deep seismic structure of the seismogenic plate boundary in the off sanriku region northeastern japan we determined the deep seismic structure of the seismogenic plate boundary zone of off sanriku the northern part of the japan trench inner slope area travel time data from aftershocks of the 1994 far off sanriku earthquake m 7 5 recorded both by ocean bottom seismographs and the local land seismic network enable us to image the three dimensional seismic velocity structure of the region along the plate boundary there exists a p wave low velocity layer of ca 7 km s 1 and with thickness 10 km as the topmost layer of the subducting pacific plate the present analysis indicates that this layer interpreted as the subducting oceanic crust continues without any velocity variations at least down to ca 40 km depth in the overriding plate we found a zone of considerable high p wave velocity ca 8 km s 1 between the pacific coastal line of the northeastern japan arc at the longitude of 142 5 e this manifests that the trenchward toe of the mantle wedge has significantly higher velocity than that beneath the land area of the arc the landward limit of the high p wave velocity toe corresponds to the down dip limit of the seismogenic plate boundary further to the east the p wave velocity is ca 6 km s 1 the structure image obtained indicates that the nature of the overriding plate is significantly different along the plate boundary in the eastern half of the seismogenic zone the island arc crust and subducting oceanic crust are in contact with each other whereas the high velocity mantle wedge makes up the hanging wall of the deeper part of the seismogenic zone

bioturbation depths rates and processes in massachusetts bay sediments inferred from modeling of 210pb and 239 240pu profiles abstract profiles of 210 pb and 239 240 pu from sediment cores collected throughout massachusetts bay water depths of 36 192 m are interpreted with the aid of a numerical sediment mixing model to infer bioturbation depths rates and processes the nuclide data suggest extensive bioturbation to depths of 25 35 cm roughly half the cores have 210 pb and 239 240 pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing bioturbation rates are reasonably well constrained by these profiles and vary from 0 7 to 40 cm 2 yr 1 as a result of this extensive reworking however sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates of 0 3 cm yr 1 can be inferred the other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide which cannot be reproduced by biodiffusive mixing models a numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima as suggested by previous work the deep penetration depths of excess 210 pb and 239 240 pu suggest either that the organisms release material over a range of 15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth additional constraints from surficial sediment 234 th data suggest that in this half of the cores the vast majority of the present day flux of recent nuclide bearing material to these core sites is transported over a timescale of a month or more to a depth of a few centimeters below the sediment surface as a consequence of the complex mixing processes surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition

deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric analysis of deep subduction zone earthquakes those at depths greater than 60 km reveals the physical and chemical properties of a descending oceanic lithosphere at mantle depths over the past five decades it has been observed that a large fraction of deep earthquakes has non double couple non dc seismic radiation patterns in contrast shallow earthquakes tend to have dc radiation patterns due to mechanisms of shear faulting these observations have been used to argue that deep earthquakes rupture differently from shallow earthquakes here we show that the observed global distribution of non dc deep earthquakes could be caused by shear faulting mechanisms but in a highly anisotropic laminated rock fabric that surrounds the deep earthquakes within subducted slabs for intermediate depth earthquakes 60 300 km we found a large shear wave anisotropy of 25 possibly caused by laminated fabric or aligned melt pockets oriented parallel to the slab interface which provides new supporting evidence for the metamorphic dehydration reactions in slabs however at deep focus depths 300 km the putative metastable phase change mechanism alone cannot explain the seismic anisotropy instead our results and those from recent experiments suggest materials such as magnesite or perhaps carbonatite melt may play a role in generating deep focus earthquakes radiation patterns for deep earthquakes could be a result of shear faulting mechanisms similar to those for shallow earthquakes but in highly anisotropic rock fabric suggest seismic analyses

seismicity of the galapagos 95 5 w propagating rift an array of ocean bottom seismometers was deployed for 21 days about the 95 5 w galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation seismicity was fairly constant 10 17 events per day with no indication of swarm activity hypocenters were determined for 304 earthquakes the largest of which is estimated to be mb 2 3 with a seismic moment of 2 9 1020 dyn cm the cumulative seismic moment released by all local earthquakes is estimated to be 6 6 1020 dyn cm a b value of 0 77 0 08 was estimated for the region implying a stress regime dominated by a broad tectonic stress field the distribution of earthquakes is consistent with the propagating rift hypothesis most earthquakes occurred south of the propagating rift within the de steiguer deep seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time the seismicity observed along the propagating rift is diffuse earthquakes were observed at the propagating rift tip but not in front of it almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip that is earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift six composite focal mechanisms are consistent with the propagating rift hypothesis first motions from earthquakes located on the propagating rift indicate extension whereas motions associated with the failing rift area indicate compression a compressional regime is also indicated west of the transform zone twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right lateral strike slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

supply and dispersal of flood sediment from a steep tropical watershed hanalei bay kaua i hawai i usa in contrast to many small mountainous watersheds in temperate coastal regions where fl uvial discharge and wave energy commonly coincide deposition and reworking of tropical fl ood sediment can be seasonally decoupled and this has important implications for coral reef ecosystems an understanding of the interaction between tropical fl ood sedimentation and wave climate is essential to identifying and mitigating effects of watershed changes on coral reefs as urbanization and climate change proceed sedimentary facies and isotopic properties of sediment in hanalei bay on the island of kaua i hawai i usa were used to assess deposition and reworking of fldeposits from the hanalei river in a case study demonstrating the potential ecosystem effects of runoff from a steep tropical watershed in hanalei bay the youngest and thickest terrigenous sediment was consistently present near the river mouth and in a bathymetric depression that acted as at least a temporary sediment sink during this 2 yr study the largest fl ood events occurred in late winter and spring 2006 substantial terrestrial sediment delivered by those fl oods still remained in the bay as of june 2006 because oceanic conditions were not suffi ciently energetic to transport all of the sediment offshore additional sediment was deposited in the bay by a summer 2006 fl ood that coincided with seasonal low wave energy in most years fl ood sediment accumulating in the bay and on its fringing reefs would be remobilized and advected out of the bay during winter when the wave climate is energetic turbidity and sedimentation on corals resulting from late spring and summer fl oods during low wave energy could have a greater impact on coral reef ecosystems than fl oods in other seasons an effect that could be exacerbated if the incidence and sediment load of tropical summer fl oods increase due to urbanization and climate change

crustal structure and seismicity beneath the forearc off northeastern japan marine seismological and other data in the japan trench area north of 38 5 n were used to infer the relation of subducting plate structure seismicity and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates previous crustal models from ocean bottom seismographic refraction obs surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data a wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting pacific plate crust its dip angle increases to about 7 110 km landward of the trench axis a large increase beneath the deep sea terrace is required to reach the well defined angle of 25 beneath the tohoku east coast a large wave speed gradient within layer 2 commonly observed under normal oceans seems to vanish beneath the inner trench slope at about 10 km depth within the overriding plate apparently brittle material with a p wave speed of 6 km s can be found as near as 35 km from the trench axis the upper limit of the seismogenic zone of interplate low angle thrust events is about 15 km depth from obs seismicity and large event analyses both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates

seismicity surveys with ocean bottom seismographs off western canada three arrays of ocean bottom seismographs have been deployed to study the seismicity at the northern end of the juan de fuca ridge system off western canada nearly 100 events were located with estimated accuracies generally better than 10 km all lying on or near the en echelon ridge transform fault plate boundaries as defined in this area by the magnetic anomalies the seafloor morphology and by other geophysical data the depths of 12 events were determined to lie between 2 and 6 km below the top of the crust the seismograms exhibit clear p and s wave arrivals along with phases that involve p to s and sometimes s to p conversion probably at the base of the sediments beneath the instruments the event magnitudes have been estimated from signal duration using four calibration events that were well recorded by a land station the magnitude estimates permit the determination of rough magnitude frequency of occurrence relations over the magnitude range of 1 to 3 that are in surprisingly good agreement with the recurrence relations for the area at larger magnitudes from 75 years of land station data the mean p wave velocity in the uppermost mantle from the earthquake datamore recorded by the sea floor arrays is 7 6 km s sup 1 and the mean v sub p v sub s ratio is 1 71 or a poisson s ratio of 0 24 less

a seismicity gap beneath the inner wall of the japan trench as derived by ocean bottom seismograph measurement abstract an ocean bottom seismograph obs array covered the area of the japan trench about 40 n off the tohoku district of northeastern honshu japan the obs array was operated for two weeks in june 1981 arrival time data of 8 obss were used to study a spatial distribution of microearthquakes around the trench during the observation period 400 500 events with a signal to noise ratio greater than 2 were detected by each obs the arrival times of 223 of these events could be picked 38 earthquakes were located in the area of the japan trench within the latitude range of 38 5 41 0 n and the longitude range of 142 0 146 0 e and about a hundred events were located beneath or near honshu island s p time distribution recorded by each obs showed seismic activities to exist on both sides of the trench axis while the seismic level was low beneath the inner wall of the trench epicentral distribution showed a seismicity gap beneath the inner wall of the trench the area of the landward activity continued eastward up to the position where the water depth is about 4000 m seismic activity existed beneath the trench axis and the seaward wall of the trench the seismicity gap suggested from the 1980 obs observations in the vicinity of the present study area was clearly confirmed by the present observations the obs array enabled us to determine the focal depth of microearthquake activity with better resolution than was possible from land observations the obs data showed that the landward activity beneath the continental slope was concentrated in the very shallow part of the lithosphere at a depth less than 30 km foci started to deepen westward with a very low dip angle of less than 10 which was the most shallow part of the upper plane of the double seismic zone the focal depth distribution showed that events just beneath the trench axis occurred to depths slightly greater than 50 km while the foci of the seaward activity were concentrated in the uppermost lithosphere at depths less than 30 km seismic cross sections clearly show that the activity beneath the trench axis and seaward wall of the trench is separated from the landward activity which is a shallow extension of the double seismic zone by the seismicity gap

lebensspuren of the bathyal mid atlantic ridge the extent of megafaunal bioturbation was characterised at flat sedimented sites on the mid atlantic ridge mar at 2500 m depth this study investigated the properties of and spatial variation in surficial bioturbation at the mar lebensspuren assemblages were assessed at four superstations either side of the mar and in two different surface productivity regimes north and south of the sub polar front high definition rov videos from these superstations were used to quantify area and abundance of 58 lebensspuren types lebensspuren area was lowest at the sw with 4 12 lebensspuren coverage and the se nw had the greatest area coverage of lebensspuren 9 69 for both all stations except the sw were dominated by epifaunal particularly track style lebensspuren infaunal mounds were more significant in the southern superstations particularly in the sw in terms of lebensspuren assemblage composition all superstations were significantly different from one another which directly corresponded with the composition of lebensspuren forming epifauna lebensspuren assemblages appeared to have been primarily influenced by local scale environmental variation and were independent of detrital flux this investigation presented a novel relationship between lebensspuren and faunal density that conflicted with the traditionally held view of inverse proportionality and suggests that at the mar megafaunal reworking was not the only significant control on lebensspuren assemblages

plutonium and210pb distributions in northeast atlantic sediments subsurface anomalies caused by non local mixing abstract the depth distributions of 210 pb and 239 240 pu measured in a suite of box cores collected from water depths of 4000 5000 m in the northeast atlantic ocean exhibit pronounced subsurface maxima caused by sediment reworking by benthic infauna small scale spatial heterogeneity in bioturbation rates is indicated by large differences in tracer profiles from duplicate cores separated only by a few centimeters 210 pb and 239 240 pu activity distributions from each subcore exhibit a high degree of correlation and most tracer profiles exhibit one or more subsurface maxima one dimensional biodiffusion analogue models do not adequately simulate the principal features of this data set however an inverse conveyer belt mixing model which simulates subsurface egestion or a functionally equivalent process of surficial material which is enriched both in organic debris and radioactive tracers can reproduce the subsurface tracer maxima single event and continuous subsurface egestion models have been formulated and solved for different feeding rates and background biodiffusive fields the single event model provides a better fit to the data and in particular ensures the observed high degree of correlation between the 210 pb and 239 240 pu activity profiles regardless of the different tracer input functions the most likely candidate responsible for subsurface tracer egestion is a large infaunal worm of the phylum sipunculida which dominates the biomass below a depth of 3 cm

benthic remineralization and burial of biogenic sio2 caco3 organic carbon and detrital material in the southern ocean along a transect at 170 west abstract we investigated the composition recycling and mass accumulation rates of sediments along a transect in the southern ocean located from 66 s to 57 s at 170 w this transect also corresponds to the location of a sediment trap mooring line the sediments at the seven sites studied range from largely terrigenous material to nearly pure 90 biogenic silica caco3 is a minor but persistent component at most sites mass accumulation rates have been determined on the basis of excess 230th in the sediments i e 230th normalized accumulation rates the influence of redistribution of sediments on the sea floor has been estimated from 14c analyses the recycling of material delivered to the sediments has been characterized on the basis of pore water studies that make extensive use of both in situ sampling and shipboard extractions the influence of the highly variable rates of input of particulate matter that characterize much of the southern ocean upon pore water gradients and fluxes across the sediment water interface has been considered we find only poor correspondence between bsio2 burial fraction burial particulate flux a quantifiable measure of preservation efficiency and bsio2 particulate rain along the transect however preservation does appear to be closely linked to a combination of sedimentation rate and particulate rain the burial fraction of bsio2 is small relative to benthic rain 5 19 despite the small fraction buried burial flux normalized to sedimentation rate 1 2 appears to provide a very consistent means of predicting benthic particulate rain over a large range of rain rates including data from a number of different studies and environments at sites with bsio2 rain 250 mmol m 2 yr 1 the average difference between predicted and observed rain is 25 30 such rain rates occur in many marine areas particularly the southern ocean with the result that this relationship potentially provides a means of estimating bsio2 benthic rain over prolonged periods in the past on the basis of readily measured sediment parameters at the southern most deep ocean station the particulate flux was characterized by an extremely high corg caco3 ratio 10 but this high ratio does not appear to have a substantial influence on caco3 burial caco3 is preserved in the sediments at this site despite a particulate flux with a 10 fold excess of corg above that required for complete dissolution in the sediments the unexpectedly high preservation of caco3 is due largely to the very steep corg oxidation rate profile at this site as a result a large fraction of the organic matter oxidized in the sediments does so in close proximity to the sediment water interface where most of the metabolic co2 is neutralized by co32 from the overlying water rather than by the dissolution of sedimentary caco3 diagenetic modeling indicates that at several of the stations the remineralization fluxes of carbonate species across the sediment water interface may not have been at steady state as a result of the highly pulsed nature of particulate rain in this environment we estimate that at the time of our sampling it is possible that near interface fluxes could have been a factor of 1 6 2 times the annual average at every site on the transect the burial fluxes of detrital material are substantially greater than the detrital particulate rain measured in the sediment traps by as much as a factor of 40 detrital burial is bimodal being greatest at the southern and northern extremes of the transect we postulate that the excess of burial over particulate rain in the south reflects the contribution of ice rafted debris at these high latitudes increases in the supply at the northern stations must have a different source we believe that the excess at these stations is material eroded from the sea floor to the west possibly on the campbell plateau and advected by currents to the northern portion of the transect at depths below the shallow traps

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern pacific abstract several long range explosion seismology experiments have been conducted in the northwestern pacific basin where one of the oldest oceanic lithospheres is postulated to exist the experiments were conducted from 1974 to 1980 highly sensitive ocean bottom seismographs which had been developed for longshot experiments were used the lengths of the profiles ranged from 1000 to 1800 km and the directions were chosen to provide wide azimuthal coverage one of the aims of this series of experiments was to test the existence of velocity anisotropy on a large regional scale the results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4 7 the anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom however the magnitude of the anisotropy may vary with depth the azimuth of the maximum velocity is 150 160 clockwise from north and coincides with the fossil direction of spreading of the pacific plate whereas it differs from the present direction of plate motion by 30 the azimuth does not seem to depend on depth in the direction of maximum velocity the lithosphere is basically two layered 8 0 8 2 and 8 6 km s 1 the depth of the interface is 50 60 km beneath the sea floor

attenuation of s waves in the lithosphere of the sea of crete according to obs observations five obs were installed in the sea of crete in 1987 during a period of nine days 430 local earthquakes were recorded hypocentres were determined for 85 microearthquakes a correlation between magnitude ml and duration of seismic events on the records of the ocean bottom seismographs obs was found from data of 14 shocks recorded simultaneously by obs and some land based greek stations a magnitude frequency relationship for earthquakes recorded by obs was obtained in the magnitude range ml 2 4 amplitude curves describing the attenuation of body waves in the lithosphere of the sea of crete were compiled assessment of quality factor qs was made by comparison of empirical and theoretical amplitude curves the values of qs for the lithosphere of the sea of crete were calculated and found equal to 200 300 low values of qs and consequently strong attenuation of s waves in the lithosphere of the sea of crete could be explained by extension of the lithosphere accompanied by its partial contamination with melted magma

three dimensional mapping of dislocation avalanches clustering and space time coupling there is growing evidence for the complex intermittent and heterogeneous character of plastic flow here we report a three dimensional mapping of dislocation avalanches during creep deformation of an ice crystal from a multiple transducers acoustic emission analysis correlation analysis shows that dislocation avalanches are spatially clustered according to a fractal pattern and that the closer in time two avalanches are the larger the probability is that they will be closer in space such a space time coupling may contribute to the self organization of the avalanches into a clustered pattern

rheology of ice ii and ice iii from high pressure extrusion rheological parameters for ice ii and ice iii needed in tectonic models of the icy satellites of the major planets are obtained from extrusion experiments and compared with the rheology of ice i at pressures 2 kbar and temperatures 240k ice ii has a higher effective viscosity by a factor 10 than ice i at similar stress levels whereas ice iii has a lower viscosity by a factor 0 01 the rheological contrasts among the ice phases are related to differences in the dielectric relaxation behavior and state of proton order disorder in the structures in a way that sheds light on the nature of dislocation motion in ice a striking transformation plasticity accompanies the ice i iii transition and could have large tectonic effects

spatial distribution of microearthquakes beneath the japan trench from ocean bottom seismographic observations summary the area of the japan trench about 40 n off the t6hoku district of north eastern honshu japan was for the first time extensively covered by an ocean bottom seismograph obs array seven obss were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 july and august during this period each obs registered between 300 and 600 earthquakes about 100 events were simultaneously recorded by at least three obss hypocentres were calculated by using a generalized inversion technique sixty five events mostly with magnitudes m from 1 5 to 3 0 which were not located by a land network were determined to be within or near the area covered by the obs array the obs array had nearly uniform detectability for earthquakes with m over 2 0 within the latitude range of 39 s0 41 5 n and the longitude range of 143 146 5 e within this area there is a highly active region of microseismicity beneath the seaward wall of the japan trench the seaward activity is concentrated within 100 km of the trench axis and is low beneath the northwest pacific basin a seismicity gap was found just westward of the trench axis beneath the landward wall of the trench at a water depth of between 6000 and 4000m seismic activity continues landward from the region west of the gap the obs array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible the results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere on the seaward side most events occur at a depth of less than 30 km beneath the trench wall while the activity on the landward side starts to deepen westward exactly at the trench axis this paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and or subducting of the oceanic plate just beneath the japan trench axis

a model for particle selective transport of tracers in sediments with conveyor belt deposit feeders conveyor belt deposit feeding organisms prevalent in both marine and freshwater systems have a profound effect on sediment properties and transport processes these organisms ingest sediments over a range of depths while depositing gut contents from tails protruding above the sediment surface this action results in particle selective transfer of buried materials to the sediment surface and imposes an accelerated rate of sediment and pore water burial within the feeding zone most previous efforts to combine sediment diagenesis with the effects of biogenic reworking characterize mixing as exclusively diffusive and ignore such major advective effects here a model is developed based on fundamental diagenetic equations for transport and reaction for the distribution of tracers in accumulating sediments subject to compaction and diffusive as well as advective redistribution by benthic organisms conveyor belt cb feeding is characterized as a first order process with a depth dependent rate constant which is either localized gaussian or distributed integrated gaussian biogenic diffusivity of bulk sediments is allowed these alternative depth dependences as well the model assumes simple linear adsorption of the tracer between solid and solution phases and uses a time dependent flux at x 0 which is a combination of that originating externally and the depth integrated contributions from feeding particle selectivity is introduced by applying mass conservation separately to transport of the tracer and bulk sediments properties of the model are illustrated for tracers in nondispersive systems strongly bound to sediment solids cb recycling gives rise to transient reflections on passage of a tracer pulse through the zone of bioturbation reflections readily disappear in the presence of various integrative processes the system time resolution is defined in terms of the downward propagation of dual tracer pulses and shown to be systematically degraded by intensified feeding and selective transport a simple expression is developed for estimating the magnitude of particle selective changes in steady state surface concentrations in terms of the particle selectivity factor and the efficiency of sediment reworking unrecognized particle selectivity can lead to false inferences about changes in the rate of delivery of tracers to sediments selective cb recycling can cause a tracer such as 210pb to be retained largely within the bioturbation zone so that relatively little activity reaches the region of sediment useful for geochronological interpretation increased selectivity accentuates the sensitivity of the profile to details of the reworking process the model applied to previous studies of cb deposit feeders in tracer labeled freshwater microcosms shows the need to include particle diffusion as well as advection in the treatment of cb transport

dramatic effects of stress on metamorphic reactions thermodynamic calculations predict mineralogy from temperature and pressure and vice versa such calculations assume that stress is isotropic despite the fact that differential stresses prevail in earth resulting from large scale tectonics and or differences between fluid and rock pressures in porous rocks new calculations show that differential stress can have significant effects on thresholds for metamorphic reactions depending on the grain scale reaction pathways a differential stress may depending on the reaction pathway have an effect equivalent to a pressure difference on the order of assemblage volume reaction volume change differential stress the multiplying factor is typically 10 or more for example the onset of a garnet clinopyroxene breakdown reaction may be offset up or down by the equivalent of 500 mpa in pressure for a 50 mpa differential stress the effect is equivalent to a temperature difference on the order of assemblage volume reaction entropy change differential stress for example the onset of muscovite quartz breakdown may be offset up or down by the equivalent of 130 c for a 50 mpa differential stress much of earth is under differential stress so the new calculations invite a reappraisal of metamorphic mineralogy and microstructure indicating that new insights into stresses and fluid pressures on earth can be gained

bioturbation and porosity gradients ubiquitous porosity gradients have a potentially important effect on the mixing of particle bound tracers such as 210pb mass depth coordinates cannot be used to deal with these effects if values of the traditional mixing coefficient db are required this paper compares and evaluates three different means of dealing directly with porosity gradients while modeling bioturbation i e mean constant porosity interphase mixing porosity mixed and intraphase mixing porosity not mixed we apply these models to 11 different 210pb profiles collected at various depths and times on the eastern canadian margin a statistical analysis of the resulting best fits shows that these models produce equivalent mixing coefficient values for 55 of the profiles for the remaining 45 of the profiles the interphase mixing model predicts the existence of well mixed near surface zones on the time scale of 210pb decay a phenomenon not predicted by the other models unfortunately our tracer dataset by itself cannot be used to establish which mixing mode is actually operative at each station

water ice phases ii iii and v plastic deformation and phase relationships several of the high pressure crystalline forms of ice exist in the interiors of the larger icy moons of the outer planets guided by the phase relations established by bridgman 75 years ago we have transformed ordinary water ice i to ices ii iii v and perhaps vi we subsequently deformed those phases in a gas deformation apparatus designed for these experiments we found ice ii to be the strongest of the high pressure phases having a strength roughly comparable to that of ice i ice ii is very weak at fixed levels of stress ice iii flows 10 sup 2 10 sup 3 times faster than ice ii we confirm bridgman s 1912 findings that ices iii and v can exist metastably within the ice ii field and in addition find that these phases may be deformed plastically within much of the metastable region without reverting to ice ii the most persistent metastable existence of ice iii occurs in the warmer portions of the ice ii field through which historical and present day temperature pressure profiles in the icy moons pass thus although the stability field of ice iii is small the weakness of this phase may have profoundly influenced the evolution and present day behavior of themore icy moons less

intragranular dynamic recrystallization in naturally deformed calcite marble diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization abstract intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains intragranular recrystallized grains within large twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study lattice distortions low angle boundaries and some high angle boundaries 15 in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb accommodated dislocation creep involving subgrain rotation recrystallization changes in the ratio of host and twin domain as the deformation zone is entered show that twin boundary migration also occurred recrystallized grains have similar sizes 10 60 m to subgrains suggesting that they formed by subgrain rotation however within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15 and misorientation axes are randomly oriented moreover recrystallized grain orientations average around the porphyroclast orientation we suggest that the recrystallized grains once formed are able to deform partly by diffusion accommodated grain boundary sliding which is consistent with predictions made from lab flow laws

detection of shallowest submarine seismicity by acoustic coupled shear waves an outstanding slow wave train similar to t wave in appearance has been observed on records of an ocean bottom seismometer array off boso central japan these slow wave trains are not observed on land are usually spindle shaped and dominant in vertical component and have apparent velocities of 1 3 km s along the array in order to understand the observations we calculate synthetic seismograms by a finite difference method for horizontally stratified layers of seawater sediment and basement which show a well defined wave train consistent with the observations only if the source is placed above the basement layer this result and normal mode calculations indicate that the slow wave train comprises essentially the sediment trapped shear waves coupled with ocean acoustic waves the observed events of slow wave trains occur near the boundary between the outer arc high and the forearc basin where vertical differential motion has been suggested to occur we suggest that these small events represent the shallowest crustal activity unique to the triple junction off boso

time averaged fluxes of lead and fallout radionuclides to sediments in florida bay recent unmixed sediments from mud banks of central florida bay were dated using 210 pb 226 ra and chronologies were verified by comparing sediment lead temporal records with pb ca ratios in annual layers of coral montastrea annularis located on the ocean side of the florida keys dates of sediment lead peaks 1978 2 accord with prior observations of a 6 year lag between the occurrence of maximum atmospheric lead in 1972 and peak coral lead in 1978 smaller lags of 1 2 years occur between the maximum atmospheric radionuclide fallout and peaks in sediment temporal records of 137 cs and pu such lags are consequences of system time averaging sta in which atmospherically delivered particle associated constituents accumulate and mix in a sedimentary reservoir before transferring to permanent sediments and coral sta model calculations using time dependent atmospheric inputs produced optimized profiles in excellent accord with measured sediment 137 cs pu lead and coral lead distributions derived residence times of these particle tracers 16 1 15 7 0 7 19 3 and 16 2 years respectively are comparable despite differences in sampling locations in accumulating media and in element loading histories and geochemical properties for a 16 year weighted mean residence time sta generates the observed 6 year lead peak lag evidently significant levels of nondegradable particle associated contaminants can persist in florida bay for many decades following elimination of external inputs present results in combination with sta model analysis of previously reported radionuclide profiles suggest that decade scale time averaging may occur widely in recent coastal marine sedimentary environments

ensuring confidence in radionuclide based sediment chronologies and bioturbation rates abstract sedimentary records of naturally occurring and fallout derived radionuclides are widely used as tools for estimating both the ages of recent sediments and rates of sedimentation and bioturbation developing these records to the point of data interpretation requires careful sample collection processing analysis and data modeling in this work we document a number of potential pitfalls that can impact sediment core records and their interpretation this paper is not intended as an exhaustive treatment of these potential problems rather the emphasis is on potential problems that are not well documented in the literature as follows 1 the mere sampling of sediment cores at a resolution that is too coarse can result in an apparent diffusive mixing of the sedimentary record at rates comparable to diffusive bioturbation rates observed in many locations 2 210 pb profiles in slowly accumulating sediments can easily be misinterpreted to be driven by sedimentation when in fact bioturbation is the dominant control multiple isotopes of different half lives and or origin may help to distinguish between these two possible interpretations 3 apparent mixing can occur due simply to numerical artifacts inherent in the finite difference approximations of the advection diffusion equation used to model sedimentation and bioturbation model users need to be aware of this potential problem solutions to each of these potential pitfalls are offered to ensure the best possible sediment age estimates and or sedimentation and bioturbation rates can be obtained

creep of water ices at planetary conditions a compilation many constitutive laws for the flow of ice have been published since the advent of the voyager explorations of the outer solar system conflicting data have occasionally come from different laboratories and refinement of experimental techniques has led to the publication of laws that supersede earlier ones in addition there are unpublished data from ongoing research that also amend the constitutive laws here we compile the most current laboratory derived flow laws for water ice phases i ii iii v and vi and ice i mixtures with hard particulates the rheology of interest is mainly that of steady state and the conditions reviewed are the pressures and temperatures applicable to the surfaces and interiors of icy moons of the outer solar system advances in grain size dependent creep in ices i and ii as well as in phase transformations and metastability under differential stress are also included in this compilation at laboratory strain rates the several ice polymorphs are rheologically distinct in terms of their stress temperature and pressure dependencies but with the exception of ice iii have fairly similar strengths hard particulates strengthen ice i significantly only at high particulate volume fractions ice iii has the potential for significantly affecting mantle dynamics because it is much weaker than the other polymorphs and its region of stability which may extend metastably well into what is nominally the ice ii field is located near likely geotherms of large icy moons

a microearthquake survey at the junction of the east pacific rise and the wilkes 9 s fracture zone summary an ocean bottom seismograph survey of the junction of the east pacific rise and the wilkes fracture zone detected only three microearthquakes beneath the rise crest during seven days of recording in contrast during the same period 41 events were detected on the fracture zone all at distances greater than 10 km from the junction these results suggest that near the rise crest the thin crust can support sufficient stress only to generate infrequent small earthquakes and that most faulting may take place by aseismic slip at 10 km from the rise axis part of the crest has become competent enough to support stress resulting in earthquakes probably at depths of up to 5 km below the sea bed gear 5 waves on the seismometer records indicate that a magma chamber if it exists near the junction is less than 10 km across

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

temporal variability of 210pb fluxes and bioturbation in shelf sediments beneath the high primary production area off concepcion central southern chile 36 s abstract we estimated monthly fluxes of 210 pb in shelf sediments beneath a high productivity area off central southern chile 36 s during 1 year september 2002 august 2003 sediment cores were obtained using a multiple corer and were analyzed mainly for 210 pb total pigments and macrofauna abundance the 210 pb inventories and fluxes were estimated for surface sediments 0 5 cm and bioturbation coefficients were inferred using chlorophyll a reactive profiles in general 210 pb content was inversely correlated with phytodetritus fluxes high photosynthetic pigment contents in surface sediments were consistently associated with lower 210 pb contents macrofaunal activity responded to oxygen and organic matter supplies at the sediment surface generally concentrated in the first centimeters but particularly so during months of high organic matter fluxes and deficient bottom water oxygen conditions at this study site several processes involved in the 210 pb surface distribution make it difficult to accurately estimate ages at the surface we postulate that the organic fluxes promote changes in the faunal activity which in combination with sediment resuspension and water circulation over the shelf produce seasonal variations in the 210 pb inventories

holocene vs anthropocene sedimentary records in a human altered estuary the pasaia case northern spain abstract this work deals with the microfaunal benthic foraminifera and geochemical trace metals 14c 210pb 137cs and 239 240pu study of two boreholes 1200 and 1400 cm length and three subtidal sediment cores 38 58 cm length collected in the pasaia estuary northern spain foraminiferal assemblages in the holocene deposits depend on the paleogeographic setting while different species appear in the anthropocene sediments as a consequence of the physical transformation of the estuary into an estuarine port regarding the geochemical composition the impact of ancient mining activities probably of roman age can be recognized in the form of peaked values of pb and zn however older sediments exhibit relatively low contents of trace metals which are conditioned by hydrodynamic patterns and lithogenic inputs in the anthropocene record the influence of these factors is displaced by the decisive role of human activities as evidenced by the enhanced concentrations of trace metals and the occurrence of artificial radioisotopes and glass microspheres additionally this work provides information about trace metal contamination that may be of interest to corresponding authorities in order to set priorities of action and to take specific measures to prevent further degradation

spatial distribution and mechanisms of earthquakes in the southern new hebrides arc from a temporary land and ocean bottom seismic network and from worldwide observations in 1977 a wide aperture seismic network of land and ocean bottom stations obs was operated for 6 weeks in the southern new hebrides island arc data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size new hebrides earthquakes of the past 17 years to study the structure and deformational processes in the new hebrides subduction zone surprisingly small differences less than about 10 km were found between the locations of shallow and intermediate depth earthquakes as located by the temporary local network and by the international seismological centre using worldwide stations analysis of the p wave travel time data from the obs station closest to the trench shows limited evidence for a high velocity zone about 5 higher than the surrounding mantle associated with the descending plate the thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks but it is not very well defined by the spatial distribution of small size events located in this study shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of erromango and tanna is divided into a series of blocks that are differentially uplifted along mainly nw se striking faults during the obs experiment several shallow events were well located beneath the coriolis trough a riftlike feature located to the east of the volcanic arc well determined depths of these events are between 11 and 22 km the well located intermediate depth events define a 20 km thick benioff zone that has a dip of about 70 the spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short term sample of small events determined by the local network and the long term sample of locations based on worldwide data several focal mechanisms of moderate to large size intermediate depth events show components of lateral extension along the strike of the arc and some show components of lateral compression both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern new hebrides arc where the trench begins to curve around to the east

modeling shear zones in geological and planetary sciences solid and fluid thermal mechanical approaches abstract shear zones are the most ubiquitous features observed in planetary surfaces they appear as a jagged network of faults at the observable brittle surface of planets and in geological exposures of deeper rocks they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of mylonitized reduced grain sizes behind the overall size of the entire shear network rarely exceeds kilometer scale at depth although mylonitic shear zones are only visible to the observer when uplifted and exposed at the surface they govern the mechanical behavior of the strongest part of the lithosphere below 10 15 km depth mylonitic shear zones dissect plates thus allowing plate tectonics to develop on the earth we review the basic multiscale physics underlying mylonitic ductile shear zone nucleation growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization we also discuss a framework for analytic and numerical modeling including the effects of thermal mechanical couplings thermal elasticity the influence of water and void volatile feedback the physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium positive feedback can amplify strength heterogeneities by cascading through different scales we define basic intrinsic length scales of strength heterogeneity such as those associated with plasticity grain size fluid inclusion and thermal diffusion length scale for an understanding ductile shear zones we need to consider the energetics of deformation shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates for the long time scale deformational energy stored inside the shear zone through plastic dilation or crystallographic and shape preferred orientation consumes only a maximum of 10 of energy dissipated in the shear zones but creates structural anisotropy shear zones become long living features with a long term memory a special role is attributed to the presence of water in nominally anhydrous minerals we show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale we have shown that fully coupled finite element calculations using mechanical data from the laboratory can reproduce the basic mode of deformation of an entire mylonitic shear zone the next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure without losing the large scale constraint numerical methods capable of fulfilling the goal are emerging these are adaptive wavelet techniques and hybrid particle finite element codes which can be run over a computational grid across the net

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

prompt transgression and gradual salinisation of the black sea during the early holocene constrained by amino acid racemization and radiocarbon dating abstract the restricted environment of the black sea is particularly sensitive to climatic and oceanographic fluctuations owing to its connection with the mediterranean sea via the narrow bosphorus strait the exact mechanism and timing of the most recent connection between these water bodies is controversial with debate on the post glacial history of the black sea being dependent on radiocarbon dating for numerical ages here we present new 23 accelerator mass spectrometer ams radiocarbon ages on peat and bivalve molluscs supported by the first amino acid racemization aar dating of bivalve molluscs n 66 in the black sea these data indicate infilling of the black sea during the early holocene from an initial depth 107 m below sea level and 72 m below that of the bosphorus sill these data combined with a review of previous radiocarbon ages has enabled a unique perspective on the post glacial black sea a sea level curve based on conventional and ams radiocarbon ages on peat and ams based ages on dreissena sp shells indicate the water level in the earlier lake phase continued until the early holocene to be lower than the bosphorus sill after the younger dryas ended however the absence of ams dated mollusc ages from the shelves of this basin older than the younger dryas is suggestive of sub aerial exposure of the shelves and comparatively lower water levels when the younger dryas began thus post glacial outflow from the black sea occurred through a lowered or open bosphorus seaway basin wide radiocarbon ages on peat indicate a prompt increase in water level from that of the pre existing and unconnected palaeo lake during the earliest holocene 9600 9200 cal a bp mass colonisation of the black sea by mediterranean taxa did not occur until salinity had risen sufficiently a process which took 1000 a or more from the initial transgressive event this gradual change in salinity contrasts with the prompt transgression which would have taken 400 a to occur

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1

239 240pu and excess210pb inventories along the shelf and slope of the northeast u s a abstract sedimentary pu and excess 210 pb inventory data are examined from a large number of sites along the u s north atlantic shelf and slope from these data an estimate can be made of the magnitude and location of pu and 210 pb accumulation in this region a major feature of the data is a decrease in the pu and 210 pb inventories with water depth which appears to reflect a decrease in the net scavenging of these elements off shore when pu and 210 pb inventories are summed over water depths less than 4000 m in the north atlantic shelf and slope region the sediments can account for 24 8 of the expected pu and 83 15 of the expected 210 pb inputs at water depths greater than 4000 m in this region we find a greater deficiency in the measured inventories of both 210 pb and pu which cannot be accounted for in the shallower sediments this suggests that the northwest atlantic continental margin is not a major site of pu or 210 pb removal and storage when compared to atmospheric in situ and lateral inputs of these radionuclides

modeling detailed sedimentary 210pb and fallout 239 240pu profiles to allow episodic events an application in chesapeake bay dependable sediment chronologies are essential to the interpretation of the sedimentary record of past environmental change in the present article we use high resolution chemical and radiochemical data as the basis for a numerical simulation of sediment accumulation bioturbation and episodic deposition or erosion in a dynamic estuary we simulate episodic events by employing a time dependent sedimentation rate that we solve by finding a set of model parameters that describes depth profiles of both excess 210pb and fallout 239 240pu we apply the model to depth distributions of these tracer nuclides in cores from upper mid and lower bay sites in chesapeake bay at the upper bay site combining chemical and radiochemical data permits us to recognize and to quantify the sediment deposition due to tropical storm agnes 1972 at the lower bay site we demonstrate nonsteady sedimentation and propose plausible scenarios to account for it given adequate data our model can provide information that is not available from steady state models