seismic investigation of the transition from continental to oceanic subduction along the western hellenic subduction zone 1 the western hellenic subduction zone whsz exhibits well documented along strike variations in lithosphere density i e oceanic versus continental subduction rates and overriding plate extension differences in slab density are believed to drive deformation rates along the whsz however this hypothesis has been difficult to test given the limited seismic constraints on the structure of the whsz particularly beneath northern greece here we present high resolution seismic images across northern and southern greece to constrain the slab composition and mantle wedge geometry along the whsz data from two temporary arrays deployed across greece in a northern line nl and southern line sl are processed using a 2d teleseismic migration algorithm based on the generalized radon transform images of p and s wave velocity perturbations reveal n60e dipping low velocity layers beneath both nl and sl the 8 km thick layer beneath sl is interpreted as subducted oceanic crust while the 20 km thick layer beneath nl is interpreted as subducted continental crust the thickness of subducted continental crust inferred within the upper mantle suggests that 10 km of continental crust has accreted to the overriding plate the relative position of the two subducted crusts implies 70 85 km of additional slab retreat in the south relative to the north overall our seismic images are consistent with the hypothesis that faster sinking of the denser oceanic portion of the slab relative to the continental portion can explain the different rates of slab retreat and deformation in the overriding plate along the whsz

the organic geochemistry of peru margin surface sediments ii paleoenvironmental implications of hydrocarbon and alcohol profiles we assess the utility of sedimentary hydrocarbons and alcohols as indicators of short term changes in depositional conditions in the coastal peruvian upwelling regime at 15 s the distribution of 35 lipids n alkanes n alkanols hopanols keto ols lycopane phytol stenols stanols sterenes and tetrahymanol in a l m dated core from 253 m water depth are interpreted by 1 a principal components analysis and 2 a consideration of individual source specific biomarkers profiles of odd carbon number n alkanes c25 c33 and even carbon number n alkanols c24 c28 reflect changes in the contribution of terrigenous sediment relative to marine sediment during deposition as indicated by the correlations between these lipids and inorganic indicators of terrigenous clastic debris the n alkane carbon preference index cpi provides a less sensitive record of fluctuations in the terrestrial input than the concentration profiles of the individual n alkanes and n alkanols and these lipids are not well correlated with the historical el nino record the similarity of all the stenol profiles measured and the lack of concordance between these profiles and inorganic indicators of terrigenous input suggest that any fluctuations in the abundance of higher plant stenols are obscured by the larger marine contribution of these compounds similarities between the profiles of total organic carbon toc and cholestanol cholesterol are consistent with stenol hydrogenation being influenced by the sediment redox conditions

temperature and salinity effects on alkenone ratios measured in surface sediments from the indian ocean abstract we compare alkenone unsaturation ratios measured on recent sediments from the indian ocean 20 n 45 s with modern sea oceanographic parameters for each of the core sites we estimated average seasonal cycles of sea surface temperature sst and salinity which we then weighted with the seasonal productivity cycle derived from chlorophyll satellite imagery the unsaturation index u37k ranges from 0 2 to 1 and correlates with water temperature but not with salinity theu37k versus sst relationship for indian ocean sediments u37k 0 033 sst 0 05 is similar to what has been observed for core tops from the pacific and atlantic oceans and the black sea a global compilation for core tops givesu37k 0 031 t 0 084 r 0 98 which is close to a previously reported calibration based on particulate organic matter from the water column for temperatures between 24 and 29 c however the slope seems to decrease to about 0 02u37k unit c for indian ocean core tops the ratios of total c37alkenones total c38alkenones and the slope of theu37k sst relationship are similar to those previously observed for cultures ofemiliania huxleyibut different from those previously published forgephyrocapsa oceanica eithere huxleyiis a major producer of alkenones in the indian ocean or strains ofg oceanicaliving in the northern indian ocean behave differently from the one cultured in contrast with coccolithophorid assemblages the ratios of c37alkenones to total c38alkenones lack clear geographic pattern in the indian ocean

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

sea surface temperature and productivity records for the past 240 kyr in the arabian sea abstract deep sea sediments of two cores from the western ty93 929 p and the southeastern md900963 arabian sea were used to study the variations of the indian monsoon during previous climatic cycles core ty93 929 p was located between the sw monsoon driven upwelling centres off somalia and oman which are characterized by large seasonal sea surface temperature sst and particle flux changes by contrast core md900963 was situated near the maldives platform an equatorial ocean site with a rather small sst seasonality less than 2 c for both cores we have reconstructed sst variations by means of the unsaturation ratio of c37 alkenones which is compared with the 18o records established on planktonic foraminifera in general the sst records follow the 18o variations with an sst maximum during oxygen isotope stage 5 5 the last interglacial at about 120 130 kyr and a broad sst minimum during isotope stage 4 and 3 3 approximately 40 50 kyr the sst difference between the holocene and the last glacial maximum lgm is of the order of 2 c in both cores the ssts during isotope stage 6 are distinctly higher by 1 2 c than the cold sst minima during the last glacial cycle lgm and stage 3 to reconstruct qualitatively the past productivity variations for the two cores we used the concentrations and fluxes of alkenones and organic carbon together with a productivity index based on coccolith species florisphaera profunda relative abundance within each core there is a general agreement between the different palaeoproductivity proxies in the southeastern arabian sea core md900963 glacial stages correspond to relatively high productivity whereas warm interstadials coincide with low productivity all time series of productivity proxies are dominated by a cyclicity of about 21 23 kyr which corresponds to the insolation precessional cycle a hypothesis could be that the ne monsoon winds were stronger during the glacial stages which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone by contrast the records are more complicated in the upwelling region of the western arabian sea core ty93 929 p this is partly due to large changes in the sedimentation rates which were higher during specific periods isotope stages 6 5 4 5 2 3 and 2 unlike core md900963 no simple relationship emerges from the comparison between the 18o stratigraphy and productivity records the greater complexity observed for core ty93 929 p could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons the sw monsoon being enhanced during interglacial periods whereas the ne monsoon was increased during glacial intervals a similar line of reasoning also could help explain the sst records by the superimposition of variations of three components global atmospheric temperature and sw and ne monsoon dynamics

the methana volcano geothermal resource greece and its relationship to regional tectonics abstract geophysical methods of analysis were applied in order to investigate the deep structure and the geothermal potential of the methana volcano ne peloponnesus greece the study is based on a re evaluation and reinterpretation of legacy magnetotelluric mt data with modern analysis methods as well as 3 d inversion of aeromagnetic data constrained by in situ measurements of magnetic susceptibility magmatic systems are located in regions of active tectonic processes that often play a controlling role the mt method is effective in delineating low resistivity functional elements of volcanic systems such as magma chambers vents thermal fluid reservoirs and thermal fluid circulation conduits the latter two of which are typically associated with active faults the aeromagnetic data can assist in mapping the configuration hence emplacement modes of volcanic rocks at depth accordingly the joint interpretation of these lines of evidence together with structural and geochemical information is expected to allow insight into the influence of contemporary tectonics on the inception and evolution of the volcano the contemporary stress field is mainly extensional nne ssw oriented and overall homogeneous in the area of methana it allows for the formation of wnw ese north easterly dipping normal faults w e faults consistent with the synthetic dextral r shear direction of riedel s shear theory and nw se faults consistent with the antithetic sinistral r shear direction all such features have been mapped on methana peninsula the magnetotelluric data imaged a significant geothermal reservoir developing around an intersection of the three active fault zones normal r and r at depths of 1 1 5 km below the centre of the peninsula as well as elongate epiphenomenal conductivity anomalies associated with the circulation of thermal fluids along all three fault zones the 3 d magnetic susceptibility model strongly suggests that the intrusion and emplacement of magmas were guided by the same active fault zones with particular reference to the r and r shears whose influence is imprinted on the configuration of volcanic rocks at depth the joint interpretation of all lines of evidence indicates that magmatism and volcanism at methana are almost completely controlled by tectonic activity in a manner analogous to the situation of the large santorini volcanic complex it also indicates that the reservoir is replenished through the weak permeable zone created by the intersection of the r and r shears which is very probably collocated with the main vent of intrusive magmatic activity and may connect with a shallow magma chamber at depths greater than 4 5 km the apparently common origin and similarities differences in the circulation paths of thermal fluids may amply explain both the individual characteristics and similarities differences in the chemical composition of thermal spring discharges which have been reported by hitherto geochemical investigations

glacial to holocene redistribution of carbonate ion in the deep sea we have reconstructed the glacial age distribution of carbonate ion concentration in the deep waters of the equatorial ocean on the basis of differences in weight between glacial and holocene foraminifera shells picked from a series of cores spanning a range of water depth on the western atlantic s ceara rise and the western pacific s ontong java plateau the results suggest that unlike today s ocean sizable vertical gradients in the carbonate ion concentration existed in the glacial age deep ocean in the equatorial pacific the concentration increased with depth and in the atlantic it decreased with depth in addition the contrast between the carbonate ion concentration in deep waters produced in the northern atlantic and deep water in the pacific appears to have been larger than in today s ocean

nonlinear 40ar 39ar age systematics along the gilbert ridge and tokelau seamount trail and the timing of the hawaii emperor bend over the last three decades the first order correlation in morphology and orientation of seamount trails has been called upon to support the concept of a fixed pacific hot spot frame of reference and to explain the hawaii emperor bend heb by a dramatic change in pacific plate motion in this paper however we present 40ar 39ar ages for the gilbert ridge and tokelau seamounts pacific that show similar changes or bends in their orientation but at different geological times up to 20 myr earlier than the heb changes in pacific plate motion alone cannot explain these observations because these asynchronous bends should have been reflected in the morphology of each of these seamount trails together with the lack of linear age progressions and inconsistent apparent local plate velocities of 131 and 87 mm yr we rule out a fixed hot spot origin for the gilbert ridge and tokelau seamount trails instead we invoke secondary or alternate processes to explain the complex age systematics and morphologies in these seamount trails we propose here that the heb type bends in these seamount trails were likely formed by short term jerk like plate extensions in the studied southwestern region of the pacific plate reactivating a preconditioned lithosphere that can be characterized by a complex structure and precursory magmatic impingements the remarkable differences observed in these colinear seamount trails fundamentally question the existence of heb type bends in the formation of pacific volcanic lineaments they also show us that applying geometric and morphologic observations alone is insufficient in constraining past plate motions nevertheless the need and search for alternate volcano tectonic mechanisms offer opportunities to better understand intraplate volcanism in general

east asian winter monsoon controlling phytoplankton productivity and community structure changes in the southeastern south china sea over the last 185 kyr abstract the south china sea scs is one of the largest marginal seas in the world and it plays an important role in regional climate change and carbon cycles however mechanisms of glacial interglacial phytoplankton productivity changes remain under studied especially for the southeastern scs in this study the productivity record for the southeastern scs core md972142 over the last 185 kyr was generated using multiple biomarkers including c37 alkenones c37 2 c37 3 brassicasterol dinosterol and alkyl diols c30 c32 as productivity proxies for haptophytes diatoms dinoflagellates and eustigmatophytes and diatoms respectively all biomarker contents indicate higher individual as well as total phytoplankton productivities during peak glacials than during interglacials our records afford a better constraint on spatial pattern of glacial productivity changes which suggest that both the southeastern and northern scs glacial productivity increases were controlled by the stronger east asian winter monsoon eawm which brought more nutrients to the surface through stronger mixing of nutrient rich deep waters biomarker ratios reveal a relatively stable phytoplankton community structure pcs but relative diatom productivity was higher during glacials the pcs record is consistent with stronger eawm forcing during glacials which supplied nutrients to the surface with relatively stable composition the decrease of c37 alkenones during the last 120 kyr is likely a global phenomenon which could be related to the evolution of the haptophytes in the scs this trend was also likely caused by changes of subsurface nutrient supply to the scs which significantly modulated the pcs by a gradual decrease of haptophyte contribution to total phytoplankton in the northern and southeastern scs since mis 5

