atmospheric radiocarbon during the younger dryas production ventilation or both a new reconstruction of past atmospheric v 14 c v 14 catm based on polish lake varved sediments has suggested that previous v 14 catm values e g from the cariaco basin record for the beginning of the younger dryas cold event yd are overestimates and that the v 14 catm rise at the yd onset could only be due to changes in atmospheric 14 c production pc 14 this result would have profound climatic implications for the yd is a paradigm example of abrupt climate change which is usually thought to have been triggered by a reduction in the northward heat flux by the atlantic thermohaline circulation here we examine results from a large number of simulations 300 based on a zonally averaged ocean circulation model to constrain the effect on v 14 catm of pc 14 changes during the yd as reconstructed from a greenland ice core record of 10 be flux our results suggest that the scatter in the lake data set is too large to exclude the probable change in deep ocean ventilation at the onset of the yd by contrast the model fit to the higher v 14 catm levels throughout the yd detected in the marine record is generally better when a substantial decrease in deep ocean ventilation is simulated the early v 14 catm drawdown that initiated during the first half of the yd however could entirely be due to production changes if this was the case the drawdown would not reflect an increasing formation of north atlantic intermediate water or southern ocean water as previously suggested the rapid v 14 catm rise at the yd onset documented in the marine record however remains unexplained fl 2001 elsevier science b v all rights reserved

mount logan ice core record of tropical and solar influences on aleutian low variability 500 1998 a d continuous high resolution paleoclimate records from the north pacific region spanning the past 1500 years are rare and the behavior of the aleutian low alow pressure center the dominant climatological feature in the gulf of alaska remains poorly constrained here we present a continuous 1500 year long calibrated proxy record for the strength of the wintertime december march alow from the mount logan summit pr col 5200 m asl ice core soluble sodium time series we show that ice core sodium concentrations are statistically correlated with north pacific sea level pressure and zonal wind speed our alow proxy record reveals a weak alow from circa 900 1300 a d and 1575 1675 a d and a comparatively stronger alow from circa 500 900 a d 1300 1575 a d and 1675 a d to present the mount logan alow proxy record shows strong similarities with tropical paleoclimate proxy records sensitive to the el nino southern oscillation and is consistent with the hypothesis that the medieval climate anomaly was characterized by more persistent la nina like conditions while the little ice age was characterized by at least two intervals of more persistent el nino like conditions the mount logan alow proxy record is significantly p 0 05 correlated and coherent with solar irradiance proxy records over various time scales with stronger solar irradiance generally associated with a weaker alow and la nina like tropical conditions however a step like increase in alow strength during the dalton solar minimum circa 1820 is associated with enhanced walker circulation furthermore rising co2 forcing or internal variability may be masking the twentieth century rise in solar irradiance

the younger dryas termination and north atlantic deep water formation insights from climate model simulations and greenland ice cores results from the gisp2 and grip ice cores show that the termination of the younger dryas yd climate event in greenland was a large and extremely fast climate change a reinitiation of north atlantic deep water formation following a shutdown and its associated winter release of heat to the atmosphere has been suggested as the most likely cause of this climate transition to test this idea two general circulation model experiments using genesis have been completed for yd time 12 000 calendar years ago one with low heat flux in the nordic seas 10 w m deep water shutdown and one with high nordic sea heat flux 300 w m active deep water formation comparison of greenland climate differences between these experiments with the ice core records shows that when deep water is turned on much of the yd termination warming is achieved the increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation which obscures the dominantly wintertime response to the specified forcing the winter storm track shift toward greenland contributes much of the climate change at the yd termination

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

characterization of late glacial continental dust in the greenland ice core project ice core twelve glacial dust samples extracted from the greenland ice core project ice core have been characterized in terms of mineralogy sr and nd isotopic composition and rare earth element concentrations the ice samples are taken from the younger dryas the bolling the last glacial maximum and both cold and mild glacial periods back to 44 kyr b p a comparison with samples from possible dust source areas determines eastern asia as being the main source area at all times hereby confirming and extending earlier results biscaye et al 1997 this finding is partly in agreement with recent atmospheric general circulation model simulations of glacial atmospheric dust transport andersen et al 1998 mahowald et al 1999 a low intersample variability in tracer characteristics is discussed in the context of a volcanogenic influence grain size dependence and variability in the conti nental dust source areas the clay mineralogy of the ice core dust shows a weak climatic de pendence while the isotopic composition of sr and nd does not correlate with climate this is interpreted as a geographical variation of source areas on a regional scale within asia two ice dust samples were size fractionated at 2 gm and the grain size dependence on tracer characteristics is in accordance with the literature

a glaciochemical survey of the summit region greenland spatial representativene ss and an understanding of controls on chemical species distribution are essential requirements of any significant ice core investigation snowpit studies provide an essential tool in this process in preparation for the central greenland deep drilling effort a series of snowpits was sampled in detail for oxygen isotopes major anions major cations total acidity and radionuclides the results of this sampling program are used to define 1 the chemical composition of the snow in the region 2 the input timing and spatial distribution of major chemical species 3 the potential dependence of

a varved lake sediment record of the 10 be solar activity proxy for the lateglacial holocene transition solar modulated variations in cosmogenic radionuclide production provide both information on past changes in the activity of the sun and a global synchronization tool however to date the use of cosmogenic radionuclides for these applications is almost exclusively based on 10be records from ice cores and 14c time series from tree rings all including archive specific limitations we present the first 10be record from annually laminated varved lake sediments for the lateglacial holocene transition from meerfelder maar we quantify environmental influences on the catchment and consequently 10be deposition using a new approach based on regression analyses between our 10be record and environmental proxy time series from the same archive our analyses suggest that environmental influences contribute to up to 37 of the variability in our 10be record but cannot be the main explanation for major 10be excursions corrected for these environmental influences our 10be record is interpreted to dominantly reflect changes in solar modulated cosmogenic radionuclide production the preservation of a solar production signal in 10be from varved lake sediments highlights the largely unexplored potential of these archives for solar activity reconstruction as global synchronization tool and thus for more robust paleoclimate studies

variations in melt layer frequency in the gisp2 ice core implications for holocene summer temperatures in central greenland the rare melt features in the gisp2 central greenland deep ice core have decreased in frequency over the most recent 7000 years calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central greenland and against modern temporal variability of temperatures in central greenland indicates that mean mid summer temperatures have cooled over the most recent 7000 years probably by slightly more than l c if variability of summer temperatures has not changed comparison to grip isotopic records from central greenland and to the melt record from the agassiz ice cap arctic canada suggests some seasonal and regional coherence for this cooling signal as well as for a cold event about 8000 8500 bp

major features and forcing of high latitude northern hemisphere atmospheric circulation using a 110 000 year long glaciochemical series the greenland ice sheet project 2 glaciochemical series sodium potassium ammonium calcium magnesium sulfate nitrate and chloride provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid low latitudes of the northern hemisphere interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants volcanic and biomass burning events storminess over marine surfaces continental aridity and biogenic source strength plus information related to the controls on both high and low frequency climate events of the last 110 000 years climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long term behavior of atmospheric circulation patterns through changes in ice volume sea level events such as the heinrich events massive discharges of icebergs first identified in the marine record that are found to operate on a 6100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics and rapid climate change events massive reorganizations of atmospheric circulation that are demonstrated to operate on 1450 year cycles changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events explanation for the exact timing and global synchroneity of these events is however more complicated preliminary evidence points to possible solar variability climate associations for these events and perhaps others that are embedded in our ice core derived atmospheric circulation records

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

stable isotope air temperature relationships in ice cores from dolleman island and the palmer land plateau antarctic peninsula whilst stable isotope analysis of ice cores yields the best quantitative evidence for past climate there remains considerable uncertainty about the detailed relationship between the isotopic composition and air temperature analysis of two ice cores from the antarctic peninsula a 47 2 m core from the palmer land plateau 74 oi s 70 38 w and a 32 m core from dolleman island 70035 2 s 60055 5 w has shown that an oxygen isotope temperature relationship exists at a resolution of inter annual variations during the period 1938 86 all the major regional temperature anomalies known from climatic records at several stations are visible in the isotope profiles including the overall temperature increase between 1960 and 1980 an isotope temperature gradient of 0 5 o 6 c is indicated for the climatic interpretation of isotopic fluctu

hudson bay river sediments and regional glaciation i iron and carbonate dispersal trains southwest of hudson and james bays sands deposited from rivers draining the southwesterly trending watershed of the cape henrietta maria area are enriched in calcite and are characterized by higher calcite dolomite ratios than the sands from rivers north and south of the cape this pattern suggests that a train of drift with distinctive carbonate composition extends southwest of cape henrietta maria indicating a dominant pattern of southwesterly ice flow in this region iron contents are highest for sands from rivers on cape henrietta maria suggesting iron enrichment in the sand fraction of tills in the cape area and the existence of a broad dispersal train extending southwest of the cape the dispersal train inferred from the calcite dolomite ratios overprints part but not all of the dispersal train inferred from the iron content

greenland ice core signal characteristics an expanded view of climate change the last millenium of earth history is of particular interest because it documents the environmental complexities of both natural variability and anthropogenic activity we have analyzed the major ions contained in the greenland ice sheet project 2 gisp 2 ice core from the present to 674 a d to yield an environmental reconstruction for this period that includes a description of nitrogen and sulfur cycling volcanic emissions sea salt and terrestrial influences we have adapted and extended mathematical procedures for extracting sporadic e g volcanic events secular trends and periodicities found in the data sets finally by not assuming that periodic components signals were stationary and by utilizing evolutionary spectral analysis we were able to reveal periodic processes in the climate system which change in frequency turn on and turn off with other climate transitions such as that between the little ice age and the medieval warm period

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

pre incan agricultural activity recorded in dust layers in two tropical ice cores in south america no documentary records survive from before the incursion of the spaniards in the mid sixteenth century knowledge of pre hispanic civilizations in this region comes largely from archaeological excavations which may now be supplemented by information recorded in two ice cores drilled in the quelccaya ice cap in southern peru fig 1 1 2 a 1 500 year record of particle concentrations and conductivity in these cores shows two major dust episodes each lasting about 130 years centred at ad 920 and 600 here we examine these dust events in the light of oxygen isotope and net accumulation records and suggest a correlation with pre incan agricultural activity the annual net accumulation record for ad 1915 1984 compares well with annual changes in lake titicaca water levels and with annual precipitation at el alto lapaz bolivia suggesting that the 1 500 year net accumulation record from quelccaya may serve as a proxy for water level changes in lake titicaca the ice core record suggests that climatic variability reflected in lake level changes has strongly influenced fluctuations in agricultural activity in southern peru

using the sunspot cycle to date ice cores for many ice cores the snow accumulation rate is too low to preserve annual stratigraphy precluding direct measurement of annual layer thickness for selected holocene sections of the taylor dome core east antarctica we instead use high resolution 10be measurements to establish a nominal 11 year thickness taking advantage of the solar cycle in cosmogenic isotope production we compare measured thicknesses with the layer thickness profile predicted by a finite element ice flow model the results are in good agreement supporting the assumption that the length of the solar cycle has remained essentially constant throughout the holocene for ice cores where annual layers are not preserved the 10be 11 year layer method can be used as an independent check on flow model estimates of layer thickness and to estimate past accumulation rates it should be possible to accurately date ice cores by counting 11 year layers detected with continuous high resolution 10be measurements

variations of younger dryas atmospheric radiocarbon explicable without ocean circulation changes the concentration of radiocarbon 14c in the atmosphere depends on its production rate by cosmic rays and on the intensity of carbon exchange between the atmosphere and other reservoirs for example the deep oceans for the holocene the past 11 500 years it has been shown that fluctuations in atmospheric radiocarbon concentrations have been caused mostly by variations in the solar magnetic field1 2 3 recent progress in extending the radiocarbon record backwards in time4 5 6 7 8 9 10 has indicated especially high atmospheric radiocarbon concentrations in the younger dryas cold period between 12 700 and 11 500 years before the present these high concentrations have been interpreted as a result of a reduced exchange with the deep ocean reservoir caused by a drastic weakening of the deep ocean ventilation7 8 9 11 12 here we present a high resolution reconstruction of atmospheric radiocarbon concentrations derived from annually laminated sediments of two polish lakes lake go ciaz and lake perespilno these records indicate that the maximum in atmospheric radiocarbon concentrations in the early younger dryas was smaller than previously believed and might have been caused by variations in solar activity if so there is no indication that the deep ocean ventilation in the younger dryas was significantly different from today s

radar attenuation and temperature within the greenland ice sheet 2015 american geophysical union all rights reserved the flow of ice is temperature dependent but direct measurements of englacial temperature are sparse the dielectric attenuation of radio waves through ice is also temperature dependent and radar sounding of ice sheets is sensitive to this attenuation here we estimate depth averaged radar attenuation rates within the greenland ice sheet from airborne radar sounding data and its associated radiostratigraphy using existing empirical relationships between temperature chemistry and radar attenuation we then infer the depth averaged englacial temperature the dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry where radar transects intersect boreholes radar inferred temperature is consistently higher than that measured directly we attribute this discrepancy to the poorly recognized frequency dependence of the radar attenuation rate and correct for this effect empirically resulting in a robust relationship between radar inferred and borehole measured depth averaged temperature radar inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice sheet model particularly in southern greenland this pattern suggests that past changes in surface boundary conditions temperature and accumulation rate affect the ice sheet s present temperature structure over a much larger area than previously recognized this radar inferred temperature structure provides a new constraint for thermomechanical models of the greenland ice sheet

methanesulfonate in the greenland ice sheet project 2 ice core in this paper we present measurements of methanesulfonate in the greenland ice sheet project 2 gisp2 ice core methanesulfonate is an atmospheric oxidation product of dimethylsulfide the gisp2 methanesulfonate record contains information about the atmospheric loading of biogenic sulfur over the past 110 kyr and its relationship to climate change the gisp2 data set supports the inferences made from the renland ice core from greenland that the glacial atmosphere over greenland had reduced concentrations of biogenic sulfur compared with the present day hansson and saltzman 1993 we conclude that the flux of biogenic sulfur from the north atlantic ocean must have been lower during glacial times and speculate that this decrease may have been related to differences in phytoplankton speciation the data suggest that changes in direct radiative forcing from biogenic sulfur aerosols would act as negative feedback to the glacial interglacial climate cycles in this region

wally was right predictive ability of the north atlantic conveyor belt hypothesis for abrupt climate change linked abrupt changes of north atlantic deep water formation north atlantic sea ice extent and widespread climate occurred repeatedly during the last ice age cycle and beyond in response to changing freshwater fluxes and perhaps other causes this paradigm developed and championed especially by w s broecker has repeatedly proven to be successfully predictive as well as explanatory with high confidence much work remains to fully understand what happened and to assess possible implications for the future but the foundations for this work are remarkably solid

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

patagonian origin of glacial dust deposited in east antarctica vostok and dome c during glacial stages 2 4 and 6 abstract the source area of continental dust deposited at both vostok and dome c sites east antarctica during the glacial stages stage 2 18 ka last glacial maximum stage 4 60 ka and stage 6 160 ka of the last two climatic cycles has remained constant the isotopic composition 87 sr 86 sr and 143 nd 144 nd of the ice core dust has been compared with the isotopic composition of the potential source areas antarctica new zealand southern africa australia and south america this comparison reveals the southern south american provenance of the dust for all three glacial periods as has already been shown for the lgm 1 we show that the patagonian loess and the marine shelf sediments from the argentine continental shelf which was variably emerged during glacial periods display different isotopic compositions and that the composition of the patagonian loess best matches the signature of the ice core dust the identification of the patagonian region as the source of the windblown dust deposited over east antarctica during all the latest glacial periods permits a better understanding of paleo atmospheric circulations of the last climatic cycles and a better constraint on the parametrization of dust sources for paleoatmospheric general circulation models

glacial interglacial changes in mineral dust and sea salt records in polar ice cores sources transport and deposition 1 sea salt and mineral dust records as represented by na and ca2 concentrations respectively in greenland and antarctic ice cores show pronounced glacial interglacial variations for the last glacial maximum lgm mineral dust sea salt concentrations in greenland show an increase of a factor of approximately 80 15 compared to the holocene and significant shifts by a factor of 15 5 during dansgaard oeschger events in antarctica the dust sea salt flux is enhanced by a factor of 15 3 during the lgm compared to the holocene and variations by approximately a factor of 8 1 2 exist in parallel to antarctic warm events primary glacial dust sources are the asian deserts for greenland and patagonia for antarctica ice core evidence and model results show that both changes in source strength as well as atmospheric transport and lifetime contributed to the observed changes in greenland ice cores in antarctica changes in ice core fluxes are in large parts related to source variations both for sea salt and dust where the formation of sea salt aerosol from sea ice may play a pivotal role summarizing our latest estimates on changes in sources transport and deposition these processes are roughly able to explain the glacial increase in sea salt in both polar regions while they fall short by at least a factor of 4 7 for mineral dust future improvements in model resolution and in the formulation of source and transport processes together with new ice core records e g on dust size distributions will eventually allow convergence of models and observations

model based constraints on interpreting 20th century trends in ice core 10be 1 beryllium 10 ice core records are useful for understanding solar magnetic field changes over time and in particular over the 20th century during which there are a variety of relevant observations however differences between 10be snow concentration records from different locations complicate the process of developing a coherent understanding of changes in cosmogenic isotope production we use the goddard institute for space studies modele general circulation model to simulate the production and transport of beryllium isotopes for this time period we compare our results with surface air observations and with ice core records from dye 3 taylor dome and south pole we find that unforced weather related internal variability causes modeled trends in 10be snow concentration to vary from the ensemble mean by 50 and greater at all three ice core locations lower levels of internal variability at taylor dome and south pole relative to dye 3 make the simulated 10be values at these locations better estimates of the ensemble mean trend in 10be snow concentration in addition the ensemble mean concentration trend at dye 3 was significantly different from the expected modeled trend based on applied production changes alone overall the results imply that during the 20th century 10be data from multiple cores are likely to be required to make meaningful inferences about 10be production changes the model simulations imply that data from antarctica are likely to be more robust

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

asian provenance of glacial dust stage 2 in the greenland ice sheet project 2 ice core summit greenland samples of dust from the greenland ice sheet project 2 gisp2 ice core summit greenland dated within marine isotope stage 2 between 23 340 and 26 180 calendar years b p around the time of the coldest local last glacial temperatures have been analyzed to determine their provenance to accomplish this we have compared them with approximately coeval aeolian sediments mostly loesses sampled in possible source areas psas from around the northern hemisphere the 5 m grain size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale which was sufficiently fine grained to be transported over long distances on the basis of comparison of the clay mineralogy and sr nd and pb isotope composition with ice dust and psas and assuming that we have sampled the most important psas we have determined that the probable source area of these gisp2 dusts was in eastern asia the dust was not derived from either the midcontinental united states or the sahara two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling except for a brief period during an interstadial when dust transport was exceptionally low for glacial times and had a mineralogical composition indicative of a slightly more southern provenance the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size this includes the high dust period that correlates with the heinrich 2 period of major iceberg discharge into the north atlantic variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size

constraining recent lead pollution sources in the north pacific using ice core stable lead isotopes 1 trends and sources of lead pb aerosol pollution in the north pacific rim of north america from 1850 to 2001 are investigated using a high resolution subannual to annual ice core record recovered from eclipse icefield 3017 masl st elias mountains canada beginning in the early 1940s increasing pb concentration at eclipse icefield occurs coevally with anthropogenic pb deposition in central greenland suggesting that north american pb pollution may have been in part or wholly responsible in both regions isotopic ratios 208 pb 207 pb and 206 pb 207 pb from 1970 to 2001 confirm that a portion of the pb deposited at eclipse icefield is anthropogenic and that it represents a variable mixture of east asian chinese and japanese emissions transported eastward across the pacific ocean and a north american component resulting from transient meridional atmospheric flow based on comparison with source material pb isotope ratios chinese and north american coal combustion have likely been the primary sources of eclipse icefield pb over the 1970 2001 time period the eclipse icefield pb isotope composition also implies that the north pacific mid troposphere is not directly impacted by transpolar atmospheric flow from europe annually averaged pb concentrations in the eclipse icefield ice core record show no long term trend during 1970 2001 however increasing 208 pb 207 pb and decreasing 206 pb 207 pb ratios reflect the progressive east asian industrialization and increase in asian pollutant outflow the post 1970 decrease in north american pb emissions is likely necessary to explain the eclipse icefield pb concentration time series when compared with low lichen and high mt logan ice core elevation pb data the eclipse ice core record suggests a gradual increase in pollutant deposition and stronger trans pacific asian contribution with rising elevation in the mountains of the north pacific rim

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

evidence for two intervals of enhanced 10 be deposition in antarctic ice during the last glacial period we have previously reported concentration profiles of cosmic ray produced cosmogenic 10be in deep ice cores from dome c and vostok antarctica1 2 in both these cores we found a concentration of 10be approximately two times larger in ice from the late glacial period than in the holocene ice we interpreted this as probably resulting from a lower precipitation rate on the antarctic plateau during glacial periods compared to interglacial periods in the vostok profile there was one sample corresponding to 60 000 yr bp which gave an unusually large 10be concentration not correlated with any obvious climatic event we suggested that this sample might be reflecting increased 10be production as for example during a period of reduced solar modulation2 we have now measured a much more detailed concentration profile for 10be in the vostok core the results confirm a 10be peak lasting 1 000 2 000 years at 60 000 yr bp and show another similar peak at 35 000 yr bp we have also observed the latter peak in the dome c core we discuss possible sources for these peaks and their potential as stratigraphic markers

global mapping of crustal and lithospheric thickness on venus 1 we have produced a high resolution global admittance map of venus using a spatiospectral localization technique calculating 64 800 admittance spectra evenly spaced in latitude and longitude we grouped the admittance spectra into 35 classes that we inverted for elastic crustal and lithospheric thickness using bottom loading top loading and short wavelength top loading models with long wavelength bottom loading hot spot models our best fit models identify 17 classes consistent with top loading 40 of the surface 7 with bottom loading 39 and 4 with a long wavelength bottom loading model 15 the remaining seven classes 6 of the surface generally had large amplitude top loading signatures that could not be fit with our models for the classes that were successfully modeled the estimated range of crustal 0 90 km and elastic thickness 0 100 km is larger than results for most previous studies primarily because of the incorporation of bottom loading nearly half of the planet has elastic thickness 20 km which we cannot distinguish from isostasy suggesting that these regions are tectonically inactive a key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions both the scale and range of crustal and elastic thickness variations indicate that venus remains an active planet and that geologic processes may be more complex than previously recognized in many regions the correlation with surface geology is not obvious and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation

continuous record of microparticle concentration and size distribution in the central greenland ngrip ice core during the last glacial period 1 a novel laser microparticle detector used in conjunction with continuous sample melting has provided a more than 1500 m long record of particle concentration and size distribution of the ngrip ice core covering continuously the period approximately from 9 5 100 kyr before present measurements were at 1 65 m depth resolution corresponding to approximately 35 200 yr particle concentration increased by a factor of 100 in the last glacial maximum lgm compared to the preboreal and sharp variations of concentration occurred synchronously with rapid changes in the 18o temperature proxy the lognormal mode of the volume distribution shows clear systematic variations with smaller modes during warmer climates and coarser modes during colder periods we find 1 7 m diameter during lgm and 1 3 m during the preboreal on timescales below several 100 years and the particle concentration exhibit a certain degree of independence present especially during warm periods when generally is more variable using highly simplifying considerations for atmospheric transport and deposition of particles we infer that 1 the observed changes of in the ice largely reflect changes in the size of airborne particles above the ice sheet and 2 changes of are indicative of changes in long range atmospheric transport time from the observed size changes we estimate shorter transit times by roughly 25 during lgm compared to the preboreal the associated particle concentration increase from more efficient long range transport is estimated to less than one order of magnitude

tree rings and ice cores reveal c 14 calibration uncertainties during the younger dryas the younger dryas interval during the last glacial termination was an abrupt return to glacial like conditions punctuating the transition to a warmer interglacial climate despite recent advances in the layer counting of ice core records of the termination the timing and length of the younger dryas remain controversial also a steep rise in the concentration of atmospheric radiocarbon at the onset of the interval recorded primarily in the cariaco basin has been difficult to reconcile with simulations of the younger dryas carbon cycle here we discuss a radiocarbon chronology from a tree ring record covering the late glacial period that has not been absolutely dated we correlate the chronology to ice core timescales using the common cosmic production signal in tree ring 14c and ice core 10be concentrations the results of this correlation suggest that the cariaco record may be biased by changes in the concentration of radiocarbon in the upper ocean during the early phase of the younger dryas climate reversal in the cariaco basin this bias in the marine record may also affect the accuracy of a widely used radiocarbon calibration curve over this interval our tree ring based radiocarbon record is easily reconciled with simulated production rates and carbon cycle changes associated with reduced ocean ventilation during the younger dryas attaching a floating tree ring chronology to ice core records that cover the abrupt younger dryas cold interval during the last glacial termination provides a better estimate of the onset and duration of the radiocarbon anomaly the chronology suggests that marine records may be biased by changes in the concentration of radiocarbon in the ocean which may affect the accuracy of a popular radiocarbon calibration program during this interval

changes in continental and sea salt atmospheric loadings in central greenland during the most recent deglaciation model based estimates by fitting a very simple atmospheric impurity model to high resolution data on ice accumulation and contaminant fluxes in the gisp2 ice core we have estimated changes in the atmospheric concentrations of soluble major ions insoluble particulates and 10 be during the transition from glacial to holocene conditions for many species changes in concentration in the ice typically overestimate atmospheric changes and changes in flux to the ice typically underestimate atmospheric changes because times of increased atmospheric contaminant loading are also times of reduced snowfall the model interpolates between the flux and concentration records by explicitly allowing for wet and dry deposition processes compared to the warm preboreal that followed we estimate that the atmosphere over greenland sampled by snow accumulated during the younger dry as cold event contained on average four seven times the insoluble particulates and nearly seven times the soluble calcium derived from continental sources and about three times the sea salt but only slightly more cosmogenic 10 be

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

spatial variation of englacial radar attenuation modeling approach and application to the vostok flowline 1 constraining the spatial variation of englacial radar attenuation is critical for accurate inference of the spatial variation of the englacial and basal properties of ice sheets from radar returned power here we evaluate attenuation models that account for spatial variations in ice temperature and chemistry and test them along the flowline that passes through the vostok ice core site antarctica the simplest model often used but rarely valid assumes a uniform attenuation rate everywhere along the flowline so that total attenuation is proportional to ice thickness the next simplest model uses spatially varying temperatures predicted by an ice flow model and assumes uniform chemistry additional models account for spatially varying chemistry using englacial stratigraphy we find that the roundtrip attenuation to the bed can easily differ by 10 db or more between the uniform attenuation rate model and models that account for variable ice temperature such differences are sufficient to confound the delineation of dry and wet beds also including spatial variations in chemistry produces smaller differences 10 db but the magnitude of these differences depends on the relative importance of dry and wet deposition of impurities in the past accounting for dry deposited impurities requires ice flow modeling and results in larger differences from all other models which assume uniform chemistry or wet deposition only these results indicate that modeling the spatial variation of attenuation requires a spatially varying temperature model in order to infer bed conditions from bed returned power accurately and that both ice core data and radar stratigraphy are also strongly desirable

paleoclimatic tracers an investigation using an atmospheric general circulation model under ice age conditions 2 water isotopes the linear relationship observed between the water isotopic contents of precipitation and surface air temperatures leads to the use of the water isotopes h218o and hdo in paleoclimatology applied to the measurements of the isotopic content of paleowaters like groundwaters and deep ice cores this relationship is used to infer paleotemperatures however this interpretation of paleo isotopic contents is only valid if the isotope temperature relationship is not affected by climate change to address this problem we have developed a water isotope modeling inside an atmospheric general circulation model agcm and performed simulations of both the present day and last glacial maximum lgm climatic conditions agcms are indeed the only appropriate tools able to account the whole complexity of the atmospheric circulation for the present day climate preliminary results for january were presented by joussaume et al 1984 and are complemented by new simulations performed for both february and august climatic conditions with a higher resolution version of the model model results are well corroborated by observations they also exhibit some effects of the atmospheric circulation on the isotopic fields for the simulated lgm climate the model results compare well with paleoclimatic data of water isotopic contents except for a higher than observed spatial variability the overall patterns of the simulated 18o temperature relationship for the lgm climate are practically unchanged which tends to comfort the use of water isotopes in paleoclimatology however concerning the deuterium excess i e the relationship between oxygen 18 and deuterium the model results are not sufficiently valid to allow a discussion of the use of deuterium excess in paleoclimatology

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes