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

temporally and spatially uniform rates of erosion in the southern appalachian great smoky mountains we measured 1 0 be in fluvial sediment samples n 27 from eight great smoky mountain drainages 1 330 km 2 results suggest spatially homogeneous sediment generation on the 10 4 10 5 yr time scale and 100 km 2 spatial scale at 73 11 t km 2 yr 1 equivalent to 27 4 m m y of bedrock erosion this rate is consistent with rates derived from fission track long term sediment budget and sediment yield data all of which indicate that the great smoky mountains and the southern appalachians eroded during the mesozoic and cenozoic at 30 m m y in contrast unroofing rates during the paleozoic orogenic events that formed the appalachian mountains were higher 10 2 m m y erosion rates decreased after termination of tectonically driven uplift enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape

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

hydrological modelling of a pleistocene landslide dammed lake in the santa maria basin nw argentina abstract around 30 000 14 c yr bp multiple landslides in the quebrada de cafayate 26 s 66 w dammed the rio de las conchas and caused a lake in the santa maria basin nw argentina since climate could have played a major role in landslide generation we used a lake balance model to compare the present day climate with the conditions during landsliding this model reveals that a hypothetical present day lake would stabilize around 70 m below the reconstructed paleolake level the volume of the water body would be one third of the corresponding paleolake volume however a minimum 10 15 increase in mean annual precipitation accompanied by at least a 3 4 c drop in temperature would result in a stable lake level at the reconstructed elevation this result implies wetter and cooler conditions at 30 000 14 c yr bp as compared to the present wetter climate in turn could help to explain enhanced landsliding in the quebrada de cafayate during this time

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

abnormal monsoon years and their control on erosion and sediment flux in the high arid northwest himalaya abstract the interplay between topography and indian summer monsoon circulation profoundly controls precipitation distribution sediment transport and river discharge along the southern himalayan mountain front shf the higher himalayas form a major orographic barrier that separates humid sectors to the south and arid regions to the north during the indian summer monsoon vortices transport moisture from the bay of bengal swirl along the shf to the northwest and cause heavy rainfall when colliding with the mountain front in the eastern and central parts of the himalaya precipitation measurements derived from passive microwave analysis ssm i show a strong gradient with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen the end of the monsoonal conveyer belt is near the sutlej valley in the nw himalaya where precipitation is lower and rainfall maxima move to lower elevations this region thus comprises a climatic transition zone that is very sensitive to changes in indian summer monsoon strength to constrain magnitude temporal and spatial distribution of precipitation we analyzed high resolution passive microwave data from the last decade and identified an abnormal monsoon year amy in 2002 during the 2002 amy violent rainstorms conquered orographic barriers and penetrated far into otherwise arid regions in the northwest himalaya at elevations in excess of 3 km asl while precipitation in these regions was significantly increased and triggered extensive erosional processes i e debris flows on sparsely vegetated steep hillslopes mean rainfall along the low to medium elevations was not significantly greater in magnitude this shift may thus play an important role in the overall sediment flux toward the himalayan foreland using extended precipitation and sediment flux records for the last century we show that these events have a decadal recurrence interval during the present day monsoon circulation hence episodically occurring amys control geomorphic processes primarily in the high elevation arid sectors of the orogen while annual recurring monsoonal rainfall distribution dominates erosion in the low to medium elevation parts along the shf

biases in thorpe scale estimates of turbulence dissipation part i assessments from large scale overturns in oceanographic data abstractoceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy this method assumes a linear scaling between the thorpe length scale lt and the ozmidov length scale lo historic evidence supporting lt lo has been shown for relatively weak shear driven turbulence of the thermocline however little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open ocean standards this study presents a direct comparison of lt and lo using vertical profiles of temperature and microstructure shear collected in the luzon strait a site characterized by topographically influenced overturns up to o 100 m in scale the comparison is also done for open ocean sites in the brazil basin and north atlantic where overturns are generally smaller and due to different processes a key result is that lt lo increases with overturn size in a fashion similar to that observed in numeric

turbulence and entrainment in a buoyant surface plume in winter tidal outflow from the leschenault estuary forms a buoyant sheared stratified plume in koombana bay on each outgoing tide the turbulence in the plume and the local entrainment into the plume are examined based on measurements collected in an intensive field program coherency measurements between horizontally separated sensors are used to examine the anisotropy of the turbulence the effects of viscosity and buoyancy on the turbulence are studied using an activity diagram which is also an indicator of the mixing efficiency and total mixing the osborne cox model is tested and found to be valid in the plume

