high resolution particle size analysis of naturally occurring very fine grained sediment through laser diffractometry abstract in this paper we present results from a large number of experiments aimed at quantifying method and instrument uncertainty associated with laser diffraction analysis we analyzed the size distribution of fine grained sediment 24 hours prior to analysis and using 60 seconds of ultrasonication during analysis 2 obscuration a measure of the concentration of the suspension during analysis produced the most reproducible results at about 20 3 variations in refractive index settings can significantly alter estimated grain size distributions 4 assumed values for absorption the degree to which sediment grains absorb the light can have a profound effect on grain size results absorption settings near 0 resulted in unexpected bimodal grain size distributions for sediments in the 10 m size fraction and significantly skewed the fine grained tail of coarser samples probably because of sub optimal diffraction by particles with a diameter similar in size to the laser wavelength absorption settings closer to 1 produced very reproducible results and unimodal grain size distributions over a wide range of refractive indexes our study has shown that laser diffraction can measure very fine grained sediments 10 m quickly with high precision 5 at 2 standard deviations and without the need for extensive mineralogical determinations these results make possible a new generation of studies in which high resolution time series data sets of sediment grain size can be used to infer subtle changes in paleohydrology

field measurements of the flux and speed of wind blown sand a field experiment was conducted to measure the flux and speed of wind blown sand under known conditions in a natural setting the experiment run at pismo beach california involved a tract 100 m long parallel with the wind by 20 m wide the site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary layer temperature probes to determine atmospheric stability and wind vanes to determine wind direction from these measurements wind friction speeds were derived for each experimental run in order to measure sand saltation flux a trench 3 m long by 10 m wide transverse to the wind direction by 0 5 m deep was placed at the downwind end of the tract and lined with 168 collector bins forming an egg box pattern the mass of particles collected in each bin was determined for four experimental runs in order to assess various sand trap systems used in previous experiments 12 leatherman traps one fryberger trap and one array of ames traps were deployed to collect particles concurrently with the trench collection particle velocities were determined from analysis of high speed 3000 and 5000 frames per second motion pictures and from a particle velocimeter sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined fluxes for each run were calculated using various previously published expressions and then compared with the flux derived from the trench collection results show that bagnold s 1941 model and white s 1979 equation most closely agree with values derived from the trench comparison of the various collector systems shows that the leatherman and ames traps most closely agree with the flux derived from the trench although these systems tended to under collect particles particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories results show that particle velocities from the velocimeter are in the range 0 5 7 0 m s 1 compared to a wind friction velocity of 0 32 0 43 m s 1 and a wind velocity of 2 7 3 9 m s 1 at the height of the particle measurements descending particles tended to exceed the speeds of ascending particles by 0 5 m s 1

aeolian dune migration along the ceara coast north eastern brazil aeolian dune dimensions and migration rates are analysed along the ceara coast north east brazil dunes that are currently mobile along the ceara coast are composed of barchans and sand sheets the results show that barchans maintain an equilibrium form which can be characterized by values of dimensionless shape parameters h w and w l where h is the dune height w is the wing to wing width and l is the dune length dunes are highly mobile with average migration rates of 17 5 m year 1 for barchans and 10 m year 1 for sand sheets the calculated migration rates were found to depend strongly on dune dimensions for both barchans and sand sheets i e the larger the dune is the lower the migration rate will be this size dependence was associated with the existence of a representative common transport rate along the dune fields which induces a different dune migration rate dependent on dune size finally from the observed dune evolution an aggregated scale aeolian sediment transport was inferred this bulk transport rate of the order of 90 100 m3 m 1 year 1 is only valid for a timescale of years to decades which is the timescale used in dune evolution analysis

lacustrine record of high magnitude flood events and climate variability during mid to late holocene in the semiarid alluvial plains western india abstract long term data on flooding events are necessary for understanding the flood frequency and magnitude variations but the available instrumental records span only a few decades recently lake sediment sequences have emerged as a relatively untapped archive for high magnitude floods over longer timescales in the present study multi proxy data from a lacustrine sequence at timbi in the dhadhar river basin were analysed to determine high magnitude floods in this semi arid region of western india in response to southwest monsoon variability the results indicate the occurrence of discreet high magnitude flood events and depositional phases corresponding to different climatic conditions during the mid to late holocene three large flash floods between 4830 and 2730 cal yr b p phase i have been identified as coeval with flood events from fluvial archives and correlated to the widespread period of aridity and weak monsoon recorded in the lakes of north western india a shift in the depositional environment from fluvial to lacustrine occurred around 2730 cal yr b p as inferred from an abrupt change in the sediment character and palynofacies caused by high intensity flash floods and high sediment influx phase ii 2730 1730 cal yr b p was marked by periodic erosion and deposition in the catchment and sediment influx during pulses of higher precipitation as evident in the dominance of glomus sp and alternaria sp and relatively low values in concentration related magnetic parameters and s ratio a high magnitude flood event occurred at 1770 cal yr b p towards the end of phase ii during phase iii 1730 880 cal yr b p the monsoon was enhanced and climate was warmer as suggested by low fd sirm s ratio higher ba sr ratio and presence of aquatic palynomorphs a high percentage of structured organic matter om low fungal remains and increased ba sr ratios between 880 and 360 cal yr b p indicate higher lake waters and better monsoon condition during phase iv the phase v 360 0 cal yr b p corresponds to lake level lowering soil formation and eutrophication the timbi lake sediments demonstrate the potential for lakes in semi arid alluvial plains of western india to preserve proxy flood records providing additional archives for generating long term datasets the flood record generated substantiates palaeoflood data from the fluvial archive suggestive of regional scale southwest monsoon variability low frequency high magnitude floods occurred during arid periods and low magnitude high frequency events during relatively strengthened monsoon and thus have implication for linking the palaeoflood events to the southwest monsoon intensity in the semi arid continental regions

longevity of aeolian megaripples abstract megaripples are distinguished from regular ripples by their larger dimensions and bimodal grain size distributions the interplay between wind grain size and ripple morphology height and wavelength controls their development two main mechanisms limit megaripple height the first megaripple flattening due to winds that are above the fluid threshold of the coarse grains destroys the armoring layer of the megaripple the second is megaripple erosion by the impacts of fast moving fine saltating grains that propel the coarse grains constituting the armoring layer for any given wind regime and grain size distribution the potential megaripple dimensions are limited by these two mechanisms here we study the first mechanism and estimate the duration of strong winds sustained above the fluid threshold needed to flatten megaripples strong gusts of wind in contrast cannot destroy the megaripples but can cause ripple migration based on data from previous works on megaripples we find a scaling law between the ripple morphology and the coarse mode of grains at the crest using this scaling relation allows us to calculate the wind velocity and duration needed for megaripple flattening in general the coarser the particles at the megaripple crest the stronger the wind needed to flatten the megaripples moreover the greater the strength of the wind required to flatten the megaripples the lower the recurrence probability taken together these findings increase the longevity of megaripples we apply the results for a megaripple field in the southern arava valley israel

