transfer of basal sliding variations to the surface of a linearly viscous glacier on etudie pour une plaque a faces paralleles d epaisseur h et de viscosite lineaire l effet en surface des fluctuations de la vitesse de derapage

variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

what are the climate controls on δd in precipitation in the zongo valley bolivia implications for the illimani ice core interpretation abstract controversy has surrounded the interpretation of the water isotopic composition d or 18o in tropical and subtropical ice cores in south america although recent modeling studies using agcm have provided useful constraints at interannual time scales no direct calibration based on modern observations has been achieved in the context of the recent ice core drilling at nevado illimani 16 39 s 67 47 w in bolivia we examine the climatic controls on the modern isotopic composition of precipitation in the zongo valley located on the northeast side of the cordillera real at about 55 km from nevado illimani monthly precipitation samples were collected from september 1999 to august 2004 at various altitudes along this valley first we examine the local and regional controls on the common d signal measured along this valley we show that 1 local temperature has definitely no control on d variations and 2 local rainout is a poor factor to explain d variations we thus seek regional controls upstream the valley potentially affecting air masses distillation based on backtrajectory calculations and using satellite data trmm precipitation noaa olr and direct observations of precipitation iaea gnip we show that moisture transport history and the degree of rainout upstream are more important factors explaining seasonal d variations analysis of a 92 yr simulation from the echam 4 model t30 version implemented with water stable isotopes confirms our observations at seasonal time scale and emphasize the role of air masses distillation upstream as a prominent factor controlling interannual d variations lastly we focus on the isotopic depletion along the valley when air masses are lifted up our results suggest that if the temperature gradient between the base and the top of the andes was higher by a few degrees during the last glacial maximum lgm less than 10 of the glacial to interglacial isotopic variation recorded in the illimani ice core could be accounted for by this temperature change it implies that the rest of the variation would originate from wetter conditions along air masses trajectory during lgm

isotopic and geochemical tracers reveal similarities in transit times in contrasting mesoscale catchments there is a need for more isotopic tracer studies at the mesoscale to extend our understanding of catchment transit times and their associated controls beyond smaller experimental sites typically 400 m with similar suites of soil coverage predominantly regosols gleys peats and podzols although the relative distribution varied as did the presence of other landscape features such as aquifers in quaternary drifts and lakes input output relationships of 18o in precipitation and runoff revealed contrasting responses and differential damping which were broadly consistent with catchment characteristics the mean transit times mtts were estimated using a convolution integral with a gamma distribution as the transfer function these varied from 528 days in the most responsive catchments to 800 days in catchments where the tracer signature was most damped shorter mtts were found in sub catchments with a higher percentage cover of responsive soils i e regosols gleys and peats whilst sub catchments with longest mtts had a higher coverage of free draining podzolic and alluvial soils as well as significant amount of stored water either in fluvio glacial aquifers or large lakes the mtt of all six catchments had the same order of magnitude this contrasts with studies in the scottish highlands with smaller 10 km2 catchments where mtt has been shown to vary between 60 and 1200 days copyright 2010 john wiley sons ltd

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

surface velocity and mass balance of ice streams d and e west antarctica over 75 000 surface velocity measurements are extracted from sequential satellite imagery of ice streams d and e to reveal a complex pattern of flow not apparent from previous measurements horizontal and vertical strain rates calculated from surface velocity indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher ten airborne radar profiles and one surface based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream improved data collection methods increase data density substantially reducing random errors in velocity however systematic errors continue to limit the ability of the flux differencing technique used here to resolve local variations in mass balance nevertheless significant local variations in mass balance are revealed while overall ice streams d and e are in approximate equilibrium an earlier estimate of the net mass balance for ice stream d is improved

aspects of precambrian palaeomagnetism with new data from the limpopo mobile belt and kaapvaal craton in southern africa palaeomagnetic results from precambrian rocks suggest that the geomagnetic field in the precambrian was similar in major respects to the recent field although there is a possibility that reversals were less frequent limitations of precision in proterozoic palaeomagnetic and geochronological data and inadequate geographic spread of these data prevent the making of complete proterozoic continental reconstructions nevertheless where the data are most abundant precambrian apparent polar wander apw paths can be used in determining the relative movement of cratons and groups of cratons relative movement between africa laurentia and australia during at least part of the proterozoic seems to be indicated but the lack of significant differences between the apw paths of several pairs of presently adjacent proterozoic cratons suggests that some proterozoic orogenic belts were not formed by the collision of previously widely separated blocks palaeomagnetic studies on metamorphic and igneous rocks from within proterozoic orogenic belts notably the grenville belt in canada show that the effects of slow post orogenic cooling are commonly detectable palaeomagnetically the problems and potential of precambrian palaeomagnetism are illustrated by studies on rocks from the kaapvaal craton and adjacent limpopo mobile belt in southern africa and new data are presented from i a stratigraphically important horizon in the waterberg supergroup ii the palabora igneous complex and iii the metamorphic rocks of the limpopo belt the results from the palabora complex indicate two distinct bursts of igneous activity at ca 2000 and ca 1900 my the rocks of the limpopo belt have two stable magnetizations these sometimes residing together in the same specimens and resolvable only by vector analysis of detailed demagnetization data component a is thought to have been acquired during cooling of the belt at about 2000 my and this result together with one previously published suggests diachronous cooling across the belt with a polarity change occurring during the cooling interval component b is thought to be a later crm but its exact age is unknown the new data assist in refining the african apw path for the interval ca 2000 to ca 1850 my this segment of the african apw path is shown to have a distinctive polarity pattern and a similar pattern is also recognised in the laurentian apw path for this interval these polarity patterns help to resolve the ambiguity of geographical polarity to be assigned to these apw paths and following from this evidence is presented which suggests that africa and laurentia were in relative motion at about 1950 my

inferring ice flow directions from single ice sheet surface images using the radon transform we present a new method for extracting the direction of surface flow for ice sheets based on the detection of flow induced features that are visible in satellite imagery the orientation of linear features is determined using a radon transform and only requires a single image the technique is demonstrated by applying it to the radarsat mosaic of antarctica over the lambert glacier amery ice shelf region of east antarctica comparisons with both existing flow direction fields and traced streamlines over the same area provide an evaluation of the method we also illustrate its application to landsat 7 imagery

steady state three dimensional ice flow over an undulating base first order theory with linear ice rheology on considere un ecoulement stationnaire d epaisseur constante en utilisant le rapport d amplitude des oscillations basales et de l epaisseur de la glace en tant que parametre de perturbation on etablit et on resout les equations du premier ordre des perturbations de vitesse et de pression

satellite imagery of the onset of streaming flow of ice streams c and d west antarctica five overlapping landsat multispectral scanner satellite images of the interior of the west antarctic ice sheet were enhanced with principal component analysis high pass filtering and linear contrast stretching and merged into a mosaic by aligning surface features in the overlap areas the mosaic was registered to geodetic coordinates to an accuracy of about 1 km using the five scene centers as control points the onset of streaming flow of two tributaries of ice stream c and one tributary of ice stream d is visible in the mosaic the onset appears to occur within a relatively short distance less than the width of the ice stream typically at a subglacial topographic feature such as a step or ridge the ice streams extend farther up into the interior than previously mapped ice stream d starts about 150 km from the ice divide at an altitude of about 1500 m approximately halfway up the convex upward dome shape of the interior ice sheet ice stream d is relatively much longer than ice stream c possibly because ice stream d is currently active whereas ice stream c is currently inactive the grounded portion of the west antarctic ice sheet is perhaps best conceptualized as an ice sheet in which ice streams are embedded over most of its area with slow moving ice converging into fast moving ice streams in a widely distributed pattern much like that of streams and rivers in a hydrologic basin a relic margin appears to parallel most of the south margin of the tributary of ice stream d separated from the active shear margin by about 10 km or less for a distance of over 200 km this means there is now evidence for recent changes having occurred in three of the five major ice streams which drain most of west antarctica b c and d two of which b and d are currently active

caribbean and pacific moisture sources on the isthmus of panama revealed from stalagmite and surface water δ18o gradients 1 we test the hypothesis that the pacific ocean contributes moisture to the intertropical convergence zone itcz over southern central america by spatial analysis of surface water 18o values from panama and costa rica the 18o values decrease with distance from the caribbean sea to the isthmian divide then gradually increase from the divide toward the pacific slope which suggests a contribution of both caribbean and pacific sourced moisture to the isthmus we estimated the pacific moisture contribution for pacific slope regions of 22 to 64 the 18o values from stalagmites from five cave systems demonstrate decreasing 18o values with distance from the caribbean implicating the atlantic basin as a dominant moisture source constraining modern moisture sources is important for the interpretation of stable isotopic proxy records of past rainfall because of the combined influence of pacific and atlantic ocean atmosphere phenomena on itcz rainfall over the isthmus of panama

paleomagnetism and age of the archean usushwana complex southern africa paleomagnetic and 40ar 39 ar age data are reported from the usushwana complex a layered gabbro and pyroxenite intrusive exposed in swaziland and eastern south africa 26 5 s 31 0 e the main gabbro body and an associated granophyre return a mean rb sr whole rock and sm nd mineral age of 2875 40 ma alternating field and thermal analysis of specimens from 18 sites 70 samples yields a mean direction d 304 i 53 k 32 95 6 this direction is significantly different from that obtained from the 2100 ma bushveld complex rare dikes of probable bushveld age cut the usushwana complex and a low coercivity overprint seen in specimens from a few sites has a bushveld direction and almost certainly reflects this event thermal overprinting is recorded by 40ar 39 ar hornblende and pyroxene ages between 2094 54 ma and 2386 58 ma that we interpret to have occurred at low temperature but over a significant period of time isothermal remanent magnetization acquisition experiments and laboratory unblocking temperatures suggest that the primary remanence carrier is single domain magnetite the paleomagnetic pole position calculated from the usushwana complex 9 n 347 e is significantly different from other late archean poles although the usushwana pole is close to the pole from the palabora igneous complex of south africa 1900 ma there is no isotopic evidence indicating a thermal event of that age in the usushwana complex

non temperate glacier flow over wavy sloping ground the mean steady state particle velocity in gravity driven glacial flow over sinusoidal sloping ground is computed using a lagrangian description of motion a newtonian viscous fluid approximation is used for the ice the glacier surface is free to move and is not subject to any stresses at the bottom the ice is frozen to the ground the non linear interaction between the basic downslope poiseuille flow and the bottom corrugations yields a mean lagrangian perturbation velocity that is always directed in the upslope direction near the ground the requirement of mass balance imposes a mean negative surface slope in the corrugated region and an associated downslope perturbation flow in the upper part of the glacier the no slip condition at the wavy bottom induces a strong velocity shear in the ice and particularly at the base analysis shows that the shear heating associated with shortwave perturbations could in the case of a marginally frozen ground lead to melting and subsequent sliding at wave crests along the bottom while the ice stays frozen at the troughs it is suggested that for glaciers the resulting high strain rates could lead to crevassing

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

paleomagnetism of the late archean nyanzian system western kenya abstract the nyanzian system lavas of western kenya are believed to be the oldest rocks of the tanzanian craton intrusive age relationships suggest an age 2850 ma although direct attempts at dating the nyanzian have produced disparate results our study involves a suite of samples collected from the nyanzian basalts pillow basalts andesites and rhyolites from sixteen sites in western kenya these rocks yield a tilt corrected paleomagnetic pole at 14 n 150 e k 59 dp 5 dm 7 this pole is constrained to be older than the first d1 deformation 2472 30 ma by positive fold conglomerate and reversal tests analysis of the paleomagnetic data base for three african cratonic nuclei tanzanian kaapvaal zimbabwe and west africa for the time period from 2 0 ga to 3 0 ga demonstrates a paucity of well dated poles although there are several poles from the kaapvaal zimbabwe and tanzanian cratons which allow spot readings of their relative positions we demonstrate based on these data that the kaapvaal zimbabwe and tanzanian cratons were drifting independently at 2875 ma 2700 ma and 2450 ma this independent motion of the tanzanian and kaapvaal zimbabwe cratons indicates that previously proposed models involving african cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 ma

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

oxygen isotope values of precipitation and surface waters in northern central america belize and guatemala are dominated by temperature and amount effects abstract an understanding of the climatic controls on precipitation 18 o is required to interpret isotopic records of paleoclimate and paleoaltimetry however variations in precipitation 18 o in time and space are only poorly known in northern central america to test the hypothesis that precipitation and surface water 18 o values are dominated by temporal and spatial amount effects we analyzed 18 o in surface waters collected from guatemala and belize and in precipitation from the global network for isotopes in precipitation database for veracruz mexico and san salvador el salvador herein we show that the dominant controls on 18 o values of precipitation and surface waters are fairly simple temporally the dominant control on precipitation 18 o values is the amount effect whereby there is an inverse correlation between rainfall amount and 18 o precipitation 18 o values decrease by 1 24 per 100 mm increase of monthly rainfall spatially only two variables distance from the coast and mean catchment altitude explain 84 of the surface water 18 o variability surface water 18 o values show an altitude effect of 1 9 to 2 4 km 1 and a continental effect of 0 69 per 100 km once corrected for altitude effects a decrease in surface water 18 o by 3 to 4 from the caribbean sea to the pacific ocean is evident as an isotopic rain shadow on the pacific slope our data also show that river waters in this humid tropical environment are good proxies for 18 o values of precipitation in northern central america the guatemala belize surface water line is defined as d 8 0 18 o 8 7 which is similar to the meteoric water line at san salvador of d 8 1 18 o 10 9 spatial variability in 18 o values is interpreted to reflect 1 progressive rainout of caribbean sourced air masses upon traverse of central america and 2 the temperature dependent equilibrium fractionation between vapor and condensate related to the altitude effect the data show that the northeast trade winds are the dominant moisture source to central america mixing with pacific derived moisture west of the cordilleran divide

seasonal variations of deuterium and oxygen 18 isotopes and their response to moisture source for precipitation events in the subtropical monsoon region deuterium and oxygen 18 are common environmental tracers in water used to investigate hydrological processes such as evaporation and groundwater recharge and to trace moisture source in this study we collected event precipitation from 01 january 2010 to 28 february 2011 at a site in changsha yangtze river basin to estimate the influence of moisture source and atmospheric conditions on stable isotope compositions the local meteoric water line established as d 8 45 0 13 18o 17 7 0 9 r2 0 97 n 189 had a higher slope and intercept than global meteoric water line temperature 18o exhibited complex correlations with positive correlations during nov apr superior to during jun sep which was attributed to distinctive moisture sources but vague the overall period amount effect examined throughout the year linear regressions between 18o and d value in different precipitation event size classes revealed progressively decreasing slope and intercept values with decreasing precipitation amount and increasing vapour pressure deficit indicating that small rainfall events 0 5 mm were subject to secondary evaporation effects during rainwater descent in contrast snowfall and heavy precipitation events exhibited high slope and intercepts for the regression equation between 18o and d high concentrations of heavy isotopes were associated with precipitation events sourced from remote westerly air masses degenerated tropical marine air masses from the bay of bengal bob and inland moisture in the pre monsoon period as determined from backward trajectories assessed in the hysplit model meanwhile low concentrations of heavy isotopes were found to correspond with remote maritime moisture from bob the south china sea and the west pacific at three different air pressures in summer monsoon and post monsoon using hysplit and records of typhoon paths these findings suggest that stable isotope compositions in precipitation events are closely associated with the meteorological conditions and respond sensitively to moisture source in subtropical monsoon climates copyright 2013 john wiley sons ltd

palaeoproterozoic assembly of the napier complex southern india and western australia implications for the evolution of the cuddapah basin abstract a detailed examination of lithostratigraphic tectonothermal geochronologic and palaeomagnetic database of the dharwar bastar cratons of south india the yilgarn craton of western australia and the east antarctic shield has shown broad similarity two palaeomagnetic key poles of 2400 ma age from the dharwar and yilgarn cratons suggest near neighbor positions matching patterns of craton outlines orientation of mafic dyke swarms and palaeo north direction for 2400 ma have established a continental assembly of siwa an acronym for south india si and western australia wa at 2400 ma in this assembly the yilgarn craton fits against the bastar craton and the singhbhum craton the available geological data from the napier complex of the eastern antarctica and the dharwar craton of south india were used to prepare the barcode style reconstruction of archaean tectonostratigraphic events of both the cratons the method of matching continental outlines the internal features of two continental blocks and the palaeomagnetic data demonstrate that the napier complex was also a part of siwa during the period 2200 1900 ma and was located at the position of the cuddapah basin of india adjacent to the dharwar craton and the yilgarn craton the separation of the napier complex from the south indian block led to the development of the cuddapah basin at 1950 ma

a method of repeat photoclinometry for detecting kilometer scale ice sheet surface evolution small scale glaciological processes can drive large scale ice sheet behavior but remain underreported due to a paucity of surface elevation measurements in remote polar regions satellite images provide a relatively long record of spatially dense surface observations which allow us to investigate changes in ice sheet topography on the spatial scales of 1 10 km inferring surface topography from satellite images is an established technique but in previous efforts strict requirements for illumination conditions and image quality have led to a great quantity of discarded data relaxing quality requirements and fitting linear trends to the time series of image derived surface topography allow inclusion of more total signal and enable blending data from multiple platforms as a proof of concept we combine 121 moderate resolution imaging spectroradiometer images to develop a 250 m resolution map of surface elevation change at totten glacier antarctica achieving a 1 sigma uncertainty of 0 22 textrm m textrm a 1 for a 15 year period our method of repeat photoclinometry agrees with repeat laser altimetry while revealing clear patterns of surface elevation change associated with ice advection channelized ice shelf basal melt subglacial lake activity and possible grounding line migration

key drivers controlling stable isotope variations in daily precipitation of costa rica caribbean sea versus eastern pacific ocean moisture sources abstract costa rica is located on the central american isthmus which receives moisture inputs directly from the caribbean sea and the eastern pacific ocean this location includes unique mountainous and lowland microclimates but only limited knowledge exists about the impact of relief and regional atmospheric circulation patterns on precipitation origin transport and isotopic composition therefore the main scope of this project is to identify the key drivers controlling stable isotope variations in daily scale precipitation of costa rica the monitoring sites comprise three strategic locations across costa rica heredia central valley turrialba caribbean slope and cano seco south pacific slope sporadic dry season rain is mostly related to isolated enriched events ranging from 5 8 to 0 9 18o by mid may the intertropical convergence zone reaches costa rica resulting in a notable depletion in isotope ratios up to 18 5 18o hysplit air mass back trajectories indicate the strong influence on the origin and transport of precipitation of three main moisture transport mechanisms the caribbean low level jet the colombian low level jet and localized convection events multiple linear regression models constructed based on random forests of surface meteorological information and atmospheric sounding profiles suggest that lifted condensation level and surface relative humidity are the main factors controlling isotopic variations these findings diverge from the recognized amount effect in monthly composite samples across the tropics understanding of stable isotope dynamics in tropical precipitation can be used to a enhance groundwater modeling efforts in ungauged basins where scarcity of long term monitoring data drastically limit current and future water resources management b improve the re construction of paleoclimatic records in the central american land bridge c calibrate and validate regional circulation models

elements of the archean thermal history and apparent polar wander of the eastern kaapvaal craton swaziland from single grain dating and paleomagnetism abstract single grains of zircon hornblende biotite and feldspar have been dated to define the thermal history of the archean mbabane pluton swaziland coincident207pb 206pb zircon and40ar 39ar hornblende ages suggest that the pluton underwent rapid cooling to about 450 c at the time of emplacement at 2690 ma because the mbabane pluton is one of a suite of granites which represent the last major archean intrusive event in the eastern kaapvaal craton this time of emplacement marks the end of cratonization much younger biotite and feldspar ages indicate argon loss perhaps due to several low temperature events related to dike intrusion at 2000 2300 ma from such a thermal history the age of magnetization of a paleomagnetic pole polelatitude 19 7 n polelongitude 105 7 e a95 9 4 k 67 obtained from 5 sites can be determined based on the unblocking temperature of the magnetic carrier and the consistent direction seen in several sites it appears that the hornblende age and coincidently the207pb 206pb zircon age most accurately represents this age of magnetization thus the mbabane pluton yields a precisely dated archean 2687 6 ma 2 hornblende plateau age paleomagnetic pole which is different than other archean poles such as that from the nearby usushwana complex 2875 40ma although the two intrusions are separated by less than 12 km and there is no evidence of relative structural rotation between them the paleomagnetic poles from the two bodies are 65 apart this implies that there was significant apparent polar wander during the late archean

evidence for a very long term trend in geomagnetic secular variation reconstructions of palaeosecular variation suggest that the earth s magnetic field reversed less frequently 2 82 to 2 45 billion years ago relative to the cenozoic era this suggests a long term trend of decreasing geodynamo stability since the archaean eon the earth s inner core is believed to inhibit rapid fluctuations in the geomagnetic field from developing into full polarity reversals1 2 consequently during the precambrian the smaller size of the inner core might suggest that polarity reversals could occur more readily it is therefore surprising that there are indications that reversals were rare during this period3 4 here we use new and existing palaeomagnetic data from three continents to examine the stability of the earth s magnetic field from 2 82 to 2 45 billion years ago we show that on average geomagnetic secular variation the field variations produced by normal geodynamo action during the late archaean and early proterozoic was different from that of the past 200 million years specifically the apparent variability of the geomagnetic pole as viewed at low and mid latitudes was reduced relative to the past 200 million years according to both dynamo simulations4 and more recent palaeomagnetic field observations5 the observed pattern of secular variation suggests a lower frequency of polarity reversals 2 5 billion years ago this may imply that the geodynamo is becoming progressively less stable over long timescales consistent with some numerical simulations6 7 possibly as a result of changing outer core geometry that has accompanied inner core growth

revisiting the paleomagnetism of the neoarchean uaua mafic dyke swarm brazil implications for archean supercratons abstract the original connections of archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes the uaua block of the northern sao francisco craton may represent a fragment of a major archean craton here we report new paleomagnetic data from the 2 62 ga uaua tholeiitic mafic dyke swarm of the uaua block in the northern sao francisco craton eastern brazil our paleomagnetic results confirm the earlier results for these units but our interpretation differs we suggest that the obtained characteristic remanent magnetization for the 2 62 ga swarm is of primary origin supported by a provisionally positive baked contact test the corresponding paleomagnetic pole 25 2 n 330 5 e a95 8 1 n 20 takes the present northern part of the sao francisco craton to moderate latitudes based on the comparison of the paleolatitudes of cratons with high quality paleomagnetic data and magmatic barcodes we suggest that the northern part of the sao francisco craton could have been part of the proposed supervaalbara supercraton during the archean supervaalbara is proposed as including but not limited to the part of the sao francisco craton as well as the superior wyoming kola karelia zimbabwe kaapvaal tanzania yilgarn and pilbara cratons

early proterozoic palaeomagnetic data from the pechenga zone north west russia and their bearing on early proterozoic palaeogeography summary an early proterozoic palaeomagnetic signature c 2125 ma verified by a positive conglomerate test is recorded in the kuetsyarvi formation pechenga group north west russia but the majority of the palaeomagnetic directions observed in the pechenga group lithologies reflect a low grade remagnetization event probably linked with the late precambrian baikalian orogeny which affected north west russia and northern norway secondary pyrrhotite is the dominant remanence carrier in the uppermost formations of the pechenga group palaeomagnetic poles from the kuetsyarvi formation differ somewhat or partially overlap with coeval palaeomagnetic poles from other tectonomagmatic provinces in northern fennoscandia but it is premature at this stage to speculate on intraplate movements during the early proterozoic besides the kuetsyarvi formation probably developed during an early phase of intracontinental rifting along the northern margin of fennoscandia similar to the present day east african rift hence younger intercontinental rifting possible seafloor spreading and subsequent convergence would remain undetected by our palaeomagnetic data palaeolatitude estimates from the kuetsyarvi formation suggest that the pechenga region was located in latitudes of around 20 to 30 during the 2100 2200ma interval these low latitude estimates are supported by the sedimentary record in the pechenga region which is characterized by red beds and evaporites comparison of fennoscandian palaeomagnetic poles with coeval poles from the slave and superior cratons laurentia questions previously publicized supercontinental configurations a close relationship between fennoscandia and early proterozoic laurentian provinces is conceivable from palaeomagnetic data but given the lack of longitudinal control as well as the choice of hemisphere such postulates are tentative at best on purely palaeomagnetic grounds

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

bed topography and surges in ice streams 1 extensive bed topography at the scale of the ice thickness occurs frequently in the form of drumlins by analogy with hard bed sliding ice flow over this type of topography leads to the generation of drag on the ice particularly when ice flow is rapid a crucial difference with classical nye kamb sliding is that the upper free surface of the ice has a significant effect on the sliding process through the formation of a standing wave using a theoretical model we demonstrate that the presence of this wave introduces a non linearity into the sliding motion which can lead to multiple sliding velocities for the same large scale ice stream geometry and that switches between these velocities can cause surging behavior

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

dating post metamorphic cooling of the eastern granulites in the mozambique belt of northern tanzania using the garnet sm nd method abstract granulites from the usambara wami river and uluguru areas in the northern part of the mozambique belt in tanzania yield sm nd garnet whole rock ages of between 580 and 634 ma with a mean of 609 11 ma 2 this mean age is only slightly younger than the previously published peak metamorphic age of 641 2 ma suggesting that contrary to some earlier arguments garnet sm nd ages can be used to closely constrain the age of peak metamorphism even in slowly cooled terranes using published peak metamorphic temperatures of 810 c and cooling rates of 1 4 c ma the mean age translates into garnet closure temperatures of 690 to 780 c the similarity in garnet ages over widely separated areas coupled with the previously established similarity in equilibrium pt conditions indicate that isolated complexes that form the eastern granulites in the tanzanian sector of the mozambique belt share the same thermal history and were formed under the same geodynamic setting a few published garnet ages of between 525 and 545 ma indicate a younger less pervasive event of granulite facies metamorphism in the belt the bimodal distribution of garnet ages supports a previously published hypothesis that the assembly of gondwana took place in two stages the 610 ma old ages most likely date cooling from granulite facies metamorphism arising from regional crustal thickening associated with the amalgamation of india madagascar parts of eastern antarctica the kalahari craton the congo craton and the arabian nubian shield forming the imslek ans collage on the other hand the 525 545 ma ages may mark cooling from a thermal event associated with the collision of australo antarctica with the imslek ans collage

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock

hydrogen isotopic compositions along a precipitation gradient of chinese loess plateau critical roles of precipitation evaporation and vegetation change as controls for leaf wax δd abstract the hydrogen isotopic compositions d of long chain plant leaf waxes can reflect changes of continental hydrology and thus have been increasingly utilized for paleoclimate reconstruction one of the unresolved major issues is whether variations of leaf wax d dwax signals along a precipitation gradient reflect changes of precipitation d precipitation amount and or evapo transpiration this ambiguity limits our interpretation of dwax in geological records as well as the quantitative reconstruction of paleohydrological variability here we systematically investigated d of soil water and waxes extracted from soil and plant leaves in the chinese loess plateau clp and its surrounding areas along a precipitation gradient with mean annual precipitation map varying from 140 mm to 676 mm the results showed that while the variation of modeled precipitation d has no significant correlation with map soil water d soil dwax and individual plant dwax exhibit negative correlations with map in relatively arid areas the d values of soil water and plant leaf waxes are significantly more positive due to much lower precipitation relative to evapo transpiration suggesting that effective precipitation p e ratio has played a crucial role in the d enrichment of soil and plant leaf waxes in this region while parallel decreasing trends in dwax are found along the increase of precipitation gradient among dominant plants including artemisia spp aster hispidus stipa bungeana and cleistogenes squarrosa in the region with similar slopes the large d offsets between plant groups suggest that plant type is an important factor in controlling plant hydrogen isotope fractionations our results indicate that dwax preserved in paleosols can be used to infer past conditions of water availability in arid and semi arid inland regions but with a mechanism that is different from the influence of amount effect in humid areas moreover vegetation changes should be constrained by independent paleobotanical data before monsoon related paleohydrology in the clp and its surrounding areas can be quantitatively reconstructed using the plant wax d proxy

the palaeomagnetism of the main zone in the western bushveld complex abstract sampling was done at thirty four sites in the main zone of the western bushveld complex south africa alternating field af and thermal demagnetization of specimens indicated a multicomponent nrm in the majority of specimens a primary magnetization direction d 357 8 i 70 9 95 2 8 k 80 was recovered from thirty two sampling sites fold tests indicate that the igneous layering was emplaced in a horizontal position and that the nrm predates tilling of the layered sequence two sampling sites situated in the lower part of the main zone yielded magnetization directions approximately antipodal to the above mentioned magnetization direction the palaeomagnetic pole position 9 2 n 27 3 e calculated for the main zone of the western bushveld complex indicates that this part of the main zone had acquired its magnetization later than the main zone in the eastern bushveld complex and later than the underlying critical zone

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

flow dynamics and iceberg calving rates of devon ice cap nunavut canada the surface velocity field of devon ice cap nunavut canada was mapped using interferometric synthetic aperture radar insar ascending european remote sensing satellite 1 and 2 ers 1 2 tandem mode data were used for the western and southeast sectors and 3 day repeat pass ers 1 imagery for the northeast sector speckle tracking procedures were used with radarsat 1 imagery to obtain surface velocities over the terminus of belcher glacier a major calving front where decorrelation between ers data occurred the insar data highlight a significant contrast in ice flow dynamics between the east and west sides of the ice cap ice movement west of the main north south divide is dominated by relatively uniform sheet flow but three fast flowing outlet glaciers that extend 14 23 km beyond the ice cap margin also drain this region several outlet glaciers that extend up to 60km inland from the eastern margin drain the eastern side of the ice cap the dominant ice flow regimes were classified based on the relationship between the driving stress averaged over a length scale often ice thicknesses and the ratio of surface velocity to ice thickness the mapped distribution of flow regimes appears to depict the spatial extent of basal sliding across the ice cap this is supported by a close relationship between the occurrence of flow stripes on the ice surface and flow regimes where basal sliding was found to be an important component of the glacier motion iceberg calving rates were computed using measured surface velocities and ice thicknesses derived from airborne radio echo sounding the volume of ice calved between 1960 and 1999 was estimated to be 20 5 4 7 km 3 or 0 57km 3 a 1 approximately 89 of this loss occurred along the eastern margin the largest single source is belcher glacier which accounts for 50 of the total amount of ice calved

water isotopic variability in response to mesoscale convective system over the tropical ocean 1 in tropical regions the empirical negative relationship between the isotopic content of precipitation and rainfall amount known as the amount effect has been used as a rationale for paleohydroclimate reconstruction from isotope records however there is still no comprehensive physical explanation for this empirical effect here we reconsider the well known amount effect using newly available isotope data for both surface water vapor and precipitation obtained from shipboard observations in this study we hypothesized that stratiform rainfall associated with mesoscale convective systems mcss is a key process in reducing tropical water isotopic concentrations and tested this hypothesis with an idealized mcs model our conceptual model reasonably accounted for several observed features and indicated that isotopic reductions in tropical oceanic regions reflect a precipitating system s change relatively high isotope ratios corresponded to disorganized convection on the other hand mcss were characterized by lower isotope ratios with increasing stratiform area in addition the amplitude of this isotopic depletion was related to the scale of the precipitation system the lowest isotopic ratios were observed during the passage of large scale disturbances corresponding to the madden julian oscillation convective envelope in which mcss are embedded this means that the frequent appearance of mcss results in further decreases in the isotopic ratio of surface vapor and precipitation from this we conclude that the amount effect can be interpreted as the development of a precipitation system from an isolated convection cell into a large scale system containing several mcss and that past large scale convective activity can be reconstructed from isotope records

diurnal to interannual rainfall δ 18 o variations in northern borneo driven by regional hydrology the relationship between climate variability and rainfall oxygen isotopic 18o variability is poorly constrained especially in the tropics where many key paleoclimate records rely on past rainfall isotopes as proxies for hydroclimate here we present a daily resolved 5 yr long timeseries of rainfall 18o from gunung mulu national park located in northern borneo 4 n 114 e in the heart of the west pacific warm pool and compare it to local and regional climatic variables daily rainfall 18o values range from 0 7 to 18 5 and exhibit a weak but significant inverse relationship with daily local precipitation amount r 0 19 p 0 05 consistent with the tropical amount effect day to day 18o variability at mulu is best correlated to regional precipitation amount averaged over the preceding week r 0 64 p 0 01 the inverse relationship between mulu rainfall 18o and local regional precipitation amount increases with increased temporal averaging reaching r 0 56 r 0 72 on monthly timescales large negative multi day rainfall 18o anomalies of up to 16 occur every 30 90 days and are closely associated with wet phases of the intraseasonal madden julian oscillation a weak semi annual seasonal cycle in rainfall 18o of 2 3 bears little resemblance to seasonal precipitation variability pointing to a complex sequence of moisture sources and or trajectories over the course of the year interannual rainfall 18o variations of 6 8 are significantly correlated with indices of the el nino southern oscillation with increased rainfall 18o during relatively dry el nino conditions and vice versa during la nina events we find that mulu rainfall 18o outperforms mulu precipitation amount as a tracer of basin scale climate variability highlighting the time and space integrative nature of rainfall 18o taken together our results suggest that rainfall 18o variability at mulu is significantly influenced by the strength of regional convective activity as such our study provides further empirical support for the interpretation of 18o based paleo reconstructions from northern borneo stalagmites as robust indicators of regional scale hydroclimate variability where higher 18o reflects regional drying

stable isotope proxy records in tropical terrestrial environments abstract the tropics refers to an area that is loosely equatorial characterized by relatively high temperatures at low altitude comparatively low seasonal contrast in temperatures and generally high or highly seasonal rainfall the stable isotopes of carbon oxygen and hydrogen are increasingly important tools of relevance to understanding trajectories and mechanisms of past environmental change in the tropics in the case of carbon this is because at the broadest level the carbon isotope composition of organic and carbon containing inorganic proxy materials formed in the tropics can be read as being derived from the balance between c3 tree and c4 grass photosynthesis hence providing a relative measure of vegetation type and indirectly climate likewise changes in the oxygen and hydrogen isotope composition of many proxy materials again at the broadest level can be read as indicative of changes in hydrological conditions and or altitude in the past this review provides an entry point into the literature relating to the use of stable isotopes as terrestrial environmental proxies for past environmental change with a focus on the measurement and interpretation of stable isotope data from the wide range of proxy materials preserved in tropical environments

the origin and longevity of flow stripes on antarctic ice streams flow stripes along the surface of rapidly moving ice streams are shown to be an expected reaction of a viscous medium flowing over an irregular bed whenever the velocity at the bed is large compared to shearing through the thickness the principal features of the process are as follows at high basal speeds the ice acts as a strongly selective band pass filter transmitting basal undulations on spatial scales of a few ice thicknesses very effectively to the surface the decay of the short scale features by diffusion is strongly retarded by horizontal stress gradients consequently localized disturbances at the bed produce topographic effects on the surface that are advected long distances downstream before decaying away this mechanism may explain many of the flow stripes on active ice streams moving by rapid basal motion

consolidation of continental crust in late archaean early proterozoic times a palaeomagnetic test abstract the proposition that continental crust consolidated during late archaean early proterozoic times as spatially distinct groups of cratons is tested using palaeomagnetic results assigned to the interval 2900 2200 ma geological data indicate the existence of two nuclei following widespread cratonisation at 2700 ma comprising ur stabilisation at 3000 ma and shallow water cover at 3000 2800 ma including central southern africa western australia and india and arctica which included laurentia fennoscandia and siberia stabilisation by 2500 ma with supracrustal cover beginning at 2400 ma a third nucleus atlantica comprising much of western africa and eastern south america consolidated later basement stabilisation at 2200 2000 ma and 2000 ma fluvial deltaic cover using this geometrical premise palaeomagnetic poles assigned to the interval 2900 2200 ma from the two older nuclei are observed to plot along two limbs of an apw swathe over 80 of the poles plot on a single segment running through the west of africa and terminating in a tight group over australia and india with ur resided in high latitudes and arctica in low latitudes the 2900 2200 ma mean poles from africa australia fennoscandia and laurentia also accord closely with one another and show that only small relative movements occurred between the cratons during these times this polar grouping does not result from datasets of similar temporal concentration and is due to low rates of continental movement 1 cm year that are probably the signature of small scale tectonic regimes the primeval continental crust consolidated as two linear belts with a configuration approximating to the geoid form and suggesting an aggregation by whole mantle convection the relative positions of arctica and ur at 2900 2200 ma were only marginally different from their positions within the putative later supercontinent of palaeopangaea 1100 550 ma and imply that the proterozoic eon was characterised by relatively small intracratonic motions close to limits of palaeomagnetic detection

topography of streaklines on an antarctic ice shelf from photoclinometry applied to a single advanced land imager ali image we investigate the usefulness of the experimental advanced land imager instrument onboard the earth observing 1 satellite and a shape from shading technique to study streaklines flow stripes on the amery ice shelf antarctica local variations in brightness in the image are directly related to surface topography because the snow surface reflectivity is mostly uniform image brightness values are combined with sun position and streakline orientation to calculate surface topography to centimeter level vertical precision the amplitude of the streaklines is typically 1 2 m and spacing is on the order of 1 km the streaklines show complex along flow changes in cross sectional shape that we interpret to be effects from temporal changes in grounding line dynamics and decay of ice topography due to flow cross sectional areas of the streaklines are calculated at various points along flow to investigate morphological evolution the study area subscene covers a 28 km spl times 90 km area and covers spl sim 350 years of ice flow

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

submarine salt flows in the central red sea the central red sea an oceanic basin floored by miocene evaporites reaching kilome ters in thickness in places is at an early stage of development where seafl oor spreading has geologically only recently replaced continental rifting surveys using a high resolution multibeam echo sounder around thetis deep a new spreading center have revealed a remarkable series of structures resembling viscous gravity fl ows which are here interpreted as originating from fl owage of the evaporites laterally unloaded by axial rifting and other processes developing the relief of the deep the fl ow margins are marked by stream wise lineaments and some apparently rotated markers their fronts in the fl oor of the deep are rounded in plan view and profi le their surfaces contain small closely spaced features resembling extensional faults in one area below declining gradients the surface contains along slope ridges and valleys typical of compression folds ogives flowparallel lineaments and extensional faults lie respectively parallel and orthogonal to the direction of maximum seabed gradient movement is apparently heterogeneous at least in part because of varied blocking by relief in underlying basement observed protruding between fl ows flowage is currently transporting materials into the fl oor of the deep where they have the potential to become incorporated into the young oceanic crust by repeated eruption of axial lavas over them in the light of these new data we reexamine the possibility and implications of fl owage in the south atlantic marginal evaporites in particular whether fl owage contaminated early oceanic crust in such areas

regional scale climate influences on temporal variations of rainwater and cave dripwater oxygen isotopes in northern borneo this study investigates the relationship between large scale climate variability rainfall oxygen isotopic composition 18 o and cave dripwater 18 o at gunung mulu and gunung buda national parks in northern borneo 4 n 115 e on intraseasonal to interannual timescales a 3 yr timeseries of rainfall 18 o contains prominent seasonal and interannual variability the seasonal cycle in rainfall 18 o is defined by lighter values of 10 during late boreal summer and heavier values of 4 during late boreal winter and is poorly correlated to local precipitation which displays very weak seasonality seasonally varying moisture trajectories likely play a key role in the observed seasonal cycle of rainfall 18 o driving enhanced fractionation during boreal summer and less fractionation during boreal winter dripwater 18 o timeseries display 2 seasonal cycles that follow the rainfall 18 o seasonal cycles with a mean 18 o value equivalent to the mean 18 o of rainfall large surveys of cave dripwaters conducted during three fieldtrips to gunung mulu buda reveal a system wide response to rainfall 18 o seasonality that supports a relatively short less than 6months response time for most drips during the weak 2005 2006 la nina event sustained positive precipitation anomalies are associated with rainfall 18 o values that are 4 to 5 lighter than previous years values consistent with the tropical amount effect observed in both models and data dripwater 18 o values are 1 to 2 lighter during the weak la nina event the importance of the amount effect in driving intraseasonal rainfall 18 o anomalies at our site is supported by an 8 increase in rainfall 18 o that occurred over the course of two weeks in response to a pronounced decrease in regional convective activity dripwater discharge rates underwent a ten fold decrease during the extended dry period but dripwater 18 o values remained constant this study supports the interpretation of stalagmite 18 o records from gunung mulu buda as paleo precipitation records that are sensitive to the location and strength of deep convection in the west pacific warm pool 2007 elsevier b v all rights reserved

climatic and environmental controls on speleothem oxygen isotope values variations in speleothem oxygen isotope values d 18 o result from a complicated interplay of environmental controls and processes in the ocean atmosphere soil zone epikarst and cave system as such the controls on speleothem d 18 o values are extremely complex an understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem d 18 o as paleoclimate and paleoenvironmental proxies and is best complemented by study of site specific cave processes such as infiltration flow routing drip seasonality and saturation state and cave microclimate among others this review is a process based summary of the multiple controls on d 18 o in the atmosphere soil epikarst and speleothem calcite illustrated with case studies primary controls of d 18 o in the atmosphere include temperature and relative humidity through their role in the multiple isotope effects variability and modifications of water d 18 o values in the soil and epikarst zones are dominated by evaporation mixing and infiltration of source waters the isotopically effective recharge into a cave system consists of those waters that participate in precipitation of caco3 resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state recent modeling experimental and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid caco3 and have implications for the hendy test to assist interpretation of speleothem d 18 o time series quantitative and semi quantitative d 18 o climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records finally several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of d 18 o to reconstruct past climate

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

the midsummer drought over mexico and central america abstract the annual cycle of precipitation over the southern part of mexico and central america exhibits a bimodal distribution with maxima during june and september october and a relative minimum during july and august known as the midsummer drought msd the msd is not associated with the meridional migration of the intertropical convergence zone itcz and its double crossing over central america but rather with fluctuations in the intensity and location of the eastern pacific itcz during the transition from intense to weak weak to intense convective activity the trade winds over the caribbean strengthen weaken such acceleration in the trade winds is part of the dynamic response of the low level atmosphere to the magnitude of the convective forcing in the itcz the intensification of the trade winds during july and august and the orographic forcing of the mountains over most of central america result in maximum precipitation along the caribbean coast and minimum precipitation along the pacific

spatial and temporal variability in the stable isotope systematics of modern precipitation in china implications for paleoclimate reconstructions the stable isotopic composition of materials such as glacial ice tree rings lake sediments and speleothems from low to mid latitudes contains information about past changes in temperature t and precipitation amount p however the transfer functions which link 18op to changes in t or p d 18op dt and d 18op dp can exhibit significant temporal and spatial variability in these regions in areas affected by the southeast asian monsoon past variations in 18o and d of precipitation have been attributed to variations in monsoon intensity storm tracks and or variations in temperature proper interpretation of past 18op variations here requires an understanding of these complicated stable isotope systematics since temperature and precipitation are positively correlated in china and have opposite effects on 18op it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions here we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord the strength of these transfer functions in china is investigated using multiple regression analysis of data from 10 sites within the global network for isotopes in precipitation gnip 18op is modeled as a function of both temperature and precipitation the magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence 18op values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature and therefore exhibit more negative values in the summer in contrast 18op values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between 18op and temperature the sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation comparison with simple linear models 18op as a function of t or p and a geographic model 18op as a function of latitude and altitude shows that the multiple regression model is more successful at reproducing 18op values at sites that are strongly influenced by the summer monsoon the fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally since the southeast asian monsoon intensity is known to exhibit large variations on a number of timescales annual to glacial interglacial and the magnitude and sign of the transfer functions is related to monsoon intensity we suggest that as monsoon intensity changes the magnitude and possibly even the sign of the transfer functions may vary therefore quantitative paleoclimate reconstructions based on 18op variations may not be valid

a neoarchean paleomagnetic pole from the kisii series of western kenya implications for crustal mobility abstract the kisii series lavas of western kenya were the target of a paleomagnetic study the kisii series is a volcano sedimentary sequence dated to 2531 3 ma u pb that rests unconformably over the mesoarchean neoarchean nyanzian and kavirondian series the paleomagnetic study reported in this paper expands on an earlier study by brock et al 1972 using modern demagnetization and analysis techniques in spite of the advanced methods the results of both our new study and the previous study are statistically indistinguishable we therefore combine the results of both investigations to arrive at a grand mean pole at 7 s 166 e a95 8 due to the relatively minor tilt corrections a fold test is inconclusive however the dual polarity magnetization passes a reversal test we argue for a primary remanence on the basis of a comparison to other paleomagnetic data in kenya and tanzania and geological information we examine the relationships between age equivalent paleomagnetic poles from the tanzanian zimbabwe and kaapvaal cratons based on this limited dataset we cannot conclude whether or not the observed apparent polar wander was due to true polar wander or modern style plate tectonics such as would be implied by the variable apparent polar wander path segments we favor the latter explanation based on the disparate lengths of the zimbabwe versus tanzanian apparent polar wander paths during the 2700 2500 ma interval the protopangea model is not consistent with paleomagnetic data from these three cratons

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream