stable isotope gradients in modern benthic foraminifera of the vema channel south atlantic abstract oxygen and carbon isotopic analyses of benthic foraminifera from core top sediment samples were made to determine the extent to which the benthic foraminiferal species planulina wuellerstorfi accurately records the isotopic signatures of the water masses in the vema channel rio grande rise region the 18o of p wuellerstorfi to parallel the present day vertical 18o gradient within the channel the 13c of p wuellerstorfi appears to accurately reflect the present 13 c gradient of total dissolved co2 in the channel and thereby provides a potential monitor of past changes in thermohaline circulation processes

exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

a geothermobarometric investigation of garnet peridotites in the western gneiss region of norway in the western gneiss region of norway are found numerous peridotite lenses which have been extensively recrystallized under amphibolite fades conditions during the caledonian orogeny however evidence for an earlier caledonian high pressure metamorphism has been recorded by abundant eclogite and granulite relicts within gneiss and by the presence of at least ten garnet perioditite bodies preserved within chlorite peridotites two garnet bearing ultramafic assemblages have been recognized olivine orthopyroxene clinopyroxene garnet and olivine ortho pyroxene pargasitic amphibole garnet

carbon isotopic changes in benthic foraminifera from the western south atlantic reconstruction of glacial abyssal circulation patterns oxygen and carbon isotopic analyses have been performed on the benthic foraminifer planulina wuellerstorfi in seven late quaternary cores from the vema channel rio grande rise region the cores are distributed over the water depth interval of 2340 to 3939 m which includes the present transition from north atlantic deep water nadw to antarctic bottom water aabw the carbon isotopic records in the cores vary as a function of water depth the shallowest and deepest cores show no significant glacial interglacial difference in 13c four of the five cores presently located in the nadw have benthic foraminiferal 13c that is lower during glacial isotopic stages based on bathymetric gradients in 13c we conclude that like today there were two water masses present in the vema channel during glacial intervals a water mass enriched in 13c overlying another water mass depleted in 13c the largest gradient of change of 13c with depth however occurred at 2 7 km 1 km shallower than the present position of this gradient on the basis of paleontologic and sedimentologic evidence we consider it unlikely that the nadw aabw transition shallowed to this level reduced carbon isotopic gradients between the deep basins of the north atlantic and pacific oceans during the last glaciation suggest that production of nadw was reduced lower production of nadw may have modified the local abyssal circulation pattern in the vema channel region

quantification of the effects of eustasy subsidence and sediment supply on miocene sequences mid atlantic margin of the united states we use backstripping to quantify the roles of variations in global sea level eustasy subsidence and sediment supply on the development of the miocene stratigraphic record of the mid atlantic continental margin of the united states new jersey delaware and maryland eustasy is a primary infl uence on sequence patterns determining the global template of sequences i e times when sequences can be preserved and explaining similarities in miocene sequence architecture on margins throughout the world sequences can be correlated throughout the mid atlantic region with sr isotopic chronology 0 6 m y to 1 2 m y eight miocene sequences correlate regionally and can be correlated to global 18 o increases indicating glacioeustatic control this margin is dominated by passive subsidence with little evidence for active tectonic overprints except possibly in maryland during the early miocene however early miocene sequences in new jersey and delaware display a patchwork distribution that is attributable to minor tens of meters intervals of excess subsidence backstripping quantifi es that excess subsidence began in delaware at ca 21 ma and continued until 12 ma with maximum rates from ca 21 16 ma we attribute this enhanced subsidence to local fl exural response to the progradation of thick sequences offshore and adjacent to this area removing this excess subsidence in delaware yields a record that is remarkably similar to new jersey eustatic estimates we conclude that sea level rise and fall is a fi rstorder control on accommodation providing similar timing on all margins to the sequence record tectonic changes due to movement of the crust can overprint the record resulting in large gaps in the stratigraphic record smaller differences in sequences can be attributed to local fl exural loading effects particularly in regions experiencing largescale progradation

ocean chemistry during glacial time abstract measurements of co2 to air ratios in the gas trapped in bubbles in ice of glacial age suggest that the co2 content of the atmosphere was considerably lower during peak glacial time than during holocene time the purpose of this paper is to show that such a change must in all likelihood be the result of alterations in the nutrient element chemistry of sea water two possible scenarios are presented one involves alternate storage and erosion of phosphorous leaving residues from shelf sediments the other involves changes in the c p ratio in the organic debris falling to the deep sea means of verifying the nutrient cycle hypothesis are also given it is shown that the 13c record as we know it in planktonic and benthic foraminifera the oxygen record as inferred from benthic foraminifera species distributions and the early post glacial caco3 preservation event as recorded by aragonitic pteropods are consistent with both of the hypotheses presented only if an early post glacial spike in the 13c record for planktonic shells could be found would it be possible to eliminate one of these hypotheses i e that involving shelf storage the implications of these nutrient hypotheses to climate theory are as follows if shelf storage is responsible for the glacial to interglacial co2 increase then the co2 change must be considered an amplifier of some primary cause the reason is that sea level changes are needed to drive deposition on and erosion from the shelves on the other hand if changes in the c p ratio for falling debris are responsible then the co2 change could either be an amplifier or a primary cause for the major glacial to interglacial climatic cycle the latter is possible as self sustained oscillations in ocean chemistry might be driven by interactions between ocean ecology and ocean nutrient chemistry

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

experimentally determined stability of clinopyroxene garnet corundum in the system cao mgo al 2 o 3 sio 2

high resolution stratigraphy in late pleistocene holocene sediments of the vema channel abstract twenty five giant gravity cores ggc were obtained from the southwest flank of the rio grande rise in the summer of 1980 and were analyzed for time slice interpretations of the late pleistocene paleoceanography of the southwestern atlantic the cores span a depth interval of 1430 4148 m encompassing each of the principal deep water masses the suite of cores contains a continuous depositional record of the past 150 000 200 000 years from calcium carbonate oxygen isotope carbon 14 and biostratigraphic analyses it is possible to determine the precision in core to core correlation which is obtainable within this region of relatively low accumulation rates 0 5 2 0 cm 10 3 yrs calcium carbonate analyses were performed on each of the 25 cores at 1 cm sampling intervals using a technique that yields an analytical precision of 0 25 the carbonate curves show glacial interglacial amplitudes of typically 10 30 caco3 higher frequency amplitudes of 2 5 are also present both the glacial interglacial and the higher frequency carbonate signals can be correlated from core to core over the entire depth interval of the transect and are essentially synchronous based on independent stratigraphic control provided in selected cores by 18o 14c and biostratigraphic analyses despite the low sedimentation rates within this particular region we obtain a precision in core to core correlation of 1000 2000 yrs over the late pleistocene holocene these high resolution studies from the vema channel have important implications for interpreting the effects of bioturbation on a given stratigraphic sequence modeling the effects of bioturbation on the abundance and isotopic records of the benthonic foraminifera planulina wuellerstorfi suggests that for the region of the vema channel rio grande rise invalid stratigraphic and paleoceanographic interpretations may result if abundance variations in this species are ignored the results of these analyses show that it is possible to generate detailed stratigraphies in regions that have not previously been selected for high resolution studies solely because of relatively low accumulation rates of sediments our results indicate that carbonate analyses in some regions may provide a distinct advantage over 18o analyses because of the ease of analysis and high precision for identifying low amplitude high frequency fluctuations which can be correlated from core to core at the same time independent stratigraphic control via 18o or 14c is essential to verify whether this technique of carbonate curve matching is valid

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

a numerical study of mineral zoning formed during the cooling of finite volume grains a numerical model is used to study compositional profiles formed by cation exchange and diffusion at constant cooling rates in spherical mineral grains fe mg zoning in garnet olivine pairs is used as an example zoned concentration profiles can be described by an exponential function changes in the parameters of the function can be used to evaluate the effects of changes in inclusion size cooling history diffusion parameters and exchange enthalpy the composition of an olivine inclusion within garnet depends strongly on inclusion size due to finite volume effects as does zoning in the garnet surrounding the inclusion thus estimates of equilibration temperatures obtained from small inclusions of one phase within another require a size correction however for a spherical garnet within an olivine matrix the effect is small for garnet radii greater than 1000 m the shapes of zoned concentration profiles are useful for bounding both the rate of cooling and the final temperature in addition the concentration ratios fe garnet fe olivine may be used to estimate the cooling rate once the final temperature is bounded