transverse aeolian ridges tars on mars aeolian processes are probably the dominant ongoing surface modification process on mars as demonstrated by observations of dunes ripples yardangs ventifacts wind streaks dust devils and dust storms large dark dunes ldds are particularly common and were first recognized in the 1960 s from mariner data however another population of smaller often brighter ripple like aeolian bed forms that are morphologically distinct from ldds has been observed in recent higher resolution images calculations of saltation path length under martian conditions suggest these small bed forms could have formed as small barchanoid dunes or as mega ripples also known as granule ripples these features have therefore been called transverse aeolian ridges tars as this label has no genetic inference we have begun a study of tars that examines all high resolution mars orbiter camera data 1 5 to 8 m pixel resolution in a 45o longitude wide pole to pole swath we aim to determine the geographic distribution of tars assess whether they are active and to identify possible sediment sources we have already made some key observations first we present a new classification of tar deposits based on crest ridge morphology of individual forms and the size and degree of topographic control of the entire sediment mass second we present associations of tars with ldds slope streaks dust devils tracks and small impact craters that provide clues as to whether tars are currently active finally we present preliminary pole to pole survey results for the northern hemisphere that show clusters of tar deposits in the low latitudes suggesting possible unique sediment sources in these regions

megaripple flattening due to strong winds abstract megaripples in nahal kasuy in the southern negev desert of israel are characterized by a mean wavelength of about 70 cm and by a bimodal distribution of coarse and fine particle sizes the latter property being a pre requisite for their formation in our three year project megaripple development was monitored using a digital elevation model dem constructed from above ground stereo digital photographs temporal dynamics of wind power drift potential dp were measured and grain size analyses were performed on samples taken from different parts of the megaripple the coarse grains that protect the crest enable the ripple to grow but when ripple height becomes too high the bed shear stress increases thus allowing the wind to move the armoring layer when this happens during strong wind storms the megaripple will flatten and even disappear as was observed in our field study we present measurements that for the first time directly show the megaripple to normal ripple transition and we suggest two possible physical processes as potential causes of this phenomenon megaripple flattening can occur either when the wind exceeds the fluid threshold for a sufficient length of time or after a sequence of storms with winds blowing from the same direction

a log normal spectral analysis of inorganic grain size distributions from a canadian boreal lake core towards refining depositional process proxy data from high latitude lakes better methods for interpreting grain size spectra will enhance current understanding of past transport depositional processes a high resolution inorganic grain size dataset has been measured from a freeze core extracted from alberta lake e a boreal fresh water lake 40 km east of the athabasca oil sands in north eastern alberta canada the grain size spectra are remarkably consistent throughout the core exhibiting a structure comprising six persistent grain size distributions below ca 250 m plus a rare medium sand distribution automated deconvolution of the grain size spectra produced poor results constraining the modes of two of the distributions produced deconvolution solutions that were statistically excellent and consistent with the structure of each spectrum statistical analysis of the constrained solutions indicates that deconvolution successfully extracted independent grain size populations conversely the multimodal spectra generate traditional measures for example mean grain size that are inconsistent combinations of different individual populations and thus are poor proxies of transport depositional processes alberta lake e is situated in a boreal wetland landscape where sediment delivery is dominated by overland flow transport during spring melt this context means that the alberta lake e grain size spectra can be interpreted to reflect i a bedload component transported during short duration high discharge events that reflect the intensity of the melt and ii a finer suspended load component representing material whose magnitude is controlled by the volume of the spring melt stratigraphically bedload and suspended load populations demonstrate different short wavelength and long wavelength cyclicity suggesting that spring melt is likely to be driven by cyclic external forcing factors the links between the grain size spectra and spring melt have potential for generating proxy records that better capture the external controls over spring melt in boreal systems and the risks associated with these energetic hydrodynamics this is exemplified by the coarsest alberta lake e distributions which indicate that more intense spring melt dynamics occurred in pre historical times this article is protected by copyright all rights reserved

wind formed gravel bed forms wright valley antarctica 1 bed forms composed of gravel size particles 50 of particles 4 mm are observed in the wright valley of the mcmurdo dry valley system in antarctica these bed forms are characterized by a very asymmetrical shape with a mean aspect ratio of 0 025 standard deviation 0 005 mean wavelength of 2 7 m 0 49 m and a mean height of 0 06 m 0 01 m particle size analysis of the bed form sediments shows bimodality with a peak near 9 mm and another between 0 5 mm and 0 25 mm time integrated sediment trap samples of horizontal saltation and creep flux indicate the flux of particles 4 mm during the two year monitoring period was extremely low measurements of the horizontal displacement of tracer particles 14 mm 12 mm 10 mm 8 mm and 6 mm diameter placed onto the bed forms corroborate the low particle flux measurements and limited movement of particles the bed forms share form and grain size characteristics with both ripples and mega ripples showing poor sorting of particles across a single wavelength except for a slight coarsening at the crest similar to ripples but their sinuosity suggest that transverse instabilities affect their formation similar to mega ripples based on the data for the prevailing environmental conditions it can be argued that the wright valley form is an expression of gravel particles moved solely by highly intermittent creep processes this also argues for the need for a very long period of time for their evolution on the order of centuries

aeolian granule ripple deposits namibia granule ripples are a common feature of most dunefields yet they have seldom been recognized in ancient deposits although granule ripples are common in erosional settings such as windward slopes of dunes or scour surfaces in interdunes they nevertheless migrate laterally and leave distinctive deposits that can be recognized in ancient rocks these deposits have characteristics of type b sand sheet deposits including poured in texture curving ripple trough tangential coarse grained foresets irregular silty layers well sorted coarse and fine layers either horizontal or within foresets and fine layers in ripple troughs wind tunnel experiments suggest that under low velocity wind conditions granule ripples grow to a significant degree as parasites dependent on saltation of fine sand grains whose impact moves the larger grains of the granule ripple although the depositional surface of granule ripples is commonly coated with a layer of coarse grains this is in most places only a few grains thick underlying deposits commonly have a poorly sorted or poured in texture this texture results from an admixture of fine grains that fall among the spaces between the larger grains during deposition

