hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

sedimentation at manop site h eastern equatorial pacific over the past 400 000 years climatically induced redox variations and their effects on transition metal cycling gravity cores recovered from manganese nodule project site h 6 33 n 92 49 w show marked downcore variations in the abundance of calcium carbonate organic carbon opal manganese and other components deposited over the past 400 000 years variations in the downcore abundance of organic carbon which ranges from 0 2 to 1 0 can be used to hindcast redox conditions in the surface sediments over this time the results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles downcore variations in solid phase mn ni and cu can be produced by such changes in redox conditions a model which predicts that solid phase mn can be trapped and buried when the mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and mn records and supports the reconstructed redox variations the history of redox variations at site h can be explained by changes with time in surface water productivity major productivity variations at the site occur over 100 kyr cycles with relatively higher productivity occurring during glacial stages thus quaternary climate changes influence surface water productivity redox conditions in sediments and the cycling of transition metals

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

development of a dynamics based statistical prediction model for the changma onset abstractthe timing of the changma onset has high impacts on the korean peninsula yet its seasonal prediction remains a great challenge because the changma undergoes various influences from the tropics subtropics and midlatitudes in this study a dynamics based statistical prediction model for the changma onset is proposed this model utilizes three predictors of slowly varying sea surface temperature anomalies sstas over the northern tropical central pacific the north atlantic and the north pacific occurring in the preceding spring season sstas associated with each predictor persist until june and have an effect on the changma onset by inducing an anticyclonic anomaly to the southeast of the korean peninsula earlier than the climatological changma onset date the persisting negative sstas over the northern tropical central pacific and accompanying anomalous trade winds induce enhanced convection over the far western tropical pacific in turn these induce a cyclonic anomaly over the south china s

formation of the holocene lake chilwa sand bar southern malawi summary a sand ridge up to 25 m high separates lake chilwa malawi from its former indian ocean outlet consisting of beach sands capped by dunes the sand bar was formed by the growth of spits across the northern end of the lake mineralogical studies of the sands indicate that they were derived from northern and western areas of the chilwa basin and were brought to the lake during a period of increased runoff and rainfall intensities the formation of the sand bar in the early holocene represented a radical change in the hydrology and biology of lake chilwa from an open fresh water lake to a closed saline lake with a marked tendency to fluctuate in level

euthecosomatous pteropods as paleohydrological and paleoecological indicators in a tyrrhenian deep sea core abstract three assemblage zones could be recognized in a core from the northern tyrrhenian sea the lowest zone is characterized by the absolute dominance of limacina retroversa the middle zone by the occurrence of diacria trispinosa and the upper zone representing the holocene by a diverse fauna with species like cavolinia inflexa and styliola subula paleohydrological interpretation indicates that the assemblage zones reflect major shifts in the position of water masses in particular the expanding influence of eastern mediterranean waters and the transitional to subtropical water masses of the eastern north atlantic ocean the presence or absence of uncommon to rare pteropod species is of paleoenvironmental significance as well paleotemperature reconstruction based on protoconch volume measurements of clio pyramidata indicate a rise in seawater temperature of some 4 to 5 c from glacial to post glacial times comparisons with pteropod data from the literature especially from the adriatic sea make it clear that similar ecological changes can be observed in detail in different parts of the mediterranean sea an ecostratigraphic zonation the quaternary mediterranean pteropod zonation q m p t is therefore proposed

seasonal signals in lake malawi sediments the sedimentary record of lake malawi one of the large rift valley lakes of east africa contains thick sequences of biologically undisturbed finely laminated muds and silts laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m the sequences consist of light dark laminae couplets with average thickness of 1 mm the light laminae are completely dominated by the diatom genus melosira with a relatively minor contribution from the genus stephanodiscus the dark laminae consist of 50 diatoms equal proportions of melosira and stephanodiscus few nitzschia and occasional littoral diatom genera and 50 terrestrial plant debris mineral grains and fine clay and organic material based on a 210pb and 14c calculated sedimentation rate of 1 mm yr 1 for lake malawi basin sediments and the well defined seasonal variations in fluvial input and diatom productivity levels our data suggest that the lamination couplets are annual varves representing biannual sedimentation sequences the light laminae result from diatom blooms occurring during the dry windy season april october when the upper 250 m of the lake is well mixed localized upwelling occurs and the epilimnion becomes enriched with nutrients the dark laminae represent sedimentation in the rainy low wind velocity season november march when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

sedimentation in lake malawi east africa during the past 10 000 years a continuous paleoclimatic record from the southern tropics a suite of piston cores recovered from lake malawi 9 14 s 34 35 e east africa in 1986 has been analyzed for major and minor elements organic c and n calcium carbonate and diatoms an internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera stephanodiscus and melosira with age control obtained primarily from 14c dating of the carbonate differences with time in fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth the depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr b p indicating less seasonality carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column the carbonate which precipitates from surface waters is most abundant during the interval from about 10 000 to 6000 yr b p this micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced sedimentary evidence suggests that lake levels were 100 150 m lower than present during this period this record is different from climatic trends in northern intertropical africa but appears to also be related to changes in insolation and monsoon circulation this is the northernmost basin in africa reported to exhibit a southern hemisphere response to the early holocene northern hemisphere summer insolation maximum the climatic hingeline north of lake malawi 9 s implied by our results is significantly south of that indicated by general circulation model simulations however the cores show evidence for periods of abrupt climate change during the interval of generally arid climate

structural evolution of lake malaŵi africa over 2 000 km of seismic reflection profiles have been collected in lake mala i nyasa the southernmost of the east african rift lakes these studies reported here reveal a severely faulted lake floor bounded by rift structures of pliocene or greater age faults that penetrate the uppermost sedimentary units and numerous earthquakes1 attest to continuing activity in the rift inter basinal differences in fault spacing orientation and sedimentary thicknesses have led to a subdivision of lake mala i into four discrete structural provinces these provinces only now are linked by faults attributed to the most recent episodes of extension an evolutionary sequence of 1 initial block faulting and subsidence 2 fragmentation of fault blocks 3 uplift and 4 renewed subsidence and rotation of fault blocks is proposed for the central part of the mala i rift

late quaternary carbon cycling responses to environmental change revealed by multi proxy analyses of a sediment core from an upland lake in southwest china stable carbon isotope 13c values of organic matter in lacustrine sediments are commonly used to trace past changes in terrestrial and aquatic carbon cycles here we use a high resolution well dated 13c record from lake tengchongqinghai tcqh in southwestern china together with other proxy indices to reconstruct the paleolimnological history over the past 18 5 ka organic matter in the sediments of lake tcqh is derived predominately from aquatic macrophytes the lacustrine primary productivity is closely linked with lake level changes affected by variations in the strength of the asian summer monsoon and modified by evapotranspiration similar to lake sediments world wide a ca 3 shift occurred in the 13c values of lake tcqh in response to the significant increase in atmospheric co2 concentration during the last deglaciation in the holocene the availability of dissolved co2 in the lake water of lake tcqh was determined by variations in hydraulic energy low water turbulence creates a thick stagnant boundary layer around aquatic plants which will restrict the rate of co2 diffusion and result in more positive 13c values of aquatic plants in contrast significant water turbulence dramatically reduces the boundary layer thickness leading to more negative 13c values of aquatic plants

discovery of sublacustrine hydrothermal activity and associated massive sulfides and hydrocarbons in the north tanganyika trough east african rift massive sulfides and carbonate mineral deposits associated with sublacustrine thermal springs were recently discovered along the zaire side of the north tanganyika trough western branch of the east african rift this hydrothermal activity investigated by scuba diving at a maximum depth of 20 m is located at the intersection of major north south normal faults and northwest southeast faults belonging to the tanganyika rukwa malawi trm strike slip fault zone the preliminary results presented here come from analyses of sulfide deposits hydrothermal fluids and associated hydrocarbons that result from geothermal activity in this part of the east african rift filled by a thick pile of sediment the north tanganyika trough

environmental changes at lake cheshi zambia since 40 000 years b p abstract sediment and microfossil analyses of a 7 5 m core from lake cheshi suggest that south central africa experienced late quaternary climate changes similar to those in east africa the lake formed around 34 000 yr b p after a prelacustrine phase of at least 6000 yr from climatic or tectonic causes ratios of precipitation to evaporation were probably similar to those of today until a decline about 15 000 13 000 yr b p when the lake shrank and became chemically concentrated maximal lake levels occurred between 8000 and 4000 yr b p and were followed by a low stand under presumably arid conditions about 3500 yr b p encroachment of sudd vegetation contributed to shallowing during the last 3000 yr a phase of microfossil dilution may reflect human activity in the basin or climatic or hydrological changes melosira valve morphology seems to reflect mixing regimes sponge and testate amoeba remains were most numerous relative to diatoms during low water phases

late quaternary paleoenvironments in the southwestern kalahari abstract landforms of the southwestern kalahari in southern africa include linear dune systems pans and associated lunette dunes and the valleys of ephemeral rivers present day climates in the region are transitional from arid to semi arid making the area sensitive to climatic change the linear dunes are part of an extensive system of fixed dunes developed in the surface kalahari sands the dunes were in existence prior to 32 000 yr b p but have been reactivated on a number of occasions since then in conditions of increased wind velocities more nw winds and reduced rainfall especially in northern and eastern parts of the region periods of dune activity probably occurred at 19 000 17 000 10 000 6000 and 4000 3000 yr b p the development of pans in the region has been strongly influenced by changes in groundwater conditions deflation from pans and formation of lunettes occurred in periods when groundwater levels were higher and fluctuated seasonally more than they do today such periods probably occurred 25 000 20 000 yr b p around 10 000 yr b p and just prior to 1000 yr b p still higher groundwater discharges promoted by increased regional rainfall resulted in the formation of spring tufas and seasonal or permanent lakes in the pans which were also fed by surface runoff during the intervals between 33 000 28 000 23 000 21 000 17 000 15 000 and 12 000 11 000 yr b p the headwaters of the major ephemeral streams lie outside the region but within the same climatic zone increased flow in the molopo river occurred prior to 19 000 and between 15 000 and 13 000 yr b p

the late pleistocene and holocene stratigraphy and paleolimnology of lakes kivu and tanganyika abstract evidence from diatoms chemistry and sedimentology shows that lakes kivu and tanganyika experienced late pleistocene low stands 86 to 300 m for kivu and 250 to 350 m for tanganyika both lakes had higher 2 10 fold sedimentation rates prior to 11 500 yr b p about 9900 yr b p upwelling became important in southern lake tanganyika which had been deepening for at least several millenia by 9400 yr b p kivu had become deep and dilute with overflow via the ruzizi river to lake tanganyika southern lake tanganyika became more stably stratified about 5200 yr b p volcanism and hydrothermal activity in kivu began about 5000 yr b p and the salinity reached 3 ppt and the lake became strongly stratified the ruzizi river outflow was interrupted from 3500 yr b p until recently the sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in kivu has probably been the cause of its impoverished fauna these results show that high lake levels in the eastern rift of africa from 10 000 to 7000 yr b p were paralleled in the western rift given the great volumes of these lakes climate has not caused great variations in salinity the diatom floras however have been remarkably sensitive to changes in stratification and nutrient supply volcanism has been a major force in the limnological and biological evolution of lake kivu while both kivu and tanganyika have responded to changes in precipitation and wind stress

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

climate change in the circum north atlantic region during the last deglaciation a survey of new and published palaeoclimate data indicates that both the high and low latitude north atlantic regions were characterized by at least three synchronous periods of abrupt climate change during the last glacial to interglacial transi tion climate model results suggest that changes in the melting history of the laurentide ice sheet may explain much of this nonlinear response of the climate system to astronomical milankovitch forcing

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

evidence of past climatic change in the namib desert abstract a preliminary survey of some areas of the central and northern namib desert was undertaken in order to identify geomorphic and sedimentary features that could potentially provide information on past climatic change in the region wherever possible samples were collected for radiocarbon dating the visits to the skeleton coast area were especially revealing practically every river valley in this tract shows a sequence of terraces and deposits that would warrant a detailed study in all about 120 new radiocarbon dates were obtained for the region these have supplied a wealth of individual results and collectively have enabled the construction of a tentative chronological sequence of the changes in climate for the past 40 000 yr three periods of increased aridity in the hinterland but with decreasing intensity are postulated during the last glacial maximum the early holocene and between a d 1200 and a d 1600 the inter pleniglacial up to 23 000 yr b p the late glacial and the period between about 4000 yr b p and 1200 yr b p on the other hand were more humid than today also with decreasing intensity other indicators of past climate were also identified but since they lie beyond the range of radiocarbon dating they cannot as yet be placed in a chronological context of significance is the fact that convincing evidence was obtained for climatic change within the last millennium and especially the rainfall is found to have been increasing since the 16th century

climatic control of turbidite deposition on the madeira abyssal plain studies of sedimentation in the southwestern part of the madeira abyssal plain have revealed sequences of turbidites correlatable over thousands of square kilometres turbidites are sediments transported by turbidity currents and represent rapidly accumulated sedimentary units1 in this case in the distal part of the flow they are very fine grained and homogeneous occasionally with silty bases individual turbidites are separated by thin pelagic units which alternate from oozes to pelagic clays a stratigraphy for these pelagic units has been produced from combining coccolith distributions and lithological changes which enables comparison to cores containing only pelagic sediments from outside the turbidite province these comparisons show that turbidity currents cause no significant erosion in the area the major turbidites occur between lithological alternations which reflect global climate changes associated with glaciations and related eustatic sea level fluctuations turbidites have been linked with eustacy on previous occasions but only with low stands of sea level2 3 our work shows that turbidites are more likely to occur during regressions and transgressions rather than in the quiet periods between

bio and lithostratigraphy of lower pleistocene marine successions in western emilia italy and their implications for the first occurrence of arctica islandica in the mediterranean sea the arda and stirone marine successions italy represent key sections for the early pleistocene they were deposited continuously within a frame of climate change recording the calabrian cooling as testified by the occurrence of the northern guests such as the bivalve arctica islandica in addition although the first occurrence of a islandica in the mediterranean sea was used as the main criterion to mark the former pliocene pleistocene boundary the age of this bioevent was never well constrained here we describe the stirone depositional environment and constrain for the first time the section age using calcareous nannofossil and foraminifera biostratigraphy we also correlate the arda and stirone sections using complementary biostratigraphic and magnetostratigraphic data our results indicate that a islandica first occurred in both the successions slightly below the top of the cnpl7 biozone dated at 1 71 ma comparisons with other lower pleistocene mediterranean marine successions indicate that the stratigraphically lowest level where a islandica first occurred in the mediterranean sea is in the arda and stirone sections these environments satisfied the ecological requirements for the establishment and the proliferation of the species which only subsequently late calabrian has been retrieved in southern italy and other areas of the mediterranean sea

climatic conditions in southern africa since the last glacial maximum inferred from pollen analysis abstract a review of late quaternary palaeoclimatic data derived from several pollen sequences between ca 22 and 34 s in southern africa shows a degree of similarity in temperature and moisture variations between the various site pollen data from sites such as wonderkrater and rietvlei transvaal tate vondo venda elim orange free state equus cave southern kalahari and boomplaas cape province suggest relative dryness during the last glacial maximum wet conditions during the late glacial and dry conditions during the early holocene ca 8000 yr b p followed by progressively moister conditions peaking soon after the development of a temperature optimum ca 6500 yr b p problems with radiocarbon dating of polleniferous spring deposits however prevent precise correlations of especially late pleistocene sequences different seasonal patterns in the late glacial and early holocene may explain palaeobotanical data and are partly compatible with simulations of past climates of kutzbach and guetter 1986

pteropods as climatic indicators in quaternary sequences a lower middle pleistocene sequence outcropping in cava puleo ficarazzi palermo italy abstract pteropod assemblages from a section outcropping at cava puleo ficarazzi palermo type locality of the sicilian stage were studied for a paleoclimatic interpretation the studied section directly overlies the subsurface lower pleistocene sequence a curve of the absolute abundance of the subarctic species limacina retroversa in the pteropod assemblage and a climatic curve based on the total amount of cumulative percentages of cold and warm pteropod indicators are proposed the former shows a climatic trend in which a strong cold peak can be easily inferred at the top of the proposed sicilian stratotype section this drastic cold event is the first one recognizable in the local lower pleistocene sequence and may be correlated with the cold event recognized by shackleton and opdyke 1976 at the top of stage 22 the cold climatic fluctuation was abruptly followed by an internal with warm temperate climate characterized by a tropical subtropical assemblage

paleoclimatic estimates from water and energy budgets of east african lakes abstract the turkana nakuru elmenteita and naivasha basins in the rift valley of east africa experienced high water levels during the period of 10 000 7000 yr b p analyses of the modern hydrologic and energy budgets for these basins along with that of lake victoria were used to infer the amount of precipitation that would have been required to maintain the enlarged paleolakes of the early holocene precipitation must have been at least 150 300 mm yr 15 to 35 above the modern average the precipitation estimates were fairly consistent among the various basins but no quantitative estimate was made for the additional precipitation required to account for overflow from the rift valley lake basins discharge from the lake victoria basin around 1880 a d was considerably above the more recent average and the increased discharge into the white nile for that period might have been similar to that of the early holocene a sensitivity analysis showed that temperature changes were probably not too important for changing the hydrologic energy budget changes of albedo bowen ratio and cloudiness were likely to have been of greater importance

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics