evidence and causes of the two stage development of the baikal rift abstract the data on the geological and deep seated structures of the baikal rift zone are briefly summarized based on the analysis of the composition of the sedimentary infill in rift basins two stages of the zone development have been recognized 1 1 slow rifting 2 2 fast rifting as a consequence of the study of the deep seated structure and construction of the thermal model of the lithosphere it was assumed that the first stage is associated with a slow uprise of the surface of the asthenospheric upwelling and the second stage connected with sideways flows of the substance involved in this upwelling after the crustal base was reached

the direct wave root in marine seismology abstract the direct wave root is an evanescent wave which is observed below the sea floor when the direct water wave is incident on the sea floor from above at supercritical angles for shallow borehole receivers it is the largest amplitude arrival on large offset seismograms the wave has been studied and observed in electromagnetics and acoustics but this paper is the first report of the direct wave root in marine seismology theoretically an expression for the direct wave root can be obtained from a branch line integral in the complex ray parameter plane for the transmission problem between two acoustic half spaces synthetic seismogram analysis based on the discrete wavenumber or reflectivity method can be applied to study the more realistic cases of elastic bottoms gradients beneath the sea floor and intrinsic attenuation steep gradients below the sea floor give amplitude depth relations which are not exponential attenuation q 40 affects amplitudes less than 1 db observations of the direct wave root have been made at the deep sea drilling project site 504b in the costa rica rift area and the observed properties are consistent with theory it is proposed that more detailed measurements of the decay of the direct wave root amplitude with depth be used to obtain high resolution measurements of upper crustal elastic properties at seismic frequencies

the variability in seismic structure of layer 2 near the east pacific rise at 12 n seismic refraction data collected by using a large air gun source and oceanbottom hydrophone receivers show that within two areas on the flanks of the east pacific rise the structure of the uppermost 2 km of the crust is laterally homogeneous the two areas are 20 and 40 km in extent on 0 5 and 4 m y old crust respectively and are separated laterally by 110 km a small but significant difference in structure exists between the two areas the younger crust exhibiting the higher velocities ray tracing through laterally heterogenous models of the shallow crust the careful calculation of corrections for seafloor topography and the objective comparison of data sets by using the slant stacking approach suggest that the established apparent variability in shallow crustal structure may in part be an artifact of experimental and interpretative techniques we propose a reasonable model for shallow crustal structure in which the isovelocity contours parallel the seafloor topography this permits determination of lateral homogeneity of the velocity gradient in the shallow crust over the subdued topography of the east pacific rise or in the deeper crust in general where vertical velocity gradients are sufficiently small that topography has negligible effect on the velocity field

seismic wave propagation in complex crust upper mantle media using 2 d finite difference synthetics summary a description is given of the numerical fd scheme used to solve the elasti wave equation including a few remarks on the source functions used our fd method has been used for computing synthetic seismograms for 2 d crust upper mantle models of size 150 400 km2 with options for free surface topography the strategy was to introduce successively more complex lithosphere models for generating the synthetics the reference model was laterally homogeneous lithosphere the interface scattering was visualized through displays of free surface synthetic waveforms and snapshots for models with a corrugated moho only and free surface topography only near the free surface the latter seems to dominate in the form of p to rg and s to rg conversions lithosphere randomizations were introduced through von karman functions of order n 0 3 with rms velocity fluctuations of 3 4 per cent and correlation distances horizontal and vertical at 2 5 or 10 km in case of a medium with only sub moho heterogeneities those with horizontal anisotropy ax 10 km az 2 5 km produced relatively strong pn and sn phases the respective codas were dominated as in most of our experiments by p to s and s to s scattering wavelets excluding rg scattering at a free surface with topography for a medium with crustal heterogeneities the distortions of the p and s wave trains with distance were clearly demonstrated for full scale heterogeneous lithosphere models characteristic features of the synthetics were quantitatively similar to observational records of local events dominant attributes were a pronounced p coda consisting mainly of p and rg scattered wavelets and a relatively strong s coda consisting mainly of p to s and s to s scattered wavelets the p and s waveforms are severely distorted pointing at the futility of reliably picking many secondary arrivals in local event recordings most of the scattering wavelets are confined to the crustal waveguide and to surface waves since coda excitations for sensors at a depth of 100 km were weak and moreover consisted mainly of s wavelets this implies that a strong teleseismic p coda does not reflect scattering within the crust in the source region but rather a complex source observational results from analysis of noress and arcess local event recordings are also presented clearly the lithosphere is not isotropically inhomogeneous the essence of our 2 d fd synthetic seismogram experiments is that a simple lithosphere model being moderately heterogeneous gives rise to complex seismograms which are grossly similar to the observational recordings in contrast complex models derived from profiling surveys but lacking the fine scale random variations give simple ray tracing like synthetics not necessarily similar to the observed records

oxygen isotope composition of modern and archaeological otoliths from the estuarine hardhead catfish ariopsis felis and their potential to record low latitude climate change abstract shells and otoliths fish ear bones from archaeological deposits of pine island southwest florida provide valuable archives of ecological patterns climate change and associated human responses many remains from these deposits that can be used for climate reconstruction are from estuarine animals the challenge in using oxygen isotope composition of their carbonate hard parts lies in deconvoluting the effects of temperature and salinity mixing of fresh and saltwater each having different isotopic compositions otoliths of the hardhead catfish ariopsis felis linneaus 1766 may provide a record of winter temperature without the complication of salinity variation because this fish spends winter months in waters of marine to near marine salinities hence 18 o water can be constrained when employing published temperature equations here we present geochemical data from modern and archaeological otoliths to evaluate whether they preserve winter temperature a modern catfish was caught near pine island sound and the otoliths lapilli removed archaeological otoliths from the 2nd 3rd century ad falling within the roman optimum ro and the 13th 14th century ad falling within the little ice age lia were obtained from the florida museum s collections oxygen isotope compositions were converted to temperature using published temperature equations and assuming 1 for gulf water isotopic compositions for the modern otolith range from 3 55 to 0 29 the lia otolith ranges from 3 92 to 0 52 and the ro otolith ranges from 1 80 to 0 84 all three otoliths record winter temperature 20 c similar to modern conditions calculated summer temperatures for the modern and lia otoliths 40 c are overestimated and reflect the combined influence of temperature and salinity when catfish inhabit brackish estuarine waters during their reproductive season in contrast summer temperature estimates for the ro otolith are similar to modern conditions indicating estuarine water during the summer was close to 1 this result suggests that seasonal rainfall patterns during the time period represented by the ro otolith were different than today such that the summer wet season was not prevalent

thermal structure of the shallow upper mantle beneath italy and neighbouring areas correlation with magmatic activity and geodynamic significance article using an advanced seismic velocity temperature conversion technique we infer the temperature and melt dis tribution in the upper mantle down to 300 km beneath italy and surroundings starting from absolute s wave velocity models the presence of melt indicated by the seismic reconstruction of the mantle structure in the tyrrhenian sea area requires an extension of the standard conversion procedure to allow for a correction of seismic velocities for the effect of hydrous melt occurrence the thermal structure of the upper mantle along four sections crossing the tyrrhenian basin and the italian peninsula is investigated some characteristics of the obtained thermal structures and melt distributions are well delineated and in accordance with recent models of the evolution of the tyrrhenian sea area the negative thermal anomalies observed in all sections clearly delineate the subduction process of the adriatic plate beneath umbria calabria and the aeolian arc or the episode of post continental convergence beneath tuscany or the thermal effect of the remnant of the adriatic plate campania temperature values at moho seem to be generally correlated with surface heat flow determinations in the tyrrhenian sea area and surroundings although most sectors of the study area like tuscan tyrrhenian area apennines and adriatic trough have not yet reached the steady state thermal regime the thermal gradients evaluated in the adria foreland are high in comparison with those of the back arc area tyrrhenian sea and could be an effect of the eastward mantle flow beneath adria lithosphere or a result of the presence of low fraction of melts 1 wt which cannot rise in compressive regimes or both the melt fraction distribution obtained by the conversion procedure is approximately correlated with the age of the magmatism the highest abundance occurring in the southern tyrrhenian sea in accordance with widespread active volcanism in this area

simulation and analysis of 1991 u s geological survey refraction data from the new madrid seismic zone two orthogonal wide aperture seismic refraction lines were acquired in the new madrid seismic zone by the u s geological survey in 1991 these n s and e w oriented lines cross the blytheville arch and adjacent portions of the reelfoot rift ten shots were fired along the e w line eight shots were fired along the n s line iterative numerical modeling including scattering and viscoelasticity has been performed for all shots on both lines using a new eighth order staggered grid two dimensional finite difference solution based on superposition of relaxation mechanisms the main features in the data can be explained by the existence of a strong 15 km wide low velocity cap lying beneath the center of the blytheville arch between 1 5 and 2 5 km depth the data do not allow estimation of its extent along the axis of the arch but it is at least 30 km long and appears to become less prominant toward the southwest the arch itself has a low velocity background that extends to at least 5 km depth an additional low velocity layer the elvins shale lies generally between 3 and 4 km depth is upwarped at the edges of the arch and thins toward the edges of the reelfoot rift there is no evidence for anomalously low intrinsic q but the data may be interpreted to indicate strong local scattering that occurs west of the low velocity cap of the blytheville arch at depths that are similar to and slightly shallower than the cap additional strong scattering occurs at the western edge of the reelfoot rift an alternative interpretation attributes the apparent scattering at the blytheville arch to instrument noise

effect of a rough seabed on the spectral composition of deep ocean infrasonic ambient noise in deep water the vertical separation of surface noise sources from the elastic bottom is too large for direct excitation of the experimentally observed seismic interface waves in the frequency regime 1 10 hz it is here proposed that these strong seismic components are generated by scattering at rough elastic interfaces in the bottom thereby coupling the noise field produced by the primary surface sources into evanescent waves in the bottom this hypothesis is investigated by combining a previously developed perturbation theory of rough interface scattering with a model for surface generated noise in a stratified ocean environment the resulting theory predicts a spectral composition of the noise field near the seabed that is consistent with the spectral structure of relevant seismic noise data recently collected in the pacific the present analysis in combination with earlier analysis of a data set gathered in a shallow water region makes it quite clear that waveguide propagation mechanisms have an

low grazing angle monostatic acoustic reverberation from rough and heterogeneous seafloors numerical wavefield modeling based on time domain finite difference solutions to the full elastic wave equation is used to quantitatively predict the sensitivity of monostatic backscatter to variations in geological properties of the seabed this article addresses the hypothesis that observed backscatter signals from the seafloor are produced by a combination of seafloor interface and subseafloor volume scattering from structures having variations at scale lengths comparable to the wavelength of the insonifying acoustic field wavelength scale seafloor roughness and subseafloor volume heterogeneity parameters are defined using stochastic spatial probability functions having gaussian autocorrelations the range of the variations in these parameters is constrained to realistic values based on estimates derived from seafloor bathymetry and other geologic data modeling results show that backscattering from rough basaltic hard bottoms in the absence of large scale seafloor slope is dominated by rough surface scattering with little contribution from volume scattering contrary to this sediment soft bottoms with subseafloor volume heterogeneity with or without seafloor roughness produce significant backscattering signals compared to a homogeneous sediment bottom

migration of a late cretaceous fish late cretaceous sediments from the western interior of north america yield exceptionally well preserved fossils1 2 that serve as proxies for the rapidly changing climate preceding the cretaceous tertiary boundary about 67 65 myr ago 3 4 here we reconstruct the ontogenetic history of a maastrichtian age fish vorhisia vulpes5 by using the carbon oxygen and strontium isotope ratios of four aragonite otoliths collected from the fox hills formation of south dakota individuals of v vulpes spawned in brackish water about 70 80 seawater and during their first year migrated to open marine waters of the western interior seaway where they remained for 3 years before returning to the estuary presumably to spawn and die the mean 18o from the marine growth phase of v vulpes yields a seawater temperature of 18 c which is consistent with leaf physiognomy and general circulation model temperature estimates for the western interior during the latest maastrichtian4 6 7

a hybrid wave propagation simulation technique for ocean acoustic problems intermediate range bottom interacting deep ocean acoustic experiments up to 50 km offset in water depths exceeding 1 km using frequencies in the 100 500 hz range encompass a variety of different scales the water column varies smoothly over many wavelengths whereas the seafloor is rough at scales from 1 m to several kilometers we present a general hybrid technique to solve the two dimensional acoustic viscoelastic equations for problems that include these different wave propagation regimes this approach makes realistic large scale computations tractable and assures high accuracy the technique which we call hybrid adaptive regime visco elastic simulation technique harvest consists of three parts a gaussian beam method is used to propagate the source wave field through a vertically varying water column to the scattering region near the seafloor this extrapolated source wave field is inserted into a viscoelastic finite difference grid on which the complex acoustic anelastic interaction of the wave field with the rough seafloor is computed the backscattered wave field collected on hypothetical vertical and horizontal lines is then extrapolated by means of the kirchhoff integral to a receiver array distant from the scattering locale the fidelity of the method is demonstrated by comparison with solutions from a wkbj approximation method and full finite difference simulations

nile behaviour and late palaeolithic humans in upper egypt during the late pleistocene abstract the reconstruction of the environment and the human population history of the nile valley during the late pleistocene have received a lot of attention in the literature thus far there seems to be a consensus that during mis2 extreme dry conditions prevailed over north eastern africa which was apparently not occupied by humans the nile valley seems to be an exception numerous field data have been collected suggesting an important population density in upper egypt during mis2 the occupation remains are often stratified in or at least related to aeolian and nile deposits at some elevation above the present day floodplain they are rich in lithics and animal bones mainly fish illustrating the exploitation of the nile valley by the late palaeolithic inhabitants the fluvial processes active during that period have traditionally been interpreted as a continuously rising highly braided river in this paper we summarize the evidence thus far available for the late pleistocene on the population densities in the nile valley and on the models of nilotic behaviour in the discussion we include data on the environmental conditions in eastern africa on the aeolian processes in the western desert of egypt derived from satellite images 14 c and osl dates in order to formulate a new model that explains the observed high remnants of aeolian and nilotic deposits and the related late palaeolithic sites this model hypothesizes that during the late pleistocene and especially the lgm dunes from the western desert invaded the nile valley at several places in upper egypt the much reduced activity of the white nile and the blue nile was unable to evacuate incoming aeolian sand and as a consequence several dams were created in the upper egyptian nile valley behind such dams the created lakes offered ideal conditions for human subsistence this model explains the occurrence of late palaeolithic hunter fisher gatherers in a very arid environment with very low nile flows even in late summer

effects of surface scattering in full waveform inversion in full waveform inversion of seismic body waves often the free surface is ignored on grounds of computational efficiency asyntheticstudywasperformedtoinvestigatetheeffectsofthissimplification intermsofsizeandfrequency the testmodelanddataconformtoareallong offsetsurveyofthe upper crust across the san andreas fault random fractal variations are superimposed on a background model with strong lateral and vertical velocity variations ranging from 1200to6800m s syntheticdatawerecomputedandinverted forthismodelanddifferenttopographies afullyviscoelastic time domain code was used to synthesize the seismograms and a viscoacoustic frequency domain code was utilized to invert them the inversion was focused on early arrivals which are dominated by p waves but also contain strong p rayleigh wave conversions from the near field of the receiver resulting waveform models show artifacts and a loss of resolution from neglecting the free surface in the inversion but the inversions are stable and they still improve the resolution of kinematic models the extent of deterioration dependsmoreonthesubsurfacethanonthesurfacestructure inversion results were improved at no additional expense by introducing a weak contrast along a staircase function above shotsandreceivers

the nature of the lithosphere beneath the sardinian continental block mantle and deep crustal inclusions in mafic alkaline lavas abstract ultramafic and mafic rocks are abundant as inclusions in plio pleistocene alkali basalt trachybasalt and basanite magmas they provide an important window into the nature of the lithosphere beneath the sardinian continental block the inclusions are predominantly porphyroclastic textured spinel peridotites although pyroxenites mafic granulite facies gneisses and augite megacrysts also occur equilibration temperatures of 956 34 c using the wells geothermometer imply that the peridotites were entrained by rising alkalic magmas somewhere near the top of the mantle lithosphere pyroxenites and augite megacrysts are more fe rich and have lower cr al ratios than clinopyroxenes from peridotite and may have precipitated from mafic magmas at mantle pressures the available evidence strongly implies considerable chemical and mineralogical heterogeneity in the upper mantle lithosphere beneath sardinia diopside mineral separates from two spinel peridotite xenoliths are characterised by la yb n 1 and a chondrite normalised u shaped ree pattern which cannot be explained by a single stage partial melting event trace element constraints imply that the rare samples of the deep crust now granulite facies gneisses were originally gabbroic cumulates

rayleigh to shear wave conversion at the tunnel face from 3d fd modeling to ahead of drill exploration for safe tunnel excavation it is important to predict lithologic and structural heterogeneities ahead of construction conventional tunnel seismic prediction systems utilize body waves p and s waves that are directly generated at the tunnel walls or near the cutter head of the tunnel boring machine tbm we propose a new prediction strategy that has been discovered by 3d elastic finite difference fd modeling rayleigh waves arriving at the front face of the tunnel are converted into high amplitude s waves propagating further ahead reflected or backscattered s waves are converted back into rayleigh waves which can be recorded along the sidewalls we name these waves rssr waves in our approach the front face acts as an s wave transceiver one technical advantage is that both the sources and the receivers may be placed behind the cutter head of the tbm the modeling reveals that the rssr waves exhibit significantly higher amplitudes than the directly reflected body waves the excavation damage zone causes dispersion of the rssr wave leading to multimodal reflection response for the detection of geologic interfaces ahead rssr waves recorded along the sidewalls are corrected for dispersion and stacked from the arrival times the distance to the s s reflection point can be estimated a recurrent application while the tunnel approaches the interface allows one to quantify the orientation of the reflecting interfaces as well our approach has been verified successfully in a field experiment at the piora adit of the gotthard base tunnel the distance to the piora fault zone estimated from stacked rssr events agrees well with the information obtained by geologic surveying and exploratory drilling

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

hydroclimate variability in the nile river basin during the past 28 000 years it has long been known that extreme changes in north african hydroclimate occurred during the late pleistocene yet many discrepancies exist between sites regarding the timing duration and abruptness of events such as heinrich stadial hs 1 and the african humid period ahp the hydroclimate history of the nile river is of particular interest due to its lengthy human occupation history yet there are presently few continuous archives from the nile river corridor and pre holocene studies are rare here we present new organic and inorganic geochemical records of nile basin hydroclimate from an eastern mediterranean em sea sediment core spanning the past 28 ka bp our multi proxy records reflect the fluctuating inputs of blue nile versus white nile material to the em sea in response to gradual changes in local insolation and also capture abrupt hydroclimate events driven by remote climate forcings such as hs1 we find strong evidence for extreme aridity within the nile basin evolving in two distinct phases during hs1 from 17 5 to 16 ka bp and from 16 to 14 5 ka bp whereas peak wet conditions during the ahp are observed from 9 to 7 ka bp we find that zonal movements of the congo air boundary cab and associated shifts in the dominant moisture source atlantic versus indian ocean moisture to the nile basin likely contributed to abrupt hydroclimate variability in northern east africa during hs1 and the ahp as well as to non linear behavior of hydroclimate proxies we note that different proxies show variable gradual and abrupt responses to individual hydroclimate events and thus might have different inherent sensitivities which may be a factor contributing to the controversy surrounding the abruptness of past events such as the ahp during the late pleistocene the nile basin experienced extreme hydroclimate fluctuations which presumably impacted paleolithic cultures residing along the nile corridor

deep seafloor arrivals an unexplained set of arrivals in long range ocean acoustic propagation receptions from a ship suspended source in the band 50 100 hz to an ocean bottom seismometer about 5000 m depth and the deepest element on a vertical hydrophone array about 750 m above the seafloor that were acquired on the 2004 long range ocean acoustic propagation experiment in the north pacific ocean are described the ranges varied from 50 to 3200 km in addition to predicted ocean acoustic arrivals and deep shadow zone arrivals leaking below turning points deep seafloor arrivals that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array are observed these deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

pacific cod gadus macrocephalus as a paleothermometer otolith oxygen isotope reconstruction stable isotope studies are increasingly important for understanding past environmental and cultural developments along the north pacific rim in this paper we present methods for using pacific cod gadus macrocephalus otoliths as a paleothermometer using a case study from kodiak island alaska the results of this study indicate that pacific cod otoliths record variable paleoenvironmental conditions during the little ice age the broad distribution of pacific cod and success in using the otoliths as a paleothermometer makes this method widely applicable to researchers working throughout the northern pacific rim

seismic energy partitioning and scattering in laterally heterogeneous ocean crust we present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust the finite difference formulation is a 2 d solution to the elastic wave equation for heterogeneous media and implicitly calculates p and sv propagation compressional to shear conversion interference effects and interface phenomena random velocity perturbations with gaussian and self similar autocorrelation functions and different correlation lengths a are presented which show different characteristics of secondary scattering heterogeneities scatter primary energy into secondary body waves and secondary stoneley waves along the water solid interface the presence of a water solid interface in the model allows for the existence of secondary stoneley waves which account for much of the seafloor noise seen in the synthetic seismograms for the laterally heterogeneous models

hydroclimatic conditions and fishing practices at late paleolithic makhadma 4 egypt inferred from stable isotope analysis of otoliths abstract the late paleolithic site of makhadma 4 located along the nile river in upper egypt yielded an important ichthyofauna characterized by a very high proportion of tilapia oreochromis niloticus we used isotopic analysis 18o of well preserved otoliths ear stones of tilapia to reconstruct the former hydrological conditions as well as the fishing strategies of the site s inhabitants otoliths from two modern fish captured in the nile river near esna were also examined to test how accurately tilapia otoliths reflect their ambient environment all otoliths were sequentially micromilled to recover high resolution isotopic profiles along the main growth axis comparison of the modern otolith profiles with environmental data shows that tilapia 18o values record seasonal variations of the modern nile hydroclimate but that their values are offset the archaeological otoliths record very large intra individual cyclical variations in 18o values with relatively consistent amplitude as well as very high seasonal maximum values up to 8 3 compared with the modern otoliths the hydrological regime of the water body in which the archaeological fish lived was characterized by a reduced nile water inflow that could not negate the effect of local evaporation during spring the reconstructed hydrological conditions are in accordance with a new model of nilotic behavior that assumes the creation of lakes by damming of the nile as a result of a high eolian activity during hyper arid periods of the late pleistocene although large seasonal evaporation may have resulted in a severe seasonal reduction in the lake s volume and extent the lake was nevertheless maintained for several years cyclic variations in otolith 18o values permit reconstruction of the period of the hydrological cycle during which the fish were captured fishing of young individuals occurred mostly after the maximum input of inflow water from the nile when evaporitic conditions were at their lowest i e during fall

scattering and attenuation abstract an extensive existence of strong small scale heterogeneity in the earth is suggested by the great variability of direct waves and remarkable stability of scattered waves both in the case of teleseismic p coda and in the case of local earthquake s coda quantitative studies of small scoale heterogeneities have been made by the following three methods a lower bound of heterogeneity in the lithosphere has been estimated by the deterministic or stochastic modeling of travel time and amplitude fluctuation of teleseismic p waves across a large aperture seismic array an upper bound of heterogeneity has been estimated by modeling the attenuation of s waves as a scattering loss due to heterogeneity these two approaches suggest the existence of heterogeneity with rms velocity fluctuations from 3 to 10 per cent the third method is based on the estimation of heterogeneity from the amplitude of coda waves we found a significant discrepancy between the result obtained by the third method and those from the first two and attribute the discrepancy to the assumption of a single scattering model adopted in the coda generation theory the need for including multiple scattering as well as for considering vector elastic waves rather than scalar waves is emphasized

seismic propagation across the east pacific rise finite difference experiments and implications for seismic tomography we use a full waveform acousto elastic finite difference technique to investigate seismic propagation across the east pacific rise at 9 30 n for a two dimensional velocity model based on that proposed by vera and others 1990 the primary feature of the model is an upper crustal low velocity region corresponding to the axial magma chamber which includes a small magma body located 1 6 km beneath the seafloor at the rise axis the high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber a qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9 30 n shows generally good agreement for paths that cross the rise axis the first arrival in the finite difference solutions diffracts above the magma chamber this phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment in such cases the first arrival with significant energy is a diffraction from below the magma chamber a high amplitude moho turning pmp phase which results from the large velocity change across the moho beneath the rise axis is apparent in both the finite difference solutions and the observations ray theoretical calculations of the paths of the diffracted arrivals are very unstable and for the diffractions above the magma chamber no solution can be found with a single precision algorithm synthetic delay time inversions using an approximate ray tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body synthetic inversions of spectral estimates of t show that q 1 models are compromised not only if the ray paths are incorrect but also if t estimates include significant contributions from more than one phase deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

lithospheric mantle evolution evidences from ultramafic xenoliths in the lessinian volcanics northern italy abstract the lower middle eocene volcanic rocks from the monti lessini area veneto region carry a suite of inclusions of mostly lherzolitic composition the magmatic activity represents the most important example of alkaline within continental plate volcanism occurred within the south alpine domain during post collisional age the lavas including xenoliths range from alkali olivine basalts to basanites their anorogenic character is evidenced by the absence of any negative anomalies for nb and ti the high mg values the ni content and the forsteritic nature of the olivine indicate that these lavas may be considered undifferentiated or very slightly differentiated mantle derived magmas the great majority of xenoliths are spinel lherzolites plus minor harzburgites and rare websterites and olivine websterites protogranular textures are common to all xenoliths pyrometamorphic textures with sio 2 1bna 2 o 1bk 2 o 1brich glassy patches or blebs are widespread and have been related to local incipient melting possibly favoured by the uprising of the mantle materials incorporated in the mafic lavas the bulk rock and mineral chemistry of the nodules outlines a depletion sequence from the nearly primitive to the most depleted mantle material however variable and anomalously high contents of the most incompatible elements such as la ce rb sr nb zr k and to a lesser extent na indicate for most nodules a more complicated petrogenetic history a petrological model based on incompatible elements at mantle level suggests that peridotite inclusions have to be considered as variably depleted mantle material accidentally incorporated into the host magma during its ascent the proposed sequence of events is a partial melting episodes responsible for the depletion of mantle materials b partial equilibration of mantle materials caused by their upwelling to shallower levels and subsequent long residence time periods possibly in connection with the rifting stage which affected the southern alps during triassic time c contamination events by alkaline fluids with variable h 2 o co 2 ratio which occurred after the mantle depletion but before and or during the incorporation of xenoliths in the alkaline host magma and d local incipient melting episodes during the ascent and subsequent decompression of the nodules resulting in their pyrometamorphic textures

lateral heterogeneity in the upper oceanic crust at deep sea drilling project site 504 the lateral variability of the upper oceanic crust can be studied by mapping the amplitude or power content of arrivals from a dense array of identical sources to a single receiver in this paper we present the results from a borehole seismic experiment at deep sea drilling project site 504 in the eastern equatorial pacific seismic amplitude anomalies of 20 db and more are present with a horizontal correlation length of 1 3 km finite difference synthetic seismogram techniques are used to predict seismic wave propagation through realistic bathymetry basement relief and lateral variations within basement a three dimensional model of the velocity structure around the borehole can be constructed the resultant model has horizontal gradients greater than 2 0 s 1 in the upper crust these gradients are comparable to the vertical gradients normally associated with oceanic crust assuming that velocity anomalies in the upper crust reflect changes in porosity the resultant structure provides constraints on the hydrothermal circulation at the site in addition the finite difference modeling indicates that polarity reversals can be generated by lateral velocity variations near the source and receiver

the influence of large scale seafloor slope and average bottom sound speed on low grazing angle monostatic acoustic scattering variations in large scale seafloor slope and average seabed sound speed account for a significant portion of the variations in scattering intensity observed in low grazing angle monostatic reverberation numerical modeling using a finite difference solution to the elastic wave equation is used to quantify the effect of these large scale parameters in interpretations of reverberation data for hard rough seafloor e g basalt the results of the modeling suggest that the monostatic backscattering strength increases with increasing large scale seafloor slope up to a slope of about 15 dipping toward the incident direction once the grazing angle of the incident wavefield exceeds the critical grazing angle for the flat reference seafloor the backscattering intensity increases only slowly with increasing grazing angle similarly average subseafloor sound speed has a significant effect seafloor with low sound speeds characteristic of soft bottoms e g sediment generate significantly weaker backscatter signals than seafloor with sound speeds characteristic of hard bottoms e g basalt the difference is that the shear waves can always be passed into soft bottoms because even for a flat seafloor there is no shear wave critical grazing angle

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

finite difference synthetic acoustic logs the finite difference method is a powerful technique for studying the propagation of elastic waves in boreholes even for the simple case of an open borehole with vertical homogeneity the snapshot format of the method displays clearly the interaction between the borehole and the rock and the origin and evolution of phases we present an outline of the finite difference method applied to the acoustic logging problem including a boundary condition formulation for liquid solid cylindrical interfaces which is correct to second order in the space increments absorbing boundaries based on the formulations of reynolds 1978 and clayton and engquist 1977 were used to reduce reflections from the grid boundaries results for a vertically homogeneous sharp interface model are compared with the discrete wavenumber method and excellent agreement is obtained the technique is also demonstrated by considering sharp and continuous transitions damaged zones at the borehole wall and by considering the effects of wa

seasonality of human site occupation based on stable oxygen isotope ratios of cod otoliths oxygen isotope ratios d 18 o were measured in micromilled samples of cod otoliths recovered from two stone age sites one from skoklefald southeastern norway and five from skipshelleren southwestern norway discrete samples of otolith material were milled from the edge of the otolith towards the centre each sample representing 3 4 weeks of growth the d 18 o values showed seasonal signals covering a period 1 2 years prior to the time of capture isotope ratios at the edge of the otoliths which is the material deposited at the time of fish capture corresponded to the coldest seasonal water temperatures in late winter or early spring these data provide independent evidence for the season of use of the skoklefald and skipshelleren sites published by elsevier ltd

wavefield separation using a volume distribution of three component recordings by making multiple three component 3c measurements around a volume so that the rate of change of the wavefield in all directions can be estimated it is possible to calculate the divergence and the curl of the wavefield which in turns corresponds to p and s waves the volume may be of arbitrary shape but the equations for wavefield separation are simplified if the volume has a known shape such as that of a tetrahedron the dimensions of the volume should be smaller than approximately one fifth of the minimum wavelength of the recorded wavefield a field test was carried out in oman in april 1998 with a lack of distinct p and s waves in the data the technique was instead evaluated on the airwave induced ground motion the airwave arrival was found to consist mainly of vertical motion that decays with depth the wavefield separation technique was successfully tested on this arrival

isotope paleontology growth and composition of extant calcareous species abstract isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms especially temperature and to obtain clues about life history and modes of growth in calcareous fossils oxygen isotopes are mainly used in the former and carbon isotopes in the latter however since physical surroundings and organism response are intimately associated both types of information are contained in each of the isotopic signals to explore the potential of isotope paleontology and to provide a basis for reconstruction a broad range of extant organisms has been studied taking the pioneering work of epstein and associates as a starting point results are summarized for a representative sampling of these studies with emphasis on work at the laboratories of the authors from the mid seventies to the present the organisms considered are nannoplankton benthic algae planktonic and benthic deep sea foraminifera larger foraminifera sponges corals bryozoans polychaetes arthropods bivalves gastropods cephalopods and vertebrates fish otoliths the survey broadly suggests that regarding oxygen isotopes materials tend to be precipitated close to equilibrium with the surrounding seawater as postulated by urey and that for carbon isotopes disequilibrium is the rule life spans growth rates differential seasonal growth and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts in detail the interpretation of isotope records of individual shells is quite complicated and simple models will fail to give satisfactory results in many or most cases

tomography reveals buoyant asthenosphere accumulating beneath the juan de fuca plate the boundary between earth s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth perhaps indicating a thin weak layer that may strongly influence plate motion dynamics the behavior of such a layer at subduction zones remains unexplored we present a tomographic model derived from on and offshore seismic experiments that reveals a strong low velocity feature beneath the subducting juan de fuca slab along the entire cascadia subduction zone through simple geodynamic arguments we propose that this low velocity feature is the accumulation of material from a thin weak buoyant layer present beneath the entire oceanic lithosphere the presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics

north american continental seasonality during the last millennium high resolution analysis of sagittal otoliths abstract 18 o caco 3 values of late holocene sagittal fish otoliths from lake erie provide a record of seasonal temperature variation for mid western north america freshwater drum aplodinotus grunniens sagittal otoliths obtained from paleo indian middens ranging in age from a d 985 to a d 1530 as well as a recent specimen were sampled at a resolution representing time averaging of as little as 3 5 days to estimate seasonal climatic variation over the last thousand years regional differences in climate as well as asynchronous variation inferred by comparison with historical records of europe greenland and iceland suggest that the climate trends of eastern north america may not always correspond to those of europe or even the north atlantic results suggest that summer temperatures of the laurentian great lakes region at the beginning of the millennium were 2 6 c warmer while winter temperatures may have been nearly 2 c cooler than the 20th century average by the late 1200s summer temperatures decreased to nearly modern values in the 1400s and through the 1500s summer maxima exhibited enhanced intra annual variation as well as temperatures which were as much as 8 c cooler while winter temperatures were 0 4 2 7 c warmer than 20th century values an additional outcome of this research is the estimation of lake water 18 o values for several time periods throughout the holocene mean annual temperatures inferred from 18 o h 2 o values display a decrease from a d 985 through at least a d 1530 concomitant with summer temperature trends the long residence time of these large lakes suggests that secular trends in 18 o values must represent long term variation in climate which can be used in concert with the short term high resolution temperature record for a comprehensive estimation of climatic trends temperature variation and paleohydrology of the great lakes region is discussed in terms of variation in storm tracks and source region with time 18 o values of lake water coupled with paleotemperature data suggest that changing climate is the result of secular variation in the position shape and strength of the circumpolar vortex

deep seafloor arrivals in long range ocean acoustic propagation ocean bottom seismometer observations at 5000 m depth during the long range ocean acoustic propagation experiment in the north pacific in 2004 show robust coherent late arrivals that are not readily explained by ocean acoustic propagation models these deep seafloor arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km the travel times for six of 16 observed deep seafloor arrivals correspond to the sea surface reflection of an out of plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers this out of plane bottom diffracted surface reflected energy is observed on the deep vertical line array about 35 db below the peak amplitude arrivals and was previously misinterpreted as in plane bottom reflected surface reflected energy the structure of these arrivals from 500 to 3200 km range is remarkably robust the bottom diffracted surface reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor

upper mantle beneath a young oceanic rift peridotites from the island of zabargad red sea information on the nature of the upper mantle in a young rift zone with lithosphere transitional from continental to oceanic was obtained from the island of zabargad probably an uplifted fragment of red sea lithosphere the island exposes exceptionally fresh mantle derived peridotite spinel iherzolite bodies the mineralogy and major element rare earth element and mineral chemistry of the spinel iherzolites suggest that they equilibrated last at depth 30 km in the mantle and that they have oceanic rather than continental affinity it is unlikely that the zabargad ultramafics are part of a late precambrian early paleozoic ophiolite as found elsewhere in circum red sea regions rather they were uplifted probably in connection with the development of the red sea rift the zabargad peridotites are in tectonic contact with continental metamorphic basement rocks it appears therefore that regions of an embryonic rift such as the red sea can be covered with continental type crust but be underlain by an upper mantle having already oceanic affinities

subsidence and sedimentation on jurassic passive continental margin southern alps italy the southern alps of italy preserve a tectonically intact array of jurassic facies that record the evolution of a part of the margin of the apulian plate from its ancestral beginnings in a complex of permian and triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs then as the new ligurian oceanic basin began to open farther west and as subsidence gradually slowed accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility carbonate dissolutio levels and the strength of bottom currents estimates of the history of seafloor depths based on a simple subsidence law of the form subsidence k age 12 provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the jurassic in this region the rapid 1 km deepening of the compensation depth for calcite during the late jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling fertile silica rich carbonate dissolving to downwelling less fertile silica poor carbonate preserving

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

upper crustal velocity structure beneath the central lucky strike segment from seismic refraction measurements we present a three dimensional velocity model of the upper crust around the central volcano of the lucky strike segment mid atlantic ridge the model constructed from a 3 d array of air gun shots 37 5 m spacing along line and 100 m between lines to ocean bottom seismometers fired during a 3 d seismic reflection survey shows an off axis velocity increase 1 km s a low velocity region within the median valley and a low velocity anomaly underneath the lucky strike volcano our observations indicate a porosity decrease of 1 9 corresponding to a velocity increase of 0 5 1 km s over a distance of 8 km from the ridge axis 0 7 ma and a porosity decrease of 4 11 corresponding to a velocity increase of 2 km s between a depth of 0 5 and 1 75 km below seafloor a sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis we favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion lava sheeted dike boundary and active extension faults fractures and fissures the porosity variation with depth probably depends on pore space collapse hydrothermal alteration and a change of stress accommodation the off axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust

seabed property estimation from ambient noise recordings part i compliance and scholte wave phase velocity measurements the ability to derive a near surface shear velocity profile from ambient noise records is useful for seismic applications such as shear wave statics estimation and geohazard prediction measurements of seafloor compliance and scholte wave velocity and amplitude are all related to the near surface shear velocity profile i analyzed a data set of 33 minutes of continuous noise records recorded by an ocean bottom cable deployed in 273 m deep water for seafloor compliance and scholte waves i failed to observe seafloor compliance because of limitations in the record length i have detected scholte waves on the inline and vertical component geophones and love waves on the crossline component using f k spectra both the scholte and love wave phase velocities can be explained by a simple 1d isotropic near surface model the scholte waves may have been excited by acoustic energy from the recording vessel while no satisfactory excitation mechanism has been found for the love waves

anisotropic seismic scattering in old atlantic crust an oblique seismic experiment ose was carried out at ocean drilling program odp hole 418a in the western north atlantic to investigate vertical lateral and azimuthal structural variations in the upper portions of relatively old ocean crust original analysis of the data for travel time anisotropy did not show systematic azimuthal trends in seismic structure except at shallow depths immediately adjacent to the borehole subsequent analysis of the ose data presented here reveals a correlation between scattering strengths versus azimuth and spreading direction of the crust analyses are performed on body and coda waves and include coda energy and normalized coda energy calculations spectral analysis coda dropoff rates and cross correlation analysis all of the analyses show trends indicating an increase in seismic scattering along directions parallel to the paleospreading direction these results suggest that evidence of crustal formation processes is still present and seismically detectable after 108 m y of crustal evolution scattering is likely caused by crustal cracks and or aligned heterogeneities indicating that either large crustal cracks are still present in the crust or that homogenization of the crust through hydrothermal deposition of secondary minerals is not complete at this site

stable isotope proxy records in tropical terrestrial environments abstract the tropics refers to an area that is loosely equatorial characterized by relatively high temperatures at low altitude comparatively low seasonal contrast in temperatures and generally high or highly seasonal rainfall the stable isotopes of carbon oxygen and hydrogen are increasingly important tools of relevance to understanding trajectories and mechanisms of past environmental change in the tropics in the case of carbon this is because at the broadest level the carbon isotope composition of organic and carbon containing inorganic proxy materials formed in the tropics can be read as being derived from the balance between c3 tree and c4 grass photosynthesis hence providing a relative measure of vegetation type and indirectly climate likewise changes in the oxygen and hydrogen isotope composition of many proxy materials again at the broadest level can be read as indicative of changes in hydrological conditions and or altitude in the past this review provides an entry point into the literature relating to the use of stable isotopes as terrestrial environmental proxies for past environmental change with a focus on the measurement and interpretation of stable isotope data from the wide range of proxy materials preserved in tropical environments

oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in the afar region of ethiopia abstract the oxygen and carbon isotope ratios in the shells of ancient freshwater gastropods coupled with geological geographical and hydrological data from their area and time of origin can yield information concerning general climatic conditions prevailing at the time the gastropod lived in addition sequential oxygen isotope analyses along the growth spiral of the shell provides indications of seasonal variability in the life span of the organisms such data were collected for some freshwater gastropods collected from two lakes and two springs in the afar region of ethiopia spanning late pleistocene to modern age episodes of high rainfall in the ethiopian highlands were clearly evident in the depleted 18o values in some shells whiel arid spells produced enriched levels in others hot springs introduced an added complication to the isotope record this study on a region of particularly complex hydrology and highly variable environmental history indicates the utility of stable isotope analyses as proxies for past climates but also emphasizes the problems involved

shifting sediment sources in the world s longest river a strontium isotope record for the holocene nile we have reconstructed long term shifts in catchment sediment sources by analysing for the first time the strontium sr and neodymium nd isotope composition of dated floodplain deposits in the desert nile the sediment load of the nile has been dominated by material from the ethiopian highlands for much of the holocene but tributary wadis and aeolian sediments in sudan and egypt have also made major contributions to valley floor sedimentation the importance of these sources has shifted dramatically in response to global climate changes during the african humid period before c 4 5 ka when stronger boreal summer insolation produced much higher rainfall across north africa the nile floodplain in northern sudan shows a tributary wadi input of 40 50 thousands of tributary wadis were active at this time along the full length of the saharan nile in egypt and sudan as the climate became drier after 4 5 ka the valley floor shows an abrupt fall in wadi inputs and a stronger blue nile atbara contribution in the arid new kingdom and later periods in palaeochannel fills on the margins of the valley floor aeolian sediments replace wadi inputs as the most important secondary contributor to floodplain sedimentation our sediment source data do not show a measurable contribution from the white nile to the floodplain deposits of northern sudan over the last 8500 years this can be explained by the distinctive hydrology and sediment delivery dynamics of the upper nile basin high strontium isotope ratios observed in delta and offshore records that were previously ascribed to a stronger white nile input during the african humid period may have to be at least partly reassessed our floodplain sr records also have major implications for bioarchaeologists who carry out sr isotope based investigations of ancient human remains in the nile valley because the isotopic signature of nile floodplain deposits has shifted significantly over time

p wave detection thresholds pn velocity estimates and t wave location uncertainty from oceanic hydrophones p wave arrivals recorded by the u s navy s sound surveillance system sosus hydrophone arrays were used to estimate earthquake detection thresholds and pn velocities in the northeast pacific ocean the navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary t waves however use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment the p wave detection threshold of the sosus hydrophones was quantified using the epicentral distance and magnitude of 250 northeast pacific ocean earthquakes earthquakes with body wave magnitudes as low as 2 have detectable p wave arrivals at epicentral distances of 500 km earthquakes with mb between 3 5 and 5 were detected 50 of the time at distances of 100 1500 km while events with mb 5 were all detected even out to distances of 1000 1500 km both p and t wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes the peak amplitude of the t wave coda and the onset of the p wave were used as the earthquake arrival times to estimate event locations t wave arrival time residuals have a gaussian distribution with zero mean which implies that using t wave peak amplitude is consistent with using the p wave onset as the arrival time there are typically 6 stations used to derive a t wave based location hence location error ellipses are not well constrained a monte carlo technique was employed to estimate t wave event location uncertainty t wave locations have error bars of 1 km in latitude and longitude when 3 hydrophones are used for a location estimate the detected p wave arrivals and earthquake locations were used to measure pn velocities pn velocity values of 7 9 0 1 and 8 0 0 1 km s were found for the pacific and juan de fuca plates respectively a pn velocity of 7 5 0 1 km s was measured for rays traveling northward from the mendocino triple junction along the cascadia subduction zone a pn velocity of 7 7 0 3 km s was estimated for ray paths originating onshore western north america and traveling to the offshore hydrophones

sediment characterization at the equatorial mid atlantic ridge from p to s teleseismic phase conversions recorded on the pi lab experiment accurate marine sediment characteristics for example thickness and seismic velocity are important for constraining sedimentation rates with implications for climate variations and for seismic imaging of deeper structures using ocean bottom seismic deployments we analyze p to s seismic phase conversions from the sediment crust boundary recorded by the passive imaging of the lithosphere asthenosphere boundary pi lab experiment to infer the sediment thickness across the mid atlantic ridge covering 0 to 80 myr old seafloor we find p d s p delay times of 0 04 0 37 s or 5 to 82 m thickness sediment thickness increases with age thickness agrees with global estimates for young 15 20 myr seafloor but is thinner on older lithosphere our result may represent a lower limit on sediment thickness given that several of our stations are on topographic highs the sedimentation rate decrease observed from 5 to 1 2 mm kyr at 10 myr suggests a recent increase in productivity related to climate change eolian dust fluxes and or biogenic marine activity

a stratigraphic framework for abrupt climatic changes during the last glacial period based on three synchronized greenland ice core records refining and extending the intimate event stratigraphy due to their outstanding resolution and well constrained chronologies greenland ice core records provide a master record of past climatic changes throughout the last interglacial glacial cycle in the north atlantic region as part of the intimate integration of ice core marine and terrestrial records project protocols have been proposed to ensure consistent and robust correlation between different records of past climate a key element of these protocols has been the formal definition and ordinal numbering of the sequence of greenland stadials gs and greenland interstadials gi within the most recent glacial period the gs and gi periods are the greenland expressions of the characteristic dansgaard oeschger events that represent cold and warm phases of the north atlantic region respectively we present here a more detailed and extended gs gi template for the whole of the last glacial period it is based on a synchronization of the ngrip grip and gisp2 ice core records that allows the parallel analysis of all three records on a common time scale the boundaries of the gs and gi periods are defined based on a combination of stable oxygen isotope ratios of the ice 18o reflecting mainly local temperature and calcium ion concentrations reflecting mainly atmospheric dust loading measured in the ice the data not only resolve the well known sequence of dansgaard oeschger events that were first defined and numbered in the ice core records more than two decades ago but also better resolve a number of short lived climatic oscillations some defined here for the first time using this revised scheme we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the last glacial period that are represented in the greenland ice records the final product constitutes an extended and better resolved greenland stratotype sequence against which other proxy records can be compared and correlated it also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations

recording teleseisms on the seafloor an example from the juan de fuca plate in 1991 during an experiment to compare low frequency seismic noise on a basaltic and a sediment covered seafloor nobs we recorded teleseisms on the juan de fuca ridge the gorda ridge and the adjacent cascadia basin with the snag ocean bottom seismometers obs these data provide an indication of the type of data that may be obtained from future experiments to record teleseisms and may be helpful in designing these experiments and analyzing the results we found that although seafloor noise is dominated by microseisms in the band 0 1 to 0 3 hz there is a well developed minimum in noise from about 0 03 to 0 1 hz the noise notch in this noise notch teleseisms can be most easily detected in the cascadia area the overall noise levels are such that only teleseismic events with magnitude greater than 6 5 were usefully recorded a magnitude 6 6 event in the new britain area 89 produced usable p and surface wave data only in this noise notch in the band 0 03 to 0 1 hz the character of compressional waves is very sensitive to water depth and the type of sensor we show that pressure sensors are especially sensitive to reverberation in the ocean and that motion sensors seismometers are less sensitive to ocean reverberations and will record teleseismic phases with less distortion than pressure sensors the cascadia data indicate enhanced p amplitudes at sites on the ridge axes that could be due to focusing caused by a low velocity lens these data suggest that amplitude information may be as or even more useful than p delay times for determining upper mantle structure

the oxygen and carbon isotopic records in holocene freshwater mollusc shells from the faiyum paleolakes egypt their paloenvironmental and paleoclimatic implications abstract stable oxygen and carbon isotopes of freshwater mollusc shells from a sequence of dated palaeolake deposits in the faiyum depression egypt provide an outline record of lake development during the holocene it is argued that the carbonate o 18 record provides a proxy for climate change and that the isotope changes are driven by variations in monsoon precipitation in nile headwaters and in local evaporation processes shells from sediments dated to ca 10 000 to 7700 cal bp had low values for oxygen and carbon isotopes indicating higher precipitation in the local area and intensified monsoonal activity leading to higher nile flood discharge and flooding of the entire faiyum depression this was followed by a transitional period characterized by marked variability but with precipitation on average less than before from ca 6000 to 4300 cal bp precipitation was decreased until a dramatic reduction in nile flood discharge occurred late in the fifth millennium bp this reduction signalled the onset of arid conditions in the faiyum partly as a consequence of changes in nile headwaters that was concomitant with declining lake levels in east africa lake level decline in the faiyum was further aggravated after 2700 bp as the nile inflow further weaken and regulation increased

depth extent of the lau back arc spreading center and its relation to subduction processes seismic tomography and wave form inversion revealed that very slow velocity anomalies 5 to 7 percent beneath the active lau spreading center extend to 100 kilometer depth and are connected to moderately slow anomalies 2 to 4 percent in the mantle wedge to 400 kilometer depth these results indicate that geodynamic systems associated with back arc spreading are related to deep processes such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab the slow regions associated with the tonga arc and the lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers suggesting that slab components of back arc magmas occur through mixing at these depths

upper crustal velocity structure in the rose area of the east pacific rise we describe a method to constrain the seismic velocity structure of the upper 500 800 m of young oceanic crust conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust the method described here requires the assumption of a linear velocity gradient and utilizes measured values of range travel time and slowness of the observed refracted waves results are reported from several refraction profiles that sample crust between 0 and 4 m y in age on the flanks of the east pacific rise at 12 n these data give an average gradient in the uppermost 800 m of the crust of 3 5 s 1 and a velocity at the seafloor of 2 5 km s ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor

millennial scale sea surface temperature changes in the eastern mediterranean nile river delta region over the last 27 000 years 1 in this study we utilize two organic geochemical proxies the u37k index and tex86 to examine past sea surface temperatures sst from a site located near the nile river delta in the eastern mediterranean em sea the u37k and tex86 records generally are in agreement and indicate sst ranges of 14 c 26 c and 14 c 28 c respectively during the last 27 cal ka during the holocene tex86 based sst estimates are usually higher than u37k based sst estimates which is likely due to seasonal differences between the timing of the haptophyte and crenarchaeota blooms in the em and is related to the onset of the modern flow regime of the nile river both records show that sst varied on centennial to millennial timescales in response to global climate events i e cooling during the last glacial maximum lgm heinrich event 1 h1 and the younger dryas yd and warming during the bolling allerod and in the early holocene during deposition of sapropel s1 the h1 cooling was particularly severe and is marked by a drop in sst of 4 5 c in comparison to pre h1 sst with temperatures 1 c cooler than during the lgm in contrast to high latitude and western mediterranean records which indicate both an abrupt onset and termination of the yd event the transition from the yd to the holocene was much more gradual in the em

on the potential of recording earthquakes for global seismic tomography by low cost autonomous instruments in the oceans 1 we describe the development and testing of an autonomous device designed to revolutionize earth structure determination via global seismic tomography by detecting earthquakes at teleseismic distances in the oceans one prototype mermaid short for mobile earthquake recording in marine areas by independent divers was constructed and tested at sea the instrument combines two readily available relatively low cost but state of the art components a sounding oceanographic lagrangian observer or solo float and an off the shelf hydrophone with custom built data logging hardware we report on the development of efficient wavelet based algorithms for the detection and discrimination of seismic events and analyze three time series of acoustic pressure collected at a depth of 700 m in pilot experiments conducted offshore san diego ca in these tests over 120 hours of data were gathered and five earthquakes of which one was teleseismic were recorded and identified quantitative estimates based on these results suggest that instruments of the mermaid type may collect up to a hundred tomographically useful teleseismic events per year the final design will also incorporate a global positioning system receiver onboard signal processing software optimized for low power chips and high throughput satellite communication equipment for telemetered data transfer with these improvements we hope to realize our vision of a global array of autonomous floating sensors for whole earth seismic tomography

composition and history of ground water in the western nile delta abstract a hydrochemical and environmental isotope investigation was conducted in the nile delta and adjacent desert fringes within the framework of a routine hydrogeological mapping project the aim of the study was to obtain additional insight into the historical groundwater flow pattern to differentiate between fossil ground water and contemporaneous recharge by nile water an inventory of existing chemical analyses was made 252 data points and an additional 27 new samples were taken which were also analysed for stable isotope 18 o and 2 h content and for nine samples 14 c and tritium in the majority of analyses irrespective of total salinity an enrichment in sodium was observed relative to the concentration resulting from direct mixing of nile water with sewater the na rich water is thought to have originated in the extreme arid period from 14000 to 10000 years b p last phase of the weichsel glaciation in which by direct evaporation of scarce rain trona minerals were formed at the surface and the ground water in phreatic aquifers gradually became saline the succeeding humid phase from 8000 to 5000 years b p when sea level was relatively high caused freshening on the desert fringes only because the delta floodplain was covered by clay the present arid phase which started at around 3500 years b p also marked the start of widespread floodplain irrigation groundwater levels on the desert fringes dropped as a result of both aquifer depletion by discharge to the qattara depression in the west and climatic effects this in turn initiated a freshening process in the quaternary and moghra aquifer of the delta nile water has since then reached wadi natrun 55 km to the west of the western floodplain border

geoacoustic scattering from seafloor features in the rose area a strong refraction branch diffraction has been observed on ocean bottom hydrophone data from the rivera ocean seismic experiment rose modeling of the interaction between seismic acoustic energy and the ocean bottom has shown that this arrival could be caused by scattering from common seafloor features seismic acoustic wave propagation was computed for models of marine seismic refraction lines using a 2d heterogeneous formulation of the elastic wave equation these models demonstrate that a significant amount of energy is diffracted from seafloor structures such as hills and valleys when both the direct water wave and the refracted compressional wave traveling through the upper crust are incident upon a structure since much of the diffracted energy travels through the crust the diffracted arrivals appear on the refraction branch of the seismograms at large ranges energy partitioning at the seafloor of the two incident wave types produce both compressional p and shear s diffracted phases in

finite difference seismic modeling of axial magma chambers we tested the feasibility of using finite difference methods to model seismic propagation at 10 hz through a two dimensional representation of an axial magma chamber with a thin liquid lid our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of p and s energy propagation the results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries we observe a high energy downward traveling shear phase observable only with borehole receivers that would be useful in studying the nature and shape of magma chambers the ability of finite difference methods to model high order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near axis drilling of the east pacific rise in order to optimize the benefits from shear wave imaging of sub axis structure

a review of finite difference methods for seismo acoustics problems at the seafloor understanding seismic wave propagation in realistic seafloor environments is essential for many problems in marine seismology finite difference methods are becoming increasingly popular in solving propagation problems as the limitations of other techniques which apply only at high frequency or for flat lying media become fully appreciated the seafloor problem a high contrast in poisson s ratio at a rough sharp interface is particularly challenging and many published formulations fail to solve it accurately the purpose of this paper is to summarize the published finite difference formulations for the elastic wave equation and to outline their applicability to seafloor problems