numerical reconstructions of lgm climate across the eurasian arctic abstract although limits of the eurasian ice sheet at the last glacial maximum lgm are known relatively well from geological evidence the palaeoclimate coexistent with this ice sheet has yet to be identified confidently a simple ice sheet model has been used to determine lgm climate conditions by finding the ice sheet mass balance configuration that permits model results to match with geological data relating to the extent of lgm ice cover these conditions are compared with atmospheric general circulation model agcm calculations of the climate at the lgm which reveal the meteorological conditions associated with the ice sheet mass balance the western margin of scandinavia is characterised by a maritime climate influenced by the nearby warm seasonally sea ice free ocean conditions however further east the warming effect of the north atlantic diminishes and an extreme polar desert climate dominates across the kara sea similar to east antarctica today the lgm climate that we establish is in contrast to many previous reconstructions involving snow laden warm westerly winds originating from the north atlantic affecting the entire eurasian arctic severely cold conditions across the eurasian arctic are compatible with a stable circulation cell over the kara sea that isolates the region climatically and or cold dry westerly winds from the fennoscandian ice sheet that flow across the rest of the eurasian arctic

new archeointensity data from nw argentina 1300 1500 ce abstract a good characterization of the geomagnetic field strength at centennial to millennial time scales in the southern hemisphere is particularly crucial to disentangle the long term evolution of the south atlantic anomaly saa an intriguing geomagnetic feature currently observed at the earth s surface here we present 59 new archeointensities obtained from four well dated groups of potteries with ages ranging between 1300 and 1500 ce and collected in northwest argentina the new data were obtained in accordance with the thellier paleointensity method including partial thermoremanent magnetization ptrm checks and trm anisotropy and cooling rate corrections we have also performed a comparative study of the efficiency of magnetic susceptibility arm and trm anisotropy tensors to correct the trm anisotropy effect upon intensity estimates our results suggest that the magnetic susceptibility tensor systematically underestimates the trm anisotropy effect by 10 30 our new data together with selected archeointensities already published confirm that the decrease of the geomagnetic field intensity in south america started around 1600 1650 ce due to the arrival of the saa

precise evaluation of atmospheric loading effects on earth s time variable gravity field 1 new space gravity missions will provide gravity measurements with unprecedented accuracy and high spatial resolution to reveal the oceanic and hydrologic signals in monthly time variable gravity field from the gravity recovery and climate experiment grace satellite tapley et al 2004 entails the removal of the atmospheric contribution which in turn requires a precise knowledge of the atmospheric mass redistribution we reconstruct the three dimensional 3 d variations of air density from vertical profiles of pressure temperature and specific humidity provided by the national centers for environmental prediction ncep atmospheric model of a realistic topography we compare our results with those from the classical thin layer 2 d approximation and show that the differences between the complete 3 d and the 2 d computations are often nonnegligible in the presence of the expected grace sensitivity up to harmonic degrees of 15 20 corresponding to wavelengths of 2000 2500 km for actual computation we recommend the use of the sigma level atmospheric data with special attention to the latitude and altitude dependence of the earth s gravity we also examine and conclude the importance of the differences with previous study which assumed a constant surface gravity acceleration without a latitudinal dependence

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

a bayesian iterative geomagnetic model with universal data input self consistent spherical harmonic evolution for the geomagnetic field over the last 4000 years abstract changes of the earth s magnetic field have been observed with varying spatial and temporal resolution since the middle ages by means of compasses and later by refined mechanical inclination and intensity instruments until the modern use of quantum mechanical devices and satellite observations ancient records on the other hand rely on the natural magnetization of archaeological artifacts and other materials such as rocks and sediments being therefore indirect less accurate and sparser the combination of such heterogeneous records into a single self consistent global model of the geomagnetic field is very challenging the highly uneven data coverage both in space and time requires a careful handling of data uncertainties and error correlations previous models dealt with this problem using separated treatments of instrumental data and indirect records respectively as well as a different handling of the dipole field component with respect to non dipole terms here we present a global geomagnetic field model based for the first time on the simultaneous inversion of historical archaeomagnetic and volcanic records using a bayesian approach with minimal committing time regularization that minimizes the energy of secular variation a detailed assessment of data uncertainty and error correlation is used to minimize artifacts generated by the appearance of an overwhelming number of incomplete mostly declination only records associated with shipboard measurements in the early colonial period our model yields lower dipole energies which better match modern values as well as higher non dipole energies and a stronger secular variation of the dipole moment some model artifacts associated with extremely heterogeneous distributions of records in time and space are not completely eliminable and might be caused by incorrect a priori uncertainty assessments alternatively they might represent an intrinsic limit that can be overcome only by adding new records at critical locations and times

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

reconstructing holocene geomagnetic field variation new methods models and implications reconstructions of the holocene geomagnetic field and how it varies on millennial timescales are important for understanding processes in the core but may also be used to study long term solar terrestrial relationships and as relative dating tools for geological and archaeological archives here we present a new family of spherical harmonic geomagnetic field models spanning the past 9000 yr based on magnetic field directions and intensity stored in archaeological artefacts igneous rocks and sediment records a new modelling strategy introduces alternative data treatments with a focus on extracting more information from sedimentary data to reduce the influence of a few individual records all sedimentary data are resampled in 50 yr bins which also means that more weight is given to archaeomagnetic data during the inversion the sedimentary declination data are treated as relative values and adjusted iteratively based on prior information finally an alternative way of treating the sediment data chronologies has enabled us to both assess the likely range of age uncertainties often up to and possibly exceeding 500 yr and adjust the timescale of each record based on comparisons with predictions from a preliminary model as a result of the data adjustments power has been shifted from quadrupole and octupole to higher degrees compared with previous holocene geomagnetic field models we find evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 yr the new models also show intermittent occurrence of reversed flux at the edge of or inside the inner core tangent cylinder possibly originating from the equator less

the last glaciation of earth size and volume of ice sheets abstract the volume and size of the late glacial ice sheets in the northern hemisphere has been estimated based on geomorphic and chronostratigraphic data limited glacial extents during the late glacial maximum characterized both arctic canada and northern eurasia in contrast to the presence of the large laurentide scandinavian and devensian ice sheets to the south glacial maxima were not synchronous throughout the northern hemisphere for example the novaya zemlya ice sheet reached its maximum extent ca 39 000 bp approximately 20 000 years earlier than the maximum extent of the contiguous scandinavian ice sheet western and central siberia and the chukhui peninsula were marked by limited glaciation during the last glacial maximum although pre sartanian events were more extensive mountain glaciation in northeast asia was similarly restricted during the last glacial maximum spatial and dynamic characteristics of the last glaciation developed in response to climatic factors the establishment and strength of high pressure systems over siberia limited precipitation and glacier formation throughout northeastern eurasia

the extent of the barents kara ice sheet during the last glacial maximum it has been a long standing discussion whether the barents kara ice sheet expanded onto mainland russia during the last glacial maximum lgm in this paper we describe many well dated by conventional and ams 14 c methods and optically stimulated luminescence sedimentary sequences in the controversial area of northern russia the sequences discussed are not covered by till and yet all predate the lgm the deposits consist mostly of aeolian or lacustrine easily deformable soft silt and fine sand two sites feature frozen mammoth carcasses and three sites contain palaeolithic artefacts and mammalian bones we emphasise that these formations show no sign of having been overridden by an ice sheet at several sites deposition of aeolian sediments and formation of ice wedges took place during the lgm time span these observations present unambiguous proof that the barents kara ice sheet did not cover mainland russia during lgm with a possible exception for the northern tip of the taimyr peninsula r 2001 elsevier science ltd all rights reserved

on the persistence of geomagnetic flux lobes in global holocene field models abstract to investigate the longevity and robustness of high latitude flux patches in the geomagnetic field at the core mantle boundary we present time dependent models of the geomagnetic field for the past 7000 years our models use the same data set as previously used for time dependent archaeomagnetic field modelling but constrained with additional priori models from time averages of field models covering the last 150 400 and 3000 years we find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years and that the northern hemisphere patches at least have highly dynamic behaviour simple averaging of the historical field may not be appropriate to obtain a characteristic time averaged model of the field for comparison with other geophysical observables our results should inform geodynamo studies of thermal core mantle coupling and of possible long term structure in the geomagnetic field

potassium argon ages from tristan da cunha south atlantic potassium argon ages measured on 11 volcanic rocks mainly lavas from tristan da cunha range from 0 21 0 01 to 0 01 0 02 ma and confirm the youthfulness of this volcano which lies on the walvis ridge just to the east of the crest of the mid atlantic ridge

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

evidence for restricted ice extent during the last glacial maximum in the koryak mountains of chukotka far eastern russia field evidence in the koryak mountains lake mainitz region of far eastern russia supports three pleistocene glacial advances the early wisconsinan and pre wisconsinan glaciations are represented by broad lobate moraines extending as much as 30 km north of the koryak mountains field evidence demonstrates that the terminal lateral and medial moraines as well as meltwater channels dead ice topography kettles and outwash plains mark the extent of ice during the last glacial maximum lgm during which glaciers reached no more than 20 km beyond their present limits those emanating from the southern koryak mountains may have reached the bering sea numerical and relative dating techniques support these results and test the theoretical models of the lgm in western beringia erratics on moraines and glaciofluvial terraces common to all valleys at 13 13 8 m above river level yield 36 cl exposure ages ranging from 10 08 to 25 78 ka the koryak mountains lake mainitz record of glaciation is spatially and temporally consistent with the glaciation pattern across central beringia found in other terrestrial and marine records glacier growth in the koryak mountains was sustained by possible increased summer sea surface temperatures and precipitation in the northwest pacific region evidence from the northern koryak mountains lat 64 n long 177 e indicates that the extent of ice in western beringia was limited to mountain and valley glaciers during the lgm this fieldbased research contradicts m g grosswald s theoretical beringian ice sheet hypothesis

palaeosecular variations of the geomagnetic field in africa during the holocene a review the importance of the full understanding of the holocene geomagnetic field spans from human history and archaeology to palaeoclimatic changes and engineering to geomagnetic field modelling and unravelling of the geomagnetic field characteristics and anomalies unfortunately the dearth of data from large under covered areas such as oceans the african and south american continents and the southern hemisphere only 4 6 of the global datasets dramatically limits our knowledge of the temporal and spatial evolution of the geomagnetic field and its application here a review of all data from the african continent is presented in order to encourage and motivate a new generation of palaeomagnetic and archaeomagnetic studies new data will sharpen the palaeomagnetism as a dating tool improve our knowledge of local global geomagnetic features and will help to finally answer some of the fundamental questions in palaeomagnetism like the temporal and spatial distribution of the palaeointensity peaks described and the origin and evolution of the south atlantic anomaly 2020 the author s published by the geological society of london all rights reserved

tide model errors and grace gravimetry towards a more realistic assessment summary the manner in which errors in tide models affect monthly gravity solutions from grace satellite data is re examined we perform a complete simulation of grace gravity estimation including allowance for accelerometer calibrations and satellite state vector adjustment errors in monthly geoid estimates are for most regions of a order of 0 5 mm or roughly 1 cm of equivalent water height when smoothed over radii of 600 km in polar regions where ocean tide models are suspect errors can grow significantly larger over 2 mm in geoid height although over very localized regions errors from diurnal tides tend to cancel but they are not completely eliminated inducing geoid errors of the order of 0 3 mm the currently accepted grace total error budget is roughly an order of magnitude larger than the component arising from tide modelling errors

elevated paleomagnetic dispersion at saint helena suggests long lived anomalous behavior in the south atlantic earth s magnetic field is presently characterized by a large and growing anomaly in the south atlantic ocean the question of whether this region of earth s surface is preferentially subject to enhanced geomagnetic variability on geological timescales has major implications for core dynamics core mantle interaction and the possibility of an imminent magnetic polarity reversal here we present paleomagnetic data from saint helena a volcanic island ideally suited for testing the hypothesis that geomagnetic field behavior is anomalous in the south atlantic on timescales of millions of years our results supported by positive baked contact and reversal tests produce a mean direction approximating that expected from a geocentric axial dipole for the interval 8 to 11 million years ago but with very large associated directional dispersion these findings indicate that on geological timescales geomagnetic secular variation is persistently enhanced in the vicinity of saint helena this in turn supports the south atlantic as a locus of unusual geomagnetic behavior arising from core mantle interaction while also appearing to reduce the likelihood that the present day regional anomaly is a precursor to a global polarity reversal

implications for the dating of wisconsinan weichselian late glacial events of systematic radiocarbon age differences between terrestrial plant macrofossils from a site in sw ireland abstract ams radiocarbon dates were obtained from salix herbacea leaves carex seeds and bulk organic detritus from a lake sediment profile of wisconsinan weichselian lateglacial age in sw ireland there is a systematic age difference between the dated series from the two types of macrofossils with ages obtained from salix herbacea leaves being 900 to 1500 14 c years younger than those obtained from carex seeds the latter tend to be more in accord with dates from the total organic detritus in the lake sediment although the bulk organic fraction invariably registered the older ages intact survival of the fragile salix leaves indicates that they are unlikely to have been physically transferred within the sediment matrix and or otherwise reworked from the surrounding catchment hence these macrofossils are the more likely to be contemporaneous with the time of deposition however there is no significant correlation between measured 14 c age and depth in the salix values which scatter over a range of 700 14 c years in contrast the age depth relationship for carex shows a significant reversal possibly reflecting the redeposition of these macrofossils and therefore giving radiocarbon ages that are anomalously old the data have important implications for the dating of lake sediment sequences by ams radiocarbon measurement of terrestrial plant macrofossils

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

luminescence chronology of late pleistocene loess paleosol and tephra sequences near fairbanks alaska abstract thermoluminescence tl and infrared stimulated luminescence irsl sediment dating methods have been applied to paleosol and tephra bearing loess sequences younger than marine oxygen isotope stage mis 7 at three important sites tl ages indicate the development of significant paleosols 75 000 and 30 000 yr ago in the loess sequence at the gold hill site relatively minor soil development occurred 70 000 and 48 000 yr ago like the 75 000 yr old soil the 30 000 yr old soil is apparently of global extent and consistent in timing with inferred warm intervals elsewhere e g greenland europe western and central china at birch hill replicate tl dating of primary loess combined with two earlier tl results from the same site and with an earlier mean fission track glass shard age of 140 000 10 000 yr for the associated old crow tephra yield a more precise numeric age of 142 300 6600 yr for this alaska yukon chronostratigraphic marker ash bed three of the tl ages at the halfway house site are difficult to interpret but combined with other evidence they indicate 1 the upper 5 6 m of loess from halfway house is not part of the gold hill loess equivalent to pre mis 5 age as long thought by t l pewe but rather is much younger 2 the regionally significant variegated tephra found in the fairbanks and klondike areas and previously thought to be older than mis 5 has an age of 77 800 4100 yr late mis 5

high resolution record of the laschamp geomagnetic excursion at the blake bahama outer ridge summary geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during normal secular variation the laschamp excursion at 41ka was one such deviation previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion accurate dating of excursions and determination of their durations from multiple locations is vital to our understanding of global field behaviour during these deviations we present here high resolution palaeomagnetic records of the laschamp excursion obtained from two ocean drilling program odp sites 1061 and 1062 on the blake bahama outer ridge odp leg 172 high sedimentation rates 30 40cmkyr 1 at these locations allow determination of transitional field behaviour during the excursion palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature across an interval of 30cm associated with a broader palaeointensity low we determine the age and duration of the laschamp excursion using a stratigraphy linked to the 18 o record from the greenland ice cores this chronology dates the laschamp excursion at the blake ridge to 41 3ka the excursion is characterized by rapid transitions less than 200yr between stable normal polarity and a partially reversed polarity state the palaeointensity record is in good agreement between the two sites revealing two prominent minima the first minimum is associated with the laschamp excursion at 41ka and the second corresponds to the mono lake excursion at 35 5ka we determine that the directional excursion during the laschamp at this location was no longer than 400yr occurring within a palaeointensity minimum that lasted 2000yr the laschamp excursion at this location is much shorter in duration than the blake and iceland basin excursions

the russian component of an arctic ice sheet during the last glacial maximum abstract the extent of late weichselian glaciation in arctic russia remains a problem our model of 1995 1998 suggests a continuous and long lived ice sheet centered on the kara sea other reconstructions suggest a smaller and diachronous glaciation the vast majority of dates support the model of restricted glaciation we show that these dates are inconsistent with the geologic record of ice flow across the kara barents divide with second order glacial geology of kola peninsula and with evidence for glacial surges they are also inconsistent with late weichselian climate of the arctic with eurasian continental paleohydrology including meltwater drainage systems cataclysmic megafloods major transgressions of the caspian sea and with the entire paleogeographic context of northern eurasia we believe that due to impeded ventilation of the pleistocene arctic ocean these dates are often too old and that recycling and contamination of the samples would also contribute to the dating errors destructive impacts of late glacial ice sheet surges and megafloods upon the arctic glacial sequences would also aggravate the situation by ignoring the dates until these problems are addressed we find that the field evidence supports continuous glaciation of arctic russia at the lgm and that the resulting ice sheet was part of an arctic ice sheet that included an ice shelf in the arctic ocean and ice sheets in greenland and north america we present 3 d reconstructions of the arctic ice sheet and with an enlarged antarctic ice sheet we show that the ice volume is equivalent to an lgm sea level that was 130 135 m lower than at present the reconstructions depict the arctic ice sheet before and after massive thawing of the bed that allows partial gravitational collapse of the overlying ice with a minimal change in ice volume

late weichselian ice sheets in arctic and pacific siberia abstract interpretation of space images coupled with field data on glacial geomorphology and glacial geology of northern eurasia have provided compelling evidence for a continuous late weichselian ice sheet covering the entire arctic margin of the continent in addition to the scandinavian ice sheet three ice sheets of the same order the karan east siberian and beringian are identified within the major eurasian ice sheet also there is evidence and other considerations suggesting a former ice sheet in the sea of okhotsk ice spreading centers of the ice sheets were situated on the continental shelves off the siberian coasts thus we can term the ice sheets siberian so far the conventional stratigraphic approach to the problem of siberian glaciation has proven fruitless and yielded nothing but uncertainty while geomorphological studies have discovered a wealth of glacial landforms in arctic siberia it was these landforms and their spacing that made the existence of former siberian ice sheets evident the east siberian ice sheet rested on the shelves of the eastern laptev and east siberian seas and its flow was directed radially including up the yana and indigirka rivers the beringian ice sheet was grounded on the chukchi bering and beaufort sea shelves overriding the chukchi and seward peninsulas from the north and flowing through the bering strait it was continued by a thick floating ice shelf buttressed by the submarine aleutian commander ridge the okhotsk ice sheet was grounded on the okhotsk shelf specific gemorphic signatures of marine ice sheets the glacially cut corners have been identified on all promontories jutting out into the bering and okhotsk seas the beringian and okhotsk ice sheets are consistent with the results of deep sea drilling in the adjacent north pacific during leg 145 of d v joides resolution

subterranean clues to the future of our planetary magnetic shield the geomagnetic field generated in the liquid outer core provides a shield from cosmic radiation that can cause damage to man made satellites electrical power grids and the ozone layer 1 in the absence of this shield the solar wind might gradually erode the atmosphere eventually robbing the planet of its water 2 but the dipole magnetic field has decreased in strength over the last 160 y at an alarming rate 3 motivating continued spirited debate 4 6 over whether we are in the early stages of a geomagnetic pole reversal some even believe we are overdue but 160 y is just a blink of the eye in geological terms the record is too short to have great confidence about what these trends might mean about causal processes and the future in pnas trindade et al 7 provide insightful context for these modern changes in the form of a magnetic record from south america that extends 1 500 y into the past we know much about the recent waning geomagnetic field since the time of gauss in 1840 we have had a rich data source in the form of geomagnetic observatory records in the satellite era we have a deluge of data from magnetometers in orbit from analyses of these data we know that the field intensity decrease is concentrated in an area known as the south atlantic anomaly saa a vast swatch stretching from south africa to chile amazingly when one estimates the field structure beneath this region at the core mantle 1 1email john tarduno at rochester edu 1 xref corresp 1 1

time variations of land water storage from an inversion of 2 years of grace geoids by delivering monthly maps of the gravity field the grace project allows the determination of tiny time variations of the earth s gravity and particularly the effects of fluid mass redistributions at the surface of the earth however grace data represent vertically integrated gravity measurements thus are the sum of all mass redistributions inside the earth s system atmosphere oceans and continental water storage plus solid earth in this paper we apply a generalized least squares inverse approach previously developed by 1 g ramillien a cazenave o brunau global time variations of hydrological signals from grace satellite gravimetry geophys j int 158 2004 813 826 to estimate from the monthly grace geoids continental water storage variations and their associated uncertainties over a 2 year time span april 2002 to may 2004 tests demonstrating the robustness of the method are presented including the separation between liquid water reservoirs surface waters soil moisture groundwaters and snow pack contributions individual monthly solutions of total land water storage from grace with a spatial resolution of 660 km are presented for the 2 year time span we also derive the seasonal cycle map we further estimate water volume changes over eight large river basins in the tropics and compare with model predictions finally we attempt to estimate an average value of the evapotranspiration over each river basin using the water balance equation which links temporal change in water volume to precipitation evapotranspiration and runoff amplitudes of the grace derived evapotranspiration are regionally consistent to the predictions of global hydrological models

ice sheets and sea level of the last glacial maximum this paper outlines the general issues regarding ice sheets and sea level of the last glacial maximum lgm which formed the basis of an epilog project workshop papers in this special issue of quaternary science reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent records of sea level change ice sheet modelling geophysical models of glacial isostatic adjustment and geochemical proxies of ice volume this new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies suggesting a net decrease in eustatic sea level at the lgm ranging from 120 to 135 m r 2001 elsevier science ltd all rights reserved

the argus multicollector noble gas mass spectrometer performance for 40ar 39ar geochronology we describe a new high sensitivity multicollector noble gas mass spectrometer argus that has been specifically designed for simultaneous collection of all ar isotopes and hence is ideally suited for 40ar 39ar geochronology the instrument uses a detector housing that holds five faraday collectors one collector is equipped with a 1011 ohm resistor for 40ar and the remaining four are fitted with 1012 ohm resistors for 36ar to 39ar argus has a mass resolution of 225 250 and a measured sensitivity of 7 10 14 mol v at 200 a trap current during the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40ar 36ar of 300 67 0 07 2 eight data points rejected and mean square weighted deviation 1 18 the ages of three mineral standards taylor creek rhyolite sanidine heidelberg biotite and alder creek sanidine are within error of the accepted ages and uncertainties for two are improvements on previously published data argus decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak jumping single collector mass spectrometers

ams radiocarbon dating of organic poor lake sediment an example from linnevatnet spitsbergen svalbard ams radiocarbon dating of specific organic fractions is used to evaluate sources of errors in dating of organic poor lake sediment from linnevatnet a proglacial arctic lake dates on un differentiated bulk organic matter are much too old because of contamination from detrital coal attempts to remove coal from other organic matter were only partially successful a consequence of the wide grain size distribution of the coal even if coal contamination is fully removed the bulk of the remaining organic matter is of aquatic origin and is unsuitable for 14c dating because the lake waters are depleted in 14 c due to dissolution of carbonate minerals terrestrial plant macrofossils provide the only reliable material to date the lake sediment however this material is present in sufficient quantity for ams radiocarbon dating only in cores proximal to the main inlet streams paired dates on terrestrial plants and aquatic insects from the same core level and paired dates on modern aquatic and terrestrial vege

re evaluation of greek archaeomagnitudes a new technique is used to determine the secular variation of the geomagnetic ancient field in athens the results indicate that published greek archaeomagnitudes were seriously affected by alteration it is shown that all intensities obtained using modifications of the thellier method may be affected by alteration and are therefore suspect

problematic 14c ams dates of pollen concentrates from lake gosciaz poland pollen concentrates and macrofossils of mid holocene age from the annually laminated sediments of lake gosciaz poland are dated by 14c ams dating the sediment by matching to the radiocarbon calibration curve is possible for the macrofossils the 14c ages of the pollen concentrates however do not support the varve chronology probably due to an as yet not fully explained reservoir effect

emergence and evolution of the south atlantic anomaly revealed by the new paleomagnetic reconstruction shawq2k abstract the south atlantic anomaly saa is one of the most outstanding features of the present geomagnetic field thus a good knowledge of the saa is clue for a better understanding of the dynamical behavior of the geomagnetic field to achieve this goal paleomagnetic data are crucial since they provide the unique way to investigate past changes in the earth s magnetic field here we present a new global geomagnetic field reconstruction the shawq2k model which is based on a critical revision of the global archeomagnetic and volcanic dataset the new model provides an improved description of the geomagnetic field during the last 2 millennia and yields surprising outcomes about the emergence and development of the saa it shows that the reversed flux patch observed at the core mantle boundary and linked to the saa emerged in the southern hemisphere from at least 950 ad this patch moved westward from the equator to southern latitudes being clearly isolated after 1400 ad in addition since 1550 ad a second reversed flux patch moving northeastward is observed in the north atlantic the new data now available for the southern hemisphere coming from africa and south america together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the saa

pleistocene ice at the bottom of the vavilov ice cap severnaya zemlya russian arctic the vavilov ice cap was perforated in 1988 by a drilling which reached the underlying frozen sediments in contrast to the overlying glacier ice the basal ice is composed of different ice layers with a variable debris load the stable isotope composition of these layers shows values much lower than everywhere else in the core or in the vavilov ice cap this is most probably the signature of a remnant of pleistocene ice which for the first time is shown to occur in the russian arctic

padm2m a penalized maximum likelihood model of the 0 2 ma palaeomagnetic axial dipole moment summary we present a new time varying model for palaeomagnetic axial dipole moment padm for the past 2 myr and compare it with earlier virtual axial dipole moment vadm reconstructions which have been based on stacking and averaging scaled relative palaeointensity records the padm is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood pml approach to recover a cubic b spline representation of axial dipole field variations on million year timescales the pml method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average vadm reconstructions we apply the pml method to a set of 96 032 published data 1800 palaeointensities from igneous rocks 3300 archaeointensities and 86 relative palaeointensity time series of variable lengths and resolutions two models are discussed padm2mp is a trial model based on a subset of the nine longest available sedimentary records padm2m uses a comprehensive data set 76 records 81 446 data 10 records were eliminated and is our preferred model padm2m has a lower mean than existing vadm reconstructions but shows similarities in long period variability some differences in timing amplitude and resolution of certain features can be attributed to variations in age assignments others result from our more comprehensive data set and a reduction in bias attributable to pml modelling padm2m has an average axial dipole moment over 0 2 ma of 5 3 1022 am2 with a standard deviation of 1 5 1022 am2 the brunhes chron average 6 2 1022 am2 is higher than for earlier epochs of matuyama 4 8 1022 am2 as seen in some previous studies the power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 myr 1 we see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour

south atlantic and north atlantic geomagnetic paleointensity stacks 0 80 ka implications for inter hemispheric correlation recent coring of high deposition rate atlantic sediments has led to the development of north atlantic napis and south atlantic sapis geomagnetic paleointensity stacks the sapis stack comprises five records from the sub antarctic south atlantic 41 471s 6 101e four are from piston cores collected during the ocean drilling program odp leg 177 site survey cruise earth planet sci lett 175 2000 145 the other is odp site 1089 leg 177 j geophys res 2001 submitted for publication all but one 4 pc03 of the sapis cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to site 1089 the site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core rc11 83 which has 14 calibrated radiocarbon ages in the 11 41 ka interval below this level the chronology is derived by matching the site 1089 benthic oxygen isotope data to the specmap stack the napis stack comprises six records from a wide area of the north atlantic philos trans r soc ser a 358 2000 1009 correlation between cores was based on marine isotopic stage boundaries augmented by millennial scale features heinrich events and fluctuations in concentration dependent magnetic parameters napis was placed on the gisp2 chronology using the marine to ice core oxygen isotope correlation proposed by voelker et al radiocarbon 40 1998 517 for one of the napis cores in the resulting age model the laschampevent recorded in five of six cores falls in a very narrow o1 kyr age range and coincides with the 10 be and 36 cl peak measured in the ice cores comparison of the two stacks placed on their own independent age models indicates that common millennial scale paleointensity features are preserved although more work is needed to define the true global content of the paleointensity record as well as the precise age of many features it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone r 2002 published by elsevier science ltd

the ice hill an example of retarded deglaciation in siberia abstract the geological structure of one of the southernmost sites of thick ice beds in the yenisei valley near the arctic circle is discussed the erosion contacts of the foliated and contorted ice abundance of glacial debris and the position between the basal till and ablation cover indicate a glacial origin for the ice layer the ice occurs in the cores of arcuate accretion ridges built of ablation materials the overlying sequence consists of a series of diamicton flows alternating with lacustrine and outwash sediments laterally the ice rock complex changes into thick lacustrine rhythmites underlain by basal tills the ice containing hummocky terrain is often lower than the adjacent lacustrine plain therefore inversion of ice controlled topography is suggested several stages of glaciokarst development are supposed to have occurred after an eastward moving ice lobe stagnated more than 50 ka bp the main depositional events are associated with the middle weichselian draining of ice dammed lake igarka and with early holocene topographic inversion the holocene warming apparently caused diapiric deformation of the buried ice probably because of the density and thickness differences between the ice and the overburden the long history of burial and preservation of the remnants of basal glacier ice was controlled by pleistocene permafrost landscape evolution in northern siberia is governed by such retarded deglaciation in contrast with atlantic environments the thick ablation sequence may be described in terms of a spitsbergen model but the siberian basal tills because of their position within the permafrost must be regarded as exfoliated layers of debris rich ice according to shantzer lavrushin s model the occurrence of buried glacier ice and the mode of distribution of the permafrost shows the relict nature of the latter in areas of modern and former retarded deglaciation intermittent diamicton flows are supposed to mark episodes of climatic amelioration like the pleistocene and holocene diamictons with driftwood at the ice hill such warm diamictons may account for some early holocene and similar puzzling interglacial ice advances

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

relating the south atlantic anomaly and geomagnetic flux patches abstract the south atlantic anomaly saa is a region of weak geomagnetic field intensity at the earth s surface which is commonly attributed to reversed flux patches rfps on the core mantle boundary cmb while the saa is clearly affected by the reversed flux region below the south atlantic we show that the relation between the intensity minimum at earth s surface and rfps is not straightforward we map a field dependent intensity kernel constable 2007a to study the relation between the radial geomagnetic field at the cmb and the field intensity at earth s surface synthetic tests highlight the role of specific patches reversed and normal in determining the location of the surface intensity minimum and demonstrate that the saa can indeed be explained by a few intense patches we show that the level of axial dipolarity of the field determines the stability of the relation between the saa minimum and rfps the present position of the saa minimum is determined by the interplay among several robust geomagnetic flux patches at the cmb the longitude of the saa minimum appears near the longitude of the patagonia rfp due to the low latitude normal flux patches nfps near africa and mid atlantic which diminish the effect of the africa rfps the latitude of the saa minimum is lower than the patagonia rfp latitude due to the south pacific high latitude nfp and the axial dipole effect the motion of the saa minimum is explained by the motions and changes in intensity of these robust geomagnetic flux patches simple secular variation sv scenarios suggest that while the saa path can be explained by advection its intensity decrease requires magnetic diffusion in addition these sv scenarios provide some speculative predictions for the saa

late holocene multi proxy records of environmental change on the south atlantic island tristan da cunha sediment stratigraphies from three sites one lake one overgrown lake and an exposed section on the island tristan da cunha in the temperate south atlantic were analysed by means of pollen analysis total carbon nitrogen and sulphur content determination magnetic susceptibility measurements and detailed radiocarbon dating the aim of these studies was to reconstruct the late holocene vegetation and climatic variations the oldest sediment sequence extends back to 2300 cal years bp the vegetation was relatively stable up to the arrival of humans in the 17th century the appearance of the introduced taxon rumex acetosa acetosella at c 300 cal years bp and a subsequent decline in forest cover on the lowland plain provide evidence of substantial human influence on the vegetation well before the establishment of the first permanent settlement in the 19th century before the first anthropogenic influence centennial scale fluctuations in the proxy records are interpreted as reflections of local hydrological changes probably caused by variations in precipitation as inferred mainly from changing proportions of pollen derived from telmatic and terrestrial taxa and corresponding changes in the deposition of mineral matter by fluvial erosion lake levels were low between c 1450 and 1050 cal years bp and high between c 1050 and 300 cal years bp these variations coincide with known climatic changes in southern africa and in the north atlantic suggesting that the inferred hydrological changes on tristan da cunha were related to large scale variations in the general oceanic and atmospheric circulation in the atlantic region

retrieval of large scale hydrological signals in africa from grace time variable gravity fields since its launch in april 2002 the gravity recovery and climate experiment grace mission is recording the earth s time variable gravity field with temporal and spatial resolutions of typically 7 30 days and a few hundreds of kilometers allowing the monitoring of continental water storage variations from both continental and river basin scales we investigate here large scale hydrological variations in africa using different grace spherical harmonic solutions using different processing strategies constrained and unconstrained solutions we compare our grace estimates to different global hydrology models with different land surface schemes and also precipitation forcing we validate grace observations through two different techniques first by studying desert areas providing an estimate of the precision then we compare grace recovered mass variations of main lakes to volume changes derived from radar altimetry measurements we also study the differences between different publicly available precipitation datasets from both space measurements and ground rain gauges and their impact on soil moisture estimates

regional four dimensional hydrological mass variations from grace atmospheric flux convergence and river gauge data 1 the spatiotemporal behavior of the earth s gravity field is traditionally modeled as a series expansion in terms of spherical harmonics with a number of time dependent coefficients in this contribution we study regional effects namely hydrological mass variations observed by the gravity recovery and climate experiment grace satellite mission since 2002 as compared to the global spherical harmonics scaling and wavelet functions are quasi compactly supported i e highly localizing with respect to space and time consequently they establish a system of base functions well suited for regional modeling we determine a spatiotemporal regional gravity model from grace using the spherical wavelet technique for the spatial part and a one dimensional b spline expansion for the temporal variations that is we end up with a four dimensional multiresolution representation mrr of the geopotential over selected regional areas and time intervals for an exemplary area consisting of three large river basins in south america we compute mass variations expressed in terms of equivalent water heights we demonstrate that a b spline expansion for modeling the temporal behavior is more appropriate than the classical fourier series approach in order to corroborate the results our storage estimates are balanced with the net effect of river discharge from gauges outflow and the difference of precipitation and evaporation inflow from the convergence of vertically integrated water vapor fluxes furthermore our results are compared with monthly mass grids from grace spherical harmonic solution data products we conclude that the mrr means a robust tool to maximize the observational accuracy for grace it considers the spatiotemporal resolution of the observations due to resolution dependent highest levels and it is applicable elsewhere globally to model hydrological or other phenomena of mass change

late weichselian glaciation of the russian high arctic a numerical ice sheet model was used to reconstruct the late weichselian glaciation of the eurasian high arctic between franz josef land and severnaya zemlya an ice sheet was developed over the entire eurasian high arctic so that ice flow from the central barents and kara seas toward the northern russian arctic could be accounted for an inverse approach to modeling was utilized where ice sheet results were forced to be compatible with geological information indicating ice free conditions over the taymyr peninsula during the late weichselian the model indicates complete glaciation of the barents and kara seas and predicts a maximum sized ice sheet for the late weichselian russian high arctic in this scenario full glacial conditions are characterized by a 1500 m thick ice mass over the barents sea from which ice flowed to the north and west within several bathymetric troughs as large ice streams in contrast to this reconstruction a minimum model of glaciation involves restricted glaciation in the kara sea where the ice thickness is only 300 m in the south and which is free of ice in the north across severnaya zemlya our maximum reconstruction is compatible with geological information that indicates complete glaciation of the barents sea however geological data from severnaya zemlya suggest our minimum model is more relevant further east this in turn implies a strong paleoclimatic gradient to colder and drier conditions eastward across the eurasian arctic during the late weichselian

tristan da cunha constraining eruptive behavior using the 40ar 39ar dating technique the historically active volcanic ocean island of tristan da cunha exhibits a complex and dynamic history with numerous often compositionally distinct parasitic centers punctuating the large edifi ce to date the temporal relationship between differing styles of activity has been unclear we have applied high precision 40ar 39ar dating to 15 carefully selected samples from tristan da cunha to ascertain spatio temporal relationships of recent volcanism explore episodicity and establish if the most recent summit activity post dated eruptions from parasitic centers lower on the fl anks this has yielded a new suite of reliable holocene ages with the youngest dated deposit at 3 1 ka 1 a recent fl ow at the summit was constrained to 5 1 ka 1 confi rming that summit and parasitic activity on the volcano s fl anks overlap in time the oldest dated deposits were 118 4 ka 1 from a parasitic cone in the southern sector and 81 10 ka 1 from one of the lowest sub aerial shield forming lava fl ows in the northern sector large scale sector collapse is bracketed between 34 1 ka and 26 5 ka 1 via dating of the youngest headwall lava fl ow and oldest sub aerial scarp fi lling deposits no systematic relationship between the new temporal framework vent location and eruptive compositions was found although magmatic fl ux has been inferred to be relatively low tristan da cunha is capable of relatively frequent eruptions from a wide variety of vent locations across a broad range of compositions

effectiveness of natural zeroing of the thermoluminescence in sediments the last exposure to sunlight of detrital feldspars and quartz grains in quaternary sediments can be dated directly by thermoluminescence tl methods but the effectiveness of zeroing of the light sensitive tl signal by daylight varies widely depending upon details of the sedimentation processes to document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures i have collected and analyzed several eolian fluvial marine deltaic and lacustrine samples from less than 100 year old deposits as well as suspensions from sediment plumes also underwater spectral irradiance was measured during recovery of suspensions tl properties of polyrnineralic feldspar dominated 2 4 m and 4 11 m size fractions were measured in the laboratory unexpectedly some light sensitive tl was found to remain in all samples however the light sensitive tl was more effectively zeroed in some samples a proximal loess a sandy braided channel deposit tidal flat and upper delta front muds and thin clayey lacustrine laminae than in others a silty braided channel deposit submarine fan muds and silt bearing lacustrine laminae for the first group the minimum relict paleodose obtained with the partial bleach technique is about 0 4 0 3 gy corresponding to a relict tl age estimate of about 200 years for these sediments thus similar sediments excluding fluvial deposits as young as about 2 ka can be accurately and routinely dated without significant correction for a zero point error if one uses the partial bleach technique coupled with integrated optical bleaching energy fluences of less than 1 j cm 2 and wavelengths only above about 550 nm also some simple optical bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments finally a simple mathematical model is presented that helps account for the gross features of the observed bleaching response curves

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

the gravity recovery and climate experiment mission overview and early results 1 the grace mission is designed to track changes in the earth s gravity field for a period of five years launched in march 2002 the two grace satellites have collected nearly two years of data a span of data available during the commissioning phase was used to obtain initial gravity models the gravity models developed with this data are more than an order of magnitude better at the long and mid wavelengths than previous models the error estimates indicate a 2 cm accuracy uniformly over the land and ocean regions a consequence of the highly accurate global and homogenous nature of the grace data these early results are a strong affirmation of the grace mission concept

non glacial paleoenvironments and the extent of weichselian ice sheets on severnaya zemlya russian high arctic abstract the extent of the barents kara sea ice sheet northern europe and russia during the last glacial maximum lgm in marine isotope stage mis 2 is controversial especially along the southern and northeastern russian high arctic margins we conducted a multi disciplinary study of various organic and mineral fractions obtaining chronologies with 14 c and luminescence dating methods on a 10 5 m long core from changeable lake 4 km from the vavilov ice cap on severnaya zemlya the numeric ages indicate that the last glaciation at this site occurred during or prior to mis 5d 4 early middle weichselian deglaciation was followed by a marine transgression which affected the changeable lake basin after the regression the basin dried up in late middle weichselian time ca 25 40 ka reworked marine sediments were deposited in a saline water body during the late weichselian mis 2 the basin was not affected by glaciation and lacustrine sediments were formed which reflect cold and arid climate conditions during the termination of the pleistocene and into the holocene warmer and wetter climate conditions than before led to a higher sediment input thus our chronology demonstrates that the northeastern margin of the lgm barents kara sea ice sheet did not reach the changeable lake basin this result supports a modest model of the lgm ice sheet in northern europe determined from numeric ice sheet modelling and geological investigations

paleogeography and geochronology in the russian eastern arctic during the second half of the quaternary abstract during the conclusion of the middle pleistocene continental glaciation was initiated in the western part of northern eurasia while alpine glaciers developed in the mountain ranges in the east small ice caps developed on wrangel and delong islands and most of the chukchi peninsula was glaciated the continental shelves of the laptev and east siberian seas remained ice free and subaerially exposed as indicated by cryogenic and aeolian sediments xerophytic herbaceous palynological assemblages and remains of mammuthus primigenius the initiation of the late pleistocene was marked by a major marine transgression over the eastern part of the arctic ocean shelf correlative with the pelukian transgression of alaska and the sangamonian interglacial isotope stage 5e of north america sea levels exceeded present values by approximately 10 15 m a subsequent regression marked the onset of sartanian glaciation ca 25 000 bp climate warming began ca 14 000 bp reaching an optimum ca 8000 bp and was accompanied by renewed marine transgression climate fluctuations persisted throughout the holocene as indicated by alternating tundra shrub tundra and taiga palynomorph assemblages

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

lateglacial and early holocene environments of novaya zemlya and the kara sea region of the russian arctic pollen and radiocarbon data from novaya zemlya and the kara sea region suggest that the hypo thetical panarctic ice sheet denton and hughes 1981 never existed in this area at least during the last 16 000 years lateglacial tundra environments were slightly cooler and drier than the present ones but there were also warmer intervals such as the allerod which were rather favourable for vegetation development better conditions existed during the early holocene when warm atlantic air masses and sea currents gradually penetrated eastwards to the kara region

assessment of gravity recovery and climate experiment grace temporal signature over the upper zambezi the temporal signature of terrestrial storage changes inferred from the gravity recovery and climate experiment grace has been assessed by comparison with outputs from a calibrated hydrological model lumped elementary watershed lew of the upper zambezi and surroundings and an inspection of the within month ground track coverage of grace together with spatial temporal rainfall patterns the comparison of the hydrological model with grace reveals temporal inconsistencies between both data sets because the lew model has been calibrated and validated with independent data sources we believe that this is a grace artifact the within month ground track coverage shows an irregular orbit behavior which may well cause aliasing in the grace monthly deconvolutions this aliasing is the most probable cause of observed temporal inconsistencies between grace and other data sets

depositional history of the north taymyr ice marginal zone siberia a landsystem approach the sediment landform associations of the northern taymyr peninsula in arctic siberia tell a tale of ice sheets advancing from the kara sea shelf and inundating the peninsula probably three times during the weichselian in each case the ice sheet had a margin frozen to its bed and an interior moving over a deforming bed the north taymyr ice marginal zone ntz comprises ice marginal and supraglacial landsystems dominated by thrust block moraines 2 3 km wide and large scale deformation of sediments and ice large areas are still underlain by remnant glacier ice and a supraglacial landscape with numerous ice walled lakes and kames is forming even today the proglacial landsystem is characterised by subaqueous e g deltas or terrestrial e g sandar environments depending on location altitude and time of formation dating results osl c 14 indicate that the ntz was initiated ca 80 kyr bp during the retreat of the early weichselian ice sheet and that it records the maximum limit of a middle weichselian glaciation ca 65 kyr bp during both these events proglacial lakes were dammed by the ice sheets part of the ntz was occupied by a thin late weichselian ice sheet 20 12 kyr bp resulting in subaerial proglacial drainage copyright c 2002 john wiley sons ltd

late pleistocene and holocene vegetation and climate on the taymyr lowland northern siberia abstract pollen records from perennially frozen sequences provide vegetation and climate reconstruction for the last 48 000 14c years in the central part of taymyr peninsula open larch forest with alnus fruticosa and betula nana grew during the kargin middle weichselian interstade ca 48 000 25 000 14c yr b p the climate was generally warmer and wetter than today open steppe like communities with artemisia poaceae asteraceae and herb tundralike communities with dwarf betula and salix dominated during the sartan late weichselian stade ca 24 000 10 300 14c yr b p the statistical information method used for climate reconstruction shows that the coldest climate was ca 20 000 17 000 14c yr b p a warming allerod interstade with mean july temperature ca 1 5 c warmer than today occurred ca 12 000 14c yr b p the following cooling with temperatures about 3 4 c cooler than present and precipitation about 100 mm lower corresponds well with the younger dryas stade tundra steppe vegetation changed to betula nana alnus fruticosa shrub tundra ca 10 000 14c yr b p larch appeared in the area ca 9400 14c yr b p and disappeared after 2900 14c yr b p cooling events ca 10 500 9600 and 8200 14c yr b p characterized the first half of the holocene a significant warming occurred ca 8500 14c yr b p but the holocene temperature maximum was at about 6000 4500 14c yr b p the vegetation cover approximated modern conditions ca 2800 14c yr b p late holocene warming events occurred at ca 3500 2000 and 1000 14c yr b p a cooling little ice age took place between 500 and 200 14c yr ago

the intensity of the earth s magnetic field over the past 160 million years in contrast to our detailed knowledge of the directional behaviour of the earth s magnetic field during geological and historical times1 2 data constraining the past intensity of the field remain relatively scarce this is mainly due to the difficulty in obtaining reliable palaeointensity measurements a problem that is intrinsic to the geological materials which record the earth s magnetic field although the palaeointensity database has grown modestly over recent years3 4 5 these data are restricted to a few geographical locations and more than one third of the data record the field over only the past 5 myr the most recent database5 covering the time interval from 5 to 160 myr contains only about 100 palaeointensity measurements here we present 21 new data points from the interval 5 160 myr obtained from submarine basalt glasses collected from locations throughout the world s oceans whereas previous estimates for the average dipole moment were comparable to that of the earth s present field6 the new data suggest an average dipole moment of 4 2 2 3 1022 a m2 or approximately half the present magnetic field intensity this lower average value should provide an important constraint for future efforts to model the convective processes in the earth s core which have been responsible for generating the magnetic field

paleomagnetic field properties at high southern latitude 1 statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales previous paleomagnetic studies have lacked high latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations these simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude whereas lower latitude observations are expressions of columnar helical flow outside the tangent cylinder this paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the erebus volcanic province evp antarctica and 21 new age determinations by the 40ar 39ar incremental heating method the new evp data are combined with previously published paleomagnetic and geochronological results providing 133 sites 91 having radioisotopic dates modified thellier thellier paleointensity estimates are reported for 47 sites 37 have dates ages for the combined data set span 0 03 to 13 42 ma the 125 high quality evp directional data selected from the merged data set have a non fisherian distribution and a mean direction with an inclination anomaly of 3 but 95 confidence limits include the prediction from a geocentric axial dipole virtual geomagnetic pole vgp dispersions for brunhes matuyama and the combined 0 5 ma data set are consistently high compared with values from middle to low latitude regions regardless of the criterion used to determine transitional fields with vgp latitude cut off at 45 the dispersion 23 9 2 1 for the combined 0 5 ma evp data set is consistent with earlier high latitude data and paleosecular variation psv in model g but not with some more recent statistical psv models mean evp paleointensity of 31 5 2 4 t derived from 41 high quality sites is about half the current value at mcmurdo 63 t the result is essentially independent of data selection criteria high vgp dispersion and low intensity values support the global observation of anticorrelation between directional variability and field strength simulations of time varying dipole strength show that uneven temporal sampling may bias the mean evp intensity estimate but the possibility of persistently anomalous field behavior at high latitude cannot be excluded

palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the south atlantic we present a dataset of a full vector palaeomagnetic study of late pleistocene lavas from the island tristan da cunha in the south atlantic ocean the current day geomagnetic field intensity in this region is approximately 25 mu t compared to an expected value of similar to 43 mu t this phenomenon is known as the south atlantic geomagnetic anomaly saa geomagnetic field models extending back to the last 10 ka find no evidence for this being a persistent feature of the geomagnetic field albeit all models are constructed from data which is particularly sparse in the southern hemisphere new ar 40 ar 39 incremental heating dating indicates the studied lavas from tristan da cunha extruded between 90 and 46 ka palaeointensity estimations of eight lava flows made using the thellier method yield an average palaeointensity of 18 6 mu t and virtual axial dipole moment vadm of 3 1 1 2 x 10 22 am 2 the lava flows demonstrate four time intervals comparable to the present day saa where the average vadm of the tristan da cunha lavas is weaker than the global vadm average this suggests a persistent or recurring low intensity anomaly to the main geomagnetic field similar to the saa existed in the south atlantic between 46 and 90 ka c 2016 elsevier b v all rights reserved

pitfalls in the ams radiocarbon dating of terrestrial macrofossils the ams 14 c technique has the advantage that small samples of late quaternary age can be dated with high accuracy and that errors due to reservoir effects can be avoided if specifically determined terrestrial micro and macrofossils are measured however to obtain such high accuracy measurements it is important how small samples are handled prior to treatment in the radiocarbon laboratories here we present a set of 51 ams 14 c measurements of which 31 dates gave expected ages and 20 dates resulted in anomalously young ages despite the fact that all samples consisted of clearly identified late weichselian terrestrial plant macrofossils to evaluate possible sources of error we compared these samples in respect to preparation methods sample storage and sample weight our results show that the long term storage of wet macrofossil samples appears to have a significant effect on the radiocarbon age obtained even when the samples are kept cool fungi or micro organisms may easily be incorporated into a sample during preparation and identification and can easily contribute to the contamination of a sample if stored cool and wet for several months 1998 john wiley sons ltd

thermoluminescence dating tests for lacustrine glaciomarine and floodplain sediments from western washington and british columbia to help further develop reliable procedures for accurate thermoluminescence tl dating of quaternary waterlaid sediments we tested tl dating procedures on sediment types rarely examined six glaciolacustrine samples three samples of glaciomarine drift and eight samples of floodplain deposits we used the partial bleach r beta gamma technique applied to fine silt polymineral grains results from our younger known age glaciolacustrine sediments confirm earlier observations that only the clayey laminae are generally suitable for tl dating a clayey lamina older than ca 140 150 ka produced no age underestimation and this result ca 300 ka suggests that such older lake sediments are probably suitable for tl dating two proximal samples of glaciomarine drift produced large tl age overestimates whereas the single distal glaciomarine drift sample yielded an expected age 177 38 ka suggesting that follow up studies are warranted our results for eight flood plain samples confirm that zeroing of li

geomagnetic secular variation and the statistics of palaeomagnetic directions in this study we examine the role of palaeosecular variation psv in the use of statistics for palaeomagnetic studies and we provide new reliability criteria for palaeomagnetic poles or directions we first conclude that fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole vgp distributions instead secondly we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets the latitude dependent properties e s k provide useful guidelines for the reliability of a palaeomagnetic data set a reliable assessment of these properties depends on the sufficient number of palaeomagnetic samples being taken therefore as an additional instrument of assessing data sets we provide a n dependent a95 envelope bounded by an upper limit a95max and a lower limit a95min that helps to ascertain whether or not a distribution has sufficiently well sampled psv and therefore geomagnetic field behaviour applying these criteria is indispensable for studies of geomagnetic behaviour or for studies aiming at using tk03 gad for inclination error correction through the elongation inclination e i method for palaeomagnetic studies aimed at geological reconstructions they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field an analysis of published eastern mediterranean data shows that the vast majority of studies do not conform to the van der voo criteria in particular with respect to n and a95 we have provided criteria that are on the one hand more lenient lower n may still provide relevant information and on the other hand more strict for high n the criterion of a95 16 should be adapted to a requirement of lower a95 e g a95 80

application of avhrr imagery and grace measurements for calculation of actual evapotranspiration over the quaternary aquifer lake chad basin and validation of groundwater models the paper presents an application of noaa avhrr and grace data for studying the regional hydrogeology of the quaternary aquifer in the lake chad basin the actual evapotranspiration et over the phreatic aquifer is computed using the s sebi algorithm and then compared to the et estimates derived from the grace land water solutions both the grace solutions and the s sebi results are similar in description of the actual et over the lake chad basin avhrr data helped to overcome non uniqueness in the modelling of the piezometry anomalies particularly in regional groundwater models the harr piezometric dome appeared to be an area of relatively high effective recharge while the chari baguirmi and the kadzell piezometric depressions and bahr el gahzal were probably areas of excess et over recharge

local and global hydrological contributions to time variable gravity in southwest niger p advances in methods of observation are essential to ensure a better understanding of changes in water resources considering climate variability and human activities the ghyraf gravity and hydrology in africa experiments aim to combine gravimetric measurements with dense hydrological surveys to better characterize the annual water storage variability in tropical west africa the first absolute gravimetric measurements were performed in southwest niger near a temporary pond where rapid infiltration to an unconfined aquifer occurs as gravity is sensitive both to local and global variations of water mass distribution the large scale hydrological contribution to time variable gravity has been removed using either grace satellite data or global hydrology models the effect of the local water storage changes was modelled using in situ measurements of the water table soil moisture and pond water level the adjustment of these simulations to residual ground gravity observations helped to constrain the specific yield to a value ranging between 1 8 and 6 2 per cent this range of value is consistent albeit on the low side with the aquifer water content 6 12 per cent estimated by magnetic resonance soundings which are known to slightly overestimate the specific yield in this geological context the comparison of these two independent geophysical methods shows their potential to constrain the local hydrogeological parameters besides this study evidences the worth of correcting the gravity signal for large scale hydrology before recovering local water storage parameters

calibration and application of marine sedimentary physical properties using a multi sensor core logger abstract we examined geophysical data from a multi sensor core logger mscl a logging device providing continuous measurements of gamma ray attenuation p wave travel time and magnetic susceptibility on marine sediment cores in the first part we focused on the gamma ray system and compared two different calibration methods from the gamma ray attenuation we calculated densities and porosities by incorporating mass weighted attenuation coefficients the application of an iteration method reduces the error of the density and porosity estimates compared to grape data in addition we derived equations to calculate water content and dry bulk density from gamma ray attenuation measurements comparison with physical properties determined on discrete samples revealed a very good correlation of both data sets r 0 99 this correlation is valid for sediments from substantially different geological settings e g turbidites hemipelagic muds and opal rich sediments in the second part we applied our data to marine geological questions for sediments from the antarctic polar frontal zone there is indication that the content of biogenic opal can be assessed using a correlation of density and p wave velocity for sediments from the bengal fan the relationship between the mscl acoustic impedance the product of density and p wave velocity and the grain size distribution in discrete samples can be used to predict clay and sand silt ratios for sediment cores from the shelf and upper continental slope

the quaternary vegetation and landscape evolution of novaya zemlya in the light of palynological records abstract the paper presents the results of paleoenvironmental pollen investigations of novaya zemlya throughout all the quaternary including eopleistocene pleistocene and holocene clear evidence of continuity between the modern flora and paleofloras confirms the lack of sharp and continuous interruptions in the vegetation development in high latitudes as well as the absence of gigantic ice sheets in the eastern arctic during the eopleistocene plant communities varied from boreal meadow coenoses in the south of novaya zemlya to arctic tundra in the north during the lower pleistocene the climate had deteriorated considerably but still remained warmer than today s there were two cold glacial and two warm interglacial stages in the upper pleistocene the older interglacial kumzha was marked by rather mild climate and extensive marine transgression during the climatic optimum southern dwarf shrub tundra grew in the southern island but forest did not penetrate there the younger interglacial krestyakhino had cooler climate and more restricted transgression extent typical tundra existed in the southern island the upper pleistocene glaciations were no more extensive than nowadays and large biota refuges existed in the coastal and even some inner areas of novaya zemlya at least between 15 000 and 10 000 bp during the holocene great change took place at the end of climatic optimum when dwarf shrub tundra was replaced by more open tundra communities

fall in earth s magnetic field is erratic earth s magnetic field has decayed by about 5 per century since measurements began in 1840 directional measurements predate those of intensity by more than 250 years and we combined the global model of directions with paleomagnetic intensity measurements to estimate the fall in strength for this earlier period 1590 to 1840 a d we found that magnetic field strength was nearly constant throughout this time in contrast to the later period extrapolating to the core surface showed that the fall in strength originated in patches of reverse magnetic flux in the southern hemisphere these patches were detectable by directional data alone the pre 1840 model showed little or no evidence of them supporting the conclusion of a steady dipole up to 1840

radiocarbon dating of pollen by accelerator mass spectrometry radiocarbon dating of bulk sediments has been the standard method for establishing chronologies in the studies of lake sediment cores which have contributed significantly to our knowledge of late quaternary paleo environments these bulk sediment dates are presumed to be direct ageindicators for the speciments e g pollen or macrofossils which are actually being studied however several recent studies have reinforced long standing apprehensions concerning this presumption in this study we demonstrate for the first time the radiocarbon dating of pollen concentrate samples by accelerator mass spectrometry the dates obtained by this method should provide more reliable radiocarbon chronologies for paleo environmental studies than have been obtainable by bulk sediment dating

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers