holocene climate variations retrieved from gun nuur lake sediment core in the northern mongolian plateau our pollen based climate reconstruction shows that both temperature and moisture were low during the early holocene from 10 000 to 8000 cal a bp and gradually climbed from 8000 to 6000 cal a bp the last 6000 years have been generally warm and wet with high amplitude fluctuations the high moisture conditions for the past 6000 years are not only corroborated by high arboreal pollen non arboreal ap nap trees and shrubs over herbs pollen ratio and high pollen concentration but also by high organic matter content and diatom data we suggest that lower winter insolation and remaining ice covers in the early holocene might have slowed down temperature rise in the continental interiors and that the resulted low temperature might have suppressed regional evaporation that was a major water vapor source for precipitation at the same time icemelt water injection into the north atlantic ocean might have also suppressed water vapor supplies to the downwind areas including the northern mongolian plateau

a freshwater old carbon offset in lake baikal siberia and problems with the radiocarbon dating of archaeological sediments evidence from the sagan zaba ii site abstract lake baikal in eastern siberia is one of the largest and deepest freshwater lakes in the world and its diverse fauna were extensively utilized by local human populations over many millennia the regional culture history models primarily are based on radiocarbon dates on human skeletal remains and in some cases also on dates on sediments from habitation sites analyses of a set of 113 radiocarbon dates from the sagan zaba holocene hunter gatherer and pastoralist habitation site 52 6825 n 106 4760 e on the western coast of lake baikal demonstrates two critical problems in the dating of these sets of materials first comparison of ams radiocarbon dates on terrestrial mammal and baikal seal bones from sagan zaba reveals the existence of a freshwater radiocarbon offset in the seal remains of at least 700 radiocarbon years radiocarbon dates on skeletal remains from humans who consumed the aquatic fauna also should be expected to carry an old carbon offset second comparison of radiocarbon dates made on sediment samples with those made on terrestrial mammal bones demonstrates major inconsistencies the primary problem with these samples is that the carbon being dated in them is of unclear relationship to human activities of interest at the site therefore the still common practice in this region of radiocarbon dating sediments to establish archaeological chronologies should be abandoned the ams radiocarbon dating of bone from species which clearly fed outside of lake baikal s aquatic environments should provide more reliable age estimates for human activities at archaeological sites

late quaternary climate and landscape evolution in arid central asia a multiproxy study of lake archive bayan tohomin nuur gobi desert southern mongolia abstract a 7 m sediment core from the bayan tohomiin nuur dry lake located in southern mongolia was investigated by a multidisciplinary study to reconstruct the history of climate and paleoenvironmental conditions during the late quaternary the geochemical granulometric palynological and ostracod results show a graduation of at least nine climatic units during the last 15 ka cal bp the climate during the late glacial changed from wet towards more dry conditions representing the beginning of the younger dryas period at 11 ka cal bp a rise in temperature and precipitation could be reconstructed marking the onset of the holocene the humid period of the holocene lasted until about 4 ka cal bp interrupted by shorter dry cold reversals at 8 5 7 5 ka cal bp and 5 4 5 ka cal bp the aridity in southern mongolia increased afterwards accompanied by the strengthening of aeolian processes dune remobilization and lake desiccation

vegetation and climate dynamics during the holocene and eemian interglacials derived from lake baikal pollen records abstract the last interglacial li and holocene changes in annual precipitation p ann the mean temperature of the warmest t w and coldest t c month and the moisture index were reconstructed from continuous pollen records from lake baikal the holocene core 52 31 n 106 09 e presented in this study was recovered from a depth of 355 m in the 25 km wide underwater buguldeika saddle separating the southern sub basin of lake baikal from its central sub basin the biome reconstruction shows that tundra and steppe biomes have highest scores during ca 15 000 13 300 cal years b p and that taiga becomes a dominant vegetation type after ca 13 300 cal years b p our quantitative reconstruction indicates an onset of relatively warm and wet conditions soon after ca 10 000 cal years b p the warmest and wettest climate with t w 16 c p ann 480 mm and 0 9 1 has been reconstructed for ca 9000 7000 cal years b p in the lake baikal region this interval is characterized by the appearance and spread of hunter communities kitoi culture consistently a hiatus in the regional archaeological record 4900 4200 years b c or 6850 6150 cal years b p coincides with the interval of a major climate deterioration which followed the climatic optimum an attempt to find a relationship between the archaeological record and a spread of steppe and meadow communities in the lake baikal region demonstrates that despite a long habitation of the area the human impact on vegetation was local rather than regional and likely did not affect the pollen record from lake baikal the reconstructed peaks in the steppe biome scores during the last 9000 years are consistent with short one to five hundred year episodes of weak pacific summer monsoon supporting our interpretation that the holocene vegetation changes around lake baikal are associated with large scale circulation processes controlling regional water balance rather than with human activities thus our study proves the suitability of lake baikal pollen data for the reconstruction of natural vegetation and climate dynamics through the whole period from the onset of the li to the present comparison of the recent and the last interglacial suggests that the holocene climatic optimum was less pronounced e g lower summer and winter temperatures and annual precipitation sums than that of the li on the other hand pollen records demonstrate that the holocene forest phase already lasts some thousand years longer than that of the li the interglacial vegetation dynamics derived from the lake baikal pollen records can be satisfactorily explained by reconstructed changes in summer and winter temperatures and in available moisture the interglacial vegetation around lake baikal is dominated by the boreal forests which are associated with a generally warm and wet climate the high sea level associated with decreased ice volume appears to have had a greater impact on the siberian environments during the last and the recent interglacial than the direct effect of lower than present winter insolation reconstructed changes in the winter temperature correlate well with changes in the sea level and global ice volume while the summer temperatures derived from the lake baikal pollen records track changes in the summer insolation

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

biological and environmental changes in lake baikal during the late quaternary inferred from carbon nitrogen and sulfur isotopes abstract an 8 m continuous sediment core approximately 250 ky old at the bottom from academician ridge in lake baikal has been analyzed for the stable isotopes of carbon nitrogen and sulfur in order to study the paleoclimatic and paleobiological changes that occurred in the eurasian continental interior these isotopic changes are closely related to changes in vertical lake water circulation between glacial and interglacial periods sedimentary organic carbon in cool periods is more enriched in 13 c 23 8 on average than that in warm periods 27 0 on average the 13 c enrichment of organic carbon suggests a decrease of land derived organic matter influx to the lake less precipitation and loss of terrestrial vegetation around lake baikal in cool periods pyrite in high total sulfur total organic carbon ts toc layers shows strong depletion in 34 s 20 8 to 32 4 during climate transitions from glacial to interglacial periods at the beginning of oxygen isotope stages ois 1 5 and 7 the 34 s depleted pyrite indicates augmentation of dissimilatory sulfate reduction by sulfate reducing bacteria srb at the sediment water interface enhancement of aqueous sulfate concentrations and limitation of oxygen circulation to the surface sediments might also occur in the climate transition periods the 15 n values of total nitrogen increase abruptly by 2 just after the 34 s negative peaks which may result from low nutrient concentrations in the euphotic zone associated with water circulation changes in lake baikal

evidence from lake baikal for siberian glaciation during oxygen isotope substage 5d abstract the paleoclimatic record from bottom sediments of lake baikal eastern siberia reveals new evidence for an abrupt and intense glaciation during the initial part of the last interglacial period isotope substage 5d this glaciation lasted about 12 000 yr from 117 000 to 105 000 yr b p according to correlation with the specmap isotope chronology lithological and biogeochemical evidence of glaciation from lake baikal agrees with evidence for the advance of ice sheet in northwestern siberia during this time period and also with cryogenic features within the strata of kazantzevo soils in southern siberia the severe 5d glaciation in siberia was caused by dramatic cooling due to the decrease in solar insolation as predicted by the model of insolation changes for northern asia according to milankovich theory coupled with western atmospheric transport of moisture from the open areas of northern atlantic and arctic seas which became ice free due to the intense warming during preceeding isotope substage 5e other marine and continental records show evidence for cooling during 5d but not for intense glaciation late pleistocene glaciations in the northern hemisphere may have begun in northwestern siberia

climate signals in sediment mineralogy of lake baikal and lake hovsgol during the lgm holocene transition and the 1 ma carbonate record from the hdp 04 drill core abstract modeling the bulk sediment xrd patterns allows insight into the environmental and depositional histories of two neighboring rift lake basins within the baikal watershed parallel 14c dated lgm holocene records in lakes baikal and hovsgol are used to discuss the mineralogical signatures of regional climate change in both basins it is possible to distinguish glacial and interglacial mineral associations clay minerals comprise in excess of 50 of layered silicates in bulk sediment the abundance of smectite expandable layers in mixed layer illite smectites and the total illite abundance are the main paleoclimatic indices in the clay mineral assemblage both indices exhibit coherent responses to the bolling allerod and the younger dryas the smectite layer index is not equivalent to the abundance of illite smectite because illite smectite tends to transform into illite repeated wetting drying cycles in soils and high abundance of expandable layers in illite smectites 42 favor the process of illitization this relationship is clearly shown in both baikal and hovsgol records for the first time the opposite late holocene trends in illite abundance in lake baikal and lake hovsgol records suggest that a sensitive optimal regime may exist for illite formation in the baikal watershed with regard to warmth and effective moisture the lake hovsgol sediments of the last glacial contain carbonates suggesting a positive trend in the lake s water budget a progressive change towards lower mg content in carbonates indicates lowering mineralization of lake waters this trend is consistent with the lithologic evidence for lake level rise in the hovsgol basin the pattern of mineralogical changes during the past 20 ka is used to interpret bulk sediment and carbonate mineralogy of the long 81 m lake hovsgol drill core hdp 04 with a basal age of 1 ma the interglacial type silicate mineral associations are confined to several thin intervals most of the sediment record is calcareous carbonates are represented by six main mineral phases calcite low mg calcite intermediate high mg calcite dolomite excess ca dolomite and metastable monohydrocalcite these mineral phases tend to form stratigraphic successions indicative of progressive changes in lake water chemistry five sediment layers with abundant mg calcites in the hdp 04 section suggest deposition in a low standing lake with high mineralization salinity and high mg ca ratios of lake waters lake hovsgol sediments contain the oldest known monohydrocalcite found tens of meters below lake bottom in sediments as old as 800 ka this unusual find is likely due to the conditions favorable to preservation of this metastable carbonate

vegetation changes and associated climate variations during the past 38 000 years reconstructed from the shaamar eolian paleosol section northern mongolia abstract this study presents a pollen based reconstruction of the vegetation changes and associated climate variations at the shaamar eolian paleosol section in the northern mongolian plateau the semi quantitative reconstructions of paleovegetation types and temperature and moisture indices show that the vegetation in the shaamar area was a taiga forest dominated by picea abies and pinus sibirica type between 38 000 and 30 000 cal bp when the reconstructed moisture level was the highest of the past 38 000 years the vegetation during the ensuing period from 30 000 to 23 000 cal bp was mainly a steppe dominated by artemisia and poaceae under low temperature and relatively low moisture conditions the vegetation during the period from 23 000 to 10 000 cal bp varied from a taiga forest through steppe dominated by poaceae taiga forest and forest steppe to a taiga forest the climate was cold and relatively dry in the early phase 23 000 to 20 000 cal bp and warmer and somewhat drier in the later phase 19 000 to 11 000 cal bp the pollen assemblages and the vegetation zonal indices suggest that the vegetation was a forest steppe during the period from 10 000 to 3000 cal bp under rising temperature and low moisture conditions a taiga forest has dominated the landscape during the past 3000 years under cooling and wetting conditions comparison of our pollen record with other paleoclimatic records suggests that the climate changes in the northern mongolian plateau have been controlled or modulated by ocean atmospheric coupling dynamics in the north atlantic region

clay mineral record in lake baikal sediments the holocene and late glacial transition abstract we analyse the clay mineralogical composition of two 1 m long cores from elevated plateaus in the southern and northern basins of lake baikal our aim is to test the limits of classically used clay proxies for climate reconstructions mineralogical signature is determined by x ray diffraction on oriented aggregates from holocene and late glacial sediments mineralogical results are presented on a palaeomagnetic derived time scale sampling at a centimeter resolution allows for a sub millenial order reconstruction the evolution of clay derived climate proxies smectite illite peak intensity ratio s i is compared between the two locations with respect to other climate reconstructions for the lake baikal area and siberia during the late glacial punctual increases of s i are related to changes of particle source rather than any climate change at both sites the most intense chemical weathering conditions occured during the subboreal lagging the eurasian middle atlantic climate optimum s i follows a gradual but irregular increase through the holocene probably related to slow warming favourable to the development of the siberian soils the different sensitivity to hydrolysing conditions in northern and southern sites can be explained by a combination of local lithological and topographical parameters

late quaternary clay mineral record in central lake baikal academician ridge siberia abstract we investigated the mineralogical composition of two cores recovered on the academician ridge central lake baikal siberia sedimentological features show that the cores are unaffected by turbidity currents however hemipelagic deposition is not continuous but intermittently disturbed by syn or post sediment reworking e g bioturbation slumps faulting such modes of deposition are consistent with the complex uplift history of the ridge bulk mineralogy suggests that terrigenous sediment supplies are constant through glacial interglacial stages and diluted by diatom rich intervals related to warmer interglacial stages the core stratigraphy is based on the correlation of the diatom zonation and opal abundance with the marine oxygen isotope reference curve specmap the 8 m cores partly recover the last four interglacial glacial cycles i e since oxygen isotope stage 8 we test the use of clay minerals as a proxy for paleoclimatic reconstruction the clays are more weathered during the diatom rich intervals in agreement with warmer climate conditions however the mean clay composition does not change significantly through glacial interglacial stages this observation implies that in the academician ridge sediments a simple smectite illite ratio s i does not alone provide a reliable indicator of climatic variation it reflects the complex clay assemblages especially the smectite group delivered to central lake baikal smectites include primarily illite smectite mixed layers made of a mixture of montmorillonite and beidellite according to their behavior after cation saturation the illite smectite mixed layers are primarily transformed smectites with some neoformed smectites intermittently observed in addition al smectites occur in minor proportions we conclude that the s i ratio has a climatic significance only if it evolves in parallel with the weathering stage of the clays and is confirmed by a change in the composition of the smectites

climate and vegetation changes around lake baikal during the last 350 000 years abstract we conducted particle size and pollen analyses on two cores extracted from lake baikal to investigate the response of vegetation to climate change and to assess the differences in vegetation changes between the northern and southern regions of lake baikal over the last 350 kyr vegetation changes in the lake baikal region corresponded well to the fluctuations in median particle size which are indicators of the climate changes in this region forest which was mainly composed of pinus and picea spread widely during the interglacials interstadials of mis 9 7 5 and 1 and open forest or desert covered most areas during the glacial stadial substages however the responses of vegetation to climate changes differed between the northern and southern regions generally vegetation expansion was restricted around the northern region during the interglacial substages equivalent to mis 9 3 7 5 and 5 5 forest spread widely around both the northern and southern regions however sparsely vegetated poor desert open forest covered the northern region during mis 5 3 and 5 1 while forest extended around the southern region in these substages moreover during glacial periods open forest composed mainly of artemisia chenopodiaceae amarantaceae betula and alnus covered the southern region while poor desert open forest covered the northern region these different vegetation responses are considered to have been caused by the geographical characteristics of the regions i e the mountains around the northern region and the plains around the selenga river in the southern region

weathering in the lake baikal watershed during the kazantsevo eemian interglacial evidence from the lacustrine clay record the clay mineralogical record of a piston core recovered on an elevated plateau in the northern basin of lake baikal has been investigated for the kazantsevo interglacial period i e eemian s s equivalent in northern europe the age model as inferred from palaeomagnetic intensity suggests that this stage spans ca 128 to 117 kyr bp relative clay mineral abundances and clay mineral ratios are used to reconstruct the weathering conditions within the baikal watershed at a sub millennial resolution and suggest that the clay record is highly variable a bimodal clay data distribution is in agreement with different clay sources and or formation between the studied glacial and interglacial periods high amounts of smectites in the taz glacial samples 128 7 136 4 kyr bp may be explained by an additional source of neoformed smectites during the glacial stage in addition to the classically used smectite illite ratio the mineralogical results are integrated by the calculation of a hydrolysis index that takes into account the abundance of all clay species and their sensitivity to chemical weathering a principal components analysis pca of the baikal clay minerals allows the comparison of the clay parameters with regard to weathering conditions clay data are further compared i with diatom and pollen profiles ii with pollen based quantitative reconstructions for the same core material and iii with other climate reconstructions for the lake baikal region and siberia several features of our record are highlighted here during the early period of the kazantsevo interglacial 128 4 125 2 kyr bp weathering processes remain controlled by physical reworking for more than 2 kyr after the initial transition from cold to warm conditions inception of chemical weathering starts only after ca 125 kyr bp a period coincident with the warmest conditions according to both the pollen record and by the strongest chernozem development in siberian soils within the interglacial interval the hydrolysis index displays a two step increase punctuated by a minimum value ca 122 kyr bp the increasing but irregular trend persists after the transition from the kazantsevo interglacial to the zyryanka glacial similar to 117 kyr bp peak chemical weathering as inferred by clay changes lags the interglacial glacial transition by at least 2 kyr this suggests that pedogenesis remains active after the interval of surface stabilization lake baikal clay minerals trace the nature of the main weathering conditions within the watershed we note any increase in physical weathering is rapidly recorded in sedimentary clay assemblages but the mineral imprint to chemical weathering changes is more gradual lagging reconstructed climate conditions over the lake by ca 2 kyr 0 2007 elsevier b v all rights reserved

late pliocene vegetation and climate of the lake baikal region southern east siberia reconstructed from palynological data abstract palynomorphs from a palaeomagnetically dated late pliocene sediment core 3 6 2 35 ma reflect highly variable climatic conditions and repeated expansion of steppe and boreo alpine vegetation mixed coniferous forests picea tsuga pinus with associated broadleaved taxa quercus corylus ulmus tilia were affected by dry and cold intervals already between 3 48 and 3 39 ma peak records of non arboreal pollen types and spores including subarctic boreo alpine selaginella selaginoides evidence advances of open vegetation which can be correlated to glacial marine oxygen isotope stages a cooling trend occurred from 3 15 ma to 2 5 ma documented by cold dry intervals and increased fluctuations of picea and pinus around 3 2 9 ma the palaeoclimate variation pattern shifted towards cold dry and warm moist oscillations cooling strengthened after 2 89 ma and severely restricted tsuga development cool and oligotrophic limnic conditions are documented by abundant gonyaulax dinoflagellate cysts in the interval 2 65 2 57 ma due to decreasing precipitation extensive areas with open steppe and rock steppe vegetation became permanently established after 2 62 ma dry conditions became a dominant environmental factor in the baikal region coeval in time with the red clay loess shift in northern china this climatic transition can be explained as a consequence of major intensification of northern hemisphere glaciation around 2 75 ma evidenced by marine records of ice rafted debris

biological nitrate utilization in south siberian lakes baikal and hovsgol during the last glacial period the influence of climate change on primary productivity abstract stable nitrogen isotope ratios 15 n in sediment cores recovered from two siberian lakes baikal and hovsgol are determined in order to evaluate the biologically mediated nitrogen cycle from the last glacial period to the postglacial period the ranges of 15 n values from the two lakes are similar varying from 2 8 to 6 7 the high values of 15 n around 6 during the last glacial maximum lgm 26 5 19 cal ka bp indicate that a larger fraction of the available nitrate in both lakes was utilised by phytoplankton a cool and dry climate during the lgm may have limited the input of terrestrial nitrate resulting in the use of a large fraction of the nitrate and low productivity in both lakes the use of a smaller fraction of the nitrate by phytoplankton in both lakes after the lgm 19 12 cal ka bp is reflected in the lower 15 n values down to 3 0 the increase in summer insolation after the lgm caused increased vertical mixing of the lake water column which transported nitrate upwards from the deeper waters and into the photic zone the reduced rate of nitrogen consumption after the lgm indicates that nitrate was more readily available to phytoplankton at this time and did not limit their activities this change suggests that other factors limited phytoplankton activity during this period at the climatic transition between oxygen isotope stages 2 and 1 around 11 5 cal ka bp the rapid increase in 15 n from 3 0 to 6 0 reflected a rapid increase in nitrate utilization at this transition increases in precipitation and lake levels which were associated with the increased intensity of the east asian monsoon may have brought a large influx of nutrients required by the phytoplankton into the lakes from the watershed this hydrological change led to increased phytoplankton activity in both lakes resulting in increased nitrate consumption the 15 n data obtained here from the sediments of lakes baikal and hovsgol suggest that the nitrogen cycle in the siberian region is strongly linked to hydrological changes across the asian continent which are related to global climate change

influence of palaeoclimate and hydrothermal activity on organic matter accumulation in lacustrine black shales from the lower cretaceous bayingebi formation of the yin e basin china abstract the lacustrine black shales of the lower cretaceous bayingebi formation are an important hydrocarbon source rock in the yin e basin it is important to understand the mechanism of organic matter accumulation in these shales to reconstruct the palaeoenvironment and palaeoclimate in the early cretaceous in northwestern china and search for potential oil and gas resources based on petrological elemental geochemical and organic geochemistry analyses of typical field profile samples the influence of palaeoclimate and hydrothermal activity on organic matter accumulation in different sedimentary units unit i ii and iii is revealed unit ii has the highest total organic matter abundance tocavg 5 95 mainly type i kerogen lamalginite which is in the immature to low mature stage organic carbon isotope data 13corg element ratios sr cu and c value and weathering indexes including cia ciw and pia together revealed that the region experienced a semihumid to semiarid palaeoclimate in the middle of the early cretaceous the increase in rainfall in unit ii enhanced the abundance of lake algae and surrounding terrestrial vascular vegetation and elevated the lake level which contributed to adequate sources and favourable preservation conditions for organic matter accumulation several lines of evidence including high sulfur contents the combination of vein pyrite and micritic dolomite positive eu anomalies and element geochemical correlations confirm the existence of intermittent low temperature white smoker type hydrothermal activity the decrease of oxygen content in lake bottom water may be related to the input of hydrothermal fluid which provided necessary nutrient elements for biological productivity according to palaeontological and radioisotope dating data it can be inferred that the black shales of the bayingebi formation are the product of the combined effect of global palaeotemperature rise in the early aptian stage of the early cretaceous and intermittent low temperature hydrothermal activity

late pliocene sedimentation in lake baikal implications for climatic and tectonic change in se siberia abstract within the framework of the baikal drilling project bdp a 192 m long sediment core bdp 96 1 was recovered from the academician ridge a submerged topographic high between the north and central basins of lake baikal sedimentological clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth corresponding to ca 2 4 3 4 ma the aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of northern hemisphere glaciation in late pliocene time a major climate change occurred in the lake baikal area at about 2 65 ma enhanced physical weathering in the catchment mirrored in the illite to smectite ratio and temporarily reduced bioproduction in the lake reflected by the diatom abundance evidence a change towards a colder and more arid climate probably associated with an intensification of the siberian high in addition the coincident onset of distinct fluctuations in these parameters and in the zr al ratio suggests the beginning of the late cenozoic high amplitude climate cycles at about 2 65 ma fluctuations in the zr al ratio are traced back to changes in the aeolian input with high values in warmer more humid phases due to a weaker siberian high assuming that the sand content in the sediment reflects tectonic pulses the lake baikal area was tectonically active during the entire investigated period but in particular around 2 65 ma tectonic movements have likely led to a gradual catchment change since about 3 15 ma from the western towards the eastern lake surroundings as indicated in the geochemistry and clay mineralogy of the sediments the strong coincidence between tectonic and climatic changes in the baikal area hints at the himalayan uplift being one of the triggers for the northern hemisphere glaciation

theoretical considerations on the anhysteretic remanent magnetization of interacting particles with uniaxial anisotropy 1 the anhysteretic remanent magnetization arm is widely used in rock magnetism and paleomagnetism because of its sensitivity to the domain state of magnetic particles and the close analogy to natural remanent magnetizations on the other hand the arm shares with other weak field magnetizations the property of being extremely sensitive to magnetostatic interactions therefore it is desirable to model the effects of interactions on natural assemblages of magnetic particles direct micromagnetic calculations of the arm acquisition process are not practicable therefore an analytical approach is used calculate the arm susceptibility of a system of interacting single domain sd particles the model is based on a statistical description of the interaction field the equations obtained have been used to evaluate the dependence of the arm on the packing fraction of the magnetic particles the effect of interactions on the anisotropy of arm aarm was evaluated as well the aarm of densely packed particles is complex and depends critically on the microcoercivity a physical interpretation of the aarm of highly interacting particles is therefore difficult

uranium isotopes as proxies of the environmental history of the lake baikal watershed east siberia during the past 150 ka abstract a new experimental method for the determination of the isotopic composition of authigenic uranium in lake baikal siberia sediments has supplied a record of 234u 238u aut ratios during the past 150 ka with a resolution of ca 200 years reconstruction of the 234u 238u aut activity ratio as a proxy for lake water composition shows that the uranium isotope disequilibrium has not stayed constant throughout this period it varied greatly in response to regional climate the 234u 238u activity ratio in lake baikal paleowaters was inferred to be high during interglacial time 1 85 2 during mis 5 5 and the holocene 1 4 1 7 during mis 5 1 and 5 3 and dropped to 1 16 0 01 during glacial time the variability in the deduced 234u 238u ratios of lake baikal waters of the past may have been caused by the climatically modulated redistribution of contributions of riverine discharges from different provinces of the very large catchment of the lake aridity during ice ages was more pronounced in the south of eastern siberia and in northern mongolia relative to that in the northern regions of the lake baikal watershed the southern contribution of riverine input into the lake was essentially absent during glacial periods records of some elements ca k sr rb si al la th u th ciw from the bottom sediments of lake baikal during the last 0 5 myr with a resolution of 2 kyr have also been obtained coupling these data with records of weathering indices shows that not only riverine waters but also terrigenous material delivered into lake baikal from different regions of the very large watershed during interglacial and glacial times glacial erosion delivered fine siliciclastic material glacial milk mainly from the nearest mountain glaciers situated in the northwestern region of lake baikal while during warmer periods terrigenous material was sourced from the surrounding river valleys a conceptual model for the simulation of the fluvial discharge gradient between the northern and southern provinces of the lake baikal watershed is proposed and a first order simulation of past changes in the contribution of the selenga river to the lake water budget is given

ams radiocarbon analyses from lake baikal siberia challanges of dating sediments from a large oligotrophic lake abstract a suite of 146 new accelerator mass spectrometer ams radiocarbon ages provides the first reliable chronology for late quaternary sediments in lake baikal in this large highly oligotrophic lake biogenic and authigenic carbonate are absent and plant macrofossils are extremely rare total organic carbon is therefore the primary material available for dating several problems are associated with the toc ages one is the mixture of carbon sources in toc not all of which are syndepositional in age this problem manifests itself in apparent ages for the sediment surface that are greater than zero however because most of the organic carbon in lake baikal sediments is algal autochthonous in origin this effect is limited to about 1000 500 years which can be corrected at least for young deposits the other major problem with dating lake baikal sediments is the very low carbon contents of glacial age deposits which makes them extremely susceptible to contamination with modern carbon this problem can be minimized by careful sampling and handling procedures the ages show almost an order of magnitude difference in sediment accumulation rates among different sedimentary environments in lake baikal from about 0 04 mm year on isolated banks such as academician ridge to nearly 0 3 mm year in the turbidite depositional areas beneath the deep basin floors such as the central basin the new ams ages clearly indicate that the dramatic increase in diatom productivity in the lake as evidenced by increases in biogenic silica and organic carbon began about 13 ka in contrast to previous estimates of 7 ka for the age of this transition holocene net sedimentation rates may be less than equal to or greater than those in the late pleistocene depending on the site this variability reflects the balance between variable terrigenous sedimentation and increased biogenic sedimentation during interglaciations the ages reported here and the temporal and spatial variation in sedimentation rates that they imply provide opportunities for paleoenvironmental reconstructions at different time scales and resolutions

a high resolution diatom record of the palaeoclimates of east siberia for the last 2 5 my from lake baikal abstract two parallel drilling cores bdp96 1 200 m and bdp96 2 100 m have been taken from top of the underwater akademichesky ridge in lake baikal and dated by paleomagnetic techniques these cores are part of the baikal drilling project kuzmin et al 1997a b diatom analysis of the first 100 m of these sediments reveals that they store a 2 5 myr high resolution continuous record of the palaeoclimates of east siberia sediments belonging to warmer climates have a high content of diatom algae frustules whereas those belonging to global glaciations are diatom barren the record of upper pleistocene 500 kyr bp strongly correlates with the oceanic specmap curve and contains distinct 100 41 23 and 19 kyr periods as revealed by fourier analysis diversity of diatom species was small in pliocene abrupt and frequent changes of the climate in pleistocene resulted in frequent dramatic changes in the diatom communities the extant dominating endemics cyclotella minuta and aulacoseira baicalensis became important in lake baikal 760 kyr and less than 120 kyr bp respectively

paleoenvironmental record in lake baikal sediments environmental changes in the last 160 ky measurement of magnetic susceptibility ms and diffuse reflectance spectra drs were used to construct environmental proxies in a 6 5 m section of the sediment core ver98 1 13 from the academician ridge lake baikal the interpretation of ms and drs was supported by x ray diffraction and microparticle voltammetry to identify the main fe bearing minerals the record of the relative paleointensity of the earth s magnetic field was used to construct an age model showing the core interval covers the last 160 ky the time resolution of the record was 1 ky the main environmental changes were recognized by a combination of drs and ms records and compared to marine isotopic stages in addition to the diatom record from 120 to 60 ky bp so as to demonstrate the importance of these inorganic proxies as paleoenvironmental records a dramatic climatic deterioration between 66 and 60 ky bp was probably preceded by a dry intermediate stage between 77 and 66 ky bp the drs based proxies of fe ii fe iii ratios in the mineral assemblage ms and or diatom records clearly reveal 1 2 ky long environmental extremes at 110 103 90 85 77 61 54 36 27 23 and 19 ky bp the majority of these extremes were contemporaneous with discharges of north atlantic and arctic ocean icebergs heinrich events documenting a teleconnection between the north atlantic ocean and east central siberia these sharp changes coincided with a major transition to the colder climatic stages of the last 100 ky d 2006 published by elsevier b v

temporal changes in the phytoplankton community of the southern basin of lake baikal over the last 24 000 years recorded by photosynthetic pigments in a sediment core abstract photosynthetic pigments and other indicators of phytoplankton were analyzed in a dated undisturbed sediment core obtained from the southern basin of lake baikal to reveal temporal changes in the phytoplankton community in the lake through the last glacial post glacial transition the sedimentation age of the core spans the last 24 14c ka chlorophyll a its derivatives carotenoids and total organic carbon toc started to increase after 15 14c ka and the onset of biogenic silica occurred at 10 14c ka this indicated that the post glacial growth of diatoms was preceded by that of other phytoplankton groups in the record of the pigments and toc a temporary decrease was observed in the period 11 5 10 5 14c ka corresponding to the younger dryas cold period the similarity found between the depth profiles of pyropheophytin a and steryl chlorin esters formed through predation of phytoplankton by zooplankton and that of toc suggested the important contribution of fecal pellets to sedimentary organic matter in the lake

lignin phenols in sediments of lake baikal siberia application to paleoenvironmental studies abstract sediments from three cores obtained from distinct depositional environments in lake baikal siberia were analyzed for organic carbon total nitrogen and lignin phenol concentration and composition results were used to examine changes in paleoenvironmental conditions during climatic cycles of the late quaternary c n ratios and organic carbon accumulation rates were significantly higher in the holocene compared with the late pleistocene reflecting overall warmer temperatures and increased runoff during the holocene a holocene maximum in organic carbon was observed at about 6 ka and may represent the warmest wettest period of the holocene at one site academician ridge pronounced late pleistocene maxima in organic carbon and biogenic silica were observed at about 80 85 ka probably indicative of an interstadial period with enhanced aquatic productivity total sedimentary lignin phenol contents were generally lower in the late pleistocene compared to the holocene but with several peaks in concentration during the late pleistocene these late pleistocene peaks in total sedimentary lignin content dated at about 80 50 and 30 ka directly precede or occur during peaks in sedimentary biogenic silica contents these periods likely represent relatively warm interstadial times with increased precipitation producing the observed increase in terrestrial runoff and aquatic productivity lignin phenol ratios s v c v and p v were used to examine changes in terrestrial vegetation type resulting from changes in paleoenvironmental conditions during the late pleistocene a degree of caution must be used in the interpretation of these ratios with regard to vegetation sources and paleoenvironmental conditions because of potential compositional changes in lignin resulting from biodegradation nevertheless results show that long glacial periods were characterized by terrestrial vegetation composed of a mix of non woody angiosperm vegetation and minor gymnosperm forest shorter interstadial periods are defined by a change to dominant gymnosperm forest and were observed at about 80 75 63 50 and 30 ka ranging from about 2 6 kyr in duration these interstadial periods of the late pleistocene defined by lignin phenol ratios generally occur during longer periods of enhanced sedimentary biogenic silica content about 10 15 ka in duration providing corroborative evidence of these warm interstadial periods

uranium series disequilibrium sedimentation diatom frustules and paleoclimate change in lake baikal abstract the large volume of water approximately one fifth of the total surface fresh water on the planet contained in lake baikal in southeastern siberia is distinguished by having a relatively high concentration of uranium ca 2 nm and together with the surface sediments an unusually high234u238u alpha activity ratio of 1 95 about 80 of the input of uranium to the lake with a234u238u ratio of 2 0 comes from the selenga river profiles of uranium as well as the extent of isotopic disequilibrium in a 9 m sediment core collected on academic ridge generally show high values during interglacial periods corresponding to high diatom frustule numbers difr and biogenic silica bsi data that have been reported elsewhere during glacial periods low difr and bsi uranium progeny 234u and230th were in secular equilibrium with low concentrations of their parent238u radionuclide distributions were interpreted in terms of a quantitative model allowing for adsorption of riverine inputs of uranium onto two classes of sedimenting particles with differing238u232th ratios and uranium progeny in secular equilibrium if the234u238u activity ratio of adsorbed uranium has remained constant mean sedimentation rates can be independently estimated as 3 6 0 6 and 3 7 0 9 cm kyr 1 for the decay of234u and in growth of230th respectively these rates are consistent with a mean rate of 3 76 cm kyr 1 calculated by optimization of the correspondence between adsorbed238u and 18o in dated oceanic sediments the adsorbed uranium apparently tracks variable river flow during interglacials and is drastically reduced during periods of glaciation evidently uranium has not been significantly redistributed within baikal sediments over at least the past 250 kyr and is a unique biologically non essential tracer for climate sensitive processes which provide their own internal geochronometers potentially useful for ages up to 1 myr bp

mineralogical signatures of lake baikal sediments sources of sediment supplies through late quaternary abstract the mineralogical composition of late quaternary sediments was investigated in three piston cores recovered on elevated plateaus in lake baikal on continent ridge a northern extension of academician ridge in the north basin on the posolsky bank near the selenga delta and on the vydrino shoulder in the south basin the sediments are alternating biogenic diatom rich muds and terrigenous silty clays with sandy layers occurring in the southern vydrino core core stratigraphy is based on ams 14 c dates on pollen diatom zonation and magnetic record correlation the 6 10 m long cores cover the last 40 kyr in vydrino 60 kyr in posolsky and 185 kyr in continent ridge the bulk clay

glaciers and meltwater flux to lake baikal during the last glacial maximum abstract maximal 3d ice extent during mis 2 around lake baikal has been reconstructed using glacial landforms in the field aerial photos satellite images and gis analysis this regional glacial maximum is correlated 18 kyr bp with the global lgm based on available 14 c dates cosmogenic 10 be exposure ages correlation of lakeshore moraines with two raised shorelines and the ird record in lake sediments total ice area was 13 000 km 2 and ice volume 2000 km 3 1800 km 3 in water equivalent ice distribution was controlled by topography and ice fields with valley glaciers spreading to adjacent depressions locally ice caps with outlet glaciers existed averaged regional ela is estimated to be 1290 240 m with the ela depression at approximately 1340 m more acceptable regional scenario of the lgm precipitation reduction by 25 50 implies that summer temperature decreased by 6 12 c along with an increased solid fraction in precipitation up to 85 there were primary climatic controls for glaciation meltwater supply as a strong controller of lgm water budget 60 of total water input to the lake is estimated to have been from 6 4 to 9 0 km 3 yr 1 7 9 times less than present day river input preliminary balance estimation shows that during full glacial stage lake baikal was overflowing with the angara river output as 4 9 km 3 yr 1 and with lake level close to that today lake level drop down to the depth of 40 m is recorded by geological and geophysical evidence perhaps explaining the abrupt increase in effective evaporation in the lake watershed at the start of mis 2 24 18 ky bp

orbital forcing of continental climate during the pleistocene a complete astronomically tuned climatic record from lake baikal se siberia abstract a new composite bdp 96 biogenic silica record over the entire pleistocene was generated by splicing bdp 96 1 and bdp 96 2 drill cores from lake baikal crosschecked against a similar record from a nearby bdp 98 drill core a new astronomically tuned age model is proposed based on correlating peak biogenic silica responses with the timing of september perihelia this target is derived from analysis of regional climate proxy responses during the holocene the last interglacial and around paleomagnetic reversals by resolving virtually every precessional cycle during the pleistocene the new age model represents a major improvement compared with previously reported lake baikal timescales the astronomically tuned ages of the pleistocene paleomagnetic reversals are consistent with published dates the minimal tuning approach we used precession only has also aligned high signal power in a narrow obliquity band confirming the strong presence of orbital forcing there are also strong ca 100 ka scale cycles but these are not aligned with the orbital eccentricity despite the location of lake baikal in a continental interior that is highly sensitive to insolation forcing the tuned biogenic silica record reveals a consistent phase difference of 32 ca 4 ka relative to insolation in the obliquity band an inherent lag embedded in a continental proxy record not driven by global ice volume is an intriguing finding another new observation is that long term changes in sedimentation rates in lake baikal appear to be related to the amplitude of orbital forcing both amplitudes and sedimentation rates undergo significant changes during mis 24 mis 19 interval corresponding to the middle pleistocene transition with potential for linking continental and marine climato stratigraphies the new baikal record serves a new benchmark correlation target in continental eurasia as an alternative to june 65 n insolation and odp correlated timescales

lignin biomarker and isotopic records of paleovegetation and climate changes from lake erhai southwest china since 18 5kabp abstract a sound knowledge of paleovegetation is fundamental to better understanding of past climate variability and provides necessary background for improving predictions of future climate changes in the present study stable isotopes 13c and 15n and lignin biomarker of sedimentary organic matter som from lake erhai 25 50 n 100 10 e southwest china were determined to reconstruct paleovegetation and to find vegetation climate relationships the stable carbon isotope signatures suggested that the organic matter mostly derived from c3 plants throughout the depositional periods since 18 5 ka bp the variability of isotopic values of carbon 13c 28 8 to 23 9 and nitrogen 15n 1 2 4 1 might be related to changes in terrestrial runoff i e precipitation compositions of the alkaline cuo oxidation products of lignin showed a significant change in terrestrial vegetation from gymnosperm rich to angiosperm rich plants at lake catchments in the early holocene the lignin phenol vegetation index lpvi also indicated changes in terrestrial vegetation with few more subtle changes during the last 18 5 ka as a single proxy of entire characteristic of terrestrial vegetation in comparison to past asian monsoon records 18o record in stalagmite of dongge cave lpvi suggested a direct linkage between vegetation changes and asian monsoon variability that might be influenced by global climate changes

depositional and tectonic framework of the rift basins of lake baikal from multichannel seismic data recent multichannel seismic reflection data from lake baikal located in a large active continental rift in central asia image three major stratigraphic units totaling 3 5 to 7 5 km thick in four subbasins a major change in rift deposition and faulting between the oldest and middle rift units probably corresponds to the change from slow to fast rifting in early pliocene time inferred from on land studies a minor modification of fault patterns characterizes the youngest unit a brief comparison of the basins of lake baikal with those of the east african rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks

long term climato limnological oscillation during the past 2 5 million years printed in lake baikal sediments the physical properties of long sediment cores bdp96 obtained from the academician ridge of lake baikal eastern siberia show that the changes in the climato limnological environment of the continental interior during the past 2 5 m y clearly reflected global climatic change oscillation in water content and grain size which are closely related to biogenic productivity coincides with marine 18o variation the values for water content and grain size are large during interglacial periods and small during glacial periods milankovitch parameters were also imprinted in the sediments over the past 2 5 m y a 400 ky period as well as a 100 ky period due to eccentricitical parameters is found in addition to other orbital parameter a 40 ky period of obliquitical and a 20 ky period of precessional parameter this indicates that change in solar insolation was closely related to long term variations in climato limnological regime in this area

biogenic silica accumulation and paleoproductivity in the northern basin of lake baikal during the holocene the effects of the holocene climatic amelioration productivity and preservation on biogenic silica accumulation in the northern basin of lake baikal are evaluated by analyzing biogenic silica organic carbon nitrogen and the stable carbon isotopic composition of organic matter in a 4 m piston core and a companion box core in the early holocene 11 000 8500 yr b p biogenic silica accumulation was low because of low productivity strong oligotrophy and the influx of nutrient limited alpine glacial meltwater in the middle holocene productivity increased significantly during 8500 7000 yr b p but little biogenic silica accumulated in sediments because the dominant primary producer was not diatom flora and because of the relatively higher productivity caused by enhanced terrestrial nutrient influx the highest accumulation rates of biogenic silica were those between 7000 and 4500 yr b p when aquatic productivity increased limnological stratification occurred a mature diatom flora developed in the late holocene the past 4500 yr biogenic silica accumulation decreased because of enhanced recycling and regeneration of silica and a low sedimentation rate

a rock magnetic record from lake baikal siberia evidence for late quaternary climate change rock magnetic measurements of sediment cores from the academician ridge region of lake baikal siberia show variations related to late quaternary climate change based upon the well dated last glacial interglacial transition variations in magnetic concentration and mineralogy are related to glacial interglacial cycles using a conceptual model interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals the variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods we also infer an increased contribution of eolian sediment during the colder windier and more arid glacial conditions when extensive loess deposits were formed throughout europe and asia eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals variations in magnetic concentration and mineralogy of lake baikal sediment correlate to the specmap marine oxygen isotope record the high degree of correlation between baikal magnetic concentration mineralogy and the specmap oxygen isotope record indicates that lake baikal sediment preserves a history of climate change in central asia for the last 250 ka this correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method future work must include providing better age control and additional climate proxy data thereby strengthening the correlation of continental and marine climate records

last glacial interglacial vegetation and environmental dynamics in southern siberia chronology forcing and feedbacks abstract radiocarbon dated pollen and diatom records from lake kotokel in southern siberia help to reconstruct the environmental history of the area since 47 kyr bp pollen spectra composition and reconstructed biome scores suggest predominance of a tundra steppe vegetation and variable woody cover 5 20 between 47 and 30 kyr bp indicating generally a harsh and unstable climate during this interval conventionally regarded as the interstadial within the last glacial the short term climate amelioration episodes in the glacial part of the records are marked by the peaks in taiga and corresponding minima in steppe biome scores and appear synchronously with the hemispheric temperature and precipitation changes recorded in the greenland ice cores and chinese stalagmites transition to full glacial environments occurred between 32 and 30 kyr bp the interval at 30 24 kyr bp was probably the driest and coldest of the whole record as indicated by highest scores for steppe biome woody coverage

lignin and fatty acid records in lake baikal sediments over the last 130 kyr a comparison with pollen records abstract a 10 m sediment core from academician ridge in lake baikal was analyzed for its molecular composition using on line tmah tetramethylammomium hydroxide thermochemolysis major products are lignin phenols n c 14 to c 30 fatty acids alkanoic acids cutin acids hydroxy acids and aliphatic dicarboxylic acids lignin phenols are abundant in warmer periods the interglacial marine isotope stage mis 5e and mis 1 but extremely low in the other colder periods this result coincides well with pollen records reported for a core near the present site where an expansion of coniferous forests in sub stage 5e and mis 1 was implied normal c 24 c 30 alkanoic acids important components of plant wax esters are abundant in 5e and mis 1 and are present in significant amounts in the other colder periods unlike the lignin phenols a high abundance of n c 24 to c 30 alkanoic acids relative to lignin phenols in the bolling allerod warm period suggests an enhanced development of herbs it is implied from comparison of the sedimentary lignin phenol record with fossil pollen records and lignin phenol analysis of modern pollen that the ratio of cinnamyl phenols to vanillyl phenols may serve as an indicator of pollen contribution to sedimentary organic matter

palaeoenvironmental history of lake baikal during the last 23000 years abstract a sediment core recovered from lake baikal was studied in an attempt to elucidate the palaeoenvironmental history of an intra continental lake under the control of last glacial to post glacial climatic fluctuations the lake conditions were characterised by subdued primary production and increased turbidity conditions during the last glacial maximum terrestrial vegetation and the development of soil layers were reduced in the drainage areas the deglaciation suggested by increased terrigenous sand influx and biological productivity took place at around 18 000 yr b p responding to the increased insolation in the northern hemisphere high latitude the onset of wet and warm conditions at ca 12 000 yr b p brought on a maximum of aquatic production and forestal expansion between 7 000 and 6 000 yr b p since 5 000 yr b p enhanced blooming of diatoms has occurred due to an increased nutrient supply and dissolved silica input into the lake

paleoenvironmental proxy records from lake hovsgol mongolia and a synthesis of holocene climate change in the lake baikal watershed abstract here we discuss paleoenvironmental evolution in the baikal region during the holocene using new records of aquatic diatom and terrestrial vegetation changes from hovsgol mongolia s largest and deepest lake we reconcile previous contradictory baikal timescales by constraining reservoir corrections of ams dates on bulk sedimentary organic carbon synthesis of the holocene records in the baikal watershed reveals a northward progression in landscape vegetation changes and an anti phase behavior of diatom and biogenic silica proxies in neighboring rift lakes in lake baikal these proxies appear to be responsive to annual temperature increases after 6 ka whereas in lake hovsgol they respond to higher precipitation runoff from 11 to 7 ka unlike around lake baikal warmer summers between 6 and 3 5 ka resulted in the decline not expansion of forest vegetation around lake hovsgol apparently as a result of higher soil temperatures and lower moisture availability the regional climatic proxy data are consistent with a series of 500 yr time slice holocene gcm simulations for continental eurasia our results allow reevaluation of the concepts of the holocene optimum and a maximum of the asian summer monsoon as applied to paleoclimate records from continental asia

vegetation response in the southern lake baikal region to abrupt climate events over the past 33calkyr abstract we reconstructed vegetation changes and fire events in the southern lake baikal region over the past 33 cal kyr using a sediment core from the buguldeika saddle southern lake baikal to examine vegetation response to past short term climate variations herbs such as artemisia and asteraceae were dominant just before the last glacial maximum lgm low pollen accumulation rates pars with little herb pollen indicate sparse herbaceous vegetation cover during the lgm picea expanded after the bolling interval suggesting expansion of riparian areas and shrub alnus rapidly increased during the following allerod interval abies and pinus subgen haploxylon began to increase at the beginning of the holocene and pinus subgen diploxylon has been highly dominant since the middle holocene the vegetation change after the lgm is similar to that from other sites in the lake baikal region but the responses to short term climate events are pronounced fluctuations in herb elements and vegetation cover as indicated by the par correspond to the younger dryas yd cooling event the decline in the par and increase in herb taxa were remarkable at the beginning of the yd but larix and picea trees were distributed in river valleys near lake baikal in the latter half the rapid expansion of betula and artemisia following frequent fires in the early holocene may have been in response to the 8 2 cal ka bp cooling event from these findings the southern lake baikal region including the selenga river basin may possibly have been more sensitive than other parts of the region to abrupt climate changes

the structure and development of the baikal rift depression abstract the baikal rift depression located in southeastern siberia is one of the best investigated intra continental rift structures in the world in addition to being the site of comprehensive studies by russian geo scientists for decades the baikal depression is now the subject of new international investigations using advanced technical means and technologies the present paper summarizes the principal results concerning the morphostructure of the baikal rift the stratigraphy of the cenozoic deposits and the stages of the baikal depression development based on cross sections which exhibit the impact of both tectonic and climatic processes the development of the baikal depression occurred through two stages i the pre crypto rift stage of the late cretaceous eocene and ii active rifting of the oligocene recent the second stage is divided into several steps and substeps separated by tectonic phases formation of the baikal rift zone appears to be related to the indo eurasian plate collision related to a model of passive rifting the uplifted shoulders of the depression have a complex morphostructure overlain by cenozoic deposits which are thin and restricted in area the near marginal zones of the rift are largely defined by the system of longitudinal blocks of different width surfaces of the blocks are inclined either to the uplifted shoulder of the rift bound by listric faults typical for western edges of the depression or to the axis of the depression cenozoic deposits on the blocks consist of accumulative plains composed primarily of thick lacustrine sequences

centennial to millennial scale climate shifts in continental interior asia repeated between warm dry and cool wet conditions during the last three interglacial states evidence from uranium and biogenic silica in the sediment of lake baikal southeast siberia abstract we observed timescale dependent changes in the correlations between biogenic silica biosi and uranium concentrations of bulk sediment from the buguldeika saddle lake baikal there was a positive correlation for the glacial interglacial 100 kyr scales over the 180 kyr with inverse correlations for centennial to millennial scales during the last three interglacial states mis 5 3 and 1 the distinction of these correlations between the time scales suggests that the sedimentation processes of biosi and u differ from each other the baikal biosi concentration record is generally regarded as a paleotemperature proxy reflecting the diatom production of the lake on the other hand we concluded that the temporal variation of the u concentration in the sediment reflected the weathering intensity in the south of lake baikal watershed associated with changes in the rainfall moisture levels based on the paleoclimate proxies from lake baikal climates in continental interior asia are identified as having dry and wet conditions during the glacial and interglacial periods respectively this accounts for the rhythms in growth and retreat of ice sheets in eurasia on glacial interglacial scales in the interglacial periods mis 5 3 and 1 the continental interior is contrary to the glacial interglacial climate changes characterized by alternating warm dry and cool wet climates with periods in tens of thousands years in particular such shifts in the climate stand out prominently at the beginning of mis 5d the detrended u record for the last 5 2 kyr shows wet events at 4 3 3 7 3 2 2 3 1 8 1 2 and 0 8 0 3 kyr these events coincide with the timing of the north atlantic ice rafted debris events which were possibly related to solar activity all this evidence suggests that climate in the continental interior followed significantly different fluctuation patterns in the glacial interglacial and shorter time scales

response of phytoplankton productivity to climate change recorded by sedimentary photosynthetic pigments in lake hovsgol mongolia for the last 23 000 years abstract we analyzed photosynthetic pigments total organic carbon toc and total nitrogen tn in two radiocarbon dated sediment cores of lake hovsgol nw mongolia covering the period of the last glacial post glacial transition since 23 14c ka the sedimentary profiles of total chlorophyll a derivatives tchl a toc and tn reflect productivity increase during the last glacial post glacial transition between 15 and 10 14c ka in response to regional climate warming and enhanced precipitation correlation between tchl a and toc was high r 2 0 934 n 74 with a zero intercept of a regression line suggests that total organic carbon in lake hovsgol sediments is of autochthonous origin the depth profiles of toc tn ratio showed increases toward the core top in parallel with other paleoproductivity proxies serving as an indicator of climatic transition quite similar to lake baikal sedimentary record the primary reason for the shifts of this parameter is still not clear however chlorophyll b derivatives and some algal carotenoids such as lutein zeaxanthin diatoxanthin and canthxanthin were detected although in concentrations lower than that of tchl a these biomarkers indicate that green algae cyanobacteria and diatoms were present in phytoplankton during the post glacial period significantly pre dating the beginning of the holocene abrupt drops of biomarker profiles at ca 1 2 and 10 14c ka due to decreased productivity in the lake or enhanced degradation of the biomarkers likely record short term climatic events

paleoproductivity in the southern peru chile current through the last 33 000 yr abstract based on a multiparameter approach including organic carbon biogenic opal carbonate and the species composition of planktic foraminifera this paper provides the first qualitative assessment of the history of paleoproductivity on glacial interglacial time scales in the southern peru chile current pcc off chile which belongs to the least studied parts of the world ocean during the last glacial maximum lgm highest relative paleoproductivity of the last 33 kyr indicated by high accumulation rates of organic carbon biogenic opal and carbonate has been found contrasted by lowest values during the early and middle holocene this shift from high to low productivity is accompanied by a major change in the species composition of planktic foraminifera from a dominance of neogloboquadrina pachyderma sin to the dominance of n pachyderma dex the temporal pattern of paleoproductivity off chile is very similar to the history of the continental paleoclimate in the region which is supposed to be driven by the position of the southern westerlies this observation points to a functional relationship between the position of the southern westerlies and the history of paleoproductivity off chile assuming that atmospheric and oceanographic circulation are closely linked a northward displacement of the antarctic circumpolar current during the lgm in line with the northward movement of the southern westerlies would bring the main nutrient source closer to our core sites resulting in increased productivity however based on the available data it is not clear if the higher productivity in the southern pcc during the lgm reflects generally higher productivity during this time or only a regional displacement of the main productivity centers

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

bound forms compositional features major sources and diagenesis of long chain alkyl mid chain diols in lake baikal sediments over the past 28 000 years abstract to elucidate the possibility of long chain alkyl mid chain diols hereafter mid chain diols in lake baikal sediments over the past 28 ka being paleoproxies we studied their i bound forms and compositional features ii major sources and iii diagenesis during sedimentation of the seven bound forms investigated i e free form free type three base hydrolysable types base 1 base 2 and base 3 and acid hydrolysable type acid 1 type in solvent extractable lipids and base hydrolysable nb and acid hydrolysable na types in non solvent extractable nonex lipids only three viz free nb and base 3 types were found the relative compositions of the three types were 70 100 0 30 and 0 8 respectively the most abundant component of the free type diols was the c 32 1 15 diol whereas that of the base 3 and nb types was the c 30 1 13 diol the average chain length ratio of total amount of even chain lengths to that of odd chain lengths the relative composition of positional isomers for each carbon number and the diol index obtained using the equation c 30 1 15 diol c 30 1 15 diol c 32 1 15 diol 100 hardly responded to reported major paleoenvironmental changes around the lake baikal region over the last 28 ka comparison of mid chain diol compositions and their bound forms with those in possible sources suggested that the major biological source of all mid chain diols in the sediments might be the eustigmatophyceae class of algae in the alga the occurrence of mid chain diols in the five bound forms free base 3 acid 1 nb and na types was suggested while the acid 1 and na types were not found in the sediments under the oxic depositional conditions of lake baikal most of the acid 1 and na type diols and part of the base 3 and nb type diols might be degraded and or released free type diols in the anoxic sediments of the lake the base 3 and nb diols seem to be resistant to diagenesis all investigated parameters for the free base 3 and nb diols in the sediments did not provide any information on paleoenvironments around the lake on the other hand among the fluxes of the three diol types the base 3 type diol flux seems to have responded to the climatic events around the lake especially over the last 15 ka hence this flux might be a potential paleoproxy for the productivity of the biological source s and associated environmental factors

post glacial history of siberian spruce picea obovata in the lake baikal area and the significance of this species as a paleo environmental indicator abstract the relative abundances of siberian spruce picea obovata and total arboreal pollen in pollen spectra from the lake baikal region and calculation of a steppe forest vegetation index place constraints on the past climatic conditions related to permafrost summer temperatures precipitation and soil moisture availability here we review the data on picea pollen in sub recent spectra the present day distribution of picea obovata and sedimentary records from a number of sites to arrive at a coherent picture of evolution of regional vegetation since the late glacial the expansion of siberian spruce along with steppe vegetation dates back to the undivided bolling allerod interval when this species became dominant in arboreal vegetation steppe landscapes persisted in a large part of the lake baikal watershed prior to 9 2 9 ka when the expansion of arboreal vegetation took place following the younger dryas which seems to have had little effect on regional vegetation and peat accumulation the role of siberian spruce steadily declined until 8 6 6 8 ka in the late holocene the abundance of p obovata rarely exceeded the present day ratio of ca 2 total arboreal pollen departures towards higher picea abundance and higher steppe forest index at ca 6 5 and 3 2 5 ka in selenga delta area appear to reflect periods with decreased temperatures and higher moisture availability in the southern part of the lake baikal watershed

continental climate response to orbital forcing from biogenic silica records in lake baikal changes in insolation caused by periodic changes in the earth s orbital parameters provide the primary forcing for global ice ages1 6 but it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets here we investigate this question using the record of biogenic silica in lake baikal as a proxy for climate change in this high latitude mid continental region we find a good correlation between this record and that of marine oxygen isotopes4 over the past 250 kyr the baikal record exhibits both a strongly nonlinear component manifested in a 100 kyr periodicity and weaker direct insolation components manifested in the 41 kyr obliquity and 23 and 19 kyr precession orbital cycles these results show that even though extreme continental climates such as this are influenced directly by insolation variations they are dominated by the nonlinear rhythm of the oceans and ice sheets

a 220ka terrestrial δ18o and deuterium excess biomarker record from an eolian permafrost paleosol sequence ne siberia the natural abundances of stable oxygen 18o 16o and hydrogen isotopes 2h 1h are valuable proxies of climate changes in the past yet to date no continuous 18o and only few 2h records are available from loess paleosol sequences taking advantage of a recently developed method based on compound specific 18o analyses of hemicellulose sugar biomarkers in soils zech and glaser 2009 rcm 23 3522 3532 we here present a first terrestrial 18o biomarker record from an eolian permafrost paleosol sequence in ne siberia that covers the last 220 ka the 18o values of the hemicellulose biomarkers arabinose and xylose range from 22 5 to 32 8 and from 21 3 to 31 9 respectively and reveal systematic glacial interglacial shifts the modern topsoil and the interglacial paleosols exhibit more positive 18o values whereas the glacial paleosols are characterized by more negative 18o values this is in agreement with the 2h record obtained for sedimentary n alkane leaf wax biomarkers we present a conceptual model for interpreting the combined 18o and 2h biomarker record based on this model we suggest that both our 18o and the 2h records primarily reflect the temperature controlled isotopic composition of paleoprecipitation modified by evaporative isotope enrichment of leaf water during transpiration considering fractionation factors during sugar and n alkane biomarker biosynthesis allows reconstructing the leaf water isotopic composition and the deuterium excess of the leaf water the deuterium excess may serve as proxy for evaporative enrichment and allows calculating relative humidity using a craig gordon model accordingly relative humidity in ne siberia was higher during marine isotope stage mis 6 compared to mis 2 4 and 5d and thus could help explaining the much larger extent of the late saalian glaciation compared to the weichselian glaciations using the craig gordon model we also calculated 18o of the plant source water 18osource water which can be assumed to primarily reflect 18o of paleoprecipitation our 220 ka 18osource water record is well in agreement with the 18odiatom record from crater lake el gygytgyn in ne siberia and enables a regional paleoclimate reconstruction and interpretation accordingly summer temperature was periodically warmer than at present during the weichselian glacial period and there is a strong july insolation forcing of the summer temperature in the extremely continental study area overall our study highlights the great potential of the novel hemicellulose biomarker 18o method for paleoclimate reconstructions especially when combined with 2h analyses of n alkane lipid biomarkers

lake baikal record of continental climate response to orbital insolation during the past 5 million years the sedimentary record of biogenic silica from lake baikal in south central siberia suggests that this region of central asia was impacted by two major cooling episodes at 2 8 to 2 6 and 1 8 to 1 6 million years ago the spectral evolution of this continental interior site parallels the evolutionary frequency spectra for various marine oxygen isotope records in the baikal record the 41 000 year obliquity cycle is particularly strong from 1 8 to 0 8 million years ago variance in the 100 000 year eccentricity band increases during the past 0 8 million years the expected precession frequency of 23 000 years is highest during the past 400 000 years the modulation of the predicted 23 000 and 41 000 year insolation forcing by the 100 000 and 400 000 year eccentricity bands indicates that the transfer of variance from the precession and obliquity frequencies to the eccentricity part of the spectrum occurred in the eurasian continental interior as well as in tropical and high latitude ocean sites

changes in the volume and salinity of lake khubsugul mongolia in response to global climate changes in the upper pleistocene and the holocene two gravity cores 1 1 and 2 2 m long of deep water bottom sediments from lake khubsugul mongolia were studied the holocene biogenic silica and organic matter rich part of the first core was subjected to ams radiocarbon dating which placed the date of dramatic increase of pelagic diatoms 40 cm below sediment surface at a calendar age of 11 5 cal ky bp icp ms analysis of weak nitric acid extracts revealed that the upper pleistocene compared to the holocene samples were enriched in ca cinorg sr mg and depleted of u w sb v and some other elements transition to the holocene resulted in an increase of total diatoms from 0 to 108 g 1 of bisi from 1 to 20 of organic matter from 6 the bolling allerod younger dryas holocene abrupt climate oscillations manifested themselves in oscillations of geochemical proxies a remarkable oscillation also occurred at 22 cm ca 5 5 ky bp the pleistocene section of the second longer core was enriched in carbonate co2 up to 10 and water extractable so42 up to 300 times greater than that in holocene pore waters all this evidence is in an accord with the earlier finding of drowned paleo deltas at ca 170 m below the modern lake surface of the lake dokl akad nauk 382 2002 261 and suggests that due to low ca 110 mm regional precipitation at the end of the pleistocene lake khubsugul was only 100 m deep and that its volume was ca 10 times less than today

quantitative reconstruction of the last interglacial vegetation and climate based on the pollen record from lake baikal russia changes in mean temperature of the coldest t c and warmest month t w annual precipitation p ann and moisture index were reconstructed from a continuous pollen record from lake baikal russia the pollen sequence con01 603 2 53 57 n 108 54 e was recovered from a 386 m water depth in the continent ridge and dated to ca 130 114 8 ky bp this time interval covers the complete last interglacial li corresponding to mis 5e results of pollen analysis and pollen based quantitative biome reconstruction show pronounced changes in the regional vegetation throughout the record shrubby tundra covered the area at the beginning of mis 5e ca 130 128 ky consistent with the end of the middle pleistocene glaciation the late glacial climate was characterised by low winter and summer temperatures t c 38 to 35 c and t w 11 13 c and low annual precipitation p ann 300 mm however the wide spread of tundra vegetation suggests rather moist environments associated with low temperatures and evaporation reconstructed 1 tundra was replaced by boreal conifer forest taiga by ca 128 ky bp suggesting a transition to the interglacial taiga dominant phase lasted until ca 117 4 ky bp e g about 10 ky the most favourable climate conditions occurred during the first half of the li p ann reached 500 mm soon after 128 ky bp however temperature changed more gradually maximum values of t c 20 c and t w 16 17 c are reconstructed from about 126 ky bp conditions became gradually colder after ca 121 ky bp t c dropped to 27 c and t w to 15 c by 119 5 ky bp the reconstructed increase in continentality was accompanied by a decrease in p ann to 400 420 mm however the climate was still humid enough 0 9 to support growth of boreal evergreen conifers a sharp turn towards a dry climate is reconstructed after ca 118 ky bp causing retreat of forest and spread of cool grass shrub communities cool steppe dominated the vegetation in the area between ca 117 5 ky and 114 8 ky bp suggesting the end of the interglacial and transition to the last glacial mis 5d shift to the new glaciation was characterised by cool and very dry conditions with t c 28 to 30 c t w 14 15 c p ann 250 mm and 0 5

holocene moisture evolution across the mongolian plateau and its surrounding areas a synthesis of climatic records abstract based on the review of 26 high standard holocene climatic reconstructions mainly pollen based from the mongolian plateau and its surrounding areas temporal and spatial patterns of the holocene moisture evolution are synthesized the regionally averaged moisture history from the summer monsoon influenced semiarid belt in china i e region a demonstrates that the moisture index curve is broadly in agreement with the synthesized east asian monsoon strength curve both following the general trend of the west tropical pacific sst that is in turn the delayed response to the northern hemispheric summer solar insolation the regionally averaged moisture indices from the winter monsoon dominated southern siberia including lake baikal area and the altai mountains i e region b exhibit a general declining trends since 10 6 9 6 cal kyr bp being largely consistent with the trends of the annual precipitation and the warm season temperature in the russian plain the consistency might be attributable to the holocene declining trend of the warm season temperature in north atlantic region the predominant feature of the regionally averaged moisture index from the westerlies affected northern xinjiang i e region c is a persistent increasing trend since 8 cal kyr bp the wetting trend of northern xinjiang during the past 8000 years might be attributable to the increasing trend of winter insolation and to the associated increasing trend of cold season temperature in northwestern europe the chronological correspondences between dry phases and warm intervals in the arid areas of the mongolian plateau i e northern mongolian plateau within mongolia and southern mongolian plateau within china region d lend a support to the proposal that the mid holocene dry phase was most likely the result of mid holocene high warm season temperature

lipid and lignin cutin compounds in lake baikal sediments over the last 37 kyr implications for glacial interglacial palaeoenvironmental change abstract a stratigraphic study of total organic carbon toc concentration carbon isotopic composition of total organic matter tom lipids n alkanes n alkanols n alkanoic acids and sterols and lignin cutin compounds has been conducted for a lake baikal core 323 pc1 over the last glacial interglacial toc concentrations mg g 1 dry sediment are low 2 4 in the glacial sections but high 10 29 in the interglacial ones the concentrations of land plant derived compounds such as n c14 to c28 hydroxyalkanoic acids lignin phenols p hydroxyl vanillyl and syringyl total cutin acids normalized to toc in the interglacial are 5 6 times higher than those in the glacial carbon isotopic compositions of tom are 24 0 9 in the glacial while they are 27 1 3 in the interglacial the heavy isotope composition of tom in the glacial might be due to a contribution from submerged plants and or grasses the light isotopic composition of tom for the interglacial may be due to a contribution from both land plant and phytoplankton mainly diatom derived om autochthonous organic carbon contents were estimated from the c n ratio corrected for the effect of inorganic nitrogen and based on the assumption of c n values of 8 for autochthonous and 25 for allochthonous om the estimation indicates that autochthonous oc comprises 74 6 of toc for the glacial and 72 9 for the interglacial respectively primary palaeoproductivity was estimated to be below 20 gc m 2 y 1 in the glacial before 18 ka but high 100 120 gc m 2 y 1 in the interglacial 11 8 ka which is 10 30 higher than the modern values

holocene millennial centennial scale multiproxy cyclicity in temperate eastern australian estuary sediments we have undertaken a comparative study of down core variation in multiproxy palaeoclimate data magnetic susceptibility calcium carbonate content and total organic carbon from two coastal water bodies myall and tuggerah lakes in temperate eastern australia to identify local regional and global forcing factors within holocene estuarine sediments the two lakes lie within the same temperate climate zone adjacent to the tasman sea but are not part of the same catchment and drain different geological provinces one is essentially a freshwater coastal lake whereas the other is a brackish back barrier lagoon despite these differences data from two sites in each of the two lakes have allowed us to investigate and compare cyclicity in otherwise uniform single facies sediments within the frequency range of 200 2000 years limited by the sedimentation rate within the lakes and our sample requirements we have auto and cross correlated strong periodicities at 360 years 500 530 years 270 290 years 420 450 years and 210 years and subordinate periods of 650 years 1200 1400 years and 1800 years our thesis is that climate is the only regionally available mechanism available to control common millennial and centennial scale cyclicity in these sediments given the geographical and other differences however regional climate may not be the dominant effect at any single time and either location within the range of frequency spectral peaks we have identified several fall within known long term periodical fluctuations of sun spot activity however feedback loops associated with short term orbital variation such as dansgaard oeschger cycles and the relationship between these and palaeo enso variation are also possible contributors copyright 2005 john wiley sons ltd

sedimentary environment changes of the ningshao plain since the later stage of the late pleistocene evidence from palynology and stable organic carbon isotopes abstract a 45 3 m long core was collected from near the hemudu civilization site luojiang county zhejiang province 29 59 064 n 121 21 754 e which is located in the ningshao plain on the southern margin of hangzhou bay the core named the hmd core was divided into seven lithological layers with clay and silt as the main lithological components the sporopollen assemblages of this core combined with toc tn and stable organic carbon isotope 13 c toc analysis revealed the sedimentary environmental changes since the later part of the late pleistocene in the ningshao plain the study area changed from terrestrial fluvial flood plain facies before marine isotope stage mis 3 to shore plain during transgression in mis3 and became a freshwater lake during regression at the time of the last glacial maximum lgm the area started to become a semi salt lake influenced by sea water during 15 900 5000 cal bp in which time period the lake was deeper and the climate became gradually warmer later the seawater withdrew the lake began desalinating and then became swampy and accumulated peat until 1000 cal bp after that the area became dry land the results also indicate that the sediments of the core are of endogenous organic origin and the 13 c toc value was controlled by change in the concentration of atmospheric co 2 and thus was a response to global changes

a re evaluation and spatial analysis of evidence for a younger dryas climatic reversal in beringia abstract an objective classification of paleoclimatic proxies from 75 lake and peat records is used to re evaluate data quality and inferred climatic patterns in beringia during the younger dryas mapped data reveal coherent but spatially complex regional patterns suggesting the younger dryas was characterized by 1 cooling in southern alaska eastern siberia and portions of northeastern siberia and 2 uniform to warmer than present conditions through most of central alaska northeastern siberia and possibly the russian far east and northern alaska the beringian patterns correspond to distinctive large scale climatic forcings although in some locations further modified by more local influences such as topography general circulation models and modern synoptic climatology provide a conceptual framework for exploring possible mechanisms responsible for the observed changes forcings and associated climatic responses consistent with the proxy data include 1 lowered sea surface temperatures in the north atlantic and pacific reducing temperature and precipitation in eastern siberia 2 intensified aleutian low and lowered sea surface temperatures causing cooler summers and higher winter precipitation in southern alaska 3 stronger pacific subtropical high and an eastward shift of the east asian trough reducing summer temperatures in southern alaska and causing relative warmth in northeastern siberia and 4 strong high pressure system producing warm dry conditions in interior alaska

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