geochemical cycling of redox sensitive metals in sediments from lake malawi a diagnostic paleotracer for episodic changes in mixing depth changes in heat balance wind stress and lake level can affect the efficiency of mixing and depth of oxygen penetration in deep tropical lakes this study examines how transient changes in these parameters as reflected by the redox state of bottom waters are recorded in the sedimentary record of lake malawi we measured concentration profiles of redox sensitive metals across a layer of oxidized sediment deposited within a sequence composed of reduced sediments in a core taken at 298 m water depth 100 m below the present day chemocline by examining a range of metals that exhibit different geochemical behaviors mobilization under reducing fe mn versus oxidizing u conditions formation of insoluble sulfides cd mo pb zn susceptibility to scavenging onto fe or mn oxyhydroxide precipitates co mo pb v zn we are able to outline a sequence of events consistent with the formation of this feature 1 at some time before 11 ky bp there was an episode of penetration of oxygenated waters to depths close to or greater than that of this core i e substantially deeper in the lake basin than at present at this time diagenetically remobilized fe and mn were oxidized and precipitated in a near surface sediment horizon 2 fe and mn oxyhydroxides that precipitated from the water column as metal rich anoxic waters were mixed upward into oxygenated waters were deposited in an immediately overlying layer of the sediment such material continued to accumulate in surface sediments for a few hundred years until oxic conditions no longer prevailed in overlying waters 3 as the depth of the chemocline rose relative to the present day lake surface this authigenic material was in turn overlain by reducing sediments 4 subsequent diagenetic remobilization at the top and bottom of this redox sandwich resulted in formation of horizons enriched in u mo and sulfide forming metals notably cd along the redox gradients both above and to a lesser extent below the layer of oxidized sediment these enrichments immobilized under reducing conditions will remain as permanent markers of the episode of deepening of the chemocline even after reduction and remobilization of the oxidized fe and mn solids in addition such markers may be useful in determining whether observed episodic interruptions in deposition of varved sediments in the deep anoxic basins of lake malawi are due to bioturbation brought on by episodes of deeper oxygen penetration in the water column

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

active surface deformation and sub lithospheric processes in the western mediterranean constrained by numerical models we present the results of dynamic modeling of the western mediterranean that accounts for observed global positioning system gps surface deformation of the alboran sea and surrounding rif and betic mountains as the result of the combined effects of relative motion of the eurasian and nubian plates low strength in the alboran sea region and sub lithospheric processes occurring beneath the external rif domain assuming that the lithosphere behaves elastically over the short time period of the gps observations an elastic plate model is considered in our study including an area of weak lithosphere factor of 10 centered on the alboran sea and in which lateral boundary conditions consist of the nubia eurasia oblique convergence sub crustal processes are modeled by application of a horizontal traction on a small area patch at the base of the elastic plate our modeling studies demonstrate the need for sub crustal or sub lithospheric southwestward directed forcing to account for observed southwestward motion of the rif and betic domains based on the location orientation and small area of the traction patch we hypothesize that forcing is associated with delamination and rollback of the subducted african continental lithospheric mantle beneath the external rif zone due to the pull of the oceanic part of the western mediterranean slab a dynamic process that may be similar to that where the over riding plate is driven toward the subduction zone during slab rollback

crustal structure beneath the rif cordillera north morocco from the rifsis wide angle reflection seismic experiment the different geodynamic models proposed since the late 1990s to account for the complex evolution of the gibraltar arc system lack definite constraints on the crustal structure of the rif orogen here we present the first well resolved p wave velocity crustal models of the rif cordillera and its southern continuation toward the atlas made using controlled source seismic data two 300 km long wide angle reflection profiles crossed the rif along ns and ew trends the profiles recorded simultaneously five land explosions of 1tn each using 850 high frequency seismometers the crustal structure revealed from 2 d forward modeling delineates a complex laterally varying crustal structure below the rif domains the most surprising feature seen on both profiles is a 50 km deep crustal root localized beneath the external rif to the east the crust thins rapidly by 20 km across the nekkor fault indicating that the fault is a crustal scale feature on the ns profile the crust thins more gradually to 40 km thickness beneath middle atlas and 42 km beneath the betics these new seismic results are in overall agreement with regional trends of bouguer gravity and are consistent with recent receiver function estimates of crustal thickness the complex crustal structure of the rif orogen in the gibraltar arc is a consequence of the miocene collision between the iberian and african plates both the abrupt change in crustal thickness at the nekkor fault and the unexpectedly deep rif crustal root can be attributed to interaction of the subducting alboran slab with the north african passive margin at late oligocene early miocene times

eolian sediment responses to late quaternary climate changes temporal and spatial patterns in the sahara abstract this paper presents a compilation of eolian based records of late quaternary climate changes in the sahara although the data are relatively sparse when viewed as a whole they reveal a general pattern of widespread eolian sediment mobilization prior to 11 000 cal years bp eolian sediment stabilization from 11 000 to 5000 cal years bp and a return to widespread eolian sediment mobilization after 5000 cal years bp furthermore an eolian based record from southern tunisia reveals the existence of millennial scale changes in eolian sediment behavior these millennial scale variations provide examples of eolian sediment responses to climate changes at a scale intermediate between seasonal and orbital milankovitch changes and they are also coincident with abrupt atmospheric and oceanic changes the general synchroneity of the eolian stratigraphic records and their coincidence with various oceanic and atmospheric changes suggest that global forcing mechanisms have influenced late quaternary eolian sediment behavior in the sahara

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

are some of the deep crustal conductive features found in sw iberia caused by graphite abstract recent results obtained from two dimensional inversion of magnetotelluric mt data acquired in sw iberia reveal high conductive features at the middle lower levels of the crust the top of these anomalous structures correlates very well with the depth 10 13 km of an important seismic interface that has been interpreted as a regional detachment horizon very shallow and relatively narrow conductors in the ossa morena zone appear to correspond to small scale fluid deposited graphite systems in the preorogenic metasedimentary sequences some of the midcrustal conductors can be ascribed to graphite bearing thrust zones the formation of graphite with variable crystallinity being a consequence of variscan shearing processes deep seated conductors are tentatively interpreted as a result of relatively continuous highly crystalline grain boundary graphite films presumably preserved in basement granulite rocks assuming that graphite occurs as interconnected films calculations indicate that a fraction of 0 006 0 02 of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of mt models the role of graphite on the thermal behaviour of the crust is also discussed the results show that low contents of graphite do not significantly change the thermal behaviour of earth materials

interhemispheric synchrony of the last deglaciation inferred from alkenone palaeothermometry the relative timings of the last deglacial warming in the southern and northern hemispheres are not well constrained but are a crucial component in understanding the mechanisms of deglaciation1 a clearer picture of the degree of interhemispheric synchrony has been obscured by a dearth of high resolution temperature records that can be tied to the absolute calendar timescale moreover the quantification of tropical temperatures during the last glacial cycle is controversial2 8 here we apply the alkenone method of sea surface temperature reconstruction9 10 to several high resolution sediment cores recovered from the tropical indian ocean between 20 n and 20 s the inferred initial sea surface temperature warming 15 000 calendar years ago at 20 s is in phase with northern hemisphere sea this study and air11 temperature changes but lags antarctic warming12 14 by several millennia this finding along with the results of recent modelling studies15 16 provides strong support for the idea that changes in the ocean s global thermohaline circulation were not the only cause of interhemispheric climate teleconnection during the last deglaciation

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

a late quaternary pollen record from the transvaal bushveld south africa pollen spectra from cores of organic spring deposits from the transvaal provide evidence for the climatic evolution of the province during the last 35 000 yr b p or more the past climatic phases are derived from palynological reconstructions of past vegetation types by comparison of fossil pollen data with modern surface pollen spectra from various localities evidence is provided for an early moist cool phase with relatively mesic bushveld and expanded montane forest in the central transvaal followed by a drier period with drier bushveld which probably lasted until approximately 25 000 yr b p during the next phase which at the latest ended about 11 000 yr b p the temperatures were probably 5 6 c cooler than at present at that time bushveld vegetation in the central transvaal was replaced by open grassland with macchia elements climatic amelioration came and semiarid savanna returned to the plains at first gradually and then developing into a warm kalahari thornveld type vegetation after 6000 yr b p it apparently became slightly wetter and a more broad leafed bushveld developed about 4000 yr b p it again became cooler and slightly wetter and the bushveld vegetation on the central and northern plains was comparable to present open upland types after 2000 yr b p conditions gradually became warner until about 1000 yr b p when the modern climate of the central transvaal bushveld originated

mantle structure under gibraltar constrained by dispersion of body waves 1 westudytheafrica iberiaplateboundaryinthevicinity of gibraltar numerous models have been proposed for that region throughout the last decades proposing mechanisms that range widely from continental delamination convective removal to subduction of oceanic lithosphere to better constrain upper mantle structure under the region we study waveforms of p waves that traverse the alboran sea region between spain and morocco these show dispersive behavior which together with early arrival times confirms the presence of an anomalous upper mantle structure under the alboran sea the dispersion is consistent with that expected from subducted lithosphere waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the alboran sea region citation bokelmann g and e maufroy 2007 mantle structure under gibraltar constrained by dispersion of body waves geophys res lett 34 l22305 doi 10 1029 2007gl030964

crustal structure of the central sector of the betic cordillera se spain deep seismic reflection gravimetric and magnetometric data allow the main features of the deep structure of the central sector of the betic cordillera to be established the moho is horizontal or dips slightly toward the s below the mountain chain and its morphology has no direct relationship with the region s topography the deep reflectors are not deformed by the neogene kilometric scale folds that produced the main topographic features probably due to the existence of detachment levels in the crust the crust is slightly thickened in the betic cordillera almost 35 km and has an abrupt transition to the thin crust of the alboran sea 15 km in the central alboran sea along an e w oriented narrow band subparallel to the coast line where the moho dips more than 60 n the neogene evolution of the mountain chain caused compressive deformation in the external zones in the internal zones pre tortonian low angle normal faults developed after the alpine metamorphism and later high and low angle normal faults and strike slip faults occurred up to the quaternary geophysical and field geology data can be used to establish a geological model for the recent evolution of the crustal deformation in this model the recent evolution of the region is probably determined by thrusting of the thin continental crust of the alboran sea toward the nw over the southern margin of the iberian massif this deformation could be a consequence of the oblique convergent movement between the african and eurasian plates which may have uplifted the entire region causing regressions in the sedimentary basins and the formation of folds and extensional structures in the upper part of the thickened crust

seasonal signals in lake malawi sediments the sedimentary record of lake malawi one of the large rift valley lakes of east africa contains thick sequences of biologically undisturbed finely laminated muds and silts laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m the sequences consist of light dark laminae couplets with average thickness of 1 mm the light laminae are completely dominated by the diatom genus melosira with a relatively minor contribution from the genus stephanodiscus the dark laminae consist of 50 diatoms equal proportions of melosira and stephanodiscus few nitzschia and occasional littoral diatom genera and 50 terrestrial plant debris mineral grains and fine clay and organic material based on a 210pb and 14c calculated sedimentation rate of 1 mm yr 1 for lake malawi basin sediments and the well defined seasonal variations in fluvial input and diatom productivity levels our data suggest that the lamination couplets are annual varves representing biannual sedimentation sequences the light laminae result from diatom blooms occurring during the dry windy season april october when the upper 250 m of the lake is well mixed localized upwelling occurs and the epilimnion becomes enriched with nutrients the dark laminae represent sedimentation in the rainy low wind velocity season november march when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum

seismic tomography from local crustal earthquakes beneath eastern rif mountains of morocco we applied a tomographic method to image an aseismic strike slip fault in north morocco and found that the occurrence of earthquakes is not only controlled by the state of tectonic stress but also by material heterogeneity in the crust we have constructed an integrated model of seismic electric magnetic and heat flow properties across northeastern morocco primarily based on a tomography inversion of local earthquake arrival times the seismic images obtained show a pronounced lowvelocity zone at 5 km depth parallels to the nekor fault coinciding with an anomalously high conductive and low gravity structure which is interpreted as a fault gouge zone and or a fluid filled subsurface rock matrix below 10 km depth a weak positive velocity zone indicates that the fault gouge is stable the seismicity and the seismic velocity results for the alhoceimas region show that the concentrations of earthquakes are confined in the high velocity area this anomaly is interpreted to be a brittle and competent layer of the upper crust that sustains seismogenic stress on the eastern coast line of morocco we infer that a high density high velocity body exists in the shallowest layers of the upper crust probably formed by miocene volcanic rocks d 2003 elsevier science b v all rights reserved

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

a rapid method to map the crustal and lithospheric thickness using elevation geoid anomaly and thermal analysis application to the gibraltar arc system atlas mountains and adjacent zones we present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness the main assumptions are local isostasy and a four layered model composed of crust lithospheric mantle sea water and the asthenosphere we consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle we perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of rms error of elevation and geoid anomaly databases the application of this method to the gibraltar arc system atlas mountains and adjacent zones reveals the presence of a lithospheric thinning zone sw ne oriented this zone affects the high and middle atlas and extends from the canary islands to the eastern alboran basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western betics eastern rif rharb basin and gulf of cadiz a number of different even mutually opposite geodynamic models have been proposed to explain the origin and evolution of the study area our results suggest that a plausible slab retreating model should incorporate tear and asymmetric roll back of the subducting slab to fit the present day observed lithosphere geometry in this context the lithospheric thinning would be caused by lateral asthenospheric flow an alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the canary island and extending as far as central europe

