carbon and nitrogen loss rates during aging of lake sediment changes over 27 years studied in varved lake sediment we used a collection of ten freeze cores of annually laminated varved lake sediment from nylandssjon in northern sweden collected from 1979 to 2007 to follow the long term loss of carbon c and

the effect of sparse vegetation on the transport of dune sand by wind the precise effect of a sparse plant cover on the transport of dune sand by wind is a critical factor in understanding the morphology evolution and global distribution of aeolian dunes1 3 it is also of considerable practical importance in sand stabilization rehabilitation and restoration ecology here i report the first detailed experimental quantification of this effect using living plants in patterns approximating natural vegetation for wind velocities v at 0 5 m elevation up to 15 m s 1 and plant covercup to 17 the rate of aeolian sand transport q g cm 1 s 1 is closely approximated by q b v 1 kc vt 3 where b is bagnold s constant4 vt is a threshold velocity and k is a constant dependent on plant geometry for typical small erect or spreading herbaceous dune plants k 0 018 for small rounded stemless plants k 0 046

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

vertical variation in porosity of nāri calcrete on chalk galilee israel a new interpretation as a tribute to dan h yaalon abstract n ri calcrete is a calcareous formation common in the semiarid mediterranean region of the levant the profile studied in nazareth galilee is 2 75 m thick and divided into the four conventional layers i e a chalky bedrock a powdery lower n ri an indurated and platy upper n ri and a laminar crust capping the profile in this study mercury intrusion porosimetry mip is used to assess the genetic processes at work in the formation of n ri calcrete based on the former work by yaalon and singer 1974 and mip results various parameters are proposed to describe the different layers total porosity n p trapped porosity n t permeability k and strength c these parameters are compared to each other for each layer as well as with their respective calcium carbonate contents an unexpected relationship shows that the more caco 3 is present the more porous the layer of the n ri consequently the lower n ri is interpreted as a weathering layer of the parent chalk whereas the upper n ri which is the most porous and the richest in caco 3 results from in situ recrystallization the upper n ri permeability precludes it from playing the role of a plugged horizon to explain the formation of the laminar crust in conclusion the n ri calcrete in nazareth questions the conventional model of calcrete genesis as its cementation is due to in situ redistribution of caco 3 rather than its translocation these results strengthen yaalon and singer 1974 s conclusions

air density effects on aeolian sand movement implications for sediment transport and sand control in regions with extreme altitudes or temperatures aeolian sand transport results from interactions between the ground surface and airflow previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity but the influence of air density which strongly constrains airflow characteristics and the resulting sand flow has not been widely considered in the present study entrainment saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from 154 m below sea level aiding lake to 5076 m above sea level tanggula mountain pass on the qinghai tibetan plateau at each site a portable wind tunnel and high speed camera system were set up and the friction wind velocity threshold friction velocity and sand flow structure were observed systematically for a given volumetric airflow lower air density increases the wind velocity low air density also creates a high threshold friction velocity the bagnold wind erosion threshold model remains valid but the value of empirical parameter a decreased with decreasing air density and ranged from 0 10 to 0 07 the smallest values reported in the literature for a given wind velocity increased altitude reduced total sand transport and creeping but the saltation rate and saltation height increased the present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude for example alpine deserts and low lying lake basins or on other planets including mars these results also provide theoretical support for improved sand control engineering measures the data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind blown sand

wind tunnel experiments on dune sands the effect of the dry bulk density time and the presence of roughness elements at the surface on the wind erodibility of a dry dune sand d50 230 250 m were evaluated in a laboratory study there is no significant correlation between the dry bulk density of the dune sand and the corresponding deflation threshold during the experiments a variation of the fluid threshold in time was noticed it increased to a value 11 per cent higher than the lowest threshold velocity selective eroding of the wind cannot be responsible for this time effect for threshold velocities do not work selectively on non uniform sands it can be explained by thinking of a dynamic surface which evolves towards a configuration of lower surface roughness the size of roughness factors e g gravel marbles placed on a sand surface and the density of the cover seem to be important factors influencing the erodibility of the underlying sand surface small cover densities are related to values of the deflation threshold which are below the fluid threshold of the sand surface furthermore dense covers protect the ground against wind erosion by increasing the threshold the size of the roughness factors determines the inversion point at which a cover becomes protective

variation in bed level shear stress on surfaces sheltered by nonerodible roughness elements 1 direct bed level observations of surface shear stress pressure gradient variability turbulence intensity and fluid flow patterns were carried out in the vicinity of cylindrical roughness elements mounted in a boundary layer wind tunnel paired corkscrew vortices shed from each of the elements result in elevated shear stress and increased potential for the initiation of particle transport within the far wake while the size and shape of these trailing vortices change with the element spacing they persist even for large roughness densities wake interference coincides with the impingement of the upwind horseshoe vortices upon one another at a point when their diameter approaches half the distance between the roughness elements while the erosive capability of the horseshoe vortex has been suggested for a variety of settings the present study shows that the fluid stress immediately beneath this coherent structure is actually small in comparison to that caused by compression of the incident flow as it is deflected around the element and attached vortex observations such as these are required for further refinement of models of stress partitioning on rough surfaces

relations between aerodynamic and surface roughness in a hyper arid cold desert mcmurdo dry valleys antarctica studies of boundary layer winds and surface roughness characteristics of unvegetated rocky desert surfaces provide new insights into the relations between aerodynamic roughness and surface roughness as described by the roughness density the description of non erodible roughness by the roughness density is shown to be valid for rocky surfaces as well as sparsely vegetated surfaces and simulated surfaces in a wind tunnel there is a good relation r 2 0 71 between aerodynamic roughness and roughness density allowing use of the threshold friction velocity ratio to predict sediment transport rates on these surfaces copyright 2004 john wiley sons ltd

comparison of aerodynamically and model derived roughness lengths zo over diverse surfaces central mojave desert california usa abstract the vulnerability of dryland surfaces to wind erosion depends importantly on the absence or the presence and character of surface roughness elements such as plants clasts and topographic irregularities that diminish wind speed near the surface a model for the friction velocity ratio has been developed to account for wind sheltering by many different types of co existing roughness elements such conditions typify a monitored area in the central mojave desert california that experiences frequent sand movement and dust emission two additional models are used to convert the friction velocity ratio to the surface roughness length zo for momentum to calculate roughness lengths from these models measurements were made at 11 sites within the monitored area to characterize the surface roughness element measurements included 1 the number of roughness species e g plants small scale topography clasts and their associated heights and widths 2 spacing among species and 3 vegetation porosity a measurement of the spatial distribution of woody elements of a plant documented or estimated values of drag coefficients for different species were included in the modeling at these sites wind speed profiles were measured during periods of neutral atmospheric stability using three 9 m towers with three or four calibrated anemometers on each modeled roughness lengths show a close correspondence correlation coefficient 0 84 0 86 to the aerodynamically determined values at the field sites the geometric properties of the roughness elements in the model are amenable to measurement at much higher temporal and spatial resolutions using remote sensing techniques than can be accomplished through laborious ground based methods a remote sensing approach to acquire values of the modeled roughness length is particularly important for the development of linked surface atmosphere wind erosion models sensitive to climate variability and land use changes in areas such as the southwestern united states where surface roughness has large spatial and temporal variations

wind tunnel modelling of the influence of vegetation structure on saltation threshold the influence of vegetation structure on saltation threshold was investigated using uniformly spaced arrays of non erodible roughness elements on a bed of erodible sand in an wind tunnel structural variables tested using arrays of solid cylinders included element aspect ratio height diameter and lateral cover total frontal silhouette area per unit ground area in agreement with previous studies increase in saltation threshold above the value for bare sand was strongly related to lateral cover increasing aspect ratio from 0 25 to 4 tended to enhance the increase in saltation threshold at a given lateral cover an approximate fit to the results could be obtained using a relation proposed by raupach et al journal of geophysical research 1993 98d 3023 3029 porous elements were constructed as clusters of narrow vertically oriented cylinders model stems forming porous model plant bodies that were cylindrical in overall shape low porosity elements were found to be approximately 50 per cent more effective in increasing saltation threshold than either solid or high porosity elements the ratio of plant body frontal silhouette area based on overall dimensions to total stem frontal silhouette area was found to be a useful measure of plant body porosity for wind erosion studies some conventional measures of vegetation amount fail to account for changes in vegetation structural attributes that may strongly influence aeolian processes

the spatial and temporal variability of sand erosion across a stabilizing coastal dune field this study aimed at quantifying the temporal and spatial variability in sand erosion and deposition over a coastal dune field in israel these were measured monthly over 2 years using 315 erosion pins over four transects that were placed perpendicular to the coastline vegetation cover was estimated based on aerial photographs and landsat satellite images whereas the relative height was based on a digital elevation model these variables were calculated for the area upwind south west of the erosion pins at various lengths ranging from 15 to 400 m nine geomorphologic units were defined five related to active units and four to stabilized units in active units at least 65 of the temporal variance in the annual absolute changes in sand level was explained by the index of resultant drift potential with most of the sand movement occurring during winter storms local rainfall had no apparent impact on sand mobility due to the low coincidence of sand carrying winds and rainfall in israel during the passage of frontal cyclones as for the spatial variables only a weak correlation was found between sand mobility with the distance from the coastline r 2 18 rather sand erosion and deposition were influenced by vegetation cover and the relative height of an area of 100 200 m upwind the values of soil adjusted vegetation index were significantly negatively correlated with annual absolute changes r 2 40 whereas the relative height was significantly positively correlated r 2 36 applying a multiple regression model 68 of the spatial variability in sand mobility was explained the resulting map of sand activity clearly shows that at this stage of the stabilization process most of the dunes are now disconnected and movement of sand grains from the beach or between the dunes is very limited these methods can be applied into spatial and temporal models of sand mobility thus assessing the impact of different management practices on coastal dunes

environmental and climatic changes during the pleistocene holocene in the bor plain central anatolia turkey abstract the bor plain lies in a round shaped flat basin in the southern part of the central anatolia it is located at the southeastern end of the tuzgolu fault zone which is the nw trending active normal fault zone and extends between the north of the tuzgolu lake tuz at nw and at kemerhisar nigde se according to geomorphologic and geologic studies this plain was occupied by lake during the pleistocene the first high lake level terrace and the latest high lake level terrace were determined at 1170 1150 m and 1110 1100 m respectively pleistocene and holocene alluvial and lacustrine deposits are composed of claystone sandstone mudstone limestone and paleosol units these units contain mostly calcite and clay minerals palygorskite smectites illite and chlorite and lesser amounts of quartz phillipsite cristobalite feldspar plagioclase goethite and muscovite the results of this study are compared with the results of sedimentologic and dating obtained from konya eregli plain according to this comparison 1 the latest high lake terraces were dated 23 000 to 17 000 yr bp the high lake level was changed 2 sandy level with carbonate determined in stratigraphy indicates that a dry phase dominated after 17 000 yr bp and 3 black and brown muds indicate that the shallow freshwater lacustrine phases and sandy levels were formed between 12 500 and 1100 yr bp during late glacial stage paleosols muddy levels and shallow lake deposits were formed in the environment of the holocene during 6000 5000 yr bp carbonate rich sandstone and mudstone deposits show that this period was interrupted by the second drought in the latest stage temperatures have increased due to hot climate conditions and salinization has begun and been still ongoing

the initiation of particle movement by wind when air blows across the surface of dry loose sand a critical shear velocity fluid threshold ut must be achieved to initiate motion however since most natural sediments consist of a range of grain sizes fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size shape sorting and packing of the surface sediment in order to investigate the initiation of particle movement by wind a series of wind tunnel tests was carried out on a range of pre screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics a sensitive laser monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep rolling or in saltation bouncing or hopping downwind as velocity continues to rise larger or less exposed grains may also be moved by fluid drag on striking the surface saltating grains impart momentum to stationary grains this impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure as a result there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities the fluid threshold range and setting in motion a rapidly increasing number of grains results of the tests showed that the progression from fluid to dynamic threshold based on grain movement can be characterized by a power function the coefficients of which are directly related to the mean size and sorting characteristics of the sediment

foredunes and blowouts initiation geomorphology and dynamics this paper reviews the initiation dynamics geomorphology and evolution of incipient foredunes established foredunes and blowouts incipient foredunes may be initiated in a variety of ways leading to the formation of ramps terraces or ridges depending on progradation rates vegetation type and cover sediment transport rates and scale of erosional processes vegetation cover morphology growth rates and types combine with aerodynamic processes to determine dune and swale morphology established foredunes are classified into five morpho ecological types and the flow behaviour over two types is detailed a new model of foredune evolutionary paths at various possible time scales is presented blowout initiation dynamics and sediment transport and evolution is examined throughout the review the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided

wind tunnel simulation of the three dimensional airflow patterns behind cuboid obstacles at different angles of wind incidence and their significance for the formation of sand shadows sand shadows commonly form downwind of obstacles in arid and semi arid regions understanding their dynamics requires insights into the airflow patterns behind the obstacles under different wind regimes here we studied models of cuboid obstacles to characterize the three dimensional responses of airflow behind obstacles with different shape ratios to variations in the incident flow in a wind tunnel simulation wind velocity was measured using particle image velocimetry piv the flow patterns behind cuboid obstacles were complicated by changes in the incidence angle of the approaching flow and in the obstacle s shape ratio the flows separated both horizontally and vertically creating reverse flow cells downwind of the obstacles the horizontal cells were characterized by two asymmetrical opposing reverse vortices at flow incidence angles greater than 60 to 65 whereas the vertical cells formed a single vortex at all angles the cells shifted position in response to changes in the incidence angle and shape ratio reflecting the interaction between outer and reverse flows the parameters of the horizontal and vertical reverse cells depended on the shape ratios and incidence angles but not on the wind velocity the sizes of the horizontal and vertical reverse cells were correlated reflecting their interactions the low velocity shadow behind the obstacle caused sand deposition and determined the sand shadow s evolution field experiments will be required to relate these airflow patterns to sand shadow development

influence of reed stem density on foredune development vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition and therefore on profile development in a long term monitoring field experiment three plots were planted with regular grids of reed bundles in three different densities 4 2 and 1 bundles per m2 this study reports on the differences in profile development under the range of vegetation densities topographic profiles were measured between may 1996 and april 1997 results indicate important differences in profile development for the three reed bundle densities in the highest density plot a distinct steep dune developed while in the lowest density a more gradual and smooth sand ramp was deposited when the stems had been completely buried differences in profile evolution vanished after a second planting of reed stems in january 1997 the process was repeated in may 1997 all plots had gained a sand volume ranging from 11 5 to 12 3 m3 m 1 indicating that the sediment budget is relatively constant regardless of the particular profile evolution the field evidence is compared with simulations of profile development generated by the foredune development model safe the model successfully reproduces the overall profile development but in general the equations used for vegetation transport interaction overestimate the effect of vegetation this causes some deviations between field and model results several reasons for this are discussed based on the experiments reported here recommendations are given for further research copyright 2001 john wiley sons ltd

a fully predictive model for aeolian sand transport abstract this paper presents the development calibration and verification of a fully predictive model for wind blown dry sand the model is based on the modification of the well known bagnold equation by including the threshold shear velocity which is also slightly modified to ensure a smoother transition from the non transport regime to the transport regime the sand transport rate is related to the static and dynamic grain roughness by using a semi empirical roughness predictor which is calibrated based on sand transport data from wind tunnel experiments with a flat mobile sand surface the model input for dry sand is the wind velocity at one height above the surface and the sand characteristics d50 d90 the effects of moisture vegetation and shells on the sand transport process are included by fairly simple expressions which are acting on the shear velocity the threshold value and the transport rate in all 105 high quality data sets have been used for verification of the proposed predictive model about 72 of the predicted values are within a factor of 2 of measured values of the transport rate

organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source we compared the burial efficiency of organic carbon buried oc deposited oc in a diverse set of 27 different sediments from 11 lakes focusing on the potential effects of organic matter source oxygen exposure and protective sorption of oc onto mineral surfaces average oc burial efficiency was high mean 48 and it was particularly high in sediments receiving high input of allochthonous organic matter mean 67 further oc burial efficiency was strongly negatively related to the oxygen exposure time again particularly so in sediments receiving high allochthonous loads on the other hand oc burial efficiency was not related to the mineral surface area which was used as a proxy of the sorption capacity of the mineral phase for oc the high oc burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes as well as to a strong dependence of oc burial efficiency on oxygen exposure time this study demonstrates that the carbon sink in lake sediments alters the oc export from the continents to the sea and that the fate of oc in lake sediments burial vs mineralization to carbon dioxide and methane is highly sensitive to environmental conditions

estimating irreducible water saturation and permeability of sandstones from nuclear magnetic resonance measurements by fractal analysis abstract understanding the relationships between the pore structure and bulk properties such as irreducible water saturation swir and permeability k of rocks is a crucial problem in geoscience in this study two types of fractal dimension were determined from nuclear magnetic resonance nmr t2 relaxation time distribution and models for estimating swir and k using fractal theory were proposed in view of the data of sandstone samples from upper paleozoic formation in china we found a good agreement between the predicted swir values based on fractal analysis and the experimental measurements and the estimated k values based on the proposed model are within one order of magnitude of measured k additionally another four frequently used k estimation models were compared with our proposed model and the result support that the proposed approach has great potential in investigating k while certain limitation still exists this procedure was applied on another set of sandstone samples collected from a different location and the predicted permeability values are reliable our study suggests that the proposed swir estimation model using nmr t2 data and fractal dimensions provides useful insights into characterizing the pore space properties of sedimentary rocks our work demonstrates that we can use nmr data to infer swir directly and the estimated swir value can be well integrated in permeability prediction model which improves the applications of nmr in characterizing the properties related to rock pores

uncoupled organic matter burial and quality in boreal lake sediments over the holocene boreal lake sediments are important sites of organic carbon oc storage which have accumulated substantial amounts of oc over the holocene epoch the temporal evolution and the strength of this holocene carbon c sink is however not well constrained in this study we investigated the temporal record of carbon mass accumulation rates cmars and assessed qualitative changes of terrestrially derived oc in the sediment profiles of seven swedish boreal lakes in order to evaluate the variability of boreal lake sediments as a c sink over time cmars were resolved on a short term centennial and long term i e over millennia of the holocene timescale using radioactive lead 210pb and carbon 14c isotope dating sources and degradation state of terrestrially derived oc were identified and characterized by molecular analyses of lignin phenols we found that cmars varied substantially on both short term and long term scales and that the variability was mostly attributed to sedimentation rates and uncoupled from the oc content in the sediment profiles the lignin phenol analyses revealed that woody material from gymnosperms was a dominant and constant oc source to the sediments over the holocene furthermore lignin based degradation indices such as acid to aldehyde ratios indicated that postdepositional degradation in the sediments was very limited on longer timescales implying that terrestrial oc is stabilized in the sediments on a permanent basis

linking organic carbon sedimentation burial efficiency and long term accumulation in boreal lakes carbon c storage in lakes is now recognized as a significant sink of c at a global scale but the pathways that lead to this storage remain poorly understood in this study we attempt to reconstruct and connect the processes that lead to long term c accumulation in boreal lakes these include the rate of particulate organic c poc sedimentation in the water column and sediment metabolism operating at a temporal scale of weeks to months organic c accumulation in the top sediment layers integrated over scales of tens of years and long term organic c burial in lake sediment integrated over hundreds to thousands of years the sinking poc flux was tenfold higher than the short term sediment c accumulation rates in all systems and we found no direct relationship between this downward c flux and either the short term or long term c accumulation rates however the resulting c burial efficiency which ranged from 5 to 62 was strongly related to lake shape which ultimately constrains the time freshly deposited material that is exposed to oxygen and thereby regulates the fraction of the carbon sinking flux that is mineralized back to the atmosphere or permanently buried in the sediments small and deep lakes act as more efficient c sinks than large and flat lakes we also show that long term burial rates are nearly identical to current centennial scale accumulation rates and that therefore little degradation occurs after a few decades sediment c storage tends to be small 5 relative to lake c emissions but that this balance is also strongly related to lake morphometry

backshore coarsening processes triggered by wave induced sand transport the critical role of storm events spatial backshore processes were investigated through field observations of topography and median sand grain size at a sandy beach facing the pacific ocean in japan a comparison of the backshore profile and cross shore distribution of the median sand grain size in 1999 and 2004 revealed an unusual sedimentary process in which sand was coarsened in a depositional area in the 5 year period although sediment is generally coarsened in erosional areas in support of these observations monthly spatial field analyses carried out in 2004 demonstrated a remarkable backshore coarsening process triggered by sedimentation in the seaward part of the backshore during a storm event in order to elucidate mechanisms involved in the backshore coarsening process thresholds of movable sand grain size under wave and wind actions a uniform parameter for both these cases in the onshore and offshore directions were estimated using wave tide and wind data the cross shore distributions of the estimated thresholds provided reasonable values and demonstrated a coarsening mechanism involving the intermediate zone around the shoreline under alternating wave and wind actions as a result of which coarse sand was transported toward the seaward part of the backshore by large waves during storms and then toward the landward part by strong onshore winds the 5 year backshore coarsening is most certainly explained by repetition of short term coarsening mechanisms caused by wave induced sand transport occurring from the nearshore to the intermediate zone copyright 2010 john wiley sons ltd

aeolian sand transport a wind tunnel model wind sand transport is an important geological process on earth and some other planets formulating the wind sand transport model has been of continuing significance majority of the existing models relate sand transport rate to the wind shear velocity based on dynamic analysis however the wind shear velocity readapted to blown sand is difficult to determine from the measured wind profiles when sand movement occurs especially at high wind velocity moreover the effect of grain size on sand transport is open to argument detailed wind tunnel tests were carried out with respect to the threshold velocity threshold shear velocity and transport rate of differently sized loose dry sand at different wind velocities to reformulate the transport model the results suggest that the relationship between threshold shear velocity and grain size basically follow the bagnold type equation for the grain size d 0 1 mm however the threshold coefficient a in the equation is not constant as suggested by bagnold but decreases with the particle reynolds number the threshold velocity at the centerline height of the wind tunnel proved to be directly proportional to the square root of grain diameter attempts have been made to relate sand transport rate to both the wind velocity and shear velocity readapted to the blown sand movement the reformulated transport model for loose dry sand follows the modified o brien rindlaub type equation q f1 d 1 ru 2 g v3 or the modified bagnold type equation q f2 d 1 rt 0 25 g u 3 where q is the sand transport rate the sand flux per unit time and per unit width in kg m 1 s 1 is the air density 1 25 kg m 3 g is the acceleration due to gravity 9 81 m s 2 ru vt v rt u t u v is the wind velocity at the centerline of the wind tunnel in m s 1 vt is the threshold velocity measured at the same height as v in m s 1 u is the shear velocity with saltating flux in m s 1 u t is threshold shear velocity in m s 1 f1 d 1 475 24 93 62d d f2 d 1 41 4 98exp 0 5 ln d 1 55d 0 57 2 d is the grain diameter in mm and d is the reference grain diameter equals 0 25 mm the bagnold s equation that asserts for a given wind drag the rate of movement of a fine sand is less than that of a coarse sand is not supported by the reformulated models

a spatio temporal reconstruction of holocene temperature change in southern scandinavia holocene mean july tjul and mean january tjan temperatures were reconstructed at lake trehorningen in southwest sweden using pollen climate calibration functions and converted into anomalies re

shear stress partitioning in sparsely vegetated desert canopies shear velocity ratios were derived for six neighbouring localities representing rangeland abandoned farmland and a playa in the semi arid region of the sonoran desert near surface and above canopy wind speeds mostly below saltation threshold were used to evaluate the partitioning of shear velocities and shear stresses between the plant canopy and the surface the technique utilized represents an alternative to the threshold shear velocity ratio rt approach of musick and gillette land degradation and rehabilitation 1990 2 87 94 and results demonstrate the need for partitioning over a range of wind speeds shear velocity ratios were variable over the playa and abandoned farmland while variance in near surface wind speeds was highest over shrub dominated sites the relationship between lateral cover lc and shear velocity r is comparable to previous findings utilizing rt although methodological differences result in mean shear velocity ratios less than shear velocity ratios derived for threshold conditions with respect to the model of raupach et al journal of geophysical research 1993 98 d2 3023 3029 this may be attributed to the difference between measured shear stresses and the average shear stress on the exposed surface at high lateral covers shear velocity ratios are lower than threshold shear velocity ratios due to increased wake interactions or decreased drag on the shrub elements at higher wind speeds

drag and shear stress partitioning in sparse desert creosote communities most research to characterize the wind erosion susceptibility and the degree of surface roughness required to suppress erosion on erodible surfaces has been empirical however a recently proposed shear velocity ratio model attempts to place shear stress partitioning in an entirely theoretical framework the purpose of this study was to directly measure components of shear stress in a sparsely vegetated environment in order to evaluate the model for the field study new instrumentation was developed to measure drag on a creosote shrub and irwin sensors were modified to measure surface shear stress in the field simultaneous measurements of total shear stress and surface shear stress were taken at four sites of different roughness densities in the eldorado valley nevada results indicate that porous shrubs had greater drag coefficients cd 0 485 than did solid elements sphere cd 0 3 and are more effective at protecting a surface values of the ratio of element to surface drag coefficients we

wind initiation thresholds of the moistened sands 1 the widely accepted bagnold type function for calculating threshold wind velocity or shear velocity was developed for dry sands but surface moisture is an extremely important variable controlling the entrainment process of sands by wind because the tensile force between the water molecules and sand grains produces cohesion here we report detailed wind tunnel experimental results on the threshold shear velocity of moistened sand of different sizes the results show that relative threshold shear velocity which is the ratio of threshold shear velocity of sand in the moistened state to that in the dry state is better related to moisture content than threshold shear velocity itself function modified after the bagnold equation has been developed to estimate the threshold shear velocities of moistened sands for a given grain size the threshold shear velocity is proportional to 1 km 1 2 where m is the moisture content and k is a coefficient depending on grain size

molecular evidence for lignin degradation in sulfate reducing mangrove sediments amazonia brazil abstract molecular lignin analyses have become a powerful quantitative approach for estimating flux and fate of vascular plant organic matter in coastal and marine environments the use of a specific molecular biomarker requires detailed knowledge of its decomposition rates relative to the associated organic matter and its structural diagenetic changes to gain insight into the poorly known processes of anaerobic lignin diagenesis molecular analyses were performed in the sulfate reducing sediment of a north brazilian mangrove organic matter in samples representing different diagenetic stages i e fresh litter a sediment core and percolating water was characterized by alkaline cuo oxidation for lignin composition element c n and stable carbon isotope analyses on the basis of these results and on a balance model long term in situ decomposition rates of lignin in sulfate reducing sediments were estimated for the first time the half life t 1 2 of lignin derived from mangrove leaf litter mainly rhizophora mangle was 150 yr in the upper 1 5 m of the sediment associated organic carbon from leaf tissue was depleted to 75 within weeks followed by a slow mineralization in the sediment t 1 2 300 yr unlike the known pathways of lignin diagenesis even highly degraded lignin did not show any alterations of the propyl or methoxyl side chains as evident from stable acid to aldehyde ratios and the proportion of methoxylated phenols vanillyl and syringyl phenols aromatic ring cleavage is probably the principal mechanism for lignin decay in the studied environment cinnamyl phenols were highly abundant in mangrove leaves and were rapidly depleted during early diagenesis thus the cinnamyl to vanillyl ratio could be used as a tracer for early diagenesis even under the sulfate reducing conditions syringyl phenols were removed from dissolved organic matter in interstitial water probably by sorption onto the sediment suspended organic matter in a mangrove creek showed a different lignin signature than its source namely sedimentary organic matter or mangrove litter with clear evidence for propyl side chain oxidation this was probably attributable to erosion of aerated thin sediment surface layers during mangrove inundation although particulate and dissolved organic matter in the mangrove creek have a common source their compositional patterns were different because of different pathways of release degradation and transport to the creek

estimating total horizontal aeolian flux within shrub invaded groundwater dependent meadows using empirical and mechanistic models 1 wind erosion is a significant environmental problem that removes soil resources from sensitive ecosystems and contributes to air pollution in regions of shallow groundwater friable puffy soils are maintained through capillary action surface evaporation of solute rich soil moisture and protection from mobilization by groundwater dependent grasses and shrubs when a reduction in vegetation cover occurs through any disturbance process there is potential for aeolian transport and dust emission we find that as mean gap size between vegetation elements scaled by vegetation height increases total horizontal aeolian sediment flux increases and explains 58 of the variation in total horizontal aeolian sediment flux we also test a probabilistic model of wind erosion based on gap size between vegetation elements scaled by vegetation height the okin model which predicts measured total horizontal aeolian sediment flux more closely than another commonly used model based on the average plant area observed in profile raupach model the threshold shear velocity of bare soil appears to increase as gap size between vegetation elements scaled by vegetation height increases reflecting either surface armoring or reduced interaction between the groundwater capillary zone and surface sediments this work advances understanding of the importance of measuring gap size between vegetation elements scaled by vegetation height for empirically estimating q and for structuring process based models of desert wind erosion in groundwater dependent vegetation

late quaternary aeolian dunes on the presently humid blue mountains eastern australia abstract sand dunes on the newnes plateau 1000 m a s l west of sydney were active during the last glacial maximum the scattered sand dunes are forested under the modern humid temperate climate regime dune types range from parabolic to transverse lee dunes and sand sheets or patches all point to the presence of conditions marginal for aeolian activity made possible through wind acceleration on windward slopes ready sand supply from the weathered sandstone of the plateau and sparse vegetation cover modern climate envelopes of sand dune activity in australia predict that unrealistically drier conditions are necessary to allow wind transport at this site only additional impediments to plant growth such as lower temperature and lower atmospheric carbon dioxide concentrations appear to allow the necessary conditions for dune formation these observations and conclusions extend our understanding of the extremes of the lgm climate in humid eastern australia confirming that the widespread treeless vegetation was also sparse even in areas that today have annual rainfall above 1000 mm

wind tunnel simulation of the three dimensional airflow patterns around shrubs 1 dunes formed around vegetation such as shrubs are common in arid and semiarid regions and some coastal regions understanding their dynamics requires insights into the airflow patterns around shrubs thus we analyzed the horizontal and vertical flow patterns around model shrubs using wind measurements obtained by particle image velocimetry in a scaled wind tunnel simulation flow patterns around shrubs were complicated by the presence of bleed flows and displaced flows below a critical shrub density between 0 05 and 0 08 bleed flow is dominant the flows separate in both horizontal and vertical planes when the shrub density equals or exceeds this critical density resulting in reverse cells in the lee of shrubs the horizontal reverse cells are characterized by two symmetrical opposing reverse eddies whereas the vertical reverse cells form a single eddy the positions of the reverse cells shift in response to changes in shrub density reflecting the interaction between bleed flow and reverse flow the parameters of horizontal reverse cells are functions of shrub density and wind velocity parameters of vertical reverse cells change mainly with respect to shrub density and vary only slightly with wind velocity the lengths of the horizontal and vertical reverse cells are correlated reflecting interactions between them field experiments on dune morphology will be required to relate these airflow patterns to dune development

integrated transfers of terrigenous organic matter to lakes at their watershed level a combined biomarker and gis analysis terrigenous organic matter tom transfer from a watershed to a lake plays a key role in contaminants fate and greenhouse gazes emission in these aquatic ecosystems in this study we linked physiographic and vegetation characteristics of a watershed with tom nature deposited in lake sediments tom was characterized using lignin biomarkers as indicators of tom sources and state of degradation geographical information system gis also allowed us to integrate and describe the landscape morpho edaphic characteristics of a defined drainage basin combining these tools we found a significant and positive relationship r 2 0 65 p 0 002 between mean slope of the watershed and the terrigenous fraction estimated by k8 in recent sediments the mean slope also correlated with the composition of tom in recent sediments as p v s and 3 5bd v ratios significantly decreased with the steepness of the watersheds r 2 0 57 p 0 021 and r 2 0 71 p 0 004 respectively more precisely areas with slopes comprised between 4 and 10 have a major influence on tom inputs to lakes the vegetation composition of each watershed influenced the composition of recent sediments of the sampled lakes the increasing presence of angiosperm trees in the watershed influenced the export of tom to the lake as k8 increased significantly with the presence of this type of vegetation r 2 0 44 p 0 019 a similar relationship was also observed with s v ratios an indicator of angiosperm sources for tom the type of vegetation also greatly influenced the degradation state of om in this study we were able to determine that low sloped areas 0 2 act as buffer zones for lignin inputs and by extension for tom loading to sediments the relative contribution of tom from the soil organic horizons also increased in steeper watersheds this study has significant implications in our understanding of the fate of tom in lacustrine ecosystems

aeolian shear stress ratio measurements within mesquite dominated landscapes of the chihuahuan desert new mexico usa a field study was conducted to ascertain the amount of protection that mesquite dominated communities provide to the surface from wind erosion the dynamics of the locally accelerated evolution of a mesquite coppice dune landscape and the undetermined spatial dependence of potential erosion by wind from a shear stress partition model were investigated sediment transport and dust emission processes are governed by the amount of protection that can be provided by roughness elements although shear stress partition models exist that can describe this their accuracy has only been tested against a limited dataset because instrumentation has previously been unable to provide the necessary measurements this study combines the use of meteorological towers and surface shear stress measurements with irwin sensors to measure the partition of shear stress in situ the surface shear stress within preferentially aligned vegetation within coppice dune development exhibited highly skewed distributions while a more homogenous surface stress was recorded at a site with less developed coppice dunes above the vegetation the logarithmic velocity profile deduced roughness length based on 10 min averages exhibited a distinct correlation with compass direction for the site with vegetation preferentially aligned while the site with more homogenously distributed vegetation showed very little variation in the roughness length this distribution in roughness length within an area defines a distribution of a resolved shear stress partitioning model based on these measurements ultimately providing potential closure to a previously uncorrelated model parameter

low frequency and high frequency changes in temperature and effective humidity during the holocene in south central sweden implications for atmospheric and oceanic forcings of climate an integrated use of independent palaeoclimatological proxy techniques that reflect different components of the climate system provides a potential key for functional analysis of past climate changes here we report a 10 000 year quantitative record of annual mean temperature tann based on pollen climate transfer functions and pollen stratigraphical data from lake flarken south central sweden the pollen based temperature reconstruction is compared with a reconstruction of effective humidity as reflected by a 18o record obtained on stratigraphy of lacustrine carbonates from lake igelsjon c 10 km from lake flarken which gives evidence of pronounced changes in effective humidity the relatively low tann and high effective humidity as reflected by a low evaporation inflow ratio suggest a maritime early holocene climate 10 000 8 300 cal year bp seemingly incompatible with the highly seasonal solar insolation configuration we argue that the maritime climate was due to the stronger than present zonal flow enhanced by the high early holocene sea surface temperatures in the north atlantic the maritime climate mode was disrupted by the abrupt cold event at 8 200 cal year bp followed at 8 000 cal year bp by a stable holocene thermal maximum the latter was characterized by tann values about 2 5 c higher than at present and markedly dry conditions indicative of stable summer time anti cyclonic circulation possibly corresponding with modern blocking anticyclonic conditions the last 4 300 year period is characterized by an increasingly cold moist and unstable climate the results demonstrate the value of combining two independent palaeoclimatic proxies in enhancing the reliability generality and interpretability of the palaeoclimatic results further methodological refinements especially in resolving past seasonal climatic contrasts are needed to better understand the role of different forcing factors in driving millennial scale climate dynamics

multi proxy records of quaternary fluvio lacustrine sediments around lakes eymir and mogan ankara central anatolia turkey this paper presents the mineralogical geochemical palynological and stable isotopic compositions of the late pleistocene holocene fluvio lacustrine sediments around lakes eymir and mogan ankara central anatolia it is based on the interpretation of the multi proxy records in conjunction with the geochronological data in eight different sections the comparison between the increase decrease in the abundances of carbonates and total clay assemblages is correlated with the changing abundance of coniferous forests and herbaceous plants mainly asteraceae and chenopodiaceae 13c and 18o values and molecular weathering ratios of the sediments revealing the hydrolysis evaporation and leaching together with chemical index of alteration cia provide insight into the climatic changes in the study area between 11 899 and 1428 cal year bp relatively higher total clay amount 18o of around 8 and higher cia 40 with higher hydrolysis suggest humid conditions during 11 899 6448 cal year bp between 6448 and 5763 cal year bp sediments show oscillations between high and relatively low values of the proxy data implying intercalation of dry and wet seasons the last period recorded between 5763 and 1428 cal year bp is realized with high calcite precipitation low total clay and low cia and relatively higher salinization which directly characterize aridity

influence of the gap ratio on variations in the surface shear stress and on sand accumulation in the lee of two side by side obstacles changes in the surface shear stress in the lee of two side by side obstacles result in sand deposition and the development of aeolian sand drifts through direct wind tunnel measurements using irwin type sensors surface shear stress behind two side by side obstacles separated by gaps with different gap ratios the ratio of the gap spacing between the obstacles to the center to center distance for the obstacles was measured particular attention to the effects of the spacing between the two obstacles on the initiation of sand drifts has been paid the results showed that the gap ratio was the key factor that determined the shear stress patterns wind speed increased through the gap but decreased a short distance downwind causing higher peak skin friction velocities and higher shear stress immediately behind the obstacles producing a range of sand deposition patterns in the sheltered lee side of the obstacles comparison of plots of surface shear stress and isoline plots of for the fraction of the total time in which a given threshold skin friction velocity was exceeded with the sand deposition patterns confirmed that sand drift may form in the lee of a gap between two adjacent obstacles when the gap ratio g 0 44 and it may disappeared when the gap ratio larger than this value

lignin biomarkers and pollen in postglacial sediments of an alaskan lake abstract we analyzed a 12 000 yr sediment core from wien lake central alaska for a suite of phenolic products from cuo oxidation of lignin polymers and compared their composition with pollen data from the same core to assess lignin phenols as sedimentary biomarkers inferences of the gross taxonomic origin of sediment organic matter from lignin phenol composition agree with vegetational reconstructions based on pollen assemblages in particular the ratios of syringyl to vanillyl phenols are consistently higher before 6500 yr bp when angiosperms dominated or codominated the regional vegetation than after 6500 yr bp when gymnosperms dominated however the ratios of cinnamyl p coumaric and ferulic acids to total vanillyl phenols c v do not show patterns expected from the abundance of woody plants c v ratios are particularly high 0 7 0 85 after 6500 yr bp when pollen spectra suggest closed boreal forests dominated by picea and the stratigraphic patterns are strikingly similar between c v and picea pollen concentrations cuo oxidation of modern pollen of p glauca and p mariana yields exceptionally high amounts of cinnamyl phenols 8 90 and 6 41 mg 100 mg oc for p glauca and p mariana respectively in particular p coumaric acid is obtained in large amounts 8 87 and 6 41 mg 100 mg oc for p glauca and p mariana respectively versus vanillyl phenols 0 25 and 0 49 mg 100 mg oc for p glauca and p mariana respectively and ferulic acid 0 03 and 0 00 mg 100 mg oc for p glauca and p mariana respectively thus lignin phenols derived from fossil picea pollen preserved in sediments likely drive the c v profile of the wien lake core these data imply that if picea pollen concentrations are sufficiently high the amount of nonwoody tissue in sediments may be grossly overestimated when the lignin composition of gymnosperm needles is used as the end member of nonwoody tissues given that pollen grains are among the most resistant components of sedimentary organic matter and that p coumaric acid is labile it is important to consider pollen as a nonwoody tissue type when lignin biomarkers are used to determine the sources of vascular plant material in sediments

the effect of vegetation patterns on wind blown mass transport at the regional scale a wind tunnel experiment abstract wind erosion is a global environmental problem re vegetating land is a commonly used method to reduce the negative effects of wind erosion however there is limited knowledge on the effect of vegetation pattern on wind blown mass transport the objective of this study was to investigate the effect of vegetation pattern on this phenomenon within a land unit and at the border between land units wind tunnel experiments were conducted with artificial shrubs representing atriplex halimus wind runs at a speed of 11 m s 1 were conducted and sand translocation was measured after 200 230 s using a graph paper prepared for this purpose this research showed that 1 the transport within a land unit is affected by the neighboring land units and by the vegetation pattern within both the unit itself and the neighboring land units 2 re vegetation plans for degraded land can take into account the streets effect zones of erosion areas similar to streets 3 the effect of neighboring land units includes sheltering effect and the regulation of sediment passing from one land unit to the neighboring land units and 4 in addition to investigation of the general effect of vegetation pattern on erosion and deposition within the region it is important to investigate the redistribution of sediment at smaller scales depending on the scope of the project

calibration and application of marine sedimentary physical properties using a multi sensor core logger abstract we examined geophysical data from a multi sensor core logger mscl a logging device providing continuous measurements of gamma ray attenuation p wave travel time and magnetic susceptibility on marine sediment cores in the first part we focused on the gamma ray system and compared two different calibration methods from the gamma ray attenuation we calculated densities and porosities by incorporating mass weighted attenuation coefficients the application of an iteration method reduces the error of the density and porosity estimates compared to grape data in addition we derived equations to calculate water content and dry bulk density from gamma ray attenuation measurements comparison with physical properties determined on discrete samples revealed a very good correlation of both data sets r 0 99 this correlation is valid for sediments from substantially different geological settings e g turbidites hemipelagic muds and opal rich sediments in the second part we applied our data to marine geological questions for sediments from the antarctic polar frontal zone there is indication that the content of biogenic opal can be assessed using a correlation of density and p wave velocity for sediments from the bengal fan the relationship between the mscl acoustic impedance the product of density and p wave velocity and the grain size distribution in discrete samples can be used to predict clay and sand silt ratios for sediment cores from the shelf and upper continental slope

effects of river regulation on aeolian landscapes colorado river southwestern usa 1 connectivity between fluvial and aeolian sedimentary systems plays an important role in the physical and biological environment of dryland regions this study examines the coupling between fluvial sand deposits and aeolian dune fields in bedrock canyons of the arid to semiarid colorado river corridor southwestern usa by quantifying significant differences between aeolian landscapes with and without modern fluvial sediment sources this work demonstrates for the first time that the flow and sediment limiting effects of dam operations affect sedimentary processes and ecosystems in aeolian landscapes above the fluvial high water line dune fields decoupled from fluvial sand supply have more ground cover biologic crust and vegetation and less aeolian sand transport than do dune fields that remain coupled to modern fluvial sand supply the proportion of active aeolian sand area also is substantially lower in a heavily regulated river reach marble grand canyon arizona than in a much less regulated reach with otherwise similar environmental conditions cataract canyon utah the interconnections shown here among river flow and sediment aeolian sand transport and biologic communities in aeolian dunes demonstrate a newly recognized means by which anthropogenic influence alters dryland environments because fluvial aeolian coupling is common globally it is likely that similar sediment transport connectivity and interaction with upland ecosystems are important in other dryland regions to a greater degree than has been recognized previously