volcano and climate driven changes in atmospheric dust sources and fluxes since the late glacial in central europe atmospheric dusts are an important part of the global climate system and play an important role in the marine and terrestrial bio geochemical cycles of major and trace nutrient elements a peat bog record of atmospheric deposition shows considerable variation in dust deposition during the past 15 k y with abrupt changes in fluxes at 12 9 2 8 4 7 2 and 6 cal kyr b p using nd isotopes and rare earth elements it is possible to clearly distinguish between volcanic inputs and those driven by climate change such as the long term aridification of the sahara and regional erosion due to forest clearing and soil cultivation activities our results indicate that a major dust event in north africa and europe preceded the 8 2 kyr b p cold event by 200 yr this dust event may have played an active role in the following climate cooling of the 8 2 kyr b p event nd isotope evidence also indicates a relatively slow change in dust regime over europe from 7 to 5 kyr b p due to sahara expansion these fi ndings show that the inorganic fraction in high resolution peat records can provide remarkably sensitive indicators of dust load and sources our study supports the priority to better identify the impact of dust loading during the holocene in terms of direct and indirect impacts on environmental and climate changes

saharan aeolian input and effective humidity variations over western europe during the holocene from a high altitude record saharan dust inputs affect present day ecosystems and biogeochemical cycles at a global scale previous saharan dust input reconstructions have been mainly based on marine records from the african margin nevertheless dust reaching western central europe is mainly transported by high altitude atmospheric currents and requires high altitude records for its reconstruction the organic and inorganic geochemical study of sediments from a southern iberia alpine lacustrine record has provided an exceptional reconstruction of saharan dust impact and regional climatic variations during the holocene after the last deglaciation results indicate that saharan dust reached western europe in a stepwise fashion from 7 0 to 6 0 cal kyr bp and increased since then until present promoting major geochemical changes in the lacustrine system effective humidity reconstruction indicates wetter conditions during the early holocene and progressive aridification during middle late holocene time boosting abrupt changes in the lacustrine system cyclostratigraphic analyses and transport mechanisms both point to solar irradiance and aridity as major triggering factors for dust supply over western europe during the holocene

geochemical composition of tanzanian shelf sediments indicates holocene climatic and sea level changes we present a high resolution geochemical and grain size record from a holocene sediment core off the pangani river mouth tanzania elemental ratios between biogenic elements and al i e ca al mg al and sr al are mainly influenced by terrigenous dilution on carbonate concentration and or limitation of carbonate production as a result of variations in the supply of fine grained terrigenous sediments of the pangani river such elemental ratios increased significantly at the end of the mid holocene between 5 and 3 5 ka demonstrating a gradual transition from the humid early and mid holocene to the arid late holocene in east africa among the elemental ratios between terrigenous elements and al si al and k al ratios correlate to grain size variation indicating a change in sedimentation regime fe al and ti al ratios show that the sediment source area has shifted from the terrestrial volcanic region of tanzania fe ti rich to the coastal and inner shelf regions fe ti poor around 7 5 ka in response to arid climate and high sea level our geochemical results correspond with a sea surface temperature record derived from the same sediment core indicating that the end of the east african humid period could have been gradual and related to the cooling water in the western indian ocean

eastern mediterranean sapropel s1 interruption an expression of the onset of climatic deterioration around 7 ka bp we discuss the palaeoclimatic interpretation of unprecedented high resolution micropalaeontological studies of shortterm 2 to 4 centuries interruptions within early holocene organic rich layer sapropel s1 from the eastern mediterranean results for cores from the adriatic and aegean seas that contain double s1 sapropels indicate that these interruptions which are centred roughly around 7000 years 14 cnc bp are genuine and related to climatic deterioration this interpretation is endorsed by a coeval dry event recorded in terrestrial records and indications of climatic deterioration affecting human migration patterns and early societies in egypt the presence of sapropel interruptions in the two major source areas of deep water for the entire eastern mediterranean likely implies that similar intervals may be found throughout the basin provided that sedimentation rates and sampling resolutions allow the detection of events with a duration of only several centuries moreover our results show that the sapropel mode of circulation comprises a delicate balance between reduced ventilation and enhanced productivity which is easily disturbed through surface water cooling triggering a short time of improved deep water ventilation 1999 elsevier science b v all rights reserved

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

post glacial arid episodes in ethiopia have implications for climate prediction several lines of geomorphological and stratigraphical evidence suggest that the widespread early post glacial wet phase in tropical africa between 12 500 and 5 000 yr bp refs 1 2 was interrupted by severe dry episodes during the intervals 11 000 10 000 8 500 6 500 and 6 200 5 800 yr bp refs 3 9 the magnitude of the second event has recently been questioned10 we present new stratigraphical and dating evidence from the ziway shala lake basin ethiopia indicating the severity and timing of dry phases during the interval 12 000 5 000 yr bp our findings have important implications for predictions of the climatic impact of increasing atmospheric levels of co2

eolian sediment responses to late quaternary climate changes temporal and spatial patterns in the sahara abstract this paper presents a compilation of eolian based records of late quaternary climate changes in the sahara although the data are relatively sparse when viewed as a whole they reveal a general pattern of widespread eolian sediment mobilization prior to 11 000 cal years bp eolian sediment stabilization from 11 000 to 5000 cal years bp and a return to widespread eolian sediment mobilization after 5000 cal years bp furthermore an eolian based record from southern tunisia reveals the existence of millennial scale changes in eolian sediment behavior these millennial scale variations provide examples of eolian sediment responses to climate changes at a scale intermediate between seasonal and orbital milankovitch changes and they are also coincident with abrupt atmospheric and oceanic changes the general synchroneity of the eolian stratigraphic records and their coincidence with various oceanic and atmospheric changes suggest that global forcing mechanisms have influenced late quaternary eolian sediment behavior in the sahara

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

grain size control on sr nd isotope provenance studies and impact on paleoclimate reconstructions an example from deep sea sediments offshore nw africa five sediment cores from offshore nw africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes sediments were taken from three time slices with well known and distinctive environmental conditions present day dry and warm similar to 6 ka wet and warm and similar to 12 ka dry and cold terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore nw africa fluvial 0 10 mu m and aeolian 10 40 mu m sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction in contrast epsilon nd values are largely unaffected by grain size minor variability in nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last similar to 12 ka variations in sr isotope ratios are interpreted to reflect major changes in the evaporation precipitation balance we suggest that the sr nd isotope data record a latitudinal shift of the northern limit of the african rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over nw africa while hyperarid conditions prevailed similar to 12 ka more humid conditions and intensified monsoonal rainfall at similar to 6 ka resulted in greater breakdown of easily weathered k bearing phases and increased sr 87 sr 86 in the detritus in late holocene times the monsoonal circulation diminished resulting in a return to arid conditions our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms if isotopic analyses are carried out on bulk sediments the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations

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

multidecadal variations in the early holocene outflow of red sea water into the arabian sea we present holocene stable oxygen isotope data from the deep arabian sea off somalia at a decadal time resolution as a proxy for the history of intermediate upper deep water these data show an overall 18o reduction by 0 5 between 10 and 6 5 kyr b p superimposed upon short term 18o variations at a decadal centennial timescale the amplitude of the decadal variations is 0 3 prior and up to 0 6 subsequent to 8 1 kyr b p we conclude from modeling experiments that the short term 18o variations between 10 and 6 5 kyr b p most likely document changes in the evaporation precipitation balance in the central red sea changes in water temperature and salinity cause the outflowing red sea water to settle roughly 800 m deeper than today

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

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

wind variability over the northern indian ocean during the past 4 million years insights from coarse aeolian dust iodp exp 359 site u1467 maldives abstract the lithogenic fraction of carbonate drift sediments from iodp exp 359 site u1467 maldives provides a unique record of atmospheric dust transport over the northern indian ocean during the past 4 myr grain size data provide proxies for dust flux controlled by source area aridity as well as wind transport capacity wind speed entrainment and long range transport of dust in the medium to coarse silt size range is linked to the strength of the arabian shamal winds and the occurrence of convective storms which prolong dust transport dust flux and the size of dust particles increased between 4 0 and 3 3 ma corresponding to the closure of the indonesian seaway and the intensification of the south asian monsoon there is no clear trend in dust flux between 3 3 and 1 6 ma whereas wind transport capacity decreased between 1 6 ma and the recent dust flux increased and shows higher variability especially during the last 500 kyr transport capacity increased between 1 2 and 0 5 ma and slightly decreased since then frequency analysis shows that dust transport varies on orbital timescales with eccentricity control being the most prominent 400 kyr throughout the record 100 kyr between 2 0 and 1 3 ma and since 1 0 ma higher frequency cycles obliquity and precession are more pronounced in wind transport capacity than in the amount of dust this indicates that the amount of coarse dust in sediments from the maldives as a far field site is more prone to changes in transport mechanisms than to changes in dust source area aridity

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

dust response to seasonal atmospheric forcing proxy evaluation and calibration eolian grain size and flux were measured on samples from 11 arabian sea sediment traps deployed 200 1250 km offshore the timing of increased grain size is coincident with the onset of strong summer monsoon winds and dust storm activity over the arabian peninsula and middle east data spanning a full annual cycle show that eolian grain size is highly correlated with barometric pressure r 0 91 and wind speed r 0 84 enabling calibration of the downcore record in terms of these primary meteorological variables eolian flux is highly correlated with organic carbon flux r 0 80 both increase 6 8 weeks after the grain size increase and summer monsoon onset this lag and the low correlation between eolian grain size and eolian flux r 0 36 likely result from the differential sinking rates of large and small dust particles in the surface waters as well as biological scavenging associated with monsoon induced productivity marily to aridity and vegetation cover at the source and 3 that grain size is primarily a function of atmospheric transport capacity these findings are based on statistical relation ships linking dust flux to glacial interglacial climate change 5 o and grain size to biological indicators of wind driven upwelling such relationships are well documented and have provided a reasonably robust framework for interpreting the downcore dust record rea 1994 provides an excellent overview however direct evaluation of links between at mospheric dynamics dust aerosols and deep sea sediments in the modem environment are needed to better understand

the provenance of sediments in the gulf of lions western mediterranean sea in this study we undertook a reconnaissance study of sediments provenance in the gulf of lions focusing over the last 16 ka we used geochemical and isotopic tracers to determine the source of sediments and give insight into the weathering conditions prevailing sediments samples were selected both onshore and offshore from the western eastern and central part of the gulf of lions we analyzed bulk sediments coarse and fine silt and clay fractions elemental and nd isotope compositions appeared to differ from one grain size fraction to another one these are interpreted in terms of zircon addition in the coarse silt fraction for the elemental concentrations and variable sources influences for the nd isotope compositions our results indicate that sediments in the gulf of lions mainly originated from the rhone river watershed although a contribution of saharan dust is seen in one sample influence of pyrenean small rivers is minor in these samples some sr isotope compositions shifts are interpreted as reflecting variable amounts of chemical weathering that are consistent with published paleoclimatic reconstructions

a 300 kyr record of upwelling off oman during the late quaternary evidence of the asian southwest monsoon in the northwest arabian sea upwelling occurs each summer driven by the strong sw monsoon winds upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off oman that are preserved in the sediments along the oman continental margin creating a geologic record of monsoon driven upwelling sediments recovered from the oman continental margin during ocean drilling program leg 117 provide an opportunity to examine how upwelling has varied during the late quaternary spanning a longer interval than piston cores recovered prior to the odp cruise variations in foraminifer shell accumulation and in the relative abundance of globigerina bulloides indicate dominant cycles of variation at 1 100 kyr the dominant frequency of glacial interglacial variations and at 1 23 kyr the frequency of precessionally driven cycles in seasonal insolation the strongest monsoon winds indicated by increased upwelling occurred during interglacial times when perihelion was aligned with the summer solstice an orbital change that increased the insolation received during summer in the northern hemisphere during glacial times upwelling was reduced and although the precessional cycles were still present their amplitude was smaller at both frequencies the upwelling cycles are in phase with minimum ice volume evidence that glacial interglacial climate changes also include changes to the climate system that influence the low latitude monsoon we attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over asia and or to changes in the areal extent and seasonal cycle in asian snow cover that decrease the summer land sea temperature contrast other mechanisms may also be involved these new upwelling time series differ substantially from previous results however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late quaternary

astronomic timescale for the pliocene atlantic δ18o and dust flux records of ocean drilling program site 659 high resolution benthic oxygen isotope and dust flux records from ocean drilling program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the atlantic from 2 85 ma back to 5 ma spectral analysis of the 18o record indicates that the 41 kyr period of earth s orbital obliquity dominates the pliocene record this is shown to be true regardless of fundamental changes in the earth s climate during the pliocene however the cycles of sahelian aridity fluctuations indicate a shift in spectral character near 3 ma from the early pliocene to 3 ma the periodicities were dominantly precessional 19 and 23 kyr and remained strong until 1 5 ma subsequent to 3 ma the variance at the obliquity period 41 kyr increased the timescale tuned to precession suggests that the pliocene was longer than previously estimated by more than 0 5 m y

origin of increased terrigenous supply to the ne south american continental margin during heinrich stadial 1 and the younger dryas we investigate the redistribution of terrigenous materials in the northeastern ne south american continental margin during slowdown events of the atlantic meridional overturning circulation amoc the compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical atlantic shows an extreme rise in sedimentation rates off the parnaiba river mouth about 2 s during heinrich stadial 1 hs1 18 15 ka sediment core geob16206 1 raised offshore the parnaiba river mouth documents relatively constant 143nd 144nd values expressed as end 0 throughout the last 30 ka whereas the homogeneous end 0 data support the input of fluvial sediments by the parnaiba river from the same source area directly onshore the increases in fe ca al si and rb sr during hs1 indicate a marked intensification of fluvial erosion in the parnaiba river drainage basin in contrast the end 0 values from sediment core geob16224 1 collected off french guiana about 7 n suggest amazon sourced materials within the last 30 ka we attribute the extremely high volume of terrigenous sediments deposited offshore the parnaiba river mouth during hs1 to i an enhanced precipitation in the catchment region and ii a reduced north brazil current which are both associated with a weakened amoc

review of recent advances in the interpretation of eastern mediterranean sapropel s1 from geochemical evidence abstract the sediments of the eastern mediterranean basin contain c org enriched layers sapropels interbedded with the c org poor sediments which form by far the greater part of the record while it is generally appreciated that different surface ocean productivity and bottom water conditions are necessary for the formation and preservation of these two sediment types less attention has been paid to diagenetic effects which are an expected consequence of transitions between dramatically different bottom water oxygenation levels a geochemical interpretation has emerged of post depositional oxidation of the most recent sapropel s1 initially based on the relationship of the mn fe c org and s concentration depth profiles observed around s1 and the characteristic shapes of these elemental profiles known from other situations this indicates that post depositional oxidation has removed approximately half of the visual evidence of the sapropel 6 cm from a total of 12 cm in the deep basin the oxidation interpretation from redox sensitive element redistribution profiles has subsequently been consolidated with evidence from pore water o 2 no 3 mn 2 and fe 2 studies from characteristic solid phase ba profiles which yield palaeoproductivity records and from oxidation sensitive indicator trace elements i and se so far these geochemical observations have been concentrated in the deeper central parts of the basin where sediment accumulation rates are lower than on the basin margins and radiocarbon dating indicates that s1 formation occurred between 5 3 and 9 0 ky uncorrected conventional radiocarbon time it remains to be demonstrated whether or not these times are applicable to the entire e mediterranean basin the implications of these findings to guide sampling in future work on the s1 productivity episode and on older sapropels for palaeoenvironmental investigations are discussed

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

synchronous holocene sea surface temperature and rainfall variations in the asian monsoon system abstract an increasing number of high resolution paleoclimate records show substantial natural variation during the holocene in order to improve climate projections on human lifetime the processes that potentially control teleconnections between different parts of the climate system need to be understood a highly suitable area to study these processes is the asian monsoon system as it is one of the most dynamic climate systems on earth and largely controls climate in asia and the indo w pacific realm here we present a holocene stable o isotope record from the summer dwelling planktic foraminifer g bulloides in core 905 off somalia initially dated by the radiocarbon method the record was tuned to atmospheric 14 c variations without violating the radiocarbon dates the o isotope variations in core 905 imply monsoon controlled holocene average summer sea surface temperature variations of up to 2 2 5 c within decades to centuries other proxy records from marine and continental settings representing key locations in the asian monsoon system were compared with the record of core 905 within the resolution of the individual age models holocene monsoonal records vary in concert for the arabian sea continental china and the south china sea and imply simultaneous warm wet and cool dry alternations in the asian monsoon within centuries to decades two potential scenarios are discussed to explain the coherent monsoonal records scenario one involves trade wind induced temperature variations in the w pacific warm pool as the driving force the second scenario advocates the controlling process to synchronous variations in trade wind strength simultaneously occurring over the pacific and indian ocean

sr isotope evidence for sources of terrigenous sediment in the southeast atlantic ocean is there increased available fe for enhanced glacial productivity 1 sr isotope ratios of the terrigenous sediments from the cape basin southeast atlantic ocean exhibit a systematic pattern of climate related variability from the holocene through the last glacial period values are high during warm climate intervals marine isotope stages miss 1 and 3 and lower during full glacial periods miss 2 and 4 the variability is large 87sr 86sr 0 717 0 723 and the rapid changes correspond temporally to abrupt climate change during the mis 5a 4 and 2 1 transitions and through mis 3 the sr isotope variability corresponds to changes in 13c of benthic foraminifera at orbital frequencies and within periods of rapid variability prior studies have suggested that benthic 13c records from the cape basin follow greenland ice core variability and thus global overturning circulation other studies suggest that these benthic 13c records contain a strong overprint from isotopically light carbon possibly associated with high fluxes of organic matter to the seabed we explore the scenario that the relationship between lower terrigenous 87sr 86sr and lighter benthic 13c may reflect high productivity during cold climatic intervals as a result of iron fertilization of the southern atlantic ocean increased supply of iron during cold periods may be associated with greater terrigenous sediment fluxes from south america characterized by a less radiogenic sr isotopic signature

variation in the indian summer monsoon intensity during the bolling allerod and holocene variations in the indian summer monsoon ism intensity during the last 16 7 ka have been studied using organic carbon corg 15 n of sedimentary organic matter caco3 sediment texture 18 oc and mg ca derived sea surface temperature 18 o of sea water and sea surface salinity in a 14 c dated sediment core from the eastern arabian sea the 18 o in water and planktonic foraminifera shells off the central west coast of india may be controlled by the ism intensity as this area receives high precipitation and land runoff also the corg and caco3 contents of sediments and 15 n of sedimentary organic matter may be linked to ism induced productivity and denitrification the results of the present study reveal that between 16 and 15 2 ka bp the ism was weak with minor fluctuations and started intensifying around 15 2 ka bp at the onset of the bolling allerod b a event the b a event is characterized by higher water column denitrification rates comparable to the present day the ism signatures observed in the 18 oc record of b a event compare well with those from timta cave of the western himalayas and also the asian summer monsoon signatures from the hulu caves in china and warming signatures in greenland ice sheet project 2 gisp2 suggesting atmospheric tele connections through intertropical convergence zone itcz the boundary between the younger dryas and the holocene is discernible with small episodes of abrupt events of increased ism intensity this decrease in 18 oc values at 11 8 ka bp is contemporary with june solar insolation maximum at 30 north and the increase in methane in the gisp2 ice core supporting episodes of warmer climate and increase in ism intensity the ism seems to have been most stable between 7 and 5 6 ka bp the core exhibits periodicity of 500 that is comparable to the atlantic water formation and the chinese monsoon

east african mid holocene wet dry transition recorded in palaeo shorelines of lake turkana northern kenya rift abstract the wet early to mid holocene of tropical africa with its enhanced monsoon ended with an abrupt shift toward drier conditions and was ultimately replaced by a drier climate that has persisted until the present day the forcing mechanisms the timing and the spatial extent of this major climatic transition are not well understood and remain the subject of ongoing research we have used a detailed palaeo shoreline record from lake turkana kenya to decipher and characterise this marked climatic transition in east africa we present a high precision survey of well preserved palaeo shorelines new radiocarbon ages from shoreline deposits and oxygen isotope measurements on freshwater mollusk shells to elucidate the holocene moisture history from former lake water levels in this climatically sensitive region in combination with previously published data our study shows that during the early holocene the water level in lake turkana was high and the lake overflowed temporarily into the white nile drainage system during the mid holocene 5270 300 cal yr bp however the lake water level fell by 50 m coeval with major episodes of aridity on the african continent a comparison between palaeo hydrological and archaeological data from the turkana basin suggests that the mid holocene climatic transition was associated with fundamental changes in prehistoric cultures highlighting the significance of natural climate variability and associated periods of protracted drought as major environmental stress factors affecting human occupation in the east african rift system

silicate rock weathering and atmospheric soil co2 uptake in the amazon basin estimated from river water geochemistry seasonal and spatial variations abstract using the data of the camrex project 1982 1984 on the water geochemistry of the amazon river and its main tributaries it was possible to assess the silicate rock weathering processes and the associated consumption of atmospheric soil co2 taking into account seasonal and spatial variations this study confirms the important role of the andes in the fluvial transport of dissolved and particulate material by the amazon and it shows for the first time that the silicate weathering rate and atmospheric soil co2 consumption are higher in the andes than in the rest of the amazon basin the seasonal variations exhibit the significant role of runoff as a major factor controlling silicate weathering processes and show that the chemical erosion rates vary greatly from low discharge to high discharge the average weathering rate estimated for the whole amazon basin 15 m my is comparable to other estimations made for other tropical equatorial environments a comparison between physical and chemical weathering rates of silicate rocks for the amazon basin and for each tributary basin show that in the andes and in the amazon trough the soil thicknesses are decreasing whereas in the shield the soil profiles are deepening

solid liquid ree fractionation in the lateritic system of goyoum east cameroon the implication for the present dynamics of the soil covers of the humid tropical regions the ree th weathering geochemistry and mineralogy has been investigated in a lateritic soil cover in relation to the close hydrographical system at goyoum east cameroon the stutied area composed of gneissic hills covered by humid tropical forest belongs to the sanaga river basin and corresponds to a transition vegetation zone between rainforest and savannah a representative soil catena was taken as a reference zone close to the sanaga river the gneissic parent rock soil and groundwater samples have been cut off in pits in the lower part of the catena waters were also sampled both in brooks rich in organic matter and in the sanaga river all the waters were sampled seasonally during the rainy and dry periods the water rock interactions and the exportation of ree and th have been discussed as a function of the evolution of specific ratios considering th as the less mobile element during laterization variation of parent rock pr normalized ratios ln th pr vs depth show that the upper ferruginous horizons are depleted in ree while the basal saprolite presents accumulation zones which are differently located for ce and the other lrees the hrees are also depleted the la accumulations are located near the saprolite gneiss weathering front associated with negative ce anomalies ce ce 0 4 the ce accumulations inducing strong positive ce anomalies ce ce 4 are located in the upper saprolite beneath the ferruginous horizons all along the soil catena the secondary lree bearing minerals which reflect lree fractionation during incipient weathering of the gneiss are hydrous phosphates rhabdophane lreepo4 nh2o and cerianite ceo2 whose precipitation depends on the eh ph changes in the soil profiles on the other hand hree depletion is controlled by the dissolution of xenotime the major hree bearer in the parent gneiss in the 0 45 m filtered acidic groundwater ph 5 5 which have flooded the saprolite for a long residence time th contents are extremely low 0 th 10 ppt while lree contents are high 90 la 450 ppt in these filtered waters th seems to be strongly controlled by minerals i e thorianite which lead to its immobilization in the soil while lree remain in the soluble and or colloidal pools moreover the groundwaters show significant negative ce anomalies throughout the year 0 1 ce ce 0 4 the mobilization of lree except redox active ce results from the dissolution and or the mobilization in colloidal form of the secondary phosphates which may present major negative ce anomalies as indicated by the bulk soil analyses at the saprolite gneiss weathering front therefore the groundwaters have the signature of saprolitic materials we propose that the strong positive ce anomalies of the upper saprolite result from ancient weathering processes these processes would have led to much stronger ce iii ce iv dissolution reprecipitation conditions than those occurring at the present time in the saprolite in the 0 45 m filtered brook acidic blackwaters lree and th contents are high 100 th 400 ppt 400 la 1700 ppt in this organic matter rich environment lree and th are highly mobilized the colloial pool controls half of the whole lree th load the geochemical signature of these waters is independent of the gneissic parent rock and soil lree contents there is no ce anomaly this signature is similar to those of other organic rich rivers draining humid tropical ecosystems in the world in the 0 45 m filtered sanaga waters lree and th contents are lower than in the brooks 15 th 40 ppt 40 la 300 ppt these concentrations are dependent on the time and on the colloidal composition however 90 of these elements are mainly controlled by the suspended pool

after the deluge mediterranean stagnation and sapropel formation in an east mediterranean marine core the upper sapropel begins soon after the start of a global event a very heavy precipitation which occurred in the equatorial latitudes during the late glacial early holocene this heavy precipitation in africa channelled by the nile river across 35 of latitude produced a low salinity surface layer in the east mediterranean in this confined basin with high bottom salinity the steep salinity gradient stratified the water column the stagnant bottom waters triggered the sapropel formation cretaceous sapropels in the tropical oceans may result from the same chain of events in warm humid climates with contrasting wet and dry seasonal rhythm

high and low latitude forcing of the nile river regime during the holocene inferred from laminated sediments of the nile deep sea fan sediments deposited on deep sea fans are an excellent geological archive to reconstruct past changes in fluvial discharge here we present a reconstruction of changes in the regime of the nile river during the holocene obtained using bulk elemental composition grain size analyses and radiogenic strontium sr and neodymium nd isotopes from a sediment core collected on the nile deep sea fan this 6 m long core was retrieved at view the mathml source water depth and is characterized by the presence of a 5 m thick section of finely laminated sediments which were deposited between 9 5 and 7 3 ka bp and correspond to the african humid period ahp the data show distinct changes in eolian dust inputs as well as variations in discharge of the blue nile and white nile sedimentation was mainly controlled by changes in fluvial discharge during the holocene which was predominantly forced by low latitude summer insolation and by the location of the eastern african rain belt the changes in relative contribution from the blue nile and white nile followed changes in low latitude spring autumn insolation which highlights the role of changes in seasonality of the precipitation on the nile river regime the relative intensity of the blue nile discharge was enhanced during the early and late holocene at times of higher spring insolation with massive erosion and runoff during the ahp at times of high summer insolation while it was reduced between 8 and 4 ka at times of high autumn insolation the gradual insolation paced changes in fluvial regime were interrupted by a short term arid event at 8 5 7 3 ka bp also associated with rejuvenation of bottom water ventilation above the nile fan which was likely related to northern hemisphere cooling events another arid event at 4 5 3 7 ka bp occurred as the apex of a gradually drier phase in ne africa and marks the end of the ahp

refining the pre industrial atmospheric pb isotope evolution curve in europe using an 8000 year old peat core from nw spain abstract pb pollution has existed for several millennia and remains relevant today by using peat cores as environmental archives it is possible to reconstruct this long history on a regional scale this is a significant contribution to the findings from ice core records the only other archive recording purely atmospheric additions without information that allows linking and comparison between sites regionally within europe and elsewhere our ability to make coherent global models of the natural pb cycle and anthropogenic forcing of this cycle is limited in this respect the characteristics of the pb pre pollution aerosol ppa are important to define globally we characterize for the first time a ppa in southern europe with pb 0 78 0 86 g g 1 net pb accumulation rates of 0 032 0 030 mg m 2 y 1 and a 206 pb 207 pb signature of 1 25470 0 02575 this ppa pb isotope signature is more radiogenic than that found thus far in western and northern europe spain is a historically important mining site using three isotope plots and a pool of potential pb isotope signatures a detailed source appointment of both natural and anthropogenic pb sources was made we found evidence of saharan aridification and its termination 4400 bp and or agricultural signals and strong local control from rock and soil of the pb ppa human impact is first recorded at 3210 bp but does not exceed 50 of deposited pb until 3005 bp mines in se spain dominate early pb pollution history at this site during the rise of roman rule contributions come from mines in n nw and sw spain with no strong indication of other european mining activities in medieval and industrial times local contributions to the peat bog are reduced

the pace of east african monsoon evolution during the holocene african monsoon precipitation experienced a dramatic change in the course of the holocene the pace with which the african monsoon shifted from a strong early to middle to a weak late holocene is critical for our understanding of climate dynamics hydroclimate vegetation interaction and shifts of prehistoric human settlements yet it is controversially debated on the basis of planktonic foraminiferal ba ca time series from the eastern mediterranean sea here we present a proxy record of nile river runoff that provides a spatially integrated measure of changes in east african monsoon eam precipitation the runoff record indicates a markedly gradual middle to late holocene eam transition that lasted over 3500 years the timing and pace of runoff change parallels those of insolation and vegetation changes over the nile basin indicating orbitally forced variation of insolation as the main eam forcing and the absence of a nonlinear precipitation vegetation feedback a tight correspondence between a threshold level of nile river runoff and the timing of occupation abandonment of settlements suggests that along with climate changes in the eastern sahara the level of nile river and intensity of summer floods were likely critical for the habitability of the nile valley egypt

record of holocene aridification 6000 7000bp in egypt ne africa authigenic carbonate minerals from laminated sediments in lake qarun abstract the origin and climate implications of authigenic carbonate minerals from lake qarun egypt were examined in relation to the mid late holocene climate changes these minerals occur as very thin 0 5 3 mm very fine well sorted white to pale yellow aragonite or calcite laminae intercalated with olive green clayey silt with sharp and horizontal contacts under the scanning electron microscope sem aragonite appears as dense euhedral and ellipsoidal rice grained crystals that range in length from 0 5 to 1 micron and in width from 0 2 to 0 5 micron the calcite crystals are stubby equant or blocky polyhedra of about 5 microns in length 18 o values range between 2 7 and 3 7 for the calcite and between 2 1 and 6 for the aragonite 13 c values range between 0 4 and 1 for the calcite and between 0 5 and 3 4 for the aragonite a very strong positive correlation is observed between carbon and oxygen isotopes for the aragonite r 2 0 9 and a more moderate positive correlation with calcite r 2 0 6 the complete absence of biological and other no carbonate grains the sharp contacts of laminae basal and the occurrence of calcite and aragonite as individual euhedral crystals rather than clusters of grain aggregation indicate that aragonite and calcite layers represent rapid inorganic precipitation from the lake water endogenic the positive values of 18 o and 13 c and the strong positive correlation between 18 o and 13 c suggest prevalence of a low lake level saline and dry climatic conditions during their formation the change from calcite to aragonite indicates a change in the formational conditions from less arid conditions in the case of calcite to severe arid conditions in the case of aragonite the relative abundance of diatoms in calcite rich laminae and the less strong positive correlation between 18 o and 13 c of calcite support this interpretation the arid conditions in which the carbonate minerals were formed could correspond to the aridification period of the north east africa between ca 6000 8500 bp due to the north south shifts of the intertropical convergence zone itcz that control the evaporation precipitation balance over north east africa and or to dry climate and desert environment conditions in egypt at the same period these results are also consistent with the aridification conditions during the mid late holocene suggesting its global trend

late glacial holocene climate variability at the south eastern margin of the aegean sea abstract new micropaleontological palynological and geochemical results from a relatively shallow 500 m sediment core ns 14 in the south eastern aegean sea provide a detailed picture of the regional expression of sapropel s1 formation in this sub basin of the eastern mediterranean sea specifically freshwater input during 10 6 10 0 ka bp has preceded the deposition of s1 further decrease in surface water salinity is evidenced between 10 0 and 8 5 ka bp at the lower part of s1a which in respect to s1b is featured by warmer 19 5 c and more productive surface waters associated with dysoxic bottom conditions a series of coolings detected within the s1 depositional interval may be linked to outbursts of cold northerly air masses and relevant pulses in the deep intermediate water ventilation that caused the s1 interruption between 7 9 and 7 3 ka bp and culminated during the deposition of s1b with the decline of deep chlorophyll maximum dcm at 6 5 ka bp the climate instability and the relevant absence of anoxia weakened the organic matter preservation in the shallow south eastern aegean margin during the s1 times ns 14 record provides evidence for a distinct mid holocene warm up to 25 c and wet phase associated with the deposition of the sapropel like layer smh sapropel mid holocene between 5 4 and 4 3 ka bp the smh layer could represent evidence of on going albeit weak african monsoon forcing only expressed at the south eastern edge of the aegean sea its end is associated with the 4 2 ka bp northern hemisphere mega drought event and the termination of the african humid period at 3 8 ka bp

pleistocene holocene environmental change in the canary archipelago as inferred from the stable isotope composition of land snail shells abstract the isotopic composition of land snail shells was analyzed to investigate environmental changes in the eastern canary islands 28 29 n over the last 50 ka shell 13c values range from 8 9 to 3 8 at various times during the glacial interval 15 to 50 ka moving average shell 13c values were 3 higher than today suggesting a larger proportion of c4 plants at those periods shell 18o values range from 1 9 to 4 5 with moving average 18o values exhibiting a noisy but long term increase from 0 1 at 50 ka to 1 6 1 8 during the lgm 15 22 ka subsequently the moving average 18o values range from 0 0 at 12 ka to 0 9 at present calculations using a published snail flux balance model for 18o constrained by regional temperatures and ocean 18o values suggest that relative humidity at the times of snail activity fluctuated but exhibited a long term decline over the last 50 ka eventually resulting in the current semiarid conditions of the eastern canary islands consistent with the aridification process in the nearby sahara thus low latitude oceanic island land snail shells may be isotopic archives of glacial to interglacial and tropical subtropical environmental change

stepwise holocene aridification in ne africa deduced from dust borne radiogenic isotope records abstract transfer of tropical heat to higher latitudes is the major driving force of the earth s climate consequently sediments in regions to the north and south of the tropics potentially retain an archive of past major climate reconfigurations the climate of one such region around the arabian sea sensitively depends on the coupled asian and african monsoons that also control the dust transport here we use the sr nd isotope ratios of the dust fraction from core 905 arabian sea off somalia as a novel tool to deduce the holocene weathering history of the horn of africa with emphasis on the climate transition that took place from a wet early to a dry late holocene the highly variable sr isotope ratios are interpreted to reflect mainly changes in the evaporation precipitation balance over ne africa whilst the nd isotope measurements record no significant variations and point to a prevailing ne african dust source the sr isotope record shows that the first aridification step occurred at 8 5 kyr bp followed by an unstable transitional period up to 6 kyr bp characterized by decadal scale high amplitude variations in the evaporation precipitation balance a second aridification step began at 6 kyr bp and ceased at 3 8 kyr bp when modern day dry climate was established the combined sr and nd isotope records probably reflect north south shifts of the intertropical convergence zone controlling the evaporation precipitation balance over ne africa

eccentricity forcing of pliocene early pleistocene climate revealed in a marine oxygen isotope record milankovitch theory that climate is controlled by variations in the earth s orbital parameters has gained wide acceptance for its ability to account for two climate cycles a 23 kyr cycle that is phase locked to the precession driven insolation cycle and a 41 kyr cycle that is phase locked to the obliquity driven insolation cycle1 6 but explaining the observed 100 kyr climate cycle in terms of milankovitch theory especially for the late pleistocene ice age cycle remains controversial in spite of a strong correlation with the 100 kyr cycle in the earth s orbital eccentricity5 one problem is that eccentricity affects insolation mainly by modulating the precession cycle its direct contribution to radiation change is too small 0 1 to effect the observed climate change directly5 7 another is the absence of a late pleistocene ice volume cycle in oxygen isotope records to match the 404 kyr component of the eccentricity cycle5 8 here we examine an oxygen isotope record spanning the interval 1 2 to 5 2 million years ago before the late pleistocene ice age regime we find 404 kyr and 100 kyr climate cycles which are coherent with eccentricity and which have amplitudes that are similar to the coexisting 23 kyr cycle analysis of these low frequency cycles suggests that they originate through an asymmetrical response mechanism that preferentially introduces variance into the climate system from the warmer portions of the eccentricity modulated precession cycle our data thus support eccentricity s role in the origin of low frequency oxygen isotope cycles before the late pleistocene ice age

the history and nature of wind erosion in deserts recently the importance of wind erosion has been reevaluated many low angle surfaces developed on susceptible materials possess closed depressions pans and these often have a distinctive morphology and lunette dunes on their lee sides it has also become apparent that in drylands there are extensive areas of stone pavements some of which have been molded by deflation however it is now recognized that other horizontal sorting processes have played a role especially movement of fines by overland flow vertical sorting associated with wetting and drying salt heave bioturbation frost action and dust accretion have also played a role in hyperarid areas especially with unidirectional winds bedrock outcrops and old lake beds have been molded to give aerodynamic forms called yardangs the study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation especially in hyperarid areas with centripetal drainage deflation hot spots such as the bo

paleoclimatic changes on the southern tibetan plateau over the past 19 000 years recorded in lake pumoyum co and their implications for the southwest monsoon evolution abstract the paleoclimatic changes in the southern region on the tibetan plateau tp after the lgm were studied by multi proxy analysis including pollen ca carbonates toc 13 c toc plant macrofossils and sands with sediment cores from lake pumoyum co close to the east end of the himalayas the following paleoclimatic changes characteristic of the southern tp were suggested by comparing with the paleoclimatic records from various regions within the southwest sw monsoon domain on the tp soon after the lgm deglaciation in the southern tp had already started at 18 5 ka bp and more actively increased after ca 16 5 ka bp with the hydrological circulation oscillating in ca 1200 2000 and 4000 year periods an abrupt increase in humidity under the reduced sw monsoon intensity occurred at 15 0 ka bp as part of further deglaciation processes subsequently the climate amelioration in the southern region started from 15 0 ka bp a few centuries earlier than other regions of the tp and then was further intensified by synchronizing with the bolling allored b a event over the b a period the sw monsoon intensity in the southern region almost continuously and sharply increased unlike many regions on the tp the southern region experienced almost no or substantially small impact of the younger dryas yd event and also other major dry cold reversals such as heinrich 1 event the 8 2 kyr event and an abrupt spell occurring between 5 and 4 ka bp the monsoon intensity in the southern region after the yd event started to rapidly increase from 11 4 ka bp and reached a maximum between 10 8 and 10 0 ka bp after the lgm which lagged about 1200 years behind the maximum insolation in the northern hemisphere from ca 9 6 ka bp the gradual decrease in humidity in the southern tp started and continued until 4 2 ka bp it s aridification trend was characterized by three stepwise transition observed at 9 6 7 7 and 5 5 ka bp however the aridification in the southern tp was rather weak and the climatic conditions were sustained fairly humid and comparably stable the humidity between 4 2 and 2 5 ka bp essentially increased despite the weakened sw monsoon intensity at that time whereas from 2 5 ka bp to the present the climatic conditions became most arid and instable during the holocene as a consequence the holocene climate optimum in the southern region lasted from 11 4 ka bp until 2 5 ka bp and was marked by the earliest start and the latest end timings within the sw monsoon domain on the tp the comparison of the start timings from various paleoclimatic records indicated that the first sw monsoon on the tp after the yd event generated at 11 4 ka bp in the southern region subsequently according to the degree of the recovery of positive vegetation albedo feedback system in each region the monsoon probably extended from the southern tp over the entire tp taking 3000 4000 years based on these results the possible key factors and mechanisms responsible for the paleoclimatic changes in the southern region and also the possible processes as to the evolution of the sw monsoon over the tp are discussed

environmental change and archaeology lake evolution and human occupation in the eastern sahara during the holocene abstract the west nubian palaeolake is the most large scale hydrographic evidence in the eastern sahara of the early to mid holocene wet phase that affected northern africa it is the result of a significant increase in local rainfall due to the northward shift of the tropical rainfall belt a series of fieldwork based differential gps dgps measurements along several profiles across the west nubian palaeolake basin provides the first precise topographic data from this up to a 5330 km2 large palaeolake feature in combination with sedimentological geochemical and archaeological results an almost complete picture of significant palaeoclimatic changes and human occupation during the early to mid holocene for this region is presented different stages of palaeolake evolution ranging from non existence of the lake through stable freshwater conditions to its extinction were identified in the period from 9400 to 3800 14c yr bp these lake stages coincide with phases of intensive human inhabitation between ca 6300 and 3500 14c yr bp and include at least four settlement phases distinguishable by style of pottery these are known from adjacent areas of the palaeolake region emphasizing strong prehistoric cultural connections in the eastern sahara during the highstands of the palaeolake in the early to mid holocene the dotted wavy line pottery relates to the early khartoum type culture with its supra regional distribution from the nile valley to the chad and possibly with slightly different forms even to the atlantic coast later in the holocene western nubia with its large palaeolakes and migration paths along palaeowadis such as wadi howar acted as an important natural and cultural link between the nile valley and the chad basin until the region was deserted during the fourth millennium bp

sediment and pollen evidence for an early to mid holocene humid period in the eastern sahara a major problem in the study of holocene palaeoenvironments of the arid and wind deflated sahara is the low preservation potential of sediments from which a record of past climatic change can be established several indirect and imprecisely dated pieces of evidence have suggested humid episodes in the holocene that may have supported more productive ecosystems and denser human populations than today searches for reliable proxy data on past climates however have failed to yield satisfactory results we report here the discovery of buried lake muds in north west sudan in the hyperarid core of the eastern sahara which yield sedimentological and palynological data clearly interpretable as recording an early to mid holocene humid episode that supported a relatively deep stratified lake surrounded by tropical savanna woodland vegetation

the initiation and evolution of the river nile abstract the nile is generally regarded as the longest river in the world knowledge of the timing of the nile s initiation as a major river is important to a number of research questions for example the timing of the river s establishment as a catchment of continental proportions can be used to document surface uplift of its ethiopian upland drainage with implications for constraining rift tectonics furthermore the time of major freshwater input to the mediterranean is considered to be an important factor in the development of sapropels yet the river s initiation as a major drainage is currently constrained no more precisely than eocene to pleistocene within the modern nile catchment voluminous cenozoic continental flood basalts cfbs are unique to the ethiopian highlands thus first detection of their presence in the nile delta record indicates establishment of the river s drainage at continental proportions at that time we present the first detailed multiproxy provenance study of oligocene recent nile delta cone sediments we demonstrate the presence of ethiopian cfb detritus in the nile delta from the start of our studied record c 31 ma by 1 documenting the presence of zircons with u pb ages unique within the nile catchment to the ethiopian cfbs and 2 using sr nd data to construct a mixing model which indicates a contribution from the cfbs we thereby show that the nile river was established as a river of continental proportions by oligocene times we use petrography and heavy mineral data to show that previous petrographic provenance studies which proposed a pleistocene age for first arrival of ethiopian cfbs in the nile delta did not take into account the strong diagenetic influence on the samples we use a range of techniques to show that sediments were derived from phanerozoic sedimentary rocks that blanket north africa arabian nubian shield basement terranes and ethiopian cfb s we see no significant input from archaean cratons supplied directly via the white nile in any of our samples whilst there are subtle differences between our nile delta samples from the oligocene and pliocene compared to those from the miocene and pleistocene the overall stability of our signal throughout the delta record and its similarity to the modern nile signature indicates no major change in the nile s drainage from oligocene to present day

holocene loess accumulation and soil development at the western edge of the chinese loess plateau implications for magnetic proxies of palaeorainfall a high resolution holocene sequence of loess palaeosols and incipient soils at the western edge of the chinese loess plateau dated usingoptically stimulated luminescence techniques is used to identify the respective influence of dust accumulation rate time and climate on soil magnetic properties dust deposition and soil formation have both been quasi continuous through the holocene at this site the soils are thus accretionary in nature the degree of soil development as indicated both by geochemical and magnetic properties which correlate strongly varied through the holocene compared with the less weathered loess units each palaeosol is enriched in nitrogen and organic carbon depleted of carbonate due to leaching and displays higher values of pedogenic magnetic susceptibility frequency dependent susceptibility remanences and magnetisation magnetic grain size indicators show that the pedogenic ferrimagnets are ultrafine of single domain and superparamagnetic dimensions sediment accumulation rates were lowest from b12 to 2 5 ka providing ample time 100 s of years for weathering and soil formation to proceed yet pedogenesis through this interval was relatively weak conversely soil development was stronger during later intervals when loess accumulation rates were higher and soil forming intervals were correspondingly shorter in terms of jenny s 1941 soil forming equation accumulation rate and thus time sediment source parent material and topography show no systematic variation between the loess and the palaeosols therefore climate seems to be the key soil forming factor which has controlled the geochemical and magnetic properties of these holocene palaeosols r 2002 elsevier science ltd all rights reserved

timing and stepwise transitions of the african humid period from geochemical proxies in the nile deep sea fan sediments large fluvial systems such as the nile river allow a basin scale integrated view of climatological and environmental changes in this study we reconstructed the nile discharge history for the last 20 ka using molecular ratios of glycerol dialkyl glycerol tetraethers gdgts and neodymium nd radiogenic isotopes by characterizing both the organic and inorganic fractions we assessed the relevance of the gdgt based proxies in deltaic environments as tracers of terrigenous origin a large increase in nile discharge is documented from 14 8 to 8 4 ka bp reflecting enhanced physical erosion and transport processes from the ethiopian traps we confirmed the primary control of insolation on precipitation on north east africa through the last 20 ka the centennial time resolution reached on the sedimentary sequence revealed a step wise onset and termination of the african humid period ahp starting at 14 8 ka and ending at 8 4 ka bp respectively our centennial millennial records allowed furthermore pinpointing the abrupt periods of arid conditions corresponding to the younger dryas these data illustrate the linkage between low and high latitude hydrological variability

development of a protocol to obtain the composition of terrigenous detritus in marine sediments a pilot study from international ocean discovery program expedition 361 abstract the geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments international ocean discovery program expedition 361 drilled sites along the eastern margin of southern africa that potentially provide archives of rainfall on the continent as well as dispersal in the agulhas current we used standard methods to remove carbonate and ferromanganese oxides and stokes settling to isolate the clay fractions in comparison to most previous studies that aimed to extract the detrital signal from marine sediments we additionally applied a cation exchange wash using cscl as a final step in the sample preparation the motivation behind the extra step not frequently applied is to remove ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure either would alter the composition of the detrital fraction if no cation exchange was applied moreover using cscl will provide an additional measure of the cation exchange capacity cec of the samples however no study so far has evaluated the potential and the limitations of such a targeted protocol for marine sediments here we explore the effects of removing and replacing adsorbed cations on the clay surfaces with cs conducting measurements of the chemical compositions and radiogenic isotopes on a set of eight clay sample pairs both sets of samples underwent the same full leaching procedure except that one batch was treated with a final cscl wash step in this study organic matter was not leached because sediments at iodp site u1478 have relatively low organic content however in general we recommend including that step in the leaching procedure as expected significant portions of elements with high concentrations in seawater were replaced by cs 2sd 2 8 from the wash including 75 of the sodium and approximately 25 of the calcium 10 of the magnesium and 8 of the potassium trace metals such as sr and nd whose isotopes are used for provenance studies are also found to be in lower concentrations in the samples after the exchange wash the exchange wash affected the radiogenic isotope compositions of the samples neodymium isotope ratios are slightly less radiogenic in all the washed samples strontium and pb isotopes showed significant deviations to either more or less radiogenic values in different samples the radiogenic isotopes from the cscl treated fractions gave more consistent correlations with each other and we suggest this treatment offers a superior measure of provenance although we observed changes in the isotope ratios the general trend in the data and hence the overall provenance interpretations remained the same however the chemical compositions are significantly different we conclude that a leaching protocol including a cation exchange wash e g cscl is useful for revealing the terrestrial fingerprint cec could with further calibration efforts be useful as a terrestrial chemical weathering proxy

late quaternary changes in climate and land cover in the northern horn of africa and adjacent areas abstract most studies of landscape dynamics in the ethiopian highlands offer linear relations of population pressure and periodic droughts as causes of environmental degradation such views rely on limited historical information of one or two centuries which is inadequate for understanding the longer term effects of climatic shifts on landscape dynamics and environmental change in the highlands in this study we conducted a meta analysis of long term climate and biogeographic change and landscape dynamics during the holocene using multi proxy evidence from lacustrine n 10 marine n 5 geomorphic n 13 and vegetation n 7 records from the highlands and the wider horn of africa and western indian ocean region based on the datasets we discern late pleistocene dry phases in the region during the last glacial maximum the heinrich 1 event and the younger dryas additionally we recognize at least six dry phases during the holocene after 6500 bce 4900 bce 3700 bce 1000 bce 450 ce and 1750 ce the hydroclimatic shifts seem associated with changing equatorial atlantic and western indian ocean sea surface temperatures further the results show nonlinear relations between major holocene climate shifts and grassland expansions in the highlands our findings challenge conventional cause effect explanations of human population growth and land degradation and point instead to potentially strong but unexplored feedbacks between climate changes human activity and landscape dynamics over longer and short term periods