climate evolution during the holocene a study with an earth system model of intermediate complexity an earth system model of intermediate complexity mobidic has been used to simulate the transient variations in continental temperature sea surface temperature sst thermohaline circulation thc and sea ice cover over the last 9000 years 9 kyr experiments were designed to determine a the deviation of the climatic system with respect to equilibrium over the last 9 kyr b the individual contributions of oceans and vegetation to climatic changes as well as the potential synergies between these components and c the relative importance of precession obliquity and co2 concentration changes during this period results show a monotonous cooling trend in the northern high latitudes between 9 kyr bp and the present day both over the oceans and the continents north of 60 n this cooling is noticed throughout the year but the largest variations appear in spring and summer up to 6 c over continents along with this cooling the model exhibits a southward shift of the northern treeline by about 600 km most of this shift takes place between 4 and 1 kyr bp during this period reorganisations of the boreal forest introduce a lag of about 200 years in the system with respect to a state in equilibrium with the external forcing sensitivity experiments illustrate the strong impact of this vegetation shift both on the oceans and the continents especially in spring and early summer however the model exhibits a weak synergy between vegetation and ocean throughout the holocene finally a sensitivity study to the forcing components shows the dominant role of the astronomical forcing with respect to co2 as well as the non linear behaviour of climate in response to obliquity and precession

pleniglacial sedimentation process reconstruction on laminated lacustrine sediments from lava dammed paleolake alf west eifel volcanic field germany abstract differentiating between regularly seasonal irregular and event based clastic sedimentation is difficult if sedimentation structures resemble and dating methods are imprecise in this study clastic light and dark laminae from lava dammed paleolake alf in the late pleistocene in the quaternary west eifel volcanic field are analyzed to clarify how they formed and if they are of annual origin and comparable to assumed periglacial varves from neighboring lake holzmaar therefore a multiproxy approach is applied combining sediment thin section analysis which focuses on composition and structure with 14c dates the results are compared to recently formed annually laminated clastic sediments of e g the high canadian arctic observed sedimentation structures reveal sediment delivery by over and interflows and deposition from suspension forming two characteristic microfacies type i graded laminae and type ii laminae with graded sublayers additionally erosional bases and event deposits indicate episodic underflows thus lamination is potentially seasonal but is significantly veiled by extreme runoff causing erosion and resuspension processes or a mixed water body preventing sediment delivery into the lake basin however sedimentation processes between watershed and lake could be reconstructed by comparing recent and paleosediment structures

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

santorini luminescence dating of a volcanic province using quartz abstract we have investigated the application of luminescence dating to the palaeosol underlying the cape riva ignimbrite from the volcanic island of santorini 36 40 n 25 40 e in the aegean sea the history of volcanism in this area is of intrinsic interest and is also of importance to archaeology in the eastern mediterranean a flourishing late bronze age settlement was buried under the volcanic products of the minoan eruption 1640 bc volcanic provinces are notoriously difficult to date using luminescence techniques because volcanic feldspars almost invariably show anomalous fading which can lead to age underestimates of more than 50 quartz does not show anomalous fading but unfortunately it is rare or non existent in volcanic sediments and so it is usually not considered as a potential chronometer we have studied the luminescence characteristics of material extracted from the cape riva palaeosol with good independent age control all samples contain detectable quartz but usually

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

direct dating of quaternary phreatic maar eruptions by luminescence methods the suitability of quartz optically stimulated luminescence osl and feldspar infrared stimulated luminescence irsl for the direct dating of phreatic eruptions was tested on examples from the eifel volcanic field germany the mean irsl age of 11 6 0 5 ka for ulmener maar tephra is in excellent agreement with the independent age control 11 0 1 ka but the mean osl age of 14 2 0 6 ka overestimates the known age by 3 ka for meerfelder maar tephra consistent irsl mean 74 9 5 0 ka and osl ages 74 9 5 5 ka have been observed the consistent results from meerfelder maar imply that the overestimation observed for ulmener maar quartz osl might not be relative to the eruption age but rather represents a small absolute offset in samples taken from deposits of the eruption of laacher see volcano no resetting of the osl signal and highly scattered irsl estimates were found this implies that phreatomagmatic eruptions are less well suited for this dating approach compared to pure phreatic maar eruptions where the effect of high pressure shock waves probably dominates the process of resetting the luminescence signal

luminescence dating of late pleistocene eruptions in the eifel volcanic field germany robust chronologies of late pleistocene and holocene volcanic eruptions are vital for hazard analysis but accurate and precise dating of these events is often difficult here we apply various luminescence techniques to quartz and polymineral extracts from heated crustal xenoliths enclosed in scoria and volcanically heated bedrock in the quaternary eifel volcanic field evf germany consistent results from red thermoluminescence rtl and optically stimulated luminescence osl of quartz and from post infrared infrared stimulated luminescence of polymineral material demonstrate complete luminescence signal resetting during lava emplacement and sufficient signal stability rtl and osl age underestimation of one sample with independently known age from the lava contact zone could be eliminated by annealing before laboratory regenerative irradiation the average luminescence age of 33 6 2 4 ka for the wartgesberg eruption is in good agreement with independent age control while the average age of 15 5 1 1 ka for the facher hohe scoria cone is much younger than previously assumed this result represents the third youngest known eruption in the evf our new data are in line with the hypothesized climate controlled triggering of eifel volcanism and confirm that active volcanism related to asthenosphere upwelling migrates from nw to se in the evf

variations in axial morphology along the galapagos spreading center and the influence of the galapagos hotspot the galapagos spreading center gsc is marked by systematic changes in axial morphology between the inca fracture zone fz at 85 5 w and the 95 5 w propagator we analyze these changes using new swath bathymetry and magnetic data acquired aboard the b o hesperides during the galapagos 96 experiment within 350 km of the galapagos hotspot the ridge axis is associated with an east pacific rise epr like axial high at increasing distance from the hotspot the axial high broadens and deepens forming a distinctive transitional axial morphology tam the axis in this transitional region is typically a broad zone 20 km wide consisting of very rough volcanic and fault generated topography west of 95 w this tam evolves into a 20 40 km wide 400 1500 m deep axial valley typical of the slow spreading mid atlantic ridge mar there is not an abrupt change from axial high to rift valley along the gsc but a distinct tam occurs over a distance of 200 300 km along axis and is accompanied by a gravity estimated crustal thickening of 1 2 km the boundary between an axial high and this tam is quite abrupt and occurs along a segment that is less than 9 km long these changes in axial morphology are primarily caused by variations in magma supply along the gsc due to the entrainment and dispersal of plume mantle from the galapagos hotspot however the changes in morphology are not symmetric about the galapagos fz at 91 w the axial high topography extends farther east of the 91 w fz than to the west and the rift valley which develops west of 94 w is not found at comparable distances along the gsc east of the hotspot axial depth variations are also asymmetric across the 91 w fz this asymmetry in both morphology and axial depth variation is attributed to a full spreading rate increase along the gsc from 46 mm yr at 97 w to 64 mm yr at 85 w off axis depth changes are symmetric about the 91 w fz and suggest that 15 40 of on axis depth variation is dynamically supported

the magnitude timing and abruptness of changes in north african dust deposition over the last 20 000 yr reconstructions of eolian dust accumulation in northwest african margin sediments provide important continuous records of past changes in atmospheric circulation and aridity in the region existing records indicate dramatic changes in north african dust emissions over the last 20 ka but the limited spatial extent of these records and the lack of high resolution flux data do not allow us to determine whether changes in dust deposition occurred with similar timing magnitude and abruptness throughout northwest africa here we present new records from a meridional transect of cores stretching from 311 nt o 191n along the northwest african margin by combining grain size endmember modeling with 230 th normalized fluxes for the first time we are able to document spatial and temporal changes in dust deposition under the north african dust plume throughout the last 20 ka our results provide quantitative estimates of the magnitude of dust flux changes associated with heinrich stadial 1 the younger dryas and the african humid period ahp 11 7 5 ka offering robust targets for model based estimates of the climatic and biogeochemical impacts of past changes in north african dust emissions our data suggest that dust fluxes between 8 and 6 ka were a factor of 5 lower than average fluxes during the last 2 ka using a simple model to estimate the effects of bioturbation on dust input signals we find that our data are consistent with abrupt synchronous changes in dust fluxes in all cores at the beginning and end of the ahp the mean ages of these transitions are 11 870 2 ka 1s and 4 970 2 ka respectively

holocene climatic evolution at 22 23 n from two palaeolakes in the taoudenni area northern mali abstract in the endorheic hydrologically isolated taoudenni depression nowadays receiving 5 mm mean annual rainfall continuous sections in fossiliferous holocene deposits from two palaeolakes with different geological evolutions have been compared the following climatic sequence is determined 9000 8300 yr b p the beginning of a humid phase 8300 6700 yr b p climatic optimum from 6700 yr b p onwards a change in regime 4500 4000 yr b p end of humid phase aridification begins the data imply a 600 km northern shift of the monsoonal belt which confirms models described in the literature

how far north did the african monsoon fringe expand during the african humid period insights from southwest moroccan speleothems we present new high resolution oxygen isotope delta o 18 records from three nw african speleothems located at 31 degrees n the present day rainfall patterns at 31 degrees n in nw africa are linked to negative winter north atlantic oscillation phases however on multimillennial time scales our delta o 18 records together with other hydroclimate records provide new evidence of humid conditions during the mid holocene a period that was presumably characterized by arid climate thus the apparent increase in moisture during the mid holocene is interpreted better as an increase in summer rainfall this is most likely linked to the expansion of the west african summer monsoon fringe during the african humid period which terminated in our record abruptly around 4 kyr bp the temporospatial difference with speleothem records from n morocco suggests that the high atlas mountains might have been a topographic barrier to further expansion of the west african summer monsoon fringe into higher latitudes plain language summary the holocene african humid period in north africa characterized by the expansion of vegetation into the green sahara has been linked to the intensification of the west african summer monsoon wasm however the temporospatial pattern of the african humid period especially the northernmost expansion of the wasm remain a matter of controversy largely owing to the lack of precisely dated and high resolution paleoclimatic records this study presents new highresolution paleoclimate data based on speleothem oxygen isotope records from a key site at 31 degrees n in nw africa our data suggest that the wasm expanded to 31 degrees n in nw africa during the mid holocene and terminated abruptly at 4 kyr bp

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

holocene palaeoclimate in the northern sahara margin jefara plain northwestern libya the palaeoclimatic evolution of northwestern libya has been deduced from the study of the palustrine and aeolian sediments the holocene environmental variations have been recognized through geological and stratigraphic surveys and by pollen analysis and malacofauna content of the sediments some organic rich horizons have been dated using the radiocarbon method the data show that in general the presence of marshes is linked with the period of higher humidity which began around 9 4 ka bp and ended around 5 0 ka bp from c 5 0 ka bp up to about 4 4 ka bp there was a simultaneous presence of marshes and vegetation of arid climate as a consequence of rainfall in the mountains and or a decrease in temperatures the holocene wet period was interrupted by two arid spells dated about 8 2 and 5 5 5 4 ka bp the end of the wet phase must have been gradual the climatic events that occurred in the jefara plain seem to be well correlated with those identified in saharan africa even if the mediterranean is only

numerical modeling of lava flow cooling applied to the 1997 okmok eruption approach and analysis 1 throughout february and march of 1997 okmok volcano in the eastern aleutian islands of alaska erupted a 6 km long lava flow of basaltic a a within its caldera a numerical model for lava flow cooling was developed and applied to the flow to better understand the nature of its cooling radiation and convection from the surface as well as conduction to the ground were used to transport the flow s heat to its surroundings in the model internally a conduction only approach moved heat from the interior outward vesiculation latent heat generation and thermal conductivity changes with temperature are among the other factors that were dynamically accounted for results indicate that ambient temperature fluctuations on the scale of days to weeks must be taken into account to create an accurate short term prediction of lava surface temperature daily data of rainfall and ambient temperature as opposed to yearly averages greatly increased the accuracy of the model furthermore convective cooling of the lava surface was observed to be a dominant heat loss process during the first 200 days indicating the convective heat transfer coefficient is a prime determinant of the accuracy of the model for predicting surface temperatures over a longer cooling period 2 years thermal conductivity and porosity proved to be among the dominating factors for heat loss because of the limiting role of conductive heat flow in the interior the model s flexibility allows application to flows other than the 1997 okmok eruption

multiple brunhes chron excursions recorded in the west eifel germany volcanics support for long held mantle control over the non axial dipole field volcanic records of excursional geomagnetic field behavior in particular paleointensity estimates are fragmentary for the pleistocene the west eifel volcanic field is unique in that 12 of 66 measured lava flow sites record virtual geomagnetic poles vgps clustered between 34 45 n latitude and 30 50 e

seamount abundances and distributions in the southeast pacific abstract sea beam bathymetry was recorded for 17 277 km of ship track in the southeast pacific and has been analyzed for seamount population characteristics all the ship tracks are over the pacific plate and most fall along 7 lines parallel to the east pacific rise between 7 s and 22 s the lines fall into three age categories one line is over 0 5 2 ma crust three are over 5 10 ma crust and three are over 30 40 ma crust seamount locations were recorded diameters were manually estimated and heights were measured if the swath crossed the seamount center over 382 features were counted along the entire ship track with heights ranging from 50 m to over 2500 m with sampling most consistent at heights between 100 m and 1000 m height to radius ratios vary considerably suggesting that seamount shapes do not scale to a single parameter the observed variety of morphological forms demonstrates that there is a wide range of seamount shapes for small features the distribution of sizes for the population was approximated by exponential dependence and by power law dependence using methods developed by smith and jordan 17 the power law distribution overestimates abundances at the smallest size ranges and both distributions fail to predict abundances of large seamounts determined from wide beam data 17 size distributions were also determined for seamounts in each seafloor age category almost all small seamounts appear to be produced on crust younger than 0 5 2 ma while the number of larger seamounts increases to 5 10 ma for all size ranges observed here the number of seamounts at 5 10 ma and at 30 40 ma is roughly identical these observations suggest that most small seamounts are formed on very young thin lithosphere that permits the passage of small volumes of magma small volumes would cool in older thicker lithosphere before reaching the surface although larger magma bodies might not numerous small sources of melt must exist near the ridge crest to supply the small seamounts probably trapped remnants of the large scale upwelling

modeling the effect of freshwater pulses on the early holocene climate the influence of high frequency climate variability freshwater 4 67 10 14 m 3 into the labrador sea at three different constant rates 1 5 sv 1 sv 1 10 6 m 3 s 1 i n 10 years 0 75 sv in 20 years and 0 3 sv in 50 years for each rate five ensemble experiments have been performed varying in initial conditions the freshwater pulses produce a weakening of the thermohaline circulation the perturbed state is in agreement with proxy evidence for the 8 2 ka event two types of recovery of the thermohaline circulation occurred differing in time scale 1 200 years and 2 200 years in the experiments with 10 year and 20 year pulses both types of recovery were observed this suggests that the model response is unpredictable in the range of parameters studied here it is hypothesized that the unpredictability is associated with annual to decadal climate variability our results demonstrate that several types of recovery may exist with the same kind of perturbation the interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere sea ice ocean system index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean keywords holocene thermohaline circulation predictability meltwater coupled climate model

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

which geomorphic processes can be informed by luminescence measurements abstract luminescence signals from quartz and feldspar contain a wealth of information about exposure to heat and sunlight while many studies have focused on quantifying individual processes such as grain transport and burial cooling during exhumation or bleaching of freshly exposed rock surfaces not much work has yet been done to illustrate how these effects might interact for example how might a geologist geomorphologist interpret the luminescence signals within a riverine cobble or within a recent rockfall from an uplifting terrain this study attempts to address this issue by first presenting a simple kinetic expression for osl signal accumulation in response to background radiation and osl loss by exposure to heat or to direct or attenuated sunlight this model is then used to simulate three example sample histories simulation results demonstrate how much information can be embedded within one sample s luminescence signal and how disparate geomorphic histories can produce strikingly similar signals the temperatures and timescales which can be examined with luminescence signals are presented in comparison to those of various geomorphic processes finally simulation results illustrate the benefit of using signals of different stability to better understand a sample s recent geologic history

oceanic spreading center hotspot interactions constraints from along isochron bathymetric and gravity anomalies we analyzed bathymetric and gravity anomalies along present and paleoaxes of oceanic spreading centers influenced by the iceland azores galapagos tristan and easter hotspots residual bathymetry up to 4 7 km and mantle bouguer gravity up to 340 km anomalies are maximum at on axis hotspots and decrease with increasing ridge hotspot separation distance d until becoming insignificant at d 500 km along isochron widths of bathymetric anomalies up to 2700 km depend inversely on paleo spreading rate reflecting the extent to which plume material will flow along axis before being swept away by the spreading lithosphere flux balance arguments suggest that the five hotspots feed material to ridges with comparable fluxes of 2 2 106 km3 m y assuming that the amplitudes of these geophysical anomalies reflect temperature dependent crustal thickness and mantle density variations we suggest that ridge temperature anomalies are maximum 150 225 c when plumes are ridge centered and decrease with increasing ridge hotspot distance due to cooling of the ridgeward migrating plume material

ubiquity of anomalous fading in k feldspars and the measurement and correction for it in optical dating anomalous fading rates were measured in k feldspars separated from 49 sediment samples mainly from north america the intensity of the optically stimulated luminescence was found to decrease linea

could gradual changes in holocene saharan landscape have caused the observed abrupt shift in north atlantic dust deposition abstract the abrupt change in north atlantic dust deposition found in sediment records has been associated with a rapid large scale transition of holocene saharan landscape we hypothesize that gradual changes in the landscape may have caused this abrupt shift in dust deposition either because of the non linearity in dust activation or because of the heterogeneous distribution of major dust sources to test this hypothesis we investigate the response of north atlantic dust deposition to a prescribed 1 gradual and spatially homogeneous decrease and 2 gradual southward retreat of north african vegetation and lakes during the holocene using the aerosol climate model echam ham in our simulations we do not find evidence of an abrupt increase in dust deposition as observed in marine sediment records along the northwest african margin we conclude that such gradual changes in landscape are not sufficient to explain the observed abrupt changes in dust accumulation in marine sediment records instead our results point to a rapid large scale retreat of vegetation and lakes in the area of significant dust sources

seychelles coral record of changes in sea surface temperature bimodality in the western indian ocean from the mid holocene to the present we report fossil coral records from the seychelles comprising individual time slices of 14 20 sclerochronological years between 2 and 6 2 kyr bp to reconstruct changes in the seasonal cycle of western indian ocean sea surface temperature sst compared to the present 1990 2003 these reconstructions allowed us to link changes in the sst bimodality to orbital changes which were causing a reorganization of the seasonal insolation pattern our results reveal the lowest seasonal sst range in the mid holocene 6 2 5 2 kyr bp and around 2 kyr bp while the highest range is observed around 4 6 kyr bp and between 1990 and 2003 the season of maximum temperature shifts from austral spring september to november to austral autumn march to may following changes in seasonal insolation over the past 6 kyr however the changes in sst bimodality do not linearly follow the insolation seasonality for example the 5 2 and 6 2 kyr bp corals show only subtle sst differences in austral spring and autumn we use paleoclimate simulations of a fully coupled atmosphere ocean general circulation model to compare with proxy data for the mid holocene around 6 kyr bp the model results show that in the mid holocene the austral winter and spring seasons in the western indian ocean were warmer while austral summer was cooler this is qualitatively consistent with the coral data from 6 2 to 5 2 kyr bp which shows a similar reduction in the seasonal amplitude compared to the present day however the pattern of the seasonal sst cycle in the model appears to follow the changes in insolation more directly than indicated by the corals our results highlight the importance of ocean atmosphere interactions for indian ocean sst seasonality throughout the holocene in order to understand holocene climate variability in the countries surrounding the indian ocean we need a much more comprehensive analysis of seasonally resolved archives from the tropical indian ocean insolation data alone only provides an incomplete picture

asthenosphere flow model of hotspot ridge interactions a comparison of iceland and kerguelen we develop a numerical model to test the asthenosphere flow paradigm in which hotspots feed the low viscosity asthenosphere and lithosphere growth consumes the asthenosphere the dynamics of this flow model are controlled by the relative position of the hotspot to the ridge absolute plate velocities lithosphere and asthenosphere rheologies and tectonic boundary conditions the model is applied to two distinct regions the iceland hotspot centered on the mid atlantic ridge and the kerguelen hotspot located near the southeast indian ridge the iceland model generates along ridge flow rates as high as 30 cm y for an asthenosphere viscosity of 5 1018 pa s these flow rates are consistent with previous interpretations of the origin of southward pointing v s south of iceland the preferred kerguelen model produces significant hotspot to ridge flow followed by along ridge flow velocities from the off axis hotspot to the ridge are 66 cm y for a model within which asthenosphere viscosity varies from 5 1018 to 5 1019 pa s the asthenosphere flow paradigm can explain major features of hotspot ridge interactions for both on axis and off axis hotspots

major element chemistry of galapagos rift zone magmas and their phenocrysts abstract basalts dredged from the galapagos rift zone between 85 w and 100 w were analyzed by electron microprobe to determine the chemistry of the glass exteriors and included phenocrysts microphenocrysts and quench minerals the basalts come from both normal mid ocean ridge segments and from ridge segments that cross the galapagos platform the basalts fall into two chemical and geographical groups group a basalts come from outside the central region of the galapagos platform i e outside 89 92 5 w and are chemically similar to basalts from normal ocean ridge segments group b basalts come from the center of the galapagos platform 89 92 5 w and are enriched in incompatible elements like plume influenced basalts from the mid atlantic ridge the spinel olivine plagioclase and clinopyroxene phenocrysts in both groups of basalts are low pressure equilibrium phases but the chemical difference among basalts from within each group indicates high pressure fractional crystallization is also responsible for the chemical evolution of some of these basalts presently no crystallization or partial melting model can relate the chemistry of the two groups of basalts and the compositional influence of a large ion lithophile elements and water rich mantle beneath the galapagos platform is a viable alternate hypothesis the eruption temperature of magmas from the normal ridge segments as determined by olivine liquid thermometry is 1217 10 c suggesting steady state conditions but on the galapagos platform the eruption temperatures are lower and more variable than on the normal ridge 1186 c 30 c suggesting a more complex plumbing system and the absence of a steady state magma chamber beneath the platform

on the cause of abrupt vegetation collapse in north africa during the holocene climate variability vs vegetation feedback the abrupt desertification over the northern africa in the mid holocene is studied in both a complex and a simple coupled climate vegetation model in contrast to the previous mechanism that relies on strong positive vegetation climate feedback and the resulted multiple equilibria we propose a new mechanism in which the abrupt desertification is caused by low frequency climate variability rather than a positive vegetation climate feedback the implication of this new mechanism to modelling and observation is also discussed

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

correlated geophysical geochemical and volcanological manifestations of plume ridge interaction along the galapagos spreading center 1 as the galapagos hot spot is approached from the west along the galapagos spreading center there are systematic increases in crustal thickness and in the k ti nb zr 3he 4he h2o and na2o content of lavas recovered from the spreading axis these increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth residual mantle bouguer gravity anomaly magma chamber depth average lava mg ca al ratio and the frequency of point fed versus fissure fed volcanism magma chamber depth and axial morphology display a threshold effect in which small changes in magma supply result in large changes in these variables these correlated variations in geophysical geochemical and volcanological manifestations of plume ridge interaction along the western galapagos spreading center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on magma supply axial thermal structure basalt chemistry and styles of volcanism

luminescence dating of archaeological materials using a new technique based on single aliquot measurements abstract a dating method based on single aliquot luminescence dose determination was applied to quartz and feldspars from heated archaeological materials the problem of changes of sensitivity to radiation resulting from signal erasure in regeneration measurements was studied in some detail and a new method based on single aliquot regeneration measurements within an added dose framework which appears to eliminate the problem of sensitivity change is proposed preliminary dating results mainly using quartz obtained with the new method are presented

holocene climate evolution in the high latitude southern hemisphere simulated by a coupled atmosphere sea ice ocean vegetation model the holocene climate is simulated in a 9000 yr long transient experiment performed with the ecbilt clio vecode coupled atmosphere sea ice ocean vegetation model this experiment is forced with annually varying orbital parameters and atmospheric concentrations of co2 and ch4 the objective is to study the impact of these long term forcings on the surface temperature evolution during different seasons in the high latitude southern hemisphere we find in summer a thermal optimum in the mid holocene 6 3 ka bp with temperatures locally 3 degrees c above the preindustrial mean in autumn the temperatures experienced a long term increase particularly during the first few thousand years the opposite trend was simulated for winter and spring with a relatively warm southern ocean at 9 ka bp in inter up to 3 5 degrees c above the preindustrial mean and a warm continent in spring 3 degrees c followed by a gradual cooling towards the present these long term temperature trends can be explained by a combination of 1 a delayed response to orbital forcing with temperatures lagging insolation by i to months owing to the thermal inertia of the system and 2 the long memory of the southern ocean this long memory is related to the storage of the warm late winter spring anomaly below the shallower summer mixed layer until next winter sea ice plays an important role as an amplifying factor through the ice albedo and ice insulation feedbacks our experiments can help to improve our understanding of the holocene signal in proxies for instance the results suggest that in contrast to recent propositions teleconnections to the northern hemisphere appear not necessarily to explain the history of southern hemisphere temperature changes during the holocene

dating volcanic and related sediments by luminescence methods a review abstract applications of luminescence dating of geological materials have undergone extensive development and refinement since 1974 when wintle first attempted to date volcanic lava using thermoluminescence tl today luminescence techniques are potentially highly useful methods for dating volcanic and related materials and events over timescales ranging from 10 2 to 10 6 years while luminescence approaches generally do not possess the precision afforded by techniques such as 40 ar 39 ar and 14 c methods they offer a useful alternative approach with advantages in terms of age range and scope of material although the application of luminescence methods has in some cases been successfully described a comprehensive study outlining and defining protocols for routine luminescence dating of volcanic materials has not been forthcoming this review surveys previous attempts to develop luminescence techniques to date volcanic products and discusses the state of knowledge of the luminescence behaviour of the four principal materials currently used for luminescence dating of volcanic events particular attention is paid to the red luminescence emissions primarily because of previous successes in using red thermoluminescence rtl of quartz and feldspar for dating volcanic products for which the upper age limit may extend beyond a million years proposals are made for future research directions these include technological developments in the measurement of optically stimulated red luminescence the systematic characterization of rtl and optically stimulated red luminescence as key luminescence parameters in volcanic quartz feldspar glass and polymineral sample fractions and optimization of luminescence dating methods to define a comprehensive protocol for routine dating of volcanic materials

the greening of the mcgill paleoclimate model part ii simulation of holocene millennial scale natural climate changes various proxy data reveal that in many regions of the northern hemisphere nh the middle holocene 6 kyr bp was warmer than the early holocene 8 kyr bp as well as the later holocene up to the end of the pre industrial period 1800 ad this pattern of warming and then cooling in the nh represents the response of the climate system to changes in orbital forcing vegetation cover and the laurentide ice sheet lis during the holocene in an attempt to better understand these changes in the climate system the mcgill paleoclimate model mpm has been coupled to the dynamic global vegetation model known as vecode see part i of this two part paper and a number of sensitivity experiments have been performed with the green mpm the model results illustrate the following 1 the orbital forcing together with the vegetation albedo feedback result in the gradual cooling of global sat from about 6 kyr bp to the end of the pre industrial period 2 the disappearance of the lis over the period 8 6 kyr bp associated with vegetation albedo feedback allows the global sat to increase and reach its maximum at around 6 kyr bp 3 the northern limit of the boreal forest moves northward during the period 8 6 4 kyr bp due to the lis retreat 4 during the period 6 4 0 kyr bp the northern limit of the boreal forest moves southward about 120 km in response to the decreasing summer insolation in the nh and 5 the desertification of northern africa during the period 8 2 6 kyr bp is mainly explained by the decreasing summer monsoon precipitation

marine productivity leads organic matter preservation in sapropel s1 palynological evidence from a core east of the nile river outflow the formation of eastern mediterranean organic matter rich deposits known as sapropels is the results of two mechanisms enhanced marine productivity and preservation of organic material at depth however their relative contribution and their leads and lags with respect to each other remain elusive here we address these questions by studying sediments deposited prior to during and after the most recent sapropel s1 10e6 calibrated ka before present bp with an integrated marine and terrestrial palynological approach combined with existing and newly generated geochemical data the studied core was retrieved from an area under strong influence of the nile outflow and has high average sediment accumulation rates allowing a high temporal resolution of several decades to centuries marine productivity as reconstructed with total dinocyst accumulation rates ars and biogenic caco3 content starts to increase 1 ka prior to sapropel formation a shift in the dinocyst taxa contributing tothe productivity signal at sapropel onset indicates the rapid development of seasonal water column stratification pollen and spore ars also increase prior to sapropel onset but a fewcenturies after the increase in marine productivity hence the first shift to a high marine productivity system before sapropel deposition may have been mostly favoured by the injection of nutrients via shoaling of the nutricline with a minor contributionofnutrientsfromlandviariverinputand floodingoftheshelves pollenassemblagesindicatea gradualchangeacrossthesapropelonsetfromasavanna like throughcoastal marshexpansion towardan open woodland assemblage which is consistent with enhanced nile influence and delta development at sapropel onset a marked shift in pollen ars could suggest increased preservation under anoxia however major shifts in pollen assemblages and signs of selective or partial decomposition of terrestrial palynomorphs are absent we therefore suggest that the high pollen ars largely result froman increased influx of pollen by enhanced nile discharge and extension of the freshwater plume three centuries after the sapropel onset dinocyst ars and caco 3 content indicate that marine productivity starts to decrease while sapropeldepositioncontinued organiccarboncontentdecreasedonlylaterandlessdramatically thismay

east african mid holocene wet dry transition recorded in palaeo shorelines of lake turkana northern kenya rift abstract the wet early to mid holocene of tropical africa with its enhanced monsoon ended with an abrupt shift toward drier conditions and was ultimately replaced by a drier climate that has persisted until the present day the forcing mechanisms the timing and the spatial extent of this major climatic transition are not well understood and remain the subject of ongoing research we have used a detailed palaeo shoreline record from lake turkana kenya to decipher and characterise this marked climatic transition in east africa we present a high precision survey of well preserved palaeo shorelines new radiocarbon ages from shoreline deposits and oxygen isotope measurements on freshwater mollusk shells to elucidate the holocene moisture history from former lake water levels in this climatically sensitive region in combination with previously published data our study shows that during the early holocene the water level in lake turkana was high and the lake overflowed temporarily into the white nile drainage system during the mid holocene 5270 300 cal yr bp however the lake water level fell by 50 m coeval with major episodes of aridity on the african continent a comparison between palaeo hydrological and archaeological data from the turkana basin suggests that the mid holocene climatic transition was associated with fundamental changes in prehistoric cultures highlighting the significance of natural climate variability and associated periods of protracted drought as major environmental stress factors affecting human occupation in the east african rift system

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

luminescence dating of the stratzing loess profile austria testing the potential of an elevated temperature post ir irsl protocol abstract feldspar infrared stimulated luminescence irsl signals saturate at 2000 gy i e 500 700 ka assuming a dose rate of 3 4 gy ka typical for loess and it is therefore theoretically possible to date middle pleistocene loess however the accuracy of age estimates is hampered by anomalous fading which leads to significant age underestimation fading corrections have been proposed but these corrections are reliant on inherent assumptions and in any case are inapplicable at large doses as the growth curve approaches saturation recent studies have identified a post ir irsl signal that shows less fading and is thus less dependent on accurate fading corrections this study builds upon these investigations and applies a post ir irsl sar dating protocol to polymineral fine grain samples of the loess palaeosol sequence in stratzing lower austria after a preheat at 320 c 60 s and an ir bleach at 50 c 200 s irsl was measured at 290 c for 200 s the same set of conditions were used to measure the response to a test dose recycling ratios recuperation and dose recovery tests show that this protocol is suitable for the loess under investigation although finite residual doses of up to 20 gy are observed for the oldest samples the natural signal is in or close to saturation indicating that fading is negligible in nature this observation is inconsistent with the measured laboratory fading rates of 1 1 5 decade and needs further investigation

high and low latitude forcing of the nile river regime during the holocene inferred from laminated sediments of the nile deep sea fan sediments deposited on deep sea fans are an excellent geological archive to reconstruct past changes in fluvial discharge here we present a reconstruction of changes in the regime of the nile river during the holocene obtained using bulk elemental composition grain size analyses and radiogenic strontium sr and neodymium nd isotopes from a sediment core collected on the nile deep sea fan this 6 m long core was retrieved at view the mathml source water depth and is characterized by the presence of a 5 m thick section of finely laminated sediments which were deposited between 9 5 and 7 3 ka bp and correspond to the african humid period ahp the data show distinct changes in eolian dust inputs as well as variations in discharge of the blue nile and white nile sedimentation was mainly controlled by changes in fluvial discharge during the holocene which was predominantly forced by low latitude summer insolation and by the location of the eastern african rain belt the changes in relative contribution from the blue nile and white nile followed changes in low latitude spring autumn insolation which highlights the role of changes in seasonality of the precipitation on the nile river regime the relative intensity of the blue nile discharge was enhanced during the early and late holocene at times of higher spring insolation with massive erosion and runoff during the ahp at times of high summer insolation while it was reduced between 8 and 4 ka at times of high autumn insolation the gradual insolation paced changes in fluvial regime were interrupted by a short term arid event at 8 5 7 3 ka bp also associated with rejuvenation of bottom water ventilation above the nile fan which was likely related to northern hemisphere cooling events another arid event at 4 5 3 7 ka bp occurred as the apex of a gradually drier phase in ne africa and marks the end of the ahp

holocene climate instability during the termination of the african humid period 1 the termination of the holocene african humid period similar to9 to similar to6 kyr bp is simulated with a three dimensional global coupled climate model that resolves synoptic variability associated with weather patterns in the simulation the potential for green and desert sahara states becomes equal between 7 5 and 5 5 thousand years ago causing the climate system to fluctuate between these states at decadal to centennial time scales this model result is supported by paleoevidence from the western sahara region showing similar paleohydrological fluctuations around that time for the present day only the desert sahara state is stable in the model

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

helium isotope ratios in easter microplate basalts abstract 3 he 4 he ratios in easter microplate basalt glasses show clear evidence of the effects of a mantle plume the east rift of the microplate between 26 and 28 s identified by la sm sr and pb isotopes and ridge crest elevation as the region of maximum plume influence has 3 he 4 he ratios spanning the entire range from 7 5 to 11 7 r a the easter microplate is the only section of the entire east pacific rise that is associated with a known hotspot track mantle plume and has elevated 3 he 4 he ratios although most of the west rift basalts contain morb helium 8 0 8 7 r a the basalt closest to the east rift has an elevated 3 he 4 he ratio 11 3 r a consistent with a significant plume component the diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur even in this region of super fast spreading the overall 3 he 4 he 206 pb 204 pb and 3 he 4 he 87 sr 86 sr trends have positive correlations although the high 3 he 4 he 87 sr 86 sr and 206 pb 204 pb component is not a single value but rather a broad band the apparent decoupling between the he and sr isotope distribution is modeled in the context of a plume source migrating ridge sink during channeling of the plume toward the ridge helium is preferentially lost from the center of the channeled plume resulting in lower he pb and he sr concentration ratios in the high 3 he 4 he component mixing trajectories in he sr isotopic space between a lile depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume ridge interactions in the context of modern theories of plume dynamics

multi emission luminescence dating of heated chert from the middle stone age sequence at sodmein cave red sea mountains egypt sodmein cave in egypt is one of the rare archaeological sites in north eastern africa conserving human occupation remains of a period most relevant for the out of africa ii hypothesis this underlines the need for establishing a chronological framework for the more than 4 m of stratified sediments ranging from the middle stone age msa to the neolithic the lowest layer j hosts huge fireplaces from which we report luminescence ages of heated chert fragments unearthed from different depths the multi emission dating approach using both the blue and red tl of each specimen as well as the osl emission of one sample allowed identifying the most reliable ages samples yield ages between 121 15 ka maximum age and 87 9 ka these data evidence human presence at the site during mis 5 for integrating sodmein cave into the actual discussion of the dispersal patterns of modern humans and to identify potential connections with other sites in the nile valley and in the middle east a sound and reliable chronology is indispensable

reconstruction of a pleistocene paleocatena using micromorphology and geochemistry of lake margin paleo vertisols olduvai gorge tanzania abstract olduvai gorge tanzania contains a rich record of pleistocene paleoclimate and paleoenvironment as well as an abundance of paleontological and archaeological data the 2 2 ma of volcaniclastic infill can be divided into time slices using dated tuffs sediments were deposited in a semi arid closed rift basin containing a shallow saline alkaline lake until 1 75 ma four trenches in uppermost bed i sample a 20 ka time slice between ng eju tuff base 1 818 0 006 ma and tuff if top 1 803 0 002 ma they form a 1 km transect of the lake margin flat that contains both stacked and cumulative paleosols that are interpreted as a heterogeneous paleocatena closer to the lake paleosols are thinner vertically stacked and separated by thin tuffs or tufa further from the lake margin there is additional volcaniclastic input and paleosols are thicker and cumulative macroscale and micromorphological features identify these clay rich paleosols as paleo vertisols abundant pedogenic slickensides and a variety of ped shapes were observed in the field as well as micro ped structures and stress cutans in thin section although weakly developed these paleo vertisols also have distinct horizons defined by soil color changes differing ped shapes and bulk geochemistry and are indicative of monsoonal precipitation seasonality paleosols and parent materials were analyzed for bulk geochemistry of major rare and trace elements geochemical proxies reveal changing hydrology not definitively identifiable in the field or micromorphology molecular weathering ratios show increased weathering and mass balance calculations indicate greater translocations positive and negative through time lower soil moisture due to a drier climate likely created better drained conditions allowing for increased pedogenesis the records of femn oxides and zeolites precipitated in rhizoliths are further evidence for changing redox conditions and water chemistry redox sensitive elements such as fe and mn were mobilized during saturated soil conditions and precipitated during drier conditions as the hydrology changed due to increased aridity zeolites precipitated as rhizoliths due to capillary rise and evaporative pumping of saline alkaline water the fluctuating hydrology identified in the paleo vertisols deposited between 1 79 and 1 81 ma is indicative of a precession cycle and consistent with records from olduvai gorge and throughout east africa this 20 ka time slice of the 2 ma drying trend identified in africa coincides with the first hominin migrations out of africa at 1 8 ma and suggests that hominins were living in a water stressed environment with annual precipitation seasonality and large variations in precipitation due to precession

neodymium isotopic evidence for galapagos hotspot spreading centre system evolution nd and sr isotope variations along the galapagos spreading centre support morgan s preferential subcrustal flow channel connecting the galapagos mantle plume with the northeastward migrating ridge axis decoupling of la sm and nd isotope variations near 91 w further suggests that basalts are derived from smaller degrees of melting and are deeper beneath this elevated part of the ridge

holocene vegetation and climate change recorded in alpine bog sediments from the borreguiles de la virgen sierra nevada southern spain high resolution pollen and magnetic susceptibility ms analyses have been carried out on a sediment core taken from a high elevation alpine bog area located in sierra nevada southern spain the earliest part of the record from 8200 to about 7000 cal yr bp is characterized by the highest abundance of arboreal pollen and pediastrum indicating the warmest and wettest conditions in the area at that time the pollen record shows a progressive aridification since 7000 cal yr bp that occurred in two steps first shown by a decrease in pinus replaced by poaceae from 7000 to 4600 cal yr bp and then by cyperaceae artemisia and amaranthaceae from 4600 to 1200 cal yr bp pediastrum also decreased progressively and totally disappeared at ca 3000 yr ago the progressive aridification is punctuated by periodically enhanced drought at ca 6500 5200 and 4000 cal yr bp that coincide in timing and duration with well known dry events in the mediterranean and other areas since 1200 cal yr bp several changes are observed in the vegetation that probably indicate the high impact of humans in the sierra nevada with pasturing leading to nutrient enrichment and eutrophication of the bog pinus reforestation and olea cultivation at lower elevations

temperature trends during the present and last interglacial periods a multi model data comparison though primarily driven by insolation changes associated with well known variations in earth s astronomical parameters the response of the climate system during interglacials includes a diversity of feedbacks involving the atmosphere ocean sea ice vegetation and land ice a thorough multi model data comparison is essential to assess the ability of climate models to resolve interglacial temperature trends and to help in understanding the recorded climatic signal and the underlying climate dynamics we present the first multi model data comparison of transient millennial scale temperature changes through two intervals of the present interglacial pig 8 1 2 ka and the last interglacial lig 123 116 2 ka periods we include temperature trends simulated by 9 different climate models alkenone based temperature reconstructions from 117 globally distributed locations about 45 of them within the lig and 12 ice core based temperature trends from greenland and antarctica 50 of them within the lig the definitions of these specific interglacial intervals enable a consistent inter comparison of the two intervals because both are characterised by minor changes in atmospheric greenhouse gas concentrations and more importantly by insolation trends that show clear similarities our analysis shows that in general the reconstructed pig and lig northern hemisphere mid to high latitude cooling compares well with multi model mean temperature trends for the warmest months and that these cooling trends reflect a linear response to the warmest month insolation decrease over the interglacial intervals the most notable exception is the strong lig cooling trend reconstructed from greenland ice cores that is not simulated by any of the models a striking model data mismatch is found for both the pig and the lig over large parts of the mid to high latitudes of the southern hemisphere where the data depicts negative temperature trends that are not in agreement with near zero trends in the simulations in this area the positive local summer insolation trend is counteracted in climate models by an enhancement of the southern ocean summer sea ice cover and or an increase in southern ocean upwelling if the general picture emerging from reconstructions is realistic then the model data mismatch in mid and high southern hemisphere latitudes implies that none of the models is able to resolve the correct balance of these feedbacks or alternatively that interglacial southern hemisphere temperature trends are driven by mechanisms which are not included in the transient simulations such as changes in the antarctic ice sheet or meltwater induced changes in the overturning circulation

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

across an early holocene humid phase in western sahara pollen and isotope stratigraphy paleohydrology and paleoecology of the region around the chemchane sabkha mauritania are reconstructed from the comparative analyses of mineralogy stable isotope ratios of bulk carbonates and palynology from sediment cores the age of the section is from ca 13 5 ka to the middle holocene maximum lake expansion between about 8 3 and 6 5 ka is recorded in a girdle of stromatolite carbonates these document a lower salinity lake concomitant with a general establishment of sahel sudan vegetation the chemchane area is the northernmost lat 21 n occurrence of humid phase elements that are related to maximum intensity of monsoon activity during the early holocene

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

decadal variability in high northern latitudes as simulated by an intermediate complexity climate model a 2500 year integration has been performed with a global coupled atmospheric sea ice ocean model of intermediate complexity with the main objective of studying the climate variability in polar regions on decadal time scales and longer the atmospheric component is the ecbilt model a spectral t21 three level quasi geostrophic model that includes a representation of horizontal and vertical beat transfers as well as of the hydrological cycle ecbilt is coupled to the clio model which consists of a primitive equation free surface ocean general circulation model and a dynamic thermodynamic sea ice model comparison of model results with observations shows that the ecbilt clio model is able to reproduce reasonably well the climate of the high northern latitudes the dominant mode of coupled variability between the atmospheric circulation and sea ice cover in the simulation consists of an annular mode for geopotential height at 800 hpa and of a dipole between the barents and labrador seas for the sea ice concentration which are similar to observed patterns of variability in addition the simulation displays strong decadal variability in the sea ice volume with a significant peak at about 18 years positive volume anomalies are caused by 1 a decrease in ice export through fram strait associated with more anticyclonic winds at high latitudes 2 modifications in the freezing melting rates in the arctic due to lower air temperature and higher surface albedo and 3 a weaker heat flux at the ice base in the barents and kara seas caused by a lower inflow of warm atlantic water opposite anomalies occur during the volume decrease phase of the oscillation

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

major element constraints on melting differentiation and mixing of magmas from the galapagos 95 5 w propagating rift system projections in the pseudo quaternary system diopside plagioclase olivine quartz di pl ol q are used in this paper as a base on which to plot additional compositional petrologic or tectonic variables used in this manner the projections provide a sensitive means of displaying and evaluating the chemical variability of mid ocean ridge basalt morb suites such as that of the galapagos 95 5 w propagating rift system and allow mantle controls on the chemical variability of morb to be distinguished from the predominant effects of shallow level crystal fractionation and mixing primitive lavas mg 63 from the 95 5 w region form a broadly linear array within the di pl ol q tetrahedron parallel to the di ol join in terms of two recent high pressure studies of morb petrogenesis stolper 1980 takahashi and kushiro 1983 the primitive liquids in this array are not primary but have evolved by less than 10 olivine fractionation from an array of primary liquids which separated from residual mantle over a range of pressures between 10 20 kb with appropriate assumptions as to residual lithology and mg of residual olivine the nature and origin of 95 5 w primary magmas can be deduced 1 lavas of the normal rift are derived from a broad range of primary magmas that separated from compositionally uniform in terms of major elements mantle over a pressure range of 10 20 kb 2 beneath the newly forming propagating rift melting is initiated perhaps by pressure release at shallow depth 9 11 kb degree of melting and source composition appear similar to those for the normal rift within a few km of the propagating rift tip this shallow source is joined by a second deeper 11 12 kb more iron rich source both sources persist for at least 100 km along the propagating rift but the deeper source appears to become dominant farther behind the tip maximum pressure for melt separation anywhere beneath the propagating rift is less than 15 kb in addition to these source effects the projections allow a more complete description of the shallow level fractionation and mixing effects along the propagating rift primitive lavas of restricted composition erupted near the rift tip give way within 2 3 km to an increasingly diverse suite of lavas characterized by variable extents of fractionation and by numerous single mixing events consistent with magmatic evolution in small only occasionally interconnected magma bodies beyond 15 km the diversity gradually declines toward a restricted population of cpx saturated lavas which characterizes the propagating rift beyond 75 km and is consistent with the presence of a large well mixed magma chamber in this region

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

seamount volcanism at the reykjanes ridge relationship to the iceland hot spot the axial zone of the reykjanes ridge is covered with small 0 5 3 km in diameter volcanoes that pile together to form larger axial volcanic ridges this style of volcanism is similar to that at the mid atlantic ridge mar and may be common to slow spreading ridges despite proximity of the reykjanes ridge to the iceland hot spot in this study we quantitatively investigate the population of seamounts in three study areas at the reykjanes ridge areas a and b are centered at 62 n and 60 n respectively area c is centered at 58 n and is located south of the transition in ridge morphology from an axial high to an axial graben using multibeam bathymetry data 541 seamounts summit height h 50 m were identified in the three areas and their size and shape statistics were compiled additionally 105 seamounts in areas b and c were recognized in deep towed side scan images and their surface morphologies hummocky or smooth were recorded on the basis of estimated population parameters we find that seamounts at the reykjanes ridge are more abundant 310 20 per 103 km2 on average than at the mar between 24 and 30 n 200 10 per 103 km2 significant along axis variations exist at the reykjanes ridge however which are not simply related to distance from the hot spot area b has nearly twice the abundance of seamounts as either area a or area c variation in the characteristic height of the seamount population is also observed between the reykjanes ridge 68 2 m and the mar 58 2 m but no significant variation is found between our three study areas a dramatic change in seamount surface morphology occurs between areas b and c there are no side scan data from area a area c has 78 hummocky seamounts similar to the proportion observed at the mar while area b has 83 smooth seamounts on the basis of these results we present a conceptual model for building the shallow crust at the slow spreading reykjanes ridge that takes into account the possible influence of the iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances sizes shapes and surface morphologies in this model we suggest that the increased seamount production and proliferation of smooth seamounts in area b may be associated with a pulse of hot spot material in the form of asthenosphere of higher temperature that has recently affected area b

comparison of modeled and reconstructed changes in forest cover through the past 8000 years eurasian perspective reproducing the tree cover changes throughout the holocene is a challenge for land surface atmosphere models here results of a transient holocene simulation of the coupled climate carbon cycle model climber2 lpj driven by changes in orbital forcing are compared with pollen data and pollen based reconstructions for several regions of eurasia in terms of changes in tree fraction the decline in tr ee fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the holocene the cooler and drier tr end at the eastern side of the eurasian continent in mongolia and china also led to a decrease in tree cover in both model and data in contrast the holocene trend towards a cooler climate in the continental interior kazakhstan is accompanied by an increase in woody cover ther e a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon affected regions in general the model data comparison demonstrates that climate driven changes during the holocene result in a non homogeneous pattern of tree cover change across the eurasian continent for the eifel region in germany the model suggests a relatively moist and cool climate and dense tree cover the holzmaar pollen r ecord agrees with the model for the intervals 8 3 ka and 1 7 1 3 ka bp but suggests great reduction of the tree cover 3 2 ka and after 1 3 ka bp when highly developed settlements and agriculture spread in the region

holocene dust records from the west african sahel and their implications for changes in climate and land surface conditions we reconstructed aeolian dust accumulation during the holocene from two radiocarbon dated lake sediment sequences from the manga grasslands in northeastern nigeria in order to investigate long term changes in the harmattan dust system over west africa and evaluate their possible causes flux values were low in the early holocene decreasing further to a minimum at around 6 2 kyr b p after which time they increased steadily until around 2 kyr b p and then more sharply after this time the long term variations in dust flux agree broadly with changes in the exposed area of the lake chad basin to the northeast of the study sites which vary inversely with the volume of paleolake megachad more proximal sources of dust including the fine fraction of local dune sand and floodplains of nearby rivers have also made a contribution to the total dust load during times of enhanced dune and fluvial activity sharp rises in dust flux over the past century may be related to human activity broad patterns of change in dust flux during the holocene agree with other reconstructions over the same period however we see no evidence for a stepped rise during the middle holocene as seen at some sites from the northeastern tropical atlantic suggesting that controls on the harmattan dust system have differed from those affecting dust deposition elsewhere across northern africa

mechanisms for the onset of the african humid period and sahara greening 14 5 11 ka bp abstract the mechanisms leading to the onset of the african humid period ahp 14 500 11 000 yr ago are elucidated using two different climate vegetation models in a suite of transient glacial interglacial simulations covering the last 21 000 yr a series of sensitivity experiments investigated three key mechanisms local summer insolation and ice sheet evolution vegetation albedo precipitation feedback and co2 increase via radiative forcing and fertilization that control the climate vegetation history over north africa during the last glacial termination the simulations showed that neither orbital forcing nor the remote forcing from the retreating ice sheets alone was able to trigger the rapid formation of the ahp only both forcing factors together can effectively lead to the formation of the ahp the vegetation albedo precipitation feedback enhances the intensity of the monsoon and further accelerates the onset of the ahp the experiments indicate that orbital forcing and vegetation albedo precipit

variations in water level for lake turkana in the past 8500 years near mt porr kenya and the transition from the african humid period to holocene aridity abstract a stratigraphic and geomorphic study of relict beaches up to 80 m above current water level of lake turkana reveal four major lake level fluctuations of 50 m between ca 8 5 and 4 5 ka during the transition from the african humid period to holocene aridity we hypothesize that high stands of lake turkana reflect complex hydrological processes with variability in atlantic and indian ocean derived moisture into east africa ultimately linked to changes in sea surface temperatures ssts lake level rise was also amplified regionally by increased precipitation causing overflow from the adjacent suguta and chew bahir basins particularly during high stands at ca 8 5 ka and at 6 4 ka the timing of transgressive and regressive events is constrained by 14c dating of carefully selected lacustrine mollusks incorporation of previous shell ages with sufficient elevational control and osl dating of quartz grains from littoral and sublittoral deposits by a multiple aliquot regeneration mar approach there was a high water level up to at least 70 m at 8 5 ka that probably reached the spillover limit 100 m a brief