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

geoelectric investigation of the hellenic subduction zone using long period magnetotelluric data abstract the strongest evidence up to date for a subduction zone in the hellenic region is a clearly identified wadati benioff zone below the central aegean sea to a maximum depth of 180 km alternative seismic tomography models suggest that subduction process continues deeper than the wadati benioff zone to a maximum depth of at least 600 km so far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the hellenic subduction zone hsz a long period magnetotelluric lmt study was carried out in the southern part of the greek mainland to study the deep electrical characteristics of the hsz and examine whether prominent modelled features correlate with structures identified by the seismic methods the study comprised collection processing and modelling of magnetotelluric mt data in the period range 100 10000 s from ten sites located along a 250 km ne sw trending profile the dimensionality of the data was examined at a pre modelling stage and it was found that they do not exhibit three dimensional 3 d features the latter enabled to construct both one dimensional 1 d and two dimensional 2 d models the proposed geoelectric model for hsz was based on 2 d modelling since it had better maximum depth resolution of about 400 km and revealed structures not detected by 1 d modelling attempts the model structure which was related to the african and euro asian lithosphere is relatively resistive 800 m and has an average thickness of 150 170 km although the bottom of the lithosphere is adequately resolved the wadati benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature the modelled structure associated with the subducting part of the african lithosphere penetrates a relatively conductive

insolation cycles as a major control of equatorial indian ocean primary production analysis of a continuous sedimentary record taken in the maldives indicates that strong primary production fluctuations 70 to 390 grams of carbon per square meter per year have occurred in the equatorial indian ocean during the past 910 000 years the record of primary production is coherent and in phase with the february equatorial insolation whereas it shows diverse phase behavior with 18o depending on the orbital frequency eccentricity obliquity or precession examined these observations imply a direct control of productivity in the equatorial oceanic system by insolation in the equatorial indian ocean productivity is driven by the wind intensity of westerlies which is related to the southern oscillation therefore it is suggested that a precession forcing on the southern oscillation is responsible for the observed paleoproductivity dynamics

study of the crustal thickness and the subducting lithosphere in greece from gravity data the area of greece has experienced a complex tectonic history dominated by the subduction of the african plate beneath eurasia in this study the variations of the crustal thickness in the area of greece were obtained by means of the multiple source werner deconvolution mswd method applied to gravity data thicknesses of 40 49 km are estimated beneath the hellenides mountain belt to the west eastward thinning of the crust to thicknesses ranging from 25 km in the north to 30 km in the south is seen in the aegean region these results are in good agreement with recent seismological results demonstrating that the mswd method successfully treated the problem using the crustal model we derived we computed the gravity effect of the crust and extracted it from the bouguer anomaly we also extracted the gravity effect of the subducting lithosphere from the bouguer anomaly producing a residual map where most of the original gravity variation has been successfully removed the remaining anomalies appear related to near surface features and an area of low velocity mantle in the central aegean sea

mid chain diols and keto ols in se atlantic sediments a new tool for tracing past sea surface water masses abstract long mid chain diols and keto ols in se atlantic surface sediments have been studied in relation to the properties of the overlying surface waters the strongest change in compound composition occurs over the angola benguela front 14 16 s south of the front 1 13 and 1 14 diols notably the 1 14c 28 and 1 13c 30 homologs increase relative to 1 15 diols the 1 13c 30 keto ol is almost absent north of the front these changes probably reflect differences in sea surface temperature persistence of stratification and upwelling at opposite sides of the front elevated relative abundances of the 1 15c 32 diol and 1 15c 32 keto ol below the congo river plume 6 s may be outflow related which should be tested on other river plumes lower relative abundances of 1 15 diols close to walvis bay might relate to a higher upwelling intensity compared to the other sites investigated the applicability of the observed surface patterns is tested down core for the penultimate deglaciation the temporal changes in diol and keto ol composition not only have their spatial counterpart in the se atlantic surface sediments they are also readily interpretable on the basis of the modern diol and keto ol assemblages as well as the palaeoenvironmental reconstructions based on other proxies

caco3 size distribution a paleocarbonate ion proxy lysocline reconstructions play an important role in scenarios purporting to explain the lowered atmospheric co2 content of glacial time these reconstructions are based on indicators such as the caco3 content the percent of coarse fraction the ratio of fragments to whole foraminifera shells the ratio of solution susceptible to solution resistant species and the ratio of coarse to fine caco3 all assume that changes with time in the composition of the input material do not bias the result however as the composition of the input material does depend on climate none of these indicators provides an absolute measure of the extent of dissolution in this paper we evaluate the reliability of the ratio of 63 m caco3 to total caco3 as a dissolution indicator we present here results that suggest that in today s tropics this ratio appears to be determined solely by co3 ion concentration and water depth i e the saturation state of bottom waters this finding offers the possibility that the size fraction index can be used to reconstruct co3 ion concentrations for the late quaternary ocean to an accuracy of 5 mol kg 1

crustal structure beneath broad band seismic stations in the mediterranean region summary we have analysed receiver functions to derive simple models for crustal structure below 12 broad band seismological stations from the midsea project and 5 permanent broad band stations in the mediterranean region including northern africa to determine an accurate moho depth we have reduced the trade off between crustal velocities and discontinuity depth using a new grid search method which is an extension of recently published methods to determine crustal thickness in this method the best fitting synthetic receiver function containing both the direct conversion and the reverberated phases is identified on a model grid of varying moho depth and varying poisson s ratio the values we found for moho depth range from around 20 km for intra oceanic islands and extended continental margins to near 45 km in regions where the eurasian and african continents have collided more detailed waveform modelling shows that all receiver functions can be well fit using a 2 or 3 layer model containing a sedimentary layer and or a mid crustal discontinuity on comparing our results with moho maps inferred from interpolated reflection and refraction data we find that for some regions the agreement between our receiver function analysis and existing moho maps is very good while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps

multimode 3 d kirchhoff migration of receiver functions at continental scale in geology and in particular in geophysics direct and indirect observations of processes occurring both at the surface of the earth and at depth are used to understand the structure and dynamics of the earth for instance seismic waves generated by large earthquakes can be used to study the structure of heterogeneities in the first few hundred kilometers inside the earth in this work we use the scattered wavefield which corresponds to energy arriving after the incident wavefield to image the earth by nature the scattered waves are linked to the scattering heterogeneities encountered along their propagation path i e the fine scale structure of the earth hence the scattered wavefield has the ability to highlight structures where rapid velocity variations would otherwise be smoothed out by tomographic regularization such as the structure of subducting slabs to extract the information from the scattered wavefield we resort to receiver function rf analysis and pre stack depth migration standard migration procedures either rely on the assumption that underlying discontinuities are horizontal such as in common conversion point stacking ccp or are computationally expensive and usually limited to 2d geometries such as in reverse time migration rtm or generalized radon transform grt here we develop a kirchhoff type teleseismic imaging method that uses fast 3d travel time calculations with minimal assumptions about the underlying structure this provides high computational efficiency without limiting the problem to 1d or 2d geometries in our method we apply elastic kirchhoff migration to transmitted and reflected teleseismic waves i e rf the approach expands on the work of cheng et al 2016 the 3d elastic kirchhoff migration is adapted to the passive seismology scattering geometry and to account for free surface multiples we use an eikonal solver based on the fast marching method fmm to compute travel times for all scattered phases 3d scattering patterns are computed to correct the amplitudes and polarities of the three component input signals we consider three different stacking methods linear phase weighted and 2 nd root to enhance the structures that are most coherent across scattering modes to showcase the efficiency and accuracy of our migration procedure we test it by conducting a series of synthetic tests in both artificially challenging and realistic scenarios results from synthetic tests show that our imaging principle can recover scattering structures accurately with minimal artifacts we show that integrating the three components of the rf into the imaging principle allows to coherently retrieve the scattering potential for arbitrarily dipping discontinuities from all back azimuths and are able to retrieve a typical 2 5d subduction zone structure we apply this novel 3d multi mode kirchhoff migration method to two different subduction zones in western greece and southern alaska

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

past changes in biologically mediated dissolution of calcite above the chemical lysocline recorded in indian ocean sediments abstract in a deep sea sediment core recovered from a site lying well above the local lysocline several organic geochemical proxies and two different calcite dissolution indicators are compared in order to evaluate the relationship between calcite dissolution and paleoproductivity over the past three glacial interglacial cycles the degree of foraminiferal break up and the caco3 particle size distribution both point to significant periods of dissolution every 22 kyr during glacial stages and substages these dissolution events are concomitant with periods of enhanced primary productivity as indicated by the abundance of several biomarkers alkenones cholesterol brassicasterol keto ol used here to indicate changes in paleoproductivity dissolution fluctuations are highly coherent and in phase with the estimated paleoproductivity variations providing strong evidence that the observed dissolution is due to organic matter remineralization within the sediments rather than to changes in co32 concentration in the overlying water column

two dimensional magnetotelluric modelling of the kos island geothermal region greece abstract the magnetotelluric mt method in the period range 0 075 42 s was applied to determine the electrical structure

the hellenic subduction beneath the peloponnesus first results of a microearthquake study abstract a preliminary examination of the 1070 earthquake locations determined from 6 weeks of recording in 1986 by 46 stations show that the seismicity is spread over a wide area of the peloponnesus and the western hellenic arc and throughout the whole crust no clear individual faults can be identified from the seismicity but clusters of activity are observed in some places seismicity is concentrated above 40 km and deeper earthquakes were not numerous only 28 of the 466 events with uncertainties in depth less than 5 km occurred deeper than 40 km seismicity deeper than 30 km defines a flat zone at a depth between 40 km and 70 km starting from the trench to about 200 km towards the northeast further northeast the dip of the seismic zone abruptly changes to 45 fault plane solutions for the deeper events generally indicate t axes plunging northeast within the subducted slab therefore we interpret the seismicity deeper than 30 km as due to the superposition of two different causes 1 the steep zone is due to the subduction of the african litospheric plate beneath the aegean and 2 the shallow flat zone located between the trench and the argolide is partly due to the loading of the overriding aegean plate which is deforming above the african plate

geochemical composition of tanzanian shelf sediments indicates holocene climatic and sea level changes we present a high resolution geochemical and grain size record from a holocene sediment core off the pangani river mouth tanzania elemental ratios between biogenic elements and al i e ca al mg al and sr al are mainly influenced by terrigenous dilution on carbonate concentration and or limitation of carbonate production as a result of variations in the supply of fine grained terrigenous sediments of the pangani river such elemental ratios increased significantly at the end of the mid holocene between 5 and 3 5 ka demonstrating a gradual transition from the humid early and mid holocene to the arid late holocene in east africa among the elemental ratios between terrigenous elements and al si al and k al ratios correlate to grain size variation indicating a change in sedimentation regime fe al and ti al ratios show that the sediment source area has shifted from the terrestrial volcanic region of tanzania fe ti rich to the coastal and inner shelf regions fe ti poor around 7 5 ka in response to arid climate and high sea level our geochemical results correspond with a sea surface temperature record derived from the same sediment core indicating that the end of the east african humid period could have been gradual and related to the cooling water in the western indian ocean

a biomarker perspective on paleoproductivity variations in two late quaternary sediment sections from the southeast atlantic ocean abstract sedimentary inventories of selected molecular biomarkers in two sediment sections from the african continental margin below highly productive surface waters in the southeast atlantic ocean each spanning the last 76 kyr were investigated and compared to each other a principal component analysis of the stratigraphic data separated groups of biomarkers representing different assemblages of biological sources and or different diagenetic histories related to their chemical structures and their metabolic role in the food chain the concentrations of long chain n alkenones n diols the major n ketool and loliolide and its epimer isololiolide are correlated with paleoproductivity estimates that were calculated from organic carbon contents this group of relatively refractory compounds very likely represents those planktonic communities which contributed significantly to surface water productivity and subsequent burial of organic carbon during the considered time interval concentrations of dust transported terrigenous long chain fatty acids c24 to c28 are interpreted to be related to the intensity of upwelling driving winds and are also significantly correlated with high contents of organic matter concentrations of cholesterol and the major short chain fatty acids are not related to the marine paleoproductivity signal most likely as a consequence of their nonspecific origin and their metabolic role during heterotrophic feeding processes abundances of 24 ethylcholest 5 en 3 ol and its saturated counterpart 24 ethyl 5 cholestan 3 ol the major steroid alcohols in both sedimentary environments appear not to be related to any of the above mentioned components and probably represent the contribution of algal species adapted to nutrient conditions different from those of other major primary producers differences between both environments the angola dome and the benguela coastal upwelling are evident in the relative distribution of major components and the stratigraphic behavior of selected compounds relative to other variables these differences are discussed in relation to specific characteristics of both oceanographic environments 1999 elsevier science ltd all rights reserved

modeling of calcite dissolution by oxic respiration in supralysoclinal deep sea sediments abstract pore water profiles of co2 ph ca2 and o2 in situ concentrations were measured at two stations on the upper continental slope off gabon the present study evaluates these measurements concerning their implications for the calcium carbonate system in deep sea sediments the model cotrem which was used to simulate the dynamics of this complex geochemical system revealed a strong dependence of calcite dissolution on oxic respiration at both sites all simulated calcite dissolution kinetics reached a dynamic equilibrium with almost equal calcite dissolution rates but different sub saturation states and ph values the latter are mainly dependent on boundary conditions and kinetic rate law parameters boundary conditions are of immense importance they define which ph deviations between measured data and the simulated equilibration instantaneous kinetics have still to be fitted by kinetic restrictions per rate law for the equilibration these ph deviations set up the range of possible values for rate constants in a given rate law to yield a well simulated ph a failure in the implementation of these boundary conditions may lead to non linearly flawed rate constants which fit only one usually the maximal ph deviation well the whole depth distribution of ph deviations has to be fitted very well by a kinetic rate law only this procedure secures that the used boundary conditions and rate laws constants are acceptable nevertheless several kinetic rate laws may be used for good ph fits featuring clear differences in parameter values for rate constants and reaction orders the connection between these rate laws with different parameters is examined and the dependence of the rate constant on the given form of the rate law is demonstrated this study achieves rate constants of 0 038 and 18 d for a dissolution rate law of 4 5th reaction order and sub saturation dependence from the term 1 these results are well within the range of rate constants obtained during former field studies for the same form of the dissolution rate law

paleoproductivity in the equatorial indian ocean for the last 260 000 yr a transfer function based on planktonic foraminifera a principal component analysis pca of planktonic foraminifera in surface sediment of the northern equatorial indian ocean defines two major assemblages the first one dominated by globigerinoides sacculifer g ruber globigerinella aequilateralis and globoquadrina conglomerata characterizes non upwelling areas the second dominated by globigerina bulloides g falconensis and globigerinita glutinata characterizes upwelling areas the distribution of planktonic foraminiferal species in the area is mainly related to primary production pp the pca applied to core md 900963 samples defines an alternation of the two assemblages through time a non upwelling one similar to that present in the core tops and an upwelling assemblage which contains in addition neogloboquadrina dutertrei globorotalia menardii and g crassaformis equatorial indian ocean core top samples are introduced as additional rows in the pca calculated on core md 900963 samples and a transfer function is constructed correlating factorial coordinates of core tops within the range of changes occurring in core md 900963 with estimated pp values applied to the last 260 000 yr the transfer function indicates that the precession is the major orbital forcing on equatorial productivity changes and that annual pp increases by about 85 from interglacial to glacial interstadials these results are in agreement with an independent productivity index based on coccoliths direct impact of insolation on equatorial productivity could be explained by the link between southern oscillation and intensity of zonal winds which seem to lead the productivity of surface waters at the location of core md 900963

preliminary magnetotelluric studies along the hellenic volcanic arc implications for the collision between the african plate and the aegea microplate abstract from middle upper miocene time to the present the african plate has subducted under the so called aegea microplate a prominent feature of this continental collision is the past and present volcanic activity along the hellenic volcanic arc hva which is sited 200 km behind the frontal compressional region since 1986 several magnetotelluric mt surveys have been undertaken along the hva and particularly on the islands of milos kos and nisyros to determine their subsurface electrical structure the derived electrical models have indicated very low resistivities less than 30 m for the uppermost 10 km which have been related to known i e milos and nisyros or potential i e kos geothermal anomalies evaluation of the existing electrical models and subsequent 1d modelling of the broadband mt data has revealed some deeper greater than 10 km depth electrical features of the lithosphere below the hva study of the azimuths of the major axes of the rotated impedance ellipses and the parkinson induction arrows in the period range 10 2 10 4 s has led to their association with the subduction and extensional processes taking place in the aegean the mt results are in agreement with the observed gravity values magnetic anomalies and the results from geotectonic deep seismic refraction and seismological studies finally proposal for the future study of the deep electrical structure of the aegean is presented

reevaluation of the caco3 size index paleocarbonate ion proxy measurements of the caco3 size index broecker and clark 1999 and of whole foraminifera shell weights lohmann 1995 on glacial age sediment from a series of water depths on the ceara rise in the west equatorial atlantic clearly demonstrate that the relationship between size index and pressure normalized carbonate ion concentration established for holocene equatorial sediments does not apply to glacial sediments rather it appears that the ratio of fine to coarse caco3 raining to the seafloor was greater during glacial time than during the holocene this finding squelches our previous enthusiasm regarding the application of the size index as a paleocarbonate ion proxy

carbonate dissolution fluctuations in the western equatorial pacific during the late quaternary planktonic foraminiferal test fragmentation in three cores along a depth transect from the western equatorial pacific erdc 93p 1619 m rc17 177 2600 m v28 238 3120 m thompson 1976 were examined for the last 500 kyr at sample intervals from 2 5 to 5 kyr to study the fluctuations of dissolution in the western equatorial pacific the age models were constructed by correlating the 18o records with the specmap stack imbrie et al 1984 results showed that intermediate and deep waters experienced the same patterns of dissolution through climatic cycles fragmentation varied with a greater amplitude and the carbonate ion concentration changed less in the deep than in the intermediate water dissolution has significant variance distributions and coherencies with 18o over the 100 41 and 23 kyr periods of orbital variations dissolution maxima lag ice volume minima by 6 to 20 kyr the dissolution variability was consistent with recent geochemical models which seek to explain the reduction of atmospheric co2 concentration at the last glacial maximum broecker 1982 boyle 1988

shell weights from the south atlantic 1 as part of our ongoing evaluation of the shell weight proxy measurements were carried out on plankton species from south atlantic cores the results portray a complex picture whereas ideally these weights should be influenced only by surface water carbonate ion concentration growth weight and deep water pressure normalized carbonate ion concentration extent of weight loss due to dissolution it appears that other environmental factors may influence growth weight and that variable pore water bottom water offsets may complicate the extent of dissolution

calcite dissolution driven by benthic mineralization in the deep sea in situ measurements of ca 2 ph pco 2 and o 2 abstract in situ measured microprofiles of ca 2 pco 2 ph and o 2 were performed to quantify the caco 3 dissolution and organic matter mineralization in marine sediments in the eastern south atlantic a numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate from the oxygen microprofiles measured at four stations along a 1300 m isobath of the eastern african margin and one in front of the river niger at a water depth of 2200 m the diffusive oxygen uptake dou and oxygen penetration depth opd was calculated dou rates were in the range of 0 3 to 3 mmol m 2 d 1 and showed a decrease with increasing water depth corresponding to an increase in opd the calculated amount of degradated organic matter is in the range of 1 to 8 5 gc m 2 a 1 the metabolic co 2 released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite supersaturated water fluxes across the sediment water interface calculated from the in situ ca 2 microprofiles were 0 6 mmol m 2 d 1 for two stations at a water depth of 1300 m the ratio of calcite dissolution flux and organic c degradation is 0 53 and 0 97 respectively the microprofiles indicate that co 2 produced within the upper oxic sediment layer dissolves up to 85 of the calcite rain to the seafloor modeling our o 2 ph and ca 2 profiles from one station predicted a calcite dissolution rate constant for this calcite poor site of 1000 mol kgw 1 a 1 mol per kg water and year which equals 95 d 1 this rate constant is at the upper end of reported in situ values

preferential detection of the lehmann discontinuity beneath continents we perform a global survey for the presence of the lehmann discontinuity using 20 000 long period ss precursors this data set is highly sensitive to upper mantle reflectors and the coverage is more complete than in previous studies our survey indicates that the lehmann discontinuity is a local feature that is observed under continents more than twice as often as it is observed under oceans we observe significant variations in travel times and waveforms associated with this shallow mantle reflector indicating its complexity and lateral depth variations little signal is detected on the continent scale stacks of the ss precursors which provides further evidence for the intermittent and variable nature of the lehmann discontinuity

the influence of organic matter diagenesis on caco3 dissolution at the deep sea floor abstract in situ benthic flux chamber measurements were performed at seafloor locations in the atlantic and pacific oceans where the bottomwaters were supersaturated approximately saturated and undersaturated with respect to calcite there was no evidence of significant dissolution at or above the saturation horizon below the saturation horizon however significant benthic fluxes of titration alkalinity and calcium out of the sediments were observed indicating appreciable dissolution where observed the rate of dissolution is consistent with a rate expression of reaction order 4 5 and a rate constant in the range of 0 05 to 0 5 per day a numerical simulation was constructed to represent organic matter and caco3 diagenesis at the seafloor at the supersaturated and near saturation sites the model over predicts the influence of metabolically produced co2 on the extent of caco3 dissolution it is possible that these results can be reconciled by a better understanding and parameterization of organic matter mineralization and porewater transport processes in surface sediments the primary factors to be considered include 1 the oxidation state of the carbon being oxidized 2 the vertical distribution of oxidation rates including the role of doc in sediment remineralization 3 the influence of nondiffusive transport of near surface porewaters and 4 the influence of episodic depositional events on organic and inorganic carbon cycling

reconstructing past particle fluxes in the tropical atlantic ocean using a research strategy analogous to modern sediment trap studies sediment accumulation patterns on submarine rises can be interpreted in terms of past ocean chemistry and circulation we have followed this research strategy to reconstruct the history of surface water productivity and deep water chemistry and circulation in the eastern equatorial atlantic sierra leone rise and western equatorial atlantic ceara rise during the last glacial maximum 18 000 b p on shallow sections of these rises at depths with little carbonate particle degradation we assume that the accumulation of skeletal carbonate approximates the carbonate production rate in surface water during the last glacial maximum the rate of carbonate productivity was lower in the eastern atlantic than it is today while the western atlantic exhibited no glacial interglacial difference in carbonate productivity based on the difference in carbonate accumulation rates between shallow and deep cores on the rises we observed greater dissolution in both basins during the last glaciation the eastern atlantic always had a lower rate of dissolution than the western atlantic despite having deep water with a lower 13c during the last glacial maximum organic carbon accumulation in the eastern atlantic increased with depth in the water column during the last glaciation suggesting that there was a bathymetric decrease in dissolved oxygen concentration at that time these observations are consistent with a glacial decrease in the production rate of northern source deep water during the last glaciation at that time the mixing zone between northern source and southern source deep water migrated to the north in the atlantic resulting in a greater proportion of corrosive southern source deep water in the western atlantic and entering the eastern atlantic through low latitude fracture zones today the ratio of northern and southern components entering the eastern atlantic is about 4 1 based on 13c and carbonate accumulation our best estimate of this ratio during the last glaciation is 1 1

modeling the calcite lysocline a numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea model results are compared with lysocline data from 13 regions in the atlantic pacific and indian oceans the model lysocline shape is sensitive to the calcite dissolution rate constant the calcite organic carbon and refractory material rain rates and the rates of oxic versus anoxic organic carbon degradation in the sediment the model is able to reproduce the observed lysocline within the constraints of the sediment trap and calcite accumulation data using a calcite dissolution rate constant of 30 100 d 1 a molar ratio of organic carbon to calcite rain rates of 0 5 1 0 and an initial caco3 fraction of 90 excluding organic carbon this rate constant is consistent with microelectrode results presented by archer et al 1989 the model predicts that 30 50 of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration consistent with previous modeling studies the range in the best fit value of the organic inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material rather than from scatter in the data lysocline data from the western equatorial atlantic are anomalous to the rest of the data this anomaly may be explained by high rates of refractory material sedimentation from the amazon river

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

on the resolving power of tomographic images in the aegean area b the imaging of upper mantle heterogeneity by seismic tomography is strongly limited by the uneven global distribution both of seismic recording stations and earthquake sources this can result in a loss of resolution and significance in the final image particularly when a sparse data set contains few ray paths which intersect at sufficiently high angles in the volume of interest in order to investigate the theoretical resolving power of a previously published tomographic image of the aegean area synthetic tests of the inversion procedure using a ray path matrix obtained in this previous study for local and teleseismic p waves were carried out the aim was to examine the extent to which the shape of a synthetic lithospheric slab penetrating to different depths is inherently distorted by the tomographic imaging procedure and to compare the synthetic tomographic images with the results from the actual inversion the distortion is found to take the form of an artificial stretching of the lithospheric slab the maximum stretching factor as indicated by the downdip displacement of the peak amplitude of the synthetic high velocity anomaly is found to be a factor of 2 or so though the distortion is usually less than this the peak amplitude of the tomographic image of a lithospheric slab is found from the inversion of traveltime data to be at depths at or below 400 km this indicates that the high velocity lithospheric slab in the aegean penetrates deeper than the benioff zone seismicity of about 200 km however no constraints of the maximum depth of penetration could be established with the data set used in the present work

assessing influence of diagenetic carbonate dissolution on planktonic foraminiferal mg ca in the southeastern arabian sea over the past 450 ka comparison between globigerinoides ruber and globigerinoides sacculifer 1 the influence of supralysoclinal calcite dissolution on the mg ca thermometer over the last 450 ka is assessed using globigerinoides ruber and globigerinoides sacculifer from a core site in the eastern arabian sea the studied site is characterized by precession cycled paleoproductivity changes that have induced calcite dissolution in the sediments although the core still contains 40 80 caco3 during high productivity periods the narrow sized test weights of both foraminiferal species tend to decrease with increasing mn ca ratio indicating corrosive pore water conditions scanning electron microscope observations show that g sacculifer maintains a better preserved test ultrastructure than g ruber microprobe mapping reveals that mg rich bands in g ruber chamber walls are preserved despite reduced ca content assuming that the low test weight and offset in mg ca temperature between g sacculifer and g ruber are exclusively produced by calcite dissolution the bias in mg ca temperature determination can be estimated at around 1 c comparable to uncertainty of this thermometry despite the evidence for test dissolution foraminiferal mg ca seems to preserve initial temperature signals under the studied conditions

a diatomaceous origin for long chain diols and mid chain hydroxy methyl alkanoates widely occurring in quaternary marine sediments indicators for high nutrient conditions for the first time a biological source for the long chain alkyl 1 14 diols and 12 hydroxy methyl alkanoates lipids widely occurring in the marine water column and sediments has been identified cultures of proboscia indica and proboscia alata rhizosolenoid diatoms belonging to the widespread diatom genus proboscia contain c28 c28 1 c30 and c30 1 alkyl 1 14 diols and c27 and c29 12 hydroxy methyl alkanoates as major neutral lipids these components form a substantial fraction of lipid fractions from sediment traps or sediments especially in areas with an elevated primary production such as upwelling regions examination of literature data reveals that as much as 20 to 35 of the total lipid flux in the arabian sea is derived from proboscia diatoms during the start of the upwelling season their rapid transfer to the water sediment interface may explain why corresponding 1 14 keto ols inferred oxidation products of diols are hardly formed these interpretations are supported by compound specific carbon isotopic analysis of long chain keto ols and diols in surface sediments of the arabian sea the data indicate that long chain alkyl 1 14 diols and 12 hydroxy methyl alkanoates can be applied as indicators for high nutrient conditions in the photic zone

combined organic and inorganic geochemical reconstruction of paleodepositional conditions of a pliocene sapropel from the eastern mediterranean sea layers of organic carbon rich sapropels in the sediment record of the mediterranean sea give evidence of repetitive changes in regional plio pleistocene climate results from biomarker molecule and major and trace element analyses of closely spaced samples are used to reconstruct the conditions leading to deposition of a pliocene sapropel at ocean drilling program odp site 969 on the mediterranean ridge organic carbon concentrations increase from 0 2 outside the sapropel and peak to more than 30 within it major and trace elemental composition and biomarker derived parameters indicate elevated productivity depletion of water column dissolved oxygen content and changes in sediment provenance in response to climatic changes budgets of rhenium thallium and other trace metals indicate that deep water exchange between the mediterranean subbasins and the atlantic ocean was not completely interrupted during sapropel formation enrichment factors of redox sensitive and sulfide forming trace metals as well as the presence of isorenieratene derivatives and high stanol sterol ratios point to an extended zone of anoxic water masses depth profiles of biomarker compositions sterols long chain alkenones alkandiols and ketols fatty acids indicate great floral diversity during deposition of a single sapropel and highlight the sensitive response of the marine community to variable environmental conditions changes in water mass circulation and eolian transport can be reconstructed by use of both lithogenic elements and average chain lengths of n alkanes acl index

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

carbonate ion concentration in glacial age deep waters of the caribbean sea 1 on the basis of the whole foraminifera shell weights for g sacculifer and n dutertrei from caribbean cores it appears that the carbonate ion concentration of water at 1800 m depth in the open atlantic i e the depth of the caribbean sill was 14 mol kg greater during glacial time than during the holocene the sense of this difference is consistent with the observation that the 13c 12c ratio for benthic foraminifera from glacial age atlantic upper deep water was higher than that for their holocene equivalents it is also consistent with fragment to whole foraminifera results however as several questions regarding the assumptions involved in converting shell weights to paleocarbonate ion concentrations remain unanswered this reconstruction must be considered tentative critical in this regard are the assumptions that shell wall thicknesses were the same as today s at any given site during glacial time and that the 0 3 g per mol kg relationship between shell weight and carbonate ion concentration is applicable over the entire deep water column the records reported here reveal no evidence for an early holocene preservation event however that for core v28 122 documents a dissolution event close to the mis 5 4 boundary

calcite dissolution in sediments of the ceara rise in situ measurements of porewater o2 ph and co2 aq in situ electrode measurements of porewater oxygen and ph from four sites on the ceara rise in the western tropical atlantic unequivocally demonstrate that calcite dissolution driven by metabolic co2 produced within the sediments is a significant part of the diagenesis of sedimentary calcite dissolving at least 20 of the calcite rain to the seafloor the first ever in situ absorbance based measurements of porewater co2 aq support the dissolution estimates based on models of the observed ph distributions and are incompatible with scenarios excluding metabolic co2 driven dissolution our dissolution estimates are consistent with those estimated by an independent study of porewater calcium and alkalinity profiles martin and sayles 1996 at this location the consistency of these disparate porewater measurements and our ability to interpret them with simple models of respiration and dissolution implies that no special mechanism is required to describe this process the calcite dissolution rate constant in the sediments of the deepest station in this study is significantly higher than determined by in situ studies at other locations values approach those determined by early laboratory measurements the dissolution rate constant here must be at least two orders of magnitude higher than for calcite in sediments on the ontong java plateau based on in situ electrode measurements there hales and emerson 1996 the calcite solubility consistent with these data is not compatible with that consistent with the ontong java data the reason for this difference is currently unresolved but may be due to the high order of the empirical dissolution rate law assumed in this study

an evaluation of lohmann s foraminifera weight dissolution index down water column traverses of core top weights for three planktonic species confirm lohmann s 1995 relationship between foraminifera shell weight loss and bottom water carbonate ion content however they also suggest that the initial shell thickness varies with growth habitat and that the offset between bottom water and pore water carbonate ion concentration varies even on small space scales

caco3 dissolution in sediments of the ceara rise western equatorial atlantic we have used porewater sampling by in situ techniques including whole core squeezing as well as by shipboard sectioning and whole core squeezing to estimate the rates of sedimentary organic matter oxidation and caco3 dissolution at seven sites on the ceara rise in the western equatorial atlantic ocean porewater no3 profiles at all sites show a pattern indicative of active organic matter oxidation in the upper 15 20 cm of the sediments and in a buried organic rich layer the organic c oxidation rate generally decreases with increasing water depth from a value of 22 mol cm2 y at the shallowest site 3279 m to 14 mol cm2 y at the deepest site 4675 m over this depth range the bottomwaters vary from moderately supersaturated with respect to calcite to strongly undersaturated high resolution alkalinity profiles measured in porewaters collected by in situ whole core squeezing yield estimated ca2 fluxes of 11 mol cm2 y at a site located at the depth of the calcite saturation horizon and 7 6 zmol cm2 y at a moderately undersaturated site ca2 fluxes calculated from profiles in porewaters collected by relatively coarse resolution in situ sampling methods clearly indicate that there is caco3 dissolution above the calcite saturation horizon the dissolution of aragonite may contribute to the dissolution flux at the shallowest site these ca2 fluxes as well as fluxes estimated from a model of sedimentary organic matter oxidation and calcite dissolution indicate that 36 66 of the caco3 rain to the seafloor dissolves at sites at and above the calcite saturation horizon while 52 75 of the rain dissolves at sites below this depth when these results are incorporated into the oceanic caco3 budget of milliman 1993 they indicate that 35 of caco3 production is preserved in the deep sea they suggest a caco3 accumulation rate that is 27 lower than that estimated by milliman 1993 our corg oxidation caco3 dissolution model indicates that a large fraction of the caco3 dissolution that is occurring on the ceara rise is attributable to the neutralization of metabolic acids produced during organic matter oxidation the efficiency with which organic matter oxidation dissolves caco3 that is the ratio caco3 dissolution attributable to organic matter oxidation organic matter oxidation rate generally increases as degree of undersaturation of bottomwaters increases however there are deviations from the general trend that can be attributed to site to site variations in the kinetics of organic matter oxidation and calcite dissolution this result indicates that the dissolution of caco3 as a result of organic matter oxidation in the deep sea may mask the effects of variations in surface water caco3 productivity and bottomwater chemistry on the accumulation rate of caco3 in deep sea sediments

carbonate dissolution in recent sediments of the eastern equatorial indian ocean preservation patterns and carbonate loss above the lysocline abstract multiple dissolution indices from 45 sediment samples show that the foraminiferal lysocline is near 3800 m in the eastern equatorial indian ocean close to the saturation horizon with respect to calcite in the water column below the lysocline the increase in foraminiferal fragmentation decrease in absolute abundance and progressive modification of planktic foraminiferal assemblages with depth is accompanied by an increasing rate of loss of carbonate from the sediments above the lysocline some dissolution must occur in order to produce the dramatic reduction in foraminiferal numbers up to 60 and the pattern of increased fragmentation with depth however the amount of carbonate actually lost from supralysoclinal sediments reaches at most 20 30 of the total at the depth of the lysocline the principal driving force behind supralysoclinal dissolution is most likely the oxidation of organic matter incorporated into the sediments dissolution above the lysocline has a greater apparent impact on relative preservation in the coarse 150 m fraction than it does on total carbonate loss because of redistribution of carbonate to the finer size fractions as fragments though the effects of dissolution on foraminiferal preservation are clearly visible at all depths the lysocline itself does appear to mark the local onset of large scale loss of dissolved carbonate from the sediments such an interpretation is broadly consistent with models regarding the saturation state of the overlying waters as the principal driving force behind the preservation or dissolution of recent carbonate sediments on the sea floor

distinguishing climatic and current effects in mid pleistocene sediments of hatton and gardar drifts ne atlantic samples taken from dsdp sites 552 and 553 on hatton drift and site 611 on gardar drift in the northeast atlantic ocean were examined by analyses of carbonate content coarse fraction amount and composition fine fraction size and composition and oxygen isotope ratios the age of both sections is the mid pleistocene ranging from the end of the jaramillo normal event to the boundary between the matuyama and brunhes epochs about 0 77 0 96 ma records of this time show a change from a dominance of 23 ka to longer periodicity at about 0 818 ma this is marked at site 552 but less so at site 611 the grain size is mainly controlled by coccoliths foraminifera and ice rafted material all of which are related to glacial interglacial changes zones barren of and abundant in coccoliths are sensitive indicators of climatic effect carbonate increase lags behind foraminiferal oxygen isotope records of surface warming in several glacial to interglacial transitions at both drifts different types of carbonates oozes occur on hatton and gardar drifts of several possible factors influencing these bottom current effects seem more significant than others deduced from size analysis of the fine fraction sedimentological results in this study further confirm that sediment drifts in the ne atlantic have maintainted their basic characteristics at least since the mid pleistocene

ecological and environmental factors affecting alkenone distributions in seawater and sediments 1 the use of alkenones to estimate past sea surface temperatures has gained wide acceptance but there are still some issues that need to be resolved before alkenone proxies can be used confidently in all environmental settings a particular concern is defining what seawater temperature is recorded by the alkenone distributions preserved in sediments does it for example reflect a seasonal average or a snapshot of bloom conditions or some other combination of environmental effects this paper reviews some of the environmental and ecological factors that might affect alkenone distributions and in particular the influence of depth and seasonality on alkenone production in the water column

evaluating the utility of b ca ratios in planktic foraminifera as a proxy for the carbonate system a case study of globigerinoides ruber b ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system however the carbonate system controls on b ca ratios are not straightforward with inline image inline image in situ inline image at saturation correlating best with b ca ratios in benthic foraminifera rather than ph inline image or inline image as a simple model of boron speciation in seawater and incorporation into caco3 would predict furthermore culture experiments have shown that in planktic foraminifera properties such as salinity and b sw can have profound effects on b ca ratios beyond those predicted by simple partition coefficients here we investigate the controls on b ca ratios in g ruber via a combination of culture experiments and core top measurements and add to a growing body of evidence that suggests b ca ratios in symbiont bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions we find that while b ca ratios in culture experiments covary with ph in open ocean sediments this relationship is not seen in fact our b ca data correlate best with inline image a previously undocumented association and in most regions salinity these findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite regardless our results underscore the need for caution when attempting to interpret b ca records in terms of the ocean carbonate system at the very least in the case of mixed layer planktic foraminifera

coarse fraction fluctuations in pelagic carbonate sediments from the tropical indian ocean a 1500 kyr record of carbonate dissolution appendix table al is available with entire article onmicrofiche order from the american geophysical union 2000florida avenue n w washington d c 20009 documentp94 001 2 50 payment must accompany order we examined coarse fraction contents of pelagic carbonates deposited between 2000 and 3700 m water depth in the tropical indian ocean using ocean drilling program odp sites 722 owen ridge arabian sea and 758 ninetyeast ridge eastern equatorial indian ocean and four giant piston cores collected by the french r v marion dufresne during the seymama expedition over the last 1500 kyr coarse fraction records display high amplitude oscillations with an irregular wavelength on the order of 500 kyr these oscillations can be correlated throughout the entire equatorial indian ocean from the seychelles area eastward to the ninetyeast ridge and into the arabian sea changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation the well known mid bruhes dissolution cycle represents the last part of this irregular long term dissolution oscillation the origin of this long term oscillation is still poorly understood our observations suggest that it is not a true cycle it has an irregular wavelength and we propose that it may result from long term changes in ca flux to the ocean sites 722 and 758 18o records provide a high resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr site 722 2030 m lies above the present and late pleistocene lysocline the lysocline shoaled to the position of site 758 2925 m only during the interglacial intervals that occurred between about 300 and 500 ka peterson and prell 1985a despite these supralysoclinal positions of the two sites short term changes in coarse fraction contents are correctable from one site to another and probably result from regional or global dissolution pulses by stacking the normalized coarse fraction records from sites 722 and 758 we constructed a composite coarse fraction index ccfi curve in which most of the local signals cancelled out the last 800 kyr of this curve appear to compare extremely well with the composite dissolution index curve from core v34 53 ninetyeast ridge which unambiguously records past variations of carbonate dissolution in the equatorial indian ocean peterson and prell 1985a in the late pleistocene the ccfi variations are mainly associated with glacial interglacial changes they show strong 100 and 41 kyr periodicities but no clear precession related periodicities as proposed earlier by peterson and prell 1985a the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation we cannot totally reject the possibility however that low temporal resolution and or bioturbation degrade somehow the precessional signal at odp sites 722 and 758 in contrast spectral density of dissolution cycles in the giant 53 m long piston core md900963 maldives area displays clear maxima centered on the precession frequencies 23 and 19 kyr 1 as well as on the kyr 1 frequency but shows little power at the 100 kyr 1 frequency these high frequency changes most probably result from changes in surface productivity associated with monsoon variability dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak sw monsoon and or increased dry ne monsoon

a study of oxic anoxic effects on degradation of sterols at the simulated sediment water interface of coastal sediments laboratory incubation experiments were conducted to determine the behavior of sterols at the sediment water interface in oxic and anoxic sediments both plankton and 4 14c cholesterol were used as tracers cholesterol was rapidly degraded at the sediment water interface 55 of 14c cholesterol was lost from sediments under anoxic conditions and 78 under oxic conditions over three and one half months about 3 of initially added free radiolabel was incorporated into a bound pool that was released only by saponification of solvent extracted sediment less than 1 of initially added radiolabel remained in pore waters after three and one half months rate constants for degradation of cholesterol in oxic and anoxic surficial sediments were estimated by tracking variations in radioactivity and sterol concentration as a function of time we discuss our results in terms of factors affecting sterol degradation in coastal marine sediments including molecular structure sediment matrix effect and redox conditions

calcite dissolution in sediments of the ontong java plateau in situ measurements of pore water o2 and ph we present in situ electrode measurements of sediment resistivity pore water oxygen and pore water ph from three stations between 2300 and 3000 m depth on the ontong java plateau in the western equatorial pacific one of these stations is also the site of a concurrent benthic chamber incubation experiment jahnke et al 1994 the pore water oxygen data and a steady state diffusion and reaction model constrain the depth dependent rate of oxic respiration in the sediments and imply a diffusive flux of oxygen to the sediments of 10 21 mol cm 2 yr 1 given these respiration rates the pore water ph data cannot be explained without calcite dissolution driven by metabolically produced co2 the dissolution necessary to explain the observations quantified by a statistical approach is 3 5 6 mol cm 2 yr 1 which corresponds to at least 20 40 of the calcite rain to these sediments over 65 of the total dissolution is driven by metabolic co2 oxygen fluxes and net calcite dissolution constrained by the electrode data are compatible with the benthic chamber measurements of jahnke et al 1994 the dissolution flux while a significant part of the early diagenesis of calcite in these sediments is less than would be predicted by earlier models of dissolution and jahnke et al 1994 probably could not distinguish it from zero with the benthic chamber technique the dissolution rates found in this study are lower than previous estimates because the respiration reaction is concentrated near the sediment water interface and the calcite dissolution rate constants are very small the statistical evaluation of the pore water ph data and model constrain the calcite dissolution rate constant to 0 005 0 16 d 1 following the general trend of lower values determined by in situ techniques rather than by laboratory methods

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

microbial degradation of the marine prymnesiophyte emiliania huxleyi under oxic and anoxic conditions as a model for early diagenesis long chain alkadienes alkenones and alkyl alkenoates to investigate the effect of bacterial diagenesis on the distributions of algal lipids the marine haptophyte emiliania huxleyi was incubated with estuarine sediment slurries under defined conditions oxic sulfate reducing and methanogenic rapid initial degradation of lipids corresponded to increased bacterial populations both of which subsequently declined under oxic conditions the algal c31 dienes were rapidly and completely degraded within 178 days extensive degradation of the characteristic c37 methyl alkenones occurred under all conditions up to 85 under oxic conditions however the u37k index remained essentially constant except for a slight increase at the longest oxic incubation time under anoxic conditions the alkyl alkenoates were preferentially degraded relative to the alkenones and changes in the aa36 index occurred these results demonstrate that lipids are part of the labile organic matter however they were generally degraded at multiple rates which slowed during incubation resulting in their partial preservation hence laboratory experiments conducted over only short periods cannot predict the preservation potential of lipid components furthermore preservation differed between sulfate reducing and methanogenic conditions so the roles of anaerobic processes need to be considered individually with regard to oxic vs anoxic preservation

burial of redox sensitive metals and organic matter in the equatorial indian ocean linked to precession authigenic metals uranium cadmium and molybdenum organic carbon oc and total c37 alkenone totc37 concentrations were measured for the last 350 kyr in core md900963 located in the eastern equatorial arabian sea authigenic metal concentrations on a carbonate free basis range between 1 and 17 ppm 0 5 and 6 ppm and 0 5 and 4 ppm for u cd and mo respectively the profiles are characterized by well defined 23 kyr cycles between oxic and mildly suboxic conditions the redox sensitive metal profiles also follow variations in the concentrations of oc 0 2 0 9 and alkenones 0 2 6 7 ppm the coupled variations in inorganic and organic constituents are attributed to a 23 kyr cycle in primary production above site md900963 as suggested by clear correlations with independent micropaleontologic proxies primary productivity indices based on foraminifera and coccoliths and fragmentation of foraminiferal shells the 23 kyr cycles do appear to be primarily driven by productivity rather than changes in bottom water oxygen comparison with other records indicates that if this interpretation is correct productivity variations across much of the indian ocean have been dominated by precessional forcing with high productivity in phase with low summer insolation in the northern hemisphere this interpretation contrasts with the traditional attribu tion of enhanced productivity in the indian ocean with periods of high summer insolation copyright 2002 elsevier science ltd

the milos geothermal field modelling and interpretation of electromagnetic induction studies abstract the magnetotelluric mt technique in the period range 10 2 to 10 4 s has been applied to determine the electrical structure of the geothermal field of milos island greece data from 39 stations have been processed using standard tensorial techniques and the data have been found to be nearly one dimensional 1d up to 10 s several 1d modelling techniques have been applied to the processed data sets subsequent two dimensional 2d finite difference modelling has revealed some deeper electrical features the 1d and 2d electrical models are in agreement with the existing geothermal model on milos within the uppermost 200 m the resistivity can be

magnetotelluric image of the fluid cycle in the costa rican subduction zone fluids entering the subduction zone play a key role in the subduction process they cause changes in the dynamics and thermal structure of the subduction zone1 and trigger earthquakes when released from the subducting plate during metamorphism fluids are delivered to the subduction zone by the oceanic crust and also enter as the oceanic plate bends downwards at the plate boundary however the amount of fluids entering subduction zones is not matched by that leaving through volcanic emissions4 or transfer to the deep mantle implying possible storage of fluids in the crust here we use magnetotelluric data to map the entire hydration and dehydration cycle of the costa rican subduction zone to 120 km depth along the incoming plate bend we detect a conductivity anomaly that we interpret as sea water penetrating down extensional faults and cracks into the upper mantle along the subducting plate interface we document the dehydration of sediments the crust and mantle we identify an accumulation of fluids at 20 30 km depth at a distance of 30 km seaward from the volcanic arc comparison with other subduction zones5 14 indicates that such fluid accumulation is a global phenomenon although we are unable to test whether these fluid reservoirs grow with time we suggest that they can account for some of the missing outflow of fluid at subduction zones

variations of oxygen minimum and primary productivity recorded in sediments of the arabian sea abstract two deep sea sediment cores from the northeastern and the southeastern arabian sea were studied in order to reconstruct the palaeoenvironments of the past glacial cycles core 136kl was recovered from the high productivity area off pakistan within the modern oxygen minimum zone omz by contrast modern primary productivity at the site of md900963 close to maldives is moderate and bottom waters are today well oxygenated for both cores we reconstructed the changes in palaeoproductivity using a set of biomarkers alkenones dinosterol and brassicasterol the main result is that primary productivity is enhanced during glacial stages and lowered during interstadials the proxies associated with productivity show a 23 kyr cyclicity corresponding to the precession related insolation cycle palaeoredox conditions were studied in both cores using a new organic geochemical parameter c35 c31 n alkane ratio developed by analysing surface sediments from a transect across the omz off pakistan the value of this ratio in core 136kl shows many variations during the last 65 kyr indicating that the omz was not stable during this time it disappeared completely during heinrich and the younger dryas events pointing to a connection between global oceanic circulation and the stability of the omz the c35 c31 ratio determined in sediments of core md900963 shows that bottom waters remained rather well oxygenated over the last 330 kyr which is confirmed by comparison with authigenic metal concentrations in the same sediments a zonally averaged circulation biogeochemical ocean model was used to explore how the intermediate indian ocean responds to a freshwater flux anomaly at the surface of the north atlantic as suggested by the geochemical time series both the abundance of southern ocean water and the oxygen concentration are significantly increased in response to this freshwater perturbation

radiocarbon calibration beyond 20 000 14c yr b p by means of planktonic foraminifera of the iberian margin we present a new set of 14c ages obtained by accelerator mass spectrometry ams on planktonic foraminifera from a deep sea core collected off the iberian margin md952042 this site at 37 n is distant from the high latitude zones where 14c reservoir age is large and variable many independent proxies alkenones magnetic susceptibility ice rafted debris foraminifera stable isotopes abundances of foraminifera pollen and dinoflagellates show abrupt changes correlative with dansgaard oeschger and heinrich events of the last glacial period the good stratigraphic agreement of all proxies from the fine to the coarse size fractions indicates that the foraminifera 14c ages are representative of the different sediment fractions to obtain reliable 14c ages of foraminifera beyond 20 000 14c yr b p we leached the shells prior to carbonate hydrolysis and subsequent analysis for a calendar age scale we matched the iberian margin u37k profile with that of greenland summit 18o both are proxies for temperature which in models varies synchronously in the two areas the match creates no spurious jumps in sedimentation rate and requires only a limited number of tie points except for ages older than 40 000 14c yr b p greenland s gisp2 and grip records yield similar calendars the 14c and imported calendar ages of the iberian margin record are then compared to data from lacustrine annual varves and from corals and speleothems dated by u th previously used to extend the calibration beyond 20 000 14c yr b p the new record follows a smooth pattern between 23 000 and 50 000 cal yr b p we find good agreement with the previous data sets between 23 000 and 31 000 cal yr b p in the interval between 33 000 and 41 000 cal yr b p for which previous records disagree by up to 5000 cal yr the iberian margin record closely follows the polynomial curve that was previously defined by an interpolation of the coral ages and runs between the lake suigetsu and the bahamian speleothem data sets

seismicity and rheology of subducted slabs since the incorporation of the inclined zones of intermediate and deep seismic activity into the theory of plate tectonics1 2 it has been generally accepted that earthquakes with focal depths 100 km occur in subducted slabs of predominantly oceanic lithosphere yet our understanding of the processes which generate intermediate and in particular deep earthquakes is still incomplete this is largely due to our limited knowledge of the rheological behaviour of subducted oceanic lithosphere at depths between 100 and 700 km however recently considerable insight has been gained into the rheology of oceanic lithosphere in near surface conditions and i investigate here whether the seismic activity at depths 100 km can be understood on the basis of this information i first present results of temperature calculations performed for all subduction zones for which adequate data on seismicity relative plate motion and age of the descending oceanic lithosphere exist sub sequently i adapt the temperature conditions governing the rheology near the surface by including depth dependence this leads to a depth dependent critical temperature tcr z above which the subducted lithosphere cannot sustain the stresses necessary to generate seismic events seismicity data and calculated temperature versus depth relationships are in good agreement with the rheological model predictions this implies that the absence of seismic activity at depths 700 km should not be interpreted as direct evidence for the hypothesis that the 650 km discontinuity in the mantle acts as a barrier to vertical motion of subducted slabs

long period magnetotelluric measurements near the central california coast a land locked view of the conductivity structure under the pacific ocean summary telluric data from hollister california were combined with magnetic data from the fresno california magnetic observatory to determine the mt response at long periods the shifted eigenstate analysis of latorraca et al 1986 was used to determine the maximum apparent resistivities and corresponding eih phases for periods from 1 to 30 h the apparent resistivity and phase at hollister are greatly influenced by the ocean with the mantle branch beginning at a period of about 12 h in typical continental areas the mantle branch usually begins at periods of 10s or less a maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters the data required a resistivity thickness product of approximately 1 x lo9 q m2 1 x lo6 q m km for the oceanic lower crust and upper mantle the oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth and a rapid conductivity increase below 640km the sensitivity analysis shows the data are most sensitive to the oceanic parameters except for the electric field amplitudes which are also sensitive to the resistivities under the observation site

planktic foraminiferal dissolution in the twilight zone abstract calcite dissolution of planktic foraminifers that settle from the surface ocean to depths has been assessed using the weight size relationship of tests and correlated to carbonate ion concentration co 3 2 of ambient sea water globigerina bulloides and globigerinita glutinata were sampled from the north atlantic and the arabian sea during different seasons spring and fall and the monsoons respectively test weight has been determined for single tests from narrow size classes 250 8 m and 300 7 m minimum test diameter initial test weight of both species in surface waters differs between regions and seasons weight loss of settling tests in the twilight zone between 100 and 1000 m water depths averaged 19 below the twilight zone 1000 2500 m no weight loss occurs on average and tests may even gain weight remineralization of settling planktic foraminiferal tests is most pronounced at maximum co 3 2 accordingly strong dissolution occurs in the twilight zone at the depth of maximum decrease in co 3 2 it is assumed that dissolution of planktic foraminiferal calcite is caused by co 3 2 under saturation inside the test buffered by diffusive exchange of co 3 2 ions with the ambient sea water through the pores and aperture consequently reconstruction of small scale differences in seawater co 3 2 from test weight alone is not feasible and systematic correlation between test weight and co 3 2 of the ambient sea water is statistically not significant at constant co 3 2 remineralization cannot be deduced from test weight neither at high nor low co 3 2 saturation in total our data suggest that the global vertical planktic foraminiferal caco 3 flux is about 19 lower than calculated by schiebel 2002 planktic foraminiferal sedimentation and the marine calcite budget global biogeochemical cycles 16 4 1065 doi 10 1029 2001gb001459

seismotectonics of the aegean region abstract fault plane solutions neotectonic field observations and in situ stress measurements have been used to determine the stress field associated with the active deformation of the lithosphere in the aegean and surrounding regions a stress gradient and a zonal pattern of different tectonism away from the trench axis has been postulated pure thrust faulting occurs in the fore arc side while the back arc side is mainly dominated by almost n s extensional tectonics a narrow belt of strike slip faults with predominantly thrust component separates these tectonic zones this belt seems to represent a transition from the thrust type faults to normal faults fault plane solutions of the whole region in question indicate that both the large and small magnitude shocks are caused by the same regional stress field a body of 33 focal mechanisms indicates that the intermediate depth shocks along the sinking mediterranean slab are associated with thrust type fracturing orientation of kinematic axes favours the suggestion that shear zones within the descending slab cause these shocks the active crustal shortening along the hellenic consuming boundary and the sinking of the mediterranean lithosphere beneath the south aegean area explain satisfactorily the seismotectonic features of the fore arc and the southern back arc sides on the contrary geophysical evidence concerning the cenozoic geodynamic evolution of the aegean is needed to interpret features of the north back arc area

caco3 dissolution in california continental margin sediments the influence of organic matter remineralization in situ benthic flux chamber and oxygen microelectrode and shipboard porewater results have been used to quantify sea floor dissolution of caco3 on the continental rise adjacent to central california usa the porewater distributions and benthic fluxes of o2 no3 ta ca2 13c and tic are interpreted using a numerical simulation of organic matter remineralization and caco3 dissolution in marine sediments the processes considered in the simulation include organic matter oxidation by o2 no3 and so42 caco3 dissolution and precipitation hs and nh4 oxidation and sediment mixing and sediment accumulation calculated benthic fluxes of o2 no3 ta tic 13c and ca2 porewater concentrations of o2 no3 and nh4 and sediment distributions of organic carbon caco3 excess 210pb and 14c agree well with the measurements benthic fluxes of alkalinity and inferred caco3 dissolution rates cannot be explained on the basis of dissolution driven solely by bottom water undersaturation if the influence of metabolically produced co2 is included benthic fluxes are fully reconciled however this is in agreement with benthic chamber ca2 and 13c results that independently imply substantial caco3 dissolution in these sediments the above observations are in contrast to those reported by jahnke et al 1994 for the west african continental rise and the western equatorial pacific where 1 g diagenetic models predict dissolution fluxes larger than observed with benthic flux chambers we conclude that the extent of metabolic caco3 dissolution may vary regionally numerous factors such as the depth of metabolic co2 production and caco3 dissolution kinetics are known or predicted to influence metabolic dissolution among the factors that should be considered in reconciling these observations are l the extent to which sulfate reduction and reoxidation reactions may influence acid base properties in surface sediments and 2 the total sedimentary caco3 content of the sites that may influence porewater acidity through surface exchange reactions at the mineral surface

postdepositional oxic degradation of alkenones implications for the measurement of palaeo sea surface temperatures free and bound long chain alkenones c37 2 and c37 3 in oxidized and unoxidized sections of four organic matter rich pliocene and miocene madeira abyssal plain turbidites one from ocean drilling program site 951b and three from site 952a were analyzed to determine the effect of severe post depositional oxidation on the value of u37k the profiles of both alkenones across the redox boundary show a preferential degradation of the c37 3 compared to the c37 2 compound because of the high initial u37k values and the way of calculating the u37k this degradation hardly influences the u37k profiles however for lower u37k values measured selective degradation would increase u37k up to 0 17 units equivalent to 5 c for most of the u37k band width much smaller degradation already increases u37k beyond the analytical error 0 017 units consequently for interpreting the u37k record in terms of past sea surface temperatures selective degradation needs serious consideration

calcite dissolution along a transect in the western tropical indian ocean a multiproxy approach three paleocarbonate ion proxies size index planktonic foraminifera shell weight and calcite crystallinity have been employed here to a set of core top samples from the western tropical indian ocean in the water depth ranges from 1086 to 4730 m all three proxies complement each other well and reveal that calcite dissolution starts to affect planktonic foraminifera from 2250 m depth onward and intense calcite dissolution begins around 3900 m depth in this sector of the tropical indian ocean three planktonic foraminifera species globigerinoides sacculifer pulleniatina obliquiloculata and neogloboquadrina dutertrei show wide variability in shell weights from core top sediments in the depth range of 3300 to 3400 m bathed by similar bottom water co3 concentrations this variability is attributed to shell calcification and shell weights are controlled by co3 concentration in the surface waters further surface water co3 concentrations override milder dissolution effects experienced by samples from shallower water depths bathed by similar bottom water co3 concentrations

latitudinal variations in intermediate depth ventilation and biological production over northeastern pacific oxygen minimum zones during the last 60 ka abstract mechanisms affecting past variability in the oxygen minimum zone omz in the eastern tropical north pacific etnp are poorly known we analyzed core md02 2524 obtained from the nicaragua margin in the present etnp omz for major and minor elements titanium ti brome br silicon si potassium k and calcium ca using an x ray fluorescence xrf core scanner and redox sensitive trace elements uranium u molybdenum mo and nickel ni determined by icp ms the u and mo content was higher during the deglaciation than during the holocene and the last glacial maximum whereas enrichment was not observed for ni an element closely associated with organic matter high resolution xrf scanning indicated that the ca based carbonate content had millennial scale variability inversely correlated with br based organic matter and si k based opal content during the last glacial period the available data suggest no clear regional trend in biological productivity during the last deglaciation but significant local variability in the coastal eastern equatorial pacific the trace element enrichment and the lack of a concomitant increase in biogenic phases indicated that an enhanced etnp omz at least between 15 n and 12 n at a water depth of 500 900 m was principally caused by a reduced oxygen supply driven by oceanic circulation to the nicaragua basin during the deglaciation the observed patterns can be interpreted as the distinct changes in the oxygenation state of northern and southern water masses at intermediate depths we also found evidence for a decoupling between local productivity and pore water oxygenation for several millennial scale events during marine isotopic stage 3 indicating that remote oxygen consumption and or oceanic ventilation impacted omz intensity multi millennial scale variations of the productivity at papagayo upwelling cell displayed an opposite trend from productivity at the costa rica dome in relation with the latitudinal shift of the inter tropical convergence zone itcz latitudinal variations of the omz intensity reflected the relative influence of the northern and southern intermediate water masses whose oxygenation was driven by high latitude climates and productivity over the intermediate water pathway

in situ measurements of calcium carbonate dissolution rates in deep sea sediments benthic fluxes of alkalinity carbon dioxide and oxygen have been measured using an in situ incubation chamber at three sites manop sites c and s and pacflux site sc in the central equatorial north pacific at two carbonate rich sites c and sc a budget for oxygen alkalinity and tco2 fluxes indicate a net caco3 dissolution rate of approximately 0 4 mmol m 2 day 1 this rate is only 20 of previous estimates but is consistent with a dissolution rate constant predicted from laboratory experiments with deep sea sediments and derived from in situ flux measurements in sediments of the southern california borderland organic matter oxidation in the sediment column provides the acid for 60 100 of the calcium carbonate dissolution occurring at these sites with the remainder derived from undersaturated bottom water at a low carbonate site s no carbonate dissolution in the sediment or on the sea floor is apparent although sediment traps indicate a rain of caco3 through 3400 m the rates of remineralization relative to resuspension or erosion at this site must differ for organic carbon and calcium carbonate so that carbonate grains reaching the sea floor are physically removed by erosion or resuspension before they dissolve while organic carbon is largely oxidized before it can be physically removed

3 d structure of the lithosphere in the aegean region summary p wave traveltime residuals from about 300 local and 400 teleseismic earthquakes recorded by the athens seismological network were used to study the structure of the lithosphere in the aegean region teleseismic data included readings from local stations which do not routinely report to the isc the large values of the residuals up to f3 s and their variation with azimuth and epicentral distance indicate significant velocity heterogeneity in the upper mantle p wave traveltime residuals were inverted simultaneously for both local and teleseismic earthquakes tomographic results which are best resolved at lithospheric depths reveal a heterogeneous lithosphere down to a depth of 100km or so a high velocity anomaly is found dipping at an average of 45 to the ne away from the hellenic trench and supports the hypothesis of subduction due to a cooler descending slab of oceanic composition an area of low velocities is observed at depths less than 80 km in the aegean back arc corresponding well with observed high heat flow values in the area in contrast an area of relatively higher velocity anomalies is resolved in the lithosphere underlying the sea of crete the tectonic structure of the crust and upper mantle in the eastern mediterranean area results from the complex interaction of several different geodynamic processes not all of which can be described by simple plate tectonic models at present the eurasian and african plates are converging at a rate of about lcmyr in an approximately n s direction minster jordan 1978 with the latter descending

orbital forcing of bottom current enhanced sedimentation on feni drift ne atlantic during the mid pleistocene feni ridge is a plastered sediment drift deposited along the northwestern flank of rockall trough ne atlantic under the influence of deep geostrophic currents formed by intermittent overflows of arctic intermediate water from the norwegian sea across the iceland scotland ridge nsow lower deep water ldw a derivative of antarctic bottom water from the south atlantic mixed with old north atlantic deep water nadw also bathes the lower flanks of the drift below 2500 m deep sea drilling project site 610 was drilled in a sediment wave field on the surface of feni drift 2417 m though the sediments recovered proved to be typically pelagic preserving a high resolution 5 cm kyr 1 quaternary record of orbitally modulated ice rafting and biogenic productivity cycles but showing little evidence of current affected deposition from particle size and compositional analyses of the biogenic and lithogenic 10 m silt which we then compared with records of the percentage and flux of ice rafted detrital ird sand foraminifera and coccoliths for the same interval cross spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume ice sheet growth and decay in response to orbitally forced insolation changes at 65 n variations in the relative proportions and fluxes of ird sand foraminifera and coccoliths all covary closely with orbitally forced ice volume fluctuations but with differing amplitude coherency and phase relationships at each milankovitch frequency however there is a significant component of variance in pelagic and ice rafted sedimentation which is controlled by bottom currents and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies the relative enrichment coarseness and flux of the contourite silt fraction of feni drift sediments all increase during glacial or cold stages of the mid pleistocene relative to warm stages suggesting that currents were stronger during these periods possible explanations for such changes in current activity include 1 an increased flux of nsow via the wyville thompson ridge into the rockall trough during glacials but not due to increased production of nsow 2 ldw presently lying below 2500 m rises during glacials by at least 50 60 m thereby flooding over the surface of feni ridge at site 610 3 the path of a major recirculation gyre in the lower nadw ldw system presently situated marginally south of site 610 encroaches upon the southern tip of feni ridge during glacials and 4 the rockall trough itself is a site of deep water formation during glacials analogous to labrador sea deep water formation today

two dimensional topographic responses in magnetotellurics modeled using finite elements we simulate the magnetotelluric response to two dimensional earth topography using finite elements linear interpolation of the secondary field parallel to strike over triangular elements allows accurate modeling of inclined resistivity boundaries including topographic surfaces to avoid discontinuities in field derivatives or resistivity care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media the nodal locations may be shifted but the derivatives still are evaluated at the field points of interest correct values may be returned at gentle breaks in slope as well as along straight surfaces the finite element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical depression and with rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization agreement with the other methods generally is excellent with t

organic matter preservation on the pakistan continental margin as revealed by biomarker geochemistry in order to assess whether the oxygen minimum zone omz in the arabian sea has an eaect on the preservation and composition of organic matter in surface sediments we investigated samples from three diaerent transects on the pakistan continental margin across the omz in addition to determining the total amount of organic carbon toc we analyzed the extractable lipids by gas chromatography combined gas chromatography mass spectrometry and compound specific stable carbon isotope measurements the extractable lipids are dominated by marine organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk and molecular isotopic composition sediments from within the omz are enriched in organic carbon and in several extractable lipids i e phytol n alcohols total sterols n c35 alkane relative to stations above and below this zone other lipid concentrations such as those of total n fatty acids and total n alkanes fail to show any relation to the omz only a weak correlation of toc with mineral surface area was found in sediments deposited within the omz in contrast sediments from outside the omz do not show any relationship between toc and surface area among the extractable lipids only the n alkane concentration is highly correlated with surface area in sediments from the hab and makran transects in sediments from outside the omz the phytol and sterol concentrations are also weakly correlated with mineral surface area the depositional environment of the indus fan oaers the best conditions for an enhanced preservation of organic matter the omz together with the undisturbed sedimentation at moderate rates seems to be mainly responsible for the high toc values in this area overall the type of organic matter and its lability toward oxic degradation the mineral surface area the mineral composition and possibly the secondary productivity by sedimentary bacteria also appear to have an influence on organic matter accumulation and composition 2000 elsevier science ltd all rights reserved

chemistry of surface sediment along a north south transect across the equator in the central indian basin an assessment of biogenic and detrital influences on elemental burial on the seafloor nineteen surface sediment samples collected along a north south transect across the equator in the central indian basin cib were analysed for their major minor and rare earth element ree composition in absence of the element flux data we demonstrate here the association of various elements and their preferential sinks using inter elemental associations in sediment exhibiting latitudinal dissimilarity biogenic sedimentation north of the equator 1 to 4 n latitudes is dominated by carbonate ooze whereas south of the equator 4 to 14 s latitudes is dominated by siliceous ooze carbonate ooze deposition 60 caco3 above 4400 m and its drastic reduction to 80 of their respective total content in all the three sediment zones these observations suggest a close link between ba and a mn oxide phase on the other hand nearly overlapping latitudinal trends of alxs with carbonate in areas north of the equator and opal south of the equator strong positive correlation of alxs with opal r 0 8 similar ratios between carbonate free opal and alxs 16 in both biogenic sediments suggest a close link between productivity and alxs the alxs proportion in both biogenic sediment types is around 28 of the total al suggesting nearly the same level of primary productivity along the transect 4 n to 14 s which supports the estimates of berger berger w h 1989 global maps of the ocean productivity in w h berger v s smetacek g wefer eds productivity of the ocean present and past dahlem workshop reports wiley chichester pp 429 455 and kabanova kabanova j g 1968 primary production of the northern part of the indian ocean oceanology 8 214 255 i e 70 g cm 2 y 1 the rees exhibit mutual coherence in the sediment r 0 7 however they display an increasing affinity towards mn r increases from 0 6 to 0 8 and decreasing affinity towards ti r decreases from 0 8 to 0 3 with increasing atomic number such a subtle but systematic variation within ree group suggests preferential burial of heavy ree by mn oxide phase and light ree by detrital phase

the carbon dioxide system in the arabian sea in 1995 the whoi c goyet and miami f j millero groups participated on a number of research cruises in the arabian sea as part of the u s joint global ocean flux study jgofs sponsored by the national science foundation nsf this paper gives the results of our total inorganic carbon dioxide tco2 total alkalinity ta and potentiometric ph measurements made on arabian sea water samples during these cruises measurements made on certified reference material crm indicate that the reproducibility of the measurements was 0 007 in ph 3 2 mol kg 1 in ta and 1 2 mol kg 1 in tco2 n 180 the surface measurements 0 30 m of ph and normalized tco2 and ta were quite uniform throughout the year ph 8 1 0 05 ntco2 1950 20 mol kg 1 and nta 2290 5 mol kg 1 the larger variations in ntco2 in the surface waters are related to changes in primary production and upwelling in the coastal waters the depth profiles of ph pco2 ta and tco2 were similar to those in the equatorial pacific ocean the components of the carbonate system co2 hco 3 co2 3 and the saturation state for calcite and aragonite were determined from the measurements of ta and tco2 the waters below 600 and 3400 m in the arabian sea were undersaturated 1 0 for aragonite and calcite respectively the co2 measurements have been combined with the nutrient data to examine the stoichiometric ratios of c n c p c o2 and c sio2 of the waters marked differences were found for the waters above and below the oxygen minimum zone the surface water results have been used to develop the following stoichiometry for phytoplankton in the arabian sea ch2o 125 nh3 14 h3po4 sio2 13 the oxidation of this material is due to reactions with o2 77 and no3 23 with the resultant formation of n2 and n2o the maximum amount of organic carbon oxidized has been estimated to be 3 1 mol kg 1 in the deep waters with as much as 0 9 mol kg 1 in the oxygen minimum zone with no3 the maximum amount of caco3 dissolved in the deep waters is 116 mol kg 1 these results together with the organic material collected from the sediment traps should be useful in characterizing the formation and degradation of plant material in the arabian sea

the geometry of the wadati benioff zone and lithospheric kinematics in the hellenic arc abstract accurate locations of 961 shallow and intermediate depth earthquakes which occurred between 1956 and 1995 in the hellenic arc 34 39 n 19 28 e are used to define the plate boundaries in this area reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc an ocean continent type of interaction occurs on a curved surface which is defined by the shallow branch 20 100 km of the wadati benioff zone the intersection of this zone with the earth s surface is a curve which follows the convex outer side of the sedimentary arc western peloponnese west of cythera south coast of crete east coast of rhodes and dips at low angle 30 to the aegean sea coupling between the subducted oceanic crust and the overriding of the aegean lithospheric plate takes place on this surface the deep branch 100 180 km of the wadati benioff zone is dipping freely without coupling at a high angle 45 beneath the south aegean trough and the volcanic arc the high shallow seismicity h 20 km which is observed in the southwestern convex outer side of the arc ionian sea is attributed to the fast southwestward motion of the aegean plate location of strong deep earthquakes h 100 km in the fore arc area of the southwestern part of the hellenic arc west of cythera indicates that oceanic crust is destroyed in this part of the hellenic trench due to roll back of the descending lithospheric slab

molecular stratigraphy a new tool for climatic assessment variations in sea surface temperatures over the past 500 000 years are inferred from the relative abundance behaviour of two organic compounds c37 alkenones over the upper 8 metres of a sediment core from the eastern equatorial atlantic this molecular record ascribed to contributions from prymnesiophyte algae correlates well with the variations in the 18 signal for the calcareous skeletons of certain planktonic foraminifera thus providing the first demonstration of a new stratigraphical technique which may be especially valuable where methods based on carbonate 18 fail

high frequency shear wave propagation across the hellenic subduction zone abstract this study investigates shear wave propagation across the hellenic subduction zone using high quality waveforms of 71 intermediate depth events 40 160 km depth the data have been recorded by a permanent seismic network consisting of 22 broadband three component stations that cover most parts of greece shear waves can be efficiently transmitted with low attenuation q s 1100 through the descending slab which extends laterally over much of the forearc area with a northwest southeast strike the slab also appears to be continuous downdip in the depth range 60 160 km even though it was found that a gap smaller than 10 km would not affect shear wave efficiency significantly two highly attenuating areas have been detected one in the central southeast aegean corresponding to the mantle wedge and the other in the northern aegean probably caused by elevated upper mantle temperatures owing to the ongoing extension in that area these results provide additional constraints for the geometry of the hellenic subduction zone and the geodynamic setting of the aegean region

greek tectonics and seismicity abstract the validity of existing tectonic models for the area of greece is examined in the light of the new recalculated parameters for earthquakes of the region makropoulos and burton 1981 relocated hypocentral positions are extracted from the catalogue to form radial and vertical distance depth cross sections centred on a reference point near the mid point of the aegean volcanic arc and these are used to form a three dimensional topography of the base of earthquake occurrence below 60 km isodepth maps are extracted from this topography as both three and two dimensional map presentations these maps reveal several significant features of deep seated tectonic processes in the region isodepths exceeding 150 km are seen in the northwest aegean and these are more closely linked to the sporadhes and gulf of thermaicos rather than the north aegean trough the 150 km isodepths are also seen in the northeast aegean straddling the dardanelles in the northeastern part of the peloponnesus gulf of saronikos and eastern gulf of corinth in the southern peloponnesus towards crete and extending from north of kos to south of rodos the largest extent of deepest activity is seen south of rodos and this continues towards southwest of turkey the subduction zone of the hellenic arc is clear but smooth benioff zones are not the norm and these data show that structural complexity is more readily observed it is concluded that none of the proposed tectonic models completely explain the observed activity over the whole area and rather than propose yet another model places where further work is still particularly necessary are identified

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator