the overkill hypothesis as a plausible explanation for the extinctions of late wisconsin megafauna abstract the pulse of large mammal extinctions at the close of the last ice age was a pronounced ecological catastrophe in the western hemisphere available paleoenvironmental paleontological and archeological data are largely concordant with the overkill hypothesis proposing that human predation was the cause of these extinctions climatic explanations fail to postulate specific mechanisms accounting for the event s timing differential impact geographic extent and stratigraphic singularity lack of provisional acceptance of the overkill hypothesis when no viable alternatives exist may be in part because this unconventional hypothesis violates a strong sociocultural bias the image of prehistoric man as the noble savage

vegetation changes and hydrological fluctuations in the central ebro basin ne spain since the late glacial period saline lake records abstract although the central ebro basin northeastern iberian peninsula is both the northernmost semi arid area in europe and one of the regions with the largest biodiversity it has been insufficiently studied in terms of past climate variability due to the scarcity of suitable sites for palaeoenvironmental analyses previous studies from ephemeral saline lakes in the area mainly based on palynological data show abrupt and rapid arid humid transitions throughout the last glacial cycle highlighting a complex palaeohydrological evolution new cores from two saline lakes la playa and la salineta in the los monegros area provide multi proxy records including sedimentology geochemistry and pollen indicators this study together with a detailed review of the main saline records from the central ebro basin enables us to reconstruct the main features of the palaeoclimate evolution during the last glacial cycle one of the main results of this study is the alternation of humid and dry phases as a characteristic of the climate evolution during the late glacial additionally the study suggests an important role of the increased flow from the pyrenean rivers during deglaciation in the hydrological balance of the central ebro basin the early holocene is the wettest period of the studied sequences contrasting with the aridity of the middle holocene interval which is frequently absent as a result of intense aeolian erosive processes the lack of archaeological remains associated with the middle holocene neolithic also supports increased aridity that would have impeded human settlements although anthropogenic activity partially masks the climate signal from the palynological data in the uppermost part of the studied sequences there is some sedimentological evidence for a climate change during the last 2000 yr resulting in a recovery of average saline lake levels in the central ebro basin

interdecadal change in the north atlantic storm track during boreal summer around the mid 2000s role of the atmospheric internal processes the first eof mode eof1 of summer storm track activity over the north atlantic is characterized by a dipole structure with negative storm track anomalies over the south coast of greenland extending northeast across iceland to the norwegian sea and positive anomalies over coastal western europe this study shows that a significant interdecadal change in the north atlantic storm track during boreal summer occurred around the mid 2000s after the mid 2000s the eof1 occurs more frequently at its positive phase which is associated with an interdecadal increase in the geopotential height anomalies around greenland on the intra seasonal timescale the anticyclonic anomalies around greenland are crucial for the occurrence of positive eof1 events via triggering eastward propagating rossby waves therefore the interdecadal increase in the geopotential height anomalies around greenland tends to facilitate the occurrence of the positive eof1 event and is therefore a key driver of the interdecadal change in the summer storm track activity over the north atlantic further analysis indicates that the development of anticyclonic anomalies around greenland is maintained by the self interaction among the low and high frequency transients themselves moreover the anomalous synoptic eddy activities associated with the change in storm tracks can in turn amplify and maintain the rossby waves triggered by the anticyclonic anomalies around greenland in the synoptic scale eddy feedback process

sedimentary and diagenetic features in saline lake deposits of the monegros region northern spain abstract the monegros region in northern spain is marked by the occurrence of a large number of ephemeral to dry lake basins occupying small karstic depressions the lacustrine sediment fill of these basins contain various carbonate and silicate minerals whose origin and palaeoenvironmental significance is poorly understood for the present study 14 lake basins were sampled in order to establish vertical lateral and regional variations in mineralogical and textural characteristics aimed at determining the mode of formation of the various mineral phases present in nearly all basins the same sequence of three lithological units is recognized including a basal clayey unit a middle magnesite bearing and gypsum rich unit and a calcite and dolomite dominated surface unit distribution patterns of carbonate minerals indicate that magnesite is a synsedimentary precipitate dolomite formed as a diagenetic authigenic phase and calcite is partly authigenic and partly allogenic all clay minerals including sepiolite and smectite appear to be allogenic regional variations are marked by similarities between groups of neighbouring basins but no overall trend related to regional drainage patterns is recognized the middle lithological unit records a lake stage with predominantly chemical sedimentation unit ii overlying a less well documented interval corresponding to a perennial lake stage with lower salinity unit iii whereas the surface unit formed during a period with predominantly clastic sedimentation based on a comparison with other regional records the middle unit is attributed to an early holocene humid stage separated from the overlying late holocene deposits by a hiatus that corresponds to a mid holocene arid stage

summer arctic atmospheric circulation response to spring eurasian snow cover and its possible linkage to accelerated sea ice decrease abstractanticyclonic circulation has intensified over the arctic ocean in summer during recent decades however the underlying mechanism is as yet not well understood here it is shown that earlier spring eurasian snowmelt leads to anomalously negative sea level pressure slp over eurasia and positive slp over the arctic which has strong projection on the negative phase of the northern annular mode nam in summer through the wave mean flow interaction specifically earlier spring snowmelt over eurasia leads to a warmer land surface because of reduced surface albedo the warmed surface amplifies stationary rossby waves leading to a deceleration of the subpolar jet as a consequence rising motion is enhanced over the land and compensating subsidence and adiabatic heating occur in the arctic troposphere forming the negative nam the intensified anticyclonic circulation has played a contributing role in accelerating the sea ice decline observed during the last two decades the results here provid

environmental controls on the late holocene carbonate sedimentation of a karstic lake in the middle atlas mountains lake afourgagh morocco the lake afourgagh sediment record and facies successions provide an outstanding example of environmentally controlled carbonate sedimentation afourgagh is a small shallow permanent lake located in the middle atlas mountains in morocco in a karstic context it is fed by ground waters that are relatively enriched in mg resulting from the leaching of the jurassic dolomitic bedrock of the catchment this eutrophic lake is episodically restricted and characterized by alkaline waters with a fluctuating high mg ca ratio the maximum extension of the holocene shoreline coincides with evidence of a lake stabilization level corresponding to the outflow of the lake through a wadi lakeshore terrace sediments deposited on an alluvial fan siltstone during the past ca 2500 cal yr bp comprise four main facies a littoral crust palaeosols palustrine silts and charophyte tufas which reflect different environments from the shoreline toward the deeper water in the more distal parts the charophyte tufas display a well expressed lamination punctuated by the development of microstromatolites on algae thalli the mineralogical composition of the carbonates is linked to the facies while the charophyte tufas are characterized by a relatively high content in aragonite in addition to low mg calcite the littoral crust is mainly composed of magnesite this pattern is related to the evolving chemistry of water due to the influence of charophyte proliferation during dry summers calcium carbonate precipitation on algae thalli both bioinduced and microbially mediated progressively induces an increase in the mg ca ratio of the lake water while the capillary evaporation of shallow ground waters causes precipitation of a magnesite precursor on the shoreline producing magnesite during early diagenesis this effect is characteristic of two episodes part of the roman warm period and the beginning of the dark age cold period the carbonate mineralogy of the different depositional sequences at afourgagh indicates lake level and water chemistry fluctuations under a climatic influence therefore among other regional records the lake afourgagh sedimentary record provides useful evidence for reconstructing these environmental changes

the 8 2 ka event and early mid holocene forests fires and flooding in the central ebro desert ne spain abstract the impact of the 8 2 ka cooling event during the early mid holocene has not been widely observed in southern europe which in contrast to northern europe was already experiencing a cooler than present climate at this time multi proxy analysis of sediment cores from two closed basin saline lakes in the central ebro desert ne spain has allowed us to investigate the impact of climatic changes around the time of this event in more detail long term changes in climate between the early and mid holocene indicate a shift in winter to a more positive nao resulting in declining lake levels in one lake sensitive to winter groundwater recharge and cooler winter temperatures reconstructed from pollen climate analysis reconstructed summer temperatures also declined over this period while annual precipitation and forest cover increased interpreted as a result of enhanced convection driven summer precipitation association with a northward displacement of the sub tropical high pressure around 8 2 ka a marked increase in fire frequency is shown between ca 8 8 and 8 0 ka bp along with an expansion of fire tolerant evergreen oak and peak in water levels in a second storm run off fed lake a maximum in fire intensity occurred with the deposition of a charcoal layer at both lake sites dated to 8150 130 and 8285 135 cal bp respectively the increase in fire is largely attributed to a temporary return southward of the summer sub tropical high pressure over the mediterranean which not only increased summer aridity but also caused a contradictory regional warming before hemispheric cooling set in

holocene paleohydrology of playa lakes in northern and central spain a reconstruction based on the mineral composition of lacustrine sediments abstract the focus of this study is the reconstruction of holocene limnic conditions of playa lakes in central spain using inorganic components of their sediments it is demonstrated that the mineralogical sediment characters provide valuable paleoenvironmental information if dolomites originate from authigenic precipitation or early diagenetic dolomitization the mg ca ratio of dolomites derived from the position of dolomite s major x ray diffraction line turns out to be an indicator for the reconstruction of paleosalinity in contrast the concentrations of carbonates and sulphatic evaporites and their changing proportion to siliciclastic sediments point to changing depositional environments the comparison of investigations from different playa lakes and drainage basin systems shows that the interpretation of mineralogical sediment features always has to consider the petrographic hydrologic and geomorphic features of the catchment as for the lacustrine sediments investigated there are no data available yet a preliminary chronological framework is derived by comparison with results from neighbouring areas based on this the hypothesis is put forward that during the so called little ice age subhumid environmental conditions occurred over the central part of the iberian peninsula while today climatic conditions are dry subhumid to subarid also for times older than the subatlantic distinct changes of environmental conditions are noticed but various hiatuses in the sediments make correlation difficult

paleohydrological fluctuations and steppe vegetation during the last glacial maximum in the central ebro valley ne spain combined analysis of sedimentary facies geochemistry and pollen from lake sediment records and sedimentological and palynological studies from slope deposits allow the characterization of vegetation and lake level status during the last glacial lgm in the central ebro valley ne spain these records show the presence of phases of increased effective moisture while regional vegetation was dominated by steppe species the longest lake record comes from la salineta one of the saline lakes in the los monegros area the other lake sequence comes from a sinkhole in the gallego river floodplain the slope deposit from valmadrid is the only periglacial deposit found in the central ebro valley our data indicate that at least for some intervals during full glacial times when cold steppe vegetation dominated the region some lakes experienced more positive water balance than today and run off was also high the data are coherent with the hypothesis that at least for some periods the ice age climate of the western mediterranean was characterized by cold winters with relatively higher effective moisture precipitation minus evaporation ratio and summer droughts increased flow from the pyrenean rivers during the early deglaciation could also have played a significant role in the paleohydrological cycle in the central ebro valley however la salineta records also show evidence for arid periods during glacial times indicating the complex evolution of hydrology and moisture availability in the central ebro valley during the lgm

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

the timing of late pleistocene mammalian extinctions in north america more than 375 14c dates from 150 fossil sites in north america have been analyzed to evaluate the question of extinction of late pleistocene megafauna when critically evaluated no 14c ages for any extinct pleistocene genera are younger than 10 000 yr b p

a depositional model for hydromagnesite magnesite playas near atlin british columbia canada this study formulates a comprehensive depositional model for hydromagnesite magnesite playas mineralogical isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite magnesite playas near atlin british columbia canada four surface environments are recognized wetlands grasslands localized mounds metre scale and amalgamated mounds composed primarily of hydromagnesite mg co oh 4ho which are interpreted to represent stages in playa genesis water chemistry precipitation kinetics and depositional environment are primary controls on sediment mineralogy at depth average 2 m ca mg carbonate sediments overlay early holocene glaciolacustrine sediments indicating deposition within a lake post deglaciation this mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water for example glacier meltwater ceased and was replaced by alkaline groundwater weathering of ultramafic bedrock in the region produces mg hco groundwater that concentrates by evaporation upon discharging into closed basins occupied by the playas an uppermost unit of mg carbonate sediments hydromagnesite mounds overlies the ca mg carbonate sediments this second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial occurring once sediments emerged from the water surface capillary action and evaporation draw mg hco water up towards the ground surface precipitating mg carbonate minerals evaporation at the water table causes precipitation of lansfordite mgco 5ho which partially cements pre existing sediments forming a hardpan as carbonate deposition continues the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds radiocarbon dating of buried vegetation at the ca mg carbonate boundary indicates that there has been ca 8000 years of continuous mg carbonate deposition at a rate of 0 4 mm yr the depositional model accounts for the many sedimentological mineralogical and geochemical processes that occur in the four surface environments elucidating past and present carbonate deposition

late holocene evolution of playa lakes in the central ebro depression based on geophysical surveys and morpho stratigraphic analysis of lacustrine terraces abstract the origin and morpho stratigraphic evolution of the largest playa lake system la playa el pueyo in the bujaraloz sastago endorheic area located in the semiarid central sector of the ebro depression are analysed the enclosed depressions are developed on gypsiferous tertiary bedrock and show a prevalent wnw ese orientation parallel to the direction of the prevalent strong local wind cierzo yardangs have been carved in bedrock and unconsolidated terrace deposits in the leeward sector of the largest lake basins a sequence of three lacustrine terrace levels has been identified by detailed geomorphological mapping the treads of the upper middle and lower terrace levels are situated at 9 m 6 m and 0 5 m above the playa lake floors respectively seismic refraction and electrical resistivity profiles acquired in la playa reveal a thin basin fill 2 m with a planar base these data allow ruling out the genetic hypothesis for the depressions involving the collapse of large bedrock cavities and support a mixed genesis of combined widespread dissolution and subsidence by groundwater discharge and eolian deflation during dry periods the 5 m thick deposit of the middle terrace was investigated in hand dug and backhoe trenches six ams radiocarbon ages from this terrace indicate an aggradation phase between 3 9 ka and ca 2 ka these numerical ages yield a maximum average aggradation rate of 2 6 mm yr and a minimum excavation rate by wind deflation of 3 mm yr subsequent to the accumulation of the middle terrace the latter figure compares well with those calculated in several arid regions of the world using yardangs carved in palaeolake deposits the aggradation phase between 4 and 2 ka is coherent with other iberian and mediterranean records showing relatively more humid conditions after 4 ka including the iron ages and the iberian roman period

response of sea ice to the arctic oscillation abstract data collected by the international arctic buoy programme from 1979 to 1998 are analyzed to obtain statistics of sea level pressure slp and sea ice motion sim the annual and seasonal mean fields agree with those obtained in previous studies of arctic climatology the data show a 3 hpa decrease in decadal mean slp over the central arctic ocean between 1979 88 and 1989 98 this decrease in slp drives a cyclonic trend in sim which resembles the structure of the arctic oscillation ao regression maps of sim during the wintertime january march ao index show 1 an increase in ice advection away from the coast of the east siberian and laptev seas which should have the effect of producing more new thin ice in the coastal flaw leads 2 a decrease in ice advection from the western arctic into the eastern arctic and 3 a slight increase in ice advection out of the arctic through fram strait taken together these changes suggest that at least part of the thinning of sea ice recently observed ov

chronologic and environmental implications of a new genus of fossil deer from late wisconsin deposits at toronto canada abstract a new cervine deer torontoceros hypogaeus described from a partial cranium with portions of the main antler beams has been recovered from deposits of early lake ontario age on the exposed bench of glacial lake iroquois at toronto ontario canada the deer was about the size of a caribou has heavy antlers that lie chiefly in a nearly horizontal plane and its beams are bowed anteriorly the tines are not flattened the brow tines are asymmetrical and no evidence of surface roughening or palmation of the beam is found a 14 c date of 11 315 325 yr b p obtained on the antler allows the date at which glacial lake iroquois drained to be revised to before 11 400 yr b p spruce picea pine pinus and sedges cyperaceae are major components of the associated pollen spectrum which implies a typically interstadial or postglacial climate in which mixed forests grew in the toronto area

a younger thinner arctic ice cover increased potential for rapid extensive sea ice loss 1 satellite derived estimates of sea ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present these data show that in addition to the well documented loss of perennial ice cover as a whole the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly the oldest ice types have essentially disappeared and 58 of the multiyear ice now consists of relatively young 2 and 3 year old ice compared to 35 in the mid 1980s ice coverage in summer 2007 reached a record minimum with ice extent declining by 42 compared to conditions in the 1980s the much reduced extent of the oldest and thickest ice in combination with other factors such as ice transport that assist the ice albedo feedback by exposing more open water help explain this large and abrupt ice loss

the recent shift in early summer arctic atmospheric circulation 1 the last six years 2007 2012 show a persistent change in early summer arctic wind patterns relative to previous decades the persistent pattern which has been previously recognized as the arctic dipole ad is characterized by relatively low sea level pressure over the siberian arctic with high pressure over the beaufort sea extending across northern north america and over greenland pressure differences peak in june in a search for a proximate cause for the newly persistent ad pattern we note that the composite 700 hpa geopotential height field during june 2007 2012 exhibits a positive anomaly only on the north american side of the arctic thus creating the enhanced mean meridional flow across the arctic coupled impacts of the new persistent pattern are increased sea ice loss in summer long lived positive temperature anomalies and ice sheet loss in west greenland and a possible increase in arctic subarctic weather linkages through higheramplitude upper level flow the north american location of increased 700 hpa positive anomalies suggests that a regional atmospheric blocking mechanism is responsible for the presence of the ad pattern consistent with observations of unprecedented high pressure anomalies over greenland since 2007 2012 american geophysical union all rights reserved

the nature and significance of variations in gypsum crystal morphology in dry lake basins depressions in semiarid regions with a gypsiferous geological substrate commonly contain highly gypsiferous sediments with gypsum that formed either as a synsedimentary precipitate during perennial or ephemeral lake stages or as a diagenetic precipitate during saline mudflat stages the dry to ephemeral lake basins of the monegros region in northern spain are depressions of this type and provide a unique opportunity to examine whether and how conditions of gypsum formation are reflected by gypsum crystal morphology yielding information that can contribute to a correct interpretation of the depositional environment of gypsum in ancient and modern formations in most lake basins of the study area the sediment fill consists of a similar sequence of three lithological units the lower unit includes fragmented gypsum crystals in some basins recording clastic reworking and in other depressions it contains large lenticular crystals formed as phreatic diagenetic precipitates the middle unit typically contains synsedimentary gypsum with a non lenticular morphology modified by postdepositional processes but it also comprises synsedimentary lenticular gypsum in some basins in the upper unit separated from the underlying interval by a marked hiatus both synsedimentary and diagenetic gypsum are present the former including layered gypsum deposits with features such as grading and variations in crystal morphology the results show that a non lenticular crystal habit of gypsum occurring within the groundmass sediment matrix indicates synsedimentary precipitation in contrast a lenticular crystal habit cannot be used in isolation as an indicator of the environment in which the gypsum initially precipitated unambiguously late diagenetic gypsum includes fine grained precipitates in voids created by soil fauna these infillings consist of initially non lenticular crystals with a tabular prismatic habit recording that conditions promoting lenticular crystal growth do not always prevail outside the groundmass in soil related settings and illustrating the importance of knowing the context of analyzed gypsum occurrences at the scale of a sample

reconstruction of palaeoenvironmental conditions by investigation of holocene playa sediments in the ebro basin spain preliminary results endorheic basins are typical landscape elements found in several parts of the ebro basin spain mineralogical and geochemical analysis of sediment cores from the endorheic basins facilitates the reconstruction of local palaeoenvironmental and palaeoclimatic conditions during the holocene the different morphogenetic and lithological environments of the areas investigated allows one to calibrate such information first results of the analysis of the geochemistry and mineralogy of sediments from two endorheic basins within the central ebro basin indicate that the climatic conditions governing sedimentation during the holocene changed from humid and subhumid to the present subarid conditions but was interrupted by a phase of increased humidity the varying character of solid bedrock caused different weathering products which are reflected in the varying geochemical and mineralogical composition of lake deposits