cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

slow regolith degradation without creep determined by cosmogenic nuclide measurements in arena valley antarctica abstract estimates of regolith degradation in the mcmurdo dry valleys of antarctica are currently based on indirect evidence and ancient ashes at or near the soil surface that suggest excellent preservation of surfaces on the other hand the existing cosmogenic nuclide surface exposure ages from many parts of the dry valleys are younger than the age of surface deposits inferred from stratigraphic relations this suggests some combination of surface erosion or past ice cover both of which would reduce the apparent exposure age this paper quantifies the regolith degradation and or past ice cover by measuring 10be and 26al from a landslide deposit that contains 11 3 ma volcanic ash the surface sample yields an apparent exposure age of only 0 4 ma however measurements of the subsurface nuclide concentrations show that the deposit has not been shielded by ice and that the age of the ash does not conflict with the apparent exposure age when slow degradation of the deposit 2 m ma 1 is taken into account soil creep which is a common degradational process in a wide variety of environments is non existent at this field site which likely reflects the persistent lack of bio and cryoturbation

rapid soil accumulation in a frozen landscape polygonal patterned ground is a dominant geomorphic feature in the mcmurdo dry valleys antarctica in central beacon valley polygons are considered to be ancient expressions of stagnant surface processes and are thought to be stable for millions of years however the actual rate at which these polygons form and alter is not known we describe a detailed cross section of a polygon make several depth profiles of meteoric 10be along this cross section and use optically stimulated luminescence to date a few key samples at the center of this polygon while confirming conclusions of previous studies that the polygon shoulders are stable on a 100 k y time scale and experience little vertical sediment mixing our results also give clear evidence of eolian transport to the polygon center leading to a sediment accumulation rate of 3 cm k y 1 over the past 15 k y moreover the data suggest that the accumulated material is locally derived and hence surface erosion of the polygon shoulders must exist that cannot be recorded by meteoric 10be we conclude that polygon modification and soil accumulation under the apparent frozen conditions of beacon valley is an active and ongoing process

depositional processes beneath coastal multi year sea ice the extent of multi year sea ice impacts climate processes worldwide such as ocean atmosphere carbon dioxide exchange and deep ocean current formation reconstructing these processes in the past and assessing the distribution of ecologically and climatically significant features such as polynas requires recognition of sediments deposited under multi year sea ice but little is known about their characteristics textural analysis of subaerial and sea floor sediment in explorers cove mcmurdo sound at the mouth of taylor valley antarctica augmented with observations of sedimentary structures and faunal components elucidates how sediment is transported to the sea floor and allows characterization of the deposits comparison of grain size characteristics of subaerial moraine delta and sea ice surface sediment and sea floor sediment from short cores taken at depths of 7 to 25 m indicates that the likely source of the moderately to poorly sorted sea floor sand is deltaic sediment small glacial meltwater streams have built deltas since taylor valley became ice free ca 7000 years ago windblown sediment accumulating on the multi year sea ice close to the coast typically is coarser grained than sediment on the sea floor this suggests that the transport of sediment through the ice to the sea floor is not the predominant mode of sediment transfer however supra sea ice sediment does move to the sea floor through local fractures the rate of sedimentation under multi year sea ice is low because of limited stream flow and biogenic sedimentation the ice cover inhibits primary productivity and dampens waves precluding physical re suspension the upper centimetres of sea floor sediment are churned by epifaunal scallops and brittle stars that leave no telltale biogenic structures and whose calcite ossicles and shells may be poorly preserved the resulting deposits under multi year sea ice are poorly sorted massive sand that provides little evidence of the bioturbators that have masked the indicators of the original physical depositional processes

dating plio pleistocene glacial sediments using the cosmic ray produced radionuclides 10be and 26al we use the cosmic ray produced radionuclides 26al and 10be to date plio pleistocene glacial sediment sequences these two nuclides are produced in quartz at a fixed ratio but have different decay constants if a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic ray flux the 26al 10be ratio is related to the duration of burial we first attempted to date pre wisconsinan tills by measuring 26al and 10be in fluvial sediments beneath them and applying the method of burial dating which previous authors have used to date river sediment carried into caves this method however requires simplifying assumptions about the 26al and 10be concentrations in the sediment at the time of burial we show that these assumptions are not valid for river sediment in glaciated regions 26al and 10be analyses of such sediment do not provide accurate ages for these tills although they do yield limiting ages in some cases we overcome this difficulty by instead measuring 26al and 10be in quartz from paleosols that are buried by tills we use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried as well as the duration of burial this technique provides a widely applicable improvement on other means of dating plio pleistocene terrestrial glacial sediments as well as a framework for applying cosmogenic nuclide dating techniques in complicated stratigraphic settings we apply it to pre wisconsinan glacial sediment sequences in southwest minnesota and eastern south dakota pre wisconsinan tills underlying the minnesota river valley were deposited 0 5 to 1 5 ma and tills beneath the prairie coteau in eastern south dakota and adjacent minnesota were deposited 1 to 2 ma

contamination of the marcy anorthosite massif adirondack mountains ny petrologic and isotopic evidence oxygen isotope analyses of 101 samples from the marcy anorthosite massif 61 from this study 40 from taylor 1969 indicate that two major and distinct processes of crustal contamination have affected the massif ninety percent of the 93 samples with over 65 plagioclase are enriched in 18o by 2 6 relative to normal anorthosites or gabbros the average 18o for 83 enriched samples is 9 5 depletions in 18o occur in 8 of the samples which have values ranging from 3 0 to 5 8 only 2 of the samples fall within the normal magmatic range for anorthosites low 18o values of 3 0 to 5 8 in the anorthosite occur only near contacts and a gradient in 18o occurs near the contact within the border zone of the massif low 18o values in both the anorthosite and adjacent wollastonite skarns with 18o down to 1 3 were probably caused by isotopic exchange with heated meteoric water when the anorthosite intruded at shallow levels prior to grenvilleage 1 1 by granulite facies metamorphism the 18o enrichment was ascribed to exchange between anorthosite and 18o rich marble by means of a pervasive h2o co2 fluid during the regional metamorphism by taylor 1969 however a number of lines of evidence argue against this hypothesis 1 the preservation of premetamorphic low 18o values in anorthosite from the border zone as well as preserved gradients in 18o from a number of localities 2 mass balance calculations of the amount of marble necessary to produce the 18o enrichment 3 metamorphic phase equilibria which buffer p text co text 2 and p text h text 2 text o to low values and 4 recent oxygen isotope analyses show homogeneity which indicates that magmatic oxygen isotope compositions have been preserved we evaluated the importance of magmatic assimilation of country rock at the present level of intrusion as an alternative cause of the 18oenrichment samples from 2 distinct lobes of the massif were analyzed the ne lobe where xenoliths of metasedimentary country rock are common and the nw lobe where xenoliths are scarce and the country rock is dominantly granitic the mean values of 18o for these two lobes are 9 6 in the ne and 9 3 in the nw thus magmatic assimilation at the present level of exposure probably had only localized and relatively minor effects on the oxygen isotope composition of the massif this conclusion is supported by rb sr data variations in rb content and 87sr 86sr i show that such crustal contamination is localized generally occurring only in samples near the border zone all of the available results suggest that the 18o enrichment is a magmatic feature acquired prior to intrusion at the present level of exposure

