palaeomagnetism and magnetic anisotropy of proterozoic banded iron formations and iron ores of the hamersley basin western australia abstract rock magnetic properties and palaeomagnetism of weakly metamorphosed banded iron formations bifs of the palaeoproterozoic hamersley group western australia and proterozoic bif derived iron ores have been investigated the bif units sampled here are slightly younger than 2500 ma at paraburdoo mount tom price and mount newman iron ore formation was completed before 1850 ma sampling was mainly from the mount tom price and paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh unweathered and unaltered bif samples has allowed the nature of the remanence of the bifs to be defined the remanence of the bifs is carried by late diagenetic low grade metamorphic magnetite after primary haematite this remanence is pre folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low determination of palaeofield directions from measured remanence directions is complicated by self demagnetization effects in strongly magnetic highly anisotropic bif specimens we present a method for correcting measured directions for the effects of self demagnetization and anisotropy for typical bifs the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes anisotropy also causes cones of confidence to be underestimated due to compression of the range of inclinations in principle deflection of post folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn folding signature with best agreement between directions from different fold limbs after partial unfolding thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors but can also mislead interpretation of the time of remanence acquisition the anisotropy of anhysteretic remanent magnetization arm probably yields an upper limit to the anisotropy of the chemical remanent magnetization crm carried by the bifs therefore from the anisotropy of arm a maximum inclination deflection of 9 is suggested for the sampled bifs this corresponds to less than 5 change of palaeolatitude the palaeomagnetic pole position calculated for bifs at paraburdoo is 40 9 s 225 0 e dp 2 9 dm 5 8 after tilt correction but without correction for anisotropy other pole positions reported include that from flat lying bifs from wittenoom at 36 4 s 218 9 e dp 4 6 dm 9 1 from mount tom price iron ore at 37 4 s 220 3 e dp 5 7 dm 11 3 and from paraburdoo ore at 36 4 s 209 9 e dp 4 7 dm 8 8 the poles from the bifs the paraburdoo ore and the part of the tom price deposit that was sampled in this study are indistinguishable from each other and from the mount jope volcanics overprint pole the magnetization of the bifs was probably acquired during burial metamorphism of the hamersley group soon before the main folding and uplift event in the southern part of the hamersley province this tectonic event exposed magnetite rich bifs to near surface oxidizing conditions producing extensive martite goethite orebodies and also appears to have produced the syn folding overprint magnetization recorded by the mount jope volcanics of the underlying fortescue group ages of magnetization are tentatively interpreted as 2200 100 ma for the bifs 2000 100 ma for the supergene enrichment of bif to martite goethite ore recorded by the parabudoo and mount tom price orebodies and 1950 100 ma for the metamorphic martite microplaty haematite ore recorded as an overprint by the tom price orebody and as the only surviving magnetization of the mount newman orebody

the introduction of neolithic resources to north africa a discussion in light of the holocene research between egypt and libya the dynamics affecting coastal areas of north africa especially the rise of neolithic economies remained largely unexplored until a few years ago there are no definitive answers for questions about how groups belonging to a very well documented late palaeolithic tradition developed locally and if and how these changes resulted from outside influences that may have affected the region during the middle and late holocene this is especially true of the provenance and success of the main domesticates in other words the plant wheat barley pulses and animal species sheep goat cattle pigs which constitute the north african neolithic complex the obvious intrusion of the domesticates from outside has prompted various authors to seek their exotic locations but reasons and circumstances that led to the displacement of groups were rarely discussed in trying to reconstruct modalities and routes of diffusion to understand and study these problems a trans regional perspective including north africa and the levant seems necessary but is very rarely adopted this paper tries to answer these questions starting from the data on the central eastern portion of mediterranean north africa egypt and libya which are closer to the levantine territories in particular it focuses on what has recently emerged from important investigations in egypt western desert and northwest libya jebel gharbi and jefara plain through a comparative approach combining chronology the characteristics of plant and animal resources used and technology both of these areas may help to understand the ways and paths followed in the introduction and propagation of food production towards northeastern africa in an attempt to trace the routes followed in the transfer from the levantine towards the western regions the paper deals also with the main theoretical and problematic issues related to the domestication of plants and animals in the near east it shows that contrary to past claims the domestication of plants and animals does not seem to have taken a short time and that instead a protracted process of domestication seems to be the more realistic model therefore the search for a core area where everything must have begun seems really complicated finally it reconsiders some specific comparisons between levant and north africa chronologically and geographically plausible that have been suggested for some time they concern the material culture repertoires within which the importance of the main component lithic production is obvious

prefolding and overprint magnetic signatures in precambrian 2 9 2 7 ga igneous rocks from the pilbara craton and hamersley basin nw australia a total of 446 oriented samples have been collected from an archaean intrusion and the fortescue group volcanics and intrusions in the pilbara and hamersley regions of northwestern australia the paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field af and thermal demagnetization the most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions a prefolding pole position from the millindinna complex is latitude 11 9 s longitude 161 3 e dp 6 8 dm 8 4 and is correlated with a radiometric age of 2 86 0 02 ga folding in the millindinna complex occurred prior to the deposition of the fortescue group new geochronological data indicate that the extrusion of the flood basalts commenced about 2 8 ga this provides a minimum age for the magnetization of the millindinna complex deformation responsible for the formation of the mobile belt separating the pilbara and yilgarn blocks during the ophthalmian orogeny has affected the fortescue group and produced a moderate degree of folding in the southern part of the hamersley basin in the region of millindinna complex outcrop the fortescue group is generally flat lying prefolding pole positions from the fortescue group are for the mount roe basalt latitude 52 4 s longitude 178 0 e dp 6 4 dm 9 1 and for the mount jope volcanics latitude 40 5 s longitude 128 7 e dp 19 9 dm 20 8 the overprint magnetization in the mount jope volcanics yields a pole position latitude 35 0 s longitude 211 5 e dp 2 8 dm 5 5 and is interpreted as having been acquired during folding relating to the formation of the ophthalmian fold belt

evidence for a 3 45 billion year old magnetic remanence hints of an ancient geodynamo from conglomerates of south africa 1 paleomagnetic and rock magnetic analyses of 3445 million year old dacite conglomerate clasts and parent body rocks from the barberton greenstone belt south africa define two dominant components of magnetization one component unblocked at low temperature is an overprint acquired 180 million years ago the other component is unblocked at high temperatures and passes a conglomerate test indicating that this component is older than the depositional age of the conglomerate 3416 ma the high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes archean overprinting and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations alternatively this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo including magnetization by an external field related to solar wind interaction with the atmosphere such exotic mechanisms can be tested with the acquisition of paleointensity data while more scattered than paleomagnetic data recording the more recent geomagnetic field the high unblocking temperature component in the dacite parent body shows some consistency and the simplest explanation of the data is that they reflect a geodynamo 3445 million years ago

volcanic degassing hydrothermal circulation and the flourishing of early life on earth a review of the evidence from c 3490 3240 ma rocks of the pilbara supergroup pilbara craton western australia abstract new data gathered during mapping of c 3490 3240 ma rocks of the pilbara supergroup in the pilbara craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism for most cherts fossil hydrothermal fluid pathways are preserved as silica barite fe bearing veins that cut through the footwall and up to the level of individual bedded chert units but not above indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events silica barite fe bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units and occurred under alternating high sulphidation and low sulphidation conditions these veins provided pathways to the surface for elements leached from the footwall e g si ba fe and volcanogenic emissions from underlying felsic magma chambers e g co 2 h 2 s hs so 2 stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of warrawoona group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and or degassing of these magmas heat from these intrusions drove episodes of hydrothermal circulation listric normal faulting during caldera collapse produced basins with restricted circulation of seawater eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components thereby providing habitats suitable for early life forms fossil stromatolites from two distinct stratigraphic units in the north pole dome grew in shallow water conditions but in two very different geological settings with different morphologies stratiform and domical stromatolites in the stratigraphically lower c 3490 ma dresser formation of the warrawoona group are intimately associated with barite and chert precipitates from hydrothermal vents suggesting that component microbes may have been chemoautotrophic hyperthermophiles evidence of shallow water to periodically exposed conditions active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water tectonically active caldera and concentrated therein to produce an extreme habitat for early life widespread conical and pseudocolumnar stromatolites in the c 3400 ma strelley pool chert at the base of the unconformably overlying kelly group occur in shallow marine platform carbonates silicification was the result of later hydrothermal circulation driven by heat from the overlying newly erupted euro basalt the markedly different morphology and geological setting of these only slightly younger stromatolites compared with the dresser formation suggests a diversity of microbial life on early earth the biogenicity of putative microfossils from this and younger hydrothermal silica veins in the warrawoona group remains controversial and requires further detailed study

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

records of geomagnetism climate and tectonics across a paleoarchean erosion surface abstract paleomagnetism has provided key constraints on the evolution of earth s climate geomagnetic field and continental geography through phanerozoic and proterozoic time extending these constraints into the archean eon has been particularly challenging due to the paucity of the ancient rock record here we report paleomagnetic measurements on the nasa astrobiology drilling project abdp 8 core drilled through one of the world s least deformed and least metamorphosed paleoarchean 3200 3600 million year old ma rock successions located in the east strelley belt of the eastern pilbara craton australia our results show that the 3350 ma euro basalt preserves a shallow magnetic inclination that appears to have formed as a result of early seafloor hydrothermal alteration suggesting that the evaporitic carbonate platform of the conformably underlying strelley pool formation was deposited in a near equatorial location this is consistent with although does not require late paleoarchean climatic zoning low orbital obliquity and a geocentric axial dipole gad field geometry similar to that of the phanerozoic the euro basalt paleopole overlaps with previously published paleoarchean poles from the east pilbara craton and with time equivalent poles reported from the barberton greenstone belt of the kaapvaal craton supporting the existence of a paleoarchean vaalbara continental aggregation

resource pressure and environmental change on the north african littoral epipalaeolithic to roman gastropods from cyrenaica libya abstract this paper discusses the marine and terrestrial shell on epipalaeolithic to classical period sites in the cyrenaican coastlands northeast libya with particular reference to the haua fteah with parallel studies at a late roman farmstead and two small caves together they provide evidence for coastal and terrestrial environments and for the continued nutritional importance of gastropods to humans during the holocene land snail evidence is consistent with regional vegetation in coastal cyrenaica becoming increasingly open through the holocene as a result of some combination of climate change and human impact marine species suggest that the coastline near the haua had been rocky throughout the holocene at hagfet al gama changing faunas provide evidence for sand encroachment onto a previously rocky shoreline in hellenistic times a biometric study of osilinus turbinatus shows that in the archaeological sites these shells are systematically smaller than modern specimens providing evidence for long term dietary stress in the human populations around the haua fteah with particularly severe stress in parts of the epipalaeolithic a biometric study of patella spp provided evidence for size selection but also seems to show evidence for resource pressure it is unlikely that variations in resource pressure seen in the mollusc biometrics are the result of climatic stress or natural ecological factors and explanations must be sought in society environment dynamics

oxygen and hydrogen isotope evidence for a temperate climate 3 42 billion years ago the widely held view that the archaean climate around 3 5 billion years ago was remarkably warm with ocean temperatures perhaps as high as 80 c has been questioned on the grounds that the established method of estimating ancient ocean temperature from the oxygen isotope ratios of sedimentary deposits is subject to significant uncertainty michael hren et al have adopted a different approach to estimating ocean temperature based on an analysis of both oxygen and hydrogen isotopes of 3 4 billion year old buck reef chert sediments from south africa the isotopes sampled are consistent with formation in waters no warmer than about 40 c suggesting that earth s early oceans may have been far cooler than previously thought the study of stable oxygen isotope ratios 18o of precambrian cherts suggests that ocean temperatures during the archaean era about 3 5 billion years ago were between 55 c and 85 c but uncertainty about the 18o of the primitive ocean has led to considerable debate regarding this conclusion here a combined analysis of oxygen and hydrogen istopes sampled from 3 42 billion year old buck reef chert rocks in south africa indicates that the ancient ocean was much cooler than previously thought stable oxygen isotope ratios 18o of precambrian cherts have been used to establish much of our understanding of the early climate history of earth1 2 3 and suggest that ocean temperatures during the archaean era 3 5 billion years ago were between 55 c and 85 c ref 2 but because of uncertainty in the 18o of the primitive ocean there is considerable debate regarding this conclusion examination of modern and ancient cherts indicates that another approach using a combined analysis of 18o and hydrogen isotopes d rather than 18o alone can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters4 5 here we show that 18o and d sampled from 3 42 billion year old buck reef chert rocks in south africa are consistent with formation from waters at varied low temperatures the most 18o enriched buck reef chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below 40 c geochemical and sedimentary evidence suggests that the buck reef chert was formed in shallow to deep marine conditions so our results indicate that the palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler 40 c than previously thought2

restricted shallow water sedimentation of early archean stromatolitic and evaporitic strata of the strelley pool chert pilbara block western australia abstract the 3 4 ga old strelley pool chert is a 25 m thick sedimentary unit near the top of the predominantly volcanic warrawoona group in greenstone belts of the eastern pilbara block western australia it is here subdivided into 5 members containing 13 lithofacies the basal member i is composed of quartzose sandstone deposited in a high energy wave or tide dominated shallow water system overlying this are members ii and iii which make up the bulk of the formation and were deposited in a low energy partially restricted hypersaline basin they record a predominantly regressive succesion of deposits including subaqueous laminite stromatolite and evaporite stromatolite carbonaceous laminite black and white banded chert evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions and subaerially deposited windblown sand evaporite and evaporite solution layers members iv and v record the progradation of a volcaniclastic alluvial fringe the strelley pool chert represents an association of sedimentary environments directly comparable to that observed in modern low energy shallow marine carbonate evaporite systems such as along the trucial coast of the persian gulf and abundantly developed in phanerozoic carbonate platform deposits there is no evidence however that uniquely identifies the environment as having been marine deposition may have taken place in either a large hypersaline lake or a restricted marine basin evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes silicified evaporites including coarsely crystalline layers resembling messinian selenite are widespread and similar to younger evaporite deposits they clearly indicate that evaporites were common within shallow water archean sequences the presence of an assemblage of biogenic deposits including organic laminite stromatolites encrusting carbonaceous mats carbonaceous granules and oncolites deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic points to the existence of an ecologically and probably biologically diverse microbial community 3 4 ga ago

the south african polymict eucrite macibini the polymict eucrite macibini is a fragmental breccia predominantly composed of eucritic materials with minor proportions maximum 2 vol of diogenitic material hence it is intermediate between the yamato 74159 type polymict eucrites which contain negligible amounts of magnesian orthopyroxene and the howardites the present study provides mineralogical and bulk compositional data for the meteorite breccia and for six clasts these clasts include both volcanic and igneous rocks and a variety of impact generated rocks a broad range of degrees of postcrystallization metamorphism affected these materials before the final aggregation of the breccia clast a is a fragment of unequilibrated eucrite with subophitic texture the edges of the zoned pyroxenes in this clast are composed of a host of fe rich augite containing vermicules blebs and lamellae composed of a mixture of fe rich olivine and silica similar features occur as fragments in lunar breccias and are attributed by some workers to the breakdown of pyroxferroite an fe rich pyroxenoid however textures and compositions of these augite olivine silica intergrowths in clast a suggest that in this case they are the result of decomposition in a series of steps of fe rich subcalcic augite among the fragments of impact generated material in macibini is clast 2 an earlier formed clastic breccia that was lithified before being broken apart and included in the meteorite breccia clast 3 is an impact melt breccia that is composed of rock and mineral fragments in a devitrified groundmass clast c is also an impact melt breccia that has a coarser grained hornfelsic groundmass that resulted from extensive metamorphism after formation

lightning remagnetization of the vredefort impact crater no evidence for impact generated magnetic fields 1 the vredefort impact crater in south africa is one of the oldest and largest craters on earth making it a unique analog for planetary basins intense and randomly oriented remanent magnetization observed in surface samples at vredefort has been attributed to impact generated magnetic fields this possibility has major implications for extraterrestrial paleomagnetism since impact generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the moon and asteroids furthermore the presence of single domain magnetite found along shock generated planar deformation features in vredefort granites has been widely attributed to the 2 02 ga impact event an alternative hypothesis is that the unusual magnetization and or rock magnetic properties of vredefort rocks are the products of recent lightning strikes lightning and impact generated fields can be distinguished by measuring samples collected from below the present surface here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes we show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact while random intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning because vredefort is the only terrestrial crater that has been proposed to contain records of impact generated fields this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos

northwest libya from the early to late holocene new data on environment and subsistence from the jebel gharbi abstract the area of jebel gharbi west of cyrenaica province and the research area of the italian libyan joint project sapienza university of rome and cassino showed a continuity of occupation even during the most difficult periods of the last glaciation marked by the abandonment of much of north africa one of the main research subjects of the project concerns the transformation of hunting societies during the holocene with the gradual transition to a food production economy and the type of neolithic organization on the coastal libyan territory this paper emphasizes this theme and takes into account especially the areas that have allowed for palaeoclimatic and geo archaeological reconstruction in a broader perspective including changes in the settlement pattern and the economic system these conditions were recognized in the jefara the plain which begins at the slopes of the jebel gharbi plateau and reaches the coast i describe the principal characteristics of the sites investigated along a belt that runs from shaksh k to el jawsh where a long period of climate stability in the mid holocene and particularly between 6 7 and 5 4 ka cal bp was recognized through the presence of large ponds and marshes that became an important focus for human occupation taking into account the palaeoenvironmental context the paper suggests a model of economic change in the jebel gharbi jefara region through three different phases of development jefara a b c that can be compared with the most recent results of research in other areas of libya and the maghreb

site formation processes in caves the holocene sediments of the haua fteah cyrenaica libya caves have yielded some of the most globally important archaeological sequences but often their interpretation has suffered from assumptions about cave sedimentary processes caves contain distinctive sedimentary environments this has major implications for the understanding of contained archaeological materials this paper describes and analyses the holocene sediments in the haua fteah a sequence regarded as essentially continuous by the original excavator 50 years after it was first excavated the haua s epipalaeolithic to post classical chronological range and rich finds make it still the key holocene archaeological site in north africa the reassessment shows however that the sequence is strongly discontinuous and this has major implications for the reinterpretation of the site as the highly resolved archaeological record is thus likely to reflect a series of brief occupations rather than continuous human activity as with many caves the sedimentary record in the haua fteah is an extremely sensitive indicator of environments and processes in the wider landscape secure understanding of sedimentary process from analysis of the highly individual records found in caves is essential for full understanding of their contained archaeology

holocene paleoenvironmental paleoclimatic and geoarchaeological significance of the sheikh el obeiyid area farafra oasis egypt abstract the present paper focuses on the combination of archaeological and geological data sets in order to obtain a synthesis of holocene occupational and environmental history in the sheikh el obeiyid region of the farafra depression egypt in this region the epipalaeolithic and saharan pastoral neolithic sites are distributed along different landscape elements of the sheikh el obeiyid plateau escarpment and bir el obeiyid depression respectively three phases of playa formation were recorded at the bir el obeiyid basin and at the top of the sheikh obeiyid plateau corresponding to humid conditions during the early mid holocene the 14c dates obtained from the excavated hearths at sheikh el obeiyid indicate the presence of three occupation phases confirmed by the different characteristics of the lithic assemblages and playa generations pi pii and piii early holocene occupation is associated with lithic artefacts of high blade and microlithic index and backed retouched tools the middle holocene is represented by two occupations the early and late middle holocene the former is linked with bifacial knives gouges discoidal side scrapers tools with foliated retouch and lens shaped arrowheads the last horizon points to a final middle holocene exploitation phase of the area probably by small groups of herders who periodically visited the region when the climate had already started to deteriorate the sheikh el obeiyid village lies on the second erosion surface of the northern plateau it is characterised by 30 circular or elliptical structures of various sizes grouped together and located near the edge of the terrace commanding the bir el obeiyid playa below radiocarbon dating of charcoal recovered from the structures indicates that the village was used between 7930 123 and 7930 144 cal bp

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

holocene climate variability and cultural evolution in the near east from the dead sea sedimentary record abstract a comprehensive record of lake level changes in the dead sea has been reconstructed using multiple well dated sediment cores recovered from the dead sea shore interpreting the lake level changes as monitors of precipitation in the dead sea drainage area and the regional eastern mediterranean palaeoclimate we document the presence of two major wet phases 10 8 6 and 5 6 3 5 cal kyr bp and multiple abrupt arid events during the holocene the arid events in the holocene dead sea appear to coincide with major breaks in the near east cultural evolution at 8 6 8 2 4 2 3 5 cal kyr bp wetter periods are marked by the enlargement of smaller settlements and growth of farming communities in desert regions suggesting a parallelism between climate and near east cultural development

geology and tectonostratigraphy of the onverwacht suite barberton greenstone belt south africa abstract a synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern barberton greenstone belt into seven units with different geological histories current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained we argue here that the status of the traditional formations of the onverwacht group of the barberton greenstone belt be changed to complexes and collectively these complexes be referred to as the onverwacht suite the total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km respectively the precise age range of the rocks present in each complex is largely unknown the original spatial relationships between the complexes can in most cases only be inferred all of the complexes include significant volumes of intrusions that were emplaced into the volcanic and volcaniclastic host rocks the ages of the intrusions in the different complexes vary in some complexes the intrusions and volcanic rocks have similar geochemistry and are probably co magmatic others provide stitching ages between complexes four of the complexes display internal relationships that resemble ophiolites basaltic pillowed and massive lavas are the most common rock types sheeted dykes are generally absent whilst multiple mafic sills are common and dominate one of the complexes the volcanology and geochemistry of the complexes suggest that they formed predominantly in oceanic spreading and intra oceanic arc environments at water depths of 1 7 4 km the complexes were tectonically stacked during the final stages of their development and during two subsequent orogenic events one complex the hooggenoeg complex was uplifted and deeply eroded between about 3460 and 3470 ma with a minimum uplift rate ca 0 5 mm yr similar to that during emplacement of phanerozoic ophiolites unconformably overlying sub aerial coarse clastic sedimentary rocks part of the newly distinguished noisy complex overlap in age with felsic intermediate igneous activity dated between 3445 and 3460 ma that in turn is genetically related to tonalite trondjemite plutons exposed farther to the south the emergence of the deep water hooggenoeg complex above sea level the start of its erosion and fluvial sedimentation on top of it at ca 3458 ma represents the onset of its obduction onto older complexes some of which are now preserved as enclaves in the granitoid terrains to the south other oceanic slabs the kromberg and the mendon complexes were subsequently emplaced onto the noisy complex during latitudinal displacements ca 12 cm yr that were not notably faster than today exhumation with up to 18 km of unroofing along the southern margin of the greenstone belt took place during a second orogenic episode of subduction and accretion between 3140 and 3230 ma this episode was associated with folding thrusting and molasse type sedimentation within the greenstone belt coupled to extension and ductile faulting at a lower crustal level during this period younger complexes were assembled and the older complexes re cycled and eroded at rates and with geotherms comparable to those in modern orogenic belts in the barberton region between 3 2 and 3 5 ga tectonic processes appear to have operated at rates similar to those within modern evolving plate boundaries

3430 to 3417 ma calc alkaline volcanism in the mcphee dome and kelly belt and growth of the eastern pilbara craton abstract the mcphee dome and kelly belt in the eastern pilbara craton contain well preserved proximal shallow marine dominantly intermediate volcanic successions these have shrimp u pb zircon ages of 3430 3 and 3417 9 ma respectively which are younger than geochemically similar 3 52 and 3 47 to 3 45 ga suites in other eastern pilbara greenstone belts basalts andesites and dacites from the mcphee dome are similar in composition to modern arc rocks whereas dacites and rhyodacites from the kelly belt have geochemical characteristics of archaean tonalite trondhjemite granodiorite ttg magmas the geochemistry of the mcphee dome basalts basaltic andesites and dacites most likely reflects infracrustal fractional crystallization of a mantle derived magma combined with assimilation of older crust or a silicic melt in contrast the kelly belt dacites and rhyodacites were most likely derived by melting of subducted mafic crust the extensional submarine environment of pre 3 4 ga dominantly mafic and intermediate volcanism in the eastern pilbara differs from that of approximately coeval granitoid magmatism the pre 3 4 ga granitoids have the major and trace element features of ttg suites and appear to have been emplaced into already thick crust culminating in core complex formation available data are most consistent with growth of the eastern pilbara craton in convergent tectonic settings involving pulses of extensional shallow marine arc like volcanism between 3 52 and 3 42 ga overlapping with and or punctuated by periods of crustal thickening and the intrusion of complex ttg granitoid batholiths

rapid precise and high sensitivity acquisition of paleomagnetic and rock magnetic data development of a low noise automatic sample changing system for superconducting rock magnetometers among earth sciences paleomagnetism is particularly linked to the statistics of large sample sets as a matter of historical development and logistical necessity because the geomagnetic field varies over timescales relevant to sedimentary deposition and igneous intrusion while the fidelity of recorded magnetization is modulated by original properties of rock units and by alteration histories ideal paleomagnetic results measure remanent magnetizations of hundreds of samples at dozens of progressive demagnetization levels accompanied by tests of magnetic composition on representative sister specimens we present an inexpensive open source system for automating paleomagnetic and rock magnetic measurements using vacuum pick and place technology and a quartz glass sample holder the system can in one hour measure remanent magnetizations as weak as a few pam2 of 30 specimens in two vertical orientations with measurement errors comparable to those of the best manual systems the system reduces the number of manual manipulations required per specimen 8 fold

eolian sand in peridesert northwestern libya and implications for late pleistocene and holocene sahara expansions abstract jebel gharbi is a semi arid mountain range on the northern boundary of the sahara desert in northwestern libya between the mediterranean coastal plain the jefara and the tripolitanian plateau at jebel gharbi quaternary strata dated by means of radiocarbon techniques and archaeological artifacts consist of two upper pleistocene oxygen isotope stages 4 and 2 eolian sand beds that interfinger with loess calcrete alluvium and colluvium the deposition of the eolian sand was a consequence of the expansions of the sahara desert into the jebel gharbi area that took place during arid cold spells during the latest pleistocene and late holocene the northern margin of the sahara desert reached its modern position and eolian activity was widespread on the jefara coastal plain and in the valleys of jebel gharbi the late pleistocene desert expansions were contemporaneous with the periods of highest sedimentation rate of non volcanic dust in polar ice cores the comparison between data on eolian activity in some different places outside of the present boundary of the sahara desert suggest that there are differences in the timing of eolian sediment mobilization and the rate of eolian sediment deposition inside and outside the hadley cell atmospheric circulation zone the present work confirms the influence of the size of the hadley cells on atmosphere dust circulation patterns