evaluation of olivine spinel geothermometry as an indicator of thermal history for peridotites olivine and spinel in peridotites from the miyamori ultramafic complex and the ichinogemata crater of northeast japan show a systematic variation in the mg mg fe ratio which is correlated mainly with the grain size of spinel this correlation can be explained by a diffusion model assuming a semi infinite composite sphere under cooling or heating conditions in order to obtain absolute temperatures of thermal events the olivine spinel geothermometer is applied to pairs of spinel core and olivine core average composition the calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel in the miyamori complex the calculated temperatures decrease monotonically with decrease in grain size of spinel whereas in the ichinomegata lherzolite nodule those of spinel smaller than 0 2 mm increase as the grain size decreases and those of spinel larger than 0 2 mm remain constant regardless of further increase in grain size these observations in the light of the diffusion model suggest that the miyamori complex may have cooled from higher than 800 c to lower than 600 c and that the lherzolite nodule from the ichinomegata crater may have been in equilibrium at 900 c before it was heated above 1 100 c for less than a few days these two examples indicate that olivine spinel pairs of peridotites do not always indicate an appropriate equilibration temperature we cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity

carbon and oxygen isotopic disequilibria of recent deep sea benthic foraminifera abstract the carbon and oxygen isotopic compositions of 149 samples of benthic foraminifera from deep sea core tops indicate that none of the nine species studied secrete calcium carbonate in isotopic equilibrium with ambient bottom water uvigerina pyrgo murrhina and oridorsalis tener are the closest to 18o equilibrium with average deviations about 0 4 while planulina wuellerstorfi and p murrhina are the closest to 13c equilibrium with average deviations about 1 p wuellerstorfi shows the most systematic relationship between 13c and bottom water apparent oxygen utilization the intraspecific variabilities in 18o and 13c suggest that estimates of bottom water paleotemperatures can be made to a precision of 0 7 c while estimates of past apparent oxygen utilization aou can be made to 35 mol kg based on intraspecific comparisons of the recent samples with fossils no temporal changes in the degree of either 18o or 13c disequilibrium have been detected for planulina wuellerstorfi uvigerina oridorsalis tener and globocassidulina subglobosa

how big was the chesapeake bay impact insight from numerical modeling the chesapeake bay impact structure virginia is the largest impact crater in the united states the important question of how large the impactor was that formed the crater remains unanswered this is primarily due to peculiarities of the crater structure revealed by seismic exploration that complicate comparisons with other terrestrial and extraterrestrial craters one potential explanation for the unusual structure of the chesapeake bay crater is that the cratering process was affected by rheologic variations in the target at the time of impact using numerical modeling we find that for a sufficient contrast in material strength between the sedimentary and crystalline units at the chesapeake bay impact site we can produce a model of the final crater that is consistent with observational constraints and hence we can infer the energy released during the impact 1 75 3 10 6 mt in the absence of target strength variations the final crater diameter is likely to have been 40 km rather than the observed 80 90 km

rapid sea level rise abstract several global and regional factors contribute to observed sea level change along any particular coast global processes include changes in ocean mass glacio eustasy from ice melt ocean volume steric effects viscoelastic land movements glacioisostatic adjustment gia and changes in terrestrial water storage regional processes often connected to steric and glacial changes include changes in ocean circulation meridional overturning circulation moc glacial melting local gia regional subsidence and others paleoclimate instrumental and modeling studies show that combinations of these factors can cause relatively rapid rates of sea level rise exceeding 3 mm yr 1 over various timescales along particular coasts this paper discusses patterns and causes of sea level rise with emphasis on paleoclimatological records it then addresses the hypothesis of late holocene pre 20th century sea level stability in light of paleoclimatic evidence notably from reconstructions of sea surface temperature and glacial activity for significant climate and sea level variability during this time the practical difficulties of assessing regional sea level sl patterns at submillennial timescales will be discussed using an example from the eastern united states

new analytical and numerical geospeedometers tested on garnet pyroxenites from braganca nappe complex ne portugal abstract two new geospeedometric approaches an analytical one phys earth planet inter 110 1999 95 and a numerical one am j sci 29 1998 30 are tested to evaluate cooling rate on upper mantle derived pyroxenites from the braganca massif ne portugal these estimations coupled with standard geobarometric techniques allow theoretically the determination of exhumation rates responsible for incorporation of these rocks within the lower crust in the present paper we discuss the most significant sources of uncertainties in geospeedometry whether numerical or analytical precision of temperature estimates and input diffusion data none of the available geospeedometrical tools when used independently is able to provide definitive results with less than a factor 10 in precision cross calibration is thus required in order to provide reliable cooling rate estimates we obtain for pyroxenites from the braganca massif a minimum cooling rate from 1000 to 750 c of the order of 250 c ma for a depth interval of 10 15 km

deep drilling into the chesapeake bay impact structure samples from a 1 76 kilometer deep corehole drilled near the center of the late eocene chesapeake bay impact structure virginia usa reveal its geologic hydrologic and biologic history we conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact melt creation and distribution transient cavity collapse and ocean water resurge comparison of post impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long term sedimentation patterns in the mid atlantic region salty connate water of the target remains in the crater fill today where it poses a potential threat to the regional groundwater resource observed depth variations in microbial abundance indicate a complex history of impact related thermal sterilization and habitat modification and subsequent post impact repopulation

the chesapeake bay bolide impact a convulsive event in atlantic coastal plain evolution abstract until recently cenozoic evolution of the atlantic coastal plain has been viewed as a subcyclical continuum of deposition and erosion marine transgressions alternated with regressions on a slowly subsiding passive continental margin their orderly succession modified mainly by isostatic adjustments occasional appalachian tectonism and paleoclimatic change this passive scenario was dramatically transformed in the late eocene however by a bolide impact on the inner continental shelf the resultant crater is now buried 400 500 m beneath lower chesapeake bay its surrounding peninsulas and the continental shelf east of delmarva peninsula this convulsive event and the giant tsunami it engendered fundamentally changed the regional geological framework and depositional regime of the virginia coastal plain and produced the following principal consequences 1 the impact excavated a roughly circular crater twice the size of rhode island 6400 km2 and nearly as deep as the grand canyon 1 3 km deep 2 the excavation truncated all existing ground water aquifers in the target area by gouging 4300 km3 of rock from the upper lithosphere including proterozoic and paleozoic crystalline basement rocks and middle jurassic to upper eocene sedimentary rocks 3 synimpact depositional processes including ejecta fallback massive crater wall failure water column collapse and tsunami backwash filled the crater with a porous breccia lens 600 1200 m thick at a phenomenal rate of 1200 m hr the breccia lens replaced the truncated ground water aquifers with a single 4300 km3 reservoir characterized by ground water 1 5 times saltier than normal sea water chlorinities as high as 25 700 mg l 4 a structural and topographic low created by differential subsidence of the compacting breccia persisted over the crater at least through the pleistocene in the depression are preserved postimpact marine lithofacies and biofacies upper eocene lower oligocene lower miocene not known elsewhere in the virginia coastal plain 5 long term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the postimpact strata many of these faults appear to reach the bay floor and may be potential hazards for motion sensitive structures in population centers around chesapeake bay near surface fracturing and faulting generated by the impact shock may extend as far as 90 km from the crater rim 6 having never completely filled with postimpact sediments the sea floor depression over the crater appears to have predetermined the location of chesapeake bay 7 as large impact craters are principal sources for some of the world s precious metals it is reasonable to expect that metal enriched sills dikes and melt sheets are present in the inner basin of the crater in addition to these specific consequences the crater and the convulsive event that produced it have widespread implications for traditional interpretations of certain structural and depositional features of the atlantic coastal plain particularly in southeastern virginia

high tide of the warm pliocene implications of global sea level for antarctic deglaciation we obtained global sea level eustatic estimates with a peak of 22 m higher than present for the pliocene interval 2 7 3 2 ma from backstripping in virginia united states new zealand and enewetak atoll north pacifi c ocean benthic foraminiferal 18 o values and mg ca 18 o estimates statistical analysis indicates that it is likely 68 confi dence interval that peak sea level was 22 5 m higher than modern and extremely likely 95 that it was 22 10 m higher than modern benthic foraminiferal 18 o values appear to require that the peak was 20 21 m our estimates imply loss of the equivalent of the greenland and west antarctic ice sheets and some volume loss from the east antarctic ice sheet and address the longstanding controversy concerning the pliocene stability of the east antarctic ice sheet

solute partitioning under continuous cooling conditions as a cooling rate indicator a mathematical model is developed to describe the concentration profiles as a function of time of a solute dissolved in two coexisting finite phases under continuous cooling conditions a temperature dependent partitioning of the solute between the two phases is included the model relates to the use of experimentally determined solute profiles to infer the rates at which various rocks cooled the model employs finite difference equations and the thomas tridiagonal method to solve the appropriate differential equations describing the solute partitioning to illustrate the application of the model the partitioning of zirconia between coexisting ilmenite and ulvospinel is considered it is shown that cooling rates can be calculated not only from solute concentration profiles within the grains but also from the ratio of the solute concentrations at a given distance on both sides of the interface or from the ratio of the average solute concentrations in the two phases the cooling rates of a suite of apollo 15 elbow crater gabbros are calculated from measured ratios of concentrations at 10 m from the interfaces between the phases a comparison is made of calculated concentration ratios 10 m from the interface and cooling rates for equilibrium versus uniform initial concentrations it is shown that the elbow crater gabbros could not have had a uniform concentration profiles at the previously suggested initial temperature of 1350 k also considered are the effects of the initial temperature the grain sizes and the diffusion coefficients of zr in the phases on the calculated cooling rates finally suggestions are made for other systems where the present model can be applied to estimate the thermal histories of rocks

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

miocene deepwater oceanography a global synthesis of miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that miocene thermohaline circulation evolved through three regimes corresponding approximately to early middle and late miocene times there is evidence for major qualitative differences between the circulation of the modern ocean and the miocene ocean prior to 11 ma the 13c 12c ratios of the benthic foraminifera cibicidoides are interpreted in terms of water mass aging i e the progressive depletion of dissolved o2 and lowering of 13c values as the result of oxidation of organic matter as water flows further from its sources at the surface of the oceans both isotopic and faunal data indicate that the early miocene regime from 22 to 15 ma was the most different from today s during that interval intermediate and deep waters of both the atlantic and the pacific oceans aged in a northward direction and the intermediate waters of the indian the south atlantic and the south pacific oceans were consistently the youngest in the global ocean we speculate that early miocene global thermohaline circulation may have been strongly influenced by the influx of warm saline water tethyan indian saline water from the tethys into the northern indian ocean the isotopic and faunal data suggest that flow from the tethyan region into the indian ocean diminished or terminated at about 14 ma isotopic and faunal data give no evidence for north atlantic deep water nadw formation prior to about 14 5 ma with the exception of a brief episode in the early miocene from 14 5 to 11 ma nadw formation was weak and circumpolar and antarctic water flooded the deep south atlantic and south pacific as the antarctic ice cap grew from about 10 ma to the end of the miocene thermohaline circulation resembled the modern circulation in many ways in latest miocene time 6 to 5 ma circulation patterns were very similar to today s except that nadw formation was greatly diminished the distribution pattern of siliceous oozes in miocene sediments is consistent with our proposed reconstruction of thermohaline circulation major changes which occurred in circulation during the middle miocene were probably related to the closing of the tethys and may have contributed to rapid middle miocene growth of the antarctic ice cap appendices 1 4 6 and 7 are available withentire article on microfiche order fromamerican geophysical union 2000 floridaavenue n w washington dc 20009 document 88p 002 5 00 payment mustaccompany order

garnet peridotite from colorado plateau ultramafic diatremes hydrates carbonates and comparative geothermometry crystal fragments of pyrope from diatremes of ultramafic microbreccia in the navajo province of the colorado plateau contain inclusions of olivine pyroxene spinel chlorite amphibole chlorapatite and dolomite the included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the plateau lithosphere garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15 20 kb no evidence was found for inclusions from greater depths temperature estimates are 800 900 c for garnet clinopyroxene equilibration but only 500 700 c for garnetolivine equilibration inherent differences between geothermometry methods account for only part of the discrepancy pronounced fe mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling the garnet olivine temperature estimates may be the best approximations to mantle temperatures before eruption our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large scale horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses

geospeedometry of metamorphic rocks examples in the rantasalmi sulkava and kiuruvesi areas eastern finland biotite garnet diffusion couples compositional zoning in biotite garnet pairs in metamorphic rocks from eastern finland has been studied the mg profiles in the garnet side of biotite garnet crystal pairs have been interpreted by means of lasaga s theory geospeedometry however the binary interdiffusion equations are first reformulated by starting from a ternary system and using the lattice fixed frame of reference this frame of reference gives the fluxes directly by means of the numbers of diffusing ions which helps to check the 1 dimensionality of the analysis assumed in lasaga s theory it is also shown that the recently argued effect of the third cation ca is negligible in our samples we were able to investigate satisfactory profiles in three samples from different areas the values for the cooling rate are a few degrees per million years if the diffusion data obtained by freer are adopted the cooling rates are in agreement with recent estimates based on the k ar ages on biotite in the same areas

deep circulation of the north atlantic over the last 200 000 years geochemical evidence variations in the cadmium calcium ratio of north atlantic deep water are recorded in the fossil shells of benthic foraminifera the oceanic distribution of cadmium is similar to that of the nutrients hence the cadmium calcium ratio in shells records temporal variations in nutrient distributions data from a north atlantic sediment core show that over the past 200 000 years there has been a continuous supply of nutrient depleted waters into the deep north atlantic the intensity of this source relative to nutrient enriched southern waters diminished by about a factor of 2 during severe glaciations this evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested

lithology dependence of porosity in slope and deep marine sediments abstract compilation of porosity data from 11 ocean drilling program legs and 53 sites show that compaction in deep marine to upper slope sediments is at least in part a function of both burial depth and lithology overall porosities range from 4 to 94 despite the broad range of porosities there is an exponential trend toward decreasing porosity with depth comparison among samples dominated 90 to 100 by each of six lithologies based on grain size and composition reveals the lithology dependence of compaction except for biogenic silt size sediments and clastic sands in the former case this may in part be due to mixing of calcareous and siliceous nannofossils clay size clastic sediments show a clear change from rapid porosity reduction in the top 172 meters porosity 82 7 e depth 430 to less pronounced loss of porosity below porosity 61 4 e depth 1671 the remainder of the lithologies silt porosity 75 5 e depth 1091 biogenic sand porosity 88 5 e depth 1338 and micrite porosity 69 5 e depth 1235 show a good to strong exponential reduction of porosity with depth these relations compare well with literature studies taking into account the less pure nature of the lithologies used in this work the silt compaction curve is unique no previous studies of porosity separated silt from other grain sizes most sediment samples are of mixed grain sizes and types the lithology based porosity depth relations were used to test four approaches to decompaction in mixed lithology sediment used in backstripping all methods generated porosity estimates that correlated strongly to the observed values with correlation coefficients ranging from 0 629 to 0 686 these methods that have been used for years are for the first time shown to be valid

subsidence along the atlantic coast of north america insights from gps and late holocene relative sea level data the atlantic coast of north america is increasingly affected by flooding associated with tropical and extratropical storms exacerbated by the combined effects of accelerated sea level rise and land subsidence the region includes the collapsing forebulge of the laurentide ice sheet high quality records of late holocene relative sea level rsl rise are now available allowing separation of long term glacial isostatic adjustment induced displacement from modern vertical displacement measured by gps we compare geological records of late holocene rsl to present day vertical rates from gps for many coastal areas there is no significant difference between these independent data exceptions occur in areas of recent excessive groundwater extraction between virginia 38 n and south carolina 32 5 n the present day subsidence rates in these areas are approximately double the long term geologic rates which has important implications for flood mitigation tide gauge records therefore should be used with caution for studying sea level rise in this region

polyphase variscan emplacement of exotic terranes morais and braganca massifs onto iberian successions evidence from 40ar 39ar mineral ages abstract the braganca and morais nappe complexes comprise internally imbricated continental tectonic units including schists deformed metagranite granulite and local eclogite and a structurally underlying ophiolite nappe high pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint by contrast rocks within the ophiolite nappe display prograde amphibolite facies assemblages hornblende concentrates from both tectonic units exposed in the braganca nappe complex record 40 ar 39 ar plateau and isotope correlation ages of c 385 390 ma a similar date is recorded by hornblende within mylonitic amphibolite ophiolite protolith along a ductile thrust which separates the two major nappe units muscovite within blastomylonitic metagranite lagoa augen gneiss in upper structural levels of the morais massif records 370 375 ma 40 ar 39 ar plateau ages muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 ma age together the 40 ar 39 ar data suggest that a regionally significant early variscan amphibolite facies metamorphism occurred prior to c 385 390 ma early devonian this was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex the composite nappe complex must have been maintained at relatively high structural levels throughout the remaining variscan evolution because there is no evidence for post 370 ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined this is in marked contrast to a significant late devonian c 330 ma locally high pressure metamorphism which is recorded in structurally underlying elements of the lower allochthon part of a foreland thrust complex composed of metasedimentary units with iberian protolith affinities together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c 330 ma with resultant deformation and metamorphism of structurally underlying components of the iberian thrust complex

core top evaluation of benthic foraminiferal isotopic ratios for paleo oceanographic interpretations abstract the benthic foraminifera cibicides wuellerstrofi oridforsalis tener and pyrgo rotalaria from twelve core tops in the norwegian greenland sea and c wuellerstorfi o tener and o umbonatus from four core tops on the rio grande rise were analysed for oxygen and carbon isotopes the norwegian greenland sea data yield statistically discrete means for both 18o and 13c for each species the interregional comparison of our data to published data from the east pacific rise shows that c wuellerstorfi 18o and 13c variations reflect oceanographic differences between sites whereas the data on oridorsalis and pyrgo do not we conclude that c wuellerstorfi is a reliable sensor of bottom water conditions whereas other taxa are of unproven utility supportive biological studies and the lack of positive covariance of 18o to 13c among deep sea benthic taxa lead us to reject models calling for incorporation of metabolic co2 to explain the isotopic variation among deep sea benthic foraminifera we propose a within sediment microhabitat hypothesis to explain the 13c variations among taxa

meteoroid mayhem in ole virginny source of the north american tektite strewn field new seismic reflection data from chesapeake bay reveal a buried 85 km wide 1 5 2 0 km deep peak ring impact crater carved through upper eocene to lower cretaceous sedimentary strata and into underlying pre mesozoic crystalline basement rocks a polymictic late eocene impact breccia composed mainly of locally derived sedimentary debris determined from four continuous cores surrounds and partly fills the crater structural and sedimentary characteristics of the chesapeake bay crater closely resemble those of the miocene ries peakring crater in southern germany we speculate that the chesapeake bay crater is the source of the north american tektite strewn field

sampling statistic limitations on benthic foraminifera chemical and isotopic data abstract analyses of benthic foraminiferal isotopic and chemical properties are typically done on a small number of individuals often 5 10 because benthic sediment mixing produces a mix of individuals whose properties were determined at times encompassing several thousand years a sampling of several individuals does not adequately represent the average mixture a quantitative statistical analysis shows that this effect places a fundamental limit on measurement precision in cores especially where rapid environmental changes occur over short vertical distances the fundamental parameters defining this limit are the rate of change of the property per unit distance and the effective mixing depth in cores found in the vema channel rio grande rise region where sedimentation rates are about 1 cm ka 1 and mixing depths are about 3 cm a single sampling of six individuals can produce a result that has an expected one sigma standard deviation of 6 25 of the observed range precision can be improved by using more individuals or running more replicate analyses but the result only improves as the square root of the number of individuals or replicates this effect is illustrated by cd ca and isotopic data from low 0 7 cm ka 1 and moderate 1 5 cm ka 1 sedimentation rate cores although precision is compromised the most in low sedimentation rate cores this effect contributes to imprecision in high sedimentation rate cores as well

stable isotope composition of benthic foraminifera from the equatorial pacific when ranked according to deviation from isotopic equilibrium deep sea benthic foraminifera exhibit no correlation between the rankings for oxygen and for carbon both rank sequences are stable from east to west in the equatorial pacific fertility independent and from glacial to post glacial time independent records for the past 15 000 yr suggest marked changes in deep sea circulation during this time interval

impact effects and regional tectonic insights backstripping the chesapeake bay impact structure the chesapeake bay impact structure is a ca 35 4 ma crater located on the eastern sea board of north america deposition returned to normal shortly after impact resulting in a unique record of both impact related and subsequent passive margin sedimentation we use backstripping to show that the impact strongly affected sedimentation for 7 m y through impact derived crustal scale tectonics dominated by the effects of sediment compaction and the introduction and subsequent removal of a negative thermal anomaly instead of the expected positive thermal anomaly after this the area was dominated by passive margin thermal sub sidence overprinted by periods of regional scale vertical tectonic events on the order of tens of meters loading due to prograding sediment bodies may have generated these events