sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in early to middle ordovician greenhouse oceans article i nfo despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal neoproterozoic era late cambrian records point to the persistence of deep water anoxia and potential for development of euxinic conditions the late cambrian spice steptoean positive carbon isotope excursion event however is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere here we present high resolution carbon and sulfur isotope profiles from early to middle ordovician carbonate rocks from the argentine precordillera and western newfoundland to constrain oceanic redox conditions in the post spice world marine c isotope profiles record relatively stable behavior excursions b3 that is characteristic of greenhouse climates ma rine s isotope profiles record short term b10 6 yr rhythmic variation superimposed over a longer term

diagenesis of plio pleistocene sediments of the colorado river delta southern california abstract interbedded mudrocks and fine grained sandstones from three wells in the plio pleistocene colorado river delta in the imperial valley southern california were examined by x ray powder diffraction methods chemical analyses and scanning electron microscopy sem to determine the extent and course of low temperature metamorphic reactions this sequence was compared to tertiary rocks from the u s gulf coast in an effort to explore the kinetics of low temperature reactions in natural systems mineralogical changes observed are 1 the progressive transformation of mixed layer illite smectite to pure illite over the temperature range 70 c to 210 c 2 the apparent appearance of chlorite as a diagenetic mineral at 180 c to 194 c coincident with the onset of r symbol 3 ordering in the illite smectites 3 the loss of kaolinite above approximately 210 c and 4 a progressive decrease in the amount of detrital potassium feldspar over the same temperature interval that brackets the illitization reaction comparison of bulk chemical analyses with analyses of the comparison of tertiary sequences with this plio pleistocene sequence indicates that kinetics significantly influence the appearance of clay mineral assemblages at temperatures below approximately 175 c at temperatures above 175 c temperature rather than time appears to govern the reactions

widespread prolonged late middle to late ordovician upwelling in north america a proxy record of glaciation late middle to late ordovician subtidal ramp carbonates of new mexico texas and oklahoma contain abundant spiculitic chert to 70 chert by volume and phosphate 1 5 wt indicating that these rocks formed in an extensive upwelling zone upwelling began in the late middle ordovician ca 454 ma and persisted until the end of the ordovician late ordovician cherty carbonates also occur along the u s cordilleran margin in board of organic rich graptolitic shale and chert the widespread occurrence of late ordovician cherty and phosphatic carbonates on southern and western laurentia in addition to phosphate rich cool water carbonates over much of the north american midcontinent suggests vigorous thermohaline circulation related to prolonged 10 14 ma gondwana glaciation

lithium isotopes in island arc geothermal systems guadeloupe martinique french west indies and experimental approach abstract we report lithium li isotopic measurements in seawater derived waters that were discharged from geothermal wells thermal springs and sub marine springs located in volcanic island arc areas in guadeloupe the bouillante geothermal field and martinique lamentin plain and the diamant areas while li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site the 7li signatures for each of these reservoirs were significantly different the first low temperature 25 250 c experiments of li isotope exchange during seawater basalt interaction confirmed that li isotopic exchange is strongly temperature dependent as previously inferred from natural studies li isotopic fractionation ranged from 19 4 solution solid at 25 c to 6 7 at 250 c these experiments demonstrated the importance of li isotopic fractionation during the formation of li bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature solution solid 7847 t 8 093 application of experimental results and literature data to the bouillante area suggested that geothermal water was in equilibrium at 250 260 c it likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust just below the transition zone between andesite volcanic flows and the basaltic dikes the upper dike section from which li is extracted by hydrothermal fluids was characterized by light li isotopic values in the rocks indicating retention of 6li by the altered rocks for the lamentin and diamant areas the geothermal fluids appeared to be in equilibrium with reservoir volcano sedimentary rocks at 90 120 c and 180 c respectively further evidence for this argument is provided by the fact that only the na li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids this suggests the importance of a sedimentary signature in these reservoir rocks altogether this study highlights that the use of li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks

surficial clay assemblage associated with the hydrothermal activity of bouillante guadeloupe french west indies abstract surficial clay minerals as indicators of present day geothermal activity were studied around bouillante guadeloupe french west indies the study is based on widespread sampling 100 surface samples in a zone extending 5 km north and 9 km south from bouillante three types of mineral associations were distinguished 1 dioctahedral smectites in which the beidellite nontronite behavior predominates with zeolites calcite quartz kaolinite 2 ordered illite smectite mixed layered clay with adularia silica calcite and 3 kaolinite smectite mixed layered clays halloysite kaolinite smectite silica the first association characterizes steaming grounds and areas subjected to hydrothermal fluid infiltrations along fractures or breccia zones in marine and subaerial environments the second association was only identified in the matrix of epithermal breccias this assemblage appears as a tracer of high temperature fluids and is a good indicator of the proximity to the geothermal reservoir the third assemblage was found both within and beyond the bounds of the known geothermal area suggesting a pedogenic origin we interpret the occurrence of the three assemblages in the bouillante geothermal area as a background of clayey tropical weathering kaolinite smectite with a hydrothermal overprint dioctahedral smectites and illite smectite

systematics of chlorite alteration at the phelps dodge massive sulfide deposit matagami quebec hydrothermal alteration at the phelps dodge archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages quartz chlorite albite epidote or sericite quartz chlorite or sericite chlorite or sericite and chlorite talc or stilpnomelane alteration at constant al was initiated by hydration addition of fe mg silicification and leaching of ca na which produced al saturated chlorites with a wide range of fe fe mg ratios within the assemblage quartz chlorite albite epidote or sericite with continued addition of fe and less mg combined with extensive leaching of quartz the chlorite became al undersaturated quartz chlorite talc assemblage and ultimately formed a rock composed essentially of chlorite the chlorite talc or stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone chlorites in the si al mg fe system form a rectangular solid solution plane with limited substitution of al for si and extensive substitution of fe for mg in si al space the solid solution plane is outlined by al saturation presence of other al rich mineral and al undersaturation presence of al poor or al free mineral boundaries al iv substitution for si in the chlorites increases from 2 3 to 3 01 formula units based on 8 si al iv with fe fe mg whereas al iv occupancy decreases from 2 88 to 2 65 the valence charge balance is compensated by octahedral site vacancies or by the conversion of fe super 2 to fe super 3 the al iv content of chlorite also increases with temperature chlorites on the al saturated boundary of the solid solution field form a potentially useful geothermometer t degrees c 106 al iv 18 al iv is corrected for changes with the fe fe mg ratio as al iv corrected al iv 0 7 fe fe mg the fe fe mg ratios of bulk rock and contained chlorite correlate well birefringence colors of the chlorite change from green to brown violet and berlin blue as fe fe mg changes from 0 18 to 0 64 the fe fe mg variation apparently depends largely on the proportions of mg rich seawater and fe rich hydrothermal fluid in the altering brine

the fossil hydrothermal system of saint martin lesser antilles geology and lateral distribution of alterations the fossil geothermal system of saint martin lesser antilles was generated by an intrusion of quartz diorite of oligocene age into eocene volcano sedimentary host rocks adjacent to the pluton the alteration pattern crops out continuously in the fort hill kool bay area over a distance of 4 km the lithology of the altered host rocks is the following 1 alternating marls limestones and minor hyaloclastite lower eocene 2 hyaloclastite and andesitic lava flows middle eocene 3 regularly bedded fine grained hyaloclastite upper eocene on the basis of alteration petrography chemistry and fluid inclusion study three alteration events have been distinguished 1 1 high temperature event 510 t 350 c accompanied by fluids with salinity higher than 35 wt nacl eq mainly represented by tourmaline quartz magnetite orthoclase apatite and sulfide veins occurring at the periphery of the pluton and along distal regional faults 2 2 moderate temperature event occurring as veins and pervasive alteration veins containing quartz phengite pyrite and minor dickite or chlorite t 300 c very low salinity are superimposed on the early high temperature veins pervasive alteration affected large concentric zones the inner zone 3 km width shows an assemblage of epidote quartz actinolite magnetite at the periphery of the pluton and epidote quartz and chlorite farther away the outer zone 1 km width shows calcite and mixed layer illite smectite i s with ordering type r3 and chlorite smectite c s fluid inclusions indicate that salinity and temperature decrease outward from the intrusion during the moderate temperature pervasive alteration event respectively from 320 to 140 c and from 30 to 5 wt nacl eq 3 3 low temperature event t 50 c showing calcite mixed layer i s ro chalcedonite or baryte in late disseminated veins this lateral distribution of alteration is very similar to alteration zoning observed in porphyry ore environment or in active deep geothermal systems as baca valles caldera it shows contrast between zones of fracture permeability which controlled active flow in the system and large zones where pervasive alteration occurred in an inactive flow regime and can be included with metamorphic process phyllic alteration seems a good indicator of the active flow paths in the system of saint martin the alteration pattern in saint martin is considered to represent the roots of a deep geothermal system porphyry ore system of oligocene age actually eroded

theory and laboratory observations of naled ice growth based on observations of naleds also called au eis or icings grown in a refr igerated laboratory flume a detailed composite description of the processes associated with naled ice growth is presen ted a spread length is derived that represents a mass balance between the water supplied to a naled s surface and the ice that freezes on to the surface herein this spread length is termed an equilib rium length guided by this concept of equilibrium length and the laboratory data length and time scales applicable to the growth of two dimensional naled are proposed these scales proved useful for normalizing the times and stream wise lengths that correspond to distinct phases of naled ice growt h it is observed that even in a laboratory flume naleds spread and thicken in a complex layer by layer manner to the best of the authors knowledge this is the first reported laboratory study on naled ice growth the descriptions and concepts presented herein should be useful to engineers concerned with the effects of naled s on engineering works and of interest to those who are planning experiments on naleds

mixed layer illite smectite as a paleotemperature indicator in the amethyst vein system creede district colorado usa ordered illite smectite is the most abundant and widespread product of argillic alteration associated with the oligocene epithermal ag pb zn cu au amethyst vein system southwest colorado hydrothermal illite smectites exhibit all reichweite from 1 to 3 and span the composition range from about 25 to about 3 illite the composition and reichweite of illite smectite vary smoothly with distance from the amethyst vein with increasing distance from the vein percent illite and the reichweite decrease when composition temperature relationships of illite smectite from modern geothermal systems are applied to the fossil amethyst system isotherms describing the thermal regime at the time of argillization can be estimated temperatures near 240 c appear to have existed near the amethyst vein these temperatures agree with homogenization temperatures obtained from fluid inclusions in vein minerals the most distal illite smectites sampled about 260 m from the vein indicate temperatures near 110 c estimated thermal gradients are on the order of 0 4 to 1 c per meter although illite smectite composition and structure vary systematically with distance from the amethyst vein there are no systematic trends associated with the numerous smaller veins and veinlets in the hanging wall of the system this indicates that temperatures of both wallrock and the fluids in all but the major amethyst vein were nearly the same during clay formation apparently the hydrothermal system had reached a fairly steady mature thermal state at the time of argillization

clay minerals related to the hydrothermal activity of the bouillante geothermal field guadeloupe abstract the geothermal field of bouillante guadeloupe fwi is a high enthalpy hydrothermal system emplaced in submarine volcanoclastic formations hyaloclastites scarce lava flows and subaerial formations andesitic lava flows pyroclastites lahars which belong to the lesser antilles arc three directional wells were drilled in 2001 to optimize the productivity of the geothermal field up to 15 mwe and to investigate the vertical distribution of clay alteration from the surface area down to a depth of 1000 m where temperatures exceed 250 c special attention has been paid to the clay signature of the fractured zones which channel the present geothermal fluids three successive zones dominated respectively by dioctahedral smectite illite and chlorite were identified at increasing depths alteration petrography indicates that these mineralogical clay zones result from the spatial superimposition of at least two successive alteration stages the first one assimilated to a propylitic alteration stage affected all parts of the system and consisted of crystallization of trioctahedral phyllosilicates chlorite or corrensite ca silicates heulandite clinoptilolite prehnite pumpelleyite wairakite and epidote quartz and minor calcite in replacement of most of the primary minerals of the intersected volcanic or volcanoclastic formations the later stage of alteration is related to the circulation of the present geothermal fluids and is assimilated to argillic or phyllic alteration it consists of a more or less intense argillization which results from the crystallization of aluminous dioctahedral clay phases smectite illite i s mixed layers and accessory kaolinite associated with quartz calcite hematite or pyrite the permeable zones which channel most of the present geothermal fluids are fracture controlled and do not contain specific clay parageneses however the illite i s mixed layers minerals differ from those of the surroundings by specific properties including both crystal structure and texture these specific properties decrease in the expandable component of the illitic material increase of the illite crystallinity can be controlled by the nucleation growth rates operating in zones of active flow regime being mainly a product of the earlier propylitic alteration stage chlorites are much less informative on the fracture controlled permeable levels however the compositional variations of chlorites recorded within the shallower fractured zone suggest a significant change in fo 2 conditions related to early circulation of fluids along the major near west striking normal faults plateau fault

carbon isotope δ13ccarb stratigraphy of the lower middle ordovician tremadocian darriwilian in the great basin western united states implications for global correlation abstract new stable carbon isotope data 13ccarb from lower middle ordovician tremadocian to darriwilian carbonate mudstone and wackestone rocks of the pogonip group are presented from two sections in the great basin region usa shingle pass east central nevada and the ibex area western utah the pogonip group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the late cambrian furongian to middle ordovician darriwilian the shingle pass and ibex area sections have been previously studied for their conodont biostratigraphy and contain a north american midcontinent conodont fauna that range from the cordylodus intermedius zone uppermost cambrian to the phragmodus polonicus zone darriwilian the 13c trend has four distinct characteristics recognized in both great basin sections 1 a drop in 13c from 1 at the base of the ordovician tremadocian to 0 7 2 a 1 to 2 positive 13c shift in the uppermost rossodus manitouensis zone during the late tremadocian 3 a gradual 13c increase from 2 to ca 0 during the end of the early ordovician floian and 4 a steady 13c decrease from 0 to 4 to 5 during middle ordovician dapingian darriwilian in the lower ordovician 13c trends reported here from the great basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies instead these lower ordovician isotope data most likely reflect primary seawater chemistry and changes in 13c on a global scale this interpretation is supported by the excellent correlation of 13c in the lower ordovician to other 13c trends reported from the sections in the argentine precordillera la silla and san juan formations and in western newfoundland st george and table head groups these correlations using 13c are consistent with published biostratigraphic data and provide an integrated and high resolution chemo biostratigraphic framework for the lower ordovician sedimentary record of the laurentian margin the middle ordovician portion of the 13c curves in the great basin represented by the kanosh and lehman formations shows significant isotopic depletion relative to the section in argentina thus although there is some indication that minima and maxima in the middle ordovician curves can be correlated the great basin sections show clear evidence of overprinting by local variables related to both diagenesis dolomitization and platform restriction

hydrothermal alteration and veins at the epithermal au ag deposits and prospects of the waitekauri area hauraki goldfield new zealand the waitekauri area of the hauraki goldfield new zealand contains several adularia sericite epithermal au ag deposits and prospects from west to east the area contains the sovereign jubilee scimitar scotia teutonic and jasper creek deposits and prospects which are hosted by andesitic and dacitic flows breccias and localized pyroclastic and air fall deposits drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3 km long composite cross section through the area most host rocks are intensely altered with 100 percent of the igneous minerals replaced by hydrothermal minerals although the alteration intensity becomes more variable and weaker toward the east alteration minerals include quartz adularia albite chlorite pyrite illite interstratified illite smectite smectite calcite hematite and minor epidote many of these minerals have zoned distributions adularia is widespread at sovereign but is restricted to shallow levels at both scotia and jasper creek albite occurs in a discrete zone below adularia at scotia and minor epidote is restricted to sovereign and jubilee illite occurs throughout sovereign and jubilee and at the western margin of scotia and scimitar where it grades eastward into interstratified illite smectite and smectite at teutonic and jasper creek veins are typically less than 10 cm wide but have diverse mineralogy with zoned distributions quartz veins dominate at sovereign and jubilee whereas calcite veins are more abundant at scotia scimitar and jasper creek laumontite occurs at scotia and locally at scimitar whereas veins of clinoptilolite and mordenite calcite occur at jasper creek and stilbite veins occur at teutonic fluid inclusions in quartz and calcite homogenized between 132 and 310 c and trapped a dilute solution with an apparent salinity of less than 2 6 wt percent nacl equiv homogenization temperatures are highest at sovereign avg 241 c jubilee avg 239 c and scimitar avg 236 c lower at scotia avg 204 c and lowest at teutonic avg 168 c and jasper creek avg 162 c estimated positions of the paleowater table above sovereign jubilee scimitar scotia jasper creekg and teutonic relative to present elevations was at least 690 750 575 450 225 and 150 m above sea level respectively the deposits and prospects therefore span a 600 m vertical interval individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west alteration intensity alteration and vein mineral distributions and fluid inclusion microthermometry are interpreted to indicate that sovereign and jubilee formed at relatively high temperatures whereas teutonic and jasper creek formed at relatively cooler temperatures several hydrologic reconstructions are possible including 1 a single hydrothermal system with an inclined water table and significant lateral outflow to the east or 2 a single low relief hydrothermal system with a flat lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10 to the east regardless of the preferred reconstruction the sovereign and jubilee deposits appear to have formed in the main zone of fluid upflow whereas the teutonic and jasper creek prospects appear to have formed toward the margin moreover the greatest erosion has occurred at the jubilee and sovereign deposits 300 400 m and these may represent the roots of a more extensive vein network that has largely been eroded

geothermal surface alteration of basalts krýsuvik iceland alteration mineralogy water chemistry and the effects of acid supply on the alteration process abstract the geothermal surface alteration of basalts and associated water chemistry at kr suvik sw iceland were studied the geothermal area was characterised with zones of intensive surface alteration steam vents mud pots and hot springs the steam heated geothermal surface waters had ph values between 1 69 and 7 67 and total dissolved solid tds concentrations between 154 and 6660 ppm with cl and so 4 concentration decreasing and increasing with decreasing ph respectively alteration mineral assemblages observed were strongly associated with the surface intensity in areas of most intensive activity the basaltic rocks were altered to amorphous silica anatase and pyrite with a crust of native sulphur at the surface with decreased activity kaolinite became important as well as iron oxyhydroxides and oxides on the flanks of the area montmorillonite was the predominant alteration product based on these observations the surface geothermal activity was divided into three groups 1 high activity areas with active steam vents and mud pots and intensive acid leaching 2 medium activity areas where the ground is hot steam vents and mud pots are uncommon and the surface alteration is less intensive and 3 low activity areas on the margins of the surface geothermal activity the primary factors influencing the steam heated acid sulphate alteration of basalts included the redox state oxidation front supply of acids and ph and the extent of reaction the formation of iron and sulphur containing minerals and the respective elemental mobility depended on the redox conditions with pyrite formation under reduced conditions and goethite and or hematite under oxidised conditions at low ph ca mg k and na were mobile and leached out whereas fe ti and al and to a large degree si were retained in the alteration product at higher ph values 5 the mobility of ca mg k and na was reduced due to the formation of clays

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

the structure and morphology of the basse terre island lesser antilles volcanic arc basse terre island is made up of a cluster of composite volcanoes that are part of the lesser antilles volcanic arc the morphology of these volcanoes and the onshore continuation of the grabens and strike slip faults that surround the island are poorly documented due to erosion and rainforest cover therefore we conducted a morphological analysis of the island using digital elevation model dem data integrated with field observations to document erosional constructional and deformational processes a dem based analysis of 1 249 lineaments and field structural measurements of 16 normal faults 3 741 veins and fractures and 46 dykes was also carried to document the structures that predominate in sub surface rocks the results indicate that the over 1 my old and elongated northern chain volcano which makes up the northern half of the island was built by high eruption rates and or a low viscosity magma injected along the n s to nnw sse striking extensional structures formed by the flexure of the lithosphere by the overall subduction regime after 1 ma the southern half of the island was shaped by an alignment of conical volcanoes likely built by a more viscous magma type that was guided by the nw se striking montserrat bouillante strike slip fault system these n to nnw and nw structural directions are however poorly expressed onshore possibly due to slow slip motion the sub surface rocks mostly contain e w striking structures which have likely guided the many flank instabilities documented in the studied area and guide hydrothermal fluids and shallow magmatic intrusions these structures are possibly part of the e w striking marie gallante offshore graben

upper ordovician mohawkian carbon isotope δ13c stratigraphy in eastern and central north america regional expression of a perturbation of the global carbon cycle abstract the guttenberg carbon isotope excursion gice documented from eastern north america demonstrates the effects that regional geochemically distinct water masses upwelling and ocean circulation have on the carbon isotope record from carbonate platforms late turinian chatfieldian carbonates from oklahoma kentucky virginia and west virginia record a positive carbon isotope excursion 3 0 the gice excursion the gice excursion has relations to established biostratigraphy beginning in the north american midcontinent phragmodus undatus conodont zone and continuing through the plectodina tenuis zone sequence and event stratigraphy previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the gice excursion where geochemically distinct water masses are defined for the upper ordovician a major eustatic sea level rise before the gice promoted a greater exchange of open ocean waters onto the carbonate platform of laurentia however restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent iapetus ocean were still apparent local variations documented in the gice excursion are directly related to upwelling of nutrient rich isotopically light waters increased primary productivity and the subsequent organic carbon production and burial

magma sources for mesozoic anorogenic granites of the white mountain magma series new england usa the magma sources for granitic intrusions related to the mesozoic white mountain magma series in northern new england usa are addressed relying principally upon nd isotopes many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts sm nd and rb sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 ma the possibility of crustal sources is further examined with analyses of representative older crust including paleozoic granitoids which serve as probes of the lower crust in the region multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations 1 in end overall mesozoic granite plutons range in end t from 4 2 to 2 3 with most 2 to 0 and in initial 87sr 86sr from 0 7031 to 0 709 the isotopic variations are roughly inversely correlated but are not obviously related to geologic geographic or age differences older igneous and metamorphic crust of the region has much lower nd isotope ratios with the most radiogenic paleozoic granitoid at end 180 ma of 2 8 these data suggest mid proterozoic separation of the crust in central northern new england moreover the bulk of the mesozoic granites cannot be explained as crustal melts but must have large mantle components the ranges of nd and sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation the results indicate production of anorogenic granites from mantle derived mafic magmas

characterising the hydrothermal alteration of the broadlands ohaaki geothermal system new zealand using short wave infrared spectroscopy abstract hydrothermal clay minerals present in the broadlands ohaaki geothermal field were characterised by field portable short wave infrared spectroscopy three major alteration zones an upper smectite a middle illite and a lower illite chlorite are spectrally separable the zoning pattern is generally consistent with the thermal structure of the geothermal field although occasionally zone boundaries cut present day isotherms the data indicate that temperature is the major control on clay zoning and permeability plays a subordinate role both beidellite and montmorillonite are common in the upper low temperature smectite zone kaolinite mainly of low crystallinity marks the margin of the field where cool acidic ground waters inflow in the middle alteration zone illite dominantly k rich shows a narrow compositional variability some highly permeable zones are characterised by illite with low octahedral al contents ammonium bearing illite and buddingtonite are present locally in permeable horizons within the illite zone where temperatures are above 200 c chlorite is most abundant in the lower alteration zone temperature 250 c although it also occurs unevenly in the upper and middle alteration zones chlorite varies from mg to fe rich varieties but mostly with mg values

error analysis of co2 and o2 estimates from the long term geochemical model geocarbsulf long term carbon and sulfur cycle models have helped shape our understanding of the phanerozoic history of atmospheric co 2 and o2 but error analyses have been largely limited to testing only a subset of input parameters singly as a result the full ranges of probable co2 and o 2 are not quantitatively known here we investigate how variation in all 68 input parameters of the geocarbsulf model both singly and in combination affect estimated co 2 and o 2 we improve formula tions for land area runoff and continental temperature the latter of which now excludes land area not experiencing chemical weathering we find our resampled model co 2 and o2 estimates are well bounded and provide high confidence for a double hump in co2 during the phanerozoic with high values during the early paleozoic and mesozoic and low values during the late paleozoic and late mesozoic to cenozoic our analyses also support a distinct atmospheric o2 peak during the late paleozoic 30 followed by low values near the triassic jurassic boundary 10 most of the spread in co2 is contributed by three factors climate sensitivity to co 2 doubling and the plant assisted chemical weathering factors life and gym c o2 estimates during the paleozoic to early mesozoic are highly concordant with indepen dent records from proxies but are offset to lower values during the globally warm late mesozoic to early cenozoic the model proxy mismatch for the late mesozoic can be eliminated with a change in gym within its plausible range but no change within plausible ranges can resolve the early cenozoic mismatch either the true value for one or more input parameters during this interval is outside our sampled range or the model is missing one or more key processes

clay minerals related to the circulation of geothermal fluids in boreholes at rittershoffen alsace france abstract two geothermal wells grt 1 and grt 2 were drilled into the granite at rittershoffen alsace france in the upper rhine graben to exploit geothermal resources at the sediment basement interface brine circulation occurs in a permeable fracture network and leads to hydrothermal alteration of the host rocks the goal of the study was to characterize the petrography and mineralogy of the altered rocks with respect to the permeable fracture zones in the granitic basement as clay minerals are highly reactive to hydrothermal alteration they can be used as indicators of present day and paleo circulation systems special attention has been paid to the textural structural and chemical properties of these minerals the fine grained clay fraction

geometry facies and development of middle ordovician carbonate buildups virginia appalachians large carbonate buildups from 1 to 60 km wide and 20 to 250 m thick in the lower part of the middle ordovician foreland basin sequence in virginia include shallow ramp and downslope types the buildups overlie peritidal and lagoonal carbonates shallow ramp buildups are overlain by deep ramp nodular limestone downslope buildups are overlain by slope and basin limestone and or shale lime wackestone and or mudstone commonly with stromatactoid structures occurs in cores of buildups and as abundant small pods enclosed in skeletal sand bindstone is rare but bryozoan bafflestone locally marine cemented is common in downslope cores and pods pelmatozoan ramose bryozoan sands and rudstone locally with abundant mud pods may form sheets beneath flanking and above core facies they are planar bedded or less commonly have large scale foresets black shaly skeletal limestone containing siliceous sponges brachiopods bryozoans and pelmatozoans forms deep flank beds surrounding downslope buildups and grades outward into basinal limestone and or shale the buildups are carbonate banks that developed in a transgressive setting involving sea level rise and subsidence of the foreland basin they formed as a result of high rates of skeletal production by bank biotas and by baffling action of stalked and branching organisms widespread marine cementation converted the loose bank sediments into rigid structures some downslope buildups were initiated on unconformity highs but most buildups overlie thick sequences of shallow water facies some shallow ramp buildups may be localized above a linear high tazewell arch shallow ramp buildups were killed by influxing mud following incipient drowning of the shallow ramp downslope buildups were killed following basin subsidence when they were covered by anoxic basin waters exploration models involving buildups on carbonate ramps should recognize the potential for both shallow ramp and downslope buildup trends such downslope buildups are particularly important because buildup facies are closely associated with potential reservoir rocks of the slope and basin

petrophysical properties of volcanic rocks and impacts of hydrothermal alteration in the guadeloupe archipelago west indies abstract the results of the geotref project presented here aim to develop high enthalpy geothermal energy in fractured reservoirs they focus on active volcanic islands especially the basse terre island guadeloupe archipelago as part of this project we measured several petrophysical properties to better understand how fluids flow in the hydrothermal system provide a set of parameters to modelers and build datasets for future interpretation of well logs the thermo physical rock properties of bulk and grain density porosity permeability compressional wave velocity thermal conductivity and diffusivity specific heat capacity of outcropping volcanic rocks from the guadeloupe archipelago lesser antilles were measured and a dataset was built according to field observations samples were grouped into three lithological classes and then separated by their degree of alteration according to macroscopic aspects and optical microscopy results geochemical data were obtained and magnetic minerals identified by combining anisotropy of magnetic susceptibility and variation of magnetic susceptibility with temperature the lithotypes investigated were lava flows debris flows and pyroclastics gathering ash pumice and scoria deposits for lavas and debris flows three degrees of alteration were defined fresh slightly to moderately altered and highly hydrothermalized only fresh pyroclastics were studied because the altered state was not outcropping typically samples are composed of andesite and belong to tholeiitic and calc alkaline series lava porosity consists of high density cooling joints and vesicles of different shapes whereas pores in debris flows and pyroclastics are located both between and within particles median porosities and permeabilities are clearly distinct for lavas 4 and 6 10 18 m2 debris flows 33 and 4 10 14 m2 and pyroclastics 34 and 8 10 13 m2 the porosity difference implies a thermal conductivity twice to three times higher for lavas 1 68 w m 1 k 1 specific heat capacity of all lithothypes is similar 0 75 0 97 kj kg 1 k 1 advancing hydrothermal alteration removes magnetic minerals and transforms initial rocks the original plagioclases and pyroxenes in neoformed chlorite white micas quartz epidote and clay minerals are replaced this modifies the shape of the pore network furthermore hydrothermal alteration results in a reduction of pore throat diameter it tends to decrease petrophysical differences between lavas and debris flows increasing the porosity and permeability of effusive rocks while the opposite effects are observed for debris flows

evolution of clay mineral assemblages in the tinguiririca geothermal field andean cordillera of central chile an xrd and hrtem aem study abstract hrtem textural evidence shows that clay minerals in the tinguiririca geothermal field andean cordillera central chile are the result of direct alteration of former volcanic glass and minerals by hydrothermal fluids at similar temperatures to the present day they show the classical pattern of diagenetic transformation from smectite at the top to illite at the bottom with the progressive formation of corrensite and chlorite the high fluid rock ratio disposability of necessary cations and absence of previous detrital phyllosilicates allow the consideration of this area as a natural laboratory to establish the extreme ideal conditions for very low t reactions transformations from smectite to r1 illite smectite i s and from these to r3 mixed layers occur respectively at 80 120 c and 125 180 c in spite of ideal genetic conditions the new formed minerals show all the defective character and lack of textural and chemical equilibrium previously described in the literature for diagenetic and hydrothermal low temperature assemblages chemistry of smectite illite phases evolves basically through a diminution of the pyrophyllitic component toward a theoretical muscovite si 4 al 3 k however a second chemical vector si 4 mg 2 al 3 al 3 that is decreasing of the tschermack component also contributes to the evolution toward the less si more al rich muscovite in relation to the original smectite residual mg and fe from the latter reaction is consumed in the genesis of chloritic phases nevertheless as a consequence of the lack of chemical equilibrium probably because of the short time scale of the geothermal alteration processes the composition of clay minerals is highly heterogeneous at the level of a single sample consequently the respective fields of smectite r1 i s and r3 i s overlap each other making the distinction among these three phases impossible based exclusively on chemical data

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal varying concentrations of atmospheric oxygen have affected the development of animals and the role of wildfire in ecosystems reconstructions of past oxygen concentrations from fossil charcoal constrain atmospheric oxygenation over the past 400 million years variations of the earth s atmospheric oxygen concentration po2 are thought to be closely tied to the evolution of life with strong feedbacks between uni and multicellular life and oxygen1 2 on the geologic timescale po2 is regulated by the burial of organic carbon and sulphur as well as by weathering3 reconstructions of atmospheric o2 for the past 400 million years have therefore been based on geochemical models of carbon and sulphur cycling4 5 6 however these reconstructions vary widely4 5 6 7 8 9 10 particularly for the mesozoic and early cenozoic eras here we show that the abundance of charcoal in mire settings is controlled by po2 and use this proxy to reconstruct the concentration of atmospheric oxygen for the past 400 million years we estimate that po2 was continuously above 26 during the carboniferous and permian periods and that it declined abruptly around the time of the permian triassic mass extinction during the triassic and jurassic periods po2 fluctuated cyclically with amplitudes up to 10 and a frequency of 20 30 million years atmospheric oxygen concentrations have declined steadily from the middle of the cretaceous period to present day values of about 21 we conclude however that variation in po2 was not the main driver of the loss of faunal diversity during the permo triassic and triassic jurassic mass extinction events

late middle ordovician environmental change and extinction harbinger of the late ordovician or continuation of cambrian patterns positive excursions in carbon isotope compositions of carbonate 3 and organic carbon 4 6 from the late middle ordovician middle caradocian of the midcontinent and the eastern united states indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric p co 2 precipitating global cooling although not necessarily ice sheet formation these climatic changes were associated with regional orogenic uplift a relative rise in sea level changes in epeiric sea circulation patterns and carbonate platform destruction that led to regional extinction of marine benthos the combination of sea level rise changing ocean circulation and extinction in the middle caradocian is similar to the suite of environmental changes described for cambrian biomere boundaries suggesting shared causes for these events in contrast middle caradocian environmental changes are markedly different from the environmental patterns associated with the late ordovician mass extinction despite the evidence for long term cooling from the middle to the late ordovician

evolution of fractures and alteration in the sierrita esperanza hydrothermal system pima county arizona the sierrita esperanza porphyry copper complex is central to a large fossil hydrothermal system located in the southeastern part of the sierrita mountains arizona abundant closely spaced fractures exert a profound control of sulfide minerals and alteration in the mineralized center of the system the extent and limits of the hydrothermal system in peripheral areas have been defined in this study by determination of the distribution and abundance of the altered and mineralized fractures mapping of these elements to their limit of occurrence has resulted in recognition of an area of about 60 km 2 where fractures related to porphyry copper mineralization occur and a contiguous area of about 40 km 2 where slightly older altered fractures related to the first stages of laramide igneous activity in the region are present within the orebody distinctive fracture types as recognized by fracture style and alteration assemblages occur in a consistent paragenetic succession this same succession has been recognized in the areas peripheral to the pits but with variations for each alteration type as influenced by wall rock mineralogies the establishment of a sequence of vein alteration commencing with quartz orthoclase alteration passing through precipitation of quartz and sulfides and ending with formation of epidote orthoclase chlorite has allowed separation of times of fracture formation in the orebody periphery fracture density fracture length sample area maps have been prepared from data collected at 1 310 stations in the study area the data were processed and transformed into maps of moving average contours of fracture abundance for different times of fracture formation as interpreted from the alteration paragenesis the distinctive differences in alteration types in crosscutting veins reveal the episodic nature of fracturing and the fracture density maps show shrinking of fractured rock volumes toward the central porphyry mass with successive fracture events evaluation of the fracture area in a 1 km thick slab of the 40 km 2 area at the time of precipitation of quartz and sulfide minerals in veins indicates that 4 x 10 5 km 2 of crack area existed a slightly smaller value of 6 x 10 4 km 2 per km thick slab existed at the time of the earliest quartz orthoclase alteration

chemical variations in assemblages including epidote and or chlorite in the fossil hydrothermal system of saint martin lesser antilles abstract epidote and or chlorite are common minerals in the roots of the fossil geothermal system of saint martin lesser antilles they appear in four distinct assemblages 1 epidote actinolite quartz magnetite near the contact between the tuffaceous host rocks andesitic modal composition and the quartz diorite intrusion of philipsburg 2 epidote chlorite quartz in host rocks as far as a lateral distance of about 3 km from the intrusion 3 epidote chlorite haematite quartz locally in iron and manganese rich host rocks 4 chlorite phengite magnetite appearing as late sealing of porosity in fracture controlled quartz veins with strongly phengitized wall rocks all these assemblages constitute a large alteration grading from propylitic alteration to thermal metamorphism actinolite bearing assemblage detailed microprobe studies of epidotes replacing plagioclases and of chlorites replacing glass and mafic minerals reveal notable compositional variations which have been studied with respect to temperature paleogradients estimated from fluid inclusions study distance from the thermal source and fo2 conditions the mean ps pm 100 fe3 mn3 al3 fe3 mn3 of epidotes vary from 21 in the presence of magnetite near the intrusion to 32 in haematite bearing iron and or manganese volcanic and sedimentary formations the intra grain chemical scattering of epidotes increases with increasing distance of the pluton and decreasing temperature of crystallization all the chlorites coexisting with epidote are mg rich xfe

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

copper mineralization and alterations in gercus basalt within the gercus formation northern iraq abstract the gercus basalt is a recently discovered body within the gercus formation which is exposed on both limbs of the large double plunging nw se trending bekhair anticline to the north of duhok city northern iraq the basalt is mostly basanitic and alkaline in nature greenish to grayish black in color and very fine grained vesicular amygdaloidal and microscopically porphyritic in texture it consists of anorthoclase diopside forsterite and accessory titaniferous magnetite ilmenite and apatite as well as their alteration products the primary minerals are pervasively altered by calcitization zeolitization serpentinization chloritization silicification iddingsitization martitization and possibly anorthoclasization copper mineralization has taken place as veins and fracture joint filling which is mostly observed near the northwestern plunge of the anticline where the basalt attains its maximum thickness of about 16 m twelve copper minerals were found within veins and fractures including primary minerals bornite chalcopyrite and an unidentified sulfide cu2 3fes3 and their secondary alteration and replacement products represented by sulfides covellite digenite and chalcocite native copper cuprite carbonates malachite and azurite silicate chrysocolla and possibly sulfates the copper minerals are in many cases zonally arranged as spherules where the primary minerals acted as cores surrounded outward by secondary minerals primary copper mineralization is of late magmatic hydrothermal origin postdating the emplacement consolidation deuteric alterations and fracturing jointing of the gercus basalt followed by the formation of secondary supergene copper minerals as a result of alteration and replacement of the primary copper sulfides along mineralized veins and fractures joints due to chemical weathering by meteoric and or groundwater during eocene time

middle upper ordovician darriwilian sandbian paired carbon and sulfur isotope stratigraphy from the appalachian basin usa implications for dynamic redox conditions spanning the peak of the great ordovician biodiversification event abstract the evans ferry section from the appalachian basin of eastern north america has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the great ordovician biodiversification event gobe paired stable isotope 13c and 34s analyses were used in this carbonate dominated locality from the appalachian basin to reconstruct the marine redox states during this key period of rapid biodiversification this succession is one of the most expanded sandbian stage upper ordovician deposits known from north america and it allows new high resolution reconstructions of long term global carbon and sulfur cycle fluctuations the integrated geochemical and sequence stratigraphic investigation presented here from the laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the middle late ordovician that may have been linked to global marine paleoredox dynamics utilizing new conodont based 87sr 86sr isotope stratigraphy along with previous conodont biostratigraphy we can confidently correlate our stable isotope profiles to other carbonate successions globally carbonate facies indicate that this area experienced partial restriction from open marine conditions during certain intervals in the sandbian these semi restricted environments record carbon 13ccarb isotope trends that are secular in nature as they can be correlated to other successions across laurentia and baltica we identify several intervals when 13ccarb and 34scas carbonate associated sulfate trends are decoupled these inverse stratigraphic trends are at times followed by parallel positive shifts in 13ccarb and 34scas causal mechanisms for the observed decoupled 13c and 34s trends may include a wide variety of factors such as more closed system local biogeochemical processes associated in part with diagenesis or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions the positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments

role of permeability versus temperature in the distribution of the fine 0 2 μm clay fraction in the chipilapa geothermal system el salvador central america abstract detailed x ray diffraction of clay minerals from the less than 0 2 m fraction of altered rocks has been carried out on cuttings and cores from three boreholes ch7 ch9 and ch7b from the geothermal field of chipilapa el salvador central america in most of the low permeable horizons the vertical distribution of i s and c s mixed layers fairly agrees with the general scheme of di smectite to illite and tri smectite to chlorite conversion series with increasing depth max 2600 m and temperature max 220 c the deepest parts of the wells are characterized by chlorite and illite rich illite smectite mixed layers i sr 1 in close agreement with the measured temperature 170 220 c within fracture controlled permeable horizons the distribution of clay minerals strongly disagrees with the classical pattern of di smectite to illite and tri smectite to chlorite conversions observed in low permeable horizons great amounts of smectites and or smectite rich mixed layers occur at temperatures which strongly exceed the current range of smectite stability in natural systems up to 205 c similar discrepancies may also be viewed from chlorite crystallinity in well ch9 and smectite content in smectite rich chlorite smectite mixed layers in well ch7b these high temperature smectites are considered as present or very recent metastable phases representing the first step of kinetically controlled conversion series toward illite and or chlorite such phenomena are restricted to the levels in which the present hydrodynamic conditions promote the crystallization process boiling and mixing of fluids with contrasting temperatures high temperature smectites and smectite rich mixed layers may be of great interest for the understanding of the very recent evolution of geothermal reservoirs they do not only indicate the present horizons of active fluid circulations in geothermal wells smectite coatings at the wall of fractures but they are also potential guides in the determination of the horizons in which permeability has recently decreased

carbon isotope record of late cambrian to early ordovician carbonates of the argentine precordillera a more than 2000 m thick cambrian ordovician carbonate platform succession developed on the exotic argentine precordillera terrane during rifting from laurentia and drifting towards gondwana on base of these carbonates a carbon isotope curve could be developed for the cambrian ordovician we measured 13ccarb and 13corg values on bulk rocks selected components and diagenetic cements whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis most subtidal carbonates exhibit primary marine values we can report reliable curves for the middle late cambrian transition and for the latest cambrian to earliest middle ordovician the 13c curve matches well with the published data from global cambrian ordovician boundary sections excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy this indicates the interdependence on sea level which rules the productivity and or preservation of organic carbon and therefore the partition between corg and ccarb burial

revised interpretations of stable c and o patterns in carbonate rocks resulting from meteoric diagenesis abstract a positive correlation between the 13c and 18o values of carbonate rocks is a screening tool widely used to identify the overprint of meteoric diagenesis on the original isotopic composition of a sample in particular it has been suggested that systematic change from negative to positive 13c and 18o values with increasing depth in the core is an indicator of alteration within the zone of mixing between meteoric and marine waters in this paper we propose that such covariance is not generated within the traditionally defined mixing zone and that positive correlations between 13c and 18o values in marine carbonates are not necessarily indicators of meteoric alteration this new interpretation is based on data collected from the shallow sub surface of the bahamas a region unequivocally influenced by meteoric waters to depths of at least 200 m below the current sediment water interface the classic interpretation of the diagenetic environments based on changes in the 13c and 18o values would suggest the maximum penetration of freshwater occurs between 65 and 100 m below seafloor below these depths a strong positive covariation between the 13c and 18o values exists and would traditionally be defined as the mixing zone however based upon known changes in sea level the penetration of the freshwater lens extends significantly below this limit we contend that the zone showing covariance of 13c and 18o values is actually altered within the freshwater lens and not the mixing zone as previously proposed the co varying trend in 13c and 18o values is the result of diagenetic processes occurring at the interface between vadose and phreatic zones significantly greater rates of recrystallization and neomorphism are driven by the increased rates of oxidation of organic matter at this transition with progressively less alteration occurring with increasing depth as sea level oscillates the position of this interface moves through the deposit causing cumulative alteration throughout the section hence we propose that the covariation between 13c and 18o values is a consequence of varying degrees of alteration rather than the result of diagenesis occurring within the zone where marine and freshwater fluids mix furthermore within the pervasively altered vadose zone there is little correlation between 13c and 18o values and therefore covariation between 13c and 18o values is not an unequivocal indicator of meteoric 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

techniques for assessing spatial heterogeneity of carbonate δ13c values implications for craton wide isotope gradients the sedimentary record of carbonate carbon isotopes 13ccarb provides one of the best methods for correlating marine strata and understanding the long term evolution of the global carbon cycle this work focuses on the late ordovician guttenberg isotopic carbon excursion a ca 2 5 positive 13ccarb excursion that is found in strata globally substantial variability in the apparent magnitude and stratigraphic morphology of the guttenberg excursion at different localities has hampered high resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle this work investigates the magnitude spatial scale and sources of isotopic variability of the guttenberg excursion in two sections from missouri usa centimetre scale isotope transects revealed variations in 13ccarb and 18ocarb greater than 2 across individual beds linear 13ccarb to 18ocarb mixing lines together with petrographic and elemental abundance data demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components these patterns facilitated objective sample screening to determine the least altered data a 18ocarb filter based on empirical 18ocarb values of well preserved carbonate mudstones allowed further sample discrimination the resulting least altered 13ccarb profile improves the understanding of regional as well as continental scale stratigraphic relations in this interval correlations with other laurentian sections strongly suggest that i small scale variability in guttenberg excursion 13ccarb values may result in part from local diagenetic overprinting ii peak guttenberg excursion 13ccarb values of the midcontinent are not distinct from their taconic equivalents and iii no primary continental scale spatial gradient in 13ccarb for example arising from chemically distinct aquafacies is required during guttenberg excursion time this study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for 13ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry

the cenomanian turonian oceanic anoxic event ii palaeoceanographic controls on organic matter production and preservation correlation of the 13c spike with the well dated occurrences of strata rich in organic carbon detailed in schlanger et al this volume indicates that a global episode of intense organic carbon orgc burial took place during the latest cenomanian earliest turonian oceanic anoxic event oae a plenus through i labiatus macrofossil zones and upper r cushmani trz through w archecretacea prz foraminiferal zones over a period of no more than 1 million years m y the shape of the 13c curve indicates that rates of orgc burial gradually increased in the early part of the late cenomanian increased more rapidly in the later cenomanian and levelled off at peak values in latest cenomanian early turonian time during the maximum rate of orgc burial the 13c values decreased nearly to pre late cenomanian levels in the early to middle turonian the decrease in 13c reflects decreasing rates of orgc burial following the cenomanian turonian oceanic anoxic event as well as the probable oxidation and return of significant amounts of orgc to the oceans following regression and re oxygenation of much of the deeper water masses in contact with the seafloor

did early land plants produce a stepwise change in atmospheric oxygen during the late ordovician sandbian 458 ma abstract a stepwise change in atmospheric oxygen o2 levels during the ordovician has been attributed to the emergence of land plants this phenomenon is tied to a major baseline shift in the stable carbon isotope 13c curve and inferred increase in nutrient delivery and enhanced primary productivity in nearshore settings which led to high organic carbon burial the timing and magnitude of this baseline shift however is still elusive in part because of the lack of high resolution 13c data that span this period much of the existing ordovician 13c literature is focused on isotopic excursions with less emphasis on identifying long term shifts in baseline pre and post excursion values this study presents new high resolution 13c data from stratigraphic sections at germany valley west virginia and union furnace pennsylvania in the central appalachian basin these sections span the entire sandbian stage and continue into the lower katian stage the 13c data from both sections are characterized by relative stability carbon isotope values mean 0 61 in the lower sandbian followed by a 1 2 shift in the upper sandbian mean 0 62 herein we propose that the positive shift represents a long term global shift in baseline 13c values of dissolved inorganic carbon the timing of this positive shift coinciding with the diversification of early land plants i e bryophytes supports earlier models that suggested enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the late ordovician

ecostratigraphical interpretation of lower middle ordovician east baltic sections based on brachiopods a detailed ecostratigraphical framework is established for the lower middle ordovician kundan regional stage of the east baltic area corresponding to the asaphus expansus a raniceps and a eichwaldi trilobite zones lower darriwilian the study is based on approximately 6200 brachiopods collected bed by bed from limestone sections in northern estonia harku trench and saka and western russia putilovo quarry lava river canyon and lynna river valley with in addition the first detailed systematic assessment of the kundan brachiopods of the east baltic these sections represent an oblique depth transect some 400 kilometres long deepening eastwards five biofacies associations have been recognized using detrended correspondence and cluster analyses a shallow water lycophoria association a transitional gonambonites association and two deeper water associations the soft substrate orthis callactis and the hard substrate orthambonites associations a separate fifth soft substrate association is present in the marl beds at the main locality of putilovo quarry the associations reflect a combination of palaeo water depth and substrate the biofacies facilitate an ecostratigraphical correlation along the transect and third and fourth order sea level curves are reconstructed reflecting mainly eustasy the sea level was relatively low early in the kundan but then rose significantly into the a raniceps biozone this corroborates the recent discovery of possible small early darriwilian ice caps on gondwana

clay mineral signatures of fossil and active hydrothermal circulations in the geothermal system of the lamentin plain martinique whereas the study of clay minerals using parameters such as structure modifications crystal chemistry has proved to be an effective approach in order to mark the circulation zones and fluid temperatures in high temperature systems there is an important lack of data dealing with such problems in moderate to low temperature systems this paper proposes to fill this lack by studying the site of the lamentin plain martinique characterized by a lowtemperature geothermal activity 50 c6 t6 90 c preliminary data acquired on the three studied exploration wells la01 la02 and la03 show that the current low temperature geothermal activity is particularly well expressed through the la03 well this low temperature activity is superimposed on evidence of a high temperature fossil activity particularly well expressed in the non productive levels of the la01 and la02 wells after a petrographic characterization this work focuses on the textural and microstructural properties of the low temperature hydrothermal clay phases utilizing x ray diffraction xrd fourier transform infrared ftir spectrometry and scanning electron microscope sem the petrographic observations have permitted successive hydrothermal events to be recognized which express the thermal collapse of the system as shown by the superposition of ancient episodes characterized by vein and pervasive alterations of high to moderate temperatures and recent episodes characterized by alterations associated with present day circulations of low temperature fluids the fossil activity results from several hydrothermal episodes with fluid temperatures ranging from 100 to over 200 c illitic phases i e i s r 1 to illite appear as the main markers of the fossil solution paths due to their abundant development close to major veins and fractures while chloritic phases chlorite corrensite seem to mark a pervasive alteration better preserved further from the fossil solution channels on the other hand the recent hydrothermal alteration is characterized by the massive presence of kaolinite dioctahedral smectite and siderite whereas smectites are only observed abundantly farther from the current solution channels kaolinite appears to be the dominant clay mineral in these channels or in the surroundings of sealed fractures which do not currently drain any more hydrothermal fluids the crystallinity estimations on xrd and ftir patterns from the 6 2 wm fraction along with sem morphological observations clearly show that the microstructural and textural properties of the kaolinites can be related to fluid temperatures indeed kaolinites with the best crystallinity or the best ordered are located in the fractures carrying the hottest fluid

cascading trend of early paleozoic marine radiations paused by late ordovician extinctions the greatest relative changes in marine biodiversity accumulation occurred during the early paleozoic the precision of temporal constraints on these changes is crude hampering our understanding of their timing duration and links to causal mechanisms we match fossil occurrence data to their lithostratigraphical ranges in the paleobiology database and correlate this inferred taxon range to a constructed set of biostratigraphically defined high resolution time slices in addition we apply capture recapture modeling approaches to calculate a biodiversity curve that also considers taphonomy and sampling biases with four times better resolution of previous estimates our method reveals a stepwise biodiversity increase with distinct cambrian and ordovician radiation events that are clearly separated by a 50 million year long period of slow biodiversity accumulation the ordovician radiation is confined to a 15 million year phase after which the late ordovician extinctions lowered generic richness and further delayed a biodiversity rebound by at least 35 million years based on a first differences approach on potential abiotic drivers controlling richness we find an overall correlation with oxygen levels with temperature also exhibiting a coordinated trend once equatorial sea surface temperatures fell to present day levels during the middle ordovician darriwilian age contrary to the traditional view of the late ordovician extinctions our study suggests a protracted crisis interval linked to intense volcanism during the middle late ordovician katian age as richness levels did not return to prior levels during the silurian a time of continental amalgamation we further argue that plate tectonics exerted an overarching control on biodiversity accumulation

use of two new na li geothermometric relationships for geothermal fluids in volcanic environments abstract this work has made it possible to obtain two new na li geothermometric relationships in addition to the three already known fouillac and michard 1981 kharaka et al 1982 and confirms that the na li geothermometer unlike the na k na k ca k mg and silica geothermometers or the isotope 18o h2o so4 geothermometer also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water one of the relationships concerns the fluids derived from seawater basalt interaction processes existing in emerged rifts such as those of iceland reykjanes svartsengi and seltjarnarnes geothermal fields and djibouti asal ghoubbet and obock geothermal areas or in numerous oceanic ridges and rises mid atlantic and middle valley ridges east pacific rise etc the best adapted na li relationship for geothermal fluids discharged from emerged rifts between 0 and 365 c is t k 920 log n a l i 1 105 r 2 0 994 n 27 where na and li are the aqueous concentrations of these elements given in mol l the other na li relationship was determined using dilute waters collected from wells located in different high temperature 200 325 c volcanic geothermal areas of iceland krafla namafjall nesjavellir and hveragerdi this relationship can be expressed as follows t k 2002 log na li 1 322 r2 0 967 n 17 these two relationships give estimations of temperature with an uncertainty close to 20 c the second na li relationship was also successfully applied to ht dilute geothermal waters from the east african rift ethiopia kenya some case studies in the literature and thermodynamic considerations suggest that the na li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite k feldspar quartz and clay minerals such as kaolinite illite or muscovite and li micas unlike the na li ratios no thermometric relationship using li isotopes could be determined for this type of water however it was noticed that 7li values higher than 16 are always associated with low to medium temperature waters

dissolution methods for strontium isotope stratigraphy guidelines for the use of bulk carbonate and phosphorite rocks diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy sis this is especially important for precambrian and cambrian studies for which no suitable skeletal material is available one proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution we applied a similar technique to dissolve a range of lithologies limestone dolostone and phosphorite and compared the strontium isotopic composition of various leaches to contemporaneous seawater our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near primary seawater 87sr 86sr composition once contaminant strontium from secondary calcite and other phases has been removed commonly applied trace element e g mn sr mg ca and isotopic c o screening cut offs were also examined in the light of these results and proved unreliable particularly in the case of dolostones and partially dolomitized limestones we recommend that rigid sample selection and sequential leaching procedures be applied to all sis studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future

characteristics and alteration origins of matrix minerals in volcaniclastic kimberlite of the muskox pipe nunavut canada abstract the matrix of volcaniclastic kimberlite vk from the muskox pipe northern slave province nunavut canada is interpreted to represent an overprint of an original clastic matrix muskox vk is subdivided into three different matrix mineral assemblages that reflect differences in the proportions of original primary matrix constituents temperature of formation and nature of the altering fluids using whole rock x ray fluorescence xrf whole rock x ray diffraction xrd microprobe analyses back scatter electron bse imaging petrography and core logging we find that most matrix minerals serpentine phlogopite chlorite saponite monticellite fe ti oxides and calcite lack either primary igneous or primary clastic textures the mineralogy and textures are most consistent with formation through alteration overprinting of an original clastic matrix that form by retrograde reactions as the deposit cools or in the case of calcite by precipitation from ca bearing fluids into a secondary porosity the first mineral assemblage consists largely of serpentine phlogopite calcite fe ti oxides and monticellite and occurs in vk with relatively fresh framework clasts alteration reactions driven by deuteric fluids derived from the juvenile constituents promote the crystallisation of minerals that indicate relatively high temperatures of formation 400 c lower temperature minerals are not present because permeability was occluded before the deposit cooled to low temperatures thus shielding the facies from further interaction with fluids the other two matrix mineral assemblages consist largely of serpentine phlogopite calcite diopside and chlorite they form in vk that contains more country rock which may have caused the deposit to be cooler upon emplacement most framework components are completely altered suggesting that larger volumes of fluids drove the alteration reactions these fluids were likely of meteoric provenance and became heated by the volcaniclastic debris when they percolated into the vk infill most alteration reactions ceased at temperatures 200 c as indicated by the absence or paucity of lower temperature phases in most samples such as saponite recognition that muskox vk contains an original clastic matrix is a necessary first step for evaluating the textural configuration which is important for reconstructing the physical processes responsible for the formation of the deposit

dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s geochemical records of the end ordovician hirnantian stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur 34spyr organic carbon 13corg and carbonate carbon 13ccarb these isotope excursions coincide with the end ordovician glaciation and mass extinction a relative increase in pyrite burial fpyr attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna an increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate 34sso4 34sso4 here we present new high resolution paired sulfur isotope data from carbonate associated sulfate 34scas and pyrite from the hirnantian stage of western anticosti island quebec canada these data document a positive 20 enrichment in 34spyr comparable in magnitude to previous reports but no parallel excursion in 34scas this pattern provides new constraints on the origin of the 34spyr excursion and the nature of carbon sulfur coupling through hirnantian time specifically these observations preclude enhanced pyrite burial as the cause of the hirnantian 34spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated rather the global 34spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling epyr the epyr record shows a strong inverse correlation with 13c suggesting a mechanistic link between carbon cycling and processes controlling epyr during the hirnantian changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation e g through syndepositional sediment reworking and or chemocline migration and resultant restricted exchange of porewater sulfate the magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates which are sensitive to the abundance type and lability of metabolically relevant substrates environmental change associated with the end ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments leading to increased microbial sulfate reduction rates and reduced epyr as such the hirnantian 34spyr excursion may be viewed as a dynamic biological response to global climate change highlighting the connections between the carbon and sulfur biogeochemical cycles

origins of calcite in a boiling geothermal system the formation of hydrothermal calcite relates to the movement of carbon dioxide in a geothermal system as governed by boiling dilution and condensation in this paper we show how these processes control the occurrence distribution and stable isotope composition of calcite based on a study at broadlands ohaaki the two principal calcite occurrences in the broadlands ohaaki geothermal system are 1 as replacement of rock forming minerals and volcanic glass and 2 as platy crystals infilling voids both are stable over a broad temperature range from 160 degrees c to 300 degrees c replacement calcite is widespread and forms through hydrolysis reactions involving calcium alumino silicates and sub boiling liquids that contain 0 3 to 0 75 m co 2 platy calcite in contrast forms over a restricted vertical interval of a few hundred meters within the upflow zone it precipitates from boiling fluids through exsolution of carbon dioxide as indicated by coeval liquid rich and vapor rich fluid inclusions and its formation in the two phase zone fluid inclusion data help to define the boiling paths of fluids from which platy calcite formed homogenization temperatures range from 160 degrees c to 310 degrees c and are consistent within the present geothermal regime ice melting temperatures range from 0 0 to 1 0 degrees c and indicate the presence of up to 0 5 m dissolved carbon dioxide model boiling curves calculated to match these data show how the concentration of dissolved carbon dioxide in the preboiled fluid dictates the depth of first boiling most fluid inclusion data lie along a model boiling path characteristic of the centre of the upflow zone in which the rising fluid initially containing 0 75 m co 2 begins to boil at approximately 320 degrees c and approximately 2000 m depth data from well br 18 instead matches a curve in which the rising fluid initially containing 0 53 m co 2 begins boiling at approximately 245 degrees c and approximately 900 m depth the shallowing of the depth of first boiling likely results from dilution of dissolved carbon dioxide in the parent chloride water as it rises and mixes with marginal waters calcite precipitates from both shallow formed steam headed groundwater and deeply derived chloride water and these waters are isotopically distinct at broadlands ohaaki the delta 18 0 values of calcite at 200 degrees c range from 0 5 to 7 5 per mil whereas delta 18 0 values of calcite at 200 degrees c range from 4 to 10 per mil taking appropriate temperature dependent fractionation factors into account these data indicate equilibration with chloride water delta 18 0 h2o 4 5 per mil and steam heated groundwater delta 18 o h2o 7 0 per mil respectively oxygen isotopes of hydrothermal calcites in the nearby wairakei and waiotapu geothermal systems show similar patterns consistent with the occurrence of both chloride and steam heated waters there calcite formation is explained by a model that describes the distribution of two phase conditions and aqueous carbon dioxide concentrations in a column of hydrothermal fluid rising through a rock matrix of isotropic permeability in this ideal situation platy calcite forms along the inner margin of the two phase zone having the shape of an inverted cone whereas replacement calcite mostly forms in the surrounding one phase liquid only zone the sparse occurrence of calcite at less than or equal to 800 m depth in the central upflow of the ohaaki sector at broadlands ohaaki is compatible with this model and appears related to the exsolution of dissolved carbon dioxide through boiling deeper in the system

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

a major drop in seawater 87sr 86sr during the middle ordovician darriwilian links to volcanism and climate a large drop in seawater 87 sr 86 sr during the middle ordovician was among the most rapid in the entire phanerozoic new 87 sr 86 sr measurements from nevada indicate that the rapid shift began in the pygodus serra conodont zone of the upper darriwilian stage we use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic sr to the oceans a close balance between volcanic outgassing and co 2 consumption from weathering produced steady p co 2 levels and climate through the middle katian consistent with recent ordovician paleotemperature estimates in the late katian outgassing was reduced while volcanic weathering continued and resulted in a cooling episode leading into the well known end ordovician glaciation

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

oxygenation history of the neoproterozoic to early phanerozoic and the rise of land plants there has been extensive debate about the history of earth s oxygenation and the role that land plant evolution played in shaping earth s ocean atmosphere system here we use the rare earth element patterns in marine carbonates to monitor the structure of the marine redox landscape through the rise and diversification of animals and early land plants in particular we use the relative abundance of cerium ceanom the only redox sensitive rare earth element in well preserved marine cements and other marine precipitates to track seawater oxygen levels our results indicate that there was only a moderate increase in oceanic oxygenation during the ediacaran average cryogenian ceanom 1 1 average ediacaran ceanom 0 62 followed by a decrease in oxygen levels during the early cambrian average cryogenian ceanom 0 90 with significant ocean anoxia persisting through the early and mid paleozoic average early cambrian early devonian ceanom 0 84 it was not until the late devonian that oxygenation levels are comparable to the modern average of all post middle devonian ceanom 0 55 therefore this work confirms growing evidence that the oxygenation of the earth was neither unidirectional nor a simple two stage process further we provide evidence that it was not until the late devonian when large land plants and forests first evolved that oxygen levels reached those comparable to the modern world this is recorded with the first modern like negative ceanom values 0 6 occurring at around 380 ma frasnian this suggests that land plants rather than animals are the engineers responsible for the modern fully oxygenated earth system

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

first plants cooled the ordovician the late ordovician period ending 444 million years ago was marked by the onset of glaciations the expansion of non vascular land plants accelerated chemical weathering and may have drawn down enough atmospheric carbon dioxide to trigger the growth of ice sheets

inclusion of the weathering of volcanic rocks in the geocarbsulf model the carbon cycle model geocarbsulf is extended by dividing the weathering of silicates into volcanic and non volcanic rocks the proportion of volcanic weathering is calculated as a function of time from the oceanic record of 87sr 86sr the volcanic proportion is then used to modify the equations for calculating atmospheric co2 by the addition of a new non dimensional volcanic weathering factor the effect of uplift and physical erosion on weathering is also modified by using only the distribution over time of the abundance of sandstones and shales and not sr isotopic data that had been used previously results indicate moderate change from geocarbsulf in the distribution of atmospheric co2 over phanerozoic time this includes an increased minimum in co2 during the late ordovician in agreement with the presence of a continental glaciation at that time and a shift of maximum mesozoic values from the jurassic to the early cretaceous

calibration of a conodont apatite based ordovician 87sr 86sr curve to biostratigraphy and geochronology implications for stratigraphic resolution the ordovician 87 sr 86 sr isotope seawater curve is well established and shows a decreasing trend until the mid katian however uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 sr 86 sr decrease may have varied which has implications for both the stratigraphic resolution possible using sr isotope stratigraphy and efforts to model the effects of ordovician geologic events we measured 87 sr 86 sr in conodont apatite in north american ordovician sections that are well studied for conodont biostratigraphy primarily in nevada oklahoma the appalachian region and ohio valley our results indicate that conodont apatite may provide an accurate medium for sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 sr 86 sr during the middle ordovician darriwilian stage our 87 sr 86 sr results suggest that sr isotope stratigraphy will be most useful as a high resolution tool for global correlation in the mid darriwilian to mid sandbian when the maximum rate of fall in sr

ordovician metre scale cycles implications for climate and eustatic fluctuations in the central appalachians during a global greenhouse non glacial to glacial transition abstract metre scale shallowing upward cycles in ordovician carbonates of the central appalachian basin record climate and eustatic fluctuations during a transition from early ordovician global greenhouse to late ordovician icehouse conditions peritidal facies and shale abundance suggest a long term trend in this area from semi arid early ordovician to more humid middle to early late ordovician conditions with a return to semi arid conditions during the late ordovician the climatic fluctuations were most likely produced by tectonics uplift and erosion related to taconic orogenesis plate motion of north america and the areal extent of water covering the shelf peritidal cyclic facies indicate that high frequency relative sea level fluctuations were of small amplitude 20 m relative sea level fluctuations that decreased into the later ordovician if these sea level changes are eustatic then the increased amplitude may mark the early initiation of continental glaciation on gondwana in the late middle ordovician followed by waning of ice sheets prior to the latest ordovician glaciation

dehydration of dioctahedral aluminous phyllosilicates thermodynamic modelling and implications for thermobarometric estimates we propose a solid solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations including hydration observed in natural smectites interlayered illite smectite illites and phengites from diagenetic to high grade metamorphic conditions the suggested formalism involves dehydrated micas and hydrated pyrophyllite like thermodynamic end members with these end members the equilibrium conditions of quartz water k bearing mica like phyllosilicates of fixed 2 1 composition are represented by a line in p t space along which the interlayer water content varies the relevant thermodynamic properties required for the calculation of equilibrium conditions were derived using a set of 250 natural data of known maximal temperature and pressure conditions which covers a range between 25 c and few mpa to 800 c and 5 gpa the temperatures calculated at fixed pressure with our model are in fair agreement with those reported in the literature for the 250 natural data at low temperature and pressure the amount of interlayer water in k deficient phengite and illite is predicted to reach 100 of the apparent vacancies which is consistent with previous values reported in the literature although the amount of interlayer water is predicted to decrease with pressure and temperature it is calculated to be significant in k deficient phengite from lt hp pelites metamorphosed at about 350 c 10 kbar the presence of molecular water in the interlayer site of such phengites has been confirmed by ftir mapping its implications for p t estimates are discussed

phase relations among smectite r1 illite smectite and illite a variety of smectitic and illitic clays were studied by tem and aem following ex pansion by l r white resin to define phase relations for clay minerals undergoing di genesis and low grade metamorphism samples included a prograde shale sequence from the gulf coast a hydrothermal bentonite from zempleni hungary and shales from the nankai trough japan michigan and the welsh sedimentary basin all samples were dominated by various proportions of only three kinds of clay minerals smectite having no 10 alayers r1 illite smectite i s 50 illite and illite with only small proportions of smectite like interlayers mixed layer phases with intermediate ratios of i s were observed only as minor components of other layer sequences lattice fringe images show that the common layer spacing of r1 i s is 21 aand that it has 0 7 0 8 k pfu its properties are not an average of those of smectite and illite which is consistent with the uniqueness of the r1 i s structure a prograde sequence of clay mineral transitions in the studied samples can be charac terized by five stages with different combinations of the three major phases i e smectite r1 i s and illite corresponding to different grades the sequence from low to high grade is 1 pure smectite 2 smectite with small proportions of discrete r1 i s and illite 3 r1 i s with small proportions of smectite and illite 4 illite with some r1 i s and smectite and 5 illite common exceptions to this scheme are inferred to be caused by the inherent metastability of all phases in occurrences that are rate or path dependent

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

contribution of multi method geophysics to the understanding of a high temperature geothermal province the bouillante area guadeloupe lesser antilles the bouillante high temperature geothermal field guadeloupe french lesser antilles is located on the west coast of basse terre island about 15 km from the currently active soufriere volcano we have studied the shallow structure of the geothermal zone using a compilation of all available gravity magnetic and resistivity data both subaerial and marine acquired during the last 30 years geophysical models have been developed based on geological constraints and previous geophysical interpretations the resistivity distribution from electromagnetic surveys is interpreted in terms of water saturation of rocks hydrothermal alteration and presence of hydrated minerals gravity is used to distinguish and characterize the denser formations from the low density areas finally we are able to differentiate structures which pre and post date various recent magnetic inversions using the polarity of the magnetic anomalies by integrating these results the previous conceptual model for the geothermal province has been improved with the layering of the geological formations as defined by geophysical parameters now being taken into account in agreement with the classical scheme of geothermal reservoirs the shallow resistive layer is explained in terms of recent massive formations the conductive intermediate layer marks the low density demagnetized clay cover of the altered geothermal system the deep resistive layer is associated with dense formations increasing in age with depth and corresponds to the deep geothermal reservoir based on our geophysical data the geothermal activity could be more developed towards the east and north compared to the previously proposed area a volcano structural control seems to affect the internal structure of the province and consequently the development of geothermal activity this current work could be used as a basis for future surveys of the bouillante province in order to improve constraints and to increase our knowledge of the island s overall evolution

gradients in the carbon isotopic composition of ordovician shallow water carbonates a potential pitfall in estimates of ancient co2 and o2 abstract the carbon isotopic composition of the global dissolved inorganic carbon dic reservoir is best estimated from open ocean pelagic carbonate sediments 13 c carb however this is not practical for most of geologic time because seafloor subduction has removed the pre jurassic record and these time periods may have lacked planktonic calcifying organisms and therefore shallow water carbonate platform or periplatform sediments are utilized shallow water deposits are susceptible to a wide range of post depositional alteration processes and syn depositional controls on 13 c carb that include carbonate mineralogy water mass restriction and a host of related variables e g ph temperature organic decomposition evaporation and co 2 solubility that can produce local gradients in dic the degree to which shallow water 13 c curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses e g endemic faunas here we integrate strontium isotope 87 sr 86 sr stratigraphy and biostratigraphy to compare 13 c curves in a case study along a depth transect in middle late ordovician carbonate platform settings the restricted tidal flat and more open marine deposits are offset by a maximum of 2 during sea level drop and 0 during highstand flooding of the platform global carbon cycle models such as geocarbsulf use published 13 c carb curves to drive organic carbon burial rates under the assumption that 13 c carb reflects a global seawater signal we show here the potential pitfalls of using a published 13 c carb curve that violates this global assumption for the 460 million year middle late ordovician time bin in geocarbsulf improper usage of our locally depleted 13 c curve to drive global organic carbon burial would result in erroneous atmospheric o 2 10 8 o 2 equal to about a 30 reduction from what it should be at 15 2 o 2 using global 13 c and co 2 400 ppm too high equal to about a 13 increase from what it should be at 2570 ppm co 2 with detailed sedimentologic analysis it may be possible to identify and exclude 13 c samples that record the influences of local carbon cycling from global carbon cycle models such as geocarbsulf

strontium isotope 87sr 86sr stratigraphy of ordovician bulk carbonate implications for preservation of primary seawater values the present study on bulk carbonate 87 sr 86 sr stratigraphy represents a companion work to earlier research that presented a conodont apatite based ordovician seawater 87 sr 86 sr curve for the tremadocian katian stages 485 445 ma here we directly compare the curve based on conodont apatite including some new data not published in earlier work with a new curve based on 87 sr 86 sr results from bulk carbonate from the tremadocian sandbian stages we sampled eight lower to upper ordovician carbonate successions in north america to assess the reliability of bulk carbonate to preserve seawater 87 sr 86 sr and its utility for 87 sr 86 sr chemostratigraphy a high resolution 87 sr 86 sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 sr 86 sr 87 sr 86 sr seawater in total 230 bulk carbonate samples that are paired to conodont samples were measured for 87 sr 86 sr in order to determine the conditions under which 87 sr 86 sr seawater is preserved in bulk carbonate results indicate that well preserved bulk carbonate can faithfully record the 87 sr 86 sr seawater trend but that its 87 sr 86 sr values are commonly more variable than those of conodont apatite on average bulk carbonate samples of the same age vary by 10 20 10 5 compared to 5 10 10 5 for conodont apatite the amount of isotopic alteration of bulk carbonate from seawater 87 sr 86 sr 87 sr 86 sr was determined by taking the difference between 87 sr 86 sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 sr 86 sr values cross plots comparing 87 sr 86 sr values to bulk carbonate sr concentration sr and conodont color alteration indices cai an estimate of the thermal history of a rock body indicate that bulk carbonate is most likely to preserve 87 sr 86 sr seawater minimally altered when either 1 bulk carbonate sr is greater than 300 ppm or 2 carbonate rocks experienced minimal thermal alteration with burial temperatures less than 150 c carbonates with intermediate sr e g between 130 and 300 ppm can also yield 87 sr 86 sr seawater values but results are less predictable and local diagenetic conditions may play a greater role modeling results support the argument that seawater 87 sr 86 sr can be preserved in bulk carbonates with low sr if pore water rock ratios are low 87 sr 86 sr is similar to the seawater 87 sr 86 sr value preserved in limestone bulk carbonate samples that meet these criteria can be useful for high resolution measurements of 87 sr 86 sr seawater with a sample variation on par with fossil materials 5 particularly for successions where well preserved fossil material i e conodonts or brachiopods is not available such as precambrian strata sequences recording mass extinction events or otherwise fossil barren facies these criteria and model predictions based on bulk carbonate sr must be considered in the context of whether a limestone accumulated under calcite seas e g ordovician with relatively high seawater sr ca or aragonite seas in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non seawater sr

precise timing of the late ordovician sandbian super eruptions and associated environmental biological and climatological events two of the largest known eruptions in the phanerozoic produced the ordovician millbrig k bentonite of north america and the kinnekulle k bentonite of scandinavia which have been previously suggested to be coeval the millbrig k bentonite from kentucky usa and the kinnekulle k bentonite from bornholm denmark yielded chemical abrasion thermal ionization mass spectrometry u pb zircon dates of 452 86 0 29 and 454 41 0 17 ma 2 analytical uncertainty respectively thus showing significant age differences contrary to what is generally held these data and four additional newly dated k bentonites directly establish the first radioisotopic age constraints for the ordovician katian sandbian global stage boundary refine global stratigraphic correlations date associated chemostratigraphic events and suggest an alternative volcanic climate hypothesis for the late ordovician supplementary material u pb radioisotopic data table and analytical methods are available at www geolsoc org uk sup18636

oxygenation as a driver of the great ordovician biodiversification event the largest radiation of phanerozoic marine animal life quadrupled genus level diversity towards the end of the ordovician period about 450 million years ago a leading hypothesis for this great ordovician biodiversification event is that cooling of the ordovician climate lowered sea surface temperatures into the thermal tolerance window of many animal groups such as corals a complementary role for oxygenation of subsurface environments has been inferred based on the increasing abundance of skeletal carbonate but direct constraints on atmospheric o2 levels remain elusive here we use high resolution paired bulk carbonate and organic carbon isotope records to determine the changes in isotopic fractionation between these phases throughout the ordovician radiation these results can be used to reconstruct atmospheric o2 levels based on the o2 dependent fractionation of carbon isotopes by photosynthesis we find a strong temporal link between the great ordovician biodiversification event and rising o2 concentrations a pattern that is corroborated by o2 models that use traditional carbon sulfur mass balance we conclude that that oxygen levels probably played an important role in regulating early palaeozoic biodiversity levels even after the cambrian explosion an increase in biodiversity 450 million years ago coincided with a rise in atmospheric oxygen concentrations suggests a geochemical analysis oxygen availability may have thus helped spur the radiation alongside climatic cooling

hydrothermal alteration of deep fractured granite effects of dissolution and precipitation abstract this paper investigates the mineralogical effects of hydrothermal alteration at depth in fractures in granite a fracture accompanied by an alteration halo and filled with clay was found at a depth of 200 m in a drill core through toki granite gifu central japan microscopic observation xrd xrf epma and sxam investigations revealed that the microcrystalline clays consist of illite quartz and pyrite and that the halo round the fracture can be subdivided into a phyllic zone adjacent to the fracture surrounded by a propylitic zone in which fe phyllosilicates are present and a distinctive outer alteration front characterized by plagioclase breakdown the processes that result in these changes took place in three successive stages 1 partial dissolution of plagioclase with partial chloritization of biotite 2 biotite dissolution and precipitation of fe phyllosilicate in the dissolution pores 3 dissolution of k feldspar and fe phyllosilicate and illite precipitation associated with development of microcracks these hydrothermal alterations of the granite proceed mainly by a dissolution precipitation process resulting from the infiltration of hydrothermal fluid along microcracks such infiltration causes locally high mobility of al and increases the ratio of fluid to rock in the alteration halo these results contribute to an understanding of how granitic rock becomes altered in orogenic fields such as the japanese island arc

hydrothermal alteration and past and present thermal regimes in the western moat of long valley caldera abstract hydrothermal alteration in caldera fill early rhyolite and moat rhyolite was investigated in six western moat drillcores in long valley caldera with an emphasis on clay mineralogy in two of the cores examined a complete sequence was observed in argillic alteration of tuffaceous rocks progressing from smectite often with kaolinite to mixed layer illite smectite i s to illite with increasing depth portions of this sequence were observed in the other four drillcores lower permeability rhyolitic lavas typically showed much sparser clay alteration than tuffs in cores where alteration progressed through smectite to i s or illite the distribution of the clay minerals and of compositional and structural variation in the i s phases allowed estimation of pre existing alteration temperatures and thermal gradients i s and illite occurred at the shallowest depths and highest elevations in the shady rest sr core and comparable alteration occurred at progressively deeper levels in plv 1 and inyo 4 presently observed temperatures are well below preexisting temperatures at comparable depths in these three holes with the exception of the deeper portion of sr paleotemperatures in the zone of i s alteration in the sr hole 200 250 m depth were 65 c higher than at present with a thermal gradient at least twice the steep gradient existing there now in plv 1 temperatures near the base of the hole have declined even further than in sr despite the steep gradient currently observed in inyo 4 hydrothermal circulation somewhat cooler than that in sr and plv 1 occurred at one time in the deeper portion of the hole and the steep thermal gradient currently observed at shallower levels in inyo 4 may be the truncated remnant of a gradient that once continued to greater depth in mlgrap 1 hydrothermal circulation occurred previously near the base of the hole but at temperatures probably not far above present ones no evidence of high temperature alteration was observed in cores from mlgrap 2 or plv 2

middle late ordovician darriwilian sandbian decoupling of global sulfur and carbon cycles isotopic evidence from eastern and southern laurentia abstract middle late ordovician sequences from the appalachian basin and arbuckle mountain regions of north america were analyzed for carbonate associated sulfate 34 s cas and pyrite 34 s pyr paired with carbonate 13 c carb and organic matter 13 c org chemostratigraphy two major negative drops in 34 s cas 12 excursions are recognized the older decline in 34 s cas occurs within the histiodella holodentata phragmodus polonicus conodont zones and the younger drop is within the cahabagnathus sweeti amorphognathus tvaerensis baltoniodus gerdae subzone zones these overall these negative shifts in 34 s cas have an antithetical relationship with positive shifts in 34 s pyr 10 and 13 c carb 2 recorded in the same successions the older negative 34 s cas shift is coincident with the widely documented mid darriwilian 13 c excursion mdice and the younger negative 34 s cas shift is coincident with another positive 13 c carb shift in the early sandbian geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative 34 s cas trends additionally a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends while increased crustal weathering is broadly consistent with a sea level lowstand and the seawater 87 sr 86 sr isotope record of change in continental weathering in the late darriwilian stage of the ordovician geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in 34 s cas these antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates pulses of oceanic ventilation would have expanded habitable environments for marine organisms and thus is broadly consistent with major increases in biodiversification during this period of the ordovician

long lived glaciation in the late ordovician isotopic and sequence stratigraphic evidence from western laurentia the timing and causes of the transition to an icehouse climate in the late ordovician are controversial results of an integrated 13c and sequence stratigraphic analysis in nevada show that in the late ordovician chatfieldian stage mid caradoc a positive 13c excursion in the upper part of the copenhagen formation was closely followed by a regressive event evidenced within the prominent eureka quartzite the chatfieldian 13c excursion is known globally and interpreted to record enhanced organic carbon burial which lowered atmospheric p co2 to levels near the threshold for ice buildup in the ordovician greenhouse climate the subsequent regressive event in central nevada previously interpreted as part of a regional tectonic adjustment is here attributed in part to sea level drawdown from the initiation of continental glaciation on gondwana this drop in sea level which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p co2 by suppressing shelf carbonate production signals the transition to a late ordovician icehouse climate 10 m y before the widespread hirnantian glacial maximum at the end of the ordovician

two dimensional spreading and thickening of aufeis the growth of two dimensional or latera lly confined flume aufeis is shown from laboratory data to depend primarily on seven independent dime nsionless para meters during the early two di mensional phase of its growth auf eis cons ists of a mixture of ice and water or ice w ater slush forming on a frigid base its early growth depends on four parameters those expressing position along aufeis period of spre ading slope of frigid base over which aufeis forms and magnitude of heat flux to air from the surface of aufeis relative to latent heat release during freez ing the influences of two of the three rema ining parameters those express ing magnitude of heat flux to air relative to heat flux to fri gid base and confin ed width of aufeis growth are not felt until after a tra nsition time has passed the tran sition time apparently coin cides with the beginning of the processes by which the ice water slush on the surface of aufeis freezes solid after a slush layer on aufeis begin s to freeze solid a new slush layer forms over its frozen surf ace the contin uing cyclic proce ss by which slush layers form and eventually freeze results in the ice laminations that are a feature of aufeis the influence of the seventh governing parameter a reynol ds number cannot be discerned in the laboratory data

a new chlorite geothermometer for diagenetic to low grade metamorphic conditions the evolution of chlorite composition with temperature and pressure serves as basis to a number of chlorite chemical thermometers for which the oxidation state of iron has been recognised as a recurrent issue especially at low temperature t a new chlorite geothermometer that does not require prior fe3 knowledge is formulated calibrated on 161 analyses with well constrained t data covering a wide range of geological contexts and tested here for low t chlorites t 350 c and pressures below 4 kbar the new solid solution model used involves six end member components the mg and fe end members of al free chlorite s sudoite and amesite and so accounts for all low t chlorite compositions ideal mixing on site is assumed with an ordered cationic distribution in tetrahedral and octahedral sites applied to chlorite analyses from three distinct low t environments for which independent t data are available gulf coast texas saint martin lesser antilles toyoha hokkaido the new pure fe2 thermometer performs at least as well as the recent models which require an estimate of fe3 content this relief from the ferric iron issue combined with the simple formulation of the semi empirical approach makes the present thermometer a very practical tool well suited for for example the handling of large analytical datasets provided it is used in the calibration range t 350 c p 4 kbar

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

carbon cycling across the southern margin of laurentia during the late ordovician abstract the guttenberg isotope carbon excursion gice a positive carbon isotope excursion that occurs near the base of the katian stage is thought to be a global event possibly related to late ordovician cooling documenting how much regional and global variability exists in carbon isotopic trends prior to and during the gice is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle paleoclimate and chemostratigraphic correlation during the ordovician to investigate carbon isotopic trends along the southern margin of the laurentian carbonate platform during the late ordovician we measured bulk carbonate 13 c carb and 18 o carb as well as organic carbon 13 c org values from four locations in alabama these sections are excellent study sites because they are well exposed contain the regionally well correlated deicke and millbrig k bentonites and are in a region where 13 c trends have not been studied carbonate 13 c carb results from one section i e fort payne in northeastern alabama record a 1 5 positive excursion above the millbrig k bentonite we interpret the 1 5 positive excursion as a primary feature and identify it as the gice the characteristic gice excursion is absent in the other study sections and we attribute this absence to the presence of unconformities either non depositional or erosional in these sections we find neither evidence for sea level control on 13 c trends nor conclusive evidence for 13 c dic gradients along the southeast margin of laurentia s epeiric sea combined with the proximity to the iapetus ocean these results suggest that carbon isotopic trends in alabama are a reliable proxy for open ocean conditions therefore despite the absence of gice excursion in all but one of our study sections increasing 13 c values in alabama prior to the gice provides evidence for a steady 13 c enrichment of the global surficial carbon reservoir consistent with a drawdown of atmospheric co 2 during the early late ordovician

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates