aridification of central asia and uplift of the altai and hangay mountains mongolia stable isotope evidence central asia has become increasingly arid during the cenozoic though the mechanisms behind this aridification remain unresolved much attention has focused on the influence and uplift history of the tibetan plateau however the role of ranges linked to india asia convergence but well north of the plateau including the altai sayan and hangay in creating the arid climate of central asia is poorly understood today these ranges create a prominent rain shadow effectively separating the boreal forest to the north from the deserts of central asia to explore the role of these mountains in modifying climate since the late eocene we measured carbon and oxygen stable isotopes in paleosol carbonates from three basins along a 650 km long transect at the northern edge of the gobi desert in mongolia and in the lee of the altai and hangay mountains we combine these data with modern air parcel back trajectory modeling to understand regional moisture transport pathways at each basin in all basins 13c increases with the largest increase in western mongolia the first 13c increase occurs in central and southwestern mongolia in the oligocene 13c again increases from the upper miocene to the quaternary in western and southwestern mongolia we use a 1 d soil diffusion model to demonstrate that these 13c increases are linked to declines in soil respiration driven by dramatic increases in aridity using modern day empirical relations between mean annual precipitation and soil respiration we estimate that precipitation has likely more than halved over the neogene given the importance of the hangay and altai in steering moisture in mongolia we attribute these changes to differential surface uplift of the hangay and altai surface uplift in the hangay began by the early oligocene blocking siberian moisture and aridifying the northern gobi in contrast surface uplift of the altai began in the late miocene blocking moisture from reaching western mongolia thus the northern gobi became increasingly arid east to west since the late eocene likely driven by orographic development in the hangay during the oligocene and the altai in the late miocene through pliocene

drivers of precipitation stable oxygen isotope variability in an alpine setting snowy mountains australia natural archives that preserve a stable isotopic signature are routinely used to reconstruct palaeoenvironmental conditions isotopic values of precipitation are known to be influenced by factors such as the amount and type of precipitation moisture pathway landscape and terrain factors and processes associated with precipitation formation and deposition this study investigates oxygen isotopic variability using real time rain and snow precipitation data from a moderate altitude 2250 m above sea level southern hemisphere alpine environment where the causes of isotopic variability are largely unknown previous research at global network of isotopes in precipitation sites skewed toward rain precipitation low altitude predominantly coastal locations identified amount effects as the dominant explanation of isotopic variability in southern australia this study based on within and between event real time sampling finds that the origin of moisture and terrain effects are the dominant cause of isotopic variability in this alpine region with little evidence of amount effects rainfall that originated from similar southern ocean latitudes showed a consistent moderate isotopic signature 18o 6 5 to 8 depleted isotopic signatures are associated with prefrontal activity and intense circulation such as east coast lows localized thunderstorms have a more neutral isotopic signature a windward to leeward depletion 0 5 18o and an elevation impact 0 5 18o 100 m 1 were found also these results have significant implications for understanding atmospheric drivers of isotopic variability from which oxygen isotope based palaeoclimate reconstruction is informed in regions with complex topography and geographically diverse moisture pathways such as the australian alps

empirical relationships between elevation and the stable isotope composition of precipitation and surface waters considerations for studies of paleoelevation change a compilation of 68 studies from throughout many of the world s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects the isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the himalayas and at elevations 5000 m there are no significant differences in isotopic lapse rates from most regions of the world 0 28 permil 100 m except at the extreme latitudes where isotopic lapse rates are higher given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals this global isotopic lapse rate can be used to place numerical constraints on the topo graphic development of some ancient mountain belts or plateaus there are many complicating factors that can confound interpretation of paleoel evation change based on stable isotopes and many of these are unique to specific mountain belts or time periods relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedo genic or authigenic mineral and the water from which it forms in cases where isotopic proxy minerals are sampled from localities where temperature will change simulta neously with elevation change the apparent change in the isotopic composition of precipitation may be dampened by several permil this suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau

temperature and vapour trajectory controls on the stable isotope signal in canadian rocky mountain snowpacks the effects of temperature and seasonal air mass trajectories on stable water isotopes in alpine snowpacks are investigated using meteorological and snow pit data at two alpine field sites in the canadian rocky mountains haig glacier alberta and opabin glacier british columbia snow pits were sampled through three accumulation seasons october june 2004 05 2005 06 and 2006 07 for 18 o d temperature and density the isotopic characteristics of precipitation over these time periods including the local meteoric waterline and average 18 o d and deuterium excess were defined using this dataset individual snowfall events over the three seasons were identified in the accumulation records from both sites and then fit to snow pit stratigraphies to determine their mean isotopic characteristics a trajectory classification was produced for all events and the key meteorological characteristics of each trajectory class were investigated using data from alpine field sites and a suite of meteorological records from the region an analysis of the relative influences of temperature and air mass trajectory on snow isotope ratios reveals some separation in mean 18 o between storm classes however the separation appears to be driven primarily by the mean temperature of each class rather then being a direct effect of vapour pathway

the evolution of hydroclimate in asia over the cenozoic a stable isotope perspective abstract links between tectonics and regional climate have been extensively studied in asia however despite decades of research the relative roles of tectonics and global climate in pacing the long term evolution of climate in asia remain disputed today climate in asia is characterized by large scale monsoonal circulation over india and southern china and exceptionally arid deserts across much of central asia that to the north yield to the wetter conditions of the steppe and taiga how why and if these boundaries have changed during the cenozoic has been limited by the lack of paleoclimate proxy data that can be broadly applied over space and time we use a large compilation of pedogenic and lacustrine carbonate stable oxygen 18o and carbon 13c isotope data 7 700 samples which record large scale atmospheric circulation and primary productivity respectively to reconstruct cenozoic hydroclimate in asia and delineate the boundaries through time of monsoonal and mid latitude westerly moisture the 18o data indicate that atmospheric circulation has remained similar to today for at least 55 million years southern tibet has always received southerly likely monsoonal moisture and this southerly moisture has likely rarely extended northward of the central tibetan plateau in contrast northern tibet and central asia have received moisture dominantly via the mid latitude westerlies which have maintained a semi arid to arid climate in central asia since the early eocene thus the spatial extent of large scale atmospheric circulation appears largely unperturbed by contemporaneous changes in india asia collisional tectonics extent of the paratethys and in global climate the 13c data support this interpretation but yield additional details particularly low 13c across a wide swath of eastern china during the eocene suggests greater primary productivity due to a strengthened east asia monsoon perhaps driven by high atmospheric pco2 and globally warm temperatures consistently higher 13c in central asia indicates that central asia has remained more arid than surrounding regions such as northern india and southern china which have received abundant precipitation due to the presence of monsoonal moisture however increases in 13c across much of central asia throughout the cenozoic including particularly pronounced increases in mongolia and kazakhstan during the neogene imply that primary productivity has declined across much of asia this large scale de greening is attributable to the combined effects of cenozoic global cooling and the late cenozoic uplift of the tian shan and altai mountains which block westerly moisture the uplift of these ranges in the late miocene is supported by decreasing neogene 18o in eastern kazakhstan and sharply increasing 13c in the lee of the altai these regional data demonstrate that the tian shan and altai reached sufficient elevations to interact with the westerly jet by the late miocene establishing a substantial rain shadow and shifting the seasonality of precipitation in western central asia we conclude that paleoclimatic changes during the cenozoic in central asia are more tightly controlled by global climate and by the interaction of the mid latitude westerlies with the tian shan and altai rather than by changes in the height or extent of the tibetan plateau or paratethys

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

hydrogen isotope study of large scale meteoric water transport in northern california and nevada abstract the depletion of deuterium in surface and shallow groundwaters along a traverse through northern california and nevada is used to model the local hydrologic balance as controlled by the eastward flux of meteoric water in a way not possible by conventional methods when plotted against distance from the pacific ocean our data indicate three hydrologic provinces which differ in their relative degree of openness isotopic variations of groundwater in the first province are governed by rayleigh distillation from the eastward moving storm clouds some 40 of the storm water vapor precipitates out before reaching the california coast and another 16 in the first 55 km across the continent the isotopic depletion in the second hydrologic province is linear with distance and this suggests extensive recycling of terrestrial water the third province constitutes the great basin characterized by the lack of significant hydrogen isotope variation of groundwater is thought to be an isotopically closed system the quantity of water crossing the coast calculated from a model that incorporates the rayleigh distillation equation is approximately equal to the quantity of precipitation along the northern california part of the transect studied this illustrates the importance of evapotranspiration in the terrestrial water budget as well as the dependency of inland areas on the eastward flux of recycled terrestrial water

