tectonic history of the lunksoos composite terrane in the maine appalachians several ordovician volcanic terranes are preserved between the paleozoic north american margin and the avalon terrane in the northern appalachians of maine they have been interpreted as remnants of an island arc on the basis of their current position in the appalachians or as ocean islands near the margin of avalon on the basis of paleontologic data a study of one of these terranes the lunksoos terrane in northern maine was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution paleomagnetic study of the pre caradoeian pre late ordovician stacyville volcanics revealed three components of magnetization the final magnetization removed d 90 i 36 95 11 8 is interpreted to be primary and yields a paleolatitude of approximately 20 s however a late ordovician remagnetization cannot be excluded the stacyville volcanics now overlie a cambrian and lower ordovician sequence the grand pitch and shin brook formations that is believed to have been deposited at the southern side of iapetus possibly near the margin of avalon during the middle and late ordovieian avalon was at paleolatitude s higher than 40 s while the volcanics were either formed or remagnetized at a paleolatitude of 20 s the pre caradocian stacyville volcanics therefore could not have been deposited directly on the grand pitch shin brook sequence but must have been tectonically emplaced the rockabema diorite which intrudes the entire cambro ordovician sequence constrains the timing of this emplacement to be pre late early silurian these relationships indicate that the lunksoos area is a composite terrane within the maine appalachians the tectonic emplacement scenario is unnecessary if the grand pitch and shin brook formations were not located on the south side of iapetus during the ordovician this alternative however poses conflicts with the interpretation of faunal affinities

hydrological response of an east saharan palaeolake nw sudan to early holocene climate we present a palaeolimnological record in western nubia nw sudan eastern sahara isolated from large scale artesian systems and thus reflecting hydrological changes resulting from local rainfall past changes in lake water chemistry and water depth are interpreted by combining results from biological indicators diatoms ostracodes and charophytes with geochemical and stable oxygen and carbon isotope data palaeolake gureinat existed from 10 9 cal ka bp until at least 7 1 cal ka bp and is devided into five lake phases for the first two millennia the lake was shallow and fluctuating salinity conditions prevailed 500 to 3000 s cm after 8 7 cal ka bp the lake level rose waters became fresh to oligosaline and the lake had a minimum depth of at least 8 m both transgressive and stable lake phases were punctuated by at least three drops in lake level with complete desiccation at around 10 9 and 8 9 cal ka bp the third of these regressions may be coeval with the 8 2 ka climatic event sediments

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians lombard a d van der pluijm b a and van der voo r 1991 acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians tectonophysics 186 279 291 samples were collected from ten sites of the late cambrian early ordovician jim pond formation for paleomagnetic study stepwise thermal demagnetization reveals three separable components of magnetization component i is typically removed by 350ac it is subparallel to the present day field 354o 76 vs 342 72o at the site location 45 3on 289 4 e and is considered to be a recent partial overprint component ii without tilt correction is a south southeasterly and shallow direction mean 165 o o k 31 4 a s5 8 6 o that is removed over an intermediate temperature range 350 600 c component iii without tilt correction is a northeasterly and shallow upward direction mean lo 24o k 21 5 ass 7 3 and is removed over the highest temperature range 480 to 69ooc though not statistically significant for components ii and iii the precision parameter k decreases and the as5 increases when tilt correction is applied suggesting that both are post folding magnetizations component ii without tilt correction has a corresponding paleomagnetic pole located at 43on 130 e dp dm 4 3 8 6 o which falls near the late carboniferous segment of the laurentian apparent polar wander path apwp component iii without tilt correction has a corresponding pole located at 32 n 98 e dp dm 4 7o 7 8 which falls near the lower middle devonian segment of the apwp we conclude that the jim pond formation has undergone two paleozoic remagnetization events one in the early to middle devonian and a second one in the late paleozoic the ages of these remagnetizations coincide with the timing of major erogenic activity in the area i e the acadian and alleghenian respectively the remagnetization event associated with the acadian pulse can be recognized in other paleomagnetic investigations in the northern appalachians

caledonian plutonism and major lineaments in the sw scottish highlands within the sw grampian highlands several ne sw trending shear zones and faults related to the caledonian transpressional collision are recognized as being distinct from an intersect ing set of nw se trending pre caledonian crustal lineaments which were reactivated during caledonian orogenesis the geochemical and isotopic characteristics of the 425 400 ma granitoids of this region show that they were derived by anatexis of the lower crust associated with a mantle component which provided an influx of mantle heat and or metasomatic fluids their spatial distribution suggests a close genetic relationship with caledonian shear zones having made the fundamental distinction between ascent transport and emplacement arrival configurations possible for granitoid magmas new data are presented for these plutons which show that i a common modal sequence of intrusive phases can be recognized ii these phases are all sited at shear zone or lineament intersections where transtensional zones allowed and facilitated ascent iii emplacement was often by a process of localized in situ ballooning the existence of such lineaments in these orientations in the sw highlands may indicate that the structure of the lower crust can be regarded as a series of blocks bounded by intersecting ductile zones of high strain the establishment of such a pattern of structural interactions within the lower crust could provide a mechanistic framework within which the location of anatectic zones siting and ascent pathways and any subsequent emplacement phenomena can be explained in orogenic belts

late quaternary floods and droughts in the nile valley sudan new evidence from optically stimulated luminescence and ams radiocarbon dating our results show that the late pleistocene nile in northern sudan was shifting position and actively aggrading at 145 20 kyr 83 24 kyr 32 8 kyr and 20 7 0 2 kyr and indicate for the first time a phase of high energy flow in the white nile at 27 8 3 2 kyr with still high but somewhat reduced flow in that river at 13 3 kyr 10 kyr and 4 8e4 0 kyr beach ridges associated with a 386 m strandline of the white nile have osl ages of 27 5 2 7 kyr and 14 5 1 6 kyr the holocene terraces and former channels of the main nile have ages of 11 kyr 6 5e5 0 kyr and 4 8e4 0 kyr after which there was a general decline in flood discharge the now arid main nile valley in northern sudan was significantly wetter during the early to middle holocene with a lake up to 450 km 2 in area fed by an overflow channel from the early holocene nile between 9 5 kyr and 7 5 kyr previously stable late pleistocene dunes were reactivated at intervals during the holocene with five samples from the white nile valley indicating brief phases of holocene dune activity at 9 9 2 0 kyr 9 0 2 8 kyr 6 6 0 9 kyr 4 8 0 9 kyr and 2 9 0 5 kyr the earliest of which occurred within periods of generally wetter climate and higher nile flow the youngest freshwater shells on the khor abu habl alluvial fan west of the white nile correspond to a time of regionally wetter climate between 1 7 and 1 0 kyr our results suggest that millennial scale climatic instability may have been characteristic of holocene climates in this region 2010 elsevier ltd all rights reserved

using titanium in quartz geothermometry and geospeedometry to recover temperatures in the aureole of the ballachulish igneous complex nw scotland titanium in quartz geothermometry suggests quartzites could yield reliable temperature estimates we here apply four calibrations of the titanium in quartz geothermometer to contact metamorphosed quartzites surrounding the ballachulish igneous complex scotland two agree broadly with thermal modelling and pre existing geothermometry two give temperatures consistently too low as reported in earlier studies the technique suffers from difficulties in analysing low titanium ti levels with high spatial and analytical precision however this study finds that the critical problem is one of ti heterogeneity which poses difficulties in constraining the chemical activity of ti during quartz growth under metamorphic conditions scanning electron microscope cathodoluminescence sem cl textures support an interpretation of extensive ti disequilibrium despite the presence of rutile indicating dynamic interplay between grain boundary diffusion fluid melt percolation and grain growth the strong zonation suggests a possible geothermometer based on apparent volume diffusion of ti in quartz to derive grain growth histories analysis of rutile quartz interaction implies peak contact temperatures of 645 12 c precise but reliant on external estimates of cooling rate from thermal models our conclusions support caution in applying ti in quartz geothermometry in aureole settings however rutile quartz juxtaposition prior to heating to 600 c defines a ti diffusion couple employable as a thermometer if cooling rates are constrained by other means

paleomagnetism of the pennington mountain terrane a near laurentian back arc basin in the maine appalachians paleomagnetic studies of volcanic terranes in the appalachians provide quantitative data on the ordovician paleogeography of the iapetus ocean new paleomagnetic results from submarine volcanics of the middle to late ordovician winterville formation of maine further constrain the evolution of iapetus ten sites yield a tilt corrected direction of d i 327 21 95 9 3 k 27 9 the corresponding paleomagnetic pole calculated for the average site location 46 8 n 291 3 e is located at 26 n 148 e d p 5 d m 10 a 95 7 7 a silurian conglomerate frenchville formation test is inconclusive due to a strong present day field overprint which obscures any primary remanence the presence of a positive tilt test however supports the conclusion that the characteristic magnetization of the winterville formation is a prefolding primary magnetization an ordovician paleolatitude of 111 5 for the pennington mountain terrane of northern maine is indistinguishable from that of the laurentian margin 15 20 during the middle to late ordovician the paleolatitude of the pennington mountain terrane is also similar to but slightly more equatorial than that of the previously studied bluffer pond formation of the nearby munsungun terrane 18 9 we conclude that the ordovician pennington mountain and munsungun volcanic terranes of maine were formed and acquired their characteristic magnetization near the laurentian margin the paleolatitudes of these terranes are also similar to that obtained from the stacyville volcanics of the more outboard lunksoos terrane 20 8 but they contrast strongly with the ordovician miramichi terrane of northern new brunswick that yielded a paleolatitude of 51 21 16 these results from maine support the presence of one or more backarc basins adjacent to the laurentian margin during the middle to late ordovician

estimating the hydrogen isotopic composition of past precipitation using leaf waxes from western africa abstract the hydrogen isotopic composition of plant leaf wax n alkanes d wax is a novel proxy for estimating d of past precipitation d p however vegetation life form and relative humidity exert secondary effects on d wax preventing quantitative estimates of past d p here we present an approach for removing the effect of vegetation type and relative humidity from d wax and thus for directly estimating past d p we test this approach on modern day late holocene 0 3 ka sediments from a transect of 9 marine cores spanning 21 n 23 s off the western coast of africa we estimate vegetation type c 3 tree versus c 4 grass using 13 c of leaf wax n alkanes and correct d wax for vegetation type with previously derived apparent fractionation factors for each vegetation type late holocene vegetation corrected d wax d vc displays a good fit with modern day d p suggesting that the effects of vegetation type and relative humidity have both been removed and thus that d vc is a good estimate of d p we find that the magnitude of the effect of c 3 tree c 4 grass changes on d wax is small compared to d p changes we go on to estimate d vc for the mid holocene 6 8 ka the last glacial maximum lgm 19 23 ka and heinrich stadial 1 hs1 16 18 5 ka in terms of past hydrological changes our leaf wax based estimates of d p mostly reflect changes in wet season intensity which is complementary to estimates of wet season length based on leaf wax 13 c

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

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

beach ridge development in lake michigan shoreline behavior in response to quasi periodic lake level events abstract strandplains of arcuate beach ridges are common in coastal embayments in parts of the great lakes similarities in beach ridge development and geomorphology are recognizable in many of the embayments in the lake michigan basin despite differences in size and shape available sediment type and supply predepositional slope and topography and hydrographic regime between the embayments these similarities are primarily a product of three scales of quasiperiodic lake level variation ranging in time from 30 to 600 years and in water level change from 0 5 to 3 7 m the interaction of these three lake level variations can be represented on a curray 1964 diagram rate of water level change versus rate of sediment supply the position of any shoreline on the diagram and the type of behavior the shoreline is experiencing is a product of the interaction of the three variations two large strandplains of late holocene beach ridges occur at opposite ends of lake michigan toleston beach and thompson embayment the two areas exhibit similar patterns of beach ridge development for the past 2600 calendar years that is both areas form beach ridges about every 30 years groups of 4 to 6 beach ridges reflect a longer term lake level variation of about 150 years only during the largest variation of about 600 years in duration do the two areas differ the rise to the 1700 cal yr b p high caused the erosion of beach ridges back to 2800 cal yrs b p in northern lake michigan in southern lake michigan no erosion occurred during this lake level high differences in shoreline development between the two areas are related to the rate of sediment supply to the shorelines as the sediment sink for the southern half of lake michigan the southern strandplain received a greater sediment flux than the northern strandplain during the latter part of the late holocene and produced a continuous record of beach ridge development

phase relations of peraluminous granitic rocks and their petrogenetic implications peraluminous granitic rocks are magmatic rocks that contain quartz po tassic feldspar and or sodic plagioclase and one or more aluminum rich minerals thus these rocks contain more aluminum than could be accom modated by feldspar in a cipw normative calculation and show normative corundum peraluminous granites are defined by their chemical nature but their recognition is often based on petrography this discrepancy thus may lead to inconsistency although peraluminous granitic rocks are com monly true granite streckeisen 1973 they range from tonalite to granite of special interest are strongly peraluminous as defi ned below granite granodiorite or in rare instances trondhjemite these rocks contain alumi nous minerals almandinic garnet cordierite tourmaline muscovite that are thought to be stable in silicate magmas only if these magmas are themselves peraluminous the origin of peraluminous granite and related intrusive rocks is of much current interest many of these form shallow level intrusions and they can reach enormous sizes examples are the batholiths in the lachlan fold belt in southeastern australia white chappell 1983 the cornub ian batholith in england exley stone 1966 the south mountain batholith in nova scotia mckenzie clarke 1975 the manaslu batho lith in the himalayas lefort 1981 much of the idaho batholith

regional differences in pb isotopic compositions of feldspars in plutonic rocks of the northern appalachian mountains u s a and canada a geochemical method of terrane correlation feldspar pb isotopic compositions from paleozoic tonalitic to leucogranitic caledonian plutons from the northern appalachian mountains of the u s a and canada range from nonradiogenic to radiogenic relative to the evolution curve of stacey and kramers 1975 pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons plutons with the least radiogenic pb isotopic signatures northern group have intruded allochthonous rocks closest to the north american craton and plutons with the most radiogenic pb southern group have intruded avalonian basement feldspars from plutons in the central group have intermediate pb isotopic compositions between the northern and southern groups and do not define an isotopically unique group the predominant source of the felsic magmas was the continental lithosphere as indicated by the general pb isotopic similarity of the plutonic feldspars with either grenville or chain lakes basement in the case of the northern group and with avalonian basement rocks in the case of the southern group the source region for plutons in the central group may have contained a mixture of grenvillian and avalonian components plutons in the northern appalachians are as a group more evolved in their pb isotopic compositions relative to caledonian plutons in the british isles

fluorine as a tool in the petrogenesis of quartz bearing magmatic associations applications of an improved f oh biotite apatite thermometer grid abstract fluorine contents of whole rock and biotites allow to discriminate quartz bearing magmatic associations through the nature of protoliths and petrogenetic processes f and rb are used to discriminate crustal and mantle derived melts with the exception of calcalkaline association in which mantle contribution may be significant the other quartz bearing associations are essentially derived from crustal melting by biotite and or amphibole vapor absent reactions based on experimental data and on an improved biotite apatite thermometer grid closure temperatures from near liquidus to near solidus conditions as well as fh2o fhf evolution during cooling are determined for the different magmatic associations it appears that the liquidus temperature and fh2o fhf decreases in the order high silica alkaline high k calcalkaline and calcalkaline associations while the solidus temperature follows an inverse order the fh2o fhf values for the peraluminous associations vary over a wide range and overlap the fh2o fhf values of the high k calcalkaline and calcalkaline associations

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

regressive and transgressive sequences in a raised holocene gravelly beach southwestern crete both a modern and a raised late holocene gravelly beach have been studied along the cliffed coast near the village of chora sfakion in southwestern crete greece the holocene beach shows the record of regional well documented relative sea level changes in the form of regressive and transgressive sequences the texture fabric and depositional architecture of these sequences are described and compared with those of the modern microtidal beach there the depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea level changes the regressive sequences of both the modern and the holocene beach resemble those of the mesotidal beaches in the sker district of southwest wales described by bluck 1967 the transgressive sequence of the holocene beach resembles the mesotidal beach sequence near newton southwest wales recognized by bluck as another type of progradational regressive beach therefore transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive the stratigraphic depositional patterns of gravelly beach systems are apparently still poorly understood

late neoproterozoic to permian tectonic evolution of the quebec appalachians canada abstract due to the unique location of the quebec appalachians in a re entrant of the paleo north american margin and to the fact that the area largely escaped late paleozoic deformation this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere despite diachronicity both along and across strike of the orogen five tectonic events of regional significance are recognized 1 the earliest event is mainly recorded by neoproterozoic to early ordovician normal faults associated with extension of the laurentian craton and the establishment of a passive continental margin 2 the second event the ordovician taconian orogeny is marked by foreland directed deformation and corresponds to the principal deformational and metamorphic events recorded by the laurentian margin 3 the third event early salinic is early silurian it is restricted to the southern part of the quebec appalachians and consists of hinterland se directed thrusts folds and retrograde metamorphism 4 the fourth event late salinic is recorded by regional scale late silurian to early devonian normal or transtensional faults and is associated with the formation of the connecticut valley gaspe trough and 5 the last event the devonian acadian orogeny affects with variable intensity all the pre carboniferous lithologies this tectonic synthesis implies that the subdivision of the peri laurentian oceanic tracts proposed in other segments of the appalachians should be applied with caution in quebec and that the structural style the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen in particular the similarities of the thetford mines and boil mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with the maximum allochtony hypothesis i e the assumption that the southern quebec ophiolites are rooted east of the chain lake massif a geodynamic model involving the taconian obduction of a large ophiolitic nappe s the docking of a peri gondwanan gander medial new england terrane in early salinic the formation of large sedimentary basins connecticut valley gaspe and merrimack troughs due to trench rollback and or delamination during the late salinic event and collision at the outer margin of composite laurentia during the acadian orogeny can account for the protracted structural and metamorphic history of the quebec appalachians

the extension of grenville basement beneath the northern appalachians results from the quebec maine seismic reflection and refraction surveys from 1983 to 1984 the united states geological survey and the geological survey of canada undertook a series of seismic experiments to determine the crustal structure of the appalachian mountain belt in maine and southeast quebec these experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high resolution refraction wide angle reflection profiles both parallel and perpendicular to strike here the data recorded in quebec and the northwest part of maine are interpreted in conjunction with existing reflection profiles from quebec synthetic seismogram calculations in laterally varying media are used to model the refraction wide angle reflection data while reprocessing migration and attribute plotting are used in the interpretation of the reflection data a major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the st lawrence lowlands southeast to about 25 km depth beneath the southeast edge of the chain lakes massif this zone of reflections is interpreted as a master decollement separating autochthonous grenville basement from overlying allochthonous rocks of the appalachian orogen including foreshortened miogeoclinal rocks the remnants of one or more magmatic arcs the connecticut valley gaspe synclinorium and the chain lakes massif in the vicinity of the chain lakes massif extension has caused grenville basement to be thinned by as much as 40 reflections from the decollement show strong layering and evidence for basement faulting associated with the guadeloupe and other faults normal incidence and wide angle reflections are observed to be almost continuous beneath the baie verte brompton line and connecticut valley gaspe synclinorium indicating that the baie verte brompton line is confined to the upper crust the connecticut valley gaspe synclinorium is characterized by p wave velocities of about 5 3 km s 1 and is shown to be a shallow structure with a maximum depth of about 3 km rock velocities immediately below the connecticut valley gaspe synclinorium are typical of chain lakes material 6 1 km s 1 a new model for the tectonic development of the region is proposed in which all pre silurian units between the baie verte brompton line and the southeastern edge of the chain lakes massif are allochthonous and confined to the upper crust we infer that the chain lakes massif volcanics of the ascot weedon formation and associated deeper crustal rocks were thrust over grenvillian basement as a composite unit and may now underlie parts of the connecticut valley gaspe synclinorium

heat flow and basement radioactivity in maine first order results and preliminary interpretations new heat flow values q for 70 boreholes in maine are in the range 33 83 mwm 2 new radioactive heat productions a for 45 sites range from 0 08 to 8 74 wm 3 the combined data for plutonic rocks determine least squares lines with slopes and intercepts in the ranges 5 0 6 5 km and 35 39 mwm 2 respectively these lines provide continued support for linear relations between q and a in continental terranes and confirm that normal reduced flux is the characteristic geothermal parameter of all of the new york new england region gravity seismic and thermal modeling suggest that complex distributions of pluton thicknesses and near surface radioactivity occur in maine thus a single q a line for this region is difficult to explain with simple distributions of vertically uniform or exponentially decreasing crustal radiogenic heat in some areas normal reduced flux in maine implies that the lithosphere has equilibrated since thermotectonic events in paleozoic and jurassic times moho temperature changes associated with 8 10 km crustal thickness changes in maine and central new hampshire are not likely to exceed 150 c and effects on mantle properties would be small

crustal differentiation processes at krakatau volcano indonesia anak krakatau is a basaltic andesite cone that has grown following the famous caldera forming 1883 eruption of krakatau it breached sea level in 1927 and since the 1950s has been growing at an average rate of similar to 8 cm a week we present new major and trace element data combined with whole rock delta o 18 sr and nd isotope data for 1883 1993 and 2002 krakatau eruptive products and the surrounding crust bombs erupted from anak krakatau during 2002 contain frothy metasedimentary and plutonic xenoliths that show variable degrees of thermal metamorphism plastic deformation and partial melting contact metamorphic minerals such as cordierite and tridymite in metasedimentary xenoliths are consistent with high temperature metamorphism and incorporation at mid to upper crustal depth energy constrained assimilation and fractional crystallization modelling of whole rock data suggests that the anak krakatau magmas have a genetic relationship with the 1883 eruption products the geochemical impact of crustal contaminants on whole rock compositions is apparently small and we conclude that low levels of assimilation of a quartzo feldspathic sediment are recorded in anak krakatau magmas plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning however and are also isotopically variable with a total range in sr 87 sr 86 of 0 center dot 7043 0 center dot 7048 as determined by in situ laser ablation inductively coupled plasma mass spectrometry this suggests that although shallow crustal assimilation appears to have had a limited effect on whole rock chemistry a complex late stage differentiation history is recorded within the magma s cargo of crystals and xenoliths

the ballachulish igneous complex scotland petrography mineral chemistry and order of crystallization in the monzodiorite quartz diorite suite and in the granite

the fate of graphite in prograde metamorphism of pelites an example from the ballachulish aureole scotland abstract graphite bearing slates and phyllites 0 4 1 2 vol graphite are progressively metamorphosed in the 3 kbar aureole of the 425 ma ballachulish intrusion scotland two major dehydration reactions are crossed the chlorite out reaction at ca 550 c forming cordierite biotite and the muscovite out reaction at 625 c forming al 2 sio 5 k feldspar graphite persists to the highest grades and shows no significant variation in abundance with grade except for a possible decrease in the highest grade rocks variable graphite abundance in rocks at the same grade reflects primary sedimentological heterogeneity texturally graphite grains and aggregates in the rock matrix become coarser grained and more widely separated as grade increases these thermally induced textural modifications of graphite are superimposed on mechanically induced features such as graphite segregations along cleavages and crenulations that formed prior to contact metamorphism mass balance modelling assuming internal fluid generation shows that the amount of graphite consumed during contact metamorphism in the aureole ranges between 0 1 and 0 3 vol depending on the amount of chlorite and muscovite in the protolith because the amount of c dissolved in a c o h fluid decreases with increasing pressure and the ballachulish aureole is at relatively low pressure these results are a maximum for regional metamorphism suggesting that graphite will persist through a regional metamorphic cycle if it is initially present in volumes ca 0 2 vol

procedures used for luminescence dating of sediments abstract an account of the procedures for thermoluminescence tl and optically stimulated luminescence osl dating of sediments used at the nordic laboratory for luminescence dating is given the topics discussed include sampling sample preparation separation of quartz and feldspars using heavy liquids dose rate estimation and methods for estimating the equivalent dose special attention is paid to the partial bleach tl technique of mejdahl 1988 which appears to give reliable results for partially bleached sediments optically stimulated luminescence dating using stimulation by infra red ir and green light has been taken up and especially added dose methods using stimulation by ir or green light have been tried with some success some sources of error are discussed including the adverse effect of shallow traps

facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration examples from lakes turkana and tanganyika abstract two african lakes turkana and tanganyika illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins they demonstrate contrasts between sediments deposited in a semi arid moderately deep mainly volcanic basin turkana and a semi humid extremely deep basin without significant volcanism tanganyika turkana deposits are dominated by rapidly accumulating organic poor terrigenous muds coarse clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers profundal coarse clastic deposition is rare sediments contain little biogenic components except in the sediment starved south basin sediments are also poor in cartonates tanganyika deposits are dominated by organic rich biogenic muds steep fault escarpments and uniform slopes along shelf platform and rift axis lake margins promote the efficient transport of sands into profundal settings total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief backslopping a limited drainage basin area and presettling in upstream lake kivu sediment ponding along margins is rare littoral carbonates are common a consequence of low net clastic sedimentation rates many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief volcanic activity and climate between the two lakes the results of this study suggest that lake deposits formed in tanganyika type rift basins are more appropriate exploration target for hydrocarbon exploration than those from turkana type basins

archean inheritance in zircon from late paleozoic granites from the avalon zone of southeastern new england an african connection abstract in southeastern new england the narragansett pier granite locally intrudes carboniferous metasedimentary rocks of the narragansett basin and yields a monazite u pb permian emplacement age of 273 2ma zircon from the narragansett pier granite contains a minor but detectable amount of an older inherited component and shows modern loss of lead zircon from the late stage aplitic westerly granite exhibits a more pronounced lead inheritance permitting the inherited component to be identified as late archean such old relict zircon has not been previously recognized in proterozoic to paleozoic igneous rocks in new england and may be restricted to late paleozoic rocks of the avalon zone we suggest that the archean crustal component reflects an african connection in which old archean crust was underplated to the avalon zone microplate in the late paleozoic during collision of gondwanaland with avalonia

155 000 years of west african monsoon and ocean thermal evolution a detailed reconstruction of west african monsoon hydrology over the past 155 000 years suggests a close linkage to northern high latitude climate oscillations ba ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the gulf of guinea in the eastern equatorial atlantic eea reveal centennial scale variations of riverine freshwater input that are synchronous with northern high latitude stadials and interstadials of the penultimate interglacial and the last deglaciation eea mg ca based sea surface temperatures ssts were decoupled from northern high latitude millennial scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low latitude solar insolation the onset of enhanced monsoon precipitation lags behind the changes in eea ssts by up to 7000 years during glacial interglacial transitions this study demonstrates that the stadial interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the west african monsoon dynamics through an atmospheric linkage

a 9000 year oxygen and carbon isotope record of hydrological change in a small ethiopian crater lake the stable isotope composition of authigenic calcites in a core from lake tilo a small crater lake in the ethiopian rift valley provides a subcentury scale record of lake response to climatic change over the last 8850 14c yrs 9850 cal yrs an unusually high range of 18o and 13c values 15 is attributed in part to major changes in hydrothermal groundwater flux although hydrothermal groundwater influx to the lake was high during the early holocene its flow rate was relatively stable and thus climatically induced changes to the water budget can be inferred from variations in 18o and 13c ratios a major decline in hydrothermal groundwater input from 5500 14c yrs bp increased lakewater residence time and led to substantial increases in 18o and 13c before the mid holocene transition to more arid conditions during the last 2700 14c years diagenetic processes have resulted in extremely varied 18o and 13c values the holocene isotope record from lake tilo challenges the widely held assumpti

climate change in the turkana basin as deduced from a 4000 year long δo18 record abstract a model of the oxygen isotopic composition of lake turkana waters sheds insight into the paleoclimatic interpretation of a 18 o profile of inorganic calcite from a core taken from the lake the model indicates that high frequency fluctuations seen in the isotopic record are probably due to shifts in lake level while the long term increase of 1 4 seen from 4 to 2 kyr bp cannot be due solely to lake level change it must be due to changes in other climatic parameters such as the isotopic composition of the inflow these findings suggests that the abrupt change observed in lake turkana and many other african lakes at 4 ka was the precursor to a 2000 year long adjustment in the climate over the ethiopian plateau the isotopic record from lake turkana agrees with other paleoclimate records from the basin including the diatom stratigraphy and radiocarbon dates on exposed shoreline deposits

partial melting of the appin quartzite driven by fracture controlled h sub 2o infiltration in the aureole of the ballachulish igneous complex scottish highlands the ballachulish igneous complex consists of an outer quartz diorite and an inner granite emplaced at about 300 mpa initially at 1000 to 1050 c the contact aureole 0 5 2 km wide occurs in metapelites and metapsammites plus minor graphitic slates carbonate rocks and metaquartzites a textural examination of the arkosic appin quartzite which was previously believed to have melted only within a few metres of the intrusion demonstrates that partial melting occurred up to 500 m away from the vertical eastern contact coupling petrographic observations with qtz ab or h2o phase relations we determined both the amounts of actual melt and the maximum possible amounts of melt in the samples melting efficiency was everywhere less than 100 and decreased with distance from the intrusion though perhaps not the only possible source of fluid throughout the aureole simple models demonstrate that h2o evolution from the pluton would have been volumetrically sufficient and persisted long enough to account for the observed partial melting a time integrated fluid flux of 7000 kg m2 from the pluton is necessary to account for the observed amounts of partial melt in the appin quartzite from its inefficiency we infer that infiltration of the appin quartzite cannot have occurred along interconnected grain edge channels rather it was controlled by hydraulic fracturing with fracture density determining melting efficiency bulk rock permeability is calculated to be 10 20 m2 an order of magnitude lower than that necessary to permit pervasive flow of all the fluid exsolving from the pluton there is little difference between the calculated time integrated fluid flux through the appin quartzite on the eastern flank and an estimate of the infiltrating flux through the pelitic leven schist on the western flank in the time interval during which both rock types were above their solidus temperature despite differences in their equilibrium quartz h2o dihedral angles at temperatures immediately below the solidus and differences in the attitude of the contact the rates of h2o expulsion from the cooling pluton are consistent with highly efficient fracture dominated flow allowing insufficient time for textural equilibration

pollen and isotopic records in late holocene sediments from lake turkana kenya abstract a 12 m piston core lt 84 8p taken in the northern basin of lake turkana has been dated by 6 ams radiocarbon dates on large ostracods and analysed for its pollen content during the past 2500 years the pollen assemblages were dominated by chenopodiaceae amaranthaceae cheno am and gramineae representing the local pollen signal of the surrounding vegetation under arid conditions percentage fluctuations of cheno am are interpreted as reflecting lake level variations in good correspondance with oxygen isotope from the same core a rise in lake level is recorded at about 1600 yr b p preceded by a relative lowstand during much of the period from 2500 to 1600 yr b p the top of the core is estimated to be about 600 yr old and shows a shift of unknown duration towards a more humid condition the high lake level documented in this paper roughly coincides with that reconstructed from other cores in the lake based on diatom and carbonate analyses

contrasting rock permeability in the aureole of the ballachulish igneous complex scottish highlands the influence of surface energy the ballachulish igneous complex in the scottish highlands part of a widespread group of late caledonian calcalkaline intrusions was emplaced at a depth of 10 km into dalradian metasediments resulting in melting of the country rocks near the intrusive contact the greatest extent of melting occurred in the leven schist in the 400 m wide so called chaotic zone which experienced infiltration of aqueous fluids from the pluton in contrast adjacent to the chaotic zone the feldspar bearing appin quartzite underwent significant melting only within a few metres of the intrusion despite both being in contact with the same fluid source as the leven schist and having a similar wet melting point the permeability of the appin quartzite and quartz horizons in the leven schist to pervasive grain edge infiltration of aqueous fluids was determined by measuring the equilibrium quartz h2o dihedral angle for the p t conditions of contact metamorphism addition of powdered samples of both rock types to the pure quartz h2o system results in a linear decrease of the quartz h2o dihedral angle with increasing temperature the rate of this decrease for the leven schist is greater than that for the appin quartzite and the angle decreases below 60 some 30 c below the wet solidus 670 c at 0 3 gpa charges bearing appin quartzite had dihedral angles greater than 60 at all temperatures below the wet solidus 690 c at 0 3 gpa these results demonstrate that quartz rich horizons in the leven schist would have been permeable to infiltration of aqueous fluids close to the solidus permitting extensive h2o fluxed melting to occur the appin quartzite would have remained impermeable to grain edge flow consistent with the observed differences in the extent of partial melting of the two lithologies

paleomagnetism of the ordovician bluffer pond formation paleogeographic implications for the munsungun terrane of northern maine the munsungun area of northern maine is an early paleozoic terrane within the central mobile belt cmb of the northern appalachians the bluffer pond formation of the munsungun terrane is composed primarily of pillow basalt and gabbro with minor intercalated sediments and has a maximum thickness of 1230 m this unit is of early late ordovician caradocian age based on paleontological data and folding most likely occurred during the devonian acadian orogeny this region has been metamorphosed to subgreenschist facies samples for paleomagnetic study have been obtained from 15 sites of pillow basalt and seven sites of gabbro within the bluffer pond formation natural remanent magnetization nrm directions are similar to the expected present day field direction and are interpreted to be strongly affected by a viscous overprint detailed stepwise thermal demagnetization fails to adequately resolve a partial overprint direction with intermediate blocking temperature but stable endpoints combined with great circles do isolate a characteristic high temperature component carried by magnetite as indicated by unblocking temperatures and rock magnetic experiments after tilt correction the formation mean calculated from pillow basalt sites only is d 339 i 33 k 12 95 14 n 11 with a corresponding paleopole of 23 n 133 e p 9 m 16 calculated for an average site location of 46 5 n 291 e when both pillow basalt and gabbro sites are used to calculate a tilt corrected formation mean the result is d 339 i 28 k 12 95 12 n 14 and this characteristic component passes a tectonic tilt test the calculated paleopole 26 n 134 e p 7 m 13 from pillows and gabbros falls near but not on the ordovician apparent polar wander path segment for north america and yields a paleolatitude 18 9 s similar to that of the lunksoos terrane which is located farther to the southeast within the cmb of maine the laurentian margin at this time was located at paleolatitudes of 15 20 s the paleogeographic implication of this result is that the munsungun terrane was located near the laurentian margin within the iapetus ocean and this near continent location is consistent with a marginal or back arc basin tectonic environment for the extrusion of the bluffer pond volcanics

interpretation of migrated seismic reflection profiles across the northern appalachians in maine summary a simple line migration technique has been developed and applied to deep crustal reflection data collected in the northern appalachians of quebec and central maine preliminary interpretation of these data combined with the results of gravity and refraction studies shows thinning of the crust in two distinct steps beneath the silurian devonian merrimack synclinorium and shows evidence for mesozoic crustal stretching and dike intrusion

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

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

zoning patterns in orthopyroxene and garnet in granulites implications for geothermometry compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the minto terrane of northern quebec the compositional maps provide clear evidence of late exchange of fe fe mg after ca in garnet and al in orthopyroxene had been quenched in the extent of late fe mg exchange was controlled by neighbouring minerals with negligible fe mg gradients against plagioclase and quartz and substantial gradients against exchangeable fe mg minerals cores of grains in contact with exchangeable fe mg neighbours are progressively more reset in fe fe mg as grain size decreases whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in fe fe mg than cores of the largest grains gradients of ca in garnet and of al in orthopyroxene in grains of uniform fe fe mg preserve a high temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular fe mg diffusion in garnet and orthopyroxene was rapid enough to homogenize fe fe mg the transition from efficient intergranular exchange at relatively high temperatures to local fe mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock possibly an internally generated dehydration melt phase implications for geothermometry of granulites include the following numerical values are particular to this study 1 core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in fe fe mg relative to the most refractory compositions by an amount equivalent to 120d c 2 fe mg exchange thermometry using even the most refractory fe fe mg compositions may not record peak granulite conditions possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock and 3 temperature sensitive net transfer equilibria involving al solubility in orthopyroxene yield temperatures up to 150d c higher than maximum fe mg exchange temperatures even in grains with flat fe fe mg compositional profiles making them a better means of estimating peak granulite temperatures than fe mg exchange thermometry

basement geology and tectonic evolution of ireland as deduced from pb isotopes in ireland lead isotope data collected on carboniferous hosted zn pb deposits can and has been interpreted to indicate that lead and other metals were derived from the underlying basement rocks in order to test this hypothesis whole rock lead isotope measurements were made on greywackes from the longford down inlier which represent the along strike continuation of the northern part of the caledonian basement underlying the base metal deposits the lead isotope data define a linear array on a 207 pb 204 pb versus 206 pb 204 pb plot with end points at 207 pb 204 pb 15 542 206 pb 204 pb 18 456 and 207 pb 204 pb 15 461 206 pb 204 pb 17 973 no evidence of lead isotope overprinting by the mineralizing event could be found in the samples studied when the lead isotope data are corrected for addition of radiogenic lead since the time of base metal mineralization c 350 ma ago there is a 1 1 correlation between the lead isotopic composition of galenas in the base metal deposits and the longford down grewyackes these results support the hypothesis that lead and other metals were derived from the greywackes underlying the base metal mineralization on a 208 pb 204 pb versus 206 pb 204 pb plot neither whole rock greywacke nor galena lead isotope data define a linear array the 208 pb 204 pb 206 pb 204 pb trend is best explained by tectonic or sedimentological mixing of three isotopically distinct lead isotope end members these end members are 1 old unradiogenic basement in nw ireland lewisian complex precursor 2 ordovician arc material from nw ireland and 3 ordovician arc material form se ireland isotopic modelling of these end members shows that the present day mixing line seen in the ordovician and silurian sediments can be calculated back to values inferred for the ordovician arc end members using the same values of as were used to back calculate the lead isotopic composition of the greywackes to the time of mineralization this information is further confirmation of the sediment mixing model presented a tectonic model developed from these and established geological data suggest a late ordovician closure of the iapetus ocean and that the silurian greywackes are a by product of eroded ordovician arc material from nw and se of the iapetus suture

cadomian lower crustal contributions to variscan granite petrogenesis south bohemian pluton austria a reply weinsberg type granite as it is doubtful whether it should preface be named according to ima rules in view of its very the comment by finger clemens 2002 nicely concomplex origin even if we look at these rocks in a firms the statement in the introduction of our paper purely descriptive sense neglecting all genetic aspects klotzli et al 2001 on p 1622 unravelling the origin the granitoids range in composition from minor quartz of a given granite still requires a major analytical effort monzodiorites to a majority of quartz monzonites and and commonly ends in some contradictory results finger granites according to the ima nomenclature the in clemens raise a number of issues both minor and terpretation of finger clemens 2002 that all the rocks major we will discuss all of them as far as they are are quartz monzodiorites is an oversimplification and relevant to the subject we will start with the problem conflicts with the geochemical composition of the pyrof nomenclature after that we will address the problem of oxene bearing rocks their k contents 4 6 wt k2o remnants of an old crustal component versus a cumulate and the constant high amount of modal k feldspar in origin and the problem of the age of the crustal remnants addition in all norm calculations the amount of normative k feldspar has to be corrected according to the measured preserved feldspar composition of or0 78 petrology ab0 20an0 01cs0 01 f koller 2000 finger clemens 2002 in their comment accept the opxand cpx bearing weinsberg type granitoid rocks formation of the weinsberg granite sensu lato magma are widespread in an area of 15 20 km surrounding under granulite facies conditions within the stability field the villages of sarleinsbach sprinzenstein and arnreit of opx as was already postulated by koller hock in the southern bohemian massif klotzli et al 2001 1993 finger clemens interpret opx and cpx as a figs 1 and 2 this type of rock shows a wide variation product of fractional crystallization of a quartz monzoin chemical and mineralogical composition the most dioritic magma after partial melting of biotite bearing conspicuous features are large k feldspars up to 20 cm lower crustal rocks consequently in their view the pyrin size as a consequence the determination of modal oxenes are cumulate phases in contrast we interpret mineralogy must take into account the large size of the the pyroxenes as part of an older magmatic event k feldspar megacrysts and be carried out on large samples only we referred to this rock as pyroxene bearing with a superimposed granulite facies re equilibration

itcz and enso like pacing of nile delta hydro geomorphology during the holocene abstract the nile valley accommodates the world s longest river and shaped the development of numerous complex societies providing a reliable source of water for farming and linking populations to sub saharan africa and the mediterranean sea its fertile delta lay at the heart of ancient egyptian civilization however little is known of its morpho sedimentary response to basin wide changes in holocene hydrology here we present two well resolved records from the nile delta based on 320 radiocarbon dates to reconstruct the timing and rhythm of catchment scale modifications during the past 8000 years on the orbital timescale we demonstrate that nilotic hydrology and sedimentation have responded to low latitude insolation forcing while on sub millennial timescales many of the major phases of deltaic modification were mediated by climate events linked to el nino southern oscillation type enso variability

stratigraphic context geochemical and isotopic properties of magmatism in the siluro devonian inliers of northern maine implications for the acadian orogeny this paper reports detailed stratigraphic analysis whole rock geochemistry and nd isotopic ratios of basalts from the northern maine inliers these data place constraints on first order controversies about the tectonic conditions leading up to and during the early stages of the acadian orogeny in northern maine late silurian and early devonian stratigraphic sequences indicate a progressive change from shallow water and subaerial exposure ripogenus and the forks formations east branch group to a rapidly subsiding basin prior to the onset of acadian deformation subsidence was accompanied by mafic to intermediate volcanism of the west branch spider lake and fish pond volcanics and that continued with intrusion of the syn deformational greenville plutonic belt including the flagstaff lake igneous complex whose properties we report in this paper trace element geochemistry indicates that magmatism is transdiscriminant showing aspects of within plate volcanic arc and back arc affinities and nd isotopic ratios are moderately positive 2 3 to 3 8 indicating either an uncontaminated and moderately enriched mantle source or a depleted mantle source contaminated by continental crust four possible tectonic models of the acadian orogeny and the siluro devonian sedimentary volcanic sequences of northern maine are evaluated in the context of a subsiding basin associated with this magmatism developed on the amalgamated laurentian plate these include 1 slab detachment during southeast directed subduction of the laurentian continental margin 2 laramide style thrust basins above a shallow northwest dipping subduction zone 3 back arc extension followed by thin skinned shortening above a northwest dipping subduction zone and 4 moluccan style dual dipping subduction zones

regional geochemical and isotopic variations of northern new england plutons implications for magma sources and for grenville and avalon basement terrane boundaries examination of 36 devonian to pennsylvanian plutons within and bordering the central maine terrane of north central new england shows that the plutons display regional variations in mineral and bulk rock compositions and in oxygen and sulfur isotopic values these variations principally correspond to the regional structural trends of the central maine terrane plutons located on that terrane9s southeast flank adjacent to the massabesic gneiss complex on the terrane9s northwest flank adjacent to the bronson hill anticlinorium and in the northeastern part of the northeast kingdom batholith of vermont have biotites with relatively low al concentrations 3 0 cations p f u per formula unit feldspars with higher an contents up to an 55 and lower 18 o 7 9 and higher 34 s average of 0 bulk rock values compared to plutons within the central parts of the central maine terrane these characteristics are most compatible with metaigneous or metavolcaniclastic sources as shown by others the plutons of vermont and northwestern maine preserve grenvillian signatures whereas those of southeastern new hampshire specifically the granite at milford new hampshire and maine sebago phillips have avalonian signatures within the interior of the central maine terrane of new hampshire and western maine and in the connecticut valley trough of vermont are distinct regions characterized by peraluminous plutons with high 18 o values 11 low 34 s values 25 al rich biotites 3 3 cations p f u and an poor plagioclases 30 these data are most consistent with metasedimentary sources or at a minimum the plutons were heavily contaminated by central maine terrane or connecticut valley trough metasedimentary rocks these peraluminous plutons do not have the unique signatures of the underlying precambrian basements several metaluminous plutons in the central maine terrane along the new hampshire maine border separate two regions of peraluminous plutons these metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the central maine terrane indicative of less paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the central maine terrane the metaluminous plutons also have trace element characteristics identical to those of plutons in vermont and northwestern maine that have a known grenville basement terrane source we suggest that the metaluminous plutons also had sources in the underlying grenville basement and that the grenville avalon boundary cuts diagonally across northern new england because the plutons with grenvillian signatures are adjacent to plutons with avalonian signatures the sebago batholith and phillips pluton we suggest that there is no medial new england basement in this region of new england some peraluminous two mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks the components of which were deposited in an anoxic basin such as the smalls falls formation of the central maine terrane but many other plutons have compositions that are inconsistent with this model we suggest that the compositions of many new hampshire plutonic suite plutons indicate greater heat input than that available from u enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust

the effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis a model abstract the initial pb isotopic composition of anatectic granites is modelled as a mixture of pb derived from major minerals that have characteristically low 238 u 204 pb and 232 th 204 pb ratios i e feldspars biopyroboles etc with pb contributed from dissolved accessory minerals with characteristically high 238 u 204 pb and 232 th 204 pb ratios i e zircon monazite etc the initial pb isotopic composition of anatectic granites is a function of a the age type modal distribution and heterogeneity in the initial u and th content of the accessory minerals b variation in melt composition and temperature during anatexis c the fraction of the source melted and d the extent to which the melt was homogenized prior to crystallization the differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the pb isotopes from other chemical and isotopic systems in granite rocks granitic plutons derived by anatexis of the same source material are not required to have the same initial pb isotopic composition low temperature peraluminous granites are more likely to exhibit homogeneous initial pb isotopic compositions dominated by the pb contained within the major mineral assemblage of the source material in contrast the initial pb isotopic composition of higher temperature metaluminous or peralkaline anatectic granites is more likely to be heterogeneous unless the melts are homogenized during emplacement and will record a larger fraction of the radiogenic pb component contained in the accessory mineral assemblage of the source material heterogeneity in the initial pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis the use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites

the crustal structure in central maine from coherency processed refraction data in 1984 the u s geological surcvey and the canadian department of energy mines and resources conducted an extensive seismic refraction experiment the quebec maine transect project as part of this project a 180 km long seismic refraction profile was recorded along the axis of the central maine synclinorium high near surface velocities of 6 0 6 2 km s made a conventional refraction analysis including first arrivals uninformative below about 10 km peak signal frequencies around 30 hz recorded at all offsets and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections to allow a reliable deep structural interpretation a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals in addition interpretation of a coincident wide angle reflection study provided constraints on the deep crustal velocities and structure the crust beneath the central maine synclinorium can be described by three major structural units the upper crust is interpreted as the metasedimentary rocks of the central maine synclinorium with velocities that range from 6 2 to 6 3 km s regions of relative lower velocity 6 0 6 1 km s within the upper crust are interpreted as granitic intrusions low velocity regions modeled at 2 7 km depth with velocities around 5 4 km s are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence at 12 15 km depth the base of the synclinorium appears alternately as a strong reflector indicating the presence of layering and also as a weak reflector especially ne of the sebago pluton a 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6 4 to 6 8 km s the crust thickens from 36 to 40 km going from refraction experiment the quebec maine transect project as part of this project a 180 km long seismic refraction profile was recorded along the axis of the central maine synclinorium high near surf ace velocities of 6 0 6 2 km s made a conventional refraction analysis including first arrivals uninformative below about 10 km peak signal frequencies around 30 hz recorded at all offsets and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections to allow a reliable deep structural interpretation a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals in addition interpretation of a coincident wide angle reflection study provided constraints on the deep crustal velocities and structure the crust beneath the central maine synclinorium can be described by three major structural units the upper crust is interpreted as the metasedimentary rocks of the central maine synclinorium with velocities that range from 6 2 to 6 3 km s regions of relative lower velocity 6 0 6 1 km s within the upper crust are interpreted as granitic intrusions low velocity regions modeled at 2 7 km depth with velocities around 5 4 km s are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence at 12 15 km depth the base of the synclinorium appears alternately as a strong reflector indicating the presence of layering and also as a weak reflector especially ne of the sebago pluton a 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6 4 to 6 8 km s the crust thickens from 36 to 40 km going from the ne to the sw end of the profile surface rocks at the sw end of the profile have a metamorphic depth of burial of at least 15 km indicating that the crust below the central maine synclinorium may have been thickened to at least 55 km during paleozoic collisions

lead isotopic fingerprinting of tectono stratigraphic terranes east central alaska common lead isotopic compositions have been determined on feldspars from meta igneous rocks from nine tectono stratigraphic terranes or subterranes in east central alaska most of the terranes have distinct and well defined signatures in terms of isotopic composition thus most can be distinguished on conventional lead isotopic diagrams lead isotopic ratios provide evidence for 1 possible sources for the igneous rocks 2 time of metamorphism 3 correlation of terrane fragments and 4 delineation of juxtaposed terranes determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono stratigraphic terranes as to mantle or crustal origin and in correlation particularly where terrane relationships are enigmatic

microdistribution of boron particle track images from samples of a caledonian igneous complex 7li recoil tracks in lexanr formed as a product of irradiation with thermal neutrons give an image of in sifu microdistribution of boron in dioritic to granitic rocks of the caledonian ballachulish zoned complex and in a carbonate rock from its contact aureole the images exhibit a polyphase incorporation of boron into primary and secondary minerals the boron microdistribution is inhomogeneous isomorphous incorporation of boron into pyroxene and plagioclase during the main stage of crystallization cannot be established because the boron distribution is blurred by secondary alteration in contrast to the intracrystalline feature the crystal interfaces are enriched in boron increased concentrations are also disclosed by overgrowths of primary crystals exsolution lamellae in alkali feldspar and alteration rims of clinopyroxene the highest amounts of boron in a siliceous dolomite marble of the contact aureole are found in the alteration products of olivine

evidence for progressive holocene aridification in southern africa recorded in namibian hyrax middens implications for african monsoon dynamics and the african humid period abstract presented here are stable nitrogen isotope data from a rock hyrax procavia capensis middens from northwestern namibia that record a series of rapid aridification events beginning at ca 3800 cal yr bp and which mark a progressive decrease in regional humidity across the holocene strong correlations exist between this record and other terrestrial and marine archives from southern africa indicating that the observed pattern of climate change is regionally coherent combined these data indicate hemispheric synchrony in tropical african climate change during the holocene with similar trends characterising the termination of the african humid period ahp in both the northern and southern tropics these findings run counter to the widely accepted model of direct low latitude insolation forcing which requires an anti phase relationship to exist between the hemispheres the combined dataset highlights 1 the importance of forcing mechanisms influencing the high northern latitudes in effecting low latitude climate change in africa and 2 the potential importance of solar forcing and variations in the earth s geomagnetic shield in determining both long term and rapid centennial scale climate changes identifying a possible mechanism for the variations marking the ahp termination in both the southern and northern tropics

monzonitic series from the variscan tormes dome central iberian zone petrogenetic evolution from monzogabbro to granite magmas abstract in the iberian massif rocks of the k rich plutonic series are not abundant but towards internal parts of the belt represented by the central iberian zone there are some sectors where shoshonitic plutonism occurs over broad areas one of these areas is the anatectic tormes dome encompassing two similar studied plutons pereruela and vitigudino a monzonitic association has been defined ranging from monzogabbros to quartz monzonites or scarce monzogranites enrichment in lree p sr and ba a high water content up to 5 5 and a high degree of oxidation ni nio buffer are the main features of magmas parental to these monzonitic rocks these petrographic and geochemical features allow these granitoids to be ascribed to the shoshonitic type sh type rather than to the i type the minimum emplacement pressure range is 410 230 mpa whereas the estimated solidus temperature range is 940 765 c i e above water saturated solidus o sr and nd isotopes point to open system processes apatite cathodoluminescence suggests that magma mixing was unlikely to have occurred for the most enriched rocks assimilation fractional crystallisation afc modelling was performed for both plutons permitting assimilation crystallization rates to be estimated between 0 16 and 0 25 different contaminants have been inferred a metapelite at upper crustal level for the vitigudino pluton and a granulitic orthogneiss for the pereruela pluton a liquid line of descent linking monzogabbroic members to quartz monzonites monzogranites can be reconstructed some loss of water and oxygen can be inferred although water remained in the system below the solidus giving rise to auto metamorphism at ca 500 c on the other hand afc processes are unlikely to have been the main factor in controlling the characteristic enrichment of the monzonitic series since the least contaminated samples are the most lile and lree enriched for both plutons thus a source controlled chemical signature can be inferred the experimental data indicate that a hybrid protholith a mixture of peridotite amphibolite and metapelite can account for the enrichment and hydrated nature of the least evolved monzogabbroic magmas late orogenic slab break off and subsequent post collisional extensional events appear to be a suitable scenario to provide the heat that triggered partial melting close to the crust mantle boundary

emsian synorogenic paleogeography of the maine appalachians abstract the acadian deformation front in the northern appalachians of maine and new hampshire can now be closely located during the early emsian early devonian 408 406 ma tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408 ma time scale tie point for the early emsian the deformation front lay between a belt of lower devonian flysch and molasse that were deposited in an acadian foreland basin and had not yet been folded and a belt of early emsian plutons that intruded folded lower devonian rocks this plutonic belt includes the newly dated ore mountain gabbro u pb 406 ma which hosts magmatic sulfide mineralization along the deformation front a 407 ma pluton that locally truncates acadian folds katahdin was the feeder to volcanic rocks traveler rhyolite 406 407 ma that are part of the foreland basin succession involved in these same folds the emsian igneous rocks thus define a syncollisional magmatic province that straddled the d

lead isotopic heterogeneities within alkali feldspars implications for the determination of initial lead isotopic compositions abstract the mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks the problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which because of its low u pb ratio should have a lead isotopic composition similar to the initial lead isotopic composition of the rock previous leaching and volatilization experiments however have demonstrated that alkali feldspars contain radio genic lead which can be removed preferentially to nonradiogenic lead this paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst this experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar loosely bound labile lead presumably associated with uranium and thorium occupying fractures and finally a radiogenic component corresponding to unidentified uranium thorium rich phases the commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir the results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching however this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars the determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars

the petrogenesis of the highlandcroft and oliverian plutonic suites new hampshire implications for the structure of the bronson hill terrane both laurentian and peri gondwanan origins for the oliverian and highlandcroft plutonic suites of new hampshire have been proposed a peri gondwanan origin places the red indian line the principle iapetus suture to the west of the bronson hill anticlinorium and requires the plutons to be allochthonous in contrast a laurentian margin emplacement setting places the red indian line farther to the east in new hampshire the combination of e nd and 207 pb 204 pb values are most compatible with the suites representing mixtures of arc derived magmas and laurentian crust indicating that both suites were derived to the west of the red indian line the tectonic setting for the arc probably was a westerly dipping subduction zone under the laurentian margin resulting from a polarity flip after the collision of the shelburne falls arc during the taconic orogeny bulk rock major and trace elements as well as nd sr and pb isotopic results indicate that the highlandcroft and oliverian plutonic suites of new hampshire are both members of the same arc complex

climatic change recorded in the sediments of the chew bahir basin southern ethiopia during the last 45 000 years abstract east african paleoenvironments are highly variable marked by extreme fluctuations in moisture availability which has far reaching implications for the origin evolution and dispersal of homo sapiens in and beyond the region this paper presents results from a pilot core from the chew bahir basin in southern ethiopia that records the climatic history of the past 45 ka with emphasis on the african humid period ahp 15 5 ka calbp geochemical physical and biological indicators show that chew bahir responded to climatic fluctuations on millennial to centennial timescales and to the precessional cycle since the last glacial maximum potassium content of the sediment appears to be a reliable proxy for aridity showing that chew bahir reacted to the insolation controlled humidity increase of the ahp with a remarkably abrupt onset and a gradual termination framing a sharply defined arid phase 12 8 11 6 ka calbp corresponding to the younger dryas chronozone the chew bahir record correlates well with low and high latitude paleoclimate records demonstrating that the site responded to regional and global climate changes

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

climatic patterns in equatorial and southern africa from 30 000 to 10 000 years ago reconstructed from terrestrial and near shore proxy data abstract as part of a wider study of last glacial and deglacial climates in the southern hemisphere continents we here review terrestrial and near shore marine records from equatorial and southern africa between 30 000 and 10 000 years ago 30 10 ka this time interval covers the lead up to the last glacial maximum lgm 21 2 ka the lgm proper and the ensuing deglacial records selected for review needed to meet three requirements continuity or near continuity over the period a well established chronology and at least one but preferably several unambiguous proxy ies we aim to show how regional climates of the sub continent have responded to orbital forcing as opposed to other global glacial interglacial boundary conditions and how they are related to high latitude climates sea and land surface conditions positions of the intertropical convergence zone itcz and of the westerly belt evidence of past climates derived from many independent proxies is given from west to southwest africa moisture from the atlantic ocean then from equatorial east africa to the southern subtropical summer rainfall domain moisture mainly from the indian ocean the lgm was cooler than today and generally drier in the tropics north of 8 9 s glacial to holocene increase in monsoonal precipitation primarily related to orbitally induced summer insolation in the northern hemisphere occurred by steps of increasing amplitude 17 16 14 5 11 5 ka major wet dry spells coincide with abrupt warm cold events in high northern latitudes and related itcz migrations in the southern tropics the main post glacial increase in tropical rainfall generally appears more gradual and in phase with antarctic warming data suggest a restricted northward migration of the itcz and concentration of tropical rainfall well south of the equator during the lgm and the younger dryas drier glacial conditions prevailed in southeastern africa while parts of southwestern africa point to enhanced humidity during the lgm suggesting that the winter westerly belt was either stronger than today or displaced further north possibly as a result of more extensive antarctic sea ice inferred african climatic fluctuations show the competing influences of tropical and high latitude climates of both hemispheres and suggest changes in both meridional and zonal circulation modes this review also reveals major geographical and methodological gaps and a number of unresolved issues providing pointers for future research

climatic instability in west equatorial africa during the mid and late holocene abstract millennial scale climatic variations have punctuated the holocene characterised by abrupt changes from warm to cool or wetter to drier conditions amongst these climatic events there is increased evidence for an abrupt multicentennial shift of climatic conditions around 3 8 3 7 kyr bp 4 1 cal kyr bp in mid to low latitude regions which had a profound impact on landscape and population migration in the mediterranean region subtropical tropical and equatorial africa a number of continental proxies lake levels pollen sequences stable isotopes record this abrupt change towards drier conditions however regionalism in climatic conditions is reflected in the vegetation records possibly in relation to orographic conditions and the influence of sea surface conditions hitherto there have been very few marine sequences that record this particular climatic shift at high resolution we present here new data from the congo deep sea fan containing integrated marine and terrestrial proxies around 5 4 cal kyr bp shifts in surface conditions off the congo river mouth are observed with possible establishment of seasonal coastal upwelling and lower sea surface temperatures in parallel pollen data indicate fluctuations of herbaceous afromontane taxa and charred grass cuticles suggesting more open vegetation in the lowland regions and an increase in cloud forest and or afromontane vegetation at higher altitudes within the congolese region

lead isotopic evidence for distinct sources of granite and for distinct basements in the northern appalachians maine lead isotopic compositions of feldspars in plutons of the coastal lithotectonic block avalonian basement central maine merrimack synclinorium and connecticut valley gaspe synclinorium in northern maine granville basement indicate the presence of three fundamentally different sources of devonian granitic rocks in maine plutons in the coastal lithotectonic block have the most radiogenic values 206 pb 204 pb 18 25 19 25 207 pb 204 pb 15 59 15 67 208 pb 204 pb 38 00 38 60 plutons in northern maine are the least radiogenic 206 pb 204 pb 18 00 18 50 207 pb 204 pb 15 51 15 55 and 208 pb 204 pb 37 80 38 38 intermediate lead isotope values characterize the plutons in central maine 206 pb 204 pb 18 20 18 40 207 pb 204 pb 15 56 15 60 and 208 pb 204 pb 38 00 38 30 all plutons show relatively radiogenic lead values for their ages and suggest the imprint of continental crustal sources especially in the coastal block and in central maine plutons from the connecticut valley gaspe synclinorium allow for a more significant isotopic input from the subcontinental mantle the three distinct lead isotope groups probably identify plutons formed in different crustal fragments in a continental collisional environment that were juxtaposed after emplacement of the granites

age of the ballachulish and glencoe igneous complexes scottish highlands and paragenesis of zircon monazite and baddeleyite in the ballachulish aureole u pb zircon ages are presented for the ballachulish igneous complex 207 pb 206 pb age 427 1 ma 206 pb 238 u age 423 0 3 ma and glencoe volcanic complex 207 pb 206 pb age 406 6 ma of the scottish highlands these ages are significantly more precise than pre existing age constraints and discriminate a previously unresolved age difference of c 20 ma between the two complexes this difference in age provides an explanation for the c 10 km difference in crustal level between the two magmatic events and constrains exhumation rates for the argyll region to be on average c 0 4 km ma 1 over the period c 425 405 ma with improved age constraints on the ballachulish igneous complex in place the associated metamorphic aureole is used as a type locality to investigate the metamorphic behaviour of the u bearing accessory minerals baddeleyite zircon and monazite baddeleyite formed in impure dolomites by the reaction of detrital zircon with dolomite and gives the same u pb age as the intrusive complex consistent with its contact metamorphic origin detrital zircon appears to be inert throughout much of the metamorphic aureole with contact metamorphic zircon growth being restricted to migmatite grade rocks a possible exception is the development of enigmatic very narrow overgrowths at temperatures between c 500 and 600 c monazite exhibits a variety of textures in lower grade parts of the aureole

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