effective medium theories for fluid saturated materials with aligned cracks there is general agreement between different theories giving expressions for the overall properties of materials with dry aligned cracks if the number density of cracks is small there is also very fair agreement for fluid filled isolated cracks however there are considerable differences between two separate theories for fluid filled cracks with equant porosity comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples however although the crack number density in the laboratory sample is such that first order theory is unlikely to apply expressions correct to second order in the number density provide a worse fit it also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective medium theories which are valid only when this ratio is large the data show dispersion effects for dry cracks and scattering neither of which will occur at sufficiently long wavelengths data from the water saturated samples indicate that the effect of equant porosity is significant although the two theories differ strongly as to just how significant once again and in spite of the reservations mentioned above a reasonable fit between theory and observation can be shown

a model based interpretation of low frequency changes in the carbon cycle during the last 120 000 years and its implications for the reconstruction of atmospheric δ14c a main caveat in the interpretation of observed changes in atmospheric d14c during the last 50 000 years is the unknown variability of the carbon cycle which together with changes in the 14c production rates determines the 14c dynamics a plausible scenario explaining glacial interglacial dynamics seen in atmospheric co2 and d13c was proposed recent ly kohler et al 2005 a similar approach and expanding its interpretation to the 14c cycle is an important step towards a deeper understanding of d14c variability this approach is based on an ocean atmosphere biosphere box model of the global carbon cycle bicycle to reproduce low freque ncy changes in atmospheric co2 as seen in antarctic ice cores the model is forced forward in time by various paleo climatic records derived from ice and sediment cores the simulation results of our proposed scenario match a compiled co2 record from various ice cores during the last 120 000 years with high accuracy r2 0 89 we analyze scenarios with different 14c production rates which are either constant based on 10be measured in greenland ice cores or the recent high resolution geomagnetic field reconstruction glopis 75 and compare them with the available d14c data covering the last 50 000 years our results suggest that during the last glacial cycle in general less than 110 o oo of the increased atmospheric d14c are based on variations in the carbon cycle while the largest part 5 6 of the variations has to be explained by other factors glacial atmospheric d14c larger than 700 o oo cannot not be explained within our framework neither through carbon cycle based changes nor through variable 14c production superimposed on these general trends might lie positive anomalies in atmospheric d14c of 50 o oo caused by millennial scale variability of the northern deep water production during heinrich events and dansgaard oeschger climate fluctuations according to our model the dominant processes that increase glacial d14c are a reduced glacial ocean circulation 40 o oo a restricted glacial gas exchange between the atmosphere and the surface ocean through sea ice coverage 20 o oo and the enrichment of dic with 14c in the surface waters through isotopic fractionation during higher glacial marine export production caused by iron fertilization 10 o oo

x ray micro computed tomography and ultrasonic velocity analysis of fractured shale as a function of effective stress abstract ultrasonic velocity is a key shale gas reservoir property especially in the context of gas production or co 2 injection for geo sequestration this ultrasonic velocity reflects the dynamic elastic properties of the rock and it thus depends on the fracture morphology which varies significantly with effective stress however the precise relationship between ultrasonic velocity and fractured shale morphology is only poorly understood we thus measured p and s wave velocities of fractured shale in two orthogonal directions and imaged the shale with x ray micro computed tomography as a function of applied effective stress and investigated how fracture morphology p and s wave velocity young s modulus shear velocity and poisson s ratio are interconnected with effective stress clearly most of the small fractures the width is around 0 1 mm closed with increasing effective stress resulting in a different fracture size distribution which again had a dramatic effect on the elastic rock properties furthermore with increasing effective stress p and s wave velocities increased significantly such that the orthogonal waves gave a similar response at 2000 psi effective stress despite significant sample heterogeneity we conclude that the fracture aperture direction and network characteristics severely influence wave propagation and thus elastic properties these results can be used to assess natural fracture networks monitor fracture development during hydraulic fracturing and predict fracture closure scenarios during production

younger dryas a data to model comparison to constrain the strength of the overturning circulation 1 the university of victoria earth system climate model uvic escm is used to compare simulated time series of radiocarbon during the younger dryas yd with paleoceanographic records i find that only a complete shut down and recovery of the atlantic meridional overturning circulation amoc can simulate both the rise in atmospheric co2 concentrations seen in ice core records and the peak and subsequent decrease in atmospheric 14c comparable to the peak recorded in the varved sediments of the cariaco basin simulated radiocarbon profiles in the western north atlantic match well with data from deep sea corals at the beginning of the yd whereas planktonic benthic foraminifera records match best with a transient state during the rapid recovery of the amoc the steepness of the increase in atmospheric 14c at 12 9 ka cal could not be simulated with oceanic circulation changes only because the response time of the climate system is too slow

atmospheric radiocarbon during the younger dryas production ventilation or both a new reconstruction of past atmospheric v 14 c v 14 catm based on polish lake varved sediments has suggested that previous v 14 catm values e g from the cariaco basin record for the beginning of the younger dryas cold event yd are overestimates and that the v 14 catm rise at the yd onset could only be due to changes in atmospheric 14 c production pc 14 this result would have profound climatic implications for the yd is a paradigm example of abrupt climate change which is usually thought to have been triggered by a reduction in the northward heat flux by the atlantic thermohaline circulation here we examine results from a large number of simulations 300 based on a zonally averaged ocean circulation model to constrain the effect on v 14 catm of pc 14 changes during the yd as reconstructed from a greenland ice core record of 10 be flux our results suggest that the scatter in the lake data set is too large to exclude the probable change in deep ocean ventilation at the onset of the yd by contrast the model fit to the higher v 14 catm levels throughout the yd detected in the marine record is generally better when a substantial decrease in deep ocean ventilation is simulated the early v 14 catm drawdown that initiated during the first half of the yd however could entirely be due to production changes if this was the case the drawdown would not reflect an increasing formation of north atlantic intermediate water or southern ocean water as previously suggested the rapid v 14 catm rise at the yd onset documented in the marine record however remains unexplained fl 2001 elsevier science b v all rights reserved

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

the preboreal climate reversal and a subsequent solar forced climate shift accurate chronologies are essential for linking palaeoclimate archives carbon 14 wiggle match dating was used to produce an accurate chronology for part of an early holocene peat sequence from the borchert the netherlands following the younger dryas preboreal transition two climatic shifts could be inferred around 11 400 cal yr bp the expansion of birch betula forest was interrupted by a dry continental phase with dominantly open grassland vegetation coeval with the pbo preboreal oscillation as observed in the grip ice core at 11 250 cal yr bp a sudden shift to a humid climate occurred this second change appears to be contemporaneous with i a sharp increase of atmospheric c 14 ii a temporary decline of atmospheric co2 and iii an increase in the grip be 10 flux the close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity which may have forced the changes in climate and vegetation at around 11 250 cal yr bp copyright c 2004 john wiley sons ltd

use of multicomponent seismic data for oil water discrimination in fractured reservoirs geophysicists have devoted great efforts to the problem of determining fluid saturation from seismic measurements with some notable successes it is commonly believed that fluid information is to be found in the p wave data with shear waves being insensitive to fluid and indeed almost all successful fluid detection methodologies have been based on analysis of the p wave