palaeoclimate kink in the thermometer temperature estimates derived from isotopes in polar ice cores reveal much about earth s past climate according to the latest analysis interglacial periods were rather warmer than previously thought

precipitation sublimation and snow drift in the antarctic peninsula region from a regional atmospheric model 1 a regional atmospheric model with a horizontal grid spacing x of 14 km is used to study the surface mass balance components precipitation sublimation and snow drift in the region of the antarctic peninsula ap an integration is performed for the 7 year period 1987 1993 using a realistic forcing at the lateral model boundaries and at the sea surface output from this integration indicates that the precipitation reaches its maximum value on the northwestern slope of the ap where the upward motion in the atmosphere is largest uplift occurs upstream of the barrier affecting the precipitation distribution over sea the effect of the barrier on the precipitation distribution over the bellingshausen sea might have important implications for the ocean circulation in this region the mean precipitation over the grounded ice of the ap 1 20 m water eq yr 1 is 6 times larger than the mean value over all the grounded ice of antarctica our estimates for the surface sublimation and wind transport of snow over the grounding line toward the sea are 9 and 6 1 of the precipitation respectively in situ data of the wind distribution at three coastal sites located on the northern eastern and western sides of the ap are used to evaluate the modeled wind field which is important for the snow drift calculations for two of the three sites considered the prevailing wind direction and bimodal wind distribution are correctly represented by the model the calculated distribution of accumulation and ablation due to snow drift shows a complex pattern the wind removes snow from the spine of the ap where the near surface flow field diverges whereas deposition occurs mainly on the eastern slopes where the near surface flow field converges an intercomparison between two 7 year integrations at different horizontal resolution x 14 km and x 55 km shows that the precipitation on the northwestern slope is very sensitive to the model resolution in the x 14 km integration precipitation on the northwestern slope is higher than in x 55 km because of higher vertical velocities resulting in a 35 increase in average precipitation over the grounded ice of the ap

a lagrangian approach to modelling stable isotopes in precipitation over mountainous terrain a lagrangian rayleigh distillation model is used to track the evolution of stable isotopes in precipitation over mountainous terrain from the pacific coast of canada to two alpine field sites in the canadian rocky mountains precipitation 18o at vancouver constrains the model and air mass back trajectories provide the water vapour pathway for 10 winter storm events isotopic values along storm pathways are modelled with a classical rayleigh model that prescribes a linear decrease in temperature and pressure from initial to final conditions and two models that account directly for orographic precipitation processes by i applying an orographic rainfall model and ii using north american regional reanalysis data to calculate the change in vapour content along storm pathways all models are significant predictors of snowpack 18o but the orographic model provides the best fit to precipitation weighted 18o for each storm the improvement in modelled 18o by accounting for terrain along storm trajectories illustrates the need to account for orographically driven moisture loss when modelling vapour transport to ice core sites with mountainous upwind terrain this finding is also applicable to isotopic studies of paleoaltimetry and source areas of groundwater recharge copyright 2011 john wiley sons ltd

orographic controls on rain water isotope distribution in the mount lofty ranges of south australia summary hydrogen and oxygen isotopes of water are common environmental tracers used to investigate hydrological processes such as evaporation vegetation water use surface water groundwater interaction and groundwater recharge the water isotope signature in surface water and groundwater evolves from the initial rain signature in mountain terrain rain water stable isotope composition spatially varies due to complex orographic precipitation processes many studies have examined the isotope elevation relationships while few have quantitatively investigate the terrain aspect and slope effect on rain isotope distribution in this paper we examine the orographic effects more completely including elevation terrain slope and aspect on stable isotope distribution in the mount lofty ranges mlr of south australia using a multivariate regression model the regression of precipitation isotope composition suggests that orographic effects are the dominant controls on isotope spatial variability about 75 of spatial variability in 18 o and deuterium excess is represented by the regression using solely orography related variables elevation terrain aspect and slope with about 25 of 18 o spatial variability attributed to the terrain aspect and slope effect the lapse rate is about 0 25 for every 100 m at both windward and leeward slopes however at the same elevation 18 o at the leeward slope eastern mlr is 0 5 larger than that at the windward slope the difference can be explained by different mechanisms continuous rain out processes on the windward side and sub cloud evaporation on the leeward side both 18 o and deuterium excess maps 1 km resolution are constructed based on the regression results for the mlr both maps are consistent with groundwater of local precipitation origin and useful to examine groundwater recharge

