a 1000 year history of large floods in the upper ganga catchment central himalaya india abstract determining the frequency magnitude and causes of large floods over long periods in the flood prone himalaya is important for estimating the likelihood of future floods a thousand year record with some information from 2600 years ago of the frequency and some estimates of velocities and discharges of large floods has been reconstructed in the upper ganga catchment india using written reports litho stratigraphy and sedimentology and dated by optical and radiocarbon methods in the upper ganga catchment rainfall triggers large landslides that dam rivers and release large amounts of water when they burst thereby amplifying the effects of rainfall the large floods in the catchment may be the result of landslide dam bursts rather than glacial lake bursts and these are likely to continue and possibly worsen as the monsoon intensifies over the next century however preliminary information suggests that the recent devastating flood of june 2013 was the result of heavy rainfall not landslide dam bursts the frequency record is non random and shows a high frequency between ad 1000 and ad 1300 omitting uncertainties then a low frequency until a cluster of floods occurred about 200 years ago then increased frequency this temporal pattern is like but not identical with that in peninsular india and both appear to be the result of variations in the monsoon

tracing the distribution of erosion in the brahmaputra watershed from isotopic compositions of stream sediments bank sediments and suspended loads of the brahmaputra river and its important tributaries were collected from the himalayan front to bangladesh along with most of the important tributaries chemical and isotopic compositions of the sediments are used to trace sediment provenance and to understand erosion patterns in the basin overall isotopic compositions range from 0 7053 to 0 8250 for sr and nd from 20 5 to 6 9 this large range derives from the variable proportions of sediments from himalayan formations with high sr isotopic ratios and low nd and transhimalayan plutonic belt with lower sr isotopic ratios and higher nd the latter are exposed to erosion in the tsangpo and in the eastern tributary drainages overall erosion of the himalayan rocks is dominant representing ca 70 of the detrital influx compositions of the brahmaputra main channel are rather stable between 0 7177 and 0 7284 for sr and between 14 4 and 12 5 for nd throughout its course in the plain from the siang tsangpo at the foot of the himalayan range down to the delta this stability despite the input of large himalayan rivers suggests that the siang tsangpo river represents the major source of sediment to the whole brahmaputra geochemical budget implies that erosion of the namche barwa zone represents about 45 of the total flux at its outflow before confluence with the ganga from only 20 of the mountain area higher erosion rates in the eastern syntaxis compared to the other himalayan ranges is related to the rapid exhumation rates of this region possibly triggered by higher precipitation over the far eastern himalaya and the high incision potential of the tsangpo river due to its very high water discharge

fashion and phases of late pleistocene aggradation and incision in the alaknanda river valley western himalaya india abstract we study the aggradation and incision of the alaknanda river valley during the late pleistocene and holocene the morphostratigraphy in the river valley at deoprayag shows the active riverbed a cut terrace and a fill terrace the sedimentary fabric of the fill terrace comprises four lithofacies representing 1 riverbed accretion 2 locally derived debris fan 3 the deposits of waning floods and 4 palaeoflood records the sedimentation style coupled with geochemical analysis and optically stimulated luminescence osl dating indicate that this terrace formed in a drier climate and the river valley aggraded in two phases during 21 18 ka and 13 9 ka during these periods sediment supply was relatively higher incision began after 10 ka in response to a strengthened monsoon and aided by increase of the tectonic gradient the cut terrace formed at 5 ka during a phase of stable climate and tectonic quiescence the palaeoflood records suggest wetter climate 200 300 yr ago when the floods originated in the upper catchment of the higher himalaya and in the relatively drier climate 1 2 ka when locally derived sediments from the lesser himalaya dominated flood deposits maximum and minimum limits of bedrock incision rate at deoprayag are 2 3 mm a and 1 4 mm a

reduced early holocene moisture availability in the bayan har mountains northeastern tibetan plateau inferred from a multi proxy lake record abstract a late glacial and holocene lake sediment record from the northeastern tibetan plateau was investigated using ostracod and geochemical analysis after lake koucha was established about 16 cal ka bp oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka bp clearly suggesting that the water body was a closed basin lake from about 10 cal ka bp the salinity of the lake rose continuously until hyperhaline levels were reached about 7 5 cal ka bp this increase in salinity was accompanied by a falling lake level extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8 5 cal ka bp causing a unique thin gravel containing layer within the continuous fine grained lake sediments the age control of our record does not allow us to correlate this event exactly to the widely recorded 8 2 cal ka bp event however the recovered sediments from lake koucha provide strong evidence that climate changes around 8 5 cal ka bp had significant environmental implications in northeastern tibet after about 7 3 cal ka bp the available moisture significantly increased causing a continuous salinity decline until the present day freshwater conditions of a through flow lake were established about 6 5 cal ka bp in lake koucha this major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte dominated to a phytoplankton dominated lake a slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4 3 and 2 0 cal ka bp in phase with numerous records from monsoonal asia the reconstructed overall change in moisture availability from dry conditions before about 7 3 cal ka bp to wetter conditions afterwards contradicts most palaeoclimate records from the tibetan plateau and its surroundings which show that wetter conditions in the early holocene were replaced by drier and probably cooler conditions in the mid and late holocene however in the semi arid region of lake koucha with a mean annual precipitation of about 500 mm the insolation driven strengthened monsoon in the early holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability our record provides further evidence for a regionally non uniform response of the available moisture evolution to the overall monsoonal signal in the holocene climate history of central asia

erosion of the tsangpo gorge by megafloods eastern himalaya at the southeastern margin of the tibetan plateau the yarlung tsangpo river plunges through the himalaya to drop 2 km through the tsangpo gorge upstream relict glacial dams and impounded lake terraces suggest that quaternary lakes as large as 800 km 3 catastrophically drained through the gorge as megafloods we report on new megaflood deposits downstream of the gorge and use detrital zircon u pb provenance data to demonstrate that these high magnitude events originated in tibet and more effectively focused erosion in the gorge than both the extremely erosive modern peak flows and one of the largest landslide dam outburst floods ever documented our findings support the proposition that in this steep narrow gorge where hillslope angles are near the threshold angle of bedrock failure megafloods provide a mechanism to rapidly evacuate hillslope material and focus erosion on channel adjacent hillslopes although megaflood frequency remains unconstrained we demonstrate the capability of these events to contribute substantially to rapid exhumation in this region

widespread aggradation in the mountainous catchment of the alaknanda ganga river system timescales and implications to hinterland foreland relationships abstract cut and fill type fluvial terraces are ubiquitous in the lesser himalayan zone of the alaknanda ganga ganges rivers system which flows perpendicular to himalayan lithotectonic units and traverses a steep climatic gradient lithofacies analysis of the sedimentary sequences of cut and fill terraces indicated that the valley aggradation took place via 1 channel bar development and excess sediment supply 2 debris flows composed of mixed rounded to sub rounded lithoclasts resulting from episodic high intensity rainfalls in the upper catchment or 3 debris flows or rockfalls generated by local landslides the luminescence chronology indicates that valley aggradation took place in two phases 49 25 ka and 18 11 ka the incision of the fill started soon after 11 ka paleoclimatic records from marine sediments indicate that the aggradation and incision in the alaknanda ganga river has oscillated in phase with global climatic variations glaciation deglaciation processes in the upper catchment produced huge amounts of sediment between 63 and 11 ka which was fluvially transferred to the lower valley via several cycles of erosion and deposition leading to extensive aggradation the climatic amelioration at 11 ka and the completion of deglaciation processes led to increased fluvial discharge and decreased sediment supply conditions conducive for incision of the alluvial fills records from the indo gangetic plain and the ganga delta demonstrate that the phase of aggradation was regional but that incision in the foreland started at least 2 3 ka later after 7 ka bedrock incision rates as calculated from dated alluvial covers of terraces that are separated by bedrock steps are spatially variable and fall within the range of rates reported from across the himalaya these estimated rates however are higher than the basin average erosion rates calculated using isotopic mass balance in riverbed sediments this study suggests that during the last 50 ka river dynamics in the himalayas were dominated by monsoon variability and the role of tectonic activity was limited to bedrock incision in few reaches only

sand petrology and focused erosion in collision orogens the brahmaputra case abstract the high relief and tectonically active himalayan range characterized by markedly varying climate but relatively homogeneous geology along strike is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments coupling of surface and tectonic processes is most evident in the eastern namche barwa syntaxis where the tsangpo siang brahmaputra river draining a large elevated area in south tibet plunges down the deepest gorge on earth here composition of river sands changes drastically from lithic to quartzofeldspathic after confluence with the lohit river draining the transhimalayan equivalent mishmi arc batholiths sediment composition remains remarkably constant across assam indicating subordinate contributions from himalayan tributaries independent calculations based on petrographical mineralogical and geochemical data indicate that the syntaxis representing only 4 of total basin area contributes 35 6 to the total brahmaputra sediment flux and 20 of total detritus reaching the bay of bengal such huge anomalies in erosion patterns have major effects on composition of orogenic sediments which are recorded as far as the bengal fan in the brahmaputra basin in spite of very fast erosion and detrital evacuation chemical weathering is not negligible sand sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon drenched assam plains where clinopyroxenes are selectively altered plagioclase instead is preferentially weathered only in detritus from the shillong plateau which is markedly enriched in microcline most difficult to assess is the effect of hydraulic sorting in bangladesh where quartz garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea high resolution petrographic and dense mineral studies of fluvial sands provide a basis for calculating sediment budgets for tracing patterns of erosion in mountain belts and for better understanding the complex dynamic feedback between surface processes and crustal scale tectonics

palaeoclimatic changes in the qinghai lake area during the last 18 000 years abstract multi proxy analysis of a sediment core from qinghai lake including pollen carbonate toc tn and 13 c of organic matter was used to document regional climatic changes during the last 18 000 years climate was very cold and dry before 16 9 cal kyr bp the warm and wet period which began at about 14 1 cal kyr bp culminated at 6 5 cal kyr bp and came to its end at about 4 5 cal kyr bp after that the climate gradually became colder and drier during the transitional period from the late glacial to the holocene the climate fluctuated frequently the short term climatic oscillations including the bolling allerod younger dryas and the cooling event at ca 8 2 cal kyr bp appear to be recorded in the qinghai lake sedimentary record the climatic evolution in the qinghai lake area during the past 18 000 years indicates that solar insolation changes on the ten thousand year scale are the likely driving force of the east asia monsoon

morpho sedimentary records at the brahmaputra river exit ne himalaya climate tectonic interplay during the late pleistocene holocene morphological and sedimentary records at the exit of brahmaputra river at pasighat in the ne himalaya inform about the climate tectonic interplay during the past ca 15 ka the geomorphology of the area comprises 1 fan terrace t3 2 a high angle fan 3 terrace t2 4 terrace t1 and 5 a low angle fan geomorphic consideration suggests that the fan terrace t3 and high angle fans are the oldest units and were coeval the low angle fan is the youngest geomorphic unit sedimentological studies and optically stimulated luminescence chronology suggest that i fan terrace t3 formed between 13 and 10 5 ka and comprised multiple events of debris flows separated by the aggradation as channel bars in a braided river environment ii the high angle fan formed during 15 10 ka and comprises channel bar aggradation in braided river conditions iii terrace t2 formed during 10 8 ka due to aggradation in a braided channel environment with lesser events of debris flows iv terrace t1 formed during 7 and 3 ka took place as bars of the braided river sudden coarsening of the sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka and v the low angle fans dated to 3 ka formed due to aggradation in a small tributary joining the brahmaputra river this implies a phase when the main channel of the brahmaputra did not flood regularly and the tributaries were actively aggrading the sedimentation style and incision of these geomorphic units responded to contemporary climatic changes and uplift in the siwalik range along the himalayan frontal fault copyright 2008 john wiley sons ltd

a passive low frequency seismic experiment in the albertine graben uganda abstract a passive seismic experiment was conducted in april may 2010 in the albertine graben region in uganda to record low frequency seismic signals and explore the possibility of their exploitation in this area as a direct hydrocarbon indicator dhi recordings were made at locations directly overlying both hydrocarbon and water bearing strata within the sedimentary basin as well as reference sites external to the basin directly on the basement contrary to findings published in some literature to date we found that spatial variations in the analysed wavefield parameters correlate with the underlying geology rather than the presence or absence of hydrocarbons inversion of the surface wave fundamental mode dispersion curve as well as the observed horizontal to vertical spectral ratio of both surface and body waves provide evidence that the observed spectral variations can be explained solely by a simple layered gradient velocity model without the presence of any kind of anomaly that could be attributed exclusively to a hydrocarbon reservoir consequently it is recommended that knowledge of the geological and velocity structure is sought when analysing passive low frequency seismic data sets this is a fundamental prerequisite in order to guard against misinterpretation of the spatial variation of seismic derived attributes as dhis

abrupt changes in indian summer monsoon strength during 33 800 to 5500 years b p speleothem proxy records from northeastern ne india reflect seasonal changes in indian summer monsoon strength as well as moisture source and transport paths we have analyzed a new speleothem record from mawmluh cave meghalaya india in order to better understand these processes the data show a strong wet phase 33 500 32 500 years b p followed by a weak dry phase from 26 000 to 23 500 years b p and a very weak phase from 17 000 to 15 000 years b p the record suggests abrupt increase in strength during the bolling allerod and early holocene periods and pronounced weakening during the heinrich and younger dryas cold events we infer that these changes in monsoon strength are driven by changes in temperature gradients which drive changes in winds and moisture transport into northeast india

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

a classification of metamorphic grains in sands based on their composition and grade abstract an operational classification of metamorphic grains in sands and sandstones is proposed with the aim of enhancing data reproducibility among operators and the potential of high resolution bulk petrography in provenance studies for each of four protolith compositions metapelite metapsammite metafelsite metacarbonate metabasite six archetype grains displaying increasing degree of recrystallization and foliation development are illustrated such a classification grid is specifically devised as a subsidiary tool for point counting with the gazzi dickinson method traditional qfr parameters can also be easily recalculated from the data set obtained thus meeting all possible needs decker and helmold 1985 suttner and basu 1985 an experiment shows that usage of visual comparison standards effectively minimizes operator variation and allows retrieval of crucial information during point counting in a reproducible way a petrogenetic grid is presented as a subsidiary tool for classifying grains that include index minerals and to help correlation with dense mineral data the metamorphic index mi is introduced as an estimator of average metamorphic grade of source rocks our classification an extension of concepts used first in the study of arc continent collision in taiwan dorsey 1988 and successfully expanded to interpret the evolution of continent continent collision in the himalayas najman and garzanti 2000 white et al 2002 proves to be fruitful in provenance analysis of foreland basin sediments shed from alpine type thick skinned collision orogens particularly when integrated with dense mineral geochemical and geochronological data

paleohydrology of late pleistocene superflooding altay mountains siberia cataclysmic flooding is a geomorphological process of planetary significance landforms of flood origin resulted from late pleistocene ice dammed lake failures in the altay mountains of south central siberia peak paleoflows which exceeded 18 x 10 6 cubic meters per second are comparable to the largest known terrestrial discharges of freshwater and show a hydrological scaling relation to floods generated by catastrophic dam failures these seem to have been earth9s greatest floods based on a variety of reconstructed paleohydraulic parameters

australasian microtektites in the south china sea and the west philippine sea implications for age size and location of the impact crater abstract microtektites from two deep sea cores in the south china sea and the west philippine sea are identified as belonging to the australasian tektite strewn field based on the morphology chronostratigraphic occurrence and geographical location of these microtektites the higher concentrations of microtektites 1000 cm2 in the marginal seas of the western pacific with the peak concentration in the south china sea support the hypothesis of a large impact crater in indochina these two new occurrences lead to a more precise dating of the impact event at 793 ka whereas the size of the australasian source crater on the indochina peninsula is estimated to be 90 1 16 km introduction the australasian strewn field covering at least one tenth of the earth s surface is the largest and the youngest 0 77 0 02 ma izett and obradovich 1992 of the four known tektite strewn fields microtektites belonging to the australasian strewn field have been found in more than 40 deep sea cores throughout much of the indian ocean the western equatorial pacific ocean and the sulu celebes and philippine seas glass

the namche barwa syntaxis evidence for exhumation related to compressional crustal folding abstract geological observation in the eastern end of the himalayas shows that the asia india suture is folded and continues southward into india and burma where the continuation of the transhimalayan plutonic complex can be identified metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating are consistent with the geological interpretation in addition they establish cooling exhumation of rocks from c 30 km depth within the last 4 ma rapid exhumation of deep crustal levels in mountain systems is generally related to extension subsequent to thermal softening of a thickened continental lithosphere however in the eastern himalayas we argue that exhumation is caused by c 10 mm yr 1 erosion coeval with crustal scale folding the general history of this syntaxis resembles the evolution of the western himalayas syntaxis in pakistan

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

tibetan plateau river incision inhibited by glacial stabilization of the tsangpo gorge much attention has focused on how and when the tibetan plateau formed but far less research has been done on the controls on river incision into the plateau itself oliver korup and david montgomery address this issue which also relates to the question of why the tibetan plateau is still there at all despite hosting one of the deepest gorges on earth the plateau edge is extremely well defined and according to widely accepted stream power theory should be heavily dissected by one of asia s most powerful rivers the yarlung tsangp or yarlung zangbo korup and montgomery propose that the southeastern edge of the plateau was preserved in part by the presence of glaciers that were large enough during the holocene to stall aggressive river downcutting along the plateau margin a considerable amount of research has focused on how and when the tibetan plateau formed in the wake of tectonic convergence between india and asia1 although far less enquiry has addressed the controls on river incision into the plateau itself2 widely accepted theory3 predicts that steep fluvial knick points river reaches with very steep gradients in the eastern himalayan syntaxis at the southeastern plateau margin should erode rapidly4 driving a wave of incision back into the plateau preservation of the plateau edge thus presents something of a conundrum that may be resolved by invoking either differential rock uplift matching erosional decay5 6 7 or other mechanisms for retarding bedrock river incision8 9 in this region where high stream power excludes the potential for aridity as a simple limit to dissection of the plateau10 here we report morphologic evidence showing that quaternary depression of the regional equilibrium line altitude where long term glacier mass gain equals mass loss was sufficient to repeatedly form moraine dams on major rivers such damming substantially impeded river incision into the southeastern edge of the tibetan plateau through the coupled effects of upstream impoundment and interglacial aggradation such glacial stabilization of the resulting highly focused river incision centred on the tsangpo gorge could further contribute to initiating and accentuating a locus of rapid exhumation known as tectonic anaeurysm6

holocene vegetation and climate histories in the eastern tibetan plateau controls by insolation driven temperature or monsoon derived precipitation changes abstract the climates on the eastern tibetan plateau are strongly influenced by direct insolation heating as well as monsoon derived precipitation change however the moisture and temperature influences on regional vegetation and climate have not been well documented in paleoclimate studies here we present a well dated and high resolution loss on ignition peat property and fossil pollen record over the last 10 000 years from a sedge dominated fen peatland in the central zoige basin on the eastern tibetan plateau and discuss its ecological and climatic interpretations lithology results indicate that organic matter content is high at 60 80 between 10 and 3 ka 1 ka 1000 cal yr bp and shows large magnitude fluctuations in the last 3000 years ash free bulk density as a proxy of peat decomposition and peatland surface moisture conditions oscillates around a mean value of 0 1 g cm 3 with low values at 6 5 4 7 ka reflecting a wet interval and an increasing trend from 4 7 to 2 ka suggesting a drying trend the time averaged mean carbon accumulation rates are 30 6 gc m 2 yr for the last 10 000 years higher than that from many northern peatlands tree pollen mainly from picea mostly reflecting temperature change in this alpine meadow forest ecotonal region has variable values from 3 to 34 during the early holocene reaches the peak value during the mid holocene at 6 5 ka and then decreases until 2 ka the combined peat property and pollen data indicate that a warm and wet climate prevailed in the mid holocene 6 5 4 7 ka representing a monsoon maximum or optimum climate for the region the timing is consistent with recent paleo monsoon records from southern china and with the idea that the interplays of summer insolation and other extratropical large scale boundary conditions including sea surface temperature and sea level change control regional climate the cooling and drying trend since the mid holocene likely reflects the decrease in insolation heating and weakening of summer monsoons regional synthesis of five pollen records along a south north transect indicates that this climate pattern can be recognized all across the eastern tibetan plateau the peatland and vegetation changes in the late holocene suggest complex and dramatic responses of these lowland and upland ecosystems to changes in temperature and moisture conditions and human activities