sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

diagenetic history of cambrian quartzarenites ras dib zeit bay area gulf of suez eastern desert egypt abstract in the northern corner of the el zeit range 370 m of sandstone overlie precambrian granite and underlie cretaceous marine strata the sandstones include the marine lower cambrian araba formation and the overlying dominantly fluvial upper cambrian naqus formation the framework compositions of both sandstones are almost entirely quartz with trace amounts of muscovite k feldspar and heavy minerals up to 21 oversize pores some filled with younger cements attest to extensive dissolution loss of detrital grains because the final mineralogical maturation of these quartzarenites reflects diagenesis they are diagenetic quartzarenites during burial diagenesis the introduction of up to 8 quartz cement inhomogeneously distributed mean 3 was followed by local pore occluding calcite cement which halted compaction sandstones without calcite cement underwent additional mechanical and chemical compaction sufficient to develop sutured quartz grain contacts and reduce porosity 27 these events were followed by 1 extensive dissolution loss of carbonate cement detrital feldspars micas and heavy minerals 2 formation of local patches of kaolinite mean 3 and 3 formation of extensive iron oxide cements including specular hematite these features suggest extensive invasion by oxidizing meteoric water the timing of this event can only be dated as post cambrian and pre cenomanian some outcrop samples contain pore occluding gypsum cement or mixtures of gypsum and halite sr87 sr86 ratios of four samples of gypsum cement have values 0 7079 and 0 7085 that indicate miocene and slightly younger seawater evaporites were evidently leached by modern meteoric water from nearby outcrops of miocene and younger marine evaporite bearing strata transported in surface and ground water to the topographically low terrain where the cambrian sandstones crop out and reprecipitated by evaporation initial primary porosity 45 was reduced to 27 by compaction in sands with minor quartz cement compaction ceased where calcite was precipitated subsequent dissolution of some calcite cement and unstable detrital grains generated considerable secondary porosity

three dimensional model of modern channel bend deposits 1 we present a three dimensional model of heterogeneous modern channel bend deposits developed through purely structure imitating interpolation kriging of hydraulic properties this model augmented with ground penetrating radar data and directional variograms agrees with detailed observations in similar modern environments and leads to a process based interpretation of the presented hydraulic conductivity structure integration of all available information permitted delineation and characterization of the modern streambed as a distinct hydrostratigraphic unit without coring or outcrop studies our results imply that the modern streambed is commonly oversimplified in available analytical and numerical models of groundwater surface water interactions where it is assumed to be homogeneous and isotropic and characterized by a constant width and thickness this three dimensional approach that integrates concepts and principles developed in sedimentology hydrogeology geophysics and geostatistics has potential implications on model development of stream aquifer systems

the palaeolithic locality schoningen germany a review of the mammalian record abstract quaternary deposits had to be removed to get access to and to explore the palaeogene lignite in the area east of the village of schoningen germany and by removing the top layers a number of archaeological sites have been discovered in 1992 the first palaeolithic artefacts were found and the locality became world famous after one of the highlights of the schoningen project the discovery in 1995 of the palaeolithic wooden throwing spears at the site scho 13 ii 4 the spears are found among remains of butchered horses the mammalian fossil record of that site comprises 12 000 larger mammal and several thousands of small mammal remains the faunal record representing at least 30 taxa includes remains that are scattered over the excavated area and interpreted as a natural background fossil record in a palaeolandscape in which early hominins operated the site scho 13 ii 4 yielded in addition to the scattered natural background fossils several thousands of horse remains many of which are smashed and butchered from the more or less complete carcasses of 20 25 horses different stratigraphic horizons in the schoningen channels i and ii correlated with respectively the holsteinian interglacial mis 11 and the succeeding reinsdorf interglacial mis 9 yielded palaeolithic artefacts and a large number of archaeozoological finds including both larger and smaller mammals the oldest fauna is scho 13 i is from the schoningen channel i deposits that overlie the elsterian glacial deposits the faunal assemblage represents at least 11 different mammalian taxa the assemblage does not include indicators for exclusive glacial or interglacial conditions the channel ii deposits yielded a number of faunal assemblages dating to the second half of the reinsdorf interglacial the faunal assemblages from channel ii levels 1 5 are from a period of changing climate spanning the transition from an interglacial optimum at the base to the beginning of the following cold stage at the top the faunal changes are however relatively minor which might indicate that the mammalian fauna has a high degree of resilience for the recorded climate changes the reinsdorf interglacial mammalian record includes relict species such as sorex drepanosorex sp and trogontherium cuvieri their occurrences as well as the evolutionary stage of the arvicola molars from schoningen channel ii are biostratigraphic markers that indicate that the faunas have a post elsterian age they are younger than the faunas such as boxgrove uk miesenheim i mauer mosbach germany and older than late middle pleistocene faunas from sites such as maastricht belvedere the netherlands and weimar ehringsdorf germany

authigenic albite in carbonate rocks a tracer for deep burial brine migration euhedral post depositional albite from the eastern and western alps the western carpathians and some greek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in carbonate rocks this study focuses on coarsely crystalline homogeneously nucleated albite in order to avoid problems related to the presence of inseparable detrital material in fine grained albite varieties all albite samples show a very restricted compositional variability and are typically 99 mol ab component unit cell parameters determined by rietveld analysis are slightly more variable than previously accepted but confirm high al si ordering characteristic of low albite the oxygen isotopic composition of albite ranges from 19 4 to 28 3 vsmow there is no direct relationship between the 18o value and the inferred temperature of albite formation nor is there one with stoichiometry the coarse crystal size up to several millimetres in diameter petrographic evidence showing albite cross cutting stylolites greater abundance of albite in carbonate rocks subject to high grade diagenetic or weak metamorphic overprinting and available fluid inclusion data suggest that albite precipitation is favoured at higher temperatures in carbonates than in sandstones pore fluids were invariably brines as suggested by the inferred high positive 18ofluid values the common association of albite bearing carbonates and evaporites and reports of saline fluid inclusions in albite the presence of authigenic albite may thus be a useful tracer of palaeobrine carbonate reactions particularly in deep burial and incipient metamorphic settings

diagenesis of dissolved aluminum in organic rich estuarine sediments abstract the sedimentary geochemistry of dissolved al is complicated by a number of different reactions in this study we show that complexation by organic matter adsorption to fe oxyhydroxides and reaction with si in solution have important effects on the distribution of dissolved al in sediments in the absence of physical resuspension of sediment into overlying waters dissolved al is rapidly consumed at the sediment water interface and is subsequently released upon reduction of fe oxyhydroxides this release does not cause noticeable perturbations in dissolved al concentrations in sediments because of rapid consumption reactions which mask the true mobility of al results suggest that one of the consumption reactions may be due to formation of an fe al silicate the amount of authigenic aluminosilicate formed in estuarine sediments must be very small relative to the detrital component in the deep sea however the long residence time of fe oxyhydroxides at the sediment water interface with resulting greater accumulation of adsorbed al may explain the abundance of al in fe smectites reported from many different areas

early diagenetic pyrite morphology in a mudstone dominated succession the lower jurassic cleveland ironstone formation eastern england abstract diagenetic pyrite in the mudstones and ironstones of the lower jurassic cleveland ironstone formation of eastern england exhibits two distinct morphologies framboidal pyrite commonly associated with organic matter and euhedral pyrite associated with detrital clay pellets these two morphologies are mutually exclusive in occurrence framboidal pyrite is present in clay rich mudstones ooidal ironstones apatite rich units and some silt rich mudstones euhedral pyrite is present in silt rich and sand rich mudstones 34s isotopic analysis of six samples of pyrite suggests that both types of pyrite morphology precipitated during early diagenesis from porewaters with open access to overlying sea water although both probably acted as sites for continued pyrite precipitation during burial it is proposed that framboidal pyrite precipitated from iron dominated porewaters at sites of sulfide supply i e in the region of organic matter as a result of bacterial sulfate reduction where locally sulfide production rates were high enough for porewaters to reach supersaturation with respect to fes euhedral pyrite also precipitated from iron dominated porewaters but sulfide production rates from organic matter was such that fes saturation was not reached at the sites of sulfide production instead euhedral pyrite was precipitated directly from porewater when fes2 saturation was reached the control over pyrite morphology was probably the amount and reactivity of the organic matter within the deposited sediments the sand rich mudstones contained less reactive organic matter due to clastic dilution and deposition in shallower environments with o2 rich bottom waters the ironstones and apatite rich units were deposited under very low sedimentation rates and as a result organic matter contents were very low and iron reduction dominated early diagenesis which inhibited sulfate reduction the presence of minor framboidal pyrite within these units however suggests that sulfide reduction took place in micro environments during early diagenesis

