criterion delineating the mode of headcut migration two modes of headcut migration are generally recognized 1 rotating headcuts that tend to flatten as they migrate and 2 stepped headcuts that tend to retain nearly vertical faces a mathematical description of the sediment detachment potential immediately upstream and downstream of the headcut is used to delineate these modes of migration the delineating parameter is the ratio of the time required to erode the headcut face from above to the time required to undermine the headcut face from below this erosional time scale ratio is a dimensionless function of flow sediment and geometry parameters for the limiting case of homogeneous cohesive soils the time scale ratio is a simple function of a froude number and the aspect ratio of drop height to normal flow depth this relationship is calibrated using original laboratory experiments of headcut migration in initially vertical headcuts and verified by independent field experiments of headcuts propagating in four different homogeneous cohesive soils

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

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

biological and environmental changes in lake baikal during the late quaternary inferred from carbon nitrogen and sulfur isotopes abstract an 8 m continuous sediment core approximately 250 ky old at the bottom from academician ridge in lake baikal has been analyzed for the stable isotopes of carbon nitrogen and sulfur in order to study the paleoclimatic and paleobiological changes that occurred in the eurasian continental interior these isotopic changes are closely related to changes in vertical lake water circulation between glacial and interglacial periods sedimentary organic carbon in cool periods is more enriched in 13 c 23 8 on average than that in warm periods 27 0 on average the 13 c enrichment of organic carbon suggests a decrease of land derived organic matter influx to the lake less precipitation and loss of terrestrial vegetation around lake baikal in cool periods pyrite in high total sulfur total organic carbon ts toc layers shows strong depletion in 34 s 20 8 to 32 4 during climate transitions from glacial to interglacial periods at the beginning of oxygen isotope stages ois 1 5 and 7 the 34 s depleted pyrite indicates augmentation of dissimilatory sulfate reduction by sulfate reducing bacteria srb at the sediment water interface enhancement of aqueous sulfate concentrations and limitation of oxygen circulation to the surface sediments might also occur in the climate transition periods the 15 n values of total nitrogen increase abruptly by 2 just after the 34 s negative peaks which may result from low nutrient concentrations in the euphotic zone associated with water circulation changes in lake baikal

absence of a regional surface thermal high in the baikal rift new insights from detailed contouring of heat flow anomalies heat flow in active tectonic zones as the baikal rift is a crucial parameter for evaluating deep anomalous structures and lithosphere evolution based on the interpretation of the existing datasets the baikal rift has been characterized in the past by either high heat flow or moderately elevated heat flow or even lacking a surface heat flow anomaly we made an attempt to better constrain the geothermal picture by a detailed offshore contouring survey of known anomalies and to estimate the importance of observed heat flow anomalies within the regional surface heat output a total of about 200 new and close spaced heat flow measurements were obtained in several selected study areas in the north baikal basin with an outrigged and a violin bow designed thermoprobe of 2 3 m length both the sediment temperature and thermal conductivity were measured the new data show at all investigated sites that the large heat flow highs are limited to local heat flow anomalies the maximum measured heat flow reaches values of 300 35000 mw m2 but the extent of the anomalies is not larger than 2 to 4 km in diameter aside of these local anomalies heat flow variations are restricted to near background values of 50 70 mw m2 except in the uplifted academician zone the extent of the local anomalies excludes a conductive source and therefore heat transport by fluids must be considered in a conceptual model where all bottom floor heat flow anomalies are the result of upflowing fluids along a conduit an extra heat output of 20 mw including advection is estimated for all known anomalies in the north baikal basin relative to a basal heat flow of 55 65 mw m2 these estimations suggest an extra heat output in the northern lake baikal of only 5 corresponding to a regional heat flow increase of 3 mw m2 the source of this heat can be fully attributed to a regional heat redistribution by topographically driven ground water flow thus the surface heat flow is not expected to bear a signal of deeper lithospheric thermal anomalies that can be separated from heat flow typical for orogenically altered crust 40 70 mw m2 the new insights on the geothermal signature in the baikal rift once more show that continental rifting is not by default characterized by high heat flow

modeling deep water renewal in lake baikal temperature dissolved oxygen nutrients and chlorofluorocarbon 12 data obtained from lake baikal are used to describe deep water renewal in a deep temperate latitude lake observations are used to propose the physical mechanism governing convection and to formulate a model of deep ventilation the key physical mechanism governing deep water renewal is the so called thermobaric instability because the temperature of maximum density decreases with depth a lake can become conditionally unstable if the base of the cold mixed layer is displaced to a depth at which its temperature matches the local temperature of maximum density thereby resulting in sinking plumes an important consequence of this phenomenon is that deep temperate lakes such as baikal do not completely mix twice yearly instead deep ventilation is episodic a two dimensional model of a wind and buoyancy driven lake shows many strong mixing events and a fairly realistic seasonal cycle indicating that the hypothesis is physically realizable a filling box model is used to deduce the annually averaged fluxes necessary to produce a steady vertical distribution of tracers as observed good fits are obtained to oxygen and chlorofluorocarbon distributions by this model about 50 of the world s liquid freshwater is contained in the large deep lakes located in the temperate latitudes between 45 and 65 n see hutchinson 1957 of fundamental importance to such lakes is their ventilation time the time scale associated with renewal of deep water for it is the deep convection process that most strongly controls the internal redistribution of scalar properties such as dissolved oxygen nutrients and anthropogenic pollutants see wiiest et al 1988 however surprisingly few attempts to study and quantify ventilation in deep

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

relationship of channel steepness to channel incision rate from a tilted and progressively exposed unconformity surface we examine the relationship of channel steepness to incision rate from channels eroding into a previously tilted planar and progressively exhumed unconformity surface channel and unconformity slopes are measured from a suite of channels developed on erosionally resistant paleozoic limestone exhumed by the removal of cenozoic sediments from the baybeiche range bordering the naryn basin in the western tian shan the compiled data set sampling 5 orders of magnitude of upstream drainage area 0 03 to 227 km2 is used to derive the exponent n relating channel steepness to channel incision rate and the ratio k v of the rate constant for channel incision of the resistant substrate k to the erosion rate v of the cover strata we show that for a typical value of intrinsic concavity slope area exponent 0 5 erosion rates that are proportional to specific stream power n 1 satisfy the data set however valley width data suggest that the intrinsic concavity is higher 0 8 and that incision data can also be fit if erosion is proportional to basal shear stress n 2 3 our results do not support values of n significantly greater than one using36cl exposure age dating of the unconformity surface we independently demonstrate that the cenozoic cover strata have been progressively stripped downward from the unconformity surface at a vertical rate of 1 to 2 m kyr using v 1 m kyr we constrain the rate constant k to between 6 1 and 9 2 10 4kyr 1 for incision of resistant limestone bedrock in this field setting key points channel incision relation to steepness calibrated from tilted bedrock surface chlorine 36 exposure age dating shows bedrock surface exhumed at 1 to 2 m kyr low n values consistent with shear stress n 2 3 satisfy field data 2014 american geophysical union all rights reserved

oxygen and hydrogen isotope systematics of lake baikal siberia implications for paleoclimate studies we interpret oxygen and hydrogen isotope data for water samples from lake baikal siberia its tributaries and other local rivers and local precipitation in terms of the known water budget for the modern lake in order to gain insight into past limnological and climatic processes that influenced the lake lake baikal is remarkably uniform in its isotopic composition i80 15 8 0 2 0 6d 123 f 2 0 and lies slightly to the right of the global meteoric water line which suggests significant evaporation water is supplied to the lake by over 300 rivers and streams the oxygen isotope values 8180 of the rivers in the baikal catchment range from 13 4 to 21 2 0 the hydrogen isotope values sd for the same area range from 103 to 156 0 both these ranges generally conform to the global meteoric water line the weighted average isotopic composition of input to the lake rivers plus precipitation is 15 2 0 for 6 0 and 116 0 for 6d values higher than those of the modern lake therefore the isotopic composition of the modem lake cannot be related to the modern input through simple evaporation instead modeling of the isotopic mass balance of the lake suggests that inputs precipitation and influx from rivers and outputs evaporation and outflow are not at a steady state equilibrium under current climate conditions we found previous input to the lake had lower zy o and 6d values than modern input which reflects cooler climates in the past compared with modern conditions under constant climate conditions steady state conditions are not expected to be reached by the lake for at least 700 yr because of its large size and the long residence time of water in the lake

drivers of deep water renewal events observed over 13 years in the south basin of lake baikal lake baikal with a depth of 1637 m is characterized by deep water intrusions that bridge the near surface layer to the hypolimnion these episodic events transfer heat and oxygen over large vertical scales and maintain the permanent temperature stratified deep water status of the lake here we evaluate a series of intrusion events that reached the bottom of the lake in terms of the stratification and the wind conditions under which they occurred and provide a new insight into the triggering mechanisms we make use of long term temperature and current meter data 2000 2013 recorded in the south basin of the lake combined with wind data produced with a regional downscaling of the global ncep ra1 reanalysis product a total of 13 events were observed during which near surface cold water reached the bottom of the south basin at 1350 m depth we found that the triggering mechanism of the events is related to the time of the year that they take place we categorized the events in three groups 1 winter events observed shortly before the complete ice cover of the lake that are triggered by ekman coastal downwelling 2 under ice events and 3 spring events that show no correlation to the wind conditions and are possibly connected to the increased spring outflow of the selenga river

processes of deep water renewal in lake baikal deep water renewal in lake baikal siberia the world s deepest lake and largest lake by volume is relatively fast water age calculated from tritium and helium as well as from chlorofluorocarbons does not exceed 19 yr relative saturation of dissolved oxygen typically exceeds 80 the equation of state of baikal water was determined including the effect of dissolved ions and silicic acid based on nearly 600 ctd profiles taken between 1993 and 1995 two mechanisms of deep water mixing were identified 1 in spring cold and relatively saline water from the selenga the major inflow to the lake forms a density plume that reaches the bottom of the central basin during april and early may due to entrainment of lake water the plume transports about 125 km of water per year to the deepest part of the basin later in spring the river water forms the thermal bar observed along the eastern shore there are indications that parts of the selenga are also plunging to the deep part of the southern basin 2 at academician ridge separating the cold and saline water of the central basin from the warmer and slightly less saline water of the northern basin horizontal mixing results in a water mass that can sink on either side of the sill whereas in the central basin the water mass stays at intcrmediatc depth in the northern basin it sinks to the decpcst part more detailed data are needed to quantify this density flux no indication of a wind induced thermobaric instability was found

bottomwater formation due to hydrothermal activity in frolikha bay lake baikal eastern siberia hydrothermal water enters frolikha bay a well known site of high geothermal heat flux in the northern part of lake baikal at 400 m depth on the basis of ctd profiles the hydrothermal water is identified as forming an anomalous bottom layer with a higher temperature 0 15 c and salinity 2 5 mg kg 1 than the overlying water due to the entrainment of lake water a distinct dense water layer up to 40 m thick stabilised by its slightly higher salinity becomes established close to the bottom of the bay the density current thus generated flows out of the bay towards the deeper parts of the basin since helium isotope analysis shows that the geochemical characteristics of the hydrothermal water are similar to those of water from nearby hot springs on land the bottomwater of frolikha bay is easily interpreted in terms of the mixing of ordinary fresh water from the lake and hydrothermal water carrying isotopically heavy he from the continental crust because of its high crustal he content a similar hydrothermal component may even be identified in the open water of the northern basin

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

deep water renewal in lake issyk kul driven by differential cooling during two field campaigns on lake issyk kul in march and august 2001 179 high resolution conductivitytemperature depth profiles were measured along with profiles of dissolved oxygen and light transmission on the basis of this extensive data set we investigated the large scale vertical advective processes responsible for deepwater renewal in lake issyk kul at some locations sampled in march variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions these intrusions have their origin in density plumes that propagate along the bottom of the channels in the eastern shelf region the cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter during summer vertical advective processes appear to be limited to the upper 200 m where upwelling in the open water leads to a doming structure the august data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water

vertical turbulent diffusion and upwelling in lake baikal estimated by inverse modeling of transient tracers vertical turbulent diffusion coefficients upwelling velocities and oxygen depletion rates are estimated by inverse modeling of the concentrations of cfc 11 ccl3f cfc 12 ccl2f2 3h 3he and dissolved oxygen for the southern central and northern basin of lake baikal a model is developed that considers two regions in each basin of lake baikal 1 a surface mixed layer sml 400 m thick and 2 a deepwater column dwc below 400 m the smls are assumed to be well mixed in each of the dwcs passive tracers are transported by vertical turbulent diffusion and upwelling upwelling is generated by a depth dependent source of water because of density plumes propagating from the sml downward to larger depths this water is considered to contain the same tracer concentrations as the sml the tracer concentrations in the smls of the three basins are coupled to the atmosphere by gas exchange including water vapor transport and precipitation to the catchment by river inflow and outflow and to the neighboring basins via diffusive exchange and advection smls and dwcs of the same basin are connected by vertical turbulent diffusion density driven water transport and upwelling beginning at the turn of this century the tracers cfc 11 cfc 12 3h and 3he are modeled simultaneously to predict modern concentrations on the basis of the tracer data the vertical diffusion coefficient k is determined to be 4 6 10 4 m2 s 1 10 for the southern 6 3 10 4 m2 s 4 10 for the central and 1 7 10 4 m2 s 4 25 for the northern basin the vertical advective flux of water at 400 m water depth is calculated as 110 km3 yr 1 in the southern 70 km3 yr 1 in the central and 290 km3 yr 1 in the northern basin concentration of dissolved molecular oxygen is modeled by using the estimated transport parameters and by fitting for the unknown consumption rate inverse modeling of oxygen suggests that o2 depletion in the dwc can be described by a volume sink of 44 3 mgo2 m 3 yr 1 combined with an areal sink at the sediment water interface of 17000 3000 mgo2 m 2 yr 1

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

self formed bedrock waterfalls waterfalls are inspiring landforms that set the pace of landscape evolution as a result of bedrock incision they communicate changes in sea level or tectonic uplift throughout landscapes or stall river incision disconnecting landscapes from downstream perturbations here we use a flume experiment with constant water discharge and sediment feed to show that waterfalls can form from a planar homogeneous bedrock bed in the absence of external perturbations in our experiment instabilities between flow hydraulics sediment transport and bedrock erosion lead to undulating bedforms which grow to become waterfalls we propose that it is plausible that the origin of some waterfalls in natural systems can be attributed to this intrinsic formation process and we suggest that investigations to distinguish self formed from externally forced waterfalls may help to improve the reconstruction of earth history from landscapes

fluvial incision into bedrock insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys western alps france bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief using digital elevation models topographic maps and field reconnaissance we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the romanche watershed french western alps longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches we reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat from morphometric analyses we find that mean channel gradients and widths as well as knickpoint retreat rates display a drainage area dependence modulated by bedrock lithology however there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints assuming a postglacial origin of these gorges our results imply high postglacial fluvial incision 0 5 15 mm yr 1 and knickpoint retreat 1 200 mm yr 1 rates that are however consistent with previous estimates numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges results from simple end member models suggest transport limited behavior of the bedrock gorges a more sophisticated model including dynamic width adjustment and sediment dependent incision rates predicts present day channel geometry only if a significant supply of sediment from the gorge sidewalls 10 mm yr 1 is triggered by gorge deepening combined with pronounced inhibition of bedrock incision by sediment transport and deposition

deglacial methane emission signals in the carbon isotopic record of lake baikal changes in the concentrations of atmospheric greenhouse gases constitute an important part of global climate forcing here we present the first continental evidence for climatically caused changes in a methane gas hydrate reservoir the organic carbon stable isotope record from lake baikal during the past 130 000 years registers regular emissions of isotopically light carbon by the occurrence of distinct negative shifts of 3 5 at every major orbitally forced cold to warm climatic transition during the past 130 000 years including marine oxygen isotope stage boundaries 6 5e 5d 5c 5b 5a and 2 1 we conclude that these emissions were associated with decomposition of sedimentary clathrates widespread in the baikal basin among potential hypotheses to account for these methane episodes the most probable appears to be hydrate dissociation due to deglacial warming of lake water we estimate that as much as 12 33 tg of methane could have been released with each episode by recording the systematically recurring episodes of massive methane clathrate decomposition closely linked with the northern hemisphere temperatures during major orbital warmings the new baikal 13c record provides further evidence for the potential involvement of clathrate reservoir in rapid deglacial rises of atmospheric methane levels

lake baikal deepwater renewal mystery solved deepwater renewal by intrusions and turbulent diffusion in lake baikal is very effective despite the enormous depth of up to 1642 m and the permanently stable stratification below similar to 300 m depth temperature time series recorded at the bottom of a mooring installed since march 2000 in the south basin of the lake indicate recurrent freshwater intrusions with volumes of 50 to 100 km 3 about one order of magnitude larger than previously observed intrusions numerous mechanisms have been proposed to explain the advective deep water renewal here we present for the first time direct observations which prove that they are caused by coastal downwelling and subsequent thermobaric instability along the steep lake shores understanding these mechanisms is an important prerequisite for studying biogeochemical cycles for predicting the effects of climate change on this unique ecosystem and for evaluating the local climate history from the extraordinary sedimentary record of lake baikal

hillslope and channel evolution in a marine terraced landscape santa cruz california a flight of marine terraces along the central california coastline provides a unique setting for the study of topographic evolution wavecut platforms mantled by 2 6 m of marine terrace cover deposits are separated by 10 50 m tall decaying sea cliffs paleocliff edges become more rounded with age yet the details of the profiles and frequent bedrock exposure on the upper slopes imply weathering limited transport five bedrock stream channels etched through the marine terrace sequence display one to three distinct convexities in their longitudinal profiles detailed hand level surveys of the hillslopes and of the stream channel longitudinal profiles constrain hillslope evolution and channel incision components of a numerical model of landscape evolution we account for regolith production as a function of regolith depth in accord with the field observation that hillslope processes are presently dominated by the activities of burrowing rodents the transport process is taken to be diffusive stream incision is assumed to be controlled by stream power for which we use the surrogate of local drainage area slope product best fits of the numerical model to field data imply hillslope diffusivity is 10 m2 kyr 1 regolith production rate on bare bedrock is 0 3 m kyr 1 and falls off rapidly with regolith cover and the constant controlling the efficiency of stream incision is 5 to 7 10 7m 1 kyr1

the evolution of a colluvial hollow to a fluvial channel with periodic steps following two transformational disturbances a wildfire and a historic flood abstract the transition of a colluvial hollow to a fluvial channel with discrete steps was observed after two landscape scale disturbances the first disturbance a high severity wildfire changed the catchment hydrology to favor overland flow which incised a colluvial hollow creating a channel in the same location this incised channel became armored with cobbles and boulders following repeated post wildfire overland flow events three years after the fire a record rainstorm produced regional flooding and generated sufficient fluvial erosion and sorting to produce a fluvial channel with periodically spaced steps an analysis of the step spacing shows that after the flood newly formed steps retained a similar spacing to the topographic roughness spacing in the original colluvial hollow prior to channelization this suggests that despite a distinct change in channel form roughness and bedform morphology the endogenous roughness periodicity was conserved variations in sediment erodibility helped to create the emergent steps as the largest particles d84 remained immobile becoming step features and downstream soil was easily winnowed away

constraining modern day silicon cycling in lake baikal constraining the continental silicon cycle is a key requirement in attempts to understand both nutrient fluxes to the ocean and linkages between silicon and carbon cycling over different time scales silicon isotope data of dissolved silica 30sidsi are presented here from lake baikal and its catchment in central siberia as well as being the world s oldest and voluminous lake lake baikal lies within the seventh largest drainage basin in the world and exports significant amounts of freshwater into the arctic ocean data from river waters accounting for 92 of annual river inflow to the lake suggest no seasonal alteration or anthropogenic impact on river 30sidsi composition the absence of a change in 30sidsi within the selenga delta through which 62 of riverine flow passes suggests a net balance between biogenic uptake and dissolution in this system a key feature of this study is the use of 30sidsi to examine seasonal and spatial variations in dsi utilization and export across the lake using an open system model against deepwater 30sidsi values from the lake we estimate that 20 24 of dsi entering lake baikal is exported into the sediment record while highlighting the impact that lakes may have upon the sequestration of continental dsi mixed layer 30sidsi values from 2003 and 2013 show significant spatial variability in the magnitude of spring bloom nutrient utilization with lower rates in the north relative to south basin

hydrological instability during the last interglacial in central asia a new diatom oxygen isotope record from lake baikal last interglacial variability is commonly used as an analogue for variability in a future warmer world pervasive cycles are increasingly apparent in last interglacial archives although studies in continental regions are under represented here we provide a new isotopic record of diatom silica 18odiatom spanning c 127 5 115 ka bp from lake baikal in central asia peak rain fed discharge occurred c 125 4 ka bp shortly after july insolation maximum and initiation of siberian soil development between 127 and 119 7 ka bp there are six marked fluctuations in 18odiatom values with a pacing of approximately 1 26 0 3 ka similar to fluctuations of within lake productivity fluctuations in 18odiatom values show good agreement with patterns in atlantic meridional overturning circulation amoc supporting hypothesis of strong teleconnections via the westerlies between the north atlantic and central asia two periods of low 18odiatom values are especially notable the earliest between c 126 5 and 126 ka bp is concurrent with the final stages of the heinrich 11 the second between 120 5 and 119 7 ka bp is also concurrent with an increase in ice rafted debris in the north atlantic aquatic productivity in lake baikal increased between 119 7 and 117 4 ka bp before declining to the top of the record 115 ka bp concomitant with a shift to predominately cool steppe catchment vegetation however isotopic composition of discharge into lake baikal provides evidence for strong penetration of westerlies into central asia during the latter stages of the last interglacial variability in 18odiatom values was compared between the last interglacial and the holocene millennial scale variability was significantly more stable during the last interglacial possibly linked to diminished influence of freshwater discharge on amoc during periods of higher global mean temperatures

self formed waterfall plunge pools in homogeneous rock waterfalls are ubiquitous and their upstream propagation can set the pace of landscape evolution yet no experimental studies have examined waterfall plunge pool erosion in homogeneous rock we performed laboratory experiments using synthetic foam as a bedrock simulant to produce self formed waterfall plunge pools via particle impact abrasion plunge pool vertical incision exceeded lateral erosion by approximately tenfold until pools deepened to the point that the supplied sediment could not be evacuated and deposition armored the pool bedrock floor lateral erosion of plunge pool sidewalls continued after sediment deposition but primarily at the downstream pool wall which might lead to undermining of the plunge pool lip sediment evacuation and continued vertical pool floor incision in natural streams undercutting of the upstream pool wall was absent and our results suggest that vertical drilling of successive plunge pools is a more efficient waterfall retreat mechanism than the classic model of headwall undercutting and collapse in homogeneous rock

recession rates of waterfalls in boso peninsula japan and a predictive equation rates of waterfall recession and major factors that influence the rate were studied using waterfalls in boso peninsula japan the mean rate of waterfall recession was estimated by determining the age and original location the principal factor in determining the rate of waterfall recession is the ratio of the erosive force of stream to the bedrock resistance this is expressed in terms of measurable variables which include the discharge drainage area and precipitation the width and height of the waterfall and the unconfined compressive strength of the bedrock an empirical equation connecting the force resistance ratios and the rates of waterfall recession is derived copyright 2003 john wiley sons ltd

ventilation and dissolved oxygen cycle in lake superior insights from a numerical model ventilation and dissolved oxygen in lake superior are key factors that determine the fate of various natural and anthropogenic inputs to the lake we employ an idealized age tracer and biogeochemical tracers in a realistically configured numerical model of lake superior to characterize its ventilation and dissolved o2 cycle our results indicate that lake superior is preferentially ventilated over rough bathymetry and that spring overturning following a very cold winter does not completely ventilate the lake interior while this is unexpected for a dimictic lake no part of the lake remains isolated from the atmosphere for more than 300 days our results also show that lake superior s oxygen cycle is dominated by solubility changes as a result the expected relationship between biological consumption of dissolved o2 and ventilation age does not manifest

formation of ferric iron crusts in quaternary sediments of lake baikal russia and implications for paleoclimate phosphate bearing ferric iron and siliceous crusts ranging in age from recent to approximately 65 000 yr b p are observed in sediments of lake baikal in younger sediments the crusts are at the base of a spectrum of secondary iron and manganese oxides that accumulate near the sediment water interface in the zone of positive oxidation potential beneath an oxygenated water column in areas where the average quaternary sedimentation rates have been slow e g 0 026 mm yr the crusts are more common and span a wider range of ages no crusts have been found where the quaternary sedimentation mode has been deltaic and rapid 0 15 mm yr independent core correlation based on magnetic properties of the sediment suggests that crusts can be correlated over most of academician ridge an area that is particularly sensitive to climatic events affecting the concentration of suspended sediment these crusts may be indicative of periods of low suspended sediment concentration which occur during sustained transitions from glacial periods of high detrital input to interglacial periods of high diatom sedimentation the crusts are dominated by iron rich and siliceous amorphous mineral phases with an feo sio2by weight of 3 1 regardless of age or location in the lake the fe phase always includes ca p and mn extensive microprobe data for these four elements recast as normalized elemental weight percent reveal linear trends of ca p and fe p with increasing p ca also increases such that the two elements maintain a linear relationship passing very close to the origin and with a mean molar ca p 0 3 too low for well characterized apatite conversely with increasing p fe decreases mean molar fe p 3 4 there is no correlation between mn and p molar fe p ratios for vivianite an fe ii phosphate mineral observed in sediments closely below some crusts are clustered around a stoichiometric composition the covariant increase in ca p and the corresponding decrease in fe p may be explained by 1 coupled adsorption of aqueous ca and p by a colloidal ferric hydrous oxide 2 loss of fe from a ca p fe phase or 3 oxidation of vivianite to a metastable mineral phase that gradually loses ca and gains fe the first explanation is favored because there is no petrographic evidence for either the existence of an originating ca p fe phase or for the oxidation of vivianite further it is suggested that by continually equalizing surface charge ca allows more phosphate to be adsorbed leading to thicker crusts and longer preservation after burial

deep ventilation of lake baikal waters due to spring thermal bars increase of density due to mixing cabbeling instability in a spring thermal bar in lake baikal results in the sinking of cold surface water t 3 2 3 4 c w 0 3 cm s 1 below the profile of tmd this density increase generates forced convection and subsequent movement of cold water along the underwater slope to maximum depths cooling of deep and near bottom waters caused by forced convection is compensated during other seasons of the year by eddy heat flux directed toward the bottom on a long term basis vertical temperature profiles in deep waters remain constant

experimental study of knickpoint and longitudinal profile evolution in cohesive homogeneous material a series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal profile evolution knickpoints are created by successive drops in base level which simulate intermittent uplift channel morphology and flow characteristics vary systematically along a knickpoint reach as the knickpoint lip is approached width decreases but depth velocity and bottom shear stress increase as a result knickpoints are rapidly destroyed through the process of 1 knickpoint replacement and 2 subsequent knickpoint inclination knickpoint replacement consists of two morphologically distinct erosional reaches the knickpoint face below the knickpoint lip which decreases in slope and an incising reach above the knickpoint lip which increases in slope knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock within experimental limits this study supports the notion that knickpoints do not undergo parallel head ward retreat in homogeneous bedrock given intermittent uplift except in those possible cases of extensive and pervasive bedrock jointing only the erosional sediment transport discontinuity above the knickpoint is developed in cohesive sediments below the knickpoint a steep bedrock channel slope low sediment load and rapid decrease in grain size of sediment load preclude development of the aggradational sediment transport discontinuity knickpoint evolution can be characterized by three general models parallel retreat replacement and inclination these models are based on the following criteria within the knickpoint reach 1 relationship between bottom shear stress 0 and critical shear stress needed to initiate motion c 2 nature of the bed load transport discontinuities and 3 spatial variability of bedrock resistance to fluvial erosion

distribution of helium and tritium in lake baikal the 3 h 3 he age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere between 1992 and 1995 a detailed study of 3 h 3 he ages was conducted in lake baikal the deepest and largest lake by volume on earth to investigate deep water renewal in its three major basins maximum 3 h 3 he ages are 14 17 years in the southern basin 16 18 years in the central basin and 10 11 years in the northern basin rates of renewal of deep water with surface water deduced from volume weighted mean 3 h 3 he ages below 250 m depth are about 10 yr 1 in the southern and central basins and 15 yr 1 in the northern basin in the southern basin the mean 3 h 3 he age below 250 m depth increased steadily from 9 6 years in 1992 to 11 years in 1995 indicating a slight diminution in deep water renewal during this time bottom water renewal by large scale advection was estimated from the mass balance of 3 he in the 200 m thick bottom layer of each basin bottom water renewal rates in the northern basin were found to be between 80 and 150 km 3 yr 1 and in the central basin between 10 and 20 km 3 yr 1 whereas in the southern basin they were practically zero correlating oxygen and dissolved helium 4 concentrations with 3 h 3 he age allowed us to determine the mean hypolimnetic oxygen depletion rate in the water column 4 5 mol l 1 yr 1 as well as mean helium fluxes from the lake bottom 2 8 x 10 11 atoms m 2 s 1 in the northern basin and 1 3 x 10 11 atoms m 2 s 1 in the central and southern basins the helium isotope ratio of the terrigenic helium component injected from the lake bottom determined from measurements of water from hydrothermal springs in the vicinity of the lake was found to be 2 2 x 10 7

lake baikal record of continental climate response to orbital insolation during the past 5 million years the sedimentary record of biogenic silica from lake baikal in south central siberia suggests that this region of central asia was impacted by two major cooling episodes at 2 8 to 2 6 and 1 8 to 1 6 million years ago the spectral evolution of this continental interior site parallels the evolutionary frequency spectra for various marine oxygen isotope records in the baikal record the 41 000 year obliquity cycle is particularly strong from 1 8 to 0 8 million years ago variance in the 100 000 year eccentricity band increases during the past 0 8 million years the expected precession frequency of 23 000 years is highest during the past 400 000 years the modulation of the predicted 23 000 and 41 000 year insolation forcing by the 100 000 and 400 000 year eccentricity bands indicates that the transfer of variance from the precession and obliquity frequencies to the eccentricity part of the spectrum occurred in the eurasian continental interior as well as in tropical and high latitude ocean sites

chemical denudation rates of the western canadian orogenic belt the stikine terrane we report in this paper major element and sr isotopic ratio analyses for 19 rivers draining the stikine province in the western canadian cordillera dissolved solutes suspended sediments and river sands were analyzed the aim of the paper is to calculate chemical denudation rates for this uplifted accretionary prism mainly dominated by volcanics and volcanic derived sedimentary rocks the major chemical features of the stikine rivers are their dilute character and the excess of ca mg and sr in the dissolved load with regards to na molar ca na ratios from 2 to 14 mean value of 5 4 are observed although the region is dominated by silicate rocks suspended sediments show ca na ratios close to 0 4 isotopically most of the rivers investigated display lower sr isotopic ratios in the dissolved load compared to the suspended load which is very unusual to account for the excess of cations three different hypotheses are tested by an inversion method allowing us to determine the origin of each major solute the first scenario assumes that the excess of ca and mg is due to the dissolution of unradiogenic carbonates the second scenario supposes that the excess of base cations is derived from the preferential dissolution of ca and mg rich minerals such as amphiboles in the third scenario all solutes are assumed to derived from silicate weathering the cationic weathering rates of silicates although imprecise are in the order of 2 5 tons km2 year for the two first hypotheses if all solutes are taken into account denudation rates are between 9 and 15 tons km2 year on a global scale these numbers are among the lowest found for rivers draining basaltic rocks they are however found to be consistent with the global law for chemical weathering rates established for basaltic rocks showing that chemical denudation rate are linearly correlated to runoff and depend on temperature according to an arrhenius law the rivers draining the complex assemblage of volcanic and volcanic derived rocks in the subduction collision geological context of the stikine province thus respond similarly to the other volcanic provinces of the world this does not seem to be true for associated co2 consumption rates the abundance of sulfate ions in the dissolved load possibly derived from oxidative weathering of sulfides may indeed provide a non negligible part of the protons necessary for chemical weathering within canada the stikine geologic province is one whose silicates weather at a high rate the order of chemical weatherabilty of canadian geological province from the lowest to the highest is established as follows granites shales of the rockies volcanics of the western cordillera shales of the interior platform carbonates

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

impact of lake level change on deep water renewal and oxic conditions in deep saline lake van turkey 1 changes in the hydrological regime of the saline closed basin lake van a large deep lake in eastern turkey resulted in a lake level increase by about 2 m between 1988 and 1995 followed by a 1 5 m decrease until 2003 and a relatively constant lake level thereafter based on measurements of transient tracers sulfur hexafluoride cfc 12 3h 3he 4he ne dissolved oxygen light transmission conductivity temperature depth profiles and thermistor data we investigate the implications associated with lake level fluctuations for deep water renewal and oxygenation our data suggest that deep water renewal was significantly reduced in lake van between 1990 and 2005 this change in mixing conditions resulted in the formation of a more than 100 m thick anoxic deep water body below 325 m depth apparently the freshwater inflows responsible for the lake level rise between 1988 and 1995 decreased the salinity of the surface water sufficiently that the generation of density plumes during winter cooling was substantially reduced compared to that in the years before the lake level rise significant renewal and oxygenation of the deep water did not occur until at least 2005 although by 2003 the lake level was back to almost the same level as in 1988 this study suggests that short term changes in the hydrological regime resulting in lake level changes of a couple of meters can lead to significant and long lasting changes in deep water renewal and oxic conditions in deep saline lakes

biogeochemical phosphorus mass balance for lake baikal southeastern siberia russia abstract extensive data for lake baikal have been synthesized into a geochemical mass balance for phosphorus p some of the p budget and internal cycling terms for baikal have been compared to similar terms for oligotrophic lake superior mesotrophic lake michigan and the baltic sea and the ocean lake baikal has a large external source of fluvial p compared to the laurentian upper great lakes and the ocean the major tributary to lake baikal has experienced substantial increases in organic p loading during the past 25 years this coupled with potential p inputs from possible phosphorite mining may threaten baikal s oligotrophic status in the future water column remineralization of particulate organic p is substantially greater in lake baikal than in the laurentian great lakes this is probably due to the great water depths of lake baikal there is a gradient in p burial efficiency with very high values 80 for lake baikal and lake superior lower values 50 for lake michigan and the baltic sea and a low value 13 for the ocean the accumulation rate of p in lake baikal sediments is somewhat greater than that in the laurentian upper great lakes and the baltic sea and much greater than in the ocean benthic regeneration rates are surprisingly similar for large lacustrine and marine environments and supply less than 10 of the p utilized for primary production in these aquatic environments

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

cosmogenic chlorine 36 from calcium spallation calcium is a major target element for cosmogenic 36cl production consequently 36cl rapidly reaches detectable levels in minerals such as calcite and calcium feldspar exposed at the earth s surface spallation of calcium isotopes typically accounts for 80 90 of 36cl production in these minerals with subsidiary contributions from negative muon capture by 40ca and thermal neutron capture by 35cl to provide a basis for surface exposure dating we have calibrated cosmogenic 36cl production in calcium feldspar from the 17 300 year old tabernacle hill basalt at an altitude of 1445 m and an effective geomagnetic latitude of 40 9 the calcium spallation rate is 152 11 atoms g ca 1 a 1 the corresponding rate at sea level and high latitude is estimated at 48 8 3 4 atoms g ca 1 a 1 the muon capture rate used to derive these values is 8 8 2 2 atoms g ca 1 a 1 at the tabernacle hill site scaled from a value of 4 8 1 2 atoms g ca 1 a 1 at sea level and high latitude the calcium spallation rate determined in this study is in excellent agreement with previous whole rock calibration measurements at tabernacle hill when these are recalculated with respect to the absolute timescale the calibration of 36c1 production from calcium underpins development of an exposure dating technique for calcite due to its high calcium content the 36cl production rate in calcite is higher than in any other common rock forming mineral measurement of 36cl in calcite with an accelerator mass spectrometric detection limit of 5 103 atoms per gram allows dating of limestone surfaces exposed for periods ranging from 102 106 years alternatively erosion rates from less than 1 to greater than 1000 m per year can be determined in the case of eroding karst surfaces though the 36cl production rate is lower in calcium feldspar than in calcite measurements on this mineral will provide a useful means of dating young basalt lavas