an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

seismic imaging of a hot upwelling beneath the british isles the iceland plume has had an important influence on vertical motions in the north atlantic the convecting mantle in this region contains a large scale low velocity seismic anomaly which correlates with a long wavelength gravity high and bathymetric feature this suggests that an arm of plume material has extended or is extending from iceland in a direction perpendicular to the mid atlantic ridge here we present the results of a detailed teleseismic traveltime study that reveals the high resolution morphology of this low velocity anomaly beneath the british isles our images provide insights into the nature of plume lithosphere interactions the low velocity anomaly imaged in this study correlates geographically with a region of high gravity anomalies and high topography that was associated with paleogene magmatism and phases of epeirogenic uplift during the cenozoic era there is evidence that the distribution of british earthquakes is also related to the low velocity anomaly the low velocity anomaly is interpreted to represent hot material from the original iceland plume head that became trapped beneath thinned regions of lithosphere ca 60 ma

analysis of the black sea sediments by evaluating dsdp leg 42b drilling data for gas hydrate potential abstract many gas seepages temperature pressure salinity anoxic environment and high source gas potential of the black sea indicates that the black sea might have huge potentials for biogenic and thermogenic gas hydrates however the last important parameter to consider gas hydrate as an energy source is the type of sediments coarse marine sands are considered as good hydrate reservoirs because of high porosity and high permeability only very limited data is available related to the types of lithology of the black sea sediments hence in this study the literature data especially the drilling and coring data of dsdp leg 42b program about gas seepages temperature gradient pressure gradient salinity anoxic environment and high source gas potential and the types of the sediments in the black sea were investigated and analyzed although gas seepages temperature gradient pressure gradient salinity anoxic environment and high source gas potential of the black sea are appropriate for producible gas hydrate reservoirs the sediments of the black sea appear to be generally fine grained with high clay content sandy silt and silty sand layers in turbidites of the black sea might be potential producible hydrate reservoirs but these sediments are fine as well as turbidites separate thin sand layers might be potential gas hydrate reservoirs as an energy source in the black sea

secular variation in seawater chemistry during the phanerozoic as indicated by brine inclusions in halite analysis of data on the chemical composition of primary fluid inclusions brine inclusions in primary bedded halite from many evaporite formations of northern pangea shows that during the phanerozoic the chemical composition of marine brines was oscillating significantly between the na k mg ca cl type and the na k mg cl so4 type we regard those changes as corresponding to the chemical evolution of the phanerozoic ocean the changes correlate in time with earlier suggested secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites in addition to those secular changes of seawater chemistry the concentrations of k mg ca and so4 ions in marine brines underwent important changes under the influence of local factors including rock water interaction these secondary changes did not influence the principal chemical type of the brine characteristic for a given time interval in the past concentration of the ca ion did not exceed the present concentration in marine water by a

vertical tectonics and the origins of bsrs along the peru margin abstract the distribution of bottom simulating reflections bsrs which mark the base of the gas hydrate zone on marine seismic records across the peru margin is examined with respect to tectonic uplift sedimentation and carbon content of sediment where accretion of only deep ocean pelagic sediment containing less than 1 carbon is seismically imaged no bsr is observed no bsr is apparent on seismic records of carbon rich 4 8 sediment of rapidly subsiding basins bsrs are abundant on records of carbon rich sediment affected by tectonic uplift and or rapid sedimentation in areas of uplift the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate during uplift free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level the strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of bsrs off peru

effect of mg ca ratio in seawater on shell composition in shallow benthic foraminifera 1 the ratio of magnesium to calcium mg ca in caco3 shells of foraminifera is widely used to determine paleotemperatures however mg ca is highly variable within and between species suggesting a strong physiological influence on the incorporation of mg2 into the shells while most field and laboratory calibrations have focused on the effect of temperature we chose to study the effect of ambient mg ca on the calcification process and the final shell composition we cultured two species of symbiont bearing benthic foraminifera amphistegina lobifera and amphistegina lessonii in seawater with different mg ca ratios electron probe analysis of the shell mg ca revealed a positive but not entirely linear correlation with mg ca in the culturing media with slightly different curves for each species partition coefficients of mg2 dmg in the calcite shells showed a decrease by a factor of roughly 2 between the lowest and highest mg ca in the ambient water this was previously demonstrated in inorganic calcite precipitation experiments however the biogenic dmg was significantly lower than the inorganic one suggesting a physiological mechanism that reduces mg ca at the calcification site unlike inorganic experiments that display a dependence of dmg on the kinetics of precipitation the biogenic dmg is not correlated with the rate of calcification both dmg and calcification rates in our experiment were sensitive to the mg ca ratio rather than the concentration of either ca2 or mg2 the largest addition of caco3 was obtained at mg ca of 1 and not at present day seawater ratio mg ca 5 this may reflect the mg ca that prevailed during the eocene mg ca 1 5 when this genus evolved

petrographic and geochemical evidence for origin of paleospeleothems new mexico implications for the application of fluid inclusions to studies of diagenesis abstract solution pockets in the mississippian lake valley formation of new mexico occur below the sub pennsylvanian unconformity large masses of columnar calcite spar form within the solution pockets the calcite shows fine growth banding stalagmitic and stalaclitic fabrics coarse detrital inclusions and truncation of crystal terminations indicating a vadose zone or speleothem origin for the spar the calcite is twinned and cross cut by both stylolites and forroan calcite filled fractures indicating deeper burial after precipitation near the surface the mean 13c of 7 29 and 18o of 3 06 pdb of the speleothem calcite defines the light carbon end member of an inverted j shaped curve of isotopic compositions of pre pennsylvanian components in the host limestone the isotopic relationship between speleothems and pre pennsylvanian components suggests a pre pennsylvanian age for the speleothems the isotopic composition and low magnesium content of speleothem calcite reflects precipitation from meteoric water altered little by stabilization of components of the host rock primary fluid inclusions are either one phase all liquid which indicate calcite precipitation below about 40 50 c or two phase with variable ratios of vapor to liquid the two phase inclusions formed by high temperature reequilibration of low temperature fluid inclusions and entrapment of air and water in the vadose zone the petrographic characteristics and one atmosphere pressure of some of the fluid inclusions suggest precipitation of speleothems in the vadose zone some secondary fluid inclusions are two phase with rather consistent ratios of vapor to liquid these inclusions indicate a late history of relatively high temperatures of the primary fluid inclusions 74 of the two phase population have been refilled with brines of deeper burial and 26 retain fresh water for the one phase fluid inclusions 100 are fresh water therefore the one phase all liquid inclusions in partially reequilibrated low temperature fluid inclusion populations should be viewed as reliable indicators of temperature and salinity of cement precipitation in this and other diagenetic studies

the major ion composition of permian seawater abstract the major ion mg2 ca2 na k so42 and cl composition of permian seawater was determined from chemical analyses of fluid inclusions in marine halites new data from the upper permian san andres formation of texas 274 272 ma and salado formation of new mexico 251 ma analyzed by the environmental scanning electron microscopy esem x ray energy dispersive spectrometry eds method along with published chemical compositions of fluid inclusions in permian marine halites from north america two formations of different ages and the central and eastern european basins eight formations of four different ages show that permian seawater shares chemical characteristics with modern seawater including so42 ca2 at the point of gypsum precipitation evolution into mg2 na k so42 cl brines and mg2 k ratios 5 permian seawater however is slightly depleted in so42 and enriched in ca2 although modeling results do not rule out ca2 concentrations close to those in present day seawater na and mg2 in permian seawater are close to slightly below their concentrations in modern seawater permian and modern seawater are both classified as aragonite seas with mg2 ca2 ratios 2 conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements the chemistry of permian seawater was modeled using the chemical composition of brine inclusions for three periods lower permian asselian sakmarian 296 283 ma lower permian artinskian kungurian 283 274 ma and upper permian tatarian 258 251 ma parallel changes in the chemistry of brine inclusions from equivalent age evaporites in north america central europe and eastern europe show that seawater underwent secular variations in chemistry over the 50 million years of the permian modeled so42 concentrations are 20 mmol per kg h2o mmolal and 19 mmolal in the asselian sakmarian and artinskian kungurian with higher concentrations in the upper permian tatarian 23 mmolal modeled ca2 is at or above its concentration in modern seawater throughout the permian mg2 is close to slightly below its concentration in modern seawater 55 mmolal in the asselian sakmarian 52 mmolal and tatarian 52 mmolal but slightly higher than modern seawater in the artinskian kungurian 60 mmolal mg2 ca2 ratios are 3 5 total range 2 7 to 5 5 in the lower permian and rose slightly to 3 7 total range 3 1 to 5 8 in the upper permian primarily due to decreases in ca2 these results are consistent with models that predict oscillations in the major ion composition of phanerozoic seawater on the basis of changes in the midocean ridge river water flux ratio driven by changes in the rate of midocean ridge crust production the permian was characterized by low sea levels icehouse conditions and southern hemisphere glaciation such conditions analogous to the present ice age and the similarities between permian seawater and modern seawater all suggest that general phanerozoic supercycles driven by mantle convection and global volcanicity also control the major ion chemistry of seawater

three dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing based on a three dimensional high resolution seismic survey off vancouver island canada we show that natural hydraulic fracturing is an efficient process to create permeable pathways for focused fluid upflow at submarine venting sites the pockmark structure examined in this study is located on top of an accreted ridge where the low permeability base of the gas hydrate stability field is also elevated and fluid overpressure will be induced tectonically elongated hydraulic fractures reveal decreased reflection amplitudes and are oriented along the maximum principal stress direction perpendicular to nearby fault planes a surface expression is created by fluid and material expulsion forming an irregular noncircular rim along the axis of the upflow zone our findings confirm a relationship between lateral variations in seismic reflectivity and sediment distribution regional tectonics episodic fluid flow and the formation and dissociation of gas hydrate they explain the presence of gas hydrate at the top of the upflow zone which requires transport of large volumes of gas through the gas hydrate stability field since hydraulic fracturing is not restricted to compressional regimes it is a likely explanation for the nature shape and orientation of upflow zones as well as for seismic blanking widely observed at both active and passive continental margins

mg ca sr ca and u ca ratios of a porites coral from sanya bay hainan island south china sea and their relationships to sea surface temperature high resolution mg ca sr ca and u ca ratios are reported in the skeleton of porites sp collected from sanya bay on the south coast of the hainan island china using inductively coupled plasma mass spectrometry combined with isotope dilution technique icp ms id the coralline mg ca ratios covary with sea surface temperatures sst for the 10 year interval giving the relationship between mg ca and sst as t degrees c 14 13 0 31 8 846 0 068 x 10 3 mg ca with the mean square weighted deviation mswd of 9 7 using this equation we are able to reconstruct sst record with a precision better than 0 5 degrees c several sr ca and u ca ratios show poor linear relationships with the measured sst this study indicates that the growth rates of the coral do not significantly affect coralline mg ca sr ca and u ca ratios however the high meteoric precipitation in this area which may cause a heavy run off from the island may change seawater sr ca and u ca ratios and ruin the relationships between coralline sr ca or u ca ratios and sst this implies that in coastal areas like hainan coralline mg ca ratios are valid for reconstructing the sst records but sr ca and u ca ratios may be suspected c 2000 elsevier science b v all rights reserved

geochemistry of modern brachiopods applications and implications for oceanography and paleoceanography abstract modern rhynchonellida terebratulida thecideida and a few inarticulated brachiopods were collected live or soon after death at 30 locations from depths of 2 3940 m from the equator to high latitudes covering the atlantic pacific indian caribbean and mediterranean oceans seas well preserved fibers of the secondary layer combined with sr 450 1928 mg kg mn 1 199 mg kg and fe 1 610 mg kg contents confirm with few exceptions the excellent preservation of the modern brachiopod material evaluation of the stable isotope compositions was conducted on 276 specimens and on 374 published samples for a total database of 650 samples from 80 locations it revealed that oxygen isotope incorporation into shell calcite secondary layer of modern articulated brachiopods in most cases occurs in equilibrium with the ambient water and as such reflects surrounding environmental conditions thecideida as a group shallow water and low latitude incorporate oxygen isotopes 3 04 to 0 35 into shell calcite in equilibrium with their ambient water a large majority of genera and species of the terebratulida shallow water and low mid latitude also incorporate oxygen 2 30 to 3 48 into shell calcite in isotopic equilibrium with their surrounding water in contrast equilibrium incorporation of oxygen isotopes into shell calcite by the rhynchonellida is uncertain but a vital effect probably applies to the craniida the question of equilibrium disequilibrium incorporation of carbon isotopes into shell calcite of modern brachiopods remains unresolved at this time isotope compositions range up to 4 5 5 6 and 1 5 for 13 c and up to 5 5 5 9 and 0 3 for 18 o for shallow water low mid and high latitude brachiopods respectively strontium isotopes of modern brachiopods as expected show no significant variation with location latitude water depth temperature salinity and general water conditions once local habitat and ecological influences of fossil brachiopods from similar latitudes are considered their isotopic elemental values should be valuable diagenetic indicators and important oceanographic chemostratigraphic proxies

spatial geochemistry of a carboniferous platform margin to basin transect balancing environmental and diagenetic factors abstract conventional one dimensional chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time whereas this approach has shown to be of value when linking isobathymetric domains usually characterised by similar facies more caution is needed when correlations are extended across different physiographic settings and hence different facies belts here the spatial geochemical record of pennsylvanian platform margin to basin transects across a bathymetric range of about 800 m is documented and discussed in a process oriented context particularly the presence of layered palaeo water masses and their potential control on slope facies distribution and geochemical properties requires attention whereas carboniferous thermo and or chemo clines most likely affected depth related slope facies zonation it was facies change and hence variances in porosity permeability properties that controlled differential early burial diagenetic alteration specifically the lower slope related breccia facies is characterised by higher volumes of early burial carbonate cements this implies that these sediments entered the shallow burial domain with a considerable open pore space and gave way to an increased rock fluid ratio whereas the 13 c record is invariant with bathymetry the more diagenesis sensitive 18 o proxy records pronounced shifts observed across major facies boundaries from this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature it is differential rock fluid ratios that control the first order spatial shifts in 18 o composition these findings show that one dimensional chemostratigraphy will severely underestimate the complexity of three dimensional bathymetric data sets across platform margins this is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general

effects of seawater carbonate ion concentration and temperature on shell u mg and sr in cultured planktonic foraminifera we investigate the sensitivity of u ca mg ca and sr ca to changes in seawater co3 and temperature in calcite produced by the two planktonic foraminifera species orbulina universa and globige rina bulloides in laboratory culture experiments our results demonstrate that at constant temperature u ca in o universa decreases by 25 7 per 100 mol co3 k g 1 as seawater co3 increases from 110 to 470 mol kg 1 results from g bulloides suggest a similar relationship but u ca is consistently offset by 40 at the same environmental co3 in o universa u ca is insensitive to temperature between 15 c and 25 c applying the o universa relationship to three u ca records from a related species globigerinoides sacculifer we estimate that caribbean and tropical atlantic co3 was 110 70 mol kg 1 and 80 40 mol kg 1 higher respectively during the last glacial period relative to the holocene this result is consistent with estimates of the glacial interglacial change in surface water co3 based on both modeling and on boron isotope ph estimates in settings where the addition of u by diagenetic processes is not a factor down core records of foraminiferal u ca have potential to provide information about changes in the ocean s carbonate concentration below ambient ph ph 8 2 mg ca decreased by 7 5 o universa t o 16 6 g bulloides per 0 1 unit increase in ph above ambient ph the change in mg ca was not significant for either species this result suggests that mg ca based paleotemperature estimates for the quaternary during which surface ocean ph has been at or above modern levels have not been biased by variations in surface water ph sr ca increased linearly by 1 6 0 4 per 0 1 unit increase in ph shell mg ca increased exponentially with temperature in o universa where mg ca 0 85 exp 0 096 t whereas the change in sr ca with temperature was within the reproducibility of replicate measurements copyright 2004 elsevier ltd

