evolution of the indian summer monsoon and terrestrial vegetation in the bengal region during the past 18 ka the indian summer monsoon ism is a major global climatic phenomenon long term precipitation proxy records of the ism however are often fragmented and discontinuous impeding an estimation of the magnitude of precipitation variability from the last glacial to the present to improve our understanding of past ism variability we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower ganges brahmaputra meghna catchment and the indo burman ranges over the last 18 000 years 18 ka the records derive from a marine sediment core from the northern bay of bengal nbob and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the bengal region the isotopic composition of terrestrial plant waxes dd and d13c of n alkanes are compared to results from an isotope enabled general atmospheric circulation model isocam for selected time slices pre industrial mid holocene and heinrich stadial 1 comparison of proxy and model results indicate that past changes in the dd of precipitation and plant waxes were mainly driven by the amount effect and strongly influenced by ism rainfall maximum precipitation is detected for the early holocene climatic optimum ehco 10 5 6 ka bp whereas minimum precipitation occurred during the heinrich stadial 1 hs1 16 9 15 4 ka bp the isocam model results support the hypothesis of a constant moisture source i e the nbob throughout the study period relative to the pre industrial period the model reconstructions show 20 more rain during the mid holocene 6 ka bp and 20 less rain during the heinrich stadial 1 hs1 respectively a shift from c4 plant dominated ecosystems during the glacial to subsequent c3 c4 mixed ones during the interglacial took place vegetation changes were predominantly driven by precipitation variability as evidenced by the significant correlation between the dd and d13c alkane records when compared to other records across the ism domain precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ism from the last glacial to the present

pre folding magnetization reconfirmed for the late ordovician early silurian dunn point volcanics nova scotia johnson r j e and van der voo r 1990 pre folding magnetization reconfirmed for the late ordovician early silurian dunn point volcanics nova scotia tectonophysics 178 193 205 paleomagnetic results have been obtained for three new sites in steeply inclined subarea1 volcanic flows of the dunn point formation from the avalon terrane of nova scotia demagnetization analysis reveals a characteristic magnetization carried by hematite which is similar to that reported in previous studies of this unit a new and improved fold test for the characteristic component combining paleomagnetic and structural data from the present study with paleomagnetic data for vertically dipping flows and laterites studied previously by the authors is significantly positive the new fold test reconfirms the prefolding age of the characteristic component a primary age of magnetization is inferred based on field evidence for early oxidation of the flows the overall mean direction for the characteristic component of d 344o i 60 k 68 6 ass 4 1 paleopole at 2on 130 e corresponds to a late ordovician earliest silurian paleolatitude for avalon of 41 s the latter is much higher than that predicted for the area on the basis of the cratonic north america apparent polar wander path implying a substantial post ordovician displacement 1700 km of avalon with respect to the craton the dunn point result supports previous paleogeographic reconstructions that show avalon together with the continental blocks of hercynian europe and northwestern gondwana

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

paleomagnetism of the cambrian moore hollow group texas evidence for a primary magnetization carried by detrital magnetite petrographic and paleomagnetic studies of the cambrian moore hollow group texas establish the presence of three magnetic minerals and several components of magnetization hematite is the dominant carrier of the magnetic remanence in most samples and is primarily a diagenetic replacement of preexisting fe bearing phases hematite dominant samples have characteristic directions which range from shallow easterly to shallow southerly this distribution suggests multiple components of magnetization acquired at various times in the paleozoic the paleopole obtained for an unmineralized breccia 135 7 e 15 4 n represents either a chemical remanent magnetization of middle ordovician to early silurian age or a vector addition of two or more cambrian through silurian components mineralized breccias at the silver creek mississippi valley type deposit yield a permian direction 117 0 e 51 6 n because hematite in this deposit predates mineralization the permian age represents a maximum age for the mineralization at silver creek many samples contain goethite as a replacement of fe dolomite glauconite and pyrite the direction of magnetization carried by goethite is near the present geomagnetic field or in a minority of samples antipodal to it rounded ti magnetite was identified by sem in magnetic extracts from siliciclastic poor facies irm acquisition and thermal demagnetization confirms the presence of magnetite in these samples some samples contain an apparent modern viscous remanent magnetization the dominant component 72 samples 10 sites has a site mean declination of 93 8 and inclination of 14 5 with an 95 8 5 corresponding to a pole position at 163 0 e 0 6 s the presence of detrital magnetite polarity changes lack of evidence for prolonged burial and the proximity of this pole position to other late precambrian cambrian poles suggest that this component of magnetization indeed reflects the late cambrian paleopole for cratonic north america

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

el nino as a mediator of the solar influence on climate 1 using a climate model of intermediate complexity we simulate the response of the el nino southern oscillation enso system to solar and orbital forcing over the holocene solar forcing is reconstructed from radiocarbon production rate data using various scaling factors to account for the conflicting estimates of solar irradiance variability as estimates of the difference since the maunder minimum range from 0 05 to 0 5 of the solar constant we consider these two extreme scenarios along with the intermediate case of 0 2 we show that for large or moderate forcings the low pass filtered east west sea surface temperature gradient along the equator responds almost linearly to irradiance forcing with a short phase lag about a decade wavelet analysis shows a statistically significant enhancement of the century to millennial scale enso variability for even a moderate irradiance forcing in contrast the 0 05 case displays no such enhancement orbitally driven insolation forcing is found to produce a long term increase of enso variability from the early holocene onward in accordance with previous findings when both forcings are combined the superposition is approximately linear in the strong scaling case overall the sea surface temperature response is of the magnitude required and is persistent enough to induce important climatic perturbations worldwide the results suggest that enso may plausibly have acted as a mediator between the sun and the earth s climate a comparison to key holocene climate records from the northern hemisphere subtropics and midlatitudes shows support for this hypothesis

ordovician paleomagnetic results from the st george and table head carbonates of western newfoundland we have studied 85 limestone and dolostone samples from 21 sites of the st george and table head groups lower and middle ordovician on port au port peninsula humber zone their gentle 15 tilt is probably devonian a steep viscous component dominant in the natural remanence is removed by thermal and less efficiently alternating field demagnetization in about 60 of all samples the thermal treatment yielded a southeasterly reverse component a which typically remained blocked to 450 500 c and then decayed to noise mean a component directions calculated from zijderveld plots and referred to bedding are d i 148 5 33 6 for n 9 st george sites and d i 150 0 i 40 7 for n 5 table head sites corresponding antipoles are 17 5 n 152 3 e dp dm 2 8 4 9 and 13 4 n 149 2 e dp dm 2 9 4 7 respectively we interpret these results as representative of the geomagnetic field in the early and mid ordovician in 14 samples mainly from the older st george strata thermal

middle cambrian paleomagnetism of the avalon terrane in cape breton island nova scotia interbedded volcanics and sediments of the bourinot group mid cambrian in central cape breton island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the avalon terrane with respect to the north american craton both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations dual polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of d 293 i 66 n 9 k 41 8 95 8 1 the sediments reveal upon detailed thermal demagnetization complex multivectorial remanences however a few samples show characteristic components and a large number of samples 38 have intersecting great circle remagnetization trajectories when combined this yields a mean direction of d 267 i 65 k 16 mean radius of error ellipse is 8 5 the mean directions for the bourinot rocks correspond to a paleolatitude of 49 s 11 which is different from the range of possible values for the predicted mid cambrian paleolatitudes for nova scotia 29 s to 18 n inferred from the north american apparent polar wander path this discrepancy between predicted paleolatitudes and the observed paleolatitude implies that nova scotia was not part of north america during the mid cambrian but rather they were separated by an ocean with a minimum width of 1000 km 20 10 this interpretation of the relative positions of nova scotia and north america based on paleomagnetism is consistent with the geological record in the northern appalachians and supports previous interpretations of the dissimilar early paleozoic faunas in avalon and north america it is paleomagnetically quite plausible that the avalon terrane formed part of a larger continent comprising africa gondwana and armorica during the cambrian

