meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

the flank margin model for dissolution cave development in carbonate platforms the bahama islands contain many abandoned dissolution caves at elevations between two and seven metres above current sea level the development of dissolution caves in tropical carbonate islands is dependent on the position and nature of the freshwater lens lens position is controlled by sea level which in stable carbonate platforms like the bahamas is a function of glacioeustatic sea level still stands caves in the bahamas that are currently subaerial must have developed during past higher sea levels during the late quaternary sea levels higher than present have been relatively short lived and that limits the amount of time that a freshwater lens could be situated at the elevation required for the cave formation the bahama islands are low lying landforms where only aeolian ridges extend to elevations higher than six metres above current sea level past high sea level events greatly reduced the exposed land area of the bahama islands thus also limiting both the catchment for and size of freshwater lenses caves must be younger than the rock in which they are developed most subaerial bahamian caves are found in limestones that are less than 150000 years old development of large dissolution caves under these limitations of time and lens size requires a powerful dissolutional mechanism the mixing of discharging freshwater with tide pulsed incoming marine water under the flanks of emergent dune ridges may have produced the conditions necessary bahamian caves formed by this process are phreatic chambers with complex interconnections and blind tubes their presence demonstrates that significant dissolution can occur rapidly as a result of the mixing of fresh and marine waters beneath small carbonate islands

diagenetic stabilization of the upper permian dalan formation persian gulf basin abstract the upper permian dalan formation is a supergiant gas reservoir in the persian gulf basin reservoir quality is controlled by the original facies type dolomitization and anhydrite cementation this study focuses on the texture petrographic analysis bulk stable isotope values and trace element composition in order to identify the original mineralogy of carbonate grains and cements and to assess the synsedimentary and diagenetic processes including the timing and setting of dolomitization the limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone and neomorphism and low mg calcite cementation blocky and drusy crystals syntaxial overgrowths in the active freshwater phreatic zone the relatively pristine 18o and 13c values low mn and relatively high sr and na contents indicate that this meteoric diagenesis was relatively limited the remaining porous space was cemented by blocky calcite later during burial four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites dolomicrite dolomicrosparite dolosparite and saddle dolomite dolomicrite formed syndepositionally from the evaporated permian seawater or by reflux of dense evaporated brines during shallow burial the second major episode of dolomitization dolomicrosparite and dolosparite was driven by reflux of warmer and more saline fluids at greater depth which affected both previously non dolomitized and dolomitized sediments the final diagenetic stabilization during deep burial is associated with extensive pressure solution fracturing vein filling of calcite and dolomite cements and crystallization of saddle dolomite

enigmatic origin of the largest known carbon isotope excursion in earth s history carbonate rocks from the middle ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in earth history this finding suggests a dramatic reorganization of earth s carbon cycle named the shuram excursion for its original discovery in the shuram formation oman the anomaly closely precedes impressive events in evolution including the rise of large metazoans and the origin of biomineralization in animals instead of a true record of the carbon cycle at the time of sedimentation the carbon isotope signature recorded in the shuram excursion could be caused by alteration following deposition of the carbonate sediments a scenario supported by several geochemical indicators however such secondary processes are intrinsically local which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe both possibilities are intriguing if the shuram excursion preserves a genuine record of ancient seawater chemistry it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern earth if however it represents secondary alteration during burial of sediments then marine sediments must have been globally preconditioned in a unique way to allow ordinary and local processes to produce an extraordinary and widespread response

δ13c stratigraphy of a turinian chatfieldian upper ordovician foreland succession ottawa embayment central canada resolving local and inter regional isotope excursions in a tectonically active basin abstract positive 13ccarb excursions are correlated through an upper turinian to lower chatfieldian carbonate platform succession along the axis of the ottawa embayment and into outliers of the northern ottawa bonnechere graben in central canada successive turinian excursions e1 and e2 are lithostratigraphically constrained by erosional surfaces and hosted within the watertown and overlying l orignal formations respectively the latter coeval with the selby formation in the adjacent northern appalachian basin the excursions coincide with periods of regional transgression but geographic patterns of 13c depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions respectively differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas with preferential corg recycling with stratigraphic condensation by early chatfieldian time segmentation of the once regional carbonate platform l orignal formation produced a regional mosaic of low energy muddy carbonate banks rockland formation and a deeper water platform lower hull formation settings subject to fluctuating high to low energy current flow excursion e3 occurs in both successions but 13c enrichment is associated only with the bank top muddy facies this may identify preferential photosynthetic drawdown of 12c across the bank tops due to limited seawater exchange across the bank deeper platform boundaries excursions e1 to e3 and a younger excursion e4 in the hull formation are correlated with varying confidence with excursions across southern laurentia excursion e3 being the local expression of the guttenberg 13c excursion our study supports local modulation of regional if not global 13c excursions arising from structurally controlled changes in oceanography and productivity

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

diagenetic salinity cycles and sea level along a major unconformity monte composauro italy abstract in shallow water carbonates position of the water table and the chemistry of early diagenetic fluids are controlled in large part by sea level and climate fluctuations these fluctuations may result in cyclic progression from marine diagenesis or sedimentation to mixing zone meteoric mixing zone and then marine diagenesis or sedimentation herein we term such a cyclic progression as a diagenetic salinity cycle this diagenetic study along a single unconformity in cretaceous limestones of monte camposauro southern apennines italy demonstrates the presence of four diagenetic salinity cycles and their link to relative changes in sea level data consist of field observations transmitted light and cathodoluminescence microscopy fluid inclusions and carbon and oxygen isotopes they show that twenty one unconformity related diagenetic features predated deposition of marine limestones above the unconformity fossiliferous internal sediments and stable isotope data from radiaxial fibrous calcite and recrystallized aragonite are used to indicate marine diagenesis fluid inclusion and stable isotope data are used to indicate precipitation of calcite cement in all mixing ratios of the mixing zone environment and in the freshwater phreatic zone paleocave morphology indicates vadose and water table dissolution and bauxite sediments support extensive vadose diagenesis in each diagenetic salinity cycle there is evidence for relative fall or rise of sea level indicated by transitions from marine sedimentation to mixing zone and freshwater phreatic diagenesis marine phreatic to freshwater vadose diagenesis vadose diagenesis to marine phreatic diagenesis and freshwater phreatic diagenesis to normal marine sedimentation during the time interval of the development of the unconformity beginning some time in the aptian and ending in the early middle cenomanian at least seven third order eustatic fluctuations have been hypothesized during an overall second order eustatic rise at a given elevation the third order eustatic fluctuations would not be able to produce four diagenetic salinity cycles without tectonic uplift of 3 5 to 8 mm ky our study illustrates that the formation and the timing of diagenetic salinity cycles are highly sensitive to paleoelevation diagenetic salinity cycles preserved along unconformities can be used to interpret relative changes of sea level and to predict the presence of depositional sequences in downdip positions conversely downdip depositional sequences tectonic history and sea level history can be used to predict some aspects of diagenetic alteration updip

aragonite to calcite transformation during fresh water diagenesis of carbonates insights from pore water chemistry dissolved strontium and calcium concentrations in fresh water lenses fwl and associated mixing zones mz on two small holocene ooid sand islands in the schooner cays bahamas were monitored during a 1 yr period to quantitatively analyze the transformation of aragonite to calcite the observed characteristics of this mass transfer are functions of climate and hydrology aragonite to calcite transformation in all hydrologic zones is primarily associated with meteoric recharge the transformation occurs throughout the fwl and in the mz to relative salinities of 19 and 36 sea water on the two islands rates of transformation are rapid in all zones and are greatest in the fwl a limestone composed of 100 calcite should form from an aragonite precursor within 4 700 to 15 600 yr in the fwl and within 8 700 to 60 000 yr in the upper mz efficiencies of transformation can vary between hydrologic zones due to pco2 effects yet the efficiency of the entire system fwl mz is high 87 this indicates that most caco3 derived from aragonite dissolution is reprecipitated as calcite somewhere in the fresh water system or upper mixing zone co2 effects fresh water sea water mixing and the differing solubilities between aragonite and calcite all drive the mass transfer the latter is the most significant accounting for up to nine times more mass transfer than co2 effects and at least ten times more mass transfer than fresh water sea water mixing differing solubilities should also cause mass transfer to occur throughout the hydrologic cycle but it apparently becomes ineffective after the rainy season possibly due to the inhibition of calcite precipitation

limitations on lowstand meteoric diagenesis in the pliocene pleistocene of florida and great bahama bank implications for eustatic sea level models data from three bahamian and south floridan cores with deeper water facies show that contrary to current models there is no phreatic meteoric diagenesis that can be attributed to large scale sea level lowstands models of meteoric diagenesis in carbonate sediments commonly assume major phreatic alteration during glacio eustatic sea level lowstands however the diagenesis previously attributed to large scale 80 m lowstands is found in shallow water facies that have been repetitively exposed in order to document diagenesis from large scale lowstands an interval is needed that was deposited in deeper water and not immediately exposed to meteoric fluids three cores with such deeper water facies indicate that the majority of phreatic meteoric diagenesis in florida and the bahamas happened during glacio eustatic highstands not during lowstands the data also suggest that diagenetically active meteoric lenses in florida and the bahamas are restricted to within 60 m and perhaps less of the land surface the most likely reasons deeper lenses are chemically inactive are 1 the greater percolation distance allows the water to reach saturation prior to entering the lens and 2 the large distance exceeds the reach of soil derived organic matter known to drive diagenesis in meteoric lenses models that currently assume a constant rate for alteration in meteoric fluids need to accommodate this variation with thickness of the vadose zone this study also questions the use of paleophreatic lenses as records of eustatic sea level as the large scale falls may not leave any record

mixing zone diagenesis in the subsurface of florida and the bahamas abstract three cores from great bahama bank and southern florida contain examples of mixing zone diagenesis where typical shallow water aragonite rich sediments were altered to low mg calcite the 40 50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric 18o 3 2 0 7 13c 1 6 1 8 to enriched marine isotopic values 18o 1 0 0 3 13c 1 9 0 7 the age of the mixing zones varies from late pliocene to pleistocene so these cores provide three independent examples of mixing zone diagenesis mixing zone diagenesis produced a limestone composed of low mg calcite with petrographic fabrics including moldic porosity blocky to dogtooth calcite cementation and recrystallization of lime mud to micrite and microspar the mixing zone diagenetic model predicts alteration in only part of the mixing zone because the alteration recorded in these cores shows continuous alteration through the entire mixing zone we consider the mixing of waters itself to be largely irrelevant rather diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter the mixing zone has been proposed as a likely location for dolomitization with the exception of some clearly sea floor dolomite in one core unda the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone the lack of dolomite in these mixing zones combined with the extensive literature questioning the chemical reasoning for mixing zone dolomite supports rejection of the mixing zone dolomite model

match between late pleistocene great bahama bank and deep sea oxygen isotope records of sea level throughout the pliocene and pleistocene a hierarchy of sea level fluctuations alternately flooded and exposed the great bahama bank controlling its sedimentary evolution the persistent rhythm of sea level is best shown in the upper pleistocene to holocene section in which 10 cycles of sea level rise and fall are identified in core borings on the lee side of the bank the same number of sea level oscillations is indicated within the brunhes chron of the deep sea oxygen isotope record in the bahamas sea level oscillations are inferred from meter scale parasequences of shallow marine carbonates interpreted largely as transgressive and highstand deposits sandwiched between discontinuity horizons interpreted as subaerial exposure horizons that developed during lower sea level subaerial exposure shifted from being largely restricted to the platform interior during the late pliocene olduvai chron to occurring across the entire lee side of the platform at the beginning of the brunhes chron this change on the platform is highly suggestive of the deep sea oxygen isotope proxy of sea level which indicates a fundamental paleoclimatic shift at approximately the same time despite dating limitations results from the bahamian cores can be used to push the calibration of the sea level record back to the late pliocene

geochemistry of meteoric diagenesis in carbonate islands of the northern bahamas 2 geochemical modelling and budgeting of diagenesis geochemical characterization and numerical modelling of surface water and ground water combined with hydrological observations provide quantitative estimates of meteoric diagenesis in pleistocene carbonates of the northern bahamas meteoric waters equilibrate with aragonite but water rather than mineral controlled reactions dominate dissolutional lowering of the undifferentiated bedrock surface is an order of magnitude slower than that within soil filled topographic hollows generating small scale relief at a rate of 65 140 mm ka 1 and a distinctive pocketed topography oxidation of organic matter within the subsoil and vadose zones generates an average pco2 of 4 0 10 3 atm which drives dissolution during vadose percolation and or at the water table however these dissolution processes together account for 60 of the average rock derived calcium in groundwaters pumped from the freshwater lens the additional calcium may derive from oxidation of organic carbon within the lens accounting for the high pco2 of the lens waters mixing between meteoric waters of differing chemistry is diagenetically insignificant but evapotranspiration from the shallow water table is an important drive for subsurface cementation porosity generation in the shallow vadose zone averages 1 6 3 2 ka 1 phreatic meteoric diagenesis is focused near the water table where dissolution generates porosity at 1 4 2 8 ka 1 maximum dissolution rates however are similar to those of evaporation driven precipitation which occludes porosity of 4 0 s 0 6 ka 1 this drives porosity inversion from primary interparticle to secondary mouldic vug and channel porosity in the deeper freshwater lens oxidation of residual organic carbon and reoxidation of reduced sulphur species from deeper anaerobic oxidation of organic carbon may generate porosity up to 0 06 ka 1 meteoric diagenesis relies critically on hydrological routing and vadose thickness controlled by sea level as well as the geochemical processes active a thin vadose zone permits direct evaporation from the water table and drives precipitation of meteoric phreatic cements even where mineral stabilization is complete copyright 2007 john wiley sons ltd

upper ocean thermal structure and the western north pacific category 5 typhoons part ii dependence on translation speed abstract using new in situ ocean subsurface observations from the argo floats best track typhoon data from the u s joint typhoon warning center an ocean mixed layer model and other supporting datasets this work systematically explores the interrelationships between translation speed the ocean s subsurface condition characterized by the depth of the 26 c isotherm d26 and upper ocean heat content uohc a cyclone s self induced ocean cooling negative feedback and air sea enthalpy fluxes for the intensification of the western north pacific category 5 typhoons based on a 10 yr analysis it is found that for intensification to category 5 in addition to the warm sea surface temperature generally around 29 c the required subsurface d26 and uohc depend greatly on a cyclone s translation speed it is observed that even over a relatively shallow subsurface warm layer of d26 60 70 m and uohc 65 70 kj cm 2 it is still possible to have a sufficient enthalpy flux to intensify the storm to category 5

dupuit ghyben herzberg analysis of strip island lenses dupuit ghyben herzberg analysis follows from combination of the continuity equation and darcy s law with the ghyben herzberg principle and the dupuit assumptions of horizontal flow the analysis is used to derive the position of the water table and salt water interface in island lenses in terms of island geometry distribution of hydraulic conductivity k and distribution of recharge r for small islands and cases for which the salt water head is zero application of dupuit ghyben herzberg analysis gives good results because the low r k ratios of natural lenses assure that height width ratios of natural lenses are extremely low 1 30 to 1 100 in earlier publications the dupuit ghyben herzberg differential equation has been integrated with the boundary condition that the water table and interface meet at sea level at the shoreline that is no outflow face the dimensions of an outflow face however are known from results of potential theory analysis integration of the dupuit ghyben herzberg equation with a boundary condition consistent with the potential theory outflow face a places the interface indistinguishably close to its position given by potential theory and b allows calculation of ghyben herzberg ratios interface depth to water table elevation which in this case depart from their usual value of 40 because equipotentials are curved close to the outflow face for natural sized lenses however such analysis is necessary only within a narrow strip within 1 to 5 of the island width of the shoreline outside this strip the regular analysis that ignores the presence of the outflow face positions the interface indistinguishably close to that of the potential theory solution analytical solutions are developed for a number of infinite strip islands it is shown by analysis of the homogeneous rectangular island case that an island can be considered an infinite strip to 0 1 accuracy if its length width ratio is larger than 4 4 asymmetric lenses occur if the island is composed of strips of different k or different r with the greater asymmetry occurring with differences in k a high k basement compresses the root of the lens and thereby decreases the water table in the island a lens perched on impermeable basement has a higher water table than would otherwise occur in the island but the volume of the lens is less

origin of caves and other solution openings in the unsaturated vadose zone of carbonate rocks a model for co2 generation the enigma that caves and other solution openings form in carbonate rocks at great depths below land surface rather than forming from the surface downward can be explained by the generation of co 2 within the aquifer system in the proposed model co 2 is generated by the oxidation of particulate and or dissolved organic carbon that is transported from the land surface deep into the unsaturated zone by recharging ground water the organic material is oxidized to co 2 by aerobic bacteria utilizing oxygen that diffuses in from the atmosphere because gas transport in the unsaturated zone is controlled largely by diffusion steady state generation of even minute amounts of co 2 deep in the unsaturated zone results in the creation of large concentrations of co 2 at depth as it establishes a concentration gradient to the surface or other sink ground water descending into these areas of high co 2 concentration becomes thermodynamically subsaturated with respect to carbonate minerals and is able to dissolve them thus forming the observed solution opening at depth

solution conduits as indicators of late quaternary sea level position abstract solution conduits or caves can provide a measure of glacio eustatic sea level position for the quaternary in high latitudes cave deposits contain artifacts fossils and sediments that reflect surface conditions with respect to climate secondary calcite deposits most notably stalagmites record growth episodes during ice minima and non growth during ice maxima they provide an indirect estimate of areal ice cover at high latitudes and are an indirect measure of sea level in low latitudes stable carbonate platforms contain caves that provide a direct measure of past sea level position solution conduits developing in a fresh water lens will record the position of the lens and therefore sea level as sea level fluctuates fresh water lens horizons and conduit development horizons will shift preserving a record of past sea level position cave wall rock and cave deposits can produce a chronology for given conduit positions secondary calcite deposits notably stalagmites record growth periods during sea level low stands or ice maxima and record growth hiatuses during flooding by sea level high stands or ice minima the inverse of high latitude stalagmites lighthouse cave san salvador island bahamas records a sea level high stand at 1 to 6 m for some time period between 85 000 and 70 000 years ago based on position and timing of conduit development and records sea level at present elevation for some time period between 49 000 and 37 000 years ago based on a stalagmite with a marine overgrowth

pleistocene speleothems of mallorca implications for palaeoclimate and carbonate diagenesis in mixing zones the pleistocene speleothems of sa bassa blanca cave mallorca are excellent indicators of palaeoclimate variations and are samples that allow evaluation of the products and processes of mixing zone diagenesis in an open water cave system integrated stratigraphic petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates meteoric marine mixing zone and meteoric vadose zone mixing zone precipitates formed at and just below the water air interface of cave pools during interglacial times when the cave was flooded as a result of highstand sea level mixing zone precipitates include bladed and dendritic high mg calcite microporous bladed calcite with variable mg content and acicular aragonite their presence suggests that calcium carbonate cementation is significant in the studied mixing zone system fluid inclusion salinities 13c and 18o compositions of the mixing zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred rather the proximity to the water table and degassing of co2 at the interface were the major controls on precipitation thus simple two end member mixing models may apply only in mixing zones well below the water table meteoric vadose speleothems include calcite and high mg calcite with columnar and bladed morphologies vadose speleothems precipitated during glacial stages when sea level was lower than present progressive increase in 13c and 18o of the vadose speleothems resulted from cooling temperatures and more positive seawater 18o associated with glacial buildup such covariation could be considered as a valid alternative to models predicting invariant 18o and highly variable 13c in meteoric calcite glacio eustatic oscillations of sea level are recorded as alternating vadose and mixing zone speleothems short term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone fluid inclusion and stable isotope data suggest that aragonite as opposed to calcite precipitated during times of reduced meteoric recharge

predicting the thickness of fresh water lenses in carbonate paleo islands abstract fresh water lenses in carbonate sand and grainstone islands can be modeled using dupuit ghyben herzberg methods the results indicate that the thickness of the lens h in a homogeneous infinite strip island will be a function of the island s width a hydraulic conductivity k and the amount of recharge r the ratio of r k in most modern carbonate islands of holocene sands and pleistocene rocks is in the range of 10 4 to 10 6 this means h should be 1 to 3 the magnitude of a however vertical and lateral variations in k variations in island shape the presence of intra island evaporitic ponds and marshes and the presence of a mixing zone result in actual h a ratios that range from 0 2 to about 2 with 1 being the most representative value the thic ness of a paleo lens in an ancient carbonate shoal or reef complex can therefore be estimated if the width of the interpreted island can be approximated from facies or isopach maps an example from the late jurassic smackover formation in oaks field louisiana indicates that the estimated thickness of paleo lenses is identical to the thickness of the porosity zones in a series of off lapping oolite shoal complexes a second example from the late permian san andres formation in yates field west texas reveals that the estimated thickness of paleo lenses is essentially equivalent to the thickness of a phreatic cave system formed on paleo islands developed along the san andres shelf margin the h a method for estimating the thickness of paleo lenses and the possible extent of island stage fresh water diagenesis in ancient carbonates is more useful than the ghyben herzberg rule h 40 times the elevation of the water table because the gh rule requires estimation of paleo water table and paleo sea level positions to within a couple of centimeters such estimates are typically impossible in ancient rocks

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

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

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

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

the hydrogeochemistry of early meteoric diagenesis in a holocene deposit of biogenic carbonates abstract this study presents the results of a quantitative hydrogeochemical investigation of early meteoric diagenesis in a deposit of biogenic mixed mineralic holocene carbonate sediments at ocean bight in the southeastern bahamas ground water samples were collected from a network of 27 wells arranged into 4 transects across the 1 5 km2 strand plain deposit four wells provided multi depth samples at 0 5 m intervals thus allowing for the construction of vertical chemical profiles analyses of the water consisted of temperature ph alkalinity ci ca2 mg2 sr2 so42 and 13c data derived from these analyses were used in a series of equilibri m and mass balance equations to calculate mineral saturation levels and magnitudes and rates of mineral transformation reactions interpretation of measured and calculated parameters provided a detailed picture of patterns and processes of meteoric diagenesis at ocean bight four features of meteoric diagenetic processes operating at ocean bight are distinct from processes documented at previously studied non biogenic and aragonite dominated holocene systems 1 lighter cementation with respect to magnitudes of calcite precipitation 2 diminished magnitudes of aragonite dissolution 3 elevated rates of mineral stabilization and 4 lower efficiencies of transformation reactions another distinction of this study which is not thought to be attributed solely to the presence of mixed biogenic carbonates is the isolation of transformation reactions to the vadose and upper phreatic zone this result contradicts the popularly held belief that metastable carbonate deposits undergo rapid equilibration throughout the freshwater phreatic zone this distribution of diagenetic reactions along with the analytical results characterizes a chemical environment dominated by carbonate mineral transformation reactions linked mainly to the oxidation of organic matter and not only to the relative solubility of the different carbonate minerals this finding may prove important in attempts to explain the preservation of metastable minerals in ancient geologic sequences despite repeated exposure to freshwater phreatic zones this finding also questions the assumption that the extent of paleo phreatic lenses is equal to the observed extent of freshwater phreatic alteration

simulating meteoric and mixing zone carbonate diagenesis with a two dimensional reactive transport model meteoric and mixing zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates most preserved pre cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing zone diagenesis however a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development here we present a new two dimensional 2d reactive transport model coupled with a 2d coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record using this model we have simulated the stratigraphic trends and relationship between isotopic records for example 13c and 18o values observed in modern recent miocene sections where the impact of meteoric diagenesis has been clearly characterized our model can also reproduce anomalous neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated further our model indicates that linear carbonate c o isotope co variations can either be generated in the mixing zone between freshwater and marine pore waters or in the freshwater phreatic zone with a downward decrease in recrystallization with no net carbonate dissolution or precipitation rate in addition numerous processes were observed to decouple 18ocarb values from other isotopic and elemental signatures during carbonate diagenesis indicating that a lack of linear correlation between 18ocarb values and other geochemical variables does not necessarily suggest limited meteoric alteration sensitivity analyses show that the steady state timescale is controlled by compositional differences between fluid endmembers the calcite water element distribution coefficient the recrystallization rate porosity and the groundwater discharge rate given that reactive transport models have proven to be powerful theoretical tools in many disciplines of earth sciences our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates

dispersed rhyolitic tephra from new zealand in deep sea sediments of the southern ocean dispersed rhyolitic tephra in deep sea piston cores collected from the south pacific sector of the southern ocean have previously been considered to originate from the balleny islands antarctica chemical composition of the materials suggests that a source in new zealand is more likely a strong correlation exists between paleomagnetically determined ages for these deep sea tephra layers and fission track ages determined on volcanic glasses and zircons from tephra sequences in wanganui basin and cape kidnappers new zealand the correlation suggests that the tephra extend more than 5000 km east and south of new zealand

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

diagenetic effects on uranium isotope fractionation in carbonate sediments from the bahamas abstract uranium isotope variations 238u recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time however the effects of diagenetic alteration on the u isotopic composition in carbonate sediments which are crucial to understand the accurate reconstruction of marine 238u are currently poorly constrained here we examine the effects of the aragonite to calcite transition in the pleistocene key largo limestone of south florida and assess the effects of vadose meteoric phreatic meteoric and phreatic marine diagenesis on u isotope fractionation in carbonate sediments from the bahamas transect including the well studied clino unda and odp site 1006 drill cores our results suggest that early diagenetic processes in bahamas carbonate sediments fractionate u isotopes by an average of 0 27 0 14 1 sd heavier than contemporaneous seawater downcore variations of 238u in slope and basin sediments display little if any correlation with u concentration and common geochemical indicators of diagenesis 13c 18o mn sr mg ca sr ca enrichments of redox sensitive elements or rare earth elements anomalies we propose two possible mechanisms to interpret the positive change in the 238u during carbonate diagenesis authigenic enrichment of isotopically positive u iv in carbonates and preferential incorporation of isotopically positive aqueous u vi species into carbonates these processes likely operate during early syndepositional diagenesis on the banktop further diagenesis during deeper burial is limited by the low solubility of u iv under reducing pore water conditions the early diagenetic behavior of u isotopes in bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past we suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater 238u from ancient carbonates furthermore early diagenesis induces significant statistical variability in sediment 238u values pointing to the need for large high resolution data sets in order to average out stochastic variations in individual bulk sediment samples

carbonate dissolution in a modern mixing zone the potential for dissolution of calcite and aragonite and precipitation of dolomite by the mixing of fresh and saline ground waters has been demonstrated theoretically1 3 although geologists have used these processes in the explanation of the diagenesis of recent4 7 and ancient8 12 carbonates there has been only limited investigation of the geochemical processes of modern mixing zones13 14 here we report the results of chemical and rock profiling through a modern mixing zone on andros island in the bahamas our results demonstrate undersaturation of the waters in the mixing zone with respect to aragonite and calcite and corresponding pervasive dissolution of the carbonate rocks at this level undersaturation is significantly enhanced by bacterial processes which are also responsible for precipitation of an iron rich crust despite chemical supersaturation precipitation of dolomite was not observed thereby casting some doubt on the mixing zone model of dolomitization

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

the nature of the δ13c of periplatform sediments implications for stratigraphy and the global carbon cycle the carbon isotopic composition y 13 c of bulk carbonates obtained from a transect of sites drilled through platform and periplatform sediments of holocene to early miocene age has been compared to ascertain whether changes in the y 13 c can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the y 13 c of the dissolved inorganic carbon in the oceans over this time period five of the sites were drilled during leg 166 of the ocean drilling project 1003 1007 in a transect ranging from five km to 25 km away from the platform margin and penetrating sediments of holocene to oligocene age that are contained in 17 depositional sequences a q two shallow water sites clino and unda were situated on a extension of the same transect on great bahama bank in a water depth of 10 15 m with the exception of unda and clino the y 13 c of the carbonates ranges from 5x in the younger sequences to 1x in the early miocene in each of the sites the y 13 c is strongly positively correlated with the percentage of aragonite as a consequence the y 13 c of sequences a through f is strongly correlated reflecting the decreasing amount of aragonite with increasing depth in the two platform sites the y 13 c is significantly lower in the younger portions of the cores as a result of the influences of meteoric diagenesis during repeated exposure during the pleistocene although the y 13 c of the individual sequences can be correlated in most instances between the odp holes the changes are not related to global changes in the y 13 c of the oceans which in contrast to the y 13 c of the platform sediments become isotopically lower towards the present day instead variations in the y 13 c appear to be related to varying mixtures of y 13 c rich banktop sediments and pelagic material

evaluating the fidelity of the cerium paleoredox tracer during variable carbonate diagenesis on the great bahamas bank abstract inferring redox conditions for ancient marine environments is critical to our understanding of biogeochemical cycles over earth history because of the redox sensitivity of cerium ce relative to other rare earth elements rees and its uptake in marine carbonates the ce anomaly ce ce is widely applied to ancient carbonates as a proxy for local redox conditions in the water column however carbonate sediments and rocks are particularly vulnerable to multiple stages and styles of post depositional diagenetic alteration where the diagenetic redox conditions and fluid compositions can vary widely from overlying seawater evaluations of the effects of this post depositional alteration for the ce anomaly have mostly been limited to ancient carbonate rocks rather than recent well characterized analog facies here we report on analyses of ree plus yttrium concentrations rey and ce anomalies in bulk carbonate samples from drill cores collected in the bahamas clino and unda that allow us to track loss or retention of primary signals of initial oxic deposition through a range of subsequent alteration scenarios mostly under anoxic conditions specifically these materials have experienced well constrained overprints linked to meteoric processes and marine burial diagenesis including dolomitization our results show that regardless of mineralogy diagenetic fluid composition and redox state the rey patterns in these carbonates including the ce anomaly are similar to those of modern oxic seawater indicating that they likely record the seawater signatures of primary deposition as such the ce anomaly in shallow marine carbonates has the potential to preserve records of primary deposition even when subject to multiple stages and styles of diagenetic alteration confirming its utility in studies of ancient marine redox

the effects of diagenesis on lithium isotope ratios of shallow marine carbonates in this study we present new data on the 7li values and li ca mg ratios of carbonate cores from the great bahama bank clino unda a deep water core off of the bank top odp leg 166 site 1007 and the coralline key largo limestone we use these samples to evaluate the influence of meteoric diagenesis marine burial diagenesis and dolomitization on the li isotope system in carbonates we find that recrystallization of aragonite to low mg calcite in the presence of meteoric fluids results in a systematic decrease of the li ca mg ratio in clino unda and key largo samples due to the lower li ca mg ratio in meteoric fluids compared to seawater for li isotopes we observe that the 7li of meteorically altered low mg calcite is 22 0 3 8 n 28 1 which is coincidentally similar to the original aragonite rich sediments 22 1 in the bahamas 18 1 in key largo but with a larger variability from 15 to 27 we interpret these features as reflecting the overprinting of primary li during meteoric alteration with a highly variable isotope signature that may be controlled by a combination of local porewater and or global climatic conditions in either case meteoric diagenesis produces isotopic signatures that are unrelated to seawater composition in contrast marine burial diagenesis and dolomitization of clino and unda sediments under fluid buffered conditions result in li isotope composition that is similar 30 2 1 5 n 36 1 to modern seawater 31 for site 1007 the 7li values range between 23 permil and 31 permil we interpret this range as reflecting a combination of varying diagenesis style fluid to sediment buffered and varying contribution of calcite derived from pelagic sediments with distinct isotopic composition due to primary mineralogy altogether our results show that diagenesis does not invalidate the use of bulk carbonates for deriving li isotope paleo records but the reliability of past carbonates as recorders of seawater 7li values will depend on carefully characterizing their diagenetic history

an integrated chemostratigraphic δ13c δ18o 87sr 86sr δ15n study of the doushantuo formation in western hubei province south china abstract high resolution isotopic analyses were conducted on core samples from the ediacaran doushantuo formation at the wangjiapeng section western hubei province in south china whereby two laterally traceable negative 13ccarb excursions en1 and en3 were recognized the magnitude and duration of these excursions permit intra basinal and inter basinal correlation which indicates that they probably represent a global change in seawater composition the occurrence of decoupled 13ccar 13corg with almost invariable 13corg values at wangjiapeng zhongling yangjiaping sections is consistent with remineralization of a dissolved organic carbon doc pool by means of sulfate reduction as recorded in en3 the synchronous presence of en3 a shift to higher 87sr 86sr and decrease of mn and fe contents and 15n values together points to a glacial influence whereby oxygenation and remineralization of reduced carbon produced 13c depleted dic glaciations cause a drop in sea level which itself leads to increased continental shelf area to be exposed to surface weathering and ultimately to enhanced delivery of radiogenic 87sr the increase of 87sr 86sr ratios sulfate and phosphate are consequences of surface runoff into oceanic environments and such perturbations induce biogeochemical changes