sills associated with the bushveld complex south africa an estimate of the parental magma composition abstract four distinct ages of sills intrude the floor rocks under the bushveld complex the first is a metadolerite and pre dates the bushveld complex the middle two are generally related to the complex itself one has quench textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the bushveld complex the second suite is a hypersthene microgabbro representative of the second major injection of magma into the complex the last is doleritic and does not obviously correlate with any portion of the layered sequence the sills genetically related to the complex display several features 1 lack of chilled margins coarse grain size ability to melt country rock xenoliths extreme heterogeneity 2 atypical of normal sills but explicable in their present context

readjustment of glacial radiocarbon chronologies by self consistent three dimensional ocean circulation modeling a critical problem in radiocarbon dating is the spatial and temporal variability of marine 14c reservoir ages this is particularly true for the time scale beyond the tree ring calibration range here we propose a method to assess the evolution of marine reservoir ages during the last deglaciation by numerical modeling we apply a self consistent iteration scheme in which existing radiocarbon chronologies can be readjusted by transient three dimensional simulations of marine and atmospheric 14c to estimate the uncertainties regarding the ocean ventilation during the last deglaciation we consider various ocean overturning scenarios which are based on different climatic background states an example readjusting 14c data from the caribbean points to marine reservoir ages varying between 200 and 900 a during the last deglaciation correspondingly the readjustment leads to enhanced variability of atmospheric 14c by 30 and increases the mysterious drop of atmospheric 14c between 17 5 and 14 5 cal ka bp by about 20

early precambrian within plate basalts from the kaapvaal craton in southern africa a case for crustally contaminated komatiites archean and early proterozoic basalts from the kaapvaal craton in southern africa have geochemical characteristics of subduction related basalts these basalts however cannot be produced in the subcontinental lithosphere if ultramafic xenoliths depleted in fe ti and ca are representative of this lithosphere geochemical and isotopic data are consistent with an origin by assimilation fractional crystallization of al undepleted komatiite in the mid to upper crust with archean granitoids as the major contaminants the proportion of granite to tonalite needed in the contaminant is higher for most of the proterozoic basalts than it is for the archean basalts widespread 2 3 ga old within plate anorogenic granites in the kaapvaal craton may be crustal melts produced by komatiitic magmas heating the crust komatiitic magmatism in southern africa could have lasted for geq 1600 ma geq 3400 to 1800 ma and many archean and early proterozoic within plate basalts from other regions also may be crust

the karoo igneous province a problem area for inferring tectonic setting from basalt geochemistry tholeiitic basalts and associated intrusives are the major component of the karoo igneous province they are of mesozoic age and constitute one of the world s classic continental flood basalt cfb provinces it has been argued that most karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the proterozoic the only exceptions to this are late stage morb like dolerites near the present day continental margins which are considered to be of asthenospheric origin when data for the southern karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting essentially all of them would be classified as calc alkali basalts cab s or low k tholeiites virtually none of them plot in the compositional fields designated as characteristic of within plate basalts there is little likelihood that the compositions of the karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a subduction component has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out it must be concluded that the mantle source areas for cab s and the southern karoo basalts have marked similarities in contrast the data for northern karoo basalts largely plot in the within plate field on geochemical discriminant diagrams available data suggest that the source composition and or the restite mineralogy and degree of partial melting are different for southern and northern karoo basalts there is no evidence for any difference in tectonic setting between the southern and northern karoo basalts at the time they were erupted this appears to be clear evidence that specific mantle source characteristics and or magmatic processes can vary within a single cfb province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks

late quaternary palynology in marine sediments a synthesis of the understanding of pollen distribution patterns in the nw african setting abstract after a review of the first steps in marine palynology we show that the understanding of the northwest african setting is crucial to evaluate the potential of marine palynological studies elsewhere we studied distribution patterns of pollen grains in recent marine sediments off nw africa and were able to clearly relate patterns to modern pollen source areas vegetation belts and operating transport systems wind belts and ocean currents in particular patterns of quercus artemisia chenopodiaceae amaranthaceae ephedra gramineae and wet forest trees are very indicative of the position of the vegetation belts on the adjacent continent aeolian pollen transport is carried out by the northeast trade winds and the african easterly jet aej in the rain forest belt transport of pollen and fern spores also occurs by rivers a detailed comparison between recent pollen rain samples from terrestrial and marine sites between 21 and 12 n showed that the latitudinal range of vegetation belts is clearly reflected in the pollen samples of both environments a migration of the southern border of the sahara is reflected by the changing ratio between chenopodiaceae amaranthaceae pollen from the desert and gramineae pollen from the savannah belt distribution patterns of pollen for 9000 and 18 000 14 c yr bp last glacial maximum time slices based on pollen records from eleven marine cores between portugal and the gulf of guinea show significant latitudinal migrations of vegetation belts but a stable position of the main wind trajectories the aej had a stable position around 21 n the belt with trade winds had a stable position from morocco southwards changing vigour of the trade winds is clearly reflected by the patterns of isopollen contours and by changes in pollen influx records

biogeography of the lord howe rise region tasman sea abstract the two principal aims of this study were to synthesise physical and biological information to characterise the lord howe rise lhr region and to use recent survey collections of benthic invertebrates mostly large benthic epifauna to describe its biogeography at regional and sub regional scales the lhr region is large 1 95 million km2 spans tropical and cool temperate latitudes 18 4 to 40 3 s and is influenced by several ocean currents notably the east australian current and the tasman front our analyses revealed that biological patterns were related to two groups of geomorphic morphotypes found in this topographically complex region subdued bathymetric features expansive soft sediment basins and plateaus and raised bathymetric features scattered seamounts guyots knolls and pinnacles raised bathymetric features in the lhr region were more likely to support richer and more abundant epifaunal assemblages dominated by suspension feeding invertebrates on hard substrata compared to subdued features which were dominated by infauna and detritivores in soft sediments however this trend does not apply to all raised bathymetric features e g gifford guyot with variations in depth elevation latitude and particularly substrata affected the composition of biological assemblages in addition some demersal fishes ophiuroids and other benthic invertebrates showed distinct north south delineations that coincide with the influence of the tasman front and thermal gradients while the lack of spatially and temporally replicated data in the region limits our interpretation of survey data paleo environmental processes and examples from other regions provide some indication of how dispersal influences migration speciation and endemism in the lhr region although our current knowledge is limited it is hoped that this review will help inform future studies in the area as equitable examination of biological geological and oceanographic characteristics will facilitate future assessments of lhr biogeography and permit the inclusion of this region in biogeographic studies with a national or global context

geochemistry and origin of late archean volcanics from the ventersdorp supergroup south africa abstract the ventersdorp supergroup 2700 2750 ma is comprised chiefly of mafic to intermediate subaerially erupted volcanics with smaller amounts of clastic sediments the lower unit the klipriviersberg group consists of basal basaltic komatiites overlain by basalts which show a progressive increase in mg number and ni content and a decrease in incompatible element contents with increasing stratigraphic height volcanics of the overlying platberg and pniel groups consist of basaltic andesites and andesites and some felsic volcanics all ventersdorp volcanics exhibit a subduction zone geochemical component and are similar in incompatible element distributions to volcanics from continental margin arc systems both tholeiitic and calc alkaline trends occur in the ventersdorp succession with the former dominating crustal contamination appears to have played a minor role in the evolution of ventersdorp magmas with the exception of basaltic komatiite the klipriviersberg lavas can be related by progressive melting of a garnet lherzolite source containing a subduction zone component followed by up to 30 of shallow fractional crystallization the basaltic komatiite can be produced from the same source at higher temperatures and the pniel lavas by shallow fractional crystallization of basaltic komatiite the platberg volcanics must come from a within plate enriched mantle source and mafic and felsic members of this group can be related by shallow fractional crystallization geochemical and geologic data are consistent with a model for ventersdorp magmas involving a subduction zone in which klipriviersberg and pniel magmas are produced from the mantle wedge and platberg magmas come from an enriched mantle lithosphere with a superimposed subduction zone component secular trends in klipriviersberg lavas can be explained by progressive adiabatic melting of an ascending plume rooted in the mantle wedge

late quaternary climate and vegetation of the sudanian zone of northeast nigeria the lake tilla crater lake in northeastern nigeria 10 23 n 12 08 e provides a ca 17 000 14c yr multiproxy record of the environmental history of a sudanian savanna in west africa evaluation of pollen diatoms and sedimentary geochemistry from cores suggests that dry climatic conditions prevailed throughout the late pleistocene before the onset of the holocene the slow rise in lake levels was interrupted by a distinct dry event between ca 10 900 and 10 500 14c yr b p which may coincide with the younger dryas episode the onset of the holocene is marked by an abrupt increase in lake levels and a subsequent spread of guinean and sudanian tree taxa into the open grass savanna that predominated throughout the late pleistocene the dominance of the mountain olive olea hochstetteri suggests cool climatic conditions prior to ca 8600 14c yr b p the early to mid holocene humid period culminated between ca 8500 and 7000 14c yr b p with the establishment of a dense guinean savanna during high lake levels frequent fires were important in promoting the open character of the vegetation the palynological and palaeolimnological data demonstrate that the humid period terminated after ca 7000 14c yr b p in a gradual decline of the precipitation evaporation ratio and was not interrupted by abrupt climatic events the aridification trend intensified after ca 3800 14c yr b p and continued until the present

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

nw african hydrology and vegetation during the last glacial cycle reflected in plant wax specific hydrogen and carbon isotopes abstract we present a hydrologic reconstruction of the sahara sahel transition covering the complete last glacial cycle 130 ka based on a combination of plant wax specific hydrogen d and carbon isotopes 13c the d and 13c signatures of long chain n alkanes from odp site 659 off nw africa reveal a significant anti correlation complementary to published pollen data we infer that this plant wax signal reflects sensitive responses of the vegetation cover to precipitation changes in the sahel region as well as varying contributions from biomes north of the sahara c3 domain by north east trade winds netw during arid phases especially the northern parts of the sahel likely experienced crucial water stress which resulted in a pronounced contraction of the vegetation cover thus reducing the amount of c4 plant waxes from the region the increase in netw strength during dry periods further promoted a more pronounced c3 plant wax signal derived from the north african c3 plant domain during humid periods the c4 dominated sahelian environments spread northward into the saharan realm in association with lower netw inputs of c3 plant waxes arid humid cycles deduced from plant wax d are in accordance with concomitant changes in weathering intensity reflected in varying major element distributions environmental shifts are generally linked to periods with large fluctuations in northern hemisphere summer insolation during marine isotope stages 2 and 3 when insolation variability was low coupling of the hydrologic regime to alkenone based estimates of ne atlantic sea surface temperatures becomes apparent

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

contrasting compositions of saharan dust in the eastern atlantic ocean during the last deglaciation and african humid period abstract during the african humid period ahp much of the modern hyperarid saharan desert was vegetated and covered with numerous lakes in marine sediments off northwestern africa the ahp is represented by markedly reduced siliciclastic sediment flux between 12 3 and 5 5 ka changes in the origin of this terrigenous sediment fraction can be constrained by sediment chemistry and radiogenic isotope tracers at ocean drilling program odp site 658 hole c 20 44 95 n 18 34 85 w 2263 mbsl the neodymium nd isotope composition of terrigenous detritus shows little variability throughout the last 25 kyr indicating that the contributing geological terranes have not changed appreciably since the last glacial period in contrast there were large and abrupt changes in strontium sr isotope ratios and chemical compositions associated with the ahp during which 87 sr 86 sr ratios were markedly less radiogenic and sediments show higher chemical indices of alteration we show that sediment geochemical changes during the ahp cannot be attributed to changes in the source terranes physical sorting or intensity of chemical weathering the low 87 sr 86 sr and high sr concentrations of ahp age samples also conflict with the interpretation of increased fine grained fluvially derived sediments we propose that the most significant compositional changes at odp 658c are due to the addition of an aluminosilicate component that has a highly altered major element signature but is enriched in soluble elements like sr and magnesium mg compared to aluminum al and has low 87 sr 86 sr relative to local terrigenous source areas we interpret these characteristics to reflect authigenic sediment supply from extensive north african paleolake basins that were prevalent during the ahp

field relations and strontium isotope systematics of the marginal rocks of the eastern bushveld complex the well exposed basal contact of the eastern bushveld complex was mapped at scales of 1 10 000 and 1 20 000 along 130 km of strike and through a 9 km vertical section field relations petrography and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups a broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone a gabbroic group occurs above this level each group consists of a number of closely related rock types most of which have sill analogs rocks of the first group b1 include pyroxenites norites and peridotites all of which may have quench textures all contain orthopyroxene en 78 plagioclase an 65 and chromite with subordinate clinopyroxene and biotite rare earth element patterns are mostly strongly fractionated ce n yb n approximately 10 and have 40x chondritic abundances at ce their initial strontium isotope ratios at 2 05 b y fall in the range 0 7032 to 0 7057 and rb sr ratios approximate 0 2 the gabbroic group consists of microcrystalline two pyroxene gabbros b2n and b3n and their coarser grained equivalents b2 and b3 that border the upper critical and main zones respectively mineral assemblages are dominated by plagioclase approximately an 55 clinopyroxene and orthopyroxene approximately en 63 with abundant magnetite and traces of ilmenite and biotite their rare earth element patterns are less fractionated than b1 rocks the b2n and b2 family have ce n yb n approximately 6 and 30x chondritic abundances at ce the b3n and b3 rocks have ce n yb n approximately 3 6x chondritic abundances at ce and pronounced positive eu anomalies strontium isotope initial ratios at 2 05 b y fall in the range 0 706 to 0 708 and the rb sr ratios are approximately 0 02 whole rock and sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial sr isotope ratio r 0 similar to that of the transition zone 0 7056 a fraction of such liquid enriched in cumulus olivine crystals when mixed with up to 30 percent of a pyroxenitic liquid with low r 0 0 7034 yields a product having the isotopic and compositional characteristics of the peridotite sills a continuum exists in the initial sr ratios through the pyroxenitic and gabbroic marginal rocks suggesting that these represent mixtures of at least two sources with distinct initial sr isotope ratio characteristics it is argued that neither magma aging nor crustal contamination adequately accounts for the sr isotope characteristics of the marginal rocks or their derived cumulates the unusual chemistry of the pyroxenitic early liquids sio 2 approximately 55 mgo approximately 13 coupled with high concentrations of incompatible elements should be considered primary

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

the concentrations of the noble metals in southern african flood type basalts and morb implications for petrogenesis and magmatic sulphide exploration concentrations of the platinum group elements have been determined in several suites of southern african flood type basalts and mid ocean ridge basalt morb covering some 3 ga of geologic evolution and including the etendeka karoo soutpansberg machadodorp hekpoort ventersdorp and dominion magmas the magmas cover a compositional range from 3 7 to 18 7 mgo 26 720 ppm ni 16 250 ppm cu and 1 255 ppb total platinum group elements pge the younger basalts etendeka karoo tend to be depleted in pge relative to cu while most of the older basalts hekpoort machadodorp ventersdorp dominion show no pge depletion relative to cu further the younger basalts tend to have lower average pt pd ratios than the older basalts and the morbs have lower average pt pd than the continental basalts within the broad groupings of old and young basalts this may reflect 1 a decreasing degree of mantle melting through geologic time and 2 source heterogeneity in that the morbs are derived from predominantly asthenospheric mantle whereas the continental basalts also contain a lithospheric mantle component enriched in pt in addition to these factors some pge fractionation also occurred during differentiation of the magmas with pd showing incompatible behaviour and the other pge variably compatible behaviour the examined southern african flood type basalts and morb appear to offer limited prospects for magmatic sulfide ores largely because they show little evidence for significant chalcophile metal depletion that could be the result of sulphide extraction during ascent and crystallization

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

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

basalt geochemistry and tectonic discrimination within continental flood basalt provinces abstract continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings in one well studied classic flood basalt province the mesozoic karoo province of southern africa magmatism occurred in the following tectonic settings a continental rifting leading to ocean floor spreading in the south atlantic ocean etendeka suite of namibia b stretched continental lithosphere and rifting not leading directly to ocean floor formation lebombo suite of southeastern africa and c an a tectonic within plate continental setting characterized by an absence of faulting or warping lesotho highlands and karoo dolerites of south africa by means of spidergrams of the elements rb ba th nb k la ce sr nd p hf zr sm ti tb y v ni and cr uncontaminated tholeiites from c above i e the lesotho type continental flood basalts ltcfb are compared with mid ocean ridge basalts morb ocean island tholeiites oit and tholeiites and calc alkali basalts from subduction environments the comparison reveals the ltcfbs are geochemically distinct the differences are reflected in relative enrichments or depletions of the more incompatible elements rb ce to less incompatible elements ce y i e the overall slope of the spidergrams and in anomalous enrichments or depletions of one or more of the elements th k nb sr ti hf and zr the distinctive geochemical character of the lesotho ltcfbs is interpreted in terms of a lithospheric mantle source for the basalts this is supported by isotopic data there are no major geochemical differences between lesotho cfbs and basalts of the rift related etendeka and lebombo suites although the latter are somewhat enriched in rb ba and k however unlike the lesotho basalts the lebombo and etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late stage dolerites having morb oit i e asthenospheric geochemical characteristics the flood basalt silicic magmatism late stage dyke swarm association is characteristic of several rift or thinned lithosphere environments e g ethiopia skye eastern greenland but in many of these the flood basalts have ocean island basalt oit geochemical characteristics the lesotho type cfb geochemistry is exhibited by the grande ronde basalt of the columbia river group a possible subduction related flood basalt province and the basic rocks associated with mesozoic rifting in the north and south atlantic basalt geochemistry alone is unhelpful in determining the tectonic setting of cfbs although the rift related environments may be identified by the petrology and geochemistry of the whole igneous suite a two source model is proposed for the mantle derived basic rocks in rift related cfb provinces early enriched basalts are derived from the lithosphere and following pronounced lithospheric attenuation or rifting later morb like melts are emplaced from the rising asthenosphere the presence of both lesotho and oit type geochemical patterns in rift related cfbs suggests that the lithosphere exhibits different styles of enrichment

the cr th ratio in precambrian pelites from the kaapvaal craton as an index of craton evolution the cr th ratio in precambrian pelites from the kaapvaal craton in southern africa correlates with pelite th sc and co th ratios and appears to monitor changes in source composition it is a more sensitive indicator of provenance than other commonly used ratios including eu eu la yb and th u pelite cr th ratios are controlled most strongly by the effects of local provenance and local tectonics secular decreases in this ratio the most prominent of which occurred at 2 4 2 3 ga in pelites of the kaapvaal craton record rapid uplift exposing granite decreases in the ratio reflect increases in the abundance of komatiite basalt in sediment sources in response to arc collisions rift hotspot volcanism or intracratonic uplift of older greenstones relatively constant cr th ratios appear to require widespread uplift or and efficient sediment mixing during tectonic stability changes in cr th ratio of kaapvaal pelites reflect a complex tectonic history of the craton from about 3 3 ga to 1 8 ga and do not favor single stage cratonization in the late archean

fundamental uniformity in the trace element patterns of the volcanics of the kaapvaal craton from 3000 to 2100 ma evidence for the lithospheric origin of these continental tholeiites

the petrology of high mg dikes from the beartooth mountains montana a search for the parent magma of the stillwater complex six geochemically distinct groups of precambrian high mg dikes have been examined to determine if any could have been a sample of or closely related to the parent magma of the stillwater complex only two groups have sufficiently magnesian olivine and orthopyroxene but neither of these has sufficiently calcic plagioclase in terms of major elements there appear to be only two distinct magma types one has a high orthopyroxene component and is petrographically similar to noritic dikes and sills near the great dyke of zimbabwe and the bushveld complex of south africa the other is similar to primitive tholeiites with higher plagioclase and diopside components than the first all of the groups are enriched in light ree and have unusually high k2o concentrations model calculations indicate that crustal contamination 30 50 of fractionating komatiitic magmas is consistent with the major and trace element characteristics of the first magma type less extensive contamination of komatiitic magmas may produce the parental magmas of the large layered intrusions

combined la ce and sm nd isotope systematics in petrogenetic studies a theoretical model is presented for the coupled evolution of cerium and neodymium isotopes in terrestrial rocks data presented for samples ranging from mid ocean ridge basalts to continental metamorphic rocks show that while mantle derived rocks are dispersed on a ce nd diagram continental crustal rocks form a clearly defined mantle array as analytical techniques improve cerium isotopes will provide valuable constraints in studies of petrogenesis and crustal evolution

the stable hydrogen isotopic composition of sedimentary plant waxes as quantitative proxy for rainfall in the west african sahel various studies have demonstrated that the stable hydrogen isotopic composition d of terrestrial leaf waxes tracks that of precipitation d precip both spatially across climate gradients and over a range of different timescales yet reconstructed estimates of d precip and corresponding rainfall typically remain largely qualitative due mainly to uncertainties in plant ecosystem net fractionation relative humidity and the stability of the amount effect through time here we present d values of the c 31 n alkane d wax from a marine sediment core offshore the northwest nw african sahel covering the past 100 years and overlapping with the instrumental record of rainfall we use this record to investigate whether accurate quantitative estimates of past rainfall can be derived from our d wax time series we infer the composition of vegetation c 3 c 4 within the continental catchment area by analysis of the stable carbon isotopic composition of the same compounds 13 c wax calculated a net ecosystem fractionation factor and corrected the d wax time series accordingly to derive d precip using the present day relationship between d precip and the amount of precipitation in the tropics we derive quantitative estimates of past precipitation amounts our data show that a vegetation composition can be inferred from 13 c wax b the calculated net ecosystem fractionation represents a reasonable estimate and c estimated total amounts of rainfall based on d wax correspond to instrumental records of rainfall our study has important implications for future studies aiming to reconstruct rainfall based on d wax the combined data presented here demonstrate that it is feasible to infer absolute rainfall amounts from sedimentary d wax in tandem with 13 c wax in specific depositional settings

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

the roles of fire in holocene ecosystem changes of west africa abstract the climate changes associated with the holocene wet phase in the sahara the african humid period are subject to ongoing debate discussing interactions between climate and vegetation and possible feedbacks between vegetation albedo desertification and dust however very little attention has been given to the role of fire in shaping the land cover although it is known that fires are important in the formation and consolidation of the african savanna to fill this gap we investigated the interaction between precipitation changes vegetation shifts and fire occurrence in west africa by combining stable isotope measurements on plant waxes with pollen and micro charcoal counts of marine sediments retrieved offshore of cape blanc our study focuses on the roles of fire at the dry limit of savanna during the holocene evolution of precipitation changes indicating that the impact of fire during a relative wet climate differs from that during aridification during the humid early holocene increased savanna extension and diversification ran parallel to increased fire occurrence in contrast after aridification of northern africa started at the end of the african humid period a maximum in fire occurrence correlated with a deterioration of the vegetation promoting desertification

middle to late holocene fire history and the impact on mediterranean pine and oak forests according to the core rf93 30 central adriatic sea the high resolution adriatic rf93 30 core shows changes in its microcharcoal record which correlate to terrestrial fires from the last 7000 years pollen and microcharcoals were transported by win

molecular proxies for paleoclimatology we summarize the applications of molecular proxies in paleoclimatology marine molecular records especially are proving to be of value but certain environmentally persistent compounds can also be measured in lake sediments loess deposits and ice cores the fundamentals of this approach are the molecular parameters the compound abundances and carbon hydrogen nitrogen and oxygen isotopic contents which can be derived by the analysis of sediment extracts these afford proxy measures which can be interpreted in terms of the conditions which control climate and also reflect its operation we discuss two types of proxy those of terrigenous and those of aquatic origin and exemplify their application in the study of marine sediments through the medium of ten case studies based in the atlantic mediterranean and pacific oceans and in antarctica the studies are mainly for periods in the present the holocene and particularly the last glacial interglacial but they also include one study from the cretaceous the terrigenous proxies which are measures of continental vegetation are based on higher plant leaf wax compounds i e long chain circa c30 hydrocarbons alcohols and acids they register the relative contributions of c3 vs c4 type plants to the vegetation in the source areas the two marine proxies are measures of sea surface temperatures sst the longer established one u37k is based on the relative abundances of c37 alkenones photosynthesized by unicellular algae members of the haptophyta the newest proxy tex86 is based on c86 glycerol tetraethers gdgts synthesized in the water column by some of the archaeal microbiota the crenarchaeota

geochemistry and origin of late archean volcanic rocks from the rhenosterhoek formation dominion group south africa abstract the rhenosterhoek formation composed chiefly of subaerial volcanic rocks is part of the dominion group 2 8 ga which rests unconformably on older continental crust in south africa chemical compositions of volcanic rocks from three drillcores of this formation do not show systematic differences either between or within cores major and immobile trace element contents indicate that the volcanics are basaltic andesites and andesites with calc alkaline affinities incompatible trace element distributions are similar to those of modern andesites from evolved island arcs or continental margin arcs geochemical modeling indicates that the basaltic andesites and andesites can be produced by open system fractional crystallization with olivine clinopyroxene and plagioclase as principal liquidus phases up to 10 contamination with continental crust is also allowed by the data this succession does not appear to have formed in an oceanic or in a continental margin arc but may have formed adjacent to a continental margin arc system in an incipient foreland basin on the kaapvaal craton

coherent high and low latitude control of the northwest african hydrological balance variability in northwest african humidity has been documented for the holocene period but less is known about the development of the hydrological balance during the pleistocene period sedimentary records and numerical simulations for the past 120 000 years show abrupt millennial scale changes in humidity which may be related to changes in the strength of the north atlantic meridional overturning circulation

diversity and endemism of the benthic seamount fauna in the southwest pacific seamounts comprise a unique deep sea environment characterized by substantially enhanced currents and a fauna that is dominated by suspension feeders such as corals1 2 3 4 the potential importance of these steep sided undersea mountains which are generally of volcanic origin to ocean biogeography and diversity was recognized over 40 years ago5 but this environment has remained very poorly explored a review3 of seamount biota and biogeography reported a total of 597 invertebrate species recorded from seamounts worldwide since the challenger expedition of 1872 most reports based on a single taxonomic group were extremely limited 5 seamounts of the estimated more than 30 000 seamounts in the world s oceans4 6 accounted for 72 of the species recorded only 15 of the species occurring on seamounts were considered potential seamount endemics here we report the discovery of more than 850 macro and megafaunal species from seamounts in the tasman sea and southeast coral sea of which 29 34 are new to science and potential seamount endemics low species overlap between seamounts in different portions of the region indicates that the seamounts in clusters or along ridge systems function as island groups or chains leading to highly localized species distributions and apparent speciation between groups or ridge systems that is exceptional for the deep sea these results have substantial implications for the conservation of this fauna which is threatened by fishing activity7