evolution of the indian summer monsoon and terrestrial vegetation in the bengal region during the past 18 ka the indian summer monsoon ism is a major global climatic phenomenon long term precipitation proxy records of the ism however are often fragmented and discontinuous impeding an estimation of the magnitude of precipitation variability from the last glacial to the present to improve our understanding of past ism variability we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower ganges brahmaputra meghna catchment and the indo burman ranges over the last 18 000 years 18 ka the records derive from a marine sediment core from the northern bay of bengal nbob and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the bengal region the isotopic composition of terrestrial plant waxes dd and d13c of n alkanes are compared to results from an isotope enabled general atmospheric circulation model isocam for selected time slices pre industrial mid holocene and heinrich stadial 1 comparison of proxy and model results indicate that past changes in the dd of precipitation and plant waxes were mainly driven by the amount effect and strongly influenced by ism rainfall maximum precipitation is detected for the early holocene climatic optimum ehco 10 5 6 ka bp whereas minimum precipitation occurred during the heinrich stadial 1 hs1 16 9 15 4 ka bp the isocam model results support the hypothesis of a constant moisture source i e the nbob throughout the study period relative to the pre industrial period the model reconstructions show 20 more rain during the mid holocene 6 ka bp and 20 less rain during the heinrich stadial 1 hs1 respectively a shift from c4 plant dominated ecosystems during the glacial to subsequent c3 c4 mixed ones during the interglacial took place vegetation changes were predominantly driven by precipitation variability as evidenced by the significant correlation between the dd and d13c alkane records when compared to other records across the ism domain precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ism from the last glacial to the present

records of the nd isotope composition of seawater from the bay of bengal implications for the impact of northern hemisphere cooling on itcz movement abstract this study presents a record of planktonic foraminiferal neodymium isotopic gradients along a north south transect in the bay of bengal during time slices of late holocene and last glacial maximum lgm age together with a record of planktonic foraminiferal variation in the northern bay of bengal 15 n over the last 195 ky in late holocene core top planktonic foraminifera the north south end gradient rises from nonradiogenic values of 12 at 20 n to 10 at 5 n in parallel with the modern surface salinity gradient controlled by discharge of himalayan rivers in the northern bay during the lgm end increased throughout the bay the contrast between northernmost and southernmost sites decreased and maximum end values of 6 5 occurred between 12 and 15 n a small part of the shift to higher mean end throughout the bay during the glacial may arise from a uniform increase in deposition of far field dust from arabian and persian gulf regions however the spatial pattern of end variation between lgm and late holocene also suggests a shift from modern dominance of nonradiogenic nd sources from the ganges brahmaputra basin to lgm dominance of more radiogenic nd sources from arakan coastal rivers over the last 195 ky at 15 n the most radiogenic end values of 7 occur at glacial maxima and the most nonradiogenic values of 11 occur during interglacials end values are highly correlated with glacial interglacial variations in planktonic foraminiferal 18o in this record shifts in river sources from the more northerly ganges brahmaputra watershed to the more southerly arakan coastal river systems respond dominantly to inter tropical convergence zone itcz movement driven by northern hemisphere cooling during 100 ky glacial interglacial cycles with a small component of variation on precessional timescales the nonlinear correlation of end with ice volume suggests that itcz movement responds to aerial coverage of ice sheets and snow rather than to ice thickness and volume these data add support to recent general circulation models of itcz response to northern hemisphere ice sheets which simulate decreased glacial precipitation in the ganges brahmaputra basin and increased glacial precipitation in the southern arakan coastal basin

a thermoconvective interpretation of heat flow data in the area of ocean drilling program leg 116 in a distal part of the bengal fan we present the results of a numerical simulation study of the thermo convective circulation in a sedimentary pile in the distal part of the bengal fan the occurrence of faults and the variability of surface heat flow data led previous workers to suggest hydrothermal circulation in this area according to the purpose of ocean drilling program survey leg 116 data were collected which are adequate to constrain a model for the comprehension and the quantification of the relationship between ocean basin heat flow and pore fluid circulation in this paper the sedimentary pile is modeled as an anisotropic and inclined porous layer interrupted by faults with a periodic distribution an important observation the model tries to reproduce is the shift of approximately 2 3 km between the surface expression of the heat flow maximum and the fault zone near site 718 the influence of permeability anisotropy and of permeability within those fault zones is analyzed when the maximum vertical permeability in the fault zones is 5 times greater than the vertical permeability of the porous block the circulation consists of longitudinal rolls i e rolls with their axis oriented downslope with more vigorous convection in fault zones when the maximum permeability of the fault zone is 10 or more times greater than the vertical permeability of the porous layer convection is primarily restricted to the fault plane in all cases maximum heat flow values are located directly above faults this paper shows that the best agreement between those data suggests a 1 m yr 1 forced convection and a permeability of the sediments of 0 5 darcy 5 10 13 m2 and 5 mdarcy respectively in the horizontal and vertical derictions

the record of sea water 187os 186os variation through the cenozoic the variation of sea water 187os 186os during the cenozoic has been determined by analyzing the hydrogenous os released by acidic hydrogen peroxide leaching of layers in an abyssal north pacific pelagic clay core overall sea water 187os 186os has increased from 3 2 at 58 ma to the present day value of 8 6 this pattern is similar to the well known 87sr 86sr evolution of sea water the increase in the proportion of continental os 187os 186os 10 could be due to enhanced weathering rates of black shales associated with the uplift of the himalayas other elements associated with black shale weathering such as u and p should show effects in the sedimentary record

the submarine delta of the ganges brahmaputra cyclone dominated sedimentation patterns the ganges brahmaputra belongs to the world s largest rivers in terms of sediment discharge but little is known yet about the processes controlling sediment distribution on the shelf and the documentation of these processes in large and small scale sedimentary structures for this study the submarine delta of the river system was investigated by high resolution seismic reflection profiles and piston and gravity coring logged physical properties of the cores were combined with visual core description grain size analysis and radiography sedimentation rates were estimated by 210pb and 137cs gamma spectrometry and cross checked with evaluation of seismic profiles the data reveal that the broad delta topset 20 m waterdepth is formed by a thick layer of sand silt highest sedimentation rates about 10 cm a 1 occur at the centre of clinoform delta foreset beds where about 20 of the total riverine sediment load is deposited and thus cause an annual progradation of the subaqueous delta front by about 15 m the foreset beds consist of graded sand silt layers which are thought to be deposited by sediment laden flows generated during the surge of tropical cyclones tempestites interbedded with silty clay layers the head of a canyon the swatch of no ground that deeply incises into the shelf forms a sediment trap along the westward cyclonic transport path where sedimentation rate increases to about 50 cm a 1 episodically earthquakes or storms destabilize especially the rapidly accumulated foreset beds in the eastern delta creating up to 8 m thick mass flows on the outer shelf in water depth beyond 80 m no holocene sediment is deposited

nd isotopic mapping of the sri lanka basement update and additional constraints from sr isotopes abstract the high grade crystalline basement of sri lanka can be subdivided into three provinces distinguished by distinct isotopic model ages this confirms the conclusions of milisenda et al 1988 made on the basis of much more limited data high grade ortho and paragneisses from the highland complex in central sri lanka yield palaeoproterozoic to archaean nd model ages from 2 0 to 3 4 ga and reflect an extended premetamorphic crustal history the wanni complex in the western and northwestern part of the island is characterized by meso to neoproterozoic nd model ages ranging from 1 0 to 2 0 ga the vijayan complex in the southeast is occupied by neoproterozoic i type granitoids with nd model ages from 1 0 to 1 8 ga this distinct crustal province appears to fit best a detached continental margin plutonic terrane the isotopic data preclude a common premetamorphic history of the wanni and vijayan complexes with the ancient highland complex the combined nd sr and pb isotope data together with recent geochronological studies strongly suggest that the distinct crustal age provinces are separated by tectonic discontinuities and that they represent terranes that were accreted just prior to the high grade metamorphic event in neoproterozoic time

constraining past seawater δ18o and temperature records developed from foraminiferal geochemistry paired measurements of magnesium to calcium ratios mg ca and the stable oxygen isotopic composition 18o in foraminifera have significantly advanced our knowledge of the climate system by providing information on past temperature and seawater 18o 18osw a proxy for salinity and ice volume however multiple sources of uncertainty exist in transferring these downcore geochemical data into quantitative paleoclimate reconstructions here we develop a computational toolkit entitled paleo seawater uncertainty solver psu solver that performs bootstrap monte carlo simulations to constrain these various sources of uncertainty psu solver calculates temperature and 18osw and their respective confidence intervals using an iterative approach with user defined errors calibrations and sea level curves our probabilistic approach yields reduced uncertainty constraints compared to theoretical considerations and commonly used propagation exercises we demonstrate the applicability of psu solver for published records covering three timescales the late holocene the last deglaciation and the last glacial period we show that the influence of salinity on mg ca can considerably alter the structure and amplitude of change in the resulting reconstruction and can impact the interpretation of paleoceanographic time series we also highlight the sensitivity of the records to various inputs of sea level curves transfer functions and uncertainty constraints psu solver offers an expeditious yet rigorous approach to test the robustness of past climate variability inferred from paired mg ca 18o measurements

structure and geodynamics of the post collision zone between the nazca antarctic spreading center and south america the chile triple junction ctj is the place where the chile ridge nazca antarctic spreading center is subducting beneath the continental south american plate sediment accretion is active to the south of the ctj in the area where the northward migrating chile ridge has collided with the continent since 14 ma at the ctj tectonic erosion of the overriding plate narrows and steepens the continental slope we present here a detailed tomographic image of the upper lithospheric antarctic south america subduction zone where the chile ridge collided with the continent 3 6 ma off golfo de penas results reveal that a large portion of trench sediment has been scraped off and frontally accreted to the forearc forming a 70 80 km wide accretionary prism the velocity depth model shows a discontinuity at 30 40 km landward of the deformation front which is interpreted as the contact between the frontal poorly consolidated sedimentary unit and middle more compacted sedimentary unit accretionary prism the formation of this discontinuity could be related to a short term episode of reduced trench sedimentation in addition we model the shape of the continental slope using a newtonian fluid rheology to study the convergence rate at which the accretionary prism was formed results are consistent with an accretionary prism formed after the collision of the chile ridge under slow convergence rate similar to those observed at present between antarctic and south america 2 0 cm a based on the kinematics of the chile ridge subduction during the last 13 ma we propose that the accretionary prism off golfo de penas was formed recently 5 ma after the collision of the chile ridge with south america

erosional history of the himalayan and burman ranges during the last two glacial interglacial cycles abstract the results of a clay mineralogy study combined with major element geochemistry strontium neodymium and oxygen isotopes and 14c ams stratigraphy are reported for deep sea gravity cores located in the bay of bengal md77 180 and the andaman sea md77 169 end 0 and 87sr 86sr from holocene and last glacial maximum lgm sediments of fifteen other cores have also been investigated to identify sediment sources and to estimate oceanic sedimentary transport the data show the contribution of three sources 1 ganges brahmaputra rivers 2 irrawaddy river and 3 sediment derived from the western part of the indo burman ranges the dispersion of the detrital material issuing from these sources has been constrained by the geographic distribution of nd and sr isotopic ratios the lgm sediments are characterized by a significant increase of 87sr 86sr small changes in end 0 and a general decrease of smectite illite chlorite which together imply a decrease in weathering intensity the increased 87sr 86sr are attributed to a decrease in chemical weathering which should release preferentially 87sr during weathering processes in the andaman sea sediments smectite illite chlorite and kaolinite quartz ratios combined with a chemical index of alteration cia indicate that the weathering intensity of the irrawaddy river basin is mainly controlled by the summer monsoon rainfall intensity the wet summer monsoons increase vegetation cover in the plains and favor soil development by the production of pedogenic clays smectite and kaolinite

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

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

salinity bias on the foraminifera mg ca thermometry correction procedure and implications for past ocean hydrographic reconstructions 1 mg ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures here we show however that the mg ca temperature relationship of the planktonic species globigerinoides ruber is significantly affected by seawater salinity with a 1 psu change in salinity resulting in a 1 6 c bias in mg ca temperature calculations if not accounted for such a bias could lead for instance to systematic overestimations of mg ca temperatures during glacial periods when global ocean salinity had significantly increased compared to today we present here a correction procedure to derive unbiased sea surface temperatures sst and 18osw from g ruber tmg ca and 18of measurements this correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the last glacial maximum lgm in the western pacific warm pool while uncorrected tmg ca data indicate a 3 c warming of the western pacific warm pool since the lgm the salinity corrected sst result in a stronger warming of 4 c

himalayan sedimentary pulses recorded by silicate detritus within a ferromanganese crust from the central indian ocean a central indian ocean deep water seamount hydrogenous ferromanganese crust ss663 crust contains variable 7 23 amounts of detrital material silicate detritus taking into account the growth rate of the authigenic component the accumulation rate of the silicate detritus has been calculated which shows an overall decrease during the past 25 7 2 ma growth history of the specimen this silicate detritus displays ond 0 between 37 7 and 312 7 and 87 sr 86 sr between 0 7083 and 0 7215 the sr nd isotopic compositions record the variation in the mixture of two detrital end members throughout the growth history of the ss663 crust while one end member clearly appears to be himalayan derived material the other component is less well defined but could be related to the volcanogenic component from the indonesian back arc basin we have unscrambled the proportion of these two components in the silicate detritus of the ss663 crust to quantify the variation in the accumulation rate of the himalayan derived silicate detritus during the last 25 ma the result shows three periods of high accumulation rates modern around 10 ma and prior to 16 ma these sedimentary pulses have already been recognised and described in the turbidites from the distal bengal fan which is located more than 1000 km away from the ss663 crust location suggesting that these sedimentary pulses are of regional significance rather than of local importance the timings of elevated contents of the himalayan derived silicate detritus in the specimen during the neogene closely superimpose the periods of the himalayan uplift along the main central thrust the main boundary thrust and the main frontal thrust this further emphasises the tectonic control of the sediment production by the erosion of an active orogenic range and its distribution over a vast area additionally close superimposition of the early miocene detritus pulse in the ss663crust on the riverine sr isotopic positive excursion suggests that the physical and chemical weathering of the himalaya was strongly coupled during the early miocene while the spatial decoupling appears to have taken place only around 12 14 ma 2002 elsevier science b v all rights reserved

vertical variation of oxygen isotope in bay of bengal and its relationships with water masses 1 we present here for the first time comprehensive depth specific near surface to 1800 m oxygen isotope 18o data set of 175 water samples collected from 22 different locations in the bay of bengal bob during july august 2009 and december january 2012 2013 the study is aimed to constrain the water mass parameters using oxygen isotope and salinity four water masses are identified on the basis of temperature salinity relationship a bay of bengal water bobw 0 50 m b mixed zone mz 60 120 m c indonesian throughflow itf 200 500 m and d indian ocean deep water idw 600 1800 m our study shows 1 18o salinity correlation slope 0 15 0 01 intercept 4 58 0 24 r2 0 89 n 54 for the bobw is found to be consistent with surface bob water reported earlier 2 relatively lower correlation r2 0 24 n 47 in underlying mixed zone suggesting mixing of various water masses 3 significant linear correlation between 18o and salinity in idw r2 0 70 indicating depleted less saline water from southern hemisphere spread up to 18 n and 4 vertical profiles at various sampling locations showing a continuous enrichment of 18o 0 6 0 8 within top 60 100 m irrespective of their proximity to coastline in conjunction with a sharp decrease in 14c data below the same depth zone 60 100 m as reported earlier we suggest the 18o value reaches to its acme at a zone of mixing between younger bobw and relatively older itf this preliminary study indicates oxygen isotope and salinity together can be used to identify different water masses in the bob

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

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

the os isotopic composition of himalayan river bedloads and bedrocks importance of black shales re os data are presented for bedrocks and bedloads of selected rivers from the himalayan range in central nepal and bangladesh these data demonstrate that the os isotopic composition of most of the bedrocks is similar to that of the average continental crust notable exceptions to this are the black shales from the lesser himalaya lh which contain large quantities of radiogenic osmium in the black shales the os is not concentrated in the sulfides and may instead be associated with organic matter bedloads collected from rivers which include lh black shales in their catchment basins have notably high os ratios 187os 188os 3 this radiogenic signature persists into the ganga river in contrast the brahmaputra river is less radiogenic suggesting an ophiolite input in the headwater region simple physical mixtures of lh black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high os ratios observed at the outflow of the range assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture respectively this observation suggests that erosion may not be occurring at steady state in addition mass balance calculations indicate that about 20 of the os and 60 of the re are carried in dissolved form implying that chemical erosion plays an important role in the osmium budget finally the contrast in os isotope ratio between ganga bedloads and bulk sediments in the bengal fan suggests that radiogenic os carried by particulate matter and later remobilized may significantly influence the os composition of seawater

climatic control of sediment transport from the himalayas to the proximal ne bengal fan during the last glacial interglacial cycle clay mineralogy siliciclastic grain size major elements 87sr 86sr and end analyses of deep sea sediments cored in the north eastern bay of bengal are used to reconstruct evolution of detrital sources and sediment transport to the proximal part of the bengal deep sea fan during the last climatic cycle end values 13 3 to 9 7 and 87sr 86sr ratios 0 721 0 733 indicate a mixture of sediments originating from the ganges brahmaputra rivers and the indo burman ranges interglacial marine isotopic stages mis 5 and 1 are associated with a higher contribution of sediments from the ganges brahmaputra river system than is the case for glacial mis 6 4 3 and 2 siliciclasitic grain size combined with si al and si fe ratios indicate coarser glacial sediments with numerous turbidite layers glacial turbidite layers display similar clay mineralogical compositions to hemipelagic sediments only few of turbidite layers mis 6 4 and 2 are slightly unradiogenic end 13 3 suggesting a higher contribution of ganges brahmaputra river sediments independently of changes in the sedimentary sources the smectite illite chlorite ratio of cores located on the ne bengal fan indicates higher inputs of primary minerals illite and chlorite from the highlands of the river basins relief during glacial mis 6 4 3 and 2 and an increased contribution of pedogenic minerals smectite and kaolinite during interglacial mis 5 and 1 maximum smectite illite chlorite ratios during the warm sub stages of mis 5 suggest an intensification of summer monsoon rainfall associated with higher rates of physical erosion of the indo gangetic flood plain and or dominant summer hydrological conditions transporting a higher proportion of sediments deriving from the ganges brahmaputra rivers to the ne bengal fan in addition a higher production of smectite in soils of the indo gangetic flood plain during periods of intensification of monsoon rainfall cannot be excluded

impure marbles of the lesser himalaya another source of continental radiogenic osmium abstract the marked rise of the 187 os 188 os ratio of seawater over the cenozoic era is generally attributed to increased continental input and particularly to increased erosion of black shales thus os isotopes are thought to trace the weathering of organic carbon here we present evidence that impure marbles lacking organic carbon provide an important source of radiogenic os to the bedload of himalayan rivers these marbles provide a critical component necessary for balancing the os isotopic budget of the bedloads the analyzed marbles have 187 os 188 os ratios as high as 194 and often contain as much 187 os as typical black shales most of this radiogenic os is not contained in the carbonate itself but rather in associated phases these phases are often of hydrothermal origin and their os may ultimately be provided by fluids circulating through black shales while these results complicate the relationship between the os isotopic ratio of seawater and erosion of organic rich rocks they also provide new information necessary for understanding the marine os isotopic record

provenance of jurassic tethyan sediments in the hp uhp zermatt saas ophiolite western alps rubidium sr and sm nd isotope data and rare earth element ree concentrations of the metasedimentary rocks within the zermatt saas zs ophiolite complex of the western alps are used to investigate element mobility and to determine the provenance of the metasediments in order to place constraints on the precollisional paleogeography of the piemont ligurian portion of the neo tethys ocean present day 8 7 sr 8 6 sr variations for the zs metasediments scatter about an early tertiary alpine metamorphic age whereas local nappes that have been interpreted to reflect african apulian and european basements scatter about a variscan like age this suggests 8 7 sr 8 6 sr isotope systematics were nearly completely homogenized for most of the zs metasediments during early tertiary metamorphism probably because they were relatively wet prior to metamorphism in contrast to the sr isotope data ree data and nd isotope compositions of the zs metasediments overlap those of average upper continental crust average shale and the local nappes and nd model ages of the zs metasediments overlap with those of variscan age rocks these relations suggest that the rees of the zs metasediments were not disturbed during high to ultra high pressure alpine metamorphism based on ree data nd isotope compositions and mixing models the zs metasediments comprise two groups that require distinct source terranes one group group i seems to be mixing of an old crustal component such as the paragneissic basement nappe samples with metasediments similar to the second group group ii the source material for group ii is dominated by homogenization of the variscan like orthogneissic basement nappe samples the provenance of group i samples is interpreted to be local where source nappes must have been proximal to the piemont ligurian basin prior to alpine convergence the similarity in dispersion of nd isotope compositions of the metasediments and likely source terranes suggests that the metasediments reflect deposition in small isolated basins early in the formation of the piemont ligurian ocean

sr nd isotopic evidence of terrigenous flux variations in the bay of bengal implications of monsoons during the last 34 000 years 1 sr and nd isotopic compositions of a sediment core from the bay of bengal are used to constrain the provenance and transport pathways of their terrigenous component during the 3 to 34 14c kyr bp the results clearly demonstrate a distinctly different source for detrital fluxes on the ninetyeast ridge from that of distal and western fan sediments there is a systematic pattern of climate related variability in 87sr 86sr and nd o values at the studied location pulses of low 87sr 86sr higher radiogenic nd and high silt content occurring at 5 7 kyr intervals suggest significant changes in the detrital source increased supply of irrawaddy derived sediments appears to be the dominant source for these pulses our results support the hypothesis that rapid cold events of north atlantic heinrich events are characterized by the intensified northeast monsoon

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

c4 plants decline in the himalayan basin since the last glacial maximum abstract the bengal fan turbiditic system supplied by the ganga brahmaputra river system provides an integrated record of himalayan system erosion thanks to minor inputs of marine organic matter and almost complete preservation of riverine organic carbon organic matter buried in bengal fan sediments is a proxy of himalayan basin paleo vegetation the active channel levee system of the middle fan documents the last 19 ka and allows the reconstruction of vegetation change in the himalayan basin since the last glacial maximum lgm we measured 13 c of organic carbon c org in order to track changes in the proportions of c3 and c4 plants in the himalayan basin from lgm to mid holocene 13 c of bulk c org shifts of 3 4 towards more negative values relative abundance of individual n alkanes reveals that terrestrial higher plant inputs represent a dominant fraction of c org buried in bengal fan sediments 13 c of higher plants biomarkers mimic that of bulk c org showing that the later can be used as continental paleo vegetation proxy 13 c negative shift from lgm to mid holocene mostly indicates the transition from a dominant input of c4 plants to a dominant input of c3 plants and therefore reveals that c4 plants were more abundant in the basin under glacial conditions vegetation repartition in the basin simulated using the caraib dynamic vegetation model is consistent with c org data the model indicates a dominance of c4 plants in the gangetic plain during the lgm while eastern part of the basin remains dominated by c3 plants the comparison between our data and proxies of regional paleo climate suggests that the large decline of c4 plants after the lgm was due to combined increase of atmospheric co 2 and humidity levels integrated record of himalayan basin paleo vegetation suggests more arid conditions during the lgm than during the mid holocene and agrees with reconstructions of the monsoon indicating stronger sw monsoon during interglacial and stronger ne monsoon during glacial periods

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

major and trace element geochemistry of bay of bengal sediments implications to provenances and their controlling factors major and trace elements of sediments from a 13 m long piston core sk187 pc33 from the western bay of bengal have been investigated in this study to infer about the changes in provenances and related controlling factors during the last glacial interglacial period factor analysis of these geochemical dataset ascertains dominant role of riverine supply of sediments in regulating the geochemistry of sk187 pc33 sediments the al normalized major k and ti and trace elemental cu and cr ratios of these marine sediments fall within the ranges observed for their major provenances viz ganga brahmaputra and the godavari krishna gk rivers and depth profiles of these ratios showed significant variations with synchronous excursion at around the last glacial maxima lgm implying a change in their provenances with relatively reduced himalayan contribution during this climatic event relatively higher cia and ti ca ratios with lower k al ratios of these sediments during the holocene hint intense chemical weathering in the river basins in response to strengthened monsoon during this period inverse model calculation of al normalized elemental ratios of the sediments estimated an average sediment contribution of 66 13 and 34 13 from the himalayan and the peninsular indian rivers to the core site respectively consistent with the depth profiles of elemental ratios the estimated sediment contributions from the himalayan rivers are observed to decrease by 20 from the himalayan rivers particularly that of the ganga to the studied location during the lgm lowering of sediment supply from the himalaya ganga during the lgm is due to weakening of south west monsoon and reduction in available exposure area for weathering due to extent of glacier cover outcomes of this study underscore the strong linkage between erosion and climate

exploring the temporal change in provenance encoded in the late quaternary deposits of the ganga plain abstract temporal analysis of sr isotopes in soil carbonates and sr and nd isotopes in silicate fractions has been carried out in a sedimentary core kalpi core 50 m long raised from the southern bank of the yamuna river ganga plain india the aim of the study is to constrain sediment provenance through comparison with the modern himalayan and peninsular river systems water and bank sediments sr isotopic data in soil carbonates 0 71874 to 0 71410 and sr nd isotopic data in silicate 0 72865 to 0 74544 and 13 9 to 17 2 respectively vary significantly with depth and are indicative of both himalayan and peninsular sources for sediments in the southern ganga plain the positive correlation between 87sr 86sr ratio and 1 sr in soil carbonate and the negative correlation between 87sr 86sr and end in silicate confirm mixing of sediments from these sources variations of 87sr 86sr ratios in soil carbonates show that at 80 and 45 ka the himalaya acted as the major source of sediments in the southern part of the ganga plain the gradual decrease in 87sr 86sr ratios after 80 and 45 ka indicates change in source to peninsular india which is also supported by limited sr and nd isotope data in silicates the change in sediment provenance corresponds well with the available climatic record and is suggestive of strong climatic control in sediment supply with high supply from the himalaya during the interglacial period and peninsular sediments during glacial period

the isotopic composition of leachable osmium from river sediments the 187os186os ratios of acidic hydrogen peroxide leaches of river sediments are inferred to be representative of the isotopic composition of os delivered to sea water from the continents the 187os186os values for river sediment leaches from the eastern and central u s the ganges river and the rio maipo range from 10 1 to 21 5 these results suggest that the 187os186os of the soluble river load delivered from the major continental areas to the oceans is on average more radiogenic than average currently eroding continental crust 10 5 sediment leaches from three rivers from the western u s that drain ophiolitic peridotites have distinctly lower 187os186os values 1 4 7 1 such rivers may be significant contributors to sea water of the relatively unradiogenic osmium required to yield the contemporary oceanic 187os186os of 8 6

a palaeo tibet myanmar connection reconstructing the late eocene drainage system of central myanmar using a multi proxy approach strain resulting from the collision of india with asia has caused fundamental changes to asian drainage patterns but the timing and nature of these changes are poorly understood one frequently proposed hypothesis involves the connection of the palaeo tsangpo drainage to a precursor to the irrawaddy river of central myanmar in the palaeogene to test this hypothesis we studied the provenance of palaeogene fluvio clastic sedimentary rocks that crop out in central myanmar namely the late middle eocene early oligocene pondaung and yaw formations isotopic analysis on bulk rock and petrographic data indicate a primary magmatic arc source and a secondary source composed of recycled metamorphosed basement material although the exact location of both sources is hardly distinguishable because burmese and tibetan provinces share common lithological features the presence of low grade metamorphic fragments the heterogeneity in sr nd isotopic values of bulk sediments and westward directed palaeoflow orientations indicate a proximal source area located on the eastern asian margin central myanmar was the locus of westward prograding deltas opening into the indian ocean supplied by the unroofing of an andean type cordillera that extended along the burmese margin we found no evidence to support a palaeo tsangpo irrawaddy river at least during the late eocene supplementary material data locations and isotopic and petrographic results are available at www geolsoc org uk sup18655

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 osmium riverine flux and the oceanic mass balance of osmium the osmium concentration os and isotopic composition were determined in a set of 17 of the largest rivers of the world os varies between 4 6 and 52 1 pg kg and the 187os 188os ratio varies between 0 64 and 2 94 measurement of rainwater samples shows that there is no input of oceanic os to the continent through rain assuming a negligible anthropogenic os input in the dissolved load the natural average river water has an os concentration of 7 9 pg kg and a mean 187os 188os ratio of 1 54 the total riverine flux of os to the ocean is estimated to be 295 kg yr the dissolved os flux from island arcs and oceanic islands represents less than 5 of the total riverine flux and is not further considered the continental os flux to the ocean is then represented by the riverine flux as dissolved os from eolian dust and glacial sediments is negligible assuming steady state it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg yr cosmic dust or mantle derived and an oceanic residence time between 2 5 104 and 5 4 104 with a mean of 3 5 104 year the estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater os would be 14 of the contribution of the unradiogenic component which means that the mantle derived flux should contribute a major part the first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and or of the ultramafic exposures contributes dominantly to the input of unradiogenic os to the seawater we show that it would be necessary to leach 1 3 of the os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic os to the ocean in which case the riverine input represents 94 of the total dissolved flux to the ocean instead of 70 the modern global dissolved os flux to the ocean would then have a 187os 188os ratio of 1 44 instead of 1 06 and the system would be far from steady state

cyclones and tides as feeders of a submarine canyon off bangladesh extremely high annual sedimentation rates of about 50 cm yr at the head of the submarine canyon swatch of no ground which connects the submarine delta of the ganges brahmaputra to the bengal deep sea fan were estimated by 210 pb and 137 cs gamma spectrometry the sediment in the canyon at 228 m water depth consists of graded sand and silt layers and laminated mud the sand and silt layers are assumed to have been deposited by tropical cyclones whereas the intercalated laminated mud is thought to have been deposited by daily variations of tidal currents the graded layers correlate well with the historical record of cyclones and allow an annual fine tuning the rapid infill of the canyon head is caused by trapping of suspended sediment that has been mobilized by storms and tides on the delta topset the sediment is stored only temporarily in the canyon head slides slumps and earthquakes occasionally remove and transfer it by turbidity currents to the deep sea fan the focusing effect of the canyon and the subsequent stepwise export of sediment probably combined with erosion of the canyon floor explain the long term regional stability of this canyon and probably other shelf canyons

link between indian monsoon rainfall and physical erosion in the himalayan system during the holocene mineralogical and geochemical analyses conducted on cores located on the active channel levee system of the northern bengal fan are used to establish changes in the weathering pattern and the sediment transport of the himalayan system and evaluate the effect of indian summer monsoon rainfall during the holocene our data indicate that during the holocene sediments from the northern bengal fan originate mainly from the g b river system without any significant changes in the relative contribution of these rivers from 9 8 to around 6 ka relatively low smectite illite1chlorite ratios and relatively high k si ratios indicate high physical denudation rates of the himalayan highlands together with a rapid transfer of the detrital material to the bengal fan the period between 9 2 and 7 ka is associated to lower values of k si and corresponds to the maximum of indian monsoon rainfall which indicates a more important chemical weathering material that rapidly transits by the g b river system without a long storage in the indo gangetic plain from 6 0 ka to present day higher smectite illite1chlorite ratio and lower k si ratio document a gradual increase of sediments originated from the indo gangetic plain characterized by higher degree of chemical weathering during the last 2 5 ka the drastic increase in the smectite illite1chlorite ratio could be associated to enhanced alteration of the plain soils due to anthropogenic activity the comparison of mineralogical and geochemical data with previous reconstructions of the indian monsoon dynamic indicates a rapid response of erosion and sediment transfer of the g b river system to changes of monsoon rainfall intensity

monsoon rainfall and contrasting source rocks influenced sediment composition of peninsular basins along the east coast of india western bay of bengal abstract the peninsular india is characterized by remarkable variation in lithological composition as well as contrasting rainfall distribution patterns the erosional and weathering history of these source rocks along with monsoonal changes are archived in sediments deposited in the basins along the east of coast of india here we present a 60 ky record of sr nd isotope ratios coupled with major elemental compositions of a sediment core off krishna godavari k g basin western bay of bengal the marked temporal variation in 87sr 86sr end and mg al ratios are attributed to the changes in relative contribution of sediments from the deccan basalts dcb and archean proterozoic gneissic complexes apgc the enhanced contribution of apgc associated with an increase in monsoon precipitation may be attributed to heightened erosion of the granitic gneissic rocks on the other hand a close correlation of 87sr 86sr ratios with rb sr ratio 18og ruber and cia profiles indicate the possible influence of weathering intensity on the sediment composition

coupled sea surface temperature seawater δ18o reconstructions in the arabian sea at the millennial scale for the last 35 ka two sediment cores from the western 905 10 46 9 n 51 56 4 e water depth 1586 m and eastern sk17 15 15 n 72 58 e water depth 840 m arabian sea were used to study past sea surface temperatures sst and seawater 18o 18ow variations for the past 35 ka we used coupled mg ca 18o calcite variability in two planktonic foraminiferal species globigerinoides ruber which thrives throughout the year and globigerina bulloides which occurs mainly when surface waters contain high nutrients during upwelling or convective mixing ssts in both areas based on mg ca in g ruber were 3 to 4 c lower during the last glacial maximum lgm 21 ka than today and the holocene period the sst records based on g bulloides also indicate general cooling down to 18 c in both areas ssts in the western arabian sea based on g bulloides were always lower than those based on g ruber indicating the presence of strong seasonal temperature contrast during the holocene and lgm we interpret the consistent presence of this seasonal temperature contrast to reflect a combination of seasonal summer upwelling sw monsoon and winter convective mixing ne monsoon in the western arabian sea in the eastern arabian sea g bulloides based ssts were slightly lower about 1 c than g ruber based ssts during the holocene indicating the almost absence of a seasonal temperature gradient similar to today however a large seasonal temperature contrast occurred during the lgm which favors the assumption that strong ne monsoon winds forced winter upwelling or convective mixing offshore goa sst and 18ow reconstructions reveal evidence of millennial scale cycles particularly in the eastern arabian sea here the stadial periods northern hemisphere cold periods such as younger dryas and heinrich events are marked by increasing ssts and salty sea surface conditions relative to those during the interstadial periods indeed the 18ow record shows evidence of low saline surface waters during interstadial periods indicating increased freshwater runoff from the western ghats as a consequence of enhanced sw monsoon intensity

re os isotope systematics in black shales from the lesser himalaya their chronology and role in the 187os 188os evolution of seawater abstract re and os abundances and os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines maldeota and durmala in the lesser himalaya the re and os concentrations in these samples exhibit wide range 0 2 to 264 ng g and 0 02 to 13 5 ng g respectively with a mean re os of 25 the 187 os 188 os of these samples also show a broad range 1 02 to 11 6 with many of them far more radiogenic than the currently eroding continental crust the black shales from the maldeota and durmala mines collected a few meters above the krol tal pc c boundary and occurring in the same stratigraphic horizon yield 187 re 187 os isochron ages of 554 16 and 552 22 ma respectively consistent with fossil evidences and those assigned for the pc c boundary at various other locations these results indicate closed system behaviour of re and os in these mine samples and reaffirm the validity of 187 re 187 os pair to date organic rich sediments the age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary re os age of 839 138 ma for black shales from theog collected from the inner belt these initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt however more analyses of black shales are needed to confirm this the role of weathering black shales from the himalaya in contributing to the os isotope evolution of seawater over the past 25 ma was assessed using a simple budget model for seawater os and assuming values for os fluxes and 187 os 188 os based on the available limited data on os concentration and its 187 os 188 os in rivers and oceans these calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187 os 188 os over the past 16 ma is difficult to satisfy by varying only the os flux from htp rivers 187 os 188 os kept constant at 1 324 or their 187 os 188 os 188 os flux constant at today s value 131 moles yr the requirements to balance os isotope budget in ocean through weathering of black shales however is easier to accommodate if both the os flux from htp himalayan tibetan plateau rivers and their 187 os 188 os are allowed to vary measurements of os abundance in htp rivers and its isotopic composition are needed to test the validity of the model results

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

himalayan uplift and osmium isotopes in oceans and rivers previous studies have shown that 187 os 188 os in seawater has become increasingly radiogenic over the last 40 ma in a manner analogous to strontium this rapid rise in the marine 187 os 188 os over the last 17 ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the himalayas and or selective weathering and erosion of highly radiogenic organic rich ancient sediments the key test of this hypothesis is the 187 os 188 os and the total osmium concentration of the himalayan rivers we report the concentration and isotopic composition of osmium in the ganges the brahmaputra and the indus rivers the 187 os 188 os of the ganges close to its source at kaudiyal 30 05 n 78 50 e is 2 65 and os 45 fm kg a second sample of the lower reaches of the ganges at patna 25 30 n 85 10 e gives 187 os 188 os 1 59 and os 171 fm kg the 187 os 188 os of the brahmaputra at guwahati 26 10 n 91 58 e is 1 07 and os 52 fm kg a sample of the indus besham 34 55 n 72 51 e has a 187 os 188 os of 1 2 and os 59 fm kg we infer that the himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans the overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the himalayas themselves appear not to be the dominant source

os sr nd results from sediments in the bay of bengal implications for sediment transport and the marine os record sr nd and os isotopic data are presented for sediments from diverse locations in the bay of bengal these data allow the samples to be divided into three groups related to their sedimentary contexts the first group mainly composed of sediments from the shelf off bangladesh and the currently active fan has sr and nd characteristics consistent with a dominantly himalayan source their 187os 188os ratios 1 2 1 5 show that the average detrital material delivered by the ganga brahmaputra g b river system is not unusually radiogenic a large difference in 187os 188os ratio exists between these bengal fan sediments and ganga bedloads 187os 188os 2 5 pierson wickmann et al 2000 this difference mainly reflects addition of a less radiogenic brahmaputra component though mineralogical sorting and loss of radiogenic os during transport may also play some role the second sample group contains sediments from elsewhere in the bay particularly those located on the continental slope they display os isotopic compositions 0 99 1 11 similar to that of present seawater and higher os and re concentrations these characteristics suggest the presence of a large hydrogenous contribution consistent with the lower sedimentation rate of these samples sr and nd ratios indicate that a significant fraction of these sediments is derived from erosion of non himalayan sources such as the indo burman range these observations could be explained by the deflection of sediments from the g b river system by westward currents in the head of the bay the third group contains only one sample but shows that in addition to a himalayan source sediment discharge from sri lanka may influence the detrital component in the distal part of the fan the similarity between the isotopic compositions of the group i r v sonne samples and those of ocean drilling program leg 116 france lanord et al 1993 reisberg et al 1997 suggests that the material eroding in the himalayas has been roughly constant since the miocene the high os isotopic ratios of leachates of both sonne group i and miocene leg 116 sediments imply that much of the leachable highly radiogenic os component was conserved during transport through the estuary or interaction with seawater in constrast samples with lower but still relatively high sedimentation rates sonne groups ii and iii and pliocene leg 116 seem to have significantly adsorbed or exchanged os and re with seawater this suggests that in some cases the os isotopic ratios of leachates of detrital sediments can be used to constrain the ancient marine os record or conversely to date unfossiliferous sediments

evidence for provenance change in deep sea sediments of the bengal fan a 7 million year record from iodp u1444a abstract previously published geochemical and mineralogical data have revealed conflicting views on the provenance of the deep sea sediments in the bengal fan here we report results of a study for a 330 m sequence consisting of both turbitide and semi pelagic sediments recovered at the site u1444a during the iodp 353 expedition a novel technique using the optically and thermally stimulated luminescence osl and tl signatures from quartz and k feldspar has been employed to characterise the mineral compositions of the sediments we show that the quartz luminescence sensitivity clearly differentiates the four sedimentary units there is a distinct increase of quartz luminescence sensitivity in unit 2 during the time interval of 3 5 0 5 ma analyses of the luminescence signal components in quartz and the ratio of the k feldspar and quartz also point to a change in unit 2 relative to the other three units we interpret such changes in the mineralogical compositions through the sequence as reflecting a change in the provenance over the last 7 million years in the western bengal fan we attribute the quartz with enhanced luminescence sensitivity in unit 2 to the contribution from the sources associated with indian rivers we infer that the delivery of the sediments from indian peninsula to iodp site u1444a is facilitated by a weakening in the turbidite deposition which is dominated by weathering products from the himalayas we speculate that the relatively high elevation of the 85 e ridge may act against the flow through and the associated deposition of turbidity current originated from the himalayas

evolution of weathering patterns in the indo burman ranges over the last 280 kyr effects of sediment provenance on 87sr 86sr ratios tracer a high resolution study of mineralogy and major element geochemistry combined with sr and nd isotopes has been conducted on high sedimentation rate cores collected off the irrawaddy river mouth in the andaman sea and the bay of bengal to reconstruct the erosional and weathering history of the irrawaddy river basin in both cores nd 0 values imply that both glacial and interglacial sediments share a common crustal source the irrawaddy river strong glacial interglacial cycles are recorded by 87sr 86sr interglacial periods yield values between 0 713 and 0 717 whereas glacial periods show higher values between 0 717 and 0 719 variations of the pedogenic clays smectite and kaolinite to primary mineral feldspar quartz illite and chlorite ratios show strong precessional cycles suggesting a control by past changes in the summer monsoon intensity each increase in pedogenic clays content is also associated with a net loss of labile elements na k and ca from the detrital minerals under chemical weathering wet periods of summer monsoon reinforcement correspond to an increase in weathering of the irrawaddy plain soils and a decrease of 87sr 86sr ratio plotting 87sr 86sr versus 87rb 86sr gives a pseudo isochrons interpreted as a mixing line representing the strength of chemical weathering during glacial stages enhanced physical erosion induced by glacier scour and frost action in the highland of the irrawaddy river basins produced high volumes of unaltered rb rich minerals the low sea level of glacial times constricted the river to the main channel in the lower reaches and permitted an efficient transport of unaltered rb rich minerals with high radiogenic sr composition from the high relief of the indo burman ranges and the tibetan plateau to the indian ocean

sr nd os evidence for a stable erosion regime in the himalaya during the past 12 myr modern erosion of the himalaya the world s largest mountain range transfers huge dissolved and particulate loads to the ocean it plays an important role in the long term global carbon cycle mostly through enhanced organic carbon burial in the bengal fan to understand the role of past himalayan erosion the influence of changing climate and tectonic on erosion must be determined here we use a 12 myr sedimentary record from the distal bengal fan deep sea drilling project site 218 to reconstruct the mio pliocene history of himalayan erosion we use carbon stable isotopes 13c of bulk organic matter as paleo environmental proxy and stratigraphic tool multi isotopic sr nd and os data are used as proxies for the source of the sediments deposited in the bengal fan over time 13c values of bulk organic matter shift dramatically towards less depleted values revealing the widespread late miocene ca 7 4 ma expansion of c4 plants in the basin sr nd and os isotopic compositions indicate a rather stable erosion pattern in the himalaya range during the past 12 myr this supports the existence of a strong connection between the southern tibetan plateau and the bengal fan the tectonic evolution of the himalaya range and southern tibet seems to have been unable to produce large re organisation of the drainage system moreover our data do not suggest a rapid change of the altitude of the southern tibetan plateau during the past 12 myr variations in sr and nd isotopic compositions around the late miocene expansion of c4 plants are suggestive of a relative increase in the erosion of high himalaya crystalline rock i e a simultaneous reduction of both transhimalayan batholiths and lesser himalaya relative contributions this could be related to an increase in aridity as suggested by the ecological and sedimentological changes at that time a reversed trend in sr and nd isotopic compositions is observed at the plio pleistocene transition that is likely related to higher precipitation and the development of glaciers in the himalaya these almost synchronous moderate changes in erosion pattern and climate changes during the late miocene and at the plio pleistocene transition support the notion of a dominant control of climate on himalayan erosion during this time period however stable erosion regime during the pleistocene is suggestive of a limited influence of the glacier development on himalayan erosion

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

the late oligocene early miocene himalayan belt constraints deduced from isotopic compositions of early miocene turbidites in the bengal fan abstract we present a mineralogical and isotopic study of early miocene turbidites from the bengal fan which are the oldest sediments recovered during odp leg 116 clay fractions are dominated by illite and chlorite and have low 18o 11 16 the clays are dominantly derived from a metamorphic precursor by physical erosion and have undergone only minor low t water rock exchange and chemical weathering sr and nd isotope analysis of silt and clay fractions and coarse muscovite biotite and feldspar fractions are remarkably similar to those of the presently exposed high himalaya crystalline hhc sequence the data show that a close analogue of the hhc was already subaerially exposed to active erosion during early miocene rb sr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for sr in biotite in the hhc ca 325 c and final sedimentation in the bengal fan despite the large uncertainties in the initial 87 sr 86 sr values of sedimentary mixtures our estimates of the elapsed time between cooling and deposition are short 4 14 ma assuming a transport time scale 1 ma deduced exhumation rates are in the same range of modern values 1 3 mm yr since ca 30 ma given that a the rocks exposed at the surface b the style of weathering and c rates of exhumation were similar to the present we infer that the factors controlling erosion of the early miocene himalayan belt were also similar to the present the data suggest that the early miocene himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range it appears that the thrusting along the main central thrust mct or similar structures has uplifted metamorphic basement analogous to the present hhc at sufficiently high rate to maintain steep relief throughout miocene time despite high average erosion rates

os isotopic compositions of leachates and bulk sediments from the bengal fan os isotopic compositions and os and re concentrations were measured in h2o2 h2so4 leachates and bulk sediment samples from holes 717c and 718c of odp leg 116 in the bengal fan os isotopic results indicate that at the sediment surface the leachable os fraction is derived from seawater in contrast leachable os from ganges river sediments has 187os 188os ratios 1 much higher than the marine value this difference suggests that the leachable radiogenic os carried by the river sediments is completely released to the oceans prior to sediment deposition in the fan a simple calculation assuming these sediments to be typical of those delivered by the ganges brahmaputra river system suggests that this process can account for a substantial part of the rise in the seawater os isotopic ratio observed over the past 16 m y bengal fan leachate 187os 188os ratios increase with increasing depositional age in contrast to the seawater os isotopic ratio which decreases with increasing age several lines of evidence suggest that at the time of sediment burial the leachate os compositions most likely reflected the seawater values thus the current divergence is probably the result of post depositional processes one such process in situ radiogenic ingrowth of187os can be excluded because the measured re concentrations of these sediments are too low similarly since most of the bulk rock os isotopic ratios were lower than those of the associated leachates the high leachate 187os 188os values cannot be explained by in situ sediment alteration instead it is proposed that the increase with age results from radiogenic os brought in by thermoconvective circulation from further upslope in the fan the ultimate source of this187os would then be alteration of radiogenic sediments or post depositional radioactive decay of re in sediments rich in organic carbon finally the divergence between the results obtained on bengal fan sediments and those obtained in the open ocean 2 by the same leaching technique suggest that os sediment leachate data must be interpreted with caution

the marine187os 186os record of the past 80 million years we report new187os 186os data and re and os concentrations in metalliferous sediments from the pacific to construct a composite os isotope seawater evolution curve over the past 80 m y analyses of four samples of upper cretaceous age yield187os 186os values of between 3 and 6 5 and187re 186os values below 55 mass balance calculations indicate that the pronounced minimum of about 2 in the os isotope ratio of seawater at the k t boundary probably reflects the enormous input of cosmogenic material into the oceans by the k t impactor s following a rapid recovery to187os 186os of 3 5 at 63 ma data for the early and middle part of the cenozoic show an increase in187os 186os to about 6 at 15 ma variations in the isotopic composition of leachable os from slowly accumulating metalliferous sediments show large fluctuations over short time spans in contrast analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data these results together with sediment leaching experiments indicate that dissolution of non hydrogenous os can occur during the hydrogen peroxide leach and demonstrate that os data from pelagic clay leachates do not always reflect the os isotopic composition of seawater new data for the late cenozoic further substantiate the rapid increase in the187os 186os of seawater during the past 15 ma we interpret the correlation between the marine sr and os isotope records during this time period as evidence that weathering within the drainage basin of the ganges brahmaputra river system is responsible for driving seawater sr and os toward more radiogenic isotopic compositions the positive correlation between87sr 86sr and u concentration the covariation of u and re concentrations and the high dissolved re u and sr concentrations found in the ganges brahmaputra river waters supports this interpretation accelerating uplift of many orogens worldwide over the past 15 ma especially during the last 5 ma could have contributed to the rapid increase in187os 186os from 6 to 8 5 over the past 15 ma prior to 15 ma the marine sr and os record are not tightly coupled the heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic os and sr to the oceans and account for the periods of decoupling of the marine sr and os isotope records

mid holocene indian summer monsoon variability off the andaman islands bay of bengal abstract a sediment core retrieved from the andamans forearc basin afb near landfall island north andaman provides a record of sediment provenance and climate change during the mid to late holocene radiocarbon ages ranging from 6078 to 1658 indicate that the core represents the interval from 6500 bp to the present grain size variation indicates a cyclic variation of wetter and drier conditions corresponding to changes in intensity of the indian summer monsoon ism which was at greatest intensity near 6400 5300 and 3300 3000 bp geochemical parameters including abundance of caco3 end and 18o in globigerinoides ruber are consistent with a long term trend from cooler wetter conditions to the warmer drier conditions at present chemical weathering intensity which lags behind climate changes on land shows a pulse of highly weathered sediment deposited at about 4000 bp during the short duration pulses of intense monsoon activity sandy sediment was supplied from the andaman islands the irrawaddy salween and sittang rivers of myanmar are the secondary sediment sources for the study area