origin of high mountains in the continents the southern sierra nevada active and passive seismic experiments show that the southern sierra despite standing 1 8 to 2 8 kilometers above its surroundings is underlain by crust of similar seismic thickness about 30 to 40 kilometers thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range but little subcrustal lithosphere is present beneath the eastern high sierra and adjacent basin and range these and other data imply the crust of both the high sierra and basin and range thinned by a factor of 2 since 20 million years ago at odds with purported late cenozoic regional uplift of some 2 kilometers

lower miocene to present stratigraphy of the equatorial pacific sediment bulge and carbonate dissolution anomalies 1 the bulge is a 600 m thick regional deposit of pelagic sediment accumulated around the equator its stratigraphy reflects a number of factors how accumulation rates have varied over time how accumulation has been spatially focused around the equator how much carbonate dissolution and reworking or nondeposition by bottom currents have occurred and how much the deposits have been translated northward by motion of the pacific tectonic plate on which they have accumulated in order to fully explore the effects of these processes a spatially continuous stratigraphic database is desirable as existing cores provide information at only discrete points and they tend to be sited in locally thicker accumulations we illustrate the utility of seismic data tied to drill cores by comparing the carbonate dissolution effect in two north south lines crossing the sediment bulge bathymetry over our easterly seismic line deepens relatively little going north whereas our westerly line deepens by 750 m from 4 n to 7 n at a common latitude the primary pelagic input to both these lines will have been similar so the difference of their sediment mass should reflect mostly their relative carbonate loss due to dissolution we use this fact to calculate the slope of the carbonate dissolution curve and find a remarkably small slope little relative carbonate dissolution compared to that expected from previous studies the techniques here may suggest a possible way forward for studying the equatorial carbonate stratigraphy more generally

drav model and its application in assessing groundwater vulnerability in arid area a case study of pore phreatic water in tarim basin xinjiang northwest china according to the characteristics of groundwater in arid area this paper proposes drav model for groundwater vulnerability assessment where d is groundwater depth r is the net recharge of aquifer a is the aquifer characteristics and v is the lithology of vadose zone as a case study the paper assesses the vulnerability of pore phreatic water in tarim basin of xinjiang china by using the drav model the results indicate that the areas of phreatic water with vulnerability index ranges of 2 4 4 6 6 8 and 8 accounting for 10 1 80 4 9 2 and 0 2 of the total plain area of the tarim basin respectively and the areas with the latter two vulnerability ranges 6 8 and 8 are mainly located in the irrigation districts with thin soil layer 20 30 cm thick surface soil of vadose zone mainly with underlying sandy gravel and with silty and fine sand layer such vadose zone generally lacks sandy loam and clayey soil and has larger recharge by infiltration of irrigation water

sediment focusing in the central equatorial pacific ocean at four sites in the central equatorial pacific ocean the flux of extraterrestrial he determined using the excess 230th profiling method is 8 10 13 cm stp cm 2 ka 1 this supply rate is constant to within 30 at these same sites however the burial rate of he determined using chronostratigraphic accumulation rates varies by more than a factor of 3 the lowest burial rates which occur north of the equator at 1 n 139 w are lower than the global average rate of supply of extraterrestrial he by 20 and indicate that sediment winnowing may have occurred the highest burial rates which are recorded at the equator and at 2 s are higher than the rate of supply of extraterrestrial he by 100 and these provide evidence for sediment focusing by analyzing several proxies measured in core pc72 sediments spanning the past 450 kyr we demonstrate that periods of maximum burial rates of 230th he 10be ti and barite with a maximum peak to trough amplitude of a factor of 6 take place systematically during glacial time however the ratio of any one proxy to another is constant to within 30 over the entire length of the records given that each proxy represents a different source 234u decay in seawater interplanetary dust upper atmosphere continental dust or upper ocean our preferred interpretation for the covariation is that the climate related changes in burial rates are driven by changes in sediment focusing

quantitative estimate of resedimentation in the pelagic sequence of the equatorial pacific abstract a study of deep sea sedimentation processes was undertaken in the east central equatorial pacific ocean near dsdp site 574 using lithologic information from the site and a grid of 250 km of high resolution seismic reflection profiles these data are used to quantitatively evaluate pelagic drape and basin filling processes in a pelagic carbonate environment sediment accumulation influenced by the relief of the depositional surface seafloor currents and sediment supply was examined in eight seismic stratigraphic intervals as expected the thinnest sediment sections occur over topographic highs and areas of steep dips while the thickest portions occur in lows and over gradual slopes syn depositional sediment redistribution varies between about 4 and 17 of the volume of the defined intervals

thorium 230 profiling in deep sea sediments high resolution records of flux and dissolution of carbonate in the equatorial atlantic during the last 24 000 years variations in carbonate flux and dissolution which occurred in the equatorial atlantic during the last 24 000 years have been estimated by a new approach that allows the point by point determination of paleofluxes to the seafloor an unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation the method is based on observations that the flux of unsupported 230th into deep sea sediments is nearly independent of the total mass flux and is close to the production rate thus excess 230th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated the study was conducted at two sites ceara rise western equatorial atlantic and sierra leone rise eastern equatorial atlantic in cores raised from three different depths at each site from measurements of 230th and caco3 changes in carbonate flux with time and depth were obtained a rapid increase in carbonate production starting at the onset of deglaciation was found in both areas this event may have important implications for the postglacial increase in atmospheric co2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters and possibly the north atlantic deep water increased carbonate dissolution occurred in the two regions during deglaciation followed by a minimum during mid holocene and renewed intensification of dissolution in late holocene during the last 16 000 years carbonate dissolution was consistently more pronounced in the western than in the eastern basin reflecting the influence of antarctic bottom water in the west this trend was reversed during stage 2 possibly due to the accumulation of metabolic co2 below the level of the romanche fracture zone in the eastern basin

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

the influence of lake morphometry on sediment focusing sediment focusing is a process whereby water turbulence moves sedimented material from shallower to deeper zones of a lake sediment focusing occurs in lakes in both the erosional coarse grained sediments and the transportational zone with the latter characterized by discontinuous sedimentation and resuspension the zone of accumulation is diefined as the zone where sedimentation is final i e no resuspension and where there is no further sediment focusing a geochemical tracer pb was used to trace sediment focusing patterns in 12 lakes of different morphomctries the area occupied by the zone of accumulation is predicted from the mean basin slope a with 86 of the variance explained only in large exposed lakes in which turbulence from waves is more severe are the resulting sediment distributions more erratic this study is the first to provide a general model of sc diment focusing patterns among lakes the study of lake bottom dynamics has received attention in the contexts of sediment contaminant and eutrophication studies the fact that sediments are major depositories for contaminants and nutrients has made them a focus for research in limnology also because sediments provide a record of past events in lakes an understanding of the manner in whllch sediments accumulate is essential for hindcasting interpretations of the historical record in sediments will be confounded if the record has been obscured by sediment resuspension the term sediment focusing was coined by likens and davis 1975 to describe the resuspension of sediments in shallower zones by waves and water currents with subsequent transport to and settling in the deeper zones of lakes much of the work

quantifying the distribution and transport of pelagic sediments on young abyssal hills the post depositional transport of pelagic sediments on rough seafloor is approximated as a nonlinear diffusive process the topographic slope distribution of the sediment water interface depends primarily on the apparent sediment diffusivity the average sediment load l and the root mean square height of the abyssal hill basement h these parameters have been determined for the arsrp corridor on the west flank of the mid atlantic ridge by fitting theoretical models to the slope distribution observed from center beam hydrosweep data the average value of the apparent diffusivity is h varies between 188 and 285 m with a mean of 230 7 m l is estimated to be as thin as 10 m on 5 ma crust near the ridge crest thickening to values greater than 50 m at 10 15 ma and then decreasing to less than 30 m on 25 ma crust in the western portion of the corridor the decrease in sediment load with age may be associated with a shallower carbonate compensation depth prior to the late miocene

equatorial pacific seismic reflectors as indicators of global oceanographic events the origin of a series of regionally correlatable seismic horizons in the neogene sediments of the central equatorial pacific is examined through seismic modeling and the detailed analyses of stratigraphic and physical property relationships in deep sea drilling project cores these regionally traceable reflectors are synchronous the younger reflectors are the direct result of carbonate dissolution events the older ones of stratigraphically selective diagenetic processes the changes in ocean chemistry associated with these events appear to be linked to global reorganizations of surface and bottom water circulation patterns the most dramatic of which are associated with reorganizations of north atlantic bottom waters these deepwater seismic horizons appear to correlate with the major events on the relative sea level curve of vail et al for the neogene

global energy dissipation rate of deep ocean low frequency flows by quadratic bottom boundary layer drag computations from current meter data the global energy dissipation rate of deep ocean low frequency flows by quadratic bottom boundary layer drag is estimated in three ways first an average over the dissipations computed from the near bottom velocities recorded by 290 moored current meters is multiplied by the world ocean area second near global maps of surface velocities derived from satellite altimetry data are used to estimate the bias due to the sparse spatial coverage of the moorings third a relationship between bottom and surface flows computed over the mooring locations is used to estimate global maps of bottom flows from the surface data all three methods suggest that at least 0 2 tw of the wind power input into geostrophic flows is dissipated in deep water by quadratic bottom drag implications for the oceanic overturning circulation and for oceanic mesoscale eddy dynamics are briefly discussed

paleogene calcite compensation depth in the eastern subtropical pacific answers and questions 1 ocean drilling program leg 199 drilled a north south transect across the eocene paleoequator in the eastern pacific permitting reconstruction the calcite compensation depth ccd since earliest eocene time the ccd was relatively shallow near the early eocene pacific equator 3200 m and unlike modern latitudinal ccd gradients deepened to the north to 3600 m paleolatitude 10 n at 41 ma the ccd underwent a brief sharp transient deepening of 700 m then remained shallow until the eocene oligocene boundary at the e o boundary the ccd deepened by 1200 m in less than 300 kyr this rapid deepening served to more than double the area of seafloor subject to caco3 deposition sea level fall associated with ice volume buildup and ensuing shelf basin fractionation is unlikely to be the sole cause of the increased deep ocean caco3 burial rather a sudden rapid increase in the amount of ca entering the ocean appears necessary to explain the observations

constraints from sea beam bathymetry on the development of normal faults on the east pacific rise we address the question of the horizontal extent of active upper crustal extension near a fast spreading ridge by applying seafloor slope criteria for normal fault identification to twelve sea beam swaths that cross the axis of the east pacific rise between 9 10 and 9 50 n both the density of and the total throw on identified fault system increase with axial distance to at least 30 km from the rise indicating that the half width of the zone of active normal faulting is 30 km or more the densities of inward and outward facing normal fault scarps in this distance range appear to be similar the distance from the rise to the nearest large throw fault zone is approximately axisymmetric to the north of 9 23 n but the mid point between nearest large throw fault zones is offset 2 3 km to the west of the bathymetric axis south of 9 23 n and thus tracks the center of the axial magma chamber reflector and presumably the axis of most recent magmatic emplacement

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

climatic change and caco3 preservation an 800 000 year bathymetric reconstruction from the central equatorial pacific ocean sixteen deep sea cores from the central equatorial pacific are used to reconstruct a continuous 800 000 year 800 kyr record of bathymetric variations in carbonate preservation as measured by calcium carbonate caco3 content the depth of the sedimentary lysocline has fluctuated markedly in conjunction with late pleistocene climate cycles while the carbonate critical depth ccrd which is the water depth where the sediments contain 10 caco3 has remained relatively constant as a result the depth of the lysocline controls the bathymetric position and thickness of the caco3 transition zone defined as the depth from the lysocline to the ccrd modern and interglacial aged sediments show poor caco3 preservation and thick caco3 transition zones glacial aged sediments show good preservation and deep thin zones due to the deepening of the lysocline detailed comparison of the caco3 preservation and oxygen isotope records from the central equatorial pacific confirms the observation that preservation maxima and minima tend to occur during the latter half of glacial and interglacial stages and on climate transitions rather than during the middle of climatic stages during the nine major glacial stages of the last 800 kyr the lysocline deepened by at least 400 to 800 m this deepening indicates an increase in the abyssal carbonate ion concentration co3 and a depression of the calcite saturation horizon best explained by the deeper presence of a more carbonate saturated water mass the bottom of the transition zone has remained at a relatively constant depth during the brunhes chron indicating a balance between caco3 sedimentation and dissolution in the deepest waters of the central equatorial pacific

the record of late pleistocene biogenic sedimentation in the eastern tropical pacific ocean we have generated 300 kyr records of biogenic opal calcite and organic carbon corg for three cores in the eastern and central equatorial pacific ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region we find that corg has been deposited in common pulses throughout the area while opal has a much more local pattern of variation calcite varies regionally but the record is shaped by superimposed dissolution and productivity processes the most intense corg peak occurs at 18 ka and can have greater than 2 times the holocene corg content other major corg peaks occur 150 ka and perhaps at 280 ka we have compared the corg record in one of the cores v19 28 to a model deepwater oxygen record developed from 13c data in the nearby v19 30 to test whether the corg record has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone we found no coherence between the two records implying that the corg record is primarily a measure of productivity by comparing the opal calcite and corg records in v19 28 a core which is at or above the lysocline we found that both increased calcite and opal deposition matches high corg accumulation we also found however that the calcite and opal records were uncorrelated so that episodes of high opal deposition do not necessarily accumulate calcite rapidly we hypothesize that at least two different plankton communities have been dominant in the waters above this site one rich in opal secreting plankton and one more dominated by calcite producers the opal rich plankton community was dominant during the intervals 10 15 ka and 35 60 ka supplement data table 2 is available with entire articleon microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document p88001 2 50 payment must accompany order

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

glacial to interglacial changes in carbonate and clay sedimentation in the atlantic ocean estimated from 230th measurements abstract the cause of the climatically controlled fluctuations in the carbonate content of deep sea sediments remains the subject of uncertainty and debate three variables are involved supply of biogenic carbonate loss by dissolution and dilution by non carbonate phases it is suggested that 230 th which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale results of a preliminary analysis based on published data indicate that for depths at and above the lysocline the carbonate fluctuations observed in cores from the north atlantic ocean are due primarily to variations in the terrigenous clay input which was 2 5 times higher during glacials than during interglacials carbonate deposition appears to have been somewhat reduced during glacials but probably not by more than a factor of 2 from published 230 th 232 th profiles it appears that the south atlantic ocean also received increased inputs of terrigenous clay during glacial periods

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

seismic interpretation of pelagic sedimentation regimes in the 18 53 ma eastern equatorial pacific basin scale sedimentation and infilling of abyssal valleys understanding how pelagic sediment has been eroded transported and deposited is critical to evaluating pelagic sediment records for paleoceanography we use digital seismic reflection data from an integrated ocean drilling program site survey amat03 to investigate pelagic sedimentation across the eastern central equatorial pacific which represents the first comprehensive record published covering the 18 53 ma eastern equatorial pacific our goals are to quantify 1 basin hill scale primary deposition regimes and 2 the extent to which seafloor topography has been subdued by abyssal valley filling sediments the eastern pacific seafloor consists of a series of abyssal hills and basins with minor late stage faulting in the basement ocean crust rarely outcrops at the seafloor away from the rise crest both hills and basins are sediment covered the carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data overall we recognized three different sedimentation regimes depositional high sedimentation rate transitional and minimal sedimentation low sedimentation rate regimes in all areas the sedimented seafloor mimics the underlying basement topography although the degree to which topography becomes subdued varies depositional regimes result in symmetric sedimentation within basins and subdued topography whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief regardless of sedimentation regime enhanced sediment deposition occurs within basins however we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills if this variation is due to sediment focusing the focusing factor in the basins as measured by 230th is no more than a factor of 1 3 of the total vertical particulate rain

the distribution of radiogenic heat production as a function of depth in the sierra nevada batholith california geochemical analyses and geobarometric determinations have been combined to create a depth vs radiogenic heat production database for the sierra nevada batholith california this database shows that mean heat production values first increase then decrease with increasing depth heat production is 2 aw m 3 within the 3 km thick volcanic pile at the top of the batholith below which it increases to an average value of 3 5 aw m 3 at 5 5 km depth then decreases to 0 5 1 aw m 3 at 15 km depth and remains at these values through the entire crust below 15 km below the crust from depths of 40 125 km the batholith s root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0 14 aw m 3 this is higher than the rates from most published xenolith studies but reasonable given the presence of crustal components in the arc root assemblages the pattern of radiogenic heat production interpreted from the depth vs heat production database is not consistent with the downward decreasing exponential distribution predicted from modeling of surface heat flow data the interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the sierra nevada could be generated within the 35 km thick crust this requires a very low heat flux from the mantle which is consistent with a model of cessation of sierran magmatism during laramide flat slab subduction followed by conductive cooling of the upper mantle for 70 m y the heat production variation with depth is principally due to large variations in uranium and thorium concentration potassium is less variable in concentration within the sierran crust and produces relatively little of the heat in high heat production rocks because silica content is relatively constant through the upper 30 km of the sierran batholith while u th and k concentrations are highly variable radiogenic heat production does not vary directly with silica content d 2005 elsevier b v all rights reserved

the potential of major ion chemistry to assess groundwater vulnerability of a regional aquifer in southern quebec canada groundwater vulnerability mapping provides useful but limited information for developing protection plans of the resource classical vulnerability ranking methods often do not take into account complex hydrostratigraphy and never consider groundwater flow dynamics the objective of this work was to test the potential of major ion chemistry to assess regional scale intrinsic groundwater vulnerability because it reflects water sediment and water rock interactions the new vulnerability index reflects both infiltration processes and groundwater hydrodynamics the method was applied on a regional fractured bedrock aquifer located in the becancour region of southern quebec canada in this region hydrogeochemistry shows that freshly recharged groundwater evolves from ca mg hco3 and ca so4 to na hco3 type with gradually increasing confinement conditions in the fractured aquifer and tends to na cl type locally by mixing with trapped marine pore water the new method identified recharge areas as those of the highest vulnerability and gradually decreasing vulnerability as confinement of the aquifer increased it also highlights local discontinuities in confinement that differ from the regional pattern results showed a good correlation between groundwater vulnerability estimated with the new method and nitrate occurrence in groundwater eighty two per cent of all samples presenting detectable nitrate concentrations were characterized by a hydrogeochemical vulnerability index greater than 9 maximum is 10 the ability of the new vulnerability method to identify areas vulnerable to detectable nitrate concentrations was much higher than that deriving from the drastic method this work confirms that major ions chemistry contains significant information about groundwater vulnerability and could be used to improve groundwater resource management

sediment size fractionation and focusing in the equatorial pacific effect on 230th normalization and paleoflux measurements we use flux dissolution and excess 230th data from the joint global ocean flux study and manganese nodule project equatorial pacific study site c to assess the extent of sediment focusing in the equatorial pacific measured mass accumulation rates mar from sediment cores were compared to reconstructed mar by multiplying the particulate rain caught in sediment traps by the 230th focusing factor and subtracting measured dissolution caco3 mar is severely overestimated when the 230th focusing factor correction is large but is estimated correctly when the focusing factor is small in contrast al fluxes in the sediment fine fraction are well matched when the focusing correction is used since caco3 is primarily a coarse sediment component we propose that there is significant sorting of fine and coarse sediments during lateral sediment transport by weak currents because caco3 does not move with 230th normalization typically overcorrects the caco3 mar and because caco3 is 80 of the total sediment 230th normalization overestimates lateral sediment flux fluxes of 230th in particulate rain caught in sediment traps agree with the water column production sorption model except within 500 m of the bottom near the bottom 230th flux measurements are as much as 3 times higher than model predictions there is also evidence for lateral near bottom 230th transport in the bottom nepheloid layer since 230th fluxes caught by near bottom sediment traps are higher than predicted by resuspension of surface sediments alone resuspension and nepheloid layer transport under weak currents need to be better understood in order to use 230th within a quantitative model of lateral sediment transport

comparing the smooth parabolic shapes of interfluves in continental slopes to predictions of diffusion transport models abstract computer models of stratigraphic development at continental margins often include elements in which sediment topography evolves according to a diffusion equation representing effects of down slope sediment transport however there has been little effort to verify the model for example by showing whether deposit geometries are as expected from it or by considering whether transport is consistent with the model s assumptions the issue is important for understanding transport erosion and deposition in continental slopes more generally and the extent to which such processes are recorded in seabed morphology examples of slope topography illustrating diffusive like characteristics are examined here in the uppermost slope of the usa central atlantic margin sandy areas between canyon heads are smoothly convex upwards and almost parabolic a morphology similar to subaerial soil mantled hillslopes known to evolve according to a diffusion equation other data of interfluves in muddy sediment from both atlantic and pacific usa margins also show this smooth convex upwards form in subaerial hill crests the upwards convex parabolic shape arises where steady erosion by streams along channels provide constant flux boundary conditions i e the parabola is the steady state solution to the diffusion equation although submarine processes will likely be different the similar morphology of submarine interfluves prompts the question as to whether erosion along channels and movements of sediment between channels can also sometimes occur to cause a diffusive like evolution of topography the discussion therefore focuses on submarine processes affecting erosion and deposition in particular to address whether the morphology results from true diffusion which requires that sediment moves down slope with a transport flux proportional to local bed gradient or is fortuitous sediment transport is instead unrelated to local bed gradient and the morphology arises from other effects on accumulation rates

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

diffusion transport model for pelagic sediments on the mid atlantic ridge the diffusion model is potentially useful for quantifying the effect of downslope gravity transport on sedimentation rate variations which are commonly found between deep sea drilling project and ocean drilling program sites if appropriate the model and stratigraphy could be used for example to constrain the amount and timing of fault block rotation deep tow profiler records from the french american mid ocean undersea study area of the mid atlantic ridge are examined to determine whether they are consistent with a diffusion model sedimentary contacts with basement show that a variety of processes may be operating some of which are inconsistent with the diffusion model for example there are moats around some contacts which are typical of scouring or nondeposition due to high current velocities more than half of the contacts however show onlapping which is qualitatively consistent with the diffusion model sediments generally fill low areas of the topography and have smooth surfaces as expected from a diffusion model where the fluxes of sediment entering a basin are constant the diffusion model predicts that the sediment surface should evolve to a parabola the steady state solution some 20 curved surfaces in the profiler data were digitized and least squares parabolas fitted to them with rms errors of less than 1 m the slopes of the model parabolas provide values for the ratio of sediment flux to diffusivity at the edges of the basins steep surfaces are produced by low diffusivity or high fluxes this ratio is combined with estimates of the fluxes to determine the apparent sediment diffusivity kapp for eacri basin flux is estimated by assuming the abyssal hill topography acts as a simple sediment trap so that the lateral flux equals the width of a basin s pelagic catchment area times the area s mean sedimentation rate s using this method median kapp is 0 04 0 11 m2 yr 1 assuming s 10 30 m m y 1 variations in kapp and the assumptions behind the diffusion model are discussed and alternative causes of sediment surface tilts are considered in particular differential compaction which is modelled using a simple porosity depth function

neogene carbonate burial in the pacific ocean 1 i have compiled caco3 mass accumulation rates mars for the period 0 25 ma for 144 deep sea drilling project and ocean drilling program drill sites in the pacific in order to investigate the history of caco3 burial in the world s largest ocean basin this is the first synthesis of data since the beginning of the ocean drilling program sedimentation rates caco3 contents and bulk density were estimated for 0 5 myr time intervals from 0 to 14 ma and for 1 myr time intervals from 14 to 25 ma using mostly data from initial reports volumes there is surprisingly little coherence between caco3 mar time series from different pacific regions although regional patterns exist a transition from high to low caco3 mar from 23 20 ma is the only event common to the entire pacific ocean this event is found worldwide the most likely cause of lowered pelagic carbonate burial is a rising sea level trend in the early miocene the central and eastern equatorial pacific is the only region with adequate drill site coverage to study carbonate compensation depth ccd changes in detail for the entire neogene the latitude dependent decrease in caco3 production away from the equator is an important defining factor of the regional ccd which shallows away from the equatorial region examination of latitudinal transects across the equatorial region is a useful way to separate the effects of changes in carbonate production productivity from changes in bottom water chemistry dissolution upon carbonate burial

intraplate volcanism of the western pacific new insights from geological and geophysical observations in the pigafetta basin understanding intraplate volcanism is a key to deciphering the earth s magmatic history one of the largest intraplate volcanic events occurred during the mid cretaceous roughly 75 to 125 ma in the western pacific to investigate the origin and effects of this volcanism on various earth systems we present the first comprehensive study of volcanism in the pigafetta basin using seismic surveys magnetic and gravity modeling and ocean drilling program drill core and well log data from site 801 our results show that intraplate volcanism in the pigafetta basin coincides with the rest of the western pacific seamount provinces supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid cretaceous volcanic events we also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the jurassic ocean crust in the pigafetta basin and hence marine magnetic anomalies recorded in the jurassic basement are preserved also the formerly identified rough smooth boundary rsb is indistinguishable from any other rough smooth topographic boundaries throughout the survey area suggesting that the rsb is unlikely to be a cretaceous sill jurassic basement boundary lastly the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the pigafetta basin indicating that hydrothermal circulation was ongoing well past 100 ma this article is protected by copyright all rights reserved

globsed updated total sediment thickness in the world s oceans we present globsed a new global 5 arc minute total sediment thickness grid for the world s oceans and marginal seas globsed covers a larger area than previously published global grids and incorporates updates for the ne atlantic arctic southern ocean and mediterranean regions which results in a 29 7 increase in estimated total oceanic sediment volume we use this new global grid and a revised global oceanic lithospheric age grid to assess the relationship between the total sediment thickness and age of the underlying oceanic lithosphere and its latitude an analytical approximation model is used to mathematically describe sedimentation trends in major oceanic basins and to allow paleobathymetric reconstructions at any given geological time this study provides a much needed update of the sediment thickness distribution of the world oceans and delivers a model for sedimentation rates on oceanic crust through time that agrees well with selected drill data used for comparison

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific 1 the paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing bioturbation and horizontal sediment transport diagenesis and bioturbation are relatively well studied but the effects of physical sedimentation have been largely ignored modeling u series isotopes e g 230th can potentially quantify horizontal sediment movement but model horizontal sediment focusing often equals or exceeds the vertical particle rain we find no evidence of this level of sediment focusing in the equatorial pacific from geophysical data or sediment core comparisons the overestimate of sediment focusing by 230th is probably caused by poor model assumptions that sediment does not fractionate does not sort according to size during transport and that 230th cannot leak from slowly accumulating sediments both assumptions are weak u series methods do hold promise to quantify sediment focusing if properly calibrated with calibration the trade offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed

vlda model and its application in assessing phreatic groundwater vulnerability a case study of phreatic groundwater in the plain area of yanji county xinjiang china based on the god method and drastic model for groundwater vulnerability assessment in this paper vlda model for assessing phreatic groundwater vulnerability is proposed where v is lithology of vadose zone l is the pattern of land use d is groundwater depth and a is the aquifer characteristics as a case study the paper assesses the vulnerability of phreatic groundwater in yanji county of xinjiang by using the vlda model as the tool and gis as the platform and matches the vulnerability with nitrate content of the phreatic groundwater in september 2009 the results indicate that areas with highly vulnerable pore phreatic groundwater account for 40 83 of the total inland area of yanji county including most of the northwestern area such as the groundwater wellfield in korla city and yanji town it is also discovered that areas with phreatic groundwater of higher vulnerability essentially coincide with areas with higher nitrate content indicating the feasibility of using vlda model to assess groundwater vulnerability

warping and cracking of the pacific plate by thermal contraction 1 lineaments in the gravity field and associated chains of volcanic ridges are widespread on the pacific plate but are not yet explained by plate tectonics recent studies have proposed that they are warps and cracks in the plate caused by uneven thermal contraction of the cooling lithosphere we show that the large thermoelastic stress produced by top down cooling is optimally released by lithospheric flexure between regularly spaced parallel cracks both the crack spacing and approximate gravity amplitude are predicted by elastic plate theory and variational principle cracks along the troughs of the gravity lineaments provide conduits for the generation of volcanic ridges in agreement with new observations from satellite derived gravity our model suggests that gravity lineaments are a natural consequence of lithospheric cooling so that convective rolls or mantle plumes are not required index terms 1208 geodesy and gravity crustal movements intraplate 8110 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1219 geodesy and gravity local gravity anomalies and crustal structure 3045 marine geology and geophysics seafloor morphology and bottom photography 8159 tectonophysics rheology crust and lithosphere keywords thermal contraction pacific plate gravity anomalies

hydrothermal pits in the biogenic sediments of the equatorial pacific ocean 1 a recent expedition to the equatorial pacific surveyed over 60 000 km2 of seafloor using swath map imaging and high resolution seismic profiling this survey revealed the common occurrence of large pits in the biogenic sediments that are usually associated with basement highs the pits range in size up to a few kilometers across and are on the order of 50 m 100 m deep they were found on seafloor ranging in age from 15 ma to 55 ma with overlying sediment cover up to 500 m thick we link their origin with hydrothermal discharge through conduits created by differential compaction and fracturing over basement highs and by basement faulting that penetrates the overlying sediments although carbonate dissolution is likely to be associated with the formation of the pits their occurrence shows no consistent relationship with the present calcite compensation depth ccd

a global analysis of mid ocean ridge axial topography summary current views of mid ocean ridges are strongly influenced by extensive mapping of the mid atlantic ridge and east pacific rise the global picture of the mid ocean ridge system particularly in the sparsely surveyed southern oceans is still based primarily on underway bathymetry profiles collected over the past 40 years this study presents a quantitative analysis of global mid ocean ridge morphology based on 156 of these underway bathymetric profiles thereby allowing commonly recognized features such as axial valleys and axial ridges to be compared on a global basis an empirical orthogonal function eof analysis is used to separate deterministic and stochastic components of axial morphology and to quantify the dependence of each on parameters such as spreading rate and axial depth it is found that approximately 50 per cent of the variance in axial morphology may be described as a linear combination of five independent symmetric and anti symmetric modes the remainder is considered stochastic maximum axial valley relief decreases with spreading rate for rates less than 80 mm yr 1 while axial ridge relief remains relatively constant for all rates greater than 50 mm yr 1 the stochastic component of the axial morphology referred to as bathymetric roughness also decreases with spreading rate for rates less than 80 mm yr 1 and remains relatively constant at higher rates although both axial valley relief and bathymetric roughness near the ridge axis show a similar spreading rate dependence they are weakly correlated at slow spreading centres the distinct differences in morphologic variability of fast and slow spreading ridges may result from the episodicity of magmatic heat input which controls the lithospheric rheology at slow spreading ridges these observations support the notion of a critical threshold separating two dynamically distinct modes of lithospheric accretion on mid ocean ridges

biogenic sediment regimes in the neogene equatorial pacific iodp site u1338 burial production and diatom community abstract the equatorial pacific experienced significant changes in productivity and microfossil assemblage since 16 million years ago ma we compiled a record of those changes from iodp site u1338 using a reconnaissance of diatom assemblages and high resolution xrf scan chemical profiles 1 2 kyr spacing productivity and caco3 dissolution intervals are defined by sediment component ratios in particular opal clay opal baso4 and caco3 baso4 or caco3 clay there are large abrupt changes in export production in the miocene especially compared to the pleistocene and pliocene and average levels of export production are higher throughout the miocene compared to the pliocene and pleistocene using diatom assemblages and bulk sediment composition the u1338 sediment record is divided into several distinct deposition regimes 1 a middle miocene regime 13 2 ma marked by low diatom numbers and a few short lived productivity intervals 2 a carbonate crash regime distinguished by an older substage 13 2 10 2 ma with relatively high abundances of productivity related diatoms and common productivity related depositional intervals interspersed with caco3 dissolution intervals and a younger carbonate crash substage 10 2 8 0 ma with weaker brief productivity intervals high caco3 dissolution and moderately high numbers of upwelling diatom species 3 the biogenic bloom regime 8 0 4 5 ma marked by extended periods of high opal and caco3 deposition with a maximum between 7 0 and 6 4 ma and 4 a pliocene modern regime 4 5 0 ma with lower productivity and high cyclic caco3 dissolution changes in production and dissolution result from reorganizations of nutrient supply to the equatorial pacific not by higher wind driven upwelling we propose that nutrients were more accessible in the miocene because of a combination of factors including higher organic matter degradation in the upper water column and deep intermediate nutrient pathways that efficiently recycled nutrients to upwelling zones closure of the central american seaway was likely a prime cause of the biogenic bloom

paleogene tropical pacific clues to circulation productivity and plate motion 1 stratigraphic data from 63 deep sea drilling project and ocean drilling program odp sites that sample the lower neogene and paleogene sediments of the tropical pacific have been compiled and put on the biostratigraphic and paleomagnetic timescale refined by odp leg 199 scientists sediment accumulation rates have been calculated for ten intervals ranging in age from 10 to 56 ma and have been plotted for the midpoint of each interval at the associated paleoposition for each site used a fixed hot spot model was used for reconstruction of the pacific lithospheric plate all such reconstructed intervals show the development of a tongue of high accumulation rates associated with the oceanographic divergence at the geographic equator the development of this equatorial band is weakest between 46 and 56 ma the time of the peak warmth in paleogene climate possible motion of the hawaiian hot spot or true polar wander between 46 and 56 ma appears to have had little effect on the plate rotation estimate of the position of the equator in addition to temporal changes in the calcite compensation depth and in productivity the biggest change in the patterns of sediment accumulation rates in the eastern tropical pacific was the development of a relatively strong divergence between 6 and 10 n near the region of divergence between the modern north equatorial current and the north equatorial counter current changes in the equatorial circulation appear to be associated in time with the opening and closing of oceanic gateways particularly the complex closing of the caribbean pacific gateway

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

the role of internal tides in mixing the deep ocean abstract internal wave theory is used to examine the generation radiation and energy dissipation of internal tides in the deep ocean estimates of vertical energy flux based on a previously developed model are adjusted to account for the influence of finite depth varying stratification and two dimensional topography specific estimates of energy flux are made for midocean ridge topography weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km the richardson number of the radiated internal tide typically exceeds unity for these motions and so direct shear instability of the generated waves is not the dominant energy transfer mechanism it also seems that wave wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux instead it appears that most of

platinum group element geochemistry of peridotite xenoliths from the sierra nevada and the basin and range california abstract the nature of pge re pge pt pd os ir ru behavior in subcontinental lithospheric mantle was investigated using new high precision pge re abundance measurements and previously published re os isotopic analyses of peridotite xenoliths from the sierra nevada and mojave province california ru ir ratios and ir concentrations are constant over a wide range in s content and major element fertility indices e g mg mg fe indicating that ru and ir are not only compatible during partial melting but also that their partitioning behaviors may not be controlled entirely by sulfide pt ir pd ir os ir and re ir ratios range from slightly superchondritic to distinctly subchondritic for all xenoliths except for one anomalous sample 1026v which is characterized by radiogenic 187os 188os low re os ratio and large enrichments in cu os pt pd and s relative to ir copps metasomatism assuming chondritic initial relative abundances the magnitudes of some of the depletions in pt pd os and re relative to ir and ru require incompatible behavior or substantial secondary loss in detail some samples which are otherwise characterized by fertile major element indices exhibit low s contents and subchondritic os ir and pd ir ratios indicating that depletions in pd and os relative to ir are not simple functions of the degree of melting as inferred from major elements possible mechanisms for depleting pt pd os and re relative to ir and ru include partitioning into chromian spinels and alloys partitioning between sulfide and sulfide liquids mobilization by aqueous fluids or secondary loss associated with late stage sulfide breakdown however it is not possible to explain all of the depletions in pt pd os and re by any single mechanism the preferential enrichment in os over re and ir in sample 1026v is somewhat paradoxical because this sample s radiogenic 187os 188os requires a metasomatic agent originating from a source with a high time integrated re os ratio the abundant garnet websterite xenoliths may be a suitable source because they have high re os ratios radiogenic os and abundant garnet which may sequester re over os during partial melting however their extremely low os contents require the processing of large amounts of garnet websterite to concentrate enough os into the metasomatic sulfides needed to enrich sample 1026v in os the homogeneity in 187os 188os ratio in the remaining xenoliths suggest that their os isotopic compositions were not significantly affected by pge metasomatism the singular nature of 1026v s composition emphasizes the rarity of copps metasomatism

ediacaran gaskiers glaciation of newfoundland reconsidered newly discovered palaeosols in the gaskiers formation of newfoundland provide a new perspective on its palaeoenvironment and are evidence that it was not a deep submarine tillite nor an anoxic deep sea floor nor a cap carbonate from extreme geochemical perturbation of the global ocean discovery of oxidized palaeosols and re evaluation of sedimentary facies and stratigraphic relationships now suggest reinterpretation as a coastal plain with glacial moraines in the forearc basin of a continental volcanic arc like modern japan ediacaran palaeosols of the gaskiers formation lack large root traces but show soil profile differentiation clay enrichment and depletion of alkali and alkaline earth elements at the expense of feldspar and rock fragments and diagnostic soil structures blocky peds argillans sepic plasmic fabric these palaeosols are evidence of a humid temperate climate and marked marine regression accompanying the gaskiers glaciation of the early ediacaran 580 ma geochemical weathering trends in the palaeosols especially phosphorus depletion are characteristic of biologically active soils ediacaran microbial earth ecoystems may have been responsible for filamentous disruption of bedding in the palaeosols

sediment mixing in the tropical pacific and radiolarian stratigraphy 1 the reworking of older radiolarian microfossils into near surface sediments of the tropical pacific has long been the source of confusion for the development of radiolarian stratigraphy and of puzzlement over the mechanism s that could effect such pervasive reworking widespread dissolution pits in the sediments of the tropical pacific are believed to be associated with hydrothermal circulation cells in the older oceanic crust and are here linked to processes which expose older sections and inject older non carbonate material into near bottom waters discharging waters of these circulation cells tend to dissolve carbonate in near surface sediments thus only the non carbonate material including radiolarians is preserved and reworked into younger sediments results from the study of two sites in the tropical pacific indicate that reworked older stratigraphically important radiolarians are less than 2 of the total radiolarian assemblage this constitutes a minimum estimate of the amount of reworked non carbonate material in the younger sediments the oldest reworked radiolarians are no more than 10 m y younger than the underlying basement and radiolarians from the entire older section above that level can be found in the reworked material a time series of the flux of reworked material at one site is not constant but instead has varied by a factor of 3 to 4 over the past 2 5 m y during times when the flux of reworked material is particularly low the proportion of older more robust radiolarians is larger

a global and regional stochastic analysis of near ridge abyssal hill morphology this paper presents the results of a global and regional stochastic analysis of near ridge abyssal hill morphology the analysis includes the use of sea beam data for the estimation of stochastic parameters up to order 4 these parameters provide important quantitative physical information regarding abyssal hills including their rms height azimuthal orientation characteristic width aspect ratio hausdorff dimension skewness tilt and peakiness the global data set consists of 64 sea beam swaths near the rivera cocos and nazca spreading sections of the east pacific rise the mid atlantic ridge and the central indian ridge in one form of analysis the parameters are averaged among spreading rate bins each of the spreading rate subsets can be identified as unique from the others in at least one aspect the slowest spreading rate subset mid atlantic data exhibit the largest scales rms height and characteristic width and length of abyssal hills these parameters generally decrease as spreading rate increases up to the fast spreading rate data pacific cocos but increase going from fast to very fast pacific nazca spreading rate data this indicates some complexity in the relationship between spreading rate and abyssal hill morphology the plan view aspect ratio is nearly twice as large for the fast spreading rate data than for any of the other subsets and is smallest for the very fast spreading rate data the fractal dimension is nearly identical for all spreading rate subsets the vertical skewness is positive for the slow and medium spreading rate data indicating larger peaks than troughs and negative for the fast spreading rate data indicating larger troughs than peaks the kurtosis or peakiness is everywhere larger than the gaussian value of 3 and tends to be larger in the atlantic than the pacific the tilting parameter provides substantial evidence indicating steeper inward facing slopes in the medium and fast spreading rate data but only marginal evidence for it in the slow spreading rate data from an analysis of correlations among parameters it is found that subsets sometimes behave differently from the entire data set in particular while over the global data set the characteristic width exhibits a well resolved positive trend when plotted versus rms height these parameters exhibit a more gradual positive trend in the mid atlantic data and a negative trend in the pacific cocos data in addition the plan view aspect ratio while generally uncorrelated with rms height for the global data set is positively correlated with rms height for the pacific cocos data set these results emphasize a strong uniqueness of the pacific cocos data relative to the rest of the data global set the pacific cocos data consist of 27 swaths concentrated between the siquieros and orozco fracture zones these data provide very good abyssal hill coverage of this well mapped and well studied region and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters in this analysis it is found that abyssal hill parameters are correlated with the depth of the adjacent ridge axis indicating that the relative abundance in magma supply which likely controls the ridge axis depth may also be an important influence on the formation of abyssal hills

timing of late quaternary productivity pulses in the panama basin and implications for atmospheric co2 high resolution percent corg and 18oforam records obtained from panama basin core atlantis ii 54 25pc and additional data from nearby core p7 show that enhanced burial of organic carbon has characterized every major glacial period for the last 500 kyr in that area both corg concentration and mass accumulation rate profiles exhibit a sawtooth pattern with maxima occurring typically in the later stages of glacial periods comparison with dust records suggests that the carbon accumulation rate profile reflects both the upwelling history and a variable rate of iron input during the late quaternary the sawtooth character may derive from increased wind velocities and rates of upwelling during glacials which are indirectly related to ice volume sarnthein et al 1988 the rapid decline in export production at the end of glacials in the equatorial pacific may be attributed to the retreat of ice sheets thus reduced wind velocities and upwelling coupled with a coincident decline in atmospheric dust load and or delivery rate the corg accumulation rate profiles do not correlate well with atmospheric co2 records for example atmospheric co2 was already at a minimum 40 kyr ago when production in the panama basin began increasing dramatically commensurate with an increase in global dust levels using the relationship between the degree of photosynthetic fractionation and the concentration of free co2 in the surface ocean postulated by popp et al 1989 13corg measurements made on core p7 show that panama basin surface waters have been supplying co2 to the atmosphere continually for at least the last 50 kyr there is no evidence for a flux of co2 into the surface ocean in this area at any time during this period despite the higher production if the panama basin cores are representative of the eastern and central equatorial pacific then these observations weaken the influence on co2 drawdown postulated for increased glacial productivity at low latitudes

fragments of the mantle and crust from beneath the sierra nevada batholith xenoliths in a volcanic pipe near big creek california a miocene volcanic pipe that cuts cretaceous granodiorite is exposed in the central sierra nevada near the town of big creek california the volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent resulting in a pseudo andesitic composition the conduit cut through the entire thickness of the sierra nevada batholith as well as the underlying lower crust and upper mantle and the magma entrained a diverse polygenetic assemblage of igneous and metamorphic xenoliths including peridotites eclogites and granulites schists and hornfels and gabbroids and granitoids some xenoliths of the high grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the sierra nevada garnet granulites the most abundant rock type of the mafic high grade metamorphic assemblage and eclogites are restricted to the central area of the pipe these xenoliths are essentially bimineralic consisting of colorless to pale green clinopyroxene and garnet eclogites contain brilliant emerald green clinopyroxene the clinopyroxenes which have a broad range of compostions mg mg fe 0 45 0 90 have a 20 jadeite component in eclogites garnets also have a range of compositions mg mg fe 0 30 0 70 with those from eclogites being the most fe rich one of the xenoliths contains ca rich garnet grs adr 90 feldspathic granulites contain plagioclase of an 45 70 and some lack garnet hornblende occurs in some granulites and a few amphibolites are present likely protoliths are ocean floor basalts for the eclogites and precambrian limestone for the grossular clinopyroxene rock the big creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western sierra nevada

pelagic sedimentation on rough seafloor topography 2 inversion results from the north atlantic acoustic reverberation corridor seafloor slope distributions are sensitive to basement topography the surface shape of sediment distributions and the average thickness of sediment within a region by comparing slope distributions computed from bathymetric data to distributions from realistic models of sedimented topography we determine how characteristic values of average sediment thickness l rms basement variability h and apparent diffusivity vary throughout the office of naval research acoustic reverberation corridor located on the western flank of the mid atlantic ridge north of the kane fracture zone 25 5 27 25 n 44 5 49 w values of l from basin centered subregions show thinner sediments near the axis thickening westward with maximum values found on 10 20 ma seafloor lower values of l are found for older late oligocene early miocene seafloor values of l on young seafloor suggest a sediment accumulation rate of 6 5 2 3 m m y since 10 ma values of h computed from hill centered regions range from 217 to 306 m with an average of 267 27 m hills are generally larger to the north of a low running through the study area values of range from 0 12 to 0 77 m2 yr with an average of 0 32 0 19 m2 yr lower values of kbetter describe the surface shapes of individual high standing ponds while higher values are needed to model the surface shapes of thick deposits in regional lows this difference in with geological setting is probably due to variations in sediment transport mechanisms

export fluxes of calcite in the eastern equatorial pacific from the last glacial maximum to present 1 the eastern equatorial pacific eep is an important center of biological productivity generating significant organic carbon and calcite fluxes to the deep ocean we reconstructed paleocalcite flux for the past 30 000 years in four cores collected beneath the equatorial upwelling and the south equatorial current sec by measuring ex230th normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for fraction of calcite preserved this method produced very similar results at the four sites and revealed that the export flux of calcite was 30 50 lower during the lgm compared to the holocene the internal consistency of these results supports our interpretation which is also in agreement with emerging data indicating lower glacial productivity in the eep possibly as a result of lower nutrient supply from the southern ocean via the equatorial undercurrent however these findings contradict previous interpretations based on mass accumulation rates mar of biogenic material in the sediment of the eep which have been taken as reflecting higher glacial productivity due to stronger wind driven upwelling

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

petrogenesis of sierra nevada plutons inferred from the sr nd and o isotopic signatures of mafic igneous complexes in yosemite valley california mafic complexes in the central sierra nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust in the intrusive suite of yosemite valley major and trace element compositions of the hornblende bearing gabbroic rocks from the rockslides mafic complex and of the mafic dikes in the north america wall are compositionally similar to high alumina basalt of these rocks two samples have higher ni and cr abundances as well as higher end values than previously recognized for the intrusive suite plagioclase crystals in rocks from the north america wall and the rockslides have prominent calcic cores and sharply defined sodic rims a texture commonly associated with mixing of mafic and felsic magmas in situ analyses of 87sr 86sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains we propose that the high 87sr 86sr 0 7067 and low end 3 4 of bulk rocks the homogeneity of 87sr 86sr in plagioclase and the high 18o values of bulk rocks 6 6 7 3 and zircon lackey et al in j petrol 49 1397 1426 2008 demonstrate that continental crust was assimilated into the sublithospheric mantle derived basaltic precursors of the mafic rocks in yosemite valley contamination 20 40 likely occurred in the lower crust as the magma differentiated to high alumina basalt prior to plagioclase and zircon crystallization as a consequence the isotopic signatures recorded by whole rocks plagioclase and zircon do not represent the composition of the underlying lithospheric mantle we conclude that the mafic and associated felsic members of the intrusive suite of yosemite valley represent 60 80 new additions to the crust and include significant quantities of recycled ancient crust

thermobarometry and p t t paths the granulite to eclogite transition in lower crustal xenoliths from eastern australia lower crustal suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c 12 kbar for exposed granulite terranes palaeogeotherms constructed from xenoliths for the lower crust beneath the phanerozoic fold belts of eastern australia sea and beneath the eastern margin of the australian craton emac indicate two distinct thermal regimes the two geotherms have similar form with the emac curve displaced c 150 c to lower temperatures reaction microstructures show the partial re equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures variations in mineral compositions and zoning are used to constrain further the history of several emac xenoliths to near isobaric trajectories detailed graphical models are constructed to predict compositional changes for isobaric p t paths at 7 14 21 kbar to transform an sea type geotherm to a cratonic geotherm the models show that for the assemblage grt cpx ky plag qtz the changes associated with falling temperature in xgr xjd increase and xan decrease will be greater at higher pressures these results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths the zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c 12 22 kbar suggests isobaric cooling of a large crustal thickness 30 35 km an isobaric cooling path is consistent with magma accretion models for the transition of a crust mantle boundary from an sea type geotherm to a cratonic geotherm the coexistence of granulite and eclogite over the depth range 35 75 km beneath the emac indicates that the granulite to eclogite transition in the lower crust is controlled by p t conditions bulk chemistry and kinetic factors at shallower crustal levels typified by exposed granulite terranes isobaric cooling may not result in the transition to eclogite

can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts 1 constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late quaternary peridotite xenoliths from the southern sierra nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle in these models two conductive melts basalt and sulfide are embedded in a resistive matrix bounds on the amount of sulfide 0 06 0 4 and the bulk conductivity 0 03 0 1 s m place constraints on the degree of interconnection between the melts because the sulfide melt is very conductive even a small fraction of well connected melt results in a bulk conductivity larger than 0 1 s m similarly completely disconnected melts result in bulk conductivities much less than 0 03 s m the only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction 1 interconnected basalt melt with a discontinuous sulfide phase such a texture is observed in laboratory experiments with much larger sulfide melt fractions but has not been reported for small melt fractions a variant of the hashin shtrikman model and a hybrid model consisting of cascaded hashin shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints with a model that appropriately simulates the melt interconnectivity we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle

patterns of caco3 deposition in the eastern tropical pacific ocean for the last 150 kyr evidence for a southeast pacific depositional spike during marine isotope stage mis 2 1 we constructed biogenic mass accumulation rate mar time series for eastern pacific core transects across the equator at 105 and 85 w and along the equator from 80 to 140 w we used empirical orthogonal function eof analysis to extract spatially coherent patterns of caco3 deposition for the last 150 kyr eof mode 1 51 variance is a caco3 mar spike centered in marine oxygen isotope stage 2 mis 2 found under the south equatorial current eof mode 2 19 of variance is high north of the equator eof mode 3 9 of variance is an east west mode centered along the north equatorial counter current the mis 2 caco3 spike is the largest event in the eastern pacific for the last 150 kyr caco3 mars are 2 3 times higher at 18 ka than elsewhere in the record including mis 6 it is caused by high caco3 production rather than minimal dissolution eof 2 while it resembles deep water flow patterns nevertheless shows coherence to corg deposition and is probably also driven by caco3 production index terms 4267 oceanography general paleoceanography 1615 global change biogeochemical processes 4805 1030 geochemistry geochemical cycles 0330 3022 marine geology and geophysics marine sediments processes and transport keywords late pleistocene paleoproductivity pacific calcite biogeochemistry carbon cycle

changes in plate motions and the shape of pacific fracture zones geosat passes the new 2 min global gravity grid smith and sandwell 1995 and shipboard bathymetry across central pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions these features are not predicted by current locked fault fracture zone models during a change in spreading direction that induces tension across the transform fault large offset greater than 500 km transforms develop multiple parallel faults spaced 50 to 100 km apart the gravity signature of small offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model seafloor formed during a period of change usually marks a transition between structural styles for example between multiple fracture zone strands and a narrower single fault fracture zone widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to but not across the old transform fault

comment on do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific by m lyle et al accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes two approaches are currently being used one consists of estimating mass accumulation rates mar between dated horizons as the product of linear sedimentation rates sediment dry bulk densities and concentrations one pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column to address this problem the method of 230th normalization was developed bacon 1984 this method is based on the assumption that the rapid scavenging of 230th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production to the extent that this assumption is correct scavenged 230th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor henderson and anderson 2003 francois et al 2004

reply to comment by r francois et al on do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific further explorations of 230th normalization

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

patterns of export production in the eastern equatorial pacific over the past 130 000 years 1 productivity at six core locations in the eastern equatorial pacific eep was reconstructed with a benthic foraminiferal transfer function the core records show strong regionality especially where affected by peru margin upwelling of deeper equatorial undercurrent water euc originally coming from the subantarctic this peru margin record differs from that seen along the equator where divergence leads to shallow upwelling and it is generally inverse to that seen in cores outside the areas of equatorial upwelling principal components analysis shows that the main productivity pattern correlates well to the global oxygen isotope record and has lowest values during isotope stages 2 and 4 in addition to this equatorial cores show a higher frequency pattern of variation which becomes much more pronounced during mis 3 and 2 the reconstructions based on benthic foraminifera were tested against those from nonaccumulation rate based inorganic chemical proxies of export production these were found to correlate well in the region influenced by peru upwelling and also to share common features for sites along the equator all the nonaccumulation rate based paleotracers are consistent with one another and differ from accumulation rate derived proxies the differences between the two classes of paleotracers may result from uncertainties in calculating actual biogenic fluxes since 230th normalized results conform more to those we obtained analysis of planktonic carbon isotope values for the eep and their comparison to the record of the pacific subantarctic indicates that the subantarctic contribution to the euc was reduced during mis 3 and 2

bioturbation in deep sea pacific sediments bioturbation structures have been recorded in sediment cores from 5000 m to 5300 m water depth collected from the pacific ocean using subcores from spade box cores shear strength and eh profiles were also taken the results have been related to pelagic fluxes and sedimentation bioturba tion structures consist of holes and chambers of various sizes running vertically and horizontally whose abundance size and shapes have been measured values did not decrease exponentially with sediment depth but end abruptly at depths ranging from 30 cm to 45 cm in many instances there were subsurface peaks of vertical and horizontal burrows there was also evidence of lateral galleries large deep in filled burrows were observed these consisted of light and dark brown sediment having different shear strengths eh profiles showed that the sediment was aerobic with slightly lower values near the sediment surface possibly associated with meiofaunal and bacterial activity the most important implications of this work for pelagic processes are likely to be increased transfer of dissolved and particulate material between the sediment and the water column due to the physical network of burrows within the sediment and increased surface roughness caused by burrow openings mucus binding of particulates and faecal pelletization are also likely to be important all of these effects will be most noticeable at high burrow densities

pelagic sedimentation on rough seafloor topography 1 forward model this is the first paper in a four part sequence exploring the factors that govern the distribution of pelagic sediments in regions of the world mid ocean ridge system where basement outcrops and sediment deposits both contribute to local topography in this paper we present a simple numerical model of pelagic sedimentation in which sediments travel from topographic highs to lows with flux regulated by a diffusivity parameter by applying the forward model to realistic models of basement topography we investigate how various basement structures sediment rain rates and values of affect the morphology of the seafloor through time we quantify the effect of sedimentation as a nonlinear smoothing filter on topography which acts to reduce spectral power at high wavenumber to decrease the root mean square variability of topography to increase the spacing between hills and to lower the relative proportion of high slope seafloor we find that local accumulation rate is highly dependent on local topography and varies greatly with time we also find that sediment compaction does not contribute directly to the surface concavity exhibited by many intrahill sediment ponds

carbonate compensation dynamics 1 carbonate saturation zsat and compensation zcc depths change with deep ocean acidification and basification we present simple explicit mechanistic formulas for the positions of these two critical depths in particular zcc is expressed as a function of the mean dissolved carbonate ion concentration of the deep ocean co3 d the supply of dissolvable caco3 fc and the dissolution rate constant at the sediment water interface kc which we show to be essentially mass transfer controlled calculations reveal that zsat and zcc are today some 0 9 km apart and will rise and separate by as much as 1 7 km with acidification conversely if co3 d increases zsat and zcc will deepen but their separation will asymptote to 0 7 km

enhanced sedimentation along the equator in the western pacific measurements on cores from the ontong java plateau demonstrate that on equator sediment accumulates more 230th than is produced in the overlying water column in contrast cores from 3 8 n accumulate an amount of excess 230th more nearly equal to that produced in the overlying water column taken together with the observation that both the caco3 and non caco3 components of the sediment accumulate at approximately twice the rate in on equator than in off equator cores broecker et al 1999 this requires either that the rain of the larger amount of organic matter generated above the equatorial upwelling plume entrains excess clay and 230th during its fall to the sea floor or that physical transport processes either in the upper ocean or near the sea bed redistribute sediments in such a way that they preferentially accumulate in a narrow 1 latitude equatorial centered belt

in situ settling behavior of marine snow1 the settling velocities of undisturbed macroscopic aggregates known as marine snow were measured with scuba in surface waters off southern california and analyzed as a function of aggregate size mass and density the mean settling velocity was 74f39 m d l for aggregates ranging from 2 4 to 75 mm in maximum length sinking rates in the field varied exponentially with aggregate size and dry weight and were consistently up to four times slower than rates measured in the laboratory the excess densities of the 80 aggregates examined were calculated from volume and dry weight and ranged over four orders of magnitude with a median of 1 4 x 1o 4 g cm 3 aggregates of marine snow sank more slowly than predicted for either solid or porous spheres of equivalent volume and density although their velocities were within the range expected for equivalent sinking prolate ellipsoids no relationships between settling velocity and either excess density or particle shape were found drag coefficients of marine snow were also higher than predicted by theory for spheres of equivalent volume and density these deviations from theoretical expectations may be partially explained by errors in the estimation of the excess densities of aggregates variability in the densities of the heterogeneous primary particles comprising marine snow fecal pellets claymineral particlcs phytoplankton molts etc and the potential for buoyancy regulation by individual phytoplankton cells inhabiting aggregates make determination of excess density especially

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

microearthquakes on and near the east pacific rise 9 10 n records from a seismic network deployed as part of an active tomography experiment at 9 30 n on the east pacific rise epr provide an opportunity to characterize local microearthquake activity over an 8 day period with the exception of the region around the 9 03 overlapping spreading center osc no events were located on the rise axis one microearthquake located from p and s wave arrival times lies adjacent to the network 18 km to the west of the rise axis five more events located from p wave and t phase data are to the south of the network three cluster around the western arm of the 9 03 osc while two are located 25 30 km to the east of the osc the seismic moments of the microearthquakes range from 5 1019 to 4 1020 dyn cm several orders of magnitude larger than those reported in other studies of shallow events along the axis of the epr these results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the epr are active off axis to distances of at least 20 km

density and size distribution of seamounts in the eastern pacific inferred from wide beam sounding data theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow beam and wide beam echo sounders the shapes of seamounts are approximated as truncated right circular cones of constant slope e height to radius ratio and flatness f i e a seamount of basal radius r has a height h r a flat top of radius fr and a slope angle of arctan e the set of all seamounts in a given physiographic province is assumed to be a poissonian ensemble specified by a function v r the average number of seamounts per unit area for which r r a simple choice for the size distribution of seamounts and one shown to be consistent with data in the eastern pacific is to specify that it decays exponentially with r v r vo exp r in the case of a profile of length l taken in water of depth d with a wide beam echo sounder half beam width the shallow slope deep water approximation e 1 e d yields that the expected number of seamounts with apparent heights h h is n h l 2 1 ed 2 fh lv0e h we apply this sampling theory to apparent height data measured from 44 020 km of wide beam profiles on pacific crust of post mesozoic age although the spatial distribution of seamounts in the eastern pacific is certainly not poissonian the abundance statistics do not involve spatial correlations and we show they are robust with respect to deviations from the poisson model provided the statistics are interpreted as averages over the area actually sampled by the track lines the observed falloff of apparent heights in the eastern pacific is very nearly exponential justifying our use of an exponential model of seamount size distribution the shape parameters are estimated from multibeam surveys of pacific seamounts an analysis of 30 features in the height range 200 4000 m yields 0 21 0 011 and f 0 21 0 028 using these parameters we obtain vo 4 0 1 1 10 9 m 2 and 6 3 0 8 10 4 m 1 these values imply that per 106 km2 in the eastern pacific there are 1600 400 seamounts with summit heights greater than 300 m and 200 60 greater than 1000 m integrated over the entire size distribution seamounts occupy about 6 of the seafloor area and constitute about 0 4 of the oceanic crustal volume our estimates of seamount abundance are appreciably greater than those of most other authors a discrepancy we attribute primarily to the sampling bias of previous studies which generally used uncorrected map counts this hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central pacific from the scripps charts above 3000 m the map counts are consistent with the predictions of the exponential model but below this height they are biased to low values

a quantitative study of the axial topography of the mid atlantic ridge some basic characteristics of ridge axis topography are related to spreading rate and distance from neighboring transform faults for example the presence of an axial depression coincides in most cases with slow spreading rates and the overall depth of the ridge axis increases toward ridge transform intersections rtis on the other hand it is also well known that the relief and width of the axial valley on say the mid atlantic ridge mar vary along strike in an unpredictable manner the purpose of the present study is to quantify how much of the observed variation in the first order topography at the axis is related to changes in other parameters such as spreading rate and distance from rtis to carry out this test the zero age depth and the relief and width of the axial valley have been estimated on 46 profiles that cross the axis of the mar between the equator and 50 n full spreading rates 22 36 km ma zero age depth is here defined to be the depth at age zero of the best fit thermal subsidence trend axial valley relief and width have been measured with respect to the ridge flanks by the least squares fit of a gaussian bell the measured axial valley relief varies between 600 and 2100 m average 1300 m while the valley width varies between 16 and 62 km average 35 km the correlation between zero age depth axial valley relief and width latitude of axial crossing spreading rate distance from nearest rtt and offset on the nearby transforms has been investigated using linear regression techniques the main results of the present study are that 1 zero age depth significantly correlates with latitude of crossing distance from nearest rti and offset on the nearby transforms and 2 the variation in axial valley relief and width is essentially uncorrelated with spreading rate zero age depth distance from nearest rtt and offset on nearby transforms the preferred explanation for the observed spatial variation in axial valley geometry is that it reflects a temporal variation in fact if the rough abyssal hill topography typical of the mar flanks is created within the axial valley the shape of the axial valley cannot be steady state although the existence of an axial valley may be a steady state phenomenon this hypothesis is supported by the observation that the variability in axial valley relief is similar to the overall amplitude of abyssal hill topography measured as the residual on the thermal subsidence trend

morphology and shallow structure of seafloor mounds in the canary basin eastern central atlantic ocean abstract the increasing volume of high resolution multibeam bathymetry data collected along continental margins and adjacent deep seafloor regions is providing further opportunities to study new morphological seafloor features in deep water environments in this paper seafloor mounds have been imaged in detail with multibeam echosounders and parametric sub bottom profilers in the deep central area of the canary basin 350 550 km west off el hierro island between 4800 and 5200 mbsl these features have circular to elongated shapes with heights of 10 to 250 m diameters of 2 24 km and with flank slopes of 2 50 based on their morphological features and the subsurface structures these mounds have been classified into five different types of mounds that follow a linear correlation between height and slope but not between height and size the first second subgroup a and third mound types show heights lower than 80 m and maximum slopes of 35 with extension ranging from 2 to 400 km2 and correspond to domes formed at the surface created by intrusions located at depth that have not outcropped yet the second subgroup b fourth and fifth mound types show higher heights up to 250 m high maximum slopes of 47 and sizes between 10 and 20 km2 and are related to the expulsion of hot and hydrothermal fluids and or volcanics from extrusive deep seated systems based on the constraints on their morphological and structural analyses we suggest that morphostructural types of mounds are intimately linked to a specific origin that leaves its footprint in the morphology of the mounds we propose a growth model for the five morphostructural types of mounds where different intrusive and extrusive phenomena represent the dominant mechanisms for mound growth evolution these structures are also affected by tectonics bulge like structures clearly deformed by faulting and mass movements slide scars and mass transport deposits in this work we report how intrusive and extrusive processes may affect the seafloor morphology identifying a new type of geomorphological feature as intrusive domes that have to date only been reported in fossil environments but might extend to other oceanic areas

bathymetric gradients of lineated abyssal hills inferring seafloor spreading vectors and a new model for hills formed at ultra fast rates abstract abyssal hill morphology provides a preliminary measure of the direction and rate of seafloor spreading however additional information e g magnetic anomaly data or a nearby mid ocean ridge is usually required to verify these estimates previous attempts to identify a unique spreading rate proxy from abyssal hill dimensions e g height length width have largely failed due to the relatively large scatter of data or the non linear character of spreading rate trends we present a new stand alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis the spreading rate is determined with the peak height parameter defined as the difference in the height maximum frequency of the two dominant modes observed in the azimuthal histograms this parameter exhibits a clear nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10 20 km myr the spreading direction is determined with the peak width parameter which compares the average width of the two dominant modes in the azimuthal histograms the wider distribution of slope azimuths is oriented away form the paleo ridge axis for all spreading rates and thus spreading direction can be determined the trends in the peak height and width parameters are used to constrain a new model of abyssal hill formation at ultra fast spreading rates which require greater off axis extensional faulting resulting in a few large throw faults on the outward facing hillsides and many smaller throw faults on the inward facing hillsides

upper mantle fragments from beneath the sierra nevada batholith partial fusion fractional crystallization and metasomatism in a subduction related ancient lithosphere

sediment focusing in the panama basin eastern equatorial pacific ocean abstract age model derived sediment mass accumulation rates mars are consistently higher than 230th normalized mars in the equatorial pacific ocean during the past 25 ka the offset being highest in the panama basin suggests a significant role for deep sea sediment redistribution i e sediment focusing in this region here we test the hypothesis that downslope transport of sediments from topographically high regions that surround the panama basin is the cause of higher than expected xs230th inventories over the past 25 ka in the deeper parts of the basin we find little difference in xs230th inventories between the highest and lowest reaches of the basin furthermore there is no correlation between xs230th derived sediment focusing factors and water depth which suggests that the topographic highs do not serve as a source of xs230th a spatial analysis suggests that there may be an enhanced scavenging effect on xs230th concentrations in sediment closest to the equator where productivity is the highest although further data is necessary to corroborate this at the equator xs230th derived focusing factors are high and range from about 1 to 5 during the holocene and about 1 to 11 during the last glacial in contrast non equatorial cores show a smaller range in variability from about 0 7 to 2 8 during the holocene and from 0 7 to 3 6 during the last glacial based on 232th flux measurements we hypothesize that the location at which eolian detrital fluxes surpass the riverine detrital fluxes is approximately 300 km from the margin while riverine fluxes from coastal margins were higher during the holocene eolian fluxes were higher during the last glacial

modeling cenozoic sedimentation in the central equatorial pacific and implications for true polar wander high productivity along the central pacific equator has left thick deposits of calcareous and siliceous ooze west of 102 w these have deposited on the pacific plate which has been drifting northwards and therefore the thickest deposits are displaced north of the equator this study attempts to simulate the sediment distribution using information from deep sea drilling project dsdp cores and models for pacific plate motion the dsdp core data are used to account for accumulation rate variations with time paleolatitude and paleodepth accounting for variable carbonate dissolution the model predicts the distribution on 35 50 ma seafloor relatively well and demonstrates that the sedimentary bulge is distorted by variable dissolution over a regional bathymetry gradient due to the northern edge of the south pacific superswell a westerly deepening of the carbonate compensation depth and sedimentary lysocline evolution are proposed which improve the model for seafloor 30 ma and older the degree of match between model and data provides a test of the amount of northward drift of the pacific plate and assuming a model for pacific hotspots motion of true polar wander tpw motion of the hotspot frame relative to the earth s spin axis assuming the duncan and clague 1985 model for pacific hotspots motion the sedimentary modeling favors tpw models which have little or no effect on the central pacific paleoequator over the past 30 ma we also explore implications of interhotspot motion and the suggestion that the bend in the hawaiian volcanic chain represents an abrupt change in pacific hotspot motion within the last 5 ma

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

thermal cracking and the deep hydration of oceanic lithosphere a key to the generation of plate tectonics 1 the development of transient thermal stress in suboceanic mantle is investigated on the basis of two dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere e g to the depth of at least 30 km and possibly down to 50 km in 100 ma old lithosphere the release of thermal stress by tension cracking is limited to the vicinity of cracks and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials possible physical and chemical consequences of deep thermal cracking are also discussed the rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere this localized weakening may help to explain why plate tectonic convection not stagnant lid convection operates in earth s mantle

distribution of groundwater arsenic in xinjiang p r china although potential contamination of groundwater as is expected to occur in xinjiang p r china few data are available for the regional distribution of groundwater as in this study the spatial distribution of groundwater as was investigated in the chepaizi cpz n and shihezi shz n areas of northern xinjiang the balikun yiwu basin by e in eastern xinjiang and the tarim trm s and yanqi yq s basins in southern xinjiang arsenic concentrations greater than 10 g l were found in 12 of analyzed groundwaters all groundwater samples collected in cpz n had as concentrations greater than 10 g l 25 185 g l 30 in shz n 10 g l was found in the eastern and southern trm s and yq s distribution of groundwater as showed a tremendous spatial variability which greatly varied over a short distance horizontally arsenic concentration generally increased with increasing sampling depth the spatial distribution of groundwater as would be regulated by as source and hydrogeochemical processes higher ph and or lower orp values were generally observed in high as groundwater 10 g l in comparison with low as groundwater 10 g l arsenic mobility in shz n and cpz n may result from as desorption under relatively high ph conditions and more tentatively from reductive dissolution of fe iii oxides in by e and trm s however detailed mechanisms of as mobilization in these regions need further investigation

mid cretaceous to early tertiary apparent polar wander path of the pacific plate pacific apparent polar wander was delineated for mid cretaceous to early tertiary time using 22 seamount paleomagnetic poles 11 paleocolatitudes from azimuthally unoriented cores five effective inclinations from two dimensional magnetic anomaly inversions and one magnetic lineation relative amplitude datum three of the seamount paleomagnetic poles were new data calculated in this study the apparent polar wander path was defined with six mean poles having ages of 39 66 76 82 85 and 94 ma an alternate path was constructed combining data from two pairs of mean poles 66 and 76 ma 85 and 94 ma that were statistically indistinguishable the apparent polar wander path is hook shaped with a nearly right angle bend at 82 ma its post 82 ma section trends north south the pre 82 ma part east west the path indicates uneven polar wander unresolvable polar motion occurred between 66 76 ma and 85 94 ma suggesting stillstands however the large distances between the 85 and 82 ma poles 15 2 3 8 and 82 and 76 ma poles 11 3 2 8 imply rapid polar wander at rates of 5 1 1 3 m y and 1 9 0 5 m y respectively both the sharp bend and rapid polar wander occurred at the end of the cretaceous normal polarity superchron the mid cretaceous paleomagnetic data show a trend suggestive of southward motion of the pacific plate that may be the end of a period of southward drift postulated by others to have begun in the jurassic the evolution of various features of the apparent polar wander path are discussed in terms of plate motion true polar wander and time varying non dipole geomagnetic fields we argue that the overall shape of the polar wander path resulted from the change of pacific plate motion from southward to northward perhaps brought about by the beginning of the subduction of the plate beneath asia