late miocene environmental change in nepal and the northern indian subcontinent stable isotopic evidence from paleosols neogene sediments belonging to the siwalik group crop out in the himalayan foothills along the length of southern nepal carbon and oxygen isotopic analyses of siwalik paleosols from four long siwalik sections record major ecological changes over the past 11 m y the carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca 7 0 ma marking the displacement of largely c3 vegetation probably semi deciduous forest by c4 grasslands by the beginning of the pliocene all the flood plains of major rivers in this region were dominated by monsoonal grasslands the floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest miocene a similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of pakistan and in terrigenous organic matter from the bengal fan showing that the floral shift was probably continentwide the latest miocene also witnessed an average change of 4 in the oxygen isotopic composition of soil carbonate as observed previously in pakistan the reasons for this are unclear if not diagenetic a major environmental change is indicated perhaps related to that driving the carbon isotopic shift recently described pollen and leaf fossils from the surai khola section show that evergreen forest was gradually displaced by semi deciduous and dry deciduous forest between 11 and 6 ma the gradual nature of this floral shift which culminated in the rapid expansion of c4 grasses starting 7 0 m y ago is difficult to explain by a decrease in atmospheric pco2 alone cerling et al 1993 but fits well with a gradual onset of monsoonal conditions in the late miocene in the northern indian subcontinent himalayan uplift driving both monsoonal intensification and consumption of co2 through weathering may be the common cause behind major late miocene environmental change globally however the decline of effective moisture associated with monsoon development has probably slowed not increased the rate of consumption of co2 by chemical weathering of himalayan sediments

constant elevation of southern tibet over the past 15 million years the uplift of the tibetan plateau an area that is 2 000 km wide to an altitude of about 5 000 m has been shown to modify global climate and to influence monsoon intensity mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the tibetan plateau but the precise history of the uplift of the plateau has yet to be confirmed by observations here we present well preserved fossil leaf assemblages from the namling basin southern tibet dated to approximately 15 myr ago which allow us to reconstruct the temperatures within the basin at that time using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves we reconstruct the elevation of the namling basin 15 myr ago to be 4 689 895 m or 4 638 847 m depending on the reference data used this is comparable to the present day altitude of 4 600 m we conclude that the elevation of the southern tibetan plateau probably has remained unchanged for the past 15 myr

effects of aridity and vegetation on plant wax δd in modern lake sediments we analyzed the deuterium composition of individual plant waxes in lake sediments from 28 watersheds that span a range of precipitation d h vegetation types and climates the apparent isotopic fractionation ea between plant wax n alkanes and precipitation differs with watershed ecosystem type and structure and decreases with increasing regional aridity as measured by enrichment of 2 h and 18 o associated with evaporation of lake waters the most negative ea values represent signatures least affected by aridity these values were 125 5 for tropical evergreen and dry forests 130 for a temperate broadleaf

functional relationships of resistance in wide flood plains with rigid unsubmerged vegetation the effect of rigid unsubmerged vegetation on flow resistance in wide flood plains was examined in this study a series of experiments was run using rigid rods inserted into the flow field of a wide shallow flume several configurations of rod diameters with various lateral and longitudinal spacing were tested over a range of discharge values head losses across the rods were measured and a resistance to momentum absorbing area concept was employed to relate density parameters to the resistance coefficient the effects of flow depth velocity rod diameter d lateral spacing r and longitudinal spacing l were estimated the results demonstrated that flow resistance is greatly impacted by both depth and velocity but that the effects are opposite in sign the friction factor increased in only a slightly nonlinear fashion with depth but it decreased in a highly nonlinear manner with increasing velocity and the velocity effects were very sensitive to vegetation density parameters of the density pa

gusev crater wind related features and processes observed by the mars exploration rover spirit 1 wind related features observed by the rover spirit in gusev crater mars include patches of soil on the surface some of which are organized into bed forms windblown grains include dust inferred to be 2 mm in diameter microscopic imager data show the sands and granules to be rounded and relatively spherical typical of grains transported long distances by the wind the interior of bed forms exposed by rover operations suggests the infiltration of dust among the grains indicating that these sands are not currently experiencing saltation orientations of 1520 features such as bed forms and ventifacts along spirit s traverse from the landing site the columbia memorial station to west spur in the columbia hills suggest primary formative winds from the north northwest which correlate with measurements of features seen in orbiter images and is consistent with afternoon winds predicted by atmospheric models a secondary wind from the southeast is also suggested which correlates with predictions for nighttime early morning winds wind abrasion is indicated by ventifacts in the form of facets and grooves cut into rocks the orientations of which also indicate prevailing winds from the north northwest orientations of many aeolian features in the west spur area however have more scatter than elsewhere along the traverse which is attributed to the influence of local topography on the patterns of wind active dust devils observed on the floor of gusev from the columbia hills demonstrate that dust is currently mobile sequential images of some dust devils show movement as rapid as 3 8 m s consistent with wind velocities predicted by atmospheric models for the afternoon when most of the dust devils were observed sands accumulated on the rover deck in the same period suggest that some sands in the columbia hills experience active saltation two toned rocks having a light band coating at their bases are considered to represent partial burial by soils and subsequent exposure while perched rocks could represent materials lowered onto other rocks by deflation of supporting soils measurements of the heights of the light bands and the perched rocks range from 1 cm to 27 cm indicating local deflation by as much as 27 cm

kinematics and tectonothermal modeling interpretation of heat flow observed on the tibetan plateau abstract thirteen heat flow values along a n s profile are employed as surface constraints on a tectonothermal model of the tibetan plateau the modeling demonstrates 40 ma of crust mantle temperature history since the india eurasia collision the main emphasis in this paper is put on the fact that a series of heat generating and heat absorbing processes such as enhancement of radiogenic heating strain and friction heating partial melting etc were induced by tectonic deformations of the lithosphere on this basis equations defining terrane deformations in forms of shortening thickening extension uplift and erosion were deduced using the principles of plate kinematics and 3 d mass balance then these equations being coupled with each other are further used as systematically defined initial and boundary conditions for integrated tectonothermal simulation the modeling results suggest heterogeneity in lithospheric thermal structure and sharp differences in tectonothermal evolution between the old northern terranes and the new southern ones however even in the warm terranes in the south the scale of the tectonically induced heat generating processes is smaller than usually expected

flow velocity measurements in vegetated channels understanding of the hydraulics of flow over vegetation is very important to support the management of fluvial processes in this paper the flow over flexible bottom vegetation is experimentally studied a two dimensional acoustic doppler velocimeter is used to measure the local flow velocities for different vegetation concentrations discharges and flume slopes the influence of vegetation concentration and the depth vegetation height ratio on the measured velocity profiles is analyzed all measured velocity distributions are s shaped and exhibit a three zone profile the relationship between the velocity distribution and the turbulence intensity distribution is also analyzed the characteristics inflection point maximum value asymptotes of the measured velocity distributions and the results of the previous investigators are used to select an analytical expression for the shape of the velocity profile a theoretical velocity profile is deduced using the classical prandtl s mixing length approach with a new expression for the mixing length the deduced four coefficient profile allows description of the flow both inside and above the vegetation the physical and geometrical meaning of the four coefficients are also shown doi 10 1061 asce 0733 9429 2002 128 7 664 ce database keywords velocity profile vegetation channel flow

application of benthic foraminiferal mg ca ratios to questions of cenozoic climate change abstract we investigate the evolution of cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater 18osw derived from benthic foraminiferal mg ca ratios to constrain the temperature effect contained in foraminiferal 18o values we have constructed two benthic foraminiferal mg ca records from intermediate water depth sites ocean drilling program sites 757 and 689 from the subtropical indian ocean and the weddell sea respectively together with the previously published composite record of lear et al science 287 2002 269 272 and the neogene record from the southern ocean of billups and schrag paleoceanography 17 2002 10 1029 2000pa000567 we obtain three almost complete representations of the 18osw for the past 52 myr we discuss the sensitivity of early cenozoic mg ca derived paleotemperatures and hence the 18osw to assumptions about seawater mg ca ratios we find that during the middle eocene 49 40 ma modern seawater ratios yield mg ca derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice free conditions intermediate waters cooled during the middle eocene reaching minimum temperatures by 40 ma the corresponding 18osw reconstructions support ice growth on antarctica beginning by at least 40 ma at the eocene oligocene boundary mg ca ratios and hence temperatures from weddell sea site 689 display a well defined maximum we caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of mg in benthic foraminifera may be sensitive to factors other than temperature throughout the remainder of the cenozoic the temporal variability among 18osw records is similar and similar to longer term trends in the benthic foraminiferal 18o record an exception occurs during the pliocene when 18osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal 18o records which provides a new perspective to the ongoing debate about the stability of the antarctic ice sheet maximum 18osw values recorded during the pleistocene at southern ocean site 747 agree well with values derived from the geochemistry of pore waters schrag et al science 272 1996 1930 1932 further highlighting the value of the new mg ca calibrations of martin et al earth planet sci lett 198 2002 193 209 and lear et al geochim cosmochim acta 66 2002 3375 3387 applied in this study we conclude that the application of foraminiferal mg ca ratios allows a refined view of cenozoic ice volume history despite uncertainties related to the geochemical cycling of mg and ca on long time scales

geology of the gusev cratered plains from the spirit rover transverse 1 the cratered plains of gusev traversed by spirit are generally low relief rocky plains dominated by impact and eolian processes ubiquitous shallow soil filled circular depressions called hollows are modified impact craters rocks are dark fine grained basalts and the upper 10 m of the cratered plains appears to be an impact generated regolith developed over intact basalt flows systematic field observations across the cratered plains identified vesicular clasts and rare scoria similar to original lava flow tops consistent with an upper inflated surface of lava flows with adjacent collapse depressions crater and hollow morphometry are consistent with most being secondaries the size frequency distribution of rocks 0 1 m diameter generally follows exponential functions similar to other landing sites for total rock abundances of 5 35 systematic clast counts show that areas with higher rock abundance and more large rocks have higher thermal inertia plains with lower thermal inertia have fewer rocks and substantially more pebbles that are well sorted and evenly spaced similar to a desert pavement or lag eolian bed forms ripples and wind tails have coarse surface lags and many are dust covered and thus likely inactive deflation of the surface 5 25 cm likely exposed two toned rocks and elevated ventifacts and transported fines into craters creating the hollows this observed redistribution yields extremely slow average erosion rates of 0 03 nm yr and argues for very little long term net change of the surface and a dry and desiccating environment similar to today s since the hesperian or 3 ga

