apatite fission track evidence for the cretaceous cenozoic cooling history of the qilian shan nw china and for stepwise northeastward growth of the northeastern tibetan plateau since early eocene abstract apatite fission track aft data from hinterland of the qilian shan at the northeastern margin of the tibetan plateau suggest this range has experienced northeastward propagation of surface uplift since early eocene and that crustal shortening occurred in the qilian shan before the late miocene thermochronometry data indicate that the qilian shan experienced a three stage cooling history including 1 rapid initial cooling during cretaceous 2 a stage of slow cooling during late cretaceous early eocene and 3 rapid stepwise cooling in a southwestern northeastern orientation since early eocene cretaceous rapid cooling may be a record of the lhasa block and eurasian collision early cretaceous denudation was followed by tectonic and quasi isothermal quiescence that continued until early eocene early eocene rapid cooling in the south qilian shan may be the first far field response in the qilian shan to the collision and convergence of the indian and eurasian continents from late eocene to middle miocene crustal shortening propagated into the central qilian shan and north qilian shan and produced surface uplift of the entire qilian shan region before the late miocene this study provides a better understanding of the tectonic evolution of the qilian shan and when the far field stress from the india eurasia collision into the northeastern tibetan plateau began

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

middle miocene ocean climate transition high resolution oxygen and carbon isotopic records from deep sea drilling project site 588a southwest pacific high resolution stable isotopic records are presented for the epi benthic foraminifer cibicidoides the inferred shallow dwelling planktonic globigerinoides quadrilobatus and the inferred deep dwelling planktonic globoquadrina dehiscens from the middle miocene 16 12 ma of deep sea drilling project site 588a lord howe rise southwest pacific high resolution multiple species oxygen and carbon isotopic data define the timing and character of the well known middle miocene climatic oceanographic transition with a resolution comparable to quaternary records the benthic foraminiferal 18o record is marked by several large fluctuations from 16 to 14 8 ma followed by a series of rapid 50 kyr 18o increases that suggest a new state of the ocean climate system after 14 8 ma the total middle miocene benthic oxygen isotopic increase of 1 2 is largely incorporated in two steps an increase of 0 8 from 14 5 to 14 0 ma and a second increase of 0 7 from 13 45 to 12 45 ma each step is comprised of a series of marked 18o increases indicative of rapid east antarctic ice sheet growth and contemporaneous deepwater cooling a strong covariance of 0 7 between the benthic and deep dwelling planktonic species from 14 5 to 14 0 ma including a rapid increase from 14 1 to 14 05 ma suggests a 0 7 increase in the 18o composition of seawater 18osw because of east antarctic ice sheet growth comparison of the 18o record of gs quadrilobatus suggests that surface waters warmed at this site by 3 c from 14 1 to 13 6 ma carbon isotopic time series for each species generally covary throughout the early to middle miocene interval 16 12 ma confirming that 13c variations in this interval largely represent reservoir changes high resolution 13c data allow improved resolution of the latter five of six 13c maxima within the well known early to middle miocene carbon isotopic excursion the monterey carbon isotopic excursion from 17 0 to 13 5 ma this is useful for global correlation the last of these maxima ends with a 1 decrease centered from 13 9 to 13 7 ma 300 kyr after the 18o increase considered to reflect east antarctic ice growth covariance between benthic 18o and 13c from 16 to 13 8 ma suggests a sensitive relation between global carbon cycling and the ocean climate system prior to 13 8 ma episodic increases in organic carbon burial may have contributed to deep sea benthic 13c maxima and synchronous global cooling the positive relationship ended at 13 8 ma indicative of changing relations between global carbon cycling and the ocean climate system brought on by the increased stability of the east antarctic ice sheet after a major growth phase from 14 5 to 14 0 ma

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

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

intensified aridity in the qaidam basin during the middle miocene constraints from ostracod stable isotope and weathering records the thick and continuous cenozoic successions in the qaidam basin provide an excellent paleoclimate archive here we focus on the ostracod fauna stable isotope records and paleoweathering indice

evidence for northeastern tibetan plateau uplift between 25 and 20 ma in the sedimentary archive of the xining basin northwestern china the growth history of the tibetan plateau provides a valuable natural laboratory to understand tectonic processes of the india asia collision and their impact on and interactions with asian and global climate change however both tibetan plateau growth and asian paleoenvironments are generally poorly documented in pre pliocene times and reflect limited temporal coverage for different parts of the plateau here we present magnetostratigraphic results from the xining basin at the ne margin of the tibetan plateau precisely dating the record between the earliest oligocene 33 ma to the middle miocene 16 ma the pattern of observed paleomagnetic polarity zones is unequivocally correlated to the geomagnetic polarity time scale gpts indicating relatively constant and low sediment accumulation rates 32 m myr except for a peculiar period of unstable accumulation between 25 3 and 19 7ma at the beginning of this interval a marked permanent increase in magnetite content of the sediments is observed and likely relates to a change in provenance we directly relate this unstable period of sediment accumulation and provenance change to the coeval exhumation recently reported by low temperature thermochronology from the laji shan range which subsequently formed the southernmargin of the xining basin evidence for ne tibet tectonismat 25 20 ma can be associated with widespread deformation over the entire himalayan tibetan orogen at this time which may be linked to the coeval appearance of monsoon climate in eastern asia and the onset of central asian desertification

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

climatic forcing of carbon oxygen isotopic covariance in temperate region marl lakes carbon and oxygen stable isotopic compositions of lacustrine carbonate from a southeastern michigan marl lake display linear covariance over a range of 4 0 peedee belemnite pdb in oxygen and 3 9 pdb in carbon mechanisms of delta 13 c delta 18 o coupling conventionally attributed to lake closure in arid region basins are inapplicable to hydrologically open lake systems thus an alternative explanation of isotopic covariance in temperate region dimictic marl lakes is required we propose that isotopic covariance is a direct record of change in regional climate in short residence time temperate region lake basins summer meteoric precipitation is enriched in 18o relative to winter values and summer organic productivity enriches epilimnic dissolved inorganic carbon in 13c thus climate change toward longer summers and or shorter winters could result in greater proportions of warm month meteoric precipitation longer durations of warm month productivity and net long term enrichment in carbonate 18o and 13c isotopic covariance observed in the michigan marl lake cores is interpreted to reflect postglacial warming from 10 to 3 ka followed by cooler mean annual temperature a shift toward greater proportions of seasonal summer precipitation a shortening of the winter season or some combination of these three factors

cenozoic paleoaltimetry of the se margin of the tibetan plateau constraints on the tectonic evolution of the region abstract an improved understanding of the elevation history of the tibetan plateau is crucial in discriminating among the various tectonic models for the evolution of the india asia continental collision we reconstruct the paleoelevation history for three cenozoic sedimentary basins from se tibet and yunnan china to provide more constraints on the tectonic processes for raising the se margin of the tibetan plateau the results presented here together with those of previous studies indicate that 1 the plateau margin of nw yunnan was near its elevation 2 6 km by the latest middle eocene 40 ma 2 the plateau margin of se yunnan reached its current elevation 1 6 km by the middle miocene 13 ma interpretations of the tectonic processes responsible for this inferred surface uplift of the region are made in the context of well documented surface geology we conclude that high landscape 2 6 km elevation in nw yunnan may represent the remnants of the eocene tibetan plateau that originally formed in the northeastern qiangtang block by crustal thickening associated with the india asia continental collision the near modern elevation of se yunnan since 13 ma probably reflects the initiation of lower crustal flow in this area by at least that time collectively our paleoaltimetric interpretations disagree with previously proposed models of middle miocene to pliocene crustal flow acting as a sole tectonic process for raising the se margin of the plateau but support a protracted history of surface uplift that most likely involved crustal thickening during the eocene southeastward extrusion of a portion of eocene tibetan plateau during the oligocene to early miocene and lower crustal flow beneath this region since at least the early miocene

cenozoic magnetostratigraphy of the xining basin ne tibetan plateau and its constraints on paleontological sedimentological and tectonomorphological evolution abstract the xining basin is well known for its thick and continuous sequence of fine grained cenozoic sediments in addition it contains important xiejia fauna which define the xiejian stage of the early miocene in the standard chinese land mammal zonation and it also provides detailed histories of eolian dust deposition which document the aridification of asia monsoon evolution and the initiation of the yellow river in relation to the uplift of the tibetan plateau however the results of magnetostratigraphic dating of the fossil bearing sequence has yielded conflicting ages for the fauna and stratigraphy hindering the use of the sequence for addressing major paleoenvironmental and paleoclimatic issues here we review the paleontological lithological and magnetostratigraphic record of the xining basin and provide a new long and continuous high resolution magnetostratigraphy from the basin center the results show that the observed magnetic polarity zones from the various sections at different sites in the basin exhibit similar magnetic polarity patterns which can be readily correlated in terms of both magnetic polarity zonation and lithofacies in addition they show that the sedimentary sequence of the basin was sub continuously deposited from 54 ma to 4 8 ma the magnetostratigraphic correlations confirm previous constraint of the xiejia fauna in the late oligocene at 25 ma challenging the current chinese mammal land zonation for the early miocene and we suggest that it provides a superior record of eolian dust deposition and river incision in the late pliocene than previous studies there is a close match of the climatic proxy records of the xiejia section dated by magnetostratigraphy with the global climatic record which corroborates the paleomagnetic correlations the evolution of the sedimentary environment provided by the lithofacies demonstrates a complete cycle of basin formation and termination which records the eastern qilian shan experienced three main phases of uplifts slow episodic uplifts at 54 ma and 22 5 ma and a late rapid episodic uplifts at 8 7 ma and since 4 8 3 6 ma this tectonic uplift in the ne tibetan plateau was mostly nearly synchronous responses to the initial and continuing collision of india and asia since 50 5 ma

an 11 ma old red clay sequence on the eastern chinese loess plateau abstract older red clay deposits on the chinese loess plateau contain information of early paleomonsoon evolution and tectonic history of the ordos platform and the tibetan plateau a well conserved eolian red clay sequence was recently discovered at shilou on the eastern chinese loess plateau paleomagnetic measurements demonstrate that polarity zones recorded in the shilou red clay profile correspond well with polarity zones between c2an 1n and c5n 2n of the gpts and the basal age of the study profile is approximately 11 0 ma field observation grain size frequency distribution and quartz grain morphology suggest a wind blown origin for this sequence this indicates that the eastern loess plateau began to accumulate dust deposits at 11 ma ago implying earlier establishment of the east asian paleomonsoon probably owing to uplifting of the tibetan plateau and ongoing global cooling it also demonstrates that the eastern loess plateau belonging to the ordos platform was slowly uplifted and suffered from erosion in the early neogene and has no conserved eolian deposits older than 11 ma as in the western chinese loess plateau

palynological evidence for vegetation development and climatic change in the sub himalayan zone neogene central nepal abstract a palynological study on miocene pleistocene sediments exposed at surai khola in central nepal yields new information on vegetation and climate change over the last 11 5 ma our results show that during the late middle miocene to early late miocene 11 5 8 ma lower siwalik the himalayan foothills and the gangetic floodplain of nepal were mainly forested with subtropical to temperate broad leafed b l taxa e g quercus lithocarpus castanopsis alnus and tropical forest taxa respectively grasses were present at the time but were not abundant between the early to late late miocene 8 6 5 ma lower middle siwalik grassland replaced subtropical and temperate b l forest this change may be related to disturbance of the vegetation on the slopes due to uplift perhaps enhanced by intensification of the monsoon between late late miocene to pliocene early pleistocene 6 5 potamogeton and better preservation of all specimens enhanced seasonal flooding in the system during the pliocene 5 5 3 5 ma top middle siwalik may have produced local lacustrine conditions on the overbanks evidenced by an abundance of algae spirogyra and pteridophytes in the pollen assemblages evidence of climatic cooling between 6 5 5 ma ago is indicated by the onset of steppe taxa decrease of tropical forest taxa and disappearance of certain dipterocarpaceae pollen assemblages from the surai khola section suggest complex vegetation changes of which the shift to c 4 grass dominance was only one global climatic cooling the intensification of the monsoon and the late miocene uplift may all have contributed to the phytogeography in the sub himalayan zone

early middle miocene topographic growth of the northern tibetan plateau stable isotope and sedimentation evidence from the southwestern qaidam basin abstract this study presents facies analysis and carbonate stable isotopic records of a 4435 m thick oligocene middle miocene 30 to 11 ma stratigraphic section from the southwestern qaidam basin which place constraints on the interplay between topographic growth and climatic hydrologic changes in the northern tibetan plateau three types of carbonate including marl limestone pedogenic carbonate and carbonate cement were collected and went through detailed screening for diagenesis carbonate cements are primary early diagenetic carbonates formed in shallow ground water setting the majority of marls limestones and pedogenic carbonates are primary micritic carbonates whereas a few of them showing alterations were sub sampled to avoid diagenetic phases a negative shift in the 18o values of marls limestones and depositional environment change from palustrine to shallow lacustrine associated with paleohydrological change at 20 ma are likely caused by tectonic activity in adjacent ranges around 15 14 ma a decrease of 1 5 in the most negative 18o values of carbonate cements is interpreted to reflect the topographic growth of surrounding mountain ranges i e altun shan east kunlun range this inference is supported by the sedimentary facies change from marginal lacustrine to fluvial that was accompanied by an abrupt increase in sedimentation rate finally a 1 5 positive shift in the most negative 18o values of carbonate cements and pedogenic carbonates enriched 13c values of pedogenic carbonates as well as sedimentary environment change from fluvial to braided river alluvial fan at 13 12 ma most likely reflect a regional aridification event observed over a large area of the central asia this aridification event may have been associated with continued topographic growth of the northern tibetan plateau to a critical elevation to block moisture e g westerlies from penetrating the interior of central asia

a high resolution stable isotopic record from the junggar basin nw china implications for the paleotopographic evolution of the tianshan mountains this study presents high resolution oxygen and carbon isotopic records of paleosol carbonates from fluvial sediments and lacustrine carbonates sampled from the jingou he and kuitun he stratigraphic sections located in the northern tianshan piedmont these sections expose remarkable outcrops of junggar foreland basin sediments that have been previously dated by high resolution magnetostratigraphy to between 23 6 and 1 ma and 10 5 and 3 1 ma a total of 216 samples of fluvio lacustrine sediments were collected from which isotopic analyses yield 18o smow values that range from 13 7 to 29 9 in the jingou he section and 16 3 to 21 0 in the kuitun he section 13c pdb values range from 12 9 to 3 0 in the jingou he section and from 7 8 to 4 0 in the kuitun he section 18o values decrease between 25 and 23 ma and then remain relatively steady with the exception of one period that contains samples with higher oxygen isotope values at 16 ma during the periods when there are samples that overlap in time from the kuitun he and jingou he sections we observe a difference of 1 7 between values from the two locales the 13c values also decrease between 25 and 23 ma and then remain relatively steady until 10 ma with again one short period of higher values at 16 ma then between 10 and 3 1 ma carbon isotope values progressively increase we interpret that 18o and 13c isotopic values during lacustrine periods 25 23 ma and 16 ma as largely controlled by evaporation and opening closing of the lake to external inputs we interpret the 18o values of paleosol carbonate in the junggar basin to be influenced by the hypsometry of the high tianshan range while the 13c values may record the uplift history of the depositional area in the foreland basin itself as well as the isotopic composition of plants consequently we conclude that the jingou he and kuitun he drainage basins in the central tianshan have remained at relatively unchanged elevations for the past 20 ma we also suggest that the elevation of the southern part of the foreland basin increased between 10 and 3 1 ma probably as a result of tectonic deformation in the piedmont and sedimentary filling of the sedimentary basin the carbon isotope record remains relatively stable through time and isotopic values suggest that there was little or no expansion of c4 plants in this region in the late miocene

late neogene environmental changes in the central himalaya related to tectonic uplift and orbital forcing the linkage between tectonic forces and climate evolution remains a matter of much debate and speculation here we present high resolution oxygen and carbon isotope data from an ancient lake basin in the central himalaya these data together with sedimentologic evidence reveal major changes in drainage systems and depositional settings at 7 2 5 5 and 3 2 ma these environmental changes appear to be driven by regional scale tectonics the oxygen isotope record also reveals alternating wet and dry climates with periodicities of 24 and 100 kyr that were likely controlled by orbital forcing paleo temperatures estimated using a fossil based oxygen isotope paleo thermometer are 21 6 ca t 7 ma which is 19 6 c higher than the present day mean annual temperature in the same area the much warmer environment inferred here is consistent with fossil mammalian and pollen assemblages and sediment clay mineralogy as well as carbon isotope data from fossil tooth enamels and paleosol carbonates the estimated temperature difference would require the study area to have been raised by 2 2 5 km since 7 ma to its current elevation of 4100 4500 m above sea level this result can be interpreted as either indicating the presence of a low altitude intermountain basin in the hanging wall of the already formed main central thrust or a protracted development of a north trending rift basin that has experienced changes in drainage system and depositional environment through time

spatial variations in paleowind direction during the last glacial period in north china reconstructed from variations in the anisotropy of magnetic susceptibility of loess deposits abstract anisotropy of magnetic susceptibility ams of chinese loess is considered to be an effective tool for determining paleowind direction however the relationship between ams and the paleowind direction is still a matter of debate this study reports the results of ams measurements of chinese loess deposited during the last glacial period on slopes of varying slope angles and orientations the sites are located on the chinese loess plateau in west qinling and on the eastern margin of qilian mountain the results show that within the same region magnetic lineations are clustered along similar orientations despite differences in slope exposure and slope angle but that different regions exhibit different directions of magnetic lineation these results suggest that the alignment of the magnetic grains during deposition of the eolian deposits was determined by air circulation rather than by water flow on the surface of the slopes and therefore that the ams of chinese loess can be used to determine paleowind directions in addition our results indicate that the ams of chinese loess is determined mainly by the patterns of regional surface wind flow that occurred during dust accumulation rather than by the uniform pattern of large scale atmospheric circulation in addition since wind direction is influenced significantly by regional topography the ams of chinese loess may have the potential to detect significant changes in past regional topography

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

the stable isotope composition of early ground water cements from sandstone in paleoecological reconstruction abstract the carbon isotope composition of soil carbonates in paleosols records the global expansion of cm4 biomass in the late miocene however soil carbonate is not present in many geological sections or if present is often difficult to distinguish from other carbonate cements here we report on the 13c values of sparry early ground water cements in sandstones from the siwalik group in pakistan that record like the soil carbonates from the same sections the dramatic late miocene expansion of c4 vegetation the early cements are confined to densely cemented nodules in the sandstones but can also be found reworked as nodules into adjacent paleo channels demonstrating their formation shortly after burial the early cements are isotopically distinct from later higher temperature calcite cements our results demonstrate that some early ground water cements can be used in a manner similar to soil carbonates to reconstruct vegetation and perhaps pco2 changes in the past

early to mid pleistocene ostracod δ18o and δ13c in the central tibetan plateau implication for indian monsoon change as one of the pioneering projects of the chinese environmental scientific drilling cesd program a 206 5 m long sediment core ce was retrieved from co ngoin co lake basin in the central tibetan plateau limnic records of the abundance of ostracod shells and their oxygen and carbon isotope measurements together with the geochemical and the sedimentological investigations were carried out on the interval between 143 and 52 m core depths corresponding to ca 2 01 0 84 myr distinct oscillations of the abundance and the chemical compositions of ostracod shells have yielded the original information about variations in lakewater chemistry in association with the change in the indian monsoon over the period of 2 01 to 0 84 myr the climate conditions during the early pleistocene were characterized by frequent oscillations and the ostracod records had expressed its synchronous response to these oscillations a major climate change occurred in the co ngoin area at about 0 93 myr a pronounced fluctuation in ostracod delta 18o and delta 13c during the mid pleistocene transition mpt is possibly associated with an increase in continental ice sheets the compositional and the sedimentological characters of the immediately overlying sediments imply a shallow aquatic environment and also a harsh salinity condition beyond the tolerance limits for ostracod survival resulting in the disappearance of the qinghaicypris crassa huang 1979 around 0 84 myr

