acoustics variability of air gun signals recorded at intermediate ranges within the lau basin 1 during january february 2009 an active source seismic survey was performed over the eastern lau spreading center in the lau back arc basin 21 s 176 s acoustic signals generated by ther v langseth s 36 gun pneumatic source array were recorded within the deep sound channel at offsets of 29 416 km the local ocean acoustic environment is everywhere bottom limited with seafloor depths within the study domain ranging from 1700 2800 m low frequency 4 125 hz sound levels are monitored using root mean square energy flux density and zero to peak measurement techniques from these field data transmission loss is found to exceed the predictions of a geometric spherical spreading model at similar ranges arrival amplitudes vary by up to 20 db and durations vary by a factor of three to six the depth of the seafloor beneath the air gun source exhibits a positive correlation with arrival duration and a negative correlation with range corrected amplitude explaining up to 30 of the observed variation in both parameters the strength of this correlation however varies for stations lying at different azimuths highlighting the importance of seafloor aspect and slope in the coupling of bottom interacting acoustic energy into the sound channel range dependent ray tracing shows that shots deployed over shallower seafloor are more likely to produce sound channel trapped signals that propagate with limited bottom interaction this results in arrivals that are more impulsive with shorter durations and higher amplitudes shots deployed in deeper water typically undergo a larger number of bounces and are characterized by more emergent longer duration and smaller amplitude arrivals

hydroacoustic events located at the intersection of the atlantis 30 n and kane 23 40 n transform faults with the mid atlantic ridge we investigate the characteristics of t phase events located at the ends of two segments of the mid atlantic ridge our motivations for the study were to understand whether t phase locations represent earthquake epicenters and thus whether accurate geological inferences can be made from their spatial patterns and to understand further the relationship between t phase event characteristics and earthquake properties we examine the characteristics of 158 t phase events with respect to both event location water depth and source to receiver distance the propagation paths of the t phases are also modeled to study the effects of encountering seafloor topography we find that existing models for t phase excitation and propagation cannot explain adequately all of our observations the amplitudes received levels of t phases at the hydrophones show no dependence on event water depths in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth the received levels are observed to decrease with increasing source to receiver distance and events from the two study areas exhibit different trends in relative received levels between hydrophones once attenuation is taken into account our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1 d topography between the event and each hydrophone but the variances in both the observed data and model are high we observe a pattern of short t phase onset times for shallow water events and long onset times for deep water events where onset time is defined as the time interval between the appearance of the t phase envelope above the ambient noise and its first peak this suggests that the onset time may be a function of several variables including efficiency of energy conversion based on local topography efficiency of propagation based on event water depth and hypocentral depth in the crust the results of this study underscore the complexity of t phase excitation and propagation and argue that current models of t phase excitation and propagation need to be improved to explain the observed characteristics of t phase data

synthesis of earthquake generated t waves t phases excited by earthquakes propagate with low transmission loss along the ocean sound channel and are characterized by long duration wavetrains often having multiple peaks associated with bathymetric highs in the source region although t waves were first identified almost fifty years ago their activation has not been adequately explained in this paper we show that for continental margin earthquakes realistic t phase coda can be computed by expressing the acoustic energy in terms of several low order modes excited by point sources distributed uniformly over the seafloor the conversion of seismic energy to acoustic energy is thus consistent with seafloor scattering

testing low very low frequency acoustic sources for basin wide propagation in the indian ocean low very low frequency acoustic signals were transmitted to distant receivers in the indian ocean the aim was to test methods for characterizing the hydroacoustic capability of the international monitoring system ims that discriminates for nuclear tests in the region several acoustic sources were deployed between seychelles and fremantle australia and the ims receivers comprised a network of hydrophones off diego garcia and australia two of the three acoustic sources tested produced basin scale propagation of impulsive signals single glass spheres imploded within the sound channel produced a clear signal at frequencies above 40 hz at ranges of hundreds to a thousand kilometers five sphere glass implosions were recorded at ranges up to 4400 km near sea surface shots from a large airgun array were recorded in several cases at ranges of hundreds to thousands of kilometers the frequency of the highest signal to noise ratio arrivals varied within the 5 100 hz band high background noise level was

geoid and topography for infinite prandtl number convection in a spherical shell geoid anomalies and surface and lower boundary topographies are calculated for numerically generated thermal convection in an infinite prandtl number boussinesq axisymmetric spherical fluid shell with constant gravity and viscosity calculations were done for heating entirely from below hfb and entirely from within hfw convective solutions were obtained for rayleigh numbers ra up to 20 times the critical ra in the hfb case and 27 times critical in the hfw case bifurcations that occur with varying ra are associated with different convective patterns boundary topography is positive at upwellings and negative at downwellings absolute topography is greater at the poles than at the equator due to poleward compression of the flow field lower boundary topography is greater than at the surface due to radial compression of the flow field the geoid is primarily dominated by the gravitational potential of surface masses due to surface boundary deformations hence the geoid parallels surface undulations in general boundary deformation increases with increasing cell wavelength this property may elucidate whether core mantle boundary cmb topography is due to mantle convection or mantle dregs chemically dense material which are only indirectly of dynamic origin and hence are unlikely to have the same wavelength dependence in its associated topography the ratios of boundary deformation and geoid to td coefficient of thermal expansion t characteristic temperature drop across the fluid shell and d shell thickness maximize with increasing ra at approximately 10 times ra critical and then steadily decrease out to the maximum ra calculated dimensionless geoid and topography in the hfb case are approximately 5 times greater than in the hfw case at the highest rayleigh numbers investigated dimensionless peak to trough amplitudes of geoid and topography are weak functions of ra which lends some justification to estimating the dimensional values of these quantities with earthlike parameters for parameter values characteristic of the earth s mantle the geoid and upper and lower boundary topograpies are approximately 1 km 10 km and 15 km for the hfb case and approximately 200 m 2 km and 3 km for the hfw case respectively the hfw values correlate with geophysical data more closely than the hfb values suggesting a predominance of internal heating in the mantle although this result is qualfied by the low rayleigh numbers and axisymmetry of the model calculations our study emphasizes that dynamically induced topography and geoid are sensitive to the mode of heating in the earth s mantle

characteristics of seismic survey pulses and the ambient soundscape in baffin bay and melville bay west greenland in 2012 a seismic survey campaign involving four vessels was conducted in baffin bay west greenland long distance 150 km pre survey acoustic modeling was performed in accordance with regulatory requirements four acoustic recorders three with hydrophones at 100 200 and 400 m depths measured ambient and anthropogenic sound during the survey additional recordings without the surveys were made from september 2013 to september 2014 the results show that 1 the soundscape of baffin bay is typical for open ocean environments and melville bay s soundscape is dominated by glacial ice noise 2 there are distinct multipath arrivals of seismic pulses 40 km from the array 3 seismic sound levels vary little as a function of depth 4 high fidelity pre survey acoustic propagation modeling produced reliable results 5 the daily sel did not exceed regulatory thresholds and were different using southall bowles ellison finneran gentry greene kastak ketten miller nachtigall richardson thomas a

three dimensional seismic array characterization study experiment and modeling in the summer of 2003 the littoral acoustic demonstration center conducted an acoustic characterization experiment for a 21 element marine seismic exploration airgun array of total volume of 0 0588m3 3590in 3 two environmental acoustic recording system buoys one with a desensitized hydrophone were deployed at a depth of 758m in a water depth of 990m near green s canyon in the gulf of mexico shots over a grid were recorded and calibrated to produce absolute broadband up to 25khz pressure time dependencies for a wide range of offsets and arrival angles in the water column experimental data are analyzed to obtain maximum received zero to peak pressure levels maximum received sound exposure levels and pressure levels in 1 3 octave frequency bands for each shot experimental data are quantitatively modeled by using an upgraded version of an underwater acoustic propagation model and seismic source modeling packages for a variety of ranges and arrival angles experimental and modeled data show good

isotopic constraints on the genesis of the rogaland anorthositic suite southwest norway demaiffe d weis d michot j and duchesne j c 1986 isotopic constraints on the genesis of the rogaland anorthositic suite southwest norway in s deutsch and a w hofmann editors isotopes in geology picciotto volume chem geol 57 167 179 isotopic data were obtained on the sveconorwegian 1200 900 ma anorthosites and associated norites and charnockites of the rogaland igneous complex sw norway the isotopic data isr 0 703 0 7077 end 0 4 to 5 5 suf 4pb 1 8 1 8 2 so 5 2 to 6 6 c suggest an upper mantle origin for the parental magma of the anorthosites and related norites and jotunites or an origin in the lower crust by melting of mantle derived basic rocks shortly after their formation in two well documented cases the egersund ogna and the hidra bodies a progressive contamination by u depleted but not rb depleted lower crustal material is apparent during the differentiation process the late stage liquids have higher i up to 0 7086 lower but still slightly positive end less radiogenic pb isotopic composition the intrusive acidic rocks some of them of the charnockitic type display variable isotopic data is 0 7055 0 709 end 0 5 to 0 8 pb isotopic compositions comparable to that of some of the surrounding gneisses suggesting that there are in general no direct cogenetic relations between the anorthosites and the charnockites our data point to an origin of some of these acidic rocks by anatectic melting of the surrounding high grade metamorphic gneisses

chaotic axisymmetrical spherical convection and large scale mantle circulation abstract this paper presents a study of high rayleigh number up to 200 times supercritical axisymmetrical convection in a spherical shell with an aspect ratio relevant for the earth s lower mantle both bottom heated and internal heated cases have been considered computations have been carried out for an infinite prandtl number isoviscous fluid with free slip isothermal boundary conditions the first part of the paper is devoted to the influence of the resolution on the accuracy of the numerical results it is shown that the resolution strongly influences the onset of time dependence recent methods of non linear physics have been used to prove that the time dependence and the chaotic behaviors of the solutions are real ones from these results we can confirm that convection is chaotic in this particular geometry even for rayleigh numbers 200 times critical aperiodic boundary layer instabilities are found to be incapable of breaking up the large scale flow owing to the shear of the global circulation spectral analysis of the power associated with the thermal anomalies shows that there is an upward cascade of energy due to small scale chaotic instabilities from l 2 to l 4 6 at the bottom boundary in agreement with new seismic observations at the core mantle boundary 1 3

tracking submarine volcanic activity at monowai constraints from long range hydroacoustic measurements monowai is a submarine volcanic center in the kermadec arc southwest pacific ocean in the past activity at the volcano had been intermittently observed in the form of fallout at the sea surface discolored water changes in seafloor topography and t phase seismicity but there is no continuous record for more recent years in this study we investigated 3 5 years of recordings at a hydrophone array of the international monitoring system ims located near juan fernandez islands for long range underwater sound waves from monowai results from direction of arrival calculations and density based spatial clustering indicate that 82 discrete episodes of activity occurred between july 2003 and march 2004 and from april 2014 to january 2017 volcanic episodes are typically spaced days to weeks apart range from hours to days in length and amount to a cumulative sum of 137 days of arrivals in total making monowai one of the most active submarine arc volcanoes on earth the resolution of the hydrophone recordings surpasses broadband network data by at least one order of magnitude identifying seismic events as low as 2 2 mb in the kermadec arc region further observations suggest volcanic activity at a location approximately 400 km north of monowai in the tonga arc and at healy or brothers volcano in the southern kermadec arc our findings are consistent with previous studies and highlight the exceptional capabilities of the ims network for the scientific study of active volcanism in the global ocean supporting information

low frequency whale and seismic airgun sounds recorded in the mid atlantic ocean beginning in february 1999 an array of six autonomous hydrophones was moored near the mid atlantic ridge 35 n 15 n 50 w 33 w two years of data were reviewed for whale vocalizations by visually examining spectrograms four distinct sounds were detected that are believed to be of biological origin 1 a two part low frequency moan at roughly 18 hz lasting 25 s which has previously been attributed to blue whales balaenoptera musculus 2 series of short pulses approximately 18 s apart centered at 22 hz which are likely produced by fin whales b physalus 3 series of short pulsive sounds at 30 hz and above and approximately 1 s apart that resemble sounds attributed to minke whales b acutorostrata and 4 downswept pulsive sounds above 30 hz that are likely from baleen whales vocalizations were detected most often in the winter and blue and fin whale sounds were detected most often on the northern hydrophones sounds from seismic airguns were recorded frequently particularly during

time dependent models of single and double layer mantle convection one outstanding problem of great geophysical importance is the vertical extent of convection in the earth s mantle1 steady state models of convection in the mantle have yet to produce a description of the mantle which is consistent with geochemical and geophysical observations here we present time dependent numerical models of two dimensional convection which simulate mantle convection starting from a fluid initially at rest with a purely conductive temperature profile we now find that for a rayleigh number ra of 107 the model experiences a transient period of double layer convection which lasts on the order of hundreds of millions of years when scaled to the earth s mantle it is suggested that transient periods of double layer convection in the earth s mantle may have existed in the past whether or not such a period exists today

a comparison between the generalized digital environmental model and levitus climatologies two ocean climatologies of temperature and salinity the generalized digital environmental model gdem and the climatological atlas of the world ocean are compared dynamic height fields are computed by season from each climatology for the north atlantic north pacific and indian oceans and are compared on a 1 latitude longitude grid large scale oceanographic features are generally found to be similarly represented in both climatologies gdem appears to render better representations of seasonal variability and regions of high current shear such as the gulf stream because of a different smoothing method and a finer grid spacing maps of dynamic heights from both climatologies are presented and their similarities and differences are discussed the methodology for the construction of gdem is also described in detail

time dependent convection in the earth s mantle abstract the range of rayleigh numbers accessible to 2 dimensional numerical models of mantle convection has been increased by almost two orders of magnitude through the use of high resolution numerical grids with up to 200 intervals in each of the horizontal and vertical directions the range of rayleigh numbers studied is believed to encompass conditions which would have existed in a hot archaean mantle although the models are highly idealized in that they neglect the compressibility and variations of viscosity in the earth s mantle they suggest that during the archaean and earlier boundary layer instabilities developed more frequently than at present and did so with a scale and separation comparable to the thickness of the thin thermal boundary layers rather than to the depth of the convecting layer itself the rising and sinking plumes which develop from these instabilities drive transient flows which nevertheless have a horizontal scale comparable to the depth of the convecting layer thus although surface motions during the archaean may have been more variable than at present in terms of speed and direction they might have occurred on a horizontal scale which is similar to that of present day plate motion the numerical models indicate that motions at the upper surface of time dependent convection cells are 1 predominantly controlled by transient sinking plumes and 2 converge directly towards the source region of the sinking plumes on the other hand rising plumes have a more diffuse influence on the upper surface and surface divergence does not always occur immediately above a rising plume if oceanic plates on earth constitute the upper surface of mantle wide convection cells the more pronounced geophysical signature of subduction zones compared to oceanic ridges may be simply due to the fact that buoyancy is most concentrated at the earth s surface in the form of sinking plumes the total surface heat flow is found to fluctuate significantly in the time dependent convection models since estimates of the present day heat loss from the earth s interior are at best instantaneous measurements it is possible that these estimates are not representative in a time averaged sense of the present thermal state of the planetary interior

on the mechanism of conversion of seismic waves to and from t waves in the vicinity of island shores high frequency seismic records from the polynesian seismic network are used to investigate in detail the processes of conversion of acoustic t wave energy from and to seismic waves at island shores on the source side we study the seismic to acoustic conversion based on t phases from hawaiian events recorded at polynesian stations located on the coral platter on the receiver side we study the acoustic to seismic conversion based on t phases from marine sources recorded across the polynesian islands in both instances our results underline the importance of steep slopes typically 50 in allowing an efficient conversion between p waves in the island structure and t waves in the water column these slopes can be coral reefs or the heads of young presumably unconsolidated basalt flows under this geometry modeling based on raytracing indicates that the seismic record of the t phase consists of a p wave at distances from the conversion point greater than 9 km at shorter distances s waves and surface waves are generated in the absence of a steep slope only surface waves are present these models can be used to compute precise and predictable travel time station corrections allowing the use of seismometers located inland to exploit the superb detection and location capabilities of t waves

fine scale structure of the mid mantle characterised by global stacks of pp precursors abstract subduction zones are likely a major source of compositional heterogeneities in the mantle which may preserve a record of the subduction history and mantle convection processes the fine scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals in this study we analyse precursors to pp by creating stacks recorded at globally distributed stations we create stacks aligned on the pp arrival in 5 distance bins with range 70 120 from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records as the energy trailing the direct p arrival the p coda interferes with the pp precursors we suppress the p coda by subtracting a best fitting exponential curve to this energy the resultant stacks show that pp precursors related to scattering from heterogeneities in the mantle are present for all distances lateral variations are explored by producing two regional stacks across the atlantic and pacific hemispheres but we find only negligible differences in the precursory signature between these two regions the similarity of these two regions suggests that well mixed subducted material can survive at upper and mid mantle depth to describe the scattered wavefield in the mantle we compare the global stacks to synthetic seismograms generated using a monte carlo phonon scattering technique we propose a best fitting layered heterogeneity model brt2017 characterised by a three layer mantle with a background heterogeneity strength 0 8 and a depth interval of increased heterogeneity strength 1 between 1000 km and 1800 km the scalelength of heterogeneity is found to be 8 km throughout the mantle since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust the detection of increased heterogeneity at mid mantle depths could be associated with stalled slabs due to increases in viscosity supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of 1000 km

airgun inter pulse noise field during a seismic survey in an arctic ultra shallow marine environment offshore oil and gas exploration using seismic airguns generates intense underwater pulses that could cause marine mammal hearing impairment and or behavioral disturbances however few studies have investigated the resulting multipath propagation and reverberation from airgun pulses this research uses continuous acoustic recordings collected in the arctic during a low level open water shallow marine seismic survey to measure noise levels between airgun pulses two methods were used to quantify noise levels during these inter pulse intervals the first based on calculating the root mean square sound pressure level in various sub intervals is referred to as the increment computation method and the second which employs the hilbert transform to calculate instantaneous acoustic amplitudes is referred to as the hilbert transform method analyses using both methods yield similar results showing that the inter pulse sound field exceeds ambient noise levels by as much as 9 db during relatively quiet conditions inter pulse noise levels are also related to the source distance probably due to the higher reverberant conditions of the very shallow water environment these methods can be used to quantify acoustic environment impacts from anthropogenic transient noises e g seismic pulses impact pile driving and sonar pings and to address potential acoustic masking affecting marine mammals

sr nd and pb isotope geochemistry of the oslo rift igneous province southeast norway abstract sr nd and pb isotopic compositions are presented for a series of 290 to 250 ma old basaltic to granitic rocks from the oslo rift southeast norway a large group of basalts larvikites and rhomb porphyry lavas cluster within a restricted range of initial isotope ratios tsr 3 to 16 tnd 3 3 to 4 2 206 pb 204 pb i 18 9 to 19 3 207 pb 204 pb i 15 59 to 15 66 208 pb 204 pb i 38 6 to 39 1 the remainder of the rocks trend towards higher initial sr and lower initial nd and pb isotopic ratios data are tested against mantle crust mixing models the oslo rift magmatic rocks originated from a somewhat heterogeneous mildly depleted source located in the subcrustal lithosphere the isotopic character of this source has been inherited by the rocks showing uniform isotopic ratios which have thus not suffered significant crustal contamination the rest of the mafic to intermediate rocks and the syenites were contaminated in the intermediate to lower crust which increased slightly their sr but decreased their nd and pb isotopic ratios as much as 40 50 contamination is suggested for some of the samples strong increases in radiogenic sr seen primarily in the granites are attributed to contamination in the upper crust the mildly depleted mantle proposed as the source for the oslo rift mafic rocks has an isotopic character which can only be explained in terms of a multistage history which included periods of relatively high u pb and th pb ratios resulting from a relatively short depletion history in the subcontinental lithosphere and metasomatic enrichment of the mantle in selected elements

mantle convection with spherical effects results of a similarity theory for spherical mantle convection are presented the single mode mean field equations are analyzed for convection which is so vigorous that temperature disturbances become localized in thin thermal boundary layers our purpose is to study effects of spherical geometry density interfaces heat source distribution and cell size steady state solutions are found for isoviscous spherical shells in which the field of motion is spatially periodic in a single spherical harmonic degree calculations are carried out over the range 2 l 40 and for various fractions of internal versus base heating three configurations are examined 1 convection in a single layer of cells extending through the whole mantle 2 convection in two layers separated by a density interface at 670 km depth and 3 convection in a single layer terminating at 670 km results of these calculations are used to give estimates of surface horizontal velocities in terms of the heat loss viscosity stratification amount of internal heating and depth of circulation the surface velocity is most strongly affected by the thickness of the convecting shell deep mantle convection can achieve surface velocities which agree with observed plate speeds while convection restricted to the upper mantle does not at least on the scale of the major plates the temperature distribution is strongly affected by the spherical geometry and by the presence of density interfaces the principal difference between convection in one and two layers is that the latter produces a hot lower mantle while the former produces a warm one

compressible convection with constant and variable viscosity the effect on slab formation geoid and topography two dimensional cartesian finite difference models of compressible convection with constant and variable viscosity and fixed bottom temperature are presented density variations according to the adams williamson equation of state are included in the case of constant viscosity convection viscous and adiabatic heating damp the flow compressible density variations hardly influence geoid undulations however the lower thermal boundary layer becomes thinner and the mean cell temperature increases in the case of variable viscosity the nonlinear coupling between compression adiabatic and viscous heating and a temperature pressure and stress dependent rheology leads to important consequences the upwelling flow broadens and plumes are retarded the flow strongly concentrates toward the downwelling region and becomes mechanically decoupled from the interior of the cell by a low viscosity region this mechanism seems to be important for the formation of subducting slabs extrapolated to mantle conditions two low viscosity regions are predicted flanking the slab on either side and inhibiting an early dispersal and mixing of slab material into the mantle this process might be aided by an increase of negative buoyancy forces with depth as observed in the models further results are as follows increasing the dissipation number in variable viscosity convection may either damp or speed up convection depending on the rheology models with internal heating and a fixed bottom temperature show that the threshold to time dependent variable viscosity convection is drastically reduced if the anelastic liquid approximation is applied instead of the extended boussinesq approximation

shallow seismicity and tectonics of the central and northern lau basin abstract the lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time as such crustal earthquakes within the lau basin derive from intrabasin tectonic volcanic and hydrothermal sources because of high mantle attenuation small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity however the 1994 labatts ocean bottom seismograph experiment in the lau basin recorded more than 100 000 local t and body phases allowing for a detailed examination of basin seismotectonics nearly 1000 events are located within the basin proper with another 2000 associated with the forearc and the aseismic front we identify two previously undocumented tectonic features a triple junction in the northwestern basin and a nanoplate in the central lau basin the single most seismically active feature within the lau basin is the lau extensional transform zone letz just north of the central lau spreading center significant seismicity also occurs near the northern extension of the eastern lau spreading center and along a southeast extension of the letz delineating the boundary of a previously unknown nanoplate swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics we see no evidence of deformation along the southern boundary of the niuafo ou microplate where 4 5 cm yr of strain is predicted it is possible that the plate boundary extends eastward from the fonualei rift fr tip towards the trench creating a niuatoputapu tonga plate division rather than westward implied by the currently accepted niuafo ou tonga plate system but more data are necessary from the fr to test that hypothesis instead we suggest that the niuafo ou tonga pole is 1 5 further north than previously proposed reducing the predicted strain in this region to

implication for mantle flow from the hawaiian emperor chain abstract assuming that rayleigh benard flow occurs within the upper mantle ascending plumes must undergo horizontal displacement during ascent that can be traced at the surface it is shown that the hawaiian emperor bend cannot be formed if the convection rate is of the same order of magnitude as the ascent rate of the plumes the plume patterns obtained by push pull or lithospheric sliding are similar to that of the hawaiian emperor chain hence the present results support the sliding model

viscosity of the lower mantle as inferred from rotational data until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters such as those associated with the surface loading in this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth s rotation axis with theoretical results from a layered viscoelastic rotating earth which has been subjected to glacial forcing our model consisting of an elastic lithosphere a two layer adiabatically stratified viscoelastic mantle and an inviscid core is essentially analytical and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciation phenomenon both sets of rotational data polar wander and nontidal deceleration of the length of the day have been employed to constrain the viscosity structure on the basis of this type of investigation the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1 and 4 1022 p our results which just rely on the second degree harmonic of the strain field also lead to a determination of an upper bound to the thickness of the globally averaged lithosphere this value ranging between 130 and 190 km suggests that strong lateral heterogeneities between oceanic and continental plates may extend a few hundred kilometers into the upper mantle

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

excitation of t phases by seafloor scattering t phases excited by suboceanic earthquakes are classified into two types abyssal phases which are excited near the earthquake epicenter at seafloor depths far below the sofar velocity channel and slope t phases which are excited at continental or ocean island slopes and ridges at distances up to several hundreds of kilometers from the epicenter in this article it is demonstrated that approximate time frequency characteristics of both classes of t phase can be synthesized under the assumption that t phases are excited by scattering from a rough seafloor seafloor scattering at shallow depths preferentially excites low order acoustic modes that propagate efficiently within the ocean sound channel minimum at greater depths scattering excites higher order modes which interact weakly with the seafloor along much of the propagation path using known variations in near source bathymetry t phase envelopes are synthesized at several frequencies for several events south of the fox islands that excited both

the petrogenesis of massif anorthosites a nd and sr isotopic investigation of the proterozoic of rogaland vest agder sw norway sm nd and rb sr isotopic analyses of charnockitic migmatite augen gneiss anorthosite leuconorite and two acid plutons from the rogaland and vest agder districts of southwest norway constrain their crustal residence ages origin and evolution the charnockitic migmatites which are a major component of the metamorphic basement complex represent the oldest and largest episode of accretion in which new crust was derived 1 5 1 9 ga ago from a mantle source of depleted nd isotopic composition the basement complex was intruded by a number of large anorthositic to granitic plutons during and after the sveconorwegian orogenic period samples from the ca 1050 ma old synorogenic haland anorthosite leuconorite massif exhibit substantial variation of initial nd of 2 1 to 4 4 at an anorthosite locality and 0 5 to 2 3 at a leuconorite locality but display significant variation of initial 87sr 86sr ratio only between the localities anorthosite mean 0 70369 leuconorite mean 0 70560 a model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of and fractional crystallization from a picritic magma derived from isotopically depleted mantle two younger acid intrusions the 950 ma old lyngdal granodiorite and the 930 ma old farsund charnockite both have initial sr and nd isotope ratios consistent with massive contamination of depleted mantle derived magma by old continental crustal material

characteristics of marine seismic survey sounds in the beaufort sea geophysical survey vessels emit very intense pulses of underwater sound at intervals of several seconds these noise pulses are sometimes detectable at very long ranges sound pulses from geophysical surveys were recorded in shallow parts of the beaufort sea during august and september of 1980 1984 hydrophones deployed from a quiet boat were the primary sensors but aircraft deployed sonobuoys were also used water depths for the boat recorded data were 9 130 m but mostly 4 km best fit equations for maximum rms received levels in db generally were obtained by assuming cylindrical spreading 10 log range and a term linear in range corresponding to an absorption loss or scattering term the coefficients of the linear term va

palaeocontinental configurations and geoid anomalies reconstructions of past continental configurations help our understanding of the evolution of planetary surface morphology and impose valuable constraints on the models for planetary dynamics the problem is that although well established palaeogeographical maps now exist for mesozoic and later times1 2 thanks to excellent agreement between palaeomagnetic and marine magnetic deductions this is not true for earlier times mainly due to the lack of independent palaeogeophysical control over otherwise longitudinally ambiguous palaeomagnetic data the prospect of relating palaeocontinental configurations to geoid anomalies recently raised by anderson3 has thus given a fresh perspective to this problem here his continental insulation model is shown to be an inadequate explanation of the undulations of subduction compensated geoid instead the geoid highs and lows are shown to correlate well with the overall divergent and convergent plate tectonism since the middle palaeozoic and are thus ascribed respectively to the ascending and descending patterns of lower mantle convection

genetic implications of pb isotopic geochemistry in the rogaland anorthositic complex southwest norway abstract 110 samples from different massifs of the rogaland igneous complex have been analyzed for whole rock pb isotopic compositions these data show no evidence of rocks older than 1500 ma this is probably the time of extraction of the magmatic precursors from the mantle the pb data indicate a mantle origin for the parental magmas of the massiftype anorthosites and of the jotunitic series nevertheless an origin by partial melting of mantle derived basic material in the lower crust cannot be excluded especially for the latter the charnockitic rocks associated with the anorthosites could either be anatectic melts of the enclosing gneisses or residual liquids of anorthosite differentiation contaminated by slightly u depleted lower crustal material although in some cases upper crustal material has also contributed to the contamination

deformation of the core mantle boundary induced by spherical shell compressible convection we derive the single mode mean field equations for spherical shell convection with variable viscosity and compressibility included core mantle boundary cmb topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at cmb results for long wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters in order to obtain excess ellipticity of about 1 km the activation energy and volume in the lower mantle should be low and the temperatures at cmb around 4 500 k in accord with recent experimental findings

surface deformation and geoid anomalies over single and double layered convective systems using a primitive variable formulation of the finite element method we have compared the differences in the surface observables such as topography and geoid produced by single and double layered thermal convection both constant and depth dependent viscosities have been considered for the same rayleigh number larger surface perturbations are produced by single cell convection for the same nusselt number the magnitudes of the surface observables are greater for double layered convection for the same surface heat flux surface topographies have similar magnitudes but the relative amplitudes of geoid anomalies depend greatly on the style of viscosity stratification this difference in the geoid between the two systems increases with greater surface heat flow regardless of viscosity structure

age relationships in the proterozoic high grade gneiss regions of southern norway discussion and comment discussion sur l article de field d et raheim a paru dans precambrian res nld 1981 14 261 275

improving epicentral and magnitude estimation of earthquakes from t phases by considering the excitation function a standard technique for locating events with t phases is to pick the peak energy of t phases as the arrival time then proceed as if it was an unscattered phase originating at the epicenter the peak energy arrival time however can shift to different parts of the wave train due to incoherent scattering we show that a 50 reduction in variance relative to picks of peak arrival times can be achieved by fitting an assumed functional shape to the log of the entire envelope of the t phase we test the stability of this approach by comparing relative event locations based on this method with those determined by cross correlating waveforms of rayleigh and love surface waves recorded teleseismically using a swarm of earthquakes at the northern end of the easter microplate as an example relative event locations show that there is no systematic bias in t phase locations the t phase location and detection can be extended to much smaller events than are detectable by surface waves we estimate t phase maximum amplitudes of about 50 events within the swarm from the amplitude of the fitted functions rather than directly from the seismograms twenty four of these events are large enough to estimate their magnitudes by surface wave analysis the empirical analysis shows the log of t phase maximum amplitude and surface wave magnitude ms exhibit a relatively uniform linear relationship with much less scatter than in previously published studies of t phase amplitude versus magnitude the reduction in scatter is due to both the stability of the functional estimate of amplitude and the use of a swarm of earthquakes with similar depth mechanism and source receiver geometry the observed t phase coda for shallow earthquakes can be synthesized using a simple model of multiple reverberation seafloor scattering the results show that scattering of energy at a rough seafloor from multiple reverberations in the water column is significant in t phase generation even where the water depths are relatively uniform

petrogenesis of monzonoritic dykes in the egersund ogna anorthosite rogaland s w norway trace elements and isotopic sr pb constraints two major monzonoritic dykes occur in the egersund ogna anorthositic massif s w norway the lomland dyke which varies from norite to monzonite and the vettaland ferronoritic dyke they are characterized by high fe ti and p contents low sio2 variable k2o 0 5 4 5 and high fe mg ratios small variations in ree distribution are observed inside the lomland dyke la yb 12 lree ca 150 180 chondrite normalized values neutral to slightly positive eu anomaly part of the vettaland dyke is severely depleted in zr rb and ree and shows a positive eu anomaly all rocks are depleted in u and th and show very low cr and ni contents as well as high sr contents 400 600 ppm variation within the lomland dyke is satisfactorily ex plained through subtraction of an apatite bearing noritic cumulate the role of apatite is predominant in controlling the ree behaviour eutectic partial melting of a fe rich noritic cumulate containing apatite under low ph2o conditions is suggested for the vettaland dyke by ree modelling sr isotopic initial ratios are different in lomland ca 0 708 and vettaland ca 0 706 dykes these values preclude direct derivation from the mantle as well as comagmatic relationship with anorthosite contamination of a mantle derived magma by deep crustal material or anatexis of the lil depleted lower crust is compatible with the low u and th contents and with the pb isotopic ratios the vettaland partial melting process is extended to all monzonoritic types the necessary heat being provided by the high temperature anorthositic crystal mush

convection in a two layer mantle with a strongly temperature dependent viscosity recent geochemical and isotopic data support the hypothesis that the earth s mantle convects in two separate layers with an interface at the 650 km seismic discontinuity this paper studies the implications of this hypothesis with regard to the thermal boundary layers that are expected to develop adjacent to the interface between the convecting layers we are particularly concerned with a possible decrease in the mantle viscosity across this interface in order to study the structure of the thermal boundary layers in a fluid with a strongly temperature dependent viscosity we utilize an approximate method developed by howard 1966 if the upper and lower mantle have the same viscosity law we find that stratified convection requires a lower mantle viscosity which is several orders of magnitude lower than the upper mantle viscosity this is not consistent with studies of postglacial rebound it is possible to obtain a viscosity law for the lower mantle that gives a viscosity nearly equal to that of the upper mantle and reasonable temperatures however the required law is not consistent with the currently available experimental data on lower mantle materials therefore we conclude that thermal arguments favor whole mantle convection

numerical models of thermally and mechanically coupled two layer convection of highly viscous fluids summary numerical solutions for thermal convection of two superposed viscous fluid layers with the same thicknesses as the earth s upper and lower mantles and separated by a fixed horizontal interface are obtained at the onset of convection and in the nonlinear regime several heating configurations and thermal boundary conditions are considered including an isothermal bottom boundary a prescribed heat flux bottom boundary and internal heating with an insulated lower boundary at the onset of convection and with the same material properties in each layer the lower layer is more unstable than the upper layer and long horizontal wavelengths proportional to the lower layer thickness predominate for layers with sufficiently different material properties the upper layer can be more unstable and short horizontal wavelengths proportional to the upper layer thickness can predominate at the onset of convection when the material properties are chosen to make the two layers equally unstable there are two stability curve rayleigh number ra versus wavelength minima with the lower minimum at long wavelength linear stability results typify weakly nonlinear convection except when both layers are equally unstable when the layers have unequal stability the stable layer has a conductive temperature field and its flow is viscously driven by the convective flow in the unstable layer when the two layers are equally unstable both layers are separately convecting even at rayleigh numbers of only twice the critical value for convection onset at large enough supercritical ra the lower layer convects with a long wavelength cell even when linear stability theory predicts short wavelength cells the upper layer always begins convecting with short wavelength cells however the upper layer cell over the lower layer downwelling plume increases in size with increasing ra thus horizontal wavelength plate dimension for example cannot be used to discriminate between whole mantle and layered mantle convection mechanical coupling of the layers downwelling plume over upwelling plume occurs for all cases with the same material properties in each layer and for the case with the same ra in each layer when the material properties of the layers are sufficiently different to produce short wavelength cells at the onset of convection the upper layer has two thermally coupled cells downwelling plume over downwelling plume and a shear zone at the interface over both lower layer plumes a large 50 per cent of total temperature difference across the interface occurs in all cases examined implying that two layer mantle convection should also have a large temperature change across the 670 km discontinuity however temperature dependent viscosity may reduce the magnitude of this temperature difference interpretations of geoid anomalies over subduction zones and seismic traveltime anomalies of slabs should account for possible thermal coupling of separate upper and lower mantle convective systems

contrasting crustal production and rapid mantle transitions beneath back arc ridges the eastern lau spreading centre located on the lau back arc basin hydrothermal vent system between fiji and tonga is a focus site of the ridge 2000 research programme of the us national science foundation the first large scale seismic study of the area reveals large and abrupt across and along strike changes in crustal properties implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes the authors conclude that stable broad triangular upwelling regions as inferred for mid ocean ridges cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which the material is rapidly released resulting in rapid changes in the character of the crust formed at the ridge this study shows that across strike and along strike changes in crustal properties at the eastern lau spreading centre are large and abrupt implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes it is concluded that stable broad triangular upwelling regions as inferred for mid ocean ridges cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which that material is rapidly released resulting in rapid changes in the character of the crust formed at the ridge the opening of back arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading1 during this process the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced with high volatile contents derived from the subducting plate 2 3 4 5 current models4 6 7 8 predict gradational rather than abrupt changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction influenced mantle in contrast here we show that across strike and along strike changes in crustal properties at the eastern lau spreading centre are large and abrupt implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes with incremental separation of the ridge axis from the volcanic front of as little as 5 km seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills upper crustal seismic velocities abruptly increase by over 20 and gravity anomalies and isostasy indicate crustal thinning of more than 1 9 km we infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge such that stable broad triangular upwelling regions as inferred for mid ocean ridges9 10 cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which that material is rapidly released from the upwelling zone resulting in rapid changes in the character of the crust formed at the ridge

the 2009 samoa tonga great earthquake triggered doublet great earthquakes having seismic magnitudes of at least 8 usually involve abrupt sliding of rock masses at a boundary between tectonic plates such interplate ruptures produce dynamic and static stress changes that can activate nearby intraplate aftershocks as is commonly observed in the trench slope region seaward of a great subduction zone thrust event1 the earthquake sequence addressed here involves a rare instance in which a great trench slope intraplate earthquake triggered extensive interplate faulting reversing the typical pattern and broadly expanding the seismic and tsunami hazard on 29 september 2009 within two minutes of the initiation of a normal faulting event with moment magnitude 8 1 in the outer trench slope at the northern end of the tonga subduction zone two major interplate underthrusting subevents both with moment magnitude 7 8 with total moment equal to a second great earthquake of moment magnitude 8 0 ruptured the nearby subduction zone megathrust the collective faulting produced tsunami waves with localized regions of about 12 metres run up that claimed 192 lives in samoa american samoa and tonga overlap of the seismic signals obscured the fact that distinct faults separated by more than 50 km had ruptured with different geometries with the triggered thrust faulting only being revealed by detailed seismic wave analyses extensive interplate and intraplate aftershock activity was activated over a large region of the northern tonga subduction zone

acoustic detection of a seafloor spreading episode on the juan de fuca ridge using military hydrophone arrays until recently no practical method has been available to continuously monitor seismicity of seafloor spreading centers the availability of the u s navy s sound surveillance system sosus for environmental research has allowed the continuous monitoring of low level seismicity of the juan de fuca ridge in the northeast pacific on june 22 1993 noaa installed a prototype system at u s naval facility whidbey island to allow real time acoustic monitoring of the juan de fuca ridge using sosus on june 26 2145 gmt a burst of low level seismic activity with accompanying harmonic tremor was observed and subsequently located near 46 15 n 129 53 w on the spreading axis of the juan de fuca ridge over the following 2 days the activity migrated to the nne along the spreading axis with the final locus of activity near 46 31 5 n 129 35 w the nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor based on this interpretation a response effort was initiated by u s and canadian research vessels and both warm water plumes and fresh lavas were subsequently identified at the site

the gulf stream and its frontal structure a quantitative representation abstract the structure of the gulf stream current is associated with the quasi permanent density front in the western north atlantic the lighter mass of warmer but saltier water of the sargasso sea is separated from the slope water by inclined isopycnals that form the front recent satellite altimeter measurements have also revealed a well defined sea surface height change across the front in this paper a model of the gulf stream cross sectional density and current structure is presented using the complete dynamical and mass conservation equations the model postulates a forcing at the interior ocean boundary by a cross stream ageostrophic circulation with inflow of light water in the upper ocean and a return flow at greater depths the model gulf stream is found to develop after initial geostrophic adjustment of several inertial periods in the quasi steady state the normalized structural results constitute a single representation of the structure of all gulf stream sections i e all sections ar

modal scattering a key to understanding oceanic t waves the excitation mechanism of oceanic t waves has been a puzzle for almost fifty years with refraction from a sloping seafloor and seafloor scattering as two of the most commonly invoked mechanisms by representing the earthquake source field as a normal mode sum it can be seen that both mechanisms are very closely related strict modal orthogonality prohibits the existence of t waves in a laterally homogeneous semi infinite half space or radially symmetric sphere as energy cannot be transferred from one mode to another in an homogeneous medium deterministic non planar bathymetry random boundary roughness upper crustal heterogeneity or a combination of these provides a physical mechanism to break the strict orthogonality we show that modal scattering from the rough seabottom in the epicentral region converts energy from the directly excited ocean crustal water column modes to the propagating acoustic modes comprising the oceanic t wave submarine earthquake fault orientation also appears to be reflected in the t wave excitation

satellite observations of a surtseyan eruption hunga ha apai tonga abstract on march 17 2009 a surtseyan eruption occurred around hunga ha apai island tonga a series of observations from the high spatial resolution advanced spaceborne thermal emission and reflection radiometer aster and the high temporal resolution moderate resolution imaging spectroradiometer modis was used to estimate the magnitude location start time and duration of the eruption and measure the evolving characteristics of the new hunga ha apai island the eruption start time was estimated to be between 01 50 and 11 10 local time on march 17 2009 i e between 12 50 and 22 10 utc march 16 the initial explosive phase lasted 3 5 days and consisted of multiple steam and tephra explosions from two distinct vent sources one on the northwest side and another about 100 m off the south shore of the pre existing island the eruption plume reached 4 0 to 7 6 km altitude above sea level and tephra added new land around each of the vents initially tripling the area of the pre existing island the next phase of steaming from newly formed crater lakes around the vents lasted a few days three warm crater lakes formed initially but disappeared with time as the shoreline eroded after 2 months vegetation that was initially buried by tephra was recovering after 10 months the size of the island had eroded down to twice that of the pre existing island and the one remaining crater lake had a temperature of 68 17 c the volume of erupted material was estimated to be 0 0176 km3 and the volcanic explosivity index vei was estimated to be vei 2

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

effects of compressibility on the temperature jump at the interface of layered spherical shell convection large temperature jumps at the interface of layered convection are important to the argument used against the likelihood of separate circulations in the upper and lower mantles we study this problem within the framework of a compressible constant viscosity spherical shell model both mechanical and thermal coupling configurations are considered although the temperature jumps are reduced by compressibility their magnitudes remain quite large 0 10 c in the case of mechanical coupling for thermal coupling the temperature jumps become smaller but still are substantial between 500 to 1000 c in layered spherical shell convection flows in the lower mantle are several times greater than the surface velocities

acoustic generation of underwater cavities comparing modeled and measured acoustic signals generated by seismic air gun arrays underwater vapor cavities can be generated by acoustic stimulation when the acoustic signals from several air guns are reflected from the sea surface the pressure drop at some locations is sufficient for cavity growth and subsequent collapse in this paper the generation of multiple water vapor cavities and their collapses are numerically modeled and the results are validated by comparing with field data from a seismic air gun array test in a first modeling attempt where cavity interaction is neglected a correspondence between measured and modeled data is found then this correspondence is improved by assuming that the acoustic signal generated by the other cavities changes the hydrostatic pressure surrounding each cavity this modeling can be used to estimate the amount and strength of high frequency signals generated by typical marine air gun arrays given that a calibration step is performed prior to the modeling

pb isotope geochemistry of a massif type anorthositic charnockitic body the hidra massif rogaland s w norway abstract the hidra massif rogaland complex s w norway is a massif type anorthositic charnockitic body it consists of undeformed anorthosites and leuconorites grading into fine grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope a stockwork of charnockitic dykes cross cuts the massif the pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites 206 pb 204 pb from 18 079 to 19 307 207 pb 204 pb from 15 568 to 15 657 and 208 pb 204 pb from 37 617 to 38 493 these values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates but show the incorporation of lower crustal material u depleted and thus less radiogenic the charnockitic dykes have significantly less radiogenic pb isotopic compositions 206 pb 204 pb from 17 472 to 19 171 207 pb 204 pb from 15 489 to 15 620and 208 pb 204 pb from 36 991 to 40 922 which can be explained by a larger proportion of lower crustal contamination material the contaminant could be the granulite facies gneisses of the metamorphic envelope this interpretation is compatible with the k rb relationships of these rocks and with the o and sr isotopic geochemistry the proportion of contaminating lead in the charnockitic dykes can be estimated at 55 15 considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant

underwater mach wave radiation from impact pile driving theory and observation the underwater noise from impact pile driving is studied using a finite element model for the sound generation and parabolic equation model for propagation results are compared with measurements using a vertical line array deployed at a marine construction site in puget sound it is shown that the dominant underwater noise from impact driving is from the mach wave associated with the radial expansion of the pile that propagates down the pile after impact at supersonic speed the predictions of vertical arrival angle associated with the mach cone peak pressure level as function of depth and dominant features of the pressure time series compare well with corresponding field observations

age relationships in the proterozoic high grade gneiss regions of southern norway abstract the proterozoic high grade gneiss regions of southern norway bamble kongsberg rogaland and ostfold historically belong to the so called sveconorwegian 1 2 0 9 ga zone this has long been correlated with the grenville province of north america and features prominently in proterozoic reconstructions across the north atlantic several recent radiometric studies have provided compelling evidence for an earlier high grade gneiss forming event at 1 6 1 5 ga ago in parts of southern norway it has also been shown that at least a section of the bamble sector suffered only a relatively low grade thermal deformational event during the sveconorwegian cycle in this paper new rb sr age determinations for metasediments and an anatectic granite from the same area confirm a pre grenvillian age for the high p t event and preclude the possibility of later high grade reworking previously published age determinations for all sectors are compared with the available radiometric data from the adjoining gneissic terrane of southwest sweden and reviewed in the light of the new data from bamble and recent evidence that relatively low grade events are capable of disturbing and possibly resetting both rb sr total rock and u th pb zircon isotope systems the conclusions are that the main high grade gneiss forming event may have been pre grenvillian in all sectors and that there may have been no regional high grade reworking during the period 1 2 0 9 ga ago

unusually loud ambient noise in tidewater glacier fjords a signal of ice melt in glacierized fjords the ice ocean boundary is a physically and biologically dynamic environment that is sensitive to both glacier flow and ocean circulation ocean ambient noise offers insight into processes and change at the ice ocean boundary here we characterize fjord ambient noise and show that the average noise levels are louder than nearly all measured natural oceanic environments significantly louder than sea ice and nonglacierized fjords icy bay alaska has an annual average sound pressure level of 120 db referenced to 1 pa with a broad peak between 1000 and 3000 hz bubble formation in the water column as glacier ice melts is the noise source with variability driven by fjord circulation patterns measurements from two additional fjords in alaska and antarctica support that this unusually loud ambient noise in icy bay is representative of glacierized fjords these high noise levels likely alter the behavior of marine mammals

broadband spectra of seismic survey air gun emissions with reference to dolphin auditory thresholds acoustic emissions from a 2120 cubic in air gun array were recorded through a towed hydrophone assembly during an oil industry 2 d seismic survey off the west wales coast of the british isles recorded seismic pulses were sampled calibrated and analyzed post survey to investigate power levels of the pulses in the band 200 hz 22 khz at 750 m 1 km 2 2 km and 8 km range from source at 750 m range from source seismic pulse power at the 200 hz end of the spectrum was 140 db re 1 pa2 hz and at the 20 khz end of the spectrum seismic pulse power was 90 db re 1 pa2 hz although the background noise levels of the seismic recordings were far in excess of ambient due to the proximity of engine propeller and flow sources of the ship towing the hydrophone seismic power dominated the entire recorded bandwidth of 200 hz 22 khz at ranges of up to 2 km from the air gun source even at 8 km range seismic power was still clearly in excess of the high background noise levels up to 8 khz acoustic observations of common dolphins during preceding seismic surveys suggest that these animals avoided the immediate vicinity of the air gun array while firing was in progress i e localized disturbance occurred during seismic surveying although a general pattern of localized disturbance is suggested one specific observation revealed that common dolphins were able to tolerate the seismic pulses at 1 km range from the air gun array given the high broadband seismic pulse power levels across the entire recorded bandwidth and known auditory thresholds for several dolphin species we consider such seismic emissions to be clearly audible to dolphins across a bandwidth of tens on kilohertz and at least out to 8 km range

metagabbros in the modum complex southern norway an important heat source for sveconorwegian metamorphism abstract the precambrian modum complex in southern norway is characterized by supracrustal rocks penetrated by numerous gabbro intrusions occurring as elongated bodies subparallel with the regional foliation these gabbroic bodies show a partial alteration to amphibolites the best preserved parts of the metagabbros show a subophitic texture with olivine and plagioclase as the major cumulus phases and clinopyroxene orthopyroxene and ilmenite as the major intercumulus phases subsequent alteration of these igneous phases has led to formation of coronas around olivine in contact with plagioclase and recrystallization of clinopyroxene the igneous phases indicate intrusion temperatures in the range of 1100 1000 c and pressures below 5 8 kbar formation of coronas and recrystallization of clinopyroxene occurred in the pressure temperature range of 7 10 kbar and 800 600 c a 1224 15 ma sm nd mineral isochron obtained from igneous phases indicates emplacement at the start of the sveconorwegian metamorphism a complex evolution is suggested for the sveconorwegian metamorphic event involving both magmatism and high grade metamorphism

dynamo waves and palaeomagnetic secular variation summary a dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000 3000yr thermal convection in the lower mantle imposes a spatially variable heat flux at the core mantle boundary if the total heat loss from the core exceeds approximately 3 x 10 2w then both i a convective layer and ii large scale quasigeostrophic shear flows are likely to exist beneath the core mantle boundary the combined effects of convection and shear at the top of the outer core can generate three dimensionally propagating magnetic field disturbances called cuw dynamo waves an analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east west shear flow nonaxisymmetric disturbances exhibit the following characteristics i dynamo waves propagate in each hemisphere with a poleward or equatorward component depending on the sign of the a effect ii both eastward and westward propagation is allowed althcugh westward propagation is preferred the apparent velocity at the earth s surface is the sum of propagation and drift velocities in the core iii dynamo wave disturbances are transient with exponential growth and decay phases iv traces of inclination versus declination of individual fourier components obey runcorn s circularity rule a comparison is made with lake sediment secular variation records from central north america measured by lund banerjee the band passed time series and the major inclination declination loops can be modelled from 2600 to 10 000 yr bp by a single dynamo wave periodic in 2400 yr propagating under the site along an azimuth of 345 between 0 and 2600 yr bp the record can be modelled by addition of a wave with 1200yr period propagating along an azimuth of 30 this indicates that the palaeomagnetic secular variation record in north america to 10 000 yr may be dominated by poleward propagating wave like disturbances

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

modeling acoustic propagation of airgun array pulses recorded on tagged sperm whales physeter macrocephalus in 2002 and 2003 tagged sperm whales physeter macrocephalus were experimentally exposed to airgun pulses in the gulf of mexico with the tags providing acoustic recordings at measured ranges and depths ray trace and parabolic equation pe models provided information about sound propagation paths and accurately predicted time of arrival differences between multipath arrivals with adequate environmental information a broadband acoustic pe model predicted the relative levels of multipath arrivals recorded on the tagged whales however lack of array source signature data limited modeling of absolute received levels airguns produce energy primarily below 250hz with spectrum levels about 20 40db lower at 1khz some arrivals recorded near the surface in 2002 had energy predominantly above 500hz a surface duct in the 2002 sound speed profile helps explain this effect and the beampattern of the source array also indicates an increased proportion of high frequency sound at near horizontal launch angles these findings indicate that airguns sometimes expose animals to measurable sound energy above 250hz and demonstrate the influences of source and environmental parameters on characteristics of received airgun pulses the study also illustrates that on axis source levels and simple geometric spreading inadequately describe airgun pulse propagation and the extent of exposure zones

a model for underwater sound levels generated by marine impact pile driving marine impact pile driving generates very high underwater sound pressures which can harm aquatic life environmental assessments for pile driving projects typically require acoustic impact zones for marine mammals and fish to be estimated in advance a computer model that predicts the radiated acoustic field from impact driving of cylindrical piles has been developed a lumped mass model of the hammer which predicts the force generated at the top of the pile is coupled to a 1 d finite difference model of radial and axial stress waves in a cylindrical pile the radiated pressure is computed by matching the velocity boundary condition at the pile wall using a superposition of monopole sources distributed over the length of the pile in a layered 2 d fluid medium the transfer function for the monopoles is computed using the near field hankel transform for radial particle velocity at the pile wall standard ocean acoustic modeling techniques are used to compute the mach wave propagating away from the pile the model considers the physical characteristics of the pile hammer system such as the hammer energy pile dimensions sediment properties and pile hammer impedance ratio the model s predictions are compared to field measurements obtained in a riverine environment

long range transmission loss of broadband seismic pulses in the arctic under ice free conditions in 2008 the louis s st laurent lssl surveyed deep arctic waters using a three airgun seismic source signals from the seismic survey were detected between 400 km and 1300 km range on a directional autonomous acoustic recorder deployed in water 53 m deep off the alaskan north slope observations of received signal levels between 10 450 hz versus lssl range roughly fit a cylindrical transmission loss model plus 0 01 db km attenuation in deep ice free waters and fit previous empirical models in ice covered waters the transition between ice free and ice covered propagation conditions shifted 200 km closer to the recorder during the survey

broadband calibration of the r v marcus g langseth four string seismic sources 1 the r v marcus g langseth is the first 3 d seismic vessel operated by the u s academic community with up to a four string 36 element source and four 6 km long solid state hydrophone arrays this vessel promises significant new insights into earth science processes the potential impact of anthropogenic sound sources on marine life is an important topic to the marine seismic community to ensure that operations fully comply with existing and future marine mammal permitting requirements a calibration experiment was conducted in the gulf of mexico in 2007 2008 results are presented from deep 1 6 km and shallow 50 m water sites obtained using the full 36 element 6600 cubic inches seismic source this array configuration will require the largest safety radii and the deep and shallow sites provide two contrasting operational environments results show that safety radii and the offset between root mean square and sound exposure level measurements were highly dependent on water depth

sounds from airguns and fin whales recorded in the mid atlantic ocean 1999 2009 between 1999 and 2009 autonomous hydrophones were deployed to monitor seismic activity from 16 n to 50 n along the mid atlantic ridge these data were examined for airgun sounds produced during offshore surveys for oil and gas deposits as well as the 20 hz pulse sounds from fin whales which may be masked by airgun noise an automatic detection algorithm was used to identify airgun sound patterns and fin whale calling levels were summarized via long term spectral analysis both airgun and fin whale sounds were recorded at all sites fin whale calling rates were higher at sites north of 32 n increased during the late summer and fall months at all sites and peaked during the winter months a time when airgun noise was often prevalent seismic survey vessels were acoustically located off the coasts of three major areas newfoundland northeast brazil and senegal and mauritania in west africa in some cases airgun sounds were recorded almost 4000 km from the survey vessel in areas that are likely occupied by fin whales and at some locations airgun sounds were recorded more than 80 days month for more than 12 consecutive months

underwater ambient noise on the chukchi sea continental slope from 2006 2009 from september 2006 to june 2009 an autonomous acoustic recorder measured ambient noise north of barrow alaska on the continental slope at 235 m depth between the chukchi and beaufort seas mean monthly spectrum levels selected to exclude impulsive events show that months with open water had the highest noise levels 80 83 db re 1 pa2 hz at 20 50 hz months with ice coverage had lower spectral levels 70 db at 50 hz and months with both ice cover and low wind speeds had the lowest noise levels 65 db at 50 hz during ice covered periods in winter spring there was significant transient energy between 10 and 100 hz from ice fracture events during ice covered periods in late spring there were significantly fewer transient events ambient noise increased with wind speed by 1 db m s for relatively open water 0 25 ice cover and by 0 5 db m s for nearly complete ice cover 75 in september and early october for all years mean noise levels were elevated by 2 8 db due to the presence of seismic surveys in the chukchi and beaufort seas

rb sr systematics of a gardar age layered alkaline monzonite suite in southern norway the morkheia monzonite suite mms is part of a layered alkaline complex of anorogenic affinity which intruded discordantly into high grade gneisses of the telemark sector in the mms one complete outcrop and low rb sr samples from other outcrops preserve a rb sr whole rock date of ca 1250 ma interpreted as approximating an intrusive age the mms is thus contemporaneous with similar alkaline complexes of early to mid gardar age in southern greenland and labrador during this period felsic alkaline magmatism was apparently a widespread expression of the anorogenic tensional regime that obtained in the north atlantic provinces mms whole rock samples from three outcrops fall below the ca 1250 ma primary isochron and define secondary ages of ca 1060 ma at this time these outcrop scale rb sr systems approached sr isotope equilibration and rb sr ratios were augmented mainly by rb addition these isotopic effects relate to minor low grade hydrous alteration of the primary minerals this low grade gree

a sr nd and pb isotopic investigation of the transition between two megacyclic units of the bjerkreim sokndal layered intrusion south norway abstract the bjerkreim sokndal layered intrusion forms part of the proterozoic south rogaland igneous complex of southwest norway the intrusion has been divided into two parts an anorthositic to gabbro noritic layered series and a generally more massive mangeritic quartz mangeritic to charnockitic upper part on the basis of repeated lithological sequences the layered series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment initial 940 ma sr nd and pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit iii and megacyclic unit iv range from 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 to 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 the isotopic compositions vary systematically with stratigraphic height and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts replenishment and post replenishment across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma the replenishing magma was characterized by 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 the positive end of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle in contrast the resident magma had 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 indicating that it had assimilated crustal material in addition the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of nd and pb in the resident magma had been strongly reduced probably by fractionation of apatite above the replenishment interval nd and pb isotopic variations are apparently decoupled from sr isotopic variations two mechanisms are proposed for achieving this the first is heterogeneous mixing of up to 20 of a third magmatic component into an already isotopically stratified magma chamber the second is the presence of 10 20 of post cumulus plagioclase that crystallized from a migrating intercumulus melt

reactions of bowhead whales balaena mysticetus to seismic exploration in the canadian beaufort sea the behavioral reactions of bowhead whales to distant seismic vessels not under our control a controlled approach by a seismic vessel and controlled tests with a single airgun were observed on 21 occasions in the summers of 1980 84 general activities of bowheads exposed to pulses of underwater noise 107 158 db re 1 mu pa from seismic vessels 6 99 km away were observed activities were indistinguishable from those without seismic noise there was no detectable avoidance hints of subtle changes in surfacing respiration and diving behavior were unconfirmed but were consistent with reactions to stronger noise pulses from closer seismic boats in a test with a full scale seismic boat 30 airguns totaling 471 source level 248 db re 1 mu pa closest point of approach 1 1 2 km bowheads began to orient away when the airgun array began to fire 7 1 2 km away however some whales continued apparent near bottom feeding until the vessel was 3 km away whales were displaced by about 2 km reactions were not much stronger than those to any conventional vessel tests with one 0 66 1 airgun showed that some bowheads move away from sources of strong seismic impulses even in the absence of boat noise and that bowheads can detect the direction from which seismic impulses arrive in general bowheads exhibit avoidance reactions when they receive seismic pulses stronger than about 160 db re 1 mu pa evidence of reactions to lower received levels remains inconclusive

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

modes of mantle convection and the removal of heat from the earth s interior earth thermal histories based on two layer and whole mantle parameterized convection models have been explored to determine the influence of the mode of mantle convection on the content and distribution of mantle radioactive heat sources and on the temperatures and heat transfer properties of the mantle two layer mantle convection thermally insulates the lower mantle and is less efficient at heat removal than whole mantle convection consequently two layer mantle convection requires a lower mantle depleted of radioactive heat sources contrary to the undepleted lower mantles of recent geochemical models whole mantle convection removes 3 times more primordial heat from the earth s interior and 6 times more heat from the core in 4 5 billion years than does two layer convection the average mantle volumetric heat generation rates for both modes of convection are comparable to that of a potassium depleted chondrite thus no potassium is necessary in the core to balance the observed present day surface heat flux however most of the radioactives are concentrated in the upper mantle for two layer convection model geotherms for whole mantle convection compare well with independent estimates of temperature at various depths including the core mantle boundary while the lower mantle is too hot for two layer convection models thermal histories of the upper mantle in two layer models are similar to those of whole mantle models thermal histories for the lower mantle in two layer models are similar to those of the core in whole mantle convection models this is because the lower mantle is thermally insulated by the upper mantle if it convects separately just as the core is thermally insulated by the whole mantle the differences between whole and two layer mantle convection are mostly due to the thermal insulation of the lower mantle and the smaller efficiency of heat removal of the upper mantle compared to that of the whole mantle which is much larger in volume a whole mantle convection model reconciling the need for distinct geochemical reservoirs with the physics of mantle convection is presented

two layer mantle convection and the depletion of radioactive elements in the lower mantle only a small percentage of the mantle s total content of radioactive elements can be in the lower mantle if this region convects steadily and separately from the upper mantle recent geochemical models of a depleted upper mantle and an undepleted lower mantle are thus inconsistent with a steady thermal model involving distinct upper and lower mantle convection cells steady two layer mantle convection requires a depleted lower mantle and an undepleted upper mantle to match the observations of surface heat flow and mantle viscosity the constraint on the abundance of radioactive elements in the lower mantle depends on the existence of at least one thermal boundary layer at the upper mantle lower mantle interface with thickness comparable to that of the surface thermal boundary layer since the temperature drop across an interface boundary layer is limited by the relatively small difference between the solidus temperature and the upper mantle temperature the amount of heat which can be transferred across this layer is small compared with the surface heat flow the upper limit on the concentration of radioactive elements in the lower mantle can be relaxed only if both the lower boundary layer of the upper mantle cell and the upper boundary layer of the lower mantle cell are extremely thin or absent altogether which we consider however highly unlikely

r v marcus g langseth seismic source modeling and calibration this paper presents analyses of calibration data of the r v marcus g langseth seismic sources that were collected in 2007 2008 the analysis includes measurements of the levels and azimuthal directivity of the langseth two string source which is used for 3 d seismic surveys and compares these data with multichannel seismic hydrophone array data in shallow water the analysis also shows the important contribution of seafloor and subseafloor reflected and refracted arrivals and the associated effects of water depth and seafloor slope as expected and predicted by modeling azimuthal directivity depends greatly on the footprint dimensions of the seismic array in shallow water where the acoustic field is dominated by near vertically traveling reflected and refracted waves data recorded using a towed mcs array are a useful proxy for single hydrophone calibration data in deeper water the easily modeled direct arrivals are exceeded in amplitude by seafloor reflected and subseafloor refracted energy at offsets which depend upon the water depth limiting the applicability of a priori modeling that does not include these interactions in addition in a seafloor sloping environment amplitudes depend greatly on whether the receivers are upslope or downslope from the source

the estimation of geoid profiles in the northwest atlantic from simultaneous satellite altimetry and airborne expendable bathythermograph sections in the nw atlantic the temporal mean circulation results in large amplitude surface topography precluding the reasonable use of mean altimetric surfaces or profiles for precise estimation of the geoid we examine a procedure which 1 uses airborne expendable bathythermograph axbt sections collected along altimeter ground tracks to estimate the single pass or instantaneous surface dynamic topography 2 employs concurrent or simultaneous satellite altimeter overflights to provide a measure of instantaneous sea level and 3 arrives at a precise estimate of the alongtrack geoid profile by differencing the instantaneous altimetric sea level and the axbt derived dynamic topography this technique is applied to several geosat exact repeat mission erm ground tracks in the area of the nw atlantic regional energetics experiment rex using axbt survey data collected in april and july 1987 geoid profile estimates are repeatable between these two independent data sets to within 10 20 cm rms this is near the estimated noise level due to barotropic variability in the gulf stream region these axbt geosat derived profiles provide a more consistent estimate of the alongtrack geoid gradient than do available gravimetric geoids analysis of the statistics of the ensemble of collinear passes provides an estimate of the geographically uncorrelated orbit error of approximately 1 2 m the total surface topography derived as the difference between adjusted instantaneous sea level profiles and the axbt geosat derived geoid profiles does not suffer significant contamination by either geographically uncorrelated or correlated orbit error

modeling of seafloor wave propagation and acoustic scattering in 3 d heterogeneous media results from a 3 d finite difference code for the elastic wave equation are used to compare scattering from volume and surface heterogeneities at the seafloor the 2 d and 3 d models for flat ocean crust containing volume heterogeneities single facets and statistically rough seafloors are compared for both hard bottom shear velocity greater than water velocity and soft bottom shear velocity less than water velocity environments heterogeneities scatter energy both forward and backward into the water column via three mechanisms primary scattering from the direct compressional wave reflecting off the seafloor heterogeneities secondary scattering from transmitted compressional and shear waves interacting with volume heterogeneities and from interface waves scattering from near surface heterogeneities and tertiary scattering from interface waves spawning additional interface waves there is significant secondary and tertiary scattering from rough hard bottoms representative of basaltic seafloors t

deep penetration of sedimentary fluids in basement rocks from southern norway evidence from hydrocarbon and brine inclusions in quartz veins abstract this paper presents evidence for fluid flow and fluid rock interaction at upper crustal levels within the crystalline basement of southern norway in the high grade modum complex postmetamorphic veins of quartz occur in association with albitisation of metagabbros and metasediments pressure temperature conditions for the formation of these veins are in the range 250 300 c and ca 1 2 kbar primary and pseudosecondary fluid inclusions in the quartz veins show two fluids 1 hydrocarbon c0 2 inclusions and 2 aqueous inclusions with variable salinities dark carbonaceous solid inclusions are also present the hydrocarbon inclusions are methane dominated ca 80 100 mot and the presence of higher complex hydrocarbons is demonstrated the aqueous inclusions in the metagabbro hosted veins show more saline compositions than the metasediment hosted veins a salinity range of ca 23 0 wt nacl eq is found some of the aqueous inclusions may contain nahco 3 crush leach analyses of inclusion fluids show na ca k ci dominated compositions with na ca k hydrocarbon poor concentrates yield br cl ratios close to seawater while hydrocarbon rich concentrates show higher values the types and chemistry of the hydrocarbons in these veins indicate a biogenic origin of the hydrocarbon fluids the inclusion fluids are interpreted as derived from an overlying sedimentary basin in late precambrian or permian times the veining is interpreted as the deep expression of the percolation of basinal fluids into the metamorphic basement during crustal extension

modelling sound propagation in the southern ocean to estimate the acoustic impact of seismic research surveys on marine mammals summary modelling sound propagation in the ocean is an essential tool to assess the potential risk of air gun shots on marine mammals based on a 2 5 d finite difference code a full waveform modelling approach is presented which determines both sound exposure levels of single shots and cumulative sound exposure levels of multiple shots fired along a seismic line band limited point source approximations of compact air gun clusters deployed by r v polarstern in polar regions are used as sound sources marine mammals are simulated as static receivers applications to deep and shallow water models including constant and depth dependent sound velocity profiles of the southern ocean show dipole like directivities in case of single shots and tubular cumulative sound exposure level fields beneath the seismic line in case of multiple shots compared to a semi infinite model an incorporation of seafloor reflections enhances the seismically induced noise levels close to the sea surface refraction due to sound velocity gradients and sound channelling in near surface ducts are evident but affect only low to moderate levels hence exposure zone radii derived for different hearing thresholds are almost independent of the sound velocity structure with decreasing thresholds radii increase according to a spherical 20 log10 r law in case of single shots and according to a cylindrical 10 log10 r law in case of multiple shots a doubling of the shot interval diminishes the cumulative sound exposure levels by 3 db and halves the radii the ocean bottom properties only slightly affect the radii in shallow waters if the normal incidence reflection coefficient exceeds 0 2

superweak asthenosphere in light of upper mantle seismic anisotropy earth s upper mantle includes a 200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background this zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself however recent seismological and electro magnetic studies indicate strong heterogeneity in thinner layers underneath the plates which if related to more extreme global viscosity reductions may require a revision of our understanding of mantle convection here i use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions the fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations i show that moderate sub oceanic viscosity reductions to 0 01 to 0 1 times the upper mantle viscosity are preferred by the fit to anisotropy and global plate motions depending on layer thickness lower viscosities degrade the fit to azimuthal anisotropy localized patches of viscosity reduction or layers of subducted asthenosphere however have only limited additional effects on anisotropy or plate velocities this indicates that it is unlikely that regional observations of sub plate anomalies are both continuous and indicative of dramatic viscosity reduction locally such weak patches may exist and would be detectable by regional anisotropy analysis for example however large scale plate dynamics are most likely governed by broad continent ocean asthenospheric viscosity contrasts rather than a thin possibly high melt fraction layer