dissolved and particulate fe in a hydrothermal plume at 9 45 n east pacific rise slow fe ii oxidation kinetics in pacific plumes production of fe iii particles in hydrothermal plumes is of fundamental importance to the long term effect of hydrothermal circulation on seawater composition to elucidate the fundamental controls on fe redox kinetics and solution particle partitioning in neutrally buoyant plumes we sampled near field 3 km plume particles at 9 45 n on the east pacific rise in 1996 returning in 1997 to sample both particulate and dissolved phases 0 40 m filter concentrations of dissolved fe varied from 320 to 20 nm in proximal 0 3 km from vent site to distal samples 1 3 km downfield constituting 85 50 of total fe respectively based on vent fluid dilution factors calculated from dissolved mn a mass balance for vent fluid fe at this site indicates that 65 of fe is lost to particulate sulfide settling in the buoyant plume and that particulate fe in distal 1 3 km samples is twice as concentrated as predicted from dilution of particles in proximal plume water these observations are consistent with a calculated fe ii oxidation half time of 3 3 h long enough that fe iii colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions preventing generation of high particulate fe concentrations 11 56 nm observed a general investigation of fe ii oxidation rates in plumes worldwide gives fe ii oxidation half lives as short as 17 min at some atlantic sites and as long as 6 h at some pacific sites the calculations indicate that the distribution of fe particles in plumes depends chiefly on inter basin differences in ambient deep water chemistry primarily ph and dissolved o2 and on local currents driving plume dilution and to a much lesser extent on variations in primary vent fluid composition long term changes in thermohaline circulation or ocean biogeochemistry may therefore alter fe dynamics and minor element fluxes associated with global hydrothermal activity independent of variations in crustal production rates

sedimentological and palaeoenvironmental study from waregi hill in the hiwegi formation early miocene on rusinga island lake victoria kenya

chemical processes in buoyant hydrothermal plumes on the east pacific rise near 21 n hydrothermal plume water containing suspended particles of precipitated black smoke has been sampled from three hydrothermal vent fields on the east pacific rise near 21 n the samples were taken within the lower 22 m of the buoyant plumes directly above 273 350 c black smoker vents using go flo bottles mounted on the submersible alvin based on li as a conservative tracer of the high temperature endmember solution mixing ratios of 102 to 104 g seawater g vent water are achieved within the lower 22 m of the plumes the particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios suggesting that the particles are settling within the lower part of the plume the major minerals in the smoke are pyrrhotite fe rich sphalerite pyrite unidentified fe s si and fe si s phases chalcopyrite amorphous silica sulfur fe oxyhydroxides including goethite and anhydrite present in trace quantities are barite isocubanite wurtzite covellite marcasite and unidentified silicates and alsilicates organic matter is common the composition of the plume solutions indicates that 35 25 of the hydrothermally injected fe remains in dissolved form i e 0 45 m within the lower 22 m of the plume in spite of a 3 to 9 fold excess of h2s in the vent solutions nearly all of the mn and si and most of the ba also remain in solution h2s by contrast has been largely 70 100 removed by precipitation as sulfides and sulfur but mainly by oxidation to dissolved species with intermediate oxidation states its early removal implies that 50 30 of the injected fe will eventually precipitate as primary oxides rather than as sulfides and that fe can be fractionated from the other chalcophile metals during sulfide precipitation compared with the vent water the particles are enriched in cu co ag cd al zn pb and ni by factors of 4 to 20 relative to fe and depleted in ba s si and mn scavenging from seawater may play a role in co and cu enrichment at the ngs field and for ag cd and ni at all three fields but is negligible for pb and zn these results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume 1 1 a dissolved component that includes about half the fe and all of the mn and will eventually precipitate as oxides and 2 2 a particulate component consisting of fine grained sulfide minerals that are enriched in ore metals by factors of 4 to 20

spatio temporal trends in normal fault segmentation recorded by low temperature thermochronology livingstone fault scarp malawi rift east african rift system the evolution of through going normal fault arrays from initial nucleation to growth and subsequent interaction and mechanical linkage is well documented in many extensional provinces over time these processes lead to predictable spatial and temporal variations in the amount and rate of displacement accumulated along strike of individual fault segments which should be manifested in the patterns of footwall exhumation here we investigate the along strike and vertical distribution of low temperature apatite u th he ahe cooling ages along the bounding fault system the livingstone fault of the karonga basin of the northern malawi rift the fault evolution and linkage from rift initiation to the present day has been previously constrained through investigations of the hanging wall basin fill the new cooling ages from the footwall of the livingstone fault can be related to the adjacent depocentre evolution and across a relay zone between two palaeo fault segments our data are complimented by published apatite fission track aft data and reveal significant variation in rock cooling history along strike the centre of the footwall yields younger cooling ages than the former tips of earlier fault segments that are now linked this suggests that low temperature thermochronology can detect fault interactions along strike that these former segment boundaries are preserved within exhumed footwall rocks is a function of the relatively recent linkage of the system our study highlights that changes in ahe and potentially aft ages associated with the along strike displacement profile can occur over relatively short horizontal distances of a few kilometres this is fundamentally important in the assessment of the vertical cooling history of footwalls in extensional systems temporal differences in the rate of tectonically driven exhumation at a given location along fault strike may be of greater importance in controlling changes in rates of vertical exhumation than commonly invoked climatic fluctuations

new oligocene early miocene microflora from the southwestern turkana basin palaeoenvironmental implications in the northern kenya rift we report five oligocene early miocene pollen assemblages from the loperot 1 exploration well drilled in the semi desert lokichar basin latitude 02 21 46 15 n longitude 35 52 23 47 e ground elevation 615 m above msl near lake turkana in northern kenya they represent the second oldest plant microfossils so far recovered from east africa and add significantly to the paleogene neogene tropical african fossil plant record the loperot pollen indicate a mosaic environment of semi deciduous forest and humid woodland whose floristic composition presents strong affinities with the vegetation occurring today in the guinea congolia zambezia phytogeographical transition zone with a rainfall more than 1000 mm year and a well defined dry season the weak representation of poaceae and herbaceous taxa characteristic of grassland dry bushland or savanna and the abundance of shade tolerant plants such as ferns all point to a vegetation composed to a variety of communities with closed forest formations predominant the lack of typical temperate mountains elements mainly podocarpus and juniperus today widespread on the east african highlands indicates that the geography of the region was different from that of today the plateaux or uplands adjacent to the lokichar basin were probably still not high enough during this period of early rifting in east africa to support the temperate coniferous forests characteristic of the plio pleistocene

helium radon and manganese at the tag and snakepit hydrothermal vent fields 26 and 23 n mid atlantic ridge abstract 3 he 4 he and 222 rn have been measured in samples of vent fluids from the tag and snakepit hydrothermal vent fields and in samples from the neutrally buoyant plume at tag the hydrothermal end member 3 he 4 he at tag normalised to the ratio in air is r r a 7 5 0 9 222 rn 187 dpm kg 1 and 226 ra 13 5 0 4 dpm kg 1 at snakepit r r a 7 86 1 85 222 rn 82 dpm kg 1 and 226 ra 10 3 0 2 dpm kg 1 helium isotope tracer systematics have been described which allow hydrothermal end member properties to be estimated from plume data or non conservative behaviour in the buoyant plume to be identified comparison of plume mn concentrations and he isotope data shows that mn behaves conservatively in the plume system over the time scale considered in contrast 222 rn is highly non conservative and there is a large 222 rn excess in the neutrally buoyant plume the hydrothermal end member accounts for less than 2 5 of the excess 222 rn enrichment at the top of the buoyant plume attributed to entrainment of 222 rn enriched fluids during buoyant plume rise 222 rn 3 he ages of the tag plume give values of around 1 2 days close to the vent field and up to 10 days in adjacent waters 222 rn mn ages are similar except for a region of anomalous water at the level of neutral buoyancy which is enriched in 222 rn and si but not 3 he and mn and which may be derived from the walls of the axial valley at the depth of the neutrally buoyant plume

trophic regions of a hydrothermal plume dispersing away from an ultramafic hosted vent system von damm vent site mid cayman rise 1 deep sea ultramafic hosted vent systems have the potential to provide large amounts of metabolic energy to both autotrophic and heterotrophic microorganisms in their dispersing hydrothermal plumes such vent systems release large quantities of hydrogen and methane to the water column both of which can be exploited by autotrophic microorganisms carbon cycling in these hydrothermal plumes may therefore have an important influence on open ocean biogeochemistry in this study we investigated an ultramafic hosted system on the mid cayman rise emitting metal poor and hydrogen sulfide methane and hydrogen rich hydrothermal fluids total organic carbon concentrations in the plume ranged between 42 1 and 51 1 m background 43 2 0 7 m n 5 and near field plume samples with elevated methane concentrations imply the presence of chemoautotrophic primary production and in particular methanotrophy in parts of the plume characterized by persistent potential temperature anomalies but lacking elevated methane concentrations we found elevated organic carbon concentrations of up to 51 1 m most likely resulting from the presence of heterotrophic communities their extracellular products and vent larvae elevated carbon concentrations up to 47 4 m were detected even in far field plume samples within the von damm hydrothermal plume we have used our data to hypothesize a microbial food web in which chemoautotrophy supports a heterotrophic community of microorganisms such an active microbial food web would provide a source of labile organic carbon to the deep ocean that should be considered in any future studies evaluating sources and sinks of carbon from hydrothermal venting to the deep ocean

diffuse venting and background contributions to chemical anomalies in a neutrally buoyant ocean hydrothermal plume plumes originating in high temperature hydrothermal discharges entrain background water and often entrain diffuse source fluids during convective ascent the mixing of fluid from those three different sources determines the chemical composition of the ensuing neutrally buoyant plumes here convection models and aluminum data from the trans atlantic geotraverse tag site are used to examine relative contributions of focused diffuse and background sources to anomalies in the plume our analysis shows that anomalously high al values observed just above the seafloor at tag which we assume stem from diffuse flows can contribute as much as 50 to the al anomaly in the neutrally buoyant plume found 350 m above bottom background water contributes nearly an equal fraction of al to the anomaly more generally entrained contributions depend on gradients in profiles external to the rising plume entrainment also depends on effective source area distributed high temperature vents entrain more fluid than a single composite high temperature vent larger effective source area is particularly significant for entrainment of diffuse source or other tracers trapped within a few tens of meters of the seafloor downstream cross stream and temporal variabilities exhibited in the three dimensional model plume also underscore problems of sampling and interpreting actual plume data

ferric ferrous ratio in silicate melts a new model for 1 atm data with special emphasis on the effects of melt composition the effect of mgo and total feo on ferric ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300 1500 c at 1 atm total pressure in air we demonstrate that the addition of these weak network modifier cations results in an increase of fe3 fe2 ratio in both mafic and silicic melts based on present and published experimental data a new empirical equation is proposed to predict the ferric ferrous ratio as a function of oxygen fugacity temperature and melt composition in contrast to previous equations the compositional effect of melts on the fe3 fe2 ratio is not only modeled by the sum of the molar fraction of the individual oxide components additional interactions terms have also been incorporated the main advantage of the proposed model is its applicability for a wide compositional range however its application to felsic melts 68 wt sio2 is not recommended other advantages of this equation and differences when compared with previous models are discussed

a model of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

hydrothermal plumes at the rodriguez triple junction indian ridge water column anomalies of light transmission mn fe al and ch4 concentrations were searched in the central southeastern and southwestern indian ridge segments centered on the rodriguez triple junction rtj 25 32 s 70 02 e for the purpose of locating hydrothermally active areas in july to august 1993 we found an active zone in the central indian ridge segment 25 18 20 s approximately 12 miles north of the rtj where significant hydrothermal plumes were observed at 2 200 2 400 m depth intensive tow yo observations using a ctd rosette multi sampling system equipped with a transmissometer revealed that the plumes show temporal as well as spatial variations discrete water samples within the plumes were enriched in mn fe and ch4 with maximum concentrations of 9 8 nm 40 2 nm and 3 3 nm respectively judging from the spatial and chemical characteristics of the plumes especially from transmission anomalies andc h4mn ratios we speculate that the hydrothermal venting site might be not in the rift valley but on the eastern off axis zone several miles distant from the rift valley

characteristics of magma driven hydrothermal systems at oceanic spreading centers 1 we use one and two limb single pass models to de termine vent field characteristics such as mass flow rate q bulk permeability in the discharge zone kd thickness of the conductive boundary layer at the base of the system magma replenishment rate and residence time in the discharge zone data on vent temperature vent field area heat output and the surface area and depth of the subaxial magma chamber amc constrain the models the results give q 100 kg s kd 10 13 m2 and 10 m essentially independent of spreading rate and detailed characteristics of the amc in addition we find no correlation between heat output at individual vent fields and spreading rate or depth to the amc we conclude that high temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of amc depth using data on partitioning of heat flow between focused and diffuse flow we find that 80 90 of the hydrothermal heat output is derived from high temperature fluid even though much of the heat output discharges as low temperature fluid in some cases diffuse flow fluids may exhibit considerable conductive cooling or heating by assuming conservative mixing of diffuse flow fluids at east pacific rise 9 50 n we find that most transport of metals such as fe and mn occurs in diffuse flow and that co2 h2 and ch4 are taken up by microbial activity

quartz solubility in the two phase region of the nacl h2o system an experimental study with application to the piccard hydrothermal field mid cayman rise

tectonic uplift and eastern africa aridification the history of eastern african hominids has been linked to a progressive increase of open grassland during the past 8 million years this trend was explained by global climatic processes which do not account for the massive uplift of eastern african topography that occurred during this period atmosphere and biosphere simulations quantify the role played by these tectonic events the reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation engendering the strong aridification and paleoenvironmental changes suggested by the data

accelerated contribution of the paleo congo river to global seawater 87 sr 86 sr change following eocene oligocene collapse of the african surface

the effect of fo on the diffusion of redox sensitive elements in haplobasaltic melt at 1 gpa and 1300 c abstract diffusivities for elements p sc v mn co zn cr nb mo la ce pr sm eu gd ta w at 1300 c and 1 gpa in basaltic melts were characterized across a range of oxygen fugacity fo2 conditions experiments were carried out using either a reducing fmq 3 1 intermediate fmq 1 2 or oxidizing fmq 6 fo2 for each fo2 three experiments were conducted for durations of 20 40 and 80 min for a given time series changes in diffusivity are typically within 3 standard error at a single fo2 the magnitudes of the elemental diffusivities can be grouped into those of the high field strength elements hfse the rare earth elements ree the transition elements and p vanadium and sc have diffusivities more similar to the rees and hfses respectively than the other transition elements the best fits of diffusivities for p also suggest that the diffusivity of this element is more in line with those of the hfses at oxidizing conditions a fractionation of nb from ta with greater magnitude than that at the other oxygen fugacities is seen across oxygen fugacities explored here eu exhibits unique changes in diffusion at more reducing conditions the diffusivity of eu increases relative to the neighboring ree elements sm and gd with this effect most pronounced at fmq 3 1 and present in experiments conducted at intermediate fo2 conditions this demonstrates that an eu anomaly can be generated by diffusion alone in oxidizing conditions because eu likely is present as mostly eu3 the signal vanishes as eu diffusivity becomes similar to that of other trivalent rees there are small systematic changes in element diffusivities for both redox sensitive and non redox sensitive elements as fo2 is varied averages of the 20 40 and 80 min diffusivities for all elements done in the intermediate fo2 experiments have the slowest diffusivities of the three oxygen fugacities explored on average the diffusivities of the entire contingent of elements studied from the more reducing fmq 3 1 conditions are faster than those from the intermediate fo2 by about a factor of 1 5 the elemental diffusivities recovered from the oxidizing experiments are on average about 2 times as fast of those recovered from the intermediate experiments for elements fit in these experiments an order of magnitude change in element diffusivities even for redox sensitive elements is never seen over the range of oxygen fugacities explored at 1300 c these experiments demonstrate that oxygen fugacity can have an important effect on the diffusivity of certain redox sensitive elements e g eu and that fo2 might play a role in element transport generally

the lamu basin deepwater fold and thrust belt an example of a margin scale gravity driven thrust belt along the continental passive margin of east africa in recent decades advances in seismic processing and acquisition of new data sets have revealed the presence of many deepwater fold and thrust belts dw ftbs often developing along continental passive margins these kinds of tectonic features have been intensively studied due to their substantial interest this work presents a regional scale study of the poorly explored lamu basin dw ftb a margin scale gravity driven system extending for more than 450 km along the continental passive margin of kenya and southern somalia east africa a 2 d seismic data set was analyzed consisting of both recently acquired high quality data and old reprocessed seismic profiles for the first detailed structural and stratigraphic interpretation of this dw ftb the system originated over an early to mid cretaceous shale detachment due to a mainly gravity spreading mechanism analysis of synkinematic strata indicates that the dw ftb was active from the late cretaceous to the early miocene but almost all of the deformation occurred before the late paleocene the fold and thrust system displays a marked n s variation in width the northern portion being more than 150 km wide and the southern portion only a few dozen kilometers wide this along strike variation is thought to be related to the complex tectonosedimentary evolution of the continental margin at the somalia kenya boundary also reflected in the present day bathymetry locally a series of volcanic edifices stopped the basinward propagation of the dw ftb a landward change in the dominant structural style from asymmetric imbricate thrust sheets to pseudo symmetric detachment folds is generally observed related to the landward thickening of the detached shales

temporal variability of the hydrothermal plume above the kairei vent field 25 s central indian ridge 1 continuous profiling of the nonbuoyant plume above a hydrothermal vent site can be used to determine the interaction between the plume formed by venting high temperature fluids and changing background temperature salinity and current fields over a tidal cycle we have conducted a 12 hour monitoring of the nonbuoyant plume above the kairei vent field 25 s central indian ridge the depth of the plume particle maximum varies significantly in the water column from 2150 m to 2350 m but remains strongly associated with the 2 36 987 potential density isopycnal we have used the interaction between two unrelated physical oceanographic phenomena to isolate the section of the time series record most suitable for determining the heat flux of the kairei source on the basis of the rise height of the plume to neutral buoyancy at the start of the time series internal waves are observed with a period similar to the calculated buoyancy period n 1 about 3 hours no consistent relationship is observed between the rise height of the plume and the calculated heat flux for the last 6 hours of the time series isopycnal surfaces are compressed above a strongly developed nonbuoyant plume such that the calculated buoyancy period is less than that of the internal waves a more constant heat flux can be calculated on the basis of the plume rise height from this part of the record

differentiation of ferro basaltic magmas under conditions open and closed to oxygen implications for the skaergaard intrusion and other natural systems because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour comparison of experimental data on an iron rich basaltic composition with predictions of the melts free energy minimization algorithm shows that although liquidus temperatures and silicate mineral equilibria are predicted relatively well the saturation of fe ti oxides is not we have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase relations mineral melt partitioning of major elements and mass balance constraints the model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar in a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids whereas systems closed to oxygen crystallize less abundant magnetite leading to a less pronounced iron depletion in the liquid predicted bulk solid compositions and variations offo with falling temperature agree well with those observed or inferred from the cumulates of the skaergaard intrusion but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the basistoppen sill which closely resemble those of skaergaard may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen a comparison of the predicted t io paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as iceland and hawaii evolve under conditions open to oxygen whereas evidence from plutonic environments e g skaergaard and kiglapait layered intrusions suggests that they evolved under conditions more closed to oxygen the compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different although the results of this study suggest that magnetite saturation will limit fe enrichment in all environments to 20wt feo consistent with enrichments reported for volcanic glasses

hydrothermal activity on the ultra slow spreading southern knipovich ridge we report first evidence for hydrothermal activity from the southern knipovich ridge an ultra slow spreading ridge segment in the norwegian greenland sea evidence comes from optical backscatter anomalies collected during a systematic side scan sonar survey of the ridge axis augmented by the identification of biogeochemical tracers in the overlying water column that are diagnostic of hydrothermal plume discharge mn ch4 atp analysis of coregistered geologic and oceanographic data reveals that the signals we have identified are consistent with a single high temperature hydrothermal source located distant from any of the axial volcanic centers that define second order segmentation along this oblique ridge system rather our data indicate a hydrothermal source associated with highly tectonized seafloor that may be indicative of serpentinizing ultramafic outcrops consistent with this hypothesis the hydrothermal plume signals we have detected exhibit a high methane to manganese ratio of 2 3 1 this is higher than that typical of volcanically hosted vent sites and provides further evidence that the source of the plume signals reported here is most probably a high temperature hydrothermal field that experiences some ultramafic influence compare to rainbow and logachev sites mid atlantic ridge while such sites have previously been invoked to be common on the sw indian ridge this may be the first such site to be located along the arctic ultra slow spreading ridge system

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

northeast african vegetation change over 12 m y intense debate surrounds the evolution of grasses using the c 4 hatch slack photosynthesis pathway and the emergence of african grasslands often assumed to be one and the same here we bring new insights with the combination of plant leaf wax carbon isotopic composition 13 c wax and pollen data from marine sediments of the gulf of aden northeast africa which show that c 4 biomass increases were not necessarily associated with regional grassland expansion we find broadly opposing trends toward more enriched 13 c wax values and decreased grass pollen proportions between 12 and 1 4 ma this apparently contradictory evidence can be reconciled if a greater proportion of the late miocene northeast african landscape were covered by c 3 grasses than previously thought such that c 4 grasses and shrubs replaced a c 3 ecosystem including trees and productive grasslands in addition 13 c wax and pollen both indicate that true rainforests were unlikely to have been extensive in northeast africa at any time in the last 12 m y although seasonally dry forests were a significant component of the regional landscape since the late miocene here we extend regionally integrative marine archives of terrestrial vegetation back to 12 ma and we evaluate them in the context of an updated compilation of pedogenic carbonate 13 c values from east african rift strata we identify two distinct phases of increasing c 4 biomass between 11 and 9 ma with a reversal by 4 3 ma and then a re expansion between 4 3 and 1 4 ma surprisingly neither was associated with grassland expansion

stratigraphy depositional history and environments of deposition of cretaceous through tertiary strata in the lamu basin southeast kenya and implications for reservoirs for hydrocarbon exploration abstract the lamu basin which is characterized by an extensional tectonic style is the failed arm of a tri radial rift system and possesses tectonic and stratigraphic elements in the post rift series that are analogous to those of the west coast of africa development of the southern part of the basin as a passive margin is closely related to considerations of the pre drift position of madagascar and formation of the indian ocean basin during mesozoic time cretaceous and tertiary strata in the basin comprise an eastward thickening gross succession of sediments on which eustatic sea level fluctuations and a sequence of unconformities related to pulses of transgressive and regressive depositional trends are superimposed recognition of these trends has provided the basis for classification of the strata into megasequences representing distinct provinces with regard to time stratigraphy sedimentation tectonics depositional environments and hydrocarbon potential megasequence i karoo group and jurassic includes strata of the permian through jurassic the discussion of which is beyond the scope of this paper megasequence ii sabaki group includes strata of the cretaceous and early paleocene deposited in tide influenced shelf and marine settings these are in ascending order the ewaso sands the walu shale the hagarso limestone the freretown limestone and the kofia sands megasequence iii tana group includes strata of the eocene through oligocene which were deposited in fluvial deltaic and restricted shelf settings these are in ascending order the kipini formation the pate limestone the linderina limestone the dodori limestone and the barren beds formation megasequence iv coastal group includes strata of the miocene through pliocene which were deposited in restricted shelf middle to outer shelf deep marine and fluvial settings these are in ascending order the baratumu formation the lamu reefs the simba shales and the marafa formation determination and description of these lithostratigraphic units their distribution and palaeohistory have been imperative for predicting reservoirs for hydrocarbon exploration

detection of an unusually large hydrothermal event plume above the slow spreading carlsberg ridge nw indian ocean about 90 of earth s volcanism occurs along the global mid ocean ridge system here sporadic volcanic and tectonic activity is thought to cause cataclysmic release of hydrothermal fluids forming event plumes each plume often contains as much hydrothermal effluent and heat as chronic hydrothermal venting from a typical vent site discharges during a year to date only a few event plumes have been detected and only above intermediate rate spreading ridges in the pacific here we report the first evidence for an unusually large event plume that originated from the slow spreading 3 cm yr full rate carlsberg ridge in the nw indian ocean at 70 km long up to 4540 km3 in volume and with up to 24 1016 j of excess heat this event plume was substantially larger than previous ones and demonstrates that dispersion of hydrothermal heat and biological products from slow spreading ridges may be more significant and effective than hitherto imagined

a new species of africanacetus odontoceti ziphiidae found on the deep ocean floor off the coast of brazil abstract a fossil skull of a beaked whale was newly collected by a manned submersible from the sao paulo ridge of the atlantic floor about 2900 m deep off the coast of brazil it was found in the middle of the manganese nodules on the seafloor the whale fossil can be dated sometime between the middle miocene and the early pliocene based on the timing of the onset of the tectonic movements of the sao paulo ridge whence the specimen came and the precipitation of the manganese layers which entirely encrusted the specimen this is the first instance of a fossil beaked whale skull retrieved from such great depths this skull belongs to a new species of africanacetus which may be a genus with a broad distribution in southern oceans in neogene times expands the geographic distribution of the genus as the northernmost record and adds the taxonomic and morphological diversity within the genus

from source to sink in central gondwana exhumation of the precambrian basement rocks of tanzania and sediment accumulation in the adjacent congo basin apatite fission track aft and u th he ahe thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated 1000 m a s l but low relief tanzanian craton coeval episodes of sedimentation documented within adjacent paleozoic to mesozoic basins of southern tanzania and the congo basin of the democratic republic of congo drc indicate that most of the cooling in the basement rocks in tanzania was linked to erosion basement samples were from an exploration borehole located within the craton and up to 2200 m below surface surface samples were also analysed aft dates range between 317 33 ma and 188 44 ma alpha ft corrected ahe dates are between 433 24 ma and 154 20 ma modelling of the data reveals two important periods of cooling within the craton one during the carboniferous triassic 340 220 ma and a later less well constrained episode during the late cretaceous the later exhumation is well detected proximal to the east african rift 70 ma thermal histories combined with the estimated geothermal gradient of 9 c km constrained by the aft and ahe data from the craton and a mean surface temperature of 20 c indicate removal of up to 9 2 km of overburden since the end paleozoic the correlation of erosion of the craton and sedimentation and subsidence within the congo basin in the paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes

circulation pathways and transports of the southern ocean in the vicinity of the southwest indian ridge circulation pathways and transports of the antarctic circumpolar current acc and agulhas return current arc around the complex bathymetry of the southwest indian ridge and del cano rise have been established from two hydrographic surveys and two year long moorings after crossing the southwest indian ridge in a 110 sv 1 sv 106 m3s 1 flow concentrated at 48 s the acc fragments before being reconcentrated into several branches by the bathymetry in particular 30 40 sv turns north around the eastern flank of the del cano rise turns west as part of anticyclonic flow round the del cano rise finally returning eastward as part of the crozet front eddy variability from both satellite data and moorings is low over the bathymetry of the del cano rise so the strong currents found between the del cano rise and the crozet plateau are believed to be permanent features this largely barotropic time independent transport is the major pathway for the acc to enter the crozet basin the anticyclonic flow also transfers water from the mozambique basin across the southwest indian ridge to the south side of the del cano rise given the evidence for fragmentation of the acc where it crosses the southwest indian ridge and the concentration of several jets into the crozet front from the acc subtropical front and agulhas return current it is misleading to relate standard frontal definitions such as the polar front and subantarctic front to the major transport pathways

kinematic evolution of a regional scale gravity driven deepwater fold and thrust belt the lamu basin case history east africa the deepwater fold and thrust belts dwftbs are geological structures recently explored thanks to advances in offshore seismic imaging by oil industry in this study we present a kinematic analysi

near field variability of the tag non buoyant plume 26 n mid atlantic ridge we report the initial results of a nine hour monitoring of the non buoyant plume above the tag hydrothermal vent field at 26 n on the mid atlantic ridge a ctd conductivity temperature and pressure and nephelometer particle sensor were continuously raised and lowered through the non buoyant plume for nine hours at a site situated 100 m south of the hydrothermal mound this method permits a detailed record of the variability of the non buoyant plume close to the vent field to be made our results show that the particulate maximum of the non buoyant plume appears 150 m lower in the water column at the start and end of the record compared with profiles recorded towards the middle of the record a corresponding structure also appears in the temperature and salinity records with the result that the plume particle maximum resides on the 27 8035 isopycnal throughout modelling of the rise height of an axisymmetric plume can only account for 95 m of the variability in the final level of neutral buoyancy plume bending due to currents is a likely explanation for the recorded temporal variability

modelling vegetation change during late cenozoic uplift of the east african plateaus abstract the present day vegetation in the tropics is mainly characterized by forests worldwide except in tropical east africa where forests only occur as patches at the coast and in the uplands these forest patches result from the peculiar aridity that is linked to the uplift of the region during the late cenozoic the late cenozoic vegetation history of east africa is of particular interest as it has set the scene for the contemporary events in mammal and hominin evolution in this study we investigate the conditions under which these forest patches could have been connected and a previous continuous forest belt could have extended and fragmented we apply a dynamic vegetation model with a set of climatic scenarios in which we systematically alter the present day environmental conditions such that they would be more favourable for a continuous forest belt in tropical east africa we consider varying environmental factors namely temperature precipitation and atmospheric co 2 concentrations our results show that all of these variables play a significant role in supporting the forest biomes and a continuous forest belt could have occurred under certain combinations of these settings with our current knowledge of the palaeoenvironmental history of east africa it is likely that the region hosted these conditions during the late cenozoic recent improvements on environmental hypotheses of hominin evolution highlight the role of periods of short and extreme climate variability during the late cenozoic specific to east africa in driving evolution our results elucidate how the forest biomes of east africa can appear and disappear under fluctuating environmental conditions and demonstrate how this climate variability might be recognized on the biosphere level

magma storage conditions and differentiation of the mafic lower pollara volcanics salina island aeolian islands italy implications for the formation conditions of shoshonites and potassic rocks crystallization experiments of basaltic andesite mafic endmember from the 24 ka lower pollara eruption salina aeolian islands italy were investigated at 200 mpa 950 1100 c in the h2o activity ah2o range 0 3 to 1 and at two ranges of oxygen fugacity fo2 between fmq to fmq 1 and fmq 2 to fmq 3 3 log bars fmq is fayalite magnetite quartz comparison of the produced phase assemblages and phase compositions with the natural sample reveals that the storage conditions were 1050 c 2 8 wt h2o in the melt ah2o 0 5 and relatively oxidizing fmq 2 5 the composition of plagioclase in the groundmass indicates a period of cooling to 950 c the overall differentiation trends of the salina volcanics can be explained by fractional crystallization close to h2o saturated conditions 5 wt h2o in the melt at 200 mpa and most likely by accumulation of plagioclase i e in basaltic andesites and by various degree of mixing mingling between the corresponding differentiates the slightly elevated k2o contents of the most mafic basaltic andesites that can be found in the lowermost unit of the lower pollara pyroclastics reveal earlier processes of moderately hydrous fractional crystallization at higher temperature 1050 c fractional crystallization with decreasing influence of h2o causes a moderate decrease of mgo and a significant increase of k2o relative to sio2 in the residual liquids it is exemplarily shown that the crystallization of sio2 rich phases at high temperature and low ah2o of only moderately k2o rich calc alkaline basalts can produce shoshonitic and high potassic rocks similar to those of stromboli and volcano this suggests that the observed transition from calc alkaline to shoshonitic and high potassic volcanism at the aeolian arc over time can be initiated by a general increase of magmatic temperatures and a decrease of ah2o in response to the extensional tectonics and related increase of heat flow and declining influence of slab derived fluids

a 17 my old whale constrains onset of uplift and climate change in east africa timing and magnitude of surface uplift are key to understanding the impact of crustal deformation and topographic growth on atmospheric circulation environmental conditions and surface processes uplift of the east african plateau is linked to mantle processes but paleoaltimetry data are too scarce to constrain plateau evolution and subsequent vertical motions associated with rifting here we assess the paleotopographic implications of a beaked whale fossil ziphiidae from the turkana region of kenya found 740 km inland from the present day coastline of the indian ocean at an elevation of 620 m the specimen is 17 my old and represents the oldest derived beaked whale known consistent with molecular estimates of the emergence of modern strap toothed whales mesoplodon the whale traveled from the indian ocean inland along an eastward directed drainage system controlled by the cretaceous anza graben and was stranded slightly above sea level surface uplift from near sea level coincides with paleoclimatic change from a humid environment to highly variable and much drier conditions which altered biotic communities and drove evolution in east africa including that of primates

effects of melt composition on fe 3 fe 2 in silicate melts a step to model ferric ferrous ratio in multicomponent systems the effect of al 2 o 3 mgo and total iron on the ferric ferrous ratio in silicate melts was investigated in model silicate melts in the temperature range 1400 1550 c at 1 atm total pressure the experiments were done mostly in air and partially in pure co 2 it is demonstrated that an increase in al2o3 concentration in a basaltic melt results in a moderate decrease of fe 3 fe 2 ratio in con trast the increase in al2o3 in more silicic melts results in a much more pronounced decrease of fe 3 fe 2 ratio the increase of mgo concentration in a basaltic melt results in a moderate increase of fe 3 fe 2 ratio but has a negligi ble effects in more silicic melts the different behavior of al 2 o 3 and mgo in basaltic and more silicic melts indicates that at constant t fo2 conditions the effects of melt com position on ferric ferrous ratio cannot be predicted accu rately as a function of dixi where di are fixed empirical coefficients and x i are mole fractions of the main oxide component in silicate melts we suggest an alternative approach which accounts for the interaction of cations in complex silicate melts based on the data obtained in this study an equation predicting the ferric ferrous ratio of ultramafic to silicic melts at air conditions with changing sio 2 tio 2 al 2 o 3 total iron mgo and p 2 o 5 is proposed

theory applied to the mid atlantic ridge hydrothermal plumes the finite difference approach a system of differential equations describing the entrainment of ambient seawater during the rise of a buoyant hydrothermal plume has been solved by an explicit finite difference method chosen for its ease of application an equation for tracer concentration has been added so that concentrations in the neutrally buoyant plume can be calculated for those elements whose background profiles vary over the depth range of the buoyant plume the height of plume rise can be related directly to the thermal and hydrothermal vent flux for the tag vent field 26 n mar five plume layers have been identified the calculated heat flux is 500 940 mw and the fluid flux is 0 210 0 392 m3 s 1 the fluid flux to the neutrally buoyant plume is 1460 2740 m3 s 1 calculated plume concentrations of mn compared with measurements made at tag in 1988 indicate that there is either 50 removal of manganese in the buoyant plume or that low temperature vent fluids contribute about 50 of the total thermal output of the tag vent field

hydrocarbon potential of the meso cenozoic turkana depression northern kenya ii source rocks quality maturation depositional environments and structural control abstract source rocks identified by drilling loperot 1 well in the lokichar basin of the northern kenya rift were evaluated for source potential using rockeval pyrolysis and palynofacies analysis results indicate moderate to good oil source potential from algal prone source rocks the original sediments accumulated in a large freshwater lake similar to some modern rift lakes in the loperot 1 well the section with the highest source potential lies within the lokhone shale member up to 17 total organic carbon toc average 2 4 toc from well log calculation the other potential source interval is the deeper loperot shale member which is highly mature to post mature r 0 1 1 average 1 2 toc from well log calculation the poorer quality of the lower shale interval while partly due to maturity is thought to be largely a result of the basin evolution the loperot shale member was restricted to the lokichar basin while the lokhone shale member was deposited during a time when displacement on the main boundary fault system was greater and adjacent basins had been established consequently lacustrine conditions appear to have extended into adjacent basins thus allowing the accumulation of relatively higher concentrations of algal material in sediment starved parts of the basin