dissolved al in sediments and waters of the east china sea implications for authigenic mineral formation abstract numerous previous studies indicate that several different authigenic aluminosilicates form in the oceans in this study we show using dissolved al distributions in sediments and waters from the nearshore regions of the east china sea that the process of aluminosilicate formation probably begins rapidly upon contact of detrital clays with seawater statistical analyses of dissolved al si h relations in surface sediments indicate that the minerals forming in east china sea sediments low in dissolved fe are dioctahedral chlorites with an average composition ex 0 91 mg 0 77 al 5 0 si 2 7 o 10 oh 8 where ex exchangeable 1 cation this composition is also consistent with dissolved al and si measurements as a function of salinity in turbid overlying waters results suggest a dissolution reprecipitation mechanism for clay mineral reconstitution this mechanism can help to explain why different authigenic clays are found in different areas of the oceans in the east china sea the total amount of authigenic clays present must constitute a very minor fraction of the bottom sediments thus the formation of these minerals has a relatively small impact upon dissolved si distributions clay mineral reconstitution in nearshore regions may provide a mechanism for buffering sediments and overlying waters with respect to ph as the composition of minerals formed should be a direct function of the h activity in the surrounding environment

texture and diagenesis of ordovician shale from the canning basin western australia implications for elastic anisotropy and geomechanical properties texture and diagenesis of ordovician shale from the canning basin western australia implications for elastic anisotropy and geomechanical properties

clay mineral transformations in tertiary and mesozoic sediments from north sea clay mineral transformations of 10 north sea wells have been studied and several transformation stages picked out and correlated with geothermal variations smectite disappears at 149 to 167 f 65 to 75 c whereas transformations in the mixed layer illite smectite clay minerals are observed between 113 and about 212 f 45 and about 100 c at temperatures above 113 to 149 f 45 to 65 c more than 30 illitic layers are found in the mixed layered clay minerals whereas temperatures from 140 to 167 f 60 to 75 c indicate the transition zone toward more than 50 dehydrated lattice positions temperatures above 176 to 212 f 80 to 100 c mark the lower limit for mixed layer clay minerals containing more than 70 illitic la ers the major part of the clay mineral transformations apparently takes place in the mesodiagenetic zone in which most of the hydrocarbons are also formed

discussiondiscussion in response to knut bjorlykke regarding jmpg_1376 open system chemical behavior in deep wilcox group mudstones texas gulf coast usa

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

degree of supersaturation and precipitation of organogenic dolomite this paper integrates the degree of carbonate saturation into a general model for the origin of dolomite in organic rich marine sediments degradation of organic matter by sulfate reducing bacteria promotes early dolomite precipitation by simultaneously increasing the carbonate alkalinity and lowering the sulfate ion concentration to near zero the interstitial water in organic rich sediments of several deep sea drilling project sites where dolomite is believed to be precipitating is up to 10 sup 3 times supersaturated with respect to near ideal dolomite and up to 10 times supersaturated with respect to calcite the highly supersaturated interstitial waters appear to overcome low temperature kinetic barriers to precipitate a nonideal fine grained calcian dolomite the degree of supersaturation with respect to dolomite decreases and the interstitial water approaches carbonate equilibrium with increasing burial depth at several of these sites removal of sulfate ion is important because sulfate ion may inhibit dolomite precipitation a calcareous sediment is not required but the presence of calcite or aragonite appears to greatly enhance dolomitization by providing an additional source of ca and carbonate ions

mass transport in sandstones around dissolving plagioclase grains the dissolution of plagioclase and precipitation of kaolinite is a common late diagenetic process in sandstones the common occurrence on a thin section scale of patchy kaolinite adjacent to a dissolving plagioclase grain is consistent with diffusional transport of dissolved constituents combined with precipitation on a favorable nucleation site the absence in thin section view of a diffusionally related halo of kaolinite around a dissolving grain can be explained by a lack of a halo of favorable nucleation sites numerical modeling results are presented for the transport of aqueous al by diffusion coupled with fluid flow in sandstones at 100 c within a few months the al in the anorthite component of plagioclase grains can be removed by diffusion during diagenesis fluid velocities needed to overcome diffusion to prevent precipitation on the upstream side of a dissolving grain depend on the unknown minimum supersaturation needed for precipitation these velocities are less than those predicted by the peclet number but are of the order of metres per year at a distance of a few tenths of a millimetre from the dissolving grain the nonlinear inverse relation of these velocities to the distance from the dissolving grain implies that mass transport to precipitation sites is dominated by fluid flow at distances greater than 1 cm

pyrite replacement of mollusc shells from the lower oxford clay jurassic of england replacement of originally aragonite mollusc shells by pyrite commonly occurs in the lower oxford clay petrographic studies show the shells to have constituted complex microenvironments in the sediment a range of replacement textures is found showing a variable amount of solution of the original aragonite three distinct textures were found in crushed pyrite replaced ammonite shells from heavily pyritized concretions 1 a texture reflecting the original shell structure due to the replacement of the organic shell matrix by pyrite 2 an ovoid texture seen at several stages of replacement reflecting processes occurring at discrete centres of sulphate reduction 3 euhedral crystals lining cracks and fractures in the shell three types of replacement are found in small gastropods and bivalves from shell bed some of which may relate to those seen in the ammonites 1 replacement of organic shell matrix by pyrite preserving good shell microstructure 2 replacement showing outwardly good preservation of morphological features but inwardly only the gross structure such as growth lines is preserved 3 replacement of the shell in a matrix of euhedral pyrite leaving only lines of carbonate inclusions marking the margins of the shell the replacement textures and types appear to be dependent on the initial structure of the shell and the access of iron and sulphate into the shell early stages of replacement appear to proceed by pyrite formation within the organic matrix of the shell with little or no solution of the carbonate this produces textures which faithfully mimic the original shell microstructure it is thought that the lack of carbonate solution is due to a limited availability of iron brought about by the less intensively reducing nature of the sediment later stages of replacement are promoted by the cracking and fracturing of the shell and are generally not as faithful to the original shell structure this is due to the greater availability of iron as the sediment becomes more reducing with burial

authigenic kaolinite and associated pyrite in chalk of the cretaceous niobrara formation eastern colorado abstract cores from the smoky hill chalk member of the cretaceous niobrara formation have several zones containing authigenic kaolinite as spherical moldic polycrystalline aggregates that occur within single or multichambered foraminiferal tests and are commonly associated with framboidal pyrite such kaolinite is inferred to result from volcanic ash deposited during chalk sedimentation shortly after burial a colloidal aluminous gel or solution formed from the unstable ash and moved into organic rich foraminiferal tests where sulfate reducing bacteria created a favorable microenvironment for the simultaneous crystallization of kaolinite and pyrite

dolomite formation in the dynamic deep biosphere results from the peru margin early diagenetic dolomite beds were sampled during the ocean drilling programme odp leg 201 at four reoccupied odp leg 112 sites on the peru continental margin sites 1227 684 1228 680 1229 681 and 1230 685 and analysed for petrography mineralogy 13c 18o and 87sr 86sr values the results are compared with the chemistry and 13c and 87sr 86sr values of the associated porewater petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate dolomite precipitation often pre dates the formation of framboidal pyrite most dolomite layers show 87sr 86sr ratios similar to the composition of quaternary seawater and do not indicate a contribution from the hypersaline brine which is present at a greater burial depth also the 13c values of the dolomite are not in equilibrium with the 13c values of the dissolved inorganic carbon in the associated modern porewater both petrography and 87sr 86sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment 30 m below the seafloor a significant depletion in the dissolved mg and ca in the porewater constrains the present site of dolomite precipitation which co occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate methane interface it has been hypothesized that microbial hot spots such as the sulphate methane interface may act as focused sites of dolomite precipitation varying 13c values from 15 to 15 for the dolomite are consistent with precipitation at a dynamic sulphate methane interface where 13c of the dissolved inorganic carbon would likewise be variable a dynamic deep biosphere with upward and downward migration of the sulphate methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter thus the study of dolomite layers in ancient organic carbon rich sedimentary sequences can provide a useful window into the palaeo dynamics of the deep biosphere

an sem study of phyllosilicates in a westphalian coal measures sandstone using back scattered electron imaging and wavelength dispersive spectral analysis abstract back scattered electron imaging bei made in a jeol jxa 733 sem was used in combination with wavelength dispersive spectral wds analysis to study phyllosilicate minerals in thin foils and polished thin sections of coal measures westphalian sandstones from the east midlands u k detrital chlorite grains up to 500 m occur intergrown with hydromuscovite and kaolinite detrital muscovite grains have undergone incipient alteration to illite and occur intergrown with chlorite and kaolinite in addition kaolinite occurs as pore filling books of plates all kaolinite is authigenic though it is not clear whether it replaces the chlorite or muscovite intergrowths or is simply intergrown with them analyses obtained from thin foils and polished thin foils are more reliable than analyses obtained from fracture surface sem samples because artefactual additions to the characteristic spectra are minimized

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial