rock physics modeling guided by depositional and burial history in low to intermediate porosity sandstones we present a new rock physics modeling approach to describe the elastic properties of low to intermediate porosity sandstones that incorporates the depositional and burial history of the rock the studied rocks have been exposed to complex burial and diagenetic history and show great variability in rock texture and reservoir properties our approach combines granular medium contact theory with inclusionbased models to build rock physics templates that take into account the complex burial history of the rock these models are used to describe well log data from tight gas sandstone reservoirs in saudi arabia and successfully explain the pore fluid rock porosity and pore shape trends in these complex reservoirs

late pleistocene time series of atmospheric and oceanic variables recorded in sediments from the subarctic pacific time series analyses of atmospheric and oceanic variables in a late pleistocene record from the northwest pacific show the complex relationship of the response of various segments of the climate system to changes in the earth s orbit most variance spectra of time series from this subarctic record contain frequency peaks with periods corresponding to at least one of the major orbital components of eccentricity obliquity or precession although the radiolarian faunal water mass assemblages have prominent spectral peaks with 41 000 year periods which are coherent with obliquity at this frequency only the transitional faunal assemblage contains variance focused at a frequency corresponding to the 100 000 year period of eccentricity three of these faunal time series also show variance concentrated at a frequency with a 20 000 year period these three time series are not coherent at a 20 000 year frequency with either of the dominant spectral peaks of precession they are coherent however with variations in the second harmonic of the obliquity cycle changes in obliquity apparently affect siliceous faunal abundances in the northwest pacific region of this high latitude site more than variations in eccentricity or precession maxima in the time series of quartz abundance occur during low values of eccentricity and high glacial ice volume because atmospheric winds serve as the major source of supply of quartz to the sediments at this site these high quartz values reflect increased aridity at the source region except for short periods during interglacials the characteristics of the surface waters in this region of the subarctic pacific during much of the last 460 000 years were similar to those which exist today in the sea of okhotsk the spectrum of winter sea surface temperature estimates derived from siliceous microfaunal abundances contains dominant frequency peaks at periods of 100 000 41 000 and 23 000 years which are coherent with eccentricity obliquity and precession respectively based upon the relationship of the subarctic front with the dominance of specific faunal asemblages the front was positioned south of its present day location throughout much of the late pleistocene

techniques for determining opal in deep sea sediments a comparison of radiolarian counts and x ray diffraction data abstract detailed records of two estimators of the biogenic silica content of deep sea sediments x ray diffraction analysis opal concentrations and quantitative radiolarians per gram counts from microscope slides were compared in downcore sediments from northwest pacific core rc14 105 to evaluate and to intercalibrate the two techniques both the degree to which the data were similar and the degree to which inferences from the data were similar were considered although both data sets are internally consistent and reflect surface water processes they are not equivalent and there is no way to convert one linearly into the other the numbers of radiolarians per gram of sediment appear to respond to the same factors which control the fluctuations in the radiolarian assemblage distrbutions in the area the opal contents respond to the factors which control the total siliceous productivity of the area

identification of density stratified waters in the late pleistocene north atlantic a faunal derivation abstract an expanded study of the radiolarian cycladophora davisiana in late pleistocene north atlantic marine sediments shows that over the last several hundred thousand years this species exhibits large variations in relative abundance the c davisiana curves in the north atlantic cores are quite similar with easily recognizable features common to all records minor deviations from the general pattern of this species abundance apparently reflect the response of c davisiana to specific oceanographic conditions characteristic of a particular area within the north atlantic c davisiana occurs today in high abundance 20 only in the sea of okhotsk extensive winter and early spring sea ice cover coupled with low surface water salinities during summer and fall is responsible for maintaining near freezing subsurface temperatures in this northwest pacific marginal sea as well as relatively stable temperatures and salinities at depths below a shallow subsurface temperature minimum during periods in the late pleistocene high c davisiana abundances 20 in the north atlantic were probably associated with oceanographic properties similar to those that exist in the sea of okhotsk today because of the relationship between relatively stable subsurface temperatures and salinities and high abundance levels of c davisiana analysis of this species abundance pattern at several locations throughout the high latitude north atlantic should assist in identifying source areas of deep water formation and determining the duration of deep convective processes at these sites

ocean atmosphere responses to climatic change in the labrador sea pleistocene plankton and pollen records abstract pleistocene climate change is examined using proxy climatic records from oxygen isotope data calcareous and organic walled marine microfossils pollen and terrestrial spores deposited during the past 0 9 ma at odp site 646 in the labrador sea 500 km north of the present polar front paleotransfer functions applied to planktonic foraminiferal assemblages show relative increases in interglacial summer 3 7 and winter 3 5 temperatures and in winter salinity 0 5 2 for the past 0 4 ma but only two earlier intervals early stage 11 and stage 17 have changes of comparable magnitude coccolith and dinoflagellate cyst accumulation rates show that primary productivity is generally correlated with temperature and salinity changes at the start of interglacials these productivity peaks lag the ice volume changes by 2 4 ka dinocyst blooms seem to precede coccolith peaks reflecting the tolerance of opportunistic species for large variations in temperature and salinity peaks in pollen and spore abundance are strongly correlated with ice volume which controls the position and stability of the polar jet stream in addition to the northern extent of forest vegetation time series analysis was made of 7 oceanographic variables sst summer and winter surface salinity coccoliths dinocysts planktonic and benthic foraminifera and 5 other variables 18 pollen spores percent sand foraminiferal test fragmentation and reworked palynomorphs most variables showed significant power peaks at 100 ka and or at 41 ka however sea surface responses showed minor peaks at 26 and 16 ka and microfossil productivity also showed significant peaks at 68 ka the high latitude labrador sea records thus display large non linear regional responses to climate changes in addition to the effects of orbital insolation forcing at 41 and 23 ka

pliocene pleistocene evolution of the north pacific ocean atmosphere system interpreted from fossil diatoms the relative abundance of diatom taxa in 216 core tops has been quantified and compared using cluster analysis with assemblages in six cores located beneath different water masses of the north pacific ocean all but one sequence span at least 1 5 m y early pleistocene to holocene and one extends to 2 6 ma the quantitative abundance of each taxon and the space time occurrence of each cluster are interpreted as tracers of oceanographic conditions in addition to high frequency glacial interglacial oscillations the records display rapid steplike changes at 2 4 ma 1 6 1 5 ma 1 0 ma 0 8 0 7 ma and 0 35 ma two secondary transitions recorded only in the northwestern cores occur at 2 0 ma and 1 1 ma we conclude that both the high frequency oscillations and the stepwise transitions reflect variations in extent of subarctic water masses under the influence of the siberian high pressure and aleutian low pressure atmospheric cells we further conclude that 1 the subarctic water mass has developed progressively and attained its modern structure only in the late pleistocene 2 some stages of this evolution may have occurred earlier in the eastern subarctic 3 the holocene is an atypical warm extreme so that the present is not a very good key to the past 4 there were several glaciations between 2 4 and 2 0 ma and 5 the subtropical gyre is more conservative than the subarctic gyre and major changes in the region have been due to forcing from the north we propose a conceptual model of oscillating metastable climatic modes maintained by internal feedback and suggest that steplike changes may occur when one set of boundary conditions is temporarily exceeded

evidence for restricted ice extent during the last glacial maximum in the koryak mountains of chukotka far eastern russia field evidence in the koryak mountains lake mainitz region of far eastern russia supports three pleistocene glacial advances the early wisconsinan and pre wisconsinan glaciations are represented by broad lobate moraines extending as much as 30 km north of the koryak mountains field evidence demonstrates that the terminal lateral and medial moraines as well as meltwater channels dead ice topography kettles and outwash plains mark the extent of ice during the last glacial maximum lgm during which glaciers reached no more than 20 km beyond their present limits those emanating from the southern koryak mountains may have reached the bering sea numerical and relative dating techniques support these results and test the theoretical models of the lgm in western beringia erratics on moraines and glaciofluvial terraces common to all valleys at 13 13 8 m above river level yield 36 cl exposure ages ranging from 10 08 to 25 78 ka the koryak mountains lake mainitz record of glaciation is spatially and temporally consistent with the glaciation pattern across central beringia found in other terrestrial and marine records glacier growth in the koryak mountains was sustained by possible increased summer sea surface temperatures and precipitation in the northwest pacific region evidence from the northern koryak mountains lat 64 n long 177 e indicates that the extent of ice in western beringia was limited to mountain and valley glaciers during the lgm this fieldbased research contradicts m g grosswald s theoretical beringian ice sheet hypothesis

evolving pliocene pleistocene climate a north pacific perspective abstract siliceous microfauna in sediments from north pacific deep sea drilling project sites record the response of the regional surface and near surface waters to global climate change over the past 3 0 ma during the late pliocene 2 7 3 0 ma the northern subtropical and subpolar pacific experienced warmer climate conditions than those of today the first predominant polar assemblage maximum occurs in the north pacific at 2 45 ma signifying this region s response to the initiation of significant northern hemisphere glaciation north pacific faunal data show that late pliocene 1 8 2 5 ma glacial maxima were only about half to two thirds as severe as those which occurred during the late pleistocene low faunal assemblage variability during a 200 ka interval in the latest pliocene 1 6 1 82 ma in the north pacific indicates minimal variations in the surface water properties and climate conditions most like those which existed just prior to 2 5 ma a distinct change in faunal assemblage registered in sediments from site 580 between 1 2 and 1 2 ma marks the first cooling in the north pacific comparable in intensity to those of the late pleistocene since 1 26 ma high amplitude high frequency variations in the north pacific polar assemblage have continued to occur providing further evidence that the interglacial glacial extremes that characterize the late pleistocene have persisted since this time the frequency distribution of north pacific winter sea surface temperature time series for the late pliocene and early pleistocene includes major concentrations of variance power centered at a frequency corresponding to a 41 ka period with variance dominance of the temperature time series for the last 800 ka shifting to lower frequencies 110 ka period

quaternary fluctuations in the northern hemisphere trade winds and westerlies the mass accumulation rate and grain size of the total colian component isolated from pelagic sediment in two north pacific cores piston core kk75 02 under the prevailing westerlies and deep sea drilling project site 503 beneath the trade winds have been used to evaluate changes in the intensity of atmospheric circulation and source area aridity over the past 700 000 yr the eolian grain size a direct indicator of wind intensity fluctuates at periodicities similar to those calculated for the earth s orbital parameters of precession obliquity and eccentricity both sites display greater variability in wind intensity prior to 250 000 yr ago eolian accumulation rates an indicator of source area aridity fluctuate at periodicities similar to those of the glacial interglacial cycles lower eolian accumulation rates during colder glacial times most likely reflect increased glacial age humidity at central asian and central american source areas

on the effects of non planar geometry for blind thrust faults on strong ground motion we quantify the effects of complex fault geometry on low frequency 1 hz strong ground motion using numerical modeling of dynamic rupture our tests include the computation of synthetic seismograms for several simple rupture scenarios with planar and curved fault approximations of the 1994 northridge earthquake we use the boundary integral equation method biem to compute the dynamic rupture process which includes the normal stress effects along the curved fault geometries the wave propagation and computation of synthetic seismograms are modeled using a fourth order finite difference method fdm the near field ground motion is significantly affected by the acceleration deceleration and arrest of rupture due to the curvature of the faults as well as the variation in directivity of the rupture for example a 6 km long hanging wall or footwall splay with a maximum offset of 1 km can change 1 hz peak velocities by up to a factor of 2 3 near the fault our tests suggest that the differences in waveform are larger on the hanging wall compared to those on the footwall although the differences in amplitude are larger in the forward rupture direction footwall the results imply that kinematic ground motion estimates may be biased by the omission of dynamic rupture effects and even relatively gentle variation in fault geometry and even for long period waves

magnetic signature of environmental changes in the last 1 2 myr at odp site 1146 south china sea abstract a detailed magnetic analysis has been done on the upper 145 m of the splice of odp site 1146 drilled during leg 184 in the south china sea first a magnetostratigraphic record was obtained the bruhnes matuyama reversal preceded by a precursor and the two jaramillo reversals were identified at 115 133 and 137 5 mcd meter composite depth respectively several known excursions during the brunhes period as well as the kamikatsura event during the matuyama chron were also identified coupled with the oxygen isotope record obtained from benthic foraminifera this allows us to establish a precise age model showing that the studied interval extends to 1 18 myr long term and short term variations have been documented in the magnetic parameters which appear at least in part related to changes in monsoon activity on the long term trend three main intervals with different magnetic trends were identified 1 2 0 7 0 7 0 2 and the most recent 0 2 myr during the oldest period the magnetic content decreases while the grain size of the magnetic mineral increases with large fluctuations the coercivity of the magnetic minerals also increases consistently with data obtained by other authors from pacific cores during the following 500 kyr all the magnetic properties are rather uniform with rather low abundance and relatively large grain sizes and low s ratio this probably illustrates a period of enhanced winter monsoon then in the most recent period the magnetic grain size decreases again with pronounced fluctuations on the short term scale a 23 kyr component is identified by power spectrum analysis in all magnetic parameters in the bottom part of the studied interval this component is correlated with the oxygen isotope record as shown by coherence analysis except for the most recent 200 kyr where the pattern changes cold warm periods coincide with low high magnetic content and magnetic grains are coarser during cold stages and finer during warm periods this is consistent with the clay mineral analysis and illustrates changes in the balance between arid humid periods corresponding to dominant winter summer monsoon and giving rise to physical erosion and wind blowing chemical weathering in the main fluvial plains and transport by oceanic current

geologic time series from deep sea sediments time scales and distortion by bioturbation abstract in order to evaluate the time and frequency domain characteristics of geologic time series obtained from deep sea sediments two important problems must be considered first an accurate time scale must be established for the sediment records to be studied an accurate time scale is a basic requirement for such techniques as spectral and cross spectral analysis secondly the distortion of a geologic time series by sedimentation processes such as bioturbation must be considered if inferences are to be made about the mechanisms controlling climatic and oceanographic processes reflected in the geologic record recently it has been suggested that the periodic components in paleoclimatic records which can be related to variations in the earth s orbital parameters can be used as a means for providing very detailed control for late pleistocene sediment sections a time scale is estimated by tuning an individual frequency component in a climate record to an orbital parameter the tuning procedure commonly uses the technique of bandpass filtering which removes all frequency components except the one equivalent to an orbital parameter and then adjusting the geologic time scale so that the filtered record and orbital record align since bandpass filtering will produce a periodic record from any data set a question is raised as to the circularity of this tuning approach a series of numerical experiments using synthetic time series are used to determine if the tuning of a geologic record artificially induces improved correlation and increased periodic character in a climatic record these experiments indicate that the tuning does not artificially induce these effects thus it is suggested that the careful use of orbital tuning provides a powerful means of defining a geologic time scale the degree to which bioturbation distorts a paleoclimatic record is studied by comparing the spectra of oxygen isotopic records obtained from cores with sedimentation rates ranging from 1 to 12 cm 1000 yr 1 it is shown that the systematic change in the variance spectra of these records that is predicted by models of bioturbation cannot be observed for sedimentation rates greater than 2 cm 1000 yr 1 this can be explained in part if bioturbation is a function of sedimentation rates if this is the case then bioturbation must proceed to depth of tens to hundreds of centimeters however it seems likely that the degree to which bioturbation alters the climate signal preserved in deep sea sediment cores has been overestimated

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

t matrix approach to shale acoustics summary the t matrix approach of quantum scattering theory is used here to place many long wavelength equivalent medium approximations for porous composites polycrystals and cracked media on a common footing and to indicate their limitations but also to derive some new results based on two point statistics in this way we have obtained an insight into the difficult problem of elastic inclusions at finite concentrations which is of foremost relevance when estimating the effective material parameters of porous cracked shales involving stacks of more or less horizontally aligned clay platelets mixed together with more rounded silt minerals and with fluid filling the spaces a rather involved perturbative analysis of the effects of interactions between or structural correlations among the various inclusions minerals and cavities making up a real shale of hexagonal symmetry was performed in an attempt to obtain a better match between theoretical predictions based on a combination of coherent and optical potential approximations and experimental results recovered from ultrasonic wave speeds for the effective elastic stiffness tensor for the particular data set considered in this study the t matrix approach was able to match the data better than the approach of hornby et al but the match was not completely satisfactory further progress in theoretical shale modelling may come from a better knowledge of the elastic properties of pure clay minerals a more detailed knowledge of the microstructure of shales the incorporation of constraints obtained from comparisons between theoretical predictions and experimental results as well as a continuing development of the t matrix approach numerical results also for the effect of bedding parallel microcracks on the elasticity of such a real shale have value in illustrating the importance of taking into account the effects of spatial distribution when trying to deal with non dilute mixtures of highly contrasting material properties

asynchronous maximum advances of mountain and continental glaciers the last maximum glacier advance in many mountain ranges appears to have predated the last maximum advance of the wisconsinan continental ice sheets 20 000 years b p published evidence from widely spaced localities in north and south america europe asia and hawai i suggests that some mountain glaciers extended farther during one or more stades early in the last glaciation roughly 115 000 30 000 years b p than at its end when low global temperature and high precipitation presumably were most favorable for the growth of glaciers variations in the timing of glacier advances in different mountain ranges suggest that alpine glaciers are dependent on regional climate as well as global conditions their study might provide sensitive tests for theories of global climate change glacial deposits are often used as time markers in neotectonic and geomorphic studies and their ages sometimes are inferred from distant measurements or from proxy histories of global climate change undated geomorphic features such as moraines sharply incised streams and river terraces which commonly have been associated with the last maximum glacier advance 20 000 years b p or last deglaciation 13 000 years b p may actually be older by a factor of 2 to 4 or more conversely deposits assigned to the maximum advances of the penultimate global glaciation 130 000 years b p may actually date from early in the most recent glacial cycle and be younger by a comparable factor rates of faulting or folding estimated from displacements or deformation and the assumed ages of such geomorphic features might be too great or too small by corresponding factors

rock physics modeling of unconsolidated sands accounting for nonuniform contacts and heterogeneous stress fields in the effective media approximation with applications to hydrocarbon exploration by treating contact stiffness as a variable one can extend the effective medium approximation used to obtain elastic stiffness of a random pack of spherical grains more specifically we suggest calibrating effective media approximation based on contact mechanics by incorporating nonuniform contact models the simple extension of the theory provides a better fit for many laboratory and field experiments and can provide insight into the micromechanical bonds associated with unconsolidated sediments this approach is motivated by repeated observations of shear wave measurements in unconsolidated sands where observed shear wave velocities are lower than predicted by the hertz mindlin contact theory we present the calibration process for well log data from a north sea well penetrating a shallow gas discovery and a deepwater well in the gulf of mexico finally we demonstrate the benefit of using this model for amplitude variation with angle ava analysis of shallow sand targets in exploration and reservoir studies

a framework for interpreting paleoclimatic variations in eastern beringia abstract paleoclimatic variations in a particular region can be viewed as the outcome of the superimposition of the effects of a number of large scale controls a framework for understanding paleoclimatic variations in a region can be established by considering the long term history of those controls jointly with climatic modeling results that illustrate how regional climates respond to changes of individual controls the relevant controls of late quaternary paleoclimatic variations in eastern beringia include 1 the size of the ice sheet which influences both atmospheric circulation and temperature 2 insolation which influences mainly temperature and seasonality 3 carbon dioxide concentration in the atmosphere which influences temperature and 4 feedbacks involving sea ice sea surface temperatures and snow cover which reinforce the insolation effects we review the results of a number of paleoclimatic experiments with general circulation models to illustrate the potential response of eastern beringia to the variations of these controls we then present a framework for understanding paleoclimatic variations in eastern beringia by describing the likely responses of the climate of this region to the known variations in the controls in addition we discuss key times during the quaternary when particular combinations of the controls give rise to natural experiments which lend themselves to the testing of particular hypotheses our understanding of the nature of climatic variations at high latitudes should increase through examination of paleoclimatic evidence that documents the response of the region at those key times

emergent marine record and paleoclimate of the last interglaciation along the northwest alaskan coast the last interglacial high sea level stand the pelukian transgression of isotope substage 5e is recorded along the western and northern coasts of alaska by discontinuous but clearly traceable marine terraces and coastal landforms up to about 10 m altitude the stratigraphy indicates that sea level reached this altitude only once during the last interglacial cycle from the type area at nome to st lawrence island in the bering sea to the eastern limit of the beaufort sea pelukian deposits contain extralimital faunas indicating that coastal waters were warmer than present amino acid ratios in molluscs from these deposits decrease to the north toward barrow consistent with the modern regional temperature gradient fossil assemblages at nome and st lawrence island suggest that the winter sea ice limit was north of bering strait at least 800 km north of its present position and the bering sea was perennially ice free microfauna in pelukian sediments recovered from boreholes indicate that atlantic water may have been present on the shallow beaufort shelf suggesting that the arctic ocean was not stratified and the arctic sea ice cover was not perennial for some period in coastal regions of western alaska spruce woodlands extended westward beyond their modern range and in northern alaska on the arctic coastal plain spruce groves may have entered the upper colville river basin the flaxman member of the gubik formation on the alaskan arctic coastal plain was deposited during marine isotope substage 5a and records the breakup of an intra stage 5 ice sheet over northwestern keewatin

forest or no forest implications of the vegetation record for climatic stability in western beringia during oxygen isotope stage 3 abstract two conflicting stratigraphic schemes describe the siberian karginskii interstade oxygen isotope stage 3 equivalent as having 1 relatively stable climate with environments more similar to the full glaciation or 2 variable climate with landscapes that more closely approximate contemporary ones new data from continuous lake cores and a nearly continuous section from western beringia wb suggest that both schemes are valid herb dominated communities possibly with isolated populations of larix characterized northern wb with only a slight shift from relatively warm to cool summers during the mid interstade in contrast herb and shrub tundra steppe forest tundra and modern larix forest occurred at various times in areas of southern wb suggesting greater climatic instability a thermal optimum is evident in the south during the mid interstade with modern vegetation in southeastern wb and larix forest tundra in the southwest variations in pinus pumila pollen indicate summer warm winter dry and summer warm winter wet conditions in southeastern wb these fluctuations contrast to other areas of wb where summers and probably winters were consistently arid although the interstade presents a unique interval within the late pleistocene paleodata and paleoclimatic models suggest that changes in marine conditions including sea level were likely key drivers in the regional climate history

towards a high resolution global deep sea chronology for the last 750 000 years abstract variations in the ratio of 18 o 16 o as measured in shells of marine calcareous microfossils are primarily dominated by changes in global ice volume hence these variations provide a set of global time lines in deep sea sediments it is likely that the timing of major changes in oxygen isotope values is strongly influenced if not controlled by variations in the geometry of the earth s orbit since the variation of orbital parameters can be accurately calculated the opportunity exists for transforming this orbital chronology into a geological chronology through careful correlation of oxygen isotope records in a set of deep sea cores from the sub antarctic south atlantic and equatorial pacific we have assembled a composite isotopic section spanning the last 750 000 years with an average sedimentation rate of 2 3 cm 1000 years a new chronology for this time period was developed by adjusting the ages of the oxygen isotope stage boundaries in this composite section so as to extend the consistent phase relationships that exist between variations in oxygen isotope ratios and changes in obliquity and precession during the last 300 000 years to the entire 750 000 year record previously identified difficulties in phase locking precession with the filtered isotopic signal between 365 000 and 465 000 years b p have been resolved with the recognition that precessional variations have an average period of 19 000 years and not 23 000 years during this interval since this new age model yields the best match between variations in obliquity and precession and their corresponding frequency components in the oxygen isotope record we believe that it presents the most accurate chronology yet developed for deep sea sediments with this new age model providing the time control power spectral analyses of south atlantic and sub antarctic chemical and biotic indices show that there is a strong tendency for variance to be concentrated at frequencies corresponding to periods of 100 000 41 000 and 23 000 years

oceanographic conditions associated with high abundances of the radiolarian cycladophora davisiana abstract the cosmopolitan radiolarian cycladophora davisiana usually comprises less than 5 of the radiolarian fauna in holocene sediments in recent sediments from the sea of okhotsk however this species frequently represents more than 20 of the radiolarian assemblage at times during the late pleistocene abundances of this species in excess of 40 are recorded in marine sediments from high latitude oceans 40 of both hemispheres the sea of okhotsk apparently represents a modern analogue of climatic and oceanographic conditions that existed throughout large portions of high latitude oceans at times during the late pleistocene the near surface water structure of the sea of okhotsk is characterized by a low salinity surface layer with a strong temperature minimum near its base the low surface salinities are responsible for maintaining near freezing subsurface temperatures as well as establishing relatively stable temperatures and salinities at depths below the temperature minimum this water structure is produced at least in part by intense freezing of sea ice in winter with subsequent summer melting the physical characteristics of the upper water column affect the abundance and activity of shallow dwelling flora and fauna while providing a stable subsurface environment for deeper dwelling fauna

rock physics diagnostics of depositional texture diagenetic alterations and reservoir heterogeneity in high porosity siliciclastic sediments and rocks a review of selected models and suggested work flows rock physics has evolved to become a key tool of reservoir geophysics and an integral part of quantitative seismic interpretation rock physics models adapted to site specific deposition and compaction help extrapolate rock properties away from existing wells and by so doing facilitate early exploration and appraisal many rock physics models are available each having benefits and limitations during early exploration or in frontier areas direct use of empirical site specific models may not help because such models have been created for areas with possibly different geologic settings at the same time more advanced physics based models can be too uncertain because of poor constraints on the input parameters without well or laboratory data to adjust these parameters a hybrid modeling approach has been applied to siliciclastic unconsolidated to moderately consolidated sediments specifically in sandstones a physical contact theory such as the hertz mindlin model combined with theoretical elastic bounds such as the hashin shtrikman bounds mimics the elastic signatures of porosity reduction associated with depositional sorting and diagenesis including mechanical and chemical compaction for soft shales the seismic properties are quantified as a function of pore shape and occurrence of cracklike porosity with low aspect ratios a work flow for upscaling interbedded sands and shales using backus averaging follows the hybrid modeling of individual homogenous sand and shale layers different models can be included in site specific rock physics templates and used for quantitative interpretation of lithology porosity and pore fluids from well log and seismic data

asian aridity and the zonal westerlies late pleistocene and holocene record of eolian deposition in the northwest pacific ocean rea d k and leinen m 1988 asian aridity and the zonal westerlies late pleistocene and holocene record of eolian deposition in the northwest pacific ocean palaeogeogr palaeoclimatol palaeoecol 66 1 8 a 30 000 year record of eolian deposition in the northwestern pacific ocean provides a history of the aridity of the asian source region and information on the changing latitude and intensity of the zonal westerlies the position of the maximum in eolian flux to the deep sea remained unchanged at 38 to 40 n during this entire time period but the dust flux record of aridity is greatest 6000 years ago at the holocene climatic optimum eolian grainsize data indicate that the northern margin of the westerly jet stream retreated northward during the holocene to a poleward extreme at about 6000 years ago and then moved back toward the equator the change in wind intensity across this margin was continually reduced throughout the holocene these sorts of data enhance our understanding of the behavior of the atmosphere during times of climate change and can be used to test computer generated models of past climatic conditions

diatom stratigraphy of the late pleistocene brunhes subarctic pacific sancetta c and silvestri s m 1984 diatom stratigraphy of the late pleistocene brunhes subarctic pacific mar micropaleontol 9 263 274 three piston cores in the subarctic region of the north pacific show consistent changes in relative abundance of diatom species throughout the brunhes magnetic epoch these events can be used both as stratigraphic markers to subdivide the interval and as indicators of oceanographic conditions the stratigraphic record shows an acme for a ochotensis from about 625 350 kyr isotope stages 15 10 a marked increase in amplitude of abundance fluctuations for r hebetata 300 0 kyr stages 8 1 the extinction of r curvirostris at 276 kyr stage 8 and abundance fluctuations of d seminae roughly in phase with the global oxygen isotope record from 450 0 kyr stages 12 1 in the subarctic pacific an abrupt change in species abundance is not a reliable indicator of hiatus occurrence although details are not clear there appears to have been a fundamental change of the subarctic gyre during the middle brunhes stages 10 8 time with more intense glacial intervals and stronger glacial interglacial contrast occurring after that time coarse clastic detritus is not restricted to glacial intervals suggesting that ice rafting has occurred throughout the brunhes interval peak levels of ice rafting may occur during ice growth and decay as well as during short intervals within a stage

an approach to combined rock physics and seismic modelling of fluid substitution effects the aim of seismic reservoir monitoring is to map the spatial and temporal distributions and contact interfaces of various hydrocarbon fluids and water within a reservoir rock during the production of hydrocarbons the fluids produced are generally displaced by an injection fluid we discuss possible seismic effects which may occur when the pore volume contains two or more fluids in particular we investigate the effect of immiscible pore fluids i e when the pore fluids occupy different parts of the pore volume the modelling of seismic velocities is performed using a differential effective medium theory in which the various pore fluids are allowed to occupy the pore space in different ways the p wave velocity is seen to depend strongly on the bulk modulus of the pore fluids in the most compliant low aspect ratio pores various scenarios of the microscopic fluid distribution across a gas oil contact goc zone have been designed and the corresponding seismic properties modelled such goc transition zones generally give diffuse reflection regions instead of the typical distinct goc interface hence such transition zones generally should be modelled by finite difference or finite element techniques we have combined rock physics modelling and seismic modelling to simulate the seismic responses of some gas oil zones applying various fluid distribution models the seismic responses may vary both in the reflection time amplitude and phase characteristics our results indicate that when performing a reservoir monitoring experiment erroneous conclusions about a goc movement may be drawn if the microscopic fluid distribution effects are neglected

a high resolution opal and radiolarian record from the subpolar north atlantic during the mid pleistocene transition 1069 779ka palaeoceanographic implications abstract a high resolution record of radiolarian assemblages from site u1314 was studied to reconstruct hydrographic and climatic changes in the north atlantic ocean during the mid pleistocene transition period 1069 779 ka besides the faunal record absolute accumulation of radiolarians and total biogenic opal are used to determine changes in surface water productivity results show that the north atlantic arctic front shifted back and forth repeatedly at a glacial interglacial timescale bringing the site under the influence of both cold arctic glacial and much warmer atlantic interglacial waters during glacial intervals and ice rafted debris ird events the deep dwelling taxon cycladophora davisiana was the greatest contributor of the radiolarian assemblage suggesting cold surface conditions melt water discharge and changes in deep and intermediate circulation interglacial intervals were characterized by abundant shallow dwelling taxa like pseudodyctiophimus gracilipes and lithomelissa setosa increased opal accumulation and higher radiolarian diversities indicating a northward flow of warm atlantic surface waters to the site u1314 area a marked change in the structure of the radiolarian assemblage occurred after mis 22 860 ka with large taxa differences between warm and cold periods probably in response to changing ocean conditions due to the higher amplitude of glacial interglacial changes as the dominant periodicity of high latitude climate oscillations shifted from 41 kyr to 100 kyr thus we conclude that the radiolarian assemblage from the north atlantic have changed drastically along with variations in ocean circulation in response to orbital and millennial scale climatic variations that occurred in the early and mid pleistocene

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr