late oligocene early miocene birth of the taklimakan desert as the world s second largest sand sea and one of the most important dust sources to the global aerosol system the formation of the taklimakan desert marks a major environmental event in central asia during the cenozoic determining when and how the desert formed holds the key to better understanding the tectonic climatic linkage in this critical region however the age of the taklimakan remains controversial with the dominant view being from 3 4 ma to 7 ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert in this study we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy we constrained the initial desertification to be late oligocene to early miocene between 26 7 ma and 22 6 ma we suggest that the taklimakan desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts both of which are closely linked to the tectonic uplift of the tibetan pamir plateau and tian shan which had reached a climatically sensitive threshold at this time

what controlled mid late miocene long term aridification in central asia global cooling or tibetan plateau uplift a review abstract debate continues over whether global cooling or uplift of the tibetan plateau provided the first order driver for the aridification moisture levels of central asia throughout the mid late miocene between about 17 and 5 ma this review attempts to throw new light on the relations between the aridification and these two key factors this paper examines the evolution of miocene climate both moisture and temperature within five separate regions of eurasia to help understand the large scale controls of long term moisture in central asia the five regions include 1 europe 2 high latitude asia 3 the east asian monsoon region 4 the south asian monsoon region and 5 central asia itself because moisture reaching central asia has to firstly cross at least one of the other four regions temperature proxy data compiled from these five regions correlate with ocean temperatures from the global deep sea oxygen isotope records furthermore compiled moisture proxy data from the four regions surrounding central asia co vary and correlate with each other this can be explained by positive feedbacks between drying and cooling and supports the assumption that global cooling provides a dominant driving factor for the drying of eurasia global cooling reduces the amount of water vapor held in the atmosphere and thereby can cause terrestrial drying however in central asia the moisture evolution shows less similarity with its surroundings the uplift of the tibetan plateau including the tianshan mountains could provide a possible explanation for this difference the changing topography resulting from uplift events over time and space strongly influenced the moisture patterns in central asia during miocene times future research on the spatial timing and amplitude of tibetan plateau uplift should be useful to understand the moisture processes in central asia during the miocene

the classification of micrometeorites due to their small size the mineralogical and chemical properties of micrometeorites mms are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites identifying which groups of mm are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical textural and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies albeit with minimal bias towards preconceived genetic models specifically the interpretation of mms requires a rigorous and meaningful classification scheme at present the classification of mms is however at best ambiguous a unified petrological chemical classification scheme is proposed in the current study and is based on observations of several thousand mms collected from antarctic ice

the cr renazzo type carbonaceous chondrite group and its implications abstract a petrologic geochemical and oxygen isotopic study of eight meteorites identified as cr renazzo type chondrites reveals that they have a set of similar properties that distinguishes them from other chondrite groups and confirms cr as a carbonaceous chondrite group characteristics of the cr group include 1 1 abundant large multilayered fe ni metal rich type i chondrules 2 2 abundant matrix and dark inclusions di that contain microchondrules microfragments and magnetite framboids and platelets 3 3 unique assemblages of serpentine and chlorite rich phyllosilicates and ca carbonates 4 4 ca carbonate rims on chondrules 5 5 abundant fe ni metal with a positive ni vs co trend and a solar ni co ratio 6 6 low abundance of refractory rich inclusions 7 7 amoeboid olivine aggregates aoa with mn rich and mn poor forsterite and 8 8 oxygen isotopic compositions that form a unique mixing line the cr chondrites are breccias consisting of two major components the large layered chondrules and the matrix dark inclusions al rais is an anomalous member in having a higher abundance of the dark inclusion component 70 vol and this accounts for its having a higher volatile content than other members of the group the conditions that prevailed during aqueous alteration of the cr chondrites differed from those of other hydrated chondrite groups and may be the result of a higher temperature environment than that for the ci and cm chondrites the cr chondrite components chondrules matrix dark inclusions etc experienced aqueous alteration to differing degrees prior to assembly of the host chondrites and dark inclusions represent a lithology that was altered by water to a greater degree than the rest of the meteorite the overall degree of hydration in cr chondrites varies among meteorites with al rais being the most hydrated the oxygen isotopic variation of the whole rocks chondrules and matrices may be related to degree of hydration indicating that cr chondrites interacted with a water rich component having heavy oxygen and that was the same as ci and cm water

paleoglaciation of the tibetan plateau and surrounding mountains based on exposure ages and ela depression estimates the tibetan plateau holds an ample record of past glaciations and there is an extensive set of glacial deposits dated by exposure dating here a compilation is presented of 10be exposure ages from 485 glacial deposits with 1855 individual samples on the tibetan plateau and ela depression estimates for the glacial deposits based on a simple toe to headwall ratio approach to recalculate the tibetan plateau exposure ages 10be production rates from 24 calibration sites across the world are compiled and recalibrated yielding an updated global reference 10be production rate the recalculated exposure ages from the tibetan plateau glacial deposits are then divided into three groups based on exposure age clustering to discriminate good well clustered from poor scattered deglaciation ages a major part of the glacial deposits have exposure ages affected by prior or incomplete exposure complicating exposure age interpretations the well clustered deglaciation ages are primarily from mountain ranges along the margins of the tibetan plateau with a main peak between 10 and 30 ka indicating glacial advances during the global lgm a large number of deglaciation ages older than 30 ka indicates maximum glaciation predating the lgm but the exposure age scatter generally prohibits accurate definition of the glacial chronology the ela depression estimates scatter significantly but the main part is remarkably low average ela depressions of 337 197 m for the lgm and 494 280 m for the pre lgm indicate restricted glacier expansion

δ13c values of loess total carbonate a sensitive proxy for asian summer monsoon in arid northwestern margin of the chinese loess plateau abstract we measured carbon isotope ratio of loess total carbonates 13 cic from the jingyuan loess profile in the arid northwestern portion of the chinese loess plateau clp in order to examine the effectiveness and sensitivity of loess total carbonate 13 cic values as a potential paleoenvironmental indicator the 13 cic record in the loess profile shows large variations ranging from 1 to 7 during the past 70 ka the most negative 13 cic peaks coincide with paleosol formation s 0 l 1 ss 1 and l 1 ss 2 which occurred under relatively wetter climatic conditions during middle holocene and marine 18 o stages 3 on the other hand the least negative 13 cic values responded to loess accumulation during relative drier phases of lgm and marine 18 o stages 4 while 13 cic variations in the loess profile in arid area demonstrate a clear correlation with changes of total organic carbon isotope 13 coc of bulk sediments which has been widely used as a proxy for the past monsoon precipitation intensity the 13 cic values in the loess profile are more sensitive to the changes of paleohabitat controlled by the asian monsoon variation in the arid area we propose that carbon isotope compositions of total carbonates in loess profile can serve as a sensitive and reliable proxy for the asian summer monsoon in the arid northwestern chinese loess plateau

mineralogical evidence of reduced east asian summer monsoon rainfall on the chinese loess plateau during the early pleistocene interglacials the east asian summer monsoon easm is an important component of the global climate system a better understanding of easm rainfall variability in the past can help constrain climate models and better predict the response of easm to ongoing global warming the warm early pleistocene a potential analog of future climate is an important period to study easm dynamics however existing monsoon proxies for reconstruction of easm rainfall during the early pleistocene fail to disentangle monsoon rainfall changes from temperature variations complicating the comparison of these monsoon records with climate models here we present three 2 6 million year long easm rainfall records from the chinese loess plateau clp based on carbonate dissolution a novel proxy for rainfall intensity these records show that the interglacial rainfall on the clp was lower during the early pleistocene and then gradually increased with global cooling during the middle and late pleistocene these results are contrary to previous suggestions that a warmer climate leads to higher monsoon rainfall on tectonic timescales we propose that the lower interglacial easm rainfall during the early pleistocene was caused by reduced sea surface temperature gradients across the equatorial pacific providing a testable hypothesis for climate models

micrometer and nanometer sized platinum group nuggets in micrometeorites from deep sea sediments of the indian ocean abstract we examined 378 micrometeorites collected from deep sea sediments of the indian ocean of which 175 180 and 23 are i type s type and g type respectively of the 175 i type spherules 13 contained platinum group element nuggets pgns the nuggets occur in two distinct sizes and have distinctly different elemental compositions micrometer m sized nuggets that are 3 m contain dominantly ir os and ru iridium platinum group element or ipge and sub m or nanometer sized 1 m nuggets which contain dominantly pt rh and pd palladium pge or ppge the m sized nuggets are found only one per spherule in the cross section observed and are usually found at the edge of the spherule by contrast there are hundreds of nanometer sized nuggets distributed dominantly in the magnetite phases of the spherules and rarely in the wustite phases both the nugget types are found as separate entities in the same spherule and apparently nugget formation is a common phenomenon among i type micrometeorites however the m sized nuggets are seen in fewer specimens 2 5 of the observed i type spherules in all we analyzed four nuggets of m size and 213 nanometer sized nuggets from 13 i type spherules for platinum group elements chemically the m sized pgns contain chondritic ratios of os ir but are depleted in the more volatile pge pt rh and pd relative to chondritic ratios on the other hand the nanometer sized nuggets contain dominantly pt and rh importantly the refractory pges are conspicuous by their absence in these nanometer nuggets palladium the most volatile pge is highly depleted 1 1 with respect to chondritic ratios in the m sized pgns and is observed in only 17 of 213 nanometer nuggets with concentrations that are just above the detection limit 0 2 distinct fractionation of the pge into ipge ir os ru and ppge seems to take place during the short span of atmospheric entry these observations suggest several implications 1 the observation of fractionated pge in an fe ni system gives rise to the possibility that earth s core could contain fractionated pge 2 the present data support the processes suggested for the fractionated pge patterns observed in the ejecta of ancient meteorite impacts 3 meteoric metals released in the troposphere could contain fractionated pgns in large numbers

chondrules in antarctic micrometeorites previous studies of unmelted micrometeorites 50 m recovered from antarctic ice have concluded that chondrules which are a major component of chondritic meteorites are extremely rare among micrometeorites we report the discovery of eight micrometeorites containing chondritic igneous objects which strongly suggests that at least a portion of coarse grained crystalline micrometeorites represent chondrule fragments six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine grained matrix these particles suggest that at least some coarse grained micrometeorites cgmms may be derived from the same parent bodies as fine grained micrometeorites the new evidence indicates that contrary to previous suggestions the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites

igneous rims on micrometeorites abstract melting of micrometeorites mms due to atmospheric entry heating causes significant changes in the textures mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials despite the abundance of melted mms the nature of melting processes in these materials is poorly constrained in this study mineralogical textural and compositional data on 77 mms with igneous rims are presented which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process the compositions of igneous rims on fine grained mms fgmms are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine grained matrix core enrichments in fe si ni si and mn si in igneous rims compared with unmelted cores within fgmms are suggested to occur due to the migration of fe s eutectic liquids from the core of the particle into the surface melt layer the sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials the presence of igneous rims on fgmms is enabled by the thermal decomposition of phyllosilicates and therefore indicates that fgmms were hydrated particles prior to atmospheric entry in contrast the compositions of igneous rims on coarse grained mms cgmms are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgmms the igneous rims of cgmms are therefore suggested to form by melting of fine grained matrix that was present on the exterior of these particles prior to atmospheric entry coarse grained mms with igneous rims thus were originally composite micrometeoroids which consist of both fine grained and coarse grained materials and are thought to be samples of chondrule like objects the abundance of composite mms to cgmms allows a first estimate of the mean chondrule radius within the parent bodies of mms of 625 m to be made

numbers types and compositions of an unbiased collection of cosmic spherules abstract abstract micrometeorites collected from the bottom of the south pole water well spww may represent a complete well preserved sample of the cosmic dust that accreted on earth from 1100 1500 a d we classified 1588 cosmic spherules in the size range 50 800 pm the collection has 41 barred olivine spherules 17 glass spheres 12 cryptocrystalline spherules 11 porphyritic olivine spherules 12 relic grain bearing spherules 3 scoriaceous spherules 2 i type spherules 1 ca al ti rich cat spherules and 1 g type spherules we also found bubbly glass spherules spherules with glass caps and ones with sulfide coatings particles that are absent from other collections a classification sequence of the stony spherules scoriaceous relic grain bearing porphyritic barred olivine cryptocrystalline glass and cat is consistent with progressive heating and evaporation of fe from chondritic materials the modem day accretion rate and size distribution measured at the spww can account for the stony spherules present in deep sea collection through preferential dissolution of glass and small stony spherules however weathering alone cannot account for the high accretion rate of i type spherules determined for two deep sea collections the spww collection provides data to constrain models of atmospheric entry heating and to assess the effects of terrestrial weathering

koronis asteroid dust within antarctic ice here i report the first discovery of large numbers of ordinary chondrite derived micro meteorites mms recovered from antarctic ice ordinary chondrite derived mms comprise 70 coarse grained igneous particles and are identified on the basis of their distribution of petrologic types accessory mineralogy and minor element compositions all of which are very similar to those of millimeter sized igneous objects known as chondrules from ordinary chondrites and largely distinct from those of other meteorite groups the majority of ordinary chondrite derived mms are unequilibrated materials however 15 are equilibrated indicating parent body metamorphism and have compositions consistent with both h high iron and l low iron chondrites the total abundance of ordinary chondrite derived mms of 18 is similar to predictions by numerical calculations for the abundance of dust generated by the recent breakup of the karin group asteroids of the koronis asteroid family suggesting that these asteroids are the main source of the particles these mms represent the first to be associated with a known asteroid and imply that the karin group progenitor asteroid was a rubble pile asteroid that sampled different depths within the original internally metamorphosed koronis parent body

making the material the formation of silt sized primary mineral particles for loess deposits abstract in 1911 e e free wrote it may be well to point out that there are really two problems of the loess a problem of the origin of material and a problem of deposition the problem of the origin of material has been relatively neglected in comparison to problems of deposition of the formation of deposits and latterly in comparison to studies of loess stratigraphy if we are to understand the true nature of loess this fascinating material we need to be aware of all the significant events in the processing of the particles which form a deposit a large amount of silt sized primary mineral material has to be produced to make a loess deposit loess is a quaternary phenomenon and we see the entire process being completed in quaternary times some powerful quaternary forces are able to produce very large amounts of granular material glacial grinding vigorously supported by smalley 1966a provides an obvious source for some loess material but the vast deposits of north china and central asia have no obvious glacial connection however they are adjacent to high asia a region of vast tectonic activity and an enormous release of terrestrial energy in the last several million years high asia is a large producer of loess silt much of which finds its way south to make the ganges delta but large amounts are moved north and east to make huge loess deposits africa presents a problem recent work e g coude gaussen 1987 suggests that there is a region of peri saharan loess but earlier observations e g penck indicate quite clearly that the sahara region is free of loess rathjens 1928 in his study of tripolitania indicated the presence of loess like deposits and described their nature but he did point out that 90 of the granular material was greater than 100 pm in diameter which would remove it from a strict loess definition on the soil taxonomy map of africa there is a belt of soil d3 which could include considerable loess deposits it stretches from the matmata loess in tunisia across northern libya to the western part of the egyptian coastal region if d3 contains loess this points to a genuine desert loess particle producing mechanism operating in the sahara

the mid pleistocene transition in the tropical pacific a sea surface temperature sst record based on planktonic foraminiferal magnesium calcium ratios from a site in the western equatorial pacific warm pool reveals that glacial interglacial oscillations in sst shifted from a period of 41 000 to 100 000 years at the mid pleistocene transition 950 000 years before the present sst changes at both periodicities were synchronous with eastern pacific cold tongue ssts but preceded changes in continental ice volume the timing and nature of tropical pacific sst changes over the mid pleistocene transition implicate a shift in the periodicity of radiative forcing by atmospheric carbon dioxide as the cause of the switch in climate periodicities at this time

rock magnetic record of core sg 3 since 1 ma in the western qaidam basin and its paleoclimate implications for the ne tibetan plateau abstract quaternary saline lake deposits are well formed and widely distributed in the qaidam basin on the northeastern tibetan plateau nw china studying these lacustrine salt mineral deposits can improve our understanding of tectonic deformation asian interior aridification and monsoon climate associated with global cooling and tibetan plateau uplift previous paleoclimate records in the qaidam basin are mostly documented by geochemical proxies pollen spectra and sedimentary characteristics however supporting evidence from the rock magnetic index of sediments with salt deposits in the basin has not been well studied here we present a continuous high resolution 1 0 myr rock magnetic record from the well dated drill core sg 3 for typical saline lake deposits in the western qaidam basin the results show that deposits with low magnetic susceptibility are caused by low contents of magnetic minerals with single domain sd grains while those with high have relatively high contents of magnetic minerals with sd and pseudo single domain psd grains in full consideration of the sedimentary environment weathering and erosion associated with the global climate since the middle pleistocene we propose that variations in the closed lake are mainly influenced by erosion intensity and precipitation by combining the changes with pollen and grain size records we infer that the drying process of central asia since 1 ma with intensified aridity at 0 6 ma and 0 13 ma was primarily induced by global cooling

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

petrology and geochemistry of antarctic micrometeorites abstract the petrology and geochemistry of twentythree chondritic dust particles with masses of 1 47 g sizes 100 400 m were recovered from blue ice near cap prudhomme antarctica and studied by inaa asem empa and optical microscopy sample selection criteria were irregular shape and for a subsample black color with the aim of studying as many unmelted micrometeorites mms as possible of thirteen unmelted mms six were phyllosilicate dominated mms and seven were coarsegrained crystalline mms consisting mainly of olivine and pyroxene the remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases scoriaceous mms thus of the black particles selected an astonishing portion 40 by number consisted of largely unmelted mms although unmelted most phyllosilicate mms have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed but not melted the original preterrestrial mineralogy is occasionally preserved and consists of serpentine like phyllosilicates with variable amounts of cronstedtite tochilinite like oxides olivine and pyroxene the crystalline mms consist of olivine low ca pyroxene tochilinite like oxides and occasional ni poor metal relics in scoriaceous mms consist of the same phases mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of cm and cr type carbonaceous chondrites however the olivine pyroxene ratio 1 and the lack of carbonates sulfates and of very fe poor refractory element rich olivines and pyroxenes sets the mms apart from cm and cr chondrites the bulk chemistry of the phyllosilicate mms is similar to that of cm chondrites however several elements are either depleted ca ni s less commonly na mg and mn or enriched k fe as br rb sb and au in mms as compared to cm chondrites similar depletions and enrichments are also found in the scoriaceous mms we suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted mms the overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere most mms are coated by magnetite of platy or octahedral habit which is rich in mg al si mn and ni we interpret the magnetites to be products of recondensation processes in the high 90 km atmosphere which are therefore probably the first refractory aerominerals identified

astronomical forcing of terrestrial climate recorded in the pleistocene of the western tarim basin nw china abstract astronomically forced variations of climate are well documented in lacustrine and marine settings however few studies have examined astronomical forced cycles within quaternary fluvial fan successions to explore whether milankovitch cyclicity might also be present in fluvial fan successions we carried out high resolution gamma ray gr from downhole logging magnetic susceptibility ms rb sr and total organic carbon toc analysis of a core from an 800 m borehole kt11 from the kashgar region in the western tarim basin china spectral analyses of the gr ms and rb sr data reveal cycles with 70 m 30 m and 14 m wavelengths these pertain to relative variations in grain size electron spin resonance esr dating and magnetostratigraphy imply that the cored interval spans the past 1 1 myr this age span constraint coupled with the comparison of the ratios of these cycle wavelengths to expected ratios of milankovitch frequencies suggests that these cycles represent 100 kyr short eccentricity 40 kyr obliquity and 20 kyr precession frequencies respectively we constructed an astronomical time scale ats spanning the past 1 13 myr using the 100 kyr short eccentricity period the average accumulation rate is about 70 cm kyr for this 800 m core succession and the evolutive spectrum emphasizes that long term sedimentation rates were stable we also applied correlation coefficient coco analysis to identify the astronomical forcing thus we conclude that orbitally forced climate change was the main driver of stratigraphic alternations between sand and clay rich intervals in the kashgar region during maxima of the 100 kyr short eccentricity cycles which correspond to interglacial intervals more chemical weathering and increased seasonal runoff delivered a larger amount of relatively coarser grained sediments to the fluvial fan in the kashgar region and vice versa the mid pleistocene transition mpt at ca 0 9 to 0 65 ma is also well recorded in our gr and rb sr data and is characterized by a shift in dominant cycle period from 41 kyr obliquity to 100 kyr eccentricity after 0 65 ma the average toc content decreases whilst rb sr values increase which suggests that there was an increase in regional aridity of the kashgar region in the western tarim basin following the mpt this regional change is synchronous with the trend toward aridification observed throughout central asia

the textures and compositions of fine grained antarctic micrometeorites implications for comparisons with meteorites abstract micrometeorites recovered from the earth s surface constitute the most abundant interplanetary dust now falling to earth we studied eighty nine fine grained antarctic micrometeorites fg amms to evaluate their state of alteration and to identify the nature of their precursor materials fine grained amms are divided into melted and unmelted groups and subdivided on the basis of textures related to atmospheric entry heating the textures of melted particles reflect crystallisation after entry heating and those of unmelted amms broadly resemble cl cm and cr chondrite matrix the matrix compositions of the fg amms closely resemble those of cm2 chondrites matrices provide a more reliable comparison to meteorites than bulk compositions because components such as chondrules and cais are not present in representative proportions within individual amms divergences from cm like matrix compositions are generally minor and probably arose during entry heating by the dissociation of volatile bearing phases at subsolidus temperatures and evaporation and loss of immiscible metallic liquids at higher temperatures depletions in ni and mg relative to cm matrix are tentatively attributed to terrestrial weathering no conclusive evidence for contamination during atmospheric residence was observed contrary to theory textural evidence suggests that bow shocks and high thermal gradients existed during deceleration of micrometeoroids in the atmosphere

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

permanent el nino like conditions during the pliocene warm period during the warm early pliocene 4 5 to 3 0 million years ago the most recent interval with a climate warmer than today the eastern pacific thermocline was deep and the average west to east sea surface temperature difference across the equatorial pacific was only 1 5 0 9 c much like it is during a modern el nino event thus the modern strong sea surface temperature gradient across the equatorial pacific is not a stable and permanent feature sustained el nino like conditions including relatively weak zonal atmospheric walker circulation could be a consequence of and play an important role in determining global warmth

seven million years of wind and precipitation variability on the chinese loess plateau abstract recent studies of the eolian deposits in northern china have extended the east asian monsoon history back to the early miocene however the relative intensities of monsoonal winds and precipitation and the extent of their coupling prior to the pleistocene epoch remain poorly constrained mainly due to uncertainties in the interpretation of proxy indices generated from the mio pliocene red clay sequences here we reconstruct east asian monsoon oscillations over the past 7 ma using magnetic susceptibility and carbonate content as summer monsoon precipitation proxies and quartz grain size as a winter monsoon wind intensity index our results suggest that precipitation and wind intensity exhibited significant orbital scale variations prior to 4 2 ma followed by slightly damped variability between 4 2 and 2 75 ma subsequently east asian monsoon circulation experienced two large shifts at about 2 75 and 1 25 ma characterized by stepwise strengthening of glacial wind and interglacial precipitation a remarkable change in east asian monsoon seasonality occurred around 3 15 2 75 ma prior to 3 15 ma strong winds were positively correlated with high effective precipitation whereas after 2 75 ma strong winds were negatively correlated with heavy precipitation this shift was probably induced by a change from an insolation forced system to one strongly influenced by the combined effects of the phased uplift of the himalaya tibetan plateau and the simultaneous development of the northern hemisphere ice sheets

extreme aridification since the beginning of the pliocene in the tarim basin western china abstract mid latitude central asia is characterized by an extreme arid landscape among the central asian deserts the taklimakan desert is the largest shifting sand desert which is located in the rain shadow of the tibetan plateau and of the other central asian high mountains the formation of this desert is important for placing widespread aridification into a regional tectonic context at the western end of the himalayan tibetan orogen however there still exists considerable controversy regarding the timing of desert formation thus impeding our understanding of the climatic effects of the tibetan uplift and regional environmental changes here we report new biostratigraphic age control and multiple high resolution climatic records from the center of the taklimakan desert our results reveal dramatic environmental changes at 5 ma suggesting that an extremely dry climate has prevailed since the beginning of the pliocene which is consistent with findings from a high resolution borehole record about 670 km to the east in the same basin the two records combined demonstrate that an extremely dry environment prevailed in the entire tarim basin from approximately 5 ma this was related to the reduced transport of water vapor by westerlies in response to the retreat of the paratethys ocean driven by global climatic cooling and the closed oceanic water vapor pathway between the pamir and the tian shan ranges driven by the ongoing india eurasia collision

timing cause and impact of the late eocene stepwise sea retreat from the tarim basin west china abstract a vast shallow epicontinental sea extended across eurasia and was well connected to the western tethys before it retreated westward and became isolated as the paratethys sea however the palaeogeography and the timing of this westward retreat are too poorly constrained to determine potential wider environmental impacts let alone understanding underlying mechanisms of the retreat such as global eustasy and tectonism associated with the indo asia collision here an improved chronostratigraphic and palaeogeographic framework is provided for the onset of the proto paratethys sea retreat at its easternmost extent in the tarim basin in western china is provided five different third order sea level cycles can be recognised from the cretaceous palaeogene sedimentary record in the tarim basin of which the last two stepped successively westwards as the sea retreated after the maximum third incursion new biostratigraphic data from the fourth and fifth incursions at the westernmost margin of the tarim basin are compared to our recent integrated bio magneto stratigraphic results on the fourth incursion near the palaeodepocentre in the south western part of the basin while the fourth incursion extended throughout the basin and retreated at 41 ma base c18r the last and fifth incursion is restricted to the westernmost margin and its marine deposits are assigned a latest bartonian early priabonian age from 38 0 to 36 7 ma near top c17n 2n to base c16n 2n similar to the fourth the fossil assemblages of the fifth incursion are indicative of shallow marine near shore conditions and their widespread distribution across eurasia suggests that the marine connection to the western tethys was maintained the lack of diachronicity of the fourth incursion between the studied sections across the southwest tarim basin suggests that the sea entered and withdrew relatively rapidly as can be expected in the case of eustatic control on a shallow epicontinental basin however the westward palaeogeographic step between the fourth and fifth incursions separated by several millions of years rather suggests the combined long term effect of tectonism possibly associated with early uplift of the pamir kunlun shan thrust belt the fourth and fifth regressions are time equivalent with significant aridification steps recorded in the asian interior thus supporting climate modelling results showing that the stepwise sea retreat from central asia amplified the aridification of the asian interior

large scale hydrological change drove the late miocene c4 plant expansion in the himalayan foreland and arabian peninsula carbon isotope changes in paleosols from siwalik pakistan and marine sediments from the bengal fan indicate a major c 4 plant expansion in the himalayan foreland during the late miocene however the timing and mechanisms behind the c 4 plant expansion remain enigmatic here we present high resolution 60 k y biomarker and compound specific isotope data spanning the past 11 m y from ocean drilling program site 722 in the arabian sea an 5 6 increase in leaf wax 13 c values indicates a marked rise of c 4 plants from 10 to 5 5 ma with accelerated expansion from 7 9 to 5 5 ma a concurrent 50 rise in leaf wax d values is attributed to a combined effect of changes in precipitation amount and evaporation indicating that source regions for the plant waxes became progressively drier from 10 to 5 5 ma in contrast to earlier reports our isotope records biomarker abundances alkenone u k 37 and globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval rather our results suggest that large scale hydrological changes drove the late miocene expansion of c 4 plants in the himalayan foreland and arabian peninsula

late amazonian glaciation at the dichotomy boundary on mars evidence for glacial thickness maxima and multiple glacial phases lineated valley fill lvf in fretted valleys at the dichotomy boundary has been interpreted as glacial in origin unknown are 1 the original thickness of the glacier ice 2 the amount of ice surface lowering through sublimation and retreat to its presently observed level and 3 whether there were multiple periods of glaciation we address these questions through analysis of an lvf glacial system the elevation difference between the upper limit of a previous highstand and the current surface of the lvf at the study site is 920 m we interpret this difference to be the minimum amount of ice surface lowering of the glacier system consistent with a general lowering of the ice surface are multiple moraines and or trimlines and changes in lvf flow patterns as the ice retreated and decreased in thickness the superposition of several lobes onto the current surface of the lvf indicates that a phase of alpine glaciation followed the lowering of the valley glacial system these data suggest that the late amazonian glaciation that produced lvf in this region involved significantly larger amounts of ice than previously thought and that subsequent alpine glaciation followed

the origins of i type spherules and the atmospheric entry of iron micrometeoroids the earth s extraterrestrial dust flux includes a wide variety of dust particles that include feni metallic grains during their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed i type spherules these particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 au this article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials the results indicate that iron micrometeoroids experience peak temperatures 300 800 k higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of 50 m the lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates which enhances their abundance among micrometeorites by a factor of 2 the abundance of i types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite derived mms arriving at earth furthermore carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the i type spherule flux events involving such objects therefore cannot be recognized from i type spherule abundances in the geological record

nature and timing of quaternary glaciation in the himalayan tibetan orogen abstract much effort has been made in recent years to define the timing and extent of quaternary glaciation throughout the himalayan tibetan orogen these studies are challenging because of the logistical and political inaccessibility of the region and the inherent problems associated with the application of numerical dating techniques nevertheless the studies are providing abundant evidence for significant glacial advances throughout the last several glacial cycles and are beginning to accurately define the extent and timing of glaciation in selected regions studies are showing that himalayan tibetan glaciers in arid regions during the last glacial cycle reached their maximum extent early in the cycle and that global last glacial maximum glacier advances were significantly less extensive however along the more monsoonal influenced greater himalaya there is increasing evidence to suggest that glaciation was more extensive later in the last glacial cycle but this has yet to be fully assessed in addition the new studies are showing that throughout most himalayan tibetan regions significant glacier advances occurred during the lateglacial and early holocene with minor advances in some regions during the mid holocene the still relatively poor chronological control in the himalayan tibetan orogen however makes it difficult to construct correlations across the region and with regions elsewhere in the world this in turn makes it hard to assess the relative importance of the different climatic mechanisms that force glaciation across the himalayan tibetan orogen and to quantify paleoclimate change in this high altitude subtropical region the lateglacial and holocene glacial records however are particularly well preserved in several himalayan tibetan regions glacial successions such as these have the greatest potential to be examined in detail using newly developing numerical dating and geomorphic and sedimentologic methods to derive high resolution terrestrial records of glaciation that will help in paleoclimatic reconstruction for high altitude subtropical regions

noble gases in fossil micrometeorites and meteorites from 470 myr old sediments from southern sweden and new evidence for the l chondrite parent body breakup event we present noble gas analyses of sediment dispersed extraterrestrial chromite grains recovered from similar to 470 myr old sediments from two quarries hallekis and thorsberg and of relict chromites in a coeval fossil meteorite from the gullhogen quarry all located ill southern sweden both the sediment dispersed grains and the meteorite gullhogen 001 were generated in the l chondrite parent body breakup about 470 myr ago which was also the event responsible for the abundant fossil meteorites previously found in the thorsberg quarry trapped solar noble gases in the sediment dispersed chromite grains have partly been retained during similar to 470 myr of terrestrial residence and despite harsh chemical treatment in the laboratory this shows that chromite is highly retentive for solar noble gases the solar noble gases imply that a sizeable fraction of the sediment dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites the grains in the oldest sediment beds were rapidly delivered to earth likely by direct injection into all orbital resonance in the inner asteroid belt whereas grains in younger sediments arrived by orbital decay due to poynting robertson p r drag the fossil meteorite gullhogen 001 has a low cosmic ray exposure age of similar to 0 9 myr based on new he and ne production rates in chromite determined experimentally this age is comparable to the ages of the fossil meteorites from thorsberg providing additional evidence for very rapid transfer times of material after the l chondrite parent body breakup

melted micrometeorites from antarctic ice with evidence for the separation of immiscible fe ni s liquids during entry heating we report the discovery of four large 50 m cosmic spherules css and a single scoriaceous micrometeorite smm that contain evidence for the separation of immiscible fe ni s liquids during atmospheric entry heating the particles contain segregated fe rich regions dominated by either ni s bearing fe oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids the oxides which may be hydrous are thought to result from alteration of metal and sulphide the compositions of the silicate portions of the css are equivalent to spherules without fe rich regions implying that metallic liquids are exsolved during the heating of most spherules but completely separate the single smm has a very different composition from other scoriaceous particles and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating the pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually c rich this smm might be therefore an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle idp the exsolution of immiscible fe ni s liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt

fe ni metal and sulfide minerals in cm chondrites an indicator for thermal history abstract cm chondrites were subjected to aqueous alteration and in some cases to secondary metamorphic heating the effects of these processes vary widely and have mainly been documented in silicate phases herein we report the characteristic features of fe ni metal and sulfide phases in 13 cm and 2 cm related chondrites to explore the thermal history of these chondrites the texture and compositional distribution of the metal in cm are different from those in unequilibrated ordinary and co chondrites but most have similarities to those in highly primitive chondrites such as ch cr and acfer 094 we classified the cm samples into three categories based on metal composition and sulfide texture fe ni metal in category a is kamacite to martensite category b is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite opaque mineral assemblages of category c are typically kamacite ni co rich metal and pyrrhotite these categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration the characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration category a cm chondrites are unheated whereas those in category b experienced small degrees of secondary heating cms in category c were subjected to the most severe secondary heating process thus opaque minerals can provide constraints on the thermal history for cm chondrites

impact of diabatic heating over the tibetan plateau on subsidence over northeast asian arid region 1 the formation mechanism of the arid climate over northeast asia is investigated by a series of regional climate model experiments precipitation there is suppressed by a prominent regional scale subsidence during summertime an experiment without diabatic heating shows very gentle vertical motion there whereas the subsidence is prominent when diabatic heating by solar radiation is included in the model additionally the other experiments indicate that an artificial heating over the tibetan plateau enhances the subsidence over the arid area which is similar to the observed distribution these results suggest that the thermal effects of the plateau induce the prominent subsidence the mechanical effects of the topography such as stationary mountain waves are not a principal reason for the subsidence nor the arid climate the stationary rossby wave driven by the diabatic heating over the plateau tends to propagate northward during boreal summer since subtropical westerly jet locates north of the plateau

late miocene global cooling and the rise of modern ecosystems a period of continental aridification and ecosystem change occurred about seven million years ago a global sea surface temperature reconstruction identifies cooling temperatures and a strengthened meridional temperature gradient at this time

characteristics and mass distribution of extraterrestrial dust from the greenland ice cap extraterrestrial grains with sizes 50 m can now be extracted in large numbers from terrestrial sediments but their origins are still uncertain at least 99 of these grains have been thought to be spherules1 2 resulting from the melting of their parent bodies in the atmosphere which destroyed their mineralogical composition here we present analyses of extraterrestrial grains extracted from six samples of cryoconite3 black dust collected from the melt zone of the greenland ice cap in addition to families of grains never reported before we have found a surprisingly high abundance of unmelted chondritic fragments in which the nonvolatile component of the parent bodies has been well preserved the mass distribution of the grains is very similar to that of the micrometeorite flux at 1 au ref 4 indicating that most of the grains are micrometeorites and not ablation products of larger meteorites these results yield new constraints on the ablation of such greenland micrometeorites as well as guidelines for using them as glaciological and geological markers

tropical ocean temperatures over the past 3 5 million years determining the timing and amplitude of tropical sea surface temperature sst change is an important part of solving the puzzle of the plio pleistocene ice ages alkenone based tropical sst records from the major ocean basins show coherent glacial interglacial temperature changes of 1 to 3 c that align with but slightly lead global changes in ice volume and deep ocean temperature over the past 3 5 million years tropical temperatures became tightly coupled with benthic 18o and orbital forcing after 2 7 million years we interpret the similarity of tropical sst changes in dynamically dissimilar regions to reflect top down forcing through the atmosphere the inception of a strong carbon dioxide greenhouse gas feedback and amplification of orbital forcing at 2 7 million years ago connected the fate of northern hemisphere ice sheets with global ocean temperatures since that time

new evidence on the age of the taklimakan desert the taklimakan desert is the world s second largest shifting sand desert located in the rain shadow of the tibetan plateau the initiation of desert formation in the asian interior is one of the most prominent climate changes in the northern hemisphere during the cenozoic era dating the earliest formation of this desert is important for understanding the climatic effects of the uplifted tibetan plateau here we report 1071 m thick neogene deposits with inter calated eolian dune sands from the only outcrop in the central taklimakan desert based on paleo magnetic measurements biostratigraphic age control and previous electron spin resonance dating results the oldest in situ eolian dune sands were confi rmed to be ca 7 ma much older than the previous reported wind blown siltstone from the southern edge of the taklimakan desert this new result indicates that desert conditions initiated ca 7 ma in the center of the tarim basin we attribute this event to the late cenozoic climatic deterioration and or to the rain shadow effect caused by late miocene uplift of the northern tibetan margin

three dimensional petrography of metal phases in equilibrated l chondrites effects of shock loading and dynamic compaction abstract to examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring we have performed a quantitative three dimensional 3d study of metal grains in a suite of increasingly shocked l chondrites with synchrotron x ray microtomography xmt our data allow rigorous quantification of size number distributions and collective morphology of fe ni metal phases in chondritic meteorites at the resolution of our xmt measurements 8 4 17 9 m voxel the number of metal particles increase with higher degrees of petrographically identified shock loading indicating a coalescing of fe ni metal at or below this scale our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact related compaction and shock loading ductile metal grains in l chondrites begin to show foliation at peak shock pressures

cometary dust in antarctic ice and snow past and present chondritic porous micrometeorites preserved on the earth s surface abstract chondritic porous interplanetary dust particles cp idps collected in the stratosphere are regarded as possibly being cometary dust and are therefore the most primitive solar system material that is currently available for analysis in laboratories in this paper we report the discovery of more than 40 chondritic porous micrometeorites cp mms in the surface snow and blue ice of antarctica which are indistinguishable from cp idps the cp mms are botryoidal aggregates composed mainly of sub micrometer sized constituents they contain two components that characterize them as cp idps enstatite whiskers and gems glass with embedded metal and sulfides enstatite whiskers appear as 2 o 6 rich high ca pyroxene roedderite k na 2 mg 5 si 12 o 30 and carbonaceous nanoglobules these components have previously been discovered in primitive solar system materials such as the cp idps matrices of primitive chondrites phyllosilicate rich mms ultracarbonaceous mms and cometary particles recovered from the 81p wild 2 comet the most outstanding feature of these cp mms is the presence of kosmochlor rich high ca pyroxene and roedderite which suggest that they have building blocks in common with cp idps and cometary dust particles and therefore suggest a possible cometary origin of both cp mms and cp idps it is therefore considered that cp mms are cp idps that have fallen to earth and have survived the terrestrial environment

the neogene de greening of central asia there remains substantial debate concerning the relative roles of tectonics and global climate in driving the evolution of climate in central asia today interior asia including the taklamakan gobi and ordos deserts is exceptionally arid and is surrounded by distinct rainfall boundaries such as those generated by the asian monsoon systems to the east and south and those generated by high topography to the north and west determining how and why these boundaries have varied over the neogene is hindered by the lack of a single proxy that can be broadly applied through space and time we construct isoscapes of pedogenic carbonate 13 c 13 c c over the neogene in asia by combining a compilation of 2236 published measurements with new data from three localities in northern central asia pedogenic carbonate 13 c records local aridity excepting localities impacted by c 4 grasslands and during large changes in atmospheric p co 2 through variations in soil respiration depth of carbonate formation and the effect of water stress on plant 13 c together these effects reflect changes in both primary productivity and mean annual precipitation throughout the neogene we find consistent and exceptionally high 13 c c in interior asia with a ring of lower 13 c c that demarcates higher precipitation this persistent ring of lower 13 c c corresponds in the south and east with the influence of the asian monsoon systems in the west and north it reflects both orographic rainfall due to uplift of the tian shan and to moisture delivery by the mid latitude westerlies finally 13 c c and hence aridity increases regionally in the latest neogene reflecting the effects of northern hemisphere glaciation and cooling this widespread de greening would have increased regional albedo and modified basin scale water balances resulting in greater dust fluxes due to reduced vegetative cover and precipitation