folsom drought and episodic drying on the southern high plains from 10 900 10 200 14c yr b p the paleoenvironments of late pleistocene and early holocene time on the southern high plains have been studied for decades but regionally extensive or long term easily recoverable proxy climate indicators are difficult to find the stratigraphy of valley fill and upland eolian deposits and stable carbon isotope data in addition to geographically limited paleontological data now provide clues to the environment during this time which includes the earliest or paleoindian period 11 200 8000 14c yr b p of human occupation during the clovis occupation 11 200 10 900 14c yr b p valleys contained perennial streams this was followed in folsom time 10 900 10 200 14c yr b p by an abrupt change to lakes and ponds with water levels fluctuating between several meters depth and no surface water and marshes and accumulation of sheet sands on uplands starting the earliest phase of construction of the regional dune fields these changing conditions indicate a shift from relatively wetter to relatively drier conditions with episodic drought stable c isotopes further indicate that warming characterized the clovis folsom transition during the rest of the paleoindian period the environment was relatively cool but fluctuated between wetter and drier conditions with an overall trend toward drying that resulted in further enlargement of the dune fields and culminated in the warm dry altithermal beginning 8000 14c yr b p clovis time probably was the wettest of any paleoindian period in terms of runoff and spring discharge the folsom period was drier and was the earliest episode of regional wind erosion and eolian deposition and may have been the warmest of paleoindian times evidence of a previously hypothesized clovis drought in this region is sparse

a slab in depth three dimensional geometry and evolution of the indo australian plate geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole the subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics plates continents subduction zones and spreading centers all exhibit motion and geometric evolution so to try and resolve the past geometries of the planet geologists have utilized plate tectonic reconstructions here we present a three dimensional image of the subducted indo australian plate below southeast asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of se asia over the past 50 ma including the collision of india with the asian continent we show how the once semicontinuous subducting indo australian plate has been segmented during collision between india australian and the subduction margin to the north thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of earth s surface as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate

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

stable isotope ratios of carbon in phytoliths as a quantitative method of monitoring vegetation and climate change the 13c 12c ratios of occluded carbon within opal phytoliths from the northern great plains show potential as a basis for paleoclimatic reconstruction a significant correlation exists between the carbon isotopic composition of a host plant and that of the organic matter in its phytoliths the 13c 12c ratios for phytoliths from surface layers of soils along climatic gradients reflect the current proportions of c3 and c4 plants variations in the 13c values of phytoliths with soil depth are caused by a variety of processes burial of soil surface by dust bioturbation and possible illuviation by percolating water also contributions of phytoliths by dust and roots have unknown isotopic effects the 13c values of phytoliths from soils increase with decreasing 14c age suggesting that the proportion of c4 plants in this region has increased during the holocene phytoliths of apparent mid holocene age suggest exclusive dominance by c4 plants which agrees with paleoclimatic interpretations of an arid middle holocene climate

radiocarbon ages on late pleistocene loess stratigraphy of nebraska and kansas central great plains u s a in the central great plains of the united states radiocarbon dating of loess and buried soils is clarifying the late quaternary loess chronology for the period 25 000 to 10 000 bp along harland lake nebraska a soil developed in the gilman canyon formation and overlain by peoria loess has radiocarbon ages on soil humates of 30 700 to 21 500 bp at one site two picea charcoal bands in the lower meter of peoria loess have radiocarbon ages of 21 250 and 19 730 bp therefore in the vicinity of harlan lake deposition of peoria loess apparently began around 21 000 bp peoria loess deposition appears to have been interrupted by an episode of river entrenchment prior to 12 600 bp spring activity shortly thereafter suggests that incision was coeval to an increase in effective moisture the termination of peoria loess deposition is marked by the brady soil which in the harlan lake area has radiocarbon ages on soil humates of 11 800 to 10 200 bp comparison of the harlan lake chronology with chronologies completed elsewhere in the central great plains reveals general synchrony among periods of pedogenesis and peoria loess deposition little is known however about vegetation conditions in the region when loess was accumulating charcoal has been noted chiefly in the basal peoria loess suggesting that some trees were present during initial peoria loess deposition the extent of this tree cover is unknown however charcoal is rare in the middle and upper peoria loess

middle to late pleistocene loess record in eastern nebraska usa and implications for the unique nature of oxygen isotope stage 2 abstract new subsurface data reveal a nearly continuous stratigraphic record of middle to late pleistocene loess sedimentation preserved beneath upland summits in eastern nebraska usa thickness and grain size trends as well as pedologic evidence indicate significant changes in loess sources accumulation rates and depositional environments the newly defined kennard formation accumulated in the middle pleistocene and may represent multiple thin increments of distal loess from nonglacial sources on the great plains the overlying loveland loess up to 18 m thick and deposited during oxygen isotope stage 6 ois 6 illinoian glaciation probably records the emergence of the missouri river valley as a major glaciogenic loess source the prominent sangamon geosol formed through long term pedogenic alteration of the upper loveland loess during ois 5 and 4 thin loess of the gilman canyon formation records slow loess accumulation and pedogenic alteration in ois 3 the peoria loess ois 2 is similar in thickness to loveland loess but may have accumulated more rapidly in an environment less favorable to bioturbation more importantly comparison of peoria and loveland loess thickness trends indicates much greater influx of nonglaciogenic loess from the great plains during ois 2 than in ois 6 suggesting colder and or drier conditions in the midcontinent during ois 2 than in earlier glacial stages

indian continental subduction and slab break off during tertiary collision high wavespeed seismic anomalies in the transition zone and uppermost lower mantle beneath the india asia collision zone imaged by body wave seismic tomography have been interpreted as subducted fragments of continental material in this study we focus on the prominent anomaly located beneath india between depths of about 450 and 900 km by combining the location of this anomaly with palaeogeographical positions of india we constrain the timing of the subduction event probably related to this anomaly we infer that a large portion of the north western margin of india initiated subduction at 35 5 ma along a 1500 km long wnw ese striking zone and ended with a progressive slab break off process this break off started most probably around 25 ma at the western end of the slab and propagated eastwards until complete break off around 15 ma this study helps to constrain better the amount of convergence between india and asia absorbed by continental subduction

large scale stabilized dunes on the high plains of colorado understanding the landscape response to holocene climates with the aid of images from space principal component analysis of landsat thematic mapper images from eastern colorado reveals stabilized dune fields that are composed of single and compound parabolic dunes some longer than 10 km these dunes have been reactivated at least four times in the past 10 000 yr at ca 9500 to 5500 yr b p 5500 to gt 4800 yr b p 4800 to gt 1000 yr b p and lt 1000 yr b p during droughts that probably exceeded the dry conditions during the 1930s dune orientations indicate that these landforms were molded by winds from the northwest which now dominate during the winter and spring with the passage of pacific air masses this study underscores the sensitivity of the landscape of the high plains to drought conditions which may become more prevalent because of the anticipated effects of greenhouse warming

isotopic and rock magnetic expression of environmental change at the pleistocene holocene transition in the central great plains abstract in a region too dry for the effective use of the traditional palynological approach loess deposits of the central great plains offer a source of proxy information for the reconstruction of past climates the brady soil a ubiquitous feature developed within the uppermost late pleistocene peoria loess recorded the character of vegetation change associated with climate shifts during the pleistocene holocene transition stable carbon isotope ratio analysis and two room temperature magnetic parameters were used to examine the nature of the change during this period 13 c values derived from soil organic matter indicate that in the late pleistocene minor populations of drought tolerant c4 plants were interspersed among the dominant cool season grasses c3 in the west while c3 grasses were accompanied by trees in the east by the early holocene c4 grasses were the dominant vegetation in the west and contributed substantially to the 13 c signal in the east magnitude of the 13 c shift suggests an increase in july mean daily temperature of 8 10 c according to existing models magnetic susceptibility and frequency dependence signal the cessation of loess deposition and development of the brady soil and indicate a spatial pattern in the intensity of weathering across the region

loess record of dry climate and aeolian activity in the early to mid holocene central great plains north america thick holocene bignell loess sections provide new evidence for the timing of early to middle holocene aridity in the central great plains the immediate source of loess in these sections was dune fields just upwind based on thickness trends and grain size data thus periods of rapid loess accumulation indicate episodes of extensive dune activity under drier than present climate in typical bignell loess sections a thick zone of coarse textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid sustained holocene loess accumulation optical ages indicate that loess in this zone was deposited beginning about 9000 10000 years ago at five study sites accumulation ended shortly after 6500 years ago at three sites but possibly earlier at the other two in two well dated sections the average rate of loess accumulation between 9000 10000 and 6500 years ago was about 1 6 3 5 times greater than in the late holocene thus we infer that the most extensive sustained holocene

geochemical variations in peoria loess of western iowa indicate paleowinds of midcontinental north america during last glaciation peoria loess deposited in western iowa during the last glacial maximum lgm shows distinct geochemical and particle size variations as a function of both depth and distance east of the missouri river geochemical and particle size data indicate that peoria loess in western iowa probably had two sources the missouri river valley and a source that lay to the west of the missouri river both sources indicate that lgm paleowinds in western iowa had a strong westerly component similar to interpretations of previous workers a compilation of loess studies in iowa and elsewhere indicates that westerly winds were dominant during loess transport over much of the midcontinent south of the laurentide ice sheet which is not in agreement with paleowinds simulated by atmospheric general circulation models agcms agcms consistently generate a glacial anticyclone with easterly or northeasterly winds over the laurentide ice sheet and the area to the south of it loess deposition in the midcontinent during the lgm may be a function of infrequent northwesterly winds that were unrelated to the presence of the glacial anticyclone

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

stratigraphy and geochronology of upper quaternary eolian sand on the southern high plains of texas and new mexico united states eolian sand in dune fields and sand sheets cover 10 000 km 2 10 of the southern high plains of northwestern texas and eastern new mexico these deposits are concentrated in three west east trending belts of dunes the muleshoe lea yoakum and andrews dune fields from north to south that appear to be eastern extensions of the mescalero monahans dune system in the pecos river valley and in the seminole sand sheet a discontinuous accumulation of sheet sands between the lea yoakum and monahans andrews dunes the most common landforms are parabolic dunes associated with blowouts coppice dunes and sand sheets all typical of sandy vegetated semiarid landscapes barchan dunes in keeping with a relatively limited sand supply and an underlying surface that is relatively hard blackwater draw formation pleistocene and fence row dunes historic dunes formed along field boundaries these eolian deposits accumulated episodically in the late pleistocene and holocene and provide clues to the history of regional drought and aridity the earliest phase of sedimentation occurred when sheet sands were deposited between 11 000 and 8000 14 c yr b p probably in several phases based on archaeological data as a result of episodic drought beginning between 11 000 and 10 000 yr b p eolian deposits dating between 8000 and 3000 yr b p are rare although eolian sediment 8000 4500 14 c yr b p is ubiquitous in the draws that cross the region and paleoenvironmental indicators show that the region was subjected to aridity throughout middle holocene time the middle holocene deposits most likely were remobilized in late holocene time most of the deposits and landforms of the dune fields and sand sheets are late holocene dating before 4000 14 c yr b p and mostly before 1500 14 c yr b p the muleshoe and lea yoakum dunes and the seminole sand sheet underwent substantial eastward expansion in late holocene time buried soils and radiocarbon ages show that the eolian sand accumulated in several stages probably in response to cyclic drought the muleshoe dunes accumulated after ca 1300 cal yr b p ca 1400 14 c yr b p after ca 750 670 cal yr b p ca 850 750 14 c yr b p just after ca 500 cal yr b p ca 450 14 c yr b p and during the nineteenth and twentieth centuries the lea yoakum dunes were active after ca 3400 cal yr b p ca 3200 14 c yr b p and historically the seminole sand sheet was active ca 430 330 cal yr b p ca 360 14 c yr b p and in the twentieth century the andrews dunes were subjected to at least two phases of eolian sedimentation after ca 2320 cal yr b p ca 2320 14 c yr b p comparisons with eolian chronologies from other regions on the great plains suggest that dune mobilization was a regional phenomenon after ca 2300 cal yr b p ca 2300 14 c yr b p after ca 1500 1400 cal yr b p ca 1650 1550 14 c yr b p after ca 700 cal yr b p ca 800 14 c yr b p between 500 and 300 cal yr b p ca 450 300 14 c yr b p and in the nineteenth century the climatic fluctuations responsible for mobilizing the dunes probably were relatively minor yet the landscape impacts were substantial resulting in widespread wind erosion and dune construction

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

thermoluminescence and new 14c age estimates for late quaternary loesses in southwestern nebraska abstract loess of late quaternary age mantles most of nebraska south of the platte river valley at least five late quaternary loesses are recognized from oldest to youngest one or more undifferentiated pre illinoian loesses the loveland loess the gilman canyon loess which exhibits a well developed soil and rests unconformably on the sangamon soil the peoria loess capped by the brady soil and the bignell loess which is distributed discontinuously previous research shows that the loveland loess is illinoian the gilman canyon loess and peoria loess are wisconsin and the bignell loess is holocene we present here the first thermoluminescence tl age estimates and new 14 c ages for these late quaternary loesses at two key sections in southwestern nebraska the eustis ash pit and the bignell hill road cut tl age estimates from all samples collected from eustis ash pit and bignell hill were internally consistent tl and 14 c age estimates from these two sections generally agree and support previous age determinations the tl age estimate on loveland loess indicates deposition at 163 ka tl and radiocarbon age estimates indicate that gilman canyon loess believed to be deposited during the farmdale interstade first began to accumulate at about 40 ka the lower part of the gilman canyon loess is 36 ka at eustis and the middle of the unit is 30 ka at bignell hill the lower and upper parts of the peoria loess give age estimates of 24 ka and 17 ka respectively tl age estimates for deposition of the bignell loess are 9 ka near the base in agreement with radiocarbon age estimates and 6 ka immediately below the modern soil substantiating its holocene age comparisons of tl age estimates with 18 o and insolation curves which show loess deposition during interglacial and interstadial as well as glacial periods indicate that loess deposition on the great plains can occur under a variety of climatic conditions

isotope evidence for pleistocene climatic changes in kashmir india stable carbon isotope ratios of organic carbon from palaeosols in loess and oxygen isotope ratios of pedogenic and lacustrine carbonates reveal significant pleistocene climatic changes in the kashmir basin the extreme 13c values of 16 and 25 reported here indicate a shift from arid to sub humid conditions the 18o values of pedogenic carbonates is 2 higher than that of the underlying lacustrine carbonates suggesting that they were deposited in different climatic conditions

crustal mass budget and recycling during the india asia collision abstract the long lasting collision between india and asia has resulted in crustal and lithospheric deformation of both continents providing unequalled opportunities to evaluate their long term mechanisms of deformation we have quantified a crustal mass budget for the collision by comparing the present day and initial crustal volumes of the indian and asian continents involved in the collision initial crustal thickness was estimated from the non deformed parts of the continents whereas their initial extent was mapped from their contours at the onset of collision using seismic tomography we quantify the portion of the initial crustal thickness of both continents stored within the currently thickened crust or redistributed due to extrusion for the indian continent this portion amounts to about 19 km far lower than the mean observed present day crustal thickness of the indian craton of about 38 km we conclude that between 40 and 50 of indian crust has been recycled into the mantle by continental subduction corresponding to a decoupling level at about 15 19 km depth for the asian continent the estimated crustal thickness stored during collision is about 33 km close to the initial asian crustal thickness we estimate that only 3 of the asian crust was recycled into the mantle this corresponds to one episode of continental subduction occurring most probably soon after the initiation of collision along the bangong suture we identify the related slab using seismic tomography the strong contrast between india and asia implies an age dependent capacity of the continental crust to be recycled into the mantle this result has to be taken into account for further analysis of global crustal recycling during earth s history

impact of holocene dust aggradation on a horizon characteristics and carbon storage in loess derived mollisols of the great plains usa the amount and vertical distribution of soil organic carbon soc storage is primarily a function of vegetation type and climate with influence from soil and landscape properties and human activities the influence of dust aggradation on landscapes during the holocene has largely been ignored as a factor in long term soc storage dust aggradation is important to soil horizonation and c sequestration since the dust deposit buries the landscape and increases solum thickness mollisols on loess covered upland tables in central nebraska have aggraded throughout the holocene as bignell loess accumulated above the late pleistocene brady soil in the loess tablelands the thickness of mollic colors a horizon properties and soc storage all decrease with distance from the source of bignell loess which is along a gradient of increasing precipitation for example along a 140 km transect of decreasing bignell loess thickness and increasing rainfall total thickness of a horizons decreases by 70 cm and the thickness of mollic colors decreases by 93 cm total soc storage decreases from 17 5 to 7 7 kg m2 a portion of the soc in subsurface horizons of modern soils was likely inherited from the brady soil which probably formed in a climate different than the present suggesting that soc storage is probably not in equilibrium with modern climatic conditions dust aggradation on a biologically active surface in a steppe environment has affected horizonation pigmentation and soc storage in central great plains loess derived mollisols

middle and late holocene dune reactivation in the nebraska sand hills usa a combination of optical dating of aeolian sand and radiocarbon dating of palaeosols and interdune sediments has allowed some refinements to the chronostratigraphy of the nebraska sand hills our results directly confirm previous inferences that multiple phases of aeolian deposition including megabarchan dune reactivation occurred within the sand hills during the middle and late holocene optical dates on stratified aeolian barchan dune sediments from two localities provide ages ranging from c 6 ka to 300 years ago weakly developed palaeosols intercalated within the dune sediments and other forms of post depositional modification indicate periods of relative stability and reduced dune mobilization optical dates on the upper 12 15 m of two dune exposures indicate repeated possibly regionally correlative reactivations which occurred between c 400 500 and 200 300 years ago comparison of late holocene aeolian activity with local and regional palaeoclimatic proxy records suggests that dune reacti

late quaternary environmental reconstructions of playa lunette system evolution on the central high plains of kansas united states playas are small ephemeral wetlands found throughout semiarid and arid regions around the world lunettes are dunes that form downwind of some large playas two playa lunette systems on the high plains were investigated to reconstruct paleoenvironments during playa lunette system evolution both playa lunette systems are composed of sediment spanning 40 k y with earliest deposits accumulating during at least marine isotope stage mis 3 and possibly starting as early as mis 5 the gilman canyon formation pedocomplex dating between 42 and 28 ka dominates this period and is ubiquitous throughout the playa lunette systems climate during mis 3 was similar to modern with warm temperatures low effective moisture and playa floors exposed long enough to allow pedogenesis during mis 2 climate was relatively cool with higher effective precipitation and playas were inundated for longer periods the regionally expressed brady soil of the pleistocene holocene transition caps these deposits and dates between 11 8 and 9 4 ka during the pleistocene holocene transition climate warmed yet moisture availability remained relatively high several distinct shifts in 13 c identify rapid climate changes associated with the bolling allerod younger dryas chronosequence warming continued into the holocene though moisture availability was highly variable holocene soils are common and date between 8 3 and 3 6 ka playa lunette system stratigraphy represents a continuum of the upland loess sequence though deposits are altered by playa hydrology geomorphic processes alternated between dissolution and fluvial eolian driven processes as climate changed and playas developed due to the aggradational environment and sensitivity of these systems high resolution records of environmental change throughout their evolution are preserved

upper mantle beneath southeast asia from s velocity tomography 1 we present a three dimensional s velocity model of the se asian western pacific upper mantle with 400 km lateral resolution using the novel automated multimode inversion technique we processed 4038 vertical component seismograms and extracted 22 708 linear equations with uncorrelated uncertainties that constrain upper mantle structure we used time frequency windows to select signal with negligible proportion of scattered energy the windows included the fundamental rayleigh mode and s and multiple s waves the observed range of s velocity variations is the widest 17 18 in the upper 150 km of the mantle high velocity continental roots can reach beyond the present extent of the overlying archean proterozoic crust by 500 km beneath some precambrian units the roots are absent which can be attributed to deformation and gradual destruction of the ancient lithosphere at 120 150 km s velocity beneath some cratons reaches 4 8 km s this can be accounted for by thermal and compositional effects beneath the hainan island area a low velocity anomaly is observed from near the surface to the bottom of our model the hot spot type volcanism here may be caused by the deep mantle hainan plume a low velocity mantle domain underlies the south central sea of japan surrounded on the surface by intraplate volcanoes a deep seismicity gap is present near 40 n in the pacific slab subducting below and may result from a plume slab interaction a high velocity anomaly is present in the transition zone beneath the northern boundary of the yangtze craton we propose that the anomaly corresponds to subducted continental lithosphere stagnant atop the 660 km discontinuity

platelet ice attachment to instrument strings beneath the amery ice shelf east antarctica oceanographic instruments suspended beneath the amery ice shelf east antarctica have recorded sporadic pressure decreases of 10 20 dbar over a few days at three sites where basal marine ice growth is expected we attribute these events to flotation due to platelet ice accretion on the instrument moorings some events were transient rapidly returning to pre event pressures probably through dislodgement of loosely attached crystals driven by these pressure changes temperatures recorded by the shallowest instruments within 20 m of the shelf base tracked in situ freezing temperatures during the events these observations provide indirect evidence for the presence of frazil ice in the sub ice shelf mixed layer and for active marine ice accretion at one site we infer that a dense layer of platelet ice 1 5 m thick was accreted to the ice shelf over a 50 day period following some permanent abrupt pressure decreases which we interpret as due to the lodgement of the uppermost instrument at the ice shelf base altered background trends in pressure suggest compaction rates of 3 4 m a 1 for the accreted basal platelet layer attachment of platelet ice and resulting displacement of moorings has ramifications for project design and instrument deployment and implications for interpretation of oceanographic data from sub ice shelf environments

the age and taphonomy of mammoths at lovewell reservoir jewell county kansas usa abstract remains of five mammoths have been excavated along the north shore of lovewell reservoir on white rock creek jewell county kansas two additional mammoths have been recorded as surface finds these seven mammoths are clustered within a distance of 2 km and are contained within sediments dating to the transition from terminal mid wisconsin to last glacial maximum this density of single adult mammoth death sites is uncommon elsewhere on the central great plains of north america radiocarbon ages from bone and decalcified organic carbon indicate the mammoths date between about 18 000 and 21 000 rcybp a discussion of the stratigraphy ages and taphonomy for the five excavated mammoth localities is presented including new taphonomic data collected from the lovewell mammoth site 14jw306 during the 2004 excavation new data indicate that the lovewell mammoth site contains three mammoths where as previously only one mammoth was reported holen s r 2006 taphonomy of two last glacial maximum mammoth sites in the central great plains of north america a preliminary report on la sena and lovewell quaternary international 142 143 30 43 results presented here support the earlier taphonomic interpretation that humans were present on the central great plains during the last glacial maximum

last glacial loess in the conterminous usa the conterminous united states contains an extensive and generally well studied record of last glacial loess the loess occurs in diverse physiographic provinces and under a wide range of climatic and ecological conditions both glacial and non glacial loess sources are present and many properties of the loess vary systematically with distance from loess sources united states midcontinent last glacial loess is probably the thickest in the world and our calculated mass accumulation rates mars are as high as 17 500 g m 2 yr at the bignell hill locality in nebraska and many near source localities have mars greater than 1500 g m 2 yr these mars are high relative to rates calculated in other loess provinces around the world recent models of last glacial dust sources fail to predict the extent and magnitude of dust flux from the mid continent of the united states a better understanding of linkages between climate ice sheet behaviour routing of glacial meltwater land surface processes beyond the ice margin and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region r 2003 elsevier ltd all rights reserved

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

latest pleistocene to holocene loess in the central great plains optically stimulated luminescence dating and multi proxy analysis of the enders loess section nebraska usa abstract loess deposits of the central great plains usa and their intercalated soils provide a detailed record of climatically driven changes within the aeolian system during the pleistocene holocene transition and the holocene here we present a detailed optically stimulated luminescence osl chronology as well as multi proxy analysis obtained for the first time on the enders section located in southwestern nebraska central great plains the section records multiple episodes of rapid loess deposition alternating with soil formation rapid accumulation of late pleistocene peoria loess was replaced around 13 14 ka by formation of the brady soil until 9 5 0 6 ka the holocene bignell loess then buried the brady soil and accumulated episodically throughout the holocene the loess paleosol stratigraphy since the late pleistocene at the enders site is very similar to that at other sites in western nebraska and the newly developed osl chronology based on three grain size classes adds new confidence to earlier dating the high resolution grain size profile from enders shares many features with similar data from the previously studied wauneta site including three peaks of fine grained material just above and within the brady soil likely representing response to millennial scale climatic changes during the pleistocene holocene transition this study demonstrates the potential for developing high resolution well dated paleoclimatic records from the loess of the central great plains contrasts between great plains and eurasian loess records reflect differences in the late pleistocene to holocene climatic evolution and other factors influencing the loess system

stratigraphy and environmental reconstruction at the middle wisconsinan gilman canyon formation type locality buzzard s roost southwestern nebraska usa abstract the middle wisconsinan gilman canyon formation at the buzzard s roost type locality in southwestern nebraska was investigated to document the stratigraphy and to reconstruct the environmental and climate record the gilman canyon formation was subdivided into three loess units and three soils with radiocarbon ages constraining it between about 40 ka and 25 ka stable carbon isotope ratios magnetic susceptibility and carbon content were used to define and characterize soils within both the gilman canyon formation and underlying illinoian loveland loess at the height of soil development within the gilman canyon formation climate was supporting c4 dominated grassland with july temperatures equal to or exceeding those of today soil forming intervals within the loveland loess including the sangamon soil also exhibited relative increases in c4 biomass climate as recorded in the gilman canyon formation is corroborated by regional proxy data the formation accumulated during mis 3 and concurrent soil formation coincided with a summer insolation maximum

taphonomy of two last glacial maximum mammoth sites in the central great plains of north america a preliminary report on la sena and lovewell abstract two mammoth sites from the central great plains of north america each containing one adult columbian mammoth mammuthus columbi were excavated from last glacial maximum lgm loess and fine grained alluvial deposits respectively taphonomic data from both sites indicate that the mammoth skeletons exhibit numerous spirally fractured limb elements dynamic loading points are present on midshafts of large limb bones bone flakes produced from the partial thickness of thick cortical bone are also present hypotheses of carnivore activity mammoth trampling and human induced fracturing are evaluated as possible causes of the fractured limb bone testing the hypotheses using modern data from actualistic taphonomic studies of elephant skeletons paleontological data from two proboscidean natural death sites experimental data from elephant bone fracturing and archaeologically derived data concerning late pleistocene human modification of mammoth limb bone indicates that the first two hypotheses can be rejected while the third hypothesis is supported

unprecedented last glacial mass accumulation rates determined by luminescence dating of loess from western nebraska abstract a high resolution chronology for peoria last glacial period loess from three sites in nebraska midcontinental north america is determined by applying optically stimulated luminescence osl dating to 35 50 m quartz at bignell hill nebraska an osl age of 25 000 yr near the contact of peoria loess with the underlying gilman canyon formation shows that dust accumulation occurred early during the last glacial maximum lgm whereas at devil s den and eustis nebraska basal osl ages are significantly younger 18 000 and 21 000 yr respectively at all three localities dust accumulation ended at some time after 14 000 yr ago mass accumulation rates mars for western nebraska calculated using the osl ages are extremely high from 18 000 to 14 000 yr much higher than those calculated for any other pre holocene location worldwide these unprecedented mars coincide with the timing of a mismatch between paleoenvironmental evidence from central north america and the paleoclimate simulations from atmospheric global circulation models agcms we infer that the high atmospheric dust loading implied by these mars may have played an important role through radiative forcing in maintaining a colder than present climate over central north america for several thousand years after summer insolation exceeded present day values

radiocarbon ages of soils and charcoal in late wisconsinan loess south central nebraska abstract the farmdale soil occurs below late wisconsinan loess throughout the u s midwest at the la sena site in the central great plains humates in the farmdale interstadial soil have a corrected age of 21 000 yr b p humates in a buried bt horizon and a bulk sample of overlying loess 2 5 m above the farmdale interstadial soil have ages of 17 000 and 19 000 yr b p respectively in the republican river valley picea spruce charcoal is common in the lower meter of peoria loess near bloomington nebraska humates from burned organic matter only 60 cm above the base of peoria loess have a corrected age of ca 19 000 yr b p

simulation of the climate of 18 000 years bp results for the north american north atlantic european sector and comparison with the geologic record of north america abstract the large ice sheets in north america and europe and the extensive sea ice cover in the north atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and in turn the location and intensity of the surface winds and jet streams a general circulation model was used to simulate the january and july patterns of temperature precipitation and wind for 18 ka bp boundary conditions for the model consisting of ice sheet location and height sea ice location and sea surface temperature were prescribed from climap 1981 the model results are illustrated and described for the north american north atlantic european sector the jet stream splits around the north american ice sheet and the southern branch strengthens considerably compared to present over the southern portion of the united states the sea ice margin of the north atlantic and the southern edge of the european ice sheet geologic evidence principally from north america of wind temperature and moisture conditions is assessed from sand dune and loess records estimates of snowline depression pollen records and lake level studies the geologic evidence is generally compatible with the model simulation