characterization of the last deglacial transition in tropical east africa insights from lake albert abstract new biomarker analyses from lake albert east africa spanning 15 9 ka show the most extreme abrupt multi stage climate and environmental shifts during the last deglacial transition of anywhere in africa records of hydroclimate expressed in compound specific d values from terrestrial leaf waxes and a tex86 paleotemperature record support multiple stages of pronounced drying and cooling from 13 8 to 11 5 ka and demonstrate the dynamic behavior of the low latitude tropics during the deglaciation the vegetation response illustrated by compound specific 13c values and fossil pollen records was an expansion of c4 grassland when the region was cool and arid these results advance our understanding of a spatially and temporally complex regional response to global climate forcing suggesting weakening of the indian ocean monsoon at the end of the pleistocene that coincides with a minor decrease in the rate of the atlantic meridional overturning circulation amoc and during a time of stepwise cooling in the northern high latitudes

late quaternary relative humidity changes from mt kilimanjaro based on a coupled 2 h 18 o biomarker paleohygrometer approach abstract our understanding of african paleoclimate hydrological history is mainly based on lake level and lake sediment studies it improved during the last decade thanks to emerging stable isotope techniques such as compound specific deuterium analysis of sedimentary leaf wax biomarkers 2hleaf wax here we present the results from a multi proxy biomarker study carried out on a 100 ka loess like paleosol sequence preserved in the maundi crater at 2780 m a s l on the southeastern slopes of mt kilimanjaro in equatorial east africa the maundi stable isotope records established for hemicellulose derived sugars lignin and pectin derived methoxyl groups leaf wax derived fatty acid and n alkane biomarkers 18osugars 2hmethoxyl groups 2hfatty acids and 2hn alkanes respectively reveal similar patterns but also some distinct differences are obvious the periods from 70 to 60 ka the last glacial maximum lgm and the younger dryas yd are characterized by more positive values whereas during the holocene and around 30 39 and 56 ka bp more negative values are determined the application of a coupled 2hn alkane 18osugar paleohygrometer approach allows us to derive information about late quaternary changes of air relative humidity at the maundi study site reconstructed changes of mean day time relative humidity rhd are in good agreement with pollen results from the study area apart from the overall regional moisture availability the intensification versus weakening of the trade wind inversion which affects the diurnal montane atmospheric circulation on the slopes of mt kilimanjaro is suggested as a local factor which may contribute to the observed variability of rhd at maundi study site the combined usage of 2hn alkanes and 18osugars allowed us to reconstruct 2h 18o of source water utilized by plants in the study area which is directly linked to local precipitation the results of this reconstruction caution against a straightforward interpretation of 2hleaf wax and 18osugars records as proxies for isotopic composition of local precipitation because variable and primarily rh dependent isotopic evaporative enrichment of leaf water can mask changes of 2hprec 18oprec in the past the biomarker based 2h 18osource water records derived for the maundi site revealed a discernible link with the reconstructed rhd record lower rhd values were generally observed during periods characterized by more negative 2h 18osource water values indicating a reverse relationship with the expected precipitation amount this indicates that the empirical relationship between amount of rainfall and its isotopic composition observed nowadays on monthly timescale in the east african region might not be valid on millennial time scale

significance of weathering himalayan metasedimentary rocks and leucogranites for the sr isotope evolution of seawater during the early miocene changes in 87 sr 86 sr ratios of seawater may reflect varying rates of chemical weathering or variations in the isotopic compositions of continental rocks undergoing weathering the unusually high dissolved sr concentrations and 87 sr 86 sr ratios that characterize the ganges and brahmaputra tsangpo river systems are a consequence of weathering early proterozoic age metasedimentary rocks and anatectic miocene granites exposed on the southern slopes of the himalayas on average the metasedimentary rocks are characterized by abundances of 87 sr about twice those of the derived granites the rapid rise in 87 sr 86 sr ratios of seawater during the early miocene was virtually coincident with metamorphism and anatexis of himalayan metasedimentary rocks model calculations indicate that 50 75 of the overall change in 87 sr 86 sr ratios for seawater between 20 and 15 ma could have resulted from exposure of himalayan metasedimentary rocks to chemical weathering rapid tectonically controlled exhumation of the metasedimentary rocks is required to expose these lithologies by 20 18 ma to generate the increase in the 87 sr flux of the trans himalayan rivers during the early miocene

on the origin of continental precipitation 1 about 9 out of 10 liters of water evaporated from the oceans every year precipitates back onto oceans however the remaining 10 that get transported to continents play an irreplaceable role feeding the land branch of the hydrological cycle here we use an objective 3 d lagrangian model flexpart to detect major oceanic moisture source areas and the associated continental regions significantly influenced by each moisture source our results reveal a highly asymmetrical supply of oceanic moisture to the continents with the northern atlantic subtropical ocean source impacting the continents considerably more than the large southern indian and north pacific sources also the small mediterranean sea and red sea basins are important moisture sources for relatively large land areas the indian subcontinent receives moisture from six different major oceanic source regions future changes in meteorological conditions over the oceanic moisture source regions may have an impact on water availability for many river basins

climate and co2 effects on the vegetation of southern tropical africa over the last 37 000 years the savanna vegetation of southern tropical africa is characterized by co dominance of c 4 grasslands and c 3 woodlands long term variations in the tropical savanna vegetation in arid and semi arid climates are commonly considered to be primarily sensitive to precipitation and atmospheric co2 concentrations the sensitivity of tropical vegetation to temperature however is often considered as secondary or negligible particularly in paleostudies due to difficulties of reconstructing terrestrial temperature in the tropics in this study we use the terrestrial vegetation model biome4 which was forced by climate simulations from the kiel climate model kcm for the holocene and by climate reconstructions for the most recent glacial period to understand reconstructed vegetation changes in southern tropical africa of the past 37 000 yr we focus on these two periods because vegetation reconstructions from a marine sediment core near the zambezi river mouth cannot be explained by precipitation changes and changes of atmospheric co2 alone for the holocene we force biome4 simulations with reconstructed atmospheric co2 concentrations and spatial and seasonal climate patterns from the early and mid holocene 9 5 and 6 ka bp simulations with the kcm for the glacial period we analyze idealized experiments based upon reconstructed temperature precipitation and co2 at 31 28 and 21 ka bp our study shows that both holocene and glacial simulations of vegetation cover exhibit good agreement with reconstructed c 4 c 3 ratios when temperature changes are taken into account while both precipitation and temperature control the c 4 c 3 ratio during the holocene atmospheric co2 and temperature variations are major factors controlling vegetation changes during the glacial period in our simulations variations in temperature along with precipitation and atmospheric co2 reconcile the evolution of vegetation observed in the zambezi catchment during the last 37 000 yr in consequence the effect of temperature variations on tropical savanna vegetation should be taken into account with respect to modeling past or future climates c 2014 elsevier b v all rights reserved

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

pollen corrected leaf wax d h reconstructions of northeast african hydrological changes during the late miocene abstract plant leaf wax hydrogen isotope dwax reconstructions are increasingly being used to reconstruct hydrological change this approach is based upon the assumption that variations in hydroclimatic variables and in particular the isotopic composition of precipitation dp dominate dwax however modern calibration studies suggest that offsets between plant types may bias the dwax hydrological proxy at times of vegetation change in this study i pair leaf wax analyses with published pollen data to quantify this effect and construct the first vegetation corrected hydrogen isotopic evidence for precipitation dcorrp in marine sediments from deep sea drilling program site 231 in the gulf of aden spanning 11 4 3 8 ma late miocene and earliest pliocene i find 77 swings in dwax that correspond to pollen evidence for substantial vegetation change similarities between dp and dcorrp imply that the hydrological tracer is qualitatively robust to vegetation change however computed vegetation corrections can be as large as 31 indicating substantial quantitative uncertainty in the raw hydrological proxy the resulting dcorrp values quantify hydrological change and allow us to identify times considerably wetter than modern at 11 09 7 26 5 71 and 3 89 ma more generally this novel interpretative framework builds the foundations of improved quantitative paleohydrological reconstructions with the dwax proxy in contexts where vegetation change may bias the plant based proxy the vegetation corrected paleoprecipitation reconstruction dcorrp represents the best available estimate as proof of concept for an approach that i hope will be refined and more broadly applied

wet phases in tropical southern africa during the last glacial period 1 a 45 000 year high resolution sedimentary record from lake masoko in tanzania shows the climate in this part of east africa to have been characterized by a short and less severe dry season during the last glacial maximum moisture and lake level proxies pollen assemblages and magnetic susceptibility indicate that rainfall in the masoko area was strongly controlled by low latitude insolation i e precessional forcing such observations contrast with the climatic patterns previously reconstructed in east africa for the last glacial indeed widespread dry conditions are generally observed and are attributed to the predominant control of high latitude glacial forcing over insolation forcing on the tropical hydrology however during the younger dryas cold event out of phase with regional insolation behaviour wetter conditions prevailed in this area suggesting that the rainfall belt over africa was probably shifted to the south bringing moisture to the southernmost tropics