the rate of granule ripple movement on earth and mars abstract the rate of movement for 3 and 10 cm high granule ripples was documented in september of 2006 at great sand dunes national park and preserve during a particularly strong wind event impact creep induced by saltating sand caused 24 granules min 1 to cross each cm of crest length during wind that averaged 9 m s 1 at a height well above 1 m which is substantially larger than the threshold for saltation of sand extension of this documented granule movement rate to mars suggests that a 25 cm high granule ripple should require from hundreds to thousands of earth years to move 1 cm under present atmospheric conditions

sedimentology of cold climate aeolian sandsheet deposits in the askja region of northeast iceland active aeolian sandsheets cover an area of 270 km2 in the askja region of ne iceland the sandsheet is divided into i a deflationary upwind part that is characterised by deflation lags ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles ii an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars and iii a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet these sinuous crested oblique bedforms attain heights of 0 35 m and downwind spacings of 6 m they are composed principally of low density pebble grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby askja volcanic centre the distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial ice margin fluvial aeolian and volcaniclastic processes under the influence of a distinctive climatic regime sediment supply at the upwind sandsheet margin adjacent to the dyngjujokull glacier increases during summer periods when rates of glacial ablation are at a maximum longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat and by periodic eruptions of volcaniclastic debris from the askja volcanic centre the availability of sediment for aeolian transport is controlled by the level of the groundwater table the depth and continuity of the permafrost layer in the proglacial area the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months the transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969 the suite of aeolian landforms developed within the askja region as defined by sediment texture and composition and bedform scale morphology and spatial distribution reflects a special set of controlling circumstances which mean that the askja sandsheet deposits differ significantly from those described from elsewhere

climate change and decadal to centennial scale periodicities recorded in a late holocene ne pacific marine record examining the role of solar forcing abstract we present a decadal scale late holocene climate record based on diatoms biogenic silica and grain size from a 12 m sediment core vec02a04 obtained from frederick sound in the seymour belize inlet complex of british columbia canada sediments are characterized by graded massive and laminated intervals laminated intervals are most common between c 2948 2708 cal yr bp and c 1992 1727 cal yr bp increased preservation of laminated sediments and diatom assemblage changes at this time suggest that climate became moderately drier and cooler relative to the preceding and succeeding intervals spectral and wavelet analyses are used to test for statistically significant periodicities in time series of proxies of primary production total diatom abundance biogenic silica and hydrology grain size preserved in the frederick sound record periodicities of c 42 53 60 70 82 89 241 243 and 380 yrs are present results are compared to reconstructed sunspot number data of solanki et al 2004 using cross wavelet transform to evaluate the role of solar forcing on ne pacific climate significant common power of periodicities between c 42 60 70 89 241 243 and of 380 yrs occur suggesting that celestial forcing impacted late holocene climate at frederick sound replication of the c 241 243 yr periodicity in sunspot time series is most pronounced between c 2900 cal yr bp and c 2000 cal yr bp broadly correlative to the timing of maximum preservation of laminated sedimentary successions and diatom assemblage changes high solar activity at the suess de vries band may have been manifested as a prolonged westward shift and or weakening of the aleutian low in the mid late holocene which would have diverted fewer north pacific storms and resulted in the relatively dry conditions reconstructed for the seymour belize inlet complex

controls on the height and spacing of eolian ripples and transverse dunes a numerical modeling investigation abstract ripples and transverse dunes in areas of abundant sand supply increase in height and spacing as a function of time grain size and excess shear velocity how and why each of these factors influence ripple and transverse dune size however is not precisely known in this paper the controls on the height and spacing of ripples and transverse dunes in areas of abundant sand supply are investigated using a numerical model for the formation of eolian bedforms from an initially flat surface this bedform evolution model combines the basic elements of werner s werner b t 1995 eolian dunes computer simulations and attractor interpretation geology 23 1107 1110 cellular automaton model of dune formation with a model for boundary layer flow over complex topography particular attention is paid to the relationship between bed shear stress and slope on the windward stoss side of evolving bedforms nonlinear boundary layer model results indicate that bed shear stresses on stoss slopes increase with increasing slope angle up to approximately 20 then decrease with increasing slope angle as backpressure effects become limiting in the bedform evolution model the linear boundary layer flow model of jackson and hunt jackson p s hunt j c r 1975 turbulent wind flow over a low hill quarterly journal of the royal meteorological society 101 929 955 generalized to 3d is modified to include the nonlinear relationship between bed shear stress and slope bed shear stresses predicted by the modified jackson and hunt flow model are then used to predict rates of erosion and deposition iteratively through time within a mass conservative framework similar to werner werner b t 1995 eolian dunes computer simulations and attractor interpretation geology 23 1107 1110 beginning with a flat bed the model forms ripples that grow in height and spacing until a dynamic steady state condition is achieved in which bedforms migrate downwind without further growth the steady state ripple spacing predicted by this model is approximately 3000 times greater than the aerodynamic roughness length of the initially flat surface which is a function of grain size and excess shear velocity once steady state ripples form they become the dominant roughness element of the surface the increase in roughness associated with ripple formation triggers the same bedform instability that created ripples causing dunes to form at a larger scale in this way the numerical model of this paper suggests that ripples and dunes are genetically linked transverse dunes in this model have a steady state height and spacing that is controlled by the effective roughness length of the rippled surface which is shown to be on the order of 500 times greater than the original roughness length but varies significantly with the details of ripple morphology the model predictions for ripple and dune spacing and their controlling variables are consistent with field measurements from the published literature the model of this paper provides a preliminary process based understanding of the granulometric control of ripples and dunes in areas of abundant sand supply and unidirectional prevailing winds and it argues for a genetic linkage between ripples and dunes via a scaling relationship between eolian bedform size and the aerodynamic roughness length

an experimental study of multiple grain size ejecta produced by collisions of saltating grains with a flat bed collision data are presented from coloured high speed films of three size fractions of sand grains saltating over a bed of the total grain population each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded the results confirm earlier findings that v3 v1 0 5 0 6 vn v1 08 and the rebound angle increases with decreasing grain size ejected grains are examined in relation to their size the impactor size ejection speed and angle and the number of ejecta per collision in addition changes in grain parameters are observed with time for fine impactors ejection speeds generally increase with a decrease in ejecta size but the fine fraction does not follow this trend for the coarse and medium impactors ejection angles are typically between 40 and 60 with coarse grains having shallower mean angles than fine ejecta the number of ejections per collision increases with a decrease in particle size for each impactor size the general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes there is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains much of the forward momentum of the saltating grains is transfered to creeping grains the composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud

evolution of luyang lake since the last 34 000 years climatic changes and anthropogenic impacts abstract a sediment section retrieved from luyang lake shaanxi province china was employed for grain size total organic carbon content toc and carbonates caco 3 analyses based on the optically stimulated luminescence osl chronology the evolution of luyang lake since the last 34 000 years was reconstructed then the historical anthropogenic information like populations and arable lands obtained from literature were incorporated into the environmental sequence to demonstrate the linkage of lake evolution with both climatic and anthropogenic forcings it indicated that the appearance of loessial sediment during ca 33 4 10 7 ka exhibited a much lower lake level associated with a cold and dry climate setting the sediment became finer with increasing toc after ca 10 7 ka implied the enhanced monsoon rainfall and relatively higher lake level until ca 4 6 ka much higher toc and lowest caco 3 at 7 2 4 6 ka suggested the holocene climate optimum in the region the rising caco 3 and declining toc at ca 4 6 2 0 ka revealed the enhanced evaporation precipitation rate and depressed paleoproductivity suggesting a drier climate toc reached its highest after ca 2 ka coinciding with the increasing population and the expanding arable lands since the last 600 years would suggest the anthropogenic impacts like land reclamation and farming rather than climate forcing briefly the evolution of luyang lake was controlled by the monsoon climate through geological times and the intensified anthropogenic interferences since the last 2 ka especially the last 600 years would account for the transformation of freshwater lake to a saline lake and wetland environment

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

late holocene lake sedimentology and climate change in southern alberta canada abstract climatic changes in southern alberta canada for the past 4000 yr are reflected in a high resolution record of lake sediment grain size the proposed mechanism for this response is that outflow discharge removes fine grained sediments but increasingly fine sediments are retained and deposited as streamflow declines at the same time coarse sediments are brought in by high discharge entering the lake the net effect of these two processes is to leave coarse clay deficient sediments during times of high streamflow and clay rich sediments during times of low flow the grain size record from pine lake reflects historic climate fluctuations as well as prehistoric fluctuations including the little ice age and the medieval warm period grain size at this site provides a simple economical and nonbiologically mediated paleoclimate proxy

late quaternary linkage of sedimentary records to three astronomical rhythms and the asian monsoon inferred from a coastal borehole in the south bohai sea china abstract the bohai sea was formed by subsidence during the cenozoic some 2000 3000 m of fluvial lacustrine and marine sediments has been deposited in this basin previous studies focused mainly on the transgression history with little examination of orbital variation in relation to other areas within the asian monsoon domain here we present the late quaternary results of a new borehole in the south bohai sea optically stimulated luminescence and radiocarbon dating which provide concordant age estimates were employed to generate an initial chronology for the borehole after refining the chronology through astronomical tuning the results showed that 1 the grain size variation represents asian monsoon intensity which was dominated by both solar insolation major and global ice volume minor forcing 2 the magnetic susceptibility indicates river incision processes which were sensitive to orbital tilt with influence from solar insolation 3 the vegetation coverage responded to global ice volume coupled obliquity changes and that 4 neither external nor internal factors could dominate the paleoenvironmental evolution on orbital timescales in an independent way and they are both integrated in a complex pattern we conclude that three different astronomical rhythms have affected coastal evolution and that the sedimentary records in the south bohai sea china result from the nonlinear interaction and the complex response to driving processes

aridification recorded by lithofacies and grain size in a continuous pliocene quaternary lacustrine sediment record in the western qaidam basin ne tibetan plateau abstract the pliocene quaternary aridification of the asian interior is key to understanding the impacts of global cooling and tibetan plateau uplift and the potential linkage to north pacific ocean biochemical processes and global changes however there is a lack of detailed continuous pliocene quaternary paleoclimatic records from the asian interior because most climatic records have been obtained from discontinuous coarse sediment outcrops around the rims of inland basins here we provide a continuous high resolution 3 1 myr record of grain size from the well dated sg 3 borehole in the western qaidam basin on the northeastern tibetan plateau nw china the results reveal a long term upward coarsening trend in grain size that can be divided into three phases i 3 1 1 1 ma laminated fine sediments fine to medium silts containing scattered gypsum crystals and interbedded by thin beds of marl ii 1 1 0 15 ma massive medium to coarse silts interbedded by beds of mirabilite iii 0 15 0 ma coarse silts containing thin layers of mirabilite halite bloedite and polyhalite this pattern reflects the upward shallowing and shrinking of the lake from a brackish lake to a salt lake and then to a playa suggesting a long term stepwise drying of the asian interior since the late pliocene probably forced by global cooling and uplift of the tibetan plateau

late holocene 1500 yr climatic periodicities and their implications fourier and nonlinear regression analysis of a 4000 yr paleoclimate proxy record in western canada shows strong periodicities of 1500 yr and several weaker century to millenial scale periodicities in conjunction with the 23 708 yr milankovitch periodicity these produce a model of climate fluctuation through the postglacial consistent with recognized paleoclimatic fluctuations of the past 15 000 yr in the northern mid latitudes these results suggest that postglacial climatic anomalies such as the little ice age and the younger dryas were at least in part periodic phenomena rather than the result of unique aperiodic events projecting these periodicities into the future suggests that even in the absence of anthropogenic climate forcing a natural warming trend will continue until ca a d 2400

a comprehensive numerical model of steady state saltation comsalt 1 the blowing of sand by wind known as saltation ejects dust aerosols into the atmosphere creates sand dunes and erodes geological features we present a comprehensive numerical model of steady state saltation comsalt that in contrast to most previous studies can reproduce a wide range of measurements and can simulate saltation over mixed soils comsalt calculates the motion of saltating particles due to gravity fluid drag particle spin fluid shear and turbulence and explicitly accounts for the retardation of the wind due to drag from saltating particles furthermore we included a novel physically based parameterization of the ejection of surface particles by impacting saltating particles which matches experimental results comsalt is the first numerical saltation model to reproduce measurements of the wind shear velocity at the impact threshold i e the lowest shear velocity for which saltation is possible and of the aerodynamic roughness length in saltation it also reproduces a range of other saltation processes including profiles of the wind speed and particle mass flux and the size distribution of saltating particles as such comsalt is the first physically based numerical model to reproduce such a wide range of experimental data since we use a minimum of empirical relations comsalt can be easily adapted to study saltation under a variety of physical conditions such as saltation on other planets saltation under water and saltating snow

a wind tunnel investigation of particle segregation ripple formation and armouring within sand beds of systematically varied texture this paper reports on a wind tunnel investigation of particle segregation ripple formation and surface armoring within sand beds of systematically varied particle size distribution from coarsely skewed to bimodal by design the system was closed with no external inputs of mass from an external particle feed particles too coarse to travel in saltation for the given range in wind speed were dyed red in order to distinguish them in optical images from finer sand particles which could be entrained into the unidirectional airflow a 3d laser scanner measured the changing bed topography at regular time intervals during 18 experiments involving varied combinations of wind speed and bed texture image classification techniques were used to investigate the coincident self organization of the two populations of particles as distinguished by their colour as soon as saltation commenced some of the red particles segregated into thin discontinuous patches particle trapping and sheltering on these rough patches was strongly favoured causing them to grow preferentially during the earliest stages of formation bedform growth coincided with i rapid coarsening of the surface texture and ii the merging of proto ripple crests to generate larger rhythmic bedforms of lower frequency consistent with previous work ripple size was observed to increase under stronger winds when not exceeding the threshold for entrainment of the coarse mode or red particles from the crest with declining rates of mass transport and particle segregation as the bed surface armoured and the consequent deceleration of ripple propagation through to the end of each experiment all surfaces eventually attained a steady state morphometry at saturation the largest ripples developed upon beds having the lowest initial concentration of red particles this article is protected by copyright all rights reserved

cross sectional profiles of sand ripples megaripples and dunes a method for discriminating between formational mechanisms cross sectional profiles of sand ripples megaripples and sand dunes provide a useful tool for discriminating between formation by ripple and dune processes feature width defined as the basal break in slope along the profile to either side of the crest represents a good standard for comparison of profile attributes across more than three orders of magnitude aspect ratio height width as a function of log width separates measurements into clusters representing differing mechanisms of formation scaling both height and distance for individual profiles by feature width facilitates comparison of profile shapes across three orders of magnitude in width the data presented here should prove useful for evaluating possible mechanisms of origin for aeolian features observed remotely including on planetary bodies copyright 2012 john wiley sons ltd

the propagation of an error in long and sharp s 1964 data on barchan dune movement a misprint in a classic study of desert dunes long and sharp geological society of america bulletin 1964 vol 75 pp 149 156 is reported its influences on subsequent literature are examined the true data of dune migration speed collected by long and sharp are found to show better correlation with dune height than seen in two recent textbooks and a paper by the authors copyright 2002 john wiley sons ltd

mechanisms limiting the growth of aeolian megaripples megaripples are distinguished from regular ripples by their larger size and bimodal sediment distribution the interplay between wind grain size and morphology controls their development but the exact mechanisms that limit the size of megaripples have been unclear using wind tunnel experiments we found two main mechanisms that limit the height of megaripples the first mechanism is megaripple flattening due to strong enough winds that drive the coarse grains into saltation the second mechanism is megaripple deflation by impacts of faster saltation grains in this latter mechanism the coarse grains are propelled by the impacts of fine saltating grains the occurrence of both these mechanisms depends on the grain size distribution and increases with both megaripple height and wind speed thus for a given wind environment and grain size distribution there exists a limit on the size of megaripples which is determined by these two mechanisms

frequency and intensity of high altitude floods over the last 3 5 ka in northwestern french alps lake anterne in central western europe several studies have shown that colder holocene periods such as the little ice age also correspond to wet periods however in mountain areas which are highly sensitive to erosion processes and where precipitation events can be localized past evolution of hydrological activity might be more complicated to assess these past hydrological changes a paleolimnological approach was applied on a 13 4 m long sediment core taken in alpine lake anterne 2063 m asl and representing the last 3 5 ka lake sedimentation is mainly composed of ood deposits triggered by precipitation events sedimentological and geochemical analyses show that oods were more frequent during cold periods while high intensity ood events occurred preferentially during warmer periods in mild temperature conditions both ood patterns are present this underlines the complex relationship between ood hazards and climatic change in mountain areas during the warmer and or dryer times of the end of iron age and the roman period both the frequency and intensity of oods increased this is interpreted as an effect of human induced clearing for grazing activities and reveals that anthropogenic interferences must be taken into account when reconstructing climatic signals from natural archives

holocene glacier fluctuations in the canadian cordillera alberta and british columbia abstract although the record of holocene glacier fluctuations in british columbia and alberta is still very fragmentary much recent progress has been made in their study dating of end moraines has been supplemented by studies of lateral moraine stratigraphy and dating of glacially overridden organic matter upstream of end moraines dates from sites close to contemporary glaciers indicate that ice had retreated to about its present limits before 10 ka bp prior to deposition of mazama tephra a minor pre hypsithermal readvance the crowfoot advance left deposits in the rockies and the interior of british columbia the earliest known neoglacial advances occurred 5 6 ka bp in the coast mountains and shortly after 4 ka bp in the rockies an advance culminating ca 4 3 ka bp has been reported from bugaboo glacier while evidence of another advance culminating in the period ca 2 5 1 8 ka bp the tiedemann advance is reported from more than one site following a recession the little ice age expansion the cavell advance began shortly after 900 bp this advance culminated in most cases in the 18th or 19th centuries the crowfoot tiedemann and little ice age advances were apparently of roughly the same magnitude the latter was most extensive in most cases but at several sites crowfoot moraines lie slightly farther downvalley than little ice age moraines during the present century ice fronts have generally retreated particularly rapidly in the period 1920 1950 a d although rates of retreat have decreased in the last three decades and some glaciers have readvanced slightly

10 000 yr record of extreme hydrologic events well dated lacustrine sediments provide a hydrologic record indicating that the frequency and magnitude of runoff events and by inference storms have varied over the past 10 k y in northern new england we used five sediment cores and radiocarbon dating to develop a chronology of holocene hydrologic events for the ritterbush pond basin northern vermont chemical and physical analyses allow us to identify 52 distinct layers of predominately inorganic sediment that represent terrestrially derived material delivered to the pond by runoff events the thickness of some layers suggests hydrologic events at least equal in size to and probably much larger than any storm or flood recorded during nearly 300 yr of written regional history layer thickness and frequency and by analogy storm size and recurrence change through the holocene the largest events occurred 2620 6840 and 9440 calibrated 14c years before present cal 14c yr b p the most frequent hydrologic events occurred in three periods 1750 to 2620 6330 to 6840 and 8600 cal yr b p the recurrence interval of layer deposition during stormy periods averages 130 100 cal yr whereas the recurrence interval during less stormy periods is longer 270 170 cal yr the ritterbush pond event record illustrates the potential of inorganic lacustrine sediment to serve as a proxy record for estimating paleoflood frequency and deciphering climate change

an equilibrium relationship for shear velocity and apparent roughness lenght in aeolian saltation abstract an equilibrium relationship between shear velocity and apparent surface roughness is established for aeolian saltation velocity profiles of wind over blowing sand were digitized from eleven published data sets the 106 profiles were analyzed to obtain estimates of shear velocity and apparent surface roughness lenghts these values were used to test charnock s relationship for suitability in aeolian environments the results indicate that z 0 0 0091 u 2 g modifying charnock s model to describe wind over an equilibrium saltation field yields z 0 2d 50 30 0 0252 u u t 2 g the latter result is used to demonstrate that the focal point concept proposed by bagnold stems from the increase in apparent roughness with increased shear velocity the results demonstrate an excellent example of threshold controlled equilibrium in dynamic systems

a note on the use of the fryberger method for evaluating potential sand transport by wind the fryberger method provides a useful and accessible way of evaluating potential sediment transport by wind in this method an equation is used to estimate relative rates of sand transport expressed as drift potential from measurements of wind velocity however the values of drift potential obtained through the calculations are dependent upon the units in which wind velocities are expressed this paper highlights some of the confusion this dependence can cause and aims to clarify the ways in which values of drift potential can be interpreted

megaripple migration on mars aeolian megaripples with 5 to 50 m spacing are abundant on the surface of mars these features were repeatedly targeted by high resolution orbital images but they have never been observed to move thus aeolian megaripples especially the bright toned ones often referred as transverse aeolian ridges tars have been interpreted as relict features of a past climate in this report we show evidence for the migration of bright toned megaripples spaced 1 to 35 m 5 m on average in two equatorial areas on mars indicating that megaripples and small tars can be active today the moving megaripples display sand fluxes that are 2 orders of magnitudes lower than the surrounding dunes on average and unlike similar bedforms on earth can migrate obliquely and longitudinally in addition the active megaripples in the two study areas of syrtis major and mawrth vallis show very similar flux distributions echoing the similarities between dune crest fluxes in the two study areas and suggesting the existence of a relationship between dune and megaripple fluxes that can be explored elsewhere active megaripples together with high sand flux dunes represent a key indicator of strong winds at the surface of mars a past climate with a denser atmosphere is not necessary to explain their accumulation and migration

field measurement and numerical modelling of aeolian mass flux distributions on a sandy beach the vertical and horizontal distributions of aeolian mass flux were measured at oceano dunes california and these data were used to evaluate a numerical model of saltation grain size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population and modelling thus focused on replicating the distributions of the mean grain size although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field both exponential and gamma distributions of launch velocity produced realistic simulations although the latter approach required the inclusion of an additional reptation component to achieve good results a range of mean launch angles and an equivalent sphere correction were also found to generate comparable results providing the other input parameters could be varied freely all the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions and underestimated the proportion occurring at the upwind end of the horizontal distributions no theoretical shortcoming that would account for these small but systematic discrepancies could be identified and experimental error thus represents a more plausible explanation the conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed and that increases in the transport rate are therefore driven primarily by the ejection of additional grains it is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation equivalent to a plastic limit hence grains of larger mass diameter rebound from the bed at lower speeds and follow shorter lower trajectories as has been widely observed previously

granule ripples in the kumtagh desert china morphology grain size and influencing factors granule ripples are found mainly in four regions of the kumtagh desert in china they are characterized by an asymmetrical shape with gentle lower slopes on both sides and abrupt crests the ripples tend to be oriented perpendicular to the prevailing winds except when they form near obstacles such as yardangs the wavelengths range between 0 31 m and 26 m and heights h range from 0 015 m to 1 m the relationship between wavelength and height can be described by a simple linear function and the mean ripple index h is about 20 4 for the study sites the sediments are poorly sorted with negative to very negative skewness at lee and stoss slopes and between ripple troughs which confirms the poured in and shadow appearance described by previous researchers the bimodal or trimodal distributions of grains with modes of 1 16 0 5 and 3 16 and the enrichment of coarse particles at the ripple surface with coarse granule contents ranging between 5 2 and 62 1 indicate that the underlying layer is the original sediment source and that the granule ripples resist erosional processes although the impact of saltating particles and consequently the creep and reptation of coarse grains are responsible for granule ripple initiation at a micro scale however the characteristics of local sediments wind regimes and topographical obstacles as well as the feedbacks among bedform and airflow more strongly affect the development and alignment of granule ripples at a macro scale

transverse instability of megaripples as a result of their inherent differences in stability sand ripples and megaripples exhibit variations in terms of their wavelengths and grain size distributions unimodal for sand ripples and bimodal for megaripples while sand ripples form almost straight lines megaripples have greater sinuosity due to their transverse instability a property that causes small megaripple undulations to grow with time the origin of the instability is due to variations in megaripple height variations that do not diminish over time and due to the inverse dependence of ripple drift velocity on the height thus the taller regions of ripples will move more slowly than the adjacent shorter portions an outcome that promotes further perturbation growth we show an example based on fi eld work of the transverse instability of megaripples the instability growth rate depends on the difference between the heights of the different segments of the megaripple in contrast to the underlying instability of megaripples normal sand ripples are essentially stable and are not affected by transverse perturbations instead reacting quickly to the wind which tends to smooth ripple height irregularities the transverse instability of megaripples derives from the composition of their crests which comprise coarse particles that allow initial perturbations in ripple height to grow further the results suggest a physical mechanism for the transverse instability of megaripples and new insight into the spatial patterns of sand ripples

eolian features produced by the december 1977 windstorm southern san joaquin valley california high wind velocities during a windstorm in december of 1977 in the southern san joaquin valley california created eolian features and effects of a magnitude not previously reported in the literature three principal mechanisms of sediment movement by wind were studied by examining the size distribution of deposits in telephone poles saltation and suspension feeder pens and abandoned buildings saltation and suspension and gravel ripples surface creep the maximum grain sizes of rocks lodged in telephone poles at heights of 240 160 and 80 cm are 7 mm x 11 mm x 6 mm 10 mm x 15 mm x 6 mm and 20 mm x 23 mm x 5 mm respectively in some areas with rocky soils lag materials formed gravel ripples

