time scales of sedimentary transfer and weathering processes from u series nuclides clues from the himalayan rivers abstract in order to define time scales of erosion and sedimentary transfer in the himalaya 238 u 234 u 230 th disequilibria have been analysed in river bank sediments and in bedloads collected along the kali gandaki river one of the main nepalese rivers and in the ghaghara and gandak rivers two major plain tributaries of the ganges the th activity ratios and u th ratios in river sediments of the two ganges tributaries constantly decrease from upstream to downstream this is related to the maturation of sediments by weathering during their transfer to the plain the u series data allow to calculate a transfer time for the sediments in the alluvial gangetic plain from the chain front to the confluence with the ganges of about 100 kyr for both rivers the kali gandaki river sediment data highlight a decrease of both the th isotopic and u th ratios which is explained by a mixing between two sources with similar u th ratios but having suffered a different u th fractionation history interpretation of the u series data in the frame of this scenario gives long time scales of weathering of several 100 s kyr for the himalayan terranes the results imply that himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers in addition occurrence of similar u sr signatures in dissolved i e

li and δ7li in himalayan rivers proxies for silicate weathering this paper presents the first systematic survey of lithium and its isotopes in the dissolved load and suspended and bed sediments of tributaries of the ganges both before and after the monsoon locations were chosen in order to cover catchments draining both silicates and carbonates at high 2000 4000 m and low 550 1300 m altitudes modelling of the dissolved composition shows that the li ca ratio of the silicate endmember in himalayan rivers is at least an order of magnitude higher than that of the carbonate endmember most of the dissolved li 90 is derived from silicates even in carbonate dominated catchments while the sr isotope composition of the dissolved load reflects that of the bedrock the main control on its li isotope composition is fractionation during weathering fractionation between the dissolved and suspended load in silicate dominated catchments is greatest at high altitude and lower at low altitude where weathering is more intense tributaries draining silicates have lower dissolved 7li values by 2 3 to 4 2 following the monsoon when weathering is more intense because of higher runoff and elevated temperatures our data suggest that riverine li fluxes largely reflect silicate weathering rates while riverine 7li varies with weathering intensity as rivers presently contribute 50 of the li input to the oceans seawater li concentrations and 7li show potential as proxies for global silicate weathering processes

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

hydrothermal source of radiogenic sr to himalayan rivers 1 k 1 and cl 2 all of which are high in the hydrothermal waters cation concentrations decrease along the lesser himalayan reach of the river hot spring dissolved co2 has a d 13 c value to 15 9 indicating that metamorphic decarbonation reactions contribute co2 to the fluids hydrothermal co2 is partially neutralized in high temperature weathering reactions which generate alkalinity and yield abundant radiogenic sr radiogenic hydrothermal carbonate can form from these solutions and later weather releasing silicate sr but imparting carbonate characteristics to the overall water chemistry

rare earth elements of carbonate rocks from the bambui group southern sao francisco basin brazil and their significance as paleoenvironmental proxies abstract rare earth elements plus y rey data obtained for carbonate rocks of the bambui group in the southern portion of the sao francisco basin match the c and sr isotope variations that subdivide the unit in three chemostratigraphic intervals ci 1 to ci 3 the lower ci 1 samples exhibit flat rey shale normalised distributions that record a high freshwater input in the basin after a glacial period the subsequent marine transgression diminished the continental influence and initiated the development of la gd and y positive anomalies in the ci 2 samples interestingly a seawater like rey pattern is present in the ci 3 samples which correspond to the closure of the sao francisco basin rather than open marine conditions the ci 3 records a decrease in the silicate chemical weathering of sources located on the marginal belts of the sao francisco craton due to higher denudation rates and a change in the dissolved influx towards carbonate rocks the resultant high concentration of carbonate ions in the restricted sea induced a rey fractionation process similarly to that of the modern ocean constant positive ce ce values throughout the shallow and deep sections suggest anoxic early diagenetic conditions inefficient circulation and stratified water column which could have worked as a barrier for life diversification in the basin at the ediacaran cambrian limit finally our data show that the rey concentrations of ancient biochemical environments are not an unequivocal tool either to track open and restricted marine settings or to distinguish proximal and distal environments

advances in global bioavailable strontium isoscapes abstract strontium isotope ratios 87sr 86sr are a popular tool in provenance applications in archeology forensics paleoecology and environmental sciences using bioavailable 87sr 86sr in provenance studies requires comparing the 87sr 86sr of a sample of interest to that of 87sr 86sr baselines historically these baselines required building empirical datasets from plants or local animals to characterize the 87sr 86sr available to local ecosystems bioavailable 87sr 86sr however researchers are increasingly relying on modeled bioavailable 87sr 86sr maps called isoscapes we review the advantages and limitations of existing approaches to mapping bioavailable 87sr 86sr for provenance studies and propose a globally applicable scalable and editable framework for creating bioavailable 87sr 86sr isoscapes this framework relies on 1 compiling global bioavailable 87sr 86sr data 2 mapping 87sr 86sr variability in rocks 3 leveraging global environmental covariates and 4 applying a random forest regression method that integrates these data to predict bioavailable 87sr 86sr when the random forest model is applied at the global scale it performs well explaining 60 of the variance of the global bioavailable 87sr 86sr dataset and accounts for geological geomorphological and atmospheric controls in data rich regions e g europe the global bioavailable 87sr 86sr isoscape can be successfully extrapolated to broad regions without bioavailable 87sr 86sr data however we also show that this extrapolation may not be valid in exceptionally geologically complex and data poor regions e g madagascar we suggest research directions to improve the accuracy of global bioavailable 87sr 86sr isoscapes which include 1 increasing the collection of bioavailable datasets in data poor regions 2 harmonizing data management practices and metadata collection for bioavailable 87sr 86sr data and 3 relying on advances in remote sensing and geological mapping techniques to improve geological covariates while significant potential to refine 87sr 86sr isoscapes remains the data products provided in this review form a basis for using 87sr 86sr data in large scale provenance studies opening new research avenues in a range of fields

dissolved osmium in bengal plain groundwater implications for the marine os budget abstract we report osmium concentrations and isotopic compositions of 40 groundwater samples from the bengal plain groundwaters have os concentrations 16 9 191 5 pg kg about 5 10 times higher than those published for most rivers or seawater 187 os 188 os varies widely from 0 96 to 2 79 and is related to the isotopic signatures of the sediments constituting local aquifers os contents are correlated with those of soluble elements such as sr mg and ca suggesting that differing extents of solid solution interaction explain most of the variation in measured os concentrations the covariation between os and sr allows us to estimate the mean os content of bengal groundwater 70 pg kg this concentration is too low to allow bengal groundwater to significantly influence the marine os isotopic composition if likely fresh groundwater discharge rates to the bay of bengal are assumed however if bengal groundwater os concentrations are typical the global os groundwater flux would be expected to be around 180 kg year making it the second largest input of os to the ocean after the river flux including this flux in the current os marine budget and assuming that this and other fluxes have remained constant with time would decrease the calculated residence time of os in the ocean by about 30

the late neogene 87sr 86sr record of lowland himalayan rivers fossil shells and paleosol carbonate from ancestral himalayan river deposits provide a 87 sr 86 sr record of lowland himalayan river water during the late neogene reconstructed 87 sr 86 sr river values increased sharply in the late miocene probably marking the beginning of exhumation of high 87 sr 86 sr metalimestones more in the central than in the western himalayas these results imply that the marine 87 sr 86 sr record may not be a proxy for silicate weathering or consumption of atmospheric co 2 resulting from that weathering

solute sources and chemical weathering in the owens lake watershed eastern california 1 chemical and strontium isotope analyses of solutes in a series of streams representing the bulk of the water reaching the 8500 km2 owens lake watershed provide a unique opportunity to evaluate factors that control stream chemistry on a regional scale the streams drain terrain that is dominated by mesozoic granitoids of the sierra nevada batholith but that also contains irregularly distributed roof pendants of paleozoic metasedimentary sequences mass balance modeling indicates that plagioclase feldspar weathering is the dominant contributor to stream chemistry and that carbonate dissolution is also a significant source of solutes for most streams the calculated ratio of plagioclase to carbonate is highly sensitive to the assumed weathering product of plagioclase kaolinite versus smectite and we suggest that the importance of carbonate could be exaggerated in cases where only the plagioclase to kaolinite reaction is considered strontium isotope 87sr 86sr ratios range from 0 70685 to 0 71958 with a flow weighted mean of 0 70911 to the extent that this value represents the water that would be flowing into owens lake were it not diverted by the los angeles aqueduct system the flow weighted mean 87sr 86sr ratio is consistent with ratios measured in owens lake core carbonates ranging in age up to 25 kyr the overall sr budget of the watershed is determined by the relative contributions of carbonate dissolution and plagioclase feldspar weathering and is dominated by reactions related to pyrite oxidation and hydrothermal fluid input from just two subbasins the impact of these processes needs to be considered when modeling regional and global riverine sr budgets climate related shifts in the relative importance of silicate mineral weathering reactions appear to be only second order effects on a regional scale

cybertectonic earth and gaia s weak hand sedimentary geology sediment cycling and the earth system tectonics climate and sea level are dominant controls on the nature and distribution of sedimentary environments by inverse logic recognition of influences on sedimentation from the stratigraphic record as a result of perturbations in these variables leads to clarification of past environmental and structural change the succession of sedimentary environments in time and place is determined by a wider combination of variables namely sediment flux biological evolution atmosphere and water body productivity and chemistry the feedbacks and interrelationships between all variables cause the complexity seen in sedimentation processes over geological time the processes of global tectonics that cause widespread mountain belt and continental plateau uplift have produced numerous severe events during earth history these often random workings out of the plate tectonic cycle define the state of cybertectonic earth from cyber after the greek kubernan to steer or govern this has worked within a usually zonally arranged series of climatic belts and within the framework provided by biological evolution at certain times in the geological past a combination of factors arose as a response to continental uplift and have acted to cause certain earth surface conditions and variables notably mean global surface temperature atmospheric p co 2 p o 2 to have varied by very large amounts 2 10 compared with present values these large fluctuations such as those responsible for the neoproterozoic late palaeozoic and late tertiary glaciations lead one to doubt the reality of homeostatic control of surface conditions as proposed in lovelock9s gaia hypothesis the gaian kuberne0te0s steersman had a weak hand on the helm frequently unable to prevent vast areas of the globe experiencing rapid fluctuations in environmental conditions and inimical conditions to life for very long periods during neoproterozoic and phanerozoic times at the same time biogenic and abiogenic processes have proved capable of returning earth to states of mean stability although biogeochemical cycling models seem alarmingly ad hoc and largely untestable as scientific hypotheses in any true geological sense

response of the ganges dispersal system to climate change a source to sink view since the last interstade the forcing of sediment dispersal systems by climate is widely accepted but there remains disagreement on the general responses to climate change such as how resulting signals propagate downstream and ultimately how they are preserved in the stratigraphic record towards a better understanding of these issues this paper presents a comprehensive overview of ganges dispersal system behavior since the last interstade which reveals major coeval responses to changes in the south asian monsoon at 3000 km from its mountain headwaters to deep sea fan the ganges dispersal system is immense and yet shows tight coupling between source area catchment basin and coastal and marine depocenters furthermore system wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1 2 ky this tight linkage of source to sink components is considered a function of the southwest summer monsoon s overwhelming control on regional hydrology about 80 of the ganges discharge and 95 of its sediment load are delivered to the margin during only 4 months making the system extremely sensitive to this seasonal forcing in addition the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents finally despite its present intensity strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global glacial boundary conditions the resulting changes in precipitation both well above and below modern values have forced system wide responses ranging from glacier advance retreat to sediment fluxes to the margin and deep sea fan the overall conclusions are 1 that this immense dispersal system responds to multimillennial scale 104 years climate change in a system wide and largely contemporaneous manner and 2 that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation furthermore these acute responses to climate change have produced sedimentary stratigraphic features that diverge from traditional sequence models in their nature and timing

evolution of carbon cycle over the past 100 million years it is generally accepted that progressive cooling of global climate since the late cretaceous results from decreasing partial pressure of atmospheric co2 pco2 however details on how and why the carbon cycle evolved and how it would affect pco2 have not been fully resolved while the long term decline of pco2 might be caused by the decrease of volcanic degassing through the negative feedback between pco2 and silicate weathering seafloor spreading the major control of co2 degassing seems to have remained relatively constant alternative explanation known as uplift driven climate change hypothesis proposes that tectonic uplift may have enhanced the sink of atmospheric co2 by silicate weathering and thus produced the decline of pco2 however increasing weathering sink of co2 could deplete atmosphere all of its co2 within several million years while holding volcanic outgassing constant in this work major fluxes of long term carbon cycle are calculated based on a reverse model constrained by marine c sr and os isotopic records and the spreading rate of sea floor weathering of island basalt and continental silicate rocks are separated in the new model the results indicate a long term decline of island basalt weathering in consistent with the global cooling trend over the past 100 million years dramatic changes of the co2 fluxes associated continental silicate weathering reverse weathering volcanic degassing and the growth of organic carbon reservoir have been observed disturbance of atmospheric co2 cycle by these fluxes seems to be maintained by the concomitant adjustments of island basalt weathering that were sensitive to the pco2 controlled environment factors such as temperature and runoff the negative feedbacks between pco2 and weathering of island basalt might have played a significant role in stabilizing the long term carbon cycle

chemical weathering in the hong red river basin rates of silicate weathering and their controlling factors abstract the hong red river drains the prominent red river fault zone that has experienced various tectonic activities intrusion of magma exhumation of basement rocks and influx of thermal waters associated with the cenozoic collision of india and eurasia we report dissolved major element and sr isotope compositions of 43 samples from its three tributary systems da thao hong main channel and lo encompassing summer and winter seasons carbonic acid ultimately derived from the atmosphere is the main weathering agent and sulfuric acid from pyrite oxidation plays a minor role seasonality is manifested in higher calcite saturation index and mg tz and lower ca mg in summer suggesting calcite precipitation and in higher si na k ratios in summer suggesting more intensive silicate weathering we quantified the input from rain evaporite carbonate and silicate reservoirs using forward and inverse models and examined the robustness of the results carbonate dissolution accounts for a significant fraction of total dissolved cations 55 97 and weathering of silicates makes a minor contribution 1 40 our best estimate of the spatially averaged silicate weathering rate in the hong basin is 170 103 mol km2 yr in summer and 51 103 mol km2 yr in winter we tested for correlations between the rate of co2 consumption by silicate weathering and various climatic air temperature precipitation runoff and potential evapotranspiration and geologic relief elevation slope and lithology parameters calculated using gis clear correlations do not emerge except for co2 and runoff in winter which we attribute to the complex geologic setting of the area the seasonal regime change from physical dominant in summer to chemical dominant in winter and the incoherent timescales involved for the different parameters tested

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

higher erosion rates in the himalaya geochemical constraints on riverine fluxes the modern erosion rate of continental scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains these two parameters are very difficult to measure directly in continental scale basins in this paper we examine the chemical composition of the suspended load bedload and dissolved load of the ganga and brahmaputra rivers and compare them with the average composition of himalayan source rocks a mass balance equation of erosion fluxes shows that a si rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock it corresponds to bedload sediment and flood plain deposits which are enriched in quartz by mineral sorting during transport the combined budget of si al and fe in the river system allows us to estimate this si rich flux by this method the total himalayan erosion is estimated to be twice the measured flux of suspended load the comparison between the brahmaputra and the ganga shows that the eastern himalaya has a higher erosion rate 2 9 mm yr than the western himalaya 2 1 mm yr this is likely the result of the higher runoff in the brahmaputra basin the intensity of the monsoon acts as a first order control of the erosion rate in the range

the strontium isotopic budget of himalayan rivers in nepal and bangladesh abstract himalayan rivers have very unusual sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic we present sr o and c isotopic data from river and rain water bedload and bedrock samples for the western and central nepal himalaya and bangladesh including the monsoon season central himalayan rivers receive sr from several sources carbonate and clastic tethyan sediments high himalayan crystalline hhc gneisses and granitoids with minor marbles carbonates and metasediments of the lesser himalaya lh and miocene recent foreland basin sediment from the siwaliks group and the modern flood plain in the tethyan himalaya rivers have dissolved sr 6 mol l and 87sr 86sr 0 717 with a large contribution from moderately radiogenic carbonate rivers draining hhc gneisses are very dilute with sr 0 2 mol l and 87sr 86sr 0 74 lesser himalayan streams also have low sr 0 4 mol l and are highly radiogenic 87sr 86sr 0 78 highly radiogenic carbonates of the lh do not contribute significantly to the sr budget because they are sparse and have very low sr in large rivers exiting the himalaya sr systematics can be modeled as a mixture between tethyan rivers where slightly radiogenic carbonates mean 87sr 86sr 0 715 are the main source of sr and lesser himalaya waters where extremely radiogenic silicates 0 8 are the main source of sr hhc waters are less important because of their low sr rivers draining the siwaliks foreland basin sediments have sr 4 mol l and 87sr 86sr 0 725 weathering of silicates in the siwaliks and the flood plain results in a probably significant radiogenic 0 72 0 74 input to the ganges and brahmaputra g b but quantification of this flux is limited by uncertainties in the hydrologic budget the g b in bangladesh show strong seasonal variability with low sr and high 87sr 86sr during the monsoon sr in the brahmaputra ranges from 0 9 mol l and 0 722 in march to 0 3 mol l and 0 741 in august we estimate the seasonally weighted flux from the g b to be 6 5 108 mol yr with 87sr 86sr 0 7295

isotope evidence for secondary sulfide precipitation along the marsyandi river nepal himalayas abstract we present sulfur and oxygen isotope data from 41 samples of dissolved riverine sulfate from along 65 km of the marsyandi river in the northern himalayas coupled sulfur and oxygen isotopic composition of riverine sulfate 34 s so 4 and 18 o so 4 respectively yield unique constraints on the source of sulfur to the river system the headwaters of the marsyandi river have light 34 s so 4 and 18 o so 4 which requires that the source of sulfate to the river is through the anoxic weathering of pyrite likely via fe3 the 34 s so 4 and 18 o so 4 of sulfate in tributaries to the marsyandi increase downstream which could result either from inputs from evaporites or bacterial sulfate reduction with subsequent sulfide precipitation in warmer and wetter catchments either of these processes could result in heavy 34 s so 4 and 18 o so 4 of the residual river sulfate elemental ratios such as sr ca and ca so4 suggest as previous studies have also concluded that evaporite weathering is not important in the marsyandi river we conclude that the isotope data is most consistent with the onset of bacterial sulfate reduction and secondary sulfide precipitation in the soils in the warmer and wetter downstream catchments our results have implications for understanding the source of sulfate to the ocean as well as the redox and acidity budget within rapidly eroding catchments

widespread basement erosion during the late paleocene early eocene in the laramide rocky mountains inferred from 87sr 86sr ratios of freshwater bivalve fossils reconstruction of the timing and pattern of exhumation of basement involved uplifts during the laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region we use reconstructions of the 87 sr 86 sr ratios of late cretaceous early cenozoic river water from fossil shells in six basins of the rocky mountains contextualized by modern river water sr chemistry studies to trace the erosion of precambrian basement cores in the laramide ranges the 87 sr 86 sr ratios and sr concentration of modern river water in the rocky mountains are controlled by river bedrock lithology weathering of precambrian silicate rocks in the cores of laramide ranges produces high 87 sr 86 sr ratios in highland rivers weathering of paleozoic and mesozoic carbonates along the basin margins reduces the 87 sr 86 sr ratios and increases sr concentration of rivers as they flow basinward lowland rivers that have headwaters in precambrian basement mostly have 87 sr 86 sr ratios 0 711 whereas rivers confined to or with very long reaches in basins have 87 sr 86 sr ratios between 0 709 and 0 711 river water 18 o values do not change in response to changes in catchment elevation our results from fossil shells show that carbonate weathering dominated the river water sr chemistry in the alberta foreland williston and crazy mountains basins and that proterozoic low grade metamorphic carbonates in the belt purcell supergroup were not exposed in the canadian rocky mountains during late cretaceous early paleocene time silicate weathering influenced the sr chemistry of surface waters in the powder river basin during the late paleocene and the washakie basin during the early eocene suggesting precambrian silicate basement rock was extensively exhumed and eroded the observation that precambrian basement was eroded earlier around the powder river basin than around the washakie basin is consistent with a previous study that suggests laramide ranges in northeast wyoming reached high elevation earlier than the ranges adjacent to the sevier thrust belt widespread basement erosion in late paleocene early eocene time was controlled by tectonic exhumation and may have been intensified by the wet and warm global climate

major ion chemistry of the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal the ganga brahmaputra one of the world s largest river systems is first in terms of sediment transport and fourth in terms of water discharge a detailed and systematic study of the major ion chemistry of these rivers and their tributaries as well as the clay mineral composition of the bed sediments has been conducted the chemistry of the highland rivers upper reaches of the ganga the yamuna the brahmaputra the gandak and the ghaghra are all dominated by carbonate weathering ca mg and hco3 account for about 80 of the cations and anions in the lowland rivers the chambal the betwa and the ken hco3 excess over ca mg and a relatively high contribution of na k to the total cations indicate that silicate weathering and or contributions from alkaline saline soils and groundwaters could be important sources of major ions to these waters the chemistry of the ganga and the yamuna in the lower reaches is by and large dictated by the chemistry of their tributaries and their mixing proportions illite is the dominant clay mineral about 80 in the bedload sediments of the highland rivers kaolinite and chlorite together constitute the remaining 20 of the clays in the chambal betwa and ken smectite accounts for about 80 of the clays this difference in the clay mineral composition of the bed sediments is a reflection of the differences in the geology of their drainage basins the highland rivers weather acidic rocks whereas the others flow initially through basic effusives the ganga brahmaputra river system transports about 130 million tons of dissolved salts to the bay of bengal which is nearly 3 of the global river flux to the oceans the chemical denudation rates for the ganga and the brahmaputra basins are about 72 and 105 tons km yr 1 respectively which are factors of 2 to 3 higher than the global average the high denudation rate particularly in the brahmaputra is attributable to high relief and heavy rainfall

carbonate versus silicate weathering in the raikhot watershed within the high himalayan crystalline series the major element and sr isotope geochemistry of surface waters bedrock and river sands was investigated in the raikhot watershed within the high himalayan crystalline series hhcs in northern pakistan mass balance calculations of mineral weathering contributions to the dissolved flux of ions from the watershed indicate that 82 of the hco 3 flux is derived from the weathering of carbonate minerals and only 18 is derived from silicate weathering even though the bedrock is predominantly quartzofeldspathic gneiss and granite with only 1 carbonate in the watershed this study demonstrates the importance of trace amounts of bedrock carbonate in controlling the water chemistry of glacial watersheds we suggest that the flux of sr with a high 87 sr 86 sr ratio in the major himalayan rivers may be derived in large part from weathering of trace amounts of calcite within the largely silicate hhcs models that use the flux of radiogenic sr from the himalayas as a proxy for silicate weathering rates may therefore overestimate the amount of co 2 consumption due to silicate weathering in the himalaya

post glacial climate forcing of surface processes in the ganges brahmaputra river basin and implications for carbon sequestration abstract climate has been proposed to control both the rate of terrestrial silicate weathering and the export rate of associated sediments and terrestrial organic carbon to river dominated margins and thus the rate of sequestration of atmospheric co2 in the coastal ocean over glacial interglacial timescales focused on the ganges brahmaputra rivers this study presents records of post glacial changes in basin scale indian summer monsoon intensity and vegetation composition based on stable hydrogen d and carbon 13 c isotopic compositions of terrestrial plant wax compounds preserved in the channel levee system of the bengal fan it then explores the role of these changes in controlling the provenance and degree of chemical weathering of sediments exported by these rivers and the potential climate feedbacks through organic carbon burial in the bengal fan an observed 40 shift in d and a 3 4 shift in both bulk organic carbon and plant wax 13 c values between the late glacial and mid holocene followed by a return to more intermediate values during the late holocene correlates well with regional post glacial paleoclimate records sediment provenance proxies sr nd isotopic compositions reveal that these changes likely coincided with a subtle focusing of erosion on the southern flank of the himalayan range during periods of greater monsoon strength and enhanced sediment discharge however grain size normalized organic carbon concentrations in the bengal fan remained constant through time despite order of magnitude level changes in catchment scale monsoon precipitation and enhanced chemical weathering recorded as a gradual increase in k si and detrital carbonate content and decrease in h2o si proxies throughout the study period these findings demonstrate a partial decoupling of climate change and silicate weathering during the holocene and that marine organic carbon sequestration rates primary reflect rates of physical erosion and sediment export as modulated by climatic changes together these results reveal the magnitude of climate changes within the ganges brahmaputra basin following deglaciation and a closer coupling of monsoon strength with oc burial than with silicate weathering on millennial timescales

sr c and o isotopes in carbonate nodules from the ganga plain evidence for recent abrupt rise in dissolved 87sr 86sr ratios of the ganga abstract the evolution of 87 sr 86 sr ratio of the ganga water during the past 100 ka has been reconstructed in this study the 87 sr 86 sr 13 c and 18 o values have been measured in carbonate nodules from different depths of two sediment cores raised from the indian institute of technology kanpur iitk campus 50 m long and jagdishpur jp 25 m long respectively the 87 sr 86 sr ratios of the carbonate nodules range from 0 7142 to 0 7189 in the iitk and 0 7142 to 0 7367 in the jp cores the nodules in general display significantly lower 87 sr 86 sr ratio compared to contemporary ganga river water at kanpur however values of 87 sr 86 sr ratio are observed in the nodules of the jp core near the surface are consistent with that of the present day ganga water at kanpur as well as groundwater samples from adjacent areas indicate recent abrupt increase in 87 sr 86 sr ratio of the ganga these findings are also consistent with the concomitant increase in 87 sr 86 sr ratio of silicates in contemporary sediments of the ganga at kanpur compared to that of past 100 ka the sudden rise of 87 sr 86 sr ratio of the ganga is probably due to increase in the relative proportion of sr from the lesser himalaya containing silicates and carbonates with higher 87 sr 86 sr ratio the cause for the recent increase in sr contribution from the lesser himalaya is not clear this could be due to enhanced agricultural activities and deforestation and or climatic variability during recent times resulting in more erosion in the lesser himalaya the 13 c and 18 o measured in the nodules from the iitk core vary from 6 8 to 1 6 and 8 3 to 5 4 respectively the co variation of 13 c and 18 o suggest the impact of paleoclimate paleovegetation petrography and chemical composition of carbonate nodules indicate little or no diagenetic alteration

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

high fluxes of radium and barium from the mouth of the ganges brahmaputra river during low river discharge suggest a large groundwater source the annual flux of sediment from the ganges brahmaputra river is among the highest in the world the desorption of226ra and ba from this sediment produces a major point source of226ra and ba input to the ocean highest226ra and ba fluxes are expected to occur during high river flow june september when most of the sediment is discharged surprisingly during low discharge of the ganges brahmaputra river fluxes of226ra and ba to the northern bay of bengal are comparable to expected river derived fluxes during peak river discharge a large non riverine source of ra and ba is required to explain the high fluxes during low river discharge i suggest that this source is submarine discharge of groundwater containing high concentrations of226ra and ba total annual fluxes of226ra and ba to the ocean from this system are significantly greater than estimated previously

chemical weathering rates in the alaknanda bhagirathi river basins in himalayas india abstract the alaknanda and bhagirathi rivers flow through the higher and lesser himalayas and confluence at devprayag which represents the origin of the ganga or ganges river in the present study a vast number of temporal and spatial samples of the river waters were collected and analyzed for major cations and anions in addition more recent and time series water flow data have been obtained and based on these inputs a more refined dissolved flux rates have been estimated the alaknanda and bhagirathi rivers show significant variations in chemical compositions during different seasons carbonate rock weathering is responsible for more than 70 of the chemical compositions in the river waters the chemical weathering rates show seasonal variations and are much higher during non monsoon season the dissolved flux of alaknanda river is much higher 1 80 106 tons yr 1 as compared to the bhagirathi river 0 34 106 tons yr 1 the chemical weathering rates in the basin vary between 85 and 155 tons km 2 yr 1 which is significantly higher compared to the global average of 24 tons km 2 yr 1

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

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

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

carbonate geochemistry and the concentrations of aqueous mg2 sr2 and ca2 western north coast of the yucatan mexico abstract we have measured concentrations of major cations sr 2 and alkalinity in groundwater pore water and surface water samples collected from the western north coast of the yucatan peninsula mexico thermodynamic calculations indicate that almost all water samples from the study area are supersaturated with respect to calcite aragonite high mg calcite hmc and dolomite analysis of the data suggests that the geochemical processes controlling mg 2 sr 2 and ca 2 concentrations differ in different environments mass balance modeling has been used to calculate the amount of dissolution and or precipitation of aragonite low mg calcite lmc and hmc and or dolomite required to produce waters with the observed mg 2 sr 2 and ca 2 concentrations mass balance calculations indicate that hmc and aragonite are dissolving whereas lmc is precipitating from most groundwater and inland surface waters the chemistry of surface waters from an extensive saline coastal swamp suggests that lmc and aragonite in surficial swamp sediments are converted to hmc or dolomite while swamp pore water chemistry suggests that hmc is dissolving and lmc and aragonite are precipitating in deeper swamp sediments the chemistry of pore waters suggests that they are derived from saline surface waters and that fresh to brackish groundwater from the shallow confined aquifer does not flow into swamp sediments

increasing chemical weathering in the himalayan system since the last glacial maximum continental chemical weathering is central in earth s surface biogeochemical cycles as it redistributes elements across reservoirs such as the crust and the oceans however the evolution of weathering through time and its response to external forcing such as changes in climate remain poorly constrained in this work a composite sediment record from the bay of bengal is used to document the evolution of chemical weathering in the himalayan system himalayan range and indo gangetic floodplain the world largest sediment conveyor to the oceans since the last glacial maximum lgm the degree of weathering of the sediments is documented using mobile to immobile ratios such as k si and h2o si as well as detrital calcite abundance robust weathering proxies are derived by correcting the chemical composition of sediment for sorting effects that occur during transport and deposition the bay of bengal record is also further compared to the chemical composition of modern river sediments from the ganga and brahmaputra basin weathering proxies all indicate that the sediments exported by the ganga and brahmaputra rivers became increasingly weathered over the past 21 kyr whereas sr nd and major elements suggest a constant sediment provenance in the system over the last 21 kyr these changes in the degree of weathering of the sediments show that the weathering flux exported by the system to the indian ocean during the lgm was significantly lower than at present and demonstrate that chemical weathering in continental scale basins such as the ganga and brahmaputra responds to late quaternary climate changes

accelerated contribution of the paleo congo river to global seawater 87 sr 86 sr change following eocene oligocene collapse of the african surface

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

tracing the sources of strontium in karst groundwater in chongqing china a combined hydrogeochemical approach and strontium isotope groundwater quality in karst regions is largely controlled by natural processes and anthropogenic activities over the past 10 years dissolved sr and its radiogenic isotope 87sr 86sr were widely used to trace the sources of solutes in groundwater however there is little research about hydrogeochemistry and sr isotopic compositions of the karst groundwater in chongqing karst area in this paper thirty five representative karst groundwater samples were collected from different aquifers limestone and dolomite and various land use types hydrochemical types of karst groundwater in chongqing were mainly of the ca hco3 type or ca mg hco3 type the dissolved sr concentrations of the studied groundwater ranged from 0 57 to 15 06 mmol l and the 87sr 86sr varied from 0 70751 to 0 71627 the groundwater samples from different aquifers and land use types showed distinctive dissolved sr concentrations and 87sr 86sr the very positive relationship between ca sr and mg sr in dolomite and limestone aquifers suggests that ca mg and sr element come mainly from the release of carbonate rock under the groundwater rock co2 gas interaction according to the 87sr 86sr ratio the sr element in karst groundwater in chongqing was controlled by the weathering of limestone dolomite and silicate rock allogenic water in a non karst area the relationship 87sr 86sr versus sr2 k na shows that the anthropogenic inputs also obviously contribute to the sr contents the research results show that the karst groundwater in chongqing is facing serious crisis of water quality and needs to be protected further

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

u th ra variations in himalayan river sediments gandak river india weathering fractionation and or grain size sorting abstract understanding the origin of u th ra variations in the ganga river sediments is a prerequisite for correctly using u series nuclides to constrain the sediment transport times in himalayan rivers for this purpose u th and ra concentrations along with 238 u 234 u 230 th 226 ra radioactive disequilibria were analyzed in bank bedload and suspended sediments from the gandak river one of the main tributaries of the ganga river the data confirm that u and th budgets of the himalayan sediments are significantly influenced by minor resistant minerals such as zircon garnet and ti bearing minerals the dissolution of which required the use of a high pressure acid digestion process most importantly the results indicate that the variations in 238 u 232 th and 230 th 232 th activity ratios and 238 u 234 u 230 th 226 ra disequilibria in sediments along the river alluvial plain mainly reflect modifications in the mineralogical and grain size compositions rather than the degree of weathering during transport the 238 u 232 th and 230 th 232 th activity ratios in the bank and bed sediments are related to variations in the minor primary minerals strongly enriched in u and th i e zircon ree bearing minerals and ti bearing minerals whereas the activity ratios in the suspended load are related to variations in the proportions of clay fe oxyhydroxides and the silt sand fraction which contains u and th bearing minor minerals the data also indicate that 238 u 234 u 230 th 226 ra disequilibria are strongly influenced by secondary mineral phases the 230 th budget is likely mainly controlled by fe oxyhydroxides and the 226 ra budget is likely mainly controlled by clay minerals therefore the variations in the 238 u 234 u 230 th 232 th system in the sediments of the gandak river cannot simply be interpreted as the result of fractionation due to chemical transformation of the bulk sediment during its transport within the alluvial plain and or the result of radioactive decay consequently they cannot be used to infer long sediment transport times within the gandak plain 10 100 ka as previously proposed such analytical and interpretative artifacts are certainly not specific to the present study on the gandak basin these issues will certainly be encountered anytime this technique is applied to alluvial systems in which the u and th budgets of the sediments are influenced by heavy minerals that can be sorted during the transport of sediments within the plain

the first large scale bioavailable sr isotope map of china and its implication for provenance studies abstract strontium isotope ratio 87sr 86sr has been widely used to trace human migrations across geologically distinct landscapes in archaeological and forensic sciences 87sr 86sr based provenance studies require a detailed strontium reference map to make valid interpretations of mobility patterns for a given region in this study we first review the previous development for 87sr 86sr isoscapes relevant to research on migration and then present the first map of bioavailable 87sr 86sr ratios for china based on 1935 measurements on biosphere samples collated from published research and supplemented by our targeted collection strategy the results show that 87sr 86sr ratios in china span a large range and exhibit regional patterns largely dependent on the divergent geological conditions of each of china s tectonic blocks we further combine river water 87sr 86sr ratios with lithology integrated elemental data and chemical weathering rates to determine which dominant weathering patterns e g silicate carbonate or evaporite have the most significant influence on 87sr 86sr ratios the low ranges of 87sr 86sr ratios 0 7125 appear in the cathaysia block as well as some orogenic belts and the southeastern margin of the tibetan plateau likely due to the influence of the metamorphic rocks and granitoid sources additionally topography differential weathering among lithologies and eolian deposits e g loess are also significant factors that influence 87sr 86sr ratios in china our study reveals that the large variation of 87sr 86sr ratios across china is predominately driven by geological conditions revealing that future sr isotope provenance studies can shed on ancient and historical migrations in china

fractionation of boron isotopes during erosion processes the example of himalayan rivers abstract the boron isotopic composition of 5 rain waters and 35 river waters running from the high himalayas 5050 m down to the bay of bengal were measured by ion microprobe to make a first survey of the boron isotopic fractionations present in a major erosion system the rains have rather homogeneous b concentrations and isotopic compositions 0 09 0 09 mol l and 5 4 1 7 at the exception of the two kathmandu samples which probably reflect a significant anthropic contamination the river waters show a large range of b concentrations from 0 09 to 28 68 mol l and of b isotopic compositions 11b values ranging from 7 0 to 29 4 no systematic correlation between the 11b values and the major ion chemistry of the river waters is present at the exception of the most b and cl rich rivers which have systematically the highest 11b values likely controlled by evaporite dissolution after correction for the evaporite contribution from 9 4 to 64 4 of dissolved b with a systematic decrease southwards the riverine b concentrations and 11b values are still highly variable from 0 10 to 5 83 mol l and from 5 8 to 24 1 respectively this variation reflects mostly different conditions of silicate weathering congruent and incongruent silicate dissolution the contribution from the dissolution of carbonates being quite negligible 5 of the dissolved b on the river b budget at variance with congruent silicate dissolution incongruent dissolution occurs with a large b isotope fractionation which depends on ph clay water 31 2 at acidic ph and 5 8 at higher ph the b systematic suggests strongly that these fractionations are produced over a wide range of ph from 7 to 9 5 in soils and don t result from reactions between the dissolved and suspended load in rivers the net result of himalayan erosion for the global b cycle is the input to the sea of waters with highly variable b concentrations and isotopic compositions 0 67 and 0 29 mol l 5 0 and 21 0 for the ganga and the brahmaputra during the monsoon period respectively

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

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

the triassic of the thakkhola nepal i stratigraphy and paleoenvironment of a north east gondwanan rifted margin the mesozoic sediments of thakkhola central nepal were deposited on a broad eastern north gondwanan passive margin at mid latitudes 28 41 s facing the southern tethys ocean to the north the facies is strikingly similar over a distance of several thousand kilometres from ladakh in the west to tibet and to the paleogeographically adjacent north west australian margin exmouth plateau odp legs 122 123 and timor in the east late paleozoic rifting led to the opening of the neo tethys ocean in early triassic times an almost uninterrupted about 2 km thick sequence of syn rift sediments was deposited on a slowly subsiding shelf and slope from early triassic to late valanginian times when break up between gondwana north west australia and greater india formed the proto indian ocean

a silicate weathering mechanism linking increases in marine 87sr 86sr with global glaciation the 87sr 86sr ratio of sea water is generally regarded as a proxy for the rate of chemical weathering of the continents interpretation of the 87sr 86sr record in marine limestones requires an understanding of how changes in this quantity are linked to climate and tectonic events a connection between the strontium isotope record and the extent of global glaciation has been proposed1 8 but without a well established mechanism glaciation may influence marine 87sr 86sr by changing global riverine sr fluxes that is rock weathering rates and or 87sr 86sr ratios that is relative mineral weathering rates here we investigate how the 87sr 86sr release from silicate weathering is affected by glaciation using the sr isotope systematics of granitoid soils on alpine glacial moraines in the wind river range wyoming we observe a negative correlation between the 87sr 86sr ratio of exchangeable sr in soils and the soil age indicating that the 87sr 86sr ratio of sr released in the early stages of weathering is significantly higher than in later stages we estimate that this mechanism can increase global riverine 87sr 86sr by an average of 0 0002 during periods of glacial interglacial cycling when superimposed on long term trends controlled by tectonic processes2 7 9 10 this shift can account for the correlation between the rate of change of marine 87sr 86sr and the intensity of glaciation over at least the past 10 myr

comparison of the sr isotopic signatures in brines of the canadian and fennoscandian shields abstract a synthesis of sr isotope data from shallow and deep groundwaters and brines from the fennoscandian and canadian shields is presented a salinity gradient is evident in the water with concentrations varying from approximately 1 75 g l 1 below 1500 m depth in the fennoscandian shield and from 10 up to 300 g l 1 below 650 m depth in the canadian shield strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water rock interaction in the systems in both shields the sr concentrations are enriched relative to cl defining a positive trend parallel to the seawater dilution line and indicative of sr addition through weathering processes the depth distribution for sr concentration increases strongly with increasing depth in both shields although the variation in sr isotope composition does not mirror that of sr concentrations strontium isotope compositions are presented for surface waters and groundwaters in several sites in the fennoscandian and canadian shields numerous mixing lines can be drawn reflecting water rock interaction a series of calculated lines links the surface end members surface water and shallow groundwater and the deep brines these mixing lines define a range of 87 sr 86 sr ratios for the deep brines in different selected sites all sites show a specific 87 sr 86 sr signature and the occurrence of large 87 sr 86 sr variations is site specific in both shields in canadian shield brines the sr isotope ratios clearly highlight large water rock interaction that increases the 87 sr 86 sr ratio from water that could have been of marine origin in contrast to the canadian shield groundwater does not occur in closed pockets in the fennoscandian and the well constrained 87 sr 86 sr signatures in deep brines should correspond to a large well mixed and homogeneous water reservoir whose sr isotope signature results from water rock interaction

hydrogen and oxygen isotope variations in the high himalaya peraluminous manaslu leucogranite evidence for heterogeneous sedimentary source abstract the manaslu granite belongs to the high himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting d and 18o values have been determined for whole rock and coexisting minerals from the 8 km thick manaslu massif and its 50 km long dyke sheet its country rocks and the formation 1 f1 paragneisses which are the source of the granite for the granite dmusc range from 70 to 85 and 18o w r from 10 9 to 12 8 h and o isotope fractionations among minerals are consistent with high temperature equilibrium and for oxygen closed system evolution a few samples coming mainly from the dyke swarm have very low d values down to 188 and biotite muscovite h isotope fractionations indicative of disequilibrium the d h ratios of associated magmatic tourmaline are essentially unmodified from the distribution of d values in the granite and its country rocks circulation of very low deuterium meteoric hydrothermal waters was extremely localized because these waters are depleted in deuterium by up to 50 relative to modern meteoric waters the manaslu area was either at an altitude substantially higher than that of today 2500 6000 m for the analyzed samples or a mountain chain once existed to the south the f1 gneisses have 18oquartz between 12 and 14 3 which confirms that the granite was generated from f1 but d values are 20 higher than in the granite such a difference can be a result of degassing of the magma and or introduction of fluid in the melting zone infiltration of low d fluid 90 into the hot but dry f1 probably triggered partial melting these fluids could have come from the dehydration of the midlands sediments which are separated from the overlying f1 by the main central thrust the correlations among 18o 87 sr 86 sr 20 ma and nd values in both f1 and the granite indicate that the variations of these isotopic ratios in the manaslu are inherited from those in f1 at the time of melting in turn these ratios in f1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of nd and o and to the quantity of radiogenic sr generated within the sediment which is a function of age and rb content amount of phyllosilicate some other himalayan leucogranites require either other crustal source rocks or the 18o and 87 sr 86 sr ratios of f1 vary along the himalaya

effects of quaternary sea level cycles on strontium in seawater abstract the effects of quaternary sea level changes on the sr budget of the ocean are investigated using coupled numerical models of the seawater sr and ca budgets glacial interglacial sea level cycles influence the sr concentration of seawater directly through the periodic exposure and weathering of aragonite on continental shelves and indirectly by modulating the location and extent of carbonate deposition in the ocean as sea level recedes brief pulses of sr are added to the ocean from recrystallization of shelf aragonite this flux at its maximum is nearly ten times larger than the combined riverine and hydrothermal influx the large shelf recrystallization fluxes during glacial maxima produce rapid increases in the sr ca ratio of seawater despite the long residence time of sr in the ocean variations in the ratio of aragonitic vs calcitic sedimentation also influence on the sr content of seawater because the sr partitioning coefficient is higher in aragonite however this effect is minor compared to shelf aragonite recrystallization based on a variety of sensitivity tests the amplitude of sr ca variations ranges from 1 to 3 and is maximized by constant rates of total carbonate deposition extensive aragonite recrystallization and carbonate budgets which include dissolution of shelf carbonates such variation is sufficient to produce up to 1 5 c errors in paleotemperatures calculated from coral sr ca ratios since the last glacial maximum

the magnesium isotope budget of the modern ocean constraints from riverine magnesium isotope ratios we have measured the magnesium mg isotope ratios in 45 rivers including 16 of the largest rivers in the world covering a range of geologic tectonic and climatic drainage basin environments the range in riverine 26mg is 2 5 half the variation in terrestrial rock although the mg isotopic composition of the source rock is important for small rivers at a global scale lithology is of limited significance for mg isotope ratios in rivers part of the variability at a global scale may be attributed to fractionation in the weathering environment the rivers analysed constitute 30 of the global mg riverine flux to the oceans and a flux weighted mg isotope composition of global runoff has been estimated at 1 09 even taking into account uncertainty this is distinct from seawater at 0 82 this difference arises either from the fractionation of mg isotope ratios in the ocean and or a mg budget which is not at steady state the difference is consistent with fractionation by carbonate precipitation in the simplest steady state scenario where the oceanic mass balance is maintained by riverine input and hydrothermal and dolomite output mg isotope ratios imply a minimum dolomite mg flux of 9 of the total output mg flux the implications of the distinct isotopic composition of the riverine flux from modern seawater is that the mg isotope ratio of seawater has probably varied outside of current analytical uncertainty mg isotope ratios may therefore provide a valuable new tracer in palaeo oceanography

predominant floodplain over mountain weathering of himalayan sediments ganga basin we present an extensive river sediment dataset covering the ganga basin from the himalayan front downstream to the ganga mainstream in bangladesh these sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column mineral sorting is the first order control on the chemical composition of river sediments taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain by comparing sediments sampled at the himalayan front with sediments from the ganga mainstream in bangladesh it is possible to budget weathering in the floodplain assuming a steady state weathering regime in the floodplain the weathering of himalayan sediments in the gangetic floodplain releases ca 189 92 10 9 and 69 22 10 9 mol yr of carbonate bound ca and mg to the dissolved load respectively silicate weathering releases 53 18 10 9 and 42 13 10 9 mol yr of na and k while the release of silicate mg and ca is substantially lower between ca 0 and 20 10 9 mol yr additionally we show that sediment hydration h2o is a sensitive tracer of silicate weathering that can be used in continental detrital environments such as the ganga basin both h2o content and the d h isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions by comparing the chemical composition of river sediments across the floodplain with the composition of the eroded himalayan source rocks we suggest that the floodplain is the dominant location of silicate weathering for na k and h2o overall this work emphasizes the role of the gangetic floodplain in weathering himalayan sediments it also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account 2012 elsevier ltd all rights reserved

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

quantifying sand provenance and erosion marsyandi river nepal himalaya we use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the marsyandi basin of the central himalaya a privileged natural laboratory in which a series of multidisciplinary geomorphological sedimentological geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic climatic and sedimentary processes in high relief orogenic belts although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the marsyandi river uncertainties are caused by potential compositional variation between the monsoon and post monsoon season estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin ourresults support focusederosion of the southern tectonically lower part of the greater himalayain the hangingwall of the mct zone where thesummer monsoon reachesitspeak intensity up to 5 m a andsediment yieldsanderosion ratesreach14 100 3400t km 2 and 5 1 1 2 mm a erosion rates sharply decrease southward in low relief lesser himalayan units 1 6 0 6 mm a and also progressivelydecreasenorthwardsinthehigh altitude tectonically upperpartofthegreaterhimalaya whererainfalldecreasesrapidly to b2 m a even areas of extreme topography such as the manaslu granite are characterized by relatively low erosion rates 2 4 0 9 mm a because precipitations become too scarce to feed significant ice flux and glacial activity monsoonal rainfall decreases further tob0 5 m a in the tethys himalayan zone farther north where erosion rates are 1 mm a coupling between erosion and peak monsoonal rainfall along the southern front of the greater himalaya is consistent with both channel flow models of tectonic extrusion

magnesium isotope systematics of the lithologically varied moselle river basin france abstract magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the moselle basin northeastern france this small basin is remarkable for its well constrained and varied lithology on a small distance scale and this is reflected in river water sr isotope compositions upstream where the moselle river drains silicate rocks of the vosges mountains waters are characterized by relatively high 87sr 86sr ratios 0 7128 0 7174 in contrast downstream of the city of epinal where the moselle river flows through carbonates and evaporites of the lorraine plateau 87sr 86sr ratios are lower down to 0 70824 magnesium in river waters draining silicates is systematically depleted in heavy isotopes 26mg values range from 1 2 to 0 7 relative to the value presently estimated for the continental crust and a local diorite 0 5 in comparison 26mg values measured in soil samples are higher 0 0 this suggests that mg isotope fractionation occurs during mineral leaching and or formation of secondary clay minerals on the lorraine plateau tributaries draining marls carbonates and evaporites are characterized by low ca mg 1 5 3 2 and low ca sr 80 400 when compared to local carbonate rocks ca mg 29 59 ca sr 370 2200 similar to other rivers draining carbonates the most likely cause of the mg and sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer 26mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year from 1 4 to 1 6 and from 1 2 to 1 4 respectively within the range defined for the underlying rocks downstream of epinal the compositions of the moselle river samples in a 26mg vs 87sr 86sr diagram can be explained by mixing curves between silicate carbonate and evaporite waters with a significant contribution from the vosgian silicate lithologies 70 temporal co variation between 26mg and 87sr 86sr for the moselle river throughout year is also observed and is consistent with a higher contribution from the vosges mountains in winter in terms of runoff and dissolved element flux overall this study shows that mg isotopes measured in waters rocks and soils coupled with other tracers such as sr isotopes could be used to better constrain riverine mg sources particularly if analytical uncertainties in mg isotope measurements can be improved in order to perform more precise quantifications

balancing the cenozoic carbon and alkalinity cycles constraints from isotopic records a simple model of the carbon alkalinity and strontium cycles is built up and used to interpret the carbon and strontium isotopic evolution of seawater over the cenozoic as recorded in marine limestones the idea that weathering fluxes have increased globally over the cenozoic in response to the himalayan uplift is critically examined it is shown that such an increase of global weathering postulated to explain the sharp increase in the 87sr 86sr ratio is not compatible with the carbon isotopic record if the budgets of carbon alkalinity and strontium are considered simultaneously another scenario of the cenozoic evolution of global weathering consistent with the carbon isotopic record is proposed in which the global weathering flux varies only slightly over the cenozoic with a small increase between 15 ma and the present this second scenario appears more consistent with the reconstructed carbonate sedimentation rate and the evolution of the carbon isotopic fractionation in continental and oceanic environments

re os isotope systematics of sediments of the brahmaputra river system re os analyses were performed on suspended loads and coarser grained bank sediments of the brahmaputra river system re and os concentrations of these sediments vary from 7 to 1154 ppt and from 3 to 173 ppt respectively 187os 188os ratios range from 0 178 to 6 8 and thus vary from nearly mantle to very radiogenic crustal values nevertheless most of the sediments have 187os 188os ratios less than 1 5 and nearly all of the samples of the brahmaputra main channel have ratios less than 1 2 thus as previously suggested the brahmaputra is much less radiogenic than the ganga the siang river the northern extension of the brahmaputra is quite radiogenic in os despite receiving sediments from the tsangpo river which flows along a suture zone with ultramafic outcrops the brahmaputra main channel has a fairly constant 187os 188os ratio even though its tributaries contribute sediments with very heterogeneous os isotopic compositions these data along with the corresponding nd isotopic compositions suggest that about 60 90 of the sediment in the brahmaputra system is derived from himalayan formations higher himalaya and lesser himalaya whereas 10 40 comes from ophiolite bearing sequences perhaps eastern equivalents of those of the transhimalayan plutonic belt os data also confirm previously published sr and nd results indicating that about half of the sediments delivered to the brahmaputra are supplied by the siang river while the himalayan and the eastern tributaries account for 40 and 10 respectively the lower 187os 188os of the brahmaputra river compared to that of the ganga is due to two factors one is the more limited presence of the lesser himalaya and hence the lower black shale content of the eastern himalaya the other is the non radiogenic os supplied by the eastern and southern tributaries reflecting the presence of mantle derived lithologies in this region despite the lower sediment supply from these tributaries they contribute greatly to the os budget of the brahmaputra river this study indicates that the brahmaputra river has little effect on the present day seawater os budget however reconsideration of this budget suggests that the ganga which provides the most radiogenic os of major rivers studied to date may have significant impact on the marine os isotopic composition the indo asian collision cannot be excluded as an important cause of the increase in the marine 187os 188os over the past 16 million years until the contributions of all of the rivers draining the himalayan tibetan plateau are known

hydrochemistry of carbonate terrains in alpine glacial settings nearly 200 analyses of meltwaters ice and snow from three alpine glacial sites in carbonate terrain are summarized and discussed in terms of sources of solutes and kinetic controls on the progress of weathering reactions most data derive from the swiss glacier de tsanfleuron which is based on cretaceous and tertiary pure and impure limestones two other sites marmolada italian dolomites and the saskatchewan glacier alberta are based on a mixed calcitic dolomitic substrate most solutes originate from carbonate dissolution moreover where pyrite is present its oxidation supplies significant sulphate and forces more dissolution of carbonate the ratios sr2 ca2 and mg2 ca2 are much higher in tsanfleuron melt waters than local bedrock a phenomenon that can be reproduced in the laboratory at small percentages of dissolution these anomalous ratios are attributed to incongruent dissolution of traces of the metastable carbonates mg calcite and aragonite these phases also provide na to solution k is argued to originate mainly by ion exchange on clays with solute ca2 quartz and very minor feldspar dissolution are also inferred locally enhanced input from atmospheric sources is recognized by high cl and associated na the progress of weathering reactions has been evaluated by the trends in the data computer modelling and some simple laboratory experiments the most dilute samples show a trend towards removal of co2 to low partial pressures c 10 5 5 atmospheres reflecting initially rapid carbonate dissolution and relatively slow dissolution of gaseous co2 later addition of atmospheric co2 or acid from pyrite oxidation allows further carbonate dissolution but solutions show a wide range of saturations and co2 pressures as high as 10 2 2 where pyrite oxidation is important in a carbonate terrain measurement of electroconductivity corrected to 25 c and alkalinity in the field allows the following preliminary deductions where meq stands for milliequivalents formula presented where s is the minimum meq ca2 mg2 produced by simple dissolution of carbonate unconnected with pyrite oxidation as with any proxy method these deductions do not remove the need for chemical analysis of waters in a given study area copyright 1994 john wiley sons ltd

mg isotopic composition of carbonate insight from speleothem formation simultaneous high precision measurement of 24 mg 25 mg and 26 mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry mc icp ms relative to the international standard srm980 data are presented on low mg calcite speleothems and their associated host rocks and waters from four caves one in the french alps and three in israel covering various climate conditions in addition data are presented on three dolostones and three limestones from the himalaya the overall variation is 4 13x and 2 14x in n 26 mg and n 25 mg respectively this is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of mg is not unique each speleothem shows a characteristic range of n 26 mg values that are attributed to the isotopic composition of the local water differences between the isotopic composition of mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to mg isotopic fractionation during carbonate precipitation in the temperature range of 4 18 c the low mg calcite is enriched in light isotopes by 1 35x amu and the dependence on temperature has been found to be less than 0 02x amu c despite various geological settings the n 26 mg of the studied dolostones is 2 0 1 2x higher than the n 26 mg of the limestones all together these results suggest a strong mineralogical control and a weak temperature effect on the mg isotopic composition of carbonate a 2002 elsevier science b v all rights reserved

transfer of the sr isotopic signature of the himalayas to the bay of bengal the sr isotopic signal of the himalayas is transferred to the sea by riverine water and carbonate particles the isotopic composition of the carbonate fraction of the sediment is used to trace the sedimentary input into the bay of bengal by the ganges brahmaputra river system carbonate with 87 sr 86 sr 0 7100 marks the main areas of recent sediment deposition the submarine delta the shelf canyon swatch of no ground and the connected active channel levee systemof the bengal fan up to a distance of at least 1100 kmfromthe mouth of the rivers in contrast the isotopic signal of the sr dissolved in the river disappears with the first mixing with the seawater sr isotopic composition of tests of surface dwelling foraminifera globigerinoides ruber prove that the average value of the seawater in the northern bay of bengal has not changed during the last 40 000 years increasing 87 sr 86 sr ratios of the total carbonate since the end of the younger dryas correlate with decreasing d 18 o values of the planktonic foraminifera which are interpreted to result fromincreasing fresh water input and supply of detrital carbonate in response to increasing intensity of the sw summer monsoon r 2003 elsevier science ltd all rights reserved

geomorphic characterization and diversity of the fluvial systems of the gangetic plains the extensive gangetic alluvial plains are drained by rivers which differ strongly in terms of hydrological and sediment transport characteristics these differences are manifested in the geomorphic diversity of the plains the western gangetic plains wgp are marked by a degradational topography with incised channels and extensive badland development in some parts while the eastern gangetic plains egp are characterized by shallow aggrading channels with frequent avulsions and extensive flooding we interpret such geomorphic diversity in terms of differences in stream power and sediment supply from the catchment areas the rivers draining the western plains are marked by higher stream power and lower sediment yield that result in degradation in comparison the rivers draining the eastern gangetic plains have lower stream power and higher sediment yield that result in aggradation the variation of stream power a function of channel slope and high sediment yield is attributed to differences in rainfall and rate of uplift in the hinterland it is suggested that such differences have resulted in a marked geomorphic diversity across the plains it is also suggested that such diversity has existed for a fairly long time because of climatic and tectonic variance

the elevation history of the tibetan plateau and its implications for the asian monsoon abstract the determination of the evolving palaeoaltitude of the tibetan plateau since the india eurasia collision underpins our understanding of how orography in central asia affects the intensity of the monsoon and hence global climate change palaeoaltitudes however cannot be measured directly and need to be inferred from proxy observations that are usually model dependent differing tectonic models for the behaviour of the lithosphere during continental collision have contrasting implications for the elevation of the plateau however two techniques recently employed for determining palaeo elevation are independent of tectonic models the first involving the variation with altitude of oxygen isotopes in precipitation and the second involving the change of leaf morphology with moist static energy of the atmosphere elevation studies have focused on southern tibet largely due to the relative ease of access to the region there is a remarkable unanimity amongst the diverse techniques applied that the altitude of the southern plateau has not significantly changed since at least the mid miocene ca 15 ma arguing for an onset of the monsoon system during or before the early miocene a range of tectonic studies suggest that the northern and eastern parts of the plateau are younger geomorphological features but there are few quantitative constraints of the timing of elevation from these regions of tibet since both the elevation and the surface area of the plateau impact on atmospheric circulation palaeoaltitude studies need to be extended to chart the increasing areas of elevated land surface through time

speckle imaging of titan at 2 microns surface albedo haze optical depth and tropospheric clouds 1996 1998 we present results from 14 nights of observations of titan in 1996 1998 using near infrared centered at 2 1 microns speckle imaging at the 10 meter w m keck telescope the observations have a spatial resolution of 0 06 arcseconds we detect bright clouds on three days in october 1998 with a brightness about 0 5 of the brightness of titan using a 16 stream radiative transfer model disort to model the central equatorial longitude of each image we construct a suite of surface albedo models parameterized by the optical depth of titan s hydrocarbon haze layer from this we conclude that titan s equatorial surface albedo has plausible values in the range of 0 0 20 titan s minimum haze optical depth cannot be constrained from this modeling but an upper limit of 0 3 at this wavelength range is found more accurate determination of titan s surface albedo and haze optical depth especially at higher latitudes will require a model that fully considers the 3 dimensional nature of titan s atmosphere

the significance of himalayan rivers for silicate weathering rates evidence from the bhote kosi tributary the significance of weathering by himalayan runoff for both the sr isotope marine record and the removal of atmospheric co2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the bhote kosi a tributary of the ganges that rises in tibet from tethyan sediment bedrock and traverses the major himalayan lithologies of eastern nepal before debouching onto the gangetic plains throughout the section the cation geochemistry of water samples is dominated by ca and mg ions suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the lesser himalayas as the river transects the metasedimentary and granitic lithologies of the high himalayas the sr isotope ratio of the bedload rises rapidly closely reflecting the isotope geochemistry of the bedrock in contrast the 87sr 86sr ratio of the dissolved load remains roughly constant 0 719 0 723 downstream of the main central thrust where the river transects the carbonate bearing lithologies of the lesser himalayas the 87sr 86sr ratio of the dissolved load rises sharply 0 768 the relative contributions of silicate and carbonate weathering from each of the main himalayan units has been estimated from major cation sr concentration and sr isotope mass balance equations these calculations suggest that the high sr and high 87sr 86sr characteristics of riverine analyses arise initially from a component dissolved from the tibetan sedimentary series which is substantially enhanced by input weathering fluxes particularly as the river traverses the lesser himalayas whilst mechanical erosion is maximised within the high himalayan crystalline series as confirmed by 143nd 144nd ratios from the bedload at least 63 of the dissolved load is acquired by chemical weathering of bedrock lithologies and or of transported particulates within the lesser himalayas enhanced by higher ambient temperatures and slower discharge rates this may involve continued dissolution of the high himalayan crystalline series particulates in addition to lesser himalayan lithologies although himalayan rivers collectively have a major influence on the sr isotope marine record the high 87sr 86sr ratios of their dissolved load results from the mixing of a small component 90 of carbonate derived material some of which is characterised by a high 87sr 86sr ratio up to 0 8 elevated 87sr 86sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates

late cenozoic environmental changes and uplift on the northern side of the central himalaya a reappraisal from field data abstract the extreme view that uplift of the tibetan plateau and the adjacent himalayan range occurred mainly during the quaternary cannot be further sustained as discussed in the light of available field data palaeobiogeography sedimentary environments and tectonic evidence comparison with other independent sources such as the loess records of central china and data on the post orogenic sedimentation and geochronology of the himalayan and south asian oceanic fans shows that the tibetan plateau certainly reached an elevation close to its present one before the climatic changes of the quaternary as such this elevated himalaya tibet ensemble was probably one of the major causes of the late tertiary general cooling of the northern hemisphere as suggested by global modelling of climate palaeoenvironmental changes observed during the quaternary were certainly governed by orbital parameters yet they were also regionally influenced by both the uplift of the greater himalaya with respect to tibet and by the disruption of the northern himalaya and south tibet into horsts and grabens uncertainty still surrounds the periods during which critical elevations were achieved to induce both the monsoon circulation and e w extension

a palaeoenvironmental investigation of sediments from the prehistoric mine of copa hill cwmystwyth mid wales this paper investigates the use of mine sediments to reconstruct the vegetational changes and atmospheric pollution history associated with prehistoric and mediaeval metal mining in the ystwyth valley dyfed mid wales pollen charcoal plant macrofossils fossil insects and chemical analyses are presented from radiocarbon dated sediments contained within a prehistoric mine situated on the upper slope of copa hill close to the village of cwmystwyth the results provide additional support to the hypothesis that prehistoric mining had a negligible impact on woodland and that deforestation took place after bronze age mining ceased although high concentrations of cu pb and zn were determined from sediments of prehistoric and roman age the patterns bear little resemblance to off site atmospheric pollution records and to the archaeological evidence for metal mining interpreting geochemical data from mine contexts is problematic as numerous factors influence the distribution and concentration of metals however an on site and off site approach to investigate human environment interactions caused by metal mining is advocated

modeling the early paleozoic long term climatic trend the early paleozoic climate has been described as warm and equable however recent data based on conodont oxygen isotopic composition reveal a large long cooling trend through the ordovician followed by an abrupt cooling during the late ordovician glaciation this long term climate change is associated with a major radiation in the earth life history nonetheless the driving mechanisms for this cooling trend remain unknown carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change however the impact of the plate motion context has not been explored yet although it might have a major impact on atmospheric co 2 levels simulations with a climate model coupled to a biogeochemical model geoclim show that the atmospheric co 2 decreased from more than 20 pal 5600 ppmv in the furongian down to approximately 10 pal 2800 ppmv in the llandovery before rising again in the early devonian we suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large co 2 drawdown that led to the onset of cooler to glacial conditions from the middle ordovician to the llandovery the weathering of fresh volcanic rocks is itself responsible for 33 of the late ordovician atmospheric co 2 decrease the rest being related to the continent motion through the intertropical convergence zone itcz mean annual continental temperature falls by 3 c in the early ordovician reaching 13 5 c during the glacial interval and rises to 16 c in the early devonian

seasonal variations in dissolved neodymium isotope composition in the bay of bengal abstract constraining the dissolved neodymium nd cycle in the ocean is paramount for using nd isotopic composition e nd as a tracer to reconstruct deep sea paleocirculations or continental weathering on different time scales dissolved end has been measured in seawater samples from six hydrological stations collected along 89 e north south transect in the bay of bengal bob in order to assess the impact of seasonal freshwater and sediment discharges from the continental river systems seawater samples collected in this study during june 2012 reveal more radiogenic end a difference of 2 epsilon units for the upper 2000 m and 0 5 epsilon unit below 2000 m and 3 8 pmol kg lower nd concentrations than the reported values of nearby seawater samples collected in november 2008 these observations are most plausibly explained by a seasonal variations in dissolved nd concentrations and end in the bob induced by seasonal variations in the freshwater and sediment discharges from the ganges brahmaputra g b river system however we cannot entirely exclude the possibility of spatial differences given that the water stations collected in this study are not exactly the same positions collected in november 2008 a two box model suggests 1 the particulate nd inputs from the g b river system mainly control the seasonal shift of end observed in the bob seawater and 2 a very rapid nd exchange exists between lithogenic particles and seawater at least on the scale of a few months seasonal changes in seawater end may also occur in other marginal seas and in the outflows of major rivers and these need to be taken into account when using the end proxy in the ocean

lithium isotope behaviour during weathering in the ganges alluvial plain the ganges river system is responsible for the transportation of a large flux of dissolved materials derived from himalayan weathering to the oceans silicate weathering driven cooling resulting from uplift of the himalayas has been proposed to be a key player in cenozoic climate variation this study has analysed li isotope 7li ratios from over 50 ganges river waters and sediments in order to trace silicate weathering processes sediments have 7li of 0 identical to bulk continental crust however suspended sediment depth profiles do not display variations associated with grain size that have been observed in other large river systems dissolved 7li are low 11 in the ganges headwaters but reach a constant value of 21 1 6 within a relatively short distance downstream which is then maintained for almost 2000 km to the ganges mouth given that li isotopes are controlled by the ratio of primary mineral dissolution to secondary mineral formation this suggests that the ganges floodplain is at steady state in terms of these processes for most of its length low 7li in the mountainous regions suggest silicate weathering is therefore at its most congruent where uplift and fresh silicate exposure rates are high however there is no correlation between 7li and the silicate weathering rate in these rivers suggesting that li isotopes cannot be used as a weathering rate tracer although they do inform on weathering congruency and intensity the close to constant 7li values for the final 2000 km of ganges flow also suggest that once the size of the alluvial plain reached more than 500 km the flow distance after which riverine 7li stops varying the ganges exerted little influence on the changing cenozoic seawater 7li because riverine 7li attained a near steady state composition

the ca isotopic composition of dust producing regions measurements of surface sediments in the black rock desert nevada dust is a relatively unconstrained flux in the geochemical cycle of ca the isotopic composition of dust derived ca has not been studied though it is an important part of any attempt at deciphering ca isotope based proxy records accordingly this study reports the elemental and calcium isotope geochemistry of 30 surface upper 0 5 cm sediments from an arid dust producer the black rock desert in northwestern nevada geochemical data for sequential water and 0 5 n hcl leaches meant to sample mobile ca and selected leached residues are presented along with x ray diffraction xrd determinations of major mineralogy bulk playa sediments have ca concentrations between 0 28 and 40 wt median 6 8 wt and calcite concentrations of 2 32 isotopically ca sampled by water leaches 60 of total ca though the degree of fractionation w a varies between 0 and 0 6 acid leaches which are the primary component of mobile ca in the sediments have 44casrm 915a values of 0 78 0 08 similar to the 44ca of modern nannofossil ooze and modern rivers this means that dust produced in closed continental basins likely has little isotopic leverage to change the ocean s isotopic composition and suggests that the ca isotopic composition of dust is tied to rivers weathering in addition while the ca concentration data in the water leach suggest that evaporative evolution controls the amount of ca in this reservoir the isotope data are inconsistent with this conclusion instead we hypothesize that adsorption of ca on clays controls the ca isotope systematics in the water leach this hypothesis requires that there is no significant isotopic fractionation during evaporite mineral precipitation and may suggest that sorption at rates appropriate for natural systems might fractionate differently than sorption at laboratory rates

u series disequilibria in minerals from gandak river sediments himalaya abstract the impact of mineralogical sorting on u th ra variations observed for the ganga sediments in the himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the u series isotopic system in himalayan minerals for this purpose u th and ra concentrations along with 238u 234u 230th 226ra radioactive disequilibria were measured in several pure mineral fractions i e monazite zircon titanite ilmenite rutile garnet magnetite quartz biotite and muscovite separated from one bank sediment of the gandak river ganga basin the data obtained through a double digestion protocol in high pressure and high temperature vessels confirm that u and th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in u and th namely monazite titanite and epidote as well as major minerals with lower u and th concentrations such as quartz more importantly the data clearly indicate that similarly to whole rock samples the u th ra systematics of individual minerals have likely recorded complex u th ra fractionations in particular the 226ra enrichment observed in the biotite and clay fractions likely results from the affinity of ra for phyllosilicate minerals while the 230th and to a lesser extent the 234u enrichment observed for the other minerals might result from a nuclide adsorption process on the fe oxide coatings present on the grains or directly on the mineral surface the linear trend outlined by the minerals phases in the 226ra 230th vs 230th 234u diagram suggests that 230th is probably mobilized from zircons the only fraction presenting 230th 234u

reconciling the elemental and sr isotope composition of himalayan weathering fluxes insights from the carbonate geochemistry of stream waters determining the relative proportions of silicate vs carbonate weathering in the himalaya is important for understanding atmospheric co2 consumption rates and the temporal evolution of seawater sr however recent studies have shown that major element mass balance equations attribute less co2 consumption to silicate weathering than methods utilizing ca sr and 87sr 86sr mixing equations to investigate this problem we compiled literature data providing elemental and 87sr 86sr analyses for stream waters and bedrock from tributary watersheds throughout the himalaya mountains in addition carbonate system parameters pco2 mineral saturation states were evaluated for a selected suite of stream waters the apparent discrepancy between the dominant weathering source of dissolved major elements vs sr can be reconciled in terms of carbonate mineral equilibria himalayan streams are predominantly ca2 mg2 hco3 waters derived from calcite and dolomite dissolution and mass balance calculations demonstrate that carbonate weathering contributes 87 and 76 of the dissolved ca2 and sr2 respectively however calculated ca sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and pco2 gradients in the himalaya the state of calcite and dolomite saturation was evaluated across these gradients and the data show that upon descending through the himalaya 50 of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas co2 stream water ca mg and ca sr ratios decrease as the degree of supersaturation with respect to calcite increases and mg2 ca2 and hco3 mass balances support interpretations of preferential ca2 removal by calcite precipitation on the basis of patterns of saturation state and pco2 changes calcite precipitation was estimated to remove up to 70 of the ca2 originally derived from carbonate weathering accounting for the nonconservative behavior of ca2 during riverine transport brings the ca sr and 87sr 86sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock thereby permitting consistency between elemental and sr isotope approaches to partitioning stream water solute sources these results resolve the dissolved sr2 budget and suggest that the conventional application of two component ca sr and 87sr 86sr mixing equations has overestimated silicate derived sr2 and hco3 fluxes from the himalaya in addition these findings demonstrate that integrating stream water carbonate mineral equilibria divalent cation compositional trends and sr isotope inventories provides a powerful approach for examining weathering fluxes