3d wave equation interferometric migration of vsp free surface multiples interferometric migration of free surface multiples in vertical seismic profile vsp data has two significant advantages over standard vsp imaging 1 a significantly larger imaging area compared to migrating vsp primaries and 2 less sensitivity to velocity estimation and static errors than other methods for migration of multiples in this paper we present a 3d wave equation interferometric migration method that efficiently images vsp free surface multiples synthetic and field data results confirm that a reflectivity image volume comparable in size to a 3d surface seismic survey around the well can be computed economically the reflectivity image volume has less fold density and lower signal to noise ratio than that obtained by a conventional 3d surface seismic survey because of the relatively weak energy of multiples and the limited number of geophones in the well however the efficiency of this method for migrating vsp multiples suggests that it might sometimes be a useful tool for 4d seismic monitoring where reflectivity images can be computed quickly for each time lapse survey

factors controlling the burial of organic carbon in laminated and bioturbated sediments off nw mexico implications for hydrocarbon preservation factors controlling the burial of organic carbon oc in late quaternary sediments on the nw mexican continental margin are assessed using a suite of box and piston cores strategically located on the shelf slope rise with respect to the intense oxygen minimum in this region an oc concentration maximum occurs on the mid slope below the core of an intense water column o2 minimum due to current winnowing on the outer shelf the preferential accumulation of organic matter in fine grained deposits and the offshore decrease in the settling flux of organic detritus the organic matter at all water depths is overwhelmingly marine hydrogen indices hi are higher on the slope 300 mg hc g toc than on the shelf 300 mg hc g toc where current winnowing has promoted organic matter degradation but there is no difference in hi in slope sediments accumulating under well oxygenated and o2 deficient conditions the degree of winnowing appears to be the primary factor affecting the preservational quality of organic matter deposited on this margin rates of accumulation of oc and opal are all higher in the interglacial intervals when compared with the glacial deposits over the last 140 000 yr however matrix corrected hi values in the mid and lower slope cores are invariant and are similar to values in the laminated intervals from the oxygen minimum site thus cyclic changes in organic carbon accumulation on this margin have been controlled by production variations rather than differential preservation hi values in late quaternary sediments from several continental margins including nw mexico and euxinic basins correspond to type ii kerogen irrespective of bottom water o2 concentrations therefore the preservation of oil prone kerogen in productive margin settings does not appear to be restricted to sediments deposited under conditions of low bottom water o2 concentrations as envisioned in models of petroleum source rock deposition

the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

a study on the regulation of the concentrations of some trace metals rb sr zn pb cu v cr ni mn and mo in saanich inlet sediments british columbia canada abstract bulk metal concentrations were measured in the sediments of saanich inlet in an attempt to establish the occurrence of trace metal enrichment in the anoxic central basin ba ni v cr zn pb cu and mo were found to be enriched in the anoxic ooze over the possible contributions from lithogenic sources spatial and seasonal variations in the chemical composition of the settling particulates collected with interceptor traps gave further indications of the mode of incorporation of these metals biogenic ba and cr appeared to be associated with opaline silica although alternative explanations are also possible particularly for ba zinc seemed to be added to the sediment essentially in association with planktonic materials while cu required an additional source directly linked to the anoxic environment similarly ni v and mo were added to the anoxic sediments by reactions occurring at the sediment water interface in the nearshore environment studied here these metals particularly ni and mo were not associated to any significant extent with planktonic materials of all the elements analyzed mo showed the largest enrichment in the anoxic sediments of saanich inlet

two dimensional three dimensional waveform modeling of subducting slab and transition zone beneath northeast asia subduction plays a fundamental role in dynamics of the mantle convection and in material circulations of the earth s interior slabs have been imaged to subduct near horizontally in the transition zone tz beneath the northeast asia in seismic tomography triplication waveform modeling is an effective tool to study the detailed seismic structure in the tz however tz triplication modeling has traditionally relied on 1 d models in this study we use the spectral element method to explore influences of 2 d 3 d slab structure on tz triplication waveforms and to model for the first time the slab and tz structures beneath the northeast asia synthetic tests suggest that for a subduction zone earthquake slab structure can have important influences on tz triplication waveforms and that even in a narrow azimuth range the effects from 2 d 3 d slab structure on the wave propagation can lead to erroneous conclusions with 1 d modeling our data are high quality triplicated sh waveforms at distances of 10 32 from a deep event below 410 km discontinuity in the pacific subducting slab our 2 d 3 d waveform modeling results suggest that a simple model of the subducting slab 5 high velocity anomaly and 100 km thick down to 560 km but normal below 560 km can match most of the observed waveforms remarkably well the bottom of the tz of the sampling region north of the yellow sea contains a patch of very slow anomaly the results indicate that subhorizontal slab above the 660 discontinuity is not everywhere beneath northeast asia and the subducting slab is not everywhere continuous to the bottom of the tz compared with the traditional 1 d modeling our new 2 d 3 d approach provides better fits to the data and allows us to constrain the slab geometry and to separate tz structure from slab structure

production and destruction control of biogenous sedimentation in the tropical atlantic 0 300 000 years b p late pleistocene signals of calcium carbonate organic carbon and opaline silica concentration and accumulation are documented in a series of cores from a zonal meridional depth transect in the equatorial atlantic ocean to reconstruct the regional sedimentary history spectral analysis reveals that maxima and minima in biogenous sedimentation occur with glacial interglacial cyclicity as a function of both 1 primary production at the sea surface modulated by orbitally forced variation in trade wind zonality and 2 destruction at the seafloor by variation in the chemical character of advected intermediate and deep water from high latitudes modulated by high latitude ice volume from these results a pattern emerges in which the relative proportion of signal variance from the productivity signal centered on the precessional 23 kyr band decreases while that of the destruction signal centered on the obliquity 41 kyr and eccentricity 100 kyr periods increases below 3600 m ocean depth

glacial interglacial variability in denitrification in the world s oceans causes and consequences the late quaternary history of water column denitrifcation of the eastern pacific margins and the arabian sea is reconstructed using sedimentary 15n measurements the 15n values in six piston cores raised from these regions show remarkably similar cyclic variations being heavy 9 10 5 during the interglacials and 2 3 lighter during the glacials this implies that denitrification in these regions decreased substantially during the glacial periods the glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone since water column denitrification in these areas accounts for about half of the fixed nitrogen loss in the modern ocean the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods because nitrate is a limiting nutrient oceanic productivity and attendant changes in co2 may therefore have been modulated on glacial interglacial timescales by variations in the oceanic no3 content

stable nitrogen isotopes in angola basin surface sediments abstract sedimentary 15n contains a record of biogeochemical processes occurring in the water column stable nitrogen isotopes were measured in the bulk sedimentary matter of 18 surface sediment samples collected in three transects in the eastern angola basin perpendicular to the coast at 6 s 12 s and 17 s and two samples from the zaire estuary relative enrichment in 15n in sediments with distance from the coast was seen in each of the three transects off northern middle and southern angola values in the zaire fan northern angola and southern angola increased by 1 9 and 1 6 respectively from shallow 100 m to deep 4500 m water sediments and in middle angola by 2 5 water depth range 73 3809 m we propose that in the angola region the degree of fractionation of nitrogen isotopes in organic matter is a function of nutrient supply to sunlit waters the shift towards lighter isotopic values in shallow water signifies that the larger nitrate pool is not utilized as extensively as in water farther away from the coastal upwelling centers southern angola sediments were more than 1 lower average 15n 5 4 than the middle and northern transects average 15n 7 2 and 6 5 respectively this difference is attributed to higher nitrate concentrations in the surface water at 17 s for which there are two reasons one is that these sediments are located south of the angola benguela front at 16 s which demarcates the convergence of warm equatorial water with cold nutrient rich benguela current water the second cause is the intense namibian coastal upwelling zone extending northward up to the front comparisons of bulk 15n with 13corg and c n ratios show that the observed sedimentary 15n variations with water depth are due neither to water column diagenesis nor to mixing of terrestrial material with marine derived organic matter the two estuary sediment samples were mainly terrigenous in origin based on 13corg and c n ratios 15n values of these two samples were 7 0 and 7 6

seismic triplication used to reveal slab subduction that had disappeared in the late mesozoic beneath the northeastern south china sea abstract we provided a new study of the seismic velocity structure of the mantle transition zone mtz beneath the northeastern south china sea using p wave triplications from two earthquakes at the central philippines recorded by the chinese digital seismic network through fitting the observed and theoretical triplications modeled by the dynamic ray tracing method for traveltimes and the reflectivity method for synthetic waveforms using grid searching method best fit velocity models based on iasp91 were obtained to constrain the p wave velocity structure of the mtz the models show that a high velocity anomaly hva resides at the bottom of mtz the hva is 215 km to 225 km thick with a p wave velocity increment of 1 0 between 450 km and 665 km or 675 km transition and increase by 2 5 3 5 at 665 km or 675 km depth the p wave velocity increment ranges from approximately 0 3 to 0 8 below the 665 km or 675 km we proposed that the hva in the mtz was caused by the broken fragments of a diving oceanic plate falling into the mtz at a high angle and or by unstable thick continental lithosphere dropping into the mtz sequentially or almost simultaneously

a complex 660 km discontinuity beneath northeast china abstract we present a detailed seismic study of the 660 km discontinuity beneath northeast china using the receiver function technique we use seismic data collected from 24 broadband stations in northeast china analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude 128 0 e to 130 5 e and latitude 40 0 n to 44 0 n and then it splits into multiple discontinuities in the surrounding regions the complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab we speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area

unchanged nitrogen isotopic composition of organic matter in the south china sea during the last climatic cycle global implications the 15n of surface and down core sediments spanning the last 20 200 kyr from the entire south china sea scs ranges only from 3 0 to 6 5 with no correlation with discernible paleoclimatic oceanographic changes detailed profiles of the uppermost sediment column including fluff samples indicate a minor diagenetic overprint of 0 3 1 2 at the sediment water interface the absence of any correlation with reconstructed glacial interglacial changes in primary production terrigenous input and or sea level related basin configuration is attributed to a complete consumption of nitrate during primary production in this marginal basin during at least the last 140 000 years this in turn implies that the 15n of the nitrate used during primary production remained approximately constant during the last climatic cycle the proposed scenario infers an unchanged nitrogen isotopic composition of the western pacific subsurface nitrate between glacial and interglacial stages as well as during terminations and thus constrains proposed changes in the oceanic n inventory

precise determination of phase relations in pyrolite across the 660 km seismic discontinuity by in situ x ray diffraction and quench experiments abstract mineral assemblage changes in a pyrolite composition with increasing pressure were observed by in situ x ray diffraction and quench experiments at pressures near that of the 660 km seismic discontinuity and at a fixed temperature of 1600 c according to results obtained by in situ x ray diffraction experiments ringwoodite rw was observed with majorite garnet and casio 3 rich perovskite at pressures of about 20 22 gpa dissociation of ringwoodite to mgsio 3 rich perovskite and magnesiowustite mw was completed at 22 0 0 2 gpa according to matsui et al s periclase pressure scale and at 21 7 0 1 gpa according to shim et al s gold pressure scale majorite garnet persisted to about 24 gpa where pyrolite transformed to a lower mantle mineral assemblage i e mgsio 3 perovskite casio 3 rich perovskite and magnesiowustite thus majorite garnet coexists with the lower mantle assemblage at pressures of about 22 24 gpa in the quench experiments an assemblage of mgsio 3 perovskite magnesiowustite casio 3 rich perovskite and majorite garnet was synthesized at 22 5 gpa and 1600 c in which mg perovskite contained 2 8 wt al 2 o 3 and was significantly poorer in fe than coexisting magnesiowustite the fe mg partition coefficient between mg perovskite and magnesiowustite including ferric iron k app 0 27 0 06 is very close to that in the al free system which suggests that these p t conditions are in the vicinity of those of ringwoodite decomposition both the results of in situ x ray diffraction and quench experiments in the present study yield a convergent result that ringwoodite decomposes into mg perovskite and magnesiowustite before the garnet to perovskite transition at 1600 c in pyrolite the relation between the al content in mg perovskite and k app in pyrolite is non linear which is consistent with the fe mg partitioning between mg perovskite and magnesiowustite previously reported for a simpler mgo feo al 2 o 3 sio 2 system

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

ocean chemistry during glacial time abstract measurements of co2 to air ratios in the gas trapped in bubbles in ice of glacial age suggest that the co2 content of the atmosphere was considerably lower during peak glacial time than during holocene time the purpose of this paper is to show that such a change must in all likelihood be the result of alterations in the nutrient element chemistry of sea water two possible scenarios are presented one involves alternate storage and erosion of phosphorous leaving residues from shelf sediments the other involves changes in the c p ratio in the organic debris falling to the deep sea means of verifying the nutrient cycle hypothesis are also given it is shown that the 13c record as we know it in planktonic and benthic foraminifera the oxygen record as inferred from benthic foraminifera species distributions and the early post glacial caco3 preservation event as recorded by aragonitic pteropods are consistent with both of the hypotheses presented only if an early post glacial spike in the 13c record for planktonic shells could be found would it be possible to eliminate one of these hypotheses i e that involving shelf storage the implications of these nutrient hypotheses to climate theory are as follows if shelf storage is responsible for the glacial to interglacial co2 increase then the co2 change must be considered an amplifier of some primary cause the reason is that sea level changes are needed to drive deposition on and erosion from the shelves on the other hand if changes in the c p ratio for falling debris are responsible then the co2 change could either be an amplifier or a primary cause for the major glacial to interglacial climatic cycle the latter is possible as self sustained oscillations in ocean chemistry might be driven by interactions between ocean ecology and ocean nutrient chemistry

minor elements in quaternary sediment from the sea of japan a record of surface water productivity and intermediate water redox conditions sediment of quaternary age from oki ridge 903 m depth in the sea of japan 3500 m deep records six episodes of high accumulation rates of cd cr cu mo ni u v and zn the high rates correspond to periods of sulfate reduction in the water column at the intermediate depth of oki ridge the intervening low values correspond to periods of denitrification and oxygen respiration the maxima have a period of 41 k y the youngest having an age of 1 10 ma the 41 k y cycle is similar to the cycle of 18 o values of open ocean plankton of the same age the similarity between the cycles of minor element accumulation in sea of japan sediment and 18 o values of atlantic ocean foraminifera indicates that redox changes in the water column of the sea of japan during the quaternary forced by major shifts in water column advection and minor shifts in photic zone productivity reflect global events

seismic thermal and compositional structures of the stagnant slab in the mantle transition zone beneath southeastern china abstract the structure of earth s mantle transition zone mtz contains important information about how oceanic plates are recycled into the mantle however morphologies of stagnant slab in the mtz inferred from seismic studies remain controversial here we conduct combined geodynamic mineral physical and seismologic modeling to infer fine features of the stagnant slab in the mtz beneath southeastern china this method applies mineral physics modeling in combination with geodynamically consistent thermal anomalies to predict seismic velocity structures in the mtz that are used in triplication waveform modeling the synthetic waveforms are systematically compared with observed data using a grid search optimization our best fit model reveals a stagnant slab lying at the bottom of the mtz with a peak temperature reduction of 350 80 k and a thickness of 85 20 km indicating that it underwent little deformation by joint modeling of the mineral physics and thermal history of the slab our approach improves estimates of both the thickness and temperature of the stagnant slab and provides a better understanding of the paleo subduction system beneath southeastern china

the inorganic geochemistry of cretaceous black shales dsdp leg 41 in comparison to modern upwelling sediments from the gulf of california

geochemical evidence for variations in delivery and deposition of sediment in pleistocene light dark color cycles under the benguela current upwelling system abstract distinctive light dark color cycles in sediment beneath the benguela current upwelling system indicate repetitive alternations in sediment delivery and deposition geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid pleistocene intervals from odp sites 1082 and 1084 concentrations of total organic carbon toc vary between 3 5 and 17 1 concentrations of caco3 vary inversely to toc and al which suggests that both carbonate dissolution and terrigenous dilution contribute to the light dark cycles opal concentrations are independent of both toc and caco3 therefore eliminating diatom production and lateral transport of shelf material as causes of the light dark cycles 13corg and 15ntot values do not vary across light dark sediment intervals implying that the extent of relative nutrient utilization did not change the stable 15ntot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the toc rich layers parallel changes in concentrations of indicator trace elements and toc imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen cu ni and zn are enriched in the darker sediment as a consequence of greater organic matter delivery redox sensitive metals vary due to loss mn and ba or enrichment mo under reducing conditions created by toc oxidation organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution sulfate reduction and the concentration of metals thus export production is considered ultimately responsible for the generation of the color cycles

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

coupled primary production benthic foraminiferal assemblage and sulfur diagenesis in organic rich sediments of the benguela upwelling system abstract episodically deposited dark organic rich pleistocene and late pliocene sediments from the lower continental slope off southwest africa reveal complex interactions between changes in primary production benthic foraminiferal assemblage and anaerobic microbial processes the organic rich layers contain diatom assemblages characteristic of intense seasonal coastal upwelling whereas stratigraphically adjacent sediments reflect pelagic primary production coastal upwelling dominated depositional intervals coincide with periods of enhanced carbon flux to the seafloor enhanced organic carbon export during dark layer deposition was accompanied by decreases in the diversity of benthic foraminifera to few opportunistic species adapted to high phytodetritus accumulation rates and low o 2 conditions in all sediments the sulfur isotopic composition of pyrite indicates redox cycling of sulfide close to the sediment water interface the sulfur isotopic evidence and the permanent presence of abundant low o 2 adapted benthic foraminifera throughout the organic rich layers suggest an oxygenated benthic environment efficient oxidation of sulfide and removal of sulfide by sulfidization of organic matter inhibited buildup of toxic hydrogen sulfide from bacterial sulfate reduction at the sediment water interface these data imply that in continental slope sediments underneath productive surface waters benthic dysoxic conditions are maintained by the lateral advection of dissolved oxygen to support a small but well adapted benthic community

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

direct evidence for the undulation of the 660 km discontinuity beneath tonga comparison of japan and california array data short period seismograms of tonga deep earthquakes recorded by japanese and californian seismic networks are stacked to identify the s p converted wave associated with the 660 km discontinuity the travel time difference between this s p converted wave and the direct p wave is used to constrain the depth of the 660 km discontinuity analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the tonga subduction zone two events which exhibit clear s p conversions in both japan and california data are selected to show directly the depth variations of the 660 km discontinuity adjacent to the subducting slab the s p conversion points on the ray paths to japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to california which represents direct evidence for a slab induced depression of the 660 km discontinuity

a sporadic low velocity layer atop the western u s mantle transition zone and short wavelength variations in transition zone discontinuities teleseismic receiver function analysis of data from six dense arrays in the western u s is used to investigate mantle transition zone mtz discontinuities and the prevalence of a low velocity layer atop the 410 km discontinuity 410 lvl negative polarity ps arrivals indicative of a low velocity layer with a top 25 60 km above the 410 are identified in 8 11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors the 410 lvl is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410 the 669 km mean depth of the 660 km discontinuity 660 and the magnitude of 660 topography suggest variable hydration locally approaching saturation in addition to 150 k lateral temperature variations beneath five arrays mean amplitudes of p410s and p660s increase monotonically with period from 2 to 10 s however greater variations are observed in the frequency dependence of p410s compared to p660s implying 410 thickness is more heterogeneous variable 410 thickness is attributed to changes in hydration modulating the width of the olivine to wadsleyite transition interval frequency dependence of p660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems sporadic detection of the 410 lvl the magnitude and length scales of mtz discontinuity topography and inferred variations in hydration support the occurrence of vigorous small scale convection in the western u s mantle comparison of receiver functions with body wave tomography suggests small scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410 lvl

a detailed map of the 660 kilometer discontinuity beneath the izu bonin subduction zone dynamical processes in the earth s mantle such as cold downwelling at subduction zones cause deformations of the solid state phase change that produces a seismic discontinuity near a depth of 660 kilometers observations of short period shear to compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the izu bonin subduction zone the discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab with a deformation profile consistent with the expected thermal signature of the slab the experimentally determined clapeyron slope of the phase transition and the regional tectonic history

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

the early matuyama diatom maximum off sw africa benguela current system odp leg 175 abstract several sites of ocean drilling program leg 175 between 20 and 30 s 1081 1082 1083 1084 and 1085 in the benguela current system show a distinct opal and diatom maximum within the late pliocene and early quaternary spanning the lower half of the matuyama reversed polarity chron mdm matuyama diatom maximum this maximum is centered around 2 0 2 6 ma and follows a rapid increase of diatom deposition near 3 1 ma within the mdm we recognized intervals dominated by the antarctic subantarctic species thalassiothrix antarctica which have the aspect of diatom mats the mdm and the mats developed during a time of sustained southern ocean influence 3 2 1 6 ma probably reflecting a period of seasonally pulsed continuous advection of subantarctic waters into the benguela oceanic current the onset and cessation of the mdm raises questions about modes in the operation of the silica cycle and the relation of these to major oceanographic events a depositional model for the late pliocene is proposed which involves positional changes in a longshore system of fronts substantial equatorward excursions of subantarctic waters during mdm time and subsurface waters greatly enriched in silicate

upper mantle tomography in the northwestern pacific region using triplicated p waves we conducted delay time tomography of the upper mantle beneath the northwestern pacific using p data from northeast china extended seismic array f net and nearby available stations to improve resolution and accuracy in the vicinity of mantle discontinuities we extracted traveltimes of both initial and secondary phases observed at triplication distances by using a waveform fitting technique compared with the model obtained by using only the initial phase the resolution just above the 410 km discontinuity is especially improved and low velocity anomalies beneath the changbai volcano are clearly observed down to the 410 km discontinuity compared with previous models low velocity anomalies atop the 410 are more pronounced the results of this study together with the previous receiver function analysis provide further support that we have hot material beneath the changbai volcano

sh velocity and compositional models near the 660 km discontinuity beneath south america and northeast asia 1 we constrain sh wave velocity structures near the 660 km discontinuity beneath south america and northeast asia using triplicated phases near the discontinuity recorded in the epicentral distance range of 10 35 for three deep events we then explore mineralogical and compositional models appropriate for explaining the inferred seismic structures between the two regions sh velocity structures near the 660 km discontinuity are found to be different in the two regions beneath south america the velocity gradient above the 660 km discontinuity is larger than that of preliminary earth reference model prem while the velocity jump across the discontinuity is the same as prem beneath northeast asia the velocity gradient above the 660 km discontinuity is the same as that of prem while the velocity jump across the discontinuity is larger than prem both regions are characterized by a large velocity gradient extending about 80 km deep below the 660 km discontinuity the different velocity structures require different mineralogical models in the transition zone in the two regions the larger velocity gradient above the 660 km discontinuity beneath south america requires existence of the ilmenite phase in the bottom of transition zone while that beneath northeast asia can be explained by the temperature and pressure dependence of elastic properties of the major mantle mineral assemblages the observed large velocity gradients in the top of the lower mantle can be explained by gradual transformation of garnet to perovskite persisting to greater depths the velocity jump across the 660 km discontinuity beneath south america in the presence of the ilmenite phase in the bottom of the transition zone requires a bulk composition of more garnet than the pyrolite model while a larger velocity jump across the discontinuity beneath northeast asia requires a larger fraction of garnet transforming to perovskite across the discontinuity than what is required for explaining prem these different mineralogical models can be caused by different mantle temperature or composition especially the aluminum content in mantle composition the presence of garnet 80 km below the 660 km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3 4 the existence of ilmenite in the bottom of the transition zone beneath south america and the absence of ilmenite beneath northeast asia can be explained by either a difference in mantle temperature of about 100 c with that beneath south america being lower between the two regions assuming a uniform mantle composition or alternatively a difference in aluminum content of about 1 with that beneath south america being lower between the two regions without invoking a temperature difference between the two regions we also discuss conditions of mantle composition and temperature that double discontinuities may occur near the 660 km depth as well as the depth separation and velocity jumps of the double discontinuities for the inferred mantle temperature and composition beneath south america and northeast asia the maximum separation of the double discontinuities is 20 km and cannot be resolved by the sh wave data

lateral variations in sh velocity structure of the transition zone beneath korea and adjacent regions 1 using seismic profiles of triplicated waveforms we show significant lateral variations in the sh velocity vs structure of the transition zone tz beneath korea and adjacent regions beneath sakhalin we detected a high vsanomaly 2 limited to middle regions of the tz mid tz and a largevsjump across the 660 km discontinuity a similar jump invs also occurs beneath the northern portion of the north china craton ncc beneath korea a high vs anomaly 2 in the lower tz is inferred accompanied by a relatively small vsjump across the 660 km discontinuity which is depressed by about 15 20 km the deep structure under the eastern part of northeast china nec also includes a slightvsanomaly 1 in the lower tz but does not exhibit significant depression of the 660 km discontinuity compared with previous study our observations reveal strong regional variations of the tz structure on a relatively short scale these variations most likely reflect the geometrical distribution of the subducting northwest pacific plate our results suggest that the subducting slab dips across the mid tz under sakhalin and becomes flattened atop of the 660 km discontinuity beneath korea while only the tip of the slab reaches the lower tz beneath the nec the tz beneath the ncc does not show evidence of the slab stagnation

multiple seismic discontinuities near the base of the transition zone in the earth s mantle the seismologically defined boundary between the transition zone in the earth s mantle 410 660 km depth and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma spinel phase of olivine to magnesium perovskite and magnesiowustite laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone but a computational study has indicated that because of the presence of garnet minerals multiple seismic discontinuities might exist near a depth of 660 km ref 4 which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary in particular garnet minerals undergo exothermic transformations near this depth acting to complicate the phase relations and possibly effecting mantle convection processes in some regions here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern california the observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma spinel phase of olivine temperature anomalies calculated from the imaged discontinuity depths using clapeyron slopes determined for the various transformations generally match those predicted from an independent p wave velocity model of the region

light dark cycles in opal rich sediments near the plio pleistocene boundary dsdp site 532 walvis ridge continental terrace diester haass l meyers p a and rothe p 1986 light dark cycles in opal rich sediments near the plio pleistocene boundary dsdp site 532 walvis ridge continental terrace mar geol 73 1 23 samples taken at 10 cm intervals from dsdp core 532b 17 contain variations in carbonate opal organic carbon and terrigenous components that correlate with light dark cycles in sediment color the core site at 1300 m water depth is well above the ccd yet the color variations appear to result largely from cyclical fluctuations in carbonate dissolution which was greater during glacial periods higher concentrations of organic carbon and of terrigenons sediment components correlate with enhanced carbonate dissolution but opal concentrations inversely correlate and suggest that biological productivity at this site diminished during glacial periods a complicated glacial interglacial picture emerges from the data in interglacial times upwelling associated with the benguela current produced abundant opaline material organic matter was fairly well preserved and carbonate was only moderately dissolved in glacial times the upwelling core shifted as sea level fell and winds intensified productivity in the waters over site 532 decreased but lateral supply of oxidizable organic matter enhanced carbonate dissolution giving rise to light dark cycles in these sediments

scavenged excess aluminum and its relationship to bulk titanium in biogenic sediment from the central equatorial pacific ocean we present results from chemical analyses of al and ti in surface sediment sampled along two cross equator latitudinal transects at 135w and 140w in the central equatorial pacific ocean although traditionally both al and ti are considered to reside exclusively within terrigenous phases in marine sediment these sediments present extremely high al ti ratios that are several times that of average shale and other potential crustal sources a sharp maximum in al ti is observed slightly south of the equator where sedimentary bulk accumulation rate bar is also highest reflecting elevated productivity in the overlying water caused by surface water divergence bulk al ti decreases sharply away 2 latitude to near crustal values at 4 north and south the latitudinal profiles of al ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of al and ti these results indicate the presence of a significant scavenged component of al sourced directly from seawater during particle settling the data from the two transects analytically and oceanographically confirms our earlier work that was based on the single 135w transect calculations of alexcess indicate that the highest al ti ratios correspond to 50 of the total al being unsupported by the small amount of terrigenous phases present these results are consistent with previous and ongoing studies of biogenic sediment suspended particulate matter and sediment trap material quantitative use of al as an index of terrigenous material may therefore lead to an overestimation by a factor of two of the true terrigenous load in marine sediment sedimentary rock and settling particles because bulk al ti appears to respond to sedimentary bar which in biogenic regimes is linked to surface water productivity downcore records of al ti in biogenic sediment may track productivity changes through time such al ti proxy records may be applicable in sediment of all ages unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr

geochemical constraints on pliocene sapropel formation in the eastern mediterranean abstract seventeen eastern mediterranean pliocene sapropels from odp sites 964 966 967 and 969 some of which are coeval have been analysed for their geochemistry the sapropels are characterized by very high organic carbon contents up to 30 which are reported to be the result of both increased productivity and improved preservation although the organic matter in the sapropels is mainly of marine origin the 13 c org values and c n ratios appear terrestrial this is the result of anaerobic organic matter degradation which preferentially removed nitrogen and 13 c rich organic components a comparison with ti al profiles which mimic the precession index and a calculation of organic carbon accumulation rates indicate that sedimentation rates were at most 30 lower or at most 50 higher during sapropel formation thus sapropel formation lasted from between 2000 and 10 000 years at site 964 to between 4500 and 12 000 years at site 967 a synthesis of new data and a comparison with existing models indicates that productivity which increased due to extra nutrients supplied as a result of winter mixing and as a result of enhanced input by the nile was the driving mechanism behind sapropel formation the resulting sapropel formation was simultaneous at different depths but lasted longer in the part of the basin closest to the nile

distinct compositional thin layers at mid mantle depths beneath northeast china revealed by the usarray abstract we observe a clear seismic arrival at 35 45 s after the direct p wave in usarray recordings of two deep earthquakes occurring beneath northeast china velocity spectrum and beam forming analyses reveal that this arrival has a lower slowness value than the direct p wave and a back azimuth slightly different from the great circle direction the measured slowness and arrival time indicate that it is a transmitted s to p conversion from structures below the sources we employ the common conversion point ccp stacking and diffraction migration methods to determine the location and geometric features of the seismic structures the ccp stacking image indicates that the structure is a localized discontinuity at 1000 km with a dimension at 200 km by 50 km along the e w and n s directions respectively it is located at 150 km northeast to the two events the 2d migrated images on the other hand indicate that the sources structure are reflectors dipping northeastwards by 17 at a slightly shallower depths the reflectors have a length scale of 100 km ant their centers are 50 km away from the epicenters of the two earthquakes forward waveform modeling suggests that the dipping reflectors may be thin layers with a thickness of few kilometers the layers have a lower shear velocity and a higher density than that of the surrounding mantle which matches well with those predicted for mid ocean ridge basalt morb at mid mantle depths according to a recent ab initio study combined with the results from previous studies our observations here suggest that the former oceanic crust may be ubiquitously present in the lower mantle beneath subduction zones

sedimentary geochemistry of manganese implications for the environment of formation of manganiferous black shales the sedimentary geochemistry of manganese is dominated by the redox control of its speciation higher oxidation states mn super 3 and super 4 occurring as insoluble oxyhydroxides in well oxygenated environments and the lower oxidation state mn super 2 being much more soluble in oxygen deficient settings its geochemical behavior is therefore quite different in oxic and anoxic environments and where oxic and anoxic conditions are juxtaposed mn is recycled between the two environments in modern marine sediments mn is present above its crustal abundance as an oxyhydroxide in all slowly accumulating pelagic sediments of the deep ocean and in surficial deposits of continental margin environments diagenetic recycling of mn in the latter causes surficial deposits to have larger mn enrichments than in many pelagic sediments bottom sediments of permanently anoxic basins show no enrichments and have mn concentrations that are controlled solely by the aluminosilicate fraction manganese carbonates kutnohorite and calcic rhodochrosite are found only in anoxic sediments accumulating beneath surface oxic horizons and therefore under oxygenated bottom waters in many nearshore environments such enrichments are due to delivery of mn by burial of surface oxyhydroxides into the subsurface anoxic environment where they are dissolved pore water mn levels can reach saturation with respect to a mixed mn ca carbonate phase in such sediments the diagenetic origin for these phases is shown by their carbon isotope compositions which typically indicate a carbon source from decomposing organic matter the presence of mn carbonates therefore signifies that the host sediment must have accumulated under oxygenated bottom waters on the basis of this information it is proposed that in contrast to several current explanations for the formation of mn carbonates kutnohorite and rhodochrosite in ancient organic rich shales limestone and marl sequences and in many mn ore deposits the occurrence of these mineral phases indicates that the sediments originally accumulated beneath oxygenated bottom waters by analogy with the present mn carbonates could not have formed in the bottom waters of anoxic basins these diagenetic phases however did form where mn was supplied at a high rate namely by the burial of oxyhydroxide enriched surface sediments to a subsurface anoxic environment this situation could only have occurred under oxygenated bottom waters the presence of mn carbonates in ancient black shales and in some carbonate rich rocks lends strong support to the notion that these rocks did not necessarily form in anoxic basins but owe their carbon richness to a high supply of organic matter to sediments deposited under oxygenated bottom waters probably in continental margin settings

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

a stagnant slab in a water bearing mantle transition zone beneath northeast china implications from regional sh waveform modelling summary sh waveforms recorded by a dense regional seismic network are used to constrain the shear wave velocity structure of the upper mantle transition zone beneath northeast china by modelling triplication waveforms we show that a 130 km thick layer of increased seismic velocities is overlying a broad slightly depressed 660 km discontinuity this anomaly can be associated with the westward extension of the stagnant pacific slab that is deflected and still cold the transition zone as a whole is featured by low shear wave velocity and high vp vs ratio which infer h2o content of 0 2 0 3 wt per cent 3 0 4 5 10 4 ppm h si atop the 660 km discontinuity we interpret that the fast eastward retreat of the japan trench facilitated the slab flattening when the subducting oceanic lithosphere hit the bottom of the transition zone and the leading edge of the slab is currently trapped in a water bearing mantle beneath northeast china

glacial to interglacial changes in surface nitrate utilization in the indian sector of the southern ocean as recorded by sediment δ15n we present a new approach for paleoceanographic reconstruction of surface nutrient utilization in the southern ocean it has been observed in the contemporary ocean that due to preferential uptake of 14no3 during photosynthesis the 15n of planktonic organic matter increases with increasing nitrate depletion in surface waters our results demonstrate that the 15n signal produced in surface waters is reflected in the underlying surface sediments core top 15n is inversely correlated with surface nitrate concentration along a transect across the subtropical convergence and the polar front in the southeast indian ocean these results are consistent with a four box model showing that the nitrogen isotopic composition of sinking organic matter depends on percent nitrate utilization in the euphotic zone by comparing the 15n of surface sediments with that measured in the glacial sections of several cores we infer changes in the intensity and latitudinal distribution of nitrate uptake in this region during the last glacial maximum these preliminary results argue against increased biological uptake of nutrients in southern polar waters as a major mechanism for glacial lowering of atmospheric co2 they also suggest that subantarctic waters in the se indian sector became more nutrient depleted as they migrated northward increased nitrate depletion might have also occurred slightly south of the glacial polar front we use a six box model to explore the possible impact of this observation on atmospheric co2

variation of transition zone high velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern kuriles to izu bonin and ryukyu the seismic structure of the transition zone beneath the northwestern pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms by modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most p wave tomography experiments these secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments we used just two one dimensional models in the waveform comparison the iasp91 slightly modified in the shallower depths as aiasp is used as a global average structure and model m3 11 is used as representative of transition zone structure near a deep subducting slab model m3 11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper 660 km discontinuity our results show strong variations in the transition zone structure beneath the northwestern pacific data that sample beneath the northwestern japanese subduction zone and the southern kuriles are well fit by model m3 11 the deep structure beneath the bohai sea to the west of korea also appears to show slab related velocity anomalies in the transition zone in contrast the seismic data sampling beneath the yellow sea to the east china sea are better modeled using the global average model iasp91 data that sampled beneath the philippine sea showed evidence for anomalously high velocity in the transition zone but there is little apparent change in the depth of the 660 km discontinuity there the overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying flat or piling up within the transition zone although we find that the region where this occurs and thus the total volume of slab within the upper mantle is considerably less than that seen in past studies the unusual structure beneath the philippine plate may be due to the relatively recent descent of slab beneath this region the slab itself is cold and thus anomalously fast but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

climatically sensitive eolian and hemipelagic deposition in the cariaco basin venezuela over the past 578 000 years results from al ti and k al al ti and k al ratios in bulk sediment are used to interpret wind blown and hemipelagic sources of deposition to a 578 kyr record in the cariaco basin venezuela ocean drilling program site 1002 graphical and cross spectral analyses indicate that these ratios vary extremely closely with planktonic foraminiferal 18o with both ratios being significantly higher during interglacials and lower during glacials k al indicates that during glacials the lower sea level that results in relative basin isolation increases the relative proportion of kaolinite derived from local rivers draining the relatively humid venezuelan margin al ti decreases during glacials suggesting greater proportions of eolian rutile sourced from the northern sahara due to increased wind strength and or aridity this interpretation is consistent with previous studies of the chemistry and mineralogy of saharan derived eolian matter in the caribbean and with a mass balance determining the effect of changes in eolian rutile accumulation on the bulk sedimentary al ti ratio

15n 14n variations in cretaceous atlantic sedimentary sequences implication for past changes in marine nitrogen biogeochemistry at two locations in the atlantic ocean dsdp sites 367 and 530 early to middle cretaceous organic carbon rich beds black shales were found to have significantly lower 15n values lower15n 14n ratios than adjacent organic carbon poor beds white limestones or green claystones while these lithologies are of marine origin the black strata in particular have 15n values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools in contrast black organic carbon rich beds at a third site dsdp site 603 contain predominantly terrestrial organic matter and have c and n isotopic compositions similar to organic matter of modern terrestrial origin the recurring15n depletion in the marine derived cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry existing isotopic and other data indicate that the low15n relative abundance is the consequence of pelagic rather than post depositional processes reduced ocean circulation increased denitrification and hence reduced euphotic zone nitrate availability may have led to cretaceous phytoplankton assemblages that were periodically dominated by n2 fixing blue green algae a possible source of this sediment15n depletion lack of parallel isotopic shifts in cretaceous terrestrially derived nitrogen site 603 argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment

distribution of biogenic silica and quartz in recent deep sea sediments all available quartz and biogenic silica concentrations from deep sea surface sediments were intercalibrated plotted and contoured on a calcium carbonate free basis the maps show highest concentrations of biogenic silica opal along the west african coast along equatorial divergences in all oceans and at the polar front in the southern indian ocean these are all areas where upwelling is strong and there is high biological productivity quartz in pelagic sediments deposited far from land is generally eolian in origin its distribution reflects dominant wind systems in the pacific but in much of the atlantic and indian oceans the distribution pattern is strongly modified by turbidite deposition and bottom current processes

glacial interglacial variations in production and nitrogen fixation in the cariaco basin during the last 580 kyr the effect of sea level change on nutrient supply to the anoxic cariaco demonstrates the fundamental importance of nitrogen n2 fixation and phosphate to oceanic production as n2 fixation produces biomass of low 15n and has been reported to be an important component of the nitrogen cycle in the modern cariaco basin we propose that it contributes to the light interglacial 15n 2 3 values observed in the ocean drilling program odp site 1002 sediment record during the glacials the sediments are bioturbated oxic conditions with low total organic carbon toc contents and sedimentary 15n values of 5 suggesting that nitrogen n2 fixation contributed little to the n nutrition of cariaco surface waters the most plausible explanation for the inferred glacial interglacial changes in n2 fixation in the cariaco is that they have occurred in response to variations in the n p ratio of the nutrient supply driven by changes in denitrification

effect of water in depleted mantle on post spinel transition and implication for 660 km seismic discontinuity abstract we have determined the post spinel transition boundary in anhydrous and hydrous mg2sio4 in a temperature range from 1173 to 2023 k at 19 3 25 4 gpa using synchrotron in situ x ray diffraction measurements the phase boundary in mg2sio4 is located at 22 gpa and 1800 k and 22 1 gpa and 1500 k which is slightly lower 0 3 0 5 gpa than that determined in the previous in situ measurements using the same pressure scale e g katsura et al 2003 post spinel transition in mg2sio4 determined by high p t in situ x ray diffractometry phys earth planet inter 136 11 24 the clapeyron slope of mg2sio4 was found to be gentle i e between 0 4 and 0 7 mpa k which is also consistent with previous in situ measurements but inconsistent with diamond anvil cell experiments and theoretical estimations the phase boundary in mg2sio4 2 wt h2o which is relevant to fe free depleted harzburgitic composition is located between 23 4 and 23 6 gpa and 1500 k which shifts the hydrous boundary to the higher pressures relative to anhydrous mg2sio4 from 1 3 to 1 0 gpa the result for hydrous mg2sio4 shows steeper clapeyron slope between 3 2 and 3 1 mpa k compared with anhydrous mg2sio4 and hydrous pyrolite system the present data suggest that water has a strong influence on 660 km discontinuity and the depressions observed at this boundary in several regions especially related to subduction zones can be explained by the presence of water in depleted harzburgite component

a petrologic anomaly accompanying outboard earthquakes beneath fiji tonga corresponding evidence from broadband p and s waveforms 1 using high resolution triplicate body waveforms we investigate strong lateral variations in p and sh wave speeds vp and vs beneath the tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr 1 under these conditions the lack of prominent anomalies of high vp and vs in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle this inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the wadati benioff zone beneath the back arc we find a sharp east west trending demarcation north of which outboard earthquakes occur and vp and vs abruptly drop by 3 thus the effect of cold temperature to raise vp and vs as indicated by the presence of outboard earthquakes must be counteracted by petrologic variations we interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes considering the density of these candidates and the strong effect of partial melt on vs volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in vp and vs meanwhile metastable olivine remains a viable candidate as a cold slab passes below the 410 km discontinuity olivine becomes metastable and makes the tip of the slab buoyant such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone

mantle transition zone shear velocity gradients beneath usarray broadband p to s scattering isolated by teleseismic receiver function analysis is used to investigate shear velocity v s gradients in the mantle transition zone beneath usarray receiver functions from 2244 stations were filtered in multiple frequency bands and migrated to depth through p and s tomography models the depth migrated receiver functions were stacked along their local 410 and 660 km discontinuity depths to reduce stack incoherence and more accurately recover the frequency dependent amplitudes of p410s and p660s the stacked waveforms were inverted for one dimensional v s between 320 and 840 km depth first a gradient based inversion was used to find a least squares solution and a subsequent monte carlo search about that solution constrained the range of v s profiles that provide an acceptable fit to the receiver function stacks relative to standard references models all the acceptable models have diminished v s gradients surrounding the 410 a local v s gradient maximum at 490 500 km depth and an enhanced v s gradient above the 660 the total 410 v s increase of 6 3 is greater than in reference models and it occurs over a thickness of 20 km however 60 of this v s increase occurs over only 6 km the 20 km total thickness of the 410 and diminished v s gradients surrounding the 410 are potential indications of high water content in the regional transition zone an enhanced v s gradient overlying the 660 likely results from remnants of subduction lingering at the base of the transition zone cool temperatures from slabs subducted since the late cretaceous and longer term accumulation of former ocean crust both may contribute to the high gradient above the 660 the shallow depth of the 520 km discontinuity 490 500 km implies that the regional mean temperature in the transition zone is 110 160 k cooler than the global mean a concentrated v s gradient maximum centered near 660 km depth and a low v s gradient below 675 km confirms that the ringwoodite to perovskite magnesiowustite reaction is the dominant cause of the 660 in the region

mapping the subducting pacific slab beneath southwest japan with hi net receiver functions abstract we have used 4th root receiver function stacks and pre stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern japan receiver functions were calculated from the quiet short period seismograms recorded by a recently deployed borehole network hi net we found that a relatively broad frequency band can be retrieved from a short period seismogram by a deconvolution of the instrument response the quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments we applied common conversion point gathering to the receiver function data to image the p to s conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence we found that the topographic anomalies of the 410 and 660 km discontinuities beneath southwest japan have very different length scales the former is characterized by a narrow 150 200 km wide topographic high while the latter exhibits a broad 400 km wide moderate topographic low together with a small scale larger amplitude depression a 2 5d pre stack depth migration of the receiver functions shows the transition zone features clearly as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660 km these observations show that the subducted pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone and is flat lying either above or across the 660 km discontinuity the flat lying slab is however restricted to the bottom of the transition zone and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities

seismic observations indicating that the 2015 ogasawara bonin earthquake ruptured beneath the 660 km discontinuity the termination of deep earthquakes at a depth of 700 km is a key feature for understanding the physical mechanism of deep earthquakes the 680 km deep may 30 2015 ogasawara bonin earthquake mw7 9 and its aftershocks were recorded by seismic stations at distances from 7 to 19 synthetic seismograms indicate that the p waveforms depend on whether the earthquake is located above or below the 660 km discontinuity in this study i show that broadband recordings indicate that the 2015 earthquake may have occurred below the 660 km velocity discontinuity recordings of the p wave from the strongest aftershock lack evidence for wave triplication expected when a subhorizontal discontinuity underlies the hypocenter theoretical waveforms computed with a 660 km discontinuity above the aftershock and mainshock match the observed waveforms more accurately these observations may indicate earthquake ruptures due to mantle minerals other than olivine or strong deformation of the 660 km phase transition

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

variations in p wave speeds and outboard earthquakes evidence for a petrologic anomaly in the mantle transition zone we investigate the nature of laterally varying p wave speeds vp and their relationship to a group of unusual deep earthquakes beneath the backarc of tonga this subhorizontal swath of outboard earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined wadati benioff zone surprisingly the seismogenic region is not associated with fast vp it follows that the effect of cold temperature as implied by the presence of large earthquakes must have been counteracted by other factors such as compositional or mineralogical effects our results are based on a sequence of seismic profiles in a fan shot geometry for a given azimuthal sector we seek the simplest model of radially varying vp that explains observed broadband waveforms and travel times the lateral extent of anomalous vp is then constrained by comparing the results from a wide range of azimuths this approach fully utilizes information from triplicate p arrivals that are sensitive to seismic wave speeds at the depths of interest our results identify prominent anomalies of fast vp 3 over average earth models both in the transition zone and in the lower mantle beneath the tonga backarc however the region of fast vp in the transition zone is offset by 300 km to the south of that in the lower mantle and the region of offset coincides with the outboard earthquakes a straightforward interpretation is that a large flux of subducted pacific lithosphere lingers in the transition zone before descending into the lower mantle within this broad remnant of subducted lithosphere the outboard earthquakes occur in a region where impounding of subducted material is most significant a condition favorable for the accumulation of met astable olivine or volatiles such materials reduce vp and are candidates for triggering earthquakes thus accounting for the absence of fast vp in the source zone of deep earthquakes

distinct velocity variations around the base of the upper mantle beneath northeast asia abstract both the global and regional p wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions in particular low velocity anomalies have been observed beneath the descending high velocity slabs in a number of subduction zones the limited resolution at large depths and possible trade off between the high and low velocities however make it difficult to substantiate this feature and evaluate the vertical extent of the low velocity structure from broadband waveform modeling of triplicated phases near the 660 km discontinuity for three deep events we constrained both the p and sh wave velocity structures around the base of the upper mantle in northeast asia for the two events beneath the southern kurile the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending pacific slab our preferred models consistently suggest normal to lower p and significantly low sh velocities above and below the 660 km discontinuity extending to about 760 km depth compared with the global iasp91 model corroborating previous observations for a slow structure underneath the slab in contrast both high p and sh velocity anomalies are shown in our preferred models for the japan subduction zone region likely reflecting the structural feature of a slab stagnant above the 660 km discontinuity the velocity jumps across the 660 km discontinuity were found to be on average 4 5 and 7 for p and s waves under the south kurile and 3 and 6 under the japan subduction zone the respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under slab areas based on mineral physics data the depth averaged 1 5 p and 2 5 sh velocity differences in the depth range of 560 760 km between the two regions could be primarily explained by a 350 450 k temperature variation although the presence of about 0 5 1 wt water might also contribute to the subtle velocity variations near the base of the transition zone in the southern kurile from our modeling results we speculate that the slow structure in the southern kurile may be correlated to the low velocity zone observed previously around the 410 km discontinuity under northern honshu if this is the case both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast asia

origin and significance of deep earthquake clusters surrounding a pronounced seismic gap in northeast china abstract from analysis of earthquakes in the global centroid moment tensor gcmt catalog we identify an intermediate depth 250 350 km seismic gap having dimensions of 300 km 500 km within the subducting northwestern pacific plate npp and surrounded by several groups of isolated earthquakes near northeast china normally groups of unusual deep earthquakes may represent a detached slab segment as in tonga vanuatu and a deep seismic gap maybe either be the aseismic continuation of a subducting slab or a possible gap in the subducting plate we evaluate the characteristics of deep earthquakes in the npp and focus on three isolated clusters particularly one directly beneath the seismic gap and northeast china this sub gap cluster has a peak in the number of quakes near depth 580 km unlike seismicity in the npp as a whole its b value 0 60 is significantly smaller than b values in two other nearby clusters 0 81 and 0 86 in the npp focal mechanisms in this sub gap cluster also show a variation with depth and projections of their t p and b axes differ from what is observed for the other isolated clusters we speculate why these sub gap earthquakes are isolated and suggest two possible models explaining the presence of the gap and the characteristics of the underlying cluster beneath northeast china in the first model the seismic gap is attributable to aseismic deformation caused by abnormally high temperatures in the subducting lithosphere and the characteristics of the sub gap clusters and its related slab structures are attributed to slab buckling caused by resistance to subduction in the second model the seismic gap represents absent subducting lithosphere and the missing material has migrated to the bottom of subducting slab although we favor the second model both models are consistent with our knowledge of deep earthquakes and the results of recent studies near northeast china

oxygenation history of bottom waters in the cariaco basin venezuela over the past 578 000 years results from redox sensitive metals mo v mn and fe we present results from analyses of the redox sensitive metals mo v mn and fe in sediment recovered from the cariaco basin ocean drilling program leg 165 site 1002 results are interpreted in the context of previous studies of 15n export production percent total organic carbon eolian input and hemipelagic deposition in the basin variations in redox metal ratios over the past 578 000 years were compared to variations in 18o at milankovitch frequencies and show a strong relationship between glacial interglacial cycles in sea level governed by the shallow sills encircling cariaco basin and bottom water oxygen content during 100 kyr and 41 kyr cyclicity enrichments of mo and v occur during highly productive interglacials indicating bottom water anoxia during glacials sediments are less depleted or enriched in mn and fe relative to the interglacials reflecting oxic conditions during 23 kyr and 19 kyr cyclicity however these redox metal patterns are not observed indicating that the cariaco basin responds differently to the higher frequency climate changes

a broad 660 km discontinuity beneath northeast china revealed by dense regional seismic networks in china 1 we examined the p wave velocity structure around the 660 km discontinuity at the tip of the subducting pacific slab beneath northeastern china by forward modeling waveform triplication data a total of 742 broadband seismograms were recorded by dense regional seismic networks in china from a deep earthquake that occurred near the border of east russia and northeast china providing an unprecedented density of ray coverage near the front edge of the subducting pacific slab multiple p waves were observed on single seismograms in the distance range of 14 29 the p wave triplication shows the following two features 1 the direct arrival traveling above the 660 km discontinuity ab branch extends as far as 29 approximately 6 further than the prediction from velocity model of international association of seismology and the physics of the earth s interior iasp91 2 the refracted wave propagating through the lower mantle cd branch appears at a distance a few degrees greater than that of the iasp91 synthetics forward waveform modeling suggests that they are best explained by a high velocity transition zone underlain by a 50 70 km thick 660 km discontinuity the broadened discontinuity is likely caused by multiple phase transitions associated with the dissolutions of olivine and garnet components

receiver functions in northeast china implications for slab penetration into the lower mantle in northwest pacific subduction zone abstract seismic studies of the subducting lithosphere and the upper mantle discontinuities in the northwest pacific subduction zone beneath japan and northeast china have suggested contrary subduction scenarios there was little consensus on the issue whether the subducting slab penetrates the upper mantle discontinuities into the lower mantle or it is deflected atop of the 660 km discontinuity over several hundred kilometers we calculate receiver functions from a recent seismic broadband station network located in northeast china and find topographic variations of the upper mantle discontinuities a deeper than normal 660 km discontinuity is observed over an area of 400 km and it coincides with the stagnant slab imaged by seismic tomography the 660 km discontinuity is locally depressed by more than 35 km and the transition zone is thickened by more than 20 km in the east of the region where it encounters the slab these observations provide evidence of the slab accumulating in the mantle transition zone and locally penetrating into the lower mantle

computation of seismic profiles from mineral physics the importance of the non olivine components for explaining the 660 km depth discontinuity the recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals the updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles these profiles are chosen to stress out the influence of non olivine minerals on seismic parameters and to represent cold and horizontally averaged temperature profiles of the earth s mantle in a first part starting compositions of pyrolite and piclogite and a single layer convection are assumed the results clearly point out the importance of the non olivine part of the mineralogy two scenarios are found to explain the 660 km depth discontinuity

velocity analysis with vertical seismic profile data using migration of surface related multiples abstractwe have developed an original method for migration velocity analysis for vertical seismic profile vsp data the method is based on vsp imaging with the primary reflected data and surface related multiples velocity updates are carried out by matching the images obtained by these two types of waves to do it automatically we have introduced an objective function of the crosscorrelation type and developed a gradient based maximization algorithm on synthetic walk away vsp data sets we found that maximization of the crosscorrelation of the images provided by the two different types of waves allowed the retrieval of the interval velocities below the borehole receivers a problem that was considered to be challenging with a vsp acquisition also the possibility of the lateral velocity gradient reconstruction was developed on a simple synthetic model on the real walk away vsp data set from the north sea we found that the method could also be used for estimation of effective anisotropy parameters

evidence of high velocity anomalies in the transition zone associated with southern kurile subduction zone we address the use of regional broad band waveform data to better constrain the upper mantle transition zone structure we then show evidence of high velocity anomalies in the transition zone associated with the southern kurile subduction zone modeling of the triplicated regional waveforms supports that high velocity anomalies exist in the deeper part of the transition zone over 500 km in depth and the 660 discontinuity may be depressed to about 690 km depth our results suggest there is some degree of flattening of the slab in the southern kuriles

topography of the 660 km discontinuity beneath northeast china implications for a retrograde motion of the subducting pacific slab 1 clear s to p converted waves at the 660 km discontinuity s660p are observed at stacked seismograms of deep earthquakes occurring in northeast china and japan sea recorded by broadband seismic arrays in north america and europe differential travel times between the s660p and p waves are used to constrain depth variations of the 660 beneath northeast china a rapid change in the depth of the 660 is observed within a narrow longitudinal range of 130 8 e 131 4 e where the lower boundary of the subducting slab encounters the 660 towards the west the 660 deepens steadily as it approaches the coldest core of the slab the maximum depression occurs at the west end of the studied region with an amplitude of 20 km to the east the 660 appears to be flat showing no obvious effect of the subducting slab these observations are consistent with a scenario for a retrograde moving pacific slab lying over the upper and lower mantle boundary progressively from west to east

late pleistocene paleoclimatology of the central equatorial pacific flux patterns of biogenic sediments high resolution records of the concentration and mass accumulation rate mar or flux of biogenic sediments from core rc11 210 in the central equatorial pacific are compared for the entire late pleistocene the fluxes of calcium carbonate and organic carbon correlate well the opal flux pattern shows no direct correlation with the other two rather opal flux maxima correlate with slopes of i e both precede and follow the organic carbon and carbonate mar peaks organic carbon and calcite therefore may both be better indicators of past productivity than opal which is an indicator of transition at this location we interpret the sequence low biogenic fluxes high opal fluxes high carbonate carbon and opal fluxes as representing the transition from low to high sea surface biological productivity the phase relationships for eccentricity of the biogenic sedimentation records were compared to those of other climatic indications including the oxygen isotope record of ice volume the ice core co2 record the pacific 13c record the deep nutrient cd ca record and the eolian grain size record of wind intensity we find that starting from maximum ice volume phase angle of 0 the phase relationships of the 100 000 year periodicity show in the first quadrant maximum wind intensity 30 and maximum calcite and carbon flux 45 occurring first followed by maximum opal flux 75 and then maximum deep nutrients 95 minimum pacific 13c occurs at a phase angle of 155 the maximum co2 in the vostok record and minimum ice volume is at 180 thus in the eccentricity climate cycle glacial conditions lead to stronger wind driven circulation which enhances productivity which in turn results in enhanced oceanic and atmospheric carbon dioxide and minimum ice volume

evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the californian continental margin peruvian continental slope north carolina continental slope and from the santa monica basin california are reported comparison of these sites indicates that o2 depleted bottomwaters enhance p regeneration from sediments diminishing overall phosphorus burial efficiency based on these observations a positive feedback linking ocean anoxia enhanced benthic phosphorus regeneration and marine productivity is proposed on shorter timescales these results also suggest that o2 depletion in coastal regions caused by eutrophication may enhance p regeneration from sediments thereby providing additional p necessary for increased biological productivity

migration methods for imaging different order multiples multiples contain valuable information about the subsurface and if properly migrated can provide a wider illumination of the subsurface compared to imaging with vsp primary reflections in this paper we review three different methods for migrating multiples the first method is model based and it is more sensitive to velocity errors than primary migration the second method uses a semi natural green s function for migrating multiples where part of the traveltimes are computed from the velocity model and part of the traveltimes i e natural traveltimes are picked from the data to construct the imaging condition for multiples the third method uses cross correlation of traces the last two methods are preferred in the sense that they are significantly less sensitive to velocity errors and statics because they use natural data to construct part of the migration imaging conditions compared with the interferometric i e crosscorrelation imaging method the semi natural green s function method is more computationally efficient and is sometimes less prone to migration artifacts numerical tests with 2 d and 3 d vsp data show that a wider subsurface coverage higher fold and more balanced illumination of the subsurface can be achieved with multiple migration compared with migration of primary reflections only however there can be strong interference from multiples with different orders or primaries when multiples of high order are migrated one possible solution is to filter primaries and different orders of multiples before migration and another possible solution is least squares migration of all events a limitation of multiple migration is encountered for subsalt imaging here the multiples must pass through the salt body more than twice which amplifies the distortion of the image