gravel mantled megaripples of the argentinean puna a model for their origin and growth with implications for mars gravel megaripples in the puna of argentina are the most extreme aeolian megaripples on earth and are useful analogs for aeolian processes on mars field observations supplemented by experimental and numerical constraints on wind characteristics and aeolian transport reveal their conditions of formation and growth to be an aeolian geomorphology perfect storm the bedforms are formed on a substrate of weakly indurated ignimbrite aeolian deflation of which yields a bimodal lag of lithics and pumice clasts onto an undulating surface under normal wind conditions in this region the lithics are organized into bedforms on local upslopes and highs through creep induced by the impact of saltating sand and pumice the gravel bedforms grow through shadowing and trap sand and silt that is gradually kinetically sieved down to lift the gravel mantle upwards to form the megaripples these observations connote that the largest features are not ripples in the sense of migrating bedforms but rather nucleation sites of wind transported sediment strong control by bedrock topography means that the largest bedform wavelengths are not a result of particle trajectories and this complicates their comparison with other ripples and may require a new classification of relevance to mars the puna megaripples are morphologically and contextually similar to small ripple like transverse aeolian ridges tars moreover the puna gravels have similar equivalent weight mg to those composing granule ripples at meridiani planum and their local origin may have implications for the origin of sediment in martian aeolian bedforms finally the stable yet dynamic character of the puna megaripples could help reconcile current models of tars with periodic bedrock ridges that may be produced by aeolian erosion

stability of armour layers in rivers the importance of both the overpassing and the embedding of river bed particles is demonstrated by experiments when a single layer thick armouring layer of uniform d d1 size particles on a bed of uniform finer d d2 size particles is broken at one location the stability of the remainder of the armour layer depends on the tendency of the d d1 particles to embed into the surface of d d2 particles if a d d1 size particle is seated in the exposed position on the surface of the d d2 sized bed sediment it may by virtue of its exposure to the flow be preferentially entrained and it may then overpass the finer bed sediment alternatively the d d1 particle may be too large for overpassing and it will become embedded armour layers composed of particles which embed tend to be more stable

a steady state model of wind blown sand transport a model for steady state eolian saltation is presented in which computation of sand grain trajectories in the wind and of the wind velocity profile modified by saltating grain drag forces is combined with experimental data on grain bed collisions in an iterative simulation scheme which resembles the time development of natural saltation qualitative features of eolian saltation are reproduced in the simulations including a height in the saltating layer at which the wind velocity remains roughly constant with changing free stream friction velocity and an approximate separation of the saltating population into successively hopping grains and reptating or creeping grains the numerical solutions give rise to more general arguments in support of the conclusions that fluid entrainment of grains is unimportant in steady state saltation that the mass flux profile obeys roughly an inverse power law dependence on height and that the vertically integrated mass flux dependence on the free stream friction velocity

a theoretical model for aeolian polydisperse sand ripples abstract a continuum model is constructed to investigate the effects of sediment heterogeneity on the formation of aeolian sand ripples and the grain size sorting over the bed forms the evolutions of small perturbations of both bed elevation and bed composition are studied analytically by a linear stability approach in this model the transport of polydisperse sediment is considered as the integrated processes of saltation reptation caused by impact driven and creep caused by gravity driven and drag driven theoretical analysis indicates that size selective sand transport leads to grain sorting phenomena which appreciably affect the dynamics of the formation of aeolian sand ripples the sediment heterogeneity is beneficial to the stability of sand bed and that the wavelength is lengthened compared with the case of uniform sand longer ripples appear when the sediment heterogeneity is characterized by a poorly sorted on the other hand the relative volume fraction of the different grain size in the sediment heterogeneity appreciably affects the characteristics of sand ripples moreover it is found that the theoretical model successfully reproduces the observed tendency of coarser grains to accumulate towards the crests of sand ripples while finer grains accumulating towards the troughs lastly a physical interpretation of the theoretical model results which qualitatively agree with field observations and wind tunnel experiments is given

largest wind ripples on earth unique wind ripples attaining heights to 2 3 m wavelengths to 43 m and a crest maximum grain size of 19 mm occur on the argentine puna plateau at 4000 m altitude these are the largest ripples reported on earth comparable only to mars counterparts they form in the presence of high proportions of low density pumice clasts 0 91 g cm 3 although crests are exclusively composed of varnished normal density clasts 2 43 g cm 3 mature ripple profiles are partly excavated on bedrock so they form by a combination of deflation winnowing of finer grains minor wind drift of fine gravel and lagging of clasts 4 cm the large ripple size appears to be related to strong winds dense saltation layers and a long time for evolution ripple sizes are smaller on obstacles as compared to flat terrain there is a lack of correlation between clast size wavelength and the extreme ripple size in spite of the thin atmosphere all of which suggest that while small scale gravel ripples may form according to a reptation model their evolution into large scale types may relate to aerodynamic instabilities originating at the saltation curtain air interface

the origin of the transverse instability of aeolian megaripples abstract flat sand beds subjected to wind stress are unstable and the wind action results in two types of aeolian sand ripples normal ripples and megaripples the distinction between the two types is based on two characteristics i the normal ripple pattern usually has a wavelength of up to 30 cm while the megaripple wavelength is on the order of meters and ii unimodal distributions of sand grain size lead to normal ripples while bimodal distributions result in megaripples on mars the distinction between the two types is more difficult to ascertain because the length scales of normal ripples and megaripples can overlap and often there is no detailed information regarding their grain size distribution unlike normal ripples megaripples show transverse instability whose mechanism remains elusive resulting in a much larger sinuosity of the crestline than normal ripples in this study we investigate the megaripples transverse instability by using field measurements wind tunnel experiments and numerical simulations of a three dimensional ripple model we show that a coarse grains accumulate at megaripple crests with a corresponding reduction of the lateral sand transport along the crest and b the transverse instability of megaripples is generated by a positive feedback between the height of the crest and the accumulation of coarse grains with more grains accumulating on the higher portions of the crest the outcomes of this positive feedback are that the thickness of the coarse grain armoring layer along the crest is not uniform and that it correlates with the crest height in turn these height differences drive the transverse instability such that higher portions of the ripple migrate more slowly than the lower sections creating a wavy crestline an analysis of martian ripple images shows variations in the sinuosity index suggesting that this characteristic can be useful in distinguishing between normal ripples and megaripples on mars

a two species model of aeolian sand transport the transport of sand by wind results from the equilibrium between the erosion of grains dragged by the flow and the resulting slow down of the wind velocity the dynamical mechanisms governing the saturation of the sand flux are investigated theoretically we first demonstrate that previous models based on the assumption that all the grains have the same trajectory are either not self consistent or lead to unstable solutions a model based on a discrete number of states is derived which solves these problems two well defined species of grain appear which correspond to saltons high energy grains and reptons grains ejected from the sand bed by the impact of saltons they play specific roles the negative feedback of the transport on the wind is limited to the reptation layer while most of the transport is due to saltation the model is further simplified benefiting from the existence of these two species and the dependencies of the threshold velocity the saturated flux the aerodynamic roughness and the saturation length are derived and compared to experimental measurements

7000 year record of lake level change on the northern great plains a high resolution proxy of past climate mineralogical and paleobotanical characteristics of the sedimentary fill in a shallow saline lake in southeastern alberta indicate that drought intervals are aperiodic and that climatic extremes unprecedented in historic time occurred earlier during the holocene when century long intervals of repeated intense droughts alternated with long periods when droughts were rare low water intervals signifying periods of drought are recorded by carbonate rich laminae containing abundant plant fossils indicative of hypersalinity in contrast highstands of relatively fresh water outlining moist periods are represented by massive silicate rich sediment with lower numbers of halophytic indicators

late holocene vegetation and fire history at the southern boreal forest margin in alberta canada abstract a transect of three shallow ponds across the aspen parkland boreal forest transition in elk island national park alberta canada shows changes in late holocene vegetation and disturbance dynamics and consequent carbon storage in response to climate change and to recent anthropogenesis pollen and charcoal analyses reveal unexpected changes in fire regime although there is the expected decline in fire activity during the historic period presumed due to agricultural clearance around the park there is also a prehistoric fluctuation in fire regime at one of the sites pollen evidence suggests that the fluctuations in fire regime may be due to changes in hydrology declining groundwater levels during the medieval warm period allowed the replacement of substantial areas of shrub birch betula glandulosa with the less fire prone aspen populus tremuloides causing a decline in fire frequency and or severity while increasing carbon storage on the landscape this is counter to the intuitive increase in fire activity with warmer and drier climate canadian national parks are currently managed under a natural processes and conditions paradigm the changes in conditions and consequent changes in processes evident due to the relatively minor climatic fluctuations of the little ice age and medieval warm period shown here suggest a need to revisit this paradigm in consideration of future anthropogenic climate change

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

evidence of increased rainfall prior to 3500 years bp as revealed by river borne terrigenous flux a study from west coast of india abstract the study has been conducted for insoluble residue and calcium carbonate content in a 4 80 m long shallow water at 22 m water depth sediment core at 22 m water depth off central west coast of india to reconstruct palaeo monsoonal precipitation linked with river discharge the down core profile of carbonate content shows that it is generally less than 28 below ca 3 00 m whereas above 3 00 m it is generally more than 28 and this showing an general upward trend on the contrary the average insoluble residue is marked by higher value 72 below ca 3 00 m while above 3 00 m it is low

is a regional flood signal reproducible from lake sediments the evolution of flood activity with global warming remains uncertain to better assess flood climate relationships lake sediments are increasingly being investigated because they could provide regional flood histories long enough to cover past climate changes however site specific sedimentary processes may bias flood reconstructions the aim of this article is to investigate these effects through the reconstruction of two distinct flood records from independent neighbouring sedimentary basins of the same lake lake allos in the mediterranean french alps i e under the same climate conditions understanding of sedimentary processes is crucial in order to adapt the sampling strategy and the flood intensity proxy to each sedimentary system and thereby reconstruct a complete and reliable flood history thanks to this detailed approach the main trends of the regional flood variability can be reproduced i e periods of high flood frequency ranges of flood frequency values and timing of the most intense events in detail some differences appeared associated to the various stream capacities to erode and transport flood sediments to the lake system implying variable sensitivity of sedimentary systems in recording floods comparing regional flood records based on independent sedimentary systems from similar environments could thus be a complementary approach to assessing past flood intensity such an approach could open particularly interesting perspectives because reconstructing the long term evolution of flood intensity is a key challenge in the geosciences

flood stratigraphies in lake sediments a review abstract records of the frequency and magnitude of floods are needed on centennial or millennial timescales to place increases in their occurrence and intensity into a longer term context than is available from gauged river flow and historical records recent research has highlighted the potential for lake sediment sequences to act as a relatively untapped archive of high magnitude floods over these longer timescales abyssal lake sediments can record past floods in the form of coarser grained laminations that reflect the capacity for river flows with greater hydrodynamic energy to transport larger particles into the lake this paper presents a framework for investigating flood stratigraphies in lakes by reviewing the conditioning mechanisms in the lake and catchment outlining the key analytical techniques used to recover flood records and highlighting the importance of appropriate field site and methodology selection the processes of sediment movement from watershed to lake bed are complex meaning relationships between measureable sedimentary characteristics and associated river discharge are not always clear stratigraphical palaeoflood records are all affected to some degree by catchment conditioning fluvial connectivity sequencing of high flows delta dynamics as well as within lake processes including river plume dispersal sediment focussing re suspension and trapping efficiency with regard to analytical techniques the potential for direct e g laser granulometry and indirect e g geochemical elemental ratios measurements of particle size to reflect variations in river discharge is confirmed we recommend care when interpreting fine resolution geochemical data acquired via micro scale x ray fluorescence xrf core scanning due to variable down core water and organic matter content altering x ray attenuation we also recommend accounting for changes in sediment supply through time as new or differing sources of sediment release may affect the hydrodynamic relationship between particle size and or geochemistry with stream power where these processes are considered and suitable dating control is obtained discrete historical floods can be identified and characterised using palaeolimnological evidence we outline a protocol for selecting suitable lakes and coring sites that integrates environmental setting sediment transfer processes and depositional mechanisms to act as a rapid reference for future research into lacustrine palaeoflood records we also present an interpretational protocol illustrating the analytical techniques available to palaeoflood researchers to demonstrate their utility we review five case studies of palaeoflood reconstructions from lakes in geographically varied regions these show how lakes of different sizes and geomorphological contexts can produce comprehensive palaeoflood records these were achieved by consistently applying site validated direct and proxy grain size measurements well established chronologies validation of the proxy process interpretation and calibration of the palaeoflood record against instrumental or historical records

millennial scale rhythms in peatlands in the western interior of canada and in the global carbon cycle abstract anatural 1450 yr global holocene climate periodicity underlies a portion of the present global warming trend calibrated basal radiocarbon dates from 71 paludified peatlands across the western interior of canada demonstrate that this periodicity regulated western canadian peatland initiation peatlands the largest terrestrial carbon pool and their carbon budgets are sensitive to hydrological fluctuations the global atmospheric carbon budget experienced corresponding fluctuations as recorded in the holocene atmospheric co 2 record from taylor dome antarctica while the climate changes following this 1450 yr periodicity were sufficient to affect the global carbon budget the resultant atmospheric co 2 fluctuations did not cause a runaway climate co 2 feedback loop this demonstrates that global carbon budgets are sensitive to small climatic fluctuations thus international agreements on greenhouse gasses need to take into account the natural carbon budget imbalance of regions with large climatically sensitive carbon pools