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

late quaternary arroyo formation and climate change in the american southwest arroyos entrenched ephemeral streams that form in desert environments first appeared in the arid and semiarid american southwest after 8000 14 c yr b p for at least 7 k y prior to that time climate vegetation and groundwater conditions were not conducive for arroyo formation along the floors of desert valleys after a hiatus in arroyo formation the frequency of arroyo cutting and filling increased dramatically after 4000 14 c yr b p the early holocene arroyos and increased frequency of arroyo incision after 4000 14 c yr b p are related to the establishment and changes in postglacial vegetation climate and groundwater conditions as a result arroyo sequences preserve a record of large scale climate change and small scale climatic perturbations that occurred during the holocene human modification of valley flood plains is an additional factor that contributed to mid nineteenth and early twentieth century arroyo cutting

bayesian chronological analyses consistent with synchronous age of 12 835 12 735 cal b p for younger dryas boundary on four continents the younger dryas impact hypothesis posits that a cosmic impact across much of the northern hemisphere deposited the younger dryas boundary ydb layer containing peak abundances in a variable assemblage of proxies including magnetic and glassy impact related spherules high temperature minerals and melt glass nanodiamonds carbon spherules aciniform carbon platinum and osmium bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled ydb age range for this event of 12 835 12 735 cal b p at 95 probability this range overlaps that of a peak in extraterrestrial platinum in the greenland ice sheet and of the earliest age of the younger dryas climate episode in six proxy records suggesting a causal connection between the ydb impact event and the younger dryas two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of the ydb layer within the limits of dating uncertainty 100 y the widespread distribution of the ydb layer suggests that it may serve as a datum layer

a high resolution record of climate variability and landscape response from kettle lake northern great plains north america abstract a decadal scale multiproxy record of minerals pollen and charcoal from kettle lake north dakota provides a high resolution record of climate and vegetation change spanning the entire holocene from the northern great plains ngp in north america the chronology is established by over 50 ams radiocarbon dates this record exhibits millennial scale trends evident in other lower resolution studies but with much more detail on short term climate variability and on the rapidity and timing of major climatic shifts as a proxy for precipitation we utilize the rate of endogenic carbonate sedimentation which depends on groundwater inflow which in turn depends on precipitation independent cluster analyses of mineral and pollen data reveal major holocene mode shifts at 10 73 ka ka cal yr bp 9 25 ka and 4 44 ka the early holocene 11 7 9 25 ka was generally wet with perhaps a trend to higher evaporation associated with warming temperatures a switch from calcite to aragonite deposition associated with a severe but brief drought occurred at 10 73 ka from 10 73 ka to 9 25 ka climate was generally humid but punctuated at 100 300 yr intervals by brief droughts including the most severe drought of the entire holocene at 9 25 ka this event was coeval with the 9 3 9 2 ka event in the greenland ice cores and observed at a number of sites worldwide in contrast the prominent 8 2 ka event in greenland is not remarkable at kettle lake the prominence of the 9 25 event locally in the ngp may be due to a major drawdown and northward retreat of lake agassiz at this time reducing its mesoclimatic effect on the ngp and thrusting the region into an insolation controlled regime the mid holocene 9 25 4 44 ka was characterized by great variability in moisture on a multi decadal scale with severe droughts alternating with more humid periods the high abundance of the weedy but drought intolerant ambrosia generally during the mid holocene and specifically during the multi decadal drought periods is seemingly paradoxical but can be explained by high interannual variability of moisture overlaid on the multi decadal variability the late holocene 4 44 ka present was also characterized by multi decadal variability in moisture but was generally wetter than the mid holocene and the magnitude of variability was less the trends in wet dry mineral pollen and charcoal proxies were similar to the mid holocene but late holocene mineral pollen assemblages are distinct from mid holocene the shift to wetter climate in the late holocene was more gradual than the abrupt shift to arid conditions 9 25 ka which may explain the asymmetric retreat and readvance of forest along the eastern margin of the ngp precipitation variations in the ngp have been linked with pacific and atlantic sea surface temperatures and mid holocene drought in the ngp has been linked with sustained la nina like conditions in the pacific these linkages may explain the decadal and millennial scale variations seen in the ngp but cause of the prominent century scale variations remains elusive

cometary airbursts and atmospheric chemistry tunguska and a candidate younger dryas event we find agreement between models of atmospheric chemistry changes from ionization for the a d 1908 tunguska siberia region russia airburst event and nitrate enhancement in greenland ice sheet project 2 gisp2h and gisp2 ice cores plus an unexplained ammonium spike we then consider a candidate cometary impact at the younger dryas onset yd the large estimated no x production and o 3 depletion are beyond accurate extrapolation but the ice core peak is much lower possibly because of insufficient sampling resolution ammonium and nitrate spikes in both greenland ice core project grip and gisp2 ice cores have been attributed to biomass burning at the onset of the yd a similar result is well resolved in tunguska ice core data but that forest fire was far too small to account for this direct input of ammonia from a comet into the atmosphere is adequate for yd ice core data but not for the tunguska data an analog of the haber process with hydrogen contributed by cometary or surface water atmospheric nitrogen high pressures and possibly catalytic iron from a comet could in principle produce ammonia accounting for the peaks in both data sets

cool wet conditions late in the younger dryas in semi arid new mexico abstract a thick alluvial sequence in central new mexico contains the scholle wet meadow deposit that traces upstream to a paleospring the wet meadow sediments contain an abundant fauna of twenty one species of freshwater and terrestrial mollusks and ten species of ostracodes the mollusks and ostracodes are indicative of a local high alluvial water table with spring supported perennial flow but without standing water pollen analysis documents shrub grassland vegetation with sedges willow and alder in a riparian community stable carbon isotopes from the wet meadow sediments have 13c values ranging from 22 8 to 23 3 indicating that 80 of the organic carbon in the sediment is derived from c3 species the wet meadow deposit is ams dated 10 400 to 9700 14c yr bp corresponding to 12 300 to 11 100 cal yr bp and overlapping in time with the younger dryas event yd the wet meadow became active about 500 yr after the beginning of the yd and persisted 400 yr after the yd ended the scholle wet meadow is the only record of perennial flow and high water table conditions in the abo arroyo drainage basin during the past 13 ka

a microbial link to weathering of postglacial rocks and sediments mount viso area western alps demonstrated through analysis of a soil paleosol bio chronosequence abstractunderstanding the mechanism associated with rates of weathering and evolution of rocks sediment soil paleosol in alpine environments raises questions related to the impact of microbial mediation versus various diverse abiotic chemical physical processes even including the overall effect of cosmic impact airburst during the early stage of weathering in late glacial lg deposits this study is of a chronosequence of soils paleosols with an age range that spans the post little ice age post lia 150 yr the little ice age lia ad 1500 1850 the middle neoglacial 3 ka younger dryas yd 12 8 ka and the lg 15 ka the goal is to elicit trends in weathering soil morphogenesis and related eubacterial population changes over the past 13 15 k yr the older lg yd paleosols in the sequence represent soil morphogenesis that started during the closing stage of pleistocene glaciation these are compared with undated soils of mid to late neoglacial age the youngest of lia and post lia age all

the younger dryas impact hypothesis a cosmic catastrophe in this paper we review the evidence for the younger dryas impact hypothesis ydih which proposes that at 12 9k cal a bp north america south america europe and the middle east were subjected to some sort of extraterrestrial event this purported event is proposed as a catastrophic process responsible for terminal pleistocene environmental changes onset of yd cooling continent scale wildfires extinction of late pleistocene mammals and demise of the clovis culture in north america the earliest well documented continent scale settlement of the region the basic physics in the ydih is not in accord with the physics of impacts nor the basic laws of physics no yd boundary ydb crater craters or other direct indicators of an impact are known age control is weak to non existent at 26 of the 29 localities claimed to have evidence for the ydih attempts to reproduce the results of physical and geochemical analyses used to support the ydih have failed or show that many indicators are not unique to an impact nor to 12 9k cal a bp the depositional environments of purported indicators at most sites tend to concentrate particulate matter and probably created many ydb zones geomorphic stratigraphic and fire records show no evidence of any sort of catastrophic changes in the environment at or immediately following the ydb late pleistocene extinctions varied in time and across space archeological data provide no indication of population decline demographic collapse or major adaptive shifts at or just after 12 9 ka the data and the hypotheses generated by ydih proponents are contradictory inconsistent and incoherent copyright 2014 john wiley sons ltd

paleoecological changes at lake cuitzeo were not consistent with an extraterrestrial impact israde alcantara et al 1 reported evidence for the younger dryas yd impact hypothesis ydih which proposes that an extraterrestrial impact triggered the yd 2 although most ydih research has focused on the impact event itself ydih proponents as in this article have argued that the ecological consequences included widespread biomass burning and contributed to the extinction of late pleistocene megafauna and to major declines in human populations to fully test the ydih the authors must evaluate evidence and mechanisms for the ecological consequences of an impact we believe there are flaws in both the interpretation of the paleoecological evidence reviewed by israde alcantara et al and in its application in testing the ydih

weathering rinds unlikely host clasts for evidence of an impact induced event abstract interhemispheric evidence of a cosmic impact 12 9 ka is known now from north to south america europe and eurasia all data supporting a cosmic event derived from cores and from geological sections most databases supporting the impact hypothesis at the younger dryas boundary ydb rely on high temperature microspherules melted minerals cracked clasts presence of nanodiamond high temperature scoria high fe ni ratios pdf s shock melted quartz high 10 be 9 be ratios and occasional presence of platinum metals controversy over the impact the so called black mat enigma and its relation to the younger dryas readvance at the end of the last ice age is fueled by arguments over whether a single extraterrestrial impact might sustain a 1 kyr long downturn in insolation engendering a substantial increase in worldwide ice volume new evidence in the form of impact microfeatures extreme breccia high crack propagation thick carbon encrustations and partial to full shock melted contorted grains in weathering rinds from the western alps france as documented here adds to the growing body of evidence that the event was truly widespread if not worldwide in effect whereas evidence of cosmic impacts may be erased by glacial and fluvial erosion in high alpine areas such events as demonstrated herein are recorded as punctuated time stratigraphic events in microcosm preserved in weathered clasts

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

desert wetlands in the geologic record abstract desert wetlands support flora and fauna in a variety of hydrologic settings including seeps springs marshes wet meadows ponds and spring pools over time eolian alluvial and fluvial sediments become trapped in these settings by a combination of wet ground conditions and dense plant cover the result is a unique combination of clastic sediments chemical precipitates and organic matter that is preserved in the geologic record as ground water discharge gwd deposits gwd deposits contain information on the timing and magnitude of past changes in water table levels and therefore are a potential source of paleohydrologic and paleoclimatic information in addition they can be important archeological and paleontological archives because desert wetlands provide reliable sources of fresh water and thus act as focal points for human and faunal activities in some of the world s harshest and driest lands here we review some of the physical sedimentological and geochemical characteristics common to gwd deposits and provide a contextual framework that researchers can use to identify and interpret geologic deposits associated with desert wetlands we discuss several lines of evidence used to differentiate gwd deposits from lake deposits they are commonly confused and examine how various types of microbiota and depositional facies aid in reconstructing past environmental and hydrologic conditions we also review how late quaternary gwd deposits are dated as well as methods used to investigate desert wetlands deeper in geologic time we end by evaluating the strengths and limitations of hydrologic and climatic records derived from gwd deposits and suggest several avenues of potential future research to further develop and utilize these unique and complex systems

late glacial sediments of the stara jimka paleolake and the first finding of laacher see tephra in the czech republic the late glacial sedimentary sequence of the stara jimka paleolake bohemian forest czech republic is characterized by the prevalence of detrital minerals with an elevated proportion of organic matter and abundant diatom valves in relatively warm periods the mineral debris originated mainly from local migmatites and mica schists or glacial and deluvial deposits derived from the same material low background contents of cao 0 20 wt and na2o 0 50 wt reflect chemical weathering correlation analysis of trace elements shows that the mineral debris concentrates rb cs ga cr zr hf light rare earth elements th u pb ba sr and weakly zn whereas mo co se as ni cu br and weakly au are allocated predominantly to sulphides and organic matter the sequence contains a continuous horizon with the laacher see tephra lst causing contrasting maxima of na ca cl and nb in bulk composition while the phonolitic tephra corresponds to the lower part of the middle lst unit it also contains phosphorus rich relicts of older sediments with small euhedral air transported volcanic crystals such sediments may have occurred in depressions at an older volcano prior to the major laacher see eruption 12 880 cal bp

nanodiamonds in the younger dryas boundary sediment layer we report abundant nanodiamonds in sediments dating to 12 9 0 1 thousand calendar years before the present at multiple locations across north america selected area electron diffraction patterns reveal two diamond allotropes in this boundary layer but not above or below that interval cubic diamonds form under high temperature pressure regimes and n diamonds also require extraordinary conditions well outside the range of earth9s typical surficial processes but common to cosmic impacts n diamond concentrations range from 10 to 3700 parts per billion by weight comparable to amounts found in known impact layers these diamonds provide strong evidence for earth9s collision with a rare swarm of carbonaceous chondrites or comets at the onset of the younger dryas cool interval producing multiple airbursts and possible surface impacts with severe repercussions for plants animals and humans in north america

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

origin and provenance of spherules and magnetic grains at the younger dryas boundary abstract one or more bolide impacts are hypothesized to have triggered the younger dryas cooling at 12 9 ka in support of this hypothesis varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the younger dryas boundary ydb we show that bulk sediment and or magnetic grains microspherules collected from the ydb sites in arizona michigan new mexico new jersey and ohio have 187os 188os ratios 1 0 similar to average upper continental crust 1 3 indicating a terrestrial origin of osmium os in these samples in contrast bulk sediments from ydb sites in belgium and pennsylvania exhibit 187os 188os ratios 1 0 and at face value suggest mixing with extraterrestrial os with 187os 188os of 0 13 however the os concentration in bulk sample and magnetic grains from belgium is 2 8 pg g and 15 pg g respectively much lower than that in average upper continental crust 31 pg g indicating no meteoritic contribution the ydb site in pennsylvania is remarkable in yielding 2 to 5 mm diameter spherules containing minerals such as suessite fe ni silicide that form at temperatures in excess of 2000 c gross texture mineralogy and age of the spherules appear consistent with their formation as ejecta from an impact 12 9 ka ago the 187os 188os ratios of the spherules and their leachates are often low but os in these objects is likely terrestrially derived the rare earth element patterns and sr and nd isotopes of the spherules indicate that their source lies in 1 5 ga quebecia terrain in the grenville province of northeastern north america

climate history of the southwestern united states based on estancia basin hydrologic variability from 69 to 10 ka abstract newly acquired u th dates on gypsum sands contained within pleistocene lake sediments allow us to extend the record of climatic change in the estancia basin of central new mexico back to 69 ka from a previous limit of 29 ka using a hydrologic balance index hbi that reflects variations in sediment mineralogy and bedding characteristics we find strong similarities to speleothem oxygen isotopic records for the southwestern united states as well as to isotopic variations in greenland ice cores intervals of wetter climate in the estancia basin generally appear to correlate to colder conditions in the north atlantic and conversely the existence of a pronounced dry period from 18 1 to 17 ka matches in timing the extrapolar climate reversal of asmerom et al 2017 at fort stanton cave in southern new mexico and confirms a different origin for this stadial like event furthermore the new u th dates support the existing estancia basin interpretation of a dry bolling allerod followed by a wet younger dryas a result consistent with speleothem and wet meadow evidence from new mexico and arizona asmerom et al 2010 2017 polyak et al 2004 hall et al 2012 wagner et al 2010 but in conflict with an interpretation of speleothem evidence from central texas feng et al 2014

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

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

black mats spring fed streams and late glacial age recharge in the southern great basin black mats are prominent features of the late pleistocene and holocene stratigraphic record in the southern great basin faunal geochemical and sedimentological evidence shows that the black mats formed in several microenvironments related to spring discharge ranging from wet meadows to shallow ponds small land snails such asgastrocopta tappanianaandvertigo berryiare the most common mollusk taxa present semiaquatic and aquatic taxa are less abundant and include catinellids fossaria parva gyraulus parvus and others living today in and around perennial seeps and ponds the ostracodescypridopsis okeechobiandscottia tumida typical of seeps and low discharge springs today as well as other taxa typical of springs and wetlands are common in the black mats several new species that lived in the saturated subsurface also are present but lacustrine ostracodes are absent the 13c values of organic matter in the black mats range from 12 to 26 reflecting contributions of tissue from both c3 sedges most shrubs and trees and c4 saltbush saltgrass plants carbon 14 dates on the humate fraction of 55 black mats fall between 11 800 to 6300 and 230014c yr b p to modern the total absence of mats in our sample between 6300 and 230014c yr b p likely reflects increased aridity associated with the mid holocene altithermal the oldest black mats date to 11 800 11 60014c yr b p and the peak in the14c black mat distribution falls at 10 00014c yr b p as the formation of black mats is spring related their abundance reflects refilling of valley aquifers starting no later than 11 800 and peaking after 11 00014c yr b p reactivation of spring fed channels shortly before 11 20014c yr b p is also apparent in the stratigraphic records from the las vegas and pahrump valleys this age distribution suggests that black mats and related spring fed channels in part may have formed in response to younger dryas yd age recharge in the region however the inception of black mat formation precedes that of the yd by at least 40014c yr and hydrological change is gradual not rapid

magma ascent and crustal accretion at ultraslow spreading ridges constraints from plagioclase ultraphyric basalts from the arctic mid ocean ridge plagioclase ultraphyric basalts pubs with up to 54 plagioclase phenocrysts were dredged in the rift valley and adjacent flanks of the ultraslow spreading mohns and knipovich ridges the pubs show large variations in crystal morphologies and zoning the large variations suggest that single basalt samples contain a mixture of plagioclase crystals that aggregated at different levels in the magma conduits resorbed crystals and repeated reverse zones suggest that the magma reservoirs were replenished and heated several times thin concentric zones with melt inclusions and sharp reductions in the anorthite content of 3 7 are common between the reverse zones these zones and skeletal crystals with distinctly lower anorthite contents than massive crystals are interpreted to be the result of rapid crystalliztion during strong undercooling the changes between short periods of cooling and longer periods with reheating are explained by multiple advances of crystal rich magma into cool regions followed by longer periods of gradual magma inflow and temperature increase the porphyritic basalts are characterizd by more depleted and more fractionated compositions than the aphyric basalts with lower la sm n k 2 oa nd mg numbers this relationship and the observation that pubs are sampled only close to segment centres along these ridges suggests that the pubs formed by higher degrees of melting and evolved in more long lived magma reservoirs we propose that the zoning patterns of plagioclase crystals and crystal morphologies of these pubs reflect the development and flow of magma through a stacked sill complex like conduit system whereas the aphyric equivalents represent later flow of magma through the conduit the formation of voluminous higher degree melts may trigger the development of the magma conduits and explain the generally depleted compositions of pub magmas

middle holocene drought on the southern high plains abstract the climate of the southern high plains during the middle holocene is indicated by data from a variety of sources stratigraphic geomorphic and pedologic research at six localities in draws several dune sites and one playa lake basin show that widespread eolian erosion and sedimentation began in some areas by 9000 yr b p and culminated 6000 4500 yr b p probably because of warmer drier conditions that reduced the vegetative cover archaeological investigations at three sites provide evidence for the excavation of wells at this time apparently because of a declining water table studies of a few vertebrate and invertebrate faunas also indicate warming and drying in the middle holocene climate models and very limited isotopic data from bison bone suggest that summers in the middle holocene were warmer than present with reduced effective precipitation all lines of evidence indicate that the southern high plains was subjected to prolonged drought in the middle holocene with a maximum between 6500 and 4500 yr b p conforming climatically and chronologically to the altithermal

megafloods and clovis cache at wenatchee washington immense late wisconsin floods from glacial lake missoula drowned the wenatchee reach of washington s columbia valley by different routes the earliest debacles nearly 19 000 cal yr bp raged 335 m deep down the columbia and built high pangborn bar at wenatchee as advancing ice blocked the northwest of columbia valley several giant floods descended moses coulee and backflooded up the columbia past wenatchee ice then blocked moses coulee and grand coulee to quincy basin became the westmost floodway from quincy basin many missoula floods backflowed 50 km upvalley to wenatchee 18 000 to 15 500 years ago receding ice dammed glacial lake columbia centuries more till it burst about 15 000 years ago after glacier peak ashfall about 13 600 years ago smaller great flood s swept down the columbia from glacial lake kootenay in british columbia the east wenatchee cache of huge fluted clovis points had been laid atop pangborn bar after the glacier peak ashfall then buried by loess clovis people came five and a half millennia after the early gigantic missoula floods two and a half millennia after the last small missoula flood and two millennia after the glacial lake columbia flood people likely saw outburst flood s from glacial lake kootenay

environmental change and seasonal behavior of mastodons in the great lakes region inferred from stable isotope analysis abstract we investigate seasonal variations in the diet and drinking water of four great lakes mastodon mammut americanum specimens using stable isotope analysis of serially sampled inner enamel bioapatite structural carbonate 13 c sc 18 o sc and previously published bulk analyses isotopic analyses and thin section measurements showed that mastodon tooth enamel extension rates 12 4 mm yr decreasing toward the cervix were lower than those of mammoths or modern elephants mastodons had distinct and highly regular seasonal variations in 13 c sc and 18 o sc which we interpret in the context of local glacial history and vegetation changes seasonal variations in 18 o were large but variations in 13 c were small and may have been obscured if coarser sampling methods than our inner enamel sampling approach were used thus our approach may be particularly useful for understanding relatively small seasonal changes in 13 c within c 3 environments the seasonal patterns though not entirely conclusive suggest that the ontario mastodons did not migrate over very long distances rather the climate and seasonal dietary patterns of mastodons within the region changed over time from 12 400 to 10 400 14 c yr bp 15 000 12 000 cal yr bp insights gained using these methods can contribute to a better understanding of megafaunal extinctions and paleoamerican lifeways

wildfire and abrupt ecosystem disruption on california s northern channel islands at the allerod younger dryas boundary 13 0 12 9 ka abstract sedimentary records from california s northern channel islands and the adjacent santa barbara basin sbb indicate intense regional biomass burning wildfire at the allerod younger dryas boundary 13 0 12 9 ka all age ranges in this paper are expressed in thousands of calendar years before present ka radiocarbon ages will be identified and clearly marked 14c years multiproxy records in sbb ocean drilling project odp site 893 indicate that these wildfires coincided with the onset of regional cooling and an abrupt vegetational shift from closed montane forest to more open habitats abrupt ecosystem disruption is evident on the northern channel islands at the allerod younger dryas boundary with the onset of biomass burning and resulting mass sediment wasting of the landscape these wildfires coincide with the extinction of mammuthus exilis pygmy mammoth the earliest evidence for human presence on these islands at 13 1 12 9 ka 11 000 10 900 14c years is followed by an apparent 600 800 year gap in the archaeological record which is followed by indications of a larger scale colonization after 12 2 ka although a number of processes could have contributed to a post 18 ka decline in m exilis populations e g reduction of habitat due to sea level rise and human exploitation of limited insular populations we argue that the ultimate demise of m exilis was more likely a result of continental scale ecosystem disruption that registered across north america at the onset of the younger dryas cooling episode contemporaneous with the extinction of other megafaunal taxa evidence for ecosystem disruption at 13 12 9 ka on these offshore islands is consistent with the younger dryas boundary cosmic impact hypothesis firestone r b west a kennett j p becker l bunch t e revay z s schultz p h belgya t kennett d j erlandson j m dickenson o j goodyear a a harris r s howard g a kloosterman j b lechler p mayewski p a montgomery j poreda r darrah t que hee s s smith a r stich a topping w wittke j h wolbach w s 2007 evidence for an extraterrestrial impact 12 900 years ago that contributed to the megafaunal extinctions and younger dryas cooling proceedings of the national academy of sciences 104 16016 16021

evidence from the northwestern venezuelan andes for extraterrestrial impact the black mat enigma abstract a carbon rich black layer encrusted on a sandy pebbly bed of outwash in the northern venezuelan andes previously considered the result of an alpine grass fire is now recognized as a black mat candidate correlative with clovis age sites in north america falling within the range of black mat dated sites 12 9 ka cal bp as such the bed at site mum7b which dates to 14 c years bp raw dates and appears to be contemporaneous with the younger dryas yd cooling event marks a possibly much more extensive occurrence than previously identified no fossils megafauna or tool assemblages were observed at this newly identified candidate site 3800 a m s l as in the case of the north american sites here evidence is presented for an extraterrestrial impact event at 12 9 ka the impact related andean bed located 20 cm above 13 7 13 3 ka cal bp alluvial and glaciolacustrine deposits falls within the sediment characteristics and age range of black mat dated sites 12 9 ka cal bp in north america site sediment characteristics include carbon glassy spherules magnetic microspherules carbon mat welded onto coarse granular material occasional presence of platinum group metals rh and ru planar deformation features pdfs in fine silt size fragmental grains of quartz as well as orthoclase and monazite with an abundance of rare earth elements rees if the candidate site is black mat correlative with the black mat sites of north america such an extensive occurrence may support the hypothesized airburst impact over the laurentide glacier which led to a reversal of allerod warming and the onset of yd cooling and readvance of glaciers while this finding does not confirm such it merits further investigation which includes the reconnaissance for additional sites in south america furthermore if confirmed such an extensive occurrence may corroborate an impact origin

late quaternary paleohydrology of desert wetlands and pluvial lakes in the soda lake basin central mojave desert california usa abstract sediment cores taken near extant springs along the western margin of soda lake playa as well as from the playa center reveal dramatic hydrologic changes that occurred in the central mojave desert during the late quaternary results of stratigraphic chronologic physical chemical and microfossil analyses of seven cores ranging in length from 5 to 23 m help refine the timing and character of the final stages of pluvial lake mojave during the late pleistocene and define distinct periods of wetland development in the early and late holocene evidence shows that an incipient lake occupied the central soda lake basin by at least 25 0 ka ka thousands of calibrated 14 c years before present and a fully developed lake mojave was present between 20 5 and 12 8 ka before receding and ultimately yielding to playa conditions by 11 0 ka organic rich black mats appear in several cores along the playa margin between 10 7 and 9 0 ka suggesting that spring fed wetlands persisted in this area long after the lake had regressed the basin remained relatively dry throughout most of the holocene until wetland ecosystems expanded along the margins between 0 73 and 0 18 ka coincident with part of the medieval climate anomaly and little ice age overall our results demonstrate that buried sediments surrounding extant and extinct springs can be used to reconstruct past hydrologic conditions in desert environments on a variety of spatial and temporal scales and provide important baseline information for effective management of limited desert resources

chronology sedimentology and microfauna of groundwater discharge deposits in the central mojave desert valley wells california during the late pleistocene emergent groundwater supported persistent and long lived desert wetlands in many broad valleys and basins in the american southwest when active these systems provided important food and water sources for local fauna supported hydrophilic and phreatophytic vegetation and acted as catchments for eolian and alluvial sediments desert wetlands are represented in the geologic record by groundwater discharge deposits which are also called spring or wetland deposits groundwater discharge deposits contain information on the timing and magnitude of past changes in water table levels and thus are a source of paleohydrologic and paleoclimatic information here we present the results of an investigation of extensive groundwater discharge deposits in the central mojave desert at valley wells california we used geologic mapping and stratigraphic relations to identify two distinct wetland sequences at valley wells which we dated using radiocarbon luminescence and uranium series techniques we also analyzed the sediments and microfauna ostracodes and gastropods to reconstruct the specifi c environments in which they formed our results suggest that the earliest episode of high water table conditions at valley wells began ca 60 ka thousands of calendar yr b p and culminated in peak discharge between ca 40 and 35 ka during this time cold 4 12 c emergent groundwater supported extensive wetlands that likely were composed of a wet sedge rush tussock meadow mixed with mesic riparian forest after ca 35 ka the water table dropped below the ground surface but was still shallow enough to support dense stands of phreatophytes through the last glacial maximum lgm the water table dropped further after the lgm and xeric conditions prevailed until modest wetlands returned briefl y during the younger dryas cold event 13 0 11 6 ka we did not observe any evidence of wet conditions during the holocene at valley wells the timing of these fl uctuations is consistent with changes in other paleowetland systems in the mojave desert the nearby great basin desert and in southeastern arizona near the border of the sonoran and chihuahuan deserts the similarities in hydrologic conditions between these disparate locations suggest that changes in groundwater levels during the late pleistocene in desert wetlands scattered throughout the american southwest were likely driven by synopticscale climate processes

weathering rinds as mirror images of palaeosols examples from the western alps with correlation to antarctica and mars weathering rinds have been used for decades as relative age indicators to differentiate glacial deposits in long quaternary sequences but only recently has it been shown that rinds contain long and extensive palaeoenvironmental records that often extend far beyond mere repositories of chemical weathering on both earth and mars when compared with associated palaeosols in deposits of the same age rinds often carry a zonal weathering record that can be correlated with palaeosol horizon characteristics with respect to both abiotic and biotic parameters as demonstrated with examples from the french and italian alps rinds in coarse clastic sediment contain weathering zones that correlate closely with horizon development in associated palaeosols of presumed late glacial age in addition to weathering histories in both rinds and palaeosols considerable evidence exists to indicate that the black mat impact 12 8 ka reached the european alps a connection with the younger dryas readvance supported by both mineral and chemical composition preliminary metagenomic microbial analysis using density gradient gel electrophoresis suggests that the eubacterial microbial population found in at least one ah palaeosol horizon associated with a rind impact site is different from that in other late glacial and younger dryas surface palaeosol horizons

chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12 800 years ago according to the younger dryas impact hypothesis ydih 12 800 calendar years before present north america experienced an extraterrestrial impact that triggered the younger dryas and devastated human populations and biotic communities on this continent and elsewhere this supposed event is reportedly marked by multiple impact indicators but critics have challenged this evidence and considerable controversy now surrounds the ydih proponents of the ydih state that a key test of the hypothesis is whether those indicators are isochronous and securely dated to the younger dryas onset they are not we have examined the age basis of the supposed younger dryas boundary layer at the 29 sites and regions in north and south america europe and the middle east in which proponents report its occurrence several of the sites lack any age control others have radiometric ages that are chronologically irrelevant nearly a dozen have ages inferred by statistically and chronologically flawed age depth interpolations and in several the ages directly on the supposed impact layer are older or younger than 12 800 calendar years ago only 3 of the 29 sites fall within the temporal window of the yd onset as defined by ydih proponents the ydih fails the critical chronological test of an isochronous event at the yd onset which coupled with the many published concerns about the extraterrestrial origin of the purported impact markers renders the ydih unsupported there is no reason or compelling evidence to accept the claim that a cosmic impact occurred 12 800 y ago and caused the younger dryas

accumulation of impact markers in desert wetlands and implications for the younger dryas impact hypothesis the younger dryas impact hypothesis contends that an extraterrestrial object exploded over north america at 12 9 ka initiating the younger dryas cold event the extinction of many north american megafauna and the demise of the clovis archeological culture although the exact nature and location of the proposed impact or explosion remain unclear alleged evidence for the fallout comes from multiple sites across north america and a site in belgium at 6 of the 10 original sites excluding the carolina bays elevated concentrations of various impact markers were found in association with black mats that date to the onset of the younger dryas black mats are common features in paleowetland deposits and typically represent shallow marsh environments in this study we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern united states and the atacama desert of northern chile at 10 of 13 sites we found elevated concentrations of iridium in bulk and magnetic sediments magnetic spherules and or titanomagnetite grains within or at the base of black mats regardless of their age or location suggesting that elevated concentrations of these markers arise from processes common to wetland systems and not a catastrophic extraterrestrial impact event

new evidence from a black mat site in the northern andes supporting a cosmic impact 12 800 years ago abstractprevious work has ascribed a cosmic impact origin to black high temperature carbon encrusted beds 2 3 cm thick associated with the younger dryas readvance of ice at 12 8 ka during the late glacial in the northern andes of venezuela the evidence for this includes carbon spherules aluminosilicate melt rocks melted coatings of glass like amorphous carbon and fe mn on sands and clasts derived from local felsic gneiss and granite these sediments have been subjected to renewed investigation using high resolution scanning electron microscopy and energy dispersive spectrometry and new data show that spherules at site mum7b exhibit unique morphologies and compositions molar oxide weight percentages prove the spherules are not volcanic and show little overlap with cosmic materials spherule microstructures display quench melting and thus could not have formed from slow geological authigenic diagenetic or metamorphic processes instead geochemical values for the venezuelan samples plot withi

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

a regional record of expanded holocene wetlands and prehistoric human occupation from paleowetland deposits of the western yarlung tsangpo valley southern tibetan plateau abstract the asian monsoon which brings 80 of annual precipitation to much of the tibetan plateau provides runoff to major rivers across the asian continent paleoclimate records indicate summer insolation and north atlantic paleotemperature changes forced variations in monsoon rainfall through the holocene resulting in hydrologic and ecologic changes in plateau watersheds we present a record of holocene hydrologic variability in the yarlung tsangpo yt valley of the southern tibetan plateau based on sedimentology and 14 c dating of organic rich black mats in paleowetlands deposits that shows changes in wetlands extent in response to changing monsoon intensity four sedimentary units indicate decreasing monsoon intensity since 10 4 ka bp wet conditions occurred at 10 4 ka bp 9 6 ka bp and 7 9 4 8 ka bp with similar to modern conditions from 4 6 2 0 ka bp and drier than modern conditions from 2 0 ka bp to present wetland changes correlate with monsoon intensity changes identified in nearby records with weak monsoon intervals corresponding to desiccation and erosion of wetlands dating of in situ ceramic and microlithic artifacts within the wetlands indicates epipaleolithic human occupation of the yt valley after 6 6 ka bp supporting evidence for widespread colonization of the tibetan plateau in the early and mid holocene during warm wet post glacial conditions

isotopic paleoecology of clovis mammoths from arizona the causes of megafaunal extinctions in north america have been widely debated but remain poorly understood mammoths mammuthus spp in the american southwest were hunted by clovis people during a period of rapid climate change just before the regional onset of younger dryas cooling and mammoth extirpation thus these mammoths may provide key insights into late pleistocene extinction processes here we reconstruct the seasonal diet and climatic conditions experienced by mammoths in the san pedro valley of arizona using the carbon 13c 12c and oxygen 18o 16o isotope compositions of tooth enamel these records suggest that clovis mammoths experienced a warm dry climate with sufficient summer rainfall to support seasonal c4 plant growth monsoon intensity may have been reduced relative to the preceding time period but there is no isotopic evidence for severe drought however it is possible that the clovis drought inferred from stratigraphic evidence occurred suddenly at the end of the animals lives and thus was not recorded in the enamel isotopic compositions unlike mammoths that lived before the last glacial maximum clovis mammoths regularly increased c4 grass consumption during summer probably seeking seasonally green grasslands farther from the river valley this predictable seasonal behavior may have made mammoths easier to locate by clovis hunters furthermore clovis mammoths probably had no previous experience of such sudden climatic change as is believed to have occurred at the time of their extinction

evidence for deposition of 10 million tonnes of impact spherules across four continents 12 800 y ago airbursts impacts by a fragmented comet or asteroid have been proposed at the younger dryas onset 12 80 0 15 ka based on identification of an assemblage of impact related proxies including microspherules nanodiamonds and iridium distributed across four continents at the younger dryas boundary ydb spherule peaks have been independently confirmed in eight studies but unconfirmed in two others resulting in continued dispute about their occurrence distribution and origin to further address this dispute and better identify ydb spherules we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry morphologies origins and processes of formation we investigated 18 sites across north america europe and the middle east performing nearly 700 analyses on spherules using energy dispersive x ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization twelve locations rank among the world s premier end pleistocene archaeological sites where the ydb marks a hiatus in human occupation or major changes in site use our results are consistent with melting of sediments to temperatures 2 200 c by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere they are inconsistent with volcanic cosmic anthropogenic lightning or authigenic sources we also produced spherules from wood in the laboratory at 1 730 c indicating that impact related incineration of biomass may have contributed to spherule production at 12 8 ka an estimated 10 million tonnes of spherules were distributed across 50 million square kilometers similar to well known impact strewnfields and consistent with a major cosmic impact event

post clovis survival of american mastodon in the southern great lakes region of north america abstract the end of the pleistocene in north america was marked by a wave of extinctions of large mammals with the last known appearances of many species falling between ca 11 000 10 000 14c yr bp temporally this period overlaps with the clovis paleoindian cultural complex 11 190 10 530 14c yr bp and with sudden climatic changes that define the beginning of the younger dryas chronozone ca 11 000 10 000 14c yr bp both of which have been considered as potential proximal causes of this extinction event radiocarbon dating of enamel and filtered bone collagen from an extinct american mastodon mammut americanum from northern indiana usa by accelerator mass spectrometer yielded direct dates of 10 055 40 14c yr bp and 10 032 40 14c yr bp indicating that the animal survived beyond the clovis time period and into the late younger dryas although the late survival of this species in mid continental north america does not remove either humans or climatic change as contributing causes for the late pleistocene extinctions neither clovis hunters nor the climatic perturbations initiating the younger dryas chronozone were immediately responsible for driving mastodons to extinction

stable carbon isotopes c3 c4 vegetation and 12 800years of climate change in central new mexico usa abstract a thick sequence of fine textured alluvium in central new mexico has provided an opportunity to reconstruct vegetation type and climate at the western edge of the great plains grassland the analysis of thirty eight ams radiocarbon ages 13 c values and modern weather data has produced a 12 800 cal yr record of changing c 3 c 4 vegetation temperature and precipitation the record begins with the younger dryas that was characterized by c 3 plants and was about 2 4 c cooler with over 100 mm rainfall than today after 11 000 cal yrs bp the climate became less cool and less wet reaching present day conditions by 9000 yrs the middle holocene was c 4 warm and dry although the 13 c record is incomplete for this interval from 3300 to 1400 cal yrs bp the climate was cool and wet with c 3 plants averaging 0 5 c cooler and 22 mm greater rainfall than today matching other records in the region for a wetter climate after 1400 cal yrs bp the local climate became warm and dry shifting to c 4 vegetation the severe second century drought first observed in tree rings in southern colorado is supported by the 13 c data and occurred during the interval a d 40 to a d 180 hiatuses in the local alluvial and 13 c record extend from 9000 to 6000 cal yrs bp and from 1000 cal yrs bp to present

nanodiamonds and wildfire evidence in the usselo horizon postdate the allerod younger dryas boundary the controversial younger dryas impact hypothesis suggests that at the onset of the younger dryas an extraterrestrial impact over north america caused a global catastrophe the main evidence for this impact after the other markers proved to be neither reproducible nor consistent with an impact is the alleged occurrence of several nanodiamond polymorphs including the proposed presence of lonsdaleite a shock polymorph of diamond we examined the usselo soil horizon at geldrop aalsterhut the netherlands which formed during the allerod early younger dryas and would have captured such impact material our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact in addition we present evidence for the occurrence of cubic diamond in glass like carbon no lonsdaleite was found the relation of the cubic nanodiamonds to glass like carbon which is produced during wildfires suggests that these nanodiamonds might have formed after rather than at the onset of the younger dryas our analysis thus provides no support for the younger dryas impact hypothesis

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

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

man and megafauna in tasmania closing the gap abstract recent discussion on the late pleistocene extinction of the australian megafauna has revolved around interpretation of several key fossil sites in tasmania it has been suggested that humans did not arrive in tasmania until after the megafauna became extinct or did not hunt now extinct megafauna and therefore that humans cannot be implicated in the extinctions radiocarbon results from these sites indicate that the youngest extinct megafauna are close to charcoal ages from the oldest archaeological deposits although difficulties have arisen in establishing chronologies because most relevant sites have ages near the limit for radiocarbon analysis we report a series of new radiocarbon ages 13 c 15 n and c n ratios on collagen and dentine fractions from skeletal remains in the mount cripps karst area and the mowbray swamp both in northwestern tasmania and discuss the reliability of ages from these and other sites we also report the discovery of an articulated simosthenurus occidentalis skeleton at mt cripps that represents only the second directly dated extinct megafaunal taxon with a reliable age our results suggest that c n ratios measured on collagen or dentine are not an infallible guide to radiocarbon age reliability we confirm previous reports of a temporal overlap between the megafaunal and archaeological records in tasmania but the presence of archaeological evidence and megafauna with the same age at the same site has not yet been demonstrated at least two megafaunal taxa the now extinct protemnodon anak and a giant pleistocene form of the extant macropus giganteus were still present in tasmania after 43 ka when human crossing of the bassian landbridge from mainland australia first became sustainable

chabazite and dolomite formation in a dolocrete profile an example of a complex alkaline paragenesis in lanzarote canary islands this paper studies the weathering and soil formation processes operating on detrital sediments containing alkaline volcanic rock fragments of the mirador del rio dolocrete profile the profile consists of a lower horizon of removilised weathered basalts an intermediate red sandy mudstones horizon with irregular carbonate layers and a topmost horizon of amalgamated carbonate layers with root traces formation occurred in arid to semiarid climates giving place to a complex mineralogical association including mg carbonates and chabazite rarely described in cal dolocretes profiles initial vadose weathering processes occurred in the basalts and in directly overlying detrital sediments producing stage 1 red smectites and dolomicrite dominant phreatic stage 2 conditions allowed precipitation of coarse zoned dolomite and chabazite filling porosities in stages 3 and 4 mostly pedogenic biogenic processes played an important role in dolomite and calcite accumulation in the profile overall evolution of the profile and its mineralogical association involved initial processes dominated by alteration of host rock to provide silica and mg rich alkaline waters suitable for chabazite and dolomite formation without a previous carbonate phase dolomite formed both abiogenically and biogenically but without a previous carbonate precursor and in the absence of evaporites dominance of calcite towards the profile top is the result of mg ca decrease in the interstitial meteoric waters due to decreased supply of mg from weathering and increased supply of ca in aeolian dust meteoric origin of the water is confirmed by c and o isotope values which also indicate lack of deep sourced co2 the dolocrete studied and its complex mineral association reveal the complex interactions that occur at surface during weathering and pedogenesis of basalt sourced rocks

shock synthesized hexagonal diamonds in younger dryas boundary sediments the long standing controversy regarding the late pleistocene megafaunal extinctions in north america has been invigorated by a hypothesis implicating a cosmic impact at the llerod younger dryas boundary or ydb 12 900 100 cal bp or 10 900 100 14c years abrupt ecosystem disruption caused by this event may have triggered the megafaunal extinctions along with reductions in other animal populations including humans the hypothesis remains controversial due to absence of shocked minerals tektites and impact craters here we report the presence of shock synthesized hexagonal nanodiamonds lonsdaleite in ydb sediments dating to 12 950 50 cal bp at arlington canyon santa rosa island california lonsdaleite is known on earth only in meteorites and impact craters and its presence strongly supports a cosmic impact event further strengthened by its co occurrence with other nanometer sized diamond polymorphs n diamonds and cubics these shock synthesized diamonds are also associated with proxies indicating major biomass burning charcoal carbon spherules and soot this biomass burning at the younger dryas yd onset is regional in extent based on evidence from adjacent santa barbara basin and coeval with broader continent wide biomass burning biomass burning also coincides with abrupt sediment mass wasting and ecological disruption and the last known occurrence of pygmy mammoths mammuthus exilis on the channel islands correlating with broader animal extinctions throughout north america the only previously known co occurrence of nanodiamonds soot and extinction is the cretaceous tertiary k t impact layer these data are consistent with abrupt ecosystem change and megafaunal extinction possibly triggered by a cosmic impact over north america at 12 900 100 cal bp

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

osl age and stratigraphy of the strauss sand sheet in new mexico usa abstract the strauss sand sheet occurs in south central new mexico usa and northern chihuahua mexico covering an area of about 4740 km2 its chronology is determined by 19 osl ages the sand sheet formed primarily during three phases of eolian deflation and deposition each phase with a separate sand source and under different climatic and environmental circumstances the first phase of eolian sedimentation occurred 45 to 15 ka with the deposition of unit 1 the sand source for the first phase was beach related features along the eastern shoreline of pluvial lake palomas in mexico the glacial age climate was cool wet and windy because of the southern path of the jet stream at that time after 15 ka with the onset of warmer conditions of the bolling allerod the shutting down of the palomas sand source and wet conditions of the younger dryas the sand sheet stabilized with weak soil development in unit 1 by 11 ka the climate shifted to holocene drying conditions and the second phase of sand accumulation began forming unit 2 the sand source was the local deflation of the previously deposited unit 1 sand the sand sheet stabilized again by 1 9 ka with slightly wetter late holocene climate a weak soil formed in unit 2 sand about a d 1500 and extending to about a d 1850 or later an a horizon formed on the sand sheet probably in response to a desert grassland vegetation during the period of wet climate of the little ice age in an anthropogenic third phase of eolian activity after a d 1850 the vegetation was likely disturbed by overgrazing and the unit 2 and a horizon unit 3 sands were deflated resulting in the deposition of a thin layer of massive eolian sand unit 4 across the sand sheet by about a d 1900 mesquite shrubs had increased in abundance and deflated sand largely from unit 2 began to accumulate around the shrubs forming coppice dunes unit 5 mesquite coppice dunes continued to increase in number and volume during the twentieth century and at present dominate most of the sand sheet this third phase of eolian deflation deposition is ongoing today

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

the mysterious onset of the younger dryas abstract the 1300 year long younger dryas cold reversal 12 900 11 600 cal bp is seen most clearly in greenland ice cores contemporaneous changes of climate and environment in more southerly regions were variably expressed the precise timing of the abrupt younger dryas onset is still uncertain it was linked to a sudden increase in 14c manifest as a cliff where 14c dates drop from 11 000 to 10 600 radiocarbon years bp within a century of real time followed by a long plateau changes in ocean and or atmospheric circulation or solar radiation and even a comet impact have been proposed as triggers but the abruptness and severity of yd onset still elude explanation

a search for shocked quartz grains in the allerod younger dryas boundary layer the younger dryas impact hypothesis suggests that multiple airbursts or extraterrestrial impacts occurring at the end of the allerod interstadial resulted in the younger dryas cold period so far no reproducible diagnostic evidence has however been reported quartz grains containing planar deformation features known as shocked quartz grains are considered a reliable indicator for the occurrence of an extraterrestrial impact when found in a geological setting although alleged shocked quartz grains have been reported at a possible allerod younger dryas boundary layer in venezuela the identification of shocked quartz in this layer is ambiguous to test whether shocked quartz is indeed present in the proposed impact layer we investigated the quartz fraction of multiple allerod younger dryas boundary layers from europe and north america where proposed impact markers have been reported grains were analyzed using a combination of light and electron microscopy techniques all samples contained a variable amount of quartz grains with sub planar microstructures often tectonic deformation lamellae a total of one quartz grain containing planar deformation features was found in our samples this shocked quartz grain comes from the usselo palaeosol at geldrop aalsterhut the netherlands scanning electron microscopy cathodoluminescence imaging and transmission electron microscopy imaging however show that the planar deformation features in this grain are healed and thus likely to be older than the allerod younger dryas boundary we suggest that this grain was possibly eroded from an older crater or distal ejecta layer and later redeposited in the european sandbelt the single shocked quartz grain at this moment thus cannot be used to support the younger dryas impact hypothesis