the tectonothermal evolution of the cadomian belt of northern brittany france a neoproterozoic volcanic arc abstract the neoproterozoic cadomian belt northern brittany france results from the accretion of crustal blocks that record distinctive p t deformation paths in addition abundant gabbro dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers as a whole the deformation history of the cadomian belt is characterised by a migration in time of the active zones which become younger towards the southeast relics of eocadomian rocks reveal that arc development took place at about 660 640 ma abundant calc alkaline magmatism intruding strongly deformed palaeoproterozoic orthogneisses characterise the northernmost unit of the cadomian belt i e the tregor unit which cooled before 600 ma the lanvollon basin a short lived intra arc rift containing abundant mafic and felsic volcanics followed by greywackes opened at about 600 580 ma the subsequent tectonothermal history of the cadomian belt is divided into two major episodes the first episode is related to the shortening of the intra arc rift and its southwestward thrusting over the guingamp unit this led to crustal thickening inducing partial melting in the guingamp unit exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the saint brieuc and guingamp units cooling of the saint brieuc unit is dated at about 570 560 ma the second episode is dominated by sinistral wrenching mainly localised in the eastern part of the cadomian belt at the boundary between the domnonean i e the arc domain and mancellian possibly a fore arc basin domains several shear zones developed during this episode some of them being associated with minor granitic intrusions dated around 550 ma the sinistral shear located southeast of the arc domain is reworking or crosscutting earlier high temperature fabrics within the saint brieuc unit in addition the vertical component of this shearing episode contributes significantly to the exhumation of the saint malo migmatites near the proterozoic cambrian boundary cordierite biotite granitoids intruded at shallow depths the fougeres unit their emplacement outlasting deformation of the sedimentary sequences minor stocks of muscovite leucogranites of cambrian age are possible equivalents of the abundant volcanism in an adjacent nw se trending rift this marks a major change in the kinematic framework with predominance of extension finally minor reactivation of the cadomian structures occurred during the carboniferous associated with the variscan orogeny

a penultimate glacial monsoon record from hulu cave and two phase glacial terminations oxygen isotope records of three stalagmites from hulu cave china extend the previous high resolution absolute dated hulu asian monsoon record from the last to the penultimate glacial and deglacial periods the penultimate glacial monsoon broadly follows orbitally induced insolation variations and is punctuated by at least 16 millennial scale events we confirm a weak monsoon interval between 135 5 1 0 and 129 0 1 0 ka prior to the abrupt increase in monsoon intensity at asian monsoon termination ii based on correlations with both marine ice rafted debris and atmospheric ch 4 records we demonstrate that most of marine termination ii the full rise in antarctic temperature and atmospheric co 2 and much of the rise in ch 4 occurred within the weak monsoon interval when the high northern latitudes were probably cold from these relationships and similar relationships observed for termination i we identify a two phase glacial termination process that was probably driven by orbital forcing in both hemispheres affecting the atmospheric hydrological cycle and combined with ice sheet dynamics

a precise late neoproterozoic u pb zircon age for the syntectonic perelle quartz diorite guernsey channel islands uk a precise u pb zircon age of 611 4 2 0 1 1 ma has been determined for the foliated quartz diorite at perelle bay guernsey uk channel islands the age is based on the analysis of single zircons half of a single grain and groups of two and three zircons the age is nearly 100 ma younger than that suggested by other workers based on 207 pb 206 pb dates of large multi grain fractions of zircon and thus rules out this intrusion as the earliest marker of the onset of cadomian magmatism in the north armorican massif a 611 ma crystallization age for the perelle quartz diorite is consistent with previous detailed fabric development studies that suggested magmatic fabrics within the pluton are largely overprinted by similarly oriented solid state deformation fabrics resulting from syntectonic emplacement during cadomian orogenesis the u pb age is approximately 8 15 ma older than 40 ar 39 ar hornblende dates for the pluton consistent with the emplacement of the intrusion followed by slow cooling in a region of moderate to high ambient crustal temperatures these new data emphasize the fact that periods of pluton emplacement are considerably briefer than periods of deformation and thus syntectonic intrusions do not date regional deformational events but only mark events within a period of regional tectonism the ability to analyse single zircons and selected portions of single zircons using ultra low blank u pb analytical techniques documents the feasibility of determining precise emplacement ages of other neoproterozoic granitoids that contain a significant percentage of xenocrystic zircon

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

holocene vegetation and climate of the alashan plateau nw china reconstructed from pollen data a set of 55 recent pollen spectra from the alashan plateau and the qilian mountains qilianshan northwestern china has been analyzed the established relationships between the pollen spectra and modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by eastern juyan palaeolake 41 89 n 101 85 e 892 m a s l several qualitative approaches e g indicator plant species artemisia chenopodiaceae and ephedra fragilis type s l ephedra distachya type pollen ratios and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the eastern juyan pollen record fossil pollen data reveal pronounced environmental changes in the studied area of the alashan plateau between ca 10 700 and 1700 cal yr b p desert taxa as chenopodiaceae nitraria calligonum reaumuria were abundant in the vegetation cover around the study site through the whole record pollen spectra dated to ca 10 700 5400 cal yr b p are characterised by highest values of chenopodiaceae e fragilis type and other desert indicating taxa suggesting rather dry climate most favourable conditions are reconstructed between 5400 and 3900 cal year b p on the basis of relative increase in abundance of artemisia pollen a return to dry conditions occurred at about 3900 cal yr b p the lake finally desiccated after 1700 cal yr b p reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal yr b p correlate well with similar events found in the published records from northern and western china and central mongolia however eastern juyan record does not show the early holocene humidity maximum as suggested by the sedimentary data from monsoon influenced areas of china

the eastern continuation of the cadomian orogen u pb zircon evidence from saxo thuringian granitoids in south western poland and the northern czech republic we report u pb single zircon ages from three pre variscan granitoids in the ne part of the bohemian massif the platerowka granodiorite from the lausitz izera unit the polish sudetes has been dated at 533 9 ma the bitouchov granite form the sw part of the south krkonose unit the czech sudetes gave an age of 540 11 10 ma and the w droze granodiorite in the fore sudetic block yielded 548 9 ma all these latest vendian early cambrian granitoids represent the post tectonic expression of a late proterozoic cadomian orogenic cycle and demonstrate the eastward extent of the cadomian basement into the variscan orogen granodiorites of similar age have so far been reported from brittany and especially from the saxo thuringian terrane to the ne and sw of the elbe fault zone we conclude that the saxo thuringian terrane extends across the elbe and sudetic marginal fault zones into the fore sudetic block

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

evolution of chaka salt lake in nw china in response to climatic change during the latest pleistocene holocene abstract the late pleistocene and holocene hydrologic balance of chaka salt lake in the eastern qaidam basin of nw china was studied based on the analysis of lithostratigraphy mineralogy total organic carbon and total nitrogen from a 9 0 m long sediment core an age depth model for the lake sediments is based on eight accelerator mass spectrometry ams 14 c measurements of organic matter and a 1700 year radiocarbon reservoir correction the pitzer model was used to calculate the sequence of minerals precipitated as a function of salinity assuming that the initial lake water was recharged from adjacent rivers and springs lake salinity values were derived from a good match between the calculated and observed mineral sequences our multi proxy based hydrologic reconstruction of chaka lake indicates that it was a clastic dominated freshwater lake between 17 2 and 11 4 cal ka bp which may have resulted from the input of glacial water into the lake at that time during the lateglacial and holocene a warm climatic regime predominated between 13 9 and 12 7 cal ka bp and then a cold climatic regime prevailed between 12 7 and 11 4 cal ka bp these warm and cold periods correlate with the bolling allerod and younger dryas events in the region beginning at 11 4 cal ka bp a saline or hypersaline lake developed which may have resulted from increased summer insolation and temperatures these conditions persisted throughout the holocene modeling indicates that lake salinity fluctuated between 66 and 223 ppt from 11 4 to 7 2 cal ka bp and then increased to 223 322 ppt between 7 2 and 6 0 cal ka bp when most regions of china recorded high moisture availability i e the so called mid holocene climatic optimum lake salinity decreased to 66 223 ppt during a short time period between 6 0 and 5 3 cal ka bp possibly caused by reduced evaporation subsequently salinity values rapidly increased to 223 322 ppt between 5 3 and 5 2 cal ka bp and a hypersaline lake with a salinity higher than 324 ppt similar to modern chaka salt lake became established around 5 2 cal ka bp this establishment may have been related to the weakening of asian monsoon regional comparisons of paleoclimatic records indicate that large temporal and spatial variations exist in the occurrence and intensity of drought over western china since the lateglacial

u pb geochronology and hf nd isotope compositions of the oldest neoproterozoic crust within the cadomian orogen new evidence for a unique juvenile terrane new u pb dates combined with nd and hf isotopic data from rocks within the port morvan area of the baie de st brieuc region of brittany identify a unique portion of the neoproterozoic cadomia terrane two gneisses near port morvan yielded u pb dates of 754 6 0 8 ma and 746 0 0 9 ma ages that are more than 130 myr older than the oldest units formed during the main phase of early cadomian magmatism two trondhjemite boulders from the monogenetic facies of the cesson conglomerate yielded identical ages of 665 2 0 5 ma and 665 5 0 7 ma and a cobble from the polygenetic facies yields a 207pb 206pb date of 637 2 ma individual detrital zircons from a sandstone associated with the cesson conglomerates yield concordant u pb dates ranging from 650 3 ma to 624 1 0 6 ma initial nd values for the rocks in this region range from 5 0 to 6 6 indicative of a substantial input from depleted mantle initial hf values determined on zircons from these neoproterozoic rocks including the detrital zircons range from 6 7 to 14 5 consistent with the nd isotopic results maximum initial hf values for two 2 ga icartian gneisses considered basement to cadomia average 8 4 and 8 7 in contrast to the results of the port morvan rocks 616 608 ma syn tectonic intrusions from normandy and the british channel islands all have negative initial nd values 10 4 to 8 3 consistent with significant contamination by ancient crust such as the 2 ga gneisses the oldest arc related magmas should have interacted most extensively with cadomian basement buffering younger mantle derived magmas that were generated in subsequent magmatic episodes the rocks within the port morvan region are thus inconsistent as examples of the earliest cadomian intrusions as they show no evidence of interaction with 2 ga basement instead the older ages and mantle like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to and independently from the cadomian arc possible terrane scale structural boundaries have recently been identified including the newly recognized port morvan thrust fault and the nw dipping main cadomian thrust

brioverian volcanism and cadomian tectonics baie de st brieuc brittany stages in the evolution of a late precambrian ensialic basin

short paper cadomian terrane tectonics and magmatism in the armorican massif the north armorican composite terrane nw france is a collage of displaced terranes which result from the amalgamation of cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone early cadomian arc activity occurred at c 700 650 ma but terrane accretion did not occur until c 540 ma and post tectonic magmatism persisted well into the palaeozoic cadomian events thus span a considerably greater period than previously supposed

the aeromagnetic map of the french cadomian belt abstract in order to obtain an image of the subsurface of the armorican massif two detailed aeromagnetic surveys were combined to obtain an aeromagnetic map at 350 m altitude with an interpolated grid of 250 m 250 m the map shows good correlation between the magnetic anomalies and known geological structures and also distinguishes between long and short wavelength anomalies the long wavelength anomalies are associated with the overall subsurface arcuate structure of the cadomian belt the short wavelength anomalies are commonly associated with the lanvollon acid volcanic rocks and syntectonic diorites with one set of short wavelength anomalies in the east of the saint malo region being associated with dykes that are exposed onshore and continue offshore

evidence for multiple high t metamorphism and regional migmatization within the cadomian belt of northern brittany france the cadomian orogeny is characterized by two major tec tonometamorphic events one that pre dates 570 ma and another at c 540 ma until recently the majority of migmatites occurring within the cadomian belt of northern brittany were interpreted as having been formed during the second event the discovery of the jouguet migmatites which have been hornfelsed by the st brieuc diorite emplaced at 533 l2 ma suggests that a migmatitic event occurred before 540 ma these data demand a reinterpretation of the structural sketch of the cadomian orogeny

geochronology and geochemistry of the pola de allande granitoids northern spain their bearing on the cadomian avalonian evolution of northwest iberia the pola de allande pre variscan tonalite granodiorite plutons are located in the narcea antiform at the boundary zone between the cantabrian and west asturian leonese zones of the iberian variscan belt these granitoids were intruded into a neoproterozoic siliciclastic sedimentary sequence with subordinate volcanic intercalations and were subsequently overprinted by variscan thrust related shear deformation u pb laser ablation inductively coupled plasma mass spectrometry icp ms dating of zircons from two plutons yielded concordant ages of intrusion of 605 10 and 580 15 ma to the authors knowledge this is the first reported u pb cadomian avalonian age for igneous rocks in this section of the iberian variscan belt these intrusions are coeval with the main episode of dominantly calc alkaline magmatic activity related to cadomian avalonian subduction major and trace element composition of the granitoids is characteristic of i type high k calc alkaline granitoids generated in continental arc se

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

behaviour of rb sr whole rock and u pb zircon systems during partial melting as shown in migmatitic gneisses from the st malo massif ne brittany france the age of regional metamorphism and partial melting in the late proterozoic succession of the st malo area has been established at around 540 ma whole rock homogenization of the sr isotope systems has been achieved only in the anatectic granites whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous at lower grade than the sillimanite isograd metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a u pb lower intercept age 536 14 ma in the sillimanite zone u pb zircon lower intercept ages are around 500 10 ma and suggest a recent loss of radiogenic pb from the observed overgrowths zircons from the anatectic granites do not yield u pb intercept ages due to the presence of inherited radiogenic pb but the finer fractions define a discordant array whose upper intercept on concordia yields 555 13 ma an age significantly older than the partial melting event indicated by the lower grade zircon ages the lower intercept defined by the fine grained anatectic zircons yields a date in the future negative ordinate value in their regional context the st malo migmatites can be considered as resulting from late stage crustal melting in the cadomian orogenic belt following genesis of calc alkaline magma at an active margin around 600 ma ago an age of 540 ma for partial melting in the st malo migmatites places the end of the cadomian orogeny near the precambrian cambrian boundary

u pb geochronological constraints on the tectonothermal evolution of the paleoproterozoic basement of cadomia la hague nw france abstract the la hague region of nw france provides some of the best exposures of paleoproterozoic basement in the northernmost segment of the cadomian orogen one of several neoproterozoic arc fragments in the circum atlantic region the basement rocks which include orthogneiss metasediments and metabsites contain a comparable series of structures with a common metamorphic history thus the basement units are regarded to have undergone the same broad tectonothermal evolution the metasedimentary succession includes migmatites that preserve relict kyanite overprinted by sillimanite associated with the development of garnet bearing leucosome biotite dehydration at a metamorphic peak of 700 800 c and 8 9 kbar is considered responsible for the anatexis evident in both the metasediments and orthogneiss units a u pb zircon crystallization age of 2061 2 7 ma for the culeron orthogneiss a member of the metamorphic suite confirms the paleoproterozoic protolith age idioblastic rims to 2 06 ga zircons from the culeron orthogneiss yield shrimp u pb dates of c 618 ma providing the first geochronological constraints on the likely age of high grade metamorphism and anatexis of the paleoproterozoic basement in cadomia foliated c 615 610 ma intrusions that intrude the metamorphic units record magmatic to solid state fabrics that parallel metamorphic fabrics within the basement consistent with deformation and metamorphism being late neoproterozoic in age

late holocene lake sedimentology and climate change in southern alberta canada abstract climatic changes in southern alberta canada for the past 4000 yr are reflected in a high resolution record of lake sediment grain size the proposed mechanism for this response is that outflow discharge removes fine grained sediments but increasingly fine sediments are retained and deposited as streamflow declines at the same time coarse sediments are brought in by high discharge entering the lake the net effect of these two processes is to leave coarse clay deficient sediments during times of high streamflow and clay rich sediments during times of low flow the grain size record from pine lake reflects historic climate fluctuations as well as prehistoric fluctuations including the little ice age and the medieval warm period grain size at this site provides a simple economical and nonbiologically mediated paleoclimate proxy

u pb geochronological constraints on the timing of plutonism volcanism and sedimentation jersey channel islands uk uranium lead dates for single and small multiple grain zircon separates are presented for the main geological units from jersey british channel islands a turbiditic sandstone from the jersey shale formation brioverian supergroup contains detrital zircons of a variety of ages ranging from c 2 6 to 0 6 ga the youngest zircons yielded nearly concordant u pb dates of 586 7 3 0 ma and 587 1 2 7 ma providing a maximum depositional age for the unit zircons from overlying volcanic rocks anne port rhyolite give an upper intercept date of 582 8 3 2 7 ma considered to be the age of eruption the undeformed sw jersey granite which intrudes and locally thermally metamorphoses the folded jersey shale formation is dated at 580 2 3 1 6 ma these dates constrain tightly the timing of deposition of the jersey shale formation and the age of deformation on jersey and indicate a dynamic arc environment during the late neoproterozoic zircons from the nw jersey granite define an upper intercept date of 482 7 1 6 ma similar to the previously reported 40 ar 39 ar hornblende date of c 473 ma confirming a renewed pulse of magmatism during the ordovician in this sector of cadomia

evolution of the western european continental crust implications from nd and pb isotopes in iberian sediments abstract this paper presents smz sbnd nd and geochemical data on fine grained sediments of the iberian massif spain together with new and published uz sbnd pb results of selected sedimentary rocks profiles from two major zones cantabrian and central iberian zone have been investigated both profiles exhibit almost complete stratigraphic successions from uppermost precambrian to lower carboniferous paleozoic shallow depth platform sediments are essentially unmetamorphosed the precambrian syn orogenic succession underwent a metamorphic overprint up to biotite zone 147 sm 144 nd ratios range from 0 0988 to 0 1457 and nd contents from 6 to 112 ppm nd today values vary from 16 5 to 7 6 pb contents from 1 7 to 97 ppm samples with high nt t 3 are found to be low in 2 330ma t dm are 1 92 ga cantabrian zone and 1 96 ga central iberian zone as juvenile cadomian crust is present in the iberian massif the oldest components have to be significantly older than 2 ga within the european variscides comparably old t dm were so far only described from the armorican massif france an active margin of the gondwana continent is the most probable provenance of the uppermost precambrian strata as indicated by their cadomian age syn orogenic facies and juvenile components precambrian as well as samples from the precambrian cambrian boundary gave nt t values 4 8 significantly higher than average paleozoic rocks contributions of juvenile cadomian crust to the syn orogenic sediments must be in the order of 30 50 to fit the nd and pb data there is no evidence for any addition of mafic material according to major or trace elements or to sm nd ratios lower cambrian samples are already below 7 3 in nd t thus compared to the total pre existing crust contribution of juvenile crust was very small nd t values of almost all samples ranging in stratigraphic age from 515 to 410 ma are strikingly negative 12 9 to 10 1 and thus indicate the absence of newly differentiated continental crust in the provenance s only the tremadocian of the southern central iberian zone gave high nt t values 4 0 to 1 0 again there is no evidence for addition of mafic material to the strata as precambrian and tremadocian nt t peaks are separated by the lowermost cambrian of nt t 7 juvenile crust may have been added related to the pre ordovician unconformity sardic phase during the middle paleozoic 410 330 ma nt t values increase from 12 to 7 indicating a continuous contribution of juvenile continental crust of 20 as major terranes of the iberian massif were not amalgamated until late paleozoic it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones nd isotope data of this study compared with published results of brittany france suggest that the medio north armoricancentro iberian domain persisted from uppermost precambrian to the variscan

holocene east asian monsoon variation inferred from species assemblage and shell chemistry of the ostracodes from hulun lake inner mongolia a sediment core from hulun lake inner mongolia was analyzed for species assemblages and shell chemistry of ostracodes to investigate changes in the hydrology and climate of the east asian summer monsoon margin during the holocene darwinula stevensoni was abundant ilyocypris spp scarce littoral ostracodes absent and mg ca sr ca and 18o were low 11 100 to 8300 yr ago indicating high lake levels and cool fresh waters darwinula stevensoni declined largely ilyocypris spp throve littoral ostracodes were rare and chemical indicators remained in low values 8300 to 6200 yr ago suggesting that the lake continued high stands but water became warm the lake then contracted and water became cool brackish 6200 to 4300 yr ago littoral ostracodes flourished 4300 to 3350 yr ago marking the lowest lake levels of the entire holocene the lake level recovered and water salinity decreased 3350 to 1900 yr ago from 1900 to 500 yr ago the lake maintained the preceding status albeit lowered stands and increased salinities 1100 to 800 yr ago during the recent 500 yr the lake expanded and water salinity decreased the data imply that the east asian summer monsoon did not intensify until 8300 yr ago and weakened dramatically 4300 to 3350 yr ago

evolution of the western approaches trough abstract the western approaches trough corresponds to a major mesozoic rift which became partly inverted during the late oligocene and miocene sediments contained in this basin range in age from permian to neogene and are some 7000 m thick the rifting stage of the western approaches trough spans permian to aptian times there is only minor evidence for volcanic activity important wrench deformations preceded the accumulation of wealden clastics in the rapidly subsiding southwest channel basin and in the brittany trough early aptian wrench deformations coincided with crustal separation in the bay of biscay during the albian to early oligocene the western approaches trough was tectonically inactive and subsided regionally late oligocene to miocene compressional deformations induced the upwarping of a major anticlinorium having a structural relief of some 3000 m

reconstruction of climate and vegetation changes of lake bayanchagan inner mongolia holocene variability of the east asian monsoon a high resolution pollen and pediastrum record spanning 12 500 yr is presented for lake bayanchagan 115 21 e 41 65 n and 1355 m a s l southern inner mongolia individual pollen taxa pt mat and the pft affinity scores pft mat were used for quantitative climatic reconstruction from pollen and algal data both techniques indicate that a cold and dry climate similar to that of today prevailed before 10 500 cal yr b p the wettest climate occurred between 10 500 and 6500 cal yr b p at which time annual precipitation was up to 30 60 higher than today the early holocene increases in temperature and precipitation occurred simultaneously but mid holocene cooling started at approximately 8000 cal yr b p 1500 yr earlier than the drying vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type the results suggest that this region was dominated by steppe vegetation throughout the holocene except for the period 9200 to 6700 cal yr b p when forest patches were relatively common inner mongolia is situated at the limit of the present east asian monsoon and patterns of vegetation and climate changes in that region during the holocene probably reflect fluctuations in the monsoon s response to solar insolation variations the early to middle holocene monsoon undoubtedly extended to more northern latitudes than at present

holocene climate changes in the mid high latitude monsoon margin reflected by the pollen record from hulun lake northeastern inner mongolia pollen assemblage data from a sediment core from hulun lake in northeastern inner mongolia describe the changes in the vegetation and climate of the east asian monsoon margin during the holocene dry steppe dominated the lake basin from ca 11 000 to 8000 cal yr bp suggesting a warm and dry climate grasses and birch forests expanded 8000 to 6400 cal yr bp implying a remarkable increase in the monsoon precipitation from 6400 to 4400 cal yr bp the climate became cooler and drier chenopodiaceae dominated the interval from 4400 to 3350 cal yr bp marking extremely dry condition artemisia recovered 3350 2050 cal yr bp denoting an amelioration of climatic conditions both temperature and precipitation decreased 2050 to 1000 cal yr bp as indicated by decreased artemisia and the development of pine forests during the last 1000 yr human activities might have had a significant influence on the environment of the lake region we suggest that the east asian summer monsoon did not become intensified until 8000 cal yr bp due to the existence of remnant ice sheets in the northern hemisphere changes in the monsoon precipitation on millennial to centennial scales would be related to ocean atmosphere interactions in the tropical pacific

variscan sourcing of westphalian pennsylvanian sandstones in the canonbie coalfield uk the zircon age spectrum in a sample from the canonbie bridge sandstone formation asturian of southern scotland contains two main peaks one is early carboniferous in age 348 318 ma and corresponds to the age of igneous activity during the variscan orogeny the other is of late neoproterozoic to early cambrian age 693 523 ma corresponding to the cadomian together these two groups comprise 70 of the zircon population the presence of these two peaks shows unequivocally that a significant proportion of the sediment was derived from the variscides of western or central europe the zircon population also contains a range of older proterozoic zircons and a small devonian component these could have been derived from the variscides but it is possible that some were locally derived through recycling of northerly derived sandstones of devonian carboniferous age the zircon age data confirm previous suggestions of variscide sourcing to the canonbie area made on the basis of petrographical heavy mineral and palaeocurrent evidence and extend the known northward distribution of variscan derived westphalian sediment in the uk

