amplified erosion above waterfalls and oversteepened bedrock reaches erosion rate at the upstream end of the flow acceleration zone above a waterfall fr is the froude number at this setting and n ranges between 0 5 1 7 this amplification expression suggests that erosion at the lip could be as much as 2 5 times higher relative to erosion at a normal setting with identical hydraulic geometry utilizing this erosion amplification expression in numerical simulations we demonstrate its impact on reach scale morphology above waterfalls amplified erosion at the lip of a waterfall can trigger the formation of an oversteepened reach whose length is longer than the flow acceleration zone provided incision wave velocity vi at the upstream edge of the flow acceleration zone is higher than the retreat velocity of the waterfall face such an oversteepened reach is expected to be more pronounced when vi increases with increasing slope the simulations also suggest that oversteepening can eventually lead to steady state gradients adjacent to a waterfall lip provided vi decreases with increasing slope flow acceleration above waterfalls can thus account at least partially for prevalent oversteepened bedrock reaches above waterfalls using the cosmogenic isotope cl 36 we demonstrate that incision wave velocity upstream of a waterfall at the dead sea western escarpment is probably high enough for freefall induced oversteepening to be feasible

sediment yield exceeds sediment production in arid region drainage basins we use 10 be and 26 al to determine long term sediment generation rates identify significant sediment sources and test for landscape steady state in nahal yael an extensively studied hyperarid drainage basin in southern israel comparing a 33 yr sediment budget with 33 paired 10 be and 26 al analyses indicates that short term sediment yield 113 138 t km 2 yr 1 exceeds long term sediment production 74 16 t km 2 yr 1 by 53 86 the difference suggests that the basin is not in steady state but is currently evacuating sediment accumulated during periods of more rapid sediment generation and lower sediment yield nuclide data indicate that 1 sediment leaving the basin is derived primarily from hillslope colluvium 2 bedrock weathers more rapidly beneath a cover of colluvium than when exposed and 3 long term erosion rates of granite schist and amphibolite are similar

desert pavement coated surfaces in extreme deserts present the longest lived landforms on earth all exposed rocks on earth s surface experience erosion the fastest rates are documented in rapidly uplifted monsoonal mountain ranges and the slowest occur in extreme cold or warm deserts millennial submeterscale erosion may be approached only in the latter the oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings thus these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape while the surrounding landscape may erode faster and be younger we present erosion rate and exposure age data from the paran plains a typical environment in the near east where vast abandoned alluvial surfaces 10 2 10 4 km 2 are covered by well developed desert pavements these surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m y major factors that reduce erosion converge in these regions extreme hyperaridity tectonic stability fl at and horizontal surfaces i e no relief and effective surface armoring by a clast mosaic of highly resistant lithology the 10 be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern negev israel representing the sahara arabia deserts indicate simple exposure ages of 1 5 1 8 ma or correspond to maximum erosion rates of 0 25 0 3 m m y 1 the 36

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

fluvial landscape response time how plausible is steady state denudation whether or not steady state topography and denudation are probable states depends on the timescale of system response to tectonic and climatic perturba tions relative to the frequency of those perturbations this paper presents analytical derivations of algebraic relations for the response time of detachment limited fluvial bedrock channel systems both to tectonic and climatic perturbations detachment limited fluvial erosion is described by the stream power incision model and the derivations are limited to the applicability of that model all factors likely to influence system response time that are not adequately captured by the stream power incision model will tend to increase the response time the calculations presented thus provide minimum estimates of landscape response time and therefore over predict the prob ability of attaining and sustaining steady state topography and denudation the central range of taiwan is used as a case study to estimate response times in a landscape often argued to be in steady state model parameters are fit to modern stream profiles by assuming that the topography represents a quasi steady state form estimated response times generally range from 0 25 to 2 5 ma depending on the non linearity of the incision rule and the magnitude and type of perturbation thus it may be reasonably argued that steady state topography and denudation are likely to prevail during periods of climatic stability response time is sufficiently short compared with plate tectonic timescales however rapid climatic fluctuation in the quaternary appears to preclude the attainment of steady state conditions in modern orogens motivation

quantifying denudation rates on inselbergs in the central namib desert using in situ produced cosmogenic 10be and 26al in situ produced cosmogenic isotope concentrations in bedrock surfaces provide valuable estimates of site specific long term rates of denudation and provide constraints for numerical landscape evolution models measurements of cosmogenic sup 10 be and sup 26 al from graphite inselbergs in the arid to hyperarid central namib desert namibia indicate a mean rate of summit lowering of 5 07 1 1 m m y over the past ge 10 sup 5 yr the persistence of an arid climate in the region suggests that a similar rate may have prevailed for the past approximately 10 m y and possibly throughout much of the cenozoic some samples have complex exposure histories that can be explained by the mode of inselberg weathering and mass wasting the denudation rates estimated here are an order of magnitude higher than those reported for inselbergs in a significantly more humid environment in south australia this difference may largely be due to active salt weathering in the central namib as a result of high levels of coastal fog precipitation

quaternary scale evolution of sequences of talus flatirons in the hyperarid negev article i nfo talus flatiron sequences are ubiquitous landforms in arid and semiarid regions characterized by horizontal erodible rocks capped by more resistant rocks alternating phases of deposition and erosion lead to the formation of generations of talus flatirons in which ancient ones are located farther from the source cliff the existing conceptual model of this systematic spatial distribution of talus flatirons is related to glacial interglacial climatic cycles and to relatively high rates of cliff retreat three groups of talus flatirons were analyzed in the northeastern hyperarid negev desert the analyses include mapping age determination using optically stimulated luminescence and cosmogenic nuclide exposure dating electrical resistivity tomography and sedimentological and soil analyses all talus flatirons contain gypsic salic soil catena typical to hyperarid climate b80 mm yr 1 no pedogenic indicators of past wetter environments such as buried calcic soil 10 be exposure ages of the talus flatirons suggest that they were deposited during the middle pleistocene 610 ka and 170 ka for the oldest and intermediate aged talus flatiron groups respectively these ages combined with the present location of the talus flatirons relative to the source cliff yield retreat rates of 6 12 and 200 m ma 1 for the bedrock cliff and talus flatiron apex respectively and in opposite directions our results show that a climatic changes at glacial interglacial time scales are not the main controls over the formation of talus flatiron generation and b that significant cliff retreat is not the main cause for the systematic spatial distribution of talus flatiron generations relative to the cliff our results indicate that the process of talus flatiron formation in the negev desert must be associated with the balance between production deposition and removal of clasts under hyperarid conditions with only a minor cliff retreat

neogene sediment thickness and miocene basin floor fan systems of the celebes sea abstract in the celebes sea basin the middle miocene turbidites were correlated from odp site 767 throughout the studied area differences in their regional thickness variations and distribution indicate two source areas the middle miocene turbidite fan complexes of the central and southern celebes sea basin are controlled by the paleo tarakan delta system the tectonic events and the basin floor morphology respectively the main source area for the time correlative turbidites along the southern sulu arc is assumed to be mindanao the correlation of the middle miocene to pleistocene sequences exhibit tentative ages for the development of the accretionary wedges along the cotabato trench and along the north sulawesi subduction a post middle miocene to pre pliocene age is inferred for the cotabato wedge and a plio pleistocene age is assumed for the north sulawesi wedge

sand residence times of one million years in the namib sand sea from cosmogenic nuclides the namib sand sea is one of the world s oldest and largest sand deserts1 yet little is known about the source of the sand in this or other large deserts2 in particular it is unclear whether the sand is derived from local sediment or comes from remote sources the relatively uniform appearance of dune sands and low compositional variability within dune fields3 make it difficult to address this question here we combine cosmogenic nuclide measurements and geochronological techniques to assess the provenance and migration history of sand grains in the namib sand sea we use u pb geochronology of detrital zircons to show that the primary source of sand is the orange river at the southern edge of the namib desert our burial ages obtained from measurements of the cosmogenic nuclides 10be 26al and 21ne suggest that the residence time of sand within the sand sea is at least one million years we therefore conclude that despite large climatic changes in the namib region associated with quaternary glacial interglacial cycles4 5 the area currently occupied by the namib sand sea has never been entirely devoid of sand during the past million years

a chemostratigraphic investigation of the prehistoric vavalaci lava sequence on mount etna simulating borehole drilling scientific drilling of volcanic successions has been suggested as a way of establishing stratigraphic sequences of unexposed lava flows on large complex volcanoes with the aim of in depth study of magmatic processes and source geochemistry of otherwise inaccessible lava sequences to simulate the core drilling of such sequences lava flows from the prehistoric vavalaci centre exposed in the south wall of the valle del bove mount etna sicily were sampled in four stratigraphic sections the fresh generally strongly porphyritic na alkaline trachybasalts and trachyandesites show diverging sub parallel trends of high and low alkali concentrations in total alkali versus silica tas diagrams whilst variations of other major and trace elements reveal two further distinct chemical groups enriched in k ree and ti which follow separate fractionation paths a set of control samples was used to establish geochemical variations within a single lava flow primitive mantle normalised incompatible element patterns demonstrate that the lavas have highly enriched oib signatures with a clear division in lree ba th nb and zr concentrations between the four different chemical groups comparison of data for the vavalaci lavas with the compositions of other prehistoric historic and recent eruptions of mt etna indicates a temporal trend towards more basic magma compositions the chemostratigraphy of the lavas was statistically analysed to give correlations between flows from different sections whilst a good number of geologically meaningful correlations were revealed we can demonstrate that only one set of lavas was actually sampled in all four sections whilst a number of unique lavas remain uncorrelated thus no individual section or simulated borehole core provided samples of the complete lava flow sequence without significant gaps in the stratigraphy the trends in lava compositions are also defined in the stratigraphy showing their evolution from low to high alkali lavas through the series which may be related to temporal decrease in degree of partial melting of the mantle rather than through simple fractionation processes or mixing of magmas

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

differential weathering and erosion in an inselberg landscape in southern zimbabwe a morphometric study and some notes on factors influencing the long term development of inselbergs abstract a morphometric analysis of inselberg systems in southern zimbabwe indicated that there are two types of inselberg systems that differ in the mode of development inselberg systems characterized by backwearing tend to occur on granitoid rocks that are foliated highly fractured and prone to grusification the inselbergs are mostly of the koppie type and are covered to a great extent with regolith or debris the second type of inselbergs consists of sparsely fractured granitoid rocks and areas underlain by these rocks are characterized by clusters of closely spaced rock domes in the case of these inselbergs backwearing is not indicated they appear to be the result of the exposure of structurally pre designed highly resistant rock compartments the hypothesis that differences in lithology and structure may influence the rate and style of inselberg development was tested by means of a simulation model the simulations are run under the same boundary conditions with differing assumptions concerning role of divergent weathering the results show that both modes of inselberg development may occur alongside one another as long as a critical relief is maintained the modelling provided also indicates that although the inselberg systems must have integrated several sequences of environmental changes during their development these changes were not able to divert the general trend of the process response system away from its steady course lithological and structural factors which are responsible for the differences in the mobility of material on the inselberg sideslopes are thus considered to be the primary source of influence of the different modes of inselberg development it is suggested that the compositional and structural diversity in granitoid rock terrains is what enables inselbergs to pass through different development modes and sequences as rocks of differing resistance become subject to weathering during the development of the landscape

late quaternary history of the chemehuevi mountain piedmont mojave desert deciphered using 10be and 26al we used the cosmogenic nuclides 10 be and 26 al as cosmic ray dosim eters to track sediment movement across the incised chemehuevi mountain piedmont surface the piedmont extends 12 km from highland source basins to an ephemeral wash at the toe of the piedmont nuclide activity in sediment from steep source basins and from the bedrock pediment allows us to estimate rates of sediment generation and thus the rate at which sediment is supplied to the piedmont from weathering rock analysis of amalgamated sediment samples collected along 4 km long transects spaced at 1 km intervals from the mountain front provide context for understanding nuclide activity in sediment samples collected from two soil pits and demonstrate that sediment steadily moves down the channel network samples from the two soil pits allow for estimates of sediment deposition rates and thus further illuminate piedmont history model results suggest that the chemehuevi mountains are eroding about 40 mm ky 1 whereas the mountain proximal bedrock pediment and the interpiedmont saw tooth range are eroding more slowly 10 to 21 mm ky 1 and 9 5 mm ky 1 respectively cosmogenic nuclide profiles in two soil pits suggest a complex history of deposition at rates between 19 to 39 mm ky 1 and stability over the past 60 ky a change in process from sediment deposition to sediment transport occurred distally on the piedmont approximately 8 ky ago while the mid piedmont piedmont surface has been stable since the mid holocene the incised chemehuevi mountain piedmont is changing at rates similar to planar and uniformly active piedmonts such as those adjacent to the iron and granite mountains all three piedmonts have average sediment grain velocities of decimeters per year long term source basin erosion rates between 35 to 40 mm ky depositional hiatuses around the pleistocene holocene climatic transition from moister to drier conditions and pleistocene deposition rates of 18 to 39 mm ky such similarity in piedmont behavior despite clear differences in morphology and appearance might suggest large scale geologic and climatic controls on the generation transport and deposition of sediment in the mojave desert

using 10be and 26al to determine sediment generation rates and identify sediment source areas in an arid region drainage basin we measured 10 be and 26 al in 64 sediment and bedrock samples collected throughout the arid 187 km 2 yuma wash drainage basin southwestern arizona from the measurements we determine long term time integrated rates of upland sediment generation 81f 5gm 2 year 1 and bedrock equivalent lowering 30f 2mm a 1 consistent with other estimates for regions of similar climate lithology and topography in a small f 8k m 2 upland sub basin differences in nuclide concentrations between bedrock outcrops and hillslope colluvium suggest weathering of bedrock beneath a colluvial cover is a more significant source of sediment 4010 4 kg year 1 than weathering of exposed bedrock surfaces 1010 4 kg year 1 mixing models constructed from nuclide concentrations of sediment reservoirs identify important sediment source areas hillslope colluvium is the dominant sediment source to the upper reaches of the sub basin channel channel cutting of alluvial terraces is the dominant source in the lower reaches similarities in nuclide concentrations of various sediment reservoirs indicate short sediment storage times 10 3 years nuclide concentrations measured in channel sediment from tributaries of yuma wash and in samples collected along the length of the wash were used to construct mixing models and determine sediment sources to the main stem channel we find an exponential decrease in the channel nuclide concentrations with distance downstream suggesting that as much as 40 of sediment discharged from yuma wash has been recycled from storage within basin fill alluvium sediment generation and denudation rates determined from the main stem are greater 25 than rates determined from upland sub basins suggesting that currently sediment may be exported from the basin more quickly than it is being generated in the uplands independence of nuclide concentration and sediment grain size indicates that channels transport sediment in discrete pulses before rapidly depositing poorly sorted material suggesting that differences in transport times for different size materials are minimal d 2002 elsevier science b v all rights reserved

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

quaternary geomorphological evolution of a granitic shore platform constrained by in situ 10 be concentrations penmarc h sw brittany france abstract the penmarc h granitic coastal domain sw brittany western france offers the opportunity to address the little investigated issue of erosional processes in a granitic coastal context from land to sea the granitic coast is composed of 1 an aerial marine terrace 10 be analyses supplies new constraints on the weathering and denudation processes recorded by the penmarc h platform in quaternary times the role of at least two denudation events is emphasized i a 1 8 m thick level of granitic material is assumed to have been removed by erosion during the mis5 sea level highstand both in the offshore domain and in the low part of the marine terrace ii an additional 0 6 m thick level of the granitic substratum was later removed during the last holocene transgression mainly in the marine domain the lack of any morphological step between the subaerial marine terrace and the shore platform is here attributed to the combined effect of multiple factors including i a quaternary tectonic quiescence ii a major eustatic transgression mis5 shaping both the shore platform and the low part of the marine terrace iii the mechanically strong behaviour of the granitic basement with regard to erosion and iv the moderate influence of the holocene sea level stillstand since 6 ka on the shore platform thanks to 10 be content analysis a range of vertical erosion rates of ca 3 6 m ma 1 is obtained for the onshore domain showing no discernable difference between the intertidal and aerial erosion rates from that result it is thus determined that the long term development of the penmarc h granitic shore platform was mainly driven by marine erosive processes during the mis5 and holocene marine transgressions whereas subaerial erosive processes prevailed after the mis5 event

topographic growth around the orange river valley southern africa a cenozoic record of crustal deformation and climatic change we reconstruct the history of topographic growth in southern africa on both sides of the orange river valley from an integrated analysis of erosion surfaces crustal deformation and climate change first we propose an inventory of erosion surfaces observed in the study area and classify them according to their most likely formative process i e chemical weathering or mechanical erosion among the various land units observed we define a new class of landform the pedivalley which corresponds to a wide valley with a flat erosional floor in the orange river valley we mapped three low relief erosion surfaces each bevelling a variety of lithologies the oldest and most elevated is 1 a stripped etchplain evolving laterally into 2 a stepped pediplain bearing residual inselbergs 3 a younger pediplain later formed in response to a more recent event of crustal deformation these are all cenozoic landforms the etchplain is associated with a late palaeocene to middle eocene weathering event and the two pediplains are older than the middle miocene alluvial terraces of the orange river landscape evolution was first driven by slow uplift 10 m ma followed by a second interval of uplift involving a cumulative magnitude of at least 200 m this event shaped the transition between the two pediplains and modified the drainage pattern a final phase of uplift magnitude 60 m occurred after the middle miocene and drove the incision of the lower terraces of the orange river climate exerted a major control over the denudation process and involved very humid conditions responsible for lateritic weathering followed by more arid conditions which promoted the formation of pedivalleys collectively these produce pediplains

scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces abstract the decrease in rates of production of cosmogenic nuclides occurs because of shielding of cosmic rays by mountains sloped surfaces and local rock formations that block them when a large part of the sky is blocked this correction is large and requires detailed model calculations this paper considers three geometries a rectangular obstruction a triangular obstruction and a sloped surface other geometries can be considered as a combination of these the results are presented in terms of formulas and graphs so that the reader can easily apply them to common field situations any use of cosmogenic nuclides in the study of geomorphic processes or forms must consider factors that introduce variations in the production of nuclides

erosion of a granite inselberg gross spitzkoppe namib desert abstract namibia with its passive margin setting long term tectonic stability and long lasting arid climate is a typical granitic landscape characterized by numerous dominating inselbergs the namib has been the focus of quantitative studies on the overall rates of bedrock exhumation escarpment retreat and sediment generation transport and deposition results from these studies indicate steady bedrock erosion rates ranging between 1 and 5 mm kyr 1 over the last 105 106 yr this rate was determined from samples collected mostly from small inselbergs with low relief how fast the large and classic inselbergs which dominate the namib desert landscape erode generate sediment and contribute to the overall sediment supply has yet to be systematically examined we document erosion rates of the gross spitzkoppe gs one of the largest inselbergs in the namib desert namibia based on measured concentrations of 10be in samples collected from bedrock boulders and surface grus the results suggest a slow lowering rate 1 2 mm kyr 1 while the overall slope processes deliver sediment to the base of the cliff with 5 6 105 atoms g 1 quartz this concentration is equivalent to an average erosion rate of 8 mm kyr 1 2 3 times faster than the bedrock lowering rate thus the inselberg has been eroding by cliff retreat the concentration of cosmogenic isotopes in slope sediment is controlled by exposure time on the slope weathering of exposed bedrock and weathering of subsurface bedrock sediment continues to accumulate cosmogenic isotopes while being transported on the flat pediments surrounding the inselberg within 1 2 km from the base of the gs the concentration doubles to 10 105 atoms g 1 quartz the increase is achieved by dosing due to exposure and mixing with highly dosed sediment eroded from near subsurface bedrock from this area grus is transported through a network of low gradient channels to the atlantic ocean 100 km away cosmogenic isotope concentration in sediment increases to 30 105 atoms of 10be g 1 quartz which is 3 4 times higher than that measured in the grus close to the gs and is controlled by the erosion rate of stable bedrock throughout the entire drainage basin dosing due to exposure during transport obviously contributes to the overall measured concentration in the sediment as it approaches the ocean

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

the research frontier and beyond granitic terrains abstract investigations of granite forms and landscapes over the past two centuries suggest that many features major and minor are shaped by fracture controlled subsurface weathering and particularly moisture driven alteration in other words etch forms are especially well represented in granitic terrains commonly referred to as two stage forms many are in reality multistage in origin for the structural contrasts exploited by weathering and erosion that are essential to the mechanism originated as magmatic thermal or tectonic events in the distant geological past fracture patterns are critical to landform and landscape development in granitic terrains but other structural factors also come into play location with respect to water table and moisture contact are also important once exposed and comparatively dry granite forms tend to stability they are developed and diversified and many are gradually destroyed as new epigene forms evolve but many granite forms persist over long ages reinforcement effects frequently play a part in landform development several granite forms are convergent i e features of similar morphology evolve under the influence of different processes frequently in contrasted environments on the other hand many landforms considered to be typical of granitic terrains are also developed in bedrock that is petrologically different but physically similar to granite and in particular is subdivided by fractures of similar pattern and density to date most of the general statements concerning the evolution of granitic terrains have been based in work in the tropics but other climatic settings and notably those of cold land are now yielding significant results future research will extend and develop these avenues but biotic factors and particularly the role of bacteria in such areas as weathering will take on a new importance structural variations inherited from the magnetic thermal and tectonic events to which granite bodies have been subjected will be more and more appreciated as offering explanations for a wide range of granite forms major and minor ancient and recent in particular investigations of rock strain including gravitational loading at a variety of scales and especially as it influences fracture patterns and susceptibility to weathering will assume a prime importance in the explanation of granitic landforms and landscapes finally there as genuine hopes that the close dating of surfaces and weathering events will allow structural and process studies to be placed in their chronilogical contexts new techniques and observations will prove important to advances in the understanding of granitic forms but as in other areas of geomorphological endeavour fresh perceptions different linkages and new ideas are critical

quantifying sediment transport on desert piedmonts using 10be and 26al in situ produced 10 be and 26 al measured in 40 sediment samples collected from the iron and granite mountain piedmonts eastern mojave desert provide a unique view of piedmont modification processes and process rates over the 10 3 to 10 5 year time scale cosmogenic nuclide based models suggest that the iron and granite mountains generate 0 11 0 13 and 0 082 0 097 m 3 of sediment per year per meter of rangefront respectively the sediment moves down the piedmont in an active transport layer atl which is 20 to 30 cm thick based on visual observations measurements of depth to a buried b horizon cosmogenic nuclide data and maximum ephemeral channel depths sediment in this layer is well mixed vertically and horizontally on the 10 2 year time scale indicating that the small ephemeral channels which dominate the piedmont surface migrate quickly interpretive models of increasing nuclide activities at depth in two pits suggest steady sediment deposition on the piedmont at rates between 17 21 and 38 45 m ma 1 until the late pleistocene epoch when a discontinuity to markedly lower nuclide activities in the isotopically well mixed active transport layer suggests that deposition stopped a significant change in piedmont behavior nuclide activities in 10 amalgamated surface samples each collected along a different 4 km long transect increase steadily away from the mountain front thus we infer that sediment is uniformly dosed by cosmic rays as it is transported down the iron and granite mountain piedmonts interpretive models suggest that long term average sediment speeds down the iron and granite mountain piedmonts are a few decimeters to a meter per year d 2002 published by elsevier science b v

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

cosmogenic exposure and erosion history of australian bedrock landforms exceptionally high activities of in situ produced 1 0 be and 2 6 al indicate that australian bedrock surfaces are some of the most stable in the world these cosmogenic nuclides demonstrate that australian granite exposed and sampled at 10 sites along a transect from the eyre peninsula 33 s to the kakadu area in the northern territory 14 s has in some places eroded only decimeters over the past million years however nuclide activities are not saturated in any of the 61 exposed samples we analyzed mandating that the surfaces we sampled are losing mass albeit slowly over time such mass loss indicates that inselbergs are dynamic landforms confounding the assignment of discreet ages to inselberg surfaces nevertheless the differential between slow rates of erosion measured on the very stable uppermost parts of these landforms and the more rapid rates measured below their flanks and below the adjacent grus covered planes suggests that the forms of many large inselbergs which stand meters to tens of meters high must be at least tertiary in age and possibly much older activities of 1 0 be and 2 6 al in 61 exposed australian samples range from 5 1 0 1 10 6 to 5 3 0 6 x 10 5 atoms g 1 and 2 1 0 1 x 10 7 to 3 4 0 5 x 10 6 atoms g 1 respectively normalized to sea level at latitudes of 60 using only neutron scaling activities of 1 0 be and 2 6 al in seven shielded samples 3 5 13 m below the surface are much lower ranging from 3 4 0 2 x 10 4 to 7 0 0 1 x 10 4 atoms g 1 and 1 6 0 1 x 10 5 to 2 8 0 2 x 10 5 atoms g 1 respectively 2 6 al 1 0 be ratios of exposed samples range from 3 89 0 19 to 6 29 0 82 if 2 uncertainty ranges and a nominal sea level high latitude 1 0 be production rate of 5 17 atoms g 1 yr 1 are considered ratio data for all but one surface sample are consistent with continuous surface or near surface exposure and or erosion rock surfaces farther north in australia where the climate is more humid have lower nuclide activities thus we infer that surface stability is inversely related to mean annual precipitation from 61 exposed samples we calculated maximum limiting single nuclide model erosion rates of 0 3 0 1 to 5 7 1 0 m m y and minimum limiting single nuclide model exposure ages of 105 16 to 1310 190 ka by assuming a sea level high latitude 1 0 be production rate of 5 17 atoms g 1 yr 1 seven shielded samples in which the nuclide inventory is muon generated also have nuclide activities consistent with similarly low rates of erosion but over even longer time scales it appears that the eyre peninsula inselbergs have been and are eroding so slowly that they may well be direct descendents of cenozoic and perhaps even mesozoic forms

rates of sediment supply to arroyos from upland erosion determined using in situ produced cosmogenic 10be and 26al using 10be and 26al measured in sediment and bedrock we quantify rates of upland erosion and sediment supply to a small basin in northwestern new mexico this and many other similar basins in the southwestern united states have been affected by cycles of arroyo incision and backfilling several times in the past few millennia the sediment generation 275 65 g m 2 yr 1 and bedrock equivalent lowering rates 102 24 m myr 1 we determine are sufficient to support at least three arroyo cycles in the past 3 000 years consistent with rates calculated from a physical sediment budget within the basin and regional rates determined using other techniques nuclide concentrations measured in different sediment sources and reservoirs suggest that the arroyo is a good spatial and temporal integrator of sediment and associated nuclide concentrations from throughout the basin that the basin is in steady state and that nuclide concentration is independent of sediment grain size differences between nuclide concentrations measured in sediment sources and reservoirs reflect sediment residence times and indicate that subcolluvial bedrock weathering on hillslopes supplies more sediment to the basin than erosion of exposed bedrock