from gullies to mountain belts a review of sediment budgets at various scales this paper reviews the state of the art in the concept as well as in the application of sediment budgets in sedimentary research sediments are a product of mass dispersal at the earth surface and take part in global cycles sediment budgets aim at quantifying this mass transfer based on the principle of mass conservation and are the key to determine ancient fluxes of solid matter at the earth surface this involves fundamental questions about the interplay of uplift climate and denudation in mountain belts and transfer of sediments from the continents to the oceans as well as applied issues such as soil and gully erosion reservoir siltation and coastal protection first after introducing basic concepts relevant scales and methodologies the different components of quaternary routing systems from erosion in headwaters river systems glacial and paraglacial systems lakes deltas estuaries coasts shelves epicontinental seas and deep sea fans are discussed in terms of their sediment budget most suitable are sedimentologically closed or semi closed depositional environments e g alluvial fans lakes deltas and deep sea fans in a second step the dynamics of passive active and collisional tectonic settings and sediment budgets in related sedimentary basins are explored and new concepts of sediment portioning at large geodynamic scales are introduced ancient routing systems are more or less incomplete and may be intensively fragmented or destroyed in active tectonic settings in terms of sedimentary basin types rifts intracontinental and epicontinental settings are preferred objects of sediment budgets because of their persistence and relatively simple overall sedimentary architecture however closing basins such as foreland forearc retroarc piggy back and wedge top basins may provide excellent snapshots of orogenic sediment fluxes in a third step the large long lived routing systems of the amazon the ganges brahmaputra and the rhine are reviewed for each system estimates of either sediment volumes mass or sediment fluxes of continental and marine subsystems have been compiled in order to receive a complete routing in terms of mass conservation for specific time periods since the late glacial maximum as well as the cenozoic following lessons can be taken from these case studies i depositional centers and fluxes show strong shifts in space and time and call for caution when simply looking at subsystems ii the response times of these large systems are within the milankovich time interval thus lower than predicted from diffusion models iii cyclic routing of sediments in continental basins is much more dominated by climate human control than by eustacy and iv at long time scales ultimate sinks win over intermittent storage it is concluded from this review that the quantitative understanding of global sediment cycling over historic and geologic time and its response to allogenic forcing is still in its infancy and further research is needed towards a holistic view of sediment routing systems at various temporal and spatial scales and their coupling with global biogeochemical cycles this includes i to better determine response times of large routing systems by linking quaternary with cenozoic sediment budgets and continental with marine sub systems ii to combine advanced provenance techniques with sediment budgets in order to reconstruct ancient systems iii to study sediment partitioning at the basin scale iv to reconcile continental supply dominated sequence stratigraphy with the eustatic dominated marine concept and iv to account for non actualism of ancient systems with respect to their erosion and transport mode in particular during glaciations and pronounced arid intervals glacial and eolian sediment routing may cross over hydrologic boundaries of drainage basins thus challenging the principle of mass conservation

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

biologically induced calcite and its isotopic composition in lake ontario we studied water sediment trap and core samples from eastern lake ontario to reconstruct the factors controlling the biologically induced production and sedimentation of calcite during so called whiting events calcite accumulation and its isotopic composition are controlled by a complex set of interrelated factors including temperature primary productivity and the abundance of pica cyanobacteria during the stratified period calcite precipitation is highly correlated to lake temperature because physical and biological factors interact to produce conditions favorable for whitings during warm years when the lake stratifies early in the seasonal cycle carbonate stratigraphies in multiple cores from eastern lake ontario revealed similar patterns of historical variation in percent carbonate an exponential rise in carbonate accumulation occurred in nine cores after 1930 culminating in peak values in the early 1980s this rise was related to historic increases in primary productivity resulting from increased phosphorus loading to lake ontario superimposed upon this rise were four peaks centered on 1940 1942 1957 1961 1971 1977 and 1983 that correlate with maxima in summer air temperature anomalies from the great lakes region and with strong el niiio events these peaks are also associated with maxima in s c values and minima in s180 values of carbonate lending support to our model that more calcite is precipitated with higher 6 c values during warm years when thermal stratitication occurs early in the seasonal cycle beginning in the mid 1980s calcite accumulation and its 6 c ratio began to decrease suggesting a reduction in primary productivity in surface waters probably related to lower phosphate concentrations in epilimnetic waters of lake ontario during the stratified period reduced summer p loading may be explained either by a lagged response to p abatement measures that began in the late 1970s or by decreased p loading from upstream lake erie beginning in the late 1980s as a result of the establishment of filter feeding zebra mussels

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

neogene himalayan weathering history and river87sr86sr impact on the marine sr record abstract clastic sediments in the bengal fan contain a neogene history of erosion and weathering of the himalaya we present data on clay mineralogy major element stable and radiogenic isotope abundances from lower miocene pleistocene sediments from odp leg 116 nd and sr isotope data show that the himalayan provenance for the eroded material has varied little since 17 ma however from 7 to 1 ma smectite replaces illite as the dominant clay while sediment accumulation decreased implying an interval of high chemical weathering intensity but lower physical erosion rates in the ganges brahmaputra gb basin o and h isotopes in clays are correlated with mineralogy and chemistry and indicate that weathering took place in the paleo gangetic flood plain the87sr86sr ratios of pedogenic clays vermiculite smectite record the isotopic composition of sr in the weathering environment and can be used as a proxy for87sr86sr in the paleo gb basin the sr data from pedogenic clays shows that river87sr86sr values were near 0 72 prior to 7 ma rose rapidly to 0 74 in the pliocene and returned to 0 72 in the middle pleistocene these are the first direct constraints available on the temporal variability of87sr86sr in a major river system the high87sr86sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate silicate weathering ratio modeling of the seawater sr isotopic budget shows that the high river87sr86sr values require a ca 50 decrease in the sr flux from the gb system in the pliocene the relationship between weathering intensity 87sr86sr and sr flux is similar to that observed in modern rivers and implies that fluxes of other elements such as ca na and si were also reduced increased weathering intensity but reduced sr flux appears to require a late miocene pliocene decrease in himalayan erosion rates followed by a return to physically dominated and rapid erosion in the pleistocene in contrast to the view that increasing seawater87sr86sr results from increased erosion mio pliocene to mid pleistocene changes in the seawater sr budget were the result of reduced erosion rates and sr fluxes from the himalaya

a pedogenic model for the magnetic enhancement of late miocene fluvial lacustrine sediments from the xining basin ne tibetan plateau

oligocene miocene 28 13 ma climato tectonic evolution of the northeastern qinghai tibetan plateau evidenced by mineralogical and geochemical records of the xunhua basin abstract changes in sediment composition in the xunhua basin provide insights into the climatic and tectonic evolution of the northeastern qinghai tibetan plateau during the oligocene and miocene here we analyzed the clay mineralogy i e the proportions of smectite illite and chlorite and clay indicators bulk mineral composition i e the amounts of calcite quartz gypsum k feldspar plagioclase dolomite and halite and trace element indicators i e rb sr and ba sr ratios of sediments deposited from 28 to 13 ma in the xunhua basin to reveal the climate and or provenance changes the results suggest that significant changes in sediment provenance occurred at 25 1 ma and 21 6 ma caused respectively by initial uplift and accelerated uplift of the laji shan the xunhua basin experienced warm and humid conditions at 28 0 25 1 ma cool and dry conditions at 25 1 21 6 ma and somewhat warmer and wetter conditions at 21 6 19 2 ma followed by aridification in two steps at 19 2 ma and 13 9 ma the climate cooling event at 25 1 ma corresponds temporally to a major phase of uplift of central tibet relatively warmer and wetter conditions during the early miocene 21 6 19 2 ma are considered to record the influence of the asian summer monsoon and accelerated uplift of the laji shan the shift toward more arid conditions on the northeastern qinghai tibetan plateau at 19 2 13 9 ma was probably driven by uplift of the tibetan plateau and further aridification since 13 9 ma may reflect a weakening of the east asian summer monsoon and global climatic cooling

the distribution and discrimination of shallow authigenic carbonate in the pliocene pleistocene palomas basin southern rio grande rift nine types of authigenic carbonate are present in the pliocene pleistocene alluvial fan and axial fluvial sediment of the palomas half graben in the southern rio grande rift pedogenic and other vadose carbonate includes 1 pedogenic carbonate of stage ii and stage iii morphology underlying bw and bt horizons 2 mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone 3 pedogenic calcic nodules and tubules in eolian sand and 4 gully bed cement of proximal fan conglomerates formed by infiltration and evaporation of surface runoff shallow groundwater carbonate exists as 5 thin 30 50 cm massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe 6 thick 1 5 3 m massive beds deposited by lateral flow of groundwater or at springs and 7 thin 30 cm calcified root mats associated with near surface water saturated sediment or springs phreatic spar cements 8 occupy the interstices of conglomerates and sandstones and locally exist as 9 oriented concretions groundwater carbonates are best developed near the toes of the large hanging wall derived alluvial fans whereas phreatic cement preferentially exists in footwall derived alluvial fan conglomerates pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons a predominantly vertical arrangement of root traces peds desiccation cracks and calcic tubules and by gradational contacts the 13 c and 18 o values are similar among pedogenic and shallow groundwater carbonate although locally nodules in eolian sand and gully bed cement have higher 18 o values perhaps due to the effects of evaporation some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of 13 c and 18 o authigenic carbonate in footwall derived alluvial fan sediment has consistently higher values of 13 c than that in hanging wall derived sediment which may reflect differences in vegetative type and or density on either side of the basin

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

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

geochemistry and isotopic study of southern bay of bengal sediments implications for provenance and paleoenvironment during the middle miocene abstract the bay of bengal bob the largest bay in the world is known to receive large amounts of fresh water and suspended particulate matter from different sources the sediments deposited in the bay serve as a repository for paleoenviromental reconstruction studies the miocene epoch is an important interval in geologic time as it is marked by major climatic events that controlled ocean productivity redox conditions and provenance of sediments however limited work has been carried out to establish paleoenviromental changes and provenance of sediments during the miocene here we present detailed geochemical and radiogenic sr nd isotopic results from predominantly middle miocene sediments of ocean drilling program odp site 758a located in the southern bob lat 5 23 n long 90 21 e water depth 2925 m to understand paleoredox conditions paleoproductivity and provenance the results suggest that these sediments are strongly linked to climatically modulated surface processes that affected the region the distribution of redox sensitive element proxies u v cr at interval 17 8 ma points to an oxic depositional environment the major trace and rare earth elements distribution supports prevalence of the indian summer monsoon ism at 12 10 ma productivity proxies babio porg show a decreasing trend during colder events east antarctic ice sheet expansion eais and advent of late miocene cooling lmc events the trace and rare earth element concentrations are generally low except at intervals where glass shards were found the 87sr 86sr and end 0 values indicate that the sediments belong to the high himalayan sedimentary series hhss throughout the middle miocene however prior to initiation of the ism these sediments underwent meteoric alteration in the lowlands before deposition in the bob our records suggest that sediments at odp site 758a may have been introduced into the bob through the irrawaddy yarlung tsangpo drainage system during the entire period of this study the irrawaddy yarlung tsangpo system drained the indus tsangpo suture zone southern tibet the eastern himalayas and the arakan coast before debouching in the bob

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

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

cenozoic shortening budget for the northeastern edge of the tibetan plateau is lower crustal flow necessary 1 two of the most popular mechanisms for thickening the crust beneath the tibetan plateau are 1 pure shear with faulting and folding in the upper crust and horizontal shortening below and 2 flow of lower or middle crust without significant shortening of the upper crust to help discriminate between the relative contributions of these two mechanisms well constrained estimates of upper crustal shortening are needed here we document the cenozoic shortening budget across the northeastern tibetan plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints these sections indicate 11 2 1 east west shortening since middle miocene time and 9 2 3 nne ssw shortening between middle eocene and middle miocene times with little subsequent shortening of this orientation shortening rates accelerate fivefold after middle miocene time given the present day crustal thickness of 56 4 km in northeastern tibet crustal restorations that remove cenozoic shortening suggest that the northeastern tibetan crust was 45 5 km thick prior to the india asia continental collision this precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the tibetan plateau 44 4 km and globally 41 6 km and suggests that pure shear alone may account for cenozoic crustal thickening in northeastern tibet in contrast to eastern tibet where in the absence of significant shortening structures crustal flow has been invoked to explain the addition of crustal material since middle miocene time our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern tibet

a critical examination of evidence for a quaternary glaciation in mt laoshan eastern china abstract accurately determining how far south quaternary glaciers existed in eastern china has important implications for understanding global quaternary climate in this study we provide evidence for a colder paleoclimate in the late pleistocene of the laoshan range on the eastern shandong peninsula at 36 05 36 19 n latitude through documentation of erosional and depositional landforms that were possibly formed by glaciation we integrate field observations documenting and mapping the existence of typical glacial cirques glacial horns moraines aretes tarn lakes and other paleoglacial features in mt laoshan with a detailed digital elevation model dem created in an arcgis environment merged with landsat etm imagery to document the spatial relations of these features other features that may or may not be glacial in origin include large boulder fields that could be moraines or debris avalanches and potholes that could have formed beneath glaciers or in river systems additionally some features previously interpreted as glacial features are here re interpreted as weathering related phenomena although some of these features alone provide only weak to moderate evidence for glaciation together the suite of indicators provides strong evidence for sea level glaciation at 36 n in the late pleistocene of eastern china we further integrate the evidence of glaciation with the late pleistocene paleogeography and climate and suggest that the glaciers formed as a result of orographic effects from moist monsoonal breezes crossing a paleolake developed in north china then rising over mt laoshan and dropping their moisture there with a wide desert formed on the leeward side of the mountains

a detailed 31 000 year record of climate and vegetation change from the isotope geochemistry of two new zealand speleothems abstract uranium series dating and stable isotope analyses of two speleothems from northwest nelson new zealand record changes in regional climate and local forest extent over the past 31 000 years oxygen isotope variation in these speleothems primarily represents changes in the meteoric waters falling above the caves possibly responding to latitudinal changes in the position of the subtropical front in the tasman sea seven positive excursions can be identified in the oxygen isotope record which coincide with periods of glacier advance known to be sensitive to northward movement of the subtropical front four glacier advances occurred during oxygen isotope stage 2 with the most extreme glacial conditions centered on 19 000 cal yr b p 2 an excursion in the oxygen isotope record from 13 800 to 11 700 cal yr b p provides support for a previously identified new zealand glacier advance at the time of the younger dryas stade but suggests it began slightly before the younger dryas as recorded in greenland ice cores carbon isotope variations in the speleothems record changes in forest productivity closely matching existing paleovegetation records on the basis of vegetation changes stage 2 glacial climate conditions terminated abruptly in central new zealand from 15 700 to 14 200 cal yr b p evidence of continuous speleothem growth at one site suggests that depression of the local treeline was limited to 600 700 m below its present altitude throughout the last 31 000 years

seasonal stable isotope evidence for a strong asian monsoon throughout the past 10 7 m y o of wet season rainfall was signi cantly morenegative 29 5 smow prior to 7 5 ma than after 26 5 smow if this change is attributable to a lessening of the amounteffect in rainfall this agrees with oral and soil geochemical datathat indicate increasing aridity beginning at 7 5 ma keywords tibetan plateau monsoon stable isotopes paleohydrology seasonality introductionthe tibetan plateau is the engine that drives the modern asianmonsoon by generating a high altitude region of low pressure in thesummer as the plateau heats and a region of high pressure in the winteras the plateau cools hastenrath 1991 during the summer warm airrises from the plateau pulling moist air off the ocean across the indiansubcontinent and into the highlands this results in heavy summer rain fall on the subcontinent the opposite occurs in the winter resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent thus the presence of a strong wet season dry season alternation implies the presence of a plateau broad and highenough to drive the monsoon the timing of the uplift of the plateau remains a matter of con siderable debate because there are few direct indicators of paleotopog raphy in the geologic record consequently past workers in tibet re lying on indirect indicators of uplift have proposed dates ranging from40 to 3 4 ma on the basis of initiation of potassic volcanism chunget al 1998 turner et al 1993 or extension on the plateau harrisonet al 1995 coleman and hodges 1995 changes in marine sedimen tation rates burbank et al 1993 sediment types rea et al 1998 or biota nigrini and caulet 1992 kroon et al 1991 and changesin stable carbon isotope and palynological patterns on the indian sub continent quade et al 1989 chen 1981 although different areasof the plateau may have risen at different times many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7 8ma by a process such as lithospheric delamination molnar et al 1993 this inference was based on the following approximately coevalphenomena a major change in plant communities of the indian sub continent quade et al 1989 and shifts in marine upwelling patternsthat are linked to an intense monsoon kroon et al 1991 althoughthere is strong evidence for signi cant climate change at 7 8 ma it isunclear whether this is the onset of the monsoon the oral transitionseems to have been a global rather than local phenomenon cerling etal 1997 and monsoonally driven upwelling may have already beenpresent by 10 12 ma nigrini and caulet 1992 kroon et al 1991 because there is an intimate association between the intense sea sonality of the modern monsoon and a high tibetan plateau and be cause evaporation can be unambiguously recognized in the d

carbonate composition and its impact on fluvial geochemistry in the ne tibetan plateau region using co variations of sr ca and mg ca we examined the carbonate compositions of various bedrocks silicate and carbonate rocks and sediments eolian and fluvial sediments sand and topsoil found in the ne tibetan plateau tp region a combined carbonate composition dataset based on our results and other reported data shows that bedrock carbonate composition on the ne tp displays a much broader range of sr ca and mg ca ratios than restricted source carbonate endmembers reported upon in previous studies this has clear implications for modern weathering studies in addition to paleo reconstructions in this tectonically active and climatically variable area during the late cenozoic bedrock carbonate compositions are characterized by disseminated carbonates with higher sr ca and mg ca ratios and sedimentary carbonates mostly marine with lower sr ca but variable mg ca ratios the mostly authigenic carbonates found in sediments show similar trends with a gradient 0 97 1 00 in a plot of log sr ca versus log mg ca suggesting that calcite precipitation processes i e the sources of the dissolved cations in the water control their chemistry based on observations and modeling we conclude that the mixing of authigenic and bedrock carbonate endmembers plus the incongruent dissolution of bedrock carbonates accounts for the bulk carbonate composition of sediments e g loess sand and topsoil a comparison of bedrock and sedimentary carbonate composition with reported fluvial water data in the ne tp suggests that weathering of carbonates in terrigenous sediments rather than in bedrock is mostly responsible for the changes in fluvial sr mg and ca compositions our study suggests that interactions between carbonates and water occur widely during the exposure transport and deposition of sediments significantly modifying regional carbonate compositions and fluvial geochemistry

population bottleneck triggering millennial scale morphospace shifts in endemic thermal spring melanopsids for more than hundred years the thermal spring fed lake pe ea near oradea romania was studied for its highly endemic subfossil and recent fauna and flora one point of focus was the species lineage of the melanopsid gastropod microcolpia parreyssii which exhibited a tremendous diversity of shapes during the earlier holocene as a consequence many new species subspecies and variety names have been introduced over time trying to categorize this overwhelming variability in contrast to the varied subfossil assemblage only a single phenotype is present today we critically review the apparent speciation event implied by the taxonomy based on the presently available information and new data from morphometric analyses of shell outlines and oxygen and carbon isotope data this synthesis shows that one turning point in morphological evolution coincides with high accumulation of peaty deposits during a short time interval of maximally a few thousand years the formation of a small highly eutrophic swamp with increased input of organic matter marginalized the melanopsids and reduced population size the presented data make natural selection as the dominating force unlikely but rather indicates genetic drift following a bottleneck effect induced by the environmental changes this claim contrasts the obvious trend and shows that great morphological variability has to be carefully and objectively evaluated in order to allow sound interpretations of the underlying mechanisms

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

formation and evolution of the badain jaran desert north china as revealed by a drill core from the desert centre and by geological survey abstract the deserts in north china are regarded as direct records of the process of inland asian aridification amongst these deserts those proximal to the chinese loess plateau clp are especially important because of their role as sediment source areas in order to investigate the formation history of the badain jaran desert the largest desert proximal to the clp a 310 m long drill core was obtained from the desert centre which reached the underlying basement rock the lithology of the core consists of an upper layer of aeolian sand with three intercalated layers of lacustrine sediments overlying alluvial fluvial deposits which are developed unconformably over red coloured basement rock a geological survey indicates that the lower red coloured basement rock is of late cretaceous age and is cut by a mesozoic erosion surface widely developed over the badain jaran area on the basis of optically stimulated luminescence and electron spin resonance dating and stratigraphic correlation a preliminary chronological framework was established which was improved by tuning the aeolian sand proportion of the drill core to earth orbital obliquity and precession this final chronology suggests that 1 the basal age of the desert strata is at least 1100 ka corresponding to the formation of the badain jaran desert and 2 that the formation of the erosion surface terminated and the surface had begun to subside by at least 1200 ka the formation of the desert can be summarized as follows the alluvial fluvial fan formed by the heihe river draining the qilian mountains provided sufficient sedimentary material to form the desert under an arid climate while the asian winter monsoon and the westerlies entrained the clastic sediments from the fan area which finally accumulated downwind more distant sediment transport was prevented by the intervening mountainous terrain these coupled processes were driven by tibetan plateau uplift and ongoing global climatic cooling after initial formation of the desert three humid phases occurred in mis 13 15 mis 5 and the holocene during which extensive lakes developed the sequential drying out of the lakes may reflect inter relationships between global climate and the intensity of the asian summer monsoon and the westerlies

plateau uplift forcing climate change around 8 6 ma on the northeastern tibetan plateau evidence from an integrated sedimentary sr record abstract the uplift of the northern tibetan plateau tp has long been regarded as one of major factors accounting for both the enhanced process of aridification recorded in central asia and the shifts in the asian monsoonal circulations that are known to have occurred at approximately 8 ma until now there have been few sedimentary records reported on which can directly and in an integrated fashion shed light on the region s tectonic and climate regimes this can be largely attributed to the fact that the influence exerted by the presence of source materials and subsequent sedimentary sorting in this tectonically active region would be likely to bias significantly any paleoclimatic interpretation of silicate derived proxies based on detailed investigations of strontium sr distributions revealed by multi step leaching in a fluvial sequence from the linxia basin which includes paleosols we reconstructed the long term late miocene regional climatic and environmental changes likely to have occurred on the northeastern tp from 12 1 to 5 2 ma we investigated sr related proxies in two typical paleosol profiles to constrain sr mobility in paleosol weathering and thus to further prompt the interpretation of sr related proxies along the whole section we would suggest that carbonate derived sr concentrations and sr ca ratios are suitable proxies for tracing regional climate change associated with weathering and pedogenesis together they exhibit weakened chemical weathering intensity and pedogenesis at 8 6 ma the concomitant appearance of immature sediments through poor sedimentary sorting revealed by silicate derived rb sr 87 sr 86 sr and eu anomaly eu eu alongside a sharp increase in sedimentation rates noted at this time suggests that it was indeed tectonic uplift that principally controlled the dramatic climatic and environmental changes this multi proxy record of concomitant tectonic uplift and climate change in the linxia basin provides direct geological evidence for uplift driven climate change in the northeastern tp region

tectonic evolution of the sibumasu indochina terrane collision zone in thailand and malaysia constraints from new u pb zircon chronology of se asian tin granitoids three principal granite provinces are defined across se asia as follows 1 the western thailand myanmar burma province consists of hornblende biotite i type granodiorite granites and felsic biotite k feldspar garnet tourmaline granites associated with abundant tin mineralization in greisen type veins new ion microprobe u pb dating results from phuket island show zircon core ages of 212 2 and 214 2 ma and a thermal overprint with rims of 81 2 1 2 and 85 75 ma 2 the north thailand west malaya main range province has mainly s type biotite granites and abundant tin mineralization resulting from crustal thickening following collision of the sibumasu plate with indochina during the mid triassic biotite granites around kuala lumpur contain extremely u rich zircons up to 38000 ppm that yield ages of 215 7 and 210 7 ma 3 the east malaya province consists of dominantly permian triassic i type hornblende biotite granites but with subordinate s type plutons and a type syenite gabbros biotite k feldspar granites from tioman island off the east coast of malaysia also yield a zircon age of 80 1 ma showing cretaceous magmatism in common with province 1 geological and u pb geochronological data suggest that two east dipping in present day coordinates subduction zones are required during the triassic one along the bentong raub palaeo tethyan suture and the other west of the phuket burma province 1 belt supplementary material a full description of u pb analytical methods used and data tables are available at www geolsoc org uk sup18523

stable isotope paleoaltimetry a critical review stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic in situ formed minerals to illustrate the usefulness as well as potential limitations of this method we review 1 processes controlling the isotopic composition of modern precipitation 2 stable isotope data from modern precipitation across regions of high topography and 3 stable isotope data from authigenic minerals that have been used to infer paleotopography from this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography with uncertainties that critically depend on a detailed understanding of local to global scale paleoclimate during the time interval of interest

paleogene high elevations in the qiangtang terrane central tibetan plateau abstract the timing of uplift of the tibetan plateau remains controversial with estimates varying from eocene to more recent than pliocene in particular the paleoaltimetry of the qiangtang terrane on the central tibetan plateau is completely unknown here we present new stable isotope results of fluvial lacustrine carbonate cement pedogenic carbonate and marl from the kangtuo and suonahu formations deposited between 51 and 28 ma in the heihuling bamaoqiongzong area of the northern qiangtang terrane the lithofacies associations indicate that the kangtuo formation was deposited in alluvial fan and fluvial floodplain environments and the suonahu formation was deposited in near shore lacustrine playa lake and channelized fluvial environments carbon and oxygen isotope values coupled with the sedimentary facies interpretations point to evaporation and low respiration rates in the eocene oligocene paleosols suggesting an arid climate in the high qiangtang area at the time the 18opsw values of paleo surface water reconstructed from the lowest i e the least evaporated 18oc pdb values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin the paleoelevation of the northern qiangtang terrane is reconstructed as above 5000 m by at least the middle oligocene 28 ma similar to the present elevation in this area the aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high lhasa terrane established by the middle oligocene blocked the northward transport of tropical moisture maintenance of high elevation 5000 m from at least the oligocene to the present suggests that the qiangtang crust was already thickened by that time and that underthrusting of india beneath asia since then has continued to provide additional material while at the same time driving asian lower crust eastward by crustal flow

magnetostratigraphic constraints on the age of the hipparion fauna in the linxia basin of china and its implications for stepwise aridification abstract late cenozoic sediments in the linxia basin of china on the northeastern tibetan plateau bear abundant fossils of the hipparion fauna from late miocene times these sections record a history of aridification in the asian interior the evolution of the east asian monsoon and multiple uplifts of the tibetan plateau in this paper a new detailed magnetostratigraphic study of a sedimentary sequence from the duikang dk section containing the hipparion fauna is presented to better understand interconnected ecologic and environmental changes the 90 m thick section reveals nine normal and nine reversed zones that span from chron c5n 2n to chron c3ar or from chron c4n 2n to chron c2ar yielding magnetostratigraphic ages of 8 1 ma 3 7 ma for the studied section and 5 3 ma for the hipparion fauna bearing bed combined with all the previous magnetostratigraphic ages of the fauna bearing sediments in the linxia basin hipparion can be distinguished as mainly having lived during five periods phase i 11 5 ma phase ii 8 2 ma phase iii 6 3 ma phase iv 5 3 ma and phase v 2 5 ma based on comparison of climate records in the linxia basin with aridification records from surrounding regions and global cooling during the late cenozoic we suggest that the evolution of hipparion from the linxia basin was mainly driven by continuous global cooling and tectonic uplift of the northeastern tibetan plateau since the late miocene

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

distribution of biomarkers in lacustrine sediments of the linxia basin ne tibetan plateau nw china significance for climate change abstract in this study n alkanes isoprenoids and n alkyl ketones were detected in lacustrine sediments in the maogou section of the linxia basin ne tibetan plateau nw china the distribution characteristics of these compounds correspond to arid climate change in inland asia from the upper oligocene to the pliocene the characteristic bimodal distribution of n alkanes which was observed in the investigated samples is centered on n c17 n c20 and has maximum values at n c18 in all samples n c27 n c31 has maximum values at n c29 in some of the samples the front mode shows a weak even carbon number predominance of short chain n alkanes cpi17 21 0 55 0 89 in contrast the back mode has a strong odd carbon number predominance of long chain n alkanes cpi25 31 1 41 2 42 changes in the nc27 nc31 ratio woody plants grassy plants along the entire section corresponded to three climate stages an arid to humid climate stage from 22 5 to 18 4 ma 6 25 to 5 5 ma and two major humid stages at 18 4 ma and 5 5 ma a warm humid climate was identified for the 10 ma to 9 ma period which turned to an arid cold climate from 8 ma to 7 5 ma by assessing changes in the n c17 n c21 n c27 n c31 ratios warm humid conditions gradually increased from 7 5 ma to 5 5 ma as has been confirmed by multiple geochemical climate indicators a distinctive climate change toward arid cold conditions at 8 ma corresponded to a striking increase in the high carbon numbered n alkanes and a rapid decrease in the n c17 n c21 n c27 n c31 and n c27 n c31 ratios of n alkanes which suggests a large input of higher plants our observation from the biomarker analysis is in agreement with previous reports that used palynofloras to determine that climate change in the investigated region responded to the uplift of the tibetan plateau at 8 ma

pulsed miocene range growth in northeastern tibet insights from xunhua basin magnetostratigraphy and provenance author s lease ro burbank dw hough b wang z yuan d abstract sedimentary rocks in tibetan plateau basins archive the spatiotemporal patterns of deformation erosion and associated climate change that resulted from cenozoic continental collision despite growing understanding of basin development in northeastern tibet during initial india asia collision as well as in the late miocene holocene surprisingly little is known about the intervening period a time when the plateau may have undergone fundamental tectonic changes to fill this gap we present new magnetostratigraphy from a g2300 m thick fluviolacustrine succession that spans ca 30 9 3 ma an integrated analysis of sedimentology subsidence and provenance from this section reveals the sequential pulsed erosion of multiple ranges bordering the xunhua basin emergence of the wnw trending laji shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 ma and a transition to coarse allu vial facies at 20 3 ma detrital zircon u pb age spectra show that these coarse sediments came from basement terranes within the laji shan together these observations suggest accelerated growth of the laji shan and its coupled foreland basin at ca 22 ma the most rapid accumulation rates in xunhua basin occur within the finest grained strata and suggest an underfilled basin during the fastest interval of laji shan deformation growth of the laji shan occurred northward of the contemporaneous plateau margin which had been defined since ca 45 50 ma by the west qinling lying 60 km farther south hence following 20 25 m y of apparent stability the deformation front in this region jumped 60 km to the north at ca 22 ma subsequently growth of the north trending jishi shan occurred at ca 13 ma and is highlighted by an acceleration in xunhua basin accumulation rates between 12 and 9 ma as well as by a significant change in detrital zircon provenance of nearby linxia basin deposits by 11 5 ma initial growth of the wnw trending laji shan in the early miocene and subsequent growth of the north trending jishi shan 10 m y later support interpretations of a middle mio cene kinematic reorganization in northeastern tibet 2012 geological society of america

paleoclimatic estimation reveals a weak winter monsoon in southwestern china during the late miocene evidence from plant macrofossils the late miocene xianfeng flora of yunnan province southwestern china was chosen to reconstruct the paleoclimate and the intensity of the asian monsoon three available quantitative climate reconstruction methods from fossil plants i e leaf margin analysis lma the climate leaf analysis multivariate program clamp and the coexistence approach ca were used in this study mats mean annual temperatures resulting from these three approaches i e lma 17 2 2 38 degrees c clamp 15 43 1 25 degrees c ca 17 2 21 7 degrees c appear to have been higher than the present 14 9 degrees c suggesting a much warmer climate in the late miocene both the growing season precipitation gsp from clamp 1908 7 217 7 mm and mean annual precipitation map from ca 1206 0 1613 0 mm estimates are higher than modern values 1003 2 mm gsp and 1038 mm map respectively indicating a more humid climate during the late miocene by comparing these with climates reconstructed from neighboring late miocene floras we conclude that the general late miocene climate appears warmer and more humid than present conditions in southwestern china furthermore the evident differences in estimated monthly temperatures between the summer and winter and precipitations between the humid and dry seasons indicate the existence of seasonality though not as strong as that of today difference in precipitation of dry season indicates a marked strengthening in the winter monsoon since the late miocene a new monsoon intensity index has also been defined based on precipitation seasonality to investigate the intensity of the asian monsoon c 2012 elsevier b v all rights reserved

a coupled environmental system formed at about 2 5 ma in east asia abstract the somewhat abrupt commencement of loess deposition on a large scale in north central china is palaeomagnetically measured at about 2 5 ma the formation of the dynamical conditions responsible for windblown loess processes implies a reorganization of the atmospheric circulation over eastern asia it is speculated that at that time the accelerated uplift of the tibetan plateau had reached a threshold height giving rise to the intensification and northward shift of the siberian high and deepening of the cyclone in the lee of the tibetan plateau the uplift of tibet and the resulting reorganization of atmospheric circulation at about 2 5 ma would initiate a coupled environmental system tibetan plateau inland desert loess plateau in eastern asia that could be a key system in understanding the onset of glaciation in northern hemisphere and climatic change in the quaternary the climatic shift from near continuous warm condition before 2 5 ma to frequent and chaotic fluctuations after is speculated to be triggered to a significant extent by the uplift of the tibetan plateau an orography cold surge albedo temperature feedback mechanism is developed in explaining the harsher climate over east asia during glacial time and is thought to be one of the major contributors in promoting glacial development on the globe

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

strengthening of the east asian summer monsoon revealed by a shift in seasonal patterns in diet and climate after 2 3 ma in northwest china article i nfo to examine climate variability in northwest china in the late cenozoic and to test hypotheses regarding the development of c4 ecosystems and the dynamics of the asian monsoons the carbon and oxygen isotopic compositions of 32 bulk and 368 serial tooth enamel samples from herbivores in the linxia basin gansu province ranging in age from 25 ma to the present were determined the results corroborate and improve the record previously obtained from the area showing that all mammals in the linxia basin lived in habitats consisting primarily of c3 vegetation prior to 2 3 ma and that c4 grasses did not become a significant component of local ecosystems until the quaternary the data also show that shifts in climate to drier and or warmer conditions after about 14 9 5 7 and 2 5 ma as indicated by positive 18 o excursions in the bulk enamel 18 o record were accompanied by increased seasonality whereas negative 18 o shifts in the bulk

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

the paleoaltimetry of tibet an isotopic perspective stable isotopes provide a valuable perspective on the timing of elevation change of the tibetan plateau we begin our paper by looking in depth at isotopic patterns in modern tibet we show that the 18 o value of surface waters decreases systematically up the himalayan front in central nepal by about 2 8 km in agreement with the patterns documented and modeled by previous research on the tibetan plateau itself there is no apparent correlation between elevation and the 18 o value of flowing surface waters both surface waters and soil carbonates display a northward increase in 18 o values of about 1 5 north of the himalayan crest even though elevation increases modestly the isotopic increase with latitude reduces the isotope elevation gradient for water in the northernmost plateau to 1 to 2 km carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau temperature estimates from clumped isotope 47 analyses of modern soil carbonates significantly exceed mean annual air t and modeled maximum summer soil temperatures by 15 8 2 8 and 9 7 2 5 c respectively similarly elevated temperatures best account for the 18 o values observed in modern soil and lake carbonates we recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude corrected isotopic values with one notable exception our revised model produces paleoelevation estimates very close to previous estimates the exception is the reconstruction from late eocene age deposits at xoh xil for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed therefore there is no evidence for northward progression through time of tibetan elevation change instead the available but admittedly very scanty evidence suggests that much of tibet attained its modern elevation by the mid eocene a truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across tibet especially in the center and north and for 15 ma

the impact of neogene grassland expansion and aridification on the isotopic composition of continental precipitation the late cenozoic was a time of global cooling increased aridity and expansion of grasslands in the last two decades numerous records of oxygen isotopes have been collected to assess plant ecological changes understand terrestrial paleoclimate and to determine the surface history of mountain belts the 18o values of these records in general increase from the mid miocene to the recent we suggest that these records record an increase in aridity and expansion of grasslands in midlatitude continental regions we use a nondimensional isotopic vapor transport model coupled with a soil water isotope model to evaluate the role of vapor recycling and transpiration by different plant functional types this analysis shows that increased vapor recycling associated with grassland expansion along with biomechanistic changes in transpiration by grasses themselves conspires to lower the horizontal gradient in the 18o of atmospheric vapor as an air mass moves into continental interiors the resulting signal at a given inland site is an increase in 18o of precipitation with the expansion of grasslands and increasing aridity matching the general observed trend in terrestrial cenozoic 18o records there are limits to the isotopic effect that are induced by vapor recycling which we refer to here as a hydrostat in the modern climate this hydrostatic limit occurs at approximately the boundary between forest and grassland ecosystems