late holocene paleoenvironmental changes inferred from manasbal lake sediments kashmir valley india abstract paleoclimate investigations and reconstruction using the lake sediment cores of kashmir valley have been rarely attempted the basin is influenced by indian summer monsoon westerlies and the local katabatic winds in the present study an 80 cm undisturbed sediment core was retrieved from the manasbal lake situated in north western himalaya sediments were dated using 14c method and the age depth model indicated a depositional history extending from 3345 yrs bp to the present grain size major and trace elements toc om contents c n ratio and diatom assemblages were applied to reconstruct the paleoenvironmental changes around the area the a cn k diagram displays that the weathering intensity of the manasbal lake sediments are moderate to highly weathered and the sediments have been derived from nearby and mixed source origin several wet and cold phases were identified during 3300 yrs bp to 2500 yrs bp and 1800 yrs bp to 1300 yrs bp due to intense precipitation reflect high lake level these wet phases are marked by higher sand and silt content c n ratio and high allochthonous input with less abundance of diatom species clay om toc contents and high cia values consequently low lake level and dry and cold period from 3345 yrs bp to 3300 yrs bp and 2500 yrs bp to 1800 yrs bp is observed these inferences are corroborated with the trace elemental ratios cia and cwi data our data indicate that the westerlies and localised katabtic winds rather than the indian summer monsoons played a major role in controlling the past climate around the manasbal lake kashmir valley

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

extreme fires under warmer and drier conditions inferred from sedimentary charcoal morphotypes from opatcho lake central british columbia canada predictions of the extent of forest fires under warmer and drier conditions in boreal regions require knowledge of long term relationships between fire and climate however many long term studies

late cretaceous intraplate silicic volcanic rocks from the lake chad region an extension of the cameroon volcanic line silicic volcanic rocks at hadjer el khamis near lake chad are considered to be an extension of the cameroon volcanic line cvl but their petrogenetic association is uncertain the silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within plate granitoids in situ u pb zircon dating yielded a mean 206pb 238u age of 74 4 1 3 ma indicating the magmas erupted 10 million years before the next oldest cvl rocks i e 66 ma the sr isotopes i e isr 0 7021 0 7037 show a relatively wide range but the nd isotopes i e 143nd 144ndi 0 51268 0 51271 are uniform and indicate that the rocks were derived from a moderately depleted mantle source thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes the silicic rocks are more isotopically similar to late cretaceous basalts identified within the late cretaceous basins i e 143nd 144ndi 0 51245 0 51285 of chad than the uncontaminated cvl rocks i e 143nd 144ndi 0 51270 0 51300 the age and isotopic compositions suggest the silicic volcanic rocks of the lake chad region are related to late cretaceous extensional volcanism in the termit basin it is unlikely that the silicic volcanic rocks are petrogenetically related to the cvl but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the central atlantic ocean and or the pan african orogeny

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

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

glacier fluctuation and tree ring records for the last millennium in the canadian rockies tree ring series and records of alpine glacier fluctuations provide complementary evidence to reconstruct decade to century climate fluctuations over the last millennium in the canadian rockies tree line ring width chronologies in this area are temperature sensitive and long ca 900 years series from sites adjacent to bennington and athabasca glaciers show synchronous declines in ring width during the periods ca 1170 1180 1280 1290 1330 1350 1430 1450 1530 1540 1690 1705 and 1810 1825 a d the latter two events are replicated in many shorter tree ring chronologies and immediately precede the two major periods of moraine development ca 1700 1725 and 1825 1875 a d during the little ice age dendrochronological dating of glacially overridden trees 0 5 1 0 km upvalley of 18th century maximum positions indicate that robson and peyto glaciers were advancing between 1150 and 1350 a d during the earliest phase of the little ice age in the canadian rockies available tree ring data suggest that the level and pattern of climate variability during the 18th and 19th centuries extends back over the last milennium it is also hypothesised that glaciers attained their maximum holocene extent during the little ice age because of the interaction between these decade to century scale fluctuations and the progressive long term decline of incoming summer solar radiation in the northern hemisphere over the last 10 000 years

hydrogen and oxygen isotope geochemistry of a type granites in the continental margins of eastern china hydrogen and oxygen isotope studies of both whole rock and mineral separates were carried out for five representative atype granites in eastern china from north to south the whole rock d d and d 18 o values vary from 2145 to 282 and from 21 2 to 5 0 for nianzishan pluton from 2135 to 2110 and from 1 6 to 6 7 for shanhaiguan pluton and from 2145 to 2111 and from 1 5 to 9 1 for laoshan massif respectively the highest whole rock dd and d 18 o values are observed in suzhou pluton and range from 281 to 259 and from 3 5 to 9 2 respectively homogeneous d d and d 18 o values are found in kuiqi massif ranging from 2113 to 299 and from 5 6 to 8 5 respectively the results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution such as magma degassing fractional crystallization as well as post magmatic alteration by local meteoric water under subsolidus conditions the initial d d and d 18 o values of the a type granites in eastern china are constrained to be about 250 5 and 7 5 1 0 respectively this is consistent with mantle like protoliths for granitoid rocks and thus precludes the possibility that the a type granites is derived from the anatexis of the 18 o rich upper crust however recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the a type granitic magmas the variation in the ddepletion of the granites with latitude implies that the paleogeographic localities of the a type granites have not changed significantly since the mesozoic q 2000 elsevier science b v all rights reserved

isotopic geochemistry o sr pb of the golda zuelva and mboutou anorogenic complexes north cameroun mantle origin with evidence for crustal contamination abstract the annular 6 8 km diameter golda zuelva and mboutou anorogenic complexes of north cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite for the mboutou complex there are two overlapping centres in the golda zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite a rb sr whole rock isochron gives an age of 66 3 ma for the golda zuelva granites with initial 87 sr 86 sr ratio of 0 7020 and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic for golda zuelva and the north mboutou centre 18 o 16 o 5 6 6 2 87 sr 86 sr 0 7030 0 7045 and pb isotopic ratios 207 pb 204 pb 15 60 15 64 support a mantle origin for the initial magmas unlike sr isotopes the o isotopic ratios of the granitic end members at golda zuelva 7 5 indicate crustal contamination post magmatic alteration was not significant for the younger south mboutou centre the o sr and pb isotopic data indicate more extensive magma crust interaction and in a different higher level crustal environment with 18 o granite 3 3 87 sr 86 sr ratios up to 0 706 and pb isotopic ratios more markedly displaced from the oceanic volcanic field the low 18 o granites probably record at least in part a magmatic process with subsequent minor post magmatic alteration effects the major and trace element systematics between the north and south mboutou centres are directly comparable the evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes

petrogenesis of the magmatic complex at mount ascutney vermont usa the ascutney mountain complex of eastern vermont usa is a composite epizonal pluton of genetically related gabbro to granite intrusives nd isotopic data are reported for mafic rocks granites and nearby country rock the parental mafic magma producing the complex 122 m y ago had 87sr 86sr 0 7039 143nd 144nd 0 512678 nd 3 8 and 18o 6 1 indicating a mantle source with time integrated lithophile element depletion uniform initial radiogenic isotope ratios for granites which are undistinguishable from those for the most primitive gabbro suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in 18o to about 7 8 is the result of fractional crystallization mafic rocks show a large range in initial 143nd 144nd ratio from about 0 51267 to 0 51236 nd 3 7 to 2 5 which is correlated with elevated 87sr 86sr ratios and 18o these data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50 crust the local country rocks include gneiss and schist and assimilation involved representatives of both rock types the isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement

neoglaciation in the southern coast mountains of british columbia chronology prior to the late neoglacial maximum moraine stratigraphy and morphology radiocarbon dates from klinaklini franklin tiedemann gilbert and bridge glaciers and related information from elsewhere in the coast mountains are used to construct a chronology for glacier fluctuations the garibaldi phase of glacier expansion 6000 5000 14c years bp at the end of the early holocene xerothermic interval is indicated by overridden tree stumps the mid neoglacial tiedemann advance 3300 1900 14c years bp is represented by moraines till and meltwater sediments at three glaciers but only tiedemann glacier attained its greatest holocene extent at this time late neoglacial expansion commenced before 900 14c years bp and continued without notable interruption until glaciers achieved their maximum post pleistocene expansion during the eighteenth and nineteenth centuries evidence for the garibaldi and tiedemann events is scarce within the coast mountains because of the more extensive late neoglacial advance however correlative advances have been

dendroglaciological evidence for holocene glacial advances in the todd icefield area northern british columbia coast mountains accelerated glacial recession and downwasting in pacific north america is exposing land surfaces and fea tures buried by glacial advances that in many locations predate the recent little ice age lia dendrochronologic analyses of increment core samples from living trees abies lasiocarpa tsuga mertensiana and samples of subfossil wood collected in the todd icefield area boundary ranges british columbia coast mountains provide the basis for a dendroglaciological and radiocarbon based reconstruction of late holocene glacier activity five intervals of glacier ex pansion were recorded by trees killed or buried by advancing glaciers 1 an advance prior to 3000 14 c years bp 2 an advance at 3000 14 c years bp that coincides with the regional tiedemann advance 3 an unattributed advance at 2300 14 c years bp 4 a two phase advance at 1700 and 1450 14 c years bp that corresponds with the regional first millennium advance 5 an advance with three phases of expansion that began prior to 750 14 c bp and is con sistent with the regional early lia interval and a two phase interval of late lia expansion culminating after 240 and 100 years bp this chronology of late holocene glaciation matches that emerging from similar investigations in the coastal cordillera of pacific north america and provides additional support for the regional significance of both the tiedemann and the first millennium advances resume une recession et une fonte glaciaire accelerees dans l ocean pacifique en amerique du nord exposent des sur faces de terrain et des traits caracteristiques enfouis sous des avancees glaciaires lesquelles dans bien des cas datent d avant le recent petit ge glaciaire des analyses dendrochronologiques d echantillons proportionnels de carottes pris

lake sediments record large scale shifts in moisture regimes across the northern prairies of north america during the past two millennia six high resolution climatic reconstructions based on diatom analyses from lake sediment cores from the northern prairies of north america show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia the predominant broad scale pattern seen at all sites is a major shift in moisture regimes from wet to dry or vice versa depending on location that occurred after a period of relative stability these large scale shifts at the different sites exhibit spatial coherence at regional scales the three canadian sites record this abrupt shift between anno domini 500 and 800 and subsequently conditions become increasingly variable all three u s sites underwent a pronounced change but the timing of this change is between anno domini 1000 and 1300 thus later than in all of the canadian sites the mechanisms behind these patterns are poorly understood but they are likely related to changes in the shape and location of the jet stream and associated storm tracks if the patterns seen at these sites are representative of the region this observed pattern can have huge implications for future water availability in this region

the pb isotopic systematics during crustal contamination of subcrustal magmas the hercynian magmatism in the serie dei laghi southern alps italy abstract the pb isotopic composition and pb u and th concentrations of a series of hercynian granite plutons and mafic intermediate stocks and dykes belonging to the serie dei laghi western southern alps have been analyzed the initial pb isotope ratios of both the granites and the dykes are typical of the lower crust and are spread over a significant range 206 pb 204 pb 280 17 883 18 530 207 pb 204 pb 280 15 597 15 710 and 208 pb 204 pb 280 38 077 38 708 four dykes all at the same locality display evidence of post emplacement fluid circulation with mobilization of u and pb in spite of the pb isotopic crustal signature of the studied rock series the 87 sr 86 sr ratios reach values as low as 0 704 in the dykes and as low as 0 707 in the granites suggesting a mantle contribution in the genesis of both lithologies indeed when compared with the metasediments from the same area the magmatic rocks have somewhat lower initial 207 pb 204 pb ratios which weakly trend towards the less radiogenic pb isotopic composition of the mantle the relationships between the pb isotopic composition and the pb content along with the pb and sr isotopic covariation suggest that the suite under examination was generated from a mantle derived primary magma that underwent extensive crustal contamination a primary magma similar to the neighbouring ivrea mafic complex has been inferred further assimilation of crustal material is particularly evident in the dykes while the granites appear to have been generated mainly by crystal fractionation processes from the intermediate magma already contaminated

possible solar forcing of century scale drought frequency in the northern great plains a 2100 yr decadal resolution salinity and aridity proxy record of lacustrine ostracode shell mg ca ratios from a closed basin lake in the northern great plains shows statistically significant periodicities of 400 200 130 and 100 yr these periodicities are similar to the three principal solar oscillation periods 420 218 143 yr as inferred from the atmospheric radiocarbon record suggesting strong solar forcing of century scale drought frequency with a fundamental 400 yr period and its 2nd 3rd and 4th harmonics our proxy record correlates visually and statistically cross spectral analysis with the atmospheric 14 c record solar proxy and the gisp2 greenland ice sheet project two 18 o record climate proxy showing that solar minima are in phase with drought periods in the northern great plains and cold periods in greenland this spectral similarity together with phase correlation indicates a possible teleconnection of century scale global climate fluctuations through common solar forcing

sr nd isotopes as tracers of fine grained detrital sediments the south barbados accretionary prism during the last 150 kyr we studied a core retrieved from the deformation area of the south barbados accretionary prism sr and nd isotopic ratios were used in conjunction with the clay mineral assemblage to determine the origin and fluxes of fined grained detrital material and the sedimentological processes in this area during the last 150 000 years during the high sea level stands of events 1 1 3 3 and stage 5 sedimentation was mainly fed by amazon material conveyed by the guiana surface current and by volcanogenic material transported by the southward flowing north atlantic deep water nadw the flux of clays were very low 0 2 g cm 2 10 3 yr except during stage 5 0 7 g cm 2 10 3 yr during the low sea level stands of events 2 2 4 2 5 2 5 4 and 6 4 and stage 5 the appearance of orinoco material 1 5 g cm 2 10 3 yr at event 2 2 primarily derived from granitic batholiths of the guiana shield and transported directly by turbidity currents indicate an important change in sedimentary processes the high flux of volcanogenic materials 0 9 g cm 2 10 3 yr at event 2 2 indicates the importance of air fall ashes from dominica guadeloupe and or martinique islands and of the nadw as the transport agent the importance of allochthonous fine grained material originating from the south american continent andes or guiana shield and the northern part of the lesser antilles arc clearly stresses the major role that climate and eustacy had on detrital sedimentation during the last climatic cycle

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

phreatomagmatic deposits and stratigraphic reconstruction at debunscha maar mt cameroon volcano the debunscha maar dm is located on the southwest flank of mount cameroon an active stratovolcano on the cameroon volcanic line cvl here we present the physical characteristics of the pyroclastic deposits at dm with the aim of deciphering tephra emplacement mechanisms evolution of water magma interaction and reconstructing the stratigraphy beneath the maar from gps measurements the crater has long and short axes of 500 m and 320 m respectively generally the pyroclastic deposits are well stratified and present a variety of depositional bed forms including structureless massive beds massive beds with faint internal stratifications inversely graded beds lens shaped units impact sags cross lamination planar beds as well as dune like beds clast sizes include ash lapilli tuff bombs and blocks pyroclastic breccia with clast lithologies consisting of entrained lithics of porous ankaramite pillow lavas lithified sediments sandstone and shale and juvenile material the porous ankaramite pillow lavas have glassy margins and vesicle zonations typical of pillow lavas formed by subaqueous eruption the pillow fragments are more common in early formed eruption products at the base of the deposit the lithified sandstones show planar laminations and together with the shales occur predominantly in stratigraphic positions above the ankaramite pillow lavas the juvenile materials include basaltic bombs with low vesicularity b15 and moderate vesicularity 15 50 the bombs have chilled surfaces and their abundance increases towards the top of the deposit the presence of accretionary lapilli fragments of country rock and juvenile clasts with ragged surfaces as well as curved and chilled margins is unambiguous evidence in support of phreatomagmatic activity of the observed lithic clasts only the pillow lavas would appear to have the porosity necessary to furnish the required amount of water to feed the phreatomagmatic maar eruption the clast stratigraphy suggests that the maar is underlain by ankaramite pillow lava that erupted on a consolidated sedimentary substratum studying deposits resulting from maar eruptions has a direct implication for hazards assessment at areas of active maar volcanism because many surface processes occur around such volcanoes well after the eruptive activities have stopped

environmental history of a closed basin lake in the us great plains diatom response to variations in groundwater flow regimes over the last 8500 cal yr bp sediment records from closed basin lakes in the northern great plains ngp of north america have contributed significantly to our understanding of regional paleoclimatology a high resolution near decadal fossil diatom record from kettle lake nd usa that spans the last 8500 cal yr bp is interpreted in concert with percent abundance of aragonite in the sediment as an independent proxy of groundwater flow to the lake and thus lake water level kettle lake has been relatively fresh for the majority of the holocene likely because of the coarse substrata and a strong connection to the underlying aquifer interpretation of diatom assemblages in four groups indicative of low to high groundwater flow based on the percent of aragonite in sediments allow interpretations of arid periods and probable meromictic lake conditions that could not be detected based on diatom based salinity reconstructions alone at the centennial millennial scale the diatom record suggests humid wet periods from 8351 to 8088 4364 to 1406 and 872 to 620 cal yr bp with more arid periods intervening during the last 4500 years decadal centennial scale periods of drought have taken place despite the generally wetter climate these droughts appear to have had similar impacts on the kettle lake hydrology as the dust bowl era droughts but were longer in duration

the nd and sr isotopic composition of i type microgranitoid enclaves and their host rocks from the swifts creek pluton southeast australia abstract initial nd and sr isotopic compositions have been determined for microgranitoid enclaves and their host rocks from the siluro devonian swifts creek pluton within the lachlan fold belt of southeastern australia enclaves are of quartz dioritic to tonalitic composition and their textures support an igneous origin host rocks range from tonalite to granite the isotopic data form an array in nd 87 sr 86 sr space with enclaves systematically displaced towards higher nd and lower 87 sr 86 sr values enclaves range in nd t 405 ma from 7 3 to 3 1 with a narrow range in 87 sr 86 sr from 0 70613 to 0 70718 whereas host rocks range from 9 0 to 6 4 and 0 70761 to 0 70946 in nd and 87 sr 86 sr respectively the isotopic relations rule out simple crystal liquid fractionation or restite unmixing alone as the major genetic link between enclaves and host rocks the data are also incompatible with an origin of the enclaves by crystal accumulation or liquid immiscibility instead mixing between a mafic mantle derived and a crustal derived end member to generate the host rock enclave spectrum is compatible with the data because the initial isotopic ratios of enclaves and host rocks are well within the range of crustal rocks we favour the alternative view that both enclave and host rock magmas are of crustal origin consequently we avoid extrapolations to hypothetical end members we suggest that the enclaves from the swifts creek pluton are derived by partial melting of a mafic lower crust and mixed at or near the site of magma generation with a chemically less primitive and isotopically more evolved component to generate the enclave and host rock spectrum the isotopic heterogeneities within the enclave and host rocks survived the superimposed effects of crystal liquid fractionation model ages tdm for enclaves and host rocks are almost identical averaging 1 7 and 1 6 ga respectively these model ages indicate that both the enclave and host rock protoliths were derived from a mantle reservoir during the proterozoic

new accelerator mass spectrometry radiocarbon ages for the mazama tephra layer from kootenay national park british columbia canada charcoal fragments recovered from the mazama air fall tephra layer in cores from dog and cobb lakes kootenay national park british columbia yielded accelerator mass spectrometry ages of 6720 70 and 6760 70 14c years bp respectively these two new ages together with other previously published radiocarbon ages on charcoal and twig fragments from mazama air fall deposits indicate that the climactic eruption of mount mazama occurred 6730 40 14c years bp

coherent high and low latitude climate variability during the holocene warm period a faunal record of sea surface temperature sst variations off west africa documents a series of abrupt millennial scale cooling events which punctuated the holocene warm period these events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling the most recent of these events was the little ice age which occurred between 1300 to 1850 a d when subtropical ssts were reduced by 3 to 4 c these events were synchronous with holocene changes in subpolar north atlantic ssts documenting a strong in phase link between millennial scale variations in high and low latitude climate during the holocene

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

oxygen isotopes in single micrometer sized quartz grains tracing the source of saharan dust over long distance atmospheric transport abstract oxygen isotope compositions were measured by ion microprobe in individual micrometer sized quartz grains extracted from one aerosol sample collected on the cape verde islands and from four surface samples three soils and one sediment representing potential source regions of aerosols in western and central africa morocco algeria niger and chad a large range of 18o values from 6 2 to 39 3 is present within the aerosol quartz grains the different size fractions of the quartz grains from the surface samples overlap nearly entirely this range but show significant differences in their 18o distributions for the different size fractions of the grains i e different modes different proportions of grains with low or high 18o these differences in 18o distributions can be related to different geological formations i e mantle derived magmatic rocks crustal magmatic rocks or sedimentary rocks outcropping in each region thus giving a fingerprint of the source region quartz grains with unusually high 18o values between 30 and 40 were attributed to lacustrine cherts formed in evaporitic environments chad basin the existence of distinct 18o distributions for the surface samples which reflect regional geology but indicate some transport of the grains enables the characterization of mixing processes during dust emission in the atmosphere particles are mixed at a regional scale in the dust reservoir but injection of fine particles into the high troposphere occurs as a discrete and localized event with no mixing during the subsequent long range transport by the easterlies the comparison of the 18o distribution of the quartz from the aerosol sample with the equivalent size fractions in surface samples shows that the niger area is the more probable source region for the aerosol although the moroccan source cannot be excluded this method gives a valuable tool to trace the source region of dust into the atmosphere or into sediment samples allowing the reconstruction of air mass circulations

hydrologic variation in the northern great plains during the last two millennia abstract reconstructions of lake water salinity at decadal resolution for the last 2000 yr are compared among three lakes in north dakota to infer regional patterns of drought the intersite comparisons are used to distinguish local variation in climate or hydrology from regional patterns of change at one lake diatom inferred salinity and lake water mg ca inferred from ostracode shell chemistry are coherent both in terms of direction and magnitude of change indicating that each is a robust technique for reconstructing lake water chemistry the data show that the last 2000 yr have been characterized by frequent shifts between high and low salinity suggesting shifts between dry and moist periods long intervals of high salinity suggest periods of multiple decades when droughts were intense and frequent thus indicating times when drought was more persistent than in the 20th century both the medieval period and little ice age were hydrologically complex and there is no clear evidence to suggest that either interval was coherent or unusual in effective moisture relative to long term patterns differences among the three sites may be attributed to variation in local hydrology and these differences emphasize the need for multiple sites in deriving regional climate interpretations from paleoecological data

expansion of alpine glaciers in pacific north america in the first millennium a d radiocarbon ages and lichen dated moraines from 17 glaciers in coastal and near coastal british columbia and alaska document a widespread glacier advance during the first millennium a d glaciers at several sites began advancing ca a d 200 300 based on radiocarbon dated overridden forests the advance is centered on a d 400 700 when glaciers along an 2000 km transect of the pacific north american cordillera overrode forests impounded lakes and deposited moraines the synchroneity of this glacier ad vance and inferred cooling over a large area suggest a regional climate forcing and to gether with other proxy evidence for late holocene environmental change during the me dieval warm period and little ice age provide support for millennial scale climate variability in the north pacific region

calendar dated early little ice age glacier advance at robson glacier british columbia canada dendrochronological studies at robson and bennington glaciers have provided the first calendar dating of an early little ice age glacier advance in north america dates derived from in situ stumps indicate that robson glacier began over riding forest between c ad 1142 and 1150 and continued until at least ad 1350 the highest rates of glacier advance c 3 8 m yr 1 occurred between ad 1214 and 1261 based on a series of kill dates from in situ stumps between c ad 1142 1214 and 1261 1350 rates were somewhat lower at c 2 2 5 m yr 1 this glacier advance terminated upvalley of the little ice age maximum position of robson glacier and there is no evidence at this site for glacial events between ad 1350 and 1783 however two smaller glaciers in the robson area and the main tongue of small river glacier have outermost little ice age moraines that date from the first decades of the ad 1700s all glaciers examined in the mount robson area have several well developed moraines dating from the period c

pb and sr isotopic systematics of some basalts and sulfides from the east pacific rise at 21 n project rita abstract tholeiitic basalts and sulfide deposits from the cyana and alvin diving programs rita project on the east pacific rise were analyzed for pb and sr isotopes the basalt data plot within the field defined previously by other east pacific rise basalts 206 pb 204 pb 18 35 18 58 207 pb 204 pb 15 48 15 53 208 pb 204 pb 37 8 38 1 87 sr 86 sr 0 7022 0 7025 pb u and sr contents 0 5 0 05 and 110 ppm respectively and values 6 are typical of morb whereas th u ratios 3 5 are significantly higher the pb isotopic ratios of the sulfide samples are very homogeneous 206 pb 204 pb 18 47 207 pb 204 pb 15 49 208 pb 204 pb 37 90 and plot in the middle of the basalt field this indicates that 1 the sulfide pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments and 2 the solutions from which the sulfides were deposited had uniform pb isotopic composition the pb contents of three sulfide samples is relatively high 170 1310 ppm the sr contents of five sulfide samples are widely scattered from 12 to 210 ppm with 87 sr 86 sr ratios intermediate between basaltic and seawater values 0 70554 0 00005 to0 70795 0 00011 leaching experiments show that both basalt derived sr and seawater sr were present in the solutions which deposited the sulfides in some cases sr was also adsorbed from seawater onto the sulfides following their deposition basalt derived sr and seawater sr are also present in associated non sulfide phases

evolution of orogenic wedges and continental plateaux insights from crustal thermal mechanical models overlying subducting mantle lithosphere the links between an early phase of orogenesis when orogens are commonly wedge shaped and a later phase with a plateau geometry are investigated using coupled thermal mechanical models with uniform velocity subductionboundary conditions applied to the base of the crust and simple frictional plastic and viscous rheologies models in which rheological properties do not change with depth or temperature are characterized by growth of back to back wedges above the subduction zone wedge taper is inversely dependent on the ramberg number rm gravity stress basal traction increasing convergence velocity or crustal strength produces narrower and thicker wedges models that are characterized by a decrease in crustal viscosity from c to b with depth or temperature leading to partial or full basal decoupling of the crust from the mantle display more complex behaviour for models with a moderate viscosity ratio b c 10 1 the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust a redaction in the viscosity ratio b c 10 2 is sufficient to cause a transition of the central wedge region to a plateau this transition depends on the basal traction therefore the thickness of the weak basal layer also affects the transition further reduction of the viscosity ratio b c 10 4 leads to full basal decoupling and the development of plateaux in all cases considered in most models the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust where the crust is fully decoupled large scale model geometries for both depth and temperature dependent rheologies are similar with gravity driven flow concentrated in the low viscosity region however strong lateral temperature gradients within these models controlled by the interaction of horizontal and vertical thermal advection diffusion and heterogeneous thickening of the radioactive crustal layer lead to differences in the velocity and deformation fields between the two cases particularly at the plateau margins the results suggest that simple depth dependent viscosity models may be reasonable approximations for describing the large scale geometry of fully developed plateaux but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

solar influences on holocene climatic changes illustrated by correlations between past lake level fluctuations and the atmospheric 14c record chronological correlations established at different time scales among the lake level fluctuations in the jura and french subalpine ranges glacier movements in the swiss and austrian alps and the atmospheric 14c record during the last 7 millennia show coincidences between lake level rises glacier advances and high 14c production and vice versa these correspondences suggest that the short term 14c variations may be an empirical indicator of holocene palaeoclimates and argue for possible origins of holocene climatic oscillations 1 the varying solar activity refers to secular climatic oscillations and to major climatic deteriorations showing a ca 2300 yr periodicity 2 a question is raised about a relationship between the earth s magnetic field and climate first the weak strength periods of the earth s dipole magnetic field between 3800 and ca 2500 b c coincide with higher climate variability and vice versa second the ca 2300 yr periods revealed by the 14c record and also by the major climatic deteriorations re corded in jurassian lakes ca 1500 a d ca 800 b c and ca 3500 b c coincide with the ca 2300 yr periods revealed by the earth s nondipole geomagnetic field the present warming induced by anthropogenic factors should be intensified during the next few centuries by natural factors of climate evolution

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

holocene environmental and climatic change in the northern great plains as recorded in the geochemistry of sediments in pickerel lake south dakota the sediments in pickerel lake northeastern south dakota provide a continuous record of climatic and environmental change for the last 12000 yr sediments deposited between 12 and 6 ka radiocarbon show extreme variations in composition oxygen and carbon isotopic composition of bulk carbonate carbon isotopic composition of organic matter and magnetic susceptibility these variations re ect changes in sources of moisture regional vegetation types precipitation evaporation balance ground and surface water in ux water residence time erosion lake productivity water level and water temperature the total carbonate content of late pleistocene sediments steadily increased from 20 at the base of the core to as much as 80 in sediments deposited between 11 and 9 ka by about 8 ka the total carbonate content of the sediments had declined to about 40 where it remained with little variation for the past 8 kyr suggesting relatively stable conditions there are marked increases in values of d13c and d18 oi n bulk carbonate and d13c of organic matter in sediments deposited between 10 and 6 ka as evaporation increased and the vegetation in the watershed changed from forest to prairie this shift toward more 18o enriched carbonate may also re ect a change in source or seasonality of precipitation during this early holocene interval the organic carbon oc content of the sediments remained relatively low 2 3 but then increased rapidly to 4 5 between 7 and 6 ka re ecting the rapid transition to a prairie lake the oc content uctuates slightly between 4 and 6 in sediments deposited over the past 6 kyr like oc and total carbonate most variables measured show little variation in the 13 m of sediment deposited over the past 6 kyr particularly when compared with early holocene variations although the magnetic susceptibility of this upper 13 m of sediment is generally low 10 si units the upper six meters of the section is marked by striking 1 m cycles ca 400 500 yr periodicity in susceptibility these cycles are interpreted as being due to variations in the in ux of eolian detrital clastic material century scale cyclic variations in di erent proxy variables for aridity and eolian activity from sediments deposited over the past 2000 yr in other lakes in the northern great plains as well as in sand dune activity suggest that aridity cycles were the dominant feature of late holocene climate of the northern great plains 2000 elsevier

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

asian monsoon oscillations in the northeastern qinghai tibet plateau since the late glacial as interpreted from visible reflectance of qinghai lake sediments abstract qinghai lake is a large saline lake on the qinghai tibetan plateau of central asia that is effected by both the indian and asian monsoons we used reflectance spectroscopy to characterize the sediments in a 795 cm long core taken from the southeastern part of the lake sediment redness which is related to iron oxide content seems to monitor paleoclimatic changes in the core iron oxides appear to be eroded from nearby red beds or loess deposits and are transported by fluvial means into the lake thus redness increases at times of increased precipitation that is as monsoon strength increases our redness monsoon proxy shows climate changes on several times scales on a millennial scale it records humid conditions during the early and mid holocene from about 4200 to 2300 yr bp low redness values suggest a two millennial long dry period which in the late holocene is followed by a more humid period on a centennial scale the redness proxy records not only the little ice age but also the medieval warm period the dark ages cool period and the roman warm period time series analysis of the redness record indicates a 200 yr frequency which corresponds to the de vries solar cycle suggesting that in addition to insolation changes resulting from orbital variations solar forcing also results from cyclic changes in the sun s luminosity

diatom pollen and chemical evidence of postglacial climatic change at big lake south central british columbia canada abstract postglacial climatic conditions were inferred from cores taken from big lake in southern british columbia low concentrations of nonarboreal pollen and pigments near the base of the core suggest that initial conditions were cool increases in both aquatic and terrestrial production suggest warmer and moister conditions until 8500 cal yr b p hyposaline diatom assemblages increases in nonarboreal pollen and increased concentrations of pigments suggest the onset of arid conditions from 8500 to 7500 cal yr b p slightly less arid conditions are inferred from 7500 until 6660 cal yr b p based on the diatoms small increases and greater variability in biogenic silica and pigments and higher percentages of arboreal pollen at 6600 cal yr b p changes in diatoms pigments biogenic silica and organic matter suggest that big lake became fresh deep and eutrophic until 3600 cal yr b p when water levels and nutrients decreased slightly our paleoclimatic inferences are similar to pollen based studies until 6600 cal yr b p however unlike these studies our multiple lines of evidence from big lake imply large changes in effective moisture since 6000 cal yr b p

neoglacial glacier fluctuations in the canadian rockies abstract the little ice age was the most extensive neoglacial glacier advance in the canadian rockies evidence of earlier less extensive neoglacial glacier advances is based on wood recovered from several glacier forefields wood flushed out of athabasca glacier 7550 8230 yr b p three dates and dome glacier 6120 6380 yr b p two dates indicates that forests occurred upvalley of present glacier termini during the hypsithermal detrital logs from peyto 14 dates saskatchewan 3 dates robson 3 dates and yoho 1 date glaciers plus in situ slumps at peyto and robson glaciers have yielded 14 c dates between 2490 and 3300 yr b p 12 dates between 2800 and 2990 this wood is derived from sources at or upvalley from present glacier termini and represents forests overridden by glaciers between ca 3100 and 2500 yr b p treeline was higher than present immediately prior to this advance this advance which did not extend beyond the little ice age maximum position is designated the peyto advance and correlated with the tiedemann advance in western british columbia earliest little ice age advances at peyto and robson glaciers are dated ca 800 600 yr b p at positions ca 500 m upvalley from little ice age limits

persistent millennial scale shifts in moisture regimes in western canada during the past six millennia inferences of past climatic conditions from a sedimentary record from big lake british columbia canada over the past 5 500 years show strong millennial scale patterns which oscillate between periods of wet and drier climatic conditions higher frequency decadal to centennial scale fluctuations also occur within the dominant millennial scale patterns these changes in climatic conditions are based on estimates of changes in lake depth and salinity inferred from diatom assemblages in a well dated sediment core after periods of relative stability abrupt shifts in diatom assemblages and inferred climatic conditions occur approximately every 1 220 years the correspondence of these shifts to millennial scale variations in records of glacial expansion recession and ice rafting events in the atlantic suggest that abrupt millennial scale shifts are important to understanding climatic variability in north america during the mid to late holocene unfortunately the spatial patterns and mechanisms behind these large and abrupt swings are poorly understood similar abrupt and prolonged changes in climatic conditions today could pose major societal challenges for many regions

late quaternary dynamics of forest vegetation on northern vancouver island british columbia canada abstract pollen analysis of radiocarbon dated lake sediment from northern vancouver island southwest british columbia reveals regional changes in forest vegetation over the last 12 200 14c yr 14 900 cal yr between at least 12 200 and 11 700 14c yr bp 14 900 13 930 cal yr bp open woodlands were dominated by pinus contorta alnus crispa and various ferns as p contorta decreased in abundance alnus rubra and more shade tolerant conifers i e picea and tsuga mertensiana increased increases in t mertensiana p contorta and a crispa pollen accumulation rates pars between 10 600 and 10 400 14c yr bp 11 660 11 480 cal yr bp reflect a cool and moist climate during the younger dryas chronozone orbitally induced warming around 10 000 14c yr bp 11 090 cal yr bp allowed the northward extension of pseudotsuga menziesii although picea tsuga heterophylla and a rubra dominated early holocene forests by 7500 14c yr bp 8215 cal yr bp shade tolerant t heterophylla was the dominant forest tree cupressaceae thuja plicata and chamaecyparis nootkatensis was present by 7500 14c yr bp but reached its maximum after 3500 14c yr bp 3600 cal yr bp when a cooler and wetter regional climate facilitated the development of temperate rainforest the highest rates of vegetation change are associated with lateglacial climate change and species with rapid growth rates and short life spans

abiotic versus biotic controls on the development of the fairmont hot springs carbonate deposit british columbia canada the relict fairmont hot springs deposit formed largely of carbonates covers an area of 0ae 5k m 2 and is up to 16 m thick the triangle shaped discharge apron which broadens down valley is divided into a proximal part with beds dipping at 10 and a distal part with beds dipping at 10 to 15 the deposit is formed of the 1 basal macrophyte 2 lower carbonate 3 middle clastic 4 upper carbonate and 5 upper clastic sequences two charcoal samples embedded in the lower carbonate sequence yielded dates of 8690 90 and 8270 70 cal yr bp indicating that much of the deposit formed post glacially during the early to mid holocene deposit aggradation ceased in the mid to late holocene when the fairmont creek valley was incised the lower and upper carbonate sequences which are the thickest sequences are composed of nearly equal parts of travertine abiotic and tufa biotic with feather dendrite travertine radiating dendrite travertine and stromatolite tufa dominating competition between calcite precipitation rates and biotic growth rates controlled the distribution of tufa and travertine across the discharge apron calcite and biotic growth rates were controlled largely by flow velocity across the apron which in turn was controlled by topography and regular fluctuations in spring water discharge volume during times of high spring discharge slow sheet flow over the proximal part of the apron promoted stromatolite growth whereas fast turbulent flow on the distal part of the apron induced rapid feather dendrite formation during times of low spring discharge quiescent shallow evaporative pools conducive to radiating dendrite formation formed on the proximal part of the apron whereas slow flow on the distal part promoted stromatolite growth facies with high calcite supersaturation experienced rapid abiotic dendrite growth that precluded most biotic growth

a new tectonic model for the cameroon line central africa abstract the cameroon line a major geological feature in central africa has been considered successively as a series of horsts and grabens a continental rift and a mega shear zone it is marked out by about 60 anorogenic complexes and a dozen volcanic centres all of which have alkaline affinity remote sensing allows us recognition of the main lineament trends n70 n s n135 and e w while autocorrelation analysis reveals a major fault zone striking n30 in western cameroon and n15 in the northern region a mega left lateral shear zone is the model that best accounts for the fracture pattern and associated features such as linear and circular structures alignment of subvolcanic complexes syntectonic leucogranites marking out older shear zones and vein dykes the n70 adamawa fault zone a pan african fracture reworked during albian aptian times is the only shear zone of continental scale that could have initiated en echelon mega tension gashes within the cameroon line during a cainozoic left lateral transcurrent movement