effects of aligned fractures on the response of velocity and attenuation ratios to water saturation variation a laboratory study using synthetic sandstones p wave to s wave ratios are important seismic characterization attributes velocity ratios are sensitive to the petrophysical properties of rocks and to the presence of gas attenuation ratios have also been shown to be sensitive to the presence of partial liquid gas saturation the relationship between liquid gas saturation and p wave and s wave ratios has been used to distinguish gas saturated rocks from liquid saturated rocks aligned fractures are common in the earth s crust and cause seismic anisotropy and shear wave splitting however most existing relationships between partial gas liquid saturation and p wave and s wave ratios are for non fractured rocks we present experimental results comparing the effects of changing water saturation on qs qp versus vp vs ratios between a non fractured rock and one containing fractures aligned parallel to wave propagation direction we also study the effects of aligned fractures on the response of vp vs to changing water saturation using synthetic fractured sandstones with fractures aligned at 45o and parallel to the wave propagation direction the results suggest that aligned fractures could have significant effects on the observed trends some of which may not be obvious fractures aligned parallel to wave propagation could change the response of qs qp versus vp vs ratios to water saturation from previously reported trends shear wave splitting due to the presence of aligned fractures results in two velocity ratios vp vs1 and vp vs2 the fluid independence of shear wave splitting for fractures aligned parallel to wave propagation direction means the difference between vp vs1 and vp vs2 is independent of water saturation for fractures aligned at oblique angles shear wave splitting can be sensitive to water saturation and consequently be frequency dependent which can lead to fluid and frequency dependent differences between vp vs1 and vp vs2 the effect of aligned fractures on vp vs ratios not only depends on the fracture effects on both p wave and s wave velocities but also on the effects of water saturation distribution on the rock and fracture stiffness and hence on the p wave and s wave velocities as such these effects can be frequency dependent due to wave induced fluid flow a simple modelling study combining a frequency dependent fractured rock model and a frequency dependent partial saturation model was used to gain valuable interpretations of our experimental observations and possible implications which would be useful for field seismic data interpretation

climatic influence in nrm and 10be derived geomagnetic paleointensity data abstract one can determine geomagnetic paleointensities from natural remanent magnetizations nrm and by inverting production rates of cosmogenic isotopes such as 10 be and 14 c recently two independently derived 200 kyr stacks y guyodo j p valet relative variations in geomagnetic intensity from sedimentary records the past 200 000 years earth planet sci lett 143 1996 23 36 m frank b schwarz s baumann p w kubik m suter a mangini a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10 be in globally stacked deep sea sediments earth planet sci lett 149 1997 121 129 were compared and the good agreement was suggested to validate the use of sedimentary cores for studies both compilations use mainly the astronomically forced and climatically controlled oxygen isotope stratigraphy to date and synchronize the sedimentary records while this very curve has several coherent features with the supposedly pure geomagnetic records an nrm relative paleointensity record which was included in the conventional paleointensity stack shows correspondence with climatic features which is explained by an inadequacy in the normalization technique therefore it is possible that the extraction of the pure paleointensity signal from marine sediments has not always been accomplished

a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10be in globally stacked deep sea sediments the reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate there is currently no generally accepted record here we present a global stacked record of 230thex normalized 10be deposition in marine sediments representing relative variations in 10be production rate which are translated into field intensity variations the record shows major periods during which the field intensity was between 10 and 40 of the present day value namely 30 42 60 75 85 110 and 180 192 kyr b p our results are compared to independently derived paleomagnetic studies and th u calibrations of 14c dates on corals during most of the observed period the geomagnetic field intensity was weaker than today resulting in an overall 30 reduced value for the last 200 kyr

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

simulation of atmospheric radiocarbon during abrupt oceanic circulation changes trying to reconcile models and reconstructions abstract an abrupt increase in the atmospheric 14 c content at the beginning of the younger dryas about 13 000 years ago has been related to a slow down of the global oceanic circulation this has been well simulated by box models but the amplitude and timing of such a 14 c change could not be represented with dynamical models we have forced a climate model of intermediate complexity with a meltwater discharge to test three model parameters that have the potential to strongly influence the simulated 14 c change and may help us to reconcile the model results with reconstructions we find that a decrease of the tropical wind speed accompanying a slow down of the oceanic circulation largely amplifies the circulation induced 14 c change and brings it closer to the reconstructions second we find that vertical mixing in a dynamical ocean model leads to a substantial damping in the 14 c response of simple models third we demonstrate that neglecting radiocarbon reservoir age variations during abrupt climate change may lead to a substantial overestimate of atmospheric 14 c fluctuations reconstructed from paleoceanographic archives

measurements of seismic anisotropy and fracture compliances in synthetic fractured media ultrasonic velocities were measured on a stack of synthetic material plexiglass at low 90 120 khz long wavelength range and high 480 khz short wavelength range frequencies plates were pressed together with uniaxial normal stress these measurements were repeated under different uniaxial pressures for two cases 1without circular rubber inclusions between synthetic fractures and 2with circular rubber inclusions between fractures calculation of anisotropy parameters and also normal and shear interface fracture compliances using linear calculus show reasonable variations anisotropy and compliances decreases as overburden pressure increases suggesting that anisotropy is caused by compliances at higher frequencies synthetic fractures seem to be stiffer than at lower frequencies for the long and short wavelength ranges similar variations were observed for anisotropy parameters and fracture compliances vs measurements in different directions show that for the case without rubber inclusions there is a good approximation of vti medium whereas for the case with inclusions there is a significant difference between vs travelling in directions parallel and perpendicular to the plates

quantitative interpretation of atmospheric carbon records over the last glacial termination 1 the glacial interglacial rise in atmospheric pco2 is one of the best known changes in paleoclimate research yet the cause for it is still unknown forcing the coupled ocean atmosphere biosphere box model of the global carbon cycle bicycle with proxy data over the last glacial termination we are able to quantitatively reproduce transient variations in pco2 and its isotopic signatures 13c 14c observed in natural climate archives the sensitivity of the box model of the isotopic carbon cycle bicycle to high or low latitudinal changes is comparable to other multibox models or more complex ocean carbon cycle models respectively the processes considered here ranked by their contribution to the glacial interglacial rise in pco2 in decreasing order are the rise in southern ocean vertical mixing rates 30 ppmv decreases in alkalinity and carbon inventories 30 ppmv the reduction of the biological pump 20 ppmv the rise in ocean temperatures 15 20 ppmv the resumption of ocean circulation 15 20 ppmv and coral reef growth 5 ppmv the regrowth of the terrestrial biosphere sea level rise and the increase in gas exchange through reduced sea ice cover operate in the opposite direction decreasing pco2 during termination i by 30 ppmv according to our model the sequence of events during termination i might have been the following a reduction of aeolian iron fertilization in the southern ocean together with a breakdown in southern ocean stratification the latter caused by rapid sea ice retreat trigger the onset of the pco2 increase after these events the reduced north atlantic deep water nadw formation during the heinrich 1 event and the subsequent resumption of ocean circulation at the beginning of the bolling allerod warm interval are the main processes determining the atmospheric carbon records in the subsequent time period of termination i we further deduce that a complete shutdown of the nadw formation during the younger dryas was very unlikely changes in ocean temperature and the terrestrial carbon storage are the dominant processes explaining atmospheric 13c after the bolling allerod warm interval

seismic attenuation and pore fluid viscosity in clay rich reservoir sandstones the frequency dependence of seismic attenuation in a suite of clay rich reservoir sandstones was investigated in the laboratory compressional and shear wave velocities vp and vs and quality factors qp and qs were measured as functions of pore fluid viscosity at an effective pressure of 50 mpa and at an experimental frequency of about 0 8 mhz using the pulse echo technique the experimental viscosity ranged from 0 3 to 1000 centipoise which gives equivalent frequencies for a water saturated sandstone of 2 6 mhz to 780 hz assuming a global flow loss mechanism two types of behavior were observed high permeability greater than 100 millidarcies sandstones tend to show variable qp and qs which are similar in magnitude to those predicted by the biot theory over the viscosity range 0 3 to about 20 centipoise equivalent frequency range 2 6 mhz to about 39 khz low permeability less than 50 millidarcies sandstones tend to show almost constant qp and qs over the experimental viscosity range that are

chlorine 36 evidence for the mono lake event in the summit grip ice core abstract a distinct peak has been discovered in the 36 cl data from the grip ice core between the dansgaard oeschger d o events 6 and 7 at approximately 32 kyr bp this peak can be attributed to a minimum of the geomagnetic dipole field associated with the mono lake event since the 36 cl peak reflects a higher production rate of all cosmogenic radionuclides it has an impact on the 14 c dating of the last ice age furthermore it provides an additional time marker similar to a peak found earlier corresponding to the laschamp event at approximately 39 kyr bp

frequency dependent anisotropy due to meso scale fractures in the presence of equant porosity fractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity this is consistent with laboratory estimates of the characteristic squirt flow frequency however these laboratory measurements are carried out on rock samples which do not contain large fractures we consider coupled fluid motion on two scales the grain scale which controls behaviour in laboratory experiments and the fracture scale our approach reproduces generally accepted results in the low and high frequency limits even under the assumption of a high squirt flow frequency we find that frequency dependent anisotropy can occur in the seismic frequency band when larger fractures are present shear wave splitting becomes dependent on frequency with the size of the fractures playing a controlling role in the relationship strong anisotropic attenuation can occur in the seismic frequency band the magnitude of the frequency dependence is influenced strongly by the extent of equant porosity with these results it becomes possible in principle to distinguish between fracture and microcrack induced anisotropy or more ambitiously to measure a characteristic fracture length from seismic data

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

causes and timing of the 8200 yr bp event inferred from the comparison of the grip be 10 and the tree ring delta c 14 record we analyse the 8200 yr bp cold event by comparing the high resolution be 10 record from the grip ice core from central greenland with the well known tree ring delta 14 c record by transferring the absolute dated tree ring chronology to the ice core time scale we show that the coldest phase in the grip record occurred around 8150 yr bp furthermore this method allows us to disentangle production and climate effects on be 10 and c 14 with important implications for the reconstruction of past solar activity and changes in the carbon cycle and be 10 transport we show that in principle it is possible to infer changes in ocean circulation by comparing be 10 and c 14 records however the duration of the 8200 yr bp event is too short to assign unambiguously a significant change in atmospheric c 14 concentration to changes in the global ocean circulation based on the comparison of be 10 with climate records one could argue that the 8200 yr bp cold event is triggered by a change towards lower solar activity however this link is questioned by the fact that around this period there are other similar and even stronger changes in solar activity that have no apparent connection to climate changes c 2004 elsevier ltd all rights reserved less

water saturation effects on elastic wave attenuation in porous rocks with aligned fractures summary elastic wave attenuation anisotropy in porous rocks with aligned fractures is of interest to seismic remote sensing of the earth s structure and to hydrocarbon reservoir characterization in particular we investigated the effect of partial water saturation on attenuation in fractured rocks in the laboratory by conducting ultrasonic pulse echo measurements on synthetic silicacemented sandstones with aligned penny shaped voids fracture density of 0 0298 0 0077 chosen to simulate the effect of natural fractures in the earth according to theoretical models our results show for the first time contrasting variations in the attenuation q 1 o fp and s

theoretical correlations between the elastic and electrical properties in layered porous rocks with cracks of varying orientations abstract cracks that are widely existing in geological formations play an important role in affecting the elastic and electrical rock properties which are remotely measured for detecting and characterizing cracked rocks joint interpretation of the complementary elastic and electrical data is expected to give a better assessment of the cracks however the correlations between the elastic and electrical rock properties which are the key to the successful joint interpretation remain poorly understood in layered porous rocks with cracks of varying orientations we reviewed and summarized the most recent advances in the theoretical models for the elastic and electrical properties of layered porous rocks with inclined and rotated cracks as well as with randomly orienting cracks the limitations of the models were identified by comparing the modeling results with available experimental data and we applied the models within their validity range to obtaining theoretically the correlations between the elastic and electrical properties in layered porous rocks with cracks of the above mentioned orientations in the horizontal and vertical directions respectively we showed that the elastic and electrical properties are systematically correlated with cracks of varying crack densities and aspect ratios different cracks orientations affect the correlations in varying ways the results not only illustrate the link between the elastic and electrical properties in cracked rocks but also have important practical applications when the seismic and electromagnetic exploration data are jointly interpreted to gain better knowledge about the subsurface earth although future work is required to extend the models to more realistic cracked formations and to experimentally test the validity of the models and the theoretically obtained joint correlations

solar forcing of climatic change during the mid holocene indications from raised bogs in the netherlands two cores of mid holocene raised bog deposits from the netherlands were 14 c wiggle match dated 14 c as a proxy for changes in solar activity is validated this paper adds to the increasing body of evidence that solar variability forced climatic changes during the holocene 14 c wiggle match dating mid holocene wet shifts

chlorine isotope composition of volcanic gases and rocks at mount etna italy and inferences on the local mantle source abstract we present the first chlorine isotope compositional data for gases and lavas from mount etna italy and to our knowledge of active mediterranean volcanism we investigated lavas erupted and gases discharged during 2008 2011 from a high temperature fumarole ht 300 c and plume gases from both north east and central craters most of the samples vary in a narrow range of chlorine isotope composition 37 cl values 0 0 7 with gases partially overlapping with rocks only ht gases sampled in 2009 have been clearly affected by secondary processes 37 cl values 15 9 resulting in partial removal of chlorine and isotopic fractionation producing a 37 cl enrichment in the residual gaseous hcl these secondary processes also affect although to a lesser extent plume gases from north east crater nec although post magmatic processes are able to modify the chlorine isotope composition 37 cl values are not affected by magma degassing for residual fractions 0 3 in the melt or any effect is within our data variability finally 37 cl values and cl k ratios of magmatic chlorine constrain the etnean source to be compatible with depleted mantle dmm contaminated by altered oceanic crust aoc in agreement with indications from more common isotopic tracers of mantle processes

hydrological changes in the european midlatitudes associated with freshwater outbursts from lake agassiz during the younger dryas event and the early holocene abstract recent studies of lake level fluctuations during the last deglaciation in eastern france jura mountains and pre alps and on the swiss plateau show distinct phases of higher water level developing at the beginning and during the latter part of greenland stade 1 i e younger dryas event and punctuating the early holocene period at 11 250 11 050 10 300 10 000 9550 9150 8300 8050 and 7550 7250 cal yr b p the phases at 11 250 11 050 and 8300 8050 cal yr b p appear to be related to the cool preboreal oscillation and the 8200 yr event assumed to be associated with deglaciation events a comparison of this mid european lake level record with the outbursts from proglacial lake agassiz in north america suggests that between 13 000 and 8000 cal yr b p phases of positive water balance were the response in west central europe to climate cooling episodes which were induced by perturbation of the thermohaline circulation due to sudden freshwater releases to oceans this probably was in response to a southward migration of the atlantic westerly jet and its associated cyclonic track moreover it is hypothesized that during the early holocene varying solar activity could have been a crucial factor by amplifying or reducing the possible effects of lake agassiz outbursts on the climate

