hysteresis of natural magnetite ensembles micromagnetics of silicate hosted magnetite inclusions based on focused ion beam nanotomography

mineralogy and geochemistry of the clay fraction of sediments from the namibian continental margin and the adjacent hinterland the continental shelf off namibia can be divided into two sectors at latitude 18 40 s based on the distinctive morphologies of the two regions and on differences in composition of their surficial sediments the kunene shelf is narrow shallow and covered mainly by terrigenous sediment whereas the walvis shelf is wide deep and supports predominantly biogenic sediment the walvis shelf also possesses a number of distinguishing features its outer region has a concave profile which is attributed to upbuilding of the inner shelf during periods of lowered sea level its inner region is covered by an extensive belt of diatomaceous mud parallel to the coast the seaward and landward flanks of this deposit differ in lithology due mainly to the advanced state of dissolution of opal in deep water and to the influx of aeolian terrigenous detritus especially illite close to shore long belts of terrigenous mud extend southwards from both the kunene and orange rivers the currents responsible for transporting this sediment are poleward directed tongues of the angola benguela front in the north and mixed poleward moving south atlantic central water in the south the kunene river carries more smectite to the sea than the much larger orange the distribution of clay minerals on the continental shelf follows that of the adjacent hinterland with smectite and to a lesser extent kaolinite being significant in the north and illite predominating in the south along the upper slope the concentrations of smectite and kaolinite increase due to their importation from the kunene river by poleward directed antarctic intermediate water the geochemical character of riverine clays reflects the rock types exposed in each of the drainage basins compared with marine sediments these clays are enriched in eight of the fourteen elements investigated namely al fe mg co mn pb rb and zn in contrast the offshore element distribution patterns show little resemblance to the patterns of individual drainage basins clays in the marine sediments are enriched relative to the river sediments in ca k cd ni and sr five lithofacies recognized in the unconsolidated marine sediments are based on the relative proportions of opal calcium carbonate phosphorite glauconite and terrigenous components average element concentrations of individual lithofacies are enriched relative to the river clays in cd diatomaceous facies ca cd ni and sr calcareous and facies k and ni slightly glauconitic facies cd and sr terrigenous facies regionally the clay fractions of unconsolidated marine sediments contain high concentrations of al fe mg k mn and rb on the kunene margin due to aluminosilicates being imported by the kunene river ca and sr reach high values on the walvis outer margin in response to upwelling along the coast maximum concentrations of cd are associated with diatomaceous mud on the walvis inner shelf cu ni and zn reach high levels at a few spot locations where organic matter morg has suffered little dilution from other components especially opal and pb is concentrated with glauconitic sediment on the kunene shelf semi consolidated sediments are exposed intermittently on the outer margin they provide evidence that the palaeoclimate became much drier in post miocene times but in the north this picture is complicated by the introduction of sediment from the kunene river during the late tertiary palaeo oceanographic evidence suggests that the northern part of the study area experienced a concomitant decrease in productivity while in the southern part productivity increased

saharan wind regimes traced by the sr nd isotopic composition of subtropical atlantic sediments last glacial maximum vs today new nd sr isotopic data on the 30 m lithic particles of surface and last glacial maximum sediments recovered along the african margin between the equator and the gibraltar strait are presented in combination with grain size measurements this 30 m size fraction allows us to eliminate any hemipelagic contribution that could occur in the coarser fractions in the eolian fraction both sr and nd isotopic tracers reveal the same major northwestern origin mauritania mali southern algeria and morocco the archaean formations of the western saharan shield could be the source of the very unradiogenic ratios observed here the more southern regions senegal guinea act only as secondary sources a similar pattern is observed for the lgm lithic particles are mostly transported by both trade and saharan air layer sal winds along an approximate ne sw axis this main feature matches the southern plume characterizing the dust transport observed during winter no significant latitudinal shift of the belt winds is observed between the lgm and today at the lgm however dust fluxes were 2 4 times higher than today leading to a more archaean type imprint in the deposits we do not observe any clear relationship between the latitudinal variability of the upwelling systems identified in this region at the lgm and the location of the major wind systems both enhanced aridity on the continent and increased wind speed probably occurred together over western tropical africa during the last glacial period

late quaternary vegetation and climate of the sahel abstract pollen and phytogeographic evidence provides a vegetational history of the sahel for the period 0 18 000 yr b p the zonal vegetation fluctuated latitudinally and its most extreme positions occurred at 18 000 and 8500 yr b p the first involved a southward shift of the sahelian wooded grassland to 10 n under the arid conditions of the last glacial maximum the second change shows a rapid northward migration of humid vegetation guinean elements reach 16 n and sahelo sudanian elements extend to the southern margin of the modern sahara 21 n when the atlantic monsoon flux increased in the middle holocene the extensive spread of sudanian elements into the modern sahelian zone suggests the appearance of a markedly dry season the modern sahelian semiarid conditions appeared abruptly at 2000 yr b p

vegetation change in equatorial west africa time slices for the last 150 ka abstract drawing on marine and terrestrial palynological data vegetation maps of equatorial west africa are presented for eleven time slices over the last 150 000 years long marine records are used as a basis for the regional picture and provide a chronology for the last glacial cycle much shorter terrestrial records help fill in the picture for the most recent periods and facilitate interpretation of regional patterns for the longer timescale temporal and spatial variation is revealed in relation to global patterns of climate change rain forest was widespread during oxygen isotope stages 1 and 5 but strongly reduced during stages 3 and 4 and especially during stages 2 and 6 when open grass rich vegetation prevailed glacial rain forest refuges are found in the southwest of the guinean mountains and along the eastern coast of the gulf of guinea miombo woodland of the zambezian vegetation zone expanded during stage 5 especially during the first half podocarpus forest had its largest expansion during substages 5d 115 to 105 ka and 5b 95 to 85 ka the last occurrence of podocarpus in the guinean mountains is during stage 5a the distribution of mangrove swamps was extremely reduced during glacial times during stage 6 savanna and open dry forest covered large areas along the northern coast of the gulf of guinea the southern saharan desert reached far to the south and the namib desert far to the north the area of rain forest was restricted mangrove swamps were strongly reduced and the area of podocarpus forest fluctuated during stage 5 large changes in the area of afromontane forest rain forest dry open forest and savanna occurred rain forest was widespread and mangroves were extensive along the coast during the last interglacial substage 5e podocarpus forest area strongly extended during substages 5d and 5b in substages 5c and 5a rain forest reclaimed areas it had lost in the previous substages 5d and 5b respectively mangrove swamps were less widespread in the later substages of stage 5 than during substage 5e during stage 4 the rain forest area was again strongly reduced and recovered only slightly during the following stage 3 also the mangrove swamp area was reduced except along the ivory coast and along the northwestern coast of the gulf of guinea podocarpus forest only occurred in angola and may be in congo during stage 4 again forest was much reduced during stage 2 and open vegetation types covered large parts of equatorial west africa mangrove swamps must have been rare at the beginning of the holocene mangrove swamps had recovered and reached their largest extension also the rain forests area increased in the early holocene and guinean and congolian rain forests were probably not separated by a savanna corridor now existing in togo and benin

a paleohydrologic reinterpretation of the homeb silts kuiseb river central namib desert namibia and paleoclimatic implications abstract slack water deposits are usually fine grained fine sand and coarse silt flood sediments deposited in areas of the floodplain that are sheltered from high velocity flood flows slack water deposits have been recognized by many investigators in a wide variety of physiographic and climatic settings late quaternary slack water deposits have not been described from the extremely arid namib desert of southwestern africa fine grained flood sediments in the kuiseb valley near homeb accumulated between 23 000 and 19 000 14c years bp have been interpreted by previous workers as i sediments deposited behind dune dams ii river endpoint accumulations and iii flood deposits of an aggrading river controlled either by a base level change in the lower reaches or a change in the hydrological regime in the catchment area furthermore the so called homeb silts could document more arid climatic conditions in the kuiseb catchment area during the last glacial maximum lgm our research shows that the homeb silts are slack water deposits and that in many valleys of the namib desert slack water sediments are observed most commonly at the junctions of major rivers and minor tributaries in bedrock canyon settings here we present evidence that these slack water deposits accumulated as a result of more humid conditions and or more intense precipitation events in the upper reaches of the river they do not document climatic changes in the desert itself as postulated by many researchers

evidence for changes in the north atlantic deep water linked to meltwater surges during the heinrich events abstract deep sea sediment records from north atlantic cores 40 n 55 n provide evidence of several massive iceberg discharges known as heinrich events during the last glacial period high resolution benthic 18 o and 13 c records from north atlantic sediment cores were used to monitor the impact of heinrich events on thermohaline circulation and to estimate the sensitivity of deep oceanic circulation to changes in freshwater input to the north atlantic surface waters our data indicate that major rearrangements of deep water masses were directly associated with these massive iceberg discharges to trace in detail the deep water conditions in the north atlantic benthic 13 c values in several cores were used to generate time slices before during and after heinrich event 4 dated at 35 ka bp although north atlantic deep water continued to form during the oxygen isotope stage 3 at 37 ka bp deep circulation was characterized by an increased incursion of deep waters of southern origin which reduced the 13 c composition of north atlantic deep waters particularly in the eastern atlantic basin north atlantic deep water production was reduced during heinrich layer 4 hl4 at 35 ka bp synchronously with the changes in the surface water hydrology deep convection processes may have occurred in areas not affected by the salinity decrease soon after hl4 33 ka bp the 13 c distribution was similar to that before the event similarly a rapid return to initial 13 c values was observed at the end of the most clearly defined heinrich events hl5 hl4 and hl1 comparison between deep circulation patterns corresponding to hl4 the last glacial maximum and heinrich event 1 indicates that each of these periods was characterized by a different circulation state associated with changes in convection sites

how dry was dry late pleistocene palaeoclimates in the namib desert abstract evidence for late quaternary changes in climate in the namib desert is geographically scattered and often poorly dated the available evidence suggests periods of increased river discharge and groundwater flow centered on 8 12 20 24 and 26 32 ka bp based on modern analogues these palaeohydrologic changes likely involved increased rainfall and runoff in the escarpment zone east of the desert there is little evidence to suggest significantly increased rainfall within the desert during these periods except perhaps during the interval prior to 26 ka bp these data reinforce earlier conclusions that the namib has experienced mostly arid to hyperarid conditions throughout the quaternary

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

on the origin of continental precipitation 1 about 9 out of 10 liters of water evaporated from the oceans every year precipitates back onto oceans however the remaining 10 that get transported to continents play an irreplaceable role feeding the land branch of the hydrological cycle here we use an objective 3 d lagrangian model flexpart to detect major oceanic moisture source areas and the associated continental regions significantly influenced by each moisture source our results reveal a highly asymmetrical supply of oceanic moisture to the continents with the northern atlantic subtropical ocean source impacting the continents considerably more than the large southern indian and north pacific sources also the small mediterranean sea and red sea basins are important moisture sources for relatively large land areas the indian subcontinent receives moisture from six different major oceanic source regions future changes in meteorological conditions over the oceanic moisture source regions may have an impact on water availability for many river basins

coupled thermal and hydrological evolution of tropical africa over the last deglaciation we analyzed the distribution of branched tetraether membrane lipids derived from soil bacteria in a marine sediment record that was recovered close to the congo river outflow and the results enabled us to reconstruct large scale continental temperature changes in tropical africa that span the past 25 000 years tropical african temperatures gradually increased from 21 to 25 c over the last deglaciation which is a larger warming than estimated for the tropical atlantic ocean a direct comparison with sea surface temperature estimates from the same core revealed that the land sea temperature difference was through the thermal pressure gradient an important control on central african precipitation patterns

wet phases in the sahara sahel region and human migration patterns in north africa the carbon isotopic composition of individual plant leaf waxes a proxy for c3 vs c4 vegetation in a marine sediment core collected from beneath the plume of sahara derived dust in northwest africa reveals three periods during the past 192 000 years when the central sahara sahel contained c3 plants likely trees indicating substantially wetter conditions than at present our data suggest that variability in the strength of atlantic meridional overturning circulation amoc is a main control on vegetation distribution in central north africa and we note expansions of c3 vegetation during the african humid period early holocene and within marine isotope stage mis 3 50 45 ka and mis 5 120 110 ka the wet periods within mis 3 and 5 coincide with major human migration events out of sub saharan africa our results thus suggest that changes in amoc influenced north african climate and at times contributed to amenable conditions in the central sahara sahel allowing humans to cross this otherwise inhospitable region

reconstruction of se trade wind intensity based on sea surface temperature gradients in the southeast atlantic over the last 25 kyr 1 a prominent feature in the southeast atlantic is the angola benguela front abf the convergence between warm tropical and cold subtropical upwelled waters at present the sea surface temperature sst gradient across the abf and its position are influenced by the strength of southeasterly se trade winds here we present a record of changes in the abf sst gradient over the last 25 kyr variations in this sst contrast indicate that periods of strengthened se trade wind intensity occurred during the last glacial maximum the younger dryas and the mid to late holocene while heinrich event 1 the early part of the bolling allerod and the early holocene were periods of weakened se trade winds

a north to south transect of holocene southeast atlantic continental margin sediments relationship between aerosol transport and compound specific δ13c land plant biomarker and pollen records 1 we examined near surface late holocene deep sea sediments at nine sites on a north south transect from the congo fan 4 s to the cape basin 30 s along the southwest african continental margin contents distribution patterns and molecular stable carbon isotope signatures of long chain n alkanes c27 c33 and n alkanols c22 c32 are indicators of land plant vegetation of different biosynthetic types which can be correlated with concentrations and distributions of pollen taxa in the same sediments calculated clusters of wind trajectories and satellite aerosol index imagery afford information on the source areas for the lipids and pollen on land and their transport pathways to the ocean sites this multidisciplinary approach on an almost continental scale provides clear evidence of latitudinal differences in lipid and pollen composition paralleling the major phytogeographic zonations on the adjacent continent dust and smoke aerosols are mainly derived from the western and central south african hinterland dominated by deserts semi deserts and savannah regions rich in c4 and cam plants the northern sites congo fan area and northern angola basin which get most of their terrestrial material from the congo basin and the angolan highlands may also receive some material from the chad region very little aerosol from the african continent is transported to the most southerly sites in the cape basin as can be expected from the present position of the phytogeographic zones the carbon isotopic signatures of the n alkanes and n alkanols both become isotopically more enriched in 13c from north to south the results of the study suggest that this combination of pollen data and compound specific isotope geochemical proxies can be effectively applied in the reconstruction of past continental phytogeographic developments

forcing of wet phases in southeast africa over the past 17 000 years a continuous record of hydrologic variability for the past 17 000 years at the mouth of the zambezi river shows that hydrologic conditions in southeast africa were controlled by variations in local insolation and migrations of the intertropical convergence zone rather than by indian ocean temperature records of hydroclimatological variability in southern africa since the last glacial maximum have produced a conflicting picture of the relative influences of events in the remote northern hemisphere and the more local indian ocean this paper presents a continuous record of hydrological variability for the past 17 000 years from marine sediment core drilled off the mouth of the zambezi river the record shows that hydrological conditions in the region were influenced by variations in local insolation and migrations of the intertropical convergence zone which were probably driven by events in the northern hemisphere rather than the indian ocean intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast africa since the last glacial maximum about 20 000 years ago in particular the relative importance of atmospheric and oceanic processes is not firmly established1 2 3 4 5 southward shifts of the intertropical convergence zone itcz driven by high latitude climate changes have been suggested as a primary forcing2 3 whereas other studies infer a predominant influence of indian ocean sea surface temperatures on regional rainfall changes4 5 to address this question a continuous record representing an integrated signal of regional climate variability is required but has until now been missing here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro climatology in southeast africa during rapid climate changes over the past 17 000 years our results are based on a reconstruction of precipitation and river discharge changes as recorded in a marine sediment core off the mouth of the zambezi river near the southern boundary of the modern seasonal itcz migration indian ocean sea surface temperatures did not exert a primary control over southeast african hydrologic variability instead phases of high precipitation and terrestrial discharge occurred when the itcz was forced southwards during northern hemisphere cold events such as heinrich stadial 1 around 16 000 years ago and the younger dryas around 12 000 years ago or when local summer insolation was high in the late holocene that is during the past 4 000 years

isotopic composition of waters from ethiopia and kenya insights into moisture sources for eastern africa 1 oxygen and deuterium isotopic values of meteoric waters from ethiopia are unusually high when compared to waters from other high elevation settings in africa and worldwide these high values are well documented however the climatic processes responsible for the isotopic anomalies in ethiopian waters have not been thoroughly investigated we use isotopic data from waters and remote data products to demonstrate how different moisture sources affect the distribution of stable isotopes in waters from eastern africa oxygen and deuterium stable isotopic data from 349 surface and near surface groundwaters indicate isotopic distinctions between waters in ethiopia and kenya and confirm the anomalous nature of ethiopian waters remote data products from the tropical rainfall measuring mission trmm and national centers for environmental prediction ncep reanalysis project show strong westerly and southwesterly components to low level winds during precipitation events in western and central ethiopia this is in contrast to the easterly and southeasterly winds that bring rainfall to kenya and southeastern ethiopia large regions of high equivalent potential temperatures e at low levels over the sudd and the congo basin demonstrate the potential for these areas as sources of moisture and convective instability the combination of wind direction data from ethiopia and e distribution in africa indicates that transpired moisture from the sudd and the congo basin is likely responsible for the high isotopic values of rainfall in ethiopia

lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern asia abstract variability in oxygen isotope ratios collected from speleothems in chinese caves is often interpreted as a proxy for variability of precipitation summer precipitation seasonality of precipitation and or the proportion of 18o to 16o of annual total rainfall that is related to a strengthening or weakening of the east asian monsoon and in some cases to the indian monsoon we use modern reanalysis and station data to test whether precipitation and temperature variability over china can be related to changes in climate in these distant locales we find that annual and rainy season precipitation totals in each of central china south china and east india have correlation length scales of 500 km shorter than the distance between many speleothem records that share similar long term time variations in 18o values thus the short distances of correlation do not support though by themselves cannot refute the idea that apparently synchronous variations in 18o values at widely spaced 500 km caves in china are due to variations in annual precipitation amounts we also evaluate connections between climate variables and 18o values using available instrumental measurements of 18o values in precipitation these data from stations in the global network of isotopes in precipitation gnip show that monthly 18o values generally do not correlate well with either local precipitation amount or local temperature and the degree to which monthly 18o values do correlate with them varies from station to station for the few locations that do show significant correlations between 18o values and precipitation amount we estimate the differences in precipitation amount that would be required to account for peak to peak differences in 18o values in the speleothems from hulu and dongge caves assuming that 18o scales with the monthly amount of precipitation or with seasonal differences in precipitation insofar as the present day relationship between 18o values and monthly precipitation amounts can be applied to past conditions differences of at least 50 in mean annual precipitation would be required to explain the 18o variations on orbital time scales which are implausibly large and inconsistent with published gcm results similarly plausible amplitudes of seasonal cycles in amounts or in seasonal variations in 18o values can account for less than half of the 4 5 difference between glacial and interglacial 18o values from speleothems in china if seasonal cycles in precipitation account for the amplitudes of 18o values on paleoclimate timescales they might do so by extending or contracting the durations of seasons a frequency modulation of the annual cycle but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average 18o values amplitude modulation allowing that several processes can affect seasonal variability in isotopic content we explore the possibility that one or more of the following processes contribute to variations in 18o values in chinese cave speleothems different source regions of the precipitation which bring different values of 18o in vapor different pathways between the moisture source and the paleorecord site along which exchange of 18o between vapor surface water and condensate might differ a different mix of processes involving condensation and evaporation within the atmosphere or different types of precipitation each may account for part of the range of 18o values revealed by speleothems and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

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

critical single domain grain sizes in elongated iron particles implications for meteoritic and lunar magnetism summary kamacite particles fe ni ni 5 per cent are very common in extra terrestrial materials such as meteorites it is normally assumed that for kamacite particles to be reliable recorders of magnetic fields they need to be magnetically uniform single domain sd and thermally stable largerparticlessubdivideintonon uniformmultidomain md magneticstructuresthat produce weaker magnetic signals while small sd particles become magnetically unstable due tothermalfluctuationsandexhibitsuperparamagneticbehaviour inthispaperwedeterminethe firstmicromagnetic calculation ofthestablesdrangedomain statephasediagram formetallic iron previous calculations were analytical there is a significant increase in the critical size for the sd md threshold size for example for cube shaped iron particles the critical sd md threshold has now been estimated to be 25 nm compared to 17 nm for previous estimates the larger critical sd md threshold size for iron agrees better with previously published nanometric observations of domain state for feni particles then early analytical models

observing thermomagnetic stability of nonideal magnetite particles good paleomagnetic recorders the thermomagnetic behavior of remanence induced magnetite fe3o4 particles in the pseudo single domain psd size range 0 1 10 m which dominate the magnetic signature of many rock lithologies is investigated using off axis electron holography construction of magnetic induction maps allowed for the visualization of the vortex domain state in an individual fe3o4 grain 200 nm in diameter as a function of temperature acquisition of a series of electron holograms at 100 c intervals during in situ heating up to 700 c demonstrates the vortex state of the fe3o4 grain in this instance remains thermally stable close to its unblocking temperature and exhibits a similar in plane remanent state upon cooling i e the particle is effectively behaving like a uniaxial single domain particle to temperatures near tc such particles are thought to be robust magnetic recorders it is suggested that evidence for psd behavior should therefore not preclude paleomagnetic investigation

spatio temporal evolution of the west african monsoon during the last deglaciation on the basis of a multi proxy data set from the gulf of guinea eastern equatorial atlantic we reconstruct the spatio temporal evolution of the west african monsoon wam and present evidence for a decoupling between latitudinal shifts of the rain belt and wam intensification the onset of deglacial monsoon invigoration at 16 600 years before present lagged northward migration of a weak rainfall zone by 2800 years conversely during the younger dryas yd time interval wam precipitation was severely reduced but we find no evidence for a large scale retreat of the rainfall front this observation is not in agreement with the hypothesis of a large scale shift of the intertropical convergence zone south of the tropical wam region during the yd our results can be better reconciled with the newly emerging concept of a strong influence of tropical easterly and african easterly jets on modern wam

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

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

primary pseudo single and single domain magnetite inclusions in quartzite cobbles of the jack hills western australia implications for the hadean geodynamo

experimental constraints on magnetic stability of chondrules and the paleomagnetic significance of dusty olivines dynamic crystallization experiments are conducted under a magnetic field to determine both magnetic and mineralogical properties of chondrules the experiment reproduced synthetic dusty olivine samples that were formed by a high temperature reduction of an initially fayalitic olivine backscattered electron microscopy observations confirmed that synthetic dusty olivine contains abundant fine submicron sized ni poor fe inclusions in the cores of mgo rich olivine grains similar to that in natural chondrules alternating field demagnetization experiments of dusty olivine samples indicate mean destructive fields of up to 80 mt suggesting the submicron sized fe inclusions are a carrier of stable remanence in natural chondrules fine fe inclusions in the dusty olivine may have been armored against chemical alteration by surrounding host olivine crystals since the fine fe inclusions were probably heated above the curie temperature during the last chondrule forming events the fine fe inclusions in dusty olivine can acquire thermal remanent magnetization during the chondrule formation event theoretical time temperature relation of such fine grained fe kamacite grains suggested that a paleomagnetic data observed above 490 c in thermal demagnetization experiments of dusty olivines is reliable despite the low grade metamorphism of unequilibrated ordinary chondrites e g ll3 0 therefore the presence of fine fe inclusions in dusty olivine in unequilibrated ordinary chondrites constrains that such dusty olivine in chondrules is a good candidate as an un altered and stable magnetic recorder of the early solar magnetic field

the provenance of vegetation and environmental signatures encoded in vascular plant biomarkers carried by the ganges brahmaputra rivers organic matter carried by rivers and deposited in continental and marine sediments contains valuable information on past environmental conditions and their impact on the terrestrial biosphere in order to use sedimentary records to reconstruct past environmental conditions on the continents such as vegetation cover and type or precipitation intensity we need to understand the provenance of the organic signatures and how they are transferred and potentially modified by fluvial transport to the sedimentary reservoir in particular we need to understand how environmental conditions are imprinted in the composition of riverine particulate organic carbon here we investigate the stable carbon and hydrogen isotopic signatures of vascular plant leaf wax biomarkers in the modern day ganges and brahmaputra river complex one of the largest fluvial systems on earth the distribution of n alkanes and n alkanoic acids in ganges brahmaputra river sediments suggests that vascular plant inputs are consistently a significant component of the organic carbon pool molecular 13c measurements reveal that c3 vegetation inputs delivered by the himalayan rivers are partly degraded and replaced by mixed c3 c4 vegetation input in the floodplain the hydrogen isotopic composition of long chain c24 n alkanoic acids in combination with their stable c isotopic composition provides constraints on the isotopic composition of the environmental water used by higher plants within the drainage basin calculated compositions compare well with the depleted d isotopic ratios of the river water during the summer monsoon suggesting that h isotopic compositions of long chain alkanoic acids are a valuable proxy for the composition of summer monsoon moisture in the ganges brahmaputra basin detailed studies of the isotopic anatomy of modern river systems are critical for informed interpretation of marine sedimentary records in river influenced continental margins

a record of rapid holocene climate change preserved in hyrax middens from southwestern africa the discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central namib desert africa is providing unprecedented insight into the region s environmental history high resolution stable carbon and nitrogen isotope records spanning 0 11 700 cal calibrated yr b p indicate phases of relatively humid conditions from 8700 7500 6900 6700 5600 4900 and 4200 3500 cal yr b p with a period of marked aridity occurring from 3500 until ca 300 cal yr b p transitions between these phases appear to have occurred very rapidly often within 200 years of particular importance are 1 the observed relationship between regional aridifi cation and the decline in northern hemisphere insolation across the holocene and 2 the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing together these elements call for a fundamental reexamination of the role of orbital forcing on tropical african systems and a reconsideration of what factors drive climate change in the region the quality and resolution of these data far surpass any other evidence available from the region and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern africa

measurement of accessible reactive surface area in a sandstone with application to co2 mineralization measurement of accessible reactive surface area in a sandstone with application to co 2 mineralization gautier landrot 1 jonathan b ajo franklin li yang stefano cabrini and carl i steefel glandrot gmail com jbajo franklin lbl gov lyang lbl gov scabrini lbl gov cisteefel lbl gov lawrence berkeley national laboratory 1 cyclotron rd berkeley ca 94720 8099 usa corresponding author glandrot gmail com tel 66 90 79 77 922 fax 66 29 42 81 06 1 present address department of environmental engineering kasetsart university 10 th floor building 14 50 ngamwongwan road jatujak bangkok 10900 thailand

separating geometry from stress induced remanent magnetization in magnetite with ilmenite lamellae from the stardalur basalts iceland

secondary magnetite in ancient zircon precludes analysis of a hadean geodynamo zircon crystals from the jack hills western australia are one of the few surviving mineralogical records of earth s first 500 million years and have been proposed to contain a paleomagnetic record of the hadean geodynamo a prerequisite for the preservation of hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon to date no images of the magnetic recorders within ancient zircon have been presented here we use high resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms magnetite is produced via a pipe diffusion mechanism whereby iron diffuses into radiation damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low temperature recrystallization of radiation damaged zircon in the presence of an aqueous fluid although these magnetites can be recognized as secondary using transmission electron microscopy they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years without microscopic evidence ruling out secondary magnetite the paleomagnetic case for a hadean eoarchean geodynamo cannot yet been made

experimental study and tem characterization of dusty olivines in chondrites evidence for formation by in situ reduction an analytical transmission electron microscopy atem study was undertaken in order to better understand the formation conditions of dusty olivines i e olivines containing abundant tiny inclusions of fe ni metal in primitive meteorites dusty olivines from type i chondrules in the bishunpur chondrite ll3 1 and from synthetic samples obtained by reduction of san carlos olivines were examined in both natural and experimental samples micron size metal blebs observed in the dusty region often show preferential alignments along crystallographic directions of the olivine grains have low ni contents typically 2 wt and are frequently surrounded by a silica rich glass layer these features suggest that dusty olivines are formed by a sub solidus reduction of initially fayalitic olivines according to the following reaction fe2sio4 in olivine 2fe metal xsio2 in melt 1 x sioin gas 3 x 2o2 in gas some volatilization of siogas may account for the apparent excess of metal relative to silica rich glass observed in both experimental and natural samples comparison with experimentally produced dusty olivines suggests that time scales of the order of minutes usually inferred for chondrule formation are also adequate for the formation of dusty olivines these observations are in agreement with the hypothesis that at least part of the metal phase in chondrites originated from reduction during chondrule formation

the flash melting of chondrules an experimental investigation into the melting history and physical nature of chondrule precursors abstract constraints placed on chondrule formation have largely been generated from experiments which use a long duration below liquidus isothermal melting minutes to hours rather than a short duration above liquidus flash melting event seconds to minutes in this paper we examine how a short duration superliquidus heat pulse can produce chondrule textures by incompletely melting material with a type of flash melting we show that the maximum temperature limit of chondrule formation was approximately 2100 c almost 400 c higher than previously constrained previous experiments also have not studied the effect of variations in precursor grain size on the formation of chondrule textures for this reason we simultaneously investigate the effect of variations in the grain size of a starting composition on the formation of chondrule textures we show how mgo rich type ia chondrules and other fine grained chondrules could only have been formed from the incomplete melting of a rather uniformly grain sized precursor of less than 63 m because fine grained mgo rich chondrules have the some of the highest chondrule liquidus temperatures we proposed that these types of textures define a minimum melting temperature for chondrule formation

a new look at old carbon in active margin sediments recent studies suggest that as much as half of the organic carbon oc undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering these estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils freshwater sediments and river deltas and thus only minimally contributes to the extraneously old 14c ages of total organic matter often observed on adjacent shelves here we test this paradigm by measuring the 14c and 13c values of individual higher plant wax fatty acids as well as the 13c values of extractable alkanes isolated from the eel river margin california the isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition a coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated with pre aged terrestrial material instead composing a considerable proportion of all organic matter if these findings are characteristic of other continental margins proximal to small mountainous rivers then the importance of petrogenic oc burial in marine sediments may need to be reevaluated 2009 the geological society of america

paleomagnetic analysis using squid microscopy 1 superconducting quantum interference device squid microscopes are a new generation of instruments that map magnetic fields with unprecedented spatial resolution and moment sensitivity unlike standard rock magnetometers squid microscopes map magnetic fields rather than measuring magnetic moments such that the sample magnetization pattern must be retrieved from source model fits to the measured field data here we present the first direct comparison between paleomagnetic analyses on natural samples using joint measurements from squid microscopy and moment magnetometry we demonstrate that in combination with a priori geologic and petrographic data squid microscopy can accurately characterize the magnetization of lunar glass spherules and hawaiian basalt the bulk moment magnitude and direction of these samples inferred from inversions of squid microscopy data match direct measurements on the same samples using moment magnetometry in addition these inversions provide unique constraints on the magnetization distribution within the sample these measurements are among the most sensitive and highest resolution quantitative paleomagnetic studies of natural remanent magnetization to date we expect that this technique will be able to extend many other standard paleomagnetic techniques to previously inaccessible microscale samples

bubble nucleation in magmas a dominantly heterogeneous process abstract bubble nucleation is at the heart of magma ascent and degassing nucleation kinetics control how easily a magma exsolves its volatiles and thereby its explosivity close to the surface homogeneous nucleation i e without assistance from crystal substrates or other phases often requires attainment of large supersaturation pressures p n 100 mpa the change in pressure required for onset of nucleation while heterogeneous nucleation on pre existing crystals can occur at significantly lower values p n p n given experimental constraints on pressure conditions at the onset of nucleation decompression rate meters e g toramaru 2006 also relate the number density of bubbles in pyroclasts a readily obtained textural characteristic to rates of pressure change dp dt during ascent all these treatments require an explicit or implicit decision as to whether magma degassing was dominated by homogeneous or heterogeneous nucleation which is guided by the choice of surface tension values this contribution exploits the current textural datasets available for eruptions involving a variety of magma compositions to test the assumptions that homogeneous and or heterogeneous nucleation dominate bubble vesiculation in natural melts the comparison between pyroclast textures and those obtained via decompression experiments in the laboratory indicates that supersaturation pressures and decompression rates required for homogeneous nucleation are unrealistically high for natural magmas i review the arguments for and against homogenous nucleation and defend the alternate view that heterogeneous nucleation may dominate in all magma types including rhyolites in order to solve this central debate our focus needs to be shifted towards understanding the pre decompression phase equilibria and textural characteristics of magnetite in addition to the state of the melt structure

terrestrial vegetation change inferred from n alkane σ13c analysis in the marine environment abstract we report gas chromatography isotope ratio monitoring mass spectrometry gc irm ms measurements of the 13c values of individual biomarker compounds n alkanes extracted from a 3 m marine sediment core taken near the mouth of the johnstone river north queensland australia the technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine derived carbon the results indicate that there has been a 2 increase in the 13c values of terrestrially derived n alkanes c29 c33 since clearing of the forested johnstone river drainage basin for sugarcane and pasture began in the late 19th century a much slower 1 increase in 13c values after 6 000 years bp and prior to european settlement may be related to a decrease in rainfall in the basin or to an increase in the abundance of c4 plants as a result of increased aboriginal burning the results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin and demonstrate that the 13c values of terrestrially derived n alkanes in the marine environment can be used to assess basin wide vegetation changes in adjacent river catchments on geological timescales

composition age and provenance of organic matter in nw african dust over the atlantic ocean 1 eolian dust deposited on a meteorology buoy over the boreal winter and spring of 1992 1993 in the northeast atlantic off northwest africa 18 n 22 w afforded sufficient material for detailed assessment of its biogeochemical characteristics and provenance the sample was subjected to microscopic examination and bulk elemental isotopic black carbon and molecular lignin lipid analyses the bulk elemental composition and organic carbon oc content 1 02 dry weight establishes the dust s origin as continental upper crust and is typical of dusts that emanate from west africa these data are in accord with the extensive satellite imagery documenting the transit of dust clouds from the sahara and sahel regions e g nasa toms aerosol index microscopic examination reveals that charcoal like particles from vegetation fires black carbon bc are the most abundant morphologically distinct organic components accelerator mass spectrometry ams 14c analysis yields conventional 14c ages for bulk oc of 1260 40 years and bc of 2070 35 years taken together with corresponding stable carbon isotopic compositions 13c 18 9 and 15 1 respectively these results suggest the presence of biomass and burning residues derived from predominantly c4 vegetation that accumulated in soils over the late holocene molecular level measurements are also consistent with this scenario for example lignin derived phenols in the dustfall sample indicate a significant contribution of angiosperm grass type tissue the altered nature of the lignin suggests storage as residues in soils solvent extractable lipid constituents include a marked leaf wax type component with a molecular and stable carbon isotopic composition similar to those observed in surface marine sediments in this region the hydrocarbon fraction contained some diesel type contamination but the n alkanes c23 c33 retained a prominent odd even distribution with 13c ranging from 26 7 to 28 5 the suite of even carbon numbered n alkanols c22 c32 is 14c dated at 647 150 years with 13c values ranging from 23 9 to 30 4 the long chain n alkanoic acids c22 c32 exhibited 13c values ranging from 22 6 to 27 4 these waxes are evidently a mixed signal derived from contemporary c3 and c4 vegetation and from ablated soils and desiccated lake sediments of middle to late holocene age this molecular approach shows promise as a tool for continental paleoenvironmental assessment particularly with respect to past vegetation cover regional aridity and wind systems

multi scale three dimensional characterization of iron particles in dusty olivine implications for paleomagnetism of chondritic meteorites dusty olivine olivine containing multiple sub micrometer inclusions of metallic iron in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence capable of maintaining a faithful record of pre accretionary magnetization acquired during chondrule formation here we show how the magnetic architecture of a single dusty olivine grain from the semarkona ll3 0 ordinary chondrite meteorite can be fully characterized in three dimensions using a combination of focused ion beam nanotomography fib nt electron tomography and finite element micromagnetic modeling we present a three dimensional 3d volume reconstruction of a dusty olivine grain obtained by selective milling through a region of interest in a series of sequential 20 nm slices which are then imaged using scanning electron microscopy the data provide a quantitative description of the iron particle ensemble including the distribution of particle sizes shapes interparticle spacings and orientations iron particles are predominantly oblate ellipsoids with average radii 242 94 199 80 123 58 nm using analytical tem we observe that the particles nucleate on sub grain boundaries and are loosely arranged in a series of sheets parallel to 001 of the olivine host this is in agreement with the orientation data collected using the fib nt and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host the shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets individual particle geometries are converted to a finite element mesh and used to perform micromagnetic simulations the majority of particles adopt a single vortex state with bulk spins that rotate around a central vortex core we observed no particles that are in a true single domain state the results of the micromagnetic simulations challenge some preconceived ideas about the remanence carrying properties of vortex states there is often not a simple predictive relationship between the major intermediate and minor axes of the particles and the remanence vector imparted in different fields although the orientation of the vortex core is determined largely by the ellipsoidal geometry i e parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids the core and remanence vectors can sometimes lie at very large tens of degrees angles to the principal axes the subtle details of the morphology can control the overall remanence state leading in some cases to a dominant contribution from the bulk spins to the net remanence with profound implications for predicting the anisotropy of the sample the particles have very high switching fields several hundred millitesla demonstrating their high stability and suitability for paleointensity studies

the tropical rainbelt and productivity changes off northwest africa a 31 000 year high resolution record abstract benthic foraminiferal assemblage compositions and sedimentary geochemical parameters were analyzed in two radiocarbon dated sediment cores from the upwelling area off nw africa at 12 n to reconstruct productivity changes during the last 31 kyr high latitude cold events and variations in low latitude summer insolation influenced humidity wind systems and the position of the tropical rain belt over this time period this in turn caused changes in intensity and seasonality of primary productivity off the southern northwest african continental margin high accumulation rates of benthic foraminifera carbonate and organic carbon during times of north atlantic melt water events heinrich 2 25 4 to 24 3 kyr bp and 1 16 8 to 15 8 kyr bp indicate high productivity dominance of infaunal benthic foraminiferal species and high numbers of deep infaunal specimens during that time indicate a strong and sustained supply of refractory organic matter reworked from the upper slope and shelf a more southerly position of the tropical rainbelt and the northeast trade wind belt during heinrich 2 and 1 may have enhanced wind intensity and almost permanent upwelling driving this scenario a phytodetritus related benthic fauna indicates seasonally pulsed input of labile organic matter but generally low year round productivity during the last glacial maximum 23 to 18 kyr bp the tropical rainbelt is more expanded to the north than during heinrich events and relatively weak ne trade winds resulted in seasonal and weak upwelling thus lower productivity high productivity characterized by a seasonally high input of labile organic matter is indicated for times of orbital forced warming such as the african humid period 9 8 to 7 kyr bp an intensified african monsoon during boreal summer and the northernmost position of the tropical rainbelt within the last 31 kyr resulted in enhanced river discharge from the northward extended drainage area initiating intense phytoplankton blooms in the late holocene 4 to 0 kyr bp strong carbonate dissolution may have been caused by even more enhanced organic matter fluxes to the sea floor increasing aridity on the continent and stronger ne trade winds induced intensive seasonal coastal upwelling

compound specific δd values of long chain n alkanes derived from terrestrial higher plants are indicative of the δd of meteoric waters evidence from surface soils in eastern china the stable carbon and hydrogen isotope composition of higher plant derived long chain n alkanes 13cn alkanes and dn alkanes from 45 surface soil samples within well characterized vegetation zones from eastern china 18 n 50 n are reported the weighted average dn alkanes value for n c27 n c29 and n c31 in the samples and the annual average d of meteoric water recorded at 12 weather stations proximal to the sampling sites show similar spatial variations the d of n alkanes shows a gradual depletion in value with increasing latitude the results demonstrate that on a large spatial scale the d values of long chain n alkanes derived from higher plants have the potential to record the d of meteoric water although many other factors can also influence the isotope values there appears to be no apparent relationship between the d of the n alkanes extracted from the surface soil and the overlying vegetation type i e forest grassland or c3 c4 composition therefore palaeoenvironmental studies utilizing dn alkanes from higher plant derived material in geological samples have the potential to provide additional information with regard to the past hydrological cycle

sahel megadroughts triggered by glacial slowdowns of atlantic meridional overturning 1 the influence of the large scale ocean circulation on sahel rainfall is elusive because of the shortness of the observational record we reconstructed the history of eolian and fluvial sedimentation on the continental slope off senegal during the past 57 000 years our data show that abrupt onsets of arid conditions in the west african sahel were linked to cold north atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with heinrich stadials climate modeling suggests that this drying is induced by a southward shift of the west african monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric african easterly jet

long lived magnetism on chondrite parent bodies abstract we present evidence for both early and late stage magnetic activity on the cv and l ll parent bodies respectively from chondrules in vigarano and bjurbole using micro ct scans to re orientate chondrules to their in situ positions we present a new micron scale protocol for the paleomagnetic conglomerate test the paleomagnetic conglomerate test determines at 95 confidence whether clasts within a conglomerate were magnetized before or after agglomeration i e for a chondritic meteorite whether the chondrules carry a pre or post accretionary remanent magnetization we found both meteorites passed the conglomerate test i e the chondrules had randomly orientated magnetizations vigarano s heterogeneous magnetization is likely of shock origin due to the 10 to 20 gpa impacts that brecciated its precursor material on the parent body and transported it to re accrete as the vigarano breccia the magnetization was likely acquired during the break up of the original body indicating a cv parent body dynamo was active 9 ma after solar system formation bjurbole s magnetization is due to tetrataenite which transformed from taenite as the parent body cooled to below 320 c when an ambient magnetic field imparted a remanence we argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution and a l ll parent body dynamo must have been active at least 80 to 140 ma after peak metamorphism primitive chondrites did not originate from entirely primitive never molten and or differentiated parent bodies primitive chondrite parent bodies consisted of a differentiated interior sustaining a long lived magnetic dynamo encrusted by a layer of incrementally accreted primitive meteoritic material the different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies and or formation location in the protoplanetary disk

sahara and sahel vulnerability to climate changes lessons from holocene hydrological data abstract the examination of more than 1500 paleohydrological dated records collected between 10 and 28 n during the last 50 years have been used to improve our knowledge and understanding of the sahara and sahel vulnerability to the atlantic monsoon changes in the long term we have analyzed the distribution of water bodies mainly lakes and wetlands over time and space the central saharan massifs played a major role in favoring water supply to the lowlands throughout the whole african humid period in addition distinct east west dynamics is recorded with humidity starting and stopping several millennia earlier to the east than to the west of the sahara a series of time lags are discussed 1 between the maximum of deep fresh water lake formation during the early holocene and the maximum of water body extensions during the mid holocene which highlight the primary role of aquifer water level in lake response to climate change 2 between the hydrological history of the sahara and the sahel and the forcings mainly insolation changes during the early and mid holocene which involves complex interactions between remnant ice sheet in the northern hemisphere open water bodies in the sahara and sahel and the atlantic monsoon system

holocene climatic optimum in southwest africa evidence from the marine clay mineral record abstract sediment composition grain size and clay mineral record of a high resolution sediment core from the continental slope off namibia was investigated to gain information on the deposition of terrigenous matter in this part of the southwest african continental margin during the last 18 k y the depositional processes involved are fluvial input by the kunene river and eolian input from the namib and kalahari deserts each supplying characteristic mineral suites during low sea level erosion of the exposed shelf yields additional material the amount of eolian or fluvial matter depends on the strength of the transport process which is a function of aridity or humidity of the source area thus allowing paleoclimatic interpretations arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short distance transport by southerly winds a dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition which is attributed to increased runoff of the kunene river and fluvial erosion of shelf sediments this period coincides with a strengthening of the monsoonal system during a precessional minimum which is observed in numerous sites in central africa and indicates an intensified influence of the monsoon at the kunene headwaters a distinct shift in clay mineralogy towards river derived material marks a second period of increased river runoff during high sea level from 9 to 5 ka indicating maximum humidity in the source area from 6 to 5 ka this corresponds to the holocene climatic optimum observed in the arid belts of northern africa the present balance between fluvial and eolian input was reached approximately at 4 ka

extratropical forcing of sahel aridity during heinrich stadials 1 in order to investigate a possible link between tropical northeast ne atlantic sea surface temperature sst atlantic meridional overturning circulation amoc and drought in the sahel during the past 44 thousand years kyr we used alkenone paleothermometry and 13c of c wuellerstorfi of a marine sediment core from the continental slope off senegal our data show periods of low sst and reduced amoc that coincided with drought in the sahel during north atlantic heinrich stadials hs the coldest period was hs1 ca 15 18 kyr before present bp when sst decreased by more than 2 c moreover the sst off senegal lagged variations in sahel aridity which is in agreement with results from a freshwater hosing experiment we conclude that variations in tropical ne atlantic sst were not the initial trigger of millennial scale sahel droughts of the past 44 kyr instead it is thought that these droughts were induced by substantial coolings of the extratropical north atlantic