hypersaline burial diagenesis delineated by component isotopic analysis late paleozoic limestones west texas abstract pennsylvanian to lower permian cyclic platform carbonates from the southwest andrews area of west texas are now 3 km below the land surface isolated samples of brachiopod shell zones of calcite cement and saddle dolomite from limestone cores of the southwest andrews oilfields have been analyzed isotopically to determine if 1 diagenesis is dominated by early meteoric processes as suggested by bulk rock isotopes 2 whether significant burial diagenesis occurred in hypersaline waters and 3 whether most burial cements are allochthonous or internally derived bulk isotopic analyses of limestones show systematic variation in 13c and 18o according to length of subaerial exposure and stratigraphic position chemically zoned calcite cements show four growth stages stage 1 and 2 cements have average 13c values of 4 6o oo and average 18o of 5 0o oo and most are interpreted to have precipitated during subaerial exposure in meteoric water a few stage 1 cements with average 13c values of 4 7o oo and 18o of 1 4o oo apparently precipitated below the sediment surface from modified seawater in a sulfate reduction zone stage 3 and 4 calcite cements precipitated after compaction and compose up to 25 of the total rock volume they have 13c values 0 2o oo to 4 4o oo that vary according to stratigraphic position and are similar to bulk rock samples of the host limestone the 13c values indicate local derivation by dissolution of adjacent rock the 18o values of stage 3 and 4 cement 5 5o oo to 3 5o oo are indistinguishable from stages 1 and 2 cements precipitated from meteoric water the decrease in 18o expected from elevated temperatures during burial present formation temperature of 55 c is negated by precipitation from a late permian brine with 18o values of 5o oo stages 3 and 4 calcite and saddle dolomite cements have 87sr 86sr values that are lower than 87sr 86sr values for contemporaneous marine carbonate 0 70830 this is probably due to incorporation of sr with low 87sr 86sr values 0 70672 to 0 70682 also from the late permian hypersaline brine the southwest andrews area has several diagenetic features that are probably more common in ancient limestone than previously recognized 1 burial cements have 18o values similar to meteoric cements when precipitated from hypersaline waters with 18o values 5o oo greater than contemporary seawater 2 deep burial cements have 13c values similar to the bulk rock because carbon is derived from pressure dissolution of nearby carbonate 3 bulk rock isotope values are characteristic of marine limestones subjected to meteoric diagenesis even though they have suffered substantial deep burial diagenesis this study clearly illustrates the value of analyzing diagenetic components to accurately understand porosity evolution in limestones

clean periodicity in secular variations of dolomite abundance in deep marine sediments abstract secular variations in dolomite abundance in deep marine sediments dadms indicate changing bio geochemical and sedimentary environments of deposition increasing dolomite abundances of the past indicate that past is the key to the present in this case spectral analyses of best available dadms record for the past 140 million years m y using a powerful clean spectral algorithm for the first time reveal a clean and statistically significant at 95 cl periodicity of about 24 5 1 5 m y maximum entropy spectral analysis mesa of the dadms record further confirms the above finding these results reinforce the general belief that changes in the earth s processes occurred in a cyclic manner interestingly 24 5 1 5 m y periodicity in the dadms record also compares extremely well within error limit with the marine mass extinction cycle 26 m y and lends credence to the cyclicity of marine processes it may be inferred that quasi periodic terrestrial records of sealevel climate and environmental changes coupled with some modest bio geochemical variations may have stimulated quasi periodic variations of dolomitization

diagenetic stabilization of the upper permian dalan formation persian gulf basin abstract the upper permian dalan formation is a supergiant gas reservoir in the persian gulf basin reservoir quality is controlled by the original facies type dolomitization and anhydrite cementation this study focuses on the texture petrographic analysis bulk stable isotope values and trace element composition in order to identify the original mineralogy of carbonate grains and cements and to assess the synsedimentary and diagenetic processes including the timing and setting of dolomitization the limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone and neomorphism and low mg calcite cementation blocky and drusy crystals syntaxial overgrowths in the active freshwater phreatic zone the relatively pristine 18o and 13c values low mn and relatively high sr and na contents indicate that this meteoric diagenesis was relatively limited the remaining porous space was cemented by blocky calcite later during burial four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites dolomicrite dolomicrosparite dolosparite and saddle dolomite dolomicrite formed syndepositionally from the evaporated permian seawater or by reflux of dense evaporated brines during shallow burial the second major episode of dolomitization dolomicrosparite and dolosparite was driven by reflux of warmer and more saline fluids at greater depth which affected both previously non dolomitized and dolomitized sediments the final diagenetic stabilization during deep burial is associated with extensive pressure solution fracturing vein filling of calcite and dolomite cements and crystallization of saddle dolomite

floristic migration in south america during the carboniferous phytogeographic and biostratigraphic implications abstract new data on the carboniferous flora of northern south america are combined with existing data for gondwanaland to re examine the floristic evolution of south america during the carboniferous the integration of paleobotanical palynological paleomagnetic paleogeographic and paleoclimatologic data makes it possible to reconstruct the floristic evolution of south america during the carboniferous as a dynamic process that was mainly controlled by the northward movement of the south american plate towards the equator during the carboniferous three carboniferous floras the cold archaeosigillaria lepidodendropsis frenguellia flora the temperate nothorhacopteris flora and the tropical euramerican flora characterize south america intra south american floristic migration is documented by the different age and composition of the nothorhacopteris flora it is of late early carboniferous age in northern south america bolivia northeastern brazil and peru whereas it is of late carboniferous age in southern south america argentina and southern brazil the flora was adapted to warm temperate climatic conditions indicating that southern south america moved into this climatic zone during the late carboniferous this migration explains the different ages recorded for the same taxa e g nothorhacopteris in northern and southern south america taxonomic differences between the northern and southern nothorhacopteris flora reflect visean westphalian evolution

marine and continental water contributions to a hypersaline basin using diatom ecology sedimentology and stable isotopes an example in the late miocene of the mediterranean hellin basin southern spain abstract a multidisciplinary approach including diatom ecology petrography and stable isotopes of carbonates and sulfates is applied to reconstruct the conditions of evaporitic deposition in the hellin basin the hellin basin is one of many similar restricted basins that evolved in this region just after the emplacement of the betic thrust complex these studies show that continental conditions prevailed in a restricted shallow basin during the evapoitic deposition episodic marine flooding however is indicated by diatom assemblages of diatomite layers intercalated in thick continental gypsum deposits which represent the last stage of concentration controlled by arid climate isotope data indicate a continental origin of the sulfate ions which were probably supplied to the basin by continental waters enriched in sulfates from the weathered triassic evaporites so during this time the basin evolved into a continental drainage system with hypersaline magnesian conditions dolomite dominant aqueous sulfate from triassic sources is affected by bacterial sulfate reduction but was further reoxydized as sulfate in oxydized fresh water the importance of sulfate bacterial reduction indicates abundant organic matter and high organic productivity it indicates anoxic condition in the bottom sediment and possible episodes of water stagnation this total sedimentologic and diagenetic picture together with excellent paleoenvironmental diatom control reflects the complexity of the basin filling within the intramontane basins of the betic chain

through water terrestrial laser scanning of gravel beds at the patch scale acquiring high resolution topographic data of natural gravel surfaces is technically demanding in locations where the bed is not exposed at low water stages often the most geomorphologically active surfaces are permanently submerged gravel beds are spatially variable and measurement of their detailed structure and particle sizes is essential for understanding the interaction of bed roughness with near bed flow hydraulics sediment entrainment transport and deposition processes as well as providing insights into the ecological responses to these processes this paper presents patch scale laboratory and field experiments to demonstrate that through water terrestrial laser scanning tls has the potential to provide high resolution digital elevation models of submerged gravel beds with enough detail to depict individual grains and small scale forms the resulting point cloud data requires correction for refraction before registration preliminary validation shows that patch scale tls through 200 mm of water introduces a mean error of less than 5 mm under ideal conditions point precision is not adversely affected by the water column the resulting dems can be embedded seamlessly within larger sub aerial reach scale surveys and can be acquired alongside flow measurements to examine the effects of three dimensional surface geometry on turbulent flow fields and their interaction with instream ecology dynamics copyright 2011 john wiley sons ltd

modeling bed load rates from fine grain size patches during small floods in a gravel bed river this study investigates the applicability of five bed load transport formulas the meyer peter and muller schoklitsch bagnold smart and jaeggi and rickenmann equations to predict bed load transport rates of frequent low magnitude flood events maximal bankfull discharge for a mountainous poorly sorted gravel bed river characterized by a bimodal sediment size distribution and spatially distributed patches for model parametrization special emphasis was placed on the spatial composition of the grain size distribution gsd to evaluate the impact of preferential removal of sediments from patches with finer sediments on bed load transport three parametrization approaches to the choice of an appropriate sediment size that considered the apparent bimodality of the gsd to varying degrees were tested the modeling study demonstrated that the incorporation of spatial structure of gsd and its bimodal character has an important impact on model performance a unimodal parametrization failed to reproduce meas

sequential evolution and diagenesis of pleistocene coral reefs south sinai egypt abstract the uplifted pleistocene coral reefs in south sinai represent two major depositional sequences they generally form two morphological terraces which locally are tilted and offset by still active faulting the major reef sequences are composed of several small scale sequences corals grew rapidly when accommodation space was created by eustatic sea level rise or fault block subsidence as the accommodation decreased coral rubble and siliciclastic sands prograded over reefal and lagoonal sediments thus creating a shallowing upward sequence terrigenous input was further stimulated by a more humid climate during interglacial times 230 th 234 u dating places the older reef cycle between 350 000 and 270 000 years b p which corresponds to the interglacial period of isotope stage 9 the younger reef cycle has been dated between 140 000 and 60 000 years b p isotope stage 5 an intermediate relic sequence found in only one outcrop can be attributed to isotope stage 7 correlation of the small scale sequences is difficult age dating is not precise enough and local tectonic activity in many cases overruled the smaller eustatic fluctuations the younger reef sequence commonly exhibits marine cements whereas the older sequence was exposed to freshwater dissolution and cementation an important feature is the locally pervasive dolomitization especially of the older reef oxygen and carbon isotope values suggest that the dolomite formed in a seawater dominated mixing zone multiple phases of dissolution cementation and dolomitization point to a very complex diagenetic history

reconstruction of palaeohydrological conditions in a lagoon during the 2nd zechstein cycle through simultaneous use of δd values of individual n alkanes and δ 18 o and δ 13 c values of carbonates for the first time 18o and 13c values from carbonates and d values of individual n alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the central european zechstein basin cezb a 12 m core covering the complete ca2 interval and adjacent anhydrites a1 and a2 was analyzed for 18o and 13c values of dolomitized carbonates and d values of individual n alkanes 18ocarb values 2 to 5 vs vpdb were strongly influenced by evaporation and temporal freshwater input into the lagoon the 13ccarb values 1 to 4 vs vpdb were controlled mainly by changes in primary production both isotopic ratios show an inverse relationship throughout most of the core contradicting diagenetic alteration since 13ccarb values are not altered significantly during dolomitization assuming a temperature range of 35 40 c in the lagoon 18ocarb values of 2 5 to 8 vs vsmow for the lagoonal water can be reconstructed the lagoon may have desiccated twice during the ca2 interval as indicated by very high 18ocarb and low 13ccarb values coinciding with increasing amount of anhydrite in the analyzed sample these events seem to reflect not just local but a regional intra ca2 cyclicity measured d values of the short chain n alkanes namely n c16 and n c18 which are widely used as indicators for photosynthetic bacterial and algal input reflect the isotopic composition of the lagoonal water assuming constant fractionation during incorporation of hydrogen into lipids of 160 an average d value of 70 vs vsmow can be reconstructed for the lagoonal water accounting for very arid conditions the long chain n alkanes n c27 n c28 n c29 and n c30 thought to be derived from the leaf waxes of terrestrial higher plants were shown to be depleted in d relative to the short chain alkanes therefore indicating a different hydrogen source terrestrial plants in arid areas mainly use water supplied by precipitation by using a smaller fractionation of 120 due to evaporation processes in the leaves reconstructed values vary between 74 and 9 vs vsmow these values are not indicating extremely arid conditions implying that the long chain n alkanes were transported trough wind and or rivers into the lagoon at the zechstein sea coast dwater values reconstructed using the n c16 alkane and 18o water values independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon altogether our results indicate that hydrogen isotopic ratios of n alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction

carbon isotope δ13ccarb stratigraphy of the lower middle ordovician tremadocian darriwilian in the great basin western united states implications for global correlation abstract new stable carbon isotope data 13ccarb from lower middle ordovician tremadocian to darriwilian carbonate mudstone and wackestone rocks of the pogonip group are presented from two sections in the great basin region usa shingle pass east central nevada and the ibex area western utah the pogonip group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the late cambrian furongian to middle ordovician darriwilian the shingle pass and ibex area sections have been previously studied for their conodont biostratigraphy and contain a north american midcontinent conodont fauna that range from the cordylodus intermedius zone uppermost cambrian to the phragmodus polonicus zone darriwilian the 13c trend has four distinct characteristics recognized in both great basin sections 1 a drop in 13c from 1 at the base of the ordovician tremadocian to 0 7 2 a 1 to 2 positive 13c shift in the uppermost rossodus manitouensis zone during the late tremadocian 3 a gradual 13c increase from 2 to ca 0 during the end of the early ordovician floian and 4 a steady 13c decrease from 0 to 4 to 5 during middle ordovician dapingian darriwilian in the lower ordovician 13c trends reported here from the great basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies instead these lower ordovician isotope data most likely reflect primary seawater chemistry and changes in 13c on a global scale this interpretation is supported by the excellent correlation of 13c in the lower ordovician to other 13c trends reported from the sections in the argentine precordillera la silla and san juan formations and in western newfoundland st george and table head groups these correlations using 13c are consistent with published biostratigraphic data and provide an integrated and high resolution chemo biostratigraphic framework for the lower ordovician sedimentary record of the laurentian margin the middle ordovician portion of the 13c curves in the great basin represented by the kanosh and lehman formations shows significant isotopic depletion relative to the section in argentina thus although there is some indication that minima and maxima in the middle ordovician curves can be correlated the great basin sections show clear evidence of overprinting by local variables related to both diagenesis dolomitization and platform restriction

microbial primary dolomite from a norian carbonate platform northern calabria southern italy the origin of fine grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the norian dolomia principale of northern calabria southern italy microbial carbonates there consist of stromatolites thrombolites and aphanitic dolomites high relief thrombolites and stromatolites characterize sub tidal facies and low relief and planar stromatolites with local oncoids typify the inter supratidal facies skeletal remains are very rare in the latter whereas a relatively rich biota of skeletal cyanophycea red algae and foraminifera is present in the sub tidal facies some 75 of the succession consists of fabric preserving dolomite especially within the microbial facies whereas the rest is composed of coarse dolomite with little fabric preservation three end members of dolomite replacement fabric are distinguished type 1 and type 2 fabric retentive with crystal size 5 and 5 60 m respectively and type 3 fabric destructive with larger crystals from 60 to several hundred microns in addition there are dolomite cements precipitated in the central parts of primary cavities during later diagenesis microbialite textures in stromatolites are generally composed of thin dark micritic laminae of type 1 dolomite alternating with thicker lighter coloured laminae of the coarser type 2 dolomite thrombolites are composed of dark micritic clotted fabrics with peloids composed of type 1 dolomite surrounded by coarser type 2 dolomite marine fibrous cement crusts are also present now composed of type 2 dolomite scanning electron microscope observations of the organic rich micritic laminae and clots of the inter supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains these probably derived from the fossilization of micron sized coccoid bacteria and spheroidal ovoidal nanometre scale dwarf type bacterial forms furthermore there are traces of degraded organic matter probably also of bacterial origin the microbial dolomites were precipitated in a hypersaline environment most likely through evaporative dolomitization as suggested by the excess ca in the dolomites the small crystal size and the positive 18o values the occurrence of fossilized bacteria and organic matter in the fabric preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn sedimentary dolomite

biotically constrained palaeoenvironmental conditions of a mid holocene intertidal lagoon on the southern shore of the arabian gulf evidence associated with a whale skeleton at musaffah abu dhabi uae whale remains a left and right mandible scapula humerus and fragmentary radius and ulna as well as parts of the cranium and rostrum belonging to a probable humpback whale megaptera cf novaeangliae were found in the well described sabkha sequence exposed in the musaffah industrial channel abu dhabi united arab emirates more precisely the whale remains were found in a series of sediments representing a range of lagoonal facies the sediments surrounding the whale bones were age dated at approximately 5200 14 c yrs bp and are therefore interpreted to correspond to the previously documented late flandrian sea level peak preceding a fall in sea level which culminated in the supratidal sabkha overprint of the carbonates associated with the whale remains is an assemblage of molluscs foraminifera and ostracods together with the inferred presence of sea grass and algae these facies are interpreted to indicate a very shallow subtidal to intertidal lagoonal environment cirripede remains found associated with the skeleton were identified as those of the whale barnacle coronula diadema and hence had their origins with the whale significantly the low species diversity of microfossils suggests that higher salinities existed in the mid holocene lagoon than are present in modern counterparts this is here inferred to be related to the onset of continental aridity in arabia during the mid holocene

defining phases of bedload transport using piecewise regression differences in the transport rate and size of bedload exist for varying levels of flow in coarse grained channels for gravel bed rivers at least two phases of bedload transport with notably differing qualities have been described in the literature phase i consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface transport rates during phase ii are considerably greater than phase i and more coarse grains are moved including material from both the channel surface and subsurface transition from phase i to phase ii indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels while the existence of different phases of transport is generally acknowledged the threshold between them is often poorly defined we present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel bed channels in colorado and wyoming usa the piecewise regression recognizes the existence of different linear relationships over different ranges of discharge the inflection where the fitted functions intersect is interpreted as the point of transition from phase i to phase ii transport this is termed breakpoint a comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during phase i transport indicating negligible movement of grains in this size range gravel larger than about d16 of the channel surface is more consistently trapped during phase ii transport the persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached even though the size of the sediment approaches the size limits of the sampler 76 mm in all cases a relative breakpoint rbr was defined by the ratio between the discharge at the breakpoint and the 1 5 year flow a surrogate for bankfull discharge expressed as a percentage the median value of rbr was about 80 percent suggesting that phase ii begins at about 80 percent of the bankfull discharge though the observed values of rbr ranged from about 60 to 100 percent variation in this value appears to be independent of drainage area median grain size sorting of bed materials and channel gradient at least for the range of parameters measured in 12 gravel bed channels data archive http dx doi org 10 2737 rds 2007 0003

sediment transport from continuous monitoring in a perennial mediterranean stream vericat d batalla r j 2010 sediment transport from continuous monitoring in a perennial mediterranean stream catena 82 2 77 86 sponsorship research project sediment export from large semi arid catchments measurements and modelling sesam funded by the deutsche forschungsgemeinschaft dgf

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

sediment supply grain protrusion and bedload transport in mountain streams 1 steep streams occupy a large fraction of mountainous drainage basins and partially control the sediment supplied to downstream rivers in these channels sediment transport equations typically over predict bedload flux by several orders of magnitude because they do not account for sediment supply limited conditions thus accurate predictions of bedload flux require an estimate of the sediment available for transport in a given event we demonstrate through field measurements that boulder step protrusion is a proxy for sediment availability protrusion is also a function of the time elapsed since an extreme event and this simple relationship can be used to estimate the relative sediment availability at any given time in addition bedload transport predictions in a steep channel were only accurate if they included this variable protrusion predictions of sedimentation hazards water quality river restoration success long term channel network evolution and channel stability may therefore require estimates of sediment availability for transport

biotic and abiotic processes in the formation and diagenesis of permian dolomitic stromatolites zechstein group ne england abstract the crinkly bed of the upper permian zechstein group succession roker formation hauptdolomit z2c in northeast england is a distinctive 1 4 meter thick unit that has been interpreted as a stromatolite of microbial origin or alternatively as the result of purely abiotic precipitation close examination of this stratigraphically significant unit shows that it is the result of both physical and microbial processes and was deposited in hypersaline shallow water immediately following lowstand evaporite deposition within the zechstein basin the internal structure of the crinkly bed consists of fine millimeter scale laminae of alternating clotted peloidal or aphanitic micritic layers and granular laminae composed of silt size peloids with rare hollow spheroids the lamination is extremely regular in thickness and laterally persistent for up to 4 meters a microbial community composed of coccoid and filamentous microorganisms inferred from fossil evidence colonized the original sediment surface it is suggested that this biofilm was responsible for the precipitation of both the micrite laminae and most of the peloids of the granular laminae the peloids are consequently interpreted as autochthonous this interpretation of a microbial origin is consistent with the microstructure and geochemical signatures of the micritic dolomite which constitutes both the continuous micritic laminae and the peloids themselves primary dolomite precipitation is hypothesized to have taken place from marine to slightly hypersaline waters mediated by bacterial metabolism similar to the process that operates in modern intertidal microbial mats the crinkly bed shows macro structures that vary along a continuum from symmetrical to asymmetrical centimeter scale ripple like structures some showing interference patterns through to domical and conical structures more typical of stromatolites the more ripple like structures have a lamination of microbial origin identical to that within the domal conical stromatolites there is no internal cross lamination their origin is equivocal they could be the result of physical current or wave induced deformation of a surficial biofilm or their form could be inherited from true mechanically deposited ripples not observed that provided a template for biofilm growth changes in the nature and strength of the wave and or current regime and possibly in the community structure of the microbial mat produced this array of microbially induced macrostructures miss the latter plus the absence of desiccation features indicative of intertidal to supratidal deposition suggest subtidal sedimentation at a depth of several meters intergranular gypsum cement precipitated during early diagenesis and ferroan sparry dolomite precipitated during burial tertiary uplift and the consequent exposure to meteoric waters resulted in the precipitation of calcite cements that replaced gypsum and pre existing dolomite probably via the biological mediation of heterotrophic bacteria the interpretation of this distinctive stratigraphic unit illustrates the difficulty in distinguishing between biotic and abiotic processes in the sedimentary record

measuring and defining bimodal sediments problems and implications gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions these can be quantified in several ways giving wide differences in derived statistics and calculations problems associated with measuring summarizing and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition no single index of bimodality serves all purposes alternatives are suggested measured near bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume this has implications for the abrupt downstream transition often found from gravel to sand bed

an upper paleozoic bio chronostratigraphic scheme for the western margin of gondwana abstract the carboniferous and permian fossiliferous sequences of the central western argentina contain abundant plant remains palynomorphs and invertebrates they include a continuous record of large distribution in the paganzo rio blanco calingasta uspallata and san rafael basins the most recent biostratigraphic schemes recognize a floristic succession represented by the biozones archaeosigillaria frenguellia af biozone frenguellia eximia nothorhacopteris kellaybelenensis cordaicarpus cesarii fnc biozone nothorhacopteris botrychiopsis ginkgophyllum nbg biozone interval biozone and gangamopteris biozone the associated palynological record is represented by the biozones reticulatisporites magnidictyus verrucosisporites quasigobbetti mq biozone raistrickia densa convolutispora muriornata dm biozone pakhapites fusus vittatina subsaccata fs biozone and lueckisporites weylandites lw biozone the precise age of the upper paleozoic western gondwanan biozones has been under discussion and remains controversial to date in some regions the main issue hampering an integrated comparison of the gondwanan biozones was its imprecise chronostratigraphic framework however new studies in some argentinian stratigraphic sections bearing floras and faunas have yielded several radiometric ages from these 206 pb 238 u zircon datings it is possible to determine the chronostratigraphic range of many fossiliferous assemblages in this sector of gondwana in this way the af and mq biozones are restricted to the late mississippian and they would be not younger than 335 ma according to radiometric ages 206 pb 238 u ages suggest that the nbg dma and dmb biozones characterize the late serpukhovian glacial deposits and persisted up to the late bashkirian beds containing the interval and dmc biozones have yielded 206 pb 238 u ages of 312 82 0 11 ma and 310 71 0 1 ma which would indicate that both zones characterize the moscovian the remains of gangamopteris biozone found in the paganzo basin overlie basalt levels ranging between 308 6 and 293 6 ma therefore the incoming of the first glossopterids was closely associated to the carboniferous permian boundary in this part of gondwana the data presented in this paper are used for establishing comparisons with other gondwanan biozones constrained by absolute ages

reevaluation of the timing and extent of late paleozoic glaciation in gondwana role of the transantarctic mountains evidence from antarctica indicates that a 2000 km long section of the transantarctic mountains including victoria land the darwin glacier region and the central transantarctic mountains was not located near the center of an enormous car boniferous to early permian ice sheet as depicted in many paleo geographic reconstructions weathering profiles and soft sediment deformation immediately below the preglacial pre permian un conformity suggest an absence of ice cover during the carbonif erous otherwise multiple glacial cycles would have destroyed these features the occurrence of glaciotectonite massive and strat ified diamictite thrust sheets sandstones containing dewatering structures and lonestone bearing shales in southern victoria land and the darwin glacier region indicate that permian sedimenta tion occurred in ice marginal periglacial and or glaciomarine set tings no evidence was found that indicates the transantarctic mountains were near a glacial spreading center during the late paleozoic although these findings do not negate carboniferous glaciation in antarctica they do indicate that gondwanan glacia tion was less widespread and therefore that glacially driven changes to other earth systems i e glacioeustatic fluctuations cli mate were much smaller than previously hypothesized

paleoecology of brachiopod communities during the late paleozoic ice age in bolivia copacabana formation pennsylvanian early permian abstract studies of modern ecological communities demonstrate that climate change may trigger changes in diversity and taxonomic composition however these studies are fundamentally limited to short timescales and therefore cannot demonstrate the full impact of major climate change understanding the ecological response of marine invertebrate communities to the late paleozoic ice age lpia the last complete transition from icehouse to greenhouse can establish a more complete picture of the climate faunal relationship we analyzed brachiopod community structure in moscovian sakmarian mid pennsylvanian to early permian samples spanning the greatest extent of the lpia collected from four localities of the copacabana formation in bolivia ancoraimes yaurichambi cuyavi and yampupata cluster analysis reveals three main groups that appear to coincide with pre syn and post glacial times genus richness was significantly greater in samples during the asselian glacial episode however the difference may be due to a combination of smaller body size and time averaged mixing of genera from different depths during more rapid glacioeustatic sea level change genera present in bolivia consistently had warm water affinities even during the main glaciation but warm water taxa increased in abundance over time and the samples became increasingly dominated by characteristically north american genera overall mean body size and the size of particular genera were smaller in the asselian cluster these size changes likely reflect variations in substrate because marine invertebrates should be larger at cooler temperatures due to oxygen limitation at higher temperatures the monotonic increase in abundance of warm water genera and increasingly north american biogeographic affinity imply that community change was most likely the result of the northward drift of bolivia rather than a response to late paleozoic glacial nonglacial cycles this lack of climate related faunal change was probably a result of bolivia s mid latitude location during the late paleozoic because both the rate of temperature change and its magnitude were likely smaller at lower latitudes reducing the impact of climate change on marine communities

coarse bed material patch evolution in low order ephemeral channels abstract in river channel beds composed of a wide range of grain sizes the bed material is often arranged in discrete patches discernable by relative texture these bed material patches are the primary source of entrainable coarse sediment within the channel system and their composition and size have been found to influence the composition and rate of sediment transport twelve coarse gravel cobble sediment patches distributed throughout the channel network within a 4 53 ha watershed in southeastern arizona were monitored for 2 years changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4 6 years while no patch fully dispersed during the study period two new patches formed most coarse patches remained relatively stable in area and grain size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream because of the grain replacement process and the effect of other sediment supply dynamics the changes in patch area and grain size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch results indicate that relative stability varies from patch to patch influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch predictive models of coarse sediment transport and yield that assume the channel bed is a fixed source of sediment supply may not adequately capture the sediment dynamics within patchy channel beds and should be used with caution when applied to these environments due to the possibility of patch instability as documented in this study

dating penecontemporaneous dolomitization in carbonate reservoirs paleomagnetic petrographic and geochemical constraints the predominant dolomitization in the mississippian debolt formation western canada sedimentary basin was hypothesized to be early in three gas fields based on stable oxygen and carbon isotopic values and on strontium isotope ratios that are similar to postulated mississippian dolomite values and seawater ratios respectively as the absolute age could not be determined by this method paleomagnetism was used to place constraints on the age of the dolomitization the magnetic analyses on debolt specimens from the same wells revealed three magnetization components that could be tied to geologic diagenetic events 1 a low temperature 350 degreesc and coercivity 80 mt c component of primary or early diagenetic origin that is found mostly in the fine grained limestone and dolomitic muds the preservation of a primary or early diagenetic magnetization combined with the preservation of primary isotopic values indicates that little or no extrabasinal fluid flow is likely to have occurred thus in this particular area orogenically induced fluid flow cannot explain the presence of the cretaceous b magnetization consideration of the potential methods for forming this b component suggests that it is likely an in situ chemical remanence resulting from either hydrocarbon migration or dissolution and reprecipitation of fe rich minerals in the original pore fluids

hyperconcentrated flow and sediment transport at steep slopes in order to simulate a fine material slurry of a debris flow a clay suspension of various concentrations was recirculated in a steep flume the effect of an increasing fluid density and viscosity on the flow behavior and the bed load transport capacity of the flow was examined viscous effects were found to become important below a limiting particle reynolds number of about 10 above this limiting value density effects cause an increase in the bed load transport rates as compared to similar conditions with clear water as transporting fluid the experimental data in this range can be described with conventional newtonian formulas and is analyzed together with other bed load transport data two different calculation schemes are proposed for the steep slope range is n n 10 where the bed load concentration is significant with respect to the total flow depth below the critical particle reynolds number of 10 the bed load transport rates decreased strongly

bedload transport in gravel bed streams with unimodal sediment bedload transport in many gravel bed streams becomes highly complicated because of the nonuniformity of the grain size and the vertical stratification of channel bed material a new relation for computing bedload transport rates in gravel bed streams is proposed here in an effort to account for the variation of the makeup of the surface material within a wide range of shields stresses the relation employs a two parameter approach one related to the material in the pavement and the other to that in the subpavement layers the mode is used to represent the grain sizes of each layer available bedload transport data from gravel bed streams with unimodal sediment are used to test the accuracy of the relation a comparison with other bedload transport relations is also considered

bedload transport in the instrumented catchment of the rio cordon part ii analysis of the bedload rate abstract sediment transport is monitored by a specially designed instrumentation installed on a steep mountain streams the rio cordon the variability of bedload rate and yield reflects the variability in flow discharge bedload and flow data for ordinary low peak flow floods and exceptional sediment transport events are used to test the range of application of the bedload equations proposed for high gradient streams such equations overestimate bedload rate for ordinary events while their reliability increases when used to predict high bedload transport rates like those recorded during the exceptional flood that affected rio cordon on september 14 1994 the best agreement between computed and measured values of cumulated bedload volumes is found by using the bagnold bagnold r a 1956 the flow of cohesionless grains in fluids philos trans r soc london 249a 235 297 smart and jaeggi smart j m jaeggi m n r 1983 sediment transport on steep slopes mitteil 64 versuchsanstalt fur wasserbau hydrologie und glaziologie eth zurich switzerland 191 pp and rickenmann rickenmann d 1991 hyperconcentrated flow and sediment transport at steep slopes j hydraul eng zurig 117 11 1419 1439 formulas the schoklitsch equation schoklitsch a 1962 handbuch des wasserbaues 3rd edn springer wien also provides good estimations for instantaneous bedload rates at high sediment transport intensity and unlimited sediment supply conditions the formal type of the last equation is still valid and can be calibrated for low intensity bedload rates

stable isotope study of carbonate sediments from the coorong area south australia the mineralogy and isotope geochemistry of carbonate minerals in the coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water the different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral in addition the dolomite was not formed by simple solid state cation exchange the occurrence of two types of dolomite was shown by isotope analysis and sem observations the dolomite which is isotopically light 13c 1 to 2 0 18o 3 to 5 0 and of fine grain size 0 5 m probably precipitated under the influence of evaporitically modified continental water coarser grained dolomite up to 4 m is isotopically heavier 13c 3 to 4 0 18o 5 to 6 0 contains mg in excess of ca and was formed in or close to equilibrium with atmospheric co2 probably by the dolomitization of aragonite

dolomite formation in the shallow seas of the alpine triassic the alpine triassic units of switzerland northern italy and western austria offer an extensive geological archive in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context recent studies clearly demonstrate that large amounts of the alpine triassic dolomites are late diagenetic or hydrothermal nevertheless as part of multiple generations of diagenetic overprint some generations of fine crystalline ca rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments in this review three cases of alpine triassic dolomites are discussed where dolomite rocks may have formed during or soon after sedimentation the sedimentary facies indicate contrasting palaeoenvironmental conditions and hence document three different possible processes of dolomite formation i in the dolomite mountains northern italy dolomite beds of the partly isolated middle triassic anisian ladinian latemar platform are confined to the very top of shallowing upward lagoonal facies cycles ii dolomite beds of the san giorgio basin southern switzerland an intra platform basin that opened during the anisian ladinian transition are associated with organic carbon rich shales which were deposited in a deeper water environment under anoxic conditions iii in the entirely dolomitized platform facies of the dolomia principale hauptdolomit formation a very early generation of fine crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles this platform extended over thousands of square kilometres along the tethys margin during the late triassic carnian and norian and large amounts of carbonate were deposited under hypersaline sabkha like conditions representing three distinct depositional environments these three different triassic systems show features in common with several dolomitization models developed from the study of modern dolomite forming environments for example the sabkha model the evaporative lagoon lake model the organogenic model and the microbial model although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation dolomite forming processes during the triassic were apparently quite different from the modern world in terms of distribution and scale recent developments in stable isotope geochemistry and high resolution geochemical probing offer the possibility to make better reconstructions of triassic palaeoceanographic conditions and suggest a non actualistic approach to better understand dolomite formation during the triassic

dedolomitization and other early diagenetic processes in miocene lacustrine deposits ebro basin spain abstract a variety of meteoric diagenetic features reveal the development of a syngenetic karst on lacustrine deposits of the ebro basin diagenetic processes that operated on lacustrine laminated and stromatolitic carbonates include the following 1 a first syndepositional stage with processes such as dolomitization desiccation and related breccia formation and sulphate precipitation either as lenticular gypsum crystals or nodules this stage took place under progressive evaporation due to lake level fall when the previous carbonate deposits became exposed as a supra littoral fringe surrounding saline mud flats of adjacent sulphate depositional environments 2 a second early diagenetic stage in which processes such as sulphate dissolution and collapse brecciation dedolomitization calcite spar cementation and silicification occurred as a result of meteoric water input that caused a progressive rise in lake level light isotopic compositions 13 c and 18 o of diagenetic calcites versus heavier compositions in primary laminated and stromatolitic limestones confirm a meteoric influence the syngenetic karst is best developed at the boundary between two allostratigraphic units and coincided with one of the extensive stages of sulphate deposition at the end of the early miocene the karst facies occurred in an area that was a low relief barrier that separated two sites of sulphate deposition during low lake levels this indicates that the karst development was controlled by topographic changes within the basin and record a shift from arid to wetter climatic conditions as suggested by the overlying freshwater carbonate deposits the presence of diagenetic features such as those described in the central ebro basin affecting saline lacustrine carbonates is relevant because they can be used as indicators of subaerial exposure periods in terrestrial environments and they also reveal important palaeogeographic and palaeoclimatic events of basinal extent

a stable isotope study of the mercia mudstones keuper marl and associated sulphate horizons in the english midlands abstract widespread evaporitic sulphate horizons occur in the frequently dolomitic mercia mudstones upper triassic of the english midlands the mudstones were deposited on an extensive peneplain which had areas of upstanding minor relief charnwood massif derbyshire pennines london platform horst and graben structures formed during the early and middle triassic controlled late triassic sedimentation rates and peneplain slope directions a stable isotope study s c o of the sulphates and dolomites of the trent formation indicates that during the deposition of the lower fauld member grabens such as the need wood basin in staffordshire and areas marginal to a graben such as newark in nottinghamshire had a marine brine regime with minor continental input using constraints applied by sulphate concentrations of modern waters in arid environments the ratio of volume of marine input to volume of continental input lay between 3 1 and 4 1 the horst area east leake nottinghamshire was strongly influenced by continental brines which derived sulphate from the exposed carboniferous hathern anhydrite series a predominantly continental brine regime existed across the whole area during deposition of the overlying hawton member periodic marine influxes gave rise to thin sulphate horizons whilst continental run off was occasionally sufficient to form temporary lakes the blue anchor formation rhaetian was deposited in a lacustrine environment with waters of mixed marine and continental origin salinities were lower than those of the preceding trent formation brines

two phase diagenetic alteration of carbonate matrix middle ordovician limestones taiyuan city area china both pseudospar and micrite are present in carbonate matrix of middle ordovician wackestones in the taiyuan city area china under the scanning electron microscope pseudospar crystals display unpitted surfaces and an absence of aragonite relics in polished and slightly etched sections micritic crystals show a similar feature with slightly pitted surfaces and absence of aragonite relics this possibly indicates that both pseudospar and micrite have a similar original mineralogy i e as calcites this is further supported by their low sr contents 417 ppm the geochemistry of pseudospar and micrite suggests that they were the products of two distinct episodes of diagenesis affecting the matrix micrite was possibly formed in near surface meteoric environments but pseudospar which is characterized by a population of more depleted 18o values 9 3 to 10 2 pdb than that of micrite 7 4 to 8 7 pdb is explained to have formed during shallow burial diagenesis because its 18o composition is too light to be accounted for by near surface meteoric origin the lower sr contents 417 ppm of both pseudospar and micrite in conjunction with their more depleted 18o values 7 4 to 10 2 pdb are interpreted to be the results of meteoric diagenesis when compared with the geochemistry of marine diagenetic calcites the more negative and wider range of 13c values of pseudospar and micrite 0 3 to 4 7 in comparison with those of other coeval meteoric calcites was attributed to the influence of the decay of organic matter in carbonate matrix as well as to meteoric diagenesis although the contents of fe and mn between pseudospar and micrine overlap widely an inverse relationship exists between the concentrations of fe and mn and the 13c values in both pseudospar and micrite this can be accounted for by variable redox by variable redox conditions during diagenesis the 18o values of void fill sparry calcite cements in micrite 8 2 to 8 6 pdb fall into the category of near surface meteoric diagenesis possibly indicating that the majority of micrites were cemented earlier and consequently were resistant to any further alterations so that only minor amount of pseudospars were formed during shallow burial diagenesis by recrystallization

marine derived dolomite in the shallowly buried temperate port campbell limestone miocene otway basin australia the miocene port campbell limestone in the otway basin port campbell embayment south eastern australia is a shallowly buried 350 m temperate carbonate grainstone which consists primarily of benthonic foraminifera bryozoans brachiopods echinoids and planktonic foraminifera volumetrically insignificant calcite cements include scalenohedral blocky and syntaxial overgrowths dolomite is present in variable amounts 1 25 scattered throughout the unit as euhedral rhombs usually comprising 2 of the whole rock volume the dolomite post dates the calcite cements and is mainly an interparticle cement with crystal size varying between 10 and 150 m mean 50 m under cathodoluminescence the dolomite rhombs have a dull core and a nonluminescent outer rim the dolomite is nonstoichiometric ca rich ca54 62mg38 46 with high trace element concentrations the mn concentrations range from 0 to 310 p p m in the crystal cores mean 140 p p m and 80 240 p p m in the crystal rims mean 140 p p m the fe concentrations increase from the crystal cores range 640 5690 p p m mean 2030 p p m to the crystal rims range 2840 9440 p p m mean 6040 p p m whereas the sr concentrations decrease from the crystal cores range 690 1510 p p m mean 1280 p p m to the crystal rims 620 1240 p p m mean 930 p p m the 13cpdb values of the dolomite range between 2 5 and 2 6 whereas the 18opdb values range from 0 3 to 0 6 this dolomite occurrence supports the idea that marine or near marine dolomite can form not only syndepositionally but also in the shallow subsurface of temperate units soon after sediment deposition under reducing conditions the fine grained low permeability nature of the port campbell limestone contributed to the reducing conditions at shallow depth the high trace element concentrations of the dolomite especially in fe and the near marine composition of the dolomitizing fluids as large volumes of meteoric water were inhibited

the geochemistry of early diagenetic dolostones from a low salinity jurassic lagoon in the middle jurassic great estuarine group of nw scotland argillaceous dolostones and dolomitic limestones occur interbedded with limestones and mudstones of low salinity 9closed lagoon9 palaeoenvironments these microcrystalline dolostones are imperfectly ordered and calcium rich and have escaped pervasive dissolution reprecipitation processes during burial diagenesis geochemical studies show that the dolostones retain a significant early diagenetic memory 18 o dol values 0 o are the heaviest reported from the great estuarine group and probably formed by evaporative fractionation from meteoric water 13 c dol values 2 o may represent precipitation from a standing body of meteoric water partially equilibrated with atmospheric co 2 calcite coexisting with the dolomite is lighter in both 13 c and 18 o by 0 8 and 3 7 o respectively it is suggested that the minerals were not co precipitates from one solution strontium is preferentially concentrated in the dolomite with 87 sr 86 sr values between 0 7095 and 0 7099 these values which are slightly more radiogenic than jurassic seawater may originate from the continental weathering of ancient limestones mixed with seawater derived from the compaction of underlying sediments sm nd model ages are consistent with weathering of dalradian rocks in the drainage hinterland dolomite formation proceeded by dolomitization of a caco 3 precursor in a warm seasonal climate during dry arid seasons the lagoon water evaporated concentrating 18 o raising mg ca and sr ca ratios and thereby promoting dolomitization during humid wet periods sr poor caco 3 precipitated with lighter 18 o and 13 c values

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

late paleozoic small foraminifers endothyrids from south america ecuador and bolivia in the pangea small foraminifers endothyrids are useful tools for studying the stratigraphy of the late paleozoic however they have been largely ignored in south america this article reports for the first time their widespread occurrence in the subcontinent and illustrates representatives of 21 families encountered from the early pennsylvanian bashkirian to the early permian artinskian fifty five foraminiferal genera are recognized in three basins of ecuador bolivia and enable us to establish a crude zonation the fauna is nonendemic all genera being previously known from eurasia and north america there are also a number of taxa derived from the arctic realm sverdrup basin and arctic alaska in association with the microflora of green and red algae the microfauna suggests temperate warm conditions this indicates that since the middle carboniferous warm climatic oscillations affected the south american part of gondwana

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

sediment entrainment and depletion from patches of fine material in a gravel bed river characteristics at shear stresses ranging from 5 to 60 n m 2 experiments demonstrate that patches of fine sediment control both the intensity and duration of bed load under hydraulic conditions characteristic of the early stages of floods the experiments allow quantifying bed load at the entrainment threshold providing the first empirical evidence that marginal bed load transport can be attributed to the mobilization of sediments from patches bed load transport was recorded consistently once shear stress exceeded 5n m 2 the experiments produced low bed load rates 6 g sm depletion of material in the patches occurred rapidly with bed load rates and particle sizes decreasing after only 5 minutes combining flume and helley smith data for the study reach a breakpoint in the relation between shear stress and bed load rate was calculated to be around 30 n m 2 this represents the transition between bed load transport phases below the breakpoint transport occurs at a low rate and is composed predominantly of fine sediment from patches but above it much higher rates occur from across the reach as a whole hydraulic conditions at the threshold are those which occur during small frequent floods 25 bankfull flow equaled or exceeded 15 of time this indicates that sediment entrainment from patches of fine material is a frequent process and the threshold change between bed load phases occurs regularly

facies analysis of the trypali carbonate unit upper triassic in central western crete greece an evaporite formation transformed into solution collapse breccias the trypali carbonate unit upper triassic which crops out mainly in central western crete occurs between the parautochthonous series plattenkalk or talea ori ida series e g metamorphic ionian series and the tripolis nappe comprising the tripolis carbonate series and including a basal phyllite quartzite unit it consists of interbedded dolomitic layers represented principally by algally laminated peloidal mudstones foraminiferal peloidal and ooidal grainstones as well as by fine grained detrital carbonate layers in which coarse baroque dolomite crystals and dolomite nodules are dispersed baroque dolomite is present as pseudomorphs after evaporite crystals nodules and rosettes which grew penecontemporaneously by displacement and or replacement of the host sediments sabkha diagenesis however portions of the evaporites show evidence of resedimentation pre existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal shaped hoppers as well as of anhydrite nodules and rosettes salt crusts all microfacies are characteristic of peritidal depositional environments such as sabkhas tidal flats shallow hypersaline lagoons tidal bars and or tidal channels along most horizons the trypali unit is strongly brecciated these breccias are of solution collapse origin forming after the removal of evaporite beds evaporite related diagenetic fabrics show that there was extensive dissolution and replacement of pre existing evaporites which resulted in solution collapse of the carbonate beds evaporite replacement fabrics including calcitized and silicified evaporite crystals are present in cements in the carbonate breccias brecciation was a multistage process it started in the triassic but was most active in the tertiary in association with uplift and ground water flow telogenetic alteration during late diagenesis in zones of intense evaporite leaching and brecciation solution collapse breccias were transformed to rauhwackes the trypali carbonate breccias trypali unit are lithologically and texturally similar to the triassic solution collapse breccias of the ionian zone continental greece the evaporites probably represent a major diapiric injection along the base of the parautochthonous series metamorphic ionian series and also along the overthrust surface separating the parautochthonous series from the tripolis nappe phyllite quartzite and tripolis series the injected evaporites were subsequently transformed into solution collapse breccias

particle friction angles in steep mountain channels sediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers one hypothesis for this observation is that particles are more stable in mountain channels due to particle particle interlocking or bridging across the channel width this hypothesis has yet to be tested however because we lack direct measurements of particle friction angles in steep mountain channels here we address this data gap by directly measuring the minimum force required to dislodge sediment pebbles to boulders and the sediment weight in mountain channels using a handheld force gauge at eight sites in california with reach averaged bed angles ranging from 0 5 to 23 and channel widths ranging from 2 m to 16 m we show that friction angles in natural streams average 68 and are 16 larger than those typically measured in laboratory experiments which is likely due to particle interlocking and burial results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps however the mean particle friction angle does not vary systematically with bed slope these results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels instead the spatial pattern and grain size dependence of particle friction angles may indirectly lower transport rates in steep narrow channels by stabilizing large clasts and channel spanning steps which act as momentum sinks due to form drag

stable isotopic compositions of carbonates from the mesoproterozoic bangemall group northwestern australia marine carbonate rocks from the mesoproterozoic bangemall group of northwestern australia show little deviation 1 3 in whole rock delta 13c carb values about a mean of 0 5 this narrow range persists despite close sampling every 10 20 m through long sections up to 2500 m that are geographically widespread up to 250 km apart over many depositional environments supralittoral to outer shelf sediment sources stromatolitic bioherms to detrital calcilutites and rock types pure limestones to dolomitic shales the only major excursions from the norm seem related to unusual environmental or post depositional processes as they are correlated with large enrichments to 3 or depletions to 16 in 18o relatively heavy delta 13c values up to 2 5 occur in a single bed of brecciated ferruginous dolostone at a single locality these abnormal values may result from local evaporitic conditions limey and shaley nodular dolostones have delta 13c values as low as 4 3 probably caused by remineralization of organic matter during late and patchy dolomitization most notably sharp negative excursions in delta 13c up to 8 4 occur in bleached kerogen free rocks with mineral assemblages of dolomite quartz calcite tremolite talc reflecting isotopic re equilibration in thick metamorphic aureoles around dolerite intrusions general environmental variations are minor with delta 13c values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative there are no strong secular trends but subtle fluctuations within the range 2 to l can be correlated along the northwestern margin of the basin this resembles the pattern seen in other mesoproterozoic successions but is markedly unlike the heavy background 5 and extreme variations up to l0 in delta 13c evident in neoproterozoic successions of similar thickness and environmental setting hence in contrast to the neoproterozoic the global rate of organic carbon burial was probably fairly constant during deposition of the bangemall group and perhaps generally during the mesoproterozoic as was the redox state of the atmosphere and hydrosphere

unraveling bed slope from relative roughness in initial sediment motion understanding incipient sediment transport is crucial for predicting landscape evolution mitigating flood hazards and restoring riverine habitats observations show that the critical shields stress increases with increasing channel bed slope and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms particle interlocking across the channel width and large bed sediment relative to flow depth relative roughness here we use scaled flume experiments with variable channel widths bed slopes and particle densities to separate these effects which otherwise covary in natural streams the critical shields stress increased with bed slope for both natural gravel s 2 65 g cm 3 and acrylic particles s 1 15 g cm 3 and adjusting channel width had no significant effect however the lighter acrylic particles required a threefold higher critical shields stress for mobilization relative to the natural gravel at a fixed slope which is unexpected because particle density is accounted for directly in the definition of shields stress a comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical shields stress in our experiments these changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion increased relative roughness can explain much of the observed heightened critical shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore pumice or ice clasts on titan

the role of bacterial sulfate reduction during dolomite precipitation implications from upper jurassic platform carbonates the early diagenetic formation of dolomite in modern aquatic environments is limited mostly to evaporitic and marine anoxic organic rich sediments dominated by bacterial sulfate reduction bsr in such environments bacterial activity lowers the energy barriers for the nucleation and growth of dolomite and thus promotes the formation of non stoichiometric highly disordered and metastable proto dolomite although the boundary conditions for the formation of modern proto dolomites are considered to be generally understood the role of bsr during limestone dolomitization in ancient marine environments remains questionable herein we present a study about the physicochemical conditions and processes which led to the formation of partly dolomitized limestone and dolostone in the presence of bsr on a stable carbonate platform during the upper jurassic at oker northern german basin the dolomite textures the spatial trace elemental patterns of the dolomite and of the surrounding limestone and the results of 18o and 13c isotope analyses reveal that the oker dolomite has been formed by the early diagenetic replacement of magnesian calcite precursors at temperatures between 26 c and 37 c we interpret the mineralizing fluids responsible for dolomitization as pristine marine to slightly evaporitic and reducing seawater being modified during shallow seepage reflux and or evaporitic tidal pumping the elevated 34scas values 17 9 to 19 7 v cdt of the oker dolomite compared to ambient upper jurassic seawater indicate that bsr facilitated dolomite formation for the first time we show that a linear anti correlation exists between decreasing carbonate associated sulfate cas contents in dolomite and increasing ordering ratio of the dolomite lattice structure with the degree of cation order in dolomite to be given by degree of cation order dol 0 018 cas dol 68 3 r2 0 98 this correlation implies that the cas content of sedimentary dolomite can be used as a measure for dolomite maturity the relationships between the ambient paleo environmental controls the resultant dolomitization pathways and subsequently the structure and the composition of the precipitating dolomite are presented and discussed in relation to the stability of modern and ancient proto dolomites throughout burial diagenesis

using spectral decomposition to identify and characterize glacial valleys and fluvial channels within the carboniferous section in bolivia the area of interest within bolivia s santa cruz basin is covered by a 3d seismic survey that covers approximately 660 km2 figure 1 shows the regional setting and the distribution of major channel trends for the upper carboniferous section

early diagenetic recrystallization of holocene 3000 years old peritidal dolomites ambergris cay belize three peritidal carbonate crusts and associated intercrust sediments total thickness of 30cm aged 3000 years bp on ambergris cay belize contain 32 100 calcian dolomite x 72 5 dolomite ranging in composition from 40 to 46 mol mgco3 x 43 3 dolomite replaced high mg calcite foraminiferal muds penecontemporaneously with sedimentation forming partially dolomitized sediments and lithified crusts dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth 0 4 m near the top of the section to 1 0 m near the base of the dolomitized section this size increase is not accompanied by any significant decrease in porosity crystal size distributions appear to be log normal and become increasingly broad and flat with depth rietveld x ray pattern fitting structure refinements indicate increasing ca and mg concentrations on their respective sites cation ordering as a function of increasing depth most of the ordering occurs within the first 15 cm of the surface stoichiometry does not increase with depth indicating no relationship between the ca mg ratio and cation ordering strong geochemical trends were observed down section in the dolomite including 1 increasing mn content 44 to 274 ppm and 2 decreasing 13c values 0 9 to 5 5 pdb oxygen isotope values range from 18o 1 3 pdb in the upper part of the section to 2 6 pdb in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend down section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy driven dissolution reprecipitation process ostwald ripening the observed trends in carbon isotopes and mn content probably result from geochemical re equilibration during recrystallization and reflect reducing conditions and an isotopically light organically derived carbon source oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level

patch dynamics and stability in steep rough streams 1 the beds of steep streams are typically composed of relatively immobile boulders and more mobile patches of gravel and cobbles little is known about how variability in flow and sediment flux affect the area thickness composition and grain mobility of sediment patches to better understand patch dynamics we measured flow sediment transport and bed properties in two steep channels patches close to the thalweg varied in area thickness and grain size whereas those outside the thalweg did not local variations in transport of several orders of magnitude occurred even on a patch with a spatially homogeneous grain size distribution during moderate flow events partial to selective transport dominated on the entire channel bed and all individual patches tracer particles moved freely between different patch classes e g fine and coarse patches exchanged particles and relatively fine sediment on all patch classes began motion at the same shear stress therefore the selective transport observed for the entire bed was not a result of the preferential transport of only fine patches but the high relative mobility of finer sediment on all patches our results suggest that local flow and sediment supply and not spatial grain size variations were the primary drivers of local bed load transport variability the use of reach averaged flow properties to understand local patch dynamics may not be applicable

two billion year old isotopically heavy carbon evidence from the labrador trough canada forty eight samples of bulk carbonates were studied from the dunphy portage alder uve denault and abner formations and the undivided pistolet subgroup in the labrador trough quebec canada the carbonate units occur as a number of thick beds up to 300 m within 100 600 m thick formations they are interbedded with siliciclastic sediments and are composed of crystalline sparry micritic allochemical and stromatolitic dolostones the carbonate rocks are recrystallized and metamorphosed up to amphibolite grade they are chemically dolostones and include chemical or biochemical laminated and massive dolostones dolorudites dolarenites and dololitites which were deposited in shallow water dunphy denault and abner formations to relatively deep water uve formation marine environments reef facies are present in the denault and abner formations the dunphy portage alder and uve formations deposited between 2 17 and 2 14 ga yield isotopically heavy values of 13c pdb ranging from 5 3 to

bed load bias comparison of measurements obtained using two 76 and 152 mm helley smith samplers in a gravel bed river 1 we assess how the size of the helley smith hs bed load sampler nozzle affects the accuracy of bed load sampling semitheoretical considerations show that the larger grains resident on the streambed can influence the sample either by blocking the sampler entrance or by causing the sampler to rest in a perched position probabilities for interference can be derived from the distribution of grain sizes but they do not capture the actual complexity of the influence of the bed on sampler performance we therefore make an empirical comparison of sediment trapped by hs samplers with 76 and 152 mm intakes during floods in the gravel bed lower ebro river most bed load rates appeared higher when sampled with the hs152 the largest clasts collected by the hs76 also tend to be smaller than those obtained with the hs152 at the same flow analyzing paired bed load samples we find the probability of a bed load sample collected with the hs152 to be biased is around 43 in the conditions of the present study whereas 65 of samples were biased when obtained with the hs76 the analysis emphasizes the influence of bed material texture over sampler performance and demonstrates that the use of samplers with intake size much larger than bed grain size i e 5d will increase the accuracy of bed load grain size distributions and the precision of annual load estimates in gravel bed rivers

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

calcium poor dolomite from the sabkhas of kuwait small quantities of a rare ca deficient dolomite have been found coexisting in thin layers and alternating with the normal ca rich variety in the holocene sabkhas of southern kuwait the ca poor dolomite has a molar composition of ca46 49 whereas the ca rich type is ca51 56 the former type has been found only in two localities of the supratidal zone that are not subjected to tidal flooding today and mostly within the fluctuating ground water table in these zones the thin layers are either carbonate mud rich or pellet rich and the amount of ca poor dolomite is never more than 4 to 6 per cent by weight of the total carbonate fraction the dolomites vary in size between 2 m and 5 m and exhibit characteristic rhombic crystal morphologies

the trace metal geochemistry of pore water brines from two hypersaline lakes abstract pore water brines from modern hypersaline lake sediments have been analyzed to determine the nature of brine trace metal interaction in these environments brine samples were collected from the surface waters and sedimentary pore waters of two hypersaline lakes solar lake eastern sinai and goto meer bonaire netherlands antilles metal concentrations in the oxic brines appear to be related to the strength of their respective chloride complexes with pb and zn more enriched than cu pore water cu zn and fe concentrations increased across the sediment water interface in both lakes all three metal profiles follow a similar pattern with depth in the pore water higher concentrations were observed in the goto meer brines strontium isotopes were utilized to qualify better the mixing of the brines in solar lake although these metals were observed at higher concentrations than sea water in these sulfidic brines their concentrations are not close to what is needed for the formation of ore deposits

stratigraphy and palynology of a late paleozoic glacial paleovalley in the andean precordillera argentina fil limarino carlos oscar consejo nacional de investigaciones cientificas y tecnicas oficina de coordinacion administrativa ciudad universitaria instituto de geociencias basicas aplicadas y ambientales de buenos aires universidad de buenos aires facultad de ciencias exactas y naturales instituto de geociencias basicas aplicadas y ambientales de buenos aires argentina

mixed water and hydrothermal dolomitization of the pliocene shirahama limestone izu peninsula central japan the early pliocene shirahama limestone is a grainstone packstone principally composed of fragments of algae bryozoa and echinoderm and subordinate volcanic rocks the limestone was variously dolomitized and the regional distribution of dolomite is patchy dolomite occurs as isolated crystals filling pores moulds and solution vugs and mosaic aggregates replacing bioclasts calcite occurs as rim and pore filling sparry cements and as calcareous skeletons isotopically the dolomites are classified into a heavy oxygen group 2 to 3 5 0 pdb and a light oxygen group 5 5 to 7 5 0 pdb calcite associated with heavy oxygen dolomite has 18o of 6 5 to 8 5 0 pdb whereas those associated with light oxygen dolomite have a wide range from 7 5 to 14 0 pdb calcite in dolomite free limestone has an oxygen isotopic composition of 2 to 8 5 0 pdb textures chemistry and isotopic evidence indicate that heavy oxygen calcite formed in freshwater and heavy oxygen dolomite in a meteoric marine mixture of 10 30 seawater light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30 65 c petrographic features and both isotopic and chemical evidence suggest that the shirahama limestone was exposed to freshwater soon after deposition subsequently blocky calcite precipitated stage i the limestone was locally submerged in the meteoric marine mixture due to gradual subsidence or eustatic movement this led to the precipitation of heavy oxygen zoned dolomite and dolospar stage ii hydrothermal alterations occurred in the area a few myr ago and related hydrothermal fluids and mixed meteoric hydrothermal waters caused dedolomitization of some zoned dolomite partial dissolution of vuggy dolomite precipitation of limpid dolomite and recrystallization of some earlier dolomites stage iii zeolites were also precipitated from these fluids finally the shirahama limestone was exposed again to freshwater and sparry calcite precipitated to plug some of the remaining pores stage iv

sr isotopic composition of paleoproterozoic 13c rich carbonate rocks the tulomozero formation se fennoscandian shield abstract rb sr systematics has been studied in 13c rich carbonate rocks of the paleoproterozoic 2 09 0 07 ga tulomozero formation in the northern onega lake area the se fennoscandian shield the formation is divided into eight members a f consisting of greenschist facies grade variegated sandstones siltstones mudstones stromatolitic dolostones and subordinate crystalline limestones samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the tulomozero formation prior to isotope analysis the rocks powders were treated with 1n ammonium acetate for partial removal of the late epigenetic carbonate phases major resetting of the rb sr systems in the tulomozero carbonate rocks appears to take place during the svecofennian regional metamorphic event and it was screened by using mn sr fe sr mg ca and 18o 16o ratios high sr content up to 2080 g g in limestones and 530 g g in dolostones coupled with low fe mn 0 40 in dolostones of the upper part of member b and of members g and h is indicative of seawater influxes sea transgression into the tulomozero basin the 87sr 86sr values in the least altered mn sr

mixing zone dolomites in the gully oolite lower carboniferous south wales the gully oolite at st lythams glamorgan is extensively dolomitized towards its base the dolomite is non ferroan and is both replacive and void filling in nature 18 o varies from 6 06 0 to 2 96 0 pdb and 13 c from 2 32 0 to 3 55 0 pdb with a marked covariation of 18 o and 13 c dolomitization is believed to have occurred within a mixing zone environment at salinities up to 50 of that of seawater the variation of dolomite isotopic composition versus depth in the sediment is explained in terms of successive upwards migration of the mixing zone and associated multiple phases of dolomitization the tops of these successive mixing zones coincide with evidence of sediment emergence and the presence of dolomite intraclasts implies that dolomitization occurred at shallow depths within the sediment this suggests dolomitization occurred in the absence of an overlying meteoric phreatic lens the high degree of dolomitization at st lythams as opposed to that occurring in modern mixing zones is related to 1 longer duration of the mixing zone environment in the sediment 2 the proximity of the mixing zone to the sediment surface 3 possible chemical differences between carboniferous and modern seawater particularly with respect to pco 2

continuous monitoring of bedload flux in a mountain gravel bed river an automatic bedload monitoring station has been established on the perennial tordera river a mediterranean mountain gravel bed stream located in ne spain bedload fluxes were obtained automatically at 1 min intervals using two birkbeck type pit bedload samplers aligned across the width of the channel flow depth and water surface slope were both measured continuously and synchronously with bedload data for five flow events show that bedload flux varies considerably bedload flux is high when it is compared with other perennial streams for a 15 min interval maximum channel average bedload flux was 0 83 kg m 1 s 1 bedload records also show that there is no direct relationship between bedload flux and grain shear stress the low correlation between bedload flux and hydraulics arises due to the different thresholds of initiation of motion for each flow event and the variable character of the bed between events for two individual floods there is a reasonable relationship between bedload flux and grain shear stress in part as a consequence of individual particle entrainment and patch movement over a stable coarse gravel bed during moderately high discharges these results demonstrate that the birkbeck system functions well in a mediterranean mountain gravel bed stream with a longitudinal slope of 2

record of a pennsylvanian cisuralian marine transgression southern bolivia a short lived event in western gondwana abstract largely terrestrial pennsylvanian strata machareti and mandiyuti groups in bolivia and northern argentina contain a brief marine incursion dominated by glacio fluvial and glacial diamictites much of the thick succession is peri glacial in origin and deposited within a basin with paleohighs in southernmost bolivia balapuca section new discoveries of poorly preserved orthotetacean brachiopods derbyoides sp document the marine systems without the brachiopod co occurrence of numerous in situ gastropods mourlonia balapucense with all ontogenetic growth stages could not be confirmed as marine palynomorphs corresponding to the tb zone di pasquo stratigraphically below the megafossil occurrence place the age of the assemblage in the kasimovian gzhelian stage many units show extensive recycling of devonian and mississippian palynomorphs retispora lepidophya and others and a brachiopod demonstrate pre gzhelian erosion and high energy deposition in diamictites during deposition of the tarija formation and through much of the succession this brief marine transgression in this basin corresponds to regionally more long lived marine transgressions in western argentina characterized by the tivertonia streptorhynchus fauna moscovian gzhelian linked with the beginning of a global major sea level rise as an interlude in the gondwana glaciations the marine transgression exceeded isostatic rebound further in bolivia the transgression from the north produced the copacabana formation carbonates coeval siliciclastics of the san telmo formation in the south show contrasting in lithologic facies and faunal composition as a result of thermal barriers warm cold waters

hydrogeologic processes in saline systems playas sabkhas and saline lakes abstract pans playas sabkhas salinas saline lakes and salt flats are hydrologically similar varying only in their boundary conditions thus in evaluating geochemical processes in these systems a generic water and solute mass balance approach can be utilized a conceptual model of a coastal sabkha near the arabian gulf is used as an example to illustrate the various water and solute fluxes analysis of this model suggests that upward flux of ground water from underlying formations could be a major source of solutes in the sabkha but contribute only a small volume of the water local rainfall is the main source of water in the modeled sabkha system with a surprisingly large recharge to rainfall ratio of more than 50 the contribution of seawater to the solute budget depends on the ratio of the width of the supratidal zone to the total width and is generally confined to a narrow zone near the shoreline of a typical coastal sabkha because of a short residence time of water steady state flow is expected within a short time 50 000 years the solute composition of the brine in a closed saline system depends largely on the original composition of the input water the high total ion content in the brine limits the efficiency of water rock interaction and absorption because most natural systems are hydrologically open the chemistry of the brines and the associated evaporite deposits may be significantly different than that predicted for hydrologically closed systems seasonal changes in temperature of the unsaturated zone cause precipitation of minerals in saline systems undergoing evaporation thus during the hot dry season months minerals exhibit retrograde solubility so that gypsum anhydrite and calcite precipitate evaporation near the surface is also a major process that causes mineral precipitation in the upper portion of the unsaturated zone e g halite and carnallite provided that the relative humidity of the atmosphere is less than the activity of water the slope of the fresh brine water interface in saline lake systems is shallower than in fresh seawater interface because of the greater density difference between the fresh brine water bodies the interface between sabkha brines and seawater slopes seaward unlike normal marine fresh water systems that slope landward moreover the brine seawater interface does not achieve steady state because it is pushed toward the sea by the sabkha s brine

timing and spatial distribution of mid holocene drying over northern china response to a southeastward retreat of the east asian monsoon 1 to determine the timing and spatial distribution of mid holocene drying over northern china the mineralogical and oxygen isotopic composition of authigenic carbonate from a closed lake at bayanchagan southern inner mongolia were measured further analysis and synthesis of the spatial geological data were performed results from lake bayanchagan show a significant drying at 6000 calendar years cal years b p indicated by dolomite precipitation and a striking rise in 18o values the synthesis of spatial data reveals a zonal distribution for timing of drying over northern china in the mid holocene which began at 9000 7000 cal years b p in deserts of north central china at 7000 5500 cal years b p drying extended into the desert steppe transitional zone and at 4500 cal years b p into northeastern and south central china this pattern indicates that the east asian summer monsoon has significantly retreated southeastward since the mid holocene which may be related to orbitally induced northern hemisphere insolation changes a retreat of 400 550 km is inferred for the front of the summer monsoon from 6500 to 4500 cal years b p

high precision u pb calibration of carboniferous glaciation and climate history paganzo group nw argentina the duration and geographic extent of carboniferous glacial events in southern gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered we report 10 high precision 2 0 1 u pb ages for the permian carboniferous paganzo group nw argentina that redefi ne the chronostratigraphy of the late paleozoic paganzo and rio blanco basins and signifi cantly refi ne the timing of glacial events and climate shifts in the western region of southern gondwana radiometric calibration of the paganzo group indicates three pulses of carboniferous glaciation in the mid visean the late serpukhovian to earliest bashkirian and between the latest bashkirian to early moscovian an abrupt shift in depositional style from high sinuosity single storied fl uvial deposits and clay rich paleosols to low sinuosity multi storied feldspathic fluvial deposits inter calated with eolianites and calcic paleosols is constrained to the latest moscovian and earliest kasimovian these constraints indicate a relatively abrupt climate shift from humid subhumid to nonseasonal semiarid regional climate conditions that occurred signifi cantly earlier than previously inferred early permian this period of high latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the pangean paleotropics in the pennsylvanian

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

late quaternary marine stratigraphy southeast of new zealand sediment cores of late quaternary age from the continental margin and deep sea bounty trough southeast of new zealand reveal an alternating sequence of glacial and interglacial sediments during glacial episodes of lowered sea level glaciation in the southern alps was at its peak and east coast rivers delivered enormous volumes of terrigenous sediments to the shelf edge at these times sediments of the deep adjacent basins were dominated by micaceous hemipelagic deposits with a low biogenic component consisting mostly of siliceous remains radiolaria and sponge spicules planktonic foraminifera although present were much reduced in abundance because of increased dissolution as in deeper water subantarctic cores farther to the south cool water forms dominated during interglacial episodes terrigenous sediment supply to the deep basins was reduced in response to higher sea levels and as glaciers retreated and deposited much of their loads in newly formed glacial lakes and on the plains terrigenous sediment dilution of the biogenic portion was thus much reduced calcium carbonate dissolution also was reduced these processes in combination led to the deposition of foraminifera rich hemipelagic sediments siliceous biogenic productivity decreased thus the late quaternary marine sediment record in the area adjacent to southeast new zealand is dominated by paleoclimatic influences that control terrigenous input calcium carbonate dissolution biogenic productivity and the migration of planktonic assemblages

dolomitization of the late cretaceous paleocene platform carbonates golkoy ordu eastern pontides ne turkey abstract late cretaceous paleocene agillar formation in the eastern pontide zone ne turkey is composed of shallow marine platform carbonates that were buried until uplift in late paleocene time the formation which is represented by 100 m thick reefal limestones around golkoy is completely dolomitized on the basis of petrographic and geochemical features four different replacive and cement dolomite phases are present the replacive dolomites which are cut by low amplitude stylolites include 1 fine crystalline planar s dolomite rd1 and 2 coarsely crystalline planar s e dolomite rd2 two types of dolomite cements were developed in minor amounts 1 a coarse dolomite cement cd1 that mostly fills molds vugs and fractures in rd1 dolomites and 2 rim dolomite cement cd2 that commonly develops on the space facing surfaces of rd2 dolomite the replacive dolomites are nearly stoichiometric ca 49 51 mg 48 50 and have 18 o values of 3 5 to 1 6 vpdb 13 c values of 1 1 to 0 8 vpdb and 87 sr 86 sr ratios of 0 707104 to 707458 sr na mn and fe concentrations of replacive dolomites are 102 171 ppm bdl 371 ppm 244 9941 ppm and 2643 8951 ppm respectively petrographic and geochemical data indicate that the replacive dolomites were formed prior to chemical compaction at shallow to intermediate burial depths from late cretaceous paleocene seawater and or seawater slightly modified by water rock interaction process and then recrystallized by meteoric waters at increasing burial depths and temperatures compared with replacement dolomites cd1 dolomites yield lower 18 o values 4 9 to 5 3 vpdb similar 13 c values 1 2 to 0 5 vpdb and higher 87 sr 86 sr ratios 0 707660 0 707609 the cd1 dolomites overlie the low amplitude stylolites and have homogenization temperatures between 94 and 112 c indicating that they were deposited from basinal brines although replacive dolomites and dolomite cements have similar na bdl 413 ppm mn 259 6990 ppm and fe 2184 6690 ppm concentrations the cements have lower sr contents 63 114 ppm like replacive dolomites cd1 dolomite is nearly stoichiometric ca 49 50 mg 47 49 but cd2 dolomite is slightly ca rich ca 51 52 mg 47 48 the cd2 dolomite is overlain by coarsely crystalline mosaic calcite cement indicates that diagenetic fluids eventually evolved to undersaturation with respect to dolomite and oversaturation with respect to calcite

testing bedload transport formulae using morphologic transport estimates and field data lower fraser river british columbia morphologic transport estimates available for a 65 km stretch of fraser river over the period 1952 1999 provide a unique opportunity to evaluate the performance of bedload transport formulae for a large river over decadal time scales formulae tested in this paper include the original and rational versions of the bagnold formula the meyer peter and muller formula and a stream power correlation the generalized approach adopted herein does not account for spatial variability in flow bed structure and channel morphology however river managers and engineers as well as those studying rivers within the context of long term landscape change may find this approach satisfactory as it has minimal data requirements and provides a level of process specification that may be commensurable with longer time scales hydraulic geometry equations for width and depth are defined using morphologic maps based on aerial photography and bathymetric survey data comparison of transport predictions with bedload transport measurements completed at mission indicates that the original bagnold formula most closely approximates the main trends in the field data sensitivity analyses are conducted to evaluate the impact of inaccuracies in input variables width depth slope and grain size on transport predictions the formulae differ in their sensitivity to input variables and between reaches average annual bedload transport predictions for the four formulae show that they vary between each other as well as from the morphologic transport estimates the original bagnold and meyer peter and muller formulae provide the best transport predictions although the former underestimates while the latter overestimates transport rates based on our findings an error margin of up to an order of magnitude can be expected when adopting generalized approaches for the prediction of bedload transport copyright 2005 john wiley sons ltd

holocene marine hardground formation in the arabian gulf shoreline stabilisation sea level and early diagenesis in the coastal sabkha of abu dhabi abstract this study provides the first comparison between a seaward and a landward section of the same diachronous hardground surface observed in the coastal sabkha of abu dhabi this hardground is described here in terms of its mode of formation its diagenetic environment and its impact on shoreline stabilisation during transgression the hardground is exposed in the intertidal zone and buried by a late holocene prograding succession of carbonates evaporites and microbial sediments in the supratidal zone the hardground itself is composed of bioclastic grains primarily of aragonitic composition that originate from intertidal depositional environments aeolian silt to sand sized quartz grains are also observed lithification occurred through the precipitation of pore filling aragonite high mg calcite and dolomite cements from sea and interstitial water that was marked by high salinities and temperatures as confirmed by stable isotope analyses high mg calcite and non stoichiometric dolomite are also observed as secondary recrystallisation products the formation of these two mineral phases as recrystallisation products was possibly microbially mediated lithification progressed in two phases the older phase of which is marked by higher amounts of non stoichiometric dolomite and high magnesium calcite as compared to the younger phase transgressive reworking of precursor siliciclastic sands was inhibited by the development of transgressive pore filling gypsum cements in the supratidal zone

variable source areas of bedload in a gravel bed stream abstract bedload texture is dependent on the spatially variable availability of mobile surface bed material distributed in patches or in other bedforms we measured bedload transport continuously with an automatic bedload station on the gravel bedded tordera river we also studied bed patchiness from pre flood vertical aerial photographs in a 40 m section upstream of the bedload station the results suggest that patches are the main source of sediment during the beginning of the flow event the median bedload size in low magnitude events depends on the percentage of total area occupied by patches of a given size patch areal extent and texture depend on the magnitude and general character of the previous flow event

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

estimating critical stream power for bedload transport calculations in gravel bed rivers the physical basis of bagnold s equation for critical stream power is examined although the general approach is well founded the original equation can be criticised for 1 failing to distinguish between the grain size in transport and the grain size that represents bed roughness 2 requiring a knowledge of critical flow depth in addition to gross channel properties 3 failing to allow for hiding and protrusion effects on the mobility of mixed size stream beds more general equations are presented which overcome these limitations they require a knowledge of channel slope but not depth the analysis suggests that the presence of form resistance in addition to grain roughness does not affect the calculation of critical power critical power is expected to depend more sensitively on grain size between reaches than within reaches

origin of siderite deposits from the lombardy valleys northern italy a carbon oxygen and strontium isotope study abstract in the lombardy valleys siderite mineralization is widespread as stratiform orebodies in lower triassic terrigenous carbonate rocks as well as vein orebodies in permian volcanics and continental sequences and in the metamorphic basement a number of these orebodies and host limestone dolomite rocks have been analysed for c o and sr isotope compositions in order to constrain geochemically the origin of the siderite the intermediate 13c 18o and 87 sr 86 sr values of stratiform siderite relative to vein siderite and host limestone and the very good linear correlations between and 87 sr 86 sr data points concur to support a metasomatic origin for stratiform siderite after calcite in addition stratiform and vein siderite should have formed from the same kind of fluids and possibly are the products of the same mineralizing event the values of carbonate rocks are discussed in terms of recrystallization dolomitization effects during diagenesis these processes occurred earlier than siderite mineralization

carbonate stromatolites from a messinian hypersaline setting in the caltanissetta basin sicily petrographic evidence of microbial activity and related stable isotope and rare earth element signatures lower messinian stromatolites of the calcare di base formation at sutera in sicily record periods of low sea level strong evaporation and elevated salinity thought to be associated with the onset of the messinian salinity crisis overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions occasionally influenced by an influx of meteoric water evidence of bacterial involvement in carbonate formation is recorded in three dolomite rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre scale lamination and small fenestrae the dominant microfabrics are i peloidal and clotted dolomicrite with calcite filled fenestrae ii dolomicrite with bacterium like filaments and pores partially filled by calcite or black amorphous matter and iii micrite in which fenestrae alternate with dark thin wispy micrite the filaments resemble beggiatoa like sulphur bacteria under scanning electron microscopy the filaments consist of spherical aggregates of dolomite interpreted to result from calcification of bacterial microcolonies the dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces biofilm like remains occur in voids between the filaments the dolomite consistently shows negative d 13 c values down to 11ae3 and very positive d 18 o mean value 7ae9 that suggest formation as primary precipitate with a substantial contribution of organic co2 very negative d 13 c values down to 31ae6 of early diagenetic calcite associated with the dolomite suggest contribution of co2 originating by anaerobic methane oxidation the shale normalized rare earth element patterns of sutera stromatolites show features similar to those in present day microbial mats with enrichment in light rare earth elements and m type tetrad effects enrichment around pr coupled to a decline around nd and a peak around sm and eu taken together the petrography and geochemistry of the sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation