upb dating of fe rich phases using a sequential leaching method a sequential leaching method was developed for upb dating of fe rich phases which uses a series of successively stronger acids the method was applied to date fe rich phases of magmatic origin in the kinsman quartz monzonite new hampshire usa and of sedimentary origin in the dales gorge member of the brockman iron formation hamersley basin australia using the composition of major element released by the acid leaching and combining it with x ray and sem analyses of the residues we were able to identify the relatively pure phases released and use these samples for dating magnetite separates from the kinsman quartz monzonite yielded upb age of 372 34 ma which is consistent with smnd and a rbsr isochron ages determined in previous studies leached whole rock samples of the brockman iron formation yielded a 207pb206pb age of 2213 250 ma which agrees with previously determined pbpb smnd and rbsr isochron ages this age is interpreted as the time of redistribution of u and pb associated with the fe rich phases during regional metamorphism while the potential of the sequential leaching method is demonstrated more work is needed in order to improve the precision of the data this requires further investigation of the nature of the pure phases

mineralogical evidence for eolian contribution to soils of late quaternary moraines wind river mountains wyoming usa abstract volcanic heavy minerals are found in the a and b horizons of soils on moraines of the western wind river mountains of wyoming volcanic minerals do not occur in the underlying moraine sediments derived from plutonic and metamorphic bedrock sources of the volcanics are the sedimentary rocks of the green river basin the presence of volcanic minerals in the green river basin and their patterns of distribution in the moraine soils indicate eolian sedimentation is an important factor of soil genesis in this region moraines affected by eolian sediments include the pinedale and temple lake indian basin type localities the presence of eolian sediments in both pinedale and holocene deposits suggests eolian addition is not simply a late glacial phenomenon but continues throughout postglacial time relations between eolian sediments and soil development must be accounted for when comparing soil development on moraine chronosequences in this and possibly all alpine regions of western north america

osmium in the rivers there is a large uncertainty in our understanding of the behavior of osmium during weathering and transport into deep oceans and the osmium budget of the oceans the problem stems chiefly from the lack of osmium data on the dissolved load in the rivers and in the estuaries in this study the concentration and isotopic composition of osmium have been determined in three north american rivers the mississippi the columbia and the connecticut and one river draining central europe and flowing into the baltic sea the vistula osmium concentration in the mississippi and the vistula is about 45 femto mol kg 1 it is about 14 and 15 femto mol kg 1 for the connecticut and the columbia respectively the 187 os 186 os ratios estimated for the mississippi and the vistula are 10 4 and 10 7 respectively for the connecticut and the columbia 187 os 186 os 8 8 and 14 4 respectively of all the rivers examined the mississippi is by far the largest supplying 1 6 of the total annual world river flow its osmium isotopic composition is identical to the upper mississippi valley loesses esser and turekian 1993a indicating 1 congruent dissolution of the bedrock and 2 little or no impact of anthropogenic sources on the osmium isotopic composition of the dissolved load the latter observation indicates that the upper limit of the anthropogenic input in the dissolved osmium load of the mississippi outflow is about 250 g yr 1 while the osmium concentration of the vistula is high the isotopic composition does not appear to have been affected by substantial pollution the river data can be used to put limits on the mean residence time of osmium in the oceans t os and on the osmium budget of the oceans if the bulk river influx of dissolved osmium into the oceans is similar to that of the mississippi we get a value for the net riverine inflow of osmium of 1680 mol yr 1 if there were no sequestering of osmium in the estuaries this would give a value of t os of 1 3 10 4 years as the estuaries may trap a significant amount of river osmium this is a lower limit so that t os 1 3 x 10 4 years using the iridium data by anbar et al 1996 the os ir ratio for the vistula is 2 8 we infer that there is no large fractionation between osmium and iridium during the transport of these elements from the continents into the oceans it follows that the high os ir ratio 22 observed in seawater must be related to the different rain out mechanisms of these elements in the estuaries or in the oceans

relation between soil age and silicate weathering rates determined from the chemical evolution of a glacial chronosequence long term weathering rates rlt were determined from base cation depletion in soil profiles developed on six granitic glacial moraines varying in age from 0 4 to 297 ka in the wind river mountains wyoming weathering rates were found to decrease with soil age t according to the power law equation rlt 15 t 0 71 where rlt is in meq m 2 yr 1 and t is in ka comparable data from the literature are consistent with this function and it is similar to power law equations describing changes in laboratory weathering rates with time on the basis of our study we predict that for a catchment unaffected by anthropogenic inputs and in a steady state with respect to the organic and cation exchange pools rlt should be higher than present day weathering rates derived from stream fluxes rpd by a factor of about 3 4

changing tectonic controls on the long term carbon cycle from mesozoic to present tectonic drivers of degassing and weathering processes are key long term controls on atmospheric co2 however there is considerable debate over the changing relative importance of different carbon sources and sinks existing geochemical models have tended to rely on indirect methods to derive tectonic drivers such as inversion of the seawater 87sr 86sr curve to estimate uplift or continental basalt area here we use improving geologic data to update the representation of tectonic drivers in the copse biogeochemical model the resulting model distinguishes co2 sinks from terrestrial granite weathering total basalt weathering and seafloor alteration it also distinguishes co2 sources from subduction zone metamorphism and from igneous intrusions we reconstruct terrestrial basaltic area from data on the extent of large igneous provinces and use their volume to estimate their contribution to degassing we adopt a recently published reconstruction of subduction related degassing and relate seafloor weathering to ocean crust creation rate revised degassing alone tends to produce unrealistically high co2 but this is counteracted by the inclusion of seafloor alteration and global basalt weathering producing a good overall fit to mesozoic cenozoic proxy co2 estimates and a good fit to 87sr 86sr data the model predicts that seafloor alteration and terrestrial weathering made similar contributions to co2 removal through the triassic and jurassic after which terrestrial weathering increased and seafloor weathering declined we predict that basalts made a greater contribution to silicate weathering than granites through the mesozoic before the contribution of basalt weathering declined over the cenozoic due to decreasing global basaltic area

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

ancient impactor components preserved and reworked in martian regolith breccia northwest africa 7034 abstract northwest africa nwa 7034 and paired stones represent unique samples of martian polymict regolith breccia multiple breccia subsamples characterized in this work confirm highly siderophile element hse re os ir ru pt pd contents that are consistently elevated e g os 9 3 18 4 ppb above indigenous martian igneous rocks mostly 187 os 188 os ratios of 0 119 0 136 record a wider variation than observed for all major chondrite types combined with the measured 187 re 188 os ratios of 0 154 0 994 the range in os isotope ratios indicates redistribution of re and os from originally chondritic components early in the history of the regolith commencing at 4 4 ga superimposed recent re mobility reflects exposure and weathering at or near the martian and terrestrial surfaces elevated os concentrations 38 0 and 92 6 ppb os superchondritic os hse ratios and 187 os 188 os of 0 1171 and 0 1197 measured for two subsamples of the breccia suggest the redistribution of impactor material at 1 5 1 9 ga possibly overlapping with a partial resetting event at 1 4 ga recorded by u pb isotope systematics in the breccia martian alteration of the originally chondritic hse host phases to form os ir rich nuggets and ni rich pyrite implies the influence of potentially impact driven hydrothermal systems multiple generations of impactor component admixture redistribution and alteration mark the formation and evolution of the martian regolith clasts and matrix of nwa 7034 and paired meteorites from the pre noachian until impact ejection to earth

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

effects of black shale weathering on the mobility of rhenium and platinum group elements radiogenic os from the continents dominates the late cenozoic marine os budget a variety of proposed radiogenic sources includes organic rich sediments precambrian shields and re rich sulfides identifying the dominant sources of radiogenic os to the oceans has a strong impact on past climate models because weathering of silicates acts as a net sink of co 2 whereas weathering of organic matter acts as a source here we investigate the effects of black shale weathering on the integrity of the re os isotope system and release of re and platinum group elements time correlative pairs of fresh drill core and weathered outcrop black shales of late ordovician age utica shale magnafacies quebec were analyzed for re os ir pt and pd as well as total s n and c as c carb and c org the results indicate that 25 64 of the initial re budget 45 90 of the os budget 14 65 of the ir budget 60 77 of the pt budget 69 86 of the pd budget and 44 96 of the c org budget are lost during weathering we use positive correlations of os pt and pd with c org and n to calculate the respective platinum group element concentrations in the unweathered organic matter end member as 3 ng g os 9 ng g pt and 11 24 ng g pd although black shales make up

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

reconstructing the evolution of the submarine monterey canyon system from os nd and pb isotopes in hydrogenetic fe mn crusts the sources of terrestrial material delivered to the california margin over the past 7 myr were assessed using 187os 188os nd and pb isotopes in hydrogenetic ferromanganese crusts from three seamounts along the central and southern california margin from 6 8 to 4 5 0 5 ma all three isotope systems show more radiogenic values at davidson seamount located near the base of the monterey canyon system than in fe mn crusts from the more remote taney and hoss seamounts at the taney seamounts approximately 225 km farther offshore from davidson seamount 187os 188os values but not pb and nd isotope ratios also deviate from the cenozoic seawater curve towards more radiogenic values from 6 8 to 4 5 0 5 ma however none of the isotope systems in fe mn crusts deviate from seawater at hoss seamount located approximately 450 km to the south the regional gradients in isotope ratios indicate that substantial input of dissolved and particulate terrestrial material into the monterey canyon system is responsible for the local deviations in the seawater nd pb and os isotope compositions from 6 8 to 4 5 0 5 ma the isotope ratios recorded in fe mn crusts are consistent with a southern sierra nevada or western basin and range provenance of the terrestrial material which was delivered by rivers to the canyon the exhumation of the modern monterey canyon must have begun between 10 and 6 8 0 5 ma as indicated by our data the age of incised strata and paleo location of the monterey canyon relative to the paleo coastline

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

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

weathering versus circulation controlled changes in radiogenic isotope tracer composition of the labrador sea and north atlantic deep water geological reconstructions and general circulation models suggest that the onset of both northern hemisphere glaciation 2 7 myr ago and convection of labrador sea water lsw were caused by the closure of the panama gateway 4 5 myr ago time series data that have been obtained from studies of ferromanganese crusts from the northwestern atlantic suggest that radiogenic isotopes of intermediate ocean residence time pb and nd can serve as suitable tracers to reconstruct these events however it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering in this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in lsw but can be explained in terms of increases of erosion levels due to the glaciation of greenland and canada furthermore we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic pb and nonradiogenic nd from continental detritus eroding into the labrador sea this can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic pb should provide such a rich supply of radiogenic pb to the oceans an important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources perhaps more importantly the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater

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

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

u pb ages for late magmatism and regional deformation in the shebandowan belt superior province canada five precise u pb zircon and titanite ages from different lithologic units of the shebandowan greenstone belt in the western wawa subprovince of the superior province put tight constraints on the time of late archean magmatism and of two major deformation events a porphyry sill from the older supracrustal sequence has an age of 2733 3 ma another porphyritic rock a trondhjemite occurring as a clast in a conglomerate of the unconformably overlying timiskaming type supracrustal sequence formed 2704 2 ma ago and defines a maximum age for the deposition of the timiskaming type sequence an alkalic volcanic rock from this sequence has been directly dated at in accord with the above constraint and with another probable maximum age of deposition given by the date of 2696 2 ma for the shebandowan lake pluton a first deformation event d1 was related to a predominantly vertical tectonic regime and occurred during or before intrusion of the shebandowan lake pluton at 2696 2 ma the second deformatio

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

reos isotopes and major and trace element geochemistry of carbonaceous shales aravalli supergroup india impact of post depositional processes abstract this study presents re os isotopes and major and trace elemental geochemistry of carbonaceous black shales and a weathering profile developed on them from the paleoproterozoic deposits of the jhamarkotra formation lower aravalli group india these measurements provide information on the impact of post depositional processes particularly hydrothermal fluid interactions on the 187re 187os geochronology and element mobility in shales in contrast to previously reported compositions for black shales the re concentrations of the jhamarkotra shales are in general lower than their corresponding os concentrations the 187os 188os ratios of the black shales vary widely from 3 5 to 10 5 without any discernible systematic trend with 187re 188os range 0 4 to 17 7 excluding an outlier these 187re 188os ratios are lower by a few orders of magnitude than those generally reported for fresh black shales these observations indicate that the jhamarkotra shales though they seem visually well preserved have undergone post depositional modification resulting in open system behavior of re and os with re more mobile than os higher sio2 tio2 and k2o na2o ratios of the jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal possibly si and k rich fluids the sio2 tio2 ratios an index for hydrothermal alteration show negative correlation with re corg and positive correlation with corg n ratios 42 154 indicating loss of re and nitrogen during hydrothermal fluid interaction earlier studies have shown the presence of re porphyrins n containing organic compounds in black shales thus the observed co variations of sio2 tio2 with re corg and corg n ratios suggest mobilization of re from the porphyrins in the shales further positive correlation between sio2 tio2 and 187os 188os ratios indicates that significant amounts of radiogenic os component were re distributed throughout the jhamarkotra shales during the post depositional processes in the weathering profile developed on black shales the al2o3 normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering on the contrary the al2o3 normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile suggesting limited mobility over the depth of the profile the depth profiles of re al2o3 and 188os al2o3 and 187os 188os ratios in the weathering profile by and large reflect compositions of their presumed parent shales which have gone through significant post depositional changes

the behavior of natural and anthropogenic osmium in long island sound an urban estuary in the eastern u s abstract the os concentration and 187 os 186 os distributions in surface sediments of long island sound eastern u s provide a way of determining the sources and estuarine transport of os the contribution of anthropogenic os from sewer outfalls from the new york city region supplies a tracer with a characteristic 187 os 186 os of about 1 the os concentration of the bulk surface sediment increases steeply moving toward new york city in the westernmost sound and generally follows the concentration of organic carbon the 187 os 186 os ratio of bulk surface sediment increases from west to east in the westernmost part of the sound and is effectively constant in the central sound we interpret these results as indicating that the surface bulk sediments of the sound contain a low 187 os 186 os component perhaps as a reduced coating associated with organic remains from sewer outfalls the acid hydrogen peroxide leach fraction has an average 187 os 186 os of 9 5 in the central sound significantly higher than both the bulk sediment value and the probable sea water value of about 8 the leach fraction in the westernmost part of the traverse is less radiogenic than the central sound and follows the os wsotope trend of the bulk sediment liquid effluent from a new york city sewer outfall contains 30 pg l 1 of dissolved os with a 187 os 186 os of about 2 5 consistent with its being an end member of the west east sediment pattern recorded in the leach fractions of the westernmost cores the leachable os from the central sound predominantly reflects os in ferromanganese oxyhydroxide coatings from continentally derived sediments with 187 os 186 os ratios more radiogenic than seawater the distribution patterns of anthropogenic and natural os with their characteristic isotopic signatures in the sound and the insights gained from the behavior of other particle reactive species indicates that very little os in solution may pass through the estuarine gauntlet

osmium isotopes and highly siderophile element fractionation in the massif type anorthosites of the mesoproterozoic kunene intrusive complex nw namibia abstract the behaviour of highly siderophile elements during differentiation of mafic melts and formation of cumulate rocks in the crust is little understood mostly because abundances of some of these elements are very low and reliable data is scarce data on massif type anorthosites and associated fe ti ore from the mesoproterozoic kunene intrusive complex nw namibia indicate the effects of fractional crystallisation crustal contamination cumulate formation and post cumulus re equilibration on the distribution of the highly siderophile elements and the osmium isotopic composition of the anorthositic cumulate rocks anorthosites and fe ti ore of the kunene intrusive complex are characterised by low concentrations and strongly fractionated chondrite normalised patterns of the highly siderophile elements the abundance of highly siderophile elements is dominated by fe ti oxides and traces of sulphides the osmium isotopic composition and highly siderophile element distribution of the anorthosites are consistent with their derivation from a mantle derived parental magma that was subject to fractionation of mafic minerals and variable extent of crustal contamination during its igneous evolution under conditions of the anorthosite formation re behaves as a compatible element in fractionating titanomagnetite the other highly siderophile elements are incorporated to a lesser extent into fe ti oxides during post cumulus re equilibration of fe ti oxides re is preferentially incorporated in magnetite whereas pt displays preference for ilmenite

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

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

glacial weathering and the hafnium isotope composition of seawater the hf isotope composition of seawater does not match that expected from dissolution of bulk continental crust this mismatch is generally considered to be due to retention of unradiogenic hf in resistant zircons during incomplete weathering of continental crust during periods of intense glacial weathering zircons should break down more efficiently resulting in the release of highly unradiogenic hf to the oceans we test this hypothesis by comparing nd and hf isotope time series obtained from nw atlantic ferromanganese crusts both isotope systems show a decrease associated with the onset of northern hemisphere glaciation the observed changes display distinct trajectories in nd hf space which differ from previously reported arrays of bulk terrestrial material and seawater such patterns are consistent with the release of highly unradiogenic hf from very old zircons facilitated by enhanced mechanical weathering

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

rhenium osmium isotope systematics of an early mesoproterozoic sedex polymetallic pyrite deposit in the north china craton implications for geological significance and the marine osmium isotopic record abstract the rhenium osmium re os isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the os composition of ancient seawater in particular the age of sedimentary exhalative sedex deposits can be determined by measuring re os isotopes in syn depositional pyrite a common constituent of sedex deposits here we report on the gaobanhe sediment hosted polymetallic pyrite deposit which is hosted within the gaoyuzhuang formation in the central part of the northern margin of the north china craton ncc re os isotopic data from syn depositional pyrite yield an isochron age of 1505 55 ma constraining the timing of primary mineralization as well as the minimum age of the host sediments to the mesoproterozoic analyzed pyrite grains indicate an initial 187os 188os osi value of 0 181 0 066 suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids with the os being primarily derived from seawater the osi value is slightly higher than that of paleoproterozoic seawater 0 13 suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased continental radiogenic os input to the oceans we propose that the re os isotopic system as recorded by these sedex deposits may provide key insights when examining changes in the os isotopic composition of seawater throughout the precambrian

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

osmium isotope systematics of ureilites abstract the 187 os 188 os for 22 ureilite whole rock samples including monomict augite bearing and polymict lithologies were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body or bodies the re os ratios of most ureilites show evidence for a recent disturbance probably related to re mobility during weathering and no meaningful chronological information can be extracted from the present data set the ureilite 187 os 188 os ratios span a range from 0 11739 to 0 13018 with an average of 0 1258 0 0023 1 similar to typical carbonaceous chondrites and distinct from ordinary or enstatite chondrites the similar mean of 187 os 188 os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites from the narrow range of the 187 os 188 os distribution in ureilite meteorites it is further concluded that re was not significantly fractionated from os during planetary differentiation and was not lost along with the missing ureilitic melt component the lack of large re os fractionations requires that re os partitioning was controlled by a metal phase and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body

climate correlated variations in seawater 187 os 188 os over the past 200 000 yr evidence from the cariaco basin venezuela abstract a high resolution record of seawater osmium isotopic composition 35 samples over the past 191 000 yr has been obtained from sediments from ocean drilling program odp leg 165 site 1002 in the cariaco basin venezuela strong correlations are observed between 187os 188os and both the local and global oxygen isotopic records with less radiogenic values observed during glacial intervals in conjunction with previous analyses of sediments from the east pacific rise the santa barbara basin and the japan sea the data demonstrate conclusively that there have been ocean wide changes in seawater 187os 188os over the pleistocene global seawater excursions to relatively unradiogenic 187os 188os indicating decreased continental inputs occur during cold intervals times of increased aridity and increased ice cover of old radiogenic continental masses the data thus indicate a direct climatic control on chemical weathering rates and or composition of weathering products on a glacial interglacial timescale

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

variations in seawater osmium isotope composition since the last glacial maximum a case study from the japan sea abstract concentrations of re and os and the isotopic composition of os have been measured in the japan sea sediments to assess the response of the japan sea to glacial interglacial climate change and associated weathering fluxes the osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg g and 187os 188os from 0 935 to 1 042 only 187os 188os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of re and os and with 80 hydrogenous os are explained in terms of seawater composition lower 187os 188os were observed for sediments deposited during the last glacial maximum lgm when planktonic foraminifera from the japan sea recorded lighter oxygen isotopic composition decrease in dissolved os fluxes from continents and or change in the composition of the dissolved load to the japan sea are suggested as the driving mechanisms for the observed lower lgm 187os 188os the results of this study coupled with lower 187os 188os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates suggest that this trend is characteristic of the global oceans data from this study show that the japan sea recorded higher 187os 188os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values this is explained in terms of preferential release of 187os during deglacial weathering and or higher continental os flux driven by warm and wet climate this study demonstrates that os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition in addition 187os 188os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi enclosed basins such as the japan sea

depositional age of the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa and implications for low level re os geochronology and paleoproterozoic global correlations abstract we present new re os isotope data from the early paleoproterozoic klipputs member nelani formation ghaap group transvaal supergroup south africa to further evaluate low level re os geochronology of organic rich sedimentary rocks and improve global correlations of paleoproterozoic sedimentary rocks the klipputs member contains re 0 3 0 8 ppb total os 34 177 ppt and common os 192 os 10 66 ppt abundances that overlap upper crustal abundances of 0 2 2 ppb 30 50 ppt and 12 21 ppt respectively five samples yield a model 1 re os date of 2479 20 ma 22 ma if the 187 re decay constant uncertainty is included 2 mean square of weighted deviates mswd 0 01 using the cr vi h 2 so 4 digestion protocol dissolution of the same sample powders using inverse aqua regia yields a model 3 re os date of 2862 190 ma 2 mswd 148 comparison of re os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital os whereas the cr vi h 2 so 4 medium did not the re os date of 2479 22 ma for the klipputs member is consistent with published geochronological ages from underlying and overlying stratigraphic units suggesting the cr vi h 2 so 4 protocol yields accurate re os depositional ages from ors containing very low levels of hydrogenous re and hydrogenous radiogenic os hence there is potential for widespread application of low level re os ors geochronology including sedimentary successions deposited before the great oxidation event the initial 187 os 188 os 0 12 0 01 from the klipputs member cr vi h 2 so 4 regression indicates that dissolved os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and or cosmic dust rather than oxidative weathering of the radiogenic upper crust in light of the re os depositional age for the klipputs member we propose that the stratigraphic units in the upper hamersley group above the brockman iron formation western australia are younger than the ghaap group diamictite from the doradale formation in the ghaap group is constrained to be older than 2479 22 ma and thus may not be correlative with glacial deposits assigned to the oldest paleoproterozoic glaciation

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

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

an osmium isotope excursion associated with the late paleocene thermal maximum evidence of intensified chemical weathering in the latest paleocene an abrupt shift to more negative 13c values has been documented at numerous marine and terrestrial sites bralower et al 1997 cramer et al 1999 kaiho et al 1996 kennett and stott 1991 koch et al 1992 stott et al 1996 thomas and shackleton 1996 zachos et al 1993 this carbon isotope event cie is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4 6 c kennett and stott 1991 and more moderate warming in the subtropics thomas et al 1999 here we report 187os 188os isotope records from the north atlantic and indian oceans which demonstrate a 10 increase in the 187os 188os ratio of seawater coincident with the late paleocene cie this excursion to higher 187os 188os ratios is consistent with a global increase in weathering rates the inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate which acts to stabilize earth s climate walker et al 1981 estimates of the duration of late paleocene cie bains et al 1999 bralower et al 1997 norris and rohl 1999 rohl et al 2000 in conjunction with the os isotope data imply that intensified chemical weathering in response to warm humid climates can occur on timescales of 104 105 years this interpretation requires that the late paleocene thermal maximum os isotope excursion be produced mainly by increased os flux to the ocean rather than a transient excursion to higher 187os 188os ratios in river runoff although we argue that the former is more likely than the latter we cannot rule out significant changes in the 187os 188os ratio of rivers

isotopic studies of the late archean plutonic rocks of the wind river range wyoming isotopic studies of the rb sr and u th pb systems in whole rock samples and the u pb systematics for zircons document the existence of two late arehean intrusive events in the wind river range all of the systems examined indicate an age of 2 630 20 m y for the louis lake batholith apparent ages for the bears ears pluton range from 2 504 40 m y to 2 575 50 m y the scatter in apparent ages for the bears ears pluton does not appear to be primarily the result of disturbance by postintrusive events but it may be explained by an isotopically inhomogenous magma at the time of intrusion data for a few samples indicate that the wind river range was affected locally by a postmagmatic hydrothermal event that was approximately tertiary in age this event lowered 18 o values and disturbed parent daughter relationships in most of the isotopic systems investigated but it was recent enough that there is no demonstrable effect in the pb pb system the bears ears pluton has some chemical and petrologic features that are similar to those reported for the granites in the granite mountains to the east these granites are spatially associated with low temperature uranium deposits of tertiary age and have been shown to have lost large amounts of uranium during the early to middle tertiary u pb systematics indicate however that the low to moderate uranium contents and highly variable th u values noted for the bears ears pluton are best interpreted as being primary features if uranium was lost after magma generation the loss most likely occurred at the time of intrusion such a loss could account for uraniferous precambrian pegmatites southwest of the main part of the range the two intrusive units apparently were derived from different protoliths that were formed during early to middle archean initial isotopic ratios and petrochemistry for the louis lake batholith are consistent with an early archean trondhjemitic to tonalitic source the protolith for the bears ears pluton must have been more evolved and somewhat younger inconsistencies as to the degree of evolution of this protolith as inferred from isotopic and trace element data suggest that the protolith may have been subjected to high grade meta morphism that caused loss of rb and u prior to generation of the magma

precise cosmogenic 10be measurements in western north america support for a global younger dryas cooling event the inner titcomb lakes moraine in the wind river mountains wyoming has been dated with a precision of 5 using accelerator mass spectrometric measurements of cosmogenic 10be in surface samples from nine boulders resting on its crest the age of this moraine is 11 4 0 5 to 13 8 0 6 ka the range in age is due to the uncertainty in the 10be production rate and uncertainties in rock erosion rate and shielding of cosmic rays by snow additional exposure ages on bedrock and boulder surfaces that are stratigraphically younger and older than the inner titcomb lakes moraine conform with this age the exposure ages of the inner titcomb lakes moraine overlap the north atlantic younger dryas event 12 8 0 2 to 11 5 0 3 ka

beryllium 10 dating of the duration and retreat of the last pinedale glacial sequence accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes cosmogenic beryllium 10 measurements in the wind river range indicate that the last glacial maximum marine oxygen isotope stage 2 was achieved there by 21 700 700 beryllium 10 years and lasted 5900 years ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12 100 500 beryllium 10 years

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

simulating the mid pleistocene transition through regolith removal abstract quaternary 18o ice volume proxy records show a transition from high frequency small amplitude glacial cycles to low frequency large amplitude glacial cycles this reorganization of the climate system termed the mid pleistocene transition mpt is thought to reflect a change in land ice response to orbital forcing despite no significant change in orbital cycles during this period one potential explanation for the mpt proposes that gradual erosion of high latitude northern hemisphere regolith by multiple cycles of glaciation caused a transition in ice sheet response to external forcing here we explore this regolith hypothesis using a complex earth system model we show that simulating a transition from deformable sediment to crystalline bedrock produces an evolution in ice volume response similar to proxy reconstructions of the mpt the simulated change in ice volume response is due to a combination of climate and ice flow changes with crystalline bedrock producing thicker colder ice sheets that accumulate more snowfall and have a smaller ablation zone further experiments that include transient eccentricity amplifying co2 forcing show only small differences in ice response compared to those with orbital forcing only suggesting that cycles of co2 were not the primary cause of the mpt

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

continental runoff of osmium into the baltic sea the isotopic composition of osmium os in continental surface waters and shallow ground waters has been determined by analyzing fresh water fe mn nodules and bog ores the isotopic composition of the continental runoff from the precambrian baltic shield is much more radiogenic 187 os 186 os 31 2 65 4 than the runoff from the phanerozoic sedimentary platform to the south of the baltic sea 187 os 186 os 8 6 11 0 fe mn nodules from the baltic sea have intermediate values 187 os 186 os 12 4 21 2 indicating mixture of north sea water and continental fresh water isotope mass balance calculations indicate that the concentration of os in the continental runoff is similar to the os concentration in seawater extrapolating these data to infer the global runoff yields 48 kg yr comparable with previous estimates the residence time of os in seawater relative to continental runoff is calculated at 32 ka the oceanic residence time of os relative to the bulk input to seawater is 16 ka if the mean 187 os 186 os of the continental runoff is 16 the os isotopic composition of the continental runoff is much more variable than previously suggested 187 os 186 os values 30 being characteristic of precambrian shields glaciation of old cratonic shields might therefore modulate riverine supply of this very radiogenic cratonic os linking accelerated increase in the marine os isotope record to glaciations of precambrian shields

climate driven glacial interglacial variations in the osmium isotope composition of seawater recorded by planktic foraminifera abstract this study presents osmium os isotope and elemental data for cleaned planktic foraminifera authigenic fe mn oxyhydroxides and pelagic carbonate host sediments from odp site 758 in the southernmost reaches of the bay of bengal the os in the bulk sediments appears to be dominantly hydrogeneous sourced by carbonate and fe mn oxyhydroxide but variations in this particular core are controlled by the presence of volcanic ash fe mn oxyhydroxide leachates of the bulk sediments from holocene samples also yield an os isotope composition close to that of seawater but the record diverges from that of foraminifera at a depth corresponding to the oxic post oxic boundary suggesting diagenetic mobilization of os at depths below this holocene planktic foraminifera cleaned using oxidative reductive techniques also give os isotope compositions indistinguishable from modern seawater but the record obtained for the past 150 kyr shows strong covaraitions of 187 os 188 os with both the local and global oxygen isotope record with less radiogenic os isotope compositions during glacial intervals these results indicate that foraminifera provide a robust record of seawater os isotope compositions and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187 os 188 os over this time interval while the covariation with oxygen isotopes suggest a process controlling the os isotope composition that is in phase with global climate cycles global excursions to relatively unradiogenic 187 os 188 os during glacial intervals are consistent with decreased input of radiogenic continental material reflecting cooler temperatures and reduced continental runoff modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an 30 decrease in the global river flux with an 5 change in river composition if the residence time of os in the oceans is 5 ka then the post glacial recovery to present day seawater values is consistent with a corresponding increase in the river flux of around 30 however if the residence time of os is closer to 40 ka as is suggested by the global river flux then this demands either significant changes in both the riverine os flux and composition of around 40 and 30 respectively that closely follow the oxygen isotope record or else a short lived post glacial pulse of weathering some 75 greater than the steady state flux in either case these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial interglacial timescales

pge re os and mo isotope systematics in archean and early proterozoic sedimentary systems as proxies for redox conditions of the early earth abstract re os data and pge concentrations as well as mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the barberton greenstone belt south africa fig tree and moodies groups 3 25 3 15 ga the belingwe greenstone belt zimbabwe manjeri formation ca 2 7 ga and shales from the witwatersrand ventersdorp and transvaal supergroups south africa ranging from 2 95 to 2 2 ga moderately oxidizing conditions are required to mobilize re and mo in the environment mo fractionation only occurs in solution and these parameters thus have potential use as paleoredox proxies for the early earth pge re abundance patterns of barberton greenstone belt sediments are uniform and very similar in shape to those of komatiites this indicates 1 that the pge came from a source of predominantly ultramafic composition and 2 that pge were transported and deposited essentially in particulate form sediments from the younger belingwe greenstone belt show more fractionated pge re patterns and have re os ratios 10 to 100 higher than those of barberton sediments their pge abundance patterns and re os ratios are intermediate between those of the mid archean shales and neoproterozoic to recent black shales they reflect scavenging of re from solution in the sedimentary environment 98 95 mo values of black shales of all ages correlate with their concentrations the barberton greenstone belt samples have 1 3 ppm mo similar to a granitoid basaltic source this mo has 98 95 mo between 1 9 and 2 4 relative to present day mean ocean water molybdenum momo and is thus not isotopically fractionated relative to such a source similar to the pge this indicates transport in solid form sediments from the belingwe greenstone belt show in part enhanced mo concentrations up to 6 ppm and mo isotope fractionation 98 95 mo up to 1 4 relative to momo the combined pge re and mo data show mainly reducing conditions in the mid archean and suggest that by 2 7 ga the atmosphere and oceans had become more oxidizing substantially younger samples from the transvaal supergroup to ca 2 2 ga surprisingly have mainly low mo concentrations around 1 ppm and show no significant mo isotope fractionation relative to the continental source among possible explanations for this are a return to reducing atmospheric conditions after 2 7 ga reservoir effects or mo removal by sulfide precipitation following sulfate reduction in early proterozoic oceans

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

lithostratigraphic correlation of blaini formation late proterozoic lesser himalaya india with other late proterozoic tillite sequences the blaini formation of the lesser himalaya has long been considered permo carboniferous in age on simple lithological correlation with gondwana tillites in southern india rare and non repeatable finds of fossils from the type area were thought to confirm this recently lower cambrian faunas have been recorded from the overlying tal and krol formations and possible metazoan trails recorded from the blaini itself in addition the blaini formation is even on a lithostratigraphic comparison much more like other late proterozoic glacial sequences the term blaini formation should only be used for rocks of the type area not for widely scattered glacial deposits which may or may not be correlatable with the type section the persistent belief in the paleozoic age of the blaini and adjacent formations has for long retarded unterstanding of lesser himalaya stratigraphy and tectonics

geochemical constraints on the regolith hypothesis for the middle pleistocene transition the transition from 41 to 100 kyr glacial cycles and concomitant increase in global ice volume 1 ma remain an enigmatic feature of late cenozoic climate here we examine the petrology mineralogy and geochemistry of the silicate fraction of tills spanning the past 2 ma from the north central united states to evaluate the hypothesis that this so called middle pleistocene transition mpt occurred by erosion of regolith and subsequent exposure of underlying canadian shield bedrock by the laurentide ice sheet these data indicate that late pliocene tills are depleted in crystalline lithologies unstable minerals and major element oxides derived from plagioclase and ferromagnesians and are enriched in kaolinite quartz iron oxides tio2bearing resistates and meteoric 10 be in contrast early and middle pleistocene tills show enrichment in crystalline lithologies stable minerals and major oxides derived from plagioclase and ferromagnesians and depletion in meteoric 10 be whereas late pleistocene tills show major element concentrations that are most similar to that of fresh shield bedrock marine isotope records of sr os and hf show significant changes around the mpt that are consistent with the removal of a regolith and the exhumation of fresh silicate bedrock these results indicate that ice sheets initially expanded on highly weathered bedrock and progressively exhumed a fresher rock source thereby supporting the hypothesis that a change in the composition of the substrate underlying ice sheets best explains the origin of the mpt d 2004 elsevier b v all rights reserved

investigation of an early pleistocene marine osmium isotope record from the eastern equatorial pacific osmium isotope composition 187os 188os and concentrations of os ir and pt are reported for an early pleistocene section from the odp site 849 in the eastern equatorial pacific using the data obtained in this study the contributions from detrital and extraterrestrial particulate matter to os concentration and 187os 188os of sediment are estimated our calculations show that detrital contributions to sedimentary os are too small 2 to significantly shift measured bulk sediment 187os 188os away from seawater values a moderate but significant negative correlation between 187os 188os and 3he 188os indicate that the average particulate extraterrestrial os flux to this site is 1 21 0 47 pg cm 2 kyr 1 which constitutes 3 of total os burial flux the estimates of detrital and extraterrestrial os are used to calculate the seawater 187os 188os in the early pleistocene the most notable features of this early pleistocene 187os 188os record are 1 glacial interglacial 187os 188os differences are insignificant within errors of estimates 2 glacial 187os 188os values are higher compared to those reported for the late pleistocene glacials comparison of 187os 188os values at site 849 to the late pleistocene records suggests that average seawater 187os 188os change has been modest 5 since the early pleistocene assuming that 187os 188os difference between the glacial periods of the late and the early pleistocene results solely from temperature dependence of weathering rates it has been calculated that average surface temperature during the late pleistocene glacials was 0 8 0 2 c lower than glacials in the early pleistocene this inference is consistent with temperature estimates based on a recent study of pco2 reconstruction in the pleistocene this observation based on limited studies of marine 187os 188os records seems to suggest that temperature played an important role in influencing chemical weathering during the pleistocene glacials however more studies are needed to confirm if this temperature weathering feedback was operational throughout the pleistocene a significant down core ir 3he co variation coupled with similar burial fluxes of ir at site 849 and at ll44 gpc 3 in the north pacific point to the utility of ir concentration as a point paleoflux tracer however a twofold difference in ir burial fluxes between the eastern and the western equatorial pacific suggests that calibration in space and time is required to use ir concentration as a robust indicator of paleoflux through time significant co variation of concentrations of os and total alkenone during the glacials coupled with lighter 13c of benthic foraminifera indicates that productivity and carbon burial played a dominant control on scavenging of os at site 849 in a broader context this data set encourages future investigation of response of pge behavior to paleoceanographic processes

dissolved rhenium in the yamuna river system and the ganga in the himalaya role of black shale weathering on the budgets of re os and u in rivers and co 2 in the atmosphere extensive measurements of dissolved re and major ion abundances in the yamuna river system yrs a major tributary of the ganga have been performed along its entire stretch in the himalaya from its source near the yamunotri glacier to its outflow at the foothills of the himalaya at saharanpur in addition re analysis has been made in granites and precambrian carbonates some of the major lithologies of the drainage basin these data coupled with those available for black shales in the lesser himalaya allow an assessment of these lithologies contributions to the re budget of the yrs the re concentrations in the yrs range from 0 5 to 35 7 pm with a mean of 9 4 pm a factor of 4 higher than that reported for its global average concentration in rivers dissolved re and cations na k ca mg are strongly correlated in the yrs indicating that they are released to these waters in roughly the same proportion throughout their course the re cations in most of these rivers are one to two orders of magnitude higher than the re na k mg ca measured in granites of the yamuna basin this leads to the conclusion that on average granites crystallines make only minor contributions to the dissolved re budget of the yrs on a basin wide scale though they may be important for rivers with low dissolved re similarly precambrian carbonates of the lesser himalaya do not seem to be a major contributor to dissolved re in these rivers as their re ca mg is much less than those in the rivers the observation that re concentrations in rivers flowing through black shales and in groundwaters percolating through phosphorite black shale carbonate layers in phosphorite mines are high and that re and so4 are significantly correlated in yrs seems to suggest that the bulk of the dissolved re is derived from black shale carbonaceous sediments material balance considerations based on average re of 30 ng g 1 in black shales from the lesser himalaya require that its abundance in the drainage basin of the yrs needs to be a few percent to yield average re of 9 4 pm furthermore the positive correlation between re and cations would require that these re rich sediments e g black shales and re poor lithologies e g crystallines precambrian carbonates contribute re and cations in roughly the same proportion throughout the drainage basin the available data on the abundance and distribution of black shales in the basin are not adequate to test if these requirements can be met the annual fluxes of dissolved re at the base of the himalaya from the yamuna are 150 mol at batamandi and 100 mol at saharanpur compared to 120 mol from the ganga at rishikesh the total flux from the yamuna and the ganga account for 0 4 of the global riverine re flux much higher than their contribution to global water discharge this is also borne out from the mobilization rate of re 1 to 3 g km 2 y 1 in the ganga and yamuna basins in the himalaya compared to the global average of 0 1 g km 2 y 1 black shale weathering can also significantly influence the budgets of os and u in rivers and co2 in rivers and the atmosphere using dissolved re in rivers as a proxy it is estimated that 6 9 108 kg y 1 of black shales are being weathered in the ganga and yamuna basins in the himalaya weathering of such amounts of black shales can account for the reported concentrations of os and u in these rivers furthermore if the weathering results in the conversion of organic carbon in the black shales to co2 it would release 2 105 mol of co2 km 2 y 1 in the yamuna and ganga basins in the himalaya comparable to the co2 consumption from silicate weathering

chemical signals for oxidative weathering predict re os isochroneity in black shales east greenland abstract re and os isotopes in shales are increasingly used for precise and accurate geochronology and for tracking rates of continental weathering in the geologic past moreover trace element compositions of organic rich shales reveal paleoredox conditions and properties of shale organic matter om useful for lithostratigraphic correlations chemical weathering alters the original shale composition and hence may compromise geologic interpretations based on geochemical data here we compare re os isotopes trace elements and om properties in correlative shales from outcrops and nearby drillcore to quantify the effects of weathering we define chemical parameters that distinguish weathered from pristine shale in samples that are macroscopically identical black shale from the ravnefjeld formation in east greenland yields disturbed re os isotope systematics for outcrop samples and highly precise re os isochron ages for drillcore geochemical data show that re os and common os are hosted in the organic matter rather than sulfides re and os both show mobility during weathering combined use of rock eval pyrolysis indices and sulfur contents documents oxidation of both om and pyrite thereby providing a valuable chemical criterion to assess invisible shale weathering further studies on chemically diverse shales will test the widespread application of these criteria as weathering indicators systematic differences in major and trace element content and kerogen quality between weathered and fresh shale samples are used to characterize and quantify weathering effects the character of the om in shales is most telling ravnefjeld data are compared with data from time correlative shales from the mid norwegian shelf georgiev et al 2011 hot acidic late permian seas stifle life in record time earth planet sci lett 310 389 400 to present a strategy for identification of re os isochroneity in advance of re os analytical work our recent compilation and comparison of upper permian shales from widely separated sequences on pangea s margins reveal exceptionally high re os ratios on a global scale we suggest upper permian shales may have significantly influenced the cenozoic e g eocene 187os 188os ratio of seawater on their exposure and weathering

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

preservation and exhumation history of the harizha halongxiuma mining area in the east kunlun range northeastern tibetan plateau china abstract the extent to which ore bodies are preserved in orogenic belts remains a poorly understood area of ore deposit research using zircon and apatite fission track analysis together with apatite u th he dating we constrained the erosion history of the ore bodies in the harizha halongxiuma mining area of the east kunlun range northern tibetan plateau china apatite fission track ages range from 114 8 to 87 6 ma with mean track lengths varying from 11 4 1 9 to 12 9 2 0 m zircon fission track ages vary from 205 14 to 142 7 ma in addition apatite u th he dating yielded ages of 60 56 ma the thermal history of jiapigou was modelled based on the apatite fission track data including ages and track lengths with constraints of zircon fission track ages and u th he ages the exhumation history of the harizha halongxiuma mining area was reconstructed with these age data revealing that since the early mesozoic the area has undergone three cooling stages 1 rapid cooling from 175 30 ma to 100 10 ma with a cooling rate and inferred exhumation of 2 0 0 8 c myr and 4 3 1 7 km respectively 2 a relatively stable stage from 100 10 ma to 40 10 ma with a cooling rate and inferred exhumation of 0 3 0 1 c myr and 0 5 0 2 km respectively and 3 rapid cooling since 40 10 ma with a cooling rate and inferred exhumation of 1 2 0 6 c myr and 1 4 0 4 km respectively this exhumation history is consistent with the subduction process of pacific plate and the strike slip movements of dunmi fault the total exhumation after main mineralization is calculated to be 7 6 3 2 km suggesting that ore bodies in the harizha halongxiuma mining area remain partially preserved

the late eocene 187os 188os excursion chemostratigraphy cosmic dust flux and the early oligocene glaciation abstract high resolution records ca 100 kyr of os isotope composition 187os 188os in bulk sediments from two tropical pacific sites odp sites 1218 and 1219 capture the complete late eocene 187os 188os excursion and confirm that the late eocene 187os 188os minimum earlier reported by ravizza and peucker ehrenbrink earth planet sci lett 210 2003 151 165 is a global feature using the astronomically tuned age models available for these sites it is suggested that the late eocene 187os 188os minimum can be placed at 34 5 0 1 ma in the marine records in addition two other distinct features of the 187os 188os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca 0 1 myr we propose a speculative hypothesis that higher cosmic dust flux in the late eocene may have contributed to global cooling and early oligocene glaciation oi 1 by supplying bio essential trace elements to the oceans and thereby resulting in higher ocean productivity enhanced burial of organic carbon and draw down of atmospheric co2 to determine if the hypothesis that enhanced cosmic dust flux in the late eocene was a cause for the 187os 188os excursion can be tested by using the paired bulk sediment and leachate os isotope composition 187os 188os were also measured in sediment leachates results of analyses of leachates are inconsistent between the south atlantic and the pacific sites and therefore do not yield a robust test of this hypothesis comparison of 187os 188os records with high resolution benthic foraminiferal 18o records across the eocene oligocene transition suggests that 187os flux to the oceans decreased during cooling and ice growth leading to the oi 1 glaciation whereas subsequent decay of ice sheets and deglacial weathering drove seawater 187os 188os to higher values although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187os 188os records are obscured by recovery from the late eocene 187os 188os excursion evidence of the global influence of glaciation on supply of os to the ocean is robust as it has now been documented in both pacific and atlantic records

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

the importance of organic rich shales to the geochemical cycles of rhenium and osmium abstract the role that marine organic rich sediments play in the surficial osmium and rhenium cycles has been assumed to be important due to the high re os ratios and high concentrations of these elements in such reducing sediments compared to the average continental crust the high re os leads to rapidly increasing 187os 188os ratios we determine how large a role these sediments play in the surficial osmium and rhenium budgets by compiling a geochemical database to quantify the mean 187re 188os and 187os 188os ratios of organic rich sediments we then utilize estimates of the abundance of organic rich sediments in the geologic record of different ages to estimate an average 187os 188os for these sediments we find that the calculated 187os 188os value of organic rich sediments varies between 2 22 and 2 27 depending on the volume age model used with error estimates of 2 07 to 2 42 and 2 12 to 2 48 respectively despite the fact that these sediments constitute a relatively small proportion of the upper continental crust they contribute a disproportionately large amount 50 to the upper continental crustal rhenium and to a lesser extent 5 10 osmium inventories owing to their high concentrations of these elements

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

simple models for the geochemical response of the ocean to climatic and tectonic forcing abstract simple models for the geochemical response of the ocean to periodic forcing are used as a framework to discuss the shortest periods of change resolvable by present analytical precision of elemental or isotopic ratio measurements of the chemical evolution of seawater for chemical properties whose budget is dominated by fluxes into and out of the ocean we show that the geochemical response to changing fluxes is strongly attenuated at periods shorter than the residence time and that the response at these periods is also phase shifted so as to lag the forcing by one quarter cycle the amplitude response as a function of period is used to estimate the periods of forcing resolvable by present analytical precision in measuring the 87sr 86sr and 234u 238u evolution of seawater and we find that the shortest periods resolvable are of the order of 1 myr if variations at periods shorter than this are found we would argue that these are very likely of local as opposed to global origin we illustrate the importance of phase shifts as a function of forcing period by discussing some recently published data that show a high degree of correlation over the last 300 kyr between changes in 87sr 86sr and 18o measured in the same sediment core at first sight this correlation might be taken as good evidence that both 87sr 86sr and 18o are responding to some common climatic forcing but given the long residence time of sr in the ocean sr and o should in fact have different phase shifts relative to any proposed common forcing thus the apparent correlation between 87sr 86sr and 18o over the past several 100 ma is not necessarily an indication that they are responding to a common cause amplitude response curves are also used to place limits on permissible global seawater 234u 238u variations and these are then compared with published data to identify local diagenetic effects application to 187os 186os and u records in seawater detectors is proposed

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

radiogenic isotope records of quaternary glaciations changes in the erosional source and weathering processes variations of global and regional silicate weathering rates and paleo ocean circulation patterns are estimated by using radiogenic isotope records but the effects of changes in provenance are generally ignored here sediment provenance has been constrained through the use of ar ar ages for individual detrital minerals from the labrador sea which can be compared directly to the radiogenic isotope compositions from the same core material dramatic changes in the radiogenic isotope composition of north atlantic deep water through the quaternary period are shown to reflect discrete changes in both sources and weathering processes accompanying northern hemisphere glaciation changes in the different radiogenic isotope systems reflect the influence of source physical weathering and chemical weathering and not simply changes in the underlying weathering rate or ocean circulation patterns that are typically inferred

the glacial interglacial variation of seawater osmium isotopes as recorded in santa barbara basin abstract the pleistocene deposits of santa barbara basin sbb show laminations during interglacial periods typical of an anoxic or suboxic depositional environment and more sporadically laminated deposits during glacial times indicating a more oxic environment of deposition the 187os 188os value of sediment leaches varies as a function of climate consistent with a more radiogenic os signal during interglacial times and less radiogenic os values more typical of glacial times the signal is confounded by contributions from a high os detrital component with a variable 187os 188os value but generally agrees with prior result from deposits from other marine environments

correlation of sturtian diamictite successions in southern australia and northwestern tasmania by re os black shale geochronology and the ambiguity of sturtian type diamictite cap carbonate pairs as chronostratigraphic marker horizons recent geochronological studies have raised concerns regarding the accuracy of regional and global correlations of neoproterozoic sedimentary successions using relative dating methods such as carbon isotope chemostratigraphy and glaciogenic diamictite cap carbonate lithological and geochemical characteristics precise and accurate radio isotopic age constraints for neoproterozoic sedimentary rocks thus remain a high priority for establishing a reliable neoproterozoic chronostratigraphy here we present a new re os age of 640 7 4 7 ma from black shales of the upper black river dolomite togari group northwestern tasmania that represents a minimum age for deposition of the immediately underlying diamictite bearing julius river member the upper black river dolomite re os age is statistically identical to a re os age of 643 0 2 4 ma previously determined for the sturtian post glacial tindelpina shale member umberatana group adelaide rift complex southern australia consistent with previous lithostratigraphic and stromatolite biostratigraphic evidence the re os ages suggest broadly coeval deposition of the julius river member and sturtian diamictites and overlying organic rich marine shales consideration of the neoproterozoic geochronological database suggests that sturtian type diamictite cap carbonate pairs cannot be used as chronostratigraphic marker horizons for global correlation of the neoproterozoic rock record

variations in the osmium isotope composition of sea water over the past 200 000 years the first osmium isotope data are presented for marine sediments spanning the past 200 000 years two full glacial cycles and establish that there have been ocean wide variations in sea water osmium isotope composition over this time while the osmium isotope composition of sea water has been relatively invariant over much of this interval 187os 186os of 8 5 to 8 6 excursions to lower isotope ratios 187os 186os 8 1 occur late in the last two glacial periods at 20 ka and 160 ka these are times of high global aridity low global temperature and maximum ice extent while various interpretations are possible a likely explanation is that the low sea water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity there is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation the response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12 000 years