tourmaline microboudinage an indicator of its host rheology abstract columnar tourmaline in cenozoic granitic mylonites from the chongshan metamorphic zone cmz yunnan china has deformed at 450 300 c by tensile fractures normal to the foliation and lineation sequential fracturing and separation of the fragments during progressive cooling and exhumation upward the brittle ductile transition produced a series of microboudins with decreasing length width ratios here we present a method to apply the microboudinage paleopiezometry combined with strain analyses microstructural observations and thermochronological data to provide constraints on strain rates e differential stresses and their correlations with temperature for the polycrystalline quartz matrix in which tourmaline grains have been fragmented into microboudins the results yield a flow law of e a nexp q rt with n 4 q 184 kj mol and a 7 180 10 7 mpa ns 1 for naturally deformed quartzite the differential stresses calibrated by this flow law are very close to the geometrical mean of the lower and upper bounds given by luan and paterson 1992 and gleason and tullis 1995 respectively utilization of this flow law to the present day crust of the cmz provides a reliable interpretation for the distribution of focal depths and electrical resistivity structure the microboudinage piezometer also permits to estimate the mechanical strengths of rocks deformed in the elastic brittle regime

paleoenvironmental implications and drainage adjustment in the middle reaches of the sabarmati river gujarat implications towards hydrological variability abstract the dryland fluvial sequences exposed in the lower part of the gujarat alluvial plain are investigated for palaeohydrological palaeoclimatic reconstruction using conventional sedimentology grain size analyses supported by optical chronology considering that the study area is cradling large scale constructional activities an attempt has also been made to assess the geotechnical properties of the sediments it has been observed that the lower reaches of the sabarmati valley have preserved record of fluvial aggradation since mis 5 90 ka to mis 3 47 ka whereas the onset of aeolian processes began at around 12 ka based on the sediment texture the alluvial sequences are divided into the lower and upper units the lower fluvial sequence is dated to 90 63 ka whereas the upper fluvial sequence is dated to 47 ka the stratigraphic marker red soil is dated to 63 ka the study indicates that the terrain experienced tectonic perturbations after 12 ka which was responsible for the shifting of the sabarmati river from regional ne sw to n s which is more pronounced towards the upstream of the sabarmati river basin while the landscape stability is observed towards the downstream consisting of the older alluvium dated to 90 ka geotechnical investigation reveals potentially liquefiable sand which however have been compacted due to large scale calcretisation since their deposition

changes in biological productivity along the northwest african margin over the past 20 000 years the intertropical convergence zone and the african monsoon system are highly sensitive to climate forcing at orbital and millennial timescales both systems influence the strength and direction of the trade winds along northwest africa and thus directly impact coastal upwelling sediment cores from the northwest african margin record upwelling related changes in biological productivity connected to changes in regional and hemispheric climate we present records of 230th normalized biogenic opal and corg fluxes using a meridional transect of four cores from 19 n 31 n along the northwest african margin to examine changes in paleoproductivity since the last glacial maximum we find large changes in biogenic fluxes synchronous with changes in eolian fluxes calculated using end member modeling suggesting that paleoproductivity and dust fluxes were strongly coupled likely linked by changes in wind strength opal and corg fluxes increase at all sites during heinrich stadial 1 and the younger dryas consistent with an overall intensification of the trade winds and changes in the meridional flux gradient indicate a southward wind shift at these times biogenic fluxes were lowest and the meridional flux gradients were weakest during the african humid period when the monsoon was invigorated due to precessional changes with greater rainfall and weaker trade winds over northwest africa these results expand the spatial coverage of previous paleoproxy studies showing similar changes and they provide support for modeling studies showing changes in wind strength and direction consistent with increased upwelling during abrupt coolings and decreased upwelling during the african humid period

hadley circulation response to orbital precession part ii subtropical continent the responseof the monsoonaland annual meanhadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with a simplified representation of land surface processes in subtropical latitudes when perihelion occurs in the summer of a hemisphere with a subtropical continent changesin the top of atmosphereenergy balance togetherwith a polewardshift ofthe monsoonalcirculation boundary lead to a strengthening of the monsoonal circulation spatial variations in surface heat capacity determine whether radiative perturbations are balanced by energy storage or by atmospheric energy fluxes althoughorbitalprecessiondoesnotaffectannual meaninsolation theannual meanhadleycirculationdoes respond to orbital precession because its sensitivity to radiative changes varies over the course of the year the monsoonal circulation in summer is near the angular momentum conserving limit and responds directly to radiative changes whereas in winter the circulation is affected by the momentum fluxes of extratropical eddies and is less sensitive to radiative changes

1982 sepm presidential address episodic sedimentation how normal is average how rare is rare does it matter abstract do sedimentary rocks record mainly average continuous day to day processes or relatively rare large magnitude ones separated by long nondepositional intervals subtle legacies from lyellian uniformitarianism may still impose a subconscious abhorrence of unique events discontinuities and large deviations from average magnitudes where repeated sharp changes of sedimentation are inescapable periodic cycles are commonly invoked to preserve uniform orderly variations from some supposed norm the sedimentary record rarely reflects such uniformity however as sedimentologists have gradually realized magnitude versus frequency of processes has long been debated in geomorphology but has received less attention in sedimentology recurrence interval recovery time and preservation potential are critical factors for evaluating their significance for the sedimentary record large magnitude processes which represent positive deviations from the norm and are rare on the human time scale must be significant over geologic time but how significant could not everyday processes have obliterated much of the evidence flood deposits have relatively low preservation potential because they lie above base level marine gravel layers dispersed by abnormal waves have greater preservation potential because most ordinary processes are not competent to modify them sandy or shelly de osits formed by large waves and displaying either hummocky cross stratification or graded bedding have a moderate preservation potential especially if too thick for burrowing animals to homogenize them turbidites which provide exceptional records of episodicity have excellent preservation potential because they lie well below base level many bedding planes are important records of episodicity too some are surfaces of erosion others of nondeposition these represent negative deviations from average process magnitudes the sedimentological importance of rare events is difficult to assess because the record of such events may be very subtle this is especially true if a deposit has been thoroughly bioturbated or if the record of an event is simply an erosional surface it has now become possible to evaluate quantitatively ancient episodic sedimentation using modern process rates as well as refined biostratigraphic and isotopic dating i predict that such evaluation will necessitate revision of our favorite depositional models which have become so important for exploration as well as for research

mediterranean moisture source for an early holocene humid period in the northern red sea paleosalinity and terrigenous sediment input changes reconstructed on two sediment cores from the northernmost red sea were used to infer hydrological changes at the southern margin of the mediterranean climate zone during the holocene between approximately 9 25 and 7 25 thousand years ago about 3 per thousand reduced surface water salinities and enhanced fluvial sediment input suggest substantially higher rainfall and freshwater runoff which thereafter decreased to modern values the northern red sea humid interval is best explained by enhancement and southward extension of rainfall from mediterranean sources possibly involving strengthened early holocene arctic oscillation patterns and a regional monsoon type circulation induced by increased land sea temperature contrasts we conclude that afro asian monsoonal rains did not cross the subtropical desert zone during the early to mid holocene

on the meaning of peak temperature profiles in inverted metamorphic sequences inverted metamorphic sequences ims are common features of main thrust systems on earth they exhibit an upwards continuous increase in peak temperature conditions and thereby constitute evidence of the close relationship between the thermal field evolution and tectonic processes heat advection and shear heating are known to allow the formation of such metamorphic signatures heat diffusion also plays an important role in temperature distribution on both sides of the thrust other advection processes such as erosion or accretion may also cause a local peak temperature inversion each one of these processes therefore affects the thermal field around the thrust however despite the crucial importance of all these processes for the interpretation of the inverted peak temperature signatures their respective influences have never been quantified and compared all together to address this issue we propose an innovative coupled approach i we use two dimensional numerical models that simulate various thrust systems allowing for a wide diversity of setups to illustrate this study we focus on intracontinental thrust systems for which all processes listed are likely to play a key role in the thermal evolution we perform a parametric study including kinematic settings i e convergence erosion and accretion thermal properties mechanical strength and heat sources ii dimensionless numbers based on parameters are used to quantify the relative contributions of each process to the thermal budget evolution hence the three thermal processes i e heat diffusion heat advection and shear heating are compared with each other via three dimensionless combinations of the peclet and brinkman numbers rdif radv and rpro respectively erosion and accretion are compared separately based on a fourth dimensionless number rea iii we analytically examine the inverted peak temperature recorded along profiles that are perpendicular to the thrust zone defined in our numerical experiments each peak temperature profile presenting an inversion can then be characterized by a function of approximation involving six meaningful parameters the location ff and width ff of the maximum peak temperature inversion the characteristic peak temperature tcte and gradient glb beneath the inversion zone and the inversion related contrasts in the peak temperature t and gradient g this coupled approach linking numerical modelling and analytical treatment allows to quantitatively interpret ims in terms of the processes involved the application of our method to intracontinental thrust systems demonstrates that shear heating and erosion support significant inversions but that the relative contributions of each process have meaningful consequences our results reveal that competition between shear heating and heat diffusion on the one hand and between erosion and accretion on the other hand have a high impact in particular the variability in the rock s mechanical strength strongly influences the features of peak temperature inversions consequently none of these processes can be ignored our results highlight the major importance of the rheology of rocks in the thermal evolution of shear zones finally our methodology is not only restricted to the analysis of numerical data but also constitutes a way of broad interest to analyse peak temperature signatures around any shear zone

water isotopes in precipitation data model comparison for present day and past climates abstract variations of hdo and h2 18 o concentrations are observed in precipitation both on a geographical and on a temporal basis these variations resulting from successive isotopic fractionation processes at each phase change of water during its atmospheric cycle are well documented through the iaea wmo network isotope concentrations are in middle and high latitudes linearly related to the annual mean temperature at the precipitation site paleoclimatologists have used this relationship to infer paleotemperatures from isotope paleodata extractable from ice cores deep groundwater and other such sources for this application to be valid however the spatial relationship must also hold in time at a given location as the location undergoes a series of climatic changes progress in water isotope modeling aimed at examining and evaluating this assumption has been recently reviewed jouzel et al 1997 with a focus on polar regions and more specifically on greenland we extend this review in comparing the results of two different isotopic agcms nasa giss and echam and in examining with a more global perspective the validity of the above assumption i e the equivalence of the spatial and temporal isotope temperature relationships these results confirm the dominating role of local temperature changes on the paleo isotope signal in most regions however the exact calibration of this valuable paleothermometer is biased by for example the seasonality of precipitation and other factors we forced the two models by the climatic boundary conditions of the mild holocene at 6 kyr bp which only slightly differs from today s climate the isotope response on this weak forcing is quite heterogeneous the only robust common response is the intensification of the hydrological cycle in low latitudes and therefore isotopically more depleted precipitation in the tropics and subtropics we also examine recent progress made in modeling the relationship between the conditions prevailing in moisture source regions for precipitation and the deuterium excess of that precipitation

late quaternary paleoclimate in the eastern mediterranean region from stable isotope analysis of speleothems at soreq cave israel abstract the eastern mediterranean continental paleoclimate during the past 25 000 years was determined by a high resolution petrographic stable isotopic and age study of speleothems from soreq cave israel 18o 13c trends indicate that all speleothems older than 7000 yr formed under conditions that differ from those of today the period from 25 000 to 17 000 yr b p was characterized by the highest 18o and 13c values which indicate deposition at temperatures of 12 16 c annual rainfall of 300 450 mm and vegetation typical of a mixed c3 c4type from 17 000 to 10 000 yr b p deglaciation in northern europe 18o values dropped progressively correlative with warming 2 3 c and a gradual increase in precipitation a simultaneous decrease in 13c gives a range expected for c3 type vegetation this period also shows significant 18o and 13c spikes which are correlatable with global events e g heinrich events and the younger dryas stade the speleothems that grew between 10 000 and 7000 yr b p have a unique petrography showing irregular thin laminae of various colors and much detritus they have the lowest 18o corresponding to 1000 mm rain coupled with the highest 13c caused by flooding events which stripped the soil cover from 7000 to 1000 yr b p conditions became closer to those of today this study demonstrates that global events which were recognized in northern europe and north africa are also evident in the eastern mediterranean and are reflected principally by large changes in the rainfall rate

mechanisms of poleward propagating intraseasonal convective anomalies in cloud system resolving models abstract an envelope of convection that propagates both poleward and eastward accounts for the largest fraction of intraseasonal variance of the tropical atmosphere during boreal summer here the mechanisms of poleward propagating convective anomalies are examined in a nonhydrostatic model with zonally symmetric boundary conditions integrated on a beta plane at resolutions high enough to explicitly represent moist convection when the domain has a narrow zonal dimension of 100 km or less the model produces a quasisteady intertropical convergence zone itcz meridionally propagating transients are produced for some prescribed sea surface temperature distributions but these transients are shallow vanish at finer resolutions and have a structure that bears little resemblance to that of observed poleward propagating anomalies this is in sharp contrast to previous studies that obtained robust poleward propagating anomalies in axisymmetric models using parameterized moist convection and it suggests that

modern precipitation δ18o and trajectory analysis over the himalaya tibet orogen from echam5 wiso simulations variations in oxygen isotope ratios 18o measured from modern precipitation and geologic archives provide a promising tool for understanding modern and past climate dynamics and tracking elevation changes over geologic time in areas of extreme topography such as the tibetan plateau the interpretation of 18o has proven challenging this study investigates the climate controls on temporal daily and 6 h intervals and spatial variations in present day precipitation 18o 18op across the tibetan plateau using a 30 year record produced from the european centre hamburg echam5 wiso global atmospheric general circulation model gcm results indicate spatial and temporal agreement between model predicted 18op and observations large daily 18op variations of 25 to 5 occur over the tibetan plateau throughout the 30 simulation years along with interannual 18op variations of 2 analysis of extreme daily 18op indicates that extreme low values coincide with extreme highs in precipitation amount during the summer monsoon vapor transport from the north and southwest of the plateau generally corresponds with high 18op whereas vapor transport from the indian ocean corresponds with average to low 18op thus vapor source variations are one important cause of the spatial temporal differences in 18op comparison of gcm and rayleigh distillation model rdm predicted 18op indicates a modest agreement for the himalaya region averaged over 86 94 e confirming application of the simpler rdm approach for estimating 18op lapse rates across himalaya

from passive continental margin to mountain belt insights from analytical and numerical models and application to taiwan abstract the physics of deformation at plate boundaries is insufficiently understood taiwan is particularly interesting in this context since geological and geophysical arguments indicate that passive margin sediments were exhumed into an active mountain belt over a relatively short period of time classically taiwan has been described as a critical wedge orogeny in which deformation is entirely restricted to the crust more recent data however indicate that deeper parts of the lithosphere also participate in deformation in order to study the physical feasibility of various endmember hypothesis we have performed two dimensional 2 d thermo mechanical numerical experiments in which both mantle lithospheric and surface processes are taken into account and in which crustal deformation evolves as a function of mantle flow our setup consists of ocean ocean subduction followed by passive continental margin deformation results indicate that in most cases the subducting slab becomes close to vertical upon arrival of continental material to the trench whereas crustal rheology deformation and surface processes do not modify mantle flow significantly they are important for crustal scale processes relatively little crustal exhumation occurs if the crust is homogeneous and erosion rates are small significant exhumation in a dome like structure is obtained if a weak lower crust and large erosion rates are present the presence of an oceanic arc in the overriding oceanic lithosphere modifies but does not control crustal deformation synthetic metamorphic facies maps and thermochronological data computed from the numerical models are in reasonable agreement with observations in southern taiwan if exhumation occurs in a dome like structure further physical insight in the doming mechanisms is obtained by a combination of analytical techniques and crustal scale numerical models this analysis indicates that the dome like exhumation mechanism can be attributed to a compressional mechanical instability facilitated by both a weak lower crust and large rates of erosion both conditions are likely to be satisfied in taiwan

variations of indian and african monsoons induced by insolation changes at 6 and 9 5 kyr bp this study investigates the role of insolation in controlling the indian and african monsoon evolutions during the holocene using coupled ocean atmosphere simulations of 0 6 9 5 kyr bp climates for which only the variations of earth s orbital configuration are considered the two monsoon systems are enhanced at 6 and 9 5 kyr bp compared to 0 kyr bp as a result of the intensified seasonal cycle of insolation in the northern hemisphere the analysis of daily climatologies indicates that even though the length of the celestial summer season is shorter at 9 5 kyr bp the rainy season is longer than at present emphasis is put on the impact of the precession on the seasonality which partly explains why the relative amplification of the indian and african monsoon varies between 9 5 and 6 kyr bp moreover the changes in snow cover over the tibetan plateau play a critical role in reinforcing the 9 5 kyr bp monsoon in india during spring the results suggest that the teleconnection between convection over india and subsidence over the mediterranean regions through the rodwell and hoskins mechanism has an impact on the development of the african monsoon at 9 5 kyr bp

relative influence of precession and obliquity in the early holocene topographic modulation of subtropical seasonality during the asian summer monsoon abstract on orbital timescales higher summer insolation is thought to strengthen the continental monsoon while weakening the maritime monsoon in the northern hemisphere through simulations using the community earth system model we evaluated the relative influence of perihelion precession and high obliquity in the early holocene during the asian summer monsoon the major finding was that precession dominates the atmospheric heating change over the tibetan plateau himalayas and maritime continent whereas obliquity is responsible for the heating change over the equatorial indian ocean thus precession and obliquity can play contrasting roles in driving the monsoons on orbital timescales in late spring early summer interior asian continental heating drives the south and east asian monsoons the broad scale monsoonal circulation further expands zonally in july august corresponding to the development of summer monsoons in west africa and the subtropical western north pacific wnp as well as a sizable increase in convection over the equatorial indian ocean tropical and oceanic heating becomes crucial in late summer over south asia indian ocean 50 e 110 e the precession maximum intensifies the monsoonal hadley cell heating with an inland highland origin which is opposite to the meridional circulation change induced by high obliquity heating with a tropical origin the existence of the tibetan plateau himalayas intensifies the precessional impact during the late summer phase of the monsoon season the effect of obliquity on tropical heating can be substantial in addition to competing with asian continental heating obliquity enhanced heating over the equatorial indian ocean also has a walker type circulation impact resulting in suppression of precession enhanced heating over the maritime continent

earthquakes and strength in the laminated lower crust can they be explained by the corset model abstract we present a model that may explain deep crustal earthquakes observed in particular in several areas of highly reflective laminated lower continental crust we combine observations from earthquake seismology crustal reflection seismics and tectonic rheological concepts the study concentrates on parts of the northern alpine foreland where many earthquakes occur inside the laminated lower crust which is generally considered to be warm and weak thin mafic ultramafic sill like intrusions and invisible dykes are assumed to form a corset like network with high strength this model can explain the observed strong and multiple reflections and the occurrence of rupture inside a stable structure within a weak lower crust tectonic stress transfer from the alpine collision zone or and the upper rhine graben and its release may follow classical friction concepts in addition the heterogeneity of the laminated lower crust may also favour various viscous instabilities

coherent pan asian climatic and isotopic response to orbital forcing of tropical insolation the oxygen 18 isotope composition of calcite in stalagmites across southern and eastern asia are highly correlated to one another on orbital time scales large negative excursions are coincident with maxima in summer insolation in the subtropics of the northern hemisphere nh these isotopic excursions reflect changes in the precipitation weighted isotopic composition of precipitation 18op we present results from two core experiments using an isotope enabled climate model the high insolation and low insolation experiments in which the model is forced by extrema in nh summer insolation compared to the low insolation experiment the high insolation climate features profound large scale changes in the pattern of monsoon precipitation spanning from africa to southeast asia that are due to changes in the relative contributions of temperature and moisture to the near surface equivalent potential temperature e under high insolation a more rapid increase in land surface temperature in early summer causes the greatest e and hence precipitation to shift from the oceans in low insolation such as today to be over land in high insolation such as the early holocene the model captures the general pattern of isotopic excursions seen in caves spanning from israel to western china including large drops in 18op over eastern tibet 7 the arabian peninsula and northeast africa 4 although there are large changes in precipitation over tibet the change in 18op is due to changes in the 18o of water vapor that is delivered and subsequently precipitated it does not inform on local precipitation amount or intensity

holocene moisture changes in western china central asia inferred from stalagmites abstract central asia lies at the convergence between the mediterranean and asian monsoon climates and there is a complex interaction between the westerlies with the monsoon to form the climate of that region and its variability the region is highly vulnerable to changes in rainfall highlighting the need to understand the underlying controls we present a stalagmite based 18 o record from kesang cave in western china using mc icp ms u series dating and stable isotope analysis stalagmite calcite 18 o largely documents changes in the 18 o of precipitation 18 o in stalagmites was low during the early and middle holocene 10 0 3 0 ka bp and shifted to higher values between 3 0 and 2 0 ka bp after 2 0 ka bp 18 o fluctuates with distinct centennial scale variations drawing from results of state of the art atmospheric general circulation model simulations for the preindustrial period and 9 ka bp we propose that changes in moisture source regions and the wetter climate both contributed to the isotopic depletion of precipitation during the early and middle holocene multiple records from surrounding regions indicate a generally wetter climate during the early and mid holocene supporting our interpretation on the speleothem 18 o changes in precipitation seasonality do not appear to be a viable explanation for the observed changes nor increased penetration of monsoonal moisture to the study site we speculate that the climatic regime shifted around 3 0 2 0 ka bp towards a drier climate resulting in temperature having dominant control on precipitation 18 o the demise of three settlements around 500ad at the margin of tarim basin coincided with a period of decreased precipitation and increased temperature that likely affected local water resources underscoring the potential impact of climate on human habitation in this region

timing and hydrological conditions of sapropel events in the eastern mediterranean as evident from speleothems soreq cave israel this paper explores the connection between the timing of sapropel events in the eastern mediterranean sea and the time of low 18o events in speleothems of the soreq cave israel a new well dated 18o and 13c profile of the soreq cave speleothems for the last 140 kyr reveals five prominent low 18o events dated at 124 to 119 kyr with the main peak at 122 kyr event v 108 to 100 kyr with peaks at 107 and 102 kyr event iv 85 to 79 kyr with peak at 80 kyr event iii 55 to 52 kyr with a peak at 54 kyr event ii and 8 5 to 7 kyr with peaks at 8 5 and 7 kyr event i these events are characteristic of period of enhanced rainfall in the eastern mediterranean area particularly so for events v and i where the marked decrease in 18o and the correspondingly marked increase in 13c indicate very wet conditions the chronology of the low 18o events v iv iii and i match the ages estimated for the formation of sapropels s5 s4 s3 and s1 which are also associated with high hydrological activity thus it is suggested that the low 18o events in the soreq cave speleothems constrain the maximum duration of the sapropel formation pollen data from the sapropel layers and the isotopic pattern of the speleothems indicate that sapropels s5 and s1 were deposited during very wet periods when there were major decrease in the sea surface salinity coupled with frost free winters and drought free summers sapropels s4 and s3 were formed during periods of increased precipitation but ones in which the general climate was mediterranean semi arid

precession and glacial cycle controls of monsoon precipitation isotope changes over east asia during the pleistocene abstract precipitation isotope reconstructions derived from speleothems and plant waxes are important archives for understanding hydroclimate dynamics their climatic significance in east asia however remains controversial here we present terrestrial plant wax stable hydrogen isotope dwax records over periods covering the last four interglacials and glacial terminations from sediment cores recovered from the northern south china sea scs as an archive of regionally integrated precipitation isotope changes in southeast china combined with previous precipitation isotope reconstructions from china we find that the scs dwax and southwest central china stalagmite 18 o records show relatively enriched and depleted isotopic values respectively during interglacial peaks but relatively similar isotopic variations during most sub interglacials and glacial periods over the past 430 thousand years during interglacial peaks strong summer insolation should have intensified the convection intensity the isotopic fractionation along moisture trajectories and the seasonality which are all in favor of causing isotopically depleted rainfall over the east asian monsoon regime these effects in combination with a relatively high proportion of indian ocean versus pacific sourced moisture influx should have resulted in strongly depleted precipitation isotopes stalagmite 18 o over most parts of china however southeast china should have been affected by a relatively low ratio of indian ocean versus pacific sourced moisture influx which dominated over effects yielding depleted precipitation isotopes and led to enriched precipitation isotopes dwax it is thus concluded that glacial boundary conditions and insolation forcing are the two most important factors for causing regional differences in precipitation isotope compositions over subtropical east asia on orbital timescales

temporal variability in the northern arabian sea oxygen minimum zone omz during the last 225 000 years the northern arabian sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen the intensity of this oxygen minimum zone omz has been reconstructed over the past 225 000 years using proxies for surface water productivity water column denitrification winter mixing and the aragonite compensation depth acd changes in omz intensity occurred on orbital and suborbital timescales lowest o2 levels correlate with productivity maxima and shallow winter mixing precession related productivity maxima lag early summer insolation maxima by 6 kyr which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer periods with a weakened or even non existent omz are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds the timing of deep winter mixing events corresponds with that of periods of climatic cooling in the north atlantic region

lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern asia abstract variability in oxygen isotope ratios collected from speleothems in chinese caves is often interpreted as a proxy for variability of precipitation summer precipitation seasonality of precipitation and or the proportion of 18o to 16o of annual total rainfall that is related to a strengthening or weakening of the east asian monsoon and in some cases to the indian monsoon we use modern reanalysis and station data to test whether precipitation and temperature variability over china can be related to changes in climate in these distant locales we find that annual and rainy season precipitation totals in each of central china south china and east india have correlation length scales of 500 km shorter than the distance between many speleothem records that share similar long term time variations in 18o values thus the short distances of correlation do not support though by themselves cannot refute the idea that apparently synchronous variations in 18o values at widely spaced 500 km caves in china are due to variations in annual precipitation amounts we also evaluate connections between climate variables and 18o values using available instrumental measurements of 18o values in precipitation these data from stations in the global network of isotopes in precipitation gnip show that monthly 18o values generally do not correlate well with either local precipitation amount or local temperature and the degree to which monthly 18o values do correlate with them varies from station to station for the few locations that do show significant correlations between 18o values and precipitation amount we estimate the differences in precipitation amount that would be required to account for peak to peak differences in 18o values in the speleothems from hulu and dongge caves assuming that 18o scales with the monthly amount of precipitation or with seasonal differences in precipitation insofar as the present day relationship between 18o values and monthly precipitation amounts can be applied to past conditions differences of at least 50 in mean annual precipitation would be required to explain the 18o variations on orbital time scales which are implausibly large and inconsistent with published gcm results similarly plausible amplitudes of seasonal cycles in amounts or in seasonal variations in 18o values can account for less than half of the 4 5 difference between glacial and interglacial 18o values from speleothems in china if seasonal cycles in precipitation account for the amplitudes of 18o values on paleoclimate timescales they might do so by extending or contracting the durations of seasons a frequency modulation of the annual cycle but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average 18o values amplitude modulation allowing that several processes can affect seasonal variability in isotopic content we explore the possibility that one or more of the following processes contribute to variations in 18o values in chinese cave speleothems different source regions of the precipitation which bring different values of 18o in vapor different pathways between the moisture source and the paleorecord site along which exchange of 18o between vapor surface water and condensate might differ a different mix of processes involving condensation and evaporation within the atmosphere or different types of precipitation each may account for part of the range of 18o values revealed by speleothems and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

sea land oxygen isotopic relationships from planktonic foraminifera and speleothems in the eastern mediterranean region and their implication for paleorainfall during interglacial intervals the oxygen and carbon stable isotope compositions of cave speleothems provide a powerful method for understanding continental climate change here we examine the question of the regionality of this isotopic record and its linkage with the marine isotopic record in the eastern mediterranean em region the study presents a new accurately dated 250 kyr 18o and 13c record determined from speleothems of the peqiin cave northern israel its comparison with the continuous 185 kyr isotopic record of the soreq cave speleothems from central israel reveals striking similarities thus a strong regional climatic signal brought about by variations in temperature and rainfall amount is reflected in both cave records low 18o minima in the peqiin profile for the last 250 to 185 kyr period interglacial marine isotopic stage 7 match the timing of sapropels 9 to 7 and are indicative of high rainfall in the em region at these times the combined soreq and peqiin 18o record for the last 250 kyr excellently matches the published globigerinoides ruber g ruber marine 18o record for the em sea with the isotopic compositional difference g ruber speleothems remaining relatively constant at 5 6 0 7 thus establishing for the first time a robust exploitable link between the land and the marine isotopic records the correspondence of low 18o speleothem values and high cave water stands with low g ruber 18o values during interglacial sapropel events indicates that these periods were characterized by enhanced rainfall in the em land and sea regions by use of sea surface temperatures derived from alkenone data as a proxy for land temperatures at the soreq cave we calculate the paleorainfall 18o values and its amounts maximum rainfall and lowest temperature conditions occurred at the beginning of the sapropel events and were followed by decrease in rainfall and increase in temperatures leading to arid conditions the record for the last 7000 yr shows a trend toward increasing aridity and agrees well with climatic and archeological data from north africa and the middle east

a 250 ka leaf wax δd record from a loess section in darai kalon southern tajikistan abstract the climate of sw central asia is influenced by strong precipitation seasonality with dry summers and winter spring precipitation delivered by mid latitude westerlies due to its geographic position sw central asia is a key locality for identifying fluctuations of westerlies siberian high and sw monsoon yet there is only limited information on the pleistocene climate history in sw central asia here we use plant wax biomarkers and their hydrogen isotope d composition from a loess section in darai kalon southern tajikistan to reconstruct environmental conditions of the past 250 thousand years ka we focus on the hydrogen isotope composition of long chain n alkanes dalk and long chain n carboxylic acids dcarb to reconstruct changes in the hydrological system moreover we infer information about past vegetation changes from proxies based on the relative abundance of these compounds results show that during glacials grasses dominated whereas during interglacials arboreal species prevailed these changes in vegetation follow the dominant glacial interglacial variability visible in the loess stratigraphy our results are in line with decreased precipitation during glacials as a result of a more extensive siberian high isotope measurements reveal a close relation of dalk and dcarb with insolation controlled by 20 ka cycles of orbital precession that is distinct from the glacial interglacial variability of the loess stratigraphy depleted enriched dalk and dcarb values coincide with low high winter spring insolation the precessional mode of our isotope records is likely influenced by variations in the precipitation seasonality between winter and spring precipitation caused by insolation induced temperature changes in comparison with speleothem records along the westerly path our results further indicate that the winter westerly storm tracks did not extend to the east of the tian shan during the last two glacial cycles

great basin hydrology paleoclimate and connections with the north atlantic a speleothem stable isotope and trace element record from lehman caves nv abstract we present a record of speleothem stable isotope 18 o 13 c and trace element mg ca sr ca variations from lehman caves nevada for an interval of time 139 128 ka that encompasses the penultimate glacial termination termination ii t ii additional growth phases provide data from about 123 ka a time that correlates with marine isotope stage mis 5e and about 84 ka and between 82 and 81 ka mis 5a chronologies from two new stalagmites are anchored by thirty six uranium thorium dates we also present new trace element data from stalagmite lc 2 which has a previously published uranium thorium chronology and stable isotope record of t ii shakun et al 2011 our t ii 18 o record broadly replicates that of the shakun et al 2011 and lachniet et al 2014 records of this time recording low values from 139 to 135 ka followed by an approximately 3 5 increase over an extended interval between 134 and 129 ka this rise in 18 o values occurs during heinrich stadial 11 and the associated weak monsoon interval observed in chinese caves our record broadly follows the marine termination rising boreal summer insolation and the rise in atmospheric co 2 we infer that this shift results from temperature increase due to increasing atmospheric co 2 and potentially a change in moisture source or precipitation seasonality from greater influence of the north american monsoon accompanying summer insolation increase it is also plausible that the melting of the ice sheet itself may have contributed to both temperature and precipitation seasonality changes trace element ratios and 13 c values are largely decoupled from 18 o values showing minimal variation between 139 and 130 ka for the duration of the chinese weak monsoon interval however these values increase sharply between 130 and 128 ka which we interpret as increased prior calcite precipitation driven by a transition from wet to dry conditions this abrupt drying event coincides within dating uncertainties with the abrupt strengthening of the east asian summer monsoon which marks the end of both the weak monsoon interval and heinrich stadial 11 our records demonstrate a link between north atlantic climate and great basin moisture during this time that is consistent with the interpretation of data from the last glacial period and may result from abrupt shifts in atlantic meridional overturning circulation affecting the strength of the aleutian low

precessional forcing of nutricline dynamics in the equatorial atlantic climate control of nutricline depth in the equatorial atlantic can be monitored by variations in the abundance of the phytoplankton species florisphaera profunda a conceptual model based on in situ evidence associates high abundances of f profunda with a deep nutricline and low abundances with a shallow nutricline a 200 000 year record of f profunda relative abundances obtained from a deep sea core sited beneath the region of maximum equatorial divergence at 10 w has 52 percent of its variance centered on the 23 000 year precessional band cross spectral analysis between the signals of f profunda and sea surface temperature independently derived from zooplankton species shows their 23 000 year cycles to be coherent and nearly in phase abundance minima of f profunda coincide with times of december perihelion whereas abundance maxima coincide with june perihelion these relations indicate that nutricline dynamics in the divergence region of the equatorial atlantic are controlled by variations in the tropical easterlies forced by the precessional component of orbital insolation on time scales greater than 10 000 years

holocene and pleistocene pluvial periods in yemen southern arabia arabia is an important potential pathway for the dispersal of homo sapiens out of africa yet because of its arid to hyper arid climate humans could only migrate across southern arabia during pluvial periods when environmental conditions were favorable however knowledge on the timing of arabian pluvial periods prior to the holocene is mainly based on a single and possibly incomplete speleothem record from hoti cave in northern oman additional terrestrial records from the arabian peninsula are needed to confirm the hoti cave record here we present a new speleothem record from mukalla cave in southern yemen the mukalla cave and hoti cave records clearly reveal that speleothems growth occurred solely during peak interglacial periods corresponding to marine isotope stages mis 1 early to mid holocene 5 1 5 3 5 5 eemian 7 1 7 5 and 9 of these humid periods highest precipitation occurred during mis 5 5 and lowest during early to middle holocene

speleothem evidence from oman for continental pluvial events during interglacial periods growth periods and stable isotope analyses of speleothems from hoti cave in northern oman provide a record of continental pluvial periods extending back over the past four of earth s glacial interglacial cycles rapid speleothem growth occurred during the early to middle holocene 6 10 5 ka b p 78 82 ka b p 120 135 ka b p 180 200 ka b p and 300 325 ka b p the speleothem calcite deposited during each of these episodes is highly depleted in 18 o compared to modern speleothems the d 18 o values for calcite deposited within pluvial periods generally fall in the range of 24 to 28 relative to the vienna peedee belemnite standard whereas modern speleothems range from 21 to 23 the growth and isotopic records indicate that during peak interglacial periods the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record the association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions and not changes in solar radiation are the primary control on continental wetness on glacial interglacial time scales

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

resolving seasonal rainfall changes in the middle east during the last interglacial period paleorainfall proxy records from the middle east have revealed remarkable patterns of variability since the penultimate glacial period 140 ka but the seasonality of this signal has been unresolvable here seasonal resolution oxygen isotope data from soreq cave speleothems suggest that summer monsoon rainfall periodically reaches as far north as israel well removed from the modern monsoon at times 125 105 ka that overlap with evidence for some of the earliest modern human migrations out of africa these seasonal proxy data are corroborated by seasonal resolution model output of the amount and oxygen isotope ratio of rainfall from an isotope enabled climate model in contrast to the modern regional climate where rainfall is delivered predominantly in winter months along westerly storm tracks the model suggests that during extreme peaks of summer insolation as occurs during the last interglacial e g 125 105 ka regional rainfall increases due to both wetter winters and the incursion of summer monsoons this interpretation brings clarity to regional paleoproxy records and provides important environmental context along one potential pathway of early modern human migration

ice age terminations 230th dated oxygen isotope records of stalagmites from sanbao cave china characterize asian monsoon am precipitation through the ends of the third and fourthmost recent ice ages as a result am records for the past four glacial terminations can now be precisely correlated with those from ice cores and marine sediments establishing the timing and sequence of major events in all four cases observations are consistent with a classic northern hemisphere summer insolation intensity trigger for an initial retreat of northern ice sheets meltwater and icebergs entering the north atlantic alter oceanic and atmospheric circulation and associated fluxes of heat and carbon causing increases in atmospheric co2 and antarctic temperatures that drive the termination in the southern hemisphere increasing co2 and summer insolation drive recession of northern ice sheets with probable positive feedbacks between sea level and co2

monsoons as eddy mediated regime transitions of the tropical overturning circulation monsoons are generally viewed as planetary scale sea breeze circulations caused by contrasts in the thermal properties between oceans and land surfaces that lead to thermal contrasts upon radiative heating1 2 but the radiative heating evolves gradually with the seasons whereas the onset of monsoon precipitation and the associated circulation changes such as reversal of surface winds occur rapidly3 4 here we use reanalysis data to show that the onset of the asian monsoon marks a transition between two circulation regimes that are distinct in the degree to which eddy momentum fluxes control the strength of the tropical overturning circulation rapid transitions of the circulation between the two regimes can occur as a result of feedbacks between large scale extratropical eddies and the tropical circulation5 using simulations with an aquaplanet general circulation model we demonstrate that rapid eddy mediated monsoon transitions occur even in the absence of surface inhomogeneities provided the planet surface has sufficiently low thermal inertia on the basis of these results we propose a view of monsoons in which feedbacks between large scale extratropical eddies and the tropical circulation are essential for the development of monsoons whereas surface inhomogeneities such as land sea contrasts are not

orbital forcing of arctic climate mechanisms of climate response and implications for continental glaciation progress in understanding how terrestrial ice volume is linked to earths orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales 10 3 10 5 years a compromise is usually made to rep resent the climate system by models that are averaged over one or more spatial dimensions or by three dimensional models that are limited to simulating par ticular snapshots in time we take advantage of the short equilibration time 310 years of a climate model consisting of a three dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in earths orbital configuration over the past 165 000 years prominent decreases in ice melt and increases in snowfall are sim ulated during three time intervals near 26 73 and 117 thousand years ago ka when aphelion was in late spring and obliquity was low there were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large aph elion was in late spring and obliquity was high or near its long term mean these glaciation friendly time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine d 18 o record both dynamical and thermal effects contribute to the increases in snowfall during these periods through increases in storm activity and the fraction of precipi tation falling as snow the majority of the mid to high latitude response to orbital forcing is organized by the properties of sea ice through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient

relationships between lake level changes and water and salt budgets in the dead sea during extreme aridities in the eastern mediterranean abstract thick halite intervals recovered by the dead sea deep drilling project cores show evidence for severely arid climatic conditions in the eastern mediterranean during the last three interglacials in particular the core interval corresponding to the peak of the last interglacial marine isotope stage 5e or mis 5e contains 30 m of salt over 85 m of core length making this the driest known period in that region during the late quaternary this study reconstructs dead sea lake levels during the salt deposition intervals based on water and salt budgets derived from the dead sea brine composition and the amount of salt in the core modern water and salt budgets indicate that halite precipitates only during declining lake levels while the amount of dissolved na and cl accumulates during wetter intervals based on the compositions of dead sea brines from pore waters and halite fluid inclusions we estimate that 12 16 cm of halite precipitated per meter of lake level drop during periods of halite precipitation the mg2 concentration increases and the na cl ratio decreases in the lake our calculations indicate major lake level drops of 170 m from lake levels of 320 and 310 m below sea level mbsl down to lake levels of 490 and 480 mbsl during mis 5e and the holocene respectively these lake levels are much lower than typical interglacial lake levels of around 400 mbsl these lake level drops occurred as a result of major decreases in average fresh water runoff to 40 of the modern value pre 1964 before major fresh water diversions reflecting severe droughts during which annual precipitation in jerusalem was lower than 350 mm y compared to 600 mm y today nevertheless even during salt intervals the changes in halite facies and the occurrence of alternating periods of halite and detritus in the dead sea core stratigraphy reflect fluctuations between drier and wetter conditions around our estimated average the halite intervals include periods that are richer and poorer in halite indicating based on the sedimentation rate that severe dry conditions with water availability as low as 20 of the present day continued for periods of decades to centuries and fluctuated with wetter conditions that spanned centuries to millennia when water availability was 50 100 of the present day these conclusions have potential implications for the coming decades as climate models predict greater aridity in the region

mullions in the high ardenne slate belt belgium numerical model and parameter sensitivity analysis in the high ardenne slate belt belgium pre existing layer perpendicular quartz veins in psammite layers acted as mechanical boundaries due to the difference in competence between vein quartz and psammite this caused the formation of mullions during layer parallel shortening in this paper the process of mullion formation is modelled using finite element techniques a parameter sensitivity analysis of the mullion model demonstrates that the stress exponent of psammite the total horizontal shortening of the psammite layers the initial aspect ratio of the psammite segments between the veins and the competence contrast between psammite and vein quartz are the controlling parameters for the shape of the mullions our results suggest that the morphology of the mullions can be used to constrain the rheology of psammite deforming in the middle crust moreover the parameter sensitivity analysis illustrates the range of layer parallel shortening extension structures associated with layer perpendicular quartz veins that can be expected in nature e g dogbones inverted mullions and creates future perspective for the use of the model as a paleorheological gauge for these structures

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

mediterranean quaternary sapropels an immediate response of the african monsoon to variation of insolation abstract in the subsurface of the east mediterranean sea eleven discrete basin wide organic rich black sapropels were deposited during the last 465 000 years they are dated by oxygen isotopic chronostratigraphy the most recent sapropel has been shown to be the consequence of heavy nile river summer floods due to increased monsoonal summer precipitation over ethiopia and not of scandinavian ice sheet meltwater influx into the east mediterranean sea as was previously believed while most sapropels were deposited during interglacials two thick sapropels formed during glacial periods as shown by their position in the oxygen isotopic stratigraphy and by their pollen record in core rc9 181 the new evidence presented here is that the sapropel sequence in this core including the two glacial sapropels correlates on a one to one basis with the highest values of an index of insolation on top of the atmosphere the concept of this index derives from an analysis of the african monsoon process the index variation is computed for the last 465 000 years from the caloric northern summer insolation in the tropical latitudes controlled by variations of the earth s orbital parameters it is conjectured that this index might be linked through the climate system to the monsoonal precipitation which in africa produces the nile river summer flood

timing and climatic impact of greenland interstadials recorded in stalagmites from northern turkey a 50 kyr long exceptionally well dated and highly resolved stalagmite oxygen 18o and carbon 13c isotope record from sofular cave in northwestern turkey helps to further improve the dating of greenland interstadials gi 1 and 3 12 timing of most gi in the sofular record is consistent within 10 to 300 years with the iconic hulu cave record larger divergences 500 years between sofular and hulu are only observed for gi 4 and 7 the sofular record differs from the most recent ngrip chronology by up to several centuries whereas age offsets do not increase systematically with depth the sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern mediterranean to gi whereas a phase lag of 100 years between climate and full ecosystem response is evident finally results of spectral analyses of the sofular isotope records do not support a 1 470 year pacing of gi

strain partitioning along the mahanadi shear zone implications for paleo tectonics of the eastern ghats province india abstract during indo antarctic collision at c 1 0 ga eastern ghats province egp granulites amalgamated with the archean indian craton the northern boundary of the egp was subsequently reworked undergoing dextral strike slip shearing at 0 5 ga this study documents a phase of dextral shearing within the egp along wnw ese trending shear planes in c 0 5 ga mylonites of the mahanadi shear zone regional structural trends in the egp show a swing from ne sw to the south of the shear zone to wnw ese to its north the mylonitic shear zone foliation has a sub horizontal lineation associated with a prominent dextral shear sense in near horizontal sections electron back scatter diffraction ebsd studies on quartz confirm that mylonitisation was associated with dextral strike slip movement in the greenschist facies north of the mahanadi shear zone strain was partitioned into narrow dextral strike slip shear zones along which the older granulite fabrics were transposed parallel to later wnw ese trending shear planes at lower grades of metamorphism this regional scale shearing at 500 ma possibly resulted in a significant dextral shift of the northern egp with respect to the south the shear zone was reactivated in the permian time during deposition of gondwana sediments in the mahanadi basin

continental oxygen isotopic record of the last 170 000 years in jerusalem abstract a long radiometrically dated oxygen isotopic record of continental climatic variations since the penultimate glaciation was obtained from a stalagmite deposited in a sealed cave in jerusalem this record shows that speleothems have the potential of assigning dates to long and short term climatic events with possible refining of milankovitch tuning of ice and marine records which themselves are not datable short term 1000 yr events are very significant in the region reaching 50 of glacial interglacial fluctuations the mediterranean sea was the most probable source of local precipitation throughout the last glacial cycle

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

climate cycles in the southern levant and their global climatic connections abstract the history of levant paleo hydrology which is expressed by the absolute water level record of the dead sea over the last 70 kyrs has been shown to be closely coupled with global climate patterns and specifically argued to be modulated by northern hemisphere climate systems here this coupling is rigorously tested and the absolute lake level curve is compared with a variety of globally distributed climate archives and used to deconvolve the processes controlling regional climate and hydrology considering that different archives are based on different sampling resolutions and hence might have inherently different recording sensitivities each of the chosen archives was interpolated and then smoothed at varying temporal resolutions the synthetic curves ranging between temporal smoothing resolutions between 10 years and 15 kyrs were compared to the similarly processed lake level record over the last 60 kyrs the best fit to the dead sea lake level was found to be with antarctica temperatures and atmospheric co2 levels greenland temperatures and proximal archives such as the soreq cave and the dead sea lithological record all yielded weaker correlations to the lake level curve an exception to the strong correlation between levant climate and atmospheric co2 concentrations is observed during marine isotope stage mis 5e at a time when the dead sea watershed was exposed to very weak mediterranean westerly systems that imposed hyperarid conditions and a superposition of southern derived systems that delivered short but intense precipitation to the region at this time antarctic temperatures cross a temperature threshold beyond 1 c above present it is suggested that this exceptional warming phase induces a reorganization of global and regional climate systems influencing the levant these results have two important implications the first is the understanding that global temperatures and atmospheric co2 concentrations impose the first order control over levant climate the second is that the antarctic records can be used as synthetic lake level proxies extending the absolute lake level curve to ca 800 ka compared with its current span of 70 kyrs considering the importance of the dead sea lake level curve as a regional paleo climatic hydrologic recorder the new synthetic curve provides a prominent proxy of levant hydro climate history

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

texture and elastic anisotropy of a mylonitic anorthosite from the morin shear zone quebec canada a sample of anorthosite from the granulite facies morin shear zone quebec canada was investigated for crystal preferred orientation and elastic anisotropy time of flight neutron diffraction data obtained with the hippo diffractometer at lansce were analyzed with the rietveld method to obtain orientation distribution functions of the principal phases plagioclase clinopyroxene and orthopyroxene texture and microstructures are compatible with the plastic deformation of the aggregate under high t conditions all mineral phases depict a significant preferred orientation that could be related to the general top to the north shearing history of the morin shear zone texture patterns suggest that 010 001 in plagioclase and 110 001 in clinopyroxene are likely dominant slip systems using preferred orientation data p and s waves velocities and elastic anisotropy were calculated and compared with previous studies to explore elastic properties of rocks with different pyroxene plagioclase mixtures p wave velocity s wave splitting and anisotropy increase with clinopyroxene content seismic anisotropy is linked to the texture symmetry which can lead to large deviations between actual anisotropy and that measured along cartesian xyz sample directions lineation foliation reference frame this is significant for the prediction and interpretation of seismic data particularly for monoclinic or triclinic texture symmetries

the end of the cretaceous sharp boundary or gradual transition evidence indicates that the cretaceous tertiary boundary is very sharp and within the limits of resolution it is apparently synchronous at the various boundary localities arguments to the contrary particularly those of officer and darke are shown to invalid

how well can we simulate past climates evaluating the models using global palaeoenvironmental datasets abstract global syntheses of palaeoenvironmental data are required to test climate models under conditions different from the present data sets for this purpose contain data from spatially extensive networks of sites the data are either directly comparable to model output or readily interpretable in terms of modelled climate variables data sets must contain sufficient documentation to distinguish between raw primary and interpreted secondary tertiary data to evaluate the assumptions involved in interpretation of the data to exercise quality control and to select data appropriate for specific goals four data bases for the late quaternary documenting changes in lake levels since 30 14 c kyr bp the global lake status data base vegetation distribution at 18 14 c kyr and 6 14 c kyr bp biome 6000 aeolian accumulation rates during the last glacial interglacial cycle dirtmap and tropical terrestrial climates at the last glacial maximum the lgm tropical terrestrial data synthesis are summarised each has been used to evaluate simulations of last glacial maximum lgm 21 calendar kyr bp and or mid holocene 6 cal kyr bp environments comparisons have demonstrated that changes in radiative forcing and orography due to orbital and ice sheet variations explain the first order broad scale in space and time features of global climate change since the lgm however atmospheric models forced by 6 cal kyr bp orbital changes with unchanged surface conditions fail to capture quantitative aspects of the observed climate including the greatly increased magnitude and northward shift of the african monsoon during the early to mid holocene similarly comparisons with palaeoenvironmental datasets show that atmospheric models have underestimated the magnitude of cooling and drying of much of the land surface at the lgm the inclusion of feedbacks due to changes in ocean and land surface conditions at both times and atmospheric dust loading at the lgm appears to be required in order to produce a better simulation of these past climates the development of earth system models incorporating the dynamic interactions among ocean atmosphere and vegetation is therefore mandated by quaternary science results as well as climatological principles for greatest scientific benefit this development must be paralleled by continued advances in palaeodata analysis and synthesis which in turn will help to define questions that call for new focused data collection efforts

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

changing moisture sources over the last 330 000 years in northern oman from fluid inclusion evidence in speleothems speleothems from hoti cave in northern oman provide a record of continental pluvial periods over the last 330 000 yr periods of rapid speleothem deposition occurred from 6000 to 10 500 78 000 to 82 000 120 000 to 135 000 180 000 to 200 000 and 300 000 to 330 000 yr ago with little or no growth during the intervening periods during each of these five pluvial periods d values of water extracted from speleothem fluid inclusions dfi are between 60 and 20 vsmow and 18o values of speleothem calcite 18oc are between 12 and 4 to vpdb these values are much more negative than modern rainfall for d or modern stalagmites for 18o previous work on the isotopic composition of rainfall in oman has shown that northern and southern moisture sources are isotopically distinct combined measurements of the d values of fluid inclusion water with calculated 18o values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern indian ocean moisture source when the monsoon rainfall belt moved northward and reached northern oman during each of these periods

a new look at sedimentation rates and the completeness of the stratigraphic record several recent papers have suggested that completeness of stratigraphic sections can be estimated by using the ratio of long term sedimentation rates to short term rates we show that the median short term sedimentation rates used in these papers are erroneous and are mostly an artifact arising from improper data manipulation to demonstrate the artificial character of short term median rates we have generated a series of plots assuming a constant sedimentation rate with fixed variance and defined limits of measurement precision these plots demonstrate that increasing variance in the time required to deposit sediments will result in spuriously high short term median sedimentation rates a new method for calculating median sedimentation rates which eliminates the problem of spurious median rates is then applied to make an improved estimate of expected completeness recalculated median values show that sedimentation rates for pelagic sediments are close to identical for both short term and long term tim

eemian tropical and subtropical african moisture transport an isotope modelling study during the last interglacial insolation maximum eemian mis 5e the tropical and subtropical african hydrological cycle was enhanced during boreal summer months the climate anomalies are examined with a general circulation model echam4 that is equipped with a module for the direct simulation of 18o and deuterium h218o and hdo respectively in all components of the hydrological cycle a mechanism is proposed to explain the physical processes that lead to the modelled anomalies differential surface heating due to anomalies in orbital insolation forcing induce a zonal flow which results in enhanced moisture advection and precipitation increased cloud cover reduces incoming short wave radiation and induces a cooling between 10 n and 20 n the isotopic composition of rainfall at these latitudes is therefore significantly altered increased amount of precipitation and stronger advection of moisture from the atlantic result in isotopically more depleted rainfall in the eemian east african subtropics compared to pre industrial climate the east west gradient of the isotopic rainfall composition reverses in the eemian simulation towards depleted values in the east compared to more depleted western african rainfall in the pre industrial simulation the modelled re distribution of 18o and d is the result of a change in the forcing of the zonal flow anomaly we conclude that the orbitally induced forcing for african monsoon maxima extends further eastward over the continent and leaves a distinct isotopic signal that can be tested against proxy archives such as lake sediment cores from the ethiopian region

large variations of oxygen isotopes in precipitation over south central tibet during marine isotope stage 5 oxygen isotope 18 o records of two stalagmites from tianmen cave on the south central tibetan plateau grew during much of marine isotope stage 5 the tianmen record the fi rst cave record from the tibetan plateau characterizes a precipitation 18 o history larger in amplitude but similar in structure to asian monsoon records from the adjacent regions providing essential evidence that the asian monsoon system including the east asian and indian monsoon subsystems responds largely to changes in northern hemisphere summer insolation extremely low 18 o values during marine isotope stages 5a 5c and 5e suggest that precipitation rather than temperature was a major factor controlling 18 o in precipitation on orbital time scales in the south central tibetan plateau the tianmen record may help in the interpretation of regional ice core 18 o records the large range of orbital scale shifts in meteoric 18 o 9 raises important considerations related to reconstructing the uplift history of the plateau

tropical precipitation and cross equatorial ocean heat transport during the mid holocene abstractsummertime insolation intensified in the northern hemisphere during the mid holocene resulting in enhanced monsoonal precipitation in this study the authors examine the changes in the annual mean tropical precipitation as well as changes in atmospheric circulation and upper ocean circulation in the mid holocene compared to the preindustrial climate as simulated by 12 coupled climate models from pmip3 in addition to the predominant zonally asymmetric changes in tropical precipitation there is a small northward shift in the location of intense zonal mean precipitation mean itcz in the mid holocene in the majority 9 out of 12 of the coupled climate models in contrast the shift is southward in simulations using an atmospheric model coupled to a slab ocean the northward mean itcz shift in the coupled simulations is due to enhanced northward ocean heat transport across the equator oht eq which demands a compensating southward atmospheric energy transport across the equator accomplished

both differential and equatorial heating contributed to african monsoon variations during the mid holocene the sahara was significantly greener 11 5 kya and during multiple earlier interglacial periods but the mechanisms related to the greening of the sahara remain uncertain as most climate models severely underestimate past wet conditions over north africa the variations in the african monsoon related to the greening of the sahara are thought to be associated with the variations in the inter hemispheric differential heating of earth caused by orbital variations however how orbital variations affect regional climate is not well understood using recent theory that relates the position of the tropical rain belt to the atmospheric energy budget we study the effect of orbital forcing during the mid holocene on the african monsoon in simulations provided by the third phase of the paleo model intercomparison project pmip3 we find that energy fluxes in the african sector are related to orbital forcing in a complex manner contrary to generally accepted theory orbital modulation of seasonal differential heating alone is shown to be a weak driver of african monsoon variations instead net atmospheric heating near the equator which modulates the intensity and extent of seasonal migrations of the tropical rain belt is an important but overlooked driver of african monsoon variations a conceptual framework that relates african monsoon variations to both equatorial and inter hemispheric differential solar heating is presented

a thermomechanical model of exhumation of high pressure hp and ultra high pressure uhp metamorphic rocks in alpine type collision belts using a fully coupled numerical thermomechanical model handling strain localization surface processes and ultra high viscosity contrasts 11 orders of magnitude we test a number of possible mechanisms of high pressure hp low temperature lt high temperature ht exhumation in continental collision zones the model considers two end member cases low or high buoyancy of the downgoing crust the first model case predicts three levels of exhumation in the same collisional context the classical corner flow lp lt low pressure low temperature exhumation in the accretionary prism deeper 70 km hp ht exhumation for the thickened subducting crustal sedimentary wedge and ultra hp ht exhumation from the lower crustal chamber forming at the depth of 100 120 km and separated from the upper one by a narrow crustal channel the width of this channel can oscillate in the process of shortening thus controlling the quantity of the crustal material exchanged between the crustal wedge and the lower crustal chamber although both zones of crustal accumulation and the narrow channel between them resemble a vortex shaped nozzle this nozzle appears to be too soft to produce any significant overpressures from the upper crustal wedge the material is exhumed following the ascending shear flow created by the overriding plate assisted by positive buoyancy of the heated crustal material from the lower crustal chamber the material is transported upward to the upper crustal wedge by a flow induced by the asthenospheric traction and a small scale convective instability forming in the lower crustal chamber due to its heating by the overriding asthenosphere in the second modelled case of high buoyancy the latter mechanisms become dominant resulting in hyper fast exhumation of the crust to the surface accelerated or slowed subduction in case of full or partial crustal decoupling respectively and upper plate extension d 2001 elsevier science b v all rights reserved

speleothem based paleoclimate record from northern oman u th age dating and stable isotope measurements of speleothems from hoti cave in northern oman yield paleoclimate information from the region extending to 125 ka the results 1 provide further confirmation of an early holocene wet period in southern arabia extending from some time prior to 9 7 ka and ending at 6 2 ka 2 demonstrate a second period of wetness closely coinciding with the last interglacial period marine isotope stage mis 5e 3 indicate that during mis 5e southern arabia was considerably wetter than during the early holocene and 4 demonstrate that periods of increased monsoon wind strength based on data from marine sediments do not always coincide with evidence of greatly increased precipitation even from nearby continental areas

new insights on chinese cave δ18o records and their paleoclimatic significance abstract in the last two decades stalagmite oxygen isotope 18o records have been widely used to study the evolution of the paleomonsoon nevertheless interpreting cave 18o records in eastern monsoonal china as an indicator of east asian summer monsoon easm rainfall has been intensively debated this study provides new insights into chinese cave 18o records and their paleoclimatic significance based on recent research progress the following points are highlighted 1 chinese cave 18o records do not reflect changes in summer rainfall throughout the whole of eastern monsoonal china which is evident by comparing monitored summer rainfall amount in eastern china and an integrated chinese cave 18osyn proxy 2 chinese cave 18o records can document millennial scale easm events due to the sensitive response to large scale atmospheric circulation changes 3 during the early holocene before 7 ka a decoupled relationship between chinese cave 18o records and easm rainfall as represented by precipitation in north china is attributed to the reduction of rainfall in north china due to the influence of remnant melting ice sheets in the northern high latitude region 4 when the ice sheet boundary conditions were stable and ice sheets were small after 7 ka there is an indirect correlation between the chinese cave 18o and rainfall changes in north china 5 the first continuous speleothem 18o record spanning the entire holocene is synthesized for the core region of the indian summer monsoon ism domain comparison of this integrated ism proxy with chinese cave 18o records reveals that in china cave 18o is primarily a signal of changes in the tropical monsoon and related large scale circulation the coupled relationship between rainfall in north china and tropical monsoon ism rainfall is attributed to the response of downstream circulation to changes in circulation upstream while their decoupled relationship is due to the possible effects of northern high latitude ice sheets on reducing rainfall in north china 6 due to the imprecise definition of monsoon intensity we suggest that cave 18o can be used as an easm proxy if monsoon intensity is defined by changes in atmospheric circulation

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

dynamics of the west african monsoon under mid holocene precessional forcing regional climate model simulations abstract the african humid period ahp about 14 800 yr ago 14 8 5 5 ka ka 1000 yr ago was a time of increased humidity over africa paleoclimate evidence suggests that the west african summer monsoon was stronger and more extensive 6 ka than today and that the saharan desert was green here a regional climate model that produces an excellent simulation of today s climate over northern africa is used to study the dynamics of the monsoon 6 ka changes in insolation atmospheric co2 and vegetation are used to impose 6 ka conditions and the role of each forcing is isolated vegetation is not interactive and the large scale circulation and ssts are fixed at present day values for the 6 ka simulations the regional model produces precipitation increases across the sahel and sahara that are in good agreement with the paleodata however unobserved drying is simulated over the guinean coast region where paleodata are sparse precipitation increases in the sahel are related to a northward shift of th

the detection and illustration of regular sedimentary cycles using walsh power spectra and filtering with examples from the lias of switzerland power spectral analysis provides an objective method for the detection of regular cyclicity in data recording some oscillating parameter or time series digitization of measured stratigraphic sections is a convenient method for generating long time series during the examination of pelagic sequences for cyclicity related to orbital variations or milankovitch cycles the application of walsh power spectral analysis to stratigraphic sections is outlined and band pass filtering using the walsh transform introduced it is shown that variations in sedimentation rate hiatuses bioturbation and variations in compaction have the effect of damping or destroying spectral peaks the wavelength ratios of spectral peaks are often used during studies of milankovitch cycles to decide which orbital cycles are involved however in some circumstances hiatuses are able to alter wavelength ratios averaged walsh power spectra were generated for three liassic limestone shale sequences from breggia gorge switzerland the 21 000 year 21 ka precession cycle was detected in all three cases the 100 ka eccentricity cycle in two cases and the 41 ka obliquity cycle in one filtering revealed that the supposed 21 ka sedimentary cycles are grouped into packets of four or five small and large amplitude cycles the packets reflect the 100 ka modulation of the 21 ka precession cycle local disruption of the packaging can be used to locate hiatuses in pelagic strata

orbitally driven east west antiphasing of south american precipitation the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales a comparison between a speleothem record of precipitation in northeast brazil and rainfall reconstructions from the rest of tropical south america shows that a similar antiphasing operated in the same hemisphere during the holocene the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales this antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the hadley cells1 2 3 4 here we compare an oxygen isotopic record recovered from a speleothem from northeast brazil for the past 26 000 years with existing reconstructions of precipitation in tropical south america5 6 7 8 during the holocene we identify a similar but zonally oriented antiphasing of precipitation within the same hemisphere northeast brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high whereas the rest of southern tropical south america shows opposite characteristics simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation driven monsoon activity our results suggest that convective heating over tropical south america and associated adjustments in large scale subsidence over northeast brazil lead to a remote forcing of the south american monsoon which determines most of the precipitation changes in the region on orbital timescales

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

early pleistocene glacial cycles and the integrated summer insolation forcing long term variations in northern hemisphere summer insolation are generally thought to control glaciation but the intensity of summer insolation is primarily controlled by 20 000 year cycles in the precession of the equinoxes whereas early pleistocene glacial cycles occur at 40 000 year intervals matching the period of changes in earth s obliquity the resolution of this 40 000 year problem is that glaciers are sensitive to insolation integrated over the duration of the summer the integrated summer insolation is primarily controlled by obliquity and not precession because by kepler s second law the duration of the summer is inversely proportional to earth s distance from the sun

orbital scale timing and mechanisms driving late pleistocene indo asian summer monsoons reinterpreting cave speleothem δ18o 1 southeast china cave 18o often interpreted as a pure east asian summer monsoon proxy lags maximum northern hemisphere summer insolation by 2 9 0 3 kyrs at the precession cycle the arabian sea summer monsoon stack lags by 8 1 kyr consistent with 13 other indian and east asian summer monsoon proxies from marine lake and terrestrial archives this 5 kyr phase difference cannot be attributed to age control inadequacies in the marine chronology it requires reconciliation in the context of proxy interpretation both of these lags are incompatible with a direct response to northern hemisphere summer insolation implicating additional forcing mechanisms analysis of heterodynes in the cave 18o spectrum demonstrates that variance contained in the arabian sea summer monsoon proxies also resides in the cave 18o record this variance is subtracted from the cave 18o record yielding a residual that is highly coherent and in phase with precession minima reflecting the impact of winter temperature change on cave 18o meteorological precipitation under cold conditions thus we argue that the timing of light cave 18o peaks cannot be interpreted as reflecting the timing of strong summer monsoons alone the 2 9 kyr precession band phase lag of cave 18o reflects the combined influence of summer monsoon forcing with a phase lag of 8 kyrs relative to precession minima and winter temperature forcing that is in phase with precession minima this interpretation is consistent with modern seasonality in the amount and isotopic composition of rainfall in southeast china

box cores from the equatorial pacific 14c sedimentation rates and benthic mixing abstract carbon 14 determinations on box cores of calcareous ooze from the western and eastern equatorial pacific suggest that patterns of mixed layer ages sedimentation rates and mixed layer thicknesses are controlled by gradients of carbonate dissolution and fertility and by small scale redeposition processes mixed layer ages range from 3000 to 7000 years with a mode between 4000 and 5000 years sedimentation rates range from 0 8 to 2 4 cm 1000 years mixed layer depths calculated according to the box model of mixing range from 7 cm to 16 cm observed thicknesses are about one fourth smaller than calculated ones

how soft is the crust in a series of thermomechanical models of the lithosphere applied to the determination of apparent elastic thickness extension and collision and exhumation burov and coworkers cf e g burov and diament 1995 cloetingh and burov 1996 burov and poliakov 2001 burov et al 2001 model the ductile behaviour of the upper crust sometimes of the whole crust assuming a quartz controlled rheology an extensive comparison of rheological input parameters used in geodynamic modelling fernandez and ranalli 1997 has shown that the dry quartz rheology employed by burov and coworkers results in a crust or upper crust which is about three orders of magnitude softer than in any other model claiming to use the same dry quartz rheology in one case cloetingh and burov 1996 a different set of parameters was included for the purpose of comparison but not used in the calculations the same conclusion is reached by comparing burov and coworkers rheology with the review of lithosphere strength estimates by kohlstedt et al 1995 although experimentally determined rheological parameters show wide scatter this usually results in uncertainties of the order of one order of magnitude or less in estimates of the ductile rheology of the crust of a given composition see ranalli 1995 1997 any larger difference may conceal some problems with the input parameters this would be particularly important in this case as the models by burov and coworkers are at the forefront in the present attempt to understand geodynamic processes and their results are widely used to interpret tectonic features the equation for the strength of a material deforming in power law creep is

chinese stalagmite δ 18 o controlled by changes in the indian monsoon during a simulated heinrich event carbonate cave deposits in india and china are assumed to record the intensity of monsoon precipitation because the 18 o of the carbonate tracks the isotopic signature of precipitation these records show spatially coherent variability throughout the last ice age and suggest that monsoon strength was altered during the millennial scale climate variations known as dansgaard oeschger events and during the heinrich cooling events here we use a numerical climate model with an embedded oxygen isotope model to assess what caused the shifts in the oxygen isotope signature of precipitation during a climate perturbation designed to mimic a heinrich event our simulations show that a sudden increase in north atlantic sea ice extent during the last glacial period leads to cooling in the northern hemisphere reduced precipitation over the indian basin and weakening of the indian monsoon the precipitation is isotopically heavier over india and the water vapour exported to china is isotopically enriched our model broadly reproduces the enrichment of 18 o over northern india and east asia evident in speleothem records during heinrich events we therefore conclude that changes in the 18 o of cave carbonates associated with heinrich events reflect changes in the intensity of indian rather than east asian monsoon precipitation