oxygen deuterium and strontium isotope characteristics of the indus river water system abstract understanding the sources and compositional characteristics of waters and sediments in the indus river system is extremely important as its water availability is one of the primary factors for sustenance of the irrigation activities and the socioeconomic status of a very densely populated region of the world here we used stable isotopic compositions d and 18o and strontium isotopic ratio 87sr 86sr in the indus river water its tributaries and its small streams nallahs in the indian territory to understand the regional hydrology water sources and catchment processes evaporation transpiration recycling and mixing the 18o values in the indus river system irs ranges from 16 9 to 12 5 and d from 122 8 to 88 5 the indus river and its major tributaries such as the zanskar nubra and shyok rivers are characterized by relatively lower 18o values whereas tangtse and other small streams contributing to the indus are relatively enriched in 18o the local meteoric water line for the irs was found to be d 7 87 18o 11 41 which is similar to the global meteoric water line gmwl indicating meteoric origin of the water and insignificant secondary evaporation in the catchment the deuterium excess d excess in the irs varies between 6 5 and 14 9 with an average of 11 7 which is mostly higher than the long term average for the indian summer monsoon 8 the higher d excess value is because of the contribution of moisture from westerlies a simple mass balance shows 26 water in the main indus channel is contributed by the westerlies originated from the mediterranean sea the sr isotope ratio in irs varies between 0 70515 and 0 71291 wherein the indus and its tributary rivers shyok and nubra are characterized by relatively high sr isotope ratios avg 0 71086 0 71243 compared to the zanskar and tangtse tributaries sr 0 709 because of the variation in silicate rock weathering component and carbonate rock weathering component ratios respectively

topography relief and trmm derived rainfall variations along the himalaya 1 along the southern himalayan topographic front the indian summer monsoon modulates erosive processes and rates to investigate the influence of topography and relief on rainfall generation and resultant erosion we processed satellite rainfall amounts for the last 8 years 1998 2005 from the tropical rainfall measurement mission trmm based upon a spatial resolution of 5 5 km for the himalaya we identify 1 the spatial distribution of rainfall and 2 the large scale relationships between topography relief and rainfall locations our results show two distinct rainfall maxima along strike in the himalaya the first outer rainfall peak occurs along the southern margin of the lesser himalaya within a narrow band of mean elevation 0 9 0 4 km and mean relief 1 2 0 2 km the second discontinuous inner band typically occurs along the southern flank of the greater himalaya elevation and relief both 2 1 0 3 km

towards defining the transition in style and timing of quaternary glaciation between the monsoon influenced greater himalaya and the semi arid transhimalaya of northern india abstract to help delineate the transition in pattern and timing of glaciation between two contrasting regions lahul to the south and ladakh to the north moraines in the puga and karzok valleys of sanskrit in the transhimalaya of northern india were mapped and dated using cosmogenic 10 be in lahul late quaternary glaciation was extensive with total valley glacial systems being 100 km in extent whereas glaciation in ladakh has been comparatively restricted with glaciers advancing only 15 km from the contemporary glaciers during the last 200 ka in the puga valley glaciers advanced 15 km at 129 ka and 10 km at 46 ka 4 2 ka and 0 6 ka in the karzok valley glaciers advanced 1 km at 3 6 ka boulder exposure ages from a large moraine complex in karzok indicate a glacial advance at 80 ka of 4 km from the present ice margin the oldest moraine in karzok is 311 ka indicating that glaciers advanced 10 km from the present ice margin during or before marine isotope stage 9 the glacial chronology of the two valleys shows a lack of early holocene glaciation and generally asynchronous glaciation between them moraines in the puga and karzok valleys broadly correlate with previous studies in the zanskar range but the paucity of data for many of the glacial stages across the zanskar region makes the correlations tentative the lack of early holocene glaciation in the puga and karzok valleys is in stark contrast to many regions of the himalaya including lahul and the restricted glacial extent in zanskar is more similar to the style of glaciation in ladakh the similarity between the glacial records in the puga and karzok study areas suggests that the transition to lahul style glaciation is to the south of the karzok valley showing that this geographical transition is abrupt

chemical processes affecting the mobility of major minor and trace elements during weathering of granitic rocks abstract the behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some portuguese granitic rocks element mobilities are calculated from geochemical data normalized with respect to ti in the fresh parent rock chemical elements are divided into two groups immobile and mobile on the basis of their geochemical distribution during weathering elements that are immobile during weathering are zr hf fe al th nb sc and the ree very mobile are ca na p k sr ba rb mg and si mobile elements are derived mainly from leachable minerals such as feldspars micas and apatites whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals the geochemical behaviour of mn cr v fe and ce is very dependent on redox conditions redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite the ree are mobilized or fractionated during late stages of weathering but not during moderate stages of weathering this fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing ree

sr and nd isotopes in river sediments from the ganga basin sediment provenance and spatial variability in physical erosion 1 sr and nd concentrations and isotope compositions in sediment of the ganga river from gangotri to rajmahal and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the ganga subbasins sr and nd in silicates range from 37 to 138 and from 10 to 36 g g with 87sr 86sr and nd of 0 7474 0 8428 and 25 5 to 15 5 respectively the results suggest that 65 of ganga mainstream sediments are derived from the higher himalayan crystallines highlighting intense physical erosion in this region the 87sr 86sr values of sediments in the gangetic plain show nearly identical trends during two seasons with a sharp and significant decrease at barauni downstream of gandak confluence this brings out the major impact of the sediment contribution from the gandak to the ganga mainstream model calculation suggests that about half of the ganga sediment at rajmahal is sourced from the gandak the erosion rates in the himalayan subbasins of the ganga range between 0 5 and 6 mm a where a is years with the gandak having the highest erosion rate high relief and intense precipitation over the headwater basins of the gandak appear to drive the rapid and focused erosion of this basin the results of this study and those in literature suggest that the eastern syntaxis brahmaputra the western syntaxis indus and the gandak have much higher physical erosion rates than the other himalayan basins focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics

late holocene climate in the lower narmada valley gujarat western india inferred using sedimentary carbon and oxygen isotope ratios we report the late holocene climate and vegetation of the lower narmada valley gujarat western india as inferred from the stable carbon and oxygen isotopic composition 13c and 18o of sediment

experimental and textural investigation of welding effects of compaction sintering and vapor phase crystallization in the rhyolitic rattlesnake tuff abstract the abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification we conducted two series of experiments using ash from the nonwelded base of the rhyolitic rattlesnake tuff of oregon usa to examine conditions of welding one series of experiments was conducted at atmospheric pressure 1 at in a muffle furnace with variable run times and temperature and another series was conducted at 5 mpa and 600 c in a cold seal apparatus with variable run times and water contents we compared the results to a suite of incipiently to densely welded natural samples of the rattlesnake tuff experiments at 1 at required a temperature above 900 c to produce welding which is in excess of the estimated pre eruptive magmatic temperature of the tuff the experiments also yielded globular clast textures unlike the natural tuff during the cold seal experiments the gold sample capsules collapsed in response to sample densification textures and densities that closely mimic the natural suite were produced at 5 mpa 600 c and 0 4 wt h2o over run durations of hours to 2 days clast deformation and development of foliation in 2 week runs were greater than in natural samples both more and less water reduced the degree of welding at otherwise constant run conditions for 5 mpa experiments changes in the degree of foliation of shards and of axial ratios of bubble shards and non bubble mainly platy shards are consistent with early densification related to compaction and partial rotation of shards into a foliation subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards sintering local fusion of shard shard contacts was increasingly important with longer run times higher temperatures and greater pressures during runs with high water concentrations sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces a few tenths wt h2o in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor phase sublimates seal permeability in the tuff

variations in the strontium isotopic ratio of seawater during the miocene stratigraphic and geochemical implications a composite strontium isotopic seawater curve was constructed for the miocene between 24 and 6 ma by combining 87sr 86sr measurements of planktonic foraminifera from deep sea drilling project sites 289 and 588 site 289 with its virtually continuous sedimentary record and high sedimentation rates 26 m m y was used for constructing the oligocene to mid miocene part of the record which included the calibration of 63 biostratigraphic datums to the sr seawater curve using the timescale of cande and kent 1992 across the oligocene miocene boundary a brief plateau occurred in the sr seawater curve 87sr 86sr values averaged 0 70824 which is coincident with a carbon isotopic maximum cm o m from 24 3 to 22 6 ma during the early miocene the strontium isotopic curve was marked by a steep rise in 87sr 86sr that included a break in slope near 19 ma the rate of growth was about 60 ppm m y between 22 5 and 19 0 ma and increased to over 80 ppm m y between 19 0 and 16 ma beginning at 16 ma between carbon isotopic maxima cm3 and cm4 of woodruff and savin 1991 the rate of 87sr 86sr growth slowed and 87sr 86sr values were near constant from 15 to 13 ma after 13 ma growth in 87sr 86sr resumed and continued until 9 ma when the rate of 87sr 86sr growth decreased to zero once again the entire miocene seawater curve can be described by a high order function and the first derivative d87sr 86sr dt of this function reveals two periods of increased slope the greatest rate of 87sr 86sr change occurred during the early miocene between 20 and 16 ma and a smaller but distinct period of increased slope also occurred during the late miocene between 12 and 9 ma these periods of steepened slope coincide with major phases of uplift and denudation of the himalayan tibetan plateau region supporting previous interpretations that the primary control on seawater 87sr 86sr during the miocene was related to the collision of india and asia the rapid increase in 87sr 86sr values during the early miocene from 20 to 16 ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans in the absence of another source of co2 these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric co2 and climatic cooling through a reversed greenhouse effect the paleoclimatic record however indicates a warming trend during the early miocene culminating in a climatic optimum between 17 and 14 5 ma we suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic co2 from the eruption of the columbia river flood basalts crfb between 17 and 15 ma the decrease in the rate of crfb eruptions at 15 ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in monterey deposits eventually led to cooling and increased glaciation between 14 5 and 13 ma the crfb hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the monterey at 17 5 ma as marked by increased 13c values and the climatic cooling and glaciation during the middle miocene as marked by the increase in 18o values which did not begin until 14 5 ma

isotopic characteristics of cryospheric waters in parts of western himalayas india present study has analysed stable isotopic composition 18o and 2h of cryospheric waters of ladakh and kashmir regions located in western himalayas india the samples of glacier ice 11 and melt water stream 10 collected altitude ranging between 3168 and 5465 m a s l have been analysed for 18o and 2h the measured 18o values of glacier ice varied between 16 5 and 13 8 in ladakh and between 12 8 and 9 6 in kashmir region study shows that isotopic characteristics of ice samples collected from snout of glaciers in ladakh and kashmir region are distinctly different from each other melt water samples collected from streams near the snout during september have shown good resemblance with the glacier ice melt of respective region the d excess of glaciers and melt water of kashmir region varied from 21 0 to 25 6 and from 18 0 to 21 5 respectively the d excess of glaciers of ladakh region have been found between 13 5 and 16 7 and of the melt water between 11 6 and 12 6 depleted 18o signature and low d excess value of glaciers of ladakh in comparison with that of kashmir region may be attributed to sublimation process triggered by cold arid climatic conditions the present study of kashmir and ladakh region suggest that isotopic composition of glacier ice is influenced by post depositional alteration such as sublimation during snow to glacier ice transformation

oxygen and hydrogen isotopic composition of major indian rivers a first global assessment nine major rivers have been sampled around the indian subcontinent to give an overview of the surface water characteristics both 18o and deuterium have been measured to determine the origin of the water and the possible evapotranspiration process the major ions have also been analysed to obtain complementary information although some basins have been studied previously mainly in the north this is the first attempt at a wider investigation of major indian rivers the results are discussed from the perspective of the hydroclimatological geographical and geological specificity of the river basins 18o values vary from light isotope enriched himalayan rivers to heavyisotope enriched peninsular indian rivers in a northwest southeast gradient across the subcontinent there is more evapotranspiration leading to heavy isotope enrichment in the peninsular southern deccan rivers compared with the light isotope enriched snow and glacier melt derived waters of the himalayan rivers the 18o values of indian rivers correspond roughly to the 18o values of the rains falling over the subcontinent however the influence of tributaries is dominant over rainfall in rivers like the narmada and tapti the cauvery and krishna rivers show maximum evapotranspiration and sodium pollution as indicated by the 18o values deuterium excess and major ion data copyright 2005 john wiley sons ltd

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

late holocene climatic record from a glacial lake in ladakh range trans himalaya india a multi proxy study using mineral magnetism sediment texture total organic content palynofacies and diatoms was conducted in one of the highest proglacial lake situated at north pulu 5098 m a s

stable isotopes as an indicator of evaporation in the river murray australia the stable isotope composition of water d and 18o and chloride concentrations were measured monthly at 26 stations along the river murray australia and several major tributaries from december 1988 to june 1989 the first four months had low natural runoff plus large irrigation diversions from storage reservoirs the following three months had much higher up to a factor of 5 river discharge deuterium and oxygen 18 compositions had a maximum range of 50 and 9 respectively from headwaters to mouth chloride concentrations increased downstream with a maximum range from 1 to 180 10 3 kg m 3 these trends can be attributed to 1 evaporation from storage facilities and river channels 2 inflows of irrigation drainage water enriched in heavy isotopes and 3 inflows of saline groundwater water losses by evaporation from the river surface and irrigation areas amounted to about 40 15 of releases from reservoir storage based on the integrated enrichment of deuterium which is comparable to the total transpiration flux within the basin about half of the chloride acquired by the murray as it traversed the middle third of the basin during summer months appears to have resulted from return flows of irrigation drainage water

evidence from lava flows for complex polarity transitions the new composite steens mountain reversal record summary geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava flow records by splicing together paleomagnetic results from earlier studies at steens mountain with those from three newly studied sections of oregon plateau flood basalts at catlow peak and poker jim ridge 70 90 km to the southeast and west respectively we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks forty five new distinguishable transitional t directions together with 30 earlier ones reveal a much more complex and detailed record of the 16 7 ma reversed r to normal n polarity transition that marks the end of chron c5cr compared to the earlier r t n t n reversal record the new record can be described as r t n t n t r t n the composite record confirms earlier features adds new west and up directions and an entire large n t r t segment to the path and fills in directions on the path between earlier directional jumps persistent virtual geomagnetic pole vgp clusters and separate vgps have a preference for previously described longitudinal bands from transition study compilations which suggests the presence of features at the core mantle boundary that influence the flow of core fluid and distribution of magnetic flux overall the record is consistent with the generalization that vgp paths vary greatly from reversal to reversal and depend on the location of the observer rates of secular variation confirm that the flows comprising these sections were erupted rapidly with maximum rates estimated to be 85 120 m ka 1 at catlow and 130 195 m ka 1 at poker jim south paleomagnetic poles from other studies are combined with 32 non transitional poles found here to give a clockwise rotation of the oregon plateau of 11 4 5 6 with respect to the younger columbia river basalt group flows to the north and 14 5 4 6 with respect to cratonic north america 95 per cent confidence interval

stable isotopes in the source waters of the yamuna and its tributaries seasonal and altitudinal variations and relation to major cations water samples from the yamuna and its tributaries one of the major river systems draining the himalaya have been analysed for their stable oxygen and hydrogen isotopes during three seasons summer monsoon and post monsoon the data show clear seasonal and altitudinal variations waters from higher altitudes and those collected during monsoon season are characterized by relatively depleted isotopic composition regression analysis of d 18 o data of samples collected during summer and monsoon seasons shows that the slope of the best fit lines are nearly identical to those of precipitation at new delhi for the same period the similarity in their slopes suggests that the isotopic composition of precipitation contributing water to these rivers are reasonably well preserved in both monsoon and non monsoon seasons however during the non monsoon period both rainfall and river waters carry signatures of evaporation the deuterium excess in river waters during the three seasons though overlap with each other the values during october are higher this can be understood in terms of recycled moisture contributions to precipitation the altitude effect for 18 o in these waters is determined to be 0 11 per 100 m a factor of about two less than that reported for the ganga source waters from similar altitudinal range the variability in altitude effects in rivers draining the himalaya seems to be controlled by the amount effect associated with the monsoon the significant spatial variability in altitude effect in these river basins which are a few hundred kilometers apart suggests that reconstruction of palaeoelevation in the himalaya based on 18 o altitude gradients would depend critically on its proper assessment in the region this study has established a relationship between total cation abundance and 18 o in waters of the yamuna mainstream total cations corrected for cyclic components double for a 1 4 km decrease in altitude as the yamuna flows downstream

chemical and strontium oxygen and carbon isotopic compositions of carbonates from the lesser himalaya implications to the strontium isotope composition of the source waters of the ganga ghaghara and the indus rivers samples of precambrian carbonate mostly dolomite outcrops collected across the lesser himalaya have been analysed for their mineralogy chemical composition and isotope ratios of sr o and c to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the ganga ghaghara and the indus their sr concentrations range from 20 to 363 ppm 18opdb 1 4 to 12 8 and mn 11 2036 ppm the petrography of the samples their low sr concentrations and wide range of 18o values are suggestive of their postdepositional alteration the 87sr 86sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0 7064 to 0 8935 and are generally more radiogenic than that of contemporaneous seawater comparison of the 87sr 86sr and sr ca ratios among the carbonates and silicates from the lesser himalaya and the source waters of the ganga ghaghara and the indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates an upper limit of carbonate sr in the various source waters is calculated to be between 6 and 43 assuming that all the ca in the rivers is of carbonate origin the results show that on the average weathering of the precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters however they can be a dominant supplier of radiogenic sr to some rivers on a regional scale the silicate sr component in some of the source waters of the ganga bhagirathi bhilangna alaknanda and ganga ghaghara kali and sarju and the indus sutlej was calculated from the ca na sr na ratios and strontium isotope compositions of these rivers and the silicate endmember these calculations suggest that 33 89 of sr in the bhagirathi bhilangna alaknanda ganga and sarju rivers is of silicate origin whereas in the kali and the sutlej it is much lower only 8 the remaining sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites this study underscores the importance of weathering of silicates carbonates and evaporites in contributing to the sr mass balance and 87sr 86sr of the source waters of the ganga ghaghara and the indus the present day silicate and carbonate sr contributions to the sr budget of the rivers vary considerably but among the major source waters of the ganga silicate sr exerts a more dominant control on their sr abundance and 87sr 86sr

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

strontium isotopes and rubidium in the ganga brahmaputra river system weathering in the himalaya fluxes to the bay of bengal and contributions to the evolution of oceanic87sr 86sr the concentrations of rb and sr and87sr 86sr isotopic ratios have been measured in the dissolved load of the ganga brahmaputra g b river system the ganga was sampled extensively from its source at gangotri in the higher himalaya to patna on the alluvial plains the brahmaputra was sampled in its stretch in assam in india the average sr concentration in the ganga at patna is 1 2 mol l and that in the brahmaputra at goalpara is 0 73 mol l the mean87sr 86sr ratios are 0 7239 and 0 7192 respectively the87sr 86sr in the ganga source waters the alaknanda the bhagirathi and their tributaries range between 0 7300 and 0 7986 considerably higher than the global average runoff value of 0 7119 the high87sr 86sr in the ganga source waters result from the intense weathering of precambrian granites and gneisses enriched in radiogenic sr the sr isotope systematics of the ganga waters is dominated by silicate weathering whereas carbonate weathering plays a significant role in their major ion chemistry the g b system transports about 910 million moles of dissolved sr annually to the bay of bengal with an average87sr 86sr of 0 7213 the sr transported by the g b system is about 2 7 of the global dissolved sr flux to the oceans via rivers model calculations reveal that the g b system has contributed significantly to the sr isotope evolution of seawater during the past 20 ma the flux of rb through the g b system is 24 million moles per year this is about 5 of the global river supply of rb to the oceans nearly twice the contribution of water via the g b system to the oceans our study suggests that the marine geochemistries of sr and rb and possibly u may have been influenced considerably by the himalayan orogeny

geology of the mid miocene rooster comb caldera and lake owyhee volcanic field eastern oregon silicic volcanism associated with grande ronde flood basalt abstract the lake owyhee volcanic field lovf of eastern oregon consists of rhyolitic caldera centers and lava fields contemporaneous with and spatially related to mid miocene columbia river flood basalt volcanism previous studies delineated two calderas in the southeastern part of lovf near owyhee reservoir the result of eruptions of two ignimbrites the tuff of leslie gulch and the tuff of spring creek our new interpretation is that these two map units are differentially altered parts of a single ignimbrite produced in a major phreatomagmatic eruption at 15 8 ma areas previously mapped as tuff of spring creek are locations where the ignimbrite contains abundant clinoptilolite mordenite which made it susceptible to erosion the resistant intracaldera tuff of leslie gulch has an alteration assemblage of albite quartz indicative of low temperature hydrothermal alteration our new mapping of caldera lake sediments and pre and post caldera rhyolitic lavas and intrusions that are chemically similar to intracaldera tuff of leslie gulch point to a single 20 25 km caldera which we name the rooster comb caldera erosion of the resurgently uplifted southern half of the caldera created dramatic exposures of intracaldera tuff of leslie gulch cut by post caldera rhyolite dikes and intrusions that are the deeper level equivalents of lava domes and flows that erupted into the caldera lake preserved in exposures to the northeast the rooster comb caldera has features in common with more southerly mid miocene calderas of the mcdermitt volcanic field and high rock caldera complex including formation in a basinal setting shortly after flood basalt eruptions ceased in the region and forming on eruption of peralkaline ignimbrite the volcanism at rooster comb caldera postdates the main activity at mcdermitt and high rock but like it begins 300 ky after flood basalt volcanism begins in the area and while flood basalts don t erupt through the silicic focus are contemporaneous with the latest stages of eruptions nearby high rock and mcdermitt rhyolites are associated with propagation of steens basalt dikes to the south and lovf rhyolites with later propagation of grande ronde basalt dikes to the north and north northwest

the steens mountain oregon geomagnetic polarity transition 3 its regional significance study of the variations of direction and intensity of the geomagnetic field as recorded by the miocene lava flows on steens mountain southeastern oregon has resulted in a detailed description of total field behavior during a reversal in polarity in addition to information about the polarity reversal itself the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15 5 ma in order to test the feasibility of using this record as a means of correlation in this part of the western united states comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the steens basalt or rocks of equivalent age despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes convincing similarities among some of the records are evident the steens basalt paleomagnetic record does indeed have potential as a correlation tool during this time of widespread basaltic volcanism additionally paleomagnetic data from flows that were sampled in detail yield a middle miocene paleomagnetic pole at 88 3 n 209 0 95 6 3 for the high lava plains of oregon this pole position is statistically indistinguishable from the earth s rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted data from selected sites within the coeval part of the columbia river basalt group yield a paleomagnetic pole at 88 7 n 171 6 e 95 4 0 the columbia river basalt group pole is statistically indistinguishable from either the rotational axis or from the high lava plains pole these findings indicate no post 20 ma differential rotation between south eastern washington and south central oregon in contrast to previous interpretations

orbital forcing and abrupt events in a continental weathering proxy from central europe most basin czech republic 17 7 15 9 ma recorded beginning of the miocene climatic optimum abstract we describe orbital forcing in siliciclastic lacustrine sediments deposited in central europe ca 50 5 n and 13 5 e the most basin the czech republic between 17 7 and 15 9 ma i e before the miocene climatic optimum mco and in its early stage chemical element analysis combined with magneto and cyclostratigraphy produced a consistent stratigraphy for the studied basin fill the palaeoenvironmental record is mainly based on variations of the chemical weathering intensity in the lake catchment normalised potassium concentration k al or k ti the most basin sedimentary profile is specific by relatively high deposition rates typically 20 cm kyr sampling density corresponded to temporal resolution of 1 2 kyr and sensitivity to a regional continental climate for age control we used the atnts2012 for younger sediments and an alternative age of the onset of c5dn chron and the end of c5dr 1r short chron in the earliest studied interval the chemical weathering intensity in the catchment was controlled by precession while between 17 5 and 17 15 ma short eccentricity was mainly driving chemical weathering intensity after 17 05 ma i e just before the onset of the mco and in the period of persistent high eccentricity between 16 95 and 16 7 ma chemical weathering intensity reached its precession controlled maxima in the subsequent period of the persistent low eccentricity at 16 65 ma the orbital signal nearly vanished in the most basin abrupt but temporary environmental change occurred in the interval between 16 5 and 16 35 ma polarity chron c5cn 2n within two cycles of high eccentricity its trigger is uncertain but it might have been local or regional the obtained composite record was compared with marine 13c and 18o data showing certain global control of the most basin deposits the most basin record thus shows climatic events of the pre mco and early mco period in terrestrial settings of central europe with changes at 17 05 16 65 and 16 5 ma possibly representing triggers of the mco

controls over the strontium isotope composition of river water abstract strontium concentrations and isotope ratios have been measured in river and ground waters from the ganges orinoco and amazon river basins when compared with major element concentrations the data set has allowed a detailed examination of the controls over the strontium isotope systematics of riverine input to the oceans in the following environments 1 1 typical drainage basins containing limestones evaporites shales and alumino silicate metamorphic and igneous rocks 2 2 shield terrains containing no chemical or biogenic sediments and 3 3 the floodplains that constitute the largest areas of many large rivers the strontium concentration and isotope composition of river waters are largely defined by mixing of strontium derived from limestones and evaporites with strontium derived from silicate rocks the strontium isotope composition of the limestone endmember generally lies within the phanerozoic seawater range which buffers the 87 sr 86 sr ratios of major rivers a major exception is provided by the rivers draining the himalayas where widescale regional metamorphism appears to have led to an enrichment in limestones of radiogenic strontium derived from coexisting silicate rocks the strontium isotope systematics of rivers draining shield areas are controlled by the intense transportlimited nature of the weathering reactions and thereby limits variations in the strontium flux from these terrains floodplains are only a minor source of dissolved strontium to river waters and precipitation of soil salts in some floodplains can reduce the riverine flux of dissolved strontium to the oceans the most effective mechanisms for altering the isotope ratio and flux of riverine strontium to the oceans are increased glaciation and large scale regional metamorphism of the type produced during continental collision both mechanisms provide a means for increasing the 87 sr 86 sr ratio of the global riverine flux

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

strontium isotopic composition of the po river dissolved load insights into rock weathering in northern italy abstract strontium isotopic composition 87sr 86sr of river water provides insight on the nature of rocks involved in the weathering processes and important constraints for reconstructing silicate versus carbonate weathering in large scale basins new data presented in the context of a geochemical map of the po river basin highlight that the po river water evolves from the upper part of the catchment where 87sr 86sr approaching 0 7097 relates to the weathering of igneous and metamorphic silicate rocks to the middle part where less radiogenic compositions 87sr 86sr 0 7089 are driven by the confluence of the tributaries draining mesozoic carbonate rocks of the south alpine domain and from inflow of apennine streams characterized by less radiogenic sr signatures due to weathering of marly sediments and mafic magmatic rocks down flow the sr isotopic compositions of po river water rises at 0 7091 either by the confluence of the last apennine tributary panaro 87sr 86sr 0 7095 and or by the possible contribution from hyporheic exchanges po river isotopic signatures reveal slightly more radiogenic 87sr 86sr in comparison to other important alpine river systems e g danube rhine and rhone likely suggesting that silicate rocks are comparatively contributing more weathering byproducts in the po basin

initial impingement of the yellowstone plume located by widespread silicic volcanism contemporaneous with columbia river flood basalts during the main phase of steens and columbia river united states flood basalt eruptions between 16 7 and 15 0 ma 3900 km 3 of silicic magma erupted from centers dispersed across 25 000 km 2 the largest and oldest silicic centers high rock mcdermitt and lake owyhee caldera complexes lie along a narrow nne trend at the transition between the craton and accreted oceanic crust suggesting that silicic magma generation was greatest where mafic magma intruded more felsic crust less voluminous and younger silicic centers track the spread of mafic dikes to the north the high rock caldera complex forms the western end of the snake river plain yellowstone trend of eastward younging silicic centers we attribute a regional lull in volcanism between ca 23 and 7 ma to lifting of the juan de fuca slab by a mantle plume beginning at ca 25 ma and the distribution of main stage flood basalts and coeval rhyolites to breakthrough of plume material at ca 17 ma to form a smaller plume head centered beneath steens mountain

assessment of the martian gravity field at short wavelength with mars express 1 the gravity part of the mars express radio science experiment consists in measuring gravity data near pericenter above selected target areas of geophysical interest the low altitude of the mars express at pericenter 263 329 km makes it a very sensitive gravity sensor at small wavelengths which can give new constraints on the local structure of the crust and lithosphere mars express gravity data can also be used to check the quality of the existing global gravity solutions in this paper we show that mars express gravity data confirm in the tharsis area the validity of existing global gravity solutions in particular the resolution of the most recent global gravity solution is excellent up to harmonic degree 73 and the amplitude at short wavelength is not biased toward zero by the power law regularization up to the same degree

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

temporal variations in sr and 87sr 86sr of the ganga headwaters estimates of dissolved sr flux to the mainstream the headwaters of the ganga the alaknanda bhagirathi and the ganga were analysed for their dissolved major ions sr and 87sr 86sr on a biweekly to monthly basis over a period of one year to determine their temporal variations and the factors contributing to them the concentrations of major ions and sr show significant seasonal variation with lower values during monsoon period in all the three rivers a similar trend is also observed for 87sr 86sr and na ca na narclr suggesting relatively lower contribution of sr and na from silicates which are more radiogenic in sr during monsoon budget calculations show that silicate derived dissolved sr srs in the river ganga alaknanda and the bhagirathi varied from 10 4 to 27 11 7 3 to 30 12 16 6 to 57 23 of measured sr respectively with lower values during monsoon the relative decrease in silicate erosion compared to carbonate during monsoon can result from several factors these include higher dissolution kinetics of the carbonates lower water rock interaction time and availability of larger area for weathering the annual discharge weighted sr flux derived from the time series data is higher by 20 from that based on peak flow sr and lower by 40 compared to that derived from lean flow sr concentration the area normalized annual flux of dissolved sr from the ganga at rishikesh is about five times its flux at rajshahi bangladesh and a few other major global rivers such as the amazon indicating higher erosion rate over the himalaya copyright 2010 john wiley sons ltd

morphological and textural diversity of the steens basalt lava flows southeastern oregon usa implications for emplacement style and nature of eruptive episodes this study focuses on middle miocene tholeiitic flood basalt lava flows from the oregon plateau northwestern usa steens basalt and is the first to comprehensively document and evaluate their morphology field observations of flows from several sections within and proximal to the main exposures at steens mountain have been supplemented with textural and geochemical data and are used to offer preliminary insights into their emplacement compound pahoehoe flows of variable thickness appear to be common throughout the study area laterally and vertically these tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as hawaii and the snake river plain classic a a flows with brecciated upper and basal crusts are not abundant but by no means uncommon flows with characters different from typical pahoehoe and a a are also common such flows display a range in morphology flows with preserved upper crusts but brecciated basal crusts as well as those displaying well developed flow top breccias and preserved basal crusts rubbly pahoehoe are observed the steens basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces the abundant compound pahoehoe flows often rich in plagioclase phenocrysts were likely emplaced during slow but sustained eruptive episodes their constituent lobes show clear evidence for endogenous growth the relatively aphyric flows with brecciated surfaces including a a hint at higher strain rates and or higher viscosity probably caused by higher effusion rates a couple of sections are characterized by compositionally similar but morphologically different flows that were possibly part of the same eruption while differences in pre eruptive topography could explain this it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system it is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems and deserve closer scrutiny

hydrochemistry and isotope geochemistry of the upper danube river abstract the upper danube river and 19 of its major tributaries were monitored for 1046 km downriver and seasonally for ca mg sr na k hco3 co2 cl po4 sio2 no3 no2 nh4 so4 13cdic 18oh2o dh2o 34sso4 18oso4 and 87sr 86sr hydrological considerations and 18o d data show that the water balance in the river particularly after its confluence with the inn is controlled by the southern tributaries draining the mesozoic carbonate complexes of the alps as a result chemical balance at bratislava is mostly a conservative product of its tributaries the concentrations of ca mg and sr in the danube are 6 to 9 times higher than in pristine rivers although all these cations are derived from dissolution of mesozoic carbonates the ultimate cause is likely the enhanced generation of soil co2 due to agricultural and forestry practices dissolution of triassic sulfates is an additional factor for ca enrichment in the headwater section of the river dissolved sulfate with a comparable enrichment factor to that of alkaline earths appears to be derived mostly from atmospheric deposition a proposition based on consideration of its sulfur and oxygen isotopic compositions na k and cl are enriched by a factor of 2 5 to 4 times mostly as a result of industrial and municipal pollution sources in contrast to the above components which behave mostly conservatively during the downriver flow of the danube biogenic elements such as nutrients and si are influenced by in river processes the photosynthesis respiration balance that impacts the carbon cycle and the oxygen balance has been discussed elsewhere pawellek and veizer 1994 no3 no2 and nh4 are enriched by a factor of 10 to 16 times from point and diffuse sources along the watercourses generating at times downflow nitrification plumes from nh4 to no3 po4 varies seasonally chiefly as a result of biologic demand during the warm periods for sio2 the biological uptake mostly for secretion of diatom frustules combined with the deficiency of this compound that results from the predominantly carbonate lithology of the catchment results in concentrations that are below those of pristine rivers overall the present day salted characteristics of the river are chiefly a consequence of the long habitation history of the upper danube watershed

silicic ash beds bracket emeishan large igneous province to 1 m y at 260 ma this research was supported by the open fund plc201404 of state key laboratory of oil and gas reservoir geology and exploitation chengdu university of technology the national natural science foundation of china 41502109 project funded by china postdoctoral science foundation 2015 m582528 and the fundamental research funds for the central universities chengdu university of technology

geology and 40ar 39ar geochronology of the middle miocene mcdermitt volcanic field oregon and nevada silicic volcanism associated with propagating flood basalt dikes at initiation of the yellowstone hotspot

late holocene flooding history of a tropical river in western india in response to southwest monsoon fluctuations a multi proxy study from lower narmada valley abstract a multi proxy study on the sediments from lower narmada valley western india records large flooding events and high sediment influx linked to southwest monsoon variations during late holocene on the basis of proxy data and chronology of preserved sediments three major environmental perturbations at 1809 1487 and 1187 cal bp have been identified that corroborate well with the regional fluctuations in southwest monsoon variability a phase of dominant marine influence between 2185 and 1809 cal bp is inferred based on the presence of marine palynomorphs mangrove pollen and amorphous organic matter and is coincident with the phase of weak monsoon in the region a transition from marine marsh environment to a phase of high fluvial influx occurred around 1809 cal bp the fluvial influence is inferred based on the replacement of amorphous organic matter with structured organic matter presence of fresh water algae decrease in the 13c values and dominance of kaolinite clay the fluvial regime stabilized between 1487 and 1187 cal bp as recorded in the overbank sediments palynomorphs and geochemical indices and sedimentation occurred under a highly oxidizing and high energy condition with long distance transport of sediments the later incision of the sediment sequence which post dates 1187 cal bp can be attributed to enhanced precipitation the response of the lower narmada valley to short term southwest monsoon fluctuations is manifest in the shifting deposition environments and a major role of terrestrial sediment influx variability is surmised leading to relative sea level variations in this part of the indian subcontinent a temporal and process based interplay between flooding periods and sediment discharge in river systems and tidal conditions is suggested for the tropical estuarine environments

a tale of two extinctions converging end permian and end triassic scenarios the end permian c 252 ma and end triassic c 201 ma mass extinction events are commonly linked to the emplacement of the large igneous provinces of the siberia traps and central atlantic magmatic province respectively accordingly scenarios for both extinctions are increasingly convergent and cross fertilization of ideas has become important here we present a synthesis of extinction scenarios based on a critical assessment of the available palaeontological sedimentological geochemical and geophysical evidence how similar were the extinction events what gaps exist in our understanding and how can a comparison of the events enhance our understanding of each event individually our focus is on the most important proximate kill mechanisms including climate change and atmospheric pollution increased soil erosion weathering and runoff forest dieback and the spread of pathogens and ocean temperature changes anoxia and acidification there is substantial evidence to suggest that very similar kill mechanisms acted upon late permian as well as late triassic ecosystems strengthening the hypothesis that the ultimate causes of the mass extinction events were similar

extrinsic forcing of plant ecosystems in a large igneous province the columbia river flood basalt province washington state usa volcanism associated with large igneous provinces lips has been implicated in both global climate and environmental change to determine the impact of lip volcanism on plant ecology we have examined plant community succession in sedimentary interbeds of the columbia river flood basalt province crbp northwest united states interbasaltic vegetation is characterized by primary succession communities that inhabit fresh lava surfaces until terminated by the next eruptive event and it is assumed that longer volcanic hiatuses should lead to more mature plant communities this expected succession trajectory is contradicted by palynological data that show that seral succession declines during the phase of waning crpb volcanism and prolonged interbed intervals frequent volcanic activity and increased deposition of snake river plain hotspot ashes during this phase resulted in ecological disturbance of intralava field vegetation together with geochemical proxies from interbed sediments this suggests that crbp flora was largely driven by extrinsic forcing and implies that lip volcanism of similar scale and magnitude to that of the crbp had a limited environmental impact this study supports the theory that past biotic extinctions were triggered by numerous factors rather than a single geological event

isotopic characteristics of indian precipitation 1 hydrogen 2h 1h and oxygen 18o 16o isotopic ratios were measured in precipitation 900 samples collected from several locations in india during the period 2003 2006 12 locations in 2003 and 18 locations in 2004 2006 the amount of rainfall along with air temperature and humidity were also measured the meteoric water line developed for india using isotopic data of precipitation samples namely 2h 7 93 0 06 18o 9 94 0 51 n 272 r2 0 98 differs slightly from the global meteoric water line regional meteoric water lines were developed for several indian regions i e northern and southern regions of india western himalayas and found to be different from each other southern indian meteoric water line slope is 7 82 intercept or d excess is 10 23 northern indian meteoric water line slope is 8 15 intercept is 9 55 which is attributed to differences in their geographic and meteorological conditions and their associated atmospheric processes i e ambient temperature humidity organ and source of vapor masses the local meteoric water lines developed for a number of locations show wide variations in the slope and intercept these variations are due to different vapor sources such as the northeast ne monsoon that originates in the bay of bengal the southwest monsoon sw that originates in the arabian sea a mixture of ne and sw monsoons retreat of ne and sw monsoons and western disturbances that originate in the mediterranean sea the altitude effect in the isotopic composition of precipitation estimated for western himalayan region also varies from month to month

using 40 ar 39 ar ages of intercalated silicic tuffs to date flood basalts precise ages for steens basalt member of the columbia river basalt group abstract to establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments highly accurate and precise ages for the flood basalts are required but flood basalts are intrinsically difficult to date we illustrate how 40ar 39ar feldspar ages for silicic tuffs intercalated with and overlying sections of steens basalt the earliest lavas of the middle miocene columbia river basalt group in the northwestern united states provide high precision ages that for the first time make it possible to resolve age differences with stratigraphic position within a section of these flood lavas the stratigraphically lowest rhyolitic tuff a fall deposit yielded an age of 16 592 0 028 ma fcs 28 02 ma and the uppermost the alkali rhyolite ignimbrite tuff of oregon canyon is 16 468 0 014 ma the argon and stratigraphic data indicate that steens basalt eruptions occurred from 16 64 to 16 43 ma in the southern end of its distribution we estimate that the steens mountain geomagnetic reversal occurred at 16 496 0 028 ma 0 18 ma total error our estimates of the timing for initiation of volcanism and volumetric eruptive rates do not seem to support volcanic forcing by the initial stages of columbia river basalt group eruptions as an explanation for the abrupt warming and carbonate dissolution at the beginning of the miocene climatic optimum

the indus river system india pakistan major ion chemistry uranium and strontium isotopes the indus river is one of the large river systems draining the himalaya we report in this paper the major ion chemistry sr and u isotope systematics of the indus system particularly its headwaters the results show that 1 on an average about a third of the cations in the waters can be from silicate weathering however most of the ca mg is likely to be from the weathering of carbonates and evaporites 2 the 87sr86sr of the waters ranges between 0 7085 and 0 7595 the higher values 0 72 are typical of the tributaries draining the precambrian granite gneissic terrains the 87sr86sr of the indus main channel throughout its entire stretch shows only minor variations 0 7104 0 7116 the indus transports 8 8 106 mol sr to the arabian sea annually with a 87sr86sr of 0 7111 if our results of sr concentration and 87sr86sr measured at thatta can be considered typical of the indus throughout the year and 3 the u concentration in the indus and its tributaries is generally high 0 37 10 3 g 1 1 the source for the high uranium can be the weathering of granites zones of uranium mineralisation and black shales the indus results when compared with our earlier data on the ganga brahmaputra show that in all these three river systems carbonate weathering is the dominant source of ca mg and hco3 and that u concentrations are high in the case of 87sr86sr the indus waters are less radiogenic however its tributaries draining the precambrian granites gneisses have 87sr86sr in excess of 0 72 similar to that in the rivers of the ganga system the low 87sr86sr of the indus 0 7111 suggests that its contribution to the sr isotope evolution of oceans since the cenozoic is less significant than that of the ganga brahmaputra

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

paleoclimate and paleovegetation of lower narmada basin gujarat western india inferred from stable carbon and oxygen isotopes abstract surface sediment samples depth 0 160 cm and a paleosol depth 10 15 m both part of a sediment profile exposed in a cliff section of narmada river western india were analyzed for stable isotopes 13 c and 18 o three organic carbon samples from top 160 cm and two paleosol samples from top and bottom of the paleosol were dated by the radiocarbon method 13 c of both soil carbonates and soil organic carbon fall in the range of mixed c3 and c4 vegetation except for the paleosol which shows differences attributable to the contribution of bedrock carbonate during calcite formation the vegetation was dominantly c3 in the late pleistocene from 15 10 ka bp which shifted to mixed c3 c4 during the holocene enhanced monsoon was observed at 12 5 and 1 6 ka bp the presence of c4 grasses in the late holocene is detectable the present day vegetation is dominantly c3 type 13 c 25 4 probably due to anthropogenic activity where grasslands c4 have been replaced by c3 vegetation recently last 50 years the enhanced atmospheric co 2 concentration in recent times could be a probable agent for the current dominance of c3 vegetation

water balance of the indus river basin and moisture source in the karakoram and western himalayas implications from hydrogen and oxygen isotopes in river water 1 stable isotope measurements of hydrogen and oxygen for surface waters from the indus river basin irb together with historical records for river discharge annual precipitation and groundwater levels are used to assess water balance for the basin the indus river presently drains 53 km3 yr 1 or roughly one eighth of the 398 km3 water that annually falls on the basin in the form of rain and snow with the remainder returned to the atmosphere by evapotranspiration monthly samples for the indus river close to its mouth for the water year march 1994 to february 1995 show a tight correlation in d and 18o space the slope of the linear regression is 7 5 which is not significantly different from the slopes of the local meteoric water lines lmwl 7 3 and 7 1 this observation argues against significant loss of water by direct evaporation from river surfaces or from soils in hydrologic continuum with surface waters an upper limit for evaporation from poorly drained soils is calculated to be 10 km3 yr 1 or only 2 5 of the annual precipitation flux groundwater storage in the entire canal command area received a maximum of 23 km3 yr 1 or 5 8 of the annual precipitation during the early stages of irrigation but modern recharge is probably balanced by discharge to rivers and well exploitation transpiration by natural vegetation and crops annually returns 83 of the precipitation flux and constitutes the largest pathway for the loss of water from the basin deuterium excess d excess in the irb ranges between 4 and 28 with values for 95 of the sample population exceeding 10 the indus main channel close to its mouth varies in d excess between 12 and 20 during low and high water stands respectively with a discharge weighted average of 18 these values are distinctly higher than the long term average for the indian monsoon 8 and reflect contributions from water vapor originating in the mediterranean 22 or other inland seas using these end member compositions and the discharge weighted average d excess isotope balance calculations require that up to 72 of the indus discharge close to its mouth must be derived from the mediterranean end member and implies that bulk of the indus discharge is the result of delayed runoff from the karakoram and the himalayas

quickly erupted volcanic sections of the steens basalt columbia river basalt group secular variation tectonic rotation and the steens mountain reversal 1 the steens basalt now considered part of the columbia river basalt group crbg contains the earliest eruptions of this magmatic episode lava flows of the steens basalt cover about 50 000 km2 of the oregon plateau in sections up to 1000 m thick the large number of continuously exposed quickly erupted lava flows some sections contain over 200 flows allows for small loops in the magnetic field direction paths to be detected for volcanic rocks this detail and fidelity are rarely found outside of the holocene and yield estimates of eruption durations at our four sections of 2 5 ka for 260 m at pueblo mountains 0 5 to 1 5 ka for 190 m at summit springs 1 3 ka for 170 m at north mickey and 3 ka for 160 m at guano rim that only one reversal of the geomagnetic field occurred during the eruption of the steens basalt the steens reversal at approximately 16 6 ma is supported by comparing 40ar 39ar ages and magnetic polarities to the geomagnetic polarity timescale at summit springs two 40ar 39ar ages from normal polarity flows 16 72 0 29 ma 16 61 and 16 92 0 52 ma 16 82 equals 2 error place their eruptions after the steens reversal while at pueblo mountains an 40ar 39ar age of 16 72 0 21 ma 16 61 from a reverse polarity flow places its eruption before the steens reversal paleomagnetic field directions yielded 50 nontransitional directional group poles which combined with 26 from steens mountain provide a paleomagnetic pole for the oregon plateau of 85 7 n 318 4 e k 15 1 a95 4 3 comparison of this new pole with a reference pole derived from crbg flows from eastern washington and a synthetic reference pole for north america derived from global data implies relative clockwise rotation of the oregon plateau of 7 4 5 0 or 14 5 5 4 respectively probably due to northward decreasing extension of the basin and range

quaternary of himalaya abstract tectonically active himalayan mountains evolves via feedbacks from deep earth and surface processes the complex interaction of various processes results into the landscape which is dynamic both at longer and shorter time scales the extreme hydrological events that possibly ride over a long term climate cycle bring the changes in the landscape that impact human societies more closely these events in the himalaya frequently cause huge damage to economy and human lives the geologist community under the umbrella of himalaya karakorum tibet hkt workshop in its 30 th edition convened a special session and deliberated on the subject this special issue quaternary of himalaya is an outcome of papers presented and discussion held during this session it consists of 18 papers in three sub themes i extreme events in himalaya ii paleoglaciation in himalaya and iii expressions of climate and neotectonics in himalaya

stable isotope evidence of dual arabian sea and bay of bengal vapour sources in monsoonal precipitation over north india abstract high resolution time series data of hydrogen d and oxygen 18o isotope values of precipitation have been generated for the first time at kolkata eastern india where the summer monsoon clouds from bay of bengal bob commence their journey over india use of a rayleigh cum two component mixing model and comparison of kolkata data with the international atomic energy agency iaea global network of isotopes in precipitation gnip data base of new delhi suggest that the precipitation at new delhi cannot be explained by simple continental effect of a bob vapour source alone traveling and raining successively along kolkata new delhi route it is necessary to invoke an admixture of 20 vapour originating from the arabian sea with the vapour coming from bob and finally causing summer monsoon rains at new delhi the findings have major implications to the regional water vapour budget over india

chemical erosion in the eastern himalaya major ion composition of the brahmaputra and δ13c of dissolved inorganic carbon abstract major ion composition of waters 13 c of its dic dissolved inorganic carbon and the clay mineral composition of bank sediments in the brahmaputra river system draining india and bangladesh have been measured to understand chemical weathering and erosion and the factors controlling these processes in the eastern himalaya the time series samples collected biweekly at guwahati from the brahmaputra mainstream were also analyzed for the major ion composition clay mineralogy and chemical index of alteration cia of sediments suggest that weathering intensity is relatively poor in comparison to that in the ganga basin this is attributed to higher runoff and associated physical erosion occurring in the brahmaputra basin the results of this study show for the first time spatial and temporal variations in chemical and silicate erosion rates in the brahmaputra basin the subbasins of the brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude the eastern syntaxis basin dominates the erosion with a rate of 300 t km 2 y 1 one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains in contrast the flat cold and relatively more arid tibetan basin undergoes much slower chemical erosion 40 t km 2 y 1 the abundance of total dissolved solids tds 102 203 mg l in the time series samples collected over a period of one year shows variations in accordance with the annual discharge except one of them cause for which is attributable to flash floods na na corrected for cyclic component shows a strong positive correlation with si indicating their common source silicate weathering estimates of silicate cations na sil k sil ca sil mg sil suggest that about half of the dissolved cations in the brahmaputra are derived from silicates a proportion higher than that for the ganga system the co 2 consumption rate due to silicate weathering in the brahmaputra watershed is 6 10 5 moles km 2 y 1 whereas that in the eastern syntaxis subbasin is 19 10 5 moles km 2 y 1 similar to the estimates for some of the basaltic terrains this study suggests that the eastern syntaxis basin of the brahmaputra is one of most intensely chemically eroding regions of the globe and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern himalaya

global perturbation of the carbon cycle at the onset of the miocene climatic optimum the miocene climatic optimum mco ca 17 14 7 ma represents one of several major interruptions in the long term cooling trend of the past 50 m y to date the processes driving high amplitude climate variability and sustaining global warmth during this remarkable interval remain highly enigmatic we present high resolution benthic foraminiferal and bulk carbonate stable isotope records in an exceptional continuous carbonate rich sedimentary archive integrated ocean drilling program site u1337 eastern equatorial pacific ocean which offer a new view of climate evolution over the onset of the mco a sharp decline in delta o 18 and delta c 13 at ca 16 9 ma contemporaneous with a massive increase in carbonate dissolution demonstrates that abrupt warming was coupled to an intense perturbation of the carbon cycle the rapid recovery in delta c 13 at ca 16 7 ma similar to 250 k y after the beginning of the mco marks the onset of the first carbon isotope maximum within the long lasting monterey excursion these results lend support to the notion that atmospheric pco 2 variations drove profound changes in the global carbon reservoir through the mco implying a delicate balance between changing co2 fluxes rates of silicate weathering and global carbon sequestration comparison with a high resolution delta c 13 record spanning the onset of the cretaceous oceanic anoxic event 1a similar to 120 m y ago reveals common forcing factors and climatic responses providing a long term perspective to understand climate carbon cycle feedbacks during warmer periods of earth s climate with markedly different atmospheric co2 concentrations

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

diverse mid miocene silicic volcanism associated with the yellowstone newberry thermal anomaly the santa rosa calico volcanic field sc of northern nevada is a complex multi vent mid miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism santa rosa calico volcanism initiated at 16 7 ma concurrent with regional steens columbia river flood basalt activity and is characterized by a complete compositional spectrum of basalt through high silica rhyolite to better understand the relationships between upwelling mafic magmatism coeval extension and magmatic system development on the oregon plateau we have conducted the first compre hensive study of santa rosa calico silicic volcanism detailed stratigraphic based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins at least five texturally distinct silicic units are found in the western santa rosa calico volcanic field including abun dant lava flows near vent deposits and shallow intrusive bodies similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present the western and eastern santa rosa calico units are characterized by abundant macro and microscopic disequilibrium textures reflecting a complex petrogenetic history additionally unlike other mid mio cene oregon plateau volcanic fields e g mcdermitt the santa rosa calico volcanic field is characterized by a paucity of caldera forming volcanism only the cold springs tuff which crops out across the central portion of the volcanic field was caldera derived major and trace element geochemical variations are present within and between eastern and western santa rosa calico silicic units and these chemical differences coupled with the observed disequilibrium textures illustrate the action of open system petrogenetic processes and melt derivation from heterogeneous source materials the processes and styles of santa rosa calico silicic magmatism are linked to three primary factors local focusing of and thermal and material contributions from the regional flood basalt event lithospheric extension within the northern portion of the northern nevada rift and interaction of mid miocene silicic magmas with pre santa rosa calico lithosphere similar processes and styles of mid miocene silicic volcanism likely occurred across the oregon plateau in regions characterized by both focused lithospheric exten sion and localized mafic magmatism

glass transition temperatures of natural hydrous melts a relationship with shear viscosity and implications for the welding process abstract glass transition temperatures tg have been determined for natural multicomponent melts using differential scanning calorimetry trachytic dacitic phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3 75 wt for each sample tg has been obtained over a range of cooling heating rates using the extrapolated onset and the peak temperatures in heat capacity temperature curves tg of all compositions are strongly reduced by increasing water content particularly for the first 1 wt added base composition also has an effect with the lowest tg occurring in the peralkaline phonolite suite for all samples a clear dependence on the cooling heating rate has been recorded these results have been compared with rheological investigations on the same samples on the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study both dry and hydrous our determinations of tg provide a lower limit for the time temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature furthermore by using the viscosity values predicted at the glass transition we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at tg

δ18o and δd of streamwaters across the himalaya and tibetan plateau implications for moisture sources and paleoelevation reconstructions abstract this study presents new 18o and d data from 191 streams across the himalaya and tibetan plateau to better constrain the spatial variability of stable isotopes in modern precipitation over this region moisture penetrating into the southeastern tibetan plateau is predominantly derived from monsoonal airmasses originating from the bay of bengal and transported into the eastern himalayan syntaxis along the brahmaputra river progressive rainout during orographic lifting and cooling results in clear relationships between 18o and d and catchment hypsometric elevation on the plateau margin however monsoonal derived moisture is progressively mixed with central asian airmasses in more western and northern parts of the tibetan plateau as a result predicted isotope elevation relationships that are based on empirical lapse rates or thermodynamic models of the isotopic evolution of an airmass produce large 1 3 km misfits between measured and predicted catchment elevations for much of the tibetan plateau including some areas directly north of the central himalayan crest this suggests that changes in the 18o or d of paleoprecipitation on the central and southwestern tibetan plateau may reflect surface uplift along moisture transport pathways or changes in the penetration of monsoonally derived moisture rather than regional surface uplift histories

the origin of present day antarctic precipitation from surface snow deuterium excess data the deuterium excess d is defined as a linear combination of the d h and 18 o 16 o ratios in natural waters we present an interpretation of deuterium excess data in surface antarctic snow using an isotopic model to derive information on the origin of present day antarctic precipitation the data come from near the coast d47 adelie land and from inland south pole 1000 km from the sea the one dimensional isotopic model belongs to the rayleigh family but fully accounts for mixed cloud processes between 0 c and 30 c modeling d in polar snowfalls meets the problem of a large sensitivity to the saturation conditions prevailing at snow formation therefore we decided to tune the saturation in the model in order to yield realistic mean d values but we use the seasonal phase of d versus which is less sensitive to the saturation conditions as an independent validation of the model predictions both inland and near the coast our model can simulate the observed phase between d and in snow with moisture of subtropical origin 40 20 s although we do not treat specifically storm precipitation near the coast this study indicates that the observed phase between d and brings up a new validation of isotopic models it supports a distant vapor source for large scale precipitation delivered to the antarctic continent

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

quaternary geology tectonics and distribution of palaeo and present fluvio glacio lacustrine deposits in ladakh nw indian himalaya a study based on field observations abstract the ladakh region of the northwestern indian himalaya is rich in quaternary deposits but it has not received much attention previous quaternary research in the region has focused on the glacial sequences and only some scattered data of the lacustrine deposits are available this article addresses the reconstruction of the palaeo lacustrine deposits and the present day lakes and their distribution in the ladakh region the region was under the influence of tectonic activity and cold climate during the late quaternary times tectonic activity at 50 000 years bp 35 000 years bp and 25 000 years bp has been recorded we report the presence of three major lakes in the region in the late quaternary these formed due to the damming of the indus river and its tributaries by debris avalanches initiated mainly by tectonic activity along the indus suture shyok suture and the karakoram fault these are the spituk leh palaeolake formed 50 000 years bp the lamayuru palaeolake dated to 35 000 years bp in the indus valley and the khalsar palaeolake 60 km in length in the shyok valley vast exposures of the palaeolake deposits ranging from 25 to 150 m in thickness are reported spituk section 25 m lamayuru section 110 m and khalsar 150 m and hundri 100 m and which have a wide lateral extent a glacial lake basin at bhaktpur city north of baralacha la is now completely filled the tsokar lake has been subdivided into smaller units now and the water level is lowering other lakes in the region e g pangong tso tso morari the twin kyun tso lakes are confined to the western side of the study area limited chemical weathering rapid erosion and cold climatic conditions in late quaternary times are suggested by the stable illite values in the lamayuru section four levels of palaeoseismic structures convolute structures sand dykes intraformational folds micro faulting are present in the khalsar section and at three levels in the tirit section at shyok nubra confluence palaeoseismic activity was also prevalent after 25 000 years bp as is evident from the three levels of palaeoseismic structures in the younger sediments