flood volcanism in the northern high latitudes of mercury revealed by messenger messenger observations from mercury orbit reveal that a large contiguous expanse of smooth plains covers much of mercury s high northern latitudes and occupies more than 6 of the planet s surface area these plains are smooth embay other landforms are distinct in color show several flow features and partially or completely bury impact craters the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement these characteristics as well as associated features interpreted to have formed by thermal erosion indicate emplacement in a flood basalt style consistent with x ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites the plains formed after the caloris impact basin confirming that volcanism was a globally extensive process in mercury s post heavy bombardment era

clustered impacts experiments and implications impact by clusters of projectiles rather than a single projectile can result from several processes atmospheric breakup tidal breakup and ejecta from a large primary impact experiments have been performed in order to establish the characteristics of such events over a wide range of impact velocities 15 m s to 6 km s at very low impact velocities 15 200 m s clustered impacts were produced by launching a grouped projectiles of aluminum shot steel shot iron filings and sand at moderate to high velocities 0 8 6 km s pyrex spheres were shattered above the target during passage through aluminum foil or paper thereby forming a well defined cluster of fragments of overall radius rc the ratio of the overall radius rc of the cluster to the radius rs of a solid impactor of the same mass provides a measure of the cluster dispersion sand and compacted pumice targets were used in order to compare qualitatively the additional effect of slight differences in target strength the experiments reveal marked contrasts between impacts by clusters of projectiles and impacts by a single solid body tight clusters rc rs 20 displace a factor of 10 less mass this reduction in cratering efficiency is largely expressed as a shallow crater with an aspect ratio diameter depth as large as 30 for open clusters the size and velocity of the cluster as well as the density and strength contrast between the impactor and target can dramatically affect crater morphology open clusters impacting compacted pumice produce a flat hummocky floor with an incipient multiring pattern whereas a tight cluster impacting the same target produces a central floor mound oblique impacts by symmetrical clusters form a characteristic array of v shaped ridges pointing uprange the apex angle of the ridges depends on the cluster dispersion and impact angle inventories of postimpact projectile material reveal that the projectile is largely retained on the surface and spread downrange from the impact direction the amount of projectile material retained on the surfaces increases with decreasing impact angle from the horizontal and with increasing strength of the target relative to the projectile clustered impact craters and lunar secondary craters bear striking similarities over a broad range of morphologic features on this basis and on the basis of reasonable models of ejecta curtain structure we suggest that these experiments provide new clues for understanding ejecta emplacement around large lunar impact craters when the ejecta curtain around large impacts is viewed as a thick wall of debris and clustered impactors are viewed as a unit section of such a curtain then experimental results indicate that the continuous ejecta facies even for lunar craters larger than 100 km could contain as much as 90 primary material such a conclusion contrasts with many existing models that derive mixing ratios implicitly based on single noninteracting impact events beyond the continuous ejecta facies impacting clusters of ejecta provide a physical basis for understanding the wide variety of secondary morphologies and the large range in spectral signatures of primary material in crater rays

planetary surface dating from crater size frequency distribution measurements partial resurfacing events and statistical age uncertainty abstract we describe the procedure to fit a cumulative production function polynomial to a partial crater size frequency distribution the technique is of particular use in deriving ages for surfaces which have undergone partial resurfacing events namely erosional or depositional events which have affected a limited diameter range of the crater population we demonstrate its use in obtaining times for both the surface formation and the resurfacing event we give a practical outline of the method for making age measurements from crater counts and how to identify resurfacing effects in the results we discuss the conversion of production function polynomials between common presentations and the statistical uncertainty of the determined ages with respect to the non linear chronology function and a minor refinement of data binning

secondary craters of tycho size frequency distributions and estimated fragment size velocity relationships 1 we examined the size frequency distribution and the spatial distribution of secondary craters around the lunar crater tycho secondary crater diameters were found to range from 0 55 to 4 0 km and their distance from tycho to range from 130 to 370 km the diameter and ejection velocity of the secondary forming fragments were also estimated from the crater size and the distance from the primary using the scaling relationships for cratering the power law index of the size frequency distribution of the tycho secondary craters ranges from 3 3 to 4 0 the same as the steep slope of the typical size frequency distribution for lunar craters of less than 4 km diameter the decay index of secondary crater densities with distance from the primary craters is not constant and is small at large distances less than 2 5 for r 2 5r the total number of secondaries forming ejecta fragments is more than twice larger than for other lunar primaries previously studied these results imply that there are many small secondary craters in fields far from the primary crater a dominant contribution of secondaries to the size frequency distribution of all lunar craters is thus strongly suggested the index range of 2 6 to 3 0 for estimated fragment size distributions is not only close to other natural and experimental craters but also to those of blocks on asteroids the relative fragment size distributions are independent of the ejection velocity this result is the first observational confirmation of the theoretical assumption and agrees with a recent result from laboratory experiments

the importance of secondary cratering to age constraints on planetary surfaces small craters less than one kilometer diameter can be primary craters produced by impact of interplanetary debris or they can be secondary craters produced by fallback of high velocity ejecta blocks from much larger but infrequent primary impacts the prevalent assumption over recent decades has been that primaries are most abundant so most small craters are independent random events and can be used for dating however recent results from europa and mars support the early theory that distant secondaries globally dominate the number of small lunar craters this would invalidate part of production functions that have been widely used for age dating crater excavation results in higher mean ejection velocities for smaller fragments resulting in a steeper size frequency distribution for secondary craters than is produced by the same size frequency distribution of interplanetary debris this review also discusses how small craters can sometimes be used to derive meaningful age constraints

devensian lateglacial environmental changes in britain a multi proxy environmental record from llanilid south wales uk abstract a multi proxy environmental record for the devensian weichselian lateglacial and early holocene periods which includes pollen plant macrofossil coleopteran geochemical and stable isotope data is described from a site at llanilid south wales uk the geochronology of the sequence is derived from two age depth models which show a broad measure of agreement over much of the profile this data set provides the basis for a detailed reconstruction of changing environmental conditions in western britain during the transition from the last cold stage to the present holocene interglacial ca 15 000 10 000 cal yr bp two marked cooling episodes are recorded during the course of the lateglacial interstadial greenland interstadial 1 with mean july temperatures falling by more than 5 c from a maximum of around 20 c during the early interstadial and by a further 4 5 c around 13 100 cal yr bp the initial drop in temperature led to a reduction in juniperus scrub while the second and more abrupt temperature decline resulted in a significant contraction in areas of betula woodland a relatively slight and short lived warming during the later interstadial enabled tree birch to expand once again mean july temperatures of 10 11 c characterised the loch lomond younger dryas stadial greenland stadial 1 between ca 12 600 and 11 400 cal yr bp during which time a scrub tundra with betula salix and a range of open habitat taxa became established locally the onset of the holocene interglacial at ca 11 400 cal yr bp is marked by an abrupt temperature rise of the order of 9 c and by the rapid expansion of betula woodland the llanilid palaeoclimate record is similar to that from the gransmoor site in northeast england and also to the climatic sequence inferred from the grip ice core particularly during the later part of the lateglacial interstadial ca 14 000 12 600 cal yr bp during the loch lomond younger dryas stadial and in the early holocene the llanilid chronology is less secure during the early part of the lateglacial interstadial pre 14 000 cal yr bp but it is possible that warming may have begun earlier in this part of western britain than in greenland

bayesian analysis of radiocarbon chronologies examples from the european late glacial although there are many late glacial ca 15 000 11 000 cal yr bp proxy climate records from northwest europe some analysed at a very high temporal resolution decadal to century scale attempts to establish time stratigraphical correlations between sequences are constrained by problems of radiocarbon dating in an attempt to overcome some of these difficulties we have used a bayesian approach to the analysis of radiocarbon chronologies for two late glacial sites in the british isles and one in the adriatic sea the palaeoclimatic records from the three sites were then compared with that from the grip greenland ice core although there are some apparent differences in the timing of climatic events during the early part of the late glacial pre 14 000 cal yr bp the results suggest that regional climatic changes appear to have been broadly comparable between greenland the british isles and the adriatic during the major part of the late glacial i e between 14 000 and 11 000 cal yr bp the advantage of using the bayesian approach is that it provides a means of testing the reliability of late glacial radiocarbon chronologies that is independent of regional chronostratigraphical climatostratigraphical frameworks it also uses the full radiocarbon inventory available for each sequence and makes explicit any data selection applied potentially therefore it offers a more objective basis for comparing regional radiocarbon chronologies than the conventional approaches that have been used hitherto copyright 2004 john wiley sons ltd

theoretical analysis of secondary cratering on mars and an image based study on the cerberus plains the issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges the approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on mars and implies the need of detailed studies of the size frequency distribution for remote secondary craters to date poorly known a key case the martian crater zunil and its secondary crater field illustrate that reasonable predictions for the secondary crater size frequency distribution at small 100 m crater diameters affected the standard model crater retention age for the cerberus plains less than the statistical uncertainty these observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range

improved age modelling approaches as exemplified by the revised chronology for the central european varved lake soppensee abstract one of the key factors in understanding past climate change is the development of robust age models that have sufficient chronological precision for comparing different palaeoclimate archives while retaining accuracy recent developments in bayesian age modelling are applied here to the swiss varve lake soppensee we develop revised age models for this sequence using the available stratigraphic information to constrain the calibrated radiocarbon ages we begin by using stratigraphical order as the only constraint and then sequentially increase the information incorporated into the model using relative varve age sample depth and varying depositional models within this bayesian framework we develop internally robust models that significantly improve the dating precision and are applicable to both varved and non varved sequences we then compare the model output to wider chronological information such as the age of the laacher see tephra in order to test the model accuracy the results of this exercise suggest that these methods can be used to build very reliable improved age models in a variety of records

small craters population as a useful geological investigative tool apollo 17 region as a case study abstract small craters below a few decametres in diameter are ubiquitous on the lunar surface and numbered in billions thanks to the expanding availability of global high resolution imagery their size frequency distribution has increasingly been employed as an investigative tool to derive relative or even absolute age estimations of resurfacing events within small regions of geological interest based on a combined survey of tens of thousands of craters this work investigates the 7 50 meter crater distributions across a wide range of lunar terrains centred around the apollo 17 exploration region data analysis using a wide array of approaches reveals distribution patterns consistent with a complex geochronological scenario confirming crater tycho as the source of the main secondary surge which was followed by seismic led modification of the small crater population within the taurus littrow valley and a possible more recent arrival of small secondaries from the fabbroni impact some 70 km to the south this study also confirms that applying standard absolute model ages fits to small craters size frequency distributions to estimate resurfacing ages will only work for surfaces no older than a few tens of ma using an application of monte carlo simulations of crater degradation based on diffusion equations we matched the cumulative sfd of each area to derive size dependent diffusivity rates the results produce equilibrium age estimates that vary among the proposed geological units and crater sizes between 10 and 100 m in diameter this work concludes that the distribution of small craters does not appear to be stochastic in terms of relative size representation and their use for mapping surface variations in terms of physical properties represents a new tool in the remote sensing investigation of planetary surfaces

lunar orientale impact basin secondary craters spatial distribution size frequency distribution and estimation of fragment size secondary impact craters features created by projectiles ejected from a primary impact contain important information about the primary cratering event and the nature and distribution of its ejecta the orientale impact basin d similar to 930km is the youngest and the least degraded large impact basin on the moon and has the most recognizable secondary impact craters we identified and mapped 2 728 secondary craters in the investigated area of similar to 1 66x10 7 km 2 covering an area from the rim of orientale to six radii secondary crater diameters range from similar to 2 to 27km and the median diameter decreases as distance increases secondary craters are concentrated predominantly in the northwest and southwest the ejecta deposit pattern inferred from secondary crater distribution suggests that the orientale basin was formed by an oblique impact in which the downrange direction was 240 degrees 265 degrees in azimuth and the incidence angle was steeper than 20 degrees the cumulative size frequency distribution of mapped secondary craters steepens as diameter increases and is very well approximated with a weibull distribution with an exponent 1 32 a widely used crater scaling relationship predicts that the fragments that produced the secondary craters were predominantly in similar to 0 5 2 km diameter range over the investigated area the diameter of the largest fragment however decreases with increasing distance from orientale on the basis of the diameter of the largest secondary crater of orientale and other craters and basins the largest secondary crater of the south pole aitken basin is estimated to be similar to 40km in diameter we explore the implications of these findings for the evolution of the megaregolith and future sample return missions

investigation of crater saturation using spatial statistics abstract conventional techniques for investigation of crater populations make use of size frequency distributions but typically disregard the statistical properties of the spatial distribution of craters cratering of a surface is generally a spatially random process so the distributions of craters on lightly cratered surfaces can be expected to be random as crater obliteration becomes important however the situation changes every random distribution has regions of sparseness and regions of clustering craters that form in areas of sparseness obliterate few older craters so the sparseness is filled in craters that form in areas of clustering obliterate more older craters relatively reducing the clustering as sparseness is filled in and clustering is reduced the spatial distribution should progress from one that is random to one that is more uniform than random we show that the spatial distribution of craters on a lightly cratered lunar maria surface is consistent with randomness but that the distributions on heavily cratered portions of rhea and callisto are markedly more uniform than random we develop a simple model of crater formation and obliteration and use it to investigate the statistical behavior of crater spatial distributions as crater saturation is approached we find that the use of spatial distributions to investigate saturation is more robust to model uncertainties than is the use of crater density alone using a simple statistical parameter the z statistic we find that 25 of the craters formed to date on our study areas on rhea and callisto have been obliterated by subsequent cratering adjusting the z statistic for crater floor area effects we find evidence that crater saturation has been reached on rhea

mercury cratering record viewed from messenger s first flyby morphologies and size frequency distributions of impact craters on mercury imaged during messenger s first flyby elucidate the planet s geological history plains interior to the caloris basin displaying color and albedo contrasts have comparable crater densities and therefore similar ages smooth plains exterior to caloris exhibit a crater density 40 less than on interior plains and are thus volcanic and not caloris impact ejecta the size distribution of smooth plains craters matches that of lunar craters postdating the late heavy bombardment implying that the plains formed no earlier than 3 8 billion years ago ga at diameters less than or equal to 8 to 10 kilometers secondary impact craters on mercury are more abundant than primaries this transition diameter is much larger than that on the moon or mars a low density of craters on the peak ring basin raditladi implies that it may be younger than 1 ga

ams radiocarbon dating and varve chronology of lake soppensee 6000 to 12000 14c years bp for the extension of the radiocarbon calibration curve beyond 10000 14c y bp laminated sediment from lake soppensee central switzerland was dated the radiocarbon time scale was obtained using accelerator mass spectrometry ams dating of terrestrial macrofossils selected from the soppensee sediment because of an unlaminated sediment section during the younger dryas 10000 11000 14c y bp the absolute time scale based on counting annual layers varves had to be corrected for missing varves the soppensee radiocarbon verve chronology covers the time period from 6000 to 12000 14c y bp on the radiocarbon time scale and 7000 to 13000 calendar y bp on the absolute time scale the good agreement with the tree ring curve in the interval from 7000 to 11450 cal y bp cal y indicates calendar year proves the annual character of the laminations the ash layer of the vasset killian tephra massif central france is dated at 8230 140 14c y bp and 9407 44 cal y bp the boundaries of the younger dryas biozone are placed at 10986 69 cal y bp younger dryas preboreal and 1212 86 cal y bp allerod younger dryas on the absolute time scale the absolute age of the laacher see tephra layer dated with the radiocarbon method at 10 800 to 11200 14c y bp is estimated at 12350 135 cal y bp the oldest radiocarbon age of 14190 120 14c y bp was obtained on macrofossils of pioneer vegetation which were found in the lowermost part of the sediment profile for the late glacial the offset between the radiocarbon 10000 12000 14c y bp and the absolute time scale 11400 13000 cal y bp in the soppensee chronology is not greater than 1000 years which differs from the trend of the u th radiocarbon curve derived from corals

lake sediments record cycles of sediment flux driven by large earthquakes on the alpine fault new zealand large earthquakes in mountain regions commonly trigger extensive landsliding and are important drivers of erosion but the contribution of this landsliding to long term erosion rates and seismic hazard remains poorly understood here we show that lake sediments record postseismic landscape response as a sequence of turbidites that can be used to quantify erosion related to large moment magnitude m w 7 6 earthquakes on the alpine fault new zealand alpine fault earthquakes caused a threefold increase in sediment flux over the 50 yr duration of each postseismic landscape response this represents considerable delayed hazard following earthquake induced strong ground motion earthquakes were responsible for 27 of the sediment flux from the lake catchment over the past 1100 yr leading us to conclude that alpine fault earthquakes are one of the most important drivers of erosion in the range front of the southern alps

effect of topography degradation on crater size frequency distributions implications for populations of small craters and age dating whether or not background secondary craters dominate populations of small impact craters on terrestrial bodies is a half century controversy it has been suggested that small craters on some planetary bodies are dominated by background secondary craters based partly on the steepened slope of crater size frequency distribution csfd towards small diameters such as the less than 1 km diameter crater population on the lunar mare here we show that topography degradation enlarges craters and increases csfd slopes with time when topography degradation is taken into account for various aged crater populations the observed steep csfd at small diameters is uniformly consistent with an originally shallower csfd whose slope is undifferentiated from the csfd slope estimated from near earth objects and terrestrial bolides the results show that the effect of topography degradation on csfd is important in dating planetary surfaces and the steepening of csfd slopes is not necessarily caused by secondary cratering but rather a natural consequence of topography degradation

proglacial lake sediment records reveal holocene climate changes in the venezuelan andes lake sediment records from the cordillera de merida in the northern venezuelan andes document the history of local glacial variability and climate changes during the holocene w12 ka to the present the valleys that contain these lakes have similar bedrock compositions and hypsometries but have different headwall elevations and aspects which makes them useful for investigating the magnitude of past glaciations there was widespread glacial retreat in the venezuelan andes during the early holocene after which most watersheds remained ice free and thus far only valleys with headwalls higher than w4400 m asl contain evidence of glaciation during the last w10 ka there was a pronounced shift in sediment composition for the montos headwall w4750 m asl and los anteojos headwall w4400 m asl records during the middle holocene from w8 0 to 7 7 ka when conditions appear to have become ice free and drier there is tentative evidence that the glacier in the mucubaji valley headwall w4609 m asl advanced from w8 1 to 6 6 ka and then retreated during the latter stages of the middle holocene clastic sediment accumulation in other nearby lake basins was either low or decreased throughout most of the middle holocene as watersheds stabilized under warmer and or drier conditions in the montos record there was another major shift in sediment composition that occurred from w6 5 to 5 7 ka similar to other regional records that suggest conditions were drier during this period overall the late holocene was a period of warmer and wetter conditions with ice extent at a minimum in the northern tropical andes there were also punctuated decadal to multi centennial periods of higher clastic sediment accumulation during the last w4 ka likely in response to periods of cooling and or local precipitation changes in watersheds with headwalls above 4600 m asl there is evidence of glacial advances during the little ice age w0 6e0 1 ka the pattern of glacial variability is generally similar in both the northern and southern tropics during the little ice age suggesting that ice margins in both regions were responding to colder and wetter conditions during the latest holocene the observed pattern of holocene climate variability in the venezuelan andes cannot be explained by insolation forcing alone and tropical ocean influences were likely associated with the observed glacial and lake level changes

martian cratering 9 toward resolution of the controversy about small craters abstract the malin et al malin m c edgett k posiolova l mccolley s noe dobrea e 2006 science 314 1573 1577 report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of hartmann hartmann w k 2005 icarus 174 294 320 and appears to contradict models that say our curves grossly overpredict primary production nonetheless such models may still predict correct order of magnitude secondary production the new observations if correct require geologically recent martian surface modification in some areas on timescales comparable to the cycles of obliquity changes suggesting episodic martian climatic environmental change driven by the obliquity cycles

high resolution stratigraphy and zircon u pb geochronology of the middle triassic buchenstein formation dolomites northern italy precession forcing of hemipelagic carbonate sedimentation and calibration of the anisian ladinian boundary interval orbitally forced cyclic variations in sedimentary sequences provide evidence for short term fluctuations of earth climate and a tool for high resolution timescale calibration we here present stratigraphic and geochronological evidence for precession forcing in middle triassic hemipelagic limestones of the buchenstein formation dolomites northern italy high resolution stratigraphy of several correlative sections of the buchenstein formation documents a coherent cycle pattern isotope dilution thermal ionization mass spectrometry zircon u pb geochronology of tuffs bracketing the cyclic interval reveals an average cycle duration of 18 5 2 1 kyr consistent with a shorter climatic precession cycle in the middle triassic compared with today this suggests a predominantly precession controlled climate in this low latitude setting of the western tethys and allows high precision calibration of the anisian ladinian boundary interval from integrating cyclostratigraphic and u pb geochronological constraints our best estimate for the age of the anisian ladinian boundary is 241 464 0 064 0 097 0 28 ma we also provide precise estimates for lithostratigraphic boundaries biostratigraphic markers and magnetic reversals within the boundary interval stratigraphic intervals with elevated sedimentation rate record a sub milankovitch signal that may be equivalent to patterns in adjacent carbonate platforms such as the latemar platform the origin of this sub milankovitch signal remains unknown but highlights the potential to investigate shorter term climatic variations in mesozoic sedimentary sequences supplementary material isotope dilution thermal ionization mass spectrometry u pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables as well as the results of the bayesian age modelling are available at https doi org 10 6084 m9 figshare c 3861817