mississippian paleocean chemistry from biotic and abiotic carbonate muleshoe mound lake valley formation new mexico u s a abstract it has been proposed that phanerozoic variation in the mg ca ratio of seawater has resulted in variation in the primary mineralogies of shallow water carbonates between aragonite and calcite dominated seas to test this hypothesis this study estimates the mississippian oceanic mg ca and sr ca ratios from the composition of marine carbonate cements and crinoidal carbonate both of which have been suggested to faithfully record seawater chemistry the early mississippian tournaisian visean muleshoe mound lake valley fm alamogordo nm u s a lends itself well to this endeavor because it contains abundant crinoids and marine carbonate cement stable isotope and elemental analyses as well as cathodoluminescence characteristics elucidate the diagenetic history of the bioherm and identify the least diagenetically altered materials for reconstruction of paleoseawater chemistry isotopic analyses document two diagenetic trends one meteoric and one burial diverging from values similar to literature estimates of mississippian marine calcite 1 5 18o 4 5 13c the burial diagenetic trend follows from this origin to more negative oxygen values 7 18o 3 13c the meteoric diagenetic trend is defined by a meteoric calcite line at 3 18o similarity of non luminescent marine cement and non luminescent crinoid isotopic and elemental compositions suggests that crinoidal carbonate was completely replaced by cement in the marine environment average elemental ratios for least altered marine cements are mg ca 11 mmol mol sr ca 0 2 mmol mol mn ca 0 2 mmol mol and fe ca 0 4 mmol mol early mississippian seawater is estimated to have had a mg ca ratio of 0 2 0 3 mol mol this mg ca ratio is compatible with the early mississippian being an interval of calcite sea conditions and compares well with the devonian estimate from marine cements 0 5 though it is 5 10 times lower than estimates from mass balance modeling the sr ca for early mississippian seawater was between 0 5 and 0 9 mmol mol which is much lower than previous estimates of 6 mmol mol from brachiopod calcite and argues for the development of a sr sink aragonite in the latest devonian to earliest mississippian

a model for calcium magnesium and sulfate in seawater over phanerozoic time a theoretical model has been constructed for calculating the concentrations of dissolved calcium magnesium and sulfate in seawater over the past 550 million years phanerozoic time it is demonstrated that the weathering of ca carbonates and silicates by carbonic acid can be generally ignored in the modeling a previous model of the carbon cycle is used to guide input to the oceans of mg from carbonate and silicate weathering by h 2 co 3 similarly a previous model of the sulfur cycle is used to guide input fluxes to the ocean by calcium sulfate weathering and the weathering of ca and mg carbonates and silicates by sulfuric acid the sulfuric acid is derived from the oxidation of pyrite during weathering plus the oxidation of sulfur gases derived from the metamorphic volcanic decomposition of pyrite and caso 4 calcium is removed from seawater as sedimentary carbonates and sulfates so 4 is removed as sedimentary pyrite and caso 4 and mg is removed by exchange for ca via basalt seawater reaction and or dolomitization plus minor burial in sediments with pyrite as anoxic dolomite by adjusting starting conditions modeling results can be found that agree well with results of studies of salt fluid inclusions for variations of ca and so 4 this is especially true of the timing when ca was more abundant in seawater than so 4 and vice versa general agreement was also found with fluid inclusion results for changes in the mg ca ratio thereby distinguishing calcite seas from aragonite seas because of this overall agreement the modeling offers a new perspective on why oceanic ca mg and so 4 have varied over phanerozoic time

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

mg ca sr ca and stable isotope δ18o and δ13c ratio profiles from the fan mussel pinna nobilis seasonal records and temperature relationships 1 we present new annually resolved 18o 13c mg ca and sr ca ratio records for two shells of the fast growing mediterranean fan mussel pinna nobilis collected from proximal spanish coast sea grass meadows the relationship between the potential geochemical proxies and ontogenetic and environmental controlling factors is investigated specifically the use of shell mg ca and sr ca ratios as potential calcification temperature proxies the latter calculated from measured shell 18o values has been assessed the 18o cycles along the growth axis indicate that our p nobilis specimens are 10 5 and 4 5 years old shell sr ca ratios do not exhibit any consistent interannual cyclicity and are not correlated to temperature a subtle ontogenetic effect on shell mg ca ratios was observed during the first 4 5 years of recorded growth but was highly evident during the organism s later growth years in p nobilis shells different mechanisms influence ontogenetic variation in shell mg ca and 18o records shell mg ca ratios from the first 4 5 years of growth correlate significantly to temperature in a best fit relationship described by the equation mg ca 17 16 1 95 exp 0 022 0 004 t p nobilis shell mg ca records therefore are a valid temperature proxy only during an early growth phase for the same range of temperatures shell mg ca ratios in p nobilis are approximately 1 3 lower than those reported for inorganic calcite but 3 to 4 times higher than in another bivalve species mytilus trossulus and 4 to 16 times higher than in foraminifera we suggest these offsets are due to a higher degree of similarity between seawater and calcification fluid composition in p nobilis than in other bivalves and foraminifera the observed shell mg ca ratio change per c of 2 2 also is lower than that observed for inorganic and other biogenic calcites our findings strongly support taxon and species specific mg ca temperature relationships for bivalves and other calcifying organisms an appreciation of the physiology and calcification mechanisms of any biogenic carbonate archive therefore is paramount prior to the application of stable isotope and element ca ratio proxies for paleotemperature reconstructions

application of redox sensitive proxies and carbonate clumped isotopes to mesozoic and palaeozoic radiaxial fibrous calcite cements abstract ancient marine radiaxial calcite cements are commonly exploited as archives of marine porewater properties based on the argument that they lack metabolic effects often assigned to biogenic carbonates here we critically test the significance of conventional versus with respect to these fabrics less conventional proxy data from pennsylvanian triassic and cretaceous case examples conventional proxies include cathodoluminescence carbon and oxygen isotope ratios main and trace elemental concentrations less conventionally applied proxies are clumped isotope 47 redox sensitive and rare earth elements sampled across a succession of triassic radiaxial fibrous calcites radiaxial calcites are subdivided in three groups based on their luminescence characteristics non luminescent patchy luminescent and bright luminescent luminescence patterns are in fair agreement with isotope ratios in particular with those of oxygen the data fall into or are close to the range of reconstructed marine seawater values and often plot to the positive end member of the isotopic range these results disagree with the commonly held view that isotope data from luminescent cements reflect a priori non marine values further evidence for this comes from ree concentration patterns and cerium anomalies suggesting normal marine porewater values for all except the very last generation of radiaxial calcites this implies that luminescent radiaxial calcites must not necessarily represent significant diagenetic resetting kinetic effects during precipitation and different activator elements must be considered marine and earliest burial porewater temperatures of 12 26 c are suggested by conventional calcite 18 o thermometry conversely the application of the clumped isotope thermometer to the same radiaxial calcites suggests temperatures of 180 200 c reflecting solid state resetting of fully cemented limestones under a low water rock ratio redox sensitive elements particularly zn cd u and cu are affected by kinetic processes overriding fluid eh manganese concentrations and ce anomaly data point to gradually decreasing marine porewater oxygen levels from outer to inner cement fringes judging from ree patterns and ce anomalies the cement layers in the central portions of the pore filling cement succession witnessed the end of marine precipitation and the onset of shallow marine diagenesis consequently radiaxial calcite precipitation is suggested to continue in the early shallow marine burial domain this study underscores the potential of radiaxial calcite successions as archives of marine porewater to shallow burial diagenetic pathways the combination of conventional and less conventional proxies is a clear strength of this study and documents that abiogenic carbonate archives are often underexplored

delta o 18 sr ca and mg ca records of porites lutea corals from leizhou peninsula northern south china sea and their applicability as paleoclimatic indicators combined seasonal to monthly resolution coral skeletal delta 18 o sr ca and mg ca records are reported for one modem and two late holocene porites lutea corals from a fringing reef at leizhou peninsula the northern coast of the south china sea scs all the profiles for the period 1989 2000 reveal annual cycles well correlated with instrumental sea surface temperatures sst and display broad peaks in summer and narrow troughs in winter reflecting seasonal growth rate variations calibration against instrumental sst yields the following equations delta 18 o 0 174 0 010 xsst degrees c 1 02 0 27 mswd 5 8 sr ca mmol mol 0 0424 0 0031 xsst degrees c 9 836 0 082 mswd 8 6 and mg ca mmol mol 0 110 0 009 xsst degrees c 1 32 0 23 mswd 55 the scatter in the mg ca sst relationship is much larger than analytical uncertainties can account for suggesting the presence of sst unrelated components in the mg ca variation calculated sr ca sst values for two later holocene porites lutea samples u series ages similar to 541 bc and similar to 487 ad respectively from the same reef suggest that sst in the scs at similar to 541 bc was nearly as warm as in the 1990s the warmest decade of the last century but at similar to 487 ad it was significantly cooler this observation is consistent with climatic data reported in chinese historic documents confirming that the sr ca sst relationship is a reliable thermometer removing the sst component in the delta 18 o variation based on calculated sr ca sst values the residual delta 18 o reflects the deviation of the holocene seawater delta 18 o from the modem value which is also a measure of the holocene sea surface salinity sss or the summer monsoon moisture level in mainland china such residual delta 18 o was close to zero at similar to 541 bc and 0 3 parts per thousand at similar to 487 ad suggesting that it was as wet as in the 1990s at similar to 541 bc but significantly drier at similar to 487 ad in mainland china which are also consistent with independent historic records calculated mg ca sst values for the two late holocene corals are significantly lower than the sr ca sst values and are also in conflict with chinese historic records suggesting that coral mg ca is not reliable proxy for sst at comparable sr ca ranges fossil corals always display negative mg ca offsets if compared with the modem coral of the same site we interpret this observation as due to preferential loss of mg during meteoric dissolution of cryptic mg calcite bearing microbialites in the exposed fossil corals microbialites mgo up to 17 sr only 100 300 ppm are ubiquitous during reef building processes and their presence in only a trace amount will have a significant impact on coral mg ca ratios without detectable influence on coral sr ca ratios c 2004 elsevier b v all rights reserved

mississippian paleocean chemistry from biotic and abiotic carbonate muleshoe mound lake valley formation new mexico u s a discussion hasiuk and lohmann 2008 present a geochemical study of cement and crinoid remains from the much studied mississippian muleshoe mud mound new mexico u s a they deduce mississippian paleocean chemistry from data on altered cement and crinoids that are assumed to have recrystallized in sea water and retained their original marine chemistry it is surprising in this context that they present data for just one brachiopod as brachiopods are accepted as one of the most reliable marine proxies they do not discuss the interpretation of muleshoe brachiopod data made by wu and chafetz 2002 the latter authors differentiate intermound from mound brachiopods by their delta 13c values higher mound values are thought to be due to seepage of 13c enriched waters produced by methanogenic bacteria the muleshoe mound is interpreted by stanton et al 2000 as being deposited in water 200 m deep in the dysoxic oxygen minimum zone omz consequently can muleshoe carbonate influenced by methane seepage and deposited in the omz be regarded as an appropriate choice of material for an open marine proxy

the role of so4 in the switch from calcite to aragonite seas throughout the phanerozoic the primary inorganic marine calcium carbonate mineralogy oscillated between calcite and aragonite reflecting changes in seawater chemistry these variations in seawater composition also appear to have influenced the evolution of calcifying organisms however the processes controlling these mineralogical and biological changes are poorly constrained previous work has focused mainly on the mg ca ratio in seawater as the primary driver here we examine the role of dissolved so 4 in these processes by performing controlled laboratory precipitation experiments and geochemical modeling of evaporite formation we show that an increase in dissolved so 4 decreases the mg ca ratio at which calcite is destabilized and aragonite becomes the dominant caco 3 polymorph our data suggest that the mg ca and so 4 thresholds for formation of calcite seas are significantly lower than previous estimates and are mutually dependent this shows that our understanding of phanerozoic changes in seawater chemistry and the models relating this to primary caco 3 mineralogy need reevaluation 2011 geological society of america

seismic triggering of catastrophic failures on shear surfaces in saturated cohesionless soils this paper is concerned with an analysis of the seismic performance of infinite slopes in undrained conditions the material assumed on the sliding surface is a loose saturated sand susceptible to a gradual loss in undrained shear strength after failure with the progress of unidirectional shear displacement the undrained monotonic and cyclic shear behavior of this sand was investigated through an experimental study based on ring shear tests with initial stresses corresponding to the static conditions on the sliding surface of the analyzed slopes these tests provide the experimental framework for a modified sliding block method to estimate the earthquake induced undrained shear displacements for conditions of no shear stress reversals on the sliding surface the proposed estimation procedure incorporates the shearing resistance obtained from undrained monotonic ring shear tests to approximate the undrained yield resistance at a certain displacement during an earthquake the term catastrophic failure is

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

mollusc and brachiopod skeletal hard parts intricate archives of their marine environment the biogenic carbonate hard parts of fossil bivalves cephalopods and brachiopods are among the most widely exploited marine archives of phanerozoic environmental and climate dynamics research the advent of novel analytical tools has led many workers to explore non traditional geochemical and petrographic proxies and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms these considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum this paper evaluates differences and similarities of seawater versus that of body fluids cephalopods bivalves and brachiopods exert a strong biological control on biomineralization that to some degree may buffer their shell geochemistry against secular changes in seawater chemistry disordered amorphous calcium carbonate precursor phases later transformed to crystalline biominerals may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases a reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo environment the impact of what is commonly referred to as biological factors on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon oxygen strontium and traditionally used element to calcium ratios in particular the often used 13ccarb or the mg ca and sr ca elemental proxies are fraught with problems an interesting new research field represents the analysis calibration and application of non traditional proxies to mollusc and brachiopod hard parts examples include the carbonate clumped isotope 47 approach and the analysis of the isotopes of ca mg n li s or element to ca ratios such as li ca or b ca and rare earth elements based on considerations discussed here a series of do s and don ts in mollusc and brachiopod archive research are proposed and suggestions for future work are presented in essence the suggestions proposed here include experimental work also field experiments making use of recent archive organisms or where possible a reasonable recent analogue in the case of extinct groups moreover the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra structures must guide sampling strategies for geochemical analyses where feasible a detailed understanding of the diagenetic pathways and the application of multi proxy and multi archive approaches should form the foundation of fossil carbonate archive research the uncritical compilation of large data sets from various carbonate shelled organisms collected at different locations is not encouraged

diagenetic origins of the calcite microcrystals that host microporosity in limestone reservoirs abstract micron size pores are common in limestones and they host significant hydrocarbon reserves in conventional and unconventional plays worldwide previous studies have shown that the vast majority of micropores in limestones occur in the interstices between micron size calcite crystals that are present in nearly all depositional environments and burial depths throughout the phanerozoic in an effort to elucidate the genetic history of these calcite microcrystals a geochemical study was undertaken to investigate a global dataset collected from seven new subsurface microporous reservoirs as well as data from 21 previous studies bulk elemental analyses indicate that the vast majority of microcrystals are composed of low mg calcite average 14 mmol mol range 1 6 90 mg ca microcrystal sr mg ratios are consistent with abiotic calcite cements though overall mg ca and sr ca are lower than modern abiotic calcite cements reported in the literature higher sr ca observed in depositional chalks implies a biotic though probably not aragonitic precursor most calcite microcrystals have oxygen isotope compositions that are 0 to 4 more negative and carbon isotope compositions just slightly more negative 1 than contemporaneous abiotic marine calcite most stable oxygen and carbon isotopic data plot either along a burial trend anchored at its most positive end in the age equivalent marine calcite composition or parallel to that trend with an offset in 13c assuming that 13c offsets are due to marine carbon isotopic excursions the isotopic data are consistent with shallow burial diagenesis as the dominant diagenetic pathway for most calcite microcrystals for samples with information on porosity and depth more negative 18o ca 1 4 is associated with increased depth and reduced porosity implying that progressive overgrowth of calcite microcrystals is modifying bulk chemistry and rock properties of the reservoir during burial diagenesis a meteoric calcite line was evident only in two datasets this demonstrates that evidence of meteoric diagenesis can be preserved in the chemistry of calcite microcrystals but that its occurrence is rare in comparison with evidence for diagenesis during shallow burial geochemical data from calcite microcrystals are bulk samples i e measurements averaged over many many calcite microcrystals which themselves may have zoned chemistries as such microcrystal data require additional interpretation compared to samples from calcite macrocrystals where superposition puts samples into clear chronological order and produces diagenetic trends on for example 18o 13c plots bulk samples of calcite microcrystals should show mixing between phases on diagenetic trends with this model in mind the assembled database suggests that most samples are mixes of primary marine and burial calcite

carbon and oxygen isotopic composition of holocene reefal carbonates carbon and oxygen isotopic compositions have been determined for holocene reefal carbonate grains and cements in samples from enewetak bikini bermuda and belize to identify components that exhibit systematic isotopic fractionation relationships these findings are compared with fractionation relationships determined from experimentally precipitated low magnesium calcite lmc aragonite and high magnesium calcite hmc to determine if these relationships are observed in naturally precipitated carbonates compositional variation within individual samples is extreme up to 6 in 13 c pdb and 3 in 18 o pdb experimental fractionation relationships fail to predict the range of observed compositions aragonite cements are enriched 1 0 to 1 4 in 13 c and 1 5 to 2 0 in 18 o relative to calcite equilibrium compositions hmc cements are enriched by 0 3 to 0 4 in 13 c and by 1 8 to 1 9 to 18 o for carbon these results are in agreement with the experimental work of rubinson and clayton for oxygen our results suggest that fractionation factors of both aragonite and hmc may have been underestimated skeletal components exhibit inconsistent fractionation relative to inorganically precipitated marine cements suggesting that marine cement compositions are more reliable indicators of the physicochemical conditions of precipitation than co occurring skeletal carbonates

early cementation during marine meteoric fluid mixing mississippian lake valley formation new mexico abstract pre pennsylvanian low magnesium calcite cements from the mississippian lake valley formation of south central new mexico comprise three major cathodoluminescent zones previous work by meyers and lohmann 1985 suggested that interzonal isotopic and minor element variation resulted from spatial and temporal exchange between meteoric water and marine carbonate within a single meteoric system that the carbonate for cementation was derived from dissolution of metastable carbonate grains and that any contribution from marine water was insignificant isotopic analysis of the first two cathodoluminescent zones using detailed micron scale sampling methods revealed a record of simultaneous variation in 18o and 13c values at the intrazonal scale that is incompatible with compositional change anticipated from the evolution of meteoric fluids in response to progressive water rock interaction rather isotopic data in conjunction with stratigraphic and petrographic constraints suggest that regionally extensive mixing between marine and meteoric water along the basal margin of the phreatic lens is the process responsible for the cementation of much of this platform succession although trends of progressive 18o and 13c depletion in cement composition are compatible with precipitation from a fluid that evolved from a marine to a meteoric dominated composition low mg and sr concentrations are problematic in the context of experimentally derived partition coefficients for calcite because of compelling evidence indicating a mixing zone origin for these cements however we attribute low mg and sr contents to temperature and or pco2 effects or to differences in conditions under which calcite is precipitated

echinoderm skeletal preservation calcite aragonite seas and the mg ca ratio of phanerozoic oceans abstract the alternation between abiotic calcite and aragonite precipitation in shallow seawater and the oscillation of seawater mg ca ratio throughout the phanerozoic are both popular hypotheses echinoderms with well preserved stereom provide new empirical evidence that supports both these hypotheses analyses are reported from 29 specimens cambrian to eocene age containing ossicles preserved either as mg calcite 3 3 to 8 mole mgco3 or as mixed calcite and dolomite 5 to 12 5 mole mgco3 echinoderms with high mole mgco3 occur in the early cambrian and late carboniferous to triassic low values come from the silurian and jurassic to cretaceous the average composition of echinoderms with preserved stereom does not fall below 3 5 mol mgco3 during the phanerozoic and reaches its highest mean value of 16 0 mol mgco3 today an empirical partition coefficient of 0 03182 for modern tropical echinoids is used to indicate the mg ca ratio of ancient seawater from the mg ca ratio of fossil stereom some assumptions are involved and a likely error is calculated from the mg2 variation of modern echinoids high mean seawater mg ca ratios are calculated for early cambrian 3 3 and the late carboniferous to triassic 2 3 but never reached today s value of 5 2 low mg ca ratios 1 1 are indicated from jurassic to cretaceous echinoderms the 29 echinoderm samples plot close to first order mg ca seawater oscillations derived from geochemical models and mg ca ratios determined from fluid inclusions but considerable discrepancies exist when shorter 106 myr time intervals are considered further data and improved understanding of mg partitioning is required before accurate secular variation in the mg ca of seawater can be determined from echinoderms however they are an underused resource in this context and provide an excellent seawater archive

geochemical conditions of deposition in the upper devonian prypiac and dnipro donets evaporite basins belarus and ukraine abstract earlier studies of individual fluid inclusions in salt minerals of some devonian evaporite basins showed that the brines in those basins were of chloride type with a high concentration of ca2 the genetic significance of this occurrence was subject to different interpretations we have studied in detail samples taken from the east european dnipro donets and prypiac evaporite basins aiming to establish the chemical composition of fluid inclusions in studied evaporites and then using other geochemical information to establish a geochemical model of the devonian evaporite basins our study showed that salt deposition in the devonian dnipro donets and prypiac evaporite basins was because of intensive evaporation of mainly marine brines of chloride type with a relatively high ca2 content in turn the ratios between na k and mg corresponded to ratios in recent marine brines salt minerals precipitated in bottom water conditions the occurrence of one phase fluid inclusions indicates the temper

model of seawater composition for the phanerozoic we present an inverse model of phanerozoic seawater com position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites the model considers step wise alteration of seawater composition via 1 variable input of river water 2 variable rates of alteration of seawater through reactions at mid ocean ridges and 3 variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general the model achieves agreement with paleoseawater fluid inclusion data for na 1 c a 2 1 s o 4 2 2 and k 1 particularly when variable runoff is considered variable rates of basalt seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater particularly the observed near constant concentration of k 1 through time

early mississippian climate based on oxygen isotope compositions of brachiopods alamogordo member of the lake valley formation south central new mexico stable isotope compositions of brachiopod shells from the alamogordo member of the lake valley formation south central new mexico provide information about early mississippian paleoclimate the alamogordo member was deposited on a ramp at the southwestern edge of the north american craton at lat 20 s it consists of cherty lime mudstone to lime packstone predominantly mudstone and wackestone strata and scattered mud cement waulsortian mounds the brachiopods analyzed lived at water depths estimated to have been between 100 and 300 m the 1 8 o values of well preserved shells of a single species of brachiopod average 2 3 relative to pdb the peedee belemnite isotope standard this average represents a temperature of 21 26 c if the isotopic composition of the seawater 1 8 o s w was between 0 and 1 as inferred from pleistocene and holocene 1 8 o s w and other considerations sea surface temperature would have been 26 31 c after we applied a 5 c correction for water depth this tropical climate for an early mississippian low latitude site agrees with previous isotopic and climate modeling results but contrasts with a temperate climate inferred from comparison of the lithology and biota of the alamogordo member with those of modern temperate carbonates

across shelf sediment transport since the last glacial maximum southern california margin correlation of continental shelf slope stratigraphy in santa monica bay southern california with ocean drilling program records for nearby slope basin sites has illuminated the timing and scale of terrigenous sediment dispersal on margin since the last glacial maximum lgm marine flooding surfaces preserved in a transgressive sequence on the santa monica shelf provide a key link between base level elevation and sediment transport across shelf sediment accumulation rates at slope basin sites were maximal ca 15 10 ka well after the lgm decreased during the 12 9 ka transition from fluvial estuarine to fully marine conditions on the shelf and decelerated throughout the holocene to 30 75 of their values at the lgm the deceleration is interpreted to manifest a landward shift in the margin depocenter with the onset of transgressive sedimentation beginning when sea level surmounted the shelf edge ca 13 ka as predicted by sequence stratigraphic models however the records make clear that factors other than base level modulated slope basin accumulation rates during the deglaciation

the δ18o record of phanerozoic abiotic marine calcite cements monomineralic abiotic marine cements formed in low latitude phanerozoic reefs provide the direction and amplitude of secular variation of delta sup 13 c and delta sup 18 o in marine calcite and defines two end member compositions 580 to 360 my minus 7 to minus 5 per thousand delta sup 18 o sub pdb and 360 to present minus 3 to 0 per thousand delta sup 18 o sub pdb sampling of the devono carboniferous transition 375 320 my at several global sites reveals a rapid change in carbonate isotopic compositions bracketed within fammenian to early visean aged strata a 7 to 15 my time interval this shift corresponds to a 2 offset in mean delta sup 13 c and 3 4 offset in delta sup 18 o the abruptness of such change and its overall correlation with variations in sup 87 sr sup 86 sr delta sup 34 s delta sup 13 c and li al ratios in marine sediments suggests a primary offset in marine water composition

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

benthic carbonate factories of the phanerozoic marine carbonate precipitation occurs in three basic modes abiotic or quasi abiotic biotically induced and biotically controlled on a geologic scale these precipitation modes combine to form three carbonate production systems or factories in the benthic environment 1 tropical shallow water factory dominated by biotically controlled mainly photo autotrophic and abiotic precipitates 2 cool water factory dominated by biotically controlled mainly heterotrophic precipitates and 3 mud mound factory dominated by biotically induced mainly microbial and abiotic precipitates sediment accumulations of the factories differ in composition geometry and facies patterns and some of these differences appear prominently in seismic data thus facilitating subsurface prediction the characteristic accumulation of the tropical factory is the flat topped often reef rimmed platform in cool water systems reefs in high energy settings are scarce and hydrodynamic influence dominates producing seaward sloping shelves and deep water sediment drifts often armored by skeletal framework the typical accumulation of the mud mound factory is groups of mounds in deeper water where the mud mound factory expands into shallow water it forms rimmed platforms similar to the tropical factory the tropical factory is most productive the mud mound factory reaches 80 90 and the cool water factory 20 30 of the tropical growth rate the three factories represent end members connected by transitions in space transitions in time are linked to biotic evolution

chemical control of carbonate phases implications from upper pennsylvanian calcite aragonite ooids of southeastern kansas abstract ooids in grainstones from the spring hill member of the pennsylvanian plattsburg limestone in southeastern kansas were precipitated as both aragonite and calcite either phase predominated in different outcrops in different units within an individual outcrop in different grains within an individual sample and in different portions of an individual ooid cortex selective diagenetic dissolution of aragonite cortices gave rise to a variety of fabrics reported from many other politic limestones these include oomoldic porosity sparry cortices strings of spastolithic deformed and elephantine trunk to tail connected ooids and displaced nuclei these and other reported occurrences of fabrics which record the former presence of aragonite are not evenly distributed through phanerozoic time rather they predominate during times of general continental emergence while this distribution supports the postulations of mackenzie and pigott 1981 and sandberg 1983 that the dominant mineralogy of ancient marine limestones may have corresponded closely to positions of global sea level fabrics of the plattsburg limestone demonstrate that evaluating changes in ooid compositions with geologic time is not a simple either aragonite or calcite proposition

seismic investigations of a vent field associated with gas hydrates offshore vancouver island 1 a three dimensional seismic reflection survey was carried out over a gas hydrate field offshore vancouver island in the area of ocean drilling program leg 146 site 889 890 the area of investigation is located on the accretionary prism of the cascadia margin in water depths of 1200 1500 m several seismic blank zones were observed in the seismic data over a frequency range from 30 hz to 3 5 khz where the degree of blanking increases with seismic frequency time slices and selected horizon slices were used to define the spatial distribution of the blank zones the zones range from 80 m to several hundred meters in width one blank zone almost circular with a diameter of 400 m has a distinct seafloor expression it shows the characteristics of a mud carbonate mound and is probably associated with free gas expulsion hydrate was found at four sites with a piston corer within this blank zone at depths of 3 8 m below the seafloor we present a tectonic model that explains the blank zones to be a result of differential uplift along thrust faults the blank zones represent conduits for fluids and gas migrating upward blanking of the seismic energy is believed to be mainly the result of increased hydrate formation in lenses within the faults the blanking effect is enhanced locally because of scattering at carbonates formed at the seafloor

geophysical and geochemical signatures associated with gas hydrate related venting in the northern cascadia margin this paper presents a comprehensive multidisciplinary study of cold vents associated with near seafloor gas hydrate several cold vents characterized by seismic blank zones have been identified on the northern cascadia margin near ocean drilling program odp site 889 890 the most prominent vent site bullseye vent has been the subject of intense geophysical and geochemical studies including two and three dimensional 2d 3d seismic imaging heat flow measurements piston coring with measurements of sediment physical properties and pore fluid geochemistry seafloor video observation and sampling with the unmanned submersible ropos the main seismically derived constraining observations are 1 blanking increases with seismic frequency 2 at low frequencies layers can be traced through the zones 3 blank zones widen with depth 4 blank zones are underlain by a bottom simulating reflector bsr and 5 no velocity anomalies were detected across the vents constraints from piston core and thermal probe analyses are 1 massive hydrate was recovered just below the seafloor at bullseye vent and 2 chemical alteration of sediments was observed by reduced magnetic susceptibility increased thermal conductivity and an elevated sulfate methane interface additional constraints are 1 no thermal anomaly was observed 2 widespread carbonates and active chemosynthetic communities were found and 3 elevated levels of methane were detected in the water column above bullseye vent we present a model for the seismic blanking at bullseye vent that honors the constraints from all observations the cold vents represent channels or networks of filamentous fractures containing hydrate and or free gas free gas can be present within the hydrate stability field only in fractures which may be coated with hydrate that prevents the inflow of water the overall concentration of hydrate or gas within the vent must be small because there was no observable velocity anomaly

mg ca sr ca and stable isotopes in modern and holocene protothaca staminea shells from a northern california coastal upwelling region this study explores the potential of intertidal protothaca staminea shells as high resolution geochemical archives of environmental change in a coastal upwelling region mg ca and sr ca ratios were analyzed by excimer laser ablation inductively coupled plasma mass spectrometry la icp ms at sub weekly temporal resolution in shells growing 1 mm per month growth patterns of a modern p staminea shell from humboldt bay california collected in december 1999 made it possible to infer a lifespan from 1993 to 1998 growth hiatuses in the shell may have excluded records of extreme events mg ca ratios appeared to be partly controlled by water temperature the correlation coefficient between temperature and mg ca was r 0 71 in one of four growth increments significant year to year differences in the sensitivity of mg ca to temperature in p staminea could not be explained however sr ca ratios appeared to be more closely related to shell growth rate oxygen isotopes measured at 2 week temporal resolution in the same shell did not show a clear relation to local temperature in summer possibly because temperatures were higher and less variable at the king salmon mudflat where the shell was collected than in the main channel of humboldt bay where water properties were monitored negative shell 13c values 0 5 marked spring and summer coastal upwelling events the mg contents of p staminea midden shells dated to 3 ka and 9 ka were significantly lower than in the modern shell this may have resulted from degradation of a mg rich shell organic matrix and precluded quantitative interpretation of the older high resolution records elevated 13c values in the 3 ka shell suggested that the individual grew in highly productive or stratified environment such as a shallow coastal embayment or lagoon

crystal habit geochemistry and cathodoluminescence of magnesian calcite marine cements from the lower slope of little bahama bank the petrography and geochemistry of marine magnesian calcite cements recovered from the sea floor on the lower thermocline lower slope of little bahama bank address three issues concerning inorganic carbonates 1 crystal habit of marine magnesian calcite cements 2 cation composition of marine precipitates and 3 origins of cathodoluminescence in calcite bladed and blocky limpid spar cements formed contemporaneously within the same intraparticle pore as shown by banded cathodoluminescence traced without interruption through both cement habits bladed and blocky cements exhibit a positively covariant linear relationship for mg and sr having low proximal early formed compositions and high distal late formed compositions linear covariance of mg and sr is interpreted to indicate that the concentrations of these cations were controlled by kinetic effects as they formed with early precipitation occurring more rapidly than later precipitation precipitation rates apparently did not affect cement habit limpid isopachous bladed spar cements the distal third of which are brightly cathodoluminescent have a wide range of mg and sr compositions which is almost completely spanned by both distal cathodoluminescent and proximal noncathodoluminescent parts of cements occurring within the same pore mg and sr do not have a linear relationship these cements are interpreted to have recrystallized in deep cool sea water during recrystallization magnesium was lost and in some cases manganese the principal cathodoluminescence excitor was presumably gained

li sr mg and na in foraminiferal calcite shells from laboratory culture sediment traps and sediment cores abstract constant temperature laboratory culture experiments of the planktonic foraminiferal species globigerinoides sacculifer brady suggest that the ratios of li and sr to ca in the shells are a function of these ratios in the culture solutions mg ca and na ca in the shells did not vary with changes of these ratios in the culture solution these are the first direct determinations of the relationship between foraminiferal shell chemistry and solution composition the possibility of temperature dependence for the minor elemental composition of foraminiferal shells was also investigated in the laboratory and by analysis of several planktonic and one benthic foraminiferal species from sediment trap and sediment core samples the sr ca mg ca and na ca ratios in the natural samples roughly correlate with calcification temperature whereas differences in the li ca ratios are small and not systematically related to temperature however laboratory culture experiments at 20 c and 30 c showed no variation in the li ca sr ca mg ca and na ca ratios with calcification temperature for the planktonic foraminifera g sacculifer and orbulina universa therefore observed differences in the sr ca mg ca and na ca ratios for the sediment trap and core foraminiferal samples cannot be ascribed to direct effects of calcification temperature but may be due to some other environmental factor which is correlated with temperature

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

coccolith strontium to calcium ratios in emiliania huxleyi the dependence on seawater strontium and calcium concentrations in recent studies the sr ca ratio of coccolithophore calcite was used as a proxy for past coccolithophore growth and calcification rates since sr and ca concentrations in seawater have not remained constant through time interpretation of sr ca data from the coccolith dominated sedimentary record requires knowledge about the incorporation of seawater sr into coccolith calcite during coccolithogenesis here we show that sr ca of emiliania huxleyi coccoliths is linearly related to seawater sr ca meaning that the sr exchange coefficient does not change with changing seawater sr ca the exchange coefficient for sr in this study 0 39 is close to values presented in the literature and is high compared with values obtained by inorganic precipitation experiments this suggests a strong effect of cell physiology on biogenic calcite precipitation in coccolithophores we present a conceptual model based on the transmembrane transport of sr and ca which explains the offset

magnesium and strontium compositions of recent benthic foraminifera from the coral sea australia and prydz bay antarctica analyses of the mg ca and sr ca ratios of the modern benthic foraminifera cibicides wuellerstorfi epifaunal and uvigerina species infaunal from the coral sea and cibicides refulgens epifaunal and trifarina angulosa infaunal from prydz bay antarctica revealed relationships with temperature that have possible applications for reconstructions of bottom water paleotemperatures a positive relationship exists between the mg ca and sr ca ratios of cibicides wuellerstorfi and cibicides refulgens and ambient temperatures at least within the range of 2 and 6 c for the correlation between mg ca compositions and temperature the r2 values range from 0 78 c wuellerstorfi alone to 0 88 c wuellerstorfi and c refulgens together at present the mg ca temperature relationship must be regarded as tentative because of significant overlap of standard error values the relationship between the sr ca compositions of c wuellerstorfi and bottom water temperature yields an r2 value of 0 95 these results indicate that sr ca and possibly mg ca ratios of cibicides wuellerstorfi may provide useful information for the assessment of paleotemperature single species data are presently insufficient to assess the influence of ambient temperature on trace element compositions of uvigerina species trifarina angulosa may have mg ca compositions which are positively related to temperature but sr ca values seem unaffected by temperature this may be due to pore water influences on infaunal tests or to vital effects although more modern data are needed our present results suggest that sr ca ratios and possibly mg ca ratios of some benthic foraminifera have the potential to be useful paleothermometers at least within a temperature range of 2 to 6 c

effect of ambient mg ca ratio on mg fractionation in calcareous marine invertebrates a record of the oceanic mg ca ratio over the phanerozoic the mg ca ratio of seawater has changed significantly over the phanerozoic primarily as a function of the rate of ocean crust production echinoids crabs shrimps and calcareous serpulid worms grown in artificial seawaters encompassing the range of mg ca ratios that existed throughout the phanerozoic exhibit a direct nonlinear relationship between skeletal and ambient mg ca specimens grown in seawater with the lowest mg ca 1 changed their mineralogy to low mg calcite 4 mol mgco3 suggesting that these high mg calcareous organisms would have produced low mg calcite in the cretaceous when oceanic mg ca was lowest 1 these results support the empirical evidence that the skeletal chemistry of calcareous organisms has varied significantly over the phanerozoic as a function of the mg ca of seawater and that the mg ca of unaltered fossils of such organisms may be a record of oceanic mg ca throughout the phanerozoic mg fractionation algorithms which relate skeletal mg ca seawater mg ca and temperature were derived from these and other experiments they can be used to estimate paleoceanic mg ca ratios and temperatures from fossil skeletal mg ca of the organisms evaluated paleoceanic mg ca ratios recalculated by using the echinoderm mg fractionation algorithm from published fossil echinoid mg ca crinoid mg ca and paleotemperature data are consistent with other estimates and models of oceanic mg ca over the phanerozoic

a cenozoic seawater sr ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes a cenozoic multi species record of benthic foraminiferal calcite sr ca has been produced and is corrected for inter specific offsets typically less than 0 3 mmol mol and for the linear relationship between decreasing benthic foraminiferal sr ca and increasing water depth the water depth correction determined from holocene late glacial maximum and eocene paleowater depth transects is not vert similar0 1 mmol mol km the corrected cenozoic benthic foraminiferal sr ca record ranges from 1 2 to 2 0 mmol mol and has been interpreted in terms of long term changes in seawater sr ca enabling issues related to higher resolution variability in sr ca to be ignored we estimate that seawater sr ca was not vert similar1 5 times modern values in the late cretaceous but declined rapidly into the paleogene following a minimum in the eocene seawater sr ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late miocene by assuming scenarios for changing seawater calcium concentration and using published carbonate accumulation rate data combined with suitable values for sr partition coefficients into carbonates the seawater sr ca record is used to estimate global average river sr fluxes these fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity tectonic outgassing to calculate changes in global average river 87sr 86sr through the cenozoic the absolute magnitude of sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties nevertheless our results suggest that river sr flux increased from 35 ma to the present day roughly two fold accompanied by an overall increase in 87sr 86sr by not vert similar0 to 0 001 between 75 and 35 ma river 87sr 86sr also increased by not vert similar0 001 to 0 002 but was accompanied by a decrease two to three fold in river sr flux

impact of atmospheric p co 2 seawater temperature and calcification rate on the δ 18 o and δ 13 c composition of echinoid calcite echinometra viridis the tropical echinoid echinometra viridis was reared in controlled laboratory experiments at temperatures of approximately 20 c and 30 c to mimic winter and summer temperatures and at carbon dioxide co2 partial pressures of approximately 487 ppm v and 805 ppm v to simulate current and predicted end of century levels spine material produced during the experimental period and dissolved inorganic carbon dic of the corresponding culture solutions were then analyzed for stable oxygen 18oe 18odic and carbon 13ce 13cdic isotopic composition fractionation of oxygen stable isotopes between the echinoid spines and dic of their corresponding culture solutions 18o 18oe 18odic was significantly inversely correlated with seawater temperature but not significantly correlated with atmospheric pco2 fractionation of carbon stable isotopes between the echinoid spines and dic of their corresponding culture solutions 13c 13ce 13cdic was significantly positively correlated with pco2 and significantly inversely correlated with temperature with pco2 functioning as the primary factor and temperature moderating the pco2 13c relationship echinoid calcification rate was significantly inversely correlated with both 18o and 13c across treatments with effects of pco2 and temperature controlled for through anova therefore calcification rate and potentially the rate of co occurring dissolution appear to be important drivers of the kinetic isotope effects observed in the echinoid spines study results suggest that echinoid 18o monitors seawater temperature but not atmospheric pco2 and that echinoid 13c monitors atmospheric pco2 with temperature moderating this relationship these findings coupled with echinoids long and generally high quality fossil record support prior assertions that fossil echinoid 18o is a viable archive of paleo seawater temperature throughout phanerozoic time and that 13c merits further investigation as a potential proxy of paleo atmospheric pco2 however the apparent impact of calcification rate on echinoid 18o and 13c suggests that paleoceanographic reconstructions derived from these proxies in fossil echinoids could be improved by incorporating the effects of growth rate

radiaxial calcite alteration product of and petrographic proxy for magnesian calcite marine cement radiaxial calcite is a common and distinctive pore fill in ancient limestones but the origin of this fabric is highly controversial we present data from the late cretaceous equatorial pacific that suggest that radiaxial calcite forms through alteration of early magnesian calcite 7 mol mgco 3 marine cement these data suggest that radiaxial calcites are not straightforward indicators of pore fluid chemistry on the other hand the presence of radiaxial calcite in limestones may provide a distinctive petrographic proxy for earlier magnesian calcite cement and thus a generally warm caco 3 saturated marine environment

13c enriched carbonate in mississippian mud mounds alamogordo member lake valley formation sacramento mountains new mexico u s a abstract the origin of the carbonate mud constituting paleozoic mid shelf and deeper water mud mounds has long been debated mississippian mud mounds and their laterally adjacent coeval inter mound strata in the lake valley formation are well exposed along cliff faces in south central new mexico the mounds originated on erosional surfaces on a gently sloping ramp in 110 to 250 m water depth carbonate muds that constitute the inter mound strata mud mounds and syndepositional neptunian dikes within the mounds are low mg calcite microspars with similar petrographic cathodoluminescent and fluorescent characteristics carbon and oxygen stable isotope analyses demonstrate that the microspars constituting the inter mound mound and neptunian dikes have average 13c values of 2 6 0 2 3 4 0 6 and 4 8 0 3 pdb respectively additionally well preserved brachiopods from the inter mound strata and from muleshoe mud mound have 13c values markedly different from each other but similar to their surrounding microspars i e brachiopod s 2 7 and 3 9 pdb respectively the stable isotope distributions within each environment similarity of values between microspar and brachiopod data from the same depositional environments as well as agreement of 13c values with other investigations of coeval strata support the conclusion that the measured 13c values are essentially primary values it is envisioned that 13c enriched fluids sourced by bacterial fermentation in the subjacent strata rose into the mounds some through neptunian dikes and influenced the precipitation of the microspars in the mounds and dikes near the sediment water interface these fluids mixed with normal marine waters thus precipitating the microspars or their precursors which have high 13c values that characterize the mud mounds the inter mound microspars and inter mound brachiopods have the lowest 13c values and are in agreement with worldwide coeval marine calcites that is the inter mound microspars formed under normal marine conditions the high 13c values measured in all of the mound related samples indicate that all mounds received a contribution of 13c enriched waters this relationship indicates that microspar precipitation in the mounds was associated with seepage of methanogenic fluids from the underlying strata

δ18o and δ13c values of modern brachiopod shells abstract researchers have not rigorously tested the hypothesis that calcite from modern brachiopod shells is precipitated in oxygen isotope equilibrium with ambient seawater isotopic variability at the intraspecimen and intertaxon levels has not been examined without such data for modern brachiopods similar data from ancient brachiopods cannot be accurately interpreted in this study a survey is made of 18 o and 13 c values of terebratulid rhynchonellid thecideidine and craniacean brachiopods from antarctica the bay of fundy curacao japan new zealand norway puget sound palau sicily and south africa this suite of samples provides a wide range of taxonomic levels temperatures salinities and depositional environments for evaluating the degree of isotopic equilibrium attained during precipitation of brachiopod calcite new data indicate that modem brachiopod calcite is not always precipitated in oxygen and carbon isotope equilibrium with ambient seawater calcite from the primary layer and specialized shell structures hinge brachidium foramen interarea muscle scars are depleted in both 18 o and 13 c a characteristic of biological fractionation or vital effects often found in other calcerous marine organisms our findings suggest that these portions of the brachiopod shell should be avoided during sampling of ancient brachiopods secondary layer calcite the material most often analyzed in ancient brachiopods has higher 18 o and 13 c values which approach and sometimes correspond with predicted equilibrium values therefore secondary layer calcite is the most suitable portion of the brachiopod shell for use as an ancient seawater proxy although near equilibrium precipitation in secondary layer calcite is encouraging to those studying the isotopic composition of ancient oceans these data come with caveats large intraspecimen variability in the 18 o values of secondary layer calcite 1 in some samples limits the use of brachiopods as precise indicators of the 18 o value or temperature of ancient seawater the 18 o and 13 c values of secondary layer calcite deviate from predicted equilibrium values toward the lower values of the primary layer and specialized shell structures indicating a measurable vital effect in most cases measured 18 o values are lower than the calculated equilibrium values dictated by seasonal variations in the 18 o value and temperature of seawater ontogenetic variations in the 13 c values of secondary layer calcite have also been measured and must be assessed when interpreting ancient data modern brachiopods occur in a wide range of seawater salinities and temperatures therefore the relation between salinity and the 18 o value of seawater must be known to correctly calculate the temperature of calcite precipitation from secondary layer 18 o values errors of up to 15 c are found if ambient seawater is assumed to have a constant 18 o value e g 0 smow and the 18 o values of secondary layer calcite are used to calculate temperature coupled with vital effects and intraspecimen variability these errors can obscure the resolution of secular variations in the rock record

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

radiaxial calcite in lower miocene strata subsurface enewetak atoll abstract radiaxial calcite is abundant in lower miocene strata 377 819 m deep on enewetak atoll these radiaxial crystals have equant bladed and fibrous morphologies and occur as isopachous crusts 0 05 2 0 mm thick radiaxial terminations consist of fibrous subcrystals bundled together into larger acute obtuse or rounded shapes enewetak radiaxial calcite ranges from clear to very inclusion rich most inclusions are microcavities caused by organic infestations videtich 1985 alternating bands of inclusion rich and inclusion poor calcite mimic existing termination patterns radiaxial calcites fill intergranular pores sponge borings fractures and aragonite molds aragonite has been pervasively dissolved from the lower miocene strata containing radiaxial calcite banded radiaxial calcit can be traced from intergranular pores into aragonite molds radiaxial calcites have a mean magnesium concentration of 3 2 mole mgco3 with a range from 1 6 to 11 1 mole mgco3 inclusion rich bands and more deeply buried radiaxial calcite tend to have lower magnesium concentrations enewetak radiaxial calcite has 13c values of 1 3 to 2 5 pdb and 18o values of 1 8 to 0 4 pdb petrographic and stable isotopic data are consistent with precipitation of radiaxial calcite in its present form directly from seawater precipitation of radiaxial calcite after aragonite dissolution indicates precipitation distinctly after deposition probably in a burial environment dissolution of aragonite by deep marine water is suggested by 1 abundant marine cement associated with aragonite dissolution 2 a lack of meteoric cements associated with aragonite dissolution and 3 no evidence of subaerial exposure in lower miocene strata between 400 and 819 m modern pacific ocean water is undersaturated with respect to aragonite at depths where aragonite dissolution is observed 375 850 m tidal fluctuations and temperature profiles indicate that lower miocene strata containing radiaxial calcite are in open communication with modern pacific ocean water therefore aragonite dissolution and radiaxial cementation are interpreted as having occurred in seawater undersaturated with respect to aragonite circulating through the atoll variations in magnesium concentration in enewetak radiaxial calcite may be the result of original heterogeneity or differential loss of magnesium during later diagenesis in seawater the first alternative is preferred precipitation of radiaxial calcite at different rates at different degrees of saturation could cause variable magnesium concentrations more deeply buried radiaxial calcite may have precipitated more slowly in deeper less supersaturated water resulting in lower magnesium concentrations likewise slower rates of precipitation may have resultedin organic infestations and lower magnesium concentrations in inclusion rich bands enewetak radiaxial calcites suggest that two factors might be critical to development of radiaxial fabric 1 precipitation at fluctuati g rates and 2 formation in waters undersaturated with respect to aragonite