rapid changes in ocean circulation and atmospheric radiocarbon a latitude depth coupled global ocean ice atmosphere model is extended to include a simple biosphere component a physically reasonable adjustment of runoff into the north atlantic is invoked to achieve a transient response to glacial meltwater perturbations which closely resembles the younger dryas climate event we then investigate the evolution of the isotopic ratio of atmospheric radiocarbon 14c due to the rapid changes of deep ocean circulation when the north atlantic branch of the conveyor belt circulation is interrupted the oceanic uptake of radiocarbon is reduced resulting in an increase of atmospheric 14c by about 35 the reduction of ventilation in the north atlantic is partly compensated by an increase of the 14c ratios of the biosphere the southern ocean and the upper ocean above 1000 m depth a plateau of the 14c year calendar year relation can be generated at the time of the rapid reinitiation of deep ocean ventilation which begins coincident with the major temperature increase and lasts for about 60 years it is hence significantly shorter than that found by analyzing tree rings during the termination of younger dryas longer than 400 years a sensitivity study reveals that the duration of the plateau depends strongly on the transient evolution of the gas exchange rate and can increase to 150 300 years if changes of pco2 or sea ice coverage are taken into account

variations of atmospheric 14 c concentrations over the allerood younger dryas transition highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation the synchronisation of two high resolution ams 14c dated records lake go ci z and a floating late weichselian glacial varve chronology at the allerod younger dryas transition allowed us to assess in detail atmospheric 14c changes between late allerod and early preboreal the combined data set shows a drastic rise in 14c during the first 200 years or so of younger dryas and the two following about 500 year long 14c plateaux model experiments which included variations in the geomagnetic field atmospheric co2 variations and a drastic reduction in north atlantic deep water flux at the onset of younger dryas allowed to reproduce the distinct rise in 14c during the first 200 years of younger dryas fairly well also the drop in 14c at the younger dryas holocene boundary seems reasonably explained by changes in north atlantic deep water circulation however the reason behind the anomalous behaviour of the 14c signal in the middle of younger dryas remains an open question

ventilation of the north atlantic ocean during the last glacial maximum a comparison between simulated and observed radiocarbon ages the distribution of radiocarbon during simulations of the last glacial maximum with a coupled ocean atmosphere sea ice model is compared with sediment core measurements from the equatorial atlantic ceara rise blake ridge caribbean sea and south china sea during these simulations we introduce a perturbation of north atlantic freshwater fluxes leading to varying strengths of the atlantic meridional overturning the best fit with the observations is obtained for an overturning weakened by 40 compared with today further we simulate the phenomenon of an age reversal found in deep sea corals but we suggest that this indicates rather a sudden interruption of deep water formation instead of an increase in ventilation which was suggested earlier

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

measurements of seismic anisotropy in synthetic rocks with controlled crack geometry and different crack densities seismic anisotropy can help to extract azimuthal information for predicting crack alignment but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development which is achieved through determining the crack density from seismic or vsp data in this research we study the dependence of seismic anisotropy on crack density using synthetic rocks with controlled crack geometries a set of four synthetic rocks containing different crack densities is used in laboratory measurements the crack thickness is 0 06 mm and the crack diameter is 3 mm in all the cracked rocks while the crack densities are 0 00 0 0243 0 0486 and 0 0729 p and s wave velocities are measured by an ultrasonic investigation system at 0 5 mhz while the rocks are saturated with water the measurements show the impact of crack density on the p and s wave velocities our results are compared to the theoretical prediction of chapman j app geophys 54 191 202 2003 and hudson geophys j r astron soc 64 133 150 1981 the comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks although chapman s model fits the experimental results better the measured anisotropy of the p and s wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density as predicted by several theoretical models the laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration

variations of younger dryas atmospheric radiocarbon explicable without ocean circulation changes the concentration of radiocarbon 14c in the atmosphere depends on its production rate by cosmic rays and on the intensity of carbon exchange between the atmosphere and other reservoirs for example the deep oceans for the holocene the past 11 500 years it has been shown that fluctuations in atmospheric radiocarbon concentrations have been caused mostly by variations in the solar magnetic field1 2 3 recent progress in extending the radiocarbon record backwards in time4 5 6 7 8 9 10 has indicated especially high atmospheric radiocarbon concentrations in the younger dryas cold period between 12 700 and 11 500 years before the present these high concentrations have been interpreted as a result of a reduced exchange with the deep ocean reservoir caused by a drastic weakening of the deep ocean ventilation7 8 9 11 12 here we present a high resolution reconstruction of atmospheric radiocarbon concentrations derived from annually laminated sediments of two polish lakes lake go ciaz and lake perespilno these records indicate that the maximum in atmospheric radiocarbon concentrations in the early younger dryas was smaller than previously believed and might have been caused by variations in solar activity if so there is no indication that the deep ocean ventilation in the younger dryas was significantly different from today s

radiocarbon and stable isotope constraints on last glacial maximum and younger dryas ventilation in the western north atlantic 1 foraminiferal abundance 14c ventilation ages and stable isotope ratios in cores from high deposition rate locations in the western subtropical north atlantic are used to infer changes in ocean and climate during the younger dryas yd and last glacial maximum lgm the 18o of the surface dwelling planktonic foram globigerinoides ruber records the present day decrease in surface temperature sst of 4 c from gulf stream waters to the northeastern bermuda rise if during the lgm the modern 18o salinity relationship was maintained this sst contrast was reduced to 2 c with lgm to interglacial 18o changes of at least 2 2 ssts in the western subtropical gyre may have been as much as 5 c colder above 2 3 km glacial 13c was higher than today consistent with nutrient depleted younger bottom waters as identified previously below that 13c decreased continually to 0 5 about equal to the lowest lgm 13c in the north pacific ocean seven pairs of benthic and planktonic foraminiferal 14c dates from cores 2 5 km deep differ by 1100 340 years with a maximum apparent ventilation age of 1500 years at 4250 m and at 4700 m apparent ventilation ages are presently unavailable for the lgm 2 5 km because of problems with reworking on the continental slope when sea level was low because lgm 13c is about the same in the deep north atlantic and the deep north pacific and because the oldest apparent ventilation ages in the lgm north atlantic are the same as the north pacific today it is possible that the same water mass probably of southern origin flowed deep within each basin during the lgm very early in the yd dated here at 11 25 0 25 n 10 conventional 14c kyr bp equal to 12 9 calendar kyr bp apparent ventilation ages 2 3 km water depth were about the same as north atlantic deep water today below 2 3 km four yd pairs average 1030 400 years the oldest apparent ventilation age for the yd is 1600 years at 4250 m this strong contrast in ventilation which indicates a front between water masses of very different origin is similar to glacial profiles of nutrient like proxies this suggests that the lgm and yd modes of ocean circulation were the same

atmospheric 14c variations derived from tree rings during the early younger dryas abstract atmospheric radiocarbon variations over the younger dryas interval from 13 000 to 11 600 cal yr bp are of immense scientific interest because they reveal crucial information about the linkages between climate ocean circulation and the carbon cycle however no direct and reliable atmospheric 14c records based on tree rings for the entire younger dryas have been available in this paper we present 1 high precision 14c measurements on the extension of absolute tree ring chronology from 12 400 to 12 560 cal yr bp and 2 high precision high resolution atmospheric 14c record derived from a 617 yr long tree ring chronology of huon pine from tasmania australia spanning the early younger dryas the new tree ring 14c records bridge the current gap in european tree ring radiocarbon chronologies during the early younger dryas linking the floating lateglacial pine record to the absolute tree ring timescale a continuous and reliable atmospheric 14c record for the past 14 000 cal yr bp including the younger dryas is now available the new records indicate that the abrupt rise in atmospheric 14c associated with the younger dryas onset occurs at 12 760 cal yr bp 240 yrs later than that recorded in cariaco varves with a smaller magnitude of 40 followed by several centennial 14c variations of 20 25 comparing the tree ring 14c to marine derived 14c and modelled 14c based on ice core 10be fluxes we conclude that changes in ocean circulation were mainly responsible for the younger dryas onset while a combination of changes in ocean circulation and 14c production rate were responsible for atmospheric 14c variations for the remainder of the younger dryas

experimental verification of the fracture density and shear wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry we present laboratory ultrasonic measurements of shear wave splitting from two synthetic silica cemented sandstones the manufacturing process which enabled silica cementation of quartz sand grains was found to produce realistic sandstones of average porosity 29 7 0 5 and average permeability 29 4 11 3 md one sample was made with a regular distribution of aligned penny shaped voids to simulate meso scale fractures in reservoir rocks while the other was left blank ultrasonic shear waves were measured with a propagation direction of 90 to the coincident bedding plane and fracture normal in the water saturated blank sample shear wave splitting the percentage velocity difference between the fast and slow shear waves of 0 5 was measured due to the bedding planes or layering introduced during sample preparation in the fractured sample shear wave splitting corrected for layering anisotropy of 2 72 0 58 for water 2 80 0 58 for air and 3 21 0 58 for glycerin saturation at a net pressure of 40 mpa was measured analysis of x ray ct scan images was used to determine a fracture density of 0 0298 0 077 in the fractured sample this supports theoretical predictions that shear wave splitting sws can be used as a good estimate for fracture density in porous rocks i e sws 100 f where f is fracture density regardless of pore fluid type for wave propagation at 90 to the fracture normal

modeling the effect of abrupt ocean circulation change on marine reservoir age abstract radiocarbon surface reservoir age is required for precise dating of marine organisms although often assumed constant changes in atmospheric radiocarbon content ocean circulation and ocean mixing imprint changes on this quantity the spatial and temporal variations of marine surface and bottom reservoir ages in response to a shutdown and subsequent recovery of atlantic meridional overturning circulation moc are analyzed using a cost efficient three dimensional ocean circulation model generally surface reservoir age changes are limited to the atlantic ocean with a reduction of about 100 yr after the moc shutdown followed by a slow increase and a peak at the time of moc resumption parameter sensitivity studies with respect to the roles of gas exchange diffusivity and north atlantic ice cover show that ice cover has the largest effect on the transient evolution of surface reservoir age during the shutdown our model results agree well with a recent reconstruction of surface reservoir age changes during the younger dryas when we reduce the rate of gas exchange in the model and include a parametrization of seasonal ice cover in the north atlantic

changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10be and 14c records the variations in atmospheric radiocarbon c 14 concentration during the last 50 000 years can be attributed to changes in the c 14 production rate due to changes in solar activity the geomagnetic field and or interstellar galactic cosmic ray flux and to changes in the global carbon cycle the relative contributions of these processes is the subject of current debate although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed c 14 variations for this period using be 10 measurements from greenland summit ice cores we show that in addition to solar and geomagnetically induced production rate changes significant changes in the carbon cycle have to be considered to explain the measured c 14 concentrations there is evidence that these changes are connected to 1 global deglaciation and 2 climate changes in the north atlantic region on centennial to millennial time scales related to changes in the ocean circulation differences between be 10 and geomagnetic field records however suggest that uncertainties of about 20 still exist in determinations of past changes in the c 14 production rate c 2004 elsevier b v all rights reserved less

temporal geographical meltwater influences on the north atlantic conveyor implications for the younger dryas the temporal and geographical roles of meltwater discharge from the laurentide ice sheet on north atlantic deep water nadw production are investigated utilizing a global realistic geometry coupled climate model which does not require the use of flux adjustments model results suggest that preconditioning by meltwater discharge to the mississippi prior to the younger dryas yd is capable of pushing nadw beyond the limit of its sustainability the diversion of meltwater to the st lawrence then merely serves to completely inhibit nadw production the modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the yd and is intimately linked to changes in nadw formation the global thermohaline circulation provides an interhemispheric teleconnection with the southern oceans while changes in the atmospheric heat transport reacting to a global redistribution of oceanic heat transport provide a mechanism for interbasin teleconnection although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model in the absence of additional feedbacks an equilibrium without the presence of nadw is possible the inclusion of the wind stress speed feedback is found to significantly contribute to the resumption of nadw production as suggested by previous studies contrary to these same studies however the coupled model indicates an advective spin up timescale is required for resumption of nadw production and hence the termination of the modeled yd like climate event as opposed to a decadal century timescale the reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies or the duration strength and geographical location of the imposed meltwater applied

glacial interglacial circulation changes inferred from 231pa 230th sedimentary record in the north atlantic region 1 studies from the subtropical western and eastern atlantic ocean using the 231pa 230th ratio as a kinematic proxy for deep water circulation provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation moc over the last deglaciation in this study we present a compilation of existing and new sedimentary 231pa 230th records from north atlantic cores between 1710 and 4550 m water depth comparing sedimentary 231pa 230th from different depths provides new insights into the evolution of the geometry and rate of deep water formation in the north atlantic during the last 20 000 years the 231pa 230th ratio measured in upper holocene sediments indicates slow water renewal above 2500 m and rapid flushing below consistent with our understanding of modern circulation in contrast during the last glacial maximum lgm glacial north atlantic intermediate water gnaiw drove a rapid overturning circulation to a depth of at least 3000 m depth below 4000 m water renewal was much slower than today at the onset of heinrich event 1 transport by the overturning circulation declined at all depths gnaiw shoaled above 3000 m and significantly weakened but did not totally shut down during the bolling allerod ba that followed water renewal rates further decreased above 2000 m but increased below our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the lgm but less accentuated moc during the younger dryas appears very similar to ba down to 2000 m and slightly slower below

the 3d shear experiment over the natih field in oman the effect of fracture filling fluids on shear propagation this is the final paper in a series on the 3d multicomponent seismic experiment in oman in this experiment a 3d data set was acquired using three component geophones and with three source orientations the data set will subsequently be referred to as the natih 9c3d data set we present for the first time evidence demonstrating that shear waves are sensitive to fluid type in fractured media two observations are examined from the natih 9c3d data where regions of gas are characterized by slow shear wave velocities one is that the shear wave splitting map of the natih reservoir exhibits much larger splitting values over the gas cap on the reservoir this increase in splitting results from a decrease in the slow shear wave velocity which senses both the fractures and the fracture filling fluid using a new effective medium model it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect secondly an anomaly was encountered on the shear wave data directly above the reservoir the thick fiqa shale overburden exhibits a low shear wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content no such effects are evident in the conventional p wave data this feature is interpreted as a gas chimney above the reservoir a conclusion supported by both effective medium modelling and the geology with this new effective medium model we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves s2 polarized perpendicular to the fracture orientation whilst leaving the vertical compressional wave velocity largely unaffected this conclusion has direct implications for seismic methods in exploration appraisal and development of fractured reservoirs and suggests that here we should be utilizing s wave data as well as the conventional p wave data as a direct hydrocarbon indicator

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

an oceanic origin for the increase of atmospheric radiocarbon during the younger dryas 1 variations in carbon 14 to carbon 12 ratio in the atmosphere 14catm provide a powerful diagnostic for elucidating the timing and nature of geophysical and anthropological change the atlantic marine archive suggests a rapid 14catm increase of 50 at the onset of the younger dryas yd cold reversal 12 9 11 7 kyr bp which has not yet been satisfactorily explained in terms of magnitude or causal mechanism as either a change in ocean ventilation or production rate using earth system model simulations and comparison of marine based radiocarbon records from different ocean basins we demonstrate that the yd 14catm increase is smaller than suggested by the marine archive this is due to changes in reservoir age predominantly caused by reduced ocean ventilation

synchronized terrestrial atmospheric deglacial records around the north atlantic on the basis of synchronization of three carbon 14 c 14 dated lacustrine sequences from sweden with tree ring and ice core records the absolute age of the younger dryas preboreal climatic shift was determined to be 11 450 to 11 390 80 years before the present a 150 year long cooling in the early preboreal associated with rising delta 14 c values is evident in all records and indicates an ocean ventilation change this cooling is similar to earlier deglacial coolings and box model calculations suggest that they all may have been the result of increased freshwater forcing that inhibited the strength of the north atlantic heat conveyor although the younger dryas may have begun as an anomalous meltwater event

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

experimental study on the effects of fractures on elastic wave propagation in synthetic layered rocks abstractfractures greatly increase the difficulty of oil and gas exploration and development in reservoirs consisting of interlayered carbonates and shales and increase the uncertainty of highly efficient development the presence of fractures or layered media is also widely known to affect the elastic properties of rocks the combined effects of fractures and layered media are still unknown we have investigated the effects of fracture structure on wave propagation in interlayered carbonate and shale rocks using physical models based on wave theory and the similarity principle we have designed and built two sets of layered physical models with randomly embedded predesigned vertically aligned fractures according to the control variate principle we have measured the p and s wave velocities and attenuation and analyzed the effects of fracture porosity and aspect ratio ar on velocity attenuation and power spectral dimension of the p and s waves the experimental results indicated that under condition

geomagnetic field intensity during the last 60 000 years based on 10be and 36cl from the summit ice cores and 14c abstract based on 10 be 36 cl and 14 c records we reconstruct past changes in the geomagnetic field intensity under the assumption that radionuclide records are mainly influenced by changes in their production rates subsequent comparison of these records with independent geomagnetic field reconstructions allows us to assess the reliability of our approach and to identify periods where our method or the geomagnetic field reconstructions might be problematic the geomagnetic field derived from the 10 be flux to summit in central greenland agrees well with independent geomagnetic field intensity reconstructions however compared to independent palaeomagnetic data the radionuclide record suggests increased geomagnetic field intensities especially for the period from 20 000 to 30 000 years bp 36 cl from the grip ice core agrees well with the 10 be record for the last ice age and hence also the reconstructed geomagnetic field based on the tree ring radiocarbon record it is possible to reconstruct geomagnetic field intensity changes during the holocene a comparison of radiocarbon based and independent geomagnetic field reconstructions indicates that changes in the carbon cycle significantly influenced the atmospheric radiocarbon concentration before 10 000 years bp

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

radiocarbon simulations for the glacial ocean the effects of wind stress southern ocean sea ice and heinrich events simulations of oceanic radiocarbon for the last glacial maximum are presented using a three dimensional global ocean circulation model forced with glacial background states according to various reconstructions we investigate the influence of sea surface temperatures sea ice margins wind stress and antarctic sea ice formation on the glacial tracer distribution and meridional overturning circulation the aim of these sensitivity studies is to reconcile available radiocarbon data from marine sediments with reconstructed sea surface temperatures and estimated sea ice production rates model runs with a modified freshwater balance in the southern ocean mimicking increased brine release due to enhanced divergence of antarctic sea ice arrive at radiocarbon values close to observations these experiments also yield abyssal temperatures and salinities which are consistent with recent inferences in the simulation with the best agreement with radiocarbon observations north atlantic deep water export is reduced by 40 compared to present day while antarctic bottom water flow is intensified to similar strength in the south atlantic transient simulations show that glacial freshwater discharge into the north atlantic can cause abrupt increases of atmospheric radiocarbon as observed during heinrich event 1 however the effect is only significant in scenarios with a massive short time discharge at the beginning which is followed by low level freshwater input for the rest of the event or if it is assumed that the meridional overturning circulation was already in a modern operational mode

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

analysis of dynamic fracture compliance based on poroelastic theory part ii results of numerical and experimental tests in part i a dynamic fracture compliance model dfcm was derived based on the poroelastic theory the normal compliance of fractures is frequency dependent and closely associated with the connectivity of porous media in this paper we first compare the dfcm with previous fractured media theories in the literature in a full frequency range furthermore experimental tests are performed on synthetic rock specimens and the dfcm is compared with the experimental data in the ultrasonic frequency band synthetic rock specimens saturated with water have more realistic mineral compositions and pore structures relative to previous works in comparison with natural reservoir rocks the fracture pore geometrical and physical parameters can be controlled to replicate approximately those of natural rocks p and s wave anisotropy characteristics with different fracture and pore properties are calculated and numerical results are compared with experimental data although the measurement frequency is relatively high the results of dfcm are appropriate for explaining the experimental data the characteristic frequency of fluid pressure equilibration calculated based on the specimen parameters is not substantially less than the measurement frequency in the dynamic fracture model the wave induced fluid flow behavior is an important factor for the fracture wave interaction process which differs from the models at the high frequency limits for instance hudson s un relaxed model

pore fluid viscosity effects on p and s wave anisotropy in synthetic silica cemented sandstone with aligned fractures ultrasonic 500 khz p and s wave velocity and attenuation anisotropy were measured in the laboratory on synthetic octagonal shaped silica cemented sandstone samples with aligned penny shaped voids as a function of pore fluid viscosity one control blank sample was manufactured without fractures another sample with a known fracture density inline image measured from x ray ct images velocity and attenuation were measured in four directions relative to the bedding fabric introduced during packing of successive layers of sand grains during sample construction and the coincident penny shaped voids fractures both samples were measured when saturated with air water viscosity 1 cp and glycerin 100 cp to reveal poro visco elastic effects on velocity and attenuation and their anisotropy the blank sample was used to estimate the background anisotropy of the host rock in the fractured sample the bedding fabric was found to show transverse isotropy with shear wave splitting sws of 1 45 1 18 i e for s wave propagation along the bedding planes in the fractured rock maximum velocity and minimum attenuation of p waves was seen at 90 to the fracture normal after correction for the background anisotropy the fractured sample velocity anisotropy was expressed in terms of thomsen s weak anisotropy parameters a theory of frequency dependent seismic anisotropy in porous fractured media was able to predict the observed effect of viscosity and bulk modulus on and in water and glycerin saturated samples and the higher and values in air saturated samples theoretical predictions of fluid independent are also in agreement with the laboratory observations we also observed the predicted polarisation cross over in shear wave splitting for wave propagation at 45 to the fracture normal as fluid viscosity and bulk modulus increases

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

the north atlantic atmosphere sea surface 14c gradient during the younger dryas climatic event abstract we attempt to quantify the 14c difference between the atmosphere and the north atlantic surface during a prominent climatic period of the last deglaciation the younger dryas event yd our working hypothesis is that the north atlantic may have experienced a measurable change in 14c reservoir age due to large changes of the polar front position and variations in the mode and rate of north atlantic deep water nadw production we dated contemporaneous samples of terrestrial plant remains and sea surface carbonates in order to evaluate the past atmosphere sea surface 14c gradient we selected terrestrial vegetal macrofossils and planktonic foraminifera neogloboquadrina pachyderma left coiling mixed with the same volcanic tephra the vedde ash bed which occurred during the yd and which can be recognized in north european lake sediments and north atlantic deep sea sediments based on ams ages from two norwegian sites we obtained about 10 300 yr bp for the atmospheric 14c age of the volcanic eruption foraminifera from four north atlantic deep sea cores selected for their high sedimentation rates 10 cm kyr 1 were dated by ams 21 samples for each core the raw 14c ages assigned to the ash layer peak is significantly older than the 14c age obtained on land part of this discrepancy is due to bioturbation which is shown by numerical modelling nevertheless after correction of a bioturbation bias the mean 14c age obtained on the planktonic foraminifera is still about 11 000 11 100 yr bp the atmosphere sea surface 14c difference was roughly 700 800 yr during the yd whereas today it is 400 500 yr a reduced advection of surface waters to the north atlantic and the presence of sea ice are identified as potential causes of the high 14c reservoir age during the yd

geomagnetic modulation of the 36cl flux in the grip ice core greenland geomagnetic field strength is expected to affect the production rate of cosmogenic isotopes such as beryllium 10 carbon 14 or chlorine 36 chlorine 36 data from the greenland ice core project grip ice core agree well with a production rate calculation based on a paleomagnetic reconstruction for the past 100 000 years over both long and short term variations a chlorine 36 peak at 38 000 years ago previously found in the beryllium 10 record from the vostok ice core can be explained by a period of low geomagnetic field intensity

understanding the causes and consequences of past marine carbon cycling variability through models abstract on geological time scales the production and degree of recycling of biogenic carbon in the marine realm and ultimately its removal to sediments exerts a dominant control on atmospheric co 2 and hence variability in climate this is a highly complex system involving a myriad of inter connected biological chemical and physical processes for this reason alone linking observations often highly abstracted in the form of proxies to the primary processes involved and ultimately to explanatory hypotheses for specific geological events and transitions is challenging the past few decades have seen a progressive improvement in theoretical and process based understanding of the various components that make up the marine carbon cycle and hand in hand with this the development of numerical model representations of the complete system models have also been designed and or adapted with paleoclimate questions in mind and applied to quantitatively explore the role of the marine carbon cycle in both perturbations and long term geologic evolutionary trends in global climate and possible feedbacks between them however we must ask whether paleoclimate models incorporate sufficiently appropriate representations of the dynamics and sensitivities of the marine carbon cycle and indeed whether in the geological context we really know what these dynamics are here we provide a comprehensive overview of how marine carbon cycling and the biological carbon pump is treated in available paleoclimate models with the aim of critically evaluating their ability to help interpret past marine carbon cycle and climate dynamics to this end we first provide an overview of commonly used paleoclimate models and some of their associated paleo applications drawing from a wide range of global carbon cycle box models and earth system models of intermediate complexity emics secondly we review and evaluate the three dominant processes involved in the cycling of organic and inorganic carbon in the marine system and how they are represented in models namely biological productivity at the ocean surface remineralisation dissolution of particulate carbon within the water column and the benthic pelagic coupling at the seafloor we generate and employ illustrative examples using the model genie to show how different parameterisations of water column and sediment processes can lead to significantly different model projections our compilation reveals the prevalence of static parameterisations of marine carbon cycling among existing paleoclimate models which are commonly empirically derived from present day observations although such approaches tend to represent carbon transfer in the modern ocean well they are potentially compromised in their ability to reflect the true degree of freedom and strength of feedbacks with respect to past climate events particularly those characterised by environmental boundary conditions that differ fundamentally from today finally we discuss the importance of using models of different complexities and how questions of model uncertainty may start to be addressed

seismic characterization of reservoirs containing multiple fracture sets natural fractures in reservoirs play an important role in determining fluid flow during production and knowledge of the orientation and density of fractures is required to optimize production variations in reflection amplitude versus offset avo are sensitive to the presence of fractures but current models used to invert the seismic response often make simplified assumptions that prevent fractured reservoirs from being characterized correctly for example many models assume a single set of perfectly aligned fractures whereas most reservoirs contain several fracture sets with variable orientation within a given fracture set in addition many authors only consider the azimuthal variation in the small offset amplitude versus offset and azimuth response the variation in avo gradient with azimuth while the effect of fractures on amplitude versus offset and azimuth increases with increasing offset in this paper the variation in the reflection coefficient of seismic p waves as a function of azimuth and offset due to the presence of multiple sets of fractures with variable orientation within any fracture set is used to determine the components of a second rank fracture compliance tensor ij the variation in the trace of this tensor as a function of position in the reservoir can be used to estimate the variation in fracture density with position in the reservoir and may be used to choose the location of infill wells in the field the principal axes of ij reveal the most compliant direction within the reservoir and may be used to optimize the trajectory of deviated wells the determination of the principal axes of ij requires wide azimuth acquisition and the use of the small offset amplitude versus offset and azimuth the azimuthal variation of the avo gradient may give misleading results

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

a laboratory study of seismic velocity and attenuation anisotropy in near surface sedimentary rocks the laboratory ultrasonic pulse echo method was used to collect accurate p and s wave velocity 0 3 and attenuation 10 data at differential pressures of 5 50 mpa on water saturated core samples of sandstone limestone and siltstone that were cut parallel and perpendicular to the vertical borehole axis the results when expressed in terms of the p and s wave velocity and attenuation anisotropy parameters for weakly transversely isotropic media q q show complex variations with pressure and lithology in general attenuation anisotropy is stronger and more sensitive to pressure changes than velocity anisotropy regardless of lithology anisotropy is greatest over 20 for velocity over 70 for attenuation in rocks with visible clay organic matter laminations in hand specimens pressure sensitivities are attributed to the opening of microcracks with decreasing pressure changes in magnitude of velocity and attenuation anisotropy with effective pressure show similar trends although they can show different signs positive or negative values of q q we conclude that attenuation anisotropy in particular could prove useful to seismic monitoring of reservoir pressure changes if frequency dependent effects can be quantified and modelled

a large drop in atmospheric 14c 12c and reduced melting in the younger dryas documented with 230th ages of corals paired carbon 14 14 c and thorium 230 230 th ages were determined on fossil corals from the huon peninsula papua new guinea the ages were used to calibrate part of the 14 c time scale and to estimate rates of sea level rise during the last deglaciation an abrupt offset between the 14 c and 230 th ages suggests that the atmospheric 14 c 12 c ratio dropped by 15 percent during the latter part of and after the younger dryas yd this prominent drop coincides with greatly reduced rates of sea level rise reduction of melting because of cooler conditions during the yd may have caused an increase in the rate of ocean ventilation which caused the atmospheric 14 c 12 c ratio to fall the record of sea level rise also shows that globally averaged rates of melting were relatively high at the beginning of the yd thus these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the mississippi to the st lawrence river triggered the yd event

experiment study of pore structure effects on velocities in synthetic carbonate rocks abstractcarbonate rocks have a complex pore structure show strong heterogeneity and have a wide range of velocities that lead to more complicated velocity porosity relationships compared with sandstones we designed and prepared 72 carbonate synthetic cores with known pore structures according to the control variate principle we measured the p and s wave velocities of these cores by an ultrasonic pulse transmission method analyzed the effects of the pore aspect ratio ar and pore size d on velocities and compared the experimental results with predictions of effective medium theories emts the matrix of our synthetic cores was consolidated mixture of carbonate cuttings and epoxy we randomly imbedded predesigned penny shaped silicone disks or expandable polystyrene balls into the matrix during the core preparation process to simulate secondary pores the experimental results indicated that han s empirical linear velocity porosity relation was a good prediction for cores with only interparticle por

analysis of the atmospheric 14c record spanning the past 50 000 years derived from high precision 230th 234u 238u 231pa 235u and 14c dates on fossil corals abstract changes in the geomagnetic field intensity solar variability and the internal changes of the carbon cycle are believed to be the three controlling factors of past atmospheric radiocarbon 14 c concentrations denoted as 14 c of these three it is believed that the field intensity is the dominant factor we analyze an atmospheric 14 c record spanning the past 50 000 years based on previously published 230 th 234 u 238 u and 14 c dates of fossil corals from kiritimati barbados araki and santo islands and identify the role of the laschamp geomagnetic field excursion on the long term trend of the 14 c record there is a general consistency between the coral 14 c record and the 14 c output from carbon cycle models based on the global 14 c production estimates high precision high accuracy 230 th 234 u 238 u dates and redundant 231 pa 235 u dates anchor the timing of this 14 c record we propose that a significant fraction of the long term 14 c trend may be due to inaccuracies in the generally accepted 14 c decay constant the uncertainty in estimating the shape of 14 c beta spectrum below 20 kev leads to one of the greatest errors in decay constant estimates once the 14 c half life is validated via redundant techniques 14 c records will provide a better opportunity to examine the roles of carbon cycle and 14 c production influences

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

modelling the concentration of atmospheric co2 during the younger dryas climate event the younger dryas yd dated between 12 7 11 6 ky bp in the grip ice core central greenland is a distinct cold period in the north atlantic region during the last deglaciation a popular but controversial hypothesis to explain the cooling is a reduction of the atlantic thermohaline circulation thc and associated northward heat flux as triggered by glacial meltwater recently a ch4 based synchronization of grip 18o and byrd co2 records west antarctica indicated that the concentration of atmospheric co2 coatm2 rose steadily during the yd suggesting a minor influence of the thc on coatm2 at that time here we show that the coatm2 change in a zonally averaged circulation biogeochemistry ocean model when thc is collapsed by freshwater flux anomaly is consistent with the byrd record cooling in the north atlantic has a small effect on coatm2 in this model because it is spatially limited and compensated by far field changes such as a warming in the southern ocean the modelled southern ocean warming is in agreement with the anti phase evolution of isotopic temperature records from grip northern hemisphere and from byrd and vostok east antarctica during the yd 13c depletion and po4 enrichment are predicted at depth in the north atlantic but not in the southern ocean this could explain a part of the controversy about the intensity of the thc during the yd potential weaknesses in our interpretation of the byrd co2 record in terms of thc changes are discussed

observations of fluid dependent shear wave splitting in synthetic porous rocks with aligned penny shaped fractures p and s wave velocity and attenuation coefficients accurate to 0 3 and 0 2 db cm respectively were measured in synthetic porous rocks with aligned penny shaped fractures using the laboratory ultrasonic pulse echo method shear wave splitting was observed by rotating the s wave transducer and noting the maximum and minimum velocities relative to the fracture direction a block of synthetic porous rock of fracture density 0 0201 0 0068 and fracture size 3 6 0 38 mm measured from image analysis of x ray ct scans was sub sampled into three 20 30 mm long 50 mm diameter core plugs oriented at 0 45 and 90 to the fracture normal transversely isotropic symmetry axis full waveform data were collected over the frequency range 500 1000 khz for both water and glycerin saturated cores to observe the effect of pore fluid viscosity at 1 cp and 100 cp respectively the shear wave splitting observed in the 90 core was 2 15 0 02 for water saturated and 2 39 0 02 for glycerin saturated in agreement with the theory that suggests that the percentage splitting should be 100 times the fracture density and independent of the saturating fluid in the 45 core by contrast splitting was 0 00 0 02 for water saturation and 0 77 0 02 for glycerin saturation this dependence on fracture orientation and pore fluid viscosity is consistent with the poro visco elastic theory for aligned meso scale fractures in porous rocks the results suggest the possible use of shear or converted wave data to discriminate between fluids on the basis of viscosity variations

relative variations in geomagnetic intensity from sedimentary records the past 200 000 years abstract eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations the accuracy and resolution of dating mostly deduced from isotopic stratigraphy were identified as significant factors of distortion we found also that different time scales have been used by authors after readjustment with respect to the same time scale the records were stacked together and the statistical significance of the results tested by a bootstrap technique seventeen records shown to be coherent were combined into a synthetic curve defined as sint 200 other stacks combining various subsets of data were found to be identical to sint 200 except for changes in dispersion in fact the dispersion around sint 200 could be reduced further by improving the correlations without violating the stratigraphy a procedure which at least for some records emphasizes the need for more detailed depth time correlations another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120 200 ka and can thus in principle be resolved however subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr finally we compared sint 200 with the records from the labrador sea and the lac du bouchet ldb both of which rely mostly on 14c chronology the two records show satisfactory agreement with sint 200 when transferred with respect to the age model established by correlating their 18o labrador and susceptibility features ldb with the 18o reference record the offset of 7 kyr at 40 ka measured in both cases between the 14c and 18o ages is higher than the value deduced from a recent model of a geomagnetic field control of 14c production 1 but is not inconsistent with recent u th dates

changes in continental and sea salt atmospheric loadings in central greenland during the most recent deglaciation model based estimates by fitting a very simple atmospheric impurity model to high resolution data on ice accumulation and contaminant fluxes in the gisp2 ice core we have estimated changes in the atmospheric concentrations of soluble major ions insoluble particulates and 10 be during the transition from glacial to holocene conditions for many species changes in concentration in the ice typically overestimate atmospheric changes and changes in flux to the ice typically underestimate atmospheric changes because times of increased atmospheric contaminant loading are also times of reduced snowfall the model interpolates between the flux and concentration records by explicitly allowing for wet and dry deposition processes compared to the warm preboreal that followed we estimate that the atmosphere over greenland sampled by snow accumulated during the younger dry as cold event contained on average four seven times the insoluble particulates and nearly seven times the soluble calcium derived from continental sources and about three times the sea salt but only slightly more cosmogenic 10 be

integrated geophysical and hydromechanical assessment for co2 storage shallow low permeable reservoir sandstones geological reservoirs can be structurally complex and can respond to co2 injection both geochemically and geomechanically hence predicting reservoir formation behaviour in response to co2 injection and assessing the resulting hazards are important prerequisites for safe geological co2 storage this requires a detailed study of thermal hydro mechanical chemical coupled phenomena that can be triggered in the reservoir formation most readily achieved through laboratory simulations of co2 injection into typical reservoir formations here we present the first results from a new experimental apparatus of a steady state drainage flooding test conducted through a synthetic sandstone sample simulating real co2 storage reservoir conditions in a shallow 1 km low permeability 1md 26 porosity sandstone formation the injected pore fluid comprised brine with co2 saturation increasing in steps of 20 brine co2 partial flow rates up to 100 co2 flow at each pore fluid stage an unload loading cycle of effective pressure was imposed to study the response of the rock under different geomechanical scenarios the monitoring included axial strains and relative permeability in a continuous mode hydromechanical assessment and related geophysical signatures ultrasonic p wave and s wave velocities vp and vs and attenuations qp 1 and qs 1 respectively and electrical resistivity on average the results showed vp and vs dropped 7 and 4 respectively during the test whereas qp 1 increased 55 and qs 1 decreased by 25 from the electrical resistivity data we estimated a maximum co2 saturation of 0 5 whereas relative permeability curves were adjusted for both fluids comparing the experimental results to theoretical predictions we found that gassmann s equations explain vp at high and very low co2 saturations whereas bulk modulus yields results consistent with white and dutta ode model predictions this is interpreted as a heterogeneous distribution of the two pore fluid phases corroborated by electrical resistivity tomography images the integration of laboratory geophysical and hydromechanical observations on representative shallow low permeable sandstone reservoir allowed us to distinguish between pure geomechanical responses and those associated with the pore fluid distribution this is a key aspect in understanding co2 injection effects in deep geological reservoirs associated with carbon capture and storage practices