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

extent age and resurfacing history of the northern smooth plains on mercury from messenger observations abstract messenger orbital images show that the north polar region of mercury contains smooth plains that occupy 7 of the planetary surface area within the northern smooth plains nsp we identify two crater populations those superposed on the nsp post plains and those partially or entirely embayed buried the existence of the second of these populations is clear evidence for volcanic resurfacing the post plains crater population reveals that the nsp do not exhibit statistically distinguishable subunits on the basis of crater size frequency distributions nor do measures of the areal density of impact craters reveal volcanically resurfaced regions within the nsp these results suggest that the most recent outpouring of volcanic material resurfaced the majority of the region and that this volcanic flooding emplaced the nsp over a relatively short interval of geologic time perhaps 100 my or less stratigraphic embayment relationships within the buried crater population including partial crater flooding and the presence of smaller embayed craters within the filled interiors of larger craters and basins indicate that a minimum of two episodes of volcanic resurfacing occurred from the inferred rim heights of embayed craters we estimate the nsp to be regionally 0 7 1 8 km thick with a minimum volume of volcanic material of 4 10 6 to 10 7 km 3 because of the uncertainty in the impact flux at mercury the absolute model age of the post plains volcanism could be either 3 7 or 2 5 ga depending on the chronology applied

ballistic sedimentation of impact crater ejecta implications for the provenance of lunar samples and the resurfacing effect of ejecta on the lunar surface

size frequency distribution of crater populations in equilibrium on the moon overprinting of craters by subsequent impacts and topographic degradation complicates crater statistics especially for old surfaces and small diameter crater populations a crater population is regarded as in equilibrium at a particular crater size when smaller craters are being produced at the same rate at which they are being destroyed evaluating the equilibrium state of crater populations is challenging and empirical equilibrium densities are frequently inferred by performing careful crater counts and cross comparisons on several lunar surfaces we study the size frequency distributions sfd for the crater populations which have portions in equilibrium the results are one of the few observational constraints on the sfd of crater populations in equilibrium showing that referring to empirical equilibrium densities is not safe for evaluating the equilibrium states of crater populations equilibrium densities are not positively correlated with the ages of crater populations and some populations in equilibrium have crater densities less than those previously believed to represent equilibrium conditions besides the sfd of the production population different crater removal rates at different diameters also affect the sfd of crater populations in equilibrium the equilibrium onset diameter deq of a crater population can be translated to model ages because older populations have larger deq and those for same aged surfaces are comparable we show that the crater populations studied here are in equilibrium at much smaller diameters than those predicted for same aged surfaces by crater degradation models thus indicating lower crater degradation rates on the moon and or younger ages of the counting areas

pleistocene to holocene transition in the central basin of the gulf of thailand based on geoacoustic survey and radiocarbon ages abstract record of the last glacial maximum and postglacial sea level rise in the central gulf of thailand some 140 km offshore of the malay peninsula was studied along a nw striking section water depth 55 82 m below the present msl by means of geoacoustic surveys shallow gravity coring and radiocarbon dating the record comprises evidence of valley incision followed by estuarine to marine transgression which was interrupted by deltaic progradation the valley incision and delta formation are attributed to the kelantan river draining the malay peninsula the delta growth was halted at 10 4 cal kyr bp by transgressive ravinement followed by emplacement of a thin condensed mantle of modern marine muds such age is compatible with the fast but diachronous nature of this transgressive event that according to previous estimates began in the central sunda shelf at 11 11 5 cal kyr bp and reached the head of the thailand bay at 8 7 cal kyr bp the presence of deltaic deposits within postglacial transgressive systems tract is uncommon on extremely wide and accommodation dominated shelves such as the sunda shelf and is here interpreted to reflect a combination of decelerating sea level rise 13 10 cal kyr bp with increased precipitation and sediment yield coupled with the reactive nature of the kelantan river catchment

holocene dust deposition in the ili basin and its implications for climate variations in westerlies dominated central asia abstract understanding holocene paleoclimate in arid central asia aca is helpful for predicting future climate however holocene climate patterns and their forcing mechanisms across aca are vigorously debated in the context of global climate change here we present a holocene loess profile sgx from the ili basin in the westerlies dominated central asia based on high resolution medium grained 38 63 m quartz optically stimulated luminescence osl dating and grain size and color measurements we reconstruct the holocene paleoclimate osl geochronology indicate that this sgx loess profile covers the last 11 5 ka and grain size analyses reveal that the sediments are sandy loess and the environmentally sensitive fractions are

crater degradation on the lunar maria topographic diffusion and the rate of erosion on the moon landscape evolution on the moon is dominated by impact cratering in the post maria period in this study we mapped 800 m to 5 km diameter craters on 30 of the lunar maria and extracted their topographic profiles from digital terrain models created using the kaguya terrain camera we then characterized the degradation of these craters using a topographic diffusion model because craters have a well understood initial morphometry these data provide insight into erosion on the moon and the topographic diffusivity of the lunar surface as a function of time the average diffusivity we calculate over the past 3 ga is 5 5 m2 myr with this diffusivity after 3 ga a 1 km diameter crater is reduced to approximately 52 of its initial depth and a 300 m diameter crater is reduced to only 7 of its initial depth and craters smaller than 200 300 m are degraded beyond recognition our results also allow estimation of the age of individual craters on the basis of their degradation state provide a constraint on the age of mare units and enable modeling of how lunar terrain evolves as a function of its topography

the self secondary crater population of the hokusai crater on mercury whether or not self secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy this issue potentially affects the age determination technique using crater statistics here the self secondary crater population on the continuous ejecta deposits of the hokusai crater on mercury is unambiguously recognized superposition relationships show that this population was emplaced after both the ballistic sedimentation of excavation flows and the subsequent veneering of impact melt but it predated the settlement and solidification of melt pools on the crater floor fragments that formed self secondaries were launched via impact spallation with large angles complex craters on the moon mercury and mars probably all have formed self secondaries populations dating young craters using crater statistics on their continuous ejecta deposits can be misleading impact melt pools are less affected by self secondaries overprint by subsequent crater populations with time reduces the predominance of self secondaries

the number of impact craters on earth any room for further discoveries abstract only 128 impact craters exposed at earth s surface have been found so far while new craters are discovered occasionally taking into account the permanent consumption of craters by erosion we present the first estimate on the number of impact craters that should be present at earth s surface our study yields no evidence for any systematic incompleteness in the available inventory of the craters larger than about 6 km in diameter exposed at the surface in contrast more than 90 craters in the diameter range from 1 km to 6 km should still be waiting to be discovered and even more than 250 between 0 25 km and 1 km diameter the transition from a probably complete inventory above 6 km to a strongly incomplete record at smaller sizes may be related to the difference between simple and complex craters beyond these results on the terrestrial crater record our findings tentatively suggest that erosion rates on the 10 to 100 million year scale may be closer to present day erosion rates than previously assumed

kilometer scale topographic roughness of mercury correlation with geologic features and units we present maps of the topographic roughness of the northern circumpolar area of 30 mercury at kilometer scales the maps are derived from range profiles obtained by the 31 mercury laser altimeter mla instrument onboard the mercury surface space 32 environment geochemistry and ranging messenger mission as measures of 33 roughness we used the interquartile range of profile curvature at three baselines 0 7 km 34 2 8 km and 11 km the maps provide a synoptic overview of variations of typical 35 topographic textures they show a dichotomy between the smooth northern plains and 36 rougher more heavily cratered terrains analysis of the scale dependence of roughness 37 indicates that the regolith on mercury is thicker than on the moon by approximately a 38 factor of three roughness contrasts within northern volcanic plains of mercury indicate a 39 younger unit inside goethe basin and inside another unnamed stealth basin these new 40 data permit interplanetary comparisons of topographic roughness

effect of an impact generated gas cloud on the acceleration of solid ejecta a large amount of petrologic geochemical isotopic and rare gas evidence strongly indicates that the shergottite nakhlite and chassigny snc meteorites originated on a parent body large enough to have sustained recent geological activity probably mars the most straightforward interpretation of the cosmic ray exposure evidence requires that the protoshergottites were at least 5 and more likely 15 m in diameter this large size combined with the 5 km s escape velocity for mars presents a formidable dynamic problem this study investigates the possibility that the entrainment of large blocks of solid ejecta within the rapidly expanding vapor cloud produced by an impact provides sufficient additional acceleration to overcome this problem a large number of impact parameters such as impactor and target material impact velocity times of ejection and crater size were varied to determine their relative effects ejection time was found to be the single most important variable rocks entrained at very early times are invariably crushed by high dynamic pressures and rocks ejected at late times after the gas density has become very low are little affected by the gas only rocks ejected within a fairly narrow window of intermediate times have their velocities significantly changed rocks initially ejected at very high velocities such as those predicted by the melosh spallation model are strongly decelerated by the gas rocks injected into the gas stream with velocities much different from the average gas velocity at times early enough that the gas density is still significant are often subjected to dynamic pressures great enough to crush them thus the net effect of a large rapidly expanding impact generated gas cloud on solid ejecta is to homogenize the velocity distribution and to reduce the average size for 30 km diameter craters the only rocks with final velocities greater than 5 km s were those whose initial velocities were greater than or approximately equal to 5 km s since the average final velocity increases with crater size it may be necessary to invoke a crater much larger than 30 km to explain the martian origin of snc meteorites if the proto sncs had diameters of greater than 1 m on the other hand if the proto sncs were only a few tens of centimeters or less gas acceleration of ejecta from a 30 km diameter crater is consistent with the martian origin of these unusual meteorites

distal ejecta from lunar impacts extensive regions of rocky deposits abstract lunar reconnaissance orbiter lro diviner radiometer mini rf and lro camera data were used to identify and characterize rocky lunar deposits that appear well separated from any potential source crater two regions are described 1 a 18 000 km 2 area with elevated rock abundance and extensive melt ponds and veneers near the antipode of tycho crater 167 5 e 42 5 n this region has been identified previously using radar and imaging data 2 a much larger and more diffuse region covering 730 000 km 2 centered near 310 e 35 s containing elevated rock abundance and numerous granular flow deposits on crater walls the rock distributions in both regions favor certain slope azimuths over others indicating a directional component to the formation of these deposits the spatial distribution of rocks is consistent with the arrival of ejecta from the west and northwest at low angles 10 30 above the horizon in both regions the derived age and slope orientations of the deposits indicate that the deposits likely originated as ejecta from the tycho impact event despite their similar origin the deposits in the two regions show significant differences in the datasets the tycho crater antipode deposit covers a smaller area but the deposits are pervasive and appear to be dominated by impact melts by contrast the nearside deposits cover a much larger area and numerous granular flows were triggered however the features in this region are less prominent with no evidence for the presence of impact melts the two regions appear to be surface expressions of a distant impact event that can modify surfaces across wide regions resulting in a variety of surface morphologies the tycho impact event may only be the most recent manifestation of these processes which likely have played a role in the development of the regolith throughout lunar history

comparisons of fresh complex impact craters on mercury and the moon implications for controlling factors in impact excavation processes the impact cratering process is usually divided into the coupling excavation and modification stages where each stage is controlled by a combination of different factors although recognized as the main factors governing impact processes on airless bodies the relative importance of gravity target and projectile properties and impact velocity in each stage is not well understood we focus on the excavation stage to place better constraints on its controlling factors by comparing the morphology and scale of crater exterior structures for similar sized fresh complex craters on the moon and mercury we find that the ratios of continuous ejecta deposits continuous secondaries facies and the largest secondary craters on the continuous secondaries facies between same sized mercurian and lunar craters are consistent with predictions from gravity regime crater scaling laws our observations support that gravity is a major controlling factor on the excavation stage of the formation of complex impact craters on the moon and mercury on the other hand similar sized craters with identical background terrains on mercury have different spatial densities of secondaries on the continuous secondaries facies suggesting that impactor velocity may also be important during the excavation stage as larger impactor velocity may also cause greater ejection velocities moreover some craters on mercury have more circular and less clustered secondaries on the continuous secondaries facies than other craters on mercury or the moon this morphological difference appears not to have been caused by the larger surface gravity or the larger median impact velocity on mercury a possible interpretation is that at some places on mercury the target material might have unique properties causing larger ejection angles during the impact excavation stage we conclude that gravity is the major controlling factor on the impact excavation stage of complex craters while impact velocity and target properties also affect the excavation stage but to a lesser extent than gravity

building and testing age models for radiocarbon dates in lateglacial and early holocene sediments the growing importance of understanding past abrupt climate variability at a regional and global scale has led to the realisation that independent chronologies of past environmental change need to be compared between various archives this has in turn led to attempts at significant improvements in the required precision at which records can be dated radiocarbon dating is still the most prominent method for dating organic material from terrestrial and marine archives and as such many of the recent developments in improving precision have been aimed at this technique these include 1 selection of the most suitable datable fractions within a record 2 the development of better calibration curves and 3 more precise age modelling techniques while much attention has been focussed on the first two items testing the possibilities of the relatively new age modelling approaches has not received much attention here we test the potential for methods designed to significantly improve precision in radiocarbon based age models wiggle match dating and various forms of bayesian analyses we demonstrate that while all of the methods can perform very well in some scenarios caution must be taken when applying them it appears that an integrated approach is required in real life dating situations where more than one model is applied with strict error calculation and with the integration of radiocarbon data with sedimentological analyses of site formation processes

rays and secondary craters of tycho abstract the large fresh crater tycho in the nearside lunar highlands has an extensive system of bright rays covering approximately 560 000 km 2 containing dense clusters of secondary craters examination of crater densities in several clusters shows that tycho produced almost 10 6 secondary craters larger than 63 m diameter this is a lower limit because small crater densities are reduced most likely by mass wasting we estimate a crater erasure rate of 2 6 cm myr varying with crater size and consistent with previous results this process has removed many small craters and it is probable that the original number of secondary craters formed by tycho was higher also we can only identify distant secondaries of tycho where they occur in bright rays craters on mars and europa also formed large numbers of secondaries but under possibly ideal conditions for spallation as a mechanism to produce high velocity ejecta fragments the results from tycho show that large numbers of such fragments can be produced even from impact into a heavily fragmented target on which spallation is expected to be less important

impact ejecta emplacement on terrestrial planets abstract impact cratering is one of the most fundamental processes responsible for shaping the surfaces of solid planetary bodies one of the principal characteristics of impact events is the formation and emplacement of ejecta deposits an understanding of which is critical for planetary exploration current models of ejecta emplacement however do not account for several important observations of ejecta deposits on the terrestrial planets in particular the presence of more than one layer of ejecta furthermore there is also no universal model for the origin and emplacement of ejecta on different planetary bodies we present a unifying working hypothesis for the origin and emplacement of ejecta on the terrestrial planets in which the ejecta are emplaced in a multi stage process the generation of the continuous ejecta blanket occurs during the excavation stage of cratering via the conventional ballistic sedimentation and radial flow model this is followed by the emplacement of more melt rich ground hugging flows the surface melt flow phase during the terminal stages of crater excavation and the modification stage of crater formation minor fallback occurs during the final stages of crater formation several factors will affect the final morphology and character of ejecta deposits the volatile content and cohesiveness of the uppermost target rocks will significantly affect the runout distance of the ballistically emplaced continuous ejecta blanket with impact angle also influencing the overall geometry of the deposits e g the production of the characteristic butterfly pattern seen in very oblique impacts ejecta deposited during the surface melt flow stage is influenced by several factors most importantly planetary gravity surface temperature and the physical properties of the target rocks topography and angle of impact play important roles in determining the final distribution of surface melt flow ejecta deposits with respect to the source crater this working hypothesis of ballistic sedimentation and surface melt flow provides a framework in which observations of ejecta at impact craters can be compared and placed in the context of the respective terrestrial planets

constraints on the depth and variability of the lunar regolith knowledge of regolith depth structure is important for a variety of studies of the moon and other bodies such as mercury and asteroids lunar regolith depths have been estimated using morphological techniques i e quaide and oberbeck 1968 shoemaker and morris 1969 crater counting techniques shoemaker et al 1969 and seismic studies i e watkins and kovach 1973 cooper et al 1974 these diverse methods provide good first order estimates of regolith depths across large distances tens to hundreds of kilometers but may not clearly elucidate the variability of regolith depth locally 100 m to km scale in order to better constrain the regional average depth and local variability of the regolith we investigate several techniques first we find that the apparent equilibrium diameter of a crater population increases with an increasing solar incidence angle and this affects the inferred regolith depth by increasing the range of predicted depths from 7 15 m depth at 100 m equilibrium diameter to 8 40 m at 300 m equilibrium diameter second we examine the frequency and distribution of blocky craters in selected lunar mare areas and find a range of regolith depths 8 31 m that compares favorably with results from the equilibrium diameter method 8 33 m for areas of similar age 2 5 billion years finally we examine the utility of using clementine optical maturity parameter images lucey et al 2000 to determine regolith depth the resolution of clementine images 100 m pixel prohibits determination of absolute depths but this method has the potential to give relative depths and if higher resolution spectral data were available could yield absolute depths

impact spallation processes on the moon a case study from the size and shape analysis of ejecta boulders and secondary craters of censorinus crater abstract impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters although theoretical spallation models were developed about three decades ago only limited geological observations have been made so far to test these models the 3 8 km censorinus crater on the moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact using the lunar reconnaissance orbiter narrow angle camera images we prepared the ejecta boulder distribution map of censorinus crater and measured the boulder sizes and shapes mapping of about 242 000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater larger size boulders dominate the crater rim areas while they become smaller away from the crater the boulder distribution exhibits a radial asymmetry suggesting censorinus is a oblique impact in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion the cumulative size frequency distribution csfd of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation the ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics there is a general decrease of axial ratios away from the crater rim rectangular boulders dominate the crater rim and they become more equant away from the crater in addition to the boulder sizes the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact small size fresh impact craters 84 000 craters are abundant on the censorinus ejecta and post date censorinus these craters are found in two morphologic types in which a large majority of craters have subdued ejecta rayless craters while some possess bright rayed ejecta bright rayed craters the csfd of rayless craters show a steep power law slope with a b value of 4 0 similar to the secondary craters produced by the impact of ejecta from primary craters we therefore interpret the rayless craters as the secondary craters of censorinus on the other hand the csfd of bright rayed craters have smaller power law slope b value 2 7 which is a characteristic of primary craters and thus provide 3 ma age for censorinus crater when the characteristics of censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target lunar highland the models generally agree with the censorinus boulders however the observed shape and size characteristics of the censorinus boulders are found to be more complex than the theoretical spallation models the ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact the spallation models accounting oblique impacts have not yet been developed therefore our censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts

diachronous evolution of sea surface conditions in the labrador sea and baffin bay since the last deglaciation assessing changes in sea surface conditions due to the effects of past freshwater outflow through baffin bay and davis strait to the labrador sea hereafter referred to as the baffin bay corridor

14c ams wiggle matching of raised bog deposits and models of peat accumulation abstract high resolution accelerator mass spectrometer ams 14 c dates of selected plant macrofossils from the raised bog engbertsdijksvenen eastern netherlands show century scale wiggles analogous to the radiocarbon calibration curve we used three relative time scales viz based on depth mass and pollen concentration respectively to match the peat ams dates to the calibration curve this procedure is repeated for one conventionally dated core for each relative time scale realistic confidence intervals are calculated depth appears to be the best time scale for certain stratigraphical units of a core this justifies using depth for wiggle matching conventionally dated mire cores published by others even when 14 c errors are larger our evidence shows four major sources of 14 c variation for mire deposits compared to treerings 1 dating error due to sample composition this includes a reservoir effect demonstrated for many bulk peat samples 2 hiatuses causing a sudden leap of peat 14 c age 3 changing accumulation rates apparent from a break in the slope of the peat 14 c ages 4 sampling error these results shed doubt on the assumed continuities in theoretical peat accumulation models both mire stratigraphy and changing accumulation rates can be explained in qualitative hydrological terms more evidence for a previously discovered reservoir effect in bog deposits is presented though the physical biological mechanisms remain uncertain

global resurfacing of mercury 4 0 4 1 billion years ago by heavy bombardment and volcanism analysis of craters on mercury s oldest most heavily cratered terrains shows that they were formed 4 0 4 1 billion years ago and that the planet s previous geological history was erased most probably by voluminous volcanism which may have been triggered by heavy asteroidal bombardment at that time the most heavily cratered terrains on mercury exhibit a lower density of craters smaller than about 100 km in diameter than on the moon a deficit that has been attributed to resurfacing by formation of ancient intercrater plains simone marchi et al used a crater areal density map based on data from the messenger spacecraft the colour coded foreground on cover with a global surface mosaic in the background to locate the oldest surfaces on mercury and interpret the crater populations in the framework of a recent lunar crater chronology they conclude that the oldest surfaces were emplaced just after the start of the late heavy bombardment 4 0 to 4 1 billion years ago the large impact basins not previously dated yield a similar surface age this agreement implies that resurfacing was global and due to volcanism perhaps aided by heavy bombardment as previously suggested the most heavily cratered terrains on mercury have been estimated to be about 4 billion years gyr old1 2 3 4 but this was based on images of only about 45 per cent of the surface even older regions could have existed in the unobserved portion these terrains have a lower density of craters less than 100 km in diameter than does the moon1 3 5 an observation attributed to preferential resurfacing on mercury here we report global crater statistics of mercury s most heavily cratered terrains on the entire surface applying a recent model for early lunar crater chronology6and an updated dynamical extrapolation to mercury7 we find that the oldest surfaces were emplaced just after the start of the late heavy bombardment lhb about 4 0 4 1 gyr ago mercury s global record of large impact basins8 which has hitherto not been dated yields a similar surface age this agreement implies that resurfacing was global and was due to volcanism as previously suggested1 5 this activity ended during the tail of the lhb within about 300 400 million years after the emplacement of the oldest terrains on mercury these findings suggest that persistent volcanism could have been aided by the surge of basin scale impacts during this bombardment

mass wasting features on the moon how active is the lunar surface abstract mass wasting features have long been noticed on the moon but there has been no systematic studies for their morphological geometrical or distribution characteristics the effect of mass wasting on topographic evolution and surface erosion on the moon is not fully understood using high resolution image and topography data obtained from the lunar reconnaissance orbiter spacecraft we found that mass wasting has been globally and continuously occurring on the moon meteor impacts and moonquakes are the two major triggering mechanisms in forming lunar mass wasting features we selected over 300 examples of lunar mass wasting features and classified them as falls flows slides slumps and creeps after studying their morphology potential mode of emplacement and sizes of displaced materials the slope and age distributions for their host terrains are studied to estimate the topographic evolutionary sequence of the lunar surface under the effect of mass wasting due to continuous reshaping by mass wasting surface topographies on the moon tend to become smoother with time with fewer mass wasting events occurring on older surfaces pre nectarian surfaces may represent the final stage of surface topographic evolution on slope angles regarding the effect of mass wasting crater densities on lunar surfaces vary with slope angles because the surface erosion rate and regolith production rate on slopes are larger than those on flat areas

localization and fault growth in layered brittle ductile systems implications for deformations of the continental lithosphere we present experiments and numerical simulations dealing with the growth of faults in thin brittle ductile systems to understand deformation modes in the continental lithosphere experiments were uniaxial shortening of layers of dry sand and silicone putties of various viscous resistances for large strength ratios between the brittle and ductile layers r 5 10 the deformation localizes into two shear bands the fault pattern is created before reaching 10 shortening and has fractal dimensions varying between 1 6 and 1 8 for small strength ratios r 5 10 deformation never localizes the fault pattern is homogeneous with a trivial dimension of 2 and grows continuously during deformation the transition between localized and homogeneous deformation occurs when the mechanical resistance of brittle layers is 5 10 times larger than the resistance of ductile layers this transition was also investigated by means of electrical analog simulations a fuse network which represents an elasto brittle layer is coupled with a capacitor layer which models strain rate dependent fluids an ac potential is applied and the fuses progressively burned out until they form a connected network the ac potential frequency is a tuning parameter similar to the applied strain rate in experiments a critical frequency is obtained marking a transition between a localization mode where the density of burned fuses decreases as the system size increases and a delocalization mode where the density of burned fuses remains constant with increasing system size the scaling dependency of the fracture process as well as the critical frequency are consistent with experimental results available information on the rheology of the continental lithosphere shows that this mechanical transition is bracketed by the possible range of brittle to ductile strength ratios

calibration of radiocarbon results pertaining to related archaeological events with the advent of the high precision radiocarbon calibration curve there is an increasing demand from archaeologists for results previously reported on the radiocarbon scale to be expressed on the calendar scale furthermore there is a general realization within the archaeological community that much more information can be derived from radiocarbon dating if the samples are taken from coherent well chosen contexts and if any available substantive archaeological information is included in the data analysis this paper describes how the bayesian approach to statistical data analysis can be used for the calibration of groups of radiocarbon results in situations where archaeological information is available a priori about the relationships between the contexts being dated recent innovations in the implementation of the bayesian paradigm are used and the methodology is illustrated by the analysis of data arising from two archaeological excavations

regional climate change from peat stratigraphy for the mid to late holocene in central ireland at the millennial scale bog surface wetness bsw records show a clear holocene climate event stratigraphy with major phases comparable with other regional climate proxies such as chironomid inferred temperature records moving towards the centennial scale however and towards the limits of chronological certainty within the records regional differences are apparent which likely reflect the more heterogeneous precipitation patterns which occur on shorter timescales the bsw data presented in this paper are reconstructed from testate amoebae assemblages from central ireland and are compared with other regional records the results suggest that this region is in phase with the north of ireland in terms of timings and durations of climate change but the comparisons are less clear with a stacked and tuned record from northern britain that shows an apparent offset compared to the irish records this may reflect variations in past regional precipitation or be a function of the tuning and stacking process the broad phases of comparison between the irish records and the extension of the central irish record back to 6000 cal bp allow comparisons with low frequency temperature reconstructions from chironomids which also show a broad level of correlation with cooler temperatures relating to wetter bsw at centennial to millennial timescales

size frequency distribution of different secondary crater populations 1 equilibrium caused by secondary impacts accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces besides primary craters the effect of widespread secondaries on the equilibrium of local crater populations is little studied here the different secondary crater populations formed by the hokusai crater on mercury are systematically studied and they are compared with those on the moon to investigate their contribution to the evolution of local crater populations self secondaries cause equilibrium on continuous ejecta deposits in a short time and the equilibrium crater population has a differential sfd slope of about 3 background secondaries are abundant on mercury and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the moon and mercury the spatial dispersion of fragments that form both near field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater sfd some clustered distant secondaries on mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same sized primaries and the sfd rollovers of these secondaries possibly reflect the inherent sfd rollovers of the impact fragments near field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments and mutual destruction of secondaries may be the major reason causing the observed sfd rollovers heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces

geological characteristics of von karman crater northwestern south pole aitken basin chang e 4 landing site region von karman crater diameter 186 km lying in the northwestern south pole aitken spa basin was formed in the pre nectarian the von karman crater floor was subsequently flooded with one or several generations of mare basalts during the imbrian period numerous subsequent impact craters in the surrounding region delivered ejecta to the floor together forming a rich sample of the spa basin and farside geologic history we studied in details the targeted landing region 45 0 46 0 s 176 4 178 8 e of the 2018 chinese lunar mission chang e 4 within the von karman crater the topography of the landing region is generally flat at a baseline of 60 m secondary craters and ejecta materials have covered most of the mare unit and can be traced back to at least four source craters finsen von karman l von karman l and antoniadi based on preferential spatial orientations and crosscutting relationships extensive sinuous ridges and troughs are identified spatially related to ba jie crater diameter 4 km reflectance spectral variations due to difference in both composition and physical properties are observed among the ejecta from various sized craters on the mare unit the composition trends were used together with crater scaling relationships and estimates of regolith thickness to reconstruct the subsurface stratigraphy the results reveal a complex geological history of the landing region and set the framework for the in situ measurements of the ce 4 mission which will provide unique insights into the compositions of farside mare basalt spa compositional zone including spa compositional anomaly and mg pyroxene annulus regolith evolution and the lunar space environment

revised constraints on absolute age limits for mercury s kuiperian and mansurian stratigraphic systems following an approach similar to that used for the moon mercury s surface units were subdivided into five time stratigraphic systems based on geologic mapping using mariner 10 images the absolute time scale originally suggested for the time periods associated with these systems was based on the assumption that the lunar impact flux history applied to mercury however we find that the duration and onset of corresponding periods in the stratigraphic sequences on mercury and the moon are not the same using high resolution and multiband image data obtained by the mercury surface space enviroment geochemistry and ranging messenger spacecraft we identify and catalog fresh impact craters interpreted to have formed during mercury s two most recent periods the mansurian and kuiperian we use the densities of the inferred kuiperian and mansurian aged crater populations to estimate new limits for the age boundaries of these time intervals results suggest that both the mansurian and kuiperian periods began more recently and extended for significantly shorter durations of time than previously suggested the kuiperian is estimated to have initiated as recently as 280 60 ma and the mansurian as recently as 1 7 0 2 ga

lateglacial and holocene environmental changes in the southern urals reflected in palynological geochemical and diatom records from the lake syrytkul sediments abstract a multi proxy study including lithology geochemistry diatom analysis and palynology of the lake syrytkul sediments provides continuous environmental reconstruction of the lateglacial and holocene in the southern urals and make it possible to compare the role of natural and anthropogenic factors in lake history the lake syrytkul sediment records reflect the main lateglacial and holocene paleoclimate changes the lateglacial holocene transition 11 600 11 500 cal bp expressed in all palaeorecords was followed by great limnological and vegetation changes the climatic changes of 7400 and 4300 cal bp were fixed in all lake sediment records vegetation and lake ecosystem were less affected by global climate events the preboreal oscillation 11 200 11 000 cal bp 8 2 ka event and regional climate events 10 350 cal bp 9750 cal bp 9000 cal bp and 2000 cal bp as follows from lake sediment records the environment deterioration after the holocene climatic optimum started in different time in the range of 6300 5100 cal bp due to various sensitivities of different proxies to paleoclimate parameters the climate events were the main drivers for the dynamics of the southern urals lake ecosystems during almost the entire lake history the role of the human factor in the change of the lake ecosystem was critical since the beginning of the twentieth century despite the adjacent karabash copper smelter as the source of considerable technogenic contamination it was a dam constructed 100 years ago that had the greatest impact on the lake system this event is reflected in all sediment records especially diatom and geochemical changes in the diatom species composition and sediments geochemistry of the shortest human impact period are comparable in magnitude to the response of the lake ecosystem to the lateglacial holocene transition

a comparison of rayed craters on the moon and mercury 1 observations of rayed craters at optical and radar wavelengths provide insight into the processes that lead to ray formation and degradation on terrestrial planets we have compared optical and s band radar data for several large 20 km diameter young craters on the moon and mercury and find evidence that secondary cratering plays a significant role in the formation of crater rays regions where rays appear bright to optical and radar sensors correspond to dense concentrations of secondary craters and the observed radar enhancement appears to be a result of the deposition of blocky immature ejecta from the secondary craters and or the rocky immature interior walls of the secondary craters we define a new optical maturity index for mercury and find that rays in radar and optical images correspond closely indicating that the rays are rich in centimeter to decimeter sized clasts rays on the moon are less prominent at radar wavelengths suggesting that they are currently composed of smaller clasts centimeter sized or less this difference suggests that secondary craters are larger on mercury and capable of excavating more decimeter sized clasts furthermore observations of rayed craters provide an opportunity to assign relative ages to the youngest craters on the moon and mercury although rayed craters on mercury appear most similar to the youngest craters on the moon the apparent ages are more likely influenced by inherent differences in impact velocity surface gravitational acceleration and target properties that result in the formation of larger secondary craters on mercury

dependence of secondary crater characteristics on downrange distance high resolution morphometry and simulations on average secondary impact craters are expected to deepen and become more symmetric as impact velocity vi increases with downrange distance l we have used high resolution topography 1 2 m pixel to characterize the morphometry of secondary craters as a function of l for several well preserved primary craters on mars the secondaries in this study n 2 643 span a range of diameters 25 m d 400 m and estimated impact velocities 0 4 km s vi 2 km s the range of diameter normalized rim to floor depth d d broadens and reaches a ceiling of d d 0 22 at l 280 km vi 1 1 2 km s whereas average rim height shows little dependence on vi for the largest craters h d 0 02 d 60 m populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent planform elongations l 110 160 km taller downrange rims l 280 km and cavities that are deeper uprange l 450 500 km populations of secondaries with lopsided ejecta were found to extend to at least l 700 km impact hydrocode simulations with isale 2d for strong intact projectile and target materials predict a ceiling for d d vs l whose trend is consistent with our measurements this study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations this has applications to understanding the chronology of planetary surfaces and the long term evolution of small crater populations

variation in ejecta size with ejection velocity a knowledge of the size velocity distribution of ejecta from impact craters is important for a number of processes that occur or may have occurred in the solar system such as planetary accretion collisional evolution of asteroids the ejection of meteorites from parent bodies larger than asteroids such as the lunar meteorites and the snc meteorites which may be from mars and massive biological extinctions on the earth in order to estimate the size velocity distribution of impact crater ejecta the sizes and ranges from the primary crater were measured for the secondary craters of twelve large craters on mercury the moon and mars the ballistic equation for spherical bodies was used to convert the ranges to velocities and the velocities and crater sizes were used in the appropriate schmidt holsapple scaling relation to estimate ejecta sizes assuming that d v lines were fitted to the log maximum size vs log ejection velocity using the least squares method to determine the velocity exponent certain problems with data collection such as partial burial of some craters and interference from other craters and topographic features made it impossible to determine a simple unique relation between size and velocity the velocity exponents are generally between 1 and 3 with an average value of approximately 1 9 the complicating effect of secondaries produced by clusters of impactors implies that the calculated lines are probably steeper than the slopes of the true fragment size velocity distributions

impact into lunar regolith inhibits high velocity ejection of large blocks 1 our data suggest that the presence of a layer of regolith covering the lunar surface results in a portion of the high velocity ejection phase occurring in the fine grained regolith reducing the population of large blocks available for ejection at high velocities this conclusion is supported by a study of boulder distributions around 10 lunar craters the boulder ejection velocities were calculated by applying crater ejecta scaling relations and assuming a ballistic trajectory regolith depths were estimated by analyzing the morphology of small craters tens of meters in diameter because both meteorites and distant secondary craters are produced by solid fragments within the highest velocity ejecta we conclude that less regolith cover at the impact site will favor the formation of meteorites and distant secondary craters

a numerical approach to 14c wiggle match dating of organic deposits best fits and confidence intervals c 14 wiggle match dating wmd of peat deposits uses the non linear relationship between c 14 age and calendar age to match the shape of a sequence of closely spaced peat c 14 dates with the c 14 calibration curve a numerical approach to wmd enables the quantitative assessment of various possible wiggle match solutions and of calendar year confidence intervals for sequences of c 14 dates we assess the assumptions advantages and limitations of the method several case studies show that wmd results in more precise chronologies than when individual c 14 dates are calibrated wmd is most successful during periods with major excursions in the c 14 calibration curve e g in one case wmd could narrow down confidence intervals from 230 to 36 yr c 2003 elsevier science ltd all rights reserved

widespread effusive volcanism on mercury likely ended by about 3 5 ga crater size frequency analyses have shown that the largest volcanic plains deposits on mercury were emplaced around 3 7 ga as determined with recent model production function chronologies for impact crater formation on that planet to test the hypothesis that all major smooth plains on mercury were emplaced by about that time we determined crater size frequency distributions for the nine next largest deposits which we interpret also as volcanic our crater density measurements are consistent with those of the largest areas of smooth plains on the planet model ages based on recent crater production rate estimates for mercury imply that the main phase of plains volcanism on mercury had ended by 3 5 ga with only small scale volcanism enduring beyond that time cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet

a multi proxy reconstruction of oceanographic conditions around the younger dryas holocene transition in placentia bay newfoundland abstract a marine sediment core from placentia bay off the south coast of newfoundland was studied to reconstruct past oceanographic conditions based on a multiproxy approach including diatoms benthic foraminifera grain size analysis ip25 x ray fluorescence calcium carbonate and total organic carbon content based on 6 radiocarbon dates the 513 5 cm long core spans the age interval from ca 13 to 10 cal kyrs bp and thus captures the transition from the younger dryas stadial into the holocene at sub decadal resolution the younger dryas was characterized by cold arctic water taxa of diatoms and benthic foraminifera and a stratified water column with high sea ice cover and low productivity due to strong influence of the labrador current during the second half of the younger dryas after ca 12 2 cal kyrs bp the influence of the labrador current gradually decreased and sea ice conditions became more variable the transition towards the holocene is characterized by a sequence of events starting with turnovers in the biogenic proxies followed by an abrupt retreat of sea ice and a rise in productivity these events are believed to be related to a northward migration of the gulf stream labrador current oceanic front the younger dryas termination is directly followed by a detrital carbonate layer associated with the final phase of the heinrich 0 event linked to laurentide ice sheet dynamics during the early holocene productivity remained high with minimal sea ice cover reduced stratification and stronger influence of warmer gulf stream derived waters

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research