evidence from u th dating against northern hemisphere forcing of the penultimate deglaciation milankovitch proposed that summer insolation at mid latitudes in the northern hemisphere directly causes the ice age climate cycles1 this would imply that times of ice sheet collapse should correspond to peaks in northern hemisphere june insolation but the penultimate deglaciation has proved controversial because june insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier2 3 4 5 6 7 8 there is a clear signature of the penultimate deglaciation in marine oxygen isotope records but dating this event which is significantly before the 14c age range has not been possible here we date the penultimate deglaciation in a record from the bahamas using a new u th isochron technique after the necessary corrections for recoil mobility of 234u and 230th and a small age correction for sediment mixing the midpoint age for the penultimate deglaciation is determined to be 135 2 5 kyr ago this age is consistent with some coral based sea level estimates but it is difficult to reconcile with june northern hemisphere insolation as the trigger for the ice age cycles potential alternative driving mechanisms for the ice age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean atmosphere system or changes in atmospheric co2 concentration controlled by a process in the southern hemisphere

changes in meridional temperature and salinity gradients in the north atlantic ocean 30 72 n during the last interglacial period eight deep sea sediment cores from the north atlantic ocean ranging from 31 to 72 n are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period using benthic foraminiferal 18o measurements and estimates of sea surface temperature sst and sea surface salinity sss we show that summer ssts and ssss decreased gradually during the interval of minimum ice volume at high latitude sites 52 72 n whereas they were stable or increased during the same time period at low latitude sites 31 41 n this increase in meridional gradients of ssts and ssss may have been due to changes in the latitudinal distribution of summer and annual average insolation and associated oceanic and atmospheric feedbacks these trends documented for the eemian ice volume minimum period are similar to corresponding changes observed during the holocene and may have had a similar origin

global ice volumes at the last glacial maximum and early lateglacial observations of sea levels during the last glacial maximum lgm at localities far from the former ice margins constrain the global change in ice volume from the lgm to the present at about 52u10 6 km 3 regional studies of sealevel change from observations near and within the margins of the former ice sheets constrain ice volumes of the individual ice sheets and have led to an imbalance between the global estimate of ice volume and the sum of the individual ice sheet volumes the latter estimates are reliable only when the observational record from localities close to the ice mass extends well into lateglacial times which is generally not the case for the major ice sheets ice volumes during the lgm and earliest part of the lateglacial period can therefore be substantially increased without affecting the predictions of lateglacial and postglacial sea level in a significant manner provided that a rapid reduction in ice volume occurred in early lateglacial time new far field data for lgm and lateglacial sea level change indicates that a rapid rise in sea level of about 15 m occurred at about 16 500 16 000 14 c or 19 200 18 700 calibrated years ago this leads to the inference that during the lgm the ice sheet volumes of the major ice sheets were greater than inferred from regional rebound analyses and that rapid reductions in volume occurred at the termination of the lgm the timing of this occurrence does not coincide with any recognised heinrich event although pulses of ice rafted debris originating from both northern ice sheets do occur in some high latitude cores an antarctic contribution to the post lgm melting event also cannot be ruled out the timing coinciding with evidence for the onset of warming in southern latitudes and the addition of meltwater into the southern ocean fl 2000 elsevier science b v all rights reserved

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

the contribution of salt fingers to vertical mixing in the north atlantic tracer release experiment abstract the north atlantic tracer release experiment natre was performed in an area moderately favorable to salt fingers however the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent this is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems despite the lack of a staircase optical shadowgraph profiles revealed that small scale tilted laminae previously observed in a salt finger staircase were abundant at the natre site using microstructure observations the strength of salt finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy the dissipation ratio by examining the dissipation ratio in a parameter space of density ratio r and richardson number ri the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred while the model for turbulence descr

acoustic modes of propagation in the borehole and their relationship to rock properties acoustic waveform measurements in boreholes have important applications in fracture hydrology and radioactive waste disposal but ambiguities in existing interpretation techniques remain a problem we have addressed the problem by using residue theory to predict the relative excitation of various modes contained in experimental waveforms a plane geometry model involving a layer of fluid between two elastic half spaces is shown to provide velocity dispersion curves for propagating modes that are very similar to those for the fluid filled borehole we use the plane geometry model to illustrate the effects of the confined borehole fluid on surface and body waves traveling along the borehole in the elastic solid we also computed excitation functions for some of the lowest order symmetric modes calculated the time domain response of the trapped modes following the shear head waves and compared them to waveforms recorded in boreholes through several homogeneous formations the insight into the mode composit

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

stratified turbulence and mixing efficiency in a salt wedge estuary abstracthigh resolution observations of velocity salinity and turbulence quantities were collected in a salt wedge estuary to quantify the efficiency of stratified mixing in a high energy environment during the ebb tide a midwater column layer of strong shear and stratification developed exhibiting near critical gradient richardson numbers and turbulent kinetic energy tke dissipation rates greater than 10 4 m2 s 3 based on inertial subrange spectra collocated estimates of scalar variance dissipation from microconductivity sensors were used to estimate buoyancy flux and the flux richardson number rif the majority of the samples were outside the boundary layer based on the ratio of ozmidov and boundary length scales and had a mean rif 0 23 0 01 dissipation flux coefficient 0 30 0 02 and a median gradient richardson number rig 0 25 the boundary influenced subset of the data had decreased efficiency with rif 0 17 0 02 0 20 0 03 and median rig 0 16 the relationship be

a dynamic model for fluid flow in open borehole fractures the attenuation of stoneley waves by an open borehole fracture is closely related to fluid flow in the fracture we consider the dynamic response of a viscous fluid in a borehole fracture to the oscillating pressure excitation of stoneley waves at the fracture opening a dynamic model has been developed to describe the fluid motion in the fracture this model relates both viscous shear effects at the fracture surface and the acoustic wave propagation in the fracture fluid the dynamic conductivity is derived to characterize the fluid conduction through the fracture this model is applied to study stoneley wave attenuation across a horizontal and along a vertical borehole fracture in the horizontal fracture case the theory is found to agree with laboratory fracture modeling data after minimizing scattering effects due to the fracture in the vertical fracture case where the scattering effects are minimal the theory is in excellent agreement with the laboratory data

fracture evaluation using reflected stoneley wave arrivals we use the low frequency reflected stoneley wave mode to locate permeable fractures intersecting a borehole and to estimate their effective apertures assuming a model in which the average aperture of the fracture is roughly constant theoretical work relates the magnitude of the stoneley wave reflectivity to an effective fracture width we treat both the case of a horizontal fracture and the case of a fracture crossing the borehole at an angle laboratory experiments verify the analytic solution for the case of a horizontal fracture full waveform array sonic data were also acquired in a wellbore with a long recording time 25 5 ms in order to capture the late stoneley wave arrivals the data processing involves computation of the stoneley wave reflectivity response using the measured direct and reflected stoneley wave arrivals a least squares fit to the arrival time of the reflected wave arrivals is used to estimate the locations of permeable fractures and the effective width of the fractures is esti

fracture compliance estimation using borehole tube waves abstractwe tested two models one for tube wave generation and the other for tube wave attenuation at a fracture intersecting a borehole that can be used to estimate fracture compliance fracture aperture and lateral extent in the tube wave generation model we consider tube wave excitation in the borehole when a p wave is incident on the fracture the amplitude ratio of the pressure due to the tube wave to that of the incident p wave is a function of fracture compliance aperture and length similarly the attenuation of a tube wave in the borehole as it crosses a fracture intersecting the borehole is also a function of fracture properties numerically solving the dispersion relation in the fracture we study tube wave generation and the attenuation coefficient as a function of frequency we observed that measuring amplitude ratios or attenuation near a transition frequency can help constrain the fracture properties the transition frequency corresponds to the regime in which the viscous skin depth in

the sequence of events surrounding termination ii and their implications for the cause of glacial interglacial co2 changes events surrounding termination ii as preserved in the vostok ice core provide a number of clues about the mechanisms controlling glacial to interglacial climate change antarctic temperature and the atmosphere s co2 content increased together over a period of 8000 years this increase is bounded by a drop in dust flux at its onset and by a drop in the 18o of trapped air at its finish a similar lag between dust flux and foraminiferal 18o is seen in a southern ocean marine record suggesting that the 18o in air trapped in vostok ice is a valid proxy for ice volume the synchronous change of atmospheric co2 and southern hemisphere temperature thus preceded the melting of the northern hemisphere ice sheets this observation coupled with the fact that nutrient reorganization in the north atlantic occurs with or after the sea level rise eliminates many scenarios proposed to explain the co2 rise including those which rely on sea level change conveyor related nutrient redistribution or north atlantic cooling southern ocean scenarios become the front runners but the most popular mechanism iron fertilization has two problems in explaining the co2 rise before termination ii first much of the dust demise occurs prior to the change in co2 so if iron is the villain a threshold value of its supply must be called upon above which productivity does not continue to increase second the co2 rise continues for some 4 5 kyr after the dust flux has fallen to close to zero these problems may be solved if the increased iron supply in dust caused higher rates of nitrogen fixation during the glacial periods in this case the residence time of oceanic nitrate of a few thousand years would enable decreasing productivity to be a global rather than a local phenomenon and would explain the slow rampup of atmospheric co2

in situ seismic measurements in borehole lb 08a in the bosumtwi impact structure ghana preliminary interpretation in order to assist in the interpretation of previous seismic refraction and reflection surveys a vertical seismic profile was acquired in the lake bosumtwi ghana hard rock core hole lb 08a no seismic reflections are seen in the up going wave field obtained and this observation is consistent with the lack of reflectivity observed in the corresponding 2 d surface seismic profile obtained in earlier studies direct down going p waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open hole hard rock section from a depth of 239 m to 451 m of lb 08a analysis of the observed travel times indicates a nearly constant p wave velocity of 1520 m s through the soft lacustrine sediments in the hard rock however the p wave velocity rapidly increases by nearly 30 from 2600 m s to 3340 m s these values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data these values are low relative to those expected for the metasedimentary protoliths an observation that has been made at other young impact structures of similar size the low velocities together with the fact that they increase so rapidly is suggestive of a decreasing density of fractures and microcracks with depth consequently the seismic velocity trend may provide a proxy measure of damage and hence the decay of the shock pressure from the impact point validation of this requires additional detailed studies of the porosity structure in the core

u th dating of marine isotope stage 7 in bahamas slope sediments in order to understand the driving forces for pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing in contrast to the last interglacial marine isotope stage mis 5 the timing of the penultimate interglacial mis 7 is poorly constrained this study constrains its timing and structure by precise u th dating of high resolution n 18 o records from aragonite rich bahamian slope sediments of odp leg 166 sites 1008 and 1009 the major glacial interglacial cycles in n 18 o are distinct within these cores and some mis 7 substages can be identified these sediments are well suited for u th dating because they have uranium concentrations of up to 12 ppm and very low initial 230 th contributions with most samples showing 230 th 232 th activity ratio of s 75 u and th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry with the latter providing dramatically better precision twenty nine of the 41 samples measured have a n 234 u value close to modern seawater suggesting that they have experienced little diagenesis ages from 27 of the 41 samples were deemed reliable on the basis of both their u and their th isotope ratios ages generally increase with depth although we see a repeated section of stratigraphy in one core extrapolation of constant sedimentation rate through each substage suggests that the peak of mis 7e lasted from v237 to 228 ka and that 7c began at 215 ka this timing is consistent with existing low precision radiometric dates from speleothem deposits the beginning of both these substages appears to be slightly later than in orbitally tuned timescales the end of mis 7 is complex but also appears to be somewhat later than is suggested by orbitally tuned timescales although this event is not particularly well defined in these cores fl 2002 elsevier science b v all rights reserved

of ice and elephants what caused the great ice ages accurate dating of the end of one of these ice ages makes us reconsider which part of the climate system drives severe glaciation

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

fracture characterization from vertical seismic profiling data in vertical seismic profiling vsp data large amplitude borehole stoneley waves are observed at depths where fractures intersect the borehole the authors present a model which predicts the amplitudes of these stoneley waves as a function of certain parameters of the fractures namely the fracture aperture width the orientation and the degree of stiffness and roughness of the fracture the proposed mechanism for the generation of these borehole stoneley waves is coupling of a guided mode induced in the fracture by an incident plane wave typically a p wave to the borehole the model expresses the borehole stoneley wave amplitude normalized by the amplitude of the direct p wave as a function of frequency in terms of the fracture parameters the model is used as the basis for an inversion scheme employing a nonlinear least squares algorithm to estimate the fracture parameters the inversion is then applied to vsp data where borehole stoneley waves are observed at depths where fractures are known to intersect the borehole the results of the inversion indicate that the aperture and vertical component of the orientation i e dip of the fracture can be accurately estimated but the horizontal component of the orientation i e strike is notmore well resolved these conclusions are based on comparisons with independent estimates of these parameters from flow tests and borehole televiewer measurements in addition stiffness and roughness of the fractures are important effects which must be considered in order to obtain realistic estimates for the other parameters especially the fracture aperture 27 refs 14 figs 21 tabs less

mixing in a coastal environment 1 a view from dye dispersion 1 dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of new england the experiments were done in 1996 and 1997 as part of the coastal mixing and optics experiment during the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km2 to more than 50 km2 sundermeyer and ledwell 2001 diapycnal diffusivities inferred from dye dispersion range from 10 6 to 10 5 m2 s at buoyancy frequencies from 9 to 28 cycles hour diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by oakey and greenan 2004 agree closely in most cases estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by duda and rehmann 2002 and rehmann and duda 2000 are fairly consistent with the dye diffusivities the dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by mackinnon and gregg 2003 from profiling microstructure measurements obtained at the same site in august 1996 all of the measurements support the general conclusion that the diffusivity averaged over several days is seldom greater than 10 5 m2 s in the stratified waters at the site and usually not much greater than 10 6 m2 s severe storms such as a hurricane that passed over the cmo site in 1996 can dramatically increase the mixing at the site however

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

mixing coefficient in stably stratified flows abstractturbulent mixing in the interior of the oceans is not as well understood as mixing in the oceanic boundary layers mixing in the generally stably stratified interior is primarily although

northern hemisphere forcing of climatic cycles in antarctica over the past 360 000 years the milankovitch theory of climate change proposes that glacial interglacial cycles are driven by changes in summer insolation at high northern latitudes the timing of climate change in the southern hemisphere at glacial interglacial transitions which are known as terminations relative to variations in summer insolation in the northern hemisphere is an important test of this hypothesis so far it has only been possible to apply this test to the most recent termination because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined here we present a new chronology of antarctic climate change over the past 360 000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the dome fuji and vostok ice cores this ratio is a proxy for local summer insolation5 and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter the accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation our results indicate that orbital scale antarctic climate change lags northern hemisphere insolation by a few millennia and that the increases in antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of northern hemisphere summer insolation these results support the milankovitch theory that northern hemisphere summer insolation triggered the last four deglaciations

fluid and solid motion in the neighborhood of a fluid filled borehole due to the passage of a low frequency elastic plane wave the exact problem of the interaction of an arbitrary plane wave in a homogeneous elastic medium with a fluid filled circularly cylindrical borehole is formulated to analyze three axis full waveform borehole seismic data the plane elastic wave which may be incident upon the borehole from any direction is partially scattered around the hole and an acoustic field is induced in the borehole fluid since seismic wavelengths tend to be much larger than diameters of boreholes the exact solution may be expanded in powers of a small number the incident wavenumber times the borehole radius in the limit as 0 the scattered field in the elastic medium is of o in this limit the borehole crosssection is undistorted and moves with the velocity of the incident wave as if no borehole were present neglecting terms proportional to 2 or higher leaves three kinds of dependent terms where is the azimuthal angle about the borehole axis measured with respect to the wavenumber vector of the incident wave

glacial cycles toward a new paradigm the largest environmental changes in the recent geological history of the earth are undoubtedly the successions of glacial and interglacial times it has been clearly demonstrated that changes in the orbital parameters of our planet have a crucial role in these cycles nevertheless several problems in classical astro nomical theory of paleoclimate have indeed been iden tified 1 the main cyclicity in the paleoclimatic record is close to 100 000 years but there is no significant orbitally induced changes in the radiative forcing of the earth in this frequency range the 100 kyr problem 2 the most prominent glacial interglacial transition occurs at a time of minimal orbital variations the stage 11 problem and 3 at 0 8 ma a change from a 41 kyr dominant periodicity to a 100 kyr periodicity occurred without major changes in orbital forcing or in the earth s configuration the late pleistocene transition prob lem additionally the traditional view states that the climate system changes slowly and continuously together with the slow evolution of the large continental ice sheets whereas recent high resolution data from ice and marine sediment cores do not support such a gradual scenario most of the temperature rise at the last termi nation occurred over a few decades in the northern hemisphere indicating a major and abrupt reorganiza tion of the ocean atmosphere system similarly huge iceberg discharges during glacial times known as hein rich events clearly demonstrate that ice sheet changes may also be sometimes quite abrupt in light of these recent paleoclimatic data the earth climate system ap pears much more unstable and seems to jump abruptly between different quasi steady states using the concept of thresholds this new paradigm can be easily integrated into classical astronomical theory and compared with recent observational evidence if the ice sheet changes are by definition the central phenomenon of glacial interglacial cycles other components of the climate sys tem atmospheric co2 concentration southern ocean productivity or global deep ocean circulation may play an even more fundamental role in these climatic cycles

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at mirror lake new hampshire results are presented from experiments carried out in conjunction with the u s geological survey at the hubbard brook experimental forest near mirror lake new hampshire the study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures the collected data set includes a four offset hydrophone vertical seismic profile full waveform acoustic logs at 5 15 and 34 khz borehole televiewer temperature resistivity and self potential logs and borehole to borehole pump test data borehole televiewer and other geophysical logs indicate that permeable fractures intersect the mirror lake boreholes at numerous depths but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis the vertical seismic profiling vsp data indicate a single major permeable fracture near a depth of 44 m corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis vsp data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m all correspond to major permeable fractures in the acoustic waveform data set pump test data confirm the presence of a hydraulic connection between the mirror lake boreholes through a shallow dipping zone of permeability at 44 m in depth effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the vsp data set this discrepancy is attributed to the effect of fracture stiffness a new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of vsps

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

glacial interglacial changes in ocean surface conditions in the southern hemisphere the stable isotope signatures of oxygen and hydrogen in the water of preserved ice and snow are both widely used to infer local temperatures of past environments a derived quantity based on these two signatures the deuterium excess 1 provides additional palaeoclimatic information2 3 4 as this parameter depends on the meteorological and oceanic characteristics of the water s source regions in particular their temperature2 3 and relative humidity4 published studies mainly focus on records from the past 40 000 years here we present a deuterium excess history obtained from ice cores from vostok east antarctica spanning the full glacial interglacial cycle of the past 150 000 years the deuterium excess record shows a strong anticorrelation with the earth s orbital obliquity 41 000 year periodicity and values are markedly higher during the cold stage 5d following the last interglacial than during the other cold stages we interpret the relationship with obliquity as resulting from changes in the latitudinal insolation gradient affecting ocean surface conditions and thus the delivery of moisture to the polar region we argue that the high 5d values relative to other cold stages are driven by relatively less moisture delivered from high latitudes and more from low latitudes the deuterium excess in antarctic precipitation thus provides long term spatially integrated information on ocean surface conditions and ocean atmosphere circulations in the southern hemisphere

elastic wave propagation in a fluid filled borehole and synthetic acoustic logs the propagation and dispersion characteristics of guided waves in a fluid filled borehole are studied using dispersion curves and modeling full wave acoustic logs by synthetic microseismograms the dispersion characteristics of the pseudo rayleigh reflected and stoneley waves in a borehole with and without a tool in the center are compared effects of different tool properties are calculated the effect of a rigid tool is to make the effective borehole radius smaller as an approximation dispersion characteristics of the guided waves in a borehole with a tool can be calculated as a purely fluid filled borehole with a smaller effective radius theoretical waveforms microseismograms of elastic waves propagating in a borehole are calculated using a discrete wavenumber integration with an appropriate choice of parameters our results look similar to the acoustic waveforms recorded in a limestone and a shale formation several factors affect the shape of an acoustic log microseismogram the effective rad

hydraulic properties of closely spaced dipping open fractures intersecting a fluid filled borehole derived from tube wave generation and scattering fluid filled fractures and fissures often determine the pathways and volume of fluid movement they are critically important in crustal seismology and in the exploration of geothermal and hydrocarbon reservoirs we introduce a model for tube wave scattering and generation at dipping parallel wall fractures intersecting a fluid filled borehole a new equation reveals the interaction of tube wavefield with multiple closely spaced fractures showing that the fracture dip significantly affects the tube waves numerical modeling demonstrates the possibility of imaging these fractures using a focusing analysis the focused traces correspond well with the known fracture density aperture and dip angles testing the method on a vsp data set obtained at a fault damaged zone in the median tectonic line japan presents evidences of tube waves being generated and scattered at open fractures and thin cataclasite layers this finding leads to a new possibility for imaging characterizing and monitoring in situ hydraulic properties of dipping fractures using the tube wavefield

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

amplitude and phase of glacial cycles from a conceptual model abstract the astronomical theory of climate in which the orbital variations of the earth are taken to drive the climate changes explains many features of the paleoclimatic records nevertheless the precise link between insolation variations and climatic changes during the quaternary remains mysterious in several aspects in particular the largest sea level changes of the past million years occurred when insolation variations were minimal like during stage 11 and vice versa like during stage 7 moreover recent data from terminations ii and iii show surprising phase lead and lag between insolation and sea level variations to explain these paradoxical amplitude and phase modulations we suggest here that deglaciations started when a combination of insolation and ice volume was large enough to illustrate this new idea we present a simple conceptual model that simulates the sea level curve of the past million years with very realistic amplitude modulations and with good phase modulations

detection of open fractures with vertical seismic profiling in vertical seismic profiling surveys tube waves are generated by compressional waves impinging on subsurface fractures or permeable zones the problem of generation of these waves by a nonnormal incident p wave for an inclined borehole intersecting a tilted parallel wall fracture is formulated theoretically the amplitude of tube waves depends on the permeability the length of the fracture and the frequency the relative effects of these parameters are studied individually the problem is also formulated for a thin oblate ellipsoidal penny shaped fracture the results for the two fracture models are compared and contrasted field data from tyngsboro massachusetts are shown for open fractures in granite from tube wave amplitudes normalized to p wave amplitudes calculated permeabilities are of the order of 100 mdarcy

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

analysis of p and s wave vertical seismic profile data from the salton sea scientific drilling project as part of the salton sea scientific drilling project at california state well 2 14 vertical seismic profile vsp data were collected from p and s wave sources at two distances from the well use of a three component geophone along with rotation of the recorded data traces into a wave front based coordinate system allows analysis of many aspects of seismic wave propagation properties around the well standard vsp analysis techniques were used to measure interval p and s wave velocities and to identify reflecting horizons both within and below the survey interval from 455 to 1735 m a reflection from below the survey interval seen with both p and s sources seems to be associated with a fractured reservoir near 2100 m indications of fracturing were observed including vertical scattering of p waves from a zone near 915 m orthogonally polarized shear waves were generated at each offset to study anisotropy by travel time measurement and particle motion analysis of the shear wave arrivals three component particle motion analysis of shear wave arrivals was found to be effective for characterizing the subtleties in the s wave splitting throughout the various zones in the well the sht source horizontal and transverse to the well produced complicated elliptical particle motion while the sv source in line with the well produced linear particle motion the difference in linearity of particle motion from orthogonally polarized shear wave sources was unexpected and may be related to regional tectonics anomalous zones may be related to transition depths in the salton sea geothermal field travel time difference between sv and sh waves while clearly observable indicates only about 1 average anisotropy

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency

zooming in on heinrich layers theories explaining the origin of the abrupt massive discharges of ice rafted detritus ird into the glacial north atlantic the heinrich layers hls generally point to the laurentide ice sheet as the sole source of these events until it was found that the irds also originated from icelandic and european ice sheets bond and lotti 1995 snoeckx et al 1999 grousset et al 2000 this apparent contradiction must be reconciled as it raises fundamental questions about the mechanism s of hl origin we have analyzed two 12 cm thick hls in an ultrahigh resolution mode 1 2 century intervals in a mid atlantic ridge piston core the 18o record n pachyderma left coiling reveals strong excursions induced by the melting of the icebergs these excursions are associated with a strong decrease in the amount of planktic foraminafersand with a 3 c cooling of the surface waters counts of coarse detrital grains reveal that ird are deposited according to a typical sequence 1 volcanic glass 2 quartz and feldspars 3 detrital carbonate that implies a chronology in the melting of the differentpan atlantic ice sheets sr and nd isotopic composition confirm that in both heinrich layers h1 and h2 precursor ird came from first europe iceland followed then by laurentide derived ird an internal cyclicity can be identified during h1 and h2 about four to six major abrupt discharges occurred roughly on a century timescale the 13c and 15n records reveal that dominant inputs of continent derived organic matter are associated with ird within the hls hiding the plankton productivity signal