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

time span of plutonism fabric development and cooling in a neoproterozoic magmatic arc segment u pb age constraints from syn tectonic plutons sark channel islands uk abstract new u pb zircon and titanite dates from syn tectonic plutons on the british channel island of sark constrain the time span of plutonism fabric development and cooling in this part of the neoproterozoic cadomian magmatic arc the tintageu leucogneiss is a mylonitic unit that was dated previously at 615 6 4 2 2 3 ma the port du moulin quartz diorite which intruded the tintageu unit contains a high strain solid state deformation fabric that is less intense than but parallel to fabrics in the leucogneiss and yields a u pb zircon date of 613 5 2 3 1 5 ma the little sark quartz diorite also displays solid state deformation fabrics in addition to relict magmatic textures and yields a u pb zircon date of 611 4 2 1 1 3 ma the north sark granodiorite is largely penetratively undeformed exhibits mainly magmatic fabrics and textures and has a u pb zircon date of 608 7 1 1 1 0 ma two fractions of titanite from each intrusion are essentially concordant and are identical within error with mean dates of 606 5 0 4 ma port du moulin quartz diorite 606 2 0 6 ma little sark quartz diorite 606 4 0 6 ma north sark granodiorite the new u pb data in combination with previous u pb and 40 ar 39 ar data and previous field studies confirm the syn tectonic nature of the sark plutons and quantify the time span ca 7 m y required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation titanite u pb and hornblende 40 ar 39 ar dates mark final cooling about 2 m y after intrusion of the last pluton

timing of post tectonic cadomian magmatism on guernsey channel islands evidence from 40ar 39ar mineral ages the channel island of guernsey exposes pre and post tectonic calc alkaline plutonic rocks attributed to late precambrian cadomian subduction activity 40 ar 39 ar mineral cooling ages presented here clarify the age and time span of post tectonic intrusions amphibole from the bon repos meladiorite yields an isotope correlation age of 605 3 4 6 ma indicating emplacement during late stages of the tectonothermal event recorded within its host rocks igneous hornblendes from the northern igneous complex record consistent isotope correlation ages st peter port gabbro 570 2 1 1 ma bordeaux diorite complex 569 0 1 1 and 565 3 2 2 ma and a comagmatic enclave within cobo granite 570 2 1 5 ma hornblende and biotite from contact metamorphosed host rocks adjacent to the cobo granite yield isotope correlation ages of 569 6 1 1 and 563 8 3 3 ma respectively these ages together with field data demonstrate that the various members of the northern igneous complex were emplaced contemporaneously at c 570 ma significantly earlier than a c 500 ma rb sr whole rock isochron age previously reported for the cobo granite the 40 ar 39 ar ages indicate that post tectonic magmatism on guernsey was broadly coeval with post metamorphic cooling around the baie de st brieuc on the adjacent french mainland and occurred c 30 ma prior to peak cadomian deformation and metamorphism in the st malo area

holocene cyclic climatic variations and the role of the pacific ocean as recorded in varved sediments from northeastern china we present an n alkane and compound specific carbon isotope record of the past 9 ka from the annually laminated sedimentary sequence of lake xiaolongwan northeastern china the n alkane distribution suggests that lake xiaolongwan has undergone a shift from an oligotrophic state with low algal production and little emergent submerged macrophytes in the early holocene to a eutrophic state with high algal production and abundant emergent submerged macrophytes since the middle holocene the pattern of variation observed in the biomarker proxies acl the n alkane average chain length paq aquatic macrophyte versus aquatic macrophyte and terrestrial plant ratio and lptp lake productivity terrigenous organic production is throughout the record similar to that of the total organic carbon the variation of compound specific carbon isotopic values in the middle and short chain alkanes was mainly regulated by lake productivity and the accumulating organic pool through time in this forested region where the vegetation is dominated by c 3 plants the long chain n alkanes c27ec31 are predominantly derived from leaf wax lipids the compound specific d 13 c27e31 value is sensitive to effective precipitation and therefore represents a useful indicator of regional monsoonal precipitation spectral analysis on the d 13 c27e31 time series reveals significant periodicities of 87e89 205e212 1020e1050 and 1750e2041 years on the centennial timescale the quasi periodicities around 88 and 210 years suggest a strong link between solar activity and monsoon rainfall the millennial monsoon cycle in northeastern china is associated with sea surface temperature sst variations in two active centers of the summer monsoon the western pacific subtropical high wpsh and the okhotsk high increasing sst in the subtropical sea may cause a northwards shift of the wpsh which extends the monsoon rain band meiyu to northeastern china and thus increasing rainfall in that region meanwhile decreasing sst in the okhotsk sea may strengthen the okhotsk high bringing more moisture into northeastern china we suggest that the pacific ocean is a main regulator for summer monsoon rainfall in northeastern china at present and at different time scales during the holocene

a multiproxy lake record from inner mongolia displays a late holocene teleconnection between central asian and north atlantic climates in order to study how the holocene central asian climate is coupled to the global climate system a 4 24 m long lake core from western inner mongolia in china was studied using a multiproxy approach sedimentology and geochemical parameters such as gypsum and dolomite content presence of lakeshore sand changing to aeolian sand and changes in paleomagnetic properties bear witness to a trend toward a generally drier climate over the late holocene aridification is linked to the southward retreat of the northern boundary of the asian summer monsoon leaving central asia under the influence of the westerly belt the weakening of the asian summer monsoon in turn was caused by an orbitally driven decrease in summer insolation the weakening summer insolation also likely increased the intensity of the siberian high pressure system further promoting aridification of central asia on a shorter time scale the multiproxy record shows the climate to have been relatively dry during the medieval warm period ad 800 1100 with the ensuing humid environment at the end of this period gradually turning to become extremely dry ad 1100 1550 at the little ice age maximum switches in the north atlantic oscillation caused these changes through a teleconnection in the form of westerlies these westerlies provided most of central asia s moisture after the retreat of the asian summer monsoon the central asian climate therefore corresponds closely with late holocene european climate changes

sedimentology stratigraphy and tectonic implications of a paleo proterozoic continental extensional basin the el sherana edith river groups northern territory australia the ca 1 83 ga el sherana edith river basin of northern territory australia contains terrestrial sedimentary and volcanic rocks deposited during continental extension or transtension braid pl

tectonic setting of cadomian low pressure metamorphism in the central ossa morena zone iberian massif sw spain abstract the upper proterozoic serie negra a monotonous pre and syntectonic slate greywacke series including volcanic episodes crops out widely in the central area of the ossa morena zone sw iberian massif iberian peninsula this unit was affected by complex late precambrian tectonothermal events the cadomian orogeny during which two phases of deformation d1 and d2 are recognized in the 610 585 ma time span the second deformational episode d2 was the most prominent and was related to progressive low pressure high temperature metamorphism lp ht reaching the highest grade in the lower structural levels where thermal evolution led to extensive migmatization and gave rise to the generation of different kinds of anatectic granitoids in this work we present field petrographic and analytical data dealing with the metamorphic evolution of a cadomian lp ht terrane from the central ossa morena zone iberian massif sw spain whose tectonic setting is unclear so far the tectonic setting proposed herein for cadomian d2 lp ht events is discussed in terms of its relationship to plate convergence or to extensional tectonic regimes modelling of the p t t trajectories undergone by a crust moderately overthickened by the thrusting of thinned sialic crust slices reproduces well the observed p t t aths if moderate or low uplift rate and high geothermal gradients are assumed for the uplift history numerical simulations combined with structural and metamorphic evidence support a compressional tectonic setting as responsible for the observed facies series distribution and deformation metamorphism relationships furthermore the regional geodynamic context coeval high pressure metamorphism in neighboring areas of the ossa morena zone and extensive calc alkaline magmatism is consistent with such a tectonic setting a correlation is tentatively proposed here between compressional cadomian deformations in the ossa morena zone upper proterozoic docking of avalonian terranes in ne america and cadomian orogenesis in the northern armorican massif

u pb zircon and sm nd geochronology of mafic and ultramafic rocks from the central part of the tauern window eastern alps geochronological and geochemical analyses were carried out in order to identify the pre variscan basement of the tauern window eastern alps maficultramafic rocks from the central part of the tauern window have been studied by ree analysis and u pb and sm nd isotopic analyses on whole rock zircons garnets and sphene u pb and sm nd zircon dating define both magmatic pan african and cambro ordovician events from 650 ma to 486 ma within the alpine fold belt this indicates a time span of 150 ma for magmatic activities in the tauern window of the eastern alps the ages of 657 ma u pb zircon and 644 sm nd zireon obtained from an amphibolite are the oldest dates of the eastern alps they may be related to the pan african orogeny and imply an early cycle of magmatic intrusion before major activity started at around 500 ma sm nd whole rock analyses of the precambrian rocks do not define an isochron reflecting heterogenities within the mantle source the initial end values 1 2 to 4 7 are very low implying an enrichment of the magma source the second main phase of magmatic activity 539 486 ma is characterized by the emplacement of mafic ultramafic rock sequences as no ophiolitic relies are observed in these domains the early paleozoic magmatism was likely associated with extensional tectonics obtained ages of 301 3 and 314 4 3 ma point to a variscan metamorphism the first combined u pb zircon sm nd zircon data for an amphibolite from the basal amphibolite formation baf favoured the sm nd zircon isochron age as a magmatic age whereas the low initial end value point to an enriched magma source as well as to heterogenities within the magma source the obtained ages suggest that parts of the pre variscan basement within the alpine fold belt were formed during the pan africa cycle the detection of pre variscan ages within the alpine basement must reffect a complex history involving significant pre variscan activity

initiation and variation of the dune fields in semi arid china with a special reference to the hunshandake sandy land inner mongolia sedimentary sequences occurring in desert dunes reflect changes in desert systems and as such may contain signals useful for recognizing spatial and temporal changes of deserts and their response to regional or even global climate fluctuations geomorphological and palaeoenvironmental studies within the dune fields of the asian middle latitudes have provided some solid evidence for interpreting the history of these sand seas using the hunshandake otindag sandy land a sandy area covered primarily by stabilized dunes and located in the semi arid zone of eastern inner mongolia china fig 1 as an example we studied the initiation and variation in the dune landscape in the eastern portion of the desert belt in northern china on the basis of physical and biochemical indicators in the sediments and osl chronology we herein argue that this dune system in the middle latitudes of eastern asia is much younger than previously assumed and that it has responded sensitively to climate change during the late quaternary geological evidence from the sandy land suggests that most of the current dunes are of late pleistocene or even holocene age palaeosols intercalated in the aeolian sequences and their osl chronology show that the climate of the hunshandake was much wetter than today between 9 6 ka and 3 ka this resulted in stabilization of the dunes in the eastern and central portions of the sandy land epochs of reworking or stabilization of the dunes are broadly consistent with the fluctuations in northern hemisphere solar radiation although with an obvious time lag because the increase rate of annual precipitation was not sufficient to fully stabilize the dunes in more arid part of the region some active dunes persisted even during this long lasting wetter epoch we conclude that periods of holocene dune stabilization due to palaeosol formation varied along the climate gradients across the various sandy lands of northern china and in general it began earlier and lasted longer in the east than in the west the general nature of the sandy lands and their counterparts in the western portion of the desert belt during the lgm and mid holocene climate optimum is discussed in comparison with their current states

timing and structure of the younger dryas event in northern china a high resolution and absolute dated stalagmite record from kulishu cave beijing characterizes asian monsoon am history in northern china between ca 14 and 10 5 ka bp thousand yrs before present present 1950 including the entire younger dryas yd event using 230 th dates and counting of annual layers the shift into the yd began at 12 850 40 yr bp and took w340 yrs and the shift out of the yd began at 11 560 40 yr bp and took 38 yrs best estimate w20 yrs broadly similar to previously reported am records from central and southeastern china the more gradual nature of the start of the yd event as observed in the am records appears to contrast with the more abrupt beginning observed in the greenland ice records the total amplitude of the am yd event is also smaller than the amplitude of the am heinrich stadial 1 event in addition the general rising trend of the am during the bolling allerod period contrasts with the general cooling trend in greenland temperature during that time the influence of rising insolation on the am may explain this observation

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

a multi proxy record of environmental changes during the holocene from the haolaihure paleolake sediments inner mongolia abstract the east central inner mongolia is located near the present day limit of the asian summer monsoon asm influence and therefore sensitive to both regional and global climate change here we present the high resolution proxy record of regional paleoclimate evolution over the past 12 2 cal ka bp based on analyses of grain size and organic geochemical proxies i e toc tn c n 13c and 15n preserved in sediments from the haolaihure paleolake the multi proxy record reveals a cool and dry period from 12 2 to 8 7 cal ka bp and a relatively warm and wet stage from 8 7 to 2 2 cal ka bp interrupted by a short interval of reduced precipitation at 4 6 3 7 cal ka bp after 2 2 cal ka bp the regional environment deteriorated as climate shifted to generally cooler and drier conditions in the area with a brief return to a warm and wet climate during the medieval warm period the record also indicates severe drought in the region during the younger dryas event comparison with other proxy paleoclimate records in inner mongolia south china and central asia suggests that the environmental conditions in the east central inner mongolia were primarily controlled by the asm in the early to mid holocene while the westerlies appeared to be a major driver of environmental changes in the late holocene six of the eight cool and dry events recorded in haolaihure paleolake sediments are in phase with the ice rafting events in the north atlantic and were possibly caused by weakening of both the asm and the westerlies

holocene moisture evolution in arid central asia and its out of phase relationship with asian monsoon history we synthesize palaeoclimate records from the mid latitude arid asian region dominated today by the westerlies arid central asia aca to evaluate spatial and temporal patterns of moisture changes during the holocene sediment records from i i takes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200 year time slices the proxies used in these records include pollen and diatom assemblages sediment lithology lake levels and geochemistry mainly isotope data the results of our synthesis show that aca as a whole experienced synchronous and coherent moisture changes during the holocene namely a dry early holocene a wetter less dry early to mid holocene and a moderately wet late holocene during the early holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka 1 ka 1000cal a bp hence the effective moisture history in aca is out of phase with that in monsoonal asia as documented by numerous palaeoclimate records in monsoonal asia a strong summer monsoon and humid climate characterized the early holocene and a weakened summer monsoon and drier climate prevailed during the late holocene which were mainly controlled by changes in low latitude summer insolation in contrast we propose that the pattern of holocene effective moisture evolution in the westerly dominated aca was mainly determined by north atlantic sea surface temperatures ssts and high latitude air temperatures that affect the availability amount and transport of water vapor also topography of the tibetan plateau and adjacent asian highlands could have contributed to the intensification of dry climate in aca during the early holocene as a result of strengthening the subsidence of dry air masses associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation summer insolation might have played a key role in directly controlling moisture conditions in aca but only after the northern hemisphere ice sheets had disappeared in the mid and late holocene c 2007 elsevier ltd all rights reserved

u pb geochronological constraints on the timing of brioverian sedimentation and regional deformation in the st brieuc region of the neoproterozoic cadomian orogen northern france during the neoproterozoic cadomian orogeny in northern france supracrustal rocks of the brioverian supergroup were deposited in marginal and back arc basins and were subsequently variably deformed and metamorphosed new u pb analyses of single and small multigrain fractions of zircon from selected plutons from the baie de st brieuc region provide robust geochronological constraints on the timing of these events the jospinet granodiorite forms part of the local basement directly overlain by brioverian metasediments and basic volcanics and yields a u pb zircon date of 625 9 3 6 1 9 2 ma the pre tectonic port moguer tonalite which has been strongly sheared along with its amphibolite facies country rocks has a crystallization age of 600 4 0 9 ma emplacement ages of 576 3 1 5 1 2 ma for the syn tectonic fort la latte quartz diorite and 574 6 1 8 1 5 ma for the late tectonic st quay quartz diorite place limits on termination of deposition and timing of subsequent regional deformation of the brioverian sequence in the baie de st brieuc region the new dates constrain the age of brioverian sedimentation to the interval 626 575 ma a range consistent with a previously published pb pb zircon evaporation age of ca 58822 ma for brioverian volcanic rocks lanvollen formation deformation within this sector of the cadomian belt is believed to have occurred shortly before 575 ma revising previously published estimates for the age of this major tectonothermal cadomian event by 10 20 my 2002 elsevier science b v all rights reserved

40ar 39ar dating of a late proterozoic palaeomagnetic pole for the armorican massif france summary part of a combined geochronologic and palaeomagnetic study of the trcgor area armorican massif we have attempted to obtain a reliable age for a late proterozoic palaeopole from experiments carried out on the keralain and petit taurel diorites a heterogeneous population of amphiboles analysed by the 40ar 39ar stepheating method gave a flat saddle shaped age spectrum characteristic of excess argon with a minimum age of 604 9f 1 2 ma which represents a maximum estimate of the age of the mineral the isochron calculation gives an initial 40ar xar ratio sensibly higher than the atmospheric ratio this indicates that the isochron age 583 f 2 ma is a better estimate of the age of the keralain diorite laser single step fusion and step heating experiments performed on single grains representative of the different components of this heterogeneous population of amphiboles show that the excess argon affects it in its entirety the minimum age of 590 8 f 2 3 ma of the flat saddle shaped age spectrum displayed by the core of a crystal of hornblende is concordant at the 20 level with the isochron age the isotopic closure temperature calculated from the step heating results is 556 f 43 c the palaeomagnetic study has shown using a thermal demagnetization procedure that the samples have a multicomponent magnetization two characteristic components a and b have been recognized respectively at keralain and the petit taurel a careful rock magnetic study has established that the magnetic carrier of the primary component a isolated at keralain is magnetite with a magnetic blocking temperature of 575 c the similarity of this temperature with the calculated isotopic closure temperature allows us to assign an age of c 583 ma to the corresponding well defined palaeopole dka located at 33 n 256 e the demagnetization experiments have also clearly shown that component b well isolated at the petit taurel is a remagnetization probably related to magmatic events at around 550 ma furthermore its magnetic behaviour as observed at keralain indicates a possible thermochemical origin the well dated dka palaeopole allows us to refine the late proterozoic polar wander path for armorica and seems to confirm that the armorican massif and southern united kingdom were linked together at that period

quaternary history of the temperate forests of china abstract pollen data from 80 sites in north china and northeast china are reviewed to document the quaternary history of the deciduous forest and the temperate mixed conifer hardwood forest of china during the late tertiary the forest in north china consisted of an admixture of temperate deciduous hardwoods subtropical broadleaved evergreen elements and ancient conifers of tropical and subtropical affinities pollen evidence from long boreholes through quaternary deposits indicates four or five glacial interglacial climatic cycles the first glacial episode known as the nangou cold period led to the development of a spruce fir forest of no modern analogue in north china deciduous forest was re established in north china during the intervening interglacials the subtropical evergreen elements and tertiary relicts were successively eliminated during the subsequent glacials in the last glacial spruce fir forests which are now confined to mountain slopes above 1500 m descended to the lowlands of north and northeast china implying a temperature depression of at least 8 10 c cold steppe occurred at least locally near beijing in the last glacial maximum but the data base is insufficient to delimit the spatial and temporal extent of this and other paleovegetation types on a regional scale or to reconstruct the locations of glacial refugia and the dynamics of plant migration the holocene pollen stratigraphies suggest a tripartite division with a period of maximum warmth the hypsithermal in the mid holocene in both north and northeast china the forest became more diverse during the mid holocene when thermophilous hardwoods expanded at the expense of pine and in more southerly locations birch unequivocal evidence for intensified summer monsoon during the early holocene as predicted by the kutzbach model based on orbital parameter changes remains to be found from the pollen records of the temperate forest regions of china

the oldest rocks of greece first evidence for a precambrian terrane within the pelagonian zone the pelagonian zone in greece represents the westernmost belt of the hellenide hinterland internal hellenides previous geochronological studies of basement rocks from the pelagonian zone have systematically yielded permo carboniferous ages in this study we demonstrate for the first time the existence of a precambrian crustal unit within the crystalline basement of the pelagonian zone the u pb single zircon and shrimp ages of these orthogneisses vary from 699 7 ma to 713 18 ma which identify them as the oldest rocks in greece these late proterozoic rocks which today occupy an area of c 20 100 km are significantly different from the neighbouring rocks of the pelagonian zone they are therefore interpreted as delineating a terrane named here the florina terrane during the permo carboniferous florina was incorporated into an active continental margin where it formed part of the basement for the pelagonian magmatic arc the activity of this arc was dated in this study by single zircon pb pb ages as having taken place at 292 5 ma and 298 7 ma during the alpine orogeny florina together with the pelagonian zone eventually became a constituent of the hellenides geochemically the florina orthogneisses represent granites formed at an active continental margin because of the late proterozoic ages this late proterozoic active continental margin can be correlated to a pan african or cadomian arc as the gneisses contain inherited zircons of late to middle proterozoic age the original location of florina was probably at the northwestern margin of gondwana similar to other gondwana derived terranes such as east avalonia florina approached the southern margin of eurasia during palaeozoic times where it became part of an active continental margin above the subducting palaeotethys these interpretations further indicate that terrane accretion was already playing an important role in the early pre alpine evolution of the hellenides

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

timing of cadomian deformation and magmatism within la hague nw france the la hague peninsula normandy nw france contains the most northerly components of the late neoproterozoic cadomian orogeny u pb dates for single grain and small multiple grain zircon separates from carefully selected plutons provide geochronological constraints on the timing of deformation and magmatism within the region the syntectonic jardeheu quartz diorite and omonville granodiorite were emplaced and contemporaneously deformed during the development of a greenschist facies fabric pervasive across the peninsula u pb zircon dates of 610 1 ma for the jardeheu quartz diorite and 608 2 ma for the omonville granodiorite are similar to a previously reported sensitive high resolution ion microprobe u pb date for metamorphism of the basement and indicate that a regionally significant period of metamorphism and magmatism occurred between 620 and 608 ma an emplacement age of 573 9 1 3 0 9 ma for the syntectonic thiebot granodiorite places limits on the timing of deformation along the thiebot shear zone zircons from the undeformed st martin monzonite give a precise u pb age of 581 3 1 1 ma the new dates point to reactivation of magmatism and localized greenschist facies deformation between 582 and 573 ma

u pb geochronology and sm nd isotopic composition of proterozoic gneisses channel islands uk gneissic units of the channel islands united kingdom have traditionally been considered to be palaeoproterozoic basement to the late neoproterozoic cadomia terrane based on lithological correlations and imprecise or ambiguous isotopic data a new precise u pb date of 2061 2 ma based on analyses of single zircons for the icart granitic orthogneiss of guernsey confirms a palaeoproterozoic age of its igneous protolith the nd depleted mantle model age t dm of this gneiss is 2220 ma only slightly older than its crystallization age indicating that it represents juvenile crust other gneisses from guernsey have similar t dm ages ranging from 2210 to 2370 ma suggesting that they are also exposures of juvenile palaeoproterozoic crust a component from a penetratively deformed orthogneiss from sark previously correlated with icartian gneisses on guernsey yields a u pb zircon date of 616 4 2 ma this crystallization age demonstrates that the protolith was a cadomian intrusion and that penetrative deformation and amphibolite facies metamorphism in the northern channel islands were cadomian in age the use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable the presence of 2170 7 ma zircon xenocrysts within the sark orthogneiss combined with its initial e nd 615 9 5 are strong evidence that pre cadomian basement was a significant source component and that documenting the extent and age of pre cadomian basement may be possible by indirect geochemical methods

late proterozoic zircon ages from a basic ultrabasic complex a possible cadomian orogenic complex in the hercynian belt of western europe the basic ultrabasic complex of belle isie en terre northwestern part of the armorican massif france has been dated at 602 4 1 m y by the u pb zircon method the calc alkaline trend of the metabasic rocks and the geologic context suggest that this ophiolite like complex is related to a back arc orogenic setting similar to the geodynamic environment that has been proposed for pan african calc alkaline ophiolite like complexes of late proterozoic age around the west african craton our results are compatible with the closing in late proterozoic time of a channel ocean north of the basement and formed during the cadomian orogeny rather than with possible caledonian or hercynian subduction