reconstruction of megalake chad using shuttle radar topographic mission data in the 2 500 000 km2 lake chad basin in central africa the 2000 shuttle radar topographic mission srtm data have been used to supplement the existing topographic data srtm data produce much sharper images of the region s topography and provide new insights into debates about the nature and extent of late quaternary lake chad this paper shows that the accuracy of srtm30 the recently released 30 arc seconds topographic data from srtm largely surpasses that of previous global digital elevation models dems available in the region using a gis we identified from srtm30 elevation data key features in the landscape topography providing further evidence for the existence of a megalake chad the srtm30 data corroborate the presence of two ancient shorelines associated with stillstands of the paleolake at the elevation of the mayo kebbi and bahr el ghazal spillovers we found a general flattening of the topography in the region covered by megalake chad which is most likely the result of wave cut action the srtm30 data show that the remains of the highest paleoshoreline have a constant elevation of 325 5 m amsl at its maximum extent megalake chad had an area of about 340 000 km2 only 8 less than the present day world s largest lake the caspian sea the srtm30 data also revealed ancient drainage networks in the sahara that lead to megalake chad we compiled available 14c dates to constrain holocene megalake chad events the results presented in this paper have significant consequences for improving our knowledge of regional paleohydrology and continental climate change this study is also the first step for a gis based reconstruction of late quaternary paleohydrology in tropical africa

late quaternary vegetation and climate of the sahel abstract pollen and phytogeographic evidence provides a vegetational history of the sahel for the period 0 18 000 yr b p the zonal vegetation fluctuated latitudinally and its most extreme positions occurred at 18 000 and 8500 yr b p the first involved a southward shift of the sahelian wooded grassland to 10 n under the arid conditions of the last glacial maximum the second change shows a rapid northward migration of humid vegetation guinean elements reach 16 n and sahelo sudanian elements extend to the southern margin of the modern sahara 21 n when the atlantic monsoon flux increased in the middle holocene the extensive spread of sudanian elements into the modern sahelian zone suggests the appearance of a markedly dry season the modern sahelian semiarid conditions appeared abruptly at 2000 yr b p

north pacific late miocene correlations using microfossils stable isotopes percent caco3 and magnetostratigraphy abstract a multidisciplinary approach to stratigraphy based on magnetostratigraphy stable isotopes percent caco 3 and microfossils provides a framework for paleoclimatic and paleoceanographic reconstruction of the equatorial and north pacific a high resolution biochronologic time scale has been achieved through integration of diatom radiolaria coccolith and planktic foraminifer datum levels with direct or indirect correlations to the paleomagnetic time scale over 70 datum levels have been identified between 12 5 and 5 ma from low and middle latitudes resulting in an unprecedented time control this high resolution biostratigraphic control combined with stable isotopic and percent caco 3 analyses permits identification of regional as well as global paleoceanographic events

plagioclase stability at elevated temperatures and water pressures the breakdown of plagioclase to zoisite kyanite quartz and a more sodic plagioclase in the system naalsi sub 3 o sub 8 caal sub 2 si sub 2 o sub 8 h sub 2 o has been investigated with reversed reactions at 8 9 and 10 kbar at these pressures all plagioclase compositions from approx an sub 40 to an sub 100 break down along the same p t curve as pure anorthite which can be described by the equation p 4590 20 4t p bars t sup 0 c the isobaric curves in the t x section are thus represented by a horizontal line constant temperature between approx an sub 40 and an sub 100 at compositions more sodic than approx an sub 40 the curves dip rather sharply to lower temperatures in the regions beneath the curves the composition of sodic plagioclase along the down dipping limb is in equilibrium with zo ky qz the remarkable behavior in the an sub 40 an sub 100 region suggests that under the hydrothermal conditions of this study regions of ordered anorthite develop in the plagioclase and subsequently react with water as would independent crystals of an partial melting of pure an in the presence of water begins at 10 2 kbar and 725 sup 0 c at 10 8 kbar a region of partial melt extends atmore nearly the same temperature from an sub 30 to an sub 100 jmt less

variability in sediment delivery and storage along river courses in bega catchment nsw australia implications for geomorphic river recovery in many catchments in southeastern australia alluvial stores have been the dominant source of sediments mobilised in the period since european settlement in bega catchment on the south coast of new south wales nsw this has been reflected by dramatic changes to river morphology extensive volumes of material have been released and efficiently flushed to the lowland plain with a sediment delivery ratio of almost 70 however only 16 of these alluvial sediments have been flushed through to the estuary as antecedent controls on valley width have resulted in the lowland plain acting as a large sediment sink the changing nature of sediment source transfer and accumulation zones has varied markedly from subcatchment to subcatchment the volume of material supplied to the lowland plain from differing subcatchments is not related to subcatchment area rather the pattern of river types dictates the spatial variability in storage and transfer over 67 of sediment released has been sourced from just 25 of the catchment from subcatchments characterised by large valley fills cut and fill river style that previously stored extensive volumes of material at the base of the escarpment these parts of bega catchment were especially sensitive to disturbance sediment exhaustion from these parts of the catchment and from river courses elsewhere has major implications for the geomorphic recovery potential of rivers constraining what can be realistically achieved in terms of river rehabilitation

composite nature of the north china granulite facies belt tectonothermal and geochronological constraints granulite facies rocks are intermittently exposed in a roughly e w trending belt that extends for approximately 2000 km across the north china craton from the helanshan qianlishan wulashan daqingshan guyang and jining complexes in the western block through the huai an hengshan xuanhua and chengde complexes in the trans north china orogen to the jianping western liaoning eastern hebei northern liaoning and southern jilin complexes in the eastern block the belt is generally referred to as the north china granulite facies belt previously interpreted as the lowest part of an obliquely exposed crust of the north china craton recent data indicate that the north china granulite facies belt is not a single terrane instead it represents components of three separate terranes the eastern and western blocks and trans north china orogen each of these units records different metamorphic histories and reflect the complex tectonic evolution of the ncc during the late archean and paleoproterozoic mafic granulites in the eastern block and the yinshan terrane western block underwent medium pressure granulite facies metamorphism at about 2 5 ga with anticlockwise p t paths involving near isobaric cooling following peak metamorphism reflecting an origin related to intrusion and underplating of mantle derived magmas pelitic granulites in the khondalite belt western block underwent medium pressure granulite facies metamorphism at about 2 0 1 9 ga with clockwise p t paths which record the paleoproterozoic amalgamation of the yinshan and ordos terranes to form the western block mafic and pelitic granulites in the trans north china orogen experienced high to medium pressure granulite facies metamorphism at 1 85 ga with clockwise p t paths involving nearly isothermal decompression following peak metamorphism which are in accord with the final collision between the eastern and western blocks to form the north china craton at 1 8 ga the ncgb cannot therefore represent a separate unique terrane instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times d 2005 international association for gondwana research published by elsevier b v all rights reserved

the neogene genus streptochilus bronnimann and resig 1971 from the gulf of california abstract four species of the planktonic foraminiferal genus streptochilus from key neogene marine localities are documented in relation to the evolution of the gulf of california s globigerus s latus s macdougallae sp nov and s inglei sp nov planktonic foraminiferal bioevents and strontium isotopes in the bouse tirabuzon carmen and ojo de buey lithostratigraphic units constrain the local distribution range between 6 and 5 3 ma for the last three species whereas s globigerus appears locally at 5 5 ma and disappears between 3 79 and 3 46 ma in the imperial and trinidad formations the last occurrence of streptochilus latus and the first and last occurrences of s globigerus in the ancient gulf of california are correlated with bioevents calibrated in the equatorial pacific therefore they can be used as reliable local biostratigraphic markers the presence of streptochilus in the ancient gulf of california seems to correlate with upwelling in a pattern similar to that observed in the modern oceans

the african humid period and the record of marine upwelling from excess 230th in ocean drilling program hole 658c using a high resolution 230 th normalized record of sediment flux we document the deglacial and holocene history of north african aridity and coastal upwelling at ocean drilling program hole 658c at both the end of the younger dryas and after the 8 2 ka event there are significant drops in terrigenous accumulation at our site indicating an increase in the monsoon moisture flux over africa at this time at 5 5 ka there is an abrupt end to the african humid period and a return to stronger upwelling conditions for carbonate and opal fluxes the 230 th normalization completely changes the shape of each record based on percentage variations alone this site is a clear example of how variations in one sediment component can obscure changes in the others and it demonstrates the need for radionuclide measurements more generally in paleoceanography by taking our new records and a large amount of previous data from this site we conclude that increases in african moisture are tightly coupled to decreases in coastal upwelling intensity

the senegal river mud belt a high resolution archive of paleoclimatic change and coastal evolution abstract fine grained sediment depocenters on continental shelves are of increased scientific interest since they record environmental changes sensitively a north south elongated mud depocenter extends along the senegalese coast in mid shelf position shallow acoustic profiling was carried out to determine extent geometry and internal structures of this sedimentary body in addition four sediment cores were retrieved with the main aim to identify how paleoclimatic signals and coastal changes have controlled the formation of this mud depocenter a general paleoclimatic pattern in terms of fluvial input appears to be recorded in this depositional archive intervals characterized by high terrigenous input high sedimentation rates and fine grain sizes occur roughly contemporaneously in all cores and are interpreted as corresponding to intensified river discharge related to more humid conditions in the hinterland from 2750 to 1900 and from 1000 to 700 cal a bp wetter conditions are recorded off senegal an observation which is in accordance with other records from nw africa nevertheless the three employed proxies sedimentation rate grain size and elemental distribution do not always display consistent inter core patterns major differences between the individual core records are attributed to sediment remobilization which was linked to local hydrographic variations as well as reorganizations of the coastal system the senegal mud belt is a layered inhomogeneous sedimentary body deposited on an irregular erosive surface early holocene deceleration in the rate of the sea level rise could have enabled initial mud deposition on the shelf these favorable conditions for mud deposition occur coevally with a humid period over nw africa thus high river discharge sedimentation started preferentially in the northern areas of the mud belt during mid holocene a marine incursion led to the formation of an embayment afterwards sedimentation in the north was interrupted in association with a remarkable southward shift in the location of the active depocenter as it is reflected by the sedimentary architecture and confirmed by radiocarbon dates these sub recent shifts in depocenters location are caused by migrations of the senegal river mouth during late holocene times the weakening of river discharge allowed the longshore currents to build up a chain of beach barriers which have forced the river mouth to shift southwards

a new model of river dynamics hydroclimatic change and human settlement in the nile valley derived from meta analysis of the holocene fluvial archive in the nile catchment a growing number of site and reach based studies employ radiocarbon and more recently osl dating to reconstruct holocene river histories but there has been no attempt to critically evaluate and synthesise these data at the catchment scale we present the first meta analysis of published and publically available radiocarbon and osl dated holocene fluvial units in the nile catchment including the delta region and relate this to changing climate and river dynamics dated fluvial units are separated both geographically into the nile delta and white blue and desert nile sub regions and into depositional environment floodplain and palaeochannel fills cumulative probability density frequency cpdf plots of floodplain and palaeochannel units show a striking inverse relationship during the holocene reflecting abrupt 100 years climate related changes in flooding regime the cpdf plot of dated floodplain units is interpreted as a record of over bank river flows whilst the cpdf plot of palaeochannel units reflect periods of major flooding associated with channel abandonment and contraction as well as transitions to multi centennial length episodes of greater aridity and low river flow this analysis has identified major changes in river flow and dynamics in the nile catchment with phases of channel and floodplain contraction at c 6150 5750 4400 4150 3700 3450 2700 2250 1350 900 800 550 cal bc and cal ad 1600 timeframes that mark shifts to new hydrological and geomorphological regimes we discuss the impacts of these changing hydromorphological regimes upon riverine civilizations in the nile valley

ultrahigh temperature mafic granulites from panrimalai south india constraints from phase equilibria and thermobarometry abstract the panrimalai area constitutes part of the granulite facies rocks of the madurai block in the southern granulite terrain sgt india garnet bearing mafic granulites in panrimalai occur as small enclaves within charnockite the common stable assemblage during peak metamorphism contains hornblende garnet orthopyroxene clinopyroxene quartz and plagioclase the resorption of garnet in various reaction textures and the development of spectacular orthopyroxene plagioclase and hornblende plagioclase symplectites characterize the subsequent stages of metamorphism application of multi equilibrium calculation procedures for mineral core compositions of the early assemblage yields near peak conditions at 900 c at 9 kbar these estimates are the highest yet reported in mafic granulites from the madurai block the post peak p t path is constructed for the mafic granulites based on observed microstructural relations and thermobarometric results is characterized by a steep clockwise decompressional p t segment from 9 to

diffusion in coronas around clinopyroxene modelling with local equilibrium and steady state and a non steady state modification to account for zoned actinolite hornblende retrograde coronas of caledonian age between clinopyroxene and plagioclase in the jotun nappe complex norway illustrate the effects of diffusion kinetics on mineral distributions among layers and on the compositions of hornblende actinolite one corona type comprises a symplectite of epidote quartz adjacent to plagioclase and a less well organized intergrowth of amphibole quartz replacing clinopyroxene the observed mineral proportions imply an open system reaction but the similarity of al si ratios in reactant plagioclase and product symplectite indicates approximate conservation of al2o3 and sio2 the largest inferred open system flux is a loss of cao mostly derived from consumption of clinopyroxene the approximate layer structure pl ep qtz hbl qtz act hbl qtz cpx is modelled using the theory of steady state diffusion controlled growth with local equilibrium to obtain a solution it is necessary to use a reactant plagioclase composition which takes into account aluminous epidote inclusions the results indicate that in terms of onsager diffusion coefficients lii ca is more mobile than al lcaca lalal 3 where means greater than or approximately equal to this behaviour of ca is comparable with that of mg in previously studied coronas around olivine si is non diffusing in the present modelling because of silica saturation oxidation of some fe2 to fe3 occurs within the corona mg diffuses towards its source clinopyroxene to maintain local equilibrium other coronas consist of two layers hornblende adjacent to plagioclase and zoned amphibole quartz adjacent to clinopyroxene in the zoned layer actinolitic hornblende forms relict patches separated from quartz blebs by more aluminous hornblende a preliminary steady state local equilibrium model of grain boundary diffusion explains the formation of low al and high al layers as due to al immobility zoning and replacement are qualitatively explained in terms of evolution of actinolite to more stable aluminous compositions this is modelled by a non steady state modification of the theory retaining local equilibrium in grain boundaries while relatively steep zoning profiles develop in grain interiors through slow intracrystalline diffusion replacement of actinolite by hornblende does not require a change in p t conditions if actinolite is a kinetically determined non equilibrium product the common preservation of a sharp contact between hornblende and actionolite layers may be explained by ineffectiveness of intracrystalline diffusion according to the theory given sufficient grain boundary al flux a metastable actinolite quartz layer in contact with hornblende may be diffusionally stable and may continue to grow in a steady state

the early holocene palaeoenvironment of ounjougou mali phytoliths in a multiproxy context abstract the site complex ounjougou on the dogon plateau mali comprises sediments up to 100 000 years old with numerous pleistocene and holocene sequences the site ravin de la mouche 11 4 10 2 ka is of special archaeological significance because in its early holocene deposits pottery sherds have been found which are among the oldest in africa for a better understanding of the environmental conditions which might have contributed to the innovation of pottery making a multi proxy approach was applied to the sediments of ravin de la mouche including phytoliths pollen palynofacies micromorphology and charcoal the multi proxy approach also allows reconstructing the complex taphonomy of the site in our phytolith study we applied a combination of the general and the indices approaches we recorded a maximum of morphotypes and used the summarized data for a calculation of the indices d p ic and iph in comparison with modern surface samples and data from other african phytolith studies with the general approach a number of morphotypes could be detected which are useful in describing the late pleistocene and the early holocene vegetation phytoliths were extracted from the pleistocene base and the early holocene layers ha1 ha2 and ha3 the pleistocene sediment samples with an age of ca 30 40 ka bp have no grass short cell phytoliths gscp and their composition is difficult to interpret ha1 is a coarse fluvial deposit with mainly redeposited phytoliths of pleistocene origin the palaeosoil in ha2 contains phytolith assemblages developed in situ from a terrestrial plant cover the vegetation was an open tropical grassland and a gallery forest with palms and marantaceae annuals from the grass subfamilies chloridoideae and panicoideae probably with a low biomass production dominated the grassland this might explain the insignificant role of fire as indicated by the very low number of micro charcoals ha3 results from a rhythmic deposition of alluvial sediments pointing to pronounced seasonality of rainfall and discharge it contains pollen charcoal and phytolith assemblages with a similar composition as in ha2 the early holocene annual grassland on the dogon plateau probably harboured a high number of species from the grass subfamily panicoideae with edible grains we suggest that the massive expansion of useful panicoid grasses during the early holocene triggered the development of important cultural innovations mainly pottery production cooking wild cereal grains in a ceramic container would have enabled a very effective exploitation of the vast sahelo sudanian grasslands which remained to be successful until modern times

contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the grenville province of western labrador contrasting coronas around olivines pyroxenes and ilmenites occur in metagabbros in two adjacent terranes in the grenville province of western labrador in the parautochthonous gagnon terrane coronas around olivines display evidence of evolution from double coronas of orthopyroxene and fluorine rich pargasitic amphibole through triple coronas of orthopyroxene garnet clinopyroxene pargasitic amphibole to double coronas of orthopyroxene garnet clinopyroxene parentheses enclose phase occurring as inclusions in previous phase in the allochthonous lac joseph terrane coronas around olivines consist of orthopyroxene clinopyroxene spinel or orthopyroxene pargasitic amphibole spinel brackets enclose phases occurring in symplectic intergrowths possible reactions that explain the development of these assemblages are investigated and it is concluded that the system was closed at the level of the corona the silica required for the olivine plagioclase reaction in gagnon terrane was gene

pan african subduction collision event evidenced by high p coronas in metanorites from the agou massif southern togo the ultrabasic basic complexes from se ghana and togo to n b enin are generally considered as an unique lithotectonic unit which may decorate the suture zone of the pan african collision belt in the southern region of togo the agou massif is one of these massifs mainly consisting of granulite facies rocks metagabbronorites and metagabbros outcrop in the innermost part of the agou igneous complex aic whereas in the outermost part relict boudins of metagabbronorites are surrounded by related retrogressed rocks such as layered grt bearing amphibolites and amphibolites the latter are intruded by leucocratic to melanocratic veins including dykes of microgabbros post solidus cooling of the metagabbronorites leads to the production of ht mp granulites m1 granulite stage opx1 cpx1 pl1 ilm sil t 850 c p 8kbar following this an evolution toward high pcoronitic granulites is indicated by early cpx and grt coronite developed around opx1 and pl1 granulite 2 stage cpx2 grt2 pl2 qtz sil rt 770 c 10kbar three successive stages of retrogression are then observed from i hornblende granulite facies 700 750 c 9kbar ii to amphibolite facies 700 c 6 7kbar and iii finally greenschist facies conditions 550 500 c 3 4kbar the inferredp tpath defines an unusual anti clockwise loop characterized by a slight pressure increase according to their calc alkaline signature and their metamorphic evolution the granulites of agou are derived from deep seated basic intrusion equilibrated in the lower continental crust under mp ht conditions and never experienced an early eclogite stage this contrasts with granulites derived from eclogites that are also described in sw togo in order to explain the existence of two types of metabasites in togo we propose that the agou massif corresponds to basic magmas emplaced and cooled at the base of an active continental margin western part of the benino nigerian shield whereas the former eclogites reflect subduction of the passive margin of the west african carton during subduction process or at the transition from subduction to collision at c 610ma burial of the edge of the active margin in the subduction zone led to the formation of coronitic granulites during the pan african orogeny at c 580ma the coronitic granulites together with the eclogites were exhumed under amphibolite facies conditions 2004 elsevier b v all rights reserved

evidence for oligocene middle miocene abyssal circulation changes in the western north atlantic previous studies of benthic foraminiferal isotopic composition have demonstrated that a latest eocene earliest oligocene benthic foraminiferal 18o increase occurred in the pacific southern and atlantic oceans1 9 a middle miocene 18o increase has been noted in the pacific southern and south atlantic oceans1 3 7 10 11 and tentatively identified in the north atlantic12 13 due to the incomplete nature of the north atlantic stratigraphical record14 15 however the oligocene to middle miocene isotopic record of this ocean is poorly understood in the modern ocean the north atlantic and its marginal seas has a critical role in abyssal circulation influencing deep and bottom water hydrography as far away as the north pacific16 18 we now report oxygen isotope measurements on oligocene to middle miocene 12 36 myr bp benthic foraminifera in the western north atlantic which show two periods of enriched 18o values early oligocene and early middle miocene these enriched intervals are interpreted as resulting in part from the build up of continental ice sheets the oligocene to middle miocene 13c record shows three cycles of enrichment and depletion of large enough magnitude to be useful for time stratigraphical correlations within the biostratigraphical age resolution 18o and 13c records correlate with records from other oceans helping to establish a useful tertiary isotopic stratigraphy an atlantic pacific 13c contrast of 0 3 0 9 during the latest oligocene to middle miocene 12 26 myr bp indicates north atlantic deep and bottom water production analogous to modern north atlantic deep water nadw

neogene and quaternary coexisting in the geological time scale the inclusive compromise removing the tertiary and quaternary periods whilst conserving the paleogene and neogene periods in the geological timescale 2004 caused a storm of protest one response was to advocate restoring an enlarged quaternary and consigning the neogene to a minor role within the tertiary amongst an array of practical traditional sentimental and anthropocentric reasons for this response the one hard core justification was that the rigidly nested hierarchy of the geological timescale must be preserved the central objective of this paper is conserving the historically legitimate miocene present neogene period and system there are two options for conserving the quaternary concurrently with the neogene i an inclusive compromise in a flexible hierarchy and ii an upgrading of pliocene and pleistocene divisions to the level of epoch in the inclusive compromise there coexist alternative pathways through the hierarchical ranks thus geohistorians and biohistorians have two options for traversing the hierarchy from era to age as in this example using the hierarchical positioning of the calabrian age and stage either cenozoic era neogene period pleistocene epoch calabrian age or cenozoic era quaternary subera pleistocene epoch calabrian age we reaffirm that the inclusive compromise is entirely viable in so doing we i challenge the necessity of the rigidly nested hierarchy which should be capable of a little flexibility ii reject all analogies of the arbitrary and conventional chronostratigraphic hierarchy with three natural biological hierarchies iii reaffirm the integrity of the neogene extending to the present and iv see no reason to doubt the harmonious coexistence of the two options preserving the quaternary and neogene traditions in an orderly working and stable time scale in the alternative schema conserving the neogene divisions of the pliocene and pleistocene are upgraded so that the late pleistocene early pleistocene and late pliocene epochs comprise the quaternary subperiod itself equivalent to late neogene the inflexibly nested hierarchy is preserved but the tertiary is lost

diffusion models for corona formation in metagabbros from the western grenville province canada metagabbro bodies in sw grenville province display a variety of disequilibrium corona textures between spinel clouded plagioclase and primary olivine or opaque oxide textural evidence favours a single stage subsolidus origin for the olivine coronas and diffusive mass transfer is believed to have been the rate controlling process irreversible thermodynamics have been used to model two different garnet symplectite bearing corona sequences in terms of steady state diffusion in the models the flux of each component is related to the chemical potential gradients of all diffusing species by the onsager or l coefficients for diffusion these coefficients are analogous to experimentally determined diffusion coefficients d but relate the flux of components to chemical potential rather than concentration gradients

large scale fluid infiltration metasomatism and re equilibration of archaean basement granulites during palaeoproterozoic thrust belt construction ungava orogen canada abstract the metamorphic history of the archaean superior province crystalline basement in the palaeoproterozoic ungava orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite granulite transition two distinct metamorphic suites separated in age by nearly one billion years are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses the older suite comprises c 2 7 ga granulite facies assemblages orthopyroxene clinopyroxene hornblende plagioclase ilmenite biotite quartz that record moderate pressures 5 kbar and high temperatures 800 c a younger c 1 8 ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal decollement of the overlying proterozoic cape smith thrust belt deformation mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re equilibration of the archaean granulites occurred during thrust belt deformation the transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet hornblende quartz cummingtonite coronas between plagioclase and orthopyroxene clinopyroxene as well as titanite coronas on ilmenite multi equilibrium thermobarometry on the coronitic assemblages documents re equilibration of the granulitic gneiss to 7 7 kbar at 644 c in the south and 9 8 kbar at 700 c in the north the variably deformed amphibolite facies domain sandwiched between the coronitic garnet zone and the basal decollement is marked by significant metasomatic changes in major element concentrations within tonalite these changes are compatible with equilibrium flow of an aqueous chloride fluid down a temperature gradient the source of fluids for basement hydration metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt with fluid flow probably focused along the basal decollement

matrix rheology effects on reaction rim growth i evidence from orthopyroxene rim growth experiments the rim forming reaction quartz olivine orthopyroxene is used to investigate the effect of matrix rheology on rim growth rates orthopyroxene rim growth around olivine grains in quartz matrix is compared to rim growth around quartz grains in an olivine matrix at constant p t within one single capsule orthopyroxene rims grow faster around quartz clasts in olivine matrix than around olivine clasts in quartz matrix fourier transform infra red spectra indicate that the entire samples are water saturated because of water adsorption on the reactant grain surfaces the increased orthopyroxene growth rates in olivine matrix as opposed to quartz matrix are interpreted in terms of matrix rheology where in the two different matrix inclusion arrangements the olivine matrix behaves softer and the quartz matrix more rigid the strain energy associated with accommodation of the negative reaction volume is higher for the quartz than the olivine matrix and reduces the free energy that drives orthopyroxene rim growth growth textures in both kinds of orthopyroxene rims indicate that the diffusivity of mgo slightly exceeds the diffusivity of sio2 the relative mobility of mgo and sio2 at given p t fh2o seems to be controlled by energy minimization during orthopyroxene growth at the compressive ol opx interface our experiments provide evidence for two previously overlooked effects relevant to rim growth reactions in metamorphic rocks i diffusivity along chemical potential gradients to reaction sites is a function of rheology and ii the relative diffusivity of components during reaction rim or corona growth is a function of local volume changes at the rim s interfaces

miocene biochronology and paleoceanography of the north pacific abstract biostratigraphic correlation based on microfossil datum levels directly or indirectly tied to the paleomagnetic time scale provides a high resolution time control for the miocene in the equatorial and middle latitude north pacific faunal changes and abundance fluctuations of planktic foraminiferal species combined with the oxygen pacific faunal changes and abundance fluctuations of planktic foraminiferal species combined with the oxygen isotope record of foraminifers reveal the paleoclimatic and paleoceanographic history the planktic foraminiferal assemblage change in the early miocene extinction of oligocene fauna and rise of a highly diverse neogene fauna appears to be related to increased water mass stratification in the world oceans presumably resulting from the establishment of circum antarctic circulation an increase in the siliceous productivity in the eastern equatorial pacific region between 20 and 18 ma suggests that the vertical and horizontal circulation was intensified at that time climates cooled rapidly during the middle miocene between 14 and 13 ma suggesting the growth of a major east antarctic ice sheet paleoclimatic conditions remained generally cool although oscillating during the late miocene in the late early to middle miocene faunal provincialism developed between low and middle latitudes and by late miocene time a distinct provincialism similar to the present was established

high pressure granulite facies metamorphism in the pan african dahomeyide orogen west africa high pressure hp granulite facies rocks are associated with eclogites in the pan african dahomeyide orogen west africa the hp granulite and eclogite facies mineral assemblages occur in mafic gneisses within the suture zone exposed in southeastern ghana and the adjoining part of togo thermobarometric calculations and phase equilibria considerations indicate that the hp metamorphic rocks preserve a record of i prograde garnet crystallization during eclogite facies metamorphism as temperature increased to a maximum of 600 c and pressure reached 18 kbar followed by ii hp granulite facies metamorphism when temperatures reached 800 900 c at p 14 kbar resulting in partial melting to produce leucocratic veins decompression along a retrograde path is inferred from granulites that yield temperatures down to 700 c at 10 kbar the inferred p t path of the metamorphic record defines a clockwise loop in the kyanite stability field compatible with orogeny involving subduction during the pan african as

chronology of nappe assembly in the pan african dahomeyide orogen west africa evidence from 40ar39ar mineral ages abstract the pan african dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the west african craton was incorporated into gondwana orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in ghana and togo external nappes include the atacora composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c 2 0 ga foreland basement the suture zone is represented by high pressure granulite facies garnet homblende gneiss we have analyzed muscovite and homblende concentrates from external and suture zone nappes in incremental heating 40 ar 39 ar experiments muscovites which define the penetrative foliation in the atacora nappes and a shear foliation in the basement parautochthon record 40 ar 39 ar plateau ages of 579 4 0 8 ma and 578 1 0 5 ma respectively in contrast c 600 km to the north muscovite in quartz schist near the base of the atacora nappes displays a spectrum which gives a minimum age of 608 1 1 2 ma whereas muscovite in the overthrust kara nappes yields a plateau age of 633 8 0 5 ma the c 30 m y difference between the atacora nappes in the northern dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication however forward imbricating thrusting is also suggested by the older age of kara nappes relative to atacora nappes in the northern segment the youngest ages are inferred to date orogen parallel nappe transport at c 575 ma hornblende from the structurally lower sections of the suture zone nappes record a 36 ar 40 ar versus 39 ar 40 ar isotope correlation age of 587 4 3 ma hornblende from a tonalitic vein in the host garnet mafic gneiss yields an isotope correlation age of 581 9 2 4 ma these ages are interpreted to date the exhumation of the suture zone nappes and thus provide a minimum age of crustal thickening in the dahomeyide orogen the age data are consistent with coeval forward and hindward propagating thrusting and nappe imbrication and constrain the amalgamation in northwest gondwana to have occurred in the latest proterozoic rather than cambrian

middle miocene pliocene stable isotope stratigraphy and paleoceanography of the mediterranean abstract stable isotope analyses of the planktonic foraminifers globigerinoides obliquus and globigerinoides trilobus from a number of sections from sicily italy yielded a rather detailed middle miocene to upper pliocene record a comparison between this record and previously published stable isotope records from the pacific and atlantic oceans suggests that at least during the time span considered the mediterranean had a paleoclimatic and paleoceanographic history that bears a close resemblance to that of the world s oceans four different local mediterranean overprints on the general isotope signal are distinguished 1 the disruption of the tethys sea way and the subsequent installation of a strong salinity stratification 2 short term eustatic sea level changes during the miocene resulting in salinity stratification and sapropel deposition 3 periodic increase of run off and subsequent salinity stratification during the pliocene also resulting in sapropel deposition 4 pliocene trends in the carbon isotope pattern deviating from the oceanic one the reason for which remained obscure

fluvial system dynamics of sudano sahelian zone during the late holocene the yame river dogon country mali abstract in west africa most palaeoenvironmental studies have focused on the role of climatic change on the environment because of the richness of its deposits the yame river mali a tributary of the niger river offers an opportunity to reconstruct the evolution of this fluvial system in comparison with both climatic and anthropogenic changes during the late holocene to investigate more closely the spatio temporal response of the fluvial system sedimentary analyses were conducted on seven reaches distributed along the 137 km length of the yame valley this approach testifies of the possibility for each time slice and river reach 1 to reconstruct the fluvial style and processes and 2 to estimate the sediment storage volume reflecting sediment distribution patterns results reveal a wide variability of the sedimentary cascade suggesting contrasting responses to external and local controls humid 4200 2900 cal bp 450 24 cal bp or arid 2350 1700 cal bp phases have been recorded in fluvial archives while an intensification in erosion and sediment supplies even during the arid period 2900 2350 cal bp and the 20th century may be associated with an increase in human pressure two other periods are related to both climate and anthropogenic factors during 1700 1400 cal bp phase huge sediment supplies can be explained mainly by the reactivation of both hydrological processes and human occupation after an intense and long arid event dispersed sediment sinks recorded during the 450 24 cal bp period originate from the combination of intensification of colluvial processes and a wet phase context originating from global climatic change thus this study provides evidence that climate is the strongest driver for the fluvial response of these semi arid and tropical rivers while human disturbance appear as a secondary factor due to the high sensitivity of the environment to climate variability in such areas

radiolarian faunal turnover across the oligocene miocene boundary in the equatorial pacific ocean abstract the global warming trend of the latest oligocene was interrupted by several cooling events associated with antarctic glaciations these cooling events affected surface water productivity and plankton assemblages well preserved radiolarians were obtained from upper oligocene to lower miocene sediments at ocean drilling program odp leg 199 sites 1218 and 1219 in the equatorial pacific and 110 radiolarian species were identified four episodes of significant radiolarian faunal changes were identified middle late oligocene 27 5 to 27 3 ma latest oligocene 24 4 ma earliest miocene 23 3 ma and middle early miocene 21 6 ma these four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in 18o values coded as oi and mi events these data indicate that antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial pacific radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity species richness and the composition of dominant species a late oligocene assemblage 27 6 to 24 4 ma a transitional assemblage 24 4 to 23 3 ma and an early miocene assemblage 23 3 to 21 2 ma the late oligocene assemblage is characterized by relatively high productivity low species richness and four dominant species of tholospyris anthophora stichocorys subligata lophocyrtis nomas and lithelius spp the transitional assemblage represents relatively low values of productivity and species richness and consists of three dominant species of t anthophora s subligata and l nomas the characteristics of the early miocene assemblage are relatively low productivity but high species richness the two dominant species present in this assemblage are t anthophora and cyrtocapsella tetrapera the most significant faunal turnover of radiolarians is marked at the boundary between the transitional early miocene assemblages we also reviewed changes in other microfossil assemblages in the low latitudes during the late oligocene through early miocene the microfossil assemblages of major groups show sequential changes near the oligocene miocene o m boundary 23 8 ma many extinction events and some first occurrences of calcareous nannofossils and many occurrences of radiolarians are found from about 24 8 to 23 3 ma and first occurrences of planktic foraminifers and diatoms followed from 23 2 through 22 ma hence the o m boundary is identified as a significant level for microfossil evolutions

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

matrix analysis of metamorphic mineral assemblages and reactions assemblage diagrams are widely used in interpreting metamorphic mineral assemblages in simple systems they can help to identify assemblages which may represent equilibrium states to determine whether differences between assemblages reflect changes in metamorphic grade or variations in bulk composition and to characterize isograd reactions in multicomponent assemblages these questions can be approached by investigating the rank composition space range and reaction space null space of a matrix representing the compositions of the phases involved singular value decomposition svd provides an elegant way of 1 finding the rank of a matrix and detemining orthonormal bases for both the composition space and the reaction space needed to represent an assemblage or pair of assemblages and 2 finding a model matrix of specified rank which is closest in a least squares sense to an observed assemblage although closely related to least squares techniques the svd approach has the advantages that it tolerates errors in all observations and is computationally simpler and more stable than non linear least squares algorithms models of this sort can be used to interpret multicomponent mineral assemblages by straightforward generalizations of the methods used to interpret assemblage diagrams in simpler systems svd analysis of mineral assemblages described by lang and rice 1985 demonstrates the utility of the approach

metamorphic reactions in dry and aluminous granulites a perple_x p t pseudosection analysis of the influence of effective reaction volume in high mg al metapelites monophase sapphirine corona occur around spinel corundum aggregates in monomineralic cordierite layers and bi phase orthopyroxene sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates p t phase topologies computed perple x software using compositions of cordierite rich layers that host the reaction textures did not match the assemblages for the discontinuous reactions spinel corundum cordierite sapphirine and sapphirine cordierite orthopyroxene sillimanite instead the reaction assemblages were reproduced using p t pseudosection analysis for micro domain reaction volumes estimated from compositions of product phases in the volume proportion they occur the results are consistent with known phase relations deduced using schreinmakers p t grids apparently the compositions of cordierite rich layers that hosted the reaction textures were inadequate chemical proxies for determining p t x relations of phase boundary controlled reactions influenced by compositions of the nearest neighbor minerals in the proportion they react effective composition and not in the proportion they existed in the layer bulk rock in other words p t x phase topologies and reconstructed p t paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures

late miocene early pliocene planktonic foraminiferal biostratigraphy and paleoceanography of low latitude marine sequences abstract plate reorganization and development of polar glaciation are closely associated with the changing climatic conditions of the cenozoic the planktonic foraminiferal fauna in three low latitude dsdp sites 224 ninety east ridge 317b manihiki plateau 366a sierra leone rise has been examined to determine how these regions responded to the late miocene climatic cooling that has been previously observed in high latitude regions it has been proposed that an expansion of antarctic glaciation and a resultant eustatic regression are associated with this cooling with the latter being at least partially responsible for the messinian salinity crisis in the mediterranean only one of the sites 366a shows any significant faunal change during the late miocene and early pliocene approximately 8 3 ma during the late miocene there is a decrease in the abundance of species considered to be tropical subtropical suggesting the incursion of a cool water mass into the sierra leone rise region during this time the lack of any major faunal changes at sites 214 and 317b indicates that the water masses of the low latitude regions of the indo pacific were relatively unaffected by this cooling in the late miocene the three sites also show no evidence for a change in the level of the ccd between the late miocene and early pliocene however this is most likely due to their equatorial position and shallow water depth higher latitude sites from both the atlantic and the pacific reveal a definite shoaling of the ccd during the late miocene

a late quaternary sedimentary shelf system under hyperarid conditions unravelling climatic oceanographic and sea level controls golfe d arguin mauritania nw africa abstract the sedimentation on the mauritanian shelf off the western saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping shallow acoustic profiling and the analysis of sediment cores coarse grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the atlantic ocean the sheltered golfe d arguin in contrast is characterized by muddy shoals in its inner area and locally restricted and wedge shaped muddy depocenters which extend contour parallel in mid shelf settings in both cases cross shelf currents control sediment distribution and accumulation not location or amount of primary eolian input a major discontinuity on the outer shelf is located in 85 110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface tidal and open shelf deposits this succession has formed during temporary lower sea levels along the last overall regression mis 4 and mis 3 the last glacial maximum and during the past deglacial sea level rise significant sediment deposition and preservation is limited to very restricted areas concerning the overall shelf architecture sea level was the main factor enabling or suppressing deposition in terms of 1 creating local erosional swales which have favored subsequent trapping and preservation of sediments 2 determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions 3 interacting with the seafloor morphology and by this controlling timing of initiation and growth geometry of mud wedges these mud wedges have formed since ca 9 cal ka under a continuous sedimentation regime these bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves in general sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input strong ravinement activity during changing sea levels significant bypassing and winnowing processes and a low preservation potential of the non cohesive materials these conditions lead to the development of almost sediment starved systems with low subsidence rates over longer geological timescales

stepwise mass extinctions and impact events late eocene to early oligocene abstract species ranges and relative abundances of dominant planktonic foraminifers of eight late eocene to early oligocene deep sea sections are discussed to determine the nature and magnitude of extinctions and to investigate a possible cause effect relationship between impact events and mass extinctions late eocene extinctions are neither catastrophic nor mass extinctions but occur stepwise over a period of about 1 2 million years four stepwise extinctions are identified at the middle late eocene boundary the upperglobigerapsis semiinvoluta zone theg semiinvoluta globorotalia cerroazulensis zone boundary and at the eocene oligocene boundary each stepwise extinction event represents a time of accelerated faunal turnover characterized by generally less than 15 species extinct and in itself is not a significant extinction event relative species abundance changes at each stepwise extinction event however indicate a turnover involving 60 of the population implying major environmental changes there microtektite horizons are present in late eocene sediments one in the upperg semiinvoluta zone 38 2 ma and two closely spaced layers only a few thousand years apart in the lower part of thegloborotalia cerroazulensis zone 37 2 ma each of the three impact events appears to have had some effect on microplankton communities however the overriding factor that led to the stepwise mass extinctions may have been the result of multiple causes as there is no evidence of impacts associated with the step preceding or the step following the deposition of the presently known microtektite horizons

tectonometamorphic evolution of ghana togo and benin in the light of the pan african brasiliano orogeny abstract the south pan african belt of west africa ghana togo and benin province can be divided into three domains 1 1 the external nappes thrust over the west african craton derived from passive margin sedimentary deposits middle to late proterozoic and displaying middle to high pressure metamorphism such as the kante and atacora nappes 2 2 the intermediate nappes including the suture zone rocks with eclogitic assemblages protoliths ca 800 ma and mainly monocyclic metasedimentary rock and orthogneiss that are found in central togo and northwest benin 3 3 the internal nappes composed of high grade commonly anatectic gneiss anatexis ca 600 ma including orthogneiss eburnian plutons elongated granulite belts reworked archean basement and an intrusive granite and charnockite association ca 600 ma which are exposed in benin and nigeria the p t t d paths calculated from rocks in these three domains show a prograde retrograde evolution 1 1 an initial clockwise evolution related to an early burial stage hp ht in the intermediate nappes followed by oblique thrusting in a sw direction that caused nappe stacking and subsequent uplift 2 2 a second anti clockwise evolution with late temperature and pressure increases only recorded in the more internal units this temperature increase was a consequence of the previous crustal thickening with subsequent doming extension and anatexis the pressure increase was related to movement along the kandi fault working as a dextral strike slip fault with a transpressional component under late e w shortening the retrograde trend corresponds to the final uplift the evolution of the belt can thus be divided into three successive stages 1 1 a first stage was characterized by an oblique collision inducing sw oblique thrusting with associated syn foliation reverse metamorphism 2 2 during a second stage anatectic doming took place followed by 3 3 a third stage at still high temperature with dextral wrench faulting and associated n s post foliation folding under late e w shortening comparison between the west african north brasiliano and central african late proterozoic belts shows very strong similarities between their lithology kinematics and metamorphism and indicates that the pan african brasiliano chain resulted from collision between three major continental domains the west african and sao francisco congo cratons and a late proterozoic mobile domain

miocene stable isotope record a detailed deep pacific ocean study and its paleoclimatic implications deep sea drilling project site 289 in the western equatorial pacific has yielded an extremely detailed record of the carbon and oxygen isotopic changes in the miocene deep ocean the isotopic record reflects major changes in paleoclimate and paleoceanography probably dominated by a major phase of antarctic ice cap growth the transition from a relatively unglaciated world to one similar to today occurred between 16 5 x 10 6 and 13 x 10 6 years before the present with the greatest change occurring between approximately 14 8 x 10 6 and 14 0 x 10 6 years before the present

thermal evolution of two textural types of mafic granulites in the north china craton evidence for both mantle plume and collisional tectonics mafic granulites from the north china craton can be divided into two textural types referred to as a and b types a type mafic granulites display garnet quartz symplectic coronas and outcrop in the eastern and western zones of the craton whereas b type mafic granulites exhibit orthopyroxene plagioclase clinopyroxene symplectites or coronas and are mainly exposed in the central zone of the craton most a type mafic granulites preserve the prograde m 1 peak m 2 and post peak near isobaric cooling m 3 assemblages which are represented respectively by inclusions of hornblende plagioclase quartz a peak mineralogy of orthopyroxene clinopyroxene plagio clase quartz garnet and overprinted by garnet quartz symplectic coronas these mineral assem blages and their p t pressure temperature estimates define anticlockwise p t evolutionary paths the b type mafic granulites preserve the peak m 1 post peak near isothermal decompression m 2 and cooling m 3 assemblages which are represented by the peak assemblage of orthopyroxene clinopyroxene plagioclase quartz garnet hornblende post peak orthopyroxene plagioclase clinopyroxene symplectites or coronas and later hornblende plagioclase magnetite symplectites respectively these mineral assemblages and their p t estimates define clockwise p t paths the anticlockwise p t paths of the a type mafic granulites in the eastern and western zones of the north china craton are consistent with a model of underplating and intrusion of mantle derived magmas in combination with lithological structural and geochronological data the eastern and west ern zones of the north china craton are considered to represent two continental blocks that devel oped through the interaction of mantle plumes with the lithosphere from the palaeoarchaean to the neoarchaean era the b type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and west ern continental blocks in the late palaeoproterozoic era the mineral reaction relations and clockwise p t paths of the b type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the north china craton at c 1800 ma

late miocene development of the western pacific warm pool planktonic foraminifer and oxygen isotopic evidence abstract the disappearance at 10 ma of the deep dwelling planktonic foraminifer globoquadrina dehiscens from the western pacific including the south china sea was about 3 myr earlier than its final extinction elsewhere accompanying this event at 10 ma was a series of faunal turnover characterized by increase in mixed layer warm water species and decrease to a minimum in deepwater species paleobiological and isotopic evidence indicates sea surface warming and a deepened local thermocline that we interpret as related to the development of an early western pacific warm pool the stepwise decline of g dehiscens and other deep dwelling species from the nw and sw pacific suggests more intensive warm water pileup than equatorial localities where surface bypass flow through the narrowing indonesia seaway appears to remain efficient during the late miocene planktonic 18o values from the south china sea consistently lighter than the tropical western pacific during the miocene also suggest similar to today more variable hydrologic conditions along the periphery than in the core of the warm pool stronger hydrologic variability affected mainly by monsoons and increased thermal gradient along the western margin of the late miocene warm pool may have contributed to the decline of deep dwelling planktonic species including the early extinction of g dehiscens from the south china sea region the late miocene warm pool became influential and paleobiologically detectable from 10 ma but the modern warm pool did not appear until about 4 ma in the middle pliocene

late neogene paleoceanography and paleoclimatology of the gulf of aden region based on calcareous nannofossils quantitative study of climatically sensitive nannofossils in the sediments of the gulf of aden has provided a climatic and oceanographic scenario for the northwestern indian ocean during the late neogene the surface water temperature was relatively cold during the latest middle miocene to late miocene 10 7 to 6 32 ma with a probable warm interval in the early late miocene 9 82 to 8 49 ma this cooling is attributed to increased upwelling in the northwestern indian ocean the surface water was relatively warm from the latest late miocene to the middle of the late pliocene 6 32 to 2 59 ma the most intense cooling took place during the late pliocene to early pleistocene 2 59 to 1 69 ma intense upwelling during this interval coincides with worldwide climatic deterioration a probable short warm interval 1 69 to 1 59 ma followed this cold interval in the pleistocene the surface water was cold from 1 59 to 1 00 ma and from 0 61 to 0 24 ma and warm from 1 00 to 0 61 ma and after 0 24 ma

palynological evidence for climatic and oceanic variability off nw africa during the late holocene pollen and organic walled dinoflagellate cyst assemblages from core geob 9503 5 retrieved from the mud belt 50 m water depth off the senegal river mouth have been analyzed to reconstruct short term palaeoceanographic and palaeoenvironmental changes in subtropical nw africa during the time interval from ca 4200 to 1200 cal yr bp our study emphasizes significant coeval changes in continental and oceanic environments in and off senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca 2900 and 2500 cal yr bp after ca 2500 cal yr bp the environment slowly became drier again as indicated by slight increases in sahelian savannah and desert elements in the pollen record around ca 2200 cal yr bp this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen dinoflagellate cysts fresh water algae and plant cuticles suggesting episodic flash flood events of the senegal river the driest phase developed after about 2100 cal yr bp

sediment yield from disturbed earth systems changes of sediment yield rates through time reflect evolutionary changes within a landscape when a drainage basin is perturbed significantly by base level climatic or tectonic change sediment yields increase dramatically but with no further disturbance they decline rapidly these sediment yield changes have been documented at all scales from small experimental studies to incised channels to the colorado river basin and to the himalaya mountains thus the shape of the sediment yield curve can be used to estimate future sediment yields and to interpret past tectonic events

miocene benthic foraminiferal isotope records a synthesis abstract 18 o 16 o and 13 c 12 c ratios of miocene benthic foraminifera from a number of atlantic pacific and indian ocean dsdp sites 71 77b 206 208 238 279 289 296 329 357 and 366a have been compiled these provide a rather detailed history of miocene deep water especially in the pacific ocean bottom water temperatures rose during the early miocene and then declined rapidly during the middle miocene this decline was accompanied by an increase in antarctic glaciation late miocene bottom temperatures and antarctic ice volumes are inferred to be similar to today s but exhibited some fluctuation the early miocene ocean was less thermally stratified at intermediate and abyssal depths while the late miocene deep ocean had a thermal structure generally similar to the modern ocean foraminiferal carbon isotope ratios at most of the sites varied quasi sympathetically throughout the miocene these variations must reflect comparable variations in the mean 13 c 12 c of marine hco 3 however the causes of such variations are not yet clear

post miocene cold water molluscan faunas from hokkaido northern japan abstract pliocene to recent molluscs from hokkaido are divided mainly into three faunas from oldest to youngest the takikawa honbetsu the setana and the oyashio faunas they correspond to the fundamental division of japanese cold water faunas in post miocene time biostratigraphic and radiometric data show that the takikawa honbetsu stage is assigned to ca 6 0 2 0 ma the setana stage to ca 2 0 0 6 ma and the oyashio stage to 0 6 ma present the takikawa honbetsu fauna characterized by fortipecten takahashii flourished under cool temperate to subarctic climatic conditions the setana fauna can be subdivided into two lower and upper by stratigraphic horizon and species composition the lower setana fauna including yabepecten tokunagai existed under subarctic climatic conditions the onset of the setana stage is marked in particular by cryophilic taxa such as chlamys islandicus this cooling event appears to have resulted from an intensification of glaciation during the early pleistocene the upper setana fauna is in turn characterized by mizuhopecten tokyoensis s s and lived under cool temperate climatic conditions the oyashio fauna is composed of extant cold water species such as mizuhopecten yessoensis that have been affected by the oyashio current however a molluscan fauna characterized by warm water elements lived in hokkaido for a short time in the middle pleistocene ca 0 4 ma during a warm interglacial period the faunal succession mentioned above is presumably controlled by environmental changes such as glacio eustacy climatic changes tectonic movements etc which may have occurred in the northwestern pacific during the late cenozoic time the appearance of these faunas in hokkaido probably resulted from the immigration of boreal elements from the northern regions

paleodepth distribution of neocene deep sea hiatuses the depth of formation of miocene to middle pliocene deep sea hiatuses nh1 to nh8 has been determined using 152 deep sea drilling project sites in the atlantic pacific and indian oceans megahiatuses representing maxima in deep sea erosion occur at three water depths below 3800 m at intermediate depths between about 2000 and 3000 m and in shallower waters above 1500 m paleodepth both the paleodepths and distribution of these hiatus maxima suggest that flow of antarctic bottom water antarctic intermediate water and north atlantic deep water masses largely caused this erosion correlation of the hiatuses with oxygen isotope carbonate and sea level records indicates that prior to 11 ma brief hiatuses including those resulting from nondeposition in high productivity equatorial regions largely correlate with global cooling episodes high carbonate content and lowstands of sea level during the last 11 m y hiatuses also seem to correlate with cooling episodes but carbonate dissolution is characteristic and sea levels may have been rising or at a lowstand during the nh4 nh7 and nh8 intervals it is possible that a rise in sea level corresponds with polar cooling but there is uncertainty in correlation brief hiatuses during rising sea levels can be explained by basin shelf fractionation of carbonates hiatuses during sea level lowstands and cooling episodes may result from intensified bottom water circulation and increased corrosiveness of bottom water due to higher levels of co2 and increased productivity during increased upwelling supplement appendix 1 is availablewith entire article on microfiche orderfrom american geophysical union 2000florida avenue n w washington dc20009 document p87001 2 50 paymentmust accompany order

models for orthopyroxene plagioclase and other corona reactions in metanorites dahomeyide orogen west africa coronal reaction textures occur in metanorite and related intrusions in the pan african dahomeyide orogen of west africa they apparently formed by retrograde subsolidus reactions during cooling of the intrusions from 900 to 700 c at c 9 1 kbar the coronas that formed around orthopyroxene opx consist of sequential layers of diopside dio hornblende hbl garnet grt and plagioclase pl with inclusions of kyanite ky or sillimanite sil three well organized mineral assemblage sequences have been identified and modelled using steady state diffusion theory for closed and open systems the mineral sequence opx hbl qtz hbl pl ky formed by diffusion controlled reactions in a closed system as layer thicknesses are very sensitive to relative component mobilities defined by onsager diffusion coefficient lii models of this corona type i that satisfy modes require lcaca lmgmg lfefe lalal lsisi however small open system fluxes involving loss of al and gain of ca are required to obtain the best fit between model and observed mineral proportions under steady state diffusion the monomineralic hornblende layer grew by replacing plagioclase whereas the hbl qtz grew by opx replacement the type ii corona which consists of the sequence opx dio grt dio pl sil is also stable under steady state diffusion in a closed system modelling results show that the diopside grew by replacing opx whereas most of grt dio grew by replacing plagioclase stable solutions to the closed system diffusion model that approximate the mode are restricted to the l ratio regions where fe mg and ca are more mobile than si and al but are unstable when lalal 100 lsisi however type ii corona mineral proportions were only closely matched when open system loss of al and gain of small amounts of fewere considered in the diffusion models and relative mobilities were lfefe lcaca lmgmg lalal lsisi the modelling results indicate that ca and mg were the most mobile elements in the formation of type i corona whereas fe and ca were the most mobile components in the growth of type ii coronas a third corona type consisting of the mineral sequence act hbl grt pl sil is only stable in open systems and requires large external fluxes involving gain of fe al mg and na and loss of ca to obtain a solution of the diffusion model that approximates the estimated mineral proportions extensive recrystallization of plagioclase to produce sillimanite or kyanite inclusions accompanying corona formation may explain the open system behaviour indicated by the diffusion models

paleo oceanography global ocean evolution paleoceanography one of the youngest branches of earth sciences is concerned with the development of oceanic circulation through geologic time this is a multifaceted and integrated endeavor since none of the subdisciplines is easily separated from the others the field is holistic in character and includes the history and effects of bottom intermediate and surface circulation patterns planktonic and benthonic biogeographic development and rates of organic evolution the history of biogenic productivity and dissolution and their effect on sediment distribution and the consequences of sea level change paleoceanography was largely born of the deep sea drilling project dsdp and continues to be nourished by it because problems are of global scope the appetite for new sediment core material is large and is being supplied by numerous expeditions of dsdp this article is a brief overview of a number of advances that have occurred in paleoceanography and related fields during the period from 1979 to 1982 because of page limitations all publications cannot be cited which is not meant to indicate any lack of importance

seasonality change and the third millennium bp rainforest crisis in southern cameroon central africa the third millennium bp crisis of the central african rainforest is not sufficiently understood the low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event and its causal climatic parameters are debated we present a high resolution pollen profile from the swamp site nyabessan in the southern cameroonian rainforest covering the period 3100 2300 cal yr bp between 3100 and 2500 cal yr bp the climate was favourable for a regional evergreen forest with caesalpiniaceae and lophira and a local raphia swamp forest around 2500 2400 cal yr bp a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations the dominance of trema orientalis a pioneer well adapted to seasonal desiccation points to a much more accentuated seasonality after 2500 cal yr bp which seems to be linked to a southwards shift of the itcz during the northern hemisphere winter months we propose that the rain forest crisis between 2500 and 2200 cal bp created favourable conditions for farming and paved the way for a major expansion of bantu speaking populations

latest cretaceous early neogene oxygen and carbon isotopic record at dsdp sites in the indian ocean abstract stable isotopic data of calcareous nannofossil monogeneric and monospecific planktic and benthic foraminifera from five indian ocean dsdp sites 212 217 220 237 and 253 leads to the following paleoclimatic and paleoceanographic conclusions the latest cretaceous oxygen isotopic record implies a cooling 3 4 c during the maastrichtian at the cretaceous tertiary boundary only a minor warming about 2 c has been recorded the parallel 13 c decrease of more than 1 indicates a significant decrease in productivity during the latest paleocene a positive 13 c excursion was detected in sites 217 and 237 this transient enrichment in 13 c may be due to productivity changes on continents and or a change in the storage rate of organic matter in marginal basins or shelf areas the most striking feature in the oxygen isotopic record is noted at the early middle eocene transition the shift towards more positive values which were probably enhanced to a certain extent by a preceding diagenetic alteration delineates a dramatic climatic deterioration at high and mid latitudes during the earlier tertiary near the eocene oligocene boundary a cooling is evident within the latest eocene interval during the earliest oligocene time a hiatus at sites 217 and 253 partially obscures the climatic record several climatic fluctuations have been noted during the oligocene a cooling at the base of zone np 23 a warming at the top of zone np 23 through np 24 and a cooling during zone np 25 the miocene oxygen isotopic record is dominated by changes in surface and bottom water environments during zone nn5 the decreasing and then increasing 18 o values together with the subsequent steepening of the vertical 18 o gradient point towards major climatic instabilities these events coincide with the mid miocene build up of antarctic ice sheets during the latest miocene to the earliest pliocene the 18 o record of planktic foraminifera indicates a significant warming of the indian ocean at mid latitudes the 13 c record during the oligocene and miocene reveals several cycles 13 c enrichments np 24 nn2 nn5 nn9 and base nn 11 which are most likely related to changes in storage rates of organic matter and biological productivity due to climatic changes and transgression regression cycles in addition changes in the circulation patterns may also have influenced the carbon isotopic record

miocene equatorial and southwest pacific paleoceanography from stable isotope evidence abstract stable isotope results from seven miocene deep sea drilling projects in the equatorial and southwest pacific ocean previously correlated using carbon isotope stratigraphy have been examined discussed and interpreted in terms of the development of the miocene pacific ocean the most obvious features of the benthonic foraminiferal stable isotopic records are a major increase in 18o 1 0 during the middle miocene a series of long term oscillations 2 3 my of amplitude 0 5 0 75 and a decrease in 13c values 0 5 1 0 during the latest miocene planktonic foraminiferal 18o records show different trends for high and low latitude regions in the equatorial pacific planktonic 18o values actually decrease during the miocene whereas in the higher southern latitudes planktonic 18o values become more positive in response to cooling surface waters planktonic 13c records show opposite trends to 18o with the high latitude values becoming more negative relative to the tropical regions the development of the miocene pacific ocean in terms of its vertical and horizontal thermal structure and isotopic composition is well illustrated by examining changes in the isotopic difference between planktonic and benthonic foraminifera 18ob p benthonic planktonic is a measure of the thermal structure of the water column 18oph pl high latitude low latitude planktonic values is a measure of the latitudinal temperature gradient 13cb p is an indirect measure of nutrient concentrations in the water column and 13cph pl measures differences in surface water nutrient concentrations between high and low latitude 18ob p increases during the miocene with the greatest increase occurring in the middel miocene at about 14 ma by the latest miocene the isotopic gradient at site 289 in the equatorial pacific approaches the present day isotopic gradient about 4 5 an increase in 18oph pl during the miocene suggests that the latitudinal temperature gradient increased by about 6 c to a value of 12 c in the latest miocene between sites 289 equator and 281 subantarctic 13cb p and 13cph pl values are relatively constant through the early miocene but begin to increase during the middle miocene bottom water 13c values respond similarly at all sites but surface water 13c values exhibit different trends because higher latitude values begin to decrease this decrease perhaps suggests that phosphate concentrations may have increased due to increased upwelling as the circum antarctic circulation system evolved its present day characteristics the isotopic data compiled in this paper suggest that the southwest pacific was responding uniformly to some global or at least pacific wide control during the early miocene in the middle miocene the response became more complex as the low and high latitudes began to show independent trends the changes in the thermal vertical and latitudinal structure probably occurred in respons to the build up of the east antarctic ice sheet intensification of bottom water circulation and an increase in zonal circulation in surface waters in the southern hemisphere the changes in 13c vertical and latitudinal gradients are due to some complex interaction of sea level continental hypsometry climate and biological processes coupled with oceanic circulation changes a strong correlation between estimated sea level changes and 13c values suggests that transgressions and regressions play a critical role in controlling the flux of oxidized organic carbon enriched in12c from the continental shelves and epicontinental seas to the open ocean

the origin of olivine plagioclase coronas in metagabbros from the adirondack mountains new york olivine plagioclase coronas in metagabbros from the adirondack mountains new york usa are spatially well organized reaction textures consisting most commonly of sequential layers of orthopyroxene clinopyroxene plagioclase and garnet the textures are characteristic of diffusion controlled reaction kinetics although similar coronas have been interpreted by previous workers in terms of an isochemical steady state diffusion model petrographical relations and material balance calculations establish that coronas in the adirondack metagabbros cannot be treated as isochemical and do not form in a single stage steady state process instead they evolve through time in a complex open system reaction in this study the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band thereby approximating the open system behaviour of the coronas the sequence and relative proportions of product minerals calculated by the open system steady state model correspond closely to those observed in coronas of the adirondacks over a wide range of values for the relative diffusivities of chemical components involved in the reaction regardless of the particular method used to determine material balance in the reaction texture despite this correspondence petrographical evidence for successive replacement of coronal product layers reveals that the adirondack coronas evolved through one or more transient states rather than forming in a single stage steady state process there is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona forming reaction proceeded this is confirmed by the fact that the evolution of the coronas over time can be replicated with the open system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the ca and si components required for reaction these simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences i orthopyroxene clinopyroxene plagioclase garnet ii orthopyroxene clinopyroxene garnet and iii orthopyroxene garnet all of these stages and the transitions between them are observed petrographically coronas in adirondack metagabbros appear therefore to have originated in a complex open system diffusion controlled reaction in which the product assemblages changed as the reaction progressed

sahara and sahel vulnerability to climate changes lessons from holocene hydrological data abstract the examination of more than 1500 paleohydrological dated records collected between 10 and 28 n during the last 50 years have been used to improve our knowledge and understanding of the sahara and sahel vulnerability to the atlantic monsoon changes in the long term we have analyzed the distribution of water bodies mainly lakes and wetlands over time and space the central saharan massifs played a major role in favoring water supply to the lowlands throughout the whole african humid period in addition distinct east west dynamics is recorded with humidity starting and stopping several millennia earlier to the east than to the west of the sahara a series of time lags are discussed 1 between the maximum of deep fresh water lake formation during the early holocene and the maximum of water body extensions during the mid holocene which highlight the primary role of aquifer water level in lake response to climate change 2 between the hydrological history of the sahara and the sahel and the forcings mainly insolation changes during the early and mid holocene which involves complex interactions between remnant ice sheet in the northern hemisphere open water bodies in the sahara and sahel and the atlantic monsoon system

dependence of reaction kinetics on h2o activity as inferred from rates of intergranular diffusion of aluminium quantitative constraints on the accelerative effects of h2o on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of al in natural systems that are fluid saturated hydrous but fluid undersaturated and nearly anhydrous widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the makhavinekh lake pluton northern labrador combined with time temperature histories from conductive thermal models yield intergranular diffusivities for al from 700 900 c under nearly anhydrous conditions those rates when extrapolated down temperature are approximately three orders of magnitude slower than rates derived from re analysis of garnet resorption coronas formed under hydrous but fluid undersaturated conditions near 575 c in rocks of the llano uplift of central texas which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid saturated conditions in rocks from the picuris range of north central new mexico thus even at constant temperature rates of intergranular diffusion of al and corresponding length scales and timescales of metamorphic reaction and equilibration may vary by as much as seven orders of magnitude across the range of h2o activities found in nature