unsaturated zone hydrostratigraphies a novel archive of past climates in dryland continental regions natural chemical tracers contained in moisture within unsaturated zone usz sediments have very significant potential as a novel archive of past climate and palaeoenvironmental conditions in dryland environments however this potential has yet to be fully realised the tracer signature is established in the near surface zone with evapotranspiration processes enriching the concentration inherited from input from atmospheric deposition predominantly rainfall this pore moisture tracer signature then moves vertically through the usz sediments toward the water table producing a hydrostratigraphy that with increasing depth records changes to the moisture balance at the surface through time the small fluxes of water through the usz in dryland climates means that this hydrostratigraphy approach is particularly valuable in these environments for providing climate records longer than the instrumental data period this paper sets out the current state of the art in the use of the usz as an archive we explain the nature of the usz the basis of the tracer technique and also the field and sampling methodologies employed examples of application worldwide by broad geographic region are also evaluated this shows that the usz hydrostratigraphy approach can be used across three key timescales i decadal to multi decadal resolution records covering hundreds of years that provide information about recent climate fluctuations and patterns of land use change ii multi decadal through to millennial scale length records which may record wetter and drier events down to decadal scale resolution and iii last interglacial through to the holocene records of lower resolution that indicate broad shifts over multi millennial timescales we also explore the questions and challenges surrounding the depositional flux of tracer inputs and the extent to which infiltration is non uniform before setting the agenda for their future potential use alongside related proxies for palaeohydrology

forces driving late pleistocene ca 77 12 ka landscape evolution in the cimarron river valley southwestern kansas abstract this study presents stratigraphic geomorphic and paleoenvironmental 13 c data that provide insight into the late pleistocene landscape evolution of the cimarron river valley in the high plains of southwestern kansas two distinct valley fills t 1 and t 2 were investigated three soils occur in the t 2 fill and five in the t 1 fill all indicating periods of landscape stability or slow sedimentation of particular interest are two cumulic soils dating to ca 48 28 and 13 12 5 ka 13 c values are consistent with regional paleoenvironmental proxy data that indicate the prevalence of warm dry conditions at these times the cimarron river is interpreted to have responded to these climatic changes and to local base level control specifically aggradation occurred during cool wet periods and slow sedimentation with cumulic soil formation occurred under warmer drier climates significant valley incision 25 m by ca 28 ka likely resulted from a lowering of local base level caused by deep seated dissolution of permian evaporite deposits

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 vegetation and fire history of lake titicaca since the last glacial maximum fine resolution fossil pollen and charcoal analyses reconstruct a vegetation and fire history in the area surrounding lake titicaca 3810 m peru bolivia since ca 27 500 cal yr bp hereafter bp time control was based on 26 accelerator mass spectrometer ams radiocarbon dates seventeen ams dates and 155 pollen and charcoal samples between ca 17 500 bp and ca 3100 bp allow a centennial scale reconstruction of deglacial and early to mid holocene events local and regional fire signals were based on the separation of two charcoal size fractions 180 m and 179 65 m charcoal abundance correlated closely with the proportion of woody taxa present in the pollen spectra little or no pollen was detected in the sedimentary record prior to ca 21 000 bp very cold climatic conditions prevailed with temperatures suggested to be at least 5 8 c cooler than present increases in pollen concentration suggest initial warming at ca 21 000 bp with a more significant transition toward deglaciation ca 17 700 bp between 17 700 bp and 13 700 bp puna brava is progressively replaced by puna and sub puna elements the most significant changes between the pleistocene and the holocene floras were largely complete by 13 700 bp providing an effective onset of near modern conditions markedly earlier than in other andean records fire first occurs in the catchment at ca 17 700 bp and becomes progressively more important as fuel loads increase no evidence is found of a rapid cooling and warming coincident with the younger dryas chron a dry event between ca 9000 bp and 3100 bp with a peak between 6000 and 4000 bp is inferred from changes in the composition of aquatics and the marsh community as pollen of cyperaceae is replaced by poaceae apiaceae plantago and the shrub polylepis human disturbance of the landscape is evident in the pollen spectra after ca 3100 bp with the appearance of weed species

origin and evolution of lunettes on the high plains of texas and new mexico lunettes isolated dunes on the lee side of playa basins are common landforms on the southern high plains of northwest texas and eastern new mexico the dunes contain calcareous 15 40 caco3 sandy loam or loamy sand with minor amounts of sepiolite deposited 25 000 800014c yr b p and derived by deflation of lacustrine carbonate in the basins the dunes also contain low carbonate 0 15 caco3 sand or loamy sand that was deposited 25 000 15 000 yr b p and 8000 5000 yr b p and was derived by deflation that created the basins or deflated from sand deposited in the basins buried soils are common in the lunettes a bk profiles characterize soils formed in the calcareous sandy loam the sandy low carbonate sediments contain a bt profiles in the oldest sand of some dunes and a bw a bt or a btk in the early and middle holocene sand the dune stratigraphy combined with carbon isotope data derived from dated a horizons in lunettes suggests the following scenario for the southern high plains the lunettes began forming as low carbonate sand dunes in the late pleistocene as playa basins were formed or deepened by wind erosion the erosion repeatedly alternated with stability the environment probably was cool and dry but one or more cool and wet intervals 25 000 15 000 yr b p resulted in a rise in the water table and deposition of lacustrine carbonate in the deepest basins there may have been short departures toward warmer and probably toward drier conditions throughout this time episodically dry conditions 15 000 8 000 yr b p resulted in deflation of the carbonate and further dune construction by repeated accretion of calcareous sandy loam or loamy sand the low carbonate sand was deposited during widespread drought and deflation 8000 5000 yr b p the dunes have been largely stable in the late holocene

fluvial response to late quaternary climatic and environmental change edwards plateau texas abstract radiocarbon controlled late pleistocene to modern stratigraphic frameworks for fluvial systems that drain the edwards plateau westcentral texas coupled with reconstruction of climatic and environmental changes permit development of a model for the evolution of fluvial landscapes during the past 20 000 years during this time large valleys of the edwards plateau were characterized by channel aggradation and sediment storage from 20 to 14 000 yrs b p deep excavation of bedrock valleys from ca 14 to 11 000 yr b p and deposition of extensive and complex valley fills during the last 11 000 yr valley fills contain three distinct unconformity bounded allostratigraphic units representing episodes of channel aggradation and floodplain construction during the early to middle ca 11 000 5000 yr b p and late holocene ca 5000 1000 yr b p and development of the incised channels and associated narrow floodplains of the last 1000 years early to middle holocene alluvial deposits were derived from proximal sources within the respective drainage basins whereas late holocene deposits include sediments derived from distal sources sediment supply to major valleys axes has been extremely limited during the last 1000 years alluvial sequences of the edwards plateau record fluvial responses to climatically driven changes in discharge regimes and the concentration of sediments along valley axes allostratigraphic units define time periods when the concentration of sediment exceeded stream power resulting in sediment storage whereas unconformities record widespread morphological and sedimentary adjustments unconformities between late pleistocene alluvium and holocene valley fills record bedrock valley incision in response to decreased sediment loads associated with slope stability in the uplands by contrast unconformities within the holocene valley fill record floodplain abandonment accompanied by continued channel migration and sediment storage but little additional bedrock valley cutting episodes of floodplain abandonment occurred as a result of decreased flood magnitudes following shifts to drier climatic conditions at ca 5000 and 1000 yr b p fluvial responses to climatic change were conditioned by a progressive degradation of upland soils that caused increases through time in the flashiness of flood events which in turn led to changes in processes of floodplain construction flood events on late pleistocene and early to middle holocene rivers were for the most part contained within channel perimeters and floodplains were constructed by lateral migration by contrast late holocene rivers were characterized by deep overbank flooding and floodplain construction by vertical accretion high magnitude floods were most significant from ca 2500 to 1000 yr b p when large chute channels were cut and filled on floodplain surfaces and soils developed on previously stable terrace surfaces were buried by up to 2 m of fine sands and muds

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

the magnitude and rapidity of the climate change marking the end of the pleistocene in the mid latitudes of south america abstract the chi squared test of independence and cluster analysis of otsuka similarity coefficients of fossil beetle assemblages from the chilean lake region in the mid latitudes of south america support the following conclusions 1 the mean summer temperature of the glacial climate was 4 5 c lower than today s climate 2 the climatic change from glacial to interglacial mode was in a single step centered on about 14 000 yr b p 3 the climatic change was rapid and within 1500 years the biota of a moorland had been completely replaced by a biota of a rain forest 4 by 12 500 yr b p the low elevation beetle fauna of the chilean lake region was similar in composition to that of the present day and 5 no reversal in the postglacial warming trend equivalent in age to the younger dryas stade was detected

an 18 ka to present pollen and phytolith based vegetation reconstruction from hall s cave south central texas usa pollen spores phytoliths and microscopic charcoal from a sedimentary column in hall s cave south central texas provide information for local and regional vegetation change during the last deglaciation and the holocene in the context of broader regional and global climatic changes the combination of paleoenvironmental proxy data from the cave indicates that between about 18 000 and 16 500 cal yr bp the cave area was dominated by an open plant community consisting of herbaceous vegetation dominated by c 3 grasses and scattered trees primarily quercus and pinus species after about 16 500 cal yr bp the arboreal component fluctuated attaining a peak between 14 000 and 13 000 cal yr bp with relatively equal proportions of c 3 and c 4 grasses including a sizable proportion of panicoideae grasses the younger dryas is marked by a conspicuous decrease in arboreal pollen with an apparent increase of c 4 grasses toward its termination early holocene recovery of arboreal vegetation is followed by a drying trend marked by the increasing dominance of c 4 drought tolerant chloridoideae grasses increasing human use of the cave in middle to late holocene times creates noise in the climatic significance of pollen phytolith and other proxies a factor to consider when interpreting paleoenvironmental proxies in other cave sedimentary records

late quaternary temperature record from buried soils of the north american great plains we present the first comprehensive late quaternary record of north american great plains temperature by assessing the behavior of the stable isotopic composition 13 c of buried soils after examining the relationship between the 13 c of topsoil organic matter and july temperature from 61 native prairies within a latitudinal range of 46 38 n we applied the resulting regression equation to 64 published 13 c values from buried soils of the same region to construct a temperature curve for the past 12 k y estimated temperatures from 12 to 10 ka 1 k y 1000 14 c yr b p fluctuated with a periodicity of 1 k y with two cool excursions between 4 5 and 3 5 c and two warmer excursions between 1 and 0 c relative to modern early holocene temperatures from ca 10 7 5 ka were 1 0 to 2 0 c before rising to 1 0 c in the middle holocene between 6 0 and 4 5 ka after a cool interlude from 4 2 to 2 6 ka when temperatures dropped to slightly below modern another warm interval ensued from 2 6 to 1 ka as temperatures increased to 0 5 c a final decline in temperature to below modern occurred beginning ca 0 5 ka cooler than present temperatures in the great plains indicate telecommunications with cool water episodes in the gulf of mexico and north atlantic potentially governed by a combination of glacial meltwater pulses and low solar irradiance

paleohydrology of southwestern texas current statistical methods may be unable to accurately predict recurrence intervals of rare large magnitude floods especially in semiarid regions having positively skewed annual flood distributions great hydrologic variability and widely spaced gaging stations current approaches rely on historical data but catastrophic floods may have recurrence intervals far greater than the length of historical records in the lower pecos and devils rivers of southwestern texas paleoflood discharge and frequency estimates are extended over 10 000 years by the study of slack water flood sediments slack water deposits are typically fine grained sand and silt that accumulate during floods in areas where current velocity is reduced i e in back flooded tributary mouths channel expansions downstream from bedrock spurs and or slump blocks and in shallow caves along bedrock walls radiocarbon dating of organic detritus in slack water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope area techniques paleoflood information extracted from slack water sediments can greatly extend flood records these floods may be weighted like historical data in log pearson type 3 calculations of flood frequency our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency this technique offers an inexpensive and rapid way to assess catastrophic flood risk

the effects of late quaternary climate and pco2 change on c4 plant abundance in the south central united states the late quaternary was a time of substantial environmental change with the past 70 000 years exhibiting global changes in climate atmospheric composition and terrestrial floral and faunal assemblages we use isotopic data and couple climate and vegetation models to assess the balance between c3 and c4 vegetation in texas during this period the carbon isotope composition of fossil bison mammoth and horse tooth enamel is used as a proxy for c3 versus c4 plant consumption and indicates that c4 plant biomass remained above 55 through most of texas from prior to the last glacial maximum lgm into the holocene these data also reveal that horses did not feed exclusively on herbaceous plants consequently isotopic data from horses are not reliable indicators of the c3 c4 balance in grassland biomes estimates of c4 percentages from coupled climate vegetation models illuminate the relative roles of climate and atmospheric carbon dioxide co2 concentrations in shaping the regional c4 signal c4 percentages estimated using observed modern climate vegetation relationships and late quaternary climate variables simulated by a global climate model are much lower than those indicated by carbon isotope values from fossils when the effect of atmospheric co2 concentration on the competitive balance between c3 and c4 plants is included in the numerical experiment however estimated c4 percentages show better agreement with isotopic estimates from late quaternary mammals and soils this result suggests that low atmospheric co2 levels played a role in the observed persistence of c4 plants throughout the late quaternary

volume and routing of late glacial runoff from the southern laurentide ice sheet melting of the laurentide ice sheet during the last deglaciation added large volumes of water to many rivers and lakes of north america and to the world s oceans the volume and routing of this meltwater not only helped shape the land s surface but also played a role in the evolution of late glacial climate a computerized model was prepared to quantify meltwater generation from seven drainage areas along the southern side of the laurentide ice sheet at 500 yr time slices between 14 000 and 8000 yr b p nearly all waters reaching the oceans flowed through the st lawrence hudson or mississippi river valleys discharge through the mississippi river to the gulf of mexico during late glacial time varied by more than a factor of 5 ranging between 17 400 m3 sec 1 550 km3 yr 1 and 98 200 m3 sec 1 3200 km3 yr 1 discharge entering the north atlantic ocean through the st lawrence and hudson valleys ranged between 20 300 m3 sec 1 640 km3 yr 1 and 65 300 m3 sec 1 2060 km3 yr 1 with very abrupt twofold changes at about 11 000 10 000 and 9500 yr b p as a result of the rerouting of water from the lake agassiz basin as the areal extent and mass of the laurentide ice sheet diminished the total volume of meltwater plus runoff due to precipitation from its southern side declined from 3800 km3 yr 1 at about 14 000 yr b p to 2100 to 2600 km3 yr 1 between 11 500 and 8000 yr b p no meltwater entered the gulf of mexico after 9500 yr b p after the demise of the ice sheet over hudson bay about 8000 yr b p the modern continental drainage network was established and flows through the st lawrence declined to modern values of about 320 km3 yr 1

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

spatial and temporal patterns of late quaternary eolian deposition eastern colorado u s a abstract eolian sediment covers about 60 of colorado east of the rocky mountains about 30 of the sediment is sand and 70 is loess initially flood plains were the principal sources of eolian sediment but during the holocene dunes formed from older eolian sand and alluvium on uplands since latest pleistocene time dominant dune forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part at present sand sheets and dunes mainly parabolic types are stable and covered with vegetation in dunes sand is commonly 20 30 m thick but elsewhere averages

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

late quaternary environmental change inferred from phytoliths and other soil related proxies case studies from the central and southern great plains usa abstract this study investigates stable carbon isotopes 13c opal phytolith assemblages burnt phytoliths microscopic charcoal and sporormiella spores from modern soils and paleosols in kansas and oklahoma grass and dicot phytoliths in combination with 13c are used as proxies for reconstructing the structure of grasslands and woodlands burnt grass phytoliths and microscopic charcoal are evaluated as proxies for reconstructing paleofire incidence concentrations of the fungal spore sporormiella are used as a proxy for assessing large herbivore activity these proxies were tested on various modern grassland communities of the central and southern great plains including areas with bison cattle and small herbivores and areas under different fire frequencies opal phytolith assemblages and 13c values show that before cal 11 ka c3 grasses and woody plants predominated in areas that today are dominated by c4 grasses the origin of the shortgrass prairie dates back to about cal 10 ka the origin of the tallgrass prairie however is not clear as phytolith data show variable assemblages throughout the holocene mixed grass tallgrass and tallgrass woodland mosaic different proxies burnt phytoliths vs charcoal reveal different fire frequencies but it is apparent that microfossil evidence for fire incidence is closely related to the abundance of woody plants in the landscape before cal 12 ka soils show somewhat elevated concentration of sporormiella but lower concentrations than the modern high density bison and cattle grazing areas throughout the holocene sporormiella frequencies are low which suggests lower large ungulate densities and perhaps high mobility

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

coherent changes in relative c4 plant productivity and climate during the late quaternary in the north american great plains evolution of the mixed and shortgrass prairie of the north american great plains is poorly understood because of limited proxies available for environmental interpretations buried soils in the great plains provide a solution to the problem because they are widespread both spatially and temporally with their organic reservoirs serving as a link to the plants than once grew on them through stable carbon isotopic analysis of soil organic carbon d

interpretation of late quaternary climate and landscape variability based upon buried soil macro and micromorphology geochemistry and stable isotopes of soil organic matter owl creek central texas usa abstract small basin floodplain buried soils formed in association with low order tributary streams are valuable archives of past climates but have not been studied extensively in central texas usa four buried soils exposed along owl creek within the larger brazos river drainage basin were examined using soil morphology and micromorphology optically stimulated luminescence osl dating soil characterization whole soil geochemical and stable isotope analyses of soil organic matter and pedogenic carbonate these buried soils provide a record of changes in paleoecological and paleo alluvial conditions spanning 14 ky morphological and geochemical differences between buried soils reflect changes in landscape attributable to climate with a distinct 5 increase in 13 c values of soil organic matter corresponding to the holocene onset and drier conditions paleoecological reconstructions coupled with depth to bk suggest possible amounts of erosion of 1 m for each of the buried soils compilation of the proxies presented shows evidence for a cooler and wetter late pleistocene climate followed by a warmer and drier climate dominating during the holocene

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

late quaternary sedimentology and geochronology of small playas on the southern high plains texas and new mexico u s a abstract playas are small circular basins forming a ubiquitous component of the southern high plains landscape they are filled with carbonaceous mud deposited since the terminal pleistocene the stratigraphy and geochronology of 30 playas was investigated to better understand the paleoenvironmental record of basin filling at the base of the fill in some playas is a well sorted eolian sand dated between 13 000 and 11 000 14c yr bp the beginning of mud deposition representing aggradation of eolian dust on a moist vegetated playa floor was largely between 12 000 and 10 500 14c yr bp playa filling slowed 9000 to 4000 14c yr bp probably due to dry conditions increased 4000 to 2000 14c yr bp then slowed again eolian sand and loam likely representing regional aridity accumulated in some basins episodically just prior to 10 700 14c yr bp between 8600 and 4700 14c yr bp and at 1300 14c yr bp stable c isotopes from one basin indicate that the playa was inundated only seasonally throughout the record beginning 11 500 14c yr bp the phytolith record in that basin indicates an abrupt shift toward cooling 11 400 to 11 200 14c yr bp and then increasing importance of xeric adapted c4 grasses through the holocene

late quaternary landscape evolution soil stratigraphy and geoarchaeology of the caprock canyonlands northwest texas usa abstract in northwest texas usa between the southern high plains to the west and the central lowlands to the east lies a geographic boundary known as the escarpment breaks or caprock canyonlands the canyonlands contain abundant springs lithic resources shelter and plant and animal food sources that attracted hunter gatherer groups a geoarchaeological study was conducted in the canyonlands to determine the effects of late quaternary landscape evolution especially intensive erosion on the region s archaeological record geomorphic and stratigraphic field research and a total of 95 new radiocarbon age determinations 94 of which were determined on paired samples aid in reconstructing an understudied dynamic and erosive landscape and explain how the landscape has changed the pattern is similar to reported data from the central plains and western rolling plains but dissimilar to the southern high plains high rates of erosion and geological controls on the south fork of the double mountain fork of the brazos river a 4th order stream have hindered the discovery of deeply buried soils and in situ paleoindian artifacts and features but a late holocene pedocomplex is relatively intact in valley fills beneath remnants of the t 2 terrace of the south fork the eroding slopes near the edge of the caprock escarpment exposed a record of in situ archaic to protohistoric aged materials the eroding slopes should be targeted for future quantification of erosion and archaeological preservation bias for the canyonlands

stable isotopic evidence for latest pleistocene and holocene climatic change in north central texas abstract a paleoclimatic record for a southern great plains locality the aubrey clovis site in north central texas has been established using stable carbon and oxygen isotopes detailed composite stratigraphic sections constrained by 14 c ages place the age of these deposits between 14 200 and 1600 yr b p calcium carbonate samples of lacustrine and pedogenic origin were analyzed oxygen isotopic compositions of most of these in situ carbonates reflect equilibrium precipitation from local meteoric waters oxygen isotope values reflect changes in the composition of meteoric waters tied to changes in the isotopic composition of moisture derived from the gulf of mexico oxygen isotopic variability at the aubrey site is coincident with marine isotopic records from the gulf that vary due to changes in laurentide ice sheet volume and meltwater influx the stable carbon isotopic record reflecting changing biomass through time corroborates humid versus arid interpretations based on sedimentology and rates of alluviation a middle holocene arid period was in contrast to moist early and late holocene climate affirming interpretations of other workers studying southern great plains holocene climate history

late quaternary vegetation and climate changes in central texas based on the isotopic composition of organic carbon stable carbon isotope analysis of organic carbon in alluvial deposits and soils of three streams in central texas reveals significant shifts in the ratio of c3 to c4 plant biomass production during the past 15 000 yr these temporal changes in vegetation appear to be in response to changes in climate during the late pleistocene c4 plants comprised only about 45 to 50 of the vegetative biomass in this area suggesting that conditions were cooler and wetter than at any time during the past 15 000 yr the time between 11 000 and 8000 yr b p is interpreted as transitional between late pleistocene conditions and warmer and drier holocene conditions based on a slight increase in the abundance of c4 plant biomass during the middle holocene between approximately 6000 and 5000 yr b p mixed c3 c4 plant communities were replaced almost completely by c4 dominated communities indicating prairie expansion and warmer and drier climatic conditions by 4000 yr b p the abundance of c4 plant biomass decreased to levels similar to the early holocene transitional period suggesting a return to cooler and wetter climatic conditions no significant shift in the ratio of c3 to c4 productivity has occurred during the last 4000 yr except for a slight increase in the abundance of c4 plant biomass around 2000 yr b p the results of this investigation correlate well with other regional late quaternary climatic interpretations for central and north texas the southern plains region and with other portions of the great plains

the micromorphology of younger dryas aged black mats from nevada arizona texas and new mexico abstract black mats are organic rich sediments and soils that form in wet environments associated with spring discharge micromorphological and geochemical analyses of 25 black mats dating to the younger dryas chronozone 12 9 11 7 ka and early holocene were conducted to determine their composition and depositional environment samples were collected from arizona new mexico texas and nevada micromorphological analyses were conducted on thin sections using polarized and blue fluorescent light these analyses determined that black mats contain humic acids fine 5 20 m plant fragments diatoms phytoliths and gastropods the dominant type of organic matter in black mats is derived from herbaceous plants contradicting previous studies that supported algal or charcoal sources differences in the micromorphological characteristics of the samples revealed that black mats formed as three different types organic horizons moist soils and ponded sediments depending on their topographic position in relation to the water table the microscopic evidence found in black mats supports the presence of widespread wet environments in nevada and arizona during the younger dryas chronozone clearly indicating a sustained period of greater effective moisture optimal for spring discharge and black mat formation

surface exposure ages of front range moraines that may have formed during the younger dryas 8 2 cal ka and little ice age events abstract surface exposure 10 be ages have been obtained on boulders from three post pinedale end moraine complexes in the front range colorado boulder rounding appears related to the cirque to moraine transport distance at each site with subrounded boulders being typical of the 2 km long chicago lakes glacier subangular boulders being typical of the 1 km long butler gulch glacier and angular boulders being typical of the few hundred m long isabelle glacier surface exposure ages of angular boulders from the isabelle glacier moraine which formed during the little ice age lia according to previous lichenometric dating indicate cosmogenic inheritance values ranging from 0 to 3 0 10 be ka 1 subangular boulders from the butler gulch end moraine yielded surface exposure ages ranging from 5 to 10 2 10 be ka we suggest that this moraine was deposited during the 8 2 cal ka event which has been associated with outburst floods from lake agassiz and lake ojibway and that the large age range associated with the butler gulch end moraine is caused by cosmogenic shielding of and or spalling from boulders that have ages in the younger part of the range and by cosmogenic inheritance in boulders that have ages in the older part of the range the surface exposure ages of eight of nine subrounded boulders from the chicago lakes area fall within the 13 0 11 7 10 be ka age range and appear to have been deposited during the younger dryas interval the general lack of inheritance in the eight samples probably stems from the fact that only a few thousand years intervened between the retreat of the pinedale glacier and the advance of the chicago lakes glacier in addition bedrock in the chicago lakes cirque area may have remained covered with snow and ice during that interval thus partially shielding the bedrock from cosmogenic radiation

quantifying the distribution of nanodiamonds in pre younger dryas to recent age deposits along bull creek oklahoma panhandle usa high levels of nanodiamonds nds have been used to support the transformative hypothesis that an extraterrestrial et event comet explosion triggered younger dryas changes in temperature flora and fauna assemblages and human adaptations firestone rb et al 2007 proc natl acad sci usa 104 41 16016 16021 we evaluate this hypothesis by establishing the distribution of nds within the bull creek drainage of the beaver river basin in the oklahoma panhandle the earlier report of an abundance spike of nds in the bull creek i younger dryas boundary soil is confirmed although no pure cubic diamonds were identified the lack of hexagonal nds suggests bull creek i is not near any impact site potential hexagonal nds at bull creek were found to be more consistent with graphene graphane an additional nd spike is found in deposits of late holocene through the modern age indicating nds are not unique to the younger dryas boundary nd distributions do not correlate with depositional environment pedogenesis climate perturbations periods of surface stability or cultural activity

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

evidence for seasonal low salinity surface waters in the gulf of mexico over the last 16 000 years oxygen isotopic analyses of individual orbulina universa from orca basin core en32 pc6 document the presence of low salinity surface waters in the northern gulf of mexico over the past 16 kyr isotopic data from an interval immediately following the younger dryas event indicate the rapid decrease in 18o values at the conclusion of the younger dryas was due to a year round return of meltwater to the gulf of mexico data indicate periodic or seasonal low salinity waters existed over the region of the orca basin prior to the initiation of the meltwater spike estimates suggest o universa grew its shell in salinities at least 4 5 below ambient since o universa may have calcified deep in the mixed layer during periods of low salinity surface salinities could have been even lower comparison of the average of individual o universa oxygen isotopic values with data from multiple shell samples of white gs ruber from the same core samples demonstrates that the two species record similar values during the late holocene in contrast o universa records lower oxygen isotopic values during the late glacial deglacial intervals possibly due to differences in seasonal distribution or shell ontogeny between the two species

routing of meltwater from the laurentide ice sheet during the younger dryas cold episode rooth1 proposed that the younger dryas cold episode which chilled the north atlantic region from 11 000 to 10 000 yr bp was initiated by a diversion of meltwater from the mississippi drainage to the st lawrence drainage system the link between these events is postulated to be a turnoff during the younger dryas cold episode of the north atlantic s conveyor belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the north atlantic region2 this turnoff is attributed to a reduction in surface water salinity and hence also in density of the waters in the region where north atlantic deep water nadw now forms here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from orca basin core en32 pc4 which reveal a significant reduction in meltwater flow through the mississippi river to the gulf of mexico from about 11 200 to 10 000 radiocarbon years ago this finding is consistent with the record for lake agassiz which indicates that the meltwater from the southwestern margin of the laurentide ice sheet was diverted to the northern atlantic ocean through the st lawrence valley during the interval from 11 000 to 10 000 years before present yr bp

paleo environmental reconstruction and bio stratigraphy oklahoma panhandle usa worldwide the period between 12 000 and 9000 radiocarbon years before present rcybp witnessed the shift from glacial to post glacial times and marks the transition from the pleistocene to the holocene on the southern plains of north america this period is marked by rapidly changing climatic conditions shifting flora and faunal associations and cultural adaptations a recently discovered locality along bull creek oklahoma panhandle provides the opportunity to study the environmental context of changing megafauna presence during this transition the paleo environment is reconstructed from sediment particle size distribution stable carbon isotope pollen and phytolith analyses from the bull creek site and compared with the megafaunal biostratigraphy in this area

high resolution seismic reflection profiles from lake titicaca peru bolivia evidence for holocene aridity in the tropical andes high resolution seismic reflection profiles of the sediments of lake titicaca peru bolivia suggest that lake levels in the recent past were considerably lower than today incised channels on the major deltas extend to depths of 85 m below modern lake level erosional truncation of onlapping seismic reflectors is found at similar depths this interpretation of the seismic data is supported by analyses of sediment cores from the lake which indicate that there was a significant drop in lake level during the early to mid holocene

the timing of the last deglaciation in cordillera oriental northern peru based on glacial geology and lake sedimentology the degree of soil development on moraines and radiocarbon ages indicate that the cordillera oriental a part of the eastern andes in northern peru 7 30 7 55 s 77 20 77 35 w was last glaciated during the last glacial maximum marine isotope stage 2 this glaciation was the most extensive late quaternary glaciation in the area radiocarbondated sedimentological changes in six sediment cores from three lakes and one bog reveal that the last deglaciation is marked by a two stepped transition from rhythmically laminated inorganic sediment to massive organic rich sediment the first step of this transition commenced before 12 1 0 2 ka 1 and probably by ca 13 5 ka glaciers covered less than 50 of their last glacial maximum area by 12 1 0 6 ka and cirques west of the main divide were probably ice free by this time a return to rhythmically laminated inorganic sediment in the lakes and the bog occurred between 12 1 0 6 and 10 3 0 4 ka this may reflect an episode of increased stream discharge and glacier expansion during which glaciers reoccupied cirques above 3 800 m this glacier expansion adds to widespread evidence of a late glacial readvance in the tropical andes all cirques have been ice free since between 10 3 0 4 and 9 7 0 5 ka

the murray springs clovis site pleistocene extinction and the question of extraterrestrial impact some of the evidence for the recent hypothesis of an extraterrestrial impact that caused late pleistocene megafaunal extinctions firestone et al 2007 proc natl acad sci usa 104 16016 16021 was based upon samples collected at murray springs a clovis archaeological site in southeastern arizona here we describe sampling and analyses of magnetic separates from within above and below the lower younger dryas boundary lydb black mat at murray springs as well as radiation measurements from the lydb at murray springs and two other well stratified clovis sites the main magnetic fraction at murray springs is maghemite magnetic microspherules have terrestrial origins but also occur as cosmic dust particles we failed to find iridium or radiation anomalies the evidence for massive biomass burning at murray springs is addressed and found to be lacking we could not substantiate some of the claims by firestone and others but our findings do not preclude a terminal pleistocene cosmic event

vegetation dynamics during the pleistocene holocene transition in the central great plains usa the holocene pleistocene transition in the upland loess mantled regions of the central great plains is punctuated by the brady soil which separates the late pleistocene peoria loess and the holocene bignell loess previous research on the brady soil at the old wauneta roadcut site in southwestern nebraska has produced paleoenvironmental information based on well constrained luminescence and radiocarbon ages stable carbon isotope data and chemical and physical data while the research indicated high effective moisture during formation of the brady soil and a shift to warm season c4 vegetation from the cool season c3 dominated vegetation of the peoria loess those data do not provide any detail as to plant community composition and significant underlying climatic inferences assemblages of phytoliths and other biosilicates extracted from the brady soil provide specific information on vegetation communities and indicate shifts of plant taxa comprising these assemblages short cell phytolith count data rev

buried paleoindian age landscapes in stream valleys of the central plains usa abstract a systematic study of late quaternary landscape evolution in the central plains documented widespread deeply buried paleosols that represent paleoindian age landscapes in terrace fills of large streams 5th order in alluvial fans and in draws in areas of western kansas with a thick loess mantle alluvial stratigraphic sections were investigated along a steep bio climatic gradient extending from the moist subhumid forest prairie border of the east central plains to the dry subhumid and semi arid shortgrass prairie of the west central plains radiocarbon ages indicate that most large streams were characterized by slow aggradation accompanied by cumulic soil development from ca 11 500 to 10 000 14c yr b p in the valleys of some large streams such as the ninnescah and saline rivers these processes continued into the early holocene the soil stratigraphic record in the draws of western kansas indicates slow aggradation punctuated by episodes of landscape stability and pedogenesis beginning as early as ca 13 300 14c yr b p and spanning the pleistocene holocene boundary the development record of alluvial fans in western kansas is similar to the record in the draws slow aggradation was punctuated by multiple episodes of soil development between ca 13 000 and 9000 14c yr b p in eastern kansas and nebraska development of alluvial fans was common during the early and middle holocene but evidence shows fan development as early as ca 11 300 14c yr b p buried soils dating between ca 12 600 and 9000 14c yr b p were documented in fans throughout the region in stream valleys across the central plains rapid alluviation after ca 9000 14c yr b p resulted in deeply buried soils that may harbor paleoindian cultural deposits hence the paucity of recorded stratified paleoindian sites in the central plains is probably related to poor visibility i e deep burial in alluvial deposits instead of limited human occupation in the region during the terminal pleistocene and early holocene the thick dark cumulic a horizons of soils representing buried paleoindian age landscapes are targets for future archaeological surveys

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

c4 plant productivity and climate co2 variations in south central texas during the late quaternary abstract a continuous record of organic carbon 13c from a buried soil sequence in south central texas demonstrates 1 strong coupling between marine and adjacent continental ecosystems in the late pleistocene as a result of glacial meltwater entering the gulf of mexico and 2 ecosystem decoupling in the holocene associated with a reduction of meltwater and a shift in global circulation patterns in the late pleistocene reduction in c4 plant productivity correlates with two well documented glacial meltwater pulses 15 000 and 12 000 14c yr b p indicating a cooler than present adjacent continental environment increased c4 production between 11 000 and 10 000 14c yr b p suggests that the younger dryas was a warm interval responding to the diversion of glacial meltwater away from the mississippi river with waning meltwater flow c4 productivity generally increased throughout the holocene culminating in peak warm intervals at 5000 and 2000 14c yr b p shifts in the abundances of c3 c4 plants through the late quaternary show no correlation to ecophysiological responses to atmospheric co2 concentration

late quaternary climate change loess sedimentation and soil profile development in the central great plains a pedosedimentary model research on soil genesis often assumes a top down model in which the soil profi le develops downward from a stable land surface this model is inapplicable to upland landscapes affected by frequent dust deposition where soils grow upward as they develop on the central great plains late quaternary loess sections proximal to immediate source areas contain the brady soil a prominent marker separating late pleistocene peoria loess from holocene bignell loess farther from immediate dust source areas the brady soil and bignell loess are not recognizable in the fi eld on loess tablelands in these distal regions surface soils typically contain a prominent clay rich b horizon below a thick silty a horizon assuming top down pedogenesis this could be interpreted as a postglacial soil profi le formed in peoria loess with the b horizon produced by weathering and clay illuviation we propose a strikingly different interpretation in which the upper b horizon at distal sites is the brady soil a horizon that has been transformed by burial organic matter loss and modern subsoil structure formation processes the overlying modern a horizon represents bignell loess properties of the brady soil at proximal sites a distinctive burrowed zone high clay content and tio 2 zro 2 and low volcanic glass content can be traced to the b horizon in distal soils a decrease in smectite abundance above the brady soil at proximal sites is identifi able at the top of the clay rich b horizon in distal soils the spatial variation of clay content in loess and soil horizons is best explained by eolian sedimentation patterns the higher clay content in the brady soil and distal b horizons defi nes a fi ne grained zone that represents a late phase of peoria loess accumulation evidence of chemical weathering is minimal and illuvial clay is rare to absent in the clay rich b horizons illuvial clay does often occur deep in the solum and is related to the depth below the top of the brady soil the depth of occurrence of illuvial clay is not related to modern climate parameters although depth to secondary carbonate appears to be in equilibrium with modern climate upland soils in the central great plains are composite soils their properties are the result of a pedosedimentary history linked to regional climate change that has infl uenced sedimentation and pedogenesis since the late pleistocene

loess record of the pleistocene holocene transition on the northern and central great plains usa various lines of evidence support conflicting interpretations of the timing abruptness and nature of climate change in the great plains during the pleistocene holocene transition loess deposits and paleosols on both the central and northern great plains provide a valuable record that can help address these issues a synthesis of new and previously reported optical and radiocarbon ages indicates that the brady soil which marks the boundary between late pleistocene peoria loess and holocene bignell loess began forming after a reduction in the rate of peoria loess accumulation that most likely occurred between 13 5 and 15 cal ka brady soil formation spanned all or part of the bolling allerod episode approximately 14 7 12 9 cal ka and all of the younger dryas episode 12 9 11 5 cal ka and extended at least 1000 years beyond the end of the younger dryas the brady soil was buried by bignell loess sedimentation beginning around 10 5 9 cal ka and continuing episodically through the holocene evidence for a brief increase in loess influx during the younger dryas is noteworthy but very limited most late quaternary loess accumulation in the central great plains was nonglacigenic and was under relatively direct climatic control thus brady soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis probably through relatively high effective moisture optical dating of loess in north dakota supports correlation of the leonard paleosol on the northern great plains with the brady soil thick loess in north dakota was primarily derived from the missouri river floodplain thus its stratig raphy may in part reflect glacial influence on the missouri river nonetheless the persistence of minimal loess accumu lation and soil formation until 10 cal ka at our north dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored brady soil formation burial of both the brady soil and the leonard paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity overall the loess paleosol sequences of the central and northern great plains record a broad peak of high effective moisture across the late pleistocene to holocene boundary rather than well defined climatic episodes corresponding to the bolling allerod and younger dryas episodes in the north atlantic region

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

sources and paleoclimatic significance of holocene bignell loess central great plains usa abstract this paper reexamines the stratigraphy sources and paleoclimatic significance of holocene bignell loess in the central great plains a broadly similar sequence of loess depositional units and paleosols was observed in thick bignell loess sections up to 300 km apart suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis new optical ages together with previously reported radiocarbon ages indicate bignell loess deposition began 9000 11 000 yr ago and continued into the late holocene some bignell loess is

paleoenvironment of the folsom archaeological site new mexico usa approximately 10 500 14c yr b p as inferred from the stable isotope composition of fossil land snail shells abstract well preserved aragonitic land snail shells vallonia from late pleistocene eolian sediment in the folsom archaeological site in new mexico exhibit an overall decrease of 18opdb from maximum values of 2 7 more positive than modern to younger samples with lower average values of about 3 6 within the modern range the age of the samples approximately 10 500 14c yr b p suggests that the decrease in 18o may manifest climatic changes associated with the younger dryas some combination of increased relative humidity and cooler temperatures with decreased 18o of precipitation during the times of snail activity can explain the decrease in shell 18o a well known paleoindian bison kill occurred at the folsom site during this inferred environmental transition average 13c values of the aragonite shells of the fossil vallonia range from 7 3 to 6 0 among different archaeological levels and are not as negative as modern values this suggests that the proportion of c4 vegetation at the folsom site approximately 10 500 14c yr b p was greater than at present a result which is consistent with other evidence for higher proportions of c4 plants in the region at that time

the deglaciation of the northern hemisphere a global perspective orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming ice albedo sealevel and greenhouse gas feedbacks together with tropical warming from weakening winds in response to polar amplification of warming caused regionalto global near synchronization of deglaciation effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly ice sheet linked changes in freshwater delivery to the north atlantic and possibly free oscillations in the climate system forced millennial climate oscillations associated with changes in north atlantic deep water nadw flow the north atlantic typically operates in one of three modes modern glacial and heinrich deglaciation occurred from a glacial mode ocean that in comparison to modern had shallower depth of penetration of nadw formed further south causing strong northern cooling and the widespread cold dry and windy conditions associated with the glacial maximum and the cold phases of the millennial dansgaard oeschger oscillations the glacial mode was punctuated by meltwater forced heinrich conditions that caused only small additional cooling at high northern latitudes but greatly reduced the formation of nadw and triggered an oceanic seesaw that warmed some high latitude southern regions centered in the south atlantic

high resolution greenland ice core data show abrupt climate change happens in few years the last two abrupt warmings at the onset of our present warm interglacial period interrupted by the younger dryas cooling event were investigated at high temporal resolution from the north greenland ice core project ice core the deuterium excess a proxy of greenland precipitation moisture source switched mode within 1 to 3 years over these transitions and initiated a more gradual change over 50 years of the greenland air temperature as recorded by stable water isotopes the onsets of both abrupt greenland warmings were slightly preceded by decreasing greenland dust deposition reflecting the wetting of asian deserts a northern shift of the intertropical convergence zone could be the trigger of these abrupt shifts of northern hemisphere atmospheric circulation resulting in changes of 2 to 4 kelvin in greenland moisture source temperature from one year to the next

primary results of glaciological studies along an 1100 km transect from zhongshan station to dome a east antarctic ice sheet the chinese national antarctic research expedition ghinare carried out three traverses from zhongshan station to dome a princess elizabeth land and inaccessible area east antarctic ice sheet during the 1996 97 to 1998 99 antarctic field seasons the expeditions are part of the chinese international trans antarctic scientific expedition program in this project glaciological investigations of mass balance ice temperature ice flow stratigraphy in snow pits and snow firn ice cores as well as the glaciochemical study of surface snow and shallow ice cores have been carried out in the 1998 99 field season chinare extended the traverse route to 1128 km inland from zhongshan station the density profiles show that firnification over princess elizabeth land and inaccessible area 290 1100 km along the route is fairly slow and the accumulation rate recovered from snow pits along the initial 460 km of the route is 4 6 21 cm 46 210 kg m 2a 1 water equivalent the initial 460 km of the route can be divided into four sections based on the differences of accumulation rate this pattern approximately coincides with the study on the lambert glacier basin lgb by australian scientists during the past 50 years the trends of both air temperature and accumulation rate show a slight increase in this area in contrast to the west side of the lgb data on surface accumulation rates and their spatial and temporal variability over ice drainage areas such as the lgb are essential for precise mass balance calculation of the whole ice sheet and are important for driving ice sheet models and testing atmospheric models

quaternary and upper pliocene glaciations and forest development in the tropical andes evidence from a long high resolution pollen record from the sedimentary basin of bogota colombia pollen analysis of 1230 samples from a 357 m long core of lake sediments from the sedimentary basin of bogota eastern cordillera colombia alt 2550 m above sea level has provided a high resolution record of changes in the andean vegetation belts and long range climatic fluctuations during the last 3 5 ma direct time control is provided by k ar datings and fission track datings of 14 intercalated volcanic ash layers the resulting pollen diagram has been divided into 55 pollen zones representing 27 complete major climatic cycles glacial interglacial cycles a tentative correlation with the marine oxygen isotope chronology and the european stratigraphical climatic subdivision could be established the altitudinal position of the forest line which corresponds with the 9 5 c isotherm shifted between minimal c 1700 1800 m and maximal c 3400 3700 m alt at present situated at c 3100 m alt which rendered the altitudinal position of the high plain of bogota the dry bed of a former lake at 2550 m a sensitive recorder of temperature changes the estimated fluctuations in mean annual temperature at the level of the high plain of bogota are from 15 c a distinct change in the nature of the climatic cycles occurs around 2 5 ma b p the interval of 3 5 2 5 ma b p shows high frequency low amplitude climatic oscillations whereas the last 2 5 ma is characterized by low frequency high amplitude climatic oscillations with a superposed high frequency oscillation it is assumed that these oscillations correspond mainly with the variations in orbital eccentricity and obliquity the major constituents making up the andean forest have been subjected to considerable changes in the course of time in the early pliocene forests vallea and myrica are prominent elements weinmannia was almost absent the role of polylepis in the timberline forests increased markedly around 3 1 ma b p and also migrated by adaptation from the upper andean forest line to higher elevations the closure of the isthmus of panama during the pliocene enabled northern hemisphere elements to migrate into south america alnus reached the area of bogota around 2 7 ma b p and dramatically changed the vegetation especially on the flats around the lake of bogota where alnus swamp forests replaced myrica carr around 2 9 ma b p weinmannia became an important constituent of the upper andean forest but slightly later slope forests of alnus replaced parts of the area which had previously been covered by weinmannia forest the marked role of vallea in the andean forest diminished definitively after 2 5 ma b p quercus migrated more recently to south america and reached the area of bogota 1 ma b p it changed for the last time drastically the aspect of the andean forest all palynologically recognizable elements of the modern andean forest were present from now on and the subsequent adaptation and speciation of quercus and many other elements must have mainly taken place at the species level

evidence for cirque glaciation in the colorado front range during the younger dryas chronozone abstract late pleistocene glacial chronologies developed for the front range mountains of colorado include two or more cirque glacier advances locally known as the satanta peak advances sediment cores were recovered from sky pond an alpine lake 3320 m located less than 100 m downvalley from a moraine that exhibits late pleistocene to early holocene relative age features and appears to correlate to the satanta peak deposits one of the cores penetrated 0 5 m of basal diamict and recovered 3 3 m of overlying sediments that are predominantly gyttja an accelerator mass spectrometry ams age of 12 040 60 yr14c b p was obtained from directly above the basal diamict and is similar to other reported ages for cirque deglaciation in the front range mountains the lower portion of the gyttja contains an interval of clastic sediments that show characteristics consistent with glacial activity in alpine catchments radiocarbon ages obtained from below and near the upper contact of this clastic interval are 11 070 50 and 9970 8014c yr b p respectively an additional ams age of 10 410 9014c yr b p was obtained from within the clastic interval in a second core the most likely source for this interval of clastic sediments is a moraine situated directly upvalley from sky pond and consequently it appears that the deposition of this moraine was coeval with the european younger dryas event 11 000 10 00014c yr b p similarities in soil development weathering features and altitude between this moraine and the type satanta peak moraines suggest that the moraines are correlative these findings are in agreement with a growing body of evidence that suggests a relatively minor advance of alpine glaciers occurred in the north american rockies during the younger dryas chron

fungus not comet or catastrophe accounts for carbonaceous spherules in the younger dryas impact layer 1 a claim attributes the onset of the younger dryas climate interval and a range of other effects 12 900 years ago to a comet airburst and or impact event one key aspect of this claim centers on the origin of carbonaceous spherules that purportedly formed during intense impact ignited wildfires samples from pleistocene holocene sedimentary sequences in the california channel islands and other sites show that carbon spherules and elongate forms are common in samples dating to before during and well after the 12 900 year time horizon including from modern samples microscopic studies show that carbon spherules have morphologies and internal structures identical to fungal sclerotia such as sclerotium and cenococcum experimental charring of fungal sclerotia shows that their reflectance increases with temperature reflectance measurements of modern and late pleistocene spherules show that the latter indicate at most low intensity burning these data cast further doubt upon the evidence suggesting a catastrophic younger dryas impact event

the mode and mechanism of the last deglaciation oceanic evidence changes in ocean temperature carbonate productivity and ice rafted detritus in the north atlantic suggest that half of the northern hemisphere ice volume at the last glacial maximum had disappeared by 13 000 yr b p despite the still extensive limits of the ice sheets this early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold dry air masses we infer that this rapid early ice disintegration 16 000 13 000 yr b p was caused by oceanic mechanisms 1 rising sea level causing increased calving along ice margins 2 the chilling of the sea surface by icebergs and meltwater reducing moisture extraction by the atmosphere and transport to the ice sheets and 3 winter freezing of the low salinity meltwater layer suppressing local moisture extraction and the regional influx of moisture bearing storms from lower latitudes in winter and hence starving the ice sheets these oceanic feedback mechanisms were strongest from 16 000 to 13 000 yr b p and weaker but still active from that date until the end of deglaciation at 6000 yr b p

geoarchaeological and paleohydrological evidence for a clovis age drought in north america and its bearing on extinction abstract at the murray springs clovis site in southeastern arizona stratigraphic and geomorphic evidence indicates that an abnormally low water table 10 900 yr b p was followed soon thereafter by a water table rise accompanied by the deposition of an algal mat the black mat that buried mammoth tracks clovis artifacts and a well this water table fluctuation correlates with pluvial lake fluctuations in the great basin during and immediately following clovis occupation of that region many elements of pleistocene megafauna in north america became extinct during the dry period oxygen isotope records show a marked decrease in 18o correlated with the younger dryas cold dry event of northern europe which ended 10 750 yr b p essentially the same time as the water table began to rise in southeastern arizona clovis hunters may have found large game animals easier prey when concentrated at water holes and under stress if so both climate and human predation contributed to pleistocene extinction in america

archaeological geology of the lubbock lake site southern high plains of texas the lubbock lake site is a well stratified archaeological locality in yellowhouse draw on the southern high plains of texas it has a cultural record that spans the past 11 000 yr that is contained within a thick sequence of sediments and soils the sequence at lubbock lake is one of the most closely integrated records of late quaternary human occupation sedimentation and soil formation documented in north america stratum 1 11000 yr b p and older was deposited by a meandering stream and contains clovis age cultural remains stratum 2 is composed of lacustrine sediments 11000 to 10000 yr b p and overlying marsh deposits 10000 to 8500 yr b p with paleoindian material in both units the firstview soil 8500 to 6300 yr b p formed in the top of stratum 2 stratum 3 6300 to 5000 yr b p contains early archaic material eolian and marsh deposits suggestive of warm arid conditions and the yellowhouse soil indicating a brief period of landscape stability stratum 4 5000 to 4500 yr b p is primarily an eolian unit that marks the second of two periods of severe drought in the middle holocene the lubbock lake soil formed in stratum 4 beginning 4500 yr b p coincides with the late archaic and ceramic periods and documents landscape stability and a return to climatic conditions similar to those of today stratum 5 contains late ceramic protohistoric and historic archaeological material within eolian and slopewash sediments and the apache and singer soils which indicate several shifts toward aridity within the past 1 000 yr

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

late quaternary sedimentation lower colorado river gulf coastal plain of texas investigations in the lower colorado river valley gulf coastal plain of texas have resulted in the development of a spatially and temporally controlled history of changes in channel and flood plain erosional and depositional processes when combined with paleoclimatic and stratigraphic data from the upper colorado river drainage and the record of glacio eustasy in the gulf of mexico this study permits evaluation of the relative influence of different external controls on channel and flood plain behavior the development of alluvial landforms and the development of alluvial stratigraphic sequences late pleistocene and holocene alluvial deposits of the lower colorado river have been subdivided into allostratigraphic units with chronological control afforded by radiocarbon ages in the bedrock confined valley up to 10 m of late pleistocene 20 000 14 000 yr b p sediments referred to as the eagle lake alloformation ela underlie a terrace at 17 20 m above the present day channel deposition of the ela was followed by bedrock valley incision then deposition of a complex holocene valley fill referred to as the columbus bend alloformation cba columbus bend allomembers 1 and 2 cba 1 and cba 2 underlie a terrace at 12 14 m above the present day channel cba 1 was deposited 12 000 5 000 yr b p whereas cba 2 was deposited 5 000 1 000 yr b p columbus bend allomember 3 cba 3 consists of channel and flood plain deposits that represent the past 600 yr of activity allostratigraphic units within the lower colorado valley correlate with allostratigraphic units in major valley axes of the upper colorado drainage and with records of climatic and environmental change suggesting that alluvial deposits record basinwide responses to climatically controlled changes in discharge regimes and sediment supply basal unconformities for holocene valley fills however appear to be 1 000 2 000 yr younger in the upper colorado drainage than they are in the lower colorado valley this time transgressioe episode of bedrock valley cutting was initiated by climatically controlled reductions in sediment supply but conditioned by limits on rates of up stream propagation of incision through a large drainage basin by contrast unconformities within holocene valley fills document time parallel episodes of flood plain abandonment and soil formation but little additional bedrock valley cutting and indicate decreased flood magnitudes following shifts to drier climatic conditions flood plain morphology and sedimentary facies changed through time in response to changes in climate coupled with a protracted degradation of upland soil mantles which altered the rate at which precipitation was transferred to stream channels as runoff during the late pleistocene through middle holocene runoff was filtered through deep upland soils floods were for the most part less flashy and contained within channel perimeters and flood plains were constructed by lateral migration without significant vertical accretion hence the ela and cba 1 contain few vertical accretion facies exposure of bedrock surfaces during the late holocene resulted in increased flood stages deep overbank flooding and construction of flood plains by vertical accretion hence cba 2 and cba 3 contain thick vertical accretion facies allostratigraphic units and bounding unconformities persist through the bedrock confined valley to the quaternary alluvial plain but stratigraphic architecture changes substantially in the downstream direction as a result of the last glacio eustatic cycle on the alluvial plain late holocene cba 2 and modern cba 3 deposited contemporaneously with the present interglacial highstand onlap and bury the ela and cba 1 which were emplaced during the last full glacial lowstand and the transgression that followed

millennial scale dynamics of valley fills over the past 12 000 14c yr in northeastern new mexico usa we studied the alluvial histories of eight small 2 watersheds in the uplands of northeastern new mexico the data come from radiocarbon dated sections exposed in the banks of arroyos and permanent channel streams results show that nine periods of valley aggradation separated by incision episodes occurred over the past 12 000 14 c yr these millennial scale cycles occurred in rough synchrony within different drainages parts of some cycles coincided with well known climatic fluctuations for example aggradation occurred during the younger dryas chronozone 11 000 10 000 14 c yr b p and valley fills persisted in incised states during the early holocene peak in milanko vitch summer insolation incision occurred early in the medieval warm period ca a d 1000 1300 aggradation during the little ice age a d 1300 1880 and incision during the last century changes in precipitation tied to the strength of the north american monsoon system probably drive these cycles of aggradation and incision when the system is strong more frequent summer thunderstorms cause increased flooding in these small watersheds wetter summers over periods of decades to centuries allow forest vegetation to spread which reduces sediment input from hillslopes at the same time floods are eroding valley fills aggradation of valley fills probably occurs when summer thunderstorms are less frequent and large floods are correspondingly rare prolonged summer drought reduces forest cover which increases erosion from slopes and causes sediment to accumulate in valleys in turn the strength of the north american monsoon system is probably controlled by fluctuating sea surface temperatures the geomorphic cycles we describe may reflect a previously unrecognized millennial scale climate oscillation that is important in shaping the landscapes of the southern great plains

variations in temperature and extent of atlantic water in the northern north atlantic during the holocene we compare six high resolution holocene sediment cores along a s n transect on the norwegian svalbard continental margin from ca 601n to 77 41n northern north atlantic planktonic foraminifera in the cores were investigated to show the changes in upper surface and subsurface water mass distribution and properties including summer sea surface temperatures sst the cores are located below the axis of the norwegian current and the west spitsbergen current which today transport warm atlantic water to the arctic sediment accumulation rates are generally high at all the core sites allowing for a temporal resolution of 10 10 2 years sst is reconstructed using different types of transfer functions resulting in very similar sst trends with deviations of no more than 71 0 1 51c a transfer function based on the maximum likelihood statistical approach is found to be most relevant the reconstruction documents an abrupt change in planktonic foraminiferal faunal composition and an associated warming at the younger dryas preboreal transition the earliest part of the holocene was characterized by large temperature variability including the preboreal oscillations and the 8 2 k event in general the early holocene was characterized by ssts similar to those of today in the south and warmer than today in the north and a smaller s n temperature gradient 0 231c 1n compared to the present temperature gradient 0 461c 1n the southern proxy records 60 691n were more strongly influenced by slightly cooler subsurface water probably due to the seasonality of the orbital forcing and increased stratification due to freshening the northern records 72 77 41n display a millennial scale change associated with reduced insolation and a gradual weakening of the north atlantic thermohaline circulation thc the observed northwards amplification of the early holocene warming is comparable to the pattern of recent global warming and future climate modelling which predicts greater warming at higher latitudes the overall trend during mid and late holocene was a cooling in the north stable or weak warming in the south and a maximum s n sst gradient of ca 0 71c 1n at 5000 cal years bp superimposed on this trend were several abrupt temperature shifts four of these shifts dated to 9000 8000 5500 3000 and 1000 and 400 cal years bp appear to be global as they correlate with periods of global climate change in general there is a good correlation between the northern north atlantic temperature records and climate records from norway and svalbard r 2007 elsevier ltd all rights reserved

absence of geochemical evidence for an impact event at the bolling allerod younger dryas transition high concentrations of iridium have been reported in terrestrial sediments dated at 12 9 ka and are interpreted to support an extraterrestrial impact event as the cause of the observed extinction in the rancholabrean fauna changes in the paleoindian cultures and the onset of the younger dryas cooling firestone rb et al 2007 proc natl acad sci usa 104 16016 16021 here we report platinum group element pge os ir ru rh pt pd gold au concentrations and 187os 188os ratios in time equivalent terrestrial lacustrine and marine sections to seek robust evidence of an extraterrestrial contribution first our results do not reproduce the previously reported elevated ir concentrations second 187os 188os isotopic ratios in the sediment layers investigated are similar to average crustal values indicating the absence of a significant meteoritic os contribution to these sediments third no pge anomalies distinct from crustal signatures are present in the marine record in either the gulf of california dsdp 480 guaymas basin or the cariaco basin odp 1002c our data show no evidence of an extraterrestrial et pge enrichment anomaly in any of the investigated depositional settings investigated across north america and in one section in belgium the lack of a clear et pge signature in this sample suite is inconsistent with the impact of a large chondritic projectile at the bolling allerod younger dryas transition

lithostratigraphy and geochronology of fills in small playa basins on the southern high plains united states playa basins are small depressions typically 1 5 km 2 on the southern high plains of northwestern texas and eastern new mexico there are about 25 000 playas in the region they lie on the blackwater draw formation pleistocene a widespread eolian deposit and locally on the ogallala formation miocene pliocene understanding the lithostratigraphy and chronostratigraphy of the fill in the basins is important because it should 1 provide clues to the origin and evolution of playas which have been long debated 2 yield a paleoenvironmental record for the region and 3 aid in understanding the history and future of the regional aquifer because playas are the principal source of recharge data from 19 playa basins combined with published data from 4 other basins show that the basin fill is composed of six distinctive facies 1 lacustrine mud 2 lacustrine carbonate 3 lacustrine delta deposits 4 eolian sand and silt 5 eolian loam and 6 accretionary eolian deposits blackwater draw formation mud deposited under ponded conditions is the most common facies and is the surficial deposit on the floors of most playas often producing vertisols the carbonate was precipitated under lacustrine conditions and is another common facies and surface deposit delta deposits are common near the basin margins well sorted layers of eolian sand and silt and poorly sorted eolian loam occur locally above within or below the lacustrine deposits the modern basins in all study areas are locally or completely inset into the blackwater draw formation supporting the interpretation that the basins are at least in part erosional features in larger basins with thicker fills generally coincident with thicker blackwater draw formation the formation interfingers with the lacustrine fill dating is based on radiocarbon ages from the fill in 12 basins and from lunettes adjacent to 5 basins all dated basins were present at the end of the pleistocene and some were present in some form throughout the pleistocene lacustrine mud and other clastic deposits accumulated in the late quaternary and locally much earlier showing that at least some basins contained water throughout the time of human occupation of the region dating of eolian sediments supports other data indicative of aridity and wind deflation in the early and late holocene the lacustrine carbonate is late pleistocene or older and its paleoenvironmental significance is unknown these lithostratigraphic and chronostratigraphic relationships show that some basins have a prolonged history as depressions persisting in more or less the same location as the high plains surface aggraded by eolian addition blackwater draw formation throughout the pleistocene sizes of the basins varied through time as they were encroached upon by the blackwater draw formation enlarged by fluvial lake margin and eolian erosion were filled and reexposed or were buried some basins are newly formed on the high plains surface and have no apparent predecessors the only evidence for subsidence beneath the basins is gently warped fill in the basins on the northern margin of the region known to be affected by salt dissolution in paleozoic bedrock pedogenic carbonates typically are absent from or beneath the basin fill due to focused recharge through the basins playa basins probably have been a ubiquitous component of the high plains landscape through much of the quaternary

glacial and postglacial pollen records from the ecuadorian andes and amazon abstract a long pollen record is derived from sediments of a lake dammed behind a low moraine of the last glaciation at 3 s latitude in the ecuadorian andes and is compared with a glacial age pollen record from the amazon rainforest immediately below lake surucucho llaviucu lies at 3180 m on the amazonian flank of the andes and above the glacial age pollen record from san juan bosco at 970 m the surucucho pollen record is interpreted as showing treeless vegetation in glacial times advance of treeline in late glacial time and holocene development of modern andean forests combining the surucucho and san juan bosco records shows that andean vegetation was affected by glacial cooling at all elevations vegetation did not move up and down slope as belts rather plant associations were reformed as temperature sensitive species found different centers of distribution with changing temperature

the younger dryas impact hypothesis a requiem abstract the younger dryas yd impact hypothesis is a recent theory that suggests that a cometary or meteoritic body or bodies hit and or exploded over north america 12 900 years ago causing the yd climate episode extinction of pleistocene megafauna demise of the clovis archeological culture and a range of other effects since gaining widespread attention in 2007 substantial research has focused on testing the 12 main signatures presented as evidence of a catastrophic extraterrestrial event 12 900 years ago here we present a review of the impact hypothesis including its evolution and current variants and of efforts to test and corroborate the hypothesis the physical evidence interpreted as signatures of an impact event can be separated into two groups the first group consists of evidence that has been largely rejected by the scientific community and is no longer in widespread discussion including particle tracks in archeological chert magnetic nodules in pleistocene bones impact origin of the carolina bays and elevated concentrations of radioactivity iridium and fullerenes enriched in 3 he the second group consists of evidence that has been active in recent research and discussions carbon spheres and elongates magnetic grains and magnetic spherules byproducts of catastrophic wildfire and nanodiamonds over time however these signatures have also seen contrary evidence rather than support recent studies have shown that carbon spheres and elongates do not represent extraterrestrial carbon nor impact induced megafires but are indistinguishable from fungal sclerotia and arthropod fecal material that are a small but common component of many terrestrial deposits magnetic grains and spherules are heterogeneously distributed in sediments but reported measurements of unique peaks in concentrations at the yd onset have yet to be reproduced the magnetic grains are certainly just iron rich detrital grains whereas reported yd magnetic spherules are consistent with the diffuse non catastrophic input of micrometeorite ablation fallout probably augmented by anthropogenic and other terrestrial spherular grains results here also show considerable subjectivity in the reported sampling methods that may explain the purported yd spherule concentration peaks fire is a pervasive earth surface process and reanalyses of the original yd sites and of coeval records show episodic fire on the landscape through the latest pleistocene with no unique fire event at the onset of the yd lastly with yd impact proponents increasingly retreating to nanodiamonds cubic hexagonal lonsdaleite and the proposed n diamond as evidence of impact those data have been called into question the presence of lonsdaleite was reported as proof of impact related shock processes but the evidence presented was inconsistent with lonsdaleite and consistent instead with polycrystalline aggregates of graphene and graphane mixtures that are ubiquitous in carbon forms isolated from sediments ranging from modern to pre yd age important questions remain regarding the origins and distribution of other diamond forms e g cubic nanodiamonds in summary none of the original yd impact signatures have been subsequently corroborated by independent tests of the 12 original lines of evidence seven have so far proven to be non reproducible the remaining signatures instead seem to represent either 1 non catastrophic mechanisms and or 2 terrestrial rather than extraterrestrial or impact related sources in all of these cases sparse but ubiquitous materials seem to have been misreported and misinterpreted as singular peaks at the onset of the yd throughout the arc of this hypothesis recognized and expected impact markers were not found leading to proposed yd impactors and impact processes that were novel self contradictory rapidly changing and sometimes defying the laws of physics the yd impact hypothesis provides a cautionary tale for researchers the scientific community the press and the broader public

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

younger dryas black mats and the rancholabrean termination in north america of the 97 geoarchaeological sites of this study that bridge the pleistocene holocene transition last deglaciation approximately two thirds have a black organic rich layer or black mat in the form of mollic paleosols aquolls diatomites or algal mats with radiocarbon ages suggesting they are stratigraphic manifestations of the younger dryas cooling episode 10 900 b p to 9 800 b p radiocarbon years this layer or mat covers the clovis age landscape or surface on which the last remnants of the terminal pleistocene megafauna are recorded stratigraphically and chronologically the extinction appears to have been catastrophic seemingly too sudden and extensive for either human predation or climate change to have been the primary cause this sudden rancholabrean termination at 10 900 50 b p appears to have coincided with the sudden climatic switch from allerod warming to younger dryas cooling recent evidence for extraterrestrial impact although not yet compelling needs further testing because a remarkable major perturbation occurred at 10 900 b p that needs to be explained

geomorphic and sedimentary responses of the bull creek valley southern high plains usa to pleistocene and holocene environmental change abstract fluvial geomorphology and stratigraphy often reflect past environmental and climate conditions this study examines the response of bull creek a small ephemeral creek in the oklahoma panhandle to environmental conditions through the late pleistocene and holocene fluvial terraces were mapped and their stratigraphy and sedimentology documented throughout the course of the main valley based on their elevations terraces were broadly grouped into a late pleistocene fill terrace t3 and two holocene fill cut terrace sets t2 and t1 terrace systems are marked by similar stratigraphies recording the general environmental conditions of the time sedimentary sequences preserved in terrace fills record the transition from a perennial fluvial system during the late glacial period and the younger dryas to a semiarid environment dominated by loess accumulation and punctuated by flood events during the middle to late holocene the highest rates of aeolian accumulation within the valley occurred during the early to middle holocene our data provide significant new information regarding the late pleistocene and holocene environmental history for this region located between the well studied southern and central high plains of north america

the holocene younger dryas transition recorded at summit greenland analysis of ice from dye 3 greenland has demonstrated that the transition between the younger dryas and holocene climate periods occurred over a 40 year period a near annually resolved multiparameter record of the transition recorded in the gisp2 core from summit greenland shows that most of the transition occurred in a series of steps with durations of about 5 years some climate proxies associated with mid latitude sources appear to have changed about 15 years before climate proxies associated with more northern regions changes in atmospheric water vapor are likely to have played a large role in the climate transition

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

high resolution proxy record of holocene climate from a loess section in southwestern nebraska usa abstract multi proxy analysis was used to produce a high resolution paleoclimatic record from an exceptionally thick section of the holocene bignell loess near wauneta southwestern nebraska in the central great plains the wauneta section has excellent age control based on optically stimulated luminescence osl and radiocarbon dating and records multiple episodes of rapid loess deposition alternating with slower deposition and soil formation the lowermost and uppermost osl ages obtained from the bignell loess are 10 250 610 years 5 9 m depth and 100 10 years 0 1 m depth respectively as a result the holocene has been temporally confined stratigraphically the bignell loess overlies the late pleistocene peoria loess deposited 21 14 ka and the two units are separated by the brady soil which is distinguished by its color and other pedogenic features l a b color parameters and organic carbon content of bignell loess are sensitive proxies to differentiate drought induced aeolian sediment layers from the intercalated soil horizons soil organic carbon derived 13 c data suggest that the c 3 dominated floral environment during peoria loess deposition shifted dramatically to a c 4 dominated environment during brady soil formation in response to a warming trend even greater c 4 abundance characterized the late holocene high resolution 13 c data support the contention that c 3 vs c 4 vegetation change in the holocene reflects ecosystem response to frequent vegetation disturbance under arid conditions time series analysis reveals that 13 c and color parameters display high frequency variation with periodicities of 103 118 years and 103 years respectively similar periodicities were also reported in studies of north dakota lakes though the physical mechanism responsible is uncertain comparison of bignell loess color and tropical pacific sea surface temperatures ssts allows evaluation of a proposed teleconnection between drought in the great plains and la nina like conditions in the tropical pacific the loess color index and eastern tropical pacific sst display broad similarities through the late pleistocene and holocene that are consistent with this teleconnection on the other hand drought centered at 3800 years ago is not consistent with the teleconnection and the end of early holocene aridity at the wauneta section around 6500 years ago is much earlier than the corresponding rise in sst and increase in el nino frequency in the eastern tropical pacific

stratigraphy of the younger dryas chronozone and paleoenvironmental implications central and southern great plains abstract the great plains of the united states was the setting for some of the earliest research in north america into patterns and changes in the character of late pleistocene environments and their effects on contemporary human populations many localities in the region have well stratified records of terminal pleistocene and early holocene human paleoindian activity and past environments these have proven important in debates over the character of the younger dryas chronozone ydc 11 000 10 000 14c bp 12 900 11 700 cal bp in the continental interior this paper reviews the lithostratigraphic record of the ydc on the central and southern great plains and summarizes paleobiological records largely isotopic the goal is to determine if there is any uniformity in the timing character direction and or magnitude of changes in depositional environments or broader geomorphic systems before during or after the ydc in order to address the question of the character of environments through this time the stratigraphic records of the late pleistocene to early holocene transition and in particular the stratigraphic records of the ydc vary through time and space the data clearly show that a host of geomorphic processes produced the terminal pleistocene and early holocene stratigraphic records of the great plains moreover the ydc is not necessarily manifest as a distinct lithostratigraphic or biostratigraphic entity in these different types of deposits and soils the various geomorphic systems of the great plains did not behave synchronously in response to any common climate driver these stratigraphic records reflect local environmental conditions and probably a complex response to the reorganization of mid latitude climates in the terminal pleistocene and early holocene

pollen studies in the junin area central peruvian andes laguna junin is a large lake 675 km long but only 4 m deep located in the grass covered altiplano at an altitude of 4 100 m northeast of lima pollen analysis of a 30 m core indicates that from about 12 000 yr ago until some time after 3 000 yr ago about 30 of the pollen was carried in by easterly winds from the east andean forests which today are at least 600 m lower in elevation contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the junin plain was dominated by local grassland types and had little contribution from this easterly source at some time after 3 000 yr ago perhaps about 1 200 yr ago the pollen influx to laguna junin from this distant forest source greatly diminished perhaps because of deforestation by increased human populations between about 12 000 and 24 000 yr ago in the junin core the sparse pollen concentration was dominated by grassland types supplemented by polylepis acaena alnus and compositae blown in from the sub puna shrublands that may have occupied much of the east andean slopes in place of the humid forest that occurs there today the silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain especially from the cordillera to the west these glaciers left a distinctive pattern of moraines and outwash fans which served to dam tine lake basin an unconformity in the sediment covers the time from about 24 000 to 39 000 yr ago when the lake was probably dry before this time the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation recorded on the landscape by an older set of moraines and outwash fans which initially dammed the lake basin

the younger dryas cool episode in the gulf of mexico data are presented from orca basin piston core en32 pc4 in the gulf of mexico that confirm the existence of surface water cooling during the younger dry as chronozone 11 10 ka late glacial planktonic foraminiferal species made a reappearance between 11 4 and 9 8 ka an episode also marked by distinctly higher oxygen isotopic values derived from the planktonic foraminifer globigerinoides ruber the presence of the younger dryas event in the gulf at 27 n demonstrates that surface water cooling extended to mid latitude regions in the north atlantic the cool surface water interval is bracketed by rapid shifts in 18o related to changes in the influx of meltwater to the gulf of mexico a chronology based upon seven accelerator radiocarbon dates indicates that cooling commenced over a 500 year period and ended in less than 200 years these results are among the first deep sea sediment data documenting the climatic transitions bracketing the younger dryas with a rapidity observed in ice core records a rapid decrease in 18o values measured in the white form of gs ruber at 10 2 ka is explained by significant meltwater influx into the gulf and rapid increase in sea surface temperatures surprisingly a similar decrease is not observed in the pink form of gs ruber a summer surface water dweller in the gulf this discrepancy may be explained by continued meltwater influx throughout the younger dryas during the summers only such that there was no change in the 18o of the pink form at the end of the episode an additional possibility is that warming at the end of the younger dryas raised year average temperatures and summer temperatures remained constant the coincidence of rapid shifts in 18o with the younger dryas strongly suggests a dynamic causal relationship and therefore supports a model for the cause of the younger dryas cooling based on changes in the routing of laurentide glacial meltwater

the younger dryas black mat from ojo de agua a geoarchaeological site in northeastern zacatecas mexico abstract new explorations in the desert of northeastern zacatecas in central northern mexico revealed dozens of archaeological and geoarchaeological sites one of them ojo de agua contains the remains of a pleistocene spring fed hydrographic system located at the southeastern end of a large elongated endorheic basin the locality yielded a particularly dark highly organic stratigraphic layer commonly known in the americas as black mat bm exposed on the natural profiles of a creek but not associated with cultural materials several radiocarbon assessments confirmed the formation of the ojo de agua black mat during the younger dryas chronozone with ten calibrated results clustering between 12 700 12 100 cal bp this multi proxy study confirmed the peculiarity of the deposit and found similarities and differences with other contexts of younger dryas age the ojo de agua black mat stratum c2 is far richer in charcoal specks than the related strata but lacks phytoliths diatoms or ostracods no further biological remains were found in it except for intrusive capillary roots clearly water lain in a shallow pond the stratum qualifies as a clayey silt with an acidic to neutral ph rich in heavy metals and with high contents of titanium the ojo de agua black mat yielded significant indicators of intense wildfires during the younger dryas but produced no carbon spherules or nanodiamonds supposedly linked to the impact theory

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

origin of the late quaternary dune fields of northeastern colorado stabilized eolian deposits mostly parabolic dunes and sand sheets cover much of the landscape of northeastern colorado and adjacent parts of southwestern nebraska in four geographically distinct dune fields stratigraphic and soil geomorphic relations and accelerator radiocarbon dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka possibly between 11 ka and 4 ka and within the past 1 5 ka thus eolian sand deposition took place under both glacial and interglacial climatic conditions in the youngest episodes of eolian sand movement holocene parabolic dunes partially buried pleistocene sand sheet deposits late holocene sands in the fort morgan and wray dune fields to the south of the south platte river have trace element ratios that are indistinguishable from modern south platte river sands but different from ogallala formation bedrock which has previously been cited as the main source of dune sand on the great plains sands in the greeley dune field to the north of the south platte river have trace element concentrations that indicate a probable laramie formation source measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields in good agreement with resultant drift directions calculated for nearby weather stations thus paleowinds were probably not significantly different from present day winds and are consistent with a south platte river source for the fort morgan and wray dune fields and a laramie formation source for the greeley dune field sand accumulated downwind of the south platte river to form the fort morgan duen field in addition sand was also transported farther downwind over the upland formed by the calcrete caprock of the ogallala formation and deposited in the lee of the upland on the southeast side because of high wind energy the upland itself served as a zone of sand transport but little or no sand accumulation took place on this surface these studies which demonstrate the importance of fluvial source sediments for dune fields in colorado may be applicable to other dune fields in north america because modern drift potentials in northeastern colorado are among the highest in the world the present stability of dunes in the region may be in part a function of the dunes being supply limited rather than solely transport limited extensive 7700 km2 late holocene dunes document that eolian sand in northeastern colorado is very sensitive to small changes in climate or fluvial source conditions