an experimental study of strontium and lead diffusion in calcite and implications for carbonate diagenesis and metamorphism chemical diffusion of pb and sr has been measured in natural calcite under anhydrous conditions the source of diffusants for the sr and pb diffusion experiments were srco3 and pbo powders respectively experiments were run in sealed silica capsules containing finely ground calcite to ensure stability of the single crystal samples during diffusion anneals rutherford backscattering spectroscopy rbs was used to measure diffusion profiles the following arrhenius relations were obtained over the temperature range 440 800 c for sr 440 650 for pb dsr 2 1 x 10 13exp 132 6 kj mol 1 rt m2 s 1 dpb 5 8 x 10 15exp 117 9 kj mol 1 rt m2 s 1 diffusion of sr and pb is relatively rapid in comparison to rates for these elements in other minerals but fine scale zoning and isotopic and chemical signatures many be retained in calcites under many conditions if diffusion is the dominant process affecting alteration

kinematics of cenozoic extension on the south china sea continental margin and its implications for the tectonic evolution of the region abstract in the northern south china sea several large sedimentary basins developed during the cenozoic characterized by episodic rifting clockwise rotation of the rifts an eastward aging breakup unconformity and intensifying crustal extension to the east these features may be summarized by a scissors model the maximum rifting occurred north of the maximum thermal subsidence which was in turn north of the seafloor opening in contrast the nansha microcontinent south of the south china sea basin was dominated by compressional deformation during the cenozoic extension was of subordinate significance and manifested mainly by block faulting and tilting in the late early oligocene maximum tilting occurred in the southern part of the nansha microcontinent at distance from the deep sea basin of the south china sea the asymmetric development of extensional structures around the south china sea basin may be explained by the simple shear model proposed by wernicke 1981 the tectonic development of the south china sea was the consequence of various interactions of three major plates and may be summarized into three stages with dominating influences of respectively the retreat of the west pacific subduction zone in the late cretaceous the hard collision and impinging of india to tibet since the late eocene and the fast northward subduction of the indian ocean australian plate since the late early miocene

the record of sea water 187os 186os variation through the cenozoic the variation of sea water 187os 186os during the cenozoic has been determined by analyzing the hydrogenous os released by acidic hydrogen peroxide leaching of layers in an abyssal north pacific pelagic clay core overall sea water 187os 186os has increased from 3 2 at 58 ma to the present day value of 8 6 this pattern is similar to the well known 87sr 86sr evolution of sea water the increase in the proportion of continental os 187os 186os 10 could be due to enhanced weathering rates of black shales associated with the uplift of the himalayas other elements associated with black shale weathering such as u and p should show effects in the sedimentary record

salt sources and water rock interaction on the yilgarn block australia isotopic and major element tracers internal drainage in the south east of western australia are underlain by predominantly granitoid rocks the regional shallow ground water are nacl brines ratios of 87sr 86sr in the brines are 0 7155 0 0015 which are close to the ratio of modern marine sr 0 7092 a two component isotopic mixing model with end members of marine sr and sr from basement granitoid rocks implies strongly that 5 of the sr is derived from rock weathering and 95 is marine the isotropic composition of s in gypsum crystallised from the brines is 19 8 0 3 cdt this value is close to that of s in recent evaporites 21 5 1cdt and confirms that weathering of archaean sulphides with 34sof0 4 cdt has contributed insignificant amounts of s to the brines the isotopic data are compatible with a late tertiary to recent age for the sr and s and by implication for the salts as a whole the formation of alunite has been a major control on brine composition while gypsum and calcrete formation have been minor influences none of the ground waters is saturated with halite which suggests the brines are immature and have evolved their present chemistry within the past few tens of thousands rather than millions of years marine aerosols are the main source of salt water sequestered in internal drainage basins by a post middle miocene regression possibly provided a subordinate fraction

strontium dissolved and particulate loads in fresh and brackish waters the baltic sea and mississippi delta a study was conducted of the isotopic composition and concentration of sr and of major elements in dissolved and suspended loads of fresh and brackish waters the purpose was to establish the contributions of different parent rocks and minerals to sr during weathering and transport and to identify the role of fe mn oxyhydroxides in the redistribution of sr in the water column during the sedimentary cycle studies were conducted on a profile across an oxic anoxic boundary in the baltic and on rivers covering behavior over an annual cycle in general the 87sr 86sr ratios differ between particulate and dissolved loads with more radiogenic sr in the particulate loads these differences are attributed to differential weathering of minerals where high rb sr minerals dominate the particulate load and low rb sr the dissolved load there is broad correlation of 87 sr 86 sr with k al in the suspended load the differences in 87 sr 86 sr between suspended and dissolved load are highly variable and are related to the fe or mn concentration on the particulates in samples with high fe al the difference becomes small a good correlation was found between sr al and fe al or mn al in the particulates both in brackish and fresh waters sr is removed from solution both in rivers and in the baltic sea whenever there is formation of fe mn oxyhydroxide particulates this precipitation greatly diminishes the difference in isotopic composition of the dissolved and suspended loads as the particles containing fe mn oxyhydroxides settle they dissolve in anoxic zones and release sr this provides a mechanism for sr redistribution in the water column sr is thus only quasi conservative in environments where fe mn oxyhydroxides form or dissolve from consideration of the isotopic differences in sr between dissolved and suspended loads it follows that the net sr input depends upon weathering characteristics of the contributing mineral phases changes in weathering mechanisms due to climate change may cause sr isotopic shifts in the marine environment

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

the assessment of ree patterns and 143nd 144nd ratios in fish remains abstract the ree content and isotopic composition of sr and nd have been determined in fish teeth ranging in age from the trias to the present and from various localities mostly around the atlantic these measurements have been carried out on selachian and teleost remains from the same locality in togo and show no appreciable difference which suggests with the help of a mass balance calculation of the ce anomaly that diagenetic effects are not responsible for the ree enrichment of biogenic phosphates one group of fossil teeth has about 3 times the ree abundances of shale and a shale normalized pattern with a minimum at sm it is thought to reflect deposition in the open sea environment a second group has ree concentration about 10 times higher than the first group with either a regular light ree enrichment or more frequently a maximum in the middle ree both being probably indicative of deposition in estuarine or near shore conditions the shape of the ree spectra and the size of the ce anomaly can be used semi quantitatively to determine the depth of deposition the results presented here on late cretaceous eocene fish teeth samples from morocco reflect an increasing influx of deep waters with a lowla yb ratio and strong negative ce anomaly which agrees well with the evolution of sediment chemistry and microfauna associations in contrast nd is typical of the water mass in which the fish debris decayed examples of nearly isolated basins identified with nd isotopes include the south atlantic prior to the lutetian nd 13 5 the miocene persian gulf nd 3 1 and bolivia during the late cretaceous nd 12 8 togo and guinea bissau results suggest that in the south atlantic the meridional oceanic circulation had not started before 45 ma ago combination of ree and nd data suggests that the assignment of jurassic cretaceous samples measured so far to open sea water masses is still ambiguous

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

orbitally tuned sr isotope chemostratigraphy for the late middle to late miocene we present a sr chemostratigraphic reference section for the late middle to late miocene 14 5 ma from ocean drilling program site 926 on the ceara rise this site combines a precise orbitally tuned timescale with a high sedimentation rate 15 m m y continuous deposition and excellent biostratigraphic control the sr isotope curve is based on measurements of cleaned planktonic foraminifera at 100 200 kyr sample intervals and it illustrates periods of rapid change in 87sr 86sr alternating with periods of little change chemostratigraphically defined ages for these intervals can be determined within 0 8 m y and 1 6 m y respectively there is excellent correlation with the published curve for site 588 hodell and woodruff 1994 however the curve for site 747 oslick et al 1994 exhibits less structure which may be due to small errors in age estimates related to slow sedimentation rates high latitude fauna and an interval of complicated magnetics late miocene data compare favorably with data from site 758 farrell et al 1995

variations in the strontium isotope composition of seawater during the paleocene and early eocene from odp leg 208 walvis ridge we refined the strontium isotope seawater curve for the paleocene and early eocene by analysis of samples recovered from the walvis ridge during ocean drilling project odp leg 208 the highest sr 87 sr 86 values occurred in the earliest paleocene at similar to 65 ma and generally decreased throughout the paleocene reaching minimum values between 53 and 51 ma in the early eocene before beginning to increase again at similar to 50 ma a plausible explanation for the sr 87 sr 86 decrease between 65 and 51 ma is increased rates of hydrothermal activity and or the eruption and weathering of large igneous provinces e g deccan traps and north atlantic strontium isotope variations closely parallel sea level and benthic delta o 18 changes during the late paleocene and early eocene supporting previous studies linking tectonic reorganization and increased volcanism to high sea level high co2 and warm global temperatures

effect of diagenesis on the sr o and c isotope composition of late cretaceous mollusks from the western interior seaway of north america evaluating the effects of diagenesis on the isotopic compositions of sr o and c in marine carbonates is critical to their use as proxies in reconstructing information on the salinity temperature and dissolved inorganic carbon of ancient oceans we have analyzed a series of samples of mollusk shells from the baculites compressus zone late campanian of the pierre shale of south dakota samples included outer shell material and septa of cephalopods collected inside and outside concretions preservation was evaluated using light microscopy scanning electron microscopy sem trace element analysis and x ray diffraction all of the material consists of aragonite based on x ray diffraction an sem preservation index pi was established based on comparison of the microstructure of the fossil material with that of modern nautilus excellent preservation pi 5 was characterized by well defined nacreous plates with discrete angular boundaries in contrast samples showing fused nacreous plates with indistinct boundaries were rated poor pi 1 87sr 86sr ratios vary with preservation and average 0 707648 000021 n 10 for excellent preservation pi 5 0 707615 000028 n 5 for good preservation pi 3 0 707404 000074 n 7 for fair preservation pi 2 and 0 707261 000053 n 8 for poor preservation pi 1 these data suggest that as the quality of the preservation declines the mean 87sr 86sr ratio decreases and the standard error of the mean increases oxygen and carbon isotope analyses of the same specimens also show decreases with preservation and 18o 13c and 87sr 86sr are well correlated suggesting that these tracers are all altered as the pi decreases the sr ca ratio increases as preservation decreases indicating that sr is added to the shell material during diagenesis in contrast mg ca shows no trend with preservation if the increasing sr concentration and decreasing 87sr 86sr of the shell material with decreasing preservation represents the addition of sr to the shell during diagenesis we calculate that the added sr had 87sr 86sr ranging from 0 707582 to 0 707032 potential sources of the added sr include older marine carbonates and weathering of volcanic ash layers present in the shale the mechanisms of alteration likely include epitaxial growth of strontianite on the original shell aragonite and isotopic exchange of c and o between alteration fluids and shell carbonate we conclude that sem preservation criteria are effective in screening shell material that records original isotopic values and that variations in sr o and c isotope composition in well preserved material can be used to assess paleoenvironmental parameters such as salinity and temperature our results also indicate that assessing preservation is a critical prerequisite to the determination of numerical ages of shell material using strontium isotope stratigraphy

variations of the187os 186os ratio of seawater over the past 28 million years as inferred from metalliferous carbonates abstract the os concentration and isotopic composition of metalliferous carbonates deposited on the east pacific rise over the past 28 ma are reported with complimentary sr isotope data variations in the os isotopic composition of these samples are interpreted as a record of past changes in the os isotopic composition of seawater these results are consistent with isotopic analyses of leachable os in pelagic clays which have also been interpreted as a record of the187os 186os ratio of seawater through time 1 the metalliferous carbonate record clearly shows that seawater os and sr isotope systems are partially decoupled from one another over the past 28 ma accelerated weathering of ancient organic rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the187os 186os ratio of seawater over the past 15 ma this rapid increase suggests that the seawater os record can potentially be used as a stratigraphic tool in some neogene marine deposits

age determinations and growth rates of pacific ferromanganese deposits using strontium isotopes abstract 87 sr 86 sr ratios trace element and ree compositions and textural characteristics were determined for three hydrogenetic fe mn crusts one hydrothermal deposit and two mixed hydrothermalhydrogenetic crusts from the pacific the sr isotope data are compared to the sr seawater curve for the cenozoic to determine the ages and growth rates of the crusts the 87 sr 86 sr in the crusts does not increase monotonically with depth as expected if the sr were solely derived from seawater and perfectly preserved since deposition this indicates post depositional exchange of sr or heterogeneous sources for the sr originally contained in the crusts textures of hydrogenetic crusts generally correlate with sr isotopic variations the highest porosity intervals commonly exhibit the highest 87 sr 86 sr ratios indicating exchange with younger seawater intervals with the lowest porosity commonly have lower 87 sr 86 sr and may preserve the original sr isotopic ratios minimum ages of crust growth inception were calculated from dense low porosity intervals growth of the hydrogenetic crusts began at or after 23 ma although their substrates are cretaceous estimated average growth rates of the three hydrogenetic crusts vary between 0 9 and 2 7 mm ma consistent with published rates determined by other techniques within the marshall islands crust growth rates for individual layers varied greatly between 1 0 and 5 4 mm ma for one crust very low 87 sr 86 sr ratios occurred in detrital rich intervals hydrothermal fe mn oxide from the active lau basin back arc spreading axis valu fa ridge has an 87 sr 86 sr ratio with a predominantly seawater signature 87 sr 86 sr 0 709196 indicating a maximum age of 0 9 ma one crust from an off axis seamount west of gorda ridge may have begun precipitating hydrogenetically at 0 5 ma 0 709211 and had increasing hydrothermal or volcanic input in the top half of the crust indicated by a significantly lower 87 sr 86 sr ratio 0 709052

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

high resolution stratigraphy with strontium isotopes the isotopic ratio of strontium 87 to strontium 86 shows no detectable variation in present day ocean water but changes slowly over millions of years the strontium contained in carbonate shells of marine organisms records the ratio of strontium 87 to strontium 86 of the oceans at the time that the shells form sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium 87 to strontium 86 with a resolution that is similar to that of other techniques used in age correlation this method may prove valuable for many geological paleontological paleooceanographic and geochemical problems

clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower miocene carbonate sediments of the bahamas platform isotopic and thermodynamic evidence abstract massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor mbsf in lower miocene well compacted carbonate periplatform sediments from the great bahama bank ocean drilling program odp leg 166 site 1007 the diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix as well as mg smectites in carbonate rich deposits the formation of the zeolite requires a supply of silica thus the objective of the study is to determine the origin of the silica supply its diagenetic evolution and consequently the related implications on interpretation of the sedimentary record in terms of local or global paleoceanographic change for lack of evidence for any volcaniclastic input or traces of si enriched deep fluids circulation an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow water carbonate sequences geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite as well as its contemporaneous formation with the carbonate deposition sr model ages of 19 6 23 2 ma the test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage seawater rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica which reacts with clay minerals the amount of silica opal a involved in the reaction has to be significant enough at least 10 wt to account for the observed content of clinoptilolite occurring at the most zeolite rich level modeling also shows that the observed amount of clinoptilolite 19 reflects an in situ and short term reaction due to the high reactivity of primary biogenic silica opal a until its complete depletion the episodic occurrence of these well lithified zeolite rich levels is consistent with the occurrence of seismic reflectors particularly the p2 seismic sequence boundary located at 1115 mbsf depth and dated as 23 2 ma the age range of most zeolitic sedimentary levels biostratigraphic ages of 21 5 22 ma correlates well with that of the early miocene glaciation mi 1 and mi 1a global events thus the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply or in other sensitive marine areas is potentially a significant new proxy for paleoproductivity and oceanic global events such as the miocene events which are usually recognized in deep sea pelagic sediments and high latitude deposits

constraining the onset of flexural subsidence and peripheral bulge extension in the miocene foreland of the southern apennines italy by sr isotope stratigraphy abstract in fold and thrust belts developing at convergent margins the migration of the advancing wedge is accompanied by bulging of the downgoing plate followed by the development of a foredeep basin filled by a thick succession of syn orogenic sediments the transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system in deep water environments the record of this transition is typically complete and progressive conversely in the shallow water continental environment of many collisional systems the uplift of the forebulge area can imply emersion and erosion obliterating the stratigraphic record of key steps of the evolution of the orogenic system the southern apennines constitute one of these collisional fold and thrust belts where the development of the forebulge has implied emersion and erosion with the development of a miocene forebulge erosional unconformity accompanied by extensional deformation associated with the bending of the lithosphere during the forebulge stage in this paper we use strontium isotope stratigraphy to constrain with unprecedented time resolution the age of the forebulge unconformity in areas presently incorporated in the northern sector of the southern apennines fold and thrust belt integration of our results and those of previous studies indicates at the regional scale a younging toward the foreland of the forebulge unconformity across the belt our high resolution ages also reveal a diachronous onset of the flexural subsidence over short distances associated with the occurrence of horst and graben structures possibly resulting from inherited paleotopography along with forebulge extension this work highlights how high resolution dating is critical to unravel the evolution of foreland basin systems at different scales

integrated stratigraphy of the oligocene pelagic sequence in the umbria marche basin northeastern apennines italy a potential global stratotype section and point gssp for the rupelian chattian boundary the oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations the pelagic succession of the umbria marche apennines central italy includes a complete and continuous sequence of marly limestones and marls with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period we present here a synthesis of detailed biostratigraphic magnetostratigraphic and chemostratigraphic studies along with geochronologic results from several biotite rich volcaniclastic layers which provide the means for an accurate and precise radiometric calibration of the oligocene time scale from this study the interpolated ages for the rupelian chattian stage boundary located in the upper half of chron 10n at meter level 188 in the monte cagnero section and corresponding to the o4 o5 planktonic foraminiferal zonal boundary are 28 36 ma paleomagnetic interpolation 28 27 0 1 ma direct radioisotopic dating and 27 99 ma astrochronological interpolation these ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations the monte cagnero section is a potential candidate for defining the chattian global stratotype section and point gssp and some reliable criteria are here proposed for marking the rupelian chattian boundary according to international union of geological sciences iugs recommendations

seawater carbon and strontium isotope variations through the late ediacaran to late cambrian in the tarim basin abstract the radiation of early animals during the cambrian explosion was accompanied by widespread perturbations in oceanic and atmospheric conditions however the cause effect relationships between evolutionary innovations carbon c cycling and continental weathering are still a matter of debate further paleoenvironmental reconstructions of poorly studied paleo continents covering the precambrian cambrian pc c transition may improve the correlation of the events and their impact on the earth system on a global scale the tarim basin was presumably located proximal to the well studied south china craton during the ediacaran cambrian boundary and is expected to contribute equally to our paleoenvironmental reconstructions here we apply carbon and strontium sr isotope records 13c and 87sr 86sr obtained from well preserved marine carbonates spanning the late ediacaran to the late cambrian from the keping region in the northwestern tarim basin nw china by comparison to other carbonate successions from various paleo continents our obtained seawater 87sr 86sr curve in this study shows a stepwise first order increase to more radiogenic values superimposed by second order presumably regional variations our obtained 13c variations exhibit four prominent negative and three positive excursions identical to the global 13c record based on existing paleontological evidence and our new tarim 13c and 87sr 86sr curves we propose a stratigraphic correlation from the late ediacaran to the late cambrian between the tarim basin and other paleo continents besides our paleoenvironmental reconstruction reveals both regional and global sea level changes that may have controlled the influx of radiogenic sr and presumably the influx of nutrients controlling primary production and the rate of organic carbon burial in the tarim basin at the time of deposition these feedbacks may have ultimately influenced the rapid diversification of metazoans and the oxygenation of the cambrian ocean and atmosphere approaching present po2 levels

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

late eocene oligocene magnetostratigraphy and biostratigraphy at south atlantic dsdp site 522 upper eocene to lowest miocene sediments recovered at deep sea drilling project dsdp site 522 in the south atlantic ocean allow direct calibration of magnetostratigraphy and calcareous plankton biostratigraphy the results from site 522 show that the eocene oligocene boundary occurs in the reversed interval of magnetic chron c13 c13r and that the oligocene imiocene boundary probably occurs in the upper part of chron c6c

meta igneous granulite xenoliths from mount ruapehu new zealand fragments of altered oceanic crust meta igneous granulite mig xenoliths in lavas from mount ruapehu taupo volcanic zone new zealand have variable but relatively high 18o 8 2 to 11 7 and 87sr 86sr 0 70506 to 0 70872 and end ranging from 1 5 to 6 2 they show a strong positive correlation between 87sr 86sr and 18o both of which are also broadly correlated with mg number but lack any correlation between 87sr 86sr and end the xenoliths have been mineralogically re equilibrated at lower crustal temperatures 800 930 c and pressures 7 10 kbar geochemical and isotopic evidence suggests they are fragments of oceanic crust that have been altered previously in an ocean floor hydrothermal system alternatively they may be igneous rocks of unknown origin hydrothermally altered in the lower crust irrespective of this uncertainty the xenoliths provide rare samples of the lower crustal basement beneath taupo volcanic zone and represent a potential source rock for the voluminous rhyolites and ignimbrites that dominate the zone

seawater sr isotope variations through time a procedure for constructing a reference curve to date and correlate marine sedimentary rocks a new curve of seawater 87 sr 86 sr variation through the past 450 m y enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties we discuss the method by which this curve was constructed strictly standardized 87 sr 86 sr ratios and biostrat igraphic data for 1300 87 sr 86 sr analyses of marine minerals were input into a data base objective criteria were used to assign each data point a reliability score these scores were used as initial weights in a robust nonparametric regression lowess to obtain a curve of seawater 87 sr 86 sr variation with rigorous 95 confidence intervals for prediction of age uncertainty precise dating and stratigraphic correlation are feasible for much of the phanerozoic the main problem with future sr isotope stratigraphy is likely to be sample quality we discuss guidelines for sample selection

boron isotope systematics of marine sediments abstract boron contents and boron isotopic compositions were determined for modern and ancient permian to miocene marine sediments including pelagic clay calcareous ooze siliceous ooze and neritic clay sediments 11b values of modern marine sediments range from 6 6 to 4 8 isotopic variation is controlled by the simple mixing of four major constituents detritus of continental origin marine smectite biogenic carbonates and biogenic silica detritus of continental origin with an average 11b value of 13 to 8 is the low 11b end member constituent of marine sediments and its boron is largely controlled by the concentration of illite which originates from wind or fluvial transport marine smectite biogenic carbonate and biogenic silica on the other hand represent the high 11b end members with 11b values of 2 3 to 9 2 8 0 to 26 2 and 2 1 to 4 5 respectively these high 11b values are the result of the equilibrium uptake of boron from seawater spatial variations in boron isotopes in the pacific sediments are essentially due to the distribution of the above four constituents although ancient argillaceous sediments shale and slate have boron contents that are identical with those of modern equivalents boron contents of limestone and chert are distinctly lower than those of modern calcareous and siliceous oozes ancient marine sediments have systematically lower 11b values 17 0 to 5 6 than those of the modern sediments the lower 11b values can be caused by diagenesis which induces 1 preferential removal of high 11b boron in calcium carbonate and silica during recrystallization and 2 boron isotopic exchange in the course of the smectite illite transition the observed boron isotopic compositions of ancient argillaceous sediments are distinctly different from those of fresh and altered morb therefore boron isotope systematics will be useful in identifying components from the descending oceanic slab involved in the formation of island arc magma and in investigating mantle crust recycling through subduction processes

the oligocene marine microfossil record age assessments using strontium isotopes a strontium isotope record for the latest eocene to earliest miocene has been produced for two well preserved deep sea sequences from the atlantic deep sea drilling project sites 516f and 563 which have magnetostratigraphic age control and four sequences from the pacific sites 77b 292 and 593 and atlantic site 529 which lack magnetostratigraphy the composite 87sr 86sr age record of sites 516f and 563 forms a comprehensive composite standard for the oligocene against which other sequences can be correlated unlike previously published 87sr 86sr oligocene sea water curves which have suggested that the sea water isotopic composition changed monotonically during the latest paleogene we note a distinct change in slope between the early oligocene 36 to 27 ma and the late oligocene to earliest miocene 27 to 20 ma the slope of the early oligocene segment is less steep than that of the late oligocene to earliest miocene we have fit two linear regression curves to the oligocene to early miocene seawater 87sr data of sites 516f and 563 which we use to establish sr isotope age relationships for the oligocene the 87sr age equations were used to evaluate the synchroneity of specific planktonic and calcareous nannofossil datum levels used to correlate oligocene sections although some datum levels appear to be synchronous between the atlantic and pacific our results show that others are diachronous along longitudinal gradients this is most evident in several last appearance datums that occurred earlier in the western equatorial pacific than in the eastern equatorial pacific this diachroneity extends into the atlantic where last occurrences were later than in the eastern equatorial pacific

the broadlands ohaaki geothermal system new zealand part 1 strontium isotope distribution in well bro 29 abstract well bro 29 at the broadlands ohaaki geothermal system new zealand penetrates a mixed volcanic sedimentary rock sequence whose 87 sr 86 sr ratios display linear distributions when plotted against 1 sr values these linear distributions do not indicate binary mixing between unaltered lithological units and a single fluid endmember the linear distribution in the volcanic pyroclastic sequences is related to the permeability of the host rocks the degree of leaching of 86 sr rich primary plagioclase laths and secondary crystallisation of the groundmass by a recharge fluid represented by modified taupo rainwater the greywacke argillite basement from the same well displays a separate linear isotopic trend when their 87 sr 86 sr ratios are plotted against 1 sr values in addition to yielding a sr sr vs rb sr errorchron with an age of 88 6 ma bp mswd 9 48 based upon supporting petrographic observations the two isotopic trends in the greywacke argillite basement are believed to be an inherited metamorphic feature from the rangitata orogeny 141 3 ma which has been modified by geothermal fluids preferentially leaching 87 sr and 87 rb from primary micas

major and minor metal composition of three distinct solid material fractions associated with juan de fuca hydrothermal fluids northeast pacific and calculation of dilution fluid samples abstract three distinct types of solid material are associated with each sample of the hydrothermal fluid that was collected from the vents of the southern juan de fuca ridge the solid materials appear to be representative of deposits on ocean floors near mid ocean ridges and interpretation of the chemistry of the hydrothermal solutions requires understanding of them sr isotopic evidence indicates that at least two and probably all three of these solid materials were removed from the solution with which they are associated by precipitation or adsorption this occurred after the pure hydrothermal fluid was diluted and thoroughly mixed with ambient seawater the three types of solid materials are respectively a coarse zn and fe rich material with small amounts of na and ca a finer material also rich in zn and fe but with alkali and alkaline earth metals and a scum composed of ba or zn with either considerable fe or si and sr mineral identification is uncertain because of uncertain anion composition only in the cases of ba and zn were metal masses greater in solid materials than in the associated fluids for all other metals measured masses in fluids dwarf those in solids the fluids themselves contain greater concentrations of all metals measured except mg than seawater we discuss in detail the relative merits of two methods of determining the mixing proportions of pure hydrothermal solution and seawater in the fluids one based on sr isotopes and another previously used method based on mg concentrations comparison of solute concentrations in the several samples shows that degree of dilution of pure hydrothermal solutions by seawater and amounts of original solutes that were removed from it as solid materials are not related there is no clear evidence that appreciable amounts of solid materials were not conserved lost either during or prior to sample collection

evidence for slowly changing 87sr86sr in runoff from freshwater limestones of southern france abstract 87 sr 86 sr ratios have been determined in modern rivers and freshwater limestones dating from 65 ma to the present over an area extending from the pyrenees to the massif central southern france about 40 ma ago the alpine mountain range of the pyrenees was strongly uplifted by the alpine orogenic activity and its pre hercynian basement dominated by schists gneisses and granites was rapidly exposed from beneath its blanket of mesozoic marine limestones meanwhile the 87 sr 86 sr ratio of the rivers has remained remarkably constant in both space with the exception of the very few northernmost samples and time in this area extreme variations through the orogenic period are small with 87 sr 86 sr lying between 0 7073 and 0 7084 at 55 65 ma and 0 7077 0 7096 for the last 30 ma river sr is controlled by a nearly binary mixing of silicate 3 4 and 1 4 end members with negligible influence of evaporites it is inferred that the silicate sediment contribution ratio of the world average rivers to seawater sr changes at a very slow rate even taking major basaltic events into account does not alter this conclusion as their contribution to the river sr is very subordinated to other components it is inferred that the seawater 87 sr 86 sr secular main trend over the phanerozoic is controlled by hydrothermal alteration of oceanic basalts and slow variation of the limestone silicate contribution ratio to the riverine sr oceanic events changes in organic productivity and carbonate compensation depth ccd may produce intense short term variations but recovery of steady state seawater 87 sr 86 sr is delayed by 40 80 ma through diagenetic sr remobilization in carbonate sediments

deep water circulation at the northern pyrenean thrust implication of high temperature water rock interaction process on the mineralization of major spring water in an overthrust area abstract in an overthrust context of the northern pyrenean area groundwater is affected by high mineralization sources a multi isotopic approach was used to track the origin of mineralization in a complex multilayer aquifer system through the study of a major regional spring the oeillal spring water osw which shows a not yet investigated geochemical singularity the aims of this study are to understand the hydrogeochemical evolution and the mineralization mechanisms of groundwater in a large scale aquifer extending over the northern pyrenean overthrust oriental corbieres based on an adequate set of geochemical and isotopic tracers including cosmogenic stable and radiogenic isotopes we have revealed complex hydrogeochemical processes that cannot be tracked with traditional hydrochemical and isotopic tracers the high 87 sr 86 sr ratios of the osw associated with relatively high concentrations in trace elements cs rb sr b li suggest the influence of brine from the triassic deposits containing saliferous clay orsaline fluids from the fractured basement the basement hypothesis may be considered regarding the hydro geochemical fingerprint of the paleozoic basement as demonstrated in previous study and further data would be required to validate the contribution of basement fluids contrasting with marine primary brine and current seawater the relatively high 37 cl 0 22 and 0 39 of osw attests the non marine origin of the chloride ion 36 cl cl shows that the osw is issued from a mixing process between recently recharged water lightly mineralized and significantly marked in 36 cl by the thermonuclear bomb pulse and a saline deep end member at secular equilibrium this indicates that cl in these springs is primarily derived from subsurface non meteoric sources the osw s low br cl molar ratio suggests the influence of simple dissolution of halite and or the contribution of more complex secondary brine the deep saline water may results from meteoric infiltration into the lower jurassic aquifer in contact with triassic deposits or deeper high temperature promotes water rock interaction enhancing leaching of mg k rich saliferous clay this is supported by environmental scanning electron microscopy esem analyses showing that i cl is found in accessory minerals from the kcl mgcl 2 h 2 o system of the triassic deposits and ii cs rb molar ratio is similar to that of osw ranging between 0 13 and 0 7 and from 0 13 to 0 4 respectively laboratory experiments carried out on triassic material show that high temperature water rock interaction is suitable to explain the relatively high 87 sr 86 sr observed in groundwater such process can be investigated to confirm the deep origin of the osw and to explain the signature of deep brines in contact with the triassic deposits under high temperature conditions basement fluid contribution may also be considered regarding the hydro geochemical fingerprint of the paleozoic basement as shown in previous studies further data would be required to validate this hypothesis

meteoric diagenesis marine diagenesis and microporosity in pleistocene and oligocene limestones enewetak atoll marshall islands abstract microporosity occurs in pleistocene and oligocene limestones cored on enewetak atoll in cores of pleistocene limestone 10 80 m deep mi microporosity is present in aragonitic skeletal grains that underwent partial intrafabric dissolution chalkification leaving a porous meshwork of aragonite needles microporous aragonite is most abundant in 5 10 m thick intervals below zones of intense diagenesis associated with paleo freshwater lenses extensive microporosity was apparently generated by slow intrafabric dissolution of aragonite in mixing zones below freshwater lenses microporous aragonite is an intermediate step in the calcitization or dissolution of aragonite most calcitized aragonite neomorphic calcite formed by precipitation of sparry calcite in over microporous aragonite aragonitic fossils were not converted to microporous microcrystalline calcite therefore data from the pleistocene of enewetak suggest that widespread microporous microcrystalline calcite does not form directly from pure aragonite at least not in open meteoric systems or mixing zones microporous micrite occurs in oligocene wackestones 930 m deep deposited in a slope environment the microporous micrite is characterized by equant and prismatic crystals of low magnesium calcite lmc equant crystals are anhedral to euhedral and 2 10 m in diameter prismatic calcite crystals are subhedral 5 10 m wide and 20 50 m long stable isotopic and trace element geochemistry suggest that microporous micrite formed by diagenetic alteration of high magnesium calcite hmc micrite in deep seawater undersaturated with respect to hmc but supersaturated with respect to lmc probably near the sediment water interface mineralogy and crystal morphology of the microporous aragonite is very different from microporous shelf limestones in the thamama group cretaceous persian gulf see moshier 1989 equant microcrystals of calcite in the oligocene micrites are similar to those in microporous parts of the thamama group widespread microporosity in lmc limestones could form by alteration of hmc micrite in mixing zones or in seawater undersaturated with respect to hmc but supersaturated with respect to lmc

rare earth elements and neodymium isotopes in ferromanganese oxide coatings of cenozoic foraminifera from the atlantic ocean abstract an evaluation has been made of the method of establishing the ree contents and patterns and nd isotopic compositions of sea water over cenozoic time from their record in the femn oxide coatings of foraminiferal calcite using 0 60 ma samples from the rio grande rise dsdp site 357 it has been established that the ree contents of the coatings are generally similar to those of recent samples however in the cenozoic samples the surface coatings have been diagenetically modified under suboxic conditions resulting in a distinctly different ree pattern although the original 143 nd 144 nd ratios appear to have been preserved the nd isotopic curve for cenozoic sea water in the s atlantic shows clear temporal trends although these are not so extreme as to show 143 nd 144 nd ratios outside the range observed in modem sea water with the principal exception of the oldest samples there is an approximate inverse relationship between the nd and sr isotopic compositions of the foraminifera it is suggested that the changes reflect both global changes in the relative proportions of nd and sr derived from continental input and from the weathering of volcanic debris together with short term and local variations to which the sr curve is insensitive reflecting the different response times of the two elements to changes in oceanic input functions the nd isotope curve appears to be a potentially useful tracer of ocean palaeochemistry

groundwater residence time and palaeohydrology in the otway basin south australia 2h 18o and 14c data stable isotopes 2h 18o and 13c and radiocarbon 14c have been used in conjunction with chemical data to evaluate recharge mechanisms groundwater residence time and palaeohydrology within the confined dilwyn sand aquifer in the gambier embayment of the otway basin this aquifer does not receive recharge down gradient of the hydraulic hinge line and data have been interpreted along two discrete flow lines the mean residence time of groundwater determined by 14c in the confined aquifer from the hydraulic hinge position to the sea a distance of about 50 km along an inferred flow path is approximately 12 800 years the corresponding hydraulic travel time calculated from darcy s law is approximately 49 000 years the apparent discrepancy may be a result of eustatic sea level lowering during the last glacial because the groundwater system is hydraulically connected to the sea lower sea level would result in increased gradients and a decrease in groundwater residence time variations in stable isotopic composition along flow lines suggest a number of recharge mechanisms stable isotope data indicate progressive depletion of 2h and 18o in the groundwaters over the past 30 000 years groundwaters older than about 10 000 years b p were recharged either during a cooler climate climatic regime lower precipitation evapotranspiration and temperature and or the vapour source s had travelled over greater continental mass than those recharged over the past 10 000 years lower chloride concentrations in some of the older groundwaters indicate lower evapotranspiration rates in the recharge areas before 10 000 years ago

modeling speleothem δ13c variability in a central sierra nevada cave using 14c and 87sr 86sr abstract carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate progressing downward from the soil zone overlying the cave these processes include soil respiration fluid rock interaction in the host limestone degassing of co2 and precipitation of calcite upflow from the speleothem drip site and calcite precipitation at the drip site here we develop a new approach to independently constrain the roles of water rock interaction and soil processes in controlling stalagmite 13c this approach uses the dead carbon proportion dcp estimated from coupled 14c and 230th u measurements in conjunction with sr isotope analyses on stalagmite calcite from a central sierra nevada foothills cave in california a region characterized by a highly seasonal mediterranean type climate to determine the roles of water rock interaction and soil processes in determining stalagmite 13c increases in stalagmite dcp between 16 5 and 8 8 ka are coincident with decreased 13c indicating a varying yet substantial contribution from the soil organic matter som reservoir likely due to significantly increased average age of som in the soil veneer above the cave during wet climatic intervals we use geochemical and isotope mixing models to estimate the host carbonate contribution throughout the 13c time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite the degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals modeled 13c values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite 13c values even during relatively wet intervals these results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem 13c records particularly those from caves that formed in seasonal semi arid to arid environments

tertiary paleoceanic chemical variability unintended consequences of simple geochemical models five lines of concrete quantitative evidence about oceanic chemical composition during the past 65 m y can provide insights into the controlling processes and their temporal variations but the system constraints are not yet fully understood the calcium carbonate compensation depth mean carbon isotopic ratio and strontium isotopic ratio have all changed significantly during this time while changes in lithium to calcium and strontium to calcium ratios have been much smaller when these tracers are considered separately simple scenarios of changes in geochemical and geological factors can be used to explain the historical records these factors include 1 increases in continental chemical weathering rates and net continental fluxes to the oceans 2 changes in the isotopic composition of material weathered from continents to the oceans 3 decreases in the proportion of calcium carbonate deposited on continental shelves 4 changes in the deposition or weathering rates of certain types of sediments and 5 changes in hydrothermal circulation because these geochemical cycles are linked to each other and to others such as nutrients and global atmospheric carbon dioxide an explanation invoked for one tracer must satisfy the direction magnitude and timing of the patterns for all the tracers several simple scenarios for geochemical variations have been evaluated for their consequences both intended and unintended and compared to the historical records despite the attractiveness and possible justifications of some simple hypotheses for example decreases in organic carbon burial rates increases in carbonate weathering rates on the continents or increases in all continental weathering rates no one simple scenario appears to be consistent with both the timing and magnitude of the changes observed during the past 65 m y for all tracers considered the contradictions uncovered can be useful in defining what knowledge is crucial to further understanding of oceanic chemical history better knowledge is needed of absolute deep ocean sediment accumulation rates and their temporal variability of the extent and time scale of variability in the net carbon isotope input to the ocean from continental weathering of steady state fluxes for oceanic mass balances and the extent to which these mass balances approach steady state behavior and of the links and interactions between the geochemical cycles of various elements

application of sr isotope stratigraphy and sedimentary analysis to the origin and evolution of the neogene basins in the isparta angle southern turkey abstract four miocene basins are located within the isparta angle in southern turkey one of these the darioren basin is well established as the foreland basin to the adjacent lycian nappes which marks the western margin of the isparta angle the tectonic origin of the other three basins aksu kopru and manavgat has remained enigmatic largely because of the difficulties of dating the lowermost part of the succession continental and shallow marine conglomerates and limestones comparison of carbonate 87 sr 86 sr with the ocean water sr isotope record provides a powerful chronological tool for marine sediments by using sr isotope ratios in this way ages of the neritic limestones have been derived despite the absence of age diagnostic fossils these data constrain the age of the underlying conglomerates as lower to middle burdigalian the timing of deepening and subsidence events suggested by these new dates taken together with palaeocurrent measurements from conglomerates suggest that the basin configuration of the isparta angle was initiated in the lower burdigalian and was directly related to movement on north south oriented faults depocentre development in the north of the aksu basin is consistent with a tectonic origin for all three basins by flexurally induced block faulting of the foreland in front of the lycian nappes along pre existing structural weaknesses

rb sr sm nd k ca o and h isotopic study of cretaceous tertiary boundary sediments caravaca spain evidence for an oceanic impact site isotopic ratios and trace element abundances were measured on samples of ir enriched clay at the cretaceous tertiary boundary and in carbonate and marl from 5 cm below and 3 cm above the boundary samples were leached with acetic acid to remove carbonate and with hydrochloric acid leachates and residues were measured the sr nd o and h isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials the data indicate that most of the clay material was derived from a terrestrial source with relatively low87sr 86sr and high143nd 144nd ratios the 18o data suggest that the detritus has been modified by submarine weathering k ca and rb sr systematics as well as o isotope ratios of k feldspar spherules within the boundary clay suggest that they are predominantly authigenic and may have formed after the time of deposition however sm nd and rb sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus the isotopic evidence for foreign terrestrial detritus in the boundary clay the low rare earth element concentrations and high ni concentration support the hypothesis of a terminal cretaceous asteroidal impact that produced a global layer of fallout the data are most easily explained if the impact site was on oceanic crust rather than continental crust and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere 3 km this would probably require a single large impactor

permian strontium isotope stratigraphy cvu acknowledges funding from the leopoldina german national academy of sciences grant no lpds 2014 08

reevaluation of early oligocene eocene and paleocene seawater strontium isotope ratios using outcrop samples from the u s gulf coast the cenozoic strontium isotope 87sr 86sr variation in seawater is largely defined using deep sea drilling project dsdp samples examination of non dsdp results particularly for the paleogene suggest that the original seawater curve of burke et al 1982 may lie above the true seawater ratio during this time to investigate this possibility we analyzed the 87sr 86sr for 230 paleogene rock and shell samples from the 83 sites in alabama mississippi and louisiana many of the samples are from classic sections on which the gulf coast paleogene stratigraphy was originally defined multiple analyses of samples from these units show very little scatter and in most cases precise agreement is obtained among geographically separated collection sites we believe that analysis of multiple samples from a unit of established age collected at more than one site yields results that most accurately define the actual seawater sr isotope ratio at any given time results from these units define a seawater variation line that tracks well within the range of dsdp data but is generally below the original seawater line of burke et al 1982 it is often assumed that samples from continental settings will exchange with high ratio sr thereby raising 87sr 86sr values our gulf coast samples which are derived from a continental setting yield individual 87sr 86sr values that are both higher and lower than the estimated contemporaneous seawater ratio an analysis of our gulf coast data and dsdp results show that the direction of isotopic exchange cannot be confidently predicted for individual samples based solely on their derivation from either continental or oceanic settings

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

late cenozoic phosphogenesis on the western shelf of south africa in the vicinity of the cape canyon abstract the strontium isotope 87 sr 86 sr composition of phosphorite carbonate fluorapatite cemented grains pcg phosphate cemented limestone pcl skeletal phosphorite grains spg and biogenic calcite mollusc shell benthic foraminifera and echinoid spines from sediment on the outer continental shelf in the vicinity of the cape canyon ranges from 0 707981 to 0 709167 and corresponds to an age range of late early oligocene to late pleistocene most biogenic carbonate sr ages agree with biostratigraphic ages but some samples contain significantly older and reworked robust 250 m benthic foraminifera although the sr ages of biogenic and phosphorite grains indicate that sedimentation was fairly continuous most sediment is reworked with only lower miocene upper pliocene and upper pleistocene successions generally preserved in the study area the initial phosphogenic episode from the latest oligocene to early early miocene 21 26 ma was the largest with additional phosphogenic episodes in the late early miocene 16 19 ma and middle miocene 10 15 ma most phosphorite occurs as pcg that formed during marine transgressions and highstands from the late oligocene warming event until the mid miocene climatic optimum a period of episodic antarctic glaciation and several positive marine 13 c excursions upper oligocene to lower miocene pcg have been extensively reworked and are associated with major erosional unconformities pcg vary from peloidal sands to gravel mollusc moulds and contain variable amounts of quartz glauconite and pyrite inclusions pcl formed from 9 to 17 ma by cementation and replacement of older calcareous sediment during the middle to late miocene marine regression phosphorite as well as associated glauconite and pyrite indicate that periods of rapid accumulation of organic rich terrigenous muds derived from the focusing of the suspended load of rivers and organic matter produced in areas of coastal upwelling have occurred episodically on the margin since the late oligocene the relative paucity of phosphorite younger than the middle miocene may result from frequent erosional events as documented by pronounced unconformities on the shelf that reflect lowered eustatic sea level tectonic uplift and high frequency high amplitude quaternary sea level fluctuations

integrated late eocene oligocene stratigraphy of the alabama coastal plain correlation of hiatuses and stratal surfaces to glacioeustatic lowerings we integrated strontium and oxygen isotopic biostratigraphic and magnetostratigraphic studies of two upper eocene oligocene boreholes drilled near bay minette and st stephens quarry ssq alabama continuous coring provided fresh unweathered material for magnetostratigraphic studies minimizing problems reported from nearby outcrops difficulties with each technique were encountered because of diagenesis absence of marker fossils and the presence of unconformities however by integrating results from isotopic stratigraphy biostratigraphy and magnetostratigraphy we correlated these relatively shallow water deposits to the geomagnetic polarity time scale gpts at the ssq borehole the upper eocene to lower oligocene section is apparently complete within our stratigraphic resolution 0 2 0 5 m y allowing us to estimate the ages of several stratal surfaces late eocene sr isotope age estimates are as expected at the ssq borehole but oligocene ages are 1 m y older than expected due to diagenesis at the bay minette borehole a latest eocene earliest oligocene and a late early oligocene hiatus were detected we correlate these two hiatuses and stratal surfaces at ssq with global 18o increases inferred to represent glacioeustatic lowerings and with evidence for hiatuses on other continental margins 1 a distinct disconformity at the base of the chickasawhay limestone at both boreholes and a hiatus at bay minette correlates with a global 18o increase we revise the age of this surface equivalent to the tb 1 1 sequence boundary making it 2 m y older than previously reported and 2 a surface at the top of the shubuta member lowermost oligocene has been interpreted both as a condensed section and a disconformity this surface at ssq and a hiatus at bay minette correlate with a sharp global 18o increase and with hiatuses on the new jersey and irish margins the timing of the hiatuses and stratal surfaces correlates with the inflection of the 18o increases and not with the maximum values supporting models that indicate that unconformities form during the maximum rates of sea level fall

sr isotopes in interstitial waters of marine sediments from deep sea drilling project cores sr isotopic compositions of interstitial sea waters have been documented in detail with cores from 37 deep sea drilling project sites the 87sr 86sr ratios are sensitive indicators of diagenetic reactions involving alteration of volcanic matter dispersed in sediments and recrystallization of carbonates effects of basalt alteration reactions except possibly for hydrothermal signals and of alteration of continental debris are minimal carbonate recrystallization has a significant effect on the geochemical mass balance of sr but diffusion of unradiogenic strontium from interstitial waters has only a small effect on the 87sr 86sr ratio of ocean water

dating phosphogenesis with strontium isotopes authigenic and replacement francolites in modern phosphorite have values of sup 87 sr sup 86 sr within error of that of modern marine sr 0 70916 plus minus 2 during the phosphatisation of carbonate sr is not inherited from the carbonate but is totally exchanged with sr in ambient water the sup 87 sr sup 86 sr in shallow marine pore waters is indistinguishable from the value for modern marine sr so authigenic francolite which precipitates from pore water accurately records the isotopic ratio of contemporary marine sr by comparing the sup 87 sr sup 86 sr in francolite with published curves for the isotopic evolution of tertiary marine sr we have dated phosphogenetic events affecting the chatham rise 4 90 plus minus 0 35 ma the nambian margin 1 15 plus minus 0 25 ma and 4 70 plus minus 0 35 ma and the sechura desert peru 11 8 plus minus 0 5 ma

a review of methane and gas hydrates in the dynamic stratified system of the blake ridge region offshore southeastern north america abstract the blake ridge region lies on the passive margin of southeastern north america and contains a large amount of methane gas hydrate the methane and methane gas hydrate are predominantly biogenic apparently produced by co2 reduction reflection seismics indicate that bottom simulating reflectors bsrs enclose 55 000 km2 with high amplitude bsrs covering 26 000 km2 ocean drilling program odp leg 164 drilled three deep holes on a 10 km long transect sites 994 995 and 997 water depth 2770 2798 m based on sampling and geochemical thermal seismic and borehole geophysical measurements gas hydrates are most likely present between 190 and 450 m in sediment column gas hydrate is most often disseminated throughout the sediment column although concentrations occur within specific sedimentary horizons within supposed fault zones and at the base of gas hydrate stability bghs where methane recycling produces more pervasive concentrations of gas hydrate estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies vertical seismic profiling electric logging and measurements of in situ methane over the entire sediment column at least 2 4 of pore space volume 1 2 sediment volume is occupied by methane gas hydrates but average and maximum estimates are 5 4 and 12 respectively extrapolation of vertical gas hydrate and methane inventory over the area containing high amplitude bsrs yields estimates of 67 406 gt gigatons 1015 g of methane gas hydrate or 9 52 gt of methane and 2 6 27 gt of methane occurring as gas bubbles below the bghs average values are 185 gt of gas hydrate and 24 gt of methane as gas hydrate any gas hydrate occurring outside the area underlain by bsrs as suggested by geochemical evidence or that associated with low amplitude bsrs may increase these estimates by an unknown factor various data give conflicting pictures of mass transport with blake ridge sediments the data can be reconciled by viewing the upper sedimentary section accumulation of gas hydrate in the blake ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone ghsz over geologic time although there are some point sources of methane loss from the sediments e g seafloor seeps odp site 996 of unknown magnitude most of the documented methane loss occurs through diffusion and consumption at the sulfate methane interface smi by anaerobic methane oxidation amo 2 108 mol year 1 methane entering the ghsz at a rate of 1 3 109 mol year 1 indicates a methane trapping efficiency of 85 129i measurements suggest that the blake ridge system has accumulated gas hydrate over as much as 55 million years

sr and 87sr 86sr in estuaries of western india impact of submarine groundwater discharge abstract dissolved sr and 87sr 86sr are measured in the narmada tapi and the mandovi estuaries linked to the eastern arabian sea the concentration of dissolved sr and 87sr 86sr in the river water endmembers show significant differences reflecting the lithologies they drain the distribution of sr in all these estuaries shows a near perfect two endmember mixing between river water and seawater suggesting that there is no discernible net addition removal of sr from the estuarine waters in contrast 87sr 86sr shows non conservative behaviour in all these estuaries its distribution exhibits significant departure from the theoretical mixing lines a likely mechanism for this difference in the behaviour between dissolved sr and its 87sr 86sr is the discharge of submarine groundwater sgd which can modify the 87sr 86sr of the estuarine waters by exchange with sediments without causing measurable changes in sr concentration the impact of such an exchange process on the 87sr 86sr of the estuaries and therefore on the sr isotope composition of dissolved sr entering the arabian sea differs among the three estuaries and also between seasons in the narmada the non conservative behaviour of 87sr 86sr provides a handle to estimate the quantum of sgd to these estuaries the sr concentration 87sr 86sr ratio and salinity of the submarine groundwater and estimate of its fluxes to the narmada estuary have been made using inverse model calculations the model derived sgd flow rates are 5 and 280 cm day during pre monsoon and monsoon respectively the more radiogenic sr isotope composition of sgd relative to the seawater suggests that sgd acts as an additional source of 87sr to the arabian sea

sr isotope constraints on the mediterranean environment at the end of the messinian salinity crisis probably the most profound event that has affected the mediter ranean is the desiccation which occurred at the end of the miocene1 8 during this period there was a very rapid change from deep open marine conditions to environments of shallow water carbonate and massive evaporite deposition1 10 here we show that ostracod valves found in messinian lago mare lake sea sediments have a uniform but generally lower sr isotopic composition than contemporary sea water thus providing evidence that some of the major mediterranean basins for example balearic and levantine basins formed large lacustrine systems that were probably interconnected towards the end of the messinian salinity crisis1 3 after the major evaporites were deposited the most likely source of fresh water is from continental rivers flowing from the eastern european inland paratethys sea or perhaps the nile river in the lower part of the lago mare some ostracods have sr isotopic compositions identical to contemporary sea water indicating a transient influence of marine derived water sr compo sitions greater than that in contemporary sea water are only found in ostracods from onshore sediments in the sorbas basin4 5 of southeastern spain suggesting that this small basin adjacent to the straits of gibraltar became isolated after the deposition of the major evaporite sequences

early cenozoic glaciation antarctic weathering and seawater 87sr 86sr is there a link abstract stable and radiogenic isotopic and sedimentological data from sub antarctic deep sea sediment cores reveal a temporal link between changes in seawater 87 sr 86 sr ratios and major episodes of late eocene early oligocene climate change the 87 sr 86 sr records show two major inflections one at 38 39 ma near the middle late eocene boundary followed by another at 33 4 ma similarly the oxygen isotope ice rafted debris and clay assemblage records indicate two important climatic events the appearance of alpine glaciers and or small ice sheets on antarctica in the late eocene at 38 39 ma followed by a rapid transition to larger and more permanent temperate ice sheets in the early oligocene at 33 4 ma moreover during the early oligocene 30 33 ma three to four inferred peaks in glacial activity appear to coincide with subtle steps in the 87 sr 86 sr record the coupled variations in climate and seawater sr isotope ratios during the eocene oligocene imply a strong causal link between the two either changes in climate directly influenced patterns of continental weathering and hence seawater chemistry and or a tectonic event e g uplift as reflected in weathering and seawater chemistry triggered relatively abrupt changes in global climate