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

looking for clues to paleoceanographic imprints a diagnosis of the gulf of cadiz contourite depositional systems a new morphosedimentary map of the gulf of cadiz is presented showing the contourite depositional system on the gulf s middle slope this map is constructed from a broad da tabase provided by the spanish research council and the u s naval research laboratory our map shows that this contourite depositional system comprises five morphosedimentary sectors 1 proximal scour and sand ribbons 2 overflow sedimentary lobe 3 channels and ridges 4 contourite deposition and 5 submarine canyons the gulf of cadiz con tourite depositional system stems directly from the interaction between mediterranean out flow water and the seafloor its morphosedimentary sectors are clearly related to the sys tematic deceleration of the mediterranean outflow waters westward branches bathymetric stress on the margin and the coriolis force the slope s depositional system can be consid ered as a mixed contourite and turbidite system i e a detached combined drift and fan

dimensions of the atlantic mediterranean connection that caused the messinian salinity crisis abstract what kind of gateway is needed to cause a salinity crisis although several reconstructions of possible atlantic mediterranean gateways are proposed for the late miocene so far the gateway that must have existed before the desiccation of the mediterranean during the messinian is unknown this study uses the theory of hydraulic control combined with the effect of bottom friction in order to find out to first order the geometrical dimensions of the connection that existed during the primary lower gypsum stage 5 97 5 61 ma of the messinian salinity crisis 5 97 5 33 ma the connecting strait is assumed to behave in a similar way as existing straits such as the strait of gibraltar or the bosphorus a salinity crisis in an enclosed basin results when its connection to the open ocean is highly restricted a strait needs to be relatively shallow narrow and or long in order to result in exchange fluxes that are of around 25 or less of the exchange at the strait of gibraltar today considering the evaporite deposits together with global sea level variations we estimate the cross section of a strait responsible of the msc to have a minimum depth of 30 45 m and a maximum width of 0 7 2 km for lengths in the range up to 500 km these dimensions are consistent with only a few of the miocene corridors identified the calculations are extended to explore the implications for sedimentary structures on the corridor floor

the gibraltar corridor watergate of the messinian salinity crisis abstract the existence and evolution of a messinian salt giant in the mediterranean sea has caused much debate in the marine science community especially the suggestion that the mediterranean was a deep desiccated basin during the messinian salinity crisis msc 5 97 5 33 ma triggered by a temporal disconnection from the global ocean made it a well known crisis beyond the scientific boundaries approximately 50 years after this provocative statement it remained unknown which mediterranean atlantic seaway delivered the 5 6 of the global ocean s salt into the mediterranean basin here we review the changes in mediterranean atlantic connectivity throughout the late miocene in order to locate date and quantify the missing messinian gateway that provided the salt water inflow during the msc we conclude that all the known pre msc gateways through southern spain and northern morocco were closed leaving the gibraltar corridor at its messinian configuration as the sole candidate we consider the possibility of longer and narrower straits existing at depth below the present gibraltar region and using strait dynamic theory we calculate its dimensions during the messinian based on the salinity changes in the mediterranean a marine messinian gateway through the gibraltar corridor is in agreement with growing evidence that atlantic waters reached the mediterranean sea during all three stages of the msc

pliocene and quaternary depositional model of the algarve margin contourite drifts gulf of cadiz sw iberia seismic architecture tectonic control and paleoceanographic insights abstract the contourite drifts off southwest iberia that formed as a result of the interaction of the mediterranean outflow water mow with the continental middle slope were studied in the algarve margin using multichannel reflection seismic lines oil wells piston cores and a bathymetric compilation of four datasets the seismostratigraphic interpretation of a dense array of oil industry seismic and stratigraphic correlation allowed the identification of five seismic units of early pliocene through holocene in the faro and albufeira drifts and four correlative seismic units in the lagos and sagres drifts and three in the portimao drift a three phased evolutionary model for the contourite formation is proposed firstly a precursory phase of pliocene age made up of sheeted drifts represents an initial phase of deposition under bottom current activity that is correlated with the first stages of an enhanced mow at about 3 5 ma secondly the building up phase of early pleistocene age is related to a strengthening of the mow close to the onset of the northern hemisphere glaciations at about 2 6 ma during which were deposited low mounded drifts thirdly the growing phase from middle pleistocene through holocene suggests the presence of a stronger mow since the beginning of the middle pleistocene transition at about 1 3 1 0 ma accounting for the deposition of mounded drifts and formation of the alvares cabral moat seismostratigraphic interpretation and isochron maps allowed for the establishment of the main oceanographic climatic morphologic and tectonic factors that controlled the drifts deposition i the pliocene and quaternary mow circulation forced by climate changes ii the sea bottom topography inherited from the late miocene mainly shaped by the portimao lagos and bartolomeu dias canyons system iii the interaction between along slope and down slope processes since the pliocene and iv pliocene quaternary fault activity and diapirism

impact of restriction of the atlantic mediterranean gateway on the mediterranean outflow water and eastern atlantic circulation during the messinian 1 messinian foraminiferal stable oxygen and carbon isotopes of the montemayor 1 core guadalquivir basin sw spain have been investigated this record is exceptional to study the mediterranean outflow water mow impact on the atlantic meridional overturning circulation amoc and global climate during the messinian because the core is near the guadalhorce corridor the last betic gateway to be closed during the early messinian our results allow dating accurately its closure at 6 18 ma constant benthic 18o values high difference between benthic and planktonic 18o and low sedimentation rates before 6 18 ma indicate the presence of a two layer water column with bottom winnowing due to an enhanced mediterranean outflow current the enhanced contribution of dense mow to the north atlantic ocean likely fostered the formation of north atlantic deep water nadw after 6 18 ma benthic 18o values parallel that of the global glacioeustatic curve the difference between benthic and planktonic 18o is low and sedimentation rates considerably increased this indicates a good vertical mixing of the water column interruption of the mow and a dominant glacioeustatic control on the isotopic signatures according to the role of mow in the modern atlantic thermohaline circulation the reduction of the mow after the closure of the guadalhorce corridor might have resulted in a decreased nadw formation rate between 6 0 and 5 5 ma weakening the amoc and promoting northern hemisphere cooling after the gibraltar strait opening the restoration of the mow and related salt export from the mediterranean could have promoted an enhanced nadw formation

diatom bound 15n 14n new support for enhanced nutrient consumption in the ice age subantarctic 1 diatom bound 15n 14n was used to reconstruct the glacial nutrient status of the subantarctic zone in the southern ocean down core records from both the pacific and indian sectors show 15n of 5 to 6 during the last glacial maximum and a decrease coincident with the glacial termination to values as low as 2 the effect of either diatom assemblage or physiological change on the diatom bound 15n 14n is unknown and cannot yet be ruled out as a possible explanation for the observed change however the consistency between indian and pacific sector records and with other paleoceanographic data suggests that the glacial interglacial difference in diatom bound 15n 14n was driven by higher consumption of nitrate in the subantarctic surface during the last ice age such a change in nutrient consumption may have resulted from atmospheric iron fertilization and or decreased glacial mixed layer depths associated with sea ice melting enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of co2 in the ice age atmosphere it also would have reduced the preformed nutrient content of the low latitude thermocline leading to decreases in low latitude productivity suboxia and denitrification

interseismic strain accumulation on the himalayan crustal ramp nepal the departement of mines and geology has been monitoring the seismicity of the central himalayas of nepal since 1985 intense microseismicity and frequent medium size earthquakes ml 4 tend to cluster beneath the topographic front of the higher himalaya this 10 20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data both microseismic and geodetic data indicate strain accumulation on a mid crustal ramp that had been previously inferred from geological and geophysical evidence this ramp connects a flat decollement under the lesser and sub himalaya with a deeper decollement under the higher himalaya and probably acts as a geometric asperity where strain and stress build up during the interseismic period the large himalayan earthquakes could nucleate there and probably activate the whole flat and ramp system up to the blind thrusts of the sub himalaya

an important current reversal influx in the rifian corridor morocco at the tortonian messinian boundary the end of tethys ocean atlantic psychrospheric and temperate mesopelagic faunas found in the lower messinian marls in morocco indicate that a strong eastward flowing bottom current was present in the rifian corridor before the salinity crisis this influx began just before diatomite deposition in the paleo mediterranean continued during a decrease in species diversity in coral reef formation and diminished with the initial stages of brine concentration in the deep water phase of the crisis the influx is most readily studied in a condensed section of marl in the bou regreg valley near rabat the beginning of this siphon event coincides with the tortonian messinian boundary 6 4 ma subchron 6n1 it is identified by 1 a change in the planktonic foraminifera from dominance of warm tropical epipelagic globorotalia menardii with globigerinoides to the temperate mesopelagic gl miotumida plexus with conomiozea 2 the sudden appearance of an upper psychrospheric ostracode fauna with agrenocythere pliocenica and oblitacythereis ruggierii 3 a change in nannoflora and 4 beginning of the 6 3 ma global carbon shift the initial strong influx stage of the siphon lasted at least 0 7 m y decreasing after the middle of chron 5 ca 5 7 to be lost ca 5 3 ma conditions for the siphon formed when the continental climate created a deficit in the water budget of the paleo mediterranean sea the reversal took place when tectonic movement in the foredeeps of the betic rif orogene changed the thresholds of the twin straits the rifian corridor and the iberian portal inflow increased rapidly in the southern corridor to draw in waters from beneath the rising atlantic pycnocline while paleo mediterranean overflow water pmow continued out of the northern iberian portal the invasion of nappes or olistoliths first into the portal and then into the corridor led to the end of the outflow of the pmow terminating the need for the siphon and then to the isolation of the paleo mediterranean

correlation of late miocene to early pliocene sequences between the mediterranean and north atlantic ocean drilling program odp site 982 in the north atlantic contains a complete latest miocene to early pliocene section that was tuned to the astronomical timescale by correlating the record of gamma ray attenuation gra bulk density to summer insolation at 65 n and the benthic 18o signal to orbital obliquity for the interval from 4 6 to 7 5 ma the astronomical tuning of the site 982 record permits a direct bed to bed correlation to the cyclostratigraphy of messinian sections in the mediterranean krijgsman et al 1999a 2001 the benthic 18o signal at site 982 records a latest miocene glacial period that lasted from 6 26 to 5 50 ma and consisted of 18 glacial to interglacial oscillations that were controlled by the 41 kyr cycle of obliquity although the intensification of glaciation at 6 26 ma may have contributed to the restriction of the mediterranean it preceded the depositional onset of the lower evaporite unit at 5 96 ma by some 300 kyr the transition from stage tg12 to tg11 at 5 5 ma marks the end of the latest miocene glacial period and precedes the miocene pliocene boundary by 170 kyr although benthic 18o values are relatively low and 18o of bulk carbonate reaches a minimum at the miocene pliocene boundary at 5 33 ma there is no single event that would indicate deglaciation and sea level rise as the cause of the reflooding of the mediterranean we conclude that glacioeustatic changes alone were not responsible for either the start or end of evaporite deposition during the messinian suggesting that tectonic or local climate changes in the mediterranean region were the dominant cause s

the abad composite se spain a messinian reference section for the mediterranean and the apts a high resolution integrated stratigraphy is presented for the abad marls of the sorbas and nijar basins in se spain preevaporitic messinian of the western mediterranean detailed cyclostratigraphic and biostratigraphic analyses of partially overlapping subsections were needed to overcome stratigraphic problems in particular encountered at the complex transition from the lower to the upper abad the resulting abad composite section contains a continuous stratigraphic record from the tortonian messinian boundary up to the transition to the messinian evaporites of the yesares member all together 18 calcareous plankton events were recognized which were shown to be synchronous throughout the mediterranean by means of detailed bed to bed cyclostratigraphic correlations the magnetostratigraphy allowed the identification of the four magnetic reversals of chron c3an in the upper abad details in the sedimentary cycle patterns allowed the abad composite to be astronomically calibrated this calibration to the 658n summer insolation curve of solution la90 1 1 yielded astronomical ages for all sedimentary cycles calcareous plankton bioevents ash layers and paleomagnetic reversals up to now the abad composite is the only astronomically well calibrated section that provided a reliable cyclostratigraphy magnetostratigraphy and calcareous plankton biostratigraphy as such it will serve as a reference section both for the pre evaporite messinian in the mediterranean as well as for the messinian interval in the astronomical polarity time scale q 2001 elsevier science b v all rights reserved

benthic foraminifera based reconstruction of the first mediterranean atlantic exchange in the early pliocene gulf of cadiz abstract upper miocene to lower pliocene sediment cores from integrated ocean drilling program iodp hole u1387c iodp expedition 339 have been studied the main goal of this study is to reconstruct initial mediterranean atlantic water exchange after the opening of the gibraltar strait in the early pliocene benthic foraminiferal assemblages are the focus of the palaeoceanographic analyses of this work a distinct faunal turnover indicates a considerable change of the depositional environment at the miocene pliocene boundary foraminiferal composition suggests high rates of organic carbon flux and poorly oxygenated sea floor environment for the basal late messinian interval in contrast earliest pliocene assemblages point to periodic advection of warm better ventilated and likely mediterranean sourced waters to the studied site parallel periodic changes in siphonina tubulosa globocassidulina subglobosa and planulina ariminensis and previously established sedimentological and geochemical records zr al 18o suggest a rather sluggish mediterranean outflow between 5 3 and 5 2 ma benthic foraminifera along with sedimentological and geochemical data thus provide the earliest indications of mediterranean atlantic water exchange following the opening of the gibraltar strait sandy layers become frequent towards the top of the interval these sediments are rich in shelf foraminifera reflecting episodes of turbidite deposition possibly caused by tectonic adjustments related to the opening of the gibraltar strait the allochthonous assemblages frequently contain cibicides lobatulus and c refulgens species that have been considered indicators of mediterranean outflow water mow in previous studies our results warrant for caution when applying these species as mow proxy in the fossil record if there is evidence for downslope transport

a family of frontal cyclones over the western atlantic ocean part ii parameter studies in this study a series of sensitivity experiments is performed to study the relative influence of latent heating surface friction and surface heat fluxes on the development of a family of frontal cyclones that occurred over the western atlantic ocean using the simulation presented in part i as a control run it is shown that dry dynamics determines the initiation and track of all the frontal cyclones and it accounts for about 59 of the deepening of a major frontal cyclone vorticity budget calculations reveal that in the absence of latent heating preexisting upper level cyclonic vorticity associated with a ring of potential vorticity provides the necessary forcing for the initiation and movement of the frontal cyclones whereas the low level thermal advection is responsible for a large portion of their amplifications as well as for their shallow circulations the impact of surface sensible and latent heat fluxes on the frontal cyclogenesis depends on the cyclones location with respect to the warm water surface in the absence of latent heating the surface fluxes have very weak impact through modifying the low level baroclinicity on the evolution and final intensity of the frontal cyclones when latent heating is allowed however the surface fluxes could result in more rapid cyclogenesis as a result of reduced static stability and increased moisture content in the maritime boundary layer the impact is as pronounced as the latent heating it is found that dry frontal cyclogenesis could still occur over a vast continental surface although it is the slowest moving and deepening system among all the sensitivity tests being conducted the results reveal that i the frontal cyclones in the present case are baroclinically driven in nature although they are markedly modulated by diabatic heating and surface fluxes and ii the rapid frontal cyclogenesis phenomena tend to occur more frequently over a warm ocean surface due to its associated weak surface friction and its generated weak static stability in the maritime boundary layer

processes leading to eclogitization densification of subducted and tectonically buried crust 1 the transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling however a complex of partially eclogitized granulites on the island of holsnoy in the western part of the norwegian caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust the rocks on holsnoy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex metamorphism then occurred in a spatially heterogeneous manner with several distinct physical processes involving feedbacks between deformation fluid infiltration and chemical reactions operating simultaneously in different parts of the rock mass field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks a reservoir flux systems model is used to represent the various processes contributing to eclogitization of the granulites which operated at vastly different rates over disparate intervals of time the model suggests that the metamorphic process may have been brief 1 myr and ultimately self limiting arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite

new insights into upper mow variability over the last 150kyr from iodp 339 site u1386 in the gulf of cadiz the upper mediterranean outflow water mow paleo oceanographic history in the gulf of cadiz is poorly constrained due to the lack of high resolution records that pre date the last glaciation existing proxy records concentrate on mow variability along the middle slope of the gulf of cadiz here we present a continuous high resolution benthic foraminifera 18o record from the upper mow core at iodp expedition 339 site u1386 in the gulf of cadiz of the past 150 000 years based on 18o grain size and zr al variability comparison of our results with existing mediterranean sea md01 2472 md95 2043 open ocean lr04 and gulf of cadiz md99 2339 records we have gathered new insights into the evolution of the upper mow core on glacial interglacial timescales the influence of the upper mow at site u1386 was strongest during mis 5 and mis 1 similar 18o variability can be seen in the levantine intermediate water liw originating from the levantine basin of the eastern mediterranean sea we found clear indication for a vertical shift of the mow from the upper to the middle slope of the gulf of cadiz during sea level lowstands coinciding with mis 4 and mis 2 but also during mis 3 additionally our results indicate an increased upper mow flow correlated with heinrich events 7 to 10 and the younger dryas and also inversely relate to precession forced monsoonal freshwater inputs into the eastern mediterranean in the context of sapropel formation we could not find conclusive evidence of the proposed mow shutdown in our data

sonic and gamma ray astrochronology cycle to cycle calibration of atlantic climatic records to mediterranean sapropels and astronomical oscillations a high resolution analysis of cyclical patterns in gamma ray and sonic logs of the lower pliocene sediments from the gulf of cadiz southern spain was carried out results suggest that these cycles can be successfully used for a bed to bed correlation with the global reference section for the astronomical time scale in capo rossello sicily cycle patterns were used to tune the logs to the astronomical solutions of j laskar the gamma ray and sonic cyclicity was interpreted in terms of precession induced oscillations of annual rainfall in southern spain that were in part responsible for the increasing input of freshwater into the mediterranean at times of sapropel formation we demonstrate that gamma ray and sonic astrochronology can be successfully used for interregional correlation and astronomical dating with errors not higher than 20 k y

magnetostratigraphic biostratigraphic and stable isotope stratigraphy of an upper miocene drill core from the sale briqueterie northwestern morocco a high resolution chronology for the messinian stage we report a high resolution stable isotope carbonate magnetostratigraphic and biostratigraphic record from a 175 m drill core from the sale briqueterie which is part of the bou regreg section in northwestern morocco the sale drill core spans the interval from paleomagnetic chron c4n partim to c3r earliest gilbert which represents the time leading up to and including the isolation and desiccation of the mediterranean i e the messinian salinity crisis during chrons c3an and c3ar 6 935 to 5 894 ma the isotope and carbonate signals display quasi periodic variations with estimated periods of 40 and 100 kyr respectively we interpret the 40 kyr 18o variations as reflecting changes in global ice volume caused by obliquity induced changes 41 kyr in solar insolation in polar regions the 100 kyr carbonate variations probably represent long term modulation of the amplitude of the precessional cycle 21 kyr which is not resolved by our sampling frequency the cyclic nature of the oxygen isotope signal permits us to extend the isotope nomenclature of shackleton et al 1994a from stage tg24 in chron c3r earliest gilbert to stage c3ar 18o 18 at the base of chron c3ar 6 935 ma a major change in paleoceanographic conditions is recorded across the tortonian messinian boundary which we correlate to chron c3bn at 7 04 ma benthic foraminiferal 18o values increased by an average of 0 4 in two steps at 7 17 ma and 6 8 ma and 13c values decreased by 0 7 0 8 between 7 1 and 6 8 ma representing the late miocene carbon shift the first step in 18o values coincides with an inferred reversal in deep water circulation through the rifian corridor and the second correlates with the base of the tripoli formation and onset of crisis conditions in the mediterranean we suggest that the increase in 18o values represents at least in part an increase in global ice volume that lowered sea level and contributed to the establishment of a negative water budget in the mediterranean average 18o values remained high throughout most of chrons c3ar and c3an reaching maximum 18o values during isotope stages tg20 and 22 in chron c3r earliest gilbert the glacio eustatic falls associated with these events may have resulted in the complete isolation of the mediterranean from the world ocean shackleton et al 1994a following stage tg12 in the sale record there exists a trend toward progressively lower 18o values that may represent a series of marine transgressions that eventually reflooded the mediterranean and ended the salinity crisis

messinian productivity changes in the northeastern atlantic and their relationship to the closure of the atlantic mediterranean gateway implications for neogene palaeoclimate and palaeoceanography the stable isotope composition of planktic and benthic foraminifera and the distribution of selected benthic foraminiferal species from a messinian record of the lower guadalquivir basin northeastern atlantic ocean show that regional productivity changes were linked to glacioeustatic fluctuations glacial periods were characterized by poorly ventilated bottom waters as a result of weak atlantic meridional overturning circulation amoc and by phases of high productivity related to intensified upwelling in contrast well ventilated bottom waters owing to strong amoc the presence of degraded organic matter in the upper slope and high input of degraded terrestrial organic matter derived from fluvial discharge to the outer shelf were recorded during interglacial periods before closure of the adjacent guadalhorce corridor at 6 18 ma which was the final active betic atlantic mediterranean gateway the study area was alternately influenced by well ventilated mediterranean outflow water mow and poorly ventilated atlantic upwelled water auw following closure of the corridor cessation of the mow reduced the amoc and promoted glacial conditions in the northern hemisphere resulting in the establishment of local upwelling cells

oxygen and hydrogen isotope signatures of northeast atlantic water masses abstract only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in ne atlantic water masses and data are especially sparse for intermediate and deep water masses the current study greatly expands this record with 527 18o values from 47 stations located throughout the mid to low latitude ne atlantic in addition d was analyzed in the 192 samples collected along the geotraces north atlantic transect ga03 ga03 e kn199 4 and the 115 iberia forams cruise samples from the western and southern iberian margin an intercomparison study between the two stable isotope measurement techniques cavity ring down laser spectroscopy and magnetic sector isotope ratio mass spectrometry used to analyze ga03 e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios the surface 0 100 m and central 100 500 m water isotope data show the typical evaporation related trend of increasing values equatorward with the exception for the zonal transect off cape blanc nw africa off cape blanc surface water isotope signatures are modified by the upwelling of fresher antarctic intermediate water aaiw that generally has isotopic values of 0 0 to 0 5 for 18o and 0 to 2 for d along the iberian margin the mediterranean outflow water mow is clearly distinguished by its high 18o 0 5 1 1 and d 3 6 values that can be traced into the open atlantic isotopic values in the ne atlantic deep water neadw are relatively low 18o 0 1 to 0 5 d 1 to 4 and show a broader range than observed previously in the northern and southern convection areas the neadw is best observed at ga03 e stations 5 and 7 in the central ne atlantic basin antarctic bottom water isotope values are relatively high indicating modification of the original antarctic source water along the flow path the reconstructed 18o salinity relationship for the complete data set has a slope of 0 51 i e slightly steeper than the 0 46 described previously by pierre et al 1994 j mar syst 5 2 159 170 for the tropical to subtropical northeast atlantic this slope decreases to 0 46 for the subtropical north atlantic central water nacw and the mow and to 0 32 for the surface waters of the upper 50 m the d salinity mixing lines have estimated slopes of 3 01 for the complete data 1 26 for the mow 3 47 for the nacw and 2 63 for the surface waters the slopes of the 18o d relationship are significantly lower than the one for the global meteoric water line with 5 6 for the complete data set 2 30 for the mow 4 79 for the nacw and 3 99 for the surface waters the lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

contourite drift molded by deep mediterranean outflow since its inception in early pliocene time the deep mediterranean outflow has eroded and sculptured the southern iberian margin the 50 km long 300 m thick faro drift constructed by this powerful bottom current has been the subject of detailed study sandy silty and muddy contourite facies can be clearly characterized and a distinctive vertical arrangement or sequence of these facies can be related to hydrodynamic fluctuation these results are important for the identification of ancient contourites the reconstruction of paleocirculation patterns and documentation of the effects of quaternary climates on the mediterranean outflow

persistent monsoonal forcing of mediterranean outflow water dynamics during the late pleistocene the mode and vigor of the global oceanic circulation critically depend on the salinity of sub surface water masses advected to the loci of deep water formation within the atlantic meridional overturning circulation amoc an important supplier of high salinity waters is the mediterranean outflow water mow discharging into the north atlantic via the strait of gibraltar despite its importance for the north atlantic salinity budget the long term dynamics of mow production have remained poorly understood here we present high resolution records of bottom current velocity from three drill sites within the gulf of cadiz that document a persistent low latitude forcing of mow flow speed over the past 150 k y we demonstrate that the african monsoon is the predominant driver of orbital scale mow variability via its influence on the freshwater budget of the eastern mediterranean sea consequently mow formation fluctuates in concert with orbital precession overprinted by centennial scale oscillations of high latitude origin we further document that northern hemisphere summer insolation minima stimulate maximal injection of mow derived salt into the north atlantic likely strengthening the intermediate amoc branch the direct coupling of mow dynamics to low latitude climate forcing represents a hitherto neglected process for propagating sub tropical climate signals into the high northern latitudes

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

a review of calcareous nannofossil astrobiochronology encompassing the past 25 million years this paper presents a review of astrobiochronological calibration of recent to late oligocene calcareous nannofossil datum events biohorizons included in the paper are those of the widely used standard nannofossil zonations of martini e 1971 standard tertiary and quaternary calcareous nannoplankton zonation in farinacci a ed proceedings of the second international conference on planktonic microfossils roma rome ed tecnosci vol 2 pp 739 785 and okada h and bukry d 1980 supplementary modification and introduction of code numbers to the low latitude coccolith biostratigraphy zonation bukry 1973 1975 marine micropaleontology 51 321 325 as well as supplementary biohorizons proposed in the literature the biohorizons have been selected on the basis of the unambiguous taxonomy of the index taxa and their biostratigraphic usefulness we emphasise the application of rigorous methodology in nannofossil studies which permits an evaluation of biohorizons in terms of reliability and calibrates their potential correlatability astrochronological age estimates rely on the geologic timescale developed by the ics in 2004 with some new calibrations included we provide an overview of the relative position of biohorizons versus the astronomically calibrated ages of magnetic reversals and reference isotope stratigraphies surprisingly there are still few high resolution quantitative biostratigraphic studies of astrochronologically tuned sections in spite of the central role of such studies in addressing fundamental problems such as the tempo and mode of plankton evolution

contourite processes associated with the mediterranean outflow water after its exit from the strait of gibraltar global and conceptual implications we characterize the eastern gulf of cadiz proximal to the strait of gibraltar using a multidisciplinary approach that combines oceanographic morphosedimentary and stratigraphic studies two terraces upper and lower were identified along the middle slope they are composed of several associated morphologic elements including two large erosive channels which allow us to determine a new and more detailed understanding of the mediterranean outflow water mow pathway and its deceleration upon exiting the strait of gibraltar there is evidence for along slope circulation and additional secondary circulation oblique to the main flow the present upper core of the mow flows along the upper terrace and the lower core flows along the lower terrace however the lower terrace shows larger and better defined erosive features on the seafloor than does the upper terrace we attribute this to a denser deeper and faster mow circulation that prevailed during past cold climates development of the present features started ca 3 8 3 9 ma but the present morphology was not established until the late pliocene early quaternary 3 2 to older than 2 0 ma when the mow was enhanced coeval with global cooling a sea level fall and an increase in thermohaline circulation we propose a direct link between the mow and the atlantic meridional overturning circulation and therefore between the mow and both the northern hemisphere and global climate our results have enabled a better understanding of a major overflow related to an oceanic gateway and are of broad interest to geologists climatologists oceanographers and petroleum geologists

no major deglaciation across the miocene pliocene boundary integrated stratigraphy and astronomical tuning of the loulja sections bou regreg area nw morocco 1 an integrated high resolution stratigraphy and orbital tuning is presented for the loulja sections located in the bou regreg area on the atlantic side of morocco the sections constitute the upward continuation of the upper messinian ain el beida section and contain a well exposed continuous record of the interval straddling the miocene pliocene m p boundary the older loulja a section which covers the interval from 5 59 to 5 12 ma reveals a dominantly precession controlled color cyclicity that allows for a straightforward orbital tuning of the boundary interval and for detailed cyclostratigraphic correlations to the mediterranean the high resolution and high quality benthic isotope record allows us to trace the dominantly obliquity controlled glacial history our results reveal that the m p boundary coincides with a minor partly precession related shift to lighter interglacial values in d18o this shift and hence the m p boundary may not correlate with isotope stage tg5 as previously thought but with an extra weak obliquity controlled cycle between tg7 and tg5 consequently the m p boundary and basal pliocene flooding of the mediterranean following the messinian salinity crisis are not associated with a major deglaciation and glacio eustatic sea level rise indicating that other factors such as tectonics must have played a fundamental role on the other hand the onset of the upper evaporites in the mediterranean marked by hyposaline conditions coincides with the major deglaciation step between marine isotope stage tg12 and tg11 suggesting that the associated sea level rise is at least partly responsible for the apparent onset of intermittently restricted marine conditions following the main desiccation phase finally the loulja a section would represent an excellent auxiliary boundary stratotype for the m p boundary as formally defined at the base of the trubi marls in the eraclea minoa section on sicily

evidence of early bottom water current flow after the messinian salinity crisis in the gulf of cadiz highlights stratigraphic framework over the miocene pliocene boundary at iodp site u1387 abrupt sedimentary changes over the miocene pliocene boundary clear hints for onset of mediterranean outflow after the messinian salinity crisis evidence of bottom water currents in contouritic sedimentation and elevated zr al quiet hemipelagic sediment deposition during the messinian in the gulf of cadiz abstract integrated ocean drilling program iodp expedition 339 cored multiple sites in the gulf of cadiz in order to study contourite deposition resulting from mediterranean outflow water mow one hole u1387c was cored to a depth of 865 6 meters below seafloor mbsf with the goal of recovering the latest miocene to pliocene transition in order to evaluate the history of mow immediately after the end of the messinian salinity crisis to understand this history an accurate age model for the succession is needed but is challenging to construct because the miocene pliocene boundary is not marked by a clear biostratigraphic event in the atlantic and coring gaps occur within the recovered stratigraphic record these limitations are overcome by combining a variety of chronostratigraphic datasets to construct an age model that fits the currently available age indicators and demonstrates that coring in hole u1387c did indeed recover the miocene pliocene boundary at around 826 mbsf this boundary is associated with a distinct and abrupt change in depositional environment during the latest messinian hemipelagic sediments exhibiting precession induced climate variability were deposited these are overlain by pliocene sediments deposited at a much higher sedimentation rate with much higher and more variable xrf scanning zr al ratios than the underlying sediment and that show evidence of winnowing particle sorting and increasing grain size which we interpret to be related to the increasing flow of mow pliocene sedimentary cyclicity is clearly visible in both the benthic 18o record and the zr al data and is probably also precessionally controlled two contouritic bigradational sandy beds are revealed above the third sedimentary cycle of the pliocene on the basis of these results we conclude that sedimentation associated with weak mediterranean atlantic exchange began in the gulf of cadiz virtually at or shortly after the miocene pliocene boundary

miocene to pleistocene osmium isotopic records of the mediterranean sediments in the late miocene the mediterranean sea experienced a salinity crisis and thick sequences of evaporites precipitated across the deep and marginal basins in this study we report os isotopic records from deep sea drilling project and ocean drilling project cores in the mediterranean the balearic sea site 372 the tyrrhenian sea site 654 the ionian basin site 374 and the florence rise sites 375 376 as well as integrated ocean drilling project site u1387 in gulf of cadiz north atlantic pliocene pleistocene sediments at all sites show 187os 188os values close to that of the coeval ocean water indicating that the mediterranean was connected to the north atlantic evaporitic sediments deposited during the latest miocene however have 187os 188os values significantly lower than coeval ocean water values the offset of the mediterranean evaporite 187os 188os is attributed to limited exchange with the north atlantic during the messinian salinity crisis the source of unradiogenic os is likely to be weathering of ultramafic rocks ophiolites cropping out in the mediterranean s drainage basins based on a box model we estimated the amount of unradiogenic os and the atlantic mediterranean exchange rate to explain this offset os isotopic ratios of the pre evaporite sediments in the western mediterranean are almost identical to that of the coeval ocean water in contrast equivalent sediments from the florence rise have significantly lower 187os 188os values the offset in the os isotopic ratio on the florence rise is attributed either to limited water exchange between eastern and western mediterranean or to local effects associated with exhumation of the troodos ophiolites cyprus

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

regional climate and glacial control on high resolution oxygen isotope records from ain el beida latest miocene northwest morocco a cyclostratigraphic analysis in the depth and time domain 1 high resolution benthic and planktonic stable oxygen isotope records are presented for the upper miocene ain el beida aeb section in northwestern morocco and reveal the clear imprint of the climatic precession obliquity and eccentricity cycles in the interval between 6 5 and 5 5 ma the 18o depth series were transformed into time series using astronomical tuning of the sedimentary color cycles to the 65 n summer insolation curve of the la93 solution with present day values for tidal dissipation and dynamical ellipticity spectral analysis and band pass filtering show that the ratios of the astronomically related spectral peaks are not consistent in the depth and time domain the inconsistencies are mainly due to variations in sedimentation rate that are positively correlated with precession amplitude i e modulated by eccentricity in addition they result from a long term trend in sedimentation rate a simple model of nonlinear response of the sedimentation rate to the eccentricity modulation of precession was used to simulate the observed shifts in the relative position of the spectral peaks in the depth domain cross spectral analysis was carried out on the 18o time series and a combined eccentricity tilt obliquity and precession etp curve the precession controlled 18o signal varies in phase with etp partly as a result of the tuning procedure while the obliquity related signal reveals a small lag of 2 3 kyr relative to obliquity this lag is slightly reduced at the time of the messinian salinity crisis msc and increases to 5 6 kyr if the tidal dissipation term in the astronomical solution is reduced to half its present day value the obliquity related lag is most likely associated with the slow buildup of ice caps whereas the precession signal is interpreted as to dominantly reflect regional climate changes comparable to those associated with mediterranean sapropels finally the benthic 18o record is compared with open ocean records and discussed in terms of an astronomically tuned oxygen isotope stratigraphic framework

recent benthic foraminiferal assemblages in deep high energy environments from the gulf of cadiz spain the benthic environment in the gulf of cadiz north eastern atlantic is strongly affected by the mediterranean outflow water undercurrent mow which flows northwards along the western iberian margin at 500 1500 m water depth foraminiferal census counts of living and dead assemblages from 27 surface samples ranging from 103 to 1917 m water depth and the examination of hard substrates reveal a close correlation of the fauna with the local hydrography and sediment facies four different faunal groups are separated by factor analysis of the living fauna assemblage 1 contains typical lower slope species and dominates samples from the lower mow core layer and in the north atlantic deep water below shelf edge foraminifera are common in assemblage 2a which shows the highest proportions in samples from 103 to 272 m assemblage 2b is dominated by upper slope species and suspension feeders that are frequent in the upper mow core layer and in distal settings between 396 and 901 m species from assemblage 3 prefer epibenthic habitats and are recorded with high proportions exclusively in the immediate flow paths of the upper mow between 496 and 881 m colonisation structures and species composition of epibenthic assemblages from the proximal facies largely differ from those in distal settings in general epibenthic foraminifers only use elevated substrates under the influence of near bottom flow under high current velocities epibenthic foraminifers prefer large and heavy objects they colonise high attachment levels where a maximum yield of advected food particles can be achieved in distal settings at lower flow velocities the elevation height does not exceed 20 mm above the surrounding sediment surface this level is related to a hydrologic transition layer with high concentrations of suspended particles the comparison of microhabitat preferences and faunal structure under high and low current velocities reveal that substrate stability may be a confining environmental variable for endobenthic and shallow epibenthic foraminifers the observations also indicate that the preferential settling height of epibenthic foraminifera is related to the highest lateral flux rates of food particles within reach from the sea floor a dynamic selection of elevated microhabitats is only used by 7 8 of all species recognised in the gulf of cadiz area

the messinian salinity crisis past and future of a great challenge for marine sciences forty years after the image of the mediterranean transformed into a giant salty lake was first conceived the fascinating history of the messinian salinity crisis msc still arouses great interest across a large and diverse scientific community early outcrop studies which identified severe palaeoenvironmental changes affecting the circum mediterranean at the end of the miocene were followed by investigations of the marine geology during the 1950s to 1970s these were fundamental to understanding the true scale and importance of the messinian event now after a long period of debate over several entrenched but largely untested hypotheses a unifying stratigraphic framework of msc events has been constructed this scenario is derived mainly from onshore data and observations but incorporates different perspectives for the offshore and provides hypotheses that can be tested by drilling the deep mediterranean basins the msc was an ecological crisis induced by a powerful combination of geodynamic and climatic drivers which had a great impact on the subsequent geological history of the mediterranean area and on the salinity of the global oceans these changed the mediterranean s connections with both the atlantic ocean and the freshwater paratethyan basins causing high amplitude fluctuations in the hydrology of the mediterranean the msc developed in three main stages each of them characterized by different palaeoenvironmental conditions during the first stage evaporites precipitated in shallow sub basins the msc peaked in the second stage when evaporite precipitation shifted to the deepest depocentres and the third stage was characterized by large scale environmental fluctuations in a mediterranean transformed into a brackish water lake the very high resolution timescale available for some late miocene intervals in the mediterranean makes it possible to consider environmental variability on extremely short time scales including in some places annual changes despite this fundamental questions remain some of which could be answered through new cores from the deepest mediterranean basins improvements in seismic imaging and drilling techniques over the last few decades make it possible to plan to core the entire basinal messinian succession for the first time the resulting data would allow us to decipher the causes of this extreme environmental change and its global scale consequences

evolution of the late miocene mediterranean atlantic gateways and their impact on regional and global environmental change marine gateways play a critical role in the exchange of water heat salt and nutrients between oceans and seas as a result changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate as well as having a profound impact on local environmental conditions mediterranean atlantic marine corridors that pre date the modern gibraltar strait closed during the late miocene and are now exposed on land in northern morocco and southern spain the restriction and closure of these miocene connections resulted in extreme salinity fluctuations in the mediterranean leading to the precipitation of thick evaporites this event is known as the messinian salinity crisis msc the evolution and closure of the mediterranean atlantic gateways are a critical control on the msc but at present the location geometry and age of these gateways are still highly controversial as is the impact of changing mediterranean outflow on northern hemisphere circulation here we present a comprehensive overview of the evolution of the late miocene gateways and the nature of mediterranean atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins we also consider the possible impact of evolving exchange on both the mediterranean and global climate and highlight the main enduring challenges for reconstructing past mediterranean atlantic exchange

crustal structure of the indian shield new constraints from teleseismic receiver functions teleseismic earthquake waveform data from 10 broadband stations spread over the indian shield and in operation since 1997 were analyzed to infer the crustal structure using the receiver function technique the south indian shield is characterized by a 33 39 km thick and remarkably simple crust with an average poisson s ratio close to 0 25 the archaean crust is devoid of any prominent intra crustal discontinuities the velocity contrast at the well developed moho is large resulting in very clear p to s conversions as well as first order multiples in contrast the predominantly proterozoic crust in the northern part of the shield exhibits a complex character due to the presence of additional seismic discontinuities moho conversions which are considerably weaker compared to the archaean terrains indicate crustal thicknesses of more than 40 km

high resolution digital image treatment to color analysis on cores from iodp expedition 339 approaching lithologic features and bioturbational influence abstract color analysis was conducted on selected cores from sites u1385 and u1389 from iodp expedition 339 based on a recently developed high resolution digital image treatment to explore its relationship with lithologic features e g percentage of sand this first step can be of potential usefulness in characterizing discriminating between different types of registered sediments in the iodp expedition 339 turbidites contourites and associated sediments and then for more general studies involving different lithologies sedimentary parameters and ichnological variations differentiation between types of deposits holds particular interest in the context of the mediterranean outflow water mow evolution and related depositional processes whose study was one of the main aims of iodp expedition 339 color characterization was executed on high resolution digital images by applying a method based on adjustment modification including levels brightness and vibrance followed by pixel measurement the applied methodology allows for a precise characterization of colors improving upon previous color descriptions by means of munsell color conducted on board during expedition 339 itself the studied interval is subdivided into four parts based on grain size analysis and color according to pixel values these parts are identified as a 120 b 90 c 140 and d 160 being well correlated with the corresponding lithology parts a and c with similar pixel values correspond to calcareous silty mud mudstone part b would correlate with calcareous silty sand sandstones and part d corresponds to calcareous mud sand content distribution throughout the studied interval bears a close relationship with the differentiated parts a to d intervals having a higher percentage of sand present a lower mean pixel value and darker color whereas the finer grained intervals contain sediments lighter in color represented by higher pixel values however as proved the proposed relationship between color sedimentological features type of sediment must be considered with some caution ichnological analysis reveals the influence of trace fossils on color variations in the sediment regardless of the size of traces and amount of bioturbation the infilling material of trace fossils being the most determinant factor thus lower bioturbated surfaces 6 by small trace fossils chondrites show higher influence on color variations 102 vs 107 than higher bioturbated surfaces 25 by large trace fossils thalassinoides 110 vs 112 due to the color of the infilling material 56 in the case of chondrites and 90 in the case of thalassinoides thus a detailed ichnological analysis should accompany the sedimentological study of deep sea modern cores in order to avoid misinterpretations of the sedimentological parameters involved

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

magnetobiochronology of lower pliocene marine sediments from the lower guadalquivir basin insights into the tectonic evolution of the strait of gibraltar area the gibraltar arc is a complex tectonic region and several competing models have been proposed to explain its evolution we studied the sedimentary fill of the guadalquivir basin to identify tectonic processes that were occurring when the reopening of the strait of gibraltar led to the reestablishment of mediterranean outflow we present a chronostratigraphic framework for the lower pliocene sediments from the lower guadalquivir basin sw spain the updated chronology is based on magnetobiostratigraphic data from several boreholes our results show that the studied interval in the la matilla core is in the early pliocene section providing better constraints on the sedimentary evolution of the basin during that period migrating depositional facies led to a younger onset of sandy deposition basinward at the northwestern passive margin a 0 7 m y period of sedimentary bypass related to a sharp decrease in sedimentation rates and lower sea levels resulted from the tectonic uplift of the forebulge in contrast high sedimentation rates with continuous deep marine sedimentation are recorded at the basin center due to continuous tectonic subsidence and west southwestward progradation of axial depositional systems the marginal forebulge uplift continuous tectonic basinal subsidence and southward progradation of clinoforms in the early pliocene can be explained by the pull of a lithospheric slab beneath the gibraltar arc as the strait of gibraltar opened these findings are to our knowledge the first reported sedimentary expression of slab pull beneath the betics related to the opening of the strait of gibraltar after the messinian salinity crisis

astronomical forcing of northwest african climate and glacial history during the late messinian 6 5 5 5 ma abstract high resolution physical chemical and biological based climate proxies from the ain el beida aeb section in atlantic morocco reveal the imprint of the three main orbital parameters precession 19 23 kyr obliquity 41 kyr and eccentricity 95 125 and 400 kyr between 6 5 and 5 5 million years before present ma the precession related variations are most prominently reflected in the color reflectance and chemical composition of the sediment showing that precession minimum configurations lead to more humid climate conditions in northwest africa probably related to the atlantic system while more arid climate conditions prevailed during precession maxima in addition precession bound changes in planktonic foraminifera and calcareous nannofossil assemblages indicate that sea surface temperature sst increased during the humid phases while productivity conditions increased during the dry periods the clear imprint of the short and long term eccentricity cycles is explained by a non linear climatic response to the precession forcing with overall more humid climate conditions during eccentricity maxima the obliquity controlled variations in sst and aridity conditions concur with glacial interglacial variability that is conspicuously recorded in both planktonic and benthic 18 o records of aeb the observed warm and wet climates during interglacial periods most likely reflect the direct ice driven thermal response to the reduced ice sheets and more active atlantic depressions similar as during the pliocene and pleistocene obliquity controlled variations in the planktonic and benthic carbon isotope 13 c records of aeb are inversely related to the oxygen isotope records with more depleted values during glacial stages although with a small lag the close correspondence between the 13 c records of aeb and other open ocean sites i e sites 982 and 926 may therefore point to large scale glacial controlled variations in the deep sea carbon reservoir superimposed on the orbital variations the imprint of the onset of the messinian salinity crisis can be traced by a marked 1 0 negative excursion in planktonic 13 c at 6 0 ma followed by a prominent peak in ti al at 5 97 ma

erosional processes and paleo environmental changes in the western gulf of lions sw france during the messinian salinity crisis abstract current interpretation of the messinian salinity crisis msc involves partial desiccation of the mediterranean sea coupled with the deposition of thick evaporites in the deep basins new sets of seismic reflection profiles in the western part of the gulf of lions confirm the basinward extension of the messinian erosion and enable the mapping of distinctive seismic markers indicating the messinian erosional surface or messinian unconformity the basin margin detrital deposits and the deep evaporite sequence the geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the msc provide new information about the evolution of the study area during the messinian the messinian erosional surface mes commonly correlated with the desiccation phase and the deposition of deep evaporites during the apogee of the event is generally interpreted as a subaerial feature in the gulf of lions it is a complex diachronic polygenic erosional surface observed at the base of the prograding plio quaternary sequence beneath the shelf and slope it extends downslope beneath the deep basin upper evaporites and the salt and possibly correlates conformably with the base of the so called deep lower evaporites the whole morphology of the mes reflects a buried drainage pattern supporting the interpretation of fluvial erosion driven by a substantial drop in sea level our results also suggest that large submarine gravity flows occurred prior to any significant accumulation of salt in the basin and prior to the upper evaporites consequently interbedded clastic deposits may partly account for the parallel reflectors of the lower evaporites since river erosion persisted throughout the msc the salt and upper evaporite units may also contain a large amount of detrital sediments the good quality of the new seismic data clearly reveals fan shaped messinian deposits in the downstream part of the main messinian valleys i e the nile var and spanish rivers the depositional scenarios generally involve a substantial sea level fall coupled with deltaic prodeltaic accumulations a chaotic seismic unit unit d filling messinian lows and extending beneath the salt within the study area is interpreted as a messinian clastic unit we propose a polyphase scenario of detrital fan deposition involving pre syn and post salt deposition in subaqueous subaerial environments in the gulf of lions a late miocene tectonic phase that affected the western shelf also played an important role in controlling a the pattern of the messinian fluvial network b the location of maximum erosion on the shelf and c the location of the detrital fan depocentre downslope

metastability mechanical strength and the support of mountain belts exhumed high pressure rocks from the caledonian root zone in norway provide analogues for processes occurring today under southern tibet allowing large scale geophysical observations from gravity and earthquakes to be linked with the mechanical properties of metastable rocks under high mountains metastability is essential for the survival of thick mountain roots and hence of high mountains and is in turn controlled by water if water is absent dry granulite formed from earlier melting episodes is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years but if hydrous fluid is introduced the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength in norway this transformation was initiated by water infiltration along fractures formed by earthquakes the same process as marked by deep earthquakes may be occurring today beneath southern tibet

cyclostratigraphy and astrochronology of the tripoli diatomite formation pre evaporite messinian sicily italy the ongoing debate about the messinian salinity crisis in the mediterranean is fuelled in part by the lach of an adequate time control the most accurate and at the same time detailed constrains are nowadays provided by the astronomical dating technique here we present an astronomical age model for the cyclically bedded tripoli diatomite formation on scily pre evaporite messinian italy based on an intergrated stratigraphic study of three key sections falconara gibliscemi and capodarso characteristic sedimentary cycle pattenrs allow i the sections to be cyclostratigraphically correlated the bed to bed correlations being confirmed by high resolution planktonic foraminiferal biostratigraphy and ii the tripoli cycles to thecalibrated to the astronomical record despite minor misfits the correctness of thetuning is evident from the match between precession obliquity interference in the astronomical target and its reflection in the sedimentary cycle record the tuning provides absolute astronomically dated at 7 005 ma indicating that the onset of diatomite formation is diachronous in the mediterranean since itstarted 300 000 years earlier on sicilt than on gavdos south of crete the top of the tripoli and thus the onset of the salinity crisis proper on sicily arrives at 5 98 ma

a quantitative analysis of the desiccation and re filling of the mediterranean during the messinian salinity crisis abstract notwithstanding the great deal of attention that the messinian evaporites of the mediterranean region have received from an observational point of view there is to date no consensus as to their mechanism of formation we aim to contribute to the investigation through a quantitative analysis of the processes of desiccation and re filling these processes are thought to have played a role in particular during the deposition of the upper part of the evaporite sequence we calculate the evolution of sea level and average salinity based on both the present day geometry and a paleogeographic reconstruction and assess the sensitivity to variations in the freshwater budget our results support previous inferences that desiccation and re filling are fast desiccation occurs on a time scale of 3 8 kyr re filling probably even faster equilibrium sea levels imply that most water has gone from the western basin while a significant water column remains in the eastern basin whether or not the eastern basin reaches the level of halite saturation depends critically on in particular the freshwater budget the fast rate of desiccation and re filling imply that temporal differences in the onset of salt precipitation between western and eastern basin and between marginal basins and basin centres are below the resolution of astronomical dating also when atlantic sea level periodically varied from below to above the level of the intervening sill the mediterranean basin will have responded with repeated desiccation and re filling fast re filling is found to require only a small connection to the atlantic ocean this in combination with the previous results suggests the mediterranean is unlikely to attain stable intermediate water levels

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

mediterranean outflow pump an alternative mechanism for the lago mare and the end of the messinian salinity crisis the final stage of the messinian salinity crisis msc was characterized by brackish water lago mare conditions in the intermediate and marginal basins of the mediterranean sea the presence of paratethyan former black sea fauna in these deposits has fueled long lasting controversies over the connectivity between the mediterranean and paratethys and contemporary sea level drops in both basins here we use the results of sub precessional climate simulations to calculate the freshwater budget of the mediterranean and paratethys in the messinian we show that during the msc the freshwater budget of paratethys was positive while the mediterranean was negative using these numerical constraints we propose a mediterranean outflow pump as an alternative scenario for the two most dramatic hydrological changes in the msc 1 the halite lago mare transition and 2 the pliocene reestablishment of marine conditions following the maximum msc lowstand during halite formation progressive mediterranean sea level rise resulting from african river runoff and overspill from both the atlantic and paratethys eventually reached the level of the paratethys sill a density contrast at this gateway caused dense mediterranean waters to flow into the paratethys driving a compensatory return flow this pump mechanism significantly enhanced paratethyan inflow to the mediterranean creating suitable conditions for the lago mare fauna to migrate and thrive when the mediterranean sea level finally reached the height of the gibraltar sill mediterranean outflow restarted there and enhanced exchange with the atlantic ocean during this reorganization of the circulation brackish and hypersaline waters were pumped out of the mediterranean and open marine conditions were reestablished without major flooding of the basin at the miocene pliocene boundary

quaternary chronostratigraphic framework and sedimentary processes for the gulf of cadiz and portuguese contourite depositional systems derived from natural gamma ray records abstract the contourite depositional systems cds in the gulf of cadiz and on the west iberian margin preserve a unique archive of mediterranean outflow water mow variability over the past 5 3 ma these cds have been recently drilled in several places during the iodp expedition 339 these drill sites now offer a new window to the internal pliocene and quaternary architecture of the cds in this study we use downhole and core gamma ray gr data acquired from 5 sites drilled in the cds along the middle slope and 1 site drilled in the deeper setting of the lower slope out of the mow path the gr data primarily tracks the clay content in the sediment and is the expression of sediment supply and for sites drilled in the cds of the bottom current processes both appear astronomically controlled as shown by spectral analysis performed on the gr data results also reveal that the gr log patterns correlate well across the sites over the last 1 4 my several gr horizons corresponding to drops in gr values were identified most of which fit with coarse grained deposits observed in cores and interpreted as contourite beds the gr horizons are interpreted as isochronous horizons providing a regional scale chronostratigraphic framework for the cds depositional records with an accuracy of 20 ky we further assess the spatial and temporal variability of the cds hiatuses at the regional scale

vertical density gradient in the eastern north atlantic during the last 30 000 years past changes in the density and momentum structure of oceanic circulation are an important aspect of changes in the atlantic meridional overturning circulation and consequently climate however very little is known about past changes in the vertical density structure of the ocean even very extensively studied systems such as the north atlantic here we exploit the physical controls on the settling depth of the dense mediterranean water plume derived from the strait of gibraltar to obtain the first robust observations based probabilistic reconstruction of the vertical density gradient in the eastern north atlantic during the last 30 000 years we find that this gradient was weakened by more than 50 relative to the present during the last glacial maximum and that changes in general are associated with reductions in amoc intensity however we find only a small change during heinrich event 1 relative to the last glacial maximum despite strong evidence that overturning was substantially altered this implies that millennial scale changes may not be reflected in vertical density structure of the ocean which may be limited to responses on an ocean overturning timescale or longer regardless our novel reconstruction of atlantic density structure can be used as the basis for a dynamical measure for validation of model based amoc reconstructions in addition our general approach is transferrable to other marginal sea outflow plumes to provide estimates of oceanic vertical density gradients in other locations

shallow marine and continental conglomerates derived from coral reef complexes after desiccation of a deep marine basin the tortonian messinian deposits of the fortuna basin se spain some miocene basins in se spain show an unusual arrangement of sedimentary facies associated with desiccated deep basins tortonian reefs and deltaic deposits are located around the basin margins but younger messinian evaporites are exposed in central areas these messinian sediments contain thick conglomerates rich in coral debris although located hundreds of metres below the tortonian reefs they show subaerial features including mud cracks vertebrate tracks and rain pits in the coral reef derived conglomerates there are aragonitic coral fragments with aragonitic fibrous cement preserved by early cementation with calcium sulphate the existence of these conglomerates within the messinian evaporites helps to identify normal salinity and desiccation stages of the 9messinian salinity crisis event9

estuarine lago mare fauna from the tertiary piedmont basin indicates episodic atlantic mediterranean exchange during the final stage of the mediterranean salinity crisis abstract the geochemical signature of fossils from lago mare deposits at moncucco torinese nw italy indicates a temporary marine ingression into the tertiary piedmont basin just before the miocene pliocene boundary isotope 18o 13c 87sr 86sr and trace element sr ca ba ca data from mollusk shells of multiple species and croaker otoliths indicate an upper estuarine environment with considerable seasonal variability in evaporation and freshwater input reconstructed salinities the presence of a paralic paleobiotope with thalassogenic water in the tertiary piedmont basin contradicts ideas of the mediterranean as a brackish lake fed by the paratethys during the latest stage of the mediterranean salinity crisis instead our data support previous notions of temporary atlantic mediterranean exchange during sea level highstands preceding the permanent re connection at the miocene pliocene transition the establishment of short lived and disjunct estuarine and lagoonal environments such as in the tertiary piedmont basin during marine ingressions also puts highly variable 87sr 86sr values and occurrences of shallow water benthic foraminifera in lago mare deposits of other mediterranean basins into a new perspective

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

deciphering bottom current velocity and paleoclimate signals from contourite deposits in the gulf of cadiz during the last 140 kyr an inorganic geochemical approach contourites in the gulf of cadiz gc preserve a unique archive of mediterranean outflow water mow variability over the past 5 3 ma in our study we investigate the potential of geochemical data obtained by xrf scanning to decipher bottom current processes and paleoclimatic evolution at two different sites drilled during iodp expedition 339 through contourites in the northern gc site u1387 which is bathed by the upper mow core and site u1389 located more proximal to the strait of gibraltar the lack of major downslope transport during the pleistocene makes both locations ideally suited for our study the results indicate that the zr al ratio representing the relative enrichment of heavy minerals zircon over less dense alumnosilicates under fast bottom current flow is the most useful indicator for a semiquantitative assessment of current velocity although most elements are biased by current related processes the bromine br record representing organic content preserves the most pristine climate signal rather independent of grain size changes hence br can be used for chronostratigraphy and site to site correlation in addition to stable isotope stratigraphy based on these findings we reconstructed mow variability for marine isotope stages mis 1 5 using the zr al ratio from site u1387 the results reveal abrupt millennial scale variations of mow strength during greenland stadials gs and interstadials gi with strong mow during gs and glacial terminations and a complex behavior during heinrich stadials millennial scale variability persisting during periods of poorly expressed gs gi cyclicities implies a strong internal oscillation of the mediterranean north atlantic climate system

sedimentary evolution of the le danois contourite drift systems southern bay of biscay ne atlantic a reconstruction of the atlantic mediterranean water circulation since the pliocene abstract the evolution of the le danois contourite depositional systems cds during the pliocene and quaternary was investigated based on high resolution seismic reflection data from old to young six seismic units u1 u6 bounded by major discontinuities h1 h6 were identified regarding variations of the bottom current circulation four evolution stages of the le danois cds were identified including onset 5 3 to 3 5 3 0 ma initial 3 5 3 0 to 2 5 2 1 ma intermediate 2 5 2 1 to 0 9 0 7 ma and drift growth 0 9 0 7 ma to present day stages the cds associated with the atlantic mediterranean water amw along the middle continental slope initiated at 3 5 3 ma and was widely built after the mid pleistocene transition mpt 0 9 07 ma at a shallower water depth a second cds associated with the eastern north atlantic central water enacw started to develop from the late quaternary 0 47 ma onwards in the amw related drift system the le danois drift was generated both under glacial and interglacial climatic oscilations repeated internal structures in unit 5 that consist of acoustically transparent lower parts moderate amplitude upper parts and high amplitude erosional surfaces at the top are compared with interglacial glacial cycles since the middle pleistocene to the present day these cyclic features suggest coarsening upward sequences of the le danois drift and processes related to enhanced amw during glacial stages the estimated sedimentation rate of the le danois cds reached a maximum during the mpt at least 27 cm ky and then decreased until present day 5 cm ky variations of sedimentary stacking patterns and processes of the le danois cds imply full domination of the intermediate water mass along the central atlantic and southwest european continental slopes from the late pliocene 3 5 3 0 ma onwards

mediterranean outflow strengthening during northern hemisphere coolings a salt source for the glacial atlantic abstract high resolution grain size and magnetic susceptibility records from the eastern gulf of cadiz site md99 2339 1170 m water depth reveal contourites formed by the mediterranean outflow mow during the last 47 kyr bp oscillations in the mow s intensity occurred in phase with greenland temperature variations with a stronger outflow during northern hemisphere coolings such as dansgaard oeschger stadials heinrich events and the younger dryas benthic 13c data implies the western mediterranean deep water as one of the main sources feeding the outflow current while differential changes in the properties of the mediterranean source and entrained north atlantic central water largely control the mow s strength detailed studies for heinrich events 1 4 and 5 show that the flow strength peaked only when subtropical surface waters prevailed in the eastern gulf of cadiz while incursions of icebergs and subpolar surface water were not favorable for the mow s intensification as the mow was strong when the atlantic s thermohaline circulation thc was weakened the heat and salt injected by the mow into the intermediate north atlantic waters might have preconditioned the thc to switch from the stadial to the interstadial mode

late miocene globorotaliid event stratigraphy and biogeography in the ne atlantic and mediterranean abstract the quantitative study of keeled and unkeeled globorotallids in several late miocene sites of the ne atlantic the north betic and south rifian gateways and the western mediterranean enabled us to recognize a sequence of planktic foraminiferal pf events useful for a high resolution correlation of the mediterranean with the global ocean for the time interval immediately prior to the salinity crisis pf event 1 6 7 ma end of subchron 7n1 is defined by the sudden disappearance or reduction ofgloborotalia menardii group i sinistral in most of the ne atlantic and mediterranean this is probably due to a southward migration of these forms to more subtropical latitudes pf event 2 6 56 ma subchron 6r2 is defined by the abundant occurrence ofgloborotalia menardii group ii dextral between pf events 1 and 2 the temperate waters of the ne atlantic ocean and mediterranean sea were almost completely lacking in keeled globorotaliids the most prominent change in the assemblages the replacement of thegloborotalia menardii group ii by thegloborotalia miotumida group pf event 3 6 4 ma subchron 6n may be correlated with the tortonian messinian boundary and is related to a widespread second southward migration of northern temperate faunas replacing the subtropical ones as the north atlantic latitudinal thermal gradients increased this event also coincides with the beginning of the mediterranean crisis recorded by isotope fluctuations in surface waters and immediately followed by the development of widespread anoxic conditions in the deep and intermediate waters during the deposition of the tripoli formation pf event 4 first abundant occurrence of dextral specimens in theneogloboquadrina acostaensis group approximately between 5 6 and 5 5 ma subchron 5r1 and pf event 5 reduction or disappearance of thegloborotalia miotumida group approximately between 5 6 and 5 5 ma subchron 5r1 pre date the lower evaporites and pf event 6 first abundant occurrence ofgloborotalia margaritae s s subchron 5n1 is contemporary to their deposition the coiling direction of unkeeled globorotaliids highly variable in the first part of the late tortonian became almost 100 dextral pf event a a short time after pf event 2 this group maintained this pattern until the late messinian with the exception of a short interval with a peak of sinistral forms just at the tortonian messinian boundary on comparing these events with the carbon isotope records of different sites of the atlantic ocean we recognized similar trends between the events maximum 13c values in planktic and benthic foraminifera coincide with pf event 1 they decrease between pf events 1 and 2 and increase again immediately after pf event 2 pf event 3 coincides with the beginning of the global late miocene carbon shift

paleoceanography of the atlantic mediterranean exchange overview and first quantitative assessment of climatic forcing 1 the mediterranean sea provides a major route for heat and freshwater loss from the north atlantic and thus is an important cause of the high density of atlantic waters in addition to the traditional view that loss of fresh water via the mediterranean enhances the general salinity of the north atlantic and the interior of the eastern north atlantic in particular it should be noted that mediterranean water outflowing at gibraltar is in fact cooler than compensating inflowing water the consequence is that the mediterranean is also a region of heat loss from the atlantic and contributes to its large scale cooling uniquely this system can be understood physically via the constraints placed on it by a single hydraulic structure the gibraltar exchange here we review the existing knowledge about the physical structure of the gibraltar exchange today and the evidential basis for arguments that it has been different in the past using a series of quantitative experiments we then test prevailing concepts regarding the potential causes of these past changes we find that 1 changes in the vertical position of the plume of mediterranean water in the atlantic are controlled by the vertical density structure of the atlantic 2 a prominent early holocene contourite gap within the gulf of cadiz is a response to reduced buoyancy loss in the eastern mediterranean during the time of sapropel 1 deposition 3 changes in buoyancy loss from the mediterranean during mis3 caused changes in the bottom velocity field in the gulf of cadiz but we note that the likely cause is reduced freshwater loss and not enhanced heat loss and 4 strong exchange at gibraltar during atlantic freshening phases implies that the gibraltar exchange provides a strong negative feedback to reduced atlantic meridional overturning given the very counterintuitive way in which the strait of gibraltar system behaves we recommend that without quantitative supporting work qualitative interpretations of how the system has responded to past external forcing are unlikely to be robust

gravity anomalies crustal structure and thermo mechanical support of the himalaya of central nepal summary we use two gravity profiles that we measured across central nepal in conjunction with existing data to constrain the mechanical behaviour and the petrological structure of the lithosphere in the himalayan collision zone the data show 1 overcompensation of the foreland and undercompensation of the higher himalaya as expected from the flexural support of the range 2 a steep gravity gradient of the order of 1 3 mgal km x1 beneath the higher himalaya suggesting a locally steeper moho and 3 a 10 km wide hinge in southern tibet we compare these data with a 2 d mechanical model in which the indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland the model assumes brittle coulomb failure and nonlinear ductile flow that depends on local temperature which is computed from a steadystate thermal model the computed moho fits seismological constraints and is consistent with the main trends in the observed bouguer anomaly it predicts an equivalent elastic thickness of 40 50 km in the foreland the flexural rigidity decreases northwards due to thermal and flexural weakening resulting in a steeper moho dip beneath the high range residuals at short wavelengths over distances of 20 30 km are interpreted in terms of 1 sediment compaction in the foreland dr 150 kg m x3 between the lower and middle siwaliks 2 the contact between the tertiary molasse and the meta sediments of the lesser himalaya at the mbt dr 220 kg m x3 and 3 the palung granite intrusion in the lesser himalaya dr 80 kg m x3 finally if petrological transformations expected from the local p t are assumed a gravity signature of the order of 250 mgal is predicted north of the lesser himalaya essentially due to eclogitization of the lower crust which is inconsistent with the gravity data we conclude that eclogitization of the indian crust does not take place as expected from a steady state local equilibrium assumption we show however that eclogitization might actually occur beneath southern tibet where it could explain the hinge observed in the gravity data we suspect that these eclogites are subducted with the indian lithosphere

glacial to interglacial changes in the settling depth of the mediterranean outflow plume we present micropalaeontological and grain size records for a set of sediment cores from the gulf of cadiz southwest spain that reflect changes in the position and strength of the mediterranean outflow mo current the cores sample a sediment drift the gil eanes drift that is positioned lower on the slope in the gulf of cadiz than the position of the main current today the data indicate that the drift is of glacial age and that the glacial mo current was positioned lower on the slope than today but also that it was active over a considerably reduced area of the slope we argue that this observation is consistent with physical constraints on the gibraltar exchange and on the likely settling and spreading behavior of the mo plume along the iberian margin under glacial environmental and sea level conditions the deeper settling of the mo is likely to have influenced the formation of glacial north atlantic intermediate water and also may have exerted indirect influence on the formation of glacial north atlantic deep water

seismic structure and evidence for eclogitization during the himalayan convergence abstract reflections on the deep moho of the tethyan himalayas have been obtained from vertical seismics at the time predicted by wide angle reflection interpretations supporting the previous inference of a low average crustal velocity a mostly felsic composition is then required through the 70 km of the thickened crust the moho appears as a heterogenous transition layer more than 10 km thick the high velocity measured beneath the moho from pn refractions cannot be attributed any more to e w oriented olivine since sks observations do not confirm such an anisotropy instead eclogite just under the seismic moho could account for the observations these three results from independent parts of the seismic data set support a model of the himalayan convergence in which the indian crust has undergone phase transformation in the granulite eclogite system upon burial the intermediate and basic composition rocks of the middle and lower layers of an initial crust of average platform type can be transformed to such high velocities that they are lost across the himalayas into the level of the moho or just under the moho defined by seismic velocity mostly upper crustal type material then remains to form the thickened crust above consistent with the observed low velocity the variation in moho depth across the himalayas as suggested from wide angle seismics accounts for the first order variation in teleseismic transit times variations in the velocity depth structure of the mantle hence do not dominate but a change in the orientation of sks splitting anisotropy would suggest deformation within the lithosphere to change across the high himalayas and the indus tsangpo suture

geochemistry of surface sediments from the fjords of northern chilean patagonia 44 47 s spatial variability and implications for paleoclimate reconstructions abstract the patagonian fjords have a clear potential to provide high resolution sedimentary and geochemical records of past climate and environmental change in the southern andes to improve our ability to interpret these proxy records we investigated the processes that control fjord sediment inorganic geochemistry through a geochemical mineralogical and sedimentological analysis of surface sediment samples from the fjords of northern chilean patagonia a simple terrestrial index based on measurements of salinity and fraction of terrestrial carbon was used to estimate the terrestrial input river discharge at each site our results demonstrate that under the cold climate conditions of patagonia chemical weathering is weak and the inorganic geochemical composition of the fjord sediments is primarily controlled by hydrodynamic mineralogical sorting i e the intensity of river discharge our results suggest that the distribution of fe ti and zr in surface sediments is controlled by their association with heavy and or coarse minerals whereas al is independent of hydrodynamic processes the elemental ratios fe al ti al and zr al are therefore well suited for estimating changes in the energy of terrestrial sediment supply into the fjords through time zr al is particularly sensitive in proximal environments while fe al is most useful in the outer fjords and on the continental margin in the most proximal environments however fe al is inversely related to hydrodynamic conditions caution should therefore be exercised when interpreting fe al ratios in terms of past river discharge the application of these proxies to long sediment cores from quitralco fjord and golfo elefantes validates our interpretations our results also emphasize the need to measure al based elemental ratios at high precision which can be achieved using simultaneous acquisition icp aes technology this study therefore constitutes a strong basis for the interpretation of sedimentary records from the chilean fjords

surface water dynamics and phytoplankton communities during deposition of cyclic late messinian sapropel sequences in the western mediterranean abstract distinctive precession scale sapropel marl sequences of late messinian age are found in several outcrops bordering the western mediterranean to examine the roles of stratification and productivity in driving this cyclic sedimentation we performed a high resolution analysis of calcareous nannofossil assemblages through four orbital sequences composed of sapropel marl diatomite marl from the sorbas basin in southern spain this effort also involved a detailed comparison between calcareous nannofossil abundances and other micropaleontological sedimentological and geochemical proxies to further understand the behaviour of some taxa for this period fluctuations in the depth and gradient of the pycnocline and the distribution and remobilization of nutrients seem to be the main factors controlling the calcareous nannoplankton assemblages with temperature of secondary importance in the sapropels the warm oligotrophic genus sphenolithus and discoaster are abundant especially significant is the progressive increase towards the top of discoaster pentaradiatus indicator of severe oligotrophic deep pycnocline conditions at the top of the sapropels peaks in reticulofenestra rotaria are interpreted here as the result of an increase of surface water temperatures although decreasing salinity may also play a role small reticulofenestra 5 m are more frequent in diatomites even considering conditions of extreme mixing and eutrophication occurred in the basin umbilicosphaera jafari peaks just after diatomites replacing small reticulofenestra probably as a response to an increase in salinity and or nutrient limitation species such as coccolithus pelagicus and helicosphaera carteri are abundant at the top of the diatomites and in the overlying marls indicating relatively mesotrophic and cool water conditions this succession shows similar characteristics in the four studied cycles allowing us to interpret surface water dynamics in this region on orbital time scales the top of the sapropel represents a warm low salinity and well stratified episode under a situation of maximum insolation a well developed pycnocline and oligotrophic conditions moderate production of calcareous phytoplankton coinciding with anoxic conditions at the bottom enhance the preservation of organic matter and consequently generate a sapropel diatomites are deposited under a situation of unstable pycnocline that produce an increase in productivity in surface waters diatoms significantly deplete limiting nutrients and consequently calcareous nannofossils start to be dominant again in the water column as result of a progressive stabilization of the pycnocline

a pliocene mixed contourite turbidite system offshore the algarve margin gulf of cadiz seismic response margin evolution and reservoir implications abstract a buried mixed contourite turbidite system has been identified in the pliocene succession of the algarve basin in the northern gulf of cadiz this margin is currently dominated by the mediterranean outflow water and associated contourite deposition analysis of seismic data along with careful geographical and oceanographic reconstructions of the margin show the gradual move from a turbidite dominated towards a contourite dominated margin and the subsequent birth of an impressive elongate mounded contourite drift system the faro albufeira drifts the contourite drift can be distinguished from down slope turbiditic and mass transport deposits based on the acoustic character distribution analysis and through careful margin reconstruction in the earliest pliocene seismic unit p1 has been interpreted as a dominant down slope most likely turbidite system sourced mainly from the northeast there is clear evidence of contourite reworking at seismic unit p2 where upslope progradation and a sheeted morphology are observed high amplitude reflections are thought to be a result of more vigorous bottom currents in the early upper pliocene that were capable to redistributing coarser sediments however in the northeast of the study area a thick sequence of chaotic seismic facies has been interpreted as mass transport deposits sourced from the north indicate that the bottom currents were unable to dominate over the entire margin due to high down slope clastic influx semi transparent seismic unit p3 indicates that the upper pliocene initially experienced a reduction in bottom current intensity however upslope progradation shows that a mixed system was maintained above the base quaternary discontinuity ca 2 6 ma highly erosive discontinuities and high amplitude seismic reflections are evidence of pronounced intensification of the mediterranean outflow water and a move to a fully contourite dominated slope mixed turbidite contourite systems such as the one identified in the algarve basin could provide impressive petroleum potential where downslope clastics are winnowed and reworked by bottom water currents to leave good reservoir properties here we present a conceptual model for sheeted drifts as hydrocarbon reservoirs in the subsurface

the impact of the mediterranean outflow water mow on benthic foraminiferal assemblages and surface sediments at the southern portuguese continental margin benthic foraminifera and surface sediments were analyzed from 23 box core and core top samples from 250 to 3600 m depth at the southern portuguese continental margin between 37 and 38 n this area is affected by the mow which flows northwards along the slope between 600 and 1500 m waterdepth sediment structures grain size distribution and clay mineral assemblages of surface sediments indicate active winnowing in the upper part of the mow and deposition of the fine fraction near its lower boundary r mode cluster analyses of foraminiferal census data from the 250 m fraction revealed four associations the shelf edge association occurs on glauconitic shelf and upper slope sands down to 268 m the upper slope association is recognized between 498 and 1300 m where sand silt clays to clayey silts are encountered the lower slope association is found between 1405 and 2985 m and differs markedly from assemblages below 3000 m deep water association where the near surface sediments are soft hemipelagic clays the boundary layer association shows no significant depth limits but abundance maxima both in the density interface above the mow and in the interval of decreasing turbulence below it the boundaries of the shelf edge and upper slope association correspond to these hydrographic boundary layers whereas the boundary between lower slope and deep water assemblage is related to threshold values of nutrient flux to the benthic community examination of hard substrates reveals 27 different species of epibenthic foraminifera six of these the epibenthos group were found attached to elevated substrates only within the mow above the mow elevated substrates were inhabited by different species whilst below the mow they were not used by any epibenthic foraminifers evidently the hydrodynamic environment of the mow current provides an ecological niche which is efficiently used by opportunistic suspension feeders the epibenthos group is more abundant in the middle part of the study area between 37 20 and 37 40 n where the sand content of surface sediments is also higher both indicate an enhanced sediment water interaction in this area which results from an obstruction of the mow flow by the principes d avis basement spur

late neogene evolution of the taza guercif basin rifian corridor morocco and implications for the messinian salinity crisis abstract magnetostratigraphic and biostratigraphic results are presented from neogene deposits in the taza guercif basin located at the southern margin of the rifian corridor in morocco this corridor was the main marine passageway which connected the mediterranean with the atlantic during messinian times correlation of the biostratigraphy and polarity sequence of the taza guercif composite section to the astronomical time scale allows an accurate dating of three subsequent events in the rifian corridor 1 the oldest marine sediments marking the opening of the rifian corridor were deposited at 8 ma at this age a deep 600 m marine basin developed in the taza guercif area marked by deposition of precession controlled turbidite marl cycles 2 paleodepth reconstructions indicate that a rapid 5 m ka shallowing of the marine corridor took place at the tortonian messinian boundary at an age of 7 2 ma this shallowing phase is primarily related to active tectonics although a small glacio eustatic sea level lowering also took place 3 the taza guercif basin was emergent at an age of 6 0 ma and subsequently continental sedimentation continued well into the early pliocene we suggest that shallowing and restricting the marine passageway through the rifian corridor actually initiated the messinian salinity crisis well before the deposition of the messinian evaporites in the mediterranean

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years