eocene tibetan plateau remnants preserved in the northwest himalaya in the northwest himalaya deeply incised mountain ranges contrast with high elevation low relief areas such as the deosai plateau in northern pakistan thermochronology data from the deosai plateau together with thermal history modelling suggest that this plateau along with other morphologically similar surfaces is a remnant of an eocene southwestern tibetan plateau

estimating long term cliff recession rates from shore platform widths coastal cliff erosion is a problem in many coastal areas however often only very limited data are available to quantify the rates of recession for the development of coastal management strategies in the soft flysch deposits of the waitemata group auckland new zealand coastal cliffs are associated with shore platforms two models exist for the profile evolution of shore platforms and associated cliffs the first suggests that an equilibrium profile develops in response to erosive processes and this profile subsequently migrates landward the second model suggests that the seaward margin of the shore platform is relatively static and the profile extends landward through a combination of cliff recession and platform lowering physical simulations and field measurements for mudstone and limestone lithologies indicate that the second model is more likely for soft flysch deposits a eustatic sea level curve for the weiti estuary auckland suggests that up to 7120f70 years have been available for shore platform development since sea level reached the present seaward margins of shore platforms shore platform widths were measured using gps at two sites in waitemata group rocks the north shore of auckland and the southern side of the tawharanui peninsula north auckland the long term cliff recession rates estimated from shore platform widths 1 4f0 1 to 14 3f0 1 mm y 1 are consistent with the lower end of the average range of cliff top and face recession rates published for waitemata group rocks using different methods 11 75 mm y 1 and in agreement with cliff base recession estimates 3 5 mm y 1 shore platform widths were qualitatively related to the rock mass characteristics of the associated cliffs and therefore platform widths could provide a method of identifying regions of potential hazard d 2005 elsevier b v all rights reserved

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

modeling the tor dotted crests bedrock edges and parabolic profiles of high alpine surfaces of the wind river range wyoming abstract broad low curvature surfaces common to many alpine landscapes and particularly well expressed in the laramide ranges of the western usa appear to be roughly in steady state with respect to the current surface processes uniform thin order 1 m regolith covers the crystalline bedrock with the exception of occasional tors at ridgecrests the surfaces are commonly edged by bedrock lips at the tops of cliffs that bound the intervening glacial troughs regolith production is low order 10 m year on bare bedrock and reaches a maximum under tens of centimeters of regolith all of these characteristics may be accounted for with a numerical model that is faithful to the periglacial surface processes presently operating on these surfaces hillslope transport is modeled assuming the dominant process is frost creep transport efficiency depends linearly upon local slope and appropriately vanishes in the absence of regolith frost frequency scales the rate of motion while the ratio of frost penetration depth to regolith thickness dictates the degree to which the process is curtailed by available regolith initial conditions include initial mean slope and bedrock roughness and a uniform regolith cover boundary conditions are imposed by dictating that the edges of the calculation space be lowered at a rate appropriate for glacial incision broad convex surfaces emerge over time scales of order 1 million years all surfaces rapidly decouple from the bounding glacial troughs as the high negative curvature at the edges disallows accumulation of regolith these bedrock edges lower at a few microns per year while the troughs lower at many times this rate as the bedrock edges therefore serve as the effective boundary condition for the remaining surface the surfaces operate independently of both lowering and widening of the glacial troughs for a reasonable set of transport and weathering rates the final steady state landscape is sensitive to the initial condition in particular the steeper the initial landscape the more likely tors are to develop as the high curvature disallows accumulation of regolith these tors in turn limit the lowering rate of the crest and the landscape retains significant local relief low relief initial conditions more likely lead to parabolic surfaces that lack tors that tors are more likely on the crests reflects the high initial curvature at the crest tors on the side slopes are eliminated the present high surfaces in the laramide ranges of the western usa imply operation of periglacial surface processes over millions of years the occasional tors imply a steeper rather than flatter initial landscape the strong decoupling of surfaces from glacial troughs justifies their use as local geomorphic markers that allow calculation of relief production since glacial incision was initiated on the other hand their present low curvature does not require a landscape history in which these surfaces were once joined as an initially widespread erosion surface

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

actively evolving microplate formation by oblique collision and sideways motion along strike slip faults an example from the northeastern caribbean plate margin abstract the pattern of folding faulting and late quaternary coral reef uplift rates in western and central hispaniola haiti and dominican republic suggest that the elongate gonave microplate a 190 000 km2 area of the northeastern caribbean plate is in the process of shearing off the caribbean plate and accreting to the north american plate late cenozoic transpression between the southeastern bahama platform and the caribbean plate in hispaniola has inhibited the eastward motion of the northeastern corner of the plate transpression is manifested in western and central hispaniola by the formation of regional scale folds that correspond to present day anticlinal topographic mountain chains continuous with offshore anticlinal ridges areas of most rapid quaternary uplift determined from onland coral reefs 125 ka and younger coincide with the axial traces of these folds offshore data suggest recent folding and faulting of the seafloor onshore reef data do not conclusively require late quaternary folding but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the northwest peninsula of haiti 0 37 mm yr to to the central part of the coast of western haiti 0 19 mm yr to the south central part of western haiti 0 mm yr formation of the 1200 km long enriquillo plantain garden walton fault zone as a bypass strike slip fault has isolated the southern edge of the gonave microplate and is allowing continued unimpeded eastward motion of a smaller caribbean plate past the zone of late neogene convergence and quaternary uplift of coral reefs in hispaniola offshore seismic reflection data from the jamaica passage the marine strait separating jamaica and haiti show that the enriquillo plantain garden fault zone forms a narrow but deep active fault bounded trough beneath the passage the active fault is continuous with active faults mapped onshore in western haiti and eastern jamaica the bathymetric deep is present because the jamaica passage fault segment represents a 50 km wide transtensional left step of the fault trace between haiti and jamaica onshore satellite imagery and field observations suggest that the enriquillo plantain garden fault forms a continuous trace extending from central hispaniola east of lake enriquillo dominican republic to the westernmost end of the southern peninsula of haiti the regional lineament corresponds to a recent fault scarp in quaternary alluvium of the clonard pull apart basin in the central part of the southern peninsula of haiti and suggests that at least this part of the lineament has undergone recent slip calmus 1983 has suggested a total offset of 30 50 km of the enriquillo plantain garden fault zone in the southern peninsula of haiti using the apparent offset of lithologic units seismic reflection data from lake enriquillo document recent deformation of quaternary lake sediments where the lineament crosses the lake leveling of the crest of a late holocene coral reef and associated algal tufa around lake enriquillo demonstrate late holocene vertical movement and tilting in a 1500 m wide zone parallel to the fault trend lateral offset is difficult to show in the enriquillo valley area because of rapid recent sedimentation into the valley the pattern of inactive strike slip faults and fold belts of cenozoic age in cuba and the yucatan basin suggest that two elongate microplates were sheared off the proto caribbean plate and accreted to the north american plate by a similar process in paleocene and eocene times age of terminal deformation in western central and eastern cuba is consistent with southeastward younging and migration of arc collision the similarity of the size and sequence of events in cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas

a demonstration of the importance of bedload transport for fluvial bedrock erosion and knickpoint propagation we provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation using aerial photographs field surveys and hydrological data we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by da an river in western taiwan this reach of the river experienced 10 m of uplift during the 1999 chi chi earthquake for five years following the earthquake bedload was prevented from entering the uplift zone the knickpoint was static and little incision took place bedload transport across the uplift zone resumed in 2004 initiating extremely rapid incision with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008 this change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation once bedload tools became available knickpoint propagation was influenced by geological structure lithology and drainage organization in particular a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity copyright 2012 john wiley sons ltd

investigating the complex interface where bedrock transforms to regolith abstract the interface where bedrock transforms to regolith is not planar but rather has a roughness that varies with the scale of observation the complexity of this surface is manifested in both element depth and fragment size depth distributions and may sometimes be related to the longitudinal profiles of watershed streams the fractal nature of the bedrock regolith interface means that the interface has a thickness which is 20 m in two ridgetop examples from pennsylvania and puerto rico such weathering thicknesses modeled as a function of one dimensional fluid flow are affected by the balance between rates of weathering and erosion one dimensional models are consistent with weathering advance rates that vary with equilibrium solubility and porefluid velocities and not reaction kinetics however fluid flow is not strictly downward and one dimensional permeability of regolith changes as particle size and bulk density changes with depth thus both downward and lateral flow occurs especially at reaction fronts where reactions change permeability the rate of weathering advance is therefore affected by the 3 dimensional distribution of reaction zones that affect permeability across the watershed quantitative models of such phenomena over a range of spatial and temporal scales are needed

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

knickpoint initiation and distribution within fluvial networks 236 waterfalls in the waipaoa river north island new zealand if knickpoints transmit signals of base level fall in river networks then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation many studies of knickpoint retreat focus on the response of trunk streams to base level fall here we examine the response of an entire fluvial network as recorded by 236 active knickpoints distributed within the waipaoa river on the north island of new zealand base level fall within the waipaoa catchment initiated 18 000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision using observations from field work aerial photo analysis and a digital elevation model dem we study the knickpoint positions within the network we find that 70 of the knickpoints are located at drainage areas between 1 10 5 m 2 and 1 10 6 m 2 and more than half are b1 km upstream of a large change in drainage area for the knickpoints b1 km upstream of large tributary junctions we find that the retreat distances were well correlated with the tributaries drainage areas in order to determine how a pulse of incision distributes itself throughout a fluvial network we develop two simple end member models and compare their behavior to the observed knickpoint distribution in the waipaoa in the first model we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area in the second model we propose that knickpoints form near a threshold drainage area below which channels cannot incise with the same efficiency as possible in downstream reaches though neither model addresses the along stream variability in substrate or knickpoint form the misfit between the modeled and the observed knickpoints along stream positions are surprisingly low b1 km 1 25 of the total stream length for knickpoints with drainage areas b1 10 6 m 2 large misfits up to 3 5 km are observed for knickpoints with present day drainage areas greater than 1 10 6 m 2 the large single step in channel elevation that characterizes knickpoints presently observed in tributaries of the waipaoa river may not characterize the base level fall signal that propagated through the trunk streams evidence for progressive rather than instantaneous incision in the trunk streams the knickpoints vicinities to tributary junctions and the equivalent success of the two end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas

the influence of bedrock geology on knickpoint development and channel bed degradation along downcutting streams in south central indiana geologic controls on knickpoint development and the mechanics of channel bed degradation were eluci dated by comparison of downcutting streams in south central indiana developed in lithologically homoge neous carbonate units and layered siliciclastic strata knickpoints formed in a single lithologic unit are stepped each step consists of a knickpoint face and tread the height of each step is equal to and controlled by bed thickness spatially knickpoints in homogeneous terrain are limited to jointed strata that dip 1 upstream or 2 downstream at angles less than the channel gradient knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units total knickpoint relief is influenced by thickness of the weaker strata knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision knickpoint distribution is also influenced by joint

linking geomorphology weathering and cation availability in the luquillo mountains of puerto rico abstract we explored the links between geomorphology weathering and cation availability across the most common forest types and geologic provinces in the el yunque national forest puerto rico our goal was to connect the controls on primary mineral supply to soil with stocks of plant available cations we established a network of 216 soil profiles comprising toposequences ridges slopes and valleys in three forest types colorado palm and tabonuco on two parent rock types quartz diorite and volcaniclastic in a factorial design the landscape underlain by quartz diorite is divided into lower slopes that are eroding rapidly in response to renewed uplift in the last 5 ma and those above a regional knickpoint where remnant landforms are eroding more slowly quartz diorite derived soils 0 80 cm below the knickpoint have lost significantly less calcium ca magnesium mg and potassium k relative to parent material 80 65 and 57 respectively than those above it which have lost 95 81 and 83 p 90 of all base cations relative to parent material while geology and geomorphology exert the most control on the degree of soil weathering as measured by total soil cation loss relative to parent material exchangeable cations are influenced by geologic biologic and meteoric factors tabonuco forests have significantly higher exchangeable cation stocks 0 80 cm than colorado forests even in the 500 700 m elevation zone where the two forest types are not correlated with elevation p 0 02 0 005 and 0 002 for ca mg and k respectively forest type explains 30 of the variance in exchangeable mg and k stocks but only 10 of the variance in exchangeable ca quartz diorite derived soils below the knickpoint have 2 more exchangeable ca than those above it and within that rapidly eroding region sites closest in elevation to the knickpoint have 2 more again the isotopic composition of bulk soil and exchangeable strontium suggests that the high ca stocks below and near the knickpoint are mostly derived from primary mineral weathering whereas above the knickpoint exchangeable ca is supplied by atmospheric deposition these data demonstrate that regional uplift lithology weathering atmospheric inputs and forest communities combine to shape cation availability across the luquillo landscape and we hypothesize that similar interactions are important in other montane tropical forests

relict landscape resistance to dissection by upstream migrating knickpoints expanses of subdued topographies are common at high elevation in mountain ranges they are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography however relict fragments could also possess intrinsic characteristics that make them resilient to dissection we document here the propagation of a wave of dissection across an uplifted relict landscape in puerto rico using 10be 26al burial dating on cave sediments we show that uplift started 4 ma and that river knickpoints have since migrated very slowly across the landscape modern detrital 10be erosion rates are consistent with these long term rates of knickpoint retreat analysis of knickpoint distribution combined with visual observations along the streambeds indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat the studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks earlier studies have shown that vegetation over the relict topography efficiently limits erosion allowing for the formation of a thick saprolite underneath such slow erosion reduces streambed load fluxes delivered to the knickpoints as well as bed load grain size both processes limit abrasion compounding the effect of slow abrasion wide joint spacing in the bedrock makes plucking infrequent thus the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream reducing the celerity at which the relict subdued landscape is dissected we conclude that similar top down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies

rate and processes of river network rearrangement during incipient faulting the case of the cahabon river guatemala deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics although the reorganization of deeply incised drainage systems has been documented the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood to address this problem we have explored the rapid drainage reorganization that affected the cahabon river in guatemala during the quaternary sediment provenance analysis field mapping and electrical resistivity tomography ert imaging are used to reconstruct the geometry of the valley before the river was captured dating of the abandoned valley sediments by the be 10 al 26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes such as the rate of tectonic deformation of the paleovalley the rate of propagation of post capture drainage reversal and the rate at which canyons that formed at the capture sites have propagated along the paleovalley transtensional faulting started 1 to 3 million years ago produced ground tilting and ground faulting along the cahabon river and thus generated differential uplift rate of 0 3 0 1 up to 0 7 0 4 mm y 1 along the river s course the river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence then the river experienced two captures and one avulsion between 700 ky and 100 ky the captures breached high standing ridges that separate the cahabon river from its captors captures occurred at specific points where ridges are made permeable by fault damage zones and or soluble rocks groundwater flow from the cahabon river down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges valley fill formation and capture occurred in close temporal succession suggesting a genetic link between the two we suggest that the aquifers accumulated within the valley fills increased the head along the subterraneous system connecting the cahabon river to its captors and promoted their development upon capture the breached valley experienced widespread drainage reversal toward the capture sites we attribute the generalized reversal to combined effects of groundwater sapping in the valley fill axial drainage obstruction by lateral fans and tectonic tilting drainage reversal increased the size of the captured areas by a factor of 4 to 6 at the capture sites 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm y 1 deepening at a rate of 0 7 to 1 5 mm y 1 at this rate 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley it is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field which involved transient sedimentation along the river s course if the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced

does decreasing paraglacial sediment supply slow knickpoint retreat in four rivers in western scotland for which there is a well constrained record of relative base level fall the rate of postglacial bedrock erosion is quantifi ed by measuring the concentration of in situ cosmogenic 10be on strath terraces downstream of headward retreating knickpoints along channel gradients in 10be exposure age show two distinct trends upstream younging and constant age which we interpret as diagnostic of knickpoint retreat and diffusive transport limited incision respectively we show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation ca 11 5 ka and that knickpoint retreat rates peaked in the early to mid holocene erosion rates have since decreased by two orders of magnitude converging in the late holocene to low rates independent of stream power per unit channel area we infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the holocene leading to a defi ciency in tools for bedrock erosion our results imply that episodes of major fl uvial erosion may be in tune with glacial cycles and that sediment depletion following glacial interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

observations on the role of lithology in strath terrace formation and bedrock channel width field observations from western washington and eastern tibet indicate a strong lithologic influence on strath terrace formation and highlight the response of bedrock channel width to spatial gradients in bedrock erodibility and or rock uplift rates measurements of local bedrock bed and bank erosion rates together with observations of the role of weathering processes on erosion of siltstones and sandstones illustrate a mechanism that underlies a conceptual model that predicts a strong lithological control on strath terrace formation direct measurements of bedrock erosion rates in lithologies susceptible to accelerated erosion upon sub aerial exposure show that lateral rates of bedrock bank erosion can substantially exceed vertical incision below the perennial flow level due to an asymmetry in erodibility between perennially submerged rock and rock exposed to cyclic wetting and drying on bedrock channel walls under such conditions the rapid weathering mediated lowering of bedrock exposed above baseflow promotes development of a beveled bedrock platform that could become a strath terrace upon abandonment in contrast to previous studies channel widths measured across a substantial gradient in lithology slope and long term erosion rates in eastern tibet do not follow conventional relations where channel width scales as a power law function of drainage area instead as they flow across a zone of rapid uplift these channels systematically narrow relative to the predicted width extrapolated from traditional power law relations field studies from a wide range of geological settings and lithologies support the interpretation that in general strath terraces tend to be more extensive in rivers flowing over weak sedimentary rock and tend to be poorly developed and or preserved where rivers flow over hard erosion resistant rock

impulse framework for unsteady flows reveals superdiffusive bed load transport 1 sediment transport is an intrinsically stochastic process and measurement of bed load in the environment is further complicated by the unsteady nature of river flooding here we present a methodology for analyzing sediment tracer data with unsteady forcing we define a dimensionless impulse by integrating the cumulative excess shear velocity for the duration of measurement normalized by grain size we analyze the dispersion of a plume of cobble tracers in a very flashy stream over two years the mean and variance of transport distance collapse onto well defined linear and power law relations respectively when plotted against cumulative dimensionless impulse data suggest that the asymptotic limit of bed load tracer dispersion is superdiffusive in line with a broad class of geophysical flows exhibiting strong directional asymmetry advection thin tailed step lengths and heavy tailed waiting times the impulse framework justifies the use of quasi steady flow approximations for long term river evolution modeling

some highly weathered soils of puerto rico 1 morphology formation and classification abstract this is part 1 of a study concerning the pedology of eleven soils of puerto rico which represent the range of highly weathered soils of this island six oxisols four ultisols and one inceptisol are discussed the environmental conditions and profiles studied are described in detail soil formation in response to pedogenetic factors is discussed in terms of accumulation of parent material and solum development particular emphasis is placed on soil landscape relationships the occurrence of oxisols on miocene pliocene and pleistocene surfaces is ascribed to particular geomorphic conditions ultrabasic rocks and pre weathered sediments respectively the ultisols are mainly developed in saprolites on backslopes of geologically young surfaces the inceptisol is found on a strongly weathered regolith in a perhumid mountain region in all soils studied profound weathering was largely accomplished prior to rather than as part of pedogenic alterations clay illuviation cutans could not always be positively identified in the field and the results of thin section studies did not conform with field observations in all instances this suggests the need for a re examination of the diagnostic value of argillans in soils with low activity clays and as a corollary a re evaluation of the definition and taxonomic role of the argillic horizon the soils were classified according to the u s soil taxonomy the legend for the fao unesco soil map of the world and the french soil classification system the rationales for placement in taxa of these systems and their correlations are discussed the correspondence of the puerto rican soils studied with those of other tropical areas is discribed in general terms

infiltration on mountain slopes a comparison of three environments abstract water is well established as a major driver of the geomorphic change that eventually reduces mountains to lower relief landscapes nonetheless within the altitudinal limits of continuous vegetation in humid climates water is also an essential factor in slope stability in this paper we present results from field experiments to determine infiltration rates at forested sites in the andes mountains ecuador the southern appalachian mountains usa and the luquillo mountains puerto rico using a portable rainfall simulator infiltrometer all three areas and a single ring infiltrometer andes we determined infiltration rates even on steep slopes based on these results we examine the spatial variability of infiltration the relationship of rainfall runoff and infiltration to landscape position the influence of vegetation on infiltration rates on slopes and the implications of this research for better understanding erosional processes and landscape change infiltration rates ranged from 6 to 206 mm h on lower slopes of the andes 16 to 117 mm h in the southern appalachians and 0 to 106 mm h in the luquillo mountains these rates exceed those of most natural rain events confirming that surface runoff is rare in montane forests with deep soil regolith mantles on well drained forested slopes and ridges apparent steady state infiltration may be controlled by the near surface downslope movement of infiltrated water rather than by characteristics of the full vertical soil profile with only two exceptions the local variability of infiltration rates at the scale of 10 m overpowered other expected spatial relationships between infiltration vegetation type slope position and soil factors one exception was the significant difference between infiltration rates on alluvial versus upland soils in the andean study area the other exception was the significant difference between infiltration rates in topographic coves compared to other slope positions in the tabonuco forest of one watershed in the luquillo mountains our research provides additional evidence of the ability of forests and forest soils to preserve geomorphic features from denudation by surface erosion documents the importance of subsurface flow in mountain forests and supports the need for caution in extrapolating infiltration rates

quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution it is well known that pebble diameter systematically decreases downstream in rivers the contribution of abrasion is uncertain in part because 1 diameter is insufficient to characterize pebble mass loss due to abrasion and 2 abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field a recent geometric theory describes abrasion as a curvature dependent process that produces a two phase evolution in phase i initially blocky pebbles round to smooth convex shapes with little reduction in axis dimensions then in phase ii smooth convex pebbles slowly reduce their axis dimensions here we provide strong evidence that two phase abrasion occurs in a natural setting by examining downstream evolution of shape and size of thousands of pebbles over 10 km in a tropical montane stream the geometric theory is verified in this river system using a variety of manual and image based shape parameters providing a generalizable method for quantifying the significance of abrasion phase i occurs over 1 km in upstream bedrock reaches where abrasion is dominant and sediment storage is limited in downstream alluvial reaches where phase ii occurs we observe the expected exponential decline in pebble diameter using a discretized abrasion model the so called box equations with deposition we deduce that abrasion removes more than one third of the mass of a pebble but that size selective sorting dominates downstream changes in pebble diameter overall abrasion is the dominant process in the downstream diminution of pebble mass but not diameter in the studied river with important implications for pebble mobility and the production of fine sediments

an examination of spatial variability in the timing and magnitude of holocene relative sea level changes in the new zealand archipelago holocene relative sea level rsl changes have been reconstructed for four regions within the new zealand archipelago the northern north island including northland auckland and the coromandel peninsula the southwest coast of the north island the canterbury coast south island and the otago coast south island in the north island the rsl highstand commenced c 8100 7240 cal yr bp when present mean sea level pmsl was first attained this is c 600 1400 years earlier than has been previously indicated for the new zealand region as a whole and is consistent with recent holocene rsl reconstructions from australia in north island locations the early holocene sea level highstand was quite pronounced with rsl up to 2 75 m higher than present in the south island the onset of highstand conditions was later with the first attainment of pmsl being between 7000 6400 cal yr bp in the mid holocene the northern north island experienced the largest sea level highstand with rsl up to 3 00 m higher than present this is demonstrably higher than the highstand recorded for the southwest north island and otago regions a number of different drivers operating at a range of scales may be responsible for the spatial and temporal variation in the timing and magnitude of rsl changes within the new zealand archipelago one possible mechanism is the north south gradient in rsl that would arise in the intermediate field around antarctica in response to the reduced gravitational attraction of the antarctic ice sheet ais as it lost mass during the holocene this gradient would be enhanced by the predicted deformation of the lithosphere in the intermediate field of the southern ocean around antarctica due to hydro isostatic loading and mass loss of the ais however no such long wavelength signals in sea surface height or solid earth deformation are evident in glacial isostatic adjustment gia model predictions for the new zealand region while research from australia has suggested that north south variations in holocene rsl changes due to hydro isostatic influences are limited or non existent at the regional to local scale post glacial meltwater loading on the continental shelf around new zealand is predicted by gia modelling to have a significant effect on the timing and magnitude of rsl changes through the phenomenon of continental levering the spatial variation in continental levering is controlled by the configuration of the coast and the width of the adjacent continental shelf with continental levering providing a robust explanation for the observed spatial and temporal variations in rsl changes further research is required to characterise the regional and local effects of different tectonic regimes wave climates and sediment regimes these are potentially very significant drivers of rsl variability at the regional to local scale however the magnitude of their potential effects remains equivocal

determination of predevelopment denudation rates of an agricultural watershed cayaguas river puerto rico using in situ produced 10be in river borne quartz accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities to model global geochemical cycles and to examine interactions among climate weathering and uplift we present a simple approach to estimate predevelopment denudation rates using in situ produced cosmogenic 10be in fluvial sediments denudation processes in an agricultural watershed cayaguas river basin puerto rico and a matched undisturbed watershed icacos river basin were compared using 10be concentrations in quartz for various size fractions of bed material the coarse fractions in both watersheds bear the imprint of long subsurface residence times fine material from old shallow soils contributes little however to the present day sediment output of the cayaguas this confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the cayaguas basin and suggests that pre agricultural erosional conditions were comparable to those of the present day icacos

lithological and fluvial controls on the geomorphology of tropical montane stream channels in puerto rico an extensive survey and topographic analysis of five watersheds draining the luquillo mountains in north eastern puerto rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels the luquillo mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels however the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment gis based topographic analysis was used to examine channel profiles and survey data were used to analyze downstream changes in channel geometry grain sizes stream power and shear stresses results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial alluvial transitions non fluvial processes such as landslides deliver coarse boulder sized sediment to the channels and may locally determine channel gradient and geometry median grain size is strongly related to drainage area and slope and coarsens in the headwaters before fining in the downstream reaches a pattern associated with a mid basin transition between colluvial and fluvial processes downstream hydraulic geometry relationships between discharge width and velocity although not depth are well developed for all watersheds stream power displays a mid basin maximum in all basins although the ratio of stream power to coarse grain size indicative of hydraulic forcing increases downstream excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size and does not vary systematically with bankfull discharge a common characteristic in self forming threshold alluvial channels the results suggest that although there is apparent bedrock and lithologic control on local reach scale channel morphology strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin scale patterns copyright 2010 john wiley and sons ltd

the quebradillas limestone miocene pliocene northern puerto rico and tectonics of the northeastern caribbean margin the uppermost formation of the limestone belt of northern puerto rico is herein redesignated the quebradillas limestone a type section is established within the original type locality north of the town of quebradillas as here redefined the quebradillas limestone consists of a limestone sequence overlying the uppermost amphistegina algal limestone of the aymamon limestone along highway 2 north of the town of quebradillas the formation consists of three types of limestone biomicrite which form the basis for subdividing the formation into three members the lower member consists of globigerine biomicrite and is 16 m thick in many areas the lower member contains thin beds of oyster shells and shell fragments inter bedded within marl both the matrix of the oyster bearing beds and the marl are rich in planktonic foraminifers the middle member is essentially operculinoides biomicrite and is slightly amphistegina algal biomicrite and its thickness is estimated to be 44 4 m the threefold subdivision has been recognized throughout the outcrop belt of the formation from the west coast of the island eastward to barceloneta it probably applies to the easternmost outcrops as well the quebradillas aymamon boundary is an erosional unconformity between the two formations the globigerine limestone of the lower member of the quebradillas fills solution cavities in the uppermost aymamon and in places the upper aymamon shows marked relief most of the quebradillas is pliocene in age although part of the lower member may be latest miocene the quebradillas limestone is a regressive sequence the lower member represents a lower neritic to upper bathyal environment the middle member an outer shelf environment and the upper member a forereef subtidal and intertidal beach environment the upper member is veneered by what appears to be a paleosol which in turn is overlain by quaternary eolianite marginal marine and marine deposits the quebradillas is not limited to only the onshore coastal area of the island but extends northward into the puerto rico trench above a water depth of 5 000 m the quebradillas covers an area of at least 4 000 km 2 on the southern slope of the trench and the total area could be as much as 9 000 km 2 seismic and dredge sample data suggest that the quebradillas consists of the same facies offshore as in both outcrop and one well onshore the presence of widespread quebradillas in the puerto rico trench has important tectonic implications for regional studies of both the puerto rico trench and the northern caribbean region

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

stream incision tectonics uplift and evolution of topography of the sierra nevada california abstract stream incision faulting thermochronologic and geobarometric data suggest that sierra nevada topography is a consequence of two periods of uplift stream incision of up to 1 km has occurred since 5 ma maximum eocene miocene incision was 150 m uplift of the sierra nevada westward tilting stream incision and east down normal and dextral faulting along the present eastern escarpment of the range began at 5 ma estimates of late cenozoic crestal rock uplift for different areas in the sierra nevada range from 1440 to 2150 m low summit erosion rates suggest that the rock uplift approximates the surface uplift of crestal summits tertiary stream gradients were lower than modern ones suggesting that the bottoms of the canyons have been uplifted in the late cenozoic and that the mean elevation of the sierra nevada has increased the elevation of pre cenozoic basement rocks above the base of tertiary paleochannels ranges from 1000 m in the south and

origin of the highly elevated pyrenean peneplain 1 peneplanation of mountain ranges is generally considered the result of long term erosional processes that smooth relief and lower elevation near sea level therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift such an interpretation has been proposed for the pyrenees where high elevation low relief erosional surfaces rose up to more than 2000 m because the pyrenean foreland basins are filled with very thick continental deposits which have buried the early jagged landscape we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering we propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief such a process allows a high elevation low relief erosional surface to develop at the scale of the range in the pyrenees occurrence of high elevation low relief erosional surface remnants does not imply a posttectonic uplift but is instead due to the dissection of the initial miocene high elevation low relief surface by the recent drainage system the erosive activity of which has been enhanced by global climate change from the late pliocene onward

effects of a tectonically triggered wave of incision on riverine exports and soil mineralogy in the luquillo mountains of puerto rico we document the long term response of a tropical mountain stream to tectonics and show how this response exerts a first order spatial control on the delivery of weatherable minerals to forest soils in its catchment these minerals in turn affect nutrient availability and biogeochemical cycles the luquillo mountains is a rainforest covered isolated massif on puerto rico island instead of displaying typical concave up long profiles of most mountain streams the rivers draining the southern flank of these mountains display a systematic downstream increase in gradient using concentrations of in situ produced cosmogenic 10be in quartz from river sediments we find that the downstream steepening is due to the propagation of an erosion wave along the rivers from the coastal plain upstream toward the headwaters decreased 10be concentration along the steepened reaches knickpoint faces or knickzones results from faster denudation of the valley sides around the steepened reaches the upper portions of the steepened reaches knickpoint lips cluster in elevation around the altitude of a flat lying dissected surface traceable around the luquillo mountains inspection of similar platforms over other parts of puerto rico and in the surrounding seas suggests that the dissected surface is an uplifted shore platform formed in the early pliocene 4 my ago upstream of the knickpoints rivers possess alluvial reaches still graded to this uplifted platform they represent relict profiles of the rivers that once drained an island that was surrounded by the now uplifted platform we conclude that the knickpoints initiated when the platform started to rise above the caribbean sea the knickpoints then started propagating upstream defining the front of a slow moving wave of erosion 10be derived catchment scale denudation rates measured above and below the knickpoints indicate a 30 210 increase in denudation associated to the passage of the knickpoints mineralogical analysis of the soils show that soils upstream of the knickpoints overlie deeply depleted saprolite and only contain recalcitrant minerals whereas soils downstream of the knickpoints experience an input of fresh weatherable minerals this influx considerably increases nutrient availability porder et al 2015 in a forest where atmospheric inputs are otherwise the main source of nutrients mcdowell et al 1990 zarin and johnson 1995 pett ridge 2009

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

chemical weathering in a tropical watershed luquillo mountains puerto rico i long term versus short term weathering fluxes abstract the pristine rio icacos watershed in the luquillo mountains in eastern puerto rico has the fastest documented weathering rate of silicate rocks on the earth s surface a regolith propagation rate of 58 m ma 1 calculated from iso volumetric saprolite formation from quartz diorite is comparable to the estimated denudation rate 25 50 ma 1 but is an order of magnitude faster than the global average weathering rate 6 ma 1 weathering occurs in two distinct environments plagioclase and hornblende react at the saprock interface and biotite and quartz weather in the overlying thick saprolitic regolith these environments produce distinctly different water chemistries with k mg and si increasing linearly with depth in saprolite porewaters and with stream waters dominated by ca na and si such differences are atypical of less intense weathering in temperate watersheds porewater chemistry in the shallow regolith is controlled by closed system recycling of inorganic nutrients such as k long term elemental fluxes through the regolith e g si 1 7 10 8 moles m 2 s 1 are calculated from mass losses based on changes in porosity and chemistry between the regolith and bedrock and from the age of the regolith surface 200 ma mass losses attributed to solute fluxes are determined using a step wise infiltration model which calculates mineral inputs to the shallow and deep saprolite porewaters and to stream water pressure heads decrease with depth in the shallow regolith 2 03 m h2o m 1 indicating that both increasing capillary tension and graviometric potential control porewater infiltration interpolation of experimental hydraulic conductivities produces an infiltration rate of 1 m yr 1 at average field moisture saturation which is comparable with libr tracer tests and with base discharge from the watershed short term weathering fluxes calculated from solute chemistries and infiltration rates e g si 1 4 10 8 moles m 2 s 1 are compared to watershed flux rates e g si 2 7 10 8 moles m 2 s 1 consistency between three independently determined sets of weathering fluxes imply that possible changes in precipitation temperature and vegetation over the last several hundred thousand years have not significantly impacted weathering rates in the luquillo mountains of puerto rico this has important ramifications for tropical environments and global climate change

neoproterozoic bimodal magmatism in the cathaysia block of south china and its tectonic significance shrimp u pb zircon age geochemical and sm nd isotopic results are reported for the mamianshan volcanic rocks in the cathaysia block of southeastern south china the mamianshan volcanic rocks are bimodal in composition and are dominantly transitional to mildly alkaline basalts and subordinate alkaline rhyolite with an eruption age of 818 9 ma the basaltic samples are characterized by lree enriched and humped trace element patterns similar to many alkali basalts in continental rifts variablend t values between 3 33 and 4 35 indicate that the primary magma of these basalts was derived from an oib like mantle source and underwent fractional crystallization plus crustal contamination the rhyolitic rocks are highly enriched in th ta nb ree zr hf and y and depleted in sr p eu and ti sharing affinity to a 1 type granites combined with their slightly positive nd t values 0 22 to 0 92 the mamianshan felsic rocks were most likely generated by partial melting of the regional paleoproterozoic mayuan amphibolites the mamianshan bimodal volcanic rocks in the cathaysia block are coeval with the widespread intraplate magmatism around the yangtze block our results support the idea that a coherent south china craton was formed during the ca 1 0 ga sibao orogeny and it subsequently underwent extensive continental rifting related to mantle plume or superplume activities beneath rodinia since ca 825 ma 2004 elsevier b v all rights reserved

three phase tectonic evolution of the northern margin of puerto rico as inferred from an integration of seismic reflection well and outcrop data abstract integration of seismic reflection well and outcrop data from the oligocene pliocene puerto rico virgin islands platform prvi platform north of puerto rico indicates that three major tectonic phases characterize this seismically active region within the north america caribbean plate boundary zone tectonic phase 1 cretaceous to eocene formation and sedimentary infilling from a southern source area into a forearc basin formed between down to the north normal faults near the present day coast of puerto rico and an outer arc ridge near the present day shelf break tectonic phase 1 concluded the last volcanic arc activity in puerto rico produced by the subduction of oceanic crust of the north america plate beneath the caribbean arc system formation of the outer arc high defining the northern limit of this elongate basin may have been related to the accretion of off scraped sedimentary rocks beneath the northern margin of puerto rico in a manner similar to that observed in active arcs the end of tectonic phase 1 is related to initial collision between the caribbean arc and the bahama carbonate platform tectonic phase 2 oligocene to pliocene formation of a 1600 m thick northward thickening prvi platform predominately formed by carbonate rocks this phase started with a period of non deposition and erosion resulting in an latest eocene oligocene unconformity the base of the prvi platform is formed by a 400 m thick middle to late oligocene basal siliciclastic sequence that prograded northward across the forearc basin depositional thicknesses of sedimentary layers deposited during phase 2 are controlled by two large arches the nnw trending guajataca arch appears to have formed as the result of tectonic activity in the mona passage area the northeast trending san juan arch cannot be related to any adjacent structure or plate boundary feature onlap relations between carbonate rocks of the platform and both arches suggest that the arches were most active in the period from middle oligocene to early miocene rocks deposited during tectonic phase 2 are surprisingly devoid of faults and folds given their central position within the north america caribbean strike slip plate boundary zone that is known from regional studies to have been active throughout the deposition of the platform rocks tectonic phase 3 pliocene to holocene northward tilting of the prvi platform submerged the northern edge of the platform to a depth of 4 km and elevated the southern edge of the platform to several hundred meters above sea level on puerto rico northward tilting of this area occurred on the northern limb of a large arch or anticline formed parallel to the long axis of the island of puerto rico and its shelf areas the arch formed in response to a post pliocene convergence between the north america and caribbean plates

temporal and spatial variations of chalk cliff retreat in east sussex 1873 to 2001 spatial and temporal variations of chalk cliff retreat rates over a 130 yr period are examined for 22 km of the east sussex chalk coastline between brighton and belle toutetout lighthouse england data are derived from maps surveyed between 1873 and 1925 28 and air photographs dating from 1973 and 2001 the long term average cliff retreat rate over the period 1873 to 1973 is 0 36 m y 1 and from 1873 to 2001 is 0 35 m y 1 over the whole 23 km study area rates calculated over shorter time slices indicate a decline in cliff retreat 0 37 m y 1 between 1873 and 1925 28 0 35 m y 1 between 1925 28 and 1973 0 27 m y 1between 1973 and 2001 temporal variations are unlikely to be due to variations in precipitation and frost days though the relationship remains uncertain due to the resolution of available cliff retreat and climatic data changes in wave climate are small and cannot be responsible for the decline in cliff retreat the study suggests that the most important factors causing the measured decline in cliff retreat rates are firstly the loss of cliff toe beaches over time and secondly widening of the rocky shore platforms that front the cliffs some of the spatial variability in retreat rates can be explained by local variations in lithology

low long term erosion rates in high energy mountain belts insights from thermo and biochronology in the eastern pyrenees abstract we relate the high elevation of flat topography in the eastern pyrenees to the resurrection of a mountain belt which prior to 12 ma was a low relief landscape or peneplain bevelling eroded stumps of the pyrenean compressional orogen new apatite fission track and u th he data together demonstrate the consistently young age

investigation of in situ weathering of quartz diorite bedrock in the rio icacos basin luquillo experimental forest puerto rico abstract the rio icacos basin in puerto rico is the site of the highest measured chemical solute fluxes for a catchment on granodiorite this is partly attributable to high annual rainfall 4300 mm high average temperature 23 c and moderate relief the bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock 0 5 1 5 m thick underlying saprolite these zones are characterized by systems of onion skin rindlets each 3 10 cm thick in which porosity development is dominated by weathering of plagioclase to kaolinite fe bearing aluminosilicate minerals hornblende augite and biotite persist in the weathered rock after plagioclase has weathered to completion but hornblende and augite disappear between the porous rindlet systems and saprolite a new watershed scale solute mass balance corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock except quartz and biotite to kaolinite and fe3 oxides our findings indicate that steady state assumptions are reasonable and useful in studying weathering at this site estimated steady state sediment yields based on net solute fluxes and average bedrock composition of 3 24 10 9 to 3 60 10 9 kg m 2 s 1 are smaller than measured values indicating that sediment generation is currently accelerated with respect to steady state however sediment generation has recently been closer to steady state since weathering within the rindlet systems occurs on multiple fronts moving upward and downward two new steady state conceptual models are developed to account for 1 multiple parallel weathering fronts and 2 differential rates of advance of weathering fronts due to porosity development based on these models in situ weathering rates for plagioclase at two sample locations of 2 7 10 15 and 5 0 10 15 mol na g 1 s 1 are calculated values comparable to bet surface area normalized rates of 2 7 10 16 to 5 0 10 15 mol na m 2 s 1

grain size effects on energy delivery to the streambed and links to bedrock erosion fluvial bedrock erosion rates are thought to be proportional to the energy delivered to the channel bed by impacting bed load particles combining methods for field measurements of impact energy and for automatic sensing of grain size distributions in the instrumented channel of the erlenbach we show that the larger a bed load grain is the more efficient it is in transferring energy to the streambed on average the largest grain size class contributes only 9 to the total bed load mass but is responsible for more than 40 of the energy delivery this effect may help to explain the predominance of extreme floods when large grains are mobile in driving long term erosion while the mechanistic saltation abrasion model accounts for hydraulic forcing of energy delivery the observed grain size effect is not captured we suggest that the discrepancy arises from differing transport modes rolling sliding and saltation not currently captured in modeling