sliding displacements due to subduction zone earthquakes abstract empirical studies of earthquake ground motions have developed relations between sliding displacement and acceleration ratio and other parameters such as arias intensity computations using strong motion records from the maule 2010 chile m 8 8 earthquake indicate that the published relations do not conform well to the computed displacements and some tend to be unconservative extensions to the empirical equations incorporating arias intensity are if anything less accurate these results suggest that these empirical relations may not apply to subduction zone events and indicate that further study using records from other recent subduction zone events is appropriate examining the analytical solutions for sliding displacements induced by sinusoidal shaking and the definition of arias intensity leads to an improved normalization for sliding displacements when this improved normalization is applied to the records from three different earthquakes in different parts of chile the results are nearly identical and the results for the chi chi and northridge earthquakes are very close to those from the chilean events suggestions for practical use of the new normalization relations are provided

spatial variability of antarctic peninsula net surface mass balance 1 measurements from ice cores and snow pits collected over the last 50 years are used to examine how net surface mass balance varies across the antarctic peninsula to give the first detailed map of mass balance for the region a total of 211 reliable mass balance measurements were available for the preparation of the map but some areas were found to be very data sparse the analysis suggests that the largest values of mass balance are found along the spine of the northern part of the peninsula where over 2 5 m yr 1 water equivalent we has been measured a secondary peak of more than 2 0 m yr 1 we is determined along the mountains of eastern alexander island precipitation minus evaporation p e fields from the european centre for medium range weather forecasts reanalysis project are compared with our analysis of in situ data the model fields are found to have peak values of p e of only half the amounts found from the measurements the greatest model values are located on the western side of the peninsula areas where a high density of in situ data is available including king george vi sound and the high south central plateau part of the peninsula show a high spatial variability of net surface mass balance suggesting that local orographic features play a major part in dictating the mass balance

investigations of the oxygen 18 content of samples from snow pits and ice cores from the filchner ronne ice shelves and ekstrom ice shelf

rational selection of critical acceleration factors for sliding stability abstract the horizontal seismic loading coefficient is an essential input in evaluating the seismic adequacy of slopes such as those in open pit mines and natural slopes in some cases the coefficient is established through dynamic finite element analyses which are time consuming and require a new analysis for each facility including a new suite of accelerograms the values of the coefficient are sometimes incorporated in design manuals but the procedures for establishing the values are seldom transparent the usual situation is that the values arise from consensus experience and previous practice in this paper the urzua christian model for normalized sliding displacement has been extended to develop the critical acceleration value corresponding to the probability of observing prescribed amounts of sliding displacement the method has been applied to two sets of data based on probabilistic seismic hazard analyses the results show that to satisfy the criterion that there must be 0 1 probability of the sliding displacement exceeding 100 cm if a maximum credible earthquake mce occurs the critical acceleration must be approximately 0 13 g this means that a slope with these parameters in this environment must be stable enough that a horizontal acceleration of 0 13 g is necessary to put it in a state of sliding motion in the case of the operational basis earthquake obe which is a much smaller ground motion the criterion of 0 1 probability of 100 cm of sliding is achieved for a slope with a critical acceleration of 0 35 g

possible linkages of late holocene drought in the north american midcontinent to pacific decadal oscillation and solar activity oxygen isotopic composition d 18 o the d 18 o record of the past 3100 years reveals that droughts of greater severity and duration than during the 20th century occurred repeatedly especially prior to 300 ad drought variability was anomalously low during the 20th century 90 of the variability values during the last 3100 years were greater than the 20th century average d 18 o is strongly correlated with the index of the pacific decadal oscillation pdo during the past 100 years and periodicities of the lateholocene d 18 o record are similar to those of the pdo furthermore time series of d 18 o and atmospheric d 14 ca re generally coherent after 700 ad both the pacific climate and solar irradiance probably played a role in drought occurrence but their effects were non stationary through the late holocene citation tian j d m nelson and f s hu 2006 possible linkages of late holocene drought in the north american midcontinent to pacific decadal oscillation and solar activity geophys res lett 33 l23702 doi 10 1029 2006gl028169

hydroclimatic change in southern manitoba since a d 1409 inferred from tree rings abstract a record of estimated annual prior august to current july precipitation derived from a regional bur oak quercus macrocarpa michx tree ring chronology indicates that southern manitoba s hydroclimate has been relatively stable over the last 200 yr although this stability was interrupted briefly by pronounced wet intervals in the late a d 1820s and 1850s hydroclimatic conditions since permanent euro canadian settlement were much less variable and persistent than those prior to a d 1790 the reconstruction indicates that the red river basin experienced extremely dry conditions between a d 1670 and 1775 with below normal precipitation occurring approximately 2 years out of 3 annual precipitation was estimated at more than two standard deviations below the mean during a d 1477 1485 1556 1595 1612 1644 1661 1743 1900 and 1980 comparisons with limnological records from north dakota and minnesota suggest that multidecadal fluctuations in regional hydroclimate have been remarkably coherent across the northeastern great plains during the last 600 yr however individual dry years in the red river basin were usually associated with larger scale drought across much of the north american interior

growth of greenland ice sheet interpretation an observed 0 23 m year thickening of the greenland ice sheet indicates a 25 to 45 excess ice accumulation over the amount required to balance the outward ice flow the implied global sea level depletion is 0 2 to 0 4 mm year depending on whether the thickening is only recent 5 to 10 years or longer term 100 years if there is a similar imbalance in the northern 60 of the ice sheet area the depletion is 0 35 to 0 7 mm year increasing ice thickness suggests that the precipitation is higher than the long term average higher precipitation may be a characteristic of warmer climates in polar regions

oxygen isotope and total beta radioactivity measurements on 10 m ice cores from the antarctic peninsula as part of the glaciology of the antarctic peninsula gap programme the snow cover at 25 stations was sampled to 10 m depth for oxygen isotope and total radioactivity analysis the mean annual oxygen isotope ratio correlates satisfactorily with 10 m temperature despite the complex topography of the area and suggests on average that climatic trends in the region are fairly systematic the relationship with temperature is similar to that derived from a simple model in which an air mass initially of maritime subtropical characteristics is progressively cooled as it moves towards the region the detailed isotope profiles show that for future deep drilling the most easily interpretable climatic information will be found in the more continental areas on the cast coast and on the plateau in the south of the region the degree of continentality of particular sites is reflected in the amplitude of the annual wave in the upper portion of the isotope ratio profile as well as in the accumulation rate

ice thickness and bedrock elevation in western neuschwabenland and berkner island antarctica in 1985 86 western neuschwabenland was surveyed with an electromagnetic reflection system emr over flight tracks totalling 6100 km the area spans a triangle from ekstrom ice shelf and heimefront range to the kraul mountains vestfjella and covers 48 000 km2 data on surface elevation ice thickness and bedrock topography are presented in maps the bedrock elevation map shows more detail than earlier compilations with a remarkable graben like structure north of the heimefront range and large areas of bedrock below sea level in the same season tracks totalling 1600 km were flown over berkner island high electromagnetic reflectivity was found over the southern part of berkner island whereas the northern part showed considerably lower reflectivity we believe that this is due to melting conditions in the southern part and freezing conditions in the northern part

the antarctic ice sheet its history and response to sea level and climatic changes over the past 100 million years abstract the development surface form flow and erosive activity of the antarctic ice sheet over the past 100 million years is related to geological evidence that indicates long term trends of global climate the glaciological description of the evolution of the ice sheet covers its interactions with the atmosphere and with the ocean discussion focuses on changes of the equilibrium line altitude ela with time of the accumulation rate with global temperature and basal melting of ice shelves by ocean waters the tendency of global climate to change rapidly or to drift slowly between two near equilibrium states involves two types of changes the former that has been widely recognized is reversible a second type involving ice sheets or ice shelves appears to be almost irreversible since they remain in their changed state unless a much larger reversal of factors driving climate occurs we refer to any rapid but reversible changes as climatic jumps and to the latter as climate ice switches variability of climate in response to changing radiation with the earth s orbital parameters and other causes is included in our model extensive glaciation of east antarctica commenced after 40 m y b p with wide fluctuations until shortly after 20 m y b p a climate ice switch then prevented any major deglaciation a further climate ice switch associated with the ela falling below sea level occurred soon after 10 m y b p this led to rapid formation of ice shelves and the marine ice sheet of west antarctica which advanced as grounded ice to the edge of the continental shelf after 5 m y b p the grounded ice started to retreat extensive ice shelves remained and most glacial debris was then deposited beneath the inner regions of ice shelves response to northern hemisphere glaciation since 2 4 m y b p during glacial periods involved extension of the grounded ice area a large decrease of mass input and decreased melting of outer parts of large ice shelves while the melting rate beneath inner regions was probably similar to the present marine core evidence suggests that retreat caused by global cooling has been dominant since 5 m y b p and that changes in extent due to northern hemisphere glaciation since 2 4 m y b p have had different effects on the major and on smaller northerly ice shelves