high times on the tibetan plateau paleoelevation of the thakkhola graben nepal east west extension in the tibetan plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift on the basis of this assumption several studies have dated east west extensional structures to determine when the plateau attained its current high elevation however independent estimates of elevation are needed to determine whether extension occurred before during or after the plateau achieved its current elevation because the isotopic composition of meteoric water decreases with increasing elevation significant change in local elevation throughout the thakkhola graben depositional history should be recorded by change in 18 o values of fluvial and lacustrine carbonates the 18 o values of 16 to 23 of thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern tibetan plateau attained its current elevation prior to eastwest extension initiation of thakkhola graben extension is constrained between 10 and 11 ma based on magnetostratigraphy of the older tetang formation the 13 c values of soil carbonates suggest an age younger than 8 ma for the base of the thakkhola formation

multiscalar model for the determination of spatially explicit riparian vegetation roughness 1 improved understanding of the connection between riparian vegetation and channel change requires evaluating how fine scale interactions among stems water and sediment affect larger scale flow and sediment transport fields we propose a spatially explicit model that resolves patch scale submeter patterns of hydraulic roughness over the reach scale caused by stands of shrubby riparian vegetation we worked in tamarisk dominated stands on the yampa and green rivers in dinosaur national monument northwestern colorado usa where questions remain regarding the role of vegetation in inducing or exacerbating documented channel changes hydraulic roughness patterns were derived from patch scale measurements made with detailed terrestrial laser scan tls data that were extrapolated to reach scales based on correlation with light detection and ranging lidar als data two dimensional patch scale hydraulic models were used to parameterize the stage dependence of hydraulic roughness of typical patch types i e sparse moderate and dense patches we illustrate the value of using this approach to characterize vegetation roughness by applying our results to a two dimensional hydraulic model of flow for one of our study sites results from this work predict that the roughness of vegetated floodplains increases with flow depth and is dependent on patch scale stem organization geomorphically relevant patterns i e areas of low or high shear stress that are likely to scour or fill during high flows become apparent with the detail introduced by spatially explicit depth dependent roughness to our knowledge the multiscalar analysis presented here is the first to mechanistically account for shrubby riparian vegetation stand structure and associated hydraulic roughness of vegetation patches at the reach scale

tertiary crustal shortening and peneplanation in the hoh xil region implications for the tectonic history of the northern tibetan plateau abstract the hoh xil basin is the largest cenozoic sedimentary basin in the hinterland of the tibetan plateau tertiary sedimentary strata 5 8 km thick comprising the fenghuoshan yaxicuo and wudaoliang groups provide compelling evidence concerning the crustal shortening erosion and peneplanation of the northern tibetan plateau the basal fenghuoshan and overlying yaxicuo groups span the eocene early oligocene stratigraphically and have been dated by magnetostratigraphy as 56 30 ma old both groups are composed of terrigenous rocks provenance analysis of sandstones and conglomerates demonstrates that permian and triassic strata in the tanggula orogenic zone in the south were the source for the fenghuoshan group in contrast the carboniferous triassic strata in the tanggula bairizhajia and heishishan gaoshan orogenic zones in the north were the source for the yaxicuo group during the late oligocene northern tibet underwent strong north south crustal shortening 43 and thickening extensive erosion which occurred over the entire plateau surface near the end of the oligocene resulted in development of a peneplain surface the latter is overlain by the early miocene wudaoliang group composed of fresh water limestones these are exposed both on summit surfaces as well as on the valley floors showing that a phase of differential uplift occurred after the deposition of the wudaoliang group this post miocene differential uplift was due to regional extension in a region of overall shortening even though we have not succeeded in obtaining conclusive data about the exact timing of phases of rapid uplift of the tibetan plateau it is most likely that the major phase of uplift occurred during the late oligocene

stable isotope based paleoaltimetry theory and validation paleoaltimetry is the quantitative estimate of the surface height above mean sea level of ancient landforms atmospheric thermodynamic modeling of the behavior of 18o relative to 16o during condensation from water vapor establish the systematic relationship that exists between 18o p and elevation where 18o p is the difference between a low altitude preferably sea level 18o p and a potentially high elevation sample comparison of model predictions with observations suggests that the model captures the first order behavior of 18o p during condensation and precipitation in orographic settings the actual relationship between 18o p and elevation depends sensitively on climate and specifically starting temperature and to a lesser degree relative humidity thermodynamic modeling allows the 18o p and elevation relationship to be explicitly calculated for any given starting climate state this makes the theoretical approach significantly more appropriate than empirically calibrated approaches based typically on quite limited samples presently available in most orographic settings today paleoaltimetry archives derive their isotopic compositions from surface and or ground water and hence it is important to understand the systematic differences between these reservoirs and precipitation surface waters and ground waters integrate not just the change in isotopic composition with altitude but also variations in hypsometry within the drainage basin and precipitation amount as functions of elevation thus these archives should reflect the precipitation amount weighted hypsometric mean elevation of the paleo drainage basin from which they derive their waters analysis of modern data from the himalayan region supports this expectation foreland basin rivers are such integrators and it is shown using an example from the siwaliks that the rivers draining the front of the himalayas in the past had isotopic compositions comparable with modern rivers draining the himalayan front suggesting little net change in himalayan hypsometry over the past 11 million years

nonrigid nonsubmerged vegetative roughness on floodplains individual pine and cedar tree saplings and branches were used to model the resistance to flow in a water flume for nonsubmerged and nonrigid vegetation to determine the amount that streamlining decreases the drag coefficient and reduces the momentum absorbing area currently vegetation on floodplains is commonly assumed to behave as rigid roughness that can lead to large errors in the relationships between velocity and drag force this presents a basic fluid mechanics problem an extreme variation of roughness with depth of flow can result due to a large increase in the momentum absorbing area in nonsubmerged vegetation as depth is increased this deems all the available roughness equations which generally are based on relative roughness approach useless for this application in this paper a dimensional analysis supported by experimental results is developed to obtain a relationship between roughness conditions i e density and flexural rigidity and flow conditions i e velocity and depth for floodplains and vegetative zones of natural waterways

channel hydrodynamics of submerged flexible vegetation with seepage abstractthe presence of aquatic vegetation changes the flow characteristics in a channel the impact of the presence of submerged flexible vegetation in a channel with an additional parameter such as downward seepage is analyzed in the present work two patterns staggered and uniform and two different vegetation spacing 15 and 10 cm center center were adopted experiments were conducted for the case of no seepage 10 seepage and 15 seepage velocity profiles show the presence of an inflection point near the top of the vegetation where maximum reynolds stress is achieved the velocity profiles measured in the unobstructed region of uniform pattern are higher compared to the velocity measured in line with the vegetation stems downward seepage increases the velocity reynolds stress and turbulent intensities in the average range of 10 15 as compared to the no seepage case higher order turbulent statistics such as moment analysis quadrant analysis and turbulent kinetic budget are derived through

hydrogen isotope ratios of recent lacustrine sedimentary n alkanes record modern climate variability abstract hydrogen isotope ratios were measured on n alkanes n c12 to n c31 extracted from recent lake surface sediments along a n s european transect to test if modern climate variability is recorded in these biomarkers d values of the n alkanes are compared to d values of meteoric water from the iaea gnip database spanning a range from 119 in northern sweden to 41 in southern italy to lake water d values and to mean annual temperatures varying between 2 0 c in the north and 13 7 c in the south d values of the short chained n alkanes n c12 to n c20 excluding algal derived n c17 and n c19 are higher in the north and lower in the south the isotopic fractionation e for hydrogen between meteoric water and the short chained n alkanes is increasing from n to s by more than 100 and is significantly correlated to mean annual temperature for n c16 and n c18 this suggests that these n alkanes may originate from a different source in the northern lakes possibly due to petroleum contamination or are synthesized using a different biochemical pathway the n c17 and n c19 alkanes of algal origin the n c21 and n c23 alkanes originating from water plants and the long chain n alkanes n c25 n c27 n c29 and n c31 of terrestrial origin clearly correlate with d values of meteoric water lake water and mean annual temperature indicating that they excellently record the d value of meteoric water the mean hydrogen isotope fractionation ec17 w of 157 sd 13 between n c17 and meteoric water is fairly constant over the wide range of different climates and lake environments suggesting only minor influence of environmental factors on this biochemical fractionation this suggests that d values of n c17 are suitable to reconstruct the isotopic composition of source water the mean fractionation between the long chain n alkanes and water is 128 sd 12 the mean difference of 31 between both e values is likely due to evaporative enrichment of deuterium in the leaf water if this is the only influence on the enrichment the difference between the d values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment

wind driven reorganization of coarse clasts on the surface of mars coarse pebble to cobble sized clasts on the intercrater plains of the mars exploration rover spirit landing site exhibit a nonrandom i e uniformly spaced distribution this pattern has been attributed to the entrainment and redistribution of coarse clasts during extreme wind events here we propose an alternative mechanism readily observable in wind tunnels and numerical models at modest wind speeds in this process coarse clasts modify the air fl ow around them causing erosion of the underlying substrate on the windward side and deposition on the leeward side until a threshold bed slope condition is reached after which the clast rolls into the windward trough clasts can migrate across an erodible substrate in repeated cycles of trough formation and clast rolling attracting or repelling one another through feedbacks between the local clast density substrate erosion and or deposition rate and substrate elevation the substrate beneath areas of locally high clast densities aggrades building up a topographic high that can cause clasts to repel one another to form a more uniform distribution of clasts through time this self organized process likely plays a signifi cant role in the evolution of mixed grain size eolian surfaces on earth and mars

uplift driven climate change at 12 ma a long δ18o record from the ne margin of the tibetan plateau carbonates from fluvial and lacustrine sediments were sampled from multiple measured sections in the linxia basin of western china based on textural and mineralogical evidence lacustrine carbonates are primary precipitates from lake water a 29 million year record of the oxygen isotope composition of meteoric water is inferred from the n 18 o values of these carbonates this inference is based on the most negative n 18 o values in the lake carbonates which represent lake waters that have experienced the least evaporative enrichment carbonate n 18 o values a proxy for rainfall n 18 o are v310 5x throughout the interval of 29 12 ma at 12 ma there is a shift to 39x a value that remains into the pliocene this implies a major reorganization of atmospheric circulation patterns and a shift to more arid conditions at the ne margin of the tibetan plateau with the post 12 ma system similar to that of today the 12 ma event may represent the time at which the tibetan plateau achieves sufficient elevation to block the penetration of moisture from the indian ocean or south pacific into western china the period of greatest aridity is from 9 6 to 8 2 ma a time interval which agrees well with other climate records 2003 elsevier b v all rights reserved

monitoring cenozoic climate evolution of northeastern tibet stable isotope constraints from the western qaidam basin china carbon and oxygen stable isotopic composition of cenozoic lacustrine carbonates from the intramontane qaidam basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes from eocene to oligocene the over all trend in the 13c composition of lacustrine carbonates shows a shift toward higher values possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived co2 at the same time the 18o composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region during the miocene distinct isotopic events can be recognized although their interpretation and linkage to a certain tectonic event remains difficult these events may be related to uplift in the himalayas to the strongest phase of uplift in the altyn mountains to pronounced subsidence of the qaidam basin or to the expansion of c4 plants on land generally cold highly evaporative conditions can be deduced from enrichment of 18o isotopic compositions during pliocene and quaternary times

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

a phenomenological model for the flow resistance over submerged vegetation 1 the bulk velocity ub in streams is conventionally estimated from manning s equation but difficulties remain in parameterizing the roughness coefficient n when the streambed is covered with vegetation a two layer velocity model is proposed to determine n and ub for the submerged vegetation case the modeled n is derived as a function of flow and vegetation properties that can be inferred from remote sensing platforms such as canopy height leaf area density and flow depth the main novelty in the proposed formulation is that the shear stress is related to the mean velocity profile by considering both ejective and sweeping motions by dominant eddies the proposed model is tested against a large data set from the literature and is shown to perform well particularly for rigid vegetation poorer model performance for flexible vegetation can be partially attributed to the shape of the assumed mean velocity profile the roughness coefficient n is found to be robust to variations in the average spacing between canopy elements allowing the model to be applied to heterogeneous canopies

stable isotopic variations in west china a consideration of moisture sources 1 in this study individual precipitation samples collected over 2 years at stations in different climatic regions of west china tibetan plateau region tianshan region and altay were analyzed for the stable isotopes of precipitation to improve our understanding of how vapor transport impacts the modern stable isotopic distribution our results identify regional patterns in both 18o and deuterium excess d excess defined as d 8 18o and in particular we have identified the northward maximum extent of the southwest monsoon over the tibetan plateau this demarcation is also the boundary for the fractionation effect of temperature on stable isotopes in precipitation the patterns we have identified are as follows 1 in the southern tibetan plateau along the southern slope of the himalayas our results show a distinct seasonality for both 18o and d excess as a result of the shift of summer monsoon moisture and winter westerly moisture transport the signals of 18o in the western tibetan plateau reveal that the region receives southwest monsoonal moisture in the east of the plateau stable isotopic variation shows alternation between monsoon intrusion and recycling of northern moisture 2 in contrast in tianshan there is an apparent temperature effect in 18o with enriched values occurring in summer and depleted values occurring in winter seasonal d excess values opposite to those observed in the southern tibetan plateau are controlled by differing seasonal evaporation conditions 3 in altay the most northern mountain region the seasonal 18o shows the same variation with that in tianshan region however d excess shows no apparent seasonal variation

expansion of c4 grasses in the late miocene of northern pakistan evidence from stable isotopes in paleosols stable isotopic clay mineralogic and bulk chemical analyses were conducted on paleosols of the neogene siwalik sections in northern pakistan in order to reconstruct floodplain environments over the past 17 ma the stable carbon isotopic composition of soil carbonate mean 13c pdb 10 2 and associated organic matter mean 13c pdb 24 1 in paleosols representing 17 7 3 ma reveal that floodplain vegetation was dominated by c3 plants at 7 3 ma a shift toward more positive carbon isotopic values began signaling the gradual expansion of c4 grasses onto the floodplain from 6 ma to present carbon isotopic values for paleosol carbonate mean 13c pdb 0 6 and organic matter mean 13c pdb 14 4 are uniformly enriched in 13c indicating the presence of nearly pure c4 grassland the scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1 0 1 25 m yr or less for the entire 17 ma of record paleosols in the lower portion of the section lack organic a horizons but have reddish b horizons often containing secondary iron oxide nodules leaching depths of soil carbonate in these older paleosols are typically greater than those in the plio pleistocene part of the section where organic a horizons are common and b horizons are markedly more yellow the combined evidence suggests that the mature paleosols in the pre 7 3 ma part of the record are dominantly calcareous alfisols or mollisols that once underlay nearly pure c3 vegetation perhaps trees and shrubs while calcareous mollisols underlying c4 grassland dominate the upper part of the record the carbon and oxygen isotopic trends in the paleosol record in pakistan are also evident in the diet of fossil mammals and in paleosols from nepal thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent wide local effects such as the development or intensification of the asian monsoon driven by uplift of the tibetan plateau may have led to the expansion of c4 grasses if however the expansion of c4 grasses proves globally synchronous then a larger scale cause such as a marked decrease in co2 may be the driving mechanism

predicting paleoelevation of tibet and the himalaya from δ18o vs altitude gradients in meteoric water across the nepal himalaya the 18o value of meteoric water varies with elevation providing a means to reconstruct paleoelevation if the 18o values of paleowater are known in this study we determined the 18o values of water 18omw from small tributaries along the seti river and kali gandaki in the nepal himalaya we found that 18omw values decrease with increasing altitude for both transects 18omw vs altitude along the kali gandaki in west central nepal fit a second order polynomial curve consistent with increasing depletion of 18o with increasing elevation as predicted by a rayleigh type fractionation process this modern 18omw vs altitude relationship can be used to constrain paleoelevation from the 18o values of miocene pliocene carbonate 18oc deposited in the thakkhola graben in the southern tibetan plateau paleoelevations of 3800 480 m to 5900 350 are predicted for the older tetang formation and 4500 430 m to 6300 330 m for the younger thakkhola formation these paleoelevation estimates suggest that by 11 ma the southern tibetan plateau was at a similar elevation to modern

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

can sedimentary leaf waxes record d h ratios of continental precipitation field model and experimental assessments abstract d h ratios of leaf waxes d wax derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology ideally d wax can be used to reconstruct precipitation d h ratios d p which is a well established paleoclimate proxy however many other factors such as vegetation and relative humidity rh also affect d wax variation how the combination of these factors affects sedimentary d wax is unclear here we use a transect of 32 lake surface sediments across large gradients of precipitation relative humidity and vegetation composition in the southwestern united states to study the natural factors affecting sedimentary d wax d values of c 28 n alkanoic acids show significant correlation with d p values r 2 0 76 with an apparent isotopic enrichment of 99 8 indicating that sedimentary d wax values track overall d p variation along the entire transect leaf waxes produced by plants grown under controlled conditions rh 80 60 40 show a small increase in d h ratios as rh decreases consistent with prediction from the craig gordon model however the isotopic effect of rh on d wax along the natural transect is partially countered by the opposing influence of vegetation changes the correlation between d wax and d p values is significantly higher r 2 0 84 in the drier portions of the transect than in the wetter regions r 2 0 64 this study suggests that d h ratios of sedimentary leaf waxes can be used as a proxy for precipitation d variations with particularly high fidelity in dry regions although more studies in other regions will be important to further test this proxy

open channel flow through different forms of submerged flexible vegetation laboratory experiments are used to explore the effect of two forms of flexible vegetation on the turbulence structure within a submerged canopy and in the surface flow region above the two simulated plant forms involve flexible rods stipes of constant height and the same rods with a frond foliage attached these plant forms were arranged in a regular staggered configuration set at the same stipe density the plant geometry and its mechanical properties have been scaled from a real aquatic plant using froudian similarity and the methods used for quantifying the bending stiffness flexural rigidity and drag force velocity relationship of the vegetation are outlined experimental results reveal that within the plant layer the velocity profile no longer follows the logarithmic law profile and the mean velocity for the rod frond canopy is less than half of that observed for the simple rod array in addition to the mean flow field the turbulence intensities indicate that the additional superficial are

the size frequency and areal distribution of rock clasts at the spirit landing site gusev crater mars 1 size frequency and nearest neighbor spacing measurements derived from the mars exploration rover spirit s panoramic camera images of the gusev landing site reveal that rock clasts on the intercrater plains are smaller better sorted and more evenly spaced than clasts found near the crater rims the even clast spacing is interpreted to be due to wind transport which acts to minimize surface roughness during periods of high velocity wind

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

paleoaltimetry of the tibetan plateau from d h ratios of lipid biomarkers abstract the past elevation of the land surface provides a unique constraint on the underlying lithospheric structure during mountain and plateau formation development of new paleoaltimetry techniques that can be applied to a wide variety of sample types is therefore of continuing importance this study evaluates organic substrates that preserve the d ratio of surface waters as a new approach to reconstruct paleoaltimetry we measured the hydrogen isotope composition of n alkanes from epicuticular plant waxes preserved in lacustrine deposits to reconstruct the d of precipitation in cenozoic basins that have been elevated as part of the tibetan plateau n alkane d and carbonate 18 o inferred water compositions from the eocene miocene lunpola basin and miocene hoh xil basin plot near or at enriched values relative to the global meteoric water line as expected for evaporative lakewater and leafwater systems that have the same precipitation source n alkane d based water compositions are nearly identical to the minimum carbonate 18 o based values demonstrating that plant wax d is minimally affected by evaporation compared to lacustrine calcite 18 o this agreement strongly supports the presence of similar precipitation isotopic compositions in both archives despite different isotope systems source water reservoirs archive materials modes of incorporation and diagenetic processes paleoelevations for each basin and time period were calculated from precipitation isotope ratios using the isotope altitude relationship derived from both a simple thermodynamic model and modern precipitation sampling from the plateau region our new results from the hoh xil basin suggest 1700 to 2600 m of uplift may have occurred some time between the late eocene and early miocene the timing of this uplift is consistent with late oligocene compressional deformation of the hoh xil basin and northward growth of the tibetan plateau however the calculated uplift is not a unique solution from the paleoisotope data because of uncertainties in eocene and miocene moisture sources and isotope gradients for the northern plateau our results demonstrate the utility of lipid based estimates of paleoelevation and expand the types of deposits amenable to paleoaltimetry analysis

early pliocene paleo altimetry of the zanda basin indicated by a sporopollen record abstract whether the uplift of the tibetan plateau tp led to the origin and development of the indian monsoon is still in dispute a helpful indicator would be greater knowledge about the chronological development of paleo altimetry the zanda basin located on the sw margin of the tp forms one of the most extensive cenozoic sedimentary basins in tibet the modern climate of this area is controlled by the indian monsoon circulation system it is thus an appropriate region for the study of paleo altimetry in addition to research into the relation between the tp and the indian monsoon sporopollen analyses of early pliocene lacustrine strata in the zanda basin show that gymnosperm pollen is dominant av 77 and mainly composed of picea av 27 2 and abies av 24 2 fern spores occupy a lesser proportion av 17 6 while angiosperm pollen including temperate sub tropical broadleaved trees shrubs and herbs accounts for only a small proportion of the assemblage this sporopollen assemblage suggests that sub alpine dark coniferous forests were distributed around the basin and grow in a cool and wet climate rather than in a modern alpine steppe environment these characteristics indicate that the zanda basin s altitude throughout the early pliocene was most probably lower than it is at present

different response of δd values of n alkanes isoprenoids and kerogen during thermal maturation abstract this study investigates the extent of post depositional alteration of d values of n alkyl lipids isoprenoids and kerogen isolated from a continuous 450 m core that covers the transition from thermally immature to early mature sediments in the lacustrine kissenda formation lower cretaceous gabon basin large variations in d values up to 40 for n c 17 and up to 30 for n c 29 alkanes as well as up to 10 for kerogen in closely spaced samples are evident throughout the core and remain preserved even at the bottom of the section d values of individual n alkanes show a slight overall d enrichment with depth and a general trend of increasing d values with increasing n alkane chain length characterizes all samples particularly in those below 600 m depth hydrogen isotopic compositions of kerogen samples overlap with those of n alkanes throughout the section d values of pristane and phytane are more negative than those of n c 17 alkane by as much as 120 at shallow depths but increase dramatically and approach d values of n c 17 alkane in the samples closest to the oil window integration of analytical and computational results indicates that 1 n alkanes and isoprenoids have the potential to preserve the original biological signal before the onset of oil generation 2 isomeric and structural rearrangements taking place at the beginning stages of oil generation do not influence significantly the d values of n alkanes and kerogen however these processes have a major effect on the isotopic composition of isoprenoids causing isotopic d enrichment up to 90

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

geochemical evaluation of fenghuoshan group lacustrine carbonates north central tibet implications for the paleoaltimetry of the eocene tibetan plateau abstract a sedimentologic petrologic and geochemical evaluation of lacustrine carbonates from the eocene oligocene fenghuoshan group of north central tibet provides insight into the paleoenvironmental and paleolimnological setting of the hoh xil basin during the mid tertiary fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones these carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial alluvial plain deposits individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward fenghuoshan carbonates also contain abundant ostracode bivalve and charaphyte fossils sedimentologic evidence suggests that carbonate deposition occurred in shallow relatively short lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems stable c and o isotopic analysis of fenghuoshan carbonates show 18o values ranging from 11 7 to 10 3 vienna pee de

palaeontology of the 1985 tibet geotraverse lhasa to golmud palaeontological collections made on the 1985 geotraverse are used to date the associated strata and provide information about palaeoenvironments a biostratigraphical and palaeoenvironmental framework for the tibetan plateau is constructed the crucial stratigraphical and palaeontological evidence for dating volcanic sequences flysch basins open oceanic sediments latest marine sediments and terrestrial red beds in each terrane is reviewed a quantitative palaeobiogeographical analysis from the carboniferous to early triassic is presented for the tibetan fauna and flora to establish the biotic relationships between the various terranes this is based largely on coral brachiopod and fusulinid distributional data but other groups have also been analysed including the permian terrestrial flora this analysis demonstrates a clear diversity gradient from equatorial or sub equatorial biotas in the north to temperate biotas in the south during the late palaeozoic no suture line consistently m arks the position of a faunal floral break during the late palaeozoic and there appears to have been no physical barrier such as a large ocean to biotic dispersal between tibetan terranes at this time climate is seen as the most likely factor dominating biotic distribution in this region and the early permian glaciation had a profound effect on marine faunal distributions in tibet

reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals the paleorelief of mountain belts can be estimated from the 18 o value of authigenic minerals development of relief during mountain building often creates lee side rain shadows in which precipitation is depleted in 18 o and d the magnitude of this rain shadow effect is strongly correlated to relief a compilation of 18 o data from surface waters throughout the globe shows a linear relationship between net elevation change and 18 o r 2 0 79 through the use of this relationship we investigated the timing and magnitude of elevation change in the southern alps of new zealand and the sierra nevada of california the 18 o values of kaolinites from new zealand show an 6 decrease in the early pliocene that corresponds to an 2 km elevation change in the southern alps the 18 o of smectites from the sierra nevada show little change since 16 ma suggesting that these mountains have been a long standing topographic feature

δd values of individual n alkanes from terrestrial plants along a climatic gradient implications for the sedimentary biomarker record abstract the compound specific hydrogen isotope composition d of sedimentary n alkanes is increasingly used as a palaeoclimate proxy to explore the origin of the hydrogen isotope signal of n alkanes from higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the n alkane composition and d values of 31 plant biomass samples birch betula beech fagus oak quercus alder alnus hornbeam carpinus and myrtle myrtus as well as non stomata containing sphagnum cladonia and moss from 14 sites along a climatic gradient from northern finland to southern italy eleven of the sites were previously investigated for the n alkane d values in adjacent lake sediments and are compared to the leaf wax n alkane d values the distribution of terrestrial n alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake evidently n alkane distributions within the same species at different sites were found to be variable these findings complicate the use of certain n alkane distributions in sediments as a palaeoenvironmental indicator d values of plant biomass n alkanes show a significant positive correlation with the precipitation d values biomass n alkane d values are in general less negative than the associated sedimentary n alkanes of terrestrial origin possibly due to the stronger evapotranspiration during the summer when sampling took place this suggests that n alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn when most leaves are incorporated into the sediment the variability of the d values from non stomata containing plants is much higher than for broadleaf trees since stomata can regulate water loss through evapotranspiration the average chain length acl of n alkanes from deciduous tree leaves increases from north to south with increasing acl the hydrogen isotopic fractionation e between source water and n alkanes also increases our results prove that terrestrial n alkane d values record the d value of precipitation modified by the amount of isotope enrichment in the leaf water due to plant anatomy stomata vs no stomata and site specific meteorological conditions such as evapotranspiration relative humidity and soil moisture therefore terrestrial n alkanes in lake sediments can deliver information on changes in these parameters over geological timescales in lake sediments where aquatic n alkanes such as n c 17 record the lake water d value the isotopic difference between terrestrial and aquatic n alkanes could therefore be used to reconstruct changes in evapotranspiration

overland flow in wetlands vegetation resistance emergent wetland vegetation frequently provides most of the resistance to flow of surface water stems are typically spaced many diameters apart therefore fluid friction should be computed from drag on single objects not channel or packed bed equations complication arises from vertical variation of vegetation density and nonoriented spatial variation of soil elevations further flows are often in the transition region between laminar and turbulent and the manning equation is therefore not appropriate a detailed approach requires knowledge of statistical distributions of wetland ground elevation depth and velocity ground slopes are typically in the range of 0 1 100 cm km however the depth range is small so the combined effects recorrelate to a simple power law equation for dense emergent vegetation with spatial uniformity on the scale of 10 m parameters in the model can be estimated with sufficient accuracy from vegetation and soil surveys combined with relatively little hydrologic data dat

vast early miocene lakes of the central tibetan plateau lacustrine strata of the wudaoliang group demonstrate that two vast lake complexes covered the central tibetan plateau during early miocene time the wudaoliang group which is characterized by dolostone limestone and marl as thick as 310 350 m lies horizontally or with a slight dip above oligocene reddish brown sandstone and is covered by upper miocene and pliocene mudstone and sandstone the extensive distribution of this group outlines interconnected lake basins of 5000 15 000 km 2 or larger in the north central tibetan plateau such as the wudaoliang beiluhe tuotuohe tongtianhe and east wenquan basins and lake basins 2000 km 2 or larger in the south central plateau such as the ando naqu bangoin lunpola and shuanghu basins these lake basins separated by mountain ranges and islands were linked by water passages in early miocene time when they apparently formed a huge lake complex that covered as much as 100 000 km 2 in the north central tibetan plateau and another one 50 000 km 2 in the south central part of the plateau such immense lakes existed for several million years between ca 23 5 and ca 13 5 ma after the tibetan plateau rose and their interconnected basins define the topography during early miocene time the fossils contained in these basins indicate a change from the warm dry climate of the oligocene to moderately cool and wet conditions at the beginning of the early miocene followed by progressive cooling and drying which suggests continuous uplift during deposition of the wudaoliang group

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

flow through trapezoidal and rectangular channels with rigid cylinders a physical model study was performed in which wood dowels were used to model rigid vegetation the dowel configurations used in the flume were intended to simulate the effects of willow post systems i e collections of rigid cylinders placed along a streambank to reduce streambank erosion in addition an analytical model is presented for predicting depth averaged velocity distributions in straight trapezoidal or rectangular channels with newly constructed willow post systems the analytical model is founded on wake theory and is applicable to channels with submerged and unsubmerged rigid cylinders data from three independent physical model studies were used to validate the analytical model depth averaged velocities predicted using the analytical model upred were compared to velocities observed in the physical models uobsd and yielded discrepancy ratios upred uobsd that were typically between 0 80 and 1 20 results from this study are that significant variables for reducing local velocitie

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

stable isotope records of cenozoic climate and topography tibetan plateau and tarim basin we seek to better understand growth of the tibetan plateau and associated climate change by presenting the first regionally extensive set of oxygen isotope data obtained from ca 40 m y of cenozoic strata from a 700 000 km2 area ranging from the northern edge of the tibetan plateau northward across the entire tarim basin our results reveal a previously unrecognized large positive isotope shift from eocene to oligocene which we suggest results from initial topographic growth of the himalayan tibetan system and attendant reorganization of climate by this growing mountain belt a negative isotopic shift recorded in miocene strata may reflect retreat of the neotethyan sea from the tarim basin as well as enhanced elevation of the himalaya tibetan plateau region and the rise of the tian shan which diverted moisture bearing westerlies and deepened aridity in the tarim basin and the northern tibetan plateau

tibetan plateau summer monsoon northward extent revealed by measurements of water stable isotopes a program of individual precipitation events and river water sampling and of water isotopic measurements d 18o was carried out during summer 1996 along a northeast southwest transect of the tibetan plateau the spatial distribution of both 18o and deuterium excess d d 8 18o of the precipitation reveals three distinct regions simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins 1 south of the himalayan mountains the moisture provided by the indian monsoon has been recycled over the indian peninsula 2 between the himalayas and the tanggula mountains the oceanic moisture is directly transported from the bay of bengal along the brahmaptra river valley 3 north of the tanggula mountains the moisture is not provided by the monsoon anymore but by continental water recycling

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain