crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

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

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

vestiges of saxothuringian crust in the central sudetes bohemian massif zircon evidence of a recycled subducted slab provenance abstract four detrital zircon concentrates from the metasediments and metavolcanics of the central sudetes bohemian massif have been dated using shrimp ii the wyszki paragneiss yielded precambrian age spectra similar to those characteristic of the cadomian terranes 1 archaean and palaeoproterozoic zircons are scattered between 2830 and 1867 ma and 2 there are abundant neoproterozoic zircons dated at 803 577 ma the estimated maximum sedimentation age for this rock is 566 4 ma the klodzko fortress pyroclastic metarhyodacite gave an igneous emplacement age of 536 2 ma the magmatic precursor to the subvolcanic gniewoszow metarhyolite was dated at 501 3 ma finally the cinawica subvolcanic rhyolite yielded an igneous emplacement age of 338 3 ma and a large population of inherited zircons dated at palaeoproterozoic neoproterozoic cambro ordovician and late devonian early carboniferous the dated samples come from two adjacent suspect terranes the orlica nieznik dome and the klodzko massif in the central sudetes that are characterised by contrasting timing of metamorphism and exhumation despite this difference the results obtained show in conjunction with earlier data a similar provenance of the studied units and their common affinity to the saxo thuringian terrane since the central sudetes are separated from saxo thuringia by a collisional suture the pre variscan rocks must represent the deformed and metamorphosed allochthonous equivalents to saxo thuringian lithologies they were probably subducted together with the saxo thuringian passive margin during a variscan collision and then exhumed within an accretionary wedge in front of brunia east avalonia the central sudetic suspected terranes must have been derived from a dismembered single accretionary complex and were then juxtaposed by strike slip faults

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

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

constraints on the timing of multiple thermal events and re equilibration recorded by high u zircon and xenotime case study of pegmatite from pilawa gorna gory sowie block sw poland abstract the application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record the alteration of the intergrown zircon and xenotime in pegmatite from the gory sowie block sw poland was studied using the electron microprobe analysis x ray wds compositional mapping micro raman analysis and la icp ms u pb dating of zircon and xenotime as well as the u th total pb dating of uraninite these microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post magmatic processes in the zircon and xenotime intergrowth and to constrain their timing textural and compositional features combined with u pb data indicate that the pegmatite related crystallization of the zircon and xenotime intergrowth occurred ca 2 09 ga 2086 35 ma for zircon and 2093 52 ma for xenotime followed by the re equilibration of zircon and xenotime ca 370 ma 373 18 ma and 368 6 ma respectively during the formation of the younger pegmatite the zircon and xenotime were most likely derived from precambrian basement rocks and emplaced in the pegmatite as a restite the zircon preserved textures related to diffusion reaction processes that affected its high u core up to ca 9 6 wt uo2 which underwent further metamictization and amorphization due to self radiation damage the zircon rim and xenotime were affected by coupled dissolution reprecipitation processes that resulted in patchy zoning age disturbance and sponge like textures xenotime was also partially replaced by fluorapatite or hingganite y and y enriched allanite ce the termination of the low temperature alteration was constrained by the u th total pb age of the uraninite inclusions that crystallized in zircon at 281 2 ma which is consistent with the age of 278 15 ma obtained from the youngest cluster of u pb ages in the re equilibrated high u zircon domains this study demonstrates the importance of the careful examination of compositional microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals

zircon geochronology and trace element characteristics of eclogites and granulites from the orlica śnieznik complex bohemian massif u pb zircon geochronology and trace element analysis was applied to eclogites and ultra high pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the orlica nieznik complex bohemian massif under favourable circumstances such data may help to unravel protolith ages and yet undetermined aspects of the metamorphic evolution for example the time span over which eclogite facies conditions were attained by means of ion probe and laser ablation techniques a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences the 206 pb 238 u dates for zircons of all samples show a large variability and no single age can be calculated the protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra the spread in apparent ages is interpreted to be mainly caused by variable and possibly multi stage pb loss further complexities are added by metamorphic zircon growth and re equilibration processes the unknown relevance of inherited components and possible mixing of different aged domains during analysis a reliable interpretation of igneous crystallization ages is not yet possible previous studies and the new data document the importance of a carboniferous metamorphic event at c 340 ma the geological significance of this age group is controversial such ages have previously either been related to peak u hp conditions the waning stages of eclogite facies metamorphism or the amphibolite facies overprint this study provides new arguments for this discussion because in both rock types metamorphic zircon is characterized by very low total ree abundances flat hree patterns and the absence of an eu anomaly these features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the carboniferous ages document late stage eclogite facies metamorphism and not amphibolite facies overprinting

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

monazite geochronology of uhp and hp metamorphism deformation and exhumation nordoyane western gneiss region norway u th pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of uhp and hp plates 820 c 39 kbar 780 c 18 kbar metamorphosed during the late silurian early devonian collision between baltica and laurentia the upper plate contains a microdiamond bearing kyanite garnet graphite gneiss and associated kyanite eclogites independently indicating probable diamond forming conditions in situ dating of monazite in the microdiamond sample using the shrimp ii at the geological survey of canada yielded ages of 415 6 8 ma for those included in garnet and 398 6 ma for those in the matrix these ages compare to 408 0 5 6 and 397 5 4 4 ma determined using the electron microprobe at the university of massachusetts both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100 950 ma and scattered grains with ages of 900 500 ma but no ages of 1680 1650 ma equivalent to the local baltica basement were found agreement between the two techniques allowed evaluation of monazite age domains 198 analyses from the microdiamond rock and a kyanite garnet sillimanite mylonite produced from it using a combination of high resolution element imaging and trace element analysis of u th pb and y this comparison yielded three mean ages of 407 0 2 1 ma 394 8 2 3 ma and 374 6 2 7 ma combining this geochronology with previous p t estimates we propose that the uhp unit reached its maximum depth of 125 km at a maximum age of 407 ma when monazite was included in garnet and experienced 65 km of exhumation at an average rate of 10 9 mm year during top southeast thrusting that brought it into contact with the hp unit following these events both units were exhumed together at an average rate of 3 8 mm year until reaching a depth of 37 km at 395 ma where these rocks experienced extensive re equilibration and top west and left lateral shearing after 395 ma these units continued to be exhumed at an average rate of 0 8 to 1 4 mm year until 375 ma the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite the exhumation histories of these units record a change in mechanism from syncollisional exhumation through late to post orogenic collapse that was a consequence of plate reorganization

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

vertical coupling and decoupling in the lithosphere recent advancements in the understanding of mountain belts have focused on vertical coupling of the lithospheric layers this volume describes the extent to which observed or inferred sub horizontal coupling or attachment zones provide vertical kinematic linkage between rheologically distinct layers in the continental lithosphere a common theme is whether the deformation which partitioned differently in each layer is linked kinematically across attachment zones and driven by flow in the deeper crust and mantle lithosphere the papers are divided into six sections the first analyses the extent to which mantle flow controls deformation of the overlying layers the vertical axis block rotations section uses geological and palaeomagnetic data to constrain the role and magnitude of basal shear across mid crustal attachment zones the lower crustal flow and topography section addresses the time dependent development of orogenic plateaux and their role in the orogenic cycle multiple examples of the spatial and temporal development of lithospheric coupling are given in both the orogenic examples and subduction examples sections finally rheological constraints of vertical coupling in the lithosphere are investigated

exhumation of eclogitized continental basement during variscan lithospheric delamination and gravitational collapse sudety mountains poland a variscan deep crustal level eclogite facies continental basement massif in western poland the snieznik complex was tectonically exhumed crustal penetrating mylonite zones record three main kinematic events early top to the north directed thrusting right slip transpression tension and late top to the south and east normal faulting thrusting resulted in extreme crustal thickening and associated eclogite facies metamorphism right slip movements produced retrogressive crystal plastic simple shear zones normal faults flank carboniferous to early permian terrigenous sedimentary basins documenting tectonic and erosional denudation of the snieznik complex during lithospheric extension sm nd isotopic dates previously reported for the in situ eclogite facies metamorphic mineral assemblages are 341 337 and 329 ma brueckner et al 1991 40 ar 39 ar isotopic dates for metamorphic hornblende 338 333 and 332 ma muscovite 329 and 329 ma and biotite 328 ma reflect times of cooling through the 500 350 and 300 c isotherms respectively these nearly concordant mineral dates document rapid cooling from 850 c eclogue facies temperatures to 300 c rapid denudation of these deep crustal rocks 19 22 kbar pressures 70 km depth is attributed to processes similar to those of metamorphic core complexes that operated during lithospheric delamination and gravitational collapse the sequence of late paleozoic alleghanian crustal thickening followed by right slip transpression tension followed by normal faulting recognized in the u s appalachians implies that this tectonic pattern may exist throughout the afleghanian variscan belt

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

constraints on the devonian carboniferous closure of the rheic ocean from a multi method geochronology study of the stare město belt in the sudetes poland and the czech republic abstract this paper attempts to clarify the complex nature of how and when the rheic ocean closed in what is now central europe and with respect to the various terranes that were involved offer a regional chronology for the associated structural metamorphic and igneous processes that accompanied and followed this closure the variscan orogen in europe originated from the multiple collisions of gondwana derived terranes the armorican terrane assemblage with laurussia however many important structural details on the timing of these collision related events remain obscure in the sudetes the stare m sto belt represents a wnw dipping part of the rheic suture that developed from the continental collision of the eastern terranes of the armorican terrane assemblage now in the bohemian massif with the brunovistulian terrane a part of laurussia old red continent in this study the results of monazite th u total pb garnet lu hf and zircon u pb geochronology were integrated into a newly established d1 d3 tectonometamorphic sequence a th u total pb age of 368 ma from a monazite that grew concurrently with d2 metasedimentary garnet as well as lu hf ages of 361 ma and 355 ma obtained from d2 metasedimentary garnets implies that the regional contractional deformation and progressive metamorphism of d2 took place mainly during the famennian late devonian and extended into the visean middle mississippian of the early carboniferous the ion probe u pb zircon ages of 355 ma and 359 ma obtained from leucocratic neosomes in migmatized amphibolites confirmed a lag in the peak temperature that followed crustal thickening during d1 d2 metamorphic monazites dated at 340 ma provide a time for the subsequent d3 dextral transpressional regime the closure of the stare m sto belt segment of the rheic ocean probably resulted from a head on hard collision between the westerly subducting brunovistulian promontory of laurussia and the eastern members of the armorican terrane assemblage thus the rheic ocean closed during the late devonian at 370 360 ma and preceded the collision of the armorican terrane assemblage with east avalonia at the western margin of what is now the bohemian massif following ocean closure the rheic slab may have broken off resulting in the suture zone becoming dominated by lateral tectonic escape movements of the colliding terranes at 340 330 ma visean syntectonic d3 intrusions of granodiorite tonalite magma acted as a hot lubricant and stitched the suture zone together

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

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

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

pervasive horizontal fabric and rapid vertical extrusion lateral overturning and margin sub parallel flow of deep crustal migmatites northeastern bohemian massif abstract in the west sudetes northeastern bohemia massif geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension instead high grade assemblages of the orlica snieznik complex have a regional sub horizontal foliation and sub horizontal lineations that trend parallel to narrow sub vertical shear zones containing exhumed high pressure assemblages mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility ams data from amphibolite facies to migmatitic meta sedimentary and meta igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the ssw on both steep and sub horizontal nne trending planar fabrics the dominant ssw nne fabric orientation is parallel to the bohemia brunia suture which marks a major boundary along the eastern margin of the massif the shape of the ams ellipsoid is predominantly oblate revealing flattened fabrics with only local prolate ellipsoids we envisage that the continental brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the bohemian margin shortly following terminal arc collision sub vertical extrusion of the orogenic root was arrested in the mid crust where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks upon reaching the crustal high strength lid the exhumed ductile mass of continental material laterally spread sub parallel to the margin underwent subsequent supra barrovian metamorphism and quickly cooled the application of ams techniques to high grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous

exhumation and metamorphism of an ultrahigh grade terrane geochronometric investigations of the sudete mountains bohemia poland and czech republic the sudete mountains ne bohemian massif czech republic and poland preserve abundant eclogitic and granulitic centimetre to decimetre scale boudins enveloped in a predominantly migmatitic matrix published geochronometry and thermobarometry from the uhp and uht rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units however the timing of stage 2 metamorphism and associated unroofing is less well constrained new in situ ion microprobe th pb monazite results together with complementary u pb zircon and electron microprobe analyser total pb monazite results on 11 amphibolite facies gneissic to migmatitic samples place important temporal constraints on the second stage of uhp and uht metamorphism exhumation the orlica snieznik dome records uhp metamorphism occurring at 375 ma and subsequent exhumation to midcrustal levels in supra barrovian conditions at c 345 330 ma in contrast the western gory sowie block preserves evidence of hp granulite conditions at c 400 ma and exhumation to mid crustal levels at 380 370 ma revealing a c 30 million years difference in exhumation events between the neighbouring terranes the eastern gory sowie block preserves ages similar to the orlica sneiznik dome suggesting that different preserved metamorphic cooling histories are juxtaposed across the sudetic marginal fault the bounding niemcza shear zone yields preliminary th pb dates that range from 380 8 ma to 283 2 ma preserving a protracted metamorphic record that spans the exhumation history of the region the distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low viscosity crust under supra barrovian conditions near the mid crustal high strength lid during oblique transpressional convergence

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

palaeomagnetism of palaeozoic ultrabasic rocks from the sudetes mts sw poland tectonic implications summary a complex palaeomagnetic rock magnetic and mineralogical study of ultrabasic rocks from the sowie gory block gsb and jordanow gogolow serpentinite massif jgsm revealed the presence of several components of natural remanent magnetization nrm the authors found three groups of palaeozoic as well as triassic and recent components of the geomagnetic field the palaeozoic components of nrm are carried mainly by magnetite of several generations formed during several serpentinization episodes permo carboniferous component a1 present overall the sudetes was isolated in one jgsm and two gsb exposures whereas the late devonian early carboniferous component a2 was found in two exposures from the gsb the corresponding remanent components were already revealed in palaeontologically dated sediments from other west sudetic units in the gsb it was probably acquired during its unroofing dated isotopically for ca 370 360 ma the newly determined group of palaeozoic directions a3 was found in three localities from jgsm and in two from gsb is interpreted as the oldest overprint in jgsm it was acquired probably shortly after the first oceanic serpentinization phase dated isotopically for ca 400 ma its acquisition in gsb corresponds to the time of emplacement of ultrabasic xenoliths dated isotopically at ca 390 ma so we suppose that the mean a3 calculated for five exposures corresponds to the 380 400 ma time span and that at that period both massifs formed one microplate mean inclination of a3 places this microplate at 380 400 ma at the palaeolatitude of 23 s whereas the west sudetes were situated during the early devonian at 16 s we suggest that during the early devonian the microplate comprising gsb and jgsm massifs was situated to the south from the west sudetes and accreted them during middle late devonian

