holocene coastal glaciation of alaska holocene uctuations of the three cirque glaciers on the seward peninsula and ve groups of tidewater and land terminating glaciers along the northernmost gulf of alaska provide a proxy record of late holocene climatic change furthermore the movements of the coastal glaciers were relevant to late holocene native american migration the earliest expansion was recorded about 6850 yr bp by hubbard glacier at the head of yakutat bay in the gulf of alaska however its down fjord advance to the bay mouth was delayed until 2700 bp similarly expansions of the icy bay bering and mccarty glaciers occurred near their present termini by 3600 3000 bp compatible with marked cooling and precipitation increases suggested by the alaskan pollen record decrease in glacier activity 2000 bp was succeeded by advances of gulf coastal glaciers between 1500 and 1300 bp correlative with early medieval expansions across the northern hemisphere a medieval optimum encompassing at least a few centuries prior to ad 1200 is recognized by general retreat of land terminating glaciers but not of all tidewater glaciers little ice age advances of land based glaciers many dated with the precision of tree ring cross dating were centered on the middle 13th or early 15th centuries the middle 17th and the last half of the 19th century a d strong synchrony of these events across coastal alaska is evident 2000 elsevier science ltd all rights reserved

a model of the western laurentide ice sheet using observations of glacial isostatic adjustment we present the results of a new numerical model of the late glacial western laurentide ice sheet constrained by observations of glacial isostatic adjustment gia including relative sea level ind

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

space geodetic and water level gauge constraints on continental uplift and tilting over north america regional convergence of the ice 6g_c vm5a vm6 models

a relative sea level history for arviat nunavut and implications for laurentide ice sheet thickness west of hudson bay abstract thirty six new and previously published radiocarbon dates constrain the relative sea level history of arviat on the west coast of hudson bay as a result of glacial isostatic adjustment gia following deglaciation sea level fell rapidly from a high stand of nearly 170 m elevation just after 8000 cal yr bp to 60 m elevation by the mid holocene 5200 cal yr bp the rate of sea level fall decreased in the mid and late holocene with sea level falling 30 m since 3000 cal yr bp several late holocene sea level measurements are interpreted to originate from the upper end of the tidal range and place tight constraints on sea level a preliminary measurement of present day vertical land motion obtained by repeat global positioning system gps occupations indicates ongoing crustal uplift at arviat of 9 3 1 5 mm yr in close agreement with the crustal uplift rate inferred from the inferred sea level curve predictions of numerical gia models indicate that the new sea level curve is best fit by a laurentide ice sheet reconstruction with a last glacial maximum peak thickness of 3 4 km this is a 30 35 thickness reduction of the ice 5g ice sheet history west of hudson bay

insar measurements of surface deformation over permafrost on the north slope of alaska 1 ground based measurements of active layer thickness provide useful data for validating calibrating remote sensing and modeling results however these in situ measurements are usually site specific with limited spatial coverage here we apply interferometric synthetic aperture radar insar to measure surface deformation over permafrost on the north slope of alaska during the 1992 2000 thawing seasons we find significantly systematic differences in surface deformation between floodplain areas and the tundra covered areas away from the rivers using floodplain areas as the reference for insar s relative deformation measurements we find seasonally varying vertical displacements of 1 4 cm with subsidence occurring during the thawing season and a secular subsidence of 1 4 cm decade we hypothesize that the seasonal subsidence is caused by thaw settlement of the active layer and that the secular subsidence is probably due to thawing of ice rich permafrost near the permafrost table these mechanisms could explain why in situ measurements on alaskan north slope reveal negligible trends in active layer thickness during the 1990s despite the fact that atmospheric and permafrost temperatures in this region increased during that time this study demonstrates that surface deformation measurements from insar are complementary to more traditional in situ measurements of active layer thickness and can provide new insights into the dynamics of permafrost systems and changes in permafrost conditions

zircon radiation damage ages abstract the suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s in the original technique radiation damage was determined by x ray diffraction however this is not suitable for the small sample analysis needed in modern geochronology it is now possible to measure radiation damage and u and th contents on micron sized areas on the polished surface of zircon grains using raman spectroscopy and sims mass spectrometry this opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re examine the technique through its application to zircons from two granitoids from the archean yilgarn craton and ancient detrital zircons from the jack hills in western australia the three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence the 420 110 ma radiation damage age for the darling range granite is coincident with 500 400 ma biotite rb sr ages in the same region of the yilgarn craton likewise a tonalite from the youanmi terrane in the craton with a 1650 150 ma zircon radiation damage age lies within the domain of a 1600 ma event recorded by biotite rb sr ages the jack hills zircons have a 1120 130 ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite ar ar ages of about 1140 ma the positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events

constraining a shrimp u pb age micro scale characterization of zircons from saxonian rotliegend rhyolites we present results of a detailed investigation of zircons from two rhyolites from st egidien and chemnitz saxony using a combination of microprobe techniques shrimp ion probe raman microprobe sem se bse and cl imaging these rhyolites belong to the so called lower volcanics which is the older of two series of late variscan volcanic rocks occurring in the saxonian sub erzgebirge basin germany the purpose of the present contribution is to demonstrate that detailed characterization of zircons as provided by the different micro techniques facilitates soundest interpretation of geochronological data the zircons at most 40 to 80 m in size show oscillatory growth zoning with reversely correlated cl and bse signal intensities these zircons are interpreted to have grown during crystallization of the rhyolite because apart from some cracking they do not appear to have experienced any alteration since the time of their growth the shapes of the zircons and their internal structures revealed by cl and bse imaging appear to be magmatic and neither annealing of the accumulated alpha decay damage nor disturbance of the u pb system is observed the shrimp ion probe measurements on the zircons gave a permian 206pb 238u age of 278 5 ma 95 confidence the concordance of this age is supported by the correlation between the low degrees of metamictization estimated from raman parameters and the accumulated alpha fluxes calculated from shrimp data the 278 ma zircon age is interpreted to represent the age of the lower rhyolites series and with that the age of postkinematic late variscan volcanism in the sub erzgebirge basin which has been related to anorogenic extension and uplift as a result of intracontinental rifting because of genetic association of rhyolites in the sub erzgebirge basin and li f granites and lamprophyres in the neighbouring erzgebirge the rhyolite age also indirectly contributes to the understanding of the geological history of the erzgebirge the 278 ma age is the first absolute dating result for rhyolites from the saxonian sub erzgebirge basin

space geodetic constraints on glacial isostatic adjustment in fennoscandia analysis of global positioning system gps data demonstrates that ongoing three dimensional crustal deformation in fennoscandia is dominated by glacial isostatic adjustment our comparison of these gps observations with numerical predictions yields an earth model that satis es independent geologic constraints and bounds both the average viscosity in the upper mantle 5 x 10 20 to 1 x 10 21 pascal seconds and the elastic thickness of the lithosphere 90 to 170 kilometers we combined gps derived radial motions with fennoscandian tide gauge records to estimate a regional sea surface rise of 2 1 0 3 mm year furthermore ongoing horizontal tectonic motions greater than 1 mm year are ruled out on the basis of the gps derived three dimensional crustal velocity field

insights into orogenic processes from drab schists and minor intrusions southern sao francisco craton brazil abstract minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the archaean and palaeoproterozoic southern sao francisco craton ssfc brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly three magmatic pulses are documented i archaean and ii palaeoproterozoic felsic intrusions both hosted by archaean protoliths and iii palaeoproterozoic felsic intrusions in palaeoproterozoic supracrustal sequences archaean felsic intrusions confirm the palaeoarchaean age of the mafic ultramafic sequence of the rio das velhas greenstone belt and rhyacian intrusions mark the collisional stage of the mineiro belt with the sfc at c 2130 ma greenstone belt schists show a wide distribution of rounded soccer ball archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 ma with an interpreted overprinting high grade metamorphic event at c 2700 to 2680 ma most high grade metamorphic rims have th u 0 1 negative ehf t values and ree pattern consistent with eclogite granulite metamorphic facies reinforcing the hypothesis of a dehydrated refractory crust formed during the stabilization of the ssfc even though no such protoliths are preserved this event links crustal thickening and partial melting of archaean lower crust archaean rutile crystals from the greenstone belt schist were reset during the palaeoproterozoic event but still preserve the early archaean high grade metamorphic signature the presence of unstable ilmenite replaced by rutile in the schist associated to felsic intrusions with the same age at c 2130 ma suggest high pressure low temperature prograde metamorphism during the collisional stage of the palaeoproterozoic orogen elongate and prismatic zircon grains from the rhyacian intrusions have low ehf t signature and crystallised from partial melting of sedimentary protoliths accretionary events produced thicker and more differentiated crust by the end of rhyacian time easily overlooked rocks in this study when studied have revealed a rich multi event history of cratonic evolution

horizontal motions due to post glacial rebound north american horizontal post glacial rebound velocities are computed for three five layered incompressible spherical maxwell earth models using a simple glacial loading cycle centered on hudson bay the reference earth model has a 120 km thick elastic lithosphere and upper and lower mantle viscosities of 1021 pa s and 2 1022 pa s respectively the second model has a lower viscosity upper mantle 5 1020 pa s and a higher viscosity lower mantle 1023 pa s and the third has a 250 km thick lithosphere the reference model and the low viscosity upper mantle model calculations are in good agreement with uplift and gravity data although they may somewhat overestimate the present day central uplift rate the thick lithosphere model fares less well because it fails to exhibit the well pronounced peripheral bulge sinking that is required by eastern north america tide gauge data the reference model generally has the largest horizontal rebound velocities reaching a maximum of 4 mm yr towards hudson bay at 2400 km from the load center the effect of a thick lithosphere or of a low viscosity upper mantle is to reduce present day horizontal velocities especially in the formerly glaciated region the sensitivity of horizontal motions to lithospheric thickness and upper mantle viscosity may assist in discriminating between different models of the earth s rheology once sufficient high quality geodetic data exist

using global positioning system derived crustal velocities to estimate rates of absolute sea level change from north american tide gauge records 1 we identified 37 tide gauges each located within 40 km of a geodetic station whose international terrestrial reference frame of 2000 itrf2000 crustal velocity had been rigorously derived from continuous global positioning system gps observations spanning from 3 to 11 years the tide gauges are located along the coasts of north america bermuda hawaii and kwajalein in the marshall islands we obtained the itrf2000 crustal velocities by averaging values from six solutions each produced by a team of investigators acting essentially independently of the other teams we then applied crustal velocities to convert rates of relative sea level change to rates of absolute sea level change in a sample containing 30 sites we found that the mean rate of absolute sea level change equals 1 80 0 18 mm yr in the 1900 1999 period the scatter about the mean for individual sites in this sample is characterized by a weighted rms value of 0 85 mm yr this scatter primarily reflects the uncertainty associated with derived crustal velocities the remaining seven sites i e five sites on the pacific coast of alaska one on dauphin island alabama and one on kwajalein an atoll in the pacific ocean experienced relatively low rates of absolute sea level change we hypothesize the low rates in alaska are caused by ongoing melting of mountain glaciers and ice masses near the stations while the low rates found for dauphin island and kwajalein remain unexplained

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

low s velocities under the tornquist teisseyre zone evidence for water injection into the transition zone by subduction a strong low s velocity anomaly at 300 500 km depth coincides with the western boundary of the russian platform the anomaly is too large to be explained by a simple temperature anomaly or by compositional variations nor is it an artifact induced by seismic anisotropy we present a model that explains the anomaly through the injection of water at the time of closure of the tomquist ocean that separated the continents of avalonia and baltica in the early paleozoic dense hydrous magnesium phases are the most likely agents for transporting water to the transition zone but an important role may also be played by nominally anhydrous clinopyroxene when these minerals are brought out of their stability field water is released it may accumulate in spinel or k amphibole and may be released when the mantle warms up after subduction halts the effect of this water is to induce weakening of the shear modulus or even creation of a heavy melt by a conservative estimate a subduction episode lasting 85 m y would inject enough water to saturate a large volume of mantle rock with 0 3 h2o low velocities or low q anomalies have also been observed in the transition zones near currently active slabs

nearly contemporaneous evolution of the a and s type fractionated granites in the krusne hory erzgebirge mts central europe abstract the krusne hory erzgebirge mountains is one of the best known provinces of granite magmatism and associated metallogenesis in europe variscan magmatic activity in the krusne hory erzgebirge area spanned the period from 330 to 295 ma for much of this time two types of magma were generated and emplaced in close proximity in this province strongly peraluminous p rich s type melts were formed alongside slightly peraluminous p poor melts a type two suites of strongly peraluminous p f li rb cs u sn rich and zr th y hree poor magmas were intruded in rapid succession over a period of about 10 ma about 330 320 ma over the entire region of the krusne hory several peraluminous rhyolitic dykes near gottesberg mark an isolated event at about 295 ma slightly peraluminous f li rb cs sn zr y hree rich and p poor magmas were intruded in several events separated in space and time 325 295 ma both magmatic episodes culminated with strongly fractionated subvolcanic granite intrusions accompanied by explosive brecciation and followed by sn w mineralization of greisen type successive volcanic eruption of rhyolites and dacites of s type and rhyolites of a type form the fill of the altenberg teplice caldera atc thus demonstrating the close temporal and spatial relationship between these two types of magma when these two geochemically distinct types of rock are in direct contact then the rocks of a type are always younger the absolute time difference between the two types can range from less than 1 ma the first eruption of the teplice rhyolite versus the schonfeld dacite in the atc to about 15 ma eruption of rhyolites at gottesberg versus emplacement of the eibenstock granite the assumed protolith of all late variscan granites in the area of krusne hory erzgebirge is a mixture of fertile quartzo feldspathic rocks with micaceous metapelites enriched in lile and sn w the differences in chemical composition of s and a type melts originated from local changes in the proportion of quartzo feldspathic and pelitic lithologies different pt conditions of melting the degree of melting and the degree of metamorphic dehydratation of the protolith prior to the melting

post glacial sediment load and subsidence in coastal louisiana 1 sea level rise in the gulf of mexico has occurred at a rate of 1 8 2 2 mm yr during the 20th century or nearly the same as observed globally due to combined steric and water mass changes tide gauges in coastal louisiana however record a substantially larger rate of rise and while a number of causal mechanisms may be responsible their specific contribution is poorly understood using a realistic viscoelastic earth model detailed geologic parameters for south louisiana and new gps data we demonstrate that holocene sedimentary loading in the gulf and mississippi river delta is capable of contributing to 1 8 mm yr of subsidence over areas of 30 0 75 103 km2

evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska 1 global positioning system gps measurements are used to examine the hypothesis of a clockwise rotating bering plate originally proposed on the basis of seismicity the bering plate encompasses the bering sea western alaska and the aleutian islands gps measurements from the bering plate s interior show south to southwest motions of 3 5 mm a where a is years we construct elastic dislocation models to determine the spatial distribution and intensity of locked patches on the aleutian subduction interface we use these to remove interseismic strain from the gps observation and determine an arc translation velocity for each region of the aleutians revealing south and southwest motions of 4 8 mm a we combine the arc translation rates with measurements from the bering plate s interior sites and estimate the euler pole for the bering plate relative to north america to be located at 42 5 n 121 3 e with an angular speed of 6 0 ma the clockwise rotation of the bering plate may cause left lateral faulting in interior alaska the bering plate s interaction with south central alaska may be responsible for the decreased slip rate on the western denali fault and for contraction across the central alaska range we analyze slip partitioning along the aleutian arc on the basis of both gps measurements and slip azimuths of thrust earthquakes and find a systematic discrepancy between plate convergence direction and slip azimuths we conclude that slip partitioning in the back arc only develops west of amchitka pass whereas slip partitioning in the fore arc is present throughout the arc

reevaluation of the viscoelastic and elastic responses to the past and present day ice changes in southeast alaska abstract we have reevaluated the viscoelastic and elastic responses to the past and present day ice changes in southeast alaska se ak based on a gps data set by combining the data previously published in 2005 with additional uplift rates compiled recently there are 22 new gps sites yielding a total of 91 sites the mean observation epoch of the data sets is the middle of 2003 here we considered three ice unloading effects due to the last glacial maximum lgm the little ice age lia and the present day ice melting pdim we also examined the dependency of the estimated lithospheric thickness and the asthenospheric viscosity on pdim models for this test two pdim models were used one is a model called here the uaf05 ice model whose mean epoch is the mid 1970s and the other is a model called here the uaf07 model whose mean epoch is the mid 1980s statistical examinations of the misfits between observations and model predictions indicate that uaf07 gives much more consistent results than uaf05 based on the magnitude of the minimum 2 and the agreement of the residual distribution with a gaussian function we have tested two different kinds of mantle viscosity structure a 2 layer model similar to the model used the paper previously published in 2005 and a 4 layer model for the 2 layer model at the 95 confidence level the optimum values for the lithospheric thickness and the asthenospheric viscosity with a thickness of 110 km are 54 45 65 km and 5 58 4 0 12 0 in unit of 10 18 pa s respectively on the other hand for the 4 layer model the best values were 60 42 63 km and 10 0 3 0 13 in unit of 10 18 pa s respectively the comparison between the analysis results for gps data and the model computations indicates that the observations detect not only the uplift rates in se ak but also their time variations which are considered to be due to the glacier changes after the 1990s

modeling 3 d crustal velocities in the united states and canada a numerical model for three dimensional 3 d crustal velocities has been derived for most of the united states and canada primarily from repeated geodetic data this model provides a foundation for a prototype of the trans4d software trans4d is being developed to enable geospatial professionals and others to transform 3 d positional coordinates across time the derived model reveals several macroscopic features of the 3 d velocity field including the pervasive presence of the glacial isostatic adjustment associated with the past melting of the ice fields that formed more than 19 000 years ago during the last glacial maximum in this study the present day 3 d velocity field associated with this melting as estimated via the recently published ice 6g c vm5a model was subtracted from this study s total 3 d velocity field to identify features of the residual velocity field in particular this study introduces the na ice 6g reference frame in which residual horizontal velocities have magnitudes that are less than 2 mm yr everywhere east of longitude 104 w and south of latitude 60 n except in southern texas residual horizontal velocities of greater magnitude are found west and or north of these two boundaries and they are due mostly to interactions among tectonic plates with localized pockets due to other geophysical phenomena large residual vertical velocities some with values exceeding 30 mm yr are found in southeastern alaska the uplift occurring here is due to present day melting of glaciers and ice fields formed during the little ice age glacial advance that occurred between 1550 a d and 1850 a d

pan arctic ice wedge degradation in warming permafrost and its influence on tundra hydrology ice wedges are common features of the subsurface in permafrost regions they develop by repeated frost cracking and ice vein growth over hundreds to thousands of years ice wedge formation causes the archetypal polygonal patterns seen in tundra across the arctic landscape here we use field and remote sensing observations to document polygon succession due to ice wedge degradation and trough development in ten arctic localities over sub decadal timescales initial thaw drains polygon centres and forms disconnected troughs that hold isolated ponds continued ice wedge melting leads to increased trough connectivity and an overall draining of the landscape we find that melting at the tops of ice wedges over recent decades and subsequent decimetre scale ground subsidence is a widespread arctic phenomenon although permafrost temperatures have been increasing gradually we find that ice wedge degradation is occurring on sub decadal timescales our hydrological model simulations show that advanced ice wedge degradation can significantly alter the water balance of lowland tundra by reducing inundation and increasing runoff in particular due to changes in snow distribution as troughs form we predict that ice wedge degradation and the hydrological changes associated with the resulting differential ground subsidence will expand and amplify in rapidly warming permafrost regions the polygonal patterns in permafrost regions are caused by the formation of ice wedges observations of polygon evolution reveal that rapid ice wedge melting has occurred across the arctic since 1950 altering tundra hydrology

gravity and uplift rates observed in southeast alaska and their comparison with gia model predictions gravity change rates unit mgal yr 1 mgal 10 8 ms 2 over the 6 sites are estimated to be 4 50 0 76 and 4 30 0 92 by only using our data and also using the 1987 data respectively we computed the uplift and gravity rates predicted by ice load models for three different time intervals last glacial maximum lgm little ice age lia and present day pd except for 1 2 examples the predictions recover the observed rates within the observation errors we also estimated the viscous portion of the ratio unit mgal mm of the observed gravity rate to the uplift rate by correcting for the effects of the present day ice mass change pdimc two pdimc models are compared which are called here as uaf05 and uaf07 mean ratios are estimated to be 0 205 0 089 and 0 183 0 052 for the cases using uaf05 and uaf07 respectively the predicted mean ratios are 0 166 0 001 and 0 171 0 002 for the cases using both the lga and lia ice models and only using the lia ice model respectively we have confirmed that our ag and gps observations detect the rates and ratios reflecting an early stage of viscoelastic relaxation mainly due to the unloading effects after the lia

dating multiply overprinted sn mineralized granites examples from the erzgebirge germany granites and primary tin mineralization in the erzgebirge were dated using 1 conventional u pb dating of uraninite inclusions in mica 2 rb sr dating of inclusions in quartz that represent highly evolved melts 3 re os dating of magmatic hydrothermal molybdenite and 4 chemical th u pb dating of uraninite conventional isotope dilution and thermal ion mass spectrometry and chemical th u pb dating of uraninite in granites from the ehrenfriedersdorf mining district provide ages of 323 9 3 5 ma 2 greifenstein granite and 320 6 1 9 and 319 7 3 4 ma 2 both sauberg mine in agreement with u pb apatite ages of 323 9 2 9 and 317 3 1 6 ms 2 both sauberg mine rb sr analysis of melt inclusions from zinnwald gives highly radiogenic sr isotopic compositions that with an assumed initial sr isotopic composition permit calculation of precise ages from single inclusions the scatter of the data indicates that some quartz hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt inclusion re os dating of two molybdenite samples from altenberg provides ages of 323 9 2 5 and 317 9 2 4 ma 2 together with age data from the literature our new ages demonstrate that primary tin mineralization and the emplacement of the large sn specialized granites in the erzgebirge fall in a narrow range between 318 and 323 ma primary sn mineralization occurred within a short interval during post collisional collapse of the variscan orogen and was essentially synchronous over the entire erzgebirge in contrast to earlier claims no systematic age difference between granites of the eastern and western erzgebirge was established furthermore our data do not support a large age range for late variscan granites of the erzgebirge 330 290 ma as has been previously suggested

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

raman scattering from metamict zircon comments on metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage by nasdala et al 2001 contribution to mineralogy and petrology 141 125 144

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

origin and geochemistry of agates in permian volcanic rocks of the sub erzgebirge basin saxony germany abstract mineralogical and geochemical investigations of agates from permian volcanic rocks of the sub erzgebirge basin saxony germany were made to constrain the genesis and characteristics of these spectacular forms of silica in acidic volcanic host rocks samples from the main agate occurrences of chemnitz hohenstein ernstthal st egidien and zwickau were selected for detailed analyses the results of the study show that agate formation can be related to volcanic activity rochlitz ignimbrite and the subsequent alteration of the volcanic rocks most agates originate from the infill of silica into cavities of lithophysae high temperature crystallization domains which formed during cooling of welded ignimbrite agate formation temperatures of probably 150 c were calculated from fluid inclusion and oxygen isotope studies which indicate that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activity the trace element composition of chalcedony and macrocrystalline quartz in agates is different from that of quartz from magmatic or metamorphic rocks and pegmatites elements of the volcanic rock matrix al ca fe na k were released during the alteration processes and accumulated in the sio 2 matrix of the agates extraordinary high contents of ge 90 ppm b 46 ppm and u 18 ppm were also detected which can exceed the clark concentration and sometimes the element concentration in the surrounding host rocks in addition chondrite normalized ree distribution patterns of the agates show strong negative eu anomalies enriched hree as well as tetrad effects these patterns are different from the ree patterns of the volcanic host rocks and point to fractionation processes during agate formation the specific geochemical features indicate interactions of the host rocks with magmatic volatile fluids and heated meteoric water and transport of sio 2 and other elements both in aqueous solution and via stable fluorine and chlorine compounds such as sif 4 bf 3 gef 4 and uo 2 f 2 the results of cathodoluminescence cl microscopy and spectroscopy revealed a microstructure of the agates that is similar for all occurrences characteristic features are irregular internal textures and sector zoning in quartz as well as luminescence colours and spectra respectively which are caused by a typical high defect density oxygen vacancies silanol groups according to these results the formation of agates can be explained by crystallization via an amorphous silica precursor under non equilibrium conditions

spectral finite element approach to viscoelastic relaxation in a spherical compressible earth application to gia modelling summary the choice of the physical model of postglacial rebound plays a decisive role to derive information about the mantle rheology and viscosity from observed data in models for the mantle rheology an incompressible maxwell material is often assumed in spite of seismic observations showing that the earth s mantle is composed of compressible material in this study in order to assess the influence of compressibility on glacial isostatic adjustment gia the spectral finite element approach proposed by martinec is extended to incorporate the effect of compressibility using this approach the present day velocity field is computed for peltier s ice5g vm2 earth model glaciation history combination considering the sea level equation in the formulation of hagedoorn et al the results show that the effect of compressibility on the vertical displacement rate is small whereas the horizontal rates are markedly enhanced for example the rate around fennoscandia and laurentide becomes twice as large when compressibility is considered this large difference between the compressible and incompressible models can be reduced by adjusting the elastic rigidity of the incompressible model so that the flexural rigidity becomes approximately the same as that in the compressible model however differences of 1 mm yr 1 still remain for wavelengths longer than 8000 km these findings show that when modelling horizontal motion induced by gia the influence of compressibility cannot be neglected

radiation damage ages practical concept or impractical vision reply to two comments on metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage and further discussion in a study of little to moderately radiation damaged zircons from various localities nasdala et al 2001 have proposed that the relation between the raman band broadening and the time integrated a dose can be used to examine the accumulation of alpha event damage in unknown zircons comparing raman dose data pairs of unknowns with those of un annealed zircons would provide an indication for whether or not the unknowns have completely stored their a damage as one among seven examples the examination of gem zircons from sri lanka gave strong evidence for partial structural reconstitution through thermal annealing experienced by these zircons two discussions on this paper were received john i garver commented ft fission track measurements on sri lankan zircons which underline the incomplete storage of radiation damage in these samples thorsten geisler and robert t pidgeon wrote a fairly critical discussion they did not question that the comparison of the present degree of metamictisation with the selfirradiation dose may provide information on complete or incomplete a damage storage which was the main conclusion of nasdala et al 2001 in fact geisler and pidgeon 2002 this volume stated that such comparison provides even more precise information and they hold to the general practicability of radiation damage ages other issues raised by these two authors however are not directly related to the content of the article by nasdala et al 2001 but criticise the conclusions of previous publications the discussion by geisler and pidgeon 2002 this volume is for the most part rather an addendum to the article of geisler et al 2001 and re discusses their observations and conclusions in replying to geisler and pidgeon 2002 this volume we are therefore obliged to refer to geisler et al 2001 too in the present paper we will reply to the two discussions and give further relevant comments our responses will focus on points raised by garver 2002 this volume and geisler and pidgeon 2002 this volume which are related to the article by nasdala et al 2001 namely the interpretation of raman spectra their use to estimate radiation damage and their potential application for geochronological purposes critical aspects in the work of geisler et al 2001 and the discussion by geisler and pidgeon 2002 this volume will only be discussed if relevant for the issues above

glacial isostatic adjustment and the radial viscosity profile from inverse modeling 1 a formal inverse procedure is used to infer radial mantle viscosity profiles from several observations related to the glacial isostatic adjustment process the data sets consist of late pleistocene and holocene sea level data from scandinavia the barents sea central europe canada and the far field as well as observations of changes in the earth s rotation and gravitational field and present day uplift and gravity changes in scandinavia inferences of mantle viscosity are robust against assumptions such as the a priori viscosity model and model discretization however the quality of ice sheet reconstruction remains crucial for the inverse inference the importance to discuss regional mantle viscosity models in view of the lateral variability in mantle properties has been evident our inference suggests a two order of magnitude increase of mantle viscosity with depth and volume averaged upper and lower mantle viscosities around 7 1020 and 2 1022 pa s respectively mantle viscosity does not need to increase sharply across the 660 km seismic discontinuity the viscosity profiles suggested are also able to reconcile the large scale geoid anomaly related to mantle convection

constraining models of postglacial rebound using space geodesy a detailed assessment of model ice 5g vm2 and its relatives summary using global positioning system very long baseline interferometry satellite laser ranging and doppler orbitography and radiopositioning integrated by satellite observations including the canadian base network and fennoscandian bifrost array we constrain in models of postglacial rebound the thickness of the ice sheets as a function of position and time and the viscosity of the mantle as a function of depth we test model ice 5g vm2 t90 rot which well fits many hundred holocene relative sea level histories in north america europe and worldwide ice 5g is the deglaciation history having more ice in western canada than ice 4g vm2 is the mantle viscosity profile having a mean upper mantle viscosity of 0 5 1021 pa s and a mean uppermost lower mantle viscosity of 1 6 1021 pa s t90 is an elastic lithosphere thickness of 90 km and rot designates that the model includes rotational feedback earth s response to the wander of the north pole of earth s spin axis towards canada at a speed of 1 myr 1 the vertical observations in north america show that relative to ice 5g the laurentide ice sheet at last glacial maximum lgm at 26 ka was 1 much thinner in southern manitoba 2 thinner near yellowknife northwest territories 3 thicker in eastern and southern quebec and 4 thicker along the northern british columbia alberta border or that ice was unloaded from these areas later thicker or earlier thinner than in ice 5g the data indicate that the western laurentide ice sheet was intermediate in mass between ice 5g and ice 4g the vertical observations and grace gravity data together suggest that the western laurentide ice sheet was nearly as massive as that in ice 5g but distributed more broadly across northwestern canada vm2 poorly fits the horizontal observations in north america predicting places along the margins of the laurentide ice sheet to be moving laterally away from the ice centre at 2 mm yr 1 in ice 4g and 3 mm yr 1 in ice 5g in disagreement with the observation that the interior of the north american plate is deforming more slowly than 1 mm yr 1 substituting vm5a t60 for vm2 t90 that is introducing into the lithosphere at its base a layer with a high viscosity of 10 1021 pa s greatly improves the fit of the horizontal observations in north america ice 4g vm5a t60 rot predicts most of the north american plate to be moving horizontally more slowly than 1 mm yr 1 in agreement with the data ice 5g vm5a t60 rot well fits both the vertical and horizontal observations in europe the space geodetic data cannot distinguish between models with and without rotational feedback in the vertical because the velocity of earth centre is uncertain and in the horizontal because the areas of the plate interiors having geodetic sites is not large enough to detect the small differences in the predictions of rotational feedback going across the plate interiors

rapid viscoelastic uplift in southeast alaska caused by post little ice age glacial retreat our observations show that extreme uplift in southeast alaska began about 1770 ad with relative sea level rsl change to 5 7 m and current uplift rates to 32 mm yr this region experienced widespread glacial melting following the little ice age lia with the collapse of the glacier bay icefield alone equivalent to 8 mm of global sea level rise geodynamic modelling links the uplift to post lia isostatic rebound with the extreme uplift signal and a priori knowledge of ice load changes requiring the presence of a low viscosity asthenosphere 3 710 18 pa s these crustal deformations are triggered by climate change

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

trace element zoning and incipient metamictization in a lunar zircon application of three microprobe techniques we have determined major si zr hf minor al y fe p and trace element ca sc ti ba ree th u concentrations and raman spectra of a zoned 200 microns zircon grain in lunar sample 14161 7069 a quartz monzodiorite breccia collected at the apollo 14 site analyses were obtained on a thin section in situ with an ion microprobe an electron microprobe and a laser raman microprobe the zircon grain is optically zoned in birefringence a reflection of variable incomplete metamictization resulting from zo nation in u and th concentrations variations in the concentrations of u and th correlate strongly with those of other high field strength trace elements and with changes in raman spectral parameters concentrations of u and th range from 21 to 55 ppm and 6 to 31 ppm respectively and correlate with lower raman peak intensities wider raman peaks and shifted si o peak positions concentrations of heavy rare earth elements range over a factor of three to four and correlate with intensities of fluorescence peaks correlated variations in trace element concentrations reflect the original magmatic differentiation of the parental melt approx 4 b y ago degradation of the zircon structure as reflected by the observed raman spectral parameters has occurred in this sample over a range of alpha decay event dose from approx 5 2 x 10 exp 14 to 1 4 x 10 exp 15 decay events per milligram of zircon as calculated from the u and th concentrations this dose is well below the approx 10 exp 16 events per milligram cumulative dose that causes complete metamictization and indicates that laser raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation induced damage in zircon

first u pb la icp ms zircon ages assessed from a volcano sedimentary complex of the mid european variscides pennsylvanian floha basin se germany nine radiometric ages constrained by u pb isotopes from magmatic zircon grains are provided for different volcanic rocks from the floha basin and its surroundings for most of them this is the first reliable data to correlate with the international chronostratigraphic chart measurements were obtained with la icp ms and revealed middle pennsylvanian ages 311 308 ma allowing first consistent correlation to other variscan volcano sedimentary basins of the saxo thuringian and tepla barrandian zones the obermuhlbach volcano formerly considered to be early permian in age is shown to be of middle pennsylvanian age in contrast the dykes from metzdorf village and oederan have shown to be not pennsylvanian but rather of early permian age biostratigraphic data obtained from the floha formation macroflora coincide with the new isotopic evidence which encompasses a likely time span from the middle to late bolsovian westphalian c up to the bolsovian asturian boundary the results not only yield a modified picture of the floha formation and their intercalated pyroclastics as new insights challenge previous geological mapping they contribute to the understanding of the complex volcano tectonic processes in the type area of the saxo thuringian zone of the european variscides

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

a benchmark study for glacial isostatic adjustment codes the study of glacial isostatic adjustment gia is gaining an increasingly im portant role within the geophysical community understanding the response of the earth to loading is crucial in various contexts ranging from the the interpretation of modern satellite geodetic measurements e g grace and goce to the projections of future sea level trends in response to climate change modern modeling approaches to gia are based on various techniques that range from purely analytical formulations to fully numerical methods despite various teams independently investigating gia we do not have a suitably large set of explicitly validated numerical results through which they may be validated a community benchmark data set would clearly be valuable following the example of the mantle convection community here we present for the first time the results of a benchmark study of codes designed to model gia the approaches benchmarked are based on significantly different codes and different techniques this effort is performed within the working group 4 wg4 of the esf cost action es0701 improved constraints on models of glacial isostatic adjustment our aims are i testing the codes currently in use by the various teams ii establish a sufficiently large set of agreed results iii correct possible systematic errors embedded in the various physical formulations or computer implementations and iv facilitate the dissemination of numerical tools for surface loading studies to the community and to young scientists the test computations are mainly based on models with spherical symmetry and maxwell rheology and include inputs from various different methods and solution techniques viscoelastic normal modes spectral finite elements and finite elements the tests involve the loading and tidal love numbers and their relaxation spectra the deformation and gravity variations driven by surface loads characterized by simple geometry and time history and the rotational fluctuations in response to glacial unloading in spite of the significant differences in the numerical methods employed the test computations show a satisfactory agreement between the results provided by the participants most of the existing misfits have been addressed during the preparation of the manuscript some others are currently the subject of analysis within the wg4 community

thermal history of northwest africa 5073 a coarse grained stannern trend eucrite containing cm sized pyroxenes and large zircon grains we report on the bulk chemical composition petrology oxygen isotopic composition trace element composition of silicates and degree of self irradiation damage on zircon grains of the eucrite northwest africa nwa 5073 to constrain its formation and postcrystallization thermal history and to discuss their implications for the geologic history of its parent body this unequilibrated and unbrecciated meteorite is a new member of the rare stannern trend eucrites it is mainly composed of elongated zoned pyroxene phenocrysts up to 1 2 cm plagioclase laths up to 0 3 cm in length and is rich in mesostasis the latter contains zircon grains up to 30 m in diameter metal sulfide tridymite and ca phosphates textural observations and silicate compositions coupled with the occurrence of extraordinary fe rich olivine veins that are restricted to large pyroxene laths indicate that nwa 5073 underwent a complex thermal history this is also supported by the annealed state of zircon grains inferred from raman spectroscopic measurements along with u and th data obtained by electron probe microanalyses

age and genesis of polymetallic veins in the freiberg district erzgebirge germany constraints from radiogenic isotopes the freiberg mining district in the erzgebirge hosts three principal types of polymetallic veins these are 1 the quartz bearing polymetallic sulfide type 2 the carbonate bearing polymetallic sulfide type and 3 the barite fluorite sulfide type we investigated the genesis of each vein type using rb sr sphalerite geochronology sm nd fluorite geochronology and pb sr and nd isotope systematics of ore and gangue minerals field relationships and the rb sr and pb isotope systematics of sulfides from quartz bearing polymetallic sulfide veins and carbonate bearing polymetallic sulfide veins confirm their close genetic affiliation and yield a combined rb sr errorchron age of 276 16 ma the high mean squared weighted deviation mswd value of 42 on the regression is considered to reflect initial isotopic heterogeneity which is probably related to fluid rock interaction during the hydrothermal mineralization process although some sphalerites from barite fluorite sulfide veins have strongly disturbed rb sr isotope systematics six sphalerites and one co genetic fahlore yield a robust isochron age of 121 3 4 2 ma with an mswd of 2 9 this age is supported by a fluorite sm nd isochron age of 101 18 ma mswd 1 3 the new ages and radiogenic isotope data place robust constraints on the long held hypothesis that veins in the freiberg district formed during two hydrothermal events the lower permian age of first stage quartz bearing polymetallic sulfide veins and carbonate bearing polymetallic sulfide veins coincides with post variscan crustal reorganization and rotliegend volcanism the mid cretaceous age of second stage barite fluorite sulfide veins coincides with opening of the north atlantic ocean during the break up of pangea

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

seismicity of the bering strait region evidence for a bering block newly compiled russian and u s seismological data support an independent bering block in motion relative to the north american plate this motion is likely to be driven by the westward extrusion of southwestern alaska resulting from compression in southern alaska due to subduction of the pacific plate and terrane accretion seismicity extends from central alaska through the bering strait and into chukotka in eastern chukotka several southwest trends are evident some of which continue through the koryak highlands to kamchatka the seismicity outlines the bering block which includes most of the bering sea chukchi peninsula seward peninsula and parts of western alaska focal mechanisms young basaltic volcanism and normal faults in western alaska and chukotka indicate that the bering strait is under northeast southwest extension this in conjunction with thrust faulting in the koryak highlands indicates that the bering block is rotating clockwise relative to the north american plate

dynamic changes and processes in the mississippi river delta research in the modern delta of the mississippi river has revealed short term changes and processes that are of significant magnitude deltaic lobes each lobe covering an area of 30 000 sq km and having an average thickness of 35 km switch sites of deposition on an average of every 1 500 yr through short periods of geologic time this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas within a single delta lobe and operating on an even higher frequency are bay fills and overbank splays bay fills having areas of 250 sq km and thickness of 15 m require only 150 yr to accumulate four major events have taken place in the modern balize delta since 1838 overbank splays are much smaller covering areas of less than 2 sq km and having thicknesses of 3 m but are associated with high floods on the river at the river mouth continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long 8 km wide and a thickness of 80 m in a period of only 200 yr differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr on the subaqueous delta platform sediment instabilities operate nearly continuously mass moving large quantities of shallow water deposits to deeper water environments via arcuate rotational slides and mudflow gullies and depositional lobes all of these changes and processes operate at differing spatial and temporal scales but all result in deposition of large volumes of sediment over extremely short periods of time

dry annealing of metamict zircon a differential scanning calorimetry study we report the results of a differential scanning calorimeter dsc study of the annealing of a metamict sri lankan zircon raman measurements on most chips of the powdered zircon starting material sri lankan zircon wz19 showed no evidence of a crystalline structure whereas a few chips retained residual raman bands typical of highly radiation damaged zircon dsc runs on aliquots of the powdered sample were heated to 850 and 1000 c at rates of 2 and 10 c min and to 1500 c at a rate of 10 c min raman spectroscopy was used to investigate the crystallinity of grains at selected temperature stages exothermal peaks were observed at about 910 and 1260 c during the dsc run to 1500 c the 910 c peak was demonstrated by raman spectroscopy to mark the crystallization of tetragonal zirconia and the exothermic peak at about 1260 c was demonstrated to represent the reaction of zirconia and amorphous silica to form crystalline zircon the degree of crystallinity of these grains was almost identical to that of highly crystalline zircons from recent gem gravels from new south wales a small number of experimental chips from dsc analyses under 1000 c were found to have zircon raman bands that indicated they had undergone partial annealing the present experimental results suggest that reconstitution of amorphous zircon to the crystalline state by dry annealing will rarely occur in terrestrial geological settings even under extreme metamorphic conditions

seasonal and long term vertical deformation in the nepal himalaya constrained by gps and grace measurements 1 we analyze continuous gps measurements in nepal southern side of the himalaya and compare gps results with grace observations in this area we find both gps and grace show significant seasonal variations further comparison indicates that the observed seasonal gps height variation and grace derived seasonal vertical displacement due to the changing hydrologic load exhibit very consistent results for both amplitude and phase for continuous gps stations whose observation time span are longer than 3 years the average wrms reduction is 45 when we subtract grace derived vertical displacements from gps observed time series the comparison for annual amplitudes between gps observed and grace derived seasonal displacements also shows consistent correlation the good seasonal correlation between gps and grace is due to the improved gps processing strategies and also because of the strong seasonal hydrological variations in nepal besides the seasonal signal grace also indicates a long term mass loss in the himalaya region assuming no gia effect this mass loss therefore will lead to crustal uplift since the earth behaves as an elastic body we model this effect and remove it from gps observed vertical rates with a 2d dislocation model most gps vertical rates especially in the central part of nepal can be interpreted by interseismic strain from the main himalayan thrust and several exceptions may indicate the complexity of vertical motion in this region and some potential local effects

glacial isostatic adjustment along the pacific coast of central north america abstract we infer the gia signal and its uncertainty along the central pacific coast of north america using 680 sea level index points and over 20 000 model runs sampling 700 1 d earth viscosity models and 29 ice sheet reconstructions due to the large spatial extent and different tectonic settings of the study area we divided it into three sub regions northern central and southern for which model parameters were inferred separately also given that this region is tectonically active the influence of this process as well as sediment isostatic adjustment was accounted for where possible by removing it from the data using published estimates our results indicate that it is not possible to produce an acceptable to for all of the rsl data with a single set of model parameters suggesting significant lateral variability in viscous structure specifically low viscosities 1019 1020 pas are inferred in the upper mantle within the northern region southwestern british columbia and northwest washington compared to those inferred 2 5 1020 pas for the central and southern regions extending from southern washington to southern california high quality model fits were obtained for all data except those from the northern region where no single parameter set was able to capture both the rapid and large rsl fall during the late glacial and the monotonic rise during the mid to late holocene at all localities this suggests the need for an earth model that incorporates departures from a linear maxwell rheology as applied here and or lateral variations in viscosity structure using our optimal model parameter sets we show that gia is a significant contributor to both contemporary vertical land motion and relative sea level change in our study region and so should be considered when interpreting observations of these signals and for making future relative sea level projections model output of present day vertical land motion at 483 gps stations and sea level change at 56 tide gauge stations is provided with estimated uncertainty so that these data can be used to study non gia processes more accurately

lithium and boron in late orogenic granites isotopic fingerprints for the source of crustal melts abstract geochemically diverse late and post variscan granites of the erzgebirge vogtland the saxon granulite massif and thuringia germany formed by anatectic melting of palaeozoic sedimentary successions and associated mafic to felsic volcanic rocks the compositional diversity of the least evolved of these granites is largely inherited from the protoliths we present li and b isotopic data of these granites and compare them with the isotopic composition of their protoliths to investigate whether i there exist systematic differences in the li and b isotopic composition among different granite types and ii li and b isotopic compositions provide information on the granite sources complementary to information from the isotopic composition of sr nd and pb and the trace element signatures low f biotite and two mica granite types have flat upper continental crust ucc normalized trace element pattern with variable enrichments in li rb cs sn and w and depletions in sr ba and eu these signatures are least pronounced for the niederbobritzsch biotite granite which has the largest contribution of mafic material and most pronounced for the two mica granites the granites show a relatively narrow range of 7li values 3 0 to 0 5 and a broad range of 11b values 13 4 to 20 1 the 11b values are lower in rocks with distinctly higher contents of li rb cs and sn the high 11b of the niederbobritzsch granite may be explained by the melting of former altered oceanic crust in its source relative to ucc intermediate f to high f low p granites show strong depletions in sr ba eu as well as zr and hf strong enrichments in li rb and cs as well as nb sn ta and w and ree pattern with stronger enrichments for hree than for lree these granites show narrow ranges of 7li 2 0 to 1 6 and 11b values 14 7 to 9 1 reflecting the smaller variability of the li and b isotopic composition in their source lithologies the anomalously high 7li value 14 7 of one granite sample burgberg which is similar to 7li values of its wall rocks up to 14 5 may indicate late magmatic fluid rock interaction with external wall rock derived fluids because of the small compositional range of most source lithologies the li and b isotopic variation in the granites is also small indicating that the isotopic composition of li and b does not represent a particularly sensitive source tracer with the exception of source lithologies characterized by extreme 7li or 11b values

palaeomagnetic dating of sub torridon group weathering profiles nw scotland verification of neoproterozoic palaeosols a palaeoplain that cuts across lewisian gneiss in the cape wrath area northwest scotland is weathered to depths of 1 6 m beneath fluvial red conglomerates and sandstones of the applecross formation of the early neoproterozoic torridon group weathered lewisian rocks are enriched in k 2 o and fe 2 o 3 and contain hematite as an alteration product of biotite and magnetite palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration thermal step demagnetization of 87 oriented core specimens of biotite gneiss and amphibolite spanning four profiles at poll a mhurain and sheigra reveal a stable high temperature component with a mean direction of remanence of d 149 1 i 57 2 95 5 0 corrected for dip of the overlying applecross strata that gives a pole at 9 8 s 199 2 e dp 5 3 dm 7 3 this direction differs significantly from that for little weathered lewisian at those two localities and also from directions for the late mesoproterozoic stoer group the remanence direction for the weathered rocks is however near the dip corrected high temperature remanence direction d 122 3 i 50 5 95 10 8 obtained by us from the thermal step demagnetization of 39 sandstone specimens of the applecross formation in the cape wrath area furthermore the direction for the weathered rocks agrees closely with other dip corrected results for the torridon group such as the detrital remanent magnetization drm mean direction d 148 3 i 59 9 95 13 8 for the applecross formation on rum and that for the main outcrop of the aultbea formation d 139 7 i 52 6 95 14 5 the concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the cape wrath area acquired their remanence penecontemporaneously with deposition of the applecross and aultbea formations the palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of applecross age c 980 ma that formed at a palaeolatitude of 38 and constitute the oldest known palaeosols in the british isles views that the weathering is substantially older or younger than applecross deposition cannot be sustained our findings carry implications for applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early neoproterozoic time

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

last glacial maximum ice sheet impacts on north atlantic climate variability the importance of the sea ice lid 1 last glacial maximum lgm ice 5g vm2 ice 6g vm5a and paleoclimate modelling intercomparison project phase 3 ice sheet reconstructions are employed in high resolution coupled climate model simulations to investigate the changes they induce in north atlantic climate variability an initial ice 5g vm2 experiment develops a rapid increase of sea ice extent once a thermal threshold is exceeded in this multimillennial simulation subpolar sea ice concentration and thickness are found to be strongly impacted by topographically induced downstream thermal effects from the laurentide ice sheet in each of the reconstructions however in the two additional lgm perturbation experiments the modeled changes in sea ice area are sufficiently similar to the lgm ice 5g vm2 experiment to lead to an equilibrium atlantic meridional overturning circulation strength that is also reduced by the same 40 from preindustrial even though we find significant variation among the models in the deep convection regions of the subpolar glacial north atlantic model predicted sea ice concentrations in this critical region exceed those based upon multiproxy reconstructions and we trace these significant differences to the intensity of the interannual variability of sea ice cover predictions

subsidence in the northeastern nile delta rapid rates possible causes and consequences holocene fluvial and marine deposits have accumulated in a graben like structure on the northeastern margin of the nile delta this part of the delta which includes lake manzala port said and the northern suez canal has subsided rapidly at rates of up to 0 5 centimeter per year since about 7500 years ago this subsidence has diverted at last four major distributaries of the nile river into this region the combined effects of continued subsidence and sea level rise may flood a large part of the northern delta plain by as much as 1 meter by the year 2100 the impact of continued subsidence now occurring when sediment input along the coast has been sharply reduced because of the aswan high dam is likely to be substantial particularly in the port said area and as far inland as south of lake manzala