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

heterogeneity and the earthquake magnitude frequency distribution regional earthquake populations are widely observed to have power law magnitude frequency statistics whether earthquake distributions on individual faults are similarly power law is presently the subject of considerable research effort recent careful observations suggest that event statistics on rough faults are power law whereas earthquake distributions on smooth faults are better described by a characteristic model in which there are more large earthquakes than would be expected by extrapolation from the distribution of small events here the relation between heterogeneity and the magnitude frequency distribution is investigated in a cellular automaton that includes new nearest neighbor stress transfer rules which produce realistic stress concentrations the results agree with the suggestion that smooth faults have more characteristic earthquake distributions and support the view that an earthquake s size is controlled by rupture arrest

correlation of seismotectonic variables and gps strain measurements in western turkey period extending from 1981 to 1998 are examined in the eastern mediterranean and western turkey area this analysis was conducted in three tectonic subdivisions corresponding to regions dominated by shear extensional and compressional deformation the region of shear coincides with the northern anatolian fault zone nafz the region of extension with the aegean back arc region and the region of compression to the aegean subduction zone between the african and anatolian plates the median b value in the region of extension is larger than those observed in the regions of shear and compression and suggests that large magnitude earthquakes are less probable in this region than along the nafz and aegean subduction zone a significant correlation is observed between d2 and d15 with dilatation over the 10 40 km range in the back arc region of extension a marginal correlation between gps derived strain and b value is observed only in the region of compression where it correlates negatively with shear and positively with dilatation a significant positive correlation is observed between d and b along the nafz a clear relationship between d and b does not exist in regions deformed by extension and compression the relationship between d and b along the nafz may also be an indicator of increased seismic hazard since the positive relationship between d and b is a recent characteristic of the nafz observed only in the 20 year period preceding the izmit earthquake index terms 7215 seismology earthquake parameters 7209 seismology earthquake dynamics and mechanics 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics keywords gps strain fractal dimension b value

evaluation of earthquake potential along the northern anatolian fault zone in the marmara sea using comparisons of gps strain and seismotectonic parameters abstract seismotectonic parameters including the gutenberg richter b value and multifractal dimensions d2 and d15 of seismicity patterns both spatial and temporal were compared to gps derived maximum shear and dilatation strains measured in the marmara sea region of western turkey along the northern anatolian fault zone nafz comparisons of seismotectonic parameters and gps derived maximum shear and dilatation strain along the nafz in the vicinity of the 1999 m7 4 izmit earthquake reveal a positive correlation r 0 5 p 0 05 between average dilatation and the gutenberg richter b value significant negative correlation r 0 56 p 0 03 and r 0 56 p 0 02 was also observed between the spatial fractal dimension d2 and gps derived maximum geodetic and shear strain this relationship suggests that as maximum geodetic and shear strains increase seismicity becomes increasingly clustered anomalous interrelationships are observed in the marmara sea region prior to the izmit event along a bend in the nafz near the eastern end of the marmara sea known as the northern boundary fault nbf an asperity is located near the northwest end of the nbf along the 50 km length of the nbf gps strains become slightly compressive the correlation between b value and gps derived dilatation suggests that regions in compression have increased probability of larger magnitude rupture the nbf appears to serve as an impediment to the transfer of strain from east to west along the nafz recurrence times for large earthquakes along the nbf are larger than in surrounding areas temporal clustering of seismicity in the vicinity of the nbf may represent foreshocks of an impending rupture

glacial aridity in central indonesia coeval with intensified monsoon circulation abstract the last glacial maximum was cool and dry over the indo pacific warm pool ipwp a key region driving global oceanic atmospheric circulation both low and high latitude teleconnections with insolation ice sheets and sea level have been suggested to explain the pervasive aridity observed in paleoecological and geomorphic data however proxies tracking the h and o isotopic composition of rainfall e g speleothems sedimentary biomarkers suggest muted aridity or even wetter conditions than the present complicating interpretations of glacial ipwp climate here we use multiproxy reconstructions from lake sediments and modern rainfall isotopic measurements from central indonesia to show that contrary to the classical amount effect intensified australian indonesian monsoon circulation drove lighter h and o isotopic composition of ipwp rainfall during the lgm while at the same time dry conditions prevailed precipitation isotopes are particularly sensitive to the apparent increase in monsoon circulation and perhaps also decreased moisture residence time implied by our data explaining contrasts among proxy records while illuminating glacial ipwp atmospheric circulation a key target for climate models

late quaternary climate change in the north eastern highlands of ethiopia a high resolution 15 600 year diatom and pigment record from lake hayk abstract multi proxy analyses of an 8 m sediment core from lake hayk a closed freshwater lake in the north central highlands of ethiopia provide a record of changing lake level and inferred regional climatic change for the last 15 6 cal ka years between ca 15 6 15 2 cal ka bp a lowstand was synchronous with heinrich event 1 and an intense drought across eastern africa at ca 15 2 cal ka bp a lake began to develop at the core site in response to wetter conditions at the onset of the african humid period ahp however in contrast to other lakes in eastern africa hayk lake level fell around ca 14 8 cal ka bp indicating a climate shift towards aridity the lake began filling again at ca 12 3 cal ka bp and reached maximum water depth between ca 12 0 10 0 cal ka bp lake level declined slowly during the holocene culminating in the termination of the ahp at hayk between ca 5 2 4 6 cal ka bp in the late holocene ca 2 2 1 3 cal ka bp lake hayk was again deep and fresh with some evidence of short term lake level variability the palaeo record from lake hayk indicates that while it experienced to a broad degree the same glacial interglacial dynamics and sub millennial shifts in climate found in other palaeolimnological records from eastern africa there are offsets in timing and rate of response these differences reflect chronological discrepancies between records as well as the varying climate sensitivities and site specific factors of individual lake basins this record highlights the different responses by lakes in a climatically vulnerable area of ethiopia

the b value as an earthquake precursor a systematic study of b values in new zealand has shown that within the vicinity of forthcoming large earthquakes there is initially an increase in b and then a return to normal the caracas and san fernando earthquakes show the same phenomenon which is related to that of seismic gaps

benthic foraminiferal response to late glacial and holocene sea level rise and rainfall variability off east africa analogous to west and north africa east africa experienced more humid conditions between approximately 12 and 5 kyr bp relative to today while timing and extension of wet phases in the north and west are well constrained this is not the case for the east african humid period here we present a record of benthic foraminiferal assemblages and sediment elemental compositions of a sediment core from the east african continental slope in order to provide insight into the regional shallow indian ocean paleoceanography and east african climate history of the last 40 kyr during glacial times the dominance of a benthic foraminiferal assemblage characterized by bulimina aculeata suggests enhanced surface productivity and sustained flux of organic carbon to the sea floor during heinrich stadial 1 h1 the nuttallides rugosus assemblage indicates oligotrophic bottom water conditions and therefore implies a stronger flow of southern sourced aaiw to the study site during the east african humid period the saidovina karreriana assemblage in combination with sedimentary c n and fe ca ratios suggest higher river runoff to the indian ocean and hence more humid conditions in east africa between 8 5 and 8 1 kyr contemporaneous to the globally documented 8 2 kyr event a severe reduction in river deposits implies more arid conditions on the continent comparison of our marine data with terrestrial studies suggests that additional moisture from the atlantic ocean delivered by an eastward migration of the congo air boundary during that time period could have contributed to east african rainfall since approximately 9 kyr the gaining influence of the millettiana millettii assemblage indicates a redevelopment of the east african fringe reefs

isotopic reconstruction of the african humid period and congo air boundary migration at lake tana ethiopia abstract the african humid period of the early to mid holocene 12 000 5000 years ago had dramatic ecological and societal consequences including the expansion of vegetation and civilization into the green sahara while the humid period itself is well documented throughout northern and equatorial africa mechanisms behind observed regional variability in the timing and magnitude of the humid period remain disputed this paper presents a new hydrogen isotope record from leaf waxes d wax in a 15 000 year sediment core from lake tana ethiopia 12 n 37 e to provide insight into the timing duration and intensity of the african humid period over northeastern africa d wax at lake tana ranges between 80 and 170 with an abrupt transition from d enriched to d depleted waxes between 13 000 11 500 years before present 13 11 5 ka a similarly abrupt transition from d depleted to d enriched waxes occurs ca 8 5 8 ka and is followed by generally d enriched waxes throughout the late holocene trends in d wax covary with changes in northern hemisphere summer insolation and reflect increased precipitation at lake tana during the ahp however the transition from d depleted to d enriched waxes occurs earlier at lake tana ca 8 ka vs 5 ka than in many other regional records and the amplitude of d depletion during the ahp is larger at lake tana as well we attribute this early enrichment to a reduction of moisture derived from westerly sources the congo basin and atlantic ocean which we suggest are d depleted relative to moisture sourced from the east indian ocean and the north red sea and mediterranean sea our new record highlights the importance of both the northward migration of the tropical rain belt as well as east west migration of the congo air boundary to precipitation source and amount during the african humid period

pre aged plant waxes in tropical lake sediments and their influence on the chronology of molecular paleoclimate proxy records sedimentary records of plant wax hydrogen ddwax and carbon d 13 cwax stable isotopes are increasingly applied to infer past climate change compound specific radiocarbon analyses however indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments the influence of these time lags on interpretations of plant wax stable isotope records is presently unconstrained and it is unclear whether such time lags also affect lacustrine sediments we present compound specific radiocarbon 14 cwax data for n alkanoic acid plant waxes n c26 to n c32 from 1 a sediment core from lake chichancanab yucatan peninsula mexico 2 soils in the lake chichancanab catchment and 3 surface sediments from three other lakes in southeastern mexico and northern guatemala 14 cwax ages in the surface sediments are consistently older than modern and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area 14 cwax ages in soils surrounding lake chichancanab increase with soil depth consistent with deep subsoil horizons being the primary source of lacustrine aged plant waxes which are likely delivered to lake sediments through subsurface transport plant waxes in the lake chichancanab core are 350 1200 years older than corresponding ages of bulk sediment deposition determined by 14 c dates on terrestrial plant macrofossils in the core a ddwax time series is in closer agreement with other regional proxy hydroclimate records when a plant wax 14 c age model is applied as opposed to the macrofossil based core chronology inverse modeling of plant wax age distribution parameters suggests that plant waxes in the lake chichancanab sediment core derive predominantly from millennial age soil carbon pools that exhibit relatively little age variance 200 years our findings demonstrate that high temporal resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils they also underscore the importance of direct radiocarbon dating of these organic molecules

temporal variations in the fractal properties of seismicity in the north anatolian fault zone between 31 e and 41 e we investigate the nature of temporal variations in the statistical properties of seismicity associated with the north anatolian fault zone between longitudes 31 41 e during the instrumental period 1900 1992 temporal variations in the seismicb value and the fractal correlation dimensiondc of earthquake epicenters are examined for earthquakes of magnitudems 4 5 using sliding windows of 100 consecutive events b varies temporally between 0 6 and 1 0 anddc between 0 6 and 1 4 both representing significant fluctuations above the errors in measurement technique a strong negative correlation r 0 85 is observed betweenb anddc consistent with previous observation of seismicity in japan and southern california major events early in this century ms 7 are associated with lowb and highdc respectively consistent with greater stress intensity and greater spatial clustering of epicenters both implying a greater degree of stress concentration at this time

aftershock activity of the 1999 izmit turkey earthquake revealed from microearthquake observations an accurate aftershock distribution of the 1999 izmit turkey earthquake was obtained by using the data from a local seismic network izinet and 10 temporary seismic stations more than 2000 aftershocks were relocated for the period of about 2 months following the mainshock from this aftershock distribution we obtained several pieces of information on the characteristics of the mainshock first the mainshock initiated fault rupture from a place adjacent to an active swarm area where many microearthquakes had been occurring for more than 20 yr prior to the mainshock second the aftershock region extended in the east west direction along the north anatolian fault zone nafz this confirms that the mainshock was caused by a slip on the nafz third the western end of the rupture caused by the mainshock is likely to have reached up to about 29 2 e in the izmit bay and hence the total length of the fault rupture caused by the mainshock amounts to about 150 km as long as the estimate of the fault rupture length is based on the aftershock distribution this information is important for the discussion on the possibility of future large earthquakes in the west of the source region of the izmit earthquake we also found a clear tendency that aftershocks occur in clusters which implies strong heterogeneity in both the rupture process and the medium along the fault zone manuscript received 7 october 2000

d h variation in terrestrial lipids from santa barbara basin over the past 1400 years a preliminary assessment of paleoclimatic relevance we analyzed d h ratios of common terrestrial leaf wax lipids in a 1400 year sediment core from the santa barbara basin sbb to test whether they accurately record terrestrial climate in southern california the d h ratios of long chain n alkanes vary substantially with depth but are poorly correlated with other terrestrial climate proxies interference from fossil hydrocarbons may be at least partly responsible long chain n alkanoic acids exhibit nearly constant downcore d h ratio values this constancy in the face of known climatic shifts presumably reflects a substantial residence time for leaf wax compounds in terrestrial soil and or on the basin flanks alternatively the isotopic composition of meteoric waters in southern california may not covary with climate particularly aridity however the d values of n c22 and n c24 fatty acids commonly attributed to terrestrial aquatic sources are partially correlated with southern california winter palmer drought severity index a tree ring based climatic proxy r 2 0 25 p 0 01 on multi centennial scales with an inferred ca 215 year time lag the improved correlation of these biomarkers can be explained by the fact that they are not stored in terrestrial soil nor are subject to interference from fossil hydrocarbons our study indicates that the sbb is unlikely to preserve high resolution leaf wax d h records that can serve as quantitative paleoclimate proxies though some qualitative information may be retained more generally the sources of lipids in marginal marine basins need to be carefully evaluated prior to attempting paleoclimate reconstruction based on the leaf wax d h proxy

hydrological changes of the past 1400 years recorded in δd of sedimentary n alkanes from poyang lake southeastern china hydrogen isotopic compositions d of plant leaf wax have been increasingly used for paleohydrological reconstructions over various geological timescales here we analyzed downcore variations of

anomalous seismicity preceding the 1999 izmit event nw turkey summary the 1999 izmit earthquake 1999 august 17 mw 7 4 was one of the largest earthquakes to occur in northwestern turkey during the past 100 yr this earthquake occurred along the izmit sapanca fault within the northern anatolian fault zone variations in the generalized fractal dimensions dq of clustering in time and space the gutenberg richter b value and earthquake frequency n are evaluated in detail over an 8 5 yr time period preceding the izmit event spatial and temporal comparisons of the variations in these parameters reveal anomalous intermediate term behaviour over shorter timescales than previously observed significant correlation is observed between changes in b value with the spatial and temporal fractal dimension of epicentre distribution these correlations oscillate back and forth over 2 3 yr time intervals and suggest occurrence of significant instability in the nature of intermediate term deformation along the fault zone seismotectonic behaviour immediately preceding the izmit event is represented by a significant rise in b value to a maximum of 2 26 accompanied by relatively small increases in d the rapid rise of b is associated with an increased frequency of low magnitude seismicity although d spatial and temporal increase slightly in the 2 3 yr period preceding rupture they remain less than 1 indicating that seismicity remains clustered prior to rupture in areas where earthquake frequency or station density do not permit resolution of short term variations of fractal scaling parameters within a few months of main rupture an accelerated period of low magnitude seismicity high b value concentrated along a fault zone d 1 may suggest heightened probability of a forthcoming large event

damage mechanisms in stressed rock from acoustic emission to better understand the phenomena leading to the failure of rock unconfined compression experiments on charcoal granite specimens were performed with the monitoring of acoustic emission ae localization in the form of spalling near the free surface was detected by the concentration of hypocenters the ae locations which look random in space and time before localization actually were clustered and have fractal structure in either space or time after localization the fractal dimensions reduced significantly in a certain range of distance and time the analysis of interaction between spatial and temporal clustering revealed the size of clusters in both space and time the cluster sizes may be related to the intrinsic properties of the rock the seismic moment tensor was evaluated through a deconvolution technique to obtain the source mechanism and orientation of each ae event the dominant mode of failure from ae was sliding on inclined planes although a significant number of source planes were parallel to the loading axis while the growth of cracks perpendicular to the loading axis was inhibited this preferential growth of microcracks is related to a tensorial measure of damage and is used to study stress induced anisotropy

wet and arid phases in the southeast african tropics since the last glacial maximum plant leaf wax carbon isotopes provide a record of c 3 versus c 4 vegetation a sensitive indicator of aridity from the southeast african tropics since the last glacial maximum wet and arid phases in southeast africa were in phase with conditions in the global tropics from 23 to 11 ka but at the start of the holocene these relationships ended and an antiphase relationship prevailed the abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the intertropical convergence zone itcz during the last glacial southward displacements of the itcz are also linked to arid conditions in southeast africa during the younger dryas and the little ice age

a comparison of biomarker records of northeast african vegetation from lacustrine and marine sediments ca 3 40 ma integrated terrestrial and marine records of northeast african vegetation are needed to provide long high resolution records of environmental variability with established links to specific terrestrial environments in this study we compare records of terrestrial vegetation preserved in marine sediments in the gulf of aden deep sea drilling project dsdp site 231 and an outcrop of lacustrine sediments in the turkana basin kenya part of the east african rift system we analyzed higher plant biomarkers in sediments from both deposits of known equivalent age corresponding to a ca 50 100 ka humid interval prior to the b tulu bor eruption ca 3 40 ma when the lokochot lake occupied part of the turkana basin molecular abundance distributions indicate that long chain n alkanoic acids in marine sediments are the most reliable proxy for terrestrial vegetation carbon preference index cpi 4 5 with more cautious interpretation needed for n alkanes and lacustrine archives marine sediments record carbon isotopic variability in terrestrial biomarkers of 2 3 roughly equivalent to 20 variability in the c3 c4 vegetation contribution the proportion of c4 vegetation apparently increased at times of low terrigenous dust input terrestrial sediments reveal much larger 2 10 shifts in n alkanoic acid d 13 c values however molecular abundance and isotopic composition suggest that microbial sources may also contribute fatty acids contaminating the lacustrine sedimentary record of terrestrial vegetation 2007 elsevier ltd all rights reserved

oxygen isotope analyses of ungulate tooth enamel confirm low seasonality of rainfall contributed to the african humid period in somalia abstract during the african humid period ahp between 14 and 5 ka north and eastern africa were much wetter and greener than they are today although the ahp has long been attributed to an increase in rainfall driven by orbital forcing many details regarding the timing pacing and contributing moisture sources remain to be determined especially for eastern africa recent research suggests that both atlantic and indian ocean moisture contributed to the ahp in eastern africa large mammalian faunas from two late pleistocene holocene rockshelter sites in southern somalia gogoshiis qabe and guli waabayo provide an unusual opportunity to investigate the ahp in an area of eastern africa that is orographically isolated from atlantic ocean moisture to track changes in aridity at these sites we used the oxygen isotope aridity index which exploits the difference in tooth enamel oxygen isotope 18o values between evaporation insensitive obligate drinkers and evaporation sensitive non obligate drinkers to calculate a water deficit value water deficit values calculated from dik dik madoqua spp non obligate drinkers and warthog phacochoerus spp obligate drinkers tooth enamel 18o values at gogoshiis qabe and guli waabayo are in agreement and progressively increase toward the present additionally two of three direct dated serially sampled warthog teeth id 4032 and 4033 from guli waabayo demonstrate low seasonality of rainfall during much of the ahp range in 18o 1 8 one tooth id 4035 with high amplitude variability in 18o values 3 1 dates to 8470 66 cal yr b p a period identified as a lowstand in several lake level records our results suggest that regions isolated from atlantic ocean moisture likely experienced a less pronounced ahp than those receiving moisture from multiple sources but indicate less seasonal variability than present our findings also support the presence of climate variability within the ahp

paleoevaporation and paleoprecipitation in the tanganyika basin at 18 000 years b p inferred from hydrologic and vegetation proxies abstract paleo hydrologic and vegetation proxy data from the tanganyika basin are integrated in energy and water balance equations to infer past evaporation and precipitation during the last glacial maximum lgm our approach is first validated on the modern system large variations are assigned to input variables to simulate the interannual precipitation variability equations are then applied to the lgm we first change those input parameters inferred from proxies basin and lake surfaces temperature and land albedo our lgm simulation suggests in percent of modern mean values decreases in evaporation from the lake el 5 between 13 and 3 and land ec 8 19 5 bodies in precipitation p 11 21 0 and p ec 42 44 40 decreases in p and e are amplified el 8 16 0 ec 14 24 2 p 17 26 6 when including empirical changes in atmospheric transmission coefficient and bowen ratio sensitivity runs suggest that even large changes in cloud cover and air humidity should not modify these trends the results suggest that the earth s glacial interglacial boundary conditions play a significant role on climate of subequatorial southern africa

tropical rainforest dynamics and palaeoclimate implications since the late pleistocene nilgiris india a multiproxy study involving sedimentology palynology radiocarbon dating stable isotopes and geochemistry was carried out on the parsons valley lake deposit nilgiris india to determine palaeoclimatic fluctuations and their possible impact on vegetation since the late pleistocene the 72 cm deep sediment core that was retrieved reveals five distinct palaeoclimatic phases 1 warm and humid conditions with a high lake stand before the last glacial maximum lgm 29 800 cal yr bp subsequently changing to a relatively cool and dry phase during the lgm 2 considerable dry conditions and lower precipitation occurred between 16 300 and 9500 cal yr bp during this period the vegetation shrank and perhaps was confined to moister pockets or was a riparian forest cover 3 an outbreak in the shift of monsoonal precipitation was witnessed in the beginning of the mid holocene around 8400 cal yr bp implying alteration in the shift toward warm and humid conditions resulting in relatively high pollen abundance for evergreen taxa 4 this phase exhibits a shift to heavier 13 c values around 1850 cal yr bp with an emergence of moist deciduous plants pointing to drier conditions 5 human activities contributed to the exceedingly high percentage of acacia and pinus pollen during the little ice age

evidence for precursory changes in the frequency magnitude b value summary a clear case has been established for a significant increase in the slope of the frequency magnitude relation for periods of up to six years before a number of earthquakes in new zealand and california in a region of about 200 km diameter in central new zealand the b value increased from near unity to more than 2 0 at the end of 1968 and remained abnormally high until the beginning of 1975 an earthquake of magnitude 6 0 followed in 1977 january within 50 km of the location which had registered the highest b value the anomaly was detected in contour maps of b values windowed in space and time then resolved more precisely in time using the cusum technique it was statistically significant at a level higher than 99 99 per cent compared with the average rate of moment release in this region there was a deficit of 1 3 x 10 nm during the anomalous period which is equivalent to an earthquake of magnitude 6 08 this phenomenon of a high b value has been found before all shallow new zealand earthquakes of magnitude 6 0 or more since 1965 wherever the seismicity rate and detection threshold were adequate to resolve it further the only statistically significant occurrences of high b value were those which were followed by large earthquakes two of magnitude 6 8 and 6 0 and a swarm of equivalent magnitude 5 3 the phenomenon has also been found before the san fernando and coalinga earthquakes in california both of which were thrust events but it is not apparent before two large californian earthquakes with strike slip mechanisms observations of high b value are consistent with reported instances of quiescence in background seismicity in fact what has been interpreted as quiescence may probably be more accurately described as a localized high b value the duration of the anomaly is not a simple function of magnitude different strain rates in the regions examined may account for this the available data suggest that careful monitoring of the b value may provide a tool for medium term earthquake prediction although perhaps not in all tectonic environments

northern hemisphere controls on tropical southeast african climate during the past 60 000 years the processes that control climate in the tropics are poorly understood we applied compound specific hydrogen isotopes d and the tex86 tetraether index of 86 carbon atoms temperature proxy to sediment cores from lake tanganyika to independently reconstruct precipitation and temperature variations during the past 60 000 years tanganyika temperatures follow northern hemisphere insolation and indicate that warming in tropical southeast africa during the last glacial termination began to increase 3000 years before atmospheric carbon dioxide concentrations d data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial scale events recorded in northern hemisphere monsoonal climate records this implies that precipitation in tropical southeast africa is more strongly controlled by changes in indian ocean sea surface temperatures and the winter indian monsoon than by migration of the intertropical convergence zone

stable isotope proxy records in tropical terrestrial environments abstract the tropics refers to an area that is loosely equatorial characterized by relatively high temperatures at low altitude comparatively low seasonal contrast in temperatures and generally high or highly seasonal rainfall the stable isotopes of carbon oxygen and hydrogen are increasingly important tools of relevance to understanding trajectories and mechanisms of past environmental change in the tropics in the case of carbon this is because at the broadest level the carbon isotope composition of organic and carbon containing inorganic proxy materials formed in the tropics can be read as being derived from the balance between c3 tree and c4 grass photosynthesis hence providing a relative measure of vegetation type and indirectly climate likewise changes in the oxygen and hydrogen isotope composition of many proxy materials again at the broadest level can be read as indicative of changes in hydrological conditions and or altitude in the past this review provides an entry point into the literature relating to the use of stable isotopes as terrestrial environmental proxies for past environmental change with a focus on the measurement and interpretation of stable isotope data from the wide range of proxy materials preserved in tropical environments

long term probabilistic forecasting of earthquakes we estimate long term worldwide earthquake probabilities by extrapolating catalogs of seismic moment solutions we base the forecast on correlations of seismic moment tensor solutions the forecast is expressed as a map showing predicted rate densities for earthquake occurrence and for focal mechanism orientation focal mechanisms are used first to smooth seismicity maps to obtain expected hazard maps and then to forecast mechanisms for future earthquakes several types of smoothing kernels are used in space domain we use the 1 distance kernel for the distribution of seismicity around any epicenter the kernel is parameterized using two adjustable parameters maximum distance and directivity distribution of seismicity around an epicenter with regard to the focal mechanism of an earthquake for temporal prediction we use the poisson hypothesis of earthquake temporal behavior we test these forecasts we use the first half of a catalog to smooth seismicity level and the second half of the catalog is used to validate and optimize the prediction to illustrate the technique we use available data in the harvard catalog of seismic moment solutions to evaluate seismicity maps for several seismic regions the method can be used with similar catalogs the technique is completely formal and does not require human operator intervention hence the prediction results can be objectively tested moreover the maps can be used as the poisson null hypothesis for testing by the likelihood method against any other prediction model which shares the same sample space the same zones time window and acceptance criteria

correlating variations of b values and crustal deformations during the 1990s may have pinpointed the rupture initiation of the mw 7 4 izmit earthquake of 1999 august 17 summary the b value increases from 1 1 along straight segments to 1 7 along a releasing bend of the north anatolian fault system between 30 1 e and 30 8 e the tensional nature of the bend is corroborated by extension rates of 0 3 strain yr 1 as obtained from global positioning system gps data fault plane solutions in this area change from abundant right lateral strike slip to oblique slip with normal components in the central part of the bend compatible with the formation of a pull apart basin we suggest that this crustal volume is highly fractured corresponding to a short mean crack length due to the superposition of horizontal and normal faulting as a consequence of low normal stress on the existing cracks frictional sliding is assumed to be the predominant mechanism for the generation of seismic events resulting in high b values the lowest b values b 0 8 in the area investigated are found at the junction between the fault bend and the adjoining segment which runs through the epicentre of the 1999 izmit earthquake at the junction a localized stress concentration is expected from numerical models of seismicity along geometrical barriers thus the site of lowest b has been considered to be the most likely place for a major earthquake a conclusion that is confirmed by the izmit earthquake with epicentre located about 13 km from the anticipated site in early 1992 the spatially averaged b value started to increase along the central and northwestern parts of the fault bend and there was a corresponding period of intensified horizontal extension since no large earthquake was recorded in 1992 it is assumed that the anomalous extension and the associated change in stress have been caused by aseismic fault creep in view of the available seismotectonic information surface displacements due to a slow dislocation at depth were calculated the gps data are reasonably explained by a source of normal fault type at a depth of 10 km and equivalent magnitude mw 5 9 stress changes at seismogenic depths are calculated under the assumption that an increase in b is most likely to be observed along those segments where both deviatoric and mean stress are reduced while the coulomb stress change is positive that is the fault is brought closer to failure for right lateral strike slip the stress condition is fulfilled at the sites of the largest b value increase the surface rupture of the izmit earthquake is associated with known fault traces only in the west and in the east of the study area along the fault bend it detaches itself from mapped fault traces and runs through the regions of positive coulomb stress change caused by the inferred 1992 slow event maximum surface slip 5 m is observed in the immediate vicinity of sites of maximum coulomb stress change 3 bar indicating that the 1992 event could have partially unclamped the rupture plane of the subsequent izmit earthquake we propose that the strong differences in b observed along the complex fault bend can be used to characterize different states of tectonic deformation in space and time

interhemispheric dynamics of the african rainbelt during the late quaternary abstract the spatial pattern of precipitation variability in tropical and subtropical africa over the late quaternary has long been debated prevailing hypotheses variously infer 1 insolation controlled asymmetry of wet phases between hemispheres 2 symmetric contraction and expansion of the tropical rainbelt and 3 independent control on moisture available in southern africa via sea surface temperatures in the indian ocean in this study we use climate model simulations covering the last glacial cycle 120 kyr with hadcm3 and the multi model ensembles from pmip3 the palaeoclimate model intercomparison project to investigate the long term behaviour of the african rainbelt and test these simulations against existing empirical palaeohydrological records through regional model data comparisons we find evidence for the validity of several hypotheses with various proposed processes occurring concurrently but with different regional emphasis e g asymmetric shifts at the seasonal extremes and symmetric expansions contractions towards west equatorial regions crucially variations in rainfall are associated with multiple forcing mechanisms that vary in their dominance both spatially and temporally over the glacial cycle an important consideration when interpreting and extrapolating from often relatively short palaeoenvironmental records

late quaternary vegetation history of southeast africa the molecular isotopic record from lake malawi abstract accurate reconstructions of past hydrological variability are essential for understanding the climate history of the tropics in tropical africa the relative proportion of vegetation utilizing the c3 vs c4 photosynthetic pathway is mainly controlled by precipitation and thus past hydrological changes can be inferred from the vegetation record in this study biomarkers of terrestrial plants lignin phenols and plant leaf wax carbon isotopes are examined from a well dated sedimentary record from lake malawi to provide a vegetation aridity record of the past 23 cal ka from southeast africa we suggest that the ratio of cinammyl to vanillyl c v lignin phenols in lake malawi sediments mainly reflects inputs of c3 trees woody tissue vs c4 grasses non woody tissue and find that changes in the c v ratio generally support variability noted in the n alkane carbon isotope record together these records provide evidence for increased c4 vegetation grasses surrounding lake malawi during last glacial maximum lgm the younger dryas in the early holocene and from 2 cal ka to the present suggesting drier conditions at these times elevated inputs of c3 vegetation are noted in the intervals from 17 13 6 cal ka and 7 7 2 cal ka indicating wet conditions in southeast africa a relationship is noted between the n alkane average chain length acl and temperature with longer acls associated with higher temperatures higher n alkane carbon preference index cpi values correlate with higher mass accumulation rates of biogenic silica and may result from periodic increased northerly winds over lake malawi which enhance upwelling and diatom productivity while simultaneously increasing erosion and transport of plant leaf waxes to the lake the molecular data produced in this study suggest that the carbon isotopic signature of bulk sediment 13ctoc in lake malawi is primarily a reflection of terrestrial inputs c3 vs c4 vegetation and may not mainly reflect changes in algal productivity as previously thought

climatic patterns in equatorial and southern africa from 30 000 to 10 000 years ago reconstructed from terrestrial and near shore proxy data abstract as part of a wider study of last glacial and deglacial climates in the southern hemisphere continents we here review terrestrial and near shore marine records from equatorial and southern africa between 30 000 and 10 000 years ago 30 10 ka this time interval covers the lead up to the last glacial maximum lgm 21 2 ka the lgm proper and the ensuing deglacial records selected for review needed to meet three requirements continuity or near continuity over the period a well established chronology and at least one but preferably several unambiguous proxy ies we aim to show how regional climates of the sub continent have responded to orbital forcing as opposed to other global glacial interglacial boundary conditions and how they are related to high latitude climates sea and land surface conditions positions of the intertropical convergence zone itcz and of the westerly belt evidence of past climates derived from many independent proxies is given from west to southwest africa moisture from the atlantic ocean then from equatorial east africa to the southern subtropical summer rainfall domain moisture mainly from the indian ocean the lgm was cooler than today and generally drier in the tropics north of 8 9 s glacial to holocene increase in monsoonal precipitation primarily related to orbitally induced summer insolation in the northern hemisphere occurred by steps of increasing amplitude 17 16 14 5 11 5 ka major wet dry spells coincide with abrupt warm cold events in high northern latitudes and related itcz migrations in the southern tropics the main post glacial increase in tropical rainfall generally appears more gradual and in phase with antarctic warming data suggest a restricted northward migration of the itcz and concentration of tropical rainfall well south of the equator during the lgm and the younger dryas drier glacial conditions prevailed in southeastern africa while parts of southwestern africa point to enhanced humidity during the lgm suggesting that the winter westerly belt was either stronger than today or displaced further north possibly as a result of more extensive antarctic sea ice inferred african climatic fluctuations show the competing influences of tropical and high latitude climates of both hemispheres and suggest changes in both meridional and zonal circulation modes this review also reveals major geographical and methodological gaps and a number of unresolved issues providing pointers for future research

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

impact of monsoons temperature and co2 on the rainfall and ecosystems of mt kenya during the common era abstract glacial and early holocene age sediments from lakes on mt kenya have documented strong responses of montane hydrology ecosystems and carbon cycling to past changes in temperature and atmospheric co2 concentrations however little is known about climate and ecosystem variations on mt kenya during the common era the past 2000 years despite mounting evidence for significant climate changes in the east african lowlands during the past millennium and recent observations of alpine glacier retreat in the east african highlands we present a new high resolution record of the hydrogen and carbon isotopic composition of terrestrial plant wax compounds dwax 13cwax preserved in the sediments of sacred lake from 200c e to the end of the 20th century we find that mt kenya s climate was highly variable during the past 1800 years droughts at sacred lake around 200c e 700c e and 1100c e align with similar droughts in central kenya and uganda congo indicating that failures of both the indian and atlantic monsoons caused widespread drought throughout equatorial east africa during the early common era in contrast dry and wet periods at sacred lake during the past 500 years show meridional and zonal contrasts with other sites in east africa suggesting strong spatial heterogeneity possibly due to independent waxing and waning of the atlantic and indian monsoons pronounced drying after 1870c e suggests that the current dry phase observed at sacred lake may have begun prior to the 20th century around the time when the retreat of mt kenya s glaciers was first observed by european explorers mt kenya s vegetation responded strongly to these recent climate changes highlighting the particular sensitivity of tropical montane climate and ecosystems to regional and global climate patterns and underscoring the critical need to understand potential impacts of future climate change scenarios on this highly sensitive region

the evolution of seismicity at parkfield observation experiment and a fracture mechanical interpretation we compare observations of local seismicity sequences with acoustic emissions during rock fracture and find similarities in the distribution of event magnitudes which lead us to consider a model of seismicity which is characterized by fracture growth laws derived from the field of fracture mechanics and which include both negative and positive feedback processes we find that the model predicts the observed negative correlation between the seismic b value and the fractal dimension of the earthquake epicentres the simple short range interactions described in the model give rise to a system with a long range order similar to current models of critical processes

a simple fracture mechanical model for the evolution of seismicity a simple model is presented in which a fault is represented by a one dimensional array of elements to which a remote stress is applied the fracture toughnesses of the fault elements are initially generated by a random walk function but the fracture system quickly becomes organized into a fractal set characterised by a box counting fractal dimension the slope b of the frequency magnitude distribution and the fractal dimension of the crack population are measured at each step and found to vary in a similar manner to the seismic b value and fractal dimension of earthquake epicentres the model also predicts the observed variation of b value of acoustic emissions in laboratory experiments

climate induced sub basin source area shifts of zambezi river sediments over the past 17 ka geochemical and mineralogical proxies for paleoenvironmental conditions have the underlying assumption that climate variations have an impact on terrestrial weathering conditions varying properties of terrigenous sediments deposited at sea are therefore often interpreted in terms of paleoenvironmental change also in gravity core geob9307 3 18 33 99 s 37 22 89 e located off the zambezi river environmental changes during heinrich stadial 1 hs 1 and the younger dryas yd are accompanied by changing properties of the terrigenous sediment fraction our study focuses on the relationship of variability in the hydrological system and changes in the magnetic properties major element geochemistry and granulometry of the sediments we propose that changes in bulk sedimentary properties concur with environmental change although not as a direct response of climate driven pedogenic processes spatial varying rainfall intensities on a sub basin scale modify sediment export from different parts of the zambezi river basin during humid phases such as hs 1 and the yd sediment was mainly exported from the coastal areas while during more arid phases sediments mirror the hinterland soil and lithological properties and are likely derived from the northern shire sub basin we propose that a de coupling of sedimentological and organic signals with variable discharge and erosional activity can occur

generalized omori law for aftershocks and foreshocks from a simple dynamics a theory of the time dependence of earthquake foreshocks and aftershocks is presented the theory involves the response to sudden forcing of a dynamics of self driven acceleration to failure the empirically observed omori law which says that the rate of aftershocks as a function of time falls as a power law in time is derived theoretically the exponent of the falloff in time is shown to generically give a value close to one for rapidly accelerating dynamics to see if the theory is consistent with other features of real data foreshocks and aftershocks of small magnitude mainshocks are analyzed in a catalogue of real earthquakes results show that the spatial and temporal distribution of aftershocks is separable into a dependence on space and a dependence on time that the spatial distribution of aftershocks is consistent with the hypothesis that stress changes from the mainshock cause aftershocks and that the number of foreshocks approaches the number of aftershocks as the magnitude of the mainshock becomes smaller

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

isotopic composition of waters from ethiopia and kenya insights into moisture sources for eastern africa 1 oxygen and deuterium isotopic values of meteoric waters from ethiopia are unusually high when compared to waters from other high elevation settings in africa and worldwide these high values are well documented however the climatic processes responsible for the isotopic anomalies in ethiopian waters have not been thoroughly investigated we use isotopic data from waters and remote data products to demonstrate how different moisture sources affect the distribution of stable isotopes in waters from eastern africa oxygen and deuterium stable isotopic data from 349 surface and near surface groundwaters indicate isotopic distinctions between waters in ethiopia and kenya and confirm the anomalous nature of ethiopian waters remote data products from the tropical rainfall measuring mission trmm and national centers for environmental prediction ncep reanalysis project show strong westerly and southwesterly components to low level winds during precipitation events in western and central ethiopia this is in contrast to the easterly and southeasterly winds that bring rainfall to kenya and southeastern ethiopia large regions of high equivalent potential temperatures e at low levels over the sudd and the congo basin demonstrate the potential for these areas as sources of moisture and convective instability the combination of wind direction data from ethiopia and e distribution in africa indicates that transpired moisture from the sudd and the congo basin is likely responsible for the high isotopic values of rainfall in ethiopia

evolving towards a critical point a review of accelerating seismic moment energy release prior to large and great earthquakes there is growing evidence that some proportion of large and great earthquakes are preceded by a period of accelerating seismic activity of moderate sized earthquakes these moderate earthquakes occur during the years to decades prior to the occurrence of the large or great event and over a region larger than its rupture zone the size of the region in which these moderate earthquakes occur scales with the size of the ensuing mainshock at least in continental regions a number of numerical simulation studies of faults and fault systems also exhibit similar behavior the combined observational and simulation evidence suggests that the period of increased moment release in moderate earthquakes signals the establishment of long wavelength correlations in the regional stress field the central hypothesis in the critical point model for regional seismicity is that it is only during these time periods that a region of the earth s crust is truly in or near a self organized critical soc state such that small earthquakes are capable of cascading into much larger events the occurrence of a large or great earthquake appears to dissipate a sufficient proportion of the accumulated regional strain to destroy these long wavelength stress correlations and bring the region out of a soc state continued tectonic strain accumulation and stress transfer during smaller earthquakes eventually re establishes the long wavelength stress correlations that allow for the occurrence of larger events these increases in activity occur over longer periods and larger regions than quiescence which is usually observed within the rupture zone of a coming large event the two phenomena appear to have different physical bases and are not incompatible with one another

gps derived strain rate field within the boundary zones of the eurasian african and arabian plates we use the gps velocity field 1988 1998 for eastern mediterranean and asia minor to determine the crustal deformation strain rate field in an area bounded by 35 n and 43 n and 20 e and 48 e we calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data uncertainties in the calculation of the strain rates reach 50 nstrain yr 1 in sparsely observed parts of anatolia whereas we estimate errors 20 nstrain yr 1 in the aegean and marmara regions the largest compressional strain rate components in the eastern part of the study area occur along the greater caucasus mountain front reaching 70 nstrain yr 1 1 nstrain yr 1 0 0317 10 15 s 1 the north anatolian fault zone is the clearest feature in the shear strain rate field it is expressed as a pronounced dextral strike slip fault zone reaching rates of up to 170 nstrain yr 1 this holds true also for the izmit area where the august 17 1999 earthquake occurred central anatolia is almost strain free whereas extension prevails in western anatolia the principal axes of extension vary around the n s direction with strain rates of up to 85 nstrain yr 1 these extensional areas coincide with graben features and normal faulting earthquakes the central and southwestern aegean sea is strain free with values far below 40 nstrain yr 1 the seismic cluster around the dodekanissa islands southeastern aegean sea coincides with nw se oriented extension attaining strain rates of up to 90 nstrain yr 1 this area of extension also exhibits recent active volcanism the entire hellenic arc shows compressional strain rates perpendicular to the arc the pliny strabo troughs along the eastern segment of the arc show left lateral shear strain rates reaching 80 nstrain yr 1 significant extension is found in central greece with a nne ssw oriented maximum of 120 nstrain yr 1 centered around the gulf of corinthos the kephalonia fault zone in nw greece is a distinct dextral fault zone separating apulia from the rapidly moving aegean microplate right lateral shear strain rates reach 150 nstrain yr 1

quasi static fault growth and cracking in homogeneous brittle rock under triaxial compression using acoustic emission monitoring this paper describes the localization of deformation acceleration in the period prior to dynamic failure in hornblende schist rock under triaxial compression using acoustic emission ae monitoring rather than stabilize the failure process by controlling axial stress to maintain a constant rate of ae for monitoring ae hypocenters as in previous works e g lockner et al 1991 we have instead developed a rapid multichannel data collection system this enables us to elucidate the dynamics of fault nucleation under condition of constant stress creep loading which is a better approximation to low strain rate condition in the earth and allows both quasi static and dynamic crack growth to occur the waveforms of more than 8000 ae events which occurred mainly during a 15 s period were recorded on 32 channels with a sampling rate of 50 ns and mask time of 200 s hypocentral locations of ae sources revealed that the fault initiated at one end of the core and then propagated into the unfaulted rock with a process zone fault front of intense cracking we found that there were two different processes operating during the quasi static nucleation of a shear fault namely a process zone in front of the fault tip and a wake of damage zone following the process zone the process zone had the following features 1 major tensile cracking 2 low b value and fewer larger events and 3 strong self excitation the mechanism of crack interaction and fault growth was therefore a mutual enhancement on dilatation due to tensile cracking on the other hand the damage zone was characterized by 1 major shear cracking 2 low b value and more larger events and 3 weak self excitation indicating that in the damage zone following the development of a shear fault linkage between cracks became the major mechanism of crack interaction and fault development the mutual changes of b value and self exciting strength observed in our experiments seem to occur as a result of the hierarchy of fault growth which was not observed under slowed down loading conditions therefore our experimental results under a realistic approximation of the dynamic condition of the earth are meaningful for the problems of earthquakes as well as rock bursts

classification of earthquake precursors from a fracture mechanics model abstract a new classification of earthquake precursors is proposed based on a fracture mechanics approach which includes the effects of stress and the acceleration of a fracture through an asperity two phases of earthquake warning associated with intermediate term and short term anomalies can be related in a simple manner to two peaks in the stress intensity the first broad peak is further split into two halves associated with precursory strain release strain hardening followed by precursory stress drop strain softening and the second peak responds to the transition from quasi static fracture and slip to dynamic rupture the model predicts a seismic b value anomaly which is in good agreement with laboratory experiment and some aspects of empirical observation of earthquakes in particular evidence is presented of the distinctive short term b value anomaly predicted by the second stress intensity peak and a critical foreshock related b value of 0 5 other earthquake precursors can also be classified into this unifying model and it is suggested that those anomalies which respond to the cumulative production of microcracks e g coda q 1 could produce a more distinctive two stage anomaly after differentiation with respect to time such a two stage anomaly especially if it were observed in several independent earthquake precursors could be a very useful method of deciding when a low level earthquake warning should be changed to a higher level warning

century to millennial scale climatic variability in lake malawi revealed by isotope records diatom based oxygen isotope data 18odiatom from lake malawi show multi centennial scale wet dry intervals spaced approximately every 2 3 ka throughout a 25 ka sequence the 18odiatom record is supported by a lower resolution deuterium dpa isotope curve derived from palmitic acid we interpret these isotope data in terms of major shifts in precipitation and evaporation moderated by seasonal controls on the host organisms dry periods marked by relatively positive isotope values represent the extension of abrupt holocene events noted from northern and equatorial africa to 10 15 s these events in lake malawi correspond to cool episodes in greenland thereby demonstrating teleconnections generated by meridional temperature gradients sea surface temperatures are likely to be the primary transmitter of deglacial climate changes although trade wind strength and direction is critical in controlling precipitation patterns in tropical regions conversely the global hydrological cycle driven by low latitude regions represents an important positive feedback amplifying deglacial processes

a high resolution paleoclimate record spanning the past 25 000 years in southern east africa high resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern lake malawi provide a climate signal for this part of tropical africa spanning the past 25 000 years the biogenic silica mass accumulation rate was low during the relatively dry late pleistocene when the river flux of silica to the lake was suppressed millennial scale fluctuations due to upwelling intensity in the late pleistocene climate of the lake malawi basin appear to have been closely linked to the northern hemisphere climate until 11 thousand years ago relatively cold conditions in the northern hemisphere coincided with more frequent north winds over the malawi basin perhaps resulting from a more southward migration of the intertropical convergence zone

atmospheric circulation patterns during late pleistocene climate changes at lake malawi africa abstract the climate of tropical africa transitioned from an interval of pronounced orbitally paced megadroughts to more humid and stable conditions approximately 70 000 years ago scholz et al 2007 the regional atmospheric circulation patterns that accompanied these climatic changes however are unclear due to a paucity of continental paleoclimate records from tropical africa extending into the last interglacial we present a new 140 kyr record of the deuterium hydrogen isotopic ratio of terrestrial leaf waxes d wax from drill cores from lake malawi southeast africa that spans this important climatic transition d wax shifts from highly variable and relatively d depleted to more stable and d enriched around 56 ka contemporary with the onset of more humid conditions in the region moisture source and transport history dominate the d wax signal at lake malawi with local rainfall amount playing a secondary role for much of the paleorecord analysis of modern moisture sources for lake malawi suggests that d depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier d depleted air mass currently located in central southern africa to the lake malawi catchment this d depleted air mass is associated with the descending limb of the hadley cell which implies significant changes in the hadley circulation during the megadroughts and related changes in the position of the intertropical convergence zone over africa these findings demonstrate the ability of d wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance and elucidate potential mechanisms for pronounced hydrological change in southeast africa during the late pleistocene

late pleistocene temperature history of southeast africa a tex86 temperature record from lake malawi we present a tex86 derived surface water temperature record for lake malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past similar to 74 kyr with millennial scale resolution average temperature during marine isotope stage mis 5a was 26 5 degrees c with a range from 25 7 to 27 3 degrees c comparable to holocene temperatures mis 4 was a relatively cold period with temperatures generally decreasing from 25 5 degrees c at 68 ka to a minimum of 20 degrees cat similar to 60 ka 1 5 2 degrees c colder than the last glacial maximum lgm termination of mis 4 is characterized by a rapid increase of 3 4 degrees c in only similar to 0 5 kyr temperatures were relatively stable throughout mis 3 at the resolution of this study with an average of 23 8 degrees c and a range from 25 1 to 22 9 degrees c the lack of millennial scale temperature variability during mis 3 suggests that lake malawi s documented response to the bipolar seesaw brown et al 2007 is not reflected in its thermal history our temperature estimates for the lgm and holocene are consistent with a previously published tex86 record from lake malawi with a temperature of similar to 22 6 degrees c for the lgm similar to 25 26 degrees c in the mid holocene and similar to 25 28 for the late holocene in general the present extended tex86 record indicates that temperature variability in tropical east africa during late mis 5 and mis 4 was as great as that associated with the deglaciation and holocene a decrease in southern hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during mis 4 but after 60 ka other factors such as the extent of the polar ice sheets or atmospheric co2 may have forced temperature in tropical africa to a greater extent than local summer insolation

a window into africa s past hydroclimates the sisal_v1 database contribution africa spans the hemispheres from temperate region to temperate region and has a long history of hominin evolution although the number of quaternary palaeoclimatic records from the continent is increasing much of the history of spatial and temporal climatic variability is still debated speleothems as archives of terrestrial hydroclimate variability can help reveal this history here we review the progress made to date with a focus on the first version of the speleothem isotopes synthesis and analysis sisal database the geology of africa has limited development of large karst regions to four areas along the northern coast bordering the mediterranean eastern africa and the horn of africa southwestern africa and southern africa exploitation of the speleothem palaeoclimate archives in these regions is uneven with long histories of research e g in south africa but large areas with no investigations such as west africa consequently the evidence of past climate change reviewed here is irregularly sampled in both time and space nevertheless we show evidence of migration of the monsoon belt with enhanced rainfall during interglacials observed in northeast africa southern arabia and the northern part of southern africa evidence from eastern africa indicates significant decadal and centennial scale rainfall variability in northwestern and southern africa precession and eccentricity influence speleothem growth largely through changing synoptic storm activity

groundwater residence time and paleorecharge conditions in the deep confined aquifers of the coastal watershed south east tanzania summary analysis of radiocarbon and stable isotopes of c h and o in groundwater samples from the coastal watershed of southeastern tanzania is used to decipher the age of groundwater and the climatic conditions at the time of recharge radiocarbon dating was corrected on the basis of stable carbon isotope ratios and hydrochemical data two main recharge periods are recognised 1 a recent recharge period with residence times dated to younger than 1 3 ka bp for the shallow aquifer and some deep groundwater aquifers and 2 a paleorecharge occurring during the late pleistocene to holocene transition between 6 and 13 ka bp for the lower confined aquifer the climatic excursions arid and humid phases identified in this study match well with those identified from studies of paleo climatic records in tanzania and east africa the oxygen 18 and radiocarbon results show that there is a potential mixing of groundwater of varying ages along flow paths in the coastal aquifer system this has also been shown by redox parameters in some of the deeper aquifer samples the data also revealed that there is relatively simple pattern of lateral groundwater flow in the deeper confined aquifer isotopic evidence from this study and related studies in paleolimnology improves our knowledge of the past climates the data suggests that the paleorecharge to the confined aquifer was from inland under wetter climatic conditions than present therefore paleorecharge from the adjacent mountain regions to coastal area like ifakara morogoro and udzungwa mountains could be important parts of recharge area of the kimbiji aquifer however the hinterland mountains of dar es salaam mainly pugu mkuranga ruvu and kisarawe could also play an important role in replenishing the kimbiji coastal aquifers in contrast recharge in the shallow unconfined aquifer is characterized by local rainfall and river water infiltration this study is the first of its kind assessing fingerprinting of paleoclimates in southeastern coastal tanzania using the isotopic composition of deep groundwater aquifers the freshwater resources in the neogene aquifer contain paleo groundwater aquifers that have been mixed with recently recharged flow systems hence the resources may be sustainable this study also provides information that may be useful for designing optimum strategies for a sustainable management of groundwater resources in the coastal aquifer and it testifies to the importance of groundwater as a paleoclimatic archive such information is also essential for consideration by modellers in their attempts to simulate the hydrogeological processes in confined aquifers over very long time spans

a new look at old carbon in active margin sediments recent studies suggest that as much as half of the organic carbon oc undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering these estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils freshwater sediments and river deltas and thus only minimally contributes to the extraneously old 14c ages of total organic matter often observed on adjacent shelves here we test this paradigm by measuring the 14c and 13c values of individual higher plant wax fatty acids as well as the 13c values of extractable alkanes isolated from the eel river margin california the isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition a coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated with pre aged terrestrial material instead composing a considerable proportion of all organic matter if these findings are characteristic of other continental margins proximal to small mountainous rivers then the importance of petrogenic oc burial in marine sediments may need to be reevaluated 2009 the geological society of america

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

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

a 3 million year index for north african humidity aridity and the implication of potential pan african humid periods abstract mediterranean sediments are valuable archives of both african monsoon variability and higher latitude climate processes and can also be used to provide an environmental context for early human migrations and settlements however the long history of mediterranean palaeoclimate studies largely pre dates the advent of widespread x ray fluorescence xrf core scanning so there are few continuous and high resolution geochemical records from this key region that extend beyond the last glacial cycle here we present xrf core scanning results for odp site 967 eastern mediterranean that have been fully calibrated into element concentrations spanning the last 3 million years my comparison with independent geochemical data from conventional xrf highlights disparities for certain element element ratios thus suggesting the need for caution when taking ratios of scanning xrf data principal component analysis of the calibrated xrf dataset reveals two dominant components detrital inputs pc1 and a sapropel monsoon run off signal pc2 which we use to establish a new orbitally tuned chronology we observe inverse covariation between pc2 and a previously published aeolian dust record from odp site 967 larrasoana et al 2003 and combine these records to produce a composite index of humidity and aridity for the wider north african region over the past 3 my we propose that by combining run off and dust signals in a single metric our index captures the effects of both strengthening northward migration increased run off and weakening southward retreat increased dust of the north african monsoon comparison of the index with published records of northwest and east african palaeohumidity suggests that it tracks the timing of green sahara periods throughout the plio pleistocene and that at least 30 of these intervals coincided with increased humidity across east africa we tentatively suggest that these specific episodes may be termed pan african humid periods as a means to highlight large scale climate trends and to provide an environmental framework for palaeo anthropological research

biogenic silica deposition in lake malawi east africa over the past 150 000years abstract we report a 150 000 year record of the abundance of biogenic silica bsi in drill cores from the northern and central basins of lake malawi the periods of highest diatom burial at both sites occurred around 65 69 and 51 60 thousand years ago ka after the termination of prolonged intense drought in the region these peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin the droughts that preceded these bsi depositional episodes occurred at precessional frequency corresponding to times of minimum austral spring or summer insolation in the region prior to 60 ka subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity however biogenic silica records do not show significant sensitivity to precessional forcing the bsi burial flux in the north basin displays strong millennial scale variability since 50 ka with peak values occurring during cold times in the northern hemisphere as reflected in the greenland ice core records we observe no relationship between bsi and deposition of volcanic tephras in this system the central basin core also displays millennial scale variability in bsi abundance during the past 50 ky of comparable magnitude to that in the north basin but without systematic correlation to the north basin bsi or greenland ice core records in general we conclude that bsi profiles from the north and probably south basins are more readily interpreted in terms of regional climate dynamics than are bsi profiles from more central locations in long narrow tropical rift lakes diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low latitude lacustrine systems

qualitative assessment of pmip3 rainfall simulations across the eastern african monsoon domains during the mid holocene and the last glacial maximum abstract in this paper we compare a compilation of multiproxy records spanning the eastern african margin with general circulation model simulations of seasonal precipitation fields for the mid holocene and the last glacial maximum lgm carried out as part of the third phase of the paleoclimate modelling intercomparison project pmip3 results show good agreement during the mid holocene the 6k experiment with palaeodata and model outputs correlating well and indicating that changes in insolation drove a stronger northern african monsoon north of 0 5 s during the terminal african humid period and a weaker southeast african monsoon for the lgm the 21k experiment however significant discrepancies exist both between model simulations and between existing palaeodata and simulated conditions both in terms of direction and amplitude of change none of the pmip3 simulations reflect the pattern inferred from the palaeodata two major discrepancies have been identified to explain this 1 the limited sensitivity of the southern monsoon domain to the colder temperatures of the indian ocean 2 c and 2 the absence of changes in the dynamic of the indian ocean walker circulation over the entire basin despite the exposure of the sahul and sunda shelves that weakened convection over the indo pacific warm pool during the lgm these results indicate that some major features of the atmospheric and oceanic teleconnections between the different monsoon regions require further consideration as models evolve

patterns of seismic activity preceding large earthquakes we analyze the patterns of seismic activity which precede large events in a mechanical model of a fault the model generates a statistical distribution of events similar to that observed for a single fault with a scaling region consistent with the gutenberg richter law at small and moderate magnitudes and an excess of events at large magnitudes we find only slight variation in the scaling behavior during a loading cycle however we do observe systematic variations in space and time of the overall rate of activity in the model the activity accelerates dramatically preceding a large event and is usually a maximum hi the neighborhood of the future epicenter these results are compared to california seismicity data where we find that activity patterns vary regionally looking at patterns of activity in the san francisco bay area since 1948 we find an increase of activity on the calaveras fault near san jose beginning in the 1980s which if our model is relevant would forecast a large earthquake in that region the 1989 loma prieta earthquake occurred on the san andreas fault within 30 km of the section of the calaveras fault showing increased activity

sedimentation patterns off the zambezi river over the last 20 000 years marine sediments from continental margins provide high resolution archives of marine and continental climate in particular near large river mouths the zambezi is one of the largest rivers in east africa discharging large amounts of fine grained sediments onto the western margin of the mozambique channel accurate reconstructions of past variations in river discharge can be retrieved from the depositional history of riverine sediments in this study sedimentation patterns along the mozambique margin are inferred from a series of sediment cores spanning the last 20 kyr these cores were retrieved off the zambezi mouth from the shelf and from various depths at the continental slope to provide detailed information on the depositional history of this region high resolution x ray fluorescence and magnetic susceptibility core scanning measurements as well as grain size distributions of the lithogenic sediment fraction and organic matter and carbonate content analyses resolve centennial millennial timescale variations in sediment transport and deposition along the mozambique margin largest changes in lithogenic sediment deposition coincide with the flooding of the mozambique shelf due to deglacial sea level rise and changes in oceanic currents fine grained mainly zambezi sediments were no longer primarily deposited south of the zambezi mouth but increasingly deposited onto the mozambique shelf and transported northwards over the shelf before spilling over to the slope in addition flooding of the shelf initiated remobilization and down slope transport of coarse grained winnowed sediments the heinrich event 1 and younger dryas are clearly indicated as events of increased fine grained sediment deposition along the mozambique margin possibly linked to increased riverine runoff from the zambezi catchment which is in harmony with a southward shift of the intertropical convergence zone itcz and associated african rainfall belts causing wetter conditions this study illustrates that interpreting marine and continental climate proxy records in the vicinity of river deltas in terms of climate change requires proper consideration of sea level and oceanic circulation induced effects on the transport and deposition patterns of fine grained sediments at continental slopes

pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical atlantic indicate that the oxygen isotopic composition 18 o of seawater at this site during the last glacial maximum was 0 8 0 1 per mil higher than it is today combined with the 18 o change in benthic foraminifera from this region the elevated ratio indicates that the temperature of deep water in the tropical atlantic ocean was 4 c colder during the last glacial maximum extrapolation from this site to a global average suggests that the ice volume contribution to the change in 18 o of foraminifera is 1 0 per mil which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from barbados corals and terrestrial climate proxies

can sedimentary leaf waxes record d h ratios of continental precipitation field model and experimental assessments abstract d h ratios of leaf waxes d wax derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology ideally d wax can be used to reconstruct precipitation d h ratios d p which is a well established paleoclimate proxy however many other factors such as vegetation and relative humidity rh also affect d wax variation how the combination of these factors affects sedimentary d wax is unclear here we use a transect of 32 lake surface sediments across large gradients of precipitation relative humidity and vegetation composition in the southwestern united states to study the natural factors affecting sedimentary d wax d values of c 28 n alkanoic acids show significant correlation with d p values r 2 0 76 with an apparent isotopic enrichment of 99 8 indicating that sedimentary d wax values track overall d p variation along the entire transect leaf waxes produced by plants grown under controlled conditions rh 80 60 40 show a small increase in d h ratios as rh decreases consistent with prediction from the craig gordon model however the isotopic effect of rh on d wax along the natural transect is partially countered by the opposing influence of vegetation changes the correlation between d wax and d p values is significantly higher r 2 0 84 in the drier portions of the transect than in the wetter regions r 2 0 64 this study suggests that d h ratios of sedimentary leaf waxes can be used as a proxy for precipitation d variations with particularly high fidelity in dry regions although more studies in other regions will be important to further test this proxy

orbital and millennial scale changes in the hydrologic cycle and vegetation in the western african sahel insights from individual plant wax δd and δ 13 c to reconstruct variability of the west african monsoon and associated vegetation changes on precessional and millennial time scales we analyzed a marine sediment core from the continental slope off senegal spanning the past 44 000 years 44 ka we used the stable hydrogen isotopic composition dd of individual terrestrial plant wax n alkanes as a proxy for past rainfall variability the abundance and stable carbon isotopic composition d 13 c of the same compounds were analyzed to assess changes in vegetation composition c3 c4 plants and density the dd record reveals two wet periods that coincide with local maximum summer insolation from 38 to 28 ka and 15 to 4 ka and that are separated by a less wet period during minimum summer insolation our data indicate that rainfall intensity during the rainy season throughout both wet humid periods was similar whereas the length of the rainy season was presumably shorter during the last glacial than during the holocene additional dry intervals are identified that coincide with north atlantic heinrich stadials and the younger dryas interval indicating that the west african monsoon over tropical northwest africa is linked to both insolation forcing and highlatitude climate variability the d 13 c record indicates that vegetation of the western sahel was consistently dominated by c4 plants during the past 44 ka whereas c3 type vegetation increased during the holocene moreover we observe a gradual ending of the holocene humid period together with unchanging ratio of c3 to c4 plants indicating that an abrupt aridification due to vegetation feedbacks is not a general characteristic of this time interval

late quaternary behavior of the east african monsoon and the importance of the congo air boundary both atlantic and indian ocean climate dynamics exert influence over tropical african hydroclimate producing complex patterns of convergence and precipitation to isolate the indian ocean influence on african paleohydrology we analyzed the deuterium hydrogen ratio of higher plant leaf waxes delta d wax in a 25 000 year sediment record from lake challa 3 degrees s 38 degrees e in the easternmost area of the african tropics whereas both the seismic record of inferred lake level fluctuations and the branched and isoprenoidal tetraether bit index proxy record changes in hydrology within the challa basin delta d wax as a proxy for the isotopic composition of precipitation delta d p is interpreted as a tracer of large scale atmospheric circulation that integrates the history of the moisture transported to the lake challa area specifically based on modern day isotope rainfall relationships we argue that lake challa delta d wax reflects the intensity of east african monsoon circulation the three hydrological proxy records show generally similar trends for the last 25 000 years but there are important differences between them primarily during the middle holocene we interpret this deviation of delta d wax from local hydrological history as a decoupling of east african monsoon intensity which heavily influences the isotopes of precipitation in east africa today from rainfall amount in the challa basin in combination the hydrological proxy data from lake challa singularly highlight zonal gradients in tropical african climate that occur over a variety of timescales suggesting that the congo air boundary plays a fundamental role in controlling hydroclimate in the african tropics

the lake natron footprint tuff northern tanzania volcanic source depositional processes and age constraints from field relations the engare sero footprint site situated on the southern shore of lake natron in northern tanzania has been reported to host one of the best preserved sets of fossilized hominid footprints in the

a leaf wax biomarker record of early pleistocene hydroclimate from west turkana kenya abstract climate is thought to play a critical role in human evolution however this hypothesis is difficult to test due to a lack of long high quality paleoclimate records from key hominin fossil locales to address this issue we analyzed organic geochemical indicators of climate in a drill core from west turkana kenya that spans 1 9 1 4 ma an interval that includes several important hominin evolutionary transitions we analyzed the hydrogen isotopic composition of terrestrial plant waxes dwax to reconstruct orbital timescale changes in regional hydrology and their relationship with global climate forcings and the hominin fossil record our data indicate little change in the long term mean hydroclimate during this interval in contrast to inferred changes in the level of lake turkana suggesting that lake level may be responding dominantly to deltaic progradation or tectonically driven changes in basin configuration as opposed to hydroclimate time series spectral analyses of the isotopic data reveal strong precession band 21 kyr periodicity indicating that regional hydroclimate was strongly affected by changes in insolation we observe an interval of particularly high amplitude hydrologic variation at 1 7 ma which occurs during a time of high orbital eccentricity hence large changes in processionally driven insolation amplitude this interval overlaps with multiple hominin species turnovers the appearance of new stone tool technology and hominin dispersal out of africa supporting the notion that climate variability played an important role in hominin evolution

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

abrupt change in tropical african climate linked to the bipolar seesaw over the past 55 000 years 1 the tropics play a major role in global climate dynamics and are vulnerable to future climate change we present a record of east african climate since 55 ka preserved in lake malawi sediments that indicates rapid shifts between discrete climate modes related to abrupt warming d o events observed in greenland although the timing of the malawi events cannot be determined exactly our age model implies that they occur prior to their greenland counterparts consistent with southward excursions of the intertropical convergence zone during greenland stadials the magnitude of each of the events recorded in malawi sediments corresponds to the scale of the subsequent greenland warming this suggests that a tropical component of climate sets a template for abrupt high northern latitude climate fluctuations associated with the bipolar seesaw

influence of terrestrial vegetation on leaf wax δd of holocene lake sediments abstract the hydrogen isotope composition of terrestrial plant leaf wax in sediments is increasingly used as a paleoclimatic indicator modern calibration studies suggest that paleoclimatic interpretation of leaf wax d values requires consideration of the differences in the apparent fractionation of hydrogen isotopes among different groups of plants however it is not common that paleoecological data are used to help interpret leaf wax d profiles here we assess the relative importance of factors influencing millennial scale shifts in d values of n alkanoic acids at steel lake minnesota usa an extensively studied site with independent records of vegetation composition d of input water to the lake and evaporation the d values of the n c 28 alkanoic acid d c28 vary between 190 and 168 and do not correlate with d of input water or the extent of evaporation however d c28 is negatively correlated with the 13 c values of the n c 28 alkanoic acid 13 c c28 the correlation along with pollen assemblage and carbonate 13 c records suggests that holocene shifts between forest and grassland and or in the water use efficiency of c 3 plants influenced the stratigraphic variation in leaf wax d thus paleoecological information such as that inferred from pollen assemblages and carbon isotopes of plant derived compounds may aid paleoclimatic interpretation of leaf wax d profiles from lake sediments

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

large temperature variability in the southern african tropics since the last glacial maximum 1 the role of the tropics in global climate change is actively debated particularly in regard to the timing and magnitude of thermal and hydrological response continuous high resolution temperature records through the last glacial maximum lgm from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments here we used the tex86 paleotemperature proxy to reconstruct mean annual lake surface temperatures through the last glacial maximum lgm in lake malawi east africa 9 14 s we find a 3 5 c overall warming since the lgm with temperature reversals of 2 c during the younger dryas 12 5 ka bp and at 8 2 ka bp maximum holocene temperatures of 29 c were found at 5 ka bp a period preceding severe drought in africa these results suggest a substantial thermal response of southeastern tropical africa to deglaciation and to varying conditions during the holocene

temporal variation of multifractal properties of seismicity in the region affected by the mainshock of the october 6 2000 western tottori prefecture japan earthquake m 7 3 1 the temporal variation of multifractal properties in spatial distribution of seismicity in the region affected by the main shock of the western tottori prefecture japan earthquake of october 6 2000 m 7 3 and its aftershock sequence are investigated we use earthquake catalog data from 1985 to 2001 in order to understand the process and mechanism preceding a large shallow earthquake a monotonous decrease of multifractal dimension dq with a wide range of q appeared clearly for about two years prior to this large shallow earthquake dq then increased to a maximum at the time of the main shock during the aftershock sequence dq oscillated between 2 0 and 0 7 the decrease and then increase of dq with respect to the spatial distribution of seismic events indicate clustering and declustering in multifractal structure respectively the temporal variation of dq reflects the accumulation and release of strain energy within the tectonic stress field

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction

vegetation response to glacial interglacial climate variability near lake malawi in the southern african tropics abstract pollen records from lake malawi africa spanning the last 135 kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity between 135 and 75 ka peaks in both the relative abundance and total production of podocarpus pollen define the first two of these drought episodes from 135 to 127 and again from 117 to 105 kyr bp podocarpus percentages remain above 16 with peak values as high as 38 indicating a period marked by a cool climate resulting in expansion of montane forest taxa to lower elevations marine palynological records from the angola margin and congo fan show similar peak podocarpus percentages at this time oxygen isotope stage 5d indicating a similar climate across the african continent at this latitude from 105 to 90 ka continuing drought resulted in total pollen accumulation rates in lake malawi to fall to less than 300 grains cm2 yr of predominately grass pollen this episode in african history was severe enough to cause the disappearance of pteridophytes and forest taxa such as uapaca and brachystegia as well as montane taxa podocarpus olea spp and ericaceae within the pollen source area of lake malawi these taxa all remain nearly absent from the surrounding vegetation for the next 18 000 years the resultant semi desert vegetation would have been inhospitable for early humans living within or traveling through the lake malawi region increasing moisture following these arid intervals allowed expansion creation and maintenance of a more diverse landscape vegetation mosaic around lake malawi including zambezian miombo woodland humid evergreen woodland and afromontane forests the relative abundance of each fluctuated in response to either cooling i e afromontane expansion from 60 to 56 ka or moisture balance i e increasing humid evergreen woodland between 75 and 65 ka notably there was no significant change in vegetation composition during the last glacial maximum lgm 30 15 ka as compared to the previous 20 000 years