record dating back 400 million years

relationship between discharge velocity and flow area for rills eroding loose non layered materials a relationship between discharge flow velocity and flow area in rills is established using data from four field and laboratory studies the proposed relationship is shown to predict successfully flow velocities measured in six other studies although slopes range from 0 035 to 0 45 and soil materials range from stony sands over silt loams to vertisols mean flow velocity can be well predicted from discharge alone thus there is no important influence of slope and or soil material characteristics on flow velocities in rills the proposed relationship may be used to improve performance of deterministic flow routing models when applied to rilled catchments furthermore it allows the calculation of unit stream power which has been shown to be related to the transporting capacity of overland flow in terms of slope and discharge

controls of temperature alkalinity and calcium carbonate reactant on the evolution of dolomite and magnesite stoichiometry and dolomite cation ordering degree an experimental approach abstract natural and synthetic dolomites exhibit large scatter of structural and compositional characteristics with respect to variations in their stoichiometry and cation ordering degree cod but the physicochemical factors controlling dolomitization reactions are still poorly understood and by far not quantified in the present study dolomite was synthesized in batch reactors at temperatures of 150 c 180 c and 220 c by reacting calcite or aragonite with an artificial solution containing mgcl2 and nahco3 over a 360 day period the obtained results on the evolution of the stoichiometry of dolomite and magnesite and on the cod of dolomite indicate that the formation of stoichiometric and well ordered dolomite and of stoichiometric magnesite follow a ripening process and proceed through dissolution and re precipitation of a sequence of intermediate phases such as ca magnesite huntite very high mg calcite vhmc and disordered dolomite the initial occurrence and the metastability of the precursor phases were dependent particularly on reaction temperature and to a lesser extent on the caco3 source used temperature is the major rate limiting parameter of the evolution of dolomite and magnesite stoichiometry and dolomite cod corroborating results from prior experiments and natural deposits from our data we determined kinetic rates for the stoichiometric ripening of magnesite and dolomite and for the dolomite cod for the first time using a first order reaction kinetic approach the kinetics increase with temperature following the linear arrhenius equation in turn allowing for activation energies of the respective processes to be determined 44 0 57 4 and 9 3 kj mol 1 respectively interestingly we found the reaction rate for magnesite stoichiometric ripening is approximately one third of that for dolomite regardless of the temperature carbonate alkalinity or caco3 phase used for reaction furthermore we see that the activation energies required for the formation of ideal dolomite decreases in sequence from the precipitation of an initial vhmc phase 133 5 kj mol 1 arvidson and mackenzie 1999 followed by stoichiometric ripening 57 4 kj mol 1 to cation ordering 9 3 kj mol 1 for perfect ordering extrapolation of the obtained dolomitization rates indicates that about 1 4 and 6 8 myr are required to approach ideal dolomite at 50 and 25 c respectively explaining the large absence of ordered dolomite in modern sedimentary successions the present dataset provides new insights into dolomitization pathways and rates and contributes to a better understanding of the wide range of stoichiometries and cod values of dolomite and its evolution versus magnesite observed in the geological record and in laboratory studies

gully head erosion processes on a semi arid valley floor in kenya a case study into temporal variation and sediment budgeting a three year monitoring programme of gully head retreat was established to assess the significance of sediment production in a drainage network that expanded rapidly by gully head erosion on the low angled alluvio lacustrine njemps flats in semi arid baringo district kenya this paper discusses the factors controlling the large observed spatial and temporal variation in gully head retreat rates ranging from 0 to 15 m a 1 the selected gullies differed in planform and in runoff contributing catchment area but soil material and land use were similar the data were analysed at event and annual timescales the results show that at annual timescale rainfall amount appears to be a good indicator of gully head retreat while at storm event timescale rainfall distribution has to be taken into account a model is proposed including only rainfall p and the number of dry days dd between storms which explains 56 per cent of the variation in retreat rate of the single headed gully of lam1 a detailed sediment budget has been established for lam1 and its runoff contributing area rca by measuring sediment input from the rca the sediment output by channelized flow and linear retreat of the gully head for nine storms it can be seen that erosion shifts between different components of the budget depending on the duration of the dry period dd between storms sediment input from the rca was usually the largest component for the smaller storms the erosion of the gully head occurred as a direct effect of runoff falling over the edge ghwaterfall and of the indirect destabilization of the adjacent walls by the waterfall erosion and by saturation ghmass storage the latter component ghmass storage was usually much larger that the former ghwaterfall the sediment output from the gully was strongly related to the runoff volume while the linear retreat because of its complex behaviour was not overall the results show that the annual retreat is the optimal timescale to predict retreat patterns more detailed knowledge about relevant processes and interactions is necessary if gully head erosion is to be included in event based soil erosion models copyright 2001 john wiley sons ltd

modern and holocene carbonate sedimentology of two saline volcanic maar lakes southern australia the basin lakes are two adjacent maar lakes located in the centre of the western volcanic plains district of victoria australia both lakes are saline and alkaline west basin lake is meromictic whereas east basin is a warm monomictic lake the carbonate mineral suite of the modern offshore bottom sediments of these basins consists mainly of dolomite and calcite with smaller amounts of hydromagnesite and magnesite in west basin and monohydrocalcite in east basin the dolomite hydromagnesite magnesite and monohydrocalcite are endogenic in origin being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment water interface the calcite is biologically precipitated as ostracod valves in addition to the carbonates in the modern offshore deep water sediments the lakes also contain a girdle of nearshore carbonate hardgrounds both beachrock and microbialites algal boundstones are present these modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics morphologies and compositions in west basin the hardgrounds are composed mainly of dolomite hydromagnesite and magnesite whereas dolomite and monohydrocalcite dominate the east basin sediments aragonite high mg calcite kutnahorite siderite and protohydromagnesite also occur in these lithified carbonate units stratigraphic variations in the carbonate mineralogy of the holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the basins these changes in conjunction with fluctuations in organic remains and fossil content indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western victoria

preliminary study of the erosion mechanisms in a biancana badland tuscany italy abstract italian badlands are present along the appennini mountains with an annual precipitation which ranges from 400 to 1200 mm the genesis of these badlands has often been studied whereas current processes and erosion rates have only rarely been examined and measured this paper describes a preliminary study on a badland developed in temperate mediterranean climate and located in sw tuscany italy the selected site is characterized by biancane dome shaped features a few meters in height generally having a vegetated nne aspect and severely eroded ssw slope with a sub horizontal basal pediment field rainfall and runoff experiments were carried out field results were substantiated by data collected in specially devised laboratory tests it was concluded that near surface pipes which develop at the point of contact between a surficial layer of weathered and severely fractured regolith and the less fractured and weathered regolith underneath are the main drainage channels while rills play a secondary role soil detachment is largely due to mechanical and chemical slaking of the regolith during infiltration the role of runoff is almost solely transport in particular it was found that the runoff detachment rate is limited by the rate at which the water front penetrates into the regolith

silicate bioweathering and biomineralization in lacustrine microbialites ancient analogues from the miocene duero basin spain the miocene dolomite chert microbialites studied here offer a complete record of the geochemical cycles of silicate weathering and the subsequent formation of secondary products the microbialites were formed in lacustrine systems during the miocene of the duero basin central spain mineralogical chemical and petrographic results provide evidence of the mediation of microbes in early weathering and by product formation processes irrespective of the composition the surfaces of the grains were subject to microbial attachment and concomitant weathering palaeo weathering textures range from surface etching and pitting to extensive physical disaggregation of the minerals extreme silicate weathering led to the complete destruction of the silicate grains whose prior existence is inferred from pseudomorphs exhibiting colonial textures like those recognized in the embedding matrix detailed petrographic and microanalytical examinations of the weathering effects in k feldspars show that various secondary products with diverse crystallinity and chemical composition can coexist in the interior of a mineral the coexistence of by products is indicative of different microenvironmental conditions likely created by microbial reactions thus the presence of varied secondary products can be used as a criterion of biogenicity intensive alteration of p bearing feldspars suggests that mineral weathering may have been driven by the nutrient requirements of the microbial consortium involved in the precipitation of dolomite the rock record provides useful information on mineral weathering mediated by microbes

sandstone diagenesis in a halite deposit from surface to high grade diagenesis haselgebirge formation eastern alps abstract the evaporitic haselgebirge formation of the northern calcareous alps was deposited in an evolving rift during the upper permian the jurassic to cretaceous alpine orogenesis caused an overprint of 200 c the aim of the study was to investigate the diagenetic evolution of these evaporite dominated sediments from intact sandstone lenses field work thin section analysis sem and xrd were completed by measurements of organic material and age dating in this paper we show that all authigenic minerals developed step by step on a prograding path but under low temperatures of

stable isotope characterisation of co existing carbonates from the holocene gallocanta lake ne spain palaeolimnological implications the sedimentary succession of gallocanta lake a closed saline lake located in the iberian range ne spain documents two successive lacustrine stages 1 brackish lake stage and 2 shallow saline lake stage the saline stage corresponds to the present day situation in which the lake water properties are mainly controlled by a strongly negative annual water balance the carbonates of the brackish lake stage have relatively constant 18o values however they are rather high 18odo 2 4 and 18occ 4 5 mean values suggesting a hydrologically closed lake with a long residence time of the waters 18o values of carbonates from the saline stage vary greatly and are lighter than in the previous stage 18odo 0 5 18occ 0 7 18omgs 2 3 mean values these carbonates also precipitated in a hydrologically closed lake but in equilibrium with a lake water of more variable isotopic composition the 13c values for carbonates of both stages reflect a mixing of different pools of carbon but during saline stage 13c values have been more controlled by the equilibrium of the lake waters with atmospheric co2 during the current stage calcite and dolomite precipitate in gallocanta lake mainly during spring and summer although dolomite precipitation is more favoured towards the summer magnesite precipitates at the beginning of autumn when the first rainfall re dissolves the saline surface crust producing saline waters with a high mg2 content the isotopic composition of lake waters sampled in 2005 are far higher than those calculated from the carbonates it is considered that this could be due to two factors either because there have not been many extremely dry years like the year 2005 during the development of the lake or because the physical and chemical characteristics of the lake waters in such conditions are not appropriate for the development of these minerals

the hydromagnesite playas of atlin british columbia canada a biogeochemical model for co2 sequestration abstract anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide co2 through the carbonation of magnesium silicate minerals to form magnesium carbonate minerals the hydromagnesite mg5 co3 4 oh 2 4h2o playas of atlin british columbia canada provide a natural model to examine mineral carbonation on a watershed scale at near surface conditions co2 is biogeochemically sequestered by microorganisms that are involved in weathering of bedrock and precipitation of carbonate minerals the purpose of this study was to characterize the weathering regime in a groundwater recharge zone and the depositional environments in the playas in the context of a biogeochemical model for co2 sequestration with emphasis on microbial processes that accelerate mineral carbonation regions with ultramafic bedrock such as atlin represent the best potential sources of feedstocks for mineral carbonation elemental compositions of a soil profile show significant depletion of mgo and enrichment of sio2 in comparison to underlying ultramafic parent material polished serpentinite cubes were placed in the organic horizon of a coniferous forest soil in a groundwater recharge zone for three years upon retrieval the cube surfaces as seen using scanning electron microscopy had been colonized by bacteria that were associated with surface pitting degradation of organic matter in the soil produced chelating agents and acids that contributed to the chemical weathering of the serpentinite and would be expected to have a similar effect on the magnesium rich bedrock at atlin stable carbon isotopes of groundwater from a well situated near a wetland in the southeastern playa indicate that 12 of the dissolved inorganic carbon has a modern origin from soil co2 the mineralogy and isotope geochemistry of the hydromagnesite playas suggest that there are three distinct depositional environments 1 the wetland characterized by biologically aided precipitation of carbonate minerals from waters concentrated by evaporation 2 isolated wetland sections that lead to the formation of consolidated aragonite sediments and 3 the emerged grassland environment where evaporation produces mounds of hydromagnesite examination of sediments within the southeastern playa wetland suggests that cyanobacteria sulphate reducing bacteria and diatoms aid in producing favourable geochemical conditions for precipitation of carbonate minerals the atlin site as a biogeochemical model has implications for creating carbon sinks that utilize passive microbial geochemical and physical processes that aid in mineral carbonation of magnesium silicates these processes could be exploited for the purposes of co2 sequestration by creating conditions similar to those of the atlin site in environments artificial or natural where the precipitation of magnesium carbonates would be suitable given the vast quantities of mg rich bedrock that exist throughout the world this study has significant implications for reducing atmospheric co2 concentrations and combating global climate change

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

hydromagnesite stromatolites and sediments in an alkaline lake salda golu turkey abstract salda lake in sw turkey is a highly alkaline ph 9 water body with waters enriched in magnesium microbial stromatolites along shorelines contain a microflora of diatoms and cyanobacteria with extensive associated biofilms together these are responsible for precipitation of hydromagnesite although distribution is not congruent with either cells or films deposition has continued over several thousand years and is independent of both lake volume and the general concentration of lake waters the lake is rimmed on three sides by serpentinites and on the fourth by dolomite the bulk of the water entering the lake is meteoric fed via extensive gravelly alluvial fan deltas with predominantly serpentinite pebbles whose total areas and catchments exceed that of the lake these waters are significantly cooler than surface waters of the lake and there is no evidence for derivation from hot springs the magnesium is thought to have been leached from the gravels extensive areas of recent sediments accumulating on shorelines have apparently formed by mechanical breakdown of microbialites similar deposits form conspicuous terraces in two areas and point to the long term effectiveness of these processes terrace deposits are modified by a variety of subaerial processes including growth of hydromagnesite cements no microbial structures are preserved in these and it is presumed that they are destroyed during reworking fine grained hydromagnesite sediment derived from shorelines and from terraces is suspended in the water column during storms and is probably redeposited as turbidites in the deeper parts of the lake sediments of this kind may be a useful analogue for some ancient and economically important stratiform magnesite deposits

palaeoproterozoic magnesite lithological and isotopic evidence for playa sabkha environments magnesite forms a series of 1 to 15 m thick beds within the 2ae 0g a palaeoproterozoic tulomozerskaya formation nw fennoscandian shield russia drillcore material together with natural exposures reveal that the 680m thick formation is composed of a stromatolite dolomite red bed sequence formed in a complex combination of shallow marine and non marine evaporitic environments dolomite collapse breccia stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds all dolomite lithologies are marked by d 13 c values from 7ae1 to 11ae6 v pdb and d 18 o ranging from 17ae4 to 26ae3 v smow magnesite occurs in different forms finely laminated micritic stromatolitic magnesite and structureless micritic crystalline and coarsely crystalline magnesite all varieties exhibit anomalously high d 13 c values ranging from 9ae0 to 11ae6 and d 18 o values of 20ae0 25ae7 laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis and replaced dolomite before the major phase of burial crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic metamorphic history magnesite apparently precipitated from sea water derived brine diluted by meteoric fluids magnesitization was accomplished under evaporitic conditions sabkha to playa lake environment proposed to be similar to the coorong or lake walyungup coastal playa magnesite magnesite and host dolostones formed in evaporative and partly restricted environments consequently extremely high d 13 c values reflect a combined contribution from both global and local carbon reservoirs a 13 c rich global carbon reservoir d 13 c at around 5 is related to the perturbation of the carbon cycle at 2ae0 ga whereas the local enhancement in 13 c up to 12 is associated with evaporative and restricted environments

concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content abstract concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate the effects of rock fragment cover rc on sediment yield for an initially wet and an initially air dry loamy topsoil the experimental results indicate that rc reduces concentrated flow erosion rates e in an exponential way i e e e brc which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet rill erosion measured on runoff plots the decay rate b of this exponential relationship increased throughout the experiments because of scour hole development and bed armouring the concentrated flow erosion rates and b values also depend on the initial moisture content of the topsoil depending on rc mean concentrated flow erosion rates were 20 to 65 less on initially wet compared to initially air dry topsoils the mean value for b was 0 032 for the initially wet but only 0 017 for the initially air dry topsoil indicating that a rock fragment cover is less efficient in reducing concentrated flow erosion rates when the topsoil is initially air dry than when it is initially wet the results help explain the data scatter in reported relationships between rc and interrill rill erosion rates they also indicate that a given surface rock fragment cover will offer more protection to wet topsoils than to dry topsoils which are very common in mediterranean environments event based water erosion models should incorporate effects of antecedent soil moisture content as well as those of rc on concentrated flow erosion rates

magnesite formation by microbial activity evidence from a miocene hypersaline lake abstract this paper provides an ancient analogue for biologically mediated magnesite in lacustrine hypersaline environments thin beds of massive to crudely laminated magnesitic marls occur interbedded with mudstone and evaporite facies deposited in a saline lake mudflat sedimentary system during the lower miocene in the madrid basin central spain exposure of this succession in a recently excavated tunnel and in collected cores offered a good opportunity to study magnesite precipitates that have been preserved in unaltered conditions by primary evaporitic minerals specifically magnesite arranged as host matrix between and enclosed by displacive halite and glauberite crystals occurs closely associated with abundant microfossils embedded in sheets of organic matter interpreted as the remains of extracellular polymeric substances eps even many magnesite clots are seen to have resulted from the agglutination of fossilized bacterial bodies forming a biofilm the close and pervasive association of microorganisms and magnesite is used as evidence that microbes played a fundamental role in the precipitation of this mineral additional traces of microorganisms and microbial activity observed in magnesite beds include the isotopic ratios in the carbonates with the 13c pdb value averaging 6 2 the presence of carbonaceous film like structures and the association with patchly distributed pyrite and celestite minerals on the basis of the combined sedimentological mineralogical chemical and morphological signatures of the magnesitic deposits it is postulated that carbonate crystals precipitated in microbial mats magnesite crystals together with celestite local barite and or pyrite have been also observed to replace silicate and sulphate minerals it is suggested that magnesite precipitation was biochemically coupled with the early dissolution of the associated sedimentary minerals microbes may have used the associated minerals as source of energy and or of essential elements