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

long term rates of denudation in t he dry valleys transantarctic mountains southern victoria land antarctica bases on in situ produced cosmogenic 21ne abstract concentrations of cosmogenic 21 ne 21 ne c measured in quartz have been used to estimate long term rates of denudation for contrasting landscape components in the dry valleys area of the transantarctic mountains southern victoria land antarctica samples of beacon supergroup sandstones and granitic basement were collected from two contrasting landscape elements low relief high elevation surfaces and rectilinear slopes to assess variations in rates of denudation with topographic position the sample sites for rectilinear slopes were selected because of proximity to 40 ar 39 ar dated lavas and ash avalanche deposits all 21 ne 20 ne ratios are significantly greater than the atmospheric value and concentrations of 21 ne c were calculated from the measured 21 ne values assuming an atmospheric composition for the trapped component apparent exposure ages calculated from the concentrations of 21 ne c assuming no denudation since exposure range from 3 78 4 66 ma for samples from the high elevation plateau surface and 0 61 2 48 ma for samples from the rectilinear slopes exposure ages for 3 he c were 2 to 42 times lower than those derived from the abundances of 21 ne c because of preferential diffusive loss of 3 he c from the quartz lattice concentrations of 3 he c were therefore not used in the calculation of rates of denudation we interpret the abundances of 21 ne c as reflecting variations in the rates of denudation rather than exposure age in view of independent evidence for prolonged exposure 15 ma of bedrock surfaces at the sample sites calculated maximum rates of denudation range from 0 26 1 02 m ma 1 for the rectilinear slopes down to only 0 133 0 164 m ma 1 for the high elevation surface sites these rates are comparable to other estimates of denudation rates for the dry valleys derived from analyses of cosmogenic isotopes but are around two orders of magnitude lower than the long term mean rate over the past 50 ma estimated from fission track thermochronology combined with the preservation of volcanic deposits dating back to the mid miocene these 21 ne c data demonstrate that only minimal modification of the landscape has occurred in the dry valleys over at least the past 15 ma this significant conclusion supports the view that the east antarctic ice sheet has been essentially stable over this period rather than experiencing major fluctuations as late as the pliocene as has previously been suggested

degradation of unconsolidated quaternary landforms in the western north america abstract one of the major goals of geomorphology is to understand the rate of landscape evolution and the constraints that erosion sets on the longevity of land surfaces the latter has also turned out to be vital in modern applications of cosmogenic exposure dating and interpretation of lichenometric data from unconsolidated landforms because the effects of landform degradation have not been well documented disagreements exist among researchers regarding the importance of degradation processes in the dating techniques applied to exposures here we show that all existing qualitative data and quantitative markers of landform degradation collectively suggest considerable lowering of the surface of unconsolidated landforms over the typical life span of quaternary moraines or fault scarps degradation is ubiquitous and considerable even on short time scales of hundreds of years on steeply sloping landforms these conservative analyses are based entirely on field observations of decreasing slope angles of landforms over the typical range of ages in western north america and widely accepted modeling of landscape degradation we found that the maximum depth of erosion on fault scarps and moraines is on average 34 of the initial height of the scarp and 25 of the final height of the moraine although our observations are limited to fault scarps and moraines the results apply to any sloping unconsolidated landform in the western north america these results invalidate the prevailing assumption of no or little surface lowering on sloping unconsolidated landforms over the quaternary period and affirm that accurate interpretations of lichen ages and cosmogenically dated boulder ages require keen understanding of the ever present erosion in our view the most important results are twofold 1 to show with a large data set that degradation affects universally all sloping unconsolidated landforms and 2 to unambiguously show that even conservative estimates of the total lowering of the surface operate at time and depth scales that significantly interfere with cosmogenic exposure and lichen dating

cosmogenic nuclide exposure ages and glacial history of late quaternary ross sea drift in mcmurdo sound antarctica 3he 10be and26al surface exposure ages were determined for rock samples from late quaternary drift sheets deposited on the coast of mcmurdo sound antarctica by the ross sea ice sheet the youngest drift previously believed to be late wisconsin in age has exposure ages of 8 106 kyr 19 samples these results suggest that the apparent late wisconsin ice margin may represent a number of ice sheet advances during the last glacial period 3he exposure ages from analogous stratigraphically older drift range from 104 to 567 kyr seven samples and clearly distinguish the youngest drift from older deposits comparison of10be and3he concentrations in ross sea drift quartz samples indicates significant3he loss in only one sample suggesting that cosmogenic3he is generally retained in young antarctic quartz samples exposure age 100 kyr of appropriate grain size in two of the five quartz samples analyzed significant excesses of26al relative to10be preclude meaningful26al exposure age calculations these excesses may be due to nucleogenic26al produced as a byproduct of uranium and thorium decay

degradation of glacial deposits quantified with cosmogenic nuclides quartermain mountains antarctica many glacial deposits in the quartermain mountains antarctica present two apparent contradictions regarding the degradation of unconsolidated deposits the glacial deposits are up to millions of years old yet they have maintained their meter scale morphology despite the fact that bedrock and regolith erosion rates in the quartermain mountains have been measured at 0 1 4 0 m ma 1 additionally ground ice persists in some miocene aged soils in the quartermain mountains even though modeled and measured sublimation rates of ice in antarctic soils suggest that without any recharge mechanisms ground ice should sublimate in the upper few meters of soil on the order of 103 to 105 years this paper presents results from using the concentration of cosmogenic nuclides beryllium 10 10be and aluminum 26 26al in bulk sediment samples from depth profiles of three glacial deposits in the quartermain mountains the measured nuclide concentrations are lower than expected for the known ages of the deposits erosion alone does not always explain these concentrations and deflation of the tills by the sublimation of ice coupled with erosion of the overlying till can explain some of the nuclide concentration profiles the degradation rates that best match the data range 0 7 12 m ma 1 for sublimation of ice with initial debris concentrations ranging 12 45 and erosion of the overlying till at rates of 0 4 1 2 m ma 1 overturning of the tills by cryoturbation vertical mixing or soil creep is not indicated by the cosmogenic nuclide profiles and degradation appears to be limited to within a few centimeters of the surface erosion of these tills without vertical mixing may partially explain how some glacial deposits in the quartermain mountains maintain their morphology and contain ground ice close to the surface for millions of years copyright 2010 john wiley sons ltd

oxygen isotope evidence for shallow emplacement of adirondack anorthosite oxygen isotopic analysis of wollastonites from the willsboro mine adirondack mountains new york reveals a 400 ft wide zone of 18o depletion at anorthosite contacts values of 18o vary more sharply with distance and are lower to 1 3 than any yet reported for a granulite fades terrain exchange with circulating hot meteoric water best explains these results and implies that the anorthosite was emplaced at relatively shallow depths 10 km in marked contrast to the depth of granulite fades metamorphism 23 km these 18o depletions offer the first strong evidence for shallow emplacement of anorthosite within the grenville province and suggest that regional metamorphism was a later and tectonically distinct event

near coastal satellite altimetry sea surface height variability in the north sea baltic sea area 1 the radiometer wet tropospheric correction is a limiting factor of the application of near coastal altimetry observations in the north sea baltic sea area using an ecmwf model based correction instead we increase the return rate from 70 to 95 with observations as close as 10 km from the coast the altimetry and 39 coastal tide gauges are referenced to the same state of the art regional geoid the two data sets are highly correlated and reveal a consistent mean dynamic topography and two distinct correlation regimes separated by the danish islands the combination of the two data sets in a simple statistical model is shown to estimate the sea surface height with the same error level as state of the art operational models indicating the potential for near coastal applications of satellite altimetry observations

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

a model for the exchange of water and salt between the baltic and the skagerrak abstract a model for the exchange of salt and water between the baltic and the sea the skagerrak is presented because of strong inter basin interactions in the baltic entrance area the model must include the kattegat and the belt sea these are modeled by horizontally homogeneous two layer sub models the most prominent dynamical properties of the sub models are wind driven entrainment flows and rotational baroclinic hydraulic controls the model is driven by a meteorologically forced barotropic transport qh calculated from the freshwater supply to the baltic qf and the sea level fluctuations in the kattegat and turbulent entrainment flows coupled to the wind speed w and in the belt sea also to the barotropic transport the most important bathymetric features of the basins are included the model equations are integrated numerically for a test period of 1 years the stratification in the kattegat as well as in the belt sea is quite well predicted it is found that approximately one half of

constraining the timing and character of crustal melting in the adirondack mountains using multi scale compositional mapping and in situ monazite geochronology

morphologic dating of fluvial terrace scarps and fault scarps near west yellowstone montana analysis of fault and fluvial terrace scarps in the west yellowstone basin south of hebgen lake montana indicates that during degradation the gradient of the scarp face and the curvatures of the basal concavity and crestal convexity of the scarp face decrease become more rounded with time the pattern of degradation is identical with that predicted by a simple analytical model for degradation by creep in which the change in elevation at a point on a hillslope is proportional to the curvature of the profile at that point the model therefore provides the basis for morphologic dating a technique for determining the age of a scarp by the extent to which its initial morphology has been degraded in order to date a scarp the coefficient of proportionality c must be known the technique is probably not suitable for use on hillslopes degraded by processes other than creep both the potential and the limitations of morphologic dating are demonstrated via measurements of a sequence of terrace scarps along the madison river the calculated relative ages are in agreement with the known relative ages despite the fact that the gradient of the youngest scarp is significantly less than that of the older scarps to calculate an absolute morphologic age c must be derived from analysis of a scarp of known age having the same aspect must be underlain by the same material and must be within the same climatic zone as the scarp to be dated in the west yellowstone basin a value of c 2 00 0 38 10 3 m 2 yr was calculated from a minimum date of 7100 50 yr b p determined by radiocarbon dating for one of the terrace scarps scarps formed by normal faulting in the west yellowstone basin are much more difficult to date than are terrace scarps because of the possible complexity of their initial morphology fault scarps formed during the 1959 yellowstone earthquake commonly consist of an assemblage of two or more smaller scarps or are superimposed on preexisting scarps formed by one or more previous faulting events by using a selective sampling procedure however one can screen out these fault scarps in this way it was possible to determine an age of 2 800 1 100 yr for a pre 1959 fault scarp in the west yellowstone basin because the fault and terrace scarps in the west yellowstone area do not vary significantly in height the morphologic dating technique proposed earlier by nash 1980a cannot be applied to them the substantially modified procedure developed for the present study does not require a wide range of heights for each scarp to be dated it can be performed using a relatively small number of scarp profiles if c is known it is easy to use and when used with considerable care it is reasonably accurate 36 59 in this study it is potentially applicable to scarps in other areas

role of in situ cosmogenic nuclides 10be and 26al in the study of diverse geomorphic processes the central premises of applications of the in situ cosmogenic dating method for studying specific problems in geomorphology are outlined for simple and complex exposure settings in the light of these general models we discuss the information that can be derived about geomorphic processes utilizing concentrations of in situ produced cosmogenic radionuclides 10be half life 1 5 ma and 26a1 half life 0 7 ma in a variety of geomorphic contexts glacial polish and tills meteorite impact craters alluvial fans paleo beach ridges marine terraces sand dunes and bedrock slopes we also compare 10be 26al data with results obtained by other dating methods we conclude that the technique of measuring in situ cosmic ray produced nuclides holds promise for quantitative studies of processes and time scales in a wide range of geomorphological problems

origin and age of adirondack anorthosites re evaluated with nd isotopes we report the sm nd systematics in seven whole rock anorthosites in plagioclase and garnet mineral separates from one anorthosite and in five mangerite norite charnockite rocks from the snowy mountain dome in the south central adirondacks the plagioclase two garnet separates and the whole rock anorthosite define a four point internal isochron of age 1 098 7 2 m y the six whole rock anorthosites and the anorthositic gabbro in conjunction with the above internal isochron define a similar age of 1 095 7 2 m y and an initial 143nd 144nd ratio of 0 51135 1 2 this age of 1 100 m y is considered to be the time of crystallization of the anorthositic rocks the initial nd isotope ratio of the combined whole rock and mineral isochron indicates an nd value of 2 5 which suggests a light rare earth element depleted source for the anorthositic parent magma in the upper mantle the trace element and isotopic data also support the view that gabbroic anorthosite is the parent magma for the anorthosites five whole rock samples of norite mangerite charnockite found in close association with the anorthosites do not define a precise isochron the initial nd values of these rocks at 1 100 m y show a range from 2 7 to 5 1 which demonstrates that the anorthositic rocks and the mangerite charnockite are not cogenetic the isotopic data further demonstrate that the mangerite charnockite rocks are mantle derived meta igneous rocks and not of sedimentary derivation

variations in parameters of extreme value distributions of water level along the eastern baltic sea coast abstract time series of water level at several sites of the eastern baltic sea contain statistically almost impossible outliers their presence may considerably modify the appearance of the relevant extreme value distributions we analyse systematically the variation in the parameters of the generalised extreme value gev gumbel and weibull distributions for extreme water levels along a 600 km coastal stretch in the north eastern baltic sea the parameters of the distributions are evaluated using a three different methods b the output of two different ocean models and c two versions of block maxima for calendar years and for stormy seasons and compared with similar parameters retrieved for measured water level time series at five sites the location and scale parameters of gev and gumbel distributions vary smoothly by about 30 from their average values the variation is modest along the open shores of the baltic sea proper all these parameters increase almost linearly along the southern coast of the gulf of finland the absolute values of the shape parameter of the gev distribution are mostly

pothole and channel system formation in the mcmurdo dry valleys of antarctica new insights from cosmogenic nuclides large pothole and channel features 15 m deep 30 m wide carved into the beacon sandstone in the upland dry valleys of antarctica have been used to infer catastrophic subglacial flooding beneath an expanded east antarctic ice sheet that overran the transantarctic mountains during the mid miocene though the age and erosion rates of these geomorphic features have not been quantified preservation of the potholes and channels has been attributed to negligible erosion under consistent polar desert conditions since the retreat of the ice sheet at 14 ma we present cosmogenic 21 ne and 10 be data

the oldest ice on earth in beacon valley antarctica new evidence from surface exposure dating abstract beacon valley antarctica contains unique remnants of glacier ice underneath a till layer covering the valley floor to constrain the age and evolution of this important indicator of antarctic paleoclimate we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon a conservative minimum exposure age of the older surface sample is 2 3 ma but taking into account erosion the true exposure age of this boulder is likely to be considerably higher the buried sample contains more than 20 times less cosmogenic noble gases than the old surface sample although its current shielding would imply only a three times lower production rate this indicates that the ice level has slowly been lowered by sublimation at the rate of a few m ma the high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in beacon valley was deposited many million years ago sugden et al nature 376 1995 412 414 and has never been thinner than at present in addition we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of 1000 c respectively

hillslope evolution by nonlinear creep and landsliding an experimental study landscape evolution models are widely used to explore links between tectonics climate and hillslope morphology yet mecha nisms of hillslope erosion remain poorly understood here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion in our experimental hillslope disturbance driven sediment transport rates increase nonlinearly with slope due to dilation driven gran ular creep and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding we use spectral analysis to quantify the variability of sediment flux and estimate the slope dependent transition from creep to landsliding the power spec trum of sediment flux steepens with hillslope gradient exhibiting fractal 1 f scaling just below the creep landsliding transition by evolving the experimental hillslope under fixed base level boundary conditions we demonstrate how disturbance driven transport gen erates hillslope convexity the transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep dominated transport

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

quantifying regolith erosion rates with cosmogenic nuclides 10be and 26al in the mcmurdo dry valleys antarctica 1 the mcmurdo dry valleys antarctica mdv are among the oldest landscapes on earth and some landforms there present an intriguing apparent contradiction such that millions of years old surface deposits maintain their meter scale morphology despite the fact that measured erosion rates are 0 1 4 m ma 1 we analyzed the concentration of cosmic ray produced 10be and 26al in quartz sands from regolith directly above and below two well documented ash deposits in the mdv the arena valley ash 40ar 39ar age of 4 33 ma and the hart ash k ar age of 3 9 ma measured concentrations of 10be and 26al are significantly less than expected given the age of the in situ air fall ashes and are best interpreted as reflecting the degradation rate of the overlying sediments the erosion rate of the material above the arena valley ash that best explains the observed isotope profiles is 3 5 0 41 10 5 g cm 2 yr 1 0 19 m ma 1 for the past 4 ma for the hart ash the erosion rate is 4 8 0 21 10 4 g cm 2 yr 1 2 6 m ma 1 for the past 1 ma the concentration profiles do not show signs of mixing creep or deflation caused by sublimation of ground ice these results indicate that the slow steady lowering of the surface without vertical mixing may allow landforms to maintain their meter scale morphology even though they are actively eroding

cosmogenic 10 be and 26 al exposure ages of glaciations in the frankland range southwest tasmania reveal a limited mis 2 ice advance abstract new mapping of the glacial geomorphology coupled with 10be and 26al exposure age dating of moraines on the flanks of the frankland range in south west tasmania indicate that glacier extent during mis 2 was far smaller than during earlier glaciations with the ice cover being confined to only the uppermost cirques of the range moraines further down the range flanks 50 150 m lower in altitude than the mis 2 dated advance indicate that glaciers were only slightly larger during earlier glaciations and depending on the interpretation of their exposure ages may range from mis 7 to mis 12 these older moraines are nested inside the maximum ice limits of an even more ancient and extensive glaciation defined by degraded valley floor moraines and coalescing glacio fluvial fans that remain undated but appear no younger than mis 12 patterns of glacial erosion and moraine deposition on the frankland range suggest that the more recent glaciations were increasingly influenced by the erosional morphology initiated by earlier glaciers microclimatic differences resulting from this earlier glacial topography were particularly influential determinants of glaciation during mis 2 these results are consistent with emerging evidence from studies of other ranges in southwest tasmania

surface exposure chronology of the waimakariri glacial sequence in the southern alps of new zealand implications for mis 2 ice extent and lgm glacial mass balance during the late quaternary the southern alps of new zealand experienced multiple episodes of glaciation with large piedmont glaciers reaching the coastal plains in the west and expanding into the eastern alpine forelands here we present a new 10be exposure age chronology for a moraine sequence in the waimakariri valley n canterbury which has long been used as a reference record for correlating glacial events across new zealand and the wider southern hemisphere our data indicate that the waimakariri glacier reached its maximum last glaciation extent prior to 26 ka well before the global last glaciation maximum lgm this was followed by a gradual reduction in ice volume and the abandonment of the innermost lgm moraines at about 17 5 ka significantly we find that during its maximum extent the waimakariri glacier overflowed the avoca plateau previously believed to represent a mid pleistocene glacial surface i e mis 8 at the same time the glacier extended to a position downstream of the waimakariri gorge some 15 km beyond the previously mapped lgm ice limit we use a simple steady state mass balance model to test the sensitivity of past glacial accumulation to various climatic parameters and to evaluate possible climate scenarios capable of generating the ice volume required to reach the full local lgm extent model outcomes indicate that under new zealand s oceanic setting a cooling of 5 c assuming modern precipitation levels or a cooling of 6 5 c assuming a one third reduction in precipitation would suffice to drive the waimakariri glacier to the eastern alpine forelands canterbury plains our findings demonstrate that the scale of lgm glaciation in the waimakariri valley and adjacent major catchments both in terms of ice volume and downvalley ice extent has been significantly underestimated our observation that high lying glacial surfaces so far believed to represent much older glacial episodes were glaciated during the lgm challenges the conventional geomorphic model of glaciation in new zealand where the vertical arrangement of glacial landform associations is used to assign successively older glaciation ages

granite emplacement during tectonic exhumation the adirondack example sensitive high resolution ion microprobe u pb zircon ages for late to posttectonic leucogranites fix the timing of extensional collapse of a portion of the mesoproterozoic grenville orogen of eastern north america plutons of lyon mountain granite lmg were emplaced within the carthage colton shear zone synchronously with formation of extensional mylonite at 1045 1037 ma leucogranite melts were generated in the hot granulite facies core of the adirondack highlands central granulite terrane that served as the lower plate for down to the northwest extension the lmg suite is associated with hightemperature hydrothermal magnetite deposits in the adirondack highlands and widespread cl 1 co2 hydrothermal alteration of upper plate rocks is localized along the carthage colton shear zone where lmg granites are present the relationships between melt generation granite intrusion high strain rates extensional collapse and hightemperature hydrothermal activity provide a framework for understanding midcrustal processes in modern and ancient orogenic belts

cosmogenic dating of fluvial terraces fremont river utah absolute dating of river terraces can yield long term incision rates clarify the role of climate in setting times of aggradation and incision and establish the rates of pedogenic processes while surface exposure dating using cosmogenic 10be and 26al would seem to be an ideal dating method the surfaces are composed of individual clasts each with its own complex history of exposure and burial the stochastic nature of burial depth and hence in nuclide production in these clasts during exhumation and fluvial transport and during post depositional stirring results in great variability in clast nuclide concentrations we present a method for dealing with the problem of pre depositional inheritance of cosmogenic nuclides we generate samples by amalgamating many individual clasts in order to average over their widely different exposure histories depth profiles of such amalgamated samples allow us to constrain the mean inheritance to test for the possible importance of stirring and to estimate the age of the surface working with samples from terraces of the fremont river we demonstrate that samples amalgamated from 30 clasts represent well the mean concentration depth profiles show the expected shifted exponential concentration profile that we attribute to the sum of uniform mean inheritance and depth dependent post depositional nuclide production that the depth dependent parts of the profiles are exponential argues against significant post depositional displacement of clasts within the deposit our technique yields 10be age estimates of 60 9 102 16 and 151 24 ka for the three highest terraces corresponding to isotope stages 4 5d and 6 respectively the mean inheritance is similar from terrace to terrace and would correspond to an error of 30 40 ka if not taken into account the inheritance likely reflects primarily the mean exhumation rates in the headwaters of order 30 m ma

quaternary downcutting rate of the new river virginia measured from differential decay of cosmogenic 26al and 10be in cave deposited alluvium the concentrations of the cosmogenic radionuclides sup 26 al and sup 10 be in quartz can be used to date sediment burial here we use sup 26 al and sup 10 be in cave deposited river sediment to infer the time of sediment emplacement sediment burial dates from a vertical sequence of caves along the new river constrain its quaternary downcutting rate to 27 3 4 5 m m y and may provide evidence of regional tectonic tilt 32 refs 3 figs 1 tab

examination of surface exposure ages of antarctic moraines using in situ produced 10be and 26al concentrations of 10be t12 1 5 106 y and 26a1 t12 0 72 106 y have been determined by accelerator mass spectrometry ams in a suite of quartz samples taken from sandstone boulders in several moraines in arena valley a dry valley adjacent to the taylor glacier in the quatermain mountains southern victoria land east antarctica these isotopes are produced in surficial quartz by cosmic ray spallation of o and si the concentrations in these samples ranged from 6 1 105 to 3 0 107 at g 1 for 10be and from 9 4 106 to 1 2 108 at g 1 for 26a1 depending upon the extent of exposure at the surface analyses of 10be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons the value of 142 96 41 g cm 2 is consistent with other published values production rates of 17 6 4 at g 1 y 1 for 10be and 113 54 16 at g 1 y 1 for 26a1 at 1300 m and 87 s and a 26al 10be production ratio of 6 5 1 3 1 3 were calculated from the data these values correspond to sea level production rates at high geomagnetic latitude of 6 4 at g 1 y 1 and 41 7 at g 1 y 1 for 10be and 26a1 respectively consistent with determinations based on 11 ky glacially polished surfaces in the sierra nevada in california these production rates imply exposure ages for the various moraines ranging from 50 ky to 2 5 my in accordance with other geological evidence the 10be and 26al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3he examination of the 3he concentrations in conjunction with the present data set yields an estimate of 3he production of 230 8540 at g 1 y 1 corresponding to 85 31 15 at g 1 y 1 at sea level this is lower than previously reported values of 240 60 at g 1 y 1 and 190 at g 1 y 1 scaled to sea level and high geomagnetic latitude based respectively on volcanic flows less than 2000 years old at low latitude and samples collected from 14 400 year old volcanic flows this discrepancy is likely due to diffusive losses which may become significant up to 50 for surface exposure ages on the order of 2 0 my nevertheless the general correlations among the three isotopes suggest that 3he does not suffer gross losses through diffusion

unraveling growth history of zircon in anatectites from the northeast adirondack highlands new york constraints on pressure temperature time paths

estimating the ages of fault scarps in the arava israel abstract systematic age estimations for the numerous fault scarps that rupture the alluvial fans in the southern arava valley israel are rare or nonexistent here we estimate the morphologic ages of several of these faults by the fault scarp degradation method and show that the ages agree well with ages estimated by the independent earlier soil geomorphic studies the values we use for the coefficient of diffusion for the fault scarp degradation process are calibrated by a few numerical ages these ages were determined by the infrared stimulated luminescence irsl method on one of the fault scarps and are the only numerical ages in the region the morphologic ages are all in a correct stratigraphic order and in agreement with field stratigraphic relationships determined by earlier studies they indicate that the studied fault scarps range in age from the latest pleistocene to very recent times the most recent fault scarps which were analyzed are less than 2000 years old and perhaps much younger they indicate that at least two earthquakes with magnitude 6 5 have affected southern israel and jordan during that period it is emphasized that so far we have studied only a limited number of fault scarps and therefore the number of large earthquakes during the last one to two millennia is a minimum

landscape evolution degrades the geologic signature of past glaciations glaciers on land leave behind characteristic landforms and bedrock topography the spatial extent of this signature and dating of the erosional and depositional features has led to a good understanding of temporal and spatial shifts of past glaciers and climate at best the landscape contains considerable information on past glacial processes in the form of moraines and other depositional landforms however evidence suggests that all unconsolidated landforms degrade manifested in the general degradation and smoothing of the topography over time to determine the relative amount and pattern of degradation on glacial moraines we use a well documented moraine degradation model that uses both local slope and a proportionality constant to determine the diamicton transport rate this formulation results in diffusion type topographic smearing and smoothing of the initially angular topographic features more specifically diamicton is eroded at the crest of a moraine transported downhill and deposited on the gentler lower flank collectively this produces moraine cross profiles that closely resemble observations in the field by assuming an initial constant boulder frequency per unit volume of the diamicton the model tracks the surface lowering and the related exhumation of boulders to the moraine surface this produces a tightly constrained set of testable predictions 1 modelled moraine cross profile 2 modelled spatial pattern of surface boulders no boulders on the lower flank and 3 relative frequency of surface boulders smallest frequency of surface boulders at the inflection point highest frequency at the crest these characteristics were measured in the field of alpine tropical and antarctic moraines the correspondence between model predictions and field data is remarkably good and strongly supports our understanding of the time dependent evolution of glacial landscapes these results highlight the evolving nature of glacial landforms and indicate the complete removal of original moraine surfaces and postglacial emergence of spatial pattern in boulder frequency

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

decoupling of erosion and precipitation in the himalayas the hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks1 2 3 most such models invoke a stream power rule in which either increased discharge or steeper channel slopes cause higher erosion rates spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain here we combine observations from a meteorological network across the greater himalaya nepal along with estimates of erosion rates at geologic timescales greater than 100 000 yr from low temperature thermochronometry across a zone of about 20 km length spanning the himalayan crest and encompassing a more than fivefold difference in monsoon precipitation significant spatial variations in geologic erosion rates are not detectable decreased rainfall is not balanced by steeper channels instead additional factors that influence river incision rates such as channel width and sediment concentrations must compensate for decreasing precipitation overall spatially constant erosion is a response to uniform upward tectonic transport of greater himalayan rock above a crustal ramp

linking growth episodes of zircon and metamorphic textures to zircon chemistry an example from the ultrahigh temperature granulites of rogaland sw norway in situ u th pb analyses by ion microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon this approach helps a to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the rogaland anorthosite norite intrusive complex and b to constrain the age of m 2 ultrahigh temperature uht metamorphism and the subsequent retrograde m 3 event all samples yield magmatic inherited zircon of c 1035 ma some an additional group at c 1050 ma this suggests that loss of pb by volume diffusion in non metamict zircon is not an important factor even under extreme crustal conditions furthermore the identical inheritance patterns in aluminous garnet cordierite osumilite bearing migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite results for the m 1 metamorphic event at c 1000 ma bp are consistent in all samples including those from outside the orthopyroxene in isograd the latter do not show evidence for zircon growth during the m 2 metamorphic episode zircon intergrown with or included within m 2 metamorphic minerals magnetite spinel orthopyroxene give an age of 927 7 ma 2 n 20 the youngest observed results are found in zircon outside m 2 minerals some overgrown by m 3 mineral assemblages late garnet coronas garnet quartz and orthopyroxene garnet symplectites and yield a slightly younger pooled age of 908 9 ma 2 n 6 these textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the uht event these youngest age groups are consistent with the emplacement age of the rogaland intrusive complex and the last magmatic activity tellnes dyke intrusion respectively this is direct and conclusive evidence for uht metamorphism in the regional aureole being caused by the intrusions and corrects earlier notions that the events are not linked trace element behaviour of zircon tb u and y content has been tracked through time in the samples and shows variations both within and between samples this heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of rules of thumb in the interpretation of zircon chemistry but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism

detrital zircon constraints on grenville sedimentation at the margin of laurentia abstract the southern grenville province is made up of metaigneous and metasedimentary rocks that were formed and metamorphosed as part of the mesoproterozoic active margin of laurentia grenville supergroup quartzites from the adirondack highlands new york the morin terrane quebec and new jersey highlands were analyzed to determine detrital and metamorphic zircon u pb ages and to help constrain mesoproterozoic tectonics these grenville quartzites have broadly similar detrital age populations as each other suggesting similar sedimentary source regions and depositional settings in back arc basin environments at the laurentian margin during geon 12 all samples contain detrital zircon with 1 45 1 25 ga ages consistent with proximal grenville source regions they also contain detrital ages that point to derivation from the mazatzal 1 7 1 6 ga yavapai 1 8 1 7 ga and penokean 1 9 1 8ga orogens of the north american midcontinent and have a diverse population of archean ages these results and published data from adirondack lowlands metasediments chiarenzelli et al 2015 2017 show distinctly laurentian sources and the related and likely connected nature of the sedimentary histories of the adirondack highlands and lowlands morin terrane and new jersey highlands metamorphic zircon formation is common in high grade quartzites and ages are variable between samples depending on the metamorphic history of each terrane and the melt productivity of different rocks peck et al 2010a adirondack highlands samples experienced metamorphic zircon grown during the shawinigan orogeny and anorthosite suite emplacement 1 19 1 14 ga morin terrane samples are from the morin shear zone which was active and caused metamorphic zircon growth during the ottawan phase of the grenvillian orogeny 1 09 1 02 ga the new jersey highlands sample has metamorphic zircon formed during the rigolet phase of the grenvillian orogeny 1010 980 ma

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle

direct dating of adirondack massif anorthosite by u pb shrimp analysis of igneous zircon implications for amcg complexes the low abundance of igneous zircon in proterozoic massif anorthosites has presented a major obstacle to the acquisition of direct absolute ages of crystallization for these important rocks indirect dating that relies on zircon ages from associated mangerite charnockite granite granitoids assumes that they have a coeval relationship with anorthosite that requires documentation shrimp sensitive high resolution ion microprobe u pb zircon dating techniques provide a powerful means for directly dating the small populations of zircons in anorthositic rocks and for resolving problems with inheritance within the adirondack mountains 10 samples of massif anorthosite have yielded more than sufficient quantities of igneous zircon to establish directly the ages of the region9s classic anorthosite occurrences e g the marcy and oregon dome massifs in addition a ferrogabbro a ferrodiorite and a coronitic olivine metagabbro all crosscutting massif anorthosite were dated the average age of this suite of 13 anorthositic samples is 1154 6 ma mswd mean square of weighted deviates 0 26 probability 0 99 in addition eight associated granitoids have been dated by shrimp techniques and complement another five previously dated by multi grain thermal ionization mass spectrometry tims methods the 13 granitoids yield an average age of 1158 5 mswd 0 89 probability 0 60 and are broadly coeval with the massif anorthosite the overlapping ages provide evidence that these rocks constitute a single composite anorthosite mangerite charnockite granite amcg suite intruded at ca 1155 ma an age corresponding to the ages of major amcg suites in the grenville province in canada e g morin and lac st jean although rocks of the adirondack amcg suite are now documented as broadly coeval it does not follow that the constituent amcg lithologies were comagmatic field relationships and mineral disequilibria in transitional zones are inconsistent with derivation from a single parental magma moreover the presence of older ca 1 2 1 3 ga inherited cores in some zircons from amcg granitoids conflicts with derivation of these rocks from magmas that formed anorthosite gabbro or ferrodiorite or jotunite in which zircons are highly soluble the slightly older ca 1158 ma average age of the mangeritic and charnockitic members of the amcg suite is consistent with an origin as early lower crustal anatectites that left behind pyroxene plagioclase restites this refractory material then reacted by assimilation fractional crystallization afc with ponded mantle derived gabbroic magmas to produce plagioclase rich crystal mushes with crustal isotopic signatures as proposed much earlier by r f emslie these magmas are considered to be parental to the adirondack anorthosite and upon ascent they were emplaced in proximity to still hot earlier mangeritic and charnockitic bodies where they underwent further fractionation the composite nature of the marcy massif documents that this process was repeated in several sequential pulses

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

regolith transport quantified by braking block mcmurdo dry valleys antarctica abstract the mcmurdo dry valleys of antarctica are today a hyper arid polar desert prior work has identified several in situ volcanic ashes 6 11 myr old resting on the surface regolith that suggests the persistent stability of the regolith surfaces and climate however our field observations of characteristic regolith bulges above and cavities below boulders that are lodged in the hillslope braking blocks contradict the apparent preservation of the regolith surfaces to quantify the regolith mobility we modeled the downslope regolith transport around large boulders in the dry valleys using a finite difference regolith transport model to guide our modeling effort we surveyed the detailed topography around one large boulder in the field model results fit the observed topography well and allow for the calculation of the minimum volume of regolith per unit width of slope that was mobilized 4 m3 1 m width to assess the general applicability of the braking block analysis on random boulders on hillslope we surveyed the topographic characteristics adjacent to 997 boulders on 10 separate hillslopes the bulge cavity development appears to be sensitive to 1 the adjacent obstructions that restrict the free movement of the regolith around the given boulder and 2 the inherent surface roughness even though theoretically boulders of all sizes should have a cavity bulge pair surrounding them the signal to noise ratio prevented us from extracting such measurements reliably on boulders whose diameter was smaller than about 1 m on boulders that sat on bedrock covered by a thin veneer of regolith and on boulders that were part of an actively aggrading talus the limiting minimum topographic diffusivity was found to be 10 6 m2 yr over time periods of millions of years this is enough to rework the surfaces and highlights the intriguing contradiction between the preserved ashes and the observed regolith transport

lyon mountain ferroan leucogranite suite magmatic response to extensional thinning of overthickened crust in the core of the grenville orogen

a consistent map of the postglacial uplift of fennoscandia a consistent map of the recent postglacial rebound of fennoscandia is constructed on the basis of sea level records lake level records and repeated high precision levellings the uplift rates calculated from the sea level series form a consistent framework of the map the sea level stations used are 56 reliable stations in the baltic sea and adjacent waters with series spanning 60 years or more many of them about 100 years using a reference station in the baltic sea and another one outside the baltic all results are reduced to a common time span the 100 year period 1892 1991 in order to eliminate oceanographic changes inland uplift differences are obtained from the repeated national levellings and in four of the large lakes from long water level series in pairs the levellings however yield less accurate land uplift values than the sea level and lake level data the resultant map shows a fairly smooth phenomenon with a maximum apparent uplift in the gulf of bothnia of 9 0 mm yr 1 the standard error is typically 0 2 mm yr i close to the sea level stations larger inland finally the pattern of the present uplift as determined here is observed to be very similar to that of the past uplift as determined from ancient shore lines of the litorina sea however the ratio between the past uplift and the present uplift rate tends to increase somewhat towards the uplift centre this might reflect a non uniform mantle viscosity also the uplift maximum seems to have migrated towards nne

mineral zoning p t x m phase relations and metamorphic evolution of some adirondack granulites new york detailed petrologic analysis of ten meta anorthosites and related introduction rocks from the adirondack highlands new york has been used to the adirondack highlands of new york are one of constrain the p t history of the region metamorphic orthopyroxene the best studied granulite facies terranes of the world and clinopyroxene occur in aggregates of 1 mm diameter grains e g bohlen et al 1985 valley et al 1990 and that are zoned concentrically with decreasing al and fe fe mg references therein based on this considerable volume and increasing ca towards the rim moreover zoning is not related of work the peak metamorphic conditions in the to the presence of garnet zoning is interpreted to have been produced adirondack highlands have been fairly well constrained during growth by the reaction igneous pyroxene plagioclase at pressures of 7 8 kbar crustal depths of 25 30 metamorphic opx metamorphic cpx along a cooling path with a km and temperatures that range from 775 c in slope of 6 bars c starting at 850 c and 6 5 8 kbar garnet the central highlands to t 675 c in the southern occurs as isolated clusters of 5 25 grains of 1 mm diameter adirondacks e g bohlen et al 1985 p t paths have crystals and as necklaces of 1 mm diameter crystals surrounding been shown to follow nearly isobaric cooling trajectories metamorphic pyroxenes or oxides plagioclase quartz typically e g bohlen 1987 because of this wealth of inseparates garnet from pyroxene but locally garnet embays metaformation some fundamental questions about the origin morphic pyroxene suggesting that garnet grew at the expense of of the adirondack granulite facies terrane have emerged pyroxene garnet clusters have slightly lower fe fe mg than e g bohlen et al 1985 how did the crust achieve garnet in necklaces fe fe mg 0 73 and 0 75 0 78 rethe relatively high temperature conditions necessary to spectively and slightly higher grossular xgrs 0 20 vs 0 19 produce granulite facies rocks at depths of 25 30 km neither is zoned in mn and zoning towards increasing fe and what is the relative importance of magmatic fe mg is restricted to contacts with biotite and locally clinoheating vs continental burial in this process pyroxene biotite adjacent to garnet is zoned towards decreasing this study will present new data on mineral comfe fe mg production of garnet in necklaces is interpreted position and element zoning in garnet pyroxene to be the result of the continuous fe mg reaction opx pl biotite amphibole and plagioclase from adirondack grt cpx qtz p t x m modeling of this reaction reveals a meta anorthosite and related rocks with the goal of p t path with a slope of 6 bars c no evidence for early prefurther constraining the metamorphic evolution of this metamorphic peak garnet is observed which rules out any possibility terrane mineral compositions will be used to construct of early higher pressure metamorphism the p t path is therefore phase diagrams in an attempt to evaluate approach to inferred to be counterclockwise with loading to 6 5 8 kbar at a chemical equilibrium theoretical analysis will be used maximum temperature of 850 c occurring during or immediately to construct p t x m phase diagrams depicting how following an influx of heat presumably from early orogenic plutons mineral composition and modes change with p and t this theoretical analysis will then be applied to the mineral zoning to constrain the p t path and the results will be discussed in the context of a tectonic

oxygen isotope thermometry using quartz inclusions in garnet oxygen isotope ratios of quartz inclusions within garnet from granulite and amphibolite facies gneisses in the adirondack mountains ny were analyzed and used to determine metamorphic temperatures primary quartz inclusions for eight of 12 samples have 18o values significantly lower than matrix quartz the primary quartz inclusions retain 18o values representative of thermal conditions during garnet crystallization whereas the 18o values of matrix quartz were raised by diffusive exchange with other matrix minerals e g mica and feldspar during cooling the 18o differences between quartz inclusions and matrix quartz show that garnet a mineral with slow diffusion of oxygen can armour quartz inclusions from isotopic exchange with surrounding matrix even during slow cooling these differences between 18o in matrix quartz and quartz inclusions can further be used to test cooling rates by fast grain boundary diffusion modelling criteria for identifying quartz inclusions that preserve primary compositions and are suitable for thermometry were developed based on comparative tests relations between 18o and inclusion size distance of inclusion to host garnet rim core rim zonation of individual inclusions and presence or absence of petrological features healed cracks in quartz inclusions inclusions in contact with garnet cracks lined by secondary minerals and secondary minerals along the inclusion grain boundary were investigated in this study 61 of quartz inclusions preserve primary 18o and 39 were associated with features that were linked to reset 18o values if 18o in garnet is homogeneous and inclusions are removed laser fluorination 18o values of bulk garnet are more precise more accurate and best for thermometry intragrain 18o grt profiles measured in situ by ion microprobe show no 18o zonation almandine rich garnet alm 60 75 from each sample was measured by laser fluorination mass spectrometry lf ms for 18o and compared with ion microprobe measurements of 18o in quartz inclusions for thermometry the 18o qz grt values for adirondack samples range from 2 66 to 3 24 corresponding to temperatures of 640 to 740 c a qz alm 2 71 out of 12 samples that were used for thermometry nine are consistent with previous estimates of peak temperature 625 to 800 c based on petrological and carbon isotope thermometry for regional granulite and upper amphibolite facies metamorphism the three samples that disagree with independent thermometry for peak metamorphism are from the anorthosite mangerite charnockite granite suite in the central adirondacks and yield temperatures of 640 to 665 c approximately 100 c lower than previous estimates these low temperatures could be interpreted as thermal conditions during late post peak crystallization of garnet on the retrograde path this article is protected by copyright all rights reserved

intragrain oxygen isotope zoning in titanite by sims cooling rates and fluid infiltration along the carthage colton mylonite zone adirondack mountains ny usa oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen bearing minerals and the dependence of mineral 18 o 16 o ratios on temperature and fluid composi tion in this study subtle intragrain oxygen isotope zoning in titanite is resolved at the 10 lm scale by secondary ion mass spectrometry the patterns of d 18 o zoning differ depending on microstructural context of individual grains and reflect multiple processes including diffusive oxy gen exchange partial recrystallization grain size reduction and grain growth using the chrono logical framework provided by structural relations these processes can be related to specific events during the grenville orogeny titanite was sampled from two outcrops within the car thage colton mylonite zone ccmz a long lived shear zone that ultimately accommodated exhumation of the adirondack highlands from beneath the adirondack lowlands during the ott awan phase 1090 1020 ma of the grenville orogeny titanite is hosted in the diana metasyenite complex which preserves three sequentially developed fabrics an early nw dipping protomylonitic fabric s1 is crosscut by near vertical ultramylonitic shear zones s2 which are locally reoriented by a nnw dipping mylonitic fabric s3 texturally early titanite pre s2 shows diffusion domi nated d 18 o zoning that records cooling from peak ottawan granulite facies conditions numerical diffusion models in the program fast grain boundary yield good fits to observed d 18 o profiles for cooling rates of 50 20 cm a 1 which are considerably faster than the 1 5 cm a 1 cooling rates previously inferred for the adirondack highlands from regional thermochronology high cool ing rates are consistent with an episode of rapid shearing and exhumation along the ccmz during gravitational collapse of the ottawan orogen at c 1050 ma texturally later titanite syn s2 h as higher overall d 18 o and shows a transition from diffusion dominated to recrystallization dominated d 18 o zoning indicating infiltration of elevated d 18 o fluids along s2 shear zones and continued shear ing below the blocking temperature for oxygen 500 c for grain sizes at the study site the tex turally latest titanite post s3 h as growth dominatedd 18 o zoning consistent with porphyroblastic grain growth following cessation of shearing along the harrisville segment of the ccmz

atmospheric 10be in an antarctic soil implications for climate change 1 concentrations of 10be 9be and salt in a soil profile from the lower wright valley reveal two distinct climatic regimes in the upper horizon of the soil profile a thin gravel lag overlies the hart ash 3 9 0 3 ma and this sits on the surface of a well developed paleosol which makes up the lower horizon of the profile the surface lag has a smaller inventory of 10be than predicted from the fallout rate at nearby taylor dome below the gravel the hart ash is virtually devoid of 10be and the calculated erosion rate for the gravel ash horizon is 2 8 m ma 1 the paleosol which has been sealed by the overlying ash has a relatively high inventory of 10be and this gives it a significantly lower erosion rate 0 5 m ma 1 than the gravel ash horizon beryllium 10 migrates into a soil profile either by solution transport or by fine particle translocation the lack of 10be in the hart ash indicates that neither of these processes have been active at this site for the past 3 9 ma however enrichment of 10be in the paleosol indicates they were active prior to deposition of the ash the most likely reason these processes were active is that there was more water which could translocate fine particles with attached 10be thus the gravel ash horizon developed in climatic conditions similar to the present while the paleosol developed in wetter and possibly warmer conditions during the early pliocene and possibly before that time

chronology of taylor glacier advances in arena valley antarctica using in situ cosmogenic 3he and 10be abstract in situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits in situ3he and 10be in quartz sandstone boulders from arena valley southern victoria land antarctica provide chronological constraints for a sequence of moraines taylor ii ivb moraines related to expansions of taylor glacier and the east antarctic ice sheet mean 3he ages are 113 000 45 000 yr 208 000 67 000 yr 335 000 187 000 yr and 1 2 0 2 myr for taylor ii iii iva and ivb moraines respectively mean 1 corresponding mean 10be ages for taylor ii and ivb moraines are 117 000 51 000 yr and 2 1 0 1 myr for the older deposits the 3he ages are probably lower limits due to diffusive loss although the exposure ages appear consistent with the few previous age estimates particularly with an isotope stage 5 age for taylor ii each moraine exhibits a broad age distribution the distribution probably results from a variety of factors which may include prior exposure to cosmic rays 3he loss erosion postdepositional boulder movement and radiogenic production of 3he nonetheless the exposure ages provide direct chronological constraints for the moraine sequence and suggest a maximum thickening of taylor glacier relative to the present ice surface of 500 m since the late pliocene early pleistocene

dating fluvial terraces with and profiles application to the wind river wyoming abstract fluvial strath terraces provide a record of river incision and the timing of climatic perturbations to the fluvial system dating depositional surfaces like terraces that are older than the range of 14 c however is difficult we employ a cosmogenic radionuclide crn profile technique that addresses a major problem of crn dating on such surfaces nuclide inheritance by measuring 10 be and 26 al profiles we constrain the exposure age and the mean crn inheritance for the deposit the crn profile also yields a self check on the assumptions underlying the method we report our attempts to date terraces along the wind river wy like many sequences of western north american fluvial terraces these are inferred to reflect oscillation between glacial and interglacial conditions in the headwaters previous dating of some of these terraces and the associated terraces and glacial deposits makes this a unique location to compare dating methods dates from five sites along the bull lake glacial correlative terrace wr 3 are 118 125 ka which agrees with dates on bull lake age moraines and independent age estimates on the terrace and is consistent with the model of terrace glacial relationship crn inheritance is significant and highly variable requiring it be considered despite the additional sampling complexity assuming all inheritance in wr 3 deposits arises during exhumation in the headwaters we obtain minimum mean rates of exhumation of 13 130 m my for the source rocks alternatively assuming the crns are inherited during clast transport the time of fluvial transport from source to terrace is 10 ka it increases downstream and is lower for sand than cobbles the crn ages for older terraces wr 7 300 ka and wr 15 510 ka are lower by 50 than previous estimates based on tephrochronology the most plausible explanation is eolian deflation of a once thicker loess cover on the terrace surfaces mean thicknesses of loess of 0 5 1 5 m are required to reconcile these concentrations of crn with the previous estimates of age difficulty in dating the older terraces emphasizes that geologic caution independent estimates of age and multiple sample sites should still be part of dating depositional surfaces with crns even when employing the inheritance correction technique

mean sea surface topography in the baltic sea and its transition area to the north sea a geodetic solution and comparisons with oceanographic models a consistent height system designed for comparisons between geodesy and oceanography is presented for the baltic sea area mean sea surface topography is computed geodetically in this height system at 42 reliable long term sea level stations connected by high precision levelings along the coasts of the baltic sea the kattegat the skagerrak and the adjacent part of the north sea based on this a map is drawn of the mean sea surface topography in the baltic sea and its transition area to the north sea the general agreement between our geodetic solution and recent oceanographic models turns out to be excellent the discrepancies rarely exceed 2 3 cm hence the mean sea surface topography in the baltic sea area to a dominating extent is caused by the distribution of salinity there are two main characteristics of the sea surface topography found here first there is a continuous increase of the sea surface height from the north sea into the baltic sea the height difference between the inner part of the gulf of bothnia and the skagerrak amounting to 35 40 cm the main reason behind this is the considerable difference in salinity close to the maximum possible one second there is a steep sea level gradient in the border zone between the kattegat and the skagerrak reaching 2 cm per 10 km this reflects the salinity front there separating the brackish baltic sea water from the saline north sea water and the associated baltic current a local maximum in the sea surface can be seen in the oslo fiord reflecting an accumulation of low salinity water there an area where there is still no oceanographic model agreeing sufficiently with the geodetic solution is the belt sea southwestern kattegat

contribution of regional climate drivers to future winter sea level changes in the baltic sea estimated by statistical methods and simulations of climate models a statistical downscaling approach is applied to the output of five different global climate model simulations driven by twenty first century future scenarios of greenhouse gas concentrations the contribution of sea level pressure slp and precipitation changes to regional future winter sea level changes is estimated for four baltic sea level stations by establishing statistical relationships between sea level as predictand and large scale climate fields as predictors using slp as predictor for the central and eastern baltic sea level stations three climate models lead to statistically significant twenty first century future trends in the range of the order of 1 2 mm year using precipitation as predictor for the stations in the southern baltic coast all five models lead to statistically significant trends with a range of the order of 0 4 mm year these numbers are smaller but of the order of magnitude as the predicted global sea level rise

late ottawan extension in the eastern adirondack highlands evidence from structural studies and zircon and monazite geochronology syn to postorogenic collapse is increasingly being recognized as a fundamental process in most orogens although the mechanism of collapse at mid to lower crustal levels is not well understood the adirondack highlands of the grenville province in northern new york are an exhumed granulite terrane and represent an ideal setting in which to investigate mid to lower crustal orogenic processes although the highlands experienced high grade metamorphism and crustal thickening during the ottawan orogeny few extensional shear zones have been recognized in this region we document a new shear zone in the eastern adirondack highlands that accommodated postorogenic extensional collapse during the late ottawan the shear zone accommodated extension via noncoaxial top to the southeast shear although we cannot rule out a significant coaxial vertical thinning component some earlier thrust motion may also have occurred in the zone u pb sensitive high resolution ion microprobe zircon and in situ electron probe u th pb monazite ages demonstrate that extension occurred from ca 1050 to 1025 ma following peak metamorphism during the ottawan orogeny our results suggest that late ottawan extension in the adirondack highlands was more widespread than previously recognized and was likely driven by gravitational collapse extension along this eastern adirondack highlands shear zone may connect with the tawachiche shear zone to the north in quebec indicating that tectonic exhumation during the ottawan occurred on a system of interconnected shear zones top to the northeast extension in the eastern adirondack highlands was synchronous with top to the northwest extension on the carthage colton shear zone in the western adirondack highlands suggesting that the highlands granulite terrane may have been partially exhumed as a large scale symmetrical gneiss dome

timing of anatexis in metapelites from the adirondack lowlands and southern highlands a manifestation of the shawinigan orogeny and subsequent anorthosite mangerite charnockite granite magmatism anatectic leucogranites are common in metapelites within both the highlands and lowlands terranes of the adirondack mountains of northern new york state the formation of these igneous bodies which are folded in the lowlands and commonly mylonitized in the highlands has been widely considered an event accompanying the ca 1050 ma ottawan orogeny during which metamorphic grade reached granulite facies in the highlands while the lowlands experienced amphibolite facies metamorphism sensitive high resolution ion microprobe shrimp analyses of zircons separated from leucosomes and melanosomes in both the southern highlands and the lowlands indicate that primary anatexis occurred ca 1180 1160 ma and is thus a manifestation of heating during the earlier shawinigan orogeny ca 1210 1160 ma and associated anorthosite mangerite charnockite granite amcg magmatism ca 1165 1150 ma the absence of ottawan overgrowths on shawinigan zircons in these leucosomes suggests that by ottawan time the rocks were too dry for further melting or zircon growth to occur however electron microprobe analyses of monazites from the southern highlands reveal multiple age zones including cores with ages of ca 1170 1180 ma consistent with primary growth during shawinigan orogenesis complex zones formed ca 1140 1155 ma during amcg magmatism and ca 1050 1020 ma formed during ottawan orogenesis and high grade metamorphism throughout the adirondacks leucosomes and melanosomes contain older ca 1320 ma zircons that are considered to be remnant detrital zircons derived from arc rocks of the elzevirian terrane the apparent absence of archean detrital zircons suggests that the protoliths of the metapelites were deposited in restricted basins that did not receive detritus from the superior craton

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

surface dating of dynamic landforms young boulders on aging moraines the dating of landforms is crucial to understanding the evolution history and stability of landscapes cosmogenic isotope analysis has recently been used to determine quantitative exposure ages for previously undatable landform surfaces a pioneering application of this technique to date moraines illustrated its considerable potential but suggested a chronology partially inconsistent with existing geological data consideration of the dynamic nature of landforms and of the ever present processes of erosion deposition and weathering leads to a resolution of this inconsistency and more generally offers guidance for realistic interpretation of exposure ages

numerical ages for plio pleistocene glacial sediment sequences by 26al 10be dating of quartz in buried paleosols abstract we describe a method for dating plio pleistocene sediments by analysing 26 al and 10 be in quartz from buried paleosols the method amounts to a generalization of the technique of burial dating which has previously been used to date river sands deposited in caves as well as fluvial terraces we a measure nuclide concentrations at multiple depths in the paleosol b use the geologic context of the sample to construct an exposure burial history an exposure model that can be used to predict the nuclide concentrations in the samples as a function of whatever unknown parameters are of geological interest e g the ages of geologic units and then c find the parameters in the exposure model that result in the best fit between predicted and observed nuclide concentrations we apply the method to a paleosol developed on the uppermost till in eastern nebraska which is buried by several middle to late pleistocene loess units the ages of some of which are known independently the lowest loess unit whose age was not previously known was deposited 0 58 0 12 ma and the till was emplaced 0 65 0 14 ma which agrees with existing age constraints from paleomagnetic measurements and the position of the lava creek ash in the regional stratigraphy the most important source of uncertainty in these age determinations is the analytical uncertainty in 26 al and 10 be measurements uncertainties in either the nuclide production rates or the geologic model are comparatively unimportant

pliocene pleistocene incision of the green river kentucky determined from radioactive decay of cosmogenic 26al and 10be in mammoth cave sediments cosmogenic 26al and 10be in sediments washed into mammoth cave kentucky record the history of 3 5 m y of water table position governed by incision and aggradation of the green river a tributary of the ohio river upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2 3 2 4 ma a brief surge of river incision ca 2 ma was followed by river stability and cave passage formation at a lower level rapid incision through 15 m of bedrock ca 1 5 ma was prompted by repositioning of the ohio river to its present course along an ice sheet margin renewed incision ca 1 2 ma and aggradation at 0 7 0 8 ma correlate with major ice advances in the ohio river basin measurements of 26al and 10be also indicate that sandstone capped uplands have maintained slow erosion rates of 2 7 m m y for the past 3 5 m y despite accelerated pleistocene river incision rates of 30 m m y

topographic and microclimatic impacts on glaciation of the denison range southwest tasmania abstract morphostratigraphic relationships between landforms and sediments comparative boulder weathering status and paired 10 be and 26 al exposure ages indicate the occurrence of at least three separate glaciations that are interpreted as dating from mis 2 6 and possibly 8 the rhona glacier remained at or close to its greatest lgm extent until 17 6 ka due to a very favourable micro climate that resulted from local topography that these factors were sufficient to over power relatively subtle zonal temperature shifts that caused significant changes in more exposed tasmanian glaciers implies a need for caution in correlating between glacier systems in markedly different topographic settings when environmental conditions are only marginally supportive of glaciation no evidence was detected for any greater ice extent during mis 3 or mis 4 than during mis 2 but much more extensive glaciations occurred during earlier glacial climatic cycles reduced local precipitation in parallel with increasing aridity of the australian continent may account for the progressively diminishing maximum extent of glaciers during the latter part of the pleistocene and it would have emphasized the importance of local topography and microclimate for the glaciers of southwest tasmania

evidence for slow late glacial ice retreat in the upper rangitata valley south island new zealand abstract a suite of cosmogenic radionuclide ages taken from boulders on lateral and latero terminal moraines in the rangitata valley eastern south island new zealand demonstrates that relatively thick ice occupied valley reaches inland of the rangitata gorge until c 21 ka thereafter ice began to thin and by c 17 ka it had retreated 33 km up valley of the rangitata gorge to the butler brabazon downs a structurally created basin in the upper rangitata valley despite its magnitude this retreat represents a minor ice volume reduction from 21 ka to 17 ka and numerous lateral moraines preserved suggest a relatively gradual retreat over that 4 ka period in contrast to records from adjacent valleys there is no evidence for an ice collapse at c 18 ka we argue that the rangitata record constitutes a more direct record of glacial response to deglacial climate than other records where glacial dynamics were influenced by proglacial lake development such as the rakaia valley to the north and the major valleys in the mackenzie basin to the south west our data supports the concept of a gradual warming during the early deglaciation in the south island new zealand

the geoid edin2000 and mean sea surface topography around the british isles summary this paper describes a new rationale for computing a gravimetric geoid and reports that the resulting geoid model edin2000 is now good enough to resolve previously contradictory estimates for mean sea surface topography msst around great britain for at least 30 yr it has been known that msst derived from tide gauges levelling and oceanography were mutually inconsistent and differed by an order of magnitude when combined with an altimetric mean sea surface edin2000 is also able to map realistic msst over the northwest european shelf and identify and quantify mean currents flowing in the deep ocean parallel to the shelf edge when compared with 11 gps referenced tide gauges on the british mainland our estimate of msst has a standard deviation of only 0 03 m and its variation with latitude is not significantly different from zero for these sites long distance levelling errors are larger than geoid errors we identify a systematic jump by 0 24 0 05 m in the levelled transfer of ordnance datum newlyn affecting all sites in great britain to the north of latitude 53 n even after this correction the variability of levelling remains larger than that of the geoid and gps implying a more complex structure to the levelling error than a simple step away from coasts we find that the best way to validate marine gravity data is their ability to predict msst with low local variability when combined with altimetric mean sea level there is a large reduction in msst variability when marine gravity data are subjected to an anti aliased adjustment of the mismatch where ship tracks cross we also report two msst model comparisons from a shelf seas and a deep ocean model both of high resolution there is quantitative and qualitative agreement between our geoid derived msst and the model predictions indicating significant improvements compared with earlier geoid models we also show that with the improved accuracy of geoid based msst it will become necessary to validate models and observations at matching epochs

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in