apatite fission track evidence for the cretaceous cenozoic cooling history of the qilian shan nw china and for stepwise northeastward growth of the northeastern tibetan plateau since early eocene abstract apatite fission track aft data from hinterland of the qilian shan at the northeastern margin of the tibetan plateau suggest this range has experienced northeastward propagation of surface uplift since early eocene and that crustal shortening occurred in the qilian shan before the late miocene thermochronometry data indicate that the qilian shan experienced a three stage cooling history including 1 rapid initial cooling during cretaceous 2 a stage of slow cooling during late cretaceous early eocene and 3 rapid stepwise cooling in a southwestern northeastern orientation since early eocene cretaceous rapid cooling may be a record of the lhasa block and eurasian collision early cretaceous denudation was followed by tectonic and quasi isothermal quiescence that continued until early eocene early eocene rapid cooling in the south qilian shan may be the first far field response in the qilian shan to the collision and convergence of the indian and eurasian continents from late eocene to middle miocene crustal shortening propagated into the central qilian shan and north qilian shan and produced surface uplift of the entire qilian shan region before the late miocene this study provides a better understanding of the tectonic evolution of the qilian shan and when the far field stress from the india eurasia collision into the northeastern tibetan plateau began

shortening of lower crust beneath the ne tibetan plateau abstract we reprocessed a high resolution deep seismic reflection profile to reveal the detailed crustal structure of the junction between the qilian shan nan shan thrust belt and the hexi corridor basin in the ne tibetan plateau the profile shows that the moho discontinuity is characterized by complex dislocations and overlapping geometries restoration of the deformed moho discontinuity reveals 50 2 km 35 5 of cenozoic lower crustal shortening along the seismic profile compression of the lower crust produces crustal thickening of 12 8 km this demonstrates that lower crustal shortening plays a significant role in topographic uplift and crustal thickening at the ne margin of the tibetan plateau thus we propose a double layered shortening mechanism with decoupled deformation of upper crust and lower crust the proposed deformation mechanism differs from previous crustal shortening models for the northern eastern and northeastern tibetan plateau our model eliminates the need for lower crustal flow to explain topographic uplift along the ne margin of the tibetan plateau

differentiating bubble free layers from melt layers in ice cores using noble gases melt layers are clear indicators of extreme summer warmth on polar ice caps the visual identification of refrozen meltwater as clear bubble free layers cannot be used to study some past warm periods because in deeper ice bubbles are lost to clathrate formation we present here a reliable method to detect melt events based on the analysis of kr ar and xe ar ratios in ice cores and apply it to the detection of melt in clathrate ice from the eemian at neem greenland additionally melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases or by altering gas transport in the firn which affects the gas chronology we find that the easily visible 1 mm thick bubble free layers in the wais divide ice core do not contain sufficient melt to alter the gas composition in the core and do not cause artifacts or discontinuities in the gas chronology the presence of these layers during winter and the absence of anomalies in soluble gases suggests that these layers can be formed by processes other than refreezing of meltwater consequently the absence of bubbles in thin crusts is not in itself proof of a melt event

co2 record between 40 and 8 kyr b p from the greenland ice core project ice core co2 ice core records show an increase in the atmospheric concentration of 80 100 parts per million by volume ppmv from the last glacial maximum lgm to the early holocene we present co2 measurements performed on an ice core from central greenland drilled during the greenland ice core project grip this co2 profile from grip confirms the most prominent co2 increase from the lgm with a mean concentration of 200 ppmv to the early holocene with concentrations between 290 and 310 ppmv some structures of the new co2 record are similar to those previously obtained from the dye 3 ice core greenland which indicated a dilemma between greenland and antarctic co2 records oeschger et al 1988 both greenland cores show high co2 values for rather mild climatic periods during the last glaciation whereas co2 records from antarctica do not show such high co2 variations during the glaciation and furthermore the co2 values in the early holocene are about 20 30 ppmv higher in the grip record than in antarctic records there is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the co2 concentration under certain conditions if in situ processes can change the co2 concentration in the air bubbles the question arises about how reliably do co2 records from ice cores reflect the atmospheric composition at the time of ice formation the discrepancies between the co2 profiles from greenland and antarctica can be explained by in situ production of excess co2 due to interactions between carbonate and acidic species since the carbonate concentration in antarctic ice is much lower than in greenland ice co2 records from antarctica are much less affected by such in situ produced co2

mesozoic and cenozoic tectonics of the northern edge of the tibetan plateau fission track constraints abstract fission track analysis on zircons and apatites yields new information about the timing of deformation of the northern tibetan plateau ages on zircons ranging from 221 22 to 96 4 ma are indicative of a general late triassic early jurassic cooling probably driven by the collision between the qiantang and kunlun blocks mid jurassic slow cooling is recorded also in the apatites in regions not affected by later cenozoic deformation this jurassic denudation was followed by a period of sedimentation during the cretaceous except along the altyn tagh fault atf zone and in some restricted areas of the western and eastern qilian shan this long and relatively quiet period ended at about 40 10 ma along the major altyn tagh and kunlun strike slip fault zones which were activated by the india asia collision this first movement along lithospheric faults resulted in the eastward extrusion of the tibet plateau which was followed in late oligocene miocene times by a major compression event initiating the formation of the high relief of north tibet a final compressional event took place at 9 5 ma and is well correlated with high sedimentation rates in the basins of this region this compression induced continental subduction in the kunlun ranges the altun shan belt and possibly the qilian shan belt

oligocene storm induced lacustrine deposits in the yaxi area of the jiuxi basin northeastern margin of the tibetan plateau abstract storm induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow thin bedded sandstone siltstone and interlayered mudstone deposits of the oligocene jianquanzi member in the yaxi area of the jiuxi basin along the northeastern margin of the tibetan plateau were formed by shallow lacustrine storm generated flows petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub mature to immature based on investigation of drill cores six storm deposit lithofacies are identified including massive or graded sandstone parallel and trough cross bedded sandstones hummocky cross stratified sandstone wave rippled sandstone and siltstone massive or horizontal bedded mudstone and structurally deformed sandstone and mudstone in a vertical succession these lithofacies occur as three fining upward lithofacies associations each of which represents different architectural elements and depositional processes of proximal to distal storm deposits the relatively coarse proximal architectural element is formed by strong density flows and unidirectional dominated combined flows the transitional architectural element is dominated by unidirectional and oscillatory dominated combined flows to pure oscillatory flows resembling tb e divisions of the classic bouma sequence the relatively fine distal architectural element is dominated by unidirectional dominated combined flows with a very weak oscillatory flow component the origin of storms responsible for these deposits is attributed to asian winter monsoons associated with the paleoclimatic and paleogeographic setting

response of atmospheric co2 to the abrupt cooling event 8200 years ago atmospheric co2 records for the centennial scale cooling event 8200 years ago 8 2 ka event may help us understand climate carbon cycle feedbacks under interglacial conditions which are important for understanding future climate but existing records do not provide enough detail here we present a new co2 record from the siple dome ice core antarctica that covers 7 4 9 0 ka with 8 to 16 year resolution we observe a small about 1 2 ppm increase of atmospheric co2 during the 8 2 ka event the increase is not significant when compared to other centennial variations in the holocene that are not linked to large temperature changes our results do not agree with leaf stomata records that suggest a co2 decrease of up to 25 ppm and imply that the sensitivity of atmospheric co2 to the primarily northern hemisphere cooling of the 8 2 ka event was limited

an immediate response to the indian eurasian collision along the northeastern tibetan plateau evidence from apatite fission track analysis in the kuantan shan hei shan abstract the tibetan plateau offers an ideal region for understanding the processes of continental collision and related intracontinental deformation however when the cenozoic deformation was initiated in the present northeastern margin of the plateau 2000 km north to the indian eurasian collision front remains an issue in debate two competing end member models have been proposed involving immediate deformation response and progressive deformation propagation solving this problem partly relies on determining whether there was contemporaneous deformation with initial indian eurasian collision along the northeastern margin of the tibetan plateau in this study we employ apatite fission track aft analysis to shed new light on the exhumation history of the kuantan shan hei shan along the northeastern margin of the tibetan plateau with a motivation to determine whether deformation related exhumation during initial collision stage occurred in the region the 22 granite samples produce aft ages ranging from 22 3 2 6 ma to 175 18 ma and mean track lengths from 11 17 2 26 m to 13 63 1 93 m thermal history modeling indicates five episodes of rapid exhumation related cooling which took place in the early jurassic early cretaceous and late cretaceous during since the eocene and since the middle miocene the exhumation events prior to the cenozoic are attributed to far field responses of successive closure of fossil oceans and assembly of blocks along the southern margin of the eurasian continent the eocene exhumation along the kuantan shan hei shan is speculated to represent an immediate response to the initial indian eurasian collision these results lend support to the immediate deformation response model along the northeastern plateau the episodic exhumation observed in this study combined with previous geophysical findings implies that the kuantan shan hei shan represents the southern margin of the alashan block serving as a significant and long lived lithospheric weak zone susceptible to intracontinental deformation during episodes of far field tectonic event

left lateral sense offset of upper proterozoic to paleozoic features across the gobi onon tost and zuunbayan faults in southern mongolia and implications for other central asian faults abstract we present a preliminary study of upper proterozoic to paleozoic features adjacent to and offset across three faults in southern mongolia restoration of four offset stratigraphic units adjacent to the gobi onon fault indicates a left lateral sense offset of 70 95 km the tost fault has a left lateral sense offset of 95 125 km defined by three offset features the zuunbayan fault has seven offset features that display left lateral sense offsets of 185 235 km deformation on these faults apparently occurred during the mesozoic prior to the late cretaceous based upon the age of offset features the age of overlap sequences and inferred timing relationships with regional contractional structures although cenozoic overprinting and reactivation has occurred along parts of these faults thus complicating their restorations we present a testable hypothesis which predicts possible connections between these faults and faults within china

new approaches to co2 analysis in polar ice cores gases trapped in polar ice sheets provide a means of obtaining samples of earth s atmosphere from times in the past we describe geochemical processes affecting gases trapped in ice and discuss how these mechanisms affect the composition of gas extracted from polar ice cores we review the methods for recovering samples of paleo atmospheres from ice cores and discuss the suitability of each method in different applications we conclude that extracting gas by mechanical disintegration e g crushing grinding shaving or milling is most easily performed on relatively young ice in which the content of the bubbles is still representative of the original atmosphere for ice recovered from greater depth where some or all of the gases have been driven into the ice matrix itself more care must be taken in interpreting the analysis of the extracted gases

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

large n s convergence at the northern edge of the tibetan plateau new early cretaceous paleomagnetic data from hexi corridor nw china abstract nine early cretaceous paleomagnetic sites have been collected in the yumen area of the hexi corridor nw china magnetic directions isolated at lower temperatures fail the fold test and lie close to the geocentric axial dipole field direction before tilt correction high temperature components are carried by magnetite and or hematite all with normal polarity and pass the fold test the average paleomagnetic pole from the nine sites is at 75 5 n 169 9 e a 95 7 7 these results are consistent with those from other areas of the north china block ncb but significantly different from those from the qaidam basin on the southern side of the qilian mountains they suggest that 1 the yumen region behaved as a rigid part of the ncb since at least the early cretaceous 2 740 500 km of north south directed convergence has taken place between the ncb and qaidam within the qilian mountains and 3 extrusion of qaidam was accompanied by a 23 5 relative rotation with respect to north china this is larger than implied by the maximum left lateral slip on the altyn tagh fault system the same data imply some 1000 800 km of cenozoic motion between the tarim and ncb blocks which were so far believed to have formed a rigid entity since at least the jurassic one interpretation could be that all tarim and qaidam cretaceous paleomagnetic samples from red beds but not those from yumen and the ncb suffered significant inclination shallowing as observed in cenozoic red beds from central asia so far we do not find support for this possibility possible tectonic interpretations include 1 the existence of a large as yet uncharted tectonic discontinuity between tarim and the ncb in the vicinity of the desert corridor near 95 100 e longitude 2 the occurrence of significant deformation within southwestern tarim to the north of yingjisha where paleomagnetic sites were obtained or 3 persistent clockwise rotation of tarim with respect to the ncb for at least 20 ma at the rate found for current block kinematics

co2 measurements on 1 cm3 ice samples with an ir laserspectrometer irls combined with a new dry extraction device abstract a new dry gas extraction and analysis method for small 1 cm3 ice samples is presented the extraction device cooled to 20 c contains two movable steel needle matrices for crushing the ice during the crushing process the gas escaping from the ice sample is continuously analyzed for co2 with an infrared laserspectrometer this method enables a fast measurement few minutes of the co2 concentration in the air bubbles with high spatial resolution in the ice core and a minimum potential contamination an important co2 contamination source due to an interaction of water vapour with surfaces is shortly discussed the reproducibility of extraction and analysis is 2 we analyzed co2 concentrations in the trapped air from different ice cores originating from the greenland ice sheet and from antarctica

the attenuation of fast atmospheric ch4 variations recorded in polar ice cores 1 to reconstruct fast atmospheric trace gas variations from polar ice cores it has to be considered that their amplitudes are attenuated during the enclosure process in the ice relevant processes for the attenuation are the molecular diffusion in the open pores of the firn column and the gradual bubble close off in the depth of the transition from firn to ice these processes depend mainly on temperature and accumulation rate and lead e g to a strong attenuation for cold sites with low accumulation rates with a diffusion and enclosure model it is possible to calculate the attenuation for a single event and to compare ice core records from different sites we investigate the atmospheric methane ch4 variation during the cold event 8200 years ago and calculate that its amplitude as recorded in the epica dome c ice core is attenuated to a magnitude between 34 and 59

mesozoic cenozoic evolution of the xining minhe and dangchang basins northeastern tibetan plateau magnetostratigraphic and biostratigraphic results 1 accurate stratigraphic ages are crucial to understanding the deformation history of the tibetan plateau prior to and during the indo asian collision efforts to quantify mesozoiccenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata magnetostratigraphic and biostratigraphic results from the xining minhe longzhong basin complex and dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern tibetan plateau 34 37 n 101 105 e analyses of 171 magnetostratigraphic levels and 24 palynological assemblages 120 species indicate late jurassic early cretaceous to mid tertiary deposition although magnetic polarity zonation is incomplete independent palynological age control partially restricts possible correlations to the geomagnetic polarity timescale the sediment accumulation record basin provenance structural geology and published thermochronological data support a history of jurassic exhumation late jurassic early cretaceous fault related basin initiation and cretaceous paleogene reduced accumulation these patterns which are compatible with late jurassic early cretaceous extension and cretaceous paleogene postrift thermal subsidence were disrupted at about 40 30 ma when shortening related to the indo asian collision induced localized range uplift vertical axis rotation and amplified subsidence indexterms 1520 geomagnetism and paleomagnetism magnetostratigraphy 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic 9609 information related to geologic time mesozoic keywords tectonics magnetostratigraphy tibetan plateau cenozoic mesozoic sedimentary basins

the co2 concentration of air trapped in greenland ice sheet project 2 ice formed during periods of rapid climate change the co2 content of air occluded in greenland ice sheet project 2 gisp2 ice formed over two separate intervals of rapidly changing climate centered at approximately 46 and 63 kyr b p is as much as 90 ppm more during warm periods interstadials than during cold periods stadials these co2 variations are superimposed on changes in annual layer thickness and 18o of the ice and do not show the 200 to 700 year offsets which would be expected for concurrent variations in the atmosphere and the ice the co2 concentrations during the stadials are similar to the atmospheric values recorded by antarctic ice of the same age so processes occurring in the ice after bubble enclosure must be enriching the air trapped in gisp2 ice formed during the interstadials this conclusion is supported by ca content and electrical conductivity measurements of the ice which show that adequate carbonate is present to produce these enrichments and that co2 content is high only when the electrical conductivity a proxy for h concentration is high high resolution mapping of one 4 cm section of ice shows a 200 ppm increase in the co2 content of the trapped air from approximately 275 to 475 ppm analyses of the total inorganic carbon of ice from both the lgm and holocene show that most of the ca in the ice is from caco3 and that the 13co2 approaches that of soil and marine carbonates these results show that the co2 record preserved in ice can be altered by in situ decarbonation reactions and that only ice containing either abundant carbonate or essentially no carbonate contains a reliable record of paleoatmospheric co2

landscape and tectonic evolution of bayin river watershed northeastern qaidam basin northern tibetan plateau implications for the role of river morphology in source analysis and low temperature thermochronology

massive ice of the tuktoyaktuk area western arctic coast canada the extensive coastal exposure of massive underground ice at peninsula point southwest of tuktoyaktuk northwest territories is believed to be intrasedimental ice the ice grew beneath a frozen d

mesozoic and cenozoic tectonic history of the central chinese tian shan reactivated tectonic structures and active deformation 1 the present day topography of the tian shan range is considered to result from crustal shortening related to the ongoing india asia collision that started in the early tertiary in this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event apatite fission track analysis and u th he dating on apatite and zircon indicate that inherited paleozoic structures were reactivated in the late paleozoic early mesozoic during a cimmerian orogenic episode and also in the late cretaceous paleogene around 65 60 ma these reactivations could have resulted from the accretion of the kohistan dras arc or lithospheric extension in the siberia mongolia zone activity resumed in the late mesozoic prior to the major tertiary orogenic phase finally the ongoing deformation which again reactivates inherited tectonic structures tends to propagate inside the endoreic basins that were preserved in the range leading to their progressive closure this study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas

atmospheric co2 concentration from 60 to 20 kyr bp from the taylor dome ice core antarctica a high resolution record of the atmospheric co2 concentration from 60 to 20 thousand years before present kyr bp based on measurements on the ice core of taylor dome antarctica is presented this record shows four distinct peaks of 20 parts per million by volume ppmv on a millennial time scale good correlation of the co2 record with temperature reconstructions based on stable isotope measurements on the vostok ice core antarctica is found

thermochronological constraints on the meso cenozoic tectonic evolution of the haiyuan liupanshan region northeastern tibetan plateau abstract the cenozoic outward growth of the tibetan plateau is traditionally considered to be responsible for the arcuate structures in the northeastern tibetan plateau however it is still unclear how pre cenozoic tectonics contributed to this unique geomorphic setting to decipher the meso cenozoic tectonic evolution we systematically summarize the published thermochronological data and perform a low temperature thermochronology study on paleozoic mesozoic samples from the haiyuan liupanshan region apatite fission track ages from the new samples vary widely ranging from 158 4 ma to 34 4 ma apatite fission track and u th sm he thermal history modeling constrained by zircon fission track data and geological evidence indicate that this area experienced multiple stages of cooling from the late triassic to the cenozoic these cooling events include i late triassic early jurassic ii late jurassic early cretaceous iii late cretaceous early cenozoic and iv late cenozoic 7 ma late triassic early jurassic cooling was driven by the collision of the nouth china plate and the sorth china plate the late jurassic early cretaceous tectonothermal event is interpreted as a record of the intracontinental orogeny in east asia it is possible that late cretaceous early cenozoic exhumation was the far field response to the neo tethyan subduction under the lhasa terrane in addition late cenozoic 7 ma rapid cooling in the yueliangshan range southern part of the haiyuan liupanshan region was a result of the intense activation of the easternmost haiyuan fault in response to the outward growth of the tibetan plateau these findings suggest that pre cenozoic tectonics played an important role in the growth of the present day northeastern tibetan plateau edifice

pan arctic ice wedge degradation in warming permafrost and its influence on tundra hydrology ice wedges are common features of the subsurface in permafrost regions they develop by repeated frost cracking and ice vein growth over hundreds to thousands of years ice wedge formation causes the archetypal polygonal patterns seen in tundra across the arctic landscape here we use field and remote sensing observations to document polygon succession due to ice wedge degradation and trough development in ten arctic localities over sub decadal timescales initial thaw drains polygon centres and forms disconnected troughs that hold isolated ponds continued ice wedge melting leads to increased trough connectivity and an overall draining of the landscape we find that melting at the tops of ice wedges over recent decades and subsequent decimetre scale ground subsidence is a widespread arctic phenomenon although permafrost temperatures have been increasing gradually we find that ice wedge degradation is occurring on sub decadal timescales our hydrological model simulations show that advanced ice wedge degradation can significantly alter the water balance of lowland tundra by reducing inundation and increasing runoff in particular due to changes in snow distribution as troughs form we predict that ice wedge degradation and the hydrological changes associated with the resulting differential ground subsidence will expand and amplify in rapidly warming permafrost regions the polygonal patterns in permafrost regions are caused by the formation of ice wedges observations of polygon evolution reveal that rapid ice wedge melting has occurred across the arctic since 1950 altering tundra hydrology

using the sunspot cycle to date ice cores for many ice cores the snow accumulation rate is too low to preserve annual stratigraphy precluding direct measurement of annual layer thickness for selected holocene sections of the taylor dome core east antarctica we instead use high resolution 10be measurements to establish a nominal 11 year thickness taking advantage of the solar cycle in cosmogenic isotope production we compare measured thicknesses with the layer thickness profile predicted by a finite element ice flow model the results are in good agreement supporting the assumption that the length of the solar cycle has remained essentially constant throughout the holocene for ice cores where annual layers are not preserved the 10be 11 year layer method can be used as an independent check on flow model estimates of layer thickness and to estimate past accumulation rates it should be possible to accurately date ice cores by counting 11 year layers detected with continuous high resolution 10be measurements

identification of an anastomosing river system in the early cretaceous khorat basin northeastern thailand using stratigraphy and paleosols abstract the phu kradung and phra wihan formations of the early cretaceous khorat group were investigated to characterize the depositional system and paleoclimate based on stratigraphy paleosol features paleosol profiles and reworked deposits the sediments of both formations are classified into 12 facies on the basis of their primary depositional attributes such as bedding grain size texture and sedimentary structures furthermore these facies are grouped into eight architectural elements based on stratigraphic relationships and outcrop scale macroforms the proportions of each architectural element indicate that the depositional system was that of an anastomosing river the anastomosing river system in the studied section is characterized by a high rate of vertical floodplain aggradation and the development of a relatively stable crevasse channel paleosols are developed mainly in floodplain deposits and are characterized by the development of calcretes a semi arid to sub humid paleoclimate is deduced from the occurrence of the calcretes in addition their microstructures suggest that these calcretes were formed under several different conditions with micritic carbonate precipitating under relatively arid climates and alveolar septal structures under semi arid to sub humid conditions using the thicknesses of the calcrete horizons we conclude that the difference in the amount of precipitation between the wet and dry periods was highly variable without any order during deposition of the studied section the thicknesses of intervals between successive calcrete horizons indicate that the inflow of fine materials occurred randomly across the floodplain partly superimposed multiple calcrete horizons were developed during periods of low sediment supply a random deposition of floodplain sediments appears to characterize the anastomosing river system of the khorat basin

atmospheric co2 over the last 1000 years a high resolution record from the west antarctic ice sheet wais divide ice core 1 we report a decadally resolved record of atmospheric co2 concentration for the last 1000 years obtained from the west antarctic ice sheet wais divide shallow ice core the most prominent feature of the pre industrial period is a rapid 7 ppm decrease of co2 in a span of 20 50 years at 1600 a d this observation confirms the timing of an abrupt atmospheric co2 decrease of 10 ppm observed for that time period in the law dome ice core co2 records but the true magnitude of the decrease remains unclear atmospheric co2 variations over the time period 1000 1800 a d are statistically correlated with northern hemispheric climate and tropical indo pacific sea surface temperature however the exact relationship between co2 and climate remains elusive due to regional climate variations and or uneven geographical data density of paleoclimate records we observe small differences of 0 2 0 6 ppm among the high precision co2 records from the law dome epica dronning maud land and wais divide antarctic ice cores however those records share common trends of co2 change on centennial to multicentennial time scales and clearly show that atmospheric co2 has been increasing above preindustrial levels since 1850 a d

atmospheric co2 and climate from 65 to 30 ka b p 1 using new and existing ice core co2 data from 65 30 ka a new chronology for co2 is established and synchronized with greenland ice core records to study how high latitude climate change and the carbon cycle were linked during the last glacial period atmospheric co2 rose several thousand years before abrupt warming in greenland associated with dansgaard oeschger events 8 12 14 17 four large warm events that follow heinrich events the co2 rise terminated at the onset of greenland warming for each of these events atmospheric co2 is strongly correlated with the antarctic isotopic temperature proxy with an average time lag of 720 370 yr mean 1 during the time interval studied the new data and chronology should provide a better target for models attempting to explain co2 variability and abrupt climate change

lacustrine fan delta deposition alongside intrabasinal structural highs in rift basins an example from the early cretaceous jiuquan basin northwestern china development of fan deltas alongside intrabasinal structural highs has been overlooked compared to those forming on basin margins however these fan deltas may provide important clues regarding the tectonic and climatic controls on deposition during rift development this paper documents fan delta deposition alongside an intrabasinal structural high within the early cretaceous xiagou formation of the jiuquan basin china using subsurface geological and geophysical data deposits observed in drill core support fan delta deposition occurring almost exclusively through subaerial and subaqueous gravity flows subsurface mapping reveals a consistent decrease in the areal extent of fan deltas from lowstand to highstand system tracts suggesting that deposition alongside the structural high is sensitive to lake level changes the temporal and spatial distribution of the fan deltas display retrogradational stacking patterns where fan deltas exhibit a decreasing lateral extent up sequence until fan delta deposition terminated and was replaced by deposition of fine grained lacustrine deposits the retrogradational stacking patterns observed alongside the intrabasinal structural high are not observed in fan deltas along the basin margin in the lower parts of the xiagou formation subsidence profiles also show differential subsidence across the basin during the earliest stages of this formation likely resulting from border fault movements these data suggest that non uniform stacking patterns in the lower parts of the xiagou formation reflect basin scale tectonic movements as the dominant control on synrift deposition patterns however later stages of xiagou formation deposition were characterized by uniform subsidence across the basin and uniform retrogradational stacking patterns for fan deltas alongside the intrabasinal structural high and border fault these observations suggest that basin scale tectonic movements played a relatively limited role in controlling sediment deposition and imply a potential change to regional scale processes affecting fan delta deposition during later synrift stages climate change is favored here as the region scale control on the uniform retrogradational fan delta stacking patterns this assertion is supported by pollen assemblages isotope signatures and organic geochemical analyses which collectively suggest a change from a humid to semi arid environment during later synrift stages we suggest that variations in stacking patterns between different fan delta systems can provide insights into the basin and regional scale processes that control rift basin deposition

mesozoic and cenozoic tectonics of the northeastern edge of the tibetan plateau evidence from modern river detrital apatite fission track age constraints abstract thermochronometry of modern river sands exploits the natural sampling of detritus in a river catchment to infer plausible regional exhumation patterns when combined with available bedrock thermochronometric information apatite fission track age distributions for six detrital samples are reported in this study to resolve the exhumation history of the upper reaches of the heihe river draining north qilian shan and tuole shan the northeastern margin of the tibetan plateau the apatite fission track aft ages include four major populations late paleozoic 300 55 293 43 ma early mesozoic 223 15 205 17 ma late jurassic cretaceous 112 6 2 84 8 5 6 ma and cenozoic 63 8 5 9 19 3 1 7 ma the ages of these populations correspond well with the deformation and cooling history of the study area the late paleozoic and early mesozoic aft age populations record cooling events resulting from continental collision between the qiantang and kunlun blocks the late jurassic cretaceous aft age populations are interpreted to record cooling events associated with exhumation of north qilian shan and tuole shan due to the collision of the lhasa block with asian continent the cenozoic aft age populations record several cooling events a cooling event is recorded corresponding to exhumation of north qilian shan and tuole shan due to late stress from the lhasa asia collision causing deformation nearly synchronous with the india asia collision another cooling event is recorded corresponding to uplift and exhumation in oligocene time due to ongoing india asia convergence north qilian shan and tuole shan were constructed as a distal effect of india asia convergence in the early miocene while approaching their present mountain river basin distributions thermochronometry applied to heihe river sediments provide convinced evidence of multi stage exhumation of most northeastern margin of the tibetan plateau in the late paleozoic cenozoic

abrupt change in atmospheric co2 during the last ice age 1 during the last glacial period atmospheric carbon dioxide and temperature in antarctica varied in a similar fashion on millennial time scales but previous work indicates that these changes were gradual in a detailed analysis of one event we now find that approximately half of the co2increase that occurred during the 1500 year cold period between dansgaard oeschger do events 8 and 9 happened rapidly over less than two centuries this rise in co2 was synchronous with or slightly later than a rapid increase of antarctic temperature inferred from stable isotopes

atmospheric composition 1 million years ago from blue ice in the allan hills antarctica here we present direct measurements of atmospheric composition and antarctic climate from the mid pleistocene 1 ma from ice cores drilled in the allan hills blue ice area antarctica the 1 ma ice is dated from the deficit in 40ar relative to the modern atmosphere and is present as a stratigraphically disturbed 12 m section at the base of a 126 m ice core the 1 ma ice appears to represent most of the amplitude of contemporaneous climate cycles and co2 and ch4 concentrations in the ice range from 221 to 277 ppm and 411 to 569 parts per billion ppb respectively these concentrations together with measured d of the ice are at the warm end of the field for glacial interglacial cycles of the last 800 ky and span only about one half of the range the highest co2 values in the 1 ma ice fall within the range of interglacial values of the last 400 ka but are up to 7 ppm higher than any interglacial values between 450 and 800 ka the lowest co2 values are 30 ppm higher than during any glacial period between 450 and 800 ka this study shows that the coupling of antarctic temperature and atmospheric co2 extended into the mid pleistocene and demonstrates the feasibility of discontinuously extending the current ice core record beyond 800 ka by shallow coring in antarctic blue ice areas

patterns and rates of riverbank erosion involving ice rich permafrost yedoma in northern alaska abstract yedoma a suite of syngenetically frozen silty ice and organic rich deposits with large ice wedges that accumulated during the late pleistocene is vulnerable to thermal degradation and erosion because of the extremely high ice contents this degradation can result in significant surface subsidence and retreat of coastal bluffs and riverbanks with large consequences to landscape evolution infrastructure damage and water quality we used remote sensing and field observations to assess patterns and rates of riverbank erosion at a 35 m high active yedoma bluff along the itkillik river in northern alaska the total volumetric ground ice content including wedge segregated and pore ice was estimated to be 86 the process of riverbank erosion and stabilization include three main stages typical of the areas with ice rich permafrost 1 thermal erosion combined with thermal denudation 2 thermal denudation and 3 slope stabilization active riverbank erosion at the main study site started in july 1995 when the itkillik river changed its channel the total retreat of the riverbank during 1995 2010 within different segments of the bluff varied from 180 to 280 m the average retreat rate for the most actively eroded part of the riverbank was almost 19 m y from august 2007 to august 2011 the total retreat varied from 10 to almost 100 m the average retreat rate for the whole 680 m long bluff was 11 m y for the most actively eroded central part of the bluff 150 m long it was 20 m y ranging from 16 to 24 m y more than 180 000 m3 of ground ice and organic rich frozen soil or almost 70 000 metric tons t of soil solids including 880 t of organic carbon were transported to the river from the retreating bank annually this study reports the highest long term rates of riverbank erosion ever observed in permafrost regions of eurasia and north america

natural and anthropogenic changes in atmospheric co2 over the last 1000 years from air in antarctic ice and firn a record of atmospheric co2 mixing ratios from 1006 a d to 1978 a d has been produced by analysing the air enclosed in three ice cores from law dome antarctica the enclosed air has unparalleled age resolution and extends into recent decades because of the high rate of snow accumulation at the ice core sites the co2 data overlap with the record from direct atmospheric measurements for up to 20 years the effects of diffusion in the firn on the co2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close off depth the uncertainty of the ice core co2 mixing ratios is 1 2 ppm 1 preindustrial co2 mixing ratios were in the range 275 284 ppm with the lower levels during 1550 1800 a d probably as a result of colder global climate natural co2 variations of this magnitude make it inappropriate to refer to a single preindustrial co2 level major co2 growth occurred over the industrial period except during 1935 1945 a d when co2 mixing ratios stabilized or decreased slightly probably as a result of natural variations of the carbon cycle on a decadal timescale

multi scale site evaluation of a relict active layer detachment in a high arctic landscape abstract investigations into the susceptibility of permafrost landscapes response to thermokarst can be performed using various approaches depending on the scale of investigation in many cases point based field measurements are extrapolated to larger scales and vice versa the integration of scales often requires some form of ground control in addition to remote sensing surveys which are at times exclusively conducted as upscaling from discrete field measurements can provide spatial coverage and landscape scale significance downscaling from remote sensing can offer insight into processes and serve as calibration or verification here we present a multiple scale evaluation of an area initially interpreted as a relict active layer detachment slide before 1950 on melville island in the high arctic where differential interferometric synthetic aperture radar dinsar showed subsidence between 2013 and 2015 ground based cryostratigraphy measurements were combined with ground penetrating radar gpr to investigate permafrost ice content the results indicate greater subsidence within the relict active layer detachment as detected by dinsar gpr surveys and permafrost coring indicated the presence of an ice rich or massive ice layer near the base of the active layer in this area in addition cryostratigraphic evidences of thaw unconformity and of massive ice depth helped validate the interpretations of the geomorphology in the active layer detachment this combination of methods indicated a localized and inherited landform subsidence association which brought further insight into the interpretation of dinsar subsidence data the framework presented in this study demonstrates the importance of site specific investigations of thermokarst signal in order to understand the processes behind the remote sensing results

electrical resistivity image of the jingsutu graben at the ne margin of the ejina basin nw china and implications for the basin development 1 magnetotelluric and transient electromagnetic data reveal the graben geometry of a nne striking depression within crystalline basement at the ne margin of the ejina basin we interpret this structure as a late mesozoic pull apart basin which is delineated by nne striking normal faults to the east and west satellite imagery srtm elevation data and previously published geophysical data of our work group suggest that the graben is part of normal fault system that continues at least 140 km farther to the south thus forming the eastern margin of the ejina basin nne striking normal faults are atypical in a regional context where nw striking and ene striking fault systems prevail thus this feature marks a distinct difference of the ejina basin when compared to other basin structures in the china mongolia border region

mass balance and accumulation rate across siple dome west antarctica snow accumulation rates and rates of ice thickness change mass balance are studied at several sites on siple dome west antarctica accumulation rates are derived from analyses of grossbeta radioactivity in shallow firn cores located along a 60 km transect spanning both flanks and the crest of the dome there is a north south gradient in snow accumulation rate across the dome that is consistent with earlier radar mappingof internalstratigraphy orographicprocessesprobablycontrolthisdistribution mass balance is inferred from the difference between global positioning system gps derived vertical velocities and snow accumulation rates for sites close to the firn core locations results indicatethatthere is virtually no netthicknesschangeat fourof thefive sites the exception is at the northernmost site where a small amount of thinning is detected that appearstobe inconsistentwith other studies a possiblecause ofthisanom alousthinning is recent retreatof the grounding line of ice stream d

cenozoic shortening budget for the northeastern edge of the tibetan plateau is lower crustal flow necessary 1 two of the most popular mechanisms for thickening the crust beneath the tibetan plateau are 1 pure shear with faulting and folding in the upper crust and horizontal shortening below and 2 flow of lower or middle crust without significant shortening of the upper crust to help discriminate between the relative contributions of these two mechanisms well constrained estimates of upper crustal shortening are needed here we document the cenozoic shortening budget across the northeastern tibetan plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints these sections indicate 11 2 1 east west shortening since middle miocene time and 9 2 3 nne ssw shortening between middle eocene and middle miocene times with little subsequent shortening of this orientation shortening rates accelerate fivefold after middle miocene time given the present day crustal thickness of 56 4 km in northeastern tibet crustal restorations that remove cenozoic shortening suggest that the northeastern tibetan crust was 45 5 km thick prior to the india asia continental collision this precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the tibetan plateau 44 4 km and globally 41 6 km and suggests that pure shear alone may account for cenozoic crustal thickening in northeastern tibet in contrast to eastern tibet where in the absence of significant shortening structures crustal flow has been invoked to explain the addition of crustal material since middle miocene time our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern tibet

stable carbon cycle climate relationship during the late pleistocene a record of atmospheric carbon dioxide co2 concentrations measured on the epica european project for ice coring in antarctica dome concordia ice core extends the vostok co2 record back to 650 000 years before the present yr b p before 430 000 yr b p partial pressure of atmospheric co2 lies within the range of 260 and 180 parts per million by volume this range is almost 30 smaller than that of the last four glacial cycles however the apparent sensitivity between deuterium and co2 remains stable throughout the six glacial cycles suggesting that the relationship between co2 and antarctic climate remained rather constant over this interval

holocene carbon cycle dynamics based on co2 trapped in ice at taylor dome antarctica a high resolution ice core record of atmospheric co2 concentration over the holocene epoch shows that the global carbon cycle has not been in steady state during the past 11 000 years analysis of the co2 concentration and carbon stable isotope records using a one dimensional carbon cycle model uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial scale changes of atmospheric co2 concentrations

thermal fractionation of air in polar firn by seasonal temperature gradients 1 air withdrawn from the top 5 15 m of the polar snowpack firn shows anomalous enrichment of heavy gases during summer including inert gases following earlier work we ascribe this to thermal diffusion the tendency of a gas mixture to separate in a temperature gradient with heavier molecules migrating toward colder regions summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion here we explore and quantify this process further in order to 1 correct for bias caused by thermal diffusion in firn air and ice core air isotope records 2 help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion severinghaus et al 1998 and 3 address whether air in polar snow convects during winter and if so whether it creates a rectification of seasonality that could bias the ice core record we sampled air at 2 m depth intervals from the top 15 m of the firn at two antarctic sites siple dome and south pole including a winter sampling at the pole we analyzed 15n 14n 40ar 36ar 40ar 38ar 18o 16o of o2 o2 n2 84kr 36ar and 132xe 36ar the results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30 although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision relative values of different gas pairs may at siple dome 40ar 4 is 66 2 as sensitive to thermal diffusion as 15n in agreement with laboratory calibration 18o 2 is 83 3 and 84kr 48 is 33 3 as sensitive as 15n the corresponding figures for summer south pole are 64 2 81 3 and 34 3 accounting for atmospheric change the figure for o2 n2 4 is 90 3 at siple dome winter south pole shows a strong depletion of heavy gases as expected however the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities except for kr constant at 34 4 suggesting the action of some other process that is not currently understood no evidence for wintertime convection or a rectifier effect is seen

influence of ground ice and permafrost on coastal evolution richards island beaufort sea coast n w t a long term sediment budget 1947 1985 for northern richards island shows that when ground ice and offshore erosion are accounted for there is a near balance between headland erosion and coastal deposition excess ice constitutes about 20 of the total volume of eroded material from the headlands with massive ground ice contributing nearly 9 and segregated ice lenses and ice wedges making up the remainder coastal response to major storms in 1987 and 1993 suggests that erosion is episodic with short periods of intense disruption followed by readjustment of cliff profiles processes characteristic of this environment include mechanical erosion of ice bonded sediments creating unstable erosional niches mechanical failure of niches along ice wedge planes and longer term thermal erosion of ice bonded sediments where ice contents are high localized thaw slumps initiated by coastal erosion may retreat at rates substantially higher than those observed at other sections of the coast cliff top retreat r

sea ice loss enhances wave action at the arctic coast 1 erosion rates of permafrost coasts along the beaufort sea accelerated over the past 50 years synchronously with arctic wide declines in sea ice extent suggesting a causal relationship between the two a fetch limited wave model driven by sea ice position and local wind data from northern alaska indicates that the exposure of permafrost bluffs to seawater increased by a factor of 2 5 during 1979 2009 the duration of the open water season expanded from 45 days to 95 days open water expanded more rapidly toward the fall 0 92 day yr 1 when sea surface temperatures are cooler than into the mid summer 0 71 days yr 1 time lapse imagery demonstrates the relatively efficient erosive action of a single storm in august sea surface temperatures have already decreased significantly by fall reducing the potential impact of thermal erosion due to fall season storm waves

abrupt climate change around 22 ka on the siple coast of antarctica a new ice core from siple dome antarctica suggests the surface temperature increased by 6 c in just several decades at approximately 22 ka bp this abrupt change did not occur 500 km away in the byrd ice core or in climate proxy records in the siple dome core indicative of the mid latitude pacific this demonstrates there was significant spatial heterogeneity in the response of the antarctic climate during the last deglaciation and draws attention to unexplained mechanisms of abrupt climate change in antarctica

early jurassic extensional basin formation in the daqing shan segment of the yinshan belt northern north china block inner mongolia abstract structural and stratigraphic studies of the western daqing shan segment of the yinshan belt have recognized an early jurassic extensional episode supported by several lines of evidence first normal faults cut the lowermost jurassic sequence and are overlapped by younger lower jurassic rocks second lower jurassic rocks include growth strata in small scale graben at the base of the jurassic basin third rapid lateral facies changes are mapped from boulder conglomerates along the basin bounding faults to lacustrine and meandering fluvial rocks in the basin center fourth paleodrainage systems provided sediment input from three sides of the basin two transverse and one axial finally there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin the early jurassic extensional episode was responsible for formation of an east trending half graben basin in the western daqing shan in which at least 1800 m of syn extensional nonmarine sediments accumulated and are preserved previous studies in the triassic and jurassic of other parts of northwest and north central china have concluded that the early mesozoic was a time of continental amalgamation and contractile deformation the recognition of an early jurassic extensional episode along the northern margin of the north china block is problematic in this context at least superficially we propose two possible explanations for the early jurassic extension transtension associated with strike slip tectonics or gravitational collapse of a pre existing late paleozoic early mesozoic contractile orogenic belt the first possibility transtensional deformation is problematic because specific strike slip faults have not been identified that could control extension in the western daqing shan however several lines of evidence allow the possibility of such a driving mechanism including documented early jurassic transtensional systems in the southwestern north china block several candidate strike slip faults along the china mongolia border region structural discontinuity between the daqing shan and southern mongolia and along strike changes in structural style within the yinshan belt the second possibility orogenic collapse is similarly difficult to establish because of the limited amount of data concerning the regional distribution and orientation of pre jurassic contractile structures and early jurassic extensional structures however documented late paleozoic triassic contractile deformation as well as the close temporal and spatial association and parallelism between early jurassic extensional structures and older contractile structures in the western daqing shan requires consideration of gravitational collapse as a driving mechanism for early jurassic extension in the yinshan belt

a high precision method for measurement of paleoatmospheric co2 in small polar ice samples we describe a high precision method now in use in our laboratory for measuring the co 2 mixing ratio of ancient air trapped in polar ice cores occluded air in ice samples weighing 8 15 g is liberated by crushing with steel pins at 35 c and trapped at 263 c in a cryogenic cold trap co 2 in the extracted air is analyzed using gas chromatography replicate measurements for several samples of high quality ice from the siple dome and taylor dome antarctic ice cores have pooled standard deviations of 0 9 ppm this high precision technique is directly applicable to high temporal resolution studies for detection of small co 2 variations for example co 2 variations of a few parts per million on millennial to decadal scales

seismic and ultrasonic velocities in permafrost we calculate the compressional and shear wave velocities of permafrost as a function of unfrozen water content and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids solid and ice matrices and a liquid unfrozen water the compressional velocity for unconsolidated sediments obtained with this theory is close to the velocity computed with wood s model since biot s theory involves a wood averaging of the moduli of the single constituents moreover the model gives lower velocities than the well known slowness averaging theory wyllie s equation for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c computing the average bulk moduli by slowness averaging the ice and solid phases and wood averaging the intermediate moduli with the liquid phase yields a fairly good fit of the experimental data the proportion of unfrozen water and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity at the whole range of temperatures provided that the average pore radius and its standard deviation are known

evolution of the minle and chaoshui basins china implications for mesozoic strike slip basin formation in central asia the minle and chaoshui basins of northern china are pull apart basins that originated during jurassic to early cretaceous strike slip and extensional deformation in the western north china block alashan and the region to its southwest known as the hexi corridor the basins are separated by the longshou shan which has been a positive topographic feature since jurassic time lower cretaceous successions in the minle and chaoshui basins display similar rift related fining upward motifs these comprise alluvial conglomerates and sandstones that pass upward into lacustrine mudrocks followed by a final fine grained alluvial phase with extensive paleosol development the upper part of each succession includes a second lacustrine interval which is related either to a short lived humid climatic period or downstream damming of the exit for the fluvial systems of both basins north south trending normal faults and rare synsedimentary rotation and slump structures affect the basin fills the minle and chaoshui basins are only two of a series of late jurassic early cretaceous strike slip related basins in this region that nucleated on three basement blocks tarim qilian shan and alashan all of the basins are characterized by a nonmarine alluvial lacustrine fill controlled by a combination of strike slip and normal faults from an analysis of the distribution orientation and motion sense of subsurface and exposed faults we conclude that three types of strike slip related basins are present pull apart basins minle huahai jinta chaoshui transtensional basins southwest badanjilin northern wuwei and a third type that developed between divergent strike slip faults of opposing motion senses we term this type extrusion fault wedge basins jiuquan bayanhaote western wuwei the cause of late jurassic to early cretaceous strike slip and extension in this region is interpreted to be the lhasa block asia collision compression arising from this event is speculated to have caused eastward extrusion of the crust of the hexi corridor and alashan deformation was accommodated by a 25 counterclockwise rotation of the alashan with respect to the remainder of the north china block thrusting occurred in the intervening helan shan the extension in the hexi corridor occurred at the same time as vast foreland basins formed to the west tarim and east ordos and while extension affected much of eastern mongolia and northeastern china

jurassic early cenozoic tectonic inversion in the qilian shan and qaidam basin north tibet new insight from seismic reflection isopach mapping and drill core data

co 2 diffusion in polar ice observations from naturally formed co 2 spikes in the siple dome antarctica ice core this is the publisher s final pdf the published article is copyrighted by the international glaciological society and can be found at http www igsoc org journal

sedimentary record and tectonic implications of mesozoic rifting in southeast mongolia the east gobi basin of mongolia is a poorly described late jurassic early cretaceous extensional province that holds great importance for reconstructions of mesozoic tectonics and paleogeography of eastern asia extension is especially well recorded in the structure and stratigraphy of the unegt and zuunbayan subbasins southwest of saynshand mongolia where outcrop and subsurface relationships permit recognition of prerift synrift and postrift mesozoic stratigraphic megasequences within the synrift megasequence three sequences developed in response to climatic and rift related structural controls on sedimentation where best exposed along the northern margin of the unegt subbasin each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone resedimented ashes and basalt flows punctuate the synrift megasequence rifting began in the unegt subbasin prior to 155 ma with coarse alluvial filling of local fault depressions subsidence generally outstripped sediment supply and fresh to saline lacustrine environments expanding southward with time dominated the unegt zuunbayan landscape for much of latest jurassic early cretaceous time episodic faulting and volcanism characterized the basin system for the balance of the early cretaceous a brief period of compressional and or transpressional basin inversion occurred at the end of the early cretaceous prior to deposition of a widespread upper cretaceous overlap sequence the driver s of late jurassic early cretaceous extension remain uncertain because southeast mongolia occupied an intraplate position by the beginning of the cretaceous extension in the east gobi basin was coeval with collapse and extension of early mesozoic contractional orogenic belts along the northern and southern borders of mongolia and probably was a linked phenomenon strike slip faulting associated with collisions on the southern asian and mongol okhotsk margins likely also played a role in late mesozoic deformation of the east gobi region perhaps partitioning the gobi from apparently coeval large magnitude contractional deformation in the yinshan yanshan orogenic belt south of the study area in inner mongolia

paleomagnetism indicates no neogene vertical axis rotations of the northeastern tibetan plateau 1 paleomagnetic data were obtained from 108 paleomagnetic sites collected in cretaceous to tertiary red beds from seven localities distributed in three general regions adjacent to the altyn tagh fault at the northern edge of the tibetan plateau in the hexi corridor 12 sites in oligocene strata at yaoquanzi 39 97 n 97 68 e yield a mean paleomagnetic direction i 33 0 d 8 6 95 6 0 with concordant declination and 36 sites in early cretaceous mudstones of the longshou shan 39 09 n 100 50 e provide a concordant paleomagnetic direction i 53 4 d 6 8 95 3 8 across the nan shan fold thrust belt miocene paleomagnetic directions from 39 sites distributed among four localities have a concordant mean declination i 40 6 d 7 2 95 5 8 in the altyn tagh range 21 sites in oligocene strata at xorkoli 38 93 n 91 43 e yield a paleomagnetic direction with concordant declination i 49 7 d 5 0 95 5 4 these results combined with existing regional paleomagnetic data indicate that 1 the hexi corridor along with the north china block has not undergone tectonic vertical axis rotation since at least early cretaceous time and is separated from the xining lanzhou basin and eastern tibet by an important post early cretaceous tectonic boundary 2 the nan shan fold thrust belt and the qaidam basin have not experienced wholesale vertical axis rotation during neogene time and 3 absence of vertical axis rotations in areas adjacent to the altyn tagh fault indicates that sinistral shear strain between the tarim basin and the northern tibetan plateau is concentrated on the fault these results are consistent with asian tectonic models that combine distributed lithospheric deformation and thickening with narrow and weak shear zones

the effect of quaternary alluvium on strong ground motion in the coyote lake california earthquake of 1979 abstract the effect of alluvium on strong ground motion can be seen by comparing two strong motion records of the coyote lake california earthquake of 6 august 1979 m l 5 9 one record at a site on franciscan bedrock had a peak horizontal acceleration of 0 13 g and a peak horizontal velocity of 10 cm sec the other at a site 2 km distant on 180 meters of quaternary alluvium overlying franciscan had values of 0 26 g and 32 cm sec amplifications by factors of 2 and 3 horizontal motions computed at the alluvial site for a linear plane layered model based on measured p and s velocities show reasonably good agreement in shape with the observed motions but the observed peak amplitudes are greater by a factor of about 1 25 in acceleration and 1 8 in velocity about 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance the remainder may represent focusing by refraction at a bedrock surface concave upward there is no clear evidence of nonlinear soil response fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model in particular the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value indicating that no significant nonlinearity occurs in the secant shear modulus computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0 5 to 1 0 or greater the data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections and more the simple effect of the low near surface velocity application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear wave velocity

synthesis of studies of palsa formation underlining the importance of local environmental and physical characteristics this review presents a synthesis of four decades of palsa studies based on field experiments and observations mainly in fennoscandia as well as laboratory measurements palsas are peat covered mounds with a permanently frozen core in finnish lapland they range from 0 5 to 7 m in height and from 2 to 150 m in diameter these small landforms are characteristic of the southern margin of the discontinuous permafrost zone palsa formation requires certain environmental conditions long lasting air temperature below 0 c thin snow cover and low summer precipitation the development and persistence of their frozen core is sensitive to the physical properties of peat the thermal conductivity of wet and frozen peat is high and it decreases significantly as the peat dries and thaws this affects the development of the active layer and makes its response to climate change complex the insulating properties of dry peat during hot and dry summers moderate the thawing of the active layer on palsas in contrast humid and wet weather during the summer causes deep thawing and may destroy the frozen core of palsas ice layers in palsas have previously been interpreted as ice segregation features but because peat is not frost susceptible the ice layers are now reinterpreted as resulting from ice growth at the base of a frozen core that is effectively floating in a mire

a record of atmospheric co2 during the last 40 000 years from the siple dome antarctica ice core 1 we have measured the co2 concentration of air occluded during the last 40 000 years in the deep siple dome a hereafter siple dome ice core antarctica the general trend of co2 concentration from siple dome ice follows the temperature inferred from the isotopic composition of the ice and is mostly in agreement with other antarctic ice core co2 records co2 rose initially at 17 5 kyr b p thousand years before 1950 decreased slowly during the antarctic cold reversal rose during the younger dryas fell to a local minimum at around 8 kyr b p and rose continuously since then the co2 concentration never reached steady state during the holocene as also found in the taylor dome and epica dome c hereafter dome c records during the last glacial termination a lag of co2 versus siple dome isotopic temperature is probable the siple dome co2 concentrations during the last glacial termination and in the holocene are at certain times greater than in other antarctic ice cores by up to 20 ppm mol co2 mol air while in situ production of co2 is one possible cause of the sporadic elevated levels the mechanism leading to the enrichment is not yet clear

reconstructing past atmospheric co2 concentration based on ice core analyses open questions due to in situ production of co2 in the ice analysis of air extracted from bubbles of polar ice showed the anthropogenic increase of atmospheric co2 concentration during the past few hundred years preindustrial concentration 280 ppmv and unexpectedly that the concentration also increased due to natural causes from 200 to 280 ppmv during the transition from the last glacial epoch to the holocene however more detailed reconstructions based on ice core analyses suggested that some of the additional observed variations were actually due to the modification of co2 concentration in the bubbles by chemical reactions between impurities in the ice detailed analyses of acidity the carbonate concentration the concentration of oxidation agents like hydrogen peroxide h2o2 and organic compounds e g formaldehyde hcho along short intervals of ice cores representing only a few annual layers allowed us to investigate these chemical reactions the records are not conclusive but they indicate that the oxidation of organic compounds is at least as important as acid carbonate reactions ice containing a low mean carbonate and a low h2o2 concentration as well as a small scattering of co2 results obtained on several adjacent samples representing a few annual layers at most is at present the best guarantee for reliable results of the atmospheric co2 concentration

paleogene crustal extension in the eastern segment of the ne tibetan plateau abstract the tibetan plateau experienced lateral growth in response to the india eurasia collision but uncertainties remain about when and how its present day margins were formed it was commonly viewed that crustal shortening in the ne tibetan plateau had commenced since the eocene and led to reverse faulting and compressional basins our recent investigations of some cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the ne tibetan plateau normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation small scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost paleogene sequences measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in nw se direction the nw se extension was consistent with paleogene stress field of the whole north china craton which were thought responsible for the ne sw trending rift basins in the interior of the north china craton such as the yinchuan weihe and bohai bay rift basin systems a broad unconformity separates paleogene and neogene sequences in the eastern segment of the ne tibetan plateau contrasting strikingly with conformable oligocene to middle miocene sequences in the neighboring ningnan and yinchuan rift basins it thus follows that the eastern segment of the ne tibetan plateau had not been formed in paleogene when it was basically dictated by crustal extension possibly as a result of far field effect of subduction of the western pacific plate end oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the ne tibetan plateau began rising at the beginning of the miocene

two phases of mesozoic north south extension in the eastern altyn tagh range northern tibetan plateau 1 the 300 km long east striking lapeiquan fault lies in the eastern altyn tagh range along the northern margin of the tibetan plateau and was interpreted as a north dipping thrust in early studies however our mapping shows that the fault is a south dipping normal fault juxtaposing archean proterozoic gneisses beneath an early paleozoic volcanic and sedimentary sequence its dip angle varies from 30 to 60 the central fault segment is expressed as a 30 50 m thick ductile shear zone with well developed mylonitic fabrics and stretching mineral lineations where the eastern and western segments are characterized by cataclastic deformation kinematic indicators such as asymmetric boudinage asymmetric folds and minor brittle and ductile faults within the fault zone consistently indicate a top south normal slip sense of shear the age of the lapeiquan fault is constrained by two types of information first a sequence of early middle jurassic sediments is locally present in the hanging wall of the lapeiquan fault the clasts of the jurassic strata particularly the stromatolite bearing cherty limestone and purple quartzite can be correlated uniquely with those in the footwall of the fault we interpret that the early middle jurassic strata were deposited in an extensional basin related to motion along the lapeiquan fault second 40ar 39ar thermochronologic analyses indicate two prominent cooling events in the lapeiquan footwall the older event occurred in the latest triassic earliest jurassic between 220 and 187 ma while the younger event occurred in the latest early cretaceous at 100 ma because the 220 187 ma cooling ages are widespread in the lapeiquan footwall we suggest it to represent the main stage of faulting we interpreted the younger phase of fault motion at 100 ma to have been related to fault reactivation the deformation was aided by motion along the south dipping qiashikan normal fault that merges with the eastern lapeiqaun fault from the regional tectonic setting it appears that mesozoic extension in northern tibet to have occurred in a back arc setting during northward subduction of the tethyan oceanic plate the findings of mesozoic extensional structures in northern tibet suggest that compressive stress induced by collision of the qiangtang and lhasa terranes with asia was not transmitted beyond northern tibet this in turn implies that the popularly inferred contractional setting for mesozoic evolution of the tian shan north of tibet needs a reevaluation based on a combination of both structural and sedimentological observations

fifty years of coastal erosion and retrogressive thaw slump activity on herschel island southern beaufort sea yukon territory canada abstract patterns of coastal erosion in the arctic differ dramatically from those coasts in more temperate environments thick sea ice and shore fast ice limit wave based erosional processes to a brief open water season however despite this permafrost coasts containing massive ice ice wedges and ice bonded sediments tend to experience high rates of erosion these high rates of erosion reflect the combined thermal mechanical processes of thawing permafrost melting ground ice and wave action climate change in the arctic is expected to result in increased rates of coastal erosion due to warming permafrost increasing active layer depths and thermokarst rising sea levels reduction in sea ice extent and duration and increasing storm impacts with the most ice rich permafrost in the canadian arctic the southern beaufort sea coast between the tuktoyaktuk peninsula and the alaskan border is subject to high rates of erosion and retrogressive thaw slump activity under many climate change scenarios this area is also predicted to experience the greatest warming in the canadian arctic this paper presents results of a remote sensing study on the long term patterns of coastal erosion and retrogressive thaw slump activity for herschel island in the northern yukon territory using orthorectified airphotos from 1952 and 1970 and an ikonos image from 2000 corrected with control points collected by kinematic differential global positioning system and processed using softcopy photogrammetric tools mean coastal retreat rates of 0 61 m yr and 0 45 m yr were calculated for the periods 1952 1970 and 1970 2000 respectively the highest coastal retreat rates are on north west facing shorelines which correspond to the main direction of storm related wave attack during the period 1970 2000 coastal retreat rates for south to south east facing shorelines displayed a distinct increase even though these are the most sheltered orientations however south to south east facing shorelines correspond to the orientations where the highest densities of retrogressive thaw slumps are observed differences in rates of headwall retreat of retrogressive thaw slumps and coastal erosion results in the formation of larger thermokarst scars and the development of polycyclic thaw slumps on south to south east exposures the number and the total area of retrogressive thaw slumps increased by 125 and 160 respectively between 1952 and 2000 as well the proportion of active retrogressive thaw slumps increased dramatically polycyclic retrogressive thaw slumps appear to develop in a periodic fashion related to retrogressive thaw slump stage and maximum inland extent

facies and basin architecture of the late carboniferous salvan dorenaz continental basin western alps switzerland france the salvan dorenaz basin formed during the late palaeozoic within the aiguilles rouges crystalline basement western alps as an asymmetric intramontane graben elongated in a ne sw direction and bounded by active faults at least 1700 m of fluvial alluvial fan and volcanic deposits provide evidence for a strong tectonic influence on deposition with long term average subsidence rates of 0ae 2m m yr 1 the early basin fill was associated with coarse grained alluvial fans that were dominated by braided channels unit i these issued from the south western margin of the basin the fans then retreated to a marginal position and were overlain by muddy floodplain deposits of an anastomosed fluvial system unit ii that drained towards the ne deposition of thick muds resulted from a reduction in the axial fluvial gradient caused by accelerated tectonic subsidence overlying sand rich meandering river deposits unit iii document a reversal in the drainage direction from the ne to the sw caused by synsedimentary tectonism reflecting large scale topographic reorganization in this part of the variscides with subsidence now preferentially in the w and sw and uplift in the e and ne coarse grained alluvial fan deposits unit iv repeatedly prograded into and retreated from the basin as documented by coarsening upward cycles tens of metres thick reflecting smaller scale tectonic cycles volcanism was active throughout the evolution of the basin and u pb isotopic dating of the volcanic deposits restricts the time of basin development to the late carboniferous 308 295 ma 40 ar 39 ar ages of detrital white mica indicate rapid tectonic movements and exhumation of the nearby basement in unit i youngest ages are close to that of the host sediment but the age spectrum is wide in unit ii high subsidence and or sedimentation rates coincide with very narrow age spectra indicating small homogeneous catchment areas in unit iii age spectra became wider again and indicate growing catchment areas

tectono thermal evolution of the northwestern edge of the tarim basin in china constraints from apatite u th he thermochronology abstract the northwestern edge of the tarim basin which contains the kalpin uplift and the north bachu uplift lies between the south tianshan and tarim continental block and its tectonic evolution is closely correlated with the tianshan the pre mesozoic units are widely exposed in the northwestern edge of the tarim basin which allow detailed geological observation and sample collection in this paper the apatite u th he thermochronology was used to study the tectono thermal evolution of the northwestern edge of the tarim basin the sandstone samples from the upper neoproterozoic silurian devonian carboniferous and permian units etc on the northwestern edge of the tarim basin were collected and used to determine apatite u th he ages the apatite he ages in the kalpin uplift reveal the tectonic uplift events during the late cretaceous 96 70 ma and the miocene 22 20 ma these uplift events are linked to collisions of the lhasa block and the india plate with the southern eurasian continental margin respectively however apatite he ages in the north bachu uplift show only the eocene event at 40 ma because the west kunlun orogenic belt thrust over the southwestern edge of the tarim basin during the eocene the thermal modeling results reveal four cooling events in the kalpin uplift since the late neoproterozoic corresponding to 460 445 ma 350 320 ma 220 210 ma and 20 10 ma but only one episode with 220 210 ma in the north bachu uplift since the devonian based on the thermal modeling results the north bachu uplift did not experience the cooling events at 460 445 ma 350 320 ma and 20 10 ma shown in the kalpin uplift this paper provides a new insight into the tectonic evolution of the tianshan and the northwestern tarim basin

mesozoic contractional deformation in the middle of the asian tectonic collage the intraplate western ordos fold thrust belt china abstract intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of strain concentrations into weak zones these weak zones may be pre existing crustal heterogeneities such as faults thermally weakened zones due to magmatism and or the effects of a thick sedimentary cover or mechanical contrasts between adjacent crustal blocks intraplate deformation is not exclusively related to continent continent collision it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries asia has many examples of active intraplate deformation that are thought to be related to the cenozoic indo asian collision although much attention is placed on active examples discerning controls on pre cenozoic asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of asia situated on either side of the yellow river along the western margin of the ordos plateau the intraplate western ordos fold thrust belt involves units as old as archean basement and as young as late jurassic the fold thrust belt is spectacularly exposed in two ranges the helan shan and the zhuozi shan cross sections drawn through the north trending fold thrust belt suggest 30 minimum shortening a paleocurrent reversal within the lower middle jurassic section is interpreted to mark the onset of contraction and mountain building a late jurassic synorogenic boulder conglomerate records the final deformation in the fold thrust belt synchronous with contraction and cutting across the fold thrust belt is an e w striking strike slip fault that displays right lateral drag and sub horizontal striae we interpret this fault as an accommodation structure in the thrust belt following e w contraction the thrust belt was dismembered by an n15 w trending left lateral strike slip fault the left lateral fault displaces the frontal portion of the thrust belt the zhuozi shan 62 km to the north relative to a more internal portion the helan shan we propose that the anomalous orientation and intraplate location of the western ordos fold thrust belt is a function of mechanical contrasts between a previously deformed area and a stable crustal block ordos to the east pre existing crustal weakness or anisotropy along the western margin of the ordos block is most likely related to either a late proterozoic early paleozoic aulacogen a triassic fault system or to both although its intraplate location and the complexities of mesozoic asian plate interactions make it difficult to relate deformation in the western ordos fold thrust belt to a specific contemporaneous plate boundary its n s trend may suggest a link with paleo pacific subduction along the eastern margin of asia

holocene sea level history of the canadian beaufort shelf new radiocarbon ages pertaining to the holocene sea level history of the canadian beaufort shelf are presented the ages were obtained on samples of freshwater and tidal marsh peat beds from offsho

the co2 concentration of air trapped in gisp2 ice from the last glacial maximum holocene transition measurements of the co2 concentration of air trapped in greenland ice sheet project two ice show an increase from approximately 190 ppm to 270 ppm during the 8000 years between the end of the last glacial maximum and the beginning of the holocene this increase is in agreement with other parallel records from both greenland and antarctica although the gisp2 data exhibit considerable variability over periods as small as 50 100 years and show that co2 can be affected by processes occurring within the ice after bubble formation the in situ reactions which contribute to the modification of the concentrations of paleoatmospheric co2 may include both co2 uptake as well as co2 production the existence of two major episodes of co2 enrichment in addition to the variability of the data prevents a precise determination of the progression of the increase in atmospheric co2 although the 80 ppm rise between 18 0 kybp and 10 4 kybp is evident these results strongly suggest that some intervals of the gisp2 co2 record must be interpreted in the context of the chemical composition of the ice and that records of atmospheric co2 in polar ice can be considered accurate only if the ice is either essentially carbonate free or contains abundant carbonate