interpretation of resistivity measurements over 2d structures1 resistivity measurements were carried out in a survey area in the south of germany this area is characterized by complicated subsurface geology schlumberger full arrays and their respective half arrays were recorded simultaneously the results obtained by the one dimensional 1d interpretation of the full array measurements were incorrect because of a resistivity discontinuity this discontinuity under a relatively thick overburden could only be located by the half array soundings its exact location and the resistivity distribution in the subsurface were ascertained by comparing the sounding curves with 2d model curves which are calculated by a finite difference method

correlation between indian ocean summer monsoon and north atlantic climate during the holocene there has been a number of investigations for the correlation between the asia monsoon and the north atlantic climate for the last glacial however little research has been done for the present interglacial the holocene here we present for the first time a high resolution composite proxy record for the indian ocean summer monsoon spanning around 12 000 years based on the 13c time series of both a single plant species carex mulieensis remains cellulose and the total plant assemblage cellulose in the hongyuan peat bog from the tibet plateau the records show that the strength of the indian ocean summer monsoon had abrupt variations during the last 12 000 years the weakest monsoon occurred in the younger dryas period following rapid strengthening from around 11 200 to 10 800 a bp the monsoon kept a generally strong level for around 5300 years from around 5500 a bp onwards the monsoon strength tended to gradual decrease in addition there are a series of abrupt variation events of the monsoon strength on centennial to millennial time scales which superimpose the general tendency of the monsoon variation in every case when the ice rafted debris events in the north atlantic occurred the summer monsoon strength decreased correspondingly these evidences show that teleconnection between the indian ocean summer monsoon and the north atlantic climate is present not only in the last glacial but also in the holocene which may be linked to abrupt reorganizations of the ocean thermohaline circulation leading to redistribution of energy changing temperature and moisture gradient over the southern subtropical indian ocean and eventually controlling the variability of the indian ocean summer monsoon

effect of anisotropy on the paleomagnetic contact test for a grenville dike we report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the precambrian grenville province of canada directions of natural remanent magnetization nrm in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric using the measured anisotropy of laboratory induced thermoremanent magnetization trm the trm ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility ams ellipsoids but the anisotropy of remanence is more pronounced than the ams anisotropy correction moves nrm directions in the unbaked zone almost 20 bringing them close to typical grenvillian paleofield directions the anisotropy seems to be controlled by multidomain rather than single domain magnetic minerals nrm directions from the baked gneiss near the dike contact after anisotropy correction agree well with the nrm directions from the chilled margin of the early cambrian 570 3 ma dike this positive baked contact test is augmented by the presence of a hybrid zone in which country rock nrms are increasingly overprinted by the dike nrm as one moves from the unbaked toward the baked zone the dike nrm is therefore primary however the virtual geomagnetic pole vgp for the dike nrm 51 n 131 w dp 10 dm 12 n 24 samples or 41 n 153 w dp 13 dm 18 n 4 chilled margin samples does not fall near most established cambrian paleopoles post emplacement tectonic movement of the dike is unlikely it may be that the dike cooled and acquired trm during a geomagnetic field excursion because of its geologically short cooling blocking period of a few tens of years

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

solar influence on the indian ocean monsoon through dynamical processes 1 the result of paleoclimate studies on the relationship between the indian monsoon and solar activity inferred from the analysis of stalagmites in oman is confirmed by using a modern meteorological dataset from 1958 1999 the present result suggests that the solar influence on monsoon activity is not due to a change in radiative heating in the troposphere but rather originates from the stratosphere through modulation of the upwelling in the equatorial troposphere which produces a north south seesaw of convective activity over the indian ocean sector during summer higher precipitation over arabia and india thus occurs during high solar activity

reduced early holocene moisture availability in the bayan har mountains northeastern tibetan plateau inferred from a multi proxy lake record abstract a late glacial and holocene lake sediment record from the northeastern tibetan plateau was investigated using ostracod and geochemical analysis after lake koucha was established about 16 cal ka bp oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka bp clearly suggesting that the water body was a closed basin lake from about 10 cal ka bp the salinity of the lake rose continuously until hyperhaline levels were reached about 7 5 cal ka bp this increase in salinity was accompanied by a falling lake level extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8 5 cal ka bp causing a unique thin gravel containing layer within the continuous fine grained lake sediments the age control of our record does not allow us to correlate this event exactly to the widely recorded 8 2 cal ka bp event however the recovered sediments from lake koucha provide strong evidence that climate changes around 8 5 cal ka bp had significant environmental implications in northeastern tibet after about 7 3 cal ka bp the available moisture significantly increased causing a continuous salinity decline until the present day freshwater conditions of a through flow lake were established about 6 5 cal ka bp in lake koucha this major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte dominated to a phytoplankton dominated lake a slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4 3 and 2 0 cal ka bp in phase with numerous records from monsoonal asia the reconstructed overall change in moisture availability from dry conditions before about 7 3 cal ka bp to wetter conditions afterwards contradicts most palaeoclimate records from the tibetan plateau and its surroundings which show that wetter conditions in the early holocene were replaced by drier and probably cooler conditions in the mid and late holocene however in the semi arid region of lake koucha with a mean annual precipitation of about 500 mm the insolation driven strengthened monsoon in the early holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability our record provides further evidence for a regionally non uniform response of the available moisture evolution to the overall monsoonal signal in the holocene climate history of central asia

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

paleomagnetism of lower cambrian volcanics and a cross cutting cambro ordovician diabase dyke from buckingham quebec the buckingham area quebec is characterized by cambrian volcanic flows 573 32 ma cut by a cambro ordovician diabase dyke which is dated at 497 14 ma by k ar method the volcanics yield multicomponent magnetizations a hard characteristic magnetization and a relatively soft one these components could be separated easily and the intensity decay curves of the different components could also be obtained separately using zijderveld s method three different groups of site mean characteristic magnetizations can be distinguished each representing a different stratigraphic level the paleopoles derived from these magnetizations are from oldest to youngest 186 e 10 s 173 e 01 n and 154 e 06 n this is interpreted as a rapid motion of the paleopole for the north american craton during the early cambrian the paleopole derived from the hard characteristic dyke direction is 123 e 03 s this magnetization direction was also found at one of the sites in the volcanics this means that the contact test

changes of the hailuogou glacier mt gongga china against the background of climate change during the holocene large changes have occurred at hailuogou glacier mt gongga china during the holocene various data indicate that it retreated by 1 88 km during the 20th century and mass balance records show that it suffered a sustained mass loss of snow and ice in the period 1959 60 2003 04 with an accumulated mass balance of 10 83 m water equivalent hydrological data demonstrate that runoff from the glacier has been increasing both seasonally and annually the principal cause being an increase of ice and snow melt water as the mean temperature in the hailuogou basin rose by 0 1 degrees c between 1994 and 2004 the runoff increased by 2 6 m 3 s 1 changes of glacier morphology which occurred during this period include a decrease of thickness enlargement of glacial caves and reduction of the size of seracs the hailuogou glacier advanced three times during the little ice age in response to three cold periods in the mt hengduan region there also were three noticeable advances in the neoglacial there was a very clear relationship between glacier change climate change and forest line change in the period 3000 a bp 100 a bp overall it is clear that changes of the hailuogou glacier in the holocene have been responded to climatic fluctuations but quantitative evaluation of the relationship is difficult owing to the sparse spatial coverage of reliable evidence and the limited available longer term data c 2008 elsevier ltd and inqua all rights reserved

oxygen and carbon isotope patterns archived in shells of the aquatic gastropod radix hydrologic and climatic signals across the tibetan plateau in sub monthly resolution abstract the tibetan plateau tp including its surrounding mountain ranges represents the largest store of ice outside the polar regions it hosts numerous lakes as well as the head waters of major asian rivers on which billions of people depend and it is particularly sensitive to climate change the moisture transport to the tp is controlled by the indian and pacific monsoon and the westerlies understanding the evolution of the interaction of these circulation systems requires studies on climate archives in different spatial and temporal contexts the objective of this study is to learn more about the interannual variability of precipitation patterns across the tp and how different hydrologic systems react to different climatic factors aragonite shells of the aquatic gastropod radix which is widely distributed in the region may represent suitable archives for inferring hydrologic and climatic signals in particularly high resolution therefore sclerochronological studies of 18 o and 13 c ratios in radix shells from seven lakes were conducted each representing a different hydrologic and climatic setting on a transect from the pamirs across the tp the shell patterns exhibit an increasing influence of precipitation and a decreasing influence of evaporation on the isotope compositions from west to east 18 o values of shells from lakes on the eastern and central tp donggi cona yamdrok yumco tarab co mirror monsoon signals indicated by more negative values and higher variabilities compared to the more western lakes karakul bangong nyak manasarovar in yadang co located on the central southern tp the monsoon rains did not reach the lake in the sampling year although it is located in a region which is usually affected by monsoon circulation the 18 o values are used to differentiate the annual hydrological cycle into ice cover period melt water period precipitation period and evaporation period 13 c compositions in the shells particularly depend on specific habitats which vary in biological productivity and in carbon sources 18 o and 13 c patterns show a positive covariance in shells originating from large closed basins the results show that radix shells mirror general climatic differences between the seven lake regions these differences reflect both regional and local climate signals in sub seasonal resolution without noticeable dependence on the particular lake system

geoelectrical model calculations for two dimensional resistivity distributions for the calculation of geoelectrical model curves for a two dimensional resistivity distribution the potential equation is transformed by means of a fourier cosine transform into a two dimensional helmholtz equation containing the separation parameter the numerical solution of this equation for different values of for an irregular grid is obtained using the method of finite differences combined with the method of overrelaxation the method by which derivatives are replaced by finite differences turned out to be very important especially for high resistivity contrasts after testing several methods designed to deal with any type of resistivity distribution a method of discretization similar to that used by brewitt taylor and weaver 1976 for magnetotelluric modeling for h polarization was found the best examples are given of model curves for schlumberger soundings over a vertical fault covered by overburden the incorrect use of horizontal layer models leads to erroneous interpretations that are more complex than the real subsurface situations

the influence of 14c reservoir age on interpretation of paleolimnological records from the tibetan plateau there is a great deal of controversy regarding the fate of glaciers and ice fields on the tibetan plateau in the face of continued anthropogenic global warming paleoclimate reconstructions and spatial analyses aimed at mapping past climate changes are the key to understanding the climatic response of the tibetan plateau to changing conditions specifically the numerous lakes distributed across the tibetan plateau can provide high resolution spatial and temporal climate reconstructions to investigate past changes in the climate system in this paper we review the primary limitation to exploiting these valuable paleoclimate archives errors in radiocarbon based age models we review the techniques that have been used to estimate 14 c reservoir ages on the tibetan plateau and compile the published 14 c reservoir ages to examine their spatial and temporal patterns and to assess the imposed chronological uncertainties using site specific evaluations of bangong co and lake qinghai we demonstrate that 14 c age model uncertainties permit equally probable and contrasting interpretations of existing paleoclimate records we also examine 14 c induced uncertainties in the spatial climatic response on the tibetan plateau to 1 the termination of the last glacial maximum and 2 the holocene thermal maximum we conclude with recommendations for reducing uncertainties in future lake based paleoclimate studies on the tibetan plateau

paleomagnetic and petrologic study of the age origin and significance of early and late paleozoic events in the long mountain granite wichita mountains oklahoma the cambrian long mountain granite exposed in the western wichita mountains oklahoma is red granophyric alkali feldspar granite the red granite abruptly transitions into a green granite in the subsurface both red and green granites have similar geochemical signatures anisotropy of magnetic susceptibility analysis shows that the green granite retains a primary magnetic fabric that is consistent with the sill like emplacement of the wichita granite group demagnetization of green granite specimens yields a characteristic remanent magnetization chrm residing in magnetite with east declinations and moderate down inclinations the paleopole 9 0 n 314 6 e is interpreted as a primary cambrian thermal remanent magnetization or an early magnetization related to deuteric hydrothermal alteration the paleopole position is consistent with some other poles for cambrian igneous rocks in southern oklahoma but is to the southwest of the cambrian part of the apparent polar wander path the red granite contains abundant secondary hematite that occurs in fractures as grain boundary coatings and along cleavage and exsolution planes in alkali feldspars the iron in the hematite appears to be sourced from the oxidation of primary magnetite and ilmenite and the breakdown of silicate ferromagnesian minerals the red granite has about two orders of magnitude lower magnetic susceptibility and natural remanent intensity than the green granite the magnetic fabric is interpreted as an alteration fabric the chrm of red granite has southeast declinations and shallow inclinations and is interpreted as a chemical remanent magnetization crm residing in hematite the paleopole 44 9 s 304 9 e falls near the 290 300 ma segment of the north american apparent polar wander path which is consistent with inferred timing of exposure of the long mountain granite the crm is interpreted to have been acquired during alteration by low temperature weathering fluids near the surface during the late paleozoic the results from the red granite are not consistent with alteration caused by widespread paleoclimatic conditions in the late permian and they are interpreted as related to local tectonic and or weathering events

a lateglacial and holocene lake record from the nianbaoyeze mountains and inferences of lake glacier and climate evolution on the eastern tibetan plateau abstract our 12 81 m long record from lake ximencuo provides insight into the lateglacial and holocene evolution of the lake and the nianbaoyeze glacier in its catchment and the regional climate history since 19 kyr lake ximencuo was a permanent deep lake throughout its history in contrast to numerous lakes on the tibetan plateau which experienced shallow lake levels or even desiccation during the lateglacial lake ximencuo was fed by large quantities of meltwater in the lateglacial the existence of glaciated upper catchment areas is apparently a prerequisite for lakes on the tibetan plateau which maintained relatively high water levels during the generally cold and dry periods following the global lgm 21 kyr a minor re advance of the nianbaoyeze glacier was recorded coeval with the greenland stadial 2a 16 4 and 14 5 kyr followed by rapid warming apparently synchronous with the greenland interstadial 1 warmest conditions were recorded at lake ximencuo during the early holocene which was punctured by a remarkable pulse of climate deterioration around 8 3 kyr this spell represents probably the 8 2 kyr event of the north atlantic region suggesting that it had a significant impact on the tibetan plateau colder conditions of longer duration occurred from 4 7 to 3 7 kyr apparently in phase with numerous records signalling colder and drier conditions on the tibetan plateau two minor spells of colder conditions and probably catchment erosion were recorded in the late holocene between 2 0 and 1 4 kyr and between 0 5 and 0 1 kyr with the latter representing the little ice age it is unclear whether human activities may have accelerated or even solely triggered the late holocene erosion events

early and late paleozoic remagnetization of the cambrian peerless formation colorado abstract the upper cambrian peerless formation in central colorado contains two secondary components of magnetization the dominant component was removed during initial af demagnetization of specimens whereas a weaker component was removed during subsequent thermal demagnetization irm acquisition experiments suggest that the dominant component has low coercivity whereas the weaker component has high coercivity the latter component has southeasterly and shallow directions and the corresponding pole position is located at 40 0 n 134 5 e the high coercivity blocking temperatures over 600 c and petrographic evidence suggest this component resides in diagenetic hematite the magnetization is interpreted as a crm acquired in the late paleozoic the low coercivity component directions fall on a great circle that passes through the modern field direction and two modes at 107 e 10 and 287 e 10 this component is interpreted as a vector sum of two antipodal crms and a modern vrm the pole position 10 n 150 e suggests acquisition in the early paleozoic and the remanence resides either in maghemite or in diagenetic magnetite

sub seasonal oxygen and carbon isotope variations in shells of modern radix sp gastropoda from the tibetan plateau potential of a new archive for palaeoclimatic studies carbon and oxygen isotope ratios have been measured for nine aragonite shells of the gastropod genus radix from the lake bangda co 30 29 n 97 04 e 4450 m a s l at the south eastern edge and from two characteristic sites at the lake kyaring co 31 09 n 88 17 e 4650 m a s l on the central tibetan plateau radix shells were sampled for isotope ratio analysis with high spatial resolution along the ontogenetic spiral of growth providing the basis of isotope records with a sub seasonal time resolution 18o values of shells from bangda co are on average 15 0 relative to pdb and the pattern exhibits a clear onset and progression of the summer monsoon precipitation indicated by a strong amount effect this pattern mirrors the precipitation pattern in the respective year and region as expected for a small surface area ca 0 3 km2 and shallow 5 m lake or habitat with short water residence times and little evaporative 18o enrichment of the lake water in contrast 18o values of radix shells from kyaring co habitat a which is connected to the deep several tens of metres and big surface area ca 660 km2 lake average at 13 0 consistent with a higher evaporation rate and longer water residence time the latter is supported by more 18o enriched water in this habitat the 18o values of radix shells from kyaring co habitat b are nearly as low as shells from bangda co due to the similar habitat characteristic but isotopic patterns of these shells exhibit a weaker amount effect in both lake systems 13c values of the shells are coupled with oxygen isotopes because a large amount of isotopically light carbon is washed from mountain slopes into the lake during the rainy season although other processes influence the isotopic patterns e g biological productivity 13c or temperature 18o these influences are minor compared with the monsoon signal or the effect of evaporation in the radix shell records the overall weaker amount effect in radix shells from kyaring co habitat b compared with shells from bangda co are consistent with a current decreasing monsoon influence from the south eastern edge towards the central tibetan plateau thus fossil shells of the gastropod genus radix are a valuable archive for reconstructing climatic and environmental changes on the tibetan plateau and provide information about former habitat sizes and depths

ages of major little ice age glacier fluctuations on the southeast tibetan plateau derived from tree ring based moraine dating abstract advances and retreats of glaciers on the tibetan plateau tp are widely regarded as indicators for climate changes sensitive to macroclimatic forcing mechanisms like the asian summer monsoon however it often remains unclear why some glaciers retreat or advance earlier than others particularly regarding the timing of the so called little ice age lia maximum within this study we present newly acquired tree ring data from four glacier forefields in the nyainqentanglha range on the southeastern tp and link them to previous studies two of the glaciers formed a double tongue moraine set during the lia with trees providing minimum ages for two different major glacier extents in combination with evidence from a third glacier three warm phases within the generally cold lia can be deduced during the first half of the 16th century and the mid 17th and 18th centuries this implies the occurrence of three preceding major cold spells a minor cool phase between 1790 and 1850 marks the end of the lia on the southeast tp minimum ages for beginning retreat from the last major lia glacial advance differ by only 40 years for all sampled monsoonal temperate valley glaciers in the nyainqentanglha range this implies a common forcing whose impact on glacier dynamics is only slightly altered by topographical factors or local climatic conditions regression analysis suggests that the delay of the glacier retreat from the maximum extent is mainly related to glacier size with minor influence of ablation area aspect all moraine ages highlight intermediate lia warm phases on the southeast tp corresponding to warm phases on the northern hemisphere and thus prove monsoonal temperate glaciers to be highly sensitive archives for northern hemispheric climatic conditions

a tibetan lake sediment record of holocene indian summer monsoon variability article history sedimentological data and hydrogen isotopic measurements of leaf wax long chain n alkanes dwax from an alpine lake sediment archive on the southeastern tibetan plateau paru co provide a holocene perspective of indian summer monsoon ism activity the sedimentological data reflect variations in lake level and erosion related to local ism rainfall over the paru co catchment whereas dwax reflects integrated synoptic scale ism dynamics our results indicate that maximum ism rainfall occurred between 10 1 and 5 2 ka during which time there were five century scale high and low lake stands after 5 2 ka the ism trended toward drier conditions to the present with the exception of a pluvial event centered at 0 9 ka the paru co results share similarities with paleoclimate records from across the tibetan plateau suggesting millennial scale ism dynamics were expressed coherently these millennial variations largely track gradual decreases in orbital insolation the southward migration of the intertropical convergence zone itcz decreasing zonal pacific sea surface temperature sst gradients and cooling surface air temperatures on the tibetan plateau centennial ism and lake level variability at paru co closely track reconstructed surface air temperatures on the tibetan plateau but may also reflect indian ocean dipole events particularly during the early holocene when enso variability was attenuated variations in the latitude of the itcz during the early and late holocene also appear to have exerted an influence on centennial ism rainfall 2014 elsevier b v all rights reserved

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

palaeoclimatic changes in the qinghai lake area during the last 18 000 years abstract multi proxy analysis of a sediment core from qinghai lake including pollen carbonate toc tn and 13 c of organic matter was used to document regional climatic changes during the last 18 000 years climate was very cold and dry before 16 9 cal kyr bp the warm and wet period which began at about 14 1 cal kyr bp culminated at 6 5 cal kyr bp and came to its end at about 4 5 cal kyr bp after that the climate gradually became colder and drier during the transitional period from the late glacial to the holocene the climate fluctuated frequently the short term climatic oscillations including the bolling allerod younger dryas and the cooling event at ca 8 2 cal kyr bp appear to be recorded in the qinghai lake sedimentary record the climatic evolution in the qinghai lake area during the past 18 000 years indicates that solar insolation changes on the ten thousand year scale are the likely driving force of the east asia monsoon

the abrupt termination of the younger dryas climate event previous studies on two deep greenland ice cores have shown that a long series of climate oscillations characterized the late weichselian glaciation in the north atlantic region1 and that the last glacial cold period the younger dryas ended abruptly 10 700 years ago2 here we further focus on this epoch defining event and present detailed heavy isotope and dust concentration profiles which suggest that in less than 20 years the climate in the north atlantic region turned into a milder and less stormy regime as a consequence of a rapid retreat of the sea ice cover a warming of 7 c in south greenland was completed in about 50 years

asynchronous evolution of the indian and east asian summer monsoon indicated by holocene moisture patterns in monsoonal central asia abstract the numerical meta analysis of 92 proxy records 72 sites of moisture and or temperature change confirms earlier findings that the dominant trends of climatic evolution in monsoonal central asia since the last glacial roughly parallel changes in northern hemisphere summer insolation i e the period following the last glacial maximum was characterized by dry and cold conditions until 15 cal kyr bp followed by a warm wet period coincident with the bolling allerod warm period and terminated by a cold dry reversal during the younger dryas period after an abrupt increase at the start of the holocene warm and wet conditions prevailed until ca 4 cal kyr bp when moisture levels and temperatures started to decrease ordination of moisture records reveals strong spatial heterogeneity in moisture evolution during the last 10 cal kyr the indian summer monsoon ism areas northern india tibetan plateau and southwest china exhibit maximum wet conditions during the early holocene while many records from the area of the east asian summer monsoon indicate relatively dry conditions especially in north central china where the maximum moisture levels occurred during the mid holocene we assign such phenomena to strengthened hadley circulation centered over the tibetan plateau during the early holocene which resulted in subsidence in the east asian monsoonal regions leading to relatively dry conditions our observations of the asynchronous nature of the two asian monsoon subsystems on millennial time scales have also been observed on annual time scales as well as implied through the spatial analysis of vertical air motion patterns after strong ascending airflows over the tibetan plateau area that were calculated from ncep ncar reanalysis data for the last 30 years analogous with the early holocene the enhancement of the ism in a future warming world will result in an increase in the asynchronous nature of the monsoon subsystems this trend is already observed in the meteorological data from the last 15 years

combined sounding profiling resistivity measurements with the three electrode arrays a combination of dc resistivity sounding and profiling measurements can be used to obtain the maximum information about distribution of resistivities in the earth combined sounding profiling measurements with the amn mnb arrays are considered the resistivity data from such measurements can be presented as a normal sounding curves b combined sounding profiling curves c profiling curves d pseudo sections or as transformations obtained by the so called gradient processing to emphasize the influence of the target objects the examples chosen from numerical modeling and field tests show the efficiency of measurements with three electrode arrays to accurately locate thin conductors and contacts of lithological units of different resistivities an interpreted cross section is compared with the results of other geophysical measurements vlf r dipole em ip showing good correlation

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

glacial interglacial indian summer monsoon dynamics the modern indian summer monsoon ism is characterized by exceptionally strong interhemispheric transport indicating the importance of both northern and southern hemisphere processes driving monsoon variability here we present a high resolution continental record from southwestern china that demonstrates the importance of interhemispheric forcing in driving ism variability at the glacial interglacial time scale as well interglacial ism maxima are dominated by an enhanced indian low associated with global ice volume minima in contrast the glacial ism reaches a minimum and actually begins to increase before global ice volume reaches a maximum we attribute this early strengthening to an increased cross equatorial pressure gradient derived from southern hemisphere high latitude cooling this mechanism explains much of the nonorbital scale variance in the pleistocene ism record

petrologic paleomagnetic and structural evidence of a paleozoic rift system in oklahoma new mexico colorado and utah a west northwest trending dike swarm consisting of at least 50 mafic dikes cuts precambrian basement rocks in the area in and south of the black canyon of the gunnison colorado and in unaweap canyon near the colorado utah border the dikes have been studied petrographically petrochemically and paleomagnetically and they have been radiometrically dated lithologically the dike rocks are very similar consisting of tholeiitic diabase composed primarily of labradorite augite and granophyric intergrowths of quartz and alkali feldspar a rb sr isochron based on mineral separates from one dike yields an age of 495 15 m y which is statistically identical with a previously reported rb sr date pooling the age data for these two dikes gives an age of 497 16 m y whole rock k ar dates on four of the dikes are scattered around the rb sr date and indicate that the dikes have been relatively unaffected by reheating or hydrothermal activity since emplacement paleomagnetic directional data from nine of the dikes are well grouped and consistent after both alternating field af and thermal demagnetization several lines of evidence indicate that the remanence reflects the dipolar geomagnetic field direction at the time the dikes were emplaced paleopoles corresponding to the mean directions are located at 37 0 n 101 0 e p 8 6 m 16 4 af demagnetization and 37 0 n 102 3 e p 4 9 m 9 4 thermal demagnetization the consistency of the paleomagnetic data corroborates the idea that the dike swarm is the result of one short pulse of mantle derived magmatic activity the dike swarm and other cambro ordovician igneous rocks in colorado powderhorn alkalic complex wet mountains alkalic complexes and dikes are aligned along a linear west northwest trend cambrian diabase dikes in northeastern new mexico and a bimodal suite basalt gabbro rhyolite granite in southern oklahoma wichita and arbuckle mountains anadarko basin are on line with this trend petrologically all of these cambrian and cambro ordovician igneous rocks represent types that are typically associated with extensional tectonic regimes from these data and consideration of documented paleozoic tectonic activity along the trend of the plutons it is concluded that throughout the paleozoic a west northwest trending tectonic zone extended from southeastern oklahoma through northeastern new mexico and colorado and into southeastern utah bimodal basalt rhyolite and alkalic igneous activity was restricted to the cambrian and cambro ordovician but tectonic activity occurred intermittently along the zone throughout the paleozoic

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

detecting small scale targets by the 2d inversion of two sided three electrode data application to an archaeological survey abstractthe detecting capabilities of some electrical arrays for the estimation of position sizeand depth of small scale targets were examined in view of the results obtained from2d inversions of apparent resistivity data the two sided three electrode apparent resistivity data are obtained by the application of left and right hand pole dipolearrays that also permit the computation of four electrode and dipole dipoleapparent resistivity values without actually measuring them synthetic apparent resistivity data sets of the dipole dipole four electrode and two sided three electrodearrays are calculated for models that simulate buried tombs the results of two dimensional inversions are compared with regard to the resolution in detecting theexact location size and depth of the target showing some advantage for the two sided three electrode array a field application was carried out in the archaeologicalsite known as alaca hoyuk a religious temple area of the hittite period the two dimensional inversion of the two sided three electrode apparent resistivity data hasled to locating a part of the city wall and a buried small room the validity of theinterpretation has been checked against the results of subsequent archaeologicalexcavations introductionthe two dimensional 2d inversion of direct current dc sounding data has become a standard tool for the investiga tion of targets buried at shallow and intermediate depths theapplications include archaeology environmental and hydro geological works mining exploration and engineeringstudies when using the traditional electrode arrays thedata acquisition is usually carried out by expanding electro des along a measurement profile the apparent resistivitydata obtained from all soundings are then used as the input ofa 2d inversion algorithm pelton rijo and swift 1978 andsasaki 1981 first introduced inversion algorithms applied tothe dipole dipole and schlumberger arrays respectively later loke and barker 1996 introduced a fast 2d inversionmethod for the wenner and dipole dipole electrode arrays this algorithm is based on a quasi newton method whichcalculates an approximate value of partial derivatives of theapparent resistivity data with respect to model parameters the finite difference technique is used for the computation ofthe model response one of the remarkable developments in dc methods is theuse of modern measuring systems with multicore cableswhich allows automatic switching between electrodes forpractical field operations the electrodes are located at equallyspaced measurement stations for example morris ronningand lile 1997 described a data acquisition system thatsimultaneously collects schlumberger and two dipole dipoledata however they did not indicate a quantitative inter pretation tool acworth and griffiths 1985 and griffithsand turnbull 1985 described wenner tripotential apparent resistivity measurements to reduce the number of ambiguitiesarising in the qualitative interpretation of apparent resistivitypseudosections the tripotential method produces threeapparent resistivity measurements for each position of acollinear equispaced quadripole of electrodes they used the

genetic implications of minor element and sr isotope geochemistry of alkaline rock complexes in the wet mountains area fremont and custer counties colorado concentrations of rb sr and ree rare earth elements and sr isotopic ratios in rocks of the cambrian alkaline complexes in the wet mountains area colorado show that rocks formed as end products of a variety of magmas generated from different source materials the complexes generally contain a bimodal suite of cumulus mafic ultramafic rocks and younger leucocratic rocks that include nepheline syenite and hornblende biotite syenite in the mcclure mountain complex nepheline syenite pegmatite in the gem park complex and quartz syenite in the complex at democrat creek the nepheline syenite and hornblende biotite syenite at mcclure mountain 535 5m y are older than the syenitic rocks at democrat creek 511 8m y ree concentrations indicate that the nepheline syenite at mcclure mountain cannot be derived from the hornblende biotite syenite which it intrudes or from the associated mafic ultramafic rocks ree also indicate that mafic ultramafic rocks at mcclure mountain have a source distinct from that of the mafic ultramafic rocks at democrat creek

pollen evidence of changing holocene monsoon climate in sichuan province china pollen evidence from lake shayema mianning county was obtained to examine postglacial vegetation and climatic change in southwestern sichuan china the sclerophyllous character of the region s warm temperate vegetation today is a reflection of extreme drought in spring before the onset of the asian monsoons the pollen record displays several changes in the vegetation over the last 11 000 yr from 11 000 to 9100 yr b p cold tolerant species such as abies betula and deciduous oaks dominated the vegetation between 9100 and 7800 yr b p the abundance of deciduous oaks decreased and evergreen oaks increased as did tsuga and mesic deciduous species this change suggests a warming climate with increased precipitation from 7800 to 4000 yr b p sclerophyllous species increased at the expense of mesic deciduous species an indication that precipitation was becoming more seasonal except for increased disturbance starting ca 1000 yr b p the predominance of sclerophyllous vegetation continued until today the pollen results are compatible with proposed global circulation hypotheses of a strengthened monsoon system during the early to mid holocene

deep resistivity structure of the dikili bergama region west anatolia revealed by two dimensional inversion of vertical electrical sounding data within the framework of the national marine geological and geophysical program we re examined deep vertical electrical sounding ves data the data measured in 1968 by the general directorate of mineral research and exploration mta of turkey with the aim of exploring the deep resistivity structure of the dikili bergama region focus on the geothermal potential the geoelectrical resistivity survey was conducted using a schlumberger array with a maximum electrode half spacing of 4 5 km the two dimensional 2d inversion was utilized to interpret the ves data that were collected along 15 to 30 km profiles the 2d resistivity depth cross sections obtained show very low resistivity values near the dikili and kaynarca hot springs the 2d inversion results also indicate the presence of fault zones striking nearly n s and e w and fault bounded graben horst structures that show promising potential for geothermal field resources the 2d gravity model which is in good agreement with the density variation of the region supports the resistivity structure revealed by 2d inversion the lithology information obtained from the borehole near kaynarca also confirms the results of the resistivity interpretation and the density model

prolonged monsoon droughts and links to indo pacific warm pool a holocene record from lonar lake central india abstract concerns about the regional impact of global climate change in a warming scenario have highlighted the gaps in our understanding of the indian summer monsoon ism also referred to as the indian ocean summer monsoon and the absence of long term palaeoclimate data from the central indian core monsoon zone cmz here we present the first high resolution well dated multiproxy reconstruction of holocene palaeoclimate from a 10 m long sediment core raised from the lonar lake in central india we show that while the early holocene onset of intensified monsoon in the cmz is similar to that reported from other ism records the lonar data shows two prolonged droughts pd multidecadal to centennial periods of weaker monsoon between 4 6 3 9 and 2 0 6 cal ka a comparison of our record with available data from other ism influenced sites shows that the impact of these pd was observed in varying degrees throughout the ism realm and coincides with intervals of higher solar irradiance we demonstrate that i the regional warming in the indo pacific warm pool ipwp plays an important role in causing ism pd through changes in meridional overturning circulation and position of the anomalous walker cell ii the long term influence of conditions like el nino southern oscillation enso on the ism began only ca 2 cal ka bp and is coincident with the warming of the southern ipwp iii the first settlements in central india coincided with the onset of the first pd and agricultural populations flourished between the two pd highlighting the significance of natural climate variability and pd as major environmental factors affecting human settlements

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

holocene vegetation history precipitation changes and indian summer monsoon evolution documented from sediments of xingyun lake south west china we present a pollen based precipitation reconstruction and multi proxy records from a 485 cm long sequence from a sediment core from xingyun lake yunnan plateau south west china which depicts the evolution of the indian summer monsoon ism during the last 8500 years pollen and other palaeoenvironmental records document several stages of vegetation history and climate change the warmest and wettest climate in the xingyun lake catchment occurred before 5500 cal a bp and subsequently the climate became gradually drier after 2000 cal a bp the regional environmental conditions became unstable and a wet medieval warm period is probably recorded our reconstruction of the ism is similar to that portrayed by holocene speleothem o 18 records from southern china but is distinctly different from the east asian summer monsoon easm evolution which features a mid holocene maximum our results support the hypothesis that the ism and easm evolved asynchronously during the holocene and imply that the chinese speleothem o 18 records from southern china may principally reflect changes in moisture source from the indian monsoon domain and thus record the history of the ism rather than the easm

late glacial and holocene vegetation indian monsoon and westerly circulation in the trans himalaya recorded in the lacustrine pollen sequence from tso kar ladakh nw india abstract palynological investigation of a 410 cm long core section from tso kar 33 10 n 78 00 e 4527 m a s l an alpine lake situated in the arid ladakh area of nw india at the limit of the present day indian summer monsoon was performed in order to reconstruct post glacial regional vegetation and climate dynamics the area was covered with alpine desert vegetation from ca 15 2 to 14 kyr bp 1 kyr 1000 cal years reflecting dry and cold conditions high influx values of long distance transported pinus sylvestris type pollen suggest prevailing air flow from the west and northwest the spread of alpine meadow communities and local aquatic vegetation is a weak sign of climate amelioration after ca 14 kyr bp pollen data e g influx values of pinus roxburghii type and quercus suggest that this was due to a strengthening of the summer monsoon and the reduced activity of westerly winds the further spread of artemisia and species rich meadows occurred in response to improved moisture conditions between ca 12 9 and 12 5 kyr bp the subsequent change towards drier desert steppe vegetation likely indicates more frequent westerly disturbances and associated snowfalls which favoured the persistence of alpine meadows on edaphically moist sites the spread of chenopodiaceae dominated vegetation associated with an extremely weak monsoon occurred at ca 12 2 11 8 kyr bp during the younger dryas interstadial a major increase in humidity is inferred from the development of artemisia dominated steppe and wet alpine meadows with gentianaceae after the late glacial early holocene transition in response to the strengthening of the summer monsoon monsoonal influence reached maximum activity in the tso kar region between ca 10 9 and 9 2 kyr bp the subsequent development of the alpine meadow steppe and desert steppe vegetation points to a moderate reduction in the moisture supply which can be linked to the weaker summer monsoon and the accompanying enhancement of the winter westerly flow from ca 9 2 to 4 8 kyr bp the highest water levels of tso kar around 8 kyr bp probably reflect combined effect of both monsoonal and westerly influence in the region an abrupt shift towards aridity in the tso kar region occurred after ca 4 8 kyr bp as evidenced by an expansion of chenopodiaceae dominated desert steppe low pollen influx values registered ca 2 8 1 3 kyr bp suggest scarce vegetation cover and unfavourable growing conditions likely associated with a further weakening of the indian monsoon

paleomagnetism of ordovician alkalic intrusives and host rocks from the pedernal hills new mexico positive contact test in remagnetized rocks jackson m van der voo r and geissman j w 1988 paleoma etism of ordovician alkalic intrusives and host rocks from the pedemal hills new mexico positive contact test in remagnetized rocks tectonophysics 147 313 323 a set of thin dikes from central new mexico dated at 469 7 ma rb sr loring and armstrong 1980 have yielded a virtual geomagnetic pole which lies on the late paleozoic segment of the north american apparent polar wander path the remanence of the dikes appears to be a product of late paleozoic hydrothermal alteration paradoxically however the magnetization of the host rocks is most simply explained in terms of a positive contact test samples collected between 0 2 and 0 5 dike widths from the contact contain a component of remanence parallel to the magnetization in the dikes with unblocking temperatures which decrease with distance from the dikes host rocks from a distance of more than 1 dike width show no evidence of the characteristic dike magnetization there are two possible resolutions of this paradox 1 the magnetization of the host rocks is secondary despite the apparent positive contact test and is a product of hydrothe al fluid migration through the dikes or along the contact zones or 2 the magnetization of the dikes is primary but not representative of the ordovician paleofield for north america possible reasons for inaccurate representation include a incomplete averaging of secular variation b tectonic rotation with respect to the stable craton or c erroneous age determination for the rocks we argue that explanation 1 is the most likely

paleomagnetism and rock magnetism of early silurian dunn point volcanics avalon zone nova scotia abstract paleomagnetic results have been obtained from the subaerial volcanics andesites rhyolites ignimbrites from the dunn point formation exposed in the antigonish highlands of the avalon zone nova scotia an early silurian age of 430 15 ma is assigned to these volcanic rocks the age is in agreement with the fossil evidence from conformably overlying sedimentary rocks the volcanic units were tightly folded and metamorphosed to sub greenschist facies during one major and one secondary phase of deformation probably associated with the mid late devonian acadian orogeny this paleomagnetic study is based on 45 samples collected from nine sites the site mean directions of the natural remanent magnetization nrm make a large deviation from the present earth s field with nrm intensities ranging from 10 2 to 10 am 1 the most commonly observed remanence direction a d 153 i 28 obtained from both alternating fields 30 75 mt and thermal 500 560 c treatments yields a corresponding direction a d 0 i 63 after correction for geological tilt a large scatter in direction up to 60 is observed in a and the fold test is not significantly positive a second and less common remanence component b d 146 i 54 which presumably postdates folding is also observed this component is mainly isolated by thermal treatment in the 540 630 c range room temperature isothermal experiments and optical examination of polished sections indicate the presence of magnetite and hematite as the carriers of both a and b directions the corresponding reversed paleomagnetic poles fall at a 163 e 52 n and b 146 e 4 n in close agreement with cratonic north american upper jurassic and late devonian poles respectively in the mid late devonian time interval the avalon zone was situated slightly to the south but close to the north american eastern margin

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

paleomagnetic reconnaissance of early mesozoic carbonates from williston lake northeastern british columbia canada evidence for late mesozoic remagnetization three classic sections of middle and late triassic fossiliferous limestones cropping out around williston lake in british columbia canada were sampled for paleomagnetic study the objective was to test the suitability of these units for detailed magnetobiostratigraphic study with the aim of improving the reference triassic geomagnetic polarity time scale the williston lake characteristic magnetizations differ however from any triassic north america cratonic reference directions a satisfactory agreement is found instead with cretaceous early cenozoic north america cratonic reference directions the exclusive occurrence of normal polarity suggests that remagnetization likely occurred during the cretaceous long normal superchron remagnetizations may have been triggered by connate brines which moved along aquifers of porous sandstones and carbonates in the early stages of laramide folding resume trois sections fossiliferes classiques de calcaires trias moyen et tardif affleurant autour du lac williston en colombie britannique ont ete echantillonnees pour une etude paleomagnetique l objectif etait de s assurer de la pertinence de ces unites pour une etude magneto bio stratigraphique detaillee visant a ameliorer l echelle referentielle de temps de la polarite geomagnetique au trias les magnetisations caracteristiques du lac williston different toutefois de toute direction cratonique referentielle au trias en amerique du nord toutefois une correlation satisfaisante est etablie avec les directions cratoniques referentielles du cretace cenozoique precoce en amerique du nord l occurrence exclusive d une polarite normale suggere qu il y eut probablement une remagnetisation au cours du long superchron normal du cretace les remagnetisations peuvent avoir ete declenchees par des saumures connees qui se sont deplacees le long d aquiferes de gres et de carbonates poreux dans les premieres phases du plissement laramide

late holocene temperature fluctuations on the tibetan plateau abstract proxy data of palaeoclimate like ice cores tree rings and lake sediments document aspects of climate changes on the tibetan plateau during the last 2000 years the results show that the tibetan plateau experienced climatic episodes such as the warm intervals during ad 800 1100 and 1150 1400 the little ice age between ad 1400 and 1900 and an earlier cold period between the 4th and 6th centuries in addition temperatures varied from region to region across the plateau a warm period from ad 800 to 1100 in the northeastern tibetan plateau was contemporaneous with cooling in the southern tibetan plateau which experienced warming between ad 1150 and 1400 large scale trends in the temperature history from the northeastern tibetan plateau resemble those in eastern china more than the trends from the southern plateau the most notable similarities between the temperature variations of the tibetan plateau and eastern china are cold phases during ad 1100 1150 1500 1550 1650 1700 and 1800 1850

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

δ13corg and n alkane evidence for changing wetland conditions during a stable mid late holocene climate in the central tibetan plateau abstract this study has examined bulk and compound specific organic carbon isotopes 13corg and lipid n alkanes of modern plants and a wetland sediment sequence from the central tibetan plateau and explores the usefulness of these two methods for palaeoenvironmental reconstructions results show that a combination of 13corg values and n alkane indices can help differentiate organic matter from c3 terrestrial plant c4 terrestrial plant and submerged macrophytes the three main sources of organic matter in the study area the analyses of total organic carbon 13corg values and n alkane indices for the sediment sequence imply that a wetland habitat was established about 8000 years ago and the wetland conditions were largely stable with dominantly c3 wetland herbs covering the wetland however small fluctuations in the dominant organic matter supply between vascular and aquatic plants are revealed suggesting short duration changes in the extent of open water area within the wetland or variations in the productivity of submerged macrophytes and aquatic plants within the wetland ecosystem which reflects variations in the strength of precipitation and evaporation

inverse phase oscillations between the east asian and indian ocean summer monsoons during the last 12 000 years and paleo el nino abstract the inverse phase variations between the east asian and indian ocean summer monsoons on the interannual timescale result from the el nino southern oscillation activity in the tropical pacific which potentially provides a new way for studying paleo enso here we reconstruct a 12 000 yr proxy record for the east asian summer monsoon from 13 c time series of the plant cellulose of the hani peat bog in the northeastern china the comparison of it with the peat proxy record of the indian ocean summer monsoon suggests that the el nino conditions are coincident with strong east asian summer monsoons weak indian summer monsoons and drift ice events in the north atlantic at both orbital and millennial time scales the orbital scale inverse phase relationships between the monsoons indicate the occurrence of a long term enso like pattern confirming the sensitivity of the monsoon systems associated with enso to insolation forcing the nine inverse phase variations on the millennial time scales may suggest the nine el nino like patterns superimposing the long term enso like pattern the inverse phase variations between the monsoons also show close correspondence to the drift ice events at high northern latitudes in every case when the abrupt ice rafted debris events occurred in the north atlantic the inverse phase relationship established and the el nino like pattern occurred in the tropical pacific correspondingly the discussions on the influence of ocean thermohaline circulation on these global linkages have been made

holocene treeline shifts and monsoon variability in the hengduan mountains southeastern tibetan plateau implications from palynological investigations abstract the upper 8 45 m of a sediment core from lake naleng were analysed for its pollen and charcoal content to detect vegetation climate and human activity changes on the southeastern tibetan plateau tp since the pleistocene holocene transition 11 7 cal kyr bp the application of pollen indicator taxa pollen sums biomisation and multivariate analysis helped to reconstruct environmental conditions during the holocene furthermore 64 surface samples from the southeastern tp and its foreland were analysed to investigate the relationship between modern pollen assemblages and vegetation the modern vegetation was recorded in the surface pollen assemblages by pollen indicator types although sediments also contain pollen spectra extra regional pollen transported from lower vegetation belts which is seen as a consequence of steep elevation gradients at the southeastern margin of the tp the relation between different vegetation belts and temperature from the modern sample sites was used to infer quantitative temperature trends for the holocene record spreading of forests at the beginning of the holocene marks the transition to warmer conditions from 10 7 to 4 4 cal kyr bp open abies betula forests were found at lake naleng probably reflecting a stronger than present summer monsoon temperature range reconstructions suggest a temperature that was about 2 3 c warmer and a treeline position that was 400 600 m higher than today a severe climate deterioration with temperatures about 1 2 c colder than before is documented by a retreating forest between 8 1 and 7 2 cal kyr bp forests retreated while subalpine rhododendron shrubs spread around lake naleng indicating colder conditions with temperatures 1 2 c below early and mid holocene level after 4 4 cal kyr bp the forest decline at 4 4 cal kyr bp is considered to reflect climatic conditions as low concentrations of charcoal particles found in this period exclude human induced fires as reason for the forest retreat however grazing indicators imply human influence on the environment since 3 4 cal kyr bp around lake naleng we therefore consider the rapid treeline shift seen at 3 4 cal kyr bp as partly generated by human activity and partly by climatic conditions

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp

late glacial vegetation and climate oscillations on the southeastern tibetan plateau inferred from the lake naleng pollen profile we present a late glacial pollen record 17 700 to 8500 cal yr bp from a lake naleng sediment core lake naleng is located on the southeastern tibetan plateau 31 10 n 99 75 e 4200 m along the upper tree line variations in the summer monsoon are evident from shifts in vegetation that correspond to late glacial climate trends from other monsoon sensitive regions alpine steppe was recorded between 17 700 and 14 800 cal yr bp indicating low effective moisture at the study site expansion of alpine meadows followed by advances in the position of tree line around lake naleng suggest that climate became warmer and wetter between 14 800 and 12 500 cal yr bp probably representing an enhancement of the asian monsoon climatic cooling and reduced effective moisture are inferred from multivariate analysis and the upward retreat of tree line between 12 500 and 11 700 cal yr bp the timing and nature of these shifts to warm wet and then cold dry climatic conditions suggest that they correspond to the bolling allerod and younger dryas intervals abies betula forests representing warm and moist conditions spread during the early holocene

spatial variability of 14c reservoir effects in tibetan plateau lakes abstract radiocarbon dating of lake sediments is often hampered by the presence of a lake reservoir effect lre also dead carbon or old carbon effect especially in dry and cold regions with a sparse plant cover in the catchment the tibetan plateau became a hotspot of the palaeoenvironmental and climate research community in recent years and the assessment of present day lres is a crucial prerequisite for the establishment of reliable radiocarbon age depth relationships for lake sediment cores this paper examines the spatial variability of lres within individual lakes through a discussion of new data for lake donggi cona and a compilation of previously published data for five additional lakes where lre data are available for different sites lake reservoir effects for lake donggi cona on the northeastern tibetan plateau were determined for shells of aquatic snails collected alive close to the lake s shore the largest determined lre of 20 000 14 c years is significantly larger than previously reported lres from the central part of the lake and larger than any previously published lre for the tibetan plateau relatively low lres in the central regions of lakes higher lres towards the margins and high lres in tributaries and spring waters are apparently a common pattern of tibetan plateau lakes the differences in lres within individual lakes or catchment areas are attributed to the more prolonged exchange of the lake water s dissolved inorganic carbon with the atmospheric co 2 in central lake regions on the one hand and the increasing influence of 14 c free or poor stream and groundwater due to the dissolution of carbonaceous basement rocks towards its margins generally higher lres were recorded in the three tectonically active lake regions of the six examined catchments and it is speculated that rising crustal co 2 further contributes to the lres in these catchments in addition to these observations and inferences elevated 14 c levels of the atmosphere as a result of nuclear bomb testing are often ignored if lres for modern materials are reported by convention relative to the atmospheric 14 c activity of the year 1950 lre data reported in this way represent unrealistic minimum estimates

possible modulation of the connection between the pacific and indian ocean variability by the solar cycle 1 the indian ocean dipole iod mode and el nino southern oscillation enso exhibit a substantial correlation during boreal autumn although they are separate phenomena coupling occurs under certain conditions this study reveals that enso related variability extends into the indian ocean led by a developed south indian ocean sio anticyclone during periods of low solar ls activity during periods of high solar hs activity anomalous sea surface temperatures are confined to the pacific with little amplification of the anticyclone in the south indian ocean the direct cause of the difference in the sio anticyclone arises from a shift in the location of the descending branch of the anomalous walker circulation this may be attributable to a change in the background indian ocean monsoon circulation which can be modulated by changes in solar irradiance

stable isotopes in bulk carbonates and organic matter in recent sediments of lake qinghai and their climatic implications abstract multi proxy indices on annual decadal scales during the past 600 years were developed from the surface sediments of lake qinghai based on 210pb and 137cs geochronology the 13ccarb 18ocarb and total carbonate content are consistent with one another in trends and their variations have been mainly ascribed to regional temperature it is suggested that the strong evaporation contributes in modifying 18o of lake water and 13c of the dissolved inorganic carbon of the surface water and appears to be responsible for the covariance between 13ccarb and 18ocarb photosynthesis of the aquatic plants appears to have played a significant role in determining 15n of organic matter the discrimination of 14n and 15n during photosynthesis may have triggered a positive linkage between temperatures and 15norg after removal of the stratigraphic trend variations of total organic carbon toc c n ratio and 13corg have been ascribed to local precipitation toc after removal of the stratigraphic trend is positively correlated with the c n ratio r 0 45

linking holocene drying trends from lonar lake in monsoonal central india to north atlantic cooling events abstract we present the results of biogeochemical and mineralogical analyses on a sediment core that covers the holocene sedimentation history of the climatically sensitive closed saline and alkaline lonar lake in the core monsoon zone in central india we compare our results of c n ratios stable carbon and nitrogen isotopes grain size as well as amino acid derived degradation proxies with climatically sensitive proxies of other records from south asia and the north atlantic region the comparison reveals some more or less contemporaneous climate shifts at lonar lake a general long term climate transition from wet conditions during the early holocene to drier conditions during the late holocene delineating the insolation curve can be reconstructed in addition to the previously identified periods of prolonged drought during 4 6 3 9 and 2 0 0 6 cal ka that have been attributed to temperature changes in the indo pacific warm pool several additional phases of shorter term climate alteration superimposed upon the general climate trend can be identified these correlate with cold phases in the north atlantic region the most pronounced climate deteriorations indicated by our data occurred during 6 2 5 2 4 6 3 9 and 2 0 0 6 cal ka bp the strong dry phase between 4 6 and 3 9 cal ka bp at lonar lake corroborates the hypothesis that severe climate deterioration contributed to the decline of the indus civilisation about 3 9 ka bp

paleomagnetism of latest cambrian early ordovician and latest cretaceous early tertiary rocks of the florida mountains southwest new mexico paleomagnetic investigations have been carried out on latest cambrian to early ordovician alkaline intrusions their contact intermediate to mafic composition metaigneous rocks and overlying lower paleozoic and early tertiary strata in the florida mountains southwestern new mexico a well defined predominantly normal polarity magnetization of high median destructive induction and high usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late cenozoic uplift combinations of stable endpoint and planar data are used in data analysis at the site level d 262 i 30 95 10 k 12 n 22 site means in addition to the presence of reverse polarity magnetizations at two sites the dispersion of site mean directions in these rocks vgp angular standard deviation of 19 6 suggests that considerable variation of the geomagnetic field has been averaged after correction for a combination of early tertiary laramide deformation and late tertiary basin range block tilting this magnetization gives a paleomagnetic pole at 6 s 169 e and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503 10 ma based on published u pb zircon isotopic age determinations a lack of well defined coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks as interpreted with rb sr isotopic data hematitic sandstones of the earliest ordovician bliss formation which directly overlie the crystalline rocks in portions of the range give dual polarity magnetizations whose directions suggest that these rocks have probably been remagnetized this remagnetization possibly occurred during deformation in late cretaceous tertiary time ordovician carbonate rocks of the el paso and montoya formations give only a holocene magnetization paleocene to earliest eocene hematite cemented clastic rocks of the lobo formation possess dual polarity magnetizations the origin of their magnetization is difficult to assess with the available data in total however paleomagnetic data from the bliss and lobo formations argue against any significant vertical axis rotation of the area since laramide deformation ending in early tertiary time the intermediate to mafic composition metaigneous rocks from the florida mountains provide a latest cambrian early ordovician paleomagnetic pole that compares favorably with other north american paleopoles satisfying several acceptance criteria the florida mountains data further support recent constructions of apparent polar wander paths for north america and thus the hypothesis that significant counterclockwise rotation of north america occurred during the ordovician

latest pleistocene and holocene vegetation and climate history inferred from an alpine lacustrine record northwestern yunnan province southwestern china abstract reconstructing past changes of the summer monsoon in southwestern china is key to understanding the paleoclimatic dynamics of the indian summer monsoon southwest monsoon a major tropical system with impacts beyond the region high resolution pollen and conifer stoma data are presented from a sediment core tiancai lake located just below treeline in the hengduan mountains northwestern yunnan province of china these data record changes in vegetation and climate for the latest pleistocene and holocene c the last 12 230 cal yr bp and by comparing these results with the pollen records from the nearby erhai lake and lugu lake a regional climate history in yunnan province can be inferred during the period of 12 230 11 510 cal yr bp open alpine meadow around tiancai lake indicates a relatively cold and dry climate corresponding to the younger dryas cold event yd between 11 510 and 10 000 cal yr bp the vegetation types around tiancai lake changed into picea and abies forest and alpine rhododendron shrubland reflecting increases in the temperature and humidity from 10 000 to 6100 cal yr bp tsuga forest expanded persistently toward tiancai lake indicating further climate changed towards warmer and wetter conditions between 6100 and 3410 cal yr bp tsuga forest was the dominant vegetation type nearest to tiancai lake denoting warm and humid climatic conditions corresponding to the holocene climatic optimum in the northwestern yunnan province tsuga forest in mountains around lugu lake shrank quickly at 3410 cal yr bp reflecting significant decline of humidity since 2930 cal yr bp tsuga forest around tiancai lake shrank gradually indicating a fluctuating decrease of temperature these climatic changes reflect changes of the southwest monsoon namely warm and humid climate indicating strong southwest monsoon cold and dry climate denoting weak southwest monsoon