cosmic impact event in lake sediments from central europe postdates the laacher see eruption and marks onset of the younger dryas abstractthe younger dryas yd climate episode 12 850 11 650 calendar years before present cal bp is an event recorded widely across the northern hemisphere we conducted multiple analyses at

impact of the late glacial eruption of the laacher see volcano central rhineland germany abstract within a period of a few weeks toward the end of the allerod interstadial the major plinian eruption of the laacher see volcano produced some 20 km3 of eruptiva covering and preserving the late glacial landscape in the german central rhineland over an area of more than 1000 km2 correlation of terrestrial archives with the greenland ice core records and improved calibration of the radiocarbon timescale permit a precise accurate age determination of the laacher see event some 200 yr before the onset of the younger dryas cold episode carbonized trees and botanical macrofossils preserved by laacher see tephra permit detailed regional paleoenvironmental reconstruction and show that open woodland were typical for the cool and humid hemiboreal climatic conditions during the late allerod this woodland provided the habitat for a large variety of animal species documented at both paleontological and final paleolithic archeological sites preserved below laacher see deposits of special interest are numerous animal tracks intercalated in middle laacher see deposits at the south of the neuwied basin this knowledge may help to evaluate possible supraregional impacts of this volcanic event on northern hemispheric environment and climate during the late allerod

acoustic backscatter measurements from littoral seabeds at shallow grazing angles at 4 and 8 khz direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters specifically returns from the air water interface typically contaminate the signals of interest to address this issue drdc atlantic has developed a sea going research system for measuring acoustic scatter from the seabed in shallow water environs the system known as the wideband sonar wbs consists of a parametric array transmitter and a superdirective receiver in this paper backscatter measurements obtained with the wbs at two sandy shallow water sites off north america s atlantic coast are presented data were collected at 4 and 8 khz at grazing angles from 3 15 the backscattering strength is similar at both sites and below about 8 it appears to be independent of frequency within the statistical accuracy of the data the measurements show reasonable agreement with model estimates of backscatter from sandy sediments a smal

very high temperature impact melt products as evidence for cosmic airbursts and impacts 12 900 years ago it has been proposed that fragments of an asteroid or comet impacted earth deposited silica and iron rich microspherules and other proxies across several continents and triggered the younger dryas cooling episode 12 900 years ago although many independent groups have confirmed the impact evidence the hypothesis remains controversial because some groups have failed to do so we examined sediment sequences from 18 dated younger dryas boundary ydb sites across three continents north america europe and asia spanning 12 000 km around nearly one third of the planet all sites display abundant microspherules in the ydb with none or few above and below in addition three sites abu hureyra syria melrose pennsylvania and blackville south carolina display vesicular high temperature siliceous scoria like objects or slos that match the spherules geochemically we compared ydb objects with melt products from a known cosmic impact meteor crater arizona and from the 1945 trinity nuclear airburst in socorro new mexico and found that all of these high energy events produced material that is geochemically and morphologically comparable including i high temperature rapidly quenched microspherules and slos ii corundum mullite and suessite fe3si a rare meteoritic mineral that forms under high temperatures iii melted sio2 glass or lechatelierite with flow textures or schlieren that form at 2 200 c and iv particles with features indicative of high energy interparticle collisions these results are inconsistent with anthropogenic volcanic authigenic and cosmic materials yet consistent with cosmic ejecta supporting the hypothesis of extraterrestrial airbursts impacts 12 900 years ago the wide geographic distribution of slos is consistent with multiple impactors

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

cosmogenic nuclide enhancement via deposition from long period comets as a test of the younger dryas impact hypothesis abstract we explore the idea that detectable excursions in 26 al may arise from direct deposition by any bolide and excursions in 14 c and 10 be abundances in the atmosphere may result from long period comet impacts this is very different from the usual processes of production by cosmic rays within earth s atmosphere long period comets experience greatly increased cosmic ray flux beyond the protection of the sun s magnetic field we report the computed amount of 14 c 10 be and 26 al present on long period comets as a function of comet mass we find that the amount of nuclide mass on large long period comets entering the earth s atmosphere may be sufficient for creating anomalies in the records of 14 c and 10 be from past impacts in particular the estimated mass of the proposed younger dryas comet is consistent with its having deposited sufficient isotopes to account for recorded 14 c and 10 be increases at that time the 26 al 10 be ratio is much larger in extraterrestrial objects than in the atmosphere and so we note that measuring this ratio in ice cores is a suitable definitive test for the younger dryas impact hypothesis even if the hypothetical bolide is not a long period comet and or did not contribute to the 14 c and 10 be increases

putting the younger dryas cold event into context abstract the younger dryas event is by far the best studied of the millennial scale cold snaps of glacial time yet its origin remains a subject of debate the long held scenario that the younger dryas was a one time outlier triggered by a flood of water stored in proglacial lake agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape the recent suggestion that the younger dryas was triggered by the impact of a comet has not gained traction instead evidence from chinese stalagmites suggests that rather than being a freak occurrence the younger dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale

gisp2 oxygen isotope ratios the gisp2 oxygen isotope record with its high resolution detail yields crucial information on past climate change the glacial 18o oscillations of the gisp2 core with their very fast onsets are templates of a prototype oscillation of variable duration with an amplitude of 3 9 the halfway mark of the cold warm transition is reached in 2 years the top is reached in 50 years the 18o time gradient of the leading front is about 7 8 per 100 yr after reaching the top 18o slowly declines by 0 14 per 100 yr the duration of 18o decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones the subsequent 18o downturn during the warm cold transition has a 18o time gradient of 3 2 per 100 yr and lasts about 80 yr

highly siderophile element depletion in the moon abstract coupled 187 os 188 os and highly siderophile element hse os ir ru pt pd re abundance data are reported for apollo 12 12005 12009 12019 12022 12038 12039 12040 apollo 15 15555 and apollo 17 70135 mare basalts along with mare basalt meteorites la paz icefield lap 04841 and miller range mil 05035 these mare basalts have consistently low hse abundances at 2 10 5 to 2 10 7 the chondritic abundance the most magnesian samples have broadly chondrite relative hse abundances and chondritic measured and calculated initial 187 os 188 os the lower abundances and fractionated hse compositions of more evolved mare basalts can be reproduced by modeling crystal liquid fractionation using rock melt bulk partition coefficients of 2 for os ir ru pt and pd and 1 5 for re lunar mare basalt bulk partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions leading to increased hse compatibility the chondritic relative abundances and chondritic 187 os 188 os of the most primitive high mgo mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface mare basalt compositions are best modeled as representing 5 11 partial melting of metal free sources with low os ir ru pd 0 1 ng g 1 pt 0 2 ng g 1 re 0 01 ng g 1 and s 75 g g 1 with sulphide melt partitioning between 1000 and 10 000 apollo 12 olivine pigeonite and ilmenite normative mare basalts define an imprecise 187 re 187 os age of 3 0 0 9 ga with an initial 187 os 188 os of 0 107 0 010 this age is within uncertainty of 147 sm 143 nd ages for the samples the initial os isotopic composition of apollo 12 samples indicates that the source of these rocks evolved with re os within 10 of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted similarity in absolute hse abundances between mare basalts from the apollo 12 15 and 17 sites and from unknown regions of the moon la paz mare basalts mil 05035 indicates relatively homogeneous and low hse abundances within the lunar interior low absolute hse abundances and chondritic re os of mare basalts are consistent with a late accretion addition of 0 02 wt of the moon s mass to the mantle prior to the formation of the lunar crust late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation 4 4 ga to enable efficient mixing and homogenization within the mantle low lunar hse abundances are consistent with proportionally 40 times more late accretion to earth than the moon disproportional late accretion to the two bodies is consistent with the small 182 w excess 21 28 ppm measured in lunar rocks compared to the silicate earth

was the younger dryas triggered by a flood draining of a huge lake into the northern atlantic may have triggered a cold period 12 900 years ago the route taken by the flood waters remains unknown

a model for the geomorphology of the carolina bays abstract geometrical analysis of the carolina bays using google earth in combination with lidar data makes it possible to postulate that the bays formed as the result of impacts rather than from eolian and lacustrine processes the carolina bays are elliptical conic sections with width to length ratios averaging 0 58 that are radially oriented toward the great lakes region the radial distribution of ejecta is one characteristic of impacts and the width to length ratios of the ellipses correspond to cones inclined at approximately 35 which is consistent with ballistic trajectories from the point of convergence these observations and the fact that these geomorphological features occur only on unconsolidated soil close to the water table make it plausible to propose that the carolina bays are the remodeled remains of oblique conical craters formed on ground liquefied by the seismic shock waves of secondary impacts of glacier ice boulders ejected by an extraterrestrial impact on the laurentide ice sheet mathematical analysis using ballistic equations and scaling laws relating yield energy to crater size provide clues about the magnitude of the extraterrestrial event an experimental model elucidates the remodeling mechanisms and provides an explanation for the morphology and the diverse dates of the bays

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

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

radiocarbon simulation fails to support the temporal synchroneity requirement of the younger dryas impact hypothesis fine scale temporal processes such as the synchronous deposition of organic materials can be challenging to identify using 14 c datasets while some events such as volcanic eruptions leave clear evidence for synchronous deposition synchroneity is more difficult to establish for other types of events this has been a source of controversy regarding 14 c dates associated with a hypothesized extraterrestrial impact at the younger dryas boundary ydb to address this controversy we first aggregate 14 c measurements from northern hemisphere ydb sites we also aggregate 14 c measurements associated with a known synchronous event the laacher see volcanic eruption we then use a monte carlo simulation to evaluate the magnitude of variability expected in a 14 c dataset associated with a synchronous event the simulation accounts for measurement error calibration uncertainty old wood effects and laboratory measurement biases the laacher see 14 c dataset is consistent with expectations of synchroneity generated by the simulation however the ydb 14 c dataset is inconsistent with the simulated expectations for synchroneity these results suggest that a central requirement of the younger dryas impact hypothesis synchronous global deposition of a ydb layer is extremely unlikely calling into question the younger dryas impact hypothesis more generally

meteoric smoke fallout over the holocene epoch revealed by iridium and platinum in greenland ice an iridium anomaly at the cretaceous tertiary boundary layer has been attributed to an extraterrestrial body that struck the earth some 65 million years ago1 it has been suggested that during this event the carrier of iridium was probably a micrometre sized silicate enclosed aggregate2 or the nanophase material of the vaporized impactor3 but the fate of platinum group elements such as iridium that regularly enter the atmosphere via ablating meteoroids remains largely unknown here we report a record of iridium and platinum fluxes on a climatic cycle timescale back to 128 000 years ago from a greenland ice core4 we find that unexpectedly constant fallout of extraterrestrial matter to greenland occurred during the holocene whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust laden atmosphere of the last glacial age we suggest that nanometre sized meteoric smoke particles5 6 formed from the recondensation of ablated meteoroids in the atmosphere at altitudes 70 kilometres are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps this implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the holocene

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

nanodiamond rich layer across three continents consistent with major cosmic impact at 12 800 cal bp abstracta major cosmic impact event has been proposed at the onset of the younger dryas yd cooling episode at 12 800 150 years before present forming the yd boundary ydb layer distributed over 50 million km2 on four continents in 24 dated stratigraphic sections in 10 countries of the northern hemisphere the ydb layer contains a clearly defined abundance peak in nanodiamonds nds a major cosmic impact proxy observed nd polytypes include cubic diamonds lonsdaleite like crystals and diamond like carbon nanoparticles called n diamond and i carbon the nd abundances in bulk ydb sediments ranged up to 500 ppb mean 200 ppb and that in carbon spherules up to 3700 ppb mean 750 ppb 138 of 205 sediment samples 67 contained no detectable nds isotopic evidence indicates that ydb nds were produced from terrestrial carbon as with other impact diamonds and were not derived from the impactor itself the ydb layer is also marked by abundance peaks in other impact related proxies including

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

synchronous records of pco2 and δ14c suggest rapid ocean derived pco2 fluctuations at the onset of younger dryas just before the onset of the younger dryas yd cold event several stomatal proxy based pco2 records have shown a sharp increase in atmospheric co2 concentration pco2 of between ca 50 and 100 ppm followed by a rapid decrease of similar or even larger magnitude here we compare one of these records a high resolution pco2 record from southern sweden with the intcal13 record of radiocarbon d 14 c the two records show broadly synchronous fluctuations at the yd onset specifically the intcal13 record documents decreasing d 14 c just before the yd onset when pco2 peaks consistent with a source of old co2 from the deep ocean we propose that this fluctuation occurred due to a major ocean flushing event the cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor we confirm that the earth system can produce such large multidecadal timescale fluctuations in pco2 through simulating an artificial ocean flushing event with the genie earth system model we suggest that sharp transitions of pco2 may have remained undetected so far in ice cores due to inter firn gas exchange and time averaging the stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change

large pt anomaly in the greenland ice core points to a cataclysm at the onset of younger dryas one explanation of the abrupt cooling episode known as the younger dryas yd is a cosmic impact or airburst at the yd boundary ydb that triggered cooling and resulted in other calamities including the disappearance of the clovis culture and the extinction of many large mammal species we tested the ydb impact hypothesis by analyzing ice samples from the greenland ice sheet project 2 gisp2 ice core across the bolling allerod yd boundary for major and trace elements we found a large pt anomaly at the ydb not accompanied by a prominent ir anomaly with the pt ir ratios at the pt peak exceeding those in known terrestrial and extraterrestrial materials whereas the highly fractionated pt ir ratio rules out mantle or chondritic sources of the pt anomaly it does not allow positive identification of the source circumstantial evidence such as very high superchondritic pt al ratios associated with the pt anomaly and its timing different from other major events recorded on the gisp2 ice core such as well understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction hints for an extraterrestrial source of pt such a source could have been a highly differentiated object like an ir poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the ydb impact hypothesis

timing and structure of the younger dryas event and its underlying climate dynamics the younger dryas yd arguably the most widely studied millennial scale extreme climate event was characterized by diverse hydroclimate shifts globally and severe cooling at high northern latitudes that abruptly punctuated the warming trend from the last glacial to the present interglacial to date a precise understanding of its trigger propagation and termination remains elusive here we present speleothem oxygen isotope data that in concert with other proxy records allow us to quantify the timing of the yd onset and termination at an unprecedented subcentennial temporal precision across the north atlantic asian monsoon westerlies and south american monsoon regions our analysis suggests that the onsets of yd in the north atlantic 12 870 30 b p and the asian monsoon westerlies region are essentially synchronous within a few decades and lead the onset in antarctica implying a north to south climate signal propagation via both atmospheric decadal time scale and oceanic centennial time scale processes similar to the dansgaard oeschger events during the last glacial period in contrast the yd termination may have started first in antarctica at 11 900 b p or perhaps even earlier in the western tropical pacific followed by the north atlantic between 11 700 40 and 11 610 40 b p these observations suggest that the initial yd termination might have originated in the southern hemisphere and or the tropical pacific indicating a southern hemisphere tropics to north atlantic asian monsoon westerlies directionality of climatic recovery

the greenland ice sheet project 2 depth age scale methods and results the greenland ice sheet project 2 gisp2 depth age scale is presented based on a multiparameter continuous count approach to a depth of 2800 m using a systematic combination of parameters that have never been used to this extent before the ice at 2800 m is dated at 110 000 years b p with an estimated error ranging from 1 to 10 in the top 2500 m of the core and averaging 20 between 2500 and 2800 m parameters used to date the core include visual stratigraphy oxygen isotopic ratios of the ice electrical conductivity measurements laser light scattering from dust volcanic signals and major ion chemistry gisp2 ages for major climatic events agree with independent ages based on varve chronologies calibrated radiocarbon dates and other techniques within the combined uncertainties good agreement also is obtained with greenland ice core project ice core dates and with the specmap marine timescale after correlation through the 18 o of o 2 although the core is deformed below 2800 m and the continuity of the record is unclear we attempted to date this section of the core on the basis of the laser light scattering of dust in the ice

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

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

comprehensive analysis of nanodiamond evidence relating to the younger dryas impact hypothesis during the end of the last glacial period in the northern hemisphere near 12 9k cal a bp deglacial warming of the bolling allerod interstadial ceased abruptly and the climate returned to glacial conditions for an interval of about 1300 years known as the younger dryas stadial the younger dryas impact hypothesis proposes that the onset of the younger dryas climate reversal pleistocene megafaunal extinctions and disappearance of the clovis paleoindian lithic technology were coeval and caused by continent wide catastrophic effects of impact bolide events in north america while there are no known impact structures dated to the younger dryas onset physical evidence of the impact bolide events is argued to be present in sediments spanning several continents at stratigraphic levels inferred to date to the bolling allerod younger dryas boundary ydb reports of nanometer to submicron sized diamonds in ydb sediments in particular the rare 2h hexagonal polytype of diamond lonsdaleite have been presented as strong evidence for shock processing of crustal materials we review the available data on diamonds in sediments and provide new data we find no evidence for lonsdaleite in ydb sediments and find no evidence of a spike in nanodiamond concentration at the ydb layer to support the impact hypothesis

study of schumann resonances based on magnetotelluric records from the western mediterranean and antarctica 1 eight independent magnetotelluric mt campaigns carried out in the western mediterranean area and antarctica have been analyzed with the aim of extracting information from schumann resonances sr the advantages and drawbacks of mt data with respect to single purpose fixed stations for sr research are evaluated two different methods of signal processing have been employed spectral analysis through the fast fourier transform fft and rescaled range analysis r s calculating the hurst exponents the first permitted a study of the source contributions the effects caused by local changes in the observation area interseasonal behavior and the relations between electric and magnetic horizontal fields the average central frequencies obtained for the first three resonances are 7 8 14 and 20 5 hz respectively but there are fluctuations in them these variations seem to respond to the characteristics of the principal active storms that generate the resonances these frequency shifts are stronger for the second and third resonances owing to the broadband registering of mt the second method of signal processing could be applied to the low noise signals from antarctica with high resolution revealing the persistent nature of sr a numerical simulation indicates a way for inferring lightning rates from r s analysis

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

a multi proxy study of changing environmental conditions in a younger dryas sequence in southwestern manitoba canada and evidence for an extraterrestrial event multi proxy analyses of a sequence spanning the younger dryas yd in the glacial lake hind basin of manitoba provides insight into regional paleohydrology and paleovegetation of meltwater rivers and lakes spanning 4000 yr the sequence is controlled by 25 new accelerator mass spectrometry ages this lake dammed by the laurentide ice sheet overflowed into lake agassiz the pre yd interval records rapid sedimentation from meltwaters that headed in proglacial lakes in the canadian prairies that are known to have been catastrophically released when ice or sediment barriers were breached pollen in this phase is dominated by pre quaternary forms eroded from paleocene bedrock at the onset of the yd at 12 8 cal ka the sudden appearance of concentrations of nanodiamonds high temperature magnetic spherules platinum and iridium provide evidence of an extraterrestrial et event that others have identified at more than 40 sites in north america major changes in oceans and climate and the catastrophic outflow of nearby lake agassiz at the onset of the yd may be related lower water levels and a reduction of souris river inflow to lake hind followed which are reflected by more clayey and organic rich sediments and a decrease in pre quaternary palynomorphs this may have resulted from the deepening of river valleys caused by the release of meltwater triggered by the et event wetlands then began to develop leading to peat deposition from 12 3 to 11 cal ka this was followed by a fluvial episode depositing sand and then by increased holocene aridity that resulted in accumulation of a thick sequence of dune sands a dry woodland environment with a mix of conifers especially picea and larix and deciduous trees especially populus and quercus covered the uplands from 13 to 10 cal ka

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

a climatic control on the accretion of meteoric and super chondritic iridium platinum to the antarctic ice cap meteoric smoke particles msps form through the vaporization of meteoroids and the subsequent re condensation of metallic species in the mesosphere recently iridium and platinum enrichments have been identified in greenland ice layers and attributed to the fallout of msps supplying polar latitudes with cosmic matter during the holocene however the msp fallout to antarctica during the earth s climatic history remains essentially unknown we have determined iridium and platinum in deep antarctic ice from dome c and vostok dated back to 240 kyrs bp we find high super chondritic fluxes during warm periods and low meteoric accretion during glacial times a pattern that is opposite to any known climatic variation in dust fallout to polar regions the proposed explanation of this accretion regime is a weaker polar vortex during warm periods allowing peripheral air masses enriched in volcanic iridium and platinum to penetrate inland to antarctica the msp signal emerges only during cold phases and is four times lower than in the greenland ice cap where more snow accumulates this suggests that wet deposition is an important route of cosmic material to the earth s surface

changes in atmospheric circulation and ocean ice cover over the north atlantic during the last 41 000 years high resolution continuous multivariate chemical records from a central greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41 000 years these chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to greenland defined here as the polar circulation index pci and in the extent of ocean ice cover massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in pci during stadials without discharge events ocean ice cover appears to reach some common maximum level the massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

extraterrestrial accretion from the gisp2 ice core the rate of extraterrestrial accretion for particles in the size range 0 45 m to 20 m was determined from dust concentrates extracted from greenland ice sheet project 2 gisp2 ice core samples using instrumental neutron activation analysis inaa we determined the iridium ir content of the dust following a core specific correction for terrestrial ir and assuming a chondritic ir abundance of 500 ppb we measure an average accretion rate for 0 45 m to 20 m particles over the entire earth of 0 22 0 11 109 g yr kton yr for 317 years of ice through the interval 6 to 20 ka this is consistent with the interplanetary dust accretion rate of 0 17 0 08 x 109 g yr that we derive from published 3he data for the gisp2 core accounting for particles that are larger and smaller than those detected by or experiment our best estimate of the total accretion rate including particle sizes up to about 4 cm in diameter is 2 5 109 g yr the uncertainty in this estimate is dominated by statistical fluctuations in the number of particles expected to end up in the ice core and not by measurement error based on monte carlo simulations we estimate the upper limit for total extraterrestrial accretion to earth of 6 25 109 g yr 95 confidence level this accretion rate is consistent with some estimates from micrometeorite concentrations in polar ice estimates from ground based radar studies and with accretion estimates of 3he bearing interplanetary dust particles assuming that 3he is correlated with particle surface area it is however lower than estimates based on platinum group element studies of marine sediments the conflict may indicate systematic errors with either the marine or the non marine samples departures from the assumed particle spectrum of grun and coauthors or time variable accretion rates with the early holocene period being characterized by low levels of accretion

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

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