structure of a frontal cyclone a diagnostic study of a mid latitude cyclone has been carried out using routinely available numerical weather prediction model products and imagery the cyclone was intense but not exceptionally so and the results are believed to have some generality the detailed description of the cyclone structure just before it began to deepen rapidly integrates concepts from a number of researchers into a common framework a dominant feature of the cyclone close to its centre was the cloud head a region of cloud with a sharp convex outer edge which formed poleward of the main polar front cloud band the cloud head was caused by two flows that entered it from the east ascending and fanning out within it one flow the cold conveyor belt brought low wet bulb potential temperature w air back into the cloud head from low levels ahead of the warm front the other flow was due to high w air that peeled off from the base of the main warm sector airflow i e part of the warm conveyor belt and travelled in the boundary layer back towards the cyclone centre first undercutting dry air that had earlier descended from the upper troposphere called a dry intrusion and then ascending at the upper boundary of the cloud head above the cold conveyor belt the transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts with a more diffuse frontal region in between frontal fracture the two sharp surface cold fronts were associated with narrow cold frontal rainbands line convection one of the line convection segments forming the southern edge of the cloud head the overrunning of the dry intrusion in the region of the frontal fracture led to a structure known as a split front in which an upper level humidity front began to run ahead of the position of the surface cold front a feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air the cold conveyor belt in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper level jet

flow dissolution and precipitation in dolomite 1 the evolution of hydraulic conductivity and porosity during the process of dedolomitization was examined in a series of laboratory experiments by analyzing the effects of concurrent dolomite dissolution and calcium carbonate precipitation linear flow experiments were performed in columns of crushed sucrosic dolomite by injecting different concentrations of hcl at various flow rates temporal changes in head gradient were used to calculate overall hydraulic conductivities of each column while chemical analyses of the effluent acid enabled estimation of porosity changes during the experiments after each experiment the rock samples were retrieved and sectioned in order to study the pore space geometry micromorphology and mineral concentrations a range of injected hcl concentrations and flow rates was identified which leads to oscillations in the effective hydraulic conductivity and porosity of the evolving rock samples in all cases however the porous medium ultimately clogged short term experiments were also used to study the formation of dissolution and precipitation bands along the columns under the experimental conditions dolomite dissolution is a reaction rate controlled process experiments indicated that as such the flow rate and the ph of the injected fluid affect dissolution only during the initial stages when calcium carbonate is dissolved on the other hand both the flow rate and the ph of the injected fluid strongly influence the precipitation process throughout the duration of the experiments because higher flow rates retard nucleation these findings are in qualitative accordance with field observations of dolomite formations

the improve 1 storm of 1 2 february 2001 part i development of a forward tilted cold front and a warm occlusion abstract analysis of observations and the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 are used to study the development of a forward tilted cold front off the coast of washington state the vertical velocity associated with the cold front produced a wide cold frontal rainband in the early stage of development the midtropospheric baroclinic zone or upper cold front moved forward with time over the warm sector to produce a structure similar to a split front the movement of the upper cold front was due to horizontal transport and frontogenetical propagation the frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis which resulted in a negative positive couplet of frontogenesis straddling the baroclinic zone the lower tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion in the initial occluding process the converging frontal zones tilted i

the genesis of dedolomites a discusion based on reactive transport modeling abstract dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of ca rich water this process has been described in the literature as either increasing preserving or decreasing porosity of the initial dolostone according to textures observed in prades ne spain dedolomitization took place due to two simultaneous processes non pseudomorphic replacement of dolomite with calcite at the wall rock of fractures and pseudomorphic replacement in the bulk rock a multicomponent reactive transport model is used to analyze the two dedolomitization processes observed diffusion of ions from the fractures was the driving force for non pseudomorphic replacement in the wall rock the dissolution of dolomite and precipitation of calcite took place in accord with a 1 2 stoichiometry as a consequence of the mineral molar volume the porosity was sealed and replacement limited to a rim on both sides of the fractures according to calculations the development of a rim took less than 500 years advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock due to the absence of empty volume for expansion replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation according to calculations pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters as the infiltrating water was also undersaturated with respect to calcite one stage of calcite dissolution postdated dedolomitization this process was responsible for the widening of open fractures where the dissolution front followed the dedolomitization front into the dolomite matrix intensive calcite dissolution was also developed following the advective flux from the top of the bed downwards drusy calcite is observed to precipitate in the pore spaces which consist of intergrains fractures and dissolution cavities this process is not predicted in detail by reactive transport calculations although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization

influence of climate and dolomite composition on dedolomitization insights from a multi proxy study in the central oman mountains abstract dedolomitization is an important diagenetic process that can yield information on the history of paleo fluids in a given aquifer or outcrop and by extension it offers a window into past hydrologic conditions dedolomitization is also relevant economically as it can alter the porosity and permeability of carbonates thus affecting reservoir quality despite considerable research the process of dedolomitization is still not entirely understood here new findings from the central oman mountains highlight the importance of dolomite chemistry and fluctuating climate on dedolomitization petrographic mineralogical and geochemical data from both altered and pristine dolomite hosted in jurassic carbonate rocks from the sahtan group and outcropping at wadi mistal in the jebel akhdar tectonic window reveal two dolomite bodies with different characteristics stratabound and fault related dolostone the ferroan dolomite crystals are larger in the fault related dolostone whereas the non ferroan dolomite crystals are smaller in the stratabound dolostone petrography reveals a complex suite of alteration textures including pristine dolomite recrystallized dolomite and calcitized dolomite dedolomite iron oxides are present pervasively in the altered rocks and different alteration textures co occur in the same sample relative to unaltered dolomite in this outcrop the recrystallized dolomite is characterized by similar positive carbon isotope values but less negative oxygen isotope values and the calcitized dolomite is characterized by even less negative oxygen isotope values and more negative carbon isotope values based on field data and petrography two phases of alteration are evident an earlier dedolomitization event affecting the stratabound dolomite only and a more pervasive pleistocene holocene alteration phase associated with surface weathering that affected both the fault related dolomite and to a lesser extent the stratabound dolomite stable isotope results further suggest that the more recent subaerial weathering phase happened under two alternating climate regimes dedolomitization was triggered by meteoric fluids that interacted with soil related carbon during humid climatic period s whereas recrystallization of ferroan fault related dolomite happened during more arid climatic period s with less developed soils and lower fluid rock interaction these results suggest that weathering of ferroan fault related dolomite can lead to formation of goethite and recrystallized non stoichiometric dolomite with a reset stable oxygen isotope signature and sometimes to dedolomitization

origin of late cements in oolitic reservoir facies a fluid inclusion and isotopic study mid jurassic southern england abstract during bathonian times a carbonate ramp was constructed over southern england by the progradation of oolitic skeletal and oncoidal grainstones and packstones the grainstones provide a significant hydrocarbon reservoir facies although these carbonates suffered early diagenesis under marine and more significantly meteoric conditions later diagenetic cements such as saddle dolomite and more particularly mildly ferroan calcite are volumetrically more important in causing the occlusion of porosity on a regional scale we have analysed systematically these late cements employing a number of techniques standard petrography including cl and uvl c and o isotopes sr isotopes and fluid inclusion studies integration of the results of these approaches constrains both the pore fluid composition and the temperature regime at the time of cement precipitation saddle dolomite was emplaced before the mildly ferroan calcite at temperatures between 75 and 85 c and from highly saline brines 18 19 5 wt nacl equivalents mildly ferroan calcite precipitated over a generally lower temperature range and from less saline brines 6 5 15 wt nacl equivalents five types of fluid inclusion have been recognised 1 monophase aqueous liquid 2 monophase oil liquid 3 two phase aqueous liquid vapour 4 two phase oil liquid hydrocarbon vapour 5 three phase aqueous oil liquid hydrocarbon aqueous liquid vapour during burial warm 75 c brines passed through the limestones and precipitated saddle dolomite later slightly cooler oil rich brines precipitated mildly ferroan calcite wherever hydrocarbon accumulation had not already occurred our sr isotopic studies suggest that both these brines were derived from underlying and more deeply buried lias shales and that these brines were generated from late jurassic times onwards brine movements are likely to have been largely complete by the late cretaceous

a review of cold fronts with prefrontal troughs and wind shifts the conceptual model of a classical surface based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface many cold fronts however do not conform to this model time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front hereafter called a prefrontal trough and prefrontal wind shift respectively although many authors have recognized these prefrontal features previously a review of the responsible mechanisms has not been performed to date this paper presents such a review ten disparate mechanisms with different frontal structures have been identified from the previous literature these mechanisms include those external to the front i e those not directly associated with the cold front itself synoptic scale forcing interaction with lee troughs drylines interaction with fronts in the mid and upper troposphere and frontogenesis associated with inhomogeneities in the prefrontal air mechanisms internal to the front i e those directly associated with the structure and dynamics of the front include the following surface friction frontogenesis acting on alongfront temperature gradients moist processes descent of air ascent of air at the front and generation of prefrontal bores gravity waves given the gaps in our knowledge of the structure evolution and dynamics of surface cold fronts this paper closes with an admonition for improving the links between theory observations and modeling to advance understanding and develop better conceptual models of cold fronts with the goal of improving both scientific understanding and operational forecasting

the sting at the end of the tail damaging winds associated with extratropical cyclones strong surface winds often accompany the low level jets that occur along the cold fronts of extratropical cyclones but there is evidence that the strongest surface winds occur in a distinctly different part of a certain class of cyclone the most damaging extratropical cyclones go through an evolution that involves the formation of a bent back front and cloud head separated from the main polar front cloud band by a dry slot when the cyclone attains its minimum central pressure the trailing tip of the cloud head bounding the bent back front forms a hook which goes on to encircle a seclusion of warm air the most damaging winds occur near the tip of this hook the sting at the end of the tail observations of the great storm of october 1987 in south east england are re examined in some detail to study this phenomenon the cloud head is shown to have a banded structure consistent with the existence of multiple mesoscale slantwise circulations air within these circulations leaves the hooked tip of the cloud head and enters the dry slot much faster than the rate of travel of the cloud head tip implying rapid evaporation and diabatic cooling immediately upwind of the area of damaging surface winds the circumstantial evidence from the observational study leads one to hypothesize that the mesoscale circulations and the associated evaporative heat sinks may play an active role in strengthening the damaging winds regardless of how important this role may be the evolution of the cloud pattern seen in satellite imagery is a useful tool for nowcasting the occurrence and location of the worst winds copyright 2004 royal meteorological society

interaction of a warm season frontal system with the coastal mountains of the western united states part i prefrontal onshore push coastal ridging and alongshore southerlies abstract observational analyses and numerical simulations are used to investigate the interaction between a warm season frontal system trough and the complex terrain of the western united states prior to frontal landfall synoptically driven offshore flow was associated with the northward development of a thermal trough and a coastal pressure ridge and associated southerly winds moved northward along the coasts of northern california oregon washington and southwestern british columbia this coastal pressure ridge coupled with the subsequent weakening of the offshore flow resulted in the onshore push of cool marine air approximately 24 h before frontal passage the onshore surge of cool marine air into warm dry continental air associated with the thermal trough produced a mesofront that moved over the coastal and cascade mountains diagnosis of a realistic simulation of the event shows that both solar heating and adiabatic warming from downslope flow over the rockies and the cascade mountains are i

the effect of large scale flow on low level frontal structure and evolution in midlatitude cyclones abstract observational and modeling studies documented in the literature indicate that the large scale flow has an important effect on the structure and evolution of low level fronts in midlatitude cyclones the purpose of this paper is to address the role of the large scale flow on low level cyclone frontal structure and evolution through a combined observational and idealized modeling approach analyses of two observed cyclone cases embedded in large scale diffluence and confluence respectively are presented to illustrate two possible cyclone frontal structures and evolutions specifically the cyclone moving into a diffluent high amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front the cold front rotates into the warm front forming an occluded front in the manner of the norwegian cyclone model as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center in contrast the cyclone moving i

oceanography of the gulf of tehuantepec mexico indicated by radiolaria remains the morphology of the gulf of tehuantepec mexico is redefined through a bathymetric analysis and this redifinition is considered to explain the distribution patterns of non carbonate sediments particularly the radiolarian remains although the distribution patterns of pelagic sediments are influenced by the gulf of tehuantepec bathymetrical morphology and the terrigenous input these reflect clearly the regional oceanography northern strong winds called the tehuanos blowing across the sea surface of this gulf when the intertropical convergence is at its southernmost position define the main sedimentologic dynamics of this region this wind field gives rise to an oceanic front at the southwestern part of the gulf as well as upwelling processes at its central eastern part in the mixing at the front there is some contribution of california current waters while in the upwelling area there is a large contribution of the tropical subtropical water which is carried by the costa rica coastal current this oceanographic framework controlls the makeup of radiolarian assemblages consequently at the seabottom under the oceanic front the presence of the radiolarian species lithomelissa thoracites and cycladophora davisiana is observed while under the upwelling region acrosphaera murrayana is conspicuous

quasigeostrophic forcing of ascent in the occluded sector of cyclones and the trowal airstream a numerical model based analysis of the quasigeostrophic forcing for ascent in the occluded quadrant of three cyclones is presented based upon a natural coordinate partitioning of the q vector into its along and acrossisentrope components qs and qn respectively the qn component describes the geostrophic contribution to the rate of change of the magnitude of pu traditional frontogenesis whereas the qs component describes the geostrophic contribution to the rate of change of direction of pu rotational frontogenesis it is shown that convergence of qs simultaneously creates the isobaric thermal ridge characteristic of the thermal structure of occluded cyclones and provides the predominant dynamical support for ascent within the occluded quadrant the absence of significant qn convergence there suggests that quasigeostrophic q g frontogenesis plays a subordinate role both in forcing vertical motions and in affecting three dimensional structural changes in the occluded sector of post mature phase midlatitude cyclones a cyclonically ascending cloud and precipitation producing airstream that originates in the warm sector boundary layer and flows through the trowal portion of the occluded structure is supported by the upward vertical motions implied by the identified q g forcing this airstream is referred to as the trowal airstream and it is shown to be responsible for the production of the wrap around cloud and precipitation commonly associated with occluded systems the relationship of the trowal airstream to previously identified cloud and precipitation producing airflows in cyclones is discussed

upper permian carbonates at the northern edge of the zechstein basin utsira high norwegian north sea abstract the first detailed description of zechstein carbonates and their diagenesis along the northern margin of the northern zechstein basin is based on core material from four wells on utsira high in the norwegian sector of the north sea it provides a generic link to the better known southern zechstein basin and northwards to the late permian east greenland basin the zechstein succession consists of shale carbonate cycles and anhydrite only occurs in the uppermost preserved cycle believed to correlate to zechstein 4 the eleven carbonate facies reflect deposition in shallow shelf and arid shoreline environments and resemble zechstein 2 and 3 carbonate facies from the southern zechstein basin the carbonates are pervasively dolomitized by an early texture preserving dolomite characterized by 18o values between 6 and 2 4 similar to dolomites in the southern zechstein basin and 13c values between 0 7 and 5 which is significantly lower than time equivalent dolomites from the southern zechstein basin later subaerial exposure and fresh water infiltration during the late permian or early triassic caused replacement of the upper part of the dolomite by crystalline fabric destructive calcite dedolomite the dedolomitized zone is up to 23 m thick and calcite crystals are characterized by moderately negative 18o values in the range 8 to 5 5 characteristic for meteoric water the utsira high carbonates thus share a depositional history with the classical zechstein carbonates whereas their diagenesis seems to have more in common with the time equivalent succession in east greenland early leaching and dolomitization enhanced porosity whereas subsequent late permian early triassic subaerial exposure and widespread dedolomitization had profound negative impact on reservoir properties of the zechstein carbonates

high resolution observations of the trowal warm frontal region of two continental winter cyclones abstract this paper compares the structure of the trough of warm air aloft trowal warm frontal region of two continental wintertime cyclones the cyclones were observed over the central great lakes region during the lake induced convection experiment snowband dynamics project field campaign the cyclones had different origins with the first forming east of the colorado rockies and the second forming over the gulf of mexico they were associated with different upper level flow regimes one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet both storms produced heavy swaths of snow across the states of illinois wisconsin and michigan high resolution observations of frontal structure were made during flights of the national center for atmospheric research electra aircraft using dropsondes and the electra doppler radar tail radar system the high resolution observations suggest a different arrangement of air masses in the trowal region compared with the

the structure and evolution of a simulated midlatitude cyclone over land abstract using output from a mesoscale model simulation this paper describes the evolution of the three dimensional temperature and humidity structures of an intense cyclone that developed over the eastern half of the united states during 14 16 december 1987 some specific findings include the following the occlusion process at the surface appeared similar to the classical paradigm with the cold front catching up with the warm front aloft a warm type occlusion structure formed as an upper baroclinic zone merged with the surface based occluded front the frontogenesis associated with the cold and warm fronts was initially continuous but as the system evolved a break in the frontogenesis along the northern section of the cold front developed a single frontogenesis feature appeared to support both the warm and occluded fronts a quasi linear region of convective activity was observed in advance of the surface cold front this convective line was associated with an upper level humidity front wh

a lagrangian based analysis of extratropical cyclones ii a detailed case study a detailed case study analysis is undertaken of the occurrence of significant coherent ensembles of trajectories cets that accompany atlantic cyclogenesis the approach is based upon the lagrangian based analysis scheme introduced in part i the analysis reveals the existence of several distinct moist ascending and dry descending cets two types of moist ascending cets associated with strong condensational processes ascend from the boundary layer of the warm sector either in the close vicinity of the cyclone centre or in the trailing cold frontal region the latter is the main contributor to the precipitation along the cold front during the early phase of the development another type of moist cets distinguished by maximum water vapour flux travels at low levels through the warm sector and ascends to mid tropospheric levels in the warm frontal region where it produces strong precipitation around the time of maximum storm intensity cets with an upper level origin are linked to the classical dry intrusion different cets correspond to strong descent of upper level tropospheric air that penetrated to the lower troposphere the movement of stratospheric air toward the centre of the maturing cyclone and an event of stratosphere to troposphere exchange upstream of the upper level trough two different visualizations of time sequences of the moist ascending cets indicate that they represent well defined flexible tubes or airstreams that exist over a time period comparable with the duration of the main cyclone intensification they provide a translating flexible but coherent river bed for the quasi continuous ascent of moist boundary layer air to the upper troposphere these tubes are akin to the concept of warm conveyor belts and the present analysis method objectively diagnoses their geometrical and physical characteristics and potential vorticity pattern some consideration is also given to the relationship of cets and tropospheric rivers and to possible shortcomings of the approach

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

the linkage between the warm and the cold conveyor belts in an idealized extratropical cyclone abstractthis study continues the investigation of airstreams in idealized moist baroclinic waves and addresses the formation of the cold conveyor belt ccb its linkage to the warm conveyor belt wcb and their impact on the development of a midlatitude cyclone the ccb is identified as a coherent bundle of trajectories characterized by weak ascent and a strong increase of potential vorticity pv along the flow in contrast to the wcb defined as the trajectories with maximum ascent the authors illuminate the role of the two conveyor belts in the formation of two strong pv anomalies that form in the upper wcb negative pv anomaly and lower troposphere ccb positive pv anomaly respectively and thereby establish a link between these airstreams and relevant aspects of the dynamics of extratropical cyclones the ccb moves close to the surface along the colder side of the bent back front and experiences a pv increase as it passes below a region of maximum latent heat release at midtropospheric level

dedolomitization reactions driven by anthropogenic activity on loessy sediments sw hungary abstract in the szigetvar area sw hungary shallow groundwaters draining upper pleistocene loess and holocene sediments are considerably contaminated by domestic effluents and leachates of farmland fertilizers the loess contains calcite and dolomite but gypsum was not recognized in these sediments the anthropogenic inputs contain significant amounts of ca and so 4 the ca from these anthropogenic inputs is promoting calcite growth with concomitant consumption of carbonate alkalinity undersaturation of the system with respect to dolomite and dolomite dissolution in brief is driving dedolomitization reactions geochemical arguments supporting the occurrence of dedolomitization reactions in the area are provided by the results of mass balance and thermodynamic analyses the mass balances predicted the weather sequence dolomite calcite plagioclase k feldspar at odds with widely accepted sequences of weatherability where calcite is the first mineral in the weathering sequence the exchange between calcite and dolomite can be a side effect of dedolomitization reactions because they cause precipitation of calcite the thermodynamic prerequisites for dedolomitization reactions are satisfied by most local groundwaters 70 since they are supersaturated or in equilibrium with respect to calcite undersaturated or in equilibrium with respect to dolomite and undersaturated with respect to gypsum the ca vs so 4 and mg vs so 4 trends are also compatible with homologous trends resulting from dedolomitization reactions

organization and structure of clouds and precipitation on the mid atlantic coast of the united states part vii diagnosis of a nonconvective rainband associated with a cold front aloft abstract a numerical simulation using the pennsylvania state university national center for atmospheric research fifth generation mesoscale model mm5 was run on a rainband associated with a cold front aloft cfa in a warm occluded structure on the u s east coast the storm originally developed in the lee of the rocky mountains as a pacific cold front overtook a rocky mountain lee trough this formed a warm type occluded structure that was essentially maintained as the storm proceeded to the east coast the cfa was a thermal front and therefore dynamically active the prominence of the cfa in the equivalent potential temperature field was due primarily to the strong upward transport of water vapor from lower levels in the updraft associated with the cfa the baroclinic zone was characterized by a tipped forward lower region where the cfa coincided with a maximum in potential temperature and a tipped backward upper region where the cfa coincided with the leading warm side edge of a zone of enhance

pore fluid and diagenetic evolution of carbonate cements in lacustrine carbonate siliciclastic rocks a case from the lower cretaceous of the erennaoer sag erlian basin ne china lower cretaceous lacustrine carbonate siliciclastic rocks including siltstones and mudstones form a unique tight oil reservoir in the erennaoer sag in the erlian basin of northeastern china that is characterized by various carbonate cements and volcanic material based on integrated analysis of the petrology mineralogy and carbon and oxygen isotope geochemistry the origin compositions and evolution of carbonate cements and pore fluids are determined five types of carbonate cements are identified from oldest to youngest penecontemporaneous micritic crystalline ferroan dolomite 1 eogenetic micro crystalline ferroan dolomite 2 sparry micro crystalline ferroan calcite 1 mesogenetic micro crystalline non ferroan dolomite 3 and calcite 2 the evolution of the pore fluids also controlled the compositions and distribution of the carbonate cements of the lacustrine mudstone during different periods this may be advantageous to predict the distribution of high quality reservoirs the geochemistry of the paleo pore fluids in these mixed lithological reservoirs are more complicated thus the origin and distribution of pore fluids and their influences on carbonate cementation fluid rock interaction and reservoir quality are complicated and certainly worth studying for hydrocarbon exploration in addition the evolution of the pore fluids of the arshan formation have experienced different salinity fluid environments under alkaline to weak acid conditions

carbonate diagenesis and feldspar alteration in fracture related bleaching zones buntsandstein central germany possible link to co 2 influenced fluid mineral reactions fracture related bleaching of lower triassic buntsandstein red beds of central germany was related to significant carbonate diagenesis and feldspar alteration caused by co2 rich fluids using cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis and stable carbon isotope study two major fluid mineral interactions were detected 1 zoned joint filling calcites and zoned pore filling calcite cements the latter replacing an earlier dolomite were formed during bleaching during the calcite formation and dolomite calcite transformation iron was incorporated into the calcite cement crystal cores due to fe availability from the coeval bleaching the dedolomitisation was ultimately associated with a volume increase the related permeability decrease implies a certain degree of sealing and increasing retention of co2 and the volume increase offers a minor co2 sink carbonate rich sandstone therefore can provide advantages for underground co2 storage especially when situated in the fringes of the reservoir 2 alkali feldspar alteration due to the bleaching fluids is reflected in cathodoluminescence spectra predominantly by the modulation of a brown luminescence emission peak 620 nm this peak represents a newly discovered effect related to alkali feldspar alteration not solely associated with bleaching its modulation by the bleaching is interpreted to be due to na depletion or a lattice defect in the si o bonds of the sio4 tetrahedron alteration reflected by this luminescence feature has a destructive effect on the feldspars implying the possibility of diminished rock integrity due to bleaching and hence co2 rich fluids two further cl spectral changes related to bleaching occur a decreased intensity between around 570 nm assigned to mn depletion and b increased amplitude and wavelength shift of the red 680 nm band converging evidence from carbonate and feldspar diagenesis stable carbon isotope data and analysis of fracture directions suggests that co2 fluids contributed to a significant extent to the bleaching phenomena and alteration in the studied buntsandstein strata

dedolomitization and calcite cementation in the middle devonian winnipegosis formation in central saskatchewan canada limestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the winnipegosis formation on the east central margin of the elk point basin where the overlying prairie evaporite deposits have been removed this type of crystalline limestone is interpreted as dedolomite based on petrographic observations the 18o and 13c values of the winnipegosis dedolomite vary from 12 8 to 11 9 vpdb vienna pee dee belemnite and from 0 5 to 1 7 vpdb respectively both values are significantly lower than those for the corresponding dolomite the 87sr 86sr ratios of the dedolomite are significantly higher between 0 7082 and 0 7087 the spatial distribution and geochemical data of the winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the prairie evaporite anhydrite during the latest mississippian to the early cretaceous when the basin was subjected to uplift and erosion the winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite including the occurrence of dedolomite restricted along fractures and adjacent areas dolomite patches floating in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts however the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the winnipegosis dedolomite coarsely to very coarsely crystalline subhedral to euhedral calcite cement is restricted in the dedolomite the petrographic features isotopic compositions and homogenization temperatures coupled with the burial history of the winnipegosis formation constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during late cretaceous to neogene time the more negative c isotopic signatures of the calcite cement 5 3 to 2 3 vpdb probably reflect a hydrocarbon derived carbon

dedolomitization review and case study of uncommon mesogenetic formation conditions abstract dedolomitization converts a dolomite into a calcite and conventionally is interpreted to form in an eogenetic or telogenetic diagenetic environment where meteoric water dissolves evaporites to supply excess calcium hydrologic connectivity between a dolostone and the surface water may then lead to a dedolomitized interval often times being a diagnostic indicator for an erosional unconformity subaerial exposure and or karst in contrast to the classic models we present a case study for pervasive burial dedolomitization unrelated to meteoric fluids dedolomitization of the zechstein 2 carbonate ca2 gas reservoir in nw germany is strongly altering reservoir quality on a regional scale the ca2 shows a textbook correlation between reservoir quality and mineralogy petrographic analyses show that approximately 80 of all observed calcite reveal a dedolomite microtexture causing a reduction of average matrix porosity by 5 to 10 compared to the dolomite dedolomite follows initial layering and abundantly forms massive concretions which are surrounded by compaction related curvature of the depositional layering in the dolomite host rock such an early burial timing is supported by a slight burial related shift of 3 18o and 1 13c towards lighter values in the dedolomites compared to dolomite an exceptionally large amount of calcium rich fluids must have been mobilized to account for 50 of the ca2 rock volume to be dedolomitized however core fabrics related to meteoric diagenesis such as karst fabrics or typical fresh water stable isotope signatures have been observed neither in the ca2 nor in over and underlying anhydrite beds instead excess calcium likely comes from gypsum to anhydrite conversion and pressure solution within over and underlying anhydrites an influx of strontium rich waters from the anhydrite beds is indicated by up to 5 times higher strontium contents measured in dedolomite fabrics compared to their dolomite host rock homogenization temperatures between 51 and 56 c measured in some early fluid inclusions in dedolomite textures further exclude a pristine meteoric water input and suggest fluid entrapment in a burial depth range of 900 to 1400 m comparing these results with literature data furthermore indicates that mesogenetic dedolomite is either volumetrically underestimated in other carbonate evaporite settings or formed under unique diagenetic conditions across the southern permian basin during zechstein times

origin of disconformity dedolomite in the martin formation late devonian northern arizona abstract calcitized dolomite dedolomite occurs along a prominent disconformity in the upper martin dolomite of north central arizona petrography stable isotopes and field associations were used in an effort to constrain the origin and timing of the dedolomite 18 o 16 o and 13 c 12 c ratios of the dedolomite show significant depletions relative to unaltered dolomite above and below the disconformity this depletion trend and dedolomitization are interpreted to have resulted from a subaerial exposure and an influx of 18 o depleted meteoric water or b descending groundwater from paleoaquifers in the lower pennsylvanian sedimentary features consistent with a subaerial exposure include detrital filled desiccation cracks and an accumulation of iron rich residual clays features suggestive of dedolomitization by paleogroundwater include calcite replaced evaporite nodules and the great volume of water inferred to have been associated with extensive continental lower pennsylvanian karstification of overlying younger carbonate units

the structure and evolution of a continental winter cyclone part i frontal structure and the occlusion process the frontal structure and occlusion process in a cyclone of moderate intensity that affected the central united states in january 1995 is examined the deep warm frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that although characterized by cold air advection near the surface had many of the characteristics of a warm front aloft in fact this feature had a structure similar to the so called bent back fronts previously documented only in association with explosively deepening maritime cyclones the development of a warm occluded structure was investigated with the aid of a numerical simulation of the event performed using the university of wisconsin nonhydrostatic modeling system the development of the warm occluded structure was asynchronous in the vertical occurring first at midtropospheric levels and later near the surface in contrast to the classical occlusion process near the surface the warm occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical coldfrontal zone at midtropospheric levels the warm occluded structure formed as a result of the cold frontal zone approaching and subsequently ascending the warm frontal zone in accord with a component of the classical occlusion mechanism the observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper level potential vorticity pv anomaly that controls the cyclogenesis it is suggested that the occlusion process begins aloft where the associated vortex strength is greatest and gradually penetrates downward toward the surface during the cyclone life cycle additionally a characteristic treble clef shape to the upper level pv anomaly is shown to be a sufficient condition for asserting the presence of a warm occluded structure in the underlying troposphere

observations of the mesoscale sub structure in the cold air of a developing frontal cyclone observations from 58 dropwindsondes released in a mesoscale array during the fronts 92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern north atlantic a conceptual model involving the intertwining of dry intrusion and cloud head flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour in the cold air two distinct dry intrusions were responsible for two cold fronts trailing south westwards from the tip of the cloud head both were surface features at the beginning of our study but the leading one evolved into an upper front with mid level convection as the dry intrusion responsible for it overran the warm sector areas of both high and low potential vorticity were indicated within the dry intrusions upon encountering a critical level of zero system relative velocity at the top of the moist boundary layer the dry intrusions arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front normal direction with a slope of typically 1 in 60 although most parts of the laminae were subsaturated their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud lines within the boundary layer these formed as an extension of the south western tip of the cloud head associated with the developing cyclone possible mechanisms for generating the observed structures are discussed

the structure and evolution of a continental winter cyclone part ii frontal forcing of an extreme snow event abstract the production of a narrow heavy occasionally convective snowband that fell within a modest surface cyclone on 19 january 1995 is examined using gridded model output from a successful numerical simulation performed using the university of wisconsin nonhydrostatic modeling system it is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower tropospheric warm frontogenesis in the presence of across front effective static stability differences as measured in terms of the equivalent potential vorticity pve the sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification model trajectories demonstrate that a stream of warm moist air ascended through the trowal portion of the warm occluded structure that developed during the cyclone life cycle the lifting of air in the trowal was in this case forced by lower tropospheric frontogenesis occ

reexamining the cold conveyor belt abstract despite the popularity of the conveyor belt model for portraying the airflow through midlatitude cyclones questions arise as to the path of the cold conveyor belt the lower tropospheric airflow poleward of and underneath the warm front some studies beginning with carlson s analysis of the eastern u s cyclone of 5 december 1977 depict the cold conveyor belt moving westward reaching the northwest quadrant of the storm turning abruptly anticyclonically rising to jet level and departing the cyclone downstream hereafter the anticyclonic path other studies depict the cold conveyor belt reaching the northwest quadrant turning cyclonically around the low center and remaining in the lower troposphere the cyclonic path to clarify the path of the cold conveyor belt the present study reexamines carlson s analysis of the cold conveyor belt using an observational and mesoscale numerical modeling study of the 5 december 1977 cyclone this reexamination raises several previously unappreciated

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

upper pennsylvanian permian naco group paleosols north central arizona field and isotopic evidence deposits of the upper naco group upper pennsylvanian permian north central arizona include features consistent with multiple episodes of soil formation the preserved paleosols are developed on shallowing upward regressive carbonate cycles that accumulated on the margins of a shallow shelf platform field evidence of paleosols consist of a monopodial taproot like and branching root and rootlet trace fossils b cylindrical carbonate nodules rhizoliths c subspherical carbonate nodules that occur as discrete entities in the paleosol and d an accumulation of red clay and quartz silt petrographic evidence consists of a plasmic fabrics analogous to those found in some modern soils cf brewer 1976 b microscopic carbonate nodule accumulations c soil nodule spar crystallaria and d alveolar septal structures paleosol profiles occur as a lithified in situ partly eroded clay rich horizons that commonly grade into unmodified parent material at depth below the disconformity and b reworked pedogenic carbonate nodule horizons oxygen and carbon stable isotope ratios of pedogenic carbonate are depleted relative to local marine carbonate this suggests that isotopically light soil co2 and meteoric water were present during pedogenic carbonate precipitation the depleted stable isotope ratios are consistent with an interpretation of soil carbonate precipitated under near surface conditions the presence of well developed calcareous rhizoliths and soil nodules in a red bed matrix and desiccation like crystallaria in host rhizoliths may qualitatively signify seasonally dry paleoclimatic conditions alteration of the underlying limestone units and development of multiple albeit truncated paleosols record a cyclical shift between marine and terrestrial paleoenvironmental conditions near the pennsylvanian permian boundary 280 m y ago

mesoscale dynamics of the trowal and warm frontal regions of two continental winter cyclones abstract the dynamic and thermodynamic structure and associated frontal circulations within the trowal and warm frontal regions of two extratropical winter cyclones are examined using numerical simulations in each cyclone the warm moist airstream originating in the warm sector was found to bifurcate upon reaching the warm front one branch of the flow turned anticyclonically eastward corresponding to the warm conveyor belt while the second branch turned cyclonically westward becoming the trowal airstream the dynamic forcing of vertical motion within the two airstreams was investigated using the fifth generation pennsylvania state university national center for atmospheric research mesoscale model mm5 both from an analysis of the sawyer eliassen se equation in two dimensions and from complete model solutions shearing deformation associated with the jet stream and the low level cyclonic flow dominated confluent deformation near the trowal in both cases the shearing deformation was accompanied

a cyclone phase space derived from thermal wind and thermal asymmetry abstract an objectively defined three dimensional cyclone phase space is proposed and explored cyclone phase is described using the parameters of storm motion relative thickness asymmetry symmetric nonfrontal versus asymmetric frontal and vertical derivative of horizontal height gradient cold versus warm core structure via the thermal wind relationship a cyclone s life cycle can be analyzed within this phase space providing substantial insight into the cyclone structural evolution an objective classification of cyclone phase is possible unifying the basic structural description of tropical extratropical and hybrid cyclones into a continuum stereotypical symmetric warm core tropical cyclone and asymmetric cold core extratropical cyclone life cycles are illustrated using 1 navy operational global atmospheric prediction system nogaps operational analyses and 2 5 ncep ncar reanalyses the transitions between cyclone phases are clearly illustrated within the phase space including extratro

the formation of a forward tilting cold front with multiple cloud bands during superstorm 1993 abstract a mesoscale model simulation is used to examine the evolution of the cold front and accompanying cloud bands in eastern mexico associated with superstorm 1993 12 14 march the simulated cold front differed in structure and evolution from a classical cold front in agreement with evidence from observations and european centre for medium range weather forecasts analyses the surface cold front as defined by the leading edge of strong northerlies and cold advection initially possessed a rearward tilt with height over southern texas within 6 h the leading edge of the front moved equatorward and developed a large scale forward tilt of greater than 200 km in the horizontal from the surface to 700 hpa this forward tilt occurred as a mid to upper tropospheric baroclinic zone arrived from over the sierra madre descended into eastern mexico and interacted with the surface cold front embedded within this large scale forward tilt was a locally enhanced horizontal potential temperature gradient tha

hollow carbonate pebbles a case study of selective secondary porosity generation neogene israel abstract neogene conglomerates are very common in israel in a few conglomerate localities in the hilly region of galilee northern israel a large number of the pebbles have internal cavities this puzzling phenomenon has not been documented previously in israel or elsewhere the studied conglomerates occur as lithified calcite cemented lenticular beds which alternate with reddish brown argillaceous mudstones the hollow pebbles constitute in fact only a special case of a wider range of related features the most relevant observations related to these occurrences are the following 1 the clasts are polymictic consisting of various limestones dolomites and minor cherts the hollow pebbles and related features however occur only in porous dolomites 2 calcitized dedolomitic hard rims characterize all the affected pebbles the thickness of these rims varies between a few mm and 1 2 cm 3 in part of the affected pebbles the dolomitic core inside the calcitic rims is preserved intact in others only the hard rim is preserved and the core is empty these are the hollow pebbles sensu stricto 4 late stage calcite spar occurs in some of the voids and in veins that cross the groundmass of the conglomerates the first step in the sequence of events leading to the production of the described occurrences is inferred to be the calcitization of porous dolomite pebbles resulting in the formation of dense rims around porous cores the next step involved the dissolution of the inner core of the pebble by acidic solutions leaving the calcitic rims almost intact the restriction of hollow pebbles and related features occurrences to conglomerate strata which are interbedded with argillaceous mudstones points to a causal relationship between the two accordingly the precipitation of calcite during the first step is attributed to the establishment of anoxic microenvironments within the porewater soon after the accumulation of the argillaceous material the reversal of conditions from carbonate precipitation to dissolution during the second step seems to be related to a change from reducing to oxidizing microenvironments following the emergence of the fluviatile sediments from the aquatic environments the critical requirements for obtaining the hollow pebbles seem therefore to be the presence of a suitable lithological type of pebble namely porous dolomite combined with a special sequence of depositional and diagenetic environments the apparent rarity of this combination seems to account for the lack of documentation on this phenomenon to this date

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c