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

role of partial melting in the evolution of the sulu eastern china ultrahigh pressure terrane strongly deformed potassium feldspar rich dikes are widely distributed in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china the fact that the crystallization ages of these dikes overlap with the age of peak uhp metamorphic conditions implies the presence of melt during metamorphism sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic sulu gneiss partial melting may be the key to solving several unusual features of the sulu and other uhp terranes such as the almost complete lack of mineralogical evidence for uhp conditions and the limited growth of zircon during uhp conditions in the dominant felsic gneiss in addition because partial melting will cause a drastic reduction in the strength of the uhp gneisses the most likely exhumation mechanism is diapiric rise of a low viscosity partially molten mass containing entrained blocks of eclogite and not a thin sheet as usually proposed

geochronology and nd sr systematics of lusatian granitoids significance for the evolution of the variscan orogen in east central europe a variety of pre variscan granitoids and two variscan monzogranites occurring in the central and western parts of the lusatian granodiorite complex lgc saxonia were dated by the single zircon evaporation method complemented by whole rock nd isotopic data and rb sr whole rock and mineral ages the virtually undeformed pre variscan granitoids constitute a genetically related mostly peraluminous magmatic suite ranging in composition from two mica granodiorite muscovitebearing biotite quartz diorite tonalite and granodiorite to biotite granodiorite and monozogranite

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

palaeozoic terrane amalgamation in central europe a ree and sm nd isotope study of the pre variscan basement ne bohemian massif

the timing of eclogite facies metamorphism and migmatization in the orlica śnieznik complex bohemian massif constraints from a multimethod geochronological study the orlica nieznik complex osc is a key geological element of the eastern variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks sporadic occurrences of eclogites and granulites record high pressure hp to ultrahigh pressure uhp metamorphic conditions a multimethod geochronological approach 40ar 39ar rb sr sm nd u pb has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks special attention was given to the unresolved significance of a 370 to 360 ma age group that was repeatedly described in previous studies efforts to verify the accuracy of c 370 ma k ar phengite and biotite dates reported for an eclogite and associated country rock gneiss from the location nowa wie suggest that these dates are meaningless due to contamination with extraneous ar extraneous ar is also considered to be responsible for a significantly older 40ar 39ar phengite date of c 455 ma for an eclogite from the location wojtowka attempts to further substantiate the importance of 370 360 ma zircon dates as an indicator for a melt forming high temperature ht episode did not provide evidence in support of anatectic processes at this time instead shrimp u pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and u hp granulites revealed two age populations 490 450 and 345 330 ma respectively that correspond to protolith ages of the magmatic precursors and late variscan anatexis the results of this study further underline the importance of late carboniferous metamorphic processes for the evolution of the osc that comprise the waning stages of hp metamorphism and lower pressure ht overprinting with partial melting eclogites and their country rocks provided no chronometric evidence for an uhp and ultrahigh temperature episode at 387 360 ma as recently suggested for granulites from the osc based on lu hf garnet ages anczkiewicz et al 2007

heterogeneous extrusion and exhumation of deep crustal variscan assembly geochronology of the western tatra mountains northern slovakia abstract the nature and style of mid crustal assembly and exhumation during continental collision has been investigated in the tatra mountains of the western carpathians the pre alpine basement of the western carpathians represents the easternmost exposure of the variscan orogen in europe which marks the collision of laurasia with gondwanian affiliated terranes during the palaeozoic the tatric crystalline unit of the western tatra in northern slovakia displays an inverted metamorphic sequence where a high grade unit comprising migmatites with relicts of eclogite has been thrust over a lower grade mica schist unit new geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the western tatra the upper unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1 6 gpa to 1 0 1 2 gpa at 750 800 c which lead to intensive re equilibration at high pressure granulite facies conditions comparable to the peak metamorphic conditions of the host migmatite both eclogite and migmatite shared a retrograde p t path following the insertion of the eclogite assemblage into the migmatites the metamorphic evolution of the lower unit mica schist is constrained to peak p t conditions of 0 6 0 8 gpa and 640 and 660 c followed by retrogression this suggests that different rock types of the western tatra metamorphic core shared only their exhumation path from mid crustal levels id tims sm nd dating of garnet from eclogite yields a whole rock garnet isochron age of 337 10 ma with an initial e nd isotopic composition of 8 3 in situ u pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c 380 ma whereas monazite from a migmatite away from the eclogite preserves a robust 340 11 ma age which is indistinguishable from sm nd garnet age and u pb age of zircons in the anatectic leucosome of the migmatite 347 7 ma a younger monazite age population from the migmatite of 300 16 ma is consistent with 40 ar 39 ar mica ages of c 310 ma this argues for a contemporaneous and likely shared exhumation path of the assemblage pair in situ monazite total pb analyses from the lower unit mica schists yields xenocrystic and c 370 ma ages but no geochronologic evidence for peak variscan tectonism exhumation of the deep crustal root occurred most probably in a two stage process the timing of the high pressure eclogite facies metamorphism before the onset of exhumation into the mid crust was likely between c 380 ma and 360 ma subsequent exhumation into the middle crust was coeval with migmatite generation at c 340 ma and garnet diffusion modeling suggest 30 c ma cooling rates the exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid crustal levels at a time coeval with anatexis and subsequently extruded with mid crustal material to the upper crust

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

reconstructing the ca 100 year flood in northern thailand the key utilities of palaeohydrological studies are the potential to extend the flood record of a catchment and improve estimates of flood frequency and magnitude in this study we use a hydraulic model to produce estimates of discharge based on psi palaeo stage indicator evidence from large floods for a study reach of the mae chaem river thailand careful calibration of this model especially manning s n was possible because of the availability of psis from several modern gauged flows organic psi evidence contained in a cave permitted dating of the largest flood for which evidence was available which is close to the 100 year flood the magnitude of this flood is twice that predicted from an extrapolation of the 49 year gauging record employing the gumbel ev1 flood frequency model officially endorsed by water management agencies in thailand this magnitude of flood is however very similar to that predicted from a power law model and suggests that extreme flood magnitude and frequency in this catchment conform to power law behaviour these results significantly increase the perception of flood risk for the area and this palaeohydrological case study represents the first for the region

age mapping and dating of monazite on the electron microprobe deconvoluting multistage tectonic histories high resolution x ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural metamorphic and tectonic analysis x ray maps commonly show complex th u and pb zoning that reflects monazite growth and overgrowth events age maps constructed from the x ray maps simplify the zoning and highlight age domains microprobe dating offers a rapid in situ method for estimating ages of mapped domains application of these techniques has placed new constraints on the tectonic history of three areas in western canada age mapping has revealed multiphase monazite with older cores and younger rims included in syntectonic garnet microprobe ages show that tectonism occurred ca 1 9 ga 700 m y later than mylonitization in the adjacent snowbird tectonic zone in new mexico age mapping and dating show that the dominant fabric and triple point metamorphism occurred during a 1 4 ga reactivation not during the 1 7 ga yavapai mazatzal orogeny in norway monazite inclusions in garnet constrain high pressure metamorphism to ca 405 ma and older cores indicate a previously unrecognized component of ca 1 0 ga monazite in all three areas microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts

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

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

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

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

new u pb monazite and zircon data from the sudetes mountains in sw poland evidence for a single cycle variscan orogeny new u pb monazite and zircon data combined with a study of their internal morphologies significantly clarifies the controversy surrounding the palaeozoic geodynamic evolution of the sudetes mountains in the northeastern bohemian massif variably thin rims of secondary magmatic overgrowth on zircons from an anatectic granite in the gory sowie block are dated by a u pb zircon lower intercept age of 353 61 ma by magmatic monazite growth at 378 2 ma and by xenotime crystallization at 383 370 ma in a late syntectonic pegmatite reflecting the anatectic event associated with a single cycle variscan orogeny in this area

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

drainage evolution in the polish sudeten foreland in the context of european fluvial archives detailed study of subsurface deposits in the polish sudeten foreland particularly with reference to provenance data has revealed that an extensive preglacial drainage system developed there in the pliocene early pleistocene with both similarities and differences in comparison with the present day odra oder system this foreland is at the northern edge of an intensely deformed upland metamorphosed during the variscan orogeny with faulted horsts and grabens reactivated in the late cenozoic the main arm of preglacial drainage of this area at least until the early middle pleistocene was the palaeo nysa klodzka precursor of the odra left bank tributary of that name significant preglacial evolution of this drainage system can be demonstrated including incision into the landscape prior to its disruption by glaciation in the elsterian sanian and again in the early saalian odranian which resulted in burial of the preglacial fluvial archives by glacial and fluvioglacial deposits no later ice sheets reached the area in which the modern drainage pattern became established the rivers incising afresh into the landscape and forming post saalian terrace systems issues of compatibility of this record with the progressive uplift implicit in the formation of conventional terrace systems are examined with particular reference to crustal properties which are shown to have had an important influence on landscape and drainage evolution in the region

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

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

transient allanite replacement and prograde to retrograde monazite re crystallization in medium grade metasedimentary rocks from the orlica śnieznik dome czech republic poland textural and geochronological arguments abstract we use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium grade metasedimentary rocks from the european variscan belt orlica nieznik dome czech republic poland the relationships between monazite allanite and other major and accessory phases are characterized for staurolite sillimanite and kyanite grade garnet bearing mica schist samples garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim allanite is virtually absent from the matrix it shows replacement textures involving a cluster of th u y rich silicates or phosphates monazite with frequent micro cracks or trimmed grain boundaries abounds in the mica rich matrix it occurs as lone grains close to partially resorbed garnet apatite or staurolite and as subhedral and compositionally zoned aggregates associated with allanite and xenotime electron microprobe analyses of monazite define pb vs th isochrons ages 363 343 ma with positive intercepts pb 0 40 120 ppm which suggest the presence of initial pb and age overestimation isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial pb which leads to variable discordance in the 206 pb 238 u 207 pb 235 u system conversely the 206 pb 238 u 208 pb 232 th system is not visibly affected analyses are concordant and 208 pb 232 th ages range from 358 to 299 ma 7 8 ma regardless of monazite texture or composition they define a main group at 330 310 ma with few older outliers all observations suggest that a first monazite generation mnzi formed via allanite breakdown at prograde to peak p t conditions 5 7 kbar 575 640 c a comparison with previous geochronological results reveals that the age of this prograde reaction is however preserved by only few domains yielding 360 340 ma instead our results emphasize the importance of retrograde processes which built the dominant age population of 330 310 ma the latter ages are ascribed to a combination of 1 transient monazite growth after allanite and controlled by p availability 2 variable recrystallization replacement of older monazite grains and 3 minor monazite neoformation due to the resorption of garnet or apatite the young monazite grains or domains mnzii mostly formed during fluid influx and chloritization they constrain the end of the orogenic history when mid crustal rocks were exhumed and re hydrated at temperatures below 300 c

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

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

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

rb sr and u pb geochronology of migmatitic gneisses from the gory sowie west sudetes poland the importance of mid late devonian metamorphism the gory sowie block west sudetes sw poland consists of a gneiss migmatite complex with minor amphibolites calc silicate rocks ultrabasic rocks and granulites migmatitic gneisses underwent a complex polyphase history of deformation and metamorphism each deformation phase d1 to d5 being associated with synkinematic amphibolite facies metamorphism m1 to m5 for a better understanding of the p t t deformation path three types of gneisses which represent distinct stages of deformation and metamorphism d2 m2 d3 m3 and d5 m5 have been studied by means of the rb sr and u pb isotopic systems u pb monazite and xenotime ages of c 380 ma for rocks migmatized and deformed during the d3 m3 and d5 m5 stages constrain the timing of the last metamorphic migmatitic event in the gory sowie block rb sr thin slab and mica whole rock dating for samples with d2 d3 and d5 characteristics provide ages between 362 and 375 ma which overlap within error 2o no correlation between deformational characteristics and apparent rb sr ages was recognized the rb sr data are interpreted as time constraints for post peak metamorphic cooling the new geochronological data indicate the significance of devonian high temperature metamorphism in the sudetes and support a similar interpretation of earlier studies the isotopic results are also consistent with the interpretation based on field structural and petrological data that at least four d2 d5 structural episodes occurred in devonian times within a very short time interval the details of which cannot be further resolved with the methods used

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

thermochronology of the west sudetes bohemian massif rapid and repeated eduction in the eastern variscides poland and czech republic the sudete mountains of northeastern bohemian massif were amalgamated during the closure of the rheic ocean culminating with variscan orogenesis and contain occurrences of high pressure granulite and small relict ultrahigh pressure eclogite formed during subduction we performed 40 ar 39 ar thermochronometry on primarily amphibolite facies gneisses and schists from two crustal blocks within the sudetes gory sowie and orlica snieznik massifs hornblende and mica plateau ages from the mountainous portion of the gory sowie reflect relatively rapid cooling between 382 1 ma and 373 0 5 ma following peak conditions at ca 400 ma kyanite sillimanite and cordierite garnet mineral growth textures denote near isothermal decompression during eduction concordant hornblende and biotite cooling ages obtained from the eastern topographically flat region adjacent to the niemcza shear zone indicate markedly younger cooling at 337 0 8 ma 40 ar 39 ar results from the snieznik mountains 50 km to the south yield plateau cooling ages for white mica and biotite between 341 1 to 337 0 6 ma and 342 1 to 334 0 6 ma respectively and are remarkably similar in age to an amphibolite facies overprinting episode mica analyses from the orlica mountains also yield cooling ages between 338 0 9 to 335 0 5 ma thermochronometry illustrates temporally disparate cooling histories but similar metamorphic evolutions between the gory sowie block and the orlica snieznik complex in both terranes it took 35 m y from the time of peak high pressure conditions to residence in the upper crust and possibly as little as 10 m y between high pressure and mid crustal supra barrovian events these results in light of regional geochronology illustrate the rapid localized and repetitious nature of eduction exhumation of the sudetic orogenic root including the eclogite and hp granulite assemblages during protracted subduction

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

epma and pixe dating of monazite in granulites from stary gieraltow ne bohemian massif poland abstract chemical th u total pb chime dating of monazite by electron probe microanalyzer epma and proton microprobe pixe was carried out on felsic granulites from stary gieraltow poland which represent part of the orlica nieznik dome in the ne bohemian massif analyzed monazite is characterized by mosaic zoning rather than simple core to rim growth and strontium contents of up to 750ppm an isochron age of 347 13ma represents timing of amphibolite facies metamorphism in agreement with previously published estimates

pre variscan variscan and early alpine thermo tectonic history of the north eastern bohemian massif an 40 ar 39 ar study the orlica snieznik and jesenik mountains correspond to three main domes from west to east the snieznik keprnfk and desna domes they are composed of a basement of autochthonous gneisses a thick series of blastomylonites and a supposed para autochthonous or allochthonous metamorphic pre devonian to devonian cover their broad direction is nne ssw 40ar 39ar radiometric measurements allow three main groups of ages to be defined 1 300 310 ma represented in the keprnik and desna domes this age is interpretated following the constraints on the age of the metamorphism which is linked with the extensional process occurring during the westphalian 2 320 340 ma represented mainly in the snieznik dome but not in the keprnfk massif the nappe structure of orlik vysoka hole in the northern area of the desna dome also exhibits this age which is interpretated as reflecting the period of the major variscan barrowian metamorphism which accompanied the compressional process it is only represented in the zones where the extensional process was not strong enough to result in a complete overprinting 3 340 440 ma corresponding to a very strictly defined area in the eastern rim of the desna dome occupied by ultramylonites and mylonites these ages obtained on muscovites result from an incomplete resetting of the minerals developed during the cooling of a granitic protolith and mylonitized during the extensional process a laser probe analysis confirms the extreme inhomogeneity of the ages of the muscovites and their different resetting from one grain to another the late alpine overprinting is more discrete but can be deciphered through the low extraction temperatures with ages between 80 and 120 ma these ages can be compared with alpine ages in the close western carpathians

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

kinematic data on major variscan strike slip faults and shear zones in the polish sudetes northeast bohemian massif the still highly disputable terrane boundaries in the sudetic segment of the variscan belt mostly seem to follow major strike slip faults and shear zones their kinematics expected to place important constraints on the regional structural models is discussed in some detail the most conspicuous is the wnw ese intra sudetic fault zone separating several different structural units of the west sudetes it showed ductile dextral activity and probably displacement magnitude of the order of tens to hundreds kilometres during late devonian to early carboniferous times in the late carboniferous to early permian the sense of motion on the intra sudetic fault was reversed in a semi brittle to brittle regime with the left lateral offset on the fault amounting to single kilometres the north south trending niemcza and north east southwest skrzynka shear zones are left lateral ductile features in the eastern part of the west sudetes similarly oriented northeast southwest to nne ssw regional size shear zones of as yet undetermined kinematics were discovered in boreholes under cenozoic cover in the eastern part of the sudetic foreland the nied wied and nysa brzeg shear zones one of these is expected to represent the northern continuation of the major stare mesto shear zone in the czech republic separating the geologically different units of the west and east sudetes the rudawy janowickie metamorphic unit assumed in some reconstructions to comprise a mostly strike slip terrane boundary is characterized by ductile fabric developed in a thrusting regime modified by a superimposed normal slip extensional deformation thrusting related deformational fabric was locally reoriented prior to the extensional event and shows present day strike slip kinematics in one of the sub units the sudetic boundary fault although prominent in the recent structure and topography of the region was not active as a variscan strike slip fault zone the reported data emphasize the importance of syn orogenic strike slip tectonics in the sudetes the recognized shear sense is compatible with a strike slip model of the northeast margin of the bohemian massif in which the kaczawa and gory sowie units underwent late devonian early carboniferous southeastward long distance displacement along the intra sudetic fault zone from their hypothetical original position within the northern phyllite zone and the mid german crystalline high of the german variscides respectively and were juxtaposed with units of different provenance southwest of the fault the intra sudetic fault zone together with the elbe fault zone further south were subsequently cut in the east and their eastern segments were displaced and removed by the younger early to late carboniferous nne ssw trending transpressional moldanubian stare mesto shear zone

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

what brought them up exhumation of the dabie shan ultrahigh pressure rocks metamorphic coesite and diamond in the dabie shan eastern china testify to subduction of continental crust to 100 km depth exhumation of these ultrahigh pressure rocks through the crust encompassed two stages 1 south dipping foliation southeast plunging stretching lineation lineation parallel isoclinal folds and boudins indicate extreme subhorizontal shortening and subvertical extension during top to northwest shearing at 200 180 ma syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability 2 northwest southeast subhorizontal extension from 133 to 122 ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the dabie shan pluton cores at deep structural levels have weak hypersolidus fabrics and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions deformation is concentrated in greenschist facies mylonites and ultramylonites along the xiaotian mozitang detachment fault at the northern topographic limit of the dabie shan our preferred exhumation model involves two stages triassic indentation vertical extrusion and erosion followed by cretaceous plate margin transtension

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

geochemistry and 40ar 39ar geochronology of the mafic metavolcanic rocks from the rýchory mountains complex west sudetes bohemian massif palaeotectonic significance the mafic metavolcanic rocks blueschists and greenschists of the r chory mountains crystalline complex west sudetes experienced sea floor hydrothermal alteration spilitization prior to regional metamorphism the metabasite geochemistry namely trace element and ree abundances indicates that the protolith was comparable in composition with 1 tholeiitic to transitional ocean floor basalts and 2 transitional and alkaline intra oceanic island basalts two main metamorphic events affected the r chory mountains metabasites in an earlier high pressure low temperature metamorphic event the rocks experienced blueschist facies metamorphism the results of 40 ar 39 ar geochronology studies on phengites from the mafic blueschists date the end of the earlier metamorphism to 360 ma the greenschist metamorphic overprint followed around 340 ma the elongated bodies of mafic metavolcanic rocks are situated within the prominent nne ssw leszczyniec shear zone following the trend of the r chory mountains and the rudawy janowickie mountains both the geochemical affinities and the blueschist facies metamorphism of the metabasites suggest that this shear zone evolved from the variscan suture dividing western and central terranes of the west sudetes according to the radiometric age for the end of the high pressure low temperature metamorphism the terranes accreted during the famennian a considerable time span between the formation of the metabasite protolith and the blueschist metamorphism may indicate long lasting subduction of a large oceanic plate between gondwana and laurussia possibly accompanied by terrane accretion prior to the variscan orogeny

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

dating granulite facies structures and the exhumation of lower crust in the moldanubian zone of the bohemian massif deformation of granulite facies rocks in the moldanubian zone of the southern bohemian massif is expressed in two intersecting planar fabrics steeply disposed s1 and flat laying s2 which correspond to two deformation stages d1 and d2 the existing sm nd garnet ages from banded granulite gneisses new u pb zircon data from deformed granite intrusions within the granulite gneisses and the p t and field structural relations constrain the ages and p t conditions of the two deformation phases the early deformation d1 was associated with a hp ht metamorphic stage with a minimum age of ca 354 ma which was followed by a near isothermal decompression a concordant u pb zircon age of 318 1 ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase d2 this phase was associated with an lp ht metamorphism dated in the region at ca 340 330 ma the available structural and isotopic data indicate that granulites in the southern bohemian massif were exhumed from lower to middle crust during compression the structural relations and p t t data for the studied granulites are consistent with their exhumation by near vertical extrusion of the softened orogenic root

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

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

ultrahigh pressure metamorphism in the śnieznik mountains sudetes poland p t constraints and geological implications the petrology and p t conditions of eclogites and associated gneisses from the nieznik mountains sudetes sw poland were investigated to establish an exhumation path for these ultrahigh pressure rocks peak pressure conditions are represented by the assemblage garnet omphacite phengite kyanite coesite rutile post dating this assemblage but still in the eclogite facies garnet with lower grossular content and a first generation of amphibole are formed under lower pressure conditions giving rise to the assemblage garnet omphacite phengite kyanite quartz rutile calcic amphibole i continuing depressurization caused the successive destabilization of omphacite garnet kyanite and phengite in some places the primary eclogite assemblage was completely transformed into amphibolite facies mineral assemblages estimated peak pressures in the nieznik and miedzygorze units lie in the coesite stability field p 27 kbar at temperatures of about 700 800 c and are not significantly different to the zlote un

lu hf geochronology and trace element distribution in garnet implications for uplift and exhumation of ultra high pressure granulites in the sudetes sw poland combining lu hf garnet geochronology with in situ trace element analyses in garnet allowed its to gain new insight into the metamorphic evolution of uhp uht rocks in the stary gieraltow region in the polish sudetes prograde garnet growth recorded by rayleigh type heavy ree hree zoning in the felsic granulites indicates that the obtained 386 6 4 9 ma lu hf age represents the time of garnet crystallization on a prograde uhp metamorphic path the surrounding rocks were metamorphosed at the same time as indicated by 381 2 6 7 ma sm nd garnet age obtained for the mid crustal metapelites the second metamorphic episode which affected most of the lower crust in the orlica snieznik massif osm occurred at ca 340 ma as determined by u pb zircon and sm nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event which took place on a retrograde path trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and hree in garnets from mafic and felsic protoliths over the course of the metamorphic evolution this had strong impact on the isotopic dating results and led to decoupling of the sm nd and lu hf clocks which recorded timing of the two different metamorphic episodes separated by as much as 40 ma moreover the preservation of the hree growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 degrees c implies that the lu hf system under relatively dry conditions does not undergo significant diffusional re equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth under hydrous conditions at least some resetting will take place as documented by the partially relaxed hree zonation profile in the amphibolitised mafic granulite which yielded a 10 ma younger age the hree distribution study appeared to be a particularly valuable and essential tool which allowed us to distinguish garnet growth from post growth complexities and hence provide improved age interpretation medium ree on the other hand did not show any obvious correlation with the isotopic signature of garnet two distinct metamorphic episodes recorded in the stary gieraltow region show that buoyancy driven uplift of uhp rocks can be arrested at the base of a continental crust if not supported by any additional force in our case study the uhp rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime the multistage discontinuous uplift revealed by the uhp rocks of the osm provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast track exhumation histories recognized in uhp terranes elsewhere c 2006 elsevier b v all rights reserved

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