tectonic control on 10 be derived erosion rates in the garhwal himalaya india erosion in the himalaya is responsible for one of the greatest mass redistributions on earth and has fueled models of feedback loops between climate and tectonics although the general trends of erosion across the himalaya are reasonably well known the relative importance of factors controlling erosion is less well constrained here we present 25 10 be derived catchment averaged erosion rates from the yamuna catchment in the garhwal himalaya northern india tributary erosion rates range between 0 1 and 0 5 mm yr 1 in the lesser himalaya and 1 and 2 mm yr 1 in the high himalaya despite uniform hillslope angles the erosion rate data correlate with catchment averaged values of 5 km radius relief channel steepness indices and specific stream power but to varying degrees of nonlinearity similar nonlinear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion and that twofold to threefold differences in annual runoff are of minor importance in this area instead the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock uplift pattern is largely controlled by the shortening rate and the geometry of the main himalayan thrust fault mht our data support a shallow dip of the mht underneath the lesser himalaya followed by a midcrustal ramp underneath the high himalaya as indicated by geophysical data finally analysis of sample results from larger main stem rivers indicates significant variability of 10 be derived erosion rates possibly related to nonproportional sediment supply from different tributaries and incomplete mixing in main stem channels

increased late pleistocene erosion rates during fluvial aggradation in the garhwal himalaya northern india the response of surface processes to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution in the himalaya most large rivers feature prominent fill terraces that record an imbalance between sediment supply and transport capacity presumably due to past fluctuations in monsoon precipitation and or effects of glaciation at high elevation here we present volume estimates chronological constraints and 10 be derived paleo erosion rates from a prominent valley fill in the yamuna catchment garhwal himalaya to elucidate the coupled response of rivers and hillslopes to pleistocene climate change although precise age control is complicated due to methodological problems the new data support formation of the valley fill during the late pleistocene and its incision during the holocene we interpret this timing to indicate that changes in discharge and river transport capacity were major controls compared to the present day late pleistocene hillslope erosion rates were higher by a factor of 2 4 but appear to have decreased during valley aggradation the higher late pleistocene erosion rates are largely unrelated to glacial erosion and could be explained by enhanced sediment production on steep hillslopes due to increased periglacial activity that declined as temperatures increased alternatively erosion rates that decrease during valley aggradation are also consistent with reduced landsliding from threshold hillslopes as a result of rising base levels in that case the similarity of paleo erosion rates near the end of the aggradation period with modern erosion rates might imply that channels and hillslopes are not yet fully coupled everywhere and that present day hillslope erosion rates may underrepresent long term incision rates

measuring middle pleistocene erosion rates with cosmic ray produced nuclides in buried alluvial sediment fisher valley southeastern utah cosmic ray produced 10 be and 26 al in riverborne quartz sediment are commonly used to estimate average catchment scale erosion rates likewise the concentrations of these nuclides in ancient sediments stored in a depositional basin carry a record of past erosion rates in the sediment source area this is important because such a record could be compared to records of climate change or tectonic events to elucidate relationships between climate tectonics and erosion if the sediments are shielded from the cosmic ray flux after deposition for example in deep water their nuclide concentrations need only be corrected for radioactive decay since deposition in order to determine past erosion rates where sediment is deposited subaerially and buried relatively slowly on the other hand the additional nuclide concentration that builds up during sediment accumulation and storage must be reconstructed and subtracted in order to recover the initial nuclide concentrations in the sediment and thence the past erosion rates we describe an example of this process for an early to middle pleistocene section of alluvial sediment in fisher valley utah we use stratigraphic observations as well as an independently known age model for the sediment section to a subtract post depositional nuclide concentrations and reconstruct past erosion rates between 0 7 and 0 6 ma and b estimate the uncertainty in the results that arises from imperfect dating of the section and the natural variability in accumulation rates the present basinaveraged erosion rate in fisher valley is near 125 m ma 1 and middle pleistocene basinaveraged erosion rates varied between 80 and 220 m ma 1 changes in the erosion rate over time do not appear to be connected to glacial interglacial climate changes but may be related to episodic subsidence of the basin uncertainties are small in the case of low erosion rates and high sediment accumulation rates and large in the opposite situation in this example we could reduce the uncertainties by increasing the sampling density or by better relating our sample locations to the small scale stratigraphy of the sedimentary section in general future attempts to reconstruct past erosion rates from cosmogenic nuclide concentrations in ancient alluvium will be most successful in situations where post depositional nuclide accumulation is minimized for example in lakes or marine basins copyright 2005 john wiley sons ltd

landscape response to pleistocene holocene precipitation change in the western cordillera peru 10be concentrations in modern sediments and terrace fills 1 the landscape response to climate change is frequently investigated with models because natural experiments on geologic timescales are rare in quebrada veladera in the western andes mountains the formation of alluvial terraces during periods of high precipitation presents opportunities for such an experiment we compare drainage average erosion rates during pleistocene terrace deposition with holocene rates using cosmogenic 10be samples for seven pairs of quartz sand taken from the trunk and tributaries of quebrada veladera and adjacent terraces each pair consists of sediment collected from the modern channel and excavated from an adjacent fill terrace the terrace fill was deposited at 16 ka and preserved an isotopic record of paleoerosion rates in the late pleistocene modern sands yield 10be concentrations between 1 68 105 and 2 28 105 atoms g corresponding to holocene erosion rates between 43 3 and 58 4 mm kyr the 10be concentrations in terrace sands range from 9 46 104 to 3 73 105 atoms g corresponding to paleoerosion rates from 27 2 to 103 8 mm kyr smaller upstream tributaries show a substantial decline in erosion rate following the transition from a wet to dry climate but larger drainage areas show no change we interpret this trend to indicate that the wetter climate drove landscape dissection which ceased with the return to dry conditions as channel heads propagated upslope erosion accelerated in low order drainages before higher order ones this contrast disappeared when the drainage network ceased to expand at that point erosion rates became spatially uniform consistent with the uniformity of modern hillslope gradients

ionospheric signatures of dayside magnetopause transients a case study using satellite and ground measurements a case study is presented of coordinated ground and space measurements featuring a set of transient auroral fragments located on the poleward side of a stable cusp cleft arc optical ground data from ny alesund svalbard and heiss island franz josef land were combined with dmsp f9 satellite measurements to examine the characteristics of these auroral features a stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the imf poleward of the red arc were several westward moving auroral jets having characteristics similar to midday auroral breakup events such events may be ground signatures of transitory magnetic merging at the dayside magnetopause if so the driven convective motion of these structures should contribute to the polar cap potential within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential

quantifying sediment supply at the end of the last glaciation dynamic reconstruction of an alpine debris flow fan in this paper we quantify the sediment dynamics in the formerly glaciated zielbach catchment in the italian alps from the end of the last glacial maximum lgm until today as a basis for our quantification we use the stratigraphic record offered by a 3 5 km2 large fan that we explore with a seismic survey stratigraphic analyses of drillhole material and 14c ages measured on organic matter encountered in these drillings in addition we calculate past denudation rate variability in the fan deposits using concentrations of cosmogenic 10be we merge this information into a scenario of how the sediment flux has changed through time and how this variability can be related to climatic variations framed within well known paraglacial models the results document a highly complex natural system from the lgm to the very early holocene ice melted discharge and climate variability promoted a high sediment flux sedimentation rate up to 40 mm yr this flux then dramatically decreased toward interglacial values 0 8 mm yr at 5 4 calibrated kyr b p however in contrast to the trend of classic paraglacial models the flux recorded at zielbach shows secondary peaks at 6 5 ka and 2 5 ka with values of 13 mm yr and 1 5 mm yr respectively paleo denudation rates also decrease from 33 mm yr at the beginning of the holocene to 0 42 mm yr at 5 ka with peaks of 6 mm yr and 1 1 mm yr at 6 5 ka and 2 5 ka high amplitude climate change is the most likely cause of the secondary peaks but anthropogenic activities may have contributed as well the good correlation between paleo sedimentation and paleo denudation rates suggests that the majority of the deglaciated material destocked from the zielbach catchment is stored in the alluvial fan

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

large scale erosion rates from in situ produced cosmogenic nuclides in european river sediments abstract we have calculated long term erosion rates of 20 100 mm kyr from quartz contained 10be in the bedload of middle european rivers for catchments ranging from 102 to 105 km2 these rates average over 10 40 kyr and agree broadly with rock uplift incision and exhumation rates historic soil erosion rates and erosion rates calculated from the measured sediment loads of the same rivers moreover our new erosion rate estimates correlate well with lithology and relief however in the regen neckar loire and meuse catchments cosmogenic nuclide derived erosion rates are consistently 1 5 4 times greater than the equivalent rates derived from measured river loads this may be due to the systematic under representation of high magnitude low frequency transport events in the gauging records which cover less than a century alternatively the discrepancy may derive from spatially non uniform erosion and preferential tapping of deeper sections of the irradiation profile a third explanation relates the high cosmogenic nuclide derived erosion rates to inheritance of an elevated pleistocene erosion signal uncertainties associated with the cosmogenic nuclide derived erosion rate estimates are not greater than the potential errors in conventional estimates therefore the cosmogenic nuclide approach is an effective tool for rapid catchment wide assessment of time integrated rates of bedrock weathering and erosion and we anticipate its fruitful application to the quaternary sedimentary record

the erosion response to quaternary climate change quantified using uranium isotopes and in situ produced cosmogenic nuclides abstract studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution operates but also predict the consequences of future climate change on soil resource availability recent years have seen the development of tools that allow a quantitative assessment of past changes in catchment erosion this work reviews the principles of the application of in situ produced cosmogenic nuclides and uranium isotopes to quantifying past erosion rates results highlight the role of periglacial processes and mass wasting in dictating how catchment erosion responds to climatic variability at the 10 kyr scale at the million year scale it is more difficult to untangle the role of climate and tectonics a strong coupling exists at the 10 kyr to 100 kyr scales between climatic cycles and the transfer time of regolith from source to sink this coupling reflects changes in sediment source that are either set by changes in vegetation cover at the catchment scale or by the storage of sediments on continental shelves at a larger scale although further analytical developments are required for these tools to reach their full potential existing works suggest that in the near future they will provide unprecedented quantitative insights on how soil and fluvial systems adapt to external perturbations climatic tectonic and or anthropic

a geologically constrained monte carlo approach to modeling exposure ages from profiles of cosmogenic nuclides an example from lees ferry arizona we present a user friendly and versatile monte carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides tcns our program available online at http geochronology earthsciences dal ca downloads models html permits the incorporation of site specific geologic knowledge to calculate most probable values for exposure age erosion rate and inherited nuclide concentration while providing a rigorous treatment of their uncertainties the simulator is demonstrated with 10be data from a fluvial terrace at lees ferry arizona interpreted constraints on erosion based on local soil properties and terrace morphology yield a most probable exposure age and inheritance of 83 9 14 1 19 1 ka and 9 49 2 52 1 21 104 atoms g 1 respectively 2 without the ability to apply some constraint to either erosion rate or age shallow depth profiles of any cosmogenic nuclide except for nuclides produced via thermal and epithermal neutron capture e g 36cl cannot be optimized to resolve either parameter contrasting simulations of 10be data from both sand and pebble sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with tcn depth profiles and when possible sand not pebbles should be used for depth profile exposure dating

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

analysis of seismic sv waves in the core s penumbra vertically polarized s waves sv attenuate rapidly as they diffract around the earth s core whereas horizontally polarized s waves sh propagate to large distances in the core shadow zone the amplitude decay of diffracted sv signals is so strong that few studies have been made of these phases despite their acute sensitivity to velocity structure just above the core mantle boundary we analyze sv signals in the penumbra of the core s shadow finding systematic waveform complexities indicative of local stratification and apparent anisotropy at the base of the mantle with implications for the dynamic processes in this internal boundary layer

spatial and temporal variability in denudation across the bolivian andes from multiple geochronometers abstract we quantify spatial and temporal variations in denudation rates across the central andean fold thrust belt in bolivia with particular focus on the holocene measured and predicted 10 be cosmogenic radionuclide crn concentrations in river sediments are used to 1 calculate catchment averaged denudation rates from 17 basins across two transects at different latitudes and 2 evaluate the sensitivity of holocene climate change on the denudation history recorded by the crn data estimated denudation rates vary by two orders of magnitude from 0 04 to 1 93 mm yr 1 with mean values of 0 40 0 29 mm yr 1 in northern bolivia and 0 51 0 50 mm yr 1 in the south results demonstrate no statistically significant correlation between denudation rates and morphological parameters such as relief slope or drainage basin size in addition the crn derived denudation rates do not reflect present day latitudinal variations in precipitation comparison to 130 previously published denudation rates calculated over long thermochronology derived 10 6 yrs medium crn derived 10 2 10 4 yrs and short timescales sediment flux derived 10 1 yrs indicate temporal variations in denudation rates that increase between 0 and 200 over the last 5 ka crn modeling results suggest that the crn derived denudation rates may not be fully adjusted to wetter climate conditions recorded in the central andes since the mid holocene we conclude that large spatial variability in crn denudation may be due to local variations in tectonics e g faulting while large temporal variability in denudation may be due to temporal variations in climate

preliminary evidence for a lower mantle shear wave velocity discontinuity beneath the central pacific abstract in a dataset consisting of long period waveforms 5 20 s and differential travel times of s scs and the arrival from the d layer scd evidence is found for a laterally varying shear wave velocity v s discontinuity at the base of the mantle two different localized d regions beneath the central pacific have been investigated predictions from a model having a v s discontinuity 180 km above the core mantle boundary cmb agree well with observations for an eastern mid pacific cmb region this thickness differs from v s discontinuity thicknesses found in other regions which average near 280 km the data presented here from a western mid pacific cmb region are more complicated and are difficult to fit with a one dimensional 1 d structure some of the data from the western region show evidence for a thicker d layer approximately 280 km though this finding is poorly constrained our data do not resolve the sharpness of the v s jump at the top of d i e the depth range over which the v s increase occurs and in fact may be modeled equally well by a lower mantle with the increase in v s at the top of d occurring over a 100 km depth range the scd s and scs s differential travel times were corrected for a published 3 d mantle model to study upper and middle mantle heterogeneity effects on these times

limits to quantifying climate driven changes in denudation rates with cosmogenic radionuclides abstract the hypothesis that climate change significantly modifies the denudation history of an orogen can be tested by reconstructing catchment wide denudation histories cosmogenic radionuclides measured in sediment deposits of known age e g a sequence of fluvial terraces offer one approach for quantifying catchment wide paleo denudation rates we present a numerical model and two case studies that evaluate the utility of this technique for reconstructing different denudation histories the concentration of cosmogenic beryllium 10 10 be is calculated for a sequence of sediments produced from climate driven variations in the mean denudation rate of a drainage basin at sea level and high latitude the model accounts for the nucleonic and muonic production and decay of 10 be as sediment is eroded and stored in deposits of different ages two scenarios for different climate driven denudation histories are considered 1 a sinusoidal variation in the input denudation rate with free parameters of periodicity 23 41 100 kyr amplitude 0 1 0 5 1 0 and mean denudation rate 0 01 0 1 1 0 mm yr 1 2 a climate driven denudation history based on oscillations of a global benthic foraminiferal oxygen isotope 18 o record over the last 2 myr results from both scenarios suggest cosmogenic radionuclides measured in a system with high denudation rates 0 5 mm yr 1 longer periodicity 100 kyr and low amplitude have the highest potential for reconstructing climate driven paleo denudation rates unfortunately the high denudation rates preferred in the theoretical consideration are not suitable for paleo denudation rate determination due to overprinting of nuclides accumulated during denudation by post depositional decay and irradiation hence a compromise between the capability to determine changes in paleo denudation rates and the determination of true denudation rates has to be accepted application of the model to previous data sets from northwest europe and southwest north america produce a good agreement but highlight the need for future work to consider how geomorphic processes e g landsliding respond to climate change and influence catchment wide denudation rates in addition other climate proxies than the benthic foraminiferal 18 o record should be used for specific sample locations

rock uplift rates in south africa from isochron burial dating of fluvial and marine terraces the high plateau of southern africa is considered to be either uplifting due to mantle driven dynamic topography or to have been stable since mesozoic rifting to address this debate we determined rock uplift in south africa from the long term incision rate of the sundays river near port elizabeth and from an uplifted marine terrace near durban we dated the terraces with cosmogenic 26 al and 10 be using both isochron and simple burial dating methods we find that the sundays river has incised at 16 1 1 3 m m y for the past 4 m y and the marine terrace yields a rock uplift rate of 9 4 2 2 m m y these results are inconsistent with rapid neogene uplift

erosion rates driven by channel network incision in the bolivian andes the bolivian andes flank one of earth s major topographic features and dominate sediment input into the amazon basin millennial scale erosion rates and dominant controls on erosion patterns in this range are poorly known to define these patterns we present 48 erosion rate estimates derived from analysis of in situ 10 be in quartz bearing alluvium collected from the upper beni river basin erosion rates corrected for the non uniform distribution of quartz in the sample basins range from 0 04 mm a 1 to 1 35 mm a 1 and thus integrate over 10 2 10 4 years mean and modal values are 0 42 standard deviation 0 29 and 0 2 0 4 mm a 1 respectively within the range of long term average erosion rates in this area derived from apatite fission track thermochronology 0 1 0 6 mm a 1 hence our data do not record any significant variation in erosion rate over the last several million years mean and modal short term erosion rates for the andes are an order of magnitude lower than rates in the ganges river headwaters in the high himalaya and an order of magnitude greater than rates typical of the european alps in the upper beni river region of the bolivian andes short term basin averaged erosion rates correlate with normalized channel steepness index a metric of relative channel gradient corrected for drainage area neither normalized channel steepness index nor basin averaged erosion rate shows strong correlation with mean basin hillslope gradient or mean basin local relief because many hillslopes in the upper beni river region are at threshold values of slope and local relief patterns of normalized channel steepness index appear primarily to reflect tectonic patterns and transient adjustment to those patterns by channel networks climate and lithology do not appear to exert first order controls on patterns of basin averaged erosion rates in the bolivian andes copyright 2005 john wiley sons ltd

day night coupling by a localized flow channel visualized by polar cap patch propagation we present unique coordinated observations of the dayside auroral oval polar cap and nightside auroral oval by three all sky imagers two super dual auroral radar network superdarn radars and defense meteorological satellite program dmsp 17 this data set revealed that a dayside poleward moving auroral form pmaf evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications pbis superdarn observations detected fast antisunward flows associated with the pmaf and the dmsp satellite whose conjunction occurred within a few minutes after the pmaf initiation measured enhanced low latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst the polar cap patch was spatially and temporally coincident with a localized antisunward flow channel the propagation across the polar cap and the subsequent pbis suggests that the flow channel originated from dayside reconnection and then reached the nightside open closed boundary triggering localized nightside reconnection and flow bursts within the plasma sheet

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

slope and climate variability control of erosion in the andes of central chile climate and topography control millennial scale mountain erosion but their relative impacts remain matters of debate confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long term erosion however there is a lack of data documenting these erosion factors here we report suspended load measurements derived decennial erosion rates and 10 be derived millennial erosion rates along an exceptional climatic gradient in the andes of central chile both erosion rates decennial and millenial follow the same latitudinal trend and peak where the climate is temperate mean runoff 500 mm yr 1 both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of 0 55 m m the comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0 in the humid zone to more than 90 in the arid zone our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

relating the spatial variability of chemical weathering and erosion to geological and topographical zones abstract the relationship between the spatial variability of soil elements weathering and erosion rates can be very complex topography abruptly changing elevation gradients and slopes of varying geological composition exert an important influence on weathering trajectories and erosion this complex interconnectivity is only rarely addressed in weathering studies the main objectives of this investigation are therefore to relate weathering and erosion to various and geomorphic units in a dry alpine and threshold landscape having steep slopes kan catchment tehran province iran a number of common weathering indices were tested using a genetic algorithm the best indices are the k na ti ratio the wip weathering index according to parker and the pi product index according to ruxton however the recently suggested 4si m r2 and m f wmin systems discriminate weathering trends more accurately our results show that these soils have a low to moderate weathering stage weathering trajectories point to the active formation of kaolinite and oxyhydroxides river sediments are slightly more weathered and contain the most weathered topsoils and suggest erosion in the catchment is predominantly due to topsoil removal chemical weathering is influenced by the dominant geomorphic units unconsolidated deposits of the quaternary and solid bedrock and landform features combination of altitude and slope aspect does not appear to have a significant impact on weathering denudation erosion rates were determined using meteoric 10be since the fluvial sediments did not contain enough quartz for in situ 10be analyses this procedure has higher uncertainties due to difficulties in estimating the depositional flux of meteoric 10be due to tectonic uplift the erosion rates are very high to maintain a soil layer soil production rates must be also high although annual precipitation is relatively low slope and soil dynamics seem particularly high which leads to a fast turnover of the soil material and maintains the weathering intensity at a rather low to intermediate level

altitude adjusted corrected geomagnetic coordinates definition and functional approximations analysis of the functional approximations used to transform between geo graphic and altitude adjusted corrected geomagnetic aacgm coordinates reveals that errors of 50 km can occur in the auroral and polar regions these errors are the result of efforts to better approximate aacgm coordinates near the magnetic equator and the south atlantic anomaly in these regions aacgm coordinates are not defined and alternate coordinates have been used this augmentation and emphasis on the so lution in regions near the equator result in spherical harmonic approximating functions that are less accurate than need be in the auroral and polar regions in response a new set of spherical harmonic coefficients have been derived that better represent aacgm coordinates in these regions these new aacgm coefficients are limited to below 2000 km in altitude in order to ensure accuracy for altitudes above 2000 km a magnetic field line tracing solution is recommended a software package developed to take advantage of the new aacgm coefficients provides the capability of tracing magnetic field lines at any altitude for improved accuracy in addition linear interpolation between 5 year epochs is used to produce coordinates that vary smoothly over the entire period from 1965 present the intent of this work is to provide a more accurate procedure for de termining aacgm coordinates in the auroral and polar regions for the study of mag netospheric and ionospheric processes

quantifying the erosional impact of a continental scale drainage capture in the duero basin northwest iberia formerly closed drainage basins provide exceptional settings for quantifying fluvial incision and landscape dissection at different time scales endorheic basins trap all the sediment eroded within the watershed which allows estimates of post basin opening erosion patterns the duero basin was a former closed basin and is presently drained by the duero river into the atlantic ocean during the cenozoic the basin experienced a long endorheic period marked by the formation of continental carbonates and evaporites the retrogressive erosion of the atlantic drainage coming from the portuguese coast subsequently captured the internal drainage and significant fluvial dissection occurred presently the basin contains a relatively well preserved sedimentary fill gridding and surface fitting in this paper provide the first attempt to reconstruct the surface of the top of the former endorheic sedimentary sequence to quantify the erosional impact of the basin opening at least 2251 524 km3 of sediment was removed from the formerly closed basin following the start of exorheism this volume represents a mean basin surface lowering of 65 13 m erosion estimates and landscape dissection patterns are consistent with geologic evidence of progressive establishment of an outward drainage system

slow rates of rock surface erosion and sediment production across the namib desert and escarpment southern africa slow erosion has characterized the namib desert the namibian escarpment and the adjacent namibian highlands over the pleistocene paired analyses n 66 of in situ produced 10be and 26al in quartz bearing samples of bedrock primarily from inselbergs of sediment from dry river and stream channels and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years bedrock samples n 47 collected in three transects from the coast across the escarpment and into the highlands show no spatial pattern in elevation normalized nuclide abundance despite a difference in mean annual precipitation map between sample sites at the coast map 400 mm yr 1 average model erosion rates inland of the escarpment 3 2 1 5 n 9 are indistinguishable from average rates seaward of the escarpment 3 6 1 9 n 38 indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands sediment samples n 3 from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster 16 m my 1 than either a basin in the highlands 5 m my 1 or a basin in the coastal plain 8 m my 1 data from large rivers n 4 constrain erosion rates averaged over 105 yrs and 104 to 105 km2 between 3 and 9 m my 1 small quartz clasts n 12 collected from four desert surfaces record extraordinarily long variable and in some cases complex exposure histories simple 10be model ages are as high as 1 8 my some minimum total histories considering both 10be and 26al and including both burial and exposure exceed 2 7 my as a group the namibian cosmogenic data do not support the model of significant and on going escarpment retreat the similarity of erosion rates calculated from 10be analysis of fluvial sediments and longer term 107 yr average mass removal rates estimated by others using fission track analysis of rock suggests that namibian erosion rates have reached a steady state and are changing little over time at outcrop scales the concordance of 10be and 26al in most bedrock samples suggests that the model of steady uniform bedrock erosion is valid there is no indication of intermittent burial shedding of thick rock slabs or stripping of previous cover at an intermediate scale a transect of bedrock samples north of gobabeb demonstrates that the northern boundary of the massive namib sand sea has been steady and unshifting similarly low cosmogenically estimated erosion rates across west and central namibia suggest that the landscape is in geomorphic steady state its overall appearance changing only slowly through time

glacial interglacial variation in denudation rates from interior texas usa established with cosmogenic nuclides abstract the brazos colorado and trinity rivers of texas drain a tectonically quiescent non glaciated and low relief landscape inland from the gulf of mexico where long term 10 3 10 5 a changes in denudation rates are probably driven largely by climate change here we use cosmogenic 10be to obtain spatially averaged denudation rates for these river catchments primarily from terrace deposits associated with glacial or interglacial intervals over the past half million years the denudation rates are 30 35 higher during interglacial periods than during glacial periods and correlate broadly with temperature the results are consistent with predictions from the bqart sediment flux model and support the hypothesis that increased weathering rates associated with warmer climates will accelerate landscape erosion furthermore by analyzing 26al 10be in these deposits we can estimate the bed load sourced from up catchment surfaces the stored coastal plain fraction varies from 10 to 30 and is greater during times of relatively lower sea level the results indicate that although sediment flux is moderated by coastal plain storage increased up catchment flux during warmer interglacial periods outpaces evacuation of stored sediment during glacial periods resulting in a net increase in sediment flux to the ocean during warm intervals if this temperature sediment flux relationship is valid beyond the plio pleistocene transition then global sediment flux to the ocean from passive non glaciated and low relief landscapes would have been greater during the pliocene than in the cooler quaternary

paleo erosion rates in central asia since 9 ma a transient increase at the onset of quaternary glaciations erosion is a fundamental player of the interactions existing between internal geodynamics and climate in particular through its influence on the carbon dioxide budget however long term 1 ma erosion rates estimated indirectly from sediment budget remain poorly constrained while some studies suggest that worldwide erosion rates increased at the plio pleistocene climatic transition 4 2 ma the validity of this observation and its significance is a matter of debate due to potential biases of the sedimentary record and to the influence of sea level fall on the global sedimentary flux to marginal basins in the present study we estimate erosion rates over the last 9 ma using in situ produced cosmogenic 10be concentrations measured in magnetostratigraphically dated continental sediments we focus on an intracontinental endorheic watershed draining the northern tianshan in central asia a key region regarding the ongoing debate while erosion rates between 0 1 and 1 mm yr 1 are derived from most of our record they reach values as high as 2 5 mm yr 1 from 2 5 to 1 7 ma then after 1 7 ma recent and modern erosion rates fell below 1 mm yr 1 this temporary increase is correlated with the onset of quaternary ice ages and suggests that global climate had a significant and transient impact on erosion

timing of european fluvial terrace formation and incision rates constrained by cosmogenic nuclide dating abstract age constraints of late cenozoic fluvial terraces are important for addressing surface process questions related to the incision rates of rivers or tectonic and climate controls on denudation and sedimentation unfortunately absolute age constraints of fluvial terraces are not always possible and many previous studies have often dated terraces with relative age constraints that do not allow for robust interpretations of incision rates and timing of terrace formation however in situ produced cosmogenic nuclides allow absolute age determination and hence incision rates of fluvial deposits back to 5 ma here we present cosmogenic depth profile dating and isochron burial dating of four different river systems in europe spanning 12 of latitude we do this to determine river incision rates and spatial variations in the timing of terrace formation isochron burial age constraints of four selected terraces from the vltava river czech republic range between 1 00 0 21 to 1 99 0 45 ma an isochron burial age derived for the allier river central france is 2 00 0 17 ma five terrace levels from the esla river nw spain were dated between 0 08 0 04 0 01 ma and 0 59 0 13 0 20 ma with depth profile dating the latter age agrees with an isochron burial age of 0 52 0 20 ma two terrace levels from the guadalquivir river sw spain were dated by depth profile dating to 0 09 0 03 0 02 ma and 0 09 0 04 0 03 ma the one terrace level from the guadalquivir river dated by isochron burial dating resulted in an age of 1 79 0 18 ma results indicate that the cosmogenic nuclide based ages are generally older than ages derived from previous relative age constraints leading to a factor 2 3 lower incision rates than previous work furthermore the timing of terrace formation over this latitudinal range is somewhat obscured by uncertainties associated with dating older terraces and not clearly synchronous with global climate variations

a 30 000 yr record of erosion rates from cosmogenic 10be in middle european river terraces abstract cosmogenic 10 be in river borne quartz sand records a time integrated erosion rate representative of an entire drainage basin when sequestered in a terrace of known age paleo erosion rates may be recovered from the nuclide content of the terrace material paleo erosion rates between 30 and 80 mm kyr are determined from terrace sediments 200 to 30 000 yr in age of the allier and dore rivers france and the meuse maas river the netherlands erosion rates determined from cosmogenic nuclides on terraces from the allier river are consistent with rates derived from the sedimentary fill of a lake in the allier catchment a strong decrease in cosmogenic nuclide derived erosion rates from terraces of the meuse river with late pleistocene to holocene age is observed the paleo erosion signal from cosmogenic nuclides records projection of the elevated late pleistocene erosion rate into the time integrated rates derived from middle european rivers

from source to sink preserving the cosmogenic 10be derived denudation rate signal of the bolivian andes in sediment of the beni and mamore foreland basins abstract the denudation rate signal of the bolivian andes as measured by cosmogenic 10 be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years this conclusion is drawn from comparing published andean source area denudation rates with new cosmogenic 10 be data as measured in the floodplains of the large beni and mamore basins for the entire beni basin including the sediment producing andes and the vast flooded plains of the foreland the cosmogenic nuclide derived denudation rate is 0 45 0 06 mm yr while that of the mamore basin is 0 55 0 19 mm yr by comparison the respective andean source areas erode at averaged rates of 0 37 0 06 mm yr upper beni and at 0 56 0 09 mm yr upper mamore we notice a decrease in variability of denudation rate with increasing spatial scale as small scale processes are averaged out sediment mixing within the floodplain damps scatter of nuclide derived denudation rates picked up in the source area on the temporal scale a remarkable agreement between cosmogenic nuclide derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide derived denudation rates capture the long term erosional characteristics of the mountain belt the sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area this finding opens the unique possibility of constraining cosmogenic nuclide derived paleo sediment budgets for these large basins as the long term spatially averaged denudation rate signal of the sediment producing area is preserved in sedimentary archives

an isochron method for cosmogenic nuclide dating of buried soils and sediments we describe an improved method for dating buried paleosols using measurements of the cosmic ray produced radionuclides 10be and 26al in quartz grains and apply it to a sequence of intercalated tills and paleosols in central missouri usa that record plio pleistocene advances of the laurentide ice sheet a buried paleosol implies a period of surface exposure and nuclide accumulation followed by burial and a halt to nuclide production if the paleosol is formed in a sedimentary unit such as till this unit may also have been emplaced with unknown 26al and 10be concentrations inherited from past surface exposure if the inherited nuclide concentrations are the same at all depths in the soil as is true for well mixed sediments such as till then the 26al and 10be concentrations at different depths in the paleosol will show a linear relationship the slope of this line depends on the duration of burial of the paleosol but not on the inherited nuclide concentrations or on the sample depths thus one can date strata overlying buried paleosols by measuring 26al and 10be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron we focus on applying this approach to till paleosol sequences but the basic idea of forming an 26al 10be burial isochron with a set of samples that share the same burial age but differ in other aspects of their exposure history applies to other stratigraphic settings as well the method yields ages for four tills in missouri that are stratigraphically consistent agree with paleomagnetic age constraints and show that ice advanced into missouri near 1 25 ma near 0 8 ma and twice between ca 0 4 and 0 2 ma

seismic anisotropy in the core mantle transition zone split s waves observed at hockley texas from events in the tonga fiji region of the southwest pacific show predominantly vertically polarized shear wave sv energy arriving earlier than horizontally polarized sh energy for rays propagating horizontally through d after corrections are made for the effects of upper mantle anisotropy beneath hockley a time lag of 1 5 to 2 0 s remains for the furthest events 93 9 100 6 while the time lags of the nearer observations 90 5 92 9 nearly disappear at closer distances the s waves from these same events do not penetrate as deeply into the lower mantle and are not split these observations suggest that a patch of d beneath the central pacific is anisotropic while the mantle immediately above the patch is isotropic the thickness of the anisotropic zone appears to be of the order of 100 200 km observations of shear wave splitting have previously been made for paths that traverse d under the caribbean and under alaska sh leads sv the reverse of the hockley observations but in these areas the fact that sv leads sh in the hkt data shown here suggests a different sort of anisotropy under the central pacific from that under alaska and the caribbean the case of sh travelling faster than sv is consistent with transverse isotropy with a vertical axis of symmetry vti and does not require variations with azimuth the case of sv leading sh is consistent with transverse isotropy with a horizontal axis of symmetry hti an azimuthally anisotropic medium and with a vti medium formed by a hexagonal crystal given that mg fe sio3 perovskite appears unlikely to form anisotropic fabrics on a large scale the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle possibly due to mantle core interactions

erosion rates on different timescales derived from cosmogenic 10 be and river loads implications for landscape evolution in the rhenish massif germany we determined erosion rates on timescales of 101 104 years for two catchments in the northeastern rhenish massif in order to unravel the quaternary landscape evolution in a variscan mountain range typical of central europe spatially averaged erosion rates derived from in situ produced 10be concentrations in stream sediment of the aabach and mohne watersheds range from 47 6 to 65 14 mm ka and integrate over the last 9 13 ka these erosion rates are similar to local rates of river incision and rock uplift in the quaternary and to average denudation rates since the mesozoic derived from fission track data this suggests that rock uplift is balanced by denudation i e the landscape is in a steady state short term erosion rates were derived from suspended and dissolved river loads subsequent to 1 correcting for atmospheric and anthropogenic inputs 2 establishing calibration curves that relate the amount of suspended load to discharge and 3 estimating the amount of bedload the resulting solid mass fluxes suspended and bedload agree with those derived from the sediment volume trapped in three reservoirs however resulting geogenic short term erosion rates range from 9 to 25 mm ka and are only about one third of the rates derived from 10be model simulations in combination with published sediment yield data suggest that this discrepancy is caused by at least three factors 1 phases with higher precipitation and or lower evapotranspiration 2 rare flood events not captured in the short term records and 3 prolonged periods of climatic deterioration with increased erosion and sediment transport on hillslopes

expression of active tectonics in erosional landscapes understanding the manner and degree to which topography in active mountain ranges reflects deformation of the earth s surface remains a first order goal of tectonic geomorphology a substantial body of research in the past decade demonstrates that incising channel systems play a central role in setting relationships among topographic relief differential rock uplift rate and climatically modulated erosional efficiency this review provides an introduction to the analysis and interpretation of channel profiles in erosional mountain ranges we show that existing data support theoretical expectations of positive monotonic relationships between channel steepness index a measure of channel gradient normalized for downstream increases in drainage area and erosion rate at equilibrium and that the transient response to perturbations away from equilibrium engenders specific spatial patterns in channel profiles that can be used to infer aspects of the forcing these aspects of channel behavior lay the foundation for a series of case studies that we use to illustrate how focused quantitative analysis of channel morphology can provide insight into the spatial and temporal dynamics of active deformation although the complexities of river response to climate lithology and uplift patterns mean that multiple interpretations of topographic data alone will always possible we show that application of stream profile analysis can be a powerful reconnaissance tool with which to interrogate the rates and patterns of deformation in active mountain belts

polar cap precursor of nightside auroral oval intensifications using polar cap arcs recent radar and optical observations suggested that localized fast flows in the polar cap precede disturbances within the nightside auroral oval however how commonly this connection occurs has been difficult to examine due to limited coverage of radar flow measurements and diffuse and dim nature of airglow patches polar cap arcs are also associated with fast flows in the polar cap and appear much brighter than patches allowing evaluation of the interaction between polar cap structures and nightside aurora more definitively we have surveyed data during six winter seasons and selected quasi steady polar cap arcs lasting 1 h thirty four arcs are found and for the majority 85 of them as they extend equatorward from high latitude their contact with the nightside auroral poleward boundary is associated with new and substantial intensifications within the oval these intensifications are localized 1 h magnetic local time mlt and statistically occur within 10 min and 1 h mlt from the contact they appear as poleward boundary intensifications in a thick auroral oval or an intensification of the only resolvable arc within a thin oval and the latter can also exhibit substantial poleward expansion when radar echoes are available they corroborate the association of polar cap arcs with localized enhanced antisunward flows that the observed oval intensifications are major disturbances that only occur after the impingement of polar cap arcs and near the contact longitude suggest that they are triggered by localized fast flows coming from deep in the polar cap

seismic anisotropy in the d layer we present observations of diffracted sv svd for a path between the fiji tonga islands and the eastern coast of north america at distances greater than 110 observed features of s diffracted suggest that coupling between svd and shd can be ruled out as a first order effect for this path arrivals of shd are late relative to iasp91 travel times by about 10 s and those of svd are late relative to shd by 3 s for most records the slope of the log svd shd spectral ratio is around 3hz 1 in the range 0 06 0 15 hz a transversely isotropic low velocity layer in the lowermost mantle with a thickness of 200 300 km may account for most of the observed properties of svd

predominant floodplain over mountain weathering of himalayan sediments ganga basin we present an extensive river sediment dataset covering the ganga basin from the himalayan front downstream to the ganga mainstream in bangladesh these sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column mineral sorting is the first order control on the chemical composition of river sediments taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain by comparing sediments sampled at the himalayan front with sediments from the ganga mainstream in bangladesh it is possible to budget weathering in the floodplain assuming a steady state weathering regime in the floodplain the weathering of himalayan sediments in the gangetic floodplain releases ca 189 92 10 9 and 69 22 10 9 mol yr of carbonate bound ca and mg to the dissolved load respectively silicate weathering releases 53 18 10 9 and 42 13 10 9 mol yr of na and k while the release of silicate mg and ca is substantially lower between ca 0 and 20 10 9 mol yr additionally we show that sediment hydration h2o is a sensitive tracer of silicate weathering that can be used in continental detrital environments such as the ganga basin both h2o content and the d h isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions by comparing the chemical composition of river sediments across the floodplain with the composition of the eroded himalayan source rocks we suggest that the floodplain is the dominant location of silicate weathering for na k and h2o overall this work emphasizes the role of the gangetic floodplain in weathering himalayan sediments it also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account 2012 elsevier ltd all rights reserved

tectonic uplift threshold hillslopes and denudation rates in a developing mountain range studies across a broad range of drainage basins have established a positive correlation between mean slope gradient and denudation rates it has been suggested however that this relationship breaks down for catchments where slopes are at their threshold angle of stability because in such cases denudation is controlled by the rate of tectonic uplift through the rate of channel incision and frequency of slope failure this mechanism is evaluated for the san bernardino mountains california a nascent range that incorporates both threshold hillslopes and remnants of pre uplift topography concentrations of in situ produced cosmogenic 10 be in alluvial sediments are used to quantify catchment wide denudation rates and show a broadly linear relationship with mean slope gradient up to 30 above this value denudation rates vary substantially for similar mean slope gradients we propose that this decoupling in the slope gradient denudation rate relationship marks the emergence of threshold topography and coincides with the transition from transport limited to detachment limited denudation the survival in the san bernardino mountains of surfaces formed prior to uplift provides information on the topographic evolution of the range in particular the transition from slope gradient dependent rates of denudation to a regime where denudation rates are controlled by rates of tectonic uplift this type of transition may represent a general model for the denudational response to orogenic uplift and topographic evolution during the early stages of mountain building

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

seismic anisotropy in the eastern united states deep structure of a complex continental plate we have analyzed shear wave splitting recorded by portable and permanent broadband and long period stations located in the eastern united states teleseismic shear waves sks skks and pks were used to retrieve the splitting parameters the orientation of the fast wave polarization plane and the delay time t in total 120 seismic events were processed allowing for more than 600 splitting measurements within the appalachians stations located in the western external part are characterized by t 1s and trending n50 70 e in the south and central regions and n30 40 e in the north closely following the trend of the orogenic belt in these areas the transition region between north and central is characterized by t 1 1 3 s and by e w trending that are at a high angle to the regional geologic trend measurements at two stations located in the eastern internal part of the belt indicate very weak anisotropy the large scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate splitting along the southern and eastern margins of the continent is consistent with that expected for grenvillian deformation an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out within the cratonic core the correlation between t and lithospheric thickness suggests a lithospheric anisotropy smaller length scale variations also argue for a significant contribution of lithospheric structures the fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern united states i e the grenville and appalachian orogenies and the subsequent rifting of the north atlantic ocean our observations in the western appalachians suggest that the anisotropy may be preserved since the grenvillian orogeny the absence of detectable splitting in the two stations in the eastern appalachians is attributed to the igneous intrusions related to the atlantic rifting the measurements in the transition between the northern and central southern appalachians constitute an intriguing anomaly whose e w have little obvious relation to the regional surface geology we suggest two possible causes 1 the local dominance of asthenospheric flow motivated by the proximity of a pervasive low velocity anomaly and 2 lithospheric deformation in a transcontinental strike slip fault zone active during the appalachian collision

seismic anisotropy in the lowermost mantle beneath the pacific onset time differences of up to 3 s are observed between transverse ssh and longitudinal ssv components of broadband s waves at distances of 85 to 120 for paths traversing the lowermost mantle d beneath the pacific after correction for upper mantle anisotropy ssh usually arrives earlier than ssv with the splitting increasing with distance from 100 to 120 the data yield two possible models of anisotropy 1 anisotropy may vary laterally with transverse isotropy existing in higher than average shear velocity regions beneath the northeastern pacific or 2 anisotropy may vary with depth with transverse isotropy concentrated in a thin 100 km thermal boundary layer at the base of d a few recordings at distances less than 105 show that ssv arrives earlier than ssh indicating that general anisotropy likely exists in shallower regions of d

shear wave splitting in the rio grande rift fast directions and lag times associated with sks shear wave splitting are determined for six digital stations in the rio grande rift rgr the mean fast direction for three stations in the central rgr is about 45 oblique to the axis of the rgr in contrast fast directions approximately parallel to the axis of the rgr are found for two stations in the southern rgr 10 and one station in the northern rgr 26 the fast directions for southern stations are not aligned with the ne sw extension direction of the oligocene to early miocene stage of extension the significant n s lateral variation in the sks shear wave splitting parameters is difficult to explain by fossil anisotropy because the mantle structure beneath the rgr is underlain by hot material with low seismic wave velocities hence the fast direction of azimuthal anisotropy is probably related to the preferred orientation of olivine created by the present deformation in the upper mantle because none of the fast directions are parallel to the present e w extension direction of the rgr the dominant upper mantle flow direction is not perpendicular to the axis of the rgr the pattern of the fast directions along the rgr provides evidence for a rising asthenosphere beneath the central to southern rgr and suggests that thermally driven small scale convection may be faster for flow parallel to the rift than perpendicular to it

denudation rates and tectonic geomorphology of the spanish betic cordillera abstract the tectonic control on landscape morphology and long term denudation is largely documented for settings with high uplift rates relatively little is known about the rates of geomorphic response in areas of low tectonic uplift here we evaluate spatial variations in denudation of the spanish betic cordillera based on cosmogenic 10be derived denudation rates denudation rates are compared to published data on rock uplift and exhumation of the betic cordillera to evaluate steady state topography the spatial patterns of catchment wide denudation rates n 20 are then analysed together with topographic metrics of hillslope and channel morphology catchments draining the betic ranges have relatively low denudation rates 64 54 mm kyr 1 but also show large variation as they range from 14 to 246 mm kyr 1 catchment wide denudation is linearly proportional to the mean hillslope gradient and local relief despite large spatial variation in denudation the magnitude and spatial pattern of denudation rates are generally consistent with longer term local uplift rates derived from elevated marine deposits fission track measurements and vertical fault slip rates this might be indicative of a steady state topography where rock uplift is balanced by denudation

spatial and temporal replicability of meteoric and in situ 10be concentrations in fluvial sediment cosmogenic isotopes short lived radionuclides elemental concentrations and thermochronometeric indicators are measured in river sand to quantify erosion rates trace sediment sources and or infer erosional processes interpretations of detrital sediment analyses are often based on the rarely tested assumption of time invariant tracer concentration better understanding of when and where this assumption breaks down and what sampling strategies minimize temporal and small scale spatial variance will improve science done using detrital river sediment here we present new and previously published spatial and temporal replicates measured for in situ and meteoric 10be 10bei and 10bem our new data include 113 replicate pairs taken from agricultural and or tectonically active watersheds in china months to millennia apart and spatial replicates taken up to 2 km apart on the same day the mean percent difference is 10 122 to 150 for both systems considered together the mode is close to 0 for both systems and 36 of pairs of samples replicate within our analytical accuracy at 2 we find that 10bei replicates better than 10bem p 0 01 10bei replicability is worse in steeper basins suggesting that stochastic processes i e landslides affect reproducibility 10bem replicability is worse in larger basins suggesting non conservative behavior of 10bem as sediment moves downstream our results are consistent with the few previously published replicate studies considering all replicate data in a wide range of landscapes in areas with deep erosional processes replicability is poor in other areas replicability is good this suggests that in steep tectonically active and or agricultural landscapes individual detrital sediment measurements do not represent upstream rates as well as larger populations of samples to ensure that measurements are representative of the upstream watershed our data suggest that samples be amalgamated either over time or from several places close by in the same channel

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

temperature and chemistry of the core mantle boundary seismic anomaly structures at the bottom of the lower mantle may be interpreted in terms of large temperature variations based on new laboratory data at very high pressure 1 sound velocity measurements at mantle pressures show an increase in ln ln p with depth this and the strong decrease in the thermal expansion coefficient yield thermal anomalies t of 1000 k using the seismically measured lateral velocity variations 2 high melting gradients in fe and fe o compounds measured at core pressures yield a temperature increase across the core mantle boundary in excess of 1300 k 3 melting temperatures of mg fe sio3 perovskite measured to deep mantle conditions extrapolate to 7500 k at the bottom of the mantle and result in ttm values between 0 3 and 0 4 this may strongly decrease the temperature dependence of viscosity if the average temperature profile of the lower mantle is nearly adiabatic and if the viscosity systematics found at low pressure are applicable at lower mantle conditions and 4 for dry conditions mgsio3 in contrast to mgo does not react with molten iron at pressures of the core mantle boundary this suggests that chemical interactions between the mantle and the core play a minor role

paleoerosion rates from cosmogenic 10be in a 1 3 ma terrace sequence response of the river meuse to changes in climate and rock uplift abstract river borne quartz carries a cosmogenic nuclide memory that is a function of the catchment wide erosion rate this record may be preserved in fluvial deposits such as river terraces if the age of a terrace is independently known and transport time in the river system is relatively short then the upstream erosion rate at the time of terrace deposition can be determined we have used cosmogenic nuclides to date river terraces in the lower meuse catchment the netherlands and to obtain a 1 3 ma record of paleoerosion rates in a 104 km2 drainage basin comprising the ardennes mountains paleoerosion rates were uniform within the range of 25 35 mm ka from 1 3 to 0 7 ma after 0 7 ma erosion rates have increased progressively to late pleistocene values of around 80 mm ka around 0 7 ma both climatic and tectonic boundary conditions changed the amplitude and duration of climate cycles increased significantly resulting in long periods of sustained low temperatures in the meuse catchment in additio

estimating rates of denudation using cosmogenic isotope abundances in sediment we propose as a testable hypothesis a basin scale approach for interpreting the abundance of in situ produced cosmogenic isotopes an approach which considers explicitly both the isotope and sediment flux through a drainage basin unlike most existing models which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on earth s surface our model is designed for interpreting isotope abundance in sediment because sediment is a mixture of materials in favourable cases derived from throughout a drainage basin we suggest that measured isotope abundances may reflect spatially averaged rates of erosion we investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed

paleo erosion rates and climate shifts recorded by quaternary cut and fill sequences in the pisco valley central peru fluvial cut and fill sequences have frequently been reported from various sites on earth nevertheless the information about the past erosional regime and hydrological conditions have not yet been adequately deciphered from these archives the quaternary terrace sequences in the pisco valley located at ca 13 s offer a manifestation of an orbitally driven cyclicity in terrace construction where phases of sediment accumulation have been related to the minchin 48 36 ka and tauca 26 15 ka lake level highstands on the altiplano here we present a 10be based sediment budget for the cut and fill terrace sequences in this valley to quantify the orbitally forced changes in precipitation and erosion we find that the minchin period was characterized by an erosional pulse along the pacific coast where denudation rates reached values as high as 600 80 mm ka600 80 mm ka for a relatively short time span lasting a few thousands of years this contrasts to the younger pluvial periods and the modern situation when 10be based sediment budgets register nearly zero erosion at the pacific coast we relate these contrasts to different erosional conditions between the modern and the minchin time first the sediment budget infers a precipitation pattern that matches with the modern climate ca 1000 km farther north where highly erratic and extreme el nino related precipitation results in fast erosion and flooding along the coast second the formation of a thick terrace sequence requires sufficient material on catchment hillslopes to be stripped off by erosion this was most likely the case immediately before the start of the minchin period because this erosional epoch was preceded by a 50 ka long time span with poorly erosive climate conditions allowing for sufficient regolith to build up on the hillslopes finally this study suggests a strong control of orbitally and ice sheet forced latitudinal shifts of the itcz on the erosional gradients and sediment production on the western escarpment of the peruvian andes at 13 during the minchin period

large scale imaging of high latitude convection with super dual auroral radar network hf radar observations the hf radars of the super dual auroral radar network superdarn provide measurements of the e b drift of ionospheric plasma over extended regions of the high latitude ionosphere with the recent augmentation of the northern hemisphere component to six radars a sizable fraction of the entire convection zone approximately one third can be imaged nearly instantaneously 2 min to date the two dimensional convection velocity has been mapped by combining line of sight velocity measurements obtained from pairs of radars within common volume areas we describe a new method of deriving large scale convection maps based on all the available velocity data the measurements are used to determine a solution for the distribution of electrostatic potential expressed as a series expansion in spherical harmonics the addition of data from a statistical model constrains the solution in regions of no data coverage for low order expansions the results provide a gross characterization of the global convection we discuss the processing of the radar velocity data the factors that condition the fitting and the reliability of the results we present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field imf in the case of a sudden polarity change from northward to southward imf the convection is seen to reconfigure globally on very short 6 min timescales

dating sediment burial with in situ produced cosmogenic nuclides theory techniques and limitations abstract dating sediment burial over million year time scales is crucial in many areas of the earth sciences and archeology but is often difficult using traditional techniques sediment burial can be dated by the radioactive decay of cosmogenic nuclides provided that the sediment was exposed to cosmic rays prior to burial dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial however the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic ray nucleons and muons this paper discusses how buried sediments can be dated over timescales up to 5 myr using 26 al and 10 be in quartz

spatial and temporal variations in denudation rates derived from cosmogenic nuclides in four european fluvial terrace sequences abstract the denudation of landscapes is affected by temporal and spatial variations in tectonics climate and vegetation however deciphering the contributions of these different processes has proven challenging in this study cosmogenic nuclide derived modern and paleo catchment wide denudation rates in four european rivers are investigated we present 12 new and 4 recalculated cosmogenic nuclide derived denudation rates from modern river sediments and 14 paleo denudation rates from terraces deposited over the last 2 ma the catchments studied are located in regions with minimal quaternary tectonic activity and span different climates over 12 o latitude results indicate that modern denudation rates range between 16 11 and 51 7 mm ka with no clear latitudinal variation modern denudation rates are compared with catchment geomorphic indices including slope fluvial steepness index and relief the denudation rates correlate better to catchment topographic indices r 2 0 4 rather than climate paleo denudation rates range from 8 7 to 56 7 mm ka and are associated with a possible increase in the average paleo denudation rates over the past 2 ma taken together the results indicate that quantification of catchment wide denudation rates over long quaternary time scales because of climate change is difficult future work to study climate influence on denudation rates should focus on the successes of previous work that document transient denudation rates over shorter and more recent time scales i e from the last glacial maximum to present

constant cosmogenic nuclide concentrations in sand supplied from the nile river over the past 2 5 m y quartz sand in the eastern mediterranean coastal plain is supplied through an extended transport system which includes the nile river east mediterranean longshore currents and inland eolian transport while the concentrations of cosmogenic nuclides 26 al and 10 be and their ratio in modern sand deposited along the coast of the eastern mediterranean reflect the combined effect of exposure and burial during transport the concentrations of these nuclides in buried sands are the result of decay of this initial dosing samples of modern exposed sand n 3 collected from the coastal plain of israel yield an average 26 al 10 be ratio of 4 8 0 2 significantly lower than the expected ratio of 6 8 for exposed quartz grains at the surface a similar ratio of 4 5 0 3 was measured in a late pleistocene sand sample indicating similar exposure burial histories during transport in spite of the difference in climatic conditions the results imply a steady preburial cosmogenic nuclide ratio related to the nile river9s ability through storage and recycling to buffer the effects of climatic and tectonic perturbations on cosmogenic nuclide concentrations in the transported quartz all ancient and buried sand samples n 11 fall on a decay path that originates from the concentrations and ratio of 26 al and 10 be in modern sand suggesting steady preburial concentrations of cosmogenic nuclides in quartz sand over the past 2 5 m y

fluvial networks of the iberian peninsula a chronological framework abstract knowledge of the evolution of spanish fluvial networks has improved during recent years as more river systems have been studied and more geochronological data has become available however the chronological framework is a major issue as the range of applications is limited by methodological constraints and spatial coverage is sparse integration of absolute dating methods with biostratigraphy and palaeomagnetism permits the recent evolution of these river systems to be reviewed the timing of incision from the late neogene to the present varies between the major iberian fluvial systems depending on the substrata and tectonic settings early pleistocene and older fluvial sequences in the core areas of the iberian peninsula provide a more extensive record of fluvial evolution and are better preserved than the terrace flights in the coastal lowlands middle pleistocene sequences are well developed in most of the major river systems in iberia particularly those of the tajo guadalquivir and aguas river and frequently represent the principal climatic cycles of that period although tectonic and sea level effects can also be seen for late pleistocene to holocene times the scheme becomes more complex our review suggests that each river system has responded differently to local and regional climate control glacial and periglacial processes in headwaters in high mountain areas glacio eustatic sea level changes and local and regional tectonic patterns

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

hillslope response to tectonic forcing in threshold landscapes hillslopes are thought to poorly record tectonic signals in threshold landscapes numerous previous studies of steep landscapes suggest that large changes in long term erosion rate lead to little change in mean hillslope angle measured at coarse resolution new lidar derived topography data enables a finer examination of threshold hillslopes here we quantify hillslope response to tectonic forcing in a threshold landscape to do so we use an extensive cosmogenic beryllium 10 10 be based dataset of catchment averaged erosion rates combined with a 500 km 2 lidar derived 1 m digital elevation model to exploit a gradient of tectonic forcing and topographic relief in the san gabriel mountains california we also calibrate a new method of quantifying rock exposure from lidar derived slope measurements using high resolution panoramic photographs two distinct trends in hillslope behavior emerge below catchment mean slopes of 30 modal slopes increase with mean slopes slope distribution skewness decreases with increasing mean slope and bedrock exposure is limited above mean slopes of 30 our rock exposure index increases strongly with mean slope and the prevalence of angle of repose debris wedges keeps modal slopes near 37 resulting in a positive relationship between slope distribution skewness and mean slope we find that both mean slopes and rock exposure increase with erosion rate up to 1 mm a in contrast to previous work based on coarser topographic data we also find that as erosion rates increase the extent of the fluvial network decreases while colluvial channels extend downstream keeping the total drainage density similar across the range our results reveal important textural details lost in 10 or 30 m resolution digital elevation models of steep landscapes and highlight the need for process based studies of threshold hillslopes and colluvial channels

evidence for anisotropy in the deep mantle beneath alaska we consider the possibility that the velocity structure of d is anisotropic the data we examined consist of seismograms from 9 deep japanese earthquakes recorded at wwssn receiver stations in north america the source receiver combinations span distances of 70 106 with associated s waves passing through d beneath alaska differential travel times of the s scd scs and sks phases are used to constrain the velocity structure in d shear waves refracted by d are observed beyond 72 2 and provide a sensitive measurement of the velocity structure in d beyond 93 the vertically polarized sv and horizontally polarized sh shear waves often appear distinctly split although at distances less than 89 the components are more nearly synchronous near 94 sh occurs as a double arrival sv in this range however remains a single arrival roughly synchronous with the second sh arrival we have been unable to reproduce these effects in isotropic model synthetics synthetics for transversely isotropic models have been computed that do match these waveforms the anisotropy was constrained to be only within d with a vertical symmetry axis we conclude that these observations may be explained by an anisotropic d layer the d discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above the core mantle boundary

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios