late cenomanian to middle turonian high resolution carbon isotope stratigraphy new data from the munsterland cretaceous basin germany abstract new high resolution carbon isotope stratigraphies from two basinal pelagic carbonate successions in northern germany halle and oerlinghausen munsterland cretaceous basin resolve late cenomanian to early mid turonian carbon cycle variations at timescales of less than 100 kyr beside the major carbon isotope excursion of the late cenomanian oceanic anoxic event oae 2 11 small scale distinct features are precisely resolved in the 13c carbonate curve and related to boreal macrofossil zonations the small scale carbon isotope events correspond to secular 13c carbonate variations identified previously in the english chalk the boreal high resolution 13c carbonate curve shows a detailed coincidence with two tethyan 13c curves from italy what demonstrates the interregional significance of the 13c dates and allows their correlation within error limits of 40 kyr furthermore the new 13c curve enables the calibration of boreal and tethyan macro and microfossil zonations accordingly the tethyan calcareous nannoplankton boundary nc13 nc14 corresponds to the boreal fo of c woollgari the index taxon for the lower middle turonian boundary the cyclic appearance and the temporal spacing of the small scale carbon isotope events suggest that orbital forcing exerted control on surface water productivity and organic matter preservation at the sea floor

cyclostratigraphy of lower triassic terrestrial successions in the junggar basin northwestern china abstract the astrochronology for the early triassic has been constructed from well preserved marine successions however astronomical analyses of terrestrial strata have continued to be rare and problematic physical logs natural gamma ray logging data from two wells in the junggar basin northwestern china cover the entire lower triassic terrestrial baikouquan formation and have been shown to be a powerful tool in cycle analysis therefore these logs offer an excellent opportunity to detect astronomically forced sedimentation in a terrestrial setting from mid to high latitudes power spectral analysis revealed that the sedimentary cycles had wavelengths of 31 3 35 9 m 8 4 10 0 m 4 0 4 6 m and 4 0 m these are interpreted to correspond to the orbital eccentricity cycles of 405 kyr and 100 kyr and to the obliquity cycles of 51 kyr and 34 kyr respectively we thus established an astronomical time scale for the studied interval assuming that the 31 3 35 9 m cycles represent the 405 kyr eccentricity cycles this astrochronology indicated that the duration of the baikouquan formation was 2 0 0 1 myr statistical evaluations of optimal sediment accumulation rates using the correlation coefficient approach showed that the rates of the investigated successions were approximately 6 0 12 0 cm kyr orbitally controlled climate change was the major dynamic mechanism for the formation of early triassic fan delta successions in the junggar basin

eustatic sea level record for the cenomanian late cretaceous extension to the western interior basin usa a combination of biostratigraphic markers ammonites inoceramid bivalves and carbon isotope excursions is employed to establish a high resolution correlation between the middle to late cenomanian successions of the western interior basin usa and the anglo paris basin southern uk sequences identified from sedimentologic criteria in the pueblo succession and elsewhere in the western interior basin are shown to coincide precisely with globally recognized sea level events and were therefore under eustatic control this evidence refutes arguments that cenomanian sequences in the western interior basin were formed by local tectonic events the interaction of longer term tectonic movements and more rapid eustatic change may have simply enhanced the amount of erosion associated with sequence boundaries a crossplot of radiometric ages derived from north american bentonites against an orbitally tuned time scale developed in the anglo paris basin provides support for the argument that the sequences were controlled by the 405 k y long eccentricity cycle

resonant interaction of acoustic waves with subaqueous bedforms sand dunes in the south china sea the large subaqueous sand dunes in the south china sea are expected to produce the coupling of energy between acoustic normal modes in this letter resonant interaction between acoustic propagating modes and subaqueous bedforms are numerically investigated as a function of bedform wavelength acoustic frequency and bedform packet length the results demonstrate that bedform wavelength impacts acoustic mode coupling behavior with the principal transfer of energy occurring between acoustic modes whose eigenvalue difference is equal to the peak value in the bedform wavenumber spectrum the observed effect of wavelength is greater than that of acoustic frequency and bedform packet length

an example of alternative correlation techniques in a low accommodation setting nonmarine hydrocarbon system the lower cretaceous mannville basal quartz succession of southern alberta chemostratigraphy and heavy mineral techniques have been applied to the lower cretaceous basal quartz in the western canada sedimentary basin the aim of the study is to demonstrate that these two techniques can be used to help understand the complex stratigraphy of reservoirs deposited in low accommodation fluvial settings the basal quartz is an ideal unit to demonstrate their applicability in stratigraphic studies of hydrocarbon reservoirs because extensive mapping and petrographic studies have enabled the establishment of a rigorous stratigraphic framework despite its complexity resulting from deposition in a low accommodation fluvial setting the three component units analyzed in the basal quartz horsefly unit bantry alderson taber bat unit and ellerslie unit each have unique geochemical and heavy mineral characteristics chemostratigraphic analysis shows that silty claystones from the horsefly bat and ellerslie units have distinctly different geochemistry from one another with the variations being caused by changes in clay mineralogy and other components such as feldspar apatite and zircon the geochemistry also suggests periodic volcanogenic input influenced the silty claystones of the basal quartz heavy mineral analysis shows that sandstones from the three units can be distinguished on the basis of ratio parameters such as apatite tourmaline rutile zircon and zircon tourmaline which are controlled by differences in provenance and intensity of weathering during transport

the cenomanian turonian oceanic anoxic event in the south atlantic new insights from a geochemical study of dsdp site 530a abstract one of the key objectives of deep sea drilling project dsdp leg 75 was to shed light on the underlying causes of cretaceous oceanic anoxia in the south atlantic by addressing two major hypotheses productivity productivity driven anoxia vs enhanced ocean stratification leading to preservation of organic matter and black shale deposition here we present a detailed geochemical dataset from sediments deposited during the cenomanian turonian c t transition and the global oceanic anoxic event 2 oae 2 at dsdp site 530a located off shore namibia southeast angola basin north of walvis ridge to characterise the succession of alternating black and green shales at this site and to reconstruct the evolution of their paleoenvironmental setting we have combined data derived from investigations on bulk organic matter biomarkers and the inorganic fraction the location of the c t boundary itself is biostratigraphically not well constrained due to the carbonate poor but organic matter rich facies of these sediments the bulk 13corg record and compound specific 13c data in combination with published as well as new biostratigraphic data enabled us to locate more precisely the c t boundary at dsdp site 530a the compound specific 13c record is the first of this kind reported from c t black shales in the south atlantic it is employed for paleoenvironmental reconstructions and chemostratigraphic correlation to other c t sections in order to discuss the paleoceanographic aspects and implications of the observations at dsdp site 530a in a broader context e g with regard to the potential trigger mechanisms of oae 2 global changes in black shale deposition and climate on a stratigraphic level an approximation and monitoring of the syndepositional degree of oxygen depletion within the sediments bottom waters in comparison to the upper water column is achieved by comparing normalised concentrations of redox sensitive trace elements with the abundance of highly source specific molecular compounds these biomarkers are derived from photoautotrophic and simultaneously anoxygenic green sulphur bacteria chlorobiacea and are interpreted as paleoindicators for events of photic zone euxinia in contrast to a number of other oae 2 sections that are characterised by continuous black shale sequences dsdp site 530a represents a highly dynamic setting where newly deposited black shales were repeatedly exposed to conditions of subtle bottom water re oxidation presumably leading to their progressive alteration into green shales the frequent alternation between both facies and the related anoxic to slight oxygenated conditions can be best explained by variations in vertical extent of an oxygen minimum zone in response to changes in a highly productive western continental margin setting driven by upwelling

the permian triassic boundary lung cam expanded section vietnam as a high resolution proxy for the gssp at meishan china the lung cam expanded stratigraphic succession in vietnam is correlated herein to the meishan d section in china the gssp for the permian triassic boundary the first appearance datum of the conodont hindeodus parvus at meishan defines the permian triassic boundary and using published graphic correlation the permian triassic boundary level has been projected into the lung cam section using time series analysis of magnetic susceptibility data it is determined that h parvus arrived at lung cam 18 kyr before the permian triassic boundary data indicate that the lung cam section is expanded by 90 relative to the gssp section at meishan given the expanded lung cam section it is possible to resolve the timing of significant events during the permian triassic transition with high precision these events include major stepped extinctions beginning at 135 kyr and ending at 110 kyr below the permian triassic boundary with a duration of 25 kyr followed by deposition of lung cam ash bed 13 which is equivalent to siberian traps volcanism is graphically correlated to a precession time series model placing onset of this major volcanic event at 242 kyr before the ptb the meishan beds 25 and 26 at 100 kyr before the permian triassic boundary in addition the elemental geochemical carbon and oxygen isotope stratigraphy and magnetostratigraphy susceptibility datasets from lung cam allow good correlation to other permian triassic boundary succession these datasets are helpful when the conodont biostratigraphy is poorly known in sections with problems such as lithofacies variability or is undefined owing possibly to lithofacies exclusions anoxia or for other reasons the lung pu permian triassic boundary section 45 km from lung cam is used to test these problems

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

dolomitization a critical view of some current views the principal models in vogue today for dolomitization are the mixing zone and the sabkha models despite the wide acceptance of these models there has been little critical assessment of their validity such an assessment is the objective of the present paper a close look at the dorag mixing zone model badiozamani 1973 reveals several serious weaknesses 1 the range of freshwater seawater mixtures that meet the dorag requirement for dolomitization shrinks to a small window if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based 2 in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed 3 in a number of dolomites interpreted to be of mixing zone origin dolomite has precipitated without dissolution of the calcite substrate evidence that negates the fundamental premise of the dorag model in addition to these weaknesses isotope and trace element data used to identify mixed water dolomite are inadmissible because 1 isotopic fractionation factors for dolomites remain unresolved 2 isotope values uncorrected for temperature for a host of dolomites interpreted to be of different origins overlap 3 nonisomorphous trace elements such as na in dolomite cannot on theoretical grounds be relied on to identify dolomitizing fluids similar overall objections can be brought against folk and land9s 1975 schizohaline version of mixing zone dolomitization in summary mixing zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization contemporaneous dolomite formation in modern sabkhas is well documented but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved in the well studied sabkha at abu dhabi brine chemistry changes have been used as evidence of a replacement origin for the dolomite but it is shown here that this evidence is far from unequivocal an alternative origin of direct precipitation of dolomite is offered an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes and in line with our laboratory experience with dolomite synthesis moreover it is suggested here that in general contemporaneous dolomite will form at low temperatures only by direct precipitation a mechanism that requires special conditions of highly super saturated waters of high mg ca ratio and elevated co 3 hco 3 concentrations this may explain why modern dolomite is mainly restricted to evaporitic environments a bias not shared by ancient dolomites in contrast replacement dolomite appears to require at low temperatures long reaction times or 10 4 yr a requirement that is mainly to be met in large regional groundwater flow systems both marine and nonmarine a third dolomitization model considered here is that of baker and kastner 1981 based on the experimental finding that sulfate ions inhibit or retard dolomitization a number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations 2 to 70 times that of seawater thus the baker kastner model should be held in abeyance until these serious contradictions are resolved the current emphasis on mixing zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem four of these avenues each of which deemphasizes the special water approach are briefly addressed and are as follows 1 influence of temperature and time 2 mass transfer processes 3 burial diagenesis of epigenetic dolomites 4 fluid inclusion studies at elevated temperatures the dolomite problem essentially disappears ordered dolomite can be made in the laboratory in days at 100 degrees c what is more at temperatures above 60 degrees c ca rich waters become dolomitizing fluids which makes most natural subsurface waters capable of dolomitization at low temperatures time may be the key element so that seawater will become a major dolomitizing fluid only where stable circulation systems such as kohout convention can drive seawater through carbonate platforms for many thousands to millions of years this paper has tried to show that currently favored models of dolomitization carry serious uncertainties enough to warn us to look more critically at the validity of these models and also it is hoped enough to spur new efforts to find new solutions to the problems of dolomitization

chemostratigraphy a method to improve interwell correlation in barren sequences a case study using onshore duckmantian stephanian sequences west midlands u k abstract chemostratigraphy has been applied to onshore duckmantian stephanian successions encountered in outcrop and penetrated by two boreholes from the west midlands u k these successions represent the onshore equivalents of the barren red measures which are important hydrocarbon bearing sequences in the southern north sea much is known about the onshore successions in terms of sedimentology mineralogy and provenance and thus they provide the ideal test for the validity of chemostratigraphy as a stratigraphic tool reliable inorganic geochemical data have been acquired from geochemical analyses of core sidewall core and cuttings samples with 19 elements being determined stratigraphic variations in elemental concentrations are compared with known variations in the mineralogical data the established lithostratigraphic units of the duckmantian stephanian intervals can also be recognized from the geochemical data and by using these data can be subdivided further this results in an independent chemostratigraphic correlation being established for the two boreholes which has been assessed statistically by discriminant function analysis from the geochemical and mineralogical data distinct changes in provenance are identified within the upper carboniferous successions the sediments of the coal measures were derived from a north westerly and westerly source caledonian whereas the etruria formation sediments came from the wales brabant massif the sediments having mixed caledonian and cadomian characteristics eventually these sediments were replaced by sediments from a southern hercynian source halesowen and salop formations

reconstruction of water column anoxia in the equatorial atlantic during the cenomanian turonian oceanic anoxic event using biomarker and trace metal proxies abstract the occurrence of the biomarker lycopane and the enrichment patterns of redox sensitive trace metals in black shales from demerara rise western tropical atlantic reveal that bottom waters were anoxic during the upper cenomanian and lower turonian at this location bottom waters were clearly oxygen depleted already before oceanic anoxic event 2 oae 2 and continued to be so afterwards furthermore the presence of fossil derivatives of isorenieratene and chlorobactene pigments derived from green sulfur bacteria demonstrates that the waters at the base of the photic zone occasionally contained sulfide i e were euxinic before and after oae 2 low concentrations of derivatives of these pigments are present while much higher concentrations occur within the oae 2 section stratification of the water column must have been strong to sustain euxinic conditions in the photic zone for prolonged periods a substantial drop in the concentrations of isorenieratene and chlorobactene derivatives demonstrates that this stratification temporarily broke down during oae 2 a simultaneous decrease in sea surface temperatures suggests that local ventilation was instrumental in this re oxygenation event a rapid decrease in seawater derived trace metal burial rates at demerara rise occurring in phase with the onset of the anoxia related to oae 2 suggests an approximately synchronous onset of oae 2 in the proto north atlantic

magnetostratigraphy susceptibility of the přidolian lochkovian silurian devonian gssp klonk czech republic and a coeval sequence in anti atlas morocco abstract the magnetosusceptibility event and cyclostratigraphy msec record for the p idolian lochkovian silurian devonian global boundary stratotype section and point gssp klonk prague basin czech republic is described and used to establish a magnetostratigraphy susceptibility profile for the gssp gssp msec data are summarized into three magnetozones msz the tma msz late p idolian with 13 msszs mssz the klonk msz latest p idolian and earliest lochkovian with 17 msszs and the voskop msz early lochkovian with 7 msszs the base of klonk mssz 2 is coincident with the base of lochkovian and the devonian the proposed magnetostratotype for the p idolian lochkovian boundary is supported by ms data from a nearby core through the gssp sequence and into the upper ludlow three additional mszs are recognized for the portion of the core sequence older than the gssp boundary sequence the extension of the mszs and msszs away from the proposed magnetostratotype is tested by comparison with a lochkovian sequence in the western sahara of southeastern morocco mszs are the result of global sea level fluctuations that alter base level and the pattern of erosion and influx of detrital paramagnetic mineral components into the marine environment large scale transgressive and regressive patterns interpreted from the msec data establish that the silurian devonian boundary in the barrandian region falls between a short lived transgressive pulse in the latest p idolian and an equally short lived regressive pulse in the earliest lochkovian msszs identified in the gssp and core sequences are the result of variation in climate cycles and the resulting periodicity can explained by either the 38 9 ka silurian and devonian equivalent of the modern 54 ka obliquity cycle or the eccentricity cycle of 106 ka periodicity use of the obliquity periodicity gives a duration for the barrandian lochkovian of 4 46 ma which agrees with the most recent geochronometric estimate of 4 5 ma for the lochkovian use of the eccentricity periodicity gives a duration for the barrandian lochkovian of 12 16 ma which agrees with the previous geochronometric estimates of 12 2 and 12 5 ma for the lochkovian in any case the cyclic nature of the msec data established by the msszs can be explained by variations in the rate of supply of weathered terrigenous paramagnetic grains to the marine system these variations in supply occurred due to climate changes resulting from the either the obliquity or eccentricity of the earth s orbit knowledge of the periodicity of lochkovian cycles in the barrandian area allows estimation of the rates of sediment accumulation the duration of fossil ranges and the rates of evolution depending on the choice of obliquity or eccentricity to explain the cyclicity in the barrandian lochkovian the estimated rate of sediment accumulation in the gssp outcrop averaged either 51 or 140 ka m the range of the graptoloid monograptus uniformis angustidens wholly contained within the lochkovian sequence at the gssp is also estimated at either 51 or 140 ka

enhanced acoustic mode coupling resulting from an internal solitary wave approaching the shelfbreak in the south china sea internal waves and bathymetric variation create time and space dependent alterations in the ocean acoustic waveguide and cause subsequent coupling of acoustic energy between propagating normal modes in this paper the criterion for adiabatic invariance is extended to the case of an internal solitary wave isw encountering a sloping bathymetry i e continental shelfbreak predictions based on the extended criterion for adiabatic invariance are compared to experimental observations from the asian seas international acoustics experiment using a mode 1 starter field results demonstrate time dependent coupling of mode 1 energy to higher adjacent modes followed by abrupt coupling of mode 5 7 energy to nonadjacent modes 8 20 produces enhanced mode coupling and higher received levels downrange of the oceanographic and bathymetric features numerical simulations demonstrate that increasing isw amplitude and seafloor slope enhance the coupling of energy to adjacent and nonadjacent modes this enhanced coupling is the direct result of the simultaneous influence of the isw and its proximity to the shelfbreak and compared to the individual effect of the isw or shelfbreak has the capacity to scatter 2 4 times the amount of acoustic energy from below the thermocline into the upper water column beyond the shelfbreak in realistic environments

facies discrimination in a mixed fluvio eolian setting using elemental whole rock geochemistry applications for reservoir characterization abstract fluvio eolian successions are generally characterized by a high degree of complexity and internal heterogeneity quantifying the rapid facies changes in time and space is a major challenge to characterization of hydrocarbon reservoirs we present a method for facies discrimination in fluvio eolian successions the method presented here is developed on the modern fluvio eolian sediments from the skeleton coast dune field namibia the examined sediments comprise eight different facies types eolian dune sands interdune fluvial channel sands intraerg mass flow deposits intraerg hyperconcentrated flow deposits fluvial channel sands fluvial mud lacustrine heterolithic sand and lacustrine heterolithic mud the contrasting provenance of the fluvial and eolian sediments results in a distinct source fingerprint which can be discriminated using elemental whole rock geochemistry a multivariate statistical technique performed on the geochemical data has enabled discrimination of seven of the eight facies types furthermore the facies discrimination method allowed a quantitative estimate of the degree of fluvial reworking of eolian sand the skeleton coast dune field was chosen as a test area for geochemical facies discrimination because of its remarkable difference between the immature fluvial source and the mature quartz rich eolian source we hope that the method presented here can be applied to subsurface data from geological settings with the same difference in fluvial and eolian sources it is furthermore hoped that the method can be used in more complex sedimentological settings

the k t boundary in oman identified using magnetic susceptibility field measurements with geochemical confirmation abstract recognizing distal ejecta marker horizons and correlating these among widely separated sections is typically difficult in the absence of visually distinctive marker beds here we propose a magnetic susceptibility ms field method to locate such horizons and explore the k t boundary interval at abat oman as a test of the method a distinctive pattern of high ms values was used to approximately locate the k t boundary interval in a sequence of platform carbonates which were then sampled in detail whole rock geochemical enrichments in ir v as ni co zn and zr and a large negative carbon isotope anomaly confirmed the inferred boundary location common microspherules whose chemistry reflects the whole rock geochemistry are associated with the boundary interval the association suggests that the microspherules formed during or as a result of the impact the geochemical record at the abat locality shows two levels where ir is high 1 ppb suggesting input from two impacts separated in the section by 1 35 m the upper level is chosen as the boundary interval based on the high ir 1 19 ppb 13c negative excursion 7 and distinctive ms pattern

dolomite crystal architecture genetic implications for the origin of the tertiary dolostones of the cayman islands abstract dolostones in the cayman formation on grand cayman and cayman brac which are the product of two and possibly three phases of dolomitization are formed of very small the recognition that individual dolomite crystals irrespective of their size may be architecturally heterogeneous carries important implications for dolomite genesis many interpretations of finely crystalline dolostones for example have tacitly assumed that they formed through one phase of dolomitization in the cayman formation however dolomite crystals as small as 10 m are characterized by internal heterogeneities that record their development through multiple time separated growth phases any model developed to explain dolomitization must take this into account the interpretation of chemical data for example must consider the multiple stage development of the dolostones that is evident from their internal zoning and discontinuities

three dimensional flow in the florida platform theoretical analysis of kohout convection at its type locality kohout convection is the name given to the circulation of saline groundwater deep within carbonate platforms first proposed by f a kohout in the 1960s for south florida it is now seen as an mg pump for dolomitization by seawater as proposed by kohout cold seawater is drawn into the florida platform from the deep straits of florida as part of a geothermally driven circulation in which the seawater then rises in the interior of the platform to mix and exit with the discharging meteoric water of the floridan aquifer system simulation of the asymmetrically emergent florida platform with the new three dimensional 3 d finite element groundwater flow and transport model sutra ms which couples salinity and temperature dependent density variations allows analysis of how much of the cyclic flow is due to geothermal heating free convection as opposed to mixing with meteoric water discharging to the shoreline forced convection simulation of the system with and without geothermal heating reveals that the inflow of seawater from the straits of florida would be similar without the heat flow but the distribution would differ significantly the addition of heat flow reduces the asymmetry of the circulation it decreases seawater inflows on the atlantic side by 8 and on the gulf of mexico side by half the study illustrates the complex interplay of freshwater saltwater mixing geothermal heat flow and projected dolomitization in complicated 3 d settings with asymmetric boundary conditions and realistic horizontal and vertical variations in hydraulic properties

fault controlled and stratabound dolostones in the late aptian earliest albian benassal formation maestrat basin e spain petrology and geochemistry constrains this study was developed under the exxonmobil fc2 alliance fundamental controls on flow in carbonates the authors wish to thank exxonmobil production company and exxonmobil upstream research company for providing funding the views in this article by sherry l stafford are her own and not necessarily those of exxonmobil this research was supported by the sedimentary geology research group of the generalitat de catalunya 2014sgr251 we would like to thank andrea ceriani and paola ronchi for their critical and valuable reviews and associated editor piero gianolla for the editorial work

magnetosusceptibility event and cyclostratigraphy method applied to marine rocks detrital input versus carbonate productivity magnetic susceptibility data from marine rocks can be used for global correlation due to synchronous variations in global erosion we show here that the magnetic susceptibility signature found in two forms resides mainly in paramagnetic and other detrital constituents in most marine rocks the first form is a short term low magnitude high frequency cyclic climate signature that is often useful for regional correlation the second form is a longer term higher magnitude low frequency signature resulting from transgressive and regressive events that can be used for global correlation fluctuations in detrital input due to eustatic based erosion are the primary cause of events these fluctuations are driven by large scale processes such as global orogenic cycles however variations in carbonate productivity cannot be ruled out when explaining the low magnitude climate driven cyclicity also observed in magnetic susceptibility data sets

geochemical and sedimentological control of the magnetic properties of hemipelagic sediments owing to their high sedimentation rates and wide areal extent hemipelagic sediments along continental borderlands are potentially important high resolution recorders of geomagnetic secular variation to assess this possibility we studied suboxic hemipelagic muds from the oregon continental slope and anoxic laminated diatomaceous oozes from the gulf of california these sediments were rapidly deposited with average sedimentation rates of 121 and 135 cm kyr respectively bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes the remanence in both environments resides in fine grained magnetite particles although minor amounts of hematite were observed in the topmost gulf of california sediments despite first order differences the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur mainly as pyrite dramatic decreases in natural nrm anhysteretic and isothermal remanent magnetization intensities and accompanying shifts in the magnetic stabilities these changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter the magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed then slowly grows finer as the remaining particles dissolve no evidence was observed for authigenic formation of magnetite provided sufficient concentration and stability of the detrital magnetic particles paleomagnetic directions can survive this dissolution diagenesis however relative paleointensity determinations based on normalization of nrm by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

fresh and saline groundwater discharge to the ocean a regional perspective 1 studies of groundwater flow in coastal aquifers often focus strongly on freshwater and investigate flow in a narrow 5 km zone surrounding the coastline this work was designed to place coastal flow in a regional context and to compare fresh and saline submarine groundwater discharge sgd for regional flow systems numerical flow and transport models were developed to estimate sgd associated with topography driven flow seawater recirculation and geothermal convection in a passive margin setting simulations were based on two cross sections of north carolina and sensitivity studies were used to explore the impact of varying hydraulic and transport parameters results suggest that saline flow associated with seawater recirculation and geothermal convection should be considered in studies of sgd studies limited to shallow topography driven flow may be justified in using small study areas but flow systems contributing to sgd may span at least 20 km surrounding the coast and continental slope

cenomanian turonian and δ13c and δ18o sea level and salinity variations at pueblo colorado stable isotopes of the surface dwelling planktic foraminifera hedbergella planispira its abundance variations and mineralogical analysis of the cenomanian turonian at pueblo co reveal cyclic variations in surface salinity due to changes in precipitation freshwater influx marine incursions and long term sea level fluctuations hedbergella planispira is a proxy for salinity variations as indicated by 2 4 more negative 18o values in intervals of peak abundances as compared to intervals with reduced populations negative 18o values reflect periods of brackish surface waters caused by freshwater influx during wet humid periods accompanied by increased clastic transport more positive 18o values reflect more normal marine salinities as a result of arid periods and or marine incursions and correlate with intervals of increased biogenic carbonate deposition the magnitude of salinity variations during the low sea level of the hartland shale is twice that during the sea level transgression of the bridge creek limestone the rapid positive 13c shift that marks the onset of oceanic anoxic event 2 oae 2 at pueblo occurred over a period of about 100 ky 93 90 94 00 ma and coincided with the major sea level transgression that culminated in the deposition of the basal bridge creek limestone a positive 13c shift also occurred in the rotalipora cushmani zone prior to oae 2 and coincided with a sea level rise and enhanced preservation of terrestrial organic matter the likely cause for oae 2 is depletion of 12c in the water column as a result of high primary productivity whereas an earlier r cushmani zone event was primarily caused by increased input of terrigenous organic matter both 13c events are associated with enhanced organic matter preservation and anoxic or dysoxic bottom waters

stepwise extinction of larger foraminifers at the cenomanian turonian boundary a shallow water perspective on nutrient fluctuations during oceanic anoxic event 2 bonarelli event a two step pattern in the extinction of larger foraminifers is recorded in the upper cenomanian shallow water carbonates of the southern apennines italy the first step eliminated the alveolinids the most extreme oligotrophs and reduced dramatically the diversity of larger foraminifers the second step wiped out the few survivors seemingly able to tolerate mesotrophic conditions leaving a disaster fauna dominated by small heterotrophs this pattern of extinction parallels the ecological succession of shallow water benthic foraminiferal assemblages along a gradient of increasing nutrient availability high resolution carbon isotope stratigraphy shows that the extinction of alveolinids was contemporaneous with the extinction of rotaliporid planktic foraminifers the drowning of certain tethyan carbonate platforms and an episode of thermal instability recorded in sea surface temperature in the open ocean ocean stratification during the first phase of oceanic anoxic event 2 would have promoted oligotrophic conditions in surface tropical waters and maximum diversity of larger foraminifers following this ocean overturning caused by surface water cooling is credited with delivering to shallow water environments the excess nutrient loads previously stored at depth triggering the environmental changes leading to stepwise extinction of larger foraminifers

the occurrence and chemical implications of geothermal convection of seawater in continental shelves 1 geothermal convection is an important driving force for fluid flow and diagenesis in carbonate platforms and the similar shape of continental shelves in general suggests that geothermal convection could occur in continental shelves around the world numerical models based on seven transects of the shelf off the east coast of north america were developed to bracket possible fluxes simulations assuming relatively permeable sediments medium grained carbonates failed to produce significant thermal variations suggesting that chemical tracers will be required to detect this flow in the field fluxes calculated assuming lower permeabilities fine grained carbonates between cape fear and the savannah river were approximately 3 orders of magnitude smaller than river discharge for the area a difference similar to that between global river discharge and hydrothermal convection at mid ocean ridges sediment diagenesis during geothermal convection in continental shelves thus has the potential to affect ocean chemical budgets including potassium calcium and magnesium

petrographic and geochemical constraints on the formation and diagenesis of anhydrite cements smackover sandstones gulf of mexico abstract anhydrite cement is common in sedimentary rocks yet its origin is poorly understood the high concentration of sulfate in sea water and the lack of appreciable sulfate in most other natural waters suggests that anhydrite cement may have a marine origin but the relatively late timing of most anhydride cement tends to preclude sea water as the source of sulfate anhydrite cement is present in upper jurassic sandstones in the gulf of mexico as poikilotopic masses in which detrital grains are replaced and as smaller patches that have replaced single detrital grains anhydrite is a relatively late cement and postdates all other volumetrically significant authigenic phases including k feldspar dolomite quartz and most calcite cements the expected isotopic composition and trace element concentration of marine derived late jurassic anhydrite cement is predicted based on analyses of bedded anhydrite and on analyses found in the literature the expected chemical signature is 34s cdt 16 18o smow 14 87sr 86sr 0 7069 sr 1500 2500 ppm ba 200 ppm when geochemical analyses of smackover anhydrite cements are compared to the predicted composition it is apparent that most of the cement is not of marine origin two generations of anhydrite cement have been identified in the east texas basin on the basis of their strontium isotopic compositions and their strontium concentrations an early cement may have been derived from slightly modified late jurassic sea water whereas a second group of cements may have precipitated later in the burial history or may represent recrystallization of the first cement the chemistry of louisiana and mississippi basin anhydrite cements indicates that the sulfate was derived from dissolved bedded anhydrite and was reprecipitated in the sandstones from fluids that had undergone extensive water rock interaction after considerable burial this study suggests that late anhydrite cements in sedimentary rocks are most likely derived from remobilized calcium sulfate de osits

cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode oceanic anoxic events oaes were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen deficient bottom waters oae2 occurring at the cenomanian turonian boundary about 93 5 myr ago is the most widespread and best defined oae of the mid cretaceous although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios the actual trigger mechanism corresponding closely to the onset of these episodes of increased carbon sequestration has not been clearly identified it has been postulated that large scale magmatic activity initially triggered oae2 refs 4 5 but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of oae2 has not yet been found here we report seawater osmium isotope ratios in organic rich sediments from two distant sites we find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of oae2 using a simple two component mixing equation we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin a approximately 30 50 fold increase relative to pre oae conditions furthermore the magmatic osmium isotope signal appears slightly before the oae2 as indicated by carbon isotope ratios suggesting a time lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial which may reflect the reaction time of the global ocean system our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of oae2 which may have triggered the subsequent deposition of large amounts of organic matter

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

hydrothermal dolomitization in a complex geodynamic setting lower palaeozoic northern spain burial hydrothermal dolomitization is a common diagenetic modification in sedimentary basins with implications for oil and gas reservoir performance outcrop analogues represent an easily accessible source of data to refine the genetic models and assess risk in hydrocarbon exploration and production the palaeozoic succession of northern spain contains numerous excellent exposures of epigenetically dolomitized limestones particularly in the carboniferous and cambrian the epigenetic dolomites in the cambrian carbonates of the lancara formation are volumetrically small but have a large aerial distribution across different tectonic units of the variscan fold and thrust belt coarse crystals abundant saddle dolomite cement negative 18o and fluid inclusion homogenization temperatures between 80 c and 120 c characterize these dolomites which are petrographically and geochemically similar to the tens of kilometre sized hydrothermal dolomites replacing the upper carboniferous succession in the same area in both cases the dolomitizing fluids are derived from highly evaporated sea water modified to a limited degree through fluid rock interaction the dolomitization events affecting both cambrian and carboniferous strata are probably related to the same post orogenic hydrothermal fluid flow the formation of the post collisional latest carboniferous cantabrian arc fostered dolomitization the extension related to bending of the arc generated deep reaching faults and strike slip movements which favoured the circulation of hot dolomitizing fluids in the outer parts of this orocline a similar dolomitization process affected other areas of europe after the main stages of the variscan orogeny dolomitization was a continuous uninterrupted isochemical process limestone replacement resulted in a major porosity redistribution and focused the fluid flow into the newly created porous zones replacement was followed immediately by partial to complete cementation of the pores including zebra fabrics and vugs with saddle dolomite the amount of porosity left depends on the volume of cement and therefore on the volume of fluids available

acoustic mode coupling due to subaqueous sand dunes in the south china sea the large subaqueous sand dunes on the upper continental slope of the south china sea are expected to couple acoustic propagating normal modes in this letter the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms using the extended criterion to examine mode propagation over the bedforms observed in the sand dune field in 2012 results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios increasing bedform amplitude enhances mode coupling numerical simulations confirm the extended criterion parameterization

formation of diagenetic dolomite in coastal sabkha along arabian persian gulf aragonitic intertidal sediments are being dolomitized in the subsurface supratidal environment of a sabkha along the south shore of the arabian persian gulf as a result of the percolation of wind driven marine derived brines the development of abundant diagenetic dolomite results from an optimum combination of high mmg mca ratio fluids a rapid flow rate which is related to the flooding frequency and sediment permeability and a shoreline configuration that maintains the proper conditions for a sufficient length of time strongly dolomitized areas are localized along the flanks of remnant channels and in the lee of islands that have been engulfed by the prograding sabkha the dolomitizing process takes about 1 000 to 1 500 years most diagenetic dolomite forms at the expense of primary aragonite according to the reaction mg 2caco3 arag rarr camg co3 2 ca dolomite forms under the following conditions a chloride concentration between 3 25 and 3 75 mcl kg b mmg mca greater than about 6 c ph between 6 3 and 6 9 d minimum pco2 of 10 2 to 10 3 atm e temperature between 25 and 40 c f saturation with respect to gypsum and g reducing environment in association with hydrogen sulfide other recent dolomite occurrences in the bahamas florida and bonaire are significantly different in terms of stratigraphy degree of lithification and evaporite mineral association from those in the arabian gulf

geophysical and geological signatures of relative sea level change in the upper wheeler formation drum mountains west central utah a perspective into exceptional preservation of fossils abstract the middle cambrian wheeler formation in utah is renowned for its exceptionally preserved fossils herein geophysical techniques gamma ray spectrometry and magnetic susceptibility carbonate analyses lithofacies and fossils are used to characterize limestones and calcareous mudrocks of the upper portion of the middle wheeler formation and the upper wheeler formation in the drum mountains west central utah this succession was deposited on a mixed carbonate siliciclastic ramp within the house range embayment an eastward incursion of deeper water created and bounded on the southeast by a normal fault geophysical data clearly reveal variations in carbonate versus clay content observed patterns among outcrop geophysical data lithofacies and fossils are interpreted within the contexts of relative sea level changes and a sequence stratigraphic model above a basal interval of rhythmically bedded limestones interpreted to have been deposited on the middle part of the ramp during a period of relatively low sea level lithofacies rising gamma ray and rising magnetic susceptibility track a transgression which culminates in a maximum flooding surface mfs this mfs is near the base of a hot zone an 6 m thick interval of exceptional preservation konservat lagerstatte highest magnetic susceptibility highest gamma ray values highest clay content and with a bulk carbonate content of 4 20 the basal layer of this zone contains abundant fully articulated specimens of agnostids and tiny polymerid trilobites exceptionally preserved fossils such as non trilobite arthropods priapulid and annelid worms hyolithids phyllocarid arthropods with soft parts algae and fragile sponges are much more abundant within this hot zone than in other strata of the succession these specimens are preserved in very thin bedded to laminated mudrock only rarely interrupted by macroscopic signs of bioturbation stratigraphically above the hot zone is a thick unit of mudrock in which both gamma ray and magnetic susceptibility decrease gradually at first and then precipitously to very low levels in concert few soft bodied specimens have been recovered from this upper interval the gradual decrease in geophysical responses is interpreted to be the result of gradual shallowing during highstand caused mainly by progradation of the carbonate factory it was followed by an abrupt regression mudrocks are capped by burrow mottled grainstone and stromatolitic boundstone representing a lowstand or the earliest transgressive phase of the next cycle these observations lead to the following conclusions 1 the upper wheeler formation represents most of a third or fourth order cycle of relative sea level change on a mixed carbonate siliciclastic ramp rather than a period of shallow water lagoonal sedimentation as previously proposed 2 superimposed on this overall cycle are several perhaps many higher order fluctuations in relative sea level 3 when the present results are coupled with results of earlier workers who interpreted the lower and middle wheeler formation as a major transgression and regression the wheeler formation of the drum mountains involves two major sea level cycles rather than one 4 the konservat lagerstatte was preserved in the deepest water early highstand portion of the upper wheeler sequence and 5 these results support earlier hypotheses that konservat lagerstatten in mixed carbonate siliciclastic successions are most likely to occur during late transgression to early highstand given that low energy anoxic conditions prevailed

the cenomanian turonian boundary event oae2 and palaeoenvironmental change in epicontinental seas new insights from the dinocyst and geochemical records abstract organic walled dinoflagellate cyst dinocyst and geochemical records across the cenomanian turonian boundary ctb are presented for the nw european reference section at eastbourne england dinocyst and nannofossil fertility indexes indicate that an upwelling driven productivity pulse accompanied a eustatic sea level fall that preceded by at least 40 kyr the global rise in 13 c values that marks the onset of oceanic anoxic event 2 oae2 and the deposition of black shales in the deep ocean a marine productivity collapse in the latest cenomanian is evidenced by the falling absolute and relative abundance of peridinioid dinocysts believed to be the product of heterotrophic dinoflagellates this biotic change accompanied transgression and sharply rising sea surface temperatures following an atlantic wide episode of short lived cooling geochemical tracers provide evidence of caribbean colombian plateau volcanism the increase in water depth caused by the latest cenomanian transgression resulted in less eutrophic waters in epicontinental seas where ctb biotic turnover was driven largely by water mass changes rather than oxygen depletion the species richness absolute abundance ratio of dinocysts is proposed as a water mass stability proxy

evidence for a change in the periodicity of tropical climate cycles at 2 4 myr from whole core magnetic susceptibility measurements the earth s orbit about the sun is influenced by gravitational interactions with the moon and other planets the resulting orbital perturbations give rise to cyclical variations in orbital eccentricity obliquity and precession with periods of 100 41 19 and 23 kyr respectively1 these variations are climatically important because they affect the seasonal and latitudinal distribution of solar radiation magnetic susceptibility measurements in deep sea sediment cores can be used as a sensitive indicator of temporal variations in the concentration of terrigenous material supplied to the sea bed here we use this approach to study the evolution of cycles of terrigenous sedimentation at ocean drilling program sites in the arabian sea and the eastern tropical atlantic we show from spectral analyses of the two susceptibility time series spanning the past 3 2 3 5 myr that the records are driven strongly by orbital forcing before 2 4 myr before present bp both records are strongly influenced by the 23 and 19 kyr periodicities but after 2 4 myr they are both significantly influenced by the 41 kyr periodicity this shift coincides with the beginning of major northern hemisphere glaciation which indicates for the indian ocean site that the supply of terrigenous sediment carried by monsoonal winds has responded to the rapid increase in ice cover at this time we cannot however exclude possible effects of changes in bottom water circulation on the susceptibility record of the atlantic site

geochemistry of pelagic and hemipelagic carbonates criteria for identifying systems tracts and sea level change the elemental si ti al mn ca zr and carbon stable isotope 13 c geochemistry of a biostratigraphically well constrained cenomanian turonian upper cretaceous chalk succession on the isle of wight southern england shows systematic variation that corresponds closely to a published sequence stratigraphic model for the cenomanian six sequences and their constituent systems tracts defined elsewhere using sedimentological criteria are clearly distinguishable from bulk sediment elemental profiles and an additional upper cenomanian sequence previously identified in spain is recognized in england from these geochemical data the manganese curve is particularly instructive exhibiting minima around sequence boundaries and through lowstands rising values from the transgressive surfaces through transgressive systems tracts maxima around maximum flooding surfaces and declining values through highstands silica and trace element ti zr aluminium ratios peak around transgressive surfaces and maximum flooding surfaces indicating pulses of increased siliciclastic input positive 13 c excursions are confirmed at the base of the middle cenomanian and spanning the cenomanian turonian boundary but are not evident in other sequences variation in mn is related to bulk sedimentation rate and detrital versus biogenic supply which control the mn flux and the efficiency of the diagenetic mn pump that leads to elevated mn contents in sediments manganese peaks do not generally correlate with positive 13 c excursions and although near coincident mn and 13 c peaks occur around the cenomanian turonian boundary the former is not necessarily linked to the oceanic anoxic event occurring at that time the global oceanic mn flux may have been enhanced during the cenomanian as a result of hydrothermal activity during rapid sea floor spreading and oceanic plateau formation elemental chemostratigraphy provides a new tool for developing sequence stratigraphic models in pelagic and hemipelagic carbonate successions

geochemical and climatic effects of increased marine organic carbon burial at the cenomanian turonian boundary perhaps the most significant event in the cretaceous record of the carbon isotope composition of carbonate1 2 other than the 1 2 5 negative shift in the carbon isotope composition of calcareous plankton at the cretaceous tertiary boundary3 is the rapid global positive excursion of 2 13c enrichment which took place between 91 5 myr and 90 3 myr late cenomanian to earliest turonian c t boundary event 1 4 5 this excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon tdc reservoir resulting from an increase in rate of burial of 13c depleted organic carbon which coincided with a major global rise in sea level5 during the so called c t oceanic anoxic event oae 6 here we present new data from nine localities which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the c t boundary7 8 is nearly synchronous with that for carbonate and is widespread throughout the tethys and atlantic basins fig 1 as well as in more high latitude epicontinental seas the postulated increase in the rate of burial of organic carbon may have had a significant effect on co2 and o2 concentrations in the oceans and atmosphere and consequent effects on global climate and sedimentary facies

bottom water anoxia inoceramid colonization and benthopelagic coupling during black shale deposition on demerara rise late cretaceous western tropical north atlantic 1 the bulk rock geochemistry and inoceramid isotopic composition from cenomanian to santonian finely laminated organic rich black shales recovered during ocean drilling program leg 207 on demerara rise western tropical north atlantic suggest persistent anoxic free h2s conditions within the sediments and short term variations within a narrow range of anoxic to episodically dysoxic bottom waters over a 15 ma time interval in addition to being organic rich the 50 90 m thick sections examined exhibit substantial bulk rock enrichments of si p ba cu mo ni and zn relative to world average shale these observations point to high organic burial fluxes likely driven by high primary production rates which led to the establishment of intensely sulfidic pore waters and possibly bottom waters as well as to the enrichments of cr mo u and v in the sediments at the same time the irregular presence of benthic inoceramids and foraminifera in this facies demonstrates that the benthic environment could not have been continuously anoxic the 13c and 15n values of the inoceramid shell organics provide no evidence of chemosymbiosis and are consistent with pelagic rain as being a significant food source demerara rise inoceramids also exhibit well defined regularly spaced growth lines that are tracked by 13c and 18o variations in shell carbonate that cannot be simply explained by diagenesis instead productivity variations in surface waters may have paced the growth of the shells during brief oxygenation events suitable for benthic inoceramid settlement these inferences imply tight benthopelagic coupling and more dynamic benthic conditions than generally portrayed during black shale deposition by invoking different temporal scales for geochemical and paleontological data this study resolves recent contradictory conclusions e g sulfidic sedimentary conditions versus dysoxic to suboxic benthic waters drawn from studies of either sediment geochemistry or fossil distributions alone on demerara rise this variability may be relevant for discussions of black shales in general

siderite formation in anoxic deep sea sediments a synergetic bacteria controlled process with improtant implications in paleomagnetism recent work on magnetic properties of limestones has demonstrated that the mineral siderite can be very important in paleomagnetism for two reasons first oxidation of siderite produces secondary daughter magnetic minerals magnetite maghemite and hematite either before during or after sampling these daughter products can completely change the magnetic properties of limestone samples and if unrecognized may be one of the primary reasons why many paleomagnetic studies of limestones especially paleozoic limestones are unsuccessful second siderite in weakly magnetized rocks may indicate the potential for successful paleomagnetic results because the presence of siderite indicates that the primary magnetic carriers are still intact appropriate demagnetization methods should yield successful results we conclude that microenvironmental conditions in anoxic marine sediments may permit the formation of siderite from iron ii produced during bacterial dissimilatory iron reduction

amino acid geochronology of individual foraminifer pulleniatina obliquiloculata tests north queensland margin australia a new approach to correlating and dating quaternary tropical marine sediment cores 1 in this study we demonstrate the utility of amino acid geochronology based on single foraminiferal tests in quaternary sediment cores from the queensland margin australia the large planktonic foraminifer pulleniatina obliquiloculata is ubiquitous in shelf slope and basin sediments of north queensland as well as pantropical oceans fossil tests are resistant to dissolution and retain substantial concentrations of amino acids 2 4 nmol mg 1 of shell over hundreds of thousands of years amino acid d and l isomers of aspartic acid asp and glutamic acid glu were separated using reverse phase chromatography which is sensitive enough to analyze individual foraminifera tests in all 462 pulleniatina tests from 80 horizons in 11 cores exhibit a systematic increase in d l ratios down core d l ratios were determined in 32 samples whose ages are known from ams 14 c analyses in all cases the asp and glu d l ratios are concordant with 14 c age d l ratios of equal age samples are slightly lower for cores taken from deeper water sites reflecting the sensitivity of the rate of racemization to bottom water temperature beyond the range of 14 c dating previously identified marine oxygen isotope stage boundaries provide approximate ages of the sediments up to about 500 000 years for this longer time frame d l ratios also vary systematically with isotope correlated ages the rate of racemization for glu and asp was modeled using power functions these equations can be used to estimate ages of samples from the queensland margin extending back at least 500 000 years this analytical approach provides new opportunities for geochronological control necessary to understand fundamental sedimentary processes affecting a wide range of marine environments index terms 4267 oceanography general paleoceanography 4850 oceanography biological and chemical organic marine chemistry 3022 marine geology and geophysics marine sediments processes and transport keywords queensland margin marine sediment amino acid racemization geochronology foraminifera quaternary stratigraphy citation hearty p j m j o leary d s kaufman m c page and j bright 2004 amino acid geochronology of individual foraminifer pulleniatina obliquiloculata tests north queensland margin australia a new approach to correlating and dating quaternary tropical marine sediment cores paleoceanography 19 pa4022 doi 10 1029 2004pa001059

stability of the astronomical frequencies over the earth s history for paleoclimate studies the expected changes over the past 500 million years in the principal astronomical frequencies influencing the earth s climate may be strong enough to be detectable in the geological records and such effects have been inferred in several cases calculations suggest that the shortening of the earth moon distance and of the length of the day back in time induced a shortening of the fundamental periods for the obliquity and climatic precession from 54 to 35 41 to 29 23 to 19 and 19 to 16 thousand years over the last half billion years at the same time the precessional constant increased from 50 to 61 arc seconds per year the changes in the frequencies of the planetary system due to its chaotic motion are much smaller their influence on the changes of the periods of climatic precession obliquity and eccentricity of the earth s orbit around the sun can be neglected eccentricity periods used for quaternary climate studies may therefore be considered to have been more or less constant for pre quaternary times

sea level and climatic induced facies variations in the middle cambrian house range embayment western laurentia abstract the middle cambrian house range embayment hre represents a deep water incursion into an extensive carbonate platform along the passive margin of western laurentia the distribution and source of hre mixed siliciclastic carbonate sediments are poorly understood because many key elements were missed during past field observations or coarse correlation this study describes the hre complex lithological variations and improves current understanding concerning the global climatic and sea level fluctuations that drove these variations magnetic susceptibility gamma radiation gr and carbonate content caco3 are used to construct high resolution correlation among three overlapping middle cambrian marine sedimentary sections 1 the drumian global boundary stratotype section and point gssp in the drum mountains western utah 2 the marjum pass section in the house range western utah and 3 the packrat section great basin national park in the snake range eastern nevada middle cambrian hre lithological variations followed broad facies patterns carbonate sediments dominated the hre edges and decreased inward while shale dominated the hre trough and decreased outward the carbonate detrital ratio varied along the area between the hre edge and trough due to 1 sea level fluctuations 2 the depositional setting at each section and 3 the allocyclic climatic variations that acted on the carbonate platform of western laurentia sea level and climatic fluctuations drove temporal facies variations that are well correlated among the three sections while the depositional setting for each section drove spatial facies variations that account for carbonate and detrital sediment sources and their trajectories into the hre the gssp is a shallow trough section marjum pass is a lower slope section of the northern hre ramp and the packrat is a lower slope section situated along the southern hre ramp sea level and climatic cycles during the middle cambrian drumian stage induced long term and short term facies variations respectively that were superimposed on each other

convective fluid flow and diagenetic patterns in domed sheets when a moving pore fluid maintains equilibrium with the host rock the flow and temperature fields can combine to produce diagenetic patterns these diagenetic patterns define source and sink regions for mass transfer within the rock and permit prediction of both the spatial distribution of diagenetic alteration and its intensity analytical solutions have been obtained for the fluid flow and temperature fields for the case of a folded porous layer embedded in an impermeable medium heated from below and held at a constant temperature above this corresponds to a layered sequence of rocks in which a permeable sand is bounded above and below by impermeable shales the present results are limited to gently folded strata small dip angles but are quite general with regard to the actual layer geometry the solutions depend on the fact that the temperature field can be uncoupled from the fluid flow when convection is weak this results in a purely conductive temperature field which drives the fluid motion through buoyancy effects it is shown that the dynamic of the system are most strongly influenced by the geometry of the porous layer and the ratio of thermal conductivities of the rocks

computation of porosity redistribution resulting from thermal convection in slanted porous layers unlike fluid displacement due to regional hydraulic head thermoconvective motions are generally slow the thermal impacts of such movements are very weak whereas their chemical impacts may be significant because of their cumulated effects over geologic time for nonhorizontal thick sedimentary reservoirs the fluid velocity due to thermal convection can be accurately approximated by an explicit function of the dip of the reservoir the permeability and the difference in thermal conductivity between the aquifer and the confining beds the latter parameter controls the rotation direction of the flow and for clastic reservoirs bounded by impervious clayey media fluid moves up the slope along the caprock layer as the fluid velocity is small the major rock forming minerals control the fluid composition by thermodynamic equilibrium thus whereas the volume of redistributed minerals depends on the volume of water circulated the localization of porosity enhancement is strongly controlled by the reservoir mineralogy with realistic values of permeability and layer thickness several per cent of secondary porosity per million years can be created or lost at shallow depth 2 km depending on the chlorinity the set of representative minerals and the temperature in sandstone reservoirs and high chlorinity calcarenite reservoirs the porosity decreases under the caprock where hydrocarbons can accumulate in chloride depleted carbonate aquifers the simultaneous control by carbonates silica and aluminosilicates can produce a decrease of porosity above the bedrock and an enhancement of porosity under the caprock however computations show that the quality of the upper part of the reservoir is mainly reduced by the precipitation of silica and clays

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

geothermal convection in the tengiz carbonate platform kazakhstan reactive transport models of diagenesis and reservoir quality a fundamental challenge in carbonate reservoir characterization is predicting the spatial distribution of diagenesis we used a reactive transport model to investigate the viability of geothermal convection and associated patterns of diagenetic porosity modification in the tengiz isolated carbonate platform reservoir before burial forced convection generates significant calcite dissolution locally up to 45 toward the platform center minor calcite cementation up to 0 4 in the slope and moderate calcite dissolution and cementation up to 1 6 in serpukhovian boundstone convective cells the patterns and rates of diagenesis proved critically sensitive to specified vertical permeability after burial with 200 m 660 ft of salt modeled subsurface temperature contrasts drive platform scale free convection flow is hydraulically closed but significant dissolution up to 7 3 after 20 m y occurred in the serpukhovian and visean platform interior and minor cementation up to 0 7 occurred toward the margin a shale filled salt withdrawal basin 500 m 1640 ft deep significantly modifies the subsurface temperature distribution and free convective flow ascending groundwaters beneath the withdrawal basin created a zone of calcite dissolution up to 24 5 in 20 m y with a mushroom geometry and minor cementation up to 2 3 in 20 m y in the distal platform interior and margin rates of diagenesis are dramatically reduced with increasing overburden as compaction retards convective flow from a generic perspective free convection persists if the salt overburden is substituted with shale although flow is reversed resulting in a different distribution of diagenesis simulations of geothermal convection provide a physically viable model for the integration of direct diagenetic observations to augment predictions of reservoir quality in tengiz and other carbonate platform reservoirs

cycle calibrated magnetostratigraphy of middle carnian from south china implications for late triassic time scale and termination of the yangtze platform abstract thick successions from the margins of the triassic yangtze platform of the south china block record a transition from carbonate rich facies zhuganpo formation and equivalents to clastic rich facies xiaowa formation and equivalents during the carnian early late triassic that mark the final phase of termination of this long lived platform cyclostratigraphy derived from spectral gamma ray sgr intensity curves was combined with magnetostratigraphy of two sections in guizhou province wayao transect and its upward extension into the geopark wayao in guanling county and the laishike section in zhenfeng county and one location in sichuan province hanwang section the cyclostratigraphy from all the guizhou sections indicates a persistent suite of ca 34 m 9 m and 1 8 m cycles which are consistent with the ratios of orbital climate oscillations caused by long eccentricity 405 kyr short eccentricity 100 kyr and precession 20 kyr the magnetostratigraphy of all sections is consistent with the cyclicity and characteristic sgr features thereby enabling a cycle tuned magnetic polarity scale spanning 2 4 myr the main feature is a 1 3 myr interval of reversed polarity containing brief normal polarity subchrons and this reversed polarity chron appears to correspond to the significant reversed polarity dominated interval spanning the upper half of the trachyceras aonoides through the lower half of the austrotrachyceras austriacum ammonoid zones of the upper julian the lower substage of carnian this magnetostratigraphic correlation implies that the termination of the yangtze platform is coeval with the beginning of the mid carnian episode of climatic disruption in europe which is locally called the carnian pluvial event or wet intermezzo and with the temporary cessation of the platform carbonates in the dolomites the cycle scaled magnetic polarity time scale supports the short tuvalian long norian age model of the late triassic in which the base of the cycle tuned polarity pattern from the newark group of eastern north america is younger than the end of the julian substage

density induced water circulations in atoll coral reefs a numerical study groundwater flow in an idealized atoll platform is simulated with castem 2000 a computer code developed at the cea commissariat a l energie atomique darcy s law and a coupled solute transport equation diffusion dispersion convection is solved by the mixed hybrid finite element method inward and upward circulations of oceanic water are shown to occur these steady state circulations result from temperature and to a lesser extent salinity gradients inclusion of a karstified dolomitic horizon observed at the base of the carbonate structure of certain atolls is necessary to account for the negative vertical temperature gradient indicated by field data the position of the platform in relation to the oceanic temperature and salinity distributions exerts a significant control on the interstitial hydraulic regime the effect of a secondary karstic layer closer to sea surface is also modeled in this case calculations show that the near surface carbonate edifice is deprived of a significant portion of the flow finally an upper limit on the possible flow rate through the platform is calculated this upper estimate is shown to be independent of the possible uncertainties on the hydraulic conductivity of the system corresponding upper limits on the nutrient fluxes of po and no are evaluated a comparison with 3 22 43 current import and export data for the surface ecosystem indicates that in every case nutrient fluxes hardly reach a few percent of the exports therefore the deep oceanic nutrient pool brought to the surface by thermally driven interstitial circulation is only a minor nutrient source for the reef ecosystem

environmental and sequence stratigraphic implications of anhydrite textures a case from the lower triassic of the central persian gulf abstract the lower triassic kangan formation in the persian gulf south pars gas field and its adjacent areas are composed of carbonate evaporite sequences these sediments were deposited in a shallow marine homoclinal ramp study of the anhydrite bearing intervals shows various structures and textures the anhydrite structures are mainly bedded massive chicken wire and nodular type and the main textures are felted sparse crystal needle shape lath shape equant and fibrous pervasive and poikilotopic cement together with replacement and porphyroblastic gypsum are accounted as the most common diagenetic features in anhydrite evaluation of anhydrite occurrences and features support both primary and secondary formations the nodular to chicken wire anhydrite formed under synsedimentary sabkha conditions whereas anhydrite cements occurred during the late stages of diagenesis shallow burial stage massive to bedded anhydrite could have been formed under subaqueous conditions or originated by coalescing and continued growth of anhydrite nodules in the sabkha zone anhydrite fabrics impose a significant control on the reservoir quality of the kangan carbonates at the south pars gas field thick massive and bedded anhydrite could have been formed as an intraformational seals and anhydrite cements occluded pore spaces and reduced the poroperm values the sequence stratigraphic analysis revealed two depositional sequences in the studied intervals which are composed of tst and hst investigation of anhydrite throughout depositional sequences indicates a change in the content and style of anhydrite texture anhydrite content volume decreases upward through transgressive system tract sea level rise whereas it enhances during highstand system tract sea level fall pervasive and poikilotopic anhydrite cements together with replacement by anhydrite are prevalent features during transgressive and early highstand system tract at the late hst with a progradational stacking pattern anhydrite value increases and felted radial equant crystalline and mosaic texture are the most common anhydrite fabrics sequence boundaries that indicate maximum sea level fall and exposure of successions are marked by the broad anhydrite deposits with massive to bedded and chicken wire structures and various textures that located in late hst package there is an unambiguous relationship between the microfacies associations the evaporite textures and the sea level fluctuations this relationship could lead to a predictable pattern that can be of use as a general guide for the sequence stratigraphic interpretations in the area

spatial patterns of diagenesis during geothermal circulation in carbonate platforms geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations we used reactive transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick in the simulations porosity and permeability decrease with depth to account for sediment compaction dolomitization of a platform consisting of medium grained 0 0 5 mm sediments occurred in a broad band ranging from 2 5 km depth near the margin to 1 5 km depth near the platform center the area of dolomitization is deep enough that temperatures exceed 50 c but not so deep that low permeabilities restrict mass transport complete dolomitization in the center of this zone is estimated to require at least 60 my incorporation of permeability contrasts permeable beds and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent dolomitization created magnesium depleted calcium rich fluids in less than 10 ky and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available

dolomitization of holocene shallow marine deposits mediated by sulfate reduction and methanogenesis in normal salinity seawater northern belize abstract dolomite constitutes an average of 12 of the holocene organic rich sediments over a 15 km2 area of the cangrejo shoals mudbank in northern belize although it defines a laterally persistent stratiform body that averages 3 m thick it is present throughout the 7 6 m thick sediment section these transgressive sediments are less than 6400 years old and were deposited in shallow marine environments of normal salinity the dolomite is dominantly cement and average crystal size is 7 m there are no significant correlations among amount of dolomite vs sediment texture mineralogy porosity or mole mgco3 in associated particulate high mg calcite depth or location on the shoals the dolomites are poorly ordered and calcic 39 5 44 5 mole mgco3 with low mean mn 210 ppm and relatively high mean sr 1034 ppm concentrations there is no evidence of recrystallization or geochemical alteration of the dolomite 18o values of the dolomites range from 0 5 to 2 8o oopdb and the mean value 2 1o oo suggests that the dolomite precipitated from normal salinity pore water dolomite 13c values range from 5 2o oo to 11 6o oopdb mean seawater 13c 0 5o oo which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water dolomitization attending these organodiagenetic reactions apparently was reversible over time and episodic rather than continual precipitation is indicated requisite mg and ca were provided by seawater and by some dissolution of host sediments the most rapid period of dolomitization may have been during early transgression when relatively high sedimentation rates sustained high levels of organodiagenesis and pore water alkalinities

a climate driven model and development of a floating point time scale for the entire middle devonian givetian stage a test using magnetostratigraphy susceptibility as a climate proxy abstract here we propose an approach to establish a preliminary age chronology for complete stages within the phanerozoic by applying cyclostratigraphic methods that employ high resolution data sets this requires use of geochemical or geophysical data known to serve as a cyclic climate proxy to demonstrate the method we use the magnetostratigraphic susceptibility technique as the basis for fitting a climate model to the givetian stage of the middle devonian system we show a milankovitch eccentricity climate zonation for the givetian that is pinned to time series analyses from outcrop samples from its upper givetian frasnian france and lower eifelian givetian morocco stage boundaries using these data sets we construct a uniform cyclicity model designed to conform to a 405 kyr cyclicity with a duration corresponding to the published duration for the givetian of 4 4 myr kaufmann 2006 to this model we fit two well established conodont zonation schemes thus allowing time estimates for conodont ranges for the givetian indicating a range from 1 8 myr to 100 kyr before extinction of an individual conodont species we then test and adjust the model using independent data sets from the eastern united states adjustments to the initial model yield a duration of 28 climate half cycles zones for the givetian stage and an increased age to 5 6 myr these zones allow high correlation among sections to better resolve timing of major bio events and provide a floating point time scale to which absolute ages can be applied

high resolution magnetic susceptibility and geochemistry for the cenomanian turonian boundary gssp with correlation to time equivalent core abstract the upper cretaceous cenomanian turonian c t boundary represents the most significant thermal climatic event in the mesozoic and it has been well studied here we present a relatively new approach for providing both independent climate based cyclostratigraphic analysis and correlation among c t sections we have collected samples for bulk initial low field magnetic susceptibility ms measurement and high resolution inorganic geochemical analyses from two c t sequences 1 the well defined c t global boundary stratotype section and point gssp located at lake pueblo colorado and deposited in the upper cretaceous western interior seaway of north america and 2 the well studied marlstone and limestone usgs 1 portland core drilled at portland colorado 40 km to the west of the c t gssp comparing the lithostratigraphy chemostratigraphy and magnetostratigraphic susceptibility developed from magnetic susceptibility ms data indicates that the usgs 1 portland core closely compares with the c t gssp sequence using any of the three measures demonstrating that intervals collected and sampled from these two sequences are highly correlated a preliminary magnetostratigraphic susceptibility zonation for the c t boundary is presented to which other ms work can be compared fourier transform ft analysis of the ms data from the gssp when compared with previous ft work using geochemical analysis of portland core samples shows close similarities to the gssp with milankovitch eccentricity obliquity and precession bands well defined in both data sets using this result we assign relative ages to the intervals sampled and evaluate changes in absolute sediment accumulation rates through the c t boundary interval we show that while sediment accumulation rate is relatively high in the upper cenomanian a change toward lower accumulation rates in the gssp section at the c t boundary is due to reduced productivity following the c t extinction events that resulted from global warming associated with oceanic anoxic event 2 oae2 that began in the uppermost cenomanian

gulf of mexico sediment sources and sediment transport trends from magnetic susceptibility measurements of surface samples abstract the magnetic properties from 200 trigger core top and van veen grab sediment samples recovered from throughout the gulf of mexico have been analyzed and used to characterize sediment source and flow pattern distributions magnetic parameters included are anhysteretic remanent magnetism arm and magnetic susceptibility ms measurements results from these measurements are compared to previously determined calcium carbonate percentages and clay and hematite influx trajectories into the gulf of mexico for the same samples reported by balsam and beeson balsam w l and beeson j p 2003 sea floor sediment distribution in the gulf of mexico deep sea res i 50 1421 1444 the arm results give an estimate of magnetic grain size distributions and by analogy grain size distributions in general whereas ms patterns show high detrital sediment accumulation zones within the gulf the dominant influx of modern high susceptibility sediment into the gulf of mexico appears to originate from the red river flow into atchafalaya river basin and out into the gulf from atchafalaya bay with significant additional contributions from the mississippi river through the southwest pass of the mississippi river delta this material then moves across the continental shelf and down through the mississippi canyon into the deep gulf where it is redistributed at depths 3600 m the eastern shelf margins in the gulf offshore from alabama and florida are accumulating calcite or quartz rich medium to fine grained sediment that has a very low or diamagnetic ms signature from the louisiana to texas gulf coast margins ms is moderate to high suggesting a river influx of magnetic constituents from the volcanic fields in new mexico and from igneous and metamorphic sources in the mississippi basin offshore from western mexico the ms is high to moderate but the yucatan shelf margin is characterized by low to diamagnetic ms values due to sediment dominated by calcite sands and oozes a trend that continues to the east onto the west florida shelf additional measurements of samples collected in association with sites characterized by hydrocarbon seepage exhibit anomalously low ms values the samples from the lower shelf and slope areas are typified by iron reduction by bacterial organisms in these samples these results produce anomalous localized lows in the ms trends observed

astronomic calibration of the late eocene early oligocene massignano section central italy we present an astronomically tuned polarity timescale for the late eocene early oligocene based on a cyclic limestone marls sedimentary succession exposed at massignano italy gssp for the eocene oligocene boundary here we applied spectral analysis techniques to the high resolution magnetic susceptibility record the results provide clear evidence of a regular and cyclic signal that allows us to discard a stochastic mechanism as controlling these fluctuations conversely these outcomes reveal the presence of stable and notable periodicities that are in tune with those associated with orbital forcing through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we refine the late eocene portion of the gpts through astronomical tuning of the rhythmic sedimentary alternations to recent astronomically calculated variations of earth s orbit we recalibrated the polarity boundaries ages and the main stratigraphic events recognized throughout the sedimentary record among these an age of 33 714 ma is proposed for the e o boundary

testing high resolution magnetic susceptibility and gamma radiation methods in the cenomanian turonian upper cretaceous gssp and near by coeval section abstract low field mass specific magnetic susceptibility and gamma radiation spectra grs field and laboratory data sets for two upper cretaceous marine sedimentary sequences representing the cenomanian turonian c t boundary in the western interior seaway in central colorado have been measured sections including the c t global boundary stratotype section and point gssp and a one kilometer distant co eval sequence the s section it has previously been established that both and grs data sets are controlled by detrital eolian fluxes into the marine environment although the effect of weathering if any on these parameters when exposed in outcrop is not well documented in addition these parameters are controlled by differing detrital eolian components that may be derived from different sources or be differentially concentrated within the marine system of great interest is 1 how well these data sets record climate cycles 2 how the and grs data are affected by weathering and 3 how well the two methods support each other reported here are the results of a number of field and laboratory experiments designed to evaluate instruments and how and grs data sets co vary and to test their usefulness as correlation tools in stratigraphy results show that both and 40k data sets from laboratory measurement on samples collected in the field where rocks are relatively fresh are generally well correlated respond closely to climate and the corresponding detrital eolian flux into the marine environment and can be used for a rough proxy for each other however weathering appears to affect the 40k data by removing 40k thus reducing correlation potential although does not appear to suffer as badly in more weathered sections and good correlations between sections remain field grs measurements produce a broadly smoothed data set that regionally can be very useful for correlation although potential global correlations are not tested here using data reported here and comparing these data to a 100 kyr cyclic eccentricity e1 climate model for the upper cretaceous at 93 9 ma shows that and 40k data measured in the laboratory can be used to build floating point time scales that produce essentially equivalent results as do other studies using absolute dates

reactive transport modelling of hydrothermal dolomitisation using the csmp gem coupled code effects of temperature and geological heterogeneity abstract reactive transport simulations using our csmp gem coupled code were applied to study the major controls on replacement dolomitisation and the development of dolomite geobodies in a hydrothermal setting a series of 2d simulations show how elevated temperature and reactive surface area increase the rate of dolomitisation and result in a dolomite replacement front that is both sharper and inclined at a higher angle from vertical this inclination an effect of gravity segregation is apparent in thick homogeneous units but in layered systems the lithological contrast determines the shape of the dolomite front the increase in permeability resulting from porosity generation upon replacement of calcite by dolomite has a major effect on accelerating the overall progress of dolomitisation in contrast the changes in fluid density due to chemical reactions and the pressure dependence of thermodynamic data have a minor influence under simulated conditions primary dolomite forms slowly after complete replacement of host calcite leading to porosity decrease and is only locally important around the source of the hydrothermal fluid for a simple layered system our model results are in excellent agreement with those obtained using toughreact code they do however show the advantage of unstructured triangular over structured rectangular meshes for resolving complex curved inclined front shapes such meshes also offer benefits in simulating fault controlled hydrothermal dolomitisation our simulations predict dolomite geobodies comparable in scale and morphology to natural examples documented at outcrops and underline the importance of understanding the permeability structure within and around the fault zone

c isotope stratigraphy and paleoenvironmental changes across oae2 mid cretaceous from shallow water platform carbonates of southern mexico the stratigraphic and geochemical record of the mid cretaceous cenomanian turonian oceanic anoxic event 2 oae2 has been studied in numerous tethyan and proto atlantic hemi pelagic pelagic successions but little data comes from nearshore carbonate successions from the proto pacific region here we present the results of a combined stratigraphic and 13c study of c t platform carbonates from southern mexico which were deposited within the proto pacific two scales of sedimentary cyclicity are recognized high frequency peritidal and subtidal cycles 0 4 8 m display little evidence of cycle capping subaerial exposure and are not correlative between sections these relationships suggest that the amplitudes of high frequency sea level changes were minimal during the peak mid cretaceous greenhouse longer term transgressive regressive sequences 18 40 m are correlated between sections and using 13c trends can be correlated with sequences developed in northern europe and india the mexican successions were sampled at a high resolution 10 ky for stable isotopes inorganic organic carbon and oxygen total organic carbon insoluble residues and trace metals the 13ccarb curve matches global trends including 6 distinct isotopic stages permitting identification of oae2 despite the lack of characteristic anoxic facies using the 13ccarb trends we tie the previously identified ammonite planktonic foram and nannofossil biostratigraphy from england and the western interior seaway of colorado into the mexican sections the initiation of oae2 defined by an abrupt positive 3 4 13c shift coincides with a long term sea level rise though the sedimentary expression of the deepening is no greater than that observed for any of the other sea level events across the studied interval oae2 termination transition from gradually decreasing to background 13c values is not associated with a particular sea level trend stratigraphic changes in insoluble residues proxy for continental sediment discharge across oae2 are not correlative between sections and do not show consistent systematic relationships with 13c or sea level variations therefore do not support the hypothesis that oae2 was associated with increased continental derived nutrient influx two peaks in trace metal concentrations coincide with the abrupt increase in 13c ratios onset of oae2 and during the transition from elevated to decreasing 13c values near the c t stage boundary these trends are similar to those recorded in coeval deposits of the western interior seaway and are consistent with the hypothesis that oae2 development was related to the release of reduced metals during the short lived 1 my caribbean oceanic plateau basalt eruption in this scenario oxidation of the metals depleted the existing low dissolved o2 concentrations and thermally buoyant plumes of seawater enriched in biolimiting elements mixed with surface waters stimulated primary productivity and further reduced o2 concentrations leading to widespread anoxia and a large positive 13c shift

paleoclimate proxies for cyclostratigraphy comparative analysis using a lower triassic marine section in south china abstract multiple proxies for paleoclimatic and paleoenvironmental change within sedimentary sequences have been developed but understanding their relationships and their relative strengths or weaknesses are required to access a wealth of untapped paleoclimate information we propose two main criteria for multiple proxy datasets to estimate their individual signal to noise levels and to decipher the relationship between multiple proxies first for proxies affected by similar processes then their time dependent changes should be similar second a paleoclimate proxy that is more sensitive to an external forcing such as astronomical forcing is more useful than those that do not have such a response we introduce two methods to evaluate these criteria 1 hierarchical cluster analysis hca probes the relationships among multiple proxies based on similarities of their oscillation patterns and 2 power decomposition analysis pda tests proxy sensitivity to external climate forcing we evaluate 16 high resolution paleoclimate proxies in detail for their applications in paleoclimatology from the marine lower triassic daye formation at the daxiakou section in the three gorges region of south china which has carbonate claystone cycles deposited on an outer ramp the proxies are spectral gamma ray gamma ray intensity potassium uranium thorium thorium uranium and thorium potassium rock color lightness l redness a and yellowness b magnetic susceptibility measured in both laboratory and outcrop anhysteretic remanent magnetization arm lithologic rank simplified lithologic rank and non carbonate fraction carbonate thickness and couplet thickness hierarchical cluster analysis hca gathers these proxies into groups likely affected by the same process the arm and thorium uranium proxies seem to reflect the hinterland weathering process during the early triassic gamma ray potassium uranium thorium magnetic susceptibility and non carbonate fraction proxies refer to terrestrial input the l a and lithologic rank proxies indicate the productivity the redox state and the relative sea level respectively at this section this case study contributes to the understanding of the sensitivity of these types of proxies within marine strata for deep time paleoclimate and astronomical tuned time scales

upper devonian carbonate platform correlations and sea level variations recorded in magnetic susceptibility abstract this paper concerns the analysis of four outcrops in the frasnian upper devonian of belgium we compare the results of a precise sedimentological analysis with magnetic susceptibility ms data this comparison allows us to improve stratigraphic correlations and to test the relationship between magnetic susceptibility and sea level changes considering the sedimentological study different microfacies have been identified from the external belt dominated by relatively argillaceous open marine facies with crinoids to the biostromal and the lagoonal belt dominated by algae rich muddy facies fourth and third order sequences have also been identified and are probably related to sea level variations magnetic susceptibility data provides us very good fourth order correlations and the link between magnetic susceptibility and different sedimentological parameters is obvious more precisely ms appears to be related to fourth and third order sequences and to microfacies

midlatitude shelf seas in the cenomanian turonian greenhouse world temperature evolution and north atlantic circulation 1 an 8 million year record of subtropical and midlatitude shelf sea temperatures derived from oxygen isotopes of well preserved brachiopods from a variety of european sections demonstrates a long term cenomanian temperature rise 16 20 c midlatitudes that reached its maximum early in the late turonian 23 c midlatitudes superimposed on the long term trend shelf sea temperatures vary at shorter timescales in relation to global carbon cycle perturbations in the mid cenomanian and the late turonian two minor shelf sea cooling events 2 3 c coincide with carbon cycle perturbations and times of high amplitude sea level falls although this evidence supports the hypothesis of potential glacioeustatic effects on cretaceous sea level the occurrence of minimum shelf sea temperatures within transgressive beds argues for regional changes in shelf sea circulation as the most plausible mechanism the major carbon cycle event in the latest cenomanian oceanic anoxic event 2 is accompanied by a substantial increase in shelf sea temperatures 4 5 c that occurred 150 kyr after the commencement of the 13c excursion and is related to the spread of oceanic conditions in western european shelf sea basins our oxygen isotope record and published 18o data of pristinely preserved foraminifera allow the consideration of north atlantic surface water properties in the cenomanian along a transect from the tropics to the midlatitudes on the basis of fossil derived 18o estimated w ranges and modeled salinities temperature salinity density ranges were estimated for tropical subtropical and midlatitude surface waters accordingly the cenomanian temperate shelf seas waters have potentially the highest surface water density and could have contributed to north atlantic intermediate to deep waters in the preopening stage of the equatorial atlantic gateway

holocene millennial scale productivity variations in the sicily channel mediterranean sea 1 the calcareous nannofossil assemblages of ocean drilling program hole 963d from the central mediterranean sea have been investigated to document oceanographic changes in surface waters the studied site is located in an area sensitive to large scale atmospheric and climatic systems and to high and low latitude climate connection it is characterized by a high sedimentation rate the achieved mean sampling resolution is 70 years that allowed the sicily channel environmental changes to be examined in great detail over the last 12 ka bp we focused on the species florisphaera profunda that lives in the lower photic zone its distribution pattern shows repeated abundance fluctuations of about 10 15 such variations could be related to different primary production levels given that the study of the distribution of this species on the sicily channel seafloor demonstrates the significant correlation to productivity changes as provided by satellite imagery productivity variations were quantitatively estimated and were interpreted on the basis of the relocation of the nutricline within the photic zone led by the dynamics of the summer thermocline productivity changes were compared with oceanographic atmospheric and cosmogenic nuclide proxies the good match with holocene master records as with ice rafted detritus in the subpolar north atlantic and the near 1500 year periodicity suggest that the sicily channel environment responded to worldwide climate anomalies enhanced northern hemisphere atmospheric circulation which has been reported as one of the most important forcing mechanisms for holocene coolings in previous mediterranean studies had a remarkable impact on the water column dynamics of the sicily channel

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

diagenesis of the devonian southesk cairn carbonate complex alberta canada marine cementation burial dolomitization thermochemical sulfate reduction anhydritization and squeegee fluid flow abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc is located at depths of about 5000 m adjacent to the disturbed belt of the rocky mountains rising to depths of 3000 m in a northeastward direction over a distance of 150 km the sccc produces oil and gas with up to about 30 h2s from several stratigraphic levels the diagenesis of the sccc was investigated by means of petrography and geochemistry stable isotope compositions were determined for fibrous calcite cements matrix dolomite saddle dolomite and sparry calcite fibrous calcites have stable isotope values of 18o 7 5 to 4 5 pdb and 13c 2 0 1 0 pdb matrix dolomite samples have stable isotope values of 18o 8 1 to 4 9 pdb and 13c 0 to 1 0 pdb saddle dolomite samples range from 18o 9 8 to 7 1 pdb and 13c 1 0 to 0 pdb sparry calcite samples have 18o 13 7 to 7 2 pdb and 13c 24 0 to 0 pdb the sr isotope values of all analyzed samples span a broad range from 0 7080 to 0 7320 showing considerable scatter but also one distinctive pattern i e values higher than 0 7220 occur only at depths in excess of about 4000 m whereas lower values scatter throughout the entire depth range furthermore the sr isotope values of sparry calcites decrease northeastward similarly the fluid inclusion homogenization temperatures of these calcites display a general decrease updip whereas the salinities show no general spatial trends the uncorrected homogenization temperatures range from about 85 to 180 c and the nacl equivalent salinities range from about 17 to 24 weight the d values of brines obtained from seven wells range from 79 to 20 smow and their 18o values range from 1 5 to 8 7 smow the sulfur isotope values of the dissolved sulfate ranges from 34s 10 1 to 24 5 cdt the sr isotope values of the brine samples range from 0 7109 to 0 7255 elemental sulfur isotope values cluster tightly around 34s 18 0 to 18 4 cdt the values of secondary anhydrites range from 34s 18 4 to 26 7 cdt values for solid bitumen vary from 34s 19 5 to 20 2 cdt integration of all data reveals a complex diagenetic evolution from the middle devonian to the recent the paragenetic sequence consists of 24 distinct diagenetic phases which have been grouped into 5 diagenetic stages stage i density driven convection in near surface settings likely superimposed on and or aided by tidal pumping facilitating marine cementation stage ii cellular convection facilitating pervasive matrix dolomitization aided by compaction stage iii hydrocarbon migration stage iv thermochemical sulfate reduction forming sour gas and regional squeegee flow forming sparry calcite cements and localized flow through faults stage v fluid flow driven by erosional rebound in the overlying mesozoic strata and remnant squeegee flow this study yielded several important results 1 the prolific repertoire of diagenetic textures found in the southesk cairn carbonate complex serves as a means of comparison for petroliferous carbonate complexes elsewhere especially in sour gas provinces close to orogenic fronts 2 the diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity 3 both thermochemical sulfate reduction and squeegee type fluid flow are recorded in late diagenetic sparry calcite cements 4 the effect of the various burial diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable specifically matrix dolomitization thermochemical sulfate reduction and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex on the other hand squeegee fluid flow was insignificant in terms of affecting the reservoir properties

geothermal springs of the west florida continental shelf evidence for dolomitization and radionuclide enrichment abstract on the sea bed of the west florida continental shelf about 45 km ssw of ft myers florida an 85 km 2 area has been discovered in which six thermal springs discharge warm chemically altered seawater from vents and seepage zones the spring water apparently originates in the subsurface ocean around the florida platform and penetrates the highly porous strata of the platform about 500 1000 meters below sea level it percolates toward the interior of the platform and is geothermally heated to about 40 c en route then it rises along more vertical flow channels and is discharged in warm submarine springs beneath the platform several chemical processes alter the percolating seawater one process seems to be a secondary dolomitization of the limestone of the platform because in the discharging seawater magnesium is lower by 2 7 mmole kg and calcium higher by 3 6 mmole kg than in normal seawater with the same chlorinity other reactions within the sediments of the platform enrich the spring effluents 1000 fold in 226 ra 10 000 fold in 222 rn and 90 fold in 228 ra compared to the seawater surrounding the platform thus the springs may be important sources of radionuclides for the gulf of mexico the percolating seawater also loses all of its oxygen and nitrate to reduction processes loses most of its phosphate and 40 of its 238 u and roughly quadruples its silica content coastal carbonate platforms are fairly common geological features thus processes like those beneath the west florida shelf may function on a world wide basis to play an important role in the diagenesis of carbonate sediments

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

compaction and decompaction algorithms for sedimentary carbonates abstract the mechanical compaction behavior of shallow marine carbonate sediments has been investigated to develop carbonate compaction algorithms and decompaction curves that can be used in stratigraphic restoration backstripping analyses and quantitative models for simulation of carbonate stratigraphy deep water chalks and calcareous oozes are not treated here empirical porosity depth data for carbonate sediments have been compiled and compared with experimentally derived compaction curves to derive appropriate porosity depth curves for shallow water carbonate muds and sands the curves are exponential in form this review indicates that mud rich shallow marine carbonates behave differently than sand rich grain supported sediments with regard to reduction in porosity and thickness quantitative analysis of compaction behavior in carbonate sediments reveals that barring early cementation carbonate muds suffer significant thickness reduction early in their burial history and at shallow depths e g 50 thickness reduction with about 150 200 m overburden however primary porosities of carbonate muds approximately 70 80 are reduced only to about 42 under that overburden thus the potential exists for significant retention of primary porosity in carbonate muds in the form of microporosity this indicates that interparticle cementation with or without chemical compaction is required to transform muddy carbonate sediments into lithified rock in the absence of early lithification carbonate sands compact more slowly than muds primary interparticle porosities of about 40 45 can be maintained in the first 200 300 m of burial with little thickness reduction

orbital time scale and new c isotope record for cenomanian turonian boundary stratotype previous time scales for the cenomanian turonian boundary ctb interval containing oceanic anoxic event ii oae ii vary by a factor of three in this paper we present a new orbital time scale for the ctb stratotype established independently of radiometric biostratigraphic or geochemical data sets update revisions of ctb biostratigraphic zonation and provide a new detailed carbon isotopic record for the ctb study interval the orbital time scale allows an independent assessment of basal biozone ages relative to the new ctb date of 93 55 ma gts04 the d 13 corg data document the abrupt onset of oae ii significant variability in d 13 corg values and values enriched to almost 222 these new data underscore the difficulty in defining oae ii termination using the new isotope curve and time scale estimates of oae ii duration can be determined and exported to other sites based on integration of well established chemostratigraphic and biostratigraphic datums the new data will allow more accurate calculations of biogeochemical and paleobiologic rates across the ctb

an integrated biostratigraphy conodonts and foraminifers and chronostratigraphy paleomagnetic reversals magnetic susceptibility elemental chemistry carbon isotopes and geochronology for the permian upper triassic strata of guandao section nanpanjiang basin south china abstract the chronostratigraphy of guandao section has served as the foundation for numerous studies of the end permian extinction and biotic recovery in south china guandao section is continuous from the permian triassic boundary to the upper triassic conodonts enable broad delineation of stage and substage boundaries and calibration of foraminifer biostratigraphy as follows changhsingian griesbachian first hindeodus parvus and first appearance of foraminifers postcladella kalhori and earlandia sp griesbachian dienerian first neospathodus dieneri and last appearance of foraminifer p grandis dienerian smithian first novispathodus waageni and late dienerian first appearance of foraminifer hoyenella ex gr sinensis smithian spathian first nv crassatus and last appearance of foraminifers arenovidalina n sp and glomospirella cf vulgaris spathian aegean first chiosella timorensis and first appearance of foraminifer meandrospira dinarica aegean bithynian first nicoraella germanica and first appearance of foraminifer pilammina densa bithynian pelsonian after last neogondolella regalis prior to first paragondolella bulgarica and first appearance of foraminifer aulotortus eotriasicus pelsonian illyrian first pg excelsa and last appearance of foraminifers meandrospira deformata and pilamminella grandis illyrian fassanian first budurovignathus truempyi and first appearance of foraminifers abriolina mediterranea and paleolituonella meridionalis fassanian longobardian first bv mungoensis and last appearance of foraminifer a mediterranea longobardian cordevolian first quadralella polygnathiformis and last appearance of foraminifers turriglomina mesotriasica and endotriadella wirzi the section contains primary magnetic signature with frequent reversals occurring around the permian triassic olenekian anisian and anisian ladinian boundaries predominantly normal polarity occurs in the lower smithian bithynian and longobardian cordevolian predominantly reversed polarity occurs in the upper griesbachian induan olenekian pelsonian and lower illyrian reversals match well with the gpts large amplitude carbon isotope excursions attaining values as low as 2 9 13c and high as 5 7 13c characterize the lower triassic and basal anisian values stabilize around 2 13c through the anisian to carnian similar signatures have been reported globally magnetic susceptibility and synthetic gamma ray logs show large fluctuations in the lower triassic and an overall decline in magnitude of fluctuation through the middle and upper triassic the largest spikes in magnetic susceptibility and gamma ray indicating greater terrestrial lithogenic flux correspond to positive 13c excursions high precision u pb analysis of zircons from volcanic ash beds provide a robust age of 247 28 0 12 ma for the olenekian anisian boundary at guandao and an age of 251 985 0 097 ma for the permian triassic boundary at taiping together the new u pb geochronology from the guandao and taiping sections suggest an estimated duration of 4 71 0 15 ma for the early triassic epoch

a 164 ka record of environmental change in the american southwest from a carlsbad cavern speleothem abstract a horizontal core 2 8 m in length drilled from the georgia giant column in carlsbad cavern provides climate information for the last 164 ka forty six alpha spectrometric u series ages determined at intervals of 7 6 cm along the core indicate five periods of deposition and five hiatuses the longest from 136 to 110 ka variations in growth rate 0 to 70 mm ka in the abundance of aragonite chalcedony and fe bearing phases and in 13c indicate that glacial intervals of the last 164 ka ois 6 4 and 2 were much wetter than today as were the colder substages 5d and 5b of ois 5 by contrast during the two warmest periods of the past 164 ka namely ois 5e and 1 there was no deposition on either side of the speleothem suggesting conditions as dry or drier than today the record from carlsbad parallels data from many other sites in the southwestern usa and northern mexico and data from marine sediments and ice cores demonstrating the extent to which ice sheet fluctuations influenced conditions in southern new mexico detailed correlation of 13c values in the georgia giant which range from 6 6 to 0 9 relative to pdb with distant speleothem records and with data from ice cores further documents the linkage of southwestern climate with global scale extent of glaciation values of 18o in the georgia giant core range from 9 7 to 4 7 relative to pdb and average 6 6 18o depleted carbonate in the georgia giant during ois 6 at a time when the world s oceans were enriched in 18o suggests that precipitation during cold intervals was brought largely by pacific air masses in fall winter and spring as a result of the southward displacement of the polar jet stream by the growth of the laurentide ice sheet termination ii marking the end of the penultimate glaciation is well defined in the core s 18o data analytical uncertainties in the radiometric ages do not preclude a start of termination ii as late as 128 ka as suggested by specmap data however data from the georgia giant core are more compatible with an earlier start like that obtained from devils hole and vostok data raising the possibility that early warming was widespread in the u s southwest region by 145 ka

the magneto susceptibility event and cyclostratigraphy msec method used in geological correlation of devonian rocks from anti atlas morocco magneto susceptibility event and cyclostratigraphy msec is used here as a means of establishing sensitive chronostratigraphic markers and chronohorizons useful for high resolution correlation msec is a composite of the magnetic susceptibility ms record of marine strata and the coeval biostratigraphic record and similar to chronozones zones based on msec data have boundaries that are isochronous ms a measure of the concentration of magnetic grains in sediments proxies for the ratio of lithogenic to biogenic components controls on the detrital input of lithogenic material include eustasy and climate however induced and sea floor basin subsidence the development of an initial msec composite reference curve for the middle upper devonian of the anti atlas region of southern morocco is presented and discussed in the context of global sea level transgressive and regressive cycles msec trends composed of increasing ms magnitudes correlate well with episodes of regression whereas trends of decreasing ms magnitudes correlate with episodes of transgression