applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

evidences for cessation of indian plate subduction in the burmese arc region the issue of whether subduction is still active in the india burma plate boundary zone has been rather controversial while the presence of an eastward dipping indian lithospheric slab is undisputed different opinions have been voiced regarding the continuance of subduction at present analysis of the harvard cmt data in comparison with major subduction zones of the world demonstrates that the burmese arc is a unique region where there is a subducted slab but the direction of plate motion is nearly perpendicular to the down dip direction we propose a major right lateral shearing of the indian plate along with its subducted slab past the burmese plate in the nne direction

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

high resolution monsoon precipitation changes on southeastern tibetan plateau over the past 2300 years abstract climate changes on southeastern tibetan plateau have important impacts on social and economic development as well as the ecosystem of southwestern china and indo china peninsula here we collected two stalagmites from shenqi miraculous cave in southern sichuan china the stalagmite 18o record shows good coherence with local instrumental rainfall record as well as tree ring and pollen based moisture reconstructions from southeastern tibetan plateau during overlapped time periods as a result we reconstructed high resolution 4 9 yrs monsoon precipitation variations on southeastern tibetan plateau over the past 2300 years by using the combined stalagmite 18o record the result reveals an overall decreasing precipitation trend with two most notable wet periods occurred in 60 280 ad and 370 510 ad the most remarkably dry period is the recent 200 years some decadal scale wet and dry intervals were also identified the abnormal drought during 1160 1245 ad might have accelerated dali kingdom s demise at 1253 ad power spectrum analysis indicated significant 373 187 22 12 and 11 yr cycles in our stalagmite record suggesting the impact of solar activity increased monsoon precipitation on southeastern tp was observed in solar activity minima during the last millennium we further synthesized an integrated precipitation record for southwestern china and discussed spatial patterns of precipitation over china during the last two millennia the comparisons confirm a dry southern and wet northern pattern in monsoonal china during the medieval warm period and a wet southern and dry northern pattern during the little ice age and dark age cold period solar activity the strength of westerly jet and summer monsoon as well as the sst of tropical indo pacific might play important roles on the rainfall spatial patterns over monsoonal china during the last 2000 years

precipitation at lake qinghai ne qinghai tibet plateau and its relation to asian summer monsoons on decadal interdecadal scales during the past 500 years knowledge of the variability of precipitation at lake qinghai and its relation to the asian summer monsoons is helpful in constraining global climatic dynamics based on the high resolution precipitation indicators of 13 c of the organic matter 13 corg c n atomic ratio and the detrended total organic carbon content tocdetrended we found that the trend of precipitation at lake qinghai is inversely correlated to that of the indian summer monsoon ism inferred from layer thickness of a stalagmite s3 in southern oman on decadal interdecadal scales the chinese drought flood d f indices which can indicate the dryness wetness over large geographic areas are also used to indicate the intensity of the monsoon rainfall the d f index of xining near lake qinghai is synchronous with those of the regions in northern china where the east asian summer monsoon easm dominates while it is anti phase with those of southwestern china where ism prevails these materials suggest that during the past 500 years the source of moisture to lake qinghai on decadal interdecadal scales is controlled mainly by the easm but not by the ism it is also suggested that the intensity of easm is inversely related to that of the ism on decadal interdecadal scales the decadal interdecadal variability of enso may be responsible for the inverse relationship between the intensity of easm and that of ism 2007 elsevier b v all rights reserved

a 780 year annually resolved record of indian ocean monsoon precipitation from a speleothem from south oman 1 meteorological records of monsoon rainfall in the indian ocean are generally less than 100 years long the relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales to determine what factors influence the intensity of rainfall on these timescales or to place possible changes in the twentieth century into a broader historical context development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward we have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern arabia our results show that monsoon variation over the past century is not outside of the range of the past 800 years decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the indian ocean spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales decadal to interannual cycles in the record appear to originate in the tropical pacific ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 3354 meteorology and atmospheric dynamics precipitation 1854 1620 global change climate dynamics 3309 1699 global change general or miscellaneous keywords monsoon paleoclimate rainfall indian ocean speleothem oxygen isotopes

the evolution of el nino past and future abstract we review forecasts of the future of el nino and the southern oscillation enso a coupled instability of the ocean atmosphere system in the tropical pacific with global impacts enso in the modern world is briefly described and the physics of the enso cycle is discussed particular attention is given to the bjerknes feedback the instability mechanism which figures prominently in enso past and future our knowledge of enso in the paleoclimate record has expanded rapidly within the last 5 yr the enso cycle is present in all relevant records going back 130 kyr it was systematically weaker during the early and middle holocene and model studies indicate that this results from reduced amplification in the late summer and early fall a consequence of the altered mean climate in response to boreal summer perihelion data from corals shows substantial decadal and longer variations in the strength of the enso cycle within the past 1000 yr it is suggested that this may be due to solar and volcanic variations in solar insolation amplified by the bjerknes feedback there is some evidence that this feedback has operated in the 20th century and some model results indicate that it will hold sway in the greenhouse future but it is very far from conclusive the comprehensive general circulation models used for future climate projections leave us with an indeterminate picture of enso s future some predict more enso activity some less with the highly uncertain consensus forecast indicating little change

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

holocene climatic fluctuations and periodic changes in the asian southwest monsoon region abstract climatic changes in the asian southwest monsoon aswm during the holocene have become a topic of recent studies it is important to understand the patterns and causes of holocene climatic changes and their relationship with global changes based on the climate proxies and wavelet analysis of lugu lake in the aswm region the climatic fluctuations and periodic changes in the aswm region during the holocene have been reconstructed with a high precision chronology the results indicate the intensification of aswm began to increase with northern hemisphere low latitude solar insolation lsi and solar activity during the early holocene and gradually decreased during the late holocene exhibiting an apparent synchrony with numerous records of aswm region meanwhile an apparent 1000 a quasi periodic signal is present in the environment proxies and it demonstrates that the environmental change in the aswm region has been driven mainly by lsi and solar activity

paleomagnetic constraints on the late cretaceous and cenozoic tectonics of southeastern asia abstract many features of the cenozoic tectonic history of central and southeastern asia can be understood as direct consequences of the thrust and penetration of india into asia recent indentation experiments with plasticine tapponnier et al 7 have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism the available cretaceous and cenozoic paleomagnetic data from this part of the world have been reviewed and a new apwp for eurasia has been constructed for reference the negligible rotation of south china and large clockwise rotation of indochina are consistent with the model i e with an history of large scale left lateral strike slip motion along the altyn tagh and red river faults data from malaya and borneo can be reconciled with the model although in a less straightforward fashion the large counter clockwise rotation of south tibet implies that it rotated in sympathy with india during the collision and suggests that future indentation experiments should include this feature finally a middle cretaceous reconstruction of the south margin of asia is proposed one interesting result is the restored continuity of geological features in tibet and indochina with active subduction of oceanic indian plate crust taking place to the south at subtropical latitudes

teleseismic shear wave splitting in se tibet insight into complex crust and upper mantle deformation we measured shear wave splitting of teleseismic xks phases i e sks skks and pks recorded by more than 300 temporary chinarray stations in yunnan of se tibet the first order pattern of xks splitting measurements shows that the fast polarization directions change at 26 27 n from dominant n s in the north to e w in the south while splitting observations around the eastern himalayan syntax well reflect anisotropy in the lithosphere under left lateral shear deformation the dominant e w to the south of 26 n is consistent with the maximum extension in the crust and suggest vertically coherent pure shear deformation throughout the lithosphere in yunnan however the thin lithosphere 80 km could account for only part 0 7 s of the observed splitting delay times t 0 9 1 5 s anisotropy in the asthenosphere is necessary to explain the nw se and nearly e w in these regions the ne sw can be explained by the counter flow caused by the subduction and subsequent retreat of the burma slab the e w is consistent with anisotropy due to the absolute plate motion in se tibet and the eastward asthenospheric flow from tibet to eastern china accompanying the tectonic evolution of the plateau our results provide new information on different deformation fields in different layers under se tibet which improves our understanding on the complex geodynamics related to the tectonic uplift and southeastward expansion of tibetan material under the plateau

temperature responses to quasi 100 yr solar variability during the past 6000 years based on δ18o of peat cellulose in hongyuan eastern qinghai tibet plateau china abstract during the past 6000 years the temperature variation trend inferred from 18o of peat cellulose in a peat core from hongyuan eastern qinghai tibet plateau southwestern china is similar to the atmospheric 14c concentration trend and the modeled solar output trend the general trend of hongyuan 18o during the past millennium also coincides well with the atmospheric 14c concentration trend the 10be concentration trend in an ice core from the south pole the reconstructed total solar irradiance trend as well as the modeled solar output trend in addition temperature events also correspond well to solar perturbations during the past 6000 years therefore the driving force of holocene temperature variations should be properly ascribed to solar activity the spectrum analysis further illustrates that quasi 100 yr fluctuation of solar activity was probably responsible for temperature variations in northeast qinghai tibet plateau during the past 6000 years

stable isotopes in bulk carbonates and organic matter in recent sediments of lake qinghai and their climatic implications abstract multi proxy indices on annual decadal scales during the past 600 years were developed from the surface sediments of lake qinghai based on 210pb and 137cs geochronology the 13ccarb 18ocarb and total carbonate content are consistent with one another in trends and their variations have been mainly ascribed to regional temperature it is suggested that the strong evaporation contributes in modifying 18o of lake water and 13c of the dissolved inorganic carbon of the surface water and appears to be responsible for the covariance between 13ccarb and 18ocarb photosynthesis of the aquatic plants appears to have played a significant role in determining 15n of organic matter the discrimination of 14n and 15n during photosynthesis may have triggered a positive linkage between temperatures and 15norg after removal of the stratigraphic trend variations of total organic carbon toc c n ratio and 13corg have been ascribed to local precipitation toc after removal of the stratigraphic trend is positively correlated with the c n ratio r 0 45

correlation between indian ocean summer monsoon and north atlantic climate during the holocene there has been a number of investigations for the correlation between the asia monsoon and the north atlantic climate for the last glacial however little research has been done for the present interglacial the holocene here we present for the first time a high resolution composite proxy record for the indian ocean summer monsoon spanning around 12 000 years based on the 13c time series of both a single plant species carex mulieensis remains cellulose and the total plant assemblage cellulose in the hongyuan peat bog from the tibet plateau the records show that the strength of the indian ocean summer monsoon had abrupt variations during the last 12 000 years the weakest monsoon occurred in the younger dryas period following rapid strengthening from around 11 200 to 10 800 a bp the monsoon kept a generally strong level for around 5300 years from around 5500 a bp onwards the monsoon strength tended to gradual decrease in addition there are a series of abrupt variation events of the monsoon strength on centennial to millennial time scales which superimpose the general tendency of the monsoon variation in every case when the ice rafted debris events in the north atlantic occurred the summer monsoon strength decreased correspondingly these evidences show that teleconnection between the indian ocean summer monsoon and the north atlantic climate is present not only in the last glacial but also in the holocene which may be linked to abrupt reorganizations of the ocean thermohaline circulation leading to redistribution of energy changing temperature and moisture gradient over the southern subtropical indian ocean and eventually controlling the variability of the indian ocean summer monsoon

a 2 326 year tree ring record of climate variability on the northeastern qinghai tibetan plateau 1 high resolution climate proxy records covering the last two millennia on the qinghai tibetan plateau are scarce yet essential to evaluation of the patterns synchroneity and spatial extent of past climatic changes including those in the medieval warm period mwp and the little ice age lia here we present a 2326 year tree ring chronology of sabina przewalskii kom for dulan area of northeastern qinghai tibetan plateau we find that the annual growth rings mainly reflect variations in regional spring precipitation the greatest change in spring precipitation during the last two millennia seems to occur in the second half of the 4th century the north atlantic mwp was accompanied by notable wet springs in the study region during a d 929 1031 with the peak occurring around a d 974 three intervals of dry springs occurred in the period of lia our tree ring data will facilitate intercontinental comparisons of large scale synoptic climate variability for the last two millennia

paleomagnetic study of cretaceous rocks from pu er western yunnan china evidence of internal deformation of the indochina block abstract cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the jingxing and nanxin formation around pu er 23 0 n 101 0 e western part of yunnan province china in order to examine quantitative amount of tectonic deformation of the simao terrane due to collision of india with asia after stepwise thermal demagnetization the high temperature component magnetization with unblocking temperature of around 680 c is isolated positive field tests show that the high temperature component magnetization from 25 sites is primary for the cretaceous formations a tilt corrected mean direction is d 59 9 i 45 2 with 95 5 1 corresponding to a paleopole at 35 8 n 173 1 e with a 95 5 6 comparison with cretaceous paleomagnetic pole of the whole china block indicates that amount of clockwise rotation in the pu er area is evaluated to be 46 9 with respect to the whole china block which is consistent with large amount of rotation in the mengla and jinggu areas 47 and 66 in the central part of the simao terrane clockwise rotation of the central part is larger by more than 20 compared to the yunlong and yongping areas of northern part of the simao terrane suggesting that the simao terrane experienced internal deformation the southward displacement of the pu er area expected from inclination data is 7 5 6 6 with respect to the chuan dian fragment which is a similar estimate to other areas in the simao terrane and the indochina block while the simao terrane displaced southward by 800 km together with clockwise rotation of 30 as a whole further clockwise rotation occurred in the central part

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

temperature variations since 1750 ce inferred from an alpine lake in the southeastern margin of the tibetan plateau abstract high altitude regions are thought to be very sensitive to global climate change especially under current global warming however due to the lack of paleoclimate archives such as ice core and tree rings above the modern tree line limited paleoclimate data have been extracted to extend instrumental data over the past few centuries which will help understand the interannual to decadal scale climate variability in high altitude regions here we present a high resolution 1 6 yr quantitative temperature record from a remote alpine lake at the southeastern margin of the tibetan plateau by applying a novel proxy based on branched glycerol dialkyl glycerol tetraethers brgdgts contrary to the often documented warming trend over the past few centuries but consistent with temperature record from the northern tibetan plateau our data show a gradual decreasing trend of 0 3 c in mean annual air temperature from 1750 to 1970 ce this result suggests a gradual cooling trend in some high altitude regions over this interval which could provide a new explanation for the observed decreasing asian summer monsoon in addition our data indicate an abruptly increased interannual to decadal scale temperature variations of 0 8 2 2 c after 1970 ce in terms of both magnitude and frequency indicating that the climate system in high altitude regions would become more unstable under current global warming

does subduction induced mantle flow drive backarc extension abstract backarc extension is a characteristic feature of many narrow subduction zones seismological and geochemical studies imply the occurrence of mantle flow around the narrow subducting slabs previous 3d models suggested that backarc extension is related to subduction induced toroidal mantle flow the physical viability of this mechanism however has never been tested using laboratory based geodynamic models in this work we present dynamic laboratory models of progressive subduction in three dimensional 3d space that were carried out to test this mechanism to achieve this we have used a stereoscopic particle image velocimetry spiv technique to map simultaneously overriding plate deformation and 3d subduction induced mantle flow underneath and around an overriding plate the results show that the strain field of the overriding plate is characterized by the localization of an area of maximum extension within its interior at 300 500 km from the trench the position of maximum extension closely coincides within 2 cm scaling to 100 km with that of the maximum trench normal horizontal mantle velocity and velocity gradient measured at a scaled depth of 15 25 km below the base of the overriding plate and the maximum horizontal gradient of the vertical mantle velocity gradient we propound that in narrow subduction zones backarc extension in the overriding plate is mainly a consequence of the trench normal horizontal gradients of basal drag force at the base of the overriding plate such shear force gradients result from a horizontal gradient in velocity in the mantle below the base of the lithosphere induced by slab rollback calculations based on our models indicate a tensional horizontal trench normal deviatoric stress in the backarc region scaling to 28 8 mpa while the overriding plate trench normal stress resulting from the horizontal component of the trench suction force is about an order of magnitude smaller scaling to 2 4 3 6 mpa

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

seismic anisotropy in eastern tibet from shear wave splitting reveals changes in lithospheric deformation abstract knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions here we focus on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china we measured shear wave splitting of core refracted phases sks and skks at a temporary array of 25 iris passcal stations we calculated splitting parameters using a multi channel and a single record cross correlation method multiple layers of anisotropy cannot be ruled out but are not required by the data a fresnel zone analysis suggests that the shallow mantle between 60 and 160 km depth is the most likely source of anisotropy the polarization directions reveal a pronounced transition from primarily north south in the north sichuan to mostly east west orientations in the south yunnan in the southern part of the study region that is south of 26 n the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields whereas gps campaigns provide evidence suggesting north south crustal flow across the red river fault the pattern of anisotropy argues against such flow in the upper mantle these observations support models that allow differential movement of upper crust relative to lithospheric mantle in the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded the interpretation of the shear wave splitting results is non unique but we suggest that the observed n s transition reflects a fundamental change in deformation regime across our study region it may be related to lateral variations in lithospheric rheology or may mark a transition from the direct impact of the continental collision to dominance of the far field strain field associated with regional subduction processes understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of eastern tibet

relation between eurasian snow cover snow depth and the indian summer monsoon an observational study abstract satellite derived snow cover data for 22 yr and snow depth data for 9 yr over eurasia have been analyzed to reexamine the possible relation of snow with the indian summer monsoon in contrast to the previous studies that use snow cover averaged over all of eurasia as a single number the frequency of occurrence of snow at each grid point over eurasia is correlated with the indian summer monsoon rainfall thus specific geographical regions over eurasia that are responsible for the well known inverse relationship between eurasian snow cover and indian monsoon rainfall are delineated it is found somewhat surprisingly that western eurasia is the only geographical region for which a significant inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall however composites for high and low snow cover over eurasia show spatially homogeneous large scale patterns of snow cover and surface temperature anomalies winters of high and low snow cover for eurasia are found t

late holocene indian summer monsoon variations recorded at lake erhai southwestern china in this study we report changes in indian summer monsoon ism intensity during the past 3500 yr inferred from proxy indices at lake erhai southwestern china both the pollen concentrations and other proxy indices including sediment grain size total organic carbon contents toc and elemental contents e g fe al clearly indicate a long term decreasing trend in ism intensity over the late holocene during the period from approximately ad 750 to ad 1200 pollen concentrations of conifer and broadleaf trees and herbs reached the lowest levels over the past 3500 yr while the pollen percentages of both herbs and broadleaf trees increased suggesting a significant medieval drought the grain size toc and elemental contents also support an arid climate during the medieval period the little ice age lia at lake erhai was characterized as cold and wet the medieval and lia climatic patterns at lake erhai were similar to those over most of the ism areas but anti phase with those over east asian summer monsoon easm areas we suspect that sea surface temperature variations in the indo pacific oceans and the related land sea thermal contrasts may be responsible for such hydroclimatic differences between easm and ism areas

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

tectonic deformation of the indochina peninsula recorded in the mesozoic palaeomagnetic results summary in order to describe features of tectonic deformation in the indochina peninsula early jurassic to early cretaceous red sandstones were sampled at three localities in the shan thai and indochina blocks stepwise thermal treatment of most samples revealed the presence of characteristic remanent magnetization which is generally unblocked by 680 c this component from phong saly 21 6 n 101 9 e and borikhanxay 18 5 n 103 8 e localities yield positive fold tests with late jurassic early cretaceous directions of dec inc 28 8 32 1 ks 15 4 95 8 8 n 22 and dec inc 42 1 46 9 ks 20 1 95 7 9 n 18 respectively additionally a syn folding mid cretaceous characteristic magnetization is observed in the samples of muang phin locality 16 5 n 106 1 e which gave a mean direction of dec inc 30 8 39 9 k 102 6 95 3 0 n 23 this reliable late jurassic to mid cretaceous palaeomagnetic directions from three different localities are incorporated into a palaeomagnetic database for shan thai and indochina blocks based on these compilations tectonic deformation of the shan thai and indochina blocks is summarized as follows 1 the shan thai and indochina blocks experienced a clockwise rotation of about 10 as a composite unit in the early stage of india asia collision and 2 following this the shan thai block underwent an internal tectonic deformation whereas the indochina block behaved as a rigid tectonic unit during the same period comparison of our palaeomagnetic results with seismic tomographic images suggests that the strength of continental lithosphere beneath these blocks played an important role in the process of deformation rather than any other tectonic regime in contrast to the shan thai block an existence of continental roots beneath the indochina block prevented its internal deformation

different responses of sedimentary δ 15 n to climatic changes and anthropogenic impacts in lakes across the eastern margin of the tibetan plateau abstract knowledge of historical variations in sedimentary 15n values and their relation to modern ones in lakes is critical in understanding n cycling in inland aquatic systems here we present late holocene sedimentary 15n variations in four lakes across the eastern margin of the tibetan plateau etp namely lake qinghai and lake chaonaqiu over north etp n etp area and lake yihai and lake lugu over south etp s etp area the results show that 1 during historical periods when anthropogenic forcing was weak the centennial multi decadal changes in 15n values were dominated by climatic changes higher precipitation brought greater nutrients supply and led to higher lacustrine primary productivity lpp and higher 15n values in lake sediments and vice versa 2 during the recent modern epoch with enhanced anthropogenic impacts sedimentary 15n values in lake qinghai and lake chaonaqiu mid high latitude lakes with low lpp showed decreasing trends possibly related to the increased global atmospheric reactive nitrogen nr deposition on the contrary sedimentary 15n values in lake lugu and lake yihai subtropical lakes with high lpp showed increasing trends most likely because more dissolved inorganic nitrogen in lake water was used during lpp processes this study highlights that lpp plays an important role in modifying sedimentary 15n values both for historical and modern times whereas atmospheric nr deposition seems to influence the modern recent sedimentary 15n trends in lakes with low lpp

decadal multi decadal temperature discrepancies along the eastern margin of the tibetan plateau abstract knowledge of the synchronicity and discrepancy of temperature variations along the eastern margin of the tibetan plateau etp is critical in understanding the driving forcing of regional temperature variations in this study we established 15 n timeseries in organic matter and 13 c timeseries in ostracod shells from sediments of lake lugu and attributed their variations to decadal multi decadal temperature variations we compared temperature variations along the etp transect during the past four centuries based on our presently developed and previously developed temperature proxy indices as well as temperature variations reconstructed by other researchers we found that 1 over the north etp area n etp the decadal multi decadal variations in temperature correlate well with each other 2 over the south etp area s etp temperature variations correlate not so well with each other while those at south to west portion of the tibetan plateau are rather local 3 the decadal variations in temperature are generally synchronous with those in precipitation over the n etp area and they are broadly anti phase out of phase with the corresponding ones over the s etp area 4 the long term temperature and precipitation trends are coupling over the n etp but decoupling over the s etp we speculate that because the n etp is located at the frontier of the asian summer monsoon asm region temperature variations there are not as strongly influenced by the asm they are most likely dominated by changes in solar activities and show general similarity to the average of the northern hemisphere over the s etp area decadal temperature variations are obviously influenced by precipitation because the decadal multi decadal precipitation variations are anti phase and or out of phase between the n etp and s etp the decadal multi decadal temperature variations between these two regions are also anti phase and or out of phase

a uniform database of teleseismic shear wave splitting measurements for the western and central united states we present a shear wave splitting sws database for the western and central united states as part of a lasting effort to build a uniform sws database for the entire north america the sws measurements were obtained by minimizing the energy on the transverse component of the pks skks and sks phases each of the individual measurements was visually checked to ensure quality this version of the database contains 16 105 pairs of splitting parameters the data used to generate the parameters were recorded by 1774 digital broadband seismic stations over the period of 1989 2012 and represented all the available data from both permanent and portable seismic networks archived at the incorporated research institutions for seismology data management center in the area of 26 00 n to 50 00 n and 125 00 w to 90 00 w about 10 000 pairs of the measurements were from the 1092 usarray transportable array stations the results show that approximately 2 3 of the fast orientations are within 30 from the absolute plate motion apm direction of the north american plate and most of the largest departures with the apm are located along the eastern boundary of the western us orogenic zone and in the central great basins the splitting times observed in the western us are larger than and those in the central us are comparable with the global average of 1 0 s the uniform database has an unprecedented spatial coverage and can be used for various investigations of the structure and dynamics of the earth

shear wave anisotropy of the northeast indian lithosphere 1 analysis of sks skks phases in the northeast india region provides the first results using digital data from the southern side of the india asia collision zone our analysis reveals detectable anisotropy in the study region contrary to negligible anisotropy reported in southern tibet the direction of anisotropy is e w within the himalaya and its foredeep n s in the indo burma convergence zone and ne sw close to the shillong plateau while lithospheric deformation due to finite strain induced by collision seems to be the source of anisotropy in the himalaya and its foredeep an asthenospheric source is invoked as a causative for the mantle strain in the plate interior

palaeoclimatic changes in the qinghai lake area during the last 18 000 years abstract multi proxy analysis of a sediment core from qinghai lake including pollen carbonate toc tn and 13 c of organic matter was used to document regional climatic changes during the last 18 000 years climate was very cold and dry before 16 9 cal kyr bp the warm and wet period which began at about 14 1 cal kyr bp culminated at 6 5 cal kyr bp and came to its end at about 4 5 cal kyr bp after that the climate gradually became colder and drier during the transitional period from the late glacial to the holocene the climate fluctuated frequently the short term climatic oscillations including the bolling allerod younger dryas and the cooling event at ca 8 2 cal kyr bp appear to be recorded in the qinghai lake sedimentary record the climatic evolution in the qinghai lake area during the past 18 000 years indicates that solar insolation changes on the ten thousand year scale are the likely driving force of the east asia monsoon

a 700 year macrophyte productivity record inferred from isotopes of macrophyte remains and bulk carbonates in lake koucha northeast qinghai tibetan plateau abstract the isotopic composition of total organic carbon toc in lakes has been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments however the main factors that affect the carbon isotopes of toc may vary in different lake sediment records limiting the ability of organic carbon isotopes to explain biogeochemical and environmental changes especially in lakes with a large number of aquatic plants in this study the 13c values of macrophyte remains and bulk carbonate in a sedimentary core from lake koucha were investigated to evaluate their paleoenvironmental implications we found that the bulk carbonate was dominated by authigenic carbonates formed in the lake and that their 18o and 13c values can be used to reconstruct environmental changes in the lake koucha area the macrophyte productivity derived from the carbon isotopic offset between the macrophyte remains and the bulk carbonate eremains bc in the lake koucha area agree well with total solar irradiance tsi records inferred from 10be and temperature records based on tree rings from the qinghai tibetan plateau although the distribution of macrophytes is related to water depth according past studies and our observations in the field our findings suggest that changes in macrophyte productivity may be primarily driven by temperature changes in the lake koucha area

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

complex trajectories of aquatic and terrestrial ecosystem shifts caused by multiple human induced environmental stresses large shifts in the isotopic compositions of organic matter om in lake sediments over the last few hundred years are commonly interpreted as representing changes in photosynthetic productivity corresponding to eutrophication or in the input of terrestrial om due to human disturbances based on multiple proxy data c n ratio d 13 c and d 15 n of om d 13 c of calcite lithology and fossil pollen from a 700 year sediment core at white lake new jersey usa we propose a new explanation that relates these large shifts in om d 13 c and d 15 n to human induced changes in aquatic om producers combined records of geochronology fossil pollen and lithology from white lake reveal that the upland forest was cleared by european settlers for farmland beginning around 1745 a d and has gradually reforested since 1930 after the abandonment of the farmlands for the pre agricultural period om had relatively constant but extremely low d 13 cvpdb 35 8 to 34 5 and d 15 nair 3 5 to 2 5 and high atomic c n ratios 13 7 to 16 7 indicating a stable anoxic lake environment with prominent microbial producers following the human disturbance since 1745 high om mass accumulation rates and abundances of the green alga pediastrum indicate an increase in aquatic photosynthetic productivity due to enhanced nutrient input from disturbed uplands however carbonate d 13 c remains constant or even decreases during this period implying that increasing productivity did not elevate the d 13 c of dissolved inorganic carbon and thus cannot explain the observed large increase in om d 13 c 7 4 and d 15 n 5 8 over this period instead d 13 c d 15 n and c n ratios of om and differences in d 13 c between calcite and om suggest that the large increase in om d 13 c and d 15 n can be attributed to a human induced ecological shift in the predominant organic source from anaerobic bacteria to autotrophic phytoplankton during the post agricultural period mass accumulation rates of om carbonate and silicate and the d 13 c of om and calcite all decreased significantly corresponding to stabilization of the uplands however over the last 70 years an intensifying aquatic stress from the deposition of 15 n enriched industrial pollutants has resulted in a steady increase of 1 9 in d 15 n proxy records for lake d 13 c and d 15 n

alkenone based reconstruction of late holocene surface temperature and salinity changes in lake qinghai china china the alkenone temperature proxy u k 0 37 indicates up to a 1 c change in mean annual air temperature or a 2 c change in summer lake water temperature during the late holocene oscillating warm and cold periods could be related to the 20th century warm period the little ice age the medieval warm period the dark ages cold period and the roman warm period the relative importance of c37 4 alkenone to total c37 alkenone production c37 4 fluctuated between 15 45 with higher values during warm periods suggesting that lake water was also fresher during these periods the coupled late holocene surface temperature and salinity changes suggest that asian monsoons strongly influenced the climate of the lake qinghai region citation liu z a c g henderson and y huang 2006 alkenone based reconstruction of lateholocene surface temperature and salinity changes in lake qinghai china geophys res lett 33 l09707 doi 10 1029 2006gl026151

indochina peninsula and the collision of india and eurasia the collision of india with asia results in a rapidly changing stress pattern around both himalayan syntaxes the maximum compressive stress which is north south north of the indenter fans radially through nearly 180 around the syntaxes to accommodate the necessary strike slip motion on the edges of the indenter thus the whole indochina peninsula was swept by the migrating stress and associated strain pattern as the eastern syntaxis moved to the north as a result the maximum horizontal component of the stress field is predicted to have been first oriented east west then north south we propose that this changing stress field controlled the opening of the sedimentary basins and of the south china sea as well as the direction of motion of the major strike slip faults

evidence for solar forcing on the indian monsoon during the last millennium abstract solar influence on the intensity of the indian monsoon is demonstrated using a sediment core from the eastern arabian sea dating back to 1200 yr through pattern matching as well as spectral analysis of proxy records of monsoon and solar activity the intensity of the indian monsoon is found to have decreased during periods of solar minima during the last millennium periodicities of 200 20 105 15 and 60 10 yr are observed in the proxy records coinciding with those known for solar cycles the 60 yr periodicity observed in the instrumental rainfall data appears to be of solar origin and supports the hypothesis of solar control on the indian monsoon on a multi decadal time scale evidence for the presence of a coupled atmospheric forcing for the indian and east african monsoons on a centennial time scale is also seen

earthquake occurrence processes in the indo burmese wedge and sagaing fault region abstract earthquakes in the indo burmese wedge and sagaing fault regions occur in response to the partitioning of the india sunda motion along these two distinct boundaries under the accretionary wedge of the indo burmese arc majority of the earthquakes occur in the depth range of 30 60 km and define an eastward gently dipping seismicity trend surface that coincides with the indian slab the dip of the slab steepens in the east direction and earthquakes occur down to a depth of 150 km though the slab can be traced up to the 660 km discontinuity although these features are similar to a subduction zone the nature of the earthquakes and our analysis of their focal mechanisms suggest that these earthquakes are of intra slab type which occur on steep plane within the indian plate and the sense of motion implies a northward relative motion with respect to the sunda plate thus these earthquakes and the stress state do not support active subduction across the indo burmese arc which is also consistent with the relative motion of india sunda plates the absence of inter plate earthquakes lack of evidence of the occurrence of great earthquakes in the historical records and non seismogenic nature of the plate interface under the accretionary wedge suggest that seismic hazard due to earthquakes along the plate boundary may be relatively low however major intra slab earthquakes at shallow and intermediate depths may still cause damage in the sediment filled valley regions of manipur and cachar in india and chittagong and sylhet regions of bangladesh in the sagaing fault region earthquakes occur through dextral strike slip motion along the north south oriented plane and the stress state is consistent with the plate motion across the sagaing fault

seismic anisotropy and mantle dynamics beneath china abstract we analyzed the shear wave splitting at 138 permanent seismograph stations to study seismic anisotropy and mantle dynamics under mainland china to obtain reliable results we used three different methods to measure the shear wave splitting parameters using core phases sks skks skiks and pks as well as the direct s waves from regional and distant earthquakes our results show that the fast orientations of the anisotropy wnw ese in eastern china are generally consistent with the absolute plate motion apm direction of the eurasian plate suggesting that the anisotropy is mainly located in the asthenosphere resulting from the lattice preferred orientation of olivine due to the shear deformation there the fast axes in western china generally agree with the strikes of the orogens and active faults while they are perpendicular to the direction of the maximum horizontal stress suggesting that the anisotropy in the lithosphere contributes significantly to the observed shear wave splitting the fast axes in western china are also consistent with the apm direction suggesting that the apm driven anisotropy in the asthenosphere is another source of the shear wave splitting there these results suggest that apm driven anisotropy commonly exists under continents similar to that under oceanic regions even though the continental lithosphere has suffered extensive deformation

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

mantle flow in subduction systems the mantle wedge flow field and implications for wedge processes 1 the mantle wedge above subducting slabs is associated with many important processes including the transport of melt and volatiles our understanding of mantle wedge dynamics is incomplete as the mantle flow field above subducting slabs remains poorly understood because seismic anisotropy is a consequence of deformation measurements of shear wave splitting can constrain the geometry of mantle flow in order to identify processes that make first order contributions to the pattern of wedge flow we have compiled a data set of local s splitting measurements from mantle wedges worldwide there is a large amount of variability in splitting parameters with average delay times ranging from 0 1 to 0 3 s up to 1 0 1 5 s and large variations in fast directions we tested for relationships between splitting parameters and a variety of parameters related to subduction processes we also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge we find that no simple model can explain all of the trends observed in the global data set mantle wedge flow is likely controlled by a combination of downdip motion of the slab trench migration ambient mantle flow small scale convection proximity to slab edges and slab morphology with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology there is also a likely contribution from b type olivine and or serpentinite fabric in many subduction zones governed by the local thermal structure and volatile distribution

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

subduction triggered magmatic pulses a new class of plumes article i nfo a variety of atypical plume like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed and seismological imaging has shown ubiquitous small scale convection in the uppermost mantle in regions such as the mediterranean region the western us and around the western pacific we argue that the three dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics the testable surface expressions of this process are the outside arc alkaline volcanism topographic swell and low velocity seismic anomalies associated with partial melt using three dimensional simplified numerical subduction models we show that focused upwellings can be generated both ahead of the slab in the back arc region though five times further inward from the trench than arc volcanism and around the lateral edges of the slab in the order of 100 km away from slab edges vertical mass transport and by inference the associated decompression melting in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities the upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation we discuss representative case histories from the pacific and the mediterranean where we find possible evidence for such slab related volcanism

aseismic plate boundary in the indo burmese wedge northwest sunda arc plate motion crustal deformation and earthquake occurrence processes in the northwest sunda arc which includes the indo burmese wedge ibw in the forearc and the sagaing fault in the backarc are very poorly constrained plate reconstruction models and geological structures in the region suggest that subduction in the ibw occurred in the geological past but whether it is still active and how the plate motion between the india and sunda plates is partitioned between motion in the ibw and sagaing fault is largely unknown recent gps measurements of crustal deformation and available long term rates of motion across the sagaing fault suggest that 20 3 mm yr of the relative plate motion of 36 mm yr between the india and sunda plates is accommodated at the sagaing fault through dextral strike slip motion we report results from a dense gps network in the ibw that has operated since 2004 our analysis of these measurements and the seismicity of the ibw suggest that the steeply dipping churachandpur mao fault in the ibw accommodates the remaining motion of 18 2 mm yr between the india and sunda plates through dextral strike slip motion and this motion occurs predominantly through velocity strengthening frictional behavior i e aseismic slip the aseismic motion on this plate boundary fault significantly lowers the seismic hazard due to major and great interplate earthquakes along this plate boundary

tree ring based spring temperature patterns over the past four centuries in western himalaya a network of 12 tree ring width chronologies of himalayan cedar cedrus deodara from the western himalayan region india has been used to reconstruct mean spring march may temperature variations back to a d 1600 the most conspicuous feature of the temperature reconstruction is the long term cooling trend since the late 17th century that ended early in the 20th century the warmest 30 yr mean for the 20th century was recorded during 1945 1974 however this warming in the context of the past four centuries is well within the range of natural variability since warmer springs of greater magnitude occurred in the later part of the 17th century 1662 1691

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

driving the upper plate surface deformation by slab rollback and mantle flow the relative contribution of crustal and mantle processes to surface deformation at convergent plate margins is still controversial conflicting models involving either extrusion mechanisms or slab rollback in particular were proposed to explain the surface strain and kinematics across the tethyan convergent domain here we present new high resolution 3d thermo mechanical numerical joint models of continental collision oceanic subduction and slab tearing which for the first time allow self consistent reproduction of first order tethyan tectonic structures such as back arc rifting and large scale strike slip faults accommodating continental escape these models suggest that mantle flow due to slab rollback and tearing can modulate the surface strain and kinematics by locally enhancing trench retreat and dragging the upper plate from below these results highlight the active role of the asthenospheric flow in driving the surface strain not only by modulating the vertical stresses and producing dynamic topography but also through sub horizontal motion we discuss the implications of these findings based on observations across the aegean anatolian and eastern indian eurasian domains though similar considerations may as well apply to other settings

late glacial erosion and pedogenesis dynamics evidence from high resolution lacustrine archives and paleosols in south bohemia czech republic abstract sediments from three paleolakes and two paleosol horizons in south bohemia czech republic provide evidence of climate change and landscape evolution in central eastern europe on millennial to centennial timescales over the late glacial 16 11 5 ky based on a combination of geochemical sedimentological and geophysical proxy indicators along with the pollen record and soil micromorphology we propose a relationship between vegetation cover soil development and erosional processes four major and two minor environmental stages identified in all investigated paleo lakes were broadly correlated with the late glacial climatostratigraphy short term decadal to centennial climatic deteriorations between the bolling and allerod and within the allerod have been correlated with the older dryas and the intra allerod cold period iacp respectively b horizons of two gleyic podzols discovered under aeolian sand dunes in the lake catchments were dated to the allerod interstadial and were parallelized with usselo soils pedostratigraphical marker horizons of west and northern central europe the upper parts of these soils have signs of colluvial processes according to the radiocarbon dating the erosion occurred at 13 155 150 cal yr bp and can be associated with the iacp event which is marked by a significant input of allogenic material into the lake basins we attribute the significant increase in the iron and consequent phosphorus content in the lake sediments during the allerod to the podzolization that occurred with the humid interstadial conditions

complex slab subduction beneath northern sumatra 1 new data provided by the 2004 2005 sumatra andaman great earthquake sequences allow us to image with improved detail the p wave velocity structure beneath sumatra and adjacent regions below northern sumatra we find that the slab is folded at depth exhibiting geometry similar to that of the volcanic arc and the trench at the surface we speculate that this fold plays a major role in the segmentation of the sumatra megathrust and may impede rupture propagation in the region north of sumatra significant slab material in the mantle transition zone is imaged for the first time and we infer the presence of a major tear between the upper mantle and transition zone there

production sedimentation and isotopic composition of organic matter in lake ontario organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the rochester basin of lake ontario to evaluate their usefulness in reconstructing historic changes in lake productivity the greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events indicating that calcite precipitation is an effective mechanism for sedimenting organic matter carbon isotopes of organic matter were low prior to the onset of stratification increased to maximum values in late summer and then decreased following fall overturn this pattern is controlled mainly by the timing of stratification and primary productivity which preferentially removes 12co2 from the epilimnion the physiological effect of decreased carbon isotopic fractionation with decreasing supplies of co2 aq may have also contributed to increased 13corgc nitrogen isotopes showed a seasonal pattern opposite to that of carbon whereby 15n values were low during the summer stratified period and high for the remainder of the year seasonal variability in 15norgn probably reflects changes in the source of sedimented organic particles which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period a comparison of organic carbon accumulation rates and 13corgc between sediment cores collected in 1987 and 1993 1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters but will not change the 13corgc values all cores analyzed for 13corgc display the reproducible pattern of a progressive increase in the 19oos peaking in the early to mid 1970s and then decreasing to the present this pattern matches the historical trends of phosphorus loading to the basin suggesting that 13c of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column the 15n of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0 3ppt per decade and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment the increase in 15norgn reflects a combination of factors including early forest clearance by europeans increased sewering by municipalities after 1940 and increased nitrate utilization as productivity increased in the lower great lakes increased rates of denitrification in the central basin of upstream lake erie from the 1930s to the early 1970s may have also contributed to the rise in 15norgn values

asian monsoon oscillations in the northeastern qinghai tibet plateau since the late glacial as interpreted from visible reflectance of qinghai lake sediments abstract qinghai lake is a large saline lake on the qinghai tibetan plateau of central asia that is effected by both the indian and asian monsoons we used reflectance spectroscopy to characterize the sediments in a 795 cm long core taken from the southeastern part of the lake sediment redness which is related to iron oxide content seems to monitor paleoclimatic changes in the core iron oxides appear to be eroded from nearby red beds or loess deposits and are transported by fluvial means into the lake thus redness increases at times of increased precipitation that is as monsoon strength increases our redness monsoon proxy shows climate changes on several times scales on a millennial scale it records humid conditions during the early and mid holocene from about 4200 to 2300 yr bp low redness values suggest a two millennial long dry period which in the late holocene is followed by a more humid period on a centennial scale the redness proxy records not only the little ice age but also the medieval warm period the dark ages cool period and the roman warm period time series analysis of the redness record indicates a 200 yr frequency which corresponds to the de vries solar cycle suggesting that in addition to insolation changes resulting from orbital variations solar forcing also results from cyclic changes in the sun s luminosity

shear wave splitting to test mantle deformation models around hawaii seismic anisotropy allows us to study mantle deformation and it can thus help to constrain mantle flow in the vicinity of hotspots hypotheses for the cause of seismic anisotropy in this environment include the parabolic asthenospheric flow paf model radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion in map view this gives a parabolic pattern of flow opening in the direction of plate motion we present new shear wave splitting observations from land and ocean stations around the hawaiian islands that can be explained by the parabolic flow model the observations suggest asthenospheric anisotropy under the hawaiian islands which may be explained if dislocation creep persists to deeper depths there than in other regions perhaps due to the higher temperatures near hotspots

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

late holocene indian summer monsoon precipitation history at lake lugu northwestern yunnan province southwestern china abstract knowledge of late holocene precipitation variability in the northwestern yunnan province is important for understanding the indian summer monsoon ism dynamics in this study we present a proxy record of the ism precipitation history at lake lugu covering the last 2 900 years based on grain size and carbon nitrogen c n ratio in the northwestern yunnan province the results show centennial variations in ism precipitation with two remarkably dry intervals from 750 to 1167 ad and from 1733 ad to the present and two relatively wet intervals from 898 bc to 750 ad and from 1167 to 1733 ad a comparison between the record at lake lugu and the pollen record at lake erhai both in the northwestern yunnan province suggests that dry climate conditions prevailed during the medieval warm period mwp and over the last 100 200 years whereas relatively humid conditions prevailed during the little ice age lia these characteristics are generally similar to those of the climate patterns in extensive areas dominated by the ism e g southwestern southern china southeastern southern tibetan plateau northeastern india himalayas southern oman and equatorial eastern africa but anti phased with the climate patterns in the wide areas dominated by the east asian summer monsoon easm e g northern northeastern china and north central japan we speculate that both the variations in the sea surface temperature sst over the tropical indo pacific ocean the ocean atmosphere coupling processes and the migration of the mean position of the intertropical convergence zone itcz should be responsible for the hydroclimatic contrasts over the asian summer monsoon asm region on centennial timescales during the last 2 000 years

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

on the weakening relationship between the indian monsoon and enso analysis of the 140 year historical record suggests that the inverse relationship between the el nino southern oscillation enso and the indian summer monsoon weak monsoon arising from warm enso event has broken down in recent decades two possible reasons emerge from the analyses a southeastward shift in the walker circulation anomalies associated with enso events may lead to a reduced subsidence over the indian region thus favoring normal monsoon conditions additionally increased surface temperatures over eurasia in winter and spring which are a part of the midlatitude continental warming trend may favor the enhanced land ocean thermal gradient conducive to a strong monsoon these observations raise the possibility that the eurasian warming in recent decades helps to sustain the monsoon rainfall at a normal level despite strong enso events

seismic anisotropy beneath the incipient okavango rift implications for rifting initiation abstract this study represents the first shear wave splitting investigation of the okavango rift zone orz an incipient continental rift belonging to the east african rift system in northern botswana analysis of broadband seismic data recorded along a 750 km long profile of 22 stations traversing the orz and adjacent congo and kalahari cratons and several precambrian orogenic zones reveals dominantly ne sw fast orientations which are parallel to both the absolute plate motion direction based on the nnr nuvel 1a model and the trend of most tectonic boundaries including that of the orz spatial coherence analysis of the splitting parameters and correspondence between the observed fast orientations and the trend of tectonic features indicate that the main source of observed anisotropy is most likely in the upper asthenosphere probably due to simple shear associated with the relative movement of the lithosphere against the asthenosphere the presence of consistently rift parallel fast orientations and normal splitting times in the orz and most parts of southern africa implies that neither an upper mantle plume nor small scale convection is the dominant source for rift initiation and development the first shear wave splitting measurements in the vicinity of the orz favor a model in which continental rifting develops in response to intra plate relative movement of continental blocks along zones of weakness produced by ancient tectonic events

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

late holocene coupled moisture and temperature changes on the northern tibetan plateau the northern tibetan plateau involves complex interactions between the mid latitude westerly circulation and the subtropical asia monsoon circulation acting as a bridge communicating high and low latitude climatic processes previous studies from the region suggest relatively wet conditions in cold periods during the late holocene for instance the little ice age lia however the inference of such temperature moisture association is subject to the large uncertainty in lacustrine 14c chronology due to the particularly large lake reservoir effect in the region here we take a different approach by reconstructing paired temperature and moisture records from the same sediment cores to assess the temperature moisture association independent of chronology uncertainty we use alkenone indices uk 37 and c37 4 to reconstruct high resolution temperature and moisture changes simultaneously from two lakes in the qaidam basin northern tibetan plateau over the last 2500 years characterized by marked climatic variability our paired records confirm the warm dry and cold wet association in arid northwestern china during the late holocene opposite to the warm wet and cold dry association in subtropical asian monsoonal regions our moisture records further suggest substantially drier conditions during the medieval warm period mwp than the current warm period lastly the temperature and moisture changes inferred from our records can be well correlated with solar irradiance changes suggesting a possible link between solar forcing and natural climate variability during the late holocene on the northern tibetan plateau

comparison of isotopic composition of planktonic foraminifera in plankton tows sediment traps and sediments planktonic foraminifera from plankton tows sediment traps and sediments in the central north atlantic were studied in order to understand what determines their oxygen and carbon isotope composition a clear separation of species and genera on a 18o vs 13c plot for all samples suggests that their isotopic composition is controlled to a certain degree by biological factors within a species population the globorotaliids show a positive linear correlation between 18o and 13c while the shallow dwelling spinose species mostly globigerinoides species do not show a definite trend the latter species when collected in plankton tows often show slight negative deviations from isotopic equilibrium with respect to oxygen all species deviate from carbon isotope equilibrium by 1 5 to 6 these deviations from equilibrium are probably caused by incorporation of isotopically light metabolic co2 into the skeleton which is enhanced by the activity of symbiotic algae during their ontogeny the average weight per individual of most species increases which indicates that calcification continues to a depth of about 100 m this additional skeleton roughly 50 by weight is isotopically heavier because temperatures are lower and photosynthesis of symbiotic algae stops below the photic zone therefore the skeleton of foraminifera collected in sediment traps below 400 m has an overall oxygen isotope composition that seems to be in equilibrium for caco3 deposited in the upper 100 m