silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits an example of meteoric dominated diagenesis from the triassic of central spain cauliflower shaped nodules are widespread in a single red mudstone bed in the buntsandstein triassic facies of the iberian range they consist mostly of quartz dolomite and calcite but other minerals such as barite kaolinite and iron oxyhydroxides are also present the nodules are spherical ovoid or elongate in shape and range from 1 to 8 cm across the surface of the nodules is irregular and some show a pedogenic coating of microspar the sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of ca2 and so4 2 in a vadose environment occasionally under the influence of pedogenic processes partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting as indicated by the presence of organic filaments on the quartz crystal surfaces in type a nodules the dissolution of the innermost anhydrite was complete and different types of quartz cement filled the porosity fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules in type b nodules the dissolution of the anhydrite was incomplete inhibiting quartz cementation and enabling later dolomitization of the anhydrite dolomitization appears to have been driven by sulphate reduction as indicated by the presence of bacterial bodies within the dolomite crystals dedolomitization and precipitation of barite kaolinite and calcite spar cements occurred later under the influence of meteoric solutions the nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions

determination of initial sr isotopic compositions of dolostones from the burlington keokuk formation mississippian constraints from cathodoluminescence glauconite paragenesis and analytical methods abstract regionally extensive fine grained dolostones from the mississippian burlington keokuk fm comprise two major generations of dolomite that can be distinguished by their cathodoluminescence chemical compositions and times of formation the petrography and geochemistry of glauconite the predominant clay mineral in the dolostones indicates that it is an authigenic phase some of which formed episodically during various stages of postmarine diagenesis this study evaluates techniques for the determination of the sr isotopic compositions of the dolomite generations through the analysis of nearly pure whole rock samples 95 soluble and mineral separates each composed predominantly of one cathodoluminescent type of dolomite sr isotopic compositions and rb and sr concentrations were measured on samples prepared by three separate methods 1 leaching with 1 25n hcl 2 leaching with 1 25n hcl and concentrated hf and 3 complete dissolution with concentrated hf and hno3 and 6n hcl leaching with hcl removes 24 60 percent of the rb in the samples compared to acid leaching complete sample dissolution yields higher 87sr 86sr ratios and rb concentrations and more reproducible results this indicates that most of the rb and radiogenic 87sr is associated with minor clay fractions in the samples and that hcl leaches rb and sr from clays to a variable extent the relatively high rb sr ratios of the dolostones necessitate an age correction of each sample s 87sr 86sr ratio the determination of such initial 87sr 86sr ratios requires contemporaneous formation of carbonate and clay closure of the rb sr system after diagenesis and constraints on the timing of diagenesis calculated initial 87sr 86sr ratios for the dolostones by method 3 differ considerably from the uncorrected measured 87sr 86sr ratios by any of the dissolution methods these results demonstrate that even minor clay components can make significant contributions to the analyses and that analysis for rb and sr and constraints on the timing and diagenetic history of both the clay and carbon te phases can be critical for the determination of the sr isotopic compositions of even nearly pure dolostones an evaluation of the uncertainties associated with the timing and type of diagenesis indicates that the initial 87sr 86sr ratios of the dolostones are representative of the isotopic composition of the dolomite generations at the time of their formation two major cathodoluminescent types of burlington keokuk dolomites preserve a record of two isotopically distinct diagenetic environments samples composed of the earliest dolomite dolomite i have initial 87sr 86sr ratios ranging from 0 70757 to 0 70808 while samples composed of dolomites ii and ii range from 0 70885 to 0 70942

reactive transport modelling of reflux dolomitization in the arab d reservoir ghawar field saudi arabia it has long been recognized that the arab d reservoir in ghawar field has been significantly dolomitized and that the distribution of dolomites is highly heterogeneous across this reservoir previous studies indicated that dolomite occurs with either a stratigraphic or non stratigraphic distribution when mapped dolomite tends to form several parallel linear trends across the field although stratigraphic dolomite was suggested to be formed early from highly evaporated pore fluids sourced from overlying evaporite deposits non stratigraphic dolomite was thought to be generated primarily from hydrothermal fluids sourced from below this study focuses primarily on these non stratigraphic dolomites and proposes that i these dolomites initially formed via seepage reflux but were reinforced by late stage hydrothermal dolomitization and ii reflux is also responsible for the formation of parallel linear trends of dolomite the reflux model hypothesizes that an evaporative lagoon which is the source of dolomitizing fluids formed during the falling stage systems tract of a depositional sequence and that with continuing sea level fall this lagoon migrated progressively towards deeper parts of an intrashelf basin adjacent to the ghawar field leaving behind lines of dolomite bodies along a series of temporary coastlines two dimensional reactive transport models have been built to test this hypothesis and have resulted in a predicted pattern of dolomite bodies that agrees with both the observed vertical distribution of non stratigraphic dolomite as well as the mapped lateral distribution of the dolomite trends in addition the major ion compositions of late jurassic seawater are calculated based on fluid inclusion data in the literature using jurassic seawater in current models leads to the absence of anhydrite cements and less potential of over dolomitization than using modern seawater

impact of early dolomitization on multi scale petrophysical heterogeneities and fracture intensity of low porosity platform carbonates albian cenomanian southern apennines italy abstract petrographic petrophysical and fracture analyses were carried out on middle cretaceous platform carbonates of the southern apennines italy that represent an outcrop analogue of the val d agri and tempa rossa reservoirs of the basilicata region the studied outcrops which are made of interlayered limestones and dolomites of inner platform environment were selected to study the impact of dolomitization on reservoir properties and the control of dolomite texture on fracture development two types of dolomites both formed during very early diagenesis were found interlayered at a metre scale with micrite rich limestones mainly mudstones and wackestones dolomite a is fine to medium crystalline and makes non planar mosaics dolomite b is coarse crystalline and makes planar s and planar e mosaics the intercrystalline space of the planar e subtype of dolomite b is either open or filled by un replaced micrite or by late calcite or saddle dolomite cement dolomite a and dolomite b have similar average porosities of 3 7 and 3 1 respectively which are significantly higher than the average porosity of limestones 1 4 their poro perm relationships are similar with the notable exception of planar e type b dolomites which generally display higher permeability values the intensity of top bounded fractures is distinctly lower in coarse crystalline dolomites than in fine crystalline dolomites and limestones both at the macro and the micro scale on the other hand neither lithology i e limestone vs dolomite nor dolomite crystal size control the intensity of perfect bed bounded fractures which is strictly controlled by the fracture layer thickness our results provide information that could be used as guidance for the characterization and modelling of fractured carbonate reservoirs made of interlayered limestones and dolomites

valanginian isotope variation in glendonites and belemnites from arctic svalbard transient glacial temperatures during the cretaceous greenhouse oxygen and carbon isotope data from cretaceous valanginian glendonites and belemnites from arctic svalbard are presented oxygen isotope data from well preserved glendonites in conjunction with the ikaite to glendonite pseudomorph transition temperature are used to provide an estimate of the oxygen isotope composition of ambient seawater calculation of such a factor is essential for robust paleotemperature estimates using this methodology our paleotemperatures calculated from the oxygen isotope compositions of coexisting belemnites yield cool temperatures 4 7 c consistent with transient glacial polar conditions during the cretaceous greenhouse cool polar temperatures during the cretaceous help reconcile geologic data with the simulations of general circulation models nevertheless beyond this postulated and transient cool event within the valanginian the remainder of the isotope data are interpretable in terms of warm polar conditions during the cretaceous greenhouse

dolomites and dolomites in mesozoic platform carbonates of the southern apennines geometric distribution petrography and geochemistry abstract a multidisciplinary study of the dolomitized bodies present among the late triassic to late cretaceous carbonate deposits of the monti lattari belt southern apennines of italy compares the different dolomitization styles present within this succession of these mesozoic platform carbonates and furnishes an overview of their formative genetic models the revealing traces of the diverse modes of diagenesis in this region include both surface and subsurface mechanisms utilizing a geometric reconstruction of the dolomitized bodies an integrated facies and petrographic analysis of the components and a geochemical characterization of the sediments including detailed mineralogy and associated stable isotope values the data presented demonstrate that some intervals are characterized by surface related dolomitization processes middle late triassic and cretaceous while others latest triassic to lower middle jurassic and are mainly dominated by later large scale subsurface dolomitization processes all of the middle late triassic norian successions are pervasively dolomitized exhibit fabric preserving textures and have a distinctly heavy stable isotope signature their origin has been related to diagenesis due to particular climatic and palaeoceanographic conditions leading to the formation of extended blankets of shallow warm saline seawater this setting was terminated by climatic and palaeoceanographic changes during the latest triassic which dramatically reduced the significance of surface related dolomitization on a regional scale the overlying latest triassic rhaetian outcrops consist of a widespread coarse fabric destructive massive dolomite with a lighter isotopic signature these dolomites seem genetically related to the ones of the lower middle jurassic which share the same geometrical petrographic and geochemical characteristics caused by large scale fluid circulation this diagenetic pattern apparently started in the early jurassic during burial driven by thermal convection due to the different temperatures between the oceanic waters and the warmer carbonate platform the middle upper jurassic units feature local episodes of early dolomitization that partially replaced the muddy carbonate facies due to tidal pumping and reflux mechanisms dolomitization is not a pervasive feature during this phase of deposition in the cretaceous portion of the succession dolomites have a totally different and distinct character and are generally related to bedding and show variability in textures and mg ca ratios local association with evaporites geometry of dolomitized bodies and stable isotope geochemistry suggest processes comprising evaporative pumping and reflux all related to the surface environment this evidence points to a renewal of a climatic context more favorable for early dolomitization thus this study permits recognition of the presence of three quite different dolomitization styles within the carbonate platform succession of the southern apennines fold thrust belt one ascribed to the norian one to the rhaetian lower jurassic and one to the cretaceous

rock composition dolomite stoichiometry and rock water reactions in dolomitic carbonate rocks dolomitic carbonate rock units from a variety of phanerozoic localities and depositional environments throughout north america were evaluated with respect to their dolomite content and dolomite composition by x ray diffraction analysis dolomite compositions range from 48 to 57 mole caco 3 with well defined modes at 51 nearly stoichiometric dolomite and 55 distinctly calcian dolomite caco 3 values of percentage of dolomite also exhibit a pronounced albeit less well defined bimodal distribution with modes at 97 dolostones and 20 dolomitic limestones dolomite similar distributions characterize most other published data on phanerozoic dolomitic carbonates these data suggest that two separate processes may lead to two distinct populations of sedimentary dolomite 1 dolomitic limestones originate in diagenetically closed systems during magnesian calcite dissolution calcite and dolomite precipitation reactions and 2 dolostones originate in diagenetically open systems during th

origin and modification of early dolomites in cyclic shallow platform carbonates yeongheung formation middle ordovician korea abstract the middle ordovician yeongheung formation korea consists of numerous metre scale shallowing upward cycles which were deposited on a shallow marine carbonate platform depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology whereas those developed on the outer platform were partially dolomitized the amount of replacement dolomite within a metre scale shallowing upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences the cross platform distribution of dolomites and their stratigraphic position in the sequences suggest that the yeongheung dolomite formed syndepositionally the common existence of pseudomorphs of evaporite minerals and nodules in the yeongheung formation indicates that dolomitization of the yeongheung formation occurred in a sabkha type tidal flat environment it also occurred during each cycle of deposition influenced by high frequency sea level oscillations the present day yeongheung replacement dolomites are characterized by near stoichiometric to stoichiometric composition 49 9 to 52 1 caco3 mole and low strontium concentration 52 to 133 ppm the 18o value of the replacement dolomites ranges from 7 7 to 5 2 pdb this is much depleted relative to that of contemporary sabkha type dolomites these geochemical characteristics of yeongheung dolomite indicate significant post depositional modification the common existence of oomolds leached skeletal grains and dissolved evaporites in the yeongheung formation illustrates invasion of meteoric water into the sediments recrystallization of yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter 18o values and lower strontium concentration

moganite as an indicator for vanished evaporites a testament reborn abstract although lutecite or length slow silica frequently is used as an indicator for evaporites its occasional presence in non evaporitic regimes has shed suspicion on its reliability this study provides evidence that lutecite actually is a newly recognized mineral called moganite powder x ray diffraction studies reveal that moganite is abundant in but not restricted to cherts that formed in evaporitic environments quantitative analysis using rietveld refinement methods indicates that non evaporitic silica typically contains between 5 and 15 wt moganite whereas evaporitic specimens may contain between 20 and 75 wt moganite no chert from non evaporitic settings has been found to contain more than 25 wt moganite consequently enhanced moganite concentrations 20 wt in m crocrystalline silica may prove a valuable indicator for vanished evaporites the frequent association of moganite with magadi type chert suggests that moganite is a diagenetic alteration product of the hydrous na silicate magadiite the transformation of magadiite to moganite occurs over hundreds to thousands of years whereas the inversion of metastable moganite to quartz often requires tens of millions of years

the barremian aptian arid phase in western europe abstract evidence is presented from nine areas in western europe speeton yorkshire southern north sea channel and weald basins southern england jeanne d arc lusitanian and aquitanian basins of the north atlantic continental margins lower saxony basin north germany the paris basin and southern france for a phase of aridity which began during the hauterivian middle barremian and continued to dominate the climate of the region until the mid aptian although conditions during this period were not uniformly dry throughout criteria for recognising arid environments include the occurrence of evaporites abundances of carbonate rich sediments red beds firmgrounds carbonate nodules and pyrite concretions depletion of kaolinite spore assemblages of limited morphological diversity and the presence of numerous coniferalean and cheirolepidiacean pollen grains the change from generally humid to dry climates during the barremian is thought to have been linked to a lowering of sea level the arid phase clearly falls within the time span of the first cretaceous oceanic anoxic event

field flume reveals aquatic vegetation s role in sediment and particulate phosphorus transport in a shallow aquatic ecosystem abstract flow interactions with aquatic vegetation and effects on sediment transport and nutrient redistribution are uncertain in shallow aquatic ecosystems here we quantified sediment transport in the everglades by progressively increasing flow velocity in a field flume constructed around undisturbed bed sediment and emergent macrophytes suspended sediment 100 m became dominant at higher velocity steps after a threshold shear stress for bed floc entrainment was exceeded shedding of vortices that had formed downstream of plant stems also occurred on that velocity step which promoted additional sediment detachment from epiphyton modeling determined that the potentially entrainable sediment reservoir 46 g m 2 was similar to the reservoir of epiphyton 66 g m 2 but smaller than the reservoir of flocculent bed sediment 330 g m 2 all suspended sediment was enriched in phosphorus by approximately twenty times compared with bulk sediment on the bed surface and on plant stems indicating that the most easily entrainable sediment is also the most nutrient rich and likely the most biologically active

source rock distribution model of the periadriatic region the periadriatic area is a mosaic of geological provinces comprised of spatially and temporally similar tectonic sedimentary cycles tectonic evolution progressed from a triassic early jurassic liassic continental rifting stage on the northern edge of the african craton through an early jurassic middle liassic late cretaceous eocene oceanic rifting stage and passive margin formation to a final continental collision and active margin deformation stage in the late cretaceous eocene to holocene this evolution occurred in a transtensional and transpressional framework resulting from the oblique separation and convergence of the african and european plates extensive shallow water carbonate platform deposits covered large parts of the periadriatic region in the late triassic platform breakup and development of a platform to basin carbonate shelf morphology began in the late triassic and extended through the cretaceous on the basis of this paleogeographic evolution the regional geology of the periadriatic region can be expressed in terms of three main upper triassic paleogene sedimentary sequences the platform sequence sequence a the platform to basin sequence sequence b and the basin sequence sequence c these sequences developed during the initial rifting and subsequent passive margin formation tectonic stages three distinct episodes of source bed deposition can be identified middle upper triassic to lower jurassic liassic organic rich oil prone and mature source beds generally are structurally controlled and are closely associated with transtensional intraplatform basins formed in the early continental rifting stage the principal triassic source basins and most of the surface hydrocarbon indications and economically important oil fields of the periadriatic region are associated with sequence b areas the second episode of organic rich rocks deposition includes both shallow and deeper water facies of jurassic cretaceous age thin immature deep water epioceanic organic rich marls and shales are generally bathymetrically controlled these units are related to basin wide anoxic events in the tethys ocean and its submerged continental margin and are associated with sequence b areas middle upper cretaceous shallow water oil prone facies occur in isolated anoxic basins within carbonate platforms these source sequences are generally thicker and are associated with sequence a areas no major hydrocarbon accumulation can be directly attributed to the jurassic cretaceous epioceanic and intraplatform source rock sequences the third episode of source bed deposition characterizes the final active margin deformation stage and is represented by upper tertiary organic rich terrigenous units mostly gas prone these are essentially associated with turbiditic and flysch sequences of foredeep basins and have generated the greater part of the commercial biogenic gases of the periadriatic region

volcanism co2 and palaeoclimate a late jurassic early cretaceous carbon and oxygen isotope record a composite tethyan late jurassic early cretaceous carbon and oxygen isotope curve is presented c isotope data provide information on the evolution and perturbation of the global carbon cycle o isotope data are used as a palaeotemperature proxy in combination with palaeontological information the resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification positive c isotope anomalies in the valanginian and aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide a major warming pulse accompanies the aptian but not the valanginian c isotope event the observed change in early aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion both c isotope anomalies are accompanied by biocalcification crises that may have been triggered by p co 2 induced changes in climate and in surface water chemistry elevated nutrient levels in river influenced coastal waters and in upwelling regions further weakened marine calcification these conditions contrast with normal trophic conditions prevailing in the latest jurassic and favouring biocalcification the c and o isotope curves record a stable mode of carbon cycling and stable temperatures we conclude that biocalcification is mostly triggered and inhibited by co 2 conditions in the atmosphere ocean system

altered carbon cycling and coupled changes in early cretaceous weathering patterns evidence from integrated carbon isotope and sandstone records of the western tethys abstract in this study we investigate if a major perturbation of the early cretaceous carbon cycle was accompanied by altered weathering and erosion rates the large aptian carbon isotope anomaly records the response of the biosphere to widespread volcanic activity and probably resulting changes in atmospheric pco 2 levels elevated pco 2 levels should also result in an accelerated hydrological cycle and increased silicate weathering creating a negative feedback loop removing co 2 from the atmosphere we propose to interpret the widespread occurrence of quartz sandstones in the tethys atlantic seaway as a result of altered weathering and erosion rates in the wake of the aptian carbon cycle excursion we challenge the traditional notion that these are flysch deposits associated with early cretaceous orogenic movements in the western tethys we propose that these sandstones were most likely part of a large conveyor belt system acting along the iberian and european margin of the tethys seaway using chemostratigraphic correlations we show that the activity of this system was only short lived and coeval with changes in coastal ecology and the aptian carbon cycle perturbations we tentatively relate the existence of this system to a transient climate regime characterized by fluctuating pco 2 levels

structural diagenesis of shallow platform carbonates role of early embrittlement on fracture setting and distribution case study of monte alpi southern apennines italy abstract bed perpendicular opening mode diffuse fractures are common features in carbonate rocks because they become brittle during the first stages of diagenesis early fractures could be independent of tectonics and form a background structural network from sub millimeter to 10 s of centimetres scale this study focuses on the outcrop to micro scales structural stratigraphic and petrographic characterization of lower cretaceous shallow water tight limestones of the inner apulian platform exposed along the axial sector of the southern apennines fold and thrust belt italy this work aims at understanding the formation of early diagenetic fractures during the first stages of sediment lithification and on their impact on subsequent deformation mechanisms associated to polyphase tectonics the results of field structural surveys of petrographic and cathodoluminescence analyses of representative samples unravel the main structural diagenetic processes in shallow water carbonates we document that early brittle fractures were intrinsically related to the host rock pore type and cementation processes and summarize the overall time dependant evolution of the structural assemblages in a conceptual model this model includes all abutting and crosscutting relationships among the various structural elements from field to thin section observation and form the now days fracture network

tying platform drowning to perturbations of the global carbon cycle with a delta13corg curve from the valanginian of dsdp site 416 the carbonate carbon isotope record of the phanerozoic is marked by repeated high amplitude excursions towards more positive values although the interpretation of c isotope events remains controversial they are regarded as a proxy of the global c cycle using 13corg measurements of samples from dsdp site 416 it is shown that a classic example of platform drowning coincided with the onset of the mid valanginian carbon cycle excursion widespread carbonate platform drowning would have contributed to the observed positive shift in the c isotope record as choking of carbonate production was compensated by increased organic production both processes favoured a shift in marine carbon partitioning from the oxidized to the reduced carbon reservoir this would have resulted in an increased flux of co2 from the atmosphere into the marine and sedimentary carbon reservoir

valanginian weissert oceanic anoxic event biotic changes in nannofossils and radiolarians associated with the valanginian 1 3 c anomaly are documented at ocean drilling program hole 1149b in the pacific ocean they are coeval and similar to those previously documented in the tethys suggesting a global perturbation of marine ecosystems a marked increase in abundance of diazomatolithus absence of nannoconids and a pantanellium peak characterize the valanginian 1 3 c excursion such changes are interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess co 2 the occurrence of organic c rich black shales in the southern alps and in the pacific in the interval corresponding to the 1 3 c excursion suggests a valanginian oceanic anoxic event oae volcanism of the parana etendeka large igneous province ca 132 ma was presumably responsible for an increase of co 2 triggering a climate change and accelerated hydrological cycling possibly causing an indirect fertilization of the oceans widespread nutrification via introduction of biolimiting metals at spreading ridges could have significantly increased during the gondwana breakup and simultaneous tectonic events in three separate oceans there is no paleontological or 1 8 o evidence of warming during the valanginian oae on the contrary both nannofossils and oxygen isotopes record a cooling event at the climax of the 1 3 c excursion weathering of basalts and burial of organic c rich black shales were presumably responsible for co 2 drawdown and establishment of reversed greenhouse conditions

a reappraisal of dolomite abundance and occurrence in the phanerozoic critical assessment of dolomite abundance throughout the phanerozoic suggests that most large scale dolomite has had an origin related to salinity elevated seawaters the distribution of such dolomite is not directly related to periods of major continental flooding but rather to periods when extensive peritidal sequences or large scale evaporite basins lagoons existed the development of such depositional systems is controlled largely by eustasy paleogeographic tectonic setting and paleoclimate extensive peritidal sequences are characteristic of mature passive margin carbonate platforms that formed during long periods of earth history generally devoid of major continental glaciation e g late precambrian through early paleozoic and mesozoic sea level fluctuations of relatively low amplitude during such periods allowed extensive cyclic peritidal dominated platforms to develop such platforms typically lack raised rims had depositional surfaces that stayed near sea level for tens of millions of years and show little evidence of major drops in sea level that would have exposed the shelf for long periods this would have allowed repeated replenishment of pore waters by mg rich marine fluids over very extensive supratidal surfaces also since the cycles are very thin the downward moving brines could penetrate several cycles on the inner platform causing massive dolomitization in contrast during major glacial periods e g middle to late mississippian to early permian and late cenozoic higher amplitude ice driven fluctuations in sea level led to extensive meteoric water flushing of shallow platforms but related dolomitization has been of minor significance under conditions of elevated mg ca ratio in hypersaline brines however large scale dolomitization could occur in response to the formation of extensive evaporite basins lagoons suitable climatic and paleogeographic tectonic conditions have allowed such dolomitization at various times during the phanerozoic regardless of specific styles of eustatic sea level change lack of massive dolomitization in holocene carbonates may be because neither extensive peritidal sequences nor large scale evaporite basins lagoons are prevalent in modern carbonate environments

high sea surface temperatures during the early cretaceous epoch the early cretaceous epoch about 145 100 million years ago is generally thought of as a greenhouse period with high atmospheric co2 concentrations1 and high global mean temperatures2 but evidence for episodes of cooler conditions and even transient glaciations has been proposed3 4 5 6 7 8 9 here we present sea surface temperature records spanning the period from 142 to 128 million years ago berriasian barremian ages from low and mid latitudes reconstructed using the tex86 palaeotemperature proxy during this period we find sea surface temperatures exceeding 32 c at 15 20 n and averaging 26 c at 53 s these temperatures substantially exceed modern temperatures at equivalent latitudes and are incompatible with the notion of consistently cooler conditions in the earliest cretaceous moreover we find little variability in the sea surface temperature records even during the valanginian carbon isotope excursion 138 135 million years ago which was thought to be associated with marked temperature fluctuations5 we conclude that the earliest cretaceous was characterized by a warm stable climate with a lower meridional temperature gradient than today

neogene patterns of relative plate motion for africa europe some implications for recent central mediterranean tectonics a detailed relative motion picture for the neogene africa europe plate kinematics is presented the kinematic reconstruction was carried out using the finite difference solution between the rotation parameters determined for anomalies 7 to 2 in the africa north america europe plate motion circuit the analysis shows a motion of africa with respect to europe which is nne directed during late oligocene to burdigalian times becoming nnw trending from the langhian to the early tortonian from upper tortonian times onward the motion changes to a clear north west directed convergence major late neogene tectonic features of the central mediterranean region can to a large extent be explained within the context of the reconstructed major plate motions late tortonian to recent africa europe slip vectors are compatible with a variety of geological phenomenoa such as north west directed subduction beneath calabria south east translation of calabria and extension in the tyrrhenian sea north west trending slip vectors from thrust earthquakes between gibraltar and sicily and dextral strike slip across the north african margin

replacement of evaporites within the permian park city formation bighorn basin wyoming usa the permian park city formation consists of cyclically bedded subtidal to supratidal carbonates cherts and siltstones early diagenesis of park city formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement syndepositional nodules and cements as well as dolomitization silicification and leaching of carbonate grains major differences are seen however in the diagenetic patterns of subsurface and surface sections of park city formation rocks subsurface samples are characterized by extensively preserved evaporite crystals and nodules and preserve evidence of significant silicification chert chalcedony and megaquartz and minor calcitization of evaporites in outcrop sections the evaporites are more poorly preserved and have been replaced by silica and calcite and also leached the resultant mouldic porosity is filled with widespread very coarse blocky calcite spar these replacements appear to be multistage phenomena field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration siliceous sponge spicules provided a major source of silica and the fluids involved in replacement were probably a mixture of marine and meteoric waters a second period of replacement and minor calcitization is inferred to have occurred during deep burial under the influence of thermochemical sulphate reduction although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval the major period of evaporite diagenesis however occurred during late stage uplift the late stage replacement and pore filling calcites have 13c values ranging from 0 5 to 25 3 and 18o values of 16 1 to 24 30 pdb reflecting extensive modification by meteoric water vigorous groundwater flow associated with mid tertiary block faulting led to migration of meteoric fluids through the porous carbonates to depths of several kilometres these waters reacted with the in situ hydrocarbon rich pore fluids and evaporite minerals and precipitated calcite cements the tosi chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history as evidenced by multiple episodes of silicification calcitization ferroan and non ferroan and hydrocarbon emplacement the multistage replacement processes described here do not appear to be restricted to the permian of wyoming similarly complex patterns of alteration have been noted in the permian of west texas new mexico greenland and other areas as well as in strata of other ages thus multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record

climate controlled early dolomite late triassic cyclic platform carbonates hungary abstract the 2 km thick late triassic hungarian carbonate platform has a completely dolomitized lower part and a limestone dominated upper part the platform succession is made up of meter scale possibly precessional 20 ky carbonate cycles in the lower platform main dolomite cycles are completely dolomitized and capped by caliche laminites and pisolites in the upper platform dachstein limestone dolomite is confined to laminite caps of cycles that are bounded by clayey paleosols most of the dolomitization in the platform interior occurred early in tidal flat settings during each high frequency cycle subtidal dolomites are slightly coarser grained are low in fe2 and mn2 and have the heaviest 18o signature this indicates they formed from evaporative oxidizing brines sourced from supratidal flats intertidal supratidal dolomites are fine grained commonly fe2 and or mn2 rich and slightly enriched in 18o compared to the marine calcite cement they formed from marine water that was weakly to moderately reducing dachstein paleosols have light 18o and 13c signatures reflecting meteoric soil waters repeated emergence stabilized the dolomites to low sr2 and na dolomite similar to cenozoic platform dolomites in contrast to these early cyclic dolomites coarse grained platform margin dolomites with very low mn2 and fe2 and light 18o signatures formed as thermally driven warm oxidizing marine water associated with jurassic rifting of the neo tethys pennini ocean circulated through the margin early dolomitization of cycles may have been controlled by high frequency sea level changes but the overall vertical distribution of early dolomite on the platform does not reflect long term eustasy rather the intense dolomitization of the lower platform reflects a semiarid hot subtropical seasonal setting and megamonsoonal climate global cooling and increased humidity toward the latest triassic and early jurassic inhibited pervasive early dolomitization leaving the upper platform largely as limestone

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

early dolomitization and recrystallization of the lower middle ordovician carbonates in western tarim basin nw china abstract the lower middle ordovician shallow marine carbonates in western tarim basin were dolomitized to different degrees the resultant dolomites are mainly concentrated in the successions of platform interior to shallow subtidal shoal and display an upward decrease in abundance based on petrographic textures three types of matrix dolomites were identified 1 very finely to finely crystalline nonplanar a to planar s dolomite md1 2 finely to medium crystalline planar e s dolomite md2 and 3 finely to coarsely crystalline nonplanar a dolomite md3 origins of matrix dolomites are interpreted on the basis of field investigation petrography and isotopic o c and sr geochemistry md1 dolomite was formed from slightly evaporated i e mesosaline to penesaline seawater in near surface to very shallow burial settings possibly facilitated by microbial mediation i e bacterial sulphate reduction bsr md2 dolomite was precipitated from connate seawater stored in coeval limestone in shallow burial realms in contrast md3 dolomite was likely formed as a consequence of intense recrystallization of early formed dolomites e g md1 at depths the formation of early matrix dolomites i e md1 was likely determined by depositional facies and sea level variations of variable orders this study provides a useful analogue to understand the dolomitization processes related to depositional facies and variable order sea level fluctuations in the tarim basin and elsewhere

repeated subaerial exposure of subtidal carbonate platforms triassic northern italy evidence for high frequency sea level oscillations on a 104 year scale triassic carbonate platforms in northern italy ladinian latemar carnian durrenstein norian dolomia principale consist of hundreds of stacked meter scale cycles each made of a subtidal unit with a cap rock that carries vadose cements pisoids and tepee structures these diagenetic caps are interpreted to be due to subaerial exposure of the subtidal sediments the absence of peritidal deposits rules out progradation as the cause of the exposure i e these are not ginsburg autocycles thus the repeated exposure must be the result of relative sea level oscillations which based on the number of cycles and time intervals involved occurred with an average frequency in the 104 year range we postulate a combination of small scale high frequency eustasy and local tectonic movements e g differential subsidence in the norian and infrequent pulses of uplift in the ladinian to explain the cyclicity and its internal variations

climate controlled fabric destructive reflux dolomitization and stabilization via marine and synorogenic mixed fluids an example from a large mesozoic calcite sea platform croatia the jurassic cretaceous dolomites adriatic platform croatia resulted from climate influenced post depositional reflux dolomitization as opposed to synsedimentary peritidal and deeper burial dolomitization and subsequent stabilization within a mesozoic calcite sea isolated platform the dolomites are stratiform 10 to 200 m thick fabric destructive 20 to over 500 m crystals nonluminescent and zoned with respect to ca bulk dolomites have low mn 10 to 30 ppm moderate sr 60 to over 200 ppm positive 13c and 18o values only moderate ordering 0 25 to 0 6 and single phase fluid inclusions temperatures 3 vpdb and sr 100 ppm composes most jurassic and 40 of cretaceous dolomites making up turbid dolomite cores and initial clear dolomite rims a first generation of low ca dolomite lcd 1 50 to 53 mol ca 18o 1 to 3 vpdb 100 180 ppm sr forms cement and variably replaces stabilizes earlier hcd cores hcd and lcd 1 formed in refluxing marine dominated pore waters under semi arid climate 55 mol phases a second generation of much younger low ca dolomite lcd 2 fracture associated more negative 18o from 1 4 to 1 vpdb sr 100 ppm enriched in radiogenic 87sr overgrows and variably replaces earlier cretaceous dolomites it formed during eocene oligocene deformation and uplift to 1 km burial depths 50 c from synorogenic mixed marine meteoric pore fluids circulating via faults fractures and local permeable zones porosity loss by dolomite cementation coupled with localized rapid cenozoic uplift variably arrested stabilization to lcd

upper cretaceous ramp limestones from the sorrento peninsula southern apennines italy micro and macrofossil associations and their significance in the depositional sequences abstract the upper cretaceous limestones of the sorrento peninsula are primarily characterized by wackestone packstone with benthic foraminifers thaumatoporellaceans and aeolisaccus sp and by rudist floatstones rudists in growth position are rare most of them appear toppled and locally oriented but not reworked in some cases they form small bouquets no more than 30 cm thick the successions analyzed in the sorrento peninsula encompass a variety of facies representative of inner shelf environments in a ramp like depositional setting on the basis of lithologic and sedimentologic characteristics and on a qualitative analysis of the biofacies three major facies associations have been recognized corresponding to a intertidal silty sand flats and shallow lagoons b subtidal mobile foraminiferal sand sheets c subtidal rudist dwelt sand plain in the lower part of the successions periodic more or less prolonged emersions subaqueous with fresh brackish waters or subaerial exposure are documented the upper part of each succession lacks any emersion evidence submarine exposure surfaces are testified by firmground related features the periodic influence of high energy regimes some storm related events is documented by wave and cross laminations hcs and the lack of fine sediments lithofacies are arranged in depositional cycles that may correspond to individual beds the boundaries of the depositional cycles show evidence of subaerial or submarine exposures the nature and position of these discontinuity surfaces in the successions provide an important clue to interpretation in terms of both depositional environment and vertical evolution the increase in thickness of the depositional cycles and the gradual change from peritidal shallow subtidal cycles to dominantly subtidal cycles document an increase in the amount of accommodation space which resulted from a long term rise in relative sea level a detailed study of benthic associations micro and macrofauna has been carried out in order to identify accurately further signs of periodicity contained within the successions as a whole these upper cretaceous limestones are abundantly fossiliferous with only a moderately diverse fauna there are no significant differences in terms of species the assemblages simply reflecting differences in abundance of the same species taphonomic studies of rudist rich beds permit seven major shell bed types to be distinguished six main foraminiferal assemblages have been detected and the distribution and relative abundance of the species have been examined within the environments inferred through facies analysis the taphonomic data and the foraminiferal assemblage abundance and diversity have been compared with lithologic and sedimentologic data in order to elucidate their distribution within depositional cycles and throughout the successions the successions show little up section changes in lithology and in taxonomic composition of the foraminiferal assemblages it is therefore difficult to establish a paleoenvironmental trend independent of the rudist shell concentrations which do change up section we noticed an overall increase in thickness and abundance of the shell beds and a shift in types of shell concentration and in taxonomic composition these characteristics associated with the upward disappearance of emersion surfaces the higher frequency of storm and or wave intercalations the increase in thickness of the depositional sequences and the gradual change from peritidal shallow subtidal cycles to dominantly subtidal cycles document a general deepening upward trend more open water conditions with a depth between fairweather and storm wave base became established as a result of relative sea level rise and to a consequent increase in the available accommodation space

controls on reflux dolomitisation of epeiric scale ramps insights from reactive transport simulations of the mississippian madison formation montana and wyoming abstract prediction of the geometry and petrophysical properties of dolomite geobodies depends on understanding both the hydrological system supplying reactive fluids and the chemistry of these fluids however patterns are complicated by the non linear response of the diagenetic system to depositional texture which controls both fluid flux via permeability architecture and reaction rate via effective surface area this study explores interactions between extrinsic controls spatial distribution of brine composition and temperature and intrinsic controls permeability and reactivity using local and regional scale reactive transport models of sequential episodes of brine reflux that resulted in partial dolomitisation of the mississippian madison ramp inter well scale models show preferential early dolomitisation of fine grained more reactive beds pervasive dolomitisation can occur most readily beneath the brine pool where flow is perpendicular to bedding and is most rapid at high brine fluxes down dip of the brine pool bedding parallel flow is focused in relatively permeable coarse grained beds providing reactants for strongly preferential alteration of intervening more reactive fine grained beds in contrast thicker sequences of fine grained beds dolomitise more slowly limited by the rate of supply of magnesium regional scale models with injection of brines of increasing salinity towards the ramp interior reproduce the observed pattern of dolomitisation however more realistic simulations in which reflux is driven by lateral density contrasts generate flow rates orders of magnitude too low for significant dolomitisation simulations suggest pervasive dolomitisation of epeiric scale ramps by a platform wide reflux circulation as often invoked is not feasible rather dolomitisation of such extensive systems critically requires local scale flow systems such as may result from topographically controlled variations in restriction of platform top seawater circulation

quantifying uncertainties in multi scale studies of fractured reservoir analogues implemented statistical analysis of scan line data from carbonate rocks in this study we performed a fracture analysis on a cretaceous bedded carbonate succession well exposed in the sorrento peninsula the studied succession includes stratigraphic units that are very similar to the productive units of the buried apulian platform reservoir rocks in southern italy we analyzed eight carbonate beds including both limestones and dolomites the basic technique used in this study consisted of measuring fractures along bedding parallel scan lines for one limestone bed a microscale scan line about 15 cm long was also analyzed using a digital microcamera provided the cumulative distribution of fracture apertures is well described by a power law our analysis shows how the uncertainty in the estimate of fracture aperture cumulative frequencies grows for large aperture values this feature results in a large uncertainty in the estimate of the slope of the least squares line in a bi logarithmic diagram approximating the data distribution which is the exponent of the power law as the latter represents a fundamental parameter characterizing a fracture set and fracture distribution over different scales reducing the uncertainty in the estimate of the slope of the curve represents an important objective of quantitative fracture analysis this is obtained in this study by the application of multi scale analysis and by integrating micro scan line data with classic outcrop based scan line analysis the quantification of uncertainties in the cumulative distribution estimates of fracture apertures is performed by analyzing in detail the spacing distribution and consequently fracture density distribution for each aperture value our results suggests that a meaningful statistical analysis of fracture attributes such as aperture or opening displacement may be effectively carried out by using properly determined confidence intervals and by the integration of outcrop based and micro scan line data sets

strontium isotope stratigraphy in the upper cenomanian shallow water carbonates of the southern apennines short term perturbations of marine 87sr 86sr during the oceanic anoxic event 2 abstract strontium isotope stratigraphy of shallow water carbonates from the southern apennines italy indicates that the late cenomanian early turonian evolution of marine 87sr 86sr is not accurately depicted by the marine reference curve using the low mg biotic calcite of well preserved rudist shells as a study material and carbon isotope stratigraphy for correlation to the standard ammonite biozonal scheme of northern europe new 87sr 86sr values are proposed for four different stratigraphic levels the middle and uppermost part of the guerangeri zone the lower and upper part of the geslinianum zone the southern apennines data suggest that a sharp positive shift at the onset of oceanic anoxic event 2 precedes the well known sr isotope negative shift the positive shift is interpreted in terms of enhanced rates of chemical weathering driven by global warming forced by volcanogenic co2 outgassing a stratified ocean is invoked to reconcile the high gradient and the short lag time of the perturbation with estimates of present day total mass and residence time of strontium in the ocean the sharp switch to the negative shift is related to the collapse of water column density gradient driven by thermal instability during a late cenomanian cool event more active ocean circulation suddenly delivered to surface waters the signal of increased submarine volcanism that had accumulated in deep waters

early dolomitization in the lower cretaceous shallow water carbonates of southern apennines italy clues about palaeoclimatic fluctuations in western tethys abstract a multidisciplinary study of the dolomitized bodies present in the lower cretaceous platform carbonates of mt faito southern apennines italy was carried out in order to explore the connection between early dolomite formation and fluctuating climate conditions the berriasian aptian investigated succession is 466 m thick and mainly consists of shallow water lagoonal limestones with frequent dolomite caps the dolomitization intensity varies along the succession and reaches its peak in the upper hauterivian lower barremian interval where it is present a completely dolomitized interval about 100 m thick field relations petrography mineralogy and geochemistry of the analyzed dolomite bodies allowed identifying two populations of early dolomites a fine medium crystalline fmdol and a coarse crystalline dolomite cdol both interpreted as the product of mesohaline water reflux according to our interpretation fmdol precipitated from concentrated brines in the very early stage of the reflux process producing typical sedimentary features as dolomite caps in the successive step of the process the basin ward latent reflux precipitated cdol from less concentrated brines a peculiar feature of the studied succession is the great consistency between stratigraphic distribution of dolomite bodies and their geochemical signature the completely dolomitized hauterivian barremian interval in fact is characterized by geochemical values suggesting an origin from distinctly saltier brines considering that the observed near surface dolomitization process is controlled by physical and chemical parameters reflecting the paleoenvironmental and paleoclimatic conditions during dolomite formation we propose that the stratigraphically controlled dolomitization intensity reflects periodic fluctuations in the salinity of dolomitizing fluid in turn controlled by long term climate oscillations the present work highlights that the stratigraphic distribution of early diagenetic dolomite may be used as proxy to define the climatic fluctuations that have influenced the sedimentary dynamics in the early cretaceous moreover considering that a comparable early dolomite distribution is present also in the dinaric platform we suggest that a regional scale climate control acted on early dolomite formation and distribution refining the knowledge of such a key control may have a significative impact on hydrocarbon reservoir characterization and exploration in the periadriatic area

a review of mesozoic climates there is overwhelming evidence based on the distribution of distinctive sediments and fossils and oxygen isotope data that the climate of the mesozoic world was appreciably more equable than that of today with no polar ice caps but precise quantitative data are not available except for an episode of late cretaceous cooling there is no good documentation of any significant change in global temperature distributions through the era the distribution of coals and evaporites together with other criteria indicates a pattern of humid arid and zones appreciably different in important respects from that of today during the triassic and jurassic western pangaea in low to middle latitudes was largely arid but in the early cretaceous the lands on the margin of the newly opening central atlantic and western tethys experienced a humid climate by late cretaceous times arid zones had become very restricted in extent because of insufficient suitable data attempts at climatic modelling have had only modest success and only to a limited extent can the major long term changes in climate between the permian and the present be explained in terms of changing geography the most probable explanation of mesozoic equability is an increased atmospheric co 2 content a number of enigmas remain such as the existence of flourishing forests in polar palaeolatitudes whereas for the late cenozoic short term climatic changes can be related successfully to variations in the geometry and mechanics of the earth sun system there is a long way to go before comparable success can be claimed for the mesozoic

stacking pattern of cyclic carbonate platform strata lower cretaceous of southern apennines italy detailed sedimentological analysis of aptian albian shallow water carbonate platform strata from monte faito southern italy reveal composite sea level change the organization of the lithofacies and their related diagenetic overprint have allowed the identification of 50 elementary cycles they often show exposure related diagenetic features superimposed on subtidal deposits suggesting drops of relative sea level elementary cycles are grouped into 16 bundles forming six superbundles the hierarchical organization of the cycles are interpreted to be related to composite eustatic sea level fluctuations driven by the earth s orbital perturbation in the milankovitch frequency band moreover the systematic variation of superbundle thickness and the general evolution of the lithofacies reflects variations of accommodation potential defining longer term environmental changes different lithofacies forming elementary cycles are frequently separated by bedding planes that when viewed in a sequence stratigraphic context are interpreted as transgressive surfaces maximum flooding surfaces or sequence boundaries this allows the recognition of lowstand transgressive and highstand deposits in the cyclic units considering both the bundle superbundle ratio and the sequence stratigraphic interpretation of the superbundles a chronostratigraphic diagram illustrating both gaps at bundle level and their expected location along the section was created it establishes that the section developed over about 2 4 ma

early diagenetic dolomitization and dedolomitization of late jurassic and earliest cretaceous platform carbonates a case study from the jura mountains nw switzerland e france early diagenetic dolomitization is a common feature in cyclic shallow water carbonates throughout the geologic record after their generation dolomites may be subject to dedolomitization re calcification of dolomites e g by contact with meteoric water during emersion these patterns of dolomitization and subsequent dedolomitization frequently play a key role in unravelling the development and history of a carbonate platform on the basis of excellent outcrops detailed logging and sampling and integrating sedimentological work high resolution sequence stratigraphic interpretations and isotope analyses o c conceptual models on early diagenetic dolomitization and dedolomitization and their underlying mechanisms were developed for the upper jurassic lower cretaceous jura platform in north western switzerland and eastern france three different types of early diagenetic dolomites and two types of dedolomites were observed each is defined by a distinct petrographic isotopic signature and a distinct spatial distribution pattern different types of dolomites are interpreted to have been formed by different mechanisms such as shallow seepage reflux evaporation on tidal flats and microbially mediated selective dolomitization of burrows depending on the type of dolomite sea water with normal marine to slightly enhanced salinities is proposed as dolomitizing fluid based on the data obtained the main volume of dolomite was precipitated by a reflux mechanism that was switched on and off by high frequency sea level changes it appears however that more than one dolomitization mechanism was active pene contemporaneously or several processes alternated in time during early diagenesis percolating meteoric waters obviously played an important role in the dedolomitization of carbonate rocks that underlie exposure surfaces cyclostratigraphic interpretation of the sedimentary succession allows for estimates on the timing of early diagenetic de dolomitization these results are an important step towards a better understanding of the link between high frequency probably orbitally forced sea level oscillations and early dolomitization under mesozoic greenhouse conditions

can humans force a return to a cretaceous climate abstract the modern pole to equator sea level temperature difference is about 50 c that of the mid cretaceous ranged from 30 c to as little as 24 c implying a much more equable climate this may have been caused by 1 reduction of the ice forced albedo of the polar regions 2 more efficient meridional energy transport by the atmosphere and ocean and 3 increased atmospheric greenhouse gas concentrations earth s icy polar regions stabilize its present inequable climate through the ice albedo feedback effect the polar ice results in permanent atmospheric highs that stabilize earth s wind systems in turn the stable winds drive the ocean currents and determine the location of the frontal systems that separate the low and high latitude oceanic gyre systems and bound the region where water sinks into the ocean interior as thermocline and intermediate water masses increased ocean heat transport can assist in making a more equable climate but unrealistic volume transports would be required to warm the polar regions to cretaceous levels the major factor forcing the equable climate of the cretaceous is now thought to be increased greenhouse gas concentrations dominated by co2 the modern rate of change in atmospheric concentration is greater than 200 ppmv per century and increasing this compares with 1 ppmv per century during the last deglaciation at current rates of fossil fuel burning atmospheric co2 levels will reach cretaceous levels of 2 times the pre industrial level about 2070 and 8 times the pre industrial level shortly after 2300 it is likely that cretaceous atmospheric co2 concentrations will last for many thousands to tens of thousands of years in addition to increased atmospheric greenhouse gas concentrations a return to climatic conditions resembling those of the cretaceous would require ice free poles and large changes in atmospheric and oceanic circulation arctic sea ice is melting much more rapidly than had been expected and the arctic ocean will soon be free of sea ice in summer the greenland ice sheet is melting more rapidly than expected because of greenhouse warming surface meltwater forms lakes and then flows down through crevasses and holes in the ice to lubricate the base allowing ice steams to flow much more rapidly the lifetime of the greenland ice sheet may be only a few hundred years the west antarctic ice sheet is inherently unstable being grounded on rock well below sea level the ice shelves blocking ice streams off west antarctica have begun to melt from beneath and break up as the southern ocean warms the east antarctic ice sheet has been regarded as highly stable but discovery of lakes beneath the ice and fast flowing ice streams raises questions about whether the ice sheet will ultimately succumb to global warming and disintegrate i conclude that a return to climatic conditions resembling those of the mid cretaceous is not only possible but also likely unless humanity can organize an effective campaign to stop co2 emissions to the atmosphere and remove some of the excess co2 already introduced

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

relative sea level changes recorded on an isolated carbonate platform tithonian to cenomanian succession southern croatia abstract superb sections of tithonian to cenomanian carbonates of the adriatic dinaric platform are exposed on the islands of southern croatia a succession approximately 1 800 m thick consists exclusively of shallow water marine carbonates limestone dolomitized limestone dolomite and intraformational breccia formed in a protected and tectonically stable part of the platform interior several phases of exposure and incipient drowning are recorded in the platform interior four are crucial for understanding the late jurassic to mid cretaceous evolution of the wider peri adriatic area 1 latest jurassic earliest cretaceous sea level fall 2 aptian drowning followed by 3 late aptian platform exposure and 4 late albian early cenomanian sea level fall deciphering these complex events from the vertical and lateral facies distribution has led to an evaluation of facies dynamics and construction of a relative sea level curve for the study area this curve shows that long term transgression during the early tithonian hauterivian early aptian and early albian resulted in generally thicker beds deposited in subtidal environments of lagoons or shoals regression was characterized by shallowing upward peritidal parasequences with well developed tidal flat laminites commonly capped by emersion breccia and or residual clay sheets early berriasian barremian late aptian late albian the southern part of the dinarides was tectonically quiet during the tithonian through aptian sea level oscillations appear to have been the primary control on facies stacking some correlation exists between local sea level fluctuations and the published global eustasy charts for the tithonian through aptian a significant departure is recognized at the albian cenomanian transition suggesting that it was influenced by tectonics associated with the disintegration of the adriatic dinaric platform

long and short term hydroclimatic variabilities in the aptian tethys clues from the orbital chronostratigraphy of evaporite rich beds in the apennine carbonate platform mt faito southern italy abstract the stratigraphic signature of the global late aptian cooling episode and sea level lowstand has been investigated in the mt faito section of the apennine carbonate platform apcp of southern italy several meter scale fine grained limestone intervals with abundant quartz pseudomorphs after anhydrite and minor calcite pseudomorphs after gypsum are present in the middle upper gargasian straddling the marly orbitolina level a regional key marker correlative with part of the fallot oceanic anoxic event the evaporite rich intervals were deposited in a mudflat saltern setting and together with frequent transgressive shallow marine interlayers alternate quite regularly with lacustrine paralic or very shallow marine deposits suggestive of overall wetter conditions three to five couplets of cyclic arid wet phases are thus identified their timing and duration determined by an original cyclostratigraphically constrained age model tuned with the astrochronology from deep water tethyan reference sections suggest fluctuating hydroclimatic conditions arid semiarid vs wet phases over time scales as brief as 100 and 400 ky they are considered to reflect astronomically induced latitudinal migrations of intertropical climate belts a secular climate shift from arid to humid conditions supra regionally correlatable across the upper gargasian of the tethyan realm is dated at 118 1 my it corresponds to the onset of persistent lacustrine deposits on the partially emerged apcp and of abundant river borne supply of siliciclastics along the opposing tethyan margin the observed climate fluctuations occurred during a major apcp subaerial exposure which lasted for 4 4 my precise supra regional correlations suggest that the platform exposure was glacioeustatic in origin and coeval with a long lived late aptian cooling episode which interrupted the mid cretaceous greenhouse biostratigraphic and astrochronological tuning from the apcp indicates that the glacioeustatic lowstand terminated in the earliest albian just above the o reicheli key marker

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

on the evolution of dolomite stoichiometry and cation order during high temperature synthesis experiments an alternative model for the geochemical evolution of natural dolomites abstract data collected from a series of high temperature dolomitization experiments in which dolomite replaces calcite in mg ca cl solutions indicate that dolomite composition mol mgco 3 and cation order evolve independently as a function of reaction progress despite a wide range of initial solution mg ca 0 43 1 50 the first product to form in all experiments is disordered dolomite both the rate of replacement and the composition of these dolomite products are strongly dependent on the initial mg ca in solution experimental solutions with lower mg ca yield less stoichiometric more ca rich dolomite and have reactions that progress at a slower rate conversely solutions with higher mg ca yield more stoichiometric dolomite and are characterized by faster overall reaction rates in all experiments dolomite composition remains constant during most of the reaction despite a solution chemistry that continually evolves to lower mg ca as magnesium is captured from solution by growing dolomite and calcium is liberated from calcite into solution despite a dolomite composition that remains relatively constant the degree of cation order increases with the percentage of dolomite shortly before complete consumption of calcite reactants when 95 of the calcite reactants have been replaced by dolomite products dolomite composition rapidly becomes more stoichiometric after all calcite reactants have been consumed dolomite is completely stoichiometric regardless of the mg ca in solution at the onset of experimental conditions however the degree of cation order in the stoichiometric dolomite products continues to increase with time in dolomitizing solutions the observation that dolomite composition remains constant throughout most of the dolomitization reaction is different than data from some previous studies this suggests that the mechanism by which dolomite replaces caco 3 is not clear and is more complex than previously recognized the data presented here suggest that the initial dolomite phase provides a compositional template for future dolomite growth thus new material being added to the crystal surface is forced to follow the established composition the data presented here support an alternative model for the commonly observed field relationship between the percent dolomite in a carbonate rock and the stoichiometry of that dolomite we propose that dolomite does not evolve toward more stoichiometric compositions simply because the percent dolomite in the rock increases i e indicating further reaction progress rather carbonate rocks with a higher dolomite to limestone ratio where the dolomite is more stoichiometric can be better explained as having been dolomitized in solutions with higher mg ca which cause both faster rates of dolomitization and more stoichiometric compositions this model explains field observations from evaporative reflux settings that show spatial variations in dolomite composition and crystal texture as the result of chemical evolution of interstitial pore fluids along a flow path the new model may also provide a new framework for investigating the chemical evolution of dolomite in reactive transport models which numerically track chemical reactions and constituents

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

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

digital classification of contact metamorphic rocks in extremadura spain using landsat thematic mapper data landsat thematic mapper tm images of an extensively cultivated part of extremadura spain have been used to distinguish soil developed on contact metamorphic rocks in aureoles around late hercynian granitic plutons from soil formed on stratigraphically equivalent late proterozoic slate and metagraywacke that has been regionally metamorphosed to the greenschist facies reflectance spectra of contact metamorphic soil have lower reflectance especially in the 1 6 mu m wavelength region and weaker al oh mg oh and fe super 3 absorption features than do spectra of the slate metagraywacke soil these spectral differences are attributed to highly absorbing carbonaceous material in the contact metamorphic soil that was subjected to high temperatures during emplacement of the plutons these spectral reflectance differences are evident in a density sliced tm band 5 image in color ratio composite images that incorporate tm 4 5 4 3 and 3 1 ratios and in principal component composite images digital classification of the numerous tilled vegetation free fields was used to map the contact metamorphic soil in an august 1984 tm scene of the caceres study area first tm 4 3 was used to identify these fields then ranges of tm 5 and tm 3 1 values were determined for selected tilled fields within and outside the contact aureoles field evaluation of a classification map based on tm 5 plus tm 3 1 shows more extensive aureoles than published geologic maps and few misclassified areas similar results were achieved using tm 4 5 instead of tm 5 this approach was subsequently used to map two linear zones of contact metamorphic rocks in the san nicolas mine area where only two small exposures of granite have been documented landsat multispectral scanner mss images can also be used to map contact aureoles in the study areas but extensive field evaluation is required because of more frequent misclassification

the evidence and implications of polar ice during the mesozoic abstract the mesozoic perhaps representing the longest period of warmth during phanerozoic earth history contains in general sparse and frequently equivocal evidence for polar ice although this time is undoubtedly punctuated by oscillations in climate whether sufficient to lead to polar cooling and the formation of polar ice an inviting mechanism to account for faunal and floral distribution patterns and large scale sea level change has been widely debated mesozoic evidence for glacial conditions includes abraded rock surfaces generally unsorted stone rich beds and the presence of dropstones rafted by ice within a finer grained host sediment faunal and floral evidence has also been utilised to determine the presence or absence of cold or sub freezing polar conditions together with more indirect evidence for glacial conditions derived from general circulation model gcm simulations of climate the analysis of clay mineral distributions glendonite abundance and palaeontological and sedimentological evidence for globally synchronous sea level change the extent of possible glacial environments during the mesozoic has been established by plotting published reports of glacial sediments on palaeogeographic reconstructions the general clustering of evidence at high palaeolatitudes suggests that the extent of polar ice during the mesozoic is likely to have been approximately one third the size of the present day based on such evidence a number of episodes of cold or sub freezing polar climates during the bajocian bathonian tithonian volgian valanginian and aptian are recognised evidence exists possibly also for a cold episode during the early jurassic pliensbachian although poorly constrained owing to limited biostratigraphical control the longevity of these events may be represented by a cold snap within a stage although undoubtedly the possible smearing of ages may have had the effect of lengthening and hence promoting the importance of these proposed events the climate regime of the earth during these times may be hypothesised to be characterised by a relatively steep pole to equator temperature gradient where low latitude regions are as warm or warmer than today the evidence to support such reasoning includes sharp increases in the areal distribution of both glendonites and deposits with affinities to glacial tillites and dropstones for these times coincidental falls of sea level arid events and an increased bipolarity of faunas at these times imparts further confidence that these events were of a magnitude to effect the earth as a whole

multiple dolomitization episodes in deep water limestones of the triassic lagonegro basin southern italy from early reflux to tectonically driven fluid flow abstract massive dolomitization does not commonly occur in deep water carbonates however this regional study on outcrops and subsurface samples of triassic lagonegro basinal carbonates from the southern apennines fold and thrust belt italy shows that the evolution of the lagonegro basin was punctuated by two events of massive dolomitization these processes resulted in two distinct dolomite types displaying different petrographic and geochemical features a medium crystalline and fabric preservative dolomite in the northern area molise campagna and san fele mount pierno contrasts with a medium to coarse crystalline and fabric destructive dolomite in the southern area from pignola to the high val d agri geochemical data increase in 18o with respect to typical late triassic seawater values high sr and low mn and fe content sr isotopes consistent with late triassic seawater indicate that the dolomite in the northern area is the result of an early replacement caused by a marine and possibly oxidizing fluid during shallow burial dolomitizing fluids likely were expelled through reflux from the norian apenninic carbonate platform towards the proximal lagonegro basin this reflux was promoted by the norian peculiar paleoceanographic and climatic conditions in the tethyan region and affected only the proximal basinal facies dolomites in the southern area display a decrease in 18o compared to late triassic seawater values and show lower sr higher mn and fe contents and higher sr isotope ratios compared to the early dolomites of the northern area fluid inclusion microthermometry reveals homogenization temperatures between 70 and 120 c and salinities of 1 9 to 6 4 wt nacl eq for saddle dolomites these geochemical signatures indicate that the southern area was affected by late diagenetic burial dolomitization interpreted to be the result of large scale tectonically driven fluid flow during the second compressional phase characterizing the southern apennines fold and thrust belt emplacement the dolomite distribution across the lagonegro basin northern and southern is possibly controlled by the paleogeographic distribution of the facies proximal vs distal as well as by their proximity to thrust faults the lagonegro case study demonstrates that basinal successions may undergo multiple dolomitization episodes in a manner similar to carbonate platform successions from early shallow to deep burial diagenesis one of the relevant implications is that in areas of restricted oceanographic circulation during periods of aridity reflux dolomitization may extend beyond the platform margin into deep water carbonates having the potential to alter their reservoir quality

tectono stratigraphic and kinematic evolution of the southern apennines calabria peloritani terrane system italy abstract temporal controls such as sedimentation ages in foredeep and wedge top basins combined with information about stratigraphic and metamorphic evolution ages and characterization of magmatic rocks deep structures burial and exhumation histories allowed us to obtain kinematic estimations of the southern apennines calabria peloritani terrane system evolution from the late oligocene to recent calculated thrust front velocities suggest to subdivide the orogenic evolution in main five kinematic stages characterized by different velocity trends nine kinematic complexes from a to i i e sets of tectonic units deformed in the same time range are determined according to foredeep ages these complexes bounded by main regional thrust faults encompass one or more tectonic sub units and successions of different paleoenvironment domains paleogeographic evolution model indicates according to available paleomagnetic data a counterclockwise rotation with a mean angle of 60 for the apennine platform carbonates whereas a mean angle of 20 for the eastern sector of the apennine platform and western side of apulian platform shortening estimations for the apennine successions show values ranging between 55 and 88 and a linear best fit indicating an increase from 60 nw sector to ca 90 se sector such high values are consistent with a deformation dominated by a thin skinned tectonics on the contrary the low value of 28 calculated for mt alpi suggests an exhumation ruled by high angle deep seated structures characterized by limited displacements tectonic vergences referred to middle late miocene late miocene and pliocene middle pleistocene when restored indicate an average eastward tectonic transport finally tectonic evolution is summarized in eleven schematic cross sections and corresponding paleogeographic maps

dolomitization of lower cretaceous peritidal carbonates by modified seawater constraints from clumped isotopic paleothermometry elemental chemistry and strontium isotopes abstract this study investigates one of the few examples of dolomitization of lower cretaceous shallow water limestones from the southern tethys carbonate platform from outcrops on the haushi huqf high in central east oman mud dominated peritidal carbonates rich in microbial mats are replaced by fine crystalline dolomite along at least 60 km in the lower 10 m of the jurf formation two meter thick beds in the overlying qishn formation are also dolomitized over lateral distances of one to two kilometers the stratabound geometry and petrographic relations of the dolomite with all other diagenetic phases indicate that the dolomite precipitated early the presence of only rare anhydrite relicts suggests that seawater was below gypsum saturation during most of the dolomitization supporting the hypothesis that cretaceous slightly evaporated water 18ofluid values range from 2 8 to 3 5 smow can affect pervasive dolomitization despite being composed of peritidal facies with features suggesting high salinity most of the carbonate succession was not dolomitized suggesting that the presence of microbial mats exerted a major control on the distribution of dolomite and that salinity is only a minor control on dolomitization clumped isotope results indicate that the early formed dolomite re equilibrated with fluids at 44 3 c in a shallow burial setting the elevated iron content on average 6011 ppm and 87sr 86sr on average 0 70782 with respect to cretaceous seawater suggests that the burial fluids interacted with permian clay and feldspars of the gharif formation that underlies the cretaceous carbonates the vertical gradients of radiogenic sr and the occurrence of small volumes of coarse crystalline dolomite in stylolites and fractures suggests that burial fluids were driven vertically and laterally by differential compaction probably during maximum burial in the late cretaceous

mapping thermal maturity in the chainman shale near eureka nevada with landsat thematic mapper images the purpose of this study was to determine if there is a correlation between measurements of organic matter om maturity and laboratory measurements of visible and near infrared spectral reflectance and if landsat thematic mapper tm images could be used to map maturity the maturity of mississippian chainman shale samples collected in east central nevada and west central utah was determined by using vitrinite reflectance and rock eval pyrolysis to establish the relations between maturity and spectral reflectance in the visible and near infrared vnir wavelength region vnir reflectance spectra of fresh and weathered whole rock sample surfaces were measured in the laboratory the spectra were convolved digitally with the landsat tm filter band passes to facilitate th analysis especially the relative sensitivities of individual band passes and ratios of band passes to spectral reflectance variations related to om maturity with increasing maturity overall vnir diffuse reflectance and mineral absorption feature intensities decrease and the shape of the spectra changes from concave downward to nearly flat the spectral shape differences between mature and supermature samples remains distinctive in reflectance spectra of weathered surfaces tm 4 tm 5 values correspond well to vitrinite reflectance and hydrogen index variations and therefore this ratio was used to evaluate a tm image of the eureka nevada area for mapping thermal maturity differences in the chainman shale first the contribution of vegetation to the tm response was minimized using a linear regression technique and then a tm 4 tm 5 density sliced image was produced field evaluation of the tm 4 tm 5 density sliced image shows that all the high values in the chainman shale which correspond to high maturity are located in the diamond mountains in contrast chainman shale in the northwestern pancake range exhibits low to moderate values these results are consistent with published local maturity determinations locally the presence of limonitic arenaceous exposures and colluvium causes anomalously low tm 4 tm 5 values but these areas can be identified in tm images because of their diagnostic vnir reflectance spectra

direct physical evidence of dolomite recrystallization the possibility of recrystallization is a long standing barrier to deciphering the genetic origin of dolomites there is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event results from 65 new high temperature dolomite synthesis experiments 1 m 1 0 mg ca ratio solutions at 218 c demonstrate dolomite recrystallization affecting stoichiometry cation ordering and nanometre scale surface texture the data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization which occur by various dissolution precipitation reactions during the first phase induction period no dolomite forms despite favourable conditions the second phase replacement period occurs when ca rich dolomite products with a low degree of cation ordering rapidly replace calcite reactants during the replacement period dolomite stoichiometry and the degree of cation ordering remain constant and all dolomite crystal surfaces are covered by nanometre scale growth mounds the third phase primary recrystallization period which occurs in the experiments between 97 and 100 dolomite is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering the end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat laterally extensive layers the fourth phase of the reaction secondary recrystallization period occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time inferences of recrystallization in natural dolomite based on cation order or stoichiometry of dolomite usually depend on assumptions about the precursor dolomite subjected to recrystallization if it is assumed that the experimental evidence presented here is applicable to natural low temperature dolomites then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

silicification of evaporites in permian guadalupian back reef carbonates of the delaware basin west texas and new mexico abstract outcrops of the seven rivers yates and tansill formations contain widespread evaporites replaced by quartz and calcite the original evaporites consisted of discrete horizons scattered nodules enterolithic layers and individual crystals or crystal fragments of gypsum and or anhydrite within a finely crystalline dolomite matrix based on field and petrographic observations silicification of evaporites proceeded from the exterior to the interior of the nodules the earliest replacement was by euhedral black megaquartz that contains abundant fluid inclusions water and hydrocarbons and solid inclusions mainly anhydrite and dolomite the siliceous replacements were followed by precipitation of equant blocky calcite spar which filled pores left by latestage dissolution of evaporites the fluid inclusions in the replacive megaquartz are primary and many contain both hydrocarbons and water daughter minerals of halite gypsum or possibly antarcticite cacl2 6h2o are also found within the aqueous inclusions homogenization temperature data for hydrocarbon and aqueous fluid inclusions average 67 7 c and 67 1 c respectively hydrocarbonbearing and aqueous inclusions are thus interpreted to have formed simultaneously from the same fluids eutectic melting and final melting temperatures for aqueous inclusions indicate that the fluids were concentrated brines consisting of cacl2 and nacl oxygen isotope values for the megaquartz replacements averaged 28 4 smow indica ing precipitation from evaporative waters with an isotopic composition of 2 9 smow evaporite silicification was coeval with or slightly postdated hydrocarbon migration the fluid inclusion data provide a record of the fluid temperatures and compositions that prevailed during silica precipitation these data coupled with regional stratigraphy and published geothermal gradients suggest a burial depth of approximately 1 3 km during silicification the source of the silica for evaporite replacement is problematic we postulate however that silica may have been derived from dissolution of siliciclastics in back reef units organic acids that form from breakdown of hydrocarbons increase the solubility of quartz by bonding with silicic acid as these products and associated brines moved into cooler oxidizing zones the organic acids broke down releasing silica into solution this organically mobilized silica eventually precipitated as megaquartz replacements of preexisting evaporites

large scale longitudinal extension in the southern apennines contractional belt italy during late cenozoic thrusting the interior of the peri tyrrhenian orogenic belt in the southern apennines underwent two episodes of nearly orthogonal extension early extension was oriented subparallel to the axis of the tectonic belt and formed in response to progressive thrust belt arcuation the length of the tectonic belt increased by 50 and the longitudinal strain was accommodated by low angle normal faults concentrated in tectonic domains recording up to 150 200 extension younger extension oriented at a high angle to the orogen was accompanied by pliocene pleistocene uplift and by southeasterly migration of tyrrhenian sea rifting

outcrop constrained hydrogeological simulations of brine reflux and early dolomitization of the permian san andres formation our hydrogeologic model tests the effectiveness of brine reflux as the mechanism for early dolomitization of the permian san andres formation brine circulation is constrained by sequence stratigraphic parameters and a heterogeneous distribution of petrophysical properties based on outcrop data the model simulates accumulation of the san andres platform and calculates fluid flow and solute transport in response to relative sea level fluctuations it tracks porosity loss caused by compaction and the concomitant permeability feedback the amount of dolomite potentially formed is calculated by means of a magnesium mass balance between brine and rock results show that 1 brine reflux is an effective mechanism to deliver magnesium to dolomitize large carbonate successions 2 relative sea level controlled transient boundary conditions result in intricate flow and salinity patterns that can generate irregular dolomite bodies with complex spatial distributions 3 pervasive dolomitization can result from several short lived reflux events by the amalgamation of brine plumes sourced in different locations and times and 4 the model successfully recreates the dolostone and limestone patterns observed in san andres outcrops

on the tectonic evolution of the ligurian accretionary complex in southern italy unraveling the deformation pattern characterizing the transition from final oceanic subduction stages to early stages of deformation of a foreland continental margin is crucial for a better understanding of the geodynamic processes taking place at convergent plate boundaries in particular the combined role of internal wedge dynamics and continental margin architecture in controlling the tectonic evolution of an accretionary complex during its final emplacement onto the foreland continent is discussed in this study to this purpose we conducted integrated structural stratigraphic and low temperature thermochronometric analyses on the ligurian accretionary complex units exposed in the campania region italy and on continental margin successions located in their footwall as well as on related foredeep and wedge top basin deposits our results point out a series of late early miocene burdigalian shortening events also involving buttressing of the accretionary wedge against the crustal ramp of the foreland continental margin emplacement of the overthickened accretionary complex onto the distal part of the continental margin was followed by horizontal extension and wedge thinning aiding the development of wedge top depocenters extension may have been either related to reduced subduction rates during the middle miocene or to a period of subduction erosion known to have occurred in the northern apennines in the same time frame early miocene nw se shortening recorded by ligurian accretionary complex units was completely unrelated to later late miocene to pleistocene ne directed thrusting in the apennines which was coeval with backarc extension in the tyrrhenian sea therefore our results emphasize the occurrence of a major discontinuity in the neogene geodynamic evolution of the southern apennines the tectonic history of which may be clearly subdivided from a kinematic point of view into pre and syn tyrrhenian backarc extension stages

onset of the mid cretaceous greenhouse in the barremian aptian igneous events and the biological sedimentary and geochemical responses basalts and biostratigraphy dated at 125 120 ma from the ontong java and manihiki plateaus in the western pacific evidence the largest volcanic event in earth history in at least the past 160 m y the intervening nova canton trough rifted at about 121 118 ma and a number of guyots and seamounts formed concurrently or slightly later geological events that probably were responses to these volcanic tectonic events occurred in the following chronostratigraphic order biotic fluctuations began at about 122 5 ma at about 122 0 ma 87sr 86sr began to decline slowly metal concentrations of co mn pb yb and cu in sediments peaked at about 121 5 121 2 ma changes in planktonic communities and sedimentation culminated in a nannoconid crisis just prior to 120 5 ma and in the selli black shale oae la at about 120 5 119 5 ma a sharp drop in 13c occurred at the beginning of the selli event and rebounded into a longer positive excursion that reached a peak after the selli event at about 119 5 118 5 ma at 120 5 ma 87sr 86sr declined rapidly and reached a minimum at about 116 113 ma we speculate that the intensity of these latter responses suggests a corresponding peak in volcanic tectonic activity at about 121 119 ma

eustatic control on early dolomitization of cyclic peritidal carbonates evidence from the early ordovician upper knox group appalachians the early ordovician upper knox group is characterized by stacked meter scale peritidal cycles that repeat at high frequencies 10 4 to 10 5 yr stacking patterns stratigraphic trends in lithofacies and cycle thickness of meter scale cycles define five depositional sequences that in conjunction with fischer plots delineate five long term relative sea level fluctuations during upper knox deposition intrabasinal and interbasinal correlation of upper knox fischer plots suggests that the third order sea level events were eustatic meter scale peritidal cycles likely formed in response to high frequency fourth and fifth order eustatic sea level fluctuations superimposed on these third order sea level events upper knox cyclic carbonates are extensively dolomitized as much as 85 of all dolomite is stratiform and consists of early dolomite exhibiting minor to extensive modification by burial dolomite synsedimentary dolomitization likely occurred in modified sea water during tidal flat progradation governed by high frequency sea level events this is suggested by the common association of dolomite with mud cracked laminites and silicified evaporite nodules the systematic decrease in dolomite abundance below laminite cycle caps and the presence of dolomite clasts in regoliths veneering high frequency cycle tops or in transgressive limestones of the overlying cycle dolomite distribution within depositional sequences shows a strong relationship to third and fourth order eustatic sea level events indicating that long term eustasy also strongly controlled early dolomitization of upper knox carbonates mass balance calculations show that the proposed sabkha model of dolomitization in concert with composite eustasy could generate stratiform dolomite of considerable vertical and lateral extent in peritidal cyclic carbonates this reflects the duration of progradation and supratidal exposure 10 4 to 10 5 yr available for dolomitization to proceed and the broad zone of active dolomitization that would develop during continued progradation throughout each cycle period

tethyan mediterranean organic carbon rich sediments from mesozoic black shales to sapropels the jurassic to holocene record of black shale deposition in the tethys mediterranean region is unrivalled by that of any other ocean basin either in land sections or drill cores the term black shale is used here broadly for sediments with elevated organic carbon concentrations 1 including the pliocene to recent sapropels most of the black shales are devoid of benthonic organisms are laminated and were deposited in distinct rhythms during periods when the deep waters of the ocean basins were anoxic or dysoxic the tethyan black shale records have become essential in studies of the transfer of organic carbon into the sediment record and for astronomical tuning and geological time scales these records have been central in understanding climate control on ocean dynamics and biogeochemical cycles the mesozoic black shales were deposited within well defined time envelopes of around 0 5 to 1 myr these black shales which were confined to certain chronostratigraphic intervals in the jurassic and cretaceous were recognized as expressions of global oceanic anoxic events in the mid 1970s and subsequently named after prominent researchers bonarelli selli goguel the black shale episodes were dated by biostratigraphic methods and by high resolution chemostratigraphy and cyclostratigraphy mesozoic black shales are now interpreted as the oceanographic expression of major perturbations of the global carbon cycle and climate research into younger pliocene to pleistocene sedimentary cycles including black shales termed sapropels exposed in land sections or found in pelagic and hemipelagic marine sediment cores of late quaternary age started in the 1950s main threads pursued from this end of the record were the climatic control of oceanic processes that permitted the development of highly detailed and precise time scales tuned to the astronomical clock of insolation changes palaeoclimate evolution of the circum mediterranean area and biogeochemical dynamics in anoxic basins after 50 years of intensive research the tethyan and mediterranean black shales remain subjects of fascination in earth science tracing the origins and building upon recent progress the current hypotheses on their formation are reviewed here it is a panorama of complex interplays between global and regional tectonics climate dynamics during both ice house and greenhouse states of global and regional climate oceanographic responses to these climate changes and biogeochemical adaptations that were all needed to shape an extraordinary archive of global change in the absence of human activity

the valanginian δ13c excursion may not be an expression of a global oceanic anoxic event abstract marine and terrestrial sediments of the valanginian age display a distinct positive 13 c excursion which has recently been interpreted as the expression of an oceanic anoxic episode oae of global importance here we evaluate the extent of anaerobic conditions in marine bottom waters and explore the mechanisms involved in changing carbon storage on a global scale during this time interval we asses redox sensitive trace element distributions rste uranium vanadium cobalt arsenic and molybdenum and the quality and quantity of preserved organic matter om in representative sections along a shelf basin transect in the western tethys in the polish basin and on shatsky rise om rich layers corresponding in time to the 13 c shift are generally rare in the tethyan sections and if present the layers are not thicker than several centimetres and total organic carbon toc contents do not surpass 1 68 wt palynological observations and geochemical properties of the preserved om suggest a mixed marine and terrestrial origin and deposition in an oxic environment in the polish basin om rich layers show evidence for an important continental component rste exhibit no major enrichments during the 13 c excursion in all studied tethyan sections rste enrichments are however observed in the pre 13 c excursion om rich barrande levels of the vocontian trough in addition all tethyan sections record higher mn contents during the 13 c shift indicating rather well oxygenated bottom waters in the western tethys and the presence of anoxic basins elsewhere such as the restricted basins of the north atlantic and weddell sea we propose that the valanginian 13 c shift is the consequence of a combination of increased om storage in marginal seas and on continents as a sink of 12 c enriched organic carbon coupled with the demise of shallow water carbonate platforms diminishing the storage capacity of 13 c enriched carbonate carbon as such the valanginian provides a more faithful natural analogue to present day environmental change than most other mesozoic oaes which are characterized by the development of ocean wide dysaerobic to anaerobic conditions

the origin of dolomites in tertiary sediments from the margin of great bahama bank abstract based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on great bahama bank we have determined three different environments of formation for the dolomites that are common throughout the pliocene and miocene parts of these cores the first environment of dolomitization occurs in association with development of nondepositional surfaces dolomite typically forms below each of these surfaces the concentration and extent of which is governed by the length of the period of nondeposition these dolomites are recognized by their association with the nondepositional surfaces characteristic positive 18o values indicative of formation from cold bottom waters and 18o and sr profiles with depth that suggest formation in the presence of diffusive temperature and sr gradients the second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes the dolomite forming here is termed background dolomite this is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space dolomitization by this mechanism uses a local source of mg2 and consequently the dolomite never constitutes more than between 5 and 10 of the sediment this type of dolomite is characterized by extremely high sr concentrations which reflect concentrations of sr2 in the pore fluids from which it formed the high concentrations of sr2 in the pore fluids arise through the continued recrystallization of metastable aragonite and high mg calcite to dolomite and low magnesium calcite driven by oxidation of organic material by sulfate not only does sulfate reduction provide an additional thermodynamic drive for recrystallization but because the absolute concentration of sr2 in the pore fluids is governed by the solubility of celestite srso4 removal of sulfate allows the sr2 ca2 ratio of the interstitial fluid to become much higher than normally encountered the final type of environment of dolomitization is associated with coarse grained reefal sediments the pervasive nature of the dolomitization and the relatively normal sr concentrations suggest the circulation of normal marine water in a relatively open system

the use of visible and near infrared reflectance spectra for estimating organic matter thermal maturity measurements of visible and near infrared spectral reflectance of 41 samples of mudstone siltstone and carbonate rocks representing two major depositional settings in nevada were compared to vitrinite reflectance r0 and hydrogen index hi measurements to determine the relation between spectral reflectance and organic matter om maturity the samples range in age from devonian to paleogene and have highly variable total organic carbon toc contents recycled om contents and kerogen compositions visible and near infrared spectral reflectance of the samples changes systematically as om maturity increases from submature to supermature r0 range of 0 28 to 4 32 therefore spectral reflectance generally can be used to estimate the thermal maturity of the contained om the sum of several ratios compound ratio used to express spectral changes in the visible and near infrared wavelength region is high for most submature samples decreases rapidly with increasing r0 into the mature range and then decreases less rapidly through the remaining mature range and the supermature range a similar trend is displayed in the plot of hi vs compound ratio some spectra are affected by iron absorption features and the presence of recycled om iron absorption generally causes the compound ratio values to be anomalously high relative to the om maturity whereas recycled om results in unusually low ratio values toc variations do not affect the ratio om maturity relation substantially iron absorption occurs in the short wavelength part of the visible and near infrared region and is readily identified in the laboratory spectra spectra affected by recycled om were identified by using a pair of ratios that expresses the difference in spectral shape between these spectra and spectra of supermature samples which they resemble samples containing recycled kerogen are much more numerous from the mississippian prodelta basin of the antler foreland basin than from the antler orogene this difference indicates derivation of much of he kerogen from the orogene

deep sea paleotemperature record of extreme warmth during the cretaceous oxygen isotope analyses of well preserved foraminifera from blake nose 30 n paleolatitude north atlantic and globally distributed deep sea sites provide a long term paleotemperature record for the late albian maastrichtian interval that is difficult to reconcile with the existence of significant cretaceous ice sheets given reasonable assumptions about the isotopic composition of cretaceous seawater our results suggest that middle bathyal water temperatures at blake nose increased from 12 c in the late albian through middle cenomanian to a maximum of 20 c during the latest cenomanian and earliest turonian bottom waters were again 12 c during the middle campanian and cooled to a minimum of 9 c during the maastrichtian correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from blake nose comparison of global bottom water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late albian through late cenomanian to a hot greenhouse phase during the latest cenomanian through early campanian then to cool greenhouse conditions during the mid campanian through maastrichtian

fate of reflux brines in carbonate platforms active reflux may occur during periods of platform top brine generation but the role and fate of these brines after reflux events are uncertain we have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform top salinity simulations suggest that reflux brines originally concentrated to gypsum saturation 150 have a relatively long platform residence time on the order of 100 times the duration of the reflux event when brine generating conditions cease brines will continue to sink through the platform entraining seawater a variant of reflux circulation we term latent reflux mesosaline brines intercepted by drilling of carbonate margins by the ocean drilling program may have originated from pleistocene reflux event s on the adjacent platform top and be currently moving by latent reflux latent reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates including mg for dolomitization

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

dynamic polar climates in a greenhouse world evidence from clumped isotope thermometry of early cretaceous belemnites sub arctic cretaceous berriasian late valanginian ca 145 134 ma marine temperatures obtained from fossil mollusks belemnites are determined using carbonate clumped isotope thermometry an approach based on the clumping of 13 c and 18 o in the carbonate mineral lattice into bonds with each other from our analyses we infer sub arctic early cretaceous marine temperatures ranging from 10 c to 20 c these possibly seasonally biased warm sub arctic temperatures are warmer than present mean summer water temperatures at 60 65 n and are therefore consistent with a warmer greenhouse world featuring a shallow equable latitudinal temperature gradient our combined temperature and 18 o belemnite data imply seawater 18 o values that have a remarkably modern character in that they are similar to modern high latitude seawater but more positive than modeled cretaceous seawater we identify a cooler late valanginian interval ca 134 ma with temperatures consistent with polar regions a few degrees above freezing and also coincident with increased 18 o seawater values thus we find evidence of intervals when polar ice was unlikely and also when polar ice was plausible both scenarios support the view of generally warm but dynamic polar climates during greenhouse intervals that were punctuated by periods of ice growth

a warm equable cretaceous the nature of the problem abstract the nature of the problem of warm equable paleoclimates is defined by investigating the mid cretaceous period the problem consists of three components 1 defining precisely the climatic state during any specified interval of geologic time 2 specifying the external climatic forcing factors which may have been important and 3 understanding the climatic response to any specific modifying influence these components are characterized by limitations of critical importance in understanding paleoclimates the nature of the problem of a warm cretaceous is defined both qualitatively and quantitatively within these limits

multistage dolomitization and distribution of dolomitized bodies in early jurassic carbonate platforms southern alps italy the early jurassic dolomitized carbonates are a hydrocarbon exploration target in northern italy of these carbonates the liassic albenza formation platform and the overlying sedrina formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub surface field work as well as analyses of well cores stable isotopes trace elements and fluid inclusions was carried out on the outcropping thrust belt and sub surface deformed foreland of the southern alps petrographic analyses showed a first pervasive replacement dolomitization phase d1 followed by volumetrically less important dolomite cement precipitation phases d2 d3 and d4 the 18o values fall between 8 2 and 0 1 vienna pee dee belemnite with the more depleted samples belonging to dolomite cement rich dolostones the 13c ranges from 2 6 to 3 7 vienna pee dee belemnite analysis of trace elements showed different fe and mn contents in the sub surface and outcropping dolostones and a higher fe in the younger dolomite cements an increase in the precipitation temperature up to 130 c from fluid inclusion data and a decrease in diagenetic fluid salinity from sea water to brackish are observed from the first pervasive replacement dolomite to the dolomite cement phases field observations indicate that in the albenza formation dolomitization was limited to palaeohighs or faulted platform margins in the early jurassic carbonates the pervasive replacement phase is interpreted based on a compaction model the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs the high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water these data along with the thermal history suggest that the dolomite cements precipitated according to the tectonic squeegee dolomitization model the dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation the dolomite precipitation temperatures 90 to 100 c were reached in the studied formations first in the thrust fold belt early tertiary 60 ma and then in the foreland succession during the late tertiary 10 ma this observation suggests that the dolomite precipitation fronts moved southwards over time recording a diagenetic wave linked to the migration of the orogenic system observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores the distribution of porous dolomitized bodies is therefore linked to the compaction dolomitization model

tex86 and stable δ18o paleothermometry of early cretaceous sediments implications for belemnite ecology and paleotemperature proxy application abstract recent studies have cast doubt on the unadjusted usage of jurassic and cretaceous 18 o paleotemperature data derived from belemnites since the latter data often reflect cooler paleotemperature estimates than would be expected in this study we address this problem by analysing rocks of barremian to early aptian age from two outcrops in northern germany using tex 86 paleothermometry along with 142 belemnite guards studied for their stable isotope 13 c 18 o and trace element composition magnesium strontium iron and manganese both tex 86 and 18 o bel indicate very warm water temperatures for a distinctive black shale sequence of late early barremian age hauptblatterton with temperatures of up to 29 c and 23 c respectively we observe a constant offset of tex 86 temperatures versus the 4 to 5 c cooler 18 o belemnite signal for this interval the late barremian sequence shows an increase of the 18 o bel values from 1 to 0 reflecting temperatures around 16 to 12 c while the contemporaneous tex 86 temperatures vary between 26 and 32 c the common occurrence of belemnites in the anoxic sediments of the early barremian implies however clearly a nektonic way of life similar to that of recent teuthids rather than a nektobenthic one like sepia this in turn suggests that the belemnites investigated here genera praeoxyteuthis aulacoteuthis oxyteuthis and neohibolites were active swimmers which inhabited a deeper habitat below the thermocline in an epicontinental sea of perhaps 100 to 250 m water depth the offset of the tex 86 and 18 o bel data is therefore interpreted to reflect temperature signals from two different depth habitats i e the tex 86 is selectively derived from warm sea surface waters and the belemnites likely occupied deeper and cooler waters with relative increasing salinities in the late barremian this study stresses the importance that the taxonomy paleobiology and ecology of the belemnite taxa genera species used for paleotemperature reconstructions must be considered before the 18 o bel signal can be further interpreted the variation of the 18 o signature gained from one belemnite population of 22 specimens by 1 1 suggests that the 18 o data of individual belemnites should be used with caution for reconstructing long termed paleotemperature trends

extremely high sea surface temperatures at low latitudes during the middle cretaceous as revealed by archaeal membrane lipids the middle cretaceous 125 88 ma greenhouse world was characterized by high atmospheric co 2 levels the general absence of polar ice caps and much higher global temperatures than at present both 1 8 o based and model based temperature reconstructions indicate extremely high sea surface temperatures ssts at high latitudes however there are a number of uncertainties with sst reconstructions based on 1 8 o isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the 1 8 o value of cretaceous seawater the paleoecology of middle cretaceous marine organisms and seawater ph here we applied a novel sst proxy i e text tetraether index of 86 carbon atoms based on the membrane lipids of marine crenarchaeota derived from middle cretaceous sedimentary rocks deposited at low latitudes the text proxy indicates that tropical ssts in the proto north atlantic were at 32 36 c during the early albian and late cenomanian early turonian this finding agrees with sst estimates based on 1 8 o paleothermometry of well preserved foraminifera as well as global circulation model calculations the text proxy indicates cooler ssts 27 32 c for the equatorial pacific during the early aptian which is in agreement with sst estimates based on 1 8 o paleothermometry

silicification of sulphate evaporites and their carbonate replacements in eocene marine sediments tunisia two diagenetic trends samples of chert nodules diagenetic carbonates and evaporites gypsum anhydrite collected from the gypsiferous limestones of the kef eddour member ypressian priabonian near metlaoui and sehib tunisia show selective silicification with great variety in the silicified by products based on d 13 c values which support an organic origin for the carbon carbonates replaced evaporites microbially through bacterial sulphate reduction observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites anhydrite gypsum in the absence of early diagenetic carbonate phases silica with d 18 o values from 25 to 28ae6 standard mean ocean water smow replaced the outer parts of anhydrite nodules at ph 9 in the innermost parts of the anhydrite nodules prevented silica precipitation the record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus the microbial diagenetic replacement of evaporites bacterial sulphate reduction by carbonates calcite aragonite and dolomite favoured silica replacement of carbonates rather than evaporites silica with d 18 o signature of 21 to 26 smow replaced carbonates on a volume for volume basis yielding a more siliceous groundmass and accounting for 90 95 of the nodules the relatively higher d 18 o values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed meteoric marine to marine water the lower d 18 o values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage the silica supply for chert formation could be derived from the reworked bio siliceous deposits diatomites to the west of the basin vestiges of an opal ct precursor undetectable by x ray diffraction xrd were revealed by d 29 si magic angle spinning nuclear magnetic resonance investigations diagenesis of the extraformational and overlying clay rich beds the host limestones are clay poor as shown by xrd measurements and minor volcanogenic and hydrothermal contributions during early diagenetic stages

palaeoenvironment and palaeoecology before and at the onset of oceanic anoxic event oae 1a reconstructions from central tethyan archives abstract in this study a new c isotope stratigraphy combined with a revised litho and biostratigraphy of barremian aptian platform carbonates from the central southern apennines italy is presented the santa maria abruzzi region and the monte faito campania region sections were investigated in order to trace platform evolution during a time of major c cycle perturbation before and during oceanic anoxic event oae 1a these successions are correlated at a regional scale with the monte raggeto reference section campania where the selli level equivalent sle and the magnetic polarity chron cm0r were recognised in earlier studies significant biotic and environmental changes are documented within the late barremian early aptian interval in particular the palorbitolina lenticularis lithocodium bacinella association followed by dark brown microbial sediments reflect considerable ecological stress upon the oligotrophic carbonate platform community mesotrophic or even eutrophic conditions which precedes the onset of sle by at least 1 2 myr moreover this integrated approach has resulted in an improved age control which offers the opportunity to correlate the apenninic sections with a succession from the french urgonian platform cluses section and with pelagic records from the italian umbria marche gorgo a cerbara and belluno cismon apticore basins pelagic biozonations and magnetostratigraphy have been projected into carbonate platform sequences a prevailing eustatic origin has been attributed to some of the lower frequency transgressive regressive cycles t rfts that reflect third order sea level oscillations the new correlations indicate that comparable biotic and environmental changes documented around the tethys are not always synchronous the type of response of biota to c cycle perturbations before and during oae1a depends upon palaeolatitude palaeogeography palaeobathymetry and regional climate patterns

the valanginian carbon isotope event a first episode of greenhouse climate conditions during the cretaceous lower cretaceous pelagic carbonates outcropping along the southern alps of northern italy provide a record of tethyan palaeoceanography as well as of low frequency fluctuations in the global carbon cycle the carbonate c isotope stratigraphy established at five selected localities in the southern alps allows an accurate picture to be drawn of the duration and amplitude of the valanginian c isotope event 13c values near 1 25 1 50 determined in berriasian and lower valanginian sediments are replaced by more pdsitive 13c values near 3 in the late valanginian the carbonate c isotope excursion ends in the early hauterivian with values fluctuating between 1 5 and 2 the carbonate c isotope excursion is accompanied by a positive excursion in the total organic carbon c isotope curve the valanginian c isotope excursion identified in tethyan sediments correlates with a c isotope excursion recorded in the western north atlantic in the gulf of mexico and in the central pacific dsdp sites 534 391 535 and 167 by analogy with the aptian stage also marked by a significant positive c isotope excursion the time of positive 13c values is regarded as a time of accelerated carbon cycling coupled with increased burial rates of organic carbon and detrital material in oceanic sediments a warm and humid climate possiblycoupled with a high atmospheric co2 content and a high global sea level may have triggered the acceleration of the global carbon cycling in this case the valanginian c isotope event would reflect a first episode of greenhouse earth conditions during the cretaceous

cyclostratigraphy of a carbonate platform as a tool for high precision correlation abstract mesozoic carbonate platforms of southern italy are thick 3000 m and well stratified bodies uplifted by the late tertiary quaternary deformation of the northern gondwanian continental margin they formed mostly in the shallow ramp open to restricted lagoon and peritidal supratidal settings and show a prevailingly cyclic organization consisting of m scale shallowing upward units we think that this cyclicity is demonstrably controlled by the earth s orbital perturbations individual cycle duration hierarchical organization numerical analysis of their sedimentological and petrophysical features the genetic mechanisms of the cyclicity have a climatic dictator allocyclicity that is coupled with eustatic oscillations evidenced by features related to subaerial exposure ranging from microkarst to more mature karst and paleosoils exposure features are widespread at the top of the cycles even when these units are formed by only subtidal deposits suggesting eustatic control this paper presents a study of three sections of the aptian in the campania region of southern italy here a total of 250 m has been studied at a cm scale sections include serra sbregavitelli central matese mountains 160 m upper aptian lower albian monte faito sorrento peninsula 55 m upper aptian lower albian and monte tobenna picentini mountains 35 m upper aptian the latter two outcropping 100 km sse of the former the three sections have a hierarchical organization composed of elementary cycles in turn grouped in bundles and superbundles that have been related respectively to the earth s short and long orbital eccentricity oscillation while the elementary cycles mostly appear to correspond to the precession or to a combination of precession and obliquity periodicities in these successions the stratal pattern shows a dominant aggradational configuration the thickness of the cycles and their groups appears environmentally controlled the thicker cycles being those formed in more open marine settings and vice versa if analysed in terms of sequence stratigraphy we may recognize in the superbundles a transgressive and a highstand system tract that together with maximum flooding surfaces and sequence boundaries was used to develop chronostratigraphic diagrams and to identify cycle omissions at the bundle and superbundle level the number and amplitude of omissions in the successions appear proportional to the environmental conditions the more restricted the water circulation in their original depositional sites the more prone they were to cycle omissions the sections of monte tobenna and monte faito were punctuated by omissions one of them at the superbundle level while in serra sbregavitelli we recognize a smaller number of gaps not reaching the superbundle level lower frequency oscillations 800 2000 k y are also evident and suggest a long term eustatic oscillation of the same periodicity during the lower cretaceous based on the hierarchical organization of the cyclic successions analysed and taking into account the recognized omissions we have correlated the serra sbregavitelli section sss with the two distant sections of monte tobenna corresponding to the lower sss and of monte faito corresponding to the upper sss using appropriate biostratigraphic markers a correlation appears possible at the scale of bundles of cycles 100 k y and sometimes also at the elementary cycle level 50 k y

authigenic euhedral megaquartz crystals in a quaternary dolomite abstract autochthonous doubly terminated euhedral megaquartz crystals as well as overgrowths on detrital quartz grains are present throughout a gypsum and anhydrite bearing pleistocene sabkha dolomite accumulation from the arabian gulf the siliceous precipitates whose source was probably recycled biogenic silica formed within meters of the sediment surface these quaternary precipitates provide the first modern analog for euhedral megaquartz crystals a relatively common constituent in ancient evaporite bearing tidal flat carbonate strata in the quaternary dolomites studied megaquartz is present as individual crystals and clusters of crystals as well as overgrowths on detrital quartz grains individual crystals commonly attain lengths of 1 mm and many show shallow dolomite mold impressions on their crystal facies and or partly or completely engulf dolomite rhombohedra the common occurrence of dolomite molds within the quartz crystals indicates that precipitation of the quartz took place either concurrently with or more likely after dolomitization of the host carbonates additionally the oft cited association of authigenic megaquartz and evaporite minerals plus the existence of void spaces into which the quartz precipitated imply a causal relationship between dissolution of the evaporites and or dolomites and precipitation of the megaquartz thus these findings demonstrate that authigenic megaquartz crystals as well as overgrowths on detrital quartz can originate under near surface temperatures and pressures and very early in the diagenetic history of evaporite associated carbonate strata