an alternative model for positive shifts in shallow marine carbonate 13c and 18o positive shifts in global seawater d 13 cdic are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in d 18 o here an alternative model is proposed for the formation of local positive isotope shifts in shallow marine settings the model involves geochemically altered platform top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition both mechanisms are active on modern sub tropical carbonate platforms and result in low carbonate d 13 ca ndd 18 o relative to typical oceanic values during high amplitude transgressive events the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced and 13 c depleted platform top water mixes with open oceanic water masses having higher isotope values both factors are recorded as a transient increase in carbonate 13 c and 18 o relative to low background values these processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas

diagenesis of devonian reefal carbonates in the oscar range canning basin western australia abstract the oscar range part of the extensive devonian reef complex of the northern canning basin is a large atoll 80 10 km in which frasnian and famennian late devonian reefs grew around an emergent island of precambrian metasedimentary rocks many sediments in the oscar range had high depositional porosities but this porosity was largely occluded by diagenetic alteration diagenesis included marine cementation dolomitization pressure solution burial cementation and karstification field mapping petrography cathodoluminescence and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected oscar range sediments earliest cementation occurred in the marine environment where reef margin and associated subfacies developed abundant non luminescing radiaxial fibrous and microcrystalline calcite cements based on convergent covariant isotopic trends marine cements had a characteristic late devonian oxygen signature of 4 5 0 5 pdb and a carbon signature of 2 0 0 5 pdb in well cemented parts of the reef complex synsedimentary neptunian fissures formed and were filled with cement and sediment cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes marine diagenesis was followed by marine burial and possible meteoric phreatic cementation and dolomitization marine cements are overgrown by non luminescing scalenohedral cements that grade into banded bright and non luminescing blocky calcite cements that are common on the platform but are missing in many reef margin and marginal slope deposits banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform the reef complexes however were generally drowned reefs with little chance for prolonged meteoric exposure this sedimentologic evidence suggests that banded luminescing cements may have formed in the marine burial environment the next cement generation consists of non luminescing non ferroan bloc y calcites that occur throughout the reef complex stratigraphic relationships combined with isotopic results 18o 6 0 to 9 0 13c 2 0 suggest that this spar is a burial cement resulting from late devonian through early carboniferous burial the reef complex was exhumed and eroded in the late carboniferous dedolomite dolo moldic porosity pisolitic calcrete crusts iron oxide stained blocky cements and deep 200 m sinkholes formed at the unconformity surface non ferroan irregularly banded moderate to bright luminescing blocky calcite precipitated throughout the reef complex unconformity related cements calcrete crusts and dedolomites have distinctive isotopic signatures that show little variation in oxygen 7 0 to 9 0 but have a broad range of carbon values 1 5 to 8 0 highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface in post carboniferous time most of the remaining porosity was filled with non ferroan bright luminescing blocky calcite that formed by pressure solution reprecipitation during burial isotopic analyses 18o 8 0 to 12 0 13c 1 5 support a burial origin for this generation of cements these cements fill primary porosity tension gashes along stylolites and hairline fractures along which hematite is leached a final generation of ferroan homogeneous dull to non luminescing blocky calcite cement formed in a late burial event that according to isotopic evidence 18o 10 5 to 15 0 13c 2 0 to 9 0 was probably related to movement of basinal brines through the reef complexes

cometary impacts into ocean their recognition and the threshold constraint for biological extinctions the montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied the impact crater is 45 km in diameter and was formed at 50 8 ma by a fall of probably an old cometary nucleus 3 4 km in diameter into shallow 600 m ocean comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim instead the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent return water flow into the excavating cavity by this process most of the fall out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land at a microscopic scale the shock metamorphism features are about the same as those for land impacts geochemically impacts of comet nucleii may not leave a recognizable signature at the impact horizon except for a minor increase in iridium thus stratigraphic horizons associated with extinctions and or major changes in biota have to be closely examined for other impact indicators like the presence of tectites glass spherules and quartz grains with shock features occurrence of megatsunami wave deposits extensive erosion on continental margins margin failures and faunal mixing above erosional unconformities are other potential impact indicators there is no single indicator that can provide sufficient proof of an impact event such interpretations have to be based on multiparameter studies of global extent since many of the impact indicators are only of regional extent the lack of extinction of any marine plankton genera or of bottom dwellers at the montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus 4 km in diameter the calculated frequency for a cometary impact which could result in a 10 extinction of marine genera is about 6 10 7 yr 1 and for the k t boundary type of extinctions about 2 10 8 yr 1 even allowing for a large degree of uncertainty in these estimates it is unlikely that the biological extinction events for the last 250 ma identified by sepkoski 1990 could have been all caused by meteorite impacts

carbon isotope chemostratigraphy and precise dating of middle frasnian lower upper devonian alamo breccia nevada usa abstract at hancock summit west nevada western usa uppermost givetian upper middle devonian and lower and middle frasnian lower upper devonian rocks of the lower guilmette formation include in stratigraphic sequence carbonate platform facies of the conodont falsiovalis transitans and punctata zones the type alamo breccia member of the middle punctata zone and slope facies of the punctata and hassi zones the catastrophically deposited alamo breccia and related phenomena record the 382 ma alamo event produced by a km scale bolide impact into a marine setting seaward of an extensive carbonate platform fringing western north america re evaluation of conodonts from the lower guilmette formation and alamo breccia member together with regional sedimentologic and conodont biofacies comparisons now firmly locates the onset of the johnson et al 1985 transgressive regressive t r cycle iic which occurred after the start of the punctata zone within a parautochthonous megablock low in the alamo breccia whole rock carbon isotope analyses through the lower guilmette formation and alamo breccia member reveal two positive 13ccarb excursions 1 a small 3 excursion which is possibly correlative with the falsiovalis event previously identified from sections in western europe and australia occurs below the breccia in the upper falsiovalis zone to early part of the transitans zone and 2 a large multi part excursion dominated by a 6 positive shift begins above the start of the punctata zone and onset of t r cycle iic and continues above the alamo breccia ending near the punctata hassi zonal boundary this large excursion correlates with the punctata event a major positive 13c excursion previously recognized in eastern laurussia and northern gondwana consistent with previous studies at hancock summit west the punctata event is apparently not associated with any regional extinctions or ecosystem reorganizations in the study area onset of the main punctata event began after the start of both the punctata zone and t r cycle iic and preceded the alamo impact by less than 650 k y as inferred from conodont biochronologic and regional rock accumulation rate estimates although complicated by the heterolithic high energy deposits of the alamo breccia the carbon isotope record of the breccia and post breccia beds does not indicate a major impact correlative perturbation to the carbon cycle this study extends recognition of the punctata event to western laurussia further reinforcing the potential global scale of the event and its potential importance to understanding early to middle frasnian marine geochemistry and palaeoenvironments based on previous models and our observations increased tectonic activity increased nutrient flux to oceans increased marine bioproductivity widespread anoxia and increased organic carbon burial were all likely key factors in driving the punctata event excursion furthermore periodic eustatic and regional relative sea level rises may have played an important role in promoting organic carbon burial and in maintaining a link between the primary open marine geochemical signal and that recorded on the shallow marine lower guilmette carbonate platform

temporal spatial reconstruction of the early frasnian late devonian anoxia in nw africa new field data from the ahnet basin algeria anoxic conditions were widespread in nw africa during the early frasnian late devonian that resulted in deposition of organic rich shales and limestones with total organic carbon toc values of up to 14 organic richness and thickness of these sediments vary laterally and organic rich vs organic poor facies boundaries are likely to have been diachronous a precise temporal spatial reconstruction of this anoxic phase in nw africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts this contribution is based on eight outcrop sections at the margin of the central algerian ahnet basin where detailed spectral gammaray measurements were carried out using a handheld instrument the pre weathering organic richness in frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that based on well studies is positively correlated with the total organic carbon content conodont biostratigraphic results from these sections suggest that the uraniumenriched interval the anoxic interval at the basin margin is most common in the basal frasnian conodont biozones 1 2 confirming previous results from the anti atlas in morocco in three of the eight localities studied the basal frasnian has not been deposited and the frasnian here commences with distinctly younger uranium enriched intervals including zones 4 11 well data from the eastern algerian berkine basin is interpreted to indicate a significantly longer anoxic phase there million yearscale diachroneities of the frasnian anoxia therefore clearly exist across the north africa shelf it is assumed that the palaeorelief might have been a major factor in controlling the onset duration and intensity of anoxia in the region d 2003 elsevier b v all rights reserved

calibrating the devonian time scale a synthesis of u pb id tims ages and conodont stratigraphy the recalibrated devonian time scale represents an important improvement of devonian chronology regarding two aspects firstly a data set of 13 biostratigraphically well bracketed u pb id tims zircon and monazite ages constitutes the framework of the scale secondly approximately time linear biostratigraphic scales have been used for the interpolation between the isotopic ages the new construction thus represents a biochronometric time scale which allows the assignment of numeric ages not only to stage boundaries but also to each biozone boundary the method of interpolation applied also enables the projection of the geochronological error onto the time scale according to the newly calibrated scale the devonian lasted 57 4 5 7 ma from 418 1 3 0 to 360 7 2 7 ma this is compared to previously published time scales the longest time span ever calculated for this period the age interpolations for the eight stage boundaries are famennian tournaisian devonian carboniferous 360 7 2 7 ma frasnian famennian 376 1 3 6 ma givetian frasnian 383 7 3 1 ma eifelian givetian 388 1 2 6 ma emsian eifelian 391 9 3 4 ma pragian emsian 409 1 3 8 ma lochkovian pragian 412 3 3 5 ma pridolian lochkovian silurian devonian 418 1 3 0 ma full size table table options view in workspace download as csv the duration of the middle devonian is calibrated here as quite short with 8 2 ma and the emsian and the famennian are the longest stages with interpolated durations of 17 2 and 15 4 ma respectively together all devonian stages contain 57 conodont zones including subzones according to the standard conodont zonation giving a mean duration of about 1 ma in the highly resolved part of the time scale mid eifelian to the devonian carboniferous boundary zonal resolution averages 0 6 ma in contrast the lowest resolution is shown in the early emsian to mid eifelian interval with zonal durations of up to 5 5 ma serotinus zone

carbon isotope geochemistry of the frasnian famennian transition abstract carbon isotope records for inorganic and total organic carbon toc as well as for individual organic biomarkers show two positive excursions in the late frasnian with comparable shape and amplitude in 13c the positive shifts in 13c can be correlated with the deposition of the kellwasser horizons the inorganic carbon isotope excursions are explained by an enhanced organic carbon burial that is expected to lower the concentration of oceanic dissolved co2 toc 13c values do not exhibit a larger shift than that recorded by inorganic carbon 13c values this is unexpected since a decrease in the dissolved co2 concentration should result in lower carbon isotope fractionation during photosynthesis ep 13c values of presumed algal biomarkers low molecular weight n alkanes pristane phytane although offset by 2 5 exhibit essentially identical records as toc and confirm this unexpected result it is proposed that high atmospheric and oceanic co2 concentrations during the devonian resulted in maximum photosynthetic fractionation any change in the co2 concentration would thus not affect ep as such 13c of primary organic carbon and 13ccarb will exhibit parallel excursions the data imply that carbon isotopes and the relationship between ep and co2 will not be effective to study changes in pco2 levels during the devonian since co2 concentrations were too high

evidence for perturbation of the carbon cycle in the middle frasnian punctata zone late devonian new carbon isotopic data from the devonian of ardennes belgium and partly from the holy cross mountains poland highlight an abrupt and high amplitude negative excursion in the punctata conodont zone published information from moravia and china suggests that this middle frasnian negative excursion jointly with the preceding large scale positive shift should be used as a global chemostratigraphic marker causation scenarios for this negative punctata event are correlated neither with major biota turnover nor major sea level changes but may be related to 1 the alamo impact event that led to 2 the massive dissociation of methane hydrates and 3 the rapid onset of global warming

conodont biofacies patterns in the kellwasser facies upper frasnian lower famennian of the eastern anti atlas morocco abstract the abundant conodont fauna of a black limestone and shale lithology kellwasser facies of late frasnian early famennian age provides a significant example of environmental control on conodont distribution and abundance conodont collections from the upper gigas and upper crepida zones of the eastern anti atlas were examined biofacies maps for the conodont genera palmatolepis polygnathus icriodus ancyrodella and ancyrognathus were constructed and their facies distribution patterns analysed no significant restriction in conodont distribution was observed to be caused by the rise and extension of oxygen depleted water over the platform during the late frasnian early famennian the abundance of conodonts however appears to have been related to optimal food supply in the oxygenated portion of the water column conodont biofacies patterns observed in the upper devonian of the eastern anti atlas do not fit a simple ecological model involving distance from shore and depth of water the faunas of two intervals investigated exhibit a similar level of species diversity the pattern of which evidently follows the platform basin configuration but the majority of species shows a more or less random distribution the onset of black sediment deposition was characterized by the dominance of polygnathus and icriodus whereas towards its termination during the upper crepida zone time interval the genus palmatolepis clearly predominated this difference is a consequence of both a rise of sea level and of evolutionary changes in the composition of the conodont fauna during the early famennian

a positive trend in seawater 87sr 86sr values over the early middle frasnian boundary late devonian recorded in well preserved conodont elements from the holy cross mountains poland abstract over the early middle frasnian boundary interval late devonian the oceans experienced major changes in global carbon cycling and nutrient dynamics with a prominent positive carbon isotope shift at the base of the palmatolepis punctata zone middle frasnian the aim of this study was to construct seawater 87 sr 86 sr ratio curves for the pa transitans pa hassi zonal interval using well preserved conodont apatite from two sections in the holy cross mountains south central poland the final curves suggest that seawater 87 sr 86 sr values were steady at 0 70794 0 70799 during the transitans zone but began to rise in the early punctata zone towards a value of 0 70814 in the late punctata zone an increase of 0 0002 the increase in seawater 87 sr 86 sr values over the early middle frasnian boundary appears to be the second of two prominent short term rises in 87 sr 86 sr values which led to an overall shift in values from a givetian plateau of between 0 70780 and 0 70785 to a late frasnian plateau of between 0 7080 and 0 7082 correlation with a regional positive 13 c anomaly and evidence for eutrophication in the marine environment suggests that the causes of these phenomena may be linked and that the positive 87 sr 86 sr shift was caused by an increase in the continental flux of sr to the oceans i e the weathering flux a possible explanation for an increase in the weathering flux is the denudation of a new acadian eovariscan orogen which would have been mostly weathered in the humid warm tropical equatorial region

anoxia pre dates frasnian famennian boundary mass extinction horizon in the great basin usa abstract major and trace metal results from three great basin stratigraphic sections with strong conodont biostratigraphy identify a distinct anoxic interval that precedes but ends approximately 100 kyr before the frasnian famennian f f mid late devonian boundary mass extinction horizon this horizon corresponds to the final and most severe step of a more protracted extinction period these results are inconsistent with data reported by others from the upper kellwasser horizon in europe which show anoxia persisting up to the f f boundary in most sections conditions returned to fully oxygenated prior to the f f boundary in the study area these data indicate that the worst part of the f f extinction was not related directly to oceanic anoxia in this region and potentially globally

shocked quartz in the alamo breccia southern nevada evidence for a devonian impact event a transmission electron microscope tem study of quartz grains strongly implies that the alamo breccia of southern nevada resulted indirectly from a late devonian hypervelocity impact event the alamo breccia is perhaps the most voluminous marine carbonate megabreccia exposed on land it covers 4 000 km2 averages 70 m thick and contains more than 250 km3 of carbonate platform debris that was deposited by a giant submarine slide the breccia is a single bed with the characteristics of a chaotic debrite at the base evolving upward to a graded turbidite at the top the bed is anomalous compared to other marine megabreccias because over a large area it is intercalated with cyclic shallow water carbonate platform rocks rather than with deep water turbidites as expected thin sections of peculiar quartz grains recovered from insoluble residues of the breccia show one to six sets of imperfect parallel lamellae and other defects suggesting shock tamorphism when studied by tem the grains clearly display planar deformation features pdfs and other defects from a high pressure shock wave straight and narrow planar microstructures consist of a high density of dislocations mostly parallel to crystal habit plane 1012 but 1013 1011 and 1121 orientations were also detected the pdfs appear identical to those in quartz grains associated with well known impact structures such as manicouagan and manson we conclude that energy from an impact triggered the epiplatform slide and the consequent sedimentary processes that formed the alamo breccia

isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans implications for secular curves isotopic analysis of conodonts and their host limestones sampled between two regionally extensive altered volcanic ash layers in eastern laurentia shows that a 454 ma epeiric sea maintained large lateral differences in nd and c isotope compositions this is consistent with inferred temperature salinity defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments because the majority of old marine fossils and sedimentary rocks are known from epeiric seas some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses rather than global scale changes in the state of the earth ocean system

carbon isotope lateral variability in a middle frasnian carbonate platform belgium significance of facies diagenesis and sea level history abstract carbon isotopic variations of frasnian shallow water carbonates from belgium are related to facies and major sea level trends the influence of the diagenetic overprint was assessed in order to determine the primary signal of the frasnian carbonates shallow water microfacies are characterized by biostromes with stromatoporoids and lagoonal deposits dominated by carbonate mud and calcareous algae with subaerial exposure surfaces the diagenetic history was controlled by three main events early meteoric diagenesis short term subaerial exposure during deposition late meteoric diagenesis major famennian regression and burial diagenesis the oxygen isotopic values are almost constant with respect to facies original material carbonate mud and cement and sedimentological units no differences before or after the main regression this homogeneity is related to resetting during late meteoric diagenesis the carbon isotopic values are related to facies with the more negative values for the shallowest facies and to major sea level variations most negative values after the main regression this pattern is interpreted as being related to primary signals this trend was enhanced by early meteoric diagenesis and the influence of more negative values from paleosols the carbon isotope patterns reflect the influence of sea level and water circulation on shallow water deposits and this influence implies that shallow water carbonates are not necessarily good material for assessing the primary isotopic values of the ocean because of the influence of long residence time aging of the platform top water and because of early diagenesis

depletion of 13c in seawater σc02 on modern carbonate platforms significance for the carbon isotopic record of carbonates seawater co2 from modern carbonate platforms bahama banks and florida is depleted in 13c by as much as 4 relative to open ocean water depletion in 13c is caused by isotopically light co2 input from respiration of marine and terrestrial organic matter during water mass residence on the bank as such depletion in 13c is related to changes in water chemistry driven by evaporation freshwater discharge and caco3 withdrawal deviation of modern platform seawater 13c values away from those of surface seawater suggests that 13c values of ancient cratonic carbonate must be cautiously interpreted initial depositional environment as well as alteration during diagenesis must be considered in extracting valid secular trends for oceanic 13c variation

persistent polybaric rests of calc alkaline magmas at stromboli volcano italy pressure data from fluid inclusions in restitic quartzite nodules a fluid inclusion study has been performed on quartzite nodules of stromboli volcano hosted by calc alkaline lavas of both the strombolicchio 200 ka and paleostromboli ii 60 ka periods the nodules are mainly composed of quartz crystals with subordinate plagioclase and k feldspar small interstitial minerals such as plagioclase k feldspar clinopyroxene biotite and quartz are also found together with glass muscovite epidote and zircon occur as accessory minerals different quartzite nodules occur 1 equigranular polygonal granoblastic quartzite nodules forming a polygonal texture with clear triple points 2 inequigranular polygonal granoblastic quartzite nodules and 3 break up nodules with strongly resorbed quartz these quartzites are restites from partial melting involving felsic crustal rocks at the magma wall rock contact restitic quartz re crystallises at variable and generally high temperatures leading to the formation of quartzites with different textures quartz grains contain five types of fluid inclusions distinguished on the basis of both fluid type and textural phase relationships at room temperature type i are two phase liquid vapour co2 rich fluid inclusions they are primary and subordinately pseudosecondary in origin and have undergone re equilibration processes type ii mono phase two phase vapour liquid vapour co2 rich fluid inclusions are the most common and based on their spatial distribution and shape they can be divided into two subclasses type iia and type iib type ii inclusions are secondary or pseudosecondary and they are assumed to have formed after decrepitation of type i inclusions and cracking of the host quartz type iii inclusions are mono phase vapour they possibly contain co2 at very low density and surround the inner rims of quartz grains type iv two phase silicate melt inclusions contain glass co2 rich fluid some of them are cogenetic with type ii inclusions finally type v two phase liquid vapour aqueous inclusions are both vapour rich and liquid rich aqueous inclusions microthermometric experiments were performed on both type i and ii inclusions type i inclusions homogenise to liquid between 20 and 30 5 c type iia inclusions homogenise to vapour in the 24 to 30 c range with a maximum peak of frequency at 29 c type iib inclusions also homogenise to vapour between 14 and 25 c there appears to be no difference in homogenisation temperature distribution between the strombolicchio and paleostromboli ii samples the trapping pressures of the fluid inclusions have been obtained by combining the microthermometric data of the strombolicchio and paleostromboli ii samples with the pressure temperature volume i e density characteristics for a pure co2 system the data on the early inclusions type i suggest an important magma rest at a pressure of about 290 mpa i e about 11 km depth type iia co2 inclusions suggest that a second magma rest occurred at a pressure of about 100 mpa i e about 3 5 km depth whereas type iib inclusions were trapped later at a shallower depth during the final magma upwelling no pressure depth differences seem to occur between the strombolicchio and paleostromboli ii periods indicating the same polybaric rests for the calc alkaline magmas of stromboli despite their significantly different ages this persistence in magma stagnation conditions from 200 to 60 ka suggests a similar plumbing system for the present day strombolian activity

flow behavior and mobility of contaminated waste rock materials in the abandoned imgi mine in korea abstract incomplete mine reclamation can cause ecological and environmental impacts this paper focuses on the geotechnical and rheological characteristics of waste rock materials which are mainly composed of sand size particles potentially resulting in mass movement e g slide or flow and extensive acid mine drainage to examine the potential for contaminant mobilization resulting from physicochemical processes in abandoned mines a series of scenario based debris flow simulations was conducted using debris 2d to identify different hazard scenarios and volumes the flow behavior of waste rock materials was examined using a ball measuring rheometric apparatus which can be adapted for large particle samples such as debris flow bingham yield stresses determined in controlled shear rate mode were used as an input parameter in the debris flow modeling the yield stresses ranged from 100 to 1000 pa for shear rates ranging from 10 5 to 102 s 1 the results demonstrated that the lowest yield stress could result in high mobility of debris flow e g runout distance 700 m from the source area for 60 s consequently the material contaminants may easily reach the confluence of the suyoung river through a mountain stream when a fast slide or debris flow occurs at or near an abandoned mine area it may result in extremely dynamic and destructive geomorphological changes even for the highest yield stress of debris flow simulation i e y 2000 pa the released debris could flow into the mountain stream therefore people living near abandoned mines may become exposed to water pollution throughout the day to maintain safety at and near abandoned mines the physicochemical properties of waste materials should be monitored and proper mitigation measures post mining should be considered in terms of both their physical damage and chemical pollution potential

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

modeling the carbon and sulfur isotope compositions of marine sediments climate evolution during the devonian abstract a model of global biogeochemical cycles coupled to an energy balance climatic model modified after the combine model godderis y joachimski m m 2004 global change in the late devonian modeling the frasnian famennian short term carbon isotope excursions palaeogeogr palaeoclimatol palaeoecol 202 309 329 is used to calculate the short term evolution of atmospheric p co 2 during the devonian the geochemical cycles for carbon alkalinity phosphorus sulfur and oxygen are included in this model with also 13 c and 34 s cycles high resolution records of 13 c of marine carbonates and 34 s of marine sulfates are used as forcing parameters of the geochemical cycles in an inverse modeling atmospheric p co 2 and p o 2 at the end of the silurian are calculated to have been 3000 ppmv and 0 165 bar 0 75 pal respectively a long term decrease in p co 2 is modeled for almost the entire devonian short term lowering of p co 2 to concentrations around 2000 ppmv is calculated for the silurian devonian transition and the pragian contents around 900 ppmv are modeled for the eifelian givetian givetian frasnian and frasnian famennian boundaries as a consequence of enhanced organic carbon burial during deposition of lochkovian eifelian and frasnian grey and black shales organic carbon burial is enhanced by the increase of phosphorus delivery to the ocean triggered by short term sea level falls the corresponding short term global climatic cooling at the silurian devonian boundary at the end of the pragian and the givetian frasnian as well as frasnian famennian boundaries reached 2 c at the equator the rapid colonization of continental surface by land plants during the middle and late devonian increasing chemical alteration of the continents and co 2 consumption by silicate weathering is assumed to have caused cooling of surface seawater as suggested by the 18 o values of biogenic apatites

facies and geochemistry across the early middle frasnian transition late devonian on south china carbonate shelf comparison with the polish reference succession abstract dongcun and longmen are two important reference sections for the study of the frasnian carbonate platform of guangxi southern south china at the dongcun section conodonts recovered in the present study provide a robust biostratigraphic framework for the lower middle frasnian interval the studied interval uppermost givetian to middle frasnian basal palmatolepis hassi zone is characterized by repeated shallower and deeper water deposits but the cyclic depositional trend was mainly deepening upward carbon isotopic investigation of bulk samples recognizes a major four step perturbation across the lower middle frasnian boundary which is also evident in the holy cross mountains poland the pattern is reinforced by correlative organic carbon isotopic secular trends from the reference polish sections a prominent long lasting increase in 13ccarb up to 4 5 event iii is evident in the palmatolepis punctata zone but is preceded and followed by rapid negative shifts events ii and iv respectively to ca 0 a weak incipient positive excursion event i is recognized in carbonate carbon only in the upper part of the palmatolepis transitans zone in chinese sections different background isotopic values indicate decoupled isotopically dissimilar hydrothermally affected and mostly oxygenated water masses over the carbonate shelf as shown by trace and rare earth elements the shallower water upslope diagenetically unaltered dongcun succession reveals lighter carbon isotopic ratios on the order of 2 while the less densely sampled and less certainly dated mostly muddy siliceous longmen downslope succession shows 13ccarb levels similar to those recorded worldwide the onset of the bipartite event iii is marked by 13c enrichment of 4 3 at dongcun event iv a light carbon excursion is especially well recorded in the shallow water facies by a 2 5 negative shift the event i excursion in the transitans zone is obscured as in some polish successions but is probably recorded in organic carbon trends slight differences in timing and magnitude of the 13ccarb oscillations may originate from geochemical decoupling between the open ocean and the tectonically active morphologically differentiated epeiric basins nevertheless a global extent for this major punctata event is clear from the similar biogeochemical signals recorded on the distantly separated south laurussian and south china shelves

magnetic susceptibility and selected geochemical mineralogical data as proxies for early to middle frasnian late devonian carbonate depositional settings in the holy cross mountains southern poland abstract the magnetic susceptibility ms and other petromagnetic features of early to middle frasnian carbonates from the holy cross mountains southern poland were studied they were correlated with the eovariscan tectonic activity and palaeoenvironmental changes the ms logs were compared with the available geochemical and mineralogical data a substantial paramagnetic contribution to the ms of the frasnian carbonates is evidenced by a poor correlation of this parameter with the isothermal remanent magnetization values and by a decrease of ms values at low temperatures an estimation of paramagnetic input indicates the dominance of this contribution to the magnetic susceptibility in 60 of studied samples the ms low widely documented in samples through the palmatolepis punctata zone is particularly due to limited input of paramagnetic mineral particles into the basin where also amounts of larger detrital grains of quartz hematite and goethite were significantly reduced this depression corresponds well to anomalously high carbon isotope 13c values for carbonates nevertheless a transient ms rise across the e mf boundary linked with an increase in detrital quartz content suggests together that both the initiation and termination phases of the punctata event indeed coincided with increased terrigenous eolian input from uplifted lands the ms high identified in the lower part of the palmatolepis hassi zone coincides with higher content of detrital quartz hematite and goethite being accompanied by low 13c values in spite of similar ms stratigraphic signatures in other basins the increased detrital input of proximal provenance is likely connected with the first variscan tectonic events that spread over the region and subsequently led to gradual closing of the saxothuringian ocean segments with basement uplift in the area of the mid german crystalline high