pseudostatic coefficient for use in simplified seismic slope stability evaluation pseudostatic slope stability procedures are commonly used in engineering practice however the selection of the seismic coefficient employed in the analysis is often based on precedence without due consideration of the amount of seismic displacement that constitutes satisfactory performance for each particular project and without incorporating the vastly different seismic exposure for sites around the world in this note a rational basis for selecting the seismic coefficient is presented the proposed procedure requires that the engineer establishes the project specific allowable level of seismic displacement the seismic response characteristics of the slope are represented by the fundamental period of the potential sliding mass and the site dependent seismic demand is characterized by the 5 damped elastic design spectral acceleration at the degraded period of the potential sliding mass the level of uncertainty in the estimates of the seismic demand and displacement can be handled through the use of different percentile estimates of these values with the proposed equations the engineer can properly incorporate the amount of seismic displacement judged to be allowable and the seismic hazard at the site in the selection of the seismic coefficient

a dendroclimatic reconstruction of annual precipitation on the western canadian prairies since a d 1505 from pinus flexilis james abstract tree ring chronologies from living and sub fossil pinus flexilis james limber pine trees extend back more than 500 yr on the western edge of the great plains in alberta living trees were found to be as old as 526 yr tree ring growth in pinus flexilis is most sensitive to annual precipitation over the annualization period august to july the longevity of the trees coupled with their sensitivity to variations in annual precipitation make them an important resource for constructing annually resolved multicentury records of paleohydrology on the northwestern great plains a 487 yr reconstruction of annual precipitation in southwestern alberta was produced from the tree ring series this is the longest dendroclimatological reconstruction of precipitation available from the northern plains the reconstruction has some similarity to reconstructions from the western great plains of the united states however there are also many divergences in the timing of droughts some of this divergence may reflect the importance of orographic uplift in producing precipitation along the eastern flanks of the rocky mountains our reconstruction shows that the frequency of droughts in alberta during the period of instrumental records about the last 100 yr has not been appreciably different from conditions of the preceding four centuries in addition the most severe drought of this century which led to widespread farm abandonment between 1918 and 1922 was not the most severe drought in the past 487 yr

a multiproxy reconstruction of hebridean nw scotland spring sea surface temperatures between ad 1805 and 2010 there is currently a deficiency of annually resolved temperature series from the marine environment we present a multiproxy reconstruction of hebridean shelf sea tiree passage nw scotland spring sea surface temperatures ssts for the period ad 1805 2010 the reconstruction is based on the growth increment series from the first absolutely dated annually resolved multi centennial glycymeris glycymeris bivalve mollusc sclerochronology coupled with previously published stable oxygen isotope data 18o from benthic foraminifera sampled from a dated sediment core from nearby loch sunart the independent series contain significant correlations with ssts across complementary frequency domains the low frequency component of the sedimentary archive was combined with the mid and high frequency components of the g glycymeris chronology indices to create a single multiproxy series split calibration verification statistics reduction of error re coefficient of efficiency ce and r2 indicate that the multiproxy record calibrated to local instrumental sea surface temperatures contains significant precision and skill at reconstructing spring ssts re 0 59 ce 0 26 r2 0 54 these data demonstrate that bivalve sclerochronologies when combined with low frequency proxies such as sediment archives can facilitate statistically robust reconstructions of palaeoceanographic variability during the late holocene for hydrographically significant regions of the temperate marine system previously void of annually resolved archives the reconstructed ssts contain a general warming trend of 0 60 0 14 c per century only four years in the reconstructed period 1999 2000 2002 and 2003 exceed temperatures greater than two standard deviations higher than the reconstructed mean sst 9 03 c whilst just three years in the first half of the 19th century 1835 1838 and 1840 fall more than 2 below the reconstructed mean 6 80 c

methods for assessing the stability of slopes during earthquakes a retrospective abstract during the twentieth century several methods to assess the stability of slopes during earthquakes were developed pseudostatic analysis was the earliest method it involved simply adding a permanent body force representing the earthquake shaking to a static limit equilibrium analysis stress deformation analysis a later development involved much more complex modeling of slopes using a mesh in which the internal stresses and strains within elements are computed based on the applied external loads including gravity and seismic loads stress deformation analysis provided the most realistic model of slope behavior but it is very complex and requires a high density of high quality soil property data as well as an accurate model of soil behavior in 1965 newmark developed a method that effectively bridges the gap between these two types of analysis his sliding block model is easy to apply and provides a useful index of co seismic slope performance subsequent modifications to sliding block analysis have made it applicable to a wider range of landslide types sliding block analysis provides perhaps the greatest utility of all the types of analysis it is far easier to apply than stress deformation analysis and it yields much more useful information than does pseudostatic analysis

late 18th century drought induced sand dune activity great sand hills saskatchewan geomorphic evidence and optical ages from seven locations indicate that widespread dune activity occurred within the last 200 years in the great sand hills region of southwestern saskatchewan optical ages n 36 define an interval of dune activity bracketed by the earliest age of back ridges in the seward sand hills 185 8 years and the average age of stabilized dune heads about 105 years during this interval parabolic dunes were active in all areas studied these ages indicate that the most recent interval of activity was initiated about ad 1800 and continued at a level higher than present for approximately 80 years the most likely cause of dune activation was lower than average precipitation relative to 1960 1991 values through the 1700s culminating in drought in the late 1700s as evidenced in dendroclimatic records from the cypress hills and from the rocky mountain foothills dunes affected by such climatically induced regional activity require many decades to restabilize historical ob

dependence of antarctic surface mass balance on temperature elevation and distance to open ocean the latest compilations of surface mass balance mean annual surface temperature and elevation for the antarctic ice sheet derived from data sets of approximately 1500 700 and 105 points respectively have been used to obtain areally integrated means for 24 ice drainage systems and 329 grid point values covering the whole ice sheet monthly summaries of remotely sensed sea ice data for 1973 1976 have been used to obtain mean annual distance to open ocean linear and second order regression analyses of surface balance on 1 temperature for the entire ice sheet 2 elevation for the conterminous grounded ice and 3 distance to the open ocean for the ice shelves including ice rises and attached islands have been made using both system means and grid point values these analyses show correlation coefficients of between 0 63 and 0 81 and they provide bases for descriptive models of the present antarctic ice sheet as well as for predictive models of the response of the ice sheet to air temperature changes and variations in meridional mass and energy transfers extrapolation to other ice sheets past and present may be possible but should be made cautiously linear models are recommended for paleoclimatic reconstructions of ice sheets in upper mid latitudes and second order models are recommended for those in high latitudes system means are shown to be reliable for these purposes incidental results are new estimates of the mean annual surface temperature for the whole ice sheet 36 c and mean surface elevation for the conterminous grounded ice 2290 m

shallow firn cores from neumayer ekstromisen antarctica a comparison of accumulation rates and stable isotope ratios since 1979 80 glaciological studies have been carried out at ekstromisen antarctica including accumulation stake measurements snow pit and shallow firn core studies snow stratigraphy chemical properties and stable isotope ratios 18 o were investigated this study focuses on three cores taken between 1982 and 1998 the 1998 core was dated using dielectric profiling 18 o profiles and stake measurements accumulation rates show high interannual and spatial variability due to the extreme wind influence no significant trend was found for the last 50 years during the first half of the 20th century accumulation decreased the high spatial and interannual variability however means that trends must be interpreted with care in spite of the highly irregular accumulation distribution stable isotope ratios show little spatial variability the mean annual 18 o values of cores b04 and fb0198 agree fairly well for the time period 1955 82 covered by both cores 18 o values have increased during most of the 20th century since the late 1980s a decrease is observed this change is not related to air temperature since mean annual air temperatures at neumayer show no significant trend over the last two decades

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

chronology and geochemistry of late holocene eolian deposits in the brandon sand hills manitoba canada abstract accelerator mass spectrometry and conventional radiocarbon age determinations of organic matter from paleosols indicate that the brandon sand hills area of southern manitoba has been subjected to recurrent intervals of eolian activity in the past 5000 years although precise regional correlations are precluded by dating uncertainties periods of most notable paleosol development occurred around 2300 to 2000 1400 to 1000 and 600 to 500 cal yr bp with eolian activity occurring before and after each of these periods episodes of eolian activity may correspond to periods of regional drought whereas paleosols mark periods of increased moisture availability and stabilization by vegetation the geochemistry of the eolian sands paleosols and source sediments indicates that partial leaching of carbonates occurs from pedogenesis during humid climatic phases and that this is probably the primary mechanism of carbonate depletion of eolian sands in this area recent trends in sand dune activity from historic aerial photography and early explorers accounts indicate that the few active dunes that presently exist have stabilized at a rate of 10 20 per decade despite several severe droughts in the 20th century this may be attributed to pre settlement droughts that were more severe than those in historic times although regional dune stabilization may also be related in part to the spread of forest cover in the past few hundred years

a century of climate variability in central dronning maud land east antarctica and its relation to southern annular mode and el nino southern oscillation stable isotope records of oxygen and hydrogen were studied from a 65 m long ice core retrieved from the central dronning maud land east antarctica in order to reconstruct the coastal antarctic climate variability during the last century and its relation to the southern annular mode sam and el nino southern oscillation enso the 18 o records showed a significant relation to the sam with a dominant 4 years variability except during specific periods 1918 1927 19381947 1989 2005 when enso teleconnection was established through the in phase relation between sam and southern oscillation index soi the combined influence of enso and sam was seen on surface air temperatures in this region mainly during the austral summer season from 1989 to 2005 further a significant relationship between 18 o and sam was observed on a decadal scale which overrides the intermittent influence of enso major shifts in the deuterium excess record were observed during periods of enso teleconnections which support a shift in moisture source regions during the periods of stronger enso teleconnections surface air temperatures estimated using the 18 o t spatial slope for this region depicted a significant warming of 1 c for the past century the study reveals that throughout the last century sam was the dominant mode of climatic variability in the coastal region of central dronning maud land on a decadal scale

the pico lightweight coring auger on decrit un nouveau carottier manuel leger pour le neve et la glace l utilisation de composes verre resine permet d obtenir un poids de l ordre de 1 kgm 1 pour des carottiers de 20 m et 50 m

multi proxy holocene palaeoclimatic record from a saline lake in the canadian subarctic multi proxy palaeolimnological analyses of a postglacial sedimentary sequence at a centennial scale resolution from an athalassic saline lake in the yukon were conducted to infer patterns of holocene climatic change in the canadian subarctic using sediment mineralogy and biostratigraphy diatoms pigments diatominferred quantitative estimates of palaeosalinity were obtained by use of transfer functions developed from a calibration set of 219 lakes from western north america the sediment mineralogy and fossil pigments at the base of the core indicated a moderately deep non stratified lake dominated by clastic influx probably in a basin fed by glacial meltwater the early holocene history c 11 000 8100 14c yr bp was characterized by a relatively deep mesosaline lake with diatom inferred salinities approximating 20 g l 1 the occurrence of both aragonite and dolomite as well as elevated concentrations of chlorophyll and carotenoid pigments support the interpretation of deepwater anoxia and possibly

subglacial preservation of valley morphology at amundsenisen western dronning maud land antarctica on the high altitude polar plateau of amundsenisen western dronning maud land east antarctica a subglacial valley with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta was studied by radio echo sounding in addition a small probably glacial valley was mapped within the same subglacial massif basal ice temperatures were calculated using field data on precipitation air temperature and ice sheet thickness discoveries of old landforms which have been preserved more or less intact beneath the former fennoscandian and laurentide ice sheets have received increasing attention during the last decade the aim of this study is to investigate whether preservation of landforms occurs under the east antarctic ice sheet and to discuss under that climatological and glaciological circumstances preservation may take place the results show that the ice sheet covering the investigated localities is frozen to bed and therefore has an insignificant erosional capability the observations suggest that a large scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold based ice sheet marginal zone for a long time period the process of glacial preservation recognized for bedrock features and tentatively observed for sediment accumulations should act on similar large scale landforms under any cold based ice sheet present or past on the basis of existing studies of the age and stability of the east antarctic ice sheet a middle pliocene age is suggested for the preserved landforms the presence of the presumed sediment filled valley further indicates that no prolonged periods of basal melting have occurred at the amundsenisen study area during the ice sheet history which includes the quaternary glaciation periods finally calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation 1997 by john wiley sons ltd

spatial variability at shallow snow layer depths in central dronning maud land east antarctica spatial variability at swallow snow layer depths in central dronning maud land east antarctic

holocene fluctuations of a meromictic lake in southern british columbia abstract holocene deposits of mahoney lake a meromictic saline lake located in a closed basin in the semi arid okanagan valley contain evidence of frequent and marked changes in lake depth up to 12 m 10014c yr probably caused by short term changes in effective precipitation we studied a 5 45 m long core comprising a basal layer of inorganic mud overlain by a succession of layers of calcareous laminated and nonlaminated organic mud marl and sand we used mazama tephra to adjust nine radiocarbon ages for the hardwater effect meromixis developed ca 900014c yr b p and the lake has been episodically meromictic for about half the time since because of close linkages between sediments and depositional environments in meromictic and saline lakes we infer that laminated sediments indicate meromictic conditions and high lake levels ca 12 m water depth whereas thick marl layers and nonlaminated sediments indicate nonmeromictic conditions and thus low lake levels

accumulation and ice core studies on filchner ronne ice shelf antarctica accumulation rates in the eastern part of ronne ice shelf were determined by isotopic stratigraphy ebo the samples were taken from snow pits dug during the filchner i and ii operations in 1984 and 1986 in general the accumulation rate decreases towards the south the greatest decrease from 21 3 to 13 3 g cm 2 a 1 was observed between filchner station and measuring point 341 sited 270 km up stream of the ice edge the slbo values of the near surface layers vary between 25 and 29 0 the 1bo content in the more southerly part is progressively depleted in the direction of mollereisstrom paralleling a decrease in the accumulation rate near the ice edge the 1bo content decreases to the west a 100 m ice core drilled in 1984 at point 340 220 km from the ice edge probably goes back to a d 1460 it has been dated by isotopic stratigraphy the accumulation rate up stream of the drilling site was deduced from the sequence of annual layers using a simple ice flow model the accumulation shows strong variations over the last 200 years which may be caused in part by local variations in the accumulation on ronne ice shelf

origins of the 1988 north american drought the 1988 summer drought in the united states was the most extensive in many years because the drought developed in different places at different times not all regional effects can be traced to the same cause along the west coast and in the northwestern united states drought conditions developed during 1987 in association with the 1986 to 1987 el nino in the tropical pacific ocean record low rainfalls from april to june 1988 led to rapid development of drought in the north central united states strong anticyclonic conditions and a northward displaced jet stream in the upper atmosphere over north america throughout this period were only part of pronounced and distinctive wavetrain of anomalies in the atmospheric circulation that appeared to emanate from the tropical pacific below average sea surface temperatures along the equator in the pacific in the northern spring of 1988 combined with warmer than normal water from 10 degrees to 20 degrees n led to a northward displaced but still active intertropical convergence zone itcz southeast of hawaii results from a steady state planetarywave atmospheric model indicate that the atmospheric heating anomalies associated with the displaced itcz can force an anomalous wavetrain across north america similar to that observed land surface processes probably contributed to the severity and persistence of the drought however the large scale atmospheric circulation perturbations associated with natural variations in the coupled atmosphere ocean system in the tropical pacific were most likely the primary cause

formation of linear and parabolic dunes on the eastern snake river plain idaho in the nineteenth century abstract geomorphic and stratigraphic evidence show periods of distinct hydrologic excess and deficit sometime in the past 500 years on the eastern snake river plain natural exposures at the current shore of mud lake contain a lacustrine silty sand beneath eolian sands the latter deposit is expressed at the surface as hairpin parabolic and linear dunes these dunes can be traced into and across lakes a clear sign of recent aridity two shallow water gastropods and two wood fragments from the lacustrine silty sand yield the respective ams 14 c ages of ca 705 and 988 year bp and 195 and 305 year bp the offset in ages indicates a lake hard water effect of aquatic gastropods with the wood ages providing the closest age constraint optically stimulated luminescence single aliquot regeneration ages of 150 30 and 120 20 year bp on quartz grains from the overlying eolian sands further constrains a drop in lake level and subsequent aridity to the 19th century this lacustrine dune sequence is probably the geomorphic expression of a distinct wet period followed by a severe drought between ad 1845 and 1856 which has been documented in the dendroclimatic records for the rocky mountains and the western great plains lake fluctuations and dune formation is associated with greater subdecadal variability in spring precipitation than recorded in the past 100 years

the little ice age in the canadian rockies abstract this paper reviews the evidence and history of glacier fluctuations during the little ice age lia in the canadian rockies episodes of synchronous glacier advance occurred in the 12th 13th early 18th and throughout the 19th centuries regional ice cover was probably greatest in the mid 19th century although in places the early 18th century advance was more extensive glaciers have lost over 25 of their area in the 20th century selective preservation of the glacier record furnishes an incomplete chronology of events through the 14th 17th centuries in contrast varve sequences provide continuous annually resolved records of sediments for at least the last millennium in some highly glacierized catchments such records have been used to infer glacier fluctuations evaluation of recent proxy climate reconstructions derived from tree rings provides independent evidence of climate fluctuations over the last millennium most regional glacier advances follow periods of reduced summer temperatures reconstructed from tree rings particularly ca 1190 1250 1280 1340 1690s and the 1800s reconstructed periods of higher precipitation at banff alberta since 1500 are 1515 1550 1585 1610 1660 1680 and the 1880s glacier advances in the early 1700s the late 1800s and in places the 1950 1970s reflect both increased precipitation and reduced summer temperatures negative glacier mass balances from 1976 to 1995 were caused by decreased winter balances the glacier fluctuation record does not contain a simple climate signal it is a complex response to several interacting factors that operate at different timescales evaluation of climate proxies over the last millennium indicates continuous variability at several superimposed timescales dominated by decade century patterns only the 19th century shows a long interval of sustained cold summers this suggests that simplistic concepts of climate over this period should be abandoned and replaced with more realistic records based on continuous proxy data series the use of the term lia should be restricted to describing a period of extended glacier cover rather than being used to define a period with specific climate conditions

surface winds over the antarctic continent a review surface winds over antarctica often classified as katabatic are intimately related to the orientation and steepness of the underlying ice terrain such flows result primarily from the strong radiational cooling of the sloping ice surface which creates a downslope directed horizontal pressure gradient force it is possible to diagnose the time averaged near surface streamline patterns of cold air drainage currents using recently compiled detailed antarctic ice topography maps and appropriate estimates of the strength of the surface temperature inversion the resulting streamlines agree with available observations and illustrate the high degree of nonuniformity of gravity driven flows off the ice sheets in certain interior sections negatively buoyant air from a large catchment area drains through a restricted section of coastline such confluence zones represent regions of enhanced supplies of cold air and are shown to be related to anomalously strong katabatic winds implications for sea ice formation are discussed

climate driven hydrologic transients in lake sediment records multiproxy record of mid holocene drought abstract oxygen and carbon isotopes trace element concentrations and ostracode paleoecology are used with ground water flow modeling to assess the magnitude of climate driven hydrologic changes during mid holocene time for elk lake grant county minnesota during mid holocene time two extended droughts were recorded in the sediment record from elk lake at 5385 and 5221 calendar yr bp a drop in lake level to 14 9 m below the 1994 level and 10 0 m below the maximum 1930s decline at about 5385 calendar yr bp is interpreted for elk lake and its water table based on heterocypris fretensis occurrence and higher 18 o 13 c and mg ca values numerical modeling suggests this low water event could have resulted from conditions drier than those of the 1930s drought sustained over a time frame of approximately a century the later drought event at approximately 5221 calendar yr bp is marked by a shift in the dominant source of water to elk lake from surface run off to ground water from the till aquifer the shift in source water is identified by lower 18 o values and the presence of limnocythere staplini estimates of climatic sensitivity for holocene lakes should not be directly based on modern conditions but must take into account the changing shape of the lake basin through time and its effect on the paleohydrology

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential