potential atmospheric impact of the toba mega eruption 71 000 years ago an approx 6 year long period of volcanic sulfate recorded in the gisp2 ice core about 71 000 5000 years ago may provide detailed information on the atmospheric and climate impact in the toba mega eruption deposition of these aerosols occur beginning of an approx 1000 year long stadial event but not immediately before the longer glacial period beginning approx 67 500 years ago total stratospheric loading estimates over this approx 6 year period range from 2200 to 4400 mt of h sub 2 so sub 4 aerosols the range in values is given to compensate for uncertainties in aerosol transport magnitude and longevity of the atmospheric loading may have led directly to enhanced cooling during the initial two centuries of this approx 1000 year cooling event 25 refs 2 fig 1 tab

evidence of the eldgja iceland eruption in the gisp2 greenland ice core relationship to eruption processes and climatic conditions in the tenth century glaciochemical studies and the evaluation of tephra in the gisp2 ice core provide information on the characteristics and potential environmental and climatic effects of the mid to late ad 930s voluminous fissure eruption of eldgja iceland the similarity in the chemical composition of basaltic glass shards found in a section of core dated at ad 938 4 compared to proximal glass from the eldgja eruption verifies the presence of eldgja debris a dacitic glass present in the same layer probably originated from eldgja as well in which case eldgja was the primary contributor of sulphur rich aerosols to the atmosphere in the late ad 930s we cannot completely exclude the possibility that another explosive eruption in the ad 930s produced this dacitic glass estimated maximum stratospheric loading is 100 x 1012 g h 2so4 over a 3 6 year period following the eruption but loading could be as low as half of that value a search of historical and proxy records for the late ad 930s to early 940s fail to show a co

quantification of holocene asian monsoon rainfall from spatially separated cave records abstract a reconstruction of holocene rainfall is presented for southwest china an area prone to drought and flooding due to variability in the east asian monsoon the reconstruction is derived by comparing a new high resolution stalagmite 18 o record with an existing record from the same moisture transport pathway the new record is from heshang cave 30 27 n 110 25 e 294 m and shows no sign of kinetic or evaporative effects so can be reliably interpreted as a record of local rainfall composition and temperature heshang lies 600 km downwind from dongge cave which has a published high resolution 18 o record wang y j cheng h edwards r l he y q kong x g an z s wu j y kelly m j dykoski c a li x d 2005 the holocene asian monsoon links to solar changes and north atlantic climate science 308 854 857 by differencing co eval 18 o values for the two caves secondary controls on 18 o e g moisture source moisture transport non local rainfall temperature are circumvented and the resulting 18 o signal is controlled directly by the amount of rain falling between the two sites this is confirmed by comparison with rainfall data from the instrumental record which also allows a calibration of the 18 o proxy the calibrated 18 o record provides a quantitative history of rainfall in southwest china which demonstrates that rainfall was 8 higher than today during the holocene climatic optimum 6 ka but only 3 higher during the early holocene significant multi centennial variability also occurred with notable dry periods at 8 2 ka 4 8 4 1 ka 3 7 3 1 ka 1 4 1 0 ka and during the little ice age this holocene rainfall record provides a good target with which to test climate models the approach used here of combining stalagmite records from more than one location will also allow quantification of rainfall patterns for past times in other regions

the application of icp ms methods to tephrochronological problems pearce nicholas westgate j a perkins w t preece s j 2004 the application of icp ms methods to tephrochronological problems applied gechemistry 19 3 pp 289 322 rae2008

assessing the importance of climate variables for the spatial distribution of modern pollen data in china to assess the importance of climate variables for the distribution of modern pollen data in china we present a continental scale dataset consisting of 1374 samples boosted regression trees and constrained ordination techniques are employed to quantify the importance of six climate variables annual precipitation pann actual potential evapotranspiration ratio alpha mean annual temperature tann mean temperature of the warmest month mtwa mean temperature of the coldest month mtco annual sum of the growing degree days above 5 c gdd5 for the distribution of individual pollen taxa and modern pollen assemblages the results show that taxon specific responses to the climate variables display a wide regional diversity and that the climate variables with low collinearity that best account for the spatial variability of modern pollen assemblages differ regionally pann is the most important variable in northwestern and northeastern china and the tibetan plateau while mtwa and mtco are the dominant variables in east central and southern china this suggests that the climate variables that can be optimally reconstructed from fossil pollen data vary in different bioclimatic regions of china this feature is typical to many continental scale modern pollen datasets and needs to be considered in pollen based climate reconstructions

holocene history of desertification along the woodland steppe border in northern china abstract the woodland steppe ecotone of the southeastern inner mongolian plateau in northern china is located at the northwestern limit of the pacific monsoon influence where the landscape may have been a sensitive recorder of past climatic changes physical chemical and biological analyses of ams 14 c dated sediment sequences from two lakes of this region were used to reconstruct the holocene vegetation and desertification history and distinguish four periods 1 a cold and humid period from 10000 to 8000 14 c yr b p 2 a warm and humid period from 8000 to 5900 14 c yr b p 3 a warm and dry period from 5900 to 2900 14 c yr b p and 4 a cool and dry period from 2900 14 c yr b p to the present the increased aridity during the middle holocene was likely caused by increased winter temperatures and enhanced winter evaporation the transition from a humid to an arid climate after 5900 14 c yr b p coincided with enhanced aeolian activity and deciduous woodlands were replaced by pine woodlands and then by steppes in response to the climatic deterioration these transitions led to the present desertification the records suggest that a simple association of thermal and moisture conditions such as warm wet or cold dry may be misleading

palaeovegetation in china during the late quaternary biome reconstructions based on a global scheme of plant functional types two previous reconstructions of palaeovegetation across the whole of china were performed using a simple classification of plant functional types pfts now a more explicit global pft classification scheme has been developed and a substantial number of additional pollen records have become available here we apply the global scheme of pfts to a comprehensive set of pollen records available from china to test the applicability of the global scheme of pfts in china and to obtain a well founded reconstruction of changing palaeovegetation patterns a total of 806 pollen surface samples 188 mid holocene mh 6000 14c yr bp and 50 last glacial maximum lgm 18 000 14c yr bp pollen records were used to reconstruct vegetation patterns in china based on a new global classification system of pfts and a standard numerical technique for biome assignment biomization the biome reconstruction based on pollen surface samples showed convincing agreement with present potential natural vegetation coherent patterns of change in biome distribution between mh lgm and present are observed in the mh cold and cool temperate evergreen needleleaf forests and mixed forests temperate deciduous broadleaf forest and warm temperate evergreen broadleaf and mixed forest in eastern china were shifted northward by 200 500 km cold deciduous forest in northeastern china was replaced by cold evergreen needleleaf forest while in central northern china cold deciduous forest was present at some sites now occupied by temperate grassland and desert the forest grassland boundary was 200 300 km west of its present position temperate xerophytic shrubland temperate grassland and desert covered a large area on the tibetan plateau but the area of tundra was reduced treeline was 300 500 m higher than present in tibet these changes imply generally warmer winters longer growing seasons and more precipitation during the mh westward shifts of the forest shrubland grassland and grassland desert boundaries imply greater moisture availability in the mh consistent with a stronger summer monsoon during the lgm in contrast cold deciduous forest cool temperate evergreen needleleaf forest cool mixed forests warm temperate evergreen broadleaf and mixed forest in eastern china were displaced to the south by 300 1000 km while temperate deciduous broadleaf forest pure warm temperate evergreen forest tropical semi evergreen and evergreen broadleaf forests were restricted or absent from the mainland of southern china implying colder winters than present strong shifts of temperate xerophytic shrubland temperate grassland and desert to the south and east in northern and western china and on the tibetan plateau imply drier conditions than present

some numerical considerations in the geochemical analysis of distal microtephra abstract the use of late quaternary tephra horizons as isochronous markers for high resolution correlation between records is of growing importance in a number of scientific fields particular advancements have been made through the identification of microtephra deposits very distal deposits of volcanic ash invisible to the naked eye the successful correlation of ash layers and volcanic eruptions depends on the acquisition of the major element chemistries of a tephra layer derived from microprobe analyses of individual glass shards this is particularly important for microtephras where aeolian fractionation has removed much of the mineral phase of the ash deposit which often aids tephra discrimination there are however difficulties in distinguishing precisely between different eruptions of the same volcano using major element composition and this may not always be resolved by the use of trace elements given the potential of tephrochronology as a correlative tool in many disciplines it is important to resolve this difficulty one way forward is to perform more robust statistical analyses on the geochemical data as in general analyses are confined to a series of bi plots of major elements here the authors explore some of the problems associated with dealing with microprobe data generated for individual tephras and pay particular attention to the unit sum problem using a subset of data generated as part of a eurodelta project on tephra distribution in the adriatic the authors demonstrate that this problem is detectable in major element data from tephras and have consequently applied the logratio method before further analyses of the data it is demonstrated that the use of logratios combined with discriminant functions analysis provides a more robust assessment of likely chemical correlations between tephras and are superior to the use of bi plots alone and obviate the need for any data normalization a particular point of contention between tephrochronologists

isotope calibrated greenland temperature record over marine isotope stage 3 and its relation to ch4 large temperature variations on millennial time scales in greenland characterised the last ice age abrupt warmings known as dansgaard oeschger do events can be traced in the 18oice record of greenland ice cores however it has been shown that 18oice is not a direct temperature proxy measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer here we present a continuous temperature record based on high resolution 15n measurements and firn model studies it covers a sequence of 9 do events 9 17 during the time period from 38 to 64 kyr bp for which temperature changes of 8 to 15 c were estimated the difference between the modern and the glacial 18oice t relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation a detailed comparison of the temperature evolution with reconstructions of the atmospheric methane ch4 concentration shows that ch4 rises lag temperature increases at the onset of do events by 25 to 70 yr within data resolution the strong correlation between greenland temperature and ch4 on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

early and mid holocene palaeoenvironments as revealed by mineral magnetic geochemical and palynological data of sediments from bai nuur and ulan nuur southeastern inner mongolia plateau china bai nuur 41 degrees 38 590 n 114 degrees 30 922 e similar to 1346 m a s l and ulan nuur 41 degrees 44 214 n 115 degrees 05 630 e similar to 1246 m a s l are two small lakes situated in the transition zone between semi humid and semi arid climate regimes that runs parallel to the present limit of the southeast monsoon along the southeastern inner mongolia plateau in north china a 215 cm deep sediment sequence was recovered from bai nuur and a 244 cm deep one from ulan nuur soils were sampled from the catchments of the two lakes and dune materials from a sandy land lying further north and northwest mineral magnetic measurements were performed on the sediments soils and dune materials ams c 14 dating and analyses of particle size toc and c n and pollen composition were also made for the sediments the mineral magnetic properties of the two sediment sequences in combination with the other proxy climate data reveal the palaeoenvironmental development during early and mid holocene around the two lakes climate began to ameliorate at 10 600 or 10 500 cal bp and maximum humidity and warmth were reached during 10 600 or 10 500 9700 or 9600 cal bp around bai nuur environments remained generally wet and or warm until 7700 or 7600 cal bp climate deterioration commenced at 7700 or 7600 cal bp around bai nuur and at 7200 or 7100 cal bp around ulan nuur this deterioration intensified from 6900 or 6800 cal bp onwards around bai nuur and from 6200 or 6100 cal bp onwards around ulan nuur the differences in timing of the reconstructed environmental changes at the two short distanced sites may be due to the differences across the narrow climatic and vegetative transition zone errors and uncertainties of dating and or different chronostratigraphic frames of the two sequences the palaeoenvironments reconstructed around the two small lakes 2 3 km 2 and 8 9 km 2 coincide with a palaeoenvironmental record from anguli nuur lake a much larger lake 47 6 km 2 in the same region these data also agree with reconstructions at several other sites in different parts of the inner mongolia plateau although the holocene amelioration took place earlier than in northern mongolia c 2011 elsevier ltd and inqua all rights reserved less

satellite and pollen based quantitative woody cover reconstructions for northern asia verification and application to late quaternary pollen data abstract accurate reconstruction of late quaternary vegetation cover is necessary for better understanding of past vegetation dynamics the role of vegetation feedbacks in glacial interglacial climate variations and for validating vegetation and climate models in this paper over 1700 surface pollen spectra from the former soviet union mongolia northern china and northern japan together with data from the advanced very high resolution radiometer avhrr were used to calibrate modern analogue method for quantitatively reconstructing past woody cover from fossil pollen data the avhrr based estimates of woody cover percentages within a 21 21 km window around pollen sampling sites were attributed to the respective modern pollen spectra reconstructions of modern woody cover using the pollen data and best modern analogues bma method matched well to the original avhrr based estimates for both total woody cover r 2 0 77 and its fractions including broad leaved r 2 0 66 needle leaved r 2 0 79 deciduous r 2 0 60 and evergreen r 2 0 76 woody cover discrepancies in the pollen avhrr cross validation may be caused by long distance transport of arboreal pollen patchy forest distributions underrepresentation of larix and populus in pollen records and errors in the avhrr classification the generally strong correlations encourage application of the modern analogue approach for reconstructing late quaternary variations in vegetation cover from northern asian fossil pollen records at the last glacial maximum lgm 21 000 cal yr bp areas presently occupied by boreal forest were much more open suggesting a reduction in total woody cover to below 20 at most modern forest sites pollen records from northern and central siberia suggest a rather quick spread of tree and shrub vegetation after 15 000 cal yr bp presumably in response to increased summer insolation woody cover histories are spatially variable in the modern forest steppe where tree growth is largely controlled by the ratio of annual actual over potential evapotranspiration which serves as an index of moisture availability in northwestern mongolia woody cover percentages decreased between 9000 and 5000 cal yr bp and again between 2000 and 1000 cal yr bp and may be linked to shifts in intensity of the pacific monsoon in contrast pollen data from the kazakhstan forest steppe where atmospheric precipitation is associated with the atlantic westerly flow suggest that woody cover density reached modern levels only during the last millennium

microbially mediated dolomite in cambrian stromatolites from the tarim basin north west china implications for the role of organic substrate on dolomite precipitation abstract dolomite camg co3 2 is abundant in sedimentary rocks throughout the geological record but it is rarely found in modern sediments also it cannot be precipitated under low temperature conditions in the laboratory without microbial mediation and as a result its origin remains a long standing enigma this study reports biologically mediated dolomite precipitation in ancient microbial mats and biofilms from the cambrian tarim basin the ambient temperature at the time of dolomite precipitation was estimated from delta o 18 values from early diagenetic dolomite and the presence of structures associated with extracellular polymeric substances eps is composed of fibres arranged in a reticular pattern would favour epitaxial crystallization of dolomite on an organic substrate in addition poorly crystallized dolomite formed nanocrystal aggregates that strongly resemble the morphology and size distribution observed in microbial culture experiments these lines of evidence confirm that microbial structures can be preserved in ancient dolomite and validate their use as biosignatures

the role of fire in the late holocene forest decline in semi arid north china based on a sediment core from lake hulun nuur in the semi arid forest steppe ecotone in north china we reconstructed the history of climate change vegetation development and fire frequency in this region during the mid to late holocene four stages were identified 1 6300 4900 cal yr bp a warm humid phase dominated by forest vegetation with moderate fire incidence and a gradual drying trend 2 4900 4250 cal yr bp a warm dry phase dominated by forest vegetation with increased aridity and high fire occurrence 3 4250 1750 cal yr bp a gradual drying phase with forest decline and low fire occurrence and 4 1750 cal yr bp present a cool dry phase dominated by steppe with some decrease in aridity building on previous studies in this region the successive timing of forest replacement by steppe along the precipitation gradient in the late holocene indicates a gradual retreat of the forest margin under climate drying vegetation succession was predominantly determined by climate but forests

trace element composition of single glass shards in distal minoan tephra from sw turkey volcanic glass from the seventeenth century bc c 3600 bp minoan eruption of santorini thera has been recovered from a 4 cm thick tephra deposit at golhisar golu a small lake in sw turkey analysis of single grains of this glass by laser ablation inductively coupled plasma mass spectrometry la icp ms provides for the first time an accurate determination of the trace element composition of the juvenile glass component of this eruption free from contaminants such as phenocrysts or detrital material individual glass shards 150 m show a large degree of variation in their trace element composition indicative of substantial small scale compositional heterogeneity in the magma this is a result of the proximity of crystallizing phases and provides insights into the chemical evolution of the minoan magma the range of elemental compositions displayed by individual glass shards combined with the consistency of many trace element ratios are characteristics of the minoan glass these features provide a trace element signature for the minoan tephra which will be of value in stratigraphic correlation across the eastern mediterranean

recognition of santorini minoan tephra in lake sediments from golhisar golu southwest turkey by laser ablation icp ms laser ablation inductively coupled plasma mass spectrometry la icp ms has been applied to the analysis of fine grained volcanic ash shards separated from a tephra layer deposited by the mid second millenniumbc minoan eruption of santorini or thera in holocene lake sediments from golhisar golu a small intramontane lake in southwest turkey in this study we have used minor isotopes of two major elements routinely determined by epma in tephrochronological studies as the internal standard in la icp ms analyses these are30si and57fe for fully quantitative and single internal standard multi element calibration so called semi quantitative analysis respectively the results show that the determination of trace elements by la icp ms is a quick and accurate method typical accuracies are better than 20 and precision measured as the mean rsd relative standard deviation is better than 15 for most petrogenetically significant trace elements these levels of precision actually reflect within sample heterogeneity as repeat analyses on the homogenous nist 612 glass standard show that instrument precision is typically 5 for most elements the level of accuracy and precision obtained during this study is quite acceptable in tephrochronological studies and provides us with the confidence to proceed to the analysis of single glass shards by using a modified small spot laser ablation system

geochemistry of historical age silicic tephras in iceland the major element chemistry of nine silicic tephras of historical age from iceland is assessed as a key step in the development of the recent tephrochronology of the north atlantic region the tephras include the largest such layers produced by each of the five central volcanoes hekla o raefajokull eyjafjallajokull torfajokull and askja since the ninth century ad h 1104 o1362 e 1821 landnam tephra c 870 a 1875 and four other tephras h 1158 h 1510 h 1947 o1727 the determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra and allows links to be made between icelandic source areas with precise dating evidence and distal deposits elsewhere in the north atlantic region although major element data can be used to discriminate between tephra layers produced by the different central volcanoes on its own it cannot be used to identify all the holocene layers produc

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

varied responses of forest at its distribution margin to holocene monsoon development in northern china abstract based on pollen analysis and other evidence in a sediment profile from huangqihai lake located at the limit of asian monsoon influence the local vegetation development and climatic change were reconstructed for the past 8600 years from 8600 to 7200 cal yr bp the local vegetation was dominated by steppe and tree cover was low then pine increased gradually and the local vegetation was pine and oak mixed forest with variable ratios of the two dominant tree taxa from 7200 to 3600 cal yr bp from 3600 to 3000 cal yr bp the forest was replaced by shrubland which was attributed to a period of intense drought from 3000 to 600 cal yr bp the tree cover declined to its lowest levels probably due to long term further drought after 600 cal yr bp steppe has become dominant again we found that the changes of vegetation type indicated by pollen percentages did not match existing evidence for monsoon dynamics changes in tree pollen concentration however well captured the changes in the asian monsoon implying different sensitivity of vegetation cover and vegetation type e g replacement of steppe by forest to the monsoon development in addition a shrubland dominated stage which has not been evident in previous studies was also clear in our pollen sequences our study implied that the responses of marginal forest to drought were variable depending on the drought regime the different responses of forest to climate drying in this study provide insights into the reliable prediction of forest dynamics in the semi arid regions

mechanisms of abrupt climate change of the last glacial period 1 more than a decade ago ice core records from greenland revealed that the last glacial period was characterized by abrupt climate changes that recurred on millennial time scales since their discovery there has been a large effort to determine whether these climate events were a global phenomenon or were just confined to the north atlantic region and also to reveal the mechanisms that were responsible for them in this paper we review the available paleoclimate observations of abrupt change during the last glacial period in order to place constraints on possible mechanisms three different mechanisms are then reviewed ocean thermohaline circulation sea ice feedbacks and tropical processes each mechanism is tested for its ability to explain the key features of the observations particularly with regard to the abruptness millennial recurrence and geographical extent of the observed changes it is found that each of these mechanisms has explanatory strengths and weaknesses and key areas in which progress could be made in improving the understanding of their long term behavior both from observational and modeling approaches are suggested finally it is proposed that a complete understanding of the mechanisms of abrupt change requires inclusion of processes at both low and high latitudes as well as the potential for feedbacks between them some suggestions for experimental approaches to test for such feedbacks with coupled climate models are given

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

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

norwegian sea tephrostratigraphy of marine isotope stages 4 and 5 prospects and problems for tephrochronology in the north atlantic region abstract this paper presents an account of the tephrostratigraphy of south eastern norwegian sea core md99 2289 for the interval spanning marine isotope stage mis 5 and part of mis 4 from 70 000 to 130 000 years ago tephra deposits from in the order of 100 mainly basaltic volcanic eruptions in iceland and on jan mayen are described in terms of their stratigraphic context and geochemical composition representing the most comprehensive record of iceland and jan mayen volcanism over this time span the most important transport mechanism of tephra to the core site is considered to have been iceberg rafting which is thought to be responsible for a centennial to millennial scale time lag between the volcanic eruption events and the subsequent deposition of tephra in deep sea sediments while this clearly limits the use of some of the tephras as isochronous markers twelve discrete tephra deposits show stratigraphical and geochemical characteristics that suggest them to have been deposited near contemporaneously with their source volcanic events and hence suitable candidates as regional isochrones four of these accords with the geochemical signatures of tephra deposits previously described from the nordic seas region potential problems for the application of tephrochronology highlighted by this work are discussed and recommendations for strategies to overcome them are proposed

tracing volcanic events in the ngrip ice core and synchronising north atlantic marine records during the last glacial period five basaltic tephra horizons have been identified in the ngrip ice core during the last glacial period geochemical analyses indicate that these tephras are of icelandic origin with one horizon identified as the faroe marine ash zone iii fmaz iii an important marker horizon in several marine records within the faroe islands margin the fmaz iii is a few millimetres thick and is constrained to 38 122 723 yr b2k before year ad 2000 according to the greenland ice core chronology gicc05 this event falls directly at the thermal peak of greenland interstadial 8 gi 8 and a marine ice correlation based on the position of this tephra suggests a close coupling of the marine and atmospheric systems at this time the remaining tephras originate from the eastern volcanic zone with the katla volcanic system pinpointed as the dominant source these are thought to represent previously unknown volcanic events which add considerably to our knowledge of icelandic volcanism during this period independent ages are assigned to these events and together with major element characterisation form the backbone of the tephrochronology framework presented here despite the identification of these tephras this study also demonstrates that the sulphate record may not always act as a suitable proxy for the identification of tephra horizons in ice core records

developments in the analysis of volcanic glass shards by laser ablation icp ms quantitative and single internal standard multielement methods abstract laser ablation inductively coupled plasma mass spectrometry la icp ms has been applied to the analysis of fine grained 125 250 m volcanic glass shards separated from tephra deposits our earlier calibration strategies used ce as an internal standard which required the use of another trace element analytical technique to determine ce followed by off line data correction by comparison with well characterised glass separates we have progressed from these earlier studies to use a minor isotope of a major element routinely determined by epma in tephrochronological studies as an internal standard for la icp ms analysis internal standards used during this study include 29si 30si or 44ca for fully quantitative determinations or 57fe for a single internal standard multi element calibration so called semiquantitative analysis no subsequent off line data correction is required the differences between each technique are minimal and choice of technique depends largely upon instrument sensitivity as little as 80 g of material is consumed during a single determination which takes approximately two minutes and detection limits are below 1 part per million accuracy is generally 15 when compared with other analytical techniques and precision may vary between 5 and 20 the worse precisions reflecting within sample variability rare earth and other trace element data can be used to correlate or distinguish individual tephra deposits with confidence and specific examples are illustrated with quaternary tephra deposits from alaska and india and cretaceous tephra deposits from banks island in arctic canada

chemical composition distribution and origin of silicic volcanic ash layers in the greenland iceland norwegian sea explosive volcanism from 10 to 300 ka as recorded in deep sea sediments explosive ocean island volcanism in the greenland iceland norwegian sea gin sea is indicated by marine tephra layers at 10 300 ka peaks of explosive volcanism occurred in oxygen isotope stages 8 7 5 and 1 the depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records geochemical analyses of the tephra layers show that all originate from iceland here we report the characteristics of tephra from these major icelandic events in 30 deep sea cores from the gin sea our findings provide constraints on the distribution of tephra from the eruption source for the vedde ash oxygen isotope stage 1 we estimate a minimum fallout area of 2 105 km2 stretching from central greenland in the west and southern sweden in the east to 71 n in the gin sea the magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards

a detailed framework of marine isotope stages 4 and 5 volcanic events recorded in two greenland ice cores sulphate records from greenland ice cores indicate that marine isotope stages 4 and 5 were charactensed by a higher incidence of large volcanic eruptions than other periods during the last glacial

palynological and avhrr observations of modern vegetational gradients in eastern north america both fossil pollen records and satellite based instruments are remote sensors of earth s vegetation with complementary properties satellites supply spatially continuous and highly resolved images for the past several decades whereas pollen records include local and regional signals of vegetation composition spanning millennia together pollen and satellite based observations measure vegetation change across a broad range of temporal scales here we compare pollen percentages of needleleaved and broadleaved plant taxa to avhrr estimates of percent tree cover for two regions in eastern north america with well defined physiognomic gradients the linear fit between the pollen percentages and percent tree cover is strongest for search window half widths of 25 75 km and unweighted or inverse distance weightings consistent with previous taxon based studies of regional pollen source area and transport variance not explained by the linear model arises primarily from differential properties of the avhrr and

tephra geochemistry a discussion of some persistent analytical problems distal tephrochronological studies are reliant upon precise and accurate geochemical quantitation this quantitation can involve difficulties not often encountered in more conventional petrology solving these problems is of particular importance in the holocene and late quaternary of northwest europe where numerous tephra layers of icelandic origin have been detected in peat bogs and lake and marine sediments electron probe microanalysis epma has proved the most applicable analytical technique when insufficient quantities of tephra are available for bulk analysis standard epma techniques require fine tuning to the tephra problem as the geochemical instability of glass is a common feature in many previous studies geochemical data of avoidably low quality have been produced and little attention appears to have been directed to modification of epma methods it is suggested that the black box approach to geochemical analysis should be avoided by the consideration of several factors these include

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

explosive silicic volcanism in iceland and the jan mayen area during the last 6 ma sources and timing of major eruptions abstract fifty three major explosive eruptions on iceland and jan mayen island were identified in 0 6 ma old sediments of the north atlantic and arctic oceans by the age and the chemical composition of silicic tephra the depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 ma and paleoclimatic proxies 18 o ice rafted debris for the last 1 ma major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off rift volcanic zones in iceland and to the jan mayen volcanic system the tholeiitic central volcanoes along the iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4 9 ma they were the source of at least 26 eruptions of dominant rhyolitic magma composition including the late pleistocene explosive eruption of krafla volcano of the eastern rift zone at about 201 ka the central volcanoes along the off rift volcanic zones in iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition with three distinct episodes recorded at 4 6 5 3 3 5 3 6 and 0 1 8 ma the longest and last episode recorded 11 pleistocene major events including the two explosive eruptions of tindfjallajokull volcano thorsmork ca 54 5 ka and katla volcano solheimar ca 11 9 ka of the southeastern transgressive zone eight major explosive eruptions from the jan mayen volcanic system are recorded in terms of the distinctive grain size mineralogy and chemistry of the tephra the tephra contain k rich glasses k 2 o sio 2 0 06 ranging from trachytic to alkali rhyolitic composition their normative trends ab q or and their depleted concentrations of ba eu and heavy ree reflect fractional crystallisation of k feldspar biotite and hornblende in contrast their enrichment in highly incompatible and water mobile trace elements such as rb th nb and ta most likely reflect crustal contamination one late pleistocene tephra from jan mayen was recorded in the marine sequence its age estimated between 617 and 620 ka and its composition support a common source with the borga pumice formation at sor jan in the south of the island

volcanic aerosol records and tephrochronology of the summit greenland ice cores the recently collected greenland ice sheet project 2 gisp2 and greenland ice core project ice cores from summit greenland provide lengthy and highly resolved records of the deposition of both the aerosol h2so4 and silicate tephra components of past volcanism both types of data are very beneficial in developing the hemispheric to global chronology of explosive volcanism and evaluating the entire volcanism climate system the continuous time series of volcanic so42 for the last 110 000 years show a strong relationship between periods of increased volcanism and periods of climatic change the greatest number of volcanic so42 signals many of very high magnitude occur during and after the final stages of deglaciation 6000 17 000 years ago possibly reflecting the increased crustal stresses that occur with changing volumes of continental ice sheets and with the subsequent changes in the volume of water in ocean basins sea level change the increase in the number of volcanic so42 signals at 27 000 36 000 and 79 000 85 000 years ago may be related to initial ice sheet growth prior to the glacial maximum and prior to the beginning of the last period of glaciation respectively a comparison of the electrical conductivity of the gisp2 core with that of the volcanic so42 record for the holocene indicates that only about half of the larger volcanic signals are coincident in the two records other volcanic acids besides h2so4 and other so42 sources can complicate the comparisons although the threshold level picked to make such comparisons is especially critical tephra has been found in both cores with a composition similar to that originating from the vatnaoldur eruption that produced the settlement layer in iceland mid a d 870s from the icelandic eruption that produced the saksunarvatn ash 10 300 years ago and from the icelandic eruption s that produced the z2 ash zone in north atlantic marine cores 52 700 years ago the presence of these layers provides absolute time lines for correlation between the two cores and for correlation with proxy records from marine sediment cores and terrestrial deposits containing these same tephras the presence of both rhyolitic and basaltic shards in the z2 ash in thegisp2 core and the composition of the basaltic grains lend support to multiple icelandic sources torfajokull area and katla for the z2 layer deposition of the z2 layer occurs at the beginning of a stadial event further reflecting the possibility of a volcanic triggering by the effects of changing climatic conditions

pollen representation source area and basin size toward a unified theory of pollen analysis abstract the concepts of pollen source area and of production and dispersal biases in pollen representation are quantified by means of a simple theoretical model source areas and relative pollen representation are shown to depend on basin size according to functions that describe the amount of pollen remaining airborne at increasing distances from single pollen sources the form of these functions is determined by physical processes standard formulas for elevated sources do not apply but the integrated form of sutton s equation for particle dispersal from a ground level source gives useful approximations applicable to pollen transport over a forest canopy simulations using this equation yielded source areas that increased realistically with basin size showed substantial differences between source areas for pollen grains with different deposition velocities and predicted that lighter pollen grains should become better represented with increasing basin size all of these predictions are qualitatively consistent with present knowledge of the characteristics of pollen assemblages in different depositional environments the model further allows parameters that can be estimated by statistical calibration methods to be predicted from underlying physical quantities this extension suggests procedures for testing the theory with quantitative data on surface pollen and forest composition preliminary results showed reasonable agreement between estimated and predicted values of dispersal indices for the most abundant taxa in pollen spectra from the northern midwestern united states

volcanic ash layers from the last glacial termination in the ngrip ice core the tephrochronological record of the 1400 1640 m depth 10 000 16 000 calendar ice core years before present of the ngrip ice core has been established by particle screening of selected samples ash was identified in 20 samples correlation with ice marine and terrestrial records from volcanic source regions in the northern hemisphere positively identifies the saksunarvatn ash and the vedde ash ash zone 1 major element chemistry of the remaining identified ash layers mainly points towards an icelandic origin this tephrochronological record provides new important marker horizons for correlating the timing of the climatic changes associated with the last glacial termination within the north atlantic region as well as outlining more details concerning the frequency and composition of volcanic eruptions occurring at this deglaciation copyright 2005 john wiley sons ltd

identification of the fugloyarbanki tephra in the ngrip ice core a key tie point for marine and ice core sequences during the last glacial period a visible tephra horizon in the ngrip ice core has been identified by geochemical analysis as the fugloyarbanki tephra a widespread marker horizon in marine cores from the faroe islands area and the northern north atlantic an age of 26 740 390 yr b2k 1 uncertainty is derived for this tephra according to the new greenland ice core chronology gicc05 based on multi parameter counting of annual layers detection of this tephra for the first time within the ngrip ice core provides a key tie point between marine and ice core records during the transition between mis 3 and 2 identification of this volcanic event within the greenland records demonstrates the future potential of using tephrochronology to precisely correlate palaeoarchives in widely separated localities that span the last glacial period as well as providing a potential method for examining the extent of the radiocarbon marine reservoir effect at this time copyright 2008 john wiley sons ltd

pollen detected altitudinal migration of forests during the holocene in the mountainous forest steppe ecotone in northern china abstract distributed on the margin of the influence of the asian monsoon the forest steppe ecotone in northern china is sensitive to climate change previous paleovegetation reconstructions in this region have led to inconsistent explanations of paleovegetation dynamics across sites we hypothesized that the patterns observed are related to altitudinal migration of forests across mountains in this region we combined eight lake sediment pollen records to explain how three forest types dominated by the tree genera pinus quercus and betula changed in response to temporal and spatial monsoon changes within the forest steppe ecotone during the holocene modern pollen assemblages showed that areas with large altitude range could support more forests which was further confirmed by the spatial patterns of late holocene forest distribution under dry climate our results confirmed that altitudinal migration of forests might exist in this region this kind of altitudinal migration of forests under different altitudinal ranges could help to explain the inconsistent patterns observed in arboreal pollen across sediment cores as well as the non linear response of horizontal forest distribution in relation to climate change

correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential pearce nicholas denton j s perkins w t westgate j a 2007 correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential journal of quaternary science 22 7 pp 721 736 rae2008

new tephra horizons from oxygen isotope stage 5 in the north atlantic correlation potential for terrestrial marine and ice core archives abstract tephra horizons of oxygen isotope stage ois 5 age from two cores in the north atlantic have been analysed geochemically the results show that several discrete tephra horizons are present which have an icelandic provenance tephras of basaltic geochemistry dominate the sequences correlation of the cores is based on the occurrence of a basaltic tephra near the base of ois 5e a rhyolitic tephra has been discovered within ois 5e sediments in a core from the se norwegian sea which is geochemically identical to the middle eemian tephra found in several cores from the nordic seas this tephra is probably widespread and thus the potential exists for its wider detection in terrestrial sequences on mainland europe and the greenland ice cores

pollen reconstruction and vegetation dynamics of the middle holocene maximum summer monsoon in northern china abstract understanding the evolution of the holocene east asian summer monsoon easm and vegetation response regimes can help predict the climatic variability and vegetation change of future warming here we reconstructed holocene vegetation and climate change based on pollen grain size and percent organic matter from a lake sediment profile in huitengxile plateau of central inner mongolia northern china we found that temperate deciduous forest steppe and temperate mixed forest steppe prevailed under maximum easm during the middle holocene 8 3 5 9 cal kyr bp with more open vegetation and a weak easm prior to and following this period the vegetation succession is consistent with regional records suggesting a regional vegetation pattern i e oak forest replacing birch and elm forest during the middle holocene and pine forest expanding at the expense of birch and oak forest since 6 0 cal kyr bp a regional synthesis suggests the maximum easm began at 8 3 7 8 cal kyr bp in northern china indicating a significant delay when compared with published records from southern china the maximum easm in northern china ended at 5 9 5 5 cal kyr bp being similar or slightly delayed from that in southern china

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

the toba volcanic event and interstadial stadial climates at the marine isotopic stage 5 to 4 transition in the northern indian ocean abstract the toba volcanic event one of the largest eruptions during the quaternary is documented in marine sediment cores from the northeastern arabian sea on the crest of the murray ridge and along the western indian continental margin we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5 4 boundary with the chemical composition of the youngest toba tuff time series of the uk 37 alkenone index planktic foraminiferal species magnetic susceptibility and sediment accumulation rates from this interval show that the toba event occurred between two warm periods lasting a few millennia using toba as an instantaneous stratigraphic marker for correlation between the marine and ice core chronostratigraphies these two arabian sea climatic events correspond to greenland interstadials 20 and 19 respectively our data sets thus depict substantial interstadial stadial fluctuations in sea surface temperature and surface water productivity we show that variable terrigenous eolian sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record within the provided stratigraphic resolution of several decades to centennials none of these proxies shows a particular impact of the toba eruption however our results are additional support that toba despite its exceptional magnitude had only a minor impact on the evolution of low latitude monsoonal climate on centennial to millennial time scales

how did the forest respond to holocene climate drying at the forest steppe ecotone in northern china abstract a sediment sequence from anguli nuur lake 41 18 24 n 114 20 27 e located at the forest steppe ecotone in the southern inner mongolia plateau of china was selected to reconstruct the holocene history of vegetation development and to determine the primary driver of forest dynamics the shifting of vegetation zones was reconstructed by pollen vegetation zone discriminant functions established by contemporary distribution of surface pollen in a transect along both precipitation and vegetation gradients in northern china pollen indicators for human disturbance were selected by comparison with a sediment sequence from west liao river basin which is also located in the forest steppe ecotone but is abundant in archeological sites changes of coarse sand percentage show great consistency with changes of regional humidity index suggesting a prominent role of climate in soil erosion the discriminant functions showed that long term replacement of the forest zone by steppe zone commenced at 2 1 ka bp pollen percentages also show that climate drying since 5 0 ka bp first led to replacement of broadleaf forest by pine forest due to soil coarsening further recession of pine forest from this region since 2 1 ka bp was primarily driven by climate deterioration because no pollen indicator of human disturbance was recognized the study also suggests an earlier arrival but later recession of pine forest in the study region implying pine pinus tabulaeformis migrated from the south and migration of pine forest in the current steppe region was driven by the pacific monsoon movement

age extent and climatic significance of the c 3400 bp aniakchak tephra western alaska usa tephra deposits at whitefish lake and cape espenberg on the northernmost seward peninsula of western alaska were derived from the aniakchak caldera on the alaska peninsula more than 1500 km to the south radiocarbon dates on the climactic caldera forming aniakchak eruption and on proximal and distal tephra indicate the aniakchak tephra was deposited about 3435 40 bp 3614 3815 cal bp and forms an isochronous marker horizon across much of western alaska the aniakchak tephra eruption and several other targe eruptions during the seventeenth century bc may have had widespread effects on climate

synchronisation of palaeoenvironmental events in the north atlantic region during the last termination a revised protocol recommended by the intimate group abstract the north atlantic intimate group of the inqua palaeoclimate commission has previously recommended an event stratigraphy approach for the synchronisation of records of the last termination using the greenland grip isotopic record as the regional stratotype and intcal98 for the calibration of radiocarbon dates lowe j j hoek w intimate group 2001 inter regional correlation of palaeoclimatic records for the last glacial interglacial transition a protocol for improved precision recommended by the intimate project group quaternary science reviews 20 1175 1187 here we present a revised protocol for time stratigraphic correlation in the north atlantic region over a more extended time period 30 8 ka this employs the new ngrip isotopic record and associated greenland ice core chronology 2005 gicc05 as the regional stratotype intcal04 for the calibration of radiocarbon dates bayesian based statistical procedures for the construction of age models and tephrochronology to validate correlations between regional site records

tephrostratigraphical evidence for the timing of pleistocene explosive volcanism at jan mayen wavelength dispersive wds electron microprobe analysis of glass shards recovered from pleistocene sediments at ocean drilling program hole 907a leg 151 has been used to define key pleistocene tephrochronostratigraphical markers for the greenland iceland norwegian gin seas two tephras have particularly distinctive geochemistry making them useful as marker horizons in cores from the iceland plateau and potentially further afield these tephras enable the construction of a sedimentation rate history that indicates reduced sediment deposition in the mid brunhes magnetochron possibly through reduced carbonate preservation no significant geochemical weathering of the iceland plateau tephra sequence has occurred the geochemical signature of a key tephra dated to 577 640 ka can be attributed to the jan mayen rock suite demonstrating unambiguously the explosive potential of jan mayen and its potential to contribute to the north atlantic tephra record copyright 2004 john wiley sons ltd

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

constraints on the dynamics of subglacial basalt eruptions from geological and geochemical observations at kverkfjoll ne iceland the kverkfjoll area ne iceland is characterised by subglacial basalt pillow lavas erupted under thick ice during the last major glaciation in iceland the water contents of slightly vesiculated glassy rims of pillows in six localities range from 0 85 0 03 to 1 04 0 03 wt the water content measurements allow the ice thickness to be estimated at between 1 2 and 1 6 km with the range reflecting the uncertainty in the co2 and water contents of the melt the upper estimates agree with other observations and models that the ice thickness in the centre of iceland was 1 5 2 0 km at the time of the last glacial maximum many of the pillows in the kverkfjoll area are characterised by vesiculated cores 40 60 vesicles surrounded by a thick outer zone of moderately vesicular basalt 15 20 vesicles the core contains 1 mm diameter spherical vesicles distributed uniformly this observation suggests a sudden decompression and vesiculation of the still molten core followed by rapid cooling the cores are attributed to a jokulhlaup in which melt water created by the eruption is suddenly released reducing the environmental pressure mass balance and solubility relationships for water allow a pressure decrease to be calculated from the observed change of vesicularity of between 4 4 and 4 7 mpa depressurization equivalent to a drop in the water level in the range 440 470 m consideration of the thickness of solid crust around the molten cores at the time of the jokulhlaup indicates an interval of 1 3 days between pillow emplacement and the jokulhlaup upper limits for ice melting rates of order 10 3 m s are indicated this interpretation suggests that jokulhlaups can reactivate eruptions

the tephrochronology of iceland and the north atlantic region during the middle and late quaternary a review the tephrochronology of iceland and the north atlantic region is reviewed in order to construct a unified framework for the last 400 kyr bp nearly all of the tephra layers described are also characterised geochemically a number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre holocene tephra layers have been given an informal denotation the tephrostratigraphy of ash zone ii is highlighted where possible the rhyolitic tephra layers found outside iceland have been correlated to known icelandic tephra layers or to the volcanic source area the application of tephra fallout in various depositional environments is described and discussed copyright 2000 john wiley sons ltd

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

patterns of local and regional grain size distribution and their application to holocene climate reconstruction in semi arid inner mongolia china abstract the semi arid temperate steppe in northern central china is one of the main areas influenced by frequent dust and sand storms and is at the same time a primary source of dust from deteriorated grasslands thus the sediment grain size distribution of inland lakes in this region can be a particularly useful indicator of palaeoenvrionmental change the local pattern of grain size suggests that aeolian activity is the most important agent for sedimentation in the lake center in this region as strong northwesterly winds prevail for most of the year and the surface runoff is very weak meanwhile the regional pattern of topsoil grain size and its close association with mean annual precipitation map allows the establishment of a statistical model for palaeo moisture reconstruction from sediment grain size in this study we reconstructed a humidity time series based on the sediment grain size sequence from anguli nuur lake on the southern inner mongolian plateau in china and found that it coincides very closely with the c n ratio carbon to nitrogen ratio and other humidity indices revealed in previous studies of this temperate steppe region and from the 18 o values of stalagmite calcite in southern monsoon dominated china this close relationship suggests that climate change in the semi arid areas of asia is strongly influenced by the pacific summer monsoon and that it is reasonable to use sediment grain size as an indicator of humidity variability in the semi arid steppe region the reconstructed humidity increased during the early holocene and generally humid conditions lasted from about 10 400 until 7000 yr bp the period from around 7000 to 5200 yr bp was a transition phase from humid to semi arid conditions and the monsoon intensity of that time may have been at the threshold for a semi arid vegetation ecosystem finally since approximately 5200 yr bp to present the climate has become more arid with corresponding vegetation deterioration and strong aeolian activity

a new continuous high resolution detection system for sulphate in ice cores sulphate so4 2 is a major ion found in polar ice cores and is related to different aerosol sources and processes explosive volcanic eruptions even far away can cause distinct sulphate peaks in ice core records thus a robust sulphate detection system which is suitable for fieldwork and which enables the measurement of sulphate at high temporal resolution is of great interest in this study we present the adaptation of a new continuous flow analysis system for sulphate that is based on a spectrophotometric method using dimethylsulfonazo iii and an inline reactor column containing barium sulphate particles the method shows a detection limit of 70 ng g 1 and a linear range up to at least 3000 ng g 1 it is simple robust and less prone to interferences compared to the previously used method

a model of pollen source area for an entire lake surface a model of pollen deposition on the surface of an entire basin is developed to estimate pollen source area and results are compared with those for a point at the center of a basin i c prentice 1985 quaternary research 23 76 86 1988 vegetation history pp 17 42 kluwer academic this model is more appropriate for approximating the source area of pollen in lake sediment since mixing in lake water and focusing of sediment redistribute pollen originally deposited over the entire surface in general the pollen source radius for the entire basin surface is 10 30 smaller than the source radius for a point at the center the difference in the source radius is more profound for heavier pollen types such as spruce and sugar maple than for lighter types such as oak and ragweed the average pollen input to the entire surface is more strongly influenced by nearby pollen sources than pollen deposition at the center the pollen record from a lake may therefore provide different spatial resolution than the record from a bog of similar radius