carbonate deposition pyramid lake subbasin nevada 2 lake levels and polar jet stream positions reconstructed from radiocarbon ages and elevations of carbonates tufas deposited in the lahontan basin most of the tufas in the pyramid lake subbasin were deposited within the last 35 000 yr including most of the mound tufas that border the existing lake many of the older tufas 21 000 yr b p contained in the mounds were formed in association with ground water discharge the radiocarbon 14c ages of the older tufas represent maximum estimates of the time of their formation lake lahontan experienced large and abrupt rises in level at 22 000 15 000 and 11 000 yr b p and three abrupt recessions in level at 16 000 13 600 and 10 000 yr b p the lake level rises that were initiated at 23 500 and 15 500 yr b p are believed to indicate the passage of the polar jet stream over the lahontan basin during expansion of the laurentide ice sheet the jet stream moved south across the basin and during the contraction of the ice sheet the jet stream moved north across the basin the bulk of the carbonate contained in the mound tufas was deposited during the last major lake cycle 23 500 12 000 yr b p indicating that ground and surface water discharges increased at 23 500 and decreased at 12 000 yr b p a lake level oscillation that occurred between 11 000 and 10 000 yr b p is represented by a 2 cm thick layer of dense laminated tufa that occurs at and below 1180 m in the low elevation tufa mounds and at 1205 m in the winnemucca lake subbasin

reduced atlantic storminess during last glacial maximum evidence from a coupled climate model the last glacial maximum lgm 21 000 yr before present was the time of maximum land ice extent during the last ice age a recent simulation of the lgm climate by a state of the art fully coupled global climate model is shown to exhibit strong steady atmospheric jets and weak transient eddy activity in the atlantic sector compared to today s climate in contrast previous work based on uncoupled atmospheric model simulations has shown that the lgm jets and eddy activity in the atlantic sector are similar to those observed today with the main difference being a northeastward extension of their maxima the coupled model simulation is shown to agree more with paleoclimate proxy records and thus is taken as the more reliable representation of lgm climate the existence of this altered atmospheric circulation state during lgm in the model has implications for understanding the stability of glacial climates for the possibility of multiple atmospheric circulation regimes and for the interpretation of paleoclimate proxy records

late quaternary water level variations and vegetation history at crooked pond southeastern massachusetts abstract sediment cores collected along a transect in crooked pond southeastern massachusetts provide evidence of water level changes between 15 000 cal yr b p and present the extent of fine grained detrital organic accumulation in the basin inferred from sediment and pollen stratigraphies varied over time and indicates low water levels between 11 200 and 8000 cal yr b p and from ca 5300 to 3200 cal yr b p this history is consistent with the paleohydrology records from nearby makepeace cedar swamp and other sites from new england and eastern canada and with temporal patterns of regional changes in effective soil moisture inferred from pollen data the similarities among these records indicate that 1 regional conditions were drier than today when white pine pinus strobus grew abundantly in southern new england 11 200 to 9500 cal yr b p 2 higher moisture levels existed between 8000 and 5500 cal yr b p possibly caused by increased meridonal circulation as the influence of the laurentide ice sheet waned and 3 drier conditions possibly contributed to the regional decline in hemlock tsuga abundances at 5300 cal yr b p although sea level rise may have been an influence moist climatic conditions during the late holocene were the primary reason for a dramatic rise in water table elevations

causes of reduced north atlantic storm activity in a cam3 simulation of the last glacial maximum abstract the aim of this paper is to determine how an atmosphere with enhanced mean state baroclinity can support weaker baroclinic wave activity than an atmosphere with weak mean state baroclinity as a case study a last glacial maximum lgm model simulation previously documented to have reduced baroclinic storm activity relative to the modern day climate simulated by the same model despite having an enhanced midlatitude temperature gradient is considered several candidate mechanisms are evaluated to explain this apparent paradox a linear stability analysis is first performed on the jet in the modern day and the lgm simulation the latter has relatively strong barotropic velocity shear it was found that the lgm mean state is more unstable to baroclinic disturbances than the modern day mean state although the three dimensional jet structure does stabilize the lgm jet relative to the eady growth rate next feature tracking was used to assess the storm track seeding and temporal growth of distur

devils hole nevada δ18o record extended to the mid holocene abstract the mid to late pleistocene devils hole 18 o record has been extended from 60 000 to 4500 yr ago the new 18 o time series in conjunction with the one previously published is shown to be a proxy of pacific ocean sea surface temperature sst off the coast of california during marine oxygen isotope stages mis 2 and 6 the devil hole and sst time series exhibit a steady warming that began 5000 to 10 000 yr prior to the last and penultimate deglaciations several possible proximate causes for this early warming are evaluated the magnitude of the peak 18 o or sst during the last interglacial lig is significantly greater 1 per mill and 2 to 3 c respectively than the peak value of these parameters for the holocene in contrast benthic 18 o records of ice volume show only a few tenths per mill difference in the peak value for these interglacials statistical analysis provides an estimate of the large shared information variation between the devils hole and eastern pacific sst time series from 41 to 2 n and enforces the concept of a common forcing among all of these records the extended devils hole record adds to evidence of the importance of uplands bordering the eastern pacific as a source of archives for reconstructing pacific climate variability

pollen based evidence of extreme drought during the last glacial 32 6 9 0 ka in coastal southern california abstract high resolution pollen analyses of sediment core ledc10 1 from lake elsinore yield the first well dated terrestrial record of sub centennial scale ecologic change in coastal southern california between 32 and 9 ka in the lake elsinore watershed the initial mesic montane conifer forests dominated by pinus and cupressaceae with trace amounts of abies and picea were replaced by a sequence of multiple extended severe mega droughts between 27 5 and 25 5 ka in which halophytic and xerophytic herbs and shrubs occupied an ephemeral lake this prolonged and extended dry interval which corresponds with warm waters offshore imply strengthening of the north pacific high and persistent below average winter precipitation the subsequent contrasting monotonic occurrence of montane conifers reflects little variation in cold mesic climate until 15 ka postglacial development of quercus woodland and chaparral mark the return to more xeric warmer conditions at this time a brief reversal at 13 1 12 1 ka as reflected by an expansion of pinus is correlative with the younger dryas and interrupts development of warm postglacial climate subsequent gradual expansion of xeric vegetation post younger dryas denotes the establishment of a winter hydroclimate regime in coastal southern california that is more similar to modern conditions pollen based reconstructions of temperature and precipitation at lake elsinore are generally correlative with pollen based paleoclimatic reconstructions and foraminifera based sea surface temperatures from santa barbara basin in marine core odp 893 the conspicuous absence of the 27 5 25 5 ka glacial mega drought in the santa barbara basin pollen record highlights the sensitivity of lake elsinore to hydroclimate change and thus the importance of this new record that indicates that mega drought can occur during the full glacial when climatic boundary conditions and forcings differed substantially from the present

pleistocene megafloods in the northeast pacific massive discharges of freshwater from the glacial lake missoula to the northeast pacific ocean are thought to have sculpted the channeled scablands of eastern washington and debouched via the columbia river near 46 n the dynamics and timing of these events and their impact on northeast pacific circulation remain uncertain here we date marine records of anomalous freshwater inputs to the ocean based on freshwater diatoms oxygen isotopes in foraminifera and radiocarbon data low salinity plumes from the columbia river reduced sea surface salinities by as much as 6 psu practical salinity units more than 400 km away between 16 and 31 cal calendar ka b p anomalously high abundances of freshwater diatoms in marine sediments from the region precede generally accepted dates for the existence of glacial lake missoula implying that large flooding or freshwater routing events were common during the advance of the cordilleran ice sheet and that such events require multiple sources

links between rossby wave breaking and the north atlantic oscillation arctic oscillation in present day and last glacial maximum climate simulations upper tropospheric rossby wave breaking processes are examined in coupled ocean atmosphere simulations of the last glacial maximum lgm and of the modern era lgm statistics of the northern hemisphere in winter computed from the paleoclimate modeling intercomparison project phase ii pmip2 dataset are compared with those from preindustrial simulations and from the 40 yr ecmwf re analysis era 40 particular attention is given to the role of wave breaking events in the north atlantic oscillation nao for each simulation anticyclonic awb and cyclonic cwb wave breaking events during lgm are shown to be less and more frequent respectively than in the preindustrial climate especially in the pacific this is consistent with the slight equatorward shift of the eddy driven jets in the lgm runs the most remarkable feature of the simulated lgm climate is that it presents much weaker latitudinal fluctuations of the eddy driven jets this is accompanied by less dispersion in the wave breaking events a physical interpretation is provided in terms of the fluctuations of the low level baroclinicity at the entrance of the storm tracks the nao in the preindustrial simulations and in era 40 is characterized by strong latitudinal fluctuations of the atlantic eddy driven jet as well as by significant changes in the nature of the wave breaking during the positive phase the eddy driven jet moves to the north with more awb events than usual and is well separated from the subtropical african jet the negative phase exhibits a more equatorward atlantic jet and more cwb events in contrast the lgm nao is less well marked by the latitudinal vacillation of the atlantic jet and for some models this property disappears entirely the lgm nao corresponds more to acceleration deceleration or extension retraction of the atlantic jet the hemispheric point of view of the arctic oscillation exhibits similar changes

pollen based continental climate reconstructions at 6 and 21 ka a global synthesis subfossil pollen and plant macrofossil data derived from 14c dated sediment profiles can provide quantitative information on glacial and interglacial climates the data allow climate variables related to growing season warmth winter cold and plant available moisture to be reconstructed continental scale reconstructions have been made for the mid holocene mh around 6 ka and last glacial maximum lgm around 21 ka allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth assessment report ar5 of the intergovernmental panel on climate change the synthesis of the available mh and lgm climate reconstructions and their uncertainties obtained using modern analogue regression and model inversion techniques is presented for four temperature variables and two moisture variables reconstructions of the same variables based on surface pollen assemblages are shown to be accurate and unbiased reconstructed lgm and mh climate anomaly patterns are coherent consistent between variables and robust with respect to the choice of technique they support a conceptual model of the controls of late quaternary climate change whereby the first order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance

post younger dryas climate interval linked to circumpolar vortex variability isotopic evidence from fayetteville green lake new york the late glacial holocene transition in the north atlantic european sectors has long been known to be a period of rapid climate change there is however a continued need for acquiring and developing paleoclimate archives spanning this interval from continental settings here we report on a lacustrine fayetteville green lake isotope record sampled at a 10 year resolution from the ne usa over the late glacial holocene interval 14 600 8000 cal year bp based on prior isotopic and hydrologic research from green lake the 18o calcite values predominantly reflect winter moisture source and thus winter atmospheric patterns furthermore we use historic ad 1948 1980 winter circulation data and 18o calcite values from varved sediments to examine the relationship between the circumpolar vortex latitude and isotopes which results in a strong r 0 79 r2 0 63 negative relationship using the linear regression from the isotope vortex relationship we model the winter vortex latitude for the late glacial holocene transition over the ne usa in addition we identify an interval from 11 600 to 10 300 cal year bp the post younger dryas climate interval wherein the mean winter vortex over the ne usa was expanded by 6 latitude 36 1 n i e 630 km from its mean historic position between ad 1948 1998 41 8 n renewal of more vigorous thermohaline circulation following the younger dryas cold event may have forced the post younger dryas climate interval increased poleward heat transport due to an active oceanic conveyor would have strengthened the thermal contrast between the ne usa and the north atlantic thereby enhancing atmospheric pressure gradients and firmly establishing the semi permanent winter trough over the ne usa consequently storms tracked more frequently up the east coast of the united states from the gulf of mexico and atlantic regions delivering precipitation with relatively high 18o values to the ne usa alternatively the relative dominance of seasonal precipitation may have changed resulting in less total winter precipitation low 18o causing lake waters to become relatively enriched in high 18o summer precipitation both hypotheses are good candidates for testing by global circulation modeling gcm the termination of the post yd interval at 10 300 cal year bp and the beginning of the holocene was likely forced by a gradual decrease in thermohaline circulation and poleward heat transport as freshwater input from melting of high latitude ice sheets increased

evidence for insolation and pacific forcing of late glacial through holocene climate in the central mojave desert silver lake ca abstract silver lake is the modern terminal playa of the mojave river in southern california usa as a result it is well located to record both influences from the winter precipitation dominated san bernardino mountains the source of the mojave river and from the late summer to early fall north american monsoon at silver lake here we present various physical chemical and biological data from a new radiocarbon dated 8 2 m sediment core taken from silver lake that spans modern through 14 8 cal ka bp texturally the core varies between sandy clay clayey sand and sand silt clay often with abrupt sedimentological transitions these grain size changes are used to divide the core into six lake status intervals over the past 14 8 cal ka bp notable intervals include a dry younger dryas chronozone a wet early holocene terminating 7 8 7 4 cal ka bp a distinct mid holocene arid interval and a late holocene return to ephemeral lake conditions a comparison to potential climatic forcings implicates a combination of changing summer winter insolation and tropical and n pacific sea surface temperature dynamics as the primary drivers of holocene climate in the central mojave desert

north atlantic eddy driven jet in interglacial and glacial winter climates the atmospheric westerly flow in the north atlantic na sector is dominated by atmospheric waves or eddies generating via momentum flux convergence the so called eddy driven jet the position of this jet is variable and shows for the present day winter climate three preferred latitudinal states a northern central andsouthernpositioninthena here theauthorsanalyzethebehavioroftheeddy drivenjetunderdifferent glacial and interglacial boundary conditions using atmosphere land only simulations with the ccsm4 climate model as state of the art climate models tend to underestimate the trimodality of the jet latitude the authors apply a bias correction and successfully extract the trimodal behavior of the jet within ccsm4 the analysis shows that during interglacial times i e the early holocene and the eemian the preferred jet positions are rather stable and the observed multimodality is the typical interglacial character of the jet during glacial times the jet is strongly enhanced its position is shifted southward and the trimodal behavior vanishes this is mainly due to the presence of the laurentide ice sheet lis the lis enhances stationary waves downstream thereby accelerating and displacing the na eddy driven jet by anomalous stationary momentum flux convergence additionally changes in the transient eddy activity caused by topography changes as well as other glacial boundary conditions lead to an acceleration of the westerly winds over the southernnaattheexpenseofmorenorthernareas consequently bothstationaryandtransienteddiesfoster the southward shift of the na eddy driven jet during glacial winter times

age and paleoclimatic significance of the stansbury shoreline of lake bonneville northeastern great basin abstract the stansbury shoreline one of the conspicuous late pleistocene shorelines of lake bonneville consists of tufa cemented gravel and barrier beaches within a vertical zone of about 45 m the lower limit of which is 70 m above the modern average level of great salt lake stratigraphic evidence at a number of localities including new evidence from crater island on the west side of the great salt lake desert shows that the stansbury shoreline formed during the transgressive phase of late pleistocene lake bonneville sometime between about 22 000 and 20 000 yr b p tufa cemented gravel and barrier beaches were deposited in the stansbury shorezone during one or more fluctuations in water level with a maximum total amplitude of 45 m we refer to the fluctuations as the stansbury oscillation the stansbury oscillation cannot have been caused by basin hypsometric factors such as stabilization of lake level at an external overflow threshold or by expansion into an interior subbasin or by changes in drainage basin size therefore changes in climate must have caused the lake level to reverse its general rise to drop about 45 m in altitude reducing its surface area by about 18 5000 km2 and later to resume its rise if the sizes of great basin lakes are controlled by the mean position of storm tracks and the jetstream which as recently postulated may be controlled by the size of the continental ice sheets the stansbury oscillation may have been caused by a shift in the jetstream during a major interstade of the laurentide ice sheet

transport direction of peoria loess in nebraska and implications for loess sources on the central great plains abstract in the midwestern united states large rivers draining the laurentide ice sheet lis were the most important sources of peoria loess deposited during the last glaciation loess deposition near those rivers may have responded primarily to ice sheet dynamics rather than direct effects of climatic change in contrast it has been proposed that thick peoria loess on the central great plains was derived mainly from unglaciated landscapes northwest of the main loess deposits in this study transport directions inferred from more than 600 measurements of peoria loess thickness in nebraska are used to test the hypothesis that much of the peoria loess on the great plains is nonglaciogenic a strong northwest to southeast thickness trend indicates that most peoria loess in nebraska was transported from one or more unglaciated northwestern source areas rather than from glacially influenced river floodplains the missouri river draining the lis the platte river draining alpine glaciers and the elkhorn river unglaciated basin were secondary sources their contribution is not detectable beyond a distance of 40 60 km peoria loess deposition on the central great plains was largely a direct response to climatic change in the unglaciated source region

pleistocene water cycle and eastern boundary current processes along the california continental margin 1 coastal marine sediments contain mixtures of terrestrial and marine paleoclimate proxies that record how the coastal water cycle has behaved over long time frames we explore a 600 kyr marine record from odp site 1018 located due west of santa cruz california to identify coastal wet and dry periods and to associate them with oceanographic processes wet periods in central california identified by increased tree pollen relative to pollen from grasslands and scrublands are found on every major deglaciation in the last 600 kyr sea surface temperature sst data were collected for the last two deglaciations wet periods are associated with a rapid rise in sst off central california sst gradients along the california margin and changes in biogenic deposition show that wet periods in central california are associated with a weakening of the california current and weakened coastal upwelling high carbonate production suggests that there was significant curl of wind stress upwelling offshore we propose that wet periods in central california are associated with a meteorological connection to the tropical pacific and weakened southward flow in the california current that shunted temperate pacific water northward into the alaska gyre we do not observe evidence for a south shifted westerly storm track at the last glacial maximum but find that wet periods are diachronous along the california margin the wettest period around the santa barbara basin peaked at 16 ka preceding the wet peak in central and northern california by 4 kyr

deglacial hydroclimate of midcontinental north america abstract during the last deglaciation temperatures over midcontinental north america warmed dramatically through the bolling allerod underwent a cool period associated with the younger dryas and then reverted to warmer near modern temperatures during the early holocene however paleo proxy records of the hydroclimate of this period have presented divergent evidence we reconstruct summer relative humidity rh across the last deglacial period using a mechanistic model of cellulose and leaf water 18o and d combined with a pollen based temperature proxy to interpret stable isotopes of sub fossil wood midcontinental rh was similar to modern conditions during the last glacial maximum progressively increased during the bolling allerod peaked during the younger dryas and declined sharply during the early holocene this rh record suggests deglacial summers were cooler and characterized by greater advection of moisture laden air masses from the gulf of mexico and subsequent entrainment over the mid continent by a high pressure system over the laurentide ice sheet these patterns help explain the formation of dark colored cumulic horizons in many great plains paleosol sequences and the development of no analog vegetation types common to the midwest during the last deglacial period likewise reduced early holocene rh and precipitation correspond with a diminished glacial high pressure system during the latter stages of ice sheet collapse

latest pleistocene to holocene hydroclimates from lake elsinore california abstract the hydroclimate of the southwestern united states us region changed abruptly during the latest pleistocene as the continental ice sheets over north america retreated from their most southerly extent to investigate the nature of this change we present a new record from lake elsinore located 36 km inland from the pacific ocean in southern california and evaluate it in the context of records across the coastal and interior southwest united states including northwest mexico the sediment core recovered from lake elsinore provides a continuous sequence with multi decadal resolution spanning 19 9 ka bp sedimentological and geochemical analyses reveal hydrologic variability in particular sand and carbonate components indicate abrupt changes at the oldest dryas od bolling allerod ba and younger dryas yd transitions consistent with the timing in greenland hydrogen isotope analyses of the c28 n alkanoic acids from plant leaf waxes dwax reveal a long term trend toward less negative values across 19 9 ka bp dwax values during the od suggest a north pacific moisture source for precipitation consistent with the dipping westerlies hypothesis we find no isotopic evidence for the north american monsoon reaching as far west as lake elsinore therefore we infer that wet dry changes in the coastal southwest were expressed through winter season precipitation consistent with modern climatology comparing lake elsinore to other southwest records notably cave of bells and fort stanton we find coincident timing of the major transitions od to ba ba to yd and hydrologic responses during the od and ba the hydrologic response however varied during the yd consistent with a dipole between the coastal and interior southwest the coherent pattern of hydrologic responses across the interior southwest us and northwest mexico during the od wet the ba drier and yd wet follows changes in the atlantic meridional overturning circulation presumably via its combined influence on north pacific winter storm tracks and the extent magnitude of the north american monsoon in contrast lake elsinore and the coastal southwest experiences a deglacial drying trend punctuated by abrupt change at the od to ba and ba to yd transitions this trend tracks rising greenhouse gases through the deglacial with an apparent southward shift in westerly moisture sources adjusting to the retreating ice sheet

last glacial interglacial environments in the southern rocky mountains usa and implications for younger dryas age human occupation abstract the last glacial interglacial transition lgit 19 9 ka was characterized by rapid climate changes and significant ecosystem reorganizations worldwide in western colorado one of the coldest locations in the continental us today mountain environments during the late glacial period are poorly known yet archaeological evidence from the mountaineer site 2625 m elev indicates that folsom age paleoindians were over wintering in the gunnison basin during the younger dryas chronozone ydc 12 9 11 7 ka to determine the vegetation and fire history during the lgit and possible explanations for occupation during a period thought to be harsher than today a 17 ka old sediment core from lily pond 3208 m elev was analyzed for pollen and charcoal and compared with other high resolution records from the southern rocky mountains widespread tundra and picea parkland and low fire activity in the cold wet late glacial period transitioned to open subalpine forest and increased fire activity in the bolling allerod period as conditions became warmer and drier during the ydc greater winter snowpack than today and prolonged wet springs likely expanded subalpine forest to lower elevations than today providing construction material and fuel for the early inhabitants in the early to middle holocene arid conditions resulted in xerophytic vegetation and frequent fire

northern hemisphere storm tracks during the last glacial maximum in the pmip2 ocean atmosphere coupled models energetic study seasonal cycle precipitation mid latitude eddies are an important component of the climatic system due to their role in transporting heat moisture and momentum from the tropics to the poles and also for the precipitation associated with their fronts especially in winter we study northern hemisphere storm tracks at the last glacial maximum lgm and their influence on precipitation using ocean atmosphere general circulation model oagcm simulations from the second phase of the paleoclimate modelling intercomparison project pmip2 the difference with pmip1 results in terms of sea surface temperature forcing fundamental for storm track dynamics is large especially in the eastern north atlantic where sea ice extends less to the south in oagcms compared to atmospheric only gcms our analyses of the physics of the eddies are based on the equations of eddy energetics all models simulate a consistent southeastward shift of the north pacific storm track in winter related to a similar displacement of the jet stream partly forced by the eddies themselves precipitation anomalies are consistent with storm track changes with a southeastward displacement of the north pacific precipitation pattern the common features of north atlantic changes in the lgm simulations consist of a thinning of the storm track in its western part and an amplification of synoptic activity to the southeast in the region between the azores islands and the iberian peninsula which reflects on precipitation this southeastward extension is related to a similar displacement of the jet partly forced by the eddies in the western north atlantic the synoptic activity anomalies are at first order related to baroclinic generation term anomalies but the mean flow baroclinicity increase due to the presence of the laurentide ice sheet is partly balanced by a loss of eddy efficiency to convert energy from the mean flow moisture availability in this region is greatly reduced due to more advection of dry polar air by stationary waves leading to less synoptic scale latent heat release and hence less precipitation also in terms of seasonality the stormy season is shifted later in the year by a few days to a month depending on the season and the model considered this shift does not directly reflect on the first order seasonal cycle of precipitation which also depends on other mechanisms especially in summer

changing amounts and sources of moisture in the u s southwest since the last glacial maximum in response to global climate change abstract the u s southwest has a limited water supply and is predicted to become drier in the 21st century an improved understanding of factors controlling moisture sources and availability is aided by reconstruction of past responses to global climate change new stable isotope and growth rate records for a central texas speleothem indicate a strong influence of gulf of mexico gom moisture and increased precipitation from 15 5 to 13 5 ka which includes the majority of the bolling allerod warming ba 14 7 12 9 ka coeval speleothem records from 900 and 1200 km to the west allow reconstruction of regional moisture sources and atmospheric circulation the combined isotope and growth rate time series indicates 1 increased gom moisture input during the majority of the ba producing greater precipitation in texas and new mexico and 2 a retreat of gom moisture during younger dryas cooling 12 9 11 5 ka reducing precipitation these results portray how late pleistocene atmospheric circulation and moisture distribution in this region responded to global changes providing information to improve models of future climate

south pacific split jet itcz shifts and atmospheric north south linkages during abrupt climate changes of the last glacial period abstract a number of key paleoclimate records in the southern hemisphere midlatitudes exhibit climate changes synchronous with abrupt climate changes in the north atlantic we advance a hypothesis argued from consideration of model evidence observational climate diagnostics and atmospheric dynamics that attributes said climate changes in the southern hemisphere to a modulation in the strength of the south pacific split jet a pronounced zonally asymmetric feature of the wintertime southern hemisphere westerlies north atlantic cooling is associated with a weaker split jet characterized by weaker south pacific subtropical and subpolar jets and a strengthened midlatitude jet it leads to climate impacts over the south pacific sector that coincides with regions with observed paleoclimate changes timed to the north atlantic these circulation changes are envisioned to operate in addition to the climate impacts resulting from the oceanic bipolar seesaw a proposed global atmospheric teleconnection links north atlantic cooling to the weakening of the split jet north atlantic cooling induces a southward shift of the marine intertropical convergence zone and weakening of the asian monsoon the resulting hadley circulation change weakens the wintertime south pacific subtropical jet and which in turn leads to a weaker south pacific split jet a weaker split jet leads to a southward shift of the zero wind stress curl line implying a shift in the same sense for the south pacific subtropical front over land it leads to winter warming over new zealand winter cooling over subtropical south america drying over western patagonia and winter warming and wetting of southernmost patagonia our hypothesis also predicts reduced storminess over west antarctica similar changes but of opposite sign occur in the northern hemisphere where a stronger wintertime north pacific subtropical jet increases precipitation over the western united states

temporal correspondence between pluvial lake highstands in the southwestern us and heinrich event 1 pluvial lakes were abundant in the southwestern united states during pleistocene glaciations particu larly in the great basin many of these lakes occupied closed basins therefore fluctuations of their water surface elevations are valuable sources of paleoclimate information histories of the largest lakes are well constrained whereas dozens of smaller lakes that were present in this region have received relatively little scientific attention given their dimensions these smaller lakes were climatically sensitive and can offer important information about quaternary climate variability here we present new ages for the highstands of three previously undated small lakes based on radiocarbon dating of gastropod shells recovered from beach ridges these results are combined with other published and unpublished 14 c ages to yield an extensive compilation of highstand shoreline ages for lakes of all sizes throughout the southwestern us the results indicate that although some lakes reached highstands during the last glacial maximum the strongest temporal correspondence is between highstands and heinrich event h1 these results are consistent with speleothem based reconstructions of effective moisture in the southwestern us which show increased precipitation during stadials of the last glacial cycle copyright 2012 john wiley sons ltd

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

hydrogen isotope ratios of palmitic acid in lacustrine sediments record late quaternary climate variations the rich paleoclimate information preserved in lacustrine sedimentary organic matter can be difficult to extract because of the mixed terrestrial and aquatic inputs herein we demonstrate that compound specific hydrogen isotope analysis of palmitic acid pa a ubiquitous compound in lacustrine sediments captures the d signals of lake water samples collected across a diverse range of 33 north american lakes show a strong correlation between water and dpa values at crooked pond massachusetts the dpa changes in a 14 k y sediment record parallel temperature trends inferred from fossil pollen downcore changes reveal differences between climatic trends in new england and in greenland that are consistent with important regional differences in climate controls

high resolution climate simulation of the last glacial maximum the climate of the last glacial maximum lgm is simulated with a high resolution atmospheric general circulation model the ncar ccm3 at spectral truncation of t170 corresponding to a grid cell size of roughly 75 km the purpose of the study is to assess whether there are significant benefits from the higher resolution simulation compared to the lower resolution simulation associated with the role of topography the lgm simulations were forced with modified climap sea ice distribution and sea surface temperatures sst reduced by 1 c ice sheet topography reduced co2 and 21 000 bp orbital parameters the high resolution model captures modern climate reasonably well in particular the distribution of heavy precipitation in the tropical pacific for the ice age case surface temperature simulated by the high resolution model agrees better with those of proxy estimates than does the low resolution model despite the fact that tropical ssts were only 2 1 c less than the control run there are many lowland tropical land areas 4 6 c colder than present comparison of t170 model results with the best constrained proxy temperature estimates noble gas concentrations in groundwater now yield no significant differences between model and observations there are also significant upland temperature changes in the best resolved tropical mountain belt the andes we provisionally attribute this result in part as resulting from decreased lateral mixing between ocean and land in a model with more model grid cells a longstanding model data discrepancy therefore appears to be resolved without invoking any unusual model physics the response of the asian summer monsoon can also be more clearly linked to local geography in the high resolution model than in the low resolution model this distinction should enable more confident validation of climate proxy data with the high resolution model elsewhere an inferred salinity increase in the subtropical north atlantic may have significant implications for ocean circulation changes during the lgm a large part of the amazon and congo basins are simulated to be substantially drier in the ice age consistent with many but not all paleo data these results suggest that there are considerable benefits derived from high resolution model regarding regional climate responses and that observationalists can now compare their results with models that resolve geography at a resolution comparable to that which the proxy data represent

bahamian speleothem reveals temperature decrease associated with heinrich stadials abstract temperature reconstructions across heinrich stadials 1 3 are presented from an absolute dated speleothem from abaco island in the bahamas to understand the nature of climate change across these intervals in the subtropical atlantic the stalagmite carbonate record dated by the u th geochronometry technique includes higher 18o and 13c values within heinrich stadials 1 2 and 3 followed by rapid declines at the end of the stadials to aid in the interpretation of these results the 18o of fluid inclusions associated with the heinrich stadials were also analyzed these measurements which allowed for the relative influence of temperature and 18o of precipitation to be distinguished demonstrate minimal changes in the 18o of fluid inclusions suggesting that changes in the 18o values of the speleothem carbonate associated with heinrich stadials 1 3 are principally driven by an average 4 c temperature decrease rather than a change in the 18o of the rainfall hence rainfall amount these findings support previous work in the north atlantic and are consistent with the climate response to a weakening of the atlantic meridional overturning circulation

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

quantifying the isotopic continental effect since the establishment of the iaea wmo precipitation monitoring network in 1961 it has been observed that isotope ratios in precipitation h2 and o18 generally decrease from coastal to inland locations an observation described as the continental effect while discussed frequently in the literature there have been few attempts to quantify the variables controlling this effect despite the fact that isotopic gradients over continents can vary by orders of magnitude in a number of studies traditional rayleigh fractionation has proven inadequate in describing the global variability of isotopic gradients due to its simplified treatment of moisture transport and its lack of moisture recycling processes in this study we use a one dimensional idealized model of water vapor transport along a storm track to investigate the dominant variables controlling isotopic gradients in precipitation across terrestrial environments we find that the sensitivity of these gradients to progressive rainout is controlled by a combination of the amount of evapotranspiration and the ratio of transport by advection to transport by eddy diffusion with these variables becoming increasingly important with decreasing length scales of specific humidity a comparison of modeled gradients with global precipitation isotope data indicates that these variables can account for the majority of variability in observed isotopic gradients between coastal and inland locations furthermore the dependence of the continental effect on moisture recycling allows for the quantification of evapotranspiration fluxes from measured isotopic gradients with implications for both paleoclimate reconstructions and large scale monitoring efforts in the context of global warming and a changing hydrologic cycle

out of the tropics the pacific great basin lakes and late pleistocene water cycle in the western united states the water cycle in the western united states changed dramatically over glacial cycles in the past 20 000 years higher precipitation caused desert lakes to form which have since dried out higher glacial precipitation has been hypothesized to result from a southward shift of pacific winter storm tracks we compared pacific ocean data to lake levels from the interior west and found that great basin lake high stands are older than coastal wet periods at the same latitude westerly storms were not the source of high precipitation instead air masses from the tropical pacific were transported northward bringing more precipitation into the great basin when coastal california was still dry the changing climate during the deglaciation altered precipitation source regions and strongly affected the regional water cycle

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

a continuous high resolution record of late pleistocene climate variability from the estancia basin new mexico lake sediments that accumulated in the estancia basin central new mexico provide a detailed record of submillennial latest pleistocene climatic oscillations sedimentologic biologic and geochemical proxies for changes in salinity and lake level were used to identify episodes of wet and dry climate that occurred between 24 and 12 k y b p as dated by 14c by means of accelerator mass spectrometry from shoreline and basin center deposits these dates determine the timing and duration of the episodes of wet and dry climate the lake expanded and contracted repeatedly during the last glacial maximum and fluctuated near its highest elevation several times during the interval ca 20 to 15 k y b p a pronounced lowstand lasting 1 k y occurred between ca 15 and 14 k y b p followed by two more highstands of the lake between ca 14 and 12 5 k y b p desiccation of the perennial lake after 12 k y b p was followed by a final poorly dated highstand at ca 10 k y b p the record of lake level fluctuations from the estancia basin provides a high resolution record of changes in water budget in a basin the comparatively simple physiography and hydrogeology of which ensured a direct response to the strong climatic fluctuations that occurred during the last glacial maximum and termination

a data calibrated distribution of deglacial chronologies for the north american ice complex from glaciological modeling article i nfo past deglacial ice sheet reconstructions have generally relied upon discipline specific constraints with no at tention given to the determination of objective confidence intervals reconstructions based on geophysical inversion of relative sea level rsl data have the advantage of large sets of proxy data but lack ice mechanical constraints conversely reconstructions based on dynamical ice sheet models are glaciologically self consistent but depend on poorly constrained climate forcings and sub glacial processes as an example of a much better constrained methodology that computes explicit error bars we present a dis tribution of high resolution glaciologically self consistent deglacial histories for the north american ice com plex calibrated against a large set of rsl marine limit and geodetic data the history is derived from ensemble based analyses using the 3d mun glacial systems model and a high resolution ice margin chronol ogy derived from geological and geomorphological observations isostatic response is computed with the vm5a viscosity structure bayesian calibration of the model is carried out using markov chain monte carlo methods in combination with artificial neural networks trained to the model results the calibration provides a posterior distribution for model parameters and thereby modeled glacial histories given the observational data sets that takes data uncertainty into account final ensemble results also account for fits between com puted and observed strandlines and marine limits given the model including choice of calibration parameters input and constraint data sets and vm5a earth rhe ology we find the north american contribution to mwp1a was likely between 9 4 and 13 2 m eustatic over a 500 year interval this is more than half of the total 16 to 26 m meltwater pulse over 500 to 700 years with lower values being more probable indicated by the barbados coral record fairbanks 1989 peltier and fairbanks 2006 if one assumes a 5 meter living range for the acropora palmata coral 20 ka ice volume for north america was likely 70 1 2 0 m eustatic or about 60 of the total contribution to eustatic sea level change we suspect that the potentially most critical unquantified uncertainties in our analyses are those related to model structure especially climate forcing deglacial ice margin chronology and earth rheology

chronology of lake bonneville 30 000 to 10 000 yr b p abstract 368 radiocarbon ages between 30 and 10 cal ka for samples collected from outcrops and shorelines from the lake bonneville basin have been compiled for this paper samples include 1 organic materials from sources outside the lake such as wood charcoal plant fragments from emergent aquatics and dispersed wetland organics and 2 carbonate materials deposited in the lake such as mollusk shells tufa charophyte debris ooids and marl in general organic materials provide rather than precise ages for lacustrine events but their interpretation is less complicated than interpretation of results from carbonate samples the data set shows evidence of contamination of different carbonate samples with both younger carbon and older carbon for example a radiocarbon reservoir at certain places within the lake during the middle transgressive phase accounts for ages of mollusk shells that are older than basal wood ages at similar altitudes the large number of ages permits an accurate reconstruction of the lake chronology conflicts between ages can be detected and reliable ages can be meaningfully integrated in interpretations if fewer ages were available the chronology might look simpler but its accuracy would be unknown

centennial scale compound specific hydrogen isotope record of pleistocene holocene climate transition from southern new england 1 northeastern north america experienced major climate shifts during the pleistocene holocene transition however there have been no high resolution isotopic records of climate change from this region here we present a centennial scale record of climate change during the transition based on d h ratios of behenic acid c22 nacid or ddba from a sediment core in blood pond massachusetts surface calibrations from a transect of 19 lakes in eastern north america show that ddba values track mean annual atmospheric temperature variations the abrupt climate events observed in blood pond records show remarkable similarity with greenland ice core d 18 o records during the pleistocene during the early holocene the northeastern north america ddba record was more variable than greenland possibly due to the close proximity of the laurentide ice sheet and impact of freshwater outbursts as the ice sheet rapidly retreated citation hou j y huang w w oswald d r foster and b shuman 2007 centennialscale compound specific hydrogen isotope record of pleistocene holocene climate transition from southern new england geophys res lett 34 l19706 doi 10 1029 2007gl030303

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

impact of the north american ice sheet orography on the last glacial maximum eddies and snowfall the present work evaluates the influence of the north american ice sheet orography on the last glacial maximum lgm 21 000 years ago atmospheric circulation and snowfall in the northern mid latitudes focusing on the north atlantic sector three atmospheric general circulation model experiments are analysed a control and an lgm climate simulation and an lgm run with flat ice sheets over northern north america this ice sheet orography affects lee cyclogenesis over north america and forces differences in stationary waves and therefore in the baroclinicity of the mean flow as a result the atlantic storm track is reinforced in the flat ice sheet experiment compared to the lgm one this in turn has a profound impact on snowfall over northern europe implying a coupling between the two ice sheets

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

lacustrine cave carbonates novel archives of paleohydrologic change in the bonneville basin utah usa records of past changes in lake levels and lake water isotopic composition in closed basins provide key insights into past variations in the hydrological cycle however these records are often limited by dating precision and temporal resolution here we present data from lacustrine cave carbonates a previously unexplored class of carbonates that comprise a promising new archive of past hydrologic changes in the bonneville basin of the northeastern great basin usa these dense carbonates precipitated within caves crevices and other protected spaces flooded by lake bonneville during its highstand in the last glacial period we report on deposits in cathedral and craners caves located 50 km apart at similar elevations approximately 100 m above the modern great salt lake and almost 200 m below lake bonneville s highstand shoreline carbonates from the two caves show similar chronologies mineralogical transitions isotopic compositions and uranium concentrations these findings suggest that lacustrine cave carbonates record changes in lake level and in the isotopic composition and chemistry of lake water importantly the deposits can be precisely dated by u th methods providing the first records of lake bonneville s water balance changes tied to precise u th ages close agreement between paired u th and calibrated 14 c ages in the deposits suggests a minimal o200 a carbon reservoir effect in the lake and allows 14 c dating to be used for age control in portions of the deposits less suitable for u th dating we use ages for the onset and cessation of lacustrine cave carbonate deposition to offer new constraints on past changes in lake level and the carbonate saturation state of lake water we also present precisely dated high resolution oxygen and uranium isotope records from the deposits within a first phase of deposition reflecting the lake s transgression between 26 and 18 ka our isotopic data suggest a large influx of freshwater during heinrich stadial 2 a hiatus in deposition beginning at 18 270 3 ka may be the result of freshening related to the lake s overflow calcite deposition resumes at cathedral cave at 16 470 2 ka suggesting that basin overflow had ceased by this time and that the lake s calcite saturation state had increased andd 18 o values increase markedly after 15 9 ka consistent with drying at this time these data imply that the lake s deglacial regression began well before the bolling warming cessation of this second phase of deposition at 14 770 2 ka coincident with the bolling warming may reflect the lake s drop below cathedral cave s elevation a final stage of aragonite deposition between 13 8 and 13 6 ka differs from the underlying facies and may not reflect deposition from lake waters filling the cave

steering of westerly storms over western north america at the last glacial maximum the last glacial maximum hydroclimate over western north america differed from the modern climate a proxy model comparison suggests that the glacial storm track was squeezed and steered by atmospheric high pressure systems

a computational approach to quaternary lake level reconstruction applied in the central rocky mountains wyoming usa abstract sediment based reconstructions of late quaternary lake levels provide direct evidence of hydrologic responses to climate change but many studies only provide approximate lake elevation curves here we demonstrate a new method for producing quantitative time series of lake elevation based on the facies and elevations of multiple cores collected from a lake s margin the approach determines the facies represented in each core using diagnostic data such as sand content and then compares the results across cores to determine the elevation of the littoral zone over time by applying the approach computationally decisions are made systematically and iteratively using different facies classification schemes to evaluate the associated uncertainty after evaluating our assumptions using ground penetrating radar gpr we quantify past lake elevation changes precipitation minus evapotranspiration p et and uncertainty in both at lake of the woods and little windy hill pond wyoming the well correlated r 0 802 0 002 reconstructions indicate that water levels at both lakes fell at 11 300 8000 5500 and 4700 1600 cal yr bp when p et decreased to 50 to 250 mm yr differences between the reconstructions are typically small 10 24 mm yr since 7000 cal yr bp and the similarity indicates that our reconstruction method can produce statistically comparable paleohydrologic datasets across networks of sites

the north american hydrologic cycle through the last deglaciation abstract while the climate evolution of north america during the last deglaciation has received considerable attention few detailed model data comparisons of the deglacial hydroclimate have been conducted at the continental scale here we use a transient climate simulation of the last deglaciation and a synthesis of hydroclimate proxies from across the continent to broadly assess the moisture budget and its evolution including the primary components and mechanisms for changes and evaluate the level of agreement between model and data in order to inform areas of major discrepancy for attention in future work at the broadest scale the simulation indicates that north america was wetter than modern through much of the last deglaciation with the exception of the pacific northwest this was principally due to increased moisture convergence by the mean flow over the west of the continent and by transient eddies over the east we find that the simulation demonstrates considerable skill in comparison to proxy records in the coastal southwest and pacific northwest with limited success in other regions the highest disagreement occurs in the north central region where the model and data disagree on the sign of changes in addition we find that the areas of agreement coincide with regions whose hydroclimate is dominated by winter precipitation while regions of disagreement are those where summer precipitation is predominant and where the model suggests dynamical changes are important these results illustrate the impact of the ice sheet on the atmospheric dynamics and therefore the importance of its accurate reconstruction as well as that of model resolution our analysis and synthesis provide context for future transient climate simulations and suggest numerous areas of priority for future research

a speleothem record of younger dryas cooling klamath mountains oregon usa awell dated d 18 o record in a stalagmite from a cave in the klamath mountains oregon with a sampling interval of 50 yr indicates that the climate of this region cooled essentially synchronously with younger dryas climate change elsewhere in the northern hemisphere the d 18 o record also indicates significant century scale temperature variability during the early holocene the d 13 c record suggests increasing biomass over the cave through the last deglaciation with century scale variability but with little detectable response of vegetation to younger dryas cooling d 2005 university of washington all rights reserved

trade wind fetch related variations in equatorial undercurrent depth speed and transport a dynamical model is developed to examine the dependence of equatorial undercurrent depth speed and volume transport on trade wind fetch the model ocean is linear continuously stratified vertically diffusive and formulated on the equatorial beta plane analytical solutions are derived in closed form for realistic choices of model parameters results indicate that the undercurrent would be significantly shallower and weaker in the atlantic than in the pacific because of the shorter atlantic trade wind fetch

variable winter moisture in the southwestern united states linked to rapid glacial climate shifts the last glacial period was characterized by large rapid climate fluctuations an analysis of a speleothem from new mexico shows that the coldest conditions over greenland coincide with increased winter precipitation in the southwestern united states which can be attributed to a southward displacement of the polar jet stream and the north american storm track

quantitative estimation of bioclimatic parameters from presence absence vegetation data in north america by the modern analog technique abstract the method of modern analogs is widely used to obtain estimates of past climatic conditions from paleobiological assemblages and despite its frequent use this method involved so far untested assumptions we applied four analog approaches to a continental scale set of bioclimatic and plant distribution presence absence data for north america to assess how well this method works under near optimal modern conditions for each point on the grid we calculated the similarity between its vegetation assemblage and those of all other points on the grid excluding nearby points the climate of the points with the most similar vegetation was used to estimate the climate at the target grid point estimates based the use of the jaccard similarity coefficient had smaller errors than those based on the use of a new similarity coefficient although the latter may be more robust because it does not assume that the fossil assemblage is complete the results of these analyses indicate that presence absence vegetation assemblages provide a valid basis for estimating bioclimates on the continental scale however the accuracy of the estimates is strongly tied to the number of species in the target assemblage and the analog method is necessarily constrained to produce estimates that fall within the range of observed values we applied the four modern analog approaches and the mutual overlap or mutual climatic range method to estimate bioclimatic conditions represented by the plant macrofossil assemblage from a packrat midden of last glacial maximum age from southern nevada in general the estimation approaches produced similar results in regard to moisture conditions but there was a greater range of estimates for growing degree days despite its limitations the modern analog technique can provide paleoclimatic reconstructions that serve as the starting point to the interpretation of past climatic conditions

influence of eolian deposition and rainfall amounts on the u isotopic composition of soil water and soil minerals 230th u dating of secondary deposits such as speleothems and pedogenic carbonate and opal provides information about the original 234u 238u composition of the waters the minerals precipitated from as a result these types of deposits often display temporal variations in initial 234u 238u that could reflect environmental variability past variations in soil water 234u 238u may reflect changes in the flux of water through the soil weathering rates or dust inputs in order to evaluate these potential influences we analyzed modern soil water chemistry and u isotopic compositions of fluids and solids from three soil sequences in nevada modern soil pore waters from diamond valley newark valley and fish lake valley nevada show significant 234u 238u differences between sites as a result of differing soil parent material newark valley pore waters are more heavily influenced by carbonate weathering and display much more enriched 234u 238u values than diamond and fish lake valley pore waters which are dominated by silicate weathering newark valley and diamond valley although located very near to each other have significantly different pore water 234u 238u values suggesting that deposition of distantly sourced dust does not exert a dominant influence on pore water 234u 238u at these sites pore water 234u 238u values also vary systematically along a rainfall gradient at fish lake valley where sites with lower rainfall display higher soil water 234u 238u values the results of this modern soil water chemistry study were used to parameterize two models that investigate the relative influence of dust content recoil and weathering on pore water 234u 238u in the upper layer of the soil and at depth under changing infiltration rates these models suggest that while the composition of soluble dust entering a soil profile is an important consideration the pore water 234u 238u at depth and thus the initial 234u 238u of minerals that precipitate from pore waters should be highly sensitive to changes in the infiltration flux through the soils as infiltration flux is closely tied to rainfall the initial 234u 238u of soil minerals at these sites should provide information about past rainfall variability

collapse of the california current during glacial maxima linked to climate change on land time series of alkenone unsaturation indices gathered along the california margin reveal large 4 to 8 c glacial interglacial changes in sea surface temperature sst over the past 550 000 years interglacial times with ssts equal to or exceeding that of the holocene contain peak abundances in the pollen of redwood the distinctive component of the temperate rainforest of the northwest coast of california in the region now dominated by the california current ssts warmed 10 000 to 15 000 years in advance of deglaciation at each of the past five glacial maxima ssts did not rise in advance of deglaciation south of the modern california current front glacial warming along the california margin therefore is a regional signal of the weakening of the california current during times when large ice sheets reorganized wind systems over the north pacific both the timing and magnitude of the sst estimates suggest that the devils hole nevada calcite record represents regional but not global paleotemperatures and hence does not pose a fundamental challenge to the orbital milankovitch theory of the ice ages

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

physiological significance of low atmospheric co2 for plant climate interactions abstract methods of palaeoclimate reconstruction from pollen are built upon the assumption that plant climate interactions remain the same through time or that these interactions are independent of changes in atmospheric co 2 the latter may be problematic because air trapped in polar ice caps indicates that atmospheric co 2 has fluctuated significantly over at least the past 400 000 yr and likely the last 1 6 million yr three other points indicate potential biases for vegetation based climate proxies first c 3 plant physiological research shows that the processes that determine growth optima in plants photosynthesis mitochondrial respiration photorespiration are all highly co 2 dependent and thus were likely affected by the lower co 2 levels of the last glacial maximum second the ratio of carbon assimilation per unit transpiration called water use efficiency is sensitive to changes in atmospheric co 2 through effects on stomatal conductance and may have altered c 3 plant responses to drought third leaf gas exchange experiments indicate that the response of plants to carbon depleting environmental stresses are strengthened under low co 2 relative to today this paper reviews the scope of research addressing the consequences of low atmospheric co 2 for plant and ecosystem processes and highlights why consideration of the physiological effects of low atmospheric co 2 on plant function is recommended for any future refinements to pollen based palaeoclimatic reconstructions

dating fluvial terraces by 230th u on pedogenic carbonate wind river basin wyoming abstract reliable and precise ages of quaternary pedogenic carbonate can be obtained with 230th u dating by thermal ionization mass spectrometry applied to carefully selected milligram size samples datable carbonate can form within a few thousand years of surface stabilization allowing ages of quaternary deposits and surfaces to be closely estimated pedogenic carbonate clast rinds from gravels of glacio fluvial terraces in the wind river basin have median concentrations of 14 ppm u and 0 07 ppm 232th with median 230th 232th 270 making them well suited for 230th u dating horizons as thin as 0 5 mm were sampled from polished slabs to reduce averaging of long 105 yr and sometimes visibly discontinuous depositional histories dense translucent samples with finite 230th u ages preserve within rind stratigraphic order in all cases ages for terraces wr4 167 000 6 400 yr and wr2 55 000 8600 yr indicate a mean incision rate of 0 26 0 05 m per thousand years for the wind river over the past glacial cycle slower than inferred from cosmogenic nuclide dating terrace wr3 which formed penecontemporaneously with the final maximum glacial advance of the penultimate rocky mountain bull lake glaciation has an age of 150 000 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum

stalagmite records of hydroclimate in central california during termination 1 abstract the last deglaciation is marked by large and abrupt hydroclimatic changes in the great basin and the american southwest however comparatively little is known about how hydroclimate varied on the western side of the sierra nevada we present new evidence for abrupt changes in precipitation amount in the central sierra nevada during the last deglaciation our new record from mclean s cave overlaps with a previously published record from nearby moaning cave oster et al 2009 and extends the record of hydroclimatic change in the region through heinrich stadial 1 mclean s cave speleothem 18o 13c mg ca and sr ca indicate a shift to drier conditions at the beginning of heinrich stadial 1 from 16 1 to 17 5 ka followed by wetter conditions at the end of heinrich stadial 1 when the majority of great basin lakes reached their deglacial highstands during the last deglaciation coincident shifts in the moaning and mclean s cave proxy records indicate drier conditions in the western sierra nevada during millennial scale intervals of warming at high latitudes such as the bolling and allerod and wetter conditions during cooler intervals such as the older and younger dryas thus the sierra nevada speleothem records are consistent with other regional records that document increased winter rainfall during millennial scale cold periods of the last deglaciation however regional differences exist in the hydroclimatic response to the younger dryas with central californian and interior southwestern sites indicating an increase in winter storms that is not apparent in southern california dynamic simulations of atmospheric circulation indicate that wetter conditions during the younger dryas relative to the bolling at mclean s cave may have resulted from a stronger storm track in the north pacific at the latitude of northern california

late pleistocene california droughts during deglaciation and arctic warming abstract recent studies document the synchronous nature of shifts in north atlantic regional climate the intensity of the east asian monsoon and productivity and precipitation in the cariaco basin during the last glacial and deglacial period yet questions remain as to what climate mechanisms influenced continental regions far removed from the north atlantic and beyond the direct influence of the inter tropical convergence zone here we present u series calibrated stable isotopic and trace element time series for a speleothem from moaning cave on the western slope of the central sierra nevada california that documents changes in precipitation that are approximately coeval with greenland temperature changes for the period 16 5 to 8 8 ka from 16 5 to 10 6 ka the moaning cave stalagmite proxies record drier and possibly warmer conditions signified by elevated 18 o 13 c mg sr and ba and more radiogenic 87 sr 86 sr during northern hemisphere warm periods bolling early and late allerod and wetter and possibly colder conditions during northern hemisphere cool periods older dryas inter allerod cold period and younger dryas moaning cave stable isotope records indicate that wet conditions persisted in this area well beyond 11 5 ka suggesting the effects of the younger dryas event may have been longer lived in the western sierra nevada than in greenland however a shifting drip center and corresponding change in seepage water routing may have influenced the trace element records between 10 6 and 9 6 ka linkages between northern high latitude climate and precipitation in the sierra nevada suggested here could indicate that under conditions of continued global warming this drought prone region may experience a reduction in pacific sourced moisture

late quaternary vegetation and climate history near little lake central coast range oregon pollen and plant macrofossils recovered from little lake oregon provide the first record of late quaternary climate and vegetation changes in oregon9s central coast range the pollen data suggest that the period from ca 42 000 to 24 770 yr b p featured an open forest of western white pine western hemlock and fir the inferred climate was cooler and wetter than today the full glacial period was characterized by a parkland of spruce lodgepole pine and mountain hemlock which suggests that the climate was colder and possibly drier than today the oregon coast range was not a glacial refugium for temperate conifers as has been proposed by 16 000 yr b p the pollen and plant macrofossil data suggest the development of a montane forest composed of pine fir western hemlock and mountain hemlock temperate taxa were present in the vegetation at 13 500 yr b p during a period of climatic amelioration a reversal in this warming trend may have occurred between 11 000 and 10 500 yr b p when pine western and mountain hemlock and spruce were slightly more abundant early holocene forests featured douglas fir red alder and bracken fern implying more severe summer drought and more frequent fires after 5600 yr b p douglas fir western hemlock and western red cedar dominated the record marking the introduction of the present day cool moist climate however during the past 2800 yr increased douglas fir and decreased cedar suggest the possibility of reduced effective moisture at little lake

representation of flora and vegetation in quaternary fossil assemblages known and unknown knowns and unknowns paleoecological inference rests on a foundation of processes and relationships that lie between the target variables of interest and the proxy data extracted from the fossil record inattention to these processes can lead to overconfidence reification of proxies and ignorance creep a process by which assumptions become taken for granted and ultimately hidden or forgotten paleoecological inference can be strengthened and its uncertainties identified and quantified by developing forward conceptual models that incorporate the processes and relationships known to lie between target variables and proxy data these may fall into four general categories source vector diagenesis and analysis the uncertainties within each of these categories are somewhat unique although interactions often arise among processes in different categories i use inference of regional species occurrence from macrofossil assemblages in woodrat neotoma middens to illustrate the effects of various processes and interactions on strength of inference and origin of uncertainties and to identify how uncertainties can be characterized reduced or quantified the kinds of issues discussed in the woodrat midden example are pervasive in paleoenvironmental and paleoecological inference and should be examined thoroughly for all proxies and archives on a regular basis

a global perspective on last glacial maximum to holocene climate change while the abrupt climate events of the last deglaciation are well defined in ice core records from the polar regions of both hemispheres their manifestation elsewhere is less well constrained here we compile 104 high resolution paleoclimate records to characterize the timing and spatial pattern of climate change during the last deglaciation this compilation indicates relatively concurrent timing of the last glacial maximum lgm peak glacial conditions and the altithermal peak interglacial conditions in the northern 22 1 4 3 ka and 8 0 3 2 ka and southern 22 3 3 6 ka and 7 4 3 7 ka hemispheres suggesting the hemispheres were synchronized by greenhouse gases local insolation and or northern hemisphere induced ocean circulation changes the magnitude of the glacialeinterglacial temperature change increases with latitude reflecting the polar amplification of climate change with a likely minimum global mean cooling of ww4 9 c during the lgm relative to the altithermal empirical orthogonal function eof analysis of 71 records spanning 19e11 ka indicates that two modes explain 72 of deglacial climate variability eof1 61 of variance shows a globally near uniform pattern with its principal component pc1 strongly correlated with changes in atmospheric co2 e of2 11 of variance exhibits a bipolar seesaw pattern between the hemispheres with its principal component pc2 resembling changes in atlantic meridional overturning circulation strength eof analysis of 90 records from 15 to 11 ka indicates that northern and southern modes of climate variability characterize the younger dryas bolling allerod interval these modes dominate at the higher latitudes of each hemisphere and exhibit a complex interaction in the tropics the magnitude of the younger dryas climate anomaly cooler drier increases with latitude in the northern hemisphere with an opposite pattern warmer wetter in the southern hemisphere reflecting a general bipolar seesaw climate response global mean temperature decreased by w0 6 c during the younger dryas therefore our analysis supports the paradigm that while the younger dryas was a period of global climate change it was not a major global cooling event but rather a manifestation of the bipolar seesaw driven by a reduction in atlantic meridional overturning circulation strength

chronologic evidence for multiple periods of loess deposition during the late pleistocene in the missouri and mississippi river valley united states implications for the activity of the laurentide ice sheet abstract the loess stratigraphy of the mid continental u s is an important proxy record for the activity of the laurentide ice sheet in north america one of the most outstanding problems is deciphering the age of loess deposits in this area during the late pleistocene radiocarbon dating of snails and thermoluminescence dating of the fine silt fraction 4 11 m from loess at the loveland loess type section loveland iowa and a recent excavation at the pleasant grove school section madison county illinois provide new chronologic control on loess deposition in the mississippi missouri river valley chronology indicates that the loveland loess is illinoian in age 135 20 ka but is not correlative with the teneriffe silt which is dated to 77 8 ka concordant radiocarbon and thermoluminescence age estimates demonstrate that the roxana silt and a correlative loess in iowa the pisgah formation is probably 40 30 ka old these age estimates in conjunction with previous results indicate that there were four periods of loess deposition during the last 150 ka at 25 12 ka 45 30 ka 85 70 ka and at ca 135 20 ka this chronology of loess deposition supports the presence of both a late illinoian and early wisconsinan loess and associated soils thus there may be more than one soil in the loess stratigraphy of the mid continental u s with morphologies similar to the sangamon soil the last three periods of loess deposition may be correlative with periods of elevated dust concentrations recorded in the dye 3 ice core from southern greenland this is particularly significant because both areas possibly had the same source for eolian particles reconstructions of atmospheric circulation for glacial periods show a southerly deflected jet stream that could have transported dust from the mid continental usa to southern greenland lastly the inferred record of loess deposition is parallel to a chronology for deglaciation of the laurentide ice sheet deciphered from chronologic and stratigraphic studies of raised glacial and marine sediments in the hudson bay lowlands canada these chronologies indicate that the laurentide ice sheet was quite dynamic during the late pleistocene advancing and retreating across north america at least four times during the last 150 ka

uranium isotopes in soils as a proxy for past infiltration and precipitation across the western united states the intermittent presence of large pleistocene lakes in the southwestern interior of north america a region that is now a semi arid desert suggests repeated oscillations between profoundly different climatic conditions the origin of these shifts is still unresolved due to inconsistencies in existing climate proxy data for example pollen lake levels and oxygen isotopes in speleothems to resolve the inconsistencies in the water balance over the last 10 to 60 kyr we use uranium isotopic variations in secondary soil minerals to quantify net infiltration and precipitation along a north south transect in western north america we show that winter infiltration increased by 30 to 100 percent and precipitation by a lesser amount in the valleys of the great basin and mojave deserts between 60 and 26 ka this increase in infiltration and precipitation preceded the last glacial maximum lgm and the timing of most lake highstands in the region by 5 to 10 kyr respectively suggesting a possible last precipitation maximum lpm that coincided with a minimum in winter insolation subsequent decreases in infiltration and precipitation after the lgm can be reconciled with the timing of lake highstands if colder summer temperatures due to a minimum in summer insolation reduced lake evaporation the soil records combined with a range of proxy data suggest that seasonal insolation is the long term driver for large shifts in both precipitation and surface water variability in the region

isotopic evidence for shifts in atmospheric circulation patterns during the late quaternary in mid north america wyoming is now at the eastern margin of westerlies originating in the pacific but in the pleistocene appears to have received moisture from elsewhere possibly the gulf of mexico oxygen isotope ratios of pedogenic carbonate in postglacial terraces correspond to ratios in equilibrium with present meteoric waters which show a strong relation to precipitation seasonality and storm sources in contrast the oxygen isotope ratios of all pre holocene soils are significantly more positive even though the carbon isotope composition of coexisting organic matter suggests that the carbonate formed in temperatures cooler than today the oxygen isotope ratios of paleowaters in mid north america appear to be more useful for identifying past storm sources than for estimating paleotemperatures

mid holocene drying of the u s great basin recorded in nevada speleothems lake level records point to dramatic changes in great basin water balance over the last 25 ka but the timing and pace of holocene drying in the region remains poorly documented here we present stable isotope and trace metal data from two lehman caves nv speleothems that provide a well dated record of latest pleistocene to mid holocene hydroclimate in the u s great basin together the stalagmites span the interval between 16 4 ka and 3 8 ka with a hiatus from 15 0 ka to 12 7 ka mg ca and d 13 c covary throughout the records consistent with control by the extent of degassing and prior calcite precipitation pcp measurements of modern cave and soil waters support pcp as the primary control on drip water trace element composition we therefore interpret mg ca and d 13 ca s reflecting infiltration rates with higher values corresponding to drier periods both mg ca and d 13 c indicate a wet period at the beginning of the record 12 7e8 2 ka followed by pronounced drying after 8 2 ka this mid holocene drying is consistent with records from around the western united states including a new compilation of great basin lake level records the strong temporal correspondence with the collapse of the laurentide ice sheet over hudson bay suggests that this drying may have been triggered by northward movement of the winter storm track as a result of ice sheet retreat however we cannot rule out an alternative hypothesis that wet early holocene conditions are related to equatorial pacific sea surface temperature regardless our results suggest that great basin water balance in the early holocene was driven by factors other than orbital changes

lgm summer climate on the southern margin of the laurentide ice sheet wet or dry abstract regional climate simulations are conducted using the polar fifth generation pennsylvania state university psu ncar mesoscale model mm5 with a 60 km horizontal resolution domain over north america to explore the summer climate of the last glacial maximum lgm 21 000 calendar years ago when much of the continent was covered by the laurentide ice sheet lis output from a tailored ncar community climate model version 3 ccm3 simulation of the lgm climate is used to provide the initial and lateral boundary conditions for polar mm5 lgm boundary conditions include continental ice sheets appropriate orbital forcing reduced co2 concentration paleovegetation modified sea surface temperatures and lowered sea level the simulated lgm summer climate is characterized by a pronounced low level thermal gradient along the southern margin of the lis resulting from the juxtaposition of the cold ice sheet and adjacent warm ice free land surface this sharp thermal gradient anchors the midtropospheric

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

stationary waves of the ice age climate abstract a linearized steady state primitive equation model is used to simulate the climatological zonal asymmetries stationary eddies in the wind and temperature fields of the 18 000 ybp climate during winter we compare these results with the eddies simulated in the ice age experiments of broccoli and manabe who used climap boundary conditions and reduced atmospheric co2 in an atmospheric general circulation model gcm coupled with a static mixed layer ocean model the agreement between the models is good indicating that the linear model can be used to evaluate the relative influences of orography diabatic heating and transient eddy heat and momentum transports in generating stationary waves we find that orographic forcing dominates in the ice age climate the mechanical influence of the continental ice sheets on the atmosphere is responsible for most of the changes between the present day and ice age stationary eddies this concept of the ice age climate is complicated by the sensitivity of t

pluvial lakes in the great basin of the western united states a view from the outcrop abstract paleo lakes in the western united states provide geomorphic and hydrologic records of climate and drainage basin change at multiple time scales extending back to the miocene recent reviews and studies of paleo lake records have focused on interpretations of proxies in lake sediment cores from the northern and central parts of the great basin in this review emphasis is placed on equally important studies of lake history during the past 30 years that were derived from outcrop exposures and geomorphology in some cases combined with cores outcrop and core records have different strengths and weaknesses that must be recognized and exploited in the interpretation of paleohydrology and paleoclimate outcrops and landforms can yield direct evidence of lake level facies changes that record details of lake level fluctuations and geologic events such as catastrophic floods drainage basin changes and isostatic rebound cores can potentially yield continuous records when sampled in stable parts of lake basins and can provide proxies for changes in lake level water temperature and chemistry and ecological conditions in the surrounding landscape however proxies such as stable isotopes may be influenced by several competing factors the relative effects of which may be difficult to assess and interpretations may be confounded by geologic events within the drainage basin that were unrecorded or not recognized in a core the best evidence for documenting absolute lake level changes lies within the shore nearshore and deltaic sediments that were deposited across piedmonts and at the mouths of streams as lake level rose and fell we review the different shorezone environments and resulting deposits used in such reconstructions and discuss potential estimation errors lake level studies based on deposits and landforms have provided paleohydrologic records ranging from general changes during the past million years to centennial scale details of fluctuations during the late pleistocene and holocene outcrop studies have documented the integration histories of several important drainage basins including the humboldt amargosa owens and mojave river systems that have evolved since the miocene within the active tectonic setting of the great basin these histories have influenced lake levels in terminal basins many pre late pleistocene lakes in the western great basin were significantly larger and record wetter conditions than the youngest lakes outcrop based lake level data provide important checks on core based proxy interpretations we discuss four such comparisons in some cases such as for lakes owens and manix outcrop and core data synthesis yields stronger and more complete records in other cases such as for bonneville and lahontan conflicts point toward reconsideration of confounding factors in interpretation of core based proxies