instability waves in the equatorial atlantic ocean abstract evidence is presented for the generation of planetary waves by barotropic instability within the cyclonic shear region of the atlantic ocean s south equatorial current sec immediately following the springtime intensification of the southeast trade wind which accelerates the sec westward a packet of waves with central periodicity of around 25 days is observed lasting for about three cycles independent wavenumber analyses on 1983 and 1984 data give newly identical zonal wavelengths and phase speed estimates of around 1100 km and 50 cm s 1 the waves are anisotropic and spatially inhomogeneous with generation confined primarily to the mixed layer an energetics analysis using 1983 data centered upon the equator at 28 w shows a rapid increase in total perturbation energy tpe reaching values of 2000 erg cm 3 within two weeks the subsequent decrease in tpe at this location is due primarily to meridional pressure work divergence baroclinic instability is negligible because both the meridional

glacial interglacial variablity in the benguela upwelling system spatial distribution and budgets of organic carbon accumulation 1 modern sedimentary total organic carbon toc content as a proxy for surface water export production was mapped on the shelf and on the upper continental slope of the benguela upwelling system using 137 core tops shelf maxima in toc can be correlated with maxima in surface water productivity on the slope high toc contents are observed offshore from sites of strong modern upwelling estimates of modern toc mass accumulation rates mar show that approximately 85 of the total is accumulating on the shelf toc mar were calculated mapped and budgeted for the holocene and for the last glacial maximum lgm using 19 sediment cores from the continental slope during the lgm centers of deposition and production have migrated offshore with respect to their holocene positions toc accumulation on the continental slope was approximately 84 higher during the lgm than during the holocene possibly reflecting enhanced productivity the toc distribution patterns and sediment echo sounding data suggest that undercurrents strongly influence the sedimentation off namibia winnowing and focusing result in great lateral heterogeneity of sedimentation rates and sediment properties individual cores therefore do not necessarily reflect general changes in export production these results highlight the need for detailed regional studies based on a large number of sediment cores for highly heterogeneous high productivity areas in order to derive general statements on total fluxes

glacial valve size variation of the southern ocean diatom fragilariopsis kerguelensis preserved in the benguela upwelling system southeastern atlantic abstract the valve size variations of the southern ocean so diatom fragilariopsis kerguelensis from the sediments of antarctic polar front and subantarctic front were shown to be strongly correlated with iron availability at the glacial interglacial timescale however recent evidence from the subtropical region of the so has suggested sea surface temperature to be a key controlling factor by mediating silica uptake other than the iron availability we present here the first record of f kerguelensis size variation in core geob3606 1 collected in the benguela upwelling system bus se atlantic 25 28 s 13 05 e where f kerguelensis valves were preserved during the glacial period 68 30 ka as a consequence of silicic acid leakage from polar to mid latitudes our study presents the first record of diatom morphology variations of f kerguelensis from a mid latitude core location to test the previous hypotheses proposed for so our results demonstrate that f kerguelensis valves at geob3606 1 were almost half of the mean size of specimens compared to the so downcore records further f kerguelensis valves were smaller and more abundant during early marine isotope stage mis 3 and conversely larger and less abundant during mis4 and late mis3 however such patterns are not supported by the linear regression analyses suggesting that iron availability could have mediated the size changes of f kerguelensis rather than the species productivity during 68 30 ka these findings agree with the recently gained knowledge on the life cycle patterns of f kerguelensis related to reproduction strategies cell size range and formation of initial cells

average seasonal variation of the atlantic equatorial currents from historical ship drifts abstract surface currents in the tropical atlantic were studied using historical ship drift data these are the only available data capable of resolving the long term seasonal fluctuations of currents over a broad geographical region the north equatorial countercurrent was found to extend as a continuous eastward flowing current across the atlantic in the band 5 8 n from july to december the maximum average eastward velocity was 30 cm s 1 during july september east of 20 w the countercurrent continued throughout the year west of 20 w and from january to june the countercurrent disappeared and westward currents were observed average westward velocity reached 20 cm 1 in april and may centered near 40 w the annual cycle in surface velocity 20 cm s 1 amplitude agrees closely with that of geostrophic velocity inferred from variations in thermocline depth across the counter current jets centered near 2 n and 4 s separated by a minimum near 1 s the northern jet has a large 14 cm s 1 semiannual v

general features of the uppermost mantle stratification from long range seismic profiles abstract long range seismic studies made by russian institutions during the last two decades reached depths of 700 800 km on the continent peace nuclear explosions were used as a source and 100 km in the ocean ocean bottom stations recording 500 kg explosions in water comparison of the wave fields of these and other seismic profiles in russia western europe and the south atlantic shows that the classical lithosphere asthenosphere model is too simple to explain the observed data a stratified medium with alternating weak and rigid layers is a more realistic model of the upper mantle one of the layers is globally observed at a depth of 80 to 100 km it is located in the thermal lithosphere beneath old platforms at the bottom of the lithosphere under active tectonic areas and inside the asthenosphere below mid oceanic ridges relatively high velocities 8 4 to 8 5 km s are typical for this boundary both in the old platforms and in the high heat flow oceanic areas a change of structural pattern where the block structure of the uppermost mantle is transformed into a horizontally homogeneous one is typical at that boundary level such features suggest that the boundary separates the brittle and more ductile upper mantle layers and may be considered as the bottom of the global mechanical lithosphere

a corollary to the silicic acid leakage hypothesis 1 the silicic acid leakage hypothesis salh attempts to explain part of the large and regular atmospheric co2 changes over the last glacial interglacial cycles it calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low latitude surface waters driven by a leakage of high si n waters from the southern ocean recent studies that present low opal accumulation rates from the glacial eastern equatorial pacific have challenged salh in a corollary to salh we argue that the key to salh is the dominance of diatoms over coccolithophorids and this does not depend on the magnitude of diatom production per se in support of our claim we show in a numerical model that atmospheric co2 can be lowered with even a reduced absolute flux of silicic acid leakage provided that si n in the leakage is elevated and that the excess si can be used by diatoms to shift the floral composition in their favor

lateral variations of thermo mechanical properties in the tyrrhenian northern apennine region abstract we present lithospheric strength profiles calculated for a transect running from the island of corsica in the west to the apennine foredeep in the east strength profiles are constructed at locations selected on the basis of the availability of seismic heat flow gravity data and earthquake foci distribution rheological modelling points to the presence of strong lateral variations in the thermo mechanical state of the lithosphere the tyrrhenian tuscany area has a low rigidity in relation with the intense stretching active from the early miocene and with very high heat flow associated with asthenospheric rise on the contrary hinterland corsica and apenninic foreland areas adriatic area show a higher level of lithospheric strength effective elastic thickness estimates confirm this scenario exhibiting a strong reduction from the northern tyrrhenian to the inner apennines and then an abrupt increase in correspondence of the adriatic plate the distribution of earthquake focal depths confirms the presence of a brittle rheology in the first kilometres of the crust in the tuscany area whereas close to the plate boundary much lower thermal gradients partly induced by the westward plunging adriatic plate enable seismic activity to occur at larger depths

oligo miocene rift of sardinia and the early history of the western mediterranean basin the geodynamic evolution of the western mediterranean basin in spite of many studies is still uncertain there is some consensus for interpreting this basin as a kind of small oceanic marginal basin its opening has generally been related to a subduction process which was active during the oligocene miocene somewhere east of sardinia corsica1 7 as the margins of the basin are deeply buried below miocene to present sediments direct lithological and stratigraphical data which could explain the events responsible for its formation are rare8 10 or missing altogether to obtain such data detailed field studies have been undertaken in sardinia fig 1 and the first results are presented here this approach is justified by the fact that in that island oligocene and miocene sediments were deposited in a rift fossa tettonica sarda of verdabasso11 which is the easternmost arm of the complex rift system that affected the european plate during oligocene and miocene times one of these arms evolved towards a small oceanic basin the western mediterranean or algero provencal basin while others such as the gulf of valencia and the sardinia rift aborted and remained at the rift stage its exceptional exposures make it possible to examine the sardinia rift to clarify the sequence of events which created it and to establish a sedimentological model which we believe is directly applicable to the western mediterranean basin

fluid geochemistry of the sardinian rift campidano graben sardinia italy fault segmentation seismic quiescence of geochemically active faults and new constraints for selection of co2 storage sites abstract sardinia is typically seismically quiescent displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity this evidence may be in agreement with the presence of a ductile layer in the northern sector of the island as suggested by the he isotopic signature in fluids rising to the surface through quiescent fault systems the fault systems have been found to be segmented and therefore isolated in fluid circulation the study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as rn indoor or to define geological structures like potential co 2 storage sites both of these have been recently requested by the exploitation in italy of the euratom directive and the evolution of the kyotoprotocol policy four water dominated hydrothermal areas of sardinia located along regional fault systems were considered campidano graben tirso valley logudoro and casteldoria a fluid geochemical survey was carried out taking into account physical chemical and environmental parameters major elements within gaseous and liquid phases a few minor and trace elements selected isotope ratios 2 h 18 o 13 c 3 he 4 he 222 rn concentration and some dissolved gases two different fluids have been recognised as regards both water chemistry and dissolved gases i co 2 rich gases poor in he and rn with a relatively high 3 he 4 he ratio up to r r a 2 32 associated with na hco 3 cl thermal and cold groundwater ii gases rich in he and n 2 poor in co 2 and rn with a low 3 he 4 he ratio associated with alkaline thermal and cold waters the distribution of these two groups of fluids characterises the sardinian tectonic systems in fact gas fluxes are not homogeneous being mainly related to the different fault segments and to the areas where quaternary basalts crop out the underground geochemical evolution of the sardinian fluids as a function of the geological and tectonic systems provides some suggestions for solving one of the most important problems co 2 geological sequestration in order to reduce the co 2 excess produced by human activity the best geological disposal sites are reservoirs with low hydraulic conductivity sealed to fluid movement or aquifers characterised by maximum ph buffering capacity of their mineralogical matrix the knowledge of the role of faults as permeability barriers or as deep fluid uprising pathways is prerequisite

rapid palaeoceanographic changes in the benguela upwelling system for the last 160 000 years as indicated by abundances of planktonic foraminifera abstract two sediment cores retrieved from the continental slope in the benguela upwelling system geob 1706 19 33 7 s 11 10 5 e and geob 1711 23 18 9 s 12 22 6 e reveal striking variations in planktonic foraminiferal abundances during the last 160 000 years these fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off namibia four species make up over 95 of the variation within the core and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages the fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment the cold water planktonic foraminifer neogloboquadrina pachyderma sinistral n pachyderma s dominates the modern day coastal upwelling centres and neogloboquadrina pachyderma dextral and globigerina bulloides characterize the fringes of the upwelling cells globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional reduced nutrient levels of the subtropical waters in the fossil record episodes of high n pachyderma s abundances are interpreted as evidence of increased upwelling intensity and the associated increase in nutrients the n pachyderma s record suggests temporal shifts in the intensity of upwelling and corresponding trophic domains that do not follow the typical glacial interglacial pattern periods of high n pachyderma s abundance describe rapid discrete events dominating isotope stages 3 and 2 the timing of these events correlates to the temporal shifts of the angola benguela front jansen et al 1997 situated to the north of the walvis ridge absence of high abundances of n pachyderma s from the continental slope of the southern cape basin indicates that southern ocean surface water advection has not exerted a major influence on the benguela current system the coincidence of increased upwelling intensity with the movement of the angola benguela front can be interpreted mainly by changes in strength and zonality of the trade wind system

submillennial to millennial variability of diatom production off mauritania nw africa during the last glacial cycle 1 the low latitude upwelling regime off the mauritanian coast in the subtropical ne atlantic accounts for a significant part of global export production although productivity variations in coastal upwelling areas are usually attributed to changes in wind stress and upwelling intensity productivity dynamics off mauritania are less straightforward because of the complex atmospheric and hydrographic setting here we integrate micropaleontological diatoms and geochemical bulk biogenic sediment components x ray fluorescence and alkenones proxies to examine on submillennial to millennial changes in diatom production that occurred off mauritania nw africa for the last 25 ka during the last glacial maximum lgm 19 0 23 0 ka b p moderate silicate content of upwelled waters coupled with weakened ne trade winds determined moderate diatom productivity no significant cooling is observed during the lgm suggesting that our alkenone based sst reconstruction represents a local upwelling related signal rather than a global insolation related one extraordinary increases in diatom and opal concentrations during heinrich event 1 h1 15 5 18 0 ka b p and the younger dryas yd 13 5 11 5 ka b p are attributed to enhanced upwelling of silica rich waters and an enlarged upwelling filament due to more intense ne trade winds the synchronous increase of caco3 and k intensity and the decreased opal and diatoms values mark the occurrence of the bolling allerod ba 13 5 15 5 ka b p due to weakened eolian input and more humid conditions on land although the high export of diatoms is inextricably linked to upwelling intensity off mauritania variability in the nutrient content of the thermocline also plays a decisive role

heat flow heat production and fission track data from the hercynian basement around the provencal basin western mediterranean abstract in the complex structural framework of the western mediterranean hercynian areas are expected to be thermally preserved from the recent tectonic evolution the thermal regime of these areas is studied using heat flow heat production and fission track data the surface heat flow is significantly higher in corsica 76 10 mw m 2 than in the maures and esterel 58 2 mw m 2 neither heat production nor erosion subsequent to the alpine orogeny in corsica can explain such a difference it is suggested that a deep thermal source related to the asymmetric evolution of the provencal basin could explain the higher heat flow in corsica a model of thermal structure based on the present day thermal regime of the maures and esterei is proposed for the stable hercynian crust in this area the mantle heat flow is 20 25 mw m 2 and the temperature at moho level is 375 500 c depending on the thermal parameter distribution with depth

late paleozoic rotations of corsica and sardinia new evidence from paleomagnetic and k ar studies abstract a detailed paleomagnetic investigation of so called permian volcanics was undertaken in southeastern and northwestern sardinia k ar dating on separate minerals representing 30 sites on corsica and sardinia showed that the first calc alkaline cycle had taken place in westphalo stephanian times and the second alkaline one at the end of the stephanian 246 paleomagnetic specimens from 28 sites of southeastern sardinia and 2 sites from the northwest were demagnetized by alternating field and thermal processes the directions of characteristic magnetizations are d 85 i 6 95 5 for the volcanics of southeastern sardinia and d 126 i 1 for the nurra samples these mean directions differ from those of northwestern corsica and northern sardinia gallura the paleomagnetic results display good grouping for each volcanic complex but exhibit a geographical dispersion of the mean directions this scattering suggests significant motions in late hercynian times

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

a magnetovariational study in sardinia abstract a study of geomagnetic field time variations with a magnetometric array was performed in sardinia during the summer of 1988 nine fluxgate magnetometers with digital acquisition systems were employed to cover the main structural features of the island the recordings do not show significant variations in the amplitudes of the horizontal components of the bay like events whereas the vertical component amplitudes show a strong southward increase fourier maps were drawn at significant periods and induction arrows and z h pseudosections were derived

a two stage rifting in the basins of the corsica sardinian straits abstract a set of seismic and field data from the corsica sardinian straits allowed us to define three well separated oligocene deep basins the castelsardo and porto torres basins forming the northern end of the sardinian graben and the bonifacio basin on the continental shelf of the bonifacio straits a seismostratigraphic investigation of these three basins revealed two major tectonic events a first oligocene to middle aquitanian rifting phase followed by oceanic spreading in the western mediterranean and then a second rifting event between corsica and sardinia in lower burdigalian time the first rifting is noticed in the three basins but the second one only in the porto torres and castelsardo basins

lateral transport controls distribution quality and burial of organic matter along continental slopes in high productivity areas in this study we demonstrate the relevance of lateral particle transport in nepheloid layers for organic carbon oc accumulation and burial across high productive continental margins we present geochemical data from surface sediments and suspended particles in the bottom nepheloid layer bnl from the most productive coastal upwelling area of the modern ocean the benguela upwelling system offshore southwest africa interpretation of depositional patterns and comparison of downslope trends in oc content organic matter composition and 14 c age between suspended particles and surface sediments indicate that lateral particle transport is the primary mechanism controlling supply and burial of oc we propose that effective seaward particle transport primarily along the bnl is a key process that promotes and maintains local high sedimentation rates ultimately causing high preservation of oc in a depocenter on the upper slope offshore namibia as lateral transport efficiently displaces areas of enhanced oc burial from maximum production at highly productive continental margins vertical particle flux models do not sufficiently explain the relationship between primary production and shallow marine oc burial on geologic time scales the widest distribution and strongest intensity of lateral particle transport is expected during periods of rapid sea level change at times in the geologic past widespread downslope lateral transport of oc thus may have been a primary driver of enhanced oc burial at deeper continental slopes and abyssal basins

cyclonic eddies formed at the pacific tropical instability wave fronts 1 sea surface temperature images and surface drifter observations are compared to the results from a high resolution numerical simulation to study the properties of cyclonic eddies generated at the density front of the tropical instability waves in the tropical pacific ocean these cyclonic eddies of which the diameter is about 30 100 km and the vertical extent is limited to the upper 100 m in depth have physical characteristics similar to those of smaller submesoscale eddies at the midlatitudes according to the model they have highly coherent structures below the surface carrying cold and salty upwelled equatorial water probably rich in marine life the stretching and tilting of the upper layer of the ocean provides the main mechanism responsible for the intense cyclonic vorticity of the eddies involving complex evolution of the density field into occluded fronts

200 000 yr diatom records from atlantic upwelling sites reveal maximum productivity during lgm and a shift in phytoplankton community structure at 185 000 yr abstract time series of diatom accumulation rates dar and assemblage composition for the last 200 000 yr from the atlantic major coastal upwelling loci of the eastern boundary currents and eastern equator are compared together with independent proxies of export production to upper ocean productivity and to nutrient concentration and utilization fluctuations in dar are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern atlantic maximum productivity about one order of magnitude higher than at present occurred at the last glacial maximum lgm despite the atlantic similarities in both coastal and equatorial upwelling types important regional and site specific variations emerge in the southern hemisphere site a contradiction of proxies diatoms and corg is noted at the base of stage 6 185 000 yr the good correlation found between the corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the n p si ratio or similar nutrient conditions associated with increased water column stratification

origins and variability of the benguela current a subset of the benguela sources and transports best 1992 1993 data is analyzed to study the magnitude and variability of the large scale transports in the area the data consist of inverted echo sounder series and conductivity temperature depth stations the mean 16 month transport values for the upper 1000 m indicate that of the 13 sv northward transport within the benguela current 50 is derived from the central atlantic which from geometry may be chiefly south atlantic water 25 comes from the indian ocean which may be chiefly agulhas water and the remaining 25 may be a blend of agulhas and tropical atlantic water a simple schematic of the transport pattern with a somewhat restricted corridor for agulhas eddies translation is envisioned to the west of the eddy corridor flows the south atlantic source for the benguela current to the east is the agulhas indian ocean source the corridor is breached by south atlantic and indian water as the transient eddy field stirs these water masses

silicic acid leakage from the southern ocean a possible explanation for glacial atmospheric pco2 1 using a simple box model we investigate the effects of a reduced si n uptake ratio by antarctic phytoplankton on the marine silica cycle and atmospheric pco2 recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added hutchins and bruland 1998 takeda 1998 franck et al 2000 the southern ocean may have supported diatoms with reduced si n uptake ratios compared to today during the dustier glacial times petit et al 1999 a similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as phaeocystis our model shows that reduced si n export ratios in the southern ocean create excess silicic acid which may then be leaked out to lower latitudes any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes caco3 production and therefore diminishes the carbonate pump in our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric co2 from the interglacial 277 to 230 242 ppm depending on where the excess silicic acid is consumed by comparison the atmospheric pco2 sensitivity of general circulation models to carbonate pump forcing is 3 5 fold greater which combined with carbonate compensation can account for peak glacial atmospheric pco2 we discuss the importance of the initial rain ratio of caco3 to organic carbon on atmospheric pco2 and relevant sedimentary records that support and constrain this silicic acid leakage scenario

late quaternary sea ice history in the indian sector of the southern ocean as recorded by diatom assemblages abstract a modern analog technique mat5201 31 has been applied to fossil diatom assemblages to provide down core estimates of february sea surface temperatures ssts and of sea ice duration over the past 220 000 years at 56 40 s 160 14 e at the core location sea ice progression lagged the sst drop by 1 ka at interglacial glacial transitions and sea ice retreat was almost synchronous to the sst increase at glacial interglacial terminations sea ice increased continuously during glacial periods to reach its maximum extent at the end of glacial times although ssts were almost constant during glacials this indicates that ssts are the major parameter determining the advance and retreat of sea ice at transitions but that the sea ice advance during glacial conditions may be related to positive feedbacks of the ice on albedo air temperature and meridional wind stress the strong correlation r 0 75 between sea ice duration at the core location and the vostok co2 record argues for a control of antarctic sea ice extent on atmospheric co2 concentration via the modification of the ocean to atmosphere gas balance

a whirling ecosystem in the equatorial atlantic 1 the equatorial pacific and atlantic oceans exhibit remarkable meridional undulations in temperature and chlorophyll fronts visible from space over thousands of kilometers and often referred to as tropical instability waves here we present new observations of an ecosystem ranging through three trophic levels phytoplankton zooplankton and small pelagic fish whirling within a tropical vortex of the atlantic ocean and associated with such undulations cold nutrient and biologically rich equatorial waters are advected northward and downward to form sharp fronts visible in all tracers and trophic levels the equatorward recirculation experiences upwelling at depth with the pycnocline and ecosystem progressively moving toward the surface to reconnect with the equatorial water mass the observations thus indicate that it is a fully three dimensional circulation that dominates the distribution of physical and biological tracers in the presence of tropical instabilities and maintains the cusp like shapes of temperature and chlorophyll observed from space

on the reconstruction of upwelling history namibia upwelling in context abstract eastern boundary upwelling is an important aspect of the modern ocean despite the relatively small area involved consequences of increased upwelling during the neogene as a result of intensifying trade winds include increased heat transport by the ocean increased mixing increased fractionation of phosphate from silicate and opportunities for evolution of new species coastal upwelling has been studied since the first decade of the 20th century the processes involved are complex and differ depending on geographic setting off namibia upwelling history has been studied by a number of drilling expeditions the last of these was odp leg 175 which occupied seven sites on the continental slope between walvis ridge and cape town during september and october of 1997 productivity proxies all along southwestern africa suggest the presence of increased upwelling and organic matter supply to the sea floor during glacial periods at the same time there is a decrease in the supply of diatoms and other siliceous plankton remains this is the walvis opal paradox established through the contrasting results of studies by diester haass 1985 and oberhansli 1991 we propose that the walvis opal paradox is fundamental to the understanding of glacial interglacial productivity fluctuations on a global scale furthermore a central feature of the history of late neogene upwelling off namibia is the matuyama opal maximum centered between gauss and olduvai magnetic chrons wefer et al proc odp 175 1998 it is due to the fact that diatom supply first increases during the gauss when the planet cools and then decreases again during additional cooling on entering the quaternary on a 400 000 year scale peak productions are coincident with or slightly lag maximum seasonal contrast potential in the high latitude insolation curve we suggest that this is further evidence that the nutrient content of thermocline waters was diminished during glacial periods the reasons why this should be so remain to be discovered

accumulation of biogenic and lithogenic material in the pacific sector of the southern ocean during the past 40 000 years the 230th normalization technique was applied to surface sediments and down core records from the pacific sector of the southern ocean between 120 w and 170 w in order to reconstruct vertical rain rates corrected for lateral redistribution of sediment preserved rain rates of opal organic carbon and biogenic ba all indicate that biological productivity south of the antarctic polar front apf was much lower than today during the last glacial maximum lgm probably in response to increased ice cover however by analogy with modern opal fluxes south of the acc we show that year round ice cover is not a prerequisite for very low rates of opal accumulation available records from north of the apf indicate little to no change in opal export in contrast to records from the indian and atlantic sectors which show enhanced opal export north of the apf during the lgm glacial records from north of 66 s in the pacific all show lithogenic fluxes that were greater than today but significantly lower than lithogenic fluxes in the glacial atlantic and indian sectors together these new records from the pacific sector show that during the lgm the three basins of the southern ocean were significantly more differentiated than today the observed inter basin distribution of lithogenic fluxes and opal burial and the presumed geographical distribution of increased dust input to the southern ocean during the lgm are consistent with an atlantic indian pacific gradient in the degree of iron stimulated diatom productivity within the zone lying between the apf and the subantarctic front previous studies which focused primarily on the atlantic sector may have overestimated the glacial increase in the strength of the biological pump in the southern ocean

using silicon isotopes to understand the role of the southern ocean in modern and ancient biogeochemistry and climate abstract the growth of siliceous phytoplankton mainly diatoms in the southern ocean influences the preformed nutrient inventory in the ocean on a global scale silicic acid use by diatoms and deep circulation combine to trap dissolved si in the southern ocean resulting in high levels of silica production and expansive diatom oozes in southern ocean sediments the analysis of the silicon isotope composition of biogenic silica or opal and dissolved silicic acid provide insight into the operation of the global marine silicon cycle and the role played by the southern ocean in nutrient supply and carbon drawdown both in the modern and in the past silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column together these and other proxies reveal variations in the northward flow of southern ocean intermediate and mode waters and how changes in southern ocean productivity altered their preformed nutrient content we present a new hypothesis the silicic acid ventilation hypothesis savh to explain the geographical variation of opal based proxy records in particular the contrasting patterns of opal burial change found in the low and high latitudes by understanding the silicon isotope systematics of opal and silicic acid in the modern we will be able to use opal based proxies to reconstruct past changes in the southern ocean and so investigate its role in global carbon cycling and climate

the three dimensional structure of an upper ocean vortex in the tropical pacific ocean in the equatorial pacific ocean easterly trade winds and the earth s rotation combine to drive surface currents away from the equator thereby causing cold nutrient rich subsurface water to upwell the front1 that forms between this upwelled water and warmer waters north of the equator is sometimes visible as a spectacular line in the sea 2 between 2 and 6 n westward propagating cusp shaped disturbances observed along this front3 have been attributed to the effect of dynamical instabilities in the system of zonal equatorial currents4 11but the connection between these phenomena remains unclear here we report extensive measurements from shipboard sensors satellite and drifting buoys which reveal the three dimensional structure of an anticyclonic eddy or vortex 500 km in diameter and centred at 4 n we suggest that cusp shaped disturbances of the front are caused by trains of large amplitude vortices which are driven by instability of the mean zonal shear we show that these vortices not only play an important role in the meridional transport of heat salt and momentum but are also associated with regions of intense horizontal convergence along the front where dramatic concentrations of marine life are observed

the gross structure of the crust under corsica and sardinia as part of the european geotraverse project egt a number of international cooperative seismic refraction surveys were carried out along the n s segment from the ligurian sea across the islands of corsica and sardinia to the sardinian channel between sardinia and north africa from 1982 to 1985 for all the experiments offshore explosions served as energy sources ranging from 200 to 1125 kg the main result is that the crust in the ligurian sea is of transitional type with a thickness of only 18 km the two continental blocks of corsica and sardinia have a maximum crustal thickness of 33 and 34 km with average crustal velocities of 6 4 and 6 3 km s respectively other results are that the internal crustal structure as derived from the available data in both islands is rather undifferentiated with only local evidence for a more steplike interface in the lower crust corsica 6 3 to 6 6 km s sardinia 6 5 to 6 8 km s there are also indications of a slight velocity inversion under the southwestern part of sardinia farther south in the sardinian channel the crustal thickness decreases again to 20 km a significant feature is the observed pn velocity this parameter increases from the relatively low value of 7 5 km s in the north ligurian sea to 7 7 km s and 7 9 km s for corsica and sardinia and finally to 8 0 km s under the sardinian channel these values reflect the tectonic evolution of that part of the mediterranean sea

interannual variations of atlantic tropical instability waves 1 observations are used to develop metrics for interannual tropical instability wave tiw variability in the atlantic and to relate that variability to larger scale processes the analysis is partitioned into different latitude bands to distinguish between off equatorial 5 s 2 n and 5 n and near equatorial 2 s and 0 tiws tiw metrics based on sea surface temperature sst and sea level anomaly sla fluctuations are compared against interannual anomalies of sst in the cold tongue region to examine the role of barotropic shear instabilities in modulating the intensity of a tiw season wind stress and near surface current indices are developed in regions where the shear between the equatorial undercurrent euc and the northern branch of the south equatorial current nsec and between the nsec and the north equatorial countercurrent necc are expected to be largest good agreement is found between the sst and sla tiw metrics along the off equatorial latitude bands and interannual variations of both metrics can largely be attributed to barotropic shear instabilities in particular years with low high tiw variance along the off equatorial latitude bands are associated with anomalously warm cold ssts in the cold tongue region weak strong wind stress divergence and curl in the euc nsec region and weak strong zonal current shear in the nsec necc region in contrast in the near equatorial latitude bands poor agreement is found between interannual tiw activity based on the sst and sla metrics and near equatorial tiw variability cannot be explained by the large scale sst wind stress divergence and curl and current shear indices

a cross section of the lithosphere along the european geotraverse southern segment from the alps to tunisia abstract the european geotraverse southern segment egt s is a 200 300 km wide strip from the alps to tunisia which crosses the youngest tectonic provinces of europe where continued alpine compression is accompanied by recent extension e g in the ligurian sea and sicily channel the geophysical research along the egt s has been nearly continuous both on land and sea from 1982 to 1987 with deep seismic sounding dss experiments and collection of heat flow gravity magnetic seismological and seismotectonic data the new dss and seismic reflection data testify to the asymmetry of the alpine edifice the deepest part of the crust mantle boundary is found beneath the internal and external crystalline massifs where the european moho disappears after a steep flexure in the seismic reflection data the continuation of the moho discontinuity into the southern alpine domain is however evident in the seismic refraction data seismic evidence and heat flow data indicate incipient rifting in the ligurian sea which is underlain by a mantle with an anomalously low pn velocity of only 7 5 km s the palaeozoic crust of the sardinia corsica block exhibits a bowl shape with a rather undifferentiated internal structure average p wave velocity of 6 3 6 4 km s crustal thickness increases from 20 km at its margins to more than 30 km beneath the centre of the islands a thinned crust with thrusted blocks in its upper part characterizes the sardinia channel finally the preliminary dss results from tunisia show an abrupt deepening of the moho from 20 to approximately 35 km in the tell area very low average crustal velocity which requires a velocity inversion within the lower crust and subcrustal reflectors revealing a structured upper mantle

miocene history of the benguela current and antarctic ice volumes evidence from rhythmic sedimentation and current growth across the walvis ridge deep sea drilling project sites 362 and 532 the history of the northward growth and of the fluctuations of the benguela current from the middle miocene ca 14 ma to the uppermost late miocene ca 5 ma has been reconstructed from analyses of the coarse and clay fractions and geochemical analyses of the organic matter in sediments from deep sea drilling project sites 362 and 532 in the middle miocene the benguela current had not yet reached the walvis ridge and consequently no local upwelling occurred carbonate preservation was good during this period of low productivity rhythmic changes related to antarctic ice volumes were weak leading to slightly higher organic matter concentrations and higher montmorillonite illite m i ratios in sediments deposited during times of greater continental ice montmorillonite was carried north from the orange river by the benguela current whereas illite originated from the nearby namib desert in the early late miocene the benguela current reached the walvis ridge in colder periods and led to well developed rhythmic increases in opal which was transported from near coastal upwelling areas organic matter concentration carbonate dissolution and m i ratios were also enhanced during periods of greater ice volume during the late miocene farther northward migration of the benguela current led to a change in the uppermost miocene sediments opal contents and m i ratios were high during times of low ice volume rather than in colder times during the cold periods local winds from the namib desert increased the illite supply to the walvis ridge and depressed the m i ratio in these periods the benguela current flowed farther to the north reaching the angola basin as it did in quaternary glacial periods throughout these changes sediment organic carbon content was always enhanced in high ice volume periods from near bottom downslope transport of organic matter from shelf regions during regressions and carbonate dissolution in the sediments was increased as a result of oxidation of this organic matter

a 350 ky radiolarian record off luderitz namibia evidence for changes in the upwelling regime abstract the study of radiolarian assemblages from core md 962086 provides new information on the variability in the upwelling intensity and origin of upwelled water masses over the past 350 ky in one of the major filamentous regions of the benguela upwelling system bus located off luderitz namibia the use of key radiolarian species to trace the source of upwelled waters and the use of a radiolarian based upwelling index uri to reconstruct the upwelling intensity represent the first use of radiolarians for paleoceanographic reconstructions in the bus these radiolarian based proxies indicate strongest upwelling during marine isotope stages mis 3 5 and 8 which compares well with other studies while during mis 3 and 8 the radiolarian based proxies indicate the influx of waters of southern ocean origin they also point to the increased influence of tropical waters during the lower portion of mis 5 during mis 2 4 and 6 the radiolarian assemblages indicate generally lower upwelling intensities although this signal is complicated by the increased occurrence of organic carbon in the sediments during these intervals during mis 2 there appears to be less of an input of southern ocean waters to the bus although during the also glacial mis 4 and 6 there is evidence for an increased influence of cold antarctic waters the comparison of the results from core md 962086 with other studies in the bus area indicates a non uniform pattern of upwelling intensity and advection of cold southern waters into this system during mis 2 weaker upwelling signaled by the radiolarian based proxy in mis 4 is in contrast to other studies that indicate higher productivity during this time period in general the data show that there is a strong spatiotemporal complexity in upwelling intensity in the bus and that the advection of water into it is not strongly tied to glacial interglacial variations in climate

sea surface chlorophyll signature in the tropical pacific during eastern and central pacific enso events 1 recent analyses of physical measurements show the existence of a central pacific type of el nino cpen with a sea surface temperature warming pattern distinct from that of the classical eastern pacific el nino epen in this study we analyze the surface chlorophyll signature of el nino southern oscillation enso using monthly maps of satellite derived chlorophyll anomalies between september 1997 and december 2010 we identify five typical enso structures the first structure describes the lonely 1997 1998 epen of the period the second and third represent la nina the fourth illustrates intermediate conditions and the fifth characterizes cpen during the 1997 1998 epen a large eastward shift of the oligotrophic warm pool and a reduction of equatorial upwelling result in negative chlorophyll anomalies east of 170 e between 10 s and 10 n during the four cpen events a reduced eastward shift yields negative chlorophyll anomalies in the equatorial band within about 160 e and 160 w only westward surface current in the central basin limits the expansion of the anomaly core negative chlorophyll anomalies that extend eastward from the equatorial anomaly core probably result from reduced upward iron fluxes linked to the deepening of the equatorial undercurrent during la nina the westward expansion of the equatorial upwelling results in positive chlorophyll anomalies west of the date line away from the equatorial band advection of oligotrophic warm pool waters by enhanced eastward countercurrents drives negative anomalies within 8 10 n and toward the marquesas islands during cpen while reduced countercurrents lead to positive chlorophyll anomaly during la nina

electrical conductivity investigation of the corso sardinian microplate area abstract the analysis of magnetovariational data from an array quasi simultaneously covering sardinia italy and corsica france from summer 1988 to spring 1991 indicates the existence of two major conducting bodies one north of corsica the other south of sardinia fourier maps also show local conductivity anomalies along the campidano graben sardinia across the bonifacio straits and in the northernmost part of corsica two dimensional 2d modelling and inversion procedures have been carried out in order to find the optimum conductivity and geometrical characteristics of the conductive bodies deduced from fourier maps induction arrows and pseudosections studies as a result of the 2d inversion two zones with enhanced electrical conductivity are found respectively in the sardinia channel and in the bonifacio straits the former can be related to the thermal characteristics of the area the latter shallower can be ascribed to an accumulation of sediments some short period magnetovariational anomalies in northern corsica can be related to recent reheating processes that occurred there as well as to the crustal thinning in the ligurian sea

a displaced neritic diatom delphineis karstenii in pelagic sediments of the southeast atlantic this paper documents the occurrence ofdelphineis karstenii a neritic diatom in pelagic sediments of the northern cape and southern angola basins southeast atlantic ocean maximum abundances are found in the southern angola basin although the source area appears to be the diatomaceous sediments which blanket much of the namibian continental shelf the observed pattern implies large scale displacement ofd karstenii valves first northward along the continental margin and then westward into deep waters transport evidently occurs in the surface waters of the benguela current which flow northward along the namibian coast and then turn westward away from the coast of angola and out into the angola basin these observations suggest thatdelphineis valves and perhaps other shallow water diatom valves may be displaced downslope on other continental margins failure to recognize such displacement could lead to incorrect ecological and paleoecological interpretations

the interplay between the surface and bottom water environment within the benguela upwelling system over the last 70 ka the benguela upwelling system bus located between 30 and 20 s is one of the fundamental high productivity systems of the world ocean the bus has previously been studied in terms of primary productivity and ecology over glacial interglacial timescales however the response and coupling with the benthic environment have received little attention here for the first time we present a high resolution reconstruction of the bus highlighting the link between surface and benthic productivity and their response to climatic and oceanographic changes over the last 70 ka the study is based on benthic foraminiferal faunal analysis together with analyses of diatom assemblages grain size of the terrigenous fraction and stable o and c isotopic and bulk biogenic components of core geob3606 1 we reveal significant shifts in benthic foraminiferal assemblage composition tight coupling existed between the surface and bottom water environment especially throughout marine isotope stages 4 and 3 mis4 and mis3 due to the high export production the site has essentially experienced continuous low oxygen conditions however there are time periods where the hypoxic conditions were even more notable two of these severe hypoxic periods were during parts of mis4 and mis3 where we find an inverse relationship between diatom and benthic foraminifera accumulation meaning that during times of extremely high phytodetritus export we note strongly suppressed benthic productivity we also stress the importance of food source for the benthos throughout the record shifts in export productivity are attributed not only to upwelling intensity and filament front position but also regional global climatic and oceanographic changes had significant impact on the bus dynamics

changes in style and intensity of production in the southeastern atlantic over the last 70 000 yr abstract accomplishing reliable paleo reconstructions of productivity and upwelling conditions in eastern boundary current systems requires the use of cores collected in a basin wide spatial pattern based on diatom assemblage analysis and the concentration and the bulk biogenic components of three gravity cores recovered from the benguela upwelling system bus between 19 and 25 s i describe rapid paleoceanographic changes that occurred during the last 70 ka b p in the southeastern atlantic the pattern of biogenic production and accumulation differs to varying degrees among the three core sites along the sw african coast the highest sedimentation and accumulation rates at 25 s off luderitz conform with the present day well known pattern of highest productivity and most intense coastal upwelling highest diatom values at 25 s during mis3 points to more intense upwelling due to the combination of strong seaward extending upwelling filaments shoaling of the upwelled water and the influence of silicate rich waters of antarctic origin productivity decreased along the central bus throughout mis2 when the siliceous calcareous productivity regime shifted toward a system dominated by calcareous producers although intensity and strength of winds created adequate conditions for upwelling during mis2 diatom production decreased the complete replacement of the upwelling associated diatom flora by a non upwelling related diatom community during mis1 reflects weakened upwelling weakened seaward extension of the upwelling filaments and dominance of warmer surface waters combining changes in the composition of the diatom assemblage and variations of the bulk biogenic components allows for reliable reconstruction of paleoproductivity and upwelling changes for the se atlantic during the last 70 ka b p

3 d shear wave velocity model of the lithosphere below the sardinia corsica continental block based on rayleigh wave phase velocities rayleigh wave dispersion curves from both ambient noise and teleseismic events allow us to provide the first high resolution 3 d shear wave velocity vs model of the crust and upper mantle below the sardinia corsica microplate an important continental block for understanding the evolution of the central western mediterranean for a wide range of periods from 3 to 30 s the phase velocities of the study area are systematically higher than those measured within the italian peninsula in agreement with a colder geotherm relative and absolute variations in the vs allow us to detect a very heterogeneous upper crust down to 8 km as opposed to a relatively homogeneous middle and lower crust the isosurface at 4 1 km s 1 is consistent with a rather flat moho at a depth of 28 0 1 8 km 2 the lithospheric mantle is relatively cold and we constrain the thermal lithosphere asthenosphere boundary at 100 km we find our estimate consistent with a continental geotherm based on a surface heat flow of 60 mw m 2 our results suggest that most of the lithosphere endured the complex history of deformation experienced by the study area and imply in general that deep tectonic processes do not easily destabilize the deeper portion of the continental lithosphere despite leaving a clear surface signature

an introduction to the european geotraverse project first results and present plans abstract the european geotraverse egt is an international multidisciplinary project focused on a north south orientated lithospheric profile 4000 km long and of varying width extending from northernmost scandinavia to north africa this profile consists of three interlinking segments northern central and southern comprising a continuous succession of tectonic provinces ranging from the oldest precambrian areas of the baltic shield to the currently active area of the western mediterranean the broad aim of the egt project is to obtain a better three dimensional picture of the structure state and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics all of the 12 major projects that constitute the egt joint programme have now been initiated and several of these projects are nearing completion along the northern segment data from the fennoscandian long range project fennolora a 2000 km long seismic profile across the precambrian baltic shield show that except beneath southern sweden the shield is characterized by a high velocity 40 50 km thick crust including a 5 10 km thick crust mantle transition zone an alternating series of 4 6 high and low velocity zones is present in the subcrustal lithosphere the base of which increases in depth from ca 110 km to ca 230 km from south to north beneath the shield the top of the mantle transition zone lies at a depth of about 450 km the second major project along this segment the eugeno s european geotraverse northern segment southern part project is a multidisciplinary study of the fennoscandian border zone and was largely completed in 1984 with the realization of a large scale seismic experiment preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the danish basin field work for a third major project a multidisciplinary transect of the archaean and early proterozoic terrains in the northernmost part of the shield the polar profile was carried out in 1985 a series of deep seismic reflection lines has so far been realized in the area of the central segment in the context of german national programmes first interpretations of the seismic data from a 260 km long profile across the two main intra variscan hercynian lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied and distinct changes in reflectivity between the main variscan tectonic zones in 1986 the entire segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies a series of seismic experiments termed egt s have been carried out across the southern segment in 1982 1983 and 1985 interpretation of data from the 1982 and 1983 experiments have led to several interesting results including 1 1 the suggestion that two crust mantle like interfaces exist beneath the po basin at depths of about 35 and 50 km and adjacent tectonic units these interfaces marking a deep contact zone between the adriatic and european plates 2 2 in the area between genoa and corsica the ligurian sea is underlain by a greatly thinned distinctly layered section of continental crust and 3 3 corsica and sardinia are underlain by bowl shaped typically variscan continental crusts the 1985 phase of seismic surveys focused on crustal structure beneath tunisia and the adjacent seas in addition two off traverse projects are being realized first a wide aperture network of autonomously recording seismic stations nars installed along the line gothenburg malaga between 1982 and 1984 is already yielding high quality data on the upper 600 700 km of the mantle second an investigation of lithospheric seismic anisotropy in the area of the iberian peninsula is being organized for 1987 1988 finally of great importance are the systematic compilation of existing data and where needed in critical regions collection of new geophysical and geological data presently being carried out for the entire area encompassed by the egt it is expected that these compilations will be completed by 1987 at about the same time that full results from the main large scale seismic experiments become available enabling the construction of an integrated lithospheric cross section along the egt requiring a final phase of intensive multidisciplinary collaboration

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

deep temperatures and lithospheric thickness along the european geotraverse abstract one dimensional steady state heat conduction models in the crust and upper mantle for 24 areas along the european geotraverse egt show that temperatures are higher beneath young geological regions at the moho they range from 450 to 825 c heat flowing out from the mantle varies from 18 to 72 mw m2 and the contribution of crustal heat flow density evaluated by the correlation between seismic velocity and the abundance of radioactive elements accounts for about 48 of the surface heat flow density the temperature gradient below moho from about 5 to 23 mk m is low where the surface heat flow density reaches its minimum values the depth at which the geotherms reach 0 85 of the mantle melting temperature is greater in the precambrian and caledonian areas with a maximum value of about 195 km under the early proterozoic scandinavian shield in the hercynian and alpine areas this depth is generally lower higher values maximum 150 km coincide with the orogenic belts and the continental block of sardinia and corsica under the ligurian sea and the sardinian channel the lithospheric thickness decreases remarkably to about 50 km globally the lithosphere thickness is controlled by the mantle heat flow density which decreases according to a power law with an increase in the lithosphere asthenosphere depth

on the annual cycle of equatorial upwelling in the central atlantic ocean abstract the annual cycle of the upper ocean s vertical velocity component w on the equator at 28 w is examined by integrating the continuity equation using current meter data from the seasonal response of the equatorial atlantic experiment the annual cycle consists in part of an intense but brief 1 month upwelling season beginning with the onset of strong easterly wind stress in boreal spring this upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress the record length averaged w profile shows that maximum upwelling 0 6 10 3 cm s 1 is located slightly above the core of the equatorial undercurrent and downwelling is located below the base of the thermocline the standard deviations are about tenfold the magnitude of the means independent evidence supporting these results are that 1 sea surface temperature sst is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacem

oscillations of the siliceous imprint in the central benguela upwelling system from mis 3 through to the early holocene the influence of the southern ocean based on a high resolution analysis of the diatom signal and biogenic bulk compo nents at site geob3606 1 25 s off namibia we describe rapid palaeoceanographic changes in the benguela upwelling system bus from early mis 3 through to the early holocene 55 000 to 7 000 14 c yr bp coastal upwelling strongly varied at 25 s from mis 3 through to mis 2 the abrupt decrease in the accumulation rate of biogenic silica and diatoms from mis 3 into mis 2 records rapid oceanographic changes in the bus off namibia during mis 3 leakage of excess h4sio4 acid from the southern ocean into low latitude surface waters as indicated by the occurrence of antarctic dia toms enhanced the production of spores of chaetoceros at the expense of calcareous phytoplankton furthermore shallower antarctic intermediate water aaiw would have enriched the thermocline off namibia with silicate transported from the southern ocean the strong decrease of the siliceous signal throughout mis 2 represents a decrease in the nutrient input to the bus even though the dia tom assemblage is still dominated by spores of the upwelling associated diatom genus chaetoceros depletion of silicate in the thermocline from the onset of mis 2 through to the early holocene reflects the shutdown of aaiw injection from the southern ocean into the bus causing upwelled waters to become reduced in silicate hence less favourable for diatom production the deglaciation and early holocene are characterised by the replacement of the upwelling associated flora by a non upwel ling related diatom community reflecting weakened upwelling retraction of the seaward extension of the chlorophyll filament off luderitz and dominance of warmer waters copyright 2003 john wiley sons ltd

silicic acid leakage from the southern ocean opposing effects of nutrient uptake and oceanic circulation 1 studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of southern ocean intermediate waters comparison of 30si records from the southern ocean to records of biogenic opal accumulation from low latitude upwelling systems indicates a strong negative correlation between the silicic acid content of southern ocean surface waters and opal deposition at low latitudes during the last 80 kyr bp this implies that si leakage was somehow rendered ineffective when the silicic acid content of southern ocean waters was high comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods this indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of antarctic surface waters in determining the magnitude of silicic acid leakage from the southern ocean

diatom response to oceanographic and climatic changes in the congo fan area equatorial atlantic ocean during the last 190ka bp changes in siliceous productivity in the eastern equatorial atlantic ocean off the western african margin over the last several glacial cycles have been either related to global to regional oceanographic changes upwelling intensity or climate changes precipitation and river discharge based on diatom assemblages in core kzai 02 located to the south of the mouth of the congo river integrated with a selection of geochemical proxies we show that siliceous productivity in the southeastern angola basin responded to non linear interactions between both oceanographic and climate changes over the last 190 000 years high diatom accumulation rates were recorded in the middle part of mis 6 in cold substage mis 5d and in mis 3 2 during these intervals high diatom productivity was sustained essentially by nutrients including dissolved silica injected by the congo river into the ocean the highest productivity was observed during mis 3 when nutrients were sourced both from the river and regional upwelling low diatom accumulation rates were recorded during early and late mis 6 mis 5e early mis 4 and during the holocene these resulted either from low river discharge and overall low nutrient stocks in the angola basin despite evidence for upwelling or from extremely high river discharge in the case of the latter the terrigenous load drastically increased the turbidity of the surface waters in the southeastern angola basin and lowered phytoplankton productivity despite the presence of sufficient dissolved silica

biogenic opal in southern ocean sediments over the last 450 000 years implications for surface water chemistry and circulation we present records of biogenic opal percentage and burial rate in 12 piston cores from the atlantic and indian sectors of the southern ocean these records provide a detailed quantitative description of changing patterns of opal deposition over the last 450 kyr the striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the southern ocean under different global climate conditions three broad depositional patterns can be distinguished northernmost records 39 42 s latitude are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity records from cores just north of the present antarctic polar front 46 49 s show even larger increases in opal burial rate during glacial intervals but have variance concentrated in the 100 and 23 kyr bands southernmost records 51 55 s are completely out of phase with those to the north with greatly reduced opal burial rates during glacial periods taken as a whole the opal records show no evidence for the increased total antarctic productivity predicted by recent geochemical models of atmospheric co2 variability the areal expansion of southern ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial interglacial opal patterns the excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region however local circulation changes may act to modify this basic signal possibly accounting for the observed differences in the opal variance spectra

asynchronous alkenone and foraminifera records from the benguela upwelling system abstract radiocarbon stratigraphy is an essential tool for high resolution paleoceanographic studies age models based on radiocarbon ages of foraminifera are commonly applied to a wide range of geochemical studies including the investigation of temporal leads and lags the critical assumption is that temporal coupling between foraminifera and other sediment constituents including specific molecular organic compounds biomarkers of marine phytoplankton e g alkenones is maintained in the sediments to test this critical assumption in the benguela upwelling area we have determined radiocarbon ages of total c37 c39 alkenones in 20 samples from two gravity cores and three multicorer cores the cores were retrieved from the continental shelf and slope off namibia and samples were taken from holocene deglacial and last glacial maximum core sections the alkenone radiocarbon ages were compared to those of planktic foraminifera total organic carbon fatty acids and fine grained carbonates from the same samples interestingly the ages of alkenones were 1000 to 4500 yr older than those of foraminifera in all samples such age differences may be the result of different processes bioturbation associated with grain size effects lateral advection of recycled material and redeposition of sediment on upper continental slopes due to currents or tidal movement are examples for such processes based on the results of this study the age offsets between foraminifera and alkenones in sediments from the upper continental slope off namibia most probably do not result from particle selective bioturbation processes resuspension of organic particles in response to tidal movement of bottom waters with velocities up to 25 cm s recorded near the core sites is the more likely explanation our results imply that age control established using radiocarbon measurements of foraminifera may be inadequate for the interpretation of alkenone based proxy data observed temporal leads and lags between foraminifera based data and data derived from alkenone measurements may therefore be secondary signals i e the result of processes associated with particle settling and biological activity

the tethys sea and the alpine himalayan orogenic belt mega elements in a new global tectonic system abstract analysis of meso cainozoic palaeomagnetic data for africa india and eurasia has led to the development of a new mobilistic alpine plate tectonic model characterized by a hierarchical system of plates in relative rotation the new model which discounts seafloor spreading implies that there have been no significant palaeogeographic changes in the overall distribution of continental and oceanic regions the mid oceanic ridges are interpreted as transpressive tectonic features caused by rotation of megaplates containing both continental and oceanic crust the isostatic uplift due to crustal lithospheric thickening giving rise to the general ridge topography as well as to the ridge parallel structural grain the new plate tectonic theory gains strong support from a variety of geophysical geological and palaeoclimatological evidence and several observations that have remained enigmatic or awkward within the context of the orthodox model can be readily accounted for in the new tectonic framework the model maintains the tethys as a relatively narrow epicontinental sea which during its maximum extent stretched latitudinally from the caribbean across the central atlantic to se asia the alpine himalayan orogenic belt developed along the boundary of two megaplates in relative rotation which provided a transpressive tectonic regime the location of the plate boundary to the north of the mediterranean has important implications for discussion of mediterranean microplates for example it now seems that italy has been subjected to 10 15 of clockwise microplate rotation previous conclusions in favour of 30 40 of anticlockwise rotation are regarded as artefacts which arise from incorrectly linking the mediterranean region to the european palaeomagnetic frame instead of to the african one the model suggests further that the indo pakistani plate was closely tied to eurasia this challenges the conventional view that the peninsula was part of an alleged gondwanaland the new pre drift configuration implies that the indo pakistani plate rotated 135 clockwise at around the cretaceous tertiary boundary before redocking with asia in approximately its present relative orientation

earthquake relocation using a 3d a priori geological velocity model from the western alps to corsica implication for seismic hazard the region between the inner zones of the alps and corsica juxtaposes an overthickened crust to an oceanic domain which makes difficult to ascertain the focal depth of seismic events using routine location codes and average 1d velocity models the aim of this article is to show that even with a rather lose monitoring network accurate routine locations can be achieved by using realistic 3d modelling and advanced location techniques previous earthquake tomography studies cover the whole region with spatial resolutions of several tens of kilometres on land but they fail to resolve the marine domain due to the absence of station coverage and sparse seismicity to overcome these limitations we first construct a 3d a priori p and s velocity model integrating known geophysical and geological information significant progress has been achieved in the 3d numerical modelling of complex geological structures by the development of dedicated softwares e g 3d geomodeller capable at once of elaborating a 3d structural model from geological and geophysical constraints and possibly of refining it by inversion processes calcagno et al 2008 then we build an arrival time catalogue of 1500 events recorded from 2000 to 2011 hypocentres are then located in this model using a numerical code based on the maximum intersection method font et al 2004 updated by theunissen et al 2012 as well as another 3d location technique the nonlinloc software lomax and curtis 2001 the reduction of arrival time residuals and uncertainties dh dz with respect to classical 1d locations demonstrates the improved accuracy allowed by our approach and confirms the coherence of the 3d geological model built and used in this study our results are also compared with previous works that benefitted from the installation of dense temporary networks surrounding the studied epicentre area the resulting 3d location catalogue allows us to improve the regional seismic hazard assessment more particularly in the south of the argentera massif and in the ligurian basin

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

european geotraverse a seismic refraction profile across the ligurian sea abstract a seismic refraction profile was shot in the ligurian sea between italy and corsica in september 1983 the profile which is part of the southern portion of the european geotraverse was shot using small dynamite charges and the seismograms were recorded by ocean bottom seismometers and land stations resulting in a number of partially overlapping reversed profiles the refraction data were interpreted by means of the velocity intercept and ray tracing methods using seismic reflection derived velocity depth information for the sedimentary section refraction velocities of 4 8 6 0 and 6 6 km s 1 were calculated for the base of the sedimentary section crystalline basement and lower crust respectively an upper mantle velocity of 7 4 km s 1 was inferred using the refraction profiles recorded on corsica the interpretation of the data indicates a prominent high in the crystalline basement in the northern part of the profile while the sedimentary section increases in thickness from about 1 5 km over the basement high to over 6 km between the basement high and corsica and includes a considerable thickness of pre miocene sediments the crust mantle boundary shows shoaling towards the centre of the profile where a minimum crustal thickness of 16 km is reached the model therefore is that of a stretched and thinned continental crust which was rifted in oligo miocene times the presence of a low velocity upper mantle may be indicative of the present renewal of subcrustal activity under the ligurian sea

crustal structure from the ligurian sea to the northern apennines a wide angle seismic transect abstract a new wide angle seismic reflection profile 250 km long was shot along a line extending from parma in the apennines to the deepest part of the ligurian sea thirty seismic land stations lobs and 25 ocean bottom seismographs obs recorded 54 shots fired at sea the resulting 55 record sections allowed the definition of the transition from continental to oceanic crust in a geologically complex and hitherto poorly understood region the subducted continental crust beneath the northern apennines has a thickness of 40 km consisting of 30 km of igneous crust overlain by 10 km of sediments two sedimentary layers are identified with vp velocities in the range from 2 8 to 5 5 km s the igneous portion of continental crust thins gradually to approx 11 km across a 120 km wide zone extending from the southwestern part of the apennines some 20 km inland into the ligurian sea an easterly thinning low velocity body 7 2 7 5 km s immediately beneath the base of the crust is interpreted as an underplated unit emplaced prior to the onset of seafloor spreading this may have been initiated by magmatic activity along the incipient continental oceanic boundary a basement high coincides with the northeastern edge of the ligurian sea and is interpreted as part of the alpine ophiolite belt indicating strike slip motion the two sedimentary layers above the transition zone are typically 5 km thick and have slightly lower velocities than those further to the northeast a sharp continent ocean break is recognised in the ligurian sea oceanic crust is 6 7 km thick and is overlain by a two layered sedimentary succession approx 5 km thick mantle velocities in the region range from 7 9 km s beneath the thinned crust to 8 1 km s beneath the thick crust the ratio c o of continental to oceanic crustal thickness at the continent ocean transition is approx 1 5 and the overall crust stretching factor at that boundary is approx 3 5 these values are similar to other continent ocean boundaries and indicate that the oceanic crust in the ligurian sea formed largely by stretching with a small pull apart component

nitrogen cycling on the namibian shelf and slope over the last two climatic cycles local and global forcings 1 in the light of new surface sediment 15n data collected over the namibian shelf and slope we examined glacial interglacial variations of n inventory in the area of luderitz 25 6s by deciphering 15n signals of three cores distributed from the upper to the lower continental slope the lower slope cores display low 15n during cold periods and high 15n during climatic optima akin to many other records from the world ocean whereas the upper slope core displays a high frequency low amplitude 15n signal without obvious glacial interglacial variability this dissimilarity results from the segregation of the upwelling structure in two cells decoupling nutrient dynamics of the shelf from those beyond the shelf edge the 15n signal of the coastal cell is relatively constant irrespective of wind strength variations and shows that nitrate was never depleted in the surface water for the deeper cores comparisons between n isotopic signals and indicators of paleoproductivity total organic carbon and upwelling intensity sea surface temperature and dust grain size reveal that over milankovitch cycles nitrate delivery to the photic zone was driven by the nutrient richness of the south atlantic central water depending in turn on aghulas water inflow and denitrification at a global scale rather than by atmospheric forcing we propose that the 15n signals of the deeper cores do not only mirror changes in relative nitrate utilization as it seems the case over annual timescales but are arguably influenced by global ocean changes in middepth nitrate 15n

variability of the atlantic off equatorial eastward currents during 1993 2010 using a synthetic method 1 we have developed validated and applied a synthetic method to monitor the off equatorial eastward currents in the central tropical atlantic this method combines high density expendable bathythermograph xbt temperature data along the ax08 transect with altimetric sea level anomalies slas to estimate dynamic height fields from which the mean properties of the north equatorial countercurrent necc the north equatorial undercurrent neuc and the south equatorial undercurrent seuc and their variability can be estimated on seasonal to interannual timescales on seasonal to interannual timescales the synthetic method is well suited for reconstructions of the necc variability reproduces the variability of the neuc with considerable skill and less efficiently describes variations of the seuc which is located in a region of low sla variability a positive correlation is found between interannual variations of the necc transport and two indices based on an interhemispheric sea surface temperature sst gradient and southeasterly wind stress in the central tropical atlantic the neuc is correlated on interannual timescales with ssts and meridional wind stress in the gulf of guinea and zonal equatorial wind stress this study shows that both altimetry and xbt data can be effectively combined for near real time inference of the dynamic and thermodynamic properties of the tropical atlantic current system

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved