mobility of rhenium platinum group elements and organic carbon during black shale weathering this study investigates the effects of black shale weathering on the re os isotope system platinum group element concentrations and the degradation of organic matter samples from a weathering profile in late devonian 365 myr ohio shale show a pronounced decrease 77 in organic carbon corg near the present soil surface relative to the interior portion of the outcrop a similar trend is observed for total n 67 loss conversely organic phosphorus porg concentrations increase by 59 near the soil surface the decrease in corg is accompanied by a pronounced decrease in re 99 and to a lesser extent os 39 palladium and pt do not appear to be significantly mobile the effects of re and os mobility on the re os isotope system are significant none of the samples plots on a 365 myr isochron rather the samples define a trend with a slope corresponding to an age of 18 myr with an initial 187os 188os of 6 1 this indicates recent disturbance of the re os system isotope mass balance calculations imply that the labile fraction of os is significantly more radiogenic 187os 188os of 7 8 than the average of the unweathered samples 187os 188os of 6 4 based on data from this study the molar ratio of labile re to corg in ohio shale is estimated at 7 10 8 we estimate the present day riverine black shale derived re flux to seawater using literature data on re burial in anoxic marine sediments and assuming steady state between re release during black shale weathering and re burial in anoxic marine sediments then the labile re corg observed in this study implies that 0 5 tmol of corg is released annually from weathering of black shales a trace lithology in the continental crust this flux corresponds to 12 of the estimated annual co2 flux from oxidative weathering of sedimentary rocks the labile molar re os of 270 indicates that black shale weathering releases 130 mol os per year which accounts for 7 of the riverine os input to seawater the data from this study support the notion that the crustal cycles of labile corg re and os are tightly coupled gray shales which are less corg re and os rich but much more abundant in the continental crust than black shales are likely to be even more important continental sources of re and os to seawater

chemistry of uranium thorium and radium isotopes in the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal abstract the most comprehensive data set on uranium thorium and radium isotopes in the ganga brahmaputra one of the major river systems of the world is reported here the dissolved 238u concentration in these river waters ranges between 0 44 and 8 32 1 and it exhibits a positive correlation with major cations na k mg ca the 238 u cations ratio in waters is very similar to that measured in the suspended sediments indicating congruent weathering of uranium and major cations the regional variations observed in the 234 u 238 u activity ratio are consistent with the lithology of the drainage basins the lowland tributaries chambal betwa ken and son draining through the igneous and metamorphic rocks of the deccan traps and the vindhyan bundelkhand plateau have 234 u 238 u ratio in the range 1 16 to 1 84 this range is significantly higher than the near equilibrium ratio 1 05 observed in the highland rivers which drain through sedimentary terrains the dissolved 226ra concentration ranges between 0 03 and 0 22 dpm 1 the striking feature of the radium isotopes data is the distinct difference in the 228ra and 226ra abundances between the highland and lowland rivers the lowland waters are enriched in 228ra while the highland waters contain more 226ra this difference mainly results from the differences in their weathering regimes the discharge weighted mean concentration of dissolved 238u in the ganga at patna and in the brahmaputra at goalpara are 1 81 and 0 63 1 respectively the ganga brahmaputra river system constitutes the major source of dissolved uranium to the bay of bengal these rivers transport annually about 1000 tons of uranium to their estuaries about 10 of the estimated global supply of dissolved uranium to the oceans via rivers the transport of uranium by these rivers far exceeds that of the amazon although their water discharge is only about 20 of that of the amazon the high intensity of weathering of uranium in the ganga brahmapura river system can also be deduced from the 232 th 238 u and 230 th 238 u activity ratios measured in the suspended sediments 230th is enriched by about 19 in the suspended sediments relative to its parent 238u the flux of excess 230th supplied to the bay of bengal via these river sediments is 980 1012 dpm a about six times more than its in situ production from seawater in the entire bay of bengal

isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

provenance and weathering control on river bed sediments of the eastern tibetan plateau and the russian far east abstract we investigated the petrography and major trace and rare earth element compositions of the bed sediments from the large rivers draining the eastern tibetan plateau etp the huang he yellow chang jiang yangtze hong he red mekong and salween the combined drainage spans tropical temperate and arid climate zones in addition samples from the lena river draining the verkhoyansk and sette daban ranges in an arctic to sub arctic climate zone in siberia were also studied for their major and trace element composition the sub angular texture the quartz feldspar rock fragment classification in the lithic arenite field and the relatively low chemical index of alteration cia 42 79 indicate that these sediments are texturally petrographically and chemically immature and immobile trace elements can be used to obtain some information about the lithologic provenance the etp river sediments are felsic and similar to the upper continental crust with the salween most felsic and the lena most mafic the sediments show a gradation in maturity that is not clearly related to present climatic or physiographic regime this suggests that maturity itself is a relict feature inherited from previous sedimentary cycles the high physical erosion in such high energy and relief systems seems to inhibit intense weathering and manifests a provenance control from local rocks on the sediment compositions

osmium geochemistry of modern estuarine sediments from the tama and yasaka rivers in japan concentrations of re and os and 187 os 188 os ratios were obtained on the modern sediment samples from two estuarine areas the tama river in tokyo and the yasaka river in oita japan the shallow depth 25 cm intervals are 99 2 105 5 ppt os that are more than 3 times higher than ucc however all samples have 187 os 188 os ratios within a relatively narrow range of 0 3029 0 4035 much lower than that of ucc 1 0 1 3 indicating a similar source of os in both parts the lower 187 os 188 os ratios and high os concentrations of the sediments in the tama river suggest the impact from human activities the sudden enrichment of os at the deep interval may be related to the redox condition change in addition the os concentration is vertically constant in either the upper and lower intervals of the sediment profile suggesting constant burial fluxes of os in the sediment the os in the yasaka river estuary sediments 9 22 29 4 ppt are generally lower than that of the tama river either similar or much lower 1 3 than the average ucc os concentrations the samples with high os concentrations have lower 187 os 188 os ratios whereas the samples with lower os concentrations have relatively higher 187 os 188 os ratios probably indicating different os sources the relatively higher 187 os 188 os ratios and lower os concentrations in some yasaka river estuarine sediments than the tama river may also suggest less pollution from human activity to the yasaka river compared to the tama river there is a large decrease of re concentrations with depth in the yasaka river estuarine sediment whereas less variations of re in the tama river estuarine sediments the ratio of re os is also sharply different between the two estuarine sediments also indicating different sedimentary environments the tama river sediments may receive much more anthropogenic influence while the yasaka river sediments get strong impact from modern volcanic activity due to their different geographical and geological surroundings the tama river flows through a heavily industrial metropolitan area whereas the yasaka river flows through a volcanic active but less industrial area

the geochemistry of rhenium and osmium in recent sediments from the black sea abstract the os isotopic composition of recent sediments from the black sea are reported with the concentrations of re os and major and minor elements osmium concentrations range from 0 23 to 0 69 ppb and re concentrations range from 21 to 85 ppb concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon 187 os 186 os ratios range from 5 7 to 7 1 all of these values are less radiogenic than the estimated 187 os 186 os ratio of seawater and average continental crust material variations in the os isotopic composition of these sediments are attributed to mixing of a hydrogenous component 187 os 186 os ratio between 7 and 9 and a less radiogenic detrital component 187 os 186 os ratio between 1 and 4 a mixing model is used to determine the fraction of os in the sediment associated with each component and the burial fluxes of re and os in the black sea are calculated in addition the data are used to estimate the magnitude of systematic error which such initial os isotopic heterogeneity may introduce into re os age determinations of ancient organic rich sediments

rejuvenation of continental lithosphere beneath an intracratonic basin abstract the effective elastic thickness of the lithosphere in the williston basin region has been determined for several time intervals by fitting an elastic flexure equation to the shape of the basin over time the elastic thickness increases from about 40 km just after the basin began subsiding 450 m y b p to about 80 km at the present these results agree well with the increasing elastic thickness with age predicted for a cooling 250 km thick plate taking elastic thickness to be depth to the 450 c isotherm if we assume a thermal age of the lithosphere of about 500 m y radiometric dates of basement in the area however generally yield ages of 1 7 b y churchill province and 2 5 b y superior province because basement ages are generally much greater than the inferred lithospheric thermal age in the williston basin region we suggest that lithosphere in this area was thermally rejuvenated about 500 m y b p by the same process es responsible for the thermal subsidence of the basin temperatures were not high enough in the upper crust to reset most radiometric clocks additional support for a thermal event 500 m y b p comes from fission track ages and a single rb sr age of basement rocks of about 500 m y lithospheric rejuvenation has been modelled using a one dimensional finite difference thermal model the base of a 250 km thick plate is heated from 1333 c to 1850 c approximately the solidus of basalt at that depth for approximately 100 m y with conduction only the depth to the 450 c isotherm slowly decreases then slowly increases after heating ceases rapid thinning and slow thickening of the lithosphere can be achieved only if convective thinning is simulated in the model the model with convection yields results consistent with the observed increase in lithospheric thickness as well as the observed subsidence of the basin over time

re os isotope systematics and weathering of precambrian crustal rocks implications for the marine osmium isotope record the effects of weathering of freshly exposed precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western wind river range in wyoming the os budget of the precambrian granitoid source rocks is dominated by os rich trace phases such as magnetite amongst the major silicates biotite or a trace phase in biotite carries most of the os and re and is also the most radiogenic mineral with 187os 186os of 113 and 187re 186os of 2 000 re os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes os with an isotopic composition similar to the isotopic composition of the bulk soils a very radiogenic fraction of os is mobilized through preferential weathering of biotite radiogenic runoff from precambrian shields inferred from osmium isotope analyses of freshwater fe mn nodules suggests that high latitude precambrian shields are important source areas of radiogenic os in seawater we propose that glacial scouring and weathering of glacial tills exposed after deglaciation of precambrian shields surrounding the north atlantic provides a mechanism for the slightly more radiogenic nature of north atlantic seawater compared to other seawater masses glacial interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial interglacial time scales in high latitude shield areas surrounding the north atlantic

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

are some domes the ghosts of basins 1 the inversion of formerly extensional basins by compressional reactivation of existing faults is believed to be an important mechanism for the exhumation of sedimentary basins we propose an additional mechanism for the exhumation of some sedimentary basins which does not require compression and which completely inverts a basin into a dome in basins formed by intra crustal density loads sublithospheric heating however caused will reduce the effective elastic thickness of the crust thereby reactivating subsidence if the crust is sufficiently weakened that it no longer supports the dense load we predict the long term result to be detachment of the load followed by uplift and exhumation of the basin finite element modeling indicates that this can occur for geologically possible load densities in high heat flow regions total upward displacement of material from depth can be as much as 10 km in some cases enough to exhume the basin completely

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

re os dating of pyrite from the tuolugou stratabound co au deposit eastern kunlun orogenic belt northwestern china abstract the tuolugou cobalt gold deposit in the northeastern margin of the qinghai tibetan plateau and central part of the eastern kunlun orogenic belt is the most recently discovered large cobalt gold discovery in qinghai province nw china exploration to date has identified a cobalt resource of greater than 20 000 t co with significant potential for expansion mineralization is largely confined to an e w striking volcano sedimentary sequence consisting of four lithological units that underwent extensive greenschist facies metamorphism and deformation all orebodies occur in stratiform or lenticular bodies within quartz albitite and sericite quartz schists and are concordant with lithological contacts the ores contain various amounts of pyrite the dominant ore mineral and display massive 80 pyrite banded 30 to 50 pyrite and disseminated re os dating shows that pyrite from different ore types has very low re and os concentrations in the range of 0 122 8 988 ppb and 0 003 0 054 ppb respectively isotope data on seven pyrite samples yield an isochron age of 429 29 ma 2 with an initial 187 os 188 os ratio of 0 2 mean square weighted deviates 7 9 on the 187 re 188 os 187 os 188 os plot which are close to the age of the host strata suggesting that primary cobalt mineralization in the tuolugou deposit may have formed from syngenetic exhalative processes in addition our results not only provide new evidence for plate rifting and mineralization taking place during the early silurian in the eastern kunlun orogenic belt but also suggest that the age of the regionally widespread nachitai group is early paleozoic

a mechanical and thermal model for the evolution of the williston basin the williston basin is a large circular intracratonic basin containing over 4 km of sediments ranging in age from cambrian to tertiary data on seven formations from over 1300 wells in montana south dakota and north dakota were used to determine the shape of the basin over time by assuming that the lithosphere behaves like an elastic plate on a fluid substrate the shape of the basin could be closely matched using a point load the match was improved by adding the equation of a plane to the equation for elastic deformation before fitting the model to the data the planar adjustment allows for the effects of regional sedimentation or erosion or eustatic sea level changes the mechanical modeling indicates that the elastic strength of the lithosphere in this area has generally increased from the ordovician to the present the geometrical center of the basin migrated less than 20 km over 450 m y the direction and magnitude of tilt of the plane removed from the data is related to tectonic events in western north america the increasing deflection of the basin over time can be explained by a simple thermal model which would explain the thickening of the lithosphere consisting of conductive cooling of a hot region in the lithosphere depths to formations in a single well differ from the subsidence predicted by the elastic model with regional tilt removed because of global sedimentation events and the development in the mesozoic of a foreland thrust basin of which the williston basin became part the mechanical and thermal models constrain possible initiating mechanisms for this basin the thermal modeling and the absence of volcanics in the stratigraphic column rule out a shallow crustal heat source neither wrench faulting nor crustal stretching were likely to have been significant in the formation of the williston basin

nd and pb isotope variability in the indus river system implications for sediment provenance and crustal heterogeneity in the western himalaya abstract the indus river system is the only major drainage system in the western himalaya and erodes not only the high himalaya but also topographically high regions within and north of the indus suture zone most notably the karakoram ion microprobe analysis of pb isotopes in detrital k feldspar grains taken from the tributaries of the indus together with bulk nd isotope analysis of those same sediments is here used to identify distinct sediment source regions these span the very radiogenic nanga parbat and associated lesser himalaya the relatively radiogenic intermediate high himalaya the unradiogenic ladakh and kohistan batholiths and intermediate values in the hindu kush karakoram and lhasa block the range of compositions reflects differing degrees of recycling of older continental crust during petrogenesis k feldspars from the ladakh and kohistan batholiths are less radiogenic than the laterally equivalent gangdese granite of tibet interpreted to reflect the preferential recycling of accreted oceanic arc units within the western transhimalaya prior to india asia collision similarly the zanskar high himalaya are less radiogenic than their equivalents in nepal isotope values from pleistocene indus fan sediment are compatible with a dominant source in the karakoram with additional important contributions from the arc batholiths and high himalaya reflecting both the area and modern rates of tectonic uplift within the drainage basin in contrast radiogenic grains are common in the lower reaches of the modern indus river possibly as a result of the damming of the main river channel where it reaches the foreland

ultramafic and mafic rocks from the garret transform fault near 13 30 s on the east pacific rise igneous petrology serpentinized peridotites metagabbros and fresh basaltic rocks were dredged from the garret transform fault near 13 s on the east pacific rise two dredge hauls taken on the northern wall from about 3100 3600 m consisted of aphyric and picritic basalts while peridotites gabbros and moderately phyric basalts were recovered in a single dredge located near the deepest part 4616 4820 m of the transform valley well preserved igneous textures and mineralogical assemblages enable us to differentiate between tectonites including harzburgites and clinopyroxene bearing harzburgites and cumulates consisting of plagioclase dunite troctolites olivine gabbros and ilmenite gabbros the harzburgites are likely to represent deep seated peridotites left after extraction of basaltic melt during upper mantle partial melting of a lherzolite the ultramafic cumulates underwent some deformation and show textural and mineralogical evidences which suggest formation at a minimum depth corresponding to the very lower crust it is also inferred that the composition of the coexisting liquid along with early cumulus crystals has a ratio mg mg fe2 of 0 7 high cao na2o ratios and a low ni content about 150 ppm when compared to similar rock types from atlantic fracture zones subsequent uplift during rifting of the oceanic lithosphere enhanced plastic deformation subsolidus recrystallization and retrogressive metamorphism of the gabbros and the ultramafics

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

phase change and thermal subsidence of the williston basin summary a mechanism of subsidence for the williston basin is proposed and tested the mechanism combines cooling and thermal contraction of the lithosphere with a phase transformation in the lower crust the cooling and the phase boundary movement following a sudden change in the thermal boundary condition at the base of the lithosphere are calculated isostatic adjustments and sea level variations are included to determine subsidence and sediment accumulation at the basin s centre which are compared with the data the subsidence record could not be explained by temperature boundary conditions the calculations show that 1 the heat flow boundary condition explains the long duration more than 350 myrs of subsidence of the williston basin and 2 the delay of the phase change subsidence about 40myrs explains an acceleration of subsidence in the early stages of the basin s evolution when isostatic adjustments and the effect of sea level variations are included the calculated sediment accumulation history fits well the sediment record of the williston basin

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

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

subsidence of the paris basin we discuss the main results of a systematic quantitative analysis of the subsidence of the paris basin made on 100 wells backstripping techniques with decompaction as well as sea level and water depth corrections are used to derive tectonic subsidence curves in the absence of water and sediment loading assuming local as well as regional compensation a best fitting sea level variations curve is derived for the deepest central part on the basis of the resulting tectonic subsidence curve the adopted amplitude of the regression since upper cretaceous is 330 m the spatial and temporal distribution of tectonic subsidence is reconstructed and discussed it is shown that rigidity of the lithosphere with an adopted 50 km flexural parameter plays a significant role these results are then quantitatively interpreted in terms of the lithospheric stretching model this interpretation is reasonable provided it is made in terms of volumetric subsidence over the whole basin because of the thermal conduction and the rigidity low pass filtering effects the subsidence can be explained if the axial zone was thinned during permo triassic by a factor of about 1 3 with respect to the adjacent lithosphere it is shown that either the initial altitude was more than 500 m above sea level or the lower part of the lithosphere was lost at the beginning of the permo triassic phase of extension resulting in a rapid uplift of several hundreds of meters the similarity of the permo triassic initiating extensional phase with active continental rifting is shown

residence time of osmium in the oceans 1 estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations 35 50 kyr appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater 3 4 kyr it is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree as the inventory of osmium in the oceans is relatively well constrained these disparate residence time estimates imply wildly different osmium input fluxes to the oceans an osmium residence time of 8 10 kyr is proposed by evaluating the uncertainties and limitations of both methods and it is argued that osmium inputs to the ocean are currently underestimated by a factor of 3 this reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified the very short residence time of 3 4 kyr inferred from the postglacial change in seawater composition assuming a single step change in input flux is rejected as it implies unreasonably high osmium input fluxes to the oceans it is concluded that a postglacial spike in osmium flux associated with a meltwater event must have driven part of the change in seawater composition however it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187os 188os

the effect of lithospheric phase transitions on subsidence of extending continental lithosphere abstract a new simple model for sedimentary basin formation which combines both stretching and phase transitions occurring together in the lithosphere is proposed and two important aspects in sedimentary basin formation are addressed 1 the explanation of the domal uplift preceding rifting and 2 the ability of the model to explain the contradiction between a small amount of crustal stretching and the large amount of thermal subsidence without the need to invoke differential stretching between the crust and mantle relationships for synrift initial and thermal post rift subsidence are derived the model predicts that extension of continental lithosphere could produce synrifting uplift up to a threshold amount of extension and then subsidence for greater extension three dimensionless parameters quantify the contribution of the phase transition to the subsidence history 1 the clapeyron slope normalized to the initial geothermal gradient 2 the ratio of depth of phase transition to the thickness of the lithosphere and 3 the ratio of density effect due to phase transition to the change in density due to thermal expansion an evaluation of these parameters has been carried out by using thermodynamical data for the garnet spinel transition for upper mantle rocks our simple model shows that by including the effects of lithospheric phase transition in existing stretching models of sedimentary basin formation we can obtain an extra significant parameter controlling the subsidence and the entire extensional tectonics of such basins

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

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

geological development origin and energy mineral resources of williston basin north dakota the williston basin of north dakota montana south dakota and south central canada manitoba and saskatchewan is a major producer of oil and gas lignite and potash located on the western periphery of the phanerozoic north american craton the williston basin has undergone only relatively mild tectonic distortion during phanerozoic time this distortion is largely related to movement of precambrian basement blocks sedimentary rocks of cratonic sequences sauk through tejas are present in the basin sauk cambrian lower ordovician tippecanoe ordovician silurian and kaskaskia devonian mississippian sequence rocks are largely carbonate as are the major oil and gas producing formations absaroka pennsylvanian triassic and zuni jurassic tertiary rocks have more clastic content but carbonates are locally important clastics of the zuni sequence fort union group contain abundant lignite tejas tertiary quaternary sequence rocks are not significant in the production of minerals or energy although glacial sediments cover much of the region oil exploration and development in the united states portion of the williston basin since 1972 have given impetus to restudy basin evolution and geologic controls for energy resource locations consequently oil production in north dakota for instance has jumped from a nadir of 19 million bbl in 1974 compared to a previous zenith of 27 million in 1966 to 32 million bbl in 1979 and 40 million bbl in 1980 geologic knowledge of carbonate reservoirs has expanded accordingly depositional environments throughout sauk tippecanoe and kaskaskia deposition were largely shallow marine subtidal and even basinal environments were developed in the basin center but sabkha deposits were abundant near the basin periphery evidence of subaerial weathering was commonly preserved in structurally high areas and on the basin periphery especially in upper kaskaskia rocks some pinnacle reefs were developed in kaskaskia deposition morphologically similar to the silurian pinnacle reefs of the michigan basin clastic sediments were transported into the southern part of the basin during absaroka sequence deposition a product of erosion of ancestral rocky mountain orogenic structures continental and shallow marine clastic sediments were deposited during zuni sedimentation until cretaceous deeper marine environments were established laramide orogenesis to the west provided detritus that was deposited in fluvial deltaic and marginal marine environments regressing to the east major lignite deposits are part of this postorogenic regressive rock body major structures in the basin and the basin itself may result from left lateral shear along the colorado wyoming and fromberg zones during pre phanerozoic time deeper drilling in the basin has revealed several major structures and given indications of others most structures probably resulted from renewed movement or tensing of pre phanerozoic faults meteorite impact events have been suggested as the origin for one or more structures

elastic thickness of the lithosphere in the central canadian shield we have used the coherence between bouguer gravity and topography to determine the elastic thickness t e of the lithosphere in central canada in order to improve the resolution we have estimated the spectra with the maximum entropy method the variations in elastic thickness delineate three main regions 1 the interior platform where t e varies between 25 and 50 km 2 the shield includes several high rigidity cores in particular in the trans hudson orogen and in the hearne province where t e 80km 3 the hudson bay basin has an elastic thickness between 50 and 70 km with higher values in the eastern part the results suggest that t e is controlled by the thermal regime in the platform there is no difference in t e between the proterozoic and archean provinces of the shield in the platform the elastic thickness is lowest where sediments are thickest indicating that low rigidity has amplified the subsidence of the basin

structural sequences and styles of subsidence in the michigan basin subsidence in the michigan basin produced 5 km of sedimentation over a period of more than 200 m y during paleozoic time utilizing well log correlations and constrained by compaction corrections and estimates of paleobathymetry we recognize four different styles of subsidence in the basin trough shaped regional tilting narrow basin centered and broad basin centered subsidence began as a trough shaped northerly extension of the illinois basin during late cambrian to early ordovician time this was followed by narrow basin centered subsidence in early to middle ordovician time basin centered subsidence ceased for 30 m y during long wavelength 1000 km eastward tilting in middle to late ordovician time a pattern incompatible with thermal contraction subsidence models basin centered subsidence resumed in silurian time but with a broader distribution a second episode of narrow basin centered subsidence occurred in latest silurian through middle devonian time and was replaced by broad basin centered subsidence at the end of middle devonian time the geometry of upper devonian and younger paleozoic deposits suggests another eastward tilting event but results remain inconclusive due to erosion of strata and uncertainties in their paleobathymetry in addition to these subsidence patterns two distinct unconformity styles are present basin wide and marginal erosion there is no evidence for significant basin centered unconformities as predicted by purely thermal mechanisms a history of episodic subsidence reactivations is interpreted as the result of a stress induced crustal weakening mechanism for the narrow basin centered subsidence whereas broad basin centered subsidence is interpreted as thermal contraction related to lower crustal attenuation during the narrow subsidence episodes recently proposed dynamic topography related to initiation of ordovician subduction provides a driving mechanism for long wavelength eastward tilting together with a temporal correlation to appalachian tectonism these mechanisms provide a plate tectonic framework for the history of the michigan basin

variations of the187os 186os ratio of seawater over the past 28 million years as inferred from metalliferous carbonates abstract the os concentration and isotopic composition of metalliferous carbonates deposited on the east pacific rise over the past 28 ma are reported with complimentary sr isotope data variations in the os isotopic composition of these samples are interpreted as a record of past changes in the os isotopic composition of seawater these results are consistent with isotopic analyses of leachable os in pelagic clays which have also been interpreted as a record of the187os 186os ratio of seawater through time 1 the metalliferous carbonate record clearly shows that seawater os and sr isotope systems are partially decoupled from one another over the past 28 ma accelerated weathering of ancient organic rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the187os 186os ratio of seawater over the past 15 ma this rapid increase suggests that the seawater os record can potentially be used as a stratigraphic tool in some neogene marine deposits

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

partitioning of ni between magnesiowustite and metal at high pressure implications for core mantle equilibrium abstract the partitioning of ni between metal and magnesiowustite is experimentally investigated up to 17 gpa from the thermodynamic analysis of the experimentally determined distribution coefficient kd m mw xni xfe metal xni xfe oxide we obtain seven thermodynamic parameters relevant to its dependency on pressure temperature and chemical composition kd m mw is strongly dependent on pressure temperature and mgo content in magnesiowustite the pressure effect is most effective and kd m mw significantly decreases with increasing pressure i e chemical affinity of ni with magnesiowustite is enhanced by high pressure kd between metal and bulk mantle silicates kd m bms is estimated on the basis of compiled data of kd m silicates and the extrapolated value of kd m mw kd m bms also decreases with pressure the chemical reaction between sinking fe alloy and mantle material at high pressure leads that ni transfer into the mantle material from fe alloy from the comparison kd m bms and kd core mantle the present high ni concentration in the earth s uppermost mantle is well explained by the equilibrium partitioning of ni during core mantle separation combined with a subsequent mantle differentiation

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

major ion chemistry of the yarlung tsangpo brahmaputra river chemical weathering erosion and co2 consumption in the southern tibetan plateau and eastern syntaxis of the himalaya the yarlung tsangpo brahmaputra river drains a large portion of the himalaya and southern tibetan plateau including the eastern himalayan syntaxis one of the most tectonically active regions on the globe we measured the solute chemistry of 161 streams and major tributaries of the tsangpo brahmaputra to examine the effect of tectonic climatic and geologic factors on chemical weathering rates specifically we quantify chemical weathering fluxes and co2 consumption by silicate weathering in southern tibet and the eastern syntaxis of the himalaya examine the major chemical weathering reactions in the tributaries of the tsangpo brahmaputra and determine the total weathering flux from carbonate and silicate weathering processes in this region we show that high precipitation rapid tectonic uplift steep channel slopes and high stream power generate high rates of chemical weathering in the eastern syntaxis the total dissolved solids tds flux from the this area is greater than 520 tons km 2 yr 1 and the silicate cation flux more than 34 tons km 2 yr 1 in total chemical weathering in this area consumes 15 2 10 5 mol co2 km 2 yr 1 which is twice the brahmaputra average these data show that 15 20 of the total co2 consumption by silicate weathering in the brahmaputra catchment is derived from only 4 of the total land

sources and sinks of unradiogenic osmium runoff from papua new guinea the fly and sepik are two of the largest rivers draining the island of new guinea and among the world s large rivers in discharge and sediment yield sediment from the upper fly river and its delta have low 187os 188os of 0 511 0 560 and sediments from the upper and lower sepik river have even lower ratios of 0 294 0 418 filtered waters 1 2 burial of os of both seawater and riverine derivation occurs in conjunction with organic carbon burial in the gulf of papua there is a negative correlation between the os isotopic composition of the bulk sediment and the fraction of organic matter of terrestrial origin based on c isotopes the systematic relationship between c and os isotopic compositions in these samples indicates that organic matter is the major host of os in tropical estuarine sediments the os corg of gulf of papua surface sediments is intermediate between that of other modern and ancient sediments approximately 1011 mol yr of corg and 20 57 mol yr of os are buried in the inner shelf 50 m depth of the gulf of papua over an area of only 21 000 km2 corresponding to 1 3 of the global marine os burial flux in 0 006 of the total sea surface area these results suggest that a large proportion of the global burial flux of os may take place in conjunction with the deposition of low corg sediments lowering of the os isotopic composition of ambient seawater due to a large flux of unradiogenic riverine os may be indicated by the os isotopic composition of water from the coral sea 187os 188os 0 844 and marine sediments within the gulf of papua the island of new guinea is estimated to provide 4 5 of the global riverine os flux given the very large particulate and dissolved flux to the oceans from rivers in the wet tropics the effect of the weathering of young arc related crust in the sw pacific is an important source of unradiogenic os to modern seawater

crustal refraction surveys across the trans hudson orogen williston basin of south central canada eight reversed seismic refraction profiles have been recorded across the phanerozoic williston sedimentary basin that overlies the margins of the superior and wyoming archean cratons and the proterozoic trans hudson orogen these data have been modeled using two dimensional ray tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals within the southern extension of the superior craton margin thompson belt a high velocity 6 4 km s lid overlies a narrow region of high electrical conductivity and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge thompson fault of the craton margin complex zones of low velocity material and the north american central plains electrical conductivity anomaly are concluded to lie within the southern extension of the reindeer south indian lakes terrain a component of the trans hudson orogen a high velocity 7 0 km s lower crustal layer which represents a zone of complexity separating normal crust from the upper mantle extends throughout the region the crust is relatively thick with depths to moho varying smoothly between 41 and 48 km there is no evidence in the relatively low resolution seismic refraction data for abrupt changes in the level of the moho but there may be a minor 3 4 km thinning of the crust near the center of the williston basin

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

thermal subsidence and generation of hydrocarbons in michigan basin temperature histories for selected stratigraphic horizons in the michigan basin are computed from three dimensional continually filled subsidence models mechanical evolution of the michigan basin is modeled as flexure of the lithosphere caused by thermal contraction results are compatible with the subsidence record of the sediments and free air gravity anomalies paleotemperature is determined from excess temperature due to the thermal anomaly plus burial temperature predicted from subsidence curves for an equilibrium temperature gradient of 22 c km 1 2 f 100 ft surface temperature of 20 c 68 f and an equilibrium surface heat flow of 1 1 hfu excess temperature paleotemperature and surface heat flow do not exceed 15 c 27 f 110 76 c 230 f and 2 5 hfu respectively these estimates are consistent with upper limits set by paleomagnetic studies the low value for excess temperature is caused by concentration of the thermal anomaly below 15 km 9 mi in agreement with gravity results the great depth of the thermal anomaly can explain the lack of evidence for an initial heating event prior to subsidence once the thermal history of the sediments is specified the oil potential of the basin can be determined from laboratory derived kinetic equations for degradation of kerogen to petroleum for the michigan basin predicted temperature conditions are sufficient for source rock 25 conversion of type i and type ii kerogen only in ordovician and older rocks in the southern peninsula of michigan by this model petroleum found in rocks younger than ordovician would have had to migrate upward from the older rocks geochemical studies of dundee devonian and trenton ordovician crude oils in michigan are compatible with this interpretation niagaran silurian crude oils appear to come from a different source than the dundee and trenton oils if their source is silurian rocks on the flanks of the basin either the scheme used to calculate maturity is not applicable to these oils or the temperature of the source rocks was significantly higher in the past than today more likely the model underestimates the depth of burial of silurian source rocks near the center of the basin especially in local regions of downfaulting

osmium isotopic variations in metalliferous sediments from the east pacific rise and the bauer basin the osmium concentration and isotopic composition of metalliferous sediments from the east pacific rise epr and bauer basin have been determined 187os186os ratios range from 7 9 to 8 6 with epr samples between 8 4 and 8 6 and bauer basin samples between 7 9 and 8 2 the osmium isotopic composition of epr samples is indistinguishable from the inferred osmium isotopic composition of seawater requiring that nearly all the osmium in these sediments is hydrogenous in origin this result is used to place broad constraints on the behavior of osmium in ridge crest hydrothermal systems while bauer basin samples also contain predominantly hydrogenous osmium an additional source of osmium with low 187os186os is required to account for the lower measured ratios calculation of total osmium burial fluxes in the bauer basin show that the background flux of cosmic osmium is adequate to produce the observed depression in bauer basin 187os186os ratios burial flux calculations also suggest that metalliferous sediment deposition is a significant sink in the marine osmium cycle

temporal variation in sr and 87sr 86sr of the brahmaputra implications for annual fluxes and tracking flash floods through chemical and isotope composition temporal variations in dissolved sr and its 87sr 86sr in the brahmaputra river were determined for the first time by analyzing biweekly samples collected over a period of about a year at guwahati sr in the brahmaputra range from 0 604 to 1 392 m with 87sr 86sr of 0 71594 to 0 71802 sr concentration decreases in monsoon except for the 15 june 2000 sample which had anomalously high concentrations of sr and major ions compared to the nonmonsoon period the decrease in elemental abundances a factor of 2 is not proportional to the increase in discharge an order of magnitude indicating enhanced weathering during monsoon this can be a cumulative effect of an increase in drainage area and in physical weathering during monsoon 87sr 86sr and other proxies of silicate weathering show a relatively lower contribution from silicate weathering to the major ion and sr budget during monsoon with a concomitant increase in carbonate weathering contribution shorter interaction time between water and minerals during monsoon coupled with the slower weathering kinetics of silicate compared to carbonate can be contributing factors to the seasonal changes in their relative contributions to the major ion budget this study shows that the annual fluxes of various ions catsil and srsil calculated on the basis of measured biweekly concentrations and monthly water discharge data are roughly within 35 of those calculated using either the lowest or the highest concentrations and annual water discharge this implies that a tropical region such as in india where river discharge is governed mostly by monsoon rains the annual elemental fluxes measured during monsoon would be a good approximation for their discharge weighted annual fluxes however an additional sampling during a drier period would be required to understand the weathering processes the geochemistry of the 15 june 2000 sample is anomalous the chemistry of this sample is similar to those from the tibetan region an inference consistent with reports of a flash flood in the brahmaputra due to a natural dam burst in the yigong river of tibet these results highlight the use of the chemical and isotopic composition of water samples to track and quantify the source and discharge of a flash flood

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

evolution of the lithosphere beneath the michigan basin using depths to formation tops from 480 wells changes in shape of the michigan basin during the phanerozoic are determined flexural rigidity of the lithosphere is then estimated for several time intervals by assuming that changes in shape were caused by deflection of an elastic plate subjected to an axisymmetric load it is found that the flexural rigidity generally increased over time from less than 10 22 n m early in the basin s history to more than 10 23 n m late in the basin s development this increase in rigidity is attributed to cooling and thickening of the elastic portion of the lithosphere as the basin was subsiding low flexural rigidity early in the basin s history indicates that the lithosphere was probably rejuvenated heated thinned and possibly uplifted prior to basin subsidence the location of the load remained approximately fixed over time the rejuvenation process is investigated using a one dimensional finite difference thermal model in which a 142 km plate is temporarily heated from below taking the depth to the 450 c isotherm to be the effective elastic thickness of the lithosphere rigidity predicted by the thermal model is consistent with the observed increase in rigidity over time furthermore phase changes produced by heating during thermal rejuvenation combined with the predicted thermal contraction following rejuvenation explain the two episodes of subsidence observed in the michigan basin we conclude that the lithosphere in this area was rejuvenated approximately 500 m y ago as the lithosphere cooled it thickened it also contracted and subsided temperatures near the surface of the crust apparently were not raised sufficiently by lithospheric rejuvenation to alter the keweenawan and older radiometric and paleomagnetic dates of basement rocks

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

rhenium in indian rivers sources fluxes and contribution to oceanic budget 1 the abundance and distribution of dissolved and particulate rhenium re has been measured in several rivers draining the himalaya and peninsular india from their origin to outflow into the bay of bengal and the arabian sea the large data set resulting from this study on rivers flowing through a variety of lithologies e g the crystallines and sediments of the himalaya deccan basalts vindhyan sediments and the indian shield significantly enhances our understanding of the aqueous geochemistry of re and also constrains its sources to rivers and fluxes to the sea the concentration of dissolved re in rivers of the himalaya and the peninsular india shows wide range 1 4 to 72 7 pmol kg mean 7 8 pmol kg and 0 5 to 122 pmol kg mean 15 pmol kg respectively the discharge weighted average annual flux of dissolved re transported by the rivers from these regions are 5800 and 15 700 mol year respectively the major source of dissolved re as determined from inter element associations is black shales for the himalayan rivers and pyrites in basalts for the east flowing deccan rivers in addition there are evidences of considerable anthropogenic supply of re to some of the rivers that have very high re concentrations estimates of anthropogenic supply based on their re k ratios suggest that this source accounts for most of the re in the peninsular rivers particularly the godavari the annual flux of anthropogenic re transported by the peninsular rivers is 14 600 mol most of which is from the godavari this anthropogenic flux accounts for 70 of the total re supply by the indian rivers to the adjacent seas and 3 4 of the global riverine flux to the oceans the global average pre anthropogenic natural concentration of dissolved re in rivers is estimated to be 3 pmol kg based on re k correlation this value is much lower than the contemporary average determined from the measured concentrations and earlier estimate of natural re based on re so4 link

behavior of osmium at the freshwater saltwater interface based on ganga derived sediments from the estuarine zone 1 leaching experiments performed on several sediments of the ganga river suggest that between 10 and 20 of the osmium is in an easily exchangeable position analyses of sediments from two estuarine rivers the pussur and the lower meghna reveal no enrichment of os in the saltwater freshwater mixing zone relative to ganga sediments of similar al2o3 sio2 ratio suggesting that osmium is not significantly trapped in this estuary however a significant decrease of the 187os 188os ratio is observed in the pussur sediments relative to the ganga composition these latter are derived entirely from the ganga or from erosion of the ganga paleodelta and thus would be expected to have similar os isotopic compositions nd isotopic results from the pussur are indistinguishable from those of the ganga while the sr isotopic results are at the lower end of the ganga range confirming the absence of a major source difference between ganga and pussur sediments it thus seems unlikely that the difference in os isotopic signature can be entirely explained by a change in provenance suggesting instead that the os compositions have been modified our results show that the less radiogenic os compositions of the pussur sediments cannot result simply from desorption of radiogenic os or from scavenging of river or seawater os instead the decrease of the 187os 188os ratio could imply a complex exchange between dissolved os derived partly from seawater and os in the leachable fraction of sediments this mechanism could therefore constitute both a source and a sink for seawater osmium and may significantly influence the osmium marine budget

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

on the thermo mechanical evolution of continental lithosphere elastic thickness of continental lithosphere is modified by both mechanical thickening and thinning events and thermal evolution a simple thermo mechanical model can account for the processes of tectonic alteration and subsequent thermal reequilibration of lithospheric thermal structure mechanical thinning or thickening of the lithosphere directly affects its thickness and flexural rigidity after a tectonic event thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness previous tectonic history and the heat flux at the base of the lithosphere heat flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery rapid heat delivery to the base of the lithosphere retards lithospheric thickening whereas slow conductive heat delivery allows lithospheric thickening at the maximum rate the relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern very thin lithosphere undergoes rapid thermal evolution such that cooling between tectonic events significantly thickens the lithosphere for thick lithosphere conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elastic thickness take place between tectonic events model results with infinite stretching agree with observations of oceanic lithospheric thickness evolution showing a divergence from a cooling half space at about 70 m y model results also agree with independent estimates of continental thermal evolution in the absence of tectonic events on the longest time scales approaching the age of the earth thermal reequilibration is attained and any tectonic effects are overprinted the time scale at which the transition between mechanically and thermally dominated elastic thickness evolution occurs depends on the heat transport efficiency of the convecting mantle for high nusselt numbers thermal effects take longer to become important even for lower nusselt numbers mechanical effects still dominate on time scales of 108 years crustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness it also introduces a dependence of equilibrium thickness on tectonic crustal thinning or thickening model results indicate that on time scales of thrust sheet and sedimentary loading in foreland basins mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere the model can be applied to the lithosphere beneath foreland basins so that observed basin geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments

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

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

the paleogene record of himalayan erosion bengal basin bangladesh a knowledge of himalayan erosion history is critical to understanding crustal deformation processes and the proposed link between the orogen s erosion and changes in both global climate and ocean geochemistry the most commonly quoted age of india asia collision is 50 ma yet the record of paleogene himalayan erosion is scant either absent or of low age resolution we apply biostratigraphic petrographic geochemical isotopic and seismic techniques to paleogene rocks of the bengal basin bangladesh of previously disputed age and provenance our data show that the first major input of sands into the basin in the 1 km thick deltaic barail formation occurred at 38 ma our biostratigraphic and isotopic mineral ages date the barail formation as spanning late eocene to early miocene and the provenance data are consistent with its derivation from the himalaya but inconsistent with indian cratonic or burman margin sources detrital mineral lag times show that exhumation of the orogen was rapid by 38 ma the identification of sediments shed from the rapidly exhuming southern flanks of the eastern central himalaya at 38 ma provides a well dated accessible sediment record 17 myr older than the previously described 21 ma sediments in the foreland basin in nepal discovery of himalayan detritus in the bengal basin from 38 ma 1 resolves the puzzling discrepancy between the lack of erosional evidence for paleogene crustal thickening that is recorded in the hinterland 2 invalidates those previously proposed evidences of diachronous collision which were based on the tenet that himalayan derived sediments were deposited earlier in the west than the east 3 enables models of himalayan exhumation e g by mid crustal channel flow to be revised to reflect vigorous erosion and rapid exhumation by 38 ma and 4 provides evidence that rapid erosion in the himalaya was coincident with the marked rise in marine 87sr 86sr values since 40 ma whether 38 ma represents the actual initial onset of vigorous erosion from the southern flanks of the east central himalaya or whether older material was deposited elsewhere remains an open question

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

phase changes and thermal subsidence in intracontinental sedimentary basins summary a model of tectonic subsidence is developed to explain the late acceleration of subsidence observed in some intracratonic sedimentary basins the proposed mechanism combines the effect of thermal contraction of an initially hot lithosphere with the effect of a subcrustal phase transformation that moves under changing pressure and temperature conditions the subsidence following a sudden change in temperature at the base of the lithosphere is calculated the calculations show that 1 phase changes if present and activated contribute substantially to the subsidence of sedimentary basins 2 because the effect of phase change is delayed subsidence accelerates after a time on the order of 20 myr and 3 the duration of the subsidence is on the order of 100 to 150 myr during the late stages of subsidence the phase change is the dominating mechanism an application to the michigan basin is presented the calculated sediment accumulation history fits the record well when the effect of sea level changes is included in the model

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

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

δ13c of organic carbon in the bengal fan source evolution and transport of c3 and c4 plant carbon to marine sediments carbon isotopic measurements on organic carbon oc in sediment cores from the bengal fan odp leg 116 show a dramatic 10 increase beginning ca 7 ma ago and a rapid decrease after 0 9 ma these shifts reflect changes in the mixing ratio of terrigenous carbon derived from c3 and c4 plants the rapid increase in 13c of bengal fan oc at 7 ma shows that late miocene expansion of c4 plants already documented in the siwaliks was widespread over all the himalayan foreland after 7 ma relations of 13c with sedimentological parameters show that c4 plants dominate in the foreland whereas c3 plants remain abundant in the mountain range variations in the source of the sediments and of the oc appear to be sensitive to climate hydrologic conditions in the basin major changes in the isotopic composition of the carbon flux in one of the worlds largest river systems modify the isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass

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

behavior of re and os during low temperature alteration results from himalayan soils and altered black shales re os analyses were performed on five soil profiles developed on silicate lithologies in the narayani drainage basin of the himalayas as well as on several altered black shales and associated alteration phases the results indicate that all of these samples have lost large amounts of re this re loss approached 100 in the black shale derived soils which also most probably lost substantial quantities of os the near constancy of os and re concentrations with depth indicates that this loss occurred quite early in the alteration process rather than progressively during the course of soil development a slight decrease in 187os 188os was observed toward the surface of the two black shale soil profiles possibly indicating a minor preferential mobility of radiogenic 187os for this lithology in the black shale derived saprolite and soils the loss of re and os was related to the loss of organic carbon the importance of this phase is underscored by the fact that organic material separated from a black shale sample accounted for nearly all of the os contained in the whole rock analysis of alteration phases from the surface of a weathered black shale showed that os liberated during alteration was partially reprecipitated in fe oxides with os isotopic compositions very similar to that of the original rock whereas re was removed with the fluid phase these results demonstrate the disproportionately large role that black shale weathering may play in determining the re and os contents of continental runoff and thus ultimately of seawater but they also emphasize the possible complexity of this process the data suggest that a large fraction of the os contained in black shales is released during chemical alteration which is apparently not the case for most other silicate rocks therefore the potential contribution of black shales to the dissolved os budget of runoff and river water may be even greater than that expected from the high os concentrations of these rocks the extent to which this os will affect the seawater os composition depends on how efficiently it is recaptured by secondary phases such as fe mn oxides in situ or during transport these results also underscore the high mobility of re with respect to os at every stage in the erosional process thus explaining the order of magnitude increase in re os ratio from the black shale source rocks to seawater

isotopic tracing of the dissolved u fluxes of himalayan rivers implications for present and past u budgets of the ganges brahmaputra system u activity ratios have been measured in the dissolved loads of selected rivers from the himalayan range in central nepal and from the bangladesh as well as in some rain waters a few european and asian rivers have also been analyzed for their u activity ratios the data confirm the negligible effect of rainwater on the budget of dissolved u in river waters the results also indicate that rivers on each himalayan structural unit have homogeneous and specific u isotope compositions i 234u 238u activity ratios slightly lower than unity in the dissolved load of the streams draining the tethyan sedimentary series tss ii values slightly higher than unity for waters from the high himalaya crystalline hhc and the lesser himalaya lh iii systematically higher 234u 238u activity ratios for waters from the siwaliks thus u activity ratios in association with sr isotopic ratios can be used to trace the sources of dissolved fluxes carried by these rivers coupling of u with sr isotope data shows 1 that the u carried by the dissolved load of the himalayan rivers mainly originates from u rich lithologies of the tss in the northern formations of the tibetan plateau and 2 that the elemental u and sr fluxes carried by the himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the himalayan chain rivers flowing on the indian plain define a different trend from that of the himalayan rivers in the u sr isotopic diagram indicating the contribution of a specific floodplain component to the u and sr budgets of the ganges and the brahmaputra the influence of this component remains limited to 10 to 15 percent for the u flux but can contribute 35 to 55 of the sr flux the variations of the sr and u fluxes of the ganges brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high altitude terrains during glacial episodes this scenario would significantly decrease the dissolved u flux of the ganges brahmaputra river system and increase its u activity ratio such a climatic dependence of the himalayan u flux could induce a periodic variation of the mean u activity ratio of the world rivers on glacial interglacial timescales

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

radiometric dating of marine influenced coal using re os geochronology abstract coal deposits are integral to understanding the structural evolution and thermal history of sedimentary basins and correlating contemporeous estuarine and fluvial delatic strata with marine sections while marine shales may readily lend themselves to re os dating due to the dominance of hydrogenous re and os the lack of a chronometer for near shore sedimentary environments hampers basinwide correlations in absolute time here we employ the re os geochronometer along with total organic carbon toc and rock eval data to determine the timing and conditions of a marine incursion at the top of the matewan coal bed kanawha formation pottsville group west virginia usa the observed range for hydrogen index hi 267 290 mg hydrocarbon gram total organic carbon for these coal samples suggests dominance of aliphatic hydrocarbons with low carbon 107 6 16 4 ng g and os 0 52 0 09 ng g concentrations of the marine influenced matewan coal are higher by few orders of magnitude than published data for terrestrial coal a re os isochron for the matewan coal provides an age of 325 14 ma model 3 mswd 12 n 19 2 this is the first re os age derived from coal samples the age overlaps a new composite re os age of 317 2 ma for the immediately overlying betsie shale member external precision for replicate os analyses carried out for several matewan coal samples shows a positive correlation with their hi the hi which is low in terrestrial organic matter reflects the degree of marine influence thus samples with the most profound marine influence also have the best analytical reproducibility equilibration of os isotopes with seawater under marine conditions overwhelms variability inherited from terrestrial plant debris decreasing scatter on the isochron the 187re 188os ratios of the matewan coal 3300 5135 are higher than most of those previously published for phanerozoic black shale mostly

why terrestrial coals cannot be dated using the re os geochronometer evidence from the finnmark platform southern barents sea and the fire clay coal horizon central appalachian basin abstract coal horizons in terrestrial sedimentary sections are key to understanding the structural and thermal evolution of sedimentary basins precise and accurate age information for coal horizons can define correlations and the temporal evolution of sedimentary successions while the biostratigraphy for coal horizons yields relative ages tied to radiometric ages through the continuously refined geological time scale direct radiometric ages of coal horizons have been derived only from tonsteins lithified ash beds the re os geochronometer has been proven to record accurate and precise depositional ages for sedimentary rocks rich in organic matter however the reliability of the re os geochronometer has not yet been assessed for coals deposited in strictly terrestrial environments here in order to test the re os geochronometer we measure the re os concentrations and isotopic composition of terrestrial coals from two distinct environments the finnmark platform southern barents sea and the central appalachian basin cab samples from the finnmark platform soldogg formation also include an adjacent organic rich shale horizon the shale and two coal horizons were also analysed for total organic carbon toc and rock eval data results show that the soldogg coals and shale reached the oil generation widow tmax 448 2 c with type iii or mixed type ii iii organic matter supporting the model for deposition in overbank deposits in a fluvial setting in contrast the fire clay coals in the cab were deposited in a broad expanse of mires average re 0 5 0 4 ng g and os 23 26 pg g concentrations of terrestrial soldogg and fire clay coals are similar to those of terrestrial maghara coal and a few orders of magnitude lower than the marine influenced matewan coal elevated re os ratio os common os os corrected for ingrowth of 187os since deposition of the sample for the fire clay coals suggests highly reducing and or euxinic conditions prevailing in the water column of the extensive carboniferous peat forming mire in which these coals were deposited in contrast lower re os for soldogg coals highlight less reducing conditions likely in a short lived mire in a dynamic braided stream system re and os concentrations of terrestrial coals are on the order of re and os of plant materials and river sediments suggesting that debris from plant materials and weathered continental crust are significant contributors of re and os to terrestrial coals with some contribution from water column of the coal mire this study is the first attempt to apply the re os geochronometer on terrestrial coals in case of the soldogg coals and shale the 187re 188os vs 187os 188os relationship displays scatter with no discernible linear trend the scatter is ascribed to heterogeneity in the initial 187os 188os ratios osi and lack of post depositional isotopic equilibration during subsequent diagenesis for the fire clay coals 187os 188os and 187re 188os are well correlated linear regression of the data however does not yield a geologically reasonable age rather the linear relationship results from binary mixing between two end members the fire clay coal and tonstein suggesting that re os systematics of terrestrial coals depositing very close to tonstein beds can be significantly disturbed we conclude that the post depositional equilibration with a large re os reservoir as with a marine incursion is necessary to record a meaningful re os radiometric age for the coals

dissolved mo and u in rivers and estuaries of india implication to geochemistry of redox sensitive elements and their marine budgets abstract dissolved molybdenum mo and uranium u concentrations were measured in five indian estuaries the narmada tapi mandovi and the mahi fall into the arabian sea and the hooghly falling into the bay of bengal riverine mo and u vary significantly in the range of 1 to 90 and 0 02 to 19 nmol kg respectively the lowest mo and u were observed in the mandovi river consistent with lateritic lithology exposed in its drainage higher runoff and lower water rock interaction the sabarmati has the highest u 19 nmol kg and sources from the groundwater having very high u concentration the highest mo 90 nmol kg is observed in the mahi river and probably results from anthropogenic sources hooghly river seems to have high concentrations of mo 14 nmol kg and u 8 nmol kg and are possibly derived from weathering of black shales of the himalaya behaviour of mo and u in all the estuaries analysed in this study is highly variable both mo and u in the narmada estuary during pre monsoon and monsoon seasons and u in the tapi estuary behave conservatively mo in the tapi shows its addition in the mid salinity ranges 4 to 12 and is sourced from anthropogenic activity probably related to effluent from industries situated along the estuary in this salinity range both dissolved mo and u show their removal at lower salinity ranges in the hooghly and the mandovi estuaries these estuaries are net sink of mo in which significant amount of oceanic mo along with riverine mo is being lost about 1 6 10 6 and 2 10 5 mol of mo are being removed annually in the hooghly and the mandovi estuaries uranium removal is order of magnitude lower compared to mo in both the hooghly and the mandovi estuaries the loss of mo and u in the estuaries are associated with mangrove swamps present in the estuaries which seems to be an important sink of the oceanic mo and could represent its significant sink in oceanic budget of mo the gulf of cambay has high dissolved mo concentration compared to that of global average seawater value which is attributed to its supply from anthropogenic sources industrial effluent waste waters along with polluted rivers supply 5 10 6 mol of mo annually to the gulf of cambay which could be derived from petrochemicals and pharmaceutical industries situated along the gulf coast

constraining the potential temperature of the archaean mantle a review of the evidence from komatiites the maximum potential temperature of the archaean mantle is poorly known and is best constrained by the mgo contents of komatiitic liquids which are directly related to eruptive temperatures however most archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid relatively fresh lavas from the saskmar suite belingwe greenstone belt zimbabwe 2 7 ga include chills of 25 6 wt mgo and olivines ranging to fo93 6 implying eruption at around 1520 c a chill sample from alexo township ontario also 2 7 ga is 28 wt mgo and associated olivines range to fo94 1 implying eruption at 1560 c however inferences of erupted liquids containing 32 33 wt mgo from lavas in the barberton greenstone belt south africa 3 45 ga and from the perseverance complex western australia 2 7 ga may be challenged on the grounds that they contain excess cumulate olivine or were enriched in mg during alteration or metamorphism re interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt mgo corresponding to an eruption temperature of 1580 c this composition is the highest liquid mgo content of an erupted lava that can be supported with any confidence the hottest modern magma on gorgona island 0 155 ga contained 18 20 mgo and erupted at circa 1400 c if 1580 c is taken as the temperature of the most magnesian known eruption then the source mantle from which the liquids rose would have been at up to 2200 c at pressures of 18 gpa corresponding to a mantle potential temperature of 1900 c these temperatures are in excess of the mantle temperatures predicted by secular cooling models and thus komatiites can only be formed in hot rising convective jets in the mantle this result requires that archaean mantle jets may have been 300 c hotter than the archaean ambient mantle temperature this temperature difference is similar to the 200 300 c temperature difference between present day jets and ambient mantle temperatures an important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions

origin and epeirogenic history of the williston basin evidence from fission track analysis of apatite we have tested the hypothesis that the lithosphere beneath the williston basin was heated uplifted and eroded prior to basin subsidence this was accomplished by fission track dating of apatite from samples of basement rock beneath and adjacent to the basin apatite fission track ages of basement rocks beneath the basin range from about 4 ma near the center to 554 ma near the eastern edge combined with stratigraphic evidence the ages suggest that at least 3 km of basement were eroded prior to subsidence of the williston basin rates of apparent uplift and erosion were on the order of 0 10 to 0 15 km m y apatite fission track ages of basement outcrop samples on the eastern flank of the basin range from about 572 ma to 441 ma these ages suggest that erosion of the flank occurred from the early cambrian to the late ordovician we conclude that timing of uplift of the basement prior to subsidence of the williston basin is consistent with the thermal origin hypothesis of basin formation

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

osmium isotope geochemistry in the mackenzie river basin the cenozoic marine osmium isotope record is largely driven by changes in the continental input we aid its interpretation by supplying direct measurements of present day riverine os in known geological and environmental settings we analyzed os concentrations and isotopic ratios in the dissolved suspended and bed materials of the mackenzie river in canada and smaller rivers draining the western cordillera into the pacific ocean rhenium content was measured for suspended and bed materials to provide further constraint dissolved 187os 188os ratios range from 0 74 to 4 5 and are controlled to first order by age and lithology of the bedrock the rivers draining jurassic volcanic rocks are unradiogenic as low as 0 74 and those draining the precambrian canadian shield are radiogenic as high as 3 5 the headwaters of the mackenzie left bank tributaries draining metamorphic carbonates in the eastern cordillera and paleozoic black and gray shales in the transition zone and the interior platform are especially radiogenic 3 0 4 5 with relatively high concentrations of os these latter rivers are responsible for the mackenzie being above world average in os concentration and yield mol os km2 year as well as in 187os 188os the dissolved load is more radiogenic than the suspended or bed loads and constitutes a significant fraction of total fluvial os

the geochemical cycle of rhenium a reconnaissance rhenium re is one of a suite of elements including uranium and molybdenum that display conservative behavior in seawater and are enriched in anoxic sediments the decay of187re to187os provides a geochronometer in ancient sedimentary rocks and gives rise to os isotopic variations in nature in order to better characterize its sources to seawater re was measured in three major rivers amazon orinoco ganges brahmaputra and some of their tributaries re concentrations span four orders of magnitude from 0 02 to 400 pmol kg with the highest concentrations found in rivers draining black shales in the venezuelan andes mainstream re levels in the three rivers are between 1 and 10 pmol kg with a flux weighted average of 2 3 pmol kg the residence time for re in the oceans is estimated to be 750 000 yr with respect to river inputs re profiles from the atlantic and pacific oceans confirm that re behaves conservatively in seawater with no significant uptake onto particles and or recycling within the water column this is also true in the anoxic water column of the black sea re removal into anoxic sediments occurs at or below the sediment water interface as demonstrated in sediment pore waters from chesapeake bay in oxic sediments re is not cycled with manganese oxides and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component or in manganese nodules burial of re in anoxic sediments which accumulate on 0 3 of the ocean floor removes approximately 50 of the riverine re flux to the oceans hence oceanic re concentrations may be very sensitive to changes in the area of anoxic sedimentation

the os and sr isotopic record of himalayan paleorivers himalayan tectonics and influence on ocean chemistry abstract weathering products in himalayan paleosols provide a unique archive of changes in 187 os 188 os and 87 sr 86 sr records of himalayan rivers in the neogene and can provide important constraints on the potential role of himalayan weathering in controlling seawater isotopic compositions covariation in the sr and os isotopic records of the paleosols from the indus and ganges river systems indicates that these elements are derived from the same source terranes the os isotopic ratios from the indus paleosols are less radiogenic than that of the paleo oceans until 5 ma whereas os in the ganges system paleosols is significantly more radiogenic than seawater ratios throughout the last 20 myr os isotopic ratios in the paleosols have increased by over 100 over the last 11 myr in both the indus and ganges river systems variations in both the os and sr records of paleosols from the ganges river system can be explained utilizing the tectonic history of the central himalayas the paleosol record provides important new constraints on the potential role of himalayan weathering in controlling seawater sr and os if the himalayan rivers systems are responsible for the observed changes in the marine sr and os records then large changes in the fluxes of both these elements must be invoked to accommodate the paleosol record future models of marine isotopic variation need to account for potentially large and rapid changes in the riverine isotopic ratios over time