droughts in the east asian summer monsoon margin during the last 6 kyrs link to the north atlantic cooling events abstract teleconnections to the high latitudes forcing by the tropical oceans and solar variability have all been suggested as dominant factors in the sub millennial global climate changes yet there is little consensus as to the relative importance of these factors for the east asian summer monsoon easm variability this study presents the results of high resolution analyses of ca and mg concentrations mg ca ratio 18 o and 13 c values of endogenic calcites from a sediment core from dali lake in the easm margin in order to investigate the sub millennial easm variability and its possible driving forces during the last 6 kyrs increases in these chemical proxy data were interpreted as drought events in the region due to the intensive evaporation losses overwhelming the water input to the lake the chemical proxy data in this study combined with multi proxy indicators including grain size component and total organic carbon concentrations from the same sediment core imply that declines in the easm intensity may have played a dominant role in triggering the drought events during the last 6 kyrs the results indicate that the easm intensity significantly declined at the intervals of 5 8 4 75 3 2 2 8 1 65 1 15 and 0 65 0 2 kyrs bp large declines in the easm intensity during the last 6 kyrs correspond in time to occurrences of ice rafted debris in the north atlantic indicating that millennial to centennial scale changes in the easm intensity were mainly controlled by climatic processes occurring in the northern high latitudes these data imply that persistent global warming may be favorable for the strengthening of the easm circulation and for the transportation of more rainfall to the semi arid regions of northern china on sub millennial scales

quantifying climatic change through the last glacial interglacial transition based on lake isotope palaeohydrology from central turkey questions remain as to the nature of climatic change through the last glacial interglacial transition in the eastern mediterranean region particularly the relative contribution of evaporation and precipitation to regional water balance here changes in oxygen isotope values through this time period from eski acigol a crater lake in central turkey are investigated using hydrological and isotope mass balance models these allow changes in evaporation and precipitation to be quantified and their relative importance evaluated we show that it is the volumetric flux rate of water passing through the lake system and not the precipitation to evaporation ratio per se which controlled the stable isotope record in eski acigol early holocene precipitation is shown to be much greater than that during both the latter part of the last glaciation and the present day we test these calculated values against other records in the same region firstly with other lake records in anatolia the konya basin and lake van and secondly with isotope inferred palaeo precipitation data from soreq cave in israel this reveals a contrast between pre and post lgm precipitation values in turkey wetter and drier respectively and also suggests that during the last glacial interglacial transition there was a more marked precipitation gradient than at present between northern interior and southern coastal parts of the eastern mediterranean region

environmental and climatic control on seasonal stable isotope variation of freshwater molluscan bivalves in the turkana basin kenya abstract we present growth incremental stable isotope records 18o and 13c of modern and 2 ma fossil bivalve shells from the semi arid turkana basin n kenya s ethiopia these data suggest that seasonal cyclicity in 18o and 13c recorded by modern and fossil shells is driven by wet dry seasonal changes in host water chemistry forced by monsoonal rainfall over the ethiopian highlands fully lacustrine shells show lower amplitude or even absent seasonal cyclicity in comparison with deltaic or riverine shells because of the buffering effect of the large water volume in the lake setting riverine shells arguably have the clearest relation to seasonally variable climate parameters riverine molluscs thus provide a potentially valuable proxy for varying rainfall 18o values in the turkana basin catchment cross plots of molluscan 18o and 13c data reveal dominant environmental control on molluscan isotope values with remarkably large isotopic differences between lacustrine deltaic and riverine environments we interpret this isotope pattern to directly result from the different mixing proportions of omo river source water with evaporated lake water in these environments the interpretation of fossil molluscan 18o and 13c data in a palaeoclimatological context is not straightforward since the potential influence of temporal changes in lake water temperature surface evaporation or river discharge on the 18o budget of the lake is smaller than the isotopic shifts caused by shifting facies patterns in the sedimentary record even though it is clear from the rich molluscan faunas that the 2 ma palaeo lake lorenyang must have been significantly less alkaline than modern lake turkana and likely provided good drinking water and abundant availability of food for the different species of hominins inhabiting the region at that time stable isotope values of molluscan bivalves are not suitable to record the difference in alkalinity between these two settings

groundwater mediated response to holocene climatic change recorded by the diatom stratigraphy of an ethiopian crater lake abstract the diatom stratigraphy of a 23 m sediment core from lake tilo a maar lake in the ethiopian rift valley provides a 10 000 yr record of lake salinity and trophic status until 5500 14c yr b p the phytoplankton was dominated by aulacoseira granulata with only minor changes in the abundance of other diatoms the lake was over 50 m deep eutrophic and oligosaline at 5500 yr b p geothermal groundwater inflow inferred from calcite and silica deposition rates declined abruptly and the lake became more oligotrophic as indicated by a rapid rise in cyclotella stelligera about 4500 yr b p lake salinity began to increase reaching approximately its present state ca 2500 yr b p but with a temporary reversal to lower salinity at 4000 3500 yr b p the record shows no evidence of salinity increases equivalent to early holocene low stands of the larger river fed rift valley lakes probably because of high rates of geothermal groundwater influx it responded to reduced rainfall at 4500 yr b p when levels of the larger lakes also fell because geothermal groundwater flux had diminished 1000 years earlier independently of climate because geothermal groundwaters can form a significant proportion of a crater lake s hydrological budget and dominate its salinity budget these results show that variable geothermal groundwater flux can override lake hydrochemical response to climate change palaeoclimatic interpretation of palaeosalinity proxies from the sediments of volcanic crater lakes should be approached with caution

new ams dates stratigraphic correlations and decadal climatic cycles for the past 4 ka at lake turkana kenya abstract new ams radiocarbon dates and stratigraphic correlations of 7 piston cores recovered from lake turkana east africa show that the frequency of cyclic variability is greater than previously reported for a single piston core almost 60 new ams dates of various carbonate fractions indicate that large 150 m ostracode carapaces yield the most accurate chronology for the sediment record due to the input of old fine grained carbonate stratigraphic correlations among the cores by carbonate abundance and magnetic susceptibility profiles provide supporting evidence for the ostracode based chronology this chronology refines earlier estimates of sediment accumulation that impacts the recurrence interval of lamination deposition from one lamination every two years to one lamination per year in two north basin cores the record of carbonate content within the north basin of the lake reflect its dilution with terrigenous sediments input from the omo river but direct paleoclimatic interpretations are complicated by lateral migrations of the delta channels with changing lake levels time series analysis of both individual cores and a combined carbonate profile show significant spectral peaks at 1000 76 32 22 18 6 and 11 yr the latter three suggest an important link between the sediments accumulating in lake turkana and global cycles of climatic variability

paleoclimate of the past 4000 years at lake turkana kenya based on the isotopic composition of authigenic calcite abstract authigenic calcite was analyzed for 13 c and 18 o in a 12 m long core from lake turkana that spans the last 4200 yr the 18 o record is interpreted to reflect lake level history and indicates high stands prior to 4000 yr b p from 1800 to 2000 yr b p and within the last century this coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for lake turkana based on 14 c dates of exposed shoreline deposits the 13 c curve roughly parallels the 18 o curve and may reflect the influence of the omo river plume both of the isotope records are strongly cyclic with periodicites common to both at 80 120 150 and 225 yr other somewhat longer periodicites are nearly equal in the two records and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same turkana core

a high resolution geochemical record from lake edward uganda congo and the timing and causes of tropical african drought during the late holocene abstract high resolution analyses of the elemental composition of calcite and biogenic silica bsi content in piston cores from lake edward equatorial africa document complex interactions between climate variability and lacustrine geochemistry over the past 5400 years correlation of these records from lake edward to other climatically forced geochemical and lake level records from lakes naivasha tanganyika and turkana allows us to develop a chronology of drought events in equatorial east africa during the late holocene major drought events of at least century scale duration are recorded in lacustrine records at about 850 1500 2000 and 4100 cal year bp of these the most severe event occurred between about 2050 and 1850 cal year bp during which time lake edward stood about 15 m below its present level numerous additional droughts of less intensity and or duration are present in the lake edward record some of which may be correlated to other lacustrine climate records from equatorial east africa these events are superimposed on a long term trend of increasingly arid conditions from 5400 to about 2000 cal year bp followed by a shift toward wetter climates that may have resulted from an intensification of the winter indian monsoon although the causes of decade to century scale climate variability in the east african tropics remain obscure time series spectral analysis suggests no direct linkage between solar output and regional rainfall rather significant periods of 725 125 63 72 31 25 and 19 16 years suggest a tight linkage between the indian ocean and african rainfall and could result from coupled ocean atmosphere variability inherent to the tropical monsoon system

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

a 15 000 year isotopic record from lake zurich of deglaciation and climatic change in switzerland abstract ostracod and pelecypod carbonate preserved since the latest pleistocene in the bottom sediments of lake zurich was probably formed under constant temperature conditions temporal shifts in the oxygen isotype ratios for those carbonates therefore correspond to shifts in the isotopic character of the net lake inflow alpine deglaciation which released isotopically light meltwaters commenced prior to ca 15 000 yr b p and was essentially completed in northern switzerland by 12 4000 250 yr b p well before substantially increased atmospheric heat was available from either increased insolation or an increased atmospheric co 2 content the early holocene climate for the region was characterized by minor short term fluctuations in mean annual air temperatures and an overall amelioration of up to 2 c contemporaneously precipitated authigenic and biogenic carbonates have recorded partitioning of carbon isotopes between surface and bottom waters during photosynthetic production and subsequent oxidation of organic matter in the lake the degree of that partitioning provides an index of the relative production rates which were low in the late pleistocene meltwater lake but considerably enhanced during the holocene

paleohydrology of lake turkana and its influence on the nile river system abstract the detailed paleohydrological record of lake turkana the largest lake in the eastern branch of the east african rift is necessary for determining the connectivity between adjacent watersheds in tropical africa the migration of both the intertropical convergence zone itcz and the congo air boundary cab constrains rainfall amount and duration for most of east africa lake turkana in northern kenya is the world s largest desert lake and experiences two itcz associated rainy seasons annually with cumulative rainfall of 200 mm yr evidence from new continuous high fidelity sediment core records and high resolution chirp seismic reflection data suggests that lake turkana received enough rainfall during the african humid period ahp to fill the lake to its sill 100 m above current lake level and spilled over into the white nile river system an atmospheric configuration with an eastward shifted cab over the turkana region and the northern kenya rift is invoked as an additional source of rainfall for the catchment this configuration began abruptly at 11 ka and lasted until 5 ka when lake turkana became a closed basin and was cut off from the white nile prior to the ahp lake turkana experienced at least two desiccation events following the last glacial maximum at 18 5 and 17 ka when the lake was at least 100 m lower than modern as evidenced by basin wide high amplitude reflections and 14 c dated shallow water facies in sediment cores lake level fluctuations generally follow trends in mean solar insolation however the onset of lake level extremes are much more abrupt than rates of insolation change lake turkana s location in relation to atmospheric convergence and dynamic rainfall patterns makes its susceptible to extreme climate change over relatively short timescales

determining the spatial and temporal patterns of climate changes in china s western interior during the last 15 ka from lacustrine oxygen isotope records china s western interior is a climatologically complex region where climatic conditions are primarily controlled by the westerly jet stream and the asian monsoon system observations reveal that the 18o signatures of these atmospheric elements as expressed in their meteoric waters are different and these differences are reflected in the lacustrine systems that are ultimately fed by the meteoric waters empirical orthogonal function eof decomposition of 17 lacustrine 18o records from this area clearly reveals the spatial and temporal structures of climate changes during the last 15 ka the first eof mode dominated by sites in the nw sector captures the temperature effect on the variability of the 18o dataset we interpret the variations through time in the amplitude of this mode to be indicative of changes in summer air temperature the second eof mode heavily weighted by sites in the se sector reveals the amount effect on 18o and thus its amplitude time series indicates changes in the intensity of the asian summer monsoon our eof based reconstructions of regional climate are generally consistent with the 18o records of ice cores cave speleothems and marine carbonates copyright 2008 john wiley sons ltd

a mass balance approach to basin sedimentation constraints on the recent history of the turkana basin abstract the turkana basin provides some excellent examples of mass balance constraints on recent basin history high sedimentation rates in the basin imply that major river captures occurred in the recent geologic past the chemistry of the lake is that to be expected for a relatively deep closed basin alkaline lake after several thousand years mass balance in the water budget shows that major changes in lake levels over a short time interval can only result from catastrophic events

climatic and tectonic effects on sedimentation in a rift valley lake evidence from high resolution seismic profiles lake turkana kenya approximately 900 km of 28 khz echo sounding and 770 km of 1 khz seismic reflection profiles were obtained from lake turkana in northern kenya between march and december 1984 the profiles penetrate as much as 60 m below the lake floor representing perhaps the last few tens of thousands of years they show that the predominant pattern of sedimentation is one of simple and rapid basin infilling the profiles suggest an abundance of gas probably methane in the holocene sediments particularly near the major rivers and in the deepest basins where sedimentation rates are relatively high high angle growth faults characteristic of extensional regimes are common and indicate that faulting is active today particularly in the central basin of the lake a major erosional unconformity indicates that lake level was about 60 m lower than present lake level probably just prior to 10 000 yr b p when unusually arid conditions prevailed throughout much of east africa

late cretaceous to early paleocene climate and sea level fluctuations the tunisian record abstract climate and sea level fluctuations across the cretaceous tertiary k t transition in tunisia were examined based on bulk rock and clay mineralogies biostratigraphy and lithology in five sections el melah el kef elles ain settara and seldja spanning from open marine to shallow inner neritic environments late campanian to early danian trends examined at el kef and elles indicate an increasingly more humid climate associated with sea level fluctuations and increased detrital influx that culminates at the k t transition this long term trend in increasing humidity and runoff in the tethys region is associated with middle and high latitude cooling results of short term changes across the k t transition indicate a sea level lowstand in the latest maastrichtian about 25 100 ka below the k t boundary with the regression marked by increased detrital influx at el kef and elles and a short hiatus at ain settara a rising sea level at the end of the maastrichtian is expressed at elles and el kef by deposition of a foraminiferal packstone a flooding surface and condensed sedimentation mark the k t boundary clay which is rich in terrestrial organic matter the p0 p1a transition is marked by a sea level lowstand corresponding to a short hiatus at ain settara where most of p0 is missing and a period of non deposition and erosion in the lower part of p1a 64 95 ma at seldja p0 and possibly the topmost part of cf1 are missing these sea level fluctuations are associated with maximum humidity these data suggest that in tunisia long term environmental stresses during the last 500 ka before the k t boundary and continuing into the early danian are primarily related to climate and sea level fluctuations within this long term climatic trend the pronounced warm and humid event within the latest maastrichtian zone cf1 may be linked to greenhouse conditions induced by deccan volcanism the absence of any significant clay mineral variations at or near the k t boundary and ir anomaly suggests that the bolide impact had a relatively incidental short term effect on climate in the tethys region

climate change in the turkana basin as deduced from a 4000 year long δo18 record abstract a model of the oxygen isotopic composition of lake turkana waters sheds insight into the paleoclimatic interpretation of a 18 o profile of inorganic calcite from a core taken from the lake the model indicates that high frequency fluctuations seen in the isotopic record are probably due to shifts in lake level while the long term increase of 1 4 seen from 4 to 2 kyr bp cannot be due solely to lake level change it must be due to changes in other climatic parameters such as the isotopic composition of the inflow these findings suggests that the abrupt change observed in lake turkana and many other african lakes at 4 ka was the precursor to a 2000 year long adjustment in the climate over the ethiopian plateau the isotopic record from lake turkana agrees with other paleoclimate records from the basin including the diatom stratigraphy and radiocarbon dates on exposed shoreline deposits

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

hydrogeochemistry of lake turkana kenya mass balance and mineral reactions in an alkaline lake abstract lake turkana in northwestern kenya is a closed basin alkaline ph 9 2 lake of moderate salinity tds 2500 ppm principal ions are na hco 3 and ci the lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters the omo river is the principal influent providing some 80 90 of water input to the lake chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water the bottom sediments are predominantly detrital and fine grained yet mineral water reactions are very important for the geochcmical budget ca 2 is precipitated as calcite na is removed as an exchangeable cation on smectite mg 2 is probably incorporated into a mg silicate phase most likely poorlycrystalline smectite as it enters the lake water k may be used in illite regradation cation exchange is a very important process in the mass balance of this lake over 40 of incoming na is removed as an exchangeable cation after cation exchange and interstitial water burial na has a response time of 2650 years which compares favorably with that of chloride these processes seem to occur rapidly within the water mass of the lake other reactions may be important in regulating interstitial water compositions several changes occur in the upper 3m of sediment interstitial water ph drops to 8 3 and alkalinity increases slightly with depth so 2 4 decreases slightly and amorphous silica saturation is approached these changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water high rates of sedimentation up to 1 cm per year may minimize the effects of diffusion between the interstitial waters and the lake water although burial of interstitial water assumes considerable importance

the holocene paleolimnology of lake issyk kul kyrgyzstan trace element and stable isotope composition of ostracodes the stable isotope and trace element geochemistry of ostracode shells from lake issyk kul kyrgyzstan provide an important new paleoclimate record for central asia the n 18 o n 13 c sr ca mg ca and u ca composition of shells from the species candona neglecta found in two piston cores constrains the paleohydrology of the lake from 8700 to 1000 calendar yr before present these data combined with robust chronologies based on 16 ams 14 c dates suggest that the region went from a relatively moist climate in the early holocene to a more arid climate in the late holocene intense freshwater input presumably glacial runoff flowed into the lake from 8700 to 8300 cal yr bp as evident from the n 18 o and n 13 c values from 8300 to 6900 cal yr bp n 18 o and sr ca values indicate that the lake was relatively fresh and an open basin and u ca values suggest that the lake was well mixed with well oxygenated bottom waters there is a transition zone between 6900 and 4900 cal yr bp when n 18 o and sr ca values rapidly increase and mg ca and u ca values rapidly decrease during this period the lake evolved from an open freshwater well mixed lake to a closed more saline less well mixed lake for the remainder of the record n 18 o sr ca mg ca and u ca are relatively constant implying conditions similar to modern conditions although the slight increasing trend in sr ca may suggest an increase in salinity over this period the n 13 c values were approximately in equilibrium with atmospheric co2 in the early part of the record but became relatively negative in the later part of the record this indicates that the dominant mechanism controlling the isotopic composition of dissolved inorganic carbon shifted from exchange with the atmosphere to variations in biological productivity fl 2001 elsevier science b v all rights reserved

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

stable isotopes of oxygen and hydrogen in the truckee river pyramid lake surface water system 3 source of water vapor overlying pyramid lake during 1988 and 1989 a series of water vapor extractions were conducted in the pyramid lake basin to determine the source of moisture that overlies the lake calculations of the isotopic and water vapor balances were made from isotopic and meteorological data from the pyramid lake and reno nevada areas the results indicate that in the warm season most of the moisture that overlies the lake is derived from evaporation as opposed to advected moisture isotopic fluxes at the lake surface can be approximated by climatic data from the lake site which simplifies the calculation of this input in numerical simulations of the isotopic evolution of the lake the stable isotopic composition of a lake is a function of the volume weighted isotopic values of each component of the lake s hydrologic balance researchers have used the stable isotope history of a lake together with the volume weighted isotopic composition of easily measured components of the hydrologic balance to estimate the stable isotope values or amounts of less easily measured components of the hydrologic balance see iaea 1970 1979 it is usually relatively simple to quantify the amounts and isotopic compositions of components of the surface water system and the isotopic composition of groundwater it is often more difficult to quantify the amount and direction of groundwater transport across the sediment water interface and the amount and isotopic composition of lake surface evaporation the isotopic value of water evaporated from a lake is highly dependent on the isotopic value acknowledgments we express our appreciation to those who assisted in sampling including jim davis dan mosley and nancy vucinich special thanks to irving friedman who loaned us the copper traps used for water vapor extraction to carol kendall and tyler coplen who oversaw the isotopic analyses to james ashby who maintained the weather stations and to steve hostetler who extracted the meteorological data from the weather station database and stratified the wind data for the sutcliffc and northshore sites we also thank zell peterman steve hostetler and pieter tans for reviews of earlier versions of this manuscript

an 11 000 year lithostratigraphic and paleohydrologic record from equatorial africa lake edward uganda congo abstract sedimentologic and geochemical analyses of four cores from lake edward uganda congo document a complex record of moisture balance in the edward basin from 11 000 cal yr bp to present highly organic diatomaceous muds provide evidence for shifts in wind intensity and stratification within an early holocene wet phase lake level variations within this period may have shifted due to tectonic lowering of the lake s outlet level the onset of mid holocene aridity as documented by the appearance of authigenic calcite at 5200 cal yr bp initiated a period of falling lake levels that culminated in a late holocene lowstand between 4000 and 2000 cal yr bp this lowstand is documented by coarse sediments whose fabric and mineralogy depend on the core site although different cores yield different ages for this regression it appears that declining lake levels culminated in a maximum lake lowstand of 14 m lake levels then rose rapidly attaining modern positions by 1700 cal yr bp this lake level history suggests that although many paleoclimatic changes in africa are apparently synchronous throughout northern hemispheric africa other events may be spatially heterogeneous these patterns highlight the need for a well dated network of paleoclimate sites within the african continent

holocene palaeovegetation of the tigray plateau in northern ethiopia from charcoal and stable organic carbon isotopic analyses of gully sediments abstract a long history of supporting sophisticated but unsustainable kingdoms makes the tigray plateau of the northern ethiopian highlands a promising region for the study of relationships between palaeoenvironmental change and the trajectories of human civilizations the natural vegetation above 2200 m elevation is thought to be forests dominated by juniperus procera nonetheless this hypothesis is not supported in the vegetation cover now and is scarcely studied in the palaeorecord to examine changes in vegetation climate and land use we identified buried charcoalized wood and estimated the percentage of organic carbon from c 4 plants c 4 carbon from 13 c values of bulk organic matter in the soils of gully walls in the boundaries of the ancient aksumite empire charcoal ranged in age from ca 13 700 to 110 cal yr bp juniperus procera occurred in even the youngest samples although at lower percentages of the total charcoal than in older samples nevertheless rapidly regenerating angiosperms usually dominated or co dominated charcoal even in some of the oldest strata a shift towards higher c 4 carbon and total organic carbon toc in soils younger than 3300 cal yr bp began during a period when agricultural uses of land may have increased in order to support the needs of growing societies a shift towards higher c 4 carbon but lower toc began at ca 6000 cal yr bp however during a period when no charcoal was found and no changes in human societal complexity are known these results indicate that juniper forest types have long been present at 2200 m in the tigray plateau but that they have rarely been the dominant natural vegetation furthermore lack of repeatable correspondence between factors suggests that the causes of similar shifts in vegetation composition were not always the same

an isotopic study of a fluvial lacustrine sequence the plio pleistocene koobi fora sequence east africa stable isotopic analyses of plio pleistocene and modern sediments in the fluvial lacustrine system occupying the turkana basin east africa provide constraints on the paleoenvironmental and diagenetic histories of the pliocene through the recent sediments in the basin the 13c values for carbonates in lacustrine sediments range from 15 to 22 relative to pdb depending on the varying proportions of co2 from the atmospheric reservoir and from various metabolic sources the 18o values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10 from the pliocene to the present the 13c values for pedogenic carbonates record paleoccologic variations and suggest that c4 plants did not become well established in the preserved depositional parts of the basin until about 1 8 myr ago the 18o values pedogenic carbonates suggest a range of over 10 for the isotopic composition of soil water during this interval they also suggest a period of major climatic instability from about 3 4 to 3 1 myr and at about 1 8 myr together the 13c and 18o values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these plio pleistocene sediments

a 725 yr cycle in the climate of central africa during the late holocene high resolution analyses of the chemical and stable isotope composition of endogenic calcite from lake edward an equatorial rift lake in central africa reveal large oscillations in african climate and moisture balance over the past 5400 yr these oscillations indicate a series of droughts from 4900 to 400 yr b p that define a drought period of 725 yr this period may be correlated to variations in the indian ocean monsoons as documented by similar periods in sea surface salinity records from the northern indian ocean the similarities in periodicity of the lake edward and indian ocean records may indicate that low frequency variability within the indian and pacific oceans dominates millennial scale african moisture balance and the holocene climate of the tropical indian ocean

oxygen and carbon isotope composition of authigenic carbonate from an ethiopian lake a climate record of the last 2000 years lamb henry leng m j telford r j ayenew t 2007 oxygen and carbon isotope composition of authigenic carbonate from an ethiopian lake a climate record of the last 2000 years the holocene 17 4 pp 517 526 rae2008

palaeoenvironments and palaeohydrology of a tropical closed lake lake asal djibouti since 10 000 yr b p in eastern africa republic of djibouti the hypersaline lake asal occupies an exceptional geological topographical and hydrological situation at 155 m below sea level and at 12 km from the sea it lies in the youngest and the deepest basin in a chain of closed depressions of central afar decreasing in elevation from west to east and represents the hydrogeological low of the whole system presently infiltration of marine water represents the major water input to the lake due to high evaporation conditions halite precipitates halocene water level fluctuations 315 m sedimentary conditions from fresh oligosaline waters to heavy brines and palaeohydrology are reconstructed field observations are supported by 85 14c dates on inorganic carbonates mollusc shells stromatolites and hydrothermal travertines stable isotope analyses on carbonates and sulphates mineralogy geochemistry and studies of biological remains discussion of the water balance for the major stages is based on reasonable assumptions about holocene conditions of precipitation and evaporation after an arid terminal pleistocene phase marked by coarse deposits due to exceptional floods the holocene cycle begins with complex strontium rich carbonate phases aragonite high and low mg calcites and dolomite these carbonates could reflect the first underground marine supply and waters concentrated through evaporation a high lake level ca 160 m a s l from ca 8600 to 6000 yr b p corresponds to the deposition of high mg calcite precipitated in a deep fresh water lake lake asal was draining the awash river system ethiopian plateau and escarpment through groundwater circulations the terminal basin of the system was probably the present day marine gulf of ghoubbat al kharab hydrologically separated from the open ocean by a sill at 4 m below sea level possibly due to fracture opening a dramatic regression in the lake level from 160 to 150 m occurred between ca 6000 and 5000 yr b p the inputs of continental water were consequently overwhelmed by marine infiltrations the carbonate phase exhibits clear resemblance with that of the transgressive episode from 5000 yr b p lake asal experienced minor water level fluctuations aragonite rich carbonates precipitated followed by calcium sulphates and halite stable isotopes of gypsum reflect its marine origin and various diagenetic processes the holocene evolution of asal which clearly differs from that of the neighbouring closed basins is due to both its specific geological and hydrological setting and to the regional climatic changes

seasonality and the isotope hydrology of lochnagar a scottish mountain lake implications for palaeoclimate research the relationship between isotope ratios in precipitation and lake waters was monitored by bi weekly measurements taken over a five and a half year period between may 2000 and september 2005 a period of 1964 days at lochnagar a remote mountain lake in eastern scotland short term changes in the oxygen isotope composition of lake water 18ol at lochnagar follow a seasonal pattern similar to isotopes in local precipitation 18op however changes in catchment residence time snow accumulation lake ice cover and lake stratification modify the temporal structure of 18ol over the seasonal cycle of particular importance is precipitation amount which controls catchment and lake residence times and determines the degree of phase lag and amplitude change between 18ol and 18op a simple mass balance model replicates these effects and demonstrates that the degree of phase lag and amplitude reduction is predictable given known input volume ratio the implications of these observations for the use of 18o

lake qinghai china closed basin like levels and the oxygen isotope record for ostracoda since the latest pleistocene abstract in 1985 a sino swiss limnogeological expedition to china s largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern tibet qinghai plateau since the last glacial maximum lake qinghai a perennial lake with an area of 4278 km2 1985 at 3194 m above sea level receives drainage from 30 000 km2 of catchment near the northwest corner of the plateau the 26 5 m deep terminal lake 1985 with brackish alkaline magnesium sulphate rich waters 14 g 1 1 tds has not overflowed since the last glacial but its levels have responded to climatic and hydrologic changes seismic reflection profiles for the sub bottom sediment structures revealed a complex topography with channels relict terraces tectonic tilting and deformation in deposits buried under a 4 6 m packet of mainly flat bedded sediments from which core samples were recovered the lowermost cored material 14 500 yr b p comprises mainly water lain loess deposited in a smaller flood plain lake of up a few metres depth under cold arid climatic conditions ams radiocarbon dating indicates average sedimentation rates for latest pleistocene and holocene deposits in central basin regions of 0 3 0 5 mm yr 1 the 18o values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation evaporation budgets which have controlled the palaeo levels for lake qinghai the precipitation evaporation budget for the lake qinghai region is largely determined by the asian monsoon system the lake qinghai sediment record shows that there was no large influx of glacoer melt waters into the lake basin during the post glacial the regional climate remained arid until at least 12 500 yr b p when an incipient monsoon was possibly initiated river transported silts first reached the basin centre at 11 800 yr b p when the lake depth may have increased to 5 m possibly a monsoonal weakening for several centuries after 10 800 yr b p europe younger dryas caused a further period of aridity at 10 200 yr b p the basin began filling towards modern lake dimensions in response to a permanent strengthening of the asian monsoon substantial lake level rises probably characterised the periods 10 200 9800 9500 8500 8300 7200 yr b p and through perhaps a century at about 6800 yr b p decreasing levels probably characterised the intervening intervals short term lake level fluctuations through the mid holocene climatic optimum were minimal indicating a very stable climatic hydrologic regime an isotopic steady state for the lake waters since 3000 500 yr b p obscures the climatic hydrologic regime dependent 18o gradient but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time three negative 18o excursions for the lake waters may correspond to known glacier advances on the plateau at 3000 2000 and 1500 yr b p during china s neoglacial

the stable isotope composition of dead sea waters abstract dead sea waters are moderately enriched in18o the degree of enrichment constitutes a balance between the dilution by freshwater influx and the isotope fractionation which accompanies evaporative water loss and vapour exchange with the atmospheric moisture modelling of the seasonal cycle and long term trends of 18o in response to the changes in the environmental parameters shows that the major control is exercised by the salinity of the surface waters through its effect on the vapour pressure gradient between the lake s surface and the atmosphere the steady state isotopic composition of the more saline brines tends towards less enriched18o values this fact can explain the relatively high 18o levels encountered in the lisan formation which was deposited from lake lisan the less saline pleistocene precursor of the dead sea

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

mineralogy and carbonate geochemistry of the dali lake sediments implications for paleohydrological changes in the east asian summer monsoon margin during the holocene abstract hydrological variations in individual plateau lakes respond from both local hydrological processes and regional climatic changes this study presents high resolution absolutely dated records of minerals in bulk samples and elements and stable isotopes of endogenic calcites from a sediment core from dali lake in inner mongolian plateau aiming to investigate the patterns and mechanisms of holocene hydrological and climatic changes in the modern northern margin of the east asian summer monsoon easm increases in the percentages of allogenic minerals quartz microcline and albite and decreases in the percentages of endogenic authigenic minerals calcite mg calcite and gypsum together with decreases in the values of ca and mg concentrations and 18o and 13c of endogenic calcites indicate an excess of water input to the lake over evaporative losses and vice versa these data suggest that the hydrological balance of dali lake was controlled by strong evaporation before 9800 cal yr bp and then the lake was supplied by significantly increased inflowing water between 9800 and 5900 cal yr bp from 5900 to 4850 cal yr bp the water input to the lake significantly decreased since 4850 cal yr bp the inflowing water maintained a low level in addition the distinct characteristics of mineralogy and carbonate geochemistry of the dali lake sediments during the periods of high lake level between 9800 7700 and 7700 5900 cal yr bp imply that the lake was mainly fed by snow ice melt water in the former period and then by regional precipitation in the latter period which was further supported by the pollen records from the same sediment core these results suggest that holocene hydrological variations in the easm margin were closely related to changes in regional temperature and precipitation which were ultimately controlled by changes in the northern hemisphere summer insolation northern high latitude ice sheets global sea level and physicochemical conditions of the north atlantic and western tropical pacific on orbital and millennial timescales the datasets in this study support that the maximum easm intensity occurred during the middle holocene in the northern easm regions

late holocene climate change in the north atlantic and equatorial africa millennial scale itcz migration 1 climate proxy data and numerical models suggest that latitudinal displacements of the intertropical convergence zone itcz occur during millennial scale cold events in the north atlantic region but the potential effects of these itcz movements on the climate of equatorial regions are unclear here we present a 5400 year geochemical record of rainfall and drought from lake edward in equatorial africa we observe a non linear correlation in which drought in equatorial africa occurs during both cold and warm extremes in the north atlantic s 1500 year quasi cycle we propose that this relationship occurs due to northward southward displacement of the itcz from its equatorial mean position during warm cold events our results show that millennial scale high latitude climate events are linked to changes in equatorial terrestrial climate even during the late holocene and suggest important constraints on the mechanisms linking tropical and extratropical climate variability

geochemical evolution of archaean granulite facies gneisses in the vestfold block and comparisons with other archaean gneiss complexes in the east antarctic shield the vestfold block like other archaean cratons in east antarctica and elsewhere consists predominantly of felsic orthogneiss mossel and crooked lake gneisses with subordinate mafic granulite tryne metavolcanics and paragneiss chelnok supracrustals two major periods of continental crust formation are represented the mossel gneiss metamorphosed about 3 000 ma ago is mainly of tonalitic composition and is similar to much of the roughly coeval napier complex in enderby land the crooked lake gneiss was emplaced under high grade conditions about 2 450 ma ago and comprises a high proportion of more potassic rocks monzodioritic and monzonitic suites as well as tonalite and minor gabbro and diorite both mossel and crooked lake gneisses are depleted in y and have moderate to high sr ce y and ti y consistent with melting of a mafic source subducted hydrated oceanic crust leaving major residual hornblende garnet most crooked lake gneisses are more enriched in incompatible elements p sr la ce and particularly rb ba and k than mossel gneisses suggesting derivation from a more enriched mafic source the vestfold block contains few orthogneisses derived by melting of older felsic crustal rocks in marked contrast to the archaean napier complex and in particular southern prince charles mountains both mossel and crooked lake tonalites are strongly depleted in rb k th and u and have very low rb sr and high k rb more potassic orthogneisses are depleted in th u and to lesser extents rb tryne metavolcanics are depleted in th and rb but appear to have been enriched in k and probably na possibly during early low grade alteration

record of palaeoenvironmental changes in a lower permian organic rich lacustrine succession integrated sedimentological and geochemical study of the rudnik member krkonose piedmont basin czech republic abstract this study presents an integrated sedimentological and geochemical analysis of the lower permian rudnik member the most extensive lacustrine deposits in the krkonose piedmont basin grey to black and variegated lacustrine mudstones laminites and carbonates of the rudnik member have a lateral extent of more than 400 km 2 in the studied sections four facies associations were recognised a anoxic offshore b suboxic to oxic offshore c nearshore and mudflat and d slope deposits in the northern part of the e w elongated basin anoxic to suboxic organic rich offshore lacustrine facies dominate and form a succession up to 130 m thick fan delta and turbidite facies occur locally along the faulted northern basin margin the central part of the basin is occupied by anoxic to oxic offshore facies interfingering with nearshore carbonate and mudflat facies of the low gradient lacustrine margin in the central part of the basin the thickness of the lacustrine deposits of the rudnik member reaches up to 60 70 m in the southern part of the basin fluvial and alluvial plain facies dominate and alternate with minor lacustrine nearshore facies the lateral facies distribution indicates that subsidence along the northern basin fault was the main mechanism generating the asymmetric infill geometry in the basin s half graben setting the 1 8 o values of primary and early diagenetic calcite range between 11 0 and 1 3 v pdb and 1 3 c values between 5 1 and 3 7 most of the data fall within the range of freshwater limestones coarser grained pure microspar laminae show more positive 1 3 c values in comparison to clayey organic rich micrite laminae and are interpreted as a record of bioinduced precipitation during seasonal eutrophication the obtained 1 3 c toc values range from 29 0 to 24 0 the total organic carbon toc content from 0 26 to 23 maceral analysis and rock eval pyrolysis indicate that most of the samples contain a mixture of aquatic and terrestrial organic matter but two minor distinctive groups of samples with algally dominated and terrestrially dominated organic matter composition respectively were also found the study of vertical changes in boron content in the clay fraction of the lacustrine mudstones shows that high lake level stages were periods of lower salinity with the lowest boron contents from 73 to 268 ppm and periods of falling lake level were followed by significant increases in salinity with much higher boron values 293 603 ppm lake level fluctuations of the rudnik lacustrine system which are recorded by shallowing up units of sedimentary facies within most of the sections throughout the basin can also be traced within the monotonous black shale dominated sections where no sedimentological evidence of these lake level changes exists good indicators for such changes seem to be the 1 8 o and 1 3 c values of primary calcite 1 3 c toc and hi these lake level fluctuations are interpreted as driven by climatic oscillations in the order of tens of thousands years which could reflect climatic changes connected with the last glaciation event of gondwana