the mt logan holocene late wisconsinan isotope record tropical pacific yukon connections the ice core recovered from prospector russell col on mt logan 5 4 km a s l in the yukon spans over 20 000 years this unique record offers a pacific view of the stable isotope and chemical record from the lateglacial to the present the timescale is based on seasonal counted years the largest known volcanic acid signatures and the major shift in stable isotopes and chemistry at the end of the younger dryas there are large and sustained changes in the stable isotopic record that are anti correlated with marine and continental chemistry series the oxygen 18 in this area is not a proxy for palaeotemperature but rather for source region the last major isotope shift in ad 1840 in 18 o and chemistry is compared with the quinn s enso record during periods of more frequent la nina stronger tropical easterlies there is more zonal flow of water vapour transport to the pacific northwest 18 o values are larger and the deuterium excess d smaller these periods coincide with periods of lower accumulation precipitation in southern yukon the holocene 18 o record indicates many large shifts between the meridional strong el nino and zonal la nina comparison of the logan isotopic record and the moisture temperature sensitive time series of peat bog inception dates for the northwest shows a strong correlation 0 36 that points to high accumulation rates coincident with low 18 o and enhanced meridional flow major changes in the core at 4200 bp and 7000 8000 bp point to enhanced meridional flow which coincide with big changes in the pacific palaeorecords of the balance between el nino and la nina 4200 bp seems to have inaugurated the modern enso world

stable isotope ratios in meteoric recharge and groundwater at mt vulture volcano southern italy summary a rain gauge network consisting of five sites located at different altitudes ranging from 320 to 1285 m a s l was installed at mt vulture volcano southern italy rain water samples were collected monthly over a two year period and their isotopic composition 18o and d was analyzed during the same period circulating groundwater was sampled from 24 springs and wells distributed throughout the study area monthly isotopic composition values were used to determine the local meteoric water line lmwl its slope is slightly lower than the relationship defined by longinelli and selmo longinelli a selmo e 2003 isotopic composition of precipitation in italy a first overall map j hydrol 270 75 88 for southern italy the groundwater samples analyzed were distributed essentially along the lmwl the weighted local meteoric water line wlmwl defined through the mean values weighted by the rainfall amount however may define in a short range the meteoric end member in the local hydrological cycle more precisely since most of the groundwater sampling locations do not show seasonal variations in their stable isotope values the flow system appears to be relatively homogeneous the mean altitude of the recharge by rainfall infiltration was estimated on the basis of the local vertical isotopic gradient 18o a few springs which show anomalous isotopic values reveal more regional circulation systems associated with tectonic structures responsible for the ascent of deeper water

origin and type of rainfall for recharge of a karstic aquifer in the western mediterranean a case study from the sierra de gador campo de dalias southeast spain isotope signatures in precipitation from the global network for isotopes in precipitation around the mediterranean basin and literature data are compared with isotopic data from a large karstic aquifer in southeast spain to explain the origin and type of the precipitation events dominating recharge analysis of the deuterium excess d at the scale of the mediterranean basin and at the regional scale allows us to understand the isotopic context of the study area campo de dalias and the sierra de gador almerfa province the origin of precipitation can be determined from its d value the d value changes as a function of the initial evaporation condition it depends on the relative humidity and temperature during the evaporation producing the water vapour of the clouds the water vapour which dominates the study area is generated in two areas the atlantic ocean d 10 parts per thousand and the western mediterranean basin d 15 parts per thousand with increasing precipitation volume the western mediterranean character dominates these heavier storms contribute mainly to recharge as illustrated by the d value of 13 6 parts per thousand in deep groundwater of the campo de dalias weighted d values increase with the volume of precipitation giving a significant relationship for the southern and eastern coasts of the iberian peninsula this selectivity of d to monthly precipitation was used to estimate the return period of precipitation leading to aquifer recharge at 0 9 4 9 years moderate rainfall which occurs more frequently still represents similar to 60 90 of the total precipitation one of the challenges to meet ever growing water demands is to increase recharge from moderate events yielding intermediate quantities per event but forming the bulk of the annual precipitation copyright c 2006 john wiley sons ltd

interpreting h2o isotope variations in high altitude ice cores using a cyclone model 1 vertical profiles of isotope 18o or d values versus altitude z from sea level to high altitude provide a link to cyclones which impact most ice core sites cyclonic structure variations cause anomalous variations in ice core time series which may obscure the basic temperature signal only one site mount logan yukon provides a complete versus z profile generated solely from data at other sites such a profile has to be constructed by supplementing field data this requires using the so called isotopic or thermometer which relates to a reference temperature t the construction of gapped sections of versus z curves requires assuming a typical atmospheric lapse rate dt dz where t is air temperature and using the slope d dt of a site derived thermometer to calculate d dz using a three layer model of a cyclone examples are given to show geometrically how changes in the thickness of the middle mixed layer leads to the appearance of anomalous values in time series producing decalibration of the thermometer there the results indicate that restrictions apply to the use of the thermometer in ice core paleothermometry according to site altitude regional meteorology and climate state

interpreting temperature information from ice cores along the antarctic peninsula era40 analysis 1 analysis of era40 temperature and accumulation data suggests that annual mean isotopic fluctuations due to temperature change will be geographically very variable across the peninsula isotopic variations of 0 4 at james ross island 0 9 at dyer and 1 3 at gomez are all likely to indicate an identical magnitude of temperature change the reduction in the magnitude of the isotopic signal in the north and east is due to climatically dependent synoptic covariance between temperature and accumulation whilst in the west and south seasonal covariance amplifies the isotopic temperature signal additionally we show that the relationship between accumulation and temperature is rather weak in the north east regions but is stronger in the central and southerly regions therefore isotopes may record 11 to 30 of the variance in annual mean temperatures in the north east 75 in central regions and 70 in the south this study enables physically based reconstructions of peninsula climate based on multi core analysis citation sime l c g j marshall r mulvaney and e r thomas 2009 interpreting temperature information from ice cores along the antarctic peninsula era40 analysis geophys res lett 36 l18801 doi 10 1029 2009gl038982

stable isotope composition of waters in the great basin united states 1 air mass trajectories 1 isentropic trajectories calculated using the noaa climate monitoring and diagnostics laboratory s isentropic transport model were used to determine air parcel origins and the influence of air mass trajectories on the isotopic composition of precipitation events that occurred between october 1991 and september 1993 at cedar city utah and winnemucca nevada examination of trajectories that trace the position of air parcels backward in time for 10 days indicated five distinct regions of water vapor origin 1 gulf of alaska and north pacific 2 central pacific 3 tropical pacific 4 gulf of mexico and 5 continental land mass deuterium d and oxygen 18 18o analyses were made of precipitation representing 99 of all cedar city events similar analyses were made on precipitation representing 66 of the precipitation falling at winnemucca during the same period the average isotopic composition of precipitation derived from each water vapor source was determined more than half of the precipitation that fell at both sites during the study period originated in the tropical pacific and traveled northeast to the great basin only a small proportion traversed the sierra nevada the isotopic composition of precipitation is determined by air mass origin and its track to the collection station mechanism of droplet formation reequilibration within clouds and evaporation during its passage from cloud to ground the rayleigh distillation model can explain the changes in isotopic composition of precipitation as an air mass is cooled pseudo adiabatically during uplift however the complicated processes that take place in the rapidly convecting environment of cumulonimbus and other clouds that are common in the great basin especially in summer require modification of this model because raindrops that form in the lower portion of those clouds undergo isotopic change as they are elevated to upper levels of the clouds from where they eventually drop to the ground

timing of the late jehol biota new geochronometric constraints from the jixi basin ne china abstract the jehol biota is a world famous early cretaceous terrestrial ecosystem in east asia and has revealed exceptionally well preserved fossils which provide significant insights into the origin and evolution of birds and feathers and the early diversifications of mammals and angiosperms after the long lived controversies over the timing of the biota varying from late jurassic to early cretaceous in last century the lifespan of the early middle jehol biota has been precisely dated from barremian to early aptian ca 131 120 ma by a series of recent radiometric analyses however few well constrained ages are available for the late jehol biota hindering our complete understanding of the evolutionary history of the terrestrial ecosystem alternating marine and non marine deposits of the chengzihe formation in eastern heilongjiang have yielded a non marine bivalve assemblage of the jehol biota basal angiosperms and cretaceous marine index bivalves which collectively offer crucial and indisputable clues against the prevailing jurassic time of the biota since 1990s here we present the first discovery of tephra layers from the chengzihe formation dated by zircon u pb geochronology as ca 116 111 ma providing the first quantitative age calibration for the early angiosperms and the jehol bivalve assemblage therefore we demonstrate that the duration of the jehol biota extends to late aptian early albian approximately 4 9 myr younger than the currently accepted age limits

on the temperature correlation of δ18o in modern precipitation reevaluation of modern precipitation temperature and isotope data permits reconciliation of previous disparate values for the correlation between 18o of modern precipitation and surface temperature past analysis has used the mean surface temperature over the time interval of sample collection e g mean weekly monthly or annual temperature to calculate temperature coefficients and different approaches at mid latitudes yield different temperature coefficients 18o t spatial correlations among geographically distinct sites yield 0 55 k seasonal variations at single sites yield 0 2 0 4 k and 12 month running averages yield 0 5 1 k however there are systematic differences in temperature during precipitation events vs time averaged surface temperature means correction for this bias using hourly weather and monthly isotope data from u s sites yields a single value of 0 55 k for all three approaches revised temperature coefficients based on surface observations are also commensurate with coefficients obtained using cloud base temperatures and with theoretical distillation models 0 5 0 7 k these coefficients provide a consistent basis for validation of general circulation models that incorporate stable isotopes of precipitation and for comparison to independent estimators of the isotopic response to climate change

appalachian stress study 3 regional scale stress variations and their relation to structure and contemporary tectonics using commercial hydrofracture data to supplement published stress data we map regional variations in least horizontal stress magnitude sh and orientation within the paleozoic sediments of the appalachian plateau systematic trends revealed by the data are examined for correlations with structure the study area includes a prominent salt detachment and tectonic history the dominant stress component present at all depths including basement throughout the region is ene oriented compression and is most likely of tectonic origin no deviations from this rule are found that are statistically significant near surface 600 m stresses are consistently greater than the overburden with strike slip regime conditions prevailing below however where the salina salt beds are present thrust regime conditions extend deeper reaching the salt in most places where data are available limited to where the salt is shallower than 1 5 km at the salt a discontinuity in the trend of sh with depth marks the abrupt transition to the underlying strike slip regime characterized by least stress ratios i e rh sh sv in the range 0 5 0 7 and extending to basement the comparatively high stress found above the salt may reflect a difference between the strain histories of the overlying and underlying sections facilitated by mechanical decoupling by the salt and perhaps includes a component of stress remnant from nw oriented alleghanian shortening of the upper section the lower subsalt stress regime extends beyond the eastern salt margin and displays remarkable uniformity over distances of hundreds of kilometers this is particularly evident across ohio where an rh value of 0 6 0 05 appears valid down to basement north of ohio values increase markedly whereas to the south of the salt pinch out in west virginia and eastern kentucky values as low as 0 35 are found in devonian shales at 1 1 5 km depth although values deeper in the section may be higher the northward increasing trend can be reasonably well modeled as the effect of persisting lithospheric flexure due to the pleistocene ice sheet however flexure was much greater in the past and as devonian shales in south ohio west virginia are currently only marginally stable a modest increase over the present flexure would be expected to cause shear failure this suggests that the very low stress in the shales of south ohio west virginia is of different origin and more recent than the pleistocene