the palaeolimnology of northern lake malawi over the last 25 ka based upon the elemental and stable isotopic composition of sedimentary organic matter abstract sediments accumulating in offshore areas of northern lake malawi typically contain 1 4 organic carbon this organic matter om is of mainly phytoplankton origin with varying mixtures of terrestrial and degraded reworked material high resolution stable carbon and nitrogen isotopic elemental and rock eval pyrolysis analyses of the bulk om contained in three cores have been used to characterise the om and provide detailed insights into the palaeolimnology and palaeoclimatology of northern lake malawi over the last 25 ka preservation of the om as reflected by its rock eval hydrogen index hi is highly variable periods characterised by notably low hi 15 n during the mid and late holocene suggest intervals of stable stratification due to significantly reduced wind driven mixing in the northern part of the lake when n fixing cyanophytes formed a major component of the phytoplankton the geochemical data support recent suggestions of lowstand conditions in lake malawi at the time of the lgm and provide additional evidence of periods of intense but localised aridity in the terminal pleistocene earliest holocene and at the time of the little ice age the change in mixing regime at ca 11 8 ka seems to have been the result of a radical reorganisation of the dominant surface wind flow over south eastern africa the transition was within the resolution of currently available chronologies apparently contemporaneous with marked changes in the circulation patterns of adjacent oceans and the climate of more distant parts of the northern hemisphere

late cretaceous and early miocene andean type plutonic activity in northern makran and central iran two calc alkaline plutonic complexes bazman and natanz intruded through the south eastern active and south western ancient continental margins of central iran have been dated by the rb sr whole rock isochron method at 74 2 and 24 4 5 ma respectively detailed trace element studies together with low 87 sr 86 sr initial ratios 87 sr 86 sr ratios and the gabbros at least were intruded some 10 ma earlier

a high electrical conductive zone at lower crustal depth beneath the betic chain spain abstract magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the central betic chain interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two dimensional resistivity model along a nw se profile from the guadalquivir foreland basin and across the boundary between the external and the internal betics elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected the conductors in the upper crust coincide with the areal distribution of the cenozoic and mesozoic sedimentary rocks from the guadalquivir basin and the external betics and the conductors in the middle crust with the major miocene extensional detachments affecting the basement rocks of the internal betic zone a high conductivity zone at lower crustal levels beneath the outer internal betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective moho this conductor was interpreted as partial melting of a southeast dipping subducted iberian lower crust the geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the alboran domain or to the iberian plate in the former case it would correspond to a major plate boundary and in the latter to an intraplate feature

rheological and tectonic modeling of salt provinces abstract technological improvements and a more rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden the fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth converted seismic sections scale models are therefore useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps although even unsealed models may help generating ideas the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes previously published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden this paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other intrinsic material properties scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics including those with synkinematic sedimentation

a revised 30 000 year paleoclimatic and paleohydrologic history of lake albert east africa abstract new geochemical and palynological data and a sedimentological reinterpretation of two cores from lake albert allow a revision of the paleoclimatic and paleohydrologic history of northwest uganda over the last 30 000 14 c yr bp our findings document one and possibly three periods of significantly reduced inflow to the nile river system from the white nile based on sediment fabric and mineralogy shifts in percentages of total organic carbon toc and total organic nitrogen ton hydrogen index hi and 13 c and 15 n as well as poor preservation of sporopollenin and siliceous microfossils we recognize two mature paleosols that formed between 18 000 and 12 500 14 c yr bp at the surface of exposed lacustrine muds a thin fossiliferous zone between the two soils documents a brief period of flooding lake albert was below the level of the coring site and thus a closed basin for several thousand years during the late pleistocene around 12 500 14 c yr bp the younger vertisol was flooded by a major rise in lake level relatively deep water conditions were established soon after this transgression but an apparent change in sedimentation rate sometime between 8 000 and 3 400 14 c yr bp may reflect another briefer period of low water which resulted in interruption of sedimentation at the coring site palynological data document a rapid and abrupt shift to drier conditions around lake albert from 11 400 to 9 900 14 c yr bp coincident with the cool younger dryas event of the northern hemisphere geochemical evidence suggests that throughout the terminal pleistocene and early holocene and again in the late holocene the water column was relatively well mixed with little tendency to persistent anoxia

a 40 000 yr pollen and diatom record from lake tritrivakely madagascar in the southern tropics abstract links between southern and northern hemisphere climates during the late quaternary are poorly known partly due to the scarcity of continuous climatic records in the southern tropics pollen and diatom evidence from lake tritrivakely 19 47 s provides information on vegetational and hydrological changes in the central highlands of madagascar over the past 40 000 yr most of the record reflects natural environmental variability since humans arrived on the island ca 2000 yr b p during glacial times the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric co2levels in the lake a positive mean annual hydrologic balance from 38 000 to 36 000 and from 17 500 to 9800 cal yr b p coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics a negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate the last glacial maximum was cool and dry the deglacial warming occurred in two steps the first step accompanied by an increase in wetness occurred abruptly at ca 17 000 cal yr b p about two millennia earlier than in the northern hemisphere it is abundantly documented in southern terrestrial data the second step at 15 000 cal yr b p was in phase with the first major temperature change in the northern hemisphere

synthetic reconstructions of southern african environments during the last glacial maximum 21 18kyr and the holocene altithermal 8 6kyr abstract the pash database has been assembling palaeoclimatic data from the southern hemisphere for a number of years the southern african sector now contains more than 100 records spanning the last 200 000 years a number of these relate to the last glacial maximum and holocene altithermal which represent the two most recent extremes in global temperatures syntheses of these records have permitted the preparation of preliminary palaeoenvironmental maps for the two intervals the results provide insights into probable changes in atmospheric circulation as temperatures across the subcontinent rose by 6 7 c these changes are in agreement with a present day model which has been developed to explain short term variations in southern african climate the results presented here should assist in calibrating models developed to predict future climate change

a high resolution paleoclimate record spanning the past 25 000 years in southern east africa high resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern lake malawi provide a climate signal for this part of tropical africa spanning the past 25 000 years the biogenic silica mass accumulation rate was low during the relatively dry late pleistocene when the river flux of silica to the lake was suppressed millennial scale fluctuations due to upwelling intensity in the late pleistocene climate of the lake malawi basin appear to have been closely linked to the northern hemisphere climate until 11 thousand years ago relatively cold conditions in the northern hemisphere coincided with more frequent north winds over the malawi basin perhaps resulting from a more southward migration of the intertropical convergence zone

multiple episodes of aridity in southern africa since the last interglacial period there is generally a dearth of evidence of the nature of quaternary climate change within desert systems which has limited previous interpretations of past environmental change at low latitudes the last glacial maximum has previously been identified as the peak of late quaternary aridity when desert systems expanded to five times their present extent1 2 3 and low latitude aridity has been described for previous glaciations4 but little evidence has been derived directly for large desert basins particularly southern africa here we report new chronological optical dating evidence of arid episodes recorded in aeolian sediments from the mega kalahari sand sea episodic aeolian activity is recorded at the northeastern desert margin whereas more sustained activity is evident from the southwestern desert core several significant arid events are apparent since the last interglacial period with dune building arid phases at 95 115 41 46 20 26 and 9 16 kyr before present existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast southwest summer rainfall gradient which are in turn related to sea surface temperatures in the southeastern atlantic ocean

diatom inferred salinity and carbonate oxygen isotopes in holocene waterbodies of the western sahara and sahel africa abstract thirteen holocene palaeolakes in the western sahara and sahel have provided diatom records with carbonate oxygen isotope profiles available from eight of them most of these palaeolakes were groundwater fed lake water chemistry is reconstructed using diatom transfer functions lake water salinity and 18 o records are assembled with some isotopic and chemical groundwater data to better understand the response of the hydrological systems to climate changes over the past 15 000 yr data are in general agreement with climate simulations using coupled atmosphere ocean vegetation models which show a mid holocene wetting over the whole of northwest africa and a rapid drying by 6 4 ka the lake record also shows that at many sites the major lake infilling lags the end of the younger dryas by 1 2 ka regional differences also appear in the timing of the lake hydrological optimum ca 10 5 8 5 and 7 5 4 5 ka in the northern sahara 10 8 5 ka in the air tenere 10 5 7 or 4 5 in the sahel and 7 5 ka in lake chad the whole of the holocene is punctuated by short term drying events changes in water isotopic composition through time are partly explained by changes in rainfall amount and air humidity during the wet holocene period however the very low values in the southern sahara also imply changes in the moisture transport pattern or rainfall mechanisms data suggest an apparent decrease in 18 o content of precipitation along the monsoon flow in contrast with modern patterns changes in water availability and quality have driven population migrations in and out of the sahara sahel but relationships between climate and cultures are complex short term dry events might have driven inventive adaptations in the sahara drying at 5 4 5 ka coincides with both the collapse of the classical neolithic civilization and the settlement of new cultures

interhemispheric comparison of deglacial sea surface temperature patterns in atlantic eastern boundary currents abstract sediment core geob 1023 5 from the eastern south atlantic was investigated at high temporal resolution for variations of sea surface temperature sst during the past 22 kyr using the alkenone u k 37 method ssts increased by 3 5 c from about 18 c during the last ice age 21 2 cal kyr bp to about 21 5 c at 14 5 cal kyr bp this warming trend associated with the deglaciation phase was followed by a cooling event with lowest ssts near 20 c persisting for about 1000 years between 13 and 12 cal kyr bp the ssts then continued to increase to about 22 5 c at the holocene climatic optimum at 7 cal kyr bp and decreased again during the late holocene to a core top value of 19 8 c that is comparable to modern annual mean sst values when compared with alkenone sst records from the eastern north atlantic our sst record indicates continuous warming throughout the deglaciation phase in the benguela current while its northern counterpart the canary current experienced prominent cooling during heinrich event 1 h1 on the other hand for the time period corresponding to the younger dryas yd cooling event the benguela sst record exhibits a cold temperature interval that corresponds to that observed in the eastern north atlantic sst records this observation suggests that interhemispheric climate response in atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with termination i for the h1 the eastern south atlantic sst record strongly supports the hypothesis that an anti phase thermal behavior in south atlantic surface waters was forced by the slowdown of the north atlantic deep water formation during cold spells in the north atlantic in contrast the abrupt cooling in the eastern south atlantic coincident with the yd period was probably induced by more vigorous global atmospheric circulation enhancing the upwelling intensity in both eastern boundary current systems this atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the south atlantic ssts during the yd which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the yd than during the h1

discussion of origins of hinterland basins to the rif betic cordillera and carpathians abstract the hinterland basins of the rif betic cordillera western alboran sea and the carpathians pannonian basin are in part composed of a thick in excess of 3 4 km thermal sag basin sequence the underlying crust has been thinned to 25 km in parts of the pannonian basin and to 15 km in parts of the alboran sea modern tectonic models for these areas envision tectonic collision and thickening of the crust followed by crustal thinning either by gravitational collapse back arc extension or a combination of the two contemporaneous with thrusting basin and range type extension has been envisioned for parts of these extensional collapse systems extensional features are common and they interact in a complex way with strike slip systems flexural cantilever modelling agrees with earlier mckenzie type pure shear modelling of the extensional systems in suggesting that extension alone cannot generate sufficient thermal decay to explain the great thickness of the sag basins associated with the alboran sea and pannonian basin consequently it is suggested that uplift of the asthenosphere accompanying subduction roll back is an important factor in the evolution of these basins roll back of the mantle lithosphere permits hot asthenospheric mantle to upwell and lie close to the base of the crust initially uplift and erosion of the crust overlying the asthenosphere occurs as the thermal perturbation cools the crust thermally subsides to produce a large sag basin

the late pleistocene and holocene stratigraphy and paleolimnology of lakes kivu and tanganyika abstract evidence from diatoms chemistry and sedimentology shows that lakes kivu and tanganyika experienced late pleistocene low stands 86 to 300 m for kivu and 250 to 350 m for tanganyika both lakes had higher 2 10 fold sedimentation rates prior to 11 500 yr b p about 9900 yr b p upwelling became important in southern lake tanganyika which had been deepening for at least several millenia by 9400 yr b p kivu had become deep and dilute with overflow via the ruzizi river to lake tanganyika southern lake tanganyika became more stably stratified about 5200 yr b p volcanism and hydrothermal activity in kivu began about 5000 yr b p and the salinity reached 3 ppt and the lake became strongly stratified the ruzizi river outflow was interrupted from 3500 yr b p until recently the sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in kivu has probably been the cause of its impoverished fauna these results show that high lake levels in the eastern rift of africa from 10 000 to 7000 yr b p were paralleled in the western rift given the great volumes of these lakes climate has not caused great variations in salinity the diatom floras however have been remarkably sensitive to changes in stratification and nutrient supply volcanism has been a major force in the limnological and biological evolution of lake kivu while both kivu and tanganyika have responded to changes in precipitation and wind stress

influences of high and low latitude processes on african terrestrial climate pleistocene eolian records from equatorial atlantic ocean drilling program site 663 high and low latitude forcing of terrestrial african paleoclimate variability is demonstrated using 900 ka eolian and biogenic component records from ocean drilling program site 663 in the eastern equatorial atlantic terrigenous eolian dust and phytolith savannah grass cuticle accumulation rate records vary predominantly at 100 and 41 kyr periodicities and spectral phase estimates implicate high latitude forcing the abundance of freshwater diatoms melosira transported from dry african lake beds varies coherently at 23 19 kyr orbital periodicities and at a phasing which implicates low latitude precessional monsoon forcing modeling studies demonstrate that african climate is sensitive to both high and low latitude boundary conditions african monsoon intensity is modulated by direct insolation variations due to orbital precession whereas remote high latitude forcing can be related to cool north atlantic sea surface temperatures ssts which promote african aridity and enhance dust transporting wind speeds the site 663 terrigenous and phytolith records covary with north atlantic sst variability at 41 n site 607 we suggest that pleistocene african climate has responded to both high latitude north atlantic sst variability as well as low latitude precessional monsoon forcing the high latitude influence dominates the sedimentary record prior to circa 2 4 ma terrigenous variations occurred primarily at precessional periodicities 23 19 kyr indicating that african climate was largely controlled by low latitude insolation variations prior to the onset of high amplitude glacial interglacial climate change

variations of stable oxygen and hydrogen isotope ratios in the cold and thermal springs of the bazman volcanic area in the southeast of iran the bazman volcanic area which comprises thermal and cold water springs with different natural characteristics was selected as the case study of this research the cold springs can be divided into western and eastern groups in total 16 water samples were collected from the thermal and cold water springs for hydrochemical and stable isotope analyses in the study area the groundwater temperature ranged from 28 1 c in the cold water to 43 9 c in the hot springs the electrical conductivity ec values of the groundwater samples were 1102 10 250 s cm the highest ec values belonged to the thermal springs known to have brackish water in this area the 18o and 2h values in the springs ranged from 5 06 to 0 04 and 24 1 to 8 6 respectively the relationship between 2h and 18o values and physicochemical characteristics was investigated in the west of the area the isotopic composition of the cold springs was close to that of the lmwl while the eastern cold springs had an isotopic composition highly enriched in 2h and 18o in contrast the isotopic composition of the thermal springs was almost on that of the gmwl and their deuterium isotope was more depleted than that of cold springs the difference between the isotopic composition of the thermal springs and the lwml can be the result of hot water rock interaction and underground evaporation however magma was not effective on this difference the results showed that the groundwater of the western cold springs moved with faster velocity through the long and deep macro pores and preferential flow channels in carbonate and igneous rocks while in the eastern springs groundwater originated from fine fractures with a low depth in shale and sandstone rocks

active tectonics of the western mediterranean geodetic evidence for rollback of a delaminated subcontinental lithospheric slab beneath the rif mountains morocco surface deformation in morocco derived from five years of global positioning system gps survey observations of a 22 station network four continuously recording gps cgps stations and four international gnss service igs stations in iberia indicates roughly southward motion 3 mm yr of the rif mountains morocco relative to stable africa motion of the rif is approximately normal to the direction of africa eurasia relative motion which is predominantly strike slip and results in shortening of the rif and subsequent crustal extension of the adjacent alboran sea region the sense and the n s asymmetry of the observed deformation i e no evidence for north directed shortening in the betic mountains north of the alboran sea cannot be easily explained in terms of crustal plate interactions suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western mediterranean the model that best fits the observations involves delamination and southward rollback of the african lithospheric mantle under the alboran and rif domains

the ghomarides nappes rif coastal range morocco a variscan chip in the alpine belt the ghomarides nappes are the uppermost basement nappes in the rifan alpine belt paleozoic rocks constitute the prominent part of these nappes together with triassic and younger cover sediments four nappes are distinguished each one with a distinct stratigraphical sequence especially for the devonian and carboniferous periods sedimentological and structural arguments plead for an initial location on the northern margin of the african continent the main folding and metamorphic events are shown to be eo variscan late acadian tectonic vergence was to the north present coordinates late variscan hercynian shortening gave upright nw se trending folds alpine ductile overprints are scarce except at the base of the lowest nappe where k ar phengite ages are about 25 ma the ghomarides nappes correlate with the malaguides nappes betic cordilleras southern spain the kabylian northern algeria and calabrian southern italy paleozoic nappes the tuscan south alpine basement and the upper austro alpine nappes the proposed correlations allow us to restaure the southern margin of the hercynian belt along northwestern africa

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

sedimentation rate dilution preservation and total organic carbon some results of a modelling study abstract a modelling exercise was undertaken to evaluate the positive preservational and negative diluent effects of sedimentation rate sr on the total organic carbon toc content of marine sediments and to use multiple regression analysis of modern data to predict toc data presented show the predicted marine toc using the carbon burial efficiency be versus sr relationship of betts and holland betts j n holland h d 1991 the oxygen content of ocean bottom waters the burial efficiency of organic carbon and the regulation of atmospheric oxygen palaeogeography palaeoclimatology palaeoecology global and planetary change section 97 5 18 calculated for a range of sr water depths primary productivities and four fixed be values the results suggest that the preservation effect of sr only enhances toc at 5 cm ka in oxic regimes after which dilution occurs if carbon supply is held constant above 35 cm ka there is no appreciable difference in toc regardless of whether the sea floor is anoxic or not below 10 cm ka there is a strong divergence in oxic and anoxic toc values observed positive correlations of sr and toc at 5 cm ka are largely an artefact of including sites from shallower depths and or higher productivities multiple regression analysis mra of modern marine sediment data suggests that once re scaled to allow for their non linear effect dissolved oxygen values may be a significant additional variable in determining the toc predicting an approximately 2 5 4 0 times higher toc in anoxic facies compared with fully oxic ones 4 ml l dissolved oxygen all other things being equal the mra also indicates a generally negative partial correlation between carbon delivery flux and be in modern sediments producing a significant moderating effect on the predicted toc

hydrogeochemistry environmental isotopes and the origin of the hamamayagi ladik thermal spring samsun turkey hamamayagi thermal spring hts is located along the north anatolian fault zone the thermal spring has a temperature of 36 c with total dissolved solids ranging from 485 6 to 508 5 mg l hard brittle and gray limestones permian aged are the reservoir rocks of the hts 18o 2h isotope ratios clearly indicate a meteoric origin for the waters the 34s value of sulfate in the thermal water is nearly 4 1 and implies a diagenetic environment characterized by reduced sulfur compounds the 13c ratio for dissolved inorganic carbonate in the hts lies between 1 78 and 1 62 showing that it originates from the dissolution of fresh water carbonates quartz geothermometry suggests a reservoir temperature of 52 85 c for the hamamayagi geothermal field but chalcedony geothermometers suggest reservoir temperatures between 30 and 53 c

co 2 record in the byrd ice core 50 000 5 000 years bp the analysis of air in polar ice cores revealed 30 lower co2 values during glacial periods than during interglacial periods at present this is confirmed by results from six different ice cores two from greenland and four from antarctica1 5 in all cores the co2 change coincides with the change in the isotopic composition of the ice expressed as either the 18o or d ratio both are indicators for the mean annual surface temperature6 to investigate the relationship between atmospheric co2 concentration changes with changes in climate the atmospheric co2 concentration during and at the end of the last glaciation has to be known in detail to achieve this we have studied a great number of samples from the deep ice core from byrd station westantarctica drilled in 1968 these measurements allow us to reconstruct the atmospheric co02 concentration in the time period 50 000 15 000 yr bp in great detail

crustal resistivity structure of the southwestern transect of the rif cordillera morocco a ne sw magnetotelluric 110 km long profile including 18 sites was acquired across the western rifcordillera along the eurasian african plate boundary allowing to constrain its poorly known deepstructure it extends from the internal zones close to the alboran coast crossing the external zones andup to the gharb foreland basin the periods recorded range from 0 001 s to 1000 s the combination ofmagnetotelluric data with available geological data provides new insight regarding the relationship

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

sodium calcium ion exchange in the weathering of shales implications for global weathering budgets unpolluted rivers and streams that drain marine shales show an excess of sodium compared to chloride and a deficiency of calcium and magnesium compared to sulfate and alkalinity this is due in part to cation exchange of sodium for divalent cations on clay minerals consideration of the global weathering budget suggests that up to 34 of the sodium in the total dissolved stream load may be due to cation exchange rather than sodium production via silicate dissolution weathering reactions these results suggest that the weathering budgets for sodium and calcium are in need of revision because of the inclusion of cation exchange processes in the weathering cycle this implies that silicate dissolution is less important in determining the composition of global river water than was previously thought

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

evidence of a large deep conductive body within the basement of the guadalquivir foreland basin betic cordillera s spain from tipper vector modelling tectonic implications abstract the betic cordillera is an alpine belt formed by the interaction of the eurasian and african plates and the westward motion of the alboran domain long period magnetotelluric observations at 26 sites in its westernmost part provide induction arrows that have been compared with 3d forward models including bathymetry and major geological bodies the results highlight the presence of a major conductive body 0 05 m unknown to date and located within the basement of the guadalquivir foreland basin aeromagnetic and field magnetic measurements further support the occurrence of magnetic anomalies related to the top of this anomalous body this major structure is interpreted as an intermediate or basic igneous rock with a high proportion of metallic mineralization its origin is discussed in the framework of the regional geological setting possibly produced in the southern iberian variscan massif by a huge concentration of volcanogenic massive sulphide vms in the prolongation of the iberian pyrite belt during devonian early carboniferous times another possibility is that the conductive anomaly is due to magmatic intrusions associated with the mesozoic fragmentation of southern iberia and the opening of the tethys