mixing at camarinal sill in the strait of gibraltar during the gibraltar experiment in 1985 1986 observations of shear stratification microstructure and acoustic backscatter revealed a complex mixing regime at camarinal sill in the strait of gibraltar the mixing is forced by strong semidiurnal tidal currents and weaker but more steady baroclinic flows flows are characterized as outflow or inflow nearly equivalent to westward or eastward rising water at gibraltar coincides with outflow at all depths we observed two modes of outflow at the sill in may 1986 the transition layer separating atlantic and mediterranean water was between 0 75 mpa and 1 mpa when we sampled east of the sill near neap tide the transition turned down just before reaching the sill crest where the flow became critical increasing shear soon destabilized the transition producing billows which grew to 30 75 m on the west flank of the sill the overturning generated intense turbulence with peak dissipation rates e exceeding 10 2 w kg 1 this turbulence rapidly thickened the transition from z 30 50 m east of the sill to z 130 150 m within 1 2 km west of the sill from the rate of thickening we estimate the rate of formation of new transitional water during neap tide as 0 8 1 2 105 m3 s 1 this outflow mode did not release eastward propagating internal bores when outflow turned to inflow but the inflow did appear to arrest the deep outflow in may the largest average dissipation rate over and just west of the sill had the same magnitude as estimates of the net energy lost by adjustment of velocity and mass across the sill during outflow other averages are smaller than the energy loss presumably because the limited sampling severely underestimates the average dissipation rate during outflow strong shears were observed to rise to 100 times those in the background internal wave field however probably owing to spatial offsets between the ship s acoustic doppler current profiler and our tethered free fall profiler the only statistically significant correlations were found between overturning scales measured directly and those estimated from e in the same profile we observed the other outflow mode in october 1985 when we profiled over the sill near spring tide and the transition was east of the sill centered near 1 5 mpa after encountering the sill the transition rose sharply and remained within 10 20 m of the surface across the sill plunging steeply downward just past the west edge of the sill this was accompanied by intense turbulence and at least one rebound of the transition we did not observe release of eastward propagating bores when inflow turned to outflow but observations by armi and farmer 1988 in april 1986 found bores released at spring tide in regimes similar to the one we observed in october 1985

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

the contribution of salt fingers to vertical mixing in the north atlantic tracer release experiment abstract the north atlantic tracer release experiment natre was performed in an area moderately favorable to salt fingers however the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent this is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems despite the lack of a staircase optical shadowgraph profiles revealed that small scale tilted laminae previously observed in a salt finger staircase were abundant at the natre site using microstructure observations the strength of salt finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy the dissipation ratio by examining the dissipation ratio in a parameter space of density ratio r and richardson number ri the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred while the model for turbulence descr

role of overturns in optimal mixing in stratified mixing layers turbulent mixing plays a major role in enabling the large scale ocean circulation the accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years primarily on the basis of adequately high resolution numerical simulations we add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes we study the inherently intermittent and time dependent nature of wave induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small scale isotropic turbulence grows the anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales we show that the characteristics of turbulent mixing depend on the relative time evolution of the ozmidov length scale compared to the so called thorpe overturning scale which represents the scale containing available potential energy upon which turbulence feeds and grows we find that when the mixing is most active and efficient since stirring by the largest overturns becomes optimal in the sense that it is not suppressed by ambient stratification we argue that the high mixing efficiency associated with this phase along with observations of in oceanic turbulent patches together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected this neglect arising through the assumption of a clear separation of scales between the background mean flow and small scale quasi isotropic turbulence leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density such an exclusion may well be significant if the mechanism of shear induced turbulence is assumed to be representative of at least some turbulent events in the ocean while our results are based upon simulations of shear instability we show that they are potentially more generic by making direct comparisons with data from ocean and lake observations which represent a much wider range of turbulence inducing physical processes

three dimensional double ridge internal tide resonance in luzon strait abstractthe three dimensional 3d double ridge internal tide interference in the luzon strait in the south china sea is examined by comparing 3d and two dimensional 2d realistic simulations both the 3d simulations and observations indicate the presence of 3d first mode semi diurnal standing waves in the 3 6 km deep trench in the strait as in an earlier 2d study barotropic to baroclinic energy conversion flux divergence and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides the resonance in the 3d simulation is several times stronger than in the 2d simulations for the central strait idealized experiments indicate that in addition to ridge height the resonance is only a function of separation distance and not of the along ridge length that is the enhanced resonance in 3d is not caused by 3d standing waves or basin modes instead the difference in resonance between the 2d and 3d simulations is attributed to the topographic blocking

large eddy simulation of flow and turbulence at the steep topography of luzon strait steep generation sites for topographic internal gravity waves can also be sites of turbulence the present work uses turbulence resolving large eddy simulation les for tidal flow over multiscale steep bathymetry patterned after a portion of the west ridge of luzon strait at 20 6 n oceanic values of the slope criticality froude number excursion number and aspect ratio are matched the amplitude and phasing of velocity overturns and dissipation are similar to observations made at a deep mooring however a tidal phase is noted in which there is a discrepancy that could arise from the resonant interaction with waves generated at the east ridge that is not included in the model turbulent dissipation originates from several mechanisms including breaking lee waves during flow reversal downslope jets critical slope boundary layer high mode internal wave beams off slope lee wave breaking and wave breaking in a valley between subridges the model baroclinic energy budget is closed with negligible residual and the local turbulence loss of 23 5 is large

local erosion rates versus active tectonics cosmic ray exposure modelling in provence south east france over the past decade in situ produced cosmogenic nuclides have revolutionised the study of landscape evolution in particular numerous studies have demonstrated that in active tectonic settings cosmic ray exposure dating of deformed or displaced geomorphic features makes it possible to quantify long term deformation rates in western european countries erosion due to climatically driven processes and human activities is probably the factor that most limits the accuracy of exposure ages and landscape modification rates in this study we present the results of a depth profiling technique applied to alluvial terraces located along the rhone and the moyenne durance rivers the expected decrease with depth of the measured 10be concentrations has been modelled using a 2 inversion method in order to constrain the exposure history of the alluvial sediments the results suggest that 1 over the quaternary the local surface erosion rates including both regional uplift and climatically driven processes acting on landforms are on the order of 30 m myr in southeastern france and 2 providing a fairly good bracketing of the exposure age the modelled abandonment age of alluvial terraces affected by the moyenne durance fault allows estimating incision rates comparing the alluvial terrace elevations with topographic river profiles and a minimum vertical slip rate value of roughly 0 02 mm yr for the southern segment of the moyenne durance fault

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

estimates of the temperature flux temperature gradient relation above a sea floor the relation between the ux of temperature or buoyancy the verti cal temperature gradient and the height above the bottom is investigatedin an oceanographic context using high resolution temperature measure ments the model for the evolution of a strati ed layer by balmforthet al 1998 is reviewed and adapted to the case of a turbulent owabove a wall model predictions are compared to the average observa tional estimates of the ux exploiting a ux estimation method proposedby winters d asaro 1996 this estimation method enables the dis entanglement of the dependence of the average ux on the height abovethe bottom and on the background temperature gradient the classicaln shaped ux gradient relation is found in the observations model andobservations show similar qualitative behaviour despite the strong sim pli cations used in the model the results shed light on the modulationof the temperature ux by the presence of the boundary and support theidea of a turbulent ux following a mixing length argument in a strati edow furthermore the results support the use of thorpe scales close to aboundary if su cient averaging is performed suggesting that the thorpescales are a ected by the boundary in a similar way as the mixing length

turbulent mixing in a deep fracture zone on the mid atlantic ridge abstractmidocean ridge fracture zones channel bottom waters in the eastern brazil basin in regions of intensified deep mixing the mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones whether affected by the local or the nonlocal canyon topography are still subject to debate to discriminate those mechanisms and to discern the canyon mean flow two moorings sampled a deep canyon over and away from a sill contraction a 2 layer exchange flow accelerated at the sill transports 0 04 0 10 sv 1 sv 106 m3 s 1 up canyon in the deep layer at the sill the dissipation rate of turbulent kinetic energy e increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy cross sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides during spring to neap tides e varies from o 10 9 to o 10 10 w kg 1 below 3500 m around the 2 layer inte

prefrontal bore mixing rainbow ridge a 1 950 m deep upthrusted ultramafic block along the axis of the mid atlantic ridge has an active hydrothermal vent system at 2 400 m on its western slope however within one kilometer from the vent excessive temperatures are barely measurable probably due to strong turbulent mixing this mixing is studied here using a 400 m long high resolution temperature sensor array moored with a 600 m ranging 75 khz acoustic doppler current profiler rich internal wave turbulence was recorded characterized by 100 200 m upshoots and 200 m large overturning in particular near the end of the warming phase of the up and down moving tide these highly nonlinear internal waves of tides interacting with buoyancy frequency waves extend up to 400 m above the sloping bottom of the ridge whilst a turbulent bottom boundary layer could barely be defined the more intense turbulence higher up in the water column is suggested to lead to the strong dispersion of the hydrothermal plume

the puncoviscana trough a large basin of late precambrian to early cambrian age on the pacific edge of the brazilian shield during the late precambrian to early cambrian a large basin with a homogeneous psammitic pelitic sediment fill existed in the area of the nw argentine andes it is now exposed in different tectonic levels this basin of meridional elongation was situated on a stable continental margin at the western edge of the brazilian shield it was underlain by segmented older continental crust according to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed four clastic facies types indicate a transport by gravity currents within a submarine fan system while massive red pelites represent cut off periods and rare carbonates deposition on rises the gravity currents derived from easterly directions the sedimentation age is proved by a rich ichnofauna and some medusoid impressions

numerical and analytical models of cosmogenic radionuclide dynamics in landslide dominated drainage basins 1 in many tectonically active regions on earth landslides dominate sediment delivery to channels while cosmogenic radionuclides crn represent a valuable tool to document basin averaged erosion rates a better understanding of crn dynamics in these systems is required before erosion rates can be reliably inferred from crn concentrations the stochastic nature of landslides results in variability of crn concentrations in both alluvial and hillslope samples that is both difficult to document and poorly understood to guide sampling strategies we developed a 2 d plan view model to simulate the influence of landslides on crn dynamics we employ a sediment mixing model in which concentrations of crns from material eroded by both landslides and steady processes are combined to evaluate how best to infer erosion rates from crn concentrations this model is further used to test a new analytical solution for the distribution of crn concentrations on hillslopes in a landslide prone system results suggest that for landscapes dominated by deep 5 m landslides only large drainage basins 100 km2 provide consistently reliable estimates of erosion rates from crn concentrations for landscapes dominated by shallow landslides 0 7 2 m smaller basins can produce reliable estimates results further suggest that the distribution of concentrations from hillslope samples could be used to deduce the relative contribution of material derived from landslide and steady processes these findings underscore the need to consider carefully the sources of natural variability in sampling and analyzing crns if properly done the variability can reveal useful information about geomorphic processes in landslide prone areas

detailed observations of a naturally occurring shear instability simultaneous profiles of microstructure horizontal velocity and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date shear increased rapidly after passing through a lateral constriction which formed a hydraulic control a kilometer long set of 20 m tall billows grew on a middepth density interface where the richardson number fell below 0 25 the velocity interface thickened steadily after the billows formed consistent with rapid momentum mixing across a shear layer with a reynolds number of 3 106 the billows generated large density overturns and dissipation rates greater than 10 5 w kg 1 even within the first large overturn indicating that these structures were fully turbulent early in their development as the billows grew a well mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods 3 times longer than in laboratory studies at low reynolds number variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed with data from the large overturns within the shear layer we find e n2 3 104 an average root mean square overturn scale lrms of 2 6 m and a buoyancy scale lb of 2 7 m despite having sampled the billows at varying stages of their evolution we find no indication that the ratio lrms lb is ever significantly different than 1 for this shear instability

chaco low level jet events characterization during the austral summer season 1 previous studies showed a relationship between convection maxima and the convergence of vapor flux over southeastern south america nicolini and saulo 2000 explored the hypothesis of an intensification of this mutual dependency during the chaco jet events they defined these events as extreme cases of low level jets east of the andes that penetrate southernmost to 25 s using eta operative products during the 1997 1998 austral warm season the interest of the present paper centers on the climatic characterization of the chaco jet events cjes and the possibility of researching this hypothesis using the european centre for medium range weather forecasts reanalyses era which cover a 15 year period 1979 1993 the cjes represent a subensemble of low level jet events east of the andes that are infrequent in the era data set their duration varies from 1 to 10 days more frequently extending from 1 to 5 days the outstanding features of the circulation and the thermodynamic field that represent this ensemble are a maximum contrast of air masses in a latitude close to 39 s the presence of a trough centered on 70 w within a baroclinic wave train penetrating from the pacific ocean and a maximum of heat and moisture over northen argentina and paraguay during the cjes there is an important flux of moisture and convergence at low and mid levels that is about 10 times more intense than the summer mean the intensity found in the water vapor flux anomaly reinforces the importance of studying these episodes for the purpose of determining the water balance over southeastern south america the statement that the cjes represent an important characteristic of the southeastern south american climate is founded on the fact that although the cjes only represent 17 of the austral summer days they account for a significant fraction of the precipitation a maximun of 55 over northeastern argentina however it is indispensable to contrast these results with future field experiments to test the hypothesis addressed in this paper

climatic controls on debris flow activity and sediment aggradation the del medio fan nw argentina in the central andes several studies on alluvial terraces and valley fills have linked sediment aggradation to periods of enhanced sediment supply however debate continues over whether tectonic or climatic factors are most important in triggering the enhanced supply the del medio catchment in the humahuaca basin eastern cordillera nw argentina is located within a transition zone between sub humid and arid climates and hosts the only active debris flow fan within this intermontane valley by combining 10be analyses of boulder and sediment samples within the del medio catchment with regional morphometric measurements of nearby catchments we identify the surface processes responsible for aggradation in the del medio fan and their likely triggers we find that the fan surface has been shaped by debris flows and channel avulsions during the last 400 years among potential tectonic climatic and autogenic factors that might influence deposition our analyses point to a combination of several favorable factors that drive aggradation these are in particular the impact of occasional abundant rainfall on steep slopes in rock types prone to failure located in a region characterized by relatively low rainfall amounts and limited transport capacity these characteristics are primarily associated with the climatic transition zone between the humid foreland and the arid orogen interior which creates an imbalance between sediment supply and sediment transfer the conditions and processes that drive aggradation in the del medio catchment today may provide a modern analog for the conditions and processes that drove aggradation in other nearby tributaries in the past

average pleistocene climatic patterns in the southern central andes controls on mountain glaciation and paleoclimate implications abstract despite elevations of 5000 6800 m modern glaciers occur along the southern puna plateau and the northern sierras pampeanas in the southern central andes the modern snowline rises from 5100 m in sierra aconquija to 5800 m in the puna as a result of a westward decrease in precipitation from 450 to less than 100 mm yr during the pleistocene these arid highlands experienced multiple cirque and valley glaciation that likely postdate the last interglacial period although lack of age control prevents an absolute chronology glaciation in the puna and along the eastern puna edge produced a 300 m pleistocene snowline psl depression while in the sierras pampeanas the psl depression was at least 900 m the greater psl depression in the sierras pampeanas is best explained by a combination of cooling and increase of easterly moisture whereas the psl depression in the puna appears more sensitive to moisture increases than temperature previously glaciations in this region have been explained by increa

mixing associated with sills in a canyon on the midocean ridge flank to close the global overturning circulation the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion hydrographic and microstructure observations from the brazil basin in the south atlantic ocean indicate that most of the abyssal mixing there takes place on the topographically rough flank of the midocean ridge in previous studies it has been suggested that the high level of abyssal mixing observed on the ridge flank is primarily caused by breaking internal waves forced by tidal currents here the results from a detailed analysis of velocity hydrographic and microstructure data from a ridge flank canyon are presented two year long current meter records indicate that within the canyon there is a significant along axial mean flow down the density gradient toward the ridge crest five hundred meters above the canyon floor the kinetic energy in the subinertial band exceeds that associated with the semidiurnal tides by approximately a factor of 2 the mean dissipation of kinetic energy inside the canyon exceeds that above the ridge flank topography by approximately a factor of 5 the largest dissipation values were observed downstream of a narrow 1000 m high sill that extends across the full width of the canyon along the entire canyon there is a strong association between the presence of sills and along axial density gradients while there is no similar association between the presence of depressions and density gradients together these observations suggest that sill related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal wave breaking which is not expected to be associated preferentially with sills while only 15 of the interfacial area between antarctic bottom water and north atlantic deep water in the brazil basin lie inside canyons the available data suggest that approximately one half of the diapycnal buoyancy fluxes take place there in comparison the region above the ridge flank topography accounts for about one third of the buoyancy fluxes the apparent importance of sill related processes for mixing in ridge flank canyons is therefore of global significance especially considering that such canyons occur on average every 50 km along 2 3 of the global midocean ridge system and that sills partially block the canyon axes every few tens of kilometers

mixing in the romanche fracture zone the romanche fracture zone is a major gap in the mid atlantic ridge at the equator which is deep enough to allow significant eastward flows of antarctic bottom water from the brazil basin to the sierra leone and guinea abyssal plains while flowing through the romanche fracture zone bottom water properties are strongly modified due to intense vertical mixing the diapycnal mixing coefficient in the bottom water of the romanche fracture zone is estimated by using the finestructure of ctd profiles the microstructure of high resolution profiler data and by constructing a heat budget from current meter data the finestructure of density profiles is described using the thorpe scaleslt it is shown from microstructure data taken in the bottom water that the ozmidov scale lo is related to lt by the linear relationship lo 5 0 95lt similar to other studies which allows an estimate of the diapycnal mixing coefficient using the osborn relation the thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream eastward of the main sill of the romanche fracture zone in this region a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water estimates of cross isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the romanche fracture zone and the nearby chain fracture zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the romanche fracture zone although the two fracture zones occupy only 0 4 of the area covered by the sierra leone and guinea abyssal plains the diffusive heat fluxes across the 1 4 8c isotherm in the romanche and chain fracture zones are half that found over the abyssal plains across the 1 88c isotherm emphasizing the role of these passages for bottom water property modifications

energy flux and dissipation in luzon strait two tales of two ridges abstractinternal tide generation propagation and dissipation are investigated in luzon strait a system of two quasi parallel ridges situated between taiwan and the philippines two profiling moorings deployed for about 20 days and a set of nineteen 36 h lowered adcp ctd time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals measurements were concentrated on a northern line where the ridge spacing was approximately equal to the mode 1 wavelength for semidiurnal motions and a southern line where the spacing was approximately two thirds that the authors contrast the two sites to emphasize the potential importance of resonance between generation sites throughout luzon strait baroclinic energy energy fluxes and turbulent dissipation were some of the strongest ever measured peak to peak baroclinic velocity and vertical displacements often exceeded 2 m s 1 and 300 m respectively energy fluxes exceeding 60 kw m 1 were measured at spring tide at the wester

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

lacustrine chronology links late pleistocene climate change and mass movements in northern new mexico well dated lacustrine deposits in northern white rock canyon new mexico record damming of the rio grande by at least four separate failures of a slump complex between about 17 5 and 12 4 ka sup 14 c linking mass movements to a period of rapid climate change in the western united states failure of metastable slumps probably resulted from removal of lateral support during down cutting and erosion by the rio grande and from a decrease in resisting forces due to increased pore pressures our chronology suggests that the lake that formed between 17 5 and 15 0 ka may record effects of both glacial melt and pluvial activity mainly enhanced rainfall the youngest lake neary 12 4 ka may record pluvial runoff and the intermediate lakes 13 7 to 13 1 ka may record pluvial and minor melt water activity we have not found lacustrine deposits younger than about 12 4 ka the record of geomorphic response along the rio grande suggests that late pleistocene climatic changes may have triggered similar mass movements elsewhere in the southwest 27 refs 4 figs

westerly jet stream controlled climate change mode since the last glacial maximum in the northern qinghai tibet plateau abstract climate change from the northern qinghai tibet plateau qtp is affected by the combined effects of the asian summer monsoon and the westerlies however changes in monsoon vapor transport and the westerly jet stream as well as their effect to climate change mode there are not clear at different time scales here we present composite research using paleoclimate reconstructions and ensemble simulations since the last glacial maximum lgm on the basis of an eolian sedimentary sequence in the northern qtp results from 317 surface sediment samples show that pedogenic carbonate 18 o is affected both by precipitation and temperature and organic carbon isotope 13 corg reflects the vegetation growth status in areas where precipitation is greater than 290 mm proxies from the sedimentary sequence and the paleoclimate modeling intercomparison project 3 pmip3 simulations show a close relationship between climate change mode in the northern qtp and the westerly jet stream whose movement is triggered by warm or cold conditions in the high latitude regions that is when the climate is in cold glacial period the westerly jet stream strengthens and moves southward and the mountains become wetter this climate change mode can be validated by records from mountains of north america and africa according to the cmip5 simulations the westerly jet stream will strengthen in the future and climate in the northern qtp and the high altitude region of the mid latitudes will become wetter with global warming

buoyancy forcing by turbulence above rough topography in the abyssal brazil basin abstract observations of turbulent dissipation above rough bathymetry in the abyssal brazil basin are presented relative to regions with smooth bathymetry dissipation is markedly enhanced above rough topography of the mid atlantic ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors furthermore mixing levels in rough areas are modulated by the spring neap tidal cycle internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation a model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry this inverse model uses both beta spiral and integrated forms of the advective budgets for heat mass and vorticity and contains sufficient information to resolve the full three dimensional flow the inverse model solution re

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

mixing dissipation rate and their overturn based estimates in a near bottom turbulent flow driven by internal tides abstractdirect numerical simulation dns and large eddy simulation les are employed to study the mixing brought about by convective overturns in a stratified oscillatory bottom layer underneath internal tides the phasing of turbulence the onset and breakdown of convective overturns and the pathway to irreversible mixing are quantified mixing efficiency shows a systematic dependence on tidal phase and during the breakdown of large convective overturns it is approximately 0 6 a value that is substantially larger than the commonly assumed value of 0 2 used for calculating scalar mixing from the turbulent dissipation rate diapycnal diffusivity is calculated using the irreversible diapycnal flux and for tall overturns of o 50 m the diffusivity is found to be almost 1000 times higher than the molecular diffusivity the thorpe overturn length scale is often used as a proxy for the ozmidov length scale and thus infers the turbulent dissipation rate from overturns the accuracy of overturn based es

millennial scale record of landslides in the andes consistent with earthquake trigger landslide deposits are often interpreted as the consequence of precipitation a millennial scale record of landslides inferred from river cobbles in the arid andes is instead consistent with earthquake triggering

observations of small scale secondary instabilities during the shoaling of internal bores on a deep ocean slope abstractthe rockall bank area located in the northeast atlantic ocean is a region dominated by topographically trapped diurnal tides these tides generate up and downslope displacements that can be locally described as swashing motions on the bank using high spatial and time resolution of moored temperature sensors the transition toward the upslope flow cooling phase is described as a rapid upslope propagating bore likely generated by breaking trapped internal waves buoyant anomalies are found during the bore propagation likely resulting from small scale instabilities the imbalance between the rate of disappearance of available potential energy and the dissipation rate of turbulent kinetic energy suggests that these instabilities are growing i e young and have high mixing potential

denudation processes and rates in the transverse ranges southern california erosional response of a transitional landscape to external and anthropogenic forcing 1 quantification of denudation in the transverse ranges of california permits reconstruction of spatial and temporal variations in erosion that represent both the response and evolution of interacting hillslopes and channels on the southern front of the san gabriel mountains observational records of the infilling of debris basins and dams define twentieth century landscape erosion rates averaging 1 6 and 0 9 mm yr 1 respectively although all major sediment transport occurs during intense winter storms debris production on hillslopes is greatly enhanced by recurrent fires consequently in this populated region anthropogenic fires have augmented the natural erosion rates we perform a global inversion to estimate the role of precipitation intensity burned areas and local slope on catchment denudation rates after subtracting the effects of anthropogenic fires we estimate landscape denudation rates given natural fire ignition rates increased fire during the past century has augmented sediment production in debris basins by an average of 60 and individual basins show increases up to 400 to identify the dominant hillslope erosion processes the volumetric contribution of landslides was estimated using repeat aerial photographs for the same time interval over which the debris basins have been operative between 1928 and 1973 landsliding produced only 10 of the sediment in debris basins even in the long term when infrequent but volumetrically important landslides occur bedrock landslides appear to contribute a maximum of 50 to the long term landscape denudation previous mapping of soil slippage in the san dimas experimental forest within the san gabriel mountains rice et al 1969 rice and foggin 1971 indicates that shallow landsliding is likely to be the dominant modern hillslope erosion process when compared to incision rates derived from a fluvial shear stress model and to exhumation rates based on low temperature thermochronological data blythe et al 2000 modern natural erosion rates are comparable to denudation rates since the pliocene comparisons of modern erosion rates suggest that debris production on hillslopes and first order channels is directly dependent on vegetation cover and precipitation intensity for higher order channels drainage areas 2 km2 only major storms convey the sediments down valley during the past century some temporal decorrelation occurs between small scale and large scale catchments because hillslope produced sediment is stored in second order and higher channels until major storms mobilize it thus the larger fluvial network damps the episodic fire induced hillslope sediment pulses that occur within small watersheds at a longer temporal scale however uplift and denudation may have been sustained sufficiently long in much of the san gabriel mountains for the topography to reach a macroscale steady state in contrast to rapidly eroding 2 mm yr 1 ranges for which quantification of bedrock landsliding will approximate the sediment flux the san gabriel mountains occupy a niche of intermediate rates 0 1 1 0 mm yr 1 in which a broad suite of hillslope processes including shallow seated and deep seated landslides debris flows and wet and dry ravel contribute to the sediment flux

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

orographic barriers high resolution trmm rainfall and relief variations along the eastern andes 1 quantifying the degree to which tectonic and erosive processes shape landscapes is key to understanding the evolution of tectonically active mountain belts here we explore the interplay of these two processes along the humid eastern flank of the south american andes we use high resolution tropical rainfall measurement mission trmm and srtm data to characterize elevation relief and hillslope angle of peak rainfall at orographic barriers over a distance of more than 3500 km along the eastern flanks we find that peak rainfall 3 5 m yr occurs at a mean elevation of 1 3 0 17 km a mean relief of 0 95 0 08 km and at moderate mean hillslope angles of 18 3 1 7 we suggest that topographic relief is the best first order rainfall predictor and we demonstrate how relief changes along strike south of the andean orocline alter rainfall distribution changes in climatic sedimentary and tectonic processes prevent the formation of high relief amounts at the mountain front and these areas are not characterized by pronounced rainfall peak

how variable is mixing efficiency in the abyss author posting american geophysical union 2020 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geophysical research letters 47 2020 e2019gl086813 doi 10 1029 2019gl086813

late pleistocene fans and terraces in the majes valley southern peru and their relation to climatic variations this study investigates the connection between sediment aggradation erosion and climate in a desert environment of the majes valley southern peru luminescence dating of terraces and fans shows that sediment aggradation correlates with wet time intervals on the altiplano suggesting a climatic influence on the aggradation degradation cycles major periods of aggradation occurred between 110 100 60 50 and 12 8 ka more precipitation in the majes catchment resulted in increased erosion and transportation of sediment from the hillslopes into the trunk river as a result the sediment loads exceeded the transport capacity of the majes river and aggradation started in the lower reaches where the river gradient is less depletion of the hillslope sediment reservoirs caused a relative increase in the capacity of the trunk river to entrain and transport sediment resulting in erosion of the previously deposited sediment consequently although climate change may initiate a phase of sediment accumulation degradation can be triggered by an autocyclic negative feedback and does not have to be driven by climatic change

explicit treatment of inheritance in dating depositional surfaces using in situ 10be and 26al we describe a new strategy for dating depositional landscape surfaces using in situ produced cosmogenic radionuclides crns that removes the complication of nuclide inheritance by clasts prior to depositon two amalgamated samples each consisting of 30 clasts one from the surface and one from a fixed depth in the subsurface constrain this crn inheritance and date the surface the inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system we test the technique using sup 10 be and sup 26 al to date the third fr3 of five terraces along the fremont river utah and the third wr3 of 15 along the wind river wyoming whereas effective ages based solely upon the surface samples yield 118 138 ka fr3 and 93 ka wr3 the subsurface samples reveal that inheritance accounts for 22 wr3 to 43 fr3 of the total crn concentration taking this into account yeilds terrace ages of nearly 61 81 ka for fr3 and nearly 67 76 ka for wr3 we explore the dependence of age estimates on the accumulation history of the terrace silt caps 22 refs 3 figs 1 tab

spatial patterns of mixing in the solomon sea the solomon sea is a marginal sea in the southwest pacific that connects subtropical and equatorial circulation constricting transport of south pacific subtropical mode water and antarctic intermediate water through its deep narrow channels marginal sea topography inhibits internal waves from propagating out and into the open ocean making these regions hot spots for energy dissipation and mixing data from two hydrographic cruises and from argo profiles are employed to indirectly infer mixing from observations for the first time in the solomon sea thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy and indicate that it is maximum in the surface and thermocline layers and decreases by 2 3 orders of magnitude by 2000 m depth estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring spatial variability of is large spanning at least 2 orders of magnitude within isopycnal layers seasonal variability of reflects regional monsoonal changes in large scale oceanic and atmospheric conditions with increased in july and decreased in march finally tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but are not clearly correlated with thermocline mixing patterns

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins