palaeo moisture evolution in monsoonal central asia during the last 50 000 years abstract the late quaternary climate history of monsoonal central asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka or part of this period wet conditions occurred during middle and late marine isotope stage 3 while the last glacial maximum lgm was characterized by dry climate conditions in the region a stepwise climate amelioration is suggested by the climate records following the lgm several climate signals of this period which were reported from high latitude ice core records are preserved in archives from monsoonal central asia as well during the early holocene high effective moisture was inferred from most records from the area dominated by the indian monsoon e g the tibetan plateau suggesting that holocene optimal climate conditions occurred there during this period in contrast areas which are dominated by the south east asian monsoon se monsoon and the westerlies in north western and north central china mongolia do not uniformly show an early holocene climate optimum for this area optimal conditions prevailed during the mid holocene these apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the holocene during the early holocene strengthened insolation possibly caused an enhanced low level convergence over the tibetan plateau which led to the intensification of the summer monsoon the strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the tibetan plateau the areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity the majority of the palaeoclimatic records suggest reduced effective moisture since the late holocene in the region

possible crustal contamination of midcontinent rift igneous rocks examples from the mineral lake intrusions wisconsin interlayered mafic telsic intrusions from the mineral lake intrusive complex in northwest wisconsin reflect the typical bimodal basalt rhyolite compositional pattern of the midcontinent rift flood

making it and breaking it in the midwest continental assembly and rifting from modeling of earthscope magnetotelluric data abstract a three dimensional lithospheric scale resistivity model of the north american mid continent has been estimated based upon earthscope magnetotelluric data details of the resistivity model are discussed in relation to lithospheric sutures defined primarily from aeromagnetic and geochronologic data which record the southward growth of the laurentian margin in the proterozoic the resistivity signature of the 1 1 ga mid continent rift system is examined in detail in particular as relates to rift geometry extent and segmentation an unrecognized expanse of concealed proterozoic deltaic deposits in kansas is identified and speculated to result from axial drainage along the southwest rift arm akin to the rio grande delta which drains multiple rift basins a prominent conductor traces out cambrian rifting in arkansas missouri tennessee and kentucky this linear conductor has not been imaged before and suggests that the cambrian rift system may have been more extensive than previously thought the highest conductivity within the mid continent is imaged in minnesota michigan and wisconsin where it is coincident with paleoproterozoic metasedimentary rocks the high conductivity is attributed to metallic sulfides and in some cases graphite the former is a potential source of sulfur for multiple mineral deposits types occurrences of which are found throughout the region finally the imprint left within the mantle following the 1 1 ga rifting event is examined variations in lithospheric mantle conductivity are observed and are interpreted to reflect variations in water content depleted versus metasomatized mantle imprinted upon the mantle by the keweenawan mantle plume

paleoenvironmental and archaeological investigations at qinghai lake western china geomorphic and chronometric evidence of lake level history abstract qinghai lake located on the northeastern qinghai tibet plateau qing zang gaoyuan is china s largest extant closed basin lake its position relative to major asian climate systems makes it sensitive to global climate change the lake has been the subject of numerous paleoenvironmental investigations including dating of shoreline features around the lake basin here we report new age estimates of shoreline features geomorphic exposures and archaeological sites that contribute to the development of a lake level history for qinghai lake and a landscape model of the qinghai lake basin lake highstands above 3230 m 36 m above the modern lake level appear to date to late mis 5 70 110 ka the lake has had much more modest highstands since then no evidence of mis 3 lake stands higher than modern were found and early holocene highstands are no more than 12 m above modern if the age of highstands greater than 3230 m is confirmed through future work then the qinghai lake basin hydrologic balance prior to 70 ka was dramatically different than after that time including during the holocene a simple hydrologic balance model provides insights into the combination of precipitation evaporation and runoff generation needed to sustain the lake at 3260 m the highest shoreline observed a range of factors may explain the difference primarily the relative strength of the east asian monsoon the basin was apparently subject to extensive alluviation during mis 3 interrupted by widespread erosion and development of cryogenic features before and during the last glacial maximum lgm loess that presently drapes much of the lower basin landscape began to be deposited after the lgm 16 18 ka the landscape model outlined here has implications for archaeological visibility of early human occupation of the qinghai tibet plateau

crust enriched mantle derived tonalites in the early proterozoic penokean orogen of wisconsin the early proterozoic 1 82 to 1 89 ga penokean orogenic province represents the initial phase of a major period of crustal growth of the north american proterozoic craton in central wisconsin the orogeny is dominated by the intrusion of variably deformed and metamorphosed tonalite trondhjemite suites in part hosted by archean gneisses and migmatites the magmatism is calc alkaline and similar in composition to modern high k orogenic andesites moderate abundances of sr 600 750 ppm and ree in the most primitive tonalites are consistent with derivation from an eclogitic altered tholeiite rather than a crustal archean amphibolitic tholeiite or enriched mantle source existing nd sr isotopic data for other tonalites of the wisconsin penokean orogen also indicate a mantle origin but with some admixture of archean crustal component this study shows that an archean component most likely was incorporated during emplacement the tonalite to trondhjemite differentiation of these magmatic suites is not

geochronology of middle and upper pleistocene loess sections in hungary abstract the application of both thermoluminescence and infrared stimulated luminescence dating to the extensively studied classical hungarian loess paleosol sequences from basaharc mende and paks provides a reliable chronological framework and climatostratigraphic reconstruction for the last interglacial glacial cycle based on this combined luminescence dating study a new chronology is proposed for the young loess in hungary luminescence dating suggests that the loess below the mf2 horizon formed during the penultimate glaciation the mf1 horizon probably formed during an interstade within oxygen isotope stage 3 for the youngest loess overlying mf1 a very high accumulation rate was determined large time gaps occur above mf2 and mf1 indicating that most of the record of the last glaciation is missing in the standard sections at basaharc mende and paks either large discontinuities or a very low accumulation rate occurred in all three type sections during the soil forming periods high resolution studies of climatic proxies using this combined luminescence dating approach provide a reliable chronological framework for loess and loess derivatives of the last glacial cycle in hungary although a precise and complete chronostratigraphic reconstruction cannot be achieved from the incomplete records found at these sites

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

development of low background vacuum extraction and graphitization systems for 14c dating of old 40 60 ka samples abstract at the university of arizona s desert laboratory we recently constructed new low background vacuum extraction and graphitization systems that are dedicated to preparing old 40 60 ka samples for 14 c dating these systems are designed to minimize the amount of contaminant carbon specifically atmospheric carbon that is introduced to a sample during laboratory processing excluding contaminants is particularly important for 14 c dating of old samples because the impact of contamination increases with sample age in this study we processed 20 pretreated and 4 untreated aliquots of ceylon graphite a naturally occurring geological graphite to determine the total procedural background level and hence the practical limit of our systems samples were heated under vacuum at 240 c for 1 h to drive off water vapor and other atmospheric gases and then combusted in ultra high purity uhp o 2 at 500 and 850 c to monitor the removal of contaminants and to ensure complete combustion after so x no x and halide species were removed sample co 2 was converted to graphite via catalytic reduction of co fe and zn powders used in the graphitization process were oxidized scrubbed and reduced with uhp o 2 he and h 2 respectively to remove sorbed atmospheric c species graphite targets were stored in uhp ar until measurement by accelerator mass spectrometry ams to avoid potential interaction with atmospheric gases based on the ams results the background level of our system is characterized by a nonlinear inverse relationship with sample mass adjusted r 2 0 75 n 24 for a 1 mg graphite target the total procedural blank including chemical pretreatment combustion cleanup graphitization storage and ams measurement is 0 05 0 01 pmc 2 equivalent to a 14 c age of 61 1 1 8 ka this should not be taken as the upper limit of our system however because if the 14 c activity of a sample is statistically indistinguishable from the appropriate mass dependent blank value at the 95 confidence level 2 then its age is considered to be infinite thus for a 1 mg target the practical limit of our system is actually 55 ka for a 0 5 mg target the practical limit is 50 ka although our extraction system can accommodate inorganic samples e g calcite aragonite the above limits are only applicable to geological graphite charcoal and organic samples that are processed via combustion future work will be directed toward determining the appropriate background levels for inorganic materials

high resolution osl dating of a late quaternary sequence from xingkai lake ne asia chronological challenge of the mis 3a mega paleolake hypothesis in china in this paper a combined osl 49 samples and 14c 14 samples dating study was conducted on a sediment core from xingkai lake ne asia a single aliquot regenerative dose protocol was used to determine the equivalent dose dede of the extracted fine grained fg quartz fraction the suitability of the measurement procedure was confirmed by a set of luminescence tests e g preheat plateau and dose recovery measurements in addition a post ir infrared stimulated luminescence protocol at 290 c pirir290 was applied to the polymineral fg fraction of seven samples our results can be summarized as follows 1 the uppermost sample was dated to 110 a indicating that the osl signal of fg quartz has been well bleached this has been further confirmed by the consistency of osl ages and pirir290 ages within 80 ka the resulting osl ages generally increased with depth and the lowermost samples of core xk08 reached back to the last interglacial although the pirir290 dating suggested that the quartz osl ages likely begin to be underestimated beyond 80 ka 2 the comparison of osl and 14c ages suggested that the radiocarbon dating technique may significantly underestimate the age of sediments for samples older than 30 cal ka bp corresponding to 25 14c ka bp and thus it is necessary to pay attention when using such old 14c dates for paleoclimatic archaeological interpretation 3 this study challenges the radiocarbon based chronology of the mis 3a mega paleolake hypothesis which had been reiterated for many years and extensively reported across northern and western china and 4 the current high resolution osl dating record one age 5 10 cm showed clear sedimentation rate changes down the core the identified variations in the sedimentation processes at the orbital timescale may be related to regional global climatic changes during the past 130 ka and the high sedimentation rate during the last 0 4 ka has probably been caused by intensified human activities in the xingkai lake catchment

age constraints on the late quaternary evolution of qinghai lake tibetan plateau dating and geomorphology of shoreline features in the qinghai lake basin of northwestern china suggest that contrary to previous interpretations the lake likely did not reach levels 66 140 m above modern within the past 90 000 yr maximum highstands of 20 66 m above modern probably date to marine isotope stage mis 5 mis 3 highstands are undated and uncertain but may have been at or below post glacial highs the lake probably reached 3202 3206 m 8 12 m during the early holocene but stayed below 3202 m after 8 4 ka this shoreline history implies significantly different hydrologic balances in the qinghai lake basin before 90 ka and after 45 ka possibly the result of a more expansive asian monsoon in mis 5

ams radiocarbon analyses from lake baikal siberia challanges of dating sediments from a large oligotrophic lake abstract a suite of 146 new accelerator mass spectrometer ams radiocarbon ages provides the first reliable chronology for late quaternary sediments in lake baikal in this large highly oligotrophic lake biogenic and authigenic carbonate are absent and plant macrofossils are extremely rare total organic carbon is therefore the primary material available for dating several problems are associated with the toc ages one is the mixture of carbon sources in toc not all of which are syndepositional in age this problem manifests itself in apparent ages for the sediment surface that are greater than zero however because most of the organic carbon in lake baikal sediments is algal autochthonous in origin this effect is limited to about 1000 500 years which can be corrected at least for young deposits the other major problem with dating lake baikal sediments is the very low carbon contents of glacial age deposits which makes them extremely susceptible to contamination with modern carbon this problem can be minimized by careful sampling and handling procedures the ages show almost an order of magnitude difference in sediment accumulation rates among different sedimentary environments in lake baikal from about 0 04 mm year on isolated banks such as academician ridge to nearly 0 3 mm year in the turbidite depositional areas beneath the deep basin floors such as the central basin the new ams ages clearly indicate that the dramatic increase in diatom productivity in the lake as evidenced by increases in biogenic silica and organic carbon began about 13 ka in contrast to previous estimates of 7 ka for the age of this transition holocene net sedimentation rates may be less than equal to or greater than those in the late pleistocene depending on the site this variability reflects the balance between variable terrigenous sedimentation and increased biogenic sedimentation during interglaciations the ages reported here and the temporal and spatial variation in sedimentation rates that they imply provide opportunities for paleoenvironmental reconstructions at different time scales and resolutions

late quaternary palaeolake levels in tengger desert nw china comprehensive field investigations and laboratory analyses show that palaeolakes including the megalake tengger and other semi connected and isolated water bodies during late pleistocene covered an area of more than 20 000 km 2 this is an area more than half the size of the tengger desert in nw china stratigraphic correlations and chronological evidence indicate that the palaeolakes started to develop around 42 000 14 c year bp uncalibrated all throughout the paper but their extent was limited until 37 000 14 c year bp based on the chronology of representative lacustrine deposits it can be deduced that the megalake tengger was established around 35 000 14 c year bp and maintained until 22 000 14 c year bp leading to the formation of the baijian hu terraces which are regarded as firm evidence of the existence of palaeolakes the formation mechanism is unclear and the climate situation at the time is still an open question the holocene palaeolakes started to develop around 12 000 14 c year bp the baijian hu terraces indicate that the high water levels in the area occurred around 8500 5400 5100 3500 and 1860 1370 14 c year bp the extent of the holocene palaeolakes primarily migratory lakes was smaller than that of the late pleistocene palaeolakes d 2004 elsevier b v all rights reserved

quantifying the timing and rate of crustal evolution global compilation of radiometrically dated detrital zircon grains abstractmultiple models steady state episodic and early growth followed by crustal reworking have been postulated to explain the evolution of earth s continental crust an independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using u pb and hf isotopes a compilation of 200 000 published zircon dates reveals a global age distribution u pb with six prominent statistically significant peaks 3 2 3 0 2 7 2 5 2 0 1 7 1 2 1 0 0 7 0 5 and 0 3 0 1 ga in most cases these peaks are recorded on all seven continents most of the peaks are detectable also in modern sediments a comparison of grains with both u pb ages and hf isotope model ages available for 5100 detrital zircons suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth the zircon age distributions and associated data provide consistent ev

late marine isotope stage 3 palaeoclimate for east asia a data model comparison abstract climate prediction under impact of anthropological greenhouse gas emission is impossible to validate but it can be inferred from past climate and modeling in east asia a general warm wet period at late marine isotope stage 3 ca 30 40 ky bp has been identified based upon extensive geological records this provides an arid humid reference for possible future warming caused by human activities based upon syntheses of geological evidence and the agcm ssib modeling this paper presents climate simulations focused on 35 ky bp using forcing of insolation glaciation and land surface conditions for east asia results of the simulation can be compared with geological records and show that 1 the climate patterns of 35 ky bp were warm wet conditions in northern china but with warm dry conditions in southern china compared to today 2 mean annual temperatures were higher in most mid low latitude areas mainly contributed to by increased winter temperatures suggesting that insolation has generated significant climate effects through the coupling in atmospheric circulation with land surface patterns 3 quaternary ice sheets in the northern hemisphere played an important role in temperature decrease at the mid high latitudes and also enhanced the south north temperature gradients which in turn increased moisture transport from low to high latitudes and increased monsoonal precipitation over the tibetan plateau and 4 vegetation changes in east asia resulted from an increased temperature in the low latitudes extended rain belt northwards into china and an enlarged area of increased precipitation inland

luminescence dating of the stratzing loess profile austria testing the potential of an elevated temperature post ir irsl protocol abstract feldspar infrared stimulated luminescence irsl signals saturate at 2000 gy i e 500 700 ka assuming a dose rate of 3 4 gy ka typical for loess and it is therefore theoretically possible to date middle pleistocene loess however the accuracy of age estimates is hampered by anomalous fading which leads to significant age underestimation fading corrections have been proposed but these corrections are reliant on inherent assumptions and in any case are inapplicable at large doses as the growth curve approaches saturation recent studies have identified a post ir irsl signal that shows less fading and is thus less dependent on accurate fading corrections this study builds upon these investigations and applies a post ir irsl sar dating protocol to polymineral fine grain samples of the loess palaeosol sequence in stratzing lower austria after a preheat at 320 c 60 s and an ir bleach at 50 c 200 s irsl was measured at 290 c for 200 s the same set of conditions were used to measure the response to a test dose recycling ratios recuperation and dose recovery tests show that this protocol is suitable for the loess under investigation although finite residual doses of up to 20 gy are observed for the oldest samples the natural signal is in or close to saturation indicating that fading is negligible in nature this observation is inconsistent with the measured laboratory fading rates of 1 1 5 decade and needs further investigation

bundled slaty cleavage in laminated argillite north central minnesota abstract exceptional bundled slaty cleavage defined herein has been found in drill cores of laminated folded weakly metamorphosed argillite at several localities in the early proterozoic animikie basin of north central minnesota the cleavage domains are more closely spaced within the cleavage bundles than outside them the mean tectosilicate grain size of siltstone layers measured normal to cleavage is less in the cleavage bundles than outside them and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra bundle segments these facts suggest that pressure solution was a major factor in bundle development if it is assumed that opaque phases have been conserved during pressure solution the modal differences in composition between intra bundle and extra bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage argillite samples from the central part of the animikie basin have been shortened a minimum of about 22 as estimated by this method these estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite and are also consistent with the regional pattern of deformation

fold axes oblique to the regional plunge and proterozoic terrane accretion in the southern lake superior region abstract the niagara fault is believed to represent the suture between a magmatic arc terrane and a passive margin assemblage which were tectonically juxtaposed about 1 85 ga ago during the penokean orogeny accretion related deformation is concentrated within an 12 km wide belt that straddles the suture a distinctive structural feature of this belt is the rotation of early fold axes toward parallelism with the direction of bulk extension resulting in a great circle stereonet distribution of fold axes within a vertical axial plane rocks of both arc and passive margin assemblages crop out in this more highly strained belt but there has been no material transfer between them across the fault structural style and metamorphic assemblages demonstrate deep level beneath the brittle ductile transition erosion of the terrane accretion boundary in the southern lake superior region there is some evidence for pure shear during the accretion event but we are unable to evaluate the amount or sense of simple shear

possible origin of n alkanes with a remarkable even to odd predominance in recent marine sediments abstract n alkane distributions with a remarkable even to odd predominance c16 c24 were found in marine surface sediments the previously proposed diagenetic reduction of the corresponding n fatty acids could not be considered as a source for these n alkane based on a comparison of compositional features of n alkane n fatty acid and n alcohol distributions carbon isotope analyses and other geochemical parameters the data indicate that the even predominant n alkanes were derived directly from marine bacteria

last glacial pollen record from lanzhou northwestern china and possible forcing mechanisms for the mis 3 climate change in middle to east asia abstract the vegetation on the northeastern margin of the tibetan plateau is highly sensitive to climatic changes and thus represents a potentially interesting environmental archive pollen samples from the fanjiaping loess section in lanzhou on the western chinese loess plateau clp were analyzed in conjunction with osl dating the results indicate that pollen zone b 60 6 46 0 ka correlative to the early mis 3 had the greatest abundances of cupressaceae tsuga gramineae and cyperaceae of the entire section suggesting a warm phase during the last glacial period these pollen taxa decreased significantly in abundance in the zones c 46 0 39 0 ka and d 39 0 27 0 ka reflecting a substantial climate cooling from the middle mis 3 to mis 2 these results correlate with climate records from the south china sea the clp baikal lake north america north atlantic ocean and other regions and probably correspond with the decline of northern high latitude insolation and the increase of global ice volume from 50 to 20 ka in particular arboreal pollen fern spore and algae abundances declined sharply since 40 ka while shrub and herb pollen reached the highest abundances conifer pollen picea and abies abundance also rose markedly and increased up the section this implies significant climate deterioration and likely corresponded with substantial growth of the polar ice sheets since 40 ka the decreasing temperature caused by an insolation decline during the last glacial period probably reinforced the cooling effect in a snow ice albedo feedback which would result in less climate sensitivity to radiative forcing meanwhile vegetation decline in the northern hemisphere during the last glacial period and tundra development at high latitudes possibly caused additional cooling enhancing the growth of polar ice sheets since 40 ka the development of polar ice sheets increased the polar to equator temperature and pressure gradients strengthening the westerlies and supplying plenty of moisture to northwest china during 40 30 ka lake sediments developed widely on the tibetan plateau during 40 30 ka probably related to an increase in the seasonality of middle to low latitude insolation which caused an enhancement of glacier melting on the plateau

reconstruction of the 30 40 ka bp enhanced indian monsoon climate based on geological records from the tibetan plateau abstract multi proxy records from ice caps lakes and pollen records from the tibetan plateau are used to reconstruct the palaeoclimate during the period 30 40 ka bp lake geomorphological and lacustrine sedimentological studies show that numerous high lake level stands and or freshwater lakes occurred in large areas of western china at this time including over the tibetan plateau where the area contains mostly shallow water and saline hypersaline water lakes under extremely dry conditions of today pollen based reconstruction of tibetan vegetation shows systematic differences from the present alpine steppe forests were shifted ca 400 km further north and the alpine conifer forest extended ca 400 800 km beyond their present western limits variations in 18 o curves from ice cores from the plateau suggest that the temperature was 2 4 c higher and precipitation was 40 to over 100 higher than today all of the evidence consistently suggests the existence of an exceedingly strong summer monsoon climate over the plateau the vigorous evaporation of the tropical ocean surface would also play an important role in promoting the penetration of moisture rich southwest monsoon over the tibetan plateau these climate patterns with strong summer monsoon circulation during the period 30 40 ka bp reflect the 20 ka earth orbital precessional cycle when the tibetan plateau region received greater solar radiation which thus enlarged the thermodynamical contrast between the plateau and the mid south portion of the indian ocean

proterozoic transcurrent movements along the kapuskasing lineament superior province canada and their relationship to surrounding structures abstract analysis of large scale geological and geophysical lineaments provide evidence for a new model to explain the kapuskasing structural zone ksz and james bay james bay is interpreted to have formed due to rifting related to lateral strike slip along the ksz the sutton arc is interpreted to be an extension from hudson bay arc which has been displaced by rifting of james bay magnetic anomalies which terminate at the ksz on the west have correlated magnetic anomalies on the east which are displaced left laterally it is proposed that these displacements and the rifting of james bay and the strike slip along the ksz were caused as a result of 160 km of left lateral motion between east and west superior terranes with an accompanying 5 of anticlockwise rotation of the west superior terrane additional support for this model comes from structural evidence drawn from previous reports from the last twenty six years

sm nd isochron age and provenance of the argillites of the gunflint iron formation in ontario canada abstract sm nd rb sr and k ar isotope analyses were made on argillaceous sediments from the gunflint iron formation in the lake superior region in order to provide new information on their sedimentation age and evolutionary history the investigations indicate that it might be possible to date the time of sedimentation by the sm nd method by studying specific material such as argillaceous whole rocks enriched in organic matter and clay fractions the spread in the sm nd ratios seems here to have been favoured by the incorporation of organic material in the argillites the sm nd isotope data yield an isochron age of 2 08 0 25 ga which seems to be a fair estimation of deposition age model calculations indicate that the main source supplying the animikie seawater with ree was located at the northern border of the basin the argillites and their clay minerals did not behave as closed systems with regard to the rb sr and k ar systems during further diagenetic evolution

hydrological and climatic changes in deserts of china since the late pleistocene article i nfo article history large areas in western china were wetlands or less arid between 40 and 30 ka corresponding to the greatest lake period on the adjacent tibetan plateau during the last glacial maximum some of these western chinese deserts again experienced wetter conditions however at the same time the sandy lands in the eastern chinese desert belt experienced an activation of aeolian dunes while interpretations of the mid holocene environment in the deserts of china are controversial it is quite likely that it was more humid not only in the eastern areas influenced by monsoon climate systems but also in the western deserts where moisture is currently associated with westerlies evaluation of lacustrine records in the lakes recharged by dryland rivers and the complex interactions of these systems as well as other paleoenvironmental proxies such as the artemisia chenopodiaceae ratio should be interpreted with greater caution facing the highlighted uncertainties in our understanding of climate changes in chinese deserts it is hoped that this special issue will improve our knowledge considerably

quaternary paleoenvironmental change on the tibetan plateau and adjacent areas western china and western mongolia abstract this paper summarises the nature of climatic change during the last glacial interglacial cycle on the tibetan plateau and adjacent areas the results are derived from inland glacier fluctuations lake level changes and dust loess records of central asia including tibet these are based on our field investigations from 1988 to 1998 and literature surveys the changing extents of different climatic controlled geomorphic landscapes and vegetation zones help to provide estimates of the magnitude of climatic change features such as ice wedge casts loess and palaeosols ela reconstruction s and palaeobotanical data are used to help reconstruct palaeoprecipitation and palaeotemperatures weathering characteristics the overlying strata and some luminescence dates indicate that there are two main glacial ice advances during the last glacial cycle these correspond to marine oxygen isotope stages ois 2 and 4 in some areas as the eastern or northern margin of the tibetan plateau remnants of older glaciations are preserved higher lake levels in the deserts of central asia and on the tibetan plateau are dated to 40 to 25 ka ois 3 and to late glacial early to mid holocene periods our work supports the view that in many areas of central asia cold phases during the last glacial correspond with the maximum extent of glaciers and the periglacial activity the last glaciation produced large alluvial fans alternating with periods of high lake stands on the tibetan plateau however at the northern margin of the plateau in the qaidam basin and in some particular desert areas in western china high lake levels occurred also during the pleistocene and are related with alluvial fans

early proterozoic tectonics and metallogeny of the lake huron region of the canadian shield abstract deposition of the huronian supergroup dominantly fluvial clastic sediments with local basal volcanic accumulations began at 2 49 ga in a broad trough that extended far south of the present lake huron area atop a large archean continent that was assembled at 2 7 ga and broken up at 2 15 ga after intrusion of widespread gabbro sheets and dykes including nipissing diabase the wyoming province which contains sequences similar in age and stratigraphy to the distinctive huronian supergroup may be a fragment of this continent the superior province is a much larger fragment tectonic controls of deposition of the huronian and its paleoplacer uranium deposits and of the pre 2 2 ga deformation of these strata were probably unrelated to processes that controlled deposition of the animikie and marquette range sequences in the lake superior region at 2 0 ga the huronian animikie and marquette range sequences were deformed during the 1 85 ga penokean orogeny which occurred when an assembly of island arc terranes collided with the superior block the sudbury structure including the whitewater group and its contained sedex deposits and the sudbury igneous complex and its ni cu pge ores were also formed at 1 85 ga the elliot lake pyritic paleoplacer uranium ores myriad lenses of radioactive pyritic gravels were deposited by early huronian fluvial systems other endogenic huronian mineral deposits include disseminated ni cu sulphides in gabbros cu in red beds high in the huronian succession and silica in quartz arenites intrusions of nipissing gabbro near sudbury contain minor ni cu mineralization these intrusions also caused recycling of metals from archean and huronian rocks to form concentrations of cu au ag fe and w in veins and other epigenetic deposits most important of these are the unconformity related ag veins of the cobalt area which had particularly complex still undeciphered histories that probably included pre huronian weathering metallogenic patterns in the early proterozoic rocks were controlled by a complex interplay of factors including the distribution of mineral deposits in the archean basement tectonically controlled variations in huronian deposition and deformation and oxyatmoversion the transition from anoxic to oxygenic atmospheric conditions that occurred during deposition of the huronian supergroup

regional cross section of the southern province adjacent to lake huron ontario implications for the tectonic significance of the murray fault zone the southern province in the lake huron area of ontario and the penokean fold belt of minnesota wisconsin and michigan have the earmarks of a collisional orogen the huronian supergroup a southerly facing passive margin sequence was deposited during early aphebian early proterozoic crustal stretching and ocean basin formation along the southern margin of the superior province archean craton it accumulated as four unconformity bounded northerly tapering onlapping clastic wedges megacycles the thickness and facies variations reflect syndepositional down to the basin south normal faulting that controlled the accumulation and preservation of the lower three huronian megacycles these are overlapped northward by the youngest megacycle an extensive sheet of clastic sediments recording post stretching regional subsidence of the cratonic margin due to cooling and thermal contraction soft sediment folds in the rocks of the youngest megaycycle to